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ADMIN: I.W
PVC TECHNOLOGY Fourth Edition
PVC TECHNOLOGY Fourth Edition W. V. TITOW M. Phil., Ph.D., C.Chem., F.R.S.C., F.P.R.I., C. Text., A. T.!. Formerly of the Yarsley Research Laboratories Ltd, Ashtead, Surrey, England
ELSEVIER APPLIED SCIENCE PUBLISHERS LONDON and NEW YORK
ELSEVIER APPLIED SCIENCE PUBLISHERS LTD Ripple Road, Barking, Essex, England Sole Distributor in the USA and Canada ELSEVIER SCIENCE PUBLISHING CO., INC. 52 Vanderbilt Avenue, New York, NY 10017, USA
First edition Second edition Reprinted Third edition Fourth edition Reprinted
1962 1966 1967 1971 1984 1986
British Library Cataloguing in Publication Data PVC technology. -4th ed. 1. Polyvinyl chloride I. Titow, W. V. 668.4'236 TP1180.V48
ISBN-13: 978-94-010-8976-0 e-ISBN-13: 978-94-009-5614-8 DOl: 10.1007/978-94-009-5614-8 WITH 171 TABLES AND 230 ILLUSTRATIONS
©
ELSEVIER APPLIED SCIENCE PUBLISHERS LTD 1984
Softcover reprint of the hardcover 1st edition 1984
Special regulations for readers in the USA This publicatiQ1l .has been registered with the Copyright Clearance Center Inc, (G;~c), Salem, Massachusetts. Information can be obtained from the CCC 'about conditions under which photocopies of parts of this publiCatio~ may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. The selection and presentation of material and the opinions expressed in this publication are the sole responsibility of the authors concerned All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher.
Preface to the Fourth Edition
This book continues the tradition of the first two editions of the late W. S. Penn's original PVC Technology, and the extensively revised third (1971) edition prepared by myself and B. J. Lanham. In the present edition the original general format, and the arrangement of chapters, have been largely preserved, but virtually nothing now remains of Penn's own text: a part of the contents is based on material from the 1971 TitowlLanham version (revised, updated and mainly rewritten): the rest is new, including, inter alia, several chapters specially contributed by experts from the plastics industry in the UK and Europe. The section listing international (ISO) and national (BS, ASTM and DIN) standards relevant to PVC, which was first introduced (as Appendix 1) in the 1971 edition, proved a popular feature: it has now been brought up to date and considerably extended. Two further appendices provide, respectively, comprehensive unit conversion" tables (with additional information on some of the most frequently encountered units, and the SI units), and a list of many properties of interest in PVC materials, with definitions, typical numerical values, and references~to relevant standard test methods. For various reasons, work on this edition involved more than the usual quota of problems: I am truly grateful to the Publisher's Managing Editor, Mr G. B. Olley, for his understanding, patience, unfailing courtesy and friendly encouragement. I am also most appreciative of the helpful attitude of all other members of the Publisher's staff who were concerned with the various aspects of processing the manuscript and bringing the book out. If my own contribution to the book has any merit, then I would like to dedicate it-respectfully and affectionately-to all my friends of the Yarsley Laboratories with whom I was priviledged to share many happy years, participating in the worthwhile work of a good team. W.V.T. v
Acknowledgements
I am much indebted to Messrs W. B. Duncker, F. J. Olivier and D. J. Sieberhagen of Vynide Ltd for their most helpful comments on the draft of Chapter 18 and for the trouble they took-individually and severally-to provide the drawing for Fig. 18.3, data for Table 18.1, and a few items of information on certain practical aspects of calendering. I am also grateful to Mr J. M. Hofmeyr of Union Carbide for the information he kindly supplied on the Ucar range of copolymer resins, and for his permission to use it in Chapter 24. It is a pleasure to record my thanks to Mr R. Coates of AECII Chlor-Alkali and Plastics Ltd for a most useful discussion of the scripts of Chapters 2 and 3, for items of information I have used in Tables 2.5 and 2.6, and for arranging his Company's permission-which I very much appreciate-to reproduce from their technical literature the contents of Tables 3.4-3.7. For the illustrations contained in the Plates my thanks are due to the companies and/or individuals identified in each caption, who kindly provided the original photographs. A small number of graphs and drawings, and one table (Table 14.1), are straight reproductions from other publications: the copyright holders' and authors' permissions to use these items-which are mentioned in each individual case-are much appreciated. A few definitions and sets of numerical data have been directly quoted (with sources clearly identified) from ISO, British and ASTM Standards. Such material from ISO specifications is reproduced by permission of the British Standards Institution granted on behalf of the International Organisation for Standardisation. The extracts from Britvii
viii
Acknowledgements
ish Standards are reproduced by permission of the British Standards Institution, 2 Park Street, London W1A 2BS, from whom complete copies of the standards concerned can be obtained. The material from ASTM Standards is copyright the American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103, and is reprinted with permission. I am particularly grateful to Mrs Rene Chizlett-whose invaluable secretarial contribution to the previous edition was greatly missed with the present one-for timely help with last-minute verification of several items of information on suppliers of commercial PVC materials. Mrs Connie von Gernet typed most of the manuscript-it is a pleasure to acknowledge her professional assistance. I am also most appreciative of Mrs Micky Kruger's secretarial help with two of the chapters and urgent correspondence. No aid on the technical side could be more important than the support and patience of my wife, Margaret Ley-Titow, during the long period, not lacking in stress, when the book was being put together. For all she has done she has my truly appreciative thanks. W.V.T.
Contents
Preface
v
Acknowledgements
vii
List of Contributing Authors
xxix
Chapter 1 Introduction-W. V. TITOW . . . . . . . . . . . . . 1.1 PVC: General Terminology and Relevant Definitions 1.2 Early History and Development of PVC . . . . . 1.3 General Statistics 1.4 Outline of the PVC Sector of the Plastics Industry 1.5 Vinyl CWoride Polymers and Copolymers . . . . 1.5.1 PVC Homopolymers: chemical structure; morphology 1.5.2 Vinyl CWoride Copolymers . . . . . . . 1.5.3 'External' Modification of PVC by Other Polym-
1 1 4 10 12 13 13 19
1.5.4 Properties of PVC Compositions 1.6 CWorinated Polyvinyl Chloride (CPVC) 1.7 Material and Test Standards References . . . . . . . . . . . . . . .
21 24 24 29 30
Chapter 2 Commercial PVC Polymers-W. V. TITOW
37
ers
ix
x
Contents
2.1 Introduction-Production and Main Types . 2.2 Polymer Characteristics Cardinal to Behaviour in Processing and/or Service Performance . 2.2.1 Composition . . . . . . . . . . . . . . 2.2.2 Molecular Weight (Viscosity Number and K Value) . 2.2.3 Polymer Particle Characteristics: particle size and size distribution; particle shape and morphology . 2.2.4 Purity . . . . . . . . . . . . . . . . 2.3 Characterisation and Designation of Commercial PVC Polymers . 2.4 Examples of Basic Properties of Commercial Polymers as Used for Some Major Applications . . 2.5 Commercial Sources of PVC Polymers References. . . . . . . . . . . . . . . . Chapter 3 Commercial PVC Compounds-W. V. TITOW 3.1 Introduction . 3.2 Commercial Sources of PVC Compounds 3.3 Types and Applications of Commercial PVC Compounds . 3.4 Properties and Designation of Commercial PVC Compounds . 3.4.1 Designation . 3.4.2 Properties Used in Characterisation of PVC Compounds ' . 3.4.3 Some Typical Properties of Commercial PVC Compounds References Chapter 4 Elementary Principles of PVC Formnlation-W. V. TITOW 4.1 Introduction . . . . . . . . . . . . . . . . . . . 4.2 The Components, and Basic Types, of a PVC Formulation . 4.3 Formulation Costing-Basic Points . . . . . . . . . . 4.4 Main General Considerations in the Selection of Principal Formulation Components . . . . . . . . . . . . . .
37
41 42 43
46 48 49 55 55 57
59 59 60
61 63 63
65 65
78
79 79
81 83 85
Contents
4.4.1 Nature and Characteristics of Individual Components of a Formulation: PVC polymer; heat stabilisers; plasticisers; lubricants; polymeric modifiers; fillers; colourants 4.4.2 Interactions and Mutual Effects of Formulation Components: compatibility effects; synergism; other mutual effects 4.4.3 Side Effects of Formulation Components: 'secondary functionality' effects; undesirable sideeffects 4.5 Some Special End-use Requirements 4.5.1 Food-contact Applications 4.5.2 Resistance to Weathering 4.5.3 Electrical Insulation 4.6 Examples of Basic Formulations 4.6.1 Film and Sheeting 4.6.2 Calendered Plasticised Vinyl/Asbestos Flooring 4.6.3 4.6.4 4.6.5 4.6.6 4.6.7 4.6.8 4.6.9
(Tile~
Pipe and Tubing Cable Covering and Insulation Gramophone Records Blow-moulded Bottles Injection Mouldings Extruded Profile Paste Formulations
Chapter 5 Theoretical Aspects of PlasticisatiOD'--D. L. BUSZARD
5.1 5.2 5.3 5.4 5.5 5.6
General Introduction . . . . . . . . . Definition of Plasticisers and Plasticisation Chemical Nature of Plasticisers Theories of Plasticisation Stages of Plasticiser Interaction with PVC Polymer Requirements for PVC Plasticisers 5.6.1 Compatibility and Miscibility: the IL value; solubility parameter 8; clear point temperature; Flory-Huggins interaction parameter x; Ap/Po ratio; loop or roll compatibility tests; maximum torque temperature
xi
86 103 105 106 106 107 107 107 107 109 110 111 112 112 113 114 114
117 117 117 119 120 122 124
125
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Contents
5.6.2 Effectivity of Plasticisers 5.6.3 Permanence of Plasticisers 5.7 General Relationships between the Structure of Plasticisers and their Behaviour in PVC 5.8 Ageing of Plasticised PVC 5.9 Antiplasticisation References . . . . . . . . .
132 134 136 138 142 143
Chapter 6 Commercial Plasticisers-D. L. BUSZARD 6.1 Introduction . . . . . . . . . . . . 6.2 Classification of Commercially Available Plasticisers 6.3 Group Characteristics of Major Plasticiser Gasses 6.4 Synonyms and Abbreviations 6.5 Group 1 Plasticisers-Phthalates . 6.5.1 Lower Phthalates . . . . 6.5.2 General-purpose Phthalates 6.5.3 Linear Phthalates 6.5.4 Higher Phthalates 6.5.5 Miscellaneous Phthalates 6.6 Group 2 Plasticisers-Phosphates 6.6.1 Triaryl Phosphates . . . 6.6.2 Trialkyl Phosphates 6.6.3 Mixed Alkyl Aryl Phosphates 6.6.4 Halogenated Alkyl Phosphates 6.7 Group 3 Plasticisers-Trimellitates . 6.8 Group 4 Plasticisers-Aliphatic Diesters 6.9 Group 5 Plasticisers-Polymeric Plasticisers 6.10 Group 6 Plasticisers-Miscellaneous Plasticisers 6.10.1 Epoxy Plasticisers 6.10.2 Chlorinated Paraffins 6.10.3 Monoesters . . . . 6.10.4 Glycol Esters 6.10.5 Hydrocarbon Extenders 6.10.6 Other Miscellaneous Plasticisers 6.11 Storage and Handling of Plasticisers 6.12 Plasticiser Manufacturers References . . . . . . . . . . . . . .
147 147 147 148 148 152 152 153 153 156 158 159 159 160 161 163 163 163 165 170 170 171 173 173 174 174 175 180 180
Conren~
Chapter 7 Properties of Plasticised PVC-D. L. BUSZARD 7.1 Introduction . . . . . . . . . . . . . . 7.2 Formulation of a Plasticised PVC Compound 7.2.1 The 'Desirability Function' 7.2.2 Computer-assisted Formulating 7.3 Softness and Tensile Properties ..... 7.3.1 Effect of Plasticiser 7.3.2 Compounding at Equal Efficiency 7.4 Low-temperature Properties 7.5 Permanence Properties 7.5.1 Extraction Resistance 7.5.2 Migration Resistance 7.5.3 Volatile Loss 7.5.4 Automotive Fogging 7.5.5 High-humidity Compatibility 7.6 Flame-retardant Properties 7.7 Electrical Properties 7.8 Weathering and Light Stability . . . 7.9 Resistance to Microbiological Attack 7.10 Resistance to Insect and Rodent Attack 7.11 Stain Resistance . . . . . . . . . . 7.12 Toxicity and Health Aspects of Plasticisers 7.12.1 Plasticisers for Food-contact Application 7.12.2 Health and Safety References . . . . . . . . . . . . . . . . . . . Chapter 8 FiDers in PVC-I. D. HOUNSHAM and W. V. TITOW
8.1 Introduction . . . . . . . . . . . . . . . 8.2 Mineral Fillers . . . . . . . . . . . . . . 8.2.1 Silicates and Silicas: asbestos; talc; clay 8.2.2 Alkaline-earth Metal Sulphates 8.2.3 Calcium Carbonates 8.3 Calcium Carbonate Fillers-Nature, Properties and Applications . . . . . . . . . . . . . . . . . . . . . 8.3.1 General Types: whiting; ground limestone, marble and calcite; ground dolomite; precipitated calcium carbonates
xili
181 181 181 183 183 184 185 185 192 195 196 199 200 202 204 204 206 206 208 209 209 210 210 211 212 215 215 216 216 219 221 224 224
xiv
Contents
8.3.2 Surface Treatments: stearate treatments; organotitanate treatments; proprietary and miscellaneous treatments 8.3.3 Filler Properties and Selection Criteria: maximum particle size; particle size distribution and mean particle size; colour (dry brightness); refractive index; oil (or plasticiser) absorption; dispersion characteristics; cost. 8.3.4 Applications, and Effects of Filler Loading: flooring; plasticised compounds; rigid PVC 8.4 Functional Fillers 8.4.1 Reinforcing Fillers: asbestos (chrysotile) fibres; inorganic microfibres; glass fibres; carbon fibres; glass spheres; fine-particle calcium carbonate 8.4.2 Flame-retardant and Smoke-suppressant Fillers 8.4.3 Miscellaneous Functional Fillers: carbon black; metal powders; wood flour; starch; synthetic silicas 8.5 Some Filler Suppliers and Trade Names References . . . . . . . . . . . . . . .
225
228 232 240 240 247 248 251 253
Chapter 9
Stabilisers: General Aspeds-W. V. TITOW 9.1 Introduction . . . . . . . . 9.2 Degradation of PVC Polymer 9.2.1 Thermal Degradation 9.2.2 Photochemical Degradation 9.3 Ideal Requirements for a Stabiliser, and General Factors Affecting Stabiliser Selection 9.4 Heat Stabilisers 9.4.1 Lead Compounds 9.4.2 Organotin Stabilisers: chemical nature and types; characteristics and applications 9.4.3 Compounds of Other Metals: metal compounds with stabilising effects in PVC; composite metal stabilisers 9.4.4 Organic (Miscellaneous) Stabilisers: esters of aminocrotonic acid; urea derivatives; epoxy compounds; organic phosphites; miscellaneous organic co-stabilisers
255 255 256 256 260 261 263 265 270 275
286
Conren~
9.5 Antioxidants and UV Absorbers 9.5.1 Antioxidants 9.5.2 UV Absorbers 9.6 Main Modes of Stabiliser Action 9.6.1 Lead Stabilisers . . . . 9.6.2 Organotin Stabilisers . . 9.6.3 Other Metal-based Stabilisers 9.6.4 Organic Stabilisers, Antioxidants, UV Stabilisers 9.7 Some General Features and Common Faults of Stabilised Compositions . . . . . 9.7.1 Plate-out . . . . . . . . . . . . 9.7.2 Sulphide Staining 9.8 Testing and Evaluation of Stabiliser Effects 9.8.1 Concept of Stability in Processing, Service and Tests 9.8.2 Heat Stability Testing 9.8.3 Light Stability Testing 9.9 Detection and Analysis of Stabilisers References . . . . . . . . . . . . . . Chapter 10 Commercial Stabillser Practice-P. S. COFFIN
10.1 10.2 10.3 10.4
Introduction Choosing a Commercial Stabiliser . . . The Importance of a Well-balanced Lubricant System One-pack Systems and the Physical Form of Stabiliser Products . . . . . . . . . . . . . . . . . . . . . 10.5 Hygiene and Environmental Considerations . . . . . 10.6 UK Stabiliser Manufacturers-Product Ranges and Applications 10.6.1 Associated Lead Manufacturers Ltd 10.6.2 Ciba-Geigy Ltd . . . . . . . . . ..... 10.6.3 Durham Chemicals Ltd 10.6.4 Diamond Shamrock Polymer Additives Division 10.6.5 Victor Wolf Ltd References . . . . . . . . . . . . . . . . . . . . . "
xv
292 292 294 299 299 300 302 304 305 305 308 311 311 315 328 330 330 335 335 337 339 340 341 342 342 346 348 351 356 356
Chapter 11 Some MisceUaneous Components of PVC Formulations-W.
V. TITOW . . . 11.1 Lubricants . . . . . . . . . . . . . . . . . . . .
359 359
xvi
Contents
11.1.1 Functions, Nature and Effects 11.1.2 Interaction and Co-action of Lubricants with Other PVC Formulation Components: lubricant/stabiliser effects; mutual effects of lubricants and plasticisers; effects of polymeric modifiers; effects of fillers and pigments 11.1.3 Assessment of Lubricant Effects 11.1.4 Sources of Information on Lubricants and their Commercial Suppliers 11.2 Polymeric Modifiers 11.2.1 Processing Aids 11.2.2 Impact Modifiers: impact resistance-its nature, significance and measurement; the impact resistance of PVC; the nature, effects and applications of polymeric impact modifiers for PVC 11.3 Colourants 11.3.1 General Nature and Functioning 11.3.2 General Classification 11.3.3 Forms in which Colourants are Available 11.3.4 Choice of Colourant-Main Considerations: general appearance and colour requirements; processability and stability in processing; stability and permanence in service; health and safety considerations 11.3.5 Some Commercial Pigments 11.4 Antistatic Agents 11.4.1 Static Electricity Charges on PVC: Phenomena and Tests 11.4.2 Nature and Use of Antistatic Agents 11.5 Flame and Smoke Retardants 11.5.1 General Mechanism of Burning of Polymers and Plastics 11.5.2 Flame Retardance and Smoke Suppression in PVC Compositions References
359
364 367 370 371 372
375 401 401 403 405
407 410 419 420 422 424 424 427 435
Chapter 12 MisceUaneous Properties of Special Interest in PVC Materials and Products-W. V. TITOW
12.1 Introduction
439 439
CQnren~
12.2 Low-temperature Properties 12.2.1 Cold Flex Temperature (Clash and Berg) 12.2.2 Cold Bend Temperature 12.2.3 Low Temperature Extensibility of Flexible PVC Sheet 12.3 Heat Resistance . . . . . . . . . . . . 12.4 Permeability 12.5 Environmental Stress Cracking and Crazing 12.6 Weathering Resistance . . . . . . . . . 12.7 Resistance to Biological Attack . . . . . 12.7.1 Microbiological Attack (Biodegradation) 12.7.2 Insect and Animal Depredations: attack by termites; attack by rodents 12.8 Chemical Resistance . . . . . . 12.9 Health Hazards . . . . . . . . 12.9.1 Vinyl Chloride Monomer 12.9.2 PVC Compounds and their Regular Constituents 12.9.3 PVC Decomposition Products 12.9.4 Peripheral Hazards 12.10 Burning Behaviour References . . . . . . . . . .
xvli
439 442 442 442 443 452 466 469 483 483 486 487 495 496 498 499 500 501 509
Chapter 13 Industrial Compounding Technology of Rigid and Plasticised
PVC-W. HENSCHEL and P. FRANZ 13.1 Introduction 13.2 Raw Materials. . . . . . . . . 13.2.1 PVC Polymer and Fillers 13.2.2 Plasticisers 13.2.3 Other Additives . . . . 13.3 Upstream Equipment (Silo Storage to Weighing) 13.3.1 Silo Storage of PVC Polymer and Fillers: silo sizes; materials of silo construction; raw material intake (silo filling); raw material discharge; dust removal system . . . . . . . . . . . . 13.3.2 Conveying of PVC Polymer and Fillers: pneumatic conveying . . . . . . . . . . . . 13.3.3 Storage of Plasticisers: tank sizes; suitable con-
513 513 514 514 519 519 519
519 525
xviii
Contents
struction materials; plasticiser delivery; pointers on pipe laying 13.3.4 Storage of Additives. . . . . . . . . . . 13.3.5 Metering and Weighing: fundamentals of metering and weighing technology; control and monitoring equipment . . . . . . . . 13.4 Mixing . . . . . . . . . . . . . . . . . . . . . . 13.4.1 Blending of Bulk Materials in Overall Solid Phase: introduction; theoretical aspects of mixing, with special reference to dry blending of PVC compositions; mixers for plastics processing; tank-type or intensive mixer . . . . . . 13.4.2 Melt Compounding: compounding and pelletising; compounding of PVC for feeding calenders; extrusion of film, sheet and board; recycling 13.4.3 Preparation of PVC Pastes: silo storage; metering; pasting-up and dispersion; filtering; degassing; ageing; colouring 13.4.4 Machinery: screw-type machines; machine drives; control and instrumentation; interlocks; materials of construction; machines for the production of pastes 13.5 Pellet Cooling and Storage . . . . . . . . . . . . . 13.5.1 Pellet Cooling: nature and outline of the operation; pellet cooler systems 13.5.2 Pellet Mixing and Storage: pellet mixer designs; handling of PVC pellets Chapter 14 Extrusion of PVC-General Aspeds--B. J. LANHAM and W. V. TITOW . . . . 14.1 Introduction 14.2 The Extruder . . . . . . . . . . . . . . . . . . . 14.2.1 Main Components and Their Functions, with Special Reference to Extrusion of PVC: the screw; the barrel; the head and die assembly; heating and cooling; the hopper . . . . . . . 14.2.2 Some General Points Relevant to Extrusion of PVC: machine outputs and energy efficiency in
530 532 532 547
547
577 603
609 660 660 664
673 673 674
674
Contents
modem extrusion practice; some features of, and aids to, modem extrusion; use of computers; some material aspects; some features and common faults of extruded products . . . . . 14.3 PVC Material Flow, Homogenisation and Gelation (Fusion) in the Extrusion Process 14.4 Single-screw Extruders . . . 14.5 Twin-screw Extruders . . . 14.6 Some Commercial Machines 14.7 Ancillary Equipment 14.8 Extrusion of Plasticised PVC 14.8.1 Normal (Relatively Slow) Extrusion 14.8.2 High-speed Extrusion . 14.8.3 Examples of Industrial Extrusion of Plasticised PVC: PVC coating of wire and cable; production of pPVC hose with braid reinforcement References Chapter 15 Injection Moulding of PVC-The late L. W. TURNER 15.1 Introduction . 15.2 Melt Properties of Particular Significance, Melt Behaviour in Relation to Moulding Conditions, and Moulding Compounds . . . . . . . . . . . . . . . . 15.2.1 Moulding compounds . 15.3 Effect of Processing Factors upon Product Properties 15.3.1 Quenching Stresses . 15.3.2 Orientation and Related Features . 15.4 The Moulding Process: Available Equipment; Process Control; Some Features of uPVC Moulding 15.4.1 Rate of Injection and Injection Pressure 15.4.2 Working Surfaces . 15.4.3 Interaction of PVC with Acetal Polymers and Copolymers 15.5 Materials and Applications 15.6 Trouble-shooting . . . . 15.6.1 Machine Selection 15.6.2 Processing Features Specific to PVC 15.6.3 General Considerations. References . . . . . . . . . . . . . . . . . .
xix
682 689 698 699 703
710 713 713 714 717 719 723 723 724 726 728 728 728 729
734 735
735 736 738 738 738
740 740
xx
Contents
Chapter 16 Sheet 'Thermoforming and Related Techniques for PVC-The late L. W. TURNER 16.1 Introduction 16.2 Materials Used . . . . . 16.3 Vacuum Forming of Sheet 16.3.1 Principal Methods: negative forming; plugassisted forming; drape forming; bubble forming; snap-back forming 16.3.2 Details of Methods .... 16.3.3 The Moulds 16.3.4 Finishing . . . . . . 16.4 Matched-mould and Related Methods 16.5 Tolerances in Dimensions and Dimensional Stability of Formed Parts . . . . . . . . . . . . . 16.6 Equipment Suppliers 16.7 Materials Assessment and Design Aspects 16.7.1 Effect on Quality of Draw Ratio and Temperature . . . . . . . . . . . 16.7.2 Thermoformability of CPVC References . . . . . . . . . . . . . . . Chapter 17 Blow Moulding of PVC-W. V. TITOW . . . . . . . . . . 17.1 Basic Features and Historical Development of Blow Moulding 17.2 Blow-moulding Processes and Their Application to PVC 17.2.1 General Characterisation and Main Features of Processes and Systems: main characteristics of extrusion; injection, and dip blow moulding; the role and effects of stretching in stretch-blow moulding; processing and equipment arrangements; cooling methods 17.2.2 Industrial Blow Moulding of PVC: some process and equipment considerations; extrusion blowmoulding equipment; injection blow-moulding equipment; dip blow-moulding equipment; sources of information on blow-moulding
743 743 745 745 745 751 753 754 755 756 757 757 759 761 761
763 763 765
765
Contents
equipment . . . . . . . . 17.3 PVC Compositions for Blow Moulding 17.3.1 The Processing Aspect . . . 17.3.2 The End-use Aspect . . . . 17.3.3 PVC Bottle Formulations: PVC polymer; stabiliser system; impact modifiers; lubrication; other additives . . 17.4 PVC Blow Mouldings 17.4.1 Applications 17.4.2 Properties and Tests References . . . . . . . . . . .
xxi
784 789 789 792 793 795 795 797 800
Chapter 18 Calendering of PVC-W. V. TITOW
18.1 Introduction 18.2 The Calender . . . . . . . . 18.3 The Calendering Operation: General Features and Their Effects on the Structure and Properties of Calendered Sheet 18.4 Calender Lines . . . . . . . . . . . . . . . . . . 18.4.1 General-purpose Line: pre-calender (compounding and feed) section; calendering; the post-calender train 18.4.2 Special Lines and Arrangements: calendered flooring lines; lamination on or at the calander 18.5 The Formulation Aspect . . . . . . . . . . . 18.6 Some Faults and Defects of Calendered Sheeting 18.6.1 Simple Dimensional Faults 18.6.2 Structural Defects . . . . . . . 18.6.3 Faults Manifested in Appearance 18.7 Further Processing of Calendered Sheet 18.7.1 Press Finishing 18.7.2 Press Lamination 18.7.3 Surface Treatments: printing; coating; embossing . . . . . . . . . . . . . . . . . 18.7.4 Continuous Lamination 18.8 Properties and Applications of Calendered Materials References . . . . . . . . . . . . . . . . . . . . .
803 803 804 808 809 809 828 830 833 833 834 835 837 837 837 838 839 840 847
xxii
Contents
Chapter 19
Rigid PVC: Main Products-Production, Properties and Applications-B. J. LANHAM and W. V. TITOW 19.1 Introduction 19.2 Some Material Properties of uPVC 19.3 uPVC Pipes 19.3.1 Types of uPVC Pipe . . . 19.3.2 Production of uPVC Pipe: equipment and process; some formulation aspects . . . . 19.3.3 Pipe Properties and Their Determination 19.3.4 Some Special Pipe Products . 19.4 uPVC Profiles 19.4.1 Main Types and Applications 19.4.2 Production . . . . . . . 19.4.3 Some Formulation Aspects 19.4.4 Testing and Specifications 19.5 uPVC Sheet and Film 19.5.1 Terminology . . . . . 19.5.2 Production . . . . . . 19.5.3 Applications and Properties 19.6 Gramophone Records . . . . . 19.7 Injection-Moulded uPVC Articles References . . . . . . . . . . . . .
849 849 856 866 867 869 878 879 883 883 884 886 889 890 890 891 893 896 897 898
Chapter 20
PVC Sheet and its Fabrication-W. V. TITOW 20.1 Introduction 20.2 Unsupported PVC Sheet Materials 20.3 Main Fabrication Techniques Applicable to PVC Sheet Materials and Parts 20.3.1 Welding: hot-gas welding; extrusion welding; high-frequency welding; heated-tool welding 20.3.2 Bonding: solvent bonding; adhesive bonding 20.3.3 Machining 20.3.4 Conversion and Manipulation of PVC Film and Sheeting for Packaging . . . . . . . . . . , 20.3.5 Surface Decoration, Marking, and Other Surface Processing of PVC Materials and Products: surface decoration; surface marking; surface
901 901 904 910 910 923 930 932
Contents
processing References Chapter 21 PVC Pastes: Properties and Formolation-W. V. TITOW 21.1 Introduction 21.2 PVC Pastes: Rheological Properties and Theory . . .... 21.2.1 Viscosity of a Simple Suspension 21.2.2 Main Compositional Factors Influencing the Apparent Viscosity of PVC Pastes . . . . 21.2.3 Expressions for the Apparent Viscosity of Pastes . . . . . . . . . . . . . . . . . . 21.2.4 Variation of Paste Viscosity with Rate of Shear, or with Time at Constant Shear Rate 21.2.5 Gelation and Fusion of PVC Pastes . . . . 21.2.6 The Measurement of Viscosity of PVC Pastes 21.3 Paste Components and Formulation . . . . . . . . 21.3.1 The Polymer: paste polymers; extender polymers 21.3.2 Plasticisers 21.3.3 Stabilisers 21.3.4 Fillers 21.3.5 Thickening Agents (for Thixotropic Plastisols and Plastigels) 21.3.6 Miscellaneous Paste Components: viscosity depressants; diluents; other minor additives . 21.4 Pastes for Rigid Products: Organosols and Rigisols . 21.4.1 Organosols 21.4.2 Rigisols References
xxiii
932 936
939 939 940 941 942 943 945 951 960 962 %2 965 969 970 973 975 975 975 976 978
Chapter 22 Preparation, Processing and Applications of Pastes-W. V.
TITOW . . . . . . . . 22.1 Introduction 22.1.1 Preparation . . . . . 22.1.2 Conversion to Products 22.2 Applications 22.2.1 Rotational Casting . .
981 981 981 982 986 986
xxiv
Contents
22.2.2 Slush Moulding 22.2.3 Paste Casting 22.2.4 Dip Coating and Moulding: hot-dip coating; hot-dip moulding; cold-dip coating 22.2.5 Spray Coating 22.2.6 Coating of Sheet Materials (Fabrics and Paper): paste coating (spreading) by doctor knife; paste coating by roller; direct-coating process; transfer (reverse) coating process; promotion of adhesion between coating and substrate; surface decoration and finishing of PVC paste coatings; testing of coated materials 22.2.7 Miscellaneous Paste Processing Methods of Minor Significance: low-pressure injection moulding; compression moulding; extrusion References
Chapter 23 PVC Latices-Revised and edited by W. V. TITOW 23.1 Introduction 23.2 Types of PVC Latices 23.2.1 Homopolymer Latices 23.2.2 Unplasticised Copolymer Latices 23.2.3 Plasticised Copolymer Latices . 23.3 Some Properties of Polymeric Products from PVC Latices 23.3.1 Mechanical Properties 23.3.2 Toxicity Considerations 23.4 Compounding 23.4.1 Latex Property Modifiers: latex stability; wetting agents; thickeners; antifoaming agents; pH modifiers and buffers . . . . . . . . 23.4.2 Polymer Property Modifiers: heat stabilisers; plasticisers; fillers; pigments 23.5 Anti-blocking Techniques 23.6 Applications 23.6.1 Textile Applications: as bonding agents in nonwoven fabrics; for coating or impregnation of fabrics . . . . . . . . . . . . . . . . . .
988 991 992 996
998 1010 1012
1013 1013 1016 1017 1017 1018 1018 1018 1019 1019 1020 1029 1039 1040 1040
Contents
xxv
23.6.2 Paper Treatments . . 23.6.3 Leather Finishes 23.6.4 Adhesive Applications References
1042 1044 1044 1045
Chapter 24
PVC Solutions and their AppUcations--W. V. TITOW 24.1 Introduction 24.2 Components of PVC Solutions 24.2.1 The PVC Polymer: homopolymers; copolymers; terpolymers 24.2.2 Solvents and Diluents 24.2.3 Other Solution Constituents 24.3 Preparation of PVC Solutions, and Solution Compositions for Particular Applications 24.4 Applications 24.5 Adhesion of Solution-applied Coatings to Substrates References . . . . . . . . . . . . . . . . . . . . .
1047 1047 1048 1048 1049 1054 1057 1060 1063 1065
Chapter 25
CeUuIar PVC Materials and Products-W. V. TITOW 25.1 Introduction 25.2 Production Methods and Processes 25.2.1 Foams: dispersed-gas blowing: 'chemical' blowing; gas entrainment (mechanical frothing); insitu gas evolution and cross-linking; solvent (monomer) blowing 25.2.2 Other Cellular PVC Materials: the 'lost filler' method; sintering of powder 25.3 Formulation and Process Factors in Foam Production 25.3.1 Effects of Formulation and Processing Variables on Foam Properties . . . . . . . . . . . . 25.3.2 Chemical Blowing Agents-Nature and Operation . . . . . . . . . . . . . . . . . . . 25.4 Some Surface Treatments-Embossing and Lacquer Coating of Flexible Cellular Sheet Materials 25.4.1 Mechankal Embossing . . . . . . . . . 25.4.2 Chemical Emboss . . . . . . . . . . . 25.4.3 Emboss Effects by Screen Printing of Paste
1067 1067 1069
1069 1078 1080 1080 1085 1092 1092 1093 1094
xxvi
Contents
25.4.4 Lacquer Coating 25.5 Examples of Basic Formulations 25.6 Evaluation and Testing References . . . . . . . . . . . . Chapter 26 Applications of PVC-W. V. TITOW . . . . . . . . . . . 26.1 Main Applications of Primary PVC Products . . . . . 26.1.1 Pipes and Tubing: rigid (uPVC) pipes; flexible tubing . . . . . . . . . . . . . . . . . . 26.1.2 Extruded Profiles and Channels . . . . . . . 26.1.3 Unsupported Sheeting and Film: rigid sheet; flexible sheet; foil and film . . . . 26.1.4 Foam: rigid foam; flexible foam . . . . . 26.2 Composite Products (Coated, Laminated, or Filled) 26.2.1 Coated Fabrics 26.2.2 Conveyor Belting 26.2.3 Sheet-type PVC Interior Wall-coverings 26.2.4 PVC Coatings and Coverings on Metal Substrates: wire and cable insulation and coverings; PVC/metal sheet laminates; 26.2.5 Laminates of PVC with Non-metallic Materials: sandwich panels; PVC/polystyrene sheet laminate; PVC/polyacetallaminated sheeting. 26.2.6 Unsupported PVC Flooring and Floor Tiles 26.3 PVC Fibres and Fibre Products . . . . 26.4 Miscellaneous Products and Applications 26.4.1 Gramophone Records 26.4.2 Blown Bottles and Containers 26.4.3 Footwear . . . . . . . . . 26.4.4 Battery Separators . . . . . 26.4.5 Powder-coated Products and Mouldings Produced by Powder-coating Methods 26.4.6 Medical Applications 26.4.7 Applications in Motor Cars . . . . . . . . . 26.4.8 Tubular-frame Furniture and Related Applications 26.5 Some Special, Unusual, or Minor Products and Applications References . . . . . . . . . . . . . . . . . . . . . . .
1094 1095 1095 1101 1103 1104 1104 1106 1107 1110 1111 1111 1112 1113 1114 1115 1116 1117 1117 1117 1118 1118 1118 1118 1120 1121 1121 1122 1125
Contents
xxvii
Appendix 1
Standards Relevant to PVC Materials and Products-Compiled by N. HERBERT and W. V. TITOW 1. Plastics Terminology, Properties and Testing: General 1.1 Terminology: general; common names and abbreviations; equivalent terms in various languages . . . . 1.2 General Test Conditions and Methods: conditioning and testing conditions; some general test methods . 2. Vinyl Polymers and Copolymers . . . . . . . . 2.1 General (Designation, Coding, Characterisation Tests) . . . . . 2.2 Viscosity . . . . . . . . . . 2.3 Chlorine Content . . . . . . . . . . 2.4 Vinyl Acetate Content in VCNA Copolymers 2.5 Ash and/or Sulphated Ash Content . 2.6 Volatile Matter (including Water) 2.7 Impurities and Foreign Matter 2.8 Bulk Density 2.9 Particle Size . . . . 2.10 Bromine Number 2.11 pH of Aqueous Extract 2.12 Miscellaneous Properties Relevant to Processing 2.13 Methanol Extract 2.14 VCM Content 3. Vinyl Compounds 3.1 General (Designation, Coding, Characterisation Tests): rigid compounds; flexible compounds, pastes; miscellaneous 3.2 Properties and Tests: bulk density and pourability; water absorption; temperature effects; mechanical properties; miscellaneous properties and analysis 4. Plasticisers 4.1 Bulk Properties . . . . . . . . . . . . . . . . 4.2 Properties in Association with PVC (Compatibility, Volatility, Migration) 4.3 Effects on PVC . 5. PVC Sheeting and Films 5.1 Rigid 5.2 Flexible 5.3 Sheet and Film Fabrication and Products
1127 1131 1131 1134 1135 1135 1136 1137 1137 1137 1137 1138 1138 1138 1139 1139 1139 1140 1140 1140 1140 1142 1144 1144 1146 1147 1148 1148 1148 1149
xxviii
Contents
6. PVC Pipes, Tubing, and Pipe Fittings 6.1 Rigid Pipes and Fittings, Including Pressure Pipes 6.2 Flexible Tubing 6.3 Miscellaneous Standards R~evant to Pipes 7. PVC-coated Materials and Products 7.1 Coated Fabrics, including Conveyor and Transmission Belting . 7.2 Other Coated Materials and Products 8. Cellular Vinyls 8.1 Rigid Cellular Materials 8.2 Flexible Cellular Materials 8.3 Miscellaneous Standards: definition and classification; physical properties-general; thermal properties-general; flammability and burning; chemical resistance and permeability; insulation materials: cushioning materials, sandwich structures 9. PVC Wire and Cable Insulation, Cable Sheathing and Jacketing 10. PVC Flooring 11. Various Product Standards and Tests 11.1 Colour Bleeding and Staining 11.2 Miscellaneous
1150 1150 1156 1157 1158 1158 1160 1160 1160 1162
1163 1165 1167 1167 1167 1167
Appendix 2 Quantities and Units: The SI System: Unit Conversion Tables--Compiled by W. V. TITOW 1169 Appendix 3 Some Material Properties of PVC Componnds-Compiled by W. V. TITOW Index 1 General
Products
and 1185
. , . . . . . . . . . . . . . . . . . . . . . . 1199
Index 2 Material and Product Trade Names . . . . . . . . . . . . 1223 Index 3 Named Equipment and Processes . . . . . . . . . . . . .
1231
List of Contributing Authors
W. V. Trrow
Formerly Manager (Special Projects), Laboratories Ltd, Ashtead, Surrey, England
Yarsley
Research
D. L. BUSZARD Market Development and Technical Service, Plastics Chemicals, Ciba-Geigy Industrial Chemicals, Tenax Road, Trafford Park, Manchester, MI71WT, England
P. S. COFFIN General Manager-Technical, Roeol Ltd, Rocol House, Swillington, Leeds, LS26 2BS, England P.
FRANZ
Manager of Process R&D Department, Buss Ltd, CH-4133 Pratteln, Switzerland W.
HENSCHEL
Manager of the Design and Construction Department, Buss Ltd, CH-4133 Pratteln, Switzerland
Miss N.
HERBERT
Head, Standards Information Centre, South African Bureau· of Standards, Private Bag X191, Pretoria 0001, Republic of South Africa xxix
xxx
List of Contributing Authors
I. D. HOUNSHAM Sales Manager, PVC Division, Croxton and Garry Ltd, Curtis Road, Dorking, Surrey, RH4 lXA, England
B. J. LANHAM European Marketing Manager, LNP Plastics Nederland BV., PO Box 13, Ottergeerde 24, Raamsdonksveer, The Netherlands The late L. W. TuRNER Formerly Senior Research Associate, Yarsley Technical Centre Ltd, Redhill, Surrey, England
CHAPTER 1
Introduction W. V.
TITOW
1.1 PVC: GENERAL TERMINOLOGY AND RELEVANT DEFINITIONS The letters 'PVC' stand for 'polyvinyl chloride'. Thus the abbreviation, like the full name, should-strictly speaking-specifically denote a homopolymer of vinyl chloride. However, both terms-and in particular the abbreviation-have acquired a different, wider meaning in common usage: to the processor and user, as well as the technologist, 'PVC' is any material or product made of a PVC composition, i.e. of an intimate mixture of a vinyl chloride polymer or copolymer with various additives, some of which (e.g. plasticisers in a flexible PVC composition) may be present in very substantial, occasionally predominant, proportion. It is usual to refer to the polymer constituent of such compositions as PVC resin or PVC polymer. The terms 'compound' and 'formulation' are also sometimes used as if they were synonymous with 'composition', although the purist may claim, with some justification, that 'formulation' is the make-up of a composition (e.g. as recorded on paper), and that the word 'compound' should be reserved for those PVC compositions which are produced by melt compounding (in contradistinction to, say, dry blends or plastisols-see Chapters 13 and 21, respectively). Some of the additives which the formulator includes in a PVC composition are heat stabilisers, necessary in all cases to counteract the inherent thermal instability of PVC resins (especially at the high processing temperatures); others also function as aids in processing (e.g. certain polymeric modifiers, lubricants), whilst still others (e.g. 1
2
W. V. Titow
plasticisers, fillers) modify the material properties to provide the wide applicational versatility that makes PVC so important among the major thermoplastics. In terms of the extent of their effect on the material properties of PVC, plasticisers are the most important group of additives. PVC compositions incorporating plasticisers (and the materials and products made from such compositions) are known as plasticised PVC (sometimes abbreviated to pPVC*); flexible PVC and soft pvc contain plasticisers in quantities high enough to impart these properties to the material. PVC compositions and products which do not incorporate plasticisers are commonly called unplasticised PVC (uPVC*) or sometimes rigid PVC, although the latter term properly extends also to PVC materials which may contain some plasticisers but in a proportion not sufficient to lower the modulus appreciably. Plasticised materials whose plasticiser contents-whilst generally low-do reduce the modulus (and usually the strength and hardness) in comparison with uPVC (but only to values still higher than those normal for flexible or soft PVC) are sometimes referred to as semi-rigid PVc. The term 'vinyl' is also used, as an adjective or noun, in the place of 'PVC' (e.g. in such expressions as 'processing of vinyls', 'vinyl composition', 'vinyl material', 'vinyl upholstery', 'vinyl foam'), especially-and most commonly-where the material concerned is a flexible or soft PVC. This terminology is quite common, and thus sanctioned by usage, but it is worth bearing in mind that it tallies neither with standard definitions in the PVC field nor with systematic chemical nomenclature. Thus the current international standard definition of vinyl resin (ISO 472-1979(E» is a resin made by polymerisation of monomers containing the vinyl group, and hence includes, for example, polystyrene (which is polyvinylbenzene), polyvinyl acetate, polyvinyl alcohol, polyvinyl fluoride and polyvinyl pyrrolidone, along with polyvinyl chloride and all the other polymers of compounds whose main structural component is the vinyl grouping CH z = CH-. Whilst the definition is sound and properly based on the relevant chemical structure, no polymer technologist would refer to, say, expanded polystyrene as 'vinyl foam': in the common parlance of
* These designations (with a space after the first, lower case letter) are prescribed by two international standards: ISO 2898/1 and ISO 1163/1 (but current revision proposals include changes from u PVC and p PVC to PVC-U and PVC-P). The letters iPVC are sometimes used to designate a high-impact compound.
i
introduction
3
the plastics industry the term 'vinyl' is firmly associated with PVC, in the way just mentioned. A few other relevant standard definitions may be noted in passing. Vinyl chloride plastic: 'A plastic based on polymers of vinyl chloride or copolymers of vinyl chloride with other monomers, the vinyl chloride being in the greatest amount by mass'. (ISO 472-1979). Rigid PVC compounds: 'Rigid plastic compounds composed of poly(vinyl chloride), chlorinated poly(vinyl chloride), or vinyl chloride copolymers, and the necessary compounding ingredients. The resin portion of copolymer compounds shall contain at least 80 percent vinyl chloride. The compounding ingredients may consist of lubricants, stabilizers, non-poly(vinyl chloride) resin modifiers, and pigments, essential for processing, property control and colouring.' (ASTM D 1784-81). Unplasticised compounds of polymers of vinyl chloride: 'Compounds based on homopolymers of vinyl chloride, or copolymers with at least 50% of vinyl chloride, or chlorinated poly(vinyl chloride), or mixtures of such polymers with each other or with other polymers, the principal ingredient being a polymer of vinyl chloride. These compounds may also contain fillers, colorants, and such small quantities of other ingredients as are necessary to facilitate fabrication, such as stabilizers and lubricants.' (ISO 1163/1-1980(E)). Non-rigid vinyl chloride polymer and copolymer moulding and extrusion compounds: Compounds based on '... nonrigid vinyl chloride polymer and copolymer classes in which the resin portion of the composition contains at least 90% vinyl chloride. The remaining 10% may include one or more monomers copolymerized with vinyl chloride or consist of other resins mechanically blended with polyvinyl chloride or copolymers thereof. These nonrigid vinyl compounds are defined by a hardness range and include the necessary stabilizers, plasticizers, fillers, dyes, and pigments to meet the designated requirements'. (ASTM D 2287-81). Flexible PVC compounds: 'Compounds ... manufactured from polyvinyl chloride or from a copolymer of vinyl chloride of which the major constituent is vinyl chloride, or from both. Such materials shall be
4
W. V. Titow
suitably compounded with plasticisers and other ingredients.' (BS 2571: 1963). Plasticised compounds of polymers of vinyl chloride* (pPVC): 'Compounds based on homopolymers of vinyl chloride or copolymers with at least 50% of vinyl chloride, or chlorinated poly(vinyl chloride), or mixtures of such polymers with each other or with other polymers, the principal ingredient of the mixtures being a polymer of vinyl chloride. These compounds contain plasticizers and may also contain fillers, colorants, and small quantities of other ingredients such as stabilizers and lubricants'. (ISO 2898/1-1980(E)).
For the sake of convenience, abbreviations (letter symbols) are used for some polymers and copolymers in many places in this book. Such abbreviations are generally in line with the recommendations of the relevant international standard (ISO 1043-1978). However, in a few cases where those recommendations are at variance with common general usage, or in order to avoid inconsistency, the ISO standard has not been followed. Two notable examples are the author's preference for EVA (over 'EN AC' recommended by the ISO standard) as an abbreviation for ethylene/vinyl acetate copolymer, and VCNA for vinyl chloride/vinyl acetate copolymer. The EVA symbol is very widely used (and indeed also recommended by another English-language standard-ASTM D 1600-83), whilst-given its use-IVA' for vinyl acetate is then more consistent than 'VAC' (the abbreviation favoured by all standards), and should be acceptable especially in contexts where there is no chance of confusion with vinyl alcohol (which is in any case usually designated by 'VAL'). It may also be noted that 'A' for acetate is recognised, though not preferred, by ISO 1043-1978.
1.2 EARLY HISTORY AND DEVELOPMENT OF PVC Although Regnault l - 3 prepared some vinyl and/or vinylidene monomers in 1838, and observed the conversion of the latter to a white powder when exposed to sunlight in sealed tubes,2 it is Baumann's polymerisation of vinyl chloride (as well as bromide) in 18724 that is often regarded as the earliest documented preparation of PVC homopolymer: this was certainly among the 'white, solid masses * NB called simply 'plasticised vinyl compounds' in an earlier version of this definition (ISO/DIS 2898, International Draft Standard, 1972).
I
Introduction
5
unaffected by solvents and acids' obtained in that work. The polymer was found to be stable on heating up to Boac, but to decompose rapidly with evolution of acid vapour at higher temperatures. 5 Early manifestations, or at least precursors, of budding practical interest in PVC came in 1912, in the form of British and German patents6 to Ostromislensky (for the production of 'rubber-like masses' from vinyl halides), and in the work in Germany by F. Klatte considered by some to have laid the foundation for the technical production of PVC: 3 Klatte took out a German patent for the production of PVC fibres,3 and Ostromislensky went on to obtain patent cover for 'polyvinyl halides' (in the USA in 1929). 2 In the meantime (c. 1928) patents were also being granted for vinyl chloride/acetate copolymers; in the USA to Du Pont and the Carbide and Carbon Chemicals Corporation, and in Germany to I. G. Farbenindustrie (now BASF).3,7 The first production of the copolymers in America (by the Carbide and Carbon Chemicals Corp.) falls in the period 1928-1930: soon after (1931) B. F. Goodrich introduced their own 'non-rigid vinyl chloride plastics'. 2 In Germany, 1931 saw the first technical-scale production of vinyl chloride polymer and copolymers, and the first preparation (by Hubert and Schonburg) of chlorinated PVC fibres (followed by the first technical production of both the CPVC polymer and the 'Pe-Ce' fibres from it in 19343). Industrial development (with emphasis eventually shifting from the vinyl chloride/acetate copolymer to the homopolymer) thereafter proceeded in both countries, with full commercial production achieved in the late 1930s. Whilst some development work was taking place in the UK in the same period, PVC was first produced there on a commercial scale in 1942-1943 (by ICI and the Distillers Company). It is thus comparatively recently that PVC became a commercial plastics material. The early interest in copolymers (in particular polyvinyl chloride/acetate) was associated with their use as the first practical solution to the problem of thermal decomposition in processing: whilst the general thermal stability of the copolymers is somewhat poorer than that of the homopolymers they can be processed at significantly lower temperatures, at which they are reasonably stable. The effects in this respect of the co-monomer units in the polymer molecules-sometimes referred to as 'internal plasticisation'-are now well understood. Effective 'external' plasticisation of PVC homopolymer by the incorporation of plasticisers first came around 1930, with the finding by several workers5 ,8 that compounding with dibutyl phthalate (DBP) and certain other esters would convert the intractable polymer to a material
6
W. V. Titow
of lower softening point; this could be processed satisfactorily at lower melt temperatures and was-in the solid state at room temperaturerelatively soft and similar to rubber in some respects. Thus, chronologically, external plasticisation came after 'internal' plasticisation by copolymerisation, although it is now the main route to the formulation of flexible and most semi-rigid PVC materials. Among early suggestions of substances for use as plasticisers, now of only historical interest, were tung oil9 and alkyd resins. 1o It was also realised at about the same time (the early 19305) that certain additives, e.g. alkaline-earth metal soaps,11 would act as heat stabilisers. The main present-day applications in which vinyl chloride/acetate copolymers are more suitable than plasticised homopolymer compositions are gramophone records and floor coverings. It is interesting to note that the first of these applications was originally disclosed in the early 1930s,8,12 i.e. around the time when other less practicable and now long defunct proposals were also current, e.g. moulded PVC dentures,13 and an adhesive, consisting of a mixture of PVC with rubber and a cellulose derivative, for sticking patches on worn places in clothing!14 It was World War II that first brought PVC into its own. It was soon realised that plasticised PVC was an effective replacement for rubber in some important applications, notably cable insulation and sheathing. Thus PVC helped to relieve the acute rubber shortage, and at the same time established itself as a material in its own right. From then on it continued growing rapidly in importance, to attain the dominant position which its properties and versatility secure for it today in so many applications. The early processing of PVC, in the pre-war period and to some extent during the war, was largely carried out by methods and on machinery originally developed for rubber or celluloid. The processes involved were mixing, calendering, compression moulding, and extrusion (including wire coating). The paddle-type (Gardner) pre-mixers were in use at an early period, but between about 1942 and 1945 open-mill mixing was widely practised. The use of internal mixers was also adopted when it was found that PVC compounds could be readily mixed in them. The open mills and other machines had to be run at temperatures higher than those appropriate for rubber: as they were normally steam-heated steam pressures had to be increased at the risk of grease melting extensively and draining away from bearings. Electrical heating, particularly for extruders, was a logical development, but one which proposals were also current, e.g. moulded PVC dentures,13 and an adhesive, consisting of a mixture of PVC with rubber and a cellulose
7
1 Introduction
was fully utilised only slowly. The need to modify the rubber extruders employed for the early production work soon became plain, and modifications were made, e.g. to enable the material to be fed-in in granular form, and to provide higher processing temperatures (by electrical heating). A special ram extruder was employed in Germany for a time to produce rigid pipe from a PVC billet. 15 Thanks to the work of Kaufman the early history of PVC polymers, compounds, and processing is well recorded and documented. 15 ,16 The development of modern PVC-processing equipment and of the many specialised processes which form such an important part of present-day PVC technology has paralleled the remarkable expansion of the production of PVC and the scope and number of its applications. The 1970s brought two unforeseen events of major significance both in their initial impact and their lasting effects upon the PVC industry-the oil crisis of 1973/74 (with its aftermath of continuing oil price rises), and the finding that vinyl chloride monomer (VCM) is a carcinogen. The oil crisis-after first causing a serious temporary shortage of the oil-derived principal feedstocks for VCM production (ethylene and acetylene-see also Chapter 2), and hence of PVC polymers (ct. the drop, c. 1974, of the curves of Fig. 1.1)-resulted in large, and continuing, increases in polymer prices. These are the outcome of higher costs of both the feedstocks and the energy (also largely oil-supplied) used to process them into monomers and thence into polymers. It may be noted that one of the developments prompted by this situation has been a refocusing of interest on coal-based raw materials and processes, with special reference to the acetylene route to the production of VCM: HC:=CH + HCI ~ CHr-CHCl
(1)
Albeit normally more energy-intensive than the ethylene route (generally favoured with oil feedstocks also because of the higher cost of acetylene from that source) it can be made completely independent of petrochemicals by producing the acetylene from coke and quicklime via calcium carbide: 3C + CaO~
Ca~
+ CO
(2)
(coke)
Ca~
+ 2H20~ HC:=CH + Ca(OH)2
(3)
i
=:
E c o
"c;l
"t:
u
+'
C
01
.
2
g3
tI
CIl
4
5
1964
I
Fig. 1.1
¥,----I
1972
x
/',/
/'
,/
I
1976
I
"
,...
./
/'
/"
"",.
/.~
I
1984
__ x
/'/'
/'
---
/'
/'
/'/'
/'
-- -- --
/"
1980
~,.-----Japan
/"
,,'/
/'
,/
Consumption of PVC polymers in the principal consuming areas.
I
1968
/
-----
--~/x--'-~
x
W~st~rn Europ~
./
./
/'
~
~
:0:::::
~
00
1 Introduction
9
The chlorine and hydrogen needed for the HCI used in reaction (1) can be produced by electrolysis of brine (with caustic soda as a saleable by-product). Some industrial plants manufacturing VCM and PVC polymer by this process have been in operation for many years (e.g. the AECI 'Coalplex' plant at Sasolburg, RSA). The discovery, in the early 1970s, that exposure to VCM could cause certain forms of cancer, coupled with the realisation that VCM concentrations in factory atmospheres and its residual contents in PVC polymers were comparatively high, had repercussions on PVC polymer production in several countries. It also caused a serious decline (especially in the USA and Japan) in the use of uPVC films for food packaging, and blow-moulded bottles for beverages and oils. The legal action for 285 million dollars brought in the USA against Borden Chemical and Goodyear Tire and Rubber Co. (two PVC polymer producers) by some supermarkets, in respect of 'damage to health' by PVC film used to wrap meat,17 is an example of the extremes of feeling in some quarters. Soft PVC was comparatively less affected, as the dilution effect of large amounts of plasticiser and greater loss in processing reduced the VCM concentration in the compounds to relatively low proportions. The considerable effort expended on investigating and remedying the situation, together with relevant regulations brought out in the major industrial countries, led to a vast reduction of VCM contents in both the factory air and PVC polymers produced by virtually all main manufacturers. The 'clean-up' brought the content levels down to values now regarded as acceptable on the basis of data obtained in extensive studies. The subject is discussed in more detail in Chapter 12 (Section 12.9.1), and also mentioned in Chapters 2 and 7. A third topic-albeit of comparatively lesser importance in the PVC context than the oil crisis and the VCM problem-which has been receiving increasing attention in recent times is the disposal of plastics waste and re-usable material. Concern with preservation of resources and conservation of the environment provides the main incentive in these two related areas. Dealing with PVC waste involves special considerations. Selective reclamation, i.e. separation from waste mixtures with other plastics (which operation is not a straightforward proposition in itself), and subsequent re-processing are complicated by the wide variety of PVC formulations, and the increased susceptibility to heat degradation in re-processing: the main factors in the latter are the 'heat history' already acquired and the possible presence of
10
W. V. Titow
polymer already partly degraded in the course of past heat treatments and/or service. Re-processing PVC-containing plastics waste without separation will normally entail dealing with mixtures in which large proportions of polyolefins (mainly polyethylene) are present: in view of the poor compatability of polyolefins with PVC this is not a particularly attractive practical proposition either with respect to processing or the resulting product. Disposal of PVC waste also has its special problems, since the polymer is not biodegradable, whilst incineration produces irritant, corrosive and toxic products (see Chapter 12, Section 12.9.3). Claims are made from time to time of successful reclamation of PVC from mixed plastics scrap and waste (e.g. by the 'Mesco' process developed in Japan by Mitsui 18) but the scale of commercial recovery is still relatively small, and the practical limitations of all existing methods are recognised. 18 ,19 The re-use of material from discarded PVC bottles is sometimes cited as a case where a certain measure of success has been achieved. In France such bottles have been processed for some time (on a limited scale) by Societe Dorlyl, to produce reclaimed PVC compounds said to be suitable for the production of certain grades of sewage and drainage pipes, and telephone cable sheathing. * The use of granulated PVC bottles as road-surfacing material in the USA has also been reportedt (as indeed has that of ground glass bottles!). Normal recirculation, in the same process, of the clean PVC scrap generated (e.g. edge trim in calendering-see Chapter 18) is widely practised, in particular with pPVC for non-critical applications. General pPVC scrap, both own and from external sources, is also converted by some processors into such products as cheap garden hose or core composition for cables (see Chapter 13, Section 13.4.2(d».
1.3 GENERAL STATISTICS Today the amount of PVC produced worldwide represents about 30% of the total production of thermoplastics: this is second only to the production of all polyolefins (i.e. low and high density polyethylene and polypropylene together). The consumption of PVC in the principal * Eur. Plast. News, (February 1979), 6(2), 3. t J. Burbidge, Chapter 8, p. 130, of the general source given in Ref. 5.
1 Introduction
11
consuming areas (where most of the production also takes place) is illustrated in Fig. 1.1. A breakdown, by main use, of PVC consumption in 1970 and 1976 is given in Table 1.1 for Western Europe and the USA. Production and consumption statistics for PVC (as well as other plastics) are published each year in the January issue of Modern Plastics International: some relevant information will also be found in the current issue of the Modern Plastics Encyclopedia. Data,
TABLE 1.1 Consumption of PVC Polymers, by Main Outlet, in Western Europe (Including UK) and the USA in 1970 and 1976 Western Europe (1000 metric tonnes)
Outlet
1970 Film and sheet (rigid and flexible) Calendered Extruded Flooring Calendered Coated
} }
430 (21'5%)
1976
} }
USA (1000 metric tonnes)
1970
1976
} }
605 (19·1%)
259} 341 82 (24,8%)
195 (6·1%)
113} 147 34 (10'7%)
550 (27'5%) 230 (11·5%)
785 (24'7%) 335 (10'6%)
215 (15'7%) 186 (13-5%)
682 (31'9%) 150 (7'0%)
Records
40 (2,0%)
78 (2'4%)
64 (4'8%)
68 (3·2%)
Blow-moulded bottles
110 (5'5%)
235 (7-4%)
32 (2·3%)
35 (1·6%)
20 (1·0%) 235 (11'8%)
82 (2'6%) 505 (15·9%)
39 (2'8%) 23 (1·7%)
104 (4'9%) 177 (8'3%)
Misc. coatings (other than flooring)
134 (6'7%)
265 (8'3%)
187 (13'6%)
190 (8'9%)
Others (including plastisol products other than coatings)
72 (3-6%)
90 (2'9%)
138 (10,1%)
283 (13-2%)
2000 (100·0%)
3175 (100,0%)
1372 (100'0%)
2139 (100,0%)
uPVC pipe, conduit and fittings Wire and cable covering
Misc. injection mouldings Misc. extruded products (including rigid profiles and cladding, flexible tubing and profiles)
Total
178 (8'9%)
200 305 105 (14,2%) 75 70
145 (6'8%)
12
W. V. Titow
predominantly for the UK and Europe, usually appear in the January issue of European Plastics News.
1.4 OUTLINE OF THE PVC SECTOR OF THE PLASTICS INDUSTRY Companies operating in the PVC sector of the plastics industry generally fall into one of four main categories, which are as follows: (i) (ii) (iii) (iv)
polymer producers; compounders; processors; companies selling finished goods consisting of or containing PVC.
Polymer importers have not been included in this sequence because they do not normally engage in technical activities. They are however, with the producers, members of the more general category of polymer suppliers. Some companies fall within more than one of the categories listed: for example, the polymer producers all produce compounds; some also produce semi-finished goods. It should be appreciated at this early stage that the number of polymer producers in any country is very small when compared with the large numbers of companies in the other categories. Category (iv) above will contain many companies for which PVC is but a small part of their interests. Nevertheless, such companies, e.g. the automobile producers, can use very large quantities of PVC and are very important to the industry. The principal processes used to convert the PVC to finished and semi-finished goods are extrusion, calendering, injection moulding and spread coating. Although some companies are concerned with more than one of these processes, most tend to specialise in one process. In some cases the processed PVC is marketed directly by the processor (e.g. unplasticised PVC pipes), whilst in other areas the processor passes on the PVC in semi-finished form to another company which employs the material in its products, e.g. vinyl automobile upholstery. In addition to the material producers, converters and users, there are many companies which specialise in the supply of additives for use in PVC compounds, e.g. plasticisers, stabilisers, lubricants, fillers, etc. It is relevant to point out that the value of the total market for some of these materials exceeds that of many other plastics materials.
I
Introduction
13
Also worthy of mention-since without them there would be no PVC industry-are the machinery manufacturers. Many companies have specialised in equipment for PVC processing, and through their development work on plant and equipment new applications for PVC have been made possible.
1.5 VINYL CHLORIDE POLYMERS AND COPOLYMERS 1.5.1 PVC Homopolymers
(a) Chemical Structure The basic repeat unit of the PVC polymer chain is HI HI ]
[
-C-C-
h tl
i
where i is the degree of polymerisation, i.e. the number of repeat units in the molecular chain. The units are linked virtually exclusively 'head-to-tail', i.e. -CHz-CHCI-CHz-CHCI-. In commercial PVC polymers the average values of i range between about 500 and 1500; this corresponds to a theoretical molecular weight range of about 31000-94000.
Note: In practice a given amount of linear, thermoplastic polymer (i.e. a test specimen, processing batch, etc.) will consist of chain molecules made up of the same basic repeat units, but differing in size (chain length). A single chain (polymer molecule) consisting of i repeat units is said to have a degree of polymerisation of i. The molecular weight of such an individual chain (neglecting any small difference due to end groups) may be designated M i , and the species may be referred to as an 'i-mer' or'ith species'. Since, at least in the ideal case, M i is the sum of the weights of all the repeat units in the chain, its value will be different for molecular chains differing in the value of i. The scatter of Mi values (the molecular weight distribution) in a given amount (i.e. sample, batch, etc.) of polymer may be wide or narrow but, as the chains are not all identical, a single molecular weight figure quoted for the whole amount can only be an average value.
14
W. V. Titow
Depending on the method of determination, this value will in practice normally be a number-average molecular weight (M o ) , weight-average molecular weight (Mw ) , or viseosityaverage molecular weight (Mv ).20 For the same batch of polymer these values are numerically in the sequence M w > Mv > Mo, with M v usually closer to Mw than to Mo. In the ideal, theoretical case of all chains being of identical length (the same value of i) M o = M w • Solution viscosity measurement is comparatively straightforward (especially with polymers soluble in convenient solvents) and the data it yields can be used to calculate either M v or-more commonly with PVC in industrial practicesuch related quantities as specific viscosity, * viscosity number, t logarithmic viscosity number or K value§ (ct. also Chapter 2, Table 2.1). The relationships between these various quantities (valid only when the relevant viscosities are determined under the same, standard conditions) are as follows:
,*
Specific viscosity (viscosity increment):
(Tf - Tfo)/Tfo
Viscosity number (formerly known as 'reduced viscosity' or'RV'): (Tf - Tfo)ITfoc Logarithmic viscosity number II (formerly known as 'inherent viscosity' or 'logarithmic reduced viscosity', Le. 'IV' or 'RV'): [In (TfITfo)]le where Tf is the viscosity of a dilute polymer solution (or its time of flow in standard conditions); Tfo is the viscosity of the solvent alone (or its time of flow in standard conditions); c is the concentration of the polymer, in g per ml of solution; and TfITfo is the viscosity ratio (formerly known as 'relative viscosity'). * Still sometimes determined by the (now superseded) Method B of ASTM D 1243-58T. t Method of ISO 174-1974. :j: Method of ASTM D 1243-66 (reapproved 1972). § For PVC normally the Fikentscher K value-ct. DIN 53 726-1983. (see also Chapter 2, Note in Section 2.2.2, and Table 2.1). II Not to be confused with the 'limiting viscosity number' (formerly known as 'intrinsic viscosity'): Iimc-+o [(fl- flo)/f/o C] or limc-+o [(In fl/flo)/c).
15
1 Introduction
The Fikentscher K values corresponding to a range of viscosity ratios of dilute solutions (0·005 g ml- 1 ) of PVC polymer in cyclohexanone at 25°C are listed in DIN 537261983. Most commercial PVC polymers have Fikentscher K values within the range 50-80, equivalent to about 50000-500 000 M w , and 3000090 000 MD. The relationship is illustrated in Fig. 1.2. Polymers of much higher molecular weight have also been made. Like those of other thermoplastics, the properties of PVC polymer are influenced by both the molecular weight and molecular weight distribution: the ratio 100000
5'8
90000
80000
70000
r{ 60000
5-2
~
I~
'"
o
.J
50
5-0+-100000 40000
30000
4·8
--50000 20000 4·6
Fig. 1.2 Relationship between the molecular weight (weight-average and number-average) and the K value of PVC polymers (the Fikentscher K value determined at 25°C on 0·005 g ml- 1 polymer solution in cyclohexanone--d. Chapter 2, Table 2.1 and Note in Section 2.2.2).
w. v.
16
Titow
Mw/M n (known as the 'dispersion of distribution') is a function of the extent of the latter (i.e. the width of the distribution curve, although it is not influenced by its shape). The evaluation of the molecular weight of vinyl chloride homo- and copolymers by gel-permeation chromatography was reviewed recently by Janca and Kolinsky?! The polymerisation temperature influences the molecular weight, and hence the dilute-solution viscosity, of PVC polymers (higher molecular weights obtained at low temperatures). This factor was examined by Ravey and Waterman 22 for the polymerisation temperature range 0-70°C. Intensive heating may be necessary to ensure complete dissolution of PVC polymers produced at sub-zero temperatures, for solution viscosity measurement and determination of M w by light scattering and gel-permeation chromatography.23 The structure of PVC polymer molecules deviates in practice from the theoretical ideal of linear chains of -CHz-CHCl- units terminating in -CHz-CHzCl and -CHCl-CH3 groups. The main differences are listed below: CHAIN BRANCHING
The chains of commercial PVC polymers are branched. Estimates of the extent of branching, based on determinations by various techniques,24,25 range from 0·5 to 20 branches per 1000 carbon atoms, and include the suggestion that 5 out of every 1000 carbons are methyl-branched whilst up to 2 per 1000 carry side-chains of more than 5 carbon atoms. Z6 The two most probable structures for a branch junction are believed to be Z5 ~CH2-CHCI-CH-CHa-CH2-CHCl~
I
R
and ~CH2-CHCI-CH2-TH-CH2-CHCl~
R
where the side-chain R is -CHz-CHCI~, or -CH3 . The presence in PVC polymers of branch junctions involving the
1 Introduction
17
existence of a tertiary chlorine ~CHz-CCl-CHz-eHCI~
I CHz I CHCI
1
has been the subject of much speculation,25 * as their formation (by appropriate radical transfer to polymer in the course of polymerisation) is a valid theoretical possibility, whilst-if present-they would constitute important sites for initiation of thermal or photo degradation of the polymer (owing to the relative ease with which the tertiary CI can be split off-see discussion in Chapter 9). However, no direct evidence of the presence of tertiary CI, or of any functional relationship between the number of chain branches and thermal stability, could be found in a number of studies concerned with these subjects. 25 ,27-30 END-GROUPS
A large variety of end-groups is encountered in commercial PVC polymers. Some are acquired by reactions, in the course of polymerisation, with fragments of initiators, emulsifying or suspending agents, or other 'external' compounds present (to which chain transfer can occur). Others are formed in terminating reactions (chain transfer to monomer or polymer, disproportionation, coupling) involving the monomer and/or any of the species generated during polymerisation. Examples of the end-groups originating in the former way are ~CH2-0C-C6H5; ~CH2-o-S02-0R; and ~CH20H (where R is an alkyl radical): those formed in the 'internal' terminating reactions are ~CHCI-CH=CH2 (an allyl-chloride type of structure in which the CI atom is activated by the unsaturated linkage at the 3,4-position relative to it31 ); ~CH CHCI; ~CCl=CH2; ~CH2-CHCI2; ~CHCI-CH2Cl; and ~CHr-CH2Cl. * cf. also, for example, the discussion by L. I. Nass of the thermal stability of PVC, in the Encyclopedia of pvc (see General Bibliography section at the end of this chapter).
18
w.
V. Titow
OTHER STRUCTURAL PEATURES
The results of several studies32 ,33 confirm the presence of double bonds, distributed randomly in the polymer chain: ordinary PVC polymers may contain up to 15 such bonds per 1000 carbon atoms. 25 Some evidence has also been obtained of the presence of oxygen: 25 apart from oxygen-containing end-groups (see above) some oxygen could be incorporated in the chain during polymerisation, or acquired through oxidation of the polymer; however, the resulting chemical groupings have not been identified.
(b) Morphology Commercial PVC polymers may be regarded as essentially amorphous,34 although crystalline material contents of about 2-10% have been reported on the basis of determinations by X-ray diffraction methods, thermal analysis (DTA, DSC, TMA) , and density measurements. 35-37 The crystallinity is associated with the stereoregular (syndiotactic) polymer fraction. 38-40 The glass transition temperatures (Tg ) of commercial homopolymers lie in the range 80-84°C (as determined by DTA, DSC and TMA).34,35 Annealing above the Tg increases crystallinity and also the crystalline melting temperature (as given by the endothermic peak on DTA curves).35,37 The density of the crystalline fraction has been reported as 1530 kg m-3 (1,530 g cm-3) and that of totally amorphous (quenched) polymer as 1337kgm- 3 (1·337gcm- 3).35 As would be expected, annealing below the Tg has no effect on crystallinity: however it can increase the density-this has been attributed to a reduction of the free volume in the polymer without ordering of its fine structure. 35 Apparently the kind of short-range non-crystalline ('domain') order which can develop in some essentially glassy, but partly crystallisable polymers (bisphenol-A polycarbonate, polyethylene terephthalate)41-44 on heating below the Tg , does not arise in PVc. Relatively highly crystalline PVC polymers (up to about 45% crystallinity) with high syndiotactic material content, have also been prepared (by polymerisation at low temperatures or in certain solvents).24,36,38,45,46 The preparation of fibres and film from such polymers has been reported47 as well as that of plasticised compounds (with about 50 phr DOp).38,47 The melting points of the crystalline polymers can be as high as 265_273°C24 ,46 (cf. commercial PVC polymer-about 210°C in the absence of decomposition). The
i
introduction
19
above-mentioned plasticised compounds of crystalline PVC had a higher modulus and hardness, and lower tensile creep at room temperature than a similar compound of ordinary commercial PVC polymer (but lower tensile strength and extension at break): below -25°C the modulus and hardness were lower (i.e. the flexibility greater) than those of the normal PVC.38 Molecular orientation of the PVC polymer, and the associated structural anisotropy, can be an important factor in the morphology (and the properties) of PVC products (especially uPVC). Thus in PVC mouldings the skin-and-core effects (a well-known, common feature also of other polymer mouldings) can involve, inter alia, a considerable degree of orientation in the skin. 48 Biaxial orientation of the polymer in PVC bottles, films and thermoformed articles increases impact resistance (ct., for example, Chapter 17) and reduces permeability (ct. Chapter 12, Section 12.4). The tensile strength and retraction on heating of extruded products is strongly influenced by longitudinal molecular orientation imparted by stretching in production (ct., for example, Chapter 12, Section 12.3), whilst the high degree of orientation produced in PVC fibres by the drawing process in manufacture is responsible for their very high tensile strength in comparison with other PVC products (cf. Appendix 3). The fracture and yield behaviour of PVC polymers and uPVC compositions is strongly influenced by the extent and nature of molecular orientation. Useful investigational work in these areas has been reported by several authors.49.50--52 1.5.2 Vinyl Chloride Copolymers
Some of these are long-established commercial materials. Others, more recent, have also become of more than academic interest. The oldest, and still most widely used, are vinyl chloride/vinyl acetate copolymers (VCNA). In most of the vinyl chloride copolymers of commercial interest the co-monomer units are in a minor proportion (and randomly distributed) in the polymer chain, i.e. most are internal, random copolymers, with the VC units predominating. Copolymers with vinylidene chloride (VCNDC) are a notable partial exception here, in that whilst those used in certain PVC compositions (e.g. some calendering compounds) for ease of heat-processing contain relatively small amounts of VDC co-monomer, in others (e.g. those for making
20
W. V. Titow
self-supporting, low-permeability films, or barrier layers in composite films) it is the VC co-monomer which is the minor constituent (usually 10-15%). Another exception is constituted by acrylic and (some) modacrylic fibres: by definition,53 the former must contain not less than 85% of acrylonitrile units in the chain (i.e. only up to 15% of a co-monomer, which may be VC), and the latter between 85% and 35%-although the material of Dynel (Union Carbide), a well-known modacrylic fibre, was in fact a 60/40 copolymer of vinyl chloride and acrylonitrile (see also Table 1.2). Some PVC graft copolymers are also noteworthy, e.g. those with ethylene/vinyl acetate copolymers (EVA),62,67 polyolefins,62,68 butadiene/acrylic ester copolymers,62 and acrylic ester polymers. Graft copolymers of the first three kinds were the subjects of early patents by, respectively, Bayer and Dynamit Nobel, Montecatini, and Pechiney-Saint-Gobain. 47 VClEVA graft copolymers are used (typically in blends with PVC homopolymer) in uPVC compositions for outdoor service, notably in window frames, where good weatherability and impact resistance at low temperature are required;3 VC/acrylate grafts are also employed for this application (see Chapter 19, Section 19.4.3). Some graft copolymers with sufficiently high polyvinyl chloride contents (e.g. VC/EVA with 50-70% EVA) can act as plasticisers for PVC homopolymer, or as processing aids. The graft copolymers are, as a rule, more expensive than main-chain copolymers. As has been mentioned in Section 1.2, the chief effects of the presence of a significant proportion of a co-monomer in the vinyl chloride polymer chain are normally similar to (but, in general, more permanent than) those of plasticising a homopolymer with 'external' plasticisers: the processing temperature is reduced (albeit the heat stability also decreases) as is the Tg (and hence the softening temperature and temperature of deflection under load); the hardness also usually decreases, and the extensibility increases.
Note: A partial exception to the general trend, which is of some practical significance, may be noted: some copolymers of vinyl chloride with N-substituted maleinimide derivatives 3,69 have Tg values and Vicat softening points significantly higher than those of vinyl chloride homopolymer: ct., e.g., Hostalit LP HT 5060 (Hoechst)-a copolymer containing 5% of Ncyclohexylmaleinimide. 69 Copolymers are normally more readily soluble than homopolymers (see Chapter 12, Section 12.8, and Chapter 24): when used as surface
1 Introduction
21
coatings they adhere better to many substrates (the adhesion may be further improved by the incorporation of a suitable third co-monomer in the chain-see Chapter 24). Broadly speaking, the morphology of most copolymers is similar to that of PVC homopolymer, except that the reduced regularity of the chain is an extra hindrance to crystallisation. This occurs also in VDCNC copolymers where VC is the minor component: the chain structure of PVDC is favourable to crystallisation and the crystallinity of the homopolymer is normally high;4o.56 the structural regularity and hence ease of crystallisation is progressively reduced as the VC unit content of the polymer chain is increased, until at VC contents;?; 30% the copolymer becomes non-crystalline. It is for this reason that VDCNC copolymer films for barrier applications contain only about 10-15% VC. This content level represents a reasonable combination of easement of processing (the highly crystalline PVDC homopolymer requires high temperatures) and retention of much of the excellent barrier effect of the homopolymer associated with its crystallinity (crystalline regions in polymers are normally impenetrable to diffusant molecules). 1.5.3 'External' Modification of PVC by Other Polymers
PVC polymers can be modified 'externally' by blending with other polymers or copolymers (including vinyl chloride copolymers-see Section 1.5.2 above). On the industrial scale this is widely practised in uPVC compositions, to improve the melt processability, and/or the toughness (impact resistance at normal and low temperatures) as well as-in some cases-the resistance to heat distortion, of the finished product. The polymeric additives incorporated for these purposes are known as processing aids and impact modifiers: they are discussed in Chapter 11. Those of the polymeric additives which are chlorinated (but not vinyl chloride) polymers-e.g. chlorinated polyethylene (see Chapter 11)as well as copolymers of vinyl chloride (e.g. VCNA, VCNDC, or the VC graft copolymers mentioned in Section 1.5.2 above) can have particularly high compatibility with PVC resins: they can sometimes be blended in such large proportions that the composition becomes a plasticised PVC (with high permanence of properties, because the plasticiser is a high polymer-see Chapter 11, Section 11.2). This can also be done with some chlorine-free polymeric additives, e.g. nitrile rubbers (see Chapter 11).
Vinyl chloride/ethylene
Vinyl chloride/propylene (VC/P)
Vinyl chloride/acrylorutrile (VClAN)
Vinyl chloride/vinylidene chloride (VClVDC)
Similar to vinyl chloride/propylene copolymers-S
2. Extruded films (packaging)-S 3. Viscosity-reducing porymer in pastes-S 4. Solution applications (esp. barrier coatingsr-SL 5. Latex applications (esp. paper and textile finishing)-E 6. Fibres, e.g. Saran" (National Plastics Products Co.) Fibres (vinyl chloride is the comonomer in some acrylic or modacrylic fibres, e.g. DyneF (Union Carbide Chemicals Co.) Extruded films (packaging); injection mouldings-S
Refs 57 and 58--polymer production, structure and properties; Ref. 59-polymer composition and density; Ref. ~ackaging film Ref. 61
Commercially available from Air Products and Chemicals Ltd, USA (Airco 400 series) Developed by Union Carbide Chemicals Co.
Refs 40 and 53
Refs 40 and 47-general nature and preparation of the copolymers; Ref. 55-packal;\ing films; Ref. 56-general revIew (with 92 references)
Literature on this copolymer is extensive-see, e.g., relevant titles in the General Bibliography section at the end of this chapter
Literature/References
Chapter 26
6. Chapter 26
5. Chapter23
2. Chapters 19 and 26 3. Chapters 21 and 22 4. Chapters 12, 24 and 26
4. Mentioned, inter alia, in Refs 53 and 54 1. Chapters 2, 3, and 18
2. Chapters 24 and 26. The copolymers used for these applications sometimes contain a third co-monomer 3. Chapters 23 and 26
2. Coatings (solution applications)-SL
3. Adhesives, finishing agents (paper and textiles)-E 4. Fibres, e.g. Vinyon H H (American Viscose Corp.) 1. U nplasticised calendered sheets; mouldings-S
1. Chapters 2, 3 and 26
1. Unplasticised mouldinss (including gramophone records) and sheeting (including sheets for thermoforming and PVC ftooring)-S
Vinyl chloride/vinyl acetate (VCNA)
Relevant chapters/Remarks
Main applications (with indication of the usual method ofproduction of copolymer for use in the applicationa)
Copolymer
TABLE 1.2 Vinyl Chloride Copolymers
o
::;j
:00:::
~
~
Surface coatings-E; SL
Vinyl chloride/vinyl isobutyl ether Vinyl chloride/acrylic ester
2. e.g. PliovicAO (Goodyear. } USA)
3. Chapter 23
b
a
Key: S = suspension polymerisation; E = emulsion polymerisation; SL = solution polymerisation, Also the generic name for vinylidene chloride copolymer fibres containing at least 80% by weight of vinylidene chloride. C Production suspended in 1975.
Vinyl ChlOride/fumariC} 1. Extruded products; injection mouldings-8 ester Vinyl chloride/maleic 2. Low-temperature fusion pastes ester Vinyl chloride/itaconic ester
2. Film and bottles (uPVC) with good transparency, heat weldabihty and impact strength-S 3. Adhesives, finishing agents, coatings-E
1. Window-frame compositions-8
Calendered sheet-S
Vinyl chloride/vinyl cetyl ether
2. Polr.mers and/or compounds avaIlable from various sources in the USA and Europe (e.g. Pantasote Inc., USA-Pantaprene L; Hiils, West Germany-Vestolit HIS 587; Wacker-Chemie, West Germany--VinnoIVl\;BIP, UK-Beetle PVC). Heat after-treatment required for good adhesion. In melt processing processability and thermal stability (of both melt and products) increase with increasing TFCE content Available from the Allied Chemical Co., USA, and some Japanese sources Developed by BASF, West Germany 1. e.g. Vinnoll\ (Wacker Chemie)-Chapter 19
2. Surface coatings; finishing agents (for paper and textiles)-E
Mainly surface coatings-E
1. Graft copolymers
I. Extrusion (profiles, esp. window frames), injection moulding-S
Vinyl chloride/triftuorochloroethylene
Vinyl chloride/EVA
Refs 40, 47, 61
Refs 47,65,66
BASF technical literature
Allied Chemical Trade literature
Ref. 64
Refs 3, 47 and 62-polymer preparation, structure and propertIes; Refs 3 and 63-applications
N
v.>
;:s
~.
f2-
~
;;-
'-
24
w.
V. Titow
Blends have also been prepared of PVC polymer with a copolymer of vinyl chloride and an unsaturated dimethacrylate compound (DMA), in which the latter component could be cross-linked (via the double bonds in the DMA) to provide a PVC material of improved strength and stability, and better processability when vacuum-formed as a sheet. 7o A further refinement of this concept (and a PVC alloy with an unusual structure) is represented by a blend of PVC polymer with a butadiene/acrylonitrile copolymer in which both components are cross-linked to form two separate but intimately interpenetrating networks. Depending on the cross-link density the properties of the material can range from those of a tough elastomer to those of a soft plastic. 71 Improved ease of processing and higher temperatures of deflection under load are claimed for blends of PVC with styrene/maleic anhydride copolymers (d. for example, Bourland and Wambach in Plastics Engineering, 1983, 39(5), 23-7). Some versions of the blends are available as commercial injection-moulding compounds (e.g. from the Arco Chemical Co., USA). 1.5.4 Properties of PVC Compositions To make their processing possible, and to achieve the required performance in service, PVC polymers are compounded with various additives to make up the compositions which are the substance of the PVC materials and products of industry and commerce. The properties of these compositions form one of the major topics dealt with in this book: almost every chapter features one or more of their aspects, including their durability, their individual and relative importance in particular contexts and applications, their measurement, the ways in which they are influenced by formulation and processing, and others. Many numerical values of properties characteristic of various compositions and products are quoted in Appendix 3, as well as throughout the text. 1.6 CHLORINATED POLYVINYL CHLORIDE (CPVC) This is an old-established material, first produced commercially in the mid-1930s in Germany by chlorination of PVC polymer in solution (in a chlorinated hydrocarbon solvent-typically tetrachloroethane or
1 Introduction
25
chloroform) at elevated temperatures (50 to about 100°C). CPVC made by this process is more soluble in solvents than the parent PVc. The early commercial materials-e.g. Igelit PC (I. G. Farbenindustrie) and Rhenoflex (Dynamit Nobel)-were used in solution-applied surface coatings and adhesives. CPVC fibres were also spun from solvent solutions. These applications still continue to some extent. Around 1960 the dispersion chlorination process came into use. In the originally patented version of this4o ,47 PVC polymer in aqueous dispersion is treated with a large excess of chlorine at relatively low temperatures (up to 60°C) in the presence of a swelling agent (a chlorinated hydrocarbon, e.g. chloroform) under UV light. The early commercial polymers produced in this way are exemplified by Trovidur HT (Dynamit Nobel) and Geon HT (Goodrich). Dispersionchlorinated CPVC polymers are less soluble than those produced by the solution process and their thermal stability is better. In both processes chlorination takes place mainly at the -CHzgroups of the PVC polymer chain (Le. the 1: 2 chlorinated configuration, -CHCI-CHCI-, is preferentially formed) so that the resulting chain structure becomes virtually that of a copolymer of vinyl chloride with 1: 2 dichloroethylene* «a) in Fig. 1.3), rather than that of a vinyl chloride/vinylidene chloride copolymer «b) in Fig. 1.3) which would be given by preferential 1: 1 chlorination. t The large preponderance of the 1: 2 chlorination is shown by the IR spectra of CPVC. It is also evidenced by the products of thermal decomposition: those generated -CHCl-CHCl-CHz-CHCl-CHCl-CHCl-CHCl-CHCl(a) Vinyl chloride/1:2 dichloroethylene copolymer or a chlorinated PVC -CHz-CClz-CHz-CHCl-CHz-CClz-CHz-CClz(b) Vinyl chloride/vinylidene chloride copolymer -CHCl-CHCl-CHCl-CHCl-CHCl-CHCl-CHCl-CHCl(c) Homopolymer of 1: 2 dichloroethylene Fig. 1.3 Simplified representation of polymer segment structures.
* Production by direct polymerisation of the monomers impracticable, although some brittle, low molecular weight products have been obtained in attempts to prepare the homopolymer. 40 t In the dispersion process tendency to 1: 1 chlorination can be increased at high temperatures if the chlorine concentration is allowed to fall. 4o
26
W. V. Titow
by CPVC contain virtually none of the aromatic hydrocarbons produced by the pyrolysis of both PVC homopolymer (see Chapter 12, Section 12.9.3) and VCIVDC copolymers. 4o Complete chlorination would give a polymer very similar to the symmetrical polydichloroethylene* «c) in Fig. 1.3); however, the degree of chlorination of commercial CPVC polymers is considerably lower than this (see Table 1.3). The density and Tg (and hence the Vicat softening point) increase with the chlorine content. TABLE 1.3 Some Properties of Commercial CPVC and PVC Polymers, and FuUy Chlorinated PVC CPVC polymer Chlorine content (weight %) Density (gcm- 3)
Tgeq
Maximum service temperature (for commercial compounds) eq Continuous exposurec Intermittent exposure c
PVC Fully chlorinated homopolymer PVC polymer
65·67 1·52-1·59b 99-123
56·8 1·40 80-84
90
65 80
110
73·2" 1·70 175
"Theoretical figure for polymer of 1 :2 dichloroethylene. b Density of commercial compounds: 1·47-1·62. This coincides almost exactly with the range specified for CPVC pipes and fittings in ISO 3514-1976. C In non-aggressive environments.
Since the same kind of site (the -CH2- group) is preferentially chlorinated in the chains of CPVC polymers prepared by either process, and the total chlorine content ranges are essentially the same, other structural factors must be responsible for the differences in solubility and thermal stability between the products of the two processes. The balance of evidence from investigations carried out since early times47 indicates that these differences are associated with the way in which the chlorine atoms substituted into the -CH2groups are distributed within the polymer chain: the distribution does vary according to the method of preparation in a manner suggesting * Production by direct polymerisation of the monomer impracticable, although some brittle, low molecular weight products have been obtained in attempts to prepare the homopolymer. 40
1 Introduction
27
that the variation is due to different accessibility of the polymer chains to the chlorine in the two processes. Solution chlorination has been reported47 to result in a uniform, random, 'statistical' distribution of the chlorine among the -CHz- groups of the molecular chain, such as would be expected if all the chains (and all segments within an individual chain) were equally accessible to the reagent. The chlorination produced by the dispersior process is believed47 to be hetrogeneous, in two senses: the chlorine contents of different molecular chains are not the same, and each individual chain contains irregularly alternating blocks of polyvinyl chloride and 1: 2 dichloroethylene polymer structures. It has also been found 47 that CPVC produced by the dispersion process from PVC polymer of high steroregularity (high syndiotactic material content) has a higher Vicat softening point than one similarly produced from polymer of low stereoregularity, whilst there is no corresponding difference between analogous solution-chlorinated materials. Chlorination of PVC polymers reduces the forces of attraction between the molecular chains, as evidenced, for example, by the comparatively greater ease and extent of stretching of CPVC films above the Tg . 72 The essentially amorphous morphology of CPVC polymers is probably a factor in this effect, as even the small amount of crystalline material present in commercial PVC polymers would have a constraining effect (with the crystallites acting as quasi cross-links) at temperatures up to the crystalline melting point. In comparison with uPVC, the effect of stretching (especially biaxial stretching) of CPVC sheet upon some of its properties (increases in Young's modulus and yield stress) is greater, although the permeability to COz of CPVC sheet was found to increase with biaxial orientation, in contrast with the reverse effect observed with uPVC: 72 as pointed out by the investigators, the increase in permeability on biaxial stretching is characteristic of essentially amorphous polymers which do not crystallise under tension. 72 ,73 However, the increase in the impact strength of the CPVC sheet, which was also achieved by biaxial stretching in the above investigation, was claimed to be greater than that attainable-in the absence of molecular orientation-through incorporation of impact modifiers. Commercial CPVC compounds are formulated on the same general lines as uPVC compounds (see Chapter 4). However their processing is influenced by the fact that the melt viscosity of the polymer increases sharply with the chlorine content. 74 The compounds are used mainly
28
W. V. Titow
for the production of pipes and pipe fittings for hot-water installations (including, increasingly, domestic central heating systems), where the general similarity of properties to uPVC (including, inter alia, suitability for jointing by solvent welding) combined with the greatly increased temperature resistance in service, are particularly advantageous. Other applications include pipes and fittings for potable water (CPVC is approved for this purpose by several professional and regulatory bodies 75), pipework and associated products (fittings, valves, tanks) for chemical plant (the general chemical resistance of CPVC is comparable with that of uPVC) , extruded profiles, sheets (including co-extruded CPVClpPVC sheets), some electrical appliances, and constructional applications. Some examples of commercial CPVC compounds are: the Lucalor range (Rhone-Poulenc, France), which includes Lucalor RB 1266 specially developed, and recently evaluated, for central heating systems; the Dekadur compounds (Deutsche Kapillar Plastik, West Germany) and the CPVC compounds in the Geon range (B. F. Goodrich, USA). Some of the properties of three Geon compounds are listed, by way of example, in Table 1.4. TABLE 1.4 Some Properties of 'Geon' CPVC Compounds
(Based on manufacturer's published data)
Property
Tensile strength (lbf in- 2 ) (ASTMD 1708) Flexural strength (lbf in- 2) (ASTMD790) Flexural modulus (lbf in- 2) (ASTMD790) Izod (notched) impact strength (ft lbf in -1) Deflection temperature under load eC): at 264lbf in- 2 (ASTMD648) Specific gravity (ASTMD792)
Geon 88933 (high temperature extrusion and injection moulding)
Geon 88934 Geon 88935 (extrusion of (profile co-extrusion pipes and with pPVC-high profiles) ductility compound)
8200
8400
7300
14500
15600
13600
387000
395000
396000
2·3
2·0
3·2
100
102
82
1·52
1·57
1·47
1 Introduction
29
It may be noted in passing that its higher chlorine content reduces the flammability of CPVC in comparison with uPVC.
1.7 MATERIAL AND TEST STANDARDS The properties of PVC materials and products, as well as methods of their characterisation and testing, are the collective subject of a very large number of standard specifications. Whilst some companies (particularly polymer manufacturers) and big user organisations (e.g. government and military procurement departments, motor car manufacturers) operate their own in some cases, the standards of by far the greatest importance to the PVC technologist and user in the Western World are those of the following four groups: (i)
Standards developed by the appropriate technical committees of the International Organisation for Standardization and published by that organisation (ISO standards). The ISO committees dealing with plastics are TC 61: Plastics and TC 138:
Plastic Pipes, Fittings and Valves for the Transport of Fluids. (ii) Standards of the British Standards Institution (BS standards). (iii) Standards of the American Society for Testing and Materials (ASTM standards) (iv) Standards of the German Institute for Standards (DIN Deutsches Institut fUr Normung: DIN standards).
Most other countries also issue their own national standard specifications. Those standards from the four main sources which relate directly to PVC are listed (by number and title) in Appendix 1. The list is divided into sections, grouping the standards by subject and also largely according to their relevance to the chapters dealing with particular topics in this book. * In addition, many 'plastics' standards not specifically or primarily directed to PVC, but nevertheless relevant to particular aspects of PVC materials, products or technology, are mentioned in the introduction to Appendix 1, in Appendix 3, and in various appropriate places in the book. The numerous references to standard specifications throughout this * For example, Section 4 of Appendix 1 lists standards dealing with various aspects of plasticisers, and is thus directly relevant to Chapters 5-7.
30
W. V. Titow
book, and the listings in Appendices 1 and 3, contain in almost every individual case not just the specification number, but also a year of issue, since this can serve as a useful point of reference. Most of the years of issue so quoted should be current at the time of going to press, but it will be appreciated that international and national standards are being periodically amended and revised, with consecutive issues appearing under newer dates. Entirely new standards are also being brought out. The introduction to Appendix 1 provides guidance on keeping up to date with proposed, new, and revised standards. The excellent book by Ives et ai. 76 served for a long time as a valuable source of information on standard tests for plastics (including PVC). An updated version, produced by an editorial team, is now available in a new edition. 77
REFERENCES 1. Regnault, V. (1838). Ann. Chim. Phys., 2,69, 151. 2. Drukker, H. L. (1944). Proc. of Symposium on Plastics, Am. Soc. for Testing Materials, Philadelphia, Pa, USA, pp. 165-77. 3. Domininghaus, H. (1976). Die Kunststoffe und Ihre Eigenschaften, VDI-Verlag GmbH, Diisseldorf, p. 566. 4. Baumann, E. (1872). Ann. Chim. Phys., 163, 308-12. 5. Tester, D. A. (1973). In Developments in PVC Technology, (Eds J. H. L. Henson and A. Whelan), Applied Science Publishers, London, Chapter 1. 6. Ostromislensky, 1. (1912). British Patent No. 6299; German Patent No. 264123. 7. Brydson, J. A. (1975). Plastics Materials, Newnes-Butterworths, London, pp. 248-9. 8. British Patent No. 408969, Carbide and Carbon Chemicals Corp., (1934). 9. US Patent No.1 938662, Du Pont, (1933). 10. British Patent No. 387928, British Thomson-Houston, (1932). 11. Canadian Patent No. 346164 (1934). 12. British Patent No. 388309 (1933); US Patent No. 1932889 (1933). 13. British Patent No. 412442 (1934). 14. German Patent No. 470 149 (1927). 15. Kaufman, M. (1969). Plast. Polym., 37(129),243-51. 16. Kaufman, M. (1969). The History of PVC, Elsevier, London. 17. Anon. (1974). Chern. Engng. News, 52(35),8. 18. Trevitt, E. W. (1976). Polym. Paint Col. J., 166(3918), 193-4. 19. Anon. (1979). Eur. Plast. News, 6(6), 8. 20. Billingham, N. C. and Jenkins, A. D. (1972). In Polymer Science, Vol. 1, (Ed. A. D. Jenkins), North-Holland Publishing Co., AmsterdamLondon, Chapter 2.
i
introduction
31
21. Janca, J. and Kolinsky, M. (1976). Plasty a Kaucuk, 13(5), 138-41. 22. Ravey, M. and Waterman, J. A. (1975). J. Polym. Sci., Polym. Chem. Ed., 13(6), 1475-8. 23. Tavan, M., Palma, G. and Carenza, M. (1975). J. Appl. Polym. ScL, 19(9),2625-7. 24. Pezzin, G. (1969). Plast. Polym., 37(130), 295-301. 25. Braun, D. (1975). In Degradation and Stabilisation of Polymers, (Ed. G. Geuskens), Applied Science Publishers, London, Chapter 2. 26. Schwenk, V., Cavagna, F., Lomker, F., Konig, I. and Streitberger, H. (1979). J. Appl. Polym. Sci., 23, 1589-93. 27. Caraculacu, A. A. (1966). J. Polym. Sci., A-i, 4, 1829, 1839. 28. Caraculacu, A. A., Bezdadea, E. C. and Istrate-Robila, G. (1970). Ibid., 8, 1239. 29. Braun, D. and Weiss, F. (1970). Angew. Makromol. Chem., 13(55), 67-71. 30. Suzuki, T., Nakamura, M., Yasuda, M. and Tatsumi, J. (1971). J. Polym. Sci., C, 33, 281. 31. Fieser, L. F. and Fieser, M. (1944). Organic Chemistry, D. C. Heath & Co., Boston, pp. 152-5. 32. Valko, L. and Tvaroska, I. (1972). Angew. Makromol. Chem., 23, 173. 33. Braun, D. and Quarg, W. (1973). Ibid., 29/30, 163. 34. Haward, R. N. (Ed.) (1973). The Physics of Glassy Polymers, Applied Science Publishers, London, pp. 201-6. 35. Gray, A. and Gilbert M. (1976). Polymer, 17(1), 44-50. 36. D'Amato, R. J. and Strella, S. (1969). Applied Polymer Symposia, No.8, 275-86. 37. Ohta, S., Kajiyama, T. and Takayanagi, M. (1976), Polym. Engng. Sci., 16(7), 465-72. 38. Gugelmetto, P., Pezzin, G., Cerri, E. and Zinelli, G. (1971). Plast. Polym., 39(144), 398-402. 39. Abdel-Alim, A. H. (1975). J. Appl. Polym. Sci., 19(8), 2179-85. 40. Brighton, C. A. (1962). In Advances in PVC Compounding and Processing (Ed. M. Kaufman), Maclaren & Sons Ltd, London, Chapter 1. 41. Titow, W. V., Braden, M., Currell, B. R. and Loneragan, R. J. (1974). J. Appl. Polym. Sci., 18,867-86. 42. Frank, W., Goddar, H. and Stuart, H. A. (1967). Polym. Lett., J. Polym. Sci.. 5,711. 43. Siegmann, A. and Geil, P. H. (1970). 1. Macromol. Sci. (Phys.), 84(2), 239. 44. Kashmiri, M. I. and Sheldon, R. P. (1969). Polym. Lett., J. Polym. Sci., B,7,51. 45. Bockman, O. C. (1965). Brit. Plast., 38(6), 364-5. 46. Gouinlock, E. V. (1975). J. Polym. Sci., Polym. Phys. Ed., 13(5),961-70, and 13(8), 1533-42. 47. Bier, G. (1965). Kunststoffe, 55(9),694-700. 48. Copsey, C. J., Gilbert, M., Marshall, D. E. and Vyvoda, J. C. (1978). 'The dependence of PVC structure and properties on injection moulding variables', paper presented at the PRI International Conference on PVC
32
w. V. Titow
Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 49. Rider, J. G. and Hargreaves, E. (1969). J. Polym. Sci., A-2, 7,829-44. 50. Miller, L. E., Puttick, K. E. and Rider, J. G. (1971). J. Polym. Sci., C, 33, 13-22. 51. Smith, K., Hall, M. G. and Hay, J. N. (1976). Polym. Lett., J. Polym. Sci., 14(12), 751-5. 52. Brady, T. E. (1976). Polym. Engng. Sci., 16(9),638-44. 53. Cook, J. G. (1964). Handbook of Textile Fibres, Merrow Publishing Co., Watford, England. 54. Dux, J. P. (1970). 'Vinyon and related fibres' in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 21, 2nd Edn, John Wiley, New York, pp. 441-51. 55. Oswin, C. R. (1975). Plastic Films and Packaging, Applied Science Publishers, London. 56. Sauntson, B. J. and Brown, G. (1971). Reports on the Progress of Applied Chemistry: Plastics, LVI, 66-76 (Society of Chemical Industry). 57. Cantow, M. J. R, Cline, C. W., Heiberger, C. A., Huibers, D. Th. A. and Phillips, R (1969). Mod. Plast., 46(6), 126-38. 58. Heiberger, C. A., Phillips, Rand Cantow, M. J. R (1969). Polym. Enging. Sci., 9(6), 445-51. 59. Ravey, M. (1975). J. Polym. Sci., Polym. Chem. Ed., 13(11),2635-7. 60. Briston, J. (1976). Packag. Rev., 96(3), 71-2. 61. Sarvetnik, H. A. (1969). Polyvinyl Chloride, Van Nostrand, New York. 62. Goebel, W., Bartl, H., Hardt, D. and Reischl, A. (1965). Kunststoffe, 55, 329-32. 63. Edser, M. H. and Bulezuik, B. W. (1974). Polym. Paint Col. J., (4th December), 1051-6. 64. Ulbricht, J. and Rassler, K. (1976). Plaste u. Kaut., 23(7),487-90. 65. Albert, W. (1963). Kunststoffe, 53(2), 86-93. 66. Bohn, L. (1963). Kunststoffe, 53(2), 93-9. 67. Edser, M. H. and Bulezuik, B. W. (1974). Loc. cit., (18th December), 1090-4. 68. Pegoraro, M., Szilagyi, L., Locati, G., Ballabio, A., Severini, F. and Natta, G. (1968). Chimica e Ind., 50(10), 1075-81. 69. Kiihne, G., Andrascheck, H. J. and Huber, H. (1973). Kunststoffe, 63(3), 139-42. 70. Sasaki, I. and Ide, F. (1975). Polym. Lett., J. Polym. Sci., 13(7), 427-32. 71. Sperling, L. H., Thomas, D. A., Lorenz, J. E. and Nagel, E. J. (1975). J. Appl. Polym. Sci., 19(8), 2225-33. 72. De Vries, A. J. and Bonnebat, C. (1976). Polym. Engng. Sci., 16(2), 93-100. 73. Hopfenberg, H. B. and Stannett, V. (1973). In The Physics of Glassy Polymers, (Ed. R N. Haward), Applied Science Publishers, London, Chapter 9. 74. Arnold, G. H. (1970). Plast. Polym., 38(133),21-6. 75. Anon. (1979). Plast. Technol., 25(9), 31. 76. Ives, G. c., Mead, J. A. and Riley, M. M. (1971). Handbook of Plastics Test Methods, Iliffe Books, London.
I
Introduction
33
77. Brown, R. P. (Ed.) (1981) Handbook of Plastics Test Methods. 2nd Edn, George Godwin Ltd. and the PRI, London.
GENERAL BIBLIOGRAPHY ON PVC SPE Vinyl Professional Activity Group (1964). A Guide to the Literature and Patents Concerning Polyvinyl Chloride Technology, SPE, Stamford, Conn., USA. Sarvetnik, H. A. (1969). Polyvinyl Chloride, Van Nostrand, New York. Canton, M. J. R. (1970). 'Vinyl Polymers (Chloride)', in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 21, 2nd Edn, John Wiley, New York, pp. 369-412. Dux, J. P. (1971). 'Vinyl Chloride Polymers', in Encyclopedia of Polymer Science and Technology, Vol. 14, (Eds H. F. Mark and N. G. Gaylord), Wiley-Interscience, New York, pp. 305-483. Matthews, G. (1971). Vinyl Chloride and Vinyl Acetate Polymers, Plastics Institute Monograph, IIiffe Books, London. Sedlacek, B. (Ed.) (1971). Polyvinyl Chloride: Its Formation and Properties, Proceedings of IUPAC Symposium, Prague 1970. Butterworths, London. Sarvetnik, H. A. (Ed.) (1972). Plastisols and Organosols. Van Nostrand, New York. Brydson, J. A. (1975). Plastics Materials, 3rd Edn, Newnes-Butterworths London, Chapter 12. Yescombe, E. R. (1976). Plastics and Rubber: World Sources of Information, Applied Science Publishers, London, pp. 151, 177-80,359. Nass, L. I. (Ed.) (1978). Encyclopedia of PVC, Marcel Dekker, New York. Burgess, R. H. (Ed.) (1981). Manufacture and Processing of PVC, Applied Science Publishers, London. Owen, E. D. (Ed.) (1984). Degradation and Stabilisation of PVC, Elsevier Applied Science Publishers, London.
~
~
..,;; 't
.
Fig. 1.4(A)
I
~I
·nn:, 1·1'
I,
wave
',,:;mml,,!T .1: . , I 'I: ;.1 fT:, IHF.!"ill'''
[A]
I'l~
I !
I
"
" .Itla·:.
I
number, em- 1
r~"
?
Ii, liI::~~ I:HiI,
I
length, lim
Infra red spectra of commercial PVC resin (Corvic D60/11-ICI Ltd).
illlEtmmnnmuI:H~:
·~H ,I",,'
wave
:to
t.
I
"... .'1: I
I,
c;;:
:::;j
:<:::
~
w -""
Fig. 1.4(B)
c
-
It
•..
E
':i
0
c·
~
wave
number. cm-'
Ltd.).
Infra red spectra of plasticised PVC (100 parts Corvic D60111, 60 parts DOP-Bisoflex 81-BP Chemicals
[8]
wave length. lim
w
V1
::: !:l Q' ::
Q..
::
a
CHAPTER 2
Commercial PVC Polymers W. V. TITaw
2.1 INTRODUCTION-PRODUCTION AND MAIN TYPES Commercial PVC polymers are the products of polymerisation (in some cases also copolymerisation) of vinyl chloride (monochloroethene; chloroethylene) CH2=CHCI. The monomer itself is commonly produced by the two reaction routes outlined schematically in Fig. 2.1. The ethylene route via dichloroethane, which is the most widely utilised, is normally operated in the form of the so-called balanced process. This combines the chlorination and oxychlorination of ethylene to dichloroethane, followed by the latter's pyrolytic dehydrochlorination to vinyl chloride. Ethylene route (Balanced Process) O/catalyst /
\.
' " high temperature
?
CH2 =CH2
02/catalyst + HO
CH2 Cl-CH2 Cl
..., ,,,
I'
pyro YSIS
~ CH2 =CHCl + HC}
,,
~------------------------------------------------~
Acetylene route CHa:CH + HC}
(excess)
catalyst So-250"C
~ CH2=CHCl
Fig. 2.1 Main reaction routes in industrial production of vinyl chloride. 37
38
W. V. Titow
Dichloroethane produced by direct catalytic chlorination of ethylene, in either vapour or liquid phase, is purified and pyrolysed directly to vinyl chloride, HCI being formed as a by-product. This is re-used in the process to make more dichloroethane by oxychlorination of some of the feed ethylene through joint reaction with oxygen (supplied directly, or as air): 2CHz=CHz + 4HCI + Oz ~ 2CHzCI-CHzCI + 2HzO The balanced process embodiment of the ethylene route, long favoured in the USA, has provided the basis of industrial production of vinyl chloride in most of the major plants set up since the mid-1960s, in which petroleum-derived ethylene is used. This feed is cheaper than acetylene; the process affords the greatest economy when run on a large scale at or near the full capacity of the plant. The acetylene route, already mentioned in Chapter 1 (Section 1.2), was of prime industrial importance until the early 1960s, and especially favoured in Europe. Although in comparison with the ethylene route it offers the advantages of lower plant cost and effective operation on a smaller scale, it is more energy-intensive, and acetylene from petroleum is more expensive than ethylene. However, as acetylene can also be readily obtained from coal (see Chapter 1, Section 1.2), this route remains relevant where raw materials can be derived from that source and-for reasons just mentioned-where relatively small-scale operation is required. Vinyl chloride, often designated by the letters VCM (vinyl chloride monomer), is a colourless gas at room temperature and normal pressure (boiling point, -13·9°q, with explosive limits of about 4-20% by volume in air,! For industrial processing it is normally kept as a liquid under pressure. It has anaesthetic properties and an odour resembling that of chloroethane. Because of its carcinogenic effects discovered in the 1970s (see Chapter 1, Section 1.2, and Chapter 12, Section 12.9.1) VCM is regarded as a health hazard and precautions must be observed in its handling and processing to prevent direct body contact and to keep the concentration in the factory air within the permitted maximum limits laid down by the relevant national authorities. Maximum content values for VCM in PVC polymers are similarly laid down, as are limits for amounts extractable from PVC products. These aspects are discussed in Section 12.9.1 of Chapter 12. The polymerisation techniques used to produce PVC polymers are, in order of commercial importance, suspension polymerisation (about
2 Commercial PVC Polymers
39
80% of total commercial polymer production), emulsion polymerisation (about 10-15 %), bulk polymerisation, also called mass polymerisation (about 10%), and solution polymerisation. The commercial application of the solution process is almost entirely confined to the manufacture of copolymers for use in surface coatings (see Chapter 24), centered on one major American producer (Union Carbide). The amount produced probably represents less than 3% of the total PVC polymer sold in the USA. Polymerisation is carried out in a liquid which may either be a solvent for both the monomer(s) and the polymer, or only for the monomer(s) so that the forming polymer precipitates out of the system ('precipitation polymerisation'). This can also happen in initially homogeneous systems above a certain degree of conversion. Initiators and selected chain-transfer agents are included in a typical solution polymerisation system. The resulting polymers can be of high purity, and the method affords good control over molecular weight. In a typical suspension polymerisation process a suspension of monomer dropiets (about 50-150.um) in water is formed by vigorous agitation in a pressure vessel (autoclave), and the monomer of the droplets, which contains a monomer-soluble free-radical initiator, is polymerised at a slightly elevated temperature, under the autogenous VCM vapour pressure corresponding to the reaction temperature (up to 1·5 MPa at 80°C). Protective colloids are included in the aqueous phase to stabilise the suspension throughout, and also buffers to counteract development of acidity. When 80-90% of the monomer has been converted to polymer the reaction is stopped by venting-off excess monomer from the reactor. The resulting slurry of polymer in water is freed from the remaining monomer by further evaporation, either in the autoclave itself or in a separate vessel. This process is known as stripping: to secure effective high degree of removal of residual VCM, stripping is assisted and completed by steam 'sparging'. Different variants, both batch-wise and continuous, of this operation exist, all essentially involving scrubbing the slurry with steam at an elevated temperature. Continuous, counterflow scrubbing in an efficient column can reduce the VCM content of the polymer to below 1 ppm. The stripped slurry is centrifuged to remove free water, and the resulting wet polymer 'cake' is dried by hot air. Note: In the so-called microsuspension polymerisation the general composition and physical state of the system are more akin to
40
w. v.
Titow
those characteristic of emulsion polymerisation, in that an emulsifying agent is used and the size of the monomer droplets is comparable with that typical for emulsion systems. Thus, although a monomer-soluble initiator is employed as in regular suspension polymerisation-and therefore initiation takes place inside the droplets-the processing is essentially as in emulsion polymerisation. The bulk polymerisation system comprises only the monomer and a free-radical initiator (typically an acyl peroxide or peroxycarbonate). Polymerisation is carried out under pressure (to keep the VCM liquid) and normally at 40-70°C. The process has two distinct stages: first polymer grains form in, and separate from, the liquid VCM phase (unlike some other polymers polyvinyl chloride is insoluble in its monomer), giving rise to a heterogeneous system. This stage is carried out under strong agitation to ensure uniformity and correct morphology of the grains. At a relatively low degree of conversion (about 8-12%) much of the VCM liquid is absorbed onto the porous polymer grains, and the viscosity, initially low in the essentially liquid system, becomes too high for effective stirring, whereas the grains develop enough strength to withstand transfer to a second-stage reactor. Here, the further polymerisation, and growth of the polymer grains, proceeds to completion in what becomes essentially a solid (powder) phase. Additional monomer and initiator required for the second stage are introduced during the transfer. At about 20% conversion the material has the appearance and consistency of damp powder, and that of a dry powder (with all the monomer absorbed into the grains) at about 40% conversion. Agitation, particularly important in the second stage, is provided by agitators of special design. Polymerisation is terminated at a predetermined level of conversion (usually about 80%) by ventingoff unreacted VCM. Residual monomer is stripped from the polymer by de-gassing (in the reactor or a separate vessel) assisted by introduction of steam or an inert gas to act as carrier. The bulk polymerisation process was developed and perfected by the French company Pechiney-Saint Gobain: subsequently in consequence of mergers and take-overs it became the property of Rhone-Poulenc Industries, which is now part of the nationalised French chemical industry conglomerate. 2 In emulsion polymerisation the system consists of the monomer and water containing emulsifier(s) and a water-soluble initiator. In batch-wise operation polymerisation is carried out in an autoclave
2 Commercial PVC Polymers
41
designed for operation at the VCM vapour pressure generated at the reaction temperature (typically 40-60 oq, say up to about 1 MPa. In the presence of the emulsifier(s), agitation of the charge in the autoclave disperses the monomer into very fine droplets (down to about O·I,um). The initiator (commonly potassium or ammonium persulphate alone or with a reducing agent, or a more complex redox system-e.g. HzOz/FeSO,Jascorbic acid) produces free radicals in the aqueous phase, where initiation takes place, at the boundary with the monomer phase. The degree of conversion is normally about 90%, the reaction being terminated by venting-off excess monomer. The final stripping of VCM from polymer produced by the emulsion process is similar in principle to that practised with suspension polymer, although it can be more difficult in practice: some spraying methods have been claimed to be particularly effective. 3 Continuous emulsion polymerisation processes are also operated by a few companies (notably in Germany). Indeed this type of process was among the earliest industrial PVC resin production processes to be developed. In principle, subject to suitability of co-monomer reactivities, all the four types of polymerisation process mentioned may be used for the production of vinyl chloride copolymers. In the case of bulk polymerisation it is also necessary that the copolymers should be insoluble in their monomers. In practice, the most important commercial copolymer-vinyl chloride/acetate-is normally produced by suspension polymerisation (which is cheaper and easier than the emulsion process) for melt processing, and by the solution process (where it is also sometimes modified by a third comonomer) for surface-coating applications (see Chapter 24). Of the other copolymers of commercial significance for melt processing, those with ethylene, propylene and vinylidene chloride are all made by the suspension process. Graft copolymers of VClEVA are also produced for outdoor applications (e.g. in window-frame profiles-see Chapter 1, Section 1.5.2 and Chapter 19, Section 19.4.3). 2.2 POLYMER CHARACTERISTICS CARDINAL TO DEHAVIOUR IN PROCESSING AND/OR SERVICE PERFORMANCE The features defining the principal differences among PVC polymers and primarily responsible for their service properties and/or processa-
w. V. Titow
42
bility are: 1. chemical composition (i.e. whether homopolymer or copolymer, and-in copolymers-the chemical nature and proportion of comonomer(s) present); 2. molecular weight (average and distribution); 3. particle characteristics (size and size distribution, morphology). The nature and amounts of extraneous impurities and adventitious functional groups in the polymer chains are also significant in some contexts.
2.2.1 Composition
In the commercial sense the most important copolymers of vinyl chloride are those with vinyl acetate. The incorporation of this co-monomer into the polymer chains reduces the melt viscosity (and hence eases melt processing) and improves solubility (a desirable feature in some solution applications), but the softening point, heat stability, and toughness are also reduced. For copolymers of comparable molecular weights these effects increase with increasing proportion of the vinyl acetate component. This proportion is in the range of 10-16% in copolymers for both melt and solution processing, although in solution polymers modified with a third component the vinyl acetate content may be as low as 2% (cf. Chapter 24, Table 24.1). Of the three main melt processing outlets for YCNA copolymers (which are mentioned also in other chapters), flooring (where ease of melt flow, and ability to accept large amounts of fillers are important) and gramophone records (again requiring good flow, and faithful reproduction of moulding surface detail) call for polymer of high vinyl acetate content (typically about 15%) and low molecular weight (K value typically about 50). On the other hand for packaging films and foils, where easy melt flow must be combined with good melt elasticity, and good mechanical properties are required in the product, a typical choice would be copolymer of about 60 K value containing about 10% vinyl acetate. In commercial copolymers of vinyl chloride with ethylene and propylene (see Chapter 1, Table 1.2), the presence of the olefin component affects the melt viscosity in the same way as that of the vinyl acetate in YCNA copolymers. However, there is comparatively very little adverse effect on heat stability and strength properties.
2 Commercial PVC Polymers
43
Nevertheless, the use of these copolymers is very limited, mainly because they are considerably more expensive than VCNA copolymers. Incorporation of the required proportion of a co-monomer into the polymer is achieved by adjusting the monomer contents of the reaction mixture in accordance with the relevant parameters (respective reactivity ratios, resonance and polarity factors 4 ), and suitably controlling the polymerisation conditions. 2.2.2 Molecular Weight (Viscosity Number and K Value) The average molecular weight of a PVC polymer is usually designated by the 'viscosity number' or the' K value'. Both are calculated from the results of determination of the viscosity of a dilute solution of the polymer. Although the methods of determination are all based on the same principle some differ in certain respects (e.g. solvent used, solution concentration), and different standards are employed in different countries despite the existence of international standards (ISO 174 and ISO 1628-see Appendix 1, Section 2.2). The values obtained by the methods in widest use are given in Table 2.1, together with certain other molecular weight indices which are sometimes determined. Note: Viscosity number (formerly known as 'reduced viscosity') is given by the expression:
(11 - 11o)/11oC equivalent to
(t - to)/toc
where: 11 = viscosity of the polymer solution; 110 = viscosity of the pure solvent; t = average flow time of the polymer solution in specified conditions; to = average flow time of the pure solvent in the same conditions; and c = solution concentration (g polymer per ml of solution). The concept of K value was first introduced by H. Fikentscher* as an index of the molecular weight of cellulosic polymers: it is dependent on the nature of the solvent, but little influenced by the concentration (in dilute solutions) and the temperature of determination. For PVC the Fikentscher K value is related to the viscosity ratio 11/110 (formerly known as 'relative viscosity') by an expression of the type
* Cellulosechemie, (1932) 13, 58.
'0
~~'-'
0·42 0·44 0·47 0·49 0·52 0·55 0·57 0·60 0·62 0·65 0·67 0·70 0·73 0·75 0·78
:::.: ..... "1
.::1
50 52 54 57 59 61 64 67 70 73 77 80 83 87 90
~~oS I\,) I\,)
0
~V):::E
Q
::
... Q""l
';> ........ ~
""-
0lY) U <".1,-... ::1
" 00 f-.., 0 .~~
Uo":S V) ::
cs .......
.S~ . . . .
. . . . <;
:: 0\ ."
E""-~ :::t K lY)
I\,)
...
~
~.?<".1
0·155 0·165 0·175 0·185 0·195 0·206 0·217 0·228 0·239 0·25 0·264 0·275 0·285 0·3 0·31
1·216 1·227 1·237 1·247 1·258 1·269 1·280 1·292 1·304 1·316 1·329 1·342 1·355 1·369 1·383
~~$ ~~ ~
I\,)
.......
"g Oo~~ ~ oS '0 ~ ~
I\,) V)
'<:j
~
;>Q
.::1 ........ ~ ~:S
]-9
8<".1'"
'r;; ~
~
I\,)
'be:;
f-..,
45 46 47 48 49 50 51 52 53 54 55 56
44
42 43
tl .....
::.::
;>
~"I ~
47 49 51 52 53 54 55 57 58 59 60 61
U
::.::c
;>
.2!~ "1-9
I\,)
~~ ....... "I
::
I\,)
lY)
... <;
c~
45·3 46·2 47·1 48·4 49·3 50·1 51·3 52·4 53·6 54·7 56·1 57·2 58·2 59·5 60·5
: .:
I\,)
~.'::1 .2!~:: "101\,) ~"ti~
. :: ~ ~"Iu
.,,0'"
eo:: '.
~I\,)Q
~"I""""
~
E:~'"
.... 0
~~
'0
-,-...
""
5
49 50 51 52 53 54 55 56·5 57·5 58·5 59·5 60·5 61·5 62·5 63·5
~~ ::.:: 'i:!
I\,)
I\,) :: "l
.2!..(:)
• ~
~I\,)
c ~
~
~
0
;>0.,
::
;>0.,
u":::~
;>
""~ .::1 U
8-9
~
0·25 0·26 0·27 0·28 0·29 0·31 0·32 0·34 0·35 0·37 0·38 0·40 0·42 0·44 0·45
~~ ~
1\,)~<".1
0·08 0·09 0·10 0·105 0·11 0·115 0·120 0·125 0·13 0·14 0·145 0·15 0·155 0·16 0·17
~ ........ ~
1\,)~lY)
~ E:~ ~E:~
u":::~
;>
·::1U'
CS~ U 0
~
~::
eol\,)
<".1 • :: 0
--
.-0"1 '-0"1~
001\,) :: • 0
."
275 310 350 380 415 450 495 525 560 600 640 680 720 760 800
~{j
1~
:u .....
........
::~ .g~ .g~
...
~
100000
70000
54000
40000
~~
.~~
~u
~~
;>
:u
I\,)
~~ "I
I\,)
45500
40000
36000
30000
26000
20000
:::t 0
<;E:
E:~
.......
"I• ... "I I\,) :::t ..(:)u
~ ~
...
~.~
I\,)~
TABLE 2.1 Some Indices of Molecular Weight of PVC Polymers (Based in part on table compiled by Matthews and Pearson of leI, first reproduced, with permission, in the previous editionS)
~
~
~
:-:::
.j:o. .j:o.
0·80 0·83 0·85 0·88 0·91 0·92 0·95 0·98 1·01 1·03 1·06 1·08 1·11 1·13 1·16 1·18 1·21 1·23 1·26 1·28 1·30 1·33 1·35 1·38
0·32 0·33 0·34 0·36 0·37 0·38 0·39 0·40 0·41 0·43 0·44 0·45 0·46 0·47 0·49 0·50 0·51 0·53 0·54 0·56 0·57 0·58 0·6 0·61
1·397 1·412 1·427 1·443 1·458 1·474 1·491 1·508 1·525 1·543 1·562 1·581 1·60 1·62 1·64 1·661 1·682 1·704 1·726 1·749 1·772 1·796 1·821 1·847 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80
62 63 64 65 66 67 68 69 70 70·5 71 72 73 74 61·7 62·9 64·1 64·9 66·1 67·1 68·2 69·2 70·2 71·5 72·4 73·3 74·3 74·9 75·8 76·7 77·5 78·3 79·1 79·9 80·7 81·5 82·2 83·0
64·5 66 67·5 68 69 70 71 72 73 74 75 76 77 78
0·47 0·49 0·51 0·53 0·55 0·57 0·59 0·61 0·63 0·65 0·67 0·69 0·71 0·73 0·75 0·77 0·79 0·81 0·83 0·85 0·87 0·89 0·90 0·93 0·175 0·18 0·19 0·195 0·20 0·205 0·21 0·22 0·225 0·23 0·235 0·24 0·25 0·255 0·26 0·27 0·275 0·28 0·29 0·295 0·30 0·31 0·315 0·32
840 885 930 975 1025 1070 1120 1175 1230 1300 1350 1420 1490 1570 1650 1720 1810 1900 1980 2070 2170 2260 2360 2460 73000
80000 82000
90000 91500
340000
480000
70000
64000
60000
55000
260000
200000
140000
50000
N
Zl
'"
3
~
<:>
~
(j
~ ~
["
'"....
3 3
g
b
a
The correct current term is logarithmic viscosity number (cf. Chapter 1, Section 1.5.1). This method involves measurement of the viscosity of 0·2 g/100 ml solution of PVC polymer in cyclohexanone at 30°e. cThis is the viscosity ratio less unity (TJITJo) -1: see Chapter 1, Section 1.5.1, and Section 2.2.2 herein. d In this method specific viscosity is calculated from the results of determination on a dilute solution of PVC polymer in nitrobenzene (0·4g/100 mI). ASTM D 1243-58T and its subsequent (1966) edition have been superseded by ASTM D 1243-79, the version currently in force. This gives only one method, whereby the viscosity of 0·2 g/100 ml solution of ... PVC polymer in cyclohexanone is determined at 30°C, and the logarithmic viscosity number calculated from the results. Vl
94 98 102 105 109 113 117 121 125 130 134 138 142 145 149 153 157 161 165 169 173 177 181 185
w.
46
V. Titow
(see DIN 53726):
75
K + ) _ cK ( In (..,.,1""'0 - 1000 1.5 cK + 1000
1)
The main parameter determining the molecular weight (K value) of PVC polymer in all the main polymerisation processes is temperature (in bulk polymerisation the temperature in the second stage). Increasing the temperature reduces the molecular weight of the polymer formed. In emulsion and solution polymerisation addition of chain-transfer agents can also play a part. In some solution systems chain transfer to the solvent may limit the maximum molecular weight obtainable in particular conditions.
2.2.3 Polymer Particle Characteristics (a) Particle Size and Size Distribution The particle size ranges characteristic for the main polymerisation processes are indicated in Table 2.2 together with the parameters which govern the particle size in each process. Further discussions of polymer particle sizes and structures, and of their role in processing, will be found in Chapters 14 (Section 14.3) and 21 (Sections 21.2.2 and 21.3.1). (b) Particle Shape and Morphology The grains of a typical suspension polymer have a substantial degree of sphericity. Under sufficient magnification in reflected light they exhibit the characteristic puckered surface. Viewed in transmitted light (especially with the grain partly transparentised, e.g. by plasticiser absorption) a particle is seen to be surrounded by a dense skin enclosing a porous interior made up of clusters of small primary particles, of the order of 11-lm in size, and the associated interstitial voids. Bulk polymer grains are similar in general overall shape and uniformity to those of suspension polymer, although they usually have a characteristic 'flat sided' appearance and a sample will usually contain some fines. Their internal structure is also closely similar, but they have no outer skin, so that their internal pores are more directly accessible. The particles of emulsion-produced polymer (spray-dried powder) are somewhat irregular aggregates of highly spherical primary particles
Depend on particle size distribution
Speed of agitation (finer particles at higher speeds) in conjunction with choice of suspension stabiliser (protective colloid) Agitator tip speed in the first (prepolymeriser) stage-lower mean particle sizes at higher speeds Type and concentration of emulsifier (for a given emulsifier primary particle size decreases with concentration); addition of pre-formed 'seed' latex; grain size affected by drying and grinding operationsb
Process factors controlling or affecting grain size
b The grains are aggregates of spherical primary particles (typically about 0'1-2 f-lm in size) which are the originarIatex particles. The aggregate grains are formed during drying. Their size may be modified by any subsequent grinding.
° Grains made up of clusters of primary particles (microgranules). Typical microgranule size of the order of about 1 f-lm.
2-70
100-150
80-200°
Bulk polymerisation
Emulsion polymerisation
120-160
50-250°
Suspension polymerisation
b
Means
Typical polymer grain size (JDn) Range
Process
TABLE 2.2 Polymer Grain Sizes Typical of the Main Polymerisation Processes, and the Relevant Process Factors
~
:ti
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3
~
;0
l)
""
~
~
'"
§
'"
48
w.
V. Titow
of about 1!J.m size. It may be noted here that commercial polymers produced by the emulsion process are of two general types. One, usually called simply 'emulsion polymer' is intended for use in some calendering and extrusion formulations (where the rapid fusion rate it offers is beneficial), and in the production of battery separators by sintering into porous sheets (see Chapter 25, Section 25.2.2). The other type is commonly referred to as 'paste polymer' since it is used in PVC pastes. It is important that the particle size and size distribution of each type should be appropriate to the application. With pastes these characteristics strongly influence paste rheology (ct. Chapter 21, Section 21.3.1). Within the gross, overall morphological characteristics, many different variants of the fine structure are possible. A good account of these has been published by Allsopp. 6 It is these variations that make individual polymer grades suitable for particular formulations and processing methods. Among the basic, general morphological features, particle porosity (amount, distribution and configuration) is of special importance to ease of processing, including rapidity and extent of absorption of such additives as plasticisers and lubricants. Indeed plasticiser absorption is the basis of one of the important standard polymer characterisation tests (see below). It may be noted in passing that veNA copolymers, which are not usually employed in plasticised formulations and which are easier to process than the homopolymers, are normally made with denser, low-porosity grain structure. In suspension polymerisation particle porosity is a function mainly of the choice of suspension stabiliser (protective colloid) and of the polymerisation temperature: effective protective colloids promote the formation of denser particles with relatively high packing density; particle density also increases with polymerisation temperature. In bulk polymerisation particle porosity decreases with increasing temperature of the first (pre-polymerisation) stage of the process, and with increasing degree of conversion in the second stage (which also increases the bulk density).
2.2.4 Purity Whilst it is not of quite such fundamental universal significance as the characteristics mentioned in Sections 2.2.1-2.2.3, polymer purity is important in many applications. In the normal contexts it is a matter of the absence (or presence in acceptably low amounts) of extraneous
2 Commercial PVC Polymers
49
contaminants, processing residues (e.g. residual protective colloids, emulsifiers, etc., on the polymer particles), and 'fish-eyes'. * In the most general sense, the presence of adventitious functional groups in the polymer chains may also be included under this heading. External impurities brought by the polymer into PVC compositions and products can impair the heat and light stability, strength properties (by acting as stress-concentrating points), appearance (e. g. in clear products freedom from fish-eyes is essential), and electrical properties (important, for example, in cable compounds). Adventitious functional groups in the polymer can also adversely affect heat stability, weathering resistance, and electrical properties. These aspects are considered at several places in this book-see, inter alia, Chapter 1, Section 1.5.1; Chapter 4, Section 4.4.1(a)); Chapter 9, Sections 9.2.1 and 9.2.2; Chapter 11, Section 11.2.2(a)); Chapter 14, Section 14.2.2(e)); and Chapter 18, Section 18.6.3. 2.3
CHARACTERISATION AND DESIGNATION OF COMMERCIAL PVC POLYMERS
Certain polymer properties are commonly determined, and their values quoted, for the purposes of characterisation, quality control, and technical specifications. Designation systems for the various commercial polymer grades normally include code indications of at least some of these properties, as well as indication of the applicational purpose, i.e. whether 'general-purpose' resin (for melt processing) or paste resin. Some principles and systems for the characterisation and designation of PVC polymers are laid down in certain standard specifications (see Appendix 1, Section 2.1). The polymer properties included in the systems of two major standards organisations-ISO and ASTM-are listed in Table 2.3. A third standard system (that contained in DIN 7746 and 7747) is in substantial technical agreement with the ISO specification. Test standards relevant to these systems are * Specks of hard polymer which persist in the finished product; called fish-eyes because of their characteristic appearance in transparent compositions. In opaque ones can show up as surface 'nibs'. For standard definition see Chapter 4, Section 4.4.1(a». Fish-eyes can originate in the course of polymer production, but they may also be formed in melt processing as a consequence of non-uniform fusion (which, in plasticised compositions, may be associated with non-uniformity of plasticiser distribution): too wide a particle size range of the polymer can be a factor in such non-uniformity.
W. V. Titow
50
TABLE 2.3 PVC Polymer Properties-Standard Characterisation Tests Test methodsa
Property ISO
Dilute solution viscosity Apparent (bulk) densitl
ISO 174 (Viscosity number) ISO 60 ~determined in gcm- )
ASTM
D 1243 (Inherent viscosity) D 1895 (test also gives bulk factor and pourability)
ISO 1068 (determined in g cm- 3) Particle size and size distri- ISO 1624 (Sieve analysis of D 1921 (Sieve analywet slurry) sis: general bution C methods standard)
Compacted bulk density
b
and
Powder mix time Flowability (dry flow)b Plasticiser absorption at room temperature Hot plasticiser absorption Chlorine content (virtually never determined on commercial polymers) Porosity
D 1705 (Sieve analysis of wet PVC powder slurry) D 2396 ISO 6168 (Time of flow D 1755 (Time of flow through standard funnel) through standard funnel) ISO 4608 (phr of DOP D 1755 (phr ofDOP sorbed in test condisorbed in test contions) ditions) ISO 4574 (phr min- 1 of DOP sorbed in test conditions) a ISO 1158 (determined as % by weight) D 2873 ~ determined in cm g-l by Hg intrusion)
Volatile matter pH of aqueous extract
ISO 1269 ISO 1264
Thermal stability Vinyl acetate contentd (of copolymers)
ISO/R 182 ISO 1159 (hydrolysis of the acetate groups, and back-titration)
a
D 1755 (determination of electrical conductivity) a
Designation requirements for comonomer (vinyl acetate and others) content stated in D 1755, but no test method given.
51
2 Commercial PVC Polymers
TABLE 2.3-contd.
Property
Test methods° ISO
Specific gravity (of copolymers) Ash and sulphated ash Preparation of a paste Apparent viscosity of a paste 'Fish-eyes' } VCMcontent
ASTM D792
ISO 1270 ISO 4612 ISO 4575 (Severs rheometer) ISO 2555 (rotating viscometer) Mentioned in ISO 1060/2 but no ISO methods yet available
° Those shown in this table are the ones specifically listed, respectively, in
ISO 1060/2, and ASTM D 1755 (homopolymers) and D 2474 (copolymers). The following ASTM methods are available but not covered by the above two ASTM standards: chlorine content-ASTM D 1303: volatile matter-ASTM D 3030; heat stability-ASTM D 793 and D 2115. b Influenced by polymer particle shape, structure and size distribution. Bulk density and compacted bulk density relate to the production rate in melt compounding, and in extrusion of rigid products (profile, pipe) from powder blends (higher rates at high bulk densities). Bulk and compacted bulk densities of PVC resins were shown by Ravey and Waterman 7 to correlate linearly with one another, and hyperbolically with the resin specific surface. Results of dry flow tests in conjunction with those for plasticiser absorption are particularly significant as a guide to hopper feeding characteristics of plasticised dry blends (ct. ASTM D 1755-81, Subsection 16.2). C Jointly with particle structure, particle size and size distribution affect the bulk density and packing characteristics of polymer powders and powder compounds, as well as dry flow in tests and in the course of conveying, metering, feeding and processing. The presence of substantial proportions of fine particles can impair dry flow and uniformity of absorption of liquid additives (plasticisers, some stabilisers) in powder blending and processing, and uniformity of fusion in melt processing. Rheological properties of PVC pastes are strongly influenced by polymer particle size and size distribution (cf. Chapter 21, Sections 21.2.2, 21.2.4 and 21.3.1). d Some manufacturers use IR spectrophotometry, involving comparison with specially prepared reference spectra. A rapid IR method for the analysis of a number of copolymers of vinyl chloride has been described by Grisenthwaite. 8
W. V. Titow
52
given in Section 2 of Appendix 1. Whilst the standard characterisation and designation systems are widely recognised, the test schemes and/or individual tests used by some polymer producers and consumers differ occasionally from the standards. By way of an example, the properties normally evaluated by two manufacturers of Corvic PVC polymers are listed, with an indication of the tests employed, in Table 2.4. TABLE 2.4 Tests Used for Routine Characterisation of AECI 'Corvic' PVC Polymers Property and units
Test method
Relevant leI test
Apparent (bulk) density (g ml- I or g I-I)
ISO 60
ISO 60
Packing density (g I-lor gml- I)
Own compaction test
Own compaction test
K value
DIN 53 726
Viscosity number determination to ISO 174: K value calculated from standard solution viscosity data obtained"
Particle size distribution
Coulter counter: results related, on the basis of preestablished correlations, to those obtainable by the method of ASTM D 1705
Wet sieving on BS mesh sieves
Particle porosityb (phr (of plasticiser absorbed»
ISO 4608 (plasticiser absorption at room temperature)
Volatile content, moisture (%)
Weight loss after 0·5 h at 135°C for homopolymers; 1 h for copolymers
Flowability
ISO 6186
Volume resistivity (Q m or Qcm)
BS 2782: 1970 Method 202A (on standard plasticised compound incorporating the polymer)
Heat stability
Heating tests on compounds made up in accordance with intended application. Stability assessed in terms of yellowness index (reflection or transmission mode)
"Some definitions of standard terms relating to viscosity are given in ISO/R 1628 (currently under revision). 3 b Sometimes also determined (in cm g-I) by the mercury intrusion method of ASTM D2873.
53
2 Commercial PVC Polymers
The two main standard polymer designation systems (ISO and ASTMJ are both based on the principle of indicating the selected polymer characteristics by appropriate code references to groups of data-ealled 'data blocks' in the ISO system. However, the systems differ in that the 'blocks' are not identical in coding, arrangement, and content. Details of the systems are given in ISO 1060/1 (for homopolymers and copolymers) and ASTM D 1755 (for homopolymers) and D 2474 (for copolymers). The following are examples of designation according to the ISO system, in which the information belonging to a 'block' is separated from that of the next 'block' by a comma (two commas after the third 'block' if the optional fourth block is also included). Homopolymer for melt processing: ISO 1060-PVC-S
G, 121-57
~ill ~
homcpolymer suspension-produced general purpose (i.e. not for paste) viscosity number - - - - - - - ' bulk density of O' 56 g rnI- 1 Copolymer for melt processing: ISO 1060-VCNAC
~
88-S,
G, 080-75,
17
vinyl chloride/acetate 88% vinyl chloride-derived suspension-produced general purpose viscosity number (80) bulk density 0·75 grnl- 1 - - - - - - - - - - - - ' particle size limits (by reference to numbered 'classes'-here classes 1 and 7) plasticiser absorption by particles (X indicates that the value is not specifically designated)
X
Applications
Apparent densiti (ISO 60) (g 1- ) Mean particle size (AECI method) (,urn) Particle porosity (plasticiser absorption, ISO 4608) (phr) Volatile content (AECI method) (%)
K value (DIN 53 726)
Type
Polymer properties
530-560
470-520 110-160 24-31 <0·5
550-580
110-160
16-22
<0·5
<0·5
17-24
110-160
60-65
Suspension
66-75
Suspension
Suspension
Rigid, calendered sheeting (for packaging)
65-68
Flexible tubing and cable compounds
Rigid pipe
<0·2
110-160
650-750
Copolymer (suspension) 47-51
VelVA copolymer (for flooring)
<0·5
10-50
350-450
Emulsion (paste grade) 65-80
Paste polymer
TABLE 2.5 Typical Basic Property Value Ranges of 'Corvic' (AECI) PVC Polymer Grades Recommended for Some Major Applications
Vl
~
0
:0:::: ::;j
~
""'"
2 Commercial PVC Polymers
55
2.4 EXAMPLES OF BASIC PROPERTIES OF COMMERCIAL POLYMERS AS USED FOR SOME MAJOR APPLICATIONS In Table 2.5 some typical property values are illustrated by reference to one commercial polymer range (Corvic-AECI). Further illustrations will be found in Table 19.2 of Chapter 19, and also in various formulations cited, and comments made, in other chapters. 2.5 COMMERCIAL SOURCES OF PVC POLYMERS Some large manufacturers of PVC polymers produce in more than one country, or make their technical know-how and backing available (under licence or other arrangements) to local producers in other countries. It is also not unusual for one producer to supply bulk quantities of monomer to another. In some countries, imported polymers account for a significant proportion of the PVC resin market. Some major PVC polymer producers are listed in Table 2.6. In the early 1980s economic factors brought about considerable reorganisation of the PVC sector of the plastics industry, especially in the UK and Continental Europe. Some PVC producers of long standing ceased operations, and many of their product ranges have been discontinued. Instances which may be cited by way of example include the following. In Europe, the take-over by ICI of Lonza, and extensive nationalisation in the French chemical industry, resulting in the concentration of PVC polymer production in the hands of two companies (Elf Aquitaine and Entreprise Miniere et Chimique) although manufacture continues through subsidiaries which were formerly independent producers (see Tables 2.6 and 2.7). In the UK, the number of manufacturers has been reduced from 4 to 2 between 1980 and 1983: the ones currently operating are ICI, who absorbed PVC polymer production activities of BP Chemicals Ltd (in exchange for their own polyethylene production operation), and the Norwegian company Norsk Hydro, who took over Vinatex Ltd, and the PVC polymer manufacturing operation of British Industrial Plastics Ltd. Some polymer trade names no longer current (in some cases as a result of the recent changes just mentioned) will continue to be of
TABLE 2.6
Some Major PVC Polymer Manufacturers Manufacturer AECI Air Products and Chemicals Inc. ATO Chimieu BASF
Country
Trade name
South Africa
Corvic
USA France West Germany
B. F. Goodrich Borden Chemical Chemische Werke Hiils Chemopetrol Chisso Corp. Conoco Chemicals Co. Denki Kagaku Dutch State Mines Electro Chern. EVCC Hoechst ICI Kanegafuchi Chemical Industry Mitsui Toatsu Chemicals Inc. Montedison
USA USA West Germany Czechoslovakia Japan USA Japan Netherlands Israel Holland West Germany UK
Airco Lacqvyl Vinoflex, Vinidur, Lutofan Geon Borden Vestolit Neralit Nipolit Conoco Denkavinyl Varlan Epivyl Various names Hostalit Corvic
Japan
Kaneka
Japan Italy
Norsk Hydro Produits Chimiques Ugine Kuhlmann u Rhone-Poulenc Ind. u Shell Shinetsu Chemical Industry Co. Shintech Inc. Singapore Polymer Corp. Societe Artesienne du Vinyle b Solvay et Cie. Stauffer Chemical Co. Tenneco Chemicals Co. Union Carbide Corp. Wacker Chemie
Norway and UK
Vinychlon Sicron, Vipla, Viplavil (copolymer) Norvinyl
France France Netherlands
Ekavyl Lucovyl Carina
Japan USA Singapore
Shinetsu Shintech SPC
France Belgium USA USA USA West Germany
Artevyl Solvic SCC Tenneco Bakelite Vinnol
Subsidiary of Elf Aquitaine. Subsidiary of Entreprise Miniere et Chimique. C European Vinyls Corporation: joint venture between ICI (UK) and Enichem (Italy).
U
b
57
2 Commercial PVC Polymers
TABLE 2.7 Some Commercial Polymer Trade Names, Past and Present Trade name
Manufacturer
Beetle Breon Dacovin Ekaryl Etinox Exon FPC Halvic Irvinil Kohinor Lonzavyl
British Industrial Plastics Ltd B.P. Chemicals International Ltd Diamond Shamrock Corp. Plastimer SA Aiscondel SA Firestone Plastics Co. Firestone Plastics Co. Halvic AG Great American Chemical Corp. Pantasote Co. Lonza
Marvinol Mirvyl Pekevic Pevikon Plaskon Pliovic Quirvyl Ravinil Reynosal Ricon Ruco Scon Trovipor Vixir Vygen
Uniroyal Inc. Rio Rodano SA Neste Oy Kema Nord Allied Chemical Co. Goodyear Tire & Rubber Co. Rumianca SpA ANIC SpA Reynolds Chemical Co. Rico Chemical Corp. Hooker Chemicals Vinatex Ltd. Dynamit Nobel Societa Italiana Resine SpA General Tire & Rubber Co.
Country
UK UK USA France Spain USA USA Austria USA USA Switzerland, West Germany USA Spain Finland Sweden USA USA Italy Italy USA USA USA UK West Germany Italy USA
interest, either purely historically or as quality technical products featured in many practical formulations and important studies. For this reason a number of such names has been included in Table 2.7.
REFERENCES 1. Hardie, D. W. F. (1964). In Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 5, 2nd Edn, John Wiley, New York, pp. 171 ff. 2. Anon. (1982). Plast. Rubb. Wkly, (964), 20th November, p. 1. 3. British Patent 1553829, ICI; US Patent 4158092, Hoechst.
58
W. V. Titow
4. Burgess, R. H. (1982). Manufacture and Processing of pvc. Applied Science Publishers, London, pp. 84-6. 5. W. V. Titow and B. J. Lanham (Eds), (1971). PVC Technology, 3rd Edn, Applied Science Publishers, London, pp. 19-22. 6. Allsopp, M. W. (1982). Chapter 7 of Ref. 4 above. 7. Ravey, M. and Waterman, J. A. (1974). J. Appl. Polym. Sci., 18, 2927-33. 8. Grisenthwaite, R. J. (1962). Plastics, 27 (January) 117-9.
CHAPTER 3
Commercial PVC Compounds W. V.
TITOW
3.1 INTRODUCTION There are two general ways in which a user of PVC compounds, i.e. the processor who converts them into products, can acquire the compositions he needs. He can do his own compounding, or he can buy compounds from a commercial producer, either as stock lines, or specially compounded to his requirements. In-house compounding by the processor has traditionally involved own compound manufacture as a separate operation: in the case of solid compositions (as distinct from pastes) this normally includes conventional batch-wise production of powder pre-mixes and dry blends. Industrial compounding technology of solid compounds and pastes is discussed in detail in Chapter 13. A paper by Adams 1 provides a useful summary of the advantages of in-house compounding of solid PVC compositions. Recently equipment has been coming into use which makes possible continuous preparation of powder compounds at the extruder, so that such compounding can, in effect, be integrated into the extrusion line. Several advantages, both technical and economic, are claimed for this kind of arrangement, including suitability for small-scale operations with relatively low outputs2 (see also Chapter 19, Section 19.3). However, many small and medium-sized extrusion operators who do not wish to, or cannot, invest in such equipment, as well as other processors in these size categories, use purchased compounds: this brings the benefits of the commercial compounder's special equipment and expertise without own capital and staff commitments. Even large processors who are equipped to produce their own compounds usually 59
60
W. V. Titow
have short runs on compositions which they find it more economical to buy-in. They may also occasionally need to supplement their own internal compound supply for regularly produced large-volume lines. The use of externally purchased masterbatches is also quite common. 3.2 COMMERCIAL SOURCES OF PVC COMPOUNDS The producers of commercial PVC compounds fall into two main groups: polymer manufacturers who also produce compounds (usually mainly solid, 'general-purpose' compositions, not pastes), and independent compounders. The latter group includes some processors with spare compounding capacity, who produce compounds to order and for general sale. In addition, imported compounds are available in most countries (normally ones produced by big foreign polymer producers and marketed by their local subsidiaries or agents). Many polymer manufacturers are listed in Chapter 2. In almost any given country the independent compounders are more numerous. By way of an example, in the UK-where there are now only two polymer producers (see Chapter 2)-the following independent compounders are currently operating: B & T Polymers Ltd; Crown Decorative Products Ltd; S. Dugdale, Son & Co. Ltd; W. R. Grace Ltd; P. W. Hall Compounding Ltd; Industrial Polymers (UK) Ltd; Phoenix Rubber Ltd; Plascoat Systems; Polycol (PVC) Ltd; Soltak Plastics Ltd; Wollaston Vulcanising Co. Ltd. If not readily available from other sources, information on compounders can often be obtained from manufacturers of compounding equipment widely used in industry, e.g. Buss AG (whose UK associate Buss-Hamilton Ltd produce a useful compounding manual),
3 Commercial PVC Compounds
61
Werner and Pfleiderer, Pappenmeier, and others (see also Chapter 13, Section 13.4.4).
3.3 TYPES AND APPLICATIONS OF COMMERCIAL PVC COMPOUNDS The compounds are available in all the physical forms in which PVC compositions are processed, viz. powder compounds (dry blends, powder coating compositions), melt-compounded pellets (both rigid and plasticised compositions), and pastes (supplied by many independent compounders). . Commercial compounds offered for applications where certain property standards are desirable or obligatory, are normally formulated to meet the relevant standard specifications, national or ISO. Some typical examples are compositions for electrical cable and wire coverings, rigid electrical conduit and junction boxes, pipes and pipe fittings for various applications. Most compounders will also produce compositions to meet special customer requirements, which may include suitability for processing on particular equipment, special processing characteristics, and conformity with stated property specifications. A general outline of the main available types of commercial PVC compounds is given in Table 3.1. Tables 3.4-3.7 provide further illustrations of the types, and the variety of grades available within each type for various particular purposes and applications. Where relevant, the variations may include such features (some illustrated in the tables) as availability in clear or opaque grades (e.g. in the case of flexible compositions for extrusion and injection moulding; blowmoulding compounds), coloured or natural grades, and-e.g. for flexible extrusion and moulding compounds (including those for profiles, electric cable and wire covering, moulded parts)-grades giving different types of surface finish (glossy or matt). Extrusion and injection-moulding compounds are available formulated for cellular products (cf., for example, Tables 3.4 and 3.7, and also Chapters 15, 19 and 25). Other special grades include compositions for applications involving contact with food, beverages (e.g. milk, beer, fruit squashes) and potable water; and compositions resistant to chemicals and particular environments (e.g. for hoses resistant to fuels
TABLE 3.1 Commercial PVC Compounds: General Outline of Types General type
Extrusion compounds
Nature of composition
Rigid
Flexible
Injection-moulding compounds
Rigid
Flexible
Blow-moulding compounds
Rigid Flexible
Masterbatches
Rigid and flexible
Powder coating compoundsb
Mainly flexible
Pastes
Common applicational types·
Pipe (various kinds) Rainwater goods (down pipe; guttering) Electrical conduit Profiles (including window frames) Sheeting General purpose Electrical wire and cable coverings (various kinds) Covering for: chain-link fencing wire clothes lines Hose Profiles Seals and gaskets General purpose Pipe fittings Electrical junction boxes Industrial mouldings General purpose Footwear Grommets, gaskets, bushes Toys General purpose Bottles, jars, other containers Sachets (for toiletries, etc.) Colour Filler Wire coating (fencing) Dishwasher baskets Weather and wear resistant (railings, outdoor furniture) General purpose Fabric and paper coating Dip-coating and moulding Rotational casting and slush moulding Casting Spraying
• Various grades available within each type. See also Tables 3.3-3.7, and relevant information on the processing and applications of PVC compositions in other chapters. b Primers required for application of the coating compounds to metal surfaces are normally available from the compound suppliers.
3 Commercial PVC Compounds
63
and oils; wire and cable covering resistant to seawater), to sulphur staining, to marring by contact with various materials, to high temperature and weathering. Some examples are given in Tables 3.5-3.7. Hot-melt compounds (in the UK originally supplied by Vinatex Ltd* under the trade name Vinamold3 ) are a special kind of composition, designed for the production of flexible moulds for the casting of concrete, plaster, castable resins (e.g. polyester), and GRP. The softening points of such compounds are, typically, within the range 13G-170°C. Thus, whilst moulds with cavities faithfully replicating the prototype of the object to be cast can be conveniently prepared by pour-coating with the molten compound, the resulting mould, once set by cooling, can withstand temperatures up to about 80°C without serious distortion. Commercial PVC compounds are sold under many trade names. Several polymer manufacturers who also market compounds use the same name (with different letter and/or number coding) for both types of product-e.g. Geon, Conoco, Sicron, Hostalit (see Table 2.6 in Chapter 2). Some, like Conoco and Ethyl (with an appropriate code) are also simply the manufacturers' names (cf. Conoco Chemicals Co.; Ethyl Corp.). Many of the compound trade names are quite free from such connections-e.g. Welvic (ICI, UK and AECI, RSA), Lucalor (Rhone-Poulenc, France), Polydon (Industrial Polymers (UK) Ltd), Vyflex (Plascoat Systems, UK).
3.4 PROPERTIES AND DESIGNATION OF COMMERCIAL PVC COMPOUNDS 3.4.1 Designation
As in the case of PVC polymers, there are standard designation systems and characterisation tests laid down in ISO, ASTM, and DIN specifications viz: for unplasticised compounds: ISO 1163/1 (Designation); ASTMD 1784; DIN 7748 * Taken over by Norsk Hydro-see Chapter 2.
64
W. V. Titow
for plasticised compounds:
ISO 2898 Parts 1 and 2; ASTM D 2287; DIN 7749 Parts 1 and 2
All these specifications are listed, with dates and titles, in Section 3.1(a) and (b) of Appendix 1 (ISO 1163/1 and 2898/1 are currently under revision). The DIN designation systems and test are similar to, but not identical with, the corresponding ISO ones. The designation systems are based on principles similar to those used for PVC polymers, as illustrated by the following examples. The examples also indicate what properties are used for the purposes of designation. ISO
DESIGNATIONS
uPVC DE 080 226: unplasticised compound; D = dry blend; E = general extrusion; 080 = Vicat softening temperature of 80 ± 2°C; 2 = 'class 2' Charpy notched impact strength-i.e. 5-20 kJ m- 2 ; 2 = 'class 2' modulus of elasticity in tension-i.e. 2000-2500 MPa; 6 = 'class 6' density-i.e. 1·35-1·40 g cm- 3 . pPVC GM A84 (XX4): plasticised compound; G = pellets (granules); M = injection moulding; A84 = Shore A hardness of 84 ± 3; X = 'class X' torsional stiffness-i.e. not designated; X = 'class X' tensile stress at 100% elongation-i.e. not designated; 4 = 'class 4' density-i.e. 1·25-1·30 g cm- 3 .
DIN
DESIGNATION OF AN UNPLASTICISED COMPOUND
DIN 7748-PVC-U, BGT, 074-30-28: U = unplasticised; B = blow moulding; G = pellets (granules); T = high clarity; 074 = 'code 74' Vicat softening temperature-i.e. softening temperature 73-75°C; 30 = 'code 30' notched impact strength-i.e. over 20 kJ m- 2 ; 28 = 'code 28' modulus of elasticity-Leo 2500-3000 N mm- 2 .
ASTM
DESIGNATION OF A PLASTICISED COMPOUND (ELECTRICAL GRADE)
Type PVC-54 6 1 5 E2 X: 5 = 'cell 5' Durometer A hardness-i.e. 75-84; 4 = 'cell 4' specific gravity-i.e. 1·30-1·34; 6 = 'cell 6' tensile strength-i.e. 17·2 MPa minimum; 1 = 'cell l' volatile loss at 105°C-
3
Commercial PVC Compounds
65
i.e. 1% maximum; 5 = 'cell 5' brittleness temperature-i.e. -20°C maximum; E2 = 'cell 2' volume resistivity at 50°C-i.e. 10 12 ohm em minimum; X = average extent and time of burning, respectively, <25 mm and
Properties Used in Characterisation of PVC Compounds
The properties widely standardised for characterisation of PVC compounds are shown in Table 3.2. Many companies use more, or fewer, tests than those referred to in the table, and different test methods in some cases, depending on the nature and intended application of the particular compound. For example, a basic characterisation scheme may comprise the following tests: Property Apparent (bulk) density Dry flow Volatile content
Test methods e.g. ISO 60 } Powder blends e.g. ISO 6186 e.g. as in Table 2.4
Melt flow
rheometry (e.g. the Macklow-Smith plastometer,' or that of ASTM D 3364--cf. Chapter 15, Section 15.2)
Tensile strength and elongation at break
ISO, BS, ASTM, or DIN method, as appropriate
Impact resistance
ISO, BS, ASTM, or DIN method, as appropriate
Density, relative density or specific volume
ISO, BS, ASTM, or DIN method, as appropriate
Volume resistivity
ISO, BS, ASTM, or DIN method, as appropriate
Durometer hardness, or BS softness
ISO, BS, ASTM, or DIN method, as appropriate
Powder blends or meltcompounded pellets
}
Cable compounds
• A. Macklow-Smith Ltd, Camberley, Surrey, England.
3.4.3
Some Typical Properties of Commercial PVC Compounds
Examples are given in Tables 3.3-3.7, in general terms, and also by reference to one manufacturer's range of PVC compounds.
Chemical resistance
ISO 178 (MPa) ISO/R 1183 (g cm -3)
Modulus of elasticity in tension Density Tensile strength Deflection temperature under load Flammability
*
* * *
ISO 306, Method B eC) ISO 179 (Charpy, notched) (kJ m- 2)
ISO [[63/1
ASTM D 638 (MPa or lbf in -2) ASTM D 648 (OC or OF) ASTM D 635 (average extent of burning-mm and average time of burning-s) ASTM D 543
*
ASTM D 256, Method A (Izod) (J m- 1 of notch or ft lbf in -1 of notch) ASTM D 638 (MPa or lbf in- 2 )
*"
ASTMD1784
*
* * * *
~
:::1 0DIN 53 457 (N mm- 2 )
~ :<:::
DIN 7748
DIN 53 460 ("C) DIN 53 453 (Charpy, notched) (kJ m- 2 )
Test specifications and property units prescribed in:
Vicat softening temperature Impact strength
Property
TABLE 3.2 Section A: Standard Tests Used in Characterisation of Unplasticised PVC Compounds
~
ISO 176, Method B(%) ISO 177 (mg) ISO 175 (%)
* * *
*
ASTM D 746, Procedure A ("C) ASTM D 1203, Method A (%) ASTM D 257 (Q cm) ASTMD635 (extent of burning-mm; burning time--s)
* *
ASTM D 792, Method A (g cm- 3) ASTMD2240 ASTM D 412 (MPa or Ibf in- 2 )
ASTM D 2287
DIN 7749 Part
* * *
* * * * *
DIN 53 455 (N mm -2)
* *
DIN 53 479 (g cm- 3 ) DIN 53505
Test specifications and property units prescribed in: ]b
b
a
An asterisk indicates determination of this property not prescribed. Numerous methods listed in DIN 7749 Part 2 (which corresponds to, but does not coincide with, ISO 2898/2), but not all are commonly used with commercial PVC compounds.
* *
Plasticiser loss Plasticiser migration Resistance to chemicals
ISO 458 ("C)
IEC Publication 93 (Q cm) Methods under study
ISO/R 1183 (g cm -3) ISO 868 ISO 527 (MPa) ISO 527 (%) ISO 527 (MPa)
ISO 2898/2
Density or specific gravity Durometer (Shore) hardness Tensile strength at break Elongation at break Tensile stress at 100% elongation Torsional siffness as a function of temperature Brittleness temperature Volatile loss at 105°C Volume resistivity Flammability
Property
Section B: Standard Tests Used in Characterisation of Plasticised PVC Compounds
~
a-
;:: ;::
~0
~
'1;l ~ ~
i'"
~ ~
~
v"
Density (g cm- 3 ) Tensile strength (MPa) Compressive strength (MPa) Flexural strength (MPa) Young's modulus (MPa) Coefficient of linear thermal expansion (per 0c) Specific heat (cal g-l °C- 1 ) (kJ kg- 1 °C- 1 ) Thermal conductivity (Wm- 1 °C- 1 ) Softening temperature (BS 2782) (0C)
1·41 41 59 83 2800
78
76
Highimpact pipe
about 0·25 about 1
1·33 1·34 38 41 48 66 55 93 76 79 3400 2400 2400 about 5 x 10- 5
1·44 48
Highimpact for close tolerance extrusion
1·37 55 66 90 3400
Pressure pipe fittings
3400
72
1·39 41 66
General purpose
2800
72
)
)
1·34 41 55
Highimpact
Injection moulding compounds
78
79
73
78
77
73
77
14 X 10- 6 14 X 10- 6 13 X 10- 6 18 X 10- 6 14x 10- 6 14 x 10- 6 14 X 10- 6
93 3400
66
1·38 45
Electrical Easy-flow General (thin-wall purpose conduit and extrusions) general purpose
16 x 10- 6 16 X 10- 6
(
(
(
1·37 52 66 93 3400
Pressure pipe
Extrusion compounds
TABLE 3.3 Section A: Properties GeneraUy D1ustrative of Some Commercial Rigid PVC Compounds
;'!;
C
:::l
~
~
0\ 00
Rotational casting and slush moulding Dipping; fabric coating Spraying or dipping Flame resistant composition for casting or spread-coating Rotational casting, slush moulding, dipping or spraying (semi-rigid products)
Application type
1·15 1·15 1·19 1·27 1·25
10-20 90-150 10-20 350-500 20-40
Phthalate Phthalate Phthalate Phosphate Adipate
Density at 25°C (gcm- 3 )
Viscosity at 25°C (P)
Main plasticiser type
Paste properties
Shore A hardness (ASTM D 2240)
42-44 47-49 72-74 56--58 96--98
Cold-bend temp. (0C) (BS 2782: 1970)
-55 -55 -35 -25 -25
Product properties
Section B: Examples of Some General Types of PVC Paste Represented Among Commercial Stock Lines
$
t}
;:: ;:,
c
.g
~
~
~
§.:
3 3 '";:::
"'" ~
b
o
42
46
2·1
2·1
1·9 40
1·46 83
1·49 82
1·47 79
Glossy finish
*
*
*
48
1·9
46
2·2
1·47 79
Good melt flow; glossy surface finish 1·42 76
Good mould filling characteristics
Pressure pipe fittings
Rigid microcellular profiles for building applications Development grade
*0
General purpose
RI7IU43
MPR/7JI70
RI71U67
Injection-moulding grades
Asterisk indicates property values strongly dependent on the density of the extruded cellular product. Property dependent on the processing history of the product: the figures quoted are conservative values.
Relative density at 23°C Softening temperature (5 kg Vicat-BS 2782) CCC) Tensile modulus at 1% strain b (GPa) Tensile strength at yield b (MPa)
Easy processing, Satin (matt) good melt surface flow finish
Characteristics
R8/M936
General purpose profiles
Profiles and conduit
R8/JI25
Applications
R71J168
Extrusion grades
TABLE 3.4 Section A: 'Welvic (AECI) uPVC Dry Blend and PeDet Compounds for Extrusion and Injection Moulding (Data reproduced, with permission, from the technical literature of AECI Chlor-Alkali and Plastics Ltd)
~
S
::;j
:0:::
~
Cl
--_._----~
115 45 6 x 10- 5 1·4 X 10- 4 5 x 10 14 1 x 1012
%
°C- 1 J m m- 2 S-l °C- 1 Qcm Q
BS 4618 AECI BS2782 BS2782
Typical value
Rockwell R scale
Hardness Limiting oxygen index Coefficient of linear thermal expansion Thermal conductivity at 23°C Volume resistivity Surface resistivity
Units
ASTM D 785 ASTM D 2863
Test method
Property
Section B: Some Representative Values of Other Properties of 'Welvic' (AECI) uPVC
-..l
-
!}
;:s
l::
~C
~
r3
"l:I
.,'" "§.:
;?! ;?!
"" ~
W. V. Titow
72
TABLE 3.5 'Welvic' (AECI) PVC Compoonds for Electric (Table reproduced, with permission, from the technical ApplicatkJn
Grade
Use
seclor
C%urs:" Specifica/ion Softness Hardness Relative (BSS No.) (Shore A) density N-Narura/ C-C%urs CL-Clear
BS2782
Hard dielectric 6·6 kV cable dielectric G.P. High speed, thin wire insulation G.P. G2lU65 GD2lU15 Fast extrusion, filled G.P. G3/U6 Cabtyre sheathing, G3/U87 filled G4lU7 G.P. GD2IJI48 Flame retardant Hame insulation retardant and sheathing Insulation DI/U21 or DIIJI91 sheathing GI/U4 GDlIJ034
High tern· perature insulation or sheathing
HIIJ180 H21J2oo H21J263 H4lU85
Telephones G4lU31 N5/U16 Clear in· sulation
X21J364
JO/1365 X4IJ366
Welding cable sheathing Y3/J331 Development
grades
G2IJ301 G2IJ302 H21J246 Y5/J357
105'C rating dielectric 105'C rating dielectric 9O'C rating sheathing (instrument wiring) 105'C rating sheathing (appliance wiring) Telephone headset wiring and grommets Retractable telephone wire sheathing aear ripcord and domestic wiring Clear ripcord and domestic wiring Clear ripcord and domestic wiring
Petrol/oil resistant Low HCI emission sheathing Low HCI emission bedding Crosslinkable hightemperature dielectric Trailing cable sheathing
Tensile properties Tensile Tear E/ongastrength strength lion at (MPa) (Nmm- J) break (%) BS2782 BS6746 BS2782
N+C SABS 150-1970 N+C N+C SABS 150-1970
10 10 9
97 97 97
1·34 l-41 1·33
21·2 20·9 23-4
104 115 116
240 200 260
N+C N+C SABS 150-1970 N+C SABS 150-1970 N+C SABS 150-1970
14 20 20 30
94 90 90 83
1·34 1·34 1·48 1·30
19·2 20·5 15·2 18·6
99 104 82 72
230
N+C SABS 150-1970 N+C SABS 150-1970
35 40
80
n
1·45 1·28
IH 17·2
45 63
350 280 330 300
350
N+C SABS 150-1970
21
90
1·47
17·8
94
220
N+C US Underwriters N+C US Underwriters
9 20
97 90
1·39 1·35
22·0 20·8
122 75
220 280
N+C US Underwriters
18
92
1·39
19·0
90
230
N+C US Underwriters
44
74
1·29
12·4
63
260
55
N+C SABS 150-1970
45
74
1·26
18·6
N+C
54
69
1·21
11·7
340
350
CL
24
87
1·30
23·2
100
210
CL
35
80
1·28
21·1
79
230
CL
45
74
1·27
18·6
59
250
N+C
36
79
1·29
N+C CES) test
20
90
1·56
N+C CES) test
20
90
1·59
N+C SABS 150-1970
48
72
1·25
14·0
330
N+C
All values are typical results measured in ACE)'s Plastics Technical Service Laboratory. " C%ur: Standard colours are Natural lOS and Black 901. Non-standard colours are available 00 request. Masterbatches are also available for colouring purposes: MBIDIOOI MBIDIOO9 MBID1201 MBIDI302 MBID/404 MBID/451
White Cream Yellow Orange Red Pink'
MBID/501 MBID/509 MBID/601 MBID/606 MBID/607 MBlDn03
Golden Brown Dark Brownt Dark Greeot Green Turquoise Blue
MBIDI803 Violet MBID1901 Black MBID1957 Grey ' Brighter than BS 6746 C Colour t Darker than BS 6746 C Colour
73
3 Commercial PVC Compounds
Cable and Wire App6cations literature of AECI Chlor-Alkali and Plastics Ltd) Retention of tensile properties :ter ageing ( )
Loss in Hot defor. mass on ageing mation (mgcm- 2 ) (%)
Tensile .strength
Elonga·
97 94 97
98 92 94
0·4 0·8 0·4
97 96 92 99
85 100 90 100
98 97
Limiting n
°fIJ:x
ASTM
Cold bend temperature ('C)
te~eratu,e
'C)
Volume resistivity at2lfC (Oem)
BS2782
BS2782
BS2782
Cold flex
(%)
Surface finish
Recommended extrusion temperatures ('C)
tion at
break SABS SABS 150·1970 150·1970
BS6746
BS6746 D2863·1976
40
3 x 1014 Glossy 6 x 1014 Satin 4 x 1014 Glossy
-30
29 39
-40 -25
10 15 8
1·0 0·6 0·6 0·7
39 50 44 68
-50
-40
5 -10 -12 -9
2·3 x 1014 1·3 x 1014 1·2 x 1014 4 x 1013
Glossy Glossy Satin Glossy
145/150115511601165 145/150115511601165 145/1501155/1601165 1401145/1501155/160
100 100
1·0 0·7
70
n
-32 -45
-10 -15
2 x 1013 Satin 2·8 x 1013 Glossy
140/145/1501155/160 135/1401145/150/155
98
94
0·6
36
-30
-5
2 x 1014 Satin
94 100
99 96
0·2 0·55
39 38
-35
-40
15 -5
1-4 x 1014 Glossy 3 x 1014 Glossy
155/160/165/1701175 145/1501155/160/165
96
90
0·7
42
-35
-I
8 x 1014 Glossy
145/150/155/1601165
6 x 1013
Glossy
1401145/150/155/160
3·5 x 1013 Glossy
135/1401145/1501155
90
100
0·6
n
100
100
1·2
70
-40
-30
33
28
25
-45
-22
-40
-20
-50 87
90
0·53
-40
-35
44
1501155/1601165/170 1501155/16011651170 1501155/1601165/170
145/1501155/16011651170
2·8
x 1010
Glossy
130/135/1401145/150
2·0
x 1013
Glossy
1401145/1501155/160/165
x 1012
91
108
0·55
50
-40
-2
4·5
Glossy
140/145/1501155/160
93
100
0·59
65
-45
-8
1·2 x 1012 Glossy
135/140/145/1501155
Glossy
1401145/150/155/160
29
Satin
135/1401145/1501155
28
Satin
135/140114511501155
70
3·3 x lO" Glossy
145/1501155/160/165
74
W. V. Titow
TABLE 3.6 'Welvic' (AECI) PVC Compounds (Table reproduced, with permission, from the technical Apptication sector
Use o
Grade
Colours: b N=Natural C= Colours B= Black
Specifications
Softness Hardness (BSSNo.) (ShoreA)
BS2782
Relative density
BS2782
Crystal-non-toxic
EX31U49 Food
Clear
37
78
1·23
G. P. extrusionopaque
GIIU4 G2JU65
Profiles Profiles Profiles Profiles Garden and mining hosing Garden and mining hosing Profiles Profiles and waterstop Profiles
N+C N+C N+C N+C N+C N+C N+C N+C N+C
9 20 30 35 35 35
97 90 83
1·33 1·34 1·30 1-45 1·26 1·35 1·28 1-41 1·26
Profiles Profiles Profiles Profies
Clear Clear Clear Clear
G.P. extrusioncrystal
G31U6 G31U87 G31J194 G31J196 G41U7 G41U88 G61U9 X2JJ364
X31J365 X41J366
X6IJ346
SABS645
40 46
80 80
80 77 73
60
66
24 35 45 63
87
74 64
1·30 1·28 1·27 1·26
N+C
4
99
1·67
B
20
90
1·31
99
1·37 1·31 1·30
80
Rawlplug extrusion
GO/J088
Rawlplugs
Antistatic extrusion
N2JU46
Antistatic hosing
Outdoor weathering
WO/U54 WllUl W2JU2
Profiles and fencing wire Profiles and fencing wire Profiles and fencing wire
N+C N+C N+C
5 10 21
97 89
Y3/J331
Y31J182
Fuel hosing Fuel hosing
Clear N+C
35 36
79
80
1·27 1·29
Oil/grease resislant
Y6IU56
Seals and gaskets
N+C
60
66
1·45
Low temperature
Y51J165
Refrigeration Gaskets
N+C N+C
56
68
1·31
Clear
42
75
PetroUoil resistant
Development grades EXSlJ265 Irradiationsterilisable WX4IJ362 Garden and mining
hosing outer
Clear
SABS645 and 1086
1·22
sheathing All values are typical results measured in ACEI's Plastics Technical Laboratory. a When specific contact with a foodslUff is involved. the advice of ACE]'s Plastics Technical Service Laboratory should
be
sought. The approval of the finished article is the fabricator's responsibility. b Colours: Standard colours are Natural 108 and Black 901 except with Y3/J331 where the standard natural is OA21. Non-standard colours are available on request. Masterbatches are also available for colouring purposes.
75
3 Commercial PVC Compounds
for Flexible Extrusions literature of AECI Chlor-Alkali and Plastics Ltd) Tensile properties Tensile strength (MPa)
Tear strength (Nmm- t )
8S2782
8S6746
Elongation at break (%)
8S2782
Loss in mass on age;nj
(mgcm- )
Hot deformation (%)
8S6746
8S6746
300
18·4
45 73
260 350 330 300 290
0·4 0·6 0·7 1·0 1·4
17-2 12·4 13·1
63 73 53
350 310 400
0·7 0·9 0·9
23·2 21·1 10·6 14·5
100 79 59 33
210 230 250 350
0·53 0·55 0·59 0·9
23·4 20·5 18·6 13·1 16·1
116 101
72
39 50
68 77
64 70 76
84
44 50 65 88
Cold bend
('C)
Cold flex temperature ('C)
8S2782
8S2782
temperature
Recommended extrusion temperatures ('C)
-45
-7
Glossy
1351140114511501155
-25 -40 -40 -32 -50
8 -10 -9 -10 -15
Glossy
-45 -35 -45
-15 -30 -34
Satin
Glossy
1501155116011651170 145/1501155/1601165 140114511501155/160 1401145115011551160 1401145/1501155/160 140/145/150/155/160 135/140114511501155 135/140114511501155 12511301135/140/145
-35 -40 -45 -45
8 -2 -8 -20
Glossy Glossy Glossy Glossy
1401145/1501155/160 140/145115011551160 1351140/145/150/155 120112511301135/140
170
10·3
Surface finish
Satin
Glossy
Satin
Glossy Glossy Glossy
Satin
1601165/1701175/180
Satin
145/150115511601165
20·7 22·7 20·7
132 117 105
260 260 320
0·4 0·4 0·5
32 38 40
-30 -30 -35
10 5 -5
Glossy Glossy Glossy
160116511701175/180 150/155116011651170 145/1501155/1601165
18·4
77
300
1·5
61
-50
-10
Glossy Glossy
140/145/1501155/160 140/145/150/1551160
Satin
120/125/130/1351140
Satin
125/13011351140/145
Glossy
135/140114511501155
10·8
-30
350
11·0 37
230
1·0
2·0%
78
-40
-18
63 70 80 87
90 95
Il/Jl09 I2IJl01 131J102 151J104 15/1341 I6IJl05 1611329 17/Jl06 191J130 1811293 19/U109 191J108
B&C B&C&Cl B&C&Cl B&C&Cl B&C B&C&CI B&C&Cl B&C&CI B&C B&C&O B&C&O B&C&CI
Heel filler blocks Translucentgeneral purpose Heel filler blocks Heel filler blocks, top pieces and thin unit soles Unit soles and general curpose Soling ~rade for footba I boots Unit so es and general put{'OSO Football boot soling requinng good low-temperature properties Unit soles and general purpose PVC/nitrile rubber blend for industrial and minin" footwear Gumboots and sports ootwear; also general pur.::.: Legging section 0 gum boots and general purpose Gumboots and slippers; also general purpose 35 50 53 60
13
65
17/J220
B only
OpaqueA cheap soling compound, cheaper footwear also general purpose
25
50 60 70 75 80
15/U75 I6IJl23 171J162 171M965 I8IJl63
52
53
54
57
64 61
80 71 69 66
95 86
63
71 66 61 59 57
1·21
1·18
1·19
1·17
1·20 1·21
1·25 1·24 1·22 1·23
1·31 1·26
1·38
1·26 1·27 1·34 1·23 1·33
Colours Relative Recommended Softness Hardness B=Black grade density (BSS No.) (Shore A) C= Colours (Natural at 23"'C) C/=Clear B&C B&C B&C B&C B&C
ParticularsQ
OpaqueThin soling for ladies shoes general purpose Unit soles and general purpose General pU!JlOSe soling Superior solmg General-purpose soling
General type
9·9
9·3
10·3
9·8
n·5 13·0
15·7 14·9 13·0 14·5
15·7 18·0
10·7
13-l 14·9 n·o 10·5 10·0
Tensile strength (MPa)
240
410
230
380
280 245
290 295 290 260
260 250
315
340
330
360
400 360
Elongation at break (%)
-40
-50
-36
-35
-35 -35
-35
-25
-29
-25
-25 -24
-8 -15 -20 -20
-25
-30 -35 -35
-3 -5
-27
-10 -20 -25 -25 -30
-15 -20
-30
-24 -25 -35 -34 -35
1 1 1 1 1
Cold Cold Loss of bend flex mass at temperature temperature l()(f'C (%) ("CJ ("CJ
TABLE 3.7 'Welvic' (AECI) PVC Compounds for Flexible Injection Moulding and Footwear (Table reproduced, with permission, from the technical literature of AECI Chlor-Alkali and Plastics Ltd)
~
PVClElvaloy blend for joggers PVClElvaloy blend for the legging section of gumboots PVClElvaloy blend for joggers
required
~~e~~l~! ::~~t~~~f i~umboots
Conducting compound formulated for use on Mauseriet soling machines. Meets BS 5451: 1971. Softer version of N3/134O PVC/nitrile rubber blend for
~~~SeBSaf~~~~t~~s.
Chunky footwear and generalpurpose applications Ladies footwear and generalpurpose applications Mens footwear and slippers and general-purpose applications Microcellular versIOn of 191J130 for lightweight industrial footwear Electrical socket boxes and general purpose Conducting compound formulated for explosive components and
17/J367 17/1336 1811368 19/1356
B&C B&C
N4IJ354
B only
B&C B&C
N31J34O
MI8I1353
B&C
B only
MI8IJ278
B&C
1211351
MI61J270
B&C
B&C
MI4IJ274
B&C 40
80 95
70 73
40
32
20
80
80
54
77
57 52
61 60
77
82
90
57
57
69
l'24(0'7)b
1·21 1-17
1-17 1-16
1·28
1·30
1·34
1'20(0·7)
1'20(0·7)
1·23(0·7)
9·5 9·0
5·5 3·5
5·7
6·0
20·0
375 220
330
310
135
150
350
-35 -40
-33
-35
-35
-25 -29
-25 -25
-10
-...l -...l
All values are typical results measured in AECI"s Plastics Technical Service Laboratory. Test methods according to BS 2782: 1970. Q Where specific contact with a foodstuff is involved, the advice of AECI's Plastics Technical Service Laboratory should be sought. The approval of the finished article is the fabricator's Iesponsibility. Properties quoted for microcellular compounds are approximate values. Densities in parenthesis are minimum blown values using suitable equipment.
Development grades
Microcellular
78
w.
V. Titow
REFERENCES 1. Adams, H. J. (1976). 34th ANTEC SPE Proceedings, pp. 330-2. 2. Weber, M. (1983). Plastics and Rubber International, 8(5),32-4. 3. Titow, W. V. and Lanham, B. J. (Eds), (1971). PVC Technology, 3rd Edn Applied Science Publishers, London, pp. 62-3.
CHAPTER 4
Elementary Principles of pvc Formulation W. V.
TITOW
4.1 INTRODUCTION As has been mentioned in Chapter 1, PVC resins-whether homopolymers or copolymers-must be modified by the incorporation of appropriate additives for satisfactory processing and end-use performance. The necessary modifications are more varied and extensive than those practised with any other polymer of major industrial importance, but so also is the applicational versatility of PVC in the form of the resulting materials. In this chapter the terms 'composition' and 'compound' are used generically for the material which results when PVC resin is combined with the necessary additives, and the operations involved in making a composition are collectively called 'compounding'. Note: Compounding may consist of, or include as one of its stages, mixing and/or melt-compounding. The former term applies either to the blending together of the constituents to produce a solid composition in the form of a free-flowing powder or powder agglomerate (either of these may be referred to as 'pre-blend' or 'dry blend', depending on the context), or to their mixing to form a liquid composition, i.e. a paste, latex, or solution. Melt-compounding is the operation (commonly carried out on a mixed pre-blend) whereby the constituents of a composition are intimately combined with the resin in the melt, under heat and shear: the resulting compound is normally pelletised (granulated). 79
w.
80
V. Titow
A composition is processed by the appropriate method to make a PVC product (e.g. sheet, film, pipe, injection-moulded component, blow-moulded container, coating on a fabric, etc.): some of the products may be used to fabricate secondary products by further processing (e.g. mouldings thermoformed from sheet, wall coverings made by laminating calendered films). A general outline of this sequence may be schematically represented as in Fig. 4.1. The material (and thus, of course, the formulation) of a product is that of the composition used to make it, but in the product the material will have had a more extensive 'heat history', having undergone at least one processing operation after the original compounding. The basic, general principles of formulating a PVC composition (Le. deciding on the nature and proportions of its constituents) are relatively straightforward: however, successful practice calls for a great deal of specialist knowledge and experience. A formulator normally designs a composition in response to a stated need for a product, Le. he starts with the knowledge of the nature and intended end-use of the product into which the composition will be processed (these factors will also determine the process to be used): in the light of this knowledge three main considerations will govern his choice of the formulation's components and their proportions, viz.
1------ --------, Compounding
: Physical forms:
JSolid: (a) Fr(l(l-flowlng powd(lr or
r---..L---,
Composition L..-_.,....----J--
powd(lr agglom(lrat(l (pr(lbl(lnd; dry bl(lndJ : (b) P(lII(lts; granulat(l I Liquid: (a) Past(l I (b) Lat(lx I (c) Solution 1-
------1
Proc(lssing
IProd"~
: 1 I
I I
1
I
l
_
I
I
Furth(lr proc(lsslng
•
I
t.I
,
IS(lcondary product 1
----------J
Fig. 4.1 Stages in the manufacture of PVC products-general outline.
4 Elementary Principles of pvc Formulation
81
Processing requirements: To meet these the composition must be in the right form (e.g. pellets for most injection mouldings, paste for spread-coating of a fabric), have the necessary heat stability under the processing conditions, and other properties appropriate for the particular process (e.g. the right melt rheology for injection moulding or extrusion, right viscosity characteristics in a paste for spread-coating, correct lubricity in a calendering compound to prevent 'plate-out'). (ii) Service requirements: These will be dictated by the end-use, and will normally include stability to the service environment. (iii) Material and process cost economy: Save in certain special cases (e.g. some military applications) this is always an important consideration, influencing the design of a formulation.
(i)
4.2 THE COMPONENTS, AND BASIC TYPES, OF A PVC FORMULATION
As already mentioned in Chapter 1, PVC materials and products fall into two broad categories: rigid (uPVC) and plasticised (pPVC). Materials of the latter group with low plasticiser content (up to about 30 phr) are often referred to as 'semi-rigid': the softest pPVC materials may contain over 100 phr of plasticiser(s). With the partial exception (not important in the context of the present discussion) of certain liquid compositions (some solutions and lattices-see Chapters 23 and 24), virtually all PVC compositions undergo heat treatment-and most experience 'internal' (frictional) heat generated by mechanical shearing-in the course of processing (including the initial compounding). Many end-uses also involve exposure to heat (albeit usually at temperatures somewhat lower than those encountered in processing) either as a normal condition of service (e.g. for flexible PVC insulation on electric wire, or rigid PVC products exposed to direct sunlight outdoors, as with, say, guttering, cladding or window frames), or as an occasional hazard. For all these reasons, the PVC polymers-which are inherently prone to heat degradation (see Chapters 1, 9, and 12)-have to be protected by the incorporation of heat stabilisers. Thus the PVC resin itself and a heat stabiliser are two essential components of virtually every PVC formulation. A simple, basic formulation for a solid uPVC composition will also
82
W. V. Titow
normally contain a processing aid (a polymeric additive, to improve fusion characteristics and melt flow), an impact modifier (also a polymeric additive, to impart toughness to the relatively brittle PVC resin), and a lubricant to counteract sticking to hot equipment surfaces in processing ('external' lubrication) and/or to reduce the frictional effects within the melt ('internal' lubrication): more than one lubricant is often used to discharge these functions. Similarly, more than one processing aid and/or impact modifier may be incorporated for best results in a particular composition, but-on the other hand-a single additive may serve in both capacities in some cases (e.g. chlorinated polyethylene in certain formulations-see Chapter 11). In a basic formulation for a pPVC composition the plasticiser (or plasticiser system, if more than one plasticiser is used) is the essential component in addition to the PVC resin and heat stabiliser. Apart from its effects on the finished material properties (imparting softness, flexibility, extensibility, toughness) it plays an important role in heat processing, combining the functions of processing aid with lubricant action (particularly internal lubrication; although some plasticisersand especially extenders-can provide external lubrication, it is not unusual to include lubricants with good 'external' effect in pPVC formulations) . In addition to the basic constituents most PVC formulations contain other components incorporated mainly to impart properties required in the PVC product for particular end-uses. The most widely used of these additives are colourants and fillers. The colourants include pigments (which impart opacity as well as colour to the PVC material) and dyes (colourants fully soluble in the composition and thus suitable for transparent materials). Fillers are often included to cheapen the cost of PVC compositions, although even when this is the primary purpose the attendant effects on some properties may also be useful, e.g. reduced shrinkage, increased hardness and stiffness, which are often brought about by the presence of a substantial proportion of finer. Some fillers have a purely functional role-e.g. glass fibres and fine-particle calcium carbonate act as reinforcements in uPVC compositions. The uses and effects of fillers are discussed in Chapter 8. Other formulation components with specific end-use functions in the PVC material are the following: Antioxidants and UV absorbers: Used to improve the resistance of PVC products (especially those for outdoor use) to photochemical degradation and weathering generally (see Chapters 11 and 12).
4
Elementary Principles of pvc Formulation
83
Antistatic agents: Incorporated to counteract the build-up of electrostatic charges (with the consequent risk of sparks in some cases, and the nuisance of dust pick-up) on the surface of PVC productse.g. pPVC conveyor belts, or uPVC mouldings. Flame retardants: Included, especially in some flexible PVC materials, to reduce their flammability (enhanced by the presence of substantial amounts of many plasticisers). uPVC is among the least flammable plastics materials (see Chapters 7, 8, 11 and 12). Smoke suppressants: Incorporated to reduce the amount of smoke generated by burning PVC materials (some additives in this category also cut down the evolution of HCI in burning)-see Chapters 8, 11 and 12. Biostats: Included in PVC compositions (especially some flexible materials) to counteract attack by micro-organisms (bacteria and fungi)-see Chapters 7 and 12. Odour control agents: Such additives are used in some formulations to impart a smell (e.g. leather-like odour to PVC handbags or upholstery) or to mask odour of another formulation component. According to their function in a particular composition they may be referred to as 'reodorants' or 'deodorants'.
Two kinds of functional components included in certain formulations to act in the processing rather than the service stage are blowing agents (which, in the production of PVC foam, create the cellular structure by evolution of gases at the processing temperature) and surface active agents used to facilitate the mechanical whipping-in of air into PVC pastes, and to control the resulting bubble formation, in the air-entrainment method of PVC foam production-see Chapters 11 and 25.
4.3 FORMULATION COSTING-BASIC POINTS It is usual to express the formulation of a composition on the basis of 100 parts by weight of the PVC resin, using the abbreviation 'phr' (parts per hundred of resin) for the content of each of the other components present. For costing purposes the volume cost is usually
w. V. Titow
84
required, and for this the volume content of each component of the formulation is worked out from the weight content and density values. These points are illustrated by the following example of a simple plasticised compound (an extrusion compound of BS softness about 20): Component
Parts by weight (g)
Density (gem- J )
Parts by volume (em3)
100·0 50·0 7·0 1·0 158·0
1·40 0·99 3·81 0·93
71·43 50·50 1·84 1·08 124·85
PVC polymer Plasticiser Stabiliser Lubricant
Since, by definition, density = weight/volume, the parts by volume are obtained by dividing the parts by weight by the density in each case. Similarly, the density of the compound is given by 158/124·85 = 1·266 gcm- 3 . The density of a PVC composition is important for costing purposes since, although it is convenient to formulate it on a weight basis, the composition will ultimately be used (and sold) on a volume basis, as a given weight will produce a certain number of mouldings or a certain volume of extrudate (e.g. a certain length of pipe of given dimensions). Thus the volume cost is of direct interest, as is the fact that, in a comparison of two formulations, it may be higher for the one cheaper on a weight basis. For example, consider two compounds, A and B, of which B is 10% more expensive on a weight basis but has lower density: Weight eost perg per kg Compound A Compound B
1·35 1-10
n 1-1n
O·OOln
O·OOlln
Volume cost (perem3) 1·35 x O·OOln = 0·00135n 1·10 x O·OOlln = 0·001 21n
Thus, in this general example, because of the density difference, the compound which is 10% more expensive on a weight basis is cheaper in roughly the same proportion on a volume basis.
4 Elementary Principles of pvc Formulation
85
The simple extrusion compound cited at the beginning of this section may be costed as illustrated below (the component prices used in the illustration are roughly in line with those applicable to tonne-lot purchases in the first half of 1980 in the UK). Formulation component
PVC polymer Plasticiser Stabiliser Lubricant
A
B
Weight proportion (kg)
Cost per kg
100 50·0 7·0 1·0 158·0
0·40 0·50 0·40 0·25
C Cost in formulation
(£)
(A
x B)
(£)
40 25 2·8 0·25 68·05
The weight cost of this compound will therefore be 68·05/158 = £0·4307 per kg, and the volume cost (given the density as 1·266 g cm- 3-see above) 1·266 x 0·0004307 = £0·000545 26 per cm3.
4.4 MAIN GENERAL CONSIDERATIONS IN THE SELECflON OF PRINCIPAL FORMULATION COMPONENTS
Each component of a particular PVC formulation (i.e. the PVC resin, stabiliser, plasticiser, etc.) is selected from a large group of available materials of its type. The choice is made-in the light of the requirements mentioned in Section 4.1-on the basis of the known characteristics of the individual candidate materials within the group, in conjunction with certain other relevant considerations (including the mutual compatability, interactions, and side effects of the formulation components when compounded into a composition). The purpose of this section is to give a general indication of the main factors concerned in the choice: a comprehensive discussion of the complex subject of the properties and effects of PVC formulation components would be far beyond its scope. Further relevant information will be found in various
86
w. v.
Titow
parts of this book (especially Chapters 2, 3, 6 to 12, and 21 to 25). A useful summary of some important considerations has been provided by K. Regan in the Buss-Hamilton Compounding Manual. *
4.4.1 Nature and Characteristics of Individual Components of a Formulation (a) PVC Polymer MOLECULAR WEIGHT
In general, the higher the molecular weight of the polymer (and hence its K value and viscosity number-see Chapters 1 and 2) the better many mechanical properties of the composition but the more difficult its processing (because the ease of fusion and melt flow decreases with increasing molecular weight). The choice of the molecular weight grade (K value) of a commercial polymer to be used in a particular formulation is normally a compromise between what is the most desirable for processing on one hand and service properties on the other. The processing considerations become less critical with adequately plasticised compositions, so that polymers of higher K value can be used in those-typically 65 to 71 t (ICI range 60-65, Fig. 4.2), as against 54 to 65t (ICI range 50-60) in uPVC (with the lowest molecular weight polymers used in those rigid compositions where the easiest melt flow is required, e.g. in some injection-moulding and bottle-blowing compounds). The molecular weight distribution of the polymer also affects physical properties and processing: it is normally maintained within certain limits in a given commercial polymer grade. KIND (PROCESS TYPE)
Except for the special case of formulating PVC solutions for surface coatings, for which polymer produced by the solution process will normally be used (see Chapters 2 and 24), the choice will be between suspension polymers, mass (bulk) polymers or emulsion polymers (i.e. PVC resins produced by those three polymerisation processes-see Chapter 2). For PVC paste compositions emulsion polymers are used
* Edited and published by Buss-Hamilton Ltd, Cheadle Hulme, Stockport, England. t K values from viscosity determinations on 0·5 g per 100 ml (polymer/cyclohexanone) solutions. Corresponding ISO viscosity number ranges are, respectively, about 105-125 and 70-105-see Table 2.1 in Chapter 2.
4 Elementary Principles of pvc Formulation
87
80
70
50
60
70
80
K value (0·59 resin In 100ml cyclohexanone)
Fig. 4.2 The relationship between ICI K value and K value determined by viscosity measurement on a solution of 0·5 g PVC resin in 100 ml cyclohexanone (ct. Chapter 1, Fig. 1.2, and Chapter 2, Table 2.1)
because of their particle size and structure characteristics, although suspension resins are sometimes added as 'extender' ('filler') polymers to modify the viscosity of pastes: the use of PVC polymers in pastes is discussed in Chapter 21. Solid compositions (sometimes referred to as 'general purpose' compounds in contradistinction to pastes) for injection moulding, extrusion, or calendering are commonly formulated with suspension or mass polymers, although mixtures of one of these with an emulsion resin, or even emulsion resins alone, are occasionally used in Europe in some uPVC compositions for better melt flow. Note: Emulsion resins are not used in electrical insulation compositions, because the residues of additives from the polymerisation process (especially emulsifying agents) which they contain lower electrical resistivity.
Suspension and mass polymers are, broadly speaking, roughly
88
W. V. Titow
equivalent from the point of view of the final material properties, although a mass polymer may contain fewer impurities than an otherwise comparable suspension polymer and hence give compounds of better clarity (see 'quality' below). The differences between the two types of polymer are more strongly manifested in processing. Mass polymers tend to have higher bulk densities than suspension polymers (for comparable polymer molecular weight and particle porosity), with greater uniformity of particle porosity and shape (and hence more uniform and rapid plasticiser absorption), as well as greater ease of gelation (i.e. ease and completeness of fusion in heat processing) which is of particular interest in the production of large mouldings. All these characteristics can offer processing advantages in certain circumstances and processes, but the fact should not be overlooked that-whilst the optimum conditions for a suspension and a mass polymer may be different in a particular process-if they are properly optimised in each individual case either polymer can be processed satisfactorily. CHEMICAL NATURE
The PVC resin on which a formulation is based may be a homopolymer, or a copolymer, or sometimes a mixture of both. The general differences between the two, their main respective applications and uses, and the copolymers of the greatest practical interest, have all been mentioned in Chapters 1 and 2. In broad summary it may be said that homopolymers have better thermal stability and impart better physical properties to the composition, whilst copolymers make for easier melt processing at relatively lower temperatures, and improve extensibility and filler acceptance in some compositions: copolymers are also normally used in PVC-based solution formulations for surface coatings-see Chapter 24. The melt-processing advantages of copolymers versus homopolymers, like those of lower versus higher molecular weight polymers (see above), are more important in rigid formulations, as in adequately plasticised compositions the fusion and melt flow characteristics are primarily governed by the plasticiser. Vinyl chloride/acetate copolymers form the basis of uPVC compounds for gramophone records and certain types of sheeting; they are also used in flooring compositions. In Europe they are sometimes included with a homopolymer to improve the ease of processing of some rigid formulations. Vinyl chloride/vinylidene chloride copolymers are employed, alone or in admixture to homopolymer, in some specialised calendering compounds (for uPVC sheeting), and as viscosity-
4 Elementary Principles of pvc Formulation
89
reducing extender polymer in some paste formulations (see Chapter
21). QUALITY
It is clearly desirable that the PVC resin used in any formulation should be of reasonable quality, and the standard grades supplied by the manufacturers are, generally speaking, reliable and acceptable in this respect for most purposes. However, some applications call for particularly high quality (and special high-grade resins are available) whilst in certain cases a lower than standard quality may be acceptable in the formulation. Apart from the molecular weight spread (which should be within appropriate limits for the given nominal K value of the polymer grade) the factors making up quality in the present context are: polymer particle size and size distribution (with special reference to coarse particle content); colour; 'fish-eye' content; and level of particulate contamination ('speck count').
Note: ISO 472-1979 defines 'fish-eye' as 'a small globular mass which has not blended completely into the surrounding material and is particularly evident in a transparent or translucent material'. In PVC polymer fish-eyes (also called gels) are hard, particulate specks of polymer which are not dispersed in processing and thus remain as discrete inhomogeneities in the finished product. A standard method of determining the number and size of fish-eyes in general-purpose PVC resins is given in ASTMD3596-77 (see also ASTMD3351-74: Gel count of plastic film). Low-grade polymer can be used, in admixture with a standard grade or even alone, in formulations for the cheaper kinds of injectionmoulding compositions, usually fairly heavily filled, used for the production of cheap moulded footwear (sandals, slipper soles) and toys. High-grade polymer is necessary for such uPVC products as transparent ('clear') packaging film and bottles, and in plasticised compositions for clear medical products (e.g. containers and tubing for saline drip sets), clear, flexible hose and packaging film (e.g. 'clingwrap') . (b) Heat Stabilisers Whilst the main mechanisms involved in the complex process of thermal degradation of PVC and its counteraction by stabilisers are
90
W. V. Titow
understood in general terms, the stabilisation of PVC formulations is still largely a practical art, relying on the specialist interpretation and application, in the specific context of a particular composition, of information and principles stemming mainly from practical experience. For this reason, and also because a great variety of stabilisers is available (many of which are not single chemicals but compositions specially developed for particular types of formulations and end-uses) even a skilled formulator will normally seek the advice of a reputable supplier when formulating a composition of which he has no previous experience. The proportion of stabiliser(s) in a PVC formulation will vary-in broad, general terms between about 0·5 and 8 phrdepending on the nature and purpose of the composition, the nature of the stabiliser itself, and the processing (including the kind of compounding to be undergone--e.g. completeness of dispersion achievable with modern compounding machinery reduces stabiliser requirement). Heat stabilisers and their effects are discussed in some detail in Chapters 9 and 10; only the most basic, general points are mentioned here. There is no rigid, standard classification of stabilisers, but-for the purposes of a brief general discussion-they may conveniently be considered under the following headings. LEAD COMPOUNDS
Several lead salts and some lead 'soaps' (notably lead stearates) are widely used as stabilisers in PVC (the soaps partly for their lubricant properties). Lead compounds are the oldest general-purpose stabilisers, comparatively cheap, and capable of imparting good long-term stability. They have always dominated the electrical insulation applications of pPVC compounds, because lead chloride, formed when a lead stabiliser reacts with HCl (a principal decomposition product of PVC-see Chapter 9), is insoluble and non-ionisable, so that the electrical resistivity remains unimpaired as the stabiliser discharges its function. In uPVC compositions lead stabilisers are used in many formulations for extruded cladding and profiles: dibasic lead phosphite is of particular interest for opaque products for outdoor use, as it imparts very good light stability. In Europe lead stabilisers are also used in uPVC pipes, although in the USA tin stabilisers (see below) are favoured for this area of application. The main limitations and disadvantages of lead stabilisers as a group are toxicity (hazards in processing, as distinct from potential risks in compounds, are reduced
4 Elementary Principles of pvc Formulation
91
to some extent by various means-see Chapter 9), opacity (or at least translucency) imparted to PVC compositions, and susceptibility to staining (through the formation of black lead sulphide) in contact with sulphur-containing materials (e.g. 'cross-staining' with certain rubbers) and atmospheres. BARIUM/CADMIUM AND BARIUM/CADMIUM/ZINC SYSTEMS
The former type of stabiliser system may consist of barium and cadmium soaps (e.g. laurates, stearates, or myristates) mixed or co-precipitated, or a liquid salt combination (e.g. barium and cadmium phenates). The zinc-containing systems incorporate in addition an organic salt of that met.al (often zinc octoate). Both types of system are extensively used-and can be considered by the formulator-for a variety of applications in both flexible and rigid PVC compositions: for example, most of the flexible calendered film and sheeting compounds are formulated with these stabilisers, as are many paste products (e.g. coatings, rotational mouldings), as well as numerous flexible and rigid extrusion compositions (including, e.g. wire-coating compounds, profiles), and injection-moulding compounds (e.g. for footwear). Both types of stabiliser system are quite widely used in conjunction with synergistic co-stabilisers-an organic phosphite and/or an epoxy compound (often an epoxidised oil) which enhance the main stabilising action and improve the weathering resistance of the composition. The epoxy compound also has a plasticising action, which may be significant in some rigid formulations, e.g. bottle compounds. Some proprietary BalCd and BalCd/Zn stabilisers include the synergistic co-stabilisers as part of the system. Both types of basic system offer good heat stability and colour control, and good light stability (when the co-stabilisers are incorporated). Clear compounds can be formulated with the liquid BalCd systems; the soap-based and zinccontaining systems have an opacifying effect and are thus more suited to translucent or opaque compositions. Although effective and versatile, the systems are not without their limitations: the presence of cadmium makes them susceptible to sulphide-staining (through the formation of yellow cadmium disulphide on contact with sulphur compounds) although this is counteracted by the presence of zinc in those of the systems which contain it: 'plate-out' can occur in the processing of compositions stabilised with these systems (i.e. the formulation of sticky deposits on the working surfaces of processing equipment-see Chapter 9); cadmium-containing systems are not
92
W. V. Titow
suitable for food-contact applications and other end-uses (e.g. medical equipment) where potential toxicity is a problem-other combined metal salt systems (usually in conjunction with epoxy compounds) are used in such cases (see below). TIN STABILISERS
These are organotin compounds, comprising methyl, butyl, octyl and lauryl tin derivatives, and including also the relatively recent 'estertins' (see Chapter 9); some contain mercaptan or thio groups (and hence are usually referred to as 'tin mercaptides' or 'thiotins'), and others sulphur-free groups derived from carboxylic acids (the tin 'carboxylates'). The tin compounds (and, within this group, especially the thiotins) are the most effective heat stabilisers for PVC, both flexible and rigid, imparting a high degree of initial and long-term stability, and outstanding clarity to transparent compounds (particularly important in, for example, packaging film and bottles). Their limitations are: relatively high cost, comparatively poor light stability of thiotinstabilised compounds, and toxicity, although a few tin stabilisers are permitted for food-contact uses (see Chapter 9). The thiotins can also impart odour to PVC compositions and cause sulphide cross-staining with heavy-metal stabilisers or pigments. Note: Some antimony mercaptide stabilisers are also available.
Whilst generally similar in their action (and limitations) to the tin stabilisers, they can be more economical in some formulations, but also make compositions somewhat more prone to sulphide cross-staining, and less stable to photodegradation. The use of calcium stearate in conjunction with antimony mercaptide stabilisers increases heat stability through a strong synergistic effect. NON-TOXIC STABILISERS
These must be used in products for food-contact and medical applications, e.g. flexible and rigid PVC packaging films, PVC bottles, flexible PVC medical tubing, and hose used in food processing. The relevant regulations and directives (and-in cases of doubt-the official bodies concerned) should always be consulted when formulating non-toxic compositions, as the requirements and restrictions applicable can differ with the application, and from country to country. As has been mentioned, some tin stabilisers are permitted (with limitations, in
4 Elementary Principles of pvc Formulation
93
some countries, on the percentage present in the formulation, and the amount extractable) for food-contact uses (see Chapter 9). Systems consisting of combinations of organic salts of calcium and magnesium, calcium and zinc, or calcium, magnesium and zinc, may also be used, commonly in conjunction with an epoxy compound (which here improves heat as well as light stability, and provides some plasticisation) and-where appropriate-also permitted antioxidants. Some esters of aminocrotonic acid are permitted food-contact stabilisers in several countries.
Note: Calcium stearate, regarded for some purposes as a non-toxic stabiliser, is used in certain compositions (e.g. pastes for dip-coating applications) which are not primarily intended for non-toxic end-uses. NON-METALLIC (ORGANIC) STABILISERS
The materials which may be grouped under this heading are the already mentioned epoxy compounds and organic phosphites, most commonly used as auxiliary, synergistic stabilisers with some metal stabiliser systems. The epoxy compounds are very often epoxidised oils (e.g. epoxidised soyabean oil), but epoxidised esters and some epoxy resins can also be used (see also Chapter 9).
(c) Plasticisers As has been mentioned, plasticisers convert the hard, inherently brittle PVC resin into compositions of varying degrees of softness and flexibility, processable into a variety of products with divers properties and uses (determined to a large extent by the nature and amount of plasticiser(s) present). Plasticisers and their effects in PVC are discussed in several chapters of this book (in particular Chapters 5 to 7): because these chapters cover the subject thoroughly, and because of its complexity and extent, only the most basic, general pointers are given in the present section. The proportion of plasticiser in a pPVC composition is always fairly substantial, and may be very high in very soft materials. For this reason cost considerations are particularly important in plasticiser selection. In the absence of special processing and/or service requirements which may dictate the choice (see below) the formulator will normally consider first a relatively inexpensive, general-purpose plasticiser. In most cases this will be a phthalate, very often dioctyl
94
W. V. Titow
phthalate (DOP) which offers good all-round compound properties. Several phthalates (including DOP) are primary plasticisers, that is they are highly compatible with PVC resins (in proportions of 100 phr and over). The cost of phthalate-plasticised formulations (and some containing other plasticisers) can often be reduced by replacing part of the main plasticiser by an extender: This is frequently a chlorinated hydrocarbon which, additionally, has a flame-retardant effect-a useful feature because substantial plasticisation with phthalates increases the flammability of PVC compositions (cf. Chapters 11 and 12). Triaryl phosphate plasticisers (which also act as flame retardants) are the second most important group of common, primary plasticisers; however, a triaryl phosphate will rarely be used as the sole plasticiser, since the low-temperature properties (cold flex, toughness) of its compounds with PVC resins are comparatively poor. This may be corrected by the inclusion of a phthalate plasticiser (in adequate proportion) or-where a stronger effect is required-of an aliphatic diester plasticiser which imparts particularly good low-temperature properties. In general, when more than one plasticiser is used, the properties of the PVC composition will represent a combination of those normally conferred by each individual plasticiser when present alone. This is an important feature of plasticiser action, widely utilised in formulating PVC compositions. Even within the phthalate group (the largest both in terms of number of members and usage volume) some plasticisers are particularly associated with the special properties they promote in the compound. For example, butyl benzyl phthalate (BBP) is one of the best solvating, quickest fusing, plasticisers for PVC (and thus promotes rapid and easy processing of its compositions); it also imparts particularly good stain resistance to PVC films and surfaces. Ditridecyl phthalate has a particularly low volatility (for a phthalate) and hence a high degree of permanence in compounds (useful, for example, in high-temperature cable coverings). For particularly high levels of certain properties, beyond what is available even with the best among general-purpose plasticisers, recourse must be had to other, special plasticiser types. Selected representatives of these types (all more expensive, as a rule, than general-purpose plasticisers) are used in PVC compositions where the particular property or group of properties they contribute is of special importance. Such special uses are indicated, in a general way, in the outline summary of plasticiser effects in Table 4.1. Table 6.1 in Chapter 6 provides a guide to the
4
Elementary Principles of pvc Formulation
95
main advantages and limitations of major types of plasticiser relevant to their usage.
(d) Lubricants The functions of lubricants in PVC compositions are: (i) to reduce the friction at, and adhesion to, working surfaces when the composition is being processed (external lubrication); and/or (ii) to lower the inter-particle and inter-molecular friction in processing (internal lubrication); this reduces the effective melt viscosity and heat build-up. Some lubricants discharge only, or mainly, one of these two functions and are, accordingly, referred to as internal or external lubricants; others act in both ways. The main factor determining the type of lubricant action is the lubricant's compatibility with PVC: a true external lubricant is poorly compatible, and a good internal lubricant highly compatible with the resin. Chemical compounds used as lubricants include paraffins, paraffin oils, polyethylene waxes, fatty acids, fatty acid amides and esters, fatty alcohols, and metal soaps. Lubricants must be used in uPVC compositions (for both internal and external effects: two or even more lubricants are often combined, up to a total content of about 3 phr) , and are beneficial in many plasticised compositions (in amounts within the range O· 2-1' 5 phr) mainly for external lubrication, as much of the internal function is discharged by the plasticiser. The nature and amount of lubricant(s) used in a formulation depend critically and in a complex way on several factors, viz. the other components (see below), the processing to be undergone by the composition (the correct choice is particularly important in many calendering compositions), and any subsequent treatments (e.g. films to be printed must have no exuded external lubricant on the surface). Possible interaction or co-action with other constituents of the formulation, especially the stabiliser(s), is a most important consideration in the choice of lubricant(s) and the amount(s) to be used. For example, some stabilisers have a lubricant action, and/or benefit from synergistic co-action with certain lubricants, whilst some lubricants also exert a stabilising effect; more lubricant may be necessary in compositions highly loaded with fine-particle fillers (because of absorption effects), but relatively less if the filler is surface-coated with a stearate (see Chapter 8); the degree of
Important features of behaviour of plasticiser in composition: (a) High compatability with PVC resin (i.e. suitability for use in high proportions in a composition) (b) Permanence (low volatility, resistance to extraction and migration in compositions)
Price economy
Characteristics required
Very soft, flexible products, including paste mouldings and coatings Shower curtains, upholstery; gaskets
Polymeric plasticisers; for some purposes: trimellitates, high molecular weight phthalates, solid blending resins (e.g. chlorinated PE, EVA copolymers, nitrile rubber)
Wide range of cheaper-grade compositions for various purposes
Examples of application
Many phthalates, triaryl phosphates
Selected phthalates, extenders
Typically relevant plasticiser type(s)
TABLE 4.1 Some General Features of Plasticiser Usage
o~
:::J
:<:::
~
~
Aliphatic diesters and extenders BBP, DBP, triaryl phosphates, polymeric plasticisers
BBP,DBP, triaryl phosphates, phthalates
End-use properties imparted to compositions: (a) Good colour Phthalates (b) Good chemical resistance Polymeric plasticisers (c) Good low-temperature properties Aliphatic diesters (sebacates, adipates, AGS esters) (d) Electrical properties high resistivity Triaryl phosphates low resistivity Sebacates (e) Food-contact applications Individual plasticisers (high purity grade) as permitted by relevant authorities (f) Mechanical properties high tensile strength Triaryl phosphates high extensibility Sebacates
(b) Effect on viscosity of pastes: low viscosity high viscosity
Processing properties: (a) Ease of solvation, fusion, and gelation
Packaging films
Clear compositions Protective clothing Tarpaulins, flexible tubing for use in cold conditions
Foamed coatings
~
5' ;::s
i:>
I::
~ ~
~
.5;,
~
~
(")
?-
is
~
f
~
W. V. Titow
98
compatibility of a lubricant with the processing aid and/or impact modifier in a uPVC composition will affect the choice of type and proportion of the lubricant. Suppliers provide lubricant systems, including 'single-pack' stabiliserllubricant combinations, for most types of PVC compositions, and even a highly skilled formulator can benefit from their advice. Lubricants and their uses are discussed in more detail in Chapter 11, Section 11.1. (e) Polymeric Modifiers The polymeric additives used in PVC compositions fall into two broad functional groups: processing aids and impact modifiers. PROCESSING AIDS
The polymers used in this capacity improve the melt-processing characteristics of PVC compositions (rapidity of homogenisation and fusion) and the properties of the melt (melt strength, cohesion and elongation are increased; in many cases the melt modulus is reduced), but they also usually increase the melt viscosity (which is already high in uPVC compositions): some can also have an external lubricant effect (lubricating processing aids). At their usual levels of incorporation (1-6 phr) processing aids do not significantly affect the end-use properties of the composition. The processing aids in commercial use may be broadly grouped under the following headings: (i) acrylic polymers (acrylates and methacrylates); (ii) styrene copolymers (with acrylonitrile or certain methacrylates). Both these general groups contain many proprietary products with varying applicability in different types of composition. The recommendations of a reputable supplier should, therefore, always be considered. Poly-a-methylstyrene and some ABS terpolymers are also used as processing aids (the former can, inter alia, actually reduce the melt viscosity of a uPVC composition). IMPACT MODIFIERS
The main function of these additives is to improve the toughness (resistance to impact at room and low temperatures) of uPVC compositions, in which they are usually incorporated in proportions of 5 to about 15 phr (the use and effects of impact modifiers are discussed
4 Elementary Principles of pvc Formulation
99
in more detail in Chapter 11, Section 11.2). In most cases there is also some processing-aid action, but this may be significant only at processing temperatures higher than those at which regular processing aids exert their effect. Some types of impact modifier (e.g. (iv), (v), and certain kinds of (vii) below) are highly compatible with PVC and may be incorporated in high proportions, to act as plasticisers (see Chapter 11, Section 11.2). Commercially available impact modifiers are polymers of the following types: (i) (ii) (iii) (iv)
acrylonitrile/butadiene/styrene terpolymers (ABS); methacrylate/butadiene/styrene terpolymers (MBS); modified acrylic polymers; ethylene/vinyl acetate copolymers (EVA) and graft copolymers of vinyl chloride and EVA (EVAIVC); (v) nitrile rubber (butadiene/acrylonitrile copolymers); (vi) polyurethane elastomers; (vii) chlorinated polyethylene (various degrees of chlorination).
While the impact modifiers find their main application in uPVC compositions (although some, as just mentioned, may be used-in large proportions-to act as plasticisers), some (notably certain ABS types) may be incorporated in semi-rigid and even flexible compounds, not primarily to contribute to toughness, but to improve melt strength in processing, emboss retention and thermoforming properties of sheet (where they can also reduce post-forming shrinkage). In some types of composition very high proportions of a polymeric modifier are included. Thus up to about 100 phr of particular processing aids, or modified ABS or MBS impact-modifier systems, may sometimes be used to improve the heat distortion properties of the material (the heat deflection temperature under load at 1·82 MPa may be raised by about 15°C in comparison with that of an unmodified lkt otherwise similar uPVC composition). Some flexible compositions may actually contain more of an ABS polymer than PVC resin, so that the latter may be regarded as a modifier for the former (see Chapter 11, Section 11.2).
(f) Fillers The use of fillers in PVC is discussed in some detail in Chapter 8, and referred to in other parts of the book in connection with various filled compositions and products. Only the most basic points relevant to the role and application of fillers in PVC formulations are briefly
100
W. V. Titow
TABLE 4.2
Effect of Particulate FiBer Content on Tensile Strength (Ibfin- z) of pPVC
Filler
Precipitated whiting Hard clay Carbon black Omya BSH
Parts by weight 30 40
0
10
20
1700 1700 1700 1700
1420 1780 1900 1615
1350 1710 1920 1590
1200 1450 1550 1580
920 1200 1750 1370
50
60
750 890
510 1110
1250
1070
mentioned here. In many compositions a filler is included principally to reduce material cost: in such 'extender' applications the filler is nowadays normally some form of particulate calcium carbonate. When present in a substantial amount this filler will-as will virtually all mineral fillers in plastics generally-affect some physical properties of the PVC composition: the common effects are reduced tensile strength (see Table 4.2), elongation at break, moulding shrinkage and thermal expansion coefficient, and increased hardness. A particularly important point concerning the general effects of mineral fillers in plastics compositions, including PVC, is that even at loadings which in many cases do not bring about substantial changes in other properties, the density of the composition may increase significantly; it will also continue rising with further increases in filler content. Since the compositions are sold on a weight basis (whilst ultimately used, in the form of products, on a volume basis-see Section 4.3 above), the overall material cost saving may drop progressively (in some cases quite rapidly) with increased filler loading. Note: e.g., if a given volume of product (say a moulding) weighs
1 kg when produced from a compound of density 1·1 gcm- 3 , the weight of compound of density 1·4 g cm -3 needed to produce it will be 1·4/1·1 = 1·27 kg (to the second decimal place).
Typical applications of calcium carbonate in plasticised PVC include its use in electrical cable coverings (insulation and sheathing), moulded footwear (especially the cheaper qualities), moulding compounds for toys, some grades of sheeting and fabric coatings (in the latter two applications plasticiser exudation can be reduced, and some improvement in weathering promoted, by the presence of the filler). In
4 Elementary Principles of pvc Formulation
101
unplasticised PVC products calcium carbonate fillers are used at relatively lower loadings in most cases. The biggest outlets are in rigid pipes and profiles, and in flooring. Over the years, calcium carbonate fillers have largely displaced clay fillers in most applications primarily because of the cost advantages they offer in virtually every case. Clays are still used, however, in certain special electrical insulation compounds, where-at loadings of about 5-15 phr-they benefit the insulation properties. Their generally low pigmentation effect (due to a refractive index closely approaching that of many PVC compositions) can also be an advantage in some cases. Some effects of an electrical grade of clay filler on the properties of a PVC composition are shown in Table 4.3. TABLE 4.3 Some effects of a Clay Finer in a pPVC Composition
Filler Composition a content and properties
Resin Di-2-ethyl hexyl phthalate Tribasic lead sulphate Paraffin wax Filler: M 501 b
M501 (phr) Nil 5 15 10 2500 2500 2500 2500 T~nsile strength (lbfin- 2) Modulus 100% extension (lbf in -2) 1500 1700 1800 1900 Elongation at break (%) 300 280 270 250 Tear strength (lb in-I) 1200 1300 1400 1400 32 38 32 30 BS softness No. Volume resistivity, conditioned x 1014 (23°C, 65% RH) 11 0·3 13 15 Volume resistivity (Qcm x 1014 , 24 h immersion) 0·2 3 7 6 Volume resistivity at 60°C x 1012 0·6 50 90 110 a Compounding
100 50
7 1 As stated
} Parts by weight
20 2400
30 40 50 2500 2400 2500
2000
2200 2300 2400
230 1400 29
170 170 120 1400 1500 1500 28 26 23
13
12
8
6
7
6
4
2
100
90
70
60
and testing was carried out according to the procedure laid down in BS 2571: 1955. b Calcined clay for electrical applications: English China Clays International Ltd.
102
W. V. Titow
In so far as a generalisation can be made, it is useful to remember that with particulate fillers the larger the particle size the greater the reduction of surface gloss of the composition, the higher the water absorption and the tendency to 'stress whitening' (see Chapter 8). However, the finer the filler the greater the plasticiser demand and the tendency to absorb lubricants except where t.he particles are effectively surface-coated (see Chapters 8 and 11). (g) C%urants A colourant must be able to discharge its function-Leo to impart the desired colour, in adequate strength, to the PVC-when present in a relatively low proportion (normally up to a few phr at most). The colourants used in PVC may be broadly divided into pigments and dyes. The pigments are fine-particle materials, intimately dispersiblebut not soluble-in the PVC composition: for this reason they have an opacifying effect and are resistant to migration (with the partial exception, in some circumstances, of certain lake pigments and organic pigments appreciably soluble in plasticisers-see below). Dyes are colourants soluble (Le. dispersible on a molecular level) in PVC compositions: they are thus non-opacifying (and hence suitable for transparent compounds) but, in general, rather prone to migration. From the standpoint of their chemical nature, dyes are organic compounds, whilst the pigments fall into three main groups: inorganic compounds (mainly metal oxides), lakes (dyestuffs precipated onto inorganic compounds, which act as bases or carriers in this combination), and organic compounds. In addition carbon black may be regarded as forming a 'group' on its own; metal powders (usually aluminium) are also occasionally used to impart a metallic colour effect. The chemical composition, classification and properties of colourants are discussed in Chapter 11, Section 11.3. Colourants of the inorganic pigment group are the most widely and commonly used in PVC: many of these combine good all-round resistance and high colour value with relatively moderate cost. Inter alia the group includes titanium dioxide ('Titanium White'), the pigment universally employed for white compositions, and also frequently incorporated to enhance the colour in coloured PVC materials. Completeness of colourant dispersion is of paramount importance for optimum colour effect in a PVC composition, whilst ease of handling and dispersibility of the colourant in compounding are equally
4 Elementary Principles of pvc Formulation
103
important from the point of view of operational efficiency and economy. Colourants are available in forms particularly suited to these requirements, viz. as premixed concentrates, or as integral components of multicomponent 'single-pack' additive systems in which they are intimately interdispersed with other additives, for direct, joint addition in the course of compounding a particular type of PVC composition (See Section 4.4.2 below). A colour concentrate may be a masterbatch (in which a high proportion of colourant is dispersed in PVC composition of the same kind as that to be coloured), or a colour paste in plasticiser(s) suitable for colouring solid plasticised compositions or pastes, or a concentrated dispersion of colourant in a carrier other than a plasticiser, compatible with (or useful in) the PVC composition to be coloured (e.g. a fatty acid, which may serve as lubricant). The compounding of colourants into PVC, including the preparation of colour concentrates, is discussed in Chapter 13, Sections 13.4.1(c), 13.4.2(a) and 13.4.3. It is self-evident that the colourant(s) chosen for a particular formulation must be suitable for (stable in) the conditions of processing and ultimate service. The chemical stability of the colourant, and its colour fastness, to heat and to any potentially aggressive constituents of the service environment are of particular importance. Mutual compatibility with, and stability to, the other components of the formulation (at room as well as elevated temperatures) is an equally important consideration: in some circumstances inorganic pigments (notably iron and zinc compounds) may promote degradation of the PVC resin-this can happen in the presence of an acid (even trace quantities remaining in the pigment itself as residues from the manufacturing process). The dyestuff components of some lakes, as well as some organic pigments, may be sufficiently soluble in plasticisers for migration or blooming to occur in some plasticised compositions. Colourant manufacturers are, as a rule, very technically minded and their advice on specific formulation problems is both readily available and worth having.
4.4.2 Interactions and Mutual Effects of Formulation Components Most PVC compositions are complex, multicomponent systems. In addition to the interactions involved in the components' exercising their primary functions in accordance with the formulation (i.e. plasticisation of the PVC resin by the plasticiser(s) and its stabilisation
104
W. V. Titow
by the stabiliser(s); lubrication of the composition by the lubricant(s); etc.), the components can also interact in certain other ways. This section gives a brief, general indication of the kinds of such secondary interactions which the formulator should take into account: more detailed discussion of some of their aspects will be found in other chapters. (a) Compatibility Effects As has been mentioned, it is a cardinal general requirement that the components of a formulation should be compatible with one another in both processing and service conditions. Possible effects of one component on the mutual compatibility of some of the others should also be borne in mind. For example, the compatibility of an external lubricant with the PVC resin (which should be relatively limited, for the lubricant to exercise its function properly) may be increased by the presence of plasticisers or certain polymeric modifiers: more external lubricant will then be required in the particular formulation than in a comparable one where the effect does not arise (cf. Chapter 11, Section 11.1.2). Similarly, the presence in the composition of an appreciable quantity of fine-particle filler (or pigment) able to absorb plasticisers or lubricants may necessitate the inclusion of these additives in proportions higher than would otherwise be required, whilst the use of a surface-coated grade of the same filler (with absorptivity reduced by the coating) would obviate the need for such compensatory increase (cf. Chapter 8, Section 8.3.3., and Chapter 11, Section 11.1.2). (b) Synergism The action of some stabilisers can be synergistically enhanced by certain other stabilisers or lubricants. Examples are: the effect of organic phosphite and epoxy co-stabilisers on the heat stability and resistance to weathering of PVC stabilised with certain stabilisers (especially BalCd-see Section 4.4.1(b) above, and Chapters 9 and 10), and the synergism of glycerol ester lubricants with sulphurcontaining tin stabilisers, or that of calcium stearate lubricant with antimony mercaptide stabilisers (see Chapter 11, Section 11.1.2(a)). (c) Other Mutual Effects Noteworthy examples are: the activating effect of some stabilisers on blowing agents in the production of cellular PVC materials (see
4 Elementary Principles of pvc Formulation
105
Chapter 25, Section 25.3), and discoloration which may result through interaction of some lead stabilisers with impurities in lubricants of the glycerol ester type (cf. Chapter 11, Section 11.1.2(a)), or through the interaction of sulphur-containing tin stabilisers with lead-containing pigments (due to formation of coloured sulphides). 4.4.3 Side Effects of Formulation Components
Some constituents of a PVC composition can, in addition to the function for which they are incorporated, also exert effects which are the primary functions of other constituents; undesirable side effects may also arise. The following examples illustrate some of the effects in each category. (a) 'Secondary Functionality' Effects Such effects are illustrated and exemplified by the following:
(i)
(ii) (iii) (iv) (v)
(vi) (vii)
Stabilising action of some lubricants and lubricant action of certain stabilisers (both permitting the use of the other component in proportions lower than would otherwise be necessary-see Section 4.4.1(d) above and Chapter 11, Section 11.1.2(a)). Flow-promoting action (Le. processing-aid effect) of some polymeric impact modifiers (see Section 4.4.1(e) above and Chapter 11, Section 11.2). Internal lubrication by plasticisers, and their impact-modifying effects. External-lubricant effect of some plasticisers and extenders (see Chapter 11, Section 11.1.2). Flame-retardant effect of phosphate plasticisers and chlorinated compounds used as plasticiser extenders (d. Section 4.4.1(c) above; Chapter 7, Section 7.6; Chapter 11, Section 11.5; and Chapter 12, Section 12.10). Stabilising action of epoxy plasticisers (cf. Section 4.4.1(b) above; Chapter 6, Section 6.10.1; and Chapter 9, Section 9.6) Light-stabilising effect of carbon black, the universal black pigment in many plastics compositions, including some PVC materials (see e.g. Chapter 8, Section 8.4.3 or Chapter 11, Section 11.3).
106
w.
V. Titow
(b) Undesirable Side-effects Examples of such effects include: (i)
(ii)
(iii) (vi) (vii)
Opacification effects of fillers, many impact modifiers and other formulation components, which must be considered when formulating clear compositions. Susceptibility to 'sulphide staining' (through formation of coloured sulphides) associated with the presence of stabilisers which are heavy-metal compounds (especially lead and cadmium-see, for example, Section 4.4.1(b) above, and Chapter 9). Detrimental effect of emulsion PVC polymer on the electrical resistance of compositions based on this type of polymer (cf. Section 4.4.1(a) above and Chapter 2). Lowering of heat stability of PVC compositions by most antistatic agents (ct. Chapter 11, Section 11.4). Tendency to 'stress whitening' promoted by the presence of some fillers (see Chapter 8, Section 8.3.3) and polymeric modifiers (see Chapter 11, Section 11.2).
4.5 SOME SPECIAL END-USE REQUIREMENTS Some end-use requirements may be described as special in that they dictate, or preclude, the incorporation of certain components in a PVC formulation. This kind of requirement may be illustrated by the following examples.
4.5.1 Food-contact Applications (e.g. food-packaging films; bottles) Non-toxicity of all formulation components is a prime requirement in such applications. Stabiliser choice will in practice be limited to selected di-n-octyl tin stabilisers (especially where the highest clarity is a requirement) and stabiliser systems comprising compounds of calcium, zinc and magnesium. Plasticisers, and all other components, should be checked with current lists of permitted materials published
by organisations concerned with the health and safety aspects of
4 Elementary Principles of pvc Formulation
107
plastics products (ct. Chapter 7, Section 7.12 and Chapter 12, Section 12.9), or-in cases of doubt-directly with such organisations. 4.5.2 Resistance to Weathering This is of paramount importance in such PVC products as rainwater goods, window frames, cladding, fencing, films for reservoir or swimming-pool lining, tarpaulins and the like. Particular heat stabiliser systems, normally incorporating synergistic co-stabilisers of the epoxy type with chelators, are highly relevant here (see Section 4.4.1(b) above; Chapter 9; and Chapter 12, Section 12.6), and-among lead stabilisers-dibasic lead phosphite (see Chapter 9). Ultra violet absorbers are also widely used in weathering-resistant formulations. Polymeric modifiers in rigid products (e.g. rain guttering and down-pipes, window frames) should be of the type which detracts least from good weatherability, such as VClEVA copolymers or selected acrylic modifiers (alternatively the PVC resin used may be a VClEVA graft copolymer-see Chapters 1 and 2). 4.5.3 Electrical Insulation The use of emulsion PVC polymer is precluded in this application (see Section 4.4.3(b)(iii) above, and Chapters 1 and 2) because of the deleterious effect of the trace impurities it contains upon electrical resistivity (and hence insulation value). Lead stabilisers are the first choice (for the stabilisation of all but transparent compositions) because they are electrically non-conductive. For insulation and cable covering resistant to high temperature selected plasticisers must be used (ct. Chapter 7, Section 7.5.3 and Chapter 12, Section 12.3).
4.6 EXAMPLES OF BASIC FORMULATIONS The following examples illustrate something of the principal features of basic outline formulations for some PVC products of the main groups listed in Table 1.1, Chapter 1.
108
W. V. Titow
4.6.1 Film and Sheeting (i) Calendered clear uPVC: food packaging grade: PVC polymer (S* or Mt, K value 60) Stabiliser: a di-n-octyl tin (e.g. Irgastab 17 MOLCiba-Geigy) Lubricants: Internal: fatty alcohol type (e.g. Irgawax 365Ciba-Geigy) Internal/external: fatty acid ester type (e.g. Irgawax 370)
100 1·1-1·5 phr 1·2-2·0phr 0·2-0·5 phr
If an impact modifier is included it should be of the MBS type (and fully compatible with the lubricant system) to maintain transparency. For maximum clarity the polymer should be of high-purity, fish-eye free grade. (ii) Calendered clear or pigmented uPVC with good stability to light: technical grade:
PVC polymer (M or S, K value 60) Stabiliser: a dibutyl tin maleate chosen for both good heat and light stability (e.g. Irgastab 1'9) UV absorber: (light stabiliser, e.g. Tinuvin 320Ciba-Geigy) Lubricants: Internal: fatty alcohol type Internal/external: fatty acid ester type
100 l'5-2'Ophr 0·2-0·3phr 1·0-1·2 phr 0·2-0·4phr
If titanium dioxide pigment is included its presence may further improve stability to light. (iii) Extruded (blown) clear uPVC film: non-toxic grade:
PVC polymer (M or S, K value 57-60) Stabiliser: a di-n-octyltin (e.g. Irgastab 17 MO) Lubricants: Internal: a glycerol monoester (e.g. Irgawax 361) Internal/external: fatty acid ester type (e.g. Irgawax 370) * Suspension type. t Mass type.
100 1·1-1·5 phr 0·5-0·8 phr 0·5-0'8 phr
4 Elementary Principles of pvc Formulation
Processing aid: an acrylic polymer (e.g. Aeryloid K-120N-Rohm and Haas)
109
0·8-2·0 phr
(iv) Calendered, clear pPVC with good stability to light: generalpurpose and horticultural applications: PVC polymer (S or M, K value 69-72) Stabiliser: BalCd liquid (e.g. Irgastab BC 26) Co-stabilisers: epoxidised soyabean oil an organic phosphite (chelator) Plasticisers: DOP triaryl phosphate UVabsorber: (light stabiliser, e.g. Tinuvin P) External lubricant: stearic acid
100 1·3-1·6 phr 5 phr 0·4-0'5 phr 55 phr 10 phr 0·2-0·3 phr 0·2-0·4 phr
(v) Extruded (blown) filled pPVC film: industrial grade: PVC polymer (S or M, K value 68-71) Stabiliser: a BalCd soap complex (e.g. Irgastab BC 247) Co-stabiliser: an organic phosphite (e.g. Irgastab CH301) Plasticiser: DOP Plasticiser extender: chlorinated paraffin (50-52% CI) Filler: whiting (a coated grade)
100 0,9-1,8 phr 0'3-0'6phr 42phr 13phr 40phr
The barium/cadmium soap stabiliser has some external lubricant action in this composition, so additional external lubrication may not be necessary. Otherwise about 0·3 phr of stearic acid may be added. It is good practice to use a chelating co-stabiliser with solid Ba/Cd stabilisers (in the respective proportions of 1 to 3) to improve the initial colour and light stability of the composition. 4.6.2 Calendered Plasticised Vinyl/Asbestos Flooring (Tiles)
Vinyl chloride/acetate copolymer (K value 55, 13-15% VA) Stabiliser: a CalZn complex powder (e.g. Irgastab
100 2'0-4'Ophr
CZ45M)
Co-stabiliser: epoxidised soyabean oil Plasticiser: DOP External lubricant: stearic acid
6·0-8·Ophr 15 phr 1·0-2·Ophr
110
w.
V. Titow
Pigment: titanium dioxide Fillers: asbestos (chrysotile) whiting
10-15 phr 100phr 160phr
Special grades of solid calcium/zinc complex stabilisers are produced by most manufacturers for vinyl/asbestos flooring compositions. These stabilisers combine good compatibility with VCNA copolymer and asbestos, with very good heat stabilisation (even when a substantial proportion of scrap is added to the compound) and freedom from sulphide staining of the flooring in contact with rubber objects in service. They have no appreciable lubricating action; an external lubricant should, therefore, be included in the formulation; stearic acid is particularly useful since, in addition to its lubricant effect it can also enhance stabilisation. The amount required will normally increase with increasing calcium carbonate filler loading (as well as when substantial amounts of pigment are incorporated): 0·1 phr stearic acid per 8 phr filler may be taken as a general guideline (but with surface-coated filler grades the effect of the presence of the coating should be taken into account-see Chapter 11, Section 11.1.2).
4.6.3 Pipe and Tubing (i) Rigid pipe:
PVC polymer (S or M, K value 66) Stabiliser: a thiotin complex Lubricants: Internal: glycerol monoester of a fatty acid (e.g. a mono-oleate, Loxiol Glo-Henkel) External: ester wax (e.g. Irgawax 360)
100 2phr 1·2 phr 0·6phr
Pigment, colourant and calcium carbonate filler (a precipitated, coated grade) may be included. (ii) Clear, ftexibile tubing (non-toxic-for medical use): PVC polymer (S or M, K value 65-70) 100 Stabiliser: Ca/Zn type 1·5-2·0 phr Co-stabiliser: epoxidised soyabean oil 2 phr Plasticisers: DOP 10 phr high molecular weight polyester (e.g. 37 phr Reoplex 43O-Ciba-Geigy) Lubricant: a glycerol monoester (e.g. Loxiol GIO) 0·0-0·6 phr
4 Elementary Principles of pvc Formulation
111
4.6.4 Cable Covering and Insulation (i) General-purpose cable sheathing:
PVC polymer (S or M, K value 68) Stabiliser: basic lead carbonate Plasticiser: DIOP Plasticiser extender: chlorinated paraffin (5052% CI) External lubricant: dibasic lead stearate Filler: whiting (a coated grade)
100 5phr 52phr 32phr 0·5-1·0phr 40phr
(ii) High-temperature cable compound:
PVC polymer (S or M, K value 68) Stabiliser: dibasic lead phthalate Plasticiser: tri-Linevol 79 trimellitate (e.g. Reomol LTM-eiba-Geigy) External lubricant: calcium stearate Flame retardant: antimony trioxide
100 7phr 70phr 1phr 6phr
In this composition calcium stearate contributes to heat stabilisation whilst acting as lubricant. Di-tridecyl phthalate (DTDP) may be used as plasticiser instead of the more expensive trimellitate for all but the most severe conditions (but the efficiency of the latter is better, so that a lower proportion is required, and trimellitate-plasticised compositions retain their properties better after heating at high temperatures). An antioxidant is desirable, especially if the compound is to be used in thin sections. Reomol LTM already contains 0·2% of an effective antioxidant (Irganox 1010). (iii) General-purpose insulation: PVC polymer (S or M, K value 68) Stabiliser: basic lead carbonate Plasticiser: DIOP Plasticiser extender: chlorinated paraffin (5052% CI) External lubricant (with stabilising action): calcium stearate Filler: whiting (a coated grade)
100 7phr 30phr 30phr 1·3 phr 70phr
W. V. Titow
112
4.6.5
Gramophone Records
Vinyl chloride/acetate copolymer (K value 47-S0, lS% VA) Stabiliser: tetrabasic lead fumarate Lubricant: a compound or system with mainly external action Pigment: carbon black
100 1·S-1·7phr 0·2-0·3 phr 1·0-1·S phr
A copolymer resin is used for ease of processing, with low K value for the best surface-detail reproduction (associated with ease of flow). The carbon black should be of a sufficiently fine and readily dispersible grade to meet this important requirement. An antistatic agent is usually incorporated in the composition: this should be selected inter alia for minimum effect on thermal stability. 4.6.6 Blow-moulded Bottles (i) For mineral water:
PVC polymer (S or M, K value S8) Stabilisers: zinc octoate calcium stearate Co-stabilisers: distearyl pentaerythritol diphosphite epoxidised soyabean oil Impact modifier: MBS type Processing aid: acrylic type
100 0·IS-0·20 phr O·OS-O·lO phr 0·4-0·S phr 3·0 phr 10·0 phr O·S-1·0 phr
All additives in a formulation for this purpos~ must be of special purity grade (as well as permitted for food contact) because mineral water picks up taints easily. For the same reason the use of permitted organotin stabilisers is not recommended. The epoxy co-stabiliser has some plasticising action. If the lubricating effect of the calcium stearate is not sufficient in a particular moulding process lubricant(s), best suited to the conditions, should be incorporated. (ii) General purpose, clear: PVC polymer (S or M, K value 54-58)
Stabiliser: liquid thiotin Impact modifier: MBS type
100 1·4-1·6phr 10-12 phr
4 Elementary Principles of pvc Formulation
Processing aid: acrylic type Lubricant (internal/external): fatty acid ester type
113
1-2 phr 1·0-1·5phr
4.6.7 Injection Mouldings (i) uPVC pipe fitting: PVC polymer (S or M, K value 55-60) Stabiliser: a solid dialkyl tin mercaptide (e.g.
100 l'0-2'Ophr
Irgastab 1'270) Impact modifier: ABS type Processing aid: acrylic type Lubricants: Internal/external: calcium stearate fatty acid ester type External: polyethylene wax Pigment: titanium dioxide Colourant: phthalocyanine type
8-lOphr 1·5-2·5 phr 0·8-1·0 phr O·5-1·0 phr 0·05-0·1 phr 3 phr 0·02-0·03 phr
In rigid injection-moulding formulations for components which may be stressed in service it is advisable to keep liquid additives to a minimum, as they can lower heat distortion temperature.
(ii) Clear, plasticised compound: PVC polymer (S or M, K value 65-67) Stabiliser: liquid Ba/Cd/Zn (e.g. Irgastab BC 206) Co-stabiliser: epoxidised soyabean oil External lubricant: liquid, non-polar type (e.g.
Irgawax 360)
100 1,5-2·5 phr 5phr 0·3-0·5 phr
The lubricant used should be chosen, inter alia, for its suitability in a clear formulation.
(iii) Oil-resistant soling compound: PVC polymer (S, K value 66-70) Stabiliser: solid Ba/Cd Co-stabiliser: epoxidised soyabean oil Plasticiser: DIOP Polymeric modifier: nitrile rubber
100 4phr 5phr 80phr 33phr
114
w.
V. Titow
4.6.8 Extruded Profile (i) High-impact uPVC: PVC polymer (S or M, K value 60) Stabilisers: tribasic lead sulphate dibasic lead stearate Lubricants (internal/external): fatty acid ester type calcium stearate Impact modifier: ABS type Processing aid: acrylic type
100 6phr 1 phr l'9-2'Ophr 0'4-0'8phr 6-10 phr 1·8-2·2phr
(ii) Flexible composition: PVC polymer (S or M, K value 68-70) Stabiliser: a BalCd soap complex (e.g. Irgastab BC 247) Co-stabiliser: an organic phosphite (e.g. decyl diphenyl phosphite, Irgastab CH 301) epoxidised soyabean oil Plasticiser: DOP External lubricant: stearic acid
100 1·2-1·5 phr 0·4-0·5 phr 5 phr 47 phr 0'1-0·4 phr
This is a non-transparent composition which may be pigmented. The BalCd stabiliser provides some lubrication, reinforced as necessary by the stearic acid.
4.6.9 Paste Formulations (i) Cold-dipping paste: PVC polymer (E, * K value 69-72) Stabiliser: basic lead carbonate Plasticisers: DOP DOS
100 2phr 65 phr 17phr
(ii) Conveyor belting: PVC polymer (E, K value 69-72) Stabiliser: tribasic lead sulphate
* Emulsion type.
100 2·0-3·0phr
4
Elementary Principles of pvc Formulation
Plasticiser: triaryl phosphate Plasticiser extender: chlorinated paraffin (45% CI) Flame retardant: antimony trioxide Antistatic agent: e.g. Lankrostat LA3 (Lankro Chemicals)
115
40phr 35phr 5 phr lOphr
(iii) Composite coating for upholstery fabric: Base coat PVC polymer (E, K value 70) Stabiliser: liquid Ca/Zn (e.g. Irgastab CZ 57) Co-stabiliser: epoxidised soyabean oil Plasticiser: Dap Filler: whiting Intermediate (expanded) coat PVC polymer (E, K value 68-70) Stabiliser/activator: liquid, zinc-containing (e.g. Irgastab ABC 2) Co-stabiliser: epoxidised soyabean oil Plasticisers: Dap BBP Blowing agent: azodicarbonamide (paste, 1: 1 in Dap) Filler: fine ground whiting
100 1·5-3·0 phr 5·0 phr 85phr 20phr
100 1·5-2·5 phr 6'0-8'Ophr 45phr 30phr 2·5-4·5 phr 5phr
The zinc-based stabiliser is selected to act also as an activator ('kicker')
for the blowing agent in this foaming composition. The BBP (butyl benzyl phthalate) is a highly solvating plasticiser, widely used in PVC foam formulations for rapid, uniform fusion of the composition. Top coat PVC polymer (E, K value 70-72) Stabiliser: liquid BalCd/Zn complex (e.g. Irgastab BC206) Co-stabiliser: epoxidised soyabean oil Plasticiser: DDP Pigment: titanium dioxide Filler: whiting Colourant: as required
100 1·5-2·5 phr 5·0 phr 52phr 0'0-3'Ophr 0·0-10·0 phr
The stabiliser is one suitable for clear PVC to enable the formulation to be used for unpigmented, unfilled compositions.
CHAPTER 5
Theoretical Aspects ofPlasticisation D. L.
BUSZARD
5.1 GENERAL INTRODUCTION
This is the first of three chapters covering various aspects of plasticisers and plasticisation. It concentrates on the more theoretical aspects of the requirements for a PVC plasticiser, theories of plasticiser action and the major effects of PVC-plasticiser interaction on physical properties. The more practical commercial aspects and the performance orientated properties are dealt with in later chapters. 5.2
DEFINITION OF PLASTICISERS AND PLASTICISATION
In September 1951, the Council of the International Union of Pure and Applied Chemistry (IUPAC) adopted the following definition: A plasticiser or softener is a substance or material incorporated in a material (usually a plastic or elastomer) to increase its flexibility, workability or distensibility. A plasticiser may reduce the melt viscosity, lower the temperature of the second-order transition or lower the elastic modulus of the melt. This may appear to be an unnecessarily elaborate definition, but it highlights the many functions a plasticiser can have in any given polymer-plasticiser system. Thus for surface coatings and thin films, a plasticiser may be defined more specifically as a compound which gives 117
118
D. L. Buszard
flexibility, impact resistance, handle, feel, etc. In rubber, plasticisers reduce stiffness, permit easier processing and may improve flexibility at lower temperatures. In thermosetting polymers and certain rigid thermoplastics, they assist processing and ease of moulding. In PVC, plasticisers convert the rigid, intractable resins into workable compounds which can exhibit a wide range of properties, depending on the type and concentration of plasticisers used. The properties required of plasticisers, or the plasticised PVC products, generally include compatibility with the resins, non-volatility, non-flammability, good heat and light stability, good low-temperature performance and non-toxicity. To some extent these requirements may conflict, and no single plasticiser can satisfy them all completely. For this reason plasticiser mixtures are used. In practice, the choice of plasticiser or even plasticiser mixture will usually involve some form of compromise. Plasticisers may be divided into two groups-primary and secondary-with the rather arbitrary division between them depending mainly on their compatibility with the resins. Primary plasticisers are highly compatible with PVC polymers up to at least 150 phr. Secondary plasticisers are less compatible, and are generally used in mixtures with a primary plasticiser in order to confer some special balance of properties. The extenders form a branch of this group; they have a very limited compatibility, but may be included in plasticised compositions as a part-replacement for other plasticisers to reduce cost. At this stage it would be useful to distinguish between 'internal' and 'external' plasticisation of PVc. The above remarks and the sections ahead apply to externally plasticised PVC, i.e. the resin compounded with the plasticiser. However, as mentioned in Chapter 1, it is possible to plasticise internally by copolymerising other monomers into the chain in order to produce a less uniform, and hence less cohesive, structure. Chain flexibility may increase, as in copolymers with vinylidene chloride, vinyl acetate, propylene or alkyl vinyl ethers, thus giving properties resembling externally plasticised PVC. The principal technological advantage of internally plasticised systems is a reduction in the processing temperature of the polymer, and the main functional benefit is that such systems cannot suffer loss of plasticiser. However, resins plasticised internally by copolymerisation are generally inferior to externally plasticised systems in strength and low-temperature properties.
5
Theoretical Aspects of Plasticisation
119
5.3 CHEMICAL NATURE OF PLASTICISERS PVC plasticisers are mainly organic esters with high boiling points. About two-thirds of the plasticisers in general use are diesters of phthalic anhydride with CCC lO alcohols. Cs alcohols offer the best balance of properties for general-purpose plasticisers. The other classes of more specialised plasticisers are triaryl phosphates, alkyl esters of dibasic alkyl acids, alkyl trimellitate esters, high molecular weight polyesters and epoxies. The majority of the extender plasticisers in common use are chlorinated paraffins or hydrocarbons. Table 5.1 gives a breakdown of the consumption of plasticisers for PVC as calculated from information gathered by the British Plastics Federation.! The expressions 'monomeric plasticiser' and 'polymeric plasticiser' are commonly and widely used. Although their meaning is well understood in the art, and rarely causes confusion, it should be remembered that neither term is strictly accurate. Monomeric plasticisers are not monomers in the accepted sense of polymerisable compounds. The word monomeric was originally, and in this context incorrectly, used as the opposite of polymeric to distinguish between high molecular weight plasticisers, many of which were indeed polymers, and compounds of considerably lower molecular weight. The latter might perhaps be better described as simple ester plasticisers or low molecular weight plasticisers, since in most cases their molecular weights lie between 300 and 500. The term polymeric plasticiser is less of a misnomer in that plasticisers produced by polymerisation (e.g. the polyesters with TABLE 5.1
Types and Relative Proportions of Plasticisers Used in the UK in 1981
Plasticiser class
Phthalates Phosphates Aliphatic dibasic acid esters Trimellitates Polymerics Epoxies Chlorinated paraffins Hydrocarbons Others
% by weight 74·0
5·0 2·0 0·5
1·9
5·0
9·5 1·9
0·2
120
D. L. Buszard
molecular weights up to 8000) are certainly included under heading. However, the name is not confined to polyesters, but is extended to other high molecular weight plasticisers which are polymers--e.g. high molecular weight complex esters with polymeric repeat units, or epoxidised oils.
this also not no
5.4 mEORIES OF PLASTICISATION Two major theories have been proposed to account for the effects of plasticisation. In the Lubricity Theory, advanced initially by Kirkpatrick2 and others,3,4 the plasticiser was considered to act as a lubricant for the sliding contact of the polymer chains, thus facilitating deformation by reducing intermolecular friction. An alternative theory was developed through the work of Doolittle,5,6 and has become known as the Gel Theory. This suggests that the resistance to deformation of amorphous polymers is due to the formation of a loose, three-dimensional honeycomb structure, with the cross-links originating from active centres along the polymer chains. These active centre cross-links continually break down and re-form in what has been termed an aggregation-disaggregation equilibrium. In a plasticised system, while the polymer molecules are continually making and breaking their contacts with each other, the plasticiser molecules are also attaching themselves to active centres. The combined effect is such that, under a given set of conditions, a proportion of active centres will be solvated or masked by the plasticiser molecules and hence will be eliminated as potential cross-linking points in the polymer chain. The second equilibrium has been termed the solvation-desolvation equilibrium, and it operates simultaneously with the aggregation-disaggregation equilibrium. It has been suggested that, in the case of PVC, these active centres are the dipoles in the polymer chain. The mechanism of dipole attraction has been discussed in a review by Leuchs. 7 He suggested two types of plasticiser action-hinge and screen (Fig. 5.1). The hinge type plasticisers are typified by the readily polarisable aromatic compounds, and the screen types by polar, aliphatic esters. An alternative, and in the light of more recent work, more plausible proposal for active centres in PVC is that the cross-links are actually areas of ordered structure or crystallites in the polymer. 8,9 This was extended by Walter,lO who investigated the crystallisation of PVC in
5 Theoretical Aspects of Plasticisation
121
+-
+Scr~~n typ~
Hing~ typ~
Fig. 5.1 Plasticiser mechanisms (after Leuchs7).
the presence of plasticiser, including studies comparing the crystallite network structure to that of vulcanised rubber. Observations by Dotyll and Hengstenberg 12 showed that in dilute solutions with most solvents PVC forms aggregates involving an average of 11-20 repeat units of the polymer chainY More recent work on PVC-plasticiser gels 14 ,15 also shows the aggregation of PVC molecules and demonstrates that the viscoelastic properties exhibited by such gels are more consistent with semi-permanent crystallite bonding than with the more labile bonding originating from polar interaction. Examination of the plasticisation and solvent swelling properties of a more crystalline syndiotactic PVC 16 ,17 again points to the importance of crystallinity in determining the physical properties of plasticised PVC. However, further work has shown the existence of other morphological features in plasticised PVC, which exhibit structural regularities unrelated to the crystallinity but which are dependent, at least in part, on processing conditions. 18-21 It would thus appear likely that the mechanism of plasticiser-PVC interaction is complex and dependent upon interaction at a number of levels. These may include energy and entropy factors resulting from the redistribution of polymer chains, an atomic level interaction of dipoles and interaction within crystallite, or possibly larger, structures. The above outline of plasticisation mechanisms has of necessity been
122
D. L. Buszard
very brief. For more detailed reviews of the theories the reader is referred to several textbooks which cover the subject in much greater depth,6,22,23 or to an extensive review of relevant literature, prepared by Stafford for the RAPRA PVC-plasticiser Group.24,25 5.5
STAGES OF PLASTICISER INTERACTION WITH PVC POLYMER
From the time a plasticiser first comes into contact with a PVC polymer until the production of the final fused product-and possibly even for some time after this-a complex series of interactions occurs. Since the precise processing conditions and heat history of a PVC plastisol are easier to control, the majority of the initial work concentrated on the liquid-solid transition. Stich studies demonstrated that increasing temperature led to an initial lowering of viscosity, followed by a rapid rise. 26 ,27 Alter26 described the vertical region of the viscosity/temperature curve as the gel region. Newton and Cronin27 refer to the attainment of 1000 P as the setting temperature. McKenna28 devised a temperature gradient bar, and, by casting and stripping of PVC films, determined gelation temperatures. Greenhoe 29 continued work on the temperature gradient bar and was able to define six transition points between liquid plastisol and the fully fused product. These were, with increasing temperature, the fluid point (where the plastisol becomes dry and putty-like), dry point (where it changes to a dry, crumbly solid), gel point (where physical strength begins), haze point (where clarity increases), elastomeric point (where tensile strength and elongation to break develop), and fusion point (where optimum tensile strengh is attained). Wheeler and Clifton30 used the concept of the temperature gradient bar to develop the Geigy Gel Block, and Critchley et ai. 31 published an extensive comparison of the gelation properties of different plasticisers determined with this apparatus. Although for practical reasons it is easier to follow PVC-plasticiser interaction in plastisols, similar studies have been carried out on melt blends, using a Brabender Plasti-Corder,32 and on dry blending. 33 ,34 Van Veersen and Dijkers34 compared published data from different sources,35-39 and demonstrated that different methods involving different processing techniques still show a comparable trend for the solvation process of PVC when mixed with plasticisers (Table 5.2).
Diethyl phthalate Dibutyl phthalate Dihexyl phthalate Dioctyl phthalate Diisodecyl phthalate
Plasticiser
92 97
127 138
56 60
80 86
-
88 86 104 118 132
eC)
eC)
72
Solid/gel transition temperature3? (0C)
Solvation point 36
Dry point 35
65 75 75 80 85
Maximum swelling temperature 38 eC)
Comparison of Plasticiser Absorption of PVC by Different Methods
TABLE 5.2
58 62 84 84 94
Relative melting temperature39 (0C)
N W
......
S" ;:.
1:;'
"l::l
~::to
~
"'"f;;
i:l
~
'" "~ ::to
<:;)
...'"
;;l
u,
124
D. L. Buszard
Sears, Darby and Touchette40 ,41 summarised the stages of plasticisation as follows: (i) Irreversible uptake of plasticiser into the porous resin. (ii) Absorption of the plasticiser, during which the total resinplasticiser volume may decrease although the resin particles swell slowly on the outside. (iii) Diffusion of the plasticiser within the particles with little or no volume change, but involving high activation energies. At this stage the plasticiser is probably present as clusters of molecules between bundles of polymer segments or molecules. (iv) The final stage of plasticisation when the plasticiser molecules penetrate the bundles of polymer molecules so that they are no longer rigidly held together but behave as a polymer in its rubbery rather than glassy state. In some processes these four steps may not be clearly defined and may overlap. With the usual plasticisers for PVC, of moderate molecular weight, plasticisation can only proceed through the first two steps at room temperature. Steps (iii) and (iv) have high activation energies thus requiring a threshold temperature to be reached. 5.6 REQUIREMENTS FOR PVC PLASTICISERS It is obviously of great importance for the plastics industry to be able
to select 'good' plasticisers. There is no standardised definition, but the term is usually concerned with the following general requirements: (i) compatibility and ease of mixing; (ii) effectivity in imparting a desirable property or properties (usually softness or flexibility) to the plasticised polymer; (iii) permanence. Unfortunately, some properties of a plasticiser helpful in one aspect may actually be detrimental in another. For example, a high rate of diffusion will increase efficiency and rate of gelation but make for poorer permanence. A good plasticiser is therefore one which fulfils simultaneously all three principal requirements to the degree necessary for a particular application. In practice, the choice is generally a compromise governed by the processing technique to be used, the end application of the plasticised material and economic factors. Neverthe-
5 Theoretical Aspects of Plasticisation
125
less, it is obviously desirable to be able to assess and compare the merits of plasticisers in relation to the above three criteria. The subject has been discussed in detail by Jones et al. 42 and in some aspects by Boyer,43 Immergut and Mark22 and Van Veersen and Meulenberg.37,44 The following methods of assessing the plasticising characteristics of plasticisers and/or their correlation with their more fundamental properties are considered worthy of interest.
5.6.1 Compatibility and Miscibility There are several theoretical techniques, as well as a number of more practical technological tests, by which likely plasticisers may be assessed for their compatibility with PVc. (a) The J1. Value This is a numerical index of the degree of interaction (or mixing) between polymer and plasticiser, and can be determined from the swelling equilibria of lightly cross-linked polymer film immersed in plasticiser. J1. is calculated from the equation 2
/-LV2 =
-In (1- V2) -
P2 VIV~
V2 - - - -
Me
where Me is the average molecular weight of the polymer segment between cross-links, P2 is the density of the polymer, V2 is the volume fraction of the polymer and VI is the molar volume of the solvent. The method has been developed by Gee45 and was successfully applied by Doty and Zable 46 to the evaluation of the miscibility of dialkyl phthalate plasticisers with PVc. Plasticisers with Jl values below 0·55 are highly miscible and those above are partly miscible or immiscible (Fig. 5.2). (b) Solubility Parameter () The Hildebrand solubility parameter () is related directly to a compound's cohesive energy density, and is a constant for any given compound. It can be shown 47 ,48 that, given certain not unduly restrictive conditions, the miscibility of a solvent and solute (or PVC polymer and plasticiser) will in general be greater the smaller the difference between their solubility parameters. These values are therefore a guide to compatibility and miscibility.49
126
D. L. Buszard
+',5
+1
0+..........;........-.__
.
~ I o~
,
~ I
I
-I·O'C----,,:OO,;:_
200
500 Molcculor Wclqhl
Fig. 5.2 Doty!J. value versus molecular weight. 1, Diethyl phthalate; 2, dibutyl phthalate; 3, dihexyl phthalate; 4, dioctyl phthalate; 5, dilauryl phthalate.
Solubility parameters can be calculated from the structural formula and density of the compound involved48 ,sQ-s2 or determined experimentally. Table 5.3 lists the solubility parameters of some common plasticisers, calculated by the method of Small. so The solubility parameter is generally a useful guide to the probable compatibility of plasticisers with a particular polymer, providing a number of factors are taken into consideration: (i)
the degree of hydrogen bonding power, and whether the polymer is a proton donor or acceptor; (ii) the degree of crystallinity of the polymer; (iii) possible steric effects in the polymer. The importance of the last, which is often disregarded, is highlighted
127
5 Theoretical Aspects of Plasticisation
TABLE 5.3
Solubility Parameters of Common Plasticisers
Plasticiser
Molecular weight
Solubility parameter t> (call/2 mrl/2)
194 278 278 362 390 390 418
10·5 9·4
Dimethyl phthalate Dibutyl phthalate Diisobutyl phthalate Dihexyl phthalate Dioctyl phthalate Diisooctyl phthalate Dinonyl phthalate Diisodecyl phthalate Benzyl butyl phthalate Tritolyl phosphate Trixylyl phosphate Dioctyl sebacate Ceredor S52 (ICI)
9·2
9·0 8·85 8·85 8·5 8·5
446
312 326 410 426
9·9
9·8 9·75 8·4 9·3 9·7
Polyvinyl chloridea a Solubility
parameter given for PVC for comparison.
by the plasticising effect of pure isomers of TXP, all of which have the same calculated solubility parameter53 of 9·75 but widely differing compatibility and plasticising effects (Table 5.4). The solubility parameter of the chlorinated paraffin, Ceredor 552, suggests that it should be much more compatible than is found in practice. TABLE 5.4
Compatibility and Plasticising Effect of Isomers of TXP
Xylenol isomer
Melting point of TXpeC)
2:6 2:5 3:4 2:3
137
2:4
Liquid at room temperature
3:5
78 71
58 40
Plasticising effect Stiffest composition Most incompatible Not compatible Borderline compatibility Compatible Compatible
D. L. Buszard
128
(c) Clear Point Temperature This has been variously termed clear point, solid-gel transition, fusion point, solution temperature and apparent melting temperature, but basically it is the temperature at which a mixture of PVC and plasticiser becomes clear or undergoes an apparent phase change. 36-38,54-56 It gives an indication of the compatibility and solvent power of a plasticiser, both of which are greater the lower the clear point temperature. Test techniques vary considerably-from the use of fairly concentrated suspensions to the use of 4% suspens\ons, as described by Van Veersen and Meulenberg,37 or 5% suspensions, as in the German Standard57 and even virtually single particles in excess plasticiser. 56 ,58 Although there is variation in the values published, no doubt because of the wide differences in experimental technique and variations in polymer and plasticiser samples, the relative ordering of plasticisers is fairly consistent. Typical clear point values for common plasticisers are shown in Table 5.5. (d) Flory-Huggins Interaction Parameter X It has been shown that the Flory theory of melting in the presence of a diluent could be applied to PVC-plasticiser interaction,56,59 and that the X values correlated well with observed compatibilities. Anagnostopoulos et al. 56,59 correlated the values of X with the clear point, using the equation -
1
Tm
=
0'002226+0'3151(1- X)/V i )
(1)
where Vi is the molar volume of the plastidser at the clear point temperature, T m' Plasticisers with X values of O·55 or above are generally regarded as incompatible, values between 0·55 and 0·3 show moderate to poor compatibility, and values below 0·3 good compatibility. It is interesting to note that arranging plasticisers by X values gives a different order from that suggested by other methods, for example solubility parameter, and that particularly in the case of Ceredor S52, the X value bears more relation to the observed compatibility (Table 5.6). Unfortunately, the molar volume of the plasticiser at the clear point temperature is not a readily determinable factor. Bigg58 proposed that
5 Theoretical Aspects of Plasticisation
129
TABLE 5.5
Clear Point Temperatures of Commercial Plasticisers Plasticiser
Clear point temperature (0C)
Dioctyl phthalate Diisooctyl phthalate Di-Linevol 79 phthalate Diisodecyl phthalate Ditridecyl phthalate Benzyl butyl phthalate Low temperature Dioctyl adipate Diisodecyl adipate plasticisers Dioctyl azelate Dioctyl sebacate Tritolyl phosphate Phosphates Trixylyl phosphate Re%s 50 (Ciba-Geigy) Santiciser 148 (Monsanto) Trioctyl phosphate Trioctyl trimellitate Trimellitates Reomol LTM (Ciba-Geigy) Morflex 525 (Pfizer) Crestapol538 (Scott Bader) Polymerics Diolpate 150 (Briggs & Townsend) Diolpate 171 Palamoll 644 (BASF) Plastolein 9503 (Unilever-Emery) Plastolein 9506 PLastoLein 9765 ReopLex 430 (Ciba-Geigy) Reoplex GL ReopLex 903 ULtramoll I (Bayer) ULtramoll II
Phthalates
117 116 114
139
150
102 138
156 154 151
98 101 100 98
118
142 132 147
138 164 137 155
141 144 151 163
123
153 155 165
since liTm is proportional to (1 - X)1V1 in eqn 1, VI could be replaced by the molecular weight, MW, of the plasticiser to give an interaction parameter, lX, as defined in eqn 2 a = (1- X)
MW
The values of
lX
X
103
(2)
are also shown in Table 5.6. These correlate
D. L. Buszard
130
TABLE 5.6 Flory-Huggins Interaction Parameters (X)
Plasticiser
X
Xb
0·53
0·62 0·52 0·48 0·32
Q
Dioetyl sebaeate Ceredor S52 (ICI) Dioetyl adipate Trioetyl trimellitate Benzyl butyl phthalate Mesamoll (Bayer) Dioetyl phthalate Dibutyl phthalate Oetyl diphenyl phosphate
O· 28 0·10 -0·03 -0·05 -0·35
0·17
0·07 0·05 0·04 -0·02
lYe
0·8 1·0 1·4 1·2 2·6 2·5 2·4 3·4 3·3
Calculated by Anagnostopoulos. 56 ,59 58 b Calculated by Bigg. 58 C Calculated by Bigg from eqn 2. Q
reasonably well with the Flory-Huggins X values calculated by both Anagnostopoulos56 ,59 and Bigg,58 as well as with observed compatibilities of plasticisers. Bigg60 also demonstrated that the interaction parameter, a, related well to the viscosity ageing characteristics of PVC plastisols. (e) AplPo Ratio This was deivsed by Van Veersen and Meulenberg37 ,44,61 as a very simplistic way of representing the polar-non-polar balance of a plasticiser by a single figure. It is calculated by dividing the number of C atoms in a plasticiser molecule by the number of ester groups present. Aromatic C atoms are not counted. For example, dioctyl azelate has two chains containing 8 C atoms and one chain with 7 C atoms-the dibasic acid. The AplPo ratio is therefore
(2x8)+7 = 11.5 2 Similarly, the dioctyl phthalates have an AplPo ratio of 8 since aromatic C atoms are ignored. The AplPo ratio of a wide range of plasticisers correlates well with a number of properties, including melting point, specific gravity, modulus, water absorption, etc. (see also Section 5.7).
5
131
Theoretical Aspects of Plasticisation
TABLE 5.7
Plasticiser Dimethyl phthalate Diethyl phthalate Dibutyl phthalate Dibutyl succinate Dihexyl phthalate Dibutyl adipate Dioctyl adipate Dibutyl sebacate Trioctyl phosphate Dihexyl azelate Dioctyl adipate Diisodecyl phthalate Dioctyl azelate Dioctyl sebacate Ditridecyl phthalate
SGTT
eC) 93 88 86 96 104 98
1 2
121 127
8 9·5 10
118 117
137
139 142
150 151
4 5
6
6 8 8
10 11·5
12 13
The AplPo ratio also correlates well with the clear point temperature, termed Solid-Gel Transition Temperature (SGTT) by Van Veersen and Meulenberg37 (Table 5.7). In the same paper they also show clear relationships with the data of Anagnostopoulos et al., 56 Darby and Graham,36 Wiirstlin and Klein,62 Jasse,63 and Doty and Zable. 46 (f) Loop or Roll Compatibility Tests These are extremely simple but very useful tests for assessing the compatibilities of plasticisers or plasticiser mixtures in a given PVC formulation. A number of test method variations exist, with differing degrees of severity, but the general principle is as follows. A test strip is moulded from the compound and rolled into a fairly tight roll,37 or bent into a loop of fixed dimensions,64 and stored under controlled conditions. Compatibility is judged on the amount of plasticiser exuding out of the compound, when the tension is released, from the inner surfaces of the PVC sample. The assessment is normally visual, and although therefore subjective, a considerable degree of accuracy can be achieved. It is well worthwhile carrying out such a test-which takes at most 24 h-on any formulation in which incompatibility can be a problem.
132
D. L. Buszard
(g) Maximum Torque Temperature This is a method of assessing the interaction between plasticisers and PVC by means of relative fusion temperature in a Brabender Plastograph. It has been described by Touchette et at. 39 and by McKinney32 (see also Section 5.5). Generally, either time to maximum torque at a fixed temperature or temperature of maximum torque (with an increasing. chamber temperature of, for example, 2°C min-I) are taken as an indication of fusion rate or relative interaction. This appears to relate reasonably well to other methods of assessing interaction. However, the temperature of maximum torque cannot be taken as an indication that fusion is complete, as has been suggested by McKinney,32 since samples of PVC removed from the chamber after the peak has occurred may still be incompletely gelled or fused, as shown by an acetone disintegration test. 65 An extensive, regularly updated bibliography of papers relating to this test method is available from Brabender. 66
5.6.2 Effectivity of Plasticisers In fundamental studies, the extent to which the glass temperature (Tg ) of the polymer is lowered by the introduction of a given amount of plasticiser is frequently used as a criterion of effectivity, since the effect can be related to the magnitude and mode of changes in polymer chain mobility.22 Changes in certain dynamic mechanical properties (modulus and damping) are also used. 22 ,41 In the technological context, plasticiser effectivity is usually expressed and compared in terms of the amount of plasticiser required to achieve a stated value of some selected property, generally of direct practical interest. Comparisons have been made on the basis of elongation, torsional modulus, resilience, 100% tensile modulus, and hardness. Jones et at. 42 calculated composite or average effective quantity values for several plasticisers from room temperature flexibility, 100% modulus, hardness, and tensile creep determinations. The results are given in Table 5.8. Many plasticiser manufacturers now include performance data on their products not only at equal plasticiser content, but also at equal efficiency as judged by hardness or 100% modulus. Alternatively, an efficiency factor may be given for a plasticiser, again judged by
5
Theoretical Aspects of Plasticisation
133
TABLE 5.8 Effectivity Quantities for Common Plasticisers
Plasticiser Tritolyl phosphate 40/42 ORD (ORD = ordinary) Tritolyl phosphate 40/42 LOC (LOC = low ortho content) Tritolyl phosphate 52/53 LOC Tritolyl phosphate HOC (HOC = high ortho content) Tritolyl phosphate (tri ortho) Tritolyl phosphate (tri meta) Tritolyl phosphate (tri para) Trixylenyl phosphate Dibutyl phthalate Diisobutyl phthalate Diheptyl phthalate Dioctyl phthalate Diisooctyl phthalate Dicapryl phthalate Dinonyl phthalate Dibutyl sebacate Diisobutyl sebacate Dioctyl sebacate Dicyclohexyl sebacate Paraplex G25 (Rohm & Haas)
Paraplex G50
Average effectivity quantity 35·0 35·8 35·3 36·3 37·2 34·4 38·1 36·6 27·7 29·7 34·9 34·0 35·3 35·3 37·5 27·5 28·9 33·7 33·2 39·2 38·7
hardness or modulus and perhaps related to the performance of a common plasticiser such as dioctyl phthalate (DOP). Table 5.9 has been extracted from the technical literature of Albright and Wilson. It may be thought that a good effectivity is an important property for a plasticiser and in many cases this is so. However, on occasion the converse is true. For example, if the plasticiser is appreciably cheaper than the PVC resin, the lower its effectivity the more will be required to reach a given modulus or hardness and hence the compound cost will be lower. In plastisol formulations lower plastisol viscosities and usually better ageing characteristics may be obtained by using larger quantities of less efficient plasticiser. (Dinonyl phthalate is particularly useful in this type of formulation.)
D. L. Buszard
134
TABLE 5.9 Efficiency" of Common Plasticisers Relative to DOP
Plasticiser Epoxidised soyabean oil Tricresyl phosphate Trixylyl phosphate Dibutyl phthalate Diisobutyl phthalate Dioctyl phthalate Diisooctyl phthalate Di- Alphanol 79 phthalate Dinonyl phthalate Diisodecyl phthalate Ditridecyl phthalate Diisooctyl adipate Diisooctyl azelate Dibutyl sebacate
Relative efficiency 1·06 1-13 1·20 0·86 1·00 1·00 1·02
0·94 1·10 1·06 1·18
0·84 0·89
0·72
a The
efficiency factor of a plasticiser is the number of parts by weight of that plasticiser per 100 parts by weight of PVC required to give a compound with a modulus, at 100% elongation at 23°C, of 1100 lbf in -2 (7·6 MN m -2) divided by 62,8, the corresponding value for dioctyl phthalate (DOP).
Many of the methods of determining the various aspects of plasticiser efficiency mentioned above are basic test methods embodied in national and international standards and specifications, and are listed in Appendix 1, Section 4. 5.6.3 Pennanence of Plasticisers It is obviously desirable that once the plasticiser is compounded with the PVC resin it should be permanently retained. Its loss would not only cause changes in the properties of the system, but may also have undesirable external side-effects. For example, plasticised PVC floor tiles bedded with a bitumen adhesive may blister or lift as a result of migration of plasticiser from the tile into the adhesive. Permanence is therefore a necessary property of a good plasticiser. In practice there are three specific modes of loss of plasticiser from a
5 Theoretical Aspects of Plasticisation
135
plasticised composition. These are: Volatilisation-in which plasticiser is lost at a surface into air. Extraction-in which plasticiser is lost at a surface into a liquid. Migration-in which plasticiser is lost by transference between two surfaces in intimate contact. Practical examples of applications for plasticised PVC where these modes of loss are important are, respectively, high temperature cable, PVC tubing and refrigerator gaskets. A fourth mode of loss which is rarely encountered in practical circumstances is exudation under pressure. 67 However, it is the critical stage of this phenomenon which is utilised in the loop and roll compatibility tests (see Section 5.6.1(f)). It has been suggested by Reed 68 that for a plasticiser to be satisfactory in general use it should have a boiling point of at least 225°C at 4 torr, so that no appreciable volatility losses occur under room temperature conditions. Studies of the factors affecting loss by volatility of the plasticiser from PVC compositions have shown that it is proportional to surface area,68 sample thickness,68 time,69,7o and increasing air flow 71 ,n (see also Chapter 12, Section 12.3). Losses by extraction are generally more complex. In a simple case, where the extractant, for example a 5% soap solution, merely removes plasticiser from the surface, the process is controlled by the solid phase diffusion of plasticiser through the PVC compound. 70 However, a number of extractants such as petrol, alcohol, etc., are absorbed and swell the PVC matrix, thus increasing rates of diffusion. Plasticiser migration is a diffusion process in which the plasticiser from one material with a high concentration diffuses into another with a lower concentration. A considerable amount of work has been carried out by Knappe 73-75 on diffusion and migration from PVC to other substrates. It can also be shown that the penetration of many plasticisers through PVC (which is closely associated with miscibility and solvent power) varies directly with the bulk density. In Fig. 5.3(a) and (b) the 'apparent' diffusion coefficients (D*) of some plasticisers, determined by Knappe,73 have been plotted against the viscosity of the same plasticisers determined at the appropriate temperatures. 76 The linear relationship is clearly evident.
D. L. Buszard
136
(a) f------"'.
...... E1""'-
"
''9
(1 )
•
10
......0
"" I
.,
-10
10
• OOA • OBP EI OOS
I
\
i
"
• OOP x TTP o ONP
E1'\
\
\ "\
A
\
\
I
!
(2)
,
".
X\
;
'{ 0
I
10
20
30
40
50
60
70
Viscosily (c S)
Fig. 5.3 Variation of diffusion of plasticisers in PVC with their bulk viscosity. (a) At 40°C; (b) at 80°C. Initial weight concentration ratios: plot 1, 30: 50 wt%; plot 2, 10:30wt%.
5.7 GENERAL RELATIONSHIPS BETWEEN THE STRUCTURE OF PLASTICISERS AND THEIR HEHAVIOUR IN PVC Several workers have drawn attention to interesting and wide-ranging regularities which may be considered to amount to significant
5
137
Theoretical Aspects of Plasticisation
(b)
I -7 10
.. • .......... e
.
...........
---
(11 .......
~
e
··,·
--
r---..
I!
..~ r--.-.
0
~ ~21
,• ,
.11>
-. 0
,
·10 10 I 3
4
5
6
Fig.5.3-contd.
8
9 ViscositJ (cS I
correlations between the structure of plasticisers and their plasticising properties. 37 ,44,77,78 The following generalisations may be made after observing the effect of shape-determining features on the performance of plasticisers: Influence Feature Molecular weight Increasing molecular weight reduces migration, softening efficiency and volatility. Polarity Incorporating extra polar groups such as ether groups in diesters, substituting halogens into the benzene ring of phthalates, or substituting aryl groups for alkyl ones, reduces softening efficiency worsens low temperature properties, improves solvation and reduces extraction by aliphatic solvents. 44,79
138
D. L. Buszard
Alkyl chain length
Alkyl chain linearity
Separation of ester grouping
Reversal of ester grouping
Increasing alkyl chain length improves the efficiency of aliphatic and phthalate diesters at equimolar concentrations. Efficiency at equal weight concentrations is, however, largely independent of chain length. 62 Increasing linearity improves lowtemperature flexibility and efficiency, and reduces electrical resistivity.80,81 Increasing the distance between ester groups at a constant Ap/Po ratio (see Section 5.6.1 and below) increases solvation, softening efficiency and effectiveness. 44 A change from a dibasic acid ester to a dihydric alcohol ester of similar chain length leads to a decrease in solvation, compatibility and efficiency 44 (e.g. di-n-octyl adipate is a better plasticiser than 1,4butylene glycol dipelargonate).
These parameters also influence the relative density and viscosity of plasticisers,82 and a plot shows that plasticisers fall into very distinct groups which are related to their plasticising properties (Fig. 5.4). Van Veersen and Meulenberg showed that in addition to correlating with the solid-gel transition temperature, SGTT, (Section 5.6.1) the AJPo ratio also showed very distinct trends relating to efficiency (by 100% modulus), low-temperature properties (Clash and BergT 135 000), extraction resistance and plastisol ageing characteristics. 3? These trends are summarised in Fig. 5.5.
5.8 AGEING OF PLASTICISED PVC As every PVC technologist knows, the hardness and modulus of a PVC compound increase measurably during the period immediately following processing. For this reason PVC test samples are usually stored at
139
5 Theoretical Aspects of Plasticisation
I
'00
.,
'00 0
.
.
0
...
~ N
50
o~elYI epoxy slearale
\
0
•• ,
~
~
"'* I~...........
.....0 p\ "l!
Ilo'Q
,~~
Oi n heptyl phthalate
ille.r
'0..
- 1----
~~
<-.,.
.o~
~ ;.",
'\; ~~<"~~" ~ ",o~ o\~\"",J ~oi"
6
,I---.
·
'"
....
• ~
'i).
.....
~
$"\~
10
~ ~
"" '" ~.. 0"
3
'\",
~..<. ~ ,.~~ ~~
10
....
.00.
(
1
0·9
0·95
1·0
t.'"
.,,'
..
• ~
$"tJ
-- f -
e\
011> 1l,,1I> ~II> 0,0"'" ~,o ,,0 ~,,, \~ 0'''' 11> ..0 ~<:J 11>*""
0",
,
0·85
• Oetyl diphenyl phosphale
"·'P4/. OBP ~.L
~
~-.y
"" "..
\..
:rp~
~
('i},
..."
~~;-f-'"TTP . .04 f-" ~~4-
B;=t~ ·'e.r
~
1>-
-
~'-t-
:OJ-I.,,,..... 1'00..
~. ~ ...
ri oelyl phosphale 0
~1iiiD
~r~,~
DOS
".1
----
"nlnll
10
,f;
~
~
~
1·05
1·1
1-15
HO
Relative density 25/25' C
Fig. 5.4 Relationships between bulk properties of plasticisers and their plasticising properties. (Graph contributed by Mr T. C. Moorshead.)
room temperature for approximately 7 days before physical properties are determined. In the past, when lower molecular weight plasticisers were used, this effect was often attributed to plasticiser volatility. However, this is impossible since the effect is reversible. Reheating to the processing temperature and subsequent cooling causes the modulus and hardness to revert to their initial values. Several workers83 ,84 have followed the changes in elastic modulus and density with storage time and temperature, and it has been proposed that the stiffening is due to crystallisation of the PVC on storage. This has been confirmed by DSC measurements which show the development of endothermic peaks accompanying the stiffening. It is suggested that the crystallites can be melted on reheating to the processing temperature and re-form again slowly on cooling.
140
D. L. Buszard
130- 60
400
300
..
>.
,..." ' tI
..
c:
~ f-
~
...c:
>. u
(»
>
~
200~
~
(»
u
5u
Q.
11
lle- ~c: 40
I-
~
ij
o
'z
c: (»
-
'u W
I'oto~-...------.----------, 6
..
7
10
11
12
'00
I
13
Fig. 5.5 Relationship between Ap/Po ratio and physical properties of plasticised PVc. 37
5
141
Theoretical Aspects of Plasticisation
90
60
-6050
60
-50-
40
50
40
.
30~ oil oil
o
-40-
U I
0 0 0
lCl 1'1
.... oil
.!! -30...
..
'-
Q.
0
. ... .
~30 oil
20~
.c:
.2
l:
... ..."x ...'"
.~
. ~ ~
u
'-
20
~
>-
'Q.
~
..-:
!!
...J
~
.2
l:
'e"n o'"
:;::;
...
10 ;
0
>
1-,0-1 6 7 8 9 ----l.~ A p Po ratio
f
Fig.
10
5.~ontd.
11
12
13
I
142
D. L. Buszard
5.9 ANTIPLASTICISATION The addition of small quantities of plasticiser (up to 20%) to a PVC compound leads to an increase in modulus and tensile strength and a reduction in impact strength and elongation at break. This is the opposite behaviour to that which might normally be expected of a plasticiser and has been termed 'antiplasticisation'.85 The phenomenon has been known for a long time. Brous and Semon86 reported the anomalous behaviour of PVC containing up to 18% TCP as early as 1935, and since then it has been examined by many other workers. 87- 99 An illustration may be seen in Fig. 5.6 from the work of Ghersa,87 which shows the effect of low concentrations of DOP on tensile strength, elongation at break, tangent modulus and impact strength. As in plasticisation, the mechanism has not yet been fully elucidated, although the major features have been well researched. Horsley88 demonstrated by X-ray diffraction that systems containing low concentrations of plasticiser possessed increased order on a molecular scale, and attributed it to increasing crystallinity caused by the increased freedom of motion induced by the presence of the plasticiser. However, more recent X-ray and IR data have suggested that only minor changes in crystallinity accompany antiplasticisation. 9o ,91 Bohn,92 using viscoelastic measurements, related the onset of brittleness to the suppression of the viscoelastic f3 relaxation process.
'"E
-
3'0
80
~f
'5
u
0'"
2·5 E ~
f
z
2.0
~ 60
'C z
g,
c:
~
400 o
o
c:
2002
-.;,
1Il 01
1Il
Q.
o
E
+----'T--"T"'""--,---+o
o
10
20
30
Plasticiser cone. 010
40
c: .Q
w
Fig. 5.6 Effect of low concentrations of DOP on the physical properties of PVC (after Ghersa87).
5
Theoretical Aspects of Plasticisation
143
He showed that the brittleness was reasonably independent of plasticiser type, whereas this had a marked effect on the concentration at which brittleness disappeared. The more efficient plasticisers reduce brittleness at lower concentrations. 92- 94 He therefore concluded that the onset of brittleness and its subsequent disappearance were due to two separate mechanisms. It is difficult to attribute the suppression of the f3 peak to structural changes since Pezzin and his co-workers95 ,96 demonstrated that the f3 relaxation was not greatly modified by increasing the crystallinity of PVC by either low-temperature polymerisation or thermal treatments. Robeson and Faucher97 ,98 considered that antiplasticisation was attributable to the filling of the polymer free volume and hence the restriction of molecular motion. Work by Mascia99 suggested that it is time, temperature and stress dependent, that is, above certain critical values the phenomenon assumes all the typical characteristics of normal plasticisation. Formulations containing less than 20% plasticiser are therefore rarely encountered because of their poor physical properties. It is also for this reason that copolymer processing aids are used for rigid PVC, rather than low concentrations of conventional plasticisers. For optimum physical properties levels of liquid epoxy and phosphite stabilisers, and pigment dispersion aids, should be kept to an absolute minimum in rigid PVC formulations.
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10. 11. 12.
British Plastics Federation. (1982). Unpublished survey. Kirkpatrick, A. (1940). J. Appl. Phys., 11, 255. Clark, F. W. (1941). Chem. Ind., 60, 255. Barron, H. (1943). Plastics (London), 7, 449. Doolittle, A. K. (1954). The Technology of Solvents and Plasticisers, John Wiley, New York, Ch. 14 and 15. Doolittle, A. K. (1965). Plasticiser Technology, (Ed. P. F. Bruins), Reinhold, New York, Ch. 1. Leuchs, D. (1956). Kunststoffe, 46,547. Stein, R. S. and Tobolsky, A. V. (1948). Text. Res. J., 18,302. Alfrey, T., Wiederhorn, W., Stein, R. S. and Tobolsky, A. V. (1949). J. Coll. Sci., 4, 211. Walter, A. T. (1965). J. Polym. Sci., 9, 207. Doty, P. M., Wagner, H. and Singer, S. (1947). J. Phys. Chem.,.51, 32. Hengstenberg, J. and Schuch, E. (1964). Makrmol. Chem., 74, 55.
144 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.
D. L. Buszard
Crugnola, A. and Danusso, F. (1968). J. Polym. Sci., B6, 535. Wales, M. (1971). J. Appl. Polym. Sci., 15, 293. Kinjo, N. and Nagakawa, T. (1973). Polym. J., 5, 316. Gugelmetto, P., Pezzin, G., Cerri, E. and Zinelli, G. (1971). Plast. Polym., 398. Gilbert, M. and Gray, A. (1975). Polymer, 16, 387. Singleton, c., Isner, J., Gezovich, D. M., Tsou, P., Geil, P. H. and Collins, E. A. (1974). Polym. Engng. Sci., 14,371. Nakamura, K. (1975). J. Polym. Sci., Polym. Phys. Ed., 13, 137. Gezovich, D. M. and Geil, P. H. (1971). Int. J. Polym. Mat., 1, 3. Tsou, P. K. C. and Geil, P. H. (1972). Int. J. Polym. Mat., 1, 233. Immergut, E. H. and Mark, H. F. (1965). Adv. in Chem. Series No. 48., ACS, Ch. 1. Paton, C. (1972). Plasticisers, Stabilisers and Fillers, (Ed. P. Ritchie), Iliffe, London, Ch. 4. Stafford, T. G. (1972). 'PVC/Plasticiser Interaction. Pt I-General Considerations', RAPRA Tech. Rev. No. 65. Stafford, T. G. (1972). 'PVClPlasticiser Interaction Pt 2-Structure and Properties', RAPRA Tech. Rev. No. 66. Alter, H. (1959). J. Appl. Polym. Sci., 6, 312. Newton, D. S. and Cronin, J. A. (1958). Brit. Plast., Oct., 426. McKenna, L. A. (1958). Mod. Plast., June, 142. Greenhoe, J. A. (1960). Plast. Technol., Oct., 43. Wheeler, A. and Clifton, B. V. (1962). Brit. Plast., Dec., 640. Critchley, S. W., Hill, A. and Paton, C. (1965). Adv. in Chem. Series No. 48, ACS, Ch. 14, 146. McKinney, P. V. (1965). J. Appl. Polym. Sci., 9, 3359. McKinney, P. V. (1967). J. Appl. Polym. Sci., 11, 193. Van Veersen, G. J. and Dijkers, J. L. C. (1974). Kunststoffe, 64,292. Mazzur, R. P. (1967). SPE Tech. Papers, 13, 177. Graham, P. R. and Darby, J. R. (1961). SPE 1., 17, 91. Van Veersen, G. J. and Meulenberg, A. J. (1972). SPE Tech. Papers, 18, 314. Luther, H., Glander, F. O. and Schleese, E. (1963). Kunststoffe, 29,409. Touchette, N. W., Seppala, H. J. and Darby, J. R. (1964). SPE 20th Ann. Tech. Conf., January. Sears, J. K., Darby, J. R. and Touchette, N. W. (1965). Paper presented at the 12th Ann. It. Tech. Meeting of Sabine Area AICHE and the Texas-Louisiana Gulf ACs, Orange, Texas. Darby, J. R. and Sears, J. K. (1969). Encyclopedia of Polymer Science and Technology, Volume 10, (Ed. H. F. Mark et al.), Interscience, New York, p.237. Jones, H., Hill, A. and Williamson, I. (1950). Trans. PI, 18. Boyer, R. F. (1951). Tappi, 34, 357. Van Veersen, G. J. and Meulenberg, A. J. (1967). Kunststoffe, 57,561. Gee, G. (1946). Trans. Faraday Soc., 42B, 33; 42, 585. Do~, P. and Zable, H. S. (1946). J. Polym. Sci. 1,90. Burrell, H. (1955). Interchem. Rev., Spring, 3.
5
Theoretical Aspects of Plasticisation
145
48. Brydson, J. A. (1961). Plastics, 26, 107. 49. Gardon, J. L. (1969). Encylcopedia of Polymer Science and Technology, Volume 3, (Ed. H. F. Mark et al.), Interscience, New York, p. 833. 50. Small, P. A. (1953). J. Appl. Chern., 3, 71. 51. Fedors, R. F. (1974). Polym. Engng. Sci., 14, 147. 52. Koenhen, D. M. and Smoulders, C. A. (1975). 1. Appl. Polym. Sci., 19, 1163. 53. Ciba-Geigy, unpublished data. 54. Severs, F. T. and Smitmans, G. (1957). Paint Varnish Prod., 47, 54. 55. Thinius, K. (1958). Plaste u. Kaut., 5,52. 56. Anagnostopoulos, C. E., Coran, A. Y. and Gamrath, H. R. (1960). J. Appl. Polym. Sci., 4(11), 181. 57. DIN 53408-1967. 58. Bigg, D. C. H. (1975). J. Appl. Polym. Sci., 19, 3119. 59. Anagnostopoulos, C. E., Coran, A. Y. and Gamrath, H. R. (1965). Mod. Plast., 43, 141. 60. Bigg, D. C. H. and Hill, R. J. (1976). J. Appl. Polym. Sci., 20, 565. 61. Van Veersen, G. J. and Meulenberg, A. J. (1966). Kunststoffe, 56,23. 62. Wiirstlin, F. and Klein, H. (1956). Kunststoffe, 46, 3. 63. Jasse, B. (1968). RGCP Plastiques, 5(6), 393. 64. Bell, K. M. and McAdam, B. W. (1967). Kunststoffe, 57, 526. 65. Paul, K. T. (1973). RAPRA Members J., November, 273. 66. Chemical Industry Bibliography. Annual Publication by Brabender OHg., Duisburg. 67. Frey, H. E. (1956). Kunststoffe, 46,81. 68. Reed, M. C. (1947). J. Polym. Sci., 2, 115. 69. Reed, M. C. and Connor, L. (1948). Ind. Engng. Chern., 40,1414. 70. Quackenbos, H. M. (1954). Ind. Engng. Chern., 46, 1335. 71. Small, P. A. (1947). J. Soc. Chern. Ind. Lond., 66, 17. 72. Royen, M. (1960). Bull. Am. Soc. Test. Mat., 243, 43. 73. Knappe, W. (1962). Kunststoffe, 52, 387. 74. Knappe, W. (1954). Z. Angew. Phys., 6, 97. 75. Hellwege, K. H., Knappe, W. and Lohe, P. (1961). Kolloid Z., 179(1), 40. 76. Titow, W. Unpublished work. 77. Moorshead, T. C. (1962). Advances in PVC Compounding and Processing, (Ed. M. Kaufman), Maclaren & Sons, London, pp. 20-31. 78. Heaps, J. M. (1972). Plasticisers, Stabilisers and Fillers, (Ed. P. D. Ritchie), Plastics Inst./Illiffe, London, Ch. 5, p. 68. 79. Lawrence, R. R. and McIntyre, E. B. (1949). Ind. Engng. Chern., 41, 689. 80. Wiirstlin, F. and Klein, H. (1955). Makromol. Chern., 16, 1. 81. Brice, R. M., Eakman, J. M. and Kaufer, D. M. (1963). SPE J., 19,984. 82. Private communication from T. C. Moorshead to W. Titow. 83. Juijn, J. A. (1972). Crystallinity in Atactic Polyvinyl Chloride, J. A. Pasmans, s-Gravenhage, Ch. 4, p. 42. 84. Leharne, S. A., Park, G. S. and Norman, R. H. (1979). Brit. Polym. J., 11, (March), 7. 85. Jackson, W. J. and Caldwell, J. R. (1967). J. Appl. Polym. Sci., 11, 211.
146
D. L. Buszard
86. Brous, S. L. and Semon, W. L. (1935). Ind. Engng. Chem., 27, 667. 87. Ghersa, P. (1958). Mod. Plast., 36(2), 135. 88. Horsley, R. A. (1957). Progress in Plastics, (Ed. P. Morgan), Iliffe, London, p. 77. 89. Shtarkman et al. (1972). Polym. Sci. USSR, 14, 1826. 90. Tabb, D. L. and Koenig, J. L. (1975). Macromolecules, 8, 929. 91. Jacobson, U. (1959). Brit. Plast., 32, 152. 92. Bohn, L. (1963). Kunststoffe, 53, 826. 93. Nakamura, K., Hashimoto, F., Nakanishi, M., Kinjo, N., Komatsu, T. and Nakagawa, T., (1970). Proc. 5th Int. Congo Rheol., p. 409. 94. Kinjo, N. and Nakagawa, T. (1973). Polym. 1., 4(2), 143. 95. Pezzin, G., Ajroldi, G. and Garbuglio, D. M. (1967). J. Appl. Polym. Sci., 11, 2553. 96. Pezzin, G., Ajroldi, G., Casiraghi, T., Carbuglio, C. and Vittadini, J. (1972). J. Appl. Polym. Sci., 16,1839. 97. Robeson, L. M. (1969). Polym. Engng. Sci., 9, 277. 98. Robeson, L. M. and Faucher, J. A. (1969). J. Polym. Sci., B, 7, 59. 99. Mascia, L. (1978). Polymer, 19, 325.
CHAPTER 6
Commercial Plasticisers D. L.
BUSZARD
6.1 INTRODUCTION
A large number of plasticisers are available to the PVC user and confusion may be caused not only by the wide diversity of chemical types but also by the widespread use of trade names. It is hoped that the following sections will help to clarify the situation.
6.2 CLASSIFICATION OF COMMERCIALLY AVAILABLE PLASTICISERS
In the past plasticisers have been classified in a number of ways: by chemical constitution, by compatibility with PVC, i.e. primary, secondary, etc., by molecular weight or by particular applicational properties. All these systems have advantages and disadvantages. The method adopted in this book is primarily a chemical classification. However, the elements of other means of classification are apparent. For example, Groups 1-4 are monomeric and Group 5 is polymeric. Also, Groups 1-3 are largely primary plasticisers, Groups 4-5 are frequently secondary plasticisers and Group 6 includes extenders. The classification used is as follows: Group Group Group Group
1-phthalate plasticisers 2-phosphate plasticisers 3--trimellitate plasticisers 4--low-temperature plasticisers 147
148
D. L. Buszard
Group 5-polymeric plasticisers Group 6-miscellaneous plasticisers: (a) epoxies, (b) chlorinated paraffins, (c) monoesters, (d) glycol esters, (e) hydrocarbons, (f) others. 6.3 GROUP CHARACTERISTICS OF MAJOR PLASTICISER CLASSES
Table 6.1 summarises the major characteristics, including the advantages and disadvantages, of the main types of plasticiser available commercially. It is intended to provide a convenient means of selecting the type or class of plasticiser which should be considered for a particular application. 6.4 SYNONYMS AND ABBREVIATIONS
There are a number of confusing synonyms and abbreviations in the plasticiser field, and for convenience the most important are summarised in Table 6.2. The preferred chemical names and abbreviations are those recommended by ISO 1043-1978(E) or, if not covered by the ISO standard, those recommended by BS 4589: 1970. It should, however, be noted that in some cases the preferred names or abbreviations differ between the two standards. For example, BS 4589: 1970 proposed the prefix 'mixo' to define commercial mixtures formed from primary, branched chain alcohols (e.g. DIOP, DIDA) since the prefix 'iso' should be limited by the rules of IUPAC on chemical nomenclature. However, this prefix (mixo) has not been included in the most recent ISO standard. The majority of the more common monomeric plasticisers are now marketed by most manufacturers under their chemical name or abbreviation, usually with a trade name prefix. The same products from different manufacturers would be expected to give similar performance, although there might be minor differences in raw materials, process, purity and specification. The following sections therefore attempt to cover these products in general terms by using chemical names or abbreviations. Manufacturers' trade names are referred to only where it will assist in defining the product more
Polymeric plasticisers Epoxy esters and oils Chlorinated paraffins
Azelates and sebacates ACJS acid esters
Adipates
DTDP BBP Triaryl phosphates Alkyl diaryl phosphates Trialkyl phosphates Trimellitates
Very good low-temperature properties and permanence CJood low-temperature properties---cheaper than other low-temperature plasticisers CJood extraction and migration resistance, low volatility Improved heat stability, oils have reasonable extraction resistance Low cost, reduced flammability
Very low volatility, excellent resistance to aqueous extractants CJood low-temperature properties
Used in the majority of applications not requiring special properties Improved low-temperature properties Lower volatility, improved aqueous extraction resistance Very low volatility Rapid gelation, stain resistance Excellent flame retardant and gelation properties, good resistance to microbial attack Moderate flame retardancy with improved lowtemperature properties CJood low-temperature properties
C g phthalates
Linear phthalates DNP, DlDP
Rapid gelation, ease of processing
Particular advantages or areas of usage
C 4 phthalates
Plasticiser type Disadvantages
May exhibit poor compatibility and poor low temperature eroperties, high viscosity Can be compatIbility problems particularly on ageing Poor efficiency and compatibility care needed on stabilisation
CJenerally poor volatility and efficiency
CJenerally poorer volatility and extraction resistance Expensive
Poor compatibility and processing
Expensive
Poor low-temperature properties
Poorer electrical properties Less efficient-but may be an advantage in some applications Poor efficiency, expensive
Very high volatility normally limits their application
TABLE 6.1 Characteristics of the Major Types of Plasticisers
150
D. L. Buszard
TABLE 6.2 Plasticiser Abbreviations and Synonyms
Preferred chemical name Q
Preferred abbreviationsQ
Other names and abbreviations
AGS esters Mixed esters of adipic, glutaric and succinic acids also known as nylonates ASE n-Alkyl sulphonate Alkyl sulphonic ester BAR Butyl-o-acetylricinolate BBP Butyl benzyl phthalate Benzyl butyl phthalate Butyl cyclohexyl phthalate BCHP Butyl isodecyl phthalate BIDP Butyl nonyl phthalate BNP Benzyl octyl adipate BOA Benzyl 2-ethIlhexyl adipate DA 79A Di-Alphanol adipate, D 79A Dialkyl (C/~) adipate Dialkyl (C/~) phthalate DA 79 P DAP, di-Alphanolb phthalate, D 79 P DA 79S Di-Alphanolb sebacate, D 79S Dialkyl (C/~) sebacate DA 79 Z Dialkyl (C/~) azelate Di-Alphanolb azelate Dibutyl phthalate DBP DBS Dibutyl sebacate DCP Dicapryl phthalate DCHP Dicyclohexyl phthalate DEP Diethyl phthalate Digol benzoate DGDP Diethylene glycol dibenzoate DHP Diheptyl phthalate Dihexyl phthalate DHXP Diisobutyl adipate DIBA Diisobutyl phthalate DIBP DIDA Diisodecyl adipate Diisodecyl phthalate DIDP Di-LinevoIC 79 phthalate D~9P Di-LinevoIC 911 phthalate D~llP Diisononyl adipate DINA Diisononyl phthalate DINP Diisooctyl adipate DIOA Diisooctyl azelate DIOZ Diisooctyl phthalate DIOP Diisooctyl sebacate DIOS DITDP Diisotridecyl phthalate DITP, ditridecyl phthalate DTDP Di(2-methoxyethyl) phtha- DMEP Di(ethylene glycol monomethyl late ether) phthalate Dimethyl phthalate DMP Dibutyl adipate DNBA DNDP Di-n-decyl phthalate Di-n-octyl adipate DNOA DNOP Di-n-octyl phthalate Dinonyl phthalate DNP Di(3,5,5-trimethylhexyl) phthalate Dinonyl sebacate DNS Di(3,5,5-trimethylhexyl) sebacate
6 Commercial Plasticisers
151
TABLE 6.2-contd.
Preferred chemical name" Dioctyl adipate Dioctyl isophthalate Dioctyl maleate Dioctyl phthalate Dioctyl terephthalate Dioctyl azelate Diphenyl cresyl phosphate Dipropylene glycol dibenzoate Diphenyl octyl phosphate
Preferred abbreviationsO
Other names and abbreviations
DOA DOIP DOM DOP DOTP DOZ DPCP DPCF DPDB
DEHA, di(2-ethylhexyl) adipate
DPOP DPOF DUP ELO ESO ODP PPA PPS TBAC TBEP
ODP, octyl diphenyl phosphate
DEHP, di-(2-ethylhexyl) phthalate Di-(2-ethylhexyl) terephthalate Di-(2-ethylhexyl) azelate CDP, cresyl diphenyl phosphate; tolyl diphenyl phosphate
Diundecyl phthalate Epoxidised linseed oil ESBO Epoxidised soyabean oil Di-Alfold 810 phthalate Octyl decyl phthalate Poly(propylene adipate) Poly(propylene sebacate) Tributyl a-acetyl citrate Tri(2-butoxyethyl) phosphate Tributyl phosphate TBP Tri(2-chlorethyl) phosphate TCEP Tricresyl phosphate TCP, TCF, Tritolyl phosphate (particularly TIP in UK) TDBP Tri(2,3-dibromopropyl) 'Tris', T23P phosphate Tri(2,3-dichloropropyl) TDCP phosphate Triethyl a-acetyl citrate TEAC Triisooctyl trimellitate TIOTM Trioctyl phosphate TOF Tri(2-ethylhexyl) phosphate Tetraoctyl pyromellitate TOPM Tetra(2-ethylhexyl) pyromellitate Trixylyl phosphate TXP, Trixylenyl phosphate TXF 3,3,5-Trimethylpentane 1,4- TXIB Texanol' isobutyrate diol diisobutyrate
° The preferred chemical names and abbreviations are those recommended in
ISO 1043-1978(E) or BS 4589: 1970, or those widely accepted in the PVC industry. b Trade name of ICI-branched chain C,C9 alcohols. C Trade name of Shell Chemicals-predominantly linear C,~ and GrC ll alcohols. d Trade name of Continental Oil Co.-mixture of linear Cg and C IO alcohols. e Trade name of Eastman Kodak.
152
D. L. Buszard
readily, or where that manufacturer has a pre-eminent position in the field. 6.5 GROUP 1 PLASTICISERS-PHTHALATES
o
(X~-O-R C~R
II
o As mentioned earlier, the phthalates are the largest single chemical group of plasticisers used in PVC, the majority being general-purpose Cs phthalates of which DOP is particularly important. There are, however, a wide range of other phthalates which offer interesting properties to the PVC formulator. Phthalates can conveniently be divided into the following groups: lower phthalates; general-purpose phthalates; linear phthalates; higher phthalates; miscellaneous phthalates. A comparison of the properties of PVC compounds plasticised with 54 phr (35%) of the more important phthalates is shown in Table 6.3. The majority of alcohols used in the production of phthalates and other alkyl diesters are manufactured by the 'OXO process' or carbonylation reaction. The principal commercial alcohols produced by this process, together with their raw materials and feedstocks, are shown in Table 6.4. 1 6.5.1 Lower Phthalates DBP and DIBP were widely used in PVC in the early 1950s. They exhibit good efficiency but unfortunately their very high volatility and poor aqueous extraction resistance lead to poor permanence properties. They have been replaced in the majority of plasticised PVC applications by higher molecular weight phthalates, the exception being certain areas where their rapid gelation properties are required and their high volatility can be tolerated. Certain plasticisers can seriously affect the growth of plants. 2 DBP and DIBP have been shown to be very bad in this respect because of
6 Commercial Plasticisers
153
their very high volatility. 3 It is important therefore that these plasticisers are excluded from any formulations which may be used in films, hoses and glazing strips, etc., for glasshouses or other horticultural applications. 6.5.2 General-purpose Phthalates The Cs phthalates are the largest class of plasticisers in use today. The most important are DOP, based on 2-ethylhexanol, and DIOP, based on isooctanol. DA79 P, which was based on a mixed C7 , Cs and ~ branched chain alcohol, has recently ceased manufacture. DOP, DIOP and DA79 P have very similar properties and are generally regarded as interchangeable, the choice being dependent on the current supply position. More recently, DINP, diisononyl phthalate, based predominantly on dimethyl-l-heptanols, has become available, giving generally lower volatilities and lower plastisol viscosities than the Cs phthalates. The other ~ phthalate, DNP, based on 3,5,5-trimethylheptanol, is markedly less efficient and finds particular application where low platisol viscosities are desirable-especially for semi-rigid dip coatings, slush mouldings and rotational castings. 6.5.3 Linear Phthalates One of the more important developments in the use of phthalates as plasticisers was the introduction and extensive use of linear or predominantly linear dialkyl phthalates to improve low-temperature performance. Since these products are available at little or no premium over the general-purpose Cg phthalates, they have replaced a large proportion of the more expensive diester market. The original linear alcohols were by-products from the manufacture of detergent alcohols by fat hydrogenation, e.g. straight chain C6-ClO alcohols from coconut oil. The majority are now manufactured from synthetic hydrocarbon feedstocks, as shown in Table 6.5. 4 Linear alcohols may of course be used in the manufacture of other non-phthalate esters. It is however in phthalates that they have had the most impact. In addition to their improved low-temperature properties linear phthalates have lower volatile losses than their branched chain counterparts, as demonstrated by the apparent plasticiser loss on
0·983 0·968 0·975
48
55
31
DA79P
DCP
DNOP
DNP
Dinonyl phthalate
0·970
0·981
53
DIOP
80
0·980
56
86
77
79
80
77
78
74
1·042
29
DIBP
DOP
72
1·035
IRHD
16
Liquid properties at 25°C Viscosity Density (cSt) (g mr 1)
DBP
Abbreviation
Dioctyl phthalate Diisooctyl phthalate Di-Alphanol 79 phthalate Dicapryl phthalate Di-n-octyl phthalate
Dibutyl phthalate Diisobutyl phthalate
Name
26
38
36
36
39
37
45
48
BS softness No.
-8
-25
-17
-18
-16
-19
-5
-19
eC)
Cold flex
0·2
0·7
1·3
1·0
1·0
1·0
13
17
Volatile loss (%)
24
25
23
23
24
24
20
23
Petrol
17
19
16
18
17
19
12
18
18
23
15
17
17
18
12
19
2·0
2·1
2·5
4
4
4
11
18
2·0
2·0
2·4
4
3
3
14
15
Extraction resistance (% mass loss) MinDetereral Olive oil Soap gent oil
TABLE 6.3 Properties of Phthalate Plasticisers in PVC Compounds at 54 phr
'"::>~
1;
~
~
r--
.".
..... Vl
DINP
Benzyl butyl phthalate Di(2-methoxyethyl) phthalate
32
DMEP
1·167
1·124
0·950
243
45
0·947
0·964
37
54
0·971
33
0·962
0·962
49
85
0·985
0·976
33
BBP
Diisodecyl phthalate DIDP Diundecyl phthalate DUP Diisotridecyl phthalate DTDP
Di-Linevol 79 phthalate Dlq9P Di-Linevol 911 phthalate D~l1P Di-Alfol610 (DROP) phthalate Di-Alfol 810 ODP phthalate
Diisononyl phthalate
76
79
95
91
84
81
77
87
76
80
40
35
13
20
26
34
38
27
42
35
3·8 10
-12
0
0·2
0·1
0·2
0·7
0·1
0·8
0·3
-7
-16
-26
-16
-25
-27
-27
-26
-14
8
12
26
21
25
20
21
26
24
16
14
16
25
18
22
18
20
23
21
16
15
18
25
22
25
18
19
30
21
15
16
14
+1
0
0·3
1·2
3
1·7
6
0·6
17
8
+0·2
+0·2
0·1
0·8
1·8
+0·8
3
0·9
Ul Ul
;;; '"
00'
~~.
'" a
'"....
3 3
<:>
0-
()
D. L. Buszard
156
TABLE 6.4 Principal Commercial OXO Alcohols Raw material
Feedstock
Propylene
(Propylene)
Propylene + butenes Propylene Isobutane Propylene
'Heptenes' 'Nonenes' 'Diisobutylene' 'Dodecenes'
Paraffin wax
Co C8 olefines
Alcohol
{n-Butanol Isobutanol 2-Ethylhexanol Isooctanol Isodecanol Nonanol Tridecanol {AIPhanol79 Linevol79
TABLE 6.5 Major Producers of Linear Plasticiser Alcohols Producer
Conoco
Raw material
Ethylene
Monsanto Ethylene
{ p"affin wox'
Shell
a
Ethylene
Process
Ethylene growth followed by oxidation Ethylene growth followed by carbonylation emking fnllow,d by }
carbonylation Ethylene growth (SHOP) followed by carbonylation
Alcohol Alfo16IO, AlfolBIO Santicizer 711 (phthalate) Linevol79 Linevol911
Recently discontinued in favour of the Shell Higher Olefin Process (SHOP).5
milling,6 Fig. 6.1, and the changes in low-temperature flexibility of plasticised PVC on ageing at 90°C,7 Fig. 6.2. The compounds for the latter were formulated to give equal Clash and Berg temperatures (3410 kg cm- 2) of -30°C. The linear phthalates do however exhibit poorer electrical properties (i.e. lower volume resistivities) which is undesirable in cable formulations. 6.5.4 Higher Phthalates
DIDP is less efficient than the Cs phthalates, but gives appreciably lower volatility and better aqueous extraction resistance.
157
6 Commercial Plasticisers 7
1,o.sl.' 2 ,ali- mill I milling lim.: 15 min
Appa,.nl plaslicis., loss (%wIJ 6
OOP
5
OL 79P
4
OIOP
3
OL 911P
2
O+--_._---~----r_---_._---___r_
110
130
150
170
190
I.mp. S.tlin9 1° C)
Fig. 6.1 Apparent plasticiser loss as a function of milling temperature for PVC compounds based on 50 phr of different plasticisers. (Reproduced by permission of Shell Chemicals International Ltd from their technical literature. )
DTDP, diisotridecyl phthalate, has a very low volatility and good ageing characteristics, but has poor efficiency. It was used in applications where the low volatility was advantageous, e.g. hightemperature cables, low-fogging leathercloth, etc. Its use in these applications is now generally being superseded by that of trimellitate plasticisers.
158
D. L. Buszard to
If
l3'
f ,.,//
e...
r,,// (//
~
:s;c
..
;;:
!
a
. l!
-20
Cl.
E
!
it
... 0
t
If
f
-10
.. ' -30
50:50
DOP/DOA
60:40
DOP/DOA
70:30
DOP
610P
_._810P
~--.~---. 1
-
DOP/DOA
2 3 Time [days]
4
5
6
Fig. 6.2 Change of low-temperature flexibility of plasticised PVC on ageing at 90°C. (Reproduced by permission of Condea Petrochemie GmbH from their technical literature.)
DUP is again used for low-volatility applications, but is more popular in North America than Europe. 6.5.5 Miscellaneous Phthalates Benzyl butyl phthalate is one of the most rapidly fusing plasticisers for PVC, and in compounding allows regular production rates at lower temperatures or higher rates at similar temperatures. In addition, it imparts good stain resistance to vinyl films and hence is frequently used for clear wear layers in vinyl flooring. More recently, dioctyl terephthalate (DOTP) has been introduced commercially in the United States by Eastman Chemicals. This product is claimed to have lower volatility, and improved permanence and lacquer mar resistance, compared with DOP. Modified phthalates, such as Hexaplas OPN (ICI) are also available from some companies. These have similar low-temperature properties
6 Commercial Plasticisers
159
to the straight chain phthalates but inferior volatility. However, their plastisol rheology and storage stability are superior and they are claimed to give volume cost savings over most other phthalates.
6.6 GROUP 2 PLASTICISERS-PHOSPHATES
Phosphate plasticisers may be divided into four classes: triaryl phosphates; trialkyl phosphates; mixed alkyl aryl phosphates; halogenated alkyl phosphates. The triaryl phosphates as a group are by far the most important, accounting for some 80-90% of the total phosphate plasticiser usage in PVc. A detailed comparison of phosphate plasticisers has been given by the writer elsewhere. 8
6.6.1 Triaryl Phosphates The original triaryl phosphate plasticisers were TIP (tritolyl or tricresyl phosphate) and TXP (trixylyl phosphate). However, in the mid-1960s, the shortage and variable quality of the coal tar cresols and xylenols, from which they were derived, led to the increasing popularity of cresol diphenyl phosphate, and later to the development of the Reofos range of synthetic triaryl phosphates based on isopropylated phenol and introduced by Geigy (now Ciba-Geigy) in 1968. The isopropylated phenyl phosphates are applicational equivalents to the older TIP (TCP) , TXP and CDP, but offer more consistent quality, much improved light fastness, better colour, lower odour and lower toxicity. These products are now available in Europe and elsewhere from Ciba-Geigy as Reofos 95, Reofos 65 and Reofos 50. They are also available in the United States from FMC as Kronitex 100 and Kronitex 50.
160
D. L. Buszard
TABLE 6.6 Comparative Performance of Triaryl Phosphates Reofos
TXP
Reofos
TTP
Reofos 50
95 1-131 87 25
95 1·134 87 24
60 1-161 85 27
60 1·158 82 31
50 1·170 83 30
+5 0·5
+8 0·3
+4 0·5
+6 0·6
+3 1·0
15
15
15 11
9
12
15 13
9
15 13 12 11
11
12
95
Viscosity at 25°C (cSt) Density at 25°C (g ml- 1) IRHD (Shore A) BS softness No. Cold flex temperature (BS 2782 method 104B) CC) Volatile loss (%) Extraction resistance: weight loss (%) Petrol Mineral oil Olive oil Soap
11 11
11 11
65
13
Formulation: PVC 100, plasticiser 54, white lead paste 4, calcium stearate 1.
A comparison of these phosphates is shown in Table 6.6. As a group triaryl phosphates offer excellent flame retardancy, good gelation properties, very good high-frequency welding characteristics, good microbial resistance, high extender tolerance and excellent compatibility even at high humidities. They do however suffer from poor low-temperature properties, but since they are seldom used as sole plasticisers, this may be offset by a careful selection of other plasticisers. The superior flame-retardant properties of triaryl phosphates in comparison with the alkyl diaryl and the trialkyl phosphates are shown in Fig. 6.3.
6.6.2 Trialkyl Phosphates Trioctyl phosphate is the most popular of this group, but in general trialkyl phosphates are little used in PVC today. They have poor compatibility and rather poor flame-retardant properties-showing no real improvement over phthalates-and are used primarily for their excellent low-temperature properties.
6 Commercial Plasticisers
161
Oxygen index (%)
46f
36
~~ ::1 28
26
24
22L__.- ' ~__-"----'----'----_..~"-:::=--~--;-!. o
10
20
30
40
50
60
70
80
90
100
Concentration of piasticiser {phrl
Fig. 6.3 Effect of plasticiser concentration on oxygen index of phosphate plasticisers. 5
6.6.3 Mixed Alkyl Aryl Phosphates
Octyl diphenyl phosphate (e.g. Santicizer 141-Monsanto Europe) and isodecyl diphenyl phosphate (e.g. Santicizer 148) are the two mixed alkyl aryl phosphates available commercially. They have properties intermediate between the triaryl and trialkyl phosphates thus offering a compromise in flame retardancy and low-temperature properties. Their physical and flammability properties are comparable with a blend of 60% triaryl phosphate/40% adipate (Table 6.7).
IRHD BS softness No. Cold flex temperature (BS 2782 method 104B) (0C) Volatile loss at 82°C (%) Extraction resistance weight loss (%) Petrol Mineral oil Olive oil Soap Oxygen index (%) 15 13 13 12 34
17 16 18 12 27
-19 1·7
-50 1·7
3 1·0
76 41
78 38
83 30
22 21 21 19 29
(octyldiphenyl phosphate)
141
Santicizer
TOF (trioctyl phosphate)
Reofos 50 (isopropylated phenyl phosphate)
20 17 16 17 29
-17 0·7
78 37
Santicizer 148 (isodecyl diphenyl phosphate)
20 17 18 12 29
-18 2·0
76 41
Phosphate blend (60% Reofos 50140% DOA)
TABLE 6.7 Performance of Different Phosphate Plasticiser Types in PVC Compounds at S4 phr
24 19 18 4 24
-19 1·1
78 37
DOP (dioctyl phthalate)
......
"'-
N
.,I:>
E;
tl:l
~
t""
Rl
6 Commercial Plasticisers
163
6.6.4 Halogenated Alkyl Phosphates
Products such as tri(2-chlorethyl)phosphate (TCEP) and the tri(monochloropropyl) phosphate for example, manufactured by Courtaulds, are effective flame retardants but exhibit poor compatibility with PVC. They are of more interest as flame retardants in other polymers (e.g. polyurethanes) but may occasionally be used as additional flame-retardant additives in PVC. 6.7 GROUP 3 PLASTICISERS-TRIMELLITATES
~
C-o-R
~ R-o-C~C-o-R II II o
0
Esters based on trimellitic anhydride have become very popular primary plasticisers for PVC compounds, for use at high temperatures or for applications requiring excellent resistance to aqueous extractants, e.g. washing machine parts. Table 6.8 gives a comparison of the properties of various trimellitate plasticisers available commercially. Since trimellitates are normally incorporated into PVC formulations for high-temperature performance, most plasticiser manufacturers supply them containing an antioxidant. Whether any additional antioxidant is required will depend on the particular grade selected and the ageing conditions to be encountered. Branched chain trimellitates, such as Reomol ATM (Ciba-Geigy) generally give better electrical properties, which are required for some Cl.ble applications, whilst straight chain trimellitates, as with phthalates, give better low-temperature properties. 6.8 GROUP 4 PLASTICISERS-ALIPHATIC nIESTERS
The aliphatic diesters are generally known as the low-temperature plasticisers since this is their prime advantage. They are secondary plasticisers, made from a number of linear dibasic acids with the general structural formula: ROOC(CHz)nCOOR. The most popular dibasic acids are adipic (n = 4), azelaic (n = 7)
Tri-Alphanol 79 trimellitate Tri-Linevol 79 trimellitate Triisooctyl trimellitate Tri(2-ethylhexyl) trimellitate Tri-Alfol810 trimellitate
Plasticisers
0·995 0·993 0·988 0·985 0·971
107
250
205 92·5
Density 25°C (gml- I )
181
Viscosity 25°C (cSt)
24 17
93
20
91 88
30
30
BS softness No.
83
84
lRHD
0·1 0·1 0·2 0·2 0·1
-15 -7 -10 -20
(%)
Volatile loss
-10
Clash and Berg ("C)
26
22
22
21
21
26
12
19
14
14
22
14
14
15
14
+1
+0·4
+0·4
+0·2
+0·1
+0·8
+0·8
+0·7
+0·3
+0·3
Extraction resistance % mass loss DeterMineral Olive Petrol oil oil Soap gent
TABLE 6.8 Properties of Trimellitate Plasticisers in PVC Compounds at 54 phr
~
N l:l
1:;
~ I:l:l
~
~
165
6 Commercial Plasticisers
and sebacic acid (n = 8). A mixture of acids, known as AGS or nylon acids since they are a by-product of nylon production, and consisting of adipic, glutaric and succinic acids, is also popular. Esters have also been made from succinic acid (n = 2) (e.g. Reomol SD) and glutaric acid (n = 3) (e.g. Plasthall DIDG-C. P. Hall), when they are commercially attractive. The majority of aliphatic diesters are manufactured from branched chain alcohols, such as isooctanol, 2-ethylhexanol or isodecanol, the latter being popular with the lower molecular weight acids. Linear alcohols are generally avoided since their esters tend to crystallise at relatively high temperatures, thus giving storage and handling difficulties. A comparison of the performance in PVC of the more important linear diesters is given in Table 6.9. 6.9 GROUP 5 PLASTICISERS-POLYMERIC PLASTICISERS
The maJonty of commercial polymeric plasticisers are saturated polyesters resulting from the reaction of a diol with a dicarboxylic acid. They differ basically from monomeric plasticisers such as DOP in that their molecular structure contains repeat units and their molecular weight is higher and can be varied. It is normal practice to include a third reactant in a polyester to 'modify' or 'endstop' the product. This is commonly either a monohydric alcohol or a monocarboxylic acid, although isocyanates and even diazomethane have been used to modify terminal hydroxyl and acid groups. Thus there are three possible general structures for polymeric plasticisers: non-terminated acid-terminated
RIf-O-C-RI-C4R-O-C-RI-e-1RIf alcohol-terminated
II
o
II
0
II
0
II
0
n
Diisodecyl succinate Isooctyl ester of AGS acids Isodecyl ester of AGS acids Dioctyl adipate Diisooctyl adipate Diisodecyl adipate Benzyl octyl adipate Diisooctyl azelate Dibutyl sebacate Dioctyl sebacate Diisooctyl sebacate
Plasticisers
79 86 74 81 85 74 78 72
0·928 0·917 0·924 0·926 0·912 0·916 0·913 0·933 0·911 0·911
13·5
23·3
12·3
13·5
22
14·2
18
8·7
19·5
22·4
80
80
85
IRHD
0·919
Density at 25°C (g ml- 1 )
22·4
Viscosity at 25°C (cSt)
0·5 0·4
-51 -46 34
48
37
35
0·4
-39
9
1·7
-48
-52
2·4
-45
0·6
1·0
-38
-47
7·5
-47
3·2
1·8
Volatile loss (%)
-38
Clash and Berg (0e)
-36
45
27
33
46
25
37
26
BS softness No.
26
24
26
24
23
27
25
26
23
26
25
Petrol
27
24
26
22
23
26
21
22
23
23
24
26
25
27
25
23
25
21
26
24
24
24
1
0·4
18
1·4
17
1·3
8
7·5
3·5
15
4·5
Extractions (%) Mineral Olive oil oil Soap
TABLE 6.9 Properties of Linear Diester Plasticisers in PVC Compounds at 54 phr
0
6
1·0
7·5
O·g
4
4·5
1
8
1·9
Detergent
.....
~
s::.
N
!;;
t:x:l
t'"'
~
~
167
6 Commercial Plasticisers
There are therefore two main parameters which can be varied in polymeric plasticisers-their constituents, including chain stoppers, and their degree of polymerisation, Le. molecular weight-and since both these factors are important in determining the resultant properties, it can be seen that there is considerable scope for variation. It also follows that polymeric plasticisers are not simple, readily definable chemicals, as are the majority of monomeric plasticisers, and hence they are invariably marketed under trade names rather than the chemical names which indicate their constitution. The most common raw materials are as follows: dicarboxylic acids-adipic, phthalic, azelaic, sebacic; diols-l ,2-propanediol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol; monocarboxylic acids-acetic, caproic, caprylic, lauric, myristic, palmitic, stearic, pelargonic, benzoic; monohydric alcohols-isooctanol, 2 ethylhexanol, isodecanol, Linevol 79 and 911 (Shell Chemicals) The effect of molecular weight on the properties of a nonendstopped polypropylene adipate has been demonstrated by Moseley and Dawkins9-Table 6.10. From these results it can be seen that increasing molecular weight gives advantages of improved permanence and lower volatility, but disadvantages of increasing viscosity, lower plasticising efficiency and poorer low-temperature properties. In addition, higher molecular weight products are more difficult to process and often exhibit reduced compatibility with PVc. TABLE 6.10
The Effect of Molecular Weight on the Properties of Polypropylene Adipate
Molecular weight Viscosity at 25°C (P) Hydroxyl value (mg KOH g-l) Compound properties BS softness No. Cold flex temperature eC) Volatile loss (%) Tensile strength (MN m- 2) Extraction loss (%) Hexane Mineral oil Soap solution (1 %)
2000 150 55
3000 250 45
4000 350 30
9000 850 25
27 -1·7 3·2 17·8
24 -1·5 2·7 19·8
23 +2 2·4 19·9
20 +5·8 2·3 20·8
0·4 1·0 9·0
0·3 0·4 6·8
0·1 0·1 4·7
0·1 0 3·8
168
D. L. Buszard
Moseley and Dawkins also demonstrated the effect of varying the reactants as well as molecular weight in typical commercially available polymeric plasticisers-Table 6.1l. Endstopping will reduce the liquid viscosity of a polymeric plasticiser of given molecular weight by reducing the hydrogen-bonding capability of the terminal hydroxyl or acid groups. Similarly, this will increase the high humidity compatibility and improve the extraction resistance to aqueous media. It does, however, have an adverse effect on the extraction resistance to non-polar media such as oils and fats. This is demonstrated in Table 6.11 by comparing polymeric plasticisers 4 and 5, which are non-endstopped, with 1 and 2, which are alcohol endstopped. Mixed diols or dicarboxylic acids are frequently used to obtain a desired balance of properties, as shown by examples 6 and 7, Example 8 may be regarded as a complex ester rather than a true polymeric since essentially n = 1and there are no repeat units. An interesting, fairly recent development in polymeric plasticisers is the use of a lactone in place of a proportion of the dibasic acid and diol. 1Q--12 These modified polyester plasticisers have the idealised structure:
The commercially available examples of this type of structure, based on caprolactone, give plasticisers which have excellent compatability with PVC, and extraction and migration-resistant properties which are comparable with those of the medium molecular weight polyesters endstopped with an alcohol. The caprolactone-based plasticisers, however, have lower viscosities and much faster gelation properties. They are therefore of particular interest in plastisols. Most manufacturers of polymeric plasticisers offer a range of products, from the low molecular weight complex ester types, such as Uraplast W2 (Urachem) and Reoplex GL (Ciba-Geigy), to the high molecular weight, harder to process products, such as Diolpate 150 (Briggs and Townsend), Reoplex 430 and Plastolein 9789 (UnileverEmery). A further group of products which may be mentioned under the classification of polymeric plasticisers is the 'solid polymerics'. These are much higher molecular weight elastomeric polymers which may be
1,3-Butylene glycol Alcohol 3400 34 -7·5
211 3·7
8 X lO"
3·9 H 0·3 5·4 Moderate
1,2-Propylene glycol Alcohol 2300 32 -6
205 4·1
1 x 1012
4·2 3·9 0·2 6·1 Moderate
Diol
Chain stopper Viscosity at 25°C (cP) BS softness No. Cold flex eq Tensile strength (kgfcm- 2 ) Volatile loss (%) Volume resistance (0 em-I) Extraction loss (%) Hexane Mineral oil Water Soap solution (1%) Humidity (compatibility)
Adipic
Adipic
Acid
2
2·7 1·8 0·1 4·7 Moderate
1
6 X 10 13
10 12
3·7 Poor
z.l
0·7 0·1
X
210 2·9
1,3-Butylene glycol None 22000 25 -0·5
Neopentyl glycol Alcohol 5500 30 0 198 3·9
Adipic
4
Adipic
3
X
10 12
0·7 0·7 2·3 4·8 Poor
5
202 3·2
1,2-Propylene glycol None 24000 24 -1·5
Adipic
5
X
1013
5·7 5·5 0·6 9·3 Very good
2
210 4·0
1,2-Propylene glycol Alcohol 800 36 -5
Adipic! phthalic anhydride
6
X
1013 2·8 3·4 0·6 6·9 Moderate
5
223 2·9
1,2-Propylene glycol Alcohol 32000 29 2
Adipic! phthalic anhydride
7
X
10 12 10·1 6·0 1·2 11·6 Excellent
3
191 6·7
Diethylene glycol Alcohol 700 33 -2
Phthalic anhydride
8
X
1013 15·1 4·3 0·3 11·4 Very good
6
210 12·0
1,2-Propylene glycol Alcohol 900 34 1·5
Phthalic anhydride
9
TABLE 6.11 The Effect of Composition on the Properties of Commercially Available Polymeric Plasticisers
X
1013 35·0 16·0 0 13·0 Excellent
2
147 22·4
45 -20
72
~OP
10
0-
\0
....
'" ;;:
:::.
f '"0;'
"tl
~
'"
3 3
~
0-
170
D. L. Buszard
more truly regarded as blending resins rather than plasticisers. However, they have recently penetrated areas previously considered to be the preserve of more conventional polymeric plasticisers by offering improved permanence in addition to other advantageous properties such as improved abrasion resistance and traction in shoe soles, etc. Typical of these products are Elvaloy (Du Pont) ethylene interpolymer resins and polyurethane elastomers such as Ultramoll PU (Bayer) and Durelast 100 (Briggs and Townsend). They are extensions of the long-established practice of blending nitrile rubbers with PVC. 6.10 GROUP 6 PLASTICISERS-MISCELLANEOUS PLASTICISERS 6.10.1 Epoxy Plasticisers This group of plasticisers contains the epoxy grouping:
They are usually derived from the reaction of unsaturated compounds with peracids. The unsaturated compounds used are generally naturally occurring oils or the alkyl esters of natural unsaturated fatty acids, although some 'synthetic' triglycerides and esters are manufactured. The resulting commercial epoxy products are primary or secondary plasticisers for PVC and also exhibit an effective stabilising action (see also Chapters 4, 9 and 10). The epoxy oils are generally derived from soyabean oil (the most popular) and linseed oil, both of which are triglycerides of unsaturated fatty acids. They exhibit a good plasticising and stabilising action, and, as suggested by molecular weights in excess of 900, their extraction and migration resistance are comparable with many of the lower molecular weight polymeric plasticisers. A disadvantage of the purer epoxy oils is their high viscosity and their tendency to crystallise out at lower temperatures, which can lead to storage difficulties. However, any precipitation rapidly re-dissolves on heating to 30--40°C for a short period with no detrimental effect on the plasticising or stabilising properties. The alkyl epoxy esters are generally manufactured from tall oil acids
6 Commercial Plasticisers
171
(a by-product of paper manufacture), which contain a significant proportion of unsaturated oleic and linoleic acids, or from purified oleic acid. The most popular are the butyl and 2-ethylhexyl esters of these acids, which are then epoxidised. The epoxy esters are characterised by their good plasticising and low-temperature properties, as well as their stabilising action. The compatibility of epoxy plasticisers with PVC is surprisingly good in comparison with the non-epoxidised equivalents. However. once the epoxy groupings are destroyed by undergoing a stabilisation reaction, their compatibility is greatly reduced. Care should therefore be taken to ensure that certain ultimate compatibility limits are not exceeded. Typical commercial products are: Epoxidised soyabean oil: Edenol D81 (Henkel) Lankroflex GE (Diamond Shamrock) Paraplex G62 (Rohm and Haas) Reoplast 39 (Ciba-Geigy) Edenol D72, B74, B35, H5235 Epoxy-alkyl esters: Lankroflex ED3, ED6 Monoplex 5-71 (Rohm and Haas) Reoplast 38, 42
6.10.2 Chlorinated Paraffins In their early years chlorinated paraffin waxes acquired a reputation for poor colour and low thermal stability. However, by employing modern chlorination techniques and the use of straight-chain liquid paraffin feedstocks, chlorinated products are now produced which are water-white and have properties comparable to those of the octyl phthalates. In the UK, ICI manufacture a range of chlorinated paraffins, under the trade name Ceredor, which have gained worldwide acceptance. Within the Ceredor range are two grades, Ceredor 552 and Ceredor 545, which are specifically recommended for PVC applications. These grades can be used as sole plasticisers in relatively hard products, such as flooring compounds, but they are more normally employed as partial replacements for phthalate or phosphate primary plasticisers. Since they are available at lower prices than primary plasticisers, their use in PVC compounds gives significant savings in raw material costs. The properties of the finished compound are not impaired and the
172
D. L. Buszard
flame retardance may be improved when Ceredor is included in a vinyl composition. The important properties of these Ceredor grades are given in Table 6.12. Ceredor 552 is the general-purpose grade, suitable for most applications, while the 45% chlorinate, Ceredor 545, was specifically developed for use in PVC plastisols, where its lower viscosity is an advantage, and for compounds having good low-temperature properties. The slightly higher volatility of Ceredor 545 still allows compounds containing this grade to pass most international ageing loss specifications. TABLE 6.12 Properties of the 'Cereclor' grades Grade
Chlorine content (%) Average chain length Density (g ml- I at 25°C) Viscosity (Pa s) Volatility (% weight loss on heating 4h at 180°C) Thermal stability (% HCI after 4h at 175°C) Toxicity
Ceredor 545
Ceredor 552
43-45 CI5 1-16 0·154l·25
50-52 CI5 1·25 1-2
2·8
1·4
0·06-0·1
0·07--0·12
Very low
Very low
The use of chlorinated paraffin secondary plasticisers is straightforward and, provided that compatibility limits are not exceeded, no exudation occurs. The compatibility limit is of the order of 25-30 phr in compounds of BS softness 40 (Shore A 75) based on octyl phthalates. It is recommended, however, to refer always to the chlorinated paraffin manufacturer's literature when reformulating. Normally PVC compounds are reformulated to the same hardness. The lower plasticising efficiency of the chlorinated paraffins and their higher density mean, however, that phthalates are not replaced part for part. Typically, 1·5 phr of Ceredor have to be included for each part of replaced octyl phthalate. Table 6.13 shows that the physical properties are relatively unaffected in compounds reformulated to contain either Ceredor 545 or 552. In addition to ICI, chlorinated products are also available from, amongst others, the following manufacturers: Hoechst, Huls, Dynamit Nobel, and Caffaro in Europe; Diamond Shamrock, Keil, and Dover
6 Commercial Plasticisers
173
TABLE 6.13 Properties of Compounds" containing 'Cereclor' Plasticiser OlOP OlDP Re%s 95 Ceredor 845 Ceredor 852
54
BS softness No. Shore A Tensile strength (MNm- Z) % elongation at break Cold flex (0C) % ageing loss (24 h at 100°C) Volume resistivity (0 cm x 1013)
40 75
40 75
40 75
40 75
40 75
17·3
17-8
17·2
15·6
17-6
38
35 62
25
51
315 -20
66
30 35
40 75
40 75
40 75
16·2
20·1
19·3
15 30
300 -16
48
320 -16
20
315 -16
325 -18
330 -16
245 -6
290
-10
0·75
0·95
0·85
0·36
0·74
0·52
0·15
0·6
2
2
2
0·6
0·5
0·6
6
3
° Formulated for constant BS softness by varying the plasticisers (amounts given in phr) in otherwise the same formulation.
in the United States; and Toya Soda, Asahi Denka, and Ajinomoto in Japan. 6.10.3 Monoesters This group comprises a wide number of different plasticisers, the majority of which are only of interest in certain specialist formulations. Examples of these are butyl oleate, phenoxyethyl oleate and tetrahydrofurfuryl oleate. They are claimed to be good lowtemperature secondary plasticisers with advantages in some plastisol applications. An exception which may be included in this class is Mesamoll, an alkyl sulphonic acid ester, manufactured by Bayer and popular in Germany. It is a good primary plasticiser with properties similar to the general-purpose phthalates. 6.10.4 Glycol Esters These also are of low interest and not very wide application. They include aromatic glycol esters such as diethylene glycol dibenzoate and
174
D. L. Buszard
dipropylene glycol dibenzoate, which are popular in the United States (e.g. Benzoflex 9·88-Velsicol Corp.) and which are used for rapid solvating properties. The unsaturated acrylic and methacrylic esters of glycols such as diethylene glycol dimethacrylate are also included in this class. These esters, together with other crosslinkable plasticisers such as dialkyl phthalate and trialkyl cyanurate, are of interest for certain speciality uses. Such applications include adhesion promoters in plastisol-coated steel and rigisols, and there is now a renewed interest in them as plasticisers for crosslinkable PVC cable insulation. 6.10.5
Hydrocarbon Extenders
A variety of hydrocarbon extenders and oils are available from a number of manufacturers, including:
Dutrex (Shell); Enerflex (BP); Electrofine (D'Electro-Chemie); Lipinol (Huls); Mobisol (Mobil); Ravolen (Manchester Oil); Kenplast (Kenrich Petrochemicals, USA). These oils are used as cheap extenders in certain PVC compounds and plastisols. They are true extenders and have very limited compatibility with PVC. 6.10.6 Other Miscellaneous Plasticisers
Citrate plasticisers such as acetyl tributyl citrate (Citraflex A 4-Pfizer) are primary plasticisers with good low-temperature properties and a low order of toxicity. They are of particular interest in products for food contact and medical use. A plasticiser of international importance not readily classifiable into the previous categories is Texanol isobutyrate (Kodaflex TXIB), manufactured by Eastman Chemicals Inc. This product is the diisobutyrate of 2,2,4-trimethyl-l,3-pentanediol. It is of a particular interest in non-stain flooring and plastisol applications such as rotational casting, where low plastisol viscosities and good viscosity stability are required. The high volatility of TXIB limits its use in some
175
6 Commercial Plasticisers
applications. Other esters of Texanol (e.g. Nuoplaz 1406-Tenneco) are available but are much less popular.
6.11 STORAGE AND HANDLING OF PLASTICISERS Plasticisers are supplied either in drums (e.g. 200 kg) or in bulk by tanker. If large quantities of a particular plasticiser are used, it is normally economically attractive to install bulk handling facilities which can then be linked into automatic or semi-automatic weighing and mixing equipment. Storage tanks and transfer lines can usually be of mild or stainless steel construction and suitable epoxy finishes can be applied if necessary. Cast iron or stainless steel gear pumps are normally preferred, although centrifugal pumps can be used for low viscosity or preheated plasticisers. Positive shut-off valves such as ball valves or plug valves should be used. Care should be taken in the selection of elastomeric materials for contact with plasticisers, e.g. hoses, flexible couplings, a-rings and seals. Table 6.14 summarises the effect of liquid plasticisers on many TABLE 6.14 Suitability of Polymeric Materials for Use as Flexible Seals, etc., in Contact with Plasticisers Material
Butyl rubber Ethylene propylene rubber Chlorosulphonated polyethylene Natural rubber Nitrile rubber Polyamide Polychloroprene Polyethylene Polypropylene Polytetrafluoroethylene Polyurethane Silicone Viton
Ratinlf
1 2 4 4 3 1 4 2
2 1 3
2 2
1 = generally recommended; 2 = acceptable; 3 = possibly suitable under certain conditions; 4 = unsuitable.
a Rating:
D. L. Buszard
176
10000
l'\.
r\ \ \
'f\.
5 OOO++-+-~~~-+l~-'--+-+--+--+--+--+--+--+--+---l 3000
\1\ r\. '\
\ \
2000
~\
\ \
\
I\.
\.
'\..
'\.
\.
1500t--t-l~......-\'~:-+--->ot--lI'Tt-+--+--+--+--+--+--+---+---i
1000
~
\
\\
-10
0
\
~\
10
\
\.1\ I\.
'\.
'\
'\..
20 30 40 50 60 70 80 90 100110120130140 Ttlmptlraturtl, ·C
Fig. 6.4 Viscosity-temperature relationships of a range of plasticisers. 1, DOA; 2, DBP; 3, DIDA; 4, L79P; 5, Reofos 65 (isopropylated phenyl phosphate); 6, DOP; 7, DIDP; 8, Reomol LTM (tri-Linevol 79 trimellitate); 9, DTDP; 10, Reoplex GL (low viscosity polymeric); 11, Reoplast 39 (ESO); 12, Reoplex 1102 (low viscosity polymeric); 13, Reoplex 903 (medium viscosity polymeric); 14, Reoplex 430 (high viscosity polymeric).
6 Commercial Plasticisers
177
TABLE 6.15 European and US Plasticiser Manufacturers Classes manufactured
Company United Kingdom Albright & Wilson B.P. Chemicals Briggs & Townsend Ciba-Geigy
1,2,4 1,3,4,6E 5 1,2,3,4,5,6A,6F
Trade names Q
Pliabrac Bisoflex, Enerflex Diolpate Reofos, Reomol, Reoplast. Reoplex
2
Courtaulds Diamond Shamrock (Europe) ICI Robinson Bros Scott Bader Shell Chemicals
1,3,4,6A 1,4,6B 5 1,3,5 1,6E
Tenneco Victor Wolf
2 5,6C
Lankroflex Ceredor, Hexaplas Arbeflex Crestapol Linevol, Dobane, Dutrex Wolflex
Austria Chemie Linz
Mollan
Belgium Argus Chemicals Essochem Europe Monsanto Europe UCB-FTAL
6A 1 1,2,4,5,6F 1,3,4,6F
Drapex Jayflex Santicizer
1,3,4
Scandinol
1 6E 1,3,4 6D,6F
Gedeflex
1 1,2,4,5,6B
Plastifiant K Garbeflex, Garbefos, Alaiflex
1,4,5,6F
Palatinol, Plastomoll, Palamoll
Denmark Scandiflex France CdF Chemie D'Electrofine-Chemie ICI Europa Nyco S.A. Products Chemie Ugine Kuhlman Rhone Poulenic Polymeres
Hexaplas Nycoflex
Germany BASF
178
D. L. Buszard
TABLE 6.15-<:ontd. Classes manufactureda
Company
Bayer
1,2,4,5, 6E, 6F
Chemische Werke Hills Deutsche Texaco Dynamit Nobel Henkel & Cie Hoechst Wacker-Chemie
1,4, 6B, 6C, 6E 1,3 1,3,4,5, 6B, 6C 1,4,5, 6A, 6C 1,6B 1,4
Trade names
Adimoll, Disftamoll. Mesamoll, Ultramoll, Unimoll Vestinol, Lipinol Reproxal Witamol, Witaclor Edenol Wacker
Holland Akzo Chemie Unilever-Emery
6A 4,5, 6A, 6C, 6F
Uraplast
1,4,5
Estabex Emery, Plastolein, Unem Uraplast (formerly Scadoplast)
Italy Akzo Italia Distillerie Italiane
4,5,6C,6F 1,4,5, 6A, 6F
Montedison Sisas Sprea
1,4 1 5
Estaftex Diplast, Diepox, Staftex Sicol Spreaftex
Sweden Berol Kemi
1,4
Switzerland Reichhold Chemie
1,4,5, 6A, 6E, 6F
Ricatyl, Contrastat
4,6C,6F 5 1,3 1 6B 6B
Kesscoftex
USA Armak Ashland Chemicals BASF Wyandotte Continental Oil Diamond Shamrock Dover Chemical Eastman Chemical International Emery Industries Exxon FMC C. P. Hall
1,3,4,5,6F 4,5, 6A, 6C, 6F 1,3 2,6A,6C 4,5,6F
Plastomoll, Palatinol Chlorowax Chlorez Kodaftex, Texanol Emery, Plastolein Jayftex Kronitex
179
6 Commercial Plasticisers
TABLE 6. 15-contd. Company
Classes manufacturedO
Harwick Hercules
1,4,6F 1,5, 6B, 6F
ICI Americas Keil Monsanto
6B,6F 6B,60 1,2,3,4,5, 6A, 60, 6E, 6F 3,4,5,6F 4, 60, 6F 1,3,4,5, 6A, 6C, 6F 4,5, 6A, 6C, 6F 5,6A 2 6A 1,2,3,4,5 2, 6A, 60 1,3,4,5,6C 1,3,4, 6A, 6C 60
Pfizer
pva
Reichold Rohm & Haas Sherex Stauffer Swift Tenneco Union Carbide Union Camp USS Velsicol
Trade names Polycizer Clorafin, Hercoflex, Hercolyn Cereclor, Hexplas Aroclor, Santicizer Citroflex, Morflex Peroxidol, Staflex Monoplex, Paraplex Admex Phosflex Epoxol Nuoplaz Flexol Uniflex PX Benzoflex
° 1, Phthalates; 2, phosphates; 3, trimellitates; 4, low-temperature plasticisers; 5, polymerics; 6A, epoxies; 6B, chlorinated paraffins; 6C, monoesters; 60, glycol esters; 6E, hydrocarbons; 6F. others.
common sealing materials. In this context, confusion may occur over the term 'compatibility'. In PVC technology it is used as has already been discussed in Chapter 5. However, 'compatibility' is also used in hydraulic fluids and liquid handling technology to indicate the resistance of a sealing or gasket material to the particular liquid in question. In practice, this is almost diametrically opposed to the conventional plastics technology definition and confusion between them could have serious consequences. If any doubt exists as to the suitability of a particular material as a seal, etc., either the plasticiser manufacturer or appropriate material manufacturer should be consulted. It may be necessary to heat plasticisers to reduce their viscosity in order to facilitate handling, particularly in colder weather. Figure 6.4 shows the viscosity-temperature relationship for a wide range of common plasticisers.
180
D. L. Buszard
6.12 PLASTICISER MANUFACTURERS A number of the more important European and US manufacturers are listed in Table 6.15, together with the types of plasticiser manufactured and their trade names. This list has been drawn from trade directories, manufacturers' literature and published literature, and whilst every effort has been made to make it so, it may not be completely comprehensive.
REFERENCES 1. Murfitt, H. C. (1970). E.C.N. Polymer Intermediates, Oct. 30th. 2. Inden, T. and Tachibana, S. (1975). Mie Diagaku Nogakubu Gakujutsu Hokoku, 1-10. 3. 'Plasticised PVC in Horticulture'. BASF Technical Bulletin. 4. Murfitt, H. C. (1979). PRI Symposium, Loughborough University, April. 5. Sherwood, M. (1982). Chern. Ind., 24,994. 6. Shell Chemical Int. Report on 'Vapour Losses During Processing of Plasticised PVC', Fig. 2. 7. Condea Petrochemie, Information bulletin No. 3151, Fig. 6. 8. Buszard, D. L. (1978). Chern. Ind., 16,610. 9. Moseley, J. and Dawkins, P. (1978). Chern. Ind., 16,620. 10. Buszard, D. L. (1983). PVC Processing II Proceedings, PRI, 22.7. 11. British Patent No. 1455196. 12. British Patent No. 1455390.
CHAPTER 7
Properties of Plasticised pvc D. L.
BUSZARD
7.1 INTRODUCTION
Chapters 5 and 6 have discussed the more theoretical aspects of plasticisation, and the types and general properties of the classes of plasticisers used commercially. It is the intention of this chapter to investigate the properties of plasticised PVC and the effect of differing plasticisers and plasticiser levels more thoroughly. The intention is to give the reader a grounding in plasticised PVC technology and a guide to formulating for specific end-use properties.
7.2 FORMULATION OF A PLASTICISED PVC COMPOUND
At one time it was usual to use only one plasticiser in a PVC compound, but this is much less common today, since a better balance of properties or a reduction in compound cost may be achieved by optimising the plasticiser system to the end-product requirements. Although this is often a complicated task, generally requiring both an intimate knowledge of and wide experience in plasticisation, there are a number of ground rules which may aid the less experienced technologist. The main points to be considered are as follows: Constraints imposed by the method of processing: The main area where this is of importance is in the processing of PVC plastisols, where there are limitations on the choice and minimum concentration 181
182
D. L. Buszard
of plasticisers. Aspects relating specifically to plastisols are dealt with in more detail in Chapters 21 and 22. Other processing techniques generally have fewer restrictions, although problems may be encountered in, for example, the dry blending of high viscosity polymeric plasticisers or in calendering operations where volatility must be a consideration. Special properties required in the final product: After an approximate level of plasticiser has been established-normally judged by compound softness or 100% modulus-reference should be made to the known properties of a general-purpose Cs phthalate, such as DOP, in PVC and any additional properties required may then be assessed. For example, these might be improved low-temperature or hightemperature performance, flame retardance or migration resistance. Appropriate classes of plasticisers to give the required improvement, either alone or as a blend, may be selected initially by reference to Table 6.1 in Chapter 6. Depending on the nature of the final product, it may be necessary to select a speciality plasticiser or merely 'shade' the properties a little by using a related phthalate from a different alcohol. Inclusion of an extender or filler: Finally the possibility of including an extender, such as a chlorinated paraffin, or a filler in the formulation should be considered. There are obvious economic advantages in doing so, providing that there would be no detrimental effect on the properties of the final product.
Once the main parameters required for a particular compound have been established, and an idea has been reached as to which, if any, plasticisers should be blended, more detailed consideration must be given to the particular properties, in order to permit plasticiser ratios, levels, etc., to be chosen. In general, when plasticisers are mixed, the resultant properties are approximately proportional to their relative concentrations-always providing that compatibility limits have not been exceeded. Although this is not an invariable rule, it is applicable to most practical situations. For highest accuracy, ratios should be determined by volume rather than by weight. Even for the experienced PVC formulator the design of a compound to fulfil a critical task is difficult because of the inevitable conflict in desirable properties. For example, good low-temperature properties in
7 Properties of Plasticised
pvc
183
a plasticiser are inevitably accompanied by poorer oil-extraction resistance and lower volume resistivities, and this obviously complicates the formulation of an oil-resistant cable for use at low temperatures. A compromise has therefore to be reached in matching the achievable properties to the desired specification whilst not overlooking other effects which might be detrimental to the compound in service. There are two 'theoretical aids' which may provide assistance in this field-the 'desirability function' and computerassisted formulation.
7.2.1 The 'Desirability Function' This useful concept has been put forward by Harrington! to assist in the selection of plasticisers for applications requiring several properties of the product, some of which may be conflicting. The method also takes account of the price factor. Briefly, the 'desirability function' is a single number representing the combination of all the important property factors-each at least at the minimum level of acceptability-for the application under consideration. For the purpose of the calculation, each of the properties being considered is allocated a value representing its desirable or admissible level in the compound. The scale of values is in arbitrary units; therefore, both the measurable properties (e.g. cold flex temperature, resistivity, etc.) and subjective ones (e.g. odour, irritation) can be accommodated. Cost can also be one of the properties. One of the attractive features of the method is that it gives a result strongly reflecting the effect of a possible particularly low desirability value in respect of a single property. This is analogous to the rejection by a prospective user of an otherwise excellent compound because of, say, poor transparency. It has been claimed for the method that compounds within the (properly calculated) range of desirability values between 0·65 and 0·90 (1·00 represents maximum desirability) are virtually certain to be suitable for the purpose for which they are being formulated.
7.2.2 Computer-assisted Formulating It is possible to handle laboratory data by computer to predict reasonably accurately the performance of PVC formulations containing a number of variables. This technique can be developed to enable the
184
D. L. Buszard
lowest cost formulation to be selected to meet a required specification. 2 ,3 It can be particularly useful at times of rapid price/availability fluctuations when a formulation originally selected for low cost may have become uneconomic. The development of this type of program is highly specialised and requires an extensive reliable data base on the effect of plasticisers, extenders and fillers on all the specification properties to be included in the system. This is generally only within the scope of the larger PVC polymer and plasticiser manufacturers and then only for the more common plasticiser combinations and properties. The following sections deal with the effect of plasticiser type and concentration on the major properties of interest.
7.3 SOFfNESS AND TENSILE PROPERTIES As was mentioned in Chapters 5 and 6, plasticisers are frequently judged by their efficiency or the degree to which they soften a PVC compound. Softness is therefore a very important property and it is worth considering briefly the more popular methods of measurement. Traditionally the rubber industry has measured it as 'hardness' using either the BS scale (in the UK) or the Shore Durometer scales* (in the USA--ef. ASTM D 2240-75) or, more recently, the International Rubber Hardness scale (Method N of BS 903, Part A 26: 1969, in technical agreement with ISO 48-1979; ASTM D 1415-68(74)). These scales range from 0 to 100; the harder the compound the higher the number. However, in the case of PVC, particularly in the UK, it is normal to employ the BS softness number scale, where the higher numbers represent greater softness (ct. BS 2782, Part 3, Method 365A: 197~formerly Method 307A of BS 2782: 1970). In practice the BS softnes~ number may be determined either with a standard 'dead-load' precision laboratory instrument (as in BS 2782, Method 365A: 1976) or with a spring-loaded pocket instrument. The use, calibration, and limitations of pocket-type instruments are covered by BS 2719: 1975. *Two scales, A and D, are in use (respectively for softer and harder materials), associated with two durometers of the same designations. Scale A is applicable to most flexible PVC materials. ASTM D 2240-75 recommends that measurements with Type D durometer should be made where values above 90 are obtained with the Type A durometer, and that Type A should be used when values less than 20 are obtained with the Type D instrument.
7 Properties of Plasticised
pvc
185
Such instruments, if properly used, are suitable for routine checks of BS softness values of PVC materials, but not for standard tests or rigid laboratory or quality control. At present, both the Shore hardness and the BS softness systems are in use, and since a certain amount of confusion can arise, a conversion graph is given in Fig. 7.1. 7.3.1 Effect of Plasticiser
The variation of softness with plasticiser level for nine common plasticisers is shown in Fig 7.2 (BS softness number) and Fig. 7.3 (Shore A). The efficiency of the different plasticisers can obviously be judged by the relative shapes of the curves (see Chapter 5, Section 5.6.2). The measurements in these graphs have been made at 23°C. However, softness varies considerably with temperature. The degree of change depends on the plasticiser type and content and it is therefore possible to modify a compound by formulating it to have a desired softness at a specific temperature. Figure 7.4 shows the effect of temperature on the softness of PVC plasticised with DOP at three concentrations. It should be noted that, as the response of different classes of plasticiser to temperature is not the same, compounds based on different plasticisers but formulated to equal softness at one temperature may differ at another. The effect of plasticiser type and concentration on tensile strength and elongation at break is shown in Figs 7.5 and 7.6, respectively. These properties also vary with temperature. 7.3.2 Compounding at Equal Efficiency
The general concept of effectivity was introduced in Chapter 5, Section 5.6.2. Comparisons between different plasticisers at equal efficiencies may differ considerably from comparisons made at equal plasticiser levels. An example of this is shown in Table 7.1 where a number of plasticisers are compared at equal efficiency as judged by 100% modulus. In particular the low-temperature properties of DA79P and DNP should be noted. At equal plasticiser level (60 phr) the former appears to be very much better. However, when compared at equal efficiency, the difference is minimal.
D. L. Buszard
186
90
t-
80
~
~
'0
.e>
70
~'"~
%..
t-
... E ....." i"
<;,
~ '.j
~
%
.Q
::J
III
?..
-0 0
60
50
.
~
4
m
30
20
10
---,L-__...L.:-_ _----:-&:--_ _
_ _....L._ _
O~
50
IlO
70 80 Shore A durometer hardness
90
~
100
Fig. 7.1 ShorelBS softness correlation graph.
7 Properties of Plasticised
pvc
187
100
80 '-
Ci
.a
§
c
60
III III Ci
.5 '5III
40
Vl
1Il
20
o
20
40
60
Plasticis~r I~V
phr
80
100
100
80
...
CI
.a E 60 :J C
III III Ci
...,c
'5III
8 40
Vl
1Il
20
o
20
40
60
Plasticis~r l~v~l,
80
phr
100
Fig. 7.2 Effect of plasticiser level on BS softness number. 1. DOP; 2, DIDP; 3, DTDP; 4, DOA; 5, Reofos 65 (Ciba-Geigy); 6, TOTM; 7, ESO; 8, PPA; 9, PPA alcohol endstopped.
D. L. Buszard
188 100
90
80
~
L-
0
.c VI
70
3
60
50
2 0
20
40
60
Plasticisar laval, phr
80
100
90
80
~
L-
0
.c VI
70
60
9 5
7
50 0
20
40
60
Plasticisar laval, phr
80
100
Fig. 7.3 Effect of plasticiser level on Shore A hardness. 1, DOP; 2, DIDP; 3, DTDP; 4, DOA; 5, Re%s 65; 6, TOTM; 7, ESO; 8, PPA; 9, PPA alcohol endstopped.
7 Properties of Plasticised
pvc
189
/10 100
~ 90l :I
Z
..
eoi
I
:
70l
:
60~
~
ID
I !
so:
4d I
I 30~ 1
20 10
1
2O=------:30~---:40'::-----:S-::·0:-------:'60'=""'"----:7:'::0:----='eo Temperature.
°c
Fig. 7.4 BS softness versus temperature.
The efficiency factor can be very useful in calculating the ratios and concentrations of plasticisers necessary to replace one with another. For example, if it is desirable to improve the low-temperature properties of a compound containing 50 phr of TIP by replacing 20 phr of it with DOA and yet retain the same modulus, then the 20 phr of TIP should be replaced by (see Table. 7.1): 0·94 20 x 1.25
= 15·0 phr DOA
190
D. L. Buszard 25
20
-
N
E
z
~
15
....Ol ~
c:
~
L
til 10 ~ ·iii
c: ~
5
o
20
40
60
80
100
40 60 80 Plasticisftr IftVftl, phr
100
Plasticisftr Iftvftl, phr
25
20
-
N
E
z
~
~15
....Ol .t:
c: ....~ Ul
.5! ·iii c: ~
10
5
o
20
Fig. 7.5 Effect of plasticiser level on tensile strength. 1, DOP; 2, DIDP; 3, DTDP; 4, DOA; 5, Reofos 65; 6, TOTM; 7, ESO; 8, PPA; 9, PPA alcohol endstopped.
7 Properties of Plasticised
pvc
191
500 4 1
2
3
1ii c o
~200 til
c
o
iii
100
o
20
40
60
Plasticiser level, phr
80
100
500
5
6
~~ 8
o
20
40
60
Plasticiser level, phr
80
100
Fig. 7.6 Effect of plasticiser level on elongation at break. 1, DOP; 2, DIDP; 3, DroP; 4, DOA; 5, Reofos 65; 6, TOTM; 7, ESO; 8, PPA; 9, PPA alcohol endstopped.
D. L. Buszard
192
TABLE 7.1 Comparison of Plasticisers at Equal Concentration and Equal Efficiency Plasticiser
Efficiency concentrationa
Efficiency factol'
Properties at 60phr Cold Tensile strength flex (MNm- 2 ) ("C)
Dibutyl phthalate Dioctyl phthalate Diisooctyl phthalate Di-Alphanol 79 phthalate Dinonyl phthalate Tritolyl phosphate Trixylyl phosphate Dibutyl sebacate Dioctyl sebacate Dioctyl adipate a b
54·0 63·5 65·5 61·2 74·2 79·3 83-1 49·5 58·8 59·9
0·85 1·0 1-03 0·97 1·17 1·25 1·31 0·78 0·93 0·94
17·0 17-4 18·4 16·5 19·4 23-1 23·5 14·7 16·1 16·6
-25 -23 -22 -25 -12·5 -1·5 +l -56 -47 -49
Properties at efficiency concentration Tensile strength (MNm- 2 ) 17·9 15·9 17-1 15·9 16·0 17·6 17·2 16·9 16·3 16·5
Cold flex ("C)
-21 -27·5 -30 -28 -24·5
-13 -]2·5 -39 -46 -49
phr to give a 100% modulus of 6·9 MN m- Z (1000 Ibf in- Z). Efficiency concentration divided by the efficiency concentration of DOP (63,5).
7.4 LOW-TEMPERATURE PROPERTIES
All plasticisers improve the low-temperature performance of PVC but some are more effective than· others. Figure 7.74 shows the effect of temperature on the rigidity modulus of PVC compounds plasticised with four common types of plasticiser. It is apparent that the different types exhibit very different responses: the highly solvating polar plasticiser, TXP, shows a rapid change in modulus over a narrow temperature range, whilst the less polar secondary plasticiser, DIOS, gives only a very gradual change. Obviously the concentration of plasticiser as well as its type affects the low-temperature flexibility of a compound. The curves in Fig. 7.7 were obtained at 54 phr. Increasing the concentration of the plasticiser displaces the curves to the left without significantly altering the gradients. A particular low-temperature performance may therefore be obtained by varying either the plasticiser itself or the concentration of a particular plasticiser. The latter will also have a significant effect on other important physical properties, such as softness, and this may be a
7 Properties of Plasticised
pvc
193
~ 3 -'
:>
o
o
:f >-
....
o
~ 2 a:
<:>
o
-'
"- .....
-50
-~o
-30
-20
-10
0
10
20
"-
30
.....
~o
50
TEMPERATURE (OC)
Fig. 7.7 Log rigidity modulus versus temperature. 4 constraint. Figure 7.8 demonstrates the effect of concentration on the 'Clash and Berg' cold flex properties (according to BS 2782, Method 150B: 1976) for a number of typical plasticisers. In this test method the temperature at which the sample is deflected through an arc oJ 2000 (equivalent to a modulus of rigidity of 1655 kg cm- z) is taken as the cold flex temperature (see also Chapter 12, Section 12.2.1). The effect of blending low-temperature plasticisers, in this case adipates, with phthalates in different ratios is shown in Table 7.2. This table also demonstrates the effect of volatility loss of plasticiser on the change in low-temperature properties and highlights the advantages of linear esters over branched chain esters in this respect (see also Fig. 6.2 in Chapter 6). In addition to cold flex tests, other types of low-temperature test methods are used in the PVC industry. These include cold bend tests, e.g. BS 2782, Method 151A: 1984, and cold crack tests, e.g. BS 3424: Method lOA: 1983. Obviously the actual results vary between the different types of test method, but in general, similar trends are observed (see also Chapter 12, Section 12.2.1).
194
D. L. Buszard 20
. u
10
o
(:I
L
;l -10 nl
L
E-20 ... (:I
(:I
>< -30
.... (:I
1J
(5 U
3
-40
2
1
-50
10
-60+----r----r----r---~~____,
o
20
40
60
100
Plasticisar laval,
20 10
.u
o
~
~ -10
... :J
nl
~ -20 a.
E
.3 -30 ><
(:I
~-40
1J
(5
U_ 50
-60+---"'T'"'"--"---"'T'"'"--"---" 100 o 20 40 60 80 Plasticisar laval, phr
Fig. 7.8 Effect of plasticiser level on 'Clash and Berg' low-temperature flexibility (BS 2782: 1970, Method 104B: NB Current revised version, BS 2782, Method 150B: 1976). 1, DOP; 2, DIDP; 3, DTDP; 4, DOA; 5, Reofos 65; 6, TOTM; 7, ESO; 8, PPA alcohol endstopped; 10, ~9P,
7 Properties of Plasticised
pvc
195
TABLE 7.2
Effect of ,Linevol' Phthalates and Adipates on Low-Temperature Properties of Identical PVC Formulationsa Linevol 79 adipatel Linevol 79 phthalate (Ph,)
Linevol 911 adipatel Linevol 911 phthalate (Ph,)
2-EH-adipatel 2-EH-phthalate (Ph,)
5010 3012020130 0150 5010 3012020130 0150 5010 3012020130 0150
Cold flex temperature
(BS 2782: 1970 M 104 B)("C) -44 -36 -31 -21 -23 -34 -30 -22 -35 -32 -24 -14 Cold flex temperature after 7 days at 100°C
(BS2782:1970MI04B)("C) -16 -15 -16 -13 -23 -30 -29 -22 +13
+9
-8
-5
a Extract
from technical literature of Shell Chemicals International Ltd, reproduced with their permission.
7.5 PERMANENCE PROPERTIES
It is obviously desirable that once a product has been manufactured from a flexible PVC formulation, it should continue to perform with a minimum of change in properties throughout its service life. Failure to perform satisfactorily may be the result of a number of factors: (i)
inaccurate initial specification, e.g. insufficient low-temperature properties; (ii) degradation by heat, light or possibly radiation; (iii) loss of plasticiser resulting in an undesirable change of properties.
The latter may be as a result of extraction, migration or volatile loss of plasticiser. The more practical aspects of these properties together with t\l. 0 important related areas, cable ageing and fogging, will be considered in this section. As was mentioned in Section 5.6.3 in Chapter 5 the rate of loss of plasticiser may be either diffusion- or surface-controlled. That is, the rate-determining step controlling the loss may be either the rate at which the plasticiser molecules can travel through the PVC matrix or the rate of loss of plasticiser from the surface. The dominant step depends on a number of factors but in general, where the overall rate of loss is slow, e.g. the volatile loss of plasticiser during service life, the process is surface-controlled. When the rate of loss is much higher,
D. L. Buszard
196
e.g. in a powerful extraction medium, the rate of plasticiser diffusion is more important. 7.5.1 Extraction Resistance One of the most important reasons for using polymeric plasticisers is their resistance to extraction by solvents. Some figures giving details of the physical properties and extraction resistance of various plasticisers are shown in Table 7.3. The polymeric plasticisers all exhibit better extraction resistance and, in general, somewhat inferior efficiency when compared with monomeric plasticisers. The high molecular weight non-endstopped Dio/pate 150 (Briggs & Townsend) possesses particularly good extraction resistance to nonpolar solvents such as hexane and oils. The endstopped polymerics, Dio/pate 214 and 917, show less resistance to nonpolar solvents, but better resistance to aqueous extractants and also superior compatibility, especially at high humidity, and greater efficiency. The endstopped, mixed adipate/phthalate polymeric Diolpate 171, exhibits reasonable extraction resistance but improved processing behaviour, both with respect to lower plastisol viscosities and faster gelation. It is worth stressing that the good extraction and migration properties of polymeric plasticisers are very dependent on achieving full gelation. The high molecular weight non-endstopped products are particularly difficult in this respect. If it cannot be guaranteed that the TABLE 7.3 Properties and Extraction Resistance of Plasticisers at 60 phr
Viscosity at 25°C (St) BS softness No. Cold flex temperature (0C) Tensile strength (MN m- 2) Elongation at break (%) Volatile loss (%) Extraction loss (%) Hexane Mineral oil Olive oil Water Soap (1%)
DOP
BBP
TXP
ESO Diolpate Diolpate Diolpate Diolpate 150 171 214 917
0·5 45 -20 17-8 335 22·5
0·6 43 -12 11·5 195 16·3
0·9 5 41 41 +2 -12 20·3 16·5 280 365 6·9 9·0
35 15·8 23·5 0·1 12·8
14·0 12·4 15·4 0·5 19·6
15·5 18·6 6·7 10·7 9-6 14·5 +0·3 0·1 21·2 4·1
130
28 -3 18·7 320 2·9 1·2 +0·5 3·1
1-6 7-2
9 42 -6 18·0 315
3-6 7-6 7·3 9·8 0·4 8·2
35 41 -7 17-4 330 H 2·9
z.t
5·8 0·3 2·7
41
44 -4 16·9 350
3-3 2·4 1·9 3·8 0·1 2·9
7 Properties of Plasticised
pvc
197
appropriate compounding/processing equipment for a particular product will achieve full gelation, then it is preferable either to use a lower molecular weight endstopped polymeric or to include a proportion of rapid-gelling plasticiser, such as triaryl phosphate, in the formulation. It can be seen that the replacement of an alkyl by an aryl group in a dialkyl phthalate reduces the nonpolar extraction resistance of the plasticiser. Triaryl phosphates and epoxidised soyabean oil exhibit intermediate extraction resistance between the dialkyl phthalates and the polymeric plasticisers. As mentioned in Section 6.9 of Chapter 6, solid polymeric resins have replaced high viscosity polymeric plasticisers in certain applications. Typical physical and extraction properties of a polyurethanebased solid elastomer are shown in Table 7.4. It will be noticed that whilst such products impart excellent extraction resistance to a PVC compound, they are far less efficient than conventional polymeric plasticisers. The thickness of a sample can also have an effect on its extraction resistance. This is shown schematically in Fig. 7.9. With very thin TABLE 7.4 Physical Properties of a PVC Compound Containing a Solid Polyurethane Elastomer-'Durelast loo,a BS softness No. Cold flex temperature eq 100% modulus (MN m- 2) Tensile strength (MN m- 2) Elongation at break (%) Tear strength (kN m- 1) Volatile loss (%) Extraction loss (%) Hexane Mineral oil Olive oil Water Soap (1%) Formulation:
a Trade
PVC Durelast 100 ESO Cd stearate
45 -22·5 5·6 15 530 59
0·2
+0·3 +0·1 -0·1 +0·7 +0·4 100 100 10 6
name of Briggs & Townsend.
D. L. Buszard
198
B
A
J.. --::l
.Q
, ._,. ,. ~
~
~ ,.'
1-
,.'
'
"
".
I I
--- ---
1:C7l .~
.
--_. '0 .,!!
~ ~
I
.Q
1
---11
~
1:
Incrllaslng sampill thlcknllss -+
Fig. 7.9 Schematic representation of the effect of sample thickness on extraction resistance.
samples the concentration gradient throughout the sample on extraction is very low (region A), whereas thicker samples exhibit a marked concentration gradient with a reservoir of plasticiser in the centre of the sample, thus resulting in the extraction being diffusion-controlled (region B). Extraction resistance is normally quoted as % weight loss of the sample (as in Table 7.3, etc.) or, less commonly, as % loss of plasticiser. However, in certain cases, it has become accepted to quote the results as actual weight loss per unit area of the sample tested. This is so in a number of national and international directives relating to the loss of additives from food-packaging materials into the contained foodstuffs. For example, the EEC draft directiveS on overall migration of plastics additives puts an upper limit of 10 mg per square decimetre of packaging material when tested by a particular method using a range of food simulants (distilled water, citric acid solution, aqueous alcohol and olive oil). Expression of the results in this manner obviously means that, as shown in Fig. 7.9, a particular compound may meet the requirements when tested at one thickness, but may fail when tested in a thicker section. In order to meet the olive oil extraction requirements of the above directives, it has been necessary to reformulate the thin PVC cling film to include a proportion of polymeric plasticiser as shown in Table 7.5.
7 Properties of Plasticised
pvc
199
TABLE 7.5 Reformulation of Stretch Wrap PVC Food Packaging Films to Include a Polymeric Plasticiser Old New formulation formulation
PVC suspension polymer DOA Reoplex 430 (Ciba-Geigy) ESO Ca/Zn stabiliser Antifogging agents Overall migration resistance into olive oil days at 40°C) (mgdm- )
pO
100 25
3 1
100 10 20 5 3 1
32
9
5
7.5.2 Migration Resistance The resistance of a plasticiser to migration from a PVC compound into another material in close contact is often a very important requirement. For example, migration of plasticiser from a PVC refrigerator gasket into a high-impact polystyrene (HIPS) refrigerator liner can lead to cracking of the HIPS in high stress areas or even softening of the HIPS resulting in it adhering to the gasket. Other examples include PVC cables in contact with plastics electrical appliance cases, self-adhesive PVC films, insulation tapes and the adhesion of print and lacquer to PVC films. The degree of migration will depend not only on the type and molecular weight of the plasticiser but also on the nature of the surface with which the PVC compound is in contact, in particular the compatibility of the plasticiser with, and its diffusion coefficient into, that material. This is highlighted in Table 7.6 where it can be seen that plasticisers generally have a greater tendency to migrate into cellulose nitrate than into natural rubber, and even less into polyethylene. It will be noted that the relative tendency of different plasticisers to migrate may be reversed when in contact with different surfaces. For example, the acid endstopping of polypropylene sebacate increases its migration into cellulose nitrate but markedly reduces its migration into natural rubber (Table 7.6).
D. L. Buszard
200
TABLE 7.6 Migration Resistance of Plasticisersa Cellulose Natural Polyethylene nitrate rubber
PPS PPA PPS/acid endstopped PPAlacid endstopped PPA/alcohol endstopped Epoxidised oil
2·7 2·2 9·1
10·7 6·4 9·1 14·1
DOP a Method
2·9 1·0
o
2·5 0·7 1·3 11·0
0·1 0·34 0·3 0·7 0·1 0·25 2·2
according to DIN 53405-1981.
Migration resistance is usually determined by DIN 53 405-1981, in which pre-weighed discs of plasticised PVC are sandwiched between discs of the relevant plastics material and placed between glass plates with a 5 kg weight on top, in an oven at 70°C. Table 7.7 shows the migration (expressed as % weight loss of the PVC disc) of a range of commercially available plasticisers into high impact polystyrene. The resistance to migration of the different plasticisers varies widely and is dependent on structure rather than molecular weight. TABLE 7.7 Migration of Polymeric Plasticisers into Polystyrene % weight loss of PVC disc a Test period (days) Wolftex b 828 Wolflex 848 Wolftex 868 Wolftex PLA 1 3
7 14
0·302 0·623 0·785 0·956
0·019 0·027 0·027 0·037
0·062 0·114 0·132 0·157
0·058 0·127 0·146 0·173
a Modified b
DIN 53 405-1981. Trade name of Victor Wolf.
7.5.3 Volatile Loss The volatile loss of plasticisers from PVC compounds is generally studied in two ways. Compounds are aged under a given set of conditions of temperature, time, airflow and sample size, and the volatilisation assessed either directly by the loss in weight of the test
7 Properties of Plasticised
pvc
201
specimens, or indirectly by the changes in physical properties occurring on ageing. The first method is very common and is the means by which the volatile loss results quoted in Chapter 6 Tables 6.3 and 6.6-6.13 have been obtained. These tests may be carried out either directly in ovens, preferably specially designed to avoid cross-contamination, or with the samples in contact with activated carbon to absorb the plasticiser vapours. Polymeric plasticisers, trimellitates and pentaerythritol esters are the classes of plasticisers exhibiting lowest volatile losses. Figure 7.10 shows the weight loss of PVC plasticised with a number of common plasticisers over an extended period. It is a characteristic of monomeric plasticisers that volatile loss tends to be reasonably linear with time up to fairly high losses. However, the volatile losses from polymeric plasticisers tend to level off at relatively low overall values, as the low molecular weight 'tails' are lost, and then continue unchanged for an extended period. The alternative method of assessing volatile loss is by following the change in physical properties on ageing. It has already been demonstrated by the changes in low-temperature properties on ageing, in Fig. 6.2 (Chapter 6) and Table 7.2. This way of assessing plasticiser volatility is obviously more compound-performance-orientated and is much favoured in the various cable specifications, particularly for high-temperature cables. The change in physical properties does not 40
30
HUG
Hcxaplal PPA
200
400 600 Houri at 100 0
800
1000
Fig. 7.10 Volatilisation of polymeric and other plasticisers.
202
D. L. Buszard
necessarily indicate just loss of plasticiser; particularly when the ageing tests are carried out at high temperature, oxidation of plasticiser and cross-linking and/or chain scission of the PVC molecules can also occur. In these cases, the inclusion of an antioxidant in the formulation can cause an apparent reduction in the plasticiser volatility. In the case of cables, the choice of plasticiser is dependent upon the conditions under which the cable is designed to operate. Generalpurpose insulation and sheathing compounds based on Cs phthalates, often with a chlorinated paraffin, are normally limited to a maximum continuous temperature of 6~5°C. For continuous operation at temperatures higher than this low-volatility plasticisers must be used. Thus for a maximum rating of 75°C, DIDP or perhaps a phosphate plastieiser are suitable, whereas for 90°C cables, DTDP is preferred. Trimellitates, polymeries or pentaerythritol esters are required for 105°C rated cables. 7.5.4 Automotive Fogging One property relating to plasticiser volatility whieh is periodically of interest to PVC technologists is automotive windscreen fogging. This problem of the build-up of an oily condensation or fog on car windscreens causing reduced light transmission, has been recognised since the 1950s and was largely attributed to volatile plasticisers. However, careful analysis of the fog by a number of laboratories suggest that in addition to the PVC plasticisers from crashpads and leathercloth, other additives may also cause problems. These include plasticisers in adhesives, pigment-dispersing media and other components of polyurethanes, antioxidants and even airborne hydrocarbons. Although fogging has been around for many years, the car manufacturing companies do not have a consistent approach to the problem. Some companies have very strict requirements, whilst others are influenced by the cost premium imposed by low-fogging leathercloth and crashpads. There are several different types of fogging tests in use, and a number of different test temperatures, e.g. 60, 75 and 90°C, depending on the part of the car for which the parts are destined. The fogging performance of a compound has been shown to be affected not only by the plasticiser but also by other constituents, e.g. stabilisers and minor impurities such as residual emulsifying agent in the polymer and free alcohol in the plasticiser. A selection of fogging results on various typical formulations is shown in Table 7.8.
7 Properties of Plasticised
pvc
203
TABLE 7.8 Fogging Test Results on Typical PVC Formulations for Automotive Use o CRASH PADS b
Breon S125/12 Blendexc 101 Chemigum d NB B1 A2 1rgastabe CH55 Titanium dioxide Antimony trioxide Reomof LTM 1rgastab 17M Fogging at 90°C
50 50 10 0·5 5 3 25 1·5 95%
CALENDERED SHEETING
Breon S125/12 Reomol LTM Palatino! 911 Reofose95 Titanium dioxide Calcium carbonate Antimony trioxide 1rgastab 17M Irgawax e 372 Fogging at 60°C Fogging at 75°C LEATHERCLOTH
Vinno[8 P70 Solvich 374 NB ReomolLTM Palatinol 911 Reofos 95 Titanium dioxide Antimony trioxide Irgastab 17M Fogging at 60°C Fogging at 75°C
100 50 2 5 5 1 0·3 98% 95%
100 30 20 2 5 1 0·3 97% 98%
33 66 70
33 66
33 66
70
2 5 1 99% 93%
2 5 1 96% 90%
45 25 2 1 98% 91%
Results by Volvo Fogging test method-minimum requirement 90% reflectance. Ii Trade name of BP Chemicals. C Trade name of Borg Warner. d Trade name of Goodyear. e Trade name of Ciba-Geigy. fTrade name of BASF. g Trade name of Wacker Chemie. h Trade name of Solvay. o
204
D. L. Buszard
Obviously the most stringent fogging test is that carried out at 90°C, which is for crashpad and window visor components. To meet the 90% reflectance requirement, it is necessary to formulate with plasticisers such as trimellitates, high molecular weight phthalates or other high molecular weight plasticisers, as well as a carefully chosen polymer and stabiliser system. Normal polymeric plasticisers are generally unsuitable since the low molecular weight 'tails' can fog severely. Trimellitates are technically preferable to high molecular weight phthalates, since it is far easier to strip Cg alcohols from the finished product than C lO-C14 alcohols.
7.5.5 High-humidity Compatibility Many plasticisers which have good compatibility under normal conditions of usage can exhibit severe incompatibility under conditions of high humidity, e.g. in refrigerators or in tropical climates. Non-endstopped polymeric plasticisers such as polypropylene adipate, are particularly bad in this respect. Phosphate plasticisers exhibit very good compatibility at high humidity. 7.6 FLAME-RETARDANT PROPERTIES
Rigid PVC is the most flame-retardant of all the thermoplastic polymers manufactured on a large scale. The addition of plasticisers reduces this flame retardance to a greater or lesser extent, as is shown in Fig. 6.3 of Chapter 6, in terms of the effect of plasticiser concentration on the oxygen index. However, even with high concentrations of non-flame-retardant plasticisers, flexible PVC exhibits a greater flame retardancy than other common polymers such as polyolefins, polystyrene, PMMA, etc., and is often self-extinguishing to small ignition sources. In recent years a greater awareness of the fire hazards associated with the extensive use of polymeric materials has led to increased requirements for more highly flame-retardant flexible PVC compositions. Such applications include cables for use in power stations and high-rise buildings, conveyor belting for coal mining, and wall coverings with low flame-spread characteristics. Flame retardancy in flexible PVC may be achieved either by (a) using a suitable plasticiser, e.g. a phosphate or chlorinated paraffin, or (b) incorporating a flame-retardant additive, such as antimony trioxide
7 Properties of Plasticised
pvc
205
or alumina trihydrate. The latter approach is often used in conjunction with a flame-retardant plasticiser rather than alone. Phosphate plasticisers, particularly triaryl phosphates which are very effective primary plasticisers as well as good flame retardants, offer a convenient means of achieving the necessary properties. They have the additional advantage of being non-pigmenting and hence clear flame-retardant formulations can be produced. Since triaryl phosphates are seldom used as sole plasticiser, their poor low-temperature properties can be offset by blending with other plasticisers. Figure 7.11 demonstrates the oxygen index and the cold flex properties of Reofos 95 (Ciba-Geigy) with di-Linevol 79 (Shell Chemicals) phthalate at overall plasticiser contents of 40, 60 and 80 phr. If the di-Linevol 79 phthalate in the blend is replaced by another non-flame-retardant plasticiser, e.g. an adipate, trimellitate, polymeric or other phthalate plasticiser, then the oxygen index of the blends would be similar, although the cold flex results would be different. The use of Fig. 7.11 35
20
o o
.
0>(
ClI
"0
.!: c ClI
~ >( 025
,/
"0
-40
8
20'+-_ _,...-_ _~_ _r-_----I o 25 50 75 100 100
Fig.7.11
75
0/0 Reofos 95
50
°/oL79P
25
o
Oxygen index and cold flex temperatures of Reofos 95/di-LinevoI79 phthalate blends.
206
D. L. Buszard
in conjunction with the data in Figs 7.2 and 7.8 enables the properties of other blends to be estimated. The use of chlorinated paraffins particularly in conjunction with phosphate plasticisers allows compositions with good flame retardancy to be formulated economically, providing that a high degree of light stability is not required. Alkyl diaryl phosphates can give formulations with reduced smoke evolution. The partial replacement of triaryl phosphates by chlorinated paraffins,6 particularly in the presence of certain fillers, such as magnesium oxide and hydroxide. is also claimed to give a reduction in smoke evolution;? see also Chapter 11, Section 11.5. 7.7 ELECTRICAL PROPERTIES
The electrical properties of greatest interest in flexible PVC are the volume and surface resistivity, and the dielectric properties. Volume resistivity and dielectric strength are naturally very important in wire and cable insulation; high values in these properties enable thinner coatings to be used. The effect of plasticiser type and concentration on the volume resistivity of a PVC compound is summarised in Fig. 7.12. 8 Since, as is apparent from its molecular structure, PVC exhibits a high dipole polarisation, the dielectric constant and power factor of its compounds are very frequency- and temperature-dependent. This limits the use of flexible PVC insulation to lower voltage and low-frequency applications. However, the power losses at high frequencies are successfully utilised in the high-frequency welding of PVC sheet (see Chapter 20). Outside the area of electrical insulation, low values of resistivity, particularly surface resistivity, are often beneficial in reducing problems of static build-up. A good review of the electrical properties of polymers and the effect of plasticisers has been made by Coulson. 8 7.8 WEATHERING AND LIGHT STABILITY
These properties depend primarily on the stabiliser system (Chapters 9 and 12) but they are also affected by the plasticisers used. The weathering and light stability of PVC compounds are usually assessed either by long-term outdoor exposure tests or by more rapid
7 Properties of Plasticised
pvc
207
Eu c:
13
> 10
>-
> >-
.r> .r>
~
~
10'2
....
:J
o >
1010L..J~_ _~:----~:----~:---
40
50
60
70
PlASTICISEIl CONTENT (phr)
Fig. 7.12 Effect of plasticiser type and level on volume resistivity at 23°C. 8
accelerated weathering techniques such as the Atlas Weatherometer* or the Xenotest. t It is very difficult to compare the results of different workers since both the climatic conditions and the accelerated ageing techniques vary so widely, and this often leads to contradictory and anomalous results. Four phenomena are generally associated with the outdoor weathering of PVc. These are: (i)
Discoloration-usually due to degradation of the PVc. This is very dependent on the stabiliser system, but photo-oxidation of plasticiser can accelerate the decomposition.
* Atlas Electrical Device Co.
t Hanau Quarzlampen GmbH.
208
D. L. Buszard
(ii) Loss of ftexibility-due to loss of the plasticiser by volatilisation, extraction Or photo-oxidation. (iii) Light-induced exudation-plasticiser migrating to the surface oxidises, leading to a discoloured, tacky surface layer. This is particularly associated with plasticisers containing carboncarbon double bonds, chlorinated paraffins or high levels of epoxy compounds. (iv) Dirt pick-up-this is often associated with (iii) and is particularly a problem in PVC-coated steel for use in outdoor cladding.
In general, aliphatic diesters impart good light stability, providing they are within their compatibility limits. Straight-chain phthalates are superior to their branched-chain counterparts, although the high linear phthalates exhibit poorer compatibility. Phosphates are poorer than phthalates, except when included in phthalate-plasticised compositions at low concentrations, when they apparently stabilise the formulations. 9 Aromatic phthalates such as BBP have markedly poorer light stability than dialkyl phthalates.
7.9 RESISTANCE TO MICROBIOLOGICAL ATIACK This is particularly important in plasticised PVC products for use outdoors, especially when in contact with soil or in warm humid areas, Examples of products particularly at risk are: buried cables, swimming pool liners and covers, foul-weather clothing, wallcoverings, and shower curtains. Plasticisation increases susceptibility to microbiological attack and no plasticiser is completely immune. Much investigational effort has been devoted to the problemy)"'15 Table 7.9 extracted from the work of Burgess and Darby13 indicates the relative resistance of a range of plasticisers to a mixture of five different fungi. Extensive soil burial tests by Decoste 12 showed that in addition to type, plasticiser concentration is also an important factor. Tests for plasticiser suitability are time-consuming and the advice of manufacturers provides a shorter route to the solution of specific practical problems of formulation. This advice may include recommendations for the use of such suitable chemical additives as fungistats or bacteriostats in the PVC formulation (e.g. Irgasan DP 300/PA-Ciba-Geigy; Estabex ABF-Akzo Chemicals; bioMET additives-M & T Chemicals).
7 Properties of Plasticised
pvc
209
TABLE 7.9 Fungal Resistance of Plasticisers13 ,a
Plasticise,P TXP D79P
DIOP
Polyester B Polyester C Polyester A
DIOA DIOS ESO
% plasticiser
% sample shrinkage
o
o o o
loss
4·19 5·24 6·21 14·58 18·28 53·13 56·43 63·83
3·2 2·2 2·7 8·5
10·7
15·0
Samples sprayed with a mixed spore suspension of: A. flavus; C. herbarum; P. funiculosum; P. pullulans; T. viride; incubated for 14 days at 28°C. b Formulation: PVC, 100; Plasticiser, 54; Epoxy, 1·5; BalCd stabiliser, 3·0. a
7.10 RESISTANCE TO INSECT AND RODENT ATTACK This is also of interest in certain building and outdoor uses of PVC and is particularly important in tropical climates. Plasticisation increases the susceptibility of PVC to attack by insects and rodents. There is some evidence that phosphate plasticisers may be more resistant than others,lO,16 but effective remedies are not primarily a matter of plasticiser selection. Special insecticides or repellants may be used in,17 or applied as coatings to, the PVC compound (e.g. Dieldrin-Shell Chemicals; bioMED·
7.11 STAIN RESISTANCE Flexible PVC is susceptible to staining by many different substances but particularly those which are oil-based (e.g. ball-point pen ink, shoe polish, tar, etc.). It has been shown that the degree of staining increases with the plasticiser content and that at equal levels of softness
D. L. Buszard
210
the type of plasticiser is also important. 18 Plasticisers which give reduced levels of staining are 2,2,4-trimethyl-l,3-pentanediol diisobutyrate (e.g. Kodaflex TXIB-Eastman Chemicals), the monoisobutyrate monobenzoate of the above diol (e.g. Nuoplaz 1046-Tenneco), benzyl butyl phthalate (e.g. Santicizer 16o-Monsanto) and triaryl phosphates (e.g. Reolos So-Ciba-Geigy). The major applications where stain resistance is important are in PVC flooring, and, to a lesser extent, in wall coverings and claddings.
'.n '.n.t
TOXICITY AND HEALTH ASPECTS OF PLASTICISERS
Plasticisers for Food-contact Application
Plasticised PVC is often used in applications in which it comes into direct contact with foodstuffs, e.g. packaging films, bottle seals, can lacquers, conveyor belting used in food preparation, etc. In such applications it is important that only those plasticisers-and indeed all other constituents-which are known to be non-toxic are used. In many parts of the world this is a requirement in law. In the USA for example, only those additives specifically permitted by the PDA 19 may be used. In other countries, such as the UK and Germany, there is currently no specified list of legally approved products, but the recommendations of the BPP20 and BGA,21 respectively, are voluntarily followed. It is likely that in Europe in the near future, the various national lists of approved additives will be replaced by a single Council of Europe or EEC directive. Table 7.10 summarises the more important plasticisers approved in a number of countries and also includes the draft proposals of the Council of Europe. However, this list is not exhaustive and in certain applications additional limitations may apply. For more detailed information a user should contact the plasticiser manufacturer or study the appropriate published national requirements/recommendations directly. Recent studies22 .23 by the US National Cancer Institute (NCI) have indicated that DOP and DOA could cause a statistically significant increase in the observed liver tumours (hepatocellular adenomas and carcinomas) when fed to certain rodents daily over a two-year period. The high levels of dosage do, however, make interpretation of these results difficult and it would not be possible on this evidence alone to
7 Properties of Plasticised
pvc
211
TABLE 7.10 Plasticisers Approved Q for PVC in Contact with Foodstuffs DBP DOP DIOP DIDP DMEP DBS DOS DOA DIDA ESO Reop/ex Reop/ex FG 430 Council of Europe Federal Republic of Germany Franceb HoUand b Italt UK USA
x
x
x x x x x x
x x x x x
x
x x x
x
x x x
x
x
x
x
x
x x
x x x x x x
x x x x x x
x x x
x x
x x
x
x x x x x x x
x x x
x x x x x
X
Limitations not indicated in the table may apply to, for example, maximum permissible concentration, type of food to be packaged, form of finished product, etc. b Subject to an overaU migration limit.
a
evaluate any potential carcinogenic hazard to man. Since it was evident that these findings could have important implications affecting the continued commercial use of these esters, the European Council of Chemical Manufacturers Federation (CEFIC) recommended that further scientific studies specifically designed to investigate the possibility of adverse health affects resulting from human exposure should be carried out. These studies sponsored by the European Plasticiser Manufacturers under the auspices of CEFIC have now been completed and reported. 24 The studies carried out on DOP included DNA binding, dose/time response and a comparative assessment in rodents (rats) and primates (monkeys). These results confirm that there is sound evidence that the NCI study is not relevant to human risk assessment and carcinogenic risk to man has not been demonstrated. Further studies involving a wide range of phthalates and adipates are currently in progress in the USA. This is a voluntary research programme funded by the Plasticiser Manufacturers section of the Chemical Manufacturers Association in conjunction with the FDA.
7.U.2 Health and Safety The majority of plasticisers manufactured today are of a low order of toxicity and constitute little hazard in use either from direct toxic effects or dermatitic effects on handling. However, as with all organic
212
D. L. Buszard
TABLE 7.11 Vapour Pressure of Dialkyl Phthalates Plasticiser Molecular weight
DBP DOP DIDP
278 390 447
Vapour pressure (rnrnHg)
Concentration in saturated air at 160"C (g rn- 3 )
1·2 0·121 0·029
12 1·75 0·48
materials, good working practice should be employed. Adequate ventilation should be ensured in the vicinity of heated equipment where plasticiser fumes may be produced. Table 7.11 indicates the concentrations of plasticiser vapour which could occur in saturated air at 160°C. Laboratory experiments25 and actual measurements on industrial plastisol coating plants indicate that 25-50 kg h- 1 of plasticiser could be lost by volatilisation alone. This indicates the need for good ventilation if the threshold limit value (TLV) of 5 mg m- 2 , generally considered to he the safe maximum concentration by the American Conference of Governmental Industrial Hygienists 26 and adopted by HSE 27 and OSHA, is not to be exceeded. REFERENCES 1. 2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Harrington, C. E. (1965). Ind. Quality Control, 21,494-8. Tang, Y. P. and Harris, E. B. (1967). SPE J. 23(11), 91-5. Pugh, D. M. and Wilson, A. S. (1976). Eur. Plast. News, 3(9), 37-42. Combey, M. (1972). Plasticisers, Stabilisers and Fillers, (Ed. P. Ritchie), Iliffe/PRI, London, Ch. 12. EEC Draft Directive on the overall migration limit for the constituents of plastics, materials and articles intended to come into contact with foodstuffs. Ref: R/1444178. Ceasar, H. J. and Davis, P. J. (1975). SPE Tech. Papers, 21, 130-4. Price, R. V. (1979). SPE Tech. Papers, 25,956--63. Coulson, S. H. (1972). Plasticisers, Stabilisers and Fillers, (Ed. P. Ritchie), Iliffe/PRI, London, Ch. 8. Dolozel, B. (1963). Chern. Prurnysl., 13(38), 3, 160-5. Wessel, C. J. (1964). SPE Trans., 4, 193-207. Berk, S., Ebert, H. and Teitell, L. (1957). Ind. Eng. Chern., 49, 1115-24. Decoste, J. B. (1968). Ind. Eng. Chern., 7(4),238-47. Burgess, R. and Darby, A. E. (1964). Brit. Plast., 37(1), 32-7. Burgess, R. and Darby, A. E. (1965). Brit. Plast., 38(2),2-6.
7 Properties of Plasticised
pvc
213
15. Wolkober, Z., Gyarmati, I. and Farkas, P. (1978). Int. Polym. Sci. Technol., 5(4), no. 16. Bultman, J. D., Southwell, C. R. and Beal, R. H. (1972). Naval Res. Lab Report No. 7417, Washington DC. 17. Anon. (1965). Mod. Plast., 42(5), 168. 18. Pinner, S. H. and Massey, B. H. (1963). Brit. Plast., 36(10), 564. 19. USA Food and Drugs Administration (FDA) Code of Federal Practice. 20. 'Plastics for Food Contact Applications-A Code of Practice for Safety in Use', The British Plastics Federation. 21. 'Kunststoffe in Lebensmittelverkehr', Kunststoff-Kommission des Bundesgesundheitsamtes, Berlin, Federal Republic of Germany. 22. NTP Technical Report series 217. 'Carcinogenesis Bioassay of Di(2ethylhexyl)phthalate (CAS No. 117-81-7) in F 344 Rats and B6C 3F1 Mice (Feed Study), NIH Publication No. 82-1773. 23. NTP Technical Report series 212. Carcinogenesis Bioassay of Di(2ethylhexyl)adipate (CAS No. 103-23-1); F 344 Rats and B6C 3F1 Mice (Feed Study), NIH Publication No. 81-1768. 24. CEFIC. (1982). 'Di-(2-ethylhexyl)phthalate (DEHP), CEFIC Plasticiser Toxicological Working Group Report on Developments in DEHP Toxicology, Avenue Louise 250, Bte 71, B-1050 Brussels. 25. Poppe, A. C. (1980). Kunststoffe, 70(1), 38-40. 26. 'Threshold Limit Values for Chemical Substances in Workroom Air for 1978', American Conf. of Govt. Ind. Hygienists, Cincinnati, Ohio. 27. 'Threshold Limit Values 1980', Health and Safety Executive, Guidance Note EH 15/80.
CHAPTER 8
Fillers in PVC I. D. HOUNSHAM and W. V. TlTow
8.1 INTRODUCTION For the purpose of this chapter fillers may be broadly defined as solid particulate or fibrous materials, substantially inert chemically, incorporated in plastics compositions (including PVC) to modify the properties or to reduce material cost. Cost reduction is often the primary reason for the use of a filler, and because of this the term is occasionally treated (incorrectly) as if it was synonymous with 'cheapening extender'. In fact all fillers-when present in significant quantitiesaffect in some measure the material and/or processing properties of the plastic, and some-which may be termed 'functional fillers'-are indeed used, often at increased cost, expressly as property modifiers, e.g. glass fibres as reinforcing filler in uPVC compositions, antimony trioxide or alumina trihydrate as flame retardants in pPVc. It may be noted that the functional aspect is emphasised in the standard definitions of a filler (ct., for example, ISO 472-1979; BS 1755: Part 1: 1967; ASTM D 883-83). In the ideal case the incorporation of a filler might confer the combined benefits of cost reduction with increased output (involving no processing difficulties and no rise in the process/production costs) and some technical advantages in the properties and service performance of the plastic. In practice, usually only one, or some, of these features can be secured, often at the expense of the others, and the selection of a filler (or filler system) will thus be a compromise, dictated by the balance of the technical requirements and cost considerations. For example, whilst such low-cost fillers as ground 215
216
I. D. Hounsham and W. V. Titow
limestone and coarse ground whitings offer the highest material cost savings in many flexible PVC compositions, they can also adversely affect the processing, and some physical properties of the end product; incorporation of glass-fibre reinforcement in uPVC upgrades mechanical properties but increases the cost and affects the processing behaviour of the material. A wide variety of materials has been evaluated as fillers for PVC compositions: in this chapter attention is centred on those which are of current technical interest. Among these, certain kinds of calcium carbonate and chrysotile (white) asbestos have attained particular commercial importance.
8.2 MINERAL FILLERS
Certain minerals, especially some naturally occurring silicates and natural (as well as synthetically produced) carbonates, provide some of the most widely used fillers for PVC. These materials may be considered under three general headings: (i) silicates and silicas; (ii) sulphates of the alkaline-earth metals; (iii) calcium carbonates. 8.2.1 Silicates and Silicas
The silicate minerals of particular interest as filler materials for PVC are asbestos, talc, and clay. Other silicate fillers used in some PVC compositions, but not on a major scale, are wollastonite (a calcium metasilicate 1 ,2 sometimes employed as a filler in floor tiles and plastisol products), nepheline syenite (an anhydrous sodium/potassium/aluminium silicate, useful in some semi-transparent compositions because of its low tinctorial power), mica (a general name for a group of complex potassium/aluminium silicates with plate-like particles, of particular interest in some electrical insulation applications), and slate flour (slate is a complex composed of muscovite mica, chlorite and quartz, i.e. a composite hydrated potassium/aluminium/magnesium silicate combined with silica). The mineral silica fillers (quartz, sand, diatomaceous earth) are of comparatively little interest for PVC compositions, although one
8 Fillers in PVC
217
form-novaculite-has been claimed to be useful in calendered sheet, rigid compositions and foams. 3 (a) Asbestos The only form of asbestos used as a filler in PVC is chrysotile (white asbestos). Chemically this is a hydrated magnesium silicate (3MgO.2SiO.2H zO). It is a fibrous material (fibre length 1-40 mm, fibre diameter 0·01-1 tlm) with a very high fusion point (1500°C) and 100% strength retention at temperatures up to 4000C. 1 Its main applications in PVC are in flooring compositions (where a short-fibre grade is used, improving melt cohesion in processing and providing some reinforcement in the product, including improved hardness and denting resistance), and as reinforcement in pressed sheet (e.g. Duraform-Turner Brothers Asbestos, UK): a longer fibre grade is used in the sheet, which finds application as internal and external cladding material for buildings (especially industrial buildings), corrosion resistant trunking and ducting, and the like. 4 ,5 Despite the increasing stringency of various health and safety regulations, the handling of asbestos and asbestos-filled PVC is still possible, if suitable precautions are observed. 1 ,4 In formulating and processing asbestoscontaining PVC compositions it should be borne in mind that their heat stability and colour may be affected by iron compounds present in chrysotile as minor chemical constituents. (b) Talc
This material also consists of hydrated magnesium and silicon oxides (with admixtures of other minerals and impurities, neither of which are completely removed in preliminary processing). The chemical composition and the particle shape of talc vary somewhat according to the source; the shape may also be influenced by the grinding process: the particles may be granular, plate-like ('platy' or 'scaly'), or needleshaped, with sizes in the range 1-50 tlm.1 Talc is used in some calendered PVC floor tile compounds to increase melt cohesion and the stiffness and hardness of the finished product: 1 its low tinctorial power and its refractive index (which is close to that of PVC resin) also make it of interest as a filler for translucent compositions. (c) Clay There are several varieties of clay, all essentially forms of complex, hydrated alumino-silicates, with varying amounts of other minor
I. D. Hounsham and W. V. Titow
218
constituents (including potassium oxide, titanium dioxide, and iron oxides). Of these only kaolinite ('kaolin', 'china clay') is of significant interest as a filler for PVC. Whilst in some other plastics ground kaolin is used directly, in PVC compositions it is normally employed in the calcined form. Calcination refines the clay (by removing some impurities and some iron compounds, which results in improved whiteness), removes the water of hydration, and improves the processing performance. The main use of china clay in PVC has been in flexible compositions for electrical applications (insulation, cable covering), and also in carpet backing compounds (in those based on PVC latices bentonite clay, as well as kaolinite, may be used-see Chapter 23), latex-based films, and plastisol products (especially coatings on fabrics). However, in all these applications clay has yielded ground to calcium carbonate fillers, which are normally cheaper, grade 90
. z ...'" ...
&>
E :>
5 50
0
Ul Ul
ID
40
30
200~-·--"I""'O---2""'0--~30'--"l---"40'-----"50->o.---<60· 0/0
by Weight of Filler
Fig. 8.1 BS softness versus filler loading. PCC: Precipitated calcium carbonate; calcium silicate: wollastonite.
219
8 Fillers in PVC 400
300
.
c 200
o
C;arbon black'
II
'"o C
iii
100
o
10
20
30
40
.,. By wllight of filler
50
60
70
Fig. 8.2 Elongation of a PVC compound versus filler loading.
for grade, whilst their effects on the properties of PVC compositions are generally similar, and can be better in some respects (see, for example, Figs 8.1 and 8.2, and Table 8.1). In broad terms, the particle size of china clay (O·2-8Ilm) is comparable with that of whiting, but the particles are platelets, and the colour imparted to PVC materials by clay fillers is comparatively poorer. 8.2.2 Alkaline-earth Metal Sulphates
Among the sulphates of the alkaline-earth metals (Be, Mg, Ca, Sr, Ba and Ra) only barium sulphate (BaS04) is of some (albeit relatively limited) practical interest as a filler for PVC. The two grades of barium sulphate in use as fillers are both produced from the mineral barytes,
220
I. D. Hounsham and W. V. Titow
TABLE 8.1 Some Effects of Varying Filler Content in a PVC Componnd Filler (% by weight)
Tensile strength (lbfin- z)
Cold flex eC)
Water absorction (increase % y wt, 48h at 500C)
1700
-21
0·6
20 30 40 50
-20
60
1300 1210 920 750 500
1·5 1·5 1·5 1·2 1·2
20 30 40 50 60
1700 1400 1210 890 1090
20 30 40 50
-17
60
1450 1150 780 820 1100
Carbon black (MPC) 20 30 40
1920 1710 1760
-14·5 -11 -7
No filler Precipitated calcium carbonate
Hard china clay
Calcium silicate (wollastonite)
-19 -18 -18 -15 -9
-16·5 -16
Q
2·8 3·7 4·9 5·1 6·5 11·3 8·3 6·8 6·5 7·1 1·2 0·9 0·9
The composition of the compound is based on 100 of polymer, 50 phr DOP, and 7 phr basic lead carbonate, with filler as stated. Q
the naturally occurring form of BaS04' They are: the natural material, ground and purified; and precipitated BaS04' known as 'blanc fixe'. Except where a high-density compound may be required for some special reason, the high density of barium sulphate (about 4,5) is a disadvantage because it makes the filler expensive on a volume-cost basis. The advantageous features include high dry brightness (up to 99·5% reflectance) and low oil absorption (see Table 8.2). Flexible PVC compositions incorporating this filler find use in specialised applications where high acid resistance or opacity to X-rays is required. Note: Calcium sulphate, found in nature as the minerals, gypsum (CaS04.2HzO) and anhydrite (CaS04), can also be prepared
8 Fillers in PVC
221
artificially in the corresponding (hydrated and anhydrous) forms, known by the same names. Gypsum loses most of its water of hydration on heating at temperatures well within the PVC melt-processing range:
Thus 'gassing' of the melt and void formation in the product is a possibility to be considered in connection with its incorporation as a filler. Anhydrous particulate CaS04 has recently been eliciting some interest as a filler for plastics (e.g. Snow White-US Gypsum Co., USA), and a microfibrous, crystalline form (Franklin Fiber-United States Gypsum Company, USA) has been suggested as a possible substitute for asbestos;8 however, neither of these materials is used in PVC on an industrial scale. 8.2.3 Calcium Carbonates This group comprises a range of naturally occurring materials, as well as prepared (precipitated) calcium carbonates. It provides the following types of fillers, widely used in PVC compositions: (i) (ii) (iii) (iv)
whiting; ground limestone, marble, and calcite; ground dolomite; precipitated calcium carbonate.
The mineral calcium carbonates are available at relatively low cost and in large quantities from abundant natural sources. The energy required for their processing (grinding and classification) is comparatively low. Their characteristics, desirable in fillers, include low plasticiser absorption, absence of water of crystallisation and generally good resistance to thermal decomposition during processing of the plastics compositions in which they are incorporated, relative softness, good white colour, and purity. The precipitated grades share these properties (although their oil and plasticiser absorption is comparatively high); they are commonly used for specialised applications, notably in rigid PVC compositions.
6 14
12
1·55 1·65 1·65
1·50 1·59 1·63 1·59 1·40
2·55
4·47 4·40
2·70
2·71
2.68
2·90 2·33
Calcite (Hydrocarb C )
Precipitated Calcium silicate: Wollastonite Precipitated
26 360
15-65
18
34
40
1·58
2·40
Alumina trihydrate (TrihydeC ) Asbestos (chrysotile) Barium Sulphate: Barytes Blanc fixe Calcium carbonate: Dry-ground whiting (stearate-coatedBritomya BSHC )
Oil absorption b (weight %)
Refractive index
Filler materiaL
Specific gravity
5·0 5·0
3·0
3·0
3·0
3·0 3·0
2·5-4·0
2·5
Moh hardness
TABLE 8.2 Some Properties of Filler Materialsa
Crystalline Crystalline
Crystalline
Crystalline
Crystalline
Granular Granular
Granular or crystalline Fibre
Particle shape
Particle size-mean 2·0 pm; top cut 15 Ilm Particle size-mean 1·5Ilm; top cut 7 Ilm
Various particle size grades
Remarks
~
1·52 1·51-1·52
1·50 1·59 1·60 1·45 1·54 1-40 1·55 1·55 1·59
2·50
2·49
0·21
2·75 2·60
2·20 2·00 2·65 2·65 2·65 2·80
Spheres, hollow (thin-walledQ-cel 300c)
d
150 55-180 120 32 20-30 27-40
47 20
35 d
30
15
36 25
6·0 7·0 5·5 1·0-1·5
-
-
3·0 6·0
6·5
6·5
6·5
2·5 2·5
Spherical Amorphous Diatom Crystalline Amorphous Platy
Platelet Crystalline
Sphere
Sphere
Amorphous
Platelet Platelet
Mean particle size 65,um (range 10-180 ,urn)
Various particle size grades
b
a
IV IV W
Based in part on data from References 1, 6 and 7. Some of the figures have been obtained by different test methods; however, most are the results of oil rub-out tests according to ASTM D 281-31 (1974), and are thus generally comparable. C Croxton and Garry Ltd, UK. d Grams of oil per 100 cc of spheres.
Mica (muscovite) Nepheline syenite Silica: Colloidal (pyrogenic) Precipitated gel Diatomaceous earth Ground quartz (sand) Novaculite Talc (Garotalc C )
1·56 1·62
2·60 2·63
Clay (kaolin): Ground Calcined ground Glass: Ground Spheres, solid (Ballotin{)
224
I. D. Hounsham and W. V. Titow
8.3 CALCIUM CARBONATE FILLERS-NATURE, PROPERTIES AND APPLICATIONS 8.3.1 General Types (a) Whiting This is a fine, white powder, produced by grinding and classifying chalk,9 which is a fairly pure natural calcium carbonate found in the form of deposits of aggregated, fossilised skeletons of microscopic marine organisms of the group Foraminifera, known as coccoliths. The deposits were produced, in the course of tens of millions of years, in the Cretaceous period of the Mesozoic era (Latin creta = chalk). Apart from its whiteness, the most characteristic features of whiting are its particle size and shape. The particles are small-typically below 10 ,urn in size: under suitable magnification some can be seen to have a fine granular structure, with the granules (in most cases about 0·2-0·6 ,urn) recognisable as elements of a coccolith skeleton; some of the particles are actually complete, ring-shaped skeletons-roughly 4-8,um in diameter-or parts of such rings. Basic dry grinding of chalk leads to an end product with the majority of particles between 1 and 5,um in size; this range is very well suited to the material's application as a filler for PVc. Of particular interest are whitings from Northern European chalks which are noted for their purity (especially the almost complete absence of silicates). (b) Ground Limestone, Marble and Calcite Limestone is a very widely occurring natural calcium carbonate formed from deposits of calcified Foraminfera, originally sedimented and later consolidated by heat and pressure. Marble and calcite are hard, compact crystalline varieties of limestone. 9,10 There are only a few areas where the deposits of limestone, marble or calcite are pure enough for grinding and processing into fillers for PVc. Of the ground end products, limestone fillers are the coarsest (and hence have the lowest oil and plasticiser absorption) and of, relatively, the poorest colour. The colour (whiteness) of ground marble and calcite is very good; their oil absorption is lower than that of whiting. The filler grades available are produced by either wet or dry processing. The wet ground materials are of higher quality, with finer particle size and more uniform size distribution, but they are more expensive.
8 Fillers in PVC
225
(c) Ground Dolomite Limestones containing about 45% magnesium carbonate, i.e. 55: 45 CaC03/MgC0 3 , are called dolomites (those with lower MgC0 3 contents are known as magnesian limestones). The filler grades of ground dolomite have very good colour, and plasticiser absorptions higher than those of their ground marble counterparts. Scandinavia has the best deposits of dolomite. (d) Precipitated Calcium Carbonates These are available in two versions-as powders specially manufactured by controlled precipitation from solutions of calcium salts, and as a by-product from water-softening plants. Both sources provide powders of fine particle size-few particles exceed 1 pm, and the average size is well below this level. For this reason the precipitated calcium carbonates are normally supplied in the coated form (see Section 8.3.2 below) to counteract agglomeration and aid dispersion in polymer compositions: it is an advantage of the precipitation processes that particle-coating treatments can be applied in the course of manufacture (and not as an additional processing step). The process also affords good control over the quality, particle size, and purity of the product (albeit that from some water-softening plants may be slightly alkaline), and the colour (whiteness) is normally very good. However, the production cost of precipitated calcium carbonate fillers is in general considerably higher than that of their ground counterparts. Precipitated calcium carbonates are sometimes referred to as 'precipitated whiting' (especially in sales literature). This nomenclature is incorrect (see description of whiting above).
8.3.2 Surface treatments Calcium carbonate powders (as well as, occasionally, other inorganic materials) used as fillers in PVC may be treated with substances which make the particle surface organophilic (hydrophobic), thereby conferring certain processing and performance advantages. Thus moisture absorption by the particles is reduced, and this improves dry flow of the powder and reduces agglomeration; it has also been claimed to improve the moisture resistance of PVC compounds filled with surface-treated fillers (in comparison with similar ones incorporating untreated grades)-an effect of interest, for example, in electrical wire
226
I. D. Hounsham and W. V. Titow
and cable coatings. The surface treatments improve interaction ('wetting') at the filler/polymer interface, promoting quicker and more intimate dispersion of the filler in the polymer melt (better dispersion, and 'wetting' of the filler particles by the melt, also make for easier melt flow in processing). Plasticiser absorption ('plasticiser demand') is reduced by the presence of the coating on the filler surface. Improvements in the mechanical properties of the filled PVC composition can also result (e.g. higher impact strength and flexural modulus in uPVC in comparison with similar compositions containing the same type of filler but without surface treatment): these may be particularly noticeable in cases where the surface treatment affords scope for actual bonding between the filler and polymer at the interface (as, for example, when the filler is treated with some organotitanate compounds-see below). However, the degree of such property improvements is not so great as to constitute the main reason for surface-treating fillers for PVC, and the substances most widely applied are not primarily intended to act as coupling agents in the way in which, say, certain silane compounds do when applied to glass fibres for use as reinforcement in some polymers! (silanes are not, in fact, used on either glass-fibre reinforcement or any other fillers for PVC). It may be noted in passing that the surface treatments of commercial fillers do not in practice attain the theoretical ideal of a complete covering of the whole surface of every filler particle with a thin (ideally molecular) layer of the coating substance. However, the general extent of coverage is normally good enough to promote in significant measure the advantages just mentioned: because the reagent in this type of treatment is thinly distributed over the particle surfaces, the amounts required are low (in general from fractions of a per cent up to about 2% on the weight of filler). The reagents used as surface treatments on calcium carbonate fillers for PVC may be considered under three headings: stearates, organotitanates, and miscellaneous (including proprietary) treatments. (a) Stearate Treatments Stearic acid and some stearates are the oldest and still most widely used reagents. They may be simply deposited on the particle surface, or also additionally made to react with the surface material-at suitably elevated temperature and/or pressure-to be positively bonded thereto. Examples of commercial CaC03 fillers with such surface-bonded stearate coatings are Polcarb S (English China Clay
8 Fillers in PVC
227
Sales Co. Ltd), Omya BLR/3 and Omyalite 95T (Pliiss-Staufer AG, or Croxton and Garry, UK and Europe), and Gama-Sperse CS 11 (Georgia Marble Co., USA). Some CaC0 3 fillers are marketed as 'double-coated' grades, e.g. Britomya BSH 30 (Croxton and Garry Ltd, UK); in comparison with the corresponding uncoated material (Britomya M) the coating reduces oil absorption by about 25% and DOP absorption by about 35%.11 (b) Organotitanate Treatments Alkoxy organotitanates, introduced comparatively recently (1974/75) in the USA as coupling agents for fillers in thermoplastics (as well as some thermoset systems )4,12 have entered the commercial field, inter alia, as surface treatments for calcium carbonate fillers and alumina trihydrate flame retardants used in PVC. On the former filler certain organotitanates (Ken-React TTOP-12 and TTOPP-38-Kenrich Petrochemicals Inc., USA) have been claimed 12 to improve substantially the extrusion characteristics and impact resistance of 40%-filled rigid PVC pipe compounds 4 ,13 and enable significant reduction in the amount of lubricant used. In flexible PVC compositions the use of TIOP-12 has been said to improve performance beyond that achievable with stearate-coated CaC03 • 12 It has also been suggested that the presence of an organotitanate coating on the CaC0 3 filler can have some flame retardant effect in a compound. 13 Among commercial calcium carbonate fillers for PVC surface-treated with organotitanates are some of the grades supplied (in the USA) by the Sylacauga Calcium Products Co., and the So/emite calcium carbonates from Solem Industries Inc. (c) Proprietary and Miscellaneous Treatments The nature of proprietary surface treatments on some commercial (ilC0 3 fillers (e.g. Super-Pflex 200-Pfizer Inc., Minerals, Pigments and Metals Division, USA) is not generally disclosed, although some are believed to be of the stearate type. Others include the so-called 'calcium resinate' treatment (as, for example, on Gama-Sperse CR-12-George Marble Co., USA) and those involving the proprietary surface-active agents of Byk-Mallinckrodt (West Germany and USA), as applied, for example, to some CaC03 filler grades of the Calcium Carbonate Co. (USA). The opacifying surface treatment (used, for example, on Omya BLR 2 and BLR 3-Croxton and Garry Ltd., UK) may also be mentioned, although its function is not to
228
I. D. Hounsham and W. V. Titow
promote the effects discussed above, but to increase the refractive index of the CaC0 3 filler and hence its whitening power in a compound.
8.3.3 FDler Properties and Selection Criteria*
In order to select a CaC03 filler for a given PVC application, the various properties of the filler should be understood in terms of its effects on the compound (some of the considerations relevant here apply also in the selection of other fillers). The most important features are: (a) (b) (c) (d) (e) (f) (g)
maximum particle size; particle size distribution and mean particle size; colour (whiteness)-dry brightness; refractive index (opacifying effect); oil and plasticiser absorption; dispersion characteristics; cost.
(a) Maximum Particle Size This is very critical in terms of both the physical properties of the filled compound and its surface appearance; the lower the maximum particle size, the better these properties become. In fact, the presence of even a small percentage of oversize particles can adversely affect performance. For most plasticised formulations for extrusions, fine-gauge calendering, and injection moulding, a maximum particle size of 10 !Jm is normally acceptable. Spreading plastisols can tolerate a still higher maximum particle size, due to higher plasticiser content and less need for good physical properties. In low-viscosity plastisols, large particles can have a tendency to settle. For rigid PVC compounds it had been long accepted that for both good physical properties and surface appearance the maximum particle size should be less than 1 !Jm. This effectively restricted this usage area to precipitated calcium carbonates. However, recent improvements in techniques for the milling and selection of natural calcium carbonates have brought such fillers into contention for rigid PVc. Furthermore, the development of better compounding equipment and improved * Section closely based on a part of Ref. 14.
8 Fillers in PVC
229
formulatory knowledge have combined to increase the use of fillers with particles of up to about 5 Ilm, with consequent cost advantages.
(b) Particle Size Distribution and Mean Particle Size The particle size distribution will determine how well the filler particles pack together. This is important in heavily filled compounds and plastisols, since the better the packing, the higher the loadings possible. In a heavily filled plastisol, not only can the correct selection of the filler or fillers control the final viscosity but it is also possible to control the rheological properties according to the final application. A good example of this would be PVC plastisols for car-body underseals. Here it is found that by using a blend of fine ground whiting and a coarser ground crystalline calcium carbonate (calcite), it is possible to achieve very high filler loadings, whilst the plastisol still remains sufficiently free-flowing to be spray-applied, with the final coating having excellent anti-sag characteristics. The mean particle size also plays a part, and the finer this is, the better the physical properties and surface appearance of the filled compound. In PVC plastisols for slush or rotational mouldings, fine fillers of mean particle size 1-3 Ilm are required to eliminate filler settlement problems.
(c) Colour (Dry Brightness) The dry brightness of a filler is normally expressed as the percentage of light reflected from the smoothed surface of the filler powder, compared with pure magnesium oxide considered as pure white with 100% reflectance. The results obtained give a useful measure of whiteness (but do not necessarily indicate the opacifying effect) of the filler. Very white fillers can afford cost savings by part replacement of titanium dioxide pigments. There is a direct relationship between particle size and opacifying effect. In general, the finer the filler, the higher the dry brightness and the effective opacity. For comparison typical dry brightness values would be: limestone, 80; UK-produced whiting, 86; dolomite, 95; and calcite, 91. (d) Refractive Index The refractive index of a PVC polymer is about 1·55 and that of a phthalate plasticiser may be in the range 1·47-1·50. Therefore, for a typical plasticised formulation containing 50 phr DOP, the refractive index of the homogeneous mix could be about 1·53. Addition of a filler
230
I. D. Hounsham and W. V. Titow
with a refractive index of 1·53 would lead to a transparent mix even though the system would be heterogeneous.
Note: If the filler has not been sufficiently 'wetted-out' by the PVC composition 'stress whitening' ('crease whitening') can occur when the material is locally stressed. This is due to the formation of minute voids as the solid phase of the filler separates from the polymer. As the refractive index of the filler increases from 1·53 the optical transparency decreases.
(e) Oil (or plasticiser) absorption This may be defined as the weight of linseed oil (or of a specified plasticiser) required to 'wet out' completely 100 g of filler to a putty-like consistency in a standard test. * The oil (or plasticiser) absorption is related to, but is not a direct measure of, the 'plasticiser demand', i.e. the extent to which the filler will absorb plasticiser when incorporated in a compound. It is an important consideration in deciding on a filler for a plasticised compound or plastisol. The 'plasticiser demand' and oil absorption tend to decrease with increasing particle size of the filler, and are directly related to the surface area of the filler particles. Fillers with good particle packing have lower oil absorption figures, and the more crystalline the particles the less they absorb internally. The presence of a coating on a filler reduces oil absorption and the actual plasticiser demand in a compound (see Table 8.3). Thus, cost savings can be. effected through the use of a coated filler, either by increasing the filler loading for the TABLE 8.3
The Effect of Surface Treatment on Plasticiser Absorption by Whiting Plasticiser (phr) Filler loading (phr) Plasticiser absorbed by ordinary whiting Plasticiser absorbed by coated whiting (Omya BSH) Reduced quantity of plasticiser permissible in formula
* cf., for example, ASTM D 281-31(1974).
50 5
50 15
50 25
1·25
3·75
6·25
0·75
2·25
3·75
49·5
48·5
47·5
8 Fillers in PVC
231
same amount of plasticiser or reducing the amount of plasticiser for the same filler loading, in comparison with an uncoated grade. Oil (plasticiser) absorption has a direct bearing on the softness of a flexible compound or the viscosity of a plastisol. Oil absorption of over 20 g per 100 g is generally regarded as high. Fillers exhibiting such absorption will cause substantially more plasticiser to be required in the compound than low-absorption fillers to achieve the same BS softness. In a plastisol the oil absorption of the filler is directly related to its effect on viscosity, whilst-in general-increased filler loadings result in an increase in viscosity. If high viscosity is required but-to limit the effect on final properties-only a relatively low filler loading can be tolerated, then a filler with high oil absorption may usefully be chosen to increase the viscosity to the required level. However, it is more common to find examples where the upper viscosity limit is reached before the limiting effect on physical properties. In such cases, low-absorption fillers will effectively mean that higher loadings can be achieved, with corresponding reductions in overall formulation cost. (f) Dispersion Characteristics Whatever the type of filler used, it is important to ensure good dispersion, and some fillers disperse more easily than others. It usually follows that fine particle size fillers are more difficult to disperse than coarse ones. The presence of stearate surface treatments prevents very fine particles from agglomerating. Where filler loadings are low, less than 20 phr, say, the main 'dry' components, polymer and filler, should be mixed together before the addition of plasticiser, to minimise the risk of agglomeration. With higher filler loadings, it is better to add plasticiser to polymer first; otherwise, the filler will tend to absorb plasticiser to an undesirable degree, making it unavailable for its plasticising function. (g) Cost In the majority of applications the primary reason for the addition of CaC0 3 and some other fillers is to reduce overall costs. It is of fundamental importance to consider not only the price of the filler but also its nett cost effectiveness in the formulation. A low-price filler may appear to fulfil its function adequately, but the price advantage could easily be offset by low output, extra pigment costs (for the requisite whiteness), increased plasticiser levels, and poor physical properties and surface finish of the product.
232
I. D. Hounsham and W. V. Titow
Therefore, in choosing a filler, its cost should be very carefully considered in relation to its effective productivity. The cheapest fillers are the untreated ground coarse whitings and ground limestone. Large quantities are used in low-cost flooring compositions but they are not normally recommended for 'critical' applications (i.e. those in which physical properties of the product are important). Their extra plasticiser demand, poorer colour, and poorer dispersibility will cause more plasticiser and pigment to be required, with longer or harder mixing conditions, and consequent increase in the overall compound cost. A coated filler will disperse more rapidly and completely than an uncoated grade and can give up to 50% higher extrusion output, as well as allowing reductions in lubricant content. Fine fillers can be used at a higher loading for the same physical property level as given by coarser fillers at lower loading, so that where special specifications have to be met, the higher loading of a fine filler will help to reduce the cost further. This is frequently the case with whitings in cable compounds, where a fine-grade filler costs only marginally more than a coarser grade, but can be used at much higher loadings, giving greater savings on polymer and plasticiser. When considering the savings of using a filler, the cost effectiveness must be related to the volume cost (see Chapter 4). The relative densities (SG) of mineral fillers are considerably higher than those of the resin/plasticiser mix, and their cost per unit volume is the product of unit weight cost and density in each case. Thus, for example, for calcium carbonate, with a relative density of 2·7, the cost of 1 ml will be 2·7 times the cost of 1 g; for barium sulphate the factor is about 4·5, and so on (see Table 8.2). Figure 8.3 shows the effect of increasing calcium carbonate loading on the specific gravity of a plasticised PVC composition. 8.3.4 Applications, and Effects of Filler Loading
The main applications of CaC0 3 fillers in PVC are summarised in Table 8.4. The following points may also be mentioned by way of amplification. (a) Flooring This is a big usage area for fillers in PVC. Calcium carbonate fillers are incorporated in flooring compositions not merely to save cost, but also because substantial filler loadings increase weight, reduce shrinkage, and improve impact indentation resistance.
233
8 Fillers in PVC 2·00
1·80
1·60
?:
'>II
tO>
1·20
1·00L.-_ _----1 o 100
----1
200
--J~_
300
CaC0 3 • phr
Fig. 8.3 Specific gravity of a plasticised PVC compound as a function of CaC03 filler concentration. Note: As has been mentioned, chrysotile asbestos is widely used as a filler in calendered flooring (sheet and tiles). This application is also referred to in Section 8.4.1 below.
In backed flooring, made by spreading a plastisol on to asbestos paper, or fabric, the correct choice of CaC03 filler type is important. Crystalline types (calcite) are widely used; finely ground limestones may also be employed. Filler loadings commonly range between 200 and 500 phr, subject to considerations of plastisol viscosity and rheology generally. Ploss-Staufer AG have found that, with compositions incorporating their special ground calcite grade (Calibrite) , maximum output could be achieved at high spreading speeds with . loadings in excess of 400 phr. Figure 8.4 illustrates some of their findings* concerning the effect of the filler loading on the plastisol viscosity at different shear rates: as can be seen, in that particular *Data available to I. D. Hounsham via Croxton and Garry Ltd.
Ground whiting, s Ground calcite
Precipitated, s
General
Dry blends
(a) Flexible:
2. Extrusion compounds 10
10 <4-
<1
10
3
3
200
30
Ground calcite (some grades of ground limestone also suitable)
2
10-20
3-5
Ground calcite (s)
Nominal maximum
60-75
Mean
Typical particle size (,mt)
Relevant filler
13-15
Ground calcite
Type"
Rotational moulding; dip- Ground calcite (s) coating
Spread-coated flooring (base coals)
1. Plastiso/s Fabric coatings; foams
Application
Good dispersion, low equipment abrasion, good flow, relatively low plasticiser demand
Fine particle size gives good surface finish and physical properties
Filler fineness and crystalline structure (and coating if present) give, respectively freedom from settlement and relatively moderate thickening
Good physical properties are not normally critical in this application, hence relatively coarse filler can be used for high loading and fast spreading rates
The finer filler gives good surface finish; obviates possible settlement problems
Low plasticiser demand, good rheological properties (for fast spreading)
Filler features and advantages/ Remarks
TABLE 8.4 Applications of CaC03 Fillers in PVC
Day/Cal;c Hakuenka b
Britomya BSHb Millicarbb
Millicarb;b Omya D2 b
Omya D40b
Millicarb;b Omya BLH b
Calibrite b
Examples of commercial products
~
C
:-::: :::J
~
s:> ~ s:> ;:, l:l..
~ I':
i:l;:>0-
~
~
~
V>
tv
General
(b) Rigid: Pipe fittings
Shoe-sole compounds
General
(a) Flexible: 10 } These fine-particle grades give good balance of physical properties, ease of 10 dispersion and processability
Somewhat coarser particles acceptable here
Small particle size and good dispersion are particularly important (for good physical properties and processability, respectively)
Good physical properties and fast output promoted by filler fineness (fastest production with coated grades). Special 'electrical' grades available
Good dispersion and physical properties
Generally as for extruded soil pipe and rain-water goods
Generally similar to those for rigid extrusion compounds, but fine particle size (especially low maximum size) even more important (because of need to ensure good physical properties of compounds based on polymer of lower K value)
Finest particle grades required for good wear properties, resistance to flex cracking, cut growth and other effects of damage in service
2
Ground whiting, s
Ground calcite Ground whiting (s) Precipitated, s
3
Ground calcite
10
2
Ground whiting, s
Soil pipe, rain-water goods
3. Injection-moulding compounds
About 7 5
<5
10
1-2
Up to 2
7-10
Precipitated, s Ground whiting, s
Ground whiting (s)
1-3
Pipe, cladding, profiles
(b) Rigid:
Cable sheathing and insulation
Ground calcite
Britomya BSHb Hydrocarb b Omyalite b
Millicarb b
Britomya BSH b
Super-Pflex 200d Omyalite 95Tb
Britomya M or BSH b
Millicarb;b Hydrocarb b
N W VI
~
~
So
<:::
~
::!1
00
-
2-3
1-2
Ground whiting (s)
Ground whiting, s
Thick sheeting
(b) Rigid sheeting
Good dispersion, physical properties, surface finish, and low plasticiser demand are the main requirements to be met
5-10
10-100
Fine grades (coated for good dispersion) required, to give good physical properties. Finest particle size for high impact strength sheeting
Physical properties usually less critical, hence less refined filler may be used. Coarse ground calcite may be included in some compositions to counteract blister formation
b
= Stearate or other suitable surface treatment (desirable but optional if shown in brackets). Croxton and Garry Ltd, UK. C Harrison Enterprises, USA. d Pfizer Inc., USA.
as
3 <5 2
Ground calcite (s) Ground limestone (s) Ground whiting, s
Thin sheeting 10 10 10
Millicarb b
:--
Britomya BSH;b Omyalite 95Tb
Morden R;b Britomya V/L b Britomya BSH b
Pfinyl402 d Britomya BSH b
;;;
15
:::'l
~
~
~
s::. ~ s::. ;:s
;:>-
'"~
~
Nominal maximum
Examples of commercial products
~
Mean
Typical particle size (JJ11I)
Filler features and advantagesl Remarks
(a) Flexible:
Type"
Relevant filler
N
w
0\
4. Calendering compounds
Application
TABLE 8.4-<:ontd.
237
8 Fillers in PVC 3
2
104 ll.
u
~ .~
~
9 8
'CIl
300
7
6 5
<:3"
10L.
4
u
5
3
500
2
1031......-~=-=-
300
~::--
-='="=--__--=-=~
400 500 Calibrite loading, phr
600
Fig. 8.4 Plastisol viscosity as a function of filler loading and shear rate. system, viscosity decreased with increasing shear rate at all loading levels; it also increased with loading levels for each individual shear rate up to about 420 phr filler, but then dropped (with increasing sharpness) at higher loadings and shear rates. The results suggest that it should be possible to increase substantially the loading level without a corresponding rise in viscosity if the shear rate (in practice the spreading speed) is suitably increased. In actual manufacturing operations the optimum loadings for maximum output should be established in production trials. (b) Plasticised Compounds Fine ground whitings and calcites are the most suitable CaC03 fillers for extrusion compounds. Filler loadings in the range 20-100 phr are normal. Some typical effects of increasing filler loading are shown in Table 8.5. As can be seen the effect of a fine whiting (in loadings up to 40 phr) on some physical characteristics of the compound is not very great. Table 8.6 shows the effects of increasing loading level of a coated whiting (Omya BSH). The coated whitings have good dispersion characteristics and improve the processability of the compound. They suffer little agglomeration and, being hydrophobic, resist moisture pick-up in storage. The coating also improves the free-flowing characteristics of the filler, making it particularly suitable
PVC resin Plasticiser Stabiliser: BalCd Lubricant: stearic acid Filler
o
100 phr 55 phr 2phr 0·5 phr 0-40 phr
-50
1·234
2585 325 79
20 2500 325 81 1·312 -35
-40
10
2605 335 80 1·271
30 315 82 1·350 -30
2465
305 83 1·384 -30
40 2410
Q
QPVC resin Plasticiser: DOP Stabiliser: dibasic lead phthalate Lubricant: dibasic lead stearate Filler
Filler (phr) Tensile strength (lbf in-2) Modulus at 100% elongation (lbf in-2) Elongation at break (%) Tear. resistance (lb in-I) Brittle temperature caq 1985 210 81 -19
2225 200 81 -20 100 phr 50phr 3phr 1 phr 0-150phr
10 2500
0 2700 1970 215 80 -19
12 2475 1965 210 78 -18
15 2425
1925 210 76 -18
20 2425
1850 200 75 -17
35 2225
1775 185 67 -14
50 2035
TABLE 8.6 Some Effects of Increased Loadings of a Coated Whiting ('Omya BSH') on a Plasticised Compound
Q
Filler (phr) Tensile strength (lbf in -2) Elongation at break (%) Hardness (Shore A) Relative density Brittle temperature (0C)
TABLE 8.5 Some Effects of Filler (3 Jim Whiting) Loading on Plasticised Compound
Q
1950 125 47 -5
150 1925
'"
C>
~
:0::::
~
:'":l '";:s"'-
~ ;,-
l::
~
~
~
~
00
239
8 Fillers in PVC
TABLE 8.7
Effect of Surface-treated Whiting on Some Properties of a Cable Compound PVC resin DOP Lead stabiliser Lubricant Uncoated whiting Coated whiting
100
Tensile strength (kg cm- z) Elongation at break (%) Brittle temperature ("C)
190 200
-17
Volume resistivity (0 cm) at 20°C
1·7xl014
Relative density
45
3 1
15
o 1·33
100 43·3 3
o o
15
195
210 -18 1·33
for bulk-handling systems. In cable compounds, especially insulation types, coated whiting offers some advantages over corresponding uncoated grades, even at relatively low loading. Table 8.7 illustrates the gain in volume resistivity: the small reduction in plasticiser content, made to preserve the same level of mechanical properties for closer comparison, demonstrates the lower plasticiser demand of the coated filler. (c) Rigid PVC Fillers are not normally included in rigid compositions to reduce raw material cost: their incorporation is usually aimed at an improvement in processing, and/or physical properties of the end product. It is known that such advantages do result at comparatively low loadings of CaC03 fillers that are suitable for rigid applications, i.e. surfacetreated precipitated calcium carbonates and the ultrafine ground, surface-treated natural chalk whitings. When selecting the latter, attention should be paid to the percentage of particles coarser than about 2 ,um: even a relatively small amount of comparatively coarse particles can have a considerable detrimental effect on the mechanical properties of the compound. The presence of a fine CaC0 3 filler at a relatively low loading increases the internal friction of the melt during compounding and hence the mixing shear: this improves the dispersion of the other additives (stabilisers, pigments, lubricants, etc.). Maximum dispersion
240
I. D. Hounsham and W. V. Titow
is important, to secure the maximum degree of stability, and optimum mechanical, and other, properties in the finished product. With precipitated calcium carbonates the dispersion characteristics (of the filler itself, and in the sense of its effect on other additives present) are very good at relatively low loadings; but as the loadings increase, say beyond 10 phr, the internal shear in the melt increases excessively due to the thickening effect of these very fine-particled materials; the production rate is slowed down and the heat stability is adversely affected. The effects are much less drastic with the so-called ultrafine ground whitings which-despite the name-are relatively coarser than the precipitated CaC03 grades. Because of this, even at loadings substantially in excess of 10 phr, the number of particles present in the melt, and the total particle surface, are much less, and hence the rise in internal shear remains within acceptable limits. Modern compounding machinery makes it possible to produce rigid compounds (e.g. for extrusion) containing up to 100 phr of such fillers. Whilst at such loading levels the use of the filler in a rigid compound can bring significant cost saving, the increased density, and the possible reduction of some physical properties (especially impact strength) must be considered. In any particular case the choice of type of CaC03 filler and the loading level will depend on the nature of the process and the end-use of the product: it will be made in the light of experience and/or advice from a reputable supplier of the filler. Final optimisation is a matter for laboratory and production trials. Table 8.8 shows some effects of fine CaC0 3 fillers in rigid PVC at 10 phr loading. 8.4 FUNCTIONAL FILLERS The fillers discussed in this section are those whose primary function in a PVC composition is to impart or modify a particular property or group of properties: their use normally increases material costs, and may also make processing more expensive. 8.4.1 Reinforcing Fillers Most of these are fibrous, although glass spheres (and even precipitated CaC03 when used in uPVC at low levels of loading) may be included under this heading. With the partial exception of chrysotile asbestos, none is of high commercial significance in PVC: however, several merit a mention for the sake of technological interest.
None Precipitated CaC03 Ultrafine ground, coated whiting Ground, coated whiting
Nature
TABLE 8.8
Filler
140 100 80 37
650 -
-
-
0·1 1·0 2·5 435
Elongation at break (%)
Tensile strength (kgfcm- Z)
(pm)
Mean particle size
Properties of compound
Some Effects of Fine CaC0 3 Fillers in a uPVC Compound at 10 phr Loading
11
12
7 12
Impact strength (kgf cm cm- Z)
~ .....
(j
-.:::
'"tl
s·
~
~
::J
00
242
I. D. Hounsham and W. V. Titow
(a) Asbestos (Chrysotile) Fibres The applications of chrysotile asbestos as a reinforcing filler for PVC have been mentioned in Section 8.2.1. It may be noted additionally that, as in the case of most reinforcing fillers, good interfacial contact (with the right degree of adhesion) between the fibre and the polymer matrix improves the initial dispersion and the reinforcing effect. Some surface treatments (notably with a polyethylene glycol) applied to the asbestos fibre were found 15 to improve the impact strength and flexural modulus of the filled PVC compound. The practical and commercial repercussions of the concern about health hazards which may be associated with the handling of asbestos in some processes and products,1,4 have made themselves felt on both sides of the Atlantic; inter alia, preoccupation is continuing with relevant safety measures and regulations. However, whilst in Western Europe and the UK emphasis is still on safer working with asbestos in its established applications, in the USA considerable attention is focussed on alternative materials and arrangements. In a non-reinforcing application of chrysotile in PVC (as a thixotropic additive for plastisols) operational safety can be improved by using the material in the form of wet 'crumb', produced by wetting-out the fibre with a plasticiser: in a commercial material of this kind (Sylodex-W. R. Grace UK Ltd), DIBP is the wetting liquid (two parts to one of asbestos).4 (b) Inorganic Microfibres Those of the materials in this category which are of some technical interest as reinforcements in PVC are all of American origin. They are:
(i) hydrated sodium/aluminium fibre (Dawsonite-ALCOA*); (ii) potassium titanate fibre (Otsuka Chemical Co. Ltd); (iii) 'Processed Mineral Fiber' (PMF-Jim Walter Resources Inc.), produced from blast furnace slag. Until 1974 a potassium titanate fibre was also available from the Du Pont organisation, under the trade name Fybex, and Fybex-reinforced rigid PVC compounds were marketed by commercial suppliers. 1 Dawsonite, in addition to its reinforcing effect, has some flameretardant and smoke-suppressant action (see Chapter 11, Section 11.5): its commercial progress has been retarded by the uncertainties * Aluminum Company of America, Pittsburgh, PA.
8 Fillers in PVC
243
of the asbestos situation in the USA. The development of PMF appears to have suffered relatively less from this factor. Some properties of Dawsonite, Fybex and chrysotile fibres are compared in Table 8.9. The effects of Dawsonite on some properties of rigid PVC are illustrated by the data of Table 8.10. TABLE 8.9 Some Properties of 'Dawsonite', 'Fybex' and Chrysotile Asbestos Relevant to their Use as Fillers4
Fibre length O (,urn) Fibre diarneterO(,um) Density (g cm-3) Refractive index Surface areab (m 2 g-l)
Dawsonite
Fybex
Chrysotile
15-20 0·4-0·6 2·44 1·53 15-17
4-7 0·1-0·16 3·20 2·35 7-10
1000-40000 0·01-1·0 2·55 1·50-1·55 3-4
° Typical dimensions of potassium titanate fibres from Otsuka Chemical Co. (the
only current source since supply of Fybex was discontinued in 1974) are: 16 length 20-30,um, average diameter 0·2,um. These fibres are claimed to have a higher heat resistance than Fybex. The dimensions of PMF (slag) fibre (cf. Section 8.4.1) are quoted!3 as: diameter 4-5 ,urn, aspect ratio 40-60. b Surface area measurement by the BET N2 method.
(c) Glass Fibres The incorporation of glass fibres in a rigid PVC compound can substantially upgrade several 'short-term' mechanical properties, as well as resistance to creep and fatigue. In PVC (and in other thermoplastics) the degree of improvement (which can be substantialsee Table 8.11) depends on the orientation of the fibres in relation to the direction of stress and-given that-also on the fibre length and the amount of fibre present. The temperature of deflection under load of a PVC composition for which the value of this property is normally low, may be substantially increased by glass-fibre reinforcement (see Table 8.11). However, the highest values that may be attained through such reinforcement are only marginally above the top figure for the general range of unreinforced uPVC compositions (see Appendix 3). In this general sense, therefore, glass-fibre reinforcement cannot be said to have a significant effect upon the deflection temperature. uPVC
21-0 (1120'3) 5647 (38·9) 0·41 (2826) 11760 (81'0) 0·75 (5170)
1()6lbf in- 2 (MN m- 2)
ASTMD790
Flexural modulus
3·39 (180'8)
6·4
153 (68)
ft Ibf in-I (J m- 1) Ibfin- 2 (MN m- 2) 1()6lbfin- 2 (MNm- 2 ) Ibfin- 2 (MN m- 2)
(J m- I)
10- 5 x °C- 1
ASTM D 696 in- 1
OF
ASTMD648
COq
ASTM D 256(E) ASTMD638 ASTMD638 ASTM D 790
4·7
0
ft Ibf
gper 10 min
Units
ASTM D 256(A)
ASTM D 1238-73F
Method of determination
Melt flow index Deflection temperature under load at 264 Ibf in- 2 (1,82 MN m- 2 ) Coefficient of linear thermal expansion lzod impact strength (notched) Izod impact strength (unnotched) Tensile strength Tensile modulus Flexural strength
Property
1-13 (7791)
18·1 (965·6) 5900 (40'6) 0·54 (3723) 12420 (85 ·6)
3·00 (160·0)
3·8
155 (69)
3·5
7·5
1·54 (10620)
16·5 (880· 2) 6100 (42-1) 1-43 (9859) 12900 (88,9)
2-62 (139·8)
3-l
160 (71)
3·2
15
Dawsonite content (% by weight)
TABLE 8.10 Some Effects of 'Dawsonite' in Rigid PVC 4
2·33 (16060)
10·4 (554,8) 6200 (42'7) 1·60 (11030) 13160 (90·7)
2·36 (125·9)
2·1
163 (73)
2·7
30
i;
~
C
:::l
:0:::
~
I'>..
.,;:.,;:~
~ ;::
~
:--
t
" Ethyl Corporation, USA.
ASTM D 648
638 638 790 256
ASTM ASTM ASTM ASTM
Tensile strength Tensile modulus Flexural modulus lin (H75mm) bar Heat distortion at 264lbf in- 2 (1,82 MN m- 2 )
D D D D
ASTM D 792
Method of determination
Specific gravity Mould shrinkage
Property
'C
in in- 1 or mmmm- 1 lbf in -2 (MN m -2) Ibfin- 2 (MNm- 2) Ibf in- 2 (MN m- 2) ft Ibfin- ' (J m- 1)
Units
86·7
0·001 14000 (96'5) 1200 000 (8273) 950 000 (6 550) 3·5 (186'71)
0·002 9000 (62·0) 650000 (4481) 600 000 (4 136) 6 (320'08)
0'003-0'004 6400 (44·1) 420000 (2 896) 375 000 (2 585) 15 (800·20) 81·7
1·53
1·45
1·40± 0·02
70·6
20% glass
1·61
30% glass
87·8
0·001 16000 (110'3) 1 300 000 (8963) 1100 000 (7584) 2 (106·69)
Reinforced (Ethyl 7042 compound)" 10% glass
Unreinforced (Ethyl 7042 compound)"
TABLE 8.11 Effect of Glass·fibre Reinforcement on Some Properties of Rigid PVC 4
~
(j
""
"1:l
S·
~
~
:::l
00
246
I. D. Hounsham and W. V. Titow
compounds reinforced with glass fibre have been available commercially for some time,! recently in special grades (e.g. from B F Goodrich) with the reinforcement coupled to the matrix. (d) Carbon Fibres If the magnitude of the reinforcing effect was the only consideration, carbon fibres would offer advantages over glass-fibre reinforcement in uPVc. However, their use in this polymer has never been an economically sensible proposition. In round figures, the cost of carbon fibre (chopped strand) is about 60 times that of PVC polymer (cf. cost factor of about 1·7 times for glass fibre). Thus the cost of base polymer in a compound containing, say, 25% carbon fibre is relatively insignificant. For applications calling for the highest performance (which would be the reason for considering carbon-fibre reinforcement in the first place) a base polymer can be afforded with inherent stability and 'engineering' properties better than those of PVC (e.g. nylon, or polycarbonate). (e) Glass Spheres Neither hollow nor solid glass spheres are used to any significant extent in PVC compositions. The former kind cannot, in any case, be regarded as a reinforcing filler in the proper sense. Purely from the technical standpoint, the main effects of incorporating solid glass spheres as a filler in uPVC would be broadly similar to those in other thermoplastics, viz. increased compressive strength, modulus, hardness and abrasion resistance, reduced creep and shrinkage: the reinforcing effects would be isotropic, as with the spherical filler orientation cannot be a factor. In processing, the effect upon the viscosity of a molten thermoplastic is normally less with glass spheres than with comparable volumes of glass fibres or even inorganic fillers of irregular particle shape.
(f) Fine-particle Calcium Carbonate Reference has already been made to the reinforcing effects of this type of filler in rigid PVC compositions, at relatively low loading levels (see Section 8.3.4). Incorporation of precipitated CaC03 and some ultra-fine ground whitings (especially when the particles have received an organotitanate or stearate treatment) can improve the impact strength and flexural modulus of uPVC compositions and products, notably pipe and extruded profile.
247
8 Fillers in PVC
8.4.2 Flame-retardant and Smoke-suppressant FiBers The flame-retardant and smoke-suppressant effects of materials incorporated for these purposes in PVC compositions are mentioned in Chapter 12 (Section 12.10) and discussed in Chapter 11 (Section 11.5). Such additives are mainly of interest for plasticised PVC, as the flammability of rigid compositions is inherently low (although smoke suppressants can be useful in some uPVC products). Some of these additives are liquids, and thus not relevant in the context of this section. Of those which are particulate solids, the ones of greatest practical significance are antimony trioxide (flame retardant), antimony trioxide/zinc (or barium) borate mixtures (flame retardant), and alumina trihydrate (flame retardant and smoke suppressant). The first two are used in sufficiently low proportions (up to about 10 phr) not to affect drastically the mechanical and physical properties of the PVC composition: however, they do have an opacifying effect (somewhat less pronounced with the borates, and generally reduced when ultra-fine particle size grades of the additives are employed). The same applies to some particulate smoke suppressants (e.g. molybdenum trioxide, Mo0 3), with the notable exception of magnesium carbonate which is used at relatively high loading levels (typically up to about 40 phr) for maximum smoke suppression effects. Substantial loading with alumina trihydrate is also necessary to realise the effects of this additive to the full (see Chapter 11, Section 11.5), and at the levels used the effect on the mechanical and certain other properties of the PVC composition can be considerable (see Figs 8.5 and 8.6), as is 25
250
IV
ll.
~~20
....r:.g'
200 •
i"
~
1ii 15 ~
'iii
c:
~ 10
o
20
40
AI (OH)3
60
80
content, phr
Fig. 8.5 Tensile strength (1) and elongation at break (2) as functions of alumina trihydrate content in a pPVC composition (50 phr plasticiser). Based on data from Ref. 7.
248
I. D. Hounsham and W. V. Titow
100
.u...
90
'a.5 80 Ol
c
'c
...'0 ~
en
Fig. 8.6 Vicat softening point (1) and hardness (2) as functions of alumina trihydrate content in a pPVC composition (50 phr plasticiser). Based on data from Ref. 7. indeed the case with other particulate fillers (ct. for example, Figs 8.1-8.3). Apart from its flame-retardant and smoke-suppressant action, which can be very effective at suitable loadings (say, broadly, between 30 and 100 phr), alumina trihydrate can be especially useful in certain compositions in that it does not impair the electrical properties (tracking resistance in particular can actually be improved) and its opacifying effect is comparatively slight (especially with ultra-fine particle grades). As has been mentioned in the preceding section, the fibrous mineral filler Dawsonite has some frame-retardant and smoke-suppressant action in PVC compositions in which it also acts as reinforcement. 8.4.3 Miscellaneous Functional Fillers
(a) Carbon Black There are various kinds and grades of carbon black available for use in a variety of application areas,17 of which incorporation in rubber and plastics is one. As a constituent of plastics compositions carbon black can have three main functions. It may be incorporated to impart black colour, or to improve resistance to photochemical and thermal
8
Fillers in PVC
249
degradation, or to reduce electrical resistivity (in order to counteract the build-up of static electricity on the surface of the plastic, or to render the whole composition conductive). In the first two of these three applications the amounts added are relatively low (commonly a few phr), and the additive may be considered to function as, respectively, a pigment (see also Chapter 11, Section 11.3) and a kind of stabiliser. The stabilising effect is particularly widely utilised in polyolefins, but significant also in some PVC compositions for outdoor use (e.g. sheeting employed as anti-seepage lining for reservoirs, ponds and canals): here too the fine, particulate carbon absorbs preferentially UV radiation of all the wavelengths normally instrumental in photodegradation of PVC polymer. It has also been claimed l7 . 18 that the carbon, in fine, intimate dispersion in a plastics composition acts as a free-radical terminator, this being the main mechanism whereby its protective effect against thermal degradation is exercised. A mediumcolour furnace black is the type of interest for the stabilising applications. The carbon black content in cases where the electrical conductivity of this filler is utilised to reduce the surface and volume resistivity of a plastic has to be relatively high, as the required effects depend upon securing enough inter-particle contact to provide-in sufficient measure---eonductive paths in the bulk and surface of the material. For a reasonable degree of conductivity (say a volume resistivity reduction down to about 1 Q cm) a carbon black loading of about 30% by volume or over may be necessary. At such loadings the effect on some mechanical properties may be similar to that observed with other fine-particle fillers at comparable content levels (it may be noted in passing that the kind of reinforcing effect that carbon black exerts in natural rubber compositions does not arise in PVC). Improvements in processability and conductivity of PVC compounds filled with carbon black have been reported to result when a vinyl chloride polymer was grafted onto the filler particle surface. 19 (b) Metal Powders A metal powder may be used as an antistatic or conductivitypromoting filler in a plastics composition (in PVC the possible effect on thermal stability is an additional practical consideration in selecting the metal filler). The application, and mechanism of operation, are similar to those just mentioned in connection with the use of carbon black, and-when ordinary compounding methods are employed-similar volume loadings of filler are required for comparable effect levels.
250
I. D. Hounsham and W. V. Titow
However, it has also been reported 20 that in compositions produced by sintering suitable mixtures of graded metal powders with PVC (also in powder form), conductive paths can be formed at relatively much lower metal loadings: for example, a volume resistivity of 10 Q cm was recorded 2o for a composition containing about 6% of nickel by volume; this kind of composition with 5-8% Ni was also found to be rendered stronger and more resistant to cracking by the presence of the filler. (c) Wood Flour Although the incorporation of wood flour in PVC compounds can result in material cost savings, this filler is not merely a cheapening extender but has a functional role in that it imparts a resemblance to wood to the filled compositions which are used mainly in the manufacture (by extrusion) of wood-substitute products, e.g. beading (extruded profile) and sheet, for use in trim and light constructional applications. Both the products (some of which are available in cellular, structural-foam form with densities down to about 0·9 g cm-3) and the compounds for their manufacture are available from commercial sources. 4 The wood flour content of the materials can be substantial (several tens phr) and, as would be expected, this can reduce some mechanical properties (in particular extensibility may be considerably lower) vis-a-vis similar unfilled compositions. However, by and large, the effects are not unduly drastic, as indicated, for example, by some values quoted 21 as typical for a well-known commercial compound (Nordxyl-Nordchem SpA, Martignacco, Italy): tensile strength at yield, 350 kg cm- 2 (34 MPa) , at break, 390 kg cm- 2 (38 MPa); elongation at yield, 0; flexural strength, 657 kg cm- 2 (64 MPa). Another consequence of the presence of the wool filler in the compound is an increased tendency to take up moisture: drying before processing is good practice (several hours at 80°C is sometimes recommended). Some modifications to the heads and dies of standard extrusion equipment may be necessary for optimum processing.
(d) Starch A technically significant early use of starch in PVC compositions has been as an extractable filler (removed by hydrolysis and dissolution with dilute acid) in the production of porous rigid PVC sheet used in the manufacture of battery separators and in some filtration applications (see Chapter 25). More recently, starch-filling of plasticised and
8 Fillers in PVC
251
semi-rigid PVC compositions was investigated as a means of promoting biodegradability.22 The effects of the starch on the physical and mechanical properties of these compositions were generally similar to those of inorganic fillers at comparable loadings, but with less opacification. The starch-filled PVC was found to be readily attacked by a mixture of micro-organisms commonly occurring in soil.
(e) Synthetic Silicas These fine-particle, amorphous silica powders include colloidal (fumed) silica, silica aerogel, and wet-process silica. Because of the very small primary particle size (in some grades within the range 0·01-0·1/-lm) these materials have an extremely large specific surface, and hence the absorption-related effects they exert are strongly pronounced even at low levels of loading (in some cases less than 1 phr). Thus some are extremely effective thickening agents for PVC pastes (see Chapter 21), improve free flow of dry blends, counteract plate-out of calendering compositions (see Chapters 9 and 18), reduce surface gloss of (Le. act as matting agent in) coatings and films, and also function as anti-blocking agent in plasticised products of this kind (Le. counteract sticking together of their surfaces, especially under pressure-e.g. in a stack).
8.5 SOME FILLER SUPPLIERS AND TRADE NAMES The list in Table 8.12 is confined to fillers available from UK sources (including some imported materials). It is neither complete nor deliberately selective, but intended to identify a few materials often referred to by their trade names. The following publications may be consulted for information on suppliers of fillers in the USA (a), (b) and (c) and Europe (d). (a) Latest edition of Modern Plastics Encyclopedia, McGraw-Hill, New York. (b) List (relevant also to additives other than fillers) published in Plastics Engineering, 33(5), 22-40 (May 1977). (c) Plastics Technology (1980), Vol. 26, No.1, Manufacturing Handbook and Buyer's Guide 1980/81. (d) Latest edition of European Plastics Buyer's Guide, IPC Business Press Ltd, London.
252
I. D. Hounsham and W. V. Titow
TABLE 8.12 Some Commercial Sources and Trade Names of Fillers
Supplier Limestone
Trade name
Ben Bennett Jr Ltd Derbyshire (Div Tarmac) Ltd Longcliffe Quarries Ltd Tilcon Gregory Ltd
Bennite Calmote Superlon
Blue Circle Industries Ltd Croxton and Garry Ltd ECC International Ltd Microfine Minerals & Chemicals Ltd
Snowcal Britomya Polycarb Microcarb
Croxton and Garry Ltd
Omya Calibrite Millicarb Omyalite Fordacal
Whiting
Calcites and Dolomites
ECC International Ltd Microfine Minerals and Chemicals Ltd Norwegian Talc (UK) Ltd Tilcon Gregory Ltd
Precipitated Calcium Carbonate
Croxton and Garry Ltd ICI Mond Division Ltd John and E. Sturge Ltd
Clays
Croxton and Garry Ltd ECC International Ltd
Alumina Trihydrate Alcoa of Great Britain Ltd B. A. Chemicals Ltd Croxton and Garry Ltd Barytes Richard Baker Harrison Ltd Vine Chemicals Ltd Zach Cartwright Ltd Tilcon Gregory Ltd Synthetic Silicas Joseph Crosfield and Sons Ltd Degussa (UK) Ltd W. R. Grace UK Ltd
Microdol Hakuenka Winnofil Calofort Calofil Sturcal Calopake Burgess MlOO, M501 Hydral BacoFRF Trihyde
Gasil Aerosil Syloid
8 Fillers in PVC
253
TABLE 8.12-contd.
Supplier Antimony Trioxide Anzon Asbestos A. A. Brazier & Company (Asbestos) Ltd Cape Asbestos Fibres Ltd Central Asbestos Company Ltd Johns Manville (GB) Ltd Marley Tile Company Ltd Turners Asbestos Fibres Ltd Cyprus Asbestos Mine Ltd Henry Kiver & Partners Ltd
Trade name Timonox
A list of glass fibre producers is given in Ref. 1, pp. 79-82. Transport cost is a factor in filler price, and it can be useful in many cases to consider the possibility of adapting a PVC formulation to make the best use of materials from local sources. At the same time it should be remembered that differences can and do occur in the performance of apparently similar fillers from different manufacturers.
REFERENCES 1. Titow, W. V. and Lanham, B. J. (1975). Reinforced Thermoplastics, Applied Science Publishers, London. 2. Fletcher, W. J. and Tully, P. R. (1967). 23rd ANTEC SPE Proceedings, p. 537. 3. Moreland, J. E. (Oct. 1971). In Modern Plastics Encyclopedia, Vol. 48, No. lOA, McGraw-Hill, New York, p. 247. 4. Titow, W. V. (1977). In Developments in PVC Production and Processing-l, (Eds A. Whelan and J. L. Craft), Applied Science Publishers, London, Chapter 4. 5. Private communication from TBA Industrial Products, Reinforced Plastics Division, Rochdale, Lancashire, England. 6. Blumberg, J. G., Falcone, J. S., Smiley, L. H. and Netting, D. I. (1980). In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edn, Vol. 10, John Wiley, New York, pp. 198-215. 7. Plastichem Ltd, Technical Bulletins and data sheets. 8. Rogan, J. (1979). Plast. Techno!., 25(8), 100.
254
1. D. Hounsham and W. V. Titow
9. 'Whiting: Notes on Origin, Manufacture, Properties and Uses', Research Council of the British Whiting Federation, April 1962. 10. Lowry, T. M. (1946). Inorganic Chemistry, Macmillan & Co., London, 2nd Edn, pp. 732-3. 11. 'Britomya BSH 30 and Britomya M', Melbourn Whiting Company Ltd. Data sheets 5.77. 12. Monte, S. J., Sugerman, G. and Seeman, D. J. (1976). 34th ANTEC SPE Proceedings, pp. 27-39. 13. Naitove, M. H. and Evans, L. (1976). Plast. Technol., 22(8). 71-4. 14. Fillers for PVC-A Guide to Selection', Melbourn Chemicals Ltd, Technical Bulletin 353/65, September 1977. 15. Axelson, J. W. and Kietzman, J. H. (1976). 34th ANTEC SPE Proceedings, pp. 601-5. 16. Anon. (1980). Plast. Technol., 26(3), 38. 17. Dannenberg, E. M. (1978). 'Carbon Black'. In Kirk-Othmer Encyclopedia or Chemical Technology, 3rd Edn, Vol. 4, John Wiley, New York, pp. 631-66. 18. Mascia, L. (1974). The Role of Additives in Plastics, Edward Arnold, London. 19. Anon, (1979). Plast. Techno/. , 25(7), 95. 20. Kusy, R. P. and Turner, D. T. (1973). SPE J., 29(7), 56-9. 21. Anon. (1976). Mod. Plast. Int., 6(10), 14 and 15. 22. Westhoff, R. P., Otey, F. H., Mehltretter, C. L. and Russell, C. R. (1974). Ind. Eng. Chem., Prod. Res. Develop., 13(2), 123-5.
GENERAL BIBLIOGRAPHY ON FILLERS Wake, W. C. (1971). Fillers for Plastics, Plastics Institute Monograph, Butterworth, London. Ritchie, P. D., Critchley, S. W. and Hill, A. (Eds) (1972). Plasticisers, Stabilisers and Fillers, Plastics Institute Monograph, Iliffe Books, London. Mascia, L. (1974). The Role of Additives in Plastics, Edward Arnold, London. Katz, H. S. and Milewski, J. V. (1978). Handbook of Fillers and Reinforcements for Plastics, Van Nostrand-Reinhold, New York.
CHAPTER 9
Stabilisers: General Aspects w.
V.
TITOW
9.1 INTRODUCTION PVC polymers and copolymers are susceptible to degradation by heat (the thermal degradation is sometimes referred to as 'thermolysis') and by light (photolysis, also called photodegradation, and-in some contexts-photochemical degradation): in both cases degradation is rapid and more severe in the presence of oxygen. In the practical context photochemical effects are of special concern in outdoor exposure of PVC materials, where they may be accompanied by those of other factors instrumental in weathering (see Chapter 12, Section 12.6) so that the overall deterioriation can be faster and more drastic than that caused by exposure to light alone. Heat stabilisers are incorporated in all PVC compositions to protect the polymer against thermal degradation at the high temperatures of processing (higher for uPVC, and generally ranging-depending on composition and process-from about 170°C to about 220°C) and also subsequently in service. UV-absorbing and antioxidant additives are included where appropriate as stabilisers against photochemical degradation. Many heat stabilisers have some antioxidant action. An antioxidant is also often incorporated as a component in composite commercial heat stabiliser systems. Formulation components other than the base polymer may also suffer degradation: e.g. chlorinated polyethylene (by dehydrochlorination and oxidation on excessive heating!), rubbery impact modifiers (by disruption and oxidation-e.g. on weathering-of the double bonds their molecules contain), and chlorinated paraffin 255
W. V. Titow
256
extenders (which can undergo dehydrochlorination with further breakdown and formation of unsaturated compounds on strong heating2). Like the PVC polymer, some of these additives can benefit from the presence of stabilisers in the composition. 9.2 DEGRADATION OF PVC POLYMER The degradation of polyvinyl chloride (and some copolymers), and the ways in which various stabilisers counteract and modify the process, have been widely studied for many years. The subject is complex and much still remains to be elucidated and verified, inter alia, in regard to the chemical and morphological effects of photochemical attack (and weathering generally), and to the mode of action of stabilisers, especially in systems involving some synergistic effects. However, at least the basic features of degradation and stabilisation are now fairly well understood, and there is reasonable general agreement as to the principal underlying mechanisms. The extensive literature of the subject includes useful reviews by Voigt,3 Silberman,4 Thinius,5 Onozuka and Asahina,6 the staff of Ciba-Geigy,7 Braun8 and Nass. 9 9.2.1 Thermal Degradation
The main outward manifestations of thermal degradation of PVC (at temperatures sensibly below those of pyrolytic decomposition and combustion, against which no stabilisation is possible) are the evolution of hydrogen chloride, development of colour (progressing with the extent of breakdown from light yellow, through reddish brown, to almost black in severe cases) and deterioration of physical, chemical and electrical properties. It is widely accepted that dehydrochlorination involves progressive 'unzipping' of neighbouring chlorine and hydrogen atoms along the polymer chain (although the actual mechanism of this process is still in some doubt*): a double bond is formed between the carbons to which the two atoms were originally attached-this constitutes an allyl chloride structure with (i.e. is in the 3,4 position in relation to) the next CI down the chain, which is thereby strongly activated (cf. Chapter 1, * An ionic mechanism is favoured by some investigators/,8 and a free-radical one by others. lO ,lI
257
9 Stabilisers: General Aspects
Section 1.5.1, and Ref. 31 in that chapter): HHHHHHHHHH
I
I
I
I
I
I
I
I
I
I
~C-c=c-e-e-e-e-e-e-e~ I @) (j)~1 I I I I I I
H
lenergy
HHHHHHHHHH
I
I
I
I
I
I
I
I
I
I
~C-C=C-;:~::;-CI-CI-el-e-C~ 'Cl-H+' I HI ~. ~CI H CI HI Cl
(1)
1energy HHHHHHHHHH
~~-W-W-W-t-t-t~ I I I I ~-----_.-:
H HCl
L~l~_!t: CI H CI
and so on. The development of colour is attributed to the conjugated double bond systems formed in this process* (d. reaction scheme (1)), and also, by some investigators,7 to the formation of strongly coloured carbonium salt complexes (possible with an ionic mechanism of dehydrochlorination) .
HH{H HfH H
I I I I -C=C C=C
H
"c+iI
H ~ry H11
Cl-
(2)
H
~+-lL--¥i* Colour appears with as few as seven conjugated double bonds in a polyene, whilst it is known 7 that sequences of up to about 30 can arise in the course of dehydrochlorination of PVc.
258
w.
V. Titow
It has been suggested 7 that the mesomeric structure indicated in eqn (2) accounts for the stability of these salts, which should also increase with the length of the sequences involved. Theoretical considerations, and the study of model compounds,7,8 indicate that-in the absence of sites of reduced stability in the chain-PVC polymer should be stable enough to resist dehydrochlorination even at the high processing temperatures. The question of which are the sites where dehydrochlorination is first initiated has been receiving much attention over the years. In the light of all the accumulated evidence it appears most likely that the process starts with a chlorine atom activated by an adjacent allylic bond configuration where that is already present in mid-chain (d. reaction scheme (1)); a chlorine in the same position relative to an allyl end-group would also be activated, albeit to a lesser extent. 8 Both mid-chain unsaturation and allyl end-groups exist in PVC polymers (see Chapter 1, Section 1.5.1). Note: That end-groups play a part in PVC stability is indicated by the fact that---other factors being equal-the heat stability increases with increasing molecular weight (K value) of the polymer, i.e. with decreasing proportion of end-groups. However, there is also considerable evidence that double bonds within the chains activate the adjacent CI atoms in the allyl chloride configuration more strongly, and hence reduce the thermal stability more markedly, than do those in allyl end-groups.
A labile 'tertiary chlorine' (i.e. a CI atom attached to a tertiary carbon in the chain), such as would be present at one possible type of junction between a branch and the main chain, is another likely starting point-the CI in this position would be activated roughly to the same extent as one adjacent to an allyl end-group.8 However, the likelihood that a tertiary chlorine is the principal type of active site in dehydrochlorination must be viewed in the light of the scarcity of evidence for the existence in PVC polymers of the type of junction which would involve its presence, or for any correlation between the number of chain branches on one hand and thermal stability on the other (see Chapter 1, Section 1.5.1). Dehydrochlorination can occur at only moderately elevated temperatures (about 100°C). It is catalysed by the HCI evolved (autocatalysis), and can also be promoted or initiated by other strong acids.
9 Stabilisers: General Aspects
259
Mathematical treatments of the dehydrochlorination process (including calculations of reaction constants for particular conditions) have been published by Woolley, 12 and by Troitskii et al. l3 In addition to dehydrochlorination, thermal degradation of PVC polymer in the presence of oxygen also involves oxidation, with the formation of hydroperoxide, cyclic peroxide, and keto groups, some of which can provide additional active sites for initiation of dehydrochlorination. 7 Chain scission and cross-linking can also take place as degradation proceeds, both in air and in an inert atmosphere (although some investigators report no scission in nitrogen at 190°C14). These effects contribute to the general deterioration in properties. If the thermal stability of PVC polymer or composition at a given temperature is defined in terms of time required for one of the main manifestations of degradation to reach a stated level (say, a certain amount of HCI evolved, or intensity of colour developed: see Section 9.8.1 below) and if the degradation is treated as a unified process, thermally activated in the classic manner (d. Chapter 12, Section 12.3), the appropriate Arrhenius-type relationship may be written in the form:
t = to exp (E/RT)
(3)
where: t is the duration of stability; to is a constant; E is the activation energy for thermal degradation of the PVC polymer in the conditions (and/or composition) concerned; R is the ideal gas constant; and Tis the absolute temperature. . The activation energy for thermal degradation of a uPVC composition is quoted by Chauffoureaux et alY as 25·9 kcal mol-i. Rice and Adam 16 give the following values, in the same units, for PVC and a few other polYII!ers: PVC approximately 20; polystyrene 55; p )lyethylene 46; polypropylene 65. This illustrates well the comparatively low thermal stability of PVc. Susceptibility to thermal degradation varies with the process of manufacture of the PVC polymer and also-even for the same process-with the source of supply. Other things being equal, the susceptibility increases (inherent stability decreases) in the sequence:
"
mass polymer ~ suspension polymer ~ emulsion polymer This is normally attributable to the amount of impurities present, which increases in the same sequence (the emulsion polymer in
260
W. V. Titow
particular contammg traces of surface-active agents used as emulsifiers), the amount of unsaturation in the chains and the kind of end-groups (both the latter factors also differ in polymers from different sources). As has been mentioned, higher molecular weight makes for greater resistance to degradation. Homopolymers are, generally speaking, more resistant than copolymers. 17 In compounds the stability of the PVC resin can also be adversely influenced by other constituents (e.g. phosphate plasticisers; antistatic agents; some colourants-see Chapter 11). The ease of stabilisation and response to particular stabilisers in particular conditions also vary with the above features. Considered in conjunction with the requirements of processing and service in any given case the variety and possible interactions of these factors provide strong support for the often expressed view that every PVC composition should be treated as in individual stabilisation problem. 9.2.2 Photochemical Degradation All the main external manifestations associated with thermal degradation of PVC appear also as a result of photolysis (i.e. dehydrochlorination, development of colour, and deterioration of properties). In plasticised compositions, exudation of plasticisers (resulting in sticky surface layers), embrittlement and cracking can also occur in varying degrees. However, in comparison with typical thermal degradation, there is greater variability in the occurrence of these effects (for example, more often than not colour development is slight or absent altogether) as well as in their onset, progress and respective intensities. The situation can be further complicated where-as is often the case in service~xposure to light is associated with weathering: this introduces a number of further destructive factors (see Chapter 12, Section 12.6). The dissociation energy of a carbon-ehlorine bond is about 77 kcal mol- 1 : this corresponds to the energy of light of wavelength 375 nm. 8 In theory, therefore, light of this wavelength (in the near-UV part of the spectrum) should be able to cause photodegradation of PVc. In practice the process requires higher photo energies (UV light of shorter wavelengths): it is known (cf. Chapter 12, Section 12.6) that the UV band between about 290 and 315 nm is mainly responsible for the photochemical degradation of plastics, including PVc. However, the mechanisms of this process in PVC, alone or as part of weathering,
9 Stabilisers: General Aspects
261
are less well understood than those of thermal degradation: the main features indicated by available evidence may be briefly summarised as follows. The first phase of photolysis of PVC in air appears to be photooxidation,7 proceeding through a free-radical mechanism8,18,19and at a general rate proportional to the intensity of radiation (in the appropriate UV region)-with the formation of hydroperoxide, keto, and aldehyde groupS?,9,18 which undergo further light-induced reactions and breakdown:? the presence of these hydrophilic groups is thought to playa part in the disruptive effects of atmospheric water in weathering of PVC materials. 18 Dehydrochlorination also occurs at an early stage7 ,8,20 (starting immediately, at a relatively fast rate, in an inert atmosphere8), but usually proceeds more slowly than in typical thermal degradation (although the rate, both in air and in nitrogen, is a function of the intensity of irradiation and the temperature8): conjugated double bond sequences are formed 7 ,8,18,19 (whose presence may be expected to increase UV absorption l9) but these are liable to be disrupted early in the process by reaction with oxygen 8,9,20 (probably resulting in the formation of carbonyl groups, themselves light-absorbent and thus capable of accelerating the photolysis8), which would counteract the formation of colour. Chain scission and cross-linking also take place. 9,19,20 Exudation of plasticisers on weathering of pPVC has been attributed 20 to their reduced affinity for the cross-linked structures. Prior degradation by heat (even if not very far advanced), e.g. such as may be allowed to arise by lack of strict attention to conditions in heat processing (or through inadequate stabilisation), can enhance and accelerate photodegradation: this should be borne in mind in the formulation and processing of PVC compositions for outdoor use. Impurities, and other adventitious 'additives' (e.g. residual solvents in films 8,21) can also have an effect in this direction.
9.3 IDEAL REQUIREMENTS FOR A STABILISER, AND GENERAL FACTORS AFFECTING STABILISER SELECTION
Consideration of the main features of degradation of PVC, coupled with the requirements of processing and service in various applications and conditions, points to the following actions and characteristics to be
262
W. V. Titow
looked for in an ideal stabiliser: rapid binding of free HCl, high ability to replace labile Cl atoms with stable groups, saturation of double bonds, antioxidant action, disruption of chromophoric groups, absorption of free radicals, neutralisation of impurities and degradation products, effective screening of UV radiation. It is self-evident that the stabiliser itself, and any reaction products resulting from the exercise by it of the above functions, should ideally be innocuous, non-migratory, non-toxic, odourless, and should not impair the colour, clarity or any other properties of the PVC compound. In addition, from the standpoint of processing and use, the ideal stabiliser should also: (i)
be readily dispersible in the PVC compound, and fully compatible with all its constituents even after prolonged service; (ii) have no adverse effect on processing properties; (iii) be equally effective in PVC resins of all types and from all sources; and (iv) be inexpensive and effective in small proportions. The ideal stabiliser does not exist. However, many of the available stabilisers, and particularly composite stabiliser systems, can be highly effective in compositions and applications for which they are appropriate. An important aspect of the suitability of a stabiliser, or stabiliser system, which may be crucial in some cases (e.g. with PVC compositions intended for food-contact or medical applications) is the question of possible health hazards, both in the general handling and processing in the factory, and in the subsequent use of the stabilised PVC material. As in the case of other additives, much attention has been given to this aspect of stabiliser usage, in particular in regard to stabilisers based on lead and cadmium. The health hazard point is mentioned briefly in connection with each group of stabilisers discussed under the classification of the next section. It may be noted in general that permitted material lists exist in several countries, and that limiting regulations or legislation have been introduced. Information and guidance on specific problems can be sought from the organisations mentioned in Chapter 7, Section 7.12 and Chapter 12, Section 12.9 (ct. also Chapter 11, Section 11.3.4). Suppliers will also advise on the acceptability of their products.
9 Stabilisers: General Aspects
263
Cost, and the related consideration of cost-effectivity, are important factors in the selection of stabilisers for PVC compositions processed on any appreciable scale. Two other points of general importance may also be mentioned. Firstly, the inter-relationship between stabiliser and lubricant (in practice most often both are systems of more than one component) is of special importance in PVC formulations: something of this has been mentioned in the discussion of lubricants in Chapter 11. It cannot be too strongly emphasised that the choice of the stabiliser system has an important bearing on that of the lubricant(s), in view of such factors as lubricant action of some stabilisers, possible synergistic or adverse interactions, and others-see Chapter 11. Secondly, the physical state of the stabiliser system can affect the processing and service properties of a PVC composition. Both liquid and solid systems are available within each of the main stabiliser type groups. In general, liquid systems tend to make for somewhat easier processing of uPVC compositions, but lower softening and heatdistortion temperature of the products: the clarity of transparent compositions is normally less affected-and may be improved-by liquid stabiliser systems. It is self-evident that a stabiliser (in practice normally stabiliser system) chosen for a particular composition should-like any constituent-be compatible with all the others, as well as otherwise suitable for the processing conditions and end-use envisaged: the importance of cost and cost-effectivity has already been mentioned. Because-as pointed out at the end of Section 9.2.1-virtually every PVC composition (and certainly every type of composition) presents an individual stabilisation problem, the choice of a stabiliser should be made in consultation with the manufacturer (in the industrial context 'stabiliser' will normally mean a proprietary product, most often composite, designed for the particular type of formulation, process, and service conditions). The stabiliser so chosen should still be finally evaluated in laboratory tests and plant trials (see also Chapter 10).
9.4 HEAT STABILISERS All heat stabilisers and stabiliser systems in industrial use are of the 'external' kind in the applicational sense, in that they are additives
264
W. V. Titow
incorporated in the PVC by physical admixing. * Permanent 'internal' stabilisation of the PVC polymer by introducing a stabilising component into the molecular chain, or by attaching it to the chain through a chemical reaction, is still more of academic interest than practical significance. Examples of this approach to stabilisation include: a vinyl chloride/lead undecylenate copolymer for which substantially increased stability (in comparison with commercial PVC polymers) has been claimed;22 treatment of PVC polymer with triphenyl aluminium to substitute phenyl groups for labile CI atoms in the chain for improved stability;17 and replacement of the labile CI by mercaptide groups through reaction of the PVC polymer with dibutyltin mercaptide salts, reported to result in improvements of thermal stability by factors of 6 to 9. 23 .24 The chemical substitution in the last-named treatment is of the same kind as those believed to constitute one of the important mechanisms of stabilisation of PVC by organotin stabilisers (see Section 9.6 below). There is no formal, rigid classification of heat stabilisers, although for the purposes of discussion or review of their properties and effects they are very often grouped on a somewhat mixed basis relating partly to the chemical nature and pa.rtly to the types and areas of application: this kind of grouping is exemplified by the headings in the brief, basic summary given in Section 4.4.1(b) of Chapter 4. The classification of the present section follows more closely the lines of division by chemical type; although no special intrinsic merit is claimed for this particular approach, it is not regarded as any less convenient, or more arbitrary, than other possible systems. On this basis, the compounds used as heat stabilisers for PVC may be divided into the following general groups: (i) (ii) (iii) (iv)
lead compounds; organotin compounds; compounds of other metals; organic stabilisers.
* This is so even in the case of so-called 'in-kettle' stabilisation,25 in which the stabiliser (normally an organotin) is added at the earliest possible stage, viz., to the reactor during the polymerisation of the PVC resin. This method has been introduced by some US companies manufacturing PVC pipes on a scale large enough to warrant producing their own polymers. The benefits are a most intimate dispersion of the stabiliser in the polymer for maximum stabilising effect, and protection against heat degradation in the drying operations concluding the polymer production cycle.
9 Stabilisers: General Aspects
265
In many instances (and with groups (iii) and (iv) predominantly) individual members of these groups are used not singly, but in combination with other compounds from the same or another group, to make up composite stabiliser systems. The majority of commercial stabilisers are such composites, specially designed for particular types of composition and application, and with a view to utilising synergistic effects between the components and avoiding undesirable interactions. Stabilisers and stabiliser systems are also available in combination with other PVC additives (lubricants, colourants, antistatic agents, etc.) in 'single-pack' additive systems marketed by suppliers for specific purposes and types of PVC composition. The advantages and limitations of such systems are mentioned in Chapters 10 (Section 10.4) and 11 (Sections 11.1.3 and 11.3.3(iii)). The mutual effects of stabilisers with lubricants (whether as members of the same single-pack system or when incorporated individually in the PVC composition) are of particular importance (see Chapter 10, Section 10.3; Chapter 11, Section 11.1.2; and Sections 9.4.1-9.4.4 below). Other things being equal, the efficiency of a stabiliser increases with the thoroughness of its dispersion in a PVC composition. Useful concise reviews of stabilisers and their applications include ones by Fernley,26 Grindley,27 and Thacker,28 as well as those in Refs 7 and 9 already cited. 9.4.1 Lead Compounds These are either lead salts or lead 'soaps' (salts with stearic acid). The main advantage of these old-established stabilisers are cost-effectivity, good heat-stabilising power (in some cases combined with UV absorption), and particular suitability for use in electrical insulation (because of their complete non-conductivity and the inert nature of the chlorides formed by reaction with HCI). However, they are not suitable for clear compositions (although some may be used in translucent ones), where freedom from sulphur staining is necessary (ct. Section 9.7), or where their toxicity presents a hazard, as, for example, in food-contact applications (e.g. packaging films, containers), products for medical use, or childrens' toys. The handling (especially at the compounding stage) of lead stabilisers can also present exposure hazards, and comes within the scope of prescribed practice and regulation limits for occupational exposure of workers to lead (e.g. in the USA 50~g of lead per cubic
266
W. V. Titow
metre of air over an 8-h weighted average is currently the maximum under OSHA regulations). Lead stabiliser powders should preferably not be used in 'open' handling and mixing operations, and certainly not unless a first-class extraction system is available. In any case, the powders may be difficult to disperse in comparison with the other physical forms in which lead stabilisers are available. These are 'dustless' powders (stabiliser powders damped down with a small proportion-varying with the absorptivity, but in general of the order of 1%-of a plasticiser or mineral oil, or treated with PTFE) , granulates, 'co-precipitates' (of lead stabilisers with lubricants: coprecipitates tend to be amorphous rather than crystalline as the lead stabilisers, and can also constitute synergistic systems), liquid dispersions (of the stabiliser(s) in a suitable plasticiser), and 'single-pack' additive systems-see also Chapter 10. In formulating with lead stabilisers, the PbO content and its proportion 'safe' for reaction (without danger of gassing,* or liberation of stearic acid from stearates) should be taken into account-see Table 9.1. Basic lead carbonate (white lead): This well-known product has been used since the early days of the industry, and it still retains its position as one of the popular, low cost, general-purpose stabilisers for PVc. Improvements in manufacturing processes have resulted in special grades of white lead becoming available, with consistent properties and exceptional purity, and having maximum heat stability for use in vinyl compounds. White lead decomposes with the evolution of water and carbon dioxide when heated to temperatures around 200°C, and this fact must be taken into account when processing compounds incorporating this stabiliser. Care should be taken to avoid excessively high temperatures, especially in the case of rigid or non-plasticised materials subject to severe shearing forces, where the heat generated by internal friction may result in temperatures high enough to cause degradation and gassing due to the evolution of carbon dioxide. White lead is suitable for use with all types of vinyl chloride polymers, but in fast-rate processing (extruding, calendering, moulding) tribasic or tetrabasic lead sulphate is now preferred. The electrical properties of white lead are good and it is therefore eminently suitable
* Release of gas, normally CO2 , during hot processing. Can be promoted or aggravated by acidity of plasticisers in plasticised compositions. 29
267
9 Stabilisers: General Aspects
TABLE 9.1 Some Characteristics of Lead Stabilisers
Stabiliser White lead
Tribasic lead sulphate
Dibasic lead phosphite
Specific gravity
PbO (totaf)
'Safe' PbO
6·4-6·8
86
28·8
7·0
86
65
6·7
89·5
60
Lead silicate
2·67
89
66·5
Lead stearate Dibasic lead stearate
1·24 1·9
31·5 55
31·5 36·5
Tribasic lead maleate
6·0
89
66·5
Quantity to use (on 100 phr) Extrusion Rapid extrusion Calendering Spreading Extrusion, plasticised Extrusion, unplasticised Calendering Spreading Extrusion, plasticised Extrusion, unplasticised Calendering Moulding Extrusion Calendering Used alone Used in combination
3-5 8-10 3-5 2-5 3-5 5-10 3-5 2-5 3-5 5-10 3-5 3-8 5-7 4-6 1·5-2·5 2-4 0·5-1 2-4
for stabilising cable compounds. As it is non-lubricating, the addition of a lubricant is necessary; 0·5-1 % dibasic lead stearate has been found satisfactory in most cases. The proportion used in PVC compounds is not critical and as it has a high degree of long-term heat stability, the problem of heat degradation during reworking scrap material can be readily overcome if sufficient white lead is used. The quantities to be used are indicated in Table 9.1. Tribasic lead sulphate: Tribasic lead sulphate is a good stabiliser for high-temperature working. It is in itself very stable to heat and can be heated without decomposition to temperatures well in excess of those normally encountered in compounding and fabricating PVc. For this
268
w.
V. Titow
reason it is suitable in such processes as high-speed extrusion or manufacture of rigid products. Under these conditions the absence of decomposition minimises the risk of porosity caused by evolution of gases. Very good electrical properties are also a feature of compounds stabilised with tribasic lead sulphate. It is especially useful for cable sheathing purposes, as when it is used with suitable electrical grade polymer, high values of volume and surface resistivity can be maintained. It is suitable for all pigmented compounds where long-period heat stability is required, for rigid profiles, and rainwater goods, guttering, downpipes, soil pipes and pipes for conveyance of gases and liquids. A mixed stabiliser of tribasic lead sulphate and dibasic lead phosphite has been found in practice to impart excellent weathering properties, the dibasic lead phosphite having excellent resistance to UV light. Tribasic lead sulphate is not self-lubricating and normally requires the addition of a lubricant, such as lead stearate, to assist fabrication. The amount of lubricant required is usually of the order of 0'5-1·0 phr. Tetrabasic lead sulphate: This stabiliser is similar in general action to tribasic lead sulphate, but somewhat more powerful. It provides the highest proportion of available lead oxide of all the lead stabilisers. Its low combined water content reduces the tendency for gassing in extrusion of rigid compositions. It is a UV absorber, giving protection to sensitive pigments. In combination with dibasic lead phosphite it gives greater protection to PVC on outdoor exposure than does an equivalent amount of tribasic lead sulphate in such combination. Dibasic lead phosphite: This is one of the best stabilisers among the lead compounds, also particularly effective against light and weathering (better than the sulphates), and hence widely employed in compositions for outdoor applications (e.g. extruded profiles, cladding, fencing), where its action as a UV-screening agent and its antioxidant functionality are particularly useful. Like the lead sulphates, dibasic lead phosphite is a strongly opacifying stabiliser: this should be considered where translucent compositions are required. Lead silicate: The refractive index of this stabiliser approximates to that of PVC polymer; it is therefore suitable for translucent compounds, promoting economy of use of colourants in translucent
9 Stabilisers: General Aspects
269
film, sheeting and fabric coatings. Lead silicate is an effective heat stabiliser, in keeping with its high lead oxide yield: as it has no lubricating action the amounts that can be added to PVC are not limited by the possibility of over-lubrication (as are those of metal soap stabilisers), whilst-by the same token-it is suitable for use in compositions to be calendered or extruded where lubrication has to be closely controlled. The main applications of lead silicate are in flexible PVC, including some plastisol products. It does not normally cause migration or blooming, which may occur with the use of metal soap stabilisers. Lead salicylate: This a good stabiliser against the action of light. It is also a chelator for iron, used in compositions incorporating iron-containing fillers to prevent the development of colour: one such common application is in asbestos-filled PVC flooring. Lead salicylate is normally employed in combination with another lead stabiliser, e.g. basic lead carbonate. Lead stearate: This is a stabiliser of only moderate power, but with strong lubricant action, which is the main reason for its use-eommonly as a component of a composite lead stabiliser system (most often containing tribasic or tetrabasic lead sulphate as the main stabiliser). It can also be used with other metal-stearate stabilisers (e.g. calcium and strontium stearates). The proportions typical for normal usage are 0·5-1·5 phr, but above about 0·75 phr it will not normally give clear compounds, so that dibasic lead stearate should be used. Over-addition can cause blooming. Dibasic lead stearate: A moderately good heat stabiliser and useful high-temperature lubricant: it is used mainly in the latter capacity, as a constituent of composite stabiliser systems (especially with the lead sulphates) for pipe, profile and sheet compositions. Dibasic lead phthalate: This is a good heat stabiliser, with some light-stabilisation effect. It is readily dispersible in PVC resins. Its refractive index is high so that its opacifying action in PVC compositions is relatively strong. Dibasic lead phthalate is recommended for use in high-quality cable covering in which it promotes good electrical properties (volume resistivity unimpaired by compounding and processing, and maintained during the life of the cable).
270
W. V. Titow
This stabiliser is also an effective activator ('kicker') for chemical blowing agents in PYC foam compositions (cf. Chapter 25).
Tribasic lead maleate: This is an effective heat stabiliser with light-stabilising (UY-screening) functionality, used in such products as pipe and sheeting. It is suitable for translucent compositions, and a good stabiliser for chlorinated PYC. Tetrabasic lead fumarate: This compound has good stabilising action and compatibility with PYC resin. Its main uses are gramophone records and cable coverings. 9.4.2 Organotin Stabilisers
(a) Chemical Nature and Types Commercial organotin stabilisers are mainly compounds of the general formula, I, with some of those represented by formula II also in use (commonly in mixtures with formula I compounds the respective proportions being adjusted for particular effects-see Section 9.4.2(b) below).
The nomenclature commonly employed in the industry for these stabilisers is largely (though not exclusively-see further on) associated with the nature of the substituent group R, as shown schematically below, so that the compounds I and II may be referred to as di- and mono-substituted respectively. The tri-substituted compounds (R3SnA) are also formed in the course of production of organotin stabilisers by the industrial processes. 25 ,30 However, because their stabilising effects in PYC are lower-and their toxicity higher-than those of compounds of types I and II, they are normally regarded as undesirable by-products and the
production processes run so as to minimise their formation. The following general classification scheme illustrates the nature and
271
9 Stabilisers: General Aspects
mutual relationship of the various types of commercial organotin stabilisers (with reference to formulae I and II). organotin stabilisers
I Alkyltin compounds
l
(R = an alkyl group)
I
Methyltin compounds
(R=-CH3)
I
Butyltin compounds (R=-C4~)
1_- - _ _, 1 Octyltin Lauryltin compounds
(R = -CsHd
I
Estertin compounds (R = an ester group: R'.OOC.CH2 ·CH2- )
compounds
(R = -C12H 2S)
The substituent groups A of formulae I and II are normally derived from either: (i) mercapto compounds (mercaptoacids and their esters; mercaptides), or (ii) carboxylic acids and their esters (e.g. maleic or lauric acids; maleic esters and half-esters). Most commonly the A groups are all mutually the same, although compounds in which they differ are not unknown among commercial organotin stabilisers (e.g. Thermolite 17, M & T Chemicals, USAdibutyltin laurate-maleate). Alkyltin stabilisers in which the A substituents are mercapto compounds are usually referred to as thiotin stabilisers, or simply 'thiotins' (occasionally 'sulphur tins'), and those with A substituents of the sulphur-free carboxylic acid or ester type as tin carboxy/ates: the most numerous members of this latter group among commercial stabilisers are dibutyltin carboxylates. The estertin stabilisers are relatively new, having been first introduced (by AKZO Chemie) in the mid-seventies. Since then several commercial representatives have been available, e.g. in the Stane/ere (AKZO Chemie), Irgastab (Ciba-Geigy), and Interstab (Interstab Chemicals) ranges. The lauryltins-a Japanese development-are even more recent. The preparation and evaluation of an interesting group of polymeric p-benzoquinone-tin compounds was described recently by Yassin and Sabaa,lO who claim a high degree of stabilising action in PVC for these compounds.
272
W. V. Titow
(b) Characteristics and Applications The organotins are powerful heat stabilisers, imparting excellent shortand long-term stability to PVC compositions, and-for the most part-a high degree of clarity in transparent products. They are relatively expensive, although the 'compounded' cost can be competitive with that of other types of stabilisers-especially when considered on the basis of performance-because of the comparatively low incorporation levels (rarely more than 2 phr, and only fractional phr in many formulations). Most organotins are toxic: however, several commercial tin stabilisers based on dioctyltin compounds* are allowed in many countries (with restriction on maximum content), because of their low level of toxicity, and good resistance to extraction. These permitted tin stabilisers are important in the production of uPVC food-packaging film and bottles for edible oils and beverages, where a combination of non-toxicity with high degrees of heat stability and clarity is a primary requirement. Some commercial estertins have also gained qualified food-contact approval. The stabilising action of organotin compounds is considerably influenced by the nature of their A substituent groups (cf. formulae I and U). In general, the thiotins-and especially dialkyl thiotins-are the most powerful and effective heat stabilisers known, suitable for use with PVC polymers of all types and grades (even emulsion resins stabilised with alkali, which can give problems with some stabilisers of other types), many copolymers, and blends with modifying polymers. However, apart from their relatively high cost, the following factors also impose some limitations on the use of the thiotins. As mercapto compounds they impart an unpleasant odour to PVC compositions: this is perceptible in the processing of both flexible and rigid PVC, and persists in pPVC products (only rarely in uPVC). Because they contain divalent sulphur, they are not normally suitable for compositions incorporating lead or cadmium stabilisers or pigments, with which they can react to form coloured metal sulphides ('cross-staining'). The thiotins have relatively little light-stabilising action. Where good stability to light, and weathering generally, is an important requirement, they are used in conjunction with tin carboxylates or other light-stabilising additives, often epoxy stabilisers. * e.g. di-n-octyltin maleate; or di-n-octyltin-bis(isooctyl thioglycollate), listed in the US Federal Register as di-n-octyl-tin-S,S'-bis-isooctyl-mercaptoacetate.
9 Stabilisers: General Aspects
273
Note: Some commercial stabilisers are mixtures of this kind, e.g. Irgastab T68 (a tin mercaptide with a dibutyltin carboxylate), or Irgastab 170 MO (di-n-octyltin-bis(2-ethylhexyl thioglycollate) with epoxidised soyabean oil). Addition of titanium dioxide (12-14 phr) to thiotin-stabilised compositions for the production of extruded products also improves weathering resistance. 25 In transparent compositions for outdoor use, further augmentation of any stabiliser system with a UV absorber is normal practice. The heat-stabilising power of the sulphur-free tin carboxylates, whilst generally good, is rather less than that of the thiotins, and their cost tends to be higher. However, they impart good stability to light (excellent in some cases, e.g. with some modified butyltin maleates). Like the thiotins they are suitable for all types and grades of PVC polymers. They are also odour-free and do not cause cross-staining with heavy-metal compounds. When used in PVC pastes (in appropriate cases: pastes are not widely stabilised with organotins) they do not increase the paste viscosity. The first-generation estertins (all of the liquid thiotin type) are comparable in performance with the established non-lubricating alkyl thiotins in the major applications (calendering, extrusion, injection moulding, blow moulding): they are also somewhat better than alkyl thiotin stabilisers in regard to weathering resistance (although inferior to tin carboxylates): low volatility and extractability are further general advantages. Organotin stabilisers are available in both liquid and solid forms. Most liquid stabiliser systems of all types, including virtually all the long-established liquid organotins, and the first generation of estertins, have no lubricant action, and-in general-non-lubricating compounds predominate among commercial tin stabilisers. Therefore lubricants are required in most tin-stabilised rigid PVC compositions. In some cases they may be introduced with the stabiliser (and possibly other additives) within a multi-component 'single-pack' additive system, but otherwise it is the task of the formulator to devise a suitable lubricant system: mutual suitability of this with the stabiliser will be the most important single factor to be considered in the general context of processing characteristics, cost, and stability in service of the composition. As mentioned in Chapter 11 (Section 11.1), inter alia, use
274
w.
V. Titow
can be made of synergistic effects that can arise between the thiotins and some lubricants, notably internal lubricants of the partial glyceride type. Conversely, these lubricants (both liquid and solid) can impair the heat stability of compositions stabilised with tin carboxylates (which give very good results when used in conjunction with lubricants of the fatty alcohol ester type). Lubricant capability can be built into methyl, butyl, and octyl tin compounds by suitable choice of the ligand (substituent group A). Thus some alkyltin mercaptides with a long aliphatic chain in the ligand have lubricating properties (e.g. dibutyltin-bis(lauryl mercaptide), used in some rigid compositions in Europe), as have members of the new group of 'reverse-ester' alkyltin mercaptides developed in the USA for rigid pipe compositions, in which the A substituents are esters of a mercaptan (such as thioethanol). In lauryltin compounds the presence of the lauryl group is conducive to lubricant action but the overall effect is governed also by the nature of the A substituent. The potential scope for imparting lubricant properties to the estertins through chemical structure modification is considerable, in that significant effects can be obtained by the appropriate choice of the R' substituent in the ester groups25 (see classification scheme above). In the USA, where lead compounds are not permitted in PVC water pipes, alkyltin stabilisers are used in this large-scale application. Their relatively high cost, and the vigorous competition among their manufacturers, prompted the development of new alkyltin stabiliser systems effective in rigid PVC pipe compositions at very low incorporation levels (down to about 0·3 phr). Among the main trends in such systems have been the increased use of monoalkyl tins, exploitation of synergists (e.g. some organotin sulphides), and introduction of the lubricating tin stabilisers. Note: The effect of the monoalkyl compounds in tin stabiliser systems has been likened 25 to that of zinc in Ba/Cd/Zn stabilisers (cf. Section 9.4.3 below), i.e. promotion of good initial colour and long-term stability improvement. In some blown-bottle and rigid-film formulations, an increased proportion of monoalkyl compound in the alkyltin stabiliser systems, coupled with suitable lubricant choice, can reduce the stabiliser requirement to below 1 phr.
9 Stabilisers: General Aspects
275
9.4.3 Compounds of Other Metals (a) Metal Compounds with Stabilising Effects in PVC Apart from those of lead and tin, certain compounds of the following metals are of practical interest as stabilisers for PVC: lithium, magnesium, aluminium, sodium, potassium, calcium, zinc, strontium, cadmium, antimony, and barium. The antimony compounds used in commercial stabilisers are antimony mercaptides. Those of the other metals may be divided into three general groups whose members are commonly known as 'soaps', 'salts', and 'complexes' (the salts and complexes are sometimes grouped together under the latter name). The 'soaps' are metal compounds of higher aliphatic carboxylic acids, typically stearic and lauric, but also some others (e.g. myristic and palmitic): they are usually solids, with some lubricating action in PVC compositions (but liquid systems, in which the metal soaps are dissolved or pasted up in a 'carrier' are available among commercial stabilisers). The 'salts' are compounds of relatively lower aliphatic acids, such as metal hexoates, heptoates and octoates, occasionally also caprates and undecylates. The 'complexes'* are metal derivatives of various aromatic acids, phenols and petroleum acids, typically naphthenates, phenates, or cresylates. Stabiliser systems based on the salts and complexes are most often liquid, and usually non-lubricating. Antimony mercaptides and calcium stearate are used alone (i.e. each as the sole stabiliser) in some PVC compositions (although they are also included as components in composite stabiliser systems-see below). All the other metal compounds of this section are virtually always employed in mixtures, compounds of two or three different metals together forming a composite stabiliser system in which they mutually supplement one another's effects (see (b) below). In most cases the mixtures are further augmented by epoxy co-stabilisers and phosphite-compound synergists ('chelators'), which are present as constituents of some proprietary mixed-metal stabiliser systems, or may be added separately to compositions stabilised with systems that * The 'complex' terminology is not strict or universal: thus a combination of metal compounds in a composite commercial stabiliser, whether in solid, liquid, or paste form, is sometimes described as a 'complex stabiliser', or 'a stabiliser complex', or simply 'a complex' (especially if synergistic additives such as chelators or epoxy co-stabilisers are also included), even if the main components are metal soaps.
276
w.
V. Titow
do not contain them (cf. (b) and Section 9.4.4 below, and Chapters 4 and 10). ANTIMONY MERCAPTIDES
Liquid commercial stabilisers of this type, based on antimony trimercaptide, were introduced in the USA in the mid-1970s. 31 Their usage and effects are, in general, similar to those of conventional liquid thiotins, which they resemble in their lack of lubricant effect and general degree of toxicity, but over which they can offer cost advantages. Some proprietary antimony mercaptide stabilisers, and antimony-stabilised uPVC compounds, are now approved by the National Sanitation Foundation (NSF) in the USA for potable-water pipes (with restrictions on maximum content fixed at relatively low levels-typically 0·3-0·4 phr).32 At such levels-which are suitable in certain compounds formulated for twin-screw pipe extrusion, but normally too low for single-screw extrusion-the antimony mercaptides are more efficient than thiotins (the efficiency advantage is reversed above about 0·8 phr content level-see Fig. 9.1); they can also be more effective in compounds experiencing moderately high temperatures over long time periods. However, the resistance to sulphur staining, and to UV radiation (especially in transparent compositions) is, in general, lower than that imparted by the organotins (in opaque compositions adequate levels of addition of titanium dioxide will promote good resistance); they may also form orange-coloured antimony sulphide when exhausted. Strong synergistic effects are possible when antimony mercaptide stabilisers are used in conjunction with calcum stearate. 31 Stabilisers based on antimony/tin combinations have also been developed for rigid pipe compounds for single-screw extrusion and for thick-walled, large diameter pipe. 32 In Europe antimony-based stabilisers are recommended, inter alia, for VCNA copolymer gramophone record compositions. The use of antimony mercaptides, whilst a relatively recent development in the commercial context, is in fact a revival 33 made possible by improvements in PVC compounding and processing techniques and by changes in the relevant material costs. At the comparatively high levels of incorporation normal in the early 1950s, when their application in PVC was first explored, antimony mercaptides were less efficient than comparable organotins. Their greater efficiency at low levels can now be utilised thanks to the general lowering of the stabiliser proportions used, made possible by the more thorough dispersion achievable with modern
277
9 Stabilisers: General Aspects
/
/
/
"'/ ~"ti-'/
2·0 L.
.c
.~-\ ~/
Co
0"0./ /
<1./
L.
~
ell
.a 11l
1ii
c i= 1·0
/
/
/
,
/
/
/
/
/
2·5
/
1~
~O
Antimony stabiliser, phr
Fig. 9.1 Amounts (phr) of an antimony mercaptide and an organotin stabiliser, at generally low incorporation levels in identical PVC compositions, required for the same degree of improvement in dynamic heat stability (torque rheometer). Marked points on the plot correspond to the improvement factors shown, i.e. x1·5: x2·0; x2·5; and x3·0. Data from Ref. 33.
compounding equipment, and more efficient processing' on modern machinery, especially twin-screw extruders. Examples of commercial antimony mercaptide stabilisers are Irgastab S 110 (Ciba-Geigy, UK) and in the USA Synpron 1027 (Synthetic Products Co.), with corresponding products in the Argus Chemical and Ferro Chemical ranges (respectively, Mark 2115 and Therm Check 1514). CALCIUM COMPOUNDS
Some PVC-stabilising calcium compounds are available individually from stabiliser manufacturers, e.g. calcium stearate and calcium laurate. However, with the partial exception of calcium stearate, their widest application is as regular constituents of composite commercial stabiliser systems in which they are variously combined with zinc, barium, and magnesium.
278
W. V. Titow
Calcium stearate can be used as a stabiliser in its own right, or as a lubricating co-stabiliser (with synergistic effects in some cases) in conjunction with other stabilisers, such as organotin and antimony compounds (e.g. in rigid pipe compositions of the American kind). Whilst it is not a highly powerful stabiliser, calcium stearate has a reasonable heat-stabilising action, although with relatively little light-stabilising effect. It is useful with emulsion-type PVC polymers and finds use in certain paste formulations (especially in the UK). Its non-toxic character also promotes certain applications. Thus uPVC compositions for potable-water pipes in France are commonly stabilised with calcium stearate, often in conjunction with organic co-stabilisers (not epoxy compounds, which are susceptible to microbiological attack). Calcium stearate is also a stabilising lubricant constituent of some uPVC bottle formulations, sometimes in conjunction with a zinc compound (e.g. zinc octoate), added as a separate component, and an epoxy co-stabiliser. Paste products for which non-toxicity is a requirement (e.g. toys) are also stabilised with calcium stearate. ZINC COMPOUNDS
Zinc compounds are never used as sole stabilisers in PVC compositions. This is because PVC polymers are sensitive to zinc, in the sense of increased susceptibility to degradation in its presence (amounting in effect to a catalytic de-stabilisation by the metal). The degree of sensitivity varies widely with the type and grade of PVC polymer, and also in different compositions (e.g. plasticised, filled compositions of vinyl chloride/acetate copolymers are among the least sensitive). The variations in response of emulsion polymers used in PVC pastes to zinc compounds and individual commercial zinc-containing composite stabilisers are well known as a general fact (although the actual effects in particular cases normally have to be ascertained by tests). Thus, whilst individual zinc compounds (e.g. zinc octoate, zinc stearate) are available within the regular ranges of many stabiliser suppliers, they are always used in conjunction with other metal compounds (e.g. those of cadmium, calcium, magnesium or barium), incorporated either singly or in pre-combination (e.g. as barium/cadmium or calcium/ magnesium stabilisers). In such made-up combinations the zinc compound improves initial colour, long-term stability, and resistance to cross-staining with sulphur compounds (which can occur, for example, with cadmium stabilisers). Liquid zinc compounds do not
9 Stabilisers: General Aspects
279
usually impair the clarity of transparent PVC composItions. Zinc compounds (e.g. octoate) are also effective as activators ('kickers') for chemical blowing agents in compositions for the production of PVC foam (see Chapter 25). The actions and effects of the zinc compounds are essentially the same where they are present as components of commercial composite stabilisers, such as proprietary barium/zinc, cadmium/zinc, calcium/ zinc, barium/cadmium/zinc and other systems of this general kind (see (b) below). Note: Calcium/zinc combinations are suitable for applications where non-toxicity is a requirement.
The amounts of a zinc compound to be employed with other suitable metal compounds or systems, as well as its proportions in proprietary composite zinc-containing stabilisers (which may be differentiated into low-zinc, medium-zinc, and high-zinc systems) can, typically, vary within the range 0·1-0·5 phr, depending on the amount and nature of the main stabiliser(s) used and the zinc-sensitivity of the PVC composition concerned. ALUMINIUM COMPOUNDS
The use of aluminium compounds in the stabilisation of PVC is relatively very limited: it is normally confined to certain paste compositions, and the aluminium compound is not the sole stabiliser. Where it is added separately (i.e. not as a constituent of a composite proprietary stabiliser) the compound will usually be aluminium stearate, and a lead compound or calcium stearate will be used as the main stabiliser in the paste. Some grades of aluminium stearate are effective as thickening (gelling) agents for PVC pastes, and are included in the formulation mainly in this capacity. In commercial mixed-compound stabilisers aluminium may, typically, be combined with calcium and zinc (d. (b) below). In general terms, the stabilising effects of aluminium stearate may be said to resemble those of zinc stearate. CADMIUM COMPOUNDS
PVC compositions stabilised solely with cadmium compounds generally [ave good initial colour hold, but their long-term stability is unsatisfactory, colour and eventual darkening developing with time. These compounds are, therefore, not used alone as PVC stabilisers, although
280
W. V. Tilow
some are available from stabiliser manufacturers (e.g. cadmium stearate, cadmium laurate, certain liquid compounds): these can be added to compositions stabilised with composite, cadmium-containing systems to increase the total cadmium content, or to some leadstabilised compositions (e.g. ones for pipe or extruded profile for outdoor use) to improve initial colour, and light stability in service. However, cadmium compounds are very important in PVC stabilisation, as one of the principal components of the widely used barium/cadmium stabiliser systems (see (b) below). A disadvantage of cadmium stabilisers is the toxicity of the compounds of this heavy metal: this not only makes cadmiumcontaining PVC compositions unsuitable for 'non-toxic' applications, but has been causing increasing concern as a hazard in the handling and processing of cadmium stabilisers and pigments (cf. Chapter 11, Section 11.3.4). BARIUM COMPOUNDS
Barium compounds (such as, for example, the stearate and laurate which are available from commercial stabiliser sources) can impart good long-term stability to PVC compositions, but the initial colour is poor and progressive yellowing usually takes place. The compounds are thus unsuitable for use as sole stabilisers, but-as would be expected from a consideration of the respective effects-they complement well the action of cadmium compounds. Moreover, when compounds of the two types are used together synergistic effects also arise, making the resulting stabilisation even more effective than would be expected from a mere addition or superposition of their individual contributions. These are the reasons underlying the wide use of composite stabilisers based on the barium/cadmium combination, often including also a zinc compound as the third component (see (b) below). Barium/zinc systems, without cadmium, are also employed, as are some barium/lead compositions in which the barium compound improves long-term stability (see, for example, Durham Chemicals' Durostabe S70 and S65, respectively, in Table 10.4, Chapter 10). Barium compounds are not as toxic as those of cadmium, but they are not suitable for use where non-toxicity is required. LITHIUM COMPOUNDS
Lithium stearate is the single compound usually readily available from stabiliser manufacturers. However, it is not used alone as a stabiliser,
9 Stabilisers: General Aspects
281
and even in combinations with other metal stabilisers its use is not widespread. Like other stearates it has lubricant properties, and may be regarded as a non-toxic substitute for other metal soaps in some formulations (but its admissibility in particular cases should be checked with the supplier or the appropriate organisations). MAGNESIUM COMPOUNDS
Here again the stearate is commonly available from stabiliser supply sources, but is not used alone in PVC stabilisation. In general, magnesium compounds are mainly encountered as constituents of composite proprietary stabilisers (usually for 'non-toxic' applications or for certain paste compositions) in which they are combined with calcium, or zinc, or both. Some commercial magnesium/zinc stabilisers are recommended for a wider range of applications (see (b) below). STRONTIUM COMPOUNDS
Strontium compounds are most commonly combined with those of zinc in composite stabiliser systems. In such combinations they may be regarded as substituents for cadmium compounds, offering lower toxicity and freedom from sulphur staining. However, the systems are less applicationally versatile than the cadmium analogues. Individual strontium soaps (typically the stearate and laurate) are available from stabiliser suppliers. COMPOUNDS OF SODIUM AND POTASSIUM
These are of minor importance, and only occasionally encountered in commercial stabilisers, as constituents of composite systems, e.g. stabiliser CS 137 (originally in the range offered by F. W. Berk & Co. Ltd in the UK)-a barium/sodium organic complex for transparent compositions. (b) Composite Metal Stabilisers
As has been indicated in the previous section, it is in certain recognised combinations with one another that the metal compounds of that section enjoy a prominent position as stabilisers for PVc. The nature and applications of these composite systems are illustrated by the examples of their commercial versions given in the tables and sample formulations of Chapter 10. Only the salient points and general features of the most important types are, therefore, briefly outlined in the present set;tion.
282
W. V. Titaw
Composite systems of the principal types are available in solid (powder, flake), liquid, or paste forms. More recently concentrates of some systems in PVC polymer have also been coming onto the market, as have low-dusting (plasticiser-dampened) powder forms. Such materials are exemplified, respectively, by some barium/cadmium powder stabilisers 'encapsulated' in PVC resin at a 1: 1 ratio (in the Poly-Chek MP series of Ferro Chemical, USA), and low-dusting powder grades of the same metal system in the Mark 8100 series of the Argus Chemical Corp., USA. The concentrates are added to PVC compositions at twice the normal levels, with one part of PVC polymer omitted from the composition for every 2 phr of concentrate. These new developments parallel the forms in which some lubricants and lead stabilisers are being offered, and involve similar advantages (ct., respectively, Section 11.1.3 in Chapter 11 and Section 9.4.1 of the present chapter). BARIUM/CADMIUM AND BARIUM/CADMIUM/ZINC STABILISERS
In terms of the amounts used this is the most important group among PVC stabilisers, although none of its members can equal the organotin systems in stabilising power and versatility (in the sense of equal suitability for use with all grades and types of PVC polymers). However, systems of this type can impart high degrees of thermal stability, good light stability, and be compatible with good clarity in transparent compounds. Their effects are improved (and especially the light stability increased) by the addition of epoxy co-stabilisers and organic phosphite 'chelators' (see Section 9.4.4 below), and these synergists are normally included either by the formulator or ab initio as constituents of the composite proprietary system. Weathering properties are further improved by addition of UV absorbers (see Section 9.5 below). Liquid BalCd and BalCd/Zn systems are particularly widely used in flexible PVC materials, typically at levels of 1-2 phr (with about O·5-1·0 phr of a chelator and up to about 5 phr of an epoxy co-stabiliser where appropriate). Solid (powder or flake) systems are combinations of the metal soaps, with or without the further additives just mentioned. Whilst they can be used in both rigid and flexible compositions they are of special interest for uPVC as they have some lubricant action, and affect the softening temperature less than do liquid stabilisers. In most cases they are suitable for translucent (as well as opaque) compositions, but only some are suitable for highly transparent ones. Typical incorporation
9 Stabilisers: General Aspects
283
levels are 2-3 phr, with a phosphite chelator (typically about 1: 3 on the main stabiliser, i.e. about 0·7-1·0 phr) , for improved initial colour, transparency and light stability: an epoxy co-stabiliser (about twice the amount of the main stabiliser, i.e. around 4-6 phr) may be included for further improvements in general stability and light and weathering resistance. Combinations of solid and liquid systems may be used for optimum flow and lubrication characteristics in particular compositions. The main general limitations of BalCd and Ba/Cd/Zn systems are: differing effectiveness in different types of PVC* (also influenced, in a given type of composition, by the ratio-and respective contents-of the Ba and Cd components, the amount of Zn component if present, and the nature of the organic parts of the compounds); variable tendency to plate-out in processing; and susceptibility to sulphurstaining (minimised in zinc-containing systems). CALCIUM/ZINC STABILISERS
Members of this system are the most widely used non-toxic stabilisers for PVC (although not all its commercial variants have approval, or the same wide approval, in various countries). Stearates and octoates of the two metals, and many of their proprietary combinations, are widely permitted in most countries for use in rigid and plasticised compositions for the production of food-packaging materials and containers, medical goods and toys. Ca/Zn combinations are not very powerful stabilisers. Wherever possible (and this is permitted in many 'non-toxic' applications) they are used in conjunction with an epoxy co-stabiliser (which is a component of many proprietary systems) to improve the stabilising effects both against heat and light, and an organic phosphite synergist (which improves initial colour and transparency, inter alia). Other organic synergists available for use (in conjunction with organic phosphites if desired) to improve the stabilising action and efficiency of Ca/Zn systems include stearoylbenzoylmethane (Rhodiastab 50Rhone-Poulenc, France) and some proprietary polyol compounds (e.g. those supplied by Perstorp AB, Sweden): stability improvements by factors up to x2 can be obtained in some compositions through the addition of these compounds (0·2-0·5% with solid CalZn systems). The CalZn stabilisers are available in both solid and liquid forms. In * Generally unsuitable for alkali-prestabilised emulsion grades.
284
W. V. Titow
addition to the conventional powders, the solid versions now include particulate concentrates in PVC polymer (e.g. Ferro-Check NT-3Ferro Chemical Corp.). Where a liquid sytem consisting of Ca and Zn soaps in a liquid carrier is to be used for a 'non-toxic' application it is necessary to ascertain that the carrier is acceptable for the purpose. Typical incorporation levels may be illustrated by the following figures: liquid Ca/Zn system for paste compositions-I'5 to 3 phr (with up to 5 phr epoxy co-stabiliser); solid (powder) system for sheeting compositions-l to 2 phr (rigid sheeting), 0·5-1·5 phr (flexible sheeting), in each case with the addition of an epoxy co-stabiliser. Outside the non-toxic field, a common use for technical grades of Ca/Zn stabiliser systems is in asbestos-filled PVC flooring. Such compositions are subject to discoloration during heat processing, due to the catalytic action of iron present in the asbestos. In compositions otherwise adequately stabilised with BalCd or lead systems the discoloration can be severe, but it can be effectively prevented by the Ca/Zn stabiliser (see also Chapter 4, Section 4.6.2). Apart from the special applicability of the non-toxic grades, the advantages of Ca/Zn stabilisers include moderate price, and virtual freedom from odour in finished products and from sulphide staining. * The soap compositions have some lubricant action. The main limitations are: the relatively low stabilising power already mentioned; rapid progress of degradation, once commenced, in Ca/Zn-stabilised compositions; unsuitability for outdoor applications; and limited suitability for crystal-clear compositions. * OTHER METAL-COMPOUND COMBINATIONS
Some of these have been mentioned in Section (a) above. Something of the nature and applications of various mixed-metal stabilisers may be illustrated by the following examples. Note: Whilst there are, as yet, no official restrictive regulations against the general use of cadmium compounds, the position is under active review in some countries (ct. Chapter 11, Section 11.3.4) and a trend has begun to consider technically acceptable but potentially safer altenatives. In the stabiliser field, certain cadmium-free metal combinations (some under current development) are seen, or offered, as such replace* Haze may develop in transparent compositions in contact with sulphur compounds-see Section 9.7.
9 Stabilisers: General Aspects
285
ments, at least in certain applications. These combinations include some CalZn systems with improved light stability as well as various BalZn, BaiCa/Zn, and MglZn compositions. In such systems, as indeed in all mixed-metal stabilisers, the principal factors governing the stabilising effects in a particular PVC composition and set of conditions are the proportions of the individual metals, total metal level, nature of the organic parts of the metal compounds, and nature and amounts of any synergistic additives included in the composite sytem. The mixed-metal stabilisers so far available as substitutes for Ba/Cd systems do not, as a group, yet match the collective applicational versatility of those systems. MgIZn: Selected combinations of this type may be used (in conjunction with phosphite chelators) for stabilising flexible and semi-rigid compositions for many calendered, extruded and paste products. A degree of UV-resistance and freedom from sulphur staining are useful general features of compositions stabilised with proprietary versions of this system. Its solid-soap variants (e.g. Synpron 1534-Synthetic Products Co., USA) also have some lubricant action. BalZn: Examples of commercial versions of this system, currently offered as replacements for Ba/Cd-based stabilisers in some applications, are listed below: Synpron 1531: a liquid, low-Ba, high-Zn system incorporating a phosphite synergist (for use in calendered, extruded and paste products, at about 2 phr); (ii) Mark 2181 and 4004 (Argus Chemical): both liquid Ba/Zn combinations, for use in paste-produced PVC flooring (at 1·5-2·5phr, with 3-lOphr of epoxy co-stabiliser) where they can confer good clarity (top 'wear' layers), initial colour hold, stability to ageing at moderate temperatures and long-term stability; (iii) Stavinor BZ-329 (Rousselot, France), also for paste flooring applications.
(i)
CdlZn: Systems of this type are useful in compositions for chemically blown cellular PVC products (e.g. injection-moulded microporous shoe soles; foamed layers of coated fabrics) in which they act both as
286
w.
V. Titow
stabilisers (alone or in conjunction with other stabilisers, usually BalCd systems) and activators ('kickers') for the blowing agent (normally azodicarbonamide). Typically (e.g. with a Ciba-Geigy commercial version-lrgastab ABC 2) about 0·5 phr would be required for the activator function, and up to about 2·5 phr for stabilising effects. SrlZn: The commercial strontium/zinc systems are exemplified by Sr/Zn laurate co-precipitates* (e.g. Lankromark M-Diamond Shamrock, USA and UK), used mainly in plasticised compositions (at about 2 phr, with about 5 phr of an epoxy co-stabiliser), where freedom from sulphur staining, and low toxicity are of interest. SrlZnlSn: This is another of the combinations offered as replacements for Ba/Cd-based systems in some applications. Resistance to sulphur staining, and low tendency to plate-out (of some commercial versions, e.g. Nuostabe V 1925-Tenneco Chemicals, USA) are advantageous features. CalSb: The already mentioned combination of antimony mercaptide with calcium stearate effected by some formulators for its synergistic and lubricant features, is also embodied in some proprietary stabilisers. An example is stabiliser SB-739 of Rousselot-a liquid system for high-fidelity gramophone records (a powder version is used in lower-quality records, where initial high sound fidelity and fidelity retention are less crucial). CaIAlIZn: This system, and some of its uses, are exemplified by the entries for Lankromark LA 105 and Lankromet LA 175 in Table 10.5 of Chapter 10. 9.4.4
OrganiclMiscellaneons Stabilisers
None of the organic, non-metal compounds used in the stabilisation of PVC compositions can be described as a powerful primary stabiliser.
* Mixed-metal systems may be physical mixtures of the components (in which the organic parts of each metal compound may be different, or mutually the same) or 'co-precipitates' in which the organic portion is normally the same for all the metal constituents present. This is the case also with composite lead stabilisers and lead-based stabiliser/lubricant systems (ct. Section 9.4.1).
9 Stabilisers: General Aspects
287
However, some are used as sole stabilisers in composItions for food-packaging products, whilst others (epoxy compounds and phosphite 'chelators') are very important as synergistic co-stabilisers used with many primary stabilisers (notably BalCd and other mixed-metal systems-see Section 9.4.3). Because some are liquids, and others melt at PVC-processing temperatures, the organic stabilisers are uniformly and intimately dispersible in PVC compositions (but some have limited compatibility with plasticisers, or particular plasticiser/resin combinations in pPVC-see below). (a) Esters of Aminocrotonic Acid Typically these are aminocrotonates of the general formula NHH 0
I
2
I
II
H 3 C-C=C-C-oR
Many commercial stabilisers based on these compounds are approved for food-packaging applications in several European countries, including the UK, West Germany, France and Italy. They are used mainly in uPVC compositions, based on suspension and emulsion polymers or vinyl chloride copolymers, for use in the production of packaging film and blow-moulded containers. Typical incorporation levels range between 1 and 2 phr, in conjunction with an epoxy co-stabiliser (up to about 4 phr) to improve both heat and light stability. Some proprietary aminocrotonate stabilisers have limited compatibility with plasticisers in pPVC-e.g. Irgastab Gl* is recommended for use mainly as a co-stabiliser in plasticised compositions, at up to 0·4 phr to avoid exudation, whereas Irgastab A70t causes no difficulties at normal incorporation levels. In most cases, uPVC compositions stabilised with the aminocrotonates require lubricants, although some proprietary versions may have lubricant action. * Ciba-Geigy: a solid (waxy flakes) mixture of esters of aminocrotonic acid with 1,4-butylene glycol and some fatty alcohols (C 16-C 1S)' t Thiodiethylene glycol-bis(f3-aminocrotonate), sold as a free-flowing powder.
288
W. V. Titow
(b) Urea Derivatives Representatives of this group in common use are H
I
0
H
II
I
phenyl urea (C6 H s)-N-e-N-H; H
0
H
I II I diphenyl urea (C6 H s)-N-C-N-(C6H s); H S H I II I diphenyl thiourea(C6 H s)-N-C-N-(C6 H s). These compounds may be used as heat stabilisers (with little light-stabilising effect) for compositions based on alkali-prestabilised emulsion polymers, and especially in plasticised compositions of this kind (yellow discoloration may occur in semi-rigid and rigid PVC). Typical incorporation levels are relatively low, around 0·3-0·5 phr, because of the low compatibility of these stabilisers' with plasticised PVC. Long-term stability can be substantially improved by the presence of an epoxy co-stabiliser (up to about 3 phr). At temperatures above 150°C some commercial urea stabilisers may start decomposing with evolution of ammonia: this point should be checked with the supplier in individual cases (and preferably also by tests) with reference to the processing method and equipment used, as such gassing can give rise to porosity in products. The main effect of stabilisers of this type is thought to be neutralisation of any hydrogen chloride evolved as a result of thermal dehydrochlorination: this mode of action would be in keeping with the basic nature of these compounds. They are also thought to have some antioxidant action. (c) Epoxy Compounds Epoxy compounds used as plasticisers for PVC are discussed in Section 6.10.1 of Chapter 6. These, as well as others (e.g. epoxy resins marketed as components for thermoset systems), also have a stabilising effect in PVC compositions. Whilst this effect is relatively mild in the absence of other stabilising additives, epoxy compounds (and in particular some of the epoxy plasticisers, e.g. epoxidised soyabean oil, epoxy esters) are very important, and widely used, as strongly synergistic co-stabilisers for metal-based stabiliser systems, as well as
289
9 Stabilisers: General Aspects
for organic stabilisers (see above). Mixed-metal stabilisers (especially BalCd systems, but also other types) benefit particularly when used in conjunction with an epoxy co-stabiliser (and a phosphite chelator-see below). In all cases both heat and light stability are improved; the latter improvement can be exceptionally marked in many instances. Moreover, the presence of epoxy co-stabilisers/plasticisers can also confer additional benefits. Thus in rigid compositions their plasticising action (even at the 'stabilising' level of only a few phr) improves flow properties in processing (although the softening point may be lowered-d. Fig. 9.2), whilst the use of a suitable epoxy plasticiser in a PVC paste can reduce viscosity where this effect is desirable in processing, in addition to improving the heat and light resistance of the product. Epoxy stabiliserslplasticisers are resistant to extraction and migration: several (but not normally epoxidised soyabean oil) improve the low-temperature flexibility and 'cold-crack' resistance of plasticised PVC. Many proprietary products (notably epoxidised soyabean oils) are approved in most countries for food-contact applications, and-at incorporation levels of up to about 3 phr-do not affect the clarity of transparent PVC compositions. The polarity of the epoxy groups makes epoxy compounds effective as dispersion aids for solid additives (pigments, fillers, polymeric modifiers) in both rigid and flexible compositions. Features which impose some limitations on the use of epoxy
....c
~
&80 Cl
c
'c Ol
~
....ru u
;;:
70
l:.- - - - - . . . . 1-----......L- - - - - - - o 1
o
1
ESBO
2
cont<2nt, phr
3
Fig. 9.2 Vicat softening point (DIN 53460) of a rigid PVC composition stabilised with 2 phr of a liquid dibutyltin mercaptide as a function of the amount of epoxidised soyabean oil co-stabiliser present.
290
W. V. TilOW
compounds in PVC compositions are their susceptibility to microbiological attack (see Section 12.7.1, Chapter 12). and the possibility of exudation, with formation of tacky surface deposits, in some cases where-in discharging its stabilising function-the compound loses its epoxy groups through the chemical reactions involved, so that its compatibility with PVC is reduced. This effect can be a potential problem, especially in weathering or ageing situations. The compositional factors which promote it are the use of the epoxy compound as the sole stabiliser and/or its presence in a relatively high proportion as a component of the plasticiser system. Thus, to prevent this kind of problem, an epoxy compound should not be used as the only stabiliser, and its level of incorporation as a plasticiser should preferably not be higher than about 25% of the total plasticiser system in PVC compositions likely to experience any but mild ageing or weathering conditions. Note: Interaction of epoxy compounds with unsaturated additives in a PVC composition (e.g. ricinoleates or oleates which might be present in stabilisers or plasticisers) can also lead to permanent chemical modification (loss) of epoxy groups, with consequences similar to those just described. This was the cause of development of surface tackiness in, for example, calendered sheeting when, in the early days, the barium component of Ba/Cd stabiliser systems then coming into use was frequently barium ricinoleate, and addition of epoxidised oils (and phosphate chelators) was beginning to be practised as an effective way of improving heat and light stability. 34 Changing to the metal soaps (e.g. Ba and Cd laurates) and, later, liquid complexes (e.g. Ba and Cd phenates) eliminated the problem. However, inferior clarity and plate-out could still be troublesome with the soap systems (especially in high-speed, high-temperature processing), and yellowish colour and susceptibility to sulphur staining with both the soaps and the complexes: incorporation of zinc compounds subsequently became a meanS of alleviating these disadvantages (see Section 9.4.3(a) above). (d) Organic Phosphites
These are not primary stabilisers, but valuable co-stabilisers, used mainly with mixed-metal stabiliser systems (and widely with the
9 Stabilisers: General Aspects
291
principal types based on BalCd and Ca/Zn combinations). Many proprietary organic phosphite co-stabilisers are acceptable (mostly at levels of up to 1 phr) in conjunction with suitable Ca/Zn combinations for use in food-packaging film and containers (e.g. blown bottles), and in other 'non-toxic' applications. Numerous commercial stabilisers are variants of this kind of composite system containing an approved organic phosphite as one of the components, and in some cases also a permitted epoxy co-stabiliser (see above). It is well known that, when used in conjunction with suitable primary stabilisers, the organic phosphites improve the clarity of transparent PVC compositions, weathering and light stability generally, as well as heat stability (especially in pPVC). Although the mechanisms of their action are not fully elucidated, the clarity improvements are widely attributed to the formation by the phosphites of PVC-soluble complexes with the normally insoluble chlorides of the stabiliser metals (CdCl z, etc.) which are themselves formed as the metal stabilisers exercise their main functions-see Section 9.6. It is because of this that the phosphite co-stabilisers are commonly known as 'chelators'. Their heat and light stabilising effects are believed to be due to their own ability to react with hydrogen chloride,35 with labile chlorine atoms, and with double bonds of conjugated systems. 36 They are also credited with antioxidant action, albeit less pronounced than that of some additives used as primary antioxidants in PVC7-ef. Section 9.5. Commercial organic phospite co-stabilisers (typically liquids, with no appreciable lubricant effect in PVC) are either individual compounds, or mixtures of compounds of the general formula:
in which R can be a simple (but relatively long-chain) alkyl, or aryl (usually phenyl), or alkyl-aryl group, or a polyhydric alcohol derivative (e.g. a partial ester of pentaerythritol); the R groups of the same compound may not all be identical. Examples of organic phosphites available as proprietary chelator co-stabilisers for PVC include triphenyl phosphite, tridecyl
w. V. Titow
292
phosphite,37 distearyl pentaerythritol diphosphite, * tris nonylphenyl phosphite, t and mixtures consisting of a dialkyl phenyl phosphite with the corresponding alkyl diphenyl phosphite, triphenyl phosphite and trialkyl phosphite (e.g. 1rgastab CH 300, CH 301, and CH 343). The levels of incorporation of phosphite chelators are, typically, about 1 phr (fractional phr in many cases). With solid BalCd systems it is fairly usual to add one part of the chelator per three parts of the metal stabiliser combination; but, in general, the optimum amount should be verified by tests.
(e) Miscellaneous Organic Co-stabilisers Stearoylbenzoylmethane and certain polyols have already been mentioned (in Section 9.4.3(b)) as synergistic co-stabilisers for use with Ca/Zn stabiliser systems. Another organic co-stabiliser of industrial interest is 2-phenyl indole:
One use of this additive is as a co-stabiliser (at a level of about O·3-{j·5 phr) in uPVC bottle compositions, frequently non-toxic ones stabilised with a Ca/Zn stabiliser and containing also an epoxy compound and phosphite chelator as the other components of the co-stabiliser system. It is also used in some pipe and sheeting formulations.
9.5 ANTIOXIDANTS AND UV ABSORBERS 9.5.1 Antioxidants As has been mentioned, oxidation (sometimes referred to as 'auto-oxidation') is one of the mechanisms instrumental in the degradation of PVC polymers by both heat and light (cf. Sections 9.2.1 * Used in some formulations for PVC bottles in place of the common combination of tris nonylphenyl phosphite and 2-phenyl indole. t Commercial versions (e.g. Irgastab CH 55-Ciba-Geigy; Phosclere P315Interstab Chemicals Inc.) approved in many countries for food-contact applications.
9 Stabilisers: General Aspects
293
and 9.2.2). Many heat stabilisers have an antioxidant action: this is particularly marked in most of those which also exert light-stabilising effects. Primary antioxidants (without heat-stabilising action) are also used as additives in PVC, either incorporated individually or as constituents of composite commercial stabiliser systems. These additives are typically phenol derivatives (with sterically protected phenolic OH groups, popularly known as 'hindered phenols'). Examples of commercial products include lrganox 1076* (Ciba-Geigy), and lonox 330t (Shell). Some aromatic amine derivatives are also available. Incorporation is usually at low-fractional phr levels. Of the so-called secondary antioxidants of general interest for thermoplastics, the ones commonly encountered in PVC compositions are the organic phosphites (which are not, however, incorporated mainly in that capacity-see Section 9.4.4 above). There is no detailed, generally accepted explanation of the mechanism of operation of antioxidants in PVC (where the major role of dehydrochlorination in the overall degradation process is a complicating factor in this respect). However, it appears that the main ways in which antioxidants act in PVC are analogous to those now widely regarded as operative in other thermoplastics, notably polyolefinsy,38,39 In line with this view, primary antioxidants are essentially scavengers of free radicals, believed to interrupt-by this type of action-the progress of oxidative degradation, generally thought to proceed through a free-radical mechanism. 39 Additives with secondary-antioxidant action, notably phosphites (including organophosphite chelators and dibasic lead phosphite), are able to remove peroxide radicals and to decompose hydroperoxide groups (which can act as free-radical initiators in the oxidative degradation process) converting them to inactive derivatives through chemical reaction. Since, in PVC, oxidation is both a degradation mechanism in its own right and a factor increasing the rate of dehydrochlorination (cf. Section 9.2.1), incorporation of antioxidants can improve the heat stability of PVC compositions as well as-in many cases and types of formulation-stability to light. Where they are added individually, and not introduced as constituents of composite proprietary stabiliser systems, primary antioxidants may typically be used at levels of about 0·1 phI.
* Octadecyi 3-(3,5-di-tert-butyi-4-hydroxyphenyi)propionate.
t 2,4,6-Tris(2,5-di-tert-butyi-4-hydroxybenzyi)-1,3 ,5-trimethyibenzene.
294
w.
V. Titow
Some antioxidants, e.g. Irganox 1010* (Ciba-Geigy), can be used in very low proportions (about 200 ppm) as additives to the polymerisation mixture in the production of PVC polymers, to improve the polymer's resistance to relatively intense heat treatment and thus improve the efficiency of heat-stripping of VCM.
9.5.2 UV Absorbers The cardinal role of UV radiation of wavelengths 290-315 nm in the photodegradation of PVC polymer has been mentioned in Section 9.2.2 (310 nm is often quoted as the wavelength most damaging to PVC, and 290-400 nm as the band instrumental in the photodegradation of plastics generally). Light in the 'damaging' wavelength range excites the PVC polymer molecules, in the sense of imparting excess energy sufficient to break bonds in the molecular chains: the free radicals formed as a result initiate, and participate in, the degradation process, which is accelerated by the presence of oxygen (cf. Section 9.2.2). Whilst many heat-stabilisers and antioxidants provide a measure of protection against photolytic and photooxidative degradation of PVC, they operate essentially after the process has started (in the case of antioxidants mainly by disposing of the free radicals formed). Moreover, they are used up as they exercise their protective functions, so that the protection is of finite duration. The UV-protective additives which are frequently included in PVC materials for outdoor use afford additional and complementary protection, in that they absorb and dissipate the incident UV radiation essentially before it can initiate degradation, without themselves undergoing de-activating chemical changes (see below). The types of organic compound in widest commercial use as UV absorbers for PVC compositions are modified benzophenones (especially certain alkoxy derivatives of 2-hydroxy or 2,2-dihydroxy benzophenone), and benzotriazole derivatives. Proprietary additives of the first type are exemplified by Cyasorb UV 9, UV 24, and UV 531 (American Cyanamid Co., Polymers and Chemicals Dept and Cyanamid of Great Britain Ltd); Uvinul D408 (BASF Wyandotte Corp.); UV-Chek AM 541A (Ferro Corp., Chemical Div.); Carstab 700 (Carstab Corporation). Benzotriazole compounds figure promin* Pentaerythrityl tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].
9 Stabilisers: General Aspects
295
ently in the Ciba-Geigy Tinuvin light-stabiliser range (e.g. Tinuvin P, 320, 326, and 327). Other organic compounds represented among proprietary UV stabilisers available for PVC include substituted cyanoacrylates (e.g. ethyl 2-cyano-3-diphenyl acrylate, cf. also Uvinul N-35 and N-559GAF Corporation), phenyl salicylate (Salol-Dow Cpemical Co.), and oxalic anilide (Sanduvor VSU-Sandoz). Polymeric UV stabilisers are exemplified by a polymer of 2,4dihydroxy benzophenone, originally prepared40 with a view to reducing the ease of diffusional migration of the additive to the surface of a PVC material (and subsequent loss by volatilisation, leaching and wear) which may be regarded as a disadvantage of non-polymeric benzophenone UV absorbers. 41 The functional suitability and effectivity of the organic UV absorbers just mentioned depends on the following combination of properties: Generally high absorption coefficients for UV radiation in the 'damaging' range and in particular much higher than those of PVC polymers. (ii) Ability to re-emit the energy absorbed in the form of UV radiation as lower-energy IR radiation (i.e. heat), relatively harmless to the PVC composition. (iii) Negligible absorption of visible light (so that there is no appreciable effect on the colour and transparency of the host PVC composition).
(i)
It is because of (i) that the organic UV stabilisers exert their effect at relatively very low incorporation levels, typically between 0·2 and 0·8 phr. In most cases the degree of UV absorption increases with concentration within the useful range. However, the protective effect is also known to be dependent on the thickness of the PVC material, in that-for the same concentration of stabiliser-it is markedly less in fine fibres, very thin films, and surface layers of thicker products. This phenomenon accords with the reported finding 42 that-at a given stabiliser concentration-the amount of UV radiation reaching a point in the substrate drops exponentially with the distance of the point from the material's surface. These facts are in line with the expectation that the degree of obstruction of the path of an incident ray by particles or molecules of evenly dispersed stabiliser (i.e. the probability that one or more particles or molecules will lie directly in the path) should increase
296
W. V. Titaw
fairly sharply with distance into the material: interaction of the radiation with the PVC material itself would also be a factor progressively reducing the amount of radiation with distance of penetration. * Note: The validity of this explanation is not affected (and is in fact indirectly supported) by the frequently cited observation42-44 that the protective effects of two other kinds of additive with light-stabilising action-viz., phenolic antioxidants, and the nickel complexes used mainly in polyolefins-are independent of material thickness. Neither of these two types of stabiliser interacts with UV radiation. As has been mentioned, phenolic antioxidants are essentially scavengers of free radicals. The nickel complexes are known as 'quenchers' because they de-activate, evidently by resonant energy transfer, some groups in the polymer chain (e.g. carbonyl groups formed in polyolefins in the course of photochemical degradation) which have become 'excited', i.e. raised to a higher energy level by absorption of UV radiation (and thereby primed to promote chain scission).42,43 Thus both types of stabiliser act on chemical species formed as a result of absorption of UV energy by the polymer (i.e. only after this has already taken place), so that, as would be expected, the only locational factors influencing their operation are the distribution and concentration of those species in the polymer, in relation to their own distribution and concentration.
Compatibility with the rest of the PVC composition, and any interactions that may occur, are considerations as relevant in the case of UV absorbers as they are for any other formulation constituent. The following practical points may be mentioned in this connection. Joint use of organic UV absorbers with antioxidants can enhance the effect of both in many PVC compositions. Their use in conjunction with a tin carboxylate stabiliser (dibutyltin maleate) has been claimed to have no beneficial effect on stability in outdoor exposure, and even to impair it * In all materials the extent of degradation by light, and weathering generally, is also dependent on thickness for purely geometrical reasons, in that whilst the surface is the most directly exposed to attack, and therefore the material of the outermost surface layer suffers the earliest and most extensive degradation, that material will also represent an increasing proportion of the total as the overall material thickness is decreased (the surface-to-volume ratio is increased) .
9 Stabilisers: General Aspects
297
in some cases. 45 Some organic UV absorbers can form metal complexes with certain metal-based stabilisers, giving rise to undesirable colour in the PVC material, as in the following cases: -Yellow coloration (overall or in spots) can develop, in the presence of alkali, in compositions (especially flexible compounds) stabilised with BalCd or BalCd/Zn systems in conjunction with certain benzophenone or benzotriazole UV absorbers (normally only those containing phenolic hydroxyl groups which are 'unhindered', i.e. not sterically protected and thus available to participate in complex-formation): the alkalinity may be introduced by the heat stabiliser system (e.g. some BalCd stabilisers, although neutral versions are widely available), or where the PVC polymer is an alkali-prestabilised emulsion resin, or adventitiously from some external source. -Pink to mauve colour can arise where a benzotriazole UV absorber and a thiotin stabiliser are used together. Some dihydroxybenzophenone compounds can develop a yellow colour in the presence of nitrogen oxides (which may be produced in the burning of gas, coal, or oil, or when air passes over incandescent wires). This phenomenon is similar to the 'gas-fume fading' long known to occur in man-made fibres dyed with aminoanthraquinone dyes and certain others. 46-48 However, the alkoxybenzophenones, benzotriazoles and salicylates used as UV absorbers in PVC are not normally susceptible. Some UV absorbers can interfere with fluorescent colourant effects. Where non-toxicity is a consideration, selection of organic UV absorbers should be made in the light of advice from the suppliers and, if relevant, also from the appropriate authorities (cf. Chapter 7, Section 7.12 and Chapter 12, Section 12.9). Carbon black and titanium dioxide, widely used and important as pigments for plastics, have a light-stabilising effect on many polymers, including PVc. Because of their particulate nature, and their mode of action, they are often referred to in this context as 'screening agents', 'screeners' or 'light screens'. Each functions as a physical barrier to radiation, both UV and visible. Carbon black absorbs the radiation over both these wavelength ranges and, like the organic UV absorbers, emits the energy in the IR region: it is also believed to act as an antioxidant by capturing free radicals (cf. Chapter 8, Section 8.4.3): selected fine-particle grades should be used, typically at a few phr.
298
w.
V. Titow
Titanium dioxide also has some UV absorption but its screening action is principally due to reflection and scattering of radiation (IR, visible, and UV). This mechanism is less efficient than that of carbon black, and higher loadings are needed (typically 5-15 phr) for appreciable effect. It is self-evident that the use of both these pigments as light stabilisers is ipso facto restricted to opaque compositions (and only black ones in the case of carbon black). Zinc oxide is another particulate, inorganic screening agent which may be considered for those PVC compositions that are not too zinc-sensitive (cf. Section 9.4.3 above): it is thought to benefit synergistically from the presence of certain antioxidants, to the point where suitable combinations of this kind can be more effective than normal amounts of organic UV stabilisers. As has been mentioned, UV absorbers incorporated in PVC compositions are less effective in the outermost surface layers than in the body of the products; yet it is the surface which needs the most protection, as it is the most directly and extensively subjected to photochemical attack on exposure. The problem is particularly relevant in transparent compositions for outdoor service, in which UV absorbers are widely used: surface application, rather than internal incorporation of the stabiliser, can be a useful solution in some cases. PVC sheeting, for example, may be surface-coated with a lacquer containing a relatively high concentration (say 1-3%) of the stabiliser: such lacquers, based on vinyl copolymer resins (cf. Chapter 24), acrylic resins, or other binders, are available from commercial sources. This approach can offer cost and protection efficiency advantages in suitable circumstances. 49 Surface absorption (from solution) of a UV stabiliser (MPB*) has also been claimed to give good results. 49 Such physical factors as the degree and uniformity of dispersion, ease of migration through the composition, extractability, and volatility, influence the performance of any additive in PVC: a useful discussion of their effects on the performance of stabilisers, including UV stabilisers, has been published by Allara. 23 Antioxidants, UV absorbers and screening agents are available individually, as composite light-stabiliser systems, and as components of polyfunctional 'single-pack' additive combinations. The individual stabilisers or stabiliser systems are also supplied as ready-made or * 2-(2'-Hydroxy-5' -methylphenyl)benzotriazole.
9 Stabilisers: General Aspects
299
custom-compounded concentrates in PVC polymer or other appropriate carriers. Many sources are given in the publications mentioned in Section 8.5 of Chapter 8.
9.6 MAIN MODES OF STABILISER ACTION The subject of stabilisation of PVC is closely interrelated with that of its degradation. Both have been investigated and discussed for a long time, and various mechanisms have been put forward to account for the observable effects of stabilisers. In the present state of knowledge none of these mechanisms can be regarded as fully proven. However, those mentioned below in connection with what might be termed the major stabilising actions are based on a substantial body of evidence, and enjoy considerable support. These actions, major in the sense of their direct bearing on the factors believed by many to be of principal significance in the degradation of PVC (see Section 9.2), are: neutralisation of hydrogen chloride evolved in the course of dehydrochlorination; elimination of labile chlorine atoms from the polymer chain (by substitution with more stably attached chemical groups); and antioxidant action (mainly through inactivation of free radicals). Other actions and mechanisms postulated in explanation of the effects of some stabiliser types and systems are mentioned, as appropriate, under the relevant sub-headings further on in this section. 9.6.1
Lead Stabilisers
Neutralisation of nascent hydrogen chloride is thought to be the main mechanism whereby these stabilisers exert their effect. 7,9,11 This view accords with their basic nature, their mode and rates of reaction with HCI,5o and the formation of lead chloride during heat treatment and service of lead-stabilised PVC compositions. Unlike the chlorides of some other stabiliser metals, this salt does not promote degradation of PVC polymer. The known fact that, in general, the performance of lead stabilisers is not much improved by the presence of co-stabilisers (especially the chelators, or metal carboxylates used as components of mixed-metal stabiliser systems-see below), is probably associated-at least to some extent-with the inactivity of lead chloride in this sense, since de-activation of stabiliser decomposition products and impurities is believed to be one of the main functions of such additives.
300
W. V. Titow
9.6.2 Organotin Stabilisers These stabilisers are believed to function in several ways. Dialkyltin chlorides have been identified among the volatiles evolved in the heat-processing of PVC compositions stabilised with dialkyltin compounds. A corresponding drop in the tin content of the compositions has also been observed, increasing with time of treatment. 7 These observations are consistent with the binding of hydrogen chloride by the stabiliser, presumably in accordance with the general reaction:
(4) where A is a mercapto or carboxylic acid substituent (see Section 9.4.2(a». Like lead chloride, the dialkyltin chlorides do not impair the stability of PVC polymer. Work with model compounds (employed as PVC polymer analogues)7,51 has shown that organotin stabilisers can replace labile (allylic) chlorine atoms with their own ligand (A) groups, whereby the stability of the thus substituted compound is significantly increased.52-54 The substitution could be either direct,l1 or possibly on the carbon originally in the 4 position to the chlorine: 7
(5)
or
It has also been suggested7 that organotin stabilisers may be able to form transitional complexes with 'instability centres' in the polymer. It would appear that free-radical scavenging, proposed in the case of some mercaptides,l1 may be among the modes of action of organotin stabilisers. Colour developed in PVC compositions as a result of heat-processing
9 Stabilisers: General Aspects
301
may be reduced, in some cases substantially, by post-treatment (re-compounding) with an organotin stabiliser. This indicates that yet another way in which these stabilisers perform their functions is to react with chromophoric groups. The chemical transformations suggested to account for the reduction of colour include reaction of the stabiliser with any carbonium groups (cf. eqn (2)) formed in the course of PVC degradation,? and with double bonds in conjugated systems. The latter type of action would interrupt the conjugated double bond sequences (thus reducing colour due to their presence) and also increase the general stability of the polymer chain: two mechanisms proposed for this are noteworthy. According to one, the carboxyl or mercapto compound generated by the organotin stabiliser in its reaction with HCl (cf. eqn (4)) may react with (presumably by adding across) a double bond in the polymer chain: 7 ,1l H H
~C=C~
+ HA -
H H
~C-C~
H A
(6)
Some support for the idea of this kind of effect is provided by an experiment in which the colour of a heat-affected PVC material was considerably reduced by re-processing with 1% ot the isooctyl ester of thioglycollic acid (even though this entailed a further heat treatment at 180°C).7 The other possibility suggested7,1l is that, where the regenerated compound (HA) is a maleic acid derivative, it may enter into a Diels-Alder reaction 55 with a diene section of the polymer chain, thereby eliminating a double bond, and introducing into the chain a relatively stable ring structure:
I H~ HC
Hn-COOR H6 + HC-COOR II
Hi
CH HC/'
'cH-COOR H~ tH-COOR . . . . . . C H/
(7)
!
The fact that dibutyltin maJeate is a better heat stabiliser than the corresponding succinate (which would not undergo a Diels-Alder reaction) has been cited7 in support of this possibility.
302
w.
V. Titow
9.6.3 Other Metal-based Stabilisers Displacement of labile chlorine atoms in the PVC polymer chain by more stable groups appears to play an important part in the action of the metal compounds of acids (salts, soaps, 'complexes', and mercaptides-see Section 9.4.3(a) above) used as stabilisers for PVc. Work on model compounds has shown that such stabilisers can substitute their acid groups for allylic (but apparently not tertiary) chlorine. 56 Substantial evidence that the acid groups of barium, cadmium and zinc carboxylates are indeed transferred to PVC polymer in conditions under which these stabilisers operate, was obtained in the radiochemical and IR-spectroscopic investigations of Frye and Horst. 57 Together with other reported evidence,l1 these findings suggest a general stabilising mechanism analogous to that of eqns (5) or (5a). Virtually all the metal compounds under the present heading react with hydrogen chloride. This reaction is regarded as one of the modes of their stabilising action. However, some of the chlorides formed, notably those of zinc and cadmium, strongly promote decomposition of PVC polymer: thus in these cases the HCI-acceptor action is not entirely beneficial. As has been mentioned, the mixed-metal systems in widest use are those combining barium and cadmium compounds, with or without a zinc component. Some calcium/zinc carboxylate combinations are also popular for non-toxic PVC compositions, and certain others (see Section 9.4.3). It appears most likely that the stabilising action of-and synergistic effects in-both these general types of composite system arise as follows. Chlorides and semi-chlorides (ct. eqns (8) and (9) below) formed from the cadmium and zinc compounds in the course of exchange of their acid groups for the labile chlorine atoms of the PVC polymer (which may well constitute the main stabilisation mechanism) are converted back to the original compounds by exchanging the chlorine for the acid groups of the barium (or calcium*) component of the system. This activity eliminates the harmful cadmium and/or zinc chlorides, and regenerates the powerful cadmium stabiliser. Direct reaction of the barium compound (or calcium compound in Ca/Zn composite systems) with nascent HCI evolved through dehydrochlorination of PVC polymer, whilst itself yielding a * The suggestion has also been put forward 58 that in Ca/Zn carboxylate systems the calcium carboxylate plays a part in the substitution of allylic chlorine atoms by the carboxylate groups of the zinc compound.
303
9 Stabilisers: General Aspects
relatively innocuous chloride, would restrict similar direct formation of cadmium and zinc chlorides: any CdCl z or ZnClz actually formed by this route would also be expected to be re-converted in the way just mentioned. These regenerative and protective processes should proceed until all the barium (or calcium) compound has been used up. Note: After heat treatment of sufficient severity and duration, PVC compositions stabilised with Ba/Cd carboxylate systems tend to darken rapidly to an almost black colour, much darker than that characteristic of normal thermal degradation of similar compositions without stabiliser. This is consistent with the expected catalytic effect of accumulation of cadmium chloride, after the regenerative capcity of the barium component of the system has been exhaused by its total conversion to BaClz. Acceleration and aggravation of the degradation of PVC polymer by the presence of CdCl z formed throughout the material is also the most likely explanation for the well-known fact that compositions stabilised solely with cadmium compounds darken badly after a relatively short time (cf. Section 9.4.3(a)). Formation and accumulation of zinc chloride accounts, in an analogous manner, for the rapid, intense darkening of PVC stabilised with Ca/Zn systems once the stabiliser is exhausted.
Considering a BalCd carboxylate system as an example, and on the assumption that the carboxylate group being exchanged for a labile chlorine attaches itself to the same carbon in the PVC polymer chain (i.e. that, in this respect, the exchanges of eqns (8) and (9) are analogous to that of eqn (5) and not (Sa)), the above explanation is illustrated by the following reactions: 7 ,1l,59,60
¥
~C~
I CI
¥
~C~
~]
/OOCR +Cd --+ '-.....OOCR
/OOCR +Cd --+
~l
¥
~C~
60CR
+ Cd
/OOCR
(8)
'-.....0
(9)
304
W. V. Titow
Cd
. . . . . . 'OOCR . . . . . . CI
. . . . . . .O OCR + Ba . . . . . . OOCR
OOCR Cd"""""'" + 'cl
Cd
. . . . . . .Cl ............
CI
OOCR
Bi
--+
"cl . . . . . . .O OCR
+ Ba
............
. . . . . . .O OCR . . . . . . .O OCR Cd +Ba (10) . . . . . . O OCR 'cl . . . . . . .O OCR . . . . . . .Cl Cd +B, .........OOCR CI . . . . . . .O OCR
------.OOCR
--+ elf
OOCR
. . . . . . .Cl
+ Ba
............
(11)
(12)
CI
In eqns (8)-(12) the R group of the original cadmium compound may be the same as, or different from, that of the barium compound. The main functions of an epoxy compound when used as co-stabiliser with mixed-metal stabiliser systems are thought to be direct binding of nascent HCI and assistance in the transfer of labile chlorine atoms to the main stabiliser (see below). Auxiliary mechanisms thought by some authors to be operative in the stabilisation of PVC with metal-compound stabilisers include catalytic oxidation of chomophoric groups,7 and 'control of the dehydrochlorination process' .61 9.6.4 Organic Stabilisers, Antioxidants, UV Stabilisers
The ways in which these additives are believed to operate have been indicated in Sections 9.4.4 and 9.5. The following additional points may be mentioned here. Epoxy compounds are widely thought to bind HCI through their oxirane groups: 7 ,9,11,62
-CH-CH- + HCI "0/
-CH-CHI I OH CI
(13)
These groups are also believed to participate in the role the epoxy co-stabilisers may play in the transfer of HCI to the main stabiliser,62 and the exchange of the PVC polymer's labile chlorine atoms for stabiliser groups. 11 The antioxidant action attributed to organic phosphite co-stabilisers (cf. Section 9.4.4) may operate through the following kind of reaction
9 Stabilisers: General Aspects
305
with free radicalsY
*
~CH-CH2~
CI*
+ P(OR)3
----+ ~TH-eH2"""""
(14)
O=P(ORh + RCl
However, the function with which these additives are most widely credited, and after which they are known as 'chelators', is the formation of complexes with the metal chlorides arising as by-products of stabilising reactions (see above). This 'complexing out' reduces the deleterious effect of the chlorides on the PVC polymer, and also improves the clarity of transparent PVC compositions (ct. Section 9.4.4). The mechanism of operation of po/yo/ co-stabilisers used, like the phosphites, with mixed-metal stabilisers (cf. Section 9.4.3), is believed to be similar. 9.7 SOME GENERAL EFFECTS AND COMMON FAULTS IN STABILISED COMPOSITIONS
Some general effects of stabiliser-associated factors upon the properties of PVC compositions are summarised in Table 9.2. The most common general faults to which certain stabilisers may give rise in PVC compositions are plate-out and sulphide staining. Both have been mentioned in passing: their principal features may be summarised as follows. 9.7.1 Plate-out
This is the build-up of sticky deposits on the working surfaces of processing equipment, which can occur in all the principal processes (extrusion, calendering, moulding) as well as in some ancillary operations (e.g. milling, embossing). The exact causes of plate-out are still not fully elucidated, * but much practical experience is available * Some interesting direct evidence has been produced by Lippoldt's78 investigation into the composition of plate-out deposited on an extruder die spider from a tin-stabilised PVC compound containing-among the other constituentsthree types of lubricant (including Ca stearate), CaC0 3 filler, and Ti0 2 pigment. The organic material content of the deposit was found to be about 12 times that of the 'parent' compound. Calcium stearate lubricant and the tin stabiliser were present (with the stearate: stabiliser ratio increased somewhat over that of the compound), as well as a substantial proportion of resinous material described as 'the result of accidental PVC particle contamination', some pigment and filler. A tentative mechanism for plate-out formation has been formulated by the author on the basis of the results (cf. pp. 687-8).
Selected octyltin compounds. Ca/Zn systems (with permitted costabilisers) . Organotin stabilisers. Selected Bal Cd systems (with epoxy and phosphite co-stabilisers). Selected Cal Zn systems (for non-toxic, clear compositions). Organotin stabilisers give best results. Some metal soaps can cause problems. In general, stabilisers should be checked for effect on viscosity. Liquid stabilisers should be used for reduction (or least increase). The pH of the stabiliser may have an effect.
Processing and service
Service (electrical insulation)
Service
Service
Heat welding
Processing
Lubrication
Electrical properties
Non-toxicity
Clarity
Weldability
Paste viscosity
In general, liquid stabilisers tend to lower the softening point of the composition. Suitable choice and balance of the stabiliser/lubricant system very important, especially in uPVc. Lead stabilisers are the usual choice (ct. Section 9.4.1).
Relevant stabiliser types, factors, and effects
Service
Significant in:
Softening point (uPVC)
Property
ct. Chapter 22.
Clarity is influenced by the refractive indices and mutual compatibility of all components of the PVC composition concerned. NB In damp conditions cloudiness can develop in some Ba/Cdstabilised clear compositions.
decoration and heat-sealing properties may be affected by stabiliser exudation. Properties important in this connection are high resistivity, low power factor (esp. for high-frequency cables) and high electric strength.
ct. Chapter 11, Section 11.2. NB Surface-
This consideration can be important with many rigid PVC products.
Remarks
TABLE 9.2 Some General Effects of Stabilisers on Properties of PVC Compositions
~
~
:-::::
;:E:
~
...,
9 Stabilisers: General Aspects
307
relevant to its incidence and prevention. Thus it is known that the nature and amount of the stabiliser is a factor, and that other components of the PVC composition-especially the lubricant (with particular reference to its balance and interaction with the stabiliser)can also playa significant part. Inter alia, plate-out can be associated with the simultaneous presence of calcium compounds with those of heavy metals (Cd, Zn, Ti, Mn, Pb).63 On the other hand it is known that PVC compositions based on emulsion polymers are less prone to plate-out than those in which the polymer is a suspension resin, and that addition of emulsion resin to a composition of the latter type can sometimes alleviate the problem. Incorporation of small amounts of fine-particle pigments or fillers (e.g. titanium dioxide, precipitated calcium carbonate, at 2-5 phr) can also be helpful, especially where the plate-out is relatively light and there is reason to think that it may be due to an imbalance of the lubricant or lubricant/stabiliser system. Some manufacturers supply special grades of silicates as anti-plate-out additives (e.g. Gasil 35-Joseph Crossfield & Sons Ltd, England). The effect of fine-particle additives is thought to be due to retention, by adsorption, of the compound responsible for plate-out deposits, and to a direct 'scrubbing' action on the working surfaces as they come into contact with the composition. 64 Pronounced plate-out can occur with some compositions stabilised with lead and other metal soaps. Those stabilised with organotins and certain liquid mixed-metal combinations (and containing suitable, properly balanced lubricant systems) are the least prone to this trouble. A colour-transfer test may sometimes be useful as a means of checking the plate-out tendency of a PVC composition. The principle of such tests is that a test mix, made up to the particular formulation and containing additionally a small amount of an appropriate colourant, is run in suitable laboratory-scale equipment (e.g. a mixer; roll mill) under conditions relevant to the intended full-scale processing of the composition. The colourant should be one that will enter and colour the plate-out deposit-a colourant supplier should be consulted regarding the choice. After the test run the mix is completely removed and a white scavenging mix run in the equipment. The plate-out from the test mix is assessed on the amount of colour picked up by the scavenging composition. One variant of this kind of procedure is exemplified by the following recommendations. 7
308
W. V. Titow
Colourant for test mix: Sico Red WRC* (0,1 weight % on the total mix). Run the test mix for 7 min at 165°C (without friction); sheet and remove from the mixer. Run the scavenging mix for 3 min: remove and judge the degree of red staining (related to the amount of plate-out) by comparison with an unused portion of this mix. For increased accuracy of assessment, the comparison specimens may be moulded into sheets of identical size. Constant, uniform conditions are essential; in particular the mixer should be in thermal equilibrium (achieved by running for some hours prior to the test): the tests should be run by one and the same operator as a continuous series, and each series completed on the same day. The recommendations include the following formulation for the scavenging composition.
100 PVC polymer: So/vic 239 (Solvay et Cie. SA, Belgium) Stabiliser: Irgastab OM 18 (Ciba-Geigy) (dibutyltin maleate) 0·5 phr Epoxy co-stabiliser: Reop/ast 39 (Ciba-Geigy) 5·0 phr Plasticiser: DOP 30·0 phr Lubricants: Irgawax 330 (Ciba-Geigy) (stearic acid) 0·8 phr Irgawax 331 0·8 phr White pigment: Kronos A (Titangesellschaft mbH, West Germany) (Ti0 2-anatase) 1·5 phr 9.7.2 Sulphide Staining PVC composItions contammg stabilisers (or other additives, e.g. pigments) based on cadmium or lead can develop colour on contact with sulphur compounds. This phenomenon is known as sulphide staining. In cadmium-containing compositions the colour is yellow, caused by the formation of yellow cadmium sulphide (CdS). Compositions incorporating lead compounds may tum black-in patches or overall-due to the formation of black lead sulphide (PbS), or sometimes dark brown ranging to black: this is probably attributable to the presence of the reddish-brown lead sulphochloride, Cl.Pb.S.Pb.Cl.
Note: Zinc-containing additives do not give rise to dark sulphide staining (zinc sulphide is white), and indeed the zinc component of a Bd/Cd/Zn stabiliser may retard colour development. However, turbidity may develop in clear compositions if enough ZnS is formed (see also Table 9.3). * BASF (originally a trade name of the Siegle company, West Germany).
Organic sulphur - free
Organic sulphur- ctg
Ca/Zn
Ba/Cd/{Zn)
Tin carboxylate
Thiotin
Lead
Organic sulphur - free Ca/Zn
~
Sulphide formation, with visible effects as indicated
No effect
Tin carboxylate
D
Ba/Cd/{Zn)
Lead
TABLE 9.3 Visible Manifestations of Interaction of the Main Types of Stabiliser in PVC Compositions
w
~
'"'0;"'
~
~
~
~
'"~ a
~
<:>-
~
'0
310
w. v.
Titow
The sulphur compounds responsible for sulphide staining may be encountered by the PVC as atmospheric pollutants, or as components of materials (e.g. rubber) with which the PVC may come into physical contact: they may also be present in the composition itself as its regular components. Although simultaneous use of sulphur-containing additives with cadmium or lead compounds (e.g., say, thiotin or antimony mercaptide stabilisers in conjunction with lead or cadmium ones) is avoided in formulating practice, it can occur in the processing of PVC scrap of different or uncertain origins. Where fresh stabiliser is to be added to a batch of scrap material of unknown composition to 'post-stabilise' it for processing and service, or where two or more such batches are to be mixed in processing, the possibility of internal sulphide staining (cross-staining) should be checked beforehand (even for black material). A simple compounding test, e.g. on the mill for about 10 min at 180°C, is regarded as satisfactory in most cases in practice,64 and easier than analysis for heavy metals and sulphur. Where the scrap material is to be post-stabilised with a lead-, cadmium-, or sulphur-containing stabiliser, the test should be carried out on a sample to which about 3% by weight of the stabiliser has been added, and the sample then examined for colour development. Absence of discoloration will indicate that the stabiliser is 'safe' for use with the material: the appearance of colour (or milky haze in transparent materials) provides not only evidence of interaction but, from the nature of the manifestation, also clues for the choice of suitable alternatives (see Table 9.3). The compounding test should also be carried out where scrap from different sources is to be processed with or without post-stabilisation. Several standard tests are available for susceptibility of PVC compositions to sulphide staining through external contact. BS 2739: 1975 (Appendix D) prescribes immersion (for 30 min) of a sheet specimen (50 mm 2) in a freshly prepared solution of 55 g of hydrated sodium sulphide (NaS.9H 20) in distilled water (1 litre), acidified with concentrated hydrochloric acid (30 ml). In the method of ASTM D 1712-65 (1977) thin sheet specimens (preferably about 100 mm by 13 mm) are half-immersed (for 15 min) in a saturated solution of hydrogen sulphide prepared by rapidly bubbling freshly produced H2S gas through 10~150 ml of water for about 5 min. The method of DIN 53378-1965 involves exposure of film specimens to gaseous H 2S (continuously generated by reaction of sodium sulphide with sulphuric acid). In all three methods any staining is detected visually, and its
9 Stabilisers: General Aspects
311
intensity determined, where relevant, in terms of colour change in comparison with untreated material or/and with the aid of the appropriate standard grey scale for stain assessment (BS 2663; DIN 54001; see also Chapter 12, Section 12.6). Note: The standard ASTM test method for the staining of PVC by rubber-compounding ingredients (ASTM D 2151-68 (1977}ct. Appendix 1, Section 3.2(c) (ii)) caters for staining resulting from actual migration of such compounds (especially certain antioxidants) into PVC, whereupon colour may develop either immediately, or after exposure to heat or UV radiation.
The development of milky cloudiness when a clear PVC composition incorporating zinc in the stabiliser system is in contact with some sulphur-containing gases or materials (see above, and Table 9.3), can cause varying degrees of loss of transparency. This effect can be quantitatively assessed with the aid of the standard methods of ASTM D 1746 or DIN 53490. For a rough, qualitative assessment a simple comparison may be made by viewing, in good light, a specimen of the clouded compound, in thin sheet form, laid on a white page carrying a few lines of sharp black print, side-by-side with a similar specimen of the original (unaffected) material. 9.8 TESTING AND EVALUATION OF STABILISER EFFECTS 9.8.1 Concept of Stability in Processing, Service and Tests PVC is subject to degradation by heat and light in ways outlined in Section 9.2. The ease and rate of degradation vary with the composition and the conditions of treatment or exposure. Therefore, in the practical context, the stability of a PVC composition may be thought of, and measured, in terms of the length of time before perceptible and/or measurable signs of degradation develop, or reach a certain level, under the relevant conditions. In such terms the well-known fact that the stability of unstabilised PVC is very poor can be quantitatively related to the ease and rapidity of its degradation on exposure to even relatively moderate heat or irradiation with light containing a UV component. Note: With regard to the effects of heat processing, it may be noted that the activation energy for degradation of PVC polymer
312
W. V. Titow
(cf. Section 9.2.1) is considerably lower than the specific energy required to generate a melt. 16 This alone would necessitate stabilisation for melt processing. Incorporation of suitable stabilisers may greatly increase the time for the onset of appreciable degradation even under comparatively severe conditions of processing and/or service. However, no practicable amount of stabilisation can entirely prevent degradation where the conditions are severe enough to promote it: the fact that primary stabilisers are used up in reactions through which they exercise their protective effects is a cardinal factor in this situation. For practical purposes the stability of PVC materials may be conveniently defined and compared in terms of stability time, or induction time for degradation. The first of these is the length of time up to the point when, under a particular set of conditions, selected manifestations of degradation reach a level set as the acceptable maximum: in tests the conditions are usually those of static heating, or mechanical working with heating, of the material at a suitable, elevated temperature, or exposure to light or weathering (see Section 9.8.2). The induction time is the period after which, in analogous circumstances, the rate of degradation (as reflected in the manifestation selected for assessment) changes from an almost negligible to a relatively high value. The definitions of stability time and induction time are illustrated by the curve of Fig. 9.3.
r-----------------Maximum
acc~ptabl~ l~v~1
of
d~gradation
I
x ~ S
I
I I
c
2ro
I
I I I
~
~ ~
a
Tim~
Fig. 9.3
i
~
Induction time (tj) and stability time (ts) of a PVC composition subjected to heat treatment: schematic representation.
9 Stabilisers: General Aspects
313
As mentioned in Chapter 12 (Section 12.3), the extent and/or progress of degradation in PVC may be evaluated by determining changes in selected physical properties (e.g. tensile strength, modulus), or by various kinds of analysis (differential thermal, thermogravimetric, IR, chemical). However, for the purposes of direct evaluation or determination of the effects of stabilisers, the degree and rate of degradation are most often measured in terms of the two main manifestitations of dehydrochlorination, viz. the development of colour, or the amount of HCI evolved in the PVC composition. Note: In stability tests in which colour development is used as an index of degradation it may be relevant to make a distinction between 'time to colour' and 'time to black'. 65 Blackening of the composition, indicating extensive degradation, may develop gradually, or occur rapidly as a result of accumulation of chlorides of stabiliser metals which can catalyse decomposition (e.g. CdClz-ef. Section 9.6.3).
Two concepts of practical significance in connection with the stability of PVC in processing (and to some extent in service, especially at elevated temperatures) are the heat life and heat history of a PVC composition. The heat life is the period during which, under given conditions (or successive sets of different conditions, as e.g. in hot compounding followed by heat-processing into a product) the composition remains substantially free of significant degradation. Thus the heat life is closely represented by the stability time (or the induction time, depending on the way in which 'significant degradation' is defined), and like the stability (or the induction) time it will be the shorter the more severe the heat treatment(s) experienced, or the less effective the stabilisation-see Figs 9.4 and 9.5. Implicit in the concept of heat life is recognition of the fact, highly important in practice, that heat treatment-or a number of consecutive heat treatments--of a given nature, intensity and duration, Le. a certain 'heat history', may be considered to use up a proportion of the heat life: the remainder then represents the residual, shorter heat life still available. In this way the extent and severity of its heat history determines how close a PVC composition is to the point of significant degradation, Le. to the limit of its total heat life. In the schematic illustration of Fig. 9.6 the three curves represent the heat life (total stability time) of the same PVC composition under the conditions of three different treatments (A, B and C) of varying severity (A> B>C). If the composition is initially
314
r
X eI .... - - -
"
W. V. Titow
B
A
c
_ ~a~i~u~ ~,=~t~b~ -.!.q::q!...O!..d!:g~~~on
.S c:
°
:;:; C1I
~ L.
Cl
eI
o
Timq--_.
Fig. 9.4 Effect of stabilisation (or severity of treatment) on the stability time (t) of a PVC composition: schematic representation. A, No stabiliser (or most drastic conditions); B, moderately effective stabilisation (or medium-severe heating); C, highly effective stabilisation (or mildest treatment).
100 1
2 3
.... 10
2·3
2·2
2·1
1/& , K- 1 x 103
2·0
Fig.9.5 Arrhenius plot (ct. Section 9.2.1) of log stability time (ts> minutes) as a function of the reciprocal of absolute temperature (1/8) for three PVC compositions of increasing stability (1) 2> 3): schematic representation (but values roughly representative of some uPVC compositions, with HCl emission as the degradation index).
9 Stabilisers: General Aspects
r
T
x
.g
.S
c
o
~
Maximum accczptablcz
+----- --- Iczvczlof dczgradation
" ~
01 (#
a
: I
j--- -- -- -
I I
A
315
B
c
---
I
I
I
I I
tA
tT
Timcz
•
Fig. 9.6 Residual heat life (stability time), tA - tT, tB - tT , tc - tT, of PVC of the same composition, processed under conditions of increasing severity (A> B > C) after an initial heat treatment (T): schematic representation.
subjected to a heat treatment (say melt compounding), under conditions A, or B, or C, or some other conditions, which uses up the amount of its heat life corresponding to the initial portion tT of the stability time, then the residual heat life (all that remains available before significant degradation sets in during subsequent heat treatment) will be represented by t A - tT for conditions A, t B - tT for conditions B, and tc - tT for conditions C. 9.8.2
Heat Stability Testing
Apart from their role in development and research work, heat stability tests on PVC compositions and products are important in the practical contexts of processing and service. Evaluation of the suitability and effectivity of stabilisers, and stabiliser/lubricant systems, in protecting PVC against degradation both under processing conditions and in use, is one of their main applications. Others include direct assessment or prediction of the stability of PVC compositions in various circumstances of treatment and/or exposure, with reference-where relevantto the effects of formulation components and/or heat history in this regard. Investigation of PVC material failures, and 'trouble shooting' generally, are related areas in which stability tests can be helpful. A stability test normally comprises a suitable treatment of the PVC material to induce degradation under controlled conditions, followed by detection, or quantitative determination, of a significant level of the
316
W. V. Titow
manifestation of degradation which is being used as the degradation index in the test. The determination methods employed in such tests can also be applied to PVC materials degraded by means other than the test treatment (e.g. in actual processing or service), but reference to a relevant standard (e.g. results of appropriate calibration tests; specimens exemplifying the effects of actual service in known, relevant conditions) will normally be necessary to characterise the extent of any degradation detected. Stability tests are of two general types: dynamic and static. The methods often used in the evaluation of stabiliser effects are summarised, in a general way, in Table 9.4. Some details of standard tests are given in Table 9.5. In dynamic tests an appropriate weight of the PVC composition is worked at an elevated temperature in sui~able equipment, typically a torque rheometer, internal mixer, extruder, or mill. Stability is assessed either by periodically checking the effect used as the degradation index (commonly colour development in the material), or~specially in a torque rheometer-determining the stability time as the period from the commencement of test processing to the ultimate rise in melt viscosity (and hence in torque) marking the onset of substantial degradation ('decomposition point'). By their nature dynamic tests are primarily relevant to the effects of processing on PVC compositions. Indeed, the test equipment, temperature, and running conditions are often chosen with a view to relating the test results to a particular process. Because PVC cannot be processed without stabilisers, dynamic methods are not suitable for the determination of the heat stability of PVC polymers alone. In a static test, the test treatment essentially consists in heating specimens of PVC material at the test temperature. The specimens are often pieces of sheet of standard size, but they may also be standard weights of powder or pellets of PVC polymer or composition. Note: Where the sheet from which specimens are cut for the test is
specially prepared, e.g. on a mill or by pressing, the preparation should be carefully standardised, so that variations in heat history do not arise to affect any comparison of results. The relevant standard test specifications (cf. Table 9.5) usually include the method and conditions of specimen preparation. In the absence of specific recommendations the following general method may be used 7 for preparing
9 Stabilisers: General Aspects
317
specimen sheet on a laboratory mill (35 x 15 cm), from about 100 g of composition. uPVC: Process for 5 min at 180°C into a sheet about 0·3 mm
thick. pPVC: Process for 5 min at a temperature between 165 and
170°C (depending on nature and amount of plasticiser) into a sheet about 0·5 mm thick. The heating equipment may, typically, be an air-circulation oven, containers for the specimens immersed in a heating bath, or-in some cases-a press with suitable arrangements for heating and cooling the platens. Stability is determined in terms of heating time to reach a certain level of the degradation index used in the test (Le. stability time in those terms): this may be the first appearance, or attainment of a certain degree, of discoloration in the composition (ct. the example below). Colour change of an indicator in continuous contact with volatiles evolved by a specimen, or the other effects mentioned in Tables 9.4 and 9.5, may also be used. Note: A test method which can give rapid results, and in which the
degradative test treatment is combined with measurement of induction time, is differential scanning calorimetry (DSC). This may be applicable where the degradation process is exothermic, so that on the usual DSC plot of heat flow rate versus time the period Of stability (induction time) at the heating temperature used is represented by a flat portion of the curve, and a subsequent drop marks the end of that period. 68 The results of static tests are more relevant to the effects of heat in service than in processing of PVC materials. However, because the equipment required and the test procedures are generally simpler than those of dynamic tests, static tests are sometimes used also to obtain indications of the likely stability in processing, albeit correlations with actual process effects (or with those of dynamic tests) may not be very good. Oven-heating involves free access of air to the PVC specimens, whilst in the heating-bath methods the containers housing the specimens can usually be continuously swept with air (cf. Table 9.5). Thus either of these two methods may be employed where accessibility to air is relevant to the purpose of the test, and their results are sometimes taken as an indication of the PVC material's likely stability
Static
Colour change (to blue) of a Congo Red paper, or universal indicator sensitive at about pH 3, in contact with volatiles evolved during the test. Reduction of colour of Ultramarine Blue incorporated in the test specimens, continuously monitored (by photo cell) during heat treatment in oven. Onset of significant degradation (end of stability time) indicated by sharp drop in blue-light reflectance curve--cf. Fig. 9.8.
Colour change
Colour change
Evolution of HCI
Evolution of HCI
Static
Both static and dynamic
Inspection after set time, or at intervals, and comparison with standards.
Method, and/or stability criteria
Test treatment for which determination method is suitable
Visual
General means
Determination of degradation effects
Development of colour in PVC material
Manifestation of degradation detected or measured
Nelson's Test: Ref. 67
ISO/R 182 (under revision); BS 2782, Method BOA: 1976; DIN 53 381 Part 1
Static: ISO 305; ASTM D 2115; DIN 53 381 Part 2 Dynamic: Refs 7,65,66
References and remarks
TABLE 9.4 Heat Stability Tests Relevant to Practical Assessment of the Effects of Stabilisers in PVC
~
'"
o
::j
:00::::
~
00
Titration
Conductivity determination
Torque-time plot
Evolution of HCI
Evolution of HCI
Melt viscosity rise with onset of degradation of composition in torque rheometer
Volatiles evolved by specimens in test treatment passed through a standard KCI solution. Time for pH (continuously monitored) to drop from about 6 to about 3·9 measured as stability time. a Volatiles evolved by specimens in test treatment absorbed in NaOH solution and HCI determined by back titration. Stability expressed as mg of HCI evolved per g of sample during heating period (30 min). Absorption in water of volatiles evolved by specimens in test treatment. Stability time measured as time for water conductivity to rise by a specified figure. a Stability time determined as time to reach point of torque rise ('decomposition point') on the torque-time graph. Dynamic
Static
Static
Static
Ref. 66
Introduction of test of this kind under consideration for a revised version of ISO/R 182
ASTMD793
ISO/R 182 (under revision); BS 2782, Method l30B: 1976; DIN 53 381 Part 3
a Volatile alkyltin chlorides are formed on heating of PVC compositions containing alkyltin stabilisers (cf. Section 9.6). These can dissociate in aqueous solution, and may thus interfere with HCI determination by conductivity and pH methods.
pH determination
Evolution of HCI
\(:)
'-0
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{;
:J;.
'";:: '" il
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is
v,
320
W. V. Titow
TABLE 9.5 Summary of
Basis of test
Standard specijications
Main particulars and features of
Q
Test equipment and reagents
Test specimens
Temperature
DeterminaISO 305-1976 Glass tubes for Discs of about As agreed individual tion of col14mm dia(consistent our developspecimens meter, about 1 with signi(with closely ment in mm thick ficant colour fitting alumiPVC comdevelopment position on nium plug inwithin the test heating serts); oil bath period) ASTMD2115- Forced-draft Squares of sheet 177 ± 1°C un67 (reapproved oven with aluabout 0·82 mm less otherwise 1974) minium foil thick; side at agreed supports for least 25·4mm PVC specimens (on oven racks) DIN 53 381, Part 2-1975
Detection or determination ofHCl evolved by PVC material on heating
Q
b
As in ISO 305
Glass tubes for ISO/R 182individual 1970b Procedure A (Congo specimens, with centrally Red Method) mounted strip of Congo Red paper 30mm x lOmm: oil bath
Essentially as As in ISO 305 ISO 305, but disc diameter 1 mm less than that of the aluminium blocks
Enough material 180°C prefersuitably comred; other minuted, to fill temperatures may be used, test tube to a depth of 50 mm consistent with completion of test within time limits stated
For specification titles see Section 3.2(c)(ii) of Appendix A. Currently under revision.
321
9 Stabilisers: General Aspects
Standard Tests for PVC Stability Remarks
specification Test treatment Time
Procedure: main points
Assessment and/ or expression of results
Visual assessment of Test duration Specimens heated in tubes immersed in oil colour developed, in limits 60bath; one withdrawn 120 min comparison with unfor inspection every treated specimen 5 min (enough should be used for completion ottest) Specimens heated in Relative heat stability Up to 120 oven and removed assessed visually, in min for inspection at suit- comparison with able intervals, e.g. af- standard sample or ter 30, 45,60,90 and control, in terms of 120 min time for particular degree of discoloration As in ISO 305 As in ISO 305 Limits as in ISO 305, but test time so selected that the last specimen to be removed is black
Stability time may be defined as the time after which unacceptable intensity of discoloration first reached
Specification based on an earlier (1963) version of ISO 305
20 min to 5 h Tube with specimen Mean of times Certain minor and indicator paper obtained in duplicate modifications in suspended above it determinations for conditions sugeach specimen is the heated at test gested to relate temperature: time in stability time (provid- the results of minutes for red coling values lie within different procesour to change to tran- ±10% of their sing methods sient violet or perma- average) nent blue recorded
TABLE 9.5Basis of test
Main particulars and features of
Standard specifications Test equipment and reagents
Test specimens Temperature
As in ISO/R BS 2782, Part 1, As in ISO/R 182 Powder, graMethod l30A: nules, frag182 ments of sheet 1976c (Congo Red Method) (5-6mm square), or other formsamount as in ISO/R 182 DIN 53 381 Part Substantially as the ISO and BS Congo Red Methods 1-1971 ISO/R 182-1970 Glass tubes for 1·0 g of commi- As in Proceindividual Procedure B nuted test dure A specimens with material (pH Method) provision for passage of a gas; heating bath; supply of gas (air or nitrogen); pH measuring cells (one for each test tube); pH meter BS 2782, Part 1, As in ISO/R 182 As in BS As in ISO/R Method BOA 182 Method l30B: 1976d (pH Method) DIN 53 381 Part Substantially as the ISO and BS pH Methods 3-1971 Specimen flasks 10 g of test ASTM D 79349 (reapproved (250 ml Erlenmaterial cut into pieces with meyer) with 1976) provision for one dimension passage of nitno larger than rogen (pre1/16 in; spread heated to test evenly on bottemp.); HCl tom of flask absorption tube with NaOH soln; oil bath C
d
In technical agreement with Procedure A of ISO/R 182-1970. In technical agreement with Procedure B of ISO/R 182-1970.
contd. specification
Remarks
Test treatment Time
Procedure: main points
Assessment and/ or expression of results
As in ISO/R 182 As in ISO/R 182
Time in minutes (mean of duplicate determinations) for first clear sign of indicator change from red to blue
As in Procedure Volatiles generated by A specimen swept by gas stream from test tube heated at test temperature into pH measuring cell; graph of pH versus time plotted: at least duplicate determinations
Induction time, in minutes, given by the period of heating for pH drop to 3·9 ± 0·1: further decomposition may be followed
As in ISO/R 182 As in ISO/R 182; gas flow 6 litre h- I
As in ISO/R 182
30 min
'Short-time stability' expressed as mg of HCl evolved per g of specimen in the test
Volatiles generated by specimen heated in flask swept into absorption tube by nitrogen bubbling through at 2-4 bubbles S-I. After 30 min Cl pptd. with AgN0 3 and determined by titration with KSCN
324
W. V. Titow
in such 'open' processes as calendering and coating. Similarly, because heating between the platens of a press substantially excludes air contact, press-heating tests may be considered more relevant (within the generally limited degree of correlation) to the stability of the material in injection moulding or extrusion. The following recommendations7 are fairly representative of conditions and methods for a press-heating test. A sheet is prepared under standard conditions from the PVC composition to be tested. Specimens of suitable size are cut from the sheet and placed in a press (preferably a multi-daylight one) preheated to the test temperature (typically 180°C), between chromium-plated metal plates, and within moulding frames to prevent thickness reduction. Pressures of about 200 MPa (29000 lbf in- z) and 100MPa (14500lbfin- Z) may be used, respectively, for uPVC and pPVC compositions. The effects of calendering are among the most difficult to relate to the results of tests, especially static tests: in-plant trials are the only complete answer, where they are practicable and not precluded by cost considerations. Note: A milling test, at mill temperatures of up to 190°C and peripheral roll speed of 120 ft min -1 (with a tight nip), developed in the late 1950s,69,70 has been claimed to give reasonable correlation with the effects of calendering at similar temperatures, and speeds up to 330 ft min -1. The degradative effects of the test treatment at temperatures within the range 17o-190°C were found to be generally more severe than those of a static (oven-heating) test at corresponding temperatures, with the difference greatest at the lower end of the range.
An oven used for static heating tests must be reliable, with good control, and even distribution, of temperature. ASTM D 2115 gives a useful summary of requirements in this regard. In the interests of the greatest uniformity of exposure, specimen carriers rotatable during the test treatment should be provided inside the oven in preference to ordinary shelves. These may take the form of self-levelling shelves carried in a horizontally mounted cylindical frame after the manner of a 'big dipper', or a turntable 67 (with the specimens placed around the periphery). An oven test, employing discoloration of the specimens as the degradation index, is illustrated by the following example. 71 Both the test and its results are fairly representative of good early practice,
325
9 Stabilisers: General Aspects
reasonably in line with current standard tests of this kind (d. ASTM D 2115; ISO 305; DIN 53381/2 in Table 9.5). A sequence of colour change was established by preparing a series of strips of PVC sheet degraded to the colours listed below. The strips served as standards for a quantitative expression of colour changes produced by the test treatment in specimens of the composition tested.
O. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
clear, barely detectable change, very slight change, slight yellow tinge, pale yellow, yellow, faint tinge of red, pale red, red, deep red, very deep red/black, black.
Four test compositions were prepared (pbw):
A B C
D
PVC resin
DOP
100 100 100 100
50 50 50 50
Barium stearate
Zinc stearate
Epoxidised oil (Pliabrac A)
2·5 2·5
0·5 0·5
5·0
Dibutyltin dilaurate
2·0
The compositions were made into sheets 0·06 in thick, and I-in squares of the sheets were used as test specimens. The squares were placed on aluminium mesh trays in an air-circulation oven at 180°C. * One specimen of each composition was taken out at IS-min intervals and its colour number determined by reference to the standards. Figure 9.7 is * Temperatures of 175-180°C are most commonly used in static stability tests, although higher ones are also sometimes recommended-e.g. 200°C by the French Centre Scientifique et Technique du Batiment. 72 For the same PVC composition and test method, stability times at different temperatures should normally conform to the Arrhenius relationship given in Section 9.2.1 (see also Fig. 9.5).
326
W. V. Titaw 11 10
9 8 01
.S 7
-g6 u L
5
54
°3
U
2
1
o
10
20
30
40
50
60
70
80
90
100
110
H<2ating tim<2, minut<2s
Fig. 9.7 Colour changes in four PVC compositions on heating at 180°C. a plot of the results. It provides an actual example of the effects illustrated in a general way in Fig. 9.4. Inter alia, the results demonstrate the stability improvement resulting from addition of an epoxy co-stabiliser to a PVC composition stabilised with a mixed-metal stabiliser. An oven test developed by Nelson 67 (originally for plastisol materials) is noteworthy for the degree of precision achievable in the determination of stability time, as well as the good control over the heating conditions. An effectively insulated, dome-shaped oven is used, in which a heated air-stream is directed, from above, onto a turntable (revolving at one r min-I) on which the PVC specimens are mounted over holes, so that they are accessible to a light beam directed at them from below. The specimens are films spread on microscope slides: they incorporate Ultramarine Blue RSl, compounded into the composition to act as indicator, in an amount sufficient to make the colour change (reduction of the blue colour) distinct when free HCI is evolved at the onset of significant degradation (0,5 phr of the indicator exemplifies the incorporation level). This degradation point, which defines the stability time, is marked by the end of a sharp drop in the reflectance curve of the blue component of the part of the light beam reflected by the specimens onto an EEL selenium cell: the reflectance is continuously measured and recorded from the start of the test (see Fig. 9.8). In a typical procedure for a dynamic stability test in a torque rheometer (Brabender Plasti-Corder), the appropriate mixing head is selected, and the mixer pre-heated to the required temperature. A weighed quantity of PVC composition is quickly introduced into the
327
9 Stabilisers: General Aspects
-! o
ts TimlZ., minutlZs
Fig. 9.8 Reflectance of blue light as a function of heating time in Nelson's stability test with Ultramarine Blue RSI indicator: 67 schematic representation. ts , stability time.
chamber (head cavity) with the rotors running. The amount of material is selected in relation to the cavity volume: if it is desired to ensure good exposure of the composition to air during mixing, the cavity should not be filled completely. If colour development is used as the degradation index, samples of the composition are withdrawn for examination at suitably frequent intevals (say every two minutes). Otherwise the time is determined from the start of mixing to the rise of torque indicating the decomposition point. The roller speed and the temperature are the principal variables; their values should be selected in the light of the purpose of the test. The following may be mentioned by way of example of combinations which have been used in investigations of stability of PVC compositions in relation to their behaviour in industrial processing: roller speeds of 35, 45, 55 and 65 r min -1 at temperatures of 165°C, 180°C and I85°C;65 or 50 and 100 r min- 1 at 140°C and I80°C. 66 Note: An interesting study by Collins et ai. 65 provided evidence for good correlation between Brabender thermal stability values and those obtained with a capillary rheometer. Data obtained with the latter instrument also demonstrated, inter alia, a
w.
328
V. Titow
correspondence between the 'time to black' of PVC compositions and a pronounced increase in melt viscosity (reflected by the commencement of an upward sweep in a plot of the extrusion load versus time). A feature of properly designed dynamic stability tests employing the Brabender torque rheometer or a suitable extruder, is that they can bring out the important role of lubrication in the heat stability of PVC in processing. The reduction of frictional heating by proper internal and external lubrication can substantially increase the stability of a generally adequately stabilised PVC composition (by reducing the heat history and hence extending the heat life), in comparison with an unlubricated, or poorly lubricated, but otherwise identical composition. This factor is not brought out by static tests, in which the stability time depends essentially on the stabiliser system and is not significantly influenced by the lubricants66 (unless these are of the stabiliserlubricant type--cf. Chapter 11, Section 11.1.2). 9.8.3 Light Stability Testing
The general format of tests for stability to light is the same as that of heat stability tests-i.e. a test treatment producing degradation, followed (or accompanied) by a determination of its extent. All the test treatments in common use are static ones. In the practical context, the ultimate significance of light stability tests is as sources of guidance to resistance of the material tested to photodegradation in service (although good correlations with actual service performance may be difficult to establish-see Chapter 12, Section 12.6): this is implicit, at least to some extent, even in the use of such tests for direct comparison of the stability of similar materials, as criteria of specification requirements, and in quality control. The usual test treatments consist of exposure of suitable specimens to 'natural' light (with or without accessibility to general weathering), or to artificial light sources in the laboratory. The laboratory tests may also include exposure to heat and water (liquid and/or vapour) to simulate the effects of these factors in outdoor weathering. The popular tests in both these categories (including international and national standard tests), their effects, applications, significance, and limitations are discussed in Section 12.6 of Chapter 12, in connection with weathering resistance of PVc. As mentioned in that section, laboratory tests which involve exposure to
9 Stabilisers: General Aspects
329
artificial light with a spectral distribution resembling that of daylight, are not necessarily accelerated tests, as the degradation effects may take as long to develop as they would on outdoor exposure. True acceleration of photodegradative effects may be achieved by using UV radiation of relatively high intensity (cf., for example, the QUV test-Section 12.6), and/or running the test at elevated temperatures or higher ambient oxygen concentrations (or both). Such accelerated tests are particularly relevant where the object is purely to achieve rapid photodegradation; or to examine the roles and effects of the three factors just mentioned in the conditions of the test (the effects may differ in magnitude or 'even nature under different conditions); or where the stability of generally similar materials (e.g. PVC of basically the same formulation but with modifications to the stabiliser system) is to be directly compared. The applicability of the test results as indication of the likely stability of the material on long-term exposure in service will, in each individual case, cardinally depend on the available evidence and records of the relevant correlations. A useful accelerated test for photochemical stability of polymers and plastics, including PVC, has been developed at the TNO. * In this method the time scale for degradation effects is reduced not only by the use-in combination-of UV light, heat, and oxygen, but also by specimen 'geometry'. The specimens are thin films: this makes for a high surface-to-volume ratio, so that a large proportion of the total material of each specimen is immediately and directly accessible to the degradative influences. The films are mounted on the outer wall of a cooling tube, of Duran 50 glass, which surrounds an inner tube housing a high-pressure mercury lamp (TQ-150, Quartzlampen GmbH, Hanau). The cooling tube also acts as a UV filter, absorbing wavelengths below about 300 nm. The assembly is contained in a cylindrical glass vessel, jacketed for temperature control by water circulation: the test temperature is maintained by regulating those of the cooling tube and the glass vessel. Test temperatures up to about 90°C can be used, with UV radiation intensities between about 200 and 1100 W m- 2 . The progress of photooxidation of the specimens is followed by continuous measurement of the uptake of oxygen from a nitrogen/oxygen mixture filling the glass vessel. The oxygen concentration in this mixture may be used as a test variable to examine its effect
* Plastics and Rubber Research Institute TNO, Delft, Holland.
330
W. V. Titow
on the degradation of the material tested. In tests on PVC, the HCI evolved is also continuously determined by absorption in water and conductivity measurement. Determinations of stability to photooxidative degradation of plastics by the TNO test can be up to ten times faster than those in some types of standard equipment (e.g. Xenotest--d. Table 12.7, Chapter 12). They have also been claimed to correlate well with Xenotest results, as well as those of some actual long-term exposures out of doors. 9.9 DETECTION AND ANALYSIS OF STABILISERS Various methods of separation and analysis are used for the identification and determination of stabilisers in PVC compositions. Infra-red spectroscopy and gas chromatography are especially noteworthy among instrumental techniques. An outline of qualitative analysis for commercial stabilisers and lubricants, published by Crowo in 1967,73 is still of some interest. Other. more extensive, sources of information include the well-known book by Haslam et at. on the analysis of plastics,74 and that by Crompton on chemical analysis of additives;75 the IR methods of identification of additives in plastics are covered in the third volume of the publication by Hummel and Scholf6 which includes many reference spectra. Extractability of stabilisers from PVC materials is of special interest in connection with the use of such products as food-wrapping film, blown bottles for beverages, potable-water pipes and medical equipment. Product specifications, where available, contain appropriate extractability tests. Useful general comments and some relevant data have been provided by Brighton. 77
REFERENCES 1. Abu-Isa, I. A. (1975). Polyrn. Engng. Sci., 15(4), 299-307. 2. Sosa, J. M. (1975). J. Polyrn. Sci., Polyrn. Chern. Ed., 13(10),2397-405. 3. Voigt, J. (1966). Die Stabilisierung der Kunststoffe gegen Licht und Wiirrne, Springer Verlag, Berlin. 4. Silberman, E. N. (1968). Preparation and Properties of Polyvinyl Chloride (in Russian), Izd. Khimia, Moscow. 5. Thinius, K. (1969). Stabilisierung und Alterung von Plastwerkstoffen, Verlag Chemie, Weinheim.
9 Stabilisers: General Aspects
331
6. Onozuka, M. and Asahina, M. (1969). J. Macromol. Sci., Revs. Macromol. Chern., C3(2), 235. 7. Manual of PVC: A,dditives. (1971). Ciba-Geigy Marienberg GmbH, Section 2. 8. Braun, D. (1975). In Degradation and Stabilisation of Polymers, (Ed. G. Geuskens), Applied Science Publishers, London, Ch. 2. 9. Nass, L. I. (Ed.) (1978). Encyclopedia of PVC, Marcel Dekker, New York. 10. Yassin, A. A and Sabaa, M. W. (1980). J. Polym. Sci., Polym. Chern. Ed., 18,2513-21 and 2523-33. 11. Mascia, L. (1974). The Role of Additives in Plastics. Edward Arnold (Publishers) Ltd, London. 12. Woolley, W. D. (1972). Plastics and Polymers, 40(148),203-8. 13. Troitskii, B. B., Dozorov, V. A, Minchuk, F. F. and Troitskaya, L. S. (1975). Eur. Polym. J., 11(3),277-81. 14. Abbas, K. B. and Sorvik, E. M. (1975). J. Appl. Polym. Sci., 19(11), 2991-3006. 15. Chauffoureaux, J. C., Dehennau, C. and van Rijckevorsel, J. (1979). J. Rheology, 23(1), 1-24. 16. Rice, P. and Adam, H. (1977). In Developments in PVC Production and Processing-I, (Eds A Whelan and J. L. Craft), Applied Science Publishers, London, Ch. 5. 17. Grassie, N. (1975). In Degradation and Stabilisation of Polymers, (Ed. G. Geuskens), Applied Science Publishers, London, Ch. 1. 18. Summers, J. W. (1976). 34th ANTEC SPE Proceedings, pp. 333-5. 19. Rabek, J. F., Canback, G., Lucky, J. and Ranby, B. (1976). J. Polym. Sci. Polym. Chern. Ed., 14(6), 1447-62. 20. Wilson, A. S., Biggin, I. S. and Pugh, D. M. (1978). 'The influence of volatility on the selection of plasticisers to meet new and developing performance requirements,' paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 21. Rabek, J. F., Shur, Y. J. and Ranby, B. (1975). 1. Polym. Sci. Polym. Chern. Ed., 13(6), 1285-95. 22. Braun, D. (1964). Kunststoffe, 54(3), 147-52. 23. Allara, D. L. (1976). 34th ANTEC SPE Proceedings, pp. 245-7. 24. Starnes, W. H. and Piitz, I. M. (1976). Macromolecules, 9(4),633-40. 25. Lanigan, D. (1978). 'Recent advances in organotin stabilisers,' paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 26. Fernley, A M. (1964). Plastics, 29(320),66-8. 27. Grindley, P. R. (1969). Chern. Processing, 15(3),8-11. 28. Thacker, G. A. (1969-1970). Modern Plastics Encyclopedia, McGraw-Hill, New York, pp. 290-4. 29. Anon. (1976). Plast. Technol., 22(2), 12. 30. Hutton, R. E. and Oakes, V. (1976). In Organotin Compounds: New Chemistry and Applications, (Ed. G. J. Zuckerman), Advances in Chemistry Series, No. 157, American Chemical Society, Ch. 8.
332
W. V. Titow
31. Dieckmann, D. (1976). 34th ANTEC SPE Proceedings, pp. 507-11. 32. Hartung, M. (1979). Plast. Techno!.. 25(8), 67-70. 33. Titow, W. V. (1977). In Developments in PVC Production and Processing-I, (Eds. A. Whelan and J. L. Croft), Applied Science Publishers, London, Ch. 4. 34. Rhys, J. A. (1962). In Advances in PVC Compounding and Processing, (Ed. M. Kaufman), Maclaren and Sons Ltd, London, Ch. 3. 35. Worschech, K. F. (1978). 'Synergistic support of various stabilisation systems during PVC processing by using lubricants,' paper presented at the PRI International Conference on PVC processing, Egham Hill, Surrey, England, 6-7 April, 1978. 36. Razuvaev, G. A., Troitskii, B. B., and Troitskaya, L. S. (1971). Mechanism of Action of Some Stabilisers in the Thermal Degradation of Polyvinyl Chloride, Proceedings of 7th IUPAC Symposium, Prague 1970, Butterworths, London. 37. Hybart, F. J. and Rowley, G. N. (1972). J. Appl. Polym. Sci., 16,715-23. 38. Thacker, G. A. (1971-1972). 'Antioxidants', in Modern Plastics Encyclopedia, Vol. 48, McGraw-Hill, New York, pp. 21~14. 39. Hageman, H. J. and de Jonge, C. R. H. I. (1972). Kunststoffe, 62(10), 681-3. 40. Bailey, D., Tirrell, D., Pinazzi, C. and Vogl, O. (1978). Macromolecules, 11(2), 312-20. 41. Johnson, M. and Houserman, R. G. (1977). 1. Appl. Polym. Sci. 21(12), 3457-63. 42. Anon. (1976). Eur. Plast. News, 3(3), 28. 43. Thacker, G. A. (1971-1972). 'UV absorbers and light stabilisers', in Modern Plastics Encyclopedia, Vol. 48, McGraw-Hill, New York, pp. 284-91. 44. Bonkowski, J. E. (1969). Text. Res. J., 39(3),243-7. 45. Szabo, E. and Lally, R. E. (1975). Polym. Engng. Sci., 15(4),277-84. 46. Asquith, R. S. and Campbell, B. (1963). 1. Soc. Dyers and Colourists, 79(12), 678-86. 47. Salvin, V. S. Ibid, pp. 687-96. 48. ISO 105-1978: Textiles-Tests for colour fastness. G05, Colour fastness to burnt gas fumes. 49. Katz, M., Shkolnik, S. and Ron, I. (1976). 34th ANTEC SPE Proceedings, pp.511-12. 50. Wypych, J. (1976). J. Appl. Polym. Sci., 20(2), 557-60. 51. Frye, A. H., Horst, R. W. and Paliobagis, M. A. (1964). J. Polym. Sci., A2, 1765, 1785 and 1801. 52. Klemchuk, P. P. (1968). In Stabilization of Polymers and Stabilizer Processes (Ed. R. F. Gould), Advances in Chemistry Series, No. 85, American Chemical Society, Ch. 1. 53. Ayrey, G., Poller, R. C. and Siddiqui, I. H. (1969). 'Reaction between organotin compounds and chlorohydrocarbons in relation to the stabilisation of poly(vinyl chloride),' paper presented at the 4th International Conference on Organometallic Chemistry, Bristol, England.
9 Stabilisers: General Aspects
333
54. Suzuki, T. and Nakamura, M. (1970). lap. Plast., 4(2), 16-21. 55. Fieser, L. F. and Fieser, M. (1944). Organic Chemistry, D. C. Heath & Co., Boston, pp. 304-6. 56. Bengough, W. J. and Onozuka, M. (1965). Polymer, 6,625. 57. Frye, A. H. and Horst, R. W. (1959). l. Polym. Sci., 40,419; (1960). Ibid, 45,1. 58. Onozuka, M. (1967).1. Polym. Sci., AS, 2229-32. 59. McBroom, J. W. and Lally, R. E. (1971-1972). In Modern Plastics Encyclopedia, Vol. 48, McGraw-Hill, New York, pp. 262-4. 60. Deanin, R. D., Foss, R. M., Gilbert, P. G., Guerard, R. F. and Muccio, E. A. (1973). Polym. Engng. Sci., 13(2), 96. 61. Fuchsman, C. H. (1968). In Stabilization of Polymers and Stabilizer Processes, (Ed. R. F. Gould), Advances in Chemistry Series, No. 85, American Chemical Society, Ch. 2. 62. Wypych, J. (1975). l. Appl. Polym. Sci., 19(12), 3387-9. 63. Nass, L. I. (Ed.) (1976). Encyclopedia of PVC, Vol. 1, Marcel Dekker, New York, pp. 344 and 670. 64. Gleissner, A. (1975). Kunststoffe-Plastics, 22(1), 11-12. 65. Collins, E. A., Metzger, A. P. and Furgason, R. R. (1976). Polym. Engng. Sci., 16(4), 240-5. 66. Menzel, G. and Polte, A. (1975). Kunststoffe, 65(3), 149-55. 67. Nelson, J. H. (1978). 'A novel method of assessing heat stability', paper presented at the PRI International Conference in PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 68. Cassel, B. and Gray, A. P. (1977). Plast. Engng., 33(5),56-8. 69. Himmler, G. G. and Nissel, F. R. (1957), Plast. Technol., 3(4), 280-2. 70. Dodgson, D. P. and Pike, M. (1960). Svenska Plastforeningens Plasttenik, Section TlF, 2. 71. Moorshead, T. C. (1957). Plastics, 22,243. 72. Fougea, D. (1971). Cahiers Centre Sci. Tech. Batiment, No. 124, p. 1070. 73. Crowo, J. A. (1967). Brit. Plast., 40(8), 84-6. 74. Haslam, J., Willis, H. A. and Squirrell, D. C. M. (1973). Identification and Analysis of Plastics, 2nd Edn, Iliffe Books, London. 75. Crompton, T. R. (1977). Chemical Analysis of Additives in Plastics, 2nd Edn, Pergamon Press, Oxford. 76. Scholl, F. (1981). Additives and Processing Aids: Spectra and Methods of Identification, Vol. 3, 2nd Edn The Hummel/Scholl Atlas of Polymer and Plastics Analysis, Carl Hanser Verlag, Munich, and Verlag Chemie International. Deerfield Beach, Fla., USA. 77. Brighton, C. A. (1968). Plastics and Polymers, 36(126), 549-54. 78. Lippoldt, R. F. (1978). 36th ANTEC SPE Proceedings, pp. 737-9.
CHAPTER 10
Commercial Stabiliser Practice P. S.
COFFIN
10.1 INTRODUCTION
There are a large number of stabiliser products available to the PVC processor. The selection of the best stabiliser system for a particular application depends very much on the specifications the PVC end-product must meet and the fabrication process to be employed. The situation is further complicated by the fact that individual stabiliser compounds are being used less and less. Instead, many processors take advantage of proprietary stabiliser brands which will provide a complete stabiliser package. This has been established practice for the 'barium/cadmium' and 'calcium/zinc' classes of stabiliser, since these systems tend to be specific to individual applications. In the lead stabiliser field the use of 'one-pack' systems, incorporating all the stabilisers and lubricants for a particular application, has become commonplace. Single-package products incorporating tin stabilisers with lubricants are now also becoming available. Although certain general principles have been mentioned in Chapter 9, it is only by reference to the proprietary materials that the great variety of PVC stabilisers available, and the relationship between their nature and applications, can be illustrated. For this reason, the product ranges of five principal UK stabiliser manufacturers are described at the end of this chapter. The list is not meant to be complete, and it does not imply that any material of any manufacturer not included is inferior to those which are included. Nor is it intended to imply or make any comparison between the materials of different suppliers. Not all the products described are of UK manufacture; part of the product 335
336
P. S. Coffin
ranges referred to may be manufactured elsewhere. The main purpose of the list is to demonstrate by reference to good commercial products the range, nature and uses of principal PVC stabilisers. For this reason, the fact that the products mentioned are mainly British is immaterial, and the chapter should be of equal use to readers in other countries. It is, in any case, a good plan for the prospective user, TABLE 10.1 Some of the Larger Stabiliser Manufacturers World-Wide (Position c 1980) UK stabiliser manufacturers Associated Lead Manufacturers Ltd Ciba-Geigy Ltd Durham Chemicals Ltd
Ferro (GB) Ltd Diamond Shamrock Ltd Victor Wolf Ltd
Stabiliser manufacturers in Western Europe. ASUA (Spain) Hondorff, Block and Braet (Holland) Akzo Chemie (Germany) Meister (Switzerland) BASF (Germany) Metallgesellschaft (Germany) BBU (Austria) Penalmex (France) Barlocher (Germany) Polytitan (France) Ciba-Geigy (Germany) Reagens (Italy) Cincinnati Milacron (Belgium) Rousselot (France) la Floridienne (Belgium) SNEA (France) Haagen Chemie (Holland) Stabital (Italy) Henkel (Germany) Swedstab (Sweden) Hoechst (Germany) Stabiliser manufacturers in the USA A and S Corp. Argus Chemical Corp. Associated Lead Inc. Cardinal Chemical Co. Ciba-Geigy Corp. Cincinnati Milacron Inc. Claremont Corp. Cyanamid Co. Eagle Picher Inc.
Emery Inc. Ferro Chemical Hammond Lead Products Inc. Interstab Chemicals Inc. M and T Chemicals Inc. Stauffer Chemical Co. Synthetic Products Co. Tenneco Chemicals Inc. Vanderbilt Co.
Stabiliser manufacturers in Japan Adeka Argus Dainippon Chemicals Katsuta Kako Kawamusa Kasei Kikuchi Chemicals Kyodo Chemicals Mizusawa Nitto Kasei
Sakai Chemicals Saukyo Chemicals Seido Chemicals Shinagawa Shin ChwQ Kagaku Tannan Kagaku Kogyo Toyko Chemicals
10
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whether in the UK or abroad-after having clarified his requirements and possibilities in the light of this chapter-to select and contact suppliers with a view to asking for relevant details and preferably a discussion. Many manufacturers will also specially formulate stabiliser systems to suit individual needs. Table 10.1 gives a list of some of the larger suppliers of stabiliser products in Western Europe, the USA and Japan. 10.2 CHOOSING A COMMERCIAL STABILISER The first step in selecting a stabiliser for a particular application i,s to consider which of the basic stabiliser systems is most appropriate for the particular conditions. Chapter 9 outlines the general performance properties of the different stabiliser systems. No one stabiliser system is equally suited to all the diverse uses of PVC, so the choice of stabiliser is very much determined by which properties are of greatest importance for the particular application in question. Cost, of course, is also usually a critical factor in the final decision. However, it must be remembered that the cheapest suitable stabiliser is not necessarily the wisest choice. It is important to consider the total PVC compound and manufacturing costs. For example, such factors as reject recycling involve large hidden costs that may overshadow differences in stabiliser prices. 1 Lead stabilisers are comparatively cheap. Although they are the heaviest (costing is often on a volume basis) and are generally used at high loadings, they can frequently prove most cost-effective. They are efficient heat stabilisers; some are good UV absorbers and others (e.g. stearates) have lubricant properties. Therefore, lead stabilisers are worth considering first. They are particularly appropriate if the PVC compound is required to have good electrical properties. However, lead stabilisers will not be suitable if clarity, non-toxicity or sulphur-staining resistance are of considerable importance. Typical application areas where lead stabilisers are used include pipes, pipe fittings, rainwater goods, interior and exterior profiles, cable coverings, conveyor belting, and insulation tape. Metal soaps* and 'complexes't are, almost without exception, most effective in synergistic mixtures involving two or more metal salts * Metal soaps used as stabilisers are typically commercial 'laurates' or 'stearates' (see Chapter 9). t Metal 'complexes' are not complexes in the chemical sense of the word but denote hexoates, octoates, benzoates, phenates, etc. (see Chapter 9).
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P. S. Coffin
together with co-stabilisers such as organophosphites and epoxy compounds. These systems, particularly those based on calcium/zinc, tend to be specific in action and it is important to use the correct system for a particular application. If the use of these stabilisers is contemplated then the supplier's advice will certainly be of value. There are many systems on the market based on various combinations of different metals. In general, the barium/cadmium or barium/ cadmium/zinc products are the most efficient in performance. However, if the use of cadmium and possibly barium is not desired on the grounds of toxicity, then the less effective barium/zinc, calcium/zinc or combinations with other metals must be considered. The metal soap systems are usually solids, and those based on stearates impart significant lubricant action. On the other hand, the metal complex systems are usually liquids. Mixed metal systems find use in a wide range of applications, e.g. barium/cadmium stabilisers in shoes, flooring, calendered sheet, leathercloth, flexible trimmings, and flexible hose; and calcium/zinc stabilisers in articles for medical or food-contact applications The organotin stabilisers are, generally speaking, the most effective of all compounds from a heat stability aspect, in particular for clarity and colour hold. Some (e.g. maleates) also contribute light stability, although UV absorbers are sometimes used in conjunction, especially for transparent PVC articles for outdoor use. However, the organotin stabilisers are the most expensive, and it is this factor that is responsible for their use not being wider than it is at present, despite their dosage level often being very low. Tin stabilisers would typically be used for rigid sheet, clear sheet, plastisols and mouldings. Some tin stabilisers (e.g. octyltins) have gained approval for food-contact applications such as packaging film and bottles. Having decided on the most suitable type of stabiliser or having narrowed the choice to two possibilities, it is necessary to consider the different forms and proprietary products within that system. As the contents of this chapter will illustrate, there are often many individual products of the same basic type within a supplier's range that have been developed to suit differing circumstances. Obviously, the supplier's description of his product will indicate its principal performance properties. Further, for specialised applications some suppliers are willing to formulate custom-fit products. However, it is important to run practical trials in the laboratory and/or the factory before the final selection is made.
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10.3 THE IMPORTANCE OF A WELL·BALANCED LUBRICANT SYSTEM
Lubricants frequently have a large influence on the action of heat stabilisers, particularly with regard to processing stability. For such manufacturing processes as extrusion, injection moulding, blow moulding and calendering, the selection of the correct lubricant system can often be as important as the choice of stabiliser. PVC has a high melt viscosity and lubricants must be added to facilitate processing. In addition, PVC will stick to hot metal surfaces unless properly lubricated. If an inefficient system is used, then the thermal history experienced by the PVC compound will be greater because of the higher shear necessary to process the material. This and the tendency to stick to the metal parts of the processing equipment will strongly encourage degradation. More of the stabiliser will be used up and the possibility of a 'burn-up' will be considerably greater. Of course some 'lubricants' such as lead stearate and calcium stearate are stabilisers in their own right. Other lubricants, e.g. glycerol monostearate and pentaerythritol esters, have been shown 2 to have stabiliser action. The lubricant system has, until recently, been the hidden art in PVC technology which would differentiate between a successful processing operation and a not so successful one. Lubricants not only influence processing and stabilisation, but also the extent of gelation and the physical properties of the fabricated article. The selection of the most suitable lubricant system is a complicated matter, particularly for rigid PVC applications. It is frequently necessary to employ a number of individual lubricants simultaneously, with each performing a different task during the processing operation and often showing subtle interactions with the other lubricants. Laboratory tests rarely give good correlation to processing conditions and it is usually necessary to carry out development work on production plant. Laboratory equipment that can give useful information, such as small scale extruders and calenders, is expensive. It is, therefore, as important to obtain recommendations from lubricant suppliers as from stabiliser suppliers. Of course, with several stabiliser suppliers providing one-pack stabiliser/lubricant products, the development work on the lubricant system can be offered as part of the package. The various types of compounds used as PVC lubricants are discussed in Chapter 11.
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10.4 ONE-PACK SYSTEMS AND THE PHYSICAL FORM OF STABILISER PRODUCTS There are many advantages in a PVC processor obtaining his stabiliser system as a one-pack product. The number of weighing operations during compound preparation is reduced considerably. Products such as antioxidants which are incorporated at low dosage levels would have involved weighing small quantities. The formulation work on stabiliser and lubricant systems is carried out by the stabiliser supplier, and the processor does not need to carry out costly development projects. The disadvantages are a loss of formulation flexibility and a higher stabiliser price. The use of a one-pack mixed metal stabiliser is almost obligatory because of the complex synergistic effects between the individual stabiliser compounds that necessitate lengthy development work to optimise any system. Such proprietary products often incorporate co-stabilisers such as organophosphites, antioxidants such as hindered phenols, and chelators such as pentaerythritol. The solid metal soap products, by virtue of the lubricant action of metal soaps, can be formulated to give as well the complete lubricant system required for the application in question. The one-pack lead stabiliser products, either co-precipitate or composite, have been developed to offer a complete stabiliser system in one product. This has been taken a stage further by some suppliers with the incorporation of fillers, processing aids, flame retardants and pigments within stabiliser composites. In the USA the one-pack concept has extended into the tin stabiliser field with the development of products containing both organotin stabiliser and a complete lubricant system. The physical form of a stabiliser product is another important aspect to consider when selecting a stabiliser system. The ancillary equipment available such as storage facilities, conveying equipment, and weighing stations must be capable of handling the stabiliser. Most mixed metal complexes and tin stabilisers are liquid and require dosing equipment. Most lead stabilisers and mixed metal soaps are solid and can be handled in equipment fundamentally the same as for the resin. However, dust is a major inconvenience in handling a solid, particularly when the dust can be harmful if inhaled. For this reason, considerable development work has been carried out by the manufacturers of solid stabilisers on forms with low dusting tendencies. Products can be
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grade from Associated Lead). Most lead stabiliser and mixed metal soaps can be supplied as pastes dispersed in a suitable plasticiser. Agglomerated forms such as granules and flakes are widely used because of their reduced dusting tendencies. More recently, this type of product has been taken a stage further with the introduction of 'strands' (see Section 10.6) and 'pearls'3 with considerably reduced dusting tendencies and free-flowing characteristics. Finally, the stabiliser can be supplied packaged within a sealed polyethylene bag for addition direct and whole into a high-speed mixer. The mixer shreds the bag and the polyethylene then constitutes part of the lubricant system. 10.5 HYGIENE AND ENVIRONMENTAL CONSIDERATIONS
Most stabiliser systems are based on heavy-metal compounds, so consideration must be given both to the handling of stabiliser products within the factory from a hygiene viewpoint and the presence of compounds of these metals within the final PVC article from an environmental aspect. Most discussions centre on products containing lead, barium and cadmium, although doubts have been expressed about certain organotin stabilisers. 4 There have been considerable improvements in the handling methods for lead-, barium- or cadmium-containing solid stabilisers within the factory environment. The previous section described the development of improved physical forms with considerably reduced dusting tendencies. It is possible for large processors to justify automatic or semi-automatic handling systems so the stabiliser can be delivered in semi-bulk and conveyed through the factory to weighing and mixing stations without any worker needing to come into direct contact with the stabiliser. Methods of monitoring such parameters as 'lead in air' and 'blood lead' levels have also improved the situation, so that now, provided that proper precautions are taken, it is possible to handle such hazardous material in the factory with safety. Information on such matters is usually readily available from stabiliser suppliers. 5 When considering the presence of heavy-metal compounds within the final fabricated PVC article, it is important to remember that the metal is present at low levels in the PVC compound and that it is generally fixed within the PVC matrix. Thus lead stabilisers are
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approved for potable water pipe in all countries except the USA and France. Extensive testing of the water extraction from lead-stabilised pipe6 ,7 has shown that only a trace of lead is leached out during the first pass through the pipe. Thereafter, the inner core is rendered passive to the extraction of lead. Cadmium compounds give overall the most problems of the various stabilisers, and at present considerable restrictions on the use of cadmium in PVC are being considered in Sweden. However, there are strong arguments for using barium/ cadmium stabilisers for certain applications, 8 and the restrictions are under review. For the applications where more stringent environmental restrictions are necessary, such as medical or food-contact articles, the calcium/zinc systems are generally acknowledged as non-toxic. The octyltin class of stabilisers have wide approval for food-contact applications from such bodies as the FDA (USA), BGA (West Germany) and BIBRA (UK). The recently introduced estertin stabilisers are also claimed to be non-toxic, but there is to date insufficient evidence for approval to be given to this type of product. 10.6 UK STABILISER MANUFACTURERS-PRODUCT RANGES AND APPLICATIONS
The types of stabiliser products available to the PVC compounder and processor are illustrated by considering the product ranges for five of the principal UK stabiliser manufactueres. No comment or comparison is intended by the discussion of the various products described here or by the omission of others. The purpose is solely to indicate the range, nature and uses of commercial stabilisers by reference to good commercial products. 10.6.1 Associated Lead Manufacturers Ltd
Associated Lead are the major UK manufacturer of lead stabilisers. The product range is described in Table 10.2. The individual stabiliser products are still used fairly widely, and these can be supplied in 'dry' form, 'D' form when damped with small smounts of plasticiser, in granule form, or as pastes dispersed in selected plasticisers. However, many PVC processors now use composite lead stabilisers, and this type of product is illustrated by the standard range of Associated Lead. Such products, or if required a formulation to the customer's
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specification, can be supplied in damped powder blend form, packaged within individual heat-sealed polyethylene bags, or in granular form. In certain cases it is possible to incorporate other additives such as fillers, pigments, processing aids and impact modifiers within the composite to constitute a total package. Associated Lead have recently launched a TABLE 10.2 The Basic Stabiliser Range of Associated Lead Manufacturers Ltd Individual stabiliser compounds
Tribasic lead sulphate Tetrabasic lead sulphate Dibasic lead phosphite Dibasic lead phthalate White lead (basic lead carbonate)
Dibasic lead stearate Normal lead stearate Calcium stearate Barium stearate Cadmium stearate
Stabiliser products for flexible PVC applications, e.g. cable covering (a) Composite stabilisers Trade Main stabiliser name present
Performance characteristics
Q
SQ, BQ or XQ
Basic lead sulphate
SQ, BQorXQ
Basic lead sulphate
SQ,BQorXQ 3103
Basic lead sulphate
SQ orBQ 3104
Basic lead phthalate
SQ orBQ
3105
Basic lead phosphite
SQ orBQ
White lead
3101
3102
3106
High stabiliser action with balanced lubricant effect for applications with high thermal stability specifications Good stabiliser action with efficient lubricant system; good general-purpose performance High lubricant action for applications where high shear and/or high linear speeds are experienced Designed for high electrical quality cable and for cables used under high ambient temperature conditions Good weathering protection for cables subject to outdoor exposure during service life High-cost efficiency where low melt temperatures are maintained, particularly when chlorparaffin extenders are used
(b) Stabiliser dispersions Individual stabiliser compounds (or mixtures thereof) dispersed in selected plasticisers to the required ratio are widely used in the manufacture of lead-stabilised flexible PVC products. Q
SQ, granular; BQ, powder blend; XQ, Strandex.
TABLE 1O.2-eontd. Stabiliser products for rigid PVC pipe extrusion Performance characteristics
Trade name
The selection of the correct stabiliser for pipe extrusion depends on the type of extruder, the overall formulation and the specifications to be met. Associated Lead offer the series of products below with varying but balanced lubricant action, while retaining similar stabiliser effect: XQ, SQ or BQ 3201 XQ, SQ or BQ 3202 XQ, SQ or BQ 3203 XQ, SQ or BQ 3204 XQ, SQ or BQ 3205
Very highly lubricating, both internally and externally Highly lubricating, both internally and externally Medium lubricant effect suitable for a wide range of processing situations Medium internal lubricant effect with low external action Low lubricant effect for when extruders must work hard for excellent physical properties
The performance of each product can be further altered by varying the dosage rate. For example, a stabiliser used at between 2·0 phr and 2·4 phr without filler would be recommended at a higher dosage, 2·6 phr to 3·0 phr in the presence of high filler loadings. Different balances between internal and external lubricant action are also available within the Associated Lead's range. Stabiliser products for convoluted drainage pipe
SQ or BQ3221
Balanced lubricant action and low PVC melt viscosity-suitable for Drossbachtype process
Stabiliser products for rigid PVC profile extrusion
SQ or BQ3251 SQ or BQ 3252 SQ or BQ3261
Interior, thin-walled profile-low external lubricant action Interior, thick-walled profile-low external lubricant action Exterior profile, e.g. cladding or window frames where balanced lubricant action is important
Stabiliser products for rigid PVC injection moulding SQ or BQ3301 SQ or BQ3303
Good early gelation performance for applications where physical properties are critical Basic lead phosphite based for mouldings subject to outdoor exposure
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TABLE 1O.2-eontd.
Mixed metal liquid stabilisers Trade name
Product type
LF037
BalCd liquid BalCd liquid CalZn liquid Ba/Zn paste Pb liquid
LF114 LH 1671 PN 1678 LLOO5
Main application Flexible PVC moulding, e.g. footwear Flexible PVC extrusion, e.g. tubing Flexible PVC extrusion and calendering Sulphur-staining resistant cable covering Alternative to powdered lead stabilisers for flexible PVC applications
new product, Strandex, which represents a significant advance towards a granulated product with non-dusting and free-flowing properties but with good dispersion characteristics into PVc. Associated Lead also offer a range of liquid, mixed metal complex, stabilisers. The following formulations illustrate the use of Associated Lead stabilisers in a number of different applications (all amounts in phr).
Pipe extrusion Pressure pipe 100 PVC (Suspension K65) CaC0 3 filler: (e.g. Omyalite 95T-Croxton and Garry Ltd) 1 XQ3204 2·2 XQ3203 XQ3221 Processing aid: Paraloid K120N (Charles Lenning Chemicals) Pigment
Soil pipe 100
Convoluted drainage pipe 100
10
3
2·8 4·0 1 As required
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Rigid PVC injection moulding
PVC (Suspension K60) 100 CaC0 3 filler: (e.g. Omyalite 95T) Ti02 pigment: (e.g. Tioxide R-CR2-BTP, Tioxide UK Ltd) SQ 3301 SQ 3303 Processing aid: Paraloid K120N Pigment Rigid PVC profile extrusion
PVC (Emulsion K70) PVC (Suspension K65) CaC0 3 filler: (e.g. Omyalite 95T) BQ 3252 Impact modifier: (e.g. Blendex 410Borg-Warner Chemicals) Pigment PVC cable covering
PVC (Suspension K65) DOP plasticiser CaC0 3 filler: (e.g. Britomya BSH-Croxton and Garry Ltd) XQ3102 White lead/DOP paste Lubricant: calcium stearate Pigment
Pipe fittings 100 3
Exterior mouldings 100
3 4·5
5·0
2
2 As required
Interior profile (twin-screw) 50 50 5 3·5
Interior profile (single-screw) 50 50 5 4·5
If required As required
System A 100
40
System B 100 39
10 4
10 5 0·5
As required
10.6.2 Ciba-Geigy Ltd Ciba-Geigy offer a wide selection of tin stabilisers. Their range (see Table 10.3) includes four out of the five commercial types of organotin compounds, namely methyltins, butyltins, octyltins and the recently introduced estertins (produced under licence from Akzo Chemie). The
TABLE 10.3 The Basic Stabiliser Range of Ciba-Geigy Ltd Heat stabiliser products Trade name
Product type
Main application
General comments
Irgastab DIM
Methyltin mercaptide
Bottles, extrusions and pipes
Irgastab T4
Butyltin
Plastisols and rigid j'VC
carboxylate Irgastab 1'9
Butyltin
Rigid PVC sheet and rigid
carboxylate Irgastab 17M
Good light stability
sheet Good light stability
PVC profile for cladding
Butyltin
Rigid PVC
Good heat stability
Non-toxic bottles and
Widely approved for
mercaptide Irgastab 17MOK
Octyltin mercaptide
sheet
food-contact applications
Irgastab T649} Irgastab T
Estertins
Food packaging materials
Assessment of non-toxic characteristics in
Irgastab BC445
BalCd/Zn
Semi-rigid and flexible
Some lubricant action,
Irgastab BCZ06
BalCd/Zn
Aexible PVC film and
liquid
sheet
BalCd
Rigid PVC for outdoor use,
progress liquid
Irgastab BCZ8
PVC film
good plate-out performance Good clarity Available in dust-
solid
e.g. window frames
Irgastab BZ505} BalZn Irgastab BZ529
Plastisols and flexible PVC calendering
Good weathering properties
reduced form
Irgastab CZ11 } Irgastab CZ113
CalZn pastes
Transparent flexible and rigid PVC
For non-toxic applications
Irgastab A70} Irgastab A80 Irgastab G1
Aminocrotonates
Rigid PVC film for food packaging
Metal-free for non-toxic applications
Irgastab S110
Antimony
PVC records
Low noise level performance
Auxiliary stabiliser products Trade name
Product type
Function
Reoplast 39 Reoplast 38
Epoxidised soyabean oil } Epoxidised octyl oleate
Co-stabiliser
Irgastab CH55 Irgastab CH300 Irgastab CH310
Tris-nonyl phenyl PhosPhite} Oi-alkyl phosphite Oi-alkyl aryl phosphite
Co-stabiliser
Tinuvin P } Tinuvin 320
Substituted benzotriazoles
UV absorbers
Irganox 1076} Irganox 1010
High-molecular-weight hindered phenols
Antioxidants
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only other class of commercial tin stabiliser, the lauryltins, are mainly used in Japan. A range of mixed metal stabilisers, both of soap and complex-type, are also manufactured. The products mentioned in Table 10.3 illustrate the range of such products available. In addition, aminocrotonate products are available as metal-free stabilisers for non-toxic applications and Irgastab SlIO is an antimony stabiliser specifically for PVC records. Ciba-Geigy's range of auxilliary stabilisers is extensive, with epoxy oils and organophosphites as co-stabilisers, as well as UV absorbers and antioxidants.
10.6.3
Durl.am Chemicals Ltd
Durham Chemicals manufacture three ranges of stabiliser products. The individual metal soaps can be provided for those who wish to formulate their own stabiliser systems. There is a Durostabe range of single-package mixed metal stabilisers in solid, paste or liquid form which have been developed by the company itself. In addition, Durham offer the Nuostabe range of liquid stabilisers which are made under licence from Tenneco Chemicals, USA. The main products in the combined range are listed in Table 10.4. There are a large number of barium/cadmium and calcium/zinc types, each developed for specific application areas. The solid stabilisers containing cadmium or lead are provided in flake or paste form to minimise dust hazards and aid handling. The recommended stabiliser systems given below demonstrate for a number of PVC applications how the use of co-stabilisers complements the main stabiliser in providing the required performance (all amounts in phr).
Flexible calendered sheet System A System B System C Ba/Ca/Zn liquid: Nuostabe 979 2 Ba/Ca/Zn flake: Durostabe 2201 2 1·5 Ba/Cd/Zn liquid: Nuostabe 1500 0·75 Epoxy oil: (e.g. Reoplast 39-Ciba-Geigy) 3 3 Organophosphite: (e.g. Lankromark LE98-Diamond Shamrock) 0·5 Lubricant: stearic acid o to 0·5
TABLE 10.4
The Basic Stabiliser Range of Durham Chemicals Ltd Individual metal soaps Aluminium stearate Barium laurate Barium stearate Cadmium laurate Cadmium stearate
Calcium laurate Calcium stearate Nonnal lead stearate Dibasic lead stearate Lithium stearate
Magnesium stearate Potassium stearate Strontium strearate Zinc stearate
Stabilisers based on solid mixed metal soaps Product type
Trade name
Main application
General comments
Durostabe S59
BalCd (high Cd) flake
Rigid and semi-rigid PVC extrusion
Durostabe S64
BalCd (low Cd) flake
Flexible PVC extrusion, injection moulding and calendering
Good general-purpose performance
Durostabe 2048
BalCd paste
Flexible PVC calendering
Good general-purpose performance
Durostabe 2176
BalCd flake
Injection moulding, e.g. footwear
Durostabe 2230
BalCd paste
Flexible PVC calendering
Durostabe 2240
BalCd flake
Flexible PVC extrusion and injection moulding
Durostabe 2201
BalCd/Zn flake
High speed calendering of flexible and semi-rigid PVC
Durostabe S70
BalZn powder
Low asbestos, highly filled flooring
Durostabe 2188
BalCd/Pb flake
Translucent and pigmented rigid PVC extrusion
Durostabe S65
BalPb flake
Flexible PVC extrusion and injection moulding
Durostabe 2090
CalZn powder
Extruded packing film, flexible PVC calendering and plastisols
Non-toxic, FDA approved ingredients
Durostabe 2147
CalZn powder
Calendered or extruded sheet, flexible or rigid
Non-toxic, FDA approved ingredients
Durostabe 2236
CalZn powder
Filled and vinyl asbestos flooring
Durostabe S67
Zn/organic
Vinyl asbestos flooring
For use with high
Cable extrusion
Effective with
powder
asbestos loadings
Durostabe 2239
Pb
Durostabe 2248
3BLS'
Injection moulding
Durostabe 2270
3BLS'
Pipe extrusion
chlorinated extenders
flake
• Based on tribasic lead sulphate.
Also used with Pb stabilisers for rigid PVC moulding
P. S. Coffin
350
TABLE lO.4-contd. Mixed metal liquid complexes Trade name
Main application
Product type
General comments
Nuostabe 979
BalCdlZn liquid
flexible PVC
General-purpose stabiliser
Nuostabe 1317
BalCdlZn liquid
flexible PVC extrusion
Effective with chlorinated extenders
Nuostabe 1500
BalCdlZn liquid
flexible PVC calendering extrusion and injection moulding
Nuostabe 1515
BalCdlZn liquid
Plastisols and organosols
Nuostabe 1842
BalCdlZn liquid
Plastisols and flexible PVC calendering, extrusion and injection moulding
Nuostabe 3060
BalCdlZn liquid
Plastisols and flexible PVC calendering
General-purpose stabiliser
Nuostabe 1471
BalZn liquid
flexible PVC
For use with phosphate plasticisers
Nuostabe 1829E
BalZn liquid
Injection moulding for
Nuostabes 1223, 1627, 3029, 3044
CdlZn liquid
Kickers for plastisols
Nuostabe 983
CalZn liquid
Plastisols and organosols
Nuostabe 1420
CalZn liquid
Plastisols and organosols
Good air release, freedom from viscosity build-up
Nuostabe 1602
CalZn liquid
Plastisols
Good viscosity stability
Nuostabe 1830
CalZn liquid
flexible and semi-rigid PVC calendering and extrusion
Nuostabe 1839
CalZn liquid
Plastisols
Good air release and mould release
Nuostabe 1844
CalZn liquid
Plastisols, and flexible and semi-rigid calendering, extrusion and injection moulding
Alternative to Cdcontaining stabilisers
Nuostabe 1858
CalZn liquid
Plastisol coatings, especially vinyl wall coverings
Nuostabe 1465
Pb liquid
flexible PVC and plastisols
Alternative to powdered Pb stabilisers
Nuostabe 1251
Chelator liquid
Auxiliary stabiliser
Composed of FDA approved ingredients
Effective at high temperatures
footwear
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351
Clear flexible extrusions of calendered sheet BalCd/Zn liquid: Nuostabe 1500 2·5 Epoxy oil: (e.g. Reoplast 39) 2 Lubricant: stearic acid 0·2-0·5 Plastisols or organosols CalZn liquid: Nuostabe 983 Epoxy oil: (e.g. Reoplast 39)
3 3
Plastisols using FDA approved ingredients CalZn paste: Durostabe S66 Organophosphite: Nuostabe 1251 Epoxy oil: (e.g. Lankroflex GE-Diamond Shamrock)
2 0·5-1·0 3
Flexible extrusions or calendered sheet using FDA approved ingredients Ca/Zn stabiliser: Durostabe 2262 2 0·5-1·0 Organophosphite: Nuostabe 1251 3 Epoxy oil: (e.g. Lankroflex GE) Clear rigid PVC extrusions or calendered sheet 2-3 BalCd flake: Durostabe S59 Organophosphite: (e.g. Irgastab CH300) 0-2 Epoxy oil: (e.g. Reoplast 39) 1 UVabsorber: (e.g. Tinuvin P-Ciba-Geigy) 0·5 Lubricant: stearic acid 0·2 10.6.4 Diamond Shamrock Polymer Additives Division
Diamond Shamrock Polymer Additives Division manufacture all stabiliser types except lead-based systems, and their range now includes the stabiliser products previously marketed by Lankro, SA Argus Chemicals (Belgium), Manchem Ltd (products previously called Manomet stabilisers) and Albright and Wilson (products previously called Mellite stabilisers). The company also manufactures plasticisers and surfactants which enables them to offer package systems in liquid form (e.g. stabiliser-antistat blends), paste form (e.g. stabiliserantifogging agent) or in pellet form (e.g. stabiliser-lubricant blends). Stabiliser development has been directed towards high quality cadmium-free stabiliser systems, in particular through the use of the matrix system. 9 the major products in the Diamond Shamrock range are described in Table 10.5.
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P. S. Coffin
TABLE 10.5 The Basic Stabiliser Range of Diamond Shamrock Ltd Flexible PVC calendering
General category (1) Stabilisers with low lubricant effect, good clarity and light stability (2) Lubricating stabilisers, good plate-out performance (3) Stabilisers for semi-rigid PVC (4) 'Do-it-yourself system using all three components as required (5) Non-cadmium stabilisers
(6) Non-toxic stabiliser (7) Highly filled PVC flooring (8) Asbestos-filled PVC flooring
Trade name
Product type
Lankromark Lankromark Lankromark Lankromark
LC 266 LC 145 LC 68 LC 442
Ba/CdlZn Ba/CdlZn BalCd Ba/Cd/Zn
Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark
LP 15 LP 48 LP 125 LC 288 LZ 22 LE 98 LZ 253 LZ 242 LZ 341 LZ 330 LN 138 LP 103 LP 147 LP 301
Ba/Cd BalCd BalCd BalCd Zn Chelator BalZn
Ba/Zn Ca/Zn Ca/Zn Ca/Zn Ba/Cd
BalZn Organic
Flexible PVC extrusion General category (1) Many of the above can also be used for flexible PVC extrusion (2) Crystal clear PVC compound (3) Stabiliser/anti-fogging agent blends for cling film (4) Sulphur-staining resistant cable covering
Trade name
Lankromark Lankromark Lankromark Lankromark Lankromark
LC 431 A 3021 A 3131 LP 114 LP 356
Product type
BalCd Ba/Cd Ba/Cd
Flexible PVC injection moulding General category (1) Many of the above can also be used for flexible PVC injection moulding
Trade name
Product type
10
353
Commercial Stabiliser Practice
TABLE 1O.5-contd. (2) Stabilisers for footwear (3) Non-cadmium stabiliser for footwear
Lankromark LP 378 Lankromark LP 257 Lankromark LP 422
Ba/Cd Ba/Cd
Plastisols General category (1) Stabiliser/activators -kickers for foamed PVC (2) Clear wear layers (good light stability) (3) Vinyl wallpaper (4) Rotational moulding (good mould release) (5) Coal Board belting (6) General-purpose paste stabiliser (7) General-purpose leathercloth stabiliser (8) Non-toxic stabiliser
Trade name
Product type
Lankromark LZ 440 Lankromark LC 101 Lankromark LC 90 Lankromark LZ 187 Lankromark LZ 473 Lankromark LZ 352 Lankromark LZ 528 Lankromark LT 41 Lankromark LT 19 Lankromark LZ 528 Lankromark LZ 110 Lankromet LA 105 Lankromet LA 175 Lankromark LA 247 Lankromark LA 105 Lankromark LP 301 Lankromark LC 310
Zn-fast action Cd/Zn-fast Cd/Zn-fast Ba/Zn-slow Ba/Zn Ba/Cd/Zn BalZn Tin maleate Tin BalZn CalZn Ca/Al/Zn Ca/Al/Zn Ca/Zn Ca/Al/Zn Organic Ba/Cd/Zn
Lankromark LZ 121
Ba/Cd/Zn
Lankromark LN 138 CalZn
Rigid PVC extrusion General category (1) Profile, e.g. PVC window frames, with good weathering properties (2) Clear sheet and profile, with good light stability (3) General-purpose extrusion, with good early colour, e.g. pipes (4) One-pack stabiliserl lubricant system for pipe
Trade name
Product type
Lankromark LP 125
Ba/Cd solid
Lankromark LT 96
Butyltin maleate
Lankromark LT 118
Butyltin mercaptide
Lankromark LN 468 Ca/Zn pellet
354
P. S. Coffin
TABLE 1O.5-contd. Rigid injection moulding General category
(1) General-purpose mouldings, e.g. pipe fittings (2) Co-stabiliser for leadstabilised mouldings for improved flow characteristics
Trade name
Product type
Lankromark LT 162} . . Lankromark LT 63 Butyltm mercaptide Lankromark LP 103 BalCd solid
Blow moulding, e.g. bottles General category
(1) Rigid PVC for food-contact applications (2) Rigid PVC for food-contact applications (3) Co-stabilisers for co-use with above Ca/Zn systems
Trade name
Lankromark Lankromark Lankromark Lankromark Lankromark Lankromark
LT74} LT 85 LN 303} LN 402 LE 87 } LE 109
Product type
Octyltin mercaptide Ca/Zn Chelators
Rigid PVC calendering
The above stabilisers (i.e. for blow moulding) would also be recommended for rigid PVC calendering, but the lubricant system would be different.
The following formulations illustrate the use of Diamond Shamrock stabilisers in a number of rigid PVC applications (all amounts in phr).
Profile extrusion for pvc window frames pvc + impact {either 5% EVA copolymer modifier or Suspension K65 + 10% Paraloid KM323B BalCd solid: Lankromark LP125 Epoxy oil: Lankromark G£ Organophosphite: Lankromark L£98 LOXiOI G3O-Hentel Lubricants: { International GmbH Loxiol G60 Processing aid: Paraloid K120N CaC03 filler: (e.g. Omyalite 951) Ti0 2 pigment: (e.g. Tioxide TC30)
100 3 2 0·5 0·5 1·0 1·0 4·0 4·0
355
10 Commercial Stabiliser Practice
Sheet extrusion for clear roofing pvc (Suspension K65) Butyltin carboxylate: Lankromark LT96 Combination lubricant: Lankroplast DP5982 . . {paraloid K175 ProcessIng aids: Paraloid K120N
100 2·5 1·4
0·8 0·7 0·7
Benzotriazole UV absorber: (e.g. Tinuvin P) Tinting violet Pipe extrusion using twin-screw extruder PVC (Suspension K65) Butyltin mercaptide: Lankromark LT118 Calcium stearate Lubricants: { Irgawax 367-Ciba-Geigy AC 629 A CaC0 3 filler: (e.g. Omyalite 951) Ti0 2 pigment: (e.g. Tioxide RCR2) Bottle blow moulding for food packaging
PVC (Suspension K60) Octyltin mercaptide: Lankromark LT85 CalZn stabiliser: Lankromark LN303 Organophosphite: Lankromark LE89 Epoxy oil: Lankroflex GE Combination lubricant: Lankroplast L234 . . {ParalOid K175 ProcessIng aids: Paraloid K120N MBS impact modifier: (e.g. Blendex BTA) Calendered foil for food packaging
PVC (Suspension K60) Octyltin mercaptide: Lankromark LT85 Ca/Zn stabiliser: Lankromark LN303 Organophosphite: Lankromark LE89 Epoxy oil: Lankroflex GE · ts: {wax E (montan ester) Lub ncan glycerol monooleate . . {paraloid K175 ProcessIng aids: Paraloid K120N
As required
100 0·5 0·8 1·2 0·15 2
1·5
Tin Ca/Zn stabilised stabilised
100 1·5
1·0 1·2 10
100 1·2 0·5 3·5 0·8 1·2 13
Tin Ca/Zn stabilised stabilised
100 1·5
0·3 1·0 1·0 0·5
100
1·2 0·8 4·0 0·1 0·5 1·0 0·5
P. S. Coffin
356
10.6.5 Victor Wolf Ltd (owned by NL Industries, USA) Victor Wolf manufacture a range of barium/cadmium/zinc, barium/ cadmium and calcium/zinc stabilisers. The principal products in their range are given in Table 10.6. These are mainly of the liquid complex TABLE 10.6 The Basic Stabiliser Range of Victor Wolf Ltd Trade name
Product type
Vinco 348
Baled liquid Vinco 249C BalCdlZn liquid BalCdlZn Vinco 265 liquid Vinco 332 BalCdlZn liquid Vinco 374B BalCdlZn liquid Vinco 2810 Vinco 268 Vinco NlO
BalCdlZn liquid CalZn liquid CalZn liquid
Vinco 654
CalZn paste
Vinco 681
Phosphite chelator
Main application
General comments
Extrusion
Good clarity performance
Calendering, extrusion and plastisols Plastisols, e.g. spreading and dipping Calendering and extrusion of film and sheet
Good general-purpose performance Good resistance to pigment plate-out
Calendering and plastisols
Good general-purpose performance
Calendering, extrusion and plastisols Plastisols, e.g. spreading and dipping Plastisols, e.g. dipping
Good sulphur-staining resistance Low toxicity, good sulphur-staining resistance Low toxicity, good sulphur-staining resistance Rigid and flexible PVC, e.g. For non-toxic applications, flexible dairy and moderate lubricant action beverage tubing Rigid and flexible PVC For non-toxic applications, for co-use with Vinco 654
type, but also included are Vinca 654 and Vinca 681 which constitute a mixed metal soap system for such applications as flexible dairy and beverage tubing.
REFERENCES 1. Press, J. B. (1978). Paper presented at the PRI International Conference on
PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978.
10 Commercial Stabiliser Practice
357
2. Worschech, K. F. (1978). Paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 3. Peter, W. (1979). Paper presented at the Krauss Maffei 8th International Extrusion Symposium, Linz, Austria, September, 1979. 4. Nass, L. I. (Ed.) (1978). Encyclopaedia of PVC, Volume 1, Marcel Dekker, New York, pp. 319, 320. 5. 'The Safe Handling of Lead Chemicals', Associated Lead Manufacturers Ltd,1979. 6. 'Health Aspects Relating to the Use of PVC Pipes for Community Water Supply', WHO International Reference Centre for Community Water Supply, Holland, 1974. 7. Phillips, I. and Marks, G. C. (1961). Brit. Plast. 34,385-90. 8. Bredereck, P. (1979). Paper presented at the 2nd International Cadmium Conference, London, February, 1979. 9. Donnelly, P. J. (1978). Paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978.
CHAPTER 11
Some Miscellaneous Components of pvc Formulations W. V. TITaw
The major constituents of PVC compositions have been discussed in some detail in the previous chapters. Others, which may be regarded as relatively minor but common, and in some cases essential, formulation components, are dealt with in this chapter, viz. lubricants, polymeric modifiers, colourants, antistatic agents, flame and smoke retardants. Still other miscellaneous minor additives, some of which were first mentioned in Chapter 4, are further considered in other relevant chapters and sections of the book (e.g. blowing agents in Chapter 25, antioxidants and UV absorbers in Chapter 9, thickening agents for plastisols in Chapter 21, etc.).
11.1 LUBRICANTS 11.1.1 Functions, Nature and Effects Lubricants are essentially processing additives-their primary role is to influence some aspects of behaviour of PVC compositions under the heat and shear experienced in the processing machinery, so that the processing is made easier, whilst certain factors promoting thermal degradation of the polymer are counteracted (see below). For example, in the extrusion of a rigid PVC composition effective lubrication can increase output for a given heat and power consumption, or reduce the heating and mechanical power required for a given output rate. As constituents of a PVC composition, lubricants can also influence some material properties. for example the thermal stability 359
360
w.
V. Titow
(either directly or through interaction with other components of the formulation-see below), impact strength of uPVC, printability (which may be impaired by the presence of a lubricant). The lubricating action and effects can be divided into external and internal, but a corresponding division of the lubricants themselves is not strictly applicable as a firm classification, since some combine both functions in varying degrees, and also because other additives in a PVC composition can have lubricant effects, as well as-in some casesstrongly affecting the performance of (and requirements for) the lubricants proper. 1 The principal external lubrication effect is reduction of friction and adhesion between the hot PVC composition and the working surfaces of processing machinery and moulds by the presence of the lubricant at the interface: this makes movement of the composition through the machinery easier, and prevents deposition of layers of PVC on the hot metal surfaces. Note: Avoidance of stagnant deposits is important in PVC ing, as the polymer in them soon undergoes decomposition, becoming a source of contamination promoting further degradation in, the rest of the hot
processthermal for, and stock.
The basic internal lubrication effect is the lowering of internal (i.e. inter-particle and inter-molecular) friction in the composition throughout processing, which reduces the effective melt viscosity and frictional heat build-up under shear. An additive, especially one of relatively low molecular weight, which is dispersible in PVC on a molecular level (i.e. capable of 'dissolving' in the PVC resin) may be expected to have some lubricating action. If the additive is fully compatible with the PVC composition when present in the amount appropriate to its purpose in the formulation (say, for example, 80 phr of a primary plasticiser) then its lubricant action will be of the internal type. Note: Those polymeric processing aids and impact modifiers used in PVC that do not lower (and in fact often raise) the melt viscosity may be regarded as an exception: their compatibility and relatively high molecular weight are factors in this behaviour. It may also be noted that poly-a-methylstyrene (a polmeric processing aid) and polymeric plasticisers do have an internal lubricant action in that they lower the melt viscosity of PVC compositions.
11
Some Miscellaneous Components of pvc Formulations
361
If the compatibility of such an additive is limited in the circumstances just mentioned, then it will have an external lubricant effect (which may be combined with a degree of internal lubrication), providing that its relevant chemical and physical properties make it an effective 'slip agent' at the hot metal/PVC interface. In broad terms, the compatibility of the common additives capable of lubricant action, and their usual level of addition in PVC compositions, decrease in the sequence: primary plasticisers > secondary plasticisers and extenders > impact modifiers* and processing aidst > stabilisers and lubricants:j: Thus the main operative features of additives incorporated specifically as lubricants are: (i) generally low compatibility with PVC (lowest for external lubricants) ; (ii) chemical nature enabling the lubricant effects to be exerted in significant measure at the low level of addition (between O· 2 and about 3·0 phr for the total lubricant system, usually comprising more than one lubricant). Since the mode of a lubricant's action (internal or external) is closely associated with its compatibility with PVC, several studies have been made of the degree of compatibility of lubricants, measured in terms of some relevant property of the lubricated PVC composition, with a view to classifying the lubricants and/or predicting their suitability in practical applications. 2 The properties measured included the amount of haze introduced by the additive into a clear PVC compound3 (haze being a manifestation of incomplete compatibility), and the reduction of the glass transition temperature (Tg ) caused by its incorporation 4 (lowest Tg with the most highly compatible lubricant). The effect of lubricants on the fusion behaviour of a PVC composition in a *Some impact modifiers (e.g. extensively chlorinated polyethylene; nitrile rubber; VC/EVA graft polymers) are highly compatible with PVC polymersee Section 11.2. t Some processing aids are occasionally used in high proportions to increase the resistance of PVC to deformation at elevated temperatures-see Section
*
11.2.
Lubricants with internal lubricant action are more compatible with PVC polymer than typically external lubricants.
362
W. V. Titow
rheometer or an extruder has also been used as the criterion of the type of lubricant action. In terms of behaviour under suitably standardised mixing conditions in a torque rheometer the most characteristic effects are: Typical external lubrication: Fusion time (i.e. time to reach peak torque-see also Section 11.1.3 below) substantially increased;5--7 torque value may be reduced (because of decrease in external friction); NB in an extruder, increase of fusion time and torque reduction are accompanied by a drop in back pressure. Typical internal lubrication: Torque significantly reduced (in consequence of drop in melt viscosity); little or no effect on fusion time.
However, the effects of an external lubricant on the torque rheometer fusion times of PVC compositions do not always correlate with the corresponding melting rates in an extruder, 7 although limited prediction of extrusion characteristics may be possible from plots of rheometer fusion time against mixing head temperature. 8 An increase of melt flow velocity at the wall face of an extruder die (slip effect) was observed directly by Chauffoureaux and co-workers9 in the case of a PVC compound containing a lubricant with effective external action: this effect was absent when the lubricant used was one with a typically internal behaviour. Various studies directed to characterising and classifying lubricant effects have been briefly reviewed by Gale,? and by Logan and Chung. lO The chemical structure of the lubricant is both the prime factor in its compatibility with PVC and the link between compatibility and the mode of lubricant action. Results of experimental studies in this area, and in particular those of the authoritative work of Illmann l l are consistent with the theoretical expectation that the molecular size (chain length) and polarity (nature and number of polar functional groups) of a lubricant are the main factors determining the compatibility and lubricant behaviour: broadly speaking, short chain length and high polarity make for good compatibility and internal lubrication, whereas relatively long-chain compounds (even with some polar groups) tend to be poorly compatible and act as external lubricants. The types of chemical compound used as lubricants in PVC are shown in Table 11.1: among those, typical examples of low compatibility and external lubrication effects are provided by, say, the polyethylene waxes and calcium stearate, whilst relatively high compatibility and
11
Some Miscellaneous Components of pvc Formulations
363
TABLE 11.1 Compounds Used as Lubricants2,4,9 General type
Class of compound
Examples
Hydrocarbons
Natural hydrocarbons Synthetic hydrocarbons
Paraffins, paraffin oils Synthetic paraffins, low molecular weight polyethylene
Hydrocarbon derivatives
Fatty acids Fatty alcohols
Stearic acid Cetyl, stearyl, octadecyl
Derivatives of organic acids
Metal salts (soaps)
Stearates of barium, calcium, aluminium, and lead
Amides Esters and partial esters
Stearamide Butyl stearate, glycerol monostearate, glycerol monoricinoleate; Stearyl esters, montan acid esters
Wax esters
associated internal lubricant action are exemplified by the partial esters of glycerol. Whereas rigid PVC compositions normally require both internal and external lubricants (unless lubricating stabilisers and/or processing aids are used-see below) only external lubricants will usually be considered for flexible compositions, as in those the plasticiser will provide internal lubrication (some external lubrication may also be supplied by plasticiser extenders if present). However, an external lubricant may be useful, or necessary, even in such heavily plasticised compositions as certain plastisols, especially for intricate mouldings. 2 Because of the need to achieve the right measure of internal and external lubrication, and to balance-in the particular, individual composition-the effects of the lubricants with those of the other additives present, more than one lubricant (i.e. a lubricant system) is normally employed in rigid PVC formulations (where up to four lubricants may be combined in some cases) and sometimes also in pPVC. The mutual effects and interactions of the lubricants with the other formulation components are important considerations, affecting both the choice of the lubricant system and the design of the formulation as a whole.
364
w.
V. Titow
11.1.2 Interaction and Co-action of Lubricants with Other PVC Formulation Components (a) Lubricant/Stabiliser effects STABILISING EFFECfS OF LUBRICANTS
Direct stabilising action: Most lubricants of the metal soap type (see Table 11.1) have some stabilising effect, and some can act as stabilisers in their own right, albeit their action in this role is not as strong as that of the more powerful 'primary' stabilisers. Thus certain metal soap lubricants (e.g. calcium stearate) can be used in either capacity in PVC formulations (but when employed as the sole stabiliser, a relatively large amount will be needed to provide a reasonable measure of long-term stability, and the overall stabilisation will not be as good as that conferred by a smaller proportion of a strong primary stabiliser). Lead stearate and dibasic lead stearate are widely used as lubricating components of lead stabiliser systems. Synergistic action with stabilisers: Some lubricants can enhance the effectivity of some stabilisers by a synergistic effect. The synergistic lubricants are of the internal kind (i.e. comparatively highly compatible with PVC) and usually contain reactive functional groups (in particular hydroxyl) in the molecule. Typical examples are partial esters of glycerol with relatively long-chain saturated or unsaturated aliphatic acids. In those compositions where it occurs, the synergistic effect depends, in a complex way, on several factors, including the PVC resin (type and grade), the nature and amount(s) of stabiliser(s) in the stabiliser system, and the amount of the synergistic lubricant present. The stabiliser systems which benefit from the effects of lubricant synergists are those based on sulphur-containing tin compounds (thiotin or tin mercaptide stabilisers) or on lead compounds. In rigid compositions incorporating calcium stearate as the sole heat stabiliser (e.g. in some pipe compounds for potable water in Europe) most lubricants, whether synergistic or 'neutral' in their effects with other stabiliser systems, can produce some stability improvement. 12 Calcium stearate used as a lubricant synergistically improves heat stabilisation by antimony mercaptide stabilisers. 13 With BalCd, Ba/Cd/Zn, CalZn and Ca/Mg/Zn stabiliser systems synergistic effects are important, but they are provided by epoxy-compound co-
11
Some Miscellaneous Components of pvc Formulations
365
stabilisers, usually in conjunction with an organic phosphite (see Chapters 4, 9 and 10), and the lubricants used do not make any significant contribution in this respect. Improvements in heat stability (mainly long-term) of compositions stabilised with sulphur-containing tin compounds (e.g. rigid films, calendered or extruded) can be promoted by the use, as lubricants, of glycerol partial esters (liquid versions, e.g. glycerol monoricinoleate, are necessary for transparency). Lubricants of this kind also enhance the stability of compositions containing lead stabiliser systems (but discoloration can arise, especially on outdoor exposure, with such inorganic lead stabilisers as lead phosphite or sulphate*) Pentaerythritollfatty acid partial ester lubricants can be particularly effective in synergistically enhancing the long- and short-term stability of lead-stabilised rigid PVC compositions, allowing significant reductions in the amount of stabiliser necessary in many cases. 12 Negative effects can also arise: for example, the presence of glycerol partial ester lubricants can reduce the thermal stability of compositions stabilised with sulphur-free tin stabilisers (whilst very good stability may be maintained if the lubricant is an ester of a monohydric alcohol and long-chain fatty acid-e.g. butyl stearate). Note: On the other hand, the heat stability of uPVC (dry blend) compositions stabilised with a butyl thiotin stabiliser can be impaired if cadmium stearate is included in the lubricant system. 8 Indirect stabilising action: In discharging their primary functions, the lubricants also affect the thermal stability of a PVC composition. By lowering frictional heat build-up (through both internal and external lubrication) and melt viscosity, and hence the effective processing temperature, as well as limiting direct contact between the stock arid hot metal surfaces whilst simultaneously preventing the formation of stagnant deposits (through external lubricant action), lubricants reduce the scope for immediate thermal degradation of the PVC in processing and limit its 'heat history': the first of these two general effects is equivalent to improving short-term thermal stability, and the second a factor enhancing the long-term stability of the composition. 5 ,8 The use
* This is attributable to a reaction of these compounds, in the presence of light, with free glycerol often contained in residual amounts in commercial glycerol esters. 12
366
W. V. Titow
of an effective lubricant system can thus reduce the demands on the stabiliser(s). LUBRICANT ACTION OF STABILISERS
Several stabilisers have some lubricating action (usually mainly of the external kind). This is greatest with some metal stearates: as indicated above, certain compounds in this group (e.g. calcium and lead stearates) may be regarded as lubricants with stabilising properties (see also Chapters 9 and 10). Some tin stabilisers also exert lubricant effects (e.g. dibutyltin dilaurate), as do Ba/Cd soap complex stabilisers. Compositions containing such stabilisers will require less lubricant(s) overall (none in some cases) and/or a different balance of the lubricant system.
(b) Mutual Effects of Lubricants and Plasticisers PRIMARY PLASTICISERS
The internal lubricating action of primary plasticisers has been mentioned (see Section 11.1.1 above): this makes the addition of internal lubricants to plasticised compositions unnecessary in most cases. However, if the external lubricant used is highly compatible with the plasticiser(s), its lubricating action in the composition will normally be reduced, necessitating an increase in the level of addition. SECONDARY PLASTICISERS AND EXTENDERS
Some of these may exert external as well as internal lubricant effects. However, the extent (or even the occurrence) of external lubrication will depend on the nature (and given that, the amount) of the plasticiser(s) or extender(s) present, and to some extent also on the process. For example, in calendering compositions many polymeric plasticisers, even when used near the compatibility limit, provide no external lubrication so that external lubricants are required to counteract 'stickiness' in processing: on the other hand, in some compositions containing a chlorinated-paraffin extender this additive can provide both internal and external lubrication in sufficient degree. (c) Effects of Polymeric Modifiers (see also Section 11.2) The polymeric additives incorporated in a PVC composition, in relatively minor proportions, as impact modifiers or processing aids, can, in any individual case, affect the total lubricant requirement
11
Some Miscellaneous Components of pvc Formulations
367
and/or that for the internaVexternal lubricant balance in a composite lubricant system, if their own compatibility with the lubricant(s) influences the latter's compatibility with the composition as a whole, or-as, for example, in the case of some processing aids (see below)-because of direct lubricant action. PROCESSING AIDS
Many of these have no lubricant effect, internal or external, in that they do not reduce the melt viscosity or the external friction and 'sticking' tendency of a PVC composition. However, some acrylicbased lubricating processing aids with pronounced external lubricant action are available,14 and poly-a--methylstyrene (of the relatively low molecular weight grade used as a processing aid) lowers the melt viscosity of PVC compositions (i.e. has an internal lubricant effect).15 IMPACf MODIFIERS
Most ABS and MBS modifiers have no lubricant action. With those highly compatible modifiers which may be incorporated in large proportions to act as permanent plasticisers (nitrile rubber, chlorinated polyethylene of high chlorine content, VClEVA graft copolymers) lubricant effects may arise. The presence of some impact modifiers increases the compatibility of external lubricants with the composition, so that the external lubricant has to be carefully selected (and a relatively high amount may have to be used) for optimum results. (d) Effects of Fillers and Pigments Fillers and pigments (especially fine-particle grades) can bind lubricants by absorption (ct. plasticiser demand-Chapters 4 and 8) so that their presence in a PVC (especially uPVC) composition can increase the lubricant requirement. However, this effect may be reversed-at least with regard to external lubrication-if the filler carries a stearate coating (see Chapter 8) as this can not only block absorption of lubricants at the particle surface, but also actually provide additional lubrication. 11.1.3 Assessment of Lubricant Effects
Reference has already been made in Section 11.1.1 to the evaluation of the effects of lubricants in PVC compositions with the aid of a torque rheometer or in an extruder; capillary rheometers are also sometimes
W. V. Titow
368
employed. The torque rheometer is widely used for this purpose (as indeed, in general, for practically oriented studies of melt-processing characteristics of PVC compositions): very popular and well-known commercial equipment of this kind is the Brabender Plasti-Corder. * In essence this consists of a thermostatically heated mixing chamber housing two rotors mounted in a measuring head and driven by a variable speed motor. The equipment is instrumented for continuous measurement of the torque on the rotors (which is a function of the resistance of the PVC composition to the mixing action) and the mix temperature. A plot of torque against mixing time typically shows a rise of the torque as the mix is fluxed, and something of a drop when the fusion point is reached (see Fig. 11.1): the 'fusion time' taken to reach this point is increased by external lubrication; the torque value can also reflect lubricant effects. A standard method for carrying out
CJl~
:J
L.
f-
~
~ ~ - ~-
- - - -
- ----=-=_::"':_=-=_:":_==--=-=-=-=-=_:-
B
A Time
Fig. 11.1 Schematic representation of a 'plastogram' recorded on a Plasti-Corder chart for a uPVC composition. A, Torque (Nm); B, mix temperature (0C); C, mixer temperature (0C).
* Marketed by C. W. Brabender Instruments Inc. (in North America), Brabender OHG (in West Germany), and agents in most countries. The Plasti-Corder is a larger, more sophisticated version of the original Brabender Plastograph. Several models are available, with torque ratings between 100 and 400 Nm and different maximum rotor speeds.
11 Some Miscellaneous Components of pvc Formulations
369
this kind of fusion test is given in ASTM D 2538-79 (see Appendix 1, Section 2.12). For determinations relevant to extrusion characteristics of a PVC composition extrusion heads can be fitted to the Plasti-Corder. 16 The RAPRA torque rheometer* is another wellknown instrument of the internal mixer type. Determinations of melt viscosity (in many cases the value determined will be the apparent viscosity, if the melt behaves as a non-Newtonian fluid) in capillary or, generally, tubular die rheometers may be used to follow the reduction in viscosity brought about by internal lubrication. A standard method, employing a piston plastometer, is given in ASTM D 3364-74 (1979) (see Appendix 1, Section 3.1(d»; this can be useful for direct comparison of internal lubricant effects in the same composition (or two closely similar compositions) but correlation with processing practice may be variable, inter alia because the shear rates imposed by the processing equipment can be different (higher) than those in the plastometer, and the effects of melt elasticity can affect the behaviour differently. Highly sophisticated equipment has been described by Chaufoureaux and co-workers,9 not only capable of demonstrating the overall effects of lubricants (as well as other constituents) on the rheology of a PVC composition, but also providing data indicative of the mechanisms of lubricant action. The concept of 'lubricant value' (LV) has been put forward 17 as a means of comparing-in a general, approximate way-the effectivity of different lubricants and lubricant systems, including the stabiliser/ lubricant combinations formulated for use in particular applications. To calculate the LV, appropriate data from standard determinations in a Brabender Plasti-Corder are used in the formula: LV = (lOOOE)/(T. md) where: E is the total weight of lubricant additive (phr); T is the stock temperature in the mixing compartment (0C); and md is the torque (kgfm). As can be seen, the LV is highest for the most effective lubricants. The LV values of many lead-based stabiliser systems (Biiropan SMS stabilisers-Otto Barlocher GmbH) were found 17 to lie between about 6 and 14. Various laboratory methods of assessment of lubricants have their * Developed in the UK by the Rubber & Plastics Research Association.
370
W. V. Titow
place in more fundamental studies as well as in the preliminary selection and comparison for the purposes of practical formulation of PVC compositions. In the latter case, however, actual processing trials should always be conducted. Much of the development work on lubricant systems for various types of compositions and processes is done by suppliers of lubricants (and stabiliser suppliers, since combined stabiliser/lubricant 'one-pack' systems properly formulated can offer the advantages of optimum compatibility, component balance and synergistic effects). A further extension of the one-pack concept is the inclusion of other additives with the stabiliser/lubricant system, so that the total additive content is tailored for particular requirements. The advantages and limitations of this approach are mentioned in Chapters 9 and 10. Another recent, interesting, line of development has been the introduction of lubricant concentrates in PVC, in the form of PVC particles heavily loaded (about 50% and over) with calcium and barium stearates. The principal advantages of such concentrates are that they are virtually dust-free, can be air-conveyed and have dry-flow properties very similar to those of PVC resins. The effects of incorrect balance, or total amount, of a lubricant system in a PVC composition may include the following, in varying degrees of severity: Processing Overlubrication:
(or wrong balance)
Underlubrication:
(or wrong balance)
Excessive slippage (resulting in lower output or even disruption of production); plate-out High shear resistance (resulting in lower output); degradation of polymer in melt
Product
Surface bloom; haze (in clear compounds); impaired printability Impaired stability (because of excessive heat history) or actual degradation
The presence of a slight excess of external lubricant can have some useful effects. Thus surface gloss may be improved, and surface friction and tendency to blocking reduced.
11.1.4 Sources of Information on Lubricants and Their Commercial Suppliers In addition to the references already quoted in this section, papers published by Jacobson,18 Riethmayer,19 and Stapfer et al. 20 are concerned with the nature, application and effects of lubricants.
11 Some Miscellaneous Components of pvc Formulations
371
Many suppliers of heat stabilisers for PVC (ct. Chapter 10, especially Table 10.1) also supply lubricants. Listings of lubricant suppliers (as well as those of most other additives) in the Western World will be found in the publications mentioned in Section 8.5 of Chapter 8. Many British suppliers are also listed in the Buyers' Guide for Plastics Additives published by the British Plastics Federation. The following may be mentioned by way of a few examples.
UK: Ciba-Geigy Plastics and Additives Co. Industrial Chemicals Division (Irgawax); Diamond Shamrock Ltd (Lankroplast, Lankromark); Croxton and Garry Ltd (Lubriol, Syntewax). Continental Europe: Henkel International GmbH, West Germany (Stenol, Ceroxin, Loxiol); Otto Barlocher GmbH, West Germany (Biiropan); Acima, Switzerland (Metawax, Metaglide). USA: Emery Industries, Inc. (Emerwax); Interstab Chemicals Inc. (Interstab); Nopco Chemical Division of the Diamond Shamrock Chemical Co. (Metasap, Nopcowax); Witco Chemical Corp. (Lubraplus); Petrochemicals Co. Inc. (Monolube). 11.2 POLYMERIC MODIFIERS As has already been mentioned in Chapter 4, the polymeric additives incorporated in PVC compositions may be broadly classified into two groups according to their functions, viz. processing aids and impact modifiers. In general terms, the main differences which form the basis of this classification are in the nature of the polymers used in each of the two capacities (see Sections 11.2.1 and 11.2.2 below), in the usual level of addition (normally substantially lower with the processing aids), and in the type of effect: processing aids-as implied by their name-serve to modify the properties of the PVC stock during heat processing (but have relatively little effect on those of the finished product), whereas the main function of impact modifiers is to improve the impact resistance of the product. However, whilst the above features do typify the group characteristics in a general way, they are not rigidly definitive. Thus there is some overlap in the types of polymer used for the two respective purposes; many impact modifiers have some processing-aid action (albeit this often tends to be manifested at temperatures somewhat higher than those at which typical processing aids exert their effect); processing aids can affect some product properties even at their usual, relatively low level of
372
W. V. Titow
addition. Certain of the polymeric additives are also used in exceptionally high proportions to upgrade the heat-distortion properties of uPVC compositions (see Section 11.2.1), or to combine a toughening effect with plasticisation of virtually ideal permanence (see Section 11.2.2). It is very important for its maximum effectivity (and hence also cost economy) that any additive (and especially one used in relatively minor proportions) should be dispersed as thoroughly as possible in the PVC polymer. This applies to the polymeric modifiers discussed in this section. In the production of PVC pre-mixes for further (melt) compounding, and dry blends for use as feedstocks in melt processing, the order of addition of the formulation components (in conjunction with the temperature at the various mixing stages) plays a significant part in the ultimate degree of dispersion and effectivity of action of the polymeric modifiers. The following guidelines for a particular procedural sequence and conditions in hot high-speed mixing of a powder blend to be used as extrusion feedstock,21 illustrate something of the points important in practice (see also Chapter 13, Section 13.4.1, for a more complete discussion of high-speed mixing): -<:harge the PVC powder to the mixer and mix at the appropriate (high) speed; -add colourant and stabilisers at 45°C, then impact modifier at 60°C; -add lubricant(s) at 80°C; -when the temperature reaches 100°C add acrylic processing aid; -<:ontinue mixing, and at l1o-112°C discharge the batch into a cold mixer (e.g. a water-jacketed ribbon blender); run till cooled to 40°C and discharge. Information on suppliers of polymeric modifiers will be found in the sources mentioned in Section 11.1.4 above. Some commercial materials are mentioned, by way of non-selective example, in Sections 11.2.1 and 11.2.2. 11.2.1 Processing Aids The polymers used as processing aids in PVC are of the following kinds: (a) acrylates and methacrylates (acrylic processing aids);
11 Some Miscellaneous Components of pvc Formulations
(b) (c) (d) (e)
373
styrene/methacrylate copolymers; acrylonitrile/butadiene/styrene (ABS) polymers; styrene/acrylonitrile (SAN) copolymers; poly-a-methylstyrene (PAMS).
The molecular weight of the PAMS grades marketed as processing aids for PVC (Amoco Resin 18-Amoco Chemicals Corp., USA) iS,low compared with those of other processing aids. Some ABS polymers are used also as impact modifiers (as are some acrylic polymers--see Section 11.2.2 below). One of their composition-related properties, viz. the modulus of elasticity, is sometimes cited as a rough index of the type of their effect in PVC compounds: those useful as processing aids have the highest moduli, whilst the moduli of ABS impact modifiers are comparatively low (less than about 1·4 GPa). Processing aids are employed in rigid PVC compositions, in relatively low proportions (about 1-6 phr) to facilitate melt processing. The typical effects of their presence on the stock are: 14 ,21-24 (i) (ii) (iii) (iv) (v)
increased rate of homogenisation and fusion, except in the case of lubricating processing aids which extend fusion time (but reduce torque and back-pressure in extrusion-cf. (v) below); improved strength, cohesion and extensibility of the melt; reduction of melt modulus and 'nerve', die-swell, and tendency to melt fracture; increased melt viscosity (except with PAMS by which it is reduced in most types of uPVC composition); tendency to increase torque and back-pressure in extrusion, attributed to the faster, higher-shear character of the fusion process 14 (PAMS is a partial exception here).
Effects (i) and (v) can be sensitive to the nature and amounts of the lubricants present in the composition (especially where the processing aid is PAMS23). Note: In view of effects (iv) and (v) the practice-occasionally encountered-of calling processing aids for PVC 'flow promoters' is questionable. It would be preferable to restrict the latter term entirely to its original use as a name for the group of additives sometimes incorporated in thermosetting resin systems to reduce the rate of interaction of reactive groups by a dilution effect. 25
374
W. V. Titow
Effect (ii) is a contributory factor in the improvement in thermoformability conferred on PVC sheeting by processing aids (especially of the acrylic type). The mechanism whereby processing aids produce their effects is not yet fully understood. In certain respects their action is similar to that of internal lubricants, but there are also important differences (e.g. the opposite effect on melt viscosity in most cases): attempts to treat the action of both these types of additive as a simple case of plasticisation at high temperature,25 fall short of a complete explanation. In addition to their role in the processing of uPVC, which is the primary reason for their use, processing aids present at the normal levels of addition can also affect some properties of the material. Typically (e.g. with acrylic processing aids) such effects may include improved surface finish (freedom from defects; improved gloss), better colour stability of coloured materials, and improved transparency of clear compositions. Incorporation in uPVC compositions of high proportions (up to about 50 phr) of some polymers with processing-aid action (e.g. certain styrene/acrylonitrile (SAN) copolymers) can improve the heat distortion temperature, increase the Vicat softening point and reduce the linear thermal expansion. However, impact strength is also usually reduced, and suitable impact modifiers should be included in the formulation to counteract this effect. Such compositions may be used in a number of applications calling for a higher than average degree of resistance to heat distortion (e.g. some pipes and injection moulded pipe fittings, some extruded profiles): transparent versions in which the polymeric modifiers (as well as the other components of the formulation) are approved for food contact are of interest for the manufacture of containers (jars, bottles) for food products which are still hot at the filling stage (e.g. jam, marmalade, some liquids). The following formulation has been cited (by Sahajpat26 ) as a good general basis for a transparent composition of this kind, with low haze and yellowness index, a Vicat softening point (DIN 53 460, 5 kgf load) of 85°C and impact strength (ASTM D 256) of 32·4 kgf cm cm- 2. Suspension PVC polymer, K value 58 Stabiliser: Irgastab 17 MOK (Ciba-Geigy) Polymeric modifiers: SAN MBS Lubricants: Sapchim Lubricant 4150 Sapchim Lubricant 6164
100
1·2 phr 20phr 10phr 0·7phr 0·5 phr
11
Some Miscellaneous Components of pvc Formulations
375
Some special ABS modifiers, * used in very high proportions, can confer similar improvements in the temperature-dependent properties, with good impact resistance and without substantial increase in flammability.21
11.2.2 Impact Modifiers (a) Impact Resistance-Its Nature, Significance and Measurement Most PVC (as indeed other plastics) products are at risk of possible damage by impact at some point during their lifetime, either through accidental hits or drops (e.g. PVC pipes during transport and installation; PVC containers in transport and storage), or in the course of normal manipulation (e.g. PVC packaging film on high-speed converting and packing machinery) and service (e.g. PVC swing doors in factories and storage premises). For these reasons the degree of resistance to breaking on impact, and its measurement, are of practical importance in the selection and comparison of plastics materials for particular applications, product design, and quality assessment and control. On a more fundamental level, understanding the causes and mechanisms of failure by fracture on impact of plastics materials and products is equally important in the context of materials science. The impact tests in general use measure directly the total energy needed to break the test specimen by impact. This is one of the two main general differences between such tests and the regular strength tests, in which the force (load) which causes the specimen to yield or break (in tension, compression, flexure or shear) is determined, normally in conjunction with the load Ideformation curve. t Note: Thus in this context 'strength' is the force (or the stress) for failure. It is for this reason that the term 'impact resistance' is replacing the formerly widely used 'impact strength' in the titles of standard impact test specifications and references to their results.
* These polymers are not primarily processing aids, and a formulation containing even, say, 80 phr of such a modifier can benefit from the inclusion of 2-4 phr of a good acrylic processing aid, e.g. Aery/aid K-120N (Rohm and Haas Co., USA-the trade name Para/aid is used by Charles Lenning Chemicals and affiliates in the UK and certain other countries: the product coding is universal). t The values of the stress at failure, and true stress/strain curves, can be derived from such data.
376
W. V. Titow
The other difference lies in the respective test speeds (and hence rates of deformation): those obtainable with ordinary strength-testing equipment are much lower than the ones occurring in impact tests, where the testing speeds are comparable with the speeds which can be experienced by freely dropped objects (about 2-7 m S-I). These differences do not mean that impact tests are fundamentally different from strength tests. Although not measured directly in the latter, the total energy to break can be computed from the area under the stress/strain curve. Whilst the value so obtained will correspond to the rate of deformation in the strength test concerned, and will thus normally be numerically different from one furnished by a relevant impact test (in which the deformation rate will be different), it has been shown27 that both values can lie on the same smooth curve in a plot of energy to break versus rate of deformation, if the mode of failure (see below) is the same in the two tests. Note: With special apparatus and instrumentation, tensile-strength
testing speeds can be brought up into the range involved in impact tests. 28 •29 One version of such a high-speed test is covered by an ASTM standard. 30 However, certain difficulties attendant upon this kind of testing, and the fact that the stresses generated differ in state and distribution from those arising in other impact tests, limit its relevance to impact properties of plastics largely to the role of a supplementary technique, of interest predominantly (though not exclusively) in research. The concept of toughness of a plastics material (or object) is based on the definition of this property as the work necessary to break a suitable specimen of the material (or the object). This is, of course, the same as the total energy to break (given by the area under a stress/deformation curve) and hence-in an impact test-the impact resistance. However, any actual numerical value of impact resistance obtained in a test is only an arbitrary index of toughness, relating specifically to the single set of conditions and specimen characteristics (including its dimensions and configuration) employed in the test. * This is so because impact resistance is not a fundamental, constant property of a plastics material (or its parent polymer) with which a product made from the material may be expected to be fully and * Such values are often referred to as 'single-point data'.
11
Some Miscellaneous Components of pvc Formulations
377
directly endowed. In both the material and its derivative products the impact resistance is influenced-in a complex way-by many factors, which may be grouped under the following headings: 'Internal' material factors: composition; morphology; fine structure (including crystallinity and molecular orientation); presence of impurities, nibs or gels. (ii) Effects of processing conditions: modification of the material properties (e.g. crystallinity, orientation, impurities, etc.) in processing; heat degradation;* incomplete fusion;* incomplete interdispersion of formulation components* (and especially impact modifiers and particulate additives in the polymer); such process-induced product features as 'skin-and-core' effects in mouldings, presence of internal stresses (cooling and packing stresses in mouldings); surface imperfections or faults; and weak sites (e.g. weak weld lines in injection mouldings, or nip closures in blow-moulded containers). (iii) Design factors: In products:t abrupt changes in section (especially sharp corners); moulded-in metal inserts; machined holes and screw threads. In standard test specimens: 'geometry'; presence and characteristics of notches. In both: material flow (direction and pattern) in the course of formation in relation to the 'geometry' of the object (position and type of gates in injection mouldings). (iv) 'External' factors:+ energy and speed of impact (and hence the rate of deformation); shape and hardness of impacting object; point of impact; temperature; nature of environment (possible presence of stress-cracking agents).
(i)
Whilst anyone of these factors can affect the magnitude of impact resistance, and the way in which fracture occurs in a given situation, the wide scope for their combined action or interactions can also complicate the fundamental interpretation of impact test results, as the same values of fracture energy may be obtained for materials or products fracturing by different mechanisms. Data on the associated stress/strain characteristics can be a great help in interpretation: some modern, suitably instrumented impact testers of the falling-weight type provide such information as well as other dynamic-property data. 31 ,32 * Of special significance in PVC.
t Features creating stress concentration
(some .crack-initiating). :t:Operative in service conditions and/or tests.
378
W. V. Titow
Standard impact tests normally yield single-point data; but even multi-point data provided by planned variation of the basic conditions of a standard test, and/or the use of more than one test, cannot provide a basis for a complete characterisation or prediction of impact performance (although a well designed, comprehensive test programme can go a long way in this direction, at least in some cases). Thus, despite the comparability of the rates of deformation in standard impact tests with those encountered in service, the degree of correlation of the test results with impact resistance in service (or between two different tests) is relatively limited. Nonetheless, a standard impact test can be of practical value if its limitations in the particular context are property recognised, and especially where-in addition-experience of its use and a body of accumulated test data relatable to service performance are available. It is on this basis that standard impact tests are mainly employed in the industry. Where a product or article can be tested in conditions realistically approximating to the most severe likely service conditions the results (even in the form of non-quantitative 'pass' or 'fail' information) can provide valuable means of material comparison and production quality control, as well as guidance on product design. This applies, for instance, to drop tests on plastics containers* (some of which are covered by standard specifications-e.g. ISO 2248 Part IV and the corresponding BS 4826 Part 4; ASTM 0 2463), bird-impact tests on aircraft canopies, or bullet-impact tests on safety screens or practice targets. Standard impact tests in general use for plastics materials or products, and hence applicable, inter alia, to PVC, are listed in Table 11.2 (see also Appendix 3, Impact Resistance). Other relevant standards, relating specifically to PVC materials and products, will be found in the appropriate sub-sections of Appendix 1. The following standard specifications give impact tests (all of the falling-weight type) for extruded PVC products (pipe and cladding):
ISO 3127-1980. Unplasticised polyvinyl chloride (PVC) pipes for the * Drop tests usually form the basis of impact resistance requirements laid down by various national and international regulations governing acceptance for transportation of filled containers (including plastics containers): ct., for example, the International Maritime Dangerous Goods (IMDG) code; US Department of Transport (DOT) regulations; International Air Transport Association (lATA) regulations; or the RID regulations (Department of Transport, UK) concerning carriage of dangerous goods by rail.
a
DIN 55448-1977
ASTM D 2289-69 (1976) (high-speed test)
ASTM D 1822-79
Plastics compositionsa
Plastics compositionsa (specimens as in ASTM D 1822)
Energy to break in J or ft lbf Energy to break per unit cross-sectional area, in J m- 2 or ftlbfin- 2 (NB Test also gives tensile strength, in Pa or lbfin- ) Energy to break, in mJ mm- 2 or kJ m- 2
Rigid plastics compositions;a rigid plastics sheet Plastics compositionsa
DIN 53 443-1975 Parts 1 and 2
In the form of standard moulded specimens.
Tensile impact
Energy to break, in J; kgf cm; or ft lbf Mean tup weight for failure, in kg or lb Energy to break, in J
Rigid plastics compositions;a rigid plastics sheet Plastics sheet or parts
BS 2782:1970: Methods 306B and C ASTM D 3029-78
Falling weight impact
Expression of results See Table 11.3
Applicable to: Rigid plastics compositions;a rigid plastics sheet
See Table 11.3
Relevant standards
Flexural impact (Izod and Charpy type)
Test type
TABLE 11.2 Types of Standard Impact Test in General Use
~
V.l
a6'
S"
l':
~ ~
~
"tI
~
t:;
~
;:
~
~c
1:;
~
;:
~
t:;.
'"a::
~ ::l
''-
380
W. V. Titow
transport of fluids-Determination and specification of resistance to external blows. BS 5255:1976. Plastics waste pipe and fittings. ASTM D 2444-70 (Reapproved 1977). Impact resistance of thermoplastic pipe and fittings by means of a tup (falling weight). ASTM D 3679-79. Rigid poly (vinyl chloride) (PVC) siding. * It is a particular advantage of falling-weight impact tests that-in the versions employing suitable equipment-actual products (mouldings; sections of pipe, profiles or cladding; etc.) can be tested. Such tests carried out on one of the modern testers with microprocessor instrumentation,t in conjunction with similar tests on suitable material specimens, can provide a great deal of information on the effects of design and processing, as well as basic data on the fracture mechanism. Impact testing of PVC pressure pipes with the aid of explosive charges33 may be mentioned as an example of the use of a special technique to obtain information on fracture characteristics under deformation rates and stresses relevant to conditions which the pipes may experience in service (e.g. in mines). Two cardinal aspects of the fracture on impact of a plastics object in a test or in service are the fracture mechanism and the mode of failure (brittle, ductile or intermediate-see below). A plastic (like its parent polymer) fractures through the initiation and propagation of cracks, and the total energy to break is the sum of the energies consumed in each of the two processes. A systematic examination of the effects of notch sharpness in notched specimens on their impact resistance (energy to break) in suitable impact tests can be a useful basis for assessing a material's resistance to crack initiation and propagation. As the sharpness of the notch increases the proportion of the total energy to failure expended on crack initiation is reduced:* tests on very sharply notched specimens (notch-tip radius about l0.um or less) can give a good indication of the crack
* This is a product specification prescribing, inter alia, a falling-weight impact test on a variable-height impact tester (Gardner Tester Model IG 1120Gardner Laboratories Inc., Bethesda, Md., USA). t e.g. the 'Dynatup' impact tester, marketed by Effects Technology, Inc., USA,31 or the CEAST Fractoscope (CEAST S.p.A., Italy). :j: The ratio of the impact resistance of unnotched specimens to that of notched specimens is generally referred to as 'notch sensitivity'. Numerical statements of notch sensitivity should always be related to the particular test (and the notch radii) used.
11 Some Miscellaneous Components of pvc Formulations
381
propagation energy (as the crack initiation energy is drastically lowered by the high local stress concentration created by the sharp notch).
Note: Stress concentrations occurring 'naturally' in plastics objects (including unnotched test specimens) can be caused, inter alia, by surface faults or damage, as well as the presence of gels, nibs or large particles (e.g. undispersed aggregates of filler or pigment particles, adventitious contaminant particles) in the material. As pointed out by Vincent,34 a notch of 0·25 mm radius approximates to the most severe defect likely to be encountered in practice, whilst, at the same time, its use in extended tests permits coverage of a wider range of temperatures than that of blunter notches which approximate better to the most common service defects. The 0·25 mm radius notch is used in several standard flexural impact tests (see Table 11.3). Evaluation of the effects of an impact modifier in a rigid PVC composition, or comparison of different modifiers, should be based on investigations of this kind rather than-as often happens in the industry~n single-point data from individual impact tests. The temperature (and-if practicable-also the speed of testing) should also preferably be varied in such investigations, to ascertain whether a ductile/brittle transition in the mode of fracture (see below) may be affecting the impact resistance within the relevant range of conditions. Finally, supplementary information on the stress/strain behaviour of the specimens at the rates of deformation and temperatures used in the test is also relevant and helpful. The validity, as well as the practical importance, of such investigations is illustrated, if illustration were needed, by reports which crop up from time to time on the inadequacy of single-point data from standard tests. The many examples which could be cited include, for instance, three recent papers35 dealing, respectively, with the insensitivity of a standard impact test (Gardner) to the presence of impact modifier in uPVC sheet if crack-promoting defects are absent; the need to vary the impactor velocity in the standard (ASTM) Izod test-and to further supplement this by high-speed tensile tests--for a better assessment of the toughness of PVC materials; and the use of a three-point bend test on notched specimens of uPVC sheet in preference to a falling dart test, for a better evaluation of increased toughness of PVC with increased modifier loading.
-
V
V V V
Specimen type A Specimen type B Specimen type C
ASTM D 256·78, Methods A andC
V V
Notch type A b Notch type B
ISO 180 (data from current revision draft document ISOIDIS 180, July 1980) BS 2782:1970: Method 306A
Izod
Shape (crosssection)
Specimen or notch designation
Standard specification
Test type
45°
45° 45° 45°
45° 45°
Included angle
0·25
1
1
1
0.25} 1·00
Tip radius (mm)
Notch characteristics
Remarks
tv
Four specimen types in specification (type 4 preferred) J or kgf cm or lbf ft per Notch moulded-in in unit width of specispecimen A, men (in m, cm and machined in Band C: width of specimen C in., respectively) is half that of A and B J or ftlbf per unit specimen width (in m and in., respectively) ckJ m- 2
Impact resistance unitsa
TABLE 11.3 Notch Characteristics and Impact Resistance Units in Standard Flexural Impact Tests
w
ClO
DIN 53 435-1977
DIN 53453-1975
ASTM D 256-78, Method B
BS 2782:Part 3: Method 351A:1977
ISO 179 (data from current revision draft document ISOIDIS 1979, July 1980)
V
-
Small standard bar
Notch width: 2mm Notch width: 0·8mm
45°
Notch width:' 2mm orO·8mm 45° 45°
Notch width: d 2mm or 0·8mm 45° 45°
Only unnotched specimens
U U
V V
Notch type B Notch type C
Standard bar
U
V V
Notch type Bb Notch type C
Notch type A
U
Notch type A
Corner radii: :/-0·1
Jm- 2
kJm- 2
kJm- 2
kJ m- 2 or mJmm- 2
J or it Ibf per m or in. (respectively) of specimen width
*o.J
*0·1 }
Com" "d;;'} :/-0·2
0·25
Com",.iiii, 0·2 1·0
l-(lO
Com,,,.dii, 0·25
b
a
w
w
00
Three specimen sizes in specification (size 1 preferred)
Four specimen types in specification (size 2 preferred)
For unit conversions see Table 11.4. This type preferred. C i.e. energy to break in kilojoules per unit area of cross-section of the part of the specimen under the notch (total cross-sectional area in un-notched specimens). d Respectively in Specimens 1 and 3. , Respectively in Specimens 1 and 2.
Dynstat
Charpy
w.
384
V. Titow
TABLE 11.4 Conversion Factors for Common Units of Flexural Impact Resistance Other units
SI unit
kJ m- 2
kgfcm cm- 2
ft [bf in- 2
ft [bf (per inch of specimen (notch) width O)
1 0·981 2·10 5·24
1·02 1 2·14 5·36
0·476 0·467 1 2·50
0,191 0·187 0·401 1
° Conversion factors for these units applicable only to rectangular specimens
with notch-tip radius of 0·25 mm and thickness of material under the notch of 1 mm (cf. ASTM D 256), on the assumption that-in general-the energy to break is proportional to this thickness. 3
In the simplest, broad terms, the way in which a plastic fractures on impact, i.e. the mode of failure, may be classified into brittle, ductile, and intermediate (between the first two). The most characteristic differences between typical cases of these three failure modes lie in the appearance of the fracture surfaces and in the stress/strain effects as shown up by the respective load/deformation curves. Typical brittle failure produces a fracture surface which is smooth and shiny (although overall it may be fragmented in the general plane of fracture). Fracture surface typical of ductile failure exhibits ridges or strands of material standing proud in varying degrees and configurations: these formations may also show stress whitening. In the intermediate mode of failure the surface characteristics are part-way between those of the other two types: in some cases there may be a progressive transition marking the original path of the developing crack. In terms of typical, basic load/deformation behaviour, (which may be demonstrated with a suitably instrumented impact tester 29 ,31) the three kinds of failure mode are characterised by the following features. Since-by definition-brittle failure occurs (i.e. a crack initiates and propagates to the point of complete fracture) before any bulk yielding of the material can take place, the load/deformation curve is of the kind shown schematically in Fig. 1l.2(A). In typical ductile failure (Fig. 11.2(B» the material yields before (in some cases well before) breaking, the yield point being reflected by a bend or peak on the curve. The shape
11 Some Miscellaneous Components of pvc Formulations
385
A
o
B
o
c
o Fig. 11.2 Load (L)/deformation (D) curves for the three general types of failure mode. Schematic representation. A, Brittle failure; B, ductile failure; C, intermediate failure.
386
W. V. Titow
of the curve after the yield point depends on the actual mechanism of plastic deformation of the material in the particular case (e.g. necking, cold drawing, etc.). With the mode of failure intermediate between brittle and ductile there is some yielding in the region of failure before or during the propagation of a crack (cf. the curve of Fig. 11.2(C)). Although the diagrams of Fig. 11.2 are schematic, they do illustrate the typical shape of the load/deformation curves for the different modes of failure, and the fact that-other factors being equal-the energy to break (as represented by the area under the curve) increases with the mode of failure in the sequence brittle ~ intermediate ~ ductile. Among the 'external' factors influencing impact resistance, the temperature and the speed of impact (rate of deformation) have a special significance, in that changes in their values which can occur within the general range of possible service conditions can alter the mode of failure of many thermoplastics from ductile to brittle (or vice versa) and thus sharply reduce (or increase) the energy to failure. The 'internal' and processing factors most strongly affecting this transition under service conditions (or in tests) are the basic composition (e.g. presence or absence of impact modifiers in uPVC) , local stress concentrations, and structural anisotropy. Lowering the temperature of a thermoplastic which initially breaks in the ductile mode will normally eventually result in a change to brittle fracture. Where the change is sufficiently sharp, * the temperature at which it occurs is the ductile/brittle transition temperature (Tb ), sometimes also referred to as 'brittleness temperature', 'brittle temperature' or 'brittle point'. Its value in any given circumstances is influenced by the various factors affecting impact resistance (see above), and in particular by the presence and magnitude of stress concentrators (surface faults or damage in service, presence and tip radius of notches in tests): even when all the main factors are kept constant the value assigned to the Tb can be affected by the choice of criterion for identifying the change of failure mode (e.g. whether sudden drop in impact strength, appearance of fracture surface, load/deformation curve, etc.), and even by the sensitivity of assessment under the same criterion (e.g. examination of fracture surface with the naked eye, under the microscope, or by electron scan). 29 However, these considerations do not invalidate
* With some plastics materials the drop in impact resistance with temperature (and the associated evidence of transition from ductile to brittle deformation on the fracture surface) may be very gradual, so that no definite Tb can be established. 29
11 Some Miscellaneous Components of pvc Formulations
387
either the concept or the relevance of Tb in connection with the understanding and assessment of impact behaviour of plastics, providing that the limitations of its applicability are properly understood. Note: Bucknall et al. 29 use the Tb (in conjunction with notch sharpness) as the basis of a classification of plastics materials (into five categories*) relevant to their impact behaviour in service.
Where no brittle/ductile transition occurs in the particular test or service situation, the impact resistance of a plastic may remain constant over a wide temperature range. This is demonstrated by the curves for polystyrene and poly(methyl methacrylate) in Fig. 11.3: both polymers 251--~·
----------., - - - - - --
pc------- ---------.
I I
i
20~ I
PPO
....cen-
c ~ 10
ti
...
.. u
a.
~
5
~~~;;~;;;;:===::::=~:==::::::-
= _.....L OL-_ _200
-150
L-_ _...L._ _---' -100 -50 0 Tast tamparatura. 'C
PSPMMA
-:'::-_ _----:::-:---_~
50
100
150
Fig. 11.3 Impact resistance (in a Charpy-type test on notched specimens with 0·25 mm notch radius) as a function of temperature for five polymers. PC, bisphenol-A polycarbonate; PPO, poly(2,6-dimethyl paraphenylene oxide); uPVC, unplasticised polyvinyl chloride; PMMA, poly(methyl methacrylate); PS, polystyrene. (Reproduced from Ref. 34 with permission of the copyright holder, ICI, and the publisher, John Wiley and Sons Ltd).
* In order of increasing impact resistance: brittle, blunt-notch-brittle, sharpnotch-brittle, tough but crack-propagating, very tough, crack-arresting.
388
w.
V. Titow
are in the glassy state, and below their brittle temperatures (which roughly coincide with the glass-transition temperatures), throughout the entire temperature range covered, and hence break in brittle fashion, with low energy to break. Note: In contrast with the impact resistance, the tensile strength of thermoplastics increases with decreasing temperature. * The brittle point may be defined in terms of this property as the temperature at which the yield strength is just equal to the brittle strength (and below which the latter becomes higher).34
The effect of increasing the speed of impact is similar to that of lowering the temperature; it has been suggested that, with thermoplastics, a ten-fold increase in impact speed may be regarded as roughly equivalent to a lOoC drop in temperature. 36 A full discussion of the impact behaviour of thermoplastics would be outside the scope of this section (and not essential to its purpose). Further information can be found in Refs 28, 29, 34 and 37, all of which list numerous book and literature sources. A useful brief summary of the main aspects of impact resistance relevant to the design and service performance of plastics products is given in Ref. 36. (b) The Impact Resistance of pvc With PVC products the possibility of fracture by impact in service is of much greater concern in the case of unplasticised compositions than with pPVc. Whilst some plasticisers (notably tricresyl phosphate) can make the plasticised material prone to embrittlement at temperatures within a moderate range for service (even above O°C for some TTP-plasticised compositions), this can easily be avoided by suitable formulation. Thus, in general, flexible PVC compositions are tough materials, which do not normally fail by fracture on impact in service or in tests (and give typically ductile breaks on failure in tension), although plasticised sheeting-particularly thin films-may be punctured by falling weights (especially sharp-ended ones, like, for example, dart impactors used in some tests). * The tensile strength of a rigid PVC composition tested by Dyment and
Ziebland 38 increased from 7·7 x 103 1bf in- 2 at 20°C to 19·7 x 103 1bf in- 2 at
-196°C (with a corresponding increase, by a factor of 2·13, in Young's modulus).
11 Some Miscellaneous Components of pvc Formulations 30
389 60
N
~~ £20
20·C 40
:::. .:L
111
N
CI
'E ....,
'-
.0
.:L
E ~10
20
'CI C
W
o
0·02 (05)
0·04 Notch radius, in (1·0) (mm, approx)
0·06
008
(1-5)
(2·0)
Fig. 11.4 Impact resistance of a uPVC composition as a function of notch-tip radius in a Charpy-type test at various temperatures.
Note: Embrittlement in consequence of changes in the material
through loss of plasticiser, heat degradation, weathering, or ageing is not directly relevant in the present context. Rigid PVC is not intrinsically a brittle material at ordinary temperatures: the brittle point of properly made un-notched uPVC specimens in a flexural impact test can be as low as - 50°C. 34 However, the material is sensitive to the presence of stressconcentrating features (as evidenced by its notch-sensitivity in impact tests-see Fig. 11.4), and also to the rate of deformation, in the sense that these factors-when operative at levels encountered in service conditions and in tests-ean, singly or jointly, bring about a ductile/brittle transition and thereby cause brittle failure to occur in impact situations even at room temperature. A plastics material susceptible in these two ways is sometimes referred to as 'notch-brittle' and 'shock-brittle'. This situation is illustrated by the curve for uPVC in Fig. 11.3: the rapid drop in the impact strength of notched specimens within a
390
W. V. Titow
relatively narrow range of common service temperatures, is attributable to the onset of brittle fracture brought about by the relatively severe 0·25 mm notches at the speed of impact employed in the test. As has been mentioned, the effects of both these factors are reasonably representative of those of stress-concentrating faults and impact speeds which may occur in service. Note: Notches, or 'natural' faults, in a polymeric material can raise the Tb and thus promote brittle failure not only by causing stress concentration, but also by limiting the amount of plastic deformation which can take place locally before fracture occurs, through a modification of the stress field at the incipient crack tip (transition from plane-stress to plane-strain conditions29 ). Contributory factors which can arise in practice include incomplete fusion (gelation) of the PVC composition (in consequence of inadequate heat processing), or degradation of the polymer (excessive heat processing).
Incorporation of impact modifiers in PVC compositions is a practical way of increasing their impact resistance. The room-temperature impact strength (notched Izod, ASTM D 256) of an unmodified PVC based on a relatively low molecular weight polymer may be as low as 0·4 ft lbf in -], and perhaps about twice that with a polymer of high molecular weight. Inclusion of, say, 15 phr of a suitable impact modifier can raise these values to about 20 ft lbf in-lor even higher in some cases, with considerable improvement also in low-temperature impact resistance. In terms of the PVC graph in Fig. 11.3 such upgrading amounts to a displacement of the curve upwards and to the left.
(c) The Nature, Effects and Applications of Polymeric Impact
Modifiers for pvc The following types of polymer are in industrial use as impact modifiers in PVC:
(i) acrylonitrile/butadiene/styrene (ABS) polymers; * (ii) methacrylate/butadiene/styrene (MBS) polymers; (iii) ethylene/vinyl acetate (EVA) copolymers and EV ANC graft copolymers; * Normally of relatively high butadiene content (and hence comparatively low modulus-see Section 11.2.1).
11
Some Miscellaneous Components of pvc Formulations
391
(iv) acrylic polymers (all-acrylic modifiers); (v) chlorinated polyethylene (CPE); (vi) acrylonitrile/butadiene copolymers (nitrile rubbers); (viii) polyurethane (PUR) elastomers. Other polymers, generally of an elastomeric nature, have been patented, examined, or used as impact modifiers for PVC, including butadiene/2-vinyl pyridine, butadiene/methyl isopropenyl ketone,39 butadiene/fumaric ester copolymers,39,4o and Hytrel 3495 (a copolyester thermoplastic elastomer produced by Du Pont).41 The normal levels of addition for straightforward impact modification of uPVC range between 5 and 20 phr (commonly 10-15 phr) but some modifier polymers are sometimes used in very high proportions to impart special properties to PVC compositions (see below). Some fine-particle fillers (especially calcium carbonate-see Chapter 8, Sections 8.3.4 and 8.4.1) and pigments (e.g. titanium dioxide-see Section 11.3 below) can, when properly dispersed, also improve the impact resistance of uPVC. At the normal levels of addition polymeric impact modifiers function by forming a disperse phase in the matrix of the PVC polymer (containing also those constituents of the composition which are fully soluble in it, e.g. processing aids, internal lubricants, some stabilisers): this interferes with crack development in the matrix, and can act as internal energy absorber (the more effectively, the more rubbery the nature of the modifier), in ways analogous to those in which elastomeric modifiers function in toughened polystyrene. 25 Useful early summaries of the mode of action of impact modifiers in PVC have been published by Bramfitt and Heaps39 and Sisson. 42 The impact-modifying particulate additives mentioned above have a broadly similar action. Where, as with ABS and many other modifiers, the refractive index of the continuous phase (PVC polymer matrix) is appreciably different from that of the modifier, the latter has an opacifying effect on the composition: hence in uPVC compositions for clear products (e.g. blow-moulded bottles, packaging films) the refractive indices should match as closely as possible at the appropriate service temperature (the respective refractive index values, and hence the closeness of match, can vary with temperature 21 ). Selected MBS modifiers are widely used in clear compositions. In cases where the refractive indices of modifier and matrix are close but not completely matched (and where all other formulation components are chosen for
392
W. V. Titow
maximum clarity), if the refractive index of the disperse phase (which may also include such components as, for example, some external lubricants and stabilisers) is lower than that of the matrix the composition will be yellowish and hazy in a degree proportional to the difference in refractive indices: if the disperse phase has the higher refractive index, blueing ('colour reversal') will result. Some polymeric impact modifiers are highly compatible with the PVC polymer, and are sometimes incorporated in very high proportions. Of these, those which interact strongly with (have a close chemical affinity for) the PVC polymer act as true plasticisers of very high permanence (e.g. EVNVC graft polymers rich in the VC component; highly chlorinated polyethylene). Others, like, for example, nitrile rubber and some ABS polymers, also modify considerably the properties of the blends they form with PVC at the high levels of loading. Side effects on the properties of uPVC materials exerted-in varying degrees-by most polymeric impact modifiers at the normal levels of addition, include the following: reduction of hardness, modulus, strength (tensile and flexural), deflection temperature under load, and chemical resistance; increased permeability; and impaired weathering resistance (except with some acrylic and EVA/VC impact modifiers which, whilst not positively improving the weathering properties, do not affect them adversely in uPVC compositions formulated for good weathering resistance). Some impact modifiers promote stresswhitening in uPVc. This phenomenon is usually attributed to the formation of microscopic voids or crazes through local separation of the disperse phase (including fillers-cf. Chapter 8, Section 8.3.3) from the polymer matrix under strain; it is particularly undesirable in clear compositions: modifiers for such compositions are selected, inter alia, for minimum effect in this respect-several suitable MBS modifiers are available. Note: As measured in tests on transparent compositions, stress whitening may be defined on the basis of the relationship
SW = 100 [1 - (Is/I)] where: SW is the stress whitening (%), Is is the % light transmission after stressing, and I is the % light transmission before stressing. In processing, many impact modifiers act as processing aids,
11
Some Miscellaneous Components of pvc Formulations
393
albeit-as has been mentioned in Section 11.2.1-this action is often at its maximum at temperatures somewhat higher than those at which regular processing aids are effective. In most cases the presence of an impact modifier also increases die-swell in extrusion (except for some acrylic modifiers, which have relatively little effect). Polymers used as impact modifiers in rigid PVC may also be incorporated in flexible compositions, where they can improve the melt properties in processing as well as modify the material properties at service temperatures, in ways which include reduction of brittle temperature, increased stiffness, and improved surface properties and emboss retention. The effects of different impact modifiers on some properties of a PVC composition are compared in Table 11.5. The following further points may be mentioned about the individual classes of polymeric impact modifiers. ABS MODIFIERS These modifiers are widely used in opaque, rigid compositions for products not required to withstand weathering (e.g. pipes and pipe fittings): in such compositions they can give high room-temperature impact resistance and improved low-temperature resistance, with relatively little effect on heat-distortion temperature and softening point. Commercial compounds containing ABS modifiers in substantial proportions are sometimes referred to as 'PVC!ABS alloys'. Note: Typically, such commercial compounds may combine roomtemperature impact resistance (notched Izod) of about 10 ft lbf in-I with v-o rating in the UL94 flammability test, and tensile strengths of about 5000lbfin- 2 .
Extrusion compounds of this kind are used, for example, for profiles, and sheets for thermoforming (with particular suitability for deep draw forming) into such products as panels, casings (e.g. computer housings), and battery covers; injection moulding compounds are used, inter alia, for electronic equipment covers and housings, television cabinets and the like. Good surface finish and low SG are among the useful features of these materials. In semi-rigid sheeting ABS modifiers can also substantially improve thermoformability and reduce post-forming shrinkage. Very high proportions of suitable ABS polymers can be incorporated in flexible PVC compositions (e.g. 65/35 ABS/PVC in some cases) if
10 15 10 15 10 15 10 15 10 15
Amount (phr)
5·8 6·2 5·4 6·1 4·5 4·8 4·8 4·9 3·6 3·4
13-8 32·5 22·0 37·0 10·1 15·2 6·4 8·2 5·1 7·2
at -36°C at 23°C
Notched impact resistanceb (kgf cm cm- 2 )
Q
PVC polymer (S grade, K value 65) 100 Modifier: as listed 10 or 15 phr Stabilisers: Ba/Cd 3'Ophr chelator 0·5 phr 3·0phr epoxidised soyabean oil 1·3 phr Lubricants b Charpy type: DIN 53453. cOIN 53460. d Determined for a composition based on 20: 80 CPE:PVC blend.
CaC03 filler
Chlorinated polyethylene EVA/VC
ABS (Blendex 31Borg-Warner Chemicals) MBS (Blendex 436)
Modifier
43 44 37 d
42 41
30-35
20-25 d 30-35 30-35
Brabender rheometer, 30 rpm
30-35
oven
Heat stability at 2000C (min)
Q
CC)
88 91 83 82 83 82 82 85 82 81
1 kg load
C
72 73 70 70 70 68 70 69 69 68
5 kg load
Vicat softening point
TABLE 11.5 Effects of Different Impact Modifiers on some Properties of a Rigid PVC Composition (Summary of selected data from Ref. 26)
~
<:;
:::;j
:<:::
~
'R
...,
11
Some Miscellaneous Components of pvc Formulations
395
the plasticisers are chosen for compatability with the ABS component. In such compositions it is the PVC which may be regarded as the modifier for ABS, improving the latter's tear and abrasion resistance, hardness and tensile strength, and reducing flammability.21 MBS MODIFIERS Whilst modifiers of this class can be used in opaque formulations, they are of particular interest for clear compositions (e.g. bottle, film and sheeting compounds), as many have refractive indices in the right range to promote good clarity: with several MBS modifiers this may be combined with good colour and surface gloss as well as resistance to stress whitening, good heat stability and low degree of odour and taste transfer in containers. Increases in the room-temperature impact strength of rigid compositions obtainable with some MBS modifiers can be as high as 25-fold (e. g. from about 1 ft lbf in -1 to about 25 ft lbf in- 1 with Blendex 436 (Borg-Warner Chemicals) in a notched Izod-type test). EVA MODIFIERS This group comprises EVA copolymers, and graft copolymers of vinyl chloride with EVA. In general, the impact-modifying effect of EVA copolymers in PVC increases with increasing vinyl acetate content. Room-temperature impact strengths of about 20 ft lbf in-1 (notched Izod, ASTM D 256) can be attained in some compositions at 10-15 phr loadings. The vinyl chloride component in EVAlVC polymers promotes compatibility with PVC. Some graft copolymers of this kind (e.g. Du Pont's Elvaloy resins 741 and 742) can be used as solid plasticisers of exceptionally high permanence, in amounts as high as 80 phr. 13 ,43 Like some all-acrylic modifiers (e.g. Aeryloid KM 323B-Rohm and Haas) EVA modifiers (e.g. Levapren 245O-Bayer; Hostalit HHoechst; Elvaloy 836 and 837-Du Pont) do not appreciably impair the retention of physical properties (and, in many cases, colour) on weathering of uPVC compositions which contain them: they are therefore applicable in such products as window frames, cladding and fencing profiles for external use, and rainwater goods. Note: As has been mentioned in Chapter 1, a suitable EVAIVC graft copolymer can be used on its own (instead of a PVC resin/modifier combination) in uPVC products of this kind.
396
W. V. Titow
Similar uses are being developed for graft copolymers of vinyl chloride on ethylene/propylene (E/P) copolymers and ethylene/propylene/diene monomer (EPDM) terpolymers;44 the graft polymers form the basis of commercial PVC compounds (Rueodur-Ruco Division of Hooker Chemical and Plastics Corp., USA)45 for injection moulding, extrusion and blow moulding which combine impact resistance in the range 15-20 ft lbf in -1 with a UL94 flammability rating of V-O. ACRYLIC MODIFIERS
This group provides impact modifiers which, in addition to their principal function in uPVC compositions, offer some processing-aid effects, low die-swell in extrusion, and in service little impairment in weatherability of products in which this is of primary importance (see preceding paragraph). The acrylic modifiers are well represented by the relevant products in the Aeryloid (Paraloid*) range of Rohm and Haas in the USA and associate companies elsewhere. CHLORINATED POLYETHYLENE
The chlorinated polyethylenes used as polymeric additives in PVC are based on high density polyethylene. They are produced by chlorination of this polymer, which may be carried out in suspension, solution, or even in the solid phase. 46 The solution process gives the greatest uniformity of distribution of the CI atoms in the polymer chains. 47 The distribution is a contributory factor in the compatibility of CPE with PVC polymer, which is mainly governed by the chlorine content. Other factors influencing the compatibility and effects of CPE in a given PVC composition are the molecular weight and degree of crystallinity of the modifier. 48 Impact-modifier grades have chlorine contents in the range 25-40%: they are used at incorporation levels normal for polymeric modifiers in uPVC (up to about 20 phr, with maximum impact resistance often reached at about 15 phr). Roomtemperature impact resistance (notched Izod ASTM D 256) of 20 ft lbf in -1 can be attained (even higher in some cases), with considerably improved low-temperature toughness: the modifier also has some processing-aid action, and lubricant effects have been reported. 49
* The trade name Paraloid is used by Charles Lenning Chemicals and affiliates in the UK and certain other countries: the product coding is universal.
11
Some Miscellaneous Components of pvc Formulations
397
CPE grades of higher chlorine contents are fully compatible in all proportions with PVC polymer, due to the close chemical similarity. They do not increase the flammability of PVC compositions, as do polymeric additives without chlorine (or with relatively low CI contents). The highly chlorinated CPEs can be used at very high incorporation levels in PVC compositions to function as plasticisers of excellent permanence. In both capacities CPE is an additive which does not substantially impair property retention on weathering of PVC compounds properly formulated for weathering resistance. An important factor in this desirable feature is the absence of double bonds (present in the molecular chains of rubbery modifiers) which constitute vulnerable sites in weathering and ageing situations (ct. also acrylic and EVA-type modifiers).
Note: In an early evaluation of a CPE impact modifier (Modifier PIM lOl-Allied Chemicals Corp.-a non-crystalline CPE with 40% CI content) O'Toole and co-workers found,49 inter alia, a 100% impact-resistance retention, with little deterioration of other properties, after natural weathering of a PVC sheet containing this impact modifier. However, incorporation of CPE modifiers does, in general, lower the strength properties of PVC in comparison with unmodified material. In such products as, for example, rigid profiles for outdoor use (and especially in cellular versions) a CPE modifier can promote filler acceptance. 13 The use of chlorinated polyethylene as plasticiser for flexible sheeting for horticultural applications or reservoir lining can be beneficial (but is restricted by cost considerations). Chlorine-rich non-crystalline CPE grades are compatible with most plasticisers, and can be incorporated-in moderate proportions-to increase the modulus and reduce brittle temperature of pPVc. Crystalline grades are said to be beneficial in plasticised flooring compositions. 50 NITRILE RUBBERS
Nitrile rubbers of suitable acrylonitrile content can be blended in virtually any proportion with PVC homopolymers. Depending on the proportion, on whether the composition has been vulcanised, and also to some extent on the application, the material based on such a blend may be regarded as rubber-modified PVC or PVC-modified rubber. In a blend the two components modify each other's properties, and the
398
W. V. Titow
properties of the blend (and any composition in which the blend is the base polymer) will reflect this mutual effect. The ratio of the components is the most important single factor governing the extent of modification in a given case, but other factors also play a significant part, e.g. the acrylonitrile content and Mooney viscosity of the rubber, and the molecular weight of the PVC polymer,5! as well as the nature and content of any other constituents of the composition (e.g. plasticisers, fillers). As a broad generalisation it may be said that in the blend the PVC polymer contributes ozone, oil and fuel resistance, strength (tensile and tear), and stiffness, as well as weatherability (with proper stabilisation), abrasion resistance, flame resistance (which may be enhanced by the presence of phosphate plasticisers and/or chlorinated plasticiser-extenders in the composition) and higher electrical resistivity. Thus these properties of a nitrile rubber will be upgraded by modification with PVC. Note: The increase in strength (and especially tear strength) can be maximised in vulcanised PVC-modified nitrile rubber compounds if the vulcanisation system is based on the total polymer blend (not the rubber component alone).
The properties of PVC mainly upgraded by incorporation of nitrile rubber are toughness (impact resistance), flex-crack resistance, lowtemperature flexibility, and resilience. Some of these effects are illustrated by the curves of Fig. 11.5. Nitrile rubber of grades used in blends with PVC is often regarded as a plasticiser (highly permanent, non-extractable and non-migratory because of its polymeric nature). In the practical context this is consistent with its complete miscibility with PVC polymer, and its principal effects on the properties of PVC compositions. The question whether nitrile rubber indeed acts as a true plasticiser is therefore more of theoretical than practical interest. It may be noted, however, that-as observed by several investigators52-intimate blending (by melt-compounding) of this modifier with PVC polymer in 'plasticising' amounts does not result in a shift of the latter's Tg to a lower temperature (as in the classic case of plasticisation) but gives rise to a single, broad glass-transition region spanning the whole range between the individual Tgs of the two components of the blend. Moreover, the fine structure of the most intimate blends prepared by meltcompounding has been reported to be heterogeneous, with rubbery domains of sub-micron size revealed by electron microscopy.39,52 With
11
Some Miscellaneous Components of pvc Formulations
399
40
(A)
o
(B)
10 20 30 40 50 60 70
°'0 PVC
o
10
20
30
0'. PVC
40
50
Fig. 11.5 Some effects of PVC content in blends of PVC homopolymer with nitrile rubber. (A) Tensile strength (TS) and modulus (M) of nitrile gum stock. (B) Oil resistance of medium-acrylonitrile rubber.
regard to the toughening effect of nitrile rubber in PVC, the point has been made39 that, whilst a rubbery additive capable of being dispersed in PVC polymer on a molecular level could increase the impact resistance by reducing the yield strength (promoting ductile failure) in the same way as a plasticiser, a two-phase system is more effective in producing a high degree of toughening. In the production of blends of PVC polymer with nitrile rubber, melt-compounding is necessary for maximum uniformity and completeness of inter-dispersion. However, availability of the rubber in powder form enables it to be included in PVC compounding processes at the pre-mix (dry blend) stage. Ready-made blends (in slab, chip, or powder form) are also available from commercial sources: e.g. the Breon 'Polyblend' 500 series (BP Chemical International Ltd-blends of butadiene/acrylonitrile copolymers of medium-to-high acrylonitrile content with PVC homopolymer). The blends may be compounded with additional nitrile rubber or PVC polymer (to adjust the ratio of the two components to a value required for a particular purpose), and/or with other materials appropriate to the intended application
400
w.
V. Titow
(PVC stabilisers, plasticisers, lubricants; vulcanising curatives and reinforcing filler for the rubber; general fillers for the composition). The main advantages claimed for the use of the commercial blends vis-a.-vis direct blending of commercially available nitrile rubber and PVC polymers, are better processing properties and reduced need for high-temperature mixing. Some typical properties of Polyblend 503* (sheet) are given in Table 11.6. TABLE 11.6 Physical Properties of 'Breon Polyblend 503' Tensile strength (lbf in -z) Elongation (%) 100% modulus (lbf in- Z) Hardness (Shore Durometer A) Specific gravity Crescent tear (lb in-I, ASTM D 624-54) Rectilinear tear (lb in-I) Low temperature brittleness (OC, ASTM D 746-64T) DC volume resistivity (Q cm)
1850 430 1100 93 1·18 320 328 -51 2·6 x 109
Typical formulation for black press polished sheetings Parts Breon Polyblend 503 100·0 Black (added as MB) 1·5 Acrawax C (Glyco Chemicals Inc.) 0·5-1·0 Lead stearate 0·2-0·3
Rubber-type processing of nitrile rubber/PVC blends and their compounds can be carried out on ordinary rubber equipment, at temperatures slightly higher than those typical for unmodified nitrile rubber compounds. Compounds based on PVC-rich blends which are not to be vulcanised are processed in ways normal for PVC compositions-by extrusion, calendering, moulding, etc. The use of nitrile rubber/PVC blends was pioneered by the cable industry, where vulcanisation of the rubber component of cable-sheath compositions based on the blends was an early development. Wire and cable coverings, which continue to provide a considerable outlet for * Described as a colloidal blend of medium acrylonitrile butadiene rubber and a PVC homopolymer in the ratio of approximately 48 pbw rubber to 52pbwPVC.
11
Some Miscellaneous Components of pvc Formulations
401
the blends, are nowadays the subject of several standard specifications (see Section 9 of Appendix 1). Other applications of nitrile rubber/PVC blend compositions include integral covers for fuel hoses, conveyor belting, and rollers, shoe soles, and flexible containers. POLYURETHANE ELASTOMERS
Like nitrile rubbers, appropriate grades of these elastomers are widely compatible with PVC, but the position with regard to their use in combination with PVC differs somewhat in two respects from that in the case of PVC/nitrile rubber blends. Thus, whereas the presence of PVC polymer in nitrile rubber can substantially upgrade the latter's resistance to ozone, oil, fuels and abrasion, the corresponding resistance of some PUR elastomers is inherently good, so that modification with PVC may not make much difference (although it might cheapen some types of composition). Conversely, as an impact modifier for uPVC, a PUR elastomer-even when potentially very effective-has to compete on price with cheaper alternative materials. Nevertheless, PUR modifiers for PVC are on the market (e.g. those in the Landex range of the Story Chemical Corp., USA, Ultramoll PU of Bayer and Durelast 100 of Briggs and Townsend, UK) and commercial PUR/PVC blends have been used for the production of shoe soles and moulded industrial boots (e.g. Ekalit M and Kombipur-VEB Chemiekombinat, Bitterfeld, E. Germany): see also Chapter 7, Table 7.4.
11.3 COLOURANTS 11.3.1 General Nature and Functioning
Colourants may be broadly divided into pigments and dyes. Both terms can have somewhat differing connotations in different industries, but for the purpose of this section-and without attempting a comprehensive description-the following working definitions are valid. Pigments are colouring materials which are insoluble (i.e. not dispersible on a molecular level) in the base polymer of a plastics composition, and therefore remain in particulate form when incorporated in the composition by proper compounding procedures: dyes are colouring materials which are soluble in the above sense.
402
W. V. Titow
Note: The primary particles of a typical organic pigment may range in size between about 0·5 and 5 ,urn, and may be made up-or consist-of pigment crystals of sizes 0·005-1,um. The
primary particles may form aggregates up to several tens, and agglomerates up to several thousands, of micrometres in size: these should be broken up into their constituent primary particles in the compounding operation. The colour of a coloured substance is due to selective absorption by its molecules of some wavelengths from the incident light; the actual colour seen by the eye is determined by the wavelengths remaining in that part of the light which is transmitted or reflected. To be useful as a colourant, a substance must be of a colour sufficiently intense to impart it in the desired degree to the material to be coloured when incorporated in relatively low proportions (in PVC compositions the colourant contents normally range from fractions of a phr to a few phr except for the special case of comparatively much higher loadings of titanium dioxide or carbon black in certain compounds-see below): it must also be compatible with the material and suitable in several other respects (see below). Where a colourant has an opacifying effect in a basically transparent plastics composition, this is due to the scattering of light at the colourant/plastic interface within the composition. It follows that the effect will arise only if the colourant is insoluble in the composition, and hence present in particulate from (i.e. acts as a pigment, not a dye), and if-in addition-the particles are: (i) large enough in relation to the visible light wavelengths (0·750-0·400,um, respectively, for the extremes of the red and violet spectrum bands) to permit scattering, as well as (ii) either opaque or sufficiently different in refractive index from the composition to enable significant internal scattering to occur. It is because they do not entirely meet condition (i) that some pigments of sufficiently small particle size can be used in translucentto-transparent compositions.
Note: Similar considerations apply to other formulation components
(fillers, flame retardants, impact modifiers): MBS modifiers provide an illustration of the case where condition (ii) is not fulfilled, i.e. where an additive forming an essentially discrete disperse phase of effective particle size large enough for
11
Some Miscellaneous Components of pvc Formulations
403
interference with light, has no opacifying effect because its refractive index matches that of the composition (cf. Section
1l.2.2(c)). 11.3.2 General Classification
As has been indicated in Chapter 4 (Section 4.4.1(g)) the following general classification of colourants used in plastics may be made on the basis of their nature and chemical composition: Dyes (organic compounds) Pigments Organic pigments: substantially insoluble organic compounds; also carbon blacks which consist essentially of elemental carbon, occasionally tinted with an organic colourant. Toners: Ba, Ca, or Mn salts of azo dyes. Lakes: complex salts of basic dyes with certain acids, especially phosphomolybdic, phosphotungstic, and phosphotungstomolybdic acids. Inorganic pigments: predominantly salts or oxides of certain metals; this group includes the important white pigment, titanium dioxide.
Several chemical types and combinations are represented among the special effect pigments, which include: (i) metallic colourants (metal flakes, usually aluminium for silver effects and copper or bronze for gold; may be tinted with organic colourants for metallic colour effects); (ii) pearlescent colourants (often based on bismuth and lead compounds; some on titanium/mica combinations, e.g. some Mearlin Luster pigments of the Mead Corporation, USA, for which superior weatherability is claimed); (iii) glossy and nacreous colour pigments (as represented, for example, by the [riodin range of E. Merck, West Germany); and (iv) fluorescent pigments, whose recent commercial representatives have much improved heat resistance (up to about 300°C in some cases) and light stability (cf., for example some Swada Z/N and Day-Glo VC fluorescent colourants of, respectively, Swada (London) Ltd, in the UK, and the Day-Glo Colour Corp. in the USA). Most of the common dye classes (including azo, anthraquinonoid, basic, nigrosine, indoline, quinophthalone, and aniline-black dyes) are represented among dye colourants for plastics. Virtually all these colourants have some solubility in common solvents, oils, fats and
404
W. V. Titow
plasticisers. For this reason they are relatively little used in flexible PVC compositions in which their solubility can give rise to troubles associated with migration ('bleeding', 'marking-off', 'blooming'). As organic compounds with high degree of unsaturation the dyes (and many of the closely related organic pigments) are fairly sensitive to light (prone to fading or changes of shade). Their molecular state of dispersion in PVC compositions also makes them more accessible to light, and thus accelerates any photochemical effects they may suffer in both pPVC and uPVc. Examples of commercial dye colourants for PVC include Red HHR and Blue B (Hoechst) used in coloured gramophone record compounds; Amaplast Yellow RRT (American Colour and Chemical Corp.)-an azo yellow for use in rigid compos~tions; and quinaphthalone yellows for uPVC (Mitsubishi Chemical Industries). Rhodamine B has been evaluated53 as a fluorescent colourant for PVC traffic cones, vacuum-formed emblems, point-of-sale displays, and the like. Commercial pigment colourants for PVC are discussed in Section 11.3.5; the main groups are listed, with some property data, in Table 11.7. In addition, the following general points may be noted. As with other additives, the highest possible degree and uniformity of dispersion* of a pigment in the PVC composition is very important, in this case for maximum colour value and uniformity of coloration: hence the form in which the colourant is used (see Section 11.3.3) should be considered, inter alia, from the point of view of dispersibility. Stability of PVC compositions may be reduced by some pigments containing iron and zinc (especially salt-type pigments in which ions of these metals are present, or pigments containing such ions as impurities). Pigments containing barium, cadmium, lead, tin, or calcium may have the opposite effect. Some pigments (notably carbon black and titanium white-see Section 11.3.5) can improve weathering resistance (d. also Chapter 12, Section 12.6, and Chapter 4, Section 4.4.1(g». Occasionally an interaction may be possible between certain dyes and fillers: this effect is not very common, but in case of doubt reliable advice can be had from the colourant supplier on this point. Any colourant used should also be resistant to HCI which may be liberated in the composition in processing or service: for example some
* A standard specification of interest in this connection is ASTM D 3015-72 (reapproved 1978). Microscopical examination of pigment dispersion in plastic compounds.
11
Some Miscellaneous Components of pvc Formulations
405
iron oxides are susceptible to attack by Hel. Stability of the colourant to the other formulation components, especially at processing temperatures, is another relevant consideration. Some organic pigments, as well as the dye components of toner or lake pigments can migrate, especially in plasticised compositions. For example, the older monoazo pigments often caused trouble by migrating to, and accumulating on, the surface of products, sometimes shortly after manufacture or early in service: such effects are known as 'blooming', 'bleeding', 'chalking' or 'bronzing'. They occur usually where the pigment is not sufficiently insolubilised, and can dissolve in the polymer/plasticiser system. In such cases, by and large, the higher the plasticiser content the worse the effect. Some monoazo pigments are still notorious in this connection, but there are newer ones which are sufficiently insolubilised to be fully suitable. Where the tendency exists, high processing temperatures will accentuate 'blooming' and the other effects; with rigid and semi-rigid products the problem would be less acute because of the absence of, or the low, plasticiser content. It may sometimes appear that where 'bleeding' occurs, transparent compounds do not bleed as much as opaque ones. However, this is not a basic difference and may be due simply to the fact that the transparent compounds will normally contain less colour. 11.3.3 Forms in which Colourants are Available A colourant is usually available in different physical forms. In any given case the choice of a particular form-like that of the kind of colourant to be used-will be made in the light of the various considerations summarised in Section 11.3.4. The following forms are available: (i) 'Dry colours': These are colourant powders with no additives, except for small amounts of lubricants, wetting agents, or dry-flow promoters (anti-caking agents) which may be present in some cases. Nowadays most dry colours are available in special non-dusting powder grades or in granulate form. (ii) Colour concentrates: A colour concentrate is a compound consisting of a high proportion of colourant dispersed in a carrier. Depending on the nature of the carrier the concentrate will be either solid or liquid.
406
W. V. Titow
Solid concentrates in which the carrier is the same polymer as that of the composition for which the colourant is intended, are known as masterbatches. The term is also often applied to solid concentrates whose carriers are polymers compatible, but not identical, with those of the compositions to be coloured. Such 'universal' polymeric carriers in some colour concentrates suitable for use in PVC can be vinyl chloride/acetate copolymers (as, for example, in the Hoechst Hostavinyl pigment concentrate series) or EVA copolymers. A widely compatible carrier is a feature of the Siscoversal concentrates (BASF) which are suitable for use in PVC, inter alia, at up to 3 phr loading. Non-polymeric carriers are also used in some concentrates (e.g. a fatty acid or a lubricant wax, respectively in the Ciba-Geigy Microlith and the Hoechst Remafin ranges). The solid masterbatch concentrates are available in the form of standard pellets containing up to about 50% colourant. Powder forms are also supplied-e.g. the Microspin dustless concentrate powders of the Hilton Davis Chemical Co. in which the colour (or titanium white) loading can be as high as 70% (in a low-melting, heat-stable 'universal' carrier). In comparison with a dry powder colourant the masterbatch offers ease and economy of handling, freedom from dusting, and ease of dispersion in processing (powder-form concentrates are, moreover, suitable for dry blending). However, whilst the colourant in a masterbatch pellet or particle is already 'wetted out' by, and intimately dispersed in, the resin carrier, because of the high colourant content the viscosity of the molten masterbatch in melt compounding is liable to be higher than that of the composition to be coloured: thus, although problems in dispersing the colourant as such are reduced, attention must be paid to ensuring that the masterbatch compound is thoroughly blended with the composition. Since compounding a masterbatch with a composition to be coloured amounts to a dilution of the former, it is common to refer to the operation as 'letting down' the masterbatch: let-down ratios may range from 100: 1 to 3: 1 (uncoloured composition or polymer to masterbatch). Liquid concentrates of interest for PVC are normally dispersions of colourants in plasticisers, often referred to as paste concentrates. (iii) Combinations of colourants with other formulation components (stabiliser/lubricant systems and occasionally also other additives, e.g. antistatic agents, fillers) in single-pack systems; 'tailored' by the supplier for particular, specified types of PVC compositions (see also Chapters 9 and 10). The main advantage
11
Some Miscellaneous Components of pvc Formulations
407
of such polyfunctional systems to the user is convenience (with elimination of individual-component storage and metering or weighing-out operations, and some pre-dispersion of colourants, fillers, etc., where present). The principal limitation is on formulation flexibility; the price of proprietary single-pack systems also reflects the supplier's blending, handling and packaging costs.
Note: Broadly speaking, similar considerations apply to the use of coloured compounds available from suppliers for direct conversion into products by appropriate processes.
11.3.4 Choice of Colourant-Main Considerations The principal considerations influencing the choice of colourant or colourant system for a PVC composition (as for most plastics compounds in general) may be grouped under the following headings.
(a) General Appearance and Colour Requirements The relevant elements here will be the optical properties of the composition (i.e. whether transparent, translucent, or opaque: ultrafine particle pigment grades may be suitable for all three categories), the actual colour required, colour strength and brightness, and any special effects (e.g. fluorescence, metallic or pearlescent effects). Where various levels of colour strength may be required but preservation of the same shade is important, the possibility of spurious shade changes associated solely with differences in colourant concentration level should not be overlooked: such changes-which can occur in PVC compositions coloured with commercial pigments-have been attributed to dichroic effects. 54 Sophisticated microprocessor-based or computerised colorimeters and colour analysers are available nowadays to assist the plastics processor with colourant quality control, rapid colour matching* and adjustments in formulation development, and *Standard colours for matching are listed in the Dictionary of Colour Standards originally contained in BS 543:1934, which no longer exists as a standard, Other BS standards of interest are BS 381C-1964 (Colours for specific purposes) and BS 4800:1972 (Paint colours for building purposes). Colours for cables are given in BS 6746C:1969: in formulating coloured cable compounds the possible effect of the colourant on the volume resistivity and power factor should be considered; colourants specially produced for this application are available.
408
W. V. Titow
the monitoring and control of colour quality and uniformity, and shade consistency in production. The following equipment may be mentioned by way of non-selective examples: the Gardner XL-80S colorimeter system;55 the Vibrochrom FFR 2 colorimeter with companion computer (Chemiefaser Lenzing AG, Austria);56 the 7842 Color Analyzer II (IBM Instrument Systems, USA); Match-Mate 3000 (Diano Corp., USA); and ACS 500 (Applied Color System, USA).57
(b) Processability and Stability in Processing Considerations here will include ease of handling, conveying and metering of the colourant; suitability for, and ease of dispersion in, the relevant process(es) and type of PVC composition; own stability under processing conditions (especially chemical and colour stability to heat at PVC processing temperatures-general range approximately 170220°C) and absence of adverse interactions with composition constituents (or their decomposition products-e.g. HCI from PVC polymer) under these conditions. An experimental scheme for the evaluation of the suitability of pigments for use in polymers processed at high temperatures proposed by Sonn58 is of interest in connection with PVC (although the author does not include it among his 'high temperature polymers'). (c) Stability and Permanence in Service The points relevant under this heading will include permanence of colour (no fading or shade changes) associated with resistance to such environmental influences as heat, photochemical action, and other weathering or ageing factors: resistance to migration and extraction (e.g. on contact with such agents as detergents, polishes, oils); no adverse interactions of the colourant (including any impurities therein) or its decomposition products with the PVC polymer or other constituents of the composition. Examples of pigments developed with a view to compatibility with appropriate stabilising systems, and own good weathering resistance in dark shades in such outdoor uPVC products as window frames, cladding and profiles, are the BASF Sicotan yellows and some pigments in the Ciba-Geigy Cromophtal range. The former are Ni/Ti and CrlTi compounds which may be combined with appropriate organic or inorganic pigments to produce dark shades. The Ciba-Geigy materials (as represented, for example, by the widely used Cromophtal Brown SR) are mainly organic pigments with low IR absorption which limits heat build-up on
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Some Miscellaneous Components of pvc Formulations
409
exposure to sunlight. Iron-free pigments recommended for outdoor applications in PVC are available from several suppliers.
(d) Health and Safety Considerations These include: (i) general safety in handling and processing (e.g. possible explosion hazards associated with fine powders); (ii) possible toxic or carcinogenic effects in handling and processing; and (iii) similar effects on direct body contact or food contact with the PVC product containing the colourant. With most colourants used on any scale in PVC (and especially when in the form of concentrates or as components of single-pack systems) (i) is not normally a cause for special concern. In the area of exposure hazards to processing operatives, and possible hazards associated with use of end products (especially in beverage and food contact applications), attention has been focusing on three topics: inorganic pigments containing heavy metals which may act as potential cumulative poisons; polychlorinated biphenyls, present as impurities in organic colourants of the phthalocyanine and diarylide type, and regarded as potentially carcinogenic; and organic colourants based on benzidine, now widely recognised as a carcinogen. Of the heavy metals present in colourants, lead is already subject to stringent exposure limits in the USA and other countries: the regulations thus affect lead chromate and lead molybdate pigments (see Table 11.7) as well as lead-based stabilisers and lubricants. The lead chromates are additionally suspect as a result of recent work in the USA on the potential hazards of chromium (especially hexavalent chromium).59,6o The question of the degree of hazard associated with cadmium compounds affects both cadmium pigments and stabilisers. Whilst the issue of appropriate official restrictions is still under debate, any limitations (or outright bans as currently proposed in Sweden 61 ) will adversely reflect on the usage of the cadmium reds and yellows (see Table 11.7) so valuable technically for their heat and light stability: many manufacturers are already looking into alternatives for cadmium pigments and stabilisers. As with other PVC formulation components, information and guidance on the latest thinking, and any specific regulations, concerning the possible health hazards associated with colourants are available from the various official and professional bodies mentioned in Chapter 7, Section 7.12 and Chapter 12, Section 12.9. Organisations directly concerned with colourants include the USA Dry Colour Manufacturers Association, the UK Society of Dyers
410
W. V. Titow
and Colourists, and Oil and Colour Chemists Association, and the German Mineralfarben-industrie eV. (e) Cost This is an important consideration, applicable no less to the colourants than to any other constituents of a PVC formulation. 11.3.5
Some Commercial Pigments*
There are many world-wide manufacturers of pigments and many different chemical types; it would be a formidable task to list them. Most of the manufacturers issue copious data on their products but correlation of one range with others is not easy, nor is it a simple matter to relate trade names with chemical constitution. Test data on the other hand are fairly well standardised. Thus a simple compound is used, e.g. PVC polymer (e.g. Corvic H65/33-ICI Ltd) Plasticiser: e.g. DOP or Reomol D79P (CibaGeigy) Stabiliser: either basic lead carbonate paste or a Baled soap and epoxidised oil in functionally equivalent proportion Colourant and titanium dioxide
100 50phr 8-10phr as indicated
to evaluate the various properties as follows. t Heat stability: A cut of hide from the mill is pressed between polished steel plates for 30 min at 170°C and the resulting sheet is compared with one pressed for 5 min. Alternatively, portions of sheet pressed for 5 min are further heated in a thermostatically controlled air oven for 30 min at 170°C or 10 min at 200°C. Any difference is assessed on the SDC Geometric Grey Scale (BS 2662: 1961) on a 5 to 1 rating,S equalling no change, 1 equalling very considerable change. These relatively long times are used to cover extremes of direct heating or the cumulative effects Of reworking.
* This section, edited and supplemented for the present" edition, was originally contributed (to the previous edition) by Mr H. G. White of ICI. tThese tests were used by the compiler of Table 11.7. Other tests (some more stringent) can be applied, e.g. those detailed by Ciba-Geigy in connection with that company's rating of its pigments for PVc. 63
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Some Miscellaneous Components of pvc Formulations
411
Migration: A specimen of coloured sheet is held in contact with a piece of white PVC under a weight of 1·0Ibfin- 2 . The assembly is placed .in an oven for 24 h at 70°C and the degree of staining assessed on a 5-1 scale. Dry rubbing: A piece of coloured sheet is rubbed with a piece of dry cotton fabric. Wet rubbing: As for 'dry rubbing' except that moist cotton is used. Soap fastness: A specimen of coloured sheet is placed between a piece of cotton and a piece of wool. The assembly is rolled and immersed in soap solution for 1 h at 60°C. Staining is assessed on the 1-5 scale. (Solution prepared by dissolving 5 g soap flakes and 2 g anhydrous sodium carbonate in 1 litre of water.) Light stability: Coloured specimens are exposed to daylight and assessed according to BS 1006: 1978 using the Blue Scale of Standards against which the light fastness is rated (upwards) from 1 to 8.
The chemistry and properties of pigments available for and used in PVC have been reviewed by White62 who originally compiled Table 11.7. The salient characteristics of the main chemical classes into which the pigments have been grouped in the table may be summarised as follows: A. Toners: Alkaline earth metal salts of organic azo dyestuffs; they give bright strong colorations but only moderate light fastness, whilst the salt linkage gives susceptibility to colour change if the PVC compound is changed. Newer types are giving better light fastness. B. Disazo pigments: Mainly derived from substituted benzidine, these are strong yellow to red pigments of good to very good light fastness but may show a trace of solubility and colour change if processed at high temperatures and low concentrations.
C. Condensed disazo pigments: These are made by condensing two monoazo pigments, giving weaker but brighter colorations than conventional disazo types with better light fastness and less solubility.
DCB/PCP
DCB-DA/PTMP
DCB/AA2MCA DCB/PMP
TCB/AAMX DCB/AAOA DCB/AAMX
(A) Toners Barium-2B Strontium-2B Manganese-2B Calcium-2B Calcium-4B Ca-monoazo Ba-monoazo Ca-ONPSA Ba-ONPSA Ba-monoazo (B) Disazos
Chemical type
P.Red 48 PoRed 48 PoRed 48 P.Red 48 P.Red 57 P.Red 134 P.Red 133 N.L. P.Yellow 62 P.Red 151 P.Yellow 81 P.Yellow 17 P.Yellow 13 N.L. PoYellow 83 Po Orange 13 N.L. P.Red 111 P.Red 38
P.V.Fast Yellow HlOG Irgalite Yellow 2GP Vynamon Yellow GRES Irgalite Yellow BAF P.V.Fast Yellow HR Vynamon Orange GS Irgalite Orange F2G Vynamon Red GES Irgalite Red PYE
C.l. Ref. (Pt 1)0
Vulcafor Red AS FW Rubine Toner BOS Rubine Toner 2BRS Irgalite Red RC Vynamon Claret YS Irgalite Red HGL Irgalite Red HBL Irgalite Yellow WSC Irgalite Yellow WSR P.V.Red H4B
Brand name
0·50 0·10 0·10 0·10 0·05 0·10 0·075 0·05 0·05
0·15 0·15 0·10 0·10 0·10 1·0 0·5 0·5 0·5 0·25
Amt for! ISD (%)
TABLE 11.7 Pigments for PVC and Some of Their Properties
0·5 0·5 0·5 0·5 0·5 0·5 0·5 0·5 0·5
0
0·5 05 0·5 0·5 0·5 1·0 1·0 2·0 2·0 0·5
F
0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01
0
0
0·01 0 01 0·01 0·01 0·01 0·02 0·02 0 04 0·04 0·01
P
% in patterns b
6-7 7 6-7 7 7 6 6 5 5
4 5-6 6D 5 4 6 6-7 6-7 7 6
F
P
6 6 5-6 6 6 4 4 3 3
5-6
-
-
-
1-2 2-3 3 3 1-2 -
Light fastness C
5 5 5 4 5 4-5 4-5 5 5
5 3-4 5 2Y 3Y 5 5 5 5 5
F
4Y 4Y 4Y 3Y 3Y 5 5 5 5 4
P
4 4B 4B 2B 5 4-5 2Y 2Y 2Y
Heat stabilityd
(E) Polycyclics Indanthrone Dioxazine Dioxazine Thio indigoid Perylene Perinone Flavanthrone Quinacridone {3 Quinacridone a (F) Isoindolinones 7th FAT/PEC Congress (1964), p. 61
(D) Insolubilised Monoazos No disclosures
(C) Condensed Disazos See: JOCCA (1963), p. 13, and (1968), p.580
0·40 0·50 0·25 0·25
P.Yellow 109 P.Yellow 110 P.Orange 42 P.Red 180
Yellow 2GLT Yellow 3RLT Orange RLT Red 2BLT
Irgazin Irgazin Irgazin Irgazin
-
0·16 0·08 0·20 0·30 0·13 0·50
0·08 0·05
0·20 0·10 0·10 0·08 0·25 0·40 0·10 0·22
0·5 0·1 0·1 0·1 0·1 0·1
P.Blue 60 P.Violet 23 P.Violet 35 P.Red 88 P.Red 149 Vat Orange 7 P.Yellow 112 P.Violet 19 P.Violet 19
P.Orange 38 P.Red 185 P.Red 183 P.Red 171 N.L. P.Yellow 105 P.Red 150 P.Red 187
P.Yellow 94 P.Orange 31 P.Red 139 P.Red 144 P.Red 140 P.Red 142
Vynamon Blue 3RS Vynamon Violet 2BS Irgazin Violet BLT Cromophtal Bordeaux RN P.V.Fast Red B P.V.Fast Orange GRL Cromophtal Yellow A2R Cinquasia Violet RT 795D P.V.Fast Red E5B
P.V.Red HFG P.V.Carmine HF4C P.V.Fast Bordeaux HFR P.V.Fast Maroon HFM P.V.Brown HFGG Vynamon Yellow 8GS P.V.Carmine HR P.Y.Pink FL
Cromophtal Yellow 6G Cromophtal Orange 4R Cromophtal Red GR Cromophtal Red BR Cromophtal Red R Cromophtal Rubine B
0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01
0·5 0·5 0·5 0·5
0·01 0·01 0·01 0·01 0·01 0·01 0·01 0·01
0·01 0·01 0·01 0·01 0·01 0·01
0·5 0·5 0·5 0·5 0·5 0·5 0·5 0·5 0·5
0·5 0·5 0·5 0·5 0·5 0·5 0·5 0·5
0·5 0·5 0·5 0·5 0·5 0·5
7 7 6 6 7 6-7 6-7 6-7 7
6 6 7 6-7 6-7 6-7 3 5
7 5 6 6 5-6 5
7D 6-7D 7D 7D 7D 7D 7D 7D
7 7 7 7 7 7 7 >7 >7
6-7 6-7 7 7 7 7 5 6
7 6-7 7 7 6-7 6-7
5 5 5 5
5 5 4-5 5 5 5 5 5 5
4-5 5 5 4-5 5 5 5 5
5 5 5 5 4-5 5
4B 5 4Y 4Y
4 5 1 3Y 4-5 3Y 4-5 4D 4Y
4Y 4-5 5 4-5 4Y 4-5 3-4 3Y
5 5 4-5 4-5 4Y 4Y
11
(G) Phthalocyanines CPC, ll'form CPC, stable ll' CPC, f3 form Chlorinated CPC Brominated CPC (H) Other Organics Iron nitroso-f3-napthol Aniline black Carbon black (I) Cadmiums Cadmium sulphide
Chemical type
11
P.Yellow 37 Yellow P 3680 Primrose P 500 Lemon Yellow P 3682 Light Orange P 4701K Deep Orange P 4702K
1·0 1·0 1·0 0·75 0·75
0·10 1·0 0·05
P.Green 8 P.Black 1 P.Black 7
Vulcafor Green LS Monolite Fast Black LS Kosmos 70
Cadmium Cadmium Cadmium Cadmium Cadmium
0·06 0·07 0·08 0·16 0·20
Amt for! ISD (%)
P.Blue 15 P.Blue 15 P.Blue 15 P.Green 7 P.Green 41
C.l. Ref. (Pt l)a
Irgalite Blue BLP Vynamon Blue LBS Monastral Fast Blue BGS Vynamon Green BES Vynamon Green 6YS
Brand name
TABLE 11.7-eontd.
5 5 5 5 5
0·5 0·5 0·5
0·5 0·5 0·5 0·5 0·5
7 7 >7 7 7
6 7 >7
0·01 0·01 0·01 0·1 0·1 0·1 0·1 0·1
>7 >7 >7 >7 >7 0·01 0·01 0·01 0·01 0·01
5 5 5 5 5
6-7 6-7 6-7 6-7 6-7
5 5 5 5 5
3 5 5
4-5 5 5 5 7 >7
P
3Y 4Y 3-4Y 4-5 5
F
Heat stabilityd
4-5 5 5 5 5 >7 >7 >7 >7 >7
P F
P
F
Light fastness C
%in patterns b
P.V.Fast Brown G
-
Chrome Green DC 3593
Vynamon Yellow 6GNS Supra Lemon Chrome 4GS Vynamon Yellow CRNS Supra Orange Chrome HYS Supra Scarlet Chrome YS Supra Scarlet Chrome MS
Light Red P 4703K Scarlet P 4704K Red P 4705K Deep Red P 4706K Crimson P 4707K Maroon P 4708K
a
b
2·0 0·75 0·3
1·0 1·0 1·0 1·0 1·0 1·5
1·0 1·0 0·75 0·75 0·75 0·75
= Darkening.
P.Green 17 P.Blue 29 P.Brown 6
P.Red 104
II
P.Yeliow 34
P.Red 108
11
Colour Index (Part I) Ref: N.L. = Not listed. Patterns: F = Full Shade; P = Pastel Shade in white plasticised PVC; D C Light fastness: Daylight, Blue Scale 8-1 ratings BS. dHeat stability: 10 min at 200°C in air oven. Grey Scale 5-1 ratings.
(K) Other Inorganics Chromic oxide Ultramarine Iron oxide
Lead molybdate
II
(J) Chromes Lead chromate
Cadmium selenide
11
Cadmium Cadmium Cadmium Cadmium Cadmium Cadmium
5 5 5
5 5 5 5 5 5
5 5 5 5 5 5
0·1 0·1 0·1
0·1 0·1 0·1 0·1 0·1 0·1
0·1 0·1 0·1 0·1 0·1 0·1
>7 7 >7
7D 7D 7D 7D 7 7
7 7 >7 >7 >7 >7
7 7 7
7D 7D 7D 7 7 7
6-7 6-7 7 7 7 7
5 5 5
5 5 5 5 5 5
5 5 5 5 5 5
5 4Y 5
5 5 5 5 5 5
5 5 5 5 5 5
416
W. V. Titow
Plate A Laboratory-scale equipment for PVC processing (Farrel Bridge Ltd.). (1) Two-roll mill (swing-side, variable friction, rolls 6 in x 13 in).
D. Insolubilised monoazo: These are types in which heavy substitution of the simple monoazo pigment has suppressed solubility to very acceptable levels; usually this is obtained at appreciable financial expense. E. Polycyclic compounds: These are offshoots of the vat dyestuffs used on textiles; this highly selected group of colours give very high strength, fastness and brilliance but at a very high cost.
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Some Miscellaneous Components of pvc Formulations
417
Plate A-<:ontd. (2) Four-roll laboratory calender ('inverted L' type, rolls 6 in x 15 in).
F. Isoindolinones: These form a new group from the polycyclic class with a shade range greenish yellow to bluish red of very good light fastness and insolubility. G. Phthalocyanines: These are a very old specialised type of polycyclic giving the most stable blue and green pigments at comparatively very low cost. H. Carbon black: This term is often used as a general name for the group of particulate carbon pigments, whose members are known under names derived from the methods of their preparation (which also affect the particle size, surface 'chemistry,64 and degree of blackness), e.g. furnace blacks, channel blacks, lamp blacks (various grades). Carbon blacks are characterised by small particle size, outstanding tinting power and hiding strength, heat and light resistance. The fine-particle furnace blacks have been increasingly widely used in PVc. As indicated in Chapter 8, Section 8.4.3, apart
418
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V. Titow
from its action as a black pigment carbon black improves the weathering resistance of plastics compositions and-in sufficiently high loadings-eounteracts static electricity collection and can confer conductive properties (see Section 11.4 below). A useful monograph on carbon black, covering both the scientific and technical aspects of the subject, has been produced by Donnet and Voet. 65 I. Cadmium sulphide and selenides: These form the main group of yellow-orange-red-maroon inorganic pigments of very high light and heat fastness with no solubility, but they are both expensive and tinctorially weak.
J. Chrome pigments: For example, lead chromate and lead molybdate form a much less expensive greenish yellow to scarlet range of pigments in which recent improvements have given light fastness comparable with cadmium pigments, but they remain susceptible to strong alkalies and to darkening by hydrogen sulphide. K. Other inorganics: These include ultramarine, iron oxides and chromic oxide, which are, respectively, the cheapest brightest reddish blue, a source of very cheap and fast fawn to brown colours provided very high processing temperatures are not involved, and the best base for green colorations for the production of camouflage effects.
The above points are illustrated in terms of numerical data in Table 11.7, in which representative members of the pigment classes discussed are listed with disclosures of their chemical constitution by reference to the Colour Index or literature. A comparison of relative tinctorial strength has been included by indicating the dosage of pigment required in a clear plasticised PVC composition with 1% titanium dioxide to give a uniformly strong coloration to 1/3 International Standard Depth (BS 1006:1978). The light fastness and heat stability ratings of the pigments as full and pastel shades in the same PVC composition (with BalCd soap-epoxidised oil stabiliser system) are then compared. Titanium dioxide, the most widely used white pigment, has not been included in Table 11.7. It is available in two crystalline forms-anatase and rutile. In general rutile is the more stable to light (and hence, broadly, preferable as a pigment for plastics, especially for outdoor applications), whilst anatase has a stronger blue undertone. Both forms are available in various grades differing in particle size and size
11
Some Miscellaneous Components of pvc Formulations
419
distribution, and in the presence (and nature) of surface treatments, so that the supplier's advice should be sought regarding the choice of grade for a particular application, although continuous development to improve the ease of dispersion, blueness of undertone, durability and resistance to moisture pick-up has made it possible to combine all these features in a fine-crystal rutile grade. In addition to its use as a white pigment, titanium dioxide is also widely employed to enhance the brightness of a colourant. In proportions appropriate to these two purposes (up to a few phr) this pigment has no appreciable effect on the physical properties of PVC compositions. At higher loadings (about 10-15 phr) it can enhance the weathering properties of some PVC products (e.g. flexible films for swimming pool and reservoir linings), or-especially when of the finest particle size-improve the impact properties of rigid materials. 34 Useful information, respectively on the nature of organic pigments, extractability of colourants (and some other additives) from PVC compositions, and on all important aspects of the colouring of plastics (including PVC), is contained in publications by Inman,66 Brighton,67 and Webber. 68
11.4 ANTISTATIC AGENTS Like many other plastics materials, PVC products and articles may readily accumulate static electricity charges under certain conditions. Friction against, and separation after surface contact with, other surfaces are common ways of charge generation. Since surface charges leak away more easily in moist conditions, the highest and most persistent charges result at lowest relative humidities. Charge build-up is drastically reduced at relative humidities above approximately 80%. Accumulated static charges can represent a serious potential hazard. For example, when present on the cover or carcass of a PVC conveyor belt, the charge may-on discharge-produce a spark which can cause a fire, or an explosion in a dust-laden atmosphere: such circumstances can arise in mines, where PVC conveyor belting is used. Charge accumulation on PVC sheeting can constitute similar hazards in other environments (including atmospheres containing flammable vapours or gases). Charges built-up on PVC polymer particles (in conveying, metering, etc.) can be troublesome; as can those acquired by dry blend particles in the course of high-speed mixing (in that they can atfect the
420
w.
V. Titow
bulk density and handling properties, although they often eventually dissipate in storage). Finally, attraction of dust and other atmospheric pollutants to charged plastics surfaces (mouldings, etc.) can result in dirty marks. For all these reasons it is necessary to have methods of preventing static electricity charge build-up. In general the methods employed may be divided into two broad groups: chemical treatments and physical treatments. The former treatments involve the incorporation in the PVC material, or external application thereto, of a chemical (an 'antistatic agent') which will prevent the build-up of charge. The latter are essentially means of ionising the atmosphere (either with the aid of a radioactive source or electrical discharge) to enable any charge build-up to be neutralised. These physical methods are mainly applicable in the handling and processing of sheet materials. It will be clear that they do not impart any antistatic properties to the material itself, and require special apparatus. For all these reasons they are not of great importance to the present subject and will not be considered further here. A recent BS Code of Practice 69 provides useful basic information on the generation of static electricity, its measurement and control. 11.4.1 Static Electricity Charges on PVC: Phenomena and Tests
In common with many other plastics PVC is a good electrical insulant: the volume and surface resistivities of the polymer (and many rigid compounds) are, typically, in the range 1015 _10 17 Q cm and Q, respectively (at room temperature and about 60% RH).7o Thus, as mentioned above, the material can readily acquire static charges, particularly in dry conditions, and retain them for long periods. The charges on PVC are predominantly negative: as with other plastics, the charge distribution may be 'patchy' and in some areas a local positive charge may be present. In their discussion of charge distribution on the surface of a plastic (polystyrene), Woodland and Zeigler71 suggest, inter alia, that the widely accepted use of surface resistivity measurements as an indication of the 'static properties' of a compound is not fully justified. However, in the PVC industry, surface resistivity measurements still provide the most popular way of assessing the effects of antistatic agents. Since conduction over the surface of the material is the principal mechanism of electrostatic charge dissipation, surface resistivity is indeed the most important single factor in 'static'
11 Some Miscellaneous Components of pvc Formulations
421
phenomena in plastics. Other ways in which static charge may be dissipated are conduction within the material itself, and ion discharge; the latter is in fact the mechanism of the previously mentioned physical methods of combating static in some processes. The scale and undesirable effects of static phenomena of interest in practice are closely associated with the magnitude of the charge and the rate of charge build-up and dissipation. Each can be measured and-in any but a superficial investigation-they should be evaluated jointly. The magnitude of static charge can be measured, in terms of field strength, by field meters and electrometers of various types. 69 ,72,73 With the proper instrumental arrangement, the field strength measured is related to the charge density by the expression: 69
a=EoK where: ais the surface charge density (Cm- 2), K is the field strength measured (V m- 1), and Eo is the permittivity of free space (8,85 x 1O- 12 Fm- 1). Field meters can also be used qualitatively to detect the presence of a charge or locate the point(s) of highest density. A convenient hand instrument is the 'Statigun',7° a gun-shaped, valve electrometer. The rate of charge decay can be measured with the aid of electrometers, but the important point that the charging method must be standard and reproducible is frequently overlooked. In more fundamental investigations charging by rubbing is not satisfactory. Measurement of charge decay in, say, factory conditions (e.g. on sheeting charged in the course of a particular process) can be meaningful in the limited context of that process and can give an indication of the efficiency of antistatic agents and treatments. The figure usually quoted in connection with charge decay results is the 'half-time', i.e. time required for the charge to be reduced by a factor of 2. Good reviews of the generation, nature and measurement of electrostatic charges on plastics have been published by Quackenbos,74 Ferraris,75 and Gale and Pacitti. 7o As already mentioned, in practice the 'static' properties of a plastic are most often gauged and indicated in terms of surface and volume resistivity, and several standards lay down their requirements in those terms. Thus, for instance, the maximum resistivity (as determined in prescribed tests) of PVC conveyor belting for underground use in coal mines is laid down as 3 x 108 Q by both BS 3289:1960 and the
422
W. V. Titow
appropriate National Coal Board specification (NCB 158/1971), whilst upper limits for antistatic products for use in hospitals (e.g. anaesthetic tubing, trolley wheels, mattresses) and industry (e.g. flooring, footwear, hose) given by BS 2050:1978 are, in many cases, about 106 Q (5 X 104 Q for flooring in explosive-handling areas). The minimum volume resistivity requirements for ordinary flexible PVC compounds covered by BS 2571: 1963 range from 5 x 109 to 1 X 1014 Q (at 23°C). Standard specifications dealing with general methods of determining resistivities of plastics are listed in Appendix 3. Methods suitable for antistatic and conductive plastics (volume resistivities up to about 105 Q cm) are given in ISO/DIS 3915-1980 and BS 2050: 1978, and for rubbers in BS 2044: 1978. The determination of antistatic properties of plastics films (three methods, including one based on charge decay) is covered by BS 2782: Methods 250 A, Band C: 1976. The attraction of particulate dirt or dust by charge-bearing plastics surfaces is the basis of a group of tests which, whilst not very precise, do give some visual indication of the magnitude of the charge. Because of their low accuracy and limited reproducibility their use should, however, be confined either to rough, practical pointer assessments or to strictly routinised checks under standardised conditions. Two common tests of this kind are the ash test and the 'dirt chamber' test. 71 In the former the plastics material is charged by rubbing and the amount of cigarette ash picked up by the surface is observed. In the popular Procter and Gamble version of the dirt chamber test the plastics article or material charged by rubbing is placed in a cabinet at 80°F (27°C) and 15% RH, and smoke (produced by burning a piece of filter paper saturated with toluene) is introduced into the chamber. The pick-up of the particulate combustion products by the material is observed. 11.4.2 Nature and Use of Antistatic Agents
Antistatic agents .are chemicals which are either incorporated in a plastics material, or applied externally, to reduce static charge build-up and promote charge dissipation, by lowering the resistivity of the material. They are cationic, anionic or non-ionic in nature, and commonly belong to one of the following groups of compounds: Amines and amides, e.g. Lubrol PE (ICI); Lankrostat LDN (Lankro Chemicals Ltd)
11
Some Miscellaneous Components of pvc Formulations
423
Quarternary ammonium compounds, e.g. Ethoquad e12 (Armour Industrial Chemical Co. (USA) or Armour Hess Chemicals Ltd (UK»; Lankrostat QA T Polyethylene glycol derivatives, e.g. Gafstat AE 610 (GAF Corp.) Sulphates and sulphonates e.g. Querton 14 ES or 16 ES (Guest Industrials Ltd) Miscellaneous ethers and esters Conductive polymers are also offered from time to time as antistatic agents, for example, Resin QX2611 (Dow)-a copolymer of a quaternary ammonium compound with styrene, or Ionac PE 100 (Ionac Chemical Co.). Typically, about 2-5 phr of an antistatic agent may be incorporated in internal application (but up to about 10 phr with some agents-see below). External application (normally to semi-products, e.g. film, or mouldings by spray, wipe or dip) is usually from solution of 0·1-2·0% concentration; but soaking the pellets of a moulding compound in a 50% solution and drying has also been recommended (e.g. with Ex-Static-Guinness Chemical Co., UK). In both kinds of application the aim is to form a layer of the antistatic agent on the surface of the product, which in turn attracts a layer of moisture, ultimately responsible-in conjunction with the conductivity of the agent-for conducting away charges. It is because of this that the effectiveness of antistatic treatments drops at low relative humidity. Internal application of antistatic agents gives more permanent results, because if the surface layer is removed (e.g. by friction against another surface, or contact with liquids) it can be reinstated by more reagent diffusing to the surface from the mass of the material. It has been claimed that this process is detectable by measurement of the contact angle of water on the plastic's surface. 76 An externally applied layer of antistatic agent, if removed, can only be restored by re-application. However, the external method may be reasonably convenient and useful in appropriate cases: inter alia it affords the user a simple means of keeping in check dust contamination of gramophone records. It is well known that most antistatic agents reduce the thermal stability of PVC (especially uPVC) compositions in which they are incorporated. However, careful formulation, particularly the selection of stabilising systems, can minimise the effect in many cases. Analytical
424
W. V. Titow
studies are claimed to indicate that nitrogen-containing antistatic agents have the strongest adverse effect on heat stability. 77 The incorporation of a conductive filler (most commonly carbon black, although metal powders-e.g. aluminium, * nickel-may be used) produces an antistatic effect, or conductivity at sufficiently high loadings. The use of carbon black for these purposes is old, having originated in rubber processing. The proportions of the filler used are high, up to about 35%. Various grades of carbon black differ in their own conductivity: in general conductivity increases with decreasing particle size, and with increasing structure, surface purity and crystallite size and orientation. High-structure furnace blacks are therefore particularly suitable. 78 t Whilst excellent permanent antistatic effect or conductivity can be achieved, the method is obviously limited to compositions acceptable in black. Two further points may be mentioned. Firstly, plasticisers, and to some extent also other constituents of PVC compositions, can affect resistivity, and this should be borne in mind both from the point of view of the resulting effect on static accumulation, and the insulation properties in electrical applications. Secondly, certain antistatic agents which have to be used in substantial proportions for their maximum effect can have an appreciable plasticising action: examples are Antistat A (Albright and Wilson) and Irgastat 51 (Ciba-Geigy), both used in flexible compositions in amounts of up to 10 phr. The effect of Antistat A on some properties of a flexible composition (Breon P13011+ plasticised with TXP and Pliabrac 987§ in the ratio 70: 30) is illustrated in Table 11.8 11.5 FLAME AND SMOKE RETARDANTS 11.5.1 General Mechanism of Burning of Polymers and Plastics
The mechanism of burning of polymers, alone or as base constituents of plastics compositions, comprises two processes-pyrolysis and * Conductive thermoplastic compounds (including PVC) filled with aluminium alloy flake are commercially available, e.g. Emiblend (Howard Industries Inc., Clark, NJ, USA). t Special proprietary brands are available for use in thermoplastics, including PVC, e.g. Conductex 975 (Columbian Chemicals Co., Tulsa, OK, USA). :j: Emulsion PVC polymer, K value 70-74 (BP Chemicals International Ltd). § Nonyl ester of saturated CC C6 dibasic acids (Albright and Wilson Ltd).
10 10 630 -33
5 20 720
-25 -29
Addition of Antistat A, (phr) Surface resistance (Q x 107 ) Modulus at 100% elongation (lbfin- Z) Cold flex temperature eC, Clash and Berg) 0 2500 830
70 phr
Plasticiser (total content)
-31
0 1000 640 -34
5 15 580 520
10
-37
80 phr 7
5 11 480
-36 -38
0 450 530
10 5 430 -40
90 phr
TABLE 11.8 Some Effects of 'Antistat A' in a pPVC Compound
-39
0 300 480
-41
5 7 420
100 phr
-43
10 4 390
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426
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combustion. Pyrolysis is the breakdown (thermal degradation) of the solid polymer material to products of lower molecular weight, including simple, volatile, hydrogen-rich compounds, and conjugated linear and cyclic compounds. Combustion is the process of exothermic oxidation of the pyrolysis products. Both processes involve, inter alia, the formation and re-combination of free radicals: it has been suggested25 ,79 that flammability is considerably influenced by the ease and rate of formation of HO* radicals. Both processes are complex, and need suitably elevated temperatures to initiate and sustain them: combustion additionally requires oxygen (pyrolysis can proceed without it, although its absence will affect the temperatures involved and the nature of degradation products formed). Pyrolysis starts first (with many organic polymers the rate is appreciable at about 300°C in air), but once the amount of breakdown products generated and the rate of their continued formation are sufficient for combustion to commence (Le. for those constituents of the pyrolysate which are present in gaseous, vapour, or fine-mist form to be ignited) and to proceed, the two processes run concurrently, with the heat produced by combustion actuating, or contributing to, further pyrolysis, which in turn provides more degradation products for the combustion process (see Fig. 11.6). The pyrolysis products may be first ignited (Le. the combustion process started) by contact with an external source of intense heat (normally a flame, but, for example, incandescent-or simply very hot-electric wires can also be the source in fire situations): this is sometimes termed 'flash ignition'. Spontaneous ignition (self-ignition) may also occur when the concentration and temperature of pyrolysis products have reached suitable values. For a given polymer or plastic these values will depend on the composition of the material and the conditions (cf. Chapter 12, Section 12.10): 450°C is fairly representative as the flash ignition temperature for rigid PVC (normal impact strength grade) in the test of ASTM 1929-78 (cf. Chapter 12, Table 12.12). In general, as has been mentioned in Chapter 12, the burning behaviour of a plastic is cardinally dependent on the conditions, to which any descriptive terms (like 'non-flammable', 'slow-burning', self-extinguishing', etc.) must relate. However, in many tests and actual service situations where the plastic is not in continuous contact with an 'external' flame, it may be properly described as self-
extinguishing if the burning process is not self-sustaining in the sense that the heat generated by the material's own combustion is not
11
Some Miscellaneous Components of pvc Formulations
427
sufficient to maintain the high temperature required for, and the rate of, pyrolysis at a level necessary to provide enough pyrolysate (in suitable form-i.e. gaseous or fine mist) to keep the combustion going. Any of the products of both pyrolysis and combustion (see Fig. 11.6) may find their way into the smoke emitted by a burning plastic. In terms of its physical nature smoke is a suspension of particles in a mixture of air and the gases and vapours generated in the burning process (and remaining uncondensed into liquid droplets or layers on existing particles).8D-82 The particles of the particulate phase may be solid or liquid (or some of each); some at least may have a solid carbon 'skeleton'.83 Gases and vapours are invisible: hence, in any given case (i.e. for a fixed total amount of matter in the smoke) the smoke density, usually defined in terms of obscuration of light, will be determined by the proportion of constituents present in the particulate phase, and to some extent also by the particle size and size distribution. In general, the greater the amount of pyrolysis products in the smoke (i.e. the less complete the combustion) the greater the visible smoke denisty (because the pyrolysis products tend to have higher molecular weights than the final products of combustion and hence condense more readily into droplets when mixed with relatively cool air). In any particular case the actual nature and proportions of the chemical constituents of the smoke will depend, in a complex way, upon the chemical composition of the burning plastic and, given that, upon the burning conditions (especially the temperature and the supply of oxygen): the same factors also influence the parameters governing smoke density. 11.5.2 Flame Retardance and Smoke Suppression in PVC Compositions It is the large proportion of chlorine they contain that is responsible for
the low intrinsic flammability of PVC polymers and those of their compositions in which the overall chlorine content has not been reduced too far by 'dilution' of the polymer with the additives used, especially flammable plasticisers (see Chapter 7, Section 7.6, and Chapter 12, Section 12.10). In terms of the mechanisms outlined above, the presence of chlorine hinders burning through the formation (mainly in the course of pyrolysis) of hydrogen chloride, which interferes with the burning process in two ways: being incombustible itself it prevents, or at least reduces, access of oxygen to the
w.
428
V. Titow
POLYMER
(or its dlZrivatlvlZ plastic)
Pyrolysis +-
Liquid or SlZmi-liquid dlZgradation products ('tar')
- - - - - - - -
GaslZous (vapour phaslZ) dlZgradation products
- - --
Solid or SlZml-solid dlZgradatlon products (primary 'char')
FurthlZr pyrolysis +-
--i I
I
Solid carbonaclZous rlZsidulZ (slZcondary 'char')
Combustion \
\ Combustion products (including CO, CO2, H20 and various oxidlZS)
\
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I
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v
/
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Fig. 11.6 Simple, schematic representation of the general mechanism of burning of polymeric materials.
11
Some Miscellaneous Components of pvc Formulations
429
combustion zone; it also reacts with free radicals (especially HO*) thus acting as terminator for the combustion propagation reactions. 25,84 Thus, by virtue of its chemical nature, PVC achieves the kind of flame-retardant effect that in other plastics has to be induced by the addition-as flame retardants-of halogen derivatives of organic compounds (sometimes in proportions so large that they affect adversely the mechanical and other properties). The effectivity of the flame retardancy mechanism just mentioned can be preserved in soft and semi-rigid PVC compositions (i.e. those containing substantial proportions of additives) by the use of chlorinated paraffin plasticiser extenders or chlorinated polyethylene. It can also be further improved by the synergistic effect of 'antimony oxide' (antimony trioxide, Sb20 3) incorporated in relatively minor proportions (up to about 10 phr). The improvement is usually explained in terms of the following effects. The antimony trioxide reacts with the hydrogen chloride generated by burning PVC polymer (and any chlorinated additives)t to form antimony trichloride, volatile at the combustion temperature. This is not only more effective than HCI as a 'barrier' against oxygen, but can actually react with oxygen in the flame zone (thus reducing the amount available even further). The reaction products are antimony trioxide-regenerated in the form of a fume (suspension of very fine particles in the combustion gases)-and atomic chlorine. The presence of the Sb20 3 particles suppresses somewhat the generation of the highly active HO* radicals by reducing the excitation energy for the oxidative process (the so-called 'wall effect'25): the particles also generally catalyse the re-combination of free radicals in the flame. 82 The chlorine is available to form more HCl. The action of zinc and tin oxides is similar to that of Sb20 3 but less effective. Zinc oxide also functions as a smoke suppressant, but it tends to impair the thermal stability of PVC. Zinc and barium borates are sometimes used in flame-retardant products, mainly in conjunction with Sb20 3 (as cheaper part-replacements13 ); synergistic effects can arise in such combinations (notably with zinc borate85 ), which may also have smoke-suppressing effects. The borates are believed to inhibit burning in a manner similar to
t In other polymers flame retardancy can be conferred by Sb20 3 in a similar way if a chlorine-containing organic compound is also incorporated.
430
W. V. Titow
that of borax (historically one of the earliest flame retardants, first used in textiles), i.e. by fusing to form a protective layer which impedes access of oxygen and heat to the PVC material;25,84 the fusion also abstracts some heat from the system. As particulate additives insoluble in PVC compositions the metal oxides and borates just mentioned all have some opacifying effect (rather lower in general with the latter compounds): this is minimised with ultra-fine (sub-micron) particle grades (d. Section 11.3.1 above) which are becoming increasingly widely available. Note: The opposite approach to the same end is represented by a version of Sb 20 3 of particle size considerably larger than in the common, regular grades (1O-40/lm as against 1-3/lm):86 this is offered as a low-reflectance 'non-opaque' grade on the basis that the comparatively lower number of coarse particles present at a given level of loading by weight causes a lower overall amount of interference with light.
Save for phosphate ester plasticisers in flexible compositions, antimony oxide is the traditional flame retardant for PVC, still most widely used today. However, its high cost has been providing incentive for development of more economical alternatives. Some examples of those already available commercially are 13 ,86 antimony-based additives cheaper than Sb20 3 (e.g. antimony and antimony/zinc silicate compositions in the Oncor and Ongard ranges of Anzon America Inc;* d. also CLarechem CLA-150o-Claremont Polychemical Corp.), and molybdenum-based compounds (e.g. a Mo/phosphate combination, MoLy FR 36-Climax Molybdenum Co; and zinc molybdate/zinc oxide combinations like MoLy White 101-Sherwin-Williams Co.). The smoke-suppressant effect of molybdenum oxide, alone or in mixtures with antimony oxide, has been known for some time (see below). As has been mentioned in Chapters 7 and 12 the phosphate ester plasticisers (especially the aryl phosphates which are primary plasticisers) are widely used to reduce the flammability of flexible PVC: they are particularly useful in transparent compositions where Sb20 3 and its above-mentioned combinations are unsuitable because of the opacifying effect. It is common to employ an aryl phosphate plasticiser in * Associated with Anzon Ltd in the UK; now owning the PVC stabiliser and fire retardant/smoke suppressant operations formerly belonging to National Lead Co., USA.
11
Some Miscellaneous Components of pvc Formulations
431
conjunction with (as part replacement for) a cheaper primary plasticiser (often DOP) or a plasticiser/extender combination, because-apart from the cost aspect-the low-temperature properties of phosphate-plasticised compositions are relatively poor, whilst their smoke emission on burning is rather high. The flame-retardant effect of phosphorus compounds in polymers (including phosphate ester plasticisers in PVC) is believed to operate mainly through the formation of phosphoric acid residues, phosphorus pentoxide and its hydrates, all of which strongly promote the generation of char during pyrolysis and thus reduce the amount of matter available for combustionZ5 ,84 (ct. Fig. 11.6). Chlorinated phosphate esters with flame-retardant and general plasticising action similar to those of alkyl phosphate plasticisers, but with lower smoke generatiOn and-in some cases-better low-temperature properties, are also noteworthy (ct. e.g. Fyroflex 2704 and 280o-Stauffer Chemical Co.): see also Chapter 6, Section 6.6.4. Cost is always a factor in the selection of a flame-retardant agent or system for a particular PVC composition: in practice it is most often flexible PVC that has to be 'flame retarded', as uPVC is inherently resistant (see above). In the absence of special requirements (e.g. high clarity), combinations of a chlorinated paraffin (plasticiser extender) with antimony oxide (alone or in conjunction with, say, zinc borate as cheaper part-replacement) can offer cost economy in pPVC compositions with good performance in flammability tests. 87 Where a phosphate plasticiser forms a substantial part of the flame-retardant system a smoke suppressant is also desirable: in filled compositions this function may be discharged in a sufficient degree by the filler-for example, calcium carbonate and talc can reduce the smoke generation (and, as inert materials, also flammability) of PVC compositions. A few outline examples of flame-retardant/smoke-suppressant systems of the above kind are given in Table 11.9. As has been mentioned in Chapter 8 (Section 8.4.2) alumina trihydrate (AI(OHh or Al z0 3 .3HzO) is another additive, with both flame-retardant and smoke-suppressing actions, of considerable use in flexible PVC compositions. The main mechanisms whereby these actions are exerted are considered to operate as follows. 88 ,89 The water of hydration present in high proportion (about 34%) in alumina trihydrate is securely retained under PVC processing conditions (the amounts lost after heating for 10 min at, respectively, 170°C and 220°C, represent about 0·5% and 2·5%88). However, at the high
Q
DOP Epoxidised oil { Aryl phosphate ester Chlorinated paraffin (52% Cl)b { Antimony trioxide Zinc borate Molybdenum complex c { Zinc/magnesium complex d
b
Q
2
2
25 2 4
25
60
5 32 5
50
2
2
1
3
24
24
60 5
5 25
60
Used for its stabilising rather than plasticising action. Cereclor S52 (lCI). c Kem-Gard 91lA (Sherwin-Williams Co., Chemical Division). d Ongard 2 (Anzon Ltd).
Smoke suppressants
Flame retardants
Plasticisers
6
5
4
2
1
3
Semi-rigid formulations (phr)
Flexible formulations (phr)
TABLE 11.9 Outline Examples of Flame-retardant/Smoke-suppressant Systems for Flexible and Semi-rigid PVC
"'"
:::'1
o;;:
~ :'"
W N
11
Some Miscellaneous Components of pvc Formulations
433
temperatures obtaining in burning it is rapidly split off as vapour or steam, which obstructs the access of oxygen to the polymer, thus hindering the combustion process. 2 AI(OH)3
h' h 18 ) temperature
Ah03 + 3HzO - 71·6 kcal
The action is supplemented by the insulating effect of the solid alumina liberated in the decomposition. The alumina also acts as a heat barrier which, in conjunction with the heat absorption in the endothermic decomposition reaction, lowers the temperature and suppresses pyrolysis, reducing smoke generation. It is also likely that adsorption of smoke constituents (especially HCI) by the finely divided alumina contributes to the reduction of smoke emission. 13 The double, flame- and smoke-retardant action of alumina trihydrate, and the fact that, weight-for-weight, it is cheaper than the main flame retardants for pPVC, are considerable advantages. However, high loadings are necessary for effective functionality (e.g. about 100 phr may be required to raise the Oxygen Index of a typical composition containing 50 phr DOP from about 22 to about 2788). At such loading levels the effects on viscosity in processing and on physical properties of the product can be substantial (see Chapter 8, Section 8.4.2). The mode of action of the mineral fibre Dawsonite (d. Chapter 8, Section 8.4.1) resembles that of AI(OHh- The material of the fibre, a hydrated sodium aluminium carbonate, decomposes at combustion temperature with essentially similar effects (supplemented in this case by the evolution of non-combustible carbon dioxide in addition to water vapour). Magnesium carbonate is another smoke suppressant used at high loading levels (up to about 40 phr) , at which its side-effects as a filler become significant. It is a component of some proprietary flameretardant/smoke-suppressant compositions, e.g. Monsanto's Phosgard LSV in which it is combined with a phosphate ester plasticiser (Santiciser 148). The compositions are recommended for use in compounds for electrical wire coatings, carpet backings and wall coverings. Molybdenum trioxide (molybdic oxide, Mo0 3) combines strong smoke-suppressant action in pPVC at all plasticiser contents with some flame-retardant effect: the latter is roughly comparable to that of antimony oxide at low levels of plasticisation (e.g. 20-30 phr DOP), but less at higher levels. The use of Mo0 3 in conjunction with Sbz0 3
434
W. V. Titow
(at 3 phr each) in medium-plasticised compositions can give good flame retardancy and smoke suppression. 13 ,82 Mo0 3 is less dense than Sb20 3 (SG respectively, 4·5 and 5,7) and has lower pigmenting strength (in comparable particle size grades); however it is an expensive additive. The smoke-suppressant effect of Mo0 3 is associated with the promotion of dehydrochlorination of the PVC polymer, and drastic reduction in generation of benzene. 82 However, the actual mechanism of its action is apparently different from that of Sb20 3 , as Mo0 3 has been reported not to yield volatile metallic species in burning PVC. 82 Molybdates (e.g. combinations of calcium and zinc derivatives) also act as smoke suppressants. Note: Selected zinc compounds, especially in combination with others, do not promote decomposition of PVC to the extent to which zinc oxide does. Such combination, therefore, most often forms the basis of commercial zinc-containing smoke suppressants (e.g. Ongard 2-ef. Table 11.9) despite the reasonable price and strong smoke-suppressing action of the oxide.
Certain derivatives of ferrocene represent another group of smoke suppressants effective in uPVC compositions. 82 Determination of the intensity of smoke generation (either by measurement of 'optical density' in a standard chamber, or by weighing the smoke substance collected on a filter in standard conditions) is covered by several of the standards listed in Table 12.12 (Chapter 12). However, only one of these-ASTM D 2843 90-is specifically and exclusively concerned with determination of smoke evolution: the others merely feature this as one of the aspects of burning behaviour under the test conditions. Another directly relevant standard (centered on the so-called NBS smoke chamber) is ASTM E 662,91 whilst an ISO standard, similar in principle, is in preparation. 92 A useful review of smoke test methods has been published in ASTM Standardisation News (August 1976, pp. 18-26). Two other useful general sources of information ~n methods of testing the flammability of, and smoke generation by, plastics materials are Flammability Test Methods for Plastics: An International List, published recently by the Chemical Industries Association Ltd, * and ISO Technical Report 3814-1975 The Development of Tests for Measuring 'Reaction to Fire' of Building Materials. * Alembic House, 93 Albert Embankment, London SEl 7TU, England.
11
Some Miscellaneous Components of pvc Formulations
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REFERENCES 1. Stapfer, C. H. (1969). Plast. World, 27(6), 76-7. 2. Pacitti, J. (1964). 'The Use of Lubricants in PVC', RAPRA Report No. 470, January. 3. Marshall, B. I. (1969). Brit. Plast., 42(8), 70-6. 4. King, L. F. and Noel, F. (1972). Polym. Engng. Sci., U(2), 112. 5. Oakes, V. (1970). 'Stabilisers and lubricants for PVC'. Plastics Institute Symposium on Additives for PVC, Manchester, England, Feb. 1970; and Oakes, V. and Hughes, B. (1966). Plastics, 31(347), 1132-4. 6. Di Francesca, A. (1966). Mat. Plast. Elast., 32(10), 1035-47. 7. Gale, G. M. (1973). In Developments in PVC Technology, (Eds J. H. L. Henson and A. Whelan), Applied Science Publishers, London, Ch. 3. 8. Andrews, K. E., Butters, C. and Wain, B. J. (1970). Brit. Plast., 43(10), 97-101; 43(11), 88-90. 9. Chauffoureaux, J. c., Dehennau, C. and van Rijckevorsel, J. (1979). J. Rheo!., 23(1),1-24. 10. Logan, M. S. and Chung, C. I. (1979). Polym. Engng. Sci., 19(15), 1110-16. 11. Illmann, G. (1967). SPE J., 23, 71-8. 12. Worschech, K. F. (1978). 'Synergistic support of various stabilisation systems during PVC processing by using lubricants', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 13. Titow, W. V. (1978). In Developments in PVC Production and Processing-I, (Eds A. Whelan and J. L. Craft), Applied Science Publishers, London, Ch. 4. 14. Petrich, R. P. (1978). 'Effect of processing aids and impact modifiers on processing characteristics of rigid PVC', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 15. Wilson, A. P. and Raimondi, V. V. (1978). 'Poly-alpha-methylstyrene as a process aid for rigid polyvinyl chloride', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 16. Blake, W. T. and Onufer, R. J. (1966). C. W. Brabender Instruments, Inc., Data Sheet 11. 17. Bottner, E. F. and Rosenthal, C. (1972). Kunststoffe. 62(10),685-7. 18. Jacobson, U. (1961). Brit. Plast., 34, 328-32. 19. Riethmayer, S. A. (1965). Gummi, Asbest, Kunst., 18(4), 425-32. 20. Stapfer, C. H., Hampson, D. G. and Dworking, R. D. (1968). SPE Technical Papers, 14, 26th ANTEC, 276-8. 21. Sahajpal, V. (1973). In Developments in PVC Technology, (Eds J. H. L. Henson and A. Whelan), Applied Science Publishers, London, Ch. 4. 22. Wilson, A. P. and Raimondi, V. V. (1976). 34th ANTEC SPE Proceedings, p. 513. 23. Wilson, A. P. and Raimondi, V. V. (1978). 'Poly-alpha-methylstyrene as a process aid for rigid polyvinyl chloride', paper presented at PRI
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24.
25. 26. 27. 28. 29. 30. 31. 32.
33. 34. 35. 36. 37. 38. 39.
40. 41. 42. 43.
44. 45. 46. 47.
W. V. Titow
International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. Moore, D. R. (1978). 'The influence of formulation on the compounding and rheological properties of PVC compositions', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. Mascia, L. (1974). The Role of Additives in Plastics, Edward Arnold (Publishers) Ltd, London. Sahajpal, V. (1976). Kunststoffe, 66(1), 18-20. Vincent, P. I. (1962). Plastics, 27(291), 115-17. Ives, G. C., Mead, J. A. and Riley, M. M. (1971). Handbook of Plastics Test Methods, Iliffe Books, London. Bucknall, c., Gotham, K. V. and Vincent, P. I. (1972). In Polymer Science, Vol. 1, (Ed. A. D. Jenkins), North Holland Publishing Co., Amsterdam, Ch. 10. ASTM D 2289-69 (Reapproved 1976): Tensile properties of plastics at high speeds. Campo, E. A. (1980). Engineering Design with Du Pont Plastics, Winter 1980, pp. 14-15, Du Pont Company, Wilmington DE 19898, USA. DIN 53 443, Sheet 2 (April 1975). Testing of plastics; multiaxial impact behaviour; impact penetration test combined with data processing by means of electronic devices. Mining-Why PVC Now, Booklet by AECI Limited, Plastics Division, PO Box 1122, Johannesburg 2000, Republic of South Africa, c. 1978. Vincent, P. I. (1974). In Thermoplastics: Properties and Design, (Ed. E. M. Ogorkiewicz), John Wiley & Sons, London, Ch. 5 and 6. Anon. (1979). Plast. Technol., 25(7), 95-6. BS 4618: Section 1.2:1972. Impact behaviour. Vincent, P. I. (1971). Impact Tests and Service Performance of Thermoplastics, Plastics Institute Monograph. Dyment, J. and Ziebland, H. (1958). J. Appl. Chem., 8, 203-6. Bramfitt, J. E. and Heaps, J. M. (1962). In Advances in PVC Compounding and Processing, (Ed. M. Kaufman), Maclaren & Sons, London, Ch. 4. Bier, G. (1965). Kunststoffe, 55(9), 694-700. Nishi, T. and Kwei, T. K. (1976). J. Appl. Polym. Sci., 20(5),1331-7. Sisson, W. B. (1968). Plastics and Polymers, 36(125), 453-63. 'Elvaloy' 741 and 742 Resin Modifiers, (1978). Du Pont Technical Booklet, Du Pont Company, Plastics Products and Resins Department, Wilmington, Delaware 19898, USA (in Europe available from Du Pont de Nemours International S.A., Plastic Products and Resins Department, CH. 1211, Geneva 24, Switzerland). Anon. (1979). Plast. Technol., 25(7), 93. Anon. (1979). Ibid, 25(9) 15, 17 and 31. Domininghaus, H. (1976). Die Kunststoffe und Ihre Eigenschaften, YDI-Verlag GmbH, Dusseldorf, p. 29. Frey, H. H. (1969). Abgehandelte Polyolefine, Kunststoff Handbuch, Bd. IV: Polyolefine, Carl Hanser Verlag, Munich, pp. 148-54.
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48. Blanchard, R. R. and Burnell, C. N. (1968). SPE J., 24(1), 74-8. 49. O'Toole, I. L., Reventas, A. A. and von Toerne, T. R. (1964). Mod. Plastics, 41(7), 149-52. 50. Nass, L. I. (Ed.), (1978). Encyclopedia of PVC, Marcel Dekker, New York. 51. Schwarz, H. F. and Edwards, W. S. (1974). Acrylonitrile in Macromolecules, Applied Polymer Symposium No. 25, pp. 243-59. 52. Landi, V. R. (1974). Ibid, pp. 223-31. 53. Hickcox, R. T. (1976). 34th ANTEC SPE Proceedings, pp. 491-3. 54. D'Amico, J. N. (1970), 28th ANTEC SPE Proceedings, pp. 678-95. 55. Gardner Laboratory Division, PO Box 5728, 5521 Landy Lane, Bethesda, MD 20014, USA. 56. Anon. (1979). Mod. Plast. Int., 9(2), 20. 57. Anon. Ibid, 9(4), 40-1. 58. Sonn, G. F. (1970). 28th ANTEC SPE Proceedings, pp. 445-9. 59. Rogan, J. (1980). Plast. Techno!. 26(8), 96-100. 60. Rogan, J. and Kirkland, C. (1979). Plast. Technol., 25(8), 81-4. 61. Anon. (1980). Mod. Plast. Int., 10(4), 41. 62. White, H. G. (1970). ICI Symposium: 'Additives for PVC', The Plastics Institute, Manchester, February, 1970. 63. Pattern Booklet: Pigments for Plasticised PVC, Geigy (UK) Ltd, June, 1969. 64. Ferch, H. (1974). Pigment Resin Technol., (November), 4-20. 65. Donnet, J. B. and Voet, A. (1976). Carbon Black, Marcel Dekker, New York. 66. Inman, E. R. (1967). Organic Pigments, No.1 in the 1967 Lecture Series, The Royal Institute of Chemistry. 67. Brighton, C. A. (1968). Plast. Polym., (December), 549-54. 68. Webber, T. G. (Ed.) (1979). Coloring of Plastics, Wiley-Intersicence, New York. 69. BS 5958: Part 1: 1980, Code of practice for control of undesirable static electricity, Part 1: General considerations. NB Part 2, currently in preparation, will contain specific recommendations on controlling static electricity in industrial operations. 70. Gale, G. M. and Pacitti, J. (1968). 'Antistatic agents: A critical review of the literature'. RAPRA Technical Review No. 43. February. 71. Woodland, P. C. and Zeigler, E. E. (1951). Mod. Plastics, 28(9), 95-106, 169-78. 72. McLaughlin, T. F. (1960). Mod. Plastics, 37(1), 120-1. 73. Langdon, S. J. (1964). Plastics, 43-6, August. 74. Quackenbos, H. M. (1968). Polym. Engng. Sci., 8(1), 24-31. 75. Ferraris, E. (1955). Materie Plastiche, 21(1), 53-61. 76. Diggwa, A. D. S. (1974). Plast. Polym., 42(159), 101-4. 77. Sheverdyaev, O. N., Slesarev, V. V., Tugov, I. I., Pudov, V. S. and Zhuravlev, V. S. (1976). Plast. Massy, No.6, 53-5. 78. Garret, M. D. (1977). Kunststoffe, 67(1), 38-40. 79. Collington, K. T. (1973). Plast. Polym., 41(151), 24-9. 80. Titow, W. V. (1961). Trans. Plast. Inst., 29(84),186-95.
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81. Gaskill, J. R. (1979). ASTM Standardisation News, 7(12),23-4. 82. Walker, A. G. (1980). 'Smoke from burning polymers', paper presented at the PISA Symposium on Plastics in Transport, Durban, RSA, 21st-23rd May, 1980. 83. Tadihisa, J. (1971). Res. [nst. Japan, 33, 40-4. 84. Grieveson, B. M. (1976). 'The fire hazard of polymers', paper presented at the Polymer Symposium, British Association for the Advancement of Science, Lancaster, England, 3rd September, 1976. 85. Cowan, J. and Manley, T. R. (1976). Brit. Polym. J., 8(2), 44-7. 86. Anon. (1980). Plast. Technol., 26(8), 71. 87. Colelli, C. C. and van Loenen, P. (1979). Plast. Technol., 25(7), 67-9. 88. Plastichem Ltd. (Croxton and Garry Ltd.), Technical Bulletin 3Ma/16, 'Trihyde' Fire Retardant Fillers in Plasticised PVC'. 89. Anon. (1977). Kunststoffe, 67(1), 34-8. 90. ASTM D 2843-77: Density of smoke from the burning or decomposition of plastics. 91. ASTM E 662-79: Specific optical density of smoke generated by solid materials. 92. ISOIDP 5924: Reaction to fire tests: Optical density of smoke using a dual chamber box.
CHAPTER 12
Miscellaneous Properties ofSpecial Interest in PVC Materials and Products W. V.
TITOW
12.1 INTRODUCTION All properties of a PVC material depend, in a more or less complex way, upon the formulation, and to some extent also on the effects of compounding and processing (including the 'heat history'). However, some properties are more directly associated with the nature and amount of a particular class of formulation component: for example, softness, extensibility, tensile strength and modulus are particularly strongly influenced by plasticisation, many mechanical properties are substantially upgraded by incorporation of fibrous reinforcing fillers, flame resistance improvement can depend directly on the incorporation of suitable flame retardants, and so on. In this book, properties linked in such a manner with individual formulation components and factors are considered in conjunction with them in most cases. Some general information and numerical data on properties are also given in Chapter 1 and Appendix 3. The properties discussed in this chapter are either those which might be described as more 'composite' in nature or origin, or which are relevant in some special context or application.
12.2 LOW-TEMPERATURE PROPERTIES Many PVC products, both rigid (e.g. pipes, containers) and flexible (e.g. PVC-coated tarpaulins and clothing, inflatables), are used in conditions and climates where they may experience low (sub-zero) 439
440
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temperatures. It is necessary, therefore, that their properties should not be affected by such conditions to the point where serviceability would be impaired. Securing this objective is essentially a matter of formulation, supplemented by proper compounding (to ensure good inter-blending of all formulation components) and care in processing. With uPVC the most important single consideration in the context of low-temperature service is to preserve the toughness (avoid embrittlement) of the material. Without appropriate modifiers uPVC can be brittle even at room temperature: suitable choice of modifier (and its correct incorporation in the right proportion) can ensure that the compound is, and remains, tough in sufficient measure. The effects obtainable are largely a matter of degree, since uPVC compounds (including the so-called high-impact grades) in most cases exhibit a definite, often regular, dependence of impact strength (which is related to toughness) on temperature, with, usually, a fairly well-defined brittle point, i.e. a temperature at which the mode of failure in an impact test changes from ductile to brittle (often with a corresponding drop in strength). Brittle point, coupled with actual impact strength values at the low temperatures concerned, can be used as a criterion of low-temperature performance. Other things being equal, the brittle point of a compound tends to vary inversely with the molecular weight of the polymer and the amount of impact modifier present (up to a practical usefulness limit in most cases). Although, in what might be termed the more academic context, and also in connection with product design, the fracture toughness approach l - 5 is playing an increasingly important role in the understanding, determination and prediction of the toughness of plastics, including PVC, in industrial practice the results of impact strength tests are still widely used as a measure of this complex property. This is so, partly because a large body of relevant test data has been built up over the years, and despite often comparatively poor correlation between the results of different tests, and between test results and effects of particular service conditions. Standard impact tests of interest in connection with the determination of brittle point of PVC materials include the following: ASTM D 759-66 (Re-approved 1976): Conducting physical property tests on plastics at subnormal and supernormal temperatures. ASTM D 746-79: Brittleness temperature of plastics and elastomers by impact:
12 Properties of Special Interest in PVC Materials and Products
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The method of this standard is technically similar to that of ISO/R 974:1969. BS 2782:Part 1: Method 150 D:1976: Cold crack temperature of film and thin sheeting. The method of this specification is essentially Method 10 of BS 3424:1973, but modified to give more precise determination. Although intended primarily for flexible film materials it can be used (with special care) for uPVC film and thin sheet that can be bent into a loop. Cold crack temperature is defined as the temperature at which cracks are caused when a loop of film or sheet is flattened by a blow of a test hammer under standard conditions. This temperature is usually related to (but not the same as) the low-temperature limit for actual service. ASTM D 1790-62 (Re-approved 1976): Brittleness temperature of plastic film by impact. Both in this method (which is suitable for films up to 0·25 mm thick) and that of ASTM D 746 (in which thicker, die-cut specimens are struck by a sharp edge), brittleness temperature is defined as that temperature at which 50% of the test specimens fail (i.e. break or shatter) under the conditions of the test. ISO 974-1980 gives a similar definition in terms of its test. In this, specimens 1·6 mm thick are struck a sharp blow which bends them round a shaped mandrel. ASTM D 3029-78: Impact resistance of rigid plastic sheeting or parts by means of a tup (falling weight).
The main effects of cooling of flexible PVC are stiffening, hardening, and embrittlement. The temperature of onset, and the extent, of these changes depend on the formulation. The stiffening and hardening are much more pronounced than with uPVC; they are also more serious in practice, since flexibility and softness are among the most important properties for which flexible PVC is formulated. It is therefore desirable that these properties should be retained under a wide range of service conditions, including low temperatures. Correct formulation, and especially choice of plasticiser(s) is again the key factor in ensuring good performance in this respect. The best low-temperature properties are conferred by plasticisers of the aliphatic ester type (see Chapters 6 and 7), whilst aromatic phosphates are among the least suitable plasticisers (e.g. compounds plasticised with TIP can be quite hard and stiff at freezing point).
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It is clearly important to have tests available, the results of which can assist in the evaluation and prediction of the stiffening and hardening of flexible PVC at low temperatures. The results of relevant standard tests are particularly useful in monitoring quality in production, or assessing the effects of formulation modifications in basically the same type of compound. Correlation between the results of tests by different methods is not always good, and none of the methods can be said to provide directly a value for the lowest temperature admissible in actual service (albeit they do give useful indications in that direction). Useful standard tests and the properties they determine include the following.
12.2.1 Cold Flex Temperature (Clash and Berg) This is the lowest temperature at which a standard strip specimen can be deflected through an angle of 200° under a fixed torque, in prescribed test conditions. Clash and Berg are the originators of the apparatus; the method is described in BS 2782: Part 1: Method 150 B: 1976. The cold flex temperature (normally quoted in 0c) is always higher (Le. 'warmer') than the 'cold bend' temperature (see below).
12.2.2 Cold Bend Temperature This is determined by the method of BS 2782:Part 1: Method 104A:1970: a strip specimen is cooled at the test temperature for 10 min and wound mechanically round a mandrel of prescribed diameter, at a prescribed rate. The cold bend temperature is the temperature at which this treatment causes the specimen to crack or fracture in the standard test.
12.2.3 Low-Temperature Extensibility of Flexible PVC Sheet This is the extension produced in a standard specimen at - 5°C under a prescribed load. It is determined by Method 150 C: 1983 of BS 2782:Part 1. The effect of temperature upon the stiffness in torsion (apparent modulus of rigidity) of plastics materials, including PVC, may be determined by measurement according to the method which forms the basis of the following standards: ISO/R 458-1965; BS 2782:Part 1: Method 150 A:1976; ASTM D 1043-72 (Re-approved 1981); DIN 53447:1966. A version of the Clash and Berg apparatus is used in the determination.
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Properties of Special1nterest in PVC Materials and Products
443
U.3 HEAT RESISTANCE This section is concerned with heat resistance of PVC materials and products in service and service-related tests. Although 'heat resistance' if often used synonymously with 'resistance to degradation by heat', in the present context it has a wider meaning embracing the following effects of heating: (i)
degradation of the PVC material (outwardly manifested in deterioration of properties and appearance); (ii) loss of volatile components (especially plasticisers from pPVC, with consequent hardening and stiffening, leading to deterioration of many properties); (iii) temporary, mainly reversible, changes in properties (especially mechanical properties) at elevated temperatures.
Degradation is the consequence of the breakdown of, and chemical changes in, the PVC polymer. In practice it is usually detected and/or monitored by detecting or measuring the evolution of HCI (the principal breakdown product), colour changes, and changes in properties. The methods used are discussed in Chapter 9, and some relevant standards are listed in Section 3.2 of Appendix 1. Thermogravimetric analysis and various analytical methods of identifying chemical changes in the polymer and its decomposition products are also employed in more fundamental studies of thermal decomposition of PVc. 6 Volatile loss is normally determined gravimetrically. Degradation and loss of volatiles are usually concurrent at least to some extent, and are often jointly termed 'heat ageing'. The greatest possible resistance to heat ageing is clearly essential in PVC materials intended for service above room temperature; it is also desirable in all PVC products, since loss of volatiles and/or deterioration of properties on heating in tests are frequently used as indication of likely long-term performance in these respects in service at room temperature. Such predictions, as well as those of the maximum acceptable service temperature, are usually obtained by the following method. A selected property (e.g. tensile or impact strength, modulus, etc.) is determined in appropriate tests. Specimens are then heated at a number of elevated temperatures suitably chosen in the light of practical experience or other relevant information or requirements. The property is re-determined at intervals during the heat treatment (several properties may also be monitored in this way) and the
w.
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V. Titow
100
c
o
~
c
~ 60------~ I ~
I I
& i!-
T(60),
I I
Fig. 12.1 'Percentage retention' of selected property as a function of heating time at various temperatures (81 ) 8z> 83 > 84 > ( 5), Schematic representation. 'percentage retention' (i.e. the value after a given heating time as a percentage of the original value) is plotted against log time, to obtain a family of curves as shown schematically in Fig. 12.1. The lowest percentage retention value of the test property acceptable for service purposes is decided (say, for example, 60%), and the corresponding times for the test temperatures from Fig. 12.1 (intercepts of the 60% retention line with the individual temperature curves) are plotted against the reciprocal of absolute temperature to produce an Arrhenius plot as shown schematically in Fig. 12.2. Note: On the assumption that heat degradation is a simple heat-activated 'reaction' the plot should represent the wellknown Arrhenius relationship, In k = -(E/RT) + constant (here expressing the effect of temperature on the rate of the notional 'degradation reaction') and should thus be rectilinear in the ideal case. In practice curves concave to the temperature axis may be obtained.
The time for the chosen test property to drop to the pre-selected percentage retention value at a particular temperature can be read off the curves of either of the two plots. The expected service time after which the test property will reach the value at room temperature can be found by extrapolating the curve of Fig. 12.2.
12 Properties of Special Interest in PVC Materials and Products
445
Reciprocal of absolute temperature, 11K
Fig. 12.2 Time to reduce percentage retention of selected property to 60%, as a function of the reciprocal of heating temperature «(j expressed in kelvin). Schematic representation. Methods of this type are used in deriving the 'temperature index' values 7-widely used temperature limits for continuous service, assigned to various polymers and plastics materials by Underwriters Laboratories Inc. (and published in the Modern Plastics Encyclopedia, McGraw-Hill Inc., and elsewhere). The mechanism of heat degradation of PVC is discussed in some detail in Chapter 9, in connection with heat stabilisation. Some standard procedures and equipment applicable in the determination and prediction of heat-ageing effects in plastics, including PVC, are described in the following specifications:
ISO I137-I975. * Plastics-Methods of test for the determination of the behaviour of plastics in a ventilated tubular oven. ISO 2578-1974: Plastics-Determination of time-temperature limits after exposure to prolonged action of heat. BS 4618: Section 4.6:1974: The presentation of plastics design data. The thermal endurance of plastics. ASTM D 1870-67 (Re-approved 1978):* Elevated temperature ageing using tubular oven.
* Mutually corresponding in technical content.
446
W. V. Titow
ASTM D 2115-67 (Re-approved 1980): Standard recommended practice for oven heat stability of poly(vinyl chloride) compositions. ASTM D 3045-79: Standard recommended practice for heat ageing of plastics without load.
It may be noted in passing that some mechanical properties of plasticised PVC (tensile strength, modulus) may appear to improve (in that their values increase) in the initial stages of accelerated heat ageing. This is associated with early loss of plasticiser: the tensile strength and modulus of pPVC normally vary inversely with the plasticiser content (except for some anomalous 'antiplasticisation' effects which may occur at very low contents); their values thus increase at first with the volatalisation of plasticiser, whilst at the same time this process uses up heat that would otherwise be available for thermal degradation (which reduces the strength and modulus). Although loss of plasticiser may thus appear to bring some benefit in the early stages of ageing of pPVC, it is in fact a contributory factor in the overall deterioration caused by this process. Plasticiser loss is accompanied by reduction of extensibility, flexibility and softness; internal stresses may be set up (especially where the loss is locally non-uniform) and these-either alone or in conjunction with any external stress that may be encountered-<:an promote cracking of the stiffened, hardened material; low-temperature properties (e.g. cold flex temperature) can also be adversely affected. Heat-ageing resistance is a significant consideration in connection with the service performance of such important PVC products as upholstery fabrics and cable compounds (insulation and sheathing). In both these applications, and perhaps even more particularly in the former, resistance to heat ageing of the PVC materials involved is of interest not only per se, but also because it is normally associated with good long-term stability in service at room temperature. 'Fogging' of motor-car windscreens and interiors by plasticiser volatalised from PVC-coated upholstery is an example of a special practical problem associated with a formulational factor (nature, volatility and compatability of plasticisers used) instrumental in heat-ageing resistance. Tests for stability to fogging used by motor-car manufacturers and plasticiser suppliers to evaluate PVC upholstery coatings in this regard are essentially heat-ageing tests at moderately elevated temperatures. The problem of fogging is discussed in Chapter 7.
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Properties of Special Interest in PVC Materials and Products
447
The performance of pPVC sheets, films and fabric coatings in tests for loss of volatiles at elevated temperatures is of particular practical significance, because it is often used as one of the criteria of technical and commercial acceptability (the implication being that it gives an indication of likely long-term behaviour in service at ordinary temperatures). Several standards exist; all give test methods, and some also lay down permitted loss limits: both the methods and the figures (where prescribed) are somewhat arbitrary and do not correlate particularly closely with actual service experience. The following standards may be mentioned: ISO 176-1976,* BS 2782:1970 Method 107F,* ASTM D 1203-67 (Re-approved 1981),* BS 2601:1973,t BS 3424:1973,t BS 4216:1970,t AS:j: 1441:1973 (Method 8) and AS 1440:1973, Canadian Standard 4-GP-149:1972. For a given formulation, the volatile loss from a plasticised PVC sheet, or coating on a fabric, in standard conditions, is a function of the specific surface (surface per unit volume) and hence the thickness. Theoretical considerations8 suggest that in the kind of test prescribed by most standards the total volatile loss in the test period (from 5 to 24 h, depending on the standard used) should conform to the expression L
= k(~+ ~)
(1)
where: L is the volatile loss; n is the thickness of the PVC sheet or coating on a fabric; P is the perimeter length and A the area of the test specimen; k is a coefficient dependent on the conditions (including heating time and temperature) of the test method; and b is a numerical factor dependent on the proportion of the specimen surface effectively available for evaporation. For a given set of test conditions the value of b may vary between 1 (for a coating on an impervious substrate) and 2 (sheet specimen with both sides freely accessible to the atmosphere). Figure 12.3 shows a plot of loss of volatiles from PVC films of the same composition but ranging in thickness between 0·1 and 0-4 mm, determined by Method B of ISO 176-1976(E). The theoretical curve (plot of eqn (1) with appropriate values) is superimposed (solid line). The general agreement is good: the moderate departures from theoretically predicted values at the extremes of the thickness range * See Appendix 1, Section 4.2. t See Appendix 1, Section 7. :t: Australian Standard.
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V. Titow
8
7
2 L =0205 ( %- .0·08)
o
01
02
03
Nominal film thicknQ:ss (n), mm
04
05
Fig. 12.3 Volatile loss from calendered PVC film in test according to Method B of ISO 176:1976(E), as a function of nominal thickness (~, dotted line). Solid line: plot of eqn (1) with appropriate constants (PIA = 0·08 mm- 1 for standard specimens in the method used; b = 2 for a sheet with both sides exposed). PVC film formulation: 100 PVC resin (emulsion type) 70phr Plasticisers: DIDP 4·5 phr Polymeric 3phr Stabiliser 0·5 phr Lubricant
covered by the tests must be ascribed to experimental error (or some other factor, possibly thickness-dependent); otherwise-if the two extreme results were valid-extrapolation of the experimental curve would predict that volatile loss should cease altogether at a film thickness of about 0·5 mm, and that it should be only about 5% for an infinitesimally thin film, when, in fact, with an almost infinite specific surface, loss of most of the monomeric plasticiser can be expected,
12 Properties of Special Interest in PVC Materials and Products
449
(i.e. up to about 40% volatile loss from this particular compositionsee caption of Fig. 12.3). Cable compounds are rated, inter alia, according to the conductor temperature to be withstood in continuous service, but the effects of still higher temperatures which may be experienced occasionally (e.g. heat shock resulting from a short-circuit) are also relevant. The pertinent standard tests typically involve heating specimens (which may be under mechanical stress) for relatively short periods at high temperatures-e.g. 1 h at 150°C (the heat-shock test of BS 6746:1976, in which no visible cracks must be caused by the treatment), or 40 to 120 min (depending on the type of insulation material) at 200°C (the thermal stability test of VDE* 0271/3.69, in which the pH indicated by a standard indicator exposed to the volatile products evolved must not be less than 3). Heat-ageing tests are also applied in which specimens are heated for longer periods at somewhat lower temperatures, with pass requirements in terms of permitted volatile loss and property change maxima (as in BS 6746:1976) or absence of cracks (as in VDE 0271/3.69 after 7 days at 120°C). Although such requirements may be coupled with particular service-related temperature rating (e.g. for a nominal 90°C cable sheath compound, suggested volatile loss limit of 2·5 mg m- 2 on heating for 7 days at 115°C9 ), the test results do not, in the main, provide a reliable basis for firm predictions of behaviour in service. Apart from the question of thermal degradation and ageing generally, the thermoplastic nature of PVC, coupled with its comparatively low softening temperature (lowered still further by plasticisation), is a limiting factor on the maximum service temperature. The limit for most types of cable compound is 70°C (conductor temperature). Some special, high-temperature cable compounds are rated at 85°C (and even up to 105°C for limited periods under certain conditions) but at high temperatures external damage to, or distortion under pressure of, the heat-softened material, as well as the possibility of outward penetration of the hot conductor through it, become serious considerations. Susceptibility to damage, at room temperature, by hot objects or materials (e.g. soldering irons, hot oil splashes) may also have to be considered. The performance of PVC cable compounds can be substantially improved in these respects by cross-linking the * Verband Deutscher Elektrotechniker (specification of the German Electrical Engineers Association).
450
W. V. Titow
cable covering, and this has been receiving attention in recent years, with special reference to irradiation techniques, including the use of electron beams. 10,11 Some of the direct (i.e. non-degradative) effects of heat on the properties of PVC are illustrated by the data of Figs 12.4-12.6. As with thermoplastics in general, mechanical properties are reduced at elevated temperatures, as is resistance to creep and fatigue; however 90 80 1... If
70
~60
Coo
::l
.
1/1
~40
C
~30
:g20 10
o
o Fig. 12.4 Effect of temperature on tensile strength and BS softness number of PVc. 6, uPVC (normal impact strength grade); 0, pPVC (50phr noP).
12
Properties of Special Interest in PVC Materials and Products
451
g·mil
1·0
100 in2 ·24h·atm
Fig. 12.5 Permeability of a uPVC (homopolymer) film to water vapour as a function of temperature. 12
A
'"'Eu 10
0
-52
..»x I:
". ' 3" "~
.... . 1i .....
8
6
Q,
u
I:
4
0
12
2
o
10
20
30
40 50 60 70 Tllmpcrraturcr. ·c
80 90
Fig. 12.6 Effect of fibrous reinforcement on the temperature dependence of tensile creep of uPVC (ICl's Darvic 110). Curve A, material with 30% glass fibres oriented parallel to the test direction; Curve B, as A, but fibre orientation perpendicular to test direction; Curve C, unreinforced material.
452
W. V. Titow
uPVC is more thermally sensitive in this sense than the other common 'engineering' thermoplastics, and its softening temperature is comparatively low (see Appendix 3). The reduction of some of the mechanical properties can be counteracted to some extent by incorporation of fibrous reinforcement. Whilst improvement in the heat deflection temperature under load is only moderate with, say, 30% glass-fibre reinforcement (increase of 5-10°C in some cases) creep at elevated temperatures can be significantly reduced (see Fig. 12.6). A non-degradative (though irreversible) effect of heat, which can be a practical problem in service, is the longitudinal shrinkage of extruded cladding and beading (finishing strip) sometimes occurring on buildings and caravans. This is attributable to retraction of stretch imparted during manufacture (extrusion and take-off). The principal ways of securing the best practicable resistance of PVC materials to heat degradation are proper stabilisation (by suitable stabilisers in appropriate quantities, properly dispersed in the compound by suitable mixing-see also Chapters 4, 9 and 10) and-where possible-the use of fillers and pigments which can afford additional protection. Volatile loss can be counteracted by proper choice of plasticisers (polymeric plasticisers are particularly useful); use of polymeric additives with plasticising action (e.g. copolymers of the Elvaloy* type, highly chlorinated polyethylene, or nitrile rubber); ensuring that plasticisers and all other constituents of the compound are of good, consistent quality; and by general attention to the mutual compatability of all constituents. 12.4 PERMEABILITY
In the broadest sense permeability may be thought of as the ease (or otherwise) with which a material will allow the passage of a penetrant. In plastics this property is of special practical interest in connection with barrier applications, Le. where these materials are used to exclude or contain potential penetrants (most often fluids, i.e. gases, vapours or liquids). For such purposes the lowest possible permeability vis-a-vis the penetrant(s) concerned is desirable: examples of barrier applica* Du Pont trade name for a group of modified ethylene/vinyl acetate copolymers.
12 Properties of Special Interest in PVC Materials and Products
453
tions of PVC are films (e.g. packaging films, rainwear, inflatables), containers (e.g. blow-moulded bottles, sachets) and protective coatings (e.g. on chain-link wire fencing, industrial gloves, tarpaulins). Such applications may be regarded as particular instances of the general case of the system penetrant(s)/plastics barrier in which low permeability is required (for example the 'passage' of water through a good tarpaulin is virtually nil), although it may be significant (as manifested, for example, in loss of contents from a shampoo sachet made from badly formulated PVC). It can readily be seen, largely as a matter of common sense, that the permeability of a plastic towards a given penetrant should be a function of the chemical nature, morphology and composition of the plastic, and that the barrier thickness and area involved must also be factors. This is indeed so, and the relevant relationships may be expressed, in a general way, by the following equation applicable in steady-state conditions: J/A
= P(F/L)
(2)
In this expression J is the flux of the penetrant, i.e. the amount (in suitable units, e.g. of mass or volume, or in moles) passing through the barrier in unit time; A is the area through which the passage takes place; F is the driving force actuating the passage, e.g. the difference between the concentrations (~c), pressures (~p), temperatures (~e) or chemical potentials (~Il), of the penetrant at the 'upstream' and 'downstream' boundaries of the barrier; L is the barrier thickness, and P is the permeability coefficient which embodies (i.e. whose value will reflect) the effects of chemical and physical interactions in the system (themselves dependent on the chemical nature and fine structure of the barrier material). As can be seen, F/L is the driving force gradient within the barrier. The various quantities in eqn (2) may be expressed in any convenient units, so long as those are mutually consistent; their choice will determine the units (and hence will influence the numerical value) of P. In practice the permeability of plastics is commonly determined by measuring-in standardised, steady-state conditions-the flux of a penetrant (usually water vapour, oxygen, carbon dioxide or nitrogen) through a film or sheet. Loss of contents (in terms of weight loss over a period of time, i.e. average flux) through the walls of a container is also often measured.
454
w.
V. Titow
The results of such determinations can normally be interpreted in terms of a version of eqn (2), or some analogous expression, but they may be (and usually are!) stated in various ways and in a variety of units. This diversity complicates comparison of results from different sources and can lead to confusion associated with, or compounded by, the following factors. (i)
Even if the same basic units are used (for flux, driving force, film thickness, etc.), expression of the result of one and the same determination (or results of separate determinations for the same plastic/penetrant system in identical conditions) can entail different combinations of the units and have different numerical values, depending on the choice of the 'index' of permeability (Le. whether permeability coefficient, rate of transmission, permeance, etc.-see Table 12.1). (ii) The same 'index' of permeability, expressing the result of the same determination, will have different numerical values if given in different units. (iii) Values are sometimes quoted simply as 'permeability' without a clear statement which 'index' is being used, so that units have to be scrutinised to ensure correct interpretation of the figures. Moreover, the same name is sometimes used for different permeability indices. This situation is illustrated by Table 12.1 in which the more common ways of expressing permeability are listed, with the associated units and an explanation of their mutual relationship (in terms of eqn (2)). Since for a given penetrant/barrier system in defined conditions the permeability coefficient should be a constant, expressing the relevant intrinsic properties of the barrier material, this coefficient should-in theory-be the best value to use as an index of permeability, especially when comparing different plastics materials. It is often used for these purposes, although here again the variety of units employed can cause some confusion. The forms of permeability coefficient which are either fairly widely used, or are of a more standard or fundamental nature, are given in Table 12.2. A useful discussion of gas permeability coefficients has been published by Yasuda. 12 However, the permeability coefficient is by no means solely or universally employed as an index of permeability, because in practice most plastics are multicomponent (often multiphase) materials; they may be heterogeneous not only by virtue of these features per se, but
12
Properties of Special1nterest in PVC Materials and Products
455
also because the distribution of the various additives in the polymer may not be uniform. Moreover, local differences in density and fine structure may occur (e.g. due to orientation, or 'skin-and-core' effects in, say, sheets and mouldings) as well as irregularities in thickness. Thus the concept of uniform flux per unit thickness of a homogeneous material, implicit in the definition of the permeability constant, may not be applicable in practice, and it may be more realistic to quote, say, transmission rates through a film or container wall of stated average thickness, in strictly defined conditions. The permeability considered so far is that of plastics materials in continuous, i.e. non-porous form: even the cellular plastics mentioned in Table 12.1 are of the kind in which the cells are separated by walls of 'solid' material, and thus are not inter-communicating pores. In its passage through a plastics barrier which is 'solid' (continuous) in the above sense, the penetrant will normally be in a molecular state of division:* it may be considered to dissolve in the plastic at the 'upstream' face of the barrier, diffuse through, and come out of solution at the 'downstream' face. In this diffusional transport mechanism the chemical nature of the penetrant and the barrier material. as well as the composition and fine structure of the latter, will be cardinal factors. Moreover, the state of the plastics material (i.e. whether glassy or rubbery) can affect the rate of diffusion. For a given composition (e.g. a particular PVC compound) the state will depend on the temperature and may also be influenced by the sorbed penetrant concentration: this effect can be especially strong if the penetrant interacts strongly with (i.e. is a good solvent or plasticiser for) the polymer. 13 For simple gases, and many other penetrants sorbed from the vapour phase, the solubility in the plastics barrier material will-under certain conditions-be proportional to the partial pressure (i.e. Henry's law will be obeyed), and the rate of diffusion will be directly proportional to the concentration gradient (i.e. Fick's law will be obeyed). In such cases permeability may be defined by the expression P=DS
(3)
where P is the permeability coefficient (as in eqn (2)), D the diffusion * Clustering may occur inside the barrier material in some cases, but this is exceptional rather than typical.
J/A
J/A
J/A
J/(A.~p)
(J.L)/(A.~p)
Transmission rate
'Permeability'
1. Transmission rate
2. Permeance
3. 'Permeability'
Film or thin sheet
Flexible sheet
Sheet up to about 1/8 in thick
Water vapour
Water vapour
Water vapour
Le. =P
J/A
In terms of eqn (2)
Transmission rate
Name
Description
19.cm m2.24 h.(mm Hg)
1 grain.in ft 2.h.(in Hg)
Perm-inch = Metric permcentimetre =
19 m2.24 h.(mm Hg)
1 grain ft2.h.(in Hg)
grain ft2.h
Metric perm =
Perm =
or
WVT
g m2.24h
g m2.24h
g m2.24h
-
-
g m2.24h
units
Constituent simple
-
Special unit name and/or symbol
htdex of permeability
Film or thin sheet
Plastics barrier
Water vapour
Penetrant
TABLE 12.1 Relevant determination methods, references and remarks
Temperature RH
Barrier thickness (L) Temperature RH
Barrier thickness (L) Pressure differance (~p) Temperature RH
Barrier thickness (L) Pressure difference (~p) Temperature RH
Barrier thickness (L) Pressure difference (~p) Temperature RH
E 96-66(1972)
~ASTM
BS 3177: 1959
BS 2782:Part 8: Methods 820 A to G (in preparation)
ISO/R 1195-1970 Barrier thickness (L) Pressure difference (~p)Q Temperature RH
Data required in conjunction
Some Common Ways of Expressing Permeability
~
:<:: :::l 0
~
.j:>.
'-" a..
J/(A.t.p)
(J.L)/(A.t.p) i.e. =p
J/(A.t.p)
J/(A.t.p)
(J.L)/(A.t.p)
1. Permeance (for boards with surface skins)
2. 'Permeability' (for boards without surface skins)
Transmission rate
Transmission rate (also termed 'permeability')
1. Transmission rate
2. Permeability coefficient
Expanded polystyrene (board)
Films and thin sheet
Films and thin sheet
Film and sheeting
Water vapour
Gas
Gas
Gas
i.e.
=P
J/(A.t.p)
J/A
Rigid cellular Transmission rate plastic (block)
Water vapour
=
=
=
Punit barrer
=
GTR unit
-
also
Barrier thickness (L) Temperature RH
Barrier thickness (L) Temperature RH
Temperature RH
Barrier thickness (L) Temperature RH
As for ISO/R 1195
10- 10 P unit
Barrier thickness (L) Temperature RH 1 cm 3 (at STP).cm T 2 emperature cm .s.(cm Hg) RH
1 cm 3 (at STP) m2 24 h.atm
ml (atSTP) m 2 .24h.atm
cm 3 (at STPj" m2 .24h.atm
--
1 fm b Pa.s
ng.m N.s
-
SI unit
ng N.s
g m2 .24h
-
WVT
J
I
ASTM 01434-75 NB This specification also recognises (and gives conversion factors for) the following units of P: (cm3 .mil)/ (m 2 .24 h.atm); (cm 3 .mil)/(lOO in 2 .24 h.atm); (cm 3 .mm)/(m2 .24 h.atm)
BS 2782:Part 5: 1970 Method 514 A (in 1979 replaced by new method 821A of BS 2782 Part 8: 1979-see also ISO 2556)
ISO 2556-1974 (identical with BS 2782 Part 8: Method 821A 1979)
BS 3837:1977 (prescribing tests by the method of BS 4370: Part 2: 1973) technically similar to ISO/R 1663
ISO/R 1663-1970
...
~
-..l
Vl
!:;
"
$::
I::>..
~
;:< I::>..
I::>
l:;-
...5'~
I::>
~
()
~
'"0
~ S·
~
[" S'
.g> '"
~
~
1'"?to
...... tv
Plastics barrier
Resistance factor Any plastics lamina (in practice film or sheet)
Permeability factor
Name
(A.t1p)/J i.e. = lI(P/L)
(J.L)/A
In terms of eqn (2)
Description
-
p1
Special unit name and/or symbol
Index of permeability
b
a
Fixed in terms of the RH in standard determinations. i.e. one femtometre per pascal per second = 8·752 em' (at STP)/(m 2 .24 h.atm). c STP: 760 mm Hg and 273 K. d i.e. permeability factor at temperature I.
Any (but mainly fluids)
Walls of Any (consti. plastics contuents tainer of contents of a plastics con· tainer)
Penetrant
N.s mol
--
g.cm m 2.day
Constituent simple units
TABLE 12.1-eontd.
Barrier thickness (L) Temperature RH
Temperature RH
Data required in conjunction
Ref. 90
ASTM D 2684-73
Relevant determination methods, references and remarks
~
:;;
0
:::J
~
~
00
Pressure" difference (dp)
m2
"Or partial pressure, as applicable. bAt STP.
g (24 h)-I
Water vapour
Pressure" difference (dp)
100in2
b
Gas or vapour
cm 3 (24 h)-l
b
Gas or vapour
Pressure" difference (dp)
Pressure" difference (dp)
cm 2
m2
mols- I
Concentration difference (dC)
Concentration difference (dC)
Nature
Pa(=Nm- 2 )
mol cm- 3
molm- 3
Units
mmHg
atm
cmHg
(F)
Driving force
cm3 S-I
cm 2
mols- I
Any
Gas or vapour
m2
Area (A) units
mols- I
Flux (J) units
Any
Physical state of penetrant in contact with barrier
(L)
cm
mil
cm
m
cm
m
units
Thickness
g (mm Hg) --= p 24h.m 2 cm
mol N -= Ps.m 2 m2 .m b cm 3 (cm Hg) --= p--s.cm 2 cm b cm 3 atm =P24 h.l00 in 2 mil
mol mol --= Ps.cm 2 cm 3 cm
mol mol -= Ps.m 2 m3 .m
Equation (2) in terms of the units
TABLE 12.2 Some Common Fonns and Units of the Permeability Coefficient
g cm (24 h)-I m- 2 (mm Hg)-I
cm 3 mil (24 h)-I (100 in 2 )-1 atm- I
cm 2 s (cm Hg)-l
"tl
molms- I N- 1
~
.,.. ID
v.
I;;;
'"
i:
~
~
;:s
l::l
;;;-
l::l
:::!.
~
~
(j
-.::::
~ S·
~ ....
S-
'"~
"-
~
~
~
".... ::>.
.g
~
......
'"
cm 2 S-I
m 2 s- 1
Resulting units of permeability coefficient (P)
w.
460
V. Titow
coefficient (assumed to be constant or dependent only on concentration) and S is a solubility factor expressing the equilibrium content of the penetrant in the plastics barrier material at the relevant temperature. If S is given in the form of a ratio, viz. S
=
equilibrium concentration of penetrant in barrier material concentration of penetrant 'upstream' of barrier
with both concentrations expressed in the same units, it becomes a dimensionless partition coefficient, and P will have the same dimensions and units as the diffusion coefficient (length2 time- 1-ef. the first two entries in Table 12.2). Solubility factors and Fickian diffusion coefficients can be determined fairly readily for plastic/penetrant systems of practical interest. However, there is still no general, complete explanation of the way in which the chemical structure and interactions of the components in such systems influence the mechanisms of solubility and diffusion. There is also no comprehensive theoretical treatment available for diffusion of penetrants in polymers below the glass temperature (Tg ), where complex time-, history- and concentration-dependence may occur. This is relevant in connection with rigid PVC compounds which are essentially glassy (non-crystalline) with Tg values well above room temperature. Flexible PVC of medium and high plasticiser c;ontents is normally rubbery, as plasticisation reduces the effective Tg • A more extensive discussion of the mechanisms and theories of diffusion in polymers would be beyond the scope of a short section in a technological book on PVC. Informative accounts are available in publications by Fujita 14 and Meares l 5-17 (theory and various features of diffusion), Crank and Park 18 (most aspects), Stafford and Braden 19 (diffusion of water in polymers). The permeability of a porous plastics material (i.e. one containing inter-communicating voids) is a matter of the rate of passage of the penetrant concerned through the pores. On the assumption that the pores are large in comparison with the size and the mean free path of the penetrant molecules (the situation of practical interest in this section), two general cases may be distinguished. (i) The penetrant may be moving through a fluid (in practice most frequently air or water) which fills the pores but does not itself flow through them to an appreciable extent. The transport
12
Properties of Special Interest in PVC Materials and Products
461
mechanism will be 'straight' diffusion, or movement under a directional driving force, e.g. the electrical force actuating the passage of ions through the water filling the pores of a PVC battery separator: apart from this example there are not many instances of this kind of permeability relevant to practical applications of PVC. Equation (2) can be applied to describe the general relationships in this case, but here the permeability coefficient will represent essentially the effects of interactions between the components of the system penetrant/pore-filling fluid integrated with those of any other factors or constraints operative in the system. (ii) The penetrant, in this case itself a fluid, may be flowing through the pores. The transport mechanism will be viscous flow (i.e. the penetrant will continue to behave as a fluid whilst in the pores): in practice the driving force will most often be a pressure difference ('pressure drop') across the porous barrier. In this case the relevant relationships can still be represented, in general terms, by an expression closely analogous to eqn (2). However, the expression must contain a viscosity factor, since the viscosity of the fluid penetrant is an important parameter. This is exemplified by the well-known D'Arcy equation which may be written in the form J/A
= K[~p/(L1])]
(4)
where 1] is the viscosity of the penetrant, K is the permeability coefficient, and the other symbols have the same meanings as in eqn (2). For a given penetrant under standard conditions K represents the combined effects of the internal structural characteristics of the porous barrier (with the total volume, configuration and size of the pores as especially important factors). As can be seen, in neither of the above two cases are the chemical and other intrinsic properties of the solid material of the barrier particularly important. Hence, say, porous uPVC may be expected to behave much like any other solid porous barrier of the same porosity characteristics. For this reason the permeability of porous PVC will not be considered further. The books by Carman20 and Scheidegger,21 and the excellent papers by Rodebush and Langmuir,22 Davies,23 Iberall,24
462
w.
V. Titow
and Thomas 25 may be referred to for further information on flow of fluids through porous media. Many aspects of the permeability of 'solid' (i.e. non-porous) plastics, with special reference to films and coatings, are discussed in a collective publication edited by Hopfenberg. 26 Two useful, earlier publications by Lebovits,27 and Hennessy et al. 28 review the permeability of polymers and plastics to fluids. The latter work deals in considerable detail with practical methods of determination. In virtually all procedures for determining the gas permeability of polymeric barriers, the amount of gas passing through in measured time is determined from the volume change at constant pressure, or the pressure change at constant volume (the manometric method), or the contents of the penetrant gas in a carrier gas 'upstream' and/or 'downstream' of the barrier. The equipment used is normally an appropriate 'gas cell' (e.g. a variant of the so-called 'Dow cell' as employed in the method of ASTM D 1434-see Table 12.1). Various types of apparatus and their commercially available embodiments are mentioned in a paper by Pye et al. ,29 which also describes a particularly versatile gas-permeability apparatus (of the manometric type) developed by the authors. Water vapour permeability is most often measured by 'dish' or 'sachet' methods, prescribed by several of the standards listed in Table 12.1. In a 'dish' method the specimen forms the closure (secured with a vapourtight seal) of a dish or beaker containing a desiccant. The container is placed in an enclosure at a constant temperature and controlled (normally high) relative humidity, and the passage of moisture through the specimen is followed by repeated weighing. In the sachet method, a sealed sachet containing water is placed in a constant-temperature enclosure in which the humidity is controlled at a low value. The sachet is weighed periodically to determine the rate of loss of water. Other methods are also available. 28 Permeability to other penetrants may be determined by methods similar to those used for gases and moisture. The container method of ASTM D 2684 (see Table 12.1) is also used. In some cases the amount of penetrant passing through a plastics barrier in measured time is determined by chemical, chromatographic or gravimetric techniques. 28 Considerable success has been claimed for predictions of the long-term barrier performance of plastics containers in service, based on permeability coefficients calculated with the aid of the 'permachor'
12
Properties of Special Interest in PVC Materials and Products
463
concept, first developed-largely on an empirical basis-in the 1950s in connection with the storage properties of blow-moulded polyethylene bottles. 30 ,31 The method is to calculate a so-called 'permachor value' for a given potential penetrant by summating appropriate values allocated to its constituent atoms and/or functional groups: in this respect the approach is somewhat similar to that involved in calculating solubility parameters. 32 Certain corrections are applied, and the final figure is substituted into the equation log P = K - Rn
(5)
where P is the permeability coefficient, K is a temperature correction factor and R is another correction factor associated with the polymer (or polymer compound). R values of 0·22 and 0·26 have been quoted 31 as appropriate for polyethylene (apparently regardless of density) and polypropylene, respectively, with values of K varying not only with temperature but also with polymer density (for polyethylene) and nature (e.g. K = 2·3 for polyethylene of density 0·945, and K = 4·2 for polypropylene, both at 23o C).31 So far no analogous data appear to have been published for PVC. Computer prediction of long-term gas transmission characteristics of polymers in service has been discussed by Horsfall and James. 33 Unplasticised PVC is a reasonably good barrier against simple gases and water vapour. Taking the relevant permeability value of unstretched uPVC 40,um film as unity in each case, some typical comparative permeabilities of similar films of other polymers would be as follows: Nylons
O2 N2 CO 2 H2O
Polyolefins
6
6·6
11
HDPE
LDPE
PP
0·3 0·7 0·6 8
0·6 0·6 0·4 2
4 2·5 7·5 0·3
22 44 36 0·1
57 146 125 0·4
22 36 30 0·3
Permeability to other penetrants depends strongly on their chemical nature. To the extent to which it is valid to generalise over a broad range of conditions and potential penetrants, the following points may also be
464
W. V. Titow
made. Other things being equal, PVC compositIOns based on homopolymers are less permeable than those based on copolymers with vinyl acetate, but more permeable than vinyl chloride/vinylidene chloride copolymer compositions (ct. Table 12.4). Blending with other polymers tends to increase permeability, but the opposite effect can occur: even with the same component in a blend the permeability can increase or decrease depending on the proportions and processing conditions. Such effects have been observed, e.g. in the case of penetration of oxygen and nitrogen through blends of PVC with ethylene/vinyl acetate copolymer (EVA):34 they can be explained in terms of the way in which composition and processing influence the morphology and phase structure of a blend, both factors affecting the rate of diffusion of penetrants. Stretch-orientation can reduce the permeability of uPVC film by a factor of 2 (more in some cases). Plasticisation, and generally incorporation of fully intermiscible additives (e.g. impact modifiers, processing aids, internal lubricants) tends to increase the permeability of PVC to most penetrants. However, transmission of water vapour may be reduced in some compositions by the inclusion of chlorinated plasticiser extenders. Addition of fillers, which form a separate phase in the compound, TABLE 12.3 General Order of Permeability of PVC to Some Common Penetrants at Room Temperature
Units: cm 3 (at STP).cm.cm- 2 .s- I.(cm Hg)-I. Units: g.mil.(lOO in 2 )-1.(24 h)-latm- 1. C Typical, approximate factors representing order of change in comparison with values for comparable uPVC compounds. a
b
12
Properties of Special Interest in PVC Materials and Products
465
may have no effect on permeability, or it may reduce or increase it, depending on the penetrant, and on the frequently complex effects of the chemical nature, particle size and shape, distribution, orientation, amount, and surface properties of the filler. The reduction in water vapour transmission sometimes brought about by the incorporation of relatively small amounts of certain waxes is probably associated with the incomplete compatability and hydrophobic nature of this type of additive. Some permeability data are given, by way of illustration, in Tables 12.3-12.5. TABLE 12.4 Permeability of PVC Homopolymer and Copolymer Compounds to Gases
TABLE 12.5 Water Vapour Transmission of Breon Films 0·004 in Thick, Containing Various Plasticisers (Room temperature, dish method at 100% RH) Plasticiser
DOP Oetyl deeyl phthalate Trioetyl phosphate Tritolyl phosphate Dioetyl adipate Cresyl diphenyl phosphate
1·15 1·35 1·61 1·70 1·77 1·91
466
W. V. Titow
The rate of transmission of water vapour (in a dish test with desiccant, at room temperature and 100% RH) through some commercial PVC sheeting products is illustrated by the following figures (grams per square metre per 24 h): Rigid sheet (homopolymer-based) 0·050 in thick: Pipe-wrapping PVC tape with polyisobutylene adhesive layer (PVC thickness 0·013 in, adhesive layer 0·008 in): General purpose adhesive tape (PVC layer 0·010 in thick):
0·1 0·6 2·5
It will be clear from what has been said in this section, that the results of permeability tests are valid and their numerical expression meaningful only for strictly standardised test conditions, which should be specified. Moreover, even when these requirements are met, and the results are given in appropriate units, comparison between sets of data from different sources should be made with caution, as materials of apparently the same kind (say two rigid PVC films of comparable thickness) may differ in composition or nature of individual constituents (e.g. different lubricants, processing aids, stabilisers, may be present), processing history and fine structure. Differences in the performance of basically the same equipment in two different laboratories may also be significant.
12.5 ENVIRONMENTAL STRESS CRACKING AND CRAZING
Plasticised PVC is not normally susceptible to failure of this kind. uPVC can be, albeit cases are not so common or frequent as to constitute a serious practical problem: however, since they do occur, the subject merits a brief discussion. Crazing is a characteristic, localised deformation of a polymeric material. A craze may be defined, after Kambour,35 as a thin, plate-like region containing apparently continuous but in fact microcellular material, usually oriented, interconnected with the surrounding 'normal' material. Craze matter has the general characteristics of open-cell foam. In most cases it is load-bearing, but-as would be expected-its mechanical properties are different from (and generally inferior to) those of the solid 'parent' material. 35,36 Thus crazes constitute regions of comparative weakness in a polymeric material, often discernible visually because of their opacity.
12
Properties of Special Interest in PVC Materials and Products
467
Cracking is a type of complete, local brittle failure. The term 'crack' needs no special definition, but it may be recalled in passing that cracks typical of environmental stress cracking of plastics are wedge-shaped, and range in size from hair-cracks to large discontinuities, depending on the material and conditions. Environmental stress cracking and crazing may jointly be termed 'environmental stress failure' (ESF). The cardinal features of ESF are that it occurs in the presence of an 'active environment' (a cracking or crazing agent, which is normally a fluid, i.e. a gas, vapour or liquid), and either in the absence of externally imposed stresses or at external stress levels significantly below the normal short-term yield strength of the material in air. In practice the stresses responsible (jointly with the active environment) for ESF can be either internal, or external or (as is often the case) a combination of both. As has been pointed out by Ziegler,37 internal stresses may arise from the following causes: (i)
differential cooling of the polymer in the final stages of moulding (thermally induced moulding stresses); (ii) molecular orientation, with or without crystallisation, arising as a result of the flow and the cooling process in the course of moulding (orientation stresses); (iii) forced introduction of additional polymer into the mould when that already present in the mould has begun to cool and contract (packing stresses); (iv) thermal shock either to the whole or to parts of the moulding, caused by treatments such as, for example, machining (thermal stresses).
The external stresses instrumental in ESF may be those encountered in any processing or handling after moulding, or in service, or those deliberately applied in tests. Some environments (e.g. certain organic liq4ids), strongly active as cracking agents for certain glassy polymers, can promote ESF under stress two orders lower than the minimum cracking or crazing stress in air at the same temperature. 36 Under given conditions the environmental stress failure of a plastic will occur when local elongation within the material exceeds the maximum such elongation that the material can accommodate without failing. This is essentially the concept of critical strain, which has been defined 38 as that minimum strain at which under a particular set of conditions stress cracking or crazing is known to start. The main
468
W. V. Titow
factors making up the 'set of conditions' are the nature of the environment (i.e. the cracking or crazing agent), the stress, the temperature, and the rate of strain. Most of the methods used to study ESF are ultimately concerned with the determination of critical strain and the associated (critical) stress, by various means and in various kinds of apparatus. 39 Practical tests, applied in production control and occasionally in 'trouble-shooting' investigations, are mainly qualitative. They usually involve immersion of a moulding in a liquid (frequently a mixture of solvents) known to be active as a cracking or crazing agent for the plastic concerned: development of cracks or crazes within a prescribed period is taken as evidence of the presence of internal stresses of unacceptable magnitude, and hence risk of failure in service under conditions conducive to ESF. 39 Standards of main, general relevance to the understanding and investigation of ESF phenomena in plastics include the following (some still in draft form at the time of writing). ISO 4600:1981: Plastics-determination of environmental-stresscracking resistance (ESCR)-ball or pin impression method. ISO 6525:1981: Plastics-determination of resistance to cracking under constant tensile force in the presence of chemical agents.
BS 4618: Subsection 1.3.3:1976: The presentation of plastics design data. Environmental stress cracking. British Draft Document 79/50196:3/79: Determination of environmental stress cracking resistance. DIN 53 449:1970: Testing of plastics; evaluation of resistance of thermoplastics to environmental stress cracking; steel ball impression method. This specification is technically similar to ISO 4600.
Other standards (largely specific to particular materials and/or products) as well as many literature sources on ESF are given in Ref. 39. Two main hypotheses have been advanced to explain the origin and mechanism of ESF in glassy polymers (of which PVC is one). According to one, the 'active environment' reduces-by wetting-the surface energy for crack or craze formation, and thus facilitates and promotes growth of holes (craze cells and cracks) from minute voids in the polymer which serve as nuclei. The second explanation is the
12
Properties of Special1nterest in PVC Materials and Products
469
so-called 'plasticisation hypothesis'. Currently this appears the more firmly supported by the available evidence, although it is possible that surface energy reduction may be a contributory factor. According to the plasticisation hypothesis, the cracking or crazing agent actually penetrates and plasticises the polymer. As is well known, plasticisation reduces the temperature of glass-to-rubber transition (Tg ) as well as the modulus and viscosity of a polymer. When operating locally at the tip of a forming crack or craze these effects should make the polymer yield more easily under the high concentration of stress obtaining at that point, but because the degree of plasticisation is still relatively low (in comparison with, say, that of bulk-plasticised flexible PVC materials) the critical strain is not substantially increased: the net result is that propagation of a craze or crack is facilitated and promoted. In flexible (bulk-plasticised) PVC the plasticiser concentration, and hence the degree of plasticisation, are relatively high. The material is well above its effective Tg at room temperature (i.e. in the rubbery state): it is thus able to relax sufficiently readily for internal stresses to dissipate rapidly, so that this important contributory factor to ESF (internal stress) does not normally exist in plasticised PVc. Moreover, pPVC is extensible and its critical strain is consequently relatively high. These are the main reasons why environmental stress failure is not normally encountered in plasticised PVc. Under suitable conditions (including sufficiently high contact concentration of agent, and magnitude of stress) some aliphatic and aromatic hydrocarbons (or their mixtures) in liquid or vapour form can act as cracking or crazing agents for uPVc. This has been demonstrated in the laboratory, e.g. for n-hexane and toluene. 4o In the author's own recent experience, local cracking (and a subsequent burst) in service of a 90-mm high-pressure uPVC water pipe occurred in a section found to have been in temporary contact with toluene (through accidental spillage). In an earlier case, investigated in Holland in 1967 by Wolf,41 the appearance of hair-cracks in a uPVC pipe conveying natural gas was identified as environmental stress cracking promoted by naphthalene found to be present in the Dutch gas in appreciable concentration.
U.6 WEATHERING RESISTANCE Weathering may be broadly defined as the overall change in a material, brought about by outdoor exposure. In PVC the change is the
470
w.
V. Titow
combined effect of several factors, interacting and affecting the material in a highly complicated manner: thus from the technical standpoint the weathering of PVC is a composite phenomenon, complex in both its mechanism and nature. Some of the principal factors instrumental in this process are listed in Table 12.6, with indications of their main effects on PVc. However, it is a cardinal characteristic of weathering that the factors never operate singly in actual service conditions, but co-act and interact in many ways: moreover their individual roles, and their contributions to the overall result, can vary considerably depending upon the place and conditions of exposure; seasonal variations at one and the same site also affect the progress of weathering. Extensive practical experience and data are available to aid producers in formulating PVC compounds for good weathering resistance, particularly important in such uPVC products as house and caravan cladding, rain gutters and down-pipes, and window frames, as well as in pPVC garden hoses, reservoir linings, tarpaulins and outerwear. However, because of the complexity of the weathering process and wide variability of service conditions, confident prediction of the performance, and in particular of the likely length of useful service life, is still very often difficult. Tests to help such predictions, and to enable different compounds to be compared or effects of formulation changes to be evaluated, are clearly desirable and important. Any test method must include two elements: (i) means of inducing in the material changes identical with, or equivalent to, those caused by actual weathering in service over relevant periods; and (ii) means of assessing or measuring the degree of changes caused (extent of deterioration). There are three general ways of inducing weathering (or similar) effects in a plastics material for test purposes. 1. Exposure of specimens at outdoor sites in locations and conditions corresponding to (or correlating with) those to be encountered in actual service, and for periods comparable with the service life contemplated. 2. Outdoor exposure at selected sites, with the time scale substantially shortened by suitable intensification of at least some of the main factors instrumental in 'natural' weathering (e.g. amount
12
Properties of Special Interest in PVC Materials and Products
471
and intensity of incident sunlight, amount or frequency of contact with water), or by increasing the susceptibility of the material specimens to weathering, or both. 3. Artificial ageing in the laboratory in conditions designed to produce, in a much shorter time, changes similar to (and/or sufficiently well correlating with) those caused by natural weathering. In all three cases the effect of exposure may be assessed in terms of changes in optical properties (e.g. colour changes-with PVC often darkening or yellowing-or changes in surface gloss, or in transparency), or differences between the 'before' and 'after' values of other properties (with PVC products often tensile and/or impact strength, modulus, elongation). Some standard methods relevant to evaluating weathering effects in this way are given in ISO 4582 ('Plastics-Determination of changes in colour and variations in properties after exposure to daylight under glass, natural weathering or artificial light') and many are listed in the Appendix to ASTM D 1435-79. In more fundamental investigations changes in some of the properties and characteristics of the PVC polymer (e.g. the molecular weight and its distribution, presence and nature of functional chemical groups, IR absorption) .may also be followed, sometimes in conjunction with microscope or electron-scan examination of material surface, and DTA or DSC determinations. Whilst the long-term exposure approach (1 above) is the most reliable it is in many cases impracticable as the sole means of testing, because of the time periods involved. However, long-term exposure at representative sites is often used to provide (eventually) reference data against which the results of accelerated tests may be evaluated. Site selection is important: sites in regions which provide severe conditions are widely favoured (e.g. Arizona, the Australian interior, the Transvaal in South Africa, for dry heat and maximum sunshine; Florida, Natal for humid heat). More than one site may advantageously be used to allow for different types of conditions, although a recent world-wide study of weathering of PVC42 has been held to suggest that a reasonable correlation can be established between results from different geographical locations. The mounting and positioning of specimens playa part: backing with a good insulator (e.g. plywood) helps to bring out the effect of temperature stresses; exposure racks are normally south-facing (north-facing in the southern hemisphere)
Mainly effects of heat and temperature fluctuation, including: (i) Heat degradation (ii) Exudation and volatalisation of components (especially plasticisers) (iii) Physical disruption by local and general stresses caused by temperature changes (i) Mechanical erosion of surface (especially by wind-borne precipitation) (ii) Leaching out of components (especially plasticisers) (iii) Mechanical disruption (especially of surface) by repeated absorption and desorption (which may be aggravated by presence of absorbent fillers in the material)
Temperature
Water (including atmospheric precipitation, i.e. rain, snow, hail, vapour and condensate)
Degradation of PVC polymer by UV component of the radiation Fading of colourants
Typical action on PVC material
Sunlight
Factor (environmental agent)
1,5,7,8
1,8,9
1,3
6,8
1, 2, 3, 4, 5
1, 2, 3, 4, 5, 6, 7, 8
Main observable effects a
Effects aggravated by the action of other factors (e.g. chemical reactions, UV, heat)
Temperature effects can promote and enhance those of other agents
Direct sunlight is also a source of heat and hence can promote temperature effects (see below)
Remarks
TABLE 12.6 Some Factors Instrumental in the Natural Weathering of PVC, and Examples of Their Main Effects
~;0
:0::::
~
~
dulling, marring and pitting of surface; cracking (in severe or advanced cases); stiffening; discoloration (e.g. yellowing or darkening); reduced strength; 6, 7, 8, 9, 10,
Effects can be accelerated by heat and sunlight
Effects promoted and enhanced by those of UV radiation (instrumental in creating reactive sites, eS{J~cially double bonds)
reduced extensibility; surface buckling or rippling; distortion (various degrees and kinds); development of microporosity; environmental stress cracking or crazing.
1,2,3,4, 5,6,8,9,10
(i) Leaching out of components (ii) Chemical reactions with the PVC polymer and possibly other components of the material (iii) Surface erosion (by wind-borne particulate pollutants)
Atmospheric pollutants (in vapour, liquid and solid particle form)
1, 2, 3, 4, 5,
1,2,4,5,6
Oxidation of reactive sites in PVC polymer and some other components
Air
a
1,2,4,9
(iv) Chemical effects of pollutants (e.g. acids) dissolved in rain
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with the specimens vertical, horizontal, or tilted at 45°. In some methods specimens are exposed under glass, so that the effects of certain weathering factors (wind, rain) are excluded (ct. ISO 877-1976, identical with BS 2782: Part 5: Method 540A: 1977; also ASTM G 24-73, and DIN 53388). Outdoor exposure procedures and effects are the subject of several international and national standards, including: ISO 4607: 1978: Plastics-methods of exposure to natural weathering. BS 4618: Section 4.2:1972: Resistance to natural weathering. Section 4.4:1973: The effect of marine exposure.
ASTM D 1435-75: Standard recommended practice for outdoor weathering of plastics. ASTM G 7-77: Atmospheric environmental exposure testing of non-metallic materials. DIN 53386:1974: Testing of plastics; testing of resistance to weathering in nature (outdoor weathering). British Draft Document 76/54334 (eventually to become method 828A of the revised version of BS 2782): Determination of resistance to natural weathering.
Intensified outdoor exposure (2 above) can be effected in various ways. A useful practical method developed by Caryl and Helmick43 involves boosting the amount of natural sunlight incident on the specimen. In their apparatus, commonly known as 'EMMA' (Equatorial Mount with Mirrors for Acceleration), 10 mirrors direct extra sunlight onto specimens, producing up to tenfold intensification. This was found to be capable of accelerating by a factor of nine the degradation of some uPVC materials: 44 the specimens are cooled during exposure to prevent undue temperature rise. A further development of this concept-involving the application of a water spray and air stream to broaden and increase the weathering effects-is embodied in the 'EMMAQUA' apparatus 45 in use in America at the Desert Sunshine Exposure Test Station in Arizona. The rate of onset of 'normal' weathering effects in PVC can also be effectively increased (and hence the time scale of outdoor exposure shortened) if very thin
12
Properties of Special1nterest in PVC Materials and Products
475
films are used as specimens: in this form the surface-to-volume ratio (specific surface) is large, so that most of the material of a specimen is immediately and directly available to the agents instrumental in weathering, and is affected by changes as soon as they begin to occur. A potential disadvantage of this approach is that the exposure period needed to bring about measurable changes may in some cases actually be too short to encompass seasonal variations and sporadic effects characteristic of the 'normal' weathering pattern in the particular locality. In accelerated artificial weathering (3 above) the aim is to reproduce or match in suitable degree the effects of natural weathering by laboratory treatment of comparatively short duration. As in the other two approaches, evaluation of the results should, in the ideal outcome, enable accurate predictions to be made of the useful service life of the material concerned under given climatic conditions. Failing that it is also of interest to be able to: (i) place materials in order of qualitative relative merit with regard to likely weathering resistance in actual service; and if possible (ii) quantify the ranking, even if still on a relative basis. The answers to (i) and (ii) would, of course, follow automatically if the ideal could be achieved of reliably equating a period of standard accelerated exposure in the laboratory to one in actual long-term outdoor exposure (say, for example, 1 h in a Wether-Ometer to 50 h outdoors in Arizona). Unfortunately this is not possible, especially where long-term predictions are concerned. This well-known fact is illustrated, for example, by the data of Kuist and Maxim,46 and Grossman. 47 The former two investigators quote correlation coefficients for results of accelerated laboratory weathering tests and those of outdoor exposure as 0·6--D·9 (i.e. unavoidable variability approximately between 20 and 70%). It is also known, moreover, that differences occur in the rates of failure between long-term outdoor exposures at the same site. However, with good equipment the relative performance of materials in accelerated weathering tests can give a reliable, at least semi-quantitative indication of the relative performance to be expected in the field (i.e. (i) and (ii) above are attainable). Three factors are employed to bring about 'artificial' weathering in accelerated laboratory tests, viz. radiation, heat, and water (as vapour,
476
w. v.
Titow
liquid condensate or spray). * Exposure to radiation (light of wavelengths extending from about 280 nm into the visible region) is the basic feature of all such tests: in most procedures this is combined with the other two factors. Radiation sources commonly used are listed in Table 12.7: the emission characteristics of such equipment are discussed in a paper by Allen et at. 48 It is generally recognised that, as far as radiation effects in the normal weathering of plastics are concerned, it is the UV component of sunlight which is the main operative factor.47-49 This is the basis of the widely held view that the spectral distribution in the UV region of the light used in accelerated laboratory weathering tests should be as similar as possible to that of sunlight, t to reduce the possibility that radiation-induced chemical (and any other) changes in the test speCimens may differ in kind from the corresponding effects of natural weathering. However, a strong case has also been made out for the use of a source (fluorescent UV lamp) emitting intensely and almost exclusively in the 290-340 nm region, where the intensity of sunlight's spectrum is in fact comparatively low. This approach is based on the view that most of the photochemical changes suffered by plastics in natural weathering are attributable to the 290-315 nm UV band,47 and that therefore exposure to a source with strong emission in this region is both sensible and particularly effective as a means of accelerating radiation effects in artificial weathering. Test apparatus employing this type of illumination in conjunction with means of subjecting the specimens to condensed moisture and elevated temperatures (see entry No.4 in Table 12.7) has been claimed47 to give particularly rapid accelerated weathering, and results which correlate well with those of outdoor exposure (within the general limitations mentioned above).
* Resistance to other agencies, of specific interest in connection with weathering in particular environments, is also sometimes assessed, in separate, additional tests. Some examples are: Resistance to salt spray (relevant to marine environments-see, for example, BS 3900:Part F4:1968); to marine exposure generally (see, for example, BS 4618: Section 4.4:1973); to microbiological attack (see Section 12.7 in this chapter); or to exposure to damp heat, water spray, and salt mist (see ISO 4611-1980). t For this purpose sometimes defined in standard terms as 'global radiation', i.e. total radiation--direct, scattered, and reflected, incident upon a horizontal plane in defined conditions (see, for example, Standard D65 of the Commission Internationale de l'Eclairage).
12 Properties of Special1nterest in PVC Materials and Products
477
The following international and national standards are concerned with the methods and apparatus of accelerated weathering of plastics: ISO/R 878-1968: Plastics-Determination of resistance of plastics to colour change upon exposure to light of an enclosed carbon arc. ISO/R 879-1968: Plastics-Determination of resistance of plastics to colour change upon exposure to light of a Xenon lamp. ISO 4892-1981: Plastics-Methods of exposure to laboratory light sources. BS 3900: Methods of test for paints. Part F3:1979: Resistance to artificial weathering (enclosed carbon arc). BS 4618:Section 4.3:1974: Resistance to colour change produced by exposure to light. ASTM D 1920-69 (Re-approved 1976): Determining light dosage in carbon-arc light ageing apparatus. ASTM D 1499-64 (Re-approved 1977): Operating light- and water-exposure apparatus (carbon-arc type) for exposure of plastics. ASTM D 1501-71: Exposure of plastics to fluorescent sunlamp. ASTM D 2565-79: Operating Xenon-arc-type (water-cooled) lightand water-exposure apparatus for exposure of plastics. ASTM G 26-77: Operating light-exposure apparatus (Xenon-arc type) with and without water for exposure of non-metallic materials. ASTM G 53-77: Operating light- and water-exposure apparatus (fluorescent-UV/condensation type) for exposure of non-metallic materials). DIN 53 387:1982: Testing of plastics; accelerated test of weathering resistance (simulation of outdoor exposure by filtered Xenon-arc radiation and artificial rain). DIN 53389:1974: Testing of plastics; short test of the light stability (simulation of global radiation behind glass by filtered Xenon-arc radiation). The results of accelerated weathering are assessed in the same way as they are for natural weathering, i.e. in terms of colour changes,
Atlas Electric Devices Co., Chicago, Ill. 60613 USA Carl Zeiss Inc. New York, USA; Quartz-Lampen GmbH, Hanau, West Germany; John Goodrich, Ludlow, Shropshire, England (ii) Xenotesta
(xenon arc)
Weather-Omete~
(i)
Close
3. Xenon arc with borosilicate glass filter
Atlas Electric Devices Co., Chicago, Ill. 60613 USA
2. 'Sunshine' carbon arc (open carbon arc with 'Corex D' filters)
Weather-Ometera (Sunshine arc)
Poor
Fairly close
Suppliers
Atlas Electric Devices Co .. Chicago, Ill. 60613 USA
Name or designation
Examples of weathering equipment in which used
Fade-Ometera
Approximation of UV spectral distribution to sunlight
1. Enclosed carbon arc
Nature
Radiation sources
TABLE 12.7 Radiation Sources in Common Use in Laboratory Weathering Equipment
ASTM D 2565
ASTM G 23; ASTM G 25.
ASTM G 23; ASTM G 25.
Remarks and References
o~
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:0::::
~
~
00
Microscal lightfastness tester
Close
Fair (more intense below 320 nm)
Fair
5. Mercury/tungsten lamp
6. Fluorescent UV lamp combined with UV fluorescent 'black-light' source (FSB unit)
7. High-pressure mercury/quartz arc with 'Corex D' filters
a
Several models available. bOriginated by American Cynamid Co., USA. C Ciba-Geigy Technical Service Bulletin PL 9.1, January, 1977. d Originated by National Starch and Chemical Corp., USA.
GP UVA ultra-violet accelerometert
GP-PS/BL b
o-U-V accelerated weathering tester
None, but source effective in producing relevant photochemical degradation (see text)
4. Fluorescent UV lamp (mercury arc lamp phosphor coated)
General Products Manufacturing Co., Morristown, NJ, USA
General Products Manufacturing Co., Morristown, NJ, USA
Microscal Ltd, Ealing, London, England
The O-Panel Co. Cleveland, Ohio 44135, USA
Refs 46 and 48
Refs 48 and 50 One hour FSB exposure sometimes approx. equated to one day outdoors in the UK c
Ref. 48
ASTM G 53; Ref. 47
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480
W. V. Titow
deterioration of surface, or changes in other properties. In assessing colour development (yellowing, darkening), colour changes, or resistance to colour fading in artificial weathering, use is often made of standard colour indices or scales, and colour fastness standards. Thus the degree (and hence the development) of yellowness may be measured and described by reference to the Yellowness Index (BS 2782:Part 5: Method 530A; ASTM D 1925·70 (Re-approved 1977)), change in colour by reference to the Grey Scale (ISO 105-A021978(E); BS 2662:1961; BS 4618: Section 4.3:1974; ASTM D 2616-1967 (Re-approved 1979)), and colour fastness in terms of the Blue Wool Standards originally set up for textiles (ISO 105-B011978(E); ISO/R 878-1968; ISO/R 879-1968; BS 1006:1971; BS 4618: Section 4.3:1974). Other relevant standards include: ISO 4582-1980: Plastics-Determination of changes in colour and variations in properties after exposure to daylight under glass, natural weathering or artificial light. BS 2782:Part 5: Method 530B:1976: Determination of the colour of near-white or near-colourless materials. * ASTM G 45-75: Standard recommended practice for specifying limits for fading and discoloration of non-metallic materials.
Thermal and thermomechanical analyses (DTA, DSC, TGA, TMA) have been used as sensitive means of detecting and evaluating the effects of artificial weathering of plastics; they have also been employed as the combined means of both the thermal ageing itself and the evaluation of its effects, useful results being claimed in the prediction of service life of plasticised PVC formulations. 51 The weathering resistance of PVC is cardinally dependent on the formulation, and in particular on the stabiliser system. The individual roles of the major formulation components are mentioned in the relevant chapters. Here the following general points may be made. Much practical experience is now available to provide guidance in the formulation of weathering-resistant PVC compounds for outdoor service. However, by the same token, the limitations of even the best formulations are recognised. Thus, in the extreme conditions encoun* This is also the subject of an ISO draft standard, ISO/DIS 3558.
12 Properties of Special Interest in PVC Materials and Products
481
tered in some hot-climate areas (e.g. in parts of Australia and South Africa) where the intensity of incident sunlight, the proportion of sunlight time, and the ambient temperature are all high, and where other factors (e.g. severe hail or rain storms, large temperature fluctuations) may also operate to aggravate the severity of exposure, the durability-and hence the use-of even the most resistant PVC compositions in long-term exposure situations is limited or precluded altogether. Elsewhere PVC is successfully used in many outdoor applications: e.g. rigid compositions for external wall cladding, 'ranch'-type fencing, road signs, rainwater goods and window frames; and pPVC as coatings on chain-link wire fencing and tarpaulins, as well as (in the form of flexible sheeting) for lining swimming pools and reservoirs (although in this last application the long-term stability is generally inferior to that of some alternative materials, e.g. butyl rubber). Among the formulational factors affecting the outdoor performance of PVC materials, the following are particularly noteworthy. The PVC polymer should have the highest molecular weight consistent with the processing requirements applicable in the particular case. Homopolymers are generally preferable, although in certain compositions (e.g. some PVC window-frame compounds) certain PVC graft copolymers are used as impact modifiers (cf. Chapter 19, Section 19.4.3). The stabiliser system should be carefully selected, and should preferably (with BalCd stabiliser systems invariably) include an epoxy co-stabiliser (typically 2-8 phr): apart from the long practical experience of the beneficial effect of this type of additive, there is investigational evidence for its useful role, inter alia, as an agent facilitating the neutralisation of nascent HCI-evolved in the course of degradation of the PVC polymer-by the main stabiIiser(s) present. 52 ,53 As the main stabilisers, selected tin carboxylates can give very good results, as can BalCd systems supplemented by epoxy co-stabilisers and phosphite chelators. The carboxylates, too, benefit from the presence of epoxy co-stabilisers, although there is some evidence to show that the effect is less pronounced than with BalCd stabilisers. 42 Where lead stabilisers are permissible, dibasic lead phosphite is especially useful because of its UV-screening action and antioxidant effect: combinations of this stabiliser with tribasic or tetrabasic lead sulphate have also proved very effective for weatheringresistant compositions. UV absorbers are commonly included (sometimes in conjunction with antioxidants) in transparent compositions for
482
w.
V. Titow
outdoor use, to provide additional protection against photolytic degradation. Polymeric impact modifiers for weathering-resistant rigid compositions should be carefully selected from among the chlorinated polyethylene, acrylic, and EVA types. Rubbery modifiers (ABS, MBS) are not suitable in this application. Two common pigments are noted for their beneficial effect on the weathering resistance of PVC: carbon black and titanium dioxide. The latter can also apparently enhance the effect of certain colourants which are known to exert a stabilising action of their own: two examples of such colourants are indanthrene blue and carbazole violet. 54 The weathering resistance of plasticised compounds can be improved by keeping the plasticiser content as low as possible, using selected high-permanence plasticisers (some polymerics can be particularly useful), and incorporating epoxy plasticisers as already mentioned. To obtain the best weathering performance possible with a particular formulation, attention should be paid to the processing conditions (in particular excessive heating should be avoided), and preferably no re-worked material should be included. Presence of solvent residues in solvent-cast films can have an effect: residual tetrahydrofuran was found to promote photodegradation of PVC in air. 55 External treatment (surface coating) may sometimes improve weathering resistance: the lacquers applied as thin top coats to flexible PVC sheet products (e.g. coatings on fabrics-see Chapter 22) can have this effect. Such coatings, conventionally applied from solvent solution, normally contain an acrylic polymer as a main component. Co-extruded protective acrylic coatings on rigid PVC products have also made their appearance: for example, improved weatherability is among the advantages claimed for the German 'Vacuplast' PVC window frame system56 produced from acrylic-surfaced extruded profiles (with aluminium reinforcement). Application of UV absorbers to the surface of transparent PVC sheeting by absorption, as well as in surface coatings, has been suggested57 as a way of obtaining more cheaply a degree of weathering protection comparable with that afforded by the conventional incorporation into the compound. The effectivity of internally incorporated stabilisers can be significantly affected by their degree of dispersion, migration through the compound, volatility and extractability. 58
12
Properties of Special Interest in PVC Materials and Products
483
12.7 RESISTANCE TO BIOLOGICAL ATTACK 12.7.1 Microbiological Attack (Biodegradation) In the plastics context this normally means attack by fungi (mould, mildew) or bacteria. Whilst infestation by algae might also be included in the term in its widest connotation, it is not a major problem even with such PVC materials as film linings for canals, reservoirs and swimming pools. The essential mechanism of microbiological attack is enzymatic degradation of the substrate on which the micro-organisms groW. Both bacteria and fungi produce enzymes capable of breaking down many carbon compounds (those containing oxygen-bearing functional groups can be particularly susceptible) to simpler substances utilisable as nutrients. Some of the products of the breakdown process can be coloured, so that the appearance of colour (in PVC often a characteristic pink stain 59) or black spots, as well as deterioration of some properties in consequence of the chemical degradation, are the main outward manifestations of microbiological attack; others include the development of offensive odours, surface tackiness (in soft pvq, or surface cracking. The resistance of PVC materials can vary widely depending on the formulation: some compositions stand up very well to long exposure in the most unfavourable conditions, such as warm, humid environments (indoors or out), soil burial or permanent immersion in water. However, even a very resistant formulation can be affected indirectly, through contact with a material that is prone to attack: thus, for example, mildew may grow on the cotton fabric backing of a PVC-coated protective glove (especially if kept moist with perspiration for long periods) or on the moist paste layer of a PVC-coated wall paper, or micro-organisms may flourish on surface contaminants (grease, dirt) on PVC cladding. In broad terms, the relevant characteristics and effects of the main components of a PVC formulation may be summarised as follows. Like many other synthetic polymers, the polymers and copolymers of vinyl chloride are resistant to attack by micro-organisms. However, some commercial PVC resins may contain residual amounts of emulsifying or suspending agents used in their production, and these may be susceptible. 44 ,60,61 Many plasticisers are vulnerable to microbiological attack, as are some stabilisers and lubricants (especially epoxy
484
W. V. Titow
compounds, some stearates and waxes), and certain antioxidants, although some organotin stabilisers and phosphite co-stabilisers actually tend to inhibit microbiological growth. 6o The general ranking of plasticisers, in order of increasing susceptibility, is: -aryl phosphates and chlorinated paraffin extenders -phthalates and trimellitates -aliphatic esters (with sebacates and ricinoleates tending to be least resistant in this group) -polyesters (with some exceptions) -epoxy esters and epoxidised oils The resistance of plasticised PVC to microbiological attack is also a function of plasticiser content in many cases. 62 Some fillers may be vulnerable, e.g. wood flour in wood-filled PVC compounds used for extruded profiles and trim. 6 Special protective additives are included in PVC formulations at risk of microbiological attack. Those with a positive ability to destroy fungi and bacteria are often referred to as fungicides and bacteriocides, respectively (or, collectively, biocides): the terms 'biostat', 'fungistat' and 'bacteriostat' are applied to substances which deter microorganism growth by whatever mechanism. Kaplan et at. 63 evaluated the action of 32 biocidal compounds in PVC film: they concluded that of those only one, 'copper 8-quinolinolate' (bis(8-quinolinolato)-Cu), provided fully satisfactory protection. N-(trichloromethylthio)phthalimide also gave good results, but was considered less effective and less widely compatible with PVC formulations. This ranking appears to reverse the order that might be inferred from industrial usage: although N-(trichloromethylthio)imides and the copper complex are both used in commercial biostats, compounds of the former group would be regarded as more versatile and possibly more effective at least in some cases. Other compounds of practical interest are phenyl mercury salicylates,6h and organic compounds of arsenic (e.g. Estabex ABF-AKZO Chemie UK Ltd). Some commercial products are claimed to give broad biostatic protection (e.g. TV-2-Sanitized Sales Co. of America Inc.; Mikro-Chek 12-Ferro Chemical Division, * The use of organomercury biocides in flexible PVC has been discontinued on toxicological and environmental grounds despite their usefulness as the only biocides of proven effectivity against Pseudomonas aeruginosa which can cause problems in pPVc.
12
Properties of Special1nterest in PVC Materials and Products
485
USA). A bacteriostat of lower than average toxicity, highly effective in plasticised PVC products (baby pants, curtains, flooring, gloves) against both gram-positive and gram-negative bacteria, is 2,4,4'-trichloro-2'-hydroxydiphenyl ether (Irgasan DP 300--CibaGeigy): it is also active against certain fungi which grow on the skin, e.g. athlete's foot. Its main function is to reduce growth and spread of bacteria and to suppress odour, rather than to protect the PVC itself against bacterial attack. Amounts of biostats used in PVC formulations vary with the nature of the reagent and the formulation itself, within the range of about 0·1-2% by weight of the formulation. The protection they afford is of interest in many applications. In addition to those already referred to in this section, electrical wire and cable coverings, PVC-coated tarpaulins and foul-weather clothing, garden hose, and some pipe formulations may also be mentioned. Testing the resistance of a plastics material to microbiological attack in the laboratory typically involves placing specimens in contact with stock cultures of selected micro-organisms under controlled conditions for a prescribed time, * and determining changes in a selected property or group of properties. The appearance of the specimens before and after the treatment is usually noted, either as part of the evaluation or additionally. Some relevant standards are listed below. Of these BS 4618 gives a short bibliography, and ASTM G 21 lists in an appendix several standard (ASTM) methods for determining changes in the properties which may be monitored in the tests. ISO 846-1978: Plastics-Determination of behaviour under the action of fungi and bacteria-Evaluation by visual examination or measurement of change in mass or physical properties. BS 4618: Section 4.5:1974: The effect on plastics of soil burial and
biological attack. ASTM G 21-70 (Re-approved 1980): Standard recommended practice for determining resistance of synthetic polymeric materials to fungi. ASTM G 22-76: Standard recommended practice for determining
resistance of plastics to bacteria. * Actual soil burial tests are also popular with investigators.
486
W. V. Titow
ASTM G 29-75: Standard recommended practice for determining algal resistance of plastic films.
u.7.2
Insect and Animal Depredations
In practice the only problems of any significance under this heading arise in connection with attack on PVC products by termites and rodents. (a) Attack by Termites Although this only occurs in tropical and sub-tropical countries it can be a problem with PVC products, especially soft PVC (e.g. electrical wire insulation, cable covering, upholstery fabrics and foam). Experience appears to indicate that termites have a preference for soft plastics generally, and hence high loading with hard fillers has been suggested as a possible way to more resistant formulations (see, for example, BS 4618). Other suggestions that have been made from time to time included the use of phosphate plasticisers (regarded as more resistant than other kinds),64 incorporation of lead naphthenate, and incorporation of insecticides in, or their application in coatings to, PVC materials. 65 The effectivity of such measures is by no means established or universal: susceptibility can differ in different localities and with different species of termite, and no PVC material can be guaranteed to be generally immune from attack, even if it has performed satisfactorily in a particular set of conditions. In some countries a metal barrier (tape) is prescribed by regulations to prevent termite and animal attack on the PVC covering of electrical cables, and this is an effective solution in this particular case. (b) Attack by Rodents PVC materials are not, in general, palatable to rodents, and are not a source of food. They are attacked, however, by mice and rats if they form an obstacle on the way to food or water. Some apparently less purposeful gnawing is also experienced from time to time on such PVC products as electrical insulation and conduit, uPVC water pipes, and reservoir linings. Although barriers may be incorporated in some products (e.g. cable coverings, see above) there is no generally applicable way of preventing these depredations. However, they are neither sufficiently frequent nor widespread to constitute a major problem.
12
Properties of Special Interest in PVC Materials and Products
487
12.8 CHEMICAL RESISTANCE At ordinary temperatures PVC homopolymer is resistant to most of the common inorganic reagents (including aqueous salt solutions), oxidising agents (with the partial exception of concentrated nitric acid) reducing agents, aqueous solutions of detergents, oils (mineral, animal and vegetable), fats, aliphatic hydrocarbons and alcohols. Its solvent resistance is, however, limited in certain respects: it can be dissolved by some ketones (tetrahydrofuran, cyclohexanone, isophorone) and swollen to varying degrees by others; some nitroparaffins can swell or even dissolve it; and chlorinated hydrocarbons, aromatic hydrocarbons, aromatic amino compounds, as well as some other reagents (e.g. acetic anhydride) are also swelling agents. Copolymers are somewhat less resistant, especially to organic solvents (cf. vinyl solutions-Chapter 24), save in the exceptional case of copolymers of vinyl chloride with maleic acid imide derivatives (cf. Chapter 1). However, their general resistance characteristics are broadly comparable with those of the homopolymer. As is usual with thermoplastic polymers, the susceptibility of PVC homo and copolymers to chemical attack increases with increasing temperature: the same behaviour is exhibited by uPVC and pPVC compounds. The resistance of compounds can also be lower (in some cases considerably so) than that of the PVC polymer alone, because of the presence of the various additives. However, uPVC compositions are not normally significantly inferior in this respect, although the presence of some impact modifiers may increase solvent susceptibility somewhat, whilst resistance to acids and alkalis may be affected by heavy loading with certain fillers (e.g. whiting and wood flour, respectively). Flexible (pPVC) compositions may be more readily attacked by solvents; the increased susceptibility depends mainly on the nature and amount of plasticiser(s) present. The general chemical resistance characteristics of PVC compositions are summarised in Table 12.8. Additional data for uPVC are given in Tables 12.9 and 12.10. ISO/DATA 7:1979 gives data on the resistance of uPVC pipes to many fluids at up to 60°C. Apart from any direct chemical action, some reagents can affect the properties of PVC materials by leaching or dissolving out important components of the formulation (e.g. plasticisers, stabilisers) even if only from the surface layer. Plasticisers may also be lost by migration into materials in close contact with pPVC (e.g. adhesives, lacquers) whose properties may be affected as a result.
S
S S S
Reducing agents Detergent solutions Inorganic salt solutions
:}
M
M
Remarks
Rigid PVC
No attack up to 60°C, but max. allowable design stress should be lowered
No attack up to 60°C; allowable design stress should be substantially reduced Allowable design stresses should be substantially reduced No attack up to 60°C
Attacked above 20°C; max. allowable design stress should be reduced substantially
:1
Q
General resistance rating
Oxidising agents
Concentrated
Organic acids Alkalis: Dilute
Oxidising (concentrated)
Concentrated
Inorganic acids: Dilute
Reagents
S S S
S
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M
U
M
S
Q
General resistance rating
No attack up to 60°C
Some fillers may be affected
Some plasticisers and fillers may be affected
No significant attack up to 20°C; plasticisers and some fillers may be affected at higher temperatures Plasticiser and some fillers may be affected Short-term contact may be acceptable in some cases
Remarks
Plasticised PVC
TABLE 12.8 General Chemical Resistance Characteristics of PVC at Room Temperature
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Rating key: S = Satisfactory. M = Moderate (dependent on formulation and conditions). U = Unsatisfactory.
S S S
U U
S
Water
Allowable design stresses should be substantially reduced Some softening possible at elevated temperatures
M-U M-U M-U
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U U U M
U U U S
U
Bromine Fluorine Iodine Aliphatic alcohols
Little attack in the absence of moisture
M
Halogens: Chlorine
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TABLE 12.9
Effects of Chemical Immersion on a High-impact uPVC Compound 30-day immersion at room temperature
Chemical
Tensile strength (lbfin- z)
Weight change
5200 4500 3600 5800 5900 5800 5300 4900 5700 5550 5450
0·81 1·99 4·48 0·50 0·43 0·22 0·32 0·40 0·48 0·04
5500 5600 5400
30-day immersion at 60"C Tensile strength (lbfin- Z)
Weight change 3·29 8·25 15·39 1·64 2·26
0·14 -0·07 -0·01
5000 2450 1450 5800 5700 4900 4700 5450 4200 4950 5250 4800 5900 5800 5700
1·48 0·98 6·59 1·72 -0·19 3·54 0·04 0·58 12·78
4600 4100 5400 5200 5600
1·20 1·21 0·14 0·02 -0·02
2150 2400 5400 5900 5400
5·14 3·17 0·22 -0·05 -0·11
Carbon tetrachloride Trichloroethylene
1850 1550
59·14 103·21
Excessive swelling Excessive swelling
Benzene Castor oil Cotton seed oil Glycerine Hexane Linseed oil Salt solutions (sat.) Barium sulphide Ferric chloride Potassium chloride Sodium dichromate Trisodium phosphate
5000 5300 5150 5450 3000 5150
73-11 0·44 0·18 0·07 4·24 0·07
Excessive swelling 5000 0·07 5400 0·26 5900 0·18
5000 5100 5000 5400 5050
0·61 0·25 0·22 0·23 0·46
5000 4700
0·80 1·53
2850 5200
3·87 0·61
Acetic acid Chromic acid Hydrochloric acid Nitric acid Oxalic acid (sat. soln) Phosphoric acid Stearic acid Sulphuric acid
Butyl alcohol Ethyl alcohol Sodium hydroxide
Formaldehyde Hydrogen peroxide Phenol Turpentine Distilled water
20% 80% (glacial) 10% 30% 40% 30% 30% 60% 75% 100% 20% 50% 80%
10% 30% 50%
(%)
(%)
4800
0·44
5450 4750 4800 5150 4800
3·11 0·32 0·20 0·18 0·78
4200 3·57 4800 1·67 Excessive swelling 1850 35·49 5100 0·94
Acetaldehyde: 100% 40% Acetone Aluminium fluoride Ammonia liquid, 100% Ammonium hydroxide, 0·88 Ammonium fluoride, 20% Ammonium sulphide Amyl acetate, 100% Aniline, 100% Barium chloride Benzaldehyde Benzine Bleach lye, 10% Bromine gas, weak Bromine liquid Bromic acid, 10% Butyl acetate Butyric acid: 20% cone. Butanol: primary secondary Calcium chlorate Calcium hypochlorite, soln Carbon disulphide Chloracetic acid, 100%
Chemical
S S U U S U S S
S S
S
M
S S S S
U
S U S
U
U S M S S S
U
S
S
U S U S S U S
M
U
-
U
-
S
M
M
U
U M U
S
60
S
U
20
(0C)
Temperature
Chloric acid, 1-20% soln Chlorine: gas moist gas liquid Chlorobenzene Chloroform Chlorosulphuric acid Chromic acid Citric acid, sat. Copper fluoride, 2% Copper cyanide Cresol Cresylic acid Cupric fluoride Cyclohexanol Cyciohexanone Dibutyl phthalate Diethylene glycol Diglycolic acid Dioctyl phthalate Ethyl acetate, 100% Ethyl alcohol Ethyl butyrate Ethyl chloride Ethyl ether Ethylene chloride Ethylene dichloride
Chemical
TABLE 12.10 Further Data on Chemical Resistance of Rigid PVCa
S U U U
M
S S U U S S S U U S
M
U U S S S S S
U
S S S
20
(0C)
U
U U U S S S S U S S U U S S S U U S U S U U U
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S S
60
Temperature
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Ethylene glycol Ethylene oxide Fatty acids Ferric sulphate Ferrous chloride Ferrous sulphate Fluosilicic acid Formaldehyde, 40% Formic acid: 50% conc. Fruit juices Fuel oils Furfural Glucose Glycerine Glycol Heptane Hydrobromic acid, 50% Hydrochloric acid, 30% + Hydrocyanic acid Hydrofluoric acid: 40% conc. Hydrogen bromide, 10% Hydrogen peroxide Hydrogen sulphide Hypochlorous acid
Chemical
S U S S S S S S S S S S U S S S S S S S S M S S S S M M S S
S
S S S
S
S U S S S
S
U U S S S S S S M U
Temperature (0C) 60 20 Chemical
Kerosene Ketones Lactic acid: 10% 90% Lead tetraethyl, 100% Magnesium chloride Magnesium hydroxide Maleic acid, sat. Mercuric chloride Mercuric cyanide Mercurous nitrate Methyl alcohol, 10% Methyl bromide Methyl chloride, 100% Methyl ethyl ketone Methyl isobutyl ketone Methylene chloride, 100% Milk Mineral oils Molasses, commercial Naphtha Naphthalene Nickel chloride Nitric acid: 1-30% 70% 98%
TABLE 12.1().......£ontd.
S U S U S S S S S S S S M M U U U S S S S U S S S U
S S S S U S M M
U U
S U M U S S S M S S S S U
Temperature eC) 20 60
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U M S M S S U M
Sodium cyanide Sodium ferrocyanide Sodium fluoride Sodium hypochlorite Sodium sulphide, sat. Stannic chloride Stannous chloride Sulphur dioxide: dry wet liquid Sulphuric acid, 90% Fuming sulphuric acid Tanning extracts Tartaric acid, sat. Tetraethyl lead Tetrahydrofuran Thionyl chloride Toluene Transformer oil Trichlorethylene Triethanolamine Turpentine Urea, up to 30% Vinegar Xylene Zinc chloride, sat. Zinc sulphate
Key: S = Satisfactory. M = Moderate (dependent on formulation and conditions). U = Unsatisfactory.
Nitrobenzene Octyl cresol Oleic acid Oxalic acid, sat. Perchloric acid, up to 70% Petrol, aliphatic Petrollbenzene, 80/20 Phenol, 90% Phenylhydrazine, 100% Phosgene, liquid Phosphoric acid Phosphorus pentoxide Phosphorus trichloride Picric acid, 1% Potassium bichromate Potassium chromate, 40% Potassium cyanide, sat. Potassium hydroxide, cone. Propyl alcohol Propylene dichloride Silver cyanide Soap solution Sodium acetate Sodium bisulphite Sodium bromide Sodium chlorate, sat. Sodium chromate
S S S S S S S S S M S U S S S U U U S U S U S S U S S
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494
W. V. Titow
The good resistance of PVC to many chemicals is utilised in such practical applications as, for example, uPVC wall cladding in chemical plants, or the ducting and fans of fume-extraction systems, and protective pPVC gloves and clothing for workers in the chemical industry, laboratories and stores. The susceptibility of PVC compositions to attack by some solvents is also used to advantage in some processes as well as for certain test purposes: apart from the various applications of PVC polymer and copolymer solutions (see Chapter 24), such uses include, for example, the incorporation of isophorone as a keying agent in printing inks for PVC sheeting and coatings, the use of solvents in tests for the completeness of gelation of PVC paste coatings on fabrics (see Chapter 22), and solvent-swelling tests (in acetone or dichloromethane) for homogeneity and structural integrity of uPVC pipe and other extruded products and mouldings (d., for example, ISO 3472-1975; BS 3506:1969; ASTM D 2152-80; SABS 791-1975; SABS 966-1976). The chemical resistance of plastics materials, including PVC, is normally tested by determining changes in appearance, dimensions, mass, and/or other properties of specimens after a period of contact (usually by immersion) with the chemical(s) concerned. Some of the relevant standards give a list of chemicals for determining the general resistance and specify the properties to be used as assessment criteria (see, for example, ISO 175 and 462; BS 4618: Section 4.1; ASTM D 543). Some basic requirements in respect of the general chemical resistance of uPVC compound are laid down in Table 2 of ASTM D 1784-1981. The international and national specifications of interest in connection with various aspects of chemical resistance of PVC include the following: ISO 175*-1981: Plastics-Determination of the effects of liquid chemicals, including water. IS0/R 462*-1965: (Later incorporated in ISO 175). Recommended practice for the determination of change of mechanical properties after contact with chemical substances. ISO 3473-1977: Unplasticised polyvinyl chloride (PVC) pipesEffect of sulphuric acid-Requirement and test method. * Essentially equivalent to parts of ASTM D 543.
12
Properties of Special1nterest in PVC Materials and Products
495
BS 2782:1970: Method 505A: Resistance to concentrated sulphuric acid of rigid polyvinyl chloride compounds. BS 4618: Section 4.1:1972: Chemical resistance to liquids. ASTM D 543-67. (Re-approved 1978): Resistance of plastics to chemical reagents. ASTM D 1239-55. (Re-approved 1982): Resistance of plastic films to extraction by chemicals. ASTM D 1784-81: Rigid poly(vinyl chloride) (PVC) compounds and chlorinated poly(vinyl chloride) (CPVC) compounds.
DIN 53476:1979: Testing of plastics; Determination of the behaviour against liquids. DIN 53756:1974: Testing of plastics; Storage in contact with chemicals. DIN 53 428:1967: Testing of cellular materials; Determination of the resistance to liquids, vapours, gases and solid materials.
U.9 HEALTH HAZARDS Health hazards arise in the production, processing, use, and disposal of most plastic materials, and PVC is no exception. Some of the hazards are of a general nature, not directly dependent on the composition of the plastic: e.g. risks of injury in operating plastics processing machinery, or the well-publicised danger of suffocation to children using plastics bags as substitute space helmets in play. This section is concerned primarily with those health hazards which are specifically associated with the chemical nature of PVC materials, although some associated 'peripheral' hazards are also briefly mentioned. The main hazard areas may be collectively identified as the risk of harmful effects on contact with the PVC materials themselves, or their individual constituents, or decomposition products, during any of the abovementioned phases of the materials' life history. The principal possible harmful effects are poisoning (in the widest sense of the term), carcinogenic action, irritation and tissue damage, and dermatitis. The forms of contact through which they can arise are ingestion, inhalation, absorption (e.g. through the skin or mucous membranes), or simple
496
w.
V. Titow
'external' contact (which may also lead to some absorption) especially if prolonged or repeated. 12.9.1 Vinyl Chloride Monomer In the case of PVC an important potential health hazard is encountered at the earliest stage of the material's life cycle, in that the vinyl chloride monomer (VCM) is a recognised carcinogen. The hazard continues wherever residual amounts of the monomer are present in PVC resins and compounds, before, during and after their conversion into end products. This situation necessitates precautions against exposure to free VCM in the production of PVC polymers and copolymers, and measures to minimise residual VCM contents of such polymers and the compounds and products based thereon. The general objective is to reduce to an acceptable level the amount of the carcinogen which can be transferred by direct contact, inhaled (or absorbed) as vapour previously volatilised into the atmosphere, or consumed in foods and beverages which can extract it from PVC packaging films or containers. Among the most important problems arising in this connection is the need to know what should be regarded as the maximum permissible concentrations of VCM in PVC materials and the atmosphere, and the associated requirement for suitable methods of determination. Although the carcinogenic activity of VCM (in animals) was first made known only in 1970,66 and links with a form of liver cancer (angiosarcoma) and a rare cancer of the mouth in humans first recognised in the mid-1970s,66,67 much effort has already been devoted to meeting both these needs. Several analytical methods for determining small amounts of VCM in PVC and in air are now available, with sensitivity in many cases better than 1 ppm, and in some down to a few parts per (American) billion. 68 Several commercial detectors and monitors are on the market.69 Gas-chromatography procedures, involving either direct or head-space sampling, can be particularly useful,70-72 although IR spectroscopy and photodetection are also utilised in monitors for VCM in air. 69 Clip-on badges have been developed for the latter purpose.73 Interest continues in possible ways of determining the actual extent of damage caused by VCM in the body: inter alia, a very sensitive method has been reported based on the alkylating action of VCM (as well as of certain other carcinogens) on amino acid constituents of haemoglobin. 74 Ideas on maximum concentrations representing 'acceptable risk'
12
Properties of Specia/1nterest in PVC Materials and Products
497
levels have undergone a considerable change in the past few years with increasing volume and availability of relevant data. The first limits recommended in the UK (in the mid-1970s) for maximum VCM concentration in factory atmospheres started with a time-weighted average figure of 25 ppm (by volume), soon to be brought down to 10 ppm with the further proviso that wherever possible zero concentration should be aimed at. 75 At the same time in West Germany (North Rhine-Westphalia) the maximum concentration limits for factories were being lowered from an initial 50 ppm to 5 ppm,76 whilst in the USA a limit of 1 ppm was being demanded, with the US Food and Drug Administration (FDA) concurrently framing regulations to prohibit the use of rigid and semi-rigid PVC for food-packaging applications (bottles, films) unless it could be shown that no migration of VCM into the contents would occur. Attention was focused on unplasticised PVC, because available evidence indicated that plasticisation reduces residual VCM contents to undetectably low levels. A temporary standard was put out in the USA by the Occupational Safety and Health Administration (OHSA) in 1974, followed by a finalised version in 1978: in the same year relevant rules, limiting VCM emission in industrial plant, were formulated by the US Environmental Protection Agency (EPA), and EEC directives issued in Europe on VCM content in food-packaging materials. These moves made themselves felt in the industry in several ways. PVC resin production, as well as that of packaging films and bottles, was curtailed by some manufacturers unwilling to face the difficulties and expense of reducing VCM concentrations in their plants and products in the face of uncertainty as to what limits might finally be laid down. Prices of some PVC resins and products were affected as production became more expensive where removal of VCM and tighter control over its concentration were being instituted. Some resins, in which the VCM content was reduced by heat treatment ('stripping'), became more glassy and harder to process as a result of this addition to their 'heat history'. On the positive side, R&D work was stimulated towards methods of reducing VCM concentrations in PVC materials and factory atmospheres, methods of determining such concentrations, and the ways in which they were affected by production conditions. Towards the end of the 1970s the practical improvements achieved in production and processing, coupled with the results of the R&D effort, led to a brighter outlook on the VCM risk. Further confirmation has been forthcoming for the relative safety of plasticised PVC
498
W. V. Titow
materials, as has evidence of a substantial drop in residual VCM levels in both PVC materials for food packaging and the foods packaged therein. 67 ,77 It is now practicable to reduce the VCM content of commercial PVC resins to a few parts per 109 (i.e. by a factor of nearly 106 since the early 1970s), and there is strong evidence (from the Ethyl Corporation in the USA) that at, or below, 2 parts per 109 VCM will not migrate into food from PVC materials at a significant rate. 78 ,79 The latest FDA estimates based on this evidence indicate potential maximum VCM levels of less than 5 parts per 1012 in PVC-packaged food. 79 Thus, whilst the fact remains that only complete avoidance of exposure to VCM can entirely eliminate all risk, a high degree of confidence in properly processed PVC as food-packaging material may soon be restored. An excellent review of the VCM problem in all its aspects was published recently by Clayton. 8o It may be noted in passing that exposure to VCM (admittedly in minute quantities) from sources unconnected with PVC may be a real possibility for large numbers of people both in the industry and outside: vinyl chloride has been reportedly found in tobacco smoke (albeit in very small concentrations-up to 0·03 ppm), and the possibility has been mentioned that it may also be formed as a combustion product of other plant materials, including vegetable refuse. 8 ! U.9.2
PVC Compounds and their Regular Constituents
Aside from the effects of VCM, the main health hazard is possible toxicity in food-contact applications involving such PVC products as films and containers: this hazard is usually considered from the point of view of the properties of the individual components of a formulation. It is normal to 'clear' these, before the formulation is finalised and made up, on the basis of experience, and/or information from the manufacturers, and/or the relevant recommendations or rules of the appropriate national authorities and organisations. In the USA the organisations most directly concerned are the ones referred to in the previous section (FDA, OHSA, EPA): the US Department of Health, Education and Welfare (HEW) may also be mentioned in this connection. In the UK and Europe the bodies with related interests and functions (albeit largely different constitutions, and scope and nature of operations) include the UK Health and Safety Executive, British Plastics Federation (BPF) , the UK Chemical Industries
12 Properties of Special1nterest in PVC Materials and Products
499
Association, the West German Federal Health Office, and corresponding organisations in many other countries. Some of these organisations (e.g. FDA, BPF) issue lists of materials (e.g. plasticisers, stabilisers, colourants) approved (or forbidden) for food-contact applications: such applications constitute the area of primary concern in the context of this section. Some aspects of the subject of toxicity of PVC materials are discussed in a brief paper by Estevez. 82 An earlier review, by Phillips and Marks,83 is also still of some interest. In the UK the BPF publishes a code of practice for safety in use of plastics for food-contact applications, based in part on extensive evaluation tests carried out by the British Industrial Biological Research Association (BIBRA). It is normally assumed that PVC homopolymers, vinyl chloride/ acetate and vinyl chloride/vinylidene chloride copolymers are non-toxic in compounds. Several lubricants (in particular stearic acid) are regarded as safe, as are some of the other two principal formulation components, plasticisers and stabilisers, when used in prescribed concentrations: acceptability, especially the concentration limits, may, however, vary according to the conditions. For example, more stringent requirements arise for food-packaging films to be used with fatty foods (e.g. bacon, butter, etc.) capable of leaching out plasticisers, than for non-fatty foods with a high water content (e.g. fruits, vegetables). The packaging of children's toys is also an area of special concern. Detailed, up-to-date information and guidance can be obtained from the organisations mentioned in this section. Some further general information is also given in the chapters on stabilisers and plasticisers.
U.9.3 PVC Decomposition Products If thermal decomposition of PVC is permitted to occur in processing, and when PVC is burned (e.g. in an accidental fire, or as a means of disposal), toxic and irritant fumes are produced. These contain a considerable proportion of hydrogen chloride (usually appearing as an acrid, highly irritant white fume), which is the principal product of thermal breakdown of vinyl chloride homopolymers and copolymers: 84 a sooty, black smoke usually arises from the combustion of plasticisers in flexible PVC compositions. Other pyrolysis products of PVC materials include benzene, toluene, xylene, naphthalene, and certain derivatives of these compounds: 6,84--86 with an adequate supply of
SOO
w.
V. Titow
oxygen, water vapour, CO and CO2 are also formed, as combustion products. 84 U.9.4 Peripheral Hazards
The kinds of hazard that may be mentioned under this heading are relevant to PVC, although not exclusive to it, * as they can arise in the production and processing of other plastics. They are: (i) fire and explosion hazards; (ii) respiratory hazards; (iii) toxic hazards; These occur in the storage and handling of additives and other formulation components (especially in powder form), and in processing operations involving the use of solvents (e.g. making up PVC solutions, printing on PVC materials, preparation and application of solvent-based lacquers for PVC sheet materials). The appropriate precautions are nowadays generally reasonably well known in the industry, but it should also be remembered that many are prescribed by law, and that the statutory requirements vary in different countries. Advice and guidance is available from the organisations mentioned in Section 12.9.2. Relevant information may also be found in the current editions of the following publications: Industrial Hygiene and Toxicology. F. A. Patty (Ed.), Interscience Publishers. Encyclopaedia of Occupational Health and Safety. International Labour Office, Geneva. Health Hazards of the Human Environment. World Health Organisation. Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons (under 'Industrial Toxicology' and other relevant headings). * However, two specific points may be made regarding PVC: under certain conditions the presence of fine PVC resin dust can lower the explosive limit of VCM/air mixtures; potentially ignitable levels of VCM may arise in high-speed mixing equipment. Guidance on safety in the operation of high-speed mixers is provided in a booklet published jointly by the British Plastics Federation and
Chemical Industries Association Ltd, 'Vinyl CWoride Monomer. Guide to the High Speed Mixing of PVC Resins and Compounds'.
12 Properties of Special Interest in PVC Materials and Products
501
Dangerous Properties of Industrial Materials. N. I. Sax, Van Nostrand Reinhold. Fire Protection Handbook. G. H. Tryon (Ed.), National Fire Protection Association, Boston, Mass., USA. Publications of the American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, USA, including (i) Documentation for Threshold Limit Values for Substances in Workroom Air; (ii) Industrial Ventilation: A Manual of Recommended Practice. Relevant HMSO Publications, (UK).
12.10 BURNING BERAVIOUR Virtually all plastics are combustible: that is they will-under suitable conditions (e.g. in a sufficiently intense fire)-undergo exothermic oxidative decomposition, accompanied by flame and/or glowing and/or smoke evolution as its main visible manifestations. However, the nature and severity of the conditions required for ignition and sustained combustion are different for different plastics and, conversely, the ignition and combustion behaviour will differ under identical conditions: in both cases the differences are governed by differences in chemical composition and physical state-e.g. a polystyrene film will burn readily in circumstances in which a uPVC one will not; a uPVC bar may have an oxygen index of 40 or only 25 depending on whether it is solid or cellular, and so on. Thus meaningful comparisons can only be made on the basis of tests relevant to the purpose of the comparison and carried out under closely standardised conditions. Moreover, because they are cardinally dependent upon the conditions, the results of laboratory tests are strictly relative (as are any comparisons based upon them) and should not be used as criteria for the prediction of the degree of hazard in actual fire situations. Similarly, such apparently definitive terms as 'self-extinguishing', 'non-flammable', 'flameresistant', 'slow-burning' can only have meaning in relation to a specified set of conditions (e.g. a particular standard test). Even when these principles are observed confusion can still occasionally arise because the terminology of the burning behaviour of plastics is not fully uniform: standardisation, and increasing awareness of the factors and concepts involved, have done much to improve matters, but even standard definitions of the same important term can still differ considerably. For example, two sources of relevant standard defini-
502
W. V. Titow
tions, ISO 3261-1975 * and ASTM E 176-82, t define 'flammable', respectively, as 'capable of undergoing combustion in the gaseous phase with emission of light during or after application of an igniting source' and 'subject to easy ignition and rapid flaming combustion'. Note: Other publications containing relevant terminology are: Addendum 2 (1983) to ISO 472, comprising definitions of terms relating to burning behaviour of plastics; and Compilation of ASTM Standard Definitions published by the American Society for Testing and Materials. Certain terms are also defined in some of the standard specifications listed in Table 12.12.
The burning behaviour of plastics is of great importance in many applications, and hence of interest to the user and technologist alike. The key aspects with which the practically oriented tests are concerned are ignitability, spread of flame, rate of heat release, and amount of smoke generated. The chemical composition of the smoke, whilst not investigated in standard tests, is also important and has been receiving increasing attention as a toxic hazard in fires. In addition, the Fennimore-Martin 'Oxygen Index,88,89 (based on the minimum concentration of oxygen required to support candle-like burning of a standard size specimen in specified conditions) provides a useful means of rating the flammability (in the sense of ease of ignition and burning) of plastics and other materials. Some typical oxygen index values for plastics, including PVC, are shown in Table 12.11. Standard burning tests relevant to (including some specifically devised for) PVC materials and products are listed in Table 12.12. A British standard covering the development, presentation and use of fire tests is now available. 91 The' flammability of PVC (resins and solid uPVC compositions) as determined in standard tests is one of the lowest among those of the common plastics. However, the smoke emission is relatively high, and the smoke is irritant and toxic (see Section 12.9.3). The low flammability is due to the large chlorine content: like the other halogens (cf., for example, PTFE in Table 12.11) chlorine acts as a retardant in the process of combustion (see Chapter 11, Section 11.5). * 'Fire tests-Vocabulary'.
t 'Standard definitions of terms relating to fire tests of building construction
and materials'.
12
Properties of Special Interest in PVC Materials and Products
503
TABLE 12.11 Oxygen Index Values of Some Plastics Materialsa
Material
PVC resin (homopolymer) uPVC compound (medium impact strength) uPVC compound containing 15% glass fibre PVC floor tile (asbestos-filled) pPVC compounds PVDC PTFE Polyamide (nylon 6.6) Polycarbonate Polymethyl methacrylate Polyethylene Polypropylene Polypropylene with flame retardant Polypropylene asbestos-filled Polystyrene uPVC foam pPVC foam Polystyrene foam Polystyrene foam with flame retardant Polyurethane foam Polyurethane foam with flame retardant Polyisocyanurate foam
Oxygen indexb (typical or representative value) 45
40 40 30
21-26
60
95 23 23-27
17-18 17-18 17-18 22 21
18
25 22
18
24 19 22 26
Table based on data from Refs 88, 92, 94 and 95. % Oxygen in the standard gas mixture, required to support candle-like combustion of standard specimen in standard conditions (ASTM 2863). a
b
The performance of PVC compositions in flammability tests falls with decreasing chlorine content (see Fig. 12.7 here, and Fig. 6.3 in Chapter 6). This is the main reason for the well-known fact that plasticisation increases flammability, albeit this effect is reduced where chlorinated extenders or phosphate plasticisers are used, since the former introduce their own chlorine, and the latter act as flame retardants in their own right (see Chapter 11, Section 11.5; and Chapter 7, Section 7.6). An expression relating the halogen content of a polymer to its carbon and hydrogen contents, known as the van Krevelen Composition Parameter, has been found to correlate well with the oxygen index for many polymers, including polyvinyl chloride. 92 The flammability of
Flammability
Plastics: rigid (selfsupporting) sheet or moulding
1. ISO 1210-1982 2. BS 2782: 1970 Method 508D Burning time and/or rate and/ 1. BS 2782:1970: or extent Method 508A 2. BS 2782:1970: Method 508B 3. ASTM D 635-81
1. ISO 871-1980 2. ASTM D 1929-77
Ignition properties
Plastics: pellets; sheet or film
Standard specifications
Incandescence resistance (be- l. ISO 181-1981 haviour during and after con- 2. BS 2782: 1970 tact with incandescent bar at Method 508E 950°C) 3. ASTM D 757-77 4. DIN 53459-1975
Property or characteristic determined
Plastics: rigid sheet or moulding
Material or product
Remarks
1. Bar specimen held horizontally 2. Relates specifically to PVC compounds 3. Bar specimen held horizontally
1. and 2. technically equivalent: self-ignition and flash-ignition temperatures determined (in a hot-air ignition furnace)
1. and 2. Intended for thermosetting plastics 3. Recommended for materials which are self-extinguishing in the test of ASTM D 635 (see below)
All four specifications closely similar technically (employ the 'Schramm! Zebrowski' method)
TABLE 12.12 Standard Burning Tests Relevant to PVC Materials
;;
is
:::'1
~
~
~
VI
Flammability and/or burning rate, and/or extent of bum
Plastics: cellular
Smoke generation Horizontal burning characteristics Smoke generation Vertical burning characteristics (flame height, burning time, mass loss)
Plastics: solid or cellular Oxygen index (applicable also to non-plastics materials e.g. wood)
Plastics: film or thin sheet
BS 5111:Part 1:1974 ASTM D 3014-76
BS 4735:1974
1. ISO 4589-1985 2. BS 2782:Part 1: Methods 141 A to C: 1978 3. ASTM D 2863-77 ASTM D 2843-77
4. ASTM D 1433-77
1. ISO/R 1326-1970 2. BS 2782:1970: Method 508Ca 3. ASTM D 568-77
4. UL subject 94 Parts A & B 5. IBM CMH 6-0430102
The 'Butler Chimney' test
2. Restricted to solid (non-cellular) specimens NB Method D for electric cable insulation or sheathing-see below Employing the XP2 smoke chamber
2. Relates specifically to thin flexible PVC sheeting 3. Vertically suspended strip specimen: test results sensitive to thickness 4. Strip specimen supported on 45° incline
4. and 5. Closely similar; vertical bar specimens ignited at lower end; effect of dripping (ignition of cotton by flaming drops) taken into account; tests more severe than 1 and 2
2S lJl
~ ~ 1:;
"'-
;::
'"
l:;'"
~.
~
r3
S' "l:I
~
§.: S'
~
~ ~
~.
.g~
..... N
Property or characteristic determined
Building materials (including plastics)
Various combustion characteristics (including smoke generation in some cases)
Combustibility
Electrical insulation and Oxygen index cable sheathing (mainly plastics) Ignition and/or spread of flame and/or rate and extent of burning
Material or product
Remarks
1. Agrees with lEC 332 (vertical specimen) 2. Test for rigid sheet and 2. ASTM D 299-82 plate insulation materials 3. Test for non-rigid PVC 3. ASTM D 876-80 tubing used for electrical insulation Test for duration of sustained ISO 1182-1979 flaming 1. ISOrrR 3814-1975 1. Report on tests being developed 2. BS 476 2. A multi-part specification 'Fire tests on building materials and structures' 3. ASTM E 84-81 3. 'Underwriters tunnel furnace test'. 25 ft specimens 4. ASTM E 286-69(1975) 4. The '8 foot tunnel' test 5. DIN 4102 5. A multi-part specification 'Behaviour of building materials and components in fire'
BS 2782:Part 1: Method 141D:1978 1. BS 4066:1969
Standard specifications
TABLE 12. 12-contd.
;e
:::J c;
:0::::
~
~
Vl
Various combustion characteristics
'Materials' (some relevance to plastics)
a
Now superseded by BS 2782:Part 1: Method 140 D:1980.
Interior materials for Burning rate and extinmotor vehicles (includ- guishing characteristics ing PVC upholstery and mouldings)
Duration of flaming and afterglow, and/or length of char (or melt), and/or flaming drips
Coated fabrics
ISO 3795-1976 (based on US Federal Motor Vehicle Safety Standard 302)
2. DIN 53 438-1977
1. ASTM E 162-81
4. DIN 54332-1975
3. ASTM D 2859-76
1. BS 3424:1973: Method 17 2. BS 5790:1979
Requirements stated in terms of rate of burning; specification much used for PVC upholstery fabrics
1. Test with radiant energy source: 'flame spread index' and smoke evolution measured
2. Specification for upholstery fabrics, including PVCcoated woven and knitted fabrics: flammability tests by the Method of 1. 3. Flammability of textile floor coverings (relevant to PVC-backed carpets) 4. Burning behaviour of textile floor coverings
1. Vertical strip specimen
~
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~ ~
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W. V. Titow
508 45
40
25
0·3
004
el,
wt fraction
0-5
0·6
Fig. 12.7 Oxygen index (01) of a PVC composition as a function of its chlorine content (fraction by weight of the total composition). Formulation of PVC composition: 100 PVC resin Plasticiser (DOP) 0-90 phr, as shown White lead 7 phr Ca stearate O· 75 phr A, B, C, D, E, F,
DOP 90 phr; all additives 97·75 phr; CI content 0·287; DOP 60 phr; all additives 67·75 phr; CI content 0·339; DOP 40 phr; all additives 47·75 phr; CI content 0·384; DOP 20 phr; all additives 27·75 phr; CI content 0·445; DOP 0; all additives 7·75 phr; CI content 0·527; PVC resin alone; CI content 0·568.
a PVC composition may be reduced, despite a reduction in the overall chlorine content, through the incorporation of a non-combustible filler (e.g. asbestos fibre), a flame-retardant compound, or a smoke suppressant. The latter two types of additive and their effects are discussed in Section 11.5 of Chapter 11. Much useful information (including an extensive list of literature references) on all aspects of combustion of polymers is contained in a recent book by Cullis and Hirschler. 93 A comprehensive (10-volume)
12
Properties of Special Interest in PVC Materials and Products
509
report * by the National Materials Advisory Board of the USA Academy of Sciences is an important source of reference on subjects falling within the ambit of its title. A list of flammability test methods for plastics (containing national standard tests of 18 countries, as well as some ISO standards and those of the Underwriters Laboratory, NCB) has been published by the Chemical Industries Association Ltd, London. Some data on the evolution of HCI and smoke from PVC (burnt with wooden cribs) are given by Edgerley and Pettett. 84
REFERENCES 1. Eftis, J. and Liebowitz, H. (1975). Engineering Fracture Mechanics, Vol. 7, Pergamon Press, Oxford, pp. 101-35. 2. Plati, E. and Williams, J. G. (1975). Polym. Engng. Sci., 15(6), 470--7. 3. Brown, H. R. (1973). J. Mat. Sci., 8,941-8. 4. Williams, J. G. (1975). 'The determination of fracture toughness from impact tests on polymers', Paper 2 at the PRI Conference on New Developments in Impact Testing, London, 2nd December. 5. Williams, J. G. (1975). 'The fracture mechanics of polymers', Ibid. Paper 14. 6. Titow, W. V. (1977). In Developments in PVC Production and Processing-l (Eds A. Whelan and J. L. Craft), Applied Science Publishers, London, Ch. 4. 7. Reymers, H. (1970). Mod. Plast., September, 78-80. 8. Titow, W. V. Unpublished work. 9. Wilson, A. S., Biggin, I. S. and Pugh, D. M. (1978). 'The influence of volatility on the selection of plasticisers to meet new and developing performance requirements', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 10. Ranney, M. W. (1975). Irradiation in Chemical Processes: Recent Developments, Noyes Data Corp. 11. Scalco, E. and Moore, W. F. (1983). Radiat. Phys. Chern., 21(4),389-96. 12. Yasuda, H. (1975). J. Appl. Polym. Sci., 19(9),2529-36. 13. Titow, W. V. (1978). In Adhesion 2, (Ed. K. W. Allen), Applied Science Publishers, London, Ch. 12. 14. Fujita, H. (1961). Fortschr. Hochpolym.-Forsch., 3, 1-47. 15. Meares, P. (1965). Polymers: Structure and Bulk Properties, Van Nostrand, London, p. 316. 16. Meares, P. (1958). J. Polym. Sci., 27, 391-404. 17. Meares, P. (1966). Eur. Polym. J., 2,95-106.
* 'Fire Safety Aspects of Polymeric Materials' (1979).
510
w.
V. Titow
18. Crank, J. and Park, G. S. (Eds) (1968). Diffusion in Polymers, Academic Press, London. 19. Stafford, G. D. and Braden, M. (1968). J. Dent. Res., 47(2), 341. 20. Carman, P. C. (1956). Flow of Gases Through Porous Media, Butterworths, London. 21. Scheidegger, A. E. (1974). The Physics of Flow Through Porous Media, 3rd Edn, University of Toronto Press, Toronto. 22. Rodebush, W. H. and Langmuir, I. (1942). 'Smokes and filters', US OSRD Report No. 865. 23. Davies, C. N. (1948). 'Fibrous filters for dust and smoke', Proc. of the IX International Congress on Industrial Medicine, London, 13-17 Sept., John Wright and Sons Ltd, Bristol. 24. Iberall, A. S. (1950). J. Res. Nat. Bur. Stds, 45, 398-406. 25. Thomas, D. J. (1952). J. Inst. Heatin!{ Ventilating Engrs. 20(201), 35-70. 26. Hopfenberg, H. B. (Ed.) (1974). Permeability of Plastics Films and Coatings, Plenum Press, New York. 27. Lebovits, A. (1966). Mod. Plast., 43 (March), 139-46, 150, 194-213. 28. Hennessy, B. J., Mead, J. A. and Stenning, T. C. (1966). The Permeability of Plastics Films, Plastics Institute. 29. Pye, D. G., Hoehn, H. H. and Panar, M. (1976). J. Appl. Polym. Sci., 20(7), 1921-31. 30. Salame, M. and Pinsky, J. (1962). Mod. Packag., (September) pp. 153-223. 31. Wilson, G. A. R. (1965). Plastics, (May), pp. 86-115. 32. Brydson, J. A. (1961). Plastics, (December), pp. 107-10. 33. Horsfall, F. and James, D. I. (1973). RAPRA Members J., (September), pp.221-7. 34. ShUT, Y. J. and Ranby, B. (1975). J. Appl. Polym. Sci., 19(7), 1337-46. 35. Kambour, R. P. (1968). Polym. Engng. Sci., 8(4),281-5. 36. Haward, R. N. (Ed.) (1973). The Physics of Glassy Polymers, Applied Science Publishers, London. 37. Ziegler, E. E. (1954). SPE J., 10(4),13-16. 38. Dempsey, L. T. (1967). Polym. Engng. Sci., 7(2),86. 39. Titow, W. V. (1975). Plast. Polym., 43(165),98-101. 40. Faulkner, P. G. and Atkinson, J. R. (1972). Plast. Polym., 40(147), 109-117. 41. Wolf, J. (1967). Gas, 87 (November), 433. 42. Szabo, E. and Lally, R. E. (1975). Polym. Engng. Sci., 15(4),277-84. 43. Caryl, C. R. and Helmick, W. E., US Patent No.2 945 417: Apparatus and Mechanism for Concentration of Solar Rays on Objects to be Tested, 19th July, 1960. 44. Ives, G. c., Mead, J. A. and Riley, M. M. (1971). Handbook of Plastics Test Methods, Iliffe Books, London. 45. Caryl, C. R. (1967). SPE J., 23(1),49. 46. Kuist, C. H. and Maxim, L. D. (1968). SPE J., 24(7), 46-51. 47. Grossman, G. W. (1977). J. Coatings Technol., 49(633),45-54. 48. Allen, N. S., McKellar, J. F. and Wood, D. G. M. (1976). Plast. Rubb.: Mat. Appln., 1(2), 57-61.
12
49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60.
61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82.
Properties of Special Interest in PVC Materials and Products
511
Summers, J. W. (1976). 34th ANTEC SPE Proceedings, pp. 333-5. Kinmonth, R. A., Jr (1964). SPE Trans., 4(3),229-335. Cassel, B. and Gray, A. P. (1977). Plast. Engng, 33(5), 56-8. Wypych, J. (1975). J. Appl. Polym. ScL, 19(12), 3387. Wypych, J. (1976). Ibid, 20(2),557. De Coste, J. B. and Hansen, R. H. (1962). SPE J., 18(4), 431-9. Rabek, J. F., Shur, Y. J. and Ranby, B. (1975), J. Polym. Sci. Polym. Chern. Ed., 13(6), 1285-95. Anon. (1979). Eur. Plast. News, 6(3), 40. Katz, M., Shkolnik, S. and Ron, I. (1976). 34th ANTEC SPE Proceedings, p.511. Allara, D. L., Ibid, p. 245. Scullin, J. P., Girard, T. A. and Koda, C. F. (1965). Rubb. Plast. Age, 46(3), 267-8. Sahajpal, V. K. (1978). 'PVC compounding for low organoleptics and controlled bacteriological growth', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. Mascia, L. (1974). The Role of Additives in Plastics, Edward Arnold, London. De Coste, J. B. (1968). Ind. Eng. Chern., 7(4), 238-47. Kaplan, A. M., Greenberger, M. and Wendt, T. M. (1970). Polym. Engng. Sci., 10(4),241-6. Wessel, C. J. (1964). SPE Trans., 4(3), 193-207. Anon. (1965). Mod. Plast., 42(5), 168. McGinty, L. (1979). New Scientist, 183 (9th Aug.), (1167), 433. Socrates, G. (1979). Plast. Rubb. Wkly, (9th March), p. 10. Anon. (1975). Ibid, (21st February) p. 70. Anon. (1974). Chern. Engng. News, (16th December), pp. 24-5. Draft German Standard DIN 53743-1977. Testing of plastics: Gas-chromatographic determination of vinyl chloride (VC) in polyvinyl chloride (PVC). Anon. (1979). Plast. Technol., 25(9), 13. Berens, A. R., Crider, L. B. and Tomanek, C. J. (1975). J. Appl. Polym. ScL, 19(12), 3169-72. Anon. (1976). Chern. Engng. News, (8th March), p. 6. Anon. (1979). New Scientist, 183 (April 19th), (1151), 185. UK Health and Safety Executive. 'Vinyl Chloride-Code of Practice for Health Precautions', Temporary Format (February 1975). Anon. (1975). Plast. Rubb. Wkly, (18th April), p. 3. Daniels, G. A. and Proctor, D. E. (1975). Mod. Packag., 48(4),45-8. Anon. (1979). Plast. Technol., 25(5), 211. Anon. (1979). Mod. Plast. Int., 9(12),28. Clayton, H. M. (1977). In Developments in PVC Production and Processing-I, (Eds A. Whelan and J. L. Craft), Applied Science Publishers, London, Ch. 3. Anon. (1977). Plast. Rubb. Wkly, (18th March), p. 15. Estevez, J. M. J. (1969). Plast. Polym., 37(129),235-42.
512
W. V. Titow
83. Phillips, I. and Marks, G. C. (1961). Brit. Plast., 34, 319 and 385. 84. Edgerley, P. G. and Pettett, K. (1981). Plast. Rubb. Proc. Appl., 1(2), 133-7. 85. Iida, T., Nakanishi, M. and Goto, K. (1975). J. Polym. Sci. Polym. Chem. Ed., 13(6), 1381-92. 86. Mitera, J. and Michal, J. (1976). Chem. Prum., 26(8),417-20. 87. Clark, C. A. (1972). SPE J., 28(7), 30-5. 88. Fennimore, C. P. and Martin, F. J. (1966). Mod. Plast., 44(3), 141-8. 89. Isaacs, J. L. (1970), J. Fire Flamm., 1(1), 36-47. 90. Oswin, C. R. (1975). Plastic Films and Packaging, Applied Science Publishers, London. 91. BS 6336:1982. Guide to the development and presentation of fire tests and their use in hazard assessment. 92. Grieveson, B. M. (1976). 'The fire hazard of polymers', paper presented at the Polymer Symposium, British Association for the Advancement of Science, Lancaster, England, 3rd September, 1976. 93. Cullis, C. F. and Hirschler, M. M. (1981). The Combustion of Organic Polymers, Clarendon Press, Oxford University Press. 94. Titow, W. V. and Lanham, B. J. (1975). Reinforced Thermoplastics, Applied Science Publishers, London. 95. Ahrens, H. W. and Zahradnik, B. (1973). 'Oxygen index rating of plastics as a guide to their behaviour in fire', CSIR Special Report BOU 29.
CHAPTER 13
Industrial Compounding Technology ofRigid and Plasticised PVC W. HENSCHEL and P. FRANZ
13.1 INTRODUCTION The compounding process represents the link between raw material production and finished-article manufacture. Its function is to combine the PVC resin with the various additives required for processing and for the service properties of the final product, in accordance with the formulation. There are five general types of industrial PVC compounding • operation (Fig. 13.1) -preparation of pre-mixes and dry blends, -melt compounding and pelletising, -compounding for the feeding of film and sheet calenders, ----,production of pastes (plastisols, organosols, plastigels), -recycling. As indicated schematically in Fig. 13.2, the equipment required can be divided into the upstream section ahead of the compounder, the compounder itself, and the downstream equipment. The upstream units are more or less identical for all the five general types of compounding operation, but the compounder and its downstream equipment have to be adapted to the specific requirements of each type. A typical line is shown in Fig. 13.3. The upstream equipment handles the raw materials: it comprises silo storage, conveying, weighing. Included in the compounding section are the PVC pre-mixing operation, the actual compounding and, where pellets are produced, the pelletising operation. 513
514
W. Henschel and P. Franz PROCESSED PVC MATERIALS
COmpounding (mixing)
Compounding
~
IU~das feedstock -, Mtllt ,~r,
,Extrusion ,Blow moulding
I
d· I compoun Ing I and .
~~~J~~_~~ i 1 1
Extrusion caltlndtlring
II
..J
Furttitlr compounding and dirtlct (in-lintl) ftltldlng to caltlndtlr
~~~~~
Iu~d asftltldstOek for~ 1
'Extrusion i,Blow Injtlction moulding moulding
Rtl-cycling
calJdtlrlng into
I I ,
~O~~~I~ ~~~..J
Fig. 13.1 Industrial compounding of PVC: general schematic outline. Table 13.1 indicates, for some important PVC products, the proportions produced, respectively, from pre-mix and from pellets as the feedstock. To interpret the table properly, one should bear in mind that the production of film, sheet and board, and of products from plastisols, involves processes with an in-line compounding step between pre-mix and final product. Thus, in these cases final-product processing follows directly on the compounding operation, and there is no need either for pelletising or for the downstream equipment that normally follows that operation. The downstream equipment normally employed for pellets and dry blends handles the cooling, conveying, storage and packaging of the compound.
13.2 RAW MATERIALS 13.2.1 PVC Polymer and Fillers
In terms of the amounts used in PVC compounding, these are the two principal solid raw material components; both are in powder form.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
515
r--T------------------, ~ RAW MATERIAL
~
STORAGE
5 a w
~
RAW MATERIAL HANDLING AND CONVEYING
!L _~_
RAW MATERIAL WEIGHING
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PVC polymer: The characteristics of PVC polymers, and their significance in formulation and processing, are discussed in Chapters 1-4. From the standpoint of the compounding or extrusion operation, it is improtant to emphasise those properties that are crucial to the production of free-flowing, dry powder blends.
2
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D Fig.13.3 Typical PVC compounding line. 1, Storage silo for PVC polymer; 2, production or holding silo for filler; 5, storage tank for plasticiser; 6, station; 8, weighing station for solid components; 9, weighing station for liquid cooling; 13, storage silo for pellets; 14, bagging and palleting; 15, plant control
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~o ~~C ~ storage silo for filler; 3, production or holding silo for PVC polymer; 4, production or holding tank for plasticiser; 7, minor components (additives) components; 10, premixer; 11, melt compounding and pelletising; 12, pellet system.
518
W. Henschel and P. Franz
TABLE 13.1 Proportions of Important PVC Products Produced, Respectively, from Pre-mix and Pelletised Compounds (World-wide)
Products
Produced from: pre-mix (%) pellets (%)
Extrusions (pipes, profiles, tubes, hoses, siding) Injection mouldings Cables Records Blow mouldings (bottles) Film and sheet Plastisol products
90 25
75
70
100 100
10 75
100 25 30
Table 13.2 lists some of the relevant properties of commercial PVC polymers produced by suspension polymerisation (S-PVC), emulsion polymerisation (E-PVC) and mass polymerisation (M-PVC). TABLE 13.2 Some Properties of PVC Polymers Polymer type
Properties
Kvalue (DIN 53726) Processing Particle shape Particle size (/lIIl)
M-PVC
S-PVC
E-PVC
57-71
55-71
65-75 Sprayed by special process Spherical, whole and broken spheres
Dried with Sprayed rolls Riven, porous up to 1000
Bulk density (g litre -1) 54~30 (DIN 53468) Good Free-flowing property 1·5-5 Plasticiser absorption (ml DOP per 5 g PVC)
Riven, porous
Bead-shaped, Flaky compact, glassy 60-250 60-250 60-500
4~20
600-700
300
~
up to 200 30Q--4()()
Good
Good
Good
Poor
Good
5
2
J-5
0·5-1
J-5
up to 60
13
Industrial Compounding Technology of Rigid and Plasticised PVC
519
Fillers: The use and effects of fillers in PVC are discussed in Chapters 4 and 8. The effect of fillers on the production of hot blends depends on the loading, particle size and plasticiser absorption. High loadings of fine-grained filler make blends flow less freely. Fillers with a porous surface often absorb too much plasticiser, which in turn results in dry mixes. One positive effect worth mentioning is the use of very small amounts of colloidal silica for powdering poorly flowing blends in the cold mixer. 13.2.2 Plasticisers
Plasticisers are the principal liquid components employed in PVC compounding. The nature and classification of plasticisers, their properties, applications, and effects in PVC, are discussed in detail in Chapters 5-7 (also, passim, in Chapters 1, 4 and others). 13.2.3 Other Additives
The other constituents of PVC compositions, which-from the point of view of compounding-may be regarded as additives to the PVC polymer, are discussed in Chapter 4: some are also dealt with in considerable detail in separate chapters or chapter sections, e.g. stabilisers in Chapters 9 and 10; lubricants, colourants, and others in Chapter 11. 13.3 UPSTREAM EQUIPMENT (SILO STORAGE TO WEIGIDNG) 13.3.1 Silo Storage of PVC Polymer and Fillers
Storage of the solid raw material components calls for buildings designed to house bagged or container goods, or for silo installations. For economic reasons, preference is normally given today to batteries of silos capable of holding several thousand tonnes. The lower limit for economical silo storage of raw material components is a percomponent consumption of about 30 tonnes per month. (a) Silo Sizes Silos with capacities of 150 to 250 m3 are generally used for storing PVC in the plastics industry. Whilst smaller silos are also used, those
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W. Henschel and P. Franz
with volumes of less than 50 m3 are regarded as uneconomical. This minimum size is set by the capacity of the rail tank cars normally employed nowadays for delivering the raw materials from production plants. The ability to discharge the entire contents of a tank car into an empty silo in a single operation is essential. Otherwise, unnecessary waiting time would result for the tank cars. On the other hand, the maximum silo size depends on the transport possibilities from the silo fabrication plant to the erection site. If finished silos have to be transported by road or rail, the acceptable volume is limited to 150 m3 . Typically, the design diameter of the silo tank is 2·4 m, though 3·5 m silos are built occasionally. Larger units-up to a volume of 400 m3 for PVC-can only be transported by water or, if this is impossible, shipped in pieces and welded together on site. In the case of fillers (notably chalk), silos of volume greater than 150 m3 are hardly ever employed because of the relatively high bulk density of the contents. The size of a battery of silos (Plate B) in a plastics plant depends on the procurement possibilities for the raw materials, raw material consumption, the plant's geographic location, and not least the market situation in the raw material sector. (b) Materials of Silo Construction Nearly all the silos erected out-of-doors today are fabricated from an aluminium/magnesium alloy (AIMg 3). It is fair to say that the steel silo with internal coating and external paint finish has been displaced by the standard aluminium alloy silo in the field of PVC compounding. Aluminium alloys are weatherproof, require no maintenance (as no paint peels off and no rust develops) and have a virtually unlimited service life. The plates used have a smooth surface, with a peak-to-valley depth normally less than 20 f.lm. Silo walls of aluminium alloy are much less prone to adhesion of contents than those of other materials. There are no problems with electrostatic charges, because unpainted aluminium is an excellent conductor of electricity. Because the external wall reflects well, there is little product heating as a result of exposure to sunlight. For some years now, silos as large as 150 m3 have also been built of glass-fibre reinforced polyester. This material is superior to aluminium in terms of chemical stability and mechanical abrasion. The disadvantage of static charges causing dust to adhere to the silo wall is countered by using antistatic additives in the material.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
521
Plate B Silo installation for PVC polymer and fillers.
(c) Raw Material Intake (Silo Filling) With increasing use of silo storage facilities, the traditional practice of purchasing solid raw material components in bags or other small containers is being increasingly replaced by bulk purchase with delivery by tanker transport. The advantages are:
-less labour; -no loss of material in transport; -lower raw material prices; -no contamination of the materials and dust-free working conditions. The filling of raw material silos is always accomplished with pneumatic conveying systems. Both road tankers and rail tank cars are used. The vehicle tanks are generally designed to resist conveying
522
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Henschel and P. Franz
pressures. Compressed air, and not a suction system, is normally used to empty the vehicle tanks. Screw compressors are used to generate the required flow of oil-free conveying air. Though most road tankers have their own compressors, a stationary compressor installation is required at the plant for emptying rail tank cars. These compressors have a working pressure between 1·5 and 2·5 bar. Air flows lie between 400 and 800Nm3 h- 1 .* With the usual pipe diameters of 80 to 100mm, depending on the material conveyed, this results in conveying capacities of 15 to 30 Mp h -1. t Generally, each silo has its own pipe leading from the connection point for the filling hose. In order to retain flexibility with regard to raw materials, and the ability to handle small batches of different formulations and qualities smoothly, most plants have an additional dumping station for filling the storage silos (or holding bins, or both) with bag goods. Because the amount of bagged goods is usually very small compared with total plant throughput, a manual dumping station is normally sufficient for this purpose. The hourly filling rate achievable by manual opening and dumping of bags is about 3 tonnes. To eliminate bag scrap, it is generally advisable to follow the dumping station with a suitably dimensioned sifting machine before the raw material is conveyed pneumatically to the storage silo through a rotary valve or a pressurised tank. The bag-dumping station must be arranged so that, instead of escaping, the dust raised during dumping is drawn off to a filter by a suitable exhaust system. As a rule these bag-dumping stations are supplied with a built-on filter, so that the filter can be cleaned mechanically after each filling operation to return the dust to the raw material. A proper exhaust system for a bag-dumping station should be laid out for an air flow of about 20 m3 min- 1 at a vacuum of 200 mm w.g. The resulting air withdrawal velocity during dumping is about 0·5ms- 1 . If, in exceptional cases, larger quantities of bagged raw materials are expected, it is advisable to plan for a semi-automatic or fully automatic bag-dumping machine. Such machines are available on the market for dumping rates of about 600 bags per hour. Maximum and minimum level monitors are necessary in storage silos * A German unit = cubic metres per hour at STP (i.e. 20°C, one bar pressure). t Megaponds per hour (i.e. tonnes per hour).
13 Industrial Compounding Technology of Rigid and Plasticised PVC
523
to prevent both overfilling and unplanned emptying. For more sophisticated demands, it is also possible to use continuously operating devices to monitor the filling level at all times. (d) Raw Material Discharge Raw material discharge is a very important factor in the operation of a silo facility. Most conical silo outlets are built with a hopper angle of 60°. Except for plastics pellets, additional discharge aids must be attached to the outlet zone for virtually all fine-grained raw materials. The familiar ability of many pulverulent products to flow freely when fluidised with air is exploited with the aid of aerating devices. The suitability of a product for aeration is determined by its bulk density, angle of repose, grain size distribution and specific surface area. Aeration plates are built based on a number of different systems. The surfaces in contact with the product are made of an air-permeable material. Nylon and polyester are generally used to cover the aeration plates, but air-permeable ceramic materials, sintered metal, and polyethylene board are sometimes used instead. Nozzles are occasionally employed to inject the air into the product, but it is important to design them in such a way that no product can enter the tiny air channels. Aeration plates are laid out to blow in the air successively in different sections, thus achieving a pulsation effect. The air must be completely free of dust or oil. Air pressures as high as 2 bar are required, depending on material depth and bulk density. The assumptions generally employed are a specific surface loading of 2-4 m3 of air per minute and m2 of aeration surface. (See Fig. 13.4). Another important mechanical discharge aid is the vibration plate. It is particularly suitable for products that tend to 'shoot over'. In such cases, it is necessary to hold the products back while metering them to the equipment that follows. From the storage silos, the raw material components are conveyed pneumatically to the weighing station. In small plants, this can be accomplished with a ring pipe and discharge flap above the scale. In larger plants, the raw materials are transported pneumatically from the storage silos to the production (or holding) silos. The latter are located directly above the weighing station in the compounding line. The raw material components are normally metered into the pneumatic conveying lines via blow-through rotary valves. Two-cycle valves are sometimes used, particularly for low-velocity pneumatic conveying or plug conveying.
524
W. Henschel and P. Franz
VI~W
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Fig. 13.4 Storage silo for PVC polymer or fillers. (e) Dust Removal System As already noted, virtually all silo storage facilities in the plastics industry are filled by pneumatic conveying systems. The product is usually blown tangentially into the tank; cyclones are very seldom used to separate the product from the air stream. The dust content of the air differs, depending on silo size, filling level and particle size of the product. Suitable filters have to be provided to remove the dust from the air. Most silo installations are laid out with a filter for each tank
13 Industrial Compounding Technology of Rigid and Plasticised PVC
525
unit. The only exceptions are cases where the same product is stored in a number of silos. The filters are provided with fully automatic or semi-automatic purging, depending on the dust loading and throughput. The difference between the two is that fully automatic filters are purged by dust-laden air during service, while semi-automatic filters are cleaned only when the filling operation is complete. Either bag or sheet filters can be used. The advantage of the sheet filter over the bag filter is that it occupies less space. For lower dust loadings and coarse-particle dusts, filters with mechanically actuated purging devices are generally sufficient. For very highly loaded filters and those handling fine dust, jet-type filters with pneumatic purging are generally used. In this case, the dust is purged from the outside of the bags by applying compressed air at about 6 bar pressure to the inside of the filter elements for back-purging. Cotton can be used for the filter fabric, but synthetics such as polyacrylonitrile or polyester are usually favoured. The filter area to be provided is governed by the admissible filter surface loading. Rule-of-thumb figures are: in the case of mechanically purged filter elements, 1 m2 of filter area can handle 1 m3 min- 1 of dust-laden air: in the case of pneumatically purged jet filters, 1 m2 of filter area can be loaded with 3 m3 min- 1 of dust-laden air. 13.3.2 Conveying of PVC Polymer and Fillers
As already indicated, any two operations in a compounding process are generally separated by a transport distance for the solid raw material components, dry blend, finished pellets, or for recycled process or start-up waste. The most suitable conveying system has to be found for each material, depending on flow rate, conveying distance, and special cleanliness requirements. It is also necessary to consider the material temperature (and whether it cools down or heats up), as well as the possibility of its segregation into various fractions. The decision whether pneumatic or mechanical conveying (by means of screws, bucket elevators, etc.) should be given preference will be made in the light of these considerations. (a) Pneumatic Conveying Pneumatic conveying, i.e. the transport of bulk materials in closed pipes with the aid of a stream of air, is standard practice in PVC processing plants just as it is in other industries. The technique has
-1500
b
Piston compressor, radial blower
Radial blower
Short to medium distances
30
150
Charging pellets into bins and machines; removing free-flowing materials from tips or containers Silo filling, suction pick-up from grinding mills
Silo filling
Filling of production or holding silos from storage silos or from bag-dumping stations Filling of holding silos
Filling of storage silos from pressureproof tankers
Application
This system offers special advantages in conjunction with such process steps as drying and cooling.
° 10 000 mm w.g. = 1 kgf cm- 2 .
Suction/pressure conveyor
Suction conveyor
Vacuum of about -5000 Piston compressor
Up to 50 (very limited)
Radial blower
<2000
b
200
Piston compressor
4000-8000
Low pressure
Long distances
Screw compressor
15 000-25 000
(m)
High pressure (generally employed for high conveying rates) Medium pressure
Conveying distance
Operating pressure range (mm w.g.)O
System
Compressor
TABLE 13.3 Main Features of Conveying Systems
Cyclone with rotary valve, filter
Cyclone with rotary valve, silo filter
Material delivery
Picked up by suction
Picked up by suction
Filter, cyclone with rotary valve
(1) Cyclone with rotary valve, filter (2) Vacuum pot with filter
Rotary valve, Cyclone with pendulum flap rotary valve, filter
Rotary valve
Pressureresistant silo
Material pick-up
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13 Industrial Compounding Technology of Rigid and Plasticised PVC
527
decisive advantages, viz. easy adaptation of the system to a given building, complete emptying of the conveying pipes, and the entirely enclosed nature of the conveying system. CONVEYING SYSTEMS
The basic division in pneumatic conveying is between suction and pressure systems. There are also combinations, which are generally known as pull/push systems. The main features of various types of conveying systems and their applications are summarised in Table 13.3. CONVEYING MODES
Flow conditions can differ widely in a conveying pipe depending on the bulk material conveying rate, air velocity, particle size, and pipe diameter. Four basic conveying modes are recognised: suspension, stream, plug, and high-density conveying. Traditional systems work on the suspension principle, with air velocities between 15 and 30 m s-1, and material loadings up to 30 kg of material per kg of air throughput. Typically, the individual particles are distributed virtually evenly over the pipe cross-section and ricochet from one side of the pipe to the other while being conveyed. At air velocities in the range 5-15 m S-l, the materials no longer ricochet; instead, they are pushed through the pipe in the form of lumps and plugs. Figure 13.5 illustrates the typical pneumatic conveying modes. Figure 13.6 summarises the limits within which each of the four conveying systems works. The basic advantages of the different conveying modes are listed below. Suspension and stream conveying:
-relatively inexpensive system, -virtually all pneumatically conveyable products can be handled, -<:onveying usually remains effective when products are modified, -appropriate in cases where no extreme demands arise in terms of operating time or conveying rates. Low-velocity conveying:
-gentle treatment of material conveyed, -avoidance of abrasion (dust, floss), -low system wear, -low energy consumption,
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W. Henschel and P. Franz
Fig. 13.5 Typical conveying modes in pneumatic conveying systems. All figures refer to a pipe of 100 mm nominal size. * Ratio of particle and air flow rates (kg h- 1 particle/kg h- 1 air). ** Particle (pellet) velocity.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
529
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-low air consumption, -low dust removal cost, -stable conveying mode, -low noise level along pipes and at separator, -no serious problem with electrostatic charges. The conveying modes usually employed in PVC processing plants are the following: E-PVC: Mostly suspension conveying, but some plug conveying, because the fine powder cannot stick to the pipe walls when so conveyed. In the case of suspension conveying, the solids loading is
= 10 kg h- 1 solids kg h- 1 air
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W. Henschel and P. Franz
S-PVC: Mainly suspension conveying; solids loading is
kg h -1 solids 20 to 25 k h-1 . g au Fillers: Mainly suspension conveying, though for abrasive materials such as untreated chalk, plug conveying is possible with solids loadings of 1
60 to 80 kg h- sol~ds kg h- 1 au PVC pellets: Suspension conveying, though ftuidised conveying (low velocity) is more effective in avoiding pellet abrasion. Pipe diameters generally lie between the nominal sizes of 80 and 125 mm. Aluminium alloys are usually used for the straight pipe sections while bends (with radii between 1000 and 1500 mm) are normally made of stainless steel.
13.3.3 Storage of Plasticisers (a) Tank Sizes The installation of storage tanks can become an economical proposition at consumption levels of about 10 tonnes per month of a plasticiser (or even at 5 tonnes per month in exceptional cases) for the following reasons: -lower purchase price for plasticisers, -easier product handling, -labour savings, -less storage space required, -easier surveillance of stocks. A storage tank should be large enough to accept the entire contents of one delivery vehicle. To ensure continuous withdrawal and complete emptying, however, it is advisable to install a second tank of the same size or possibly somewhat smaller. Typical storage tanks have capacities of 6, 12 or 24 m3 . If possible, the tanks should be set up in the production plant to allow withdrawal of plasticiser entirely by gravity, without using any pump or compressed air. In the case of larger storage tanks, though, it is more advisable to install the tanks at ground level: then the plasticiser is pumped into the production or holding tank with a centrifugal pump (chemical pump), which is set up in the compounding area above the metering and weighing station.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
531
(b) Suitable Construction Materials
Tanks -corrosion-resistant steel; -steel with paint finish resistant to the plasticiser; -steel, sand-blasted, internal walls sprayed with plasticiser; -aluminium; -glass-fibre reinforced Palatal.
Pipes -corrosion-resistant steel; -steel; -aluminium; -glass-fibre reinforced Palatal.
Valves, ball valves -corrosion-resistant steel; -steel; -aluminium; -cast iron.
Pumps: -cast iron. The pumps should be equipped with single-acting mechanical seals, because packing cords are attacked by the plasticiser. (c) Plasticiser Delivery Small consumers normally receive the plasticisers in drums, and large-scale production plants in road tankers holding l(~20 tonnes (or, under certain conditions, in tanker compartments holding 5 tonnes), or in rail tank cars holding between 15 and 30 tonnes. Road tankers normally have an outlet nozzle of 80 mm nominal size, and are equipped with a pump or a compressor for emptying the plasticiser. Maximum delivery head is 10 m, provided the piping is straight. Most rail tank cars have outlet nozzles of 100 mm nominal size. Here the customer must provide the pump for emptying the car. A properly dimensioned pump will drain a tank car in about one hour. The storage tanks should be installed in the immediate vicinity of the siding.
532
vv. lfenscheland P. Franz
(d) Pointers on Pipe Laying All piping should be laid so that it will run empty into the storage tank. It is advisable to use 80 mm nominal size piping for empyting the storage tanks, i.e. for pumping the plasticiser into the production or holding tanks. If the plasticisers are highly viscous, the piping must be heated. 13.3.4 Storage of Additives Processing additives are delivered, and subsequently stored in the processing plant, in small containers such as drums, bags or the like. Additives required in the production line are emptied at dust-free bagor drum-dumping stations into the production or holding silos with capacities between 0·5 and 2 m3 . These silos are normally made of stainless steel. The additives are generally withdrawn with the aid of metering screws, which feed them into the material flow as required by particular formulations. 13.3.5 Metering and Weighing
(a) Fundamentals of Metering and Weighing Technology If a material must be removed from a silo or tank at a steady rate, or if
a specified weight or volume must be removed, metering equipment is required. Accurate metering of the individual raw materials is a crucial prerequisite of precise formulation maintenance, and therefore of final product quality. Generally, the raw materials and the final product are the factors determining whether to meter gravimetrically or volumetrically, continuously or intermittently. CONTINUOUS VOLUMETRIC METERING
The following volumetric metering equipment is available for raw material components in powder or paste form, depending on the physical properties of the product. -Metering screws (in single-or twin-screw designs): Single screws are employed in the form of simple helical screws, helical ribbons and worms, sometimes with supplementary vibration or concentric spirals in the inlet to break up the product; -Belt metering devices; -Vibrating chutes; -Rotary valves.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
533
Liquid raw material components are metered by means of positive displacement pumps (piston or gear). In volumetric metering, high accuracy can be achieved only if the physical properties of the metered material remain constant. Particulate materials should not be subject to bulk density variation such as can occur through compaction or fluidisation in the course of filling and emptying of the silos, or even inside the metering device itself. In the case of liquids, temperature fluctuations can cause substantial changes in volume. Another requirement is that the mechanical and geometrical parameters of the metering device remain constant. The quantity of material delivered per unit time is constant if its conveying speed and cross-section in transit are likewise kept constant. Depending on the physical properties of the individual materials, volumetric metering entails weight tolerances between ±2 and 5%, or even higher. Specified formulation accuracies normally do not permit the use of volumetric metering for assembly of components, particularly where powders are involved. CONTINUOUS GRAVIMETRIC METERING
In the case of gravimetric metering-and in contrast with volumetric metering-neither the various product properties, such as particle size, particle shape, bulk density or flow behaviour, nor the specified throughput rate, make any difference whatever. For many years, it has been standard practice to use belt weighers for this work. Because the materials can differ widely (pellets, beads, surging or poorly flowing powders, fibres, etc.), the belt weighers are nearly always equipped with feeding units that present a pre-metered stream of material to the weigher. Belt loading is detected continuously by a load cell, and the metering control system (set-point/actual comparison) keeps the product weight (belt loading x belt speed) constant at a certain, pre-selected level. Belt weigher systems work with a short-term deviation from set-points of the order of ±0·3 to 0·5%. Metering screw weighers operate on the same principle. The operating principle of a differential metering weigher is totally different from that of the continuous belt weigher or metering screw weigher. It is a refinement of the subtraction-type container scale that has been modified to suit continuous operation. The material being metered is withdrawn from a supply container by a discharge unit-a metering screw, conveyor belt, vibration chute or
534
W. Henschel and P. Franz
pump. The metering control system operates this discharge device in such a way that the material removed per unit time corresponds to a pre-established set-point curve. When the container is empty, the system switches automatically to a volumetric phase and the container is refilled with product. During the volumetric phase, the scale's discharge device operates at the last-established speed. Once the container is back to maximum filling level, the system switches back to gravimetric operation. Despite its obvious advantages, continuous gravimetric metering has yet to break into the PVC processing industry. One reason is the great variety of formulations. Besides the large quantities involved (PVC, fillers, some plasticisers), very small formulation portions, between 0·5 and 5% by weight, must be processed as well. But even more important is the fact that the pre-mixing for PVC compoundingdry blend and agglomerate production-still works on the batch principle. This makes it unnecessary to use continuous gravimetric metering systems. Figure 13.7 summarises the different batch and continuous weighing systems. BATCH WEIGHING SYSTEMS
As a rule, a number of choices is available to a planner of batch weighing installations, in terms of both weighing and metering techniques. The decision is not always easy as the relevant evaluation criteria may vary. Wherever final products are being produced in automatically functioning systems, accuracy is almost always at the top of the specification list. In other words, the components going into the final product, often in widely varying proportions by weight, have to be added as accurately as possible. Because modern weighing systems can be produced with very high resolution, offering high weighing accuracy with static measuring methods, preference is often given to batch weighing in the layout of new plants. We have already seen that this is the case in the PVC compounding field. Three systems are available in current batch weighing practice (see Fig. 13.8). Together with the various types of scales, these systems-mechanical, hybrid and electromechanical-give the user a broad range of alternatives. With the different transducer systems available, i.e. mechanical and mechanical-electrical, and the choice of analog or digital technology for analysis and display equipment, the tendency in modern weighing and control practice is towards the following combinations:
13 Industrial Compounding Technology of Rigid and Plasticised PVC
535
BATCH WEIGHING
I
mechanical
I
batch scales
II II
hybrid
Ilelectromechanical
floor scales
Ir
vehicle scales
road vehicle scales
I I
container scales
solids
I
I
railway scales
I bagging scales I
II
liquids
gross weight baqqinq
I
net weight baqqinq
CONTINUOUS WEIGHING
I
I
I I differential weighers I I electromechanical
ICOOJE!'y'OI" belt weighers I
mechanical
I
belt weighers
I
lmetering belt weighers I
Fig. 13.7 Batch and continuous weighing systems.
-digital display of weight readings, -handling separated from the weighing station, -automatic control functions and calculations with the use of microcomputers, -data processing and display unit that can be placed anywhere, -compatibility with peripheral computer and data processing installations.
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a I
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c Fig. 13.8 Weighing systems. A, Mechanical; B, electromechanical; C, hybrid. Key: a, load receptacle; b, lever system; c, force detector; d, junction box; e, cable; f, weight display.
Solid raw material components: Depending on the formulation called for by the final product, specific quantities of each raw material are metered into a scale container. In this operation, the smallest possible batch metering error is determined primarily by the accuracy of the container scale. The only demands placed on the scale charging equipment are that it should deliver a roughly continuous flow of bulk material, and that this should be cut off virtually instantaneously when the preset weight is reached. The metering accuracy of the piece of
13 Industrial Compounding Technology of Rigid and Plasticised PVC
537
equipment used to charge the scale has no influence on the batch metering error. Container scales are charged with volumetric metering devices such as rotary valves, vibrating chutes, single and double metering screws, metering screws with agitators, or vibrating screws. To prevent the scale reading from being falsified by the impact of the falling bulk material, most container scale manufacturers recommend that the momentary quantity of material during filling be limited to an hourly rate that corresponds to 60 to 70 times the maximum scale weight. But if peak accuracies are required in weighing, the maximum product rate during filling should not exceed 10 times the maximum scale figure per hour. In other words, a 100 kg scale could be filled with a metering screw having a maximum product throughput of 1000 kg h- 1 . In practice, weighing cycles are kept short (1-1·5 min), and high accuracy is achieved at the same time by filling in about 90 to 95% of the prescribed batch weight at the maximum admissible throughput rate. Then the remaining 5 to 10% of the bulk material is metered into the scale container at reduced throughput until the final weight is reached. This approach is known as coarse/fine metering. It is achieved by equipping the metering screws with a change-pole motor. The cut-off of product flow upon reaching the preset weight is obtained by mounting a pneumatically actuated closure flap on the outlet of the metering screw. It is also essential that the scale container should be emptied completely after its hatch is opened. This is done by mounting a beater on the container or installing a flexible venting cloth inside it. Many weighing errors are traceable to incorrectly installed scale containers. Every scale container must be able to move freely in the vertical direction, i.e. all incoming and outgoing pipes must be attached to the scale container with elastic collars (Fig. 13.9). Efficient aeration and venting of the scale container are also necessary. Most PVC compounding plants have several batch weighing installations. One is required to weigh-in the solid raw material components according to the formulation (PVC, fillers, possibly regrind). Scales with weighing ranges from 0 to 500 kg, or 0 to 1000 kg, are normally found in this part of the plant. The individual components are withdrawn from the production or holding bins according to a preset programme in coarse and fine streams, and metered into the scale container one after the other. The entire process-metering of the raw material components in a specific order, weighing (set-point/ actual comparison), emptying of the scale into a holding container or
538
W. Henschel and P. Franz
Fig. 13.9 Weighing system for solids. 1, Container for solids; 2, discharge plate; 3, metering screw, 4, pneumatic closure flap; 5, container scale; 6, beater, 7, elastic collar; 8, vent.
preparation for pneumatic conveying or emptying of the scale directly into the premixer, initiation of the mixed weighing process, batch counting, and finally the pneumatic refilling of the production or holding bins from the storage silos-runs completely automatically. The batch weighing installation is rightly called the 'heart' of the overall production line.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
539
Liquids: Figure 13.10 shows the main metering methods for liquids. The most accurate metering is obtained by weighing the liquids as indicated in Fig. 13.lO(A). In this case the material being metered flows by gravity out of a storage tank into the scale tank, or else is pumped into it from a lower level. Temperature, density and viscosity fluctuations have no effect on metering accuracy here. Highly viscous liquids are usually heated. Where liquids are metered by volume, as illustrated in Fig. 13.lO(B), they are again conveyed by gravity or a pump into a collecting tank. In this case, the feed is cut off when a certain filling level is reached. This is a very simple metering method, but density fluctuations result in metering errors. Flow measurement, as illustrated in Fig. 13.10(C), is based on measuring the volume of liquid delivered by a pump. Because most flow meters react sensitively to fluctuations of the liquid's
A.
Weoighing
B.
Volumetric measuremeont
- -T-
flowmeter
\ C.
metering pump
Flow rat~ m~asur~ment
D.
Volumeotric displac~ment
Fig. 13.10 Metering methods for liquids.
540
w.
Henschel and P. Franz
viscosity, metering installations of this type are sometimes heated up to a constant temperature. In the approach shown in Fig. 13.1O(D), the liquid pump itself is the metering device. It delivers a specific volume of liquid at each stroke with high accuracy. The number of pump strokes is exactly proportional to the quantity of liquid metered. However, here again metering failures can arise as a result of density changes in the liquids. In a modern PVC compounding plant the second batch weighing installation is found in the plasticiser metering area. The various plasticisers are circulated from the production or holding tanks via piping loops by gear-type pumps. Automatically controlled ball valves direct the flow of plasticiser into the weighing tank. As soon as the prescribed plasticiser formulation has been assembled in the weighing tank, it is drained automatically into the downstream mixer or into a holding tank ahead of the mixer. The plasticiser is called up for charging into the mixer by the automatic programme control system. Minor additives: These are the additives whose quantities assembled for each batch are smaller than the quantity of PVC polymer by one or two powers of ten, i.e. their total content in the overall formulation is not more than 10% (and commonly 0·5-5%). For this reason, the additives are handled in a number of different ways in the PVC compounding field. Often they are weighed by hand by one person and then filled individually into PVC pouches for incorporation into the particular composition. The pouches are added in the production line at a minor component station, where the mixing station calls them up automatically for each batch of PVC. Labour shortages, rising material and wage costs, ecological problems, and not least the increasing quality requirements have fostered the development of fully automatic batch weighing installations for handling the minor additives. Figure 13 .11 shows a concept developed by the Waschle Company. Conceived along modular lines, the system provides for the metering and weighing of up to 16 components at a batch weighing station. The 1 m3 containers arranged in a circle around a container scale are filled manually from bags or drums by means of dust-free bag or drum dumpers. Refilling of the containers can also be carried out pneumatically from storage silos or bins. Individually adjustable discharge and metering screws, which are equipped with a rapidclosing flap, meter the products into a container scale to satisfy the particular formulation. Scales with several different weighing ranges are used to enable the batch weighing installation to work optimally.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
541
View A- A
level devIce
A
A 3000 d,a up to 16 contaIners of O'S m3 each 3800 d,a up to 16 contaIners of 1·0 m3 each
Fig. 13.11 Fully automatic weighing installation for additives.
542
W. Henschel and P. Franz
Control of the weighing procedure, i.e. input of the formulation, can be accomplished via selector switches or punched cards. The additives weighed in this manner are called up automatically by the pre-mixing station for each batch of PVc. Another possibility is to fill the additives automatically into PVC pouches, after which the pouches are automatically heat-sealed. This approach has the advantage of allowing the additive weighing to take place away from the production line. Here again, the additive pouches are fed into the production line automatically at a minor component station. (b) Control and Monitoring Equipment SYSTEM ACCURACY IN COMPOUNDING PLANTS
Even in a thoroughly planned metering installation, sizeable deviations can occur under certain conditions. Figure 13.12 illustrates the control loop for a weighing system. Very roughly stated, the following sources of error exist in the individual functions of the control loop: -mechanical errors in the scale, -cut-off errors in metering, -discharge errors during emptying of the scale, --errors in data input or acquisition.
Ulhmal•. conl,oll.d ,",I .al~
Fig. 13.12 Control loop of a weighing installation.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
543
These potential sources of error deserve a great deal of attention in the planning of batch metering and weighing systems, because they represent the most frequent cause of malfunctions. Ultimately, however, the crucial factors are the extent to which the material weight obtained from a container weigher agrees with the set-point, and the extent to which the recorded weight of a batch agrees with the quantity drawn from the scale. The system error of a weighing installation is obtained by comparing the deviation between set-point and the amount actually weighed out with the maximum value of the weigher's dial. System errors less than 2%0 are possible in weighing installations with mechanical scale filling. BATCH ASSEMBLY
As illustrated in Fig. 13.13 the control requirements for the actual weighing of the components are relatively modest. The output signals coming from the weighing installation comprise only the momentary weight readings in digital or analogue form. The only input signals required are the variables acting on the positioning elements, M1 and M2, and on the closing device, M3, of the scale container. Only two connections are required for the set-point input and the real value recording. Of course the inputs and outputs multiply with the number of components being metered. But the factor that really complicates the control system is the multitude of interlocks among the various functions and pieces of equipment in a PVC compounding plant. For one thing, the three batch-weighing installations described in the foregoing have to be integrated into a master control complex. The metering procedure in a plant designed along modern lines can run as follows: -Switch on plant in general. -Set all scales to 'automatic'. If necessary, zero taring can be carried out from the control desk. Automatic starting is only possible when all scales are at 'zero'. -Place punched cards in the reader. -Release mixer filling system. -Formulation is called for by the mixer control system. -Metering of the individual components in coarse/fine stream with 'finished' report following the last components. -Emptying of the individual scales in predetermined order and with predetermined timing.
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544
Henschel and P. Franz
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-Transfer of batches (solid components, liquids, additives) to the mixer calling for them. -All scales move to 'zero', and the next weighing procedure can be initiated. Analog and digital remote displays at the control desk make it possible to supervise the weighing installations.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
545
Table 13.4 provides a comparison of the main control functions of a metering and weighing installation with conventional control technology and microprocessor technology. The simplest type of control system is the one in which all batch weight inputs are set manually. In this case, the operators perform the metering and weighing of the materials, i.e. the accuracy with which the individual components are added to the mixing operation depends entirely on the reliability of the personnel. Though this system requires a minimum of electrical equipment, the role of the human factor makes the error quota relatively high. In control systems working entirely automatically, all figures required for the individual formulations are stored in punched cards. A card reader then transmits these figures to the measuring heads of the scales and to other measuring instruments and counters. Whilst other storage systems, such as crossbar distributors, may also be used, punched cards have proved effective in practice and are the most common form of storage. In a fully automatic control system like the one described above, the human factor is largely excluded. Moreover, the punched cards can be run through once before the actual production process starts, for verification of the punched figures. Manual operation is necessary even in the case of fully automatic control systems as a back-up, however, to allow operators to take a hand in the process if things go wrong. One of the most modern types of system is the freely programmable control system under which all of the data leaving and entering the weighing installation are fed to a computer unit. The logical sequence of the successive switching operations, measurements, monitoring operations and data outputs are laid down in the form of a computer program. With this system, one can alter or adapt the control procedure from one's desk. Virtually unlimited possibilities for control, data acquisition, data storage and data processing are offered by microprocessors, which are essentially miniaturised computers and data memories. Like a freely programmable control system, which is built around a computer, a control system based on microprocessors is largely indifferent to the particular application. The functional sequence of the particular compounding line is described with a computer program and then programmed. Control changes require nothing more than a program modification. In microprocessor technology, the number of data items stored is practically unlimited. Furthermore, a memory can take on the data for several weighing installations and many different
Simplest scale control system for 1 weigher and few formulations
Material balances
Output of process data
Storage of process data Arithmetic data processing Supply of process data
Formulation input, Digital switch or potentiometer weight input Formulation storage Selector switch Component or digital switch preselection Display on weigher Acquisition of process data scale
Control functions
Printout after each batch
Remote digital display of real weight
Punch card Punch card
Keypunch
Conventional scale control system for a number of weighers and formulations
After every batch on demand Sum of cumulated increments, total Sum of cumulated increments, each component Printout after each batch Printout on demand Output of process data on screen Balancing of daily, weekly and monthly consumption Inventory control Quality control Production control
Microcomputer
Remote digital display, total quantity Remote ditital display, real weight each component Remote digital display, set-point each component Batch counter with digital display Microcomputer
Microcomputer, call-up via code Microcomputer Material matrix
General possibilities
Supplementary possibilities
Data exchange with external computers
Error acquisition and diagnosis
Extension of control system for peripheral functions
Microprocessor technology
Hardware possibilities
TABLE 13.4 Possibilities of Conventional Control Systems and Microprocessor Technology
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13 Industrial Compounding Technology of Rigid and Plasticised PVC
547
components. Formulations can be called up in extremely simple fashion with code numbers. Individual weighings for a given component can be added as desired to provide a balance of the daily, weekly or monthly raw material consumption at any time. These figures can provide the basis for overall production control as well as stock control. With all of these possibilities, it is apparent that nothing remains to stand in the way of the all-automatic, unmanned production operation, but, like all other technical innovations, microprocessor technology is still very expensive and accessible only to a few highly skilled specialists. At the present time, the freely programmable control system is simpler to operate and offers nearly the same possibilities by connecting process computers. The big advantage of both types of control system is that modifications to the computer programs allow them to be used indefinitely, regardless of changes to equipment and processes. 13.4 MIXING 13.4.1 Blending of Bulk Materials in Overall Solid Phase
(a) Introduction The processing of PVC compositions for the manufacture of products (e.g. mouldings) or semi-products (e.g. sheeting) always involves powder blending as an early process step. During this operation PVC polymer powder is blended with the additives (fillers, lubricants, stabilisers, pigments, plasticisers etc.) called for by the particular formulation, plasticated to a greater or lesser extent and gelled, and possibly agglomerated or pelletised. Depending on the way the operation is carried out, the resulting blend of particulate formulation components (and any liquid components present) can take one of the following three forms, which are then processed further as indicated: (i)
A free-flowing powder blend, subsequently used directly for processing into products or semi-products. (ii) A free-flowing agglomerate, suitable for subsequent direct processing into products or semi-products. (iii) A powder blend that is only relatively free-flowing: blends of this type may subsequently be either melt-compounded and pelletised, with the pellets then being used in the production of
548
W. Henschel and P. Franz
products or semi-products; or melt-compounded for subsequent direct conversion of the compound-by calendering-into sheet or film. A basic distinction is made between cold blends and hot blends. The cold mixing of rigid PVC composition merely results in interdispersion of the individual components of the mixture. In the cold mixing of plasticised compositions, absorption of the plasticiser by the PVC particles takes place in addition to the interdispersion process. The mixing temperature in both these cases generally remains below 50°C. Cold blends are produced in slow-speed, unheated mixers. They require long mixing times and are not dry, particularly where the plasticiser content is relatively high. For this reason, their use is recommended only in cases where the blend does not have to be free-flowing or thoroughly dispersed, Le. where further plastication will take place in subsequent melt processing on mixing rolls, in an internal mixer, screw-type kneader or extruder. The mixing with simultaneous heating of a PVC composition to a point below the softening temperature is referred to as hot mixing, and the product as dry blend, dry powder, fluidised powder, powder compound, pre-mix or dry mix. These terms are used rather loosely, and there are no standard definitions. In some cases it may be clear what mixing state is meant, but considerable confusion can also arise. In hot mixing, the temperature is raised to 100°C or higher. In the case of rigid PVC this enables lubricants and liquid stabilisers to be absorbed by the PVC polymer particles: in the case of plasticised PVC, plasticisers are also similarly absorbed. The result in either case is a dry, free-flowing blend. The production of hot mixes involves the use of heatable, slow- or high-speed mixers, in either case in conjunction with a cold mixer. If the temperature of the hot mix is raised so that it is heated above its softening temperature, a PVC agglomerate is formed as groups of primary particles clump together to form enlarged secondary particles. Agglomerate formation does not, however, involve a melt stage. Agglomeration is obtained in mixers working with high-speed mixing tools: it becomes attractive where difficult mixing functions are involved, such as the incorporation of large percentages of plasticiser or filler, and where the possibility of segregation before the next process step is to be minimised.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
549
CONTINUOUS DRY BLENDING OF PVC COMPOSITIONS Conventional dry blending in the hot or hot/cold mixers is a batch process. One-step, continuous dry blending of uPVC and pPVC compositions has now also been introduced (inter alia for direct feeding of extruders-d. Chapter 19, Section 19.3). Equipment developments include an adaptation for powder blending of the Buss MT Turbine (see pp. 657-9), and the Thyssen Henschel horizontal mixer Kontinuum TKlD250.
(b) Theoretical Aspects of Mixing, with Special Reference to Dry Blending of pvc Compositions In the simplest case, mixing implies the presence of at least two substances with different characteristics. These may be totally different substances, or variants of the same substance differing merely in physical properties (e.g. particle size), temperature, or the like. In more complex cases, a number of constituent substances may be required for the desired final mixture. The job of the mixer is to combine the substances in such a way that extremely small samples of the mixture (of a size appropriate to the particular application) do in fact contain the constituents in proportions as close as possible to those aimed at. Wherever mixing is done on a commercial basis, i.e. by machine, the actual proportions will always differ to some extent from the theoretical ideal. The optimum mixture achievable in mechanical mixing is referred to as the ideal random mixture: this can be calculated for specific materials and mixing ratios. Samples can be taken from an actual mechanically mixed composition to determine whether the ideal random mixture has been approximated in the desired degree. The methods of statistical probability are an important element of this sample-taking and checking, and are integrated into the calculations employed. PHYSICAL STATE OF THE MIXTURE COMPONENTS
Table 13.5 shows how the difficulty of mixing materials in plastics processing varies as a function of the physical state of the mixture components. EFFECT OF THE PHYSICAL PROPERTIES OF THE MIXTURE MATERIAL
The mixing process is greatly affected by the physical properties of the mixture material, such as particle size, structure, angle of repose,
w.
550
Henschel and P. Franz
TABLE 13.5 Effect of the Physical State of Components upon the State of the Mixture and Difficulty of Mixing Physical state Main components
Additives
Mixture
Solid Solid, coarse-grained (granulate) Solid Solid Solid
Solid Solid, fine-grained (powder) Liquid, viscous Liquid, thin Liquid
Liquid
Solid
Liquid
Liquid, viscous
Liquid, viscous Liquid
Liquid, viscous
Liquid
Liquid
Liquid
Liquid
Liquid
Degree of difficulty in mixing
Solid Solid
Easy Fairly difficult
Solid Solid Liquid
Difficult Fairly difficult Easy to difficult, depending on particle size of solid component(s) Easy to fairly difficult Fairly difficult to difficult Easy to fairly difficult Easy
cohesion and adhesion to other components, adhesion to the working surfaces of the mixer, melting point, softening point, electrostatic properties, and moisture absorption. Any of these properties can be decisive in the selection of a particular machinery system, and all influence the level of sophistication called for. DETERMINATION OF MIXING EFFECTIVITY: SAMPLING AND SAMPLE SIZE
In the plastics industry the effectivity of mixing is often judged merely on the basis of final product reject rate, or material behaviour during final processing. But since many other variables are involved, it is impossible to tell on this basis whether difficulties have arisen as a result of inadequate mixing accuracy or as a result of other factors outside the mixing operation. Special measures should therefore be taken to monitor the quality of mixing, particularly as the effect of this on the final product can be much greater than generally assumed. The obvious approach is to take samples and determine the percentages of the individual components. This usually means separating the components again. If such a separation is no longer
13 Industrial Compounding Technology of Rigid and Plasticised PVC
551
possible, colorimetry can be employed in cases where a new colour is produced by mixing the components. Another method is to add test components, but these should behave very similarly to the components being mixed. Very reliable results are also obtained by mixing-in isotopes, which are later counted with suitable equipment. Sample size should be suited to the particular end product involved. The larger the samples can be, the less critical are any errors, starting with metering and moving on to mixing technology, sample taking and evaluation. Basically, to ensure comparability the sampling point should be located within the mixer and the gross size of the sample should be kept constant. The direction in which the sampling device is inserted into the mixture also affects the results, particularly in the case of large-scale mixing. When samples are examined, it is generally sufficient to show that the minimum acceptable percentage of each prescribed component is present. SIDE EFFECTS OF MIXING
Energy consumption: The efficiency with which the energy employed in mixing is used, as measured in terms of the uniformity of the final blend, is extremely low-just as it is in size-reduction operations. The bulk of the energy is converted into acceleration energy and heat. The objective should always be to do the necessary job with the lowest possible energy consumption per unit of final blend. Temperature rise: The effects of heating as a result of energy conversion differ greatly from one type of machine to another, as do the final blend temperatures reached in different mixers. The shorter the mixing time, i.e. the time required to impart the necessary mixing energy to the material, the less time is available for loss of heat, and the sharper the temperature rise. It has been found that suitable intensification of the mixing operation in high-speed mixers is an excellent way of heating up the material, so that heat need not be introduced from external sources. Energy conversion in the mixture can be increased by speeding up the mixing tools and altering their configuration. Particle size reduction: In the ideal case mixing should not change the type or size of the particles being mixed. But because any
552
W. Henschel and P. Franz
commercial mixing process involves mutual abrasion or collision of the particles, even the gentlest one results in a certain amount of particle size reduction. Seldom occurring as actual particle breakage, this phenomenon is generally a matter of extremely fine particle abrasion as a result of 'circular grinding' of the particles. If there is a tendency towards agglomeration in a mixture of fine powders, the forces responsible for size reduction can break down the agglomerates under certain conditions, which is entirely desirable.
Machine wear: During the mixing process, the material moves in relation to the mixing chamber wall and the mixing tools. Wear of these machine parts is therefore unavoidable. The degree of wear depends partly on the material being mixed and very much on the speed of the mixing tools. In general, the higher the speed the shorter the mixing time but the greater the wear, because the wear usually increases more than in direct proportion with the speed of the moving, wear-inducing material particles. Virgin polymers normally cause very little wear, but many fillers speed it up sharply. Wear and the desired mixing tool speed are criteria in the selection of a mixer for a particular process. Bulk density change: Because of particle size reduction and abrasion, some densification takes place in many cases, i.e. the bulk density of the mixed material rises. Where such densification is desirable, high-speed mixing machines are usually employed; but it can also be achieved by partial agglomeration of the particles as a result of product heating through energy conversion.
Particle segregation: In the mixing of particulate solid components, continuous segregation is superimposed on the mixing operation proper. Where components of sharply differing particle size are being mixed (coarse granulate with powdered colourants or coarse granulate with extremely small beads), the effect can be serious enough to make successful mixing nearly impossible. In many cases it becomes necessary to use certain additives in order to reduce or eliminate the tendency to segregation. Other side effects: Other side effects inevitable in any mixing
operation are those of adhesive, cohesive, and repulsive forces.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
553
Because these forces tend to act selectively, they hinder the actual mixing process. In some cases they can even render optimal mixing impossible.
(c) Mixers for Plastics Processing Virtually every type of mixer system on the market finds some application in plastics processing. This section deals with mixers for bulk materials in the solid overall phase, applicable to PVC compositions. Mixer systems for primarily liquid phases used in the production of plastisols are discussed in a later section. A mixing operation is integrated into every kneading process for compounds in the highly viscous, viscous/plastic, or paste-like overall phases handled with internal mixers, screw-type kneaders or extruders. This aspect, too, is dealt with in a later section. GENERAL CLASSIFICAnON
Fundamentally, one can distinguish between two types of mlXlng: mechanical mixing and pneumatic mixing. Mechanical mixers may be subdivided into two groups: those with rotating containers and those with rotating mixing tools. The static mixer may also be mentioned at this point for the sake of completeness. Another classification can be based on the mode of operation, i.e. whether the mixer operates continuously or intermittently. Figure 13.14 lists and illustrates mixers working on the batch principle. MECHANICAL MIXERS
Mixers with rotating containers: Operating principle: In rotating containers, the free-flowing bulk material is carried along by the tank wall by means of friction, and raised up, until it flows back down the incline formed (bulk cone). The cascade motion produced at low speeds becomes a cataract motion at higher speeds, whereby the particles are intermixed more intensively and the mixing time is considerably reduced. All mixers of this type produce a continuous interleaving and re-forming of the material, which is thus repeatedly divided and reunited in a different arrangement. The mixing process takes place virtually without shearing forces and affords gentle product handling.
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13 Industrial Compounding Technology of Rigid and Plasticised PVC
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Applications in the processing of PVC: -Pre-mixers for mixing plastic pellets with masterbatch pellets for the colouring of PVC on injection extruders or cable insulation extruders. -Post-mixers for mixing PVC pellets following compounding and prior to packaging for the purpose of batch homogenisation. This makes it possible to equalise production-related quality variations. Large-volume mixers such as double cone, tumbler or vee mixers are used for this purpose. Table 13.6 lists the main technical features of the mixers with rotating containers shown in Fig. 13.14. Mixers with rotating mixing tools:
Operating principle: The different mixer designs can be divided into three groups on the basis of peripheral speed (Vu ): slow-speed 'push' mixers with V u < 2 m S-l slow-speed 'throw' mixers with Vu = 2-12 m S-l high-speed intensive mixers with Vu = 12-50 m S-l In the slow-speed push mixers, the mixing tool displaces the material to the front and the side, so that it flows back into the resulting empty space behind the tool and becomes mixed. The material is treated gently and hardly any size reduction occurs. If the speed of the mixing tool is raised until the centrifugal force exceeds gravity, we have the throw mixer. The material particles are thrown upwards by the mixing tools, follow intersecting trajectories, and are thus mixed together. Mixing times are much shorter than they are in push mixers. Unlike push mixers, which are nearly filled with product, throw mixers are filled to only about 60 or 70% of their volume, to provide sufficient free space into which the mixture components can be thrown. Velocity differences between the various trajectories produce frictional forces that can help break up agglomerates, but friction-related product stressing and heating remains within reasonable limits. Still higher mixing tool speeds do not result in shorter mixing times, but raise the energy absorption, with resultant heating of the material, an effect that is desirable in many cases. Peripheral speeds between 30 and 50 m S-l are common in high-speed intensive mixers. The mixing
Twin shell mixer (vee)
Wobble miX« aod } double cone mixer
Gyrowheel mixer
PVC application
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Name
50-10
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5-10
50-75
30-20
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5-10
(min)
Speed of mixing container (r min-i)
50-75
(%)
(litres)
Mixing time
50-300
Filling level
Size, total volume
TABLE 13.6 Technical Data for Rotating-tank Mixers
0·1--40
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0·1-1·0 (up to 3·0)
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(kW)
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Industrial Compounding Technology of Rigid and Plasticised PVC
557
tool produces an intensive impact effect, which tends to break up particles besides dispersing the material components. Applications in PVC processing (see Fig. 13.14): -As hot or cold mixers for the blending of raw material components. This function can be performed by helical ribbon mixers, ribbon bar mixers, paddle mixers or ploughshare mixers. It is also often carried out by tank-type mixers, alternatively known as intensive mixers, fluidising mixers or turbo-mixers. -Change-can mixers and orbiting vertical screw mixers are used for the preparation of PVC pastes. -Vertical-screw silo mixers are used for after-mixing of PVC pellets following compounding for batch equalisation. Table 13.7 and Fig. 13.15 contain the main technical data for the mixers with rotating mixing tools shown in Fig. 13.14. Because tank-type or intensive mixers are extremely important in the processing of PVC, a detailed description of this type is provided in the next section.
(d) Tank-type or Intensive Mixer DESIGN AND OPERATING PRINCIPLE
Tank-type or intensive mixers, which are also known as turbo, high-speed, or fluidising mixers, are used mainly as batch mixers for the pre-mixing of the PVC raw material components to form a free-flowing powder blend. Among the best-known designs are those marketed by the following companies: Batagion, Caccia, Covema, Diosna, Fielder, Thyssen, Mixaco, Moritz, MTI, Papenmeier, Spangenberg. Tank-type mixers with high-speed mixing tools are generally built with cylindrical tanks arranged vertically (Fig. 13.16) or horizontally. Inside the tank, the mixing tools are mounted on a vertical or horizontal mixing shaft. The tools, which generally operate at peripheral speeds between 20 and 50 m s -1 differ depending on the mixing job and manufacturer. They may take the shape of radial flights in the form of bars, knives or propellers, or they may be ring-shaped or paddle-shaped. In the vertical design, the tank bottom is either flat or dished, and the ratio of tank diameter to height is approximately 1: 1. The Diosna design employs two cylindrical tanks joined together to form a figure-of-eight shell, which is equipped with two separate drives
Category
Helical ribbon mixer Cold mixer or hot mixer Ribbon bar mixer } Cold mixer in combination Paddle mixer with hot mixer Ploughshare mixer Hot mixer High-speed tank mixer Cold mixer Low-speed tank mixer
Name
60-70 40-50 80-90 40-50
200{}-{)000 100-3500 10-1500 25-4500
5-12
5-10
5-12
15-30
Size, Filling level Mixing time total volume (litres) (%) (min)
TABLE 13.7 Technical Data for Rotating-tool Mixers
350-50
1000-250
350-100
50-15
rotational (r min-I)
(10) 8-4
50-20
10-4
1·5
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Industrial Compounding Technology of Rigid and Plasticised PVC
559
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and two separate, counter-rotating sets of mixing flights. This doubles the useful volume with equal or better mixing quality, whilst staggered starting of the two drive motors avoids high current surges. Another interesting design is the Moritz 'Turbosphere' mixer (Fig. 13.17). In this, the use of a spherical tank and large-diameter agitator forces the material to run along the sphere wall in uniformly thin layers. The result is a thorough comminution of the individual layers, which promotes the dispersion and mixing effect. It also makes for much-improved heat transfer between the material and the heating or cooling medium circulating in the jacket. The spherical configuration
560
W. Henschel and P. Franz
Fig. 13.16 Tank or high-intensity mixer.
Paths of product
mov.m.nl
Fig. 13.17 The Turbosphere intensive mixer (Moritz).
13 Industrial Compounding Technology of Rigid and Plasticised PVC
561
affords positive self-release of the material from the tank wall: build-up on the internal wall surface is thus made more difficult, and cleaning of the tank is facilitated. The mixing tank of an intensive mixer is usually provided with a jacket for heat control, and can be equipped with degassing attachments. High-speed intensive mixers of the tank type are sometimes built with a removable mixing tank or with a machine superstructure that can be raised and lowered. The latter includes the tank cover, rotor with mixing tool, and the entire drive. The mode of operation of the intensive mixer stems from the speed and shape of the mixing tool. The material particles are accelerated so sharply by the mixing tool that they exceed the aeration velocity of the fixed bed and are fluidised. The high peripheral speed of the mixing tool imparts impact and friction forces to the material in the tangential direction and centrifugal forces in the radial direction. The latter drive the material against the tank wall in such a way that it can only escape upwards: upon reaching the cover, it falls back into the centre of the tank, which has been emptied of material. The result is the familiar whorl of material, i.e. a circulating flow that is otherwise found only in liquids. Intensive mixers with long mixer shafts and high-set mixing tools provide larger useful volumes (up to 90% of the total volume) than low-set tool combinations (Fig. 13.18). The high-speed mixers just described are generally used for batch mixing, with drive ratings of 0·4 to 0·6 kW per kg of material. For emulsion PVC blends with high percentages of plasticiser, drive ratings as high as 1 kW per kg of material are employed. Extrapolation of
I
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MIXing tool set low
Fig. 13.18 Product whorl resulting from mixing tool setting.
562
W. Henschel and P. Franz
these drive power levels indicates that high-speed mixers are economical only up to certain sizes. The largest known high-speed mixers on the market have a tank volume of 1000 to 1500 litres, which corresponds to a PVC batch size of about 800 kg. In order to convey dry blend or agglomerate efficiently, and to be able to store it at all, one must cool the heated product down to storage temperature. Theoretically it would be possible to cool the batch down in the high-speed mixer, but this would be unsuitable and unprofitable in production practice. Moreover, the alternating stresses on the machine in continuous operation would shorten its service life considerably. Since it is obviously desirable to cool the dry blend or agglomerate down to the storage temperature (40°C to 50°C) in the shortest possible time to avoid thermal degradation, and because cold shock is known to improve the free-flowing properties, it becomes necessary to employ separate mixers in series, the so-called hot/cold mixer combinations (Figures 13.19 and 13.20). Optimisation of this Hot mixer 5PHd at 1001 Vu =20-S0 ms"
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Fig. 13.19 Hot/cold mixer combination with hot mixer vertical and cold mixer horizontal.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
563
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combination in the production process dictates that both mixers should operate with the same cycle times, and hence-because PVC is a poor heat conductor-the cooling mixer must have a much larger volume: this is usually two to four times that of the hot mixer, and the tank is a cylinder arranged either vertically (whorl or plate mixer) or horizontally (throw mixer). The overall arrangement of the two units is generally stepped, with the hot mixer placed high up on a machine frame and the cold mixer set in front of it. Unlike the hot mixers (intensive mixers), the cold mixers operate with tool peripheral speeds of the order of only 4 to 8 m S-1. Cold mixers are equipped with jackets for circulation of water as cooling medium. Their mixing tools are relatively simple, and are sometimes hollow to permit circulation of cooling water. The cooling area is increased by installing water-cooled jacketed segments or baffle rings inside the tank, i.e. directly in the flow of the PVC blend undergoing agitation (Fig. 13.20). These inserts also direct the material onto the cooled bottom at the centre of the tank. Some cold mixer designs employ cooling air blown into the mixer with fans, in addition to the water-cooling system. Whilst providing about 30% of the cooling capacity, and thus reducing costs, the cooling air also serves to
W. Henschel and P. Franz
564
eliminate the undesirable condensation that often occurs when water is used as the sole cooling medium. The design principle of the cold mixer is aimed at providing as much cooling surface as possible for rapid cooling. Relatively little product movement is required for the heat transfer. The amount of heat dissipated per unit time declines with rising mixing tool speed and product velocity: when these reach sufficiently high values the material is no longer cooled at all, because the amount of heat generated by the energy of motion becomes equal to or greater than that dissipated through the cooling surfaces. Hot/cold mixer combination units are offered with various degrees of automation. Frequently the basic control system is designed for manual operation, but can be switched to the semi-automatic mode for emptying the unit when the preset material temperature is reached. More sophisticated systems are equipped with fully automatic process control based on material temperature and time, and linked to the overall plant control system. Data input is read-in either with punched cards or programmable process control systems. PROCESS ENGINEERING
Variables in the preparation of blends: Besides producing the PVC composition, the mixing operation is intended to impart those properties to it which will facilitate problem-free storage, conveying and processing in the particular process. The most important of these properties include:
(i) (ii)
(iii) (iv) (v) (vi) (vii)
homogeneous dispersion of the individual components throughout the blend; blend dryness, i.e. all additives, such as stabilisers, lubricants, etc., should be comminuted, homogeneously distributed and melted, and liquid additives such as plasticiser should be worked into the PVC polymer particles: any existing moisture should also be driven off; good free-flowing properties; low moisture absorption; no tendency to segregate; increased bulk density; good shelf life of final blend.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
565
The main variables affecting the achievement of these properties are: mixing tool shape, mixing tool speed, temperature of the material, filling level of tank, and pressure in tank. The mutual suitability of PVC and high-speed mixer processing is remarkable. The polymer has to be combined with a great variety of additives, into a wide range of compositions, and yet-because of its thermal sensitivity-its 'heat history' must be kept to a minimum. These objectives are achieved simply, efficiently and economically in high-speed mixers, utilising the polymer in the form in which it is produced (i.e. powder). The importance of external plasticisation, coupled with the polymer's ability to absorb plasticisers on heating, have been prime factors in the development of high-speed mixers. Slow-speed mixers-which work with mixing tool peripheral speeds only up to 10 m s-l-are not a viable alternative, as they are unable to deliver a dry, free-flowing and sufficiently homogeneous blend after a short period of mixing. Mixing tool shape and setting: Mixing tool profiles have to be designed to prevent undue overheating of the material and build-up on the mixing flights. Tool profile is extremely important in high-speed mixers in conjunction with the peripheral speed, which-as already mentioned~an lie between 20 and 50 m s-1. Because the peripheral speed determines the energy transmittable to the product, it is an important factor to consider when selecting a mixing system. As the peripheral speed of the mixing tools rises, so do the overall velocity of the material particles, their velocities relative to one another, and the resulting friction. This creates a solids-laden air stream that permits sharp impact between PVC particles and metal if sharp-edged or non-aerodynamic tools are used. Poorly designed tool profiles therefore make it possible for build-up of mix components to occur at the flight tips and their trailing edges where the material becomes overheated. Scorching can also occur if the individual PVC particles are heated by impact against the mixing tool: as this continues, the stabilisation of the PVC may prove inadequate and flecks or streaks of scorched material will appear in the final product. To prevent overheating, it is important that flow along the mixing tool surface should be uniform, enabling material particles lying directly in the path of a mixing tool flight to evade it and follow the flight profile without undergoing excessive impact or friction. For these
566
w.
Henschel and P. Franz
Fig. 13.21 Mixing tool cross-section. reasons, the profile of an efficient mixing tool should be aerodynamic (Figure 13.21), so that its flights allow the particles to flow off rapidly, and overheating of material on its surface is consistently avoided. The aerodynamic profiling and peripheral speed of the mixing tools are matched, to provide rapid and intensive energy transfer to the product, so that little time is required to produce fine blends and homogeneous agglomerates with high bulk density and good flow properties. The angular position, projected surface and spacing of the individual flights should be set optimally in relation to the mixing tank diameter, so that the resulting flow forces prevent any material build-up on the mixing tool and tank wall. Power input and temperature pattern; general aspects of the process sequence: The sequence of events in a high-speed mixer can be described as follows: When the machine is started, turbulent movement of material is set up immediately and the mixing of the individual components begins. The material is spun against the tank wall by the mixing tools and forced upwards. Thus it leaves the area of the mixing tools, following a toroidal path to the centre of the tank, to the bottom and back to the mixing tool area. Whilst undergoing this
13 Industrial Compounding Technology of Rigid and Plasticised PVC
567
horizontal-vertical motion in the direction of tool rotation, the material starts to heat up. Unlike some softer materials, particulate PVC is not comminuted to any great extent when subjected to impact, friction, and high mechanical stressing. The main effect is on the particle surfaces: sharp corners and protruding fibres are flattened or bent over. As a result the particles become smoother and rounder. This reduces the flow resistance (and-to some extent-the size) of the individual particle. The mechanisms contributing to heat build-up are: -friction between material and mixing tool; -inter-particle friction; -friction between air and mixing tool; -friction between material and tank wall; -heat transfer from heated tank wall to air and material; -heat generation by moving air and product; -radiation heating. The mixing progress is quasi-adiabatic, as the material is heated primarily by the mechanical energy imparted by the effective surfaces of the mixing tools (measured as the projections of flight surfaces at right-angles to the direction of rotation) and by the mutual impacts of the particles. Compared with these effects, the other heating mechanisms contribute relatively little to the heat build-up. If the mixing tools are properly designed, external heating (by means of a heating jacket) is required only in certain cases; for example it is advantageous where emulsion polymer is being mixed with a high percentage of plasticiser. Formulae are available describing the relationship between, on one hand the power imparted by the mixing tool, and on the other, such factors as the mass of material in the tank, the active tool-flight surface, the height of material in the tank, the difference between tool speed and product velocity, and the actual speed of the mixing tool. Such a formula shows that the power input (and hence also the mixing time) can be changed by modifying the effective mixing tool surfaces, and even more sharply by altering the tool speed (see also Fig. 13.22). As the quality demands specified for final products become ever more stringent, gentle treatment becomes increasingly important at every stage of processing, and above all during mixing. The best way to make mixing more gentle is to reduce the peripheral speed of mixing
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13 Industrial Compounding Technology of Rigid and Plasticised PVC
569
tools. In this case, of course, the shape and configuration of the tools must be adapted to the lower mixing speeds. The mixing of a rigid PVC composition will normally proceed as follows. The associated changes in power input (motor amperage) are shown in Fig. 13.23. The particulate components are homogenised by the turbulent mixing motion immediately after the high-speed mixer motor is switched on. As this occurs, the mechanical energy of the mixing tool is converted into heat by material friction. The friction, and particle impacts, bring about the particle surface changes previously mentioned and the material flows more freely, causing the energy consumption level to drop until a temperature of about 8SoC is reached. Around this temperature the lubricants melt, and the material becomes sticky andflows less freely, which again raises the energy consumption; this peaks when the liquefied lubricants are being absorbed by the PVC grains: thereafter the material again flows more freely and the energy consumption falls. Another energy peak, this time a small one, is observed at temperatures above 12SoC when the metallic stearates melt. A
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Fig. 13.23 Power input during the blending of rigid PVC in a high-speed mixer. Phase I, mixing and abrading; II, increase in free-flowing property; III, melting of lubricants; IV, dry mixing; V, melting of metallic stearates; VI, hot mixing; VII, cooling of blend in cold mixer.
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13 Industrial Compounding Technology of Rigid and Plasticised PVC
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When the motor amperage levels off, the mixing operation is over and it is time to discharge the material into the cold mixer. Continued mixing in this temperature range, but below the melting point of the PVC, would merely impair the flow properties as a result of further size reduction, accompanied-perhaps surprisingly-by a drop in bulk density. Thermal degradation would also be initiated. Changes in material temperature in the course of mixing are illustrated, for various PVC compositions, in Figs 13.24-13.27. Figure 13.24(A) shows the energy input and temperature curves of an emulsion PVC blend with 35% plasticiser. In this case the cold plasticiser was fed continuously into the preheated mixture. The start O(OC)
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Fig. 13.25 Temperature change in a high-speed mixer as a function of blending time. 1, S rigid PVC; 2, E rigid PVC; 3, S plasticised PVC 30% pi; 4, S plasticised PVC 40% pi; 5, S plasticised PVC 50% pi; 6, E plasticised PVC 40% pI. (Source: Herfeld Mixaco.)
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and finish of plasticiser feeding are apparent from the energy input curve. Figure 13.24(B) is a similar plot for suspension polymer, but with the cold plasticiser charged all at once into the mixture. Again, the point at which this took place is reflected in the temperature and energy curves. Both diagrams indicate that the quality of a blend is affected by the method and timing of the plasticiser addition. To obtain homogeneous dispersion of the plasticiser, it is necessary to achieve a mixer temperature of 70 to 80°C. In this temperature range, the PVC particles swell and absorb the continuously fed, and possibly preheated, plasticiser more easily. Figure 13.25 shows the temperature changes as a function of mixing time for rigid and plasticised PVC with various amounts of plasticiser. Figures 13.26(A) and (B) show the temperature curves, respectively, for a rigid PVC pipe blend and a plasticised (60:40) PVC shoe compound blend, both processed in a hot/cold mixer combination. Figure 13.27 illustrates, in a generalised and highly simplified form, the temperature changes of rigid and plasticised PVC compositions as functions of mixing time in hot/cold mixer combinations. Among factors contributing substantially to the achievement of optimal properties in a PVC composition are the timing, sequence and method of additive admixing, as well as the speed programme of the high-speed mixer, mixing time and final temperature. It is therefore a serious mistake to underestimate mixing technology and to regard the mixing step as an unimportant preliminary to subsequent processing. Influence of raw material components and processing conditions on the properties of hot blends: -PVC polymer. Particle structure, shape and size affect the blend flow properties. Suspension and bulk polymers with porous particles absorb the plasticiser easily. Fine-grained emulsion polymers containing emulsifier, and their large-grained clusters which are broken down in the mixer, usually result in pasty stock when the plasticiser is added: this can only be used to produce agglomerate. -Type of plasticiser. Dry mixes are difficult to produce with plasticisers of limited compatibility (which are poorly absorbed by the polymer particles), and with slow-gelling plasticisers. Primary plasticisers with good gelation effect give the best results. -Plasticiser quantity. This is a crucial factor in producing a hot
13 Industrial Compounding Technology of Rigid and Plasticised PVC
575
dry blend. The maximum amount of plasticiser that the hot mixer will tolerate depends on the type and K value of the polymer, as well as the plasticiser type. In some cases high percentages of plasticiser can only be handled by hot mixers to produce agglomerate. The temperature of incipient agglomerate formation, and then that of lump formation, decreases with increasing plasticiser content. In other words: the higher the plasticiser content, the lower the admissible final temperature. Cold mixing at high plasticiser contents will normally result in damp or wet mixtures lacking the ability to flow freely. -Plasticiser viscosity. This mainly influences the diffusion rate into the PVC particles (see Fig. 5.3 in Chapter 5). Medium- to high-viscosity plasticisers therefore require longer mixing times, unless their viscosity is reduced by heating before addition. -Admixing the plasticiser. Understandably, it makes a difference whether the plasticiser is placed in the mixer first and the solid components then added or whether it flows into the cold or hot pre-mix of the other components, and whether its introduction into the running mixer is made in one shot, in portions, or continuously. The best results are obtained by allowing the plasticiser to flow slowly into the pre-mix of the solid components at an elevated temperature (about 7o-S0oq. The feed rate should be set in such a way that the amperage drawn by the drive motor does not rise sharply. After the plasticiser has been added, mixing should be continued for about one minute to make sure all of it has been completely absorbed. If plasticiser is added to suspension or bulk PVC of medium absorption capacity faster than it can be absorbed by the particles, a highly viscous paste will form which will then gel to produce large, tough lumps if mixing is continued. This may cause the drive to cut out because of overloading: in any case, the mixture is unusable and cleaning of the mixer is unpleasant and time-consuming. -Mixing temperature and mixing time. Mixing time depends upon the required final temperature, which is given in the temperature/ time diagrams earlier in this section. For high-speed mixers it ranges between 10 and 20 min, depending on the formulation and whether a hot or hot/cold mix is involved. A reliable rule-ofthumb is that a temperature rise of more than 15°C min -1 should be avoided during mixing: otherwise local overheating can damage the mix. Furthermore, the correct final temperature of the mix is
576
W. Henschel and P. Franz
also affected by the type and quantity of plasticiser employed, as well as by the K value and particle structure of the polymer. It is set correctly when the cooled blend emerging from the cold mixer is dry and free-flowing. -Lubricant: The effects of lubricants in the hot mix depend upon their physical state (solid, liquid, paste), compatibility, melting characteristics and quantity. A liquid lubricant (e.g. paraffin oil) can produce a sticky, non-flowing hot mix if it is poorly compatible with PVC and becomes concentrated on the particle surface. PVC-compatible liquid lubricants are absorbed into the particles with the plasticiser, unless the amount added exceeds the compatibility limit. The melting range of solid lubricants should be chosen in such a way that (1) the lubricant is molten before the final mixture temperature is reached and is dispersed homogeneously throughout all of the material, and (2) the solid physical state has already been regained after the material has cooled down to the cold mixer temperature (about 40°C). In the case of rigid PVC blends, all formulation components except the lubricant are mixed at low speed: then the speed is raised, and the lubricant is added about 20°C below the desired final temperature. For the production of hot mixes, it is therefore advisable to combine PVC-compatible internal lubricants with solid external lubricants in the required melting range. -Fillers: The effect of a filler on the production of hot mixes depends on its amount, particle size and plasticiser absorption. High percentages of very fine-grained fillers impair the freeflowing property. Fillers with porous surfaces usually have undesirably high plasticiser absorptions, but, on the other hand, promote dry mixes. Mixes poorly flowing in the cold mixer can be improved by the addition of very small amounts of colloidal silica. APPLICATION OF INTENSIVE MIXERS
Either high-speed mixers (hot mixers) or hot/cold mixer combinations are used, depending on the production route followed for the final product. Hot mixers (high-speed mixers): Hot mixers are normally used alone ahead of pelletising machines and for direct processing of PVC powder blends. The advantages of hot mixing of PVC powders in high-speed
13 Industrial Compounding Technology of Rigid and Plasticised PVC
577
mixers are: -optimal mix quality and homogeneity; -short cycle times and high output rates; -very free-flowing blends; -pneumatic conveying of dry blends or agglomerates without product segregation; -up to 20-40% increase in bulk density as a result of sintering, thus raising the output rates of processing machines; -low-cost and effective lowering of the residual VC content, partly because of the high mixing temperatures of more than 80°C; -nearly complete elimination of residual moisture in the material.
Hot/cold mixer combinations: Hot/cold mixer combinations have won wide acceptance among PVC compounders in the past 15 years. As already mentioned, they enable the hot mix from the high-speed mixer to be transferred directly into the cold mixer and cooled down to about 4Q-50°C, whereby unnecessary 'heat history' (and hence thermal degradation) of the material is avoided, tendency to agglomeration during storage is counteracted, and the free-flowing property improved by the cold shock. Hot/cold mixer combinations are used both for feeding pelletising extruders and for direct processing of the PVC powder blend for use in injection moulding and blow moulding. Virtually all rigid PVC blends are produced on hot/cold mixer combinations, even if pelletising follows. During mixing of suspension and bulk polymers, high friction sometimes produces an electrostatic charge in the material, which impairs flow properties, especially in batches with relatively low bulk densities. One way to eliminate the charge is to cool the mix rapidly to about 40°C in the cold mixer. For certain purposes, e.g. the production of large pipes, crystal-clear blown films and similar sensitive products, which have to be absolutely free of moisture, the PVC batch can also be de-gassed during mixing. This can be done by using vacuum mixers or by blowing or drawing dry, filtered air under the hot-mixer cover during the final phase of the mixing process. 13.4.2 Melt Compounding In some production processes, such as the manufacture of records or the extrusion of pipe and profiles, it is sometimes possible to use
W. Henschel and P. Franz
578
pre-mixes, produced as described in Section 13.4.1, directly as feedstock for the processing machines. However, where only limited homogenisation is achievable in the processing equipment (such as for example, simple extruders, light-duty and heavy-duty calenders), or if a simple pre-mix is not acceptable as feedstock (by reason of particular material-handling arrangements or feed requirements), or where stringent quality specifications for the final product necessitate the highest degree of homogenisation of the composition, the material must be melt-compounded. Table 13.1 indicates the percentage of melt-compounded PVC used in the main application areas. While dry blending interdisperses the formulation components uniformly on a macroscopic, and down to the microscopic scale, the level of interdispersion produced by melt compounding may be described as microscopic to sub-microscopic, and largely intermolecular for those components which are sufficiently compatible. This degree of dispersion optimises the effectivity of additives combined with the PVC resin in the composition. It can only be achieved when the resin is in an elasto-viscous (melt) state during the compounding: this is brought about by thermomechanical means. PVC is a heat-sensitive material. The extent of thermal decomposition in processing depends on stock temperature and residence time, as illustrated by Fig. 13.28; therefore both must be carefully controlled in the compounding process. In this, thermomechanical stressing of the PVC polymer changes the structure of the original particles and hence
104.11 T.mp.,otur.
UNACCEPTABLE FOR EXTRUSION
(OC)
EXTRUDABLE AND WEATHERABLE 2S
50
75
Uillmot. Proc.sslng TI_ (1041.....1••)
100
AREA OF HEAT HISTORY IN BUSS COMPOUNDING AND PELLETISING LINES TYPE IIG
Fig. 13.28 Influence of processing conditions ('heat history') on the processability and performance of melt-compounded PVc.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
579
the rheological behaviour of the material. Raising the stock temperatures and residence times results in more severe structural disruption, and unravelling of the macromolecules, ultimately leading to a higher degree of gelation. Where pellets are being fed to an extruder, for instance, a higher degree of gelation requires higher melting energies during extrusion. Because modern final-processing extruders are pushed to the performance limit, raising the melting power may make certain extrusion operations unprofitable as a result of reduced output. Thus the compounding process has to be matched to the operating conditions in the final processing (production) step, whilst achieving its fundamental objective, viz. the maximum homogenisation and degree of dispersion of the additives in the PVC resin, with the lowest possible input of thermomechanical energy and minimum residence time, so that the 'heat history' (and hence degradation) of the resin is kept to a minimum. Besides homogenising the composition, compounding converts it into a physical form suitable for subsequent processing. In recent years, too, sharply increased processing speeds on calenders and extruders have added effective de-gassing of the PVC compoundoriginally effected on mixing rolls and in internal mixers-to the list of the functions of compounding. Whilst they are still in use in laboratories and in some small-scale operations, mixing rolls and internal mixers find only limited application in present-day industrial melt-compounding of PVC, which is now largely the domain of screw-type compounding machines. Continuous compounding on such equipment was introduced about 30 years ago with a view to better control of residence times, improved efficacy of mixing, reduction of compound quality fluctuation (and hence better reproducibility), process rationalisation, and elimination of the effects of chance in batch and intermittent operation. In view of this, the discussion in this section is confined to compounding processes involving continuously operating screw-type machines. In such machines, 80-90% of the energy required for fluxing the polymers and homogenising the mixture is obtained by the conversion of mechanical shearing energy. Only 10-20% of the total energy requirement is provided by heating the barrels and screws. The main job of the heating system is to ensure that the screw and barrel surfaces in contact· with the material are kept at a desired set-point temperature; inter alia this prevents overheating and scorching of the PVC stock as a result of wall slippage: thus the heating system must be
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W. Henschel and P. Franz
capable of both supplying and dissipating heat. For this reason systems based on the circulation of liquid heat-transfer media, such as water or HT oil, in screws and separate barrel zones have gained wide acceptance in industrial practice. (a) Compounding and pelletising Both rigid and plasticised PVC compositions are melt-compounded. The general types of compound produced are shown in Fig. 13.29. The characteristic curves of various rigid and plasticised PVC compositions, obtained on a high-pressure capillary rheometer (Fig. 13.30), demonstrate the wide differences in rheological behaviour that have to be coped with during the compounding process to obtain the necessary degree of gelation. The graph shows that with plasticised PVC compositions high outputs are obtained within a relatively narrow (and generally low) pressure range. This applies in melt compounding, so that-in the context of this process-plasticised PVC may be regarded as relatively insensitive to pressure and shear. For rigid PVC the graph shows a pronounced influence of pressure on output, i.e. in this sense rigid PVC compositions are sensitive to pressure and shear. Because energy dissipation in screw-type machines is essentially a function of shear rate and system pressure, adjustment of the degree of gelation of rigid PVC calls for special elements to control these parameters in this kind of equipment (see also further discussion below).
MELTCOMPOUNDED PVC COMPOSITIONS
FLEXIBLE
BOTTLE EXTRUSION CABLE COMPOUNDS COMPOUNDS COMPOUNDS
INJECTION MOLDING COMPOUNDS
EXTRUSION COMPOUNDS
Fig. 13.29 Melt-compounded PVC compositions.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
plasticised PVC [k~] Oulpul
2
581
rigid PVC 6
3
21)
1,5
1,0
0,5
o
10
20
30
40
so
60
70
Fig. 13.30 Characteristic flow curves of some PVC compositions in a capillary rheometer. 1, Shoe compounds; 2, water hose compounds; 3, general-purpose extrusion grades; 4, cable compounds (insulation, sheathing); 5, 6, 7, blow-moulding compounds; 8, general-purpose extrusion grades; 9, 10, profile compounds; 11, window profile and siding compounds. Both rigid and plasticised PVC are compounded for pelletising by essentially the same process (Fig. 13.31). The free-flowing PVC blend from the pre-mixer passes through a holding bin before being charged into the feed hopper which serves as a product surge bin and a volumetric metering element. In some cases the hopper is designed additionally for powder de-aeration, but this does not eliminate the need for de-gassing the fluxed stock. An agitator with suitably shaped arms is employed in the upper part to prevent bridging (Fig. 13.32). A separately driven twin-screw metering device is sometimes used below the hopper (as, for example, in the feed section of the Plastifikator machines). An alternative is to add a vertical metering screw to the bottom of the agitator, a concept employed, for instance, in the design of the Buss-Kneader and the Kombiplast machine. In either case, the speed of the metering screws determines the volumetric flow of product fed into the screw-type machine. In the intake zone of the machine the metered blend is picked up and carried along to the plasticating and homogenising zone. In this zone, part of the total shear energy is converted into heat and the PVC is fluxed (plasticated).
582
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Fig. 13.31 PVC pelletising: flow sheet. 1, Silos for solid components (resin, filler); 2, tanks for plasticisers; 3, discharge and conveying devices; 4, plasticiser supply pumps; 5, batch weighing station for solid components; 6, batch weighing station for plasticiser; 7, pre-mixer; 8, compounding and pelletising unit; 9, vacuum pump; 10, pellet cooler.
FEED HOPPER WITH HORIZONTAL TWIN SCREW
FEED HOPPER WITH VERTICAL 51 GLE SCREW
Fig. 13.32 Typical feed hopper designs.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
583
Another portion of the shear energy is used for breaking down and dispersing the additives. The ways and means of imparting the energy to the material are discussed in Section 13.4.4(a) ('screw-type machines'). Homogenisation is one of the three objectives of the compounding process, the other two being de-gassing and conversion of the compound into a form suitable for further processing. In practice, cylindrical pellets with diameters of 2·5-4 mm and lengths from 1 to 4 mm have proved optimal for feeding final processing machines. To produce the pellets, it is necessary to force the plastic PVC stock through a multi-hole die plate. One way to do this is to incorporate in the screw-type machine a suitable metering and pumping zone, and to mount the pelletising die plate directly on the end of such a machine. This approach represents a rigid combination of compounding and forming (i.e. pelletising). Research and development work started at the end of the 1960s demonstrated, however, that the optimal design and operating parameters for compounding and forming, respectively, are often diametrically opposed, so that it makes more sense to separate the two stages clearly from one another in high-production compounding lines. This approach resulted in the development of two-stage compounding lines, with one screw-type machine for compounding and a second one for pelletising. Usually the pelletising screw is set up in cascade fashion following the compounding screw. Pelletising machines are normally single-screw designs, but the compounding step may be handled by either single-screw or multi-screw machines. In some cascade compounding units, the homogeneously fluxed PVC stock drops in lump form through a connecting tube from the outlet end of the compounding stage into the feed opening of the pelletising screw. Vacuum can be applied to the connecting tube to de-gas the PVC stock. The pelletising screw proceeds to build up the pressure required for extruding the compounded stock through the pelletiser's die plate. With this separate arrangement, design and process parameters such as speed, shear and temperature profile can be selected in such a way that a minimum amount of energy is dissipated during the forming phase, which is probably the most critical phase of the entire compounding process. As already mentioned, when compounding rigid PVC compositions it is necessary to control effectively the system pressure in the
584
W. Henschel and P. Franz BUSS-KNEADER
REGULA TlNG SCREW
PELLETISER
Fig. 13.33 Back-pressure control by regulating screw.
compounding stage in order to maintain the desired degree of gelation in the PVC pellets. This control may be achieved in various ways. Buss-Kneader compounding lines of the KG type employ a regulating screw in place of the connecting tube between the compounding and pelletising stages (Fig. 13.33). The speed of this 2·5 LID single screw is separately and infinitely variable: its adjustment provides accurate control over back-pressure in the kneader, and hence over energy dissipation in the homogenisation zone and therefore over the stock temperature and degree of gelation. On the twin-screw ZSK Kombiplast machines both screws can be shifted axially with a gear motor and adjusting spindle. This moves the kneading elements at the discharge end (Fig. 13.34, shaded area) into the outlet orifice to a greater or lesser extent, so that the back-pressure can be varied infinitely without stopping production. Another approach to controlling pressure conditions is adopted in the twin-shaft MPC/v system. Two 'barrel valves' are employed (Fig. 13.35), one in the kneading disk area and the other at the discharge
Fig. 13.34 Back-pressure control by adjustable outlet orifice.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
585
Centreline olugilulol shults - -
Fig. 13.35 MPCN barrel valve.
end. This kind of valve constitutes a movable saddle-shaped cut-out in the barrel. When the valve body is turned manually, the saddle is set at right angles to the direction of product flow, which closes the gusset space between the two shafts and causes the pressure to build up. In the open position the saddle piece in the valve body is lined up with the barrel saddle, thus offering the least resistance to stock flow. All three arrangements allow the composition to be de-gassed in transit to the pelletising screw. Two basic cutting arrangements are available for pelletising~entral cutting and side cutting (Fig. 13.36). Side cutting has gained wide
KNIFE SHAFT At-() DIE PL ATE CONCENTRIC
KNIFE SHAFT NEXT TO DIE PLATE
Fig. 13.36 Basic cutting arrangements in pelletising equipment.
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W. Henschel and P. Franz
acceptance among compounders because of the favourable cutting conditions it provides. In both methods the plasticated PVC strands forced through the multi-hole die plate by the pelletising screw are cut into pellets by rotating knives on the outer surface of the die plate. Pellet length can be changed by altering the speed of knife rotation. The die head is enclosed in a collector casing that catches the pellets chopped off on the die plate, pre-cools them, and feeds them to the pneumatic conveying system. Because of their relatively high melt viscosity and low stickiness, most rigid and plasticised PVC formulations can be pelletised 'dry' by the hot die-face cutting method, i.e. room air is sufficient for cooling and conveying. In special cases, such as heavily plasticised formulations for the shoe industry or for medical purposes, record compounds with high vinyl acetate content, or bottle compounds with low K value polymer, it may be necessary to counteract pellet sticking by supplementing the pre-cooling in the collector casing and the conveying line with low levels of water mist. In this case, water and compressed air are mixed in a mixing valve (Fig. 13.37) and injected into the collector casing. The spontaneous evaporation of the water affords an additional cooling effect, and prevents any direct contact between the PVC pellets and free water. A flow meter is used to control the amount of water atomised. Particularly in the cable industry, impurities such as fibres, wood chips, paper cuttings, etc., which enter the compounding process in the raw materials, can seriously impair product quality and affect profitability. It is therefore becoming common practice in the cable
COMPRESSED AIR
~+-"'T'"-DC::.t---
WATER
PNEUMATIC PELLET CONVEYING
Fig. 13.37 Water-mist cooling of cut pellets.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
587
DIE PLATE
PACK
Fig. 13.38 Die plate with strainer. industry to filter such impurities out of the composition during compounding. Because the contamination level is generally relatively low, it does not always pay to use an automatic screen changer. An economical solution is to insert behind a pelletising die plate a sandwich screen pack consisting, for instance, of two 20 mesh support screens flanking a 60 mesh strainer screen (Fig. 13.38). The die plate must be designed to accept this screen pack. Experience has shown that the screen pack has to be changed once per working shift. The line is shut down, the die head swung out, and the contaminated screen removed. Wire and cable insulation is normally coloured all the way through. The cable industry prefers to colour the cable with pigment concentrates or masterbatches, which are normally prepared from a complete pre-mix with all components. A compounding unit must therefore be capable, in addition to its normal compounding duties, of dispersing pigments in high concentrations uniformly without streaking. One aspect of masterbatch production is the preparation of semiconductor PVC compounds, in which carbon black loadings as high as 30% are dispersed in the PVC mixture. In this case, too, a complete pre-mix of all components is supplied to the compounding unit. Apart from cable applications, these compounds are also used for producing antistatically finished PVC film and sheet. Another special demand placed on compounding technology by the cable industry arises from the trend towards increased use of
588
W. Henschel and P. Franz
crosslinkable polyethylenes, either the traditional, peroxidically crosslinkable polyethylene or Sioplas (developed by Dow Corning, UK). A modern compounding line must therefore be sufficiently versatile to compound and pelletise these crosslinkable polyethylene compounds as well as a broad range of PVC formulations. The specific energy normally dissipated in the compounding step is about 0·06-0·08 kWh kg- 1 for rigid PVC formulations and 0·040·06 kWh kg- 1 for plasticised PVC formulations. In the forming step, i.e. in the pelletising screw, additional specific energy is expended for pressure build-up and extrusion of the PVC stock through the die plate. A rule-of-thumb from actual practice indicates that the specific energy requirement for extrusion through the die plate equals about 1/10 to 1/5 of the energy dissipated during compounding. The introduction of energy causes the stock temperature to rise as it moves through the kneader. Figure 13.39 shows typical stock temperature profiles for various PVC formulations. Compounding lines now available on the market cover a production band from 100 kg h -1 to 6 t h -1. Besides these production units, scaled-down laboratory units are offered for formulation development and colour matching as a basis for production. Such units must employ
T (·e) 200
4
180
3
160
2
140
1
120 100
eo 60 40
20
~
~
~
~
I.ENGlH
('tttot )
Fig. 13.39 PVC pelletisiing: temperature profiles. 1, Record compounds; 2, plasticised PVC; 3, rigid PVC; 4, rigid PVC, US grades.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
589
the same mixing and homogenisation process as their production counterparts if the results are to be convertible. (b) Compounding of pvc for Feeding Calenders Calendering is a major process for the production of continuous film and sheet. In this process the compounding step is arranged in-line, i.e. the process runs from raw material through compounding to film or sheet forming on a continuous basis. The operation stages of a calender line as outlined in Fig. 13.40 must therefore be well matched to form an integral, interconnected system. These stages are common to regular calender lines, coating lines and light-duty calender lines, so the following remarks on the calendering process are intended to cover those processes as well. Because the calender is essentially a forming machine with no substantial mixing or homogenising effect, the compounding section must fulfil the following functions in the interconnected system: -fluxing of the polymers and homogenisation of the stock; -achievement of an optimal degree of gelation for the calendering process; -de-gassing of the stock and straining (where necessary); -conversion of the fluxed stock into a form suitable for feeding the calender. The provision of feedstock by the compounding section must, at any given time, match the calender's throughput rate.
Fig. 13.40 Block diagram of a calender line.
590
W. Henschel and P. Franz
These requirements apply to the entire range of PVC film and sheet formulations, which can be classified into three main groups according to their plasticiser content: -rigid (substantially without plasticiser); -semi-rigid (10-25% plasticiser-Shore A hardness 95-85); -plasticised (25-40% plasticiser-Shore A hardness 85-60). For plasticised PVC film and sheet, a standardised calendering practice has emerged in which a 'rolling bank' is set up in the calender nip. In the case of rigid PVC, processed at temperatures higher than those in plasticised PVC lines, one of two calendering methodsrespectively known as the high- and low-temperature processes-may be used, depending on the type of sheet or film being produced. In the high-temperature process, the calender bowls are heated to progressively higher temperatures with-typically-the intake bowls at about 160°C, and the last nip at about 210°C. The film or sheet is then passed through the usual cooling roll train. In the low-temperature process the reverse heating order is used. The temperature drops from, say, 175°C at the intake to 145°C at the last bowls, i.e. the fluxed stock is formed in a relatively cool state. Normally, the films produced by this method are fed directly to an orienting calender, where they are heated rapidly on melting rolls heated to about 240°C, and then cooled sharply down to 110°C. This provides optimal film orientation. Known under the name of the 'Luvitherm process', this low-temperature process has gained acceptance in Europe for the production of tearproof films. The calendering method also determines how the temperature will be controlled in the compounding step. For the high-temperature process, the PVC stock must be delivered to the calender fully plasticated; for the low-temperature process, it should be barely plasticated (crumbly), i.e. it has to be fed to the calender at a lower temperature. The calender design is determined essentially by the type of film and sheet to be produced on it. The design determines, in turn, the overall layout, position of the compounding unit relative to the calender, and the necessary conveying and feeding elements. Figure 13.41 shows the bowl arrangements in various four-roll calender types. At the present time, the 'L' and inverted 'L' (also called 'F') types are dominant.
13
(i)
(iv)
Industrial Compounding Technology of Rigid and Plasticised PVC
(iI)
(v)
591
(Iii)
(vi)
Fig. 13.41 Bowl arrangements in 4-bowl calenders. (i) Inverted 'L' type (sometimes referred to as 'F' type); (ii) 'z' type; (iii) inclined '5' (or 'Z') type, down-stack; (iv) inclined '5' (or 'Z') type, up-stack; (v) 'L' type; (vi) vertical stack. Arrangements (i), (ii) and (iii) are widely used in PVC sheet production.
From a development history rich in tradition, two basic methods for feeding calenders have emerged: -direct feeding; -feeding through a set of mixing rolls (two-roll mill) set up between compounding unit and calender. The mixing rolls are still justified where sport runs and a broad range of formulations are handled on a given calender line. In this case the mixing rolls serve as a material-surge unit, keeping the stock hot. This makes it possible to change formulations without any interruption of the calender operation in some cases. By passing through the roll nip repeatedly, the stock can also be de-gassed to a limited extent. However, it is necessary to keep the stock temperature in the
592
W. Henschel and P. Franz
compounding step lower because of the additional work done on the mixing rolls and the longer retention time of the PVC stock at elevated temperature. The endless ribbon of PVC emerging from the mixing rolls is slit and fed to the calender in endless strips. With direct calender feeding, which is undoubtedly the most economical solution in terms of capital investment, labour and space requirements, no mixing rolls are used. This type of feeding is appropriate mainly for the processing of large runs and relatively limited formulation ranges. The job of the compounding unit in this case is to gel the PVC stock to the degree required for the calender and to de-gas it effectively. Whether the calender is fed directly or via mixing rolls, the stock is delivered to the intake nip by conveyor belts. A separate wig-wag belt is sometimes used just before the calender to achieve better distribution of the stock acrosss the intake. These conveyor belts have to be selected carefully with regard to thermal and abrasion resistance; otherwise the stock can be contaminated and the quality of the PVC film or sheet impaired. Another point to watch is that bits of metal can sometimes enter the calender line with the raw materials. The consequences of even the smallest metal particles entering the rolls of the mixing bank or the calender can be much more serious for the machinery than for film or sheet quality. To avoid such damage, it is essential to set up a metal detector, capable of picking up non-magnetic as well as magnetic particles, between the compounding unit and the calender. Normally, the metal detector is located at the beginning of the conveyor belt. Whenever it responds to a bit of metal in the PVC stock, the belt is thrown into reverse for a preset period of time to eject the foreign object. It is a feature of the calendering process that the throughput varies widely. The variation may be deliberate or random (sporadic). The deliberate variation is a consequence of changes in (range of) formulations and qualities handled on the calender line. These determine calendering speeds and film or sheet gauges: since the full width of the calender is normally used, the throughput (and hence feed supply) required can be calculated roughly from calendering speed and film or sheet gauge. The random, momentary throughput variations stem from the nature of the calendering operation. To ensure trouble-free running, a certain amount of stock should always be maintained on the mixing rolls or at the calender intake. Because degradation is a function of the intensity and duration of heating (retention time) this buffer should be kept to a
13 Industrial Compounding Technology of Rigid and Plasticised PVC
593
minimum. If it starts increasing in size, the throughput of the compounding line must be cut back, and vice versa. Despite various attempts, no satisfactory method has been found as yet to measure the buffer stock volume on the mixing rolls or at the calender intake automatically, and to employ the resulting signal to control the throughput of the compounding unit. Such monitoring and throughput regulation must, therefore, still remain among the operator's duties. The compounding unit must be capable of delivering stock of unvarying degree of gelation over the entire output range. The overall variation can cover a ratio of 1: 10, i.e. the minimum throughput equals one tenth of the maximum throughput. As already described for PVC compounding generally, in compounding for the calender a free-flowing blend is first produced from the individual raw material components, and then discharged into a holding silo (Fig. 13.42). The blend leaves the holding silo-normally
Fig. 13.42 A typical compounding train for PVC calendering: flow sheet. 1, Truck unloading station for solid components; 2, debagging station for solid components; 3, pneumatic conveying system; 4, silos for solid components (resin, filler); 5, day bins for solid components; 6, day bins for plasticisers; 7, batch weighing station for solid components; 8, batch weighing station for liquid components; 9, minor ingredients proportioning station; 10, pre-mixer; 11, buffer silo; 12, compounder; 13, conveying belt with metal separator; 14, two-roll mill; 15, conveying belt; 16, calender; 17, edge trim cutter; 18, silo for cut edge trimmings.
594
W. Henschel and P. Franz
under gravity-to enter a feed hopper serving as buffer bin and volumetric metering element (see Section 13.4.2(a)). The metered blend flows into the screw-type kneader, where it is densified, plasticated and homogenised. To simplify operation and make it possible to maintain optimal compounding conditions when the throughput changes, systems have been developed for synchronising the feeds of the feed hopper shaft and the kneading screw. The speed ratio characteristic of the particular formulation is set on a potentiometer. When it is necessary to alter the throughput the operator needs only to raise or reduce the kneading screw speed by pressing a single pushbutton. The synchronisation system adjusts the metering rate automatically, so that the specific plastication rate remains constant regardless of the momentary throughput. For effective de-gassing, where necessary, the screw-type kneader is equipped with a de-gassing zone (Fig. 13.43), to which vacuum (70-200 mbar) is applied. If allowed to remain, the air, moisture and other volatiles introduced with the pre-mix can cause blistering of the calendered film or sheet. Their withdrawal from the stock is promoted and enhanced by the continuous creation and exposure of new surface in the course of compounding. Figure 13.44 demonstrates the effect of de-gassing on PVC stock. A die on the end of the kneader forms the stock into one or more strands. These can be fed either to mixing rolls or the calender. For
-~ ~
BUSS-KNEADER
Fig. 13.43 De-gassing arrangements in screw-type compounders.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
595
Fig. 13.44 Effect of de-gassing of PVC film/sheet stock (crystal-clear, rigid composition). Samples taken at the outlet of a screw-type kneader. Left, material processed without de-gassing (high bubble content); right, de-gassed, bubble-free material.
simpler and more uniform nip feeding, the strands can be sliced into chips with a cutting device working in conjunction with the die (Fig. 13.45). Straining of the stock may be desirable, particularly for the production of plasticised PVC sheet and film, to remove foreign objects such as fibres, paper cuttings, wood chips, etc., which may have entered the mixture from the raw material packages.
Fig. 13.45 Kneader die with cutting device.
596
w.
Henschel and P. Franz
This is done by setting up a 'strainer extruder' following the compounding unit. Normally a single-screw machine, it is equipped with screen pack and die. Three basic alternatives exist for straining, depending on the average degree of raw material contamination expected: -Use of automatic screen changer for high contamination levels requiring an hourly screen change. -Use of a tandem strainer head, i.e. a twin strainer head arrangement permitting the contaminated strainer to be swung out and the clean head swung in in a matter of minutes. This solution is practical in cases where medium to low contamination of the raw materials calls for just one screen change per shift. -Insertion of a screen pack in the extruder head, a possibility that is justified only in cases of minimal contamination, i.e. where a screen change is required only at the end of a production run. During the production of PVC film and sheet, the product is trimmed twice: a hot trim on the calender, and a cold trim to the desired width. The resulting trimmings are recycled. Mixing rolls and internal mixers were widely used for compounding in the early years of calendering. They are still sometimes included in a compounding section which may be used intermittently, for particular runs or formulations, as an alternative to the main compounding set-up (see Chapter 18). The edge trim can be fed back to these machines just as it is. However, attempts to feed it directly into continuously operating screw-type machines have usually failed because of various problems, including difficulties in metering. The usual current practice is to grind the trim in grinding mills together with unsaleable starting film and sheet to obtain 'regrind' particles with edge lengths of 5-10 mm. As a rule, this regrind is added to the virgin blend in the pre-mixing operation at rates of about 10-20%. In isolated cases-particularly where semi-rigid or plasticised film or sheet is being produced-the regrind can be charged directly into the kind of feed hopper shown in Fig. 13.32. The mixing effect of the agitator arms designed to prevent bridging in the hopper is sufficient to intermix the regrind with the powder blend adequately. The specific energy required for compounding is in the 0·060·10 kWh kg- I range. Typical stock temperature profiles resulting from this energy input are shown in Fig. 13.46. The compounding units available on the market cover a throughput range from 100 kg h -1 to 5th- I .
13
Industrial Compounding Technology of Rigid and Plasticised PVC
T (OC)
597
2
ZXl 180
1iO
v.o lZ)
m
eo 60
IIJ Z)
}1.
Fig. 13.46 Temperature profiles, PVC sheet and film stock. 1, Plasticised PVC; 2, rigid PVc.
In the early 1970s some leading companies attempted to set up the entire calender line, from pre-mixing to finished film and sheet, in the form of an integral control loop employing a central process computer, with the idea of saving labour and reducing out-of-tolerance film and sheet (with consequent raw material and other savings). These projects demonstrated, however, that the entire system is so complex and the mutual interaction of the parameters involved so varied, that it makes more sense to divide the calender line into three subloops, pre-mixing, compounding, and calendering. Automation and control can be applied in goal-oriented land comprehensible fashion in these subloops. Automation in the pre-mixing operation is aimed at producing the desired blend with the desired temperature, bulk density and flow properties for delivery to the compounding line. The compounding control system is responsible for maintaining the throughput called for at any given moment in conjunction with the prescribed outlet temperature and fluidity of the stock. The control loops on the calender are essentially responsible for· guaranteeing film and sheet thickness and thickness tolerances in the lengthwise and transverse directions.
Fig. 13.47
r
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PVC cascade extruder with sheeting line. 1, Extruder; 2, flat sheet die; 3, triple-roll smoothing system; 4, roller conveyor; 5, edge trimmer; 6, twin-roll haul-off unit; 7, length cutter; 8, sheet deposit.
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13 Industrial Compounding Technology of Rigid and Plasticised PVC
599
(c) Extrusion of Film, Sheet and Board Considerable progress has recently been made in work on production of rigid and plasticised PVC film, sheet and board by extrusion. Film as thin as 0·1 mm and board as thick as 10 mm have been produced on a commercial scale. Maximum finished width at present is 1·2 m. As in the case of calendering, these systems employ compounding equipment in line with the extrusion set-up. In the example shown in Fig. 13.47, a cascade extruder (here equipped with a slit die for film formation) is used as the combined melt compounding and extrusion unit (see also Plate Cl). The operation of this machine is discussed in Section 13.4.4(a)). Production systems of the type illustrated by Fig. 13.47 and Plate C2 are designed for outputs up to 500 kg h- 1 for film and 600 kg h- 1 for sheet and board.
(d) Recycling At the time of writing, no general strategy for recovery of plastics scrap has yet gained wide acceptance. Undoubtedly this is attributable
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Plate C.l Cascade extruder (Barmag).
600
W. Henschel and P. Franz
Plate C.2 Sheeting line employing a cascade extruder.
to questions such as the possible applications and quality profile of recycled plastics scrap, price and profitability, technological potential and limits, the consumer mentality, and not least the problem of communication among various industries. Nevertheless, the current ecological situation is making it necessary to assess technological progress not only according to increased sales and profitability, but also according to recycling possibilities. We shall probably wait a long time for a satisfactory solution to the problem of reclaiming valuable substances from municipal refuse. From the standpoint of PVC compounding technology, however, several interesting examples can be cited of recovery of scrap produced during a manufacturing process. The simplest and easiest problem to solve is the handling of in-line scrap, so it is not surprising that recovery of the edge trim and starting scrap produced in the manufacture of PVC film and sheet has been practised (as described in Section 13.4.2(b» virtually since the beginning of calendering technology. Recycling practice is similar in PVC pelletising operations when products with the wrong composition or colour shade are produced. Recently, two possibilities have been investigated for the recycling of die-cutting scrap in the processing of PVC sheet, viz. production of
13 Industrial Compounding Technology of Rigid and Plasticised PVC
601
secondary sheet and production of extruded profiles or pipes. First the scrap is ground up in a grinding mill. The stabiliser used in the original process is added again in a pre-mixer, and pigments are often added to achieve a uniform, desired colour. This pre-mix is fed into the compounding unit. If the regrind includes rigid, semi-rigid and plasticised PVC, the rigid and semi-rigid fractions can be charged into the compounding unit through a first inlet opening and plasticated in an initial kneading zone. The plasticised portion is then fed into this fluxed stock, which ensures the gentlest and most homogeneous processing (Fig. 13.48). Because the plasticiser cannot diffuse in the PVC regrind within a reasonable time, as it can in the case of virgin PVC, any additional plasticiser required is injected directly into the kneading zone of the compounding unit by a pump. It is advisable to use a strainer in order to remove any contamination from the stock. Afterwards, the calendering process is carried out as usual. For the production of profiles and pipes, the homogeneous stock is pelletised (as described in Section 13.4.2(a» following compounding. Then the pellets are fed to an extrusion line. In the cable sector, compounders were confronted with the problem of recycling copperless insulation and sheathing scrap. The approach taken in this case was to use this scrap for producing filling core mixtures. The purpose of the filling cores is to fill out the cavities between a cable's conductors (Fig. 13.49). Since their composition is not subject to any special electrical or mechanical specifications, it is normally made as inexpensive as possible, usually receiving up to 700 RIGID PVC SCRAP
PLASTICISED PVC SCRAP
PLASTlC~ ~
~~Wl STRAINER
Fig. 13.48 Recycling of film scrap.
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INNER SHEATHING REINFORCEMENT Fig. 13.49 Cable design.
parts of chalk filler. At this level of loading the PVC acts mainly only as binder for the filler. The reground PVC scrap, with a particle size of 5-10 mm, is fed into the first inlet of a compounding unit designed specifically for this application (Fig. 13.50). The regrind is plasticated homogeneously in the first kneader zone, enabling it to absorb the high filler loading metered in at the second inlet opening without any problems. To plasticise the filler cores further (for increased flexibility) plasticiser is injected into the kneading chamber by a pump. The homogeneous stock is pelletised as described in Section 13.4.2(a). PVC SCRAP
FILLER
PlASTIC~ ~ Fig. 13.50 Compounding of cable filler cores.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
603
Another feature of the equipment shown is that virgin blends can be compounded on the same line. Cork flour-obtained as a waste product in the manufacture of bottle corks~an be admixed to either a PVC virgin blend or a PVC regrind blend and compounded as described in Section 13.4.2(b) for producing flooring materials on a calender. The presence of the cork is believed to increase the resilience of such flooring.
13.4.3 Preparation of PVC Pastes The preparation of PVC pastes consists in dispersing suitable PVC polymer grades in plasticisers, or sometimes plasticisers and solvents, together with fillers, colourants and stabilisers. Paste viscosities of the order of 103-1·5x104 cP are required for ultimate processing. The nature, properties and applications of PVC pastes are discussed in detail in Chapters 21 and 22. Because the pastes are viscous liquids suitable for further processing and giving rise to homogeneous products when fused (gelled) by heating, melt compounding is not relevant or appropriate. Indeed care must be taken during paste preparation to keep any heat development or temperature rise to a minimum in order to prevent premature gelation. Thus screw-type machines of the type used for melt compounding are not applicable. For quite a long time, PVC pastes were prepared exclusively in slow-speed mixers (see Table 13.8), as it was felt that the compositions would gel if handled in high-speed mixers. In the mid-1950s, however, TABLE 13.8 Mixing Equipment for PVC Pastes Continuous
Batch Vertical Low speed
Horizontal
Horseshoe mixer Ribbon blender Planetary mixer Paddle mixer Perl mill High speed
Dissolver
Roll mills
Vertical
3-Roll mill
Dissolver Buss mixing turbine MT
604
W. Henschel and P. Franz
rationalisation efforts led to the first trials with such mixers: the use of dissolvers with infinitely variable speed control eliminated such recognised shortcomings as limitations on mixing tool speed and inadequate coverage of the entire mixture volume, and marked a real improvement in the efficiency of producing pastes on a batch basis. Today, changing economic conditions have again evoked demands for improved efficiency and automation, for better working conditions and anti-pollution measures, for improved space utilisation and higher per capita outputs. To meet these demands, continuously operating units such as continuous dissolvers and the Buss Mixing Turbine were developed. Regardless of whether the continuous or batch approach is used, industrial preparation of stock pastes can be broken down into the following stages (Fig. 13.51): -silo storage of components; -metering of components; -pasting-up and dispersion; -filtering; -de-gassing (de-aeration); -ageing. The preparation of pigment pastes, and stock paste colouring, run in parallel with the preparation of stock pastes. (a) Silo Storage This is practised as described in Section 13.3. (b) Metering Because of the special nature and mixing requirements of PVC pastes it is necessary to charge the components into the mixer in fractions. In the simplest case, the components are batch-weighed and the portions charged into the mixer manually. In automated processes this manual operation has been replaced by gravimetric metering devices for the bulk materials and metering pumps for the plasticisers. For the bulk materials, either belt weighers or differential weighers (loss-in-weight feeders) can be used. While the metering devices used for batch operation operate intermittently, i.e. are switched off after the desired amount of a component has been fed, metering devices for continuous operation supply an uninterrupted flow of material that remains constant per unit time.
TANK)
pASTE BUFFER
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DE-GASSING
FIL TERING
DISPERSING)
(WETTING,
PREPARATIONI
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~
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Flow diagram for industrial preparation of PVC paste.
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For batch operation, the gravimetric metering devices can be program-controlled to deliver the desired metering sequence without any difficulty. Metering accuracy is approximately ±0·5%, which is better than the accuracy normally obtained in manual weighing. (c) Pasting-up and Dispersion Although a distinction is made between these two operations one follows the other in continuous sequence. In batch operation, the liquid components are placed in the dissolver first, and the portionwise addition of the solid components is then commenced, with the mixing tool rotating. This practice largely avoids the formation of lumps, and promotes wetting-out of the solid particles. When the last solids fraction has been charged and mixed into the plasticiser, the pasting-up step is over. Dispersion is initiated by speeding up the mixing tool to step up the stock's turbulence and hence the shearing effect. The upper limit on speed is determined by the drive motor rating and depends on the viscosity of paste. In the case of the continuous-preparation system shown in Fig. 13.51, the bulk components are not charged in portions, but are comminuted in the inlet head of the Buss Mixing Turbine by suitable rotor elements. The plasticiser is sprayed in, so that the solid and liquid phases intermix in finely dispersed form. Thorough wetting-out of the surfaces of the solid particles is the result. When low-viscosity plastisols are being produced by the batch process, only part of the total amount of plasticiser is placed in the mixer at the start: otherwise the shearing forces for the dispersion operation would not be sufficiently high. The remainder is added, to effect final dilution, only after the dispersion operation has been completed. In the continuous-preparation equipment of Fig. 13.51, the same result is obtained by spraying-in the plasticiser at various points along the mixing turbine axis. In the batch process, the pasting-up operation in the dissolver takes 8-10 min depending on batch size, and the dispersion operation about 5-8 min (i.e. 13-18 min total mixing time). The peripheral speeds of the mixing tool during dispersion are between 20 and 30 m s-1. In the continuous process employing the Buss Mixing Turbine, the average residence times for the entire mixing operation lie between 15 and 30 s at rotor peripheral speeds between 10 and 25 m S-I, depending on the throughput and formulation. The very large time savings, at generally comparable peripheral speeds, offered by the continuous process arise
13 Industrial Compounding Technology of Rigid and Plasticised PVC
607
because in the Buss Mixing Turbine the mix components come into contact at individual particle (or plasticiser droplet) level. This accelerates both the pasting-up and dispersion operations (particularly the latter). Batch mixing in the dissolver and continuous mixing in a turbine both produce pastes so smooth that they are ready for use and normally require no additional grinding step on mixing rolls (as practised in some traditional, or small-scale, paste preparation methods). (d) Filtering Paste quality can be impaired by the kind of contaminants mentioned in the discussion of melt compounding, or by the presence of isolated undispersed lumps. In cases where the paste is passed through mixing rolls for grinding, partly gelled particles (small lumps) of paste can cling to the ends of the rolls and get into the product at the end of the grinding operation. All such impurities must be filtered out. Although a number of filter devices are available, the vacuum filter described below is the one most frequently used. It can either be moved around the workshop on a portable frame and fitted onto the vacuum-tight mixing and transport tanks as required, or set up for stationary use between mixer and de-gassing tank. The PVC paste fed into the hopper of the device (Fig. 13.52) passes through a screen pack mounted in a quick-change frame: the screen packs are much the same as those employed in automatic screen changers for thermoplastics
Fig. 13.52 Vacuum filter for PVC paste.
608
W. Henschel and P. Franz
processing. One pack is in operation while the second waits its turn. The paste is drawn through the screen by vacuum in the tank below. When the screens become clogged, the two packs are reversed and the contaminated one replaced by a new pack. Clogging of a screen is indicated by an increase in tank vacuum. The screen pack mesh must be fine enough to retain particles larger than 200 tlm. The filtered paste passes directly on to the de-gassing step. (e) De-gassing Air enters the mixing unit with the bulk materials. In addition, the PVC particles contain air and moisture as a result of their structure. Steadily rising quality requirements have been forcing producers of imitation leather, floorings, toys, etc., to de-gas their PVC pastes effectively, and this has how become standard practice. Low-viscosity pastes are de-gassed by the application of a 5-50 mbar vacuum for about 10 min (about 20 min for high-viscosity pastes). Effective de-gassing requires that the surface of the stock be continuously renewed, thus shortening the diffusion paths and the evacuation time. That is why vacuum dissolvers or vacuum planetary mixers are employed for the de-gassing of PVC pastes. For continuous operation, appropriate continuous degassing units are available on the market. (f) Ageing The procedure just described is typical of the preparation of medium to large batches of stock (uncoloured) PVC pastes. Directly after preparation the stock paste is pumped into a storage tank where it is kept-under slow stirring-to serve as buffer stock between paste production and subsequent processing. The storage may also serve as an ageing period (of up to about 24 h) during which the paste viscosity attains a desired value (see also Chapter 21). (g) Colouring PVC pastes are coloured by the addition of colour pastes, produced by dispersing colourants in plasticisers (batchwise in planetary mixers or dissolvers, or by the continuous method in mixing turbines), and homogenising by milling (in ball or roll mills). The method of preparation is thus similar to that used in the paint and varnish industry. Colour pastes are added to PVC stock pastes in high-speed mixers or dissolvers with average mixing tool peripheral speeds of 10-12 m S-l,
13 Industrial Compounding Technology of Rigid and Plasticised PVC
609
peaking as high as 22-25ms- 1 . The stock paste is first stirred briefly, and then the batch of colourant, appropriate to the paste formulation, is added slowly into the vortex forming around the mixing tool. In cases where smoothness is of cardinal importance (e.g. coating pastes for the production of artificial leather) the paste may be milled after colouring. If this is done on 3-roll mills, it is advisable to pass the milled paste through a vacuum filter to preserve the high quality of the ultimate coating by removing any gelled particles that may have formed on the rolls. 13.4.4
~achinery
(a) Screw-type Machines The compounding of PVC is a particularly difficult operation because of the need to minimise the material's 'heat history', which is a function of the time and temperature of treatment. PVC compositions must not be compounded at high stock temperatures for long periods. Consequently, the energy required for plasticisation and homogenisation must be supplied to the system by the gentlest possible shearing within the shortest practicable stock residence time. Another complicating factor is the tendency of PVC compositions to slip on processing surfaces. For all of these reasons, PVC compounding is normally carried out by screw-type machines that operate with relatively little wall slippage and have a sufficiently effective mixing action to plasticate and homogenise the stock at the lowest possible level of energy conversion. Some machines typical of equipment embodying these features are listed in Table 13.9 and briefly discussed below. The list is given by way of illustration only, and is not exhaustive. THE PLASTIFIKATOR
Applications: Compounding and pelletising of plasticised PVC compositions (cable, shoe, profile, hose, flooring compounds). Equipment and operational sequence (Fig. 13.53): The pre-mix passes through a holding tank to reach the stirred feed hopper, 1. The variable-speed twin feed screws, 2, (intermeshing and co-rotating) determine the rate of material feed to the shear cone in the plastication section, 3. In smaller Plastifikator models, the homogeneous stock is picked up by a discharge screw, 4, arranged coaxially with the plastication cone and driven by the same shaft; this
Werner u. Pfleiderer, Stuttgart West Germany Buss Ltd, Basle, Switzerland Barmag, Remscheid-Lennep, West Germany
Manufacturer
Werner u. Pfleiderer, Stuttgart, West Germany FCM continuous mixer {Farrel Corp., Ansonia, CT, USA David Bridge Co. Ltd, Rochdale, UK Pomini-Farrel, Castellanza, Italy Baker Perkins Chern. Machinery MPCN Ltd, Hanley, Stoke-an-Trent, UK Bitruder BT Reifenhauser KG, Troisdorfsiglar, West Germany Planetary EKK Kleinewefers Kunstoff PWE and ZSE Maschinen GmbH, Bochum, West Germany Hermann Berstorff Maschinenbau WE GmbH, Hannover, West Germany
Twin screw Kombiplast
Buss-Kneader Cascade extruder
Single screw Plastifikator
Machine type and designation
X X X X X
X X X X
X X
Rigid PVC
Process
X
X
X
Extrusion
X
feeding
+ pelletising Calender-
X
X X
X
Plasticised PVC
Compounding
TABLE 13.9 Typical Screw-type Compounding Machines
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Fig. 13.53 Plastifikator PK400. (See text for details.) screw forces the stock through the pelletiser die plate. In larger models, the discharge screw constitutes a separate unit with its own drive, gear box, bearings and supports. Cutting knives, 5, offset from the pelletiser die plate, chop the emerging strands into pellets by the dry cutting process. The pellets are gathered in a collector casing, 6, and conveyed pneumatically to the pellet cooler. Working principle (Fig. 13.54): The plastication and homogenisation comprises a shear cone, 1, rotating inside the conical housing, 2. The tapered part of the shear cone is fitted with spiral fins, 3. A closed ring of powder formed, in the first part of the shearing gap between the shear cone and housing, becomes sintered under the influence of shear forces, and gels in the next section. The fins and increasing shear cone diameter serve to divide the product into individual strands; these are squeezed in the constricting gaps between fins and housing wall to form thin layers, only to be re-formed again. Axial displacement of the shear cone in relation to the housing varies the shear gap and therefore the amount of shear energy dissipated. s~tion
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Fig. 13.54 Working principle of the Plastifikator. (See text for details.) Temperature control system: The cone housing is equipped with an electric heater. The barrel of the discharge screw is also electrically heated, but is fitted with an air cooling system in addition. If the machines are separate, the discharge screw can be cooled with a separate heat transfer medium. De-gassing: Where the plastication machine and discharge screw are separate, a vacuum of 400 to 550 mbar can be applied to the transition section between the two to de-gas the stock. Operation and cleaning: During starting, a diverter can be placed between the plastification section and discharge screw to remove any PVC powder emerging; this prevents clogging of the die plate or screen pack (if any). For cleaning, the plastification section and discharge screw are run empty and purged of residual powder by passing warm pellets through. The pelletiser head can be swung out to permit easy removal of any product remaining on the screw tip or back of the die plate. For extreme colour or formulation changes that call for a thorough cleaning, the discharge screw with pelletising head of the smaller models can be rolled away on an assembly dolly. Then the shear cone can be pulled out in front of the barrel on a bar sliding in the main shaft. On the larger models, the cone is rolled away together with the transfer tube on guides, making the parts in contact with the product freely accessible. Energy input:
-cold pre-mix: 0·1-0·12 kWh kg-I; -hot/cold blend (50°C): 0·08-0·1 kWh kg-I; -hot blend (85-90°C): 0·06-0·08 kWh kg-I. Output rates and technical data: See Table 13.10.
Production rate: Plasticised PVC (kg h -i) Drive rating: Feed screw (kW) Main drive (kW) Pelletising (kW) Fan (kW) Separate pelletising screw (kW) Speed range: Feed screw (r min-i) Shearing (r min-i) Pelletising (r min-i) Fan (r min-i) Separate pelletising screw (r min-i) Heating: Cone barrel (kW) Extruder barrel (kW) Pelletising (kW) Cooling: Fan (m 3 h- i) Water cooling, bearing (litre h- 1) Water cooling, cone barrel (optional) (litre h- 1) Water cooling, gear-box (optional) (litre h- 1) Weight: Plastifikator (kg) Control cabinet (kg)
4 2·5 1·25 800
-
4 2·5 1·25 800
-
3200 270
-
3500 270
-
60
20-120 52 240-900 2800
20-120 42 204-900 2800
60
4 30 0·75 0·37
4 22 0·75 0·37
-
250-350
-
Model
PKlOOIV11
150-250
PK lOOIVI
TABLE 13.10 Technical Data for the Plastifikator
10 4 2
4500 500
-
800 100 -
-
30-150 88 230-1150 2800
-
4 55 1·5 0·37
450-900
PK 400/111
13100 700
2 x 800 200 500 500
20 16 4·8
29-117 75 312-1560 2800 35
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THE BUSS·KNEADER
Applications: -compounding and pelletising of (i) plasticised PVC blends (cable, shoe, profile, hose, flooring compounds), and (ii) rigid PVC formulations (bottle, profile, record compounds); -compounding of rigid, semi-rigid and plasticised PVC blends for calender feeding (packaging, deep-drawing, decoration, upholstery and engineering film and sheet).
Equipment and operational sequence (Plates D.l and D.2): The pre-mix moves through a holding silo to the feed hopper, which is equipped with an agitator. The bottom section of the agitator is designed in the form of a vertical screw, so that speed variation of the agitator shaft feeds product continuously at the desired rate into the kneading section. If pellets are being produced (Plate D.l), the stock homogenised in the kneader is transferred cascade-fashion to the pelletising screw, which forces the product through the pelletising die plate. Cutting knives supported next to the die plate chop the emerging strands into pellets by the dry cutting process. The pellets are caught in a collector casing and conveyed pneumatically to the pellet cooler. In the case of calender feeding (Plate D.2), the homogeneous stock is sliced into chips in a die mounted on the kneader; the chips are then fed to the calender. Working principle (Figs 13.55 and 13.56): The Buss-Kneader is a continuously operating single-screw machine of special screw design. Conventional single-screw machines have an uninterrupted Archimedes' screw, which merely rotates around its longitudinal axis (Fig. 13.55(a)). In the Buss-Kneader, each turn of the screw helix is interrupted by three gaps to form the screw kneading tools, or 'screw flights'. Their counterparts in the barrel, the kneading pins or teeth, are arranged in three rows (Fig. 13.55(b)). An axial oscillation is superimposed on the rotation of the screw. A special gear box generates this characteristic Buss-Kneader motion. The mechanism ensures that each turn of the screw is accompanied by one back-and-forth movement. The operating principle can be explained by reference to a projection of the screw profile onto a flat plane (Figure 13.56(A)). The
13
Industrial Compounding Technology of Rigid and Plasticised PVC
615
616
W. Henschel and P. Franz
Plate D.2 Buss-Kneader PRKJE.
screw flight shaded in the figure is considered for this purpose, together with the four marked kneading teeth which work in conjunction with it. The sequence of drawings in Fig. 13.56(A)-(F) shows the tracks of the kneading teeth relative to the screw flights. Each consecutive drawing represents a quarter-turn of the screw shaft, i.e. the rotation positions of 0, 90, 180, 270 and 360 angular degrees. In the drawings, the screw rotation corresponds to movement of the screw flights from bottom to top; the product is conveyed from right to left. The starting position is shown in the first drawing. In the course of a turn through 90 (Part (B», the dark kneading tooth, which started at the top flight tip, has wiped past the long left-hand flank of the screw flight. As it did so, the product was sheared in the gap between the kneading tooth and shaded screw flight, and the flank of this flight was cleaned at the same time. The other marked teeth have not yet 'moved into action with the shaded screw flight, but they have been working with other flights. 0
13 Industrial Compounding Technology of Rigid and Plasticised PVC
617
Fig. 13.55 The screw and barrel of: a, conventional single-screw machine; b, the Buss-Kneader.
As the screw turns through another 90° (Part (e», so that it is describing a half turn in relation to the starting position, the right-hand kneading tooth of the two shown at the top of the projection in the starting position wipes past the short right-hand flank of the shaded flight, thus shearing the product and cleaning this flank. During this rotary motion, the middle and the two upper kneading teeth pass through their respective gaps between the screw flights and push the product back into the next turn of the helix. Note: Thus, superimposed on the conveying effect of the screw pitch and right-to-Ieft movement, there is a backward movement of material. This combined motion has been referred to in the literature as 'pilgrim's step' (two steps forward and one back).
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13 Industrial Compounding Technology of Rigid and Plasticised PVC
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Naturally, the forward conveying effect produced by the pitch and stroke of the screw is greater than the backward movement of material actuated by the passage of the kneading teeth through the gaps. The net material movement is thus in the direction of the outlet of the machine. The backward movement of material adds the crucial benefit of axial mixing to the radial mixing effect. This is why the homogenisation process in the Buss-Kneader takes less time than in conventional screw-type machines. In completing the 180° turn the screw has sheared the material and performed a cleaning operation on both the left-hand (long) flank of the screw-flight and the right-hand (short) flank. In the course of the next 90° turn (Part (D» the shearing of the material proceeds as the left-hand (short) flank of the left-hand tooth of the pair shown at the top in the starting position is being cleaned. Axial product exchange takes place again as the kneading tooth passes through the gap. The result of the last 90° turn (Part (E», which completes one full revolution of the screw, is tha,t the kneading tooth shown at the lower tip of the shaded flight in the starting position (Part (A» wipes past the right-hand (long) flank of the flight, again shearing the material and cleaning the flank. The interaction of the four kneading teeth with the shaded screw flight during one full turn of the screw has thus sheared the material on all four flanks of the flight, mixed it both axially and radially, and mutually cleaned the screw flight and teeth. As can be seen from the diagrams the same events-though with the roles of the collaborating flanks and kneading teeth exchanged-take place on all of the other screw flights. This is demonstrated by the enlarged detail of the relevant projection: Part (F) shows that the coverage of the kneading tooth tracks in relation to the screw surface is complete, with no surfaces or spaces left unwiped. This is the basis of the thorough mixing, kneading and self-cleaning action of the Buss-Kneader. On the KG models, the regulating screw can be used to set the back-pressure at the kneader outlet steplessly by remote control. This provides finger-tip control of the energy input. On PRK models for feeding calenders, the same back-pressure control is achieved with the aid of a die with infinitely variable cross-section.
Temperature control system: The barrels of both the kneader and the
13 Industrial Compounding Technology of Rigid and Plasticised PVC
621
pelletising screws are fitted with jackets divided into several zones. The screws are hollow right up to the tip. This design permits accurate control of screw and barrel temperatures by means of liquid heat-transfer media, usually HT oil. Pelletising die plates are heated either electrically or with liquid. De-gassing: On the KG and WKG models, which are used for compounding and pelletising, de-gassing of the stock takes place at the transition point between Buss-Kneader and pelletising screw. On PRKlE models for feeding calenders, the Buss-Kneader is equipped with its own de-gassing zone at the outlet end. The de-gassing systems provide for vacuum as high as 50-60 mbar. Operation and cleaning: When the KG and WKG pelletisers are started up, the initial material can be eliminated with a diverter between the Buss-Kneader and pelletising screw. In calender installations, the starting material can be eliminated at the kneader outlet. Buss-Kneaders and pelletising screws are cleaned by running them empty and purging residual powder with cleaning pellets. The pelletising head can be swung out to allow easy removal of material remaining between screw tip and die plate. For extreme product changes requiring a thorough cleaning, the Buss-Kneader barrel flaps open (Plate E), so that both screw and barrel are freely accessible. The barrel of the pelletising screw can be drawn off the screw on an integral roll-away chassis. Because all parts are fastened to the machine base and therefore remain properly positioned, this cleaning procedure requires no special tools, hoists or specially trained personnel. Energy input:
-plasticised PVC: 0.04-0·06 kWh kg-I; -rigid PVC: 0·05--0·08 kWh kg-I. Output rates and technical data: See Tables 13.11 and 13.12, and Fig. 13.57. THE CASCADE EXTRUDER
Applications:
-rigid and plasticised PVC film, sheet and board extrusion; -pelletising.
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W. Henschel and P. Franz
Plate E
The split barrel of a Buss-Kneader.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
623
Equipment and operational sequence (Fig. 13.58 and Plate Cl): The cascade extruder is made up of two single-screw extruders, the fluxing extruder and the discharge extruder. PVC pre-mix is fed into the former through a simple inlet hopper without any agitator or metering device. The fluxing screw mixes the blend and partly agglomerates the stock into particles of approximately uniform size. The production rate depends on the momentary bulk density of the pre-mix and the screw speed. When the agglomerated stock emerges from the fluxing extruder, it drops through a de-gassing tube (to which vacuum as high as 80 mbar can be applied)' into the intake section of the discharge extruder. Retention time in the de-gassing tube can be regulated by altering the agglomerate level in the tube. The discharge extruder's job is to complete plastication of the partially fluxed material, homogenise it, and force it through the outlet die. Working principle (Fig. 13.59): The energy required for agglomeration and plastication of the PVC stock is supplied by heating the barrel and by shearing the stock between screw core and barrel wall. The screws are provided with a 'heat pipe' to prevent overheating and scorching. To improve its intake characteristics, the fluxing extruder is fitted with a cooled, grooved bushing in the inlet area. The intake section of the discharge extruder is designed in the form of a tapered slot. Temperature control: The barrels are heated electrically. Screw temperature is controlled by the heat pipe system. De-gassing: In the transition tube between fluxing and discharge extruders. Output and technical data: See Table 13.13 and Fig. 13.60. THE KOMBIPLAST
Applications: Compounding and pelletising of plasticised and rigid PVC. Equipment and operational sequence (Fig. 13.61): The pre-mixed material is fed into the feed zone of the twin-screw unit (ZSK or ZDS-K) by a vertical force-feed screw. The compounded stock drops cascade-fashion into the ES-A pelletising screw which forces it through
Rigid PVC· (kg b- I) Plasticised PVC (kg h- I) (at elevated PR screw speed)
Production rate:
Total weigbt' (kg) No. of beating units Heating capacity (approx. kW)
Dtive rating (kW) Screw speed (r min-I) Screw diameter (mm) Screw lengtb (LID) Heating zones PeUetising: Drive rating (kW) Knife speed (r min-I) No. of knives Size of die boles (mm dia.) Die beater (kW)
15 40-300 46 7 4
0·6 16-80 46 2
40 75
1000 3 30
PR
ET
GS
1·1 120-600 2 3/3·6 0·5
3 20-80 70 6 3
WKG4·6-7
Technical data for KGIWKG (rigid and plasticised PVC)
1·8 7-47 80 2
ET
400 750
4500 3 30
70 40-240 100 7 4
PR
GS
1·5 120-600 2 3/3·6 1·7
11 10-47 140 4 3
WKG1Q-14
3 4-30 140 2·5
ET
1500
BOO
1ססoo
3 60
107 40-120 140 7 4
PR 5·8 10-40 140 2·5 2
RS
KG 14-18
2·2 12(}..6()() 2 3/3·6 3
24 8-40 180 6 3
GS 4·5 4-30 215 2
ET
1600 3000
17500 3 60
210 40-120 200 7 4
PR
12·5 5-39 200 2·5 2
RS
KG2Q-25
TABLE 13.11 Technical Data For Buss-Kneader Models KG and WKG
3 120-600 2 3/3·6 4
52 8-40 250 6 3
GS
7·5 4-25 300 2
ET
3200 S-7ooo
38500 6 120
510 40-120 300 7 4
PR
23 5-35 300 2·5 2
RS
GS
7·5 100-1000 2 3/3·6 7
95 6-30 320 6 3
KG3Q-32
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ET PR 70 40-240 100 7 4
750
1·8 7-47 80 2
75
1·1 12lHiOO 2 3 0·5
3 20-80 70 6 3
4500 3 30
IS 40-300 46 7 4 1·5 12Q-{;()(J 2 3 1·7
11 10-47 140 4 3
GS
WKGIO-14
1000 3 30
PR
ET
0·6 16-80 46 2
WKG4·6-7 ET 3 4-30 140 2·5
1500
8700 3 30
107 40-240 140 7 4
PR
2·2 12lHiOO 2 3 3
24 8-40 180 4 3
GS
WKGl4-18
Approximate figures, not including heating/cooling units, pellet cooling installation, control cabinet or main drive motor. Approximate figures; may vary depending on formulation and operating conditions.
Total weight" (kg) No. of heating units Heating capacity (approx. kW) Production rate: b Plasticised PVC (kg h- I)
Drive tating (kW) Sctew speed (r min-I) Screw diameter (mm) Screw length (LID) Heating zones Pelletising: Drive rating (kW) Knife speed (r min-I) No. of knives Size of die holes (mm dia.) Die heater (kW)
Technical data for WKG (plastic/sed PVC)
ET 4·5 4-30 215 2
3000
60
15800 3
210 40-240 200 7 4
PR
3 12lHiOO 2 3 4
52 8-40 250 4 3
GS
WKG20-25 ET 7·5 4-25 300 2
6-7000
35000 4 120
510 40-120 300 7 4
PR
7·5 100-1000 2 3 7
95 6-30 320 4 3
GS
WKG30-32
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TABLE 13.12 Technical Data for Buss-Kneader Models PRK and PRK/E PRK, PRKIE
PRK, PRKIE
PRK, PRKIE
100
140
200
Feed hopper: Capacity (litre) Screw diameter (mm) Screw drive (kW) Screw speed range (r min-I)
150 100 1·8 6·7-47
280 or 430 140 4·4 2·2-44
430 or 830 215 5·5 1·4-28
Buss-Kneader: Screw diameter (mm) Screw length PRK (LID) Screw length PRKlE (LID) Screw speed range (r min-I) Gear box model Kneader drive (kW)
100 7 12 120 G20 60
140 7 11 120 G45 120
200 7 11 120 G130 200
Heating/cooling equipment: No. of units Installed heating capacity (kW)
2 20
2 36
2 36
3800
6300
11700
Approx. total weight (kg) (not including heating/cooling units, control station or kneader drive equipment)
the pelletising die. Cutting knives mounted next to the die plate chop the emerging strands into pellets, which are conveyed pneumatically to the pellet cooler. Working principle: The ZSK is a twin-screw machine with intermeshing, co-rotating screws equipped with kneading discs (Fig. 13.62) which shear and knead the product and change the local direction of stock flow as it passes the barrel wall. The shearing occurs, and energy is imparted to the stock, as a result of the velocity drop between the kneading discs and the barrel wall and in the saddle area between the two screws. Shearing intensity is determined both by the speed of the screws and the inherent resistance of the kneading discs. The configuaration and arrangement of the discs shown schematically in Fig. 13.62 give rise to squeezing forces which pass the material forward along the screws while it is subjected to shearing and mixing by the consecutive disc elements. The design ensures that the root of each screw is wiped by the flight tip of the other. This self-cleaning
13
Industrial Compounding Technology of Rigid and Plasticised PVC
Output in kg h- 1 Model PRK/E 2000
Output in kg h-1 Model PRK 3000
/
1670 PRK/E 200 1350
1000
/~
670
/
330
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627
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2500
2000
1500 PRK/E 140 ~
J/" ~ PRK140
-- -
20 40 60 80 Speed of Buss-Kneader screw
PRK/~ 100 I
1000
500
PRK100
100 r min- 1
120
0
Fig. 13.57 Output diagram for Buss-Kneader models PRK and PRKlE.
profile eliminates dead spots and normalises the residence time of the material. Temperature control system: The ZDS-K and ES-A barrels are electrically heated. The ES-A unit is cooled by an air blower, whilst the barrel of the ZDS-K unit has channels for cooling with liquid heat-transfer media. De-gassing: De-gassing is carried out at the transition point between the ZDS-K and ES-A units. Operation and maintenance: The starting product can be diverted between the ZDS-K and ES-A units. For cleaning, both units are run empty, and residual powder is purged with a cleaning material. The pelletising head can be rolled away, which facilitates the removal of product between screw tip and die plate. The barrel sections of the ZDS-K and the barrel of the ES-A can be dismantled for more thorough cleaning work.
628
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13 Industrial Compounding Technology of Rigid and Plasticised PVC
629
Standard Extruder n
Process Steps
IVonabieland Fixed Operating Conditions
Fig. 13.59 Process steps and operating conditions of a standard cascade extruder.
Energy input: 0·04-0·08 kWh kg-I. Output and technical data: See Table 13.14. THE FCM CONTINUOUS MIXER
Applications:
--<:ompounding and pelletising of plasticised and rigid PVC; --<:ompounding of rigid and plasticised PVC for feeding calenders.
W. Henschel and P. Franz
630
TABLE 13.13 Cascade Extruder: Technical Data Data
Screw diameter, D (mm) Barrel length extruder (mm) Screw speed: Extruder (r min-I) 1 and 2 Drive power: Extruder (kW)
Model
G
G
Number of heating zones Heating power total (kW) Number of cooling zones: Barrel Vacuum chute Screws Tempering devices-cooling capacity: For barrel and vacuum chute (kcal h-I) For screws (kcal h- I) Vacuum pump: Pumping capacity (with 60 torr), (m 3 h- I ) Drive power (kW) Water consumption (with 15°C input temperature) (m 3 h- I)
1000 kg/h
900
E6K
E9K
E12K
2 x 60
2 x 90
2 x 120
2 x 150
9D 13D
9D 13D
9D 13D
9D 13D
100
80
70
60
20 12 5 14·4
48 32 5 27·3
87 60 5 48·6
140 100 5 74·0
4 1 2
4 1 2
4 1 2
4 1 2
20000 4000
30000 8000
40000 12000
50000 12000
36 1·2 0·23
46 1·9 0·26
97 2·2 0·45
97 2·2 0·45
~
Material' rigid PVC
800 700
.... 600 ....5.500 ::l
~
0400
300 200
100
~ E9K
E15K
E12K
E15K
Extruder size
Fig. 13.60 Output of the cascade extruder.
Fig. 13.61 The Kombiplast machine. 1, Screw barrel; 2, two co-rotating screw shafts; 3, thyristor controlled DC motor; 4, reduction gear; 5, duo gear which distributes the drive capacity between the two screw shafts; 6, screw barrel; 7, single screw; 8, eccentric pelletiser; 9, die plate; 10, neck connecting the two machines; 11, quick acting lock; 12, pole-changing three-phase motor; 13, spur gear.
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Fig. 13.62 Working principle of the Kombiplast ZSK and ZDS-K.
TABLE 13.14 Technical KPl(}(}
Production rate: Plasticised PVC (kgh- I ) Rigid PVC (kg h- I ) Screw: Diameter (mm) Length (UD) Drive rating: Main drive (kW) Fan (kW) Pelletiser (kW) Speed ranges: Screw (r min-I) Fan (r min-I) Pelletiser (r min-I) Temperature control: Barrel (kW) Die plate (kW) Heating cabinet (kW) Cooling capacity of beating cabinet (kcal/h) Water consumption (m 3 h- I) Air consumption (m 3 h- 1) Weight: Machine (kg) Control cabinet (kg)
KP5(}(}
200-350 100-200
500-1100 350-700
ZSK57M23
ES-Al(}(}
ZSK90M58
ES-A150
57 13
100 6
89 11·5
150 6
3·&-17
7/10 0·37 1·1
11-:>-50
17f27
82-360
21/42 2800 156-780
82-360
16/32 2800 23o-U50
9·6
3·0 3-15 4·5 10000
18·0
7·0 5·0 4·5 10000
4·5 14000
0·37 1·5
4·5 22000
0·4
0·1 800
0·7
0·15 800
1400 500
1500
2400 600
1900
13
Industrial Compounding Technology of Rigid and Plasticised PVC
633
Equipment and operational sequence: The FCM is fed not from a hopper, but from one or more belt weighers, depending on the number of components. The metered material is plasticated and homogenised in the mixing chamber and discharged through a variable flap which provides control over the outlet cross-section. For calender feeding, the ribbon of product is passed to mixing rolls or a strainer extruder. For the production of pellets, a suitable ribbon can be discharged for subsequent dicing, or the product emerging through the FCM flap may be passed to a pelletising extruder for hot die-face cutting. Working principle (Fig. 13.63): Since the two screws (sometimes referred to as rotors) of the FCM do not mesh, the operating principle is comparable to that of the batch-type Banbury mixer. In the inlet section, the shafts are designed in the form of Archimedes' screws. In the mixing zone they are shaped like kneading paddles. The two shafts are counter-rotating and are run at variable, different speeds. Besides the shear occurring between barrel bore and screws, this also produces shear and mixing action between the screws. Energy dissipation and mixing effect can be controlled with the aid of the flap for changing the outlet cross-section. Data for the Kombiplast Model KP800
KP 1500
KP 2500
KP 3500
1200-2200 700-1300
1800-3600 1100-2100
3000-5000 1400-2800
4500-7500 2100-3800
ZSK 130M325
ES-A250
ZSK130M325
ES-A3oo
ZSK130M650
ES·A350
89 11·5
200 6
130 14
250 6
130 14
300 6
130 14
350 6
10-100
25/36 2 x 0·37 2·2
1ll-180
40/56 2 x 1·5 4
24-240
62/90 2 x 1·5 5·5
35·5
90/132
36-360
16132
26·6-266
15130
36-360
15130
30-300
15130 2800 155-750
ASK90M120 ES-A200
2800
18·0 4·5 22000
16·0 7·2 4·5 10000
2800
200-1000
230-1150 32·0 4·5 22000
20·0 8·4 12·0 14000
2800
7·5
270-780 32·0 4·5 22000
22·0 9·6 15·0 14000
32·0 12·0 40000
12·0 36·0 + 18·0 30 000 + 14000
1·0
0·3 1600
1·5
0·5 2000
2·0
0·7 2000
3·0
3·0
3900 700
2500
5500 700
7500
5500 700
12000
6500 800
18000
634
w.
Henschel and P. Franz
A
Section A-A
B
Section B-B
Mixing
Feeding
Fig. 13.63 Working principle of the FCM compounder.
Temperature control system: The shearing forces generated by the rotors and the mixing chamber walls permit adiabatic operation of the FCM. To avoid product overheating, the rotor shafts are hollow and the mixing chamber is provided with a jacket, so that water cooling can be carried out. Since the outlet flap is plated for PVC products, there is no need to control its temperature.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
635
De-gassing: Vacuum can be applied to the connecting tube between the FCM and pelletising extruder. Operation and cleaning: The starting product can be eliminated at the flap outlet. Because the machine is not self-purging, it is necessary to dismantle the system for formulation changes where no intermixing can be tolerated. This is done by shifting the mixing chamber barrel away hydraulically and swinging it 60° away from the kneader axis. This makes the rotors and barrel bores freely accessible for cleaning. Energy input:
-plasticised PVC: 0·06--0·14 kWh kg-l; -rigid PVC: 0·05-Q·12 kWh kg- l . Output and technical data: See Table 13.15.
TABLE 13.15
Technical Data for the FCM Model
3
Volume (dm ) Max. torque (Nm) Standard gear box: Max. rating (kW) Max. screw speed (r min-l) Special gear box: Max. rating (kW) Max. screw speed (r min-I) Dimensions with drive: Length (m) Width (m) Height (m)
2FCM
4FCM
6FCM
9FCM
0·344 187
2·70 1432
9·13 7162
30·8 28640
22 1150
73·5 100
257 350
882 300 1102 375
Weight without drive (kg)
2·0 1·5 2·8 2050
Production rate: Plasticised PVC (kg h- I) Rigid PVC (kg h- l)
165 90
4·2 1·6 2730
6·0 1·5 1·3 6700
7·0 2·0 1·6 9600
550 300
2000 1100
5000 3750
1-1
THE MPCIV
Applications: Compounding and pelletising of plasticised and rigid PVc.
636
W. Henschel and P. Franz DIscharge
SCrews
Vacuum
Connection
Extruder
To Pellellser
Fig. 13.64 MPCN design.
Equipment and operating sequence: PVC pre-mix is fed into the twin-screw MPCN. The discharge screw elements (Fig. 13.64) mounted on the end of the shafts push the homogeneous stock into the pelletising screw, which is attached at right angles to the MPCN. Hot die-face cutting is used for pelletising, but, in contrast to the other processes, the pellets are cooled by being whirled into a cooling water film. Finally, the pellets are dried. Working principle: Shearing and mixing are carried out with the aid of kneading disks similar to those described above for the Kombiplast. A barrel valve (Fig. 13.35), located just ahead of the screw discharge elements in the barrel, is used to control the back-pressure in the kneading zone and hence the energy dissipation. Temperature control system: MPCN systems are designed for adiabatic operation. De-gassing: De-gassing takes place in the pelletising screw immediately following the inlet opening (see Fig. 13.64). Operation and cleaning: With this design there is no way to eliminate
13
Industrial Compounding Technology of Rigid and Plasticised PVC
637
starting product between the kneader and pelletising screw. Cleaning is facilitated by the horizontally split barrel, which can be flapped open. Output: See Table 13.16. TABLE 13.16 MPCIV Output Rates MPCIV model
50mm 80mm 100mm 125mm 160mm
PVC
Drive
rigid flexible (kg h- 1 )
(kW)
130 500 700 1400 2800
250 1000 1500 3000 6000
11 45 55 110 225
THE BITRUDER BT
Applications: Compounding and pelletising of rigid and plasticised PVc. Equipment and operational sequence: Unlike the systems described thus far, Bitruders do not employ the cascade layout. In this case, the operation steps of material intake, plastication, homogenisation and discharge are carried out in a single, twin-screw machine (Plate F). The PVC pre-mix enters the Bitruder either through a simple inlet hopper without any internal devices, or through a feed hopper with agitator and forcing screw. A barrel de-gassing arrangement is provided ahead of the discharge zone as standard equipment. From the discharge zone the screws force the homogenised stock out through a multi-hole pelletiser die plate equipped with a torpedo. The pellets are produced by the hot die-face cutting method and are conveyed pneumatically to the pellet cooler. Working principle: Bitruders are equipped with meshing, counterrotating screws. The energy required for plastication and homogenisation is dissipated primarily in the area where the screws mesh. Temperature control: The barrel zones are heated by electrical
638
W. Henschel and P. Franz
Plate F
Bitruder GTlOO-18G.
heating circuits. The screws can be heated or cooled with a liquid heat-transfer medium (HT oil). De-gassing: De-gassing takes place in a section between the agglomeration zone and the plastication/discharge zone. In this section the screws are only partially filled and smooth product flow is aided by a special insert. Operation and cleaning: In this system start-up material cannot be diverted ahead of the pelletiser die plate. The machine is cleaned by running it empty and chasing with purging pellets. For major cleaning operations the barrel is withdrawn with the aid of a rack-and-pinion mechanism. Output rates and technical data: see Table 13.17.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
639
TABLE 13.17
Technical Data for the Bitmder Range Extruder
BT80-12G BT 80-16G BT 100-14G BT 100-18G BT 150-17G
Screw diameter (mm)
Screw length (LID)
77 77
12 16 14 18 17
98 98 150
Screw Heating rating capacity (kW) (kW)
25 25 33 37 133
17 17 31 36 88
Output rates (kg h- J )
120-230 200-400 300-550 450-750 650-1200
THE PLANETARY EXTRUDER
Because the two makes listed in Table 13.9 are basically very similar, they are discussed together in this section. Applications: -compounding and pelletising of rigid and plasticised PVC: ZSE (EKK Kleinewefers); -compounding of rigid and plasticised PVC film/sheet formulations for calender feeding: PWE-AK and PWE-EV (EKK Kleinewefers); WE (Hermann Berstorff). Equipment and operational sequence (Fig. 13.65): A force-feed hopper feeds the PVC pre-mix into the filling zone; the material is then passed into the planetary screw section for plastication and homogenisation. This section is closed off with a retention ring which prevents axial displacement of the planet screws (Fig. 13.66). Models PWE-AK and WE typify the machines used for feeding calenders without any de-gassing; the homogenised stock emerges from the retention ring of the planetary screw section in the form of helical strands, which are fed to the calender. If intensive de-gassing of the stock is required, model PWE-EV is employed (Fig. 13.66). Basically, this is a PWE-AK machine with a single-screw extension with de-gassing and discharge zones. Here again, the planetary screw section is terminated by a retention ring. ZSE lines (Fig. 13.67), widely used for compounding and pelletising rigid and plasticised PVC compositions, are basically cascade systems made up of a PWE-AK machine followed by a single-screw discharge extruder, with a de-gassing tube between the two units. The strands of
W. Henschel and P. Franz
640
Fig. 13.65 Planetary kneader design. 5
3
4
2
1
Fig. 13.66 The screw and barrel arrangement of planetary compounder PWE-EV. 1, Feed zone 3L1D; 2, plastication zone (planetary system) 5L1D; 3, discharge zone 2LID; 4, de-gassing zone 4L1D; 5, retention ring.
compound forced through a multi-hole die plate are cut into pellets by the hot die-face cutting method and conveyed pneumatically to a cooler. Working principle (Fig. 13.68): The plastication and homogenisation process takes place in the planetary screw section, which comprises four interacting elements, viz. the driving main spindle, the follower planet screws, stationary barrel, and retention ring.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
641
5
30
2
1
4
Fig. 13.67 Planetary compounder ZSE. 1, Feed zone; 2, plastication zone (planetary system); 3, de-gassing zone; 4, discharge screw; 5, retention ring.
When the main spindle is rotated, the planet screws rotate along with it and roll around the inside of the barrel. As a result, the PVC stock is sheared and squeezed between the tooth flanks and roots of the main spindle, planet screws and barrel. Mechanical energy is converted into heat and the stock is plasticated. External heating is employed to aid the plastication work.
Fig. 13.68 Operating principle of planetary-type machines. 1, Main screw; 2, planetary screw; 3, barrel; 4, retention ring; 5, discharge orifice.
642
W. Henschel and P. Franz
The replaceable retention ring serves two purposes: -it prevents the planet screws from shifting axially, i.e. from being screwed out of the barrel; -it controls the level of back-pressure (and thus also the energy conversion) in the planetary screw section: the control is exercised by pre-selecting the internal diameter of the ring opening and hence the proportion of outlet cross-section left open.
Anywhere from 6 to 18 planet screws can be employed, depending on machine size and desired level of energy conversion.
Temperature control: The barrels have several temperature control zones heated by means of a liquid heat-transfer medium (HT oil). The main spindle and discharge extruder screw have the same type of thermal control. De-gassing: As mentioned under 'Operational sequence' above. Output rates and technical data: See Table 13.18. (b) Machine Drives Present-day requirements in compounding technology call for variablespeed driving motors for the screw-type machines. The selection is made from AC commutator motors, DC motors, and frequencycontrolled AC motors. All three types make it possible to alter screw speed by remote control. In Europe, DC motors are priced more favourably than commutator motors for drive ratings above about 80 kW. The price advantage begins at even lower ratings in the USA. This is one of the main reasons why DC drives are preferred for most production installations. Although such drives become expensive where explosion protection is required, drive motors for PVC compounding seldom have to meet this standard. Dustproof and splashproof motors are usually sufficient. An interesting development is taking place in the area of frequency-controlled AC motors, which exhibit DC-motor characteristics (such as constant torque over the entire speed range) despite the use of AC. Another advantage is that explosion protection is no more expensive than it is for commutator motors. To protect drive motors from dust, it is advisable to draw cooling air for self-ventilation through a generously dimensioned filter or to use filtered fresh air in the case of separate ventilation.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
643
Small drives for peripheral equipment such as feed hoppers, cutting knives, etc., are normally designed with single-speed AC motors. A mechanical variator gear unit makes it possible to alter speeds. Where the speeds of the main screw and the feed hopper are synchronised (see Section 13.4.2(b», however, the hopper drive must also employ a DC motor for technical reasons. Power transmission from motor to gear box can be accomplished in any of four different ways, viz. by direct drive with an elastic coupling, V-belt drive, chain drive, or flat belt drive. Direct driving by means of an elastic coupling is preferred for machines with screw speeds higher than 200 r min -1 and in cases where V-belt drives are impractical for technical reasons. Transmission with V-belts, which provide an elastic link between motor and gear box, is standard practice in the 50-250 r min- 1 screw speed range. The V-belts should travel no faster than 30-40 m S-I. With today's materials and V-belt structures, up to 35 kW can be transmitted per belt. Since it is feasible to combine as many as 10 or 12 V-belts, drive ratings as high as about 400 kW can be handled. Drive chains are used typically for slow-turning screws, such as pelletising screws. They are capable of transmitting high forces at low speeds, Le. high torques. Chain speed is limited to 10-15 m S-I, however. Modern materials and improved designs are also opening up new possibilities for flat belt drives, which date back to the beginnings of industrialisation. Their low inertia forces and good elasticity are desirable qualities for power levels up to about 40 kW. (c) Control and Instrumentation Process control of screw-type machines requires that the following data be measured and recorded, and in some cases controlled automatically: -temperature of the individual heating zones; -material metering rate and screw speeds; -energy input; -stock temperature; -pressures. HEATING ZONE TEMPERATURE
Where liquid heat-transfer media are employed, a heat-exchanger unit is provided for each heating zone. The temperature of the liquid is maintained at the set-point adjusted on the electronic controller by a
Number of planetary screws Main screw speed (min-I)
Planetary screw length (mm)
Screw length
Screw diameter, D (mm)
6 to 12 2·5-50 (70, 100, 120 and 150 EV) 3-60 (200 EV) 3·5-70 (170 EV)
-
6 to 18 2-40 (ZSE 300) to 3·5-70 (ZSE 100)
5D1twin helix
6D (ZSE 300) to 8·5D (ZSE 150) 500-1000
1-20 (ZSE 300) to 3-60 (ZSE 150)
-
13D (170EV) to 16·5D (100 EV) 260-1000
150 to 350
150 to 315
70 to 206
2nd stage Single-screw system
PWE 70 EV to 200 Eva (6 models)
1st stage Planetary system
,ZSE 100 to 300n (4 models)
WE 100 to 2900 (5 models) 138 (120 AK) to 315 (300 AK-N) 6·2D (300 AK-N) to 8·8D(150 AK) 500 (100 AK-N); 700 (120 AK); 900 (150 AK); 1000 (all other models) 7 to 18 2-40 (300 AK-N) 3-60 (all models) to 3·5-70 (100 AK-N)
PWE 100 to 300n Two series: AK-N (4 models) and AK (4 models)
TABLE 13.18 Technical Data for Planetary Compounders: Summary Table
9 to 40 6 to 22
12 to 49 9 to 31
Screw Heating capacity (kW): Barrel
85 to 2300 40 to 700
60 to 1400
7·7 to 81
3 (70 EV); 4 (100, 120 and 200 EV); 5 (170 EV) or 6 (150 EV)
12 to 280
• Range limit data apply respectively to these range limit models, unless otherwise indicated. b Approximate ranges. c Figure for PWE 120 AK (the output of PWE 100 AK-N is the next lowest).
400 to 2800 500 to 3300 700 to 4000
2
1 to 2
Screw Output ratesb (kg h- 1 ): Rigid PVC Semi-rigid PVC Plasticised PVC PVClABS blend
23 to 130
60 to 320
Drive rating (kW) No. of heating/cooling zones: Barrel
300c to 2800 350c to 3300 450c to 4000
21 (100 AK) to 72 (170 AK and 200 AK)
2 (100 AK) to 3 (all other models)
60 to 320
36 to 90
3 (all models)
50 to 400
646
W. Henschel and P. Franz
comparison in the controller between the desired and actual tempertures and subsequent correction, either by cutting-in electric heaters or passing cooling water through the heat exchanger. Electric heating systems are controlled in similar fashion. Thermocouples detect the temperature of the barrel wall in each zone, which is then compared with the set-point. If the wall is too cool the electric heater is switched on; if it is too hot, a liquid cooling medium is passed through the particular jacket or else fans are switched on to provide air cooling. Each heating zone represents a closed control loop with the set-point temperature acting as command variable. In contrast to final processing systems, compounding operations generally do not require the command variable, i.e. the set-point temperature, to be altered in line with the momentary stock temperature. Depending on the physical layout of a compounding operation, it may be advisable to display the various zone temperatures in the central control room or even to record them. METERING RATE AND SCREW SPEEDS
In the equipment described in the preceding sections, in which screw-type machines are fed via feed hoppers, the speed of the feed screw governs the production rate. This rate, and the speed of the main screw, are two of the variables on which the energy input depends. If conditions are unfavourable, a screw-type machine can be overfed so much that the feed screw or the main screw jams. An added factor in the case of pellet production is that the pelletising screw must extrude the stream of product emerging from the compounder through the multi-hole die plate; again, this is a question of screw speed. It is therefore important to be able to preselect and control these speeds from the central control station. Normally it is sufficient to employ analog indicating instruments for showing the speeds picked off by tacho-generators. Where DC drives are used, the preset speed is automatically held constant regardless of momentary motor load until the maximum admissible amperage is reached. This, then, constitutes another closed electronic control loop. In the compounding operation, there is no need to control speeds on the basis of stock temperature and stock pressure as is the case, for instance, in injection moulding.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
647
ENERGY INPUT
The level of energy input is the main determinant of product quality. It is expressed as specific kneading energy (work), i.e. that imparted to a unit weight of stock, calculated by dividing the momentary driving power by the output rate: kWh) e (kg
N(kW)
= G (kgh-I)
Hence the driving power levels of the kneading and pelletising screws are among the most important variables requiring monitoring in a compounding installation. Experience has shown that the input power reacts more quickly and reliably to changes in process conditions than do temperatures or pressures. In the case of AC drives, the energy reading must be compensated for reactive power to show only the active power imparted to the product. STOCK TEMPERATURE
After the specific kneading energy, heat history is the second important determinant of product quality. Thus it is extremely important to know the temperature profile of the stock as it moves through the line. A technical problem arises in this connection. In the screw-type machines described, the temperature probes (usually thermocouples) have to be inserted into the barrel wall without coming into contact with the PVC stock (Fig. 13.69). However, the barrel wall is heavily affected thermally by the heat-transfer liquid flowing through it, or by electric heaters. Thus the temperature probe picks up not the true temperature of the stock, but merely a reference temperature lying somewhere between it and the heating zone temperature. To obtain more accurate readings, probe rings (Fig. 13.70) have been developed. However these can only be placed ahead of the screw tip and thus do not provide a temperature profile along the barrel. Moreover, although the measuring shoulder is streamlined, they raise the resistance at the outlet of the machine and may even cause scorching. An exception to the foregoing is the temperature measurement in the Buss-Kneader system described in Section 13.4.4(a). In this case the barrel kneading tools ('teeth') are completely surrounded by the
648
W. Henschel and P. Franz
HEAT-TRANSFER MEDIUM
T
Fig. 13.69 Temperature probe in extruder barrel wall.
PVC stock (Fig. 13.71) and virtually take on its temperature. Thus the thermocouples, which are inserted in the teeth and reach close to their ends, are able to pick up the actual stock temperature at any point along the barrel. Because the stock temperatures are very important in assessing heat history--or even only as reference data-it is advisable to record them at the central control station. Multicolour chopper-bar recorders are normally used for this purpose.
A-A
Fig. 13.70 Probe ring.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
649
Fig. 13.71 Temperature measurement in the Buss-Kneader.
PRESSURES
Measurement of pressure is not as important in compounding as in final processing, because the compounding process can be controlled with sufficient accuracy on the basis of power input and temperature profile. The diaphragms of the pressure probes normally employed are highly susceptible to inadvertent damage during cleaning operations, and disastrous errors can result if this damage remains undiscovered. For this reason, pressure probes are normally used in compounding only for monitoring the clogging of strainer screens. (d) Interlocks A compounding installation, that reaches from pre-mix feeding all the way to pellet bagging or storage, represents a complex, integral system that may well cover several different sections or storeys of a production plant. It is therefore of utmost importance that switching sequences be maintained without exception both during starting and in the event of an unplanned shutdown. A mimic diagram is therefore provided at the central control station, particularly in large scale production systems, to facilitate operation of the equipment. Moreover, the individual pieces of equipment are interlocked electrically on the counter-flow principle, so that start-up must begin with the last link in the chain-e.g. the conveying air fan for
650
W. Henschel and P. Franz
carrying the cooled pellets to a storage silo-and move upstream, with each unit being switched on in succession. If a given element in the chain is omitted, none of the rest can be switched on. Should any element stop because of a malfunction, all equipment upstream from it is switched off according to a preprogrammed schedule. Trouble of this kind is signalled acoustically and shown visually on the mimic diagram. (e) Materials of Construction Two main factors must be considered in the selection of materials for PVC compounding machinery: corrosion and abrasion. All PVC formulations are a possible source of corrosion. There is always a latent hazard that traces of hydrochloric acid will form as a result of power fajlure or incorrect plant operation. Hence all parts of a compounding machine in contact with the product are made of corrosion-resistant steels. For parts subject to severe mechanical loading, designers prefer steels with 17% Cr because of their high yield point and good fatigue strength. Chrome nickel steels are more resistant to corrosion, but are suitable only for elements with lower mechanical loading because the yield point is only half as high. In the case of highly filled formulations, such as flooring compounds, abrasion must be taken into consideration along with corrosion. It has proven advantageous to employ element-type screws for such applications, i.e. the kneading tools are carried on short elements that are mounted on a cylindrical arbour. When wear occurs, only the worn elements need replacing. Abrasion resistance can be increased by cladding the screw elements with high-wear armour materials by welding, plasma spraying or cold spraying. Barrels are protected against wear in similar fashion. Figure 13.72 illustrates the two basic possibilities. In machines with a one-piece barrel (Fig. 13.72(a)), bimetallic liners are forced into the bore. As soon as wear exceeds admissible limits, the barrel is taken off and the liners are replaced. In the case of the vertically split barrel of the Buss-Kneader (Fig. 13.72(b)), similarly split liner sections are installed in each barrel half. These semi-cyclindrical liners are held in place by the kneading teeth mounted in the barrel. Worn-out liner elements can be replaced without taking the barrel off the machine; those not yet worn are left in place.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
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(f) Machines for the Production of Pastes PLANETARY MIXERS
Planetary mixers (Fig. 13.73) are suitable for mixing stock pastes and for colouring virgin pastes with colour concentrates. Because little energy is imparted, no temperature problems arise, but this mixer is not capable of dispersing solids thoroughly: additional sifting and grinding steps are required for exacting specifications. Planetary mixers are usually built as vacuum mixers to de-aerate the paste prior to final processing. Anywhere from 6 to 40 mbar of vacuum is sufficient for this purpose. Mixing aids, such as internal pin mills (Fig. 13.73(b)), can be added without difficulty. Mixing tools can be raised up to permit removal of the mixing tank and to facilitate cleaning. Planetary mixers are particularly suitable for medium-size operations with a broad product range. HORSESHOE MIXER
This vertical mixer has a centrally mounted, slow-turning agitator, shaped like a horseshoe. This sweeps the entire tank wall to make sure all the contents are involved in the mixing process. Baffles must be provided to prevent the paste from rotating along with the agitator in laminar fashion, which would impair the mixing effect. The simple version of the horseshoe mixer is limited to use as a holding vessel for pastes. Lately efforts have been made to achieve a mixing effect similar to that of the dissolver by adding a fast-rotating second agitator mounted eccentrically in relation to the first (Fig. 13.74). BALL MILLS
In these mills, the friction gaps required to achieve dispersion are provided by balls moving past one another. Where ball diameters are relatively large, it is sufficient to tumble them in drums. To meet more exacting dispersion requirements, however, small spheres or beads of glass, ceramic materials or steel, or silica sand, are used, kept in motion by agitators (Fig. 13.75). Because energy input is relatively high, ball mills are used mainly for producing colour pastes. In a recent development cooled agitating disks and cooled housings are being tried to eliminate excessive heating. Ball wear and screening of the balls deserve special attention.
Because of cleaning expense, these mills are used only for long production runs.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
653
b
a
Fig. 13.73 Planetary mixer. a, Basic; b, with internal pin mill.
I
~ .~ L.--
L.-.
I
.
I
~
t'--- L-----" Fig. 13.74 Horseshoe mixer.
RIBBON BLENDERS
Ribbon blenders (Fig. 13.76) have a slow-turning agitator that works in conjunction with counter-flow screws or counter-flow baffles to mix the product axially and tangentially. Because hardly any kinetic energy is converted to heat, these mixers require relatively low driving power.
654
W. Henschel and P. Franz
Fig. 13.75 Ball mill.
On the other hand, the energy is insufficient to produce the fineness of dispersion called for in many cases. Sifting and grinding operations are therefore used following, or between, mixing processes of this type. These mixers make it possible to prepare large quantities of uniform stock or finished pastes, but they suffer from the drawback of being hard to clean. Axially removable agitators have improved matters lately, however.
Fig. 13.76 Ribbon blender.
13
Industrial Compounding Technology of Rigid and Plasticised PVC
655
Ribbon blenders are used preferably for very long runs with infrequent colour changes. PADDLE MIXERS
The mixing tools of the paddle mixers (Fig. 13.77) dip down and bring large, cohesive quantities of paste to the surface. In addition, the tool surfaces beat alternately into the paste. The kneading and squeezing processes combine to produce the mixing effect. However, for effective operation the mixer should be no more than about 70% full. Because the energy input is at the thermally tolerable limit, the mixing tank is equipped with a jacket for cooling. To reduce the labour required for cleaning, the mixing tools can normally be withdrawn axially. Though pin mills may be installed to improve the efficiency of mixing, they should be used cautiously because of the energy dissipation involved. If the mixing tool speed is changed, paddle mixers can also be operated under vacuum. Conversely, pressure can be applied to accelerate emptying (even with the mixing tool stopped) and to feed the contents to pressure or vacuum strainer units.
Fig. 13.77 Paddle mixer.
656
W. Henschel and P. Franz
ROLL MILLS
Three-roll mills with two shearing nips (Fig. 13.78) are normally used for producing PVC pastes on this type of equipment. Nip pressure, roll speed and roll speed differentials (which produce the shear) must be set appropriately for PVC pastes. If the settings are not appropriate to their rheological properties, the paste films carried on the rolls may be broken in the nips.
,
I
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t\
I
f\
./
Fig. 13.78 Three-roll mill.
Unlike single-roll mills, three-roll mills have no sifting effect. Roll adjustment to maintain uniform nip clearance can be made automatic (and thus independent of the operator) with the aid of hydraulic systems so sophisticated that they are even capable of correcting automatically for changes in nip characteristics resulting from thermal expansion or other factors. Current standard practice is to feed the raw paste to the roll mill freely with a cut-off element or with the aid of pumps, regardless of viscosity. The feed rate is controlled by the paste buffer in the inlet nip. DISSOLVERS
Dissolvers (Fig. 13.79) are mixers with high-speed, infinitely variable disk agitators. They are used for the production of stock pastes and for dispersion. Most dissolvers are vertical machines that work-especially in large-scale production-with portable tanks. The main element is the agitator disk, whose diameter should be half that of the tank. Desirable peripheral speeds during the dispersion phase are in the
13
Industrial Compounding Technology of Rigid and Plasticised PVC
657
Fig. 13.79 Dissolver. a. Disc agitator; b, scraper. range 22-25 m S-1. To achieve laminar flow, the final viscosity of the paste should reach about 25 P. If low-viscosity pastes are being produced, they should be mixed at a higher viscosity level (with some of the plasticiser left out) and then diluted after the dispersion phase. The mixing tanks are smooth on the inside, preferably with vertical walls. Circulating scrapers (wipers) can be installed (Fig. 13.79) to return powder or poorly wetted paste adhering to the tank wall to the bulk stock. Dissolvers with a vacuum facility make it possible to produce, de-aerate and colour the paste in a single operation. In such cases vacuum is applied only after the resin has been wetted-out. Continuous dissolvers are being developed. BUSS MIXING TURBINE MT
Unlike the mixers described thus far, the Buss mixing turbine is a continuously operating unit for the production of stock, finished and coloured pastes. The equipment (Fig. 13.80) consists of a vertical stator containing a high-speed rotor. The solids are fed by a metering device through the inlet tube, 2, into the atomisation zone, a, where they are aerated and disintegrated for passage to the centre of the top rotor, 4. While the solids are spun out radially by the rotor, the liquid components are injected through nozzles on the bottom of the stator
658
W. Henschel and P. Franz
Fig. 13.80 Buss mixing turbine MT. 1, Drive motor; 2, inlet for solids; 3, rotor shaft; 4, rotor; 5, stator ring; 6, casing; 7, spray nozzles for liquids; 8, product outlet; a, atomisation zone.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
659
rings. The rotors are shaped in such a way as to propel the product downwards as well as radially outwards. The next stator ring directs the already wetted or partially dissolved solid material toward the centre of the following rotor, and the process is repeated. In some applications the entire quantity of liquid is metered in through the top nozzle ring. In others, it is fed in fractions to the successive mixing chambers. The mixing turbine can be connected directly to a vacuum filter system, in which case de-gassing can take place at the same time. The finished PVC paste is then pumped continuously out of the vacuum tank to the next operation. The two-piece rotor can be swung open for easy cleaning when drastic changes of formulation or colour are scheduled. Because the machine is compact and easy to clean, it is economical for both short and long production runs. Some technical data are given in Fig. 13.81. I
r i1
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i
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H(mm) B(mm) T(mm)
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Weight (kg) Output (kg h- 1)
._.*
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T MTIOO
MT250
1200 300 500 110 lOG-2oo
2000 700
MT400
2800 1200 900 1100 1300 2000 60(}-1000 150G-25oo
Fig. 13.81 Technical data for the Buss mixing turbine.
660
W. Henschel and P. Franz
13.5 PELLET COOLING AND STORAGE 13.5.1 Pellet Cooling (a) Nature and Outline of the Operation It is necessary to cool down the hot pellets rapidly in order to prevent sticking, deformation and possible thermal degradation. Since most PVC compositions are in a dry, highly viscous state on leaving the pelletiser, there is no need to employ water cooling (as practised, for example, with polyolefins). Cooling in a strong air stream is sufficient. This has the big advantage that there is no need to superimpose a pellet-drying operation on the cooling process. Ambient air is usually all that is needed, although in a tropical climate it may sometimes be necessary to pass it through a cooler or even an air drier before blowing it into the pellet cooler. From the pelletiser head (where their temperature may be anywhere from 130 to 190°C depending on PVC type and formulation) the pellets are conveyed pneumatically through a pipe to the pellet cooler. En route, the pellet temperature will fall (by 40-50°C depending on air flow, pellet loading and distance): in the cooler it is lowered further, to about 40 to 60°C, to permit trouble-free storage. Most pellet coolers operate on the fluidised bed principle, i.e. the pellets are cooled while being held in turbulent suspension in a rising stream of air. (b) Pellet Cooler Systems Figure 13.82 shows two systems developed by Buss. In the first (Fig. 13.82(A)), the pellets leaving the pelletiser are collected in a casing and carried to the pellet cooler through a pipe operating on the pneumatic suction principle. A cyclone separator built into the cooler separates the pellets from the conveying air. A rotary valve attached to the cyclone then feeds the pellets into the cylindrical cooling chamber, where they are distributed by a sheet-metal cone. The conical bottom of the cooling chamber is made of perforated sheet-metal. A second fan, which is connected to the cooling chamber by a pipe, draws the cooling air (room air) through the perforated bottom so that it passes upward through the slowly sinking pellets. The cooled pellets are discharged by a second, variable-speed rotary valve attached to the conical bottom of the cooling chamber. An equilibrium condition, i.e. a constant fluid-bed height in the cooling chamber, is obtained by
13
Industrial Compounding Technology of Rigid and Plasticised PVC
661
A or
I coolpr
rolar
YClIVPS
B
Fig. 13.82 Granulate discharge and cooling units. A, Suction conveying unit; B, pressure feed unit.
662
W. Henschel and P. Franz
altering the rotary valve speed to match the momentary output of the compounding line. Each of the suction conveying pipes contains a butterfly valve for adjustment of optimal air conditions in the conveying pipe and cooling chamber. The alternative design (Fig. 13.82(B» is similar to the first, except that there are no mechanically moved parts (rotary valves) in the cooler. Here again, the pellets are collected in a casing as they leave the pelletiser. They are then blown by a fan directly into the cooling chamber through a pipe attached to the casing (pneumatic 'push' conveying). The conveying air escapes to the atmosphere through an oversized pipe capped by a filter basket. A second fan blows cooling air into the cylindrical cooling chamber up through the perforated, conical base. Because the air velocity and pressure in the cooling chamber are lower than those of the conveying air in the pipe, cooling air cannot penetrate the pipe projecting into the cooling chamber. The cooling air also escapes through the exhaust air nozzle at the top of the cooler. A butterfly valve is provided to permit adjustment of the air flow so that the pellets are kept in a state of gradually sinking suspension. The cooled pellets leave the cooler through an adjustable slide valve at the bottom. Two other pellet cooler designs are shown in Fig. 13.83. Both are chute conveyors employing the ftuidised bed principle, and both use a cyclone separator ahead of the cooler to isolate the pellets from the conveying air, a rotary metering valve at the bottom of the cyclone, and adjustable diverter plates over which the pellets pass on their way to the chute. The two designs differ in the mechanism employed to obtain forward movement of the pellets in the chute. In the version shown in Fig. 13.83(B), the chute, which has a finely perforated sheet-metal base, is mounted on elastic vibration elements (such as vibration mounts or helical springs): it thus functions as a vibrating chute conveyor (here actuated by an eccentric motor) in which the pellets are moved forward by micro-propulsion under both horizontal and vertical acceleration, being whirled periodically upward from the bottom of the chute in parabolic trajectories whose size and shape depend upon the frequency and amplitude of the drive. A fan connected by a pipe to the top of the chute conveyor draws cooling air up through the perforated base, at right angles to the conveying direction. Because the air flows over the entire surface of each individual pellet, an intensive cooling effect is achieved. Pellet residence time on the chute is adjustable over a broad range by changing the chute's inclination or altering the amplitude of the
13
Industrial Compounding Technology of Rigid and Plasticised PVC
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Fig. 13.83 Horizontal granulate cooling units with: A, feeding system using pulsation of air; B, feeding system using mechanical vibrations.
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664
Henschel and P. Franz
eccentric motor. The rate of cooling is regulated by varying the air flow in conjunction with the pellet residence time. The pellet temperatures normally achieved are some 5 to 10°C above the ambient air temperature. In the Biihler-Miag design shown in Fig. 13.83(A), the pellets are moved along the chute by means of pulsating cooling air, which is blown into the chute by a fan, through a finely perforated base plate. A pulsator in the form of a butterfly valve (driven by a variable-speed motor) in the intake duct between the fan and the perforated base produces an intermittent air flow that moves the pellets along the chute. The cooling air leaves the chute through an oversized exhaust pipe. Pellet residence time can be varied by altering chute inclination or pulsation rate. 13.5.2 Pellet Mixing and Storage
The mixing of pellets with different characteristics or the homogenisation of pellets of a given type from different production batches calls for large containers with useful capacities up to 1000 m3 .
(a) Pellet Mixer Designs MECHANICAL MIXERS
A broad selection of mixers is available for mixing very small batches of pellets up to about 10 m3 • The main types of mixers used in the plastics industry for larger batches are listed in Fig. 13.84 which gives their capacities and corresponding driving power levels.
Note: Another type of mixer, not included in Fig. 13.84, is the pure gravity mixer: the bulk material is withdrawn simultaneously from various levels of the mixing silo and fed to a common chamber, or else silo cells formed by vertical partitions are filled successively and then simultaneously emptied. These mixers, which require no energy input, cannot produce an ideal (random) mixture but are adequate for certain purposes. Mechanical mixers such as the orbiting vertical screw mixer (Fig. 13.85) are built with useful volumes as large as 75 to 80 m3 . This type of mixer has a hopper-shaped container with a screw set parallel to the hopper wall. The screw is supported at the bottom of the hopper and is
13
Industrial Compounding Technology of Rigid and Plasticised PVC
665
10000
1000 ~
~
01 C
15 L-
a.o
10
~
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a
. 0.1
10lXlO
Capacity
m3
Fig. 13.84 Drive rating as a function of mixing volume in pellet mixers. A, Orbiting vertical screw mixer; B, double cone mixer; C, vertical screw silo mixer; D, pneumatic mixer.
driven by a crank arm at the top so that it both rotates and circles the inner wall of the hopper simultaneously. (In the epicycloidal mixer, the screw describes an epicyclic curve between the hopper wall and centre instead of travelling in a circle around the periphery.) The PVC pellets are carried upward by the screw to reach ever higher levels within the hopper. Double-cone mixers are built with capacities as high as 100 m3 . So are vertical screw silo mixers of the type shown in Fig. 13.86. In vertical screw silo mixers, the pellets are raised to the upper part of the silo by a screw conveyor in the central pipe. Spreader arms distribute the material evenly over the silo cross-section. The material then proceeds to move downward in the silo, at a rate which is a function of the conveying rate of the screw in the bottom part. A hopper-shaped insert divides the pellets as they slide downward. Ultimately they pass through the outer and inner annular spaces to the bottom of the screw and are raised once again.
W. Henschel and P. Franz
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US4!'fui capacIty Standard siz4!'s
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OrIY4!' rohng 1.51010 kW/m 3
- To sloraQ4!' - To shipping
Fig. 13.85 Orbiting vertical screw mixer.
PNEUMATIC MIXERS
Pneumatic misers, which include both the fluidised-bed mixer for powders (Fig. 13.87) and the recirculating mixer for coarse bulk materials and pellets (Fig. 13.88), can be built in sizes up to about 1000 m3 . Pneumatic mixers are built with higher drive ratings and larger capacities than mechanical mixers. They also handle the pellets more gently. Figure 13.89 shows the operating pressure as a function of mixer height for both types of pneumatic mixer.
13 Industrial Compounding Technology of Rigid and Plasticised PVC
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Fig. 13.86 Vertical screw silo mixer.
Fine bulk materials with particle sizes between about 30 and 200 J.lm can be homogenised very economically in fluidised-bed silo mixers of the kind originally developed for the cement industry, in which the mixing process depends on different degrees of aeration of (different rate of air supply to) different sections of the base of the silo. The main design parameter is the fluidisation line. If a fluidised bed is aerated at an air velocity above the point of incipient fluidisation, the bed expands. The base of the silo mixer is designed to permit individual sections to be aerated more strongly (or 'actively') than other ('inactive') sections. This causes the bulk material above the actively aerated sections to expand and rise higher in the silo, so that it flows down onto the material in the inactive sections. The material in the
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bottom of the 'inactive columns' also tends to flow into the actively aerated sections, because the actively fluidised 'columns' have lower bulk density. The resulting circulation produces mixing, which can be promoted by activating and de-activating a given section at regular intervals. Fluidised-bed mixers of this type are used for homogenising PVC powder following polymerisation. Because the rate of air flow required for fluidisation rises with the square of particle size, fluidised bed mixers become uneconomical for coarse bulk materials with particle size larger than 500 pm, including
13 Industrial Compounding Technology of Rigid and Plasticised PVC
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plastics pellets. For instance, pellets 3 mm in diameter with a solids density of 1000 kg m -3 and a bulk density of 500 kg m -3, mixed with an air fluidisation velocity of 1 m s -1 in a silo 5 m in diameter, would require a minimum air flow rate of 70 000 Nm 3 h -1. * The pneumatic circulating mixer provides a way of obtaining satisfactory pellet homogenisation with less air. In this case the PVC pellets are withdrawn at two points in the mixer outlet and conveyed * A German unit = cubic metres per hour at STP.
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11'1
30m Mixer height
Fig. 13.89 Operating pressures for pneumatic mixers. a, Fluidised-bed mixer; b, pellet recirculation mixer. (Source: Waschle leaflet.) pneumatically back up to the top. In addition-as shown in Fig. 13.88-a mixing hopper (b) is built around the riser pipe (a). After the pellet streams flowing between mixing hopper and riser and between mixing hopper and mixing cone (c) have passed through a bottleneck (d), in the form of annular metering gaps s1 and s2, they are metered into the air flow for upward conveying. Together with the mixer geometry, this metering step affords effective horizontal and vertical mixing of the entire mixer contents. The metering gap s2 can be optimised by shifting the riser pipe (a) vertically. A single-stage fan generates the air pressure for the vertical conveying. It blows the air into the mixing head (f), in which the PVC pellets are metered in through the gaps s1 and s2. Because air velocities are low (about 12 m S-l), the pellets are conveyed very gently. Loadings of 7-12 kg of pellets per kg of air are optimal. At the upper end of the riser pipe, the pellets are spread on top of the mixer contents by a diverter cone (h). In view of the low conveying velocity, this change in direction is also very gentle. The conveying air leaves the silo through a central capped opening in the mixer lid. Intelligent design of the riser pipe enables the single-stage fan to raise the pellets at rates of 100 Mp h-h and higher. • Megaponds per hour (i.e. tonnes per hour).
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The mixing process requires the pellets to be circulated 3 or 4 times through the silo. The uniform random mixture obtained has a standard deviation of the order of 2 to 3%. Pneumatic circulating mixers have the following features: -no moving parts in the mixing chamber; -high-quality mixing; -achievement of uniform, random mixture; -same mixing quality throughout entire mixture; -no segregation of different particle sizes; ---easy adaptation to process because of flexible mixing time; -low-cost installation; -virtually unlimited batch size; -low energy consumption, even for large particle diameters; -gentle product handling; -low noise level; -spray nozzles making cleaning easy at product changeover. (b) Handling of pvc Pellets Having been homogenised in the mixers, the batches of pellets can now be delivered for processing. Most mixer silos are set on frames to permit road tankers, container trailers or rail tankers to be driven underneath for gravity filling. If the pellets are to be bagged, semiautomatic or fully automatic bagging installations are supplied directly from the mixers. Fully automatic systems normally provide automatic stacking of the bagged goods on pallets followed by fixation of the unit load with PVC shrink film. If the compounding operation is part of a final processing plant, the pellets are conveyed to the injection machines or extruders through distribution pipes or in boxes carried by fork lifts.
CHAPTER 14
Extrusion ofPVC-General Aspects B. J.
LANHAM
and W. V. TITOW
14.1 INTRODUCTION
In the widest general sense, extrusion is a process for the conversion of a plastics feed material into a continuous product with cross-section shaped and dimensioned to particular requirements. In some operations the primary object is compounding and thorough mixing of the plastics material (extrusion compounding-see Chapter 13). Historically, plasticised compositions were the first PVC compounds to be processed into products by extrusion. Single-screw extruders, designed for rubber compounds, were used: these machines had very low length-to-diameter (LID) ratios (down to 4: 1 in some cases), with no provision for effective heating at the temperatures appropriate for PVC (higher than those required for rubber processing) and for temperature measurement. Improvements in these respects, together with provision for acceptance of granular feedstocks, may be regarded as the earliest steps in the evolution of PVC extrusion equipment (see Chapter 1, Section 1.2). Even with these modifications the early extruders were crude by modern standards: nevertheless, cable coatings, tubes and sections were being produced towards the end of, and shortly after, World War II. Since those days the single-screw extruder has developed into advanced equipment of a high standard of efficiency and reliability. The growth of the use of rigid PVC-first extruded in the mid-1950sand especially rigid pipe production, was a prime factor in the development of sophisticated twin-screw machines (and certain types of multi-screw compounding extruders-see Chapter 13). 673
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The use of extruders in the compounding of PVC is discussed in Chapter 13. Apart from its role in that application, and indeed even more importantly, extrusion is a major production process, whereby about one-third of all PVC polymers used are converted into products of high industrial and applicational significance. The main rigid PVC products made by extrusion are pipes (including water, soil, and drainage pipes, rainwater goods, and electrical conduit), cladding for buildings, profiles (including window frames), sheeting (including packaging films), and blow-moulded containers. These products are reviewed, together with others (including plasticised extrusions) in Chapter 26; some are also discussed in Chapters 17 and 19. The most important plasticised PVC extrusion products are electrical wire and cable coverings, flexible film, flexible tubing and profiles. Protective wire coating for chain-link fencing may also be mentioned. 14.2 THE EXTRUDER 14.2.1 Main Components and Their Functions, with Special Reference to Extrusion of PVC
Figure 14.1 is a simple, generalised, schematic representation of the basic processing parts of a single-screw extruder. The PVC feedstock (pellets or powder~f. Chapter 13, especially Section 13.4.1(a); and also Section 14.2.2 below) is charged into the hopper, whence it passes to the feed section of the screw through an opening in the barrel (the feed throat). The stock is conveyed forward by the screw and at a
Hopper
- - t -....
Fig. 14.1 General arrangement of a single-screw extruder.
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Fig. 14.2 A typical twin-screw arrangement. (Intermeshing screws mounted in horizontal plane; actual screw configuration of a Schleomann machine, ct. Table 14.2.) certain stage-depending on the composition, screw design and operating conditions-fuses to a homogeneous melt. The conversion of the solid PVC feed material to this state, and in particular the morphological changes involved (see Section 14.3), is usually referred to as 'gelation' or 'plasticisation'. The molten material is forced out through a die at the outlet end. This extrudate is cooled, whereupon it solidifies in the cross-sectional shape imparted by the die. In a twin-screw extruder the two screws are positioned in the barrel side-by-side, in a horizontal plane (cf. Fig. 14.2). The screws may be either the regular cylindrical overall shape. or they may be conical, tapering towards the die end of the extruder. In general. the screws may be either co-rotating or counter-rotating (to the inside or the outside)-in PVC processing commonly the latter. Twin-screw extruders for PVC usually have intermeshing screws: with segmented screw construction (see (a) below) the flights may intermesh fully along some segments, and only partially in others (some screw configurations may also include non-intermeshing sections). It is essentially a question of semantics, and in any case a largely academic point, whether the twin-screw machine should be classified as the simplest version of the multi-screw extruder. As a matter of practical fact, twin-screw extruders-of various designs and makesare in wide industrial use for the processing of PVC (both melt-compounding and manufacture of important products like pipe, profiles, sheeting, etc.), whereas truly multiple-screw machines (e.g. planetary extruders-cf. Chapter 13, Section 13.4.4, and Ref. 1 here) are less common and employed principally in a compounding role. For the purposes of this chapter, twin-screw extruders are regarded as a group distinct from multi-screw extruders proper, and the latter are not
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discussed in any detail because of their essentially compounding function, already covered in Chapter 13. The heat necessary to fuse the stock to a melt, and to maintain that at the appropriate temperature for the degree of fluidity required, is supplied in part by heaters on the barrel (in some machines the screw temperature may also be controlled by the circulation of a fluid), and partly generated in the material itself by the shear and friction to which it is subjected in the machine. In otherwise comparable operations the work-heat generation is greater in single-screw machines, where it may also lead to a temperature distribution in the material less uniform than that obtainable in twin-screw extrusion. (a) The Screw Figure 14.3 illustrates some basic general structural features of a screw. In broad terms, the functions of a screw-in conjunction with the confining barrel-are to convey the feed forward, melt ('plasticise') and mix (homogenise) it, and to meter the resulting melt, under
Fig. 14.3 Some general features of a screw and associated definitions. Depth: the perpendicular distance from the top of the thread to the root surface. Flight: the space enclosed by the thread and the surface of the root in one complete turn of the screw. Helix Angle: the angle between the screw thread and the transverse plane of the screw. Land: the surface at the radial extremity of the screw thread constituting the periphery or outside diameter of the screw. Lead: the horizontal distance travelled by the material in one complete revolution of the screw, assuming 100% efficiency. It is equal to the pitch multiplied by the number of starts. Number of starts: the number of separate threads traced along the length of the screw. Pitch: the horizontal distance between corresponding points of two successive lands. Root: the continuous central shaft, usually of cylindrical or conical shape.
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pressure, to the die. Originally PVC was processed with screws of relatively simple design, comprising essentially three sections, each associated with one of the above functions, viz. a plain(parallel)channelled feed section, a transition (compression) section with gradually increasing root diameter (reducing flight cross-section) in which the composition is melted and compacted, and a metering section where the mixing of the composition is completed, and which acts as a metering pump delivering the melt to the die at constant volume and pressure. 2 Today, more intricate screw designs are widely used, involving shear sections, special mixing sections, and sections forming decompression zones in the screw/barrel assembly, from which volatiles are vented off. Complex flight configurations and-in some cases-special processing elements (e.g. mixing pins on the pin-type screw of a single-screw extruder) are found in some of the sections. In certain designs the elements may be in the form of demountable bushings on an essentially plain screw shaft, so that screw configuration can be varied for different extrusion or compounding operations: one example is provided by the screws of the Werner Pfleiderer ZSK compounding extruder3 (d. also Chapter 13, Section 13.4.4(a). Other facts being equal, the ratio of the effective length of the screw (the distance between the forward edge of the feed opening in the barrel and the forward end of the barrel bore) to its diameter (taken as equal to the barrel bore diameter) affects its processing capacity, as well as the amount of heat generated and its transfer in the system screw/material/barrel. Note: This ratio, known as the LID ratio, is customarily expressed in relation to unity: e.g. the LID ratio for a screw of 100 mm
diameter and effective length 2·5 m (i.e. 2500/100 = 25) would be given as 25: 1.
LID ratios of screws in modern single-screw extruders for PVC can range up to about 35: 1 (or even higher in some cases), although 24: 1 is fairly common. With twin-screw machines lower values are practicable, because the co-action of the screws makes it possible to attain the requisite material pressures and degree of homogenisation at shorter screw length (aided also by the greater degree of contact between the working surfaces and the stock in comparison with the single-screw extruder). Modern screws have internal channels for the circulation of cooling liquid. With some thermoplastics only the feed section of the screw
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needs to be cored for cooling. For PVC this facility should be available along the whole length. As mounted in place in the extruder, the screw is driven by a motor through a set of reduction gears to ensure sufficient torsional power and to transform the high rotational speed of the motor into the much lower rotational speeds required of the screw. A thrust bearing at the rear end serves to accommodate the substantial back-thrust experienced by the screw as a result of back-pressure built up in operation, by the restricting action of the die upon the forward movement of the melt which the screw actuates. Screws are made of steel, and surface-hardened (most commonly by nitriding) for increased resistance to wear and some corrosion protection. For increased protection against corrosion in PVC processing the screw may be chromium-plated (at least on the flights 4 ): hard chroming will also confer some wear resistance. Other means of increasing resistance to abrasive wear are nitrogen ion implantation (a relatively new treatment developed in the UK at AERE, Harwell, and now in commercial useS), or coating with special alloys (of the kind also used for lining extruder barrels-see below). Note: It has been claimed that a Z-Core (Z-Core Inc., Trenton, NJ, USA) alloy coating can increase the life of screw and barrel by a factor of 3·5 over that of an otherwise comparable nitrided set in a twin-screw extruder processing PVC pipe compound and regrind containing untreated calcium carbonate filler. 6
The extent of mutual compatibility of the screw and barrel facing materials can be a factor in the resistance to wear in operation (apart from the effects in this regard of the plastics stock). The significance and role of this factor have been discussed by McCandless and Maddy. 7 The mechanics of wear on plastics extrusion equipment, and the main methods of improving wear resistance (as well as resistance to corrosion in some cases) are listed in a useful brief review by Avery and Csongor. 8 These authors point out, inter alia, that hard facings (applied by various welding methods) of cobalt-ehromium-tungsten alloys, although expensive, offer a very high degree of resistance to surface wear (with particularly good stability at local frictional temperatures high enough to soften conventonally hardened steels) combined with high resistance to corrosion, particularly by hydrogen chloride. Such facings can be applied in relatively thick layers resistant
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to penetration and undermining in service. Nitriding provides some corrosion protection, whilst hard chromium plating is both very hard and corrosion resistant but comparatively thin and-if not carefully applied-subject to cohesion and adhesion problems. (b) The Barrel The barrel is often a solid forging of high grade mild steel which is prepared externally to receive the heaters and internally to receive the liner. The latter is of the greatest importance and is usually made as a one-piece, hardened and ground, nitralloy component, honed to a mirror finish. The clearance between the screw and liner will often be as low as 0·001-0·003 in. It may be necessary to run them in and not place too great a load on them at the start. The barrel should be provided with suitable positions for temperature sensors (usually deep-seated thermocouples), and venting port(s): venting, which is standard in modern PVC extrusion, is particularly important with powder feed because of its relatively low bulk density (high air content). Each venting port (which in operation will usually be connected to a vacuum system) is normally associated with a section in the corresponding part of the screw where the material is not under compression (a decompression zone) to ensure that it is not extruded through the vent. Barrel and screw lengths have increased considerably over the years, particularly in single-screw extruders. Several factors have been instrumental in this change. Thus, in both single- and twin-screw machines, some length extension was the direct result of the incorporation of venting zones: the introduction, in many cases, of additional or modified screw sections-in consequence of advances in screw design-was also a contributory factor. In single-screw extruders, the main route to attaining the desired higher output rates was by increasing the screw diameter (as, other things being equal, the output rate goes up in proportion to the square of this-see eqn (1) below). A simultaneous increase in length was also necessary, however, to prevent a reduction in the residence time of the material in the barrel which would otherwise result as a consequence of the faster output (and hence throughput) rate: such a reduction would entail the need for a higher temperature and/or shear energy input in working the material to ensure adequate fusion and homogenisation. Note: With PVC, and especially uPVC, which is susceptible to
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thermal degradation, and whose heat history should always be kept to a practicable minimum, the fact that the extended residence time in a longer barrel enables the total amount of energy necessary for the requisite plastification and gelation to be imparted under less drastic conditions of temperature and shear, is particularly significant in both single- and twin-screw extrusion. The barrel length is also, in its own right, a factor in the output rate. Moreover, with a longer barrel, the output is less sensitive to changes in back-pressure, as well as more closely linear with screw speed. The following expression9 is a fairly good representation of the general relationship between the rate of output of a single-screw extruder and the main structural parameters (although the degree of its quantitative applicability can be somewhat variable in practice):
Qex:D 2 LhN
(1)
where Q = output rate (weight per unit time); D = barrel (screw) diameter; L = effective barrel (screw) length; h = flight depth in the last metering section of the screw; and N = rotational speed of the screw. In practice, the volume (v) of molten material in the foremost flight of the screw (which is proportional to h) is also a power function of D: to a first approximation v ex: D n , where the index n depends on melt viscosity, and can have values of about 0·8 for uPVC and 0·6 for pPVC. 9 Since if v ex: D n then h ex: D n , it follows that the output per revolution of the screw is proportional to iY+ n . Equation (1) suggests that the output could be increased simply by speeding up the screw: whilst this is correct, there is a top limit on practicable screw speeds, imposed by the attendant shear and frictional effects on the material. (c) The Head and Die Assembly The melt leaving the screw at the front of the extruder is fed through the die head and die to form the extrudate. Most dies (the actual shaping sections) need to be held in a head which provides the passageway(s) for the melt delivered by the screw to flow evenly to the die opening. Dies vary greatly in size and complexity, from very large pipe dies which can dwarf the extruder, down to small dies for profiles of simple, small cross-section. Dies for use with rigid PVC must have
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smooth, fully streamlined flow channels, free from hang-up points for the material. Requirements are less critical in this respect with flexible PVC compositions, and the use of crosshead dies (that turn the melt through 900 in relation to the extruder screw axis) is quite common. Dies may be of the fixed type, in which no adjustment of the aperture is possible, or they may be so constructed as to allow adjustment during extrusion. Adjustment is usually necessary on dies of annular cross-section (e.g. for pipe or channelled profile), where it is achieved by displacement of either the control mandrel or the outer ring (the two elements defining the aperture-see Chapter 19, Fig. 19.8). On flat-sheet dies the die lips are normally adjustable, in automatically controlled lines on signal from on-line product control devices (d. Chapter 19, Sections 19.3-19.5). For PVC processing, especially uPVC, the walls of the die channel must be protected against acid corrosion, e.g. by chromium plating. (d) Heating and Cooling In extruders used for PVC processing, barrel heating is generally achieved by electrical resistance heaters cast into aluminium or aluminium/bronze blocks clamped around the barrel. The blocks also contain cast-in stainless steel tubes for circulation of cooling liquid. Some resistance heaters still in use are insulated in ceramic material supported in ceramic wedges.
Note: The heaters form a number of heating zones along the barrel, with temperatures separately controlled. Fairly typically, there may be five heating zones for a 24: 1 LID extruder, and six for a 30: 1 LID. Heaters on the die adapter and the die form separate heating zones. Mains frequency induction heaters-which entail a continuous coil round the barrel-are also occasionally encountered, but they are not common: such heaters give very good control of temperature, but are expensive and can fail by short-circuiting initiated by various causes. The barrel can be cooled by passing mains water through the cooling pipes, incorporated in the heater blocks as just mentioned, or situated in the barrel wall under the heaters. A closed-circuit liquid system may also be used: in this, the cooling water (or another liquid) is circulated through the barrel-cooling pipes and any channels in the screw, and is itself cooled by passage through a heat exchanger. 10 Air cooling, which is also practised on some machines, is effected by blowing cold air over the extruder barrel from blowers situated underneath.
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Like the cooling and heating systems, the temperature control instruments have grown in sophistication with the growth in scope and extent of extrusion of products which must be kept to close tolerances, e.g. pipe, film, complex profiles. Most temperature controls in use today are of the relay-proportioning (anticipatory) type. Such instruments are an advance on the simple on/off (two-step) controllers so often used in the past, in that they damp down temperature variation. (e) The Hopper This is fitted over the feed throat of the barrel. The feedstock may be dropped directly into it, but is more often introduced via a vibratory delivery chute, or a pneumatic, vacuum, or screw metering device to ensure a constant rate of feed (see also Chapter 13, Section 13.4.2(a)). Feeds can be completely automated, with the material conveyed to a number of extruders from a central point. Pre-heaters for the feedstock, usually interposed between the hopper and the feed throat, are available for use with twin-screw extruders ll ,12 (e.g. Kraus Maffei, Maize, Kan., USA; Maplan, Vienna, Austria). The devices raise the temperature of the material, so that the extruder's work in producing a melt is reduced: the resulting advantages claimed include increased extruder output (by up to 50%), reduced wear on screw, barrel and gears, as well as maintenance of constant feed temperature irrespective of ambient conditions (see also Chapter 19, Section 19.3).
Note: In an early test run over 5000 h the output of a twin-screw 90 mm Kraus Maffei extruder (KMD 90) producing uPVC pipe from powder feed was reported to be increased by about 38% (from 705 to 970 lb h- 1 ) when a pre-heater was fitted and operated to raise the feedstock temperature to 80°C. 14.2.2 Some General Points Relevant to Extrusion of PVC
(a) Machine Outputs and Energy Efficiency in Modern Extruder Practice Figures exemplifying outputs of some extruders when used as compounding machines for PVC are given in Chapter 13 (Tables 13.10-13.18). In the manufacture of PVC products by extrusion, the regular production capacity of a good, modern twin-screw extruder may be as high as about 800 lb h -1 on uPVC pipe or profile, and up to
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about 12001b h -1 on flexible profile. The typical energy efficiency of a modern single-screw extruder operating on PVC (averaged over a range of common LID ratios, drive powers and screw designs) was quoted (December 1980)13 as lO-lllb per horsepower per hour. Figures from the same source, as well as others, show that the average output of extruders of comparable size increased by factors of 3-4 between the late 1950s and 1980. (b) Some Features of, and Aids to, Modern Extrusion VENTING
This has already been briefly mentioned in Sections 14.2.1(a) and (b). With many plastics materials vented extruders are used in order that volatiles and moisture may be extracted before the melt is metered to the die. With PVC, however, removal of moisture and voaltiles, though relevant, is not the major consideration: venting is important primarily in the extrusion of rigid compositions from powder blend feeds, where its main function is to get rid of occluded air. The general arrangement of a vented single-screw extruder is schematically shown in Fig. 14.4: it will be seen that the screw is of the two-stage type, with a decompression section immediately following the first metering section. At this point the stock fills the flights of the screw only partially, and is subjected to a vacuum applied through the venting port. In a vented twin-screw extruder the vent may be positioned before the compression zone. Two venting ports spaced along the barrel are provided in some cases.
Decompression zone
r---
2nd----j1-oo41----- 1st - - - - - - - 1__1 Stlge Stlg.
Fig. 14.4 Principle of a vented extruder.
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Note: Machines with the so-called 'by-pass' venting arrangementinvolving a seal at the end of the first metering zone and a separate channel in the barrel wall through which the melt passes to the decompression zone-are not recommended for use with rigid PVC. In such systems the by-pass channelling of the melt entails the possibility of material hang-up and consequent decomposition. Moreover, the amount of working undergone by the material is increased, although an experimental investigation 14 indicated that the increase can be relatively small. GEAR PUMPS
The metering performance of a single-screw extruder (and hence the output and-in some cases-product uniformity) can be improved by interposing a gear pump between the extruder head and the die. Gear pumps have been used inman-made fibre spinning (a form of extrusion) since the early days of cellulose acetate fibre production 15 for the even, positive, smooth delivery of the spinning solution or melt they effect owing to their positive-displacement action. In single-screw extrusion of a thermoplastic (where the pumping and metering action is less positive than in twin-screw extruders) a gear pump can similarly serve to eliminate pressure surges, including those that tend to occur at screw speeds above the normal optimum for a particular material and set of conditions. This stabilising effect can enable screw speeds to be increased with consequent increase of the production rate, whilst maintaining or even improving product uniformity, and possibly effecting net energy savings. With an extruder of sufficient capacity one pump can be used to feed several dies. Microprocessor controls are available to coordinate automatically the pump temperature and intake/discharge pressures with extrusion speeds. In a case, quoted by Rice,16 of extrusion of a uPVC precision profile on a 4·5 in extruder, the installation of a gear pump* doubled the output by enabling the screw speed to be substantially increased without unacceptable rise in back-pressure, screen pack clogging, or material degradation, which prior to the use of the gear pump was caused by such an increase. Gear pumps can benefit extrusion of sheet and blown film,17 as well as profile and pipe. * A Thermorex gear pump, Luwa Corp., Charlotte, NC, USA.
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COEXTRUSION
Production of multi-layer plastics packaging films by coextrusion has been increasingly practised, with continuing technical advance, during the past decade. The technique is also employed in the manufacture of some blown plastics containers (where the resulting composite structure provides the required combinations of mechanical and barrier properties), as well as some pipes and profiles,18,19 and sheeting, where it provides an advantageous alternative to coating and lamination. 2o Specifically with regard to PVC, some sheets and profiles (in particular for window frames) are 'capped' by coextrusion with protective or decorative layers of acrylic polymers or-in some cases-highly pigmented PVC compositions. Such applications are referred to in Chapter 19 (Sections 19.4 and 19.5), in Chapter 26, and alsopassim----elsewhere in this book. PURGING COMPOSITIONS
Effective and rapid cleaning of extrusion equipment is important for the reduction in machine down-time it can effect, prevention of colour and other contamination, and for reduction of material waste. The plastics materials suitable for purging injection-moulding machines used in PVC processing, which have been mentioned in Chapter 15 (Section 15.4), can also be used on extruders. The purging agents specially developed for this purpose are also of interest for both these kinds of equipment: most commercial sources supply a special grade for use with PVc. Two such compounds may be mentioned by way of non-selective example. In using the RapidPurge (Rapid Purge Corp., Trumbull, Conn., USA) pre-mix blend for PVC purging to purge a rigid PVC pipe composition, the barrel and die are left at a temperature near the operating temperature (say about 160°C), the barrel is emptied, and then filled with the purging compound. After this starts emerging from the die, purging is continued until most of the original PVC composition has been cleared out (say for about 15 s). The screw is then stopped and the material allowed to 'soak' for about 5 min. This is followed by another 15-s purge and 5-min 'soak', the cycle being repeated until all traces of contamination disappear, whereupon the barrel is emptied and the new extrusion compound purged through until the purging agent has been thoroughly removed. 21 Like RapidPurge, ChemPurge (special grade for PVCEngineering Chemicals BV, Steenbergen, Holland; UK agent, Normandy Plastics, Sidcup, Kent) is said to be based on a thermally
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degradable nitrogen compound claimed to depolymerise by free-radical attack the material being purged to products of low viscosity which are readily removable: the use of this purging agent is said to involve no health hazards,22 with full cleanliness achievable in about 15 min. The nature, applications and sources of commercially available purging agents of American origin have been reviewed recently by Brockschmidt: 23 some of those of special interest for PVC, like Polyvin (A. Schulman Inc., Akron, Ohio, USA) or 50-FT-50 (Wilson-Fiberfil, Neshanic Station, NJ, USA)-the former re-usable, and suitable for being left in machinery during shut-down and start-up-contain scouring additives and stabilisers. (c) Use of Computers This has been growing in extent and importance in three areas:
computer modelling of the extrusion process 24 ,25 and of the strategies of its automation;26 (ii) use of computer models and programs in the design of extruder screws (d. Section 14.4 below), dies (cL Chapter 19, Section 19.3.2), and other parts; (iii) practical application of microprocessors in automation of extrusion lines, including automatic on-line monitoring of product dimensions and quality, with necessary corrections by automatic running adjustment of die apertures, as well as automatic control of 'downstream' operations (e.g. cutting and machining of pipes and profiles): several examples of this kind of automation are given in Chapter 19.
(i)
(d) Some Material Aspects FEED TYPE
The feedstocks used in PVC extrusion are of two physical forms: powder and pellets. The powder feeds are usually dry blends, produced by hot mixing of the formulation components, but powders mixed without substantial heating-often called pre-mixes-are also used in some extrusion operations: in a pre-mix the composition constituents are less thoroughly and intimately blended with the PVC resin than in a dry blend. Therefore the amount of overall homogenisation work that must be done on the feed by the extruder decreases (whilst the cost of the feed rises) in the sequence pre-mix> dry blend> pellets (which will
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have been melt-compounded). The preparation of these types of PVC compound is described in Chapter 13, Sections 13.4.1 (pre-mix and dry blend powders) and 13.4.2 (pellets). Their use as extruder feeds is further discussed in the introduction to Chapter 19. PVC COMPOSITIONS FOR EXTRUSION As in injection moulding of PVC, processing by extrusion is easier with plasticised compositions, so that, in formulating, the processing considerations (as against those associated with service requirements) are less critical than in the case of uPVc. With the latter, the type of extruder used (i.e. whether single- or twin-screw) also influences the formulation. This, and other aspects of formulating uPVC compositions for use in the manufacture of major extruded products, are discussed in Sections 19.3-19.5 of Chapter 19, in Chapter 4, and Chapters 9 and 11 (in connection with the roles of stabilisers, lubricants and other formulation constituents): they are also mentioned at various other points in the book.
FORMULATION OF
PLATE-OUT
As mentioned in Section 9.7 of Chapter 9, plate-out-i.e. persistent, sticky deposits on working surfaces of processing machinery--<:an occur in extrusion, as well as calendering and other PVC processing. In extruders, the deposits tend to appear and accumulate particularly in areas where pressure or flow changes occur, as, for example, on the wall and legs of the spider. Appreciable deposit build-up on these sites causes surface marks on the extrudate: plate-out in the die can also affect the extrudate dimensions. Cleaning-up operations on surfaces so affected interrupt production, causing loss of time and output, and material wastage (or at least creation of extra re-work) associated with shutting down and starting up again. Moreover, cleaning merely temporarily removes the symptoms, without remedying the cause of the problem: this, albeit still not completely elucidated, is known to lie in the interactions of some formulation components (certain lubricants, stabilisers, pigments and fillers) in the particular processing conditions. The first comprehensive explanation of the nature of plate-out and the mechanism of its formation in extrusion to be proposed was put forward by Lippoldt,27,28 on the basis of an analytical study of actual plate-out deposits, and experimental preparation of 'synthetic' plateout material. The proposed explanation postulates a fairly complex mechanism, involving six sequential steps, whereby a gel is formed by
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components of the lubricant and stabiliser system (one of which-a hydrocarbon in this particular study-acts as a temporary solvent above the 'critical solution temperature') occluding particles of pigment and filler (Ti0 2 and CaC0 3), and is deposited on surfaces whose temperature is below the critical value (put by Lippoldt as 175°C for the system examined). Loss of the temporary solvent after deposition results in transformation of the deposit into a solid film, resistant to re-dispersion in the PVC composition and forming a receptive surface for further deposit build-up. Lippoldt suggests28 the following measures for the prevention of plate-out: maintenance of temperature in pressure-change zones well above the critical solution temperature (CTS); reduction of the CTS by suitable selection of lubricants and stabilisers; modification of the potential solvent/solute system of the gel precursor of the plate-out deposit so as to hinder its formation, e.g. by selection of a stabiliser with low capacity for solubilising fatty acid salts (such as calcium stearate). (e) Some Features and Common Faults of Extruded Products MANIFESTATIONS OF INCOMPLETE GELATION
These can take various forms. Inadequate overall gelation (see Section 14.3 below) impairs the mechanical properties of the product. Inhomogeneity of the melt arising from variation in the degree of gelation produces corresponding differences in melt rheology, whichif sufficiently pronounced-ean be manifested as transverse undulations (in more drastic cases lines of transverse weakness) in the product: such effects can show up, for example, as waviness in the bore of an extruded pipe. This can take the form of a regular pattern, being associated with release of strain (whose degree will be influenced by melt rheology) imposed on the melt in the screw flights, and hence related to the flight pattern. Given that the required degree of gelation and homogenisation of the melt is being achieved in the process, the pattern can be reduced or eliminated altogether by ensuring that the die design and adjustment is fully suited to the pipe size being made and the production rate (d. Chapter 19, Section 19.3.2). Fine porosity may sometimes appear in the extruded product even with adequate venting of the machine: this is usually caused by the screw speed being too high, which can cause either some decomposition of the polymer (or of an additive) due to excessive shear, or incomplete gelation (because the material moves out of the barrel
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before full gelation has been attained). Reduction of screw speed, or-in the case of porosity associated with incomplete gelation (which can be shown up by appropriate tests)-also possibly increase of the back-pressure (by suitable die design), are the remedial measures. Longitudinal streaks or weld lines can sometimes arise if the melt separated in its flow (e.g. by the spider legs) has failed to knit properly: this kind of fault which results from incorrect die design is rare in good modern extrusion practice (see also Chapter 19, Section 19.3.2). IRREGULARITIES OF PARTICULATE APPEARANCE
These may be visible on the surface of an opaque extruded product, or on the surface and in the body of a clear one (say film or sheet). Dark-coloured specks or particles, especially if of irregular shape, may be adventitious contaminants, or points where the polymer has been degraded through overheating. More regularly shaped particles ('nibs') roughly circular or oval in appearance, may be 'fish-eyes' introduced into the composition with the polymer (see Chapter 4, Section 4.4.1(a)) or lumps of incompletely dispersed material remaining either because the composition-itself of basically good quality-has not been properly homogenised in processing, or because it contained· added material (possibly scrap or re-work) not completely dispersible by normal processing. In the latter case the regularly shaped nibs may also be discoloured if the added material has had an excessive heat history. VARIATION OR FLUcrUATION IN DIMENSIONS
These may arise from several causes, most commonly inadequate extrusion die setting or adjustment: sizing die problems, take-off irregularities, poor melt uniformity, and even operator error, can also be factors. The sizing and uniformity control of some extruded products are discussed in Chapter 19 (Sections 19.3.2, 19.4.2, 19.5.2). 14.3 PVC MATERIAL FLOW, HOMOGENISATION AND GELATION (FUSION) IN THE EXTRUSION PROCESS
The PVC resins used in compositions processed by extrusion are either of the suspension- or the mass-polymerised type. A particle (sometimes referred to as a 'grain') of either kind of polymer is an aggregate
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of much smaller primary particles (microgranules). The microgranules within a particle form mutually adhering clusters with voids of varying size between them: these voids may thus be regarded as internal pores in the particle. The microgranules, which are the primary structural units of a particle, are closely comparable in size between the suspension and the mass polymer, ranging in both from about 0·2 to about 2 J1.m (with the majority around, and below, 1 J1.ffi). The mean sizes of the aggregate particles are also similar for both kinds of polymer, typically around 150 J1.ffi, but the particle size distribution of a suspension polymer is much wider-say about 95% of particles between around 50 and 250 J1.m, as compared with an analogous range of perhaps 100-170 J1.ffi for a mass polymer, for typical polymer samples. In any given case the actual mean and range values are influenced by polymerisation conditions: 29 this is the main reason for differences between figures quoted by different investigators, as illustrated by the following examples.
Microgranule (primary particle) size 'Approaching 1 JJrn'* 'Subrnicron' 'About 2 JJrn' 0·2-1·5 JJrn 0·5 JJrn Approx. 1 JJrn
Grain (aggregate particle) size About 100-500 JJrn* 50-250 JJffi 100-150 JJrn 100 JJrn 100 JJrn Approx. 200-300 JJrn
Type of polymer
Author
Mass Suspension Suspension Unspecified Suspension Suspension
Marks29 ChartofPO Surnrners et al. 31 Parey and Menges 32 Menges et al. 33 Benjamin34
A typical mass polymer particle has a higher degree of sphericity than its suspension polymer counterpart, and its porosity is, on the average, more uniform. The greatest morphological difference between the two types is the presence of a 'skin' on the suspension polymer particles. This has been likened to a bag enveloping the
* Values cited as generally representative of those obtainable at high conversion rates in polymerisation. Particle size distribution graphs also given-as typical examples-in this reference indicate a total range of about 30-200 Iffil (with a mean of about 130,um) for a mass polymer, and about 4O-26O,um (with a mean of about 160 Iffil) for a suspension polymer.
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microgranule clusters constituting the particle. 29 The skin is formed at the boundary between the original monomer droplet (to which the polymer particle corresponds) and the aqueous suspending medium employed in the polymerisation process: hence, whilst the skin may not be complete (seam-like gaps are a fairly common feature, for example), and may vary in thickness (from about 0·5 to 5 fl1ll), it is always present in suspension polymer particles and absent from those of mass polymer, being clearly visible in photomicrographs of suspension polymer particle sections and in electron-scan micrographs of whole particles. Some excellent photographs of both kinds have been published by Marks. 29 An interesting method for the determination of the internal particle structure of suspension PVC polymers by analysis of the desorption rate of vinyl chloride has been described by Daniels and Longeway.35 A uPVC powder compound (pre-mix or dry blend) will normally be extrusion-processed for one of two purposes: to melt-compound it into pellets for use as feedstock in further processing, or to convert it directly into an extruded product (e.g. pipe, profile, etc.). In either case, the powder-which originally consists of polymer particles mixed with the additives called for by the particular formulation (in a dry blend the mixture may have been heat-fluxed to some extent; cf. Section 14.2.2(d))-must be transformed into a uniform, fused solid containing the additives in intimate, thorough dispersion. The degree of fusion (also referred to as 'gelation') actually achieved, and the homogeneity (including uniformity of interdispersion of polymer and additives) are important factors in the properties of the resulting product. * The progress of the transformation of powder to uniform, fused solid material, and the degree of fusion actually attained in the latter in particular processing conditions, can be characterised in terms of changes undergone by the polymer particles as the stock is moved by the screw(s) down the extruder barrel. The transformation should be completed within the dwell period of the material in the machine, i.e. the time taken by a particle to traverse the barrel from feed port to die outlet, which it should leave as an integral, no longer distinguishable part of the uniformly fused extrudate. In very round figures, this time may be about 1·5 min. In the light of a large body of relevant evidence obtained in * A useful discussion of the effects of these factors on the quality of rigid PVC pipes has been published by Benjamin. 34
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experiments and practical investigations30,33,34,3~2 the explanation of the phenomenology of the transformation outlined below has gained wide recognition. The central theme of this explanation is that the cardinal feature of the transformation-in extrusion, and indeed in all processes in which a melt is produced from PVC polymer powder by mechanical working and heat-is progressive breakdown of the polymer particles. For this reason, the mechanism formulated in these terms has been referred to as the 'comminution mechanism'. Recently it has been claimed* that this mechanism operates (or at least predominates) only in equipment which subjects the material to very vigorous mechanical working under high shear, such as internal Banbury-type mixers and Brabender torque rheometers. Otherwise, for what they regard as less vigorous processing in extruders and on two-roll mills, a group of investigators* proposes a very different mechanism, involving not breakdown but compaction, densification, fusion and elongation of the polymer grains, followed directly by melting (the 'CDFE mechanism'). According to the orthodox comminution mechanism, as the powder compound is conveyed by the screw(s) from the feed zone of the extruder, the temperature of the polymer particles is raised by the friction and shearing they experience, and the heat provided by the barrel heaters. Note: Whilst friction is a factor in this heating process it (and in particular that which occurs between the particles and the working surfaces of the machine) also affects the conveying efficiency in the screw/barrel system (especially in a singlescrew extruder, where there is no positive mechanical pumping action). A study by Huxtable et at. 43 indicated that, for several polymers, temperature was the most important single physical variable controlling the particle/surface friction. The data of Isherwood and Katwiremu44 underline the role of this 'wall' friction in the degree of compaction undergone by the polymer particles. Riley and Klein 45 point out that the coefficient of friction of pPVC is high and rises with surface velocity: however, because of the material's heat sensitivity, under some conditions the coefficient of friction on the barrel surface may be lower than that on the screw * ct. M. W. Allsopp In Manufacture and Processing of PVC, (Ed. R. H. Burgess), Applied Science Publishers, 1982, Chapter 8.
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surface, so that in the single-screw extruder no driving force for conveying the stock forward will exist (see Section 14.5). At this initial stage, the forward movement of the stock may be envisaged as the sum total of the movements of the individual particles as they tumble and slide over one another and over the working surfaces in their progress. It is in this sense that the particles have been said to constitute the 'units of flow'. 32 When the particles have been heated to a high enough temperature they begin to break down under the shearing and frictional forces they experience, into their constituent microgranules, which then become the units of flow. The temperature above which this breakdown proceeds depends on the shear rate. Most investigators regard 160°C as this threshold in a typical extrusion process (mainly on the basis of morphological examination of under-processed stocks), but values as low as 9Q-lOO°C have been quoted. 46 It is at this stage that the additives in the composition become more intimately dispersed, among and onto the microgranules (having previously been attached to the surface of the polymer particles or, in the case of liquid stabilisers, or plasticisers in pPVC powder compositions, possibly partly penetrated into the pores). With some solid stabilisers and lubricants the relatively high temperature reached may result in melting, and coating of the microgranules by the molten substance. The breakdown of the aggregate particles into their constituent microgranules is sometimes identified as the first of the two stages in the gelation process: the rate at which it proceeds (dependent on the temperature and the amount of mechanical working, both of which also influence the rate of heat input) is certainly an important parameter in the achievement of complete gelation in the particular processing conditions. The second main stage is the breakdown of the microgranules themselves and their merging into a uniform mass. The main mechanisms involved are melting (thermal disruption) of these primary particles followed by the inter-diffusion-under the same thermal activation-of the molecular chains initially confined within the boundaries of a particle, with those similarly liberated by its neighbours. With uPVC compositions, this second stage proceeds at temperatures above about 190°C. Note: This stage should not be regarded as demarcated with absolute sharpness. Thus, depending on the processing conditions, and in particular the amount of shear experienced, the microgranules can undergo partial surface coalescence-at
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temperatures from about 175°C upwards-before final disruption. 31 ,41 It is significant, however, that a large increase in the flow activation energy of a PVC compound has been found 47 to take place at 195°C indicating a change in the flow mechanism. 48 The change is in line with morphological changes noted in another investigation. 31 With pPVC powder compounds the penetration of plasticisers into the microgranules is a factor in the gelation mechanism: inter alia, it tends to lower the temperatures at which the morphological changes just mentioned take place. The second stage also involves further homogenisation with respect to the additives (which-from their positions on the surfaces of the microgranules-are now being distributed through the polymer mass) to attain a still more uniform and intimate stage of dispersion in that, if full gelation is reached, the polymer is in a molecular state of division. The degree of completeness of the breakdown of the microgranules and their inter-merging is the true 'degree of gelation' (or 'fusion') in the context of melt processing. In this regard the concept is thus virtually the same as that applicable in the case of fusion of PVC pastes* (see Chapter 21, Section 21.2.5) given that there the plasticiser is a substantial component of the system, so that when complete fusion has been attained, its molecules are uniformly interspersed with those of the polymer, jointly forming the matrix within which the additives are distributed. If the temperature is allowed to rise too high above 190°C, or if the stabilisation is poor, or the residence time of the melt in the machine too long, the melting and fusion may be complete but degradation of the polymer (in some cases also possibly some of the additives-d. Chapter 9, Section 9.1) may also occur.
Note: The small crystalline fraction present in commercial PVC polymers (see Chapter 1, Section 1.5.1) is assumed to remain * It should be noted, however, that in the correct current paste-processing terminology (ct. Chapter 21, Section 21.2.5) 'gelation' and 'fusion' are no longer interchangeable to the extent to which they still are in the context of melt processing: with pastes, gelation is the first stage of the conversion of the paste to a homogeneous thermoplastic solid, whilst the progress of that conversion is substantially different from that in melt processing, because of differences in the structure of the polymer particles, absence of shear from the heating stage in paste processing and-most importantly-the cardinal role of the plasticiser in the conversion.
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undisrupted after what is normally regarded as complete fusion at safe processing temperatures, because of the relatively very high melting point of crystalline polyvinyl chloride (about 270°C-cf. the section of Chapter 1 just mentioned). It is also a reasonable supposition that the crystalline material is distributed (first in the fused melt and then in the resulting solid) in the form of the crystallites (crystalline microdomains about 0·01 f1.m in size42 ,49) thought to exist in the original polymer, and that these contribute to the strength of the melt-processed product, possibly acting as quasi cross-links after the manner of crystallites in other polymers (cf. also Chapter 1, Section 1.5.1). However, such contribution can certainly be outweighed by the adverse effects of incomplete gelation; conversely, complete gelation (without polymer degradation) ensures good properties of the material. The ideal result aimed at in the extrusion (and all melt processing) of PVC is to achieve full gelation of the material, coupled with freedom from voids, degraded polymer and external contaminants, any of which can act as stress-concentrating flaws (ct. Chapter 11, Section 11.2.2). As pointed out by Marshall and Birch,5o the correct balance between good gelation (which optimises resistance to fracture) and introduction of flaws (which reduce that resistance) is the essence of good processing. As in the case of PVC pastes, the completeness of gelation (fusion) of melt-processed PVC (and especially uPVC compositions) is most directly characterisable in terms of the material's morphology. The morphological effects of processing just outlined have in fact been worked out and demonstrated on the basis of examinations of the fine structure of compositions, after various degrees of processing, by optical and electron microscopy techniques. 30 ,31,34,36,37.39,41,49 On such a basis, full gelation may be sensibly equated with complete absence of detectable granular structure in the material (including any vestigial traces of such structure in the fracture surfaces). Similarly, degrees of completeness of gelation may be assigned in terms of the nature and extent of persisting granularity. Other factors (and in particular the molecular weight of the resin) being equal, increasingly perceptible granularity is associated with reduced mechanical strength, impact resistance, and resistance to weathering. This is because the
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number of molecular chains participating in 'ties' across a perceptible inter-grain boundary is smaller than that in a corresponding volume in the interior of an incompletely dispersed granule: hence the boundaries are planes of actual relative weakness. Note: Moreover, the weakness may be accentuated by the presence at the boundary of additives (in concentrations higher than elsewhere in the material) which were originally reposing on the microgranule surfaces.
This lower density of molecular ties is also the main factor in the lower weathering resistance: photochemical disruption by incident UV light of a number of these, even though small in absolute terms can, because of their relative scarcity, cause cracking at the inter-grain boundary under, say, normal thermal stresses. As with PVC paste products, the relatively sophisticated methods of morphological study are too time-consuming and elaborate to be widely used in monitoring the completeness of gelation in industrial operations. It is largely for this reason that more straightforward, direct determination of some property of the product, influenced (like some of those just mentioned) by the degree of completeness of fusion, is usually resorted to. Whilst such determinations have the merit of relative simplicity-and the property determined may be directly relevant to service requirements (e.g. the hoop strength of extruded pipe, which is sensitive to the completeness of gelation)-the functional relationships of their numerical results to the fine structure of the product are not normally linear. Thus such results are only relative indices of the degree of gelation, and-whilst each may be satisfactory for a particular purpose-they are not readily quantitatively relatable to one another. This is well brought out in the results obtained in a study by Benjamin34 of the degree of gelation of an extruded pipe of a relatively simple formulation. Benjamin's data are reproduced in Table 14.1. For practical purposes the completeness of gelation of extruded products (pipe, profile) is usually assessed in terms of a solvent immersion test (ct. Chapter 19, Sections 19.3.3 and 19.4.4), or a strength or impact test. A rheological method, giving numerical values for the degree of gelation, has been developed,32,34,51 foreshadowed by ideas of Gonze 52 and Lamberty.53 With a series of melt-flow determinations in a capillary rheometer (a standard melt-flow index apparatus may be used
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TABLE 14.1
Degree of Gelation of a PVC Pipe as Reflected in its Various Properties
(Data of Benjamin, reproduced from Ref. 34 with permission of the author and the copyright holder, The Plastics and Rubber Institute.) Gelation level" (%)
32
44
68
90
Resistance to attack by methylene chioride b
Severe attack
Light attack
No attack
No attack
Homogeneity of microtome section C
Inhomogeneous
Some inhomogeneity
Homogeneous
Homogeneous
Tensile strength, 20°C: yield (N mm- Z) Elongation at break (%)
54 108
55 133
56 115
56 58
Tensile impact energy:d O°C, energy (Nmm mm- Z) Elongation after break (%) 20°C, energy (Nmm mm- Z) Elongation after break (%)
381 3 624 15
706 15 763 19
711 16 733 18
656 12 697 16
Resistance to stress cracking' 20°C Critical strain, Ec(%)
0·8
1·5
5·1
3-6
"Determined by the rheological method. b By the method of KIWA KE 49. C Section (approximately 10 J.LITl thick) taken across the pipe wall. dDetermined by the falling weight method of KIWA KE 49. • Determined by the steel ball indentation test of DIN 53449.
in one variant5 !) a 'gelation curve' is constructed, representing changes in flow characteristics with increasing degree of gelation, to serve as a standard in determining the degree of gelation of a given specimen. Where there is no prior knowledge of the processing which the specimen has experienced (and thus a complete reference curve cannot be obtained) it is still possible to assess the degree of gelation by the following procedure. Part of the specimen is used for the flow measurement under the appropriate conditions; the rest is heatprocessed (with periodical testing) until no further increase in gelation is indicated by the rheological behaviour (e.g. the appropriate extrusion pressure in the standard test conditions used remains constant). On the assumption that the 'steady' value reached
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corresponds to 100% gelation, the gelation level similarly corresponding to the result obtained with the specimen in its original state can be calculated.
14.4 SINGLE-SCREW EXTRUDERS This section deals briefly with some features of, and developments in, extruder design with special reference to those of interest in connection with extrusion of PVc. Whilst twin-screw extruders have come into their own in the compounding and extrusion of PVC, especially uPVC from powder feeds, the single-screw machine is widely used, particularly for processing plasticised PVc. The main advantages of this type of extruder over the twin-screw machine are economic: lower equipment capital cost; lower maintenance costs; on the average, shorter down-time for repairs; lower rate of wear of screw, barrel and gearbox. The theory of design and operation is also better developed for single-screw machines. Most suppliers provide a range of machines, varying from small screw diameter (say 30 mm) to large (say 250 mm). It is also common practice for a supplier to offer a range of screws not only for specific materials but also for specific applications, e.g. uPVC pipe extrusion, paper coating with polyethylene. It is in the area of screw design that research and development have been most concentrated for a considerable time. 54 The design of screws for the single-screw extruder has been increasingly benefiting from the use of computers. An important early step in this direction was the work carried out at the University of Cambridge under a Ministry of Technology contract, aimed at providing a suite of computer programs to model accurately the single-screw extrusion process, and mathematical models for prediction of the performance of screws of various configurations. 55 At about the same time a company (Scientific Process and Research Inc.) was being founded (by Klein and Tadmor, two eminent workers in the field of screw design) to offer to industry a computer service for the design of extruder screws. A feature of the service was a technique whereby screw configuration and plasticating performance could be simulated without need for any empirical information, prior experience or laboratory experiments. 56 Initially, screws for flexible PVC have usually been of the constant-pitch, increasing-root types; with a compression ratio of 2: 1 (up to about 3: 1 for softer grades). This type
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of screw (at the lower end of the compression range quoted) had also been used for uPVC. Subsequently metering screws with mixing rings fitted in the metering section came to be employed,S7 and thereafter many variants of 'mixing' screws which may be broadly classified into three groups: screws containing 'smearing' sections designed to improve homogenisation of the stock; 'pin screws' with lines of pins in the metering section9 designed to break up the flow pattern of the stock for better homogenisation; twin-channel screws in which the material already melting is periodically temporarily separated ('drained away') from that still predominantly solid, for more effective working of the latter and generally better overall homogenisation. Sophisticated designs of the last-named type are represented by the 'barrier' screw, * and the 'double-wave' screw;t the latter is claimed to be more efficient than pin-type screws, to operate effectively with stock temperatures significantly lower than those typical of a conventional single-screw machine, and to make the single-screw extruder competitive with a twin-screw one in the extrusion of pipe from dry blend. 58 Among the other developments in single-screw extruder features (some already mentioned in Section 14.2 above) the following should be mentioned: increase of barrel length, screw speeds and torques; low noise level drives; improvements in power transmission; increase in heat output of barrel heaters. Some modern single-screw machines are among those discussed in Sections 13.4.2 and 13.4.4 of Chapter 13.
14.5 TWIN-SCREW EXTRUDERS As mentioned in the introduction, the development of twin-screw extruders has been primarily associated with the processing of rigid PVC, in particular, from dry blends: in this area the twin-screw machine rapidly established supremacy in Europe and eventually also a strong position in the USA, although there its advance vis-a-vis the single-screw extruder has been rather slower. Twin-screw machines used for the manufacture of extruded PVC products are normally of the intermeshing screw type, with cylindrical or conical screws. It is relevant to consider briefly the main differences in the way in which the single-screw and intermeshing twin-screw extruders operate.
* Davis Standard-cf. Table 14.3. t Patented by the HPM Corporation, USA-ef. Table 14.3.
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Single-screw machines work by virtue of the fact that the friction between the stock and the extruder barrel is greater than that between the stock and the screw: 2 this causes the material to be conveyed forward. The twin-screw machine, on the other hand, moves the material forward-on and between the screws-by positive displacement, i.e. it exerts a positive pumping action. Note: A good illustration of the essential difference between these two modes of operation is provided by the experiences of Wyman's59 imaginary spider who finds himself in the screw flight of a machine in motion (but empty of stock). If the machine is a single-screw one, and the spider does not touch the internal surface of the barrel (which-if he sits on the screw root-he will see as a moving ceiling), he can relax undisturbed on the spot he is occupying. Alternatively, when he is ready, he can walk forward along the continuous helical passage formed by the screw flights and the barrel bore surface until he strolls out at the die. In either case he is not impelled in any way by the movement of the machinery. If, however, the spider sits on the root of a screw of an intermeshing twin-screw extruder his situation will not be so comfortable. He will realise that he is enclosed in a chamber formed by the floor (screw root), two side walls (screw thread walls), moving ceiling (internal surface of the barrel) and two curved, convex walls at each end (lands of the other, intermeshing screw). Moreover, he will see one of the end walls rolling towards him and-if he is nimble enough to avoid being crushed-soon pushing him along, unless he runs fast enough ahead of it. If he runs too fast, he will catch up with the other end wall (the 'rear' side of the second land of the other screw) which is rolling away from him. He will thus be forced to move forward (i.e. be positively displaced) at a pace determined by the speed of movement of the end walls of his prison (governed by the rate of rotation of the screws). When he reaches the end of the barrel, the curved wall receding in front of him will 'open up' and he will be forced out through the die opening. The difference between the material's friction against the screw and
barrel surfaces in a single-screw machine-whilst enabling the stock to be moved forward-is also associated with some unevenness of the
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stock temperature in the screw flight prior to complete homogenisation: the material is coolest in the centre of the flight and-particularly in the absence of the kind of special mixing arrangements mentioned above-that part fuses last. Relatively high screw speed is necessary to promote uniformity of temperature distribution by the resultant high shear, but this also generates considerable shear heating making it necessary, in some cases, to run the machine with cooling constantly on to keep the temperature to a safe level. The twin-screw extruders produce much less in the way of shear heating and rely more on the input of heat from the barrel. Single-screw machines are dependentparticularly for any mixing action-on the development of relatively high back-pressures. Hence, since they have no positive pumping action, with die openings of large cross-sectional area the achievement of such pressures may be difficult: also in practice-for a given screw speed-the output tends to decrease with decreasing die opening. The performance of the twin-screw machine is little affected by the cross-sectional area of the die. With reference to the above points, and generally to its performance and operation in PVC (and especially upvq processing, the advantages of a twin-screw machine in comparison with a single-screw one may be listed as follows: (i) (ii)
lower melt temperature; more uniform melt flow (cf. also the paragraph on gear pumps in Section 14.2.2(b) above); (iii) less product variation.
These three features are consequent upon the positive pumping action of the screws. (iv) (v)
higher outputs with less degradation; possibility of using cheaper formulations (less expensive stabilisation) .
These two points arise from the generally lower melt temperature in processing. (vi) better temperature control; (vii) better mixing action and removal of entrapped air (hence greater suitability for processing bulky feeds like dry blends); (viii) insensitivity to the cross-sectional area of die opening in operation.
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Various views have been expressed about the respective merits of twin-screw extruders with conical (tapering) and cylindrical (parallel) screws. Two points may be mentioned. The taper on conical screws tends to produce a slight increase in compression, unless counterbalanced by suitable adjustment of the pitch and/or lead. The fact that-with conical screws-the centres of the screw shafts at the rear are more widely spaced than the screw tips, can be utilised in bearing construction so that higher loads can be accommodated by the bearings. The design concepts of twin-screw extruders have been well described by Prause,60--62 with references to machines produced by various manufacturers. Some experimental work on this type of machine, and its theoretical interpretation, have been published by Jewmenow et al. 63 ,64 A theoretical model for the intermeshing twin-screw extruder (including a comparison with a single-screw machine) is developed in an interesting paper by Wyman. 59 An important difference among commercial twin-screw machines is their choice of screw design. Table 14.2 indicates, in very brief summary, the general nature and mutual direction of rotation of screws TABLE 14.2
Types of Screw Used on Some Commercial Twin-Screw Machines Manufacturer
AGM Anger APM Kesterman LMP Nouvelle Mapre Schloemann Windsor
Screw rotation
Type of screws
Tapered screws, large diameter at feed, small diameter at meter Three separated sets of flights, those in the feed area having the largest pitch, a reduced pitch on the second set and the smallest on the third Features a single-flighted feed zone with a change Counterto triple flights in the transition zone rotating Co-rotating Three-section screw with a different diameter in each section, proceeding from large diameter at feed to small diameter at meter Single-flighted screws with increasing flight thickCounterness towards the die rotating CounterScrews are three-sectioned, constant pitch within rotating section, but decreasing in pitch from feed to meter Co-rotating Three-section screws are employed, having a different diameter in each section, the large diameter at feed and the small at meter Counterrotating Counterrotating
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703
in some extruders. In general terms, it is usually claimed that counter-rotating screws provide a better mixing effect and that co-rotating screws offer superior self-cleaning performance. Another area of twin-screw design which aroused a good deal of controversy at one time, is the service life and general robustness of the thrust bearings. Where two shafts are running in close proximity, the space available for the thrust bearings is clearly limited: the advantage in this connection of the conical twin screws has already been mentioned. Twin-screw machine manufacturers have developed various bearing designs which permit higher loads to be borne than was possible in the early days of twin-screw extrusion. Whilst-as has been mentioned-most twin-screw machines are not operated at the high back-pressures experienced with single-screw extruders, the loads that the bearings are called upon to withstand have been increasing with the progressive increase in operational screw speeds effected to increase outputs. Among the principal modern developments in twin-screw extruders (which in many respects paralleled those in single-screw machines) must be counted: increase in barrel and screw length (to secure better controlled energy transfer with gentler heating, and ensure completeness of gelation at higher output rates); introduction and refinement of barrel venting; increase in the speed range and torque capacity of the screws; development of screw design (in particular the introduction of mixing and decompression zones); improvement in the temperature control systems for barrel and screws; improvement in drives and reduction gears. 14.6 SOME COMMERCIAL MACHINES
Twelve extruder manufacturers from various countries, whose machines are widely used in processing PVC, are listed-by way of non-selective examples-in Table 14.3, together with brief indications of the machines supplied. A comprehensive, worldwide survey of extrusion machinery has been published in Plastics and Rubber Weekly, Nos 828 (15th March, 1980) and 829 (22nd March, 1980), and a shorter one, with emphasis on Europe, in European Plastics News for April, 1982 (Vol. 9, No.4). Other sources of information include the current issues of the Modern Plastics Encyclopedia (McGraw-Hili), the Plastics Technology Plastics Manufacturinf( Handbook and Buyers Guide. and
HPM Corp., Mount Gilead, Ohio, USA
Battenfeld Maschinenfabrik GmbH, Meinerzhagen, Westfalen, West Germany Herman Berstorff Maschinenbau GmbH, Hanover, West Germany Cincinnati Milacron, Plastics Machinery Division, Batavia, Ohio, USA Davis-Standard, Div. of Crompton and Knowles Corp., Pawcatuck, CT, USA EMS Industrie, Bobigny, France
Manufacturer
S;T;M
Berstorff Ltd, Lancaster
S;T
S
International Corporation Ltd, Farnham, Surrey
Loreburn Engineering Ltd, Sheffield
D. Marks,b 2 Spicer Place, S Bilton, Rugby, T Warwickshire Davis Standard, Alderley S Edge, Cheshire
S;T;M; cascade
General type of machineO
Battenfeld (England) Ltd, Chesham, Bucks
-
UK associate or agent
Claimed to be largest manufacturer in Europe; many models of each main type of machine
Remarks
Specially designed complete production lines and turn-key factories, with appropriate types and sizes of extruders 50-90 Specialist in twin-screw 40-90 (the (conical) extruders 'CM'range) ~30 Wide range of extruders and extrusion lines; screw design specialists-designers of 'barrier' screw Extensive extruder range 30-300 includes special machines combining single- and twinscrew construction Provide custom-design control 50-250 systems; patentees of 'double wave'screw
~40
Range of screw sizes (mm)
TABLE 14.3 Some Commercial Extrusion Equipment Widely Used for PVC Processing
~20
50-250
S
Francis Shaw and Co. Ltd, Manchester, England
Range of general-purpose PVC extruders and downstream equipment Special gear-box design for high screw torque performance Incorporates former Anton Anger operation; single- and twin-screw (conical) extruders with dual-circuit screw temperature control; downstream equipment and complete lines Inter alia, supplies a complete, compact twin-screw line for the production of PVC window profiles (continuous output up to 130 kg h- 1) Extruder range, and complete lines for sheet extrusion, cable and wire covering, and compounding
b
Nearest representative: Cincinnati Milacron Austria AG, Vienna.
as = single-screw machines; T = twin-screw machines; M = multiple-screw machines (including planetary extruders).
60-120
S T
60-100 50-90
S T
Reifenhauser GmbH, Trois- Reifenhauser Ltd, dorf, West Germany Tewkesbury, Gloucs
~70
52-132
~40
~20
T
David Dryburgh and Co. Ltd, Epsom, Surrey Mr E. P. Hartmann, Wakefield, Yorks
Mapre SA, Diekirch, Luxembourg Oswag Plastics Machines, Linz, Austria
Maplan Maschinen GmbH, Vienna, Austria
Regis Machinery (Sales) S Ltd, Bognor Regis, T Sussex Speedex Engineering Ltd, T Bradford
Kaufman, Le Havre, France
706
B. 1. Lanham and W. V. Titow
the British Plastic Federation's Buyers Guide for Plastics Processing Machinery and Equipment. Information on some compounding extruders used for PVC is given in Chapter 13, Section 13.4.4. Plates G and H show two twin-screw extruders. Technical data for a cascade extruder and the Bitruder machine range are given in Chapter 13, Section 13.4.4(a). A few data are also given for the Reifenhaser Bitruder BT 701-2-80-16V (shown in Plate H) in Table 14.4 together with information on a PVC window profile extrusion line incorporating this machine. Apart from their compounding uses the Bitruders are employed in a wide range of extrusion lines for sheeting (single and multi-layer), pipes, profiles and coatings. Twin-screw extruders designed basically for PVC are exemplified by the Maplan r';lnge (see Table 14.3). All are produced to EUROMAP sizes, with an extrusion height of 1 m. The screws are parallel, counter-rotating, and temperature-controlled over the range S().....200°C by oil circulation. Barrel heat control-in the range SO-300°C-is
Plate G Mapre twin-screw extruder Type E2.78.100: screw diameter 100 mm; two models available, with LID ratios respectively 22:1 and 16:1. (Courtesy Mr David Dryburgh-D. Dryburgh & Co Ltd., Epsom, Surrey, England.)
707
14 Extrusion of PVC-General Aspects
Plate H Bitruder BT 701-2-80-16V twin-screw extruder. (Courtesy of A. Reifenhiiuser Ltd.) operated by five heating zones (electrical heaters) and three oil-cooling zones. Feedstock pre-heaters are supplied for the machines (cf. Section 14.2.1(e)). High, uniform screw torque is claimed to be provided by the special gear transmission design, and a throughput per revolution up to 50% higher than with conventional designs, so that the screws can be run at relatively low speeds with consequent lower shear heat generation and machine wear. The following data 12 relate to two of the models.
Screw diameter (mm) Screw LID ratio Screw speed range (r min-I) Torque (Nm) Drive power (kW) Output: rigid PVC pipe (kg h- 1)
Map/an DS 80
Map/an DS 100
80 24: 1 1·6--31·5 2 x 4000 1·3-26 250-300
24: 1 1·6--32 2 x 7500 2·6--52 400-500
100
708
B. J. Lanham and W. V. Titow
TABLE 14.4 Bitmder BT 701·2-80·16V Twin Screw Extruder: Some Technical Data for this Model and for a Reifenhauser Window Profile Extrusion Line Based upon It (Extracted, with the permission of A. Reifenhauser Ltd, from the technical literature of Reifenhauser GmbH, Troisdorf, West Germany) Technical and output data for Bitruder BT 70I·2·80-I6V Screw diameter length (D, mm) (D)
Nominal torque (daNm) 700
16
81
Drive rating (kW)
Screw speed range (rmin- I )
Typical output (kg h- 1) Corrugated pipes
Sections
4·2-25·0
5·2-31·0
220
160
Compressed air with supply pressure 5-6 bar (Nm3 h- 1 )
Net weight (kg)
uPVC window profile extrusion plant data Line components Type designation
BT701-2-80-16V
KKE-P0500-15-V3
A-P420D-2-175L TSQ-P516A-220-450
ST-P710-00-300 Total
Description
Feed hopper Twin-screw extruder type 'Bitruder' Temperature con· troller cabinet, 8 controllers Profile die head and sizing Sizing unit base with vacuum sizing tank Haul-off unit Length cutting saw with swarf extraction Discharge unit Rail track
Installed electric rating (kW)
Cooling water consumption with supply water 12-I5°C (Nm 3 h- 1 )
70·0
0·5-0·8
10 3800 250
4·5 23·0
240 2·5-3·0
3·0 3·0
104"
3·0-3·8
2400
0·3 0·3-0·9
1800 520
0·1-0·3
450 250
0·7-1'2
9720
"The effective consumption for drives is up to approx. 75% and for heaters approx. 30% of the installed rating.
A specification for a 4·5-in extruder is exemplified by the data given in Table 14.5. The machine (produced by Melville Plastics Engineering (Scotland) Ltd) is equipped with a one-pipe, bimetallic steel barrel lined with a corrosion-resistant 'Brux' alloy, ground and honed to the
Screw diameter Length/diameter ratio Number of heating zones Total heating load Cooling system Barrel zones (optional) Individual blowers rated at Feed section Gear ratio Gear type Lubrication Thrust bearing Dynamic load capacity B-10 life at 100 r rnin- 1 and 5000 lbf in- 2 head pressure Motor power (suggested) Screw speed Installation area (approx.) Extruder (excluding drive) Control cabinet Weight (approx). Extruder (excluding drive) Control cabinet
Specification £S450-24
4·50 in 24:1 4 60·1 kW Air 595 ft 3 min- 1 Water 10:1 Single helical Force feed 482 000 lb 56 000 h 1OQ-150hp Up to 125 r min- 1 184 x 33 in 47 x 22 in 7900lb 850lb
£S450-20
4·50 in 20: 1 3 50·1 kW Air 595 ft 3 min- 1 Water 10: 1 Single helical Force feed 482000lb 56000h 100--150 hp Up to 125 r min- 1 165 x 33 in 47 x 22in 7100lb 850lb
Imperial
3225 kg 386 kg
420 x 84cm 120 x 56 cm
56000 h 74-112 kW Up to 125 r min- 1
219000 kg
Air 17 m3 min- 1 Water 10: 1 Single helical Force feed
115mm 20: 1 3 50·1 kW
£S450-20
TABLE 14.5 Melville Plastics Engineering (Scotland) Ltd 4·5 in Extruder
219000 kg
Air 17 m 3 min- 1 Water 10: 1 Single helical Force feed
115mm 24:1 4 60·1 kW
£S450-24
3590 kg 386 kg
468 x 84cm 120)( 56 cm
56000 h 74-112 kW Up to 125 r min- 1
Metric
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final bore diameter. Heating zones are individually controlled by proportioning-type pyrometer controllers having a temperature range of o-800°F (0-400°C). Each zone is equipped with a resistance-type heater and air blower for cooling. Lightweight fibreglass-insulated steel covers, designed for ease of removal, are fitted. The extrusion screw is machined from high-grade alloy steel bored for fluid heating and cooling, and fitted with a rotary union. Flights are flame-hardened and the screw is plated with industrial hard chrome on all working surfaces.
14.7 ANCILLARY EQUIPMENT For all extrusion processes, equipment in addition to the extruder and its die is necessary to form, handle and collect the product. The major uses to which the extruder is put in processing PVC are as follows: (i) (ii) (iii) (iv) (v) (vi)
wire covering; hose and flexible pipe production; blown and cast film; rigid pipe; rigid sheet; profiles, rigid and flexible.
Common to most of these processes are the following items of general ancillary equipment: (a) sizing dies; (b) a means of cooling the extrudate; (c) a haul-off system. The ancillary equipment required for wire covering is mentioned in Section 14.8 below. The equipment used in the extrusion of uPVC pipe and other rigid products is discussed in Chapter 19. In the case of hose and profile65-67 the extrudate is passed into a shaping device dimensioned to the size required for the final product: the device is usually cooled in order to bring the extrudate to a temperature that will ensure the retention of dimensions. The methods of cooling and shaping of PVC film and sheeting depend on the extrusion process employed, i.e. blowing or casting (see Chapter 19, Section 19.5). A good description of both processes appears in ICI Technical Service Note C 108.
14 Extrusion of PVC-General Aspects
711
Cooling of extrudates is usually achieved by the use of one or more water baths or water sprays. Film is again a special case, in that with blown film, air is used to cool the product: with cast sheeting the cold metal surface of the casting roll achieves the required cooling. Take-off systems may be of the caterpillar type, in which two or more soft tracks or belts press against the extrudate and draw it off
Plate I Equipment for production of tubular (blown) film. (Courtesy of Battenfeld Gloenco Extrusion Systems Ltd, Droitwich, England.) (1) Take-off unit showing film bubble.
B. J. Lanham and W. V. Titow
712
Plate I-contd.
(2) Ring die with internal bubble cooling.
(see Chapter 19), or consist of wheels arranged in such a manner as to exert a steady pull on the extrudate. Screens are often employed in the extrusion of flexible PVc. These screens are positioned on a metal support plate (the breaker plate) fitted between the end flange of the extruder and the die, i.e. immediately after the screw tip. The purpose of the screen is to
14 Extrusion of PVC-General Aspects
713
prevent foreign matter or ungelled particles from passing to the die, and to provide a means of increasing back-pressure, thus improving plastication of the melt. Typical screen selection might be as follows: Medium grade PVC:
3 x 60 mesh
Hard grade PVC:
3 x 120 mesh
The use of screens has led to the development of an interesting piece of ancillary equipment, viz. the automatic screen changer. Before the advent of these units the changing of a screen involved stopping the extrusion, opening up the head and replacing the screen. There are many different screen changers available, working on several different principles. The most common type simply has a slide block which carries two breaker plates and screen packs, with a pneumatic or hydraulic ram to slide the clean unit into position. The slide ram is arranged at right angles to the barrel. Other methods use a continuous screen which is fed very slowly across the face of the breaker unit.
14.8 EXTRUSION OF PLASTICISED PVC 14.8.1 Nonnal (Relatively Slow) Extrusion The basics of extrusion procedures are common to all processing by this technique, and in this sense all extrusion operations may be considered to be normal. However, it is convenient to distinguish those operations which require a marked change in common operational procedure. For the purpose of this section, therefore, extrusions at fairly low rates, say 5-200 ft min-I, will be classified as normal, and those at rates significantly higher as fast extrusions. It is impossible to be dogmatic about extrusion conditions in PVC, or any other material for that matter. Every compound and even every machine is different and extrusion conditions for each set of circumstances must be worked out. However, it is possible to give general guidance. The overall principle for slow extrusions is to have a temperature gradient from the hopper end of the barrel to the die. Some typical figures are given in Table 14.6. The general principles are that the softer the compound the less heat required, and the smaller the die orifice the more the heat required. This is because the speed of extrusion will normally be greater the
B. J. Lanham and W. V. Titow
714
TABLE 14.6
Temperature Gradients for Slow Extrusions
eC)
Position in extruder Type of extrusion
Soft compound, medium orifice Hard compound, medium orifice Soft compound, small orifice Hard compound, small orifice
Hopper end
Middle of barrel
Head end
110 120 130 140
125 140 140 155
135 150 150 165
Head Die
150 160 160 175
160 170 180 190
smaller the orifice so that greater heat input will be required. The rotational speed of the screw will not yet be great enough to create an excessive amount of heat. Higher temperatures are required on twin-screw machines. The cooling of the screw has to be controlled. Cooling is usually unnecessary at the start of the extrusion since the aim is to build up heat, not to take it away. After a time, however, a trickle of coolant in the screw is often advantageous. Before extrusion begins it is necessary to allow the extruder to warm up to the required temperatures. The compound should then be introduced gradually into the machine, the ammeter being watched continually to ensure that the cold compound does not cause overloading. If tubes are being made and a torpedo is in use, this may also be capable of being heated. This is most advantageous and permits really sensitive extrusion control, also allowing the barrel temperatures to be kept lower than usual.
14.8.2 High-speed Extrusion The principal difference between slow and high-speed extrusion is the amount of mechanical heat developed. This can be so great that, once extrusion has started, no external heat is required in the barrel (the so-called adiabatic heating). This type of extrusion is fairly common, particularly in the cable industry, and its successful practice is a matter of obtaining a delicate balance between heating and cooling. Table 14.7 gives some typical temperatures for high-speed extrusion, particularly associated with 20-24: 1 LID machines.
'715
14 Extrusion of PVC-General Aspects TABLE 14.7 Temperature Gradients for Fast Extrusions
Type of extrusion
Medium compound, small orifice (3,5 in extruder) Medium compound, large orifice (3,5 in extruder) Medium compound and orifice (2,5 in extruder) Hard compound, medium orifice (2,5 in extruder)
eq
Position in extruder Hopper Head end Middle end Head Die 300
300
300
300
300
110
120
130
130
150
150
150
160
150
150
140
140
150
150
160
There are some noteworthy points about Table 14.7 in that they are contrary to convention. First, in general, the temperature gradient has virtually been eliminated. Sometimes a little extra heat is necessary just behind the head. The die temperature, however, can be varied quite independently of all the other heaters and this is purely to control the surface finish. This temperature can be quite high without causing decomposition of the compound, largely due to the high speed of extrusion. With slow extrusions it is necessary to heat the hard compounds more than the soft ones. With fast extrusions the opposite is the case since the internal heat developed with the hard grade is much greater. Cooling almost always needs to be applied. Its application should, however, be judicious: it is not good enough just to turn the coolant full on. The correct flow must be found by trial and error and, once established, should be kept fairly constant. A flow meter can and should be installed in the line so that the rate of flow can be regulated. The amount of heat is the prime factor which determines whether or not a good or poor extrusion will be obtained. There are many symptoms of poor extrusions, two of the most common being 'lumps' in the surface, or a 'grey' background instead of a fine deep colour. Both of these troubles are caused by external overheating, usually of the barrel. If an extruder is stripped after obtaining lumpy extrudate, unplasticised lumps will be found behind the screen-the trouble is this pronounced. The natural tendency on seeing lumps is to increase the heat to
716
B. J. Lanham and W. V. Titow
obtain greater plasticisation. This has the opposite effect. The compound is made more plastic so that less mechanical work is put into it and less heat is developed. The external heating should be reduced to increase the shear energy input. All temperatures nowadays are controlled automatically, but great care should be taken in interpreting the temperatures given by automatic equipment. If when troubles of the above type arise and overheating is suspected, a casual glance at the temperature recorders indicates that this is not so, one of two things can have happened: (a) the thermostatic control has gone wrong, or (b) the recording instruments are such that the indicator needle cannot rise more than a few degrees above the temperature at which the thermostat has been set. If the restriction is momentarily removed it will frequently be found that the temperature recorded will rise 20-30°C above the value at which it was set. Whilst (a) above is not the usual cause of overheating troubles, if the thermostat does break down and allows unrestricted heating at a point, polymer degradation will occur, frequently to the point of carbonisation. However, this kind of problem is ultimately one for the maintenance electricians. In the case of (b) on the other hand, the situation is quite different: it is also relatively common. The only answer is to switch off all the heat and to apply cooling until a reasonable temperature is reached. Often it is advisable to set the initial heats rather lower than those theoretically required so that with the heat developed within the PVC itself, a reasonable balance will be achieved. The recommendation given above about setting the extruder at a lower temperature than normal may give rise to certain difficulties. The principal one is that at the start of the extrusion cold compound is fed to the extruder and with the low heats this is likely to place undue strain on the extruder motor, and cut-outs may even be experienced. The answer is to add a relatively small amount of stock at a time so that the screw has a chance to work on it and develop some heat. In other words, the screw should not be filled from an open hopper right at the start of the extrusion. In actual practice, a schedule of operations can easily be laid down to guide the extruder operator. The information given above may appear contrary to theory and usual recommendations. The conditions may not even be common to all operators of high-speed modern extruders. Nevertheless, it is in line with the experience of many users. The comments do emphasise the point that in starting with a new extruder and/or raw material, it will be
14 Extrusion of PVC-General Aspects
717
necessary to find the best heat levels by a process of trial and error. In doing this it should be appreciated that an increase or decrease in temperature may achieve the desired results. These comments rarely apply to the die (and often the head heats), which may be operated independently of other heats to give the best finish. 14.8.3 Examples of Industrial Extrusion of Plasticised PVC
(a) PVC Coating of Wire and Cable Plasticised PVC compositions are widely used as insulation on domestic and industrial wiring cables, high-voltage transmission cables (up to about 10 kV), short-haul telecommunication cables (switchboard and inter-phone cables), and also as protective sheathing on most types of cable. pPVC re-work and reclaimed material are employed as filling core for composite cables. pPVC has maintained its firm position in these major applications by virtue of its good general dielectric properties, great formulational versatility and low flammability. For very high voltage cables, and high-frequency cables (e.g. telephone and coaxial cables) where dielectric loss is of cardinal significance, polyethylene is preferentially used because of its better dielectric properties (and its electrical breakdown resistance, important in high-voltage applications). The compounding of PVC compositions for the applications just mentioned is discussed in Chapter 13, Sections 13.4.2 and 13.4.4. The compositions are applied by extrusion-coating, involving the use of crosshead dies in which the melt is turned through a 90° angle to form the covering around the cable passing through the die. Useful mathematical analyses of flow of the melt in wire-coating dies have been published by Carley et al., 68 and by Fenner and Nadiri. 69 There are two basic kinds of internal die configuration, corresponding to two general ways of forming the insulation around the wire or cable: these ways are pressure extrusion, in which the melt is fed more or less directly onto the cable, and tubing extrusion in which the melt is first formed into a tube inside the die and this is then collapsed onto the cable. The latter method has operational advantages and is often favoured, although pressure extrusion can give particularly good tightness of the insulation or sheath, with effective filling of interstices in composite cables. A schematic diagram of a typical building-wire insulation and sheathing line is shown in Fig. 14.5.
-
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•
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~~
.1l'O'''~, ~
.~
i
"" ""~ ;: ""!:>. is
:::1
~
~
;:-
;:
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Fig. 14.5 A typical building-wire insulation and sheathing line. 1, Dual reel flyer pay-off for insulation or earth wire ;;; when sheathing; 2, wire straightener; 3, rotating pay-offs for insulated core to make flat twin-sheathed cable; 4, cable guides; 5, extruder; 6, hopper loader and colour masterbatch feeder; 7, control panel; 8, embossing unit; 9, multi-pass capstan/cooling trough; 10, diameter control; 11, spark tester; 12, accumulator to control take-up speed; 13, high-speed dual reel take-up; 14, stripping extruder. (Reproduced, with permission, from Ref. 9.)
~! ~-
, .."
8
;::J 00
14 Extrusion of PVC-General Aspects
719
Good accounts of extrusion-coating of wire and cable with PVC have been published by Barnett,9 and Burton and Clarke. 7o (b) Production of pPVC Hose with Braid Reinforcement An example of a modern, continuous production operation is the Maillefer line (Meillefer SA, Ecublens, Switzerland).?l A primary extruder (operating on either pellet or powder feed) extrudes a tube (the ultimate inner part of the hose): this is vacuum-sized, cooled and dried, and thereupon enters an in-line winding section where filament reinforcement is applied-in a braid configuration-by a counterrotating, multi-strand helical winder. The tube next passes through an adhesive-coating station and a pre-heater, and then enters the cross-head die of a second extruder, where it is coated with an outer PVC layer. The composite tube emerging from the die is passed through a cooling bath and caterpillar haul-off (which provides traction for the whole operation), to a winding and cutting station. A typical production speed (with 60-mm screw extruders) for 20-mm hose is quoted as 15-20 m min- l depending on the PVC compound and the reinforcement winding density.
REFERENCES 1. Anders, D. (1978). 36th ANTEC SPE Proceedings, pp. 726-31. 2. Fisher, E. G. (1964). Extrusion of Plastics, Illiffe Books Ltd, and The Plastics Institute, London. 3. Titow, W. V. and Lanham, B. J. (1975). Reinforced Thermoplastics, Applied Science Publishers, London. 4. Burbridge, J. (1973). In Developments in PVC Technology, (Eds J. H. L. Henson and A. Whelan), Applied Science Publishers, London, Ch. 8. 5. Anon. (1982). Eur. Plast. News, 9(11), 3~. 6. Anon. (1982). Mod. Plast. Int., U(lO), 24. 7. McCandless, W. W. and Maddy, W. D. (1981). Plast. Technol., 27(2), 89-93. 8. Avery, D. H. and Csongor, D. (1978). 36th ANTEC SPE Proceedings, pp. 446-8. 9. Barnett, G. P. (1977). In Developments in PVC Production and Processing, (Eds A. Whelan and J. L. Craft), Applied Science Publishers, London, Ch. 8. 10. Grant, D. and Wilkinson, P. (1968). Plast. Polym., 36(124), 33~1. 11. Anon. (1979). Mod. Plast. Int., 9(1), 30-1. 12. Anon. (1982). Mod. Plast. Int.. 12(3), 20. 13. Anon. (1980). Plast. Technol.. 26(13). 62-4.
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B. J. Lanham and W. V. Titow
14. Carley, J. F. (1968). SPE J., 24(2),36-41. 15. Yarsley, V. E. and Flavell, W. (1956). Cellulosic Plastics: Part 1: Cellulose Acetate, Cellulose Esters, and Regenerated Cellulose, Plastics Monograph No. C. 6., The Plastics Institute, London. 16. Rice, W. T. (1980). Plast. Technol., 26(2), 87-91. 17. Anon. (1982). Mod. Plast. Int., 12(11), 24-5. 18. Anon. (1981). Mod. Plast. Int., 11(11), 35. 19. Anon. (1981). Eur. Plast. News, 8(11), 37. 20. Kautz, G. and Schumacher, F. (1977). Kunststoffe, 67(10),585. 21. Brockschmidt, A. (1982). Plast. Techno!., 28(5), 35-7. 22. Anon. (1982). Eur. Plast. News, 9(11), 37. 23. Brockschmidt, A. (1982). Plast. Techno!., 28(3), 73-6. 24. Fenner, R. T. (1979). Plast. Rubb. Int., 4(5), 219-22. 25. Fischer, P. (1981). Plast. Rubb. News, January, 39-45. 26. Parnaby, J., Kochhar, A. K. and Wood, B. (1975). Polym. Engng. Sci., 15(8), 594-605. 27. Lippoldt, R. F. (1978). 36th ANTEC SPE Proceedings, pp. 737-9. 28. Lippoldt, R. F. (1978). Plast. Engng, 34(9), 37-9. 29. Marks, G. C. (1973). Developments in PVC Technology (Eds J. H. L. Henson and A. Whelan), Applied Science Publishers, London, Ch. 2. 30. Chartoff, R. P. (1976). 34th ANTEC SPE Proceedings, pp. 347-9. 31. Summers, J. W., Isner, J. D. and Rabinovitch, E. B. (1978), 36th ANTEC SPE Proceedings, pp. 757-9. 32. Parey, J. and Menges, G. (1981). J. Vinyl Technol., 3(3), l52-{). 33. Menges, G., Berndtsen, N. and Opfermann, J. (1979). Kunststoffe, 69(9), 562-9. 34. Benjamin, P. (1978). 'The influence of processing on the properties of PVC pipe,' paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978; and (1980) Plast. Rubb.: Mat. Appln, November, 151-{)0. 35. Daniels, C. A. and Longeway, G. D. (1979). Polym. Engng. Sci., 19(3), 181-9. 36. Hattori, T., Tanaka, K. and Matsuo, M. (1972). Polym. Engng. Sci., 12, 199. 37. Faulkner, P. G. (1975). J. Macromol. Sci. (phys.) , B11,251. 38. Khanna, R. (1977). Pigment Resin Techno!., 6(7), 11-14. 39. Krzewki, R. J. and Sieglaff, C. L. (1978). Polym. Engng. Sci., 18, 1174. 40. Kulas, F. R. and Thorshaug, N. P. (1979). J. App!. Polym. Sci, 23, 1781-94. 41. Summers, J. W. and Rabinovitch, E. B. (1981). J. Macromo!. Sci. (Phys.) , B20(2), 219. 42. Krzewki, R. J. and Collins, E. A. (1981). J. Macromo!. Sci. (phys.) , B20(4), 443. 43. Huxtable, J., Cogswell, F. N. and Wriggles, J. D. (1981). Plast. Rubb. Process. Appln, 1(1), 87-93. 44. Isherwood, D. P. and Katwiremu, J. B. (1982). Plast. Rubb. Process. Appln, 2(3), 253-63. 45. Riley, D. W. and Klein, I. (1978). 36th ANTEC SPE Proceedings, po. 525-8.
14 Extrusion of PVC-General Aspects
721
46. Press, J. B. (1978). 'The selection of PVC polymers, additives and fillers and the choice of equipment to maximise output and quality and to minimise cost,' paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, fr-7 April, 1978. 47. Collins, E. A. and Metzger, A. P. (1970). Polym. Engng. Sci., 10, 57. 48. Logan, M. S. and Chung, C. I. (1979). Polym. Engng. Sci., 19(15), 1110-16. 49. Summers, J. W. (1981). J. Vinyl Technol., 3, 107-9. 50. Marshall, G. P. and Birch, M. W. (1982). Plast. Rubb. Process. Appln, 2(4), 369-79. 51. Parey, J. and Zajchowski, S. (1981). Plastverarbeiter, 32(6), 724-6. 52. Gonze, A. (1971). Chim. Ind., 104(4/5),422-7. 53. Lamberty, M. (1974). Plast. Mod. Elast., December, 82-9. 54. Lanham, B. J. (1969). Plast. Rubb. Wkly, (286), 10-11. 55. Martin, B. (1970). Plast. Polym., 38(134), 113-19. 56. Klein, I. and Tadmor, Z. (1969). Mod. Plast., 48(9), 16fr-70. 57. Sweetapple, L. (1968). Plast. Technol., 14(11), 75-83. 58. Anon. (1981). Plast. Techno!., 27(7), 15-17. 59. Wyman, C. E. (1975). Polym. Engng. Sci., 15(8), 60fr-11. 60. Prause, J. J. (1967). Plast. Techno!., 13(11), 41-5. 61. Prause, J. J. (1968). Plast. Technol., 14(2),29-33. 62. Prause, J. J. (1968). Plast. Technol., 14(3), 52-7. 63. Jewmenow, S. D. and Kim, W. S. (1973). Plaste u. Kaut. 20, 356. 64. Kim. W. S.,Statschkow, W. W. and Jewmenow, S. D. (1973). Plasteu. Kaut., 20,696. 65. Korney, A. F. Jr. (1969). SPE J., 25(7),27-9; 25(9), 28-31. 66. Kramer, A. (1969). Kunststoffe, 59(7),409-16. 67. Anon. (1967). Brit. Plast., 40(12), 57-61. 68. Carley, J. F., Endo, T. and Krantz, W. B. (1978). 36th ANTEC SPE, Proceedings, pp. 453-61. 69. Fenner, R. T. and Nadiri, F. (1979). Polym. Engng. Sci., 19(3),203-10. 70. Burton, V. A. C. and Clarke, J. J. (1978). 'Cable extrusion machinery for PVC', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, fr-7 April, 1978. 71. Anon. (1979). Mod. Plast. Int., 9(1),20.
CHAPTER 15
Injection Moulding of pvc The late L. W. TURNER
15.1 INTRODUCTION Injection moulding is a commonly operated melt process, more extensively applied to thermoplastics other than PVC than to PVC itself, chiefly because of two inherent features especially appertaining to the unplasticised PVC compounds. These features are thermal instability and high melt viscosity, both of which create situations which those responsible for the process must anticipate and control carefully. There is little doubt that as moulding technology improves and as the position with regard to VCM becomes less hazardous the injection moulding of PVC will extend. Injection moulding is a cyclic process in which precision in repetition yields significant applicational, economic and quality returns. The procedure by which an injection machine is used to produce adequate components can best be envisaged in distinct stages: feeding the melting cylinder with compound; preparing plastic melt by thermal and mechanical effects; forcing the melt from the melting cylinder into the mould; cooling the contents of the mould; finally emptying the mould. While post-ejection processes may need to be applied-jigging, trimming, decorating, assembling, cementing-these will not be dealt with here. The two features that have been emphasised above as being particularly significant with PVC, are especially so during melt preparation and mould filling. For this reason attention will be largely devoted to the behaviour of PVC in these two stages of the overall cycle of operations. It is here that the plastic melt reaches its maximum temperature and is forced to flow through restricting features such as cylinder nozzle, sprue, runners and gates. 723
724
L. W. Turner
Concerning the other stages of the cyclic process of injection moulding, it is generally satisfactory to consider PVC as one would other thermoplastics. Thus, in the section below on trouble-shooting, this approach is taken specifically, and the account emphasises the effects which might be met with due to instability under melt conditions.
15.2 MELT PROPERTIES OF PARTICULAR SIGNIFICANCE, MELT BEHAVIOUR IN RELATION TO MOULDING CONDITIONS, AND MOULDING COMPOUNDS While, in many respects, PVC responds to the process of injection moulding as do other thermoplastics, it is notable for two specific reactions to moulding conditions which tend rather to set it apart. These are: development of decomposition, in times of the order of moulding cycles, at those levels of temperature found to be necessary; and a tendency for the melt to have a higher viscosity than many other materials at permissible temperature levels, when the polymer is unaided by the presence of plasticisers or other flow promoters. It is often stated that PVC shows a marked tendency to decompose by reason of loss of HCI from the molecular chain accompanied by darkening-at temperatures from, say, I80 a C upwards. But such a description of its behaviour during moulding must be regarded as oversimplistic. In fact degradative effects can become apparent over a range of temperatures, including levels below I80 a C, in relation to the time for which the melt is held at these levels. This leads to the concept of residence time (often applied by moulders when setting up a trouble-shooting programme) which also tends to be employed in too simple a fashion in considering moulding performance. What has to be taken into account is the integrated thermal input to the material (from any input source) during both melt preparation and mould-filling stages. The shear-work being done on a highly viscous polymer melt can result in reduction of the molecular entanglements, thus aiding flow; but this effect will not normally give rise to difficulties in production control or in product quality. Rather more serious is the molecular damage which high shear levels can cause, especially to longer chains present in the melt. This latter degradative effect is separate from, and additional to, the thermal degradative effect and is
15 Injection Moulding of pvc
725
almost always much less prominent and camouflaged by thermal effects. Firstly, to illustrate the straightforward effect of material temperature, the graph in Fig. 15.1 has been prepared from data given in the literature! for three types of moulding compound (referred to again in Section 15.6). The graph defines reasonable limits for residence times: i.e. to the right of a given curve the residence time is too long for safe operation with the compound concerned at a selected temperature; to the left conditions are safe (no thermal/time decomposition). The graph gives data for specific time-temperature pairs, but from the discussion above it will be realised that to deal adequately with degradative effects occurring during moulding, the broader picture of integrated thermal input has to be considered. Secondly, it is necessary to realise that the degradation which might occur during moulding cannot be attributed to one specific timetemperature value pair. The melt reaches the mould cavity by what might be regarded as an
230
220
V.
210
''"-
::J
,"200 '-
'"
a. E
~190 180
lima,s
Fig. 15.1 Time-temperature behaviour of typical PVC compounds. A, Pipe-fitting compound; B, rigid general-purpose compound; C, flexible compound. Generalised representation, based on data from Ref. 1.
726
L. W. Turner
integration of a series of temperature-time steps which vary continuously as the plastic material proceeds from hopper to mould entry. Shear-work, dissipated as thermal energy, raising the melt temperature, takes place in the melt in the forward zones of the screw and barrel, in the nozzle, and finally in the gate. Thermal conductivity is poor, and non-homogeneous build-up of temperature will occur. Poor streamlining in the nozzle-to-gate region can exacerbate the situation by causing local stagnation of flow, which adds seriously to the time component of the thermal input. The rheological behaviour of PVC2 is important in relation to injection moulding because melt preparation usually involves an extrusion screw and barrel arrangement. Melting and flow behaviour during this heating process and flow during the moulding operation are dependent on the rheology, which in turn depends on the composition, i.e. the PVC polymer and the additives employed. Shear-work is another feature dependent upon composition via slip and flow behaviour. 15.2.1 Moulding Compounds
In the case of plasticised compositions, the general difficulties associated with the high melt viscosity and susceptibility to thermal degradation of PVC compounds are, to some extent, relieved by the presence of the plasticiser(s) which makes for easier melt flow (lowers melt viscosity) and hence for processability at somewhat lower heat inputs and temperatures. It is thus the uPVC compounds that present the greatest problems-a circumstance responsible for their general unpopularity with moulders, and long operative as a restricting factor on the scope and volume of the injection moulding of 'rigids'. However, the problems can be considerably eased by suitable formulation, and in particular by the use of special PVC homopolymer resins of relatively low molecular weight (cf. also Chapter 4, Section 4.4.1(a)). 'Easy flow' rigid injection-moulding compounds* available from most big suppliers are commonly based on such resins (in which the lower molecular weight is also usually combined with fine particle size and medium porosity). With good equipment and moulding practice easy flow compounds permit production of relatively thinwalled mouldings of large surface area; deep draws, as well as normally
* e.g. Geon 110 x 346 (B. F. Goodrich); Ethyl 7042 (Ethyl Corp.).
15 1njection Moulding of pvc
727
difficult surface detail, can also be achieved in many cases. Although it is sometimes claimed that the compounds can be successfully run-with only a few modifications-on equipment set up for the processing of materials less difficult than uPVC (e.g. ABS) it is important for effective operation with any uPVC composition to pay attention to the equipment and process considerations summarised in Section 15.4. Compounds based on vinyl chloride copolymers, or on combinations of homopolymer with flow-promoting resins, may be used for ease of processing, where the ultimate properties of the resulting products are consonant with the end-use requirements. Note: A standard flow test3 in a piston plastometer (melt index apparatus with a modified die) provides a useful method of comparison of PVC compounds with particular reference to the effects of compositional factors and thermal and rheological stability. Whilst some correlation of the test results with actual processing behaviour is also claimed3 (especially for compounds of generally similar composition) the validity of this claim in particular cases must be subject to such factors as the differences in shear rates between test and processing conditions (shear generally higher in the latter) and the effects of the elastic nature of the melt (not normally brought out in a plastometer test).
The mould shrinkage of rigid PVC moulding compounds is relatively low: typically 0·1-0·7%. That of plasticised compositions is generally more variable and much higher. Note: Shrinkage of plastics mouldings can vary with the rate of operation (cycle time). Useful general comments on the practical aspects of mould shrinkage calculations for optimum processing rates have been published by W. B. Glenn. 4
uPVC moulding compounds are not very sensitive to moisture and do not require drying before use, except where significant moisture pick-up may have resulted from prolonged explosure to humid atmospheres. Even in such cases a hopper drier on the injection machine may be sufficiently effective. Any oven pre-drying should normally be done on shallow layers (up to about 4 cm), with the temperature and time of drying preferably not exceeding 65°C and 3 h, respectively.
728
L. W. Turner
15.3 EFFECT OF PROCESS FACTORS UPON PRODUCT PROPERTIES Some reference to process factors affecting product properties, aside from the effects attributable to thermal decomposition, is necessary. PVC polymer, in common with other thermoplastics, by reason of its characteristic long chain-like structure, yields melts of high viscosity and poor thermal conductivity. These physical attributes give rise to mouldings in which quenching stresses and orientation both exist. These can have important effects upon product properties, though it is not uncommon for moulders to be aware of and concerned about orientation while neglecting to consider the effects of quenching stresses. Other morphological features of mouldings, e.g. skin and core effects, also arise, and influence the properties. A useful summary of methods of characterisation of injection mouldings was published recently by Haworth et al. 5
15.3.1 Quenching Stresses Injection into a mould which is much cooler than the melt results in rapid chilling of surface layers of the moulding. Inner layers, by reason of poor conductivity in the melt and the solid, cool more slowly and stresses are set up within the moulding cross-section. Commonly, compressive stresses of considerable magnitude arise in the surface layers and balancing tensile stresses in the central zone; together these are sufficient to affect mechanical behaviour and environmental resistance of the moulding.
15.3.2 Orientation and Related Features The flow pattern and the effect of the shear involved in melt flow give rise to anisotropy of mechanical and other properties. Mouldings are most resistant to fracture when deformed across the line of orientation and less so along this direction. This situation can be exaggerated if fillers are present in the compound, especially if they are fibrous. Orientation is not uniform either along a flow path, or, at any given point, through the moulding thickness. Development of orientation relates to the shear stress required to fill the mould cavity and through this physical parameter becomes related
15 1njection Moulding of pvc
729
to gate size, mould temperature, wall thickness, injection rate, melt temperature, and so on. Thus the effective level of applied shear stress can often be employed as an overriding control parameter and guide in troubleshooting, enabling some of the complex inter-relationships between various machine settings to be more simply recognised in so far as moulding properties are being affected. A useful reference to the effects of process conditions on injection-moulded product properties is due to Gilbert et al., 6 who give a description of the behaviour of a selected rigid formulation involving typical ingredients. A low level of crystallinity in mouldings made from this formulation is observed, as are differences between the skin and core, layered structure developing during mould filling and cooling. The distinct skin layer is highly oriented as shown by its strong birefringence (when a thin cut section is viewed through the thickness), whereas the core is largely unoriented. The skin layer decreases in thickness from, say, 0·4 mm down to around 0·1 mm as moulding thickness increases from 1·6 to 6 mm. Additionally, the thickness of this oriented skin is governed by each of the commonly varied process factors: filling rate, mould temperature, material temperature. Unfortunately no reference is made to the quenching stresses which must have developed in parallel with the oriented skin layer and for this reason the consideration of the effects on product properties is not too helpful to the reader. In referring to this paper it has to be recognised that mould shrinkage data are not given though the word 'shrinkage' is used. Such 'shrinkage' data as are given relate to recovery at an elevated temperature (130°C).
15.4 THE MOULDING PROCESS: AVAILABLE EQUIPMENT; PROCESS CONTROL; SOME FEATURES OF uPVC MOULDING
The elementary principles of injection moulding (including machine features, mould design and processing conditions) are outlined in an leI publication. 7 Useful practical guidance on the equipment, the basic process, process operation and control, and diagnosis and cure of moulding faults is provided in a recent book by Brown. s A brief general discussion of the main considerations in injection machine selection has been published by Ireland and Smith. 9
730
L. W. Turner
Plate J Injection unit for the Peco-Loewy 20/90R injection-moulding machine: shot weight 30 lb; 2500 ton locking unit. (Courtesy of the designer Mr T. Seklecki, formerly of Peco, now with Gay's (Hampton) Ltd, Hampton, Middlesex, England.) The modern injection-moulding machine is a sophisticated piece of equipment but, at the present moment, its design details are in a transitional stage. Quite extensive changes are taking place, not so much in the actual mechanics of the separate process stages which it provides, but almost entirely in regard to the detailed control of these process steps, their balance and integration. This is due to a background already developed in monitoring and feed-back control during the last decade, plus the well-nigh revolutionary possibilities inherent in the use of microprocessors. By the use of these, inexpensive integrated control can be achieved. Moulders involved in large runs of closely related products (e.g. pipe fittings) can set up processing units which are unique to the product in satisfying, at optimum level, specific processing requirements. Having in mind the nature of PVC and the specific nature of some of its applications, the development in dedicated process control is most welcome to moulders. Others, wishing to make a wide range of products, can gain advantage from the greater ease of setting up and re-establishing process conditions, thus achieving greater consistency of operation, less rejects and less down-time.
15 Injection Moulding of pvc
731
Extensive surveys of injection-moulding machines are published from time to time in the technical literature. Note: Two recent surveys appeared, respectively, in the March 1983 issues of Plastics and Rubber Weekly and in European Plastics News for June 1983.
The data provided for the various manufacturers' machines normally relate to machine characteristics and construction, with the wide range of equipment available divided into four groups according to the mould-clamping force. Whilst manufacturers will advise how particular machines can best be set up for PVC working, the following general points may be mentioned by way of a few broad guidelines. For moulding uPVC, the machine should preferably have a clamping pressure capacity of up to 3·5 tons (UK) per square inch of proj~cted area of moulding (i.e. about 55 MN m- 2 , in round figures), although anything from about 2 tons upwards (i.e. about 30 MN m- 2 plus) should normally be sufficient with most easy-flow compounds. In many cases a shot size between about two-thirds and three-quarters of barrel capacity will represent the practical optimum for melt residence times sufficiently short to avoid undesirable heat effects or excessive heat history when operating near the top limit of the relatively narrow melt temperature range (see further on) desirable for ease of flow. With screws specially designed for uPVC processing ('PVC screws') shot sizes up to full capacity may be practicable. With decreasing shot sizes (preferably not below about one-fifth of barrel capacity) the balance of the effects of melt temperature, residence time, and cycle timealways important with PVC-becomes progressively more critical. Several features of equipment and operation desirable or essential for the best results in PVC moulding reflect the need to cater for the two special factors which have already been emphasised, viz. the susceptibility of the material to heat degradation, and the relatively high melt viscosity (of uPVC). Thus streamlining right up to the nozzle exit is important to avoid creation of points of increased friction in the flowing melt, and pockets of stagnant material where thermal decomposition may occur with consequent contamination of, and destabilising effect on, the melt. For analogous reasons it is usually considered essential for rigid PVC to have a conical extension (smear-head tip) on the screw which-with a suitably shaped barrel head-virtually empties the barrel at the finish of mould-fill, as it attains its most forward position. Some relevant comments about screw
732
L. W. Turner
tip patterns for different circumstances have been published by Tulley and Harris,I and about the moulding of rigid PVC generally by Huber. lO The screw should preferably be of a design intended for uPVC processing: LID ratios in the range 14-24: 1, and compression ratios of 1·5-2·0: 1, may be regarded as optimal, although higher compression ratios (up to about 3: 1) can be used if the effects of the associated greater shear-heat generation are properly monitored and controlled (by lowering the rotational speed and back-pressure of the screw). Conversely, lower compression ratios allow a wider range, and higher values, of back-pressures and rotational speeds. As an example, the following values have been recommended for an easy-flow high-impact uPVC compound (Ethyl 7042).11 Screw compression ratio Back-pressure range (Ibf in -2) Rotational speed range (r min-I)
l'S:l 2·0:1 2·S: 1 G-1000 0-400 G-100 2G-100 2G-80 2G-SO
3·0: 1 G-SO 2G-30
It can be worthwhile to set the machine to operate under zero cushion mode: otherwise a minimum cushion (say 2~ mm) is desirable. The flow routes (nozzle, runners) should be as short and generous as possible. The nozzle should preferably be reverse-tapered, to reduce friction-heating possibilities in this area. Gates should also be generous (and round wherever possible), with small lands and edges radiused towards the component, for ease of flow, rapid mould filling and reduction of pressure losses. Pin-gating is undesirable, although it may be used in some cases (in particular for small parts moulded with easy flow compounds). Adequate venting of moulds is important. Maintenance of optimum stock temperatures at the various stages of the material's processing in the machine is particularly important with uPVC, for ease of flow, avoidance of decomposition, and minimum heat history. Close attention should be paid to the proper setting and control of all heater temperatures, as well as-and in conjunction with-the factors influencing shear heating in the barrel (the back-pressure and rotational speed of the screw-see above), and frictional heating thereafter (size and configuration of nozzle, flow channels and gates; rate of injection). The melt temperature should be monitored directly, either through intermittent checks (e.g. with a thermocouple inserted into melt flushing out of the nozzle while the barrel is retracted from the mould) or continuously by a sensor (thermocouple) so positioned that it does not cause additional friction
15
733
Injection Moulding of PVC
or stagnant spot in the flowing melt. It is generally advantageous to set the barrel and nozzle heaters for a temperature profile rising fairly sharply from the rear (feed) zone of the barrel to the nozzle. The set temperatures should preferably lag somewhat behind the desired stock temperatures, the heat needed to make up the difference coming from the right amount of mechanical working of the material in the barrel secured by appropriate speed and back-pressure of the screw. For a given machine and mould set-up (and with a particular screw type, speed and back-pressure) the heater temperature settings will vary somewhat according to the composition processed (see examples in Table 15.1), but an arrangement of this general kind offers the following advantages. The final fusion of the stock takes place well forward in the barrel, so that the amount and dwell time of the hottest material are minimised, whilst any volatiles have an escape route through the interstices between incompletely fused compound particles in the rear of the barrel, with consequent reduction of the possibility of porosity in moulding arising from this source. During a mechanical stoppage, the material in contact with the cylinder walls (especially the fused melt in the front part of the cylinder) will be hotter, at least initially, than the wall surface, so that the immediate effect should be its cooling-and not continued heating-with consequent delay of the onset of decomposition. TABLE 15.1
Examples of Recommendations for Temperature Regimes in the Processing of Some uPVC Compounds Type of compound
Temperature settings
eC)
Barrel zone heaters
General-purpose easy flow, high-impact Pipe-fitting compound (Type 1°) Universal pipe-fitting compound (Type 1/2°) ° See Section 15.5.
Nozzle heater
Melt temperature (0C)
Rear (feed section)
Middle
Front
135
160
170
170-175
200-210
140
155
170
170-175
190-200
145
160
175
175-180
200-205
734
L. W. Turner
Note: In the case of stoppages of appreciable length, the heater temperature settings should be reduced, and the barrel should be pulled back from the mould and purged out slowly. If the stoppage is prolonged, or when shutting down, the barrel should be completely emptied and purged with a purging compound (polystyrene, ABS, acrylic, or HDPE) at a relatively low temperature (about 160°C): uPVC should never be allowed to solidify in the barrel. If the temperature settings on the rear and centre zone heaters are too low, the torque on the screw trying to work the relatively cold material may be excessive: to prevent overloading the motor, an upward adjustment should be made (say in steps of 5°C) until the screw is running normally. The melt temperature should preferably not exceed 205°C, and should never be higher than 215°C: the range 18Q-205°C may be regarded as typical for most processing. The mould (inlet water) temperatures used are normally within the general range 2Q-70°C, with 2Q--40°C usual for many easy-flow compounds (the actual optimum depending on the composition, and the wall thickness, flow length and moulding size), and the higher temperatures for the older types of uPVc. Temperature control techniques for injection-moulding machines are summarised in a recent paper by Ingham. 12 The following further points may be mentioned.
15.4.1
Rate of Injection and Injection Pressure
The rate of injection should be the optimum for rapid, complete filling of the mould, but consistent with avoidance of decomposition (first manifested in the appearance of discoloration at and near the gate point-d. Section 15.6) through excessive frictional heating. Note: If the injection rate is reduced too far, weak weld lines, sink marks, or even short shots may result. When the characteristic signs of decomposition appear at a generally reasonable injection rate, consideration should be given to enlarging (and streamlining) at points where melt flow may be unduly restricted (nozzle orifice, sprue, runners, gate). As a general principle, automatic adjustment of injection rate (via a closed-loop control arrangement) is highly desirable.
15 1njection Moulding of pvc
735
Injection pressure is interdependent with injection rate, and also influenced by other factors, notably melt fluidity and the geometry and temperature of the mould. As a general guide, for uPVC the first-stage injection pressure can advantageously be within one-half to threequarters of the total pressure available on the equipment, and the holding (dwell) pressure will usually be between one-half and two-thirds of the first-stage pressure. It is desirable that the equipment should be able to provide pressure up to 25000 lbf in -2 (about 172MNm- 2). 15.4.2 Working Surfaces
The working surfaces of machine and moulds must be resistant to acid corrosion. Stainless steel moulds are desirable wherever the type of work and run lengths can justify the initial expense.
Note: Galling can be a serious problem where stainless steel surfaces in contact undergo repeated relative movement. For this reason any moving parts in a stainless steel mould (ejector pins, sliding cores) are usually of chromium-plated hardened steel. The mould surfaces, as well as those of the screw, may also be of chromium-plated hardened steel, or of hardened steel nickel-plated (by the electroless process) over-plated with chromium (the electroless nickel finish alone may not provide sufficient corrosion protection, especially where the mould contains deep recesses). Deep nitriding may be adequate as the sole protective finish in some cases. It is strongly preferable that all surfaces (those of the platens, etc.) accessible to any gaseous products of decomposition of the PVC material should also be suitably protected.
Note: For out-of-use periods (following the end of a run, before a shut-down) the surfaces of the mould, including those of the moving parts, should be treated to neutralise acidity and inhibit corrosion. Suitable treatments (neutralisers and inhibitors, usually in sprayable form) are available on the market. 15.4.3 Interaction of PVC with Acetal Polymers and Copolymers
PVC and the acetal resins (e.g. Delrin-Du Pont; HostaformHoechst; Kematal-Amcel UK; Celcon-Amcel USA) interact strong-
736
L. W. Turner
ly at elevated temperatures, with resultant vigorous mutual degradation. The two materials should on no account be allowed to come together in plastics processing equipment (injection-moulding machines, extruders, melt-compounding mixers, etc.). It is highly preferable that the same machine should not be used for the processing-especially consecutive processing-of these materials: at the very least extremely thorough purging must be carried out (with a suitable purging compound-see above) before a change-over. Two brief general summaries of considerations important in the moulding of uPVC have been published recently by Whelan,13 and Murrey and Dito. 14 15.5 MATERIALS AND APPLICATIONS It should be obvious from what has been said concerning the rather
unique aspects of PVC moulding that the moulder, prospective or current, is greatly aided if he has available to him property details of the compounds he is intending to use. By and large, manufacturers can help but it is not too wise to rely entirely on such sources and the moulder would do well to look at some of the more fundamental aspects of his activity. The purposes for which PVC moulding compounds are used are numerous and the compounds will range in type. It is, however, possible to classify and thus to understand better what the main types are and how they will mould. An excellent account of such a classification is given in Ref. 1. The classification recognises two basic types of unplasticised material, and a general category of plasticised compounds: Rigid (unplasticised) Flexible (plasticised)
1. Typical for pipes and pipe fittings 2. Typical general-purpose General: used for flexible demands
The differences. between the two types of rigid composition arise in consequence of the requirements to be met. Type 1: Mouldings usually thick-sectioned; high strength and stiffness, high rupture strength, good corrosion resistance. Minimum cost because of large number of items used, runner stabilised. Type 2: Designed for thinner sections and a wide range of
15 1njection Moulding of pvc
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conditions. Higher level of stabilisation and better flow properties for thinner, larger-area products. The flexible (plasticised) grades, while considered as one broad type, will vary widely in respect of types and amounts of plasticisers used. In the moulding of PVC-and especially uPVC-pellets are used as the feedstock in the majority of cases. This is mainly because they consist of melt-compounded material which is already well homogenised and uniformly fused (d. Chapter 13), and thus-unlike powder (dry-blend) feeds-does not have to rely for final homogenisation on the treatment received in the barrel of the injection machine which is not primarily designed as a mixing unit. This consideration normally outweighs that of the more extensive heat history the pellets acquire in consequence of the melt-compounding operation. The most important area for the extension of injection moulding of PVC undoubtedly lies in rigid mouldings of thinner sections. The technical problems will be clear from the text above: the successful development of new applications has depended upon improving processing characteristics by increasing thermal stability and promoting easier flow through the use of suitable PVC polymers and additives of the appropriate types. Availability of screw/ram machines has created better processing conditions in making for better thermal homogeneity and reducing residence times. It is not likely that piston machines will be employed in the future in any significant way. Where really thin disposable items are required (as in the packaging field where PVC is regaining acceptance now that VCM concentration hazards are controlled) these will not be made by injection moulding but by forming of extruded sheet. A process sometimes referred to as 'flow moulding'* enables large mouldings to be produced on relatively small injection machines, by utilising pressure developed by the. rotation of the screw (instead of that produced by the injection stroke) as mould-filling pressure. Inter alia, large (30 lb) PVC pipe fittings have been produced on a 250 ton injection machine 15 by this process, which may be considered as a combination of extrusion (as the means of supplying melt to the mould) and injection moulding (in that a conventional-though modified-injection-moulding equipment is used). The modifications include adjustment of the hydraulics to keep the screw in the forward * Developed by Durapipe Ltd, Cannock, UK.
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position (feeding the stock through a gap between the screw tip and the rear face of the nozzle); provision for adjustment of the size of this gap (where the main working of the material to produce plastication is effected through the shear imparted) to regulate the amount of shear; special provisions for screw and barrel cooling; and overall control of temperatures and pressures. The process is claimed to be particularly suited to heat-sensitive materials such as PVc.
15.6 TROUBLE-SHOOTING Many processing and material factors have to be balanced in order to achieve high outputs of good quality mouldings. Achieving an acceptable balance can sometimes be a difficult task requiring experience and skill. Two principles are here emphasised: good knowledge of the material concerned (this is perhaps more important with PVC compounds than with any other thermoplastic) and use of such control aids as the equipment can provide. Three groups of considerations which arise in aiming at continuous in-balance quality mouldings are distinguished below. They relate to machine selection; to those processing features pertaining more specifically to PVC; and to those features which are of general significance in injection moulding of thermoplastics.
15.6.1 Machine Selection The moulder with a limited range of machines faces problems not necessarily met with by the one who can match more closely machine size to moulding shot-weight and cooling time. Here the essential feature is 'residence time', or the 'temperature-time integral' to be more accurate. During operation it is important to maintain observation of heater control indications (watching out, inter alia, for persistent and increasing override), filling rate (to spot any tendency to increase with running time), and changes in thickness of cushion layer.
15.6.2 Processing Features Specific to PVC These relate almost entirely to the behaviour of the material, there being an inevitable interaction between these features and those mentioned in Section 15.6.1 above.
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TABLE 15.2
Common Faults in PVC Mouldings
Observation 1. Splay marks or
silvery streaks emanating from the gate position
2. Dark centre to sprue or internally in gate area
Significance May be due to condensed moisture but, more important, could be initial signs of decomposition due to thermal input increased above acceptable level Progressive overheating of material (sometimes associated with hot spot at screw tip)
3. Dark, streaky areas on surface of sprue
Increasing overheating of material
4. More prominent areas of darkened material on sprue surface often extending from sprue into the moulding
Some decomposition of material due to overheating
5. Dark streaks spreading from gate point
High shear or stagnation in this region
6. Dark decomposition
Local 'burning'
spot at the same point in each moulding (sometimes in weld line)
Adjustment Choice of reduction of forward band heater settings (especially nozzle temperature), screw speed, back-pressure, filling rate, singly or in various combinations
Adjust barrel temperature and/or reduce backpressure, and/or screw speed; if fault persists attend to shape or smooth runs of screw tip; perhaps change to distorted tip to give an increase of local shear Reduce band-heater settings, and/or backpressure and speed of the screw Retract barrel and make a few air shots to remove overheated stock. Return barrel, reduce heater settings; increase cooling, lower screw speed and/or backpressure; consider residence time Better streamlining required: slow down input rate, attend to streamlining if necessary; consider enlarging nozzle and/or gate(s) First try reducing injection rate; if fault persists attend to proper mould venting and radiusing of corners
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Observation of the product provides the clues from which process adjustments can be decided upon. Table 15.2 lists the most frequent manifestations of problems in the moulding of uPVC (1-4 may be regarded as a sequence in order of increasing severity). 15.6.3 General Considerations
General moulding faults such as warping, sink marks, short shots, flashing, weld lines and others can be treated as they would be for other thermoplastics, so long as it is consistently borne in mind that adjustments covering an increase in the temperature-time input must be undertaken with caution by careful observation of signs of incipient decomposition. Mould filling rate should also be capable of being adjusted and maintained at a specific level. Pressure control via feedback of data obtained from pressure sensors in the mould cavity(ies) is a useful way of controlling mould filling, since rates and pressures are interrelated. Some control experts favour sensors mounted elsewhere. Mould temperatures should be accurately maintained. This calls for correct provision of cooling channels in the mould and control of the fluid from circulating units of adequate performance. Regarding the improvement of older equipment (again neglecting piston machinery), in addition to improvements to the screw by removal of unstreamlined non-return valves and the introduction of conical extensions to the screw tip, it is often a simple matter to add proprietary control features. The simplest and the best control improvement which can be introduced by the addition of a single item is cavity pressure control, though more specifically for PVC, screw-advance rate control has some advantages especially where forward-zone streamlining has proved difficult.
REFERENCES 1. Tulley, F. T. and Harris, B. C. (1979). In Encyclopedia of PVC, (Ed. L. I.
Nass), Marcel Dekker, New York, Ch. 24, p. 1313. 2. Shah, P. L. (1979). Ibid, Ch. 22, p. 1153. 3. ASTM D 3364-74 (Reapproved 1979). Flow rates for poly (vinyl chloride) and rheologically unstable thermoplastics. 4. Glenn, W. B. (1980). Plast. Technol., 26(10), 73-5. 5. Haworth, B., Sandilands, G. J. and White, J. R. (1980). Plast. Rubb. Int., 5(3), 109-13.
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6. Gilbert, M., Marshall, D. E., Voyvoda, J. C. and Copsey, C. J. (1979). Plast. Rub.: Process., 3,96. 7. 'The Principles of Injection Moulding,' ICI Technical Service Note G 103 (2nd edn), ICI Plastics Division, Welwyn Garden City, Herts, England. 8. Brown, J. (1979). Injection Moulding of Plastic Components-A Guide to Efficiency, Fault Diagnosis and Cure. McGraw-Hill Book Co. (UK) Ltd, London. 9. Ireland, R. A. and Smith, J. B. (1977). Plast. Rubb. Int., 2(5),225-7. 10. Huber, M. (1977). Plast. Rubb. Wkly, 18th February, pp. 2~23. 11. Technical Bulletins 8/73, Ethyl Corporation, Polymer Division, Baton Rouge, Louisiana, USA. 12. Ingham, P. H. J. (1979). Plast. Rubb. Int., 4(5), 211-15. 13. Whelan, A. (1981). Brit. Plast. Rubb., April, p. 25. 14. Murrey, J. L. and Dito, A. J. (1981). Plast. Techno/., 27(12),79-82. 15. Anon. (1981). Mod. Plast. Int., 11(11), 36.
CHAPTER 16
Sheet Thermoforming and Related Techniques for pvc
The late L. W. TURNER
16.1 INTRODUCTION Sheet thermoforming has been described as the one method whose development sprang uniquely from plastics technological history and was not picked up from metallurgy. This contrasts with the case of such solid-phase forming processes as forging, stamping, and certain others, originally developed for metals and only relatively recently explored in the plastics context. 1 Now super-plastic metals technology has taken up the sheet-forming idea from the plastics industry. Plastic sheet thermoforming is a secondary process, depending on, as primary process, sheet extrusion to supply its starting material. In fact it is well to bear in mind that extruded forms other than sheet (e.g. tubes), or sheet produced by methods other than extrusion (e.g. cast as is PMMA sheet, or laminated) can be thermoformed. It was in the early Celluloid or Xylonite days of the plastics industry that methods of shaping various sheet, rod or tube forms first came into industrial use. For PVC it is largely the forming of extruded and calendered sheet that must be considered. Additionally, though, there are combination processes which start by converting granules to a dimensioned pre-form of planar solid sheet geometry and then form this into a 'drawn' article (the 'Topformer,2,3 process for example). . There are three basic methods of sheet thermoforming: (i) vacuum forming; (ii) pressure forming; (iii) matched-mould forming. 743
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In each the sheet is softened by raising its temperature to an optimum point in the 'rubbery' region to enable large deformations to be achieved. A persistent technological goal for forming operations with various sheet materials is to save energy and promote rapid-cycle operations by forming below the softening point. This is still not a widely established technique and material formulation is especially important. The methods of traditional sheet forming are quite distinct as a group from injection moulding and blow moulding, although in certain respects sometimes rival both. The advantages are that plant of lighter construction can be used, which reduces capital investment in moulds and equipment. Production can be very rapid indeed and it is comparatively simple and cheap to change designs. Apart from any financial considerations, there are a number of technical advantages. It is possible to make quite large articles cheaply and simply from thin sheets covering large areas. Design details not involving sharp changes in thickness can be reproduced and preprinting of sheet is possible; this latter has no equivalent in injection moulding and blow moulding, though in these processes in-the-mould decoration is possible,4,5 but not too common. It should not be thought that thermoforming is free from difficulties. One of the major problems is that of the inevitable development of considerable orientation as the 'rubbery' material is stretched, and cooled in the stretched form. With deep forming, in particular, orientation can be substantial and the objects produced will distort excessively above certain temperatures. Extensive thinning of the material can occur and control of distribution of thickness to maintain stiffness and strength is a persistent difficulty. Some quenching stresses may also be developed due to rapid cooling of the warm sheet. This aspect is discussed in some detail later in this section. Sheet forming was perhaps at one stage envisaged as being primarily suitable for comparatively short runs of large objects. The scope has gradually widened, however, and today fully automatic machines are available which will produce small objects, e.g. beakers, at a high rate, and such forming may be integrated with the sheet extrusion stage. Sheet-forming equipment until recently has tended to be rather poorly controlled, but this situation has now begun to change rapidly due to the employment of improved engineering and, very recently, microprocessor-based process monitoring and control. Good process control is important in order to achieve economic operations with high yields of good quality products.
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16.2 MATERIALS USED Almost any thermoplastic which can be made into a satisfactory sheet may be used for thermoforming provided that: (i) it does not decompose at the processing temperatures, and (ii) it has properties which make it suitable for the finished application. Materials which are used do differ, however, in their response to the process conditions. Many materials are used: commercially, the commoner ones are ABS, HIPS, CA, PVC, PE, PP, and PMMA. In general, the thermoforming performance of vinyl chloride homopolymer sheet tends to be only moderate compared to ABS and HIPS, although suitable formulation (and especially inclusion of appropriate polymeric modifiers) can make a substantial difference. Whilst a copolymer-based sheet (again without special compositional modification) will normally be much easier to form than an otherwise comparable but homopolymer-based one, the lower softening point (and related ease of distortion after processing) tends to make it less acceptable. In general, chlorinated PVC is a better thermoforming material than unmodified uPVC. It also offers better dimensional stability (at room and elevated temperatures) of the finished product (see also Section 16.7). Rigid, homopolymer-based PVC compositions incorporating polymeric modifiers (including some regular high-impact compositions) are widely used for thermoforming. It is this kind of composition that is the main material considered here. One commercial example is Kydex sheeting (Rohm and Haas-ef. Chapter 20), an acrylic-modified uPVC material specially formulated for thermoforming. Some thermoformable composite sheet materials, incorporating PVC as one of the components, may also be mentioned. These are exemplified by ABS/PVC solid-sheet laminates (represented, inter alia, by some grades of Raya/ite sheeting3-Uniroyal Plastics Division), and foam sandwich laminates with a PVC foam core and ABS sheet facings, used for the production-by thermoforming-of interior fittings for aircraft (side panels, window and door surrounds, side-liners). 6 16.3 VACUUM FORMING OF SHEET 16.3.1 Principal Methods A useful brief summary of the general types of thermoforming processes and the main advantages and limitations of their industrial
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L. W. Turner
embodiments was published recently by O'Neill,? and one of thermoforming techniques and set-ups by Lubin et al. 8 Here forming methods relating to sheet are described in some detail since sheet is by far the commonest prepared form processed in this way; however-as has been mentioned-other forms, also made by extrusion, or cast, can be shaped by adapted thermoforming techniques. There are five basic methods of vacuum forming. These are: (a) (b) (c) (d) (e)
negative forming (or straight vacuum forming); plug-assist forming; drape forming; bubble or blister forming; and snap-back forming.
The essentials of each method are given below. (a) Negative Forming (Fig. 16.1) In this method the sheet is first fixed in a clamp frame over the mould. It is heated from above and once it has reached the required temperature is dropped onto the mould and sealed around the edges; vacuum is applied from underneath and this draws the sheet down into the shape of the female mould. When the sheet has cooled it may either be removed by hand, or ejected by air pressure from underneath. Method (b) is used for objects where drawing is greater than 2 in. in depth.
(b) Plug-assist Forming (Fig. 16.2) This technique is really an extension of method (a). A female mould is again used but the drawing process is assisted by a plug very roughly conforming to the shape of the moulding. The plug is forced down onto the sheet, thus starting the forming process which is completed by vacuum as in method (a). The plug has to be heated as well as the sheet and the temperature of both has to be controlled fairly closely. The plug also has to descend quite quickly. (c) Drape Forming (Fig. 16.3) This method is used for deeper drawing and thicker wall sections and involves the use of a male mould. The male former can either be pushed up into the heated sheet or alternatively the latter may be pulled onto it. Vacuum is applied to complete the process once a seal
16 Sheet Thermoforming and Related Techniques for
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Stoge I
Key
Heeter B Clomping frome C Plostics moteriol o Former E Gosket F Vocuum box G Pressure line H Vocuum line A
Stoge I-Heeter ·pleced over sheet Stege 2-Vocuum opplied Stoge 3-Heotor removed from sheet. which is eliowed to cool before vocuum is releosed
Fig. 16.1 Vacuum forming-negative forming. (BX Plastics Ltd.)
has been made. As in method (b) the temperature of the mould and speed of ascent or descent is important. Speed is also essential in the rate of air evacuation. (d) Bubble Forming (Fig. 16.4) In this method the sheet is heated, clamped over the vacuum box and a specific air pressure applied to blow it into a bubble. A shaped male plug is then forced into the bubble and vacuum is applied in the usual way.
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L. W. Turner
DT-.....IIIIIIIil_~iLE
'r-rSloge I
Stege 2
C-
c:..
Sloge 3
A B C D E F G
Heater Plug Plastics material Clamping frame Gasket Former Vacuum box
Stoge 4
Key H Air inlet I Air outlet J Vacuum line Stage 1 - Heater placed over sheet Stage 2 - Plug partly down Stage 3 - Plug fully down - no vacuum Stage 4 - Vacuum applied
Fig. 16.2 Vacuum forming-plug assisted. (BX Plastics Ltd.)
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Stage 2
Key A Heater B Clamping frame C
o E F
G H I
Plastics material Gaskets Former Vacuum box Air inlet Air outlet Vacuum pipe
Stage I-Heater over sheet Stage 2-Drope up, no vacuum Stage 3-Vocuum applied
Fig. 16.3 Vacuum forming-drape forming. (BX Plastics Ltd.)
Fairly deep draws with reasonably uniform wall thicknesses can be achieved by this method.
(e) Snap-back Forming This method is really the reverse of method (d). As before, the heated sheet is clamped over a vacuum box and is sucked into the box by vacuum, thus forming an 'upside down' blister or bubble. A male plug
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L. W. Turner
A
Steg. 2
A B C D E F G
Heater Clamping frame Plastics material Gaskets Former Vacuum box Air inlet
Key H Air outlet I Vacuum pipe Stage 1 - Heater over sheet Stage 2 - Bubble blown Stage 3 - Drape up, no vacuum Stage 4 - Vacuum applied
Fig. 16.4 Vacuum forming-bubble forming. (BX Plastics Ltd.)
16 Sheet Thermoforming and Related Techniques for
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or mould then enters this from above until it seals onto the sheet. The vacuum is then released on the box side and applied to the male plug side and the sheet 'snaps back' onto the mould.
16.3.2 Details of Methods One of the important practical points is to apply vacuum as rapidly as possible. If, however, a pump of sufficient self-capacity were used this would be an extremely costly operation. The standard method everywhere is to use an accumulator tank with a smaller pump than would otherwise be required, as the vacuum is required intermittently. In the case of some of the methods, particularly plug-assist forming, a slip ring is used for clamping. This enables the material to slip through the ring during the forming process, which has several advantages. It makes more material available, which prevents edge-tearing, buckling or undue thinning. When clamps are used there must be a good seal between the mould edge and the material. The drape and plug-assisted methods are ideally suitable for producing sections with sharply radiused corners and intricate designs. In the snap-back forming method the material has to be sucked down to a predetermined length which nowadays is controlled by a photoelectric eye. The various aspects of heat control are most important in vacuum-forming methods. Pre-heating may be effected by means of radiant heaters or convection ovens. The former are almost invariably used for thin sheets, particularly in continuous production, and a bank of heaters placed about 2 in from the sheet has been found to be very satisfactory. Convection ovens are usually used for thicker sheets in view of the poor thermal conductivity of rigid PVC. It has been generally established that the temperature at which the sheet should be heated for optimum results is that temperature which gives maximum elongation at break. Figure 16.5 illustrates the principle for a calendered high-impact PVC sheet. In this particular case the optimum temperature is about B2°C, to which temperature the sheet should therefore be uniformly heated. It may be noted that although the material does not have a peak the curve is fairly sharp, and some rigid vinyls give curves sharper than this. These would, therefore, rapidly deteriorate if heated more than a degree or two above the optimum elongation temperature. Once the material has been brought to the correct temperature it
752
L. W. Turner 700
.. o c
o
i.,. 400 c o
iii
20'Cl!:--=--~~-----:-'----""""o:---~""""-_--:-:! ioo /10 120 130 140 150
Temperature,OC Fig. 16.5 Variation of elongation at break with temperature.
should be formed as quickly as possible. A maximum forming time of 60 s should be imposed but it should preferably be below this, particularly with sheets 0·050 in. thick or less, when 30 s or shorter is preferable. Although moulds can be heated (and often are for some materials) it is not general practice with rigid PVC, the relatively low softening point being significant in this respect; whatever the choice it should be consistent. Another point of detail is a method of assisting the flow into the required positions. Interference bars or plates can be placed at strategic points to guide the material in the desired directions. Early descriptions of some methods of producing vacuum formings of uniform wall thickness have been published by Lee and Welham9 and Bretton and Welham. lo
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16.3.3 The Moulds The moulds are made from many types of materials; obviously the cheapest consistent with adequate strength and inertness for service is used. Any metal except copper or brass may be used and other materials include phenolic resins and laminates, laminated wood, hardwood, or even plaster for short runs. Light alloy castings are frequently used for long runs. As already explained, both male and female moulds are used, although only one or the other is normally employed for a given method. This keeps mould costs down to a minimum. The choice depends on the method and other factors. For example, deeper forming can be obtained better over a male than a female mould. However, a female mould is best used if a design is incorporated on the outside of the moulding, as this gives much sharper impressions. It is also used for low depth of draw and where a number of cavities need to be placed close together for large-scale production. On the other hand a male mould is used if internal dimensions are of importance. Some guidance can be given regarding the dimensions of the mould compared with the depth of draw. In the case of a male mould the height can be equal to the narrowest dimension and often greater. In the case of a female mould the depth of draw should not exceed half the minimum moulding width. Wooden moulds, etc., are made by machining. Other types, which are cast from metal for long production runs, are made by the use of a wooden pattern. Here shrinkage considerations are of the utmost importance; not only does the shrinkage of the metal used for the mould itself have to be taken into account but also the shrinkage of the formed article coming off the mould. Such moulds as these sustain no damage, except by carelessness, and they are likely to last for a long time. Those made from thermosetting plastics and the like are likely to sustain much more damage even with metal spraying, metal fillers or electroforming. All the moulds must be vented to apply vacuum at various points. The closer the detail, or wherever really close conformity to the mould is required, the greater the number of vents needed. At most the holes will not exceed 0·030 in. in diameter, although half the thickness of the material may sometimes be used as a guide. On the side away from the material, the holes can be opened much wider and tapered to give more rapid evacuation. Where moulds are made by any sort of casting
754
L. W. Turner
process the vent holes can be made by inserting wires into the wooden pattern. With large male moulds, particularly where large production runs are involved, it may be necessary to have some method of cooling. The mould is then cored for water circulation. Walls can never be exactly upright; all must have a draft with a minimum of 3°, a larger one giving a better product. The radii should be as large as possible, certainly at least the thickness of the material being moulded. As with many moulding processes undercuts should preferably be avoided. However, the comparatively simple moulds allow removable parts to be used so that undercuts can be included. Inserts can easily be incorporated in the mould surface and so can embossed lettering on the surface. In the case of large runs it is clearly preferable to trim the article in the mould to avoid a subsequent deflashing process. For this purpose a cut-off is required and the mould must be metal, preferably hardened at the cutting edges.
16.3.4 Finishing The finishing and decoration of vacuum-formed parts is frequently necessary. In particular, it is necessary to remove flash, which can be done by saw, trimmers, guillotine, or male and female dies in toggle or clicking presses. Precisely which method is chosen will depend on the size of the object and the rate of production. Apart from deflashing, the cutting of various shapes in the component is frequently necessary. It is advisable to employ .jigs for such purposes to guarantee accuracy and speed up production; low- or high-speed routers can be used for the cutting. Other finishing operations are often necessary. Various parts have often to be joined together; if high-frequency welding is not possible (see Chapter 20) then parts have to be joined by adhesives. These may simply consist of polymer solutions (in, say, cyclohexanone) or some of the many proprietary adhesives available today (see Chapter 20). Some impact adhesives may well be necessary if dissimilar materials are used. Another type of finishing operation is the folding of edges, e.g. in containers which have to receive a sliding top; for this purpose it is possible to obtain heated folding or bending machines. Decoration sometimes presents problems, but there is no problem in decorating the finished object by masking and spraying or vacuum-
16 Sheet Thermoforming and Related Techniques for
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metallising by conventional techniques. The problem arises when the sheets are decorated before forming. This means that inks and paints must be able to stretch, and stretch without much loss of colour. Far more important, however, is the problem of allowing for the distortion which takes place during forming. One way of overcoming this is to form an object in the usual way without decoration. It is then decorated or printed and heated, when, because of the residual strain or elastic memory of the material, it returns to its flat state. This gives a pattern of the distorted decoration which can be converted into the printing block.
16.4 MATCHED·MOULD AND RELATED METHODS An early review of these methods has been made by Mottram and LeverY It is, of course, possible to use female and matching male dies employing either conventional pressures or relatively low pressures of say 200-100Ibfin- 2 . The actual pressure will depend on the flow characteristics and thickness of the sheet. In some instances it is possible to use a female die only. Shallow mouldings can be made in this manner with thin sheet, examples being 0·030-in. thick material and maximum draws of about 0·250 in. Thicker sheets could be used, but this would reduce the possible deformation. A good example of the possibilities here is the production of printing plates, illustrating that extremely accurate reproductions can be made. The pressures used are only 100 Ibfin- 2 and the opposite side of the material is backed with a rubber blanket. A considerable number of small containers are made from rigid PVC by the use of matched metal dies. Such methods are usually continuous; the stock of sheet being fed from a roll, heated continuously by radiant heat, stamped and removed by means of a take-up roll. Because the dies are continually subjected to heat it is necessary to core them for water cooling. In the design of the dies for this purpose the corners and edges should be rounded if possible to assist flow. The clearances between the surfaces of the male and female dies should be at least equal to the thickness of the material, as this avoids undue wear. Finally, it is preferable to introduce cut-offs at the edges of the die to eliminate the necessity for a finishing operation. It is necessary to employ clamping frames, particularly with deep drawn objects. If the depth of draw is not great. then the edges of the
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L. W. Turner
material may be clamped at once. If the draw is deep, some of the drawing must be allowed to occur before the edges are clamped. If this is not done, excessive thinning, undue strain or even tearing may occur. With both vacuum- and pressure-forming attention to detail is of the utmost importance. Uniformity of heating is essential and the material must reach the correct temperature just as it is removed from the oven for pressing. The forming stroke should be quite rapid, but controlled, and at the bottom of its stroke it should stay there long enough to cool the material below its heat distortion point, usually a few seconds only. Conditions will have to be varied according to the material and thickness, and should be established at the start of a production run. If the sheet used is too hot it will cause loss of embossing on an embossed sheet; if it is too cold or the clamping is insufficient then wrinkled mouldings will result. Even the cut-off must not take place too soon, since, if there is insufficient cooling, warped mouldings will result, or alternatively the edges will pull away. The cutting edges must also be kept in good condition or rough trimming will result. The dies must be properly aligned and be close together, otherwise air trapping may occur with consequent shiny spots which look most unpleasant. It is quite clear from all these methods that great care must be exercised in establishing correct details of production. It is for this reason that fully automatic production is so valuable, since conditions can be set and maintained. 16.5 TOLERANCES IN DIMENSION AND DIMENSIONAL STABILITY OF FORMED PARTS It has already been indicated that if a thermoformed article is heated it
tends to revert to a flat sheet. This is not important for many applications, where the components produced are unlikely to be subjected to elevated temperatures. However, there are certain applications where the problem can be serious. Some of these are car parts, such as fascia panels, which are liable to be affected by strong sunlight, illuminated displays and lighting fittings, etc. A careful study of the subject is therefore required. It has been established by experiment that the two factors which most closely control the degree of distortion are the softening point of the material used and the material temperature during the forming
16 Sheet Thermoforming and Related Techniques for
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process. The amount of distortion which occurs (during a test, say, of 2 h at 70°C) depends on the amount of distortion put into the component in the first place. Thus shallow-formed articles will distort much less than deep-drawn ones. Having established the two most important factors influencing distortion the next problem is what to do about it. There is no complete answer but certain precautions can be taken. First, the higher the softening point of the material used, the less the distortion; this is a very important factor. Thus in choosing a vinyl compound, the one with the highest softening point must be used. The second important point is to use as high a forming temperature as possible, for as long as possible. This allows more random movement of molecules, i.e. relaxation to reduce excessive orientation. Unfortunately there are several limits to the temperature which can be used, not least of which is rate of production which has to be extended when temperatures are high. High temperatures will destroy surface finish and even partially destroy embossing, although in some cases embossed sheet that can resist forming temperatures can be supplied. The optimum temperature from the forming point of view is that at which the elongation at break reaches a maximum. Obviously the forming temperature cannot be allowed to exceed this. 16.6 EQUIPMENT SUPPLIERS A number of thermoforming machines are made3,12-many automatic-as well as some completely integrated, computerised thermoforming lines. * A list of some suppliers is given in Table 16.1. 16.7 MATERIALS ASSESSMENT AND DESIGN ASPECTS The term 'optimum temperature' was used earlier when the processes of sheet thermoforming were being introduced and commented upon. This relates to the fact that in uniaxial creep-type tests carried through to rupture and in similar tests carried out in a biaxial-stretching mode at different temperatures, the materials under discussion show
* e.g. the Shelley 'Linear Series' (M. L. Shelley and Partners Ltd, Huntingdon in the UK, and Deacon North America, Bristol, cr in the USA).
TABLE 16.1
Some British and European Manufacturers of Sheet Thermoforming Equipment Anchor Plastics Machinery, Weirvale Industrial Estate, Denham Way, Maple Cross, Herts, UK
Stewart Machinery Sales, The Old George, Lavendon, Olney, Bucks, UK
Leesona Plastics Machinery, Falkland Close, Coventry, CV48AU, UK
Engineering Developments (Farnborough), Belmore Road, Farnborough, Hants, GUI47NW, UK
Ridat Engineering, Fishponds Road, Wokingham, Berks, UK
Mortimer Plastics Machinery, Coronation Road, London, NWlO 7PT, UK
Ataroth Plastics Machinery, Unit A3, Wem, Salop, UK
Technoimpex, Hungarian Foreign Machine Ind. Trade Co., PO Box 183, Dorottya u. 6 H, 1390 Budapest 62, Hungary
Maschinenbau Gabler, Niels-Bohr-Ring 5a, PO Box 1690, D-2400 Lubeck 1, West Germany
Hanwood Engineering Services, Unit B3, Stafford Park 2, Telford, Salop, UK
M. L. Shelley and Partners, St. Peters Road, Huntingdon, Cambridgeshire, UK
OMV Officine Meccaniche, Veronesi Spa, Lungadige Attiraglio 34, Parona, Verona, Italy
Bone Craven Daniels, Bath Road, Stroud, Gloucestershire, GL5 3TL, UK
VTM, Ing Rudolf Wybranietz, 435 Recklinghausen, Hubertusstrasse 41, Postfach 1268, West Germany
Meaf, Groeninx van Zoelenstraat 33 (industrieterrein) , Yerseke 3622, Netherlands
16 Sheet Thermoforming and Related Techniques for pvc
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maximum elongation (before rupture) within particular 'optimum' temperature regions depending on the material formulation. A broader peak is preferable to a narrow one as the latter obviously allows less processing tolerance. Various relatively simple techniques can be used to compare materials and to a degree assess forming characteristics. In Ref. 13 rapid creep tests are described comparing the forming behaviour of two PVC sheet formulations. The tests consist of applying a predetermined dead load to strips of sheet material at different temperatures, the load being such that extension to high strains in the range 4-5 or up to break-point can be achieved in about 1·4 s. As described the creep curve is observed optically, but simpler means of recording deformation can be employed to provide a useful evaluatory procedure. In such a test a peak elongation at a certain temperature can be observed (Fig. 16.6). The manner in which the data curve falls away above and below this peak (operating region) gives useful guidance as to the behaviour of the test material. The peak position on the temperature scale, its sharpness and the shape of the overall curve can be correlated with actual forming experiments. A similar rupture test has been described,14 demonstrating the effect of temperature on strain development. In this it is shown that rapid uniaxial creep can be factorised into stress and time factors, and a fracture envelope can be readily obtained. Correlation between such creep data and actual thermoforming can usefully be made as in Refs 13 and 15. For this purpose the draw ratio becomes the significant process variable to observe at various temperatures to correlate with laboratory creep data. 16.7.1 Effect on Quality of Draw Ratio and Temperature
While it can be seen that acceptable forming can be conducted over a reasonable range of temperatures, other factors can obtrude (process rate, energy consumed, handling difficulties, discoloration and embritdement) so that, in practice, a processor would aim to conduct his operation close to the shaded area in Fig. 16.7 which typifies the product quality/forming temperature relationship. Overall the most significant comparison between materials illustrated in this sketch is the heavy-line boundary between good and bad forming; this can closely be observed from uniaxial creep data, with due regard to the differences between stretching one way and two.
L. W. Turner
760
optimum Strain l~v~1
c .~
[Crl'Kld
Ar~a of ruptur~
tim~
iii Incr~sing
straining load
rat~ und~r fix~d T~mperatur~
Fig. 16.6 Effect of temperature on rapid strain (based on Ref. 13).
Biaxial creep data, such as might be obtained by blowing up a clamped disc of sheet material, can be employed for similar correlations, but uniaxial tests have the advantage of being the simplest conceivable test pattern. It must be added that care should be taken if the intention is to use creep data to indicate the effect of the pressure employed in forming.
(Cold) rupture
Holes
o
~
~ o
Discoloration degradation Details not reproduced
Temperature
Fig. 16.7 Quality aspects of correlation between draw ratio and temperature (based on Ref. 15).
16 Sheet Thermoforming and Related Techniques for
pvc
761
16.7.2 Thermoformability of CPVC
The extensibility, uniaxial and biaxial, of chlorinated PVC (a simple composition without polymeric modifiers) is good over a relatively wide range of temperatures above the Tg • This behaviour, and the associated improvement in thermoformability over uPVC, must be largely due to the reduction of inter-chain attraction brought about by the presence of additional chlorine atoms in CPVC (ct. Chapter 1, Section 1.6). These points are supported by the results of an experimental study by De Vries and Bonnebat,16 which also demonstrated the molecular orientation imparted by stretching, and the considerable improvements in some properties, notably impact resistance and barrier effects (i.e. permeability reduction), in biaxially stretched CPVC sheet. Note: Similar effects of biaxial orientation have been reported by Brady17 for uPVC, where biaxial stretching under optimum conditions to about 2 x 2 final extension could substantially improve the impact resistance in the absence of polymeric modifiers.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Kulkarni, K. M. (1979). Polym. Engng. Sci., 19(7), 474--81. 'Topformer', Literature from Bone Craven Daniels, Stroud, Glos, UK. Anon. (1979). Plast. Rubb. Wkly, 23rd November, pp. 17-26. Titow, W. V. and Turner, L. W. (1966). 21st ANTEC SPE Proceedings, 'Plastics in Europe', Paper No.2, pp. 1-5. Titow, W. V. (1966). Plast. Technol., U(8), 3~0. Anon. (1979). Mod. Plast. Int., 9(1), 35. O'Neill, W. A. (1980). Plast. Rubb. Int., 5(5), 185-7. Lubin, G., Mele, J. J. and Sheehan, M. J. (1978). 36th ANTEC SPE Proceedings, pp. 50-6. Lee, D. J. A. and Welham, F. A. (1959). Brit. Plast., 32(6), 265. Bretton, R. H. and Welham, F. A. (1961). Ibid, 34(5), 244. Mottram, S. and Lever, D. A. (1957). The Industrial Chemist, May. Anon. (1981). Eur. Plast. News, 8(9), 66--74. Harris, B. L. and Bruins, P. F. (1973). In Basic Principles of Thermoforming, (Ed. P. F. Bruins), Gordon and Breach, New York, p. 81. Turner, L. W. (1958). Society of Chemical Industry, Monograph No. 17.
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15. Malpass, V. E. and White, C. H. (1973). In Basic Principles of Thermoforming, (Ed. P. F. Bruins), Gordon and Breach, New York, p. 103. 16. De Vries, A. J. and Bonnebat, C. (1976). Polym. Engng Sci., 16(2), 93-100. 17. Brady, T. E. (1976). Ibid, 16(9),638-44.
CHAPTER 17
Blow Moulding of pvc W. V.
TITOW
17.1 BASIC FEATURES AND HISTORICAL DEVELOPMENT OF BLOW MOULDING
The general principle of blow moulding may be stated as follows: a gas under pressure is introduced into the interior of an envelope of heat-softened material, which is thereby expanded into conformity with the cavity of a containing mould, to form a hollow article when solidified on cooling. In the blow moulding of plastics the envelope is normally tubular. It is self-evident that to be useful for blow moulding, a material must-when suitably softened by heating-have the right rheological properties and sufficient cohesion at the processing temperature. It is equally obvious that if the moulding is to be fit for use the material must also possess the appropriate service properties. Other characteristics are also needed, e.g. sufficient thermal stability under processing conditions. The thermoplastics first available on a large scale (cellulose acetate, polystyrene) did not adequately meet the combination of requirements relevant in the production and use of blow moulded containers (a natural application for blow mouldings, first embodied in glass bottles). Moreover, the earliest attempts at blow moulding these materials, which were closely modelled on the blowing of glass bottles (from a 'gob' of hot melt), were not very successful: as frequently happens, direct transplantation of a technique from one technology into another did not provide the best route. A combination of two factors ultimately made possible, and initiated, the development of blow moulding of plastics and its rapid growth. The factors were: 763
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(i) the advent of polyethylene-a thermoplastic with processing characteristics well suited to blow moulding, and end-use properties acceptable in containers for many purposes; (ii) the introduction of the extrusion-blow method, pioneered after the Second World War by the Plax Corporation and others in the USA, l and by Kautex in Europe. * Blow-moulded uPVC bottles (produced by the extrusion-based process) first made their commercial-scale appearance around 1960. In the UK, for example, a prestigious chain store began to market fruit squashes in such containers in 1962. 2 As practised today, all the numerous variants of blow moulding include the following essential elements: The production of a tube of the thermoplastic material used. If made by extrusion, the tube (which is initially open-ended) is called a parison. If injection or dip moulded (in which case it is always produced with one end closed) the tube is correctly termed a preform. (ii) Expansion of the hot tube (at this stage always sealed at one end; sometimes at both ends) with compressed air inside a mould. (iii) Cooling the article so formed and removal from the mould. (i)
Individual processes differ in the ways in which these basic operations are performed, and in the number and nature of associated process steps (see Section 17.2). Some methods in which no parison or preform is used were explored early in the development of blow moulding, but none has attained industrial importance. In one example,3 two hot thermoplastic sheets are clamped in a mould and thereby edge-sealed in the outline of the shape required in the ultimate moulding. The envelope so formed is expanded in the mould with compressed air, and the resulting moulding cooled in the mould, removed and trimmed. uPVC is one of the group of plastics important as blow-moulding materials. The other members of this group are polyethylene * Until the appearance of Kautex machines in the late 1950s blow-moulding equipment in industrial use was custom-made for patented versions of the process worked by the patent holders and their licensees. For example, in the UK the patented Plax process (British Patent No. 516262) was for many years operated, under an exclusive licence, by British Xylonite.
17 Blow Moulding of pvc
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(especially the high density polymer), polyethylene terephthalate, certain modified acrylonitrile polymers, polypropylene, and polycarbonate. Some of these-notably HDPE and PET-eompete with uPVC in many applications (see Section 17.4.1). A few examples of their non-competitive uses are: HDPE petrol tanks for certain makes of motor car, large cans and drums; PP header tanks for truck radiators; PC baby-feed bottles. Blow-moulded containers with composite walls made up of layers of different materials are produced from coextruded parisons or-more recently-eo-injection-moulded preforms. This is done to secure sets of properties not obtainable with a single material: often the combination of properties achieved in this way is low permeability with good mechanical strength and stiffness. Typical examples of such composites are HDPE with polyvinyl alcohol or with an acrylonitrile polymer (in both cases an intermediate adhesive layer is normally also present). PVC is not used as a component of such products.
17.2 BLOW-MOULDING PROCESSES AND THEIR APPLICATION TO PVC 17.2.1 General Characterisation and Main Features of Process and Systems A blow-moulding process can be characterised by reference to three features: the method of production of the first tubular precursor of the ultimate moulding, the presence or absence of a stretching step in the operation(s) whereby the tube is transformed into the moulding, and the nature of the processing and equipment arrangements. A broad classification on the basis of the first two features may be illustrated by the simple schematic flow diagrams of Fig. 17.1. It is this kind of classification that is implicit in such widely used terms as 'extrusion blow moulding', 'injection stretch-blow moulding', 'dip blow moulding', and the like. The processing and equipment arrangements comprise a number of elements which may be combined in various ways in a particular blow-moulding set-up. The nature of these elements and the way in which they are combined may collectively be regarded as the third main distinguishing feature of a blow-moulding process (cf. (c) below).
PLASTICS
extrusion
Fig. 17.1
FEEDSTOCK
PLASTICS
FEEDSTOCK
PLASTICS
by piston push on melt
in cavity: ) complete silJlpmg of pretonn round mandrel
dip mandrel into melt
mould, stretch and blow, 0001
)
MOULDED ARTICLE
PREFORM (CLOSE-ENDED TUBEWrrn FULLY FORMED NECK)
MOULDED (ORIENTED) ARTICLE
blow,oool
transfer to second (article) mould,
transfer to second (article)
HOT MELT IN CAVrrY
I
PREFORM (CLOSE-ENDED TUBE WITH FULLY FORMED NECK)
MOULDED ARTICLE
(article) mould,) blOW, 0001
transfer to second
MOULDED ARTICLE
PREFORM transfer to second (CLOSE-ENDED (article) mould, ) MOULDED TUBE WITH (ORIENTED) stretch and blow, FULLY FORMED ARTICLE 0001 NECK)
enclose in mould. blow, 0001
enclose in first (pretonn) mould, blow (possibly 0001)
OPEN-ENDED TUBE (PARISON)
DIP (DISPLACEMEN1) BLOW MOULDING
INJECTION STRETCH-BLOW MOULDING
INJECTION BLOW MOULDING
EXTRUSION STRETCH-BLOW MOULDING
EXTRUSION BLOW MOULDING
Characteristic operational elements in the main types of blow moulding process. Schematic outline.
or injection
~
moulding
injection
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17 Blow Moulding of PVC
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(a) Main Characteristics of Extrusion, Injection, and Dip Blow Moulding EXTRUSION BLOW MOULDING
The typical sequence of operations in simple extrusion blow moulding of a bottle is shown in Fig. 17.2A. As can be seen, a tube is extruded, and immediately enclosed-while still hot-in a split mould. In the action of closing upon it, the mould seals one end of the tube, leaving outside a 'tail' of waste material (and in some cases also pinched-off pieces or flash in other areas). Note: In the arrangement shown in Fig. 17.2A the tube is closed at the bottom, the neck being formed at the top, and the bottle subsequently blown 'right-side-up'. The alternative arrangement is to have the bottom end of the parison tube descending over a blowing mandrel: the tube is then closed at the top and the bottle blown in the 'upside-down' position. See also Section (c) below.
The parison tube is severed (before, during, or after the closing of the mould) and expanded inside the mould by air pressure; the resulting moulding is cooled in the mould and ejected. In some versions of the process the flash (always formed in extrusion blow moulding) is trimmed off in the mould; in others trimming is a separate, post-moulding operation. In some cases such operations as printing or labelling, filling, and closure application, are carried out in-line. In a typical extrusion stretch-blow moulding process, schematically illustrated in Figs 17.2B and 17.3, the extruded parison is first blow-moulded in one mould into a complete preform (undersize in relation to the ultimate moulding), and this is then stretched and blown to final shape in a second mould. The stretching (longitudinal extension) of the preform is commonly effected mechanically, by an extending telescopic pin, and the radial extension by the final blow (cf. Fig. 17.3). The object of stretch-blowing is to produce biaxial molecular orientation in the moulding-the effects and advantages of this are discussed in Section (b) below. For the best results the orientation should be imparted at a temperature somewhat below the extrusion temperature. In most industrial equipment provision is made for temperature conditioning of the preform during and/or after its cooling in the course of formation in the pre-blowing mould: the
W. V. Titow
768 A - Feed·hopper B - Extruder C - ElI1ru ,on head
D Mould buring d'stributor E - Blower head.
Stege I - EXTRUSION The parilon is ell1ruded between two half·mouldl.
Stage 3 • BLOW·MOULDING The blown perison upands end Ilanans out on tha mould ~vity wall•.
Stege 2 • CLOSING·IN The parison Is clamped into the mould.
Stege 4 • EJECTION Once cooled. tha bonia i. immediately ajacted from the mould.
Fig. 17.2A Extrusion blow moulding of PVC: schematic representation. Direct blow moulding: a machine, and operational sequence. (SideI equipment c. 1970: Courtesy of Sidel through their UK agents Engelmann and Buckham Ltd.)
17 Blow Moulding of pvc
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Fig. 17.2B A modern Sidel continuous-extrusion machine for stretch-blow moulding of bottles. 1, Parison; 2, preform mould; 3, temperature conditioning of preforms; 4, conveyor; 5, bottle mould; 6, demoulding.
conditioning may take place at a separate station (as, for example, on the Sidel MSF machines 4). Extrusion blow moulding was chronologically the first to come into industrial use: stretch-blowing is a relatively recent development in this method as well as in injection blow moulding. Apart from this refinement, which is now available as an integral part of most leading manufacturers' equipment (in some cases also as a retrofit modification-e.g. for the older versions of Kautex KEB machines 4 ), a modern extrusion blow-moulding set-up usually includes parison programming. This term is widely used for programmable adjustment of the wall thickness of a parison in a number of zones along its length. However, in its broadest sense, the expression may also be considered to include certain other ways of parison modification and control now available. In variable-thickness dies, both the central mandrel ('pin') and the die ring interior are tapered, so that a relatively slight vertical movement of the former (actuated by a suitable control mechanism) alters significantly the annular gap between them at the die face. The number of points at which parison thickness can be varied depends on the particular programming system used. Many are available, from various manufacturers: some of these systems are incorporated, as standard components, in particular makes of blow-moulding equipment. The
W. V. Titow
770
1
2
5
3
6
Fig. 17.3 Operational sequence in a typical extrusion stretch-blow moulding process. Schematic representation. 1, Extrusion of parison; 2, blow moulding of preform; 3, transfer of preform to article mould; 4, mechanical stretching (by extensible pin) and blowing of preform; 5, completion of formation of article; 6, removal of article from mould.
17 Blow Moulding of pvc
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systems range from relatively inexpensive, simple thickness programmers offering control at a limited number of points, adequate for long-run production of simple blow mouldings (e.g. the Moog* system for to-point thickness programming), to sophisticated computerised, fully automatic systems for control at up to 50 points (e.g. the 49-point parison programming facility of the MACa Vlt integrated microprocessor control system for extrusion blow-moulding equipment5). Other examples are the 30-point parison programmer used on some Bekum+ machines,6 and the Hunkar§ programmers (some with 32 set points and higher). In the modern thickness programming systems operated by microprocessors, the required thickness values at the number of points available are pre-set on the control unit, which then continuously automatically computes and executes the necessary adjustments of the movable die elements. Programming capabilities other than parison thickness control offered by some of the modern systems include parison length control, parison stretch compensation, parison ovalisation, and what is sometimes called 'deformable die control'-provision for programming (through automatic control of mutual movement of the die mandrel and bushing) for minimising the amount of material in the pinch-off area(s) of the moulding. It is largely self-evident that the main direct object of parison programming is optimisation of the wall thickness and thickness distribution in the finished moulding. The advantages this brings are two-fold: better structure (with the attendant improvement in strength properties and resistance to damage), and cost savings. The savings are in two areas-reduction of the amount of material, and more effective processing (especially easier and quicker cooling of mouldings which contain no unnecessarily thick areas, and generally only the necessary minimum amount of material). In an informative article (published in 1976), Hunkar7 gives, inter alia, the following figures (based on p~rformance studies of over 800 blow-moulding machines) for the economic gains resulting from parison thickness programming in the * Moog Inc., Electronics and Systems Div., USA, and Moog GmbH, West Germany: the group also supplies more advanced systems with full microprocessor control, e.g. the Moog 40-point analog parison programmer. t Barber-Colman Co. Industrial Instruments Div., Rockford, Ill., USA. :\: Bekum Maschinenfabriken GmbH, Berlin, West Germany. § Hunkar Laboratories Inc., Cincinnati, OH, USA.
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W. V. Titow
production of PVC bottles: Nominal size Average weight (g) Material savings (%) Production increase (%)
1 quart 54·6 15·8 13·3
1 litre 43·5 11·5
9·6
16-220z 22·3 19·3 29·8
Another source! quotes estimated savings of 20-30% in part weight and cycle time with electronic parison-programming systems. The above advantages of parison programming must be considered in relation to the greater equipment cost and die complexity (undesirable in principle with a heat-sensitive material like PVC). Thus, whilst parison programmers are operated, and useful, with PVC, possibilities sometimes exist-and should not be overlooked-of saving material and securing good thickness distribution simply by suitable design of the moulding. INJECTION BLOW MOULDING
In this type of process a preform is injection-moulded around a metal mandrel (core rod) in a closed mould. It is then transferred to a second mould where it is blown to final shape, cooled and removed. In some versions of the process the sequence is continuous, the temperature of the preform being maintained for the blowing step which follows directly after moulding. In certain others the preforms are cooled, and may be stored, to be reheated and blown in a separate operation. In either of these two general variants of the process the hot preform may be stretched in the blowing mould to produce orientation in the ultimate article in essentially the same way as in extrusion stretch-blow moulding. In comparison with extrusion blow moulding, the advantages of the injection-based process may be summarised as follows. Exact, pre-designed shaping of the preform with the attendant improvement in product quality and material and processing cost savings, analogous to the advantages of accurate parison programming in extrusion blow moulding; lack of waste; virtually constant weight of mouldings (but less easy to alter than in the extrusion-based process); a moulded neck finish in bottles, giving accurate dimensions (and particularly useful for crown closures); better quality finish, with no nip closure line (pinch weld). The main disadvantages are: ratio of neck diameter of bottles to the length limited in practice to about 1 : 12 (due to core rod deflection
17 Blow Moulding of pvc
773
problems); containers with handles not readily produced; highly oval cross-sectional shapes not as successful as with extrusion blow moulding; normally higher tooling costs. Injection blow moulding originated roughly at the same time as extrusion blow moulding, and some containers were being made-in polystyrene and polyethylene-by early versions of this process (involving manual transfer of a single preform on its core from the injection to the blowing mould) from about 1948 onwards. 8 However, large-scale production of injection blow-moulded PVC containers came considerably later than industrial extrusion blow moulding of this material, largely because of heat stability problems. DIP BLOW MOULDING
Commercial equipment for this process (also called 'displacement blow moulding') first appeared in the 1970s. The operational features of the available machines differ in some respects, but the general essential elements of the method are common to all. These may be outlined as follows. A predetermined quantity of hot plastics melt is introduced into a heated cavity, kept at a controlled, optimum temperature. This may be done by horizontal extrusion (as in the original SchloemannSiemag equipment9 ), or vertical (downward) extrusion or injection (as in the early Saum Systems design lO and its adapted version represented by the Leesona displacement injection blow-moulding machine ll ), but in all cases the cavity is not closed, so that the melt enters at relatively low pressure. A metal mandrel (core rod), kept at the required temperature by circulating heat-exchange liquid, is pushed---eentrally-into the melt in the cavity: this is done either by lowering the mandrel or by raising the cavity. As it moves into the cavity, the mandrel displaces the melt upwards, shaping it into an annulus defined by the mandrel body and the cavity walls. The annulus being moulded in this way is the body of the ultimate preform. The shaping of the latter is finally completed-when full mandrel penetration has been reached-by the upward movement of a piston positioned at the bottom of the cavity, which ensures that the melt fills all available space, thereby positively moulding the top (neck portion) of the preform (inside a split mould, sometimes referred to as 'thread mould', enclosing the top of the mandrel) and finalising the material distribution. The mandrel carrying the preform just produced is withdrawn (or the cavity lowered away from it), and then enclosed in the article
774
W. V. Titow
(blowing) mould. Air at normal blowing pressure is admitted through a channel in the mandrel, blow-moulding the body of the preform to the shape of the cavity. The resulting article is cooled and ejected when the blowing mould and the neck mould round the top of the mandrel have opened. The advantages claimed for dip blow moulding are: moderate cost of machine and moulds (because they do not have to withstand high pressures during the moulding of the preform); precision moulding of the neck; no nip closure or gate marks on the article (which are characteristic of, respectively, extrusion and injection blow-moulded containers); virtual absence of scrap; ease of production of wide necks in containers; close control over wall thickness by virtue of the method of producing the preform and the fact that the preform diameter can be close to that of the ultimate moulding. (b) The Role and Effects of Stretching in Stretch-blow Moulding The combination of longitudinal stretching with transverse expansion effected in stretch-blow moulding imparts to the product a considerable degree of biaxial orientation. This improves several of the properties particularly important in containers, which constitute the vast majority of commercially produced PVC blow mouldings. The properties concerned are: impact resistance (as measured in drop impact tests, this property can be better by factors of 2-4 in stretch-blow-moulded PVC bottles as compared with similar bottles produced by simple blowing); bursting pressure; resistance to compression and to deformation by top loading (important in stacking); rigidity; clarity of transparent containers; and permeability (which is reduced by biaxial orientation). Some properties of biaxially oriented PVC are discussed, in relation to those of CPVC, in a paper by De Vries and Bonnebat. 12 Brady's study of the effect of biaxial stretching upon the mechanical properties of uPVC13 (O·015-in thick calendered sheet) indicated, inter alia, that optimum property improvements are obtainable at a stretch factor of x2 in each direction, and that lOO°C was the optimum stretching temperature. The improvements in properties obtained by stretch-blowing make possible significant savings in the amount of material used (because container wall thickness can be reduced without sacrificing performance) and in processing costs (mainly because thinner walls make for faster cooling). Furthermore, the material cost can be lowered in many cases by 'reducing or eliminating impact modifiers. Savings-in
17 Blow Moulding of pvc
775
comparison with otherwise similar but unstretched blow mouldingsquoted as typical include, for example, weight reduction of 20-25% on 1·5litre mineral water bottles produced in rigid PVC on a Bekum BMO-4D machine,14 an average material cost saving of about 10% with typical bottle formulations through elimination of impact modifiers,15 and possible overall production cost savings of up to 20%, despite the somewhat higher equipment cost of stretch-blow moulding. 16 At the same time it should not be overlooked that stretch-blow moulding is applicable primarily to simple, symmetrical shapes (e.g. not to containers with handles), and that stretch-blown PVC bottles can still be no cheaper than bottles conventionally blown in, say, high-density polyethylene which-despite their inferiority in some properties-may be acceptable for many applications where clarity is not a requirement.
(c) Processing and Equipment Arrangements These arrangements comprise a number of elements which may be combined in various ways in a particular blow-moulding set-up. The elements and their particular combination collectively constitute another general distinguishing feature of a blow-moulding process. SINGLE- AND TWO-STAGE OPERATIONS
A blow-moulding process-whether based on extrusion or Injection moulding-may be of the single-stage or two-stage type. That is, the complete production may be carried out in the same machine, or on two separate machines: in the latter case it is normally the blowing (of a separately made parison or preform) that constitutes the second stage. Industrial dip-moulding processes are normally all single-stage. The version of simple extrusion blow moulding where the parison is blown directly under the extruder head is an example of a single-stage, single-station process. It may be noted that in this kind of operation extrusion must be intermittent, to allow for completion of blowing, cooling and ejection of the moulding, before the mould receives the next parison. In the widely used multi-station extrusion blow-moulding systems, a mould receives the parison (which is severed at the appropriate point in the sequence) and is moved out of the way while another is presented to the extruder die. Here extrusion can be continuous, as parison blowing and subsequent cooling of the resulting moulding in a
776
w.
V. Titow
direct blowing process, or-in a stretch-blowing process-the blowing of a preform, its transfer to a blowing mould, and the stretch-blowing, cooling and ejection of the final article, take place at stations away from the extruder head. Continuous extrusion systems are preferable for PVC (and other heat-sensitive materials) because they do not entail stationary residence of the hot melt in the extruder, whilst optimum melt temperature and uniform temperature distribution are easier to maintain under constant, controlled flow conditions, so that the risk of overheating is reduced. The simplest multi-station system consists of two reciprocating moulds which are presented alternately to the extruder die. More complex systems employ a number of moulds mounted on a rotary support which may revolve in a horizontal or vertical plane. Blow moulding on a multi-station system (even one with many stations and process steps) will still be a single-stage process if complete production takes place on the same machine, in a continuous sequence of operations. If, however, the sequence is interrupted and the moulding finished on separate equipment, then the process becomes a two-stage one. Note: In the context of blow-moulding processes, the terms 'step' and 'stage' are sometimes used as if they were synonymous. However, the usage adopted in this section is the most common; it is also logical and avoids ambiguity if consistently followed.
Examples of a two-stage process are the Marrick process and the Corpoplast process. The former, developed in the 1960s,17 is now mainly of historical interest. In its first stage PVC tubes were extruded, cooled, and cut into parison lengths. The prefabricated parisons could then be stored and transported to different locations as required. For blow moulding, which took place on separate, special equipment, the parisons were heated (on mandrels) to the required processing temperature. The heating was carried out in ovens: as can be seen, very close temperature control was essential for successful operation, as well as accurate allowance for dimensional changes on heating. The plant and 'know-how' for this stage of the process were provided, under licence, by the Marrick Manufacturing Company Ltd (UK), from whom parison tubes were also available. The advantages claimed for the process included a low scrap percentage, and a potentially high output (for the 1960s) made possible by the use of multi-cavity blowing units: in a single-stage process these would have to be fed by
17 Blow Moulding of pvc
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multi-head extrusion with its attendant increased risk of heat degradation of the material. Thus the heat stability problem was eased whilst the production rate at the moulding stage was not limited by the rate of extrusion. However, the Marrick process, apart from lacking the optimisation of wall thickness and economy of material use available through parison programming in modern extrusion blowmoulding systems, was also generally less competitive in present-day terms. None the less, it had played its part in the industrial production of PVC bottles. It has also paved the way for the Corpoplast process in which similar ideas have been given a sophisticated, modern format. The Corpoplast system for the production of stretch-blown bottles in PVC, polyacrylonitrile, and polyethylene terephthalate, originated in West Germany. * In the first stage, preforms are produced either by injection moulding, or from lengths of extruded tube (by heating the ends and moulding a neck on one side and a rounded, closed bottom on the other: 18 machines are available for either kind of operation. The preforms are cooled for storage or transport. In the second stage each preform is heated (on a mandrel telescopically extensible and channelled for subsequent stretching and blowing operations) by IR radiation. The heaters are specially designed for quick, effective heating of the body of the preform to the optimum temperature for orientation by stretch-blowing: one of the operational features is the matching of the peak emission wavelengths to those at which the plastics material absorbs most strongly.t The heating is uniform, as it does not depend on conduction through the material. During the heating step the pre-moulded neck is screened from the radiation. The heated preform is stretch-blown in the way already mentioned. Preform production and blowing may also be run in direct sequence. The advantages claimed for the Corpoplast process are: high output rates; versatility of equipment, permitting different arrangements with good control over process variables; relative ease of setting up and running bottle production at the filling plant (with preforms produced at a different site transported more conveniently and cheaply than
* Initially marketed by Gildermeister Corpoplast GmbH: now KruppCorpoplast Maschinenbau GmbH, Hamburg. UK agent: Ritter Plastics Machinery, Bracknell, Berks. In the USA exclusive rights held by Owens Illinois, Toledo, Ohio. t An early discussion of this way of increasing the efficiency of radiant heating of polymeric materials was published by Grant and Foster. 19
778
w. v.
Titow
finished bottles); waste re-processing minimised (with injectionmoulded preforms); advantages associated with stretch-blowing (improved container properties, material and process economies). However, the costs of equipment and tooling are relatively high, and licence fees and royalties are a factor. Apart from the version of the Corpoplast process in which the preforms are injection-moulded, injection blow moulding as commonly practised is a single-stage operation. It must also, of necessity, involve at least two stations, because the preform has to be transferred to another mould for blowing. Indeed, the first machine designs to come into industrial use operated on a two-station system. 1 Currently the most popular configuration is a rotary three- or four-station arrangement: in a typical version, core rods are mounted on a turret which indexes to the stations in turn. With a three-station turret these will be the injection station, the blowing station, and the ejection station. A four-station system may be set up to operate in various ways; some of the common ones are given below. Station 4 Station 1 Station 2 Station 3 Injection ~ Pre-blowing ~ Final blowing ------+) Stripping off the core and ejection Injection ~ Temperature----+ Stretch-blowing ----~ Stripping off the conditioning * core and ejection ) Surface decoration Stripping off the Injection ~ Blowing· (e.g. printing) core and ejection
Dip blow-moulding equipment currently available is single-stage and typically one- or two-station,1° although the process involves several steps (see (a) above). BLOWING ARRANGEMENTS
In extrusion blow moulding of containers, such as bottles, the parison is most often blown by means of a blowing mandrel (blowing 'pin') through the neck opening, with the bottle formed either in the upright position ('top blow') or upside-down in the mould ('bottom blow'). A
* This can be useful with PVC (ct. Fig. 17.2B), but not absolutely necessary because PVC can be cooled directly to optimum orientation temperature, unlike PET which needs a separate 'tempering' treatment of the preform (normally at a separate station) for greatest property improvement on stretching, or PP which must be conditioned for stretching within a narrow temperature range.
17 Blow Moulding of pvc
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hollow needle may also be used, being introduced-at any convenient point-into a completely sealed parison through a suitable opening in the mould. Needle blowing is useful with some container designs, e.g. ones with neck openings too small to admit a regular blowing pin, or where a portion is trepanned out after moulding (e.g. a domesometimes called a 'lost blowhead'-moulded, for operational convenience, over a very wide opening) in which case the blowing needle is introduced through that portion. As with bottom blow, needle blowing can start as the mould closes. The bottom-blow arrangement is the simplest mechanically. The open lower end of the parison descends over a blowing pin (often called a spigot in this context). The mould closes around the parison, its neck part (at the bottom) clamping on the lowest portion of the tube which encloses the pin, and the top pinch-sealing the opposite end. Blowing is commenced immediately, sometimes even before the mould has completely closed (this can counteract any tendency for parison sag, and help in the formation of a strong nip weld). The general advantages of bottom blowing include fast operation, positive forming of the outside of the neck (by the neck part of the mould cavity against the blow pin), smooth internal neck surface, and considerable latitude in the relationships between the thicknesses and diameters of neck and body. The main limitations are that the internal diameter of the parison must be large enough to fit, with reasonable clearance, over the blowing pin, the external diameter must be greater than the internal diameter of the neck portion of the mould, and that-as a result-there is always flash on the sides and upper part of the neck. This has to be removed, and the finish in this area is never completely 'clean'. In a top-blowing arrangement the parison must be severed from the parent extrudate and clamped in a mould, completely closed, before the blowing pin is brought into action. Typically (but not invariably) the cutting follows the closing of the mould. If extrusion is continuous, the mould enclosing the parison is moved away from the extruder die. A blowing pin is introduced into the open top of the parison: the pin is usually shaped and dimensioned to participate in forming the neck of the bottle. Blowing, cooling and demoulding then proceed in the usual way (often at separate, consecutive stations). It is an advantage of top-blow arrangements that the bottle neck can be free from flash (if the parison diameter is not greater than that of the neck part of the mould cavity). However, the time lapse between parison formation
780
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and commencement of blowing increases the length of the cycle: the possibilities are also increased (although these are adequately counteracted in good modern equipment) of parison sag and undesirable surface cooling of material in the neck region. In injection and dip blow moulding, blowing takes place through the core rod on which the preform is moulded. The arrangement thus broadly corresponds (but is not strictly analogous) to top blowing in extrusion blow moulding. The blowing air pressures used in all blow-moulding processes are typically within the general range of 0·5-1·0 MPa. OTHER FEATURES
Mould venting: This is necessary to avoid impairment of surface finish of the moulding by air trapped in the mould (in extreme cases a pronounced 'orange peel' effect may be caused). Good surface contact between the moulding and the cavity-without intervening air layers-is also necessary for effective cooling. Vent slits are usually provided along the mould parting line, and-if required-at other points, according to the shape and design features of the article. Slight plate-out (which may not be otherwise discernible) may collect in the vents with some formulations. Such blockage should be removed with a suitable solvent (e.g. a chlorinated hydrocarbon). Multicavity moulds: These are used on some versions of blowmoulding equipment. In extrusion blow moulding they are operated in conjunction with multiple heads on the extruder. In injection blow moulding the number of cavities that can be operated has been increasing over the years: equipment with as many as 8, 12 and 14 cavity moulds is in current industrial use. 8 ,20 Waste material removal: In extrusion blow moulding some waste is always created as the mould grips the parison. This usually consists of the 'tail' beyond the bottom nip closure, often with flash in the neck region and-in containers with handles-handle pinch-off waste and flash. The waste has to be removed. The removal operations are automated to a large extent in modern equipment. 'Top and tail' waste is trimmed in the mould in several systems: in some the removal is a secondary operation carried out in-line-this is often the case with handle pinch-off waste. In some equipment for stretch-blow moulding
17 Blow Moulding of pvc
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of PVC the tail waste is torn off the preforms clamped in their moulds, by sliding grippers. Examples of systems with automatic waste trimming are the Hoover* Uniloy 300 CE line, the Automat Speed 3000 D and Maximat machines, and the Krupp-Kautex* KEB machine range. Part removal (take-off) systems: These too are largely automated, especially in modern extrusion blow-moulding equipment. Typically, mouldings trimmed of waste material (which may be automatically routed to a granulator and from there to the extruder) are positively deposited, in the required orientation, on a conveyor on which they proceed to leak-testing, decorating or labelling, and packing. (d) Cooling Methods The blown article must be cooled in the mould before ejection. Cooling is effected by circulation of a heat-exchange liquid (commonly water and glycol) through drilled channels or milled labyrinth cavities in the mould block. Blowing pins in extrusion blow moulding, and the core rods in injection and dip blow moulding, are also channelled for coolant flow. Note: Preform temperatures prior to stretch-blowing can be controlled via the mould cooling system. As in any moulding process, rapid cooling of the blow-moulded article is desirabie for maximum output. However, in the interest of product quality, the cooling should also be uniform and not so fast as to set up stresses in the moulding. The relatively low wall thickness of blow mouldings is conducive to quick cooling, whilst stresses arise less readily in thin than in thick sections. The rate of cooling of uPVC is faster than that of many other thermoplastics (including PET and PP which, as materials of stretch-blown bottles, compete with uPVC in some applications). Note: The principal factor in this is uPVC's comparatively high thermal diffusivity, which is the parameter governing the rate of transfer of heat through a plastics material in transient (i.e. non-steady) flow conditions, such as obtain during the cooling *Hoover Universal, Plastics Machinery Div., Manchester, Michigan, USA. t Automa SpA, Bologna, Italy. t Krupp-Kautex Maschinenbau GmbH, Bonn, West Germany.
w.
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V. Titow
of a moulding when the temperature is continually falling. Heat flow in steady-state conditions is governed by thermal conductivity. The thermal diffusivity (D) of a plastic can be calculated from the relationship: D
= (thermal conductivity)/(specific heat
x density)
The following calculated average values of D (in cm2 S-1) are fairly representative for the temperature range over which a moulding would be cooling. uPVC, 1·2 x 10- 3 ; PP (homopolymer), 0·6 x 10- 3 ; PET, 0·8 x 10- 3 . Another practical advantage of uPVC over the polyolefins and PET is its substantially lower mould shrinkage. The typical ranges are: 0·1--0·7% for uPVC; 1·5-5% for PE (all densities); 1·0-2·5% for PP (homo- and copolymers); 2,0-2·5% for PET (blow-moulding grades). It can be shown 21 that, under given moulding conditions, with no undue fluctuation of the relevant operational parameters, the cooling time (t) of a part in a mould will be proportional to the square of the wall thickness (wZ), * i.e. t = const. x w2 . A plot of this equation (t versus w) for a real situation (i.e. only positive values of w) will be the positive arm of a parabola whose axis is the t-axis and whose vertex is at the origin. A typical plot-given in Ref. 21-of actual cooling time versus part thickness for preforms injection-moulded on an injection blow-moulding machine has this form. Removal of heat from a moulding via the mould walls by means of a circulating coolant is the traditional and still the basic method of cooling. Chilled water, at temperatures down to about 5°C, can be successfully used with PVC blow mouldings. However, the colder the mould the higher the risk-especially in humid atmospheres-of moisture 'sweating' on the cavity surface, with consequent impairment of the surface finish of the moulding. Factors in the efficiency of such cooling, more important than mould temperature alone, are the rate and nature of the flow of coolant through the mould channels: for * The derivation of this relationship in Ref. 21 is based on thermal conductivity rather than thermal diffusivity (which is the proper parameter to consider). However, in this particular case, this does not invalidate the method or the equation derived. The author also suggests a practical way of using the relationship to predict the cooling cycle for any thickness of moulding (in his example an injection-moulded preform) on the basis of a demonstration with a given thickness.
17 Blow Moulding of pvc
783
effective heat transfer the flow should be fast and turbulent. These flow characteristics are achieved by a combination of appropriately high pressures in pumping the coolant through, with suitable design of the channels. Since the shortest practicable cooling time is desirable for increased outputs, various methods are available of supplementing the mould cooling with internal cooling of the moulding. Reductions of cooling time by up to 30% are claimed for some of these. The cost of the techniques has been a restricting factor on their industrial use, although they are of definite practical interest, particularly in the blow moulding of large articles which normally require long cooling dwell times in the mould. The following internal cooling methods may be mentioned. Liquid nitrogen or carbon dioxide cooling: In this, N2 or CO2 is injected into the interior of the moulding directly after normal blowing. Vaporisation and expansion of the originally liquified gas, and its warming to the temperature of the cavity, abstract a substantial amount of heat from the moulding. Chilled-air cooling: With this system, a shot of air, cooled to a low sub-zero temperature (about -50°C), is injected under pressure into the blown article through a special insulated nozzle. Cooling with air and water mist: In the Hunkar variant of this method (the Hunkar ILC process)* an amount of highly compressed air and a metered quantity of water are simultaneously injected into the interior of the moulding. The rapid expansion of the air as it leaves the nozzle causes sharp cooling: the atomised water freezes, and further heat is then extracted from the moulding as the ice particles melt and the resulting water droplets evaporate. Air flushing: There are several embodiments of this approach to internal cooling. In the 'Interval Blowing' systemt a minimum air pressure is maintained inside the moulding, whilst cool, compressed air is additionally introduced and then vented out at intervals, 'flushing out' * Hunkar Laboratories Inc. Also relevant here: the 'Frost Air' system (Ryder Associates, Whippany, NJ, USA). t Battenfeld-Fischer Blastformtechnik GmbH, Lohmar, West Germany.
784
W. V. Titow
some heat each time. In a simple continuous flushing system blowing air is circulated through the moulding (after that has been blown in the normal way): by controlled air feed and venting, a pressure near the blowing pressure is maintained during the cooling period whilst heat is continually removed. The necessary modifications to the blowing circuit are relatively simple. A continuous cooling system has also been developed* which can reduce substantially the time for cooling the neck of a blow-moulded bottle. 17.2.2 Industrial Blow Moulding of PVC
(a) Some Process and Equipment Considerations Both in the commercial and the technical sense, bottles are by far the most important among blow-moulded PVC articles. Extrusion blow moulding is the predominant method, a considerable proportion of the mouldings being stretch-blown. Injection blow moulding of PVC is less widespread, principally because the greater thermal severity of this process has delayed and complicated its application to all heat-sensitive plastics materials from which bottles are blown (modified polyacrylonitrile, multi-acrylic copolymers and PVC). However, PVC bottles are being produced by this technique on an increasing scale, and the problems are eased by the advent of special moulding compounds (see Chapter 15). Relatively recent machine developments include injection units in which the preform mould is filled at low pressure by rotating the screw during injection:t this makes the treatment received by the stock less drastic, and also reduces cycle time because the next consecutive shot can be commenced before full screw recovery. Multi-cavity injection blow moulding of PVC bottles has been operated commercially for several years, with, for example, eight-cavity production of round, 120 ml bottles at the rate of 48 per minute as early as 1978. 8 The extruders used in the extrusion blow moulding of PVC are of the general kind suitable for PVC extrusion (d. Chapter 14): LID ratios around 25: 1 and compression ratios of 1· 8 : 1-2·5 : 1 are fairly typical for single-screw machines. The advantages and disadvantages (mainly higher cost) of twin-screw extruders in the processing of PVC are discussed in Chapter 14: the points made apply also to their use in * By FGH Systems Inc., Dover, NJ, USA. tRainville Co. Inc., Middlesex, NI, USA.
17 Blow Moulding of pvc
785
blow moulding. The relatively gentler processing and good stock temperature control they afford are of particular interest in the production of high-clarity bottles. Some blow-moulding machines are available with a choice of twin-screw extruder (for PVC) or a single-screw one (for polyolefins). One example is the Cincinnati Milacron* BB3 equipment. Some equipment suppliers make a special point of their twin-screw extruders' ability to process bottle compounds based on PVC polymer of relatively high molecular weight (e.g. up to K value 65 with the conical twin-screw machine of the Bellt blow-moulding plant6). Certain features of equipment arrangement or design, serving to reduce the risk of material hold-up and decomposition, are characteristic of extrusion blow moulding. Thus, in some equipment, the extruder is mounted end-down for straight-through vertical extrusion. With the more common horizontal extrusion set-up, axial-flow crosshead dies are usual, equipped with a 'swan neck' curve (cf. Fig. 17.2A) which preserves smooth melt flow at a constant rate whilst changing the flow direction from the horizontal to the vertical. Side-entry crosshead dies as used with polyolefins are not suitable for PVC. The general, basic configuration of a typical die is similar to that in pipe extrusion, with a spider~supported core to form the parison tube. As in pipe extrusion, the spider legs should be suitably profiled and the melt temperature high enough to ensure full merging of the melt after its passage through the spider zone: otherwise flow lines can result in the parison and the blown article (see Section 19.3.2(a) of Chapter 19). The die head temperature is usually kept slightly below that of the melt. All internal parts of the die should be streamlined and smooth, with the number of joints kept to a minimum. A channel in the core, opening into the interior of the parison, serves as a passage for air which is blown in (usually through a connecting channel in one of the spider legs) to provide shape support during extrusion. Multi-parison extrusion-whether by means of multiple heads, each producing one parison, or one multi-parison head-is attractive in principle as a means of increasing production without a proportional increase in machine space requirements. However, in practice it also presents special problems of melt temperature and pressure control. Close, accurate control is necessary, not only because of the thermal sensitivity of PVC, but also to ensure even parison lengths (a hotter
* See Table 14.3 in Chapter 14.
t Bell Engineering Works Ltd,
Lucerne, Switzerland.
786
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portion of the melt will normally flow faster), and to prevent parison curling (outward splaying) which may occur if the outside of the tube is colder than the inside. The flow paths and lengths must also be suitably balanced. Multi-parison heads are available on modern extrusion blow-moulding equipment for PVC, e.g. four-parison ones on some Bell* and Plastimact models. A production rate of 2400 I-litre bottles per hour is claimed for the Bell two-station machine with the multi-parison head. 6 'Universal' die heads suitable for both PVC and polyethylene are also available, in single- or multi-parison versions. Resistance of working surfaces to acid corrosion is as important in blow moulding as it is in other melt-processing of PVc. Thus the relevant points made in the chapters on extrusion and injection moulding also apply here. With regard to the material of moulds for blow moulding of PVC articles (including preform moulds), good heat transfer and wear resistance are additional considerations. An appropriate grade of stainless steel:j: would be the first choice for its combination of very good resistance to corrosion and wear, unless cost (high for stainless steel moulds), and/or the highest possible thermal conductivity, and/or light weight were paramount considerations in the particular conditions. In such cases the alternatives would be an aluminium alloy (for light weight and fast heat transfer), or a zinc alloy (for similar reasons) or beryllium/copper (for high heat transfer rate with some corrosion resistance). The following examples illustrate something of the features and performance of some equipment in current industrial use for the blow moulding of PVC containers. (b) Extrusion Blow-moulding Equipment An example of the smaller-size, continuous-extrusion machine is the Bekum BMO 8, the smallest unit in the BMO§ range. Equipped with a 50-mm (2-in), 24: I LID extruder with a capacity of 40 kg (90 lb) of PVC per hour it can produce single containers of up to I litre, or-with a twin head-two containers up to about 113 litre. Production in a
*Bell Engineering Works Ltd, Lucerne, Switzerland. t Plastimac SpA, Milan, Italy. fA useful review of mould steels has been published recently by Hoffmann. 22 § High capacity extrusion stretch-blow-moulding machines for the production of biaxially oriented PVC and polyacrylonitrile bottles of up to 2 litre capacity. The BM and BAE series comprise single-station machines producing unstretched blow mouldings in the range 5-20Iitres.
17 Blow Moulding of pvc
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typical run, with a twin head, of 230 ml (8 oz) PVC bottles (material weight 12 g) can be at the rate of up to 2100 bottles per hour (3,4 s cycle: dry cycle claimed 1·4 s). The containers are fully de-flashed in the machine and leave in upright position for the secondary operations of testing, surface decoration and filling and/or packing. The single-station machines of the Battenfeld-Fischer VKl series range from the VK1 07 model (maximum blown container size 0·7 litre) to the 30 litre VK130 model. Capable of operation with a single extrusion head or twin heads, these machines also provide complete in-machine trimming, and delivery in upright position for secondary operations. A typical output for the smallest unit (with twin extrusion heads) on 12 g PVC bottles would be about 2000 per hour. The Automa Speed 1500 SB machine, designed primarily for PVC, can blow mould biaxially oriented containers up to 1·5 litre with single-head and up to 1 litre with twin-head extrusion. Typical output of I-litre (32 g weight) PVC bottles in a single-cavity mould would be 480 per hour. Other machines produced by Automa (in the Speed and Maximat ranges) produce containers between 2 and 10 litres. Some interesting features are embodied in the fully automatic blow-moulding line produced by Sidel* (see also Table 17.1), and in the Krupp-Kautex KEB 2-2 unit. In the former, rotary arrangements of 24 moulds are operated to mould and temperature-condition preforms (produced from continuously extruded parisons) and then to blow them into bottles. The process is run under closed-loop computer control involving automatic monitoring and adjustment of extrusion rate, blowing pressure, temperatures, and product wall thickness. The KEB 2-2 machine is a two-station unit equipped for both the conventional blow moulding and stretch-blow moulding of PVC bottles (typically about 1 litre capacity). In the former mode of operation, twin heads are used with two two-cavity moulds: the problems of constant parison delivery and uniform wall thickness and temperature distribution which-as has been mentioned-ean arise in multi-head extrusion of PVC, are overcome by the use of two extruders, each feeding one head. (c) 1njection Blow-moulding Equipment Two examples of commercial equipment of this kind suitable for PVC are the Battenfeld-Fischer FiB injection stretch-blow-moulding equip-
* The Side) Division of SMTP, Paris, France.
b
a
VK1-2ESB BM04 BM04D KEB 2 MSFBO 3000 MSF BO 5000
b
Model
Material weight, about 30 g. Can be converted for direct blow moulding.
Battenfeld-Fischer Bekum Bekum Krupp-Kautex Sidel (SMTP) Sidel (SMTP)
Producer
Equipment
2
1
2
1 2
Cavities per mould
24 preform moulds and 24 bottle moulds on two rotary wheels
2
1
2
1 1
Number
Moulds operated
500 900 1800 500 2600 10000
Typical production rate: I-litre bottles per hour
TABLE 17.1 Typical Performance of Some Commercial Extrusion Stretch-blow-moulding Machines in Producing 1-litre PVC Bottlesa
--.)
~
o
:::l
~
~
ClO ClO
17 Blow Moulding of pvc
789
ment range, and the Tri-Delta machines (Tri-Delta Technology, Inc., Middlesex, NJ, USA). Both types are four-station units. In, for example, the Fischer FIB 517 machine the preform is temperatureconditioned at a station between the injection and the blowing stations: the longitudinal stretch is imparted by pre-blowing (not mechanically): the output (for bottle sizes up to 0·5 litre) is 500-600 per hour. (d) Dip Blow-moulding Equipment Industrial equipment for this process has been mentioned in Section 17.2.1(a) above. Another example is the Gamma series of machines produced by Plastimac: this equipment is intended primarily for the manufacture of small containers for pharmaceutical products. The version specifically recommended for PVC (and polycarbonate) is a three-cavity unit in which the melt shots are deposited by three vertically mounted 35-mm screw extruders each driven by a 5-hp variable-speed motor. (e) Sources of Information on Blow-moulding Equipment A survey of blow-moulding machines (with indications, inter alia, of suitability for PVC proces&ing) has been published in Plastics and Rubber Weekly for the 13th September, 1980. Up-to-date information will also be found in the general sources mentioned in Section 14.6 of Chapter 14. 17.3 PVC COMPOSmONS FOR BLOW MOULDING
The discussion in this section is centered on compositions for blow-moulded bottles, since bottles are the most important PVC products manufactured by this process. Like any other PVC composition, a bottle compound is formulated in the light of three key considerations: behaviour in processing, properties for service, and cost. The formulation is normally a compromise reflecting the relative importance of these three factors in a particular situation.
17.3.1 The Processing Aspect The PVC compositions used as feedstock in the production of extrusion blow-moulded bottles may be in powder (dry blend) or pellet form. A twin-screw extruder is preferable with powder feeds (see Section 19.1 of Chapter 19). Pellet feeds are more common in injection
790
W. V. Titow
blow moulding of good quality bottles. The respective merits of the two forms of feedstock are mentioned in the introduction to Chapter 19 and, passim, in Chapter 14 (Section 14.2.2(d». Ambient conditions (especially possible high atmospheric humidity in transport and/or storage) can be a factor in the choice of feed form, in that powder blends absorb moisture more readily than pellets, and this can affect their dry-flow properties as well as cause bubbles in the mouldings (unless completely effective de-gassing can be guaranteed). The basic processing considerations in formulating a bottle compound are substantially as those applicable to PVC extrusion or injection-moulding compounds generally (see Chapters 14, 15 and 19), although some points may acquire particular emphasis. These include the use of relatively low K value polymer for ease of processing; need for particularly good dynamic heat stability of the stock in the manufacture of clear products; and desirability of complete freedom from plate-out to avoid blockage of vents in blowing moulds. Recommendations of the suppliers of the polymer and other formulation constituents (or of the compound if purchased ready made) can be useful as initial guidance on processing. In the light of such recommendations the compound should be tested for processing behaviour (as well as product quality) to establish the best operating conditions. Some of the preliminary tests may usefully be conducted on a suitable torque rheometer. The following properties are of direct interest (especially in connection with extrusion blow moulding, or dip blow moulding with extrusion feed). Dynamic heat stability: A useful assessment of this can be made in a torque rheometer test of the kind outlined in Section 9.8.2 of Chapter 9. The test conditions (temperature, shearing rate, and duration) should be worked out in preliminary trials so that they relate to those of production. Melt viscosity: This determines the ease of melt flow in processing (and the temperature for good flow: increasing temperature reduces viscosity). It is also a factor in the amount of working the stock will experience (which in turn influences the work-heat generation, degree of homogenisation and fusion). Determinations can be carried out in a torque rheometer or another suitable instrument. * * A concise review of melt rheometry (with 78 references) has been published recently by Dealy.z4
17 Blow Moulding of pvc
791
Rate of fusion: A bottle compound should be fast-fusing for good homogeneity of melt with minimum heat history. Rates of fusion can be determined in a torque rheometer (see Chapter 11, Section 11.1). Melt swell: This is relevant in extrusion blow moulding, where the extent to which the walls of a parison expand on leaving the die is a factor in the ultimate weight of the moulding. The degree of swell is determined by comparing the diameter of a rod produced under relevant conditions from an appropriate die, with that of the die. Melt fracture: This can occur, usually within a range of relatively high rates of shear, in the extrusion of plastics compositions. In extrusion blow moulding of PVC bottles it can cause varying degrees of surface roughness. Tests may be carried out directly on the extrusion equipment, or the relevant range of shear rates may be covered in determinations with a rheometer. Evolution of volatiles: The extent to which this occurs is a function of the composition of the material and, to some extent, the processing conditions. If the conditions are not so excessively severe as to cause appreciable decomposition, the volatiles are often vapours of the more labile components of the stabiliser system. The volatiles can condense on the cold surface of the mould cavity: this can cause marring of the bottle surface, with possibly also staining if the condensate darkens in colour, as occasionally happens. Condensation of the fumes inside the bottle can lead to tainting of the ultimate contents: this can be counteracted by flushing with clean air directly after moulding; 'natural' airing in the course of storage can also help to reduce the smell in some cases. However, the composition should in any case be formulated with a view to preventing such problems. A test for evolution and nature of volatiles can be run: this may consist in processing the compound, under relevant conditions, in a torque rheometer, collecting the condensate on a cold metal surface, determining the amount produced, and taking an IR spectrum to identify its nature. A version of the test, employing a Brabender Plasti-Corder fitted for condensate collection, has been described by Latham and Mendham. 23 These authors also give some information on the application of this rheometer in the evaluation of the other processing characteristics just mentioned.
792
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17.3.2 The End-use Aspect Rigidity is a cardinally important property in blow-moulded containers, since their relatively thin-walled structure is the better able to resist distortion in service (under external forces, or the pressure of own contents) the more rigid the walls. Moreover, the higher the rigidity of the wall material the lower the thickness necessary (Le. the less material need be used) to achieve the required degree of distortion resistance. This is one of the main reasons why only unplasticised PVC compositions are used for the production of PVC bottles. Other important considerations are the better barrier properties and general chemical resistance of uPVC in comparison with pPVC, and its greater stability to extraction of formulation constituents by the bottle's contents. The only plasticisers used commonly in PVC bottle compounds are epoxy compounds (usually epoxidised soyabean oil) which are incorporated, in low phr, as components of some stabiliser systems: thus whilst they do ease melt flow to some extent, plasticisation is not their primary function. Small proportions of dibutyl phthalate are also occasionally included in some PVC compositions for the production of bottles for mineral water: the functions of this additive in such formulations (in which it may be associated with benzoic acid) are to act as co-stabiliser and fungistat, and to assist fusion. Properly formulated uPVC has a number of properties which qualify it as a good material for bottles for a large number of uses (see Section 17.4 below). These include a high degree of rigidity, toughness (in impact-modified or biaxially stretched mouldings), resistance to environmental stress cracking, high clarity and 'sparkle' of transparent compositions, good barrier properties, and suitability (when formulated with approved constituents) for food contact. The points on which uPVC compares less favourably with PET, its main competitor in many blown-container applications, are relatively low maximum service temperature, susceptibility to ketone and chlorinated hydrocarbon solvents, lower impact strength, greater susceptibility to stress whitening (of some impact-modified grades) and permeability to some penetrants. PVC bottle compounds for the packaging of oil, beverages, and pharmaceuticals must fulfil the requirements of the appropriate regulatory authorities with regard to the suitability of all formulation components. They must also be formulated to minimise odour and taste effects and to resist microbiological infestation.
17 Blow Moulding of pvc
793
17.3.3 PVC Bottle Formulations The basic formulation will comprise the following. (a) PVC Polymer Usually a suspension (sometimes mass) polymer, with particle structure and size characteristics suited to ease of processing and rapid fusion. Relatively low K value: normally 50-60 (number-average molecular weight approximately 36000-55 OOO)-for low melt viscosity. The polymer should be of good quality to secure freedom from gels ('fish-eyes' or 'nibs'), and its VCM content must be acceptably low (in most countries under 1 ppm).
Note: The gel content is sometimes classified with processing features. Its determination in bottle compounds can be carried out on a suitably fitted torque rheometer, e.g. a Plasti-Corder with extrusion attachment and bubble die assembly. A free-blown thin-walled bubble is produced, in which the gels should be plainly visible. 23 For a quantitative determination the bubble size may be standardised and the number of gels counted. Whilst gels may not be readily visible from the outside in an opaque production bottle, they usually show up as small protrusions on the inside surface. Their presence may significantly impair strength properties, as they can act as stress concentrators. (b) Stabiliser System This is usually liquid, to avoid increasing the melt viscosity. Most often either a thiotin for good heat stability and high clarity (permitted compounds for food contact), or a Ca/Zn system for reasonable clarity and non-toxicity. In either case an epoxy co-stabiliser is usually included, sometimes with a phosphite or another synergist. (c) Impact Modifiers Although in some cases the impact resistance of a conventionally blown uPVC bottle of a particular design and wall thickness may be considered sufficient for a particular end-use (and intended contents) without impact modifiers in the formulation, these toughening additives are important ingredients of most PVC compositions for the blow moulding of bottles without biaxial stretching. Their nature, and
794
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V. Titow
effects in uPVC, are discussed in Section 11.2.2 of Chapter 11. As in the compositions for other transparent products, the impact modifiers in clear bottle compounds are of the MBS type. ABS, and some of the other types, may be used for opaque bottles. Impact modifiers can adversely affect, in varying degrees, such properties as permeability and colour ,25,26 softening point, chemical resistance, resistance to stress whitening,25 and resistance to photochemical degradation. It is, therefore, an important selection criterion that the modifier chosen should combine the best toughening effect with the least impairment of those of the properties which are the most significant in the particular situation. In most cases, and especially with clear bottle compositions, stress whitening potential will be a factor in the selection. The impact resistance of blown bottles normally increases with the proportion of modifier in the composition, up to about 15 phr. Higher loadings produce no further significant improvement, and may be increasingly detrimental to other properties. As has been mentioned, biaxial orientation imparted by stretchblowing can have a pronounced toughening effect, so that the amount of impact modifier in compositions for stretch-blown bottles may be substantially reduced, or the additive left out altogether in some cases. (d) Lubrication Both internal and external lubricants are employed in bottle formulations. Some proprietary composite lubricants, specially developed for this application (e.g. Irgawax 368-Ciba-Geigy), can be particularly useful. In general, all the points of lubricant action (and interactions with other formulation components, especially stabilisers) discussed in Section 11.1 of Chapter 11 are relevant to the formulation of lubricant systems for bottle compositions. The question of permanence (i.e. resistance to extraction and migration) is important: in this connection (and quite apart from cost considerations) it is a good general principle to keep the lubricant content to the necessary minimum. Impact strength can also be significantly reduced by excess of internal lubricant, whilst avoidance of the risk of plate-out through proper choice of the nature, amounts and balance of the components of the lubricant system is a further important consideration. Internal lubrication is often provided in bottle compositions by calcium stearate, whether incorporated solely for that purpose or as part of a Ca/Zn stabiliser system: the addition level should preferably not exceed 1 phr, and the effect on transparency of the moulding should
17 Blow Moulding of pvc
795
not be overlooked. Among lubricants with external action, polyethylene waxes, synthetic waxes, and some fatty acid esters and alcohols are of interest. (e) Other Additives Processing aids (commonly acrylic) are often included in bottle compound formulations at levels of up tl) about 1 phr. At such levels they can effectively discharge their functions in processing without significant effect on the properties of the finished mouldings, except for possible slight clouding of transparent products. For this reason any processing aid to be used in a clear composition should have a refractive index as close as possible to that of the PVC resin. Colourants and pigments are incorporated in some bottle compounds, as appropriate. Two examples of basic bottle formulations are given in Section 4.6.6 of Chapter 4. These, and the further examples in Table 17.2, illustrate some of the points made in the present section. The effects of some formulation and processing factors on the quality of bottles blow-moulded from powder blends based, respectively, on a PVC homopolymer, and vinyl chloride/alkyl vinyl ether and vinyl chloride/propylene copolymers, were investigated-with the aid of dynamic stability tests-by Taylor and King. 27 These workers found that the copolymer-based compositions gave bottles with a lower light absorbance and yellowness index, and that certain product imperfections were related to the particle size distribution and volatiles content of the resins. Suppliers of PVC compounds market some blow-moulding grades. One example is compound 9275 CI38 of the Ethyl Corp., formulated, like others of its kind, for good heat stability and low melt viscosity in processing, and for such product properties as good clarity (light transmittance about 74%), impact strength and UV barrier action (to protect light-sensitive contents in bottles).
17.4 PVC BLOW MOULDINGS 17.4.1 Applications The vast majority of PVC blow mouldings comprises containers, used for the packaging of a variety of products. The containers are mostly
Corvic D50116a Stanclere 80C 2·2 phr Paraplex G62f 2·8 phr a--phenyl indole 0·6 phr Kane Ace Bl2g 11 phr Estol294h Synthetic wax
Medium impact
1 phr 2phr
Kane Ace Bl2 14 phr 1·1 phr 0·1 phr
100 parts Mel/ite 831c;d Paraplex G62
High-clarity, high-impact
Fruit squash bottles
MBS lrgawax 368; PE wax
lOphr 1·2 phr 0·05 phr
Breon S90110 b 100 parts Irgastab 17 Moe 1·2 phr
Clear bottles (general purpose)
a
ICI. K value 50. Low MW polymer for blow moulding and film production. A special grade with low 'fish-eye' level available. b BP Chemicals. K value 57-60. A blow-moulding resin. C AKZO. Liquid octyltin. BPF (British Plastics Federation) and BGA (Bundesgesundheitsamt-West German Federal Health Office) approved for food contact. d Albright and Wilson. Liquid octylin. BPF, BGA, and FDA (US Food and Drug Administration) approved. e Ciba-Geigy. Di-n-octyltin bis(2-ethylhexyl thioglycolate) with 25% epoxidised soyabean oil. f Rohm and Haas. Epoxidised soyabean oil (high MW grade). BPF approved. g Kanegafuchi Chemical Industry Co. h Joseph Crosfield and Sons Ltd, Warrington, Lancs. ; Ciba-Geigy. Composite lubricant for blow-moulding applications.
Impact modifier Lubricants
PVC polymer (suspension type) Stabilisers
Formulation components
TABLE 17.2 Examples of Basic Outline Formulations for Bottle Compounds
~
:::'! 0'
:0:::
~
~
-.J
17 Blow Moulding of pvc
797
bottles of various kinds, but wide-mouthed jars and the like are also blow-moulded and find diverse applications in the packaging of some foodstuffs (e.g. dried vegetables, syrup, honey), cosmetics and toiletries (e.g. creams and ointments), small hardware items (e.g. nails, screws) and others. Large-scale application areas of PVC bottles comprise their use for packaging consumable liquids (including fruit squashes, edible oils, vinegar, cheaper varieties of table wine, mineral and natural spring water, and carbonated drinks), liquids for household use (detergents, bleaches, disinfectants, cleaners), cosmetics and toiletries (e.g. lotions, oils, shampoos and other hair-care preparations, liquid soaps), and pharmaceuticals. In several of these outlets PVC competes with polyethylene terephthalate (e.g. bottles for carbonated drinks, edible oil, some cosmetics and toiletries), polyethylene (e.g. detergent and other household liquid bottles, bottles for pharmaceuticals), polypropylene (bottles for household liquids, some toiletries and pharmaceuticalse.g. mouthwash), and modified polyacrylonitrile (applications in which barrier properties are of particular importance). As an illustrative example, it may be mentioned that the consumption of uPVC for the production of blow-moulded containers in France in 1972 and 1973 was reported 28 as, respectively, 110000 and 130000 metric tonnes. The main items making up the figure for the former year were stated28 to be bottles for still mineral water (70000 tonnes), for cooking oil (17000), for table wine (14000), for vinegar (3000), for cosmetics (2000), and miscellaneous (1000). 17.4.2 Properties and Tests
The material properties of uPVC blow-moulding compositIOns, which-together with the relatively moderate price-make containers blown in this material attractive for many applications, have already been mentioned. Several processing factors also play a part in the properties of a blow-moulded PVC container. Among these, perhaps the most important single effcd is that of the nature and extent of molecular orientation, especially when that is deliberately imparted in a substantial degree by biaxial stretching. Some other factors have been mentioned in Sections 17.2.1(c) and 17.3.1. Still others of considerable importance are the temperature of the parison or preform (and
798
W. V. Titow
the temperature distribution within it) during moulding, and the rate and efficiency of cooling. The former factor can affect the dimensional conformity and strength of the article and the surface finish; in extrusion blow moulding it is instrumental in the strength of the nip weld (also affected by the speed of mould closure, shape of the nipping edges, and the blowing time and pressure). The cooling factor can also influence the strength and dimensions. Two properties normally monitored in the course of routine quality control in production are the integrity (freedom from leaks) and wall thickness of containers. Microprocessor-operated leak detectors are available from several suppliers for on- or off-line use, in some cases as part of the regular blow-moulding equipment (e.g. with Bekum or Battenfeld-Fischer machines). A typical leak tester applies accurately controlled air pressure to the interior of the container and measures the rate of decay: this indicates the presence and size of any leaks. Apparatus of this kind can also be used to test the dimensions of small moulded orifices. An example of a fully automatic, computerised leak tester is the BLT 1000 Leak Finder (South Bend Lathe Inc., USA 5 ), capable of testing up to 50 containers per minute (test pressure about O·llbf in- 2): the device can be programmed for automatic removal of rejects from the line. Thickness gauges, working on the basis of absorption of IR radiation, are available (e.g. from Oy G. W. Sohlberg AB., Espoo, Finland): these can measure and record the wall thickness of a blow-moulded container at 100 points within 2 s. Service-related quality control tests widely performed on blowmoulded containers are those for impact resistance, resistance to crushing, and permeability (barrier effect). Many manufacturers have their own test specifications. ASTM test methods are also available in the following standards: ASTM D 2463 (drop impact resistance), ASTM D 2659 (column crush test), ASTM D 2684 (container permeability). Another ASTM standard (D 2911) lays down dimensional tolerances for blow-moulded containers. Compatibility with the intended contents is also sometimes determined, usually by relevant mechanical property tests on containers before and after a prescribed period of contact. The above tests are normally carried out on samples of actual production containers, although where the object is to compare or evaluate moulding compounds the container specimens may be ones specially produced in a prescribed design and size (e.g. the standard 750 cc Lesieur test bottle25 ). Compound property tests involved in such
17 Blow Moulding of pvc
799
comparison or evaluation may also include determinations of some or all of the following properties, carried out on appropriate standard moulded test specimens: deflection temperature under load; brittle temperature; Vicat softening point; tensile strength and elongation; flexural strength and modulus; density; hardness; and light transmittance (for clear compositions). In the practical service context, the drop impact resistance is possibly the most significant among the properties of a container. It is usually determined by dropping the container-filled with water and closedonto a standard, hard surface. Note: Some specifications include the same kind of test for a multi-container pack, if the containers involved are normally transported and stored in such groups.
In some versions of the drop test the height of the drop is fixed; in others it is increased by prescribed increments until a stated percentage (often 50%) of the drops results in failures, or-in some variants of the method-until all specimens fail, the failure criterion being fracture or defined damage. Both approaches are represented in ASTM D 2463 (together with a third procedure, known as the 'Bruceton staircase method'). It may be noted that, as illustrated, for example, by Sisson's data,25 whilst increasing the K value of the polymer (within the range acceptable for processing) does raise somewhat the impact resistance in a drop test, that rise is much less than one brought about by incorporating in the composition an effective impact modifier in suitable proportion. The design of the container is also a factor in drop impact tests. A non-destructive dynamic compression test of a few seconds' duration may also be used to obtain a numerical index of the resistance to flexure in the test conditions. With appropriate allowance for the design, wall thickness distribution, and material of construction, the index is claimed to be a reasonable measure of a bottle's relevant mechanical properties, and to correlate well with the results of some conventional destructive tests. 7 Bottles for carbonated drinks have to combine resistance to internal pressure with a low enough permeability to CO2 to provide a sufficiently long shelf life before 'carbonation loss' occurs. For a 2-litre bottle in PET (a material somewhat superior to PVC in this application) many soft-drink companies specify a carbonation-loss shelf life of 16 weeks (with 8-12 weeks for a !-litre bottle). Biaxial
800
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Titow
orientation, combined with pressure-resistant design and material weight for extra wall thickness, are necessary to meet such specifications with PVC bottles, especially where they are to be used in a hot climate (PET carbonated drink bottles are also biaxially oriented). An example is a round-shouldered, heavy weight (29 g as against 23 g for a conventional design), biaxially oriented O'33-litre clear PVC bottle with integrally moulded, internally rounded base (incorporating four 'feet' for stable standing without external support). This is capable of containing-without distortion-drinks with up to 8 g litre- 1 CO 2 at temperatures up to SO°c. The bottles, and injection stretch-blowmoulding equipment on which they are produced, have been developed by Voith-Fischer (now Battenfeld-Fischer) in collaboration with 4P Rube Gottingen GmbH. 16,29 Apart from the pressure resistance and barrier requirements, bottles for carbonated drinks and mineral water (as indeed those for wine and other beverages) must be made from PVC compositions formulated for the greatest possible resistance to development of taints and odours, through the highest purity and resistance to extraction of all constituents, and discouragement of bacterial growth. Some comments on this formulation aspect are provided in a paper by Sahajpal. 30 REFERENCES 1. Anon. (1980). Plast. Techno!., 26(13), 66-7. 2. Couzens, E. G. and Yarsley, V. E. (1968). Plastics in the Modern World, Penguin Books Ltd, Harmondsworth, Middlesex, England, p. 298. 3. British Patent No. 821 173. 4. Smoluk, G. R. (1981). Mod. Plast. Int., 11(2), 25-7. 5. Brockschmidt, A. (1982). Plast. Technol., 28(5), 78-81. 6. Anon. (1980). Mod. Plast. Int., 10(1), 25-7. 7. Hunkar, D. (1976). Plast. Rubb. Wkly, 12th November, pp. 30-1. 8. Crabtree, D. R. and Hart, R. J. (1978). Plast. Rubb. Int., 3 (6), 247-8. 9. Anon. (1975). Plast. Rubb. Wkly, 7th November, pp. 22-3. 10. Anon. (1979). Plast. Techno!., 25(2), 13-17. 11. Anon. (1982). Eur. Plast. News, 9(7), 8-9. 12. De Vries, A. J. and Bonnebat, C. (1976). Polym. Engng. Sci., 16(2), 93-100. 13. Brady, T. E. (1976). Polym. Engng. Sci., 16(9), 638-44. 14. Anon. (1981). Eur. Plast. News, 8(9), 64. 15. Anon. (1981). Plast. Rubb. Wkly, 26th September, p. 12. 16. Anon. (1980). Mod. Plast. Int.. 10(4), 36-8. 17. Anon. (1967). Packaging, 38(11), 95-9.
17 Blow Moulding of pvc
18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
801
Anon. (1976). Plast. Rubb. Wkly, 12th November, p. 37. Grant, R. and Foster, R. (1965). Mod. Plast., 43(2), 122-6,129,199. Brockschmidt, A. (1981). Plast. Technol., 27(13), 55-60. Valyi, E. I. (1981). Plast. Technol., 27(10),133-45. Hoffmann, M. (1982). Plast. Techno!., 28(4), 67-72. Latham, J. R. and Mendham, W. E. (1973). 31st ANTEC SPE Proceedings, pp. 458-60. Dealy, J. M. (1983). Plast. Engng, 39(3), 57-61. Sisson, W. B. (1968). Plast. Polym., 36(125), 453-63. 'The blow moulding of Welvic PVC', ICI Technical Service Note W. 110. Taylor, W. and King, L. F. (1970). Polym. Engng. Sci., 10(4),204-8. Anon. (1974). Plast. Rubb. Wkly, 26th July, p. 19. Anon. (1983). Eur. Plast. News, 10(1), 29. Sahajpal, V. K. (1978). 'PVC compounding for low organoleptics and controlled bacterial growth', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978.
CHAPTER 18
Calendering of pvc W. V. TITaw
18.1 INTRODUCTION According to Griffin 1 the word 'calender' shares a common Greek antecedent with 'roller', and the first recorded reference to calendering appears to be a description, in an 18th century publication, of a smoothing treatment for textile fabrics, carried out by means of individually operated, weighted rolls. The multi-roll machine of the general type still represented by the modern calender, was developed for the processing of rubber around the mid-1800s. Today, in the thermoplastics context, calendering is very much a PVC-processing operation, in that relatively little in the way of other thermoplastics is processed on calenders, although some thermoplastic rubbers, certain polyurethane compositions, talc-filled polypropylene, and ABS have been calendered into sheeting. 2 ,3 More recently chlorinated polyethylene sheeting produced by calendering has been finding application as roof-lining material. In this context, therefore, calendering may be defined, in general terms, as a process whereby a hot mass of a thermoplastic is fashioned into a continuous sheet by passage through a system of heated rolls (the calender). The sheet may then simply be cooled (possibly after embossing-see below) and wound up, or it may be deposited, while still hot, on a continuous base material (e.g. fabric, paper) fed through the appropriate part of the calender, to form an adherent coating. A calender may also be used to laminate together externally fed sheets: however, this kind of operation is lamination by calender, not calendering proper. The general thickness range for typical calendered PVC products is 803
804
W. V. Titow
about 75-900,urn, although sheet up to about 1·5 mm thick can be produced. Thus, whilst the products are often referred to as 'film and sheet', according to the systematic terminology discussed and adopted in Chapters 19 and 20, all the unsupported products of this kind should properly be described as sheeting (or sheet), the top thickness limit for PVC film being about 75/lom (and its normal method of production the blown film variant of the extrusion process-d. Chapter 19, Section 19.5.2). The main advantages of calendering over extrusion as a method of sheet production are high outputs and production rates, good product thickness control (and its relatively ready automation), and suitability for long, continuous runs. These features are largely responsible for the fact that most PVC sheeting is manufactured by calendering despite the much higher capital cost of equipment and lower operational flexibility for short runs in comparison with extrusion. Suppliers of calenders and calender line equipment are included in most of the main sources of information on commercially available machinery for plastics processing. These sources are mentioned elsewhere in this book (see, for example, Chapter 14, Section 14.6). The following may be quoted as non-selective examples: The Berstorff group: Berstorff Corp., Charlotte, NC, USA; Herman Berstorff Maschinenbau GmbH, Hanover, West Germany. BKMllndustrieanlagen GmbH, Munich, West Germany. The Farrel group: Farrel Bridge Ltd, Rochdale, Lancs, UK; Farrel Conn. Div., Emhart Machinery Group, Ansonia, CT, USA.
18.2 THE CALENDER The machine comprises the arrangement ('stack') of rolls (also known as bowls) mounted in bearing blocks supported by side frames ('gables'), equipped with roll drives, nip-adjusting gear, and heating arrangements. Calenders used for the general production of PVC sheeting are commonly four-roll machines. Whilst there is no basic reason (other than cost, space, and structural complexity considerations) for limiting the number of rolls to four, five-roll calenders are not common, and
18 Calendering of pvc
805
only used for special purposes, such as the production of some types of thin rigid sheeting where the extra nip can substantially improve the surface finish. 1 ,4 Note: Three-roll (two-nip) calenders find some use in the manufacture of PVC flooring, and sequences of two-roll (single-nip) units are used for processing certain highly filled flooring compositions-see Section 18.4.2(a) below.
The various roll arrangements of a four-roll calender are shown in Fig. 13.41 of Chapter 13. Their respective merits and drawbacks have been discussed in some detail by Elden and Swan. 1 The advantages of the 'L' configuration, widely favoured (especially in the inverted form-d. Fig. 18.1(i» for PVC processing, are usually quoted as reasonable cost; good visibility and accessibility of all the rolls in the stack; good rigidity imparted by the vertical superimposition of three of the four rolls; good range of the available total lap ('wrap') of the materials round the rolls (up to 540°, depending on the positioning of the offset roll of the stack and the stripper rolls); and ability to apply thickness corrections at two material banks by crossing just one roll (No.3 roll-the middle one in the vertical arrangement).
(ij)
(i)
' Cf3 1
(iii)
2
"""
(iv)
3
Fig. 18.1 Customary roll numbering in calenders. (i) Inverted 'V type; (ii) inclined 'z' Jype, downstack; (iii) 'V type; (iv) three-roll offset.
806
w.
V. Titow
Note: It is customary to designate the rolls of a calender by
numbers, starting from the first roll at the feed nip and ending with the last one at the take-off (ct. Fig. 18.1). Another roll configuration popular with manufacturers of PVC sheeting is the inclined Z type (cf. Fig. 18.1(ii)): this too offers a relatively wide range of the degree of lap, as well as easy aCCtSS for fabric coating. Note: In principle, the 'L', inverted 'L', and 'Z' configurations are
also convenient for bringing an embossing unit close up to the last nip. Formerly this constituted an advantage, but is less significant with the multi-roll stripper arrangements favoured in modern practice (see Section 18.4.1(c) below). The following typical features of modern calenders are instrumental in their operational versatility and effectivity (including certain aspects of product quality). Roll face lengths up to about 3 m are available: large working face length (sheet width) is a factor in output rate, and provides the option of slitting the product into any lower widths required. Individual roll drives, standard on modern machines, enable the roll speed ratios to be widely varied, so that the calender can cope with a range of compositions differing in rheological behaviour: the corresponding available variation of friction ratios between the rolls makes possible a high degree of control over the material in the rolling nip which, inter alia, promotes a good surface finish in the product. In the modern drilled rolls, the close proximity of the drilled channels for circulation of the heating medium (high-pressure hot water or special heatexchange liquid), both to the working surface and the two ends of the roll provides good temperature control, with fast response to adjustments, which makes for improved processing and product quality in comparison with those obtainable with older type cored rolls. Another important feature is the provision for counteracting the deflection of the rolls by the PVC compound being passed through the nips. Because of its relatively high viscosity and compression resistance the material tends to force the rolls apart, the effect being most pronounced in the middle where the restraining effect of the bearings is least, and also increasing with roll face length (sometimes also called face width). The distorting forces developed can be quite high-up to about 1 MN per linear metre of working face length in the production
a
18 Calendering of pvc
807
of thin rigid sheeting: 1·5 MN over the roll face of a 1·68-m (i.e. 66-in) calender has been quoted as not unusual in such conditions. 1 ,2 With a typical plasticised composition a representative range would be about O· 2-0·5 MN m -1. These forces actually deflect the rolls, so that-if the deflection is not corrected or compensated for-the sheet produced can be substantially thicker at the centre than at the edges. The rolls can be suitably contoured (crowned) to counteract this effect on the product, where the deflecting forces are known and remain reasonably constant. In practice this means that roll crowning can be sufficiently effective only over a rather limited range of compositions and processing conditions. Outside this range the crowning will either undercom}.lensate (i.e. with deflecting forces significantly higher than those for which the crowning was designed, the sheet will again be thicker in the middle) or overcompensate (i. e. with deflecting forces significantly lower than those allowed for, the sheet will get thicker towards the edges). Systems are also available in which-to cope with a wider range of deflecting force values-roll crowning is associated with special arrangements for nip adjustment by a combination of lateral and vertical movements of some of the rolls in the stack. * However, the means of counteracting the effects of roll distortion that are the most operationally flexible and widely used in current industrial practice are the techniques known as roll bending and roll crossing (sometimes also referred to as roll skewing or cross-axis roll adjustment). A modern calender is normally equipped with facilities for both these procedures. Roll bending is effected by applying a hydraulic load to the roll journal ends (extended for this purpose beyond the regular bearings); this exerts leverage on the roll (against the bearings) which makes it slightly concave or convex at the nip, the direction of the imparted curvature depending on that of load application. The maximum extent of effective compensatory crown increase or decrease with this method is limited (typically to about 0·075 mm 1 ) by the extra load that may be safely exerted on the bearings. Roll crossing is an angular shift of one or both rolls of a nip-forming pair so that their axes, whilst remaining in their original horizontal planes, are no longer parallel, but form a slight angle: this increases the end clearance between the rolls and hence the amount of material * e.g. the ES Profit Steering calender, developed by Wiik and Hoeglund, Finland. s
808
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V. Titow
that can be accommodated in the resulting enlarged gaps, making for a thickening towards the edges of the sheet being formed. The degree of correction achievable in this way can-in particular conditions-be equivalent to an apparent crown increase on radius of about 0·75 mm. 1 The roll-crossing facility is usually available at least on the last nip of the calender (the gauge-determining or 'gauging' nip).
18.3 THE CALENDERING OPERATION: GENERAL FEATURES, AND THEIR EFFECTS ON THE STRUCTURE AND PROPERTIES OF CALENDERED SHEET The operation of a four-roll calender has been said6 to constitute a form of extrusion with rotating die lips, the material being moved forward by friction against the roll surfaces in its passage through the three consecutive nips, said to function, respectively, as the feed, metering, and final sheet forming and finishing stages. Whilst the calendering operation may indeed be considered in this way in general terms, there is no close analogy with screw extrusion as, quite apart from the different mechanical formats of the two processes, there are essential differences between them in the state of the PVC material and the way in which this is influenced by the processing. Thus, as pointed out in Section 13.4.2(b) of Chapter 13, unlike the extruder, the calender is not required to de-aerate, homogenise and fuse the PVC composition: these functions are performed by the compounding (upstream) part of the calender line, whose make-up and operation are discussed in some detail in the section just mentioned. The calender itself essentially only forms into sheet the homogenised fused (gelled), hot PVC composition which it receives as feed. Note: The amount of shearing and the temperature of treatment on the calender can, in principle, be sufficient to homogenise and gel even a powder feed, and some experiments have been carried out in that direction. 1 However, quite apart from such problems as ensuring adequate, uniform intake of powder feedstock, and effective removal of entrapped air, the residence time required to achieve a homogeneous, fully gelled melt makes for a relatively slow rate of material passage through the machine, and thus works against one of the main advantages of calendering, viz. a high output rate.
18 Calendering of pvc
809
Furthermore, because of the way the final product is formed by the calender roll nips it differs from extruded sheeting in some morphological features and in certain properties associated therewith. This is brought out by the following brief consideration of the material's passage through an inverted 'L' calender. In operation, three banks of material are maintained at the three nips (d. Fig. 18.1): a large relatively narrow feed bank at the first nip (between No.1 and No.2 rolls) and progressively smaller but wider banks at the subsequent two nips (with a full width bank between rolls No. 3 and 4). Because the material is shaped essentially by surface contact with the rolls. the surface of the ultimate product may be regarded as having undergone two re-forming treatments (in the second and the last nips), while the 'core' material inside the sheet is not substantially re-worked after being given its laminar shape by the first nip. In this sense, therefore, the core may be considered to be formed by this nip. The properties of the core-and hence of the whole product-are thus influenced by the uniformity of feed to the calender, and uniformity and homogeneity of the first material bank. Differential cooling of the sheet· surfaces and the core tends to set up stresses in the latter. These can be aggravated locally by non-uniform feed and irregularities in the feed bank, such as, for example, parts of the feed strip finding their way directly (without equalising residence in the bank) into the material being formed by the feed nip: this can give rise to cold streaks in the core with consequent creation of local stress. The extent of development of skin-and-core morphology together with the additional adventitious local strains and stresses, are factors in the strength properties of the sheet and affect its ability to lie flat. They can also cause problems in service (e.g. wrinkling) due to strain recovery. 18.4 CALENDER LINES 18.4.1 General-purpose Line A fairly typical calender line is schematically represented in Fig. 13.40 of Chapter 13, and something of its operation mentioned-with special detailed reference to the pre-calender compounding train-in Section 13.4.2(b) of that chapter. Line arrangements, and in particular those of the pre-calender
W. V. Titow
810
compounding and feed section, can differ in some respects, depending on the type and range of calendering operations undertaken. The schematic flow diagram of Fig. 18.2 illustrates a versatile arrangement (of the kind shown in part in Plate K(1)) of a pre-calender section which incorporates two alternative material paths for respective use with plasticised and rigid (or semi-rigid) compositions. Polymer storage
Plasticiser storage
~
I
Other additives storage
Metering (weighing) devices
111
High-speed (hot) mixer
1 1
Cooling mixer
Feed (metering) hopper
0
r
...... ....
Internal mixer
1
l
®
BussKneader
j
Two-roll mill
1
Metering extruder/strainer
Two-roll mill
.)
~
feed conveyor (with metal detector)
1
CALENDER Fig. 18.2 An arrangement of the compounding and feed section of a calender line providing two alternative material routes. A, Plasticised compound processing route; B, rigid and semi-rigid compound processing route.
18 Calendering of pvc
811
(a) Pre-calender (Compounding and Feed) Section Typical features of a compounding section of a calender line, and main variations of the arrangements used, include the following. The PVC formulation components can be metered in various ways for the initial mixing (blending) operation (see Chapter 13). Of the two main materials, the polymer is normally dispensed by weighing: plasticisers can be metered by volume, but weights should also be monitored for any dispensing adjustments necessitated by variation of density with temperature. The efficiency of the initial mixing, in which a powder blend is produced, is a factor in the uniformity and homogeneity of the calendered sheet. For this reason high-speed mixers are widely employed with both rigid and plasticised compositions. However, ribbon blenders are also sometimes used as the means of first combining the polymer with plasticiser(s) and the other, minor components of the formulation, especially where the subsequent melt-compounding operation (sometimes also referred to as 'fluxing' in this context) is carried out in an internal mixer. As indicated in Fig. 18.2, the melt-compounding unit may be a continuous-operation one, like a Buss-Kneader (or another one of the suitable machines discussed in Chapter 13), or an internal mixer of the Banbury type. Modern versions of these batch-processing mixers are equipped with microprocessor controls which automate the mixing cycle. Various basic models are available, with chamber capacities within the approximate range 40-5001itres: in many cases a standard model can be modified by the makers to suit a client's special requirements. In comparison with a continuous compounder, the advantages of an internal mixer in this application have been listed7 as a relatively wide capacity range (although the output of a batch mixer can be matched by that of a continuous compounder of appropriate size); ease of loading re-work* (promoted by the large loading port and ram operation) and its thorough re-dispersion; generally greater heat transfer capacity; and comparatively moderate routine maintenance costs. The main comparative disadvantages include more drastic working of the stock (a factor instrumental in confining the use of these batch mixers in calender lines to those processing pPVC), and the need to handle large discharged material batches; the connected-power requirements are also relatively high, although the total energy *In some operations re-work is freeze-ground and added to the high-speed mixer-see Chapter 13, Section 13.4.2(b).
812
W. V. Titow
consumption is roughly comparable with that of an equivalent continuous compounder. In general, the batch mixers are best suited to calendering operations involving relatively short runs on different formulations. In an arrangement much used with flexible compositions in the past, but now no longer very common, an internal mixer is followed by two two-roll mills. The batch of compounded material is dumped directly onto the first ('dump') mill where it is sheeted and taken off as an unsupported strip into the nip of the second mill (the 'strip' mill). The strip taken off this mill is carried on a conveyor to the feed bank of the calender. A metal detector would be positioned either over the strip between the two mills, or over that on the conveyor. Note: In the early days of PVC calendering the material off the dump mill might be fed to the calender manually in the form of 'dollys'*, and even the melt-compounding might be carried out on a mill.
Nowadays the arrangement of Fig. 18.2(A) would be fairly typical for processing flexible compositions. In this, the two-roll mill which receives the hot batches from the internal mixer links the batch-wise operation of the latter with the continuous operation of the calender by maintaining a reserve bank of hot, homogenised stock (the working the material receives on the mill can further enhance homogeneity). A continuous strip of material from the mill is fed to a short-barrelled extruder (the extruder/strainer): the functions of this are to remove from the stock extraneous contaminants and any material lumps and particles (e.g. of pigment, filler) that may be present; to maintain the material at a uniform, correct temperature; and to providecontinuously and at the appropriate rate-ealender feed in the form of a flat or round strip (see also Section 13.4.2(b), Chapter 13). Continuous compounders are equally suitable-and widely usedfor both plasticised and rigid PVC calendering compositions. With uPVC this is the type of unit normally employed, in preference to an internal mixer, ,because of its less severe processing action and very good stock temperature control afforded by such machines as the Buss* This spelling is sometimes favoured over 'dollies' (also called 'pigs')-pieces of hot hide cut from the mill and tightly rolled up to reduce heat loss: can cause excessive bowl deflection in the first nip of the calender if not fully integrated into the feed bank.
18 Calendering of pvc
813
Kneader. In some pre-calender line arrangements the compounding machine may feed the calender directly, without the interposition of another unit (see Chapter 13, Sections 13.4.2(b) and 13.4.4(a) for a discussion of this and many other relevant points). It is quite usual, however, for a continuous compounder to be followed in the line by a metering extruder (extruder/strainer) or sometimes a mill (ct. Fig. 18.2(B», either acting in the capacity already mentioned. Where a mill is used it makes a convenient addition point for edge-trim that is to be re-worked, because of the manipulative ease, and to keep down the heat history of such material (which has already experienced a full heat processing cycle). The Buss-Kneader is very popular as the continuous compounder in modern pre-calender trains. Planetary extruders have also proved their worth in this application (see Chapter 13). An interesting example of the use of a planetary extruder in a purpose-designed but fairly basic calendering set-up is the Berstorff* 'Rollex' line,3 intended for what by general calendering standards is comparatively small-scale manufacture of rigid calendered sheet for thermoforming (at a quoted typical rate of up to 500 kg h -1), speciality sheeting, and a wide range of rigid and flexible PVC sheeting for limited local markets (e.g. in developing countries). The meltcompounding and feed section of the line comprises a planetary extruder feeding directly into a single-screw metering extruder (LID ratio 8: 1) which feeds its hot extrudate (formed into a strip by a simple slot die) into the nip of a three-roll calender. Both extruders are available in several sizes (the planetary screw size ranging from 100 mm to 290 mm, with rated outputs for the latter size of about 3300 kg h- 1 for uPVC and 4000 kg h- 1 for pPVq,3 but the metering extruder always has the larger screw size (e.g. 200 mm if the planetary machine's screw is a 140 mm one): this permits running at screw speeds low enough to avoid substantial work-heat input into the stock. The compounding extruder takes a powder blend feed introduced by a hopper-mounted vertical force-feeder. The hopper also houses a metal detector, particularly important if scrap is being processed (although metal detectors are standard equipment in pre-calender trains-see below). A special screw design featured by the planetary extruder is said to make for good output rates with compositions based on PVC polymers of high K value, and with highly filled compounds. * Berstorff Maschinenbau GmbH, Hanover, West Germany-see also Table 14.3 in Chapter 14.
814
W. V. Titaw
Plate K Calender train (AECI Ltd Vinyl Products Division Midland Factory, RSA-eourtesy Mr. W. B. Duncker). (1) Part of the compounding and feed section.
As indicated in Fig. 18.2 (and discussed also in Section 13.4.2(b) of Chapter 13), the final member of a typical pre-calender train is a conveyor which carries the feed strip up to the calender feed bank. The conveyor may consist of one or more sections. The final section is usually swivel-mounted, so that the end can traverse the length of the feed nip for uniform feed distribution. To help maintain its temperature, the strip should have the lowest practicable specific surface (i.e. it should preferably have a circular cross-section or, if flat, be thick and narrow rather than thin and wide), and/or should be heated by some
18 Calendering of pvc
Plate K-contd.
815
(2) Rear view of calender.
means (for example by IR heaters) if it is carried over a long distance (say more than about 2 m). Where heating is employed the material of the conveyor should be suitably heat resistant. The metal detector, always employed in the pre-calender train to guard against damage to the calender rolls by any fragments of metal that may find their way into the PVC composition, is commonly positioned over the feed conveyor (although-as mentioned in passing above-other locations may also be used): see also discussion of the role of metal detectors in Section 13.4.2(b) of Chapter 13.
816
W. V. Titow
Plate L Calender train: part of the cooling-roll and take-off section (AECI Ltd Vinyl Products Division Midland Factory, RSA-eourtesy Mr W. B. Duncker).
(b) Calendering Several features of the calender and its operation have already been discussed in Sections 18.2 and 18.3 above. A few further important points should be mentioned. As the PVC composition fed to the calender is normally already gelled and molten, the machine's main task is to form it into a uniform sheet of the required thickness (although the material also receives a certain amount of mechanical working-see below). The forming is carried out gradually, in the course of passage through the consecutive
18 Calendering of pvc
817
roll nips. Two factors actuate the passage, their operation also governing the route the material follows through the machine (referred to as the 'sheet path') in that they determine whether the sheet is or is not transferred from one roll of a nip-forming pair to the other. These factors are the material's adhesion to the roll surfaces, and the ratio of the roll speeds at a nip, usually called the friction ratio. Unless it is grossly over-lubricated, a hot PVC calendering compound will adhere to a hot roll surface: the adhesion is always stronger to a matt than to a polished surface; it also usually increases with the roll temperature, although some compositions may be formulated to minimise or even reverse this effect by suitable selection of the lubricant system. Where the speeds of the nip-forming rolls are different, the material will run on the faster roll, if both have the same surface finish. Otherwise the effect of the finish is strongly dominant, that is to say if one of the nip-forming rolls is polished and the other matt, the hot sheet will tend to remain on, or transfer to, the matt roll irrespective of differences in speed and/or temperature. In practice the sheet path is conveniently established-and changed where necessary-by suitable settings of roll temperatures and friction ratios. Save in the special case of the so-called low-temperature mode of operation (see Section 13.4.2(b) of Chapter 13) it is usual to have the roll temperatures (as well as the speeds) going up with roll numbers. Note: If the sheet does not lap the last roll (e.g. roll No.4 in the
arrangement of Fig. 18.1(ii» then that roll will normally be run at a lower temperature, and usually a lower speed, than its partner (roll No.3 in the figure) which is required to retain the sheet. Some examples of roll temperature settings in particular runs on particular machines are given in Tables 18.1 and 18.2. As can be seen, the individual settings for a nip-forming pair can be very close: as a general rule, the maximum difference between them should not normally exceed lO°e. If, with optimum friction ratio, a difference of this order still does not make for satisfactory operation, the temperature measuring and control equipment should be checked and/or the lubricant system of the composition re-formulated. The nature of the compound, and in particular its lubrication, is always a factor in the roll temperature settings: in general, for normal industrial
TABLE 18.1
Some mustrative Features of Industrial Production of PVC Sheeting on an 'L' Type Calender with Highly Polished RoDs (Based on data supplied by Mr D. J. Sieberhagen of Vynide Ltd.) Composition type Rigid (crystal clear)
Flexible (opaque)
Basic formulation PVC homopolymer resin Stabilisers Lubricants Plasticiser Processing aid Impact modifier Filler Pigment
Processing conditions High-speed mixer Tool tip speed Mix temperature on discharge Cooler mixer Tool tip speed Mix temperature on discharge Buss-Kneader Compound temperature at die head Internal mixer Discharge temperature Two-roll mill Roll 1 temperature Roll 2 temperature Calender Roll 1 temperature Roll 2 temperature Roll 3 temperature Roll 4 temperature Stripping device Cieneral temperature Embossing roll Temperature Cooling train Temperature
87 pbw (K value 57) 2·5 pbw (organotin) 1·5 pbw (internal! external)
100pbw (K value 71) 2·5 pbw (BalCd liquid) 0·5 pbw (external) 46 pbw (DIDP)
5pbw 8pbw 5 pbw (coated CaC0 3) 8pbw
TABLE 18.1-eontd. Composition type Rigid (crystal clear)
Flexible (opaque)
Sheet properties
Tensile strength (yield)" Elongation at breaka •b Tear strength b (ASTM D 1922): in machine direction in transverse direction Tensile impact strength (DIN 53448)
49-60MPa
22-29MPa 206-273%
4·5--6·9 kgfmm- 1 6·(}-8·4 kgf mm- 1
8·7-11·3 kgf mm- 1 12·1-12·9 kgf mm- 1
323--555 kgf cm cm- 2
a Dumbbell b
specimens, with parallel central portion 10 mm wide. Sheet 251-350 11m.
TABLE 18.2 Some Wustrative Features of a Laboratory Preparation of Flexible PVC Sheeting on an Inverted 'L' Type, 61-cm (24-in) Calender Producing at 3mmin- 1 Formulation
PVC resin-Corvic R65181 a Stabilisers: BalCd liquid complex epoxidised oil Plasticiser: DAP Lubricant: stearic acid Calender
Offset (No.1) roll temperature (0C) Top (No.2) roll temperature COC) Middle (No.3) roll temperature (0C) Bottom (No.4) roll temperature (0C) Mechanical properties of sheeting
Tensile strength (MPa): in machine direction in transverse direction Elongation at break (%) in machine direction in transverse direction Tear strength (N per mm of thickness): in machine direction in transverse direction
100 phr 2 phr 3 phr 47phr 0'5phr For 0·125 mm sheeting
For 0·500 mm sheeting
155 155 160 165
158 164 168 170
0·125mm sheeting
0·500 mm sheeting
16·2 16·4
20·5 19·4
225 250
305 300
63 63
67 68
A high molecular weight VCNA copolymer containing 2% VA; porous, easyprocessing particles; recommended for extrusion and calendering of thick sheet.
a
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W. V. Titow
production, these will be within the extreme overall range of 15O-195°C. The material of the banks at the roll nips experiences an amount of shearing and friction increasing with roll speeds and speed ratios: when these are sufficiently high the energy input can result in substantial temperature rises. Such work-heating must be allowed for in the roll temperature settings, to ensure that the composition is not overheated (in some cases the rolls may have to be kept at temperatures below the material temperature aimed at). Subject to this general consideration, and any special ones that may arise in individual cases, it is normally desirable to operate at the highest practicable material temperature for ease of processing (lowest material viscosity) and good ultimate sheet properties. With roll temperature settings and friction ratios increasing from feed to delivery, the material temperature also rises: in the final nip it can, for a short time, be as high as 200°C (although accurate measurement is difficult).8 For material coming off the last bowl a fairly typical range is 17o-175°C. (c) The Post-calender Train The post-calender section of a modern line is shown schematically in Fig. 18.3, with the principal components clearly labelled. The figure is largely self-explanatory, but the following additional points may be mentioned. STRIPPER ROLLS, AND OFF-THE-CALENDER STRETCHING
The hot sheet is removed from the calender bowl on which it runs after the final nip by a stripper (pick-off) roll. Whereas formerly the use of a single pick-off roll was quite common, in modern practice a set of rolls is employed, in which the first stripper roll is backed by a large number of similar rolls (see Fig. 18.3), temperature-controlled in groups of two or more: this makes for good control over sheet thickness and for uniform, gradual cooling, or close maintenance of temperature (with provision for an extra heat boost by an IR heater as shown in Fig. 18.3, or by passage over a hot drum) if the sheet is to be embossed in-line. The fact that each side of the sheet is in contact with alternate rollers (see Fig. 18.4) is a factor in the uniformity of temperature control. In the common arrangement where the first pick-off roll counter-rotates with respect to the calender roll, the stripping action takes place by virtue of the greater peripheral speed of the stripper and the lapping of the sheet round it (and the other rolls of the set-see Fig. 18.4).
18 Calendering of pvc
821
The running of the stripper rolls may be so regulated that the sheet coming off the calender is stretched to a predetermined extent whilst it is still at a temperature significantly higher than any that will be reached in subsequent processing or in service (in conditions where retraction of the stretch with consequent undesirable distortion is possible). This procedure offers the following advantages. Greater operational flexibility, in that a calender profiled, roll-loaded and set to produce one thickness of sheet can also be used for a range of lower thicknesses: furthermore, the lowest thickness limit is effectively brought down below that for which the machine can be set. Note: For example, sheet of thickness about 75 J.lIIl (a typical lower thickness limit for calendered sheeting) would normally be produced by stretching down a sheet calendered at a higher thickness, say about 100 11m.
A setting of the final calender nip (the gauging nip) wider than would be required for direct production of the sheet thickness that is ultimately achieved by stretching, means lower distorting forces on the rolls and somewhat less drastic working (and heating) of the material. Calendering at greater thickness can also reduce the overall power requirements. Stretching of the sheet should not be allowed to cause a substantial reduction in width, or be carried to the point of excessive uniaxial orientation of the ultimate product: uniformity of thickness should also be maintained. These considerations are factors which-together with the composition of the sheet and the actual calendered thickness-limit the extent of sensible stretch in a given situation. For suitably formulated sheet, calendered at a thickness reasonably above the minimum for which the machine can be set, stretching (and the corresponding thickness reduction) by a factor of about 2 would not be uncommon: say a sheet calendered at a nominal 280l1m might be stretched down to 14(}-120l1m. EMBOSSING
Although calendered sheeting can be embossed away from the calendering line in an entirely separate operation (which involves re-heating to a suitable temperature), embossing is often carried out in-line. This obviates the need to set up separate equipment (which, inter alia, has to duplicate sheet heating and cooling), saves heating energy (as the sheeting does not have to be re-heated, but merely kept
822
W. V. Titow
Fig. 18.3 An 'L' calender with post-calender section. Schematic hot on leaving the last nip of the calender), and avoids the acquisition of extra heat history. Before the advent of multiple stripping rolls with close temperature control it was usual to have the embossing station positioned as closely as possible to the calender, for minimum heat loss from the sheet, although a heat boost would still normally be given at least to semi-rigid or rigid material which, for optimum embossability,
A
B
Fig. 18.4 Schematic representation of some stripper roll arrangements. A, The lapping of hot sheet around the stripper rolls; B, a reverse stripping arrangement.
18 Calendering of pvc
T'''''
823
r
,....-,--------,
representation. (Courtesy of Mr D. J. Sieberhagen, Vynide Ltd.) must be at a higher temperature than that suitable for flexible sheeting. A typical embossing unit consists of a chilled, metal roller engraved with the emboss pattern, and a back-up roller-usually of substantially larger diameter-covered with a synthetic rubber to provide the requisite degree of resilience. It is normal for the backing roller to be also cooled-internally and/or externally-to prevent distortion and possibly deterioration of the rubber, and to maintain a reasonably constant level of resilience. Other factors being equal, the definition of the emboss pattern is the better the lower the temperature of the embossing roller and the higher the nip pressure, although the optimum pressure will vary with sheet formulation, as well as its temperature and rate of passage through the unit. One (either one) of the unit's two rollers is positively driven: the drive is independent of those of the other parts of the line, but must be suitably synchronised to avoid stretching the sheet. Note: Prevention of uncontrolled stretching, and generally of introduction of strain into the sheet in any part of the post-calender train (after the initial, planned hightemperature stretching off the calender-see above), and especially avoidance of such straining at relatively low temperature (below about 100°C), is an important considera-
824
W. V. Titow
tion in the running of the train. The presence of lowtemperature strain impairs the dimensional stability of the sheeting in any subsequent heat processing (e.g. heat lamination) and in service. A typical example4 of a problem that may be caused by such strain is the wrinkling or puckering of interior car door trim produced from calendered PVC sheeting: during its manufacture, or in subsequent use, the temperature inside a car can reach about 80a C (or even higher in hot countries), giving rise to these unsightly faults as a result of the reversion of any substantial strains originally introduced at or below such temperatures.
COOLING
Since the early days of calendering the most common method of cooling the sheet has been by passage around cooled drums (also known as 'cans'), although air-cooling on conveyors has been tried, and thick sheet has been cooled by passage through a water bath. l The temperature of the drums is kept at the required level by circulation of water through the annular space between the outer wall (which constitutes the working surface) and an inner shell. Hollow drums cooled internally by water sprays have been used in the past, but both their temperature control and their balance in running are less satisfactory for the precise cooling rate adjustment and fast, stable rotation required with high production rates. Apart from the wide adoption of the 'double-skinned' drum just mentioned, the other-and indeed the main-developments over the years in the typical cooling section of the post-calender train have been an increase in the number of cooling drums, with improvement in their temperature control (in small groups, or even individually), and improvements in the speed drive. These developments were aimed at a better, more complete attainment of the objects of the cooling operation, which are to bring the temperature of the sheet gradually and evenly down to that of the surroundings (or even somewhat below ambient), without thermal shock (which can impair the sheet properties, especially with rigid compositions) and without introducing low-temperature strain. In typical modern practice these objects are achieved by passing the sheet around a large number of cooling drums, running uniformly (under an independent drive but in synchronisation with the rest of the train), and temperature-controlled at gradually decreasing values: as an
18 Calendering of pvc
825
example, in the cooling section shown in Fig. 18.3, drums 1-3 might be kept at 50°C, drums 4-6 at 40°C, drums 7-9 at 25°C, and drums 10-12 at a temperature about 5-lOoC below ambient. As indicated in the figure, the sheet is lapped around the drums in such a way that each of its surfaces is alternately in contact with consecutive drum surfaces: this promotes even, uniform cooling. SHEET THICKNESS MEASUREMENT AND CONTROL
The monitoring of sheet thickness is a necessary part of production control. For many years a ~gauge has been virtually standard equipment for on-line thickness measurement. This device determines continuously the extent to which the passage of a beam of electrons from a radioactive source scanning across the moving sheet is obstructed by the sheet material. Thus the sheet property actually measured is the mass per unit area; but as this is directly proportional to thickness (since the density of the material is fixed by the composition) the read-out is in terms of thickness (and its variation across the sheet).
Note: The nature of the measurement also means that the thickness obtained is gravimetric thickness. * For a plain sheet this is the same as the directly measurable 'geometrical' thickness. For an embossed sheet-where the geometrical thickness cannot be measured because of the surface contouring produced by the emboss-the gravimetric thickness is the appropriate one, and it is thus particularly useful that this is what is measured by the ~gauge. More recently, the scanning ~gauge has been integrated into a complete, computerised on-line control system incorporating feedback control loops: the thickness data from the gauge are utilised by microprocessor-based controls which automatically make continuous adjustments to the roll-crossing and roll-bending devices to keep the * The notional average thickness (t) which, for a given piece or section of uniform sheet is determined by the area (A), mass (m) and density (p), in accordance with the relationships: m/At = p;
t = m/Ap
Standard laboratory methods of measuring gravimetric thickness9-normally prescribed for embossed sheeting-involve weighing a specimen of known area and determining, separately, the density of the material.
826
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V. Titow
sheet thickness and profile within acceptable tolerances around the appropriate set target values. This kind of sophisiticated, integrated control system is exemplified by the well-known Measurex 2001/25 Vinyl Calender Control System *.10 In addition to the automatic closedloop control this provides multi-colour visual data displays and print-outs of production and control data (including sheet profile plots and figures in relation to the target values), process summaries, trend plots, etc. A particular feature is the 'target adaptive control' (TAD): this automatically adjusts the sheet thickness very close to a specified percentage-of-nominal limit, so that the finished sheeting area or weight per roll is optimised. EDGE TRIMMING
The edges of the sheet are trimmed, to eliminate unevenness which commonly arises in calendering. The trimming is normally done towards the end of the line, after cooling (cold trimming), although hot trimming on the calender is also possible. It is sometimes claimed that the latter procedure is advantageous because of the particular ease of re-circulating the hot trim by feeding it directly to the final two-roll mill or metering extruder of the calender feed section, or even straight into the calender feed nip. However, with a feed section of the kind illustrated in Fig. 18.2 there is normally no great problem about feeding the cold trim from rigid sheeting directly to the two-roll mill (of the B route in the flow diagram), or that from flexible sheeting into the internal mixer. In the former case, an end of the strip of rigid trim (which is wound into a roll during the trimming operation) is fed into the nip of the mill, where it is continuously melted and incorporated into the bank of hot stock as it is being unwound. The rolls of flexible trim strip can simply be dropped, without unwinding, into the internal mixer, where the heat and shearing action during the mixing operation will normally be sufficient to disperse them completely into the charge of virgin material, providing that the proportion of re-work so introduced is not excessive. Thus hot trimming on the calender is not really essential for fairly direct re-processing of trim, whilst it does present certain problems. One is that a bead tends to form on the hot-trimmed sheet edges: this can cause difficulties in the ultimate winding and-in any case-makes the edge finish less neat than that produced by cold trimming. Another *Measurex Corp., USA.
18 Calendering of pvc
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disadvantage is marking of the calender roll surface by the trimming knives. Cold trim can also be comminuted (e.g. by freeze grinding) and re-circulated by addition, in suitable proportions, to the high-speed mixer, where it is incorporated into the powder blend being produced from the components of the formulation (see Chapter 13, Section 13.4.2(b)). WIND-UP
Most calendered sheeting is wound up into rolls for subsequent storage, handling in post-calendering operations (e.g. printing or lamination), transport, etc. Note: The sheeting may occasionally be slit in-line prior to winding, where a number of specified, narrower widths is required. It is also sometimes cut into lengths or panels for further processing (e.g. press polishing or lamination) or for use: in such cases the roll-winding gear is replaced by cutting devices (rotary kn:ves or guillotines) followed by any necessary stacking and/or take-off arrangements.
The winding arrangement widely used in modern practice for both rigid and flexible sheeting is a centre core wind-up unit, in which the sheeting is wound onto a core (of wood or thick cardboad) mounted on a mandrel turned by a constant-torque drive that keeps the winding tension in the sheet at a constant, set value despite the rise of peripheral speed as the roll builds up on the core. It is important that the tension should be both constant and as low as possible (consistent with the production of a reasonably tight, stable roll), to minimise the introduction of low-temperature strain into (and its fluctuation in) the sheeting. It is self-evident that the drive of any wind-up unit, whilst independent, must be synchronised ('tracked-in') with the calender speed and the other drives in the post-calender train. A cheaper system, formerly in common use (and still employed in some lines) for flexible PVC sheeting is contact batching. In this, the core is not rotated directly, but driven round by surface friction between the roll of sheeting being built up upon it and a revolving wind-up drum. With this arrangement the tension in the sheet is more variable, and the control over introduction of low-temperature strain less good, than with a centre-core wind-up. Rigid sheeting is not wound up by contact batching, because the surface friction between
828
W. V. Titow
the driving drum and the roll of sheeting is not sufficient to maintain-in the relatively stiff material-even tension at the required level, so that slipping occurs which disrupts and impairs the operation. 18.4.2 Special Lines and Arrangements
(a) Calendered Flooring Lines Heavily filled vinyl/asbestos flooring compositions are difficult to process on a conventional calender because of their stiffness, hardness and relatively low resin content. Moreover, the thickness in which the material is required may range up to 5 mm. For these reasons material of this kind (for use as continuous flooring or cut into tiles) is calendered on a sequence of two or three individual two-roll calendering units, with vertical or inclined roll arrangement. The compounding and feed section of a typical line may be similar to that shown in Fig. i8.2(A), but with two mills between the internal mixer and the first calendering unit. In the production of the familiar mottled PVC flooring or tiles, multi-coloured vinyl chips are added either on the first (dumping) mill, or the second (sheeting) mill which feeds the first calendering unit. The functions of this and the subsequent units is essentially to roll out the sheet as it makes its straight passage through the nip, so that the sheet thickness is progressively reduced. The operation is thus somewhat akin to the rolling out of ingots in sheet metal production. The sheet emerging from the nip of the final unit is cooled and wound or cut up, as required. Fully flexible flooring compositions, whose relative resin content is substantially higher, can be processed in the normal way: three-roll calenders are quite often used, or four-roll calenders operated with a 'spinning' bank or a 'spewing' bank of stock before the second nip (both these promote maintenance of material and temperature homogeneity in the stock). Sheets of suitable flooring compositions calendered in the normal way on a three- or four-roll calender can be laminated-by various techniques-to produce multi-layer flooring of the type conventionally, if somewhat incongruously, styled 'homogeneous'. The individual plies may be of the same or different composition. For example, a three-layer laminate (which is fairly common, although flooring with a higher number of plies is also produced), may be made up of a highly filled base layer, a medium-filled middle layer with a decorated surface, and a clear, tough, top (wear) layer: alternatively, three layers
18 Calendering of pvc
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of sheeting of the same reasonably wear-resistant formulation, and calendered to the same thickness (fairly typically about 0·5 mm), may be laminated together, the sheeting with the best looking surface being used as the top layer. (b) Lamination on or at the Calender Calendered PVC films can be laminated-on the calender or in-line-to other continuous web materials, such as textile fabrics, felt, paper and films. Lamination in an entirely separate operation is mentioned in Section 18.7 below. In principle, the range of PVC-coated fabrics and papers which can be produced by lamination with flexible calendered film is equivalent to that of similar materials made by paste coating. In practice the coating route offers greater formulation versatility and operational flexibility, especially for relatively short runs, for multi-layer coatings, and where frequent formulation changes are made. Calender lamination can be economically attractive where long, continuous runs without formulation changes are involved. Product properties are broadly comparable, but the range and quality of cellular coating layers are better in paste-produced coatings, whilst solid coatings of this kind are completely free from orientation and strain, which may not invariably be the case with some laminated film coatings. There are three basic ways in which a freshly calendered PVC film can be laminated to a continuous web of another material. These are schematically illustrated in Fig. 18.5, which indicates the modifications to the normal calender set-up entailed by each method. Additional equipment is also normally necessary for unwinding the web to be laminated (substrate to be coated) and its conveyance to the laminating nip, as well as-especially with fibrous webs (fabrics and paper)arrangements for drying and pre-heating the web. The in-line lamination method shown in Fig. 18.5(C) is of special interest with heat-sensitive SUbstrates, or where an adhesive layer is to be applied prior to lamination. In the calender nip lamination arrangement illustrated in Fig. 18.5(A) the degree of penetration of the hot film into a porous substrate, say a fabric being coated by this method, increases with decreasing gap between the nip-forming rolls and with increasing friction ratio between them: zero friction and a suitably wide gap should be employed for minimum penetration. Similar considerations apply to fabric lamination on the calender roll with a squeeze roller, as shown in Fig. 18.5(B).
830
W. V. Titow
A
B
Fig. 18.5 Lamination or web coating with PVC film on an inverted 'L' calender. Schematic representation. A, Nip lamination; B, lamination, with a squeeze roller, against calender bowl; C, in-line lamination.
18.5 THE FORMULATION ASPECT
In formulating PVC compositions for calendering-as indeed with any kind of PVC formulation for industrial processing-it is necessary to take into account the considerations and requirements arising from the nature and conditions of the process, together with the end-use requirements and cost considerations, and to arrive at the best practicable compromise. The nature of the calendering process makes the lubricant system of cardinal importance, especially the extent and balance of external lubrication which is the formulation factor directly instrumental in the degree of adhesion of the composition to (and hence also ease of its release from) the calender rolls in the course of processing. External lubrication is necessary in both plasticised and unplasticised formulations. The role of internal lubricant additives in rigid compositions, whilst important, is more in line with the relevant general requirements of melt processing (see Chapter 11, Section 11.1). The same is broadly true of processing aids.
18 Calendering of pvc
831
Much calendered sheeting is surface printed, whilst some largeoutlet applications (e.g. as reservoir lining, upholstery material, luggage covering) involve welding of the sheeting (ct. Chapter 20). Print adhesion and bond formation in welding can both be impaired by excessive external lubrication (as can the fusion in press laminationsee Section 18.7.2 below). The intensive working and relatively high temperatures experienced by calendering compositions in modern high-rate production entail the need for good heat stabilisation. This is especially important with rigid compounds, and most particularly those used in the production of high-clarity transparent colourless foil and sheeting for packaging applications. For these reasons clear, rigid PVC calendering compositions (as well as some pigmented ones) are commonly stabilised with organotin stabilisers (where relevant, ones permitted for food contact applications) . Barium/cadmium systems, with or without zinc, are the stabilisers most widely used in flexible calendering compositions. The liquid versions are of particular interest for ease of metering for compounding, and good dispersibility. However, in highly plasticised compositions a solid system may be preferable to avoid lowering the melt viscosity. This is the current position, although the question of toxic hazards associated with the use of cadmium compounds has been receiving increasing attention and development effort, which has already produced some viable (albeit still not really fully operationally equivalent) alternative stabiliser systems (see Chapter 9, Section 9.4.3(b)). Lead stabilisers, historically the first to be used in calendering formulations, are no longer employed on any scale because of their toxicity, somewhat inferior effectivity in high-speed processing, and generally limited suitability for clear formulations. The PVC resins used in calendering compositions are grades of suspension or mass polymers (the latter favoured for some high-clarity sheeting), of particle size, size distribution, porosity, and bulk density appropriate to the requisite high-rate processing suitability, inter alia, with regard to such features as ease and uniformity of dry flow in metering and dispensing, ready blending with other formulation components in high-speed mixing (including fast, uniform plasticiser absorption), and ease of fluxing and gelation. Flexible compositions are normally based on high K value resins (see, for example, Tables 18.1 and 18.2). This promotes good physical properties important in
832
W. V. Titow
service, whilst the associated increase of melt viscosity in processing (which prevents the use of high molecular weight resins in rigid compositions) is actually helpful-especially with highly plasticised formulations-as it compensates, at least to some extent, for the opposite effect of plasticisers. Indeed, one of the functions of a filler in a strongly plasticised material may be to bulk ('sludge') it up for higher melt viscosity. In rigid compositions, the customary use of polymers of relatively low K value is dictated (as also in other melt processes, e.g. injection moulding, blow moulding-d. Chapters 15 and 17) by the need to ease processing behaviour by all available means. The same consideration lies behind the use of copolymer resins in some types of composition-e.g. VCN A copolymers in calendered flooring, or in sheeting for thermoforming-(although acrylic-modified, homopolymer-based compositions are also widely used for the latter), and copolymers with vinylidene chloride (e.g. Breon CS lOOl30-BP Chemicals Ltd) in some special formulations. The choice of plasticiser(s) for a flexible calendering composition is primarily influenced by end-use and cost requirements, but consideration should also be given to correct melt rheology in processing and to possible effects (desirable or otherwise) of the plasticiser(s) upon the degree and balance of lubrication. Chlorinated polyethylene of appropriate (relatively high) chlorine content (cf. Chapter 11, Section 11.2.2) is being increasingly used as a solid plasticiser in calendered sheeting, because of the resulting combination of permanence of plasticisation with good resistance to weathering, important in such products as, for example, reservoir linings. The cost of blends of this kind can be reduced-without too drastic an impairment of properties-by the incorporation of phthalate plasticisers. An interesting study by Young l l indicated that some compositions based on a blend of PVC resin of relatively high molecular weight (Diamond 450Diamond Shamrock Chemical Co.) with a chlorinated HOPE (containing 42% or 46% chlorine: respectively, XO 2243.49 and 2243.51-Dow Chemical Co.) incorporating diundecyl phthalate (Monsanto) or linear phthalate esters (Santicizer 71l-Monsanto) compared favourably in mechanical properties and long-term performance with ones plasticised with polymeric plasticisers. The fillers most commonly encountered in calendered products are asbestos fibres in PVC flooring, and calcium carbonate, also used as a particulate filler in some flooring compositions, and as a cheapening filler with some processing-aid and reinforcing effects in certain other
18 Calendering of pvc
833
types of sheeting (e.g. supported or unsupported vinyl upholstery material). The general use and effects of both these kinds of filler are discussed in other chapters (see in particular Chapter 4, Section 4.6.2; and Chapter 8, Sections 8.2.1, 8.3 and 8.4), with reference, inter alia, to the special stabilisation requirements and rheology of asbestoscontaining flooring compositions, and to the principal effects of the surface treatment and particle size characteristics of calcium carbonate fillers. With regard to the latter, it may be recalled that whilst with untreated grades plasticiser demand (and such reinforcing effects as may arise) is normally an inverse function of paticle size, this effect can be counteracted by surface treatment with stearic acid, some stearates, and certain other substances. Apart from reducing plasticiser absorption by the filler (and hence the amount of plasticiser required in the formulation) surface treatment can improve the filler's dry-flow characteristics and dispersibility in dry blending and melt compounding. However, possible lubricating effects of the surface coating must be considered and allowed for in the choice and formulation of the lubricant system. Some useful data on the effects of particle surface area and surface treatment of CaC0 3 fillers on the properties of calendered flexible PVC sheeting have been published by Mathur et al. 12 The basic make-up of some calendering compositions is indicated by the formulation examples in Section 4.6 of Chapter 4, and in Tables 18.1 and 18.2 here. However, successful formulating for the calender calls for considerable skill on the part of the formulator as well as knowledge not only of calendering and PVC materials generally, but also of the special requirements (especially with regard to lubrication) of the particular equipment concerned, as these may vary significantly from one calender line to another. 18.6 SOME FAULTS AND DEFECTS OF CALENDERED SHEETING
Several of these can also occur in extruded sheeting, in the same or similar form (see Chapter 14, Section 14.2.2(e)). 18.6.1 Simple Dimensional Faults
These are rare with good modern equipment and correct operation. However, the following may be mentioned for the sake of completeness.
834
W. V. Titow
Excessive thickness variation and profile irregularity: This fault can result, possibly temporarily, from faulty operation of the monitoring and/or control equipment, but would not continue undetected for long on a modern line properly run. Local dimensional irregularities: These are associated with 'bagging' of the sheet in the transverse direction, or longitudinal sagging, on stripping or in further passage through the post-calender train. These faults, which are due to uneven or inadequate support of the sheet while still hot, do not normally arise with modern equipment properly operated. Thickness generally too low or too high: Such faults can be caused by incorrect calender setting, or-with plasticised compositions-may sometimes arise as a result of, respectively, over- or underplasticisation, even if the setting is basically correct (for a properly plasticised composition). 18.6.2 Structural Defects
Presence of strain imparted at relatively low temperature: The origin and consequences of this fault are mentioned in various parts of Section 18.4.1(c) above. Unsatisfactory lay-flat behaviour: This can cause problems in winding and in subsequent handling, processing and use of the sheet. The fault may be associated with development of excessive skin-and-core structure in the sheet (and/or irregularities of such structure) during processing on the calender and subsequent cooling (see Section 18.3 above); it may also be caused or contributed to, by the kind of uneven stretching or inadequate support of the hot sheet that leads to bagging and sagging (ct. Section 18.6.1). Excessive uniaxial orientation: This can be caused by over-stretching the sheet on stripping from the calender. Although imparted at a temperature which should be high enough not to lead to retraction troubles in subsequent use (as in the case of low-temperature strain) such stretch can result in unduly high differences in strength in the machine and transverse directions (low tear strength lengthways; low transverse tensile strength) and excessive longitudinal retraction at
18 Calendering of pvc
835
high temperature (high-temperature strain release). This fault is rare in modern calendering practice. Below-par mechanical properties: These are manifested as generally low values of the properties most commonly measured, viz. tensile strength, extensibility, and tear strength of sheeting (both flexible and rigid), and low impact strength of rigid sheeting. The most likely cause of this fault is insufficiently high processing temperature on the calender. 18.6.3 Faults Manifested in Appearance 'Fish-eyes' (nibs): These hard, undispersed particles of polymer may be introduced with substandard PVC resin, or persist in consequence of incomplete gelation (see also Section 14.2.2(e) of Chapter 14). Flecking ('fleck marking'): The flecks are a characteristic manifestation of an incorrectly functioning lubricant system (often excess of external lubricant). The fault may be aggravated by (and in extreme cases even due to) under-gelation of the composition, and/or incorrect calendering temperatures. Plate-out, on sheeting and/or equipment: The origins and nature of this fault are discussed in Chapter 9 (Section 9.7) and Chapter 14 (Section 14.2.2(d». Some of the main factors responsible are the same as those involved in flecking, although plate-out is a more complex phenomenon. Pluck marks: These are manifestations of poor release of the hot sheet from the calender roll surface. The release difficulty can be c~used by under-lubrication of the composition, or sometimes by overheating in compounding and/or too high roll temperatures in calendering. Heat lines: These marks take the form of continuous fairly close-lying lines running in the machine direction. They are usually associated with difficulty of sheet release, commonly due to excessively high calender roll temperatures; over-processing of the composition in the compounding section may also be a factor. In severe cases heat lines may be succeeded by pluck marks.
836
W. V. Titow
Surface roughness: In severe cases this may take the form of pronounced rough surface marks. The fault is usually due to under-gelation and attendant incomplete homogenisation of the sheet material. The manifestation is broadly in line with extrusion experience, in that surface roughness in extruded products (if not caused by purely rheological factors) can be reduced by increasing the processing temperature and residence time, both of which promote gelation. Bareich's discussion 13 of the form and measurement of surface roughness in pPVC extrudates, and of the processing factors instrumental in its origin, is of some interest here as part of the relevant background. In some cases surface roughness may be aggravated, or even caused, by incorrect operation of the calender. Bank marks: Typically these have the form of irregular areas of slight surface roughness, reminiscent of water marking of paper. In severe cases the marks may merge into an overall orange peel effect. The fault is caused by patches of compound cooler than the bulk of the stock going through the roll nips: this can be due to incorrect size (too large or too small) of the stock banks, or to temperature variation in the stock as delivered to the feed nip. Colour streaking (in coloured sheet): The streaks are due to poor homogenisation of the composition, usually associated with undergelation (which may also be manifested in some of the ways mentioned above). Overall discoloration: Commonly this is a result of incipient or substantial polymer degradation (the severity being reflected in the depth of colour developed), attributable to inadequate stabilisation, or overheating of the composition in compounding. Dark specks: These may be either foreign particles small enough to pass through any straining device employed in the feed section (e.g. the screen of an extruder/strainer), or particles of partly degraded material resulting from overheating at the compounding stage. 'Crow foot' marks: The marks (in some cases also referred to as 'pine trees'), which resemble a bird's footprint, are usually attributable to poor dispersion of particulate additives (fillers or pigments).
18 Calendering of pvc
837
Pinholes: This is a fairly common fault in calendered sheeting, especially the flexible type, often due to faulty gelation. Pinholing adversely affects the barrier, strength and other properties of the sheeting (including the appearance, in more severe cases). Large pinholes, or substantial incidence of smaller ones, are normally detectable at the inspection panel before the final wind-up (see Fig. 18.3). However, special automatic inspection by laser scanning 14 offers a particularly effective means of detecting the presence and loation of pinholes, as well as gauging their size; the time elapsing before any necessary remedial action can be taken is also minimised. 18.7 FURTHER PROCESSING OF CALENDERED SHEET 18.7.1 Press Finishing Substantial quantities of sheets produced by cutting up calendered sheeting (especially rigid and semi-rigid) are finished by hot pressing between suitably surfaced metal plates (polished or matt). A stack of sheets, interleaved with the plates, is usually processed in one cycle.
Note: A typical cycle, which comprises a heating and a cooling period of roughly equal durations, may last about 40 min, with the set temperature peaking at about 175°C, and the actual material temperature only slightly lower. The operation is sometimes referred to as press surfacing. The term 'planishing' has also made its appearance: 15 the question of its purely linguistic appeal and merit apart, it is the more accurately descriptive in the technical sense, in that the hot pressing not only imparts particularly good surface finish to the sheets, but also regularises sheet thickness and relieves internal stresses and residual strains. Pressfinished sheets are used for products in which the resulting properties are important, e.g. offset printing plates, draftsman's instruments (transparent curves, set squares, etc.), name plates, calculator cases, computer floppy discs, and some thermoformed products.
18.7.2 Press Lamination As will be mentioned in Chapter 20 (Section 20.1) this method is used to combine a number of calendered sheets into one of greater thickness
838
w. v.
Titow
than the top limit for direct calendering. The process is similar to press finishing, except that only two metal plates are used, on the top and bottom of the stack, so that the sheets forming the stack are laminated together and the surfaces of the resulting thick sheet acquire the desired high-quality finish. The sheets to be combined, including the outermost ones, need not be of particularly good quality except for uniformity of thickness, necessary for straight, parallel-faced product. Moreover, the pressed sheet may be matt on one side and glossy on the other-a surface finish combination not obtainable directly by calendering. This combination can also be obtained on individual thin sheets by press finishing. 18.7.3 Surface Treatments
(a) Printing Much calendered sheeting is printed with decorative designs, e.g. in the production of vinyl wall coverings, self-adhesive decorative surface-facing sheets (including imitation veneers), vinyl shower curtaining, etc. The methods used are mentioned in Section 20.3.5(a) of Chapter 20. Of these, rotogravure printing (with solvent-based inks) is the most widely used, especially for plasticised sheeting. (b) Coating
Coatings are applied to calendered sheeting-by variants of the common roller or doctor blade techniques-for various purposes. Plasticised sheeting is often coated with a lacquer for surface protection (especially against soiling and abrasion) and/or to provide a decorative finish. In general, the objects of lacquer coating, and the lacquers and coating methods used, are the same as those described for paste-produced PVC layers on fabric and other support in Chapter 22 (Section 22.2.6) and mentioned in Chapter 25 (Section 25.4). Various adhesives are also coated onto sheeting as a preliminary to lamination with certain other materials (see below), and in the manufacture of self-adhesive surfacing sheeting, tapes, plasters, flocked wall-coverings, and the like. (c) Embossing In some operations it is convenient to emboss away from the calender line. The two rollers forming the embossing nip are essentially like
18 Calendering of pvc
839
those used in the post-calender train: the sheeting must be pre-heated to a suitable temperature, and subsequently cooled evenly with minimum strain. 18.7.4 Continuous Lamination
Continuous lamination of calendered sheeting to similar sheeting, other plastics sheet materials, and various fibrous substrates (fabric, paper) is often carried out on laminating equipment entirely separate from the calendering line. This alternative to lamination on the calender in many cases offers greater convenience and operational versatility. The key element in a typical operation is the bringing together of the sheeting and the other web material and their joint passage, under suitable pressure, through a laminating roller assembly. In heat lamination the sheeting (and, where appropriate-as, for example, with a textile fabric-also the other component) is pre-heated, and the lamination takes place under further heating. With adhesive lamination, the adhesive layer may also require pre-heating (e.g. for activation of a melt adhesive, or gelation and softening of a PVC paste coating serving as a bonding layer). An example of heat lamination of two calendered PVC sheets is provided by a common method of production of veneer-style facing sheeting for furniture and panels. In this, flexible sheeting with a wood-grain pattern printed on the surface is laminated with transparent sheeting to serve as a wear layer and give good surface finish. Typically, the clear facing sheet is pre-heated by passage over a hot roller, and then brought into contact-under suitable tension and/or pressure-with the base sheet, on the surface of a drum kept at about 170°C. The resulting laminate, in which the two components should be completely merged (so that it has the appearance and characteristics of a single-layer sheet) is stripped off the drum by stripper rolls and may be either cooled directly or first passed through an embossing station. Cellular leathercloth may be produced by lamination of a base fabric with calendered sheeting (as an alternative to the paste coating methods discussed in Chapters 22 and 25). Various procedures are used. In one type of process, specially formulated thin sheeting which will act as a wear layer is assembled with sheeting containing a blowing agent, and with the base fabric: the three are laminated together under pressure at a suitable high temperature at which the blowing agent is
840
W. V. Titow
activated and controlled expansion initiated of the film constituting the middle layer between the surface 'skin' and the fabric. Special machines are available for this process, the 'Lembo' laminating unit being a typical example. In this kind of composite lamination the two PVC layers merge at the interface, but the fabric is usually bonded to the middle layer with the aid of a thin coating of PVC paste deposited on the surface of the middle-layer sheeting by a roller and fused in the laminating step. Note: The expandable sheeting may also be calendered directly onto the base fabric and the 'skin' layer applied in a separate operation. Expansion of the middle layer may also be carried out after lamination, in a separate passage through an oven.
An interesting example of lamination by adhesive bonding is the facing of calendered PVC sheeting (with or without fabric support) with thin polyvinyl fluoride sheet. Outstanding resistance to soiling and weathering is claimed for such laminates in use as, for example, wall coverings, greenhouse sheeting and other outdoor applications.
18.8 PROPERTIES AND APPLICATIONS OF CALENDERED MATERIALS In addition to the continuous on-line monitoring and control of sheet thickness and profile, and inspection for faults on- and off-line, some properties of calendered sheeting are commonly determined in the course of production quality control and for characterisation purposes generally. These properties are listed in Tables 18.1-18.4 together with numerical values that are either fairly typical of the product to which they relate (Tables 18.1 and 18.2) or representative of minimum requirements laid down in relevant standards (Tables 18.3 and 18.4). Similar information relating to PVC-coated fabrics is contained in Table 18.5. Other properties are also assessed in tests, where they are of interest in connection with the quality or suitability for service of particular kinds of sheeting. Some examples are: surface hardness of plasticised sheeting; print adhesion on printed sheeting; resistance to blocking of flexible sheeting; ply adhesion of laminated sheeting; emboss retention by embossed sheeting; electrical resistivity of sheeting for hospital use. Standard test methods are available and-in some cases-minimum
15 50
8 50
21 9 (plain-surfaced sheet) 11 (other surfaces, or laminated sheet)
160
250-900 14 180 21 8
Up to 400 (inclusive) 13
8
11 (laminated sheeting)
21 9 (single sheeting)
160
Up to 900 (inclusive) 13
7
35
75-250 14·5-15·9 150
b
a
Each numerical property value quoted relates to the determination method appropriate to the standard specification concerned. Medium stiffness general-purpose single or laminated thick sheeting with plain or embossed surface (one of the eight types of sheeting covered by the specification). c PVC sheeting for hospital-use; production method not specified. d The calendered sheeting covered by this specification, which also covers extruded sheeting (Type II) and cast sheeting (Type III).
Low-temperature extensibility (%, minimum) Extensibility after heat ageing (%, minimum)
Thickness (JlIIl) Tensile strength (MPa, minimum) Elongation at break (%, minimum) Tear strength (N mm- 1 , minimum) Dimensional stability: change in a linear dimension (%, maximum)
BS 3878:1982 c
(Type 1 sheetingd )
sheetin!/,)
(Type 103 BS 1763:1975
ASTM D 1593-81
BS 2739:1975
TABLE 18.3 Some Standard Property Requirements for Calendered Flexible PVC Sheeting
00 .f>.
~
"1:l
~
I
~
..... 00
Softening point (minimum) Dimensional change at 120°C (maximum) Heat deflection temperature (at 264lbf in- 2 fibre stress) (maximum) Hardness (Rockwell R) (minimum)
Tensile strength (minimum) Flexural strength (minimum) Elastic modulus in f1exure c (minimum) Impact resistance d
-
%
MPa MPa Number of failures in test °C
Units
--
-
-
60 15
38 2000 0
45 2500 Negotiable'
70 15
Type C3
Type Cl
-
50 Negotiable'
38 2000 Negotiable'
Type D
Numerical value for sheet type b
BS 3757:1978"
Ibf in- 2 Ibf in- 2 Ibf in- 2 ft-Ib per in. of notch (Izod)
Units
70 110
7000 11 000 400000 0·5
Type I
66 100
5000 8500 300000 3·0
Type Il
Numerical value for sheet type b
CS201-55"
°C R scale units
TABLE 18.4 Some Standard Property Requirements for Rigid PVC Sheeting
No failure in the mechanical splitting test prescribed in the specification
% % % %
% % % % increase decrease increase decrease
increase decrease increase} decrease
0 0 55
10
15
5 5
20 0 0 80
25
15 0
No delamination or disintegration in the acetone immersion test prescribed
a Both standards are directed to rigid sheeting generally, but Types Cl, C3 and 0 of BS 3757 are specified as calendered or extruded sheets. CS 201 is a US commercial standard. b BS 3757: Type Cl-general-purpose sheet suitable for most applications and fabricating techniques. Type C3--similar to Cl but with specific impact strength and possibly lower chemical resistance. Type D-particularly suitable for deep vacuum forming. CS 201: Type I-1:hemical resistant, normal impact resistance. Type II-1:hemical resistant, high impact resistance. C In BS 3757 applicable to sheet of minimum nominal thickness of 0·5 mm. dIn BS 3757 applicable to sheet of minimum nominal thickness of 1·0 mm. e Between supplier and purchaser.
Change in flexural strength (maximum)
After immersion in 100% acetic acid Change in weight (maximum)
Change in flexural strength (maximum)
Property retention After immersion in 80% sulphuric acid Change in weight (maximum)
Resistance to delamination
Total mass per unit area (gm- Z , minimum) Base cloth mass per unit area (g m- z , minimum) Coating mass per unit area (gm- Z , minimum) Tear strength (N per 50 mm, minimum) Lengthways Transverse
Property"
Standard
~
-
-
-
75 685
110 480
-
760
Grade V
-
-
110 685
795
Grade X
Type 2 (with PVC coating incorporating an expanded layer)
590
Type 1 (with solid PVC coating
BS 5790:Part 1:1979 (for PVC-coated knitted fabrics)
TABLE 18.5 Some Standard Property Requirements for PVC-coated Upholstery Fabrics
40 40
300
550
Grade A
29 29
240
420
Grade B
BS 5790:Part 2: 1979 (for PVC-coated woven fabrics)
1·27 1·14 700
1·09 0·97 700
-
-
-
3
3
33 400000 30 5
10
40
26
690
3
-
33 400000 30 5
26
380
33 400000 30 5
15 50
26
690
10 40
-
0·4
3
400000 30 5
580 26
0·4
3
300000 30 5
450 26
a Properties tested by the relevant methods of BS 3424, except for print wear, for which modified test methods are specified in both parts of BS 5790. b Requirements applicable to both lengthways and transverse strengths. COn a Martindale-type abrasion apparatus, under prescribed test conditions.
Bursting strength (kPa, minimum) Breaking strength (N, minimum b ) Coating adhesion (N per 50 mm, minimum) Elongation (%, minimum) Lengthways Transverse Tension set (% of actual elongation, maximum) Flex cracking (cycles, minimum) Surface drag angle (degrees, maximum) Heat ageing (% coating mass loss, maximum) Print wear (chan~e of appearance) (grey scale rating, mmimum) Thickness (mm, minimum) Mean Individual reading _ Abrasion resistance (cycles,C minimum)
w.
846
V. Titow
requirement specifications (d., for example, BS 1763, BS 2739, BS 3878, and the relevant standards among those listed in Appendix 1 and Appendix 3). However, producers, processors and purchasers of calendered sheeting sometimes use their own tests. For example, whereas a standard test for emboss retention (BS 1763 and 2739) prescribes immersion in water at 100°C for 10 min as the test treatment (whereupon the emboss pattern should remain substantially unaffected), other test treatments, involving different conditions and higher temperatures (up to 180°C in some cases), are also in use where they are relevant to particular processing or service conditions that the embossed sheet may experience. 1 The properties which are of interest in flexible calendered sheeting for reservoir lining are listed in Table 18.6, together with some relevant standard test methods and typical minimum requirements. TABLE 18.6 Calendered Flexible PVC Sheeting 1 IDOl Thick, for Reservoir Lining and Similar Applications: Typical Minimum Property Requirements Property
Specific gravity Tensile properties: Tensile strength Elongation at break Modulus at 100% elongation Tear resistance Brittleness temperature Dimensional stability Q
Test method
ASTM D 792, Method A ASTM 882
ASTM D 1004 (Die C) ASTM D 1790 ASTM D 1204 (15 min at 100°C)
Volatile loss
ASTM D 1203 Method A
Water extraction
ASTM D 3083
ASTM D 751 Method A Hydrostatic resistance Resistance to soil burial: original property retention Tensile strength Elongation at break Modulus at 100% elongation Q
In both the machine and transverse directions.
Numerical value
1·20 17MPa 300% 9MPa 50Nmm- 1 -30°C Linear dimensional change Q not to exceed 5% Not to exceed 0·5% by weight Weight loss not to exceed 0·35% 690 kPa
95% 80% 90%
18 Calendering of pvc
847
The following are some of the main application areas of calendered flexible PVC sheeting: Seepage barriers (swimming pool liners; lining for water reservoirs, effluent lagoons and the like); film-packaging applications; production of inflatables (both supported and unsupported film); production of baby pants; production of adhesive tapes and labels; motor car trim (door panels, head liners, crash pads); decorative surface coverings; awnings; furniture-facing sheet (veneer effects and others); wall coverings; facing sheet for metal and board panels for partitioning and building applications; production of book bindings, document cases, folders; shower curtains; tablecloths; mattress covers; floor coverings (continuous or tiles); luggage. Rigid calendered PVC sheeting finds substantial outlets in thermoformed packs and containers (blister packs; packs and trays for confectionery and sweets, pharmaceuticals, margarine tubs); lining and trim for public transport vehicles, aircraft and marine craft; display signs; production of venetian blinds; film-packaging applications. Press-laminated sheeting is used in chemical plant construction; wall cladding; tank lining; corrosion-resistant ducting; tunnel lining. Uses of PVC sheeting are also discussed in Chapter 26.
REFERENCES 1. Elden, R. A. and Swan, A. D. (1971). Calendering of Plastics, Iliffe Books and The Plastics Institute, London. 2. Stackhouse, N. (1978). 'Calendering and paste processing,' paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 3. Anon. (1977). Mod. Plast. Int., 7(7), 18. 4. Elden, R. A. (1977). In Developments in PVC Production and Processing-I, (Eds A. Whelan and J. A. Craft), Applied Science Publishers, London, Ch. 10. 5. Anon. (1980). Plast. Rubb. Wkly, 13th December, p. 7 and (1981). Plast. Rubb. Wkly, 24th January, p. 6. 6. Eighmy, G. W. (Jr) (1982). In Modern Plastics Encyclopedia 1982-1983, pp.220-2. 7. Watkins, W. D. (1976). Plast. Engng, 32(6),23-5. 8. Private communication from Mr T. Hodgson, Storey Brothers and Co. Ltd, Lancaster, England (1978). 9. ISO 4591-1979. Plastics-Film and sheeting-Determination of average thickness of a sample and average thickness and yield of a roll by gravimetric techniques (gravimetric thickness). BS 2782: Part 6: Method 631A: 1982. Determination of gravimetric thickness and yield of flexible sheet. (Identical with ISO 4591.)
848
10. 11. 12. 13. 14. 15.
w.
V. Titow
ASTM D 1593-81. Non-rigid vinyl chloride plastic sheeting. ASTM E 252-78. Thickness of thin foil and film by weighing. Anon. (1979). Plast. Techno!., 25(5), 24-9 and 49; and Technicalliteature of Measurex Corporation, Cupertino, California 95014, USA. Young, W. L. (1978). 36th ANTEC SPE Proceedings, pp. 750-3. Mathur, K. K., Greenzweig, J. E. and Driscoll, S. B. (1978). Ibid., pp. 732-5. Bareich, G. (1970). 28th ANTEC SPE Proceedings, pp. 569-72. Anon. (1982). Eur. Plast. News, 9(10), 44. Rusincovitch, G. (1982). Plast. Engng, 38(11), 31-3.
CHAPTER 19
Rigid PVC: Main Products-Production, Properties and Applications B. J.
LANHAM
and W. V. TITOW
19.1 INTRODUCTION The properties, processing and applications of rigid PVC are mentioned, in various contexts, in several parts of this book. Those references are accessible via the Index. In the present chapter, some salient technological and applicational aspects of rigid PVC materials are focused more directly in a brief review, with special reference to the most important products. The rigid PVC compositions discussed are in fact almost exclusively unplasticised (uPVC) materials. Although many compositions containing plasticisers in relatively low amounts might be classified as rigid PVC on a strict, formal definition, * and could thus qualify for inclusion here, the ways in which the presence of plasticisers affects the properties of a PVC composition (including 'antiplasticisation' by small proportions of plasticisers) are adequately covered in other parts of the book (see, in particular, Chapters 5,6 and 7, and Appendix 3). It may be noted in passing that in semi-rigid PVC compositions (which may typically contain up to about 20-30 phr of plasticiser) the
* See Chapter 1, Section 1.1 in conjunction with ISO 472-1979 and ASTM D 883-80: both these standards define a 'rigid plastic' essentially as a plastics material which, at a standard temperature and humidity (about 23°C and 50% RH respectively), has a modulus of elasticity (in flexure or in tension) greater than 700 MPa (= 1Q5 Ibfin- 2). A 'semi-rigid' plastic is similarly defined as one having a modulus of elasticity between 70 and 700 MPa (approximately 2 1Q4-1Q5 Ibfin- ). Typical modulus values for the material of main uPVC products are given in Appendix 3. 849
850
B. J. Lanham and W. V. Titow
relationships between plasticiser content and degree of modification of mechanical properties are often relatively complex and non-linear ('antiplasticisation' can be a factor in this). An early summary of some of these relationships was published by Jacobson. 1 The effect of the proportion of plasticiser in a semi-rigid composition (based on Pevikon R335 PVC resin-Kema Nord, Sweden) upon the impact strength is illustrated, for four plasticisers, in Fig. 19.1. Note: Whilst, as shown by the curves of Fig. 19.1, the impact strengths of these compositions pass through minima (not quite reached by the TIP curve within the plasticiser content range covered), the corresponding tensile strength plots pass through maxima (at about 20 phr for TIP, and 5-10 phr for DOP, DOA and DBS). The curves for elongation at break in the tensile test also exhibit minima at about 10-15 phr plasticiser.
As has been mentioned elsewhere in this book, uPVC compositions
120
DBS
11
DOA
:550 01
c
~40
+' Ul
+,30 u
E C1J
20 10
o
10
20
30
Plasticisar conttlnt I phr
Fig. 19.1 Effect of some plasticisers, at low content levels, on the impact strength of a PVC composition.
19 Rigid PVC: Main Products-Production, Properties and Applications
851
may be based on homopolymers (in conjunction with suitable modifiers where shear and heat processing is involved) or copolymers, which are easier to process (but normally have lower softening temperatures and are somewhat inferior in certain properties, especially mechanical and thermal). The copolymers most frequently used are VCNA (e.g. in gramophone records, flooring compositions, some vacuum-forming sheet especially in the UK) and VCNDC (e.g. in some calendered sheeting, some packaging films, and as extender polymer in special paste compositions for rigid products). Copolymers of vinyl chloride with propylene are also used in some uPVC compositions for ease of processing (see Chapter 1, Section 1.5.2, and Sections 19.3.1 and 19.5.3 here). Chlorinated PVC is often the polymer in rigid compositions for use at temperatures higher than the service temperature of ordinary uPVC (ct. Chapter 1, Section 1.6). The feedstocks in the processing of uPVC are nowadays mainly dry-blend powders, although melt-compounded pellet feeds are also used, especially in the injection moulding of parts, and compression moulding of gramophone records. Some extruded and blow-moulded products are also made from pellet stock, particularly where processing is by single-screw extruder, albeit dry blends are successfully run on single-screw machines (suitably vented). For the production of-for example-pipe or window frame profiles from powder (dry blend) feedstocks, this kind of equipment may advantageously incorporate: such features as special screw design (e.g. a 'wave' screw-see Chapter 14), and special feed arrangements (e.g. cram-feeders and vacuum hoppers). A few examples of the uses of commercial uPVC compounds representing both kinds of feedstock are given in Table 19.1. The principal advantage of pellet feedstocks is that, having been melt-compounded, they are fully homogenised (gelled) and thus the processing they undergo in the course of conversion into products (by extrusion or moulding) is not required to effect homogenisation (with complete gelation-see Chapter 14, Section 14.3) but only melting: this places less exacting demands on the processing effectivity of the equipment. The advantages of a dry-blend feed over pellets are: (i) lower cost of the compound; (ii) lower capital cost of production equipment for the compound; (iii) less extensive heat history acquired by the compound.
Q
Powder Pellets Powder
BS 3504 and 3506 BS 4514 BS 4576
Pressure pipe
Soil pipe fittings
Rainwater goods
Pellets Powder
Pellets
BS 4607 Part 1}
High-impact conduit High-impact profiles
Pellets Powder
Form
-
Relevant standard
Profiles (normal impact)
Nature
Product
--
B.I.P. Ltd: VX 105 B.I.P. Ltd: VI 109
B.I.P. Ltd: VX 115 B.I.P. Ltd: VI 109
BP Chemicals Ltd: PA 182
BP Chemicals Ltd: RA 166
BP Chemicals Ltd: RA 126 BP Chemicals Ltd: PA 181
Source and gradeQ
PVC feedstock
1·44-1·46 1·48-1·52
1·47-1·49 1·48-1·52
1·42
1·48
1·48 1·46
SG
By way of non-exclusive, illustrative example. Numerous feedstock compounds available from many different sources.
Injection moulding
Extrusion
Process
TABLE 19.1 Examples of the Uses of Some Commercial uPVC Feedstocks
'"
is
:::1
~ :0:::
;::, l:>...
l:>
;:
l:>
;::, ;::,-
t-< l:>
~
~
~
00
19 Rigid PVC: Main Products-Production, Properties and Applications
853
Various formulation aspects of uPVC compositions and their significance in processing, material properties, and product performance, are considered in many of the other chapters. However, two points may be reiterated here, viz. that in the absence of plasticisers the molecular weight (K value) of the polymer (as well as its nature-i.e. whether a homo- or copolymer) acquires additional importance vis-a-vis processing, whilst the role of polymeric modifiers (and hence their correct choice) with regard to effects in processing and service also assumes primary significance, especially in homopolymer-based compositions. Note: The physical characteristics of the polymer powder (including
particle structure, size and size distribution, bulk density) are also important in such connections as flow and mixing behaviour in dry blending, ease of gelation in melt compounding or melt processing from dry blend. Other factors being equal, the K value of the PVC polymer influences the ease of fusion and the melt rheology of a uPVC composition. Hence relatively low K values are chosen where ease of flow at reasonably non-degradative processing temperatures is particularly important (e.g. in injection moulding, extrusion blow moulding), although a high K value is always desirable for best mechanical properties of the product. Copolymers (in preference to homopolymers) are also used for ease of processing and good melt flow (as required, for example, of gramophone record compositions), and special purity grades of resin where high quality (high clarity in transparent products) is a requirement-see Chapter 4, Sections 4.4-4.6 for more detailed discussion. Some of the characteristics of PVC resins used in common types of uPVC compositions are illustrated by the examples given in Table 19.2. The role and functions of polymeric modifiers in uPVC compositions are discussed in some detail in Chapter 11 (Section 11.2). Both the increased ease of processing imparted by polymeric processing aids and the improved impact resistance conferred on the ultimate product by impact modifiers are important in all uPVC compositions. In the case of some products for outdoor use-e.g. window-frame profiles, cladding, some kinds of pipe-it is particularly desirable that the polymeric modifier(s) should provide maximum process-aid effects at the high rates of extrusion normally aimed at, with the greatest weathering resistance in service. Some acrylic modifiers achieve this
100 9 0·2 0·15
H S or M 57 580
Injection or blow moulding; calendered sheeting
-
99·5 10 0·5
65-67 500-550
Sb
Hb
Largediameter pressure pipe
100 5 0·2 0·22
He se 66 54{)
General extrusion
100 99 0·2
H E 72 480
b
D
99·9 5 0·2
C S 62 510
Extruded or calendered sheeting (for thermoforming)
= emulsion polymer.
Battery separators
H = homopolymer; C = copolymer (VCNA); S = suspension polymer; M = mass polymer; E e.g. Breon 5110110 (BP Chemicals Ltd). e e.g. Corvic 5 6617 (AECI). dAfter 30 min at 135°C.
Polymer natureD Polymer typeD K value (Fikentscher: DIN 53726) Apparent density (ISO 60) (g litre-I) Particle size (ASTM D 1705): % below 250 /lm % below 75 /lm Heat IOSSd (%) Porosity (ASTM D 2873) (cm 3 g-I)
Polymer characteristics
Typical application
TABLE 19.2 Some Characteristics of PVC Polymers Typically Chosen for Various Applications (Based largely on data from Ref. 2)
99·7 15 1·8
C S 47 780
Gramophone records; flooring
'l:
0
:::1
:<::::
~
;:, l:l..
l:l
3
l:l
::.-
;:,
l:l
t--
~
~
"""
00 U1
19 Rigid PVC: Main Products-Production, Properties and Applications
855
TABLE 19.3 Impact Resistance and Tensile Strength of Typical uPVC Compounds at Different Temperatures Temperature
Impact resistance" (notched lzod: ft lbfin- I )
COC)
Tensile strength b (lbfin- Z )
Normal-impact composition
High-impact composition
Normal-impact composition
High-impact composition
0·3 0·4 0·9 4·0 16·0
1·0 4·0 16·0 18·0 19·0
12200 8900 6400 4100 2300
10400 7000 5500 2900 1500
-40
o
25 60 82
"ASTM D 256. b ASTM D 638.
combination in a particularly high degree (see also Section 19.5 below). The way in which the impact resistance and tensile strength of uPVC compositions can vary with temperature is demonstrated by the data of Table 19.3. The comparison of room-temperature impact resistance of uPVC compositions based on two copolymer resins, given in Table 19.4, illustrates something of the effects on this property of the nature and molecular weight of the resin. TABLE 19.4 Influence of the Nature and Molecular Weight of the Copolymer Resin Used upon the Room-temperature Impact Resistance of uPVC PVC resin
VCNDC copolymer (Breon CS 100/30) VCNA copolymer (Breon AS 70/42)
Relative density
Specific viscositya
K value
Impact resistanceb (J cm- 2 )
1·41
0·55
66
1·55
1·37
0·36
55
0·73
a 0.5% w/w in cyclohexanone.
In a modified Charpy test on notched specimens: the quoted values are means over ranges representing considerable scatter of results.
b
856
B. J. Lanham and W. V. Titow
19.2 SOME MATERIAL PROPERTIES OF uPVC
Typical numerical values or value ranges for many properties of uPVC materials are quoted in Appendix 3, together with corresponding figures for pPVC. Further numerical data will also be found throughout the book in connection with discussions of aspects of PVC to which they are relevant (e.g. permeability of films in Chapter 12, Section 12.4; effects of impact modifiers on the toughness and other properties of uPVC in Chapter 11, Section 11.2; analogous effects of fillers in Chapter 8; etc.). The data given in the present section are offered as additional, complementary information, put forward by way of general illustration or representative examples: in view of the great variety of uPVC compositions and their derivative products a comprehensive treatment cannot be contemplated within the space and scope available. PHYSICAL AND MECHANICAL ('SHORT-TERM') PROPERTIES
Many of these are represented, together with certain others, in Tables 19.5 and 19.6. Table 8.11 in Chapter 8 (Section 8.4.1) summarises some typical properties of uPVC compositions reinforced with glass fibres. In several of its properties uPVC approaches the so-called engineering plastics (nylon, polyacetal, polycarbonate). The factors which mainly restrict its use in 'engineering' applications are its relatively low softening (and heat deflection) temperature and its susceptibility to degradation by heat. THERMAL PROPERTIES
Some of these are featured in Tables 19.5 and 19.6. The following values have also been quoted 7 for We/vic (ICI) rigid PVC moulding material: Specific heat at 20°C: 110 J kg- 1 (oq-l Thermal conductivity at 20°C: 0·17Wm- 1 COq-l Approximate total heat (moulding): 270 x 106 J m- 3 ELECfRICAL PROPERTIES
Some electrical properties of two uPVC compounds, including some effects of temperature, are illustrated by the figures of Table 19.7.
c
b
Q
-
-
-
7·0 X 10- 5 Shore D: 85 0·02
5 x 10- 5
-
ASTM D 696 ASTM D 2240 ASTM D 570
73
77
-
-
1·40 6600 30 480000 11500 -
ASTM D 648 BS 2782
9500
-
490000 10500
1·39 6000
8
792 638 638 638 790 790 695
Injection moulding: general purpose, easY-flow b
-
D D D D D D D
Injection moulding: general purposeQ
Compound type
ASTM D 256
ASTM ASTM ASTM ASTM ASTM ASTM ASTM
Method of determination
Breon RA 142/A (BP Chemicals International Ltd). Ethyl 7042 (Ethyl Corporation: Polymer Division). Ethyl 7020 (Ethyl Corporation: Polymer Division).
Relative density Tensile strength (yield) (lbf in -Z) Elongation at break (%) Tensile modulus (lbfin- Z) Flexural strength (yield) gbf in -Z) Flexural modulus (lbf in- ) Compressive strength (lbf in-z) Impact resistance, Izod (notched, 1/4 in bar) (ftlbfin- 1) Deflection temperature under load (264lbfin- z stress) (0C) Vicat softening point eC) Coefficient of linear thermal expansion (mm mm- 1 °C- 1) Hardness Water absorption (24 h) (%)
Property and units
TABLE 19.5 Some Properties of uPVC Compounds for Melt Processing
71
17
6·3 X 10- 5 Shore D: 75 0·1
-
-
1·34 6500 30 400000 13000 375000
Extrusion: high-impact (for thin-section products)C
VI -J
00
~
:::.
0
~
~
~
"'-
;:s
l::>
~
'"3-.
"'tl ., .g
j:
~ "';:: :"::. 0
I
~ s· ~ "';:: !:;' "
0
"'tl "<:::
"'-
~:
::I:l
..... '0
-
ASTM D 2863
0·10 (0·20) 38"
60 R102-R113
6G--65 R115"
ASTM D 785
40-
7·7-8·3 0·15"
6·(}...6·5 0·17"
ASTM 696 ASTM C 177
-
70"
8-18
1·34-1·36 6·G-7·0 2-30 3,2-3,9 11·0" 3·2" 188
75"
1·37-1-45 7·G-8·5 2-10 3·5-5·5 13·5" 4·0" 235
ASTM D 648
792 638 638 638 790 790
High-impact
0,7-3,0
D D D D D D
Ordinary
upvc
ASTM D 256
-
ASTM ASTM ASTM ASTM ASTM ASTM
Method of determination
" Representative values (particular compounds). b Homopolymer-eopolymer. e Absorption after 24 h (ASTM D 570).
Impact resistance, Izod (notched) (ft Ibfin- 1) Deflection temperature under load (264lbfin- 2 stress) (0C) Coefficient of linear thermal expansion (mm mm- t °C- 1 X lO- S) Thermal conductivity (W m- 1 K-') Maximum temperature for continuous service ("C) Hardness (Rockwell) Water absorption at equilibrium (% at 20·C (lOOOC» Oxygen index
Specific gravity Tensile strength (lbfin- 2 x 10') Elongation at break (%) Tensile modulus (lbf in- 2 x 10") Flexural strength (lbf in- 2 x 10') Flexural modulus (lbf in- 2 x lOS) Creep modulus at 1000 h" (lbf in- 2 )
Property and units
0·05 e 50"
90 Rl18-R120
3·8-6·5 0·14
1OG-105
2-3
-
1·52-1·57 8·2-8·5 8-15 4·G-5·5 14·5-16·0 3·8-4·3
cpvc
-
80
70
0·00 (0·01) 17-18
R1G-R20
11-13 0·33--0·47
43--55
3--20
0·941--{)·965 3·G-5·0 2G-8oo 0·6--1·8 3·G-3·5 1·G-2·0 40
HDPE
12-17 O· 33--0· 35
32-40
no break
0·91--{)·93 1·G-2·0 9G--650 0·2--{)·4 1·G-2·0 0·1--{)·6 -
LDPE
0·00 (0'01) -18
110 R8G-R110
6·8-11·0 0·10--{)·22
55-60
2_lOb
0·90--{)·91 3·G-5·0 2G--600 1·G-2·2 5·G-8·0 1·7-2·5 60 (copolymer)
PP
1·05 (1·40) -20
100 R3G-R118
6·G-ll·0 0·19--0·34
85-105
2·5-12·0
1·03--1·07 5,5-8·0 5-80 2·G-4·0 6·G-12·0 2·G-3·5 172
ABS
TABLE 19.6 uPVC and Other Plastics Materials with Some Overlapping Applications (Especially in Pipes and Pipe Fittings)-A General Comparison of Some Typical Properties (Based in part on data from Refs 3 to 6)
;E
:::-J C
:0::::
~
.,.,
s:> ;::
;:l
s:> ;:: ;::s:>
t-<
~
~
00 1Il 00
19 Rigid PVC: Main Products-Production, Properties and Applications
859
TABLE 19.7
Some Electrical Properties of Two Rigid PVC Compounds (Breon RA 124 and RA 170") Volume resistivity (Qcm)
Temperature eC)
RA 124 23 40
60 80
90 100 aA
1·0 1·0 1·0 1·0 1·0 1·5
x 1014 x 1014 X
1014
x 1014 x 1014 x 1013
RA 170 1·0 X 2·5 X 7·0 X 2·0 X 9·0 X 5·0 X
1014 1013 1012 1012 1011 1011
Power factor at
Permittivity at
800 Hz
800 Hz
RA 124
RA 170
RA 124
RA 170
0·02 0·01 0·12 0·10 0·10 0·13
0·02 0·01 0·07 0·12 0·14 0·15
3·0 3·9 8·5 7·9 7·9
3·3 3·0 3·5 5·1 6·1 7·5
7-8
high-impact composition.
CHEMICAL PROPERTIES
These are discussed in Chapter 12 (Section 12.8), where data on resistance of PVC to various chemicals are tabulated. OPTICAL PROPERTIES
The light transmission characteristics of transparent uPVC materials are similar to those of PMMA plastics. Compositions of high clarity and 'sparkle' can be formulated (for use in, for example, the production of transparent bottles or packaging films and sheeting). Light transmission curves for clear and translucent (white) pigmented uPVC (mass-polymerised resin) films, 0·025 in thick, are shown in Fig. 19.2. LONG-TERM MECHANICAL PROPERTIES
With the use of thermoplastics for engineering applications (i.e. ones entailing service under stress, continuous or intermittent) now widespread, their long-term mechanical properties-creep (including creep rupture) and fatigue-have been extensively studied and the results used to aid design, and prediction of behaviour in service.
Creep: Because of their viscoelastic rheological behaviour, polymers-and the plastics materials based upon them-are subject to creep, i.e. increase of strain (deformation) with time under continued stress. For a given polymeric material the creep strain will normally be the higher the greater the applied stress, and the higher the temperature.
860
B. J. Lanham and W. V. Titow 100 A
80
..
c"60
o "iii
III
'EIII c
......~40 ~
01
:J
20
o0""""'3~:--."L-,---0~''='5---0~"'="6---0~.=7---='0'-=8 WavlZllZngth.l!m
Fig. 19.2 Light transmission of O·025-in uPVC films. A, Clear formulation; B, translucent formulation (Ti0 2). The creep rate-i.e. the slope at a given time of a plot of creep strain versus time (cf., for example, Fig. 19.3)-ean also increase with increasing temperature and/or stress. The creep strain or rate can sometimes be influenced by other factors, e.g. relative humidity (which can affect the moisture content of the material). For many thermoplastic materials the rate of creep can increase relatively suddenly after remaining virtually constant for a considerable period under the same applied load (cf., for example, the top curve in Fig. 19.4). Creep can take place in any of the modes of deformation encountered by polymeric materials in service, or employed in tests, i.e. tension, flexure, compression, shear or torsion. Formal definitions of creep and creep strain differ somewhat depending on whether the context is scientific or engineering. 9 The 'engineering' type of definition is favoured in standard plastics terminology, as represented by the following versions. Creep: 'The time-dependent strain resulting from stress'. 10 'The time-dependent increase in strain in a solid resulting from force' Y
c
o~
I&l
>C
~
2
o iii 3·0 c
4·
Fig. 19.3
0'1
100
Time in Hours
'000
0'1
10
At 51°C
100
Creep of a high-impact uPVC composition (Breon RA 170) at two temperatures.
10
4·
5·0
5.0
1
6·0
6·0
1000
g;
-
i;
5"
2
'<:5
~
l:l.
;:s
l:l
~.
.g
~
j:s
g.
~~
1.:t
~
~ s· ~
~
~
l:l.
~:
~
'0
......
862
B. J. Lanham and W. V. Titow
300kglcm 2
(42661bl i,;2)
100kglcm 2
(1422 Ibl i,;2)
1 10- L....,--...&.....,---'-:,..-----'-:----.J..,,---.....L..::,..----J
10- 2
10- 1
100 101 10 2 Timt und..r load, h
10 3
10 4
Fig. 19.4 Creep curves for a uPVC pipe composition at different loads. Reproduced with permission from Ref. 8. Creep strain: 'The strain at any given time produced by the applied stress during a creep test'. 10 'The total strain, which is time-dependent, resulting from an applied stress or system of stresses'. 12 'The total strain, at any given time, produced by the applied stress during a creep test'. 13 Two further definitions are particularly relevant to data most frequently quoted and used in connection with creep of thermoplastics. Creep modulus: 'The ratio of initial stress to creep strain. It is given by the equation Eo = alet, where a is the initial stress in megapascals, and et is the creep strain at time t,.10 'The ratio of applied stress to creep strain' .12 Creep rupture strength (sometimes called 'stress rupture strength'): '(FL -2)-the stress that will cause fracture in a creep test at a given time, in a specified constant environment' .11 Early work by Turner 14 provided useful information on the creep of rigid PVC at small levels of strain. The results showed, inter alia, that the time at which the creep rate starts to increase sharply is dependent
19 Rigid PVC: Main Products-Production, Properties and Applications
863
on the heat history of the uPVC composition concerned. Later work by Ogorkiewicz and Bowyer15 compared the tensile creep of rigid PVC with that of some other thermoplastics: the findings indicated that at low stresses and for relatively short times of their operation uPVC is superior to polyacetal and polycarbonate, and that whilst it falls below the performance of polycarbonate at higher stresses operating over longer periods, some rigid compositions can still out-perform polyacetal. Some creep characteristics of uPVC are illustrated in Figs 19.3-19.5 and in Table 19.8.
Fatigue: Fatigue has been broadly defined 16 as '... the progressive weakening of a test peice or component with increasing time under load, such that loads which are satisfactorily accommodated at short times produce failure at long times'. In the plastics context 'fatigue' usually means dynamic fatigue, Le. fatigue resulting from the application of periodically varying (cyclic) loads. Rupture (or other failure) occurring as a result of fatigue under a steady, continuous load, is-in relation to plastics, and especially thermoplastics-correctly termed creep rupture (or failure): the term 'stress rupture' is also sometimes used. Two relevant standard definitions 17 may be noted. 40--------,------.------,.-------,-
A
B
c
en
c
~~ 10F======::f~=""'---+. . . ==:;;;;::;;~~D===d ell ell
..,~ VJ
o
10
10
2
Time,h
Fig. 19.5 Creep of a uPVC pipe composition: stress/time relationship for a given strain. A, 3% strain; B, 2% strain; C, 1% strain; D, 0·5% strain.
864
B. J. Lanham and W. V. Titow
TABLE 19.8 Typical Values of Applied Stress to Produce Given Average Rates of Tensile Creep in Some Plastics Materials
(Based on data from Ref. 6)
Material
Polyester resin with 60% glass fibre Polycarbonate, glass fibre reinforced Nylon 6, glass fibre reinforced: as moulded conditioned Polycarbonate uPVC Polyacetal Q
Tensile stress (MPa) producing average creep rate of
115·7 44·1 10·8
54·9 46·1 Q 34·3Q 30·4 17·7
9·0
These figures illustrate the effect of moisture absorbed during conditioning. Fatigue: 'The process of progressive localised permanent structural change occurring in a material subjected to conditions which produce fluctuating stresses and strains at some point or points and which may culminate in cracks or complete fracture after a sufficient number of fluctuations'. Fatigue life: The number of cycles of stress or strain of a specified character that a given specimen sustains before failure of a specified nature occurs.
In plastics, fatigue can be aggravated by temperature rises under cyclic loading due to the high mechanical hysteresis and relatively poor thermal conductivity of these materials. In some cases fatigue failure can actually be caused by heat effects in the polymeric material (thermal fatigue). The technical aspects offatigue in plastics have been reviewed by Andrews. 18 ,19 A more recent series of brief review articles20 deals with research on, and tests for, fatigue in materials generally. The molecular weight (K value) of the PVC polymer is a significant factor in the fatigue resistance of a uPVC composition (the resistance increasing with the molecular weight). However, this effect can be overshadowed by that of processing, poor processing reducing fatigue endurance. As would be expected, the presence of cracks and other
19 Rigid PVC: Main Products-Production, Properties and Applications
865
faults in a uPVC product (or notches in a test specimen) also reduces fatigue resistance. In general, the fatigue behaviour and resistance of uPVC-in a given deformation mode, and for a given molecular weight of the polymer and constant processing effects-are influenced by the wave form of cyclic load application,21,22 the magnitude and amplitude of applied stresses, and the cycle frequency. Note: At high frequencies, the already mentioned factors of high mechanical hysteresis and poor thermal conductivity of the material are brought into play: this can cause a substantial temperature rise, with consequent ready failure of the heat-softened material.
The general appearance of a fatigue curve for uPVC subjected to cyclic loading at a relatively low frequency (of the order of 1 Hz) is schematically illustrated by the solid-line curve of Fig. 19.6, with indications of the effects of such factors as the wave form of load application, increased cycling frequency, poor processing, etc. The range of stress or stress amplitude values (ordinates) and the number of cycles to failure (abscissae) within which the curve might
Number of cycles to failure
Fig. 19.6 Schematic representation of a typical fatigue curve for uPVC. a, Shift with lower cycling frequency, or higher K value of polymer; b, shift with poorer processing, or introduction of notches or faults, or change from sinusoid to square wave form of loading.
866
B. J. Lanham and W. V. Titow
typically lie would be, respectively, about 10-60 MPa and 10-106 cycles. It is usual to present fatigue data for plastics in the form illustrated by Fig. 19.6, i.e. as a plot of stress (or stress amplitude) versus the number of cycles to failure. This is the form of presentation used for results' obtained with a sinusoid loading wave form which is widely employed in experiments. However, with a square-wave form of loading, the data can also be represented on a stress-time basis, * for direct comparison with creep failure ('static fatigue') curves obtained for the same material under continuous loading. The strength of uPVC is normally higher under the latter conditions, so that, for the same composition, the creep failure curve will lie above the corresponding fatigue curve, as has been shown by Gotham 21 for calendered uPVC sheet, and by Gotham and Hitch 22 for rigid pipe material. Two further general points (made by Gotham 21 ) may be noted: (i) Under cyclic loading a ductile-brittle transition in the failure mode of uPVC may be expected after relatively short times, whereas it may not occur at all under continuous loading (any failure being of a ductile nature). (ii) Brittle fracture occurs under cyclic loading at stresses much lower than the corresponding yield stress. The role of plastic deformation in the failure of PVC under stress has been considered by Gotham and Turner;23 some data on fatigue and creep behaviour of uPVC, and a discussion of their significance in predicting long-term service behaviour of pipe materials, have been published by Gotham and Hitch,22 and Moore et ai. 24 Fatigue (as well as creep) of uPVC pipes (especially those operating under pressure) is of particular interest because of their large-scale use in a number of important application areas. Cyclic loading arises in service, e.g. as fluids are pumped through the pipes, or in consequence of repeated opening and shutting of valves.
19.3 uPVC PIPES The acceptance of rigid PVC pipes in a number of important applications, coupled with their increasing utilisation, has been
* By summing up the time under load in the fatigue experiment over the number of cycles to failure, so that a figure for time to failure is obtained. 21
19 Rigid PVC: Main Products-Production, Properties and Applications
867
cardinally instrumental in the general growth rate of usage of uPVc. The fact that the pipes are produced by extrusion is a substantial factor in the importance of this technique in uPVC processing. The usual feedstock for pipe extrusion nowadays is a powder blend (dry blend): most large-scale producers prepare their own powder compositions on site, and feed them pneumatically to the extruders, usually via a drier. Extruder-mounted compounding units are also available (e.g. compounder DCE/MSG-Colortronics, USA), which produce dryblend powder and feed it in directly without cooling, at temperatures up to about Boac. This arrangement offers the possibility of the following savings and improvements: lower energy demand in the extruder and reduced heat history of material (as the stock does not have to be completely reheated); increased throughput and reduced screw wear for the same rate of rotation, in comparison with operating on off-line compounded feedstock.
19.3.1 Types of uPVC Pipe These are associated with the main application areas, as follows: (i)
pressure pipes for potable water supply (including water reticulation systems below and above ground); (ii) pressure pipes for irrigation systems; (iii) pressure pipes for cold water services (including services in mines); (iv) pressure pipes for gas supply; (v) non-pressure pipes for sanitary pipework in buildings: discharge pipes-main and branch-for soil water (toilets) and waste water (bathrooms, showers, sinks); house drains; ventilation pipes-main and branch; (vi) non-pressure pipes for underground sewerage and waste-water systems; (vii) non-pressure, permeable drainage pipes (land and road drainage); (viii) pipes for chemical plant installations; (ix) rainwater goods (guttering and down-pipes); (x) conduit for electrical wiring and cables. Several grades of pipe and appropriate fittings (injection moulded from suitable uPVC compositions) are available for use in each of the above applications, according to the particular conditions and
868
B. J. Lanham and W. V. Titow
requirements: relevant international and national standards abound (see Appendix 1, Section 6). Even in non-aggressive environments, uPVC pipes are not normally suitable for continuous service at temperatures above 60-65°C. Rigid CPVC pipes extend the service temperature range to about 90°C (cf. Table 19.6). A few additional points may be noted concerning the performance of uPVC pipes in some applications. Pipes made of compositions unmodified for impact resistance have been used for town or natural gas distribution systems operating at pressures up to 1 bar. However, the possibility of brittle fracture (especially at low temperatures), susceptibility to environmental stress cracking under the influence of trace contaminants present in the gas25-27 (see also Chapter 12, Section 12.5), and the desirability-in some cases-of operating at pressures up to 4 bar, have limited (and in some countries excluded) the use of ordinary uPVC pipes in this application. Pipes of impact-resistant 'ductile' uPVC (modified with chlorinated polyethylene or ethylene/propylene copolymers26 ) have proved more resistant to environmental stress cracking, and of satisfactory toughness: they are acceptable in some countries (generally for use at pressures up to 1 bar), but are not used in others (e.g. the UK) on any significant scale. Like other plastics pipes, uPVC ones have considerable structural flexibility, that is to say they are able to withstand substantial axial deformation without fracture or gross permanent distortion, and-by flexing under load-can 'shed' a considerable part by transmitting it to the soil28 side-fill in a trench. Their interior surface is smooth initially and to a large extent remains so in service, minimising 'furring' which can greatly impede flow in non-plastics pipes (in metal ones the effect can also be aggravated by corrosion)-cf. Table 19.9. These features represent important service advantages of uPVC pipes over such competitive products as pitch-impregnated fibre pipes, concrete pipes (including asbestos/cement pipes) and clay pipes, used in drainage (sewage and waste) systems and sub-soil drainage. As clean-water conduits uPVC pipes score over the metal pipe alternatives in corrosion resistance and ease of flow. Note: For sub-soil drainage, non-porous pipes (uPVC, pitch-fibre)
pipes must be suitably perforated: the clay pipes used in this application are permeable, in contrast with glazed clay sewage
19 Rigid PVC: Main Products-Production, Properties and Applications
869
TABLE 19.9 Typical Ease-of-flow Characteristics of Pipes (AU Values Relate to Flow Through a New uPVC Pipe, Taken as Unity) Pipe material
uPVC Cast iron Steel Asbestos/cement
Relative ease of flow a New pipe
After 30 years' operation
1 0·87 0·93 1
0·93 0·27 0·29 0·73
% retention of original ease of flow
93 31 31 73
Based on the Hazen-Williams hydraulic roughness factors for 100-mm bore pipes carrying water.
a
pipes. Particular effectivity has been claimed for uPVC drainage pipes with perforations in the form of circumferential slots. * From the point of view of transport, handling and installation, the much lower weight of PVC in comparison with other, non-plastics pipe materials is a great advantage. uPVC pipes can stand up to interior wear by some mineral process slurries, e.g. a large-diameter pipe line has been used to carry away slurry from a tin mine. 19.3.2 Production of uPVC Pipe (a) Equipment and Process A typical basic pipe extrusion set-up is schematically shown in Fig. 19.7, and sections of actual extrusion lines (twin-screw machines) in Plate M. The elements of the pipe extrusion line are labelled in Fig. 19.7. Note: An on-line inspection unit may be included to monitor the pipe wall thickness. This is usually an ultrasonic scanner continuously measuring the time lag between reflections of an ultra-sound beam from the outer and inner surfaces of the pipe. The unit may form part of a computerised control set-up
* Extrudex uPVC pipes-Turner and Newall Ltd, Ayecliffe, UK.
SAW MACHINE
MARKING
OFF
HAUL
WATER BATH
DIE AND SIZING DEVICE EXTRUDER
HEATER OlE BODY
BREAKER PlATE
TORPEDO
BARREL
MELT FROM - - THE EXTRUDE:R
Fig. 19.8 Typical pipe extrusion die. (Reproduced, with permission, from Ref. 8.)
APERTURE
MANDREL
OlE RING
THERMOCOUPLE
Fig. 19.7 A typical pipe extrusion line: schematic representation. (Reproduced, with permission, from Ref. 8.)
TABLE
DISCHARGE
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19 Rigid PVC: Main Products-Production, Properties and Applications
871
Plate M Pipe extrusion line (Superocla (Pty) Ltd, Rosslyn, Tv!., RSA). Top, general view. Bottom, extruder (Cincinnati Milacron CM65 conical twin-screw extruder), die, and spray cooling unit.
872
B. J. Lanham and W. V. Titow
which converts the readings into wall thickness and pipe concentricity data (that may be displayed, printed out, and/or stored on tape), and automatically applies corrections, via a closed-loop system, by changing the haul-off speed or other settings. With some control equipment it is possible to set the controls for running close to the minimum wall thickness limit for material economy. The Buhl controller* is an example of such a complete control system, and the Rota-Sonict scanner of a commercial thickness-monitoring unit. Features of computer-aided automation of pipe production (from raw materials intake and compounding to the monitoring of final product and requisite process adjustments) are outlined in an article by Smith. 30 THE DIE
As shown in Fig. 19.8, a typical die for pipe extrusion consists of an outer member comprising the die ring/die body assembly, concentrically enclosing a mandrel (with a torpedo-shaped end on the inlet side) supported in position by three or more 'legs' of a 'spider'. The internal annular gap between these two main components of the die is the flow channel for the melt. Appropriately shaped and dimensioned, it tapers conically towards the exit end, terminating in a parallel-sided annulus defined by the mandrel body and the die ring: it is this annulus which determines the cross-sectional dimensions of the tubular extrudate issuing from the die and hence to a large extent also those of the finished pipe, although the annulus is normally slightly oversized in relation to the pipe dimensions required, to allow for the effects of draw-down and of the sizing/cooling operation. At the spider, the die channel is spanned radially, at intervals, by the spider legs. The reduction factor in the cross-sectional area of the channel between the spider and the die orifice (outlet), resulting from the conical taper, may-as a rough guide-be about X 8: a high degree of reduction is necessary to ensure that the melt is considerably compressed, and thus enabled to 'knit' thoroughly, after being divided by the spider legs. To the same end, the legs should be kept to a minimum size consistent with securing the necessary strength and rigidity of the mandrel mounting, and hence of the whole die assembly, and should be suitably streamlined. Failure of the melt to re-unite properly after passing * Buhl Automatic, 24 Topstykket, 3460 Birkeroed, Denmark.
t Sonic Instruments Inc., USA. 29
19 Rigid PVC: Main Products-Production, Properties and Applications
873
round the spider legs can result in the presence of weld lines in the pipe, which may become failure sites under impact or pressure. Note: This potential cause of weld-line formation does not present any serious problems in good, modern pipe production practice. In some extrusion operations even the possibility can be eliminated by the use of a spiderless die. A commercial example is the Cuvar* straight-flow die (with mandrel support from the rear) originally designed for the production of co-extruded plastics pipes. 31 Other considerations important in die design include the following. The die 'geometry' is responsible for a substantial part of the back-pressure generated in the extruder, and hence affects such important process features influenced by the back-pressure as energy consumption, extrusion rate, and the completeness (as well as the rate) of gelation of the PVC stock. The constructional factors principally instrumental in the generation of pressure in the die are (a) the configuration and dimensions of the conically tapering section of the channel, and (b) the length and cross-sectional area of the annulus: this length is usually referred to as the land length, and its ratio to the width of the aperture as the land length ratio. Values of 8-12 are fairly typical for land length ratios of pipe extrusion dies. Obviously, the smaller the cross-section and the greater the land length, the higher the pressure developed. The die design must promote generation of back-pressure at the appropriate level: sufficient to secure the required throughput and good gelation of the stock, but not so high as to result in excessive generation of frictional heat in the material or cause abnormal wear of screw thrust bearings. In relation to the throughput aimed at, the die channel volume and configuration must be such that the dwell time of the material in the die is optimised to ensure that the homogenised melt is uniformly distributed in the parallel-sided annular gap which imparts its shape to the emergent pipe. The pressure that is likely to be developed in the die head can be calculated if the melt viscosity of the compound at the extrusion temperature, the die channel geometry and dimensions, and the output rate are known. The calculation (especially with regard to the section of the die channel between the spider and the annulus) is complex: computer programs are available for use in working out the best die
* Cuvar Inc., Rexdale, Ont., Canada.
874
B. J. Lanham and W. V. Titow
configuration and dimensions to suit the combination of a particular pipe size and output rate required. Ideally, for optimum economy of operation and product quality at a given throughput, each pipe grade (representing a given combination of outside diameter and wall thickness) should be produced with a die designed for that particular grade. However, this would entail a heavy outlay on equipment not normally justifiable on grounds of overall process economy. In practice modern die designs extend the effective and economical working range of one die over a number (typically 3 or 4) of standard pipe diameters and wall thicknesses. In some cases the adjacent pipe dimension ranges covered in this way by two dies can overlap to some extent, so that for pipes of certain sizes either die can be used. Equipment manufacturers provide charts and tables setting out the effective working ranges and performance characteristics of their dies. SIZING (CALIBRAnON) SYSTEMS
Sizing is an operation of cardinal importance in pipe manufacture, in that it finalises the bore, or the outside diameter, and circularity of the pipe. It is performed on the hot tubular extrudate emerging from the die orifice. The four main methods of calibrating uPVC pipe are as follows: (i) Mandrel sizing: An extended mandrel protrudes from the die and is cooled with circulating water. This system yields pipes with very smooth, calibrated bore, but can be troublesome to run and is less versatile than the other systems. (ii) Pressure sizing: In this system the tubular extrudate passes directly into a metal tube (the sizing die or 'sleeve') having a bore equal to the OD required for the finished pipe. Air pressure is applied through the mandrel via one of the spider legs. The tube being extruded is sealed by means of a plug attached by a wire to the mandrel, and an internal pressure is built up sufficient to keep the pipe pressed against the metal tube (about 2lbf in-2). The tube is usually double-walled and cooling water is passed through the cavity. Such sizing dies are usually quite short (6-18 in) and the pipe being extruded passes directly from the die into a cooling bath.
(iii) Vacuum tube sizing: A similar arrangement to the pressure
19 Rigid PVC: Main Products-Production, Properties and Applications
875
sizing die is used, but in this method a vacuum is applied to the outside of the extruded hot pipe via a number of small holes in the sizing tube. This obviates the need for a sealing plug. (iv) Vacuum ring and cooling tank: In this system the tubular extrudate is passed through a vacuum ring (a short vacuum tube) which effects the initial sizing of the pipe whilst cooling proceeds in a water bath kept under reduced pressure. The atmospheric pressure inside the pipe maintains its circularity during its passage through the bath. Modern vacuum sizing tanks can handle a range of pipe sizes, up to about 900 mm diameter. 32 As can be seen, mandrel sizing calibrates directly the bore of the pipe, and the other systems, the external diameter (the dimension particularly important vis-a.-vis pipe fittings and jointing requirements). COOLING BATH
High-velocity, finely atomised water sprays are used as the cooling medium in modern practice. Flooded immersion water baths are suitable only at very low take-off speeds: otherwise inordinate bath lengths would be necessary to achieve the requisite heat transfer. In some modern take-off trains for the production of uPVC guttering (e.g. the Technoform PPZ process by Reifenhauser) the extruded pipe can be slit longitudinally and post-formed in the cooling tank, the two halves forming guttering profiles of the desired shape. HAUL-OFF (TAKE-OFF)
The main functions of this equipment are to pull the pipe away from the die and through the sizing and cooling units at the required rate and in such a way that it is kept straight during calibration and cooling; it also feeds the cooled pipe forward to the marking and cutting sections of the line. The haul-off is usually of the caterpillar or pneumatic-tyre type, employing these elements to grip the pipe securely and impart the necessary traction. The haul-off speed is normally set slightly in excess of extrusion speed, to achieve the requisite degree of draw-down. As is largely self-evident, the wall thickness of the pipe will decrease with increasing draw-down (increasing take-off speed), whilst the uniformity of take-off rate is an important factor in wall thickness uniformity. For these reasons close controllability and fine adjustment of haul-off speed are of cardinal importance in the process.
876
B. J. Lanham and W. V. Titow
MARKING EQUIPMENT
This is normally a unit which prints on the pipe, at intervals, such information as the manufacturer's trade mark and/or name, grade designation, batch number, any standardisation mark, etc. CUT-OFF EQUIPMENT
Pipes are usually cut by so-called flying saws. A flying saw is a circular saw which travels along with the pipe as it makes the cut. The saw is generally activated by a limit switch touched off by the end of the pipe when a desired length has passed under the saw position. On large pipes (diameter> about 150 mm) peripheral ('planetary') saws are used, which do not cut right across the pipe but move around the circumference, cutting through the wall: such saws may be associated with a chamfering device. Complete 'downstream handling' systems for pipe or profile extrusion operations are available, comprising take-off and all subsequent units (including on-demand cutters, transport, packaging and automatic control equipment) for use on- or off-line. Automatic perforating equipment may be used as part of an extrusion train for the production of sub-soil drainage pipe. 32 For certain kinds of jointing (rubber-ring joints in common use in some large-diameter pipe lines; solvent-cemented joints of the spigot-and-socket type) a shaped socket is required at one end of a length of pipe, with internal diameter equal to (or slightly greater than) the external diameter of the pipe. Such sockets are produced by a 'belling' operation, in which the pipe end is heated to a suitable temperature (quite commonly 160-165°C), moulded on an expanding mandrel of appropriate shape, and cooled. The operation may be partly or fully automated. Some automatic belling equipment can operate in-line with the extrusion train. 32 (b) Some Formulation Aspects
In very round figures, the cost of raw materials can amount to as much as 60-70% of the total production cost of uPVC pipe. This is the main reason behind the importance of material cost economy in pipe compound formulation. At the same time, the fact that the fastest practicable production rates are desirable for the process economy they offer, imposes a need for considerable sophistication in formulation to ensure stability, completeness of gelation and good rheological
19 Rigid PVC: Main Products-Production, Properties and Applications
877
behaviour in processing, as well as the requisite service properties: in many cases these requirements preclude the use of the cheapest formulation constituents, or introduction of cheapening fillers. The most economical approach may involve formulating with not the cheapest, but the most functionally effective additives for the PVC resin, so that they may be used in minimum quantities. An illustration of this is the trend towards increased effectivity, permitting low levels of incorporation (fairly typically 0·3-0·4 phr) of organotin stabilisers (as well as some antimony ones) used in uPVC pipe compositions in the USA (ct. Chapter 9, Sections 9.4.2 and 9.4.3). Similar considerations are also behind the careful selection and balancing of lubricant and lubricant/stabiliser systems for pipe formulations, including the finer points of the merits in this connection of lubricating stabilisers vis-a-vis combinations of non-lubricating ones with particular lubricant systems.
Note: Some high-efficiency methyltin stabilisers combine relatively moderate cost with functional effectivity-and a degree of lubricity-in uPVC pipe compositions at incorporation levels down to 0·25 phr in some cases (e.g. Mark 1939-Argus Chemical, USA: a liquid methyltin). The crucial importance of the lubricant system, well understood from practical experience, is brought out by the results of an experimental study by Zechinati et al. 33 In this, the effects of the seven additives in the following American-type* formulation were investigated in terms of changes in extruder response (drive amperage, thrust-bearing load, and output rate), Brabender torque curve, and properties of pipe produced. PVC polymer (pipe-extrusion grade) Stabiliser: methyltin mercaptide
lOOpbw 0·3 phr
* A fairly typical basic formulation for a pressure pipe (non-impact-resistant grade) in countries (or for applications) where lead stabilisers are permissible, might be: PVC polymer (pipe-extrusion grade) Stabiliser/lubricant (co-precipitate): tetrabasic lead sulphate calcium stearate polyethylene wax Filler: CaC03
100 pbw 1·2 phr 1·2 phr 0,1 phr 2-3 phr
878
B. J. Lanham and W. V. Titow
Lubricant system: External lubricants: paraffin wax (MP 71 0c) partially oxidised polyethylene (MP 104°C) Internalluhricant: calcium stearate Acrylic processing aid Pigment: Ti02 Filler: CaC0 3
1·2 phr 0·2 phr 0·8 phr 1·0 phr 1·5 phr 2·5 phr
The investigators found, inter alia, that the greatest changes in processing behaviour and product quality resulted from varying the amounts and proportions of the components of the lubricant system. 19.3.3 Pipe Properties and Their Determination The importance of some properties of uPVC pipes is relative to the end-use: e.g. high bursting strength and fatigue resistance are required in pressure pipes, but not in electrical conduit (which, on the other hand, should have a relatively high degree of impact resistance), etc. However, mechanical properties relevant to all applications are strongly influenced by the degree of gelation of the pipe material. For this reason, this structural characteristic may be regarded as the most important single property of uPVC pipe. The mechanism and role of gelation in extrusion of uPVC are discussed in Chapter 14 (Section 14.3). In pipes the completeness of gelation is usually assessed by a solvent immersion test (although inadequate gelation will, of course, also be shown up in several mechanical property tests-e.g. those for impact resistance, bursting strength, tensile strength, resistance to stress cracking, and others). The solvents used in such tests are acetone (ISO 3472; BS 3505 and 3506; ASTM D 2152) or dichloromethane (KIWA * KE 49; SABS 966). Typically, a section of pipe is immersed for about 30 min in the solvent and, after removal, examined (especially at the edges) for signs of white efflorescence, delamination or disintegration (respective manifestations of increasingly serious under-gelation). A desirable refinement, employed in many versions of the test, is to cut at the end of the specimen, across the full wall thickness, a continuous taper of an acute included angle (say 5-10°). This helps reveal any layer-wise differences in homogeneity within the pipe wall. Examination under the microscope for grainy appearance and inhomogeneity (frequently involving comparison with standard specimens or photo-
* Keuringsinstituut voor Waterleiding Artikelen, Delft, The Netherlands.
19 Rigid PVC: Main Products-Production, Properties and Applications
879
micrographs) is also often carried out as a means of (or aid to) assessment of completeness of gelation. Note: On the basis of a study of the mechanism of failure of uPVC pipe materials, Marshall and Birch34 concluded, inter alia, that the combination of the dichloromethane test on tapered specimens with fracture toughness determinations provides a good means of evaluating the completeness of gelation of such materials.
The property tests commonly carried out on uPVC pipes for some typical applications are cited in Table 19.10. Analogous or corresponding tests are also performed on the appropriate fittings. Some of the tests are applicable only to the fittings or only to pipes (see the list of relevant specifications in Appendix 1, Section 6.1). As has been mentioned, resistance to creep (including creep rupture) and fatigue are long-term properties very important in the service performance of uPVC pipes operating under internal pressure. Pipe design, and prediction of behaviour in service on the basis of evaluation tests rely to a considerable degree on extrapolation-in accordance with established procedures-of plots of relevant test data (cf., for example, ASTM D 1598 and D 2837, and Refs 35 and 36). Much is now also available in the way of actual results obtained in long-term service. 35 ,37 Note: Evidence has been produced35 indicating that the bursting strength of uPVC pipe can increase significantly after a long period under steady internal pressure (as experienced in a creep test or in service). It appears possible that such improvement might be attributable to molecular orientation effected by the prolonged stress, andlor possibly also to some annealing effects. 19.3.4 Some Special Pipe Products FOAMED uPVC PIPE Rigid pipes with cellular (structural foam) walls are not produced or used on a large scale because of the unfavourable difference in mechanical properties in comparison with solid-walled pipes. However, in some areas where the property requirements are less critical (e.g. certain types of electrical conduit), or where the superior heat insulation performance of cellular wall structure is relevant, foamed
V V
-
V V
v v
Solvent immersion (acetone or CH2 C12 ) Chemical resistance
Water-extractable content
Water absorption
V
-
V
v'
-
-
V
V
v
V
V
V
V -
Vb
Vb
V
Vb
Vb
-
V
V
-
-
-
-
-
v
v
v
v
v v -
Rainwater goods
Above-ground sanitary piping
Sewerage and drains
Industrial uses
Application area of pipe
Cold-water services"
Temperature cycling
Vicat softening point Temperature of deformation underload Resistance to heat Thermal reversion
Nature of test, or property tested
e
-
V
-
-
V
v
-
Electric wire conduit
TABLE 19.10 Tests Commonly Performed on uPVC Pipes for Various Applications
Decrease in length on heating Alternating hot and cold water cycling Test for completeness of gelation Weight change after immersion in acid and alkali, sometimes also other reagents In some cases checked for Sn and Pb
Remarks
t-<
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(tensile impact resistance in some cases)
Including pressure piping in mines. Check for blistering or splitting in the heat reversion test, or after special heating test. Ball indentation (BS 4607). Particularly for impact-modified grades.
Electrical insulation properties Stiffness flammability
Resistance to creep and creep rupture under hydraulic pressure Suitability for end-beUing
1/
V
v
1/
-
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Resistance to flattening or crushing Bursting strength Tensile properties (of pipe material)
v
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(often only high-impact grades)
V
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Impact resistance d
v
v'
v'
v
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v v v
-
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Absence of splitting when heated end expanded on standard conical former
Normally tensile strength and elongation
Usually a falling-weight test method, but sometimes special tests (ct. Chapter 11, Section 11.2.2(a))
00 00
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882
B. J. Lanham and W. V. Titow
pipes are of interest. They can certainly offer cost advantages over the solid-walled pipes in view of the already mentioned high share of material cost in the total cost of pipe production. A good summary of the main aspects of production of foamed pipes (including formulation, extrusion conditions, and economics) and their properties has been published by Szamborski and Marcelli. 38 'HOLLOW-WALLED' uPVC PIPE Sewer pipes have been produced incorporating closely spaced longitudinal channels in the wall (formed by the extrusion die of special design used to produce the pipe):* weight reduction of up to about 30% in comparison with a solid pipe has been claimed, with corresponding cost reduction of around 20%, and only moderate sacrifice of the relevant mechanical properties. 39 This last feature-also said to make the strength superior to that of foamed-wall pipes-has been attributed to the fact that the structure of the pipe wall (as readily demonstrated by a cross-sectional view) resembles a circumferential assembly of 'I-beams' joined at the edges: this beam shape is well known for its high strength-to-weight ratio. HIGH-STRENGTH, ORIENTED uPVC PIPE Controlled, partially biaxial molecular orientation induced in the wall material by modifications of the production process involving a stretching operation, can result in substantial improvements in bursting strength, impact resistance and fatigue resistance of the pipe in comparison with standard pipe of similar diameter and pressure rating but nearly double wall thickness. The first commercially available pipe of this kind was developed and produced in the late 1970s. t
uPVC PIPE Large-diameter pipes for culverts and sewers, having a corrugated wall, are manufactured (in PVC and other thermoplastics) by extrusion directly followed by air-pressure expansion against corrugated sectional mould blocks. In a variant of the process a continuously extruded strip is helically wound and fused together to form a continuous pipe. The patented* production process can also be used to make double-walled CORRUGATED
* Wavihol pipe-Wavin Overseas BV, Zwolle, The Netherlands.
t Superpolyroc pipe-Yorkshire Imperial Plastics, UK. :j: By
the Corma company (Toronto, Canada).
19 Rigid PVC: Main Producls-Producliun, Properlies and Applicalions
883
pipe (with a smooth inside wall and corrugated outside wall, or vice versa), or conventional smooth-walled pipe. RIBBED PIPE
Pipes with integral external ribs (circumferential flanges)* are produced-in a range of diameters including ones of 1 m and over-by a novel injection-moulding technique. The process has been called 'grow-moulding', because the pipe is made stepwise, each consecutive section being moulded (over a mandrel positioned in the mould) so that it welds onto the end of the previous section (left in the mould when that section has been partially pushed out by an ejector ram). Of the materials so far used in the manufacture of these pipes (polyethylene, polypropylene, uPVC) , uPVC is-as would be expected-the most difficult to mould, especially in the large sections for big pipes. The problem is eased somewhat by using foaming compositions producing structural-foam pipe walls with relative density down to about 1 (from about 1·4 for solid uPVC). The main general advantage claimed for the ribbed structure is that the pipe's ability to withstand external loads within the normal service range is on a par with that of a conventional pipe containing over twice the amount of material per unit length. Also, when laid in a back-filled trench, a ribbed pipe has been found to be more resistant to deflection or damage under an external load (such as might be imposed by the passage of a heavy vehicle on the surface) than steel, or steelreinforced concrete sewer pipes of the same diameter. 40 Applications of 'Ribstruct' pipes include non-pressurised sewers and effluent drains, ventilation ducting, multi-core cable ducting, shuttering for concrete, chemical silos, industrial box gutters, pneumatic conveyance of materials, and others. 19.4 uPVC PROFILES 19.4.1 Main Types and Applications
Extruded profiles range from simple strips for use as beading or trim, e.g. with PVC panelling, through more intricate cross-sectional shapes (including channels and the like) for various trimming, finishing and * Ribstruct pipe-Duropenta (Pty) Ltd, New Germany, RSA.
884
B. J. Lanham and W. V. Titow
mounting purposes, to very complex hollow-section multi-channel products for use in modern uPVC window-frame systems. Note: uPVC casings have been extruded over timber profiles for use as window- and door-frame components, where the covering serves as a permanent protective and decorative surface finish. uPVC window-frame profiles covered by an integral layer of acrylate resin have been produced by coextrusion (e.g. the Trocal Colour profile-Dynamit Nobel, West Germany41). The acrylate 'skin', which may be coloured, provides protection, inter alia, against weathering.
Among the uses of solid (i.e. non-cellular) uPVC profiles, window-frame systems are the fastest growing and possibly technically the most interesting, representing as they do a combination of sophisticated design, material formulation and extrusion technology. Other products include components of demountable partitioning systems (cloaking head channels and mitre pieces, angle trim and skirting, some of which may incorporate a flexible PVC lip extruded onto the rigid section to provide a dust and draft seal), solid rods and various angle sections for general applications. Cellular (structural foam) profiles have been replacing traditional wood products in such applications as skirting boards and architraves; furniture, boat, and caravan trim; and picture frames. Wood-flour fillers are often incorporated in products of this kind. The foam density and skin thickness is governed by the intended application and the design of the profile. In general, the thinner the cross-section and the greater the stresses and impacts expected in service, the higher the density. The approximate overall density range within which the cellular profiles fall is 0·SD-O·80gcm- 3 , with thick-sectioned purely decorative beading fairly typically near the lower limit and, say, relatively thin-section skirting board around O· 70 g cm -3. Note: Foam-filled uPVC window- and door-frame profiles (of relatively simple cross-section) are produced by the 'Coexcel' process, * in which a foaming composition for the core is coextruded within the one forming the solid profile walls.
* Developed by GKN. Commercial 'Coexcel Thermal Clad' profiles manufactured by a subsidiary, Scope Aluminium Products Division of BKL Extrusion Ltd, UK.
19 Rigid PVC: Main Products-Production, Properties and Applications
885
19.4.2 Production The production process and typical extrusion line for profiles are generally similar to those for pipes, the greatest degree of equipment complexity and refinement being called for in the manufacture of hollow multi-channel profiles of complex cross-section used for modern uPVC window frames: sophisticated extrusion dies and calibrating and cooling equipment (with the associated pumping gear) are employed in lines for this product, which also include-as do extrusion trains for all kinds of profiles-haul-off, marking, and cutting equipment. Whole lines specially designed for window-frame profile are available from manufacturers of this type of machinery, incorporating advanced control systems and cutters. In the USA, production of window-frame profiles from pellet stock on single-screw extruders is still common, whilst dry blend extrusion on twin-screw machines is widely practised in Europe. Typical production rate capabilities are in the range 1·5-3 m min- 1 , with pellet extrusion on single-screw machines near the lower end. The main restricting factors on speed of production are the sizing and cooling operations: both require elaborate arrangements and sufficient time to ensure effective heat removal from the complex, multi-channel profile, and its accurate dimensioning to close tolerances. Foamed profiles are normally extruded from powder feedstocks to avoid subjecting the blowing agent to extra heat processing at the high temperatures involved in the melt-compounding of pellet feeds. Since foaming takes place on emergence from the extrusion die (cf. Chapter 25, Section 25.2.1), a sizing die ('former') is used-normally positioned in the cooling bath-to control the final size: the sizing operation also determines the density of the finished profile. As a rough general guide, the respective cross-sectional areas of the extrusion die orifice and the sizing die should be in the same ratio as the product density aimed at and the density of the solid (unexpanded) uPVC composition. The density of the final product is also influenced by other factors, in particular the distance the extrudate travels before entering the cooling bath and sizing die. Torpedoes and spreaders used in the extrusion of uPVC compositions into solid pipes and profiles are not normally necessary with foaming compounds. A useful discussion of foamed profile extrusion has been published by Davies,42 and one of methods of production of structural foam in general (by extrusion and injection moulding) by Harris. 43
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B. J. Lanham and W. V. Titow
Among refinements available for modern extrusion lines are special foam-faced traction belts in haul-off units, for delicate profiles,32 and on-line tooling and fabrication systems (mounted on travelling work tables), for automatically routering, milling, drilling, notching and punching the moving profile before it is cut. 44 19.4.3 Some Formulation Aspects
In general, the processing considerations in the formulation of compositions for simple profiles are similar to those arising with pipe compositions. However, vis-a-vis the end-uses, virtually all profile formulations must provide for some degree of impact resistance: in many cases provision is also necessary for good resistance to weathering. Formulations for window-frame profiles embody the greatest degree of sophistication, and also, incidentally, provide a good illustration of the importance of harmonising the properties and effects of the individual components. Broadly speaking, the formulations are designed principally to satisfy the following cardinal technical requirements: good processing at fast rates of production, high impact resistance of the product, and the highest possible degree of resistance to weathering. Other considerations, including the usual economic ones, are also involved. Window-frame profiles have been produced for well over a decade, but the expansion in their use (notably in the UK and the USA) is more recent. Development and improvement of formulations is still going on and is far from complete, although some very good compositions are available. The best current formulations are, naturally, not made public by their users, but the general trends are known, as are the main differences in the stabiliser systems favoured in Europe (where the window profile technology originated and first came into large-scale practice, especially in Germany) and the USA. The subject is extensive, and only some salient points can be indicated within the scope of the present section: useful general reviews have been published recently dealing with several aspects in reasonable detail. 45-47 There are three general formulation routes-based on three different impact modification systems*-used to obtain compounds * uPVC compositions without impact modifier have also been used (chiefly in France), based on a Solvay suspension homopolymer. Whilst such compositions give stiffer, harder and higher-softening profiles, they tend to be more brittle than modified types.
19 Rigid PVC: Main Products-Production, Properties and Applications
887
with the requisite combination of good impact resistance and weathering performance, coupled with adequate levels of other relevant properties (including sufficiently high softening point and temperature of deflection under load; low thermal expansion; and good processing characteristics).* These approaches are represented by the following types of composition. (i)
Compositions modified with EVA copolymers, commonly graft copolymers of EVA and vinyl chloride (e.g. Levapren-Bayer; Elvaloy-Du Pont). (ii) Compositions incorporating conventional acrylic impact modifiers (e.g. Aeryloid KM 323 B-Rohm and Haas), or ones modified with acrylatet/vinyl chloride graft copolymers (e.g. Vestolit P1982K-Hiils, or Vinnol K-Wacker Chemie). (iii) Compositions modified with chlorinated polyethylene (e.g. Hostalit Z-Hoechst).
The stabiliser systems commonly used in Europe in window-frame compositions are BalCd combinations (often selected liquid grades, for ease of processing) usually incorporating also some dibasic lead phosphite, or all-lead systems, again normally including dibasic lead phosphite. In the USA organotins are employed-ehiefly methyltins, but also butyltin and some estertin stabilisers. Whilst one of the cardinal criteria for the use of the particular impact modifier types is that they do not significantly impair weatherability, the stabilisers are certainly chosen with a view to enhancing this important property: the objective is also further promoted by incorporation of titanium dioxide (typically about 4-12 phr, depending on the stabiliser-Iower proportions with the BalCd/Pb and all-lead systems, highest with organotins). Acrylic processing aids are also used: e.g. the well-known Aeryloid K120N or K125 (Rohm and Haas Co., USA), sometimes in conjunction with a lubricating grade (e.g. Aeryloid 175). Lubricant systems vary: some involve calcium stearate for internal lubrication, and a wax, e.g. paraffin or ethylene bisstearamide ('bisamide'). * An example of some property levels acceptable in practice is provided by values for an industrial powder compound (RP lDO--Conoco Chemicals Co., USA) claimed to meet the specification for rigid PVC compound of the Window and Door Committee of the SPI. 48 Tensile strength, 6200 lbf in -2; tensile modulus, 329800 lbf in -2; Izod impact resistance, 20 ft lbf in- \ Vicat softening point, 82°C; deflection temperature under load (264lbf in -2 stress), 72°C; Shore hardness, D 83; water absorption (24 h), 0·2%. (All results obtained by the appropriate ASTM test methods.) tUsually butyl acrylate. .
888
B. J. Lanham and W. V. Titow
Calcium carbonate fillers are included in some formulations, the usual claims being made regarding impact strength improvement with the very fine grades normally used: low loadings (up to about 4 phr) are favoured, but much higher ones (about 20 phr) are also occasionally encountered. The actual proportions of constituents in a particular formulation depend on their nature. As regards the popularity of the three basic formulation types, acrylic-modified compositions have been gaining ground largely because of their tolerance for temperature and pressure variation in processing and tractability at high production rates. However, profiles made from this type of composition can have an excessively shiny surface (this tendency may have to be counteracted by incorporation of matting agents). Compositions with EVA-type modifiers-for a long time the leaders in Europe-have been suffering some loss in popularity in the face of claims, challenged by the modifier suppliers, that these modifiers have an excessive plasticising effect when present in amounts optimal for the required high impact resistance (especially the relatively high proportions needed for this purpose in cold climates) and that they are inferior to the best acrylics in weathering performance: the generally somewhat better processing of acrylic-modified compositions, especially at high production rates, is 100
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Fig. 19.9 Percentage shares of the three basic types of impact-modified PVC window profile compositions in the West German market. (Reproduced from Ref. 46 by courtesy of the editor of Plastics and Rubber Weekly.)
19 Rigid PVC: Main Products-Production, Properties and Applications
889
also a factor. The use of CPE-containing compositions-historically the earliest type-has also been declining: need for close control to avoid over-processing, and alleged susceptibility to yellowing of white profiles in some circumstances, are usually cited as the main reasons. Figure 19.9 shows the respective market shares of the three types of composition in West Germany. 19.4.4 Testing and Specifications The tests carried out on profiles are normally related to the end-use. The properties evaluated may be broadly divided into material properties (determined on standard specimens moulded from the profile composition) and product properties, evaluated in tests on the finished profile. The material properties frequently determined include tensile and flexural strengths with the corresponding moduli, impact resistance (commonly Izod or Charpy), softening point, temperature of deflection under load, and coefficient of linear thermal expansion. Product property tests often include those for impact resistance (usually by a falling weight method), thermal reversion, and completeness of gelation (by solvent immersion), as well as any special determinations particularly relevant to end-use. Window profiles are also tested for property retention after heat ageing and weathering as well as for weldability and weld strength. Made-up window assemblies are subjected to the various tests prescribed by relevant building specifications (e.g. tests for thermal insulation, air and water ingress, sound insulation, etc.). The national standards of direct interest to uPVC window profiles are: DIN 53 419 (1977) -see Appendix 1, Section 11.2. NEN 7034 (1976) -Unplasticized PVC profiles for constructing window frames, windows and doors in facades. 41-GP-19 Ma: 1978-Rigid vinyl extrusions for windows and doors. Work has been under way since August 1980 on a British standard for uPVC windows. Some of the numerical requirements included in the draft standard prepared by the Plastics Windows Group of the British Plastics Federation are listed in Ref. 45. A technical schedule to BS 5750 Part 1 deals, inter alia, with the design and properties of plastics windows, including the related hollow uPVC profiles. The ASTM specification (D 3679) for uPVC siding is also of some interest.
890
B. J. Lanham and W. V. Titow
19.5 uPVC SHEET AND FILM 19.5.1 Terminology The meanings of these terms in industrial usage and their standard definitions are discussed in the introduction to Chapter 20. As pointed out there, the common usage and standard terminology do not fully coincide, the demarcation in thickness between thick film and thin sheet is not very precise, and the description of certain kinds of thin sheet as 'foil' is not free from ambiguity. However, if the context in which they are being used is clear, the terms-though not associated with strict, universally accepted thickness limits-are fairly consistent and understood; with regard to PVC, common usage is reasonably in line* with the following classification, which is adopted for the purpose of this chapter. She~ A
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19.5.2 Production uPVC film is usually made by the blown-film extrusion process. Some solution-cast film is also produced, for applications where high clarity and/or freedom from orientation are important (see Chapter 24). Sheeting is made by calendering, or by extrusion (from a flat sheet die into the nip of cooling and polishing rolls-sometimes referred to as extrusion casting). Calendering of PVC is discussed in Chapter 18: in comparison with this process, extrusion involves lower equipment cost * It may be recalled that the relevant British nomenclature standard puts the demarcation point between PVC film and sheet at 0·003 in (76Ilm), whilst other standards give 250 Ilm as the upper thickness limit for film for all plastics (ct. Chapter 20). It may also be noted that in industry the thickness range for PVC foils is sometimes quoted as, or implied to be, 0·1--0·6 mm, or 0·1--0·7 mm.
19 Rigid PVC: Main Products-Production, Properties and Applications
891
and is more flexible, in the sense that calendering is best suited to long runs on the same thickness and width of product. Very thick sheets (up to about 25 mm and even thicker in some cases) for such applications as, for example, glazing, chemical plant, etc., are made by compression-laminating a number of thinner sheets (see Chapter 20, Section 20.1).
Note: Equipment combining elements of both extrusion and calendering is also available for the manufacture of uPVC sheet (e.g. for thermoforming applications). In the 'Calandrette'* system, a planetary extruder homogenises the composition-at a melt temperature somewhat below that usual for conventional extrusion-the extrudate is automatically cut into lumps (to increase the specific surface for ease of subsequent de-gassing) and dropped into the feed zone of a vented single-screw metering extruder. This re-forms the melt and extrudes it as a series of strands, which are spaced out horizontally by an adjustable comb and passed on to form a bead in the nip of calendering rolls (a stack of three, of diameter up to 50 cm). The rolls form the bead into a continuous web whose width (which may range between 60 and 140 cm) is determined by the number and original spread of the strands. The spaced-strand form of feed to the rolls is claimed to reduce the thermal stressing of the material, giving a better flow, and to facilitate die adjustment. Blown-film extrusion of PVC follows the general pattern of this type of process. The melt is extruded through a tubular die, and the resulting tube is taken off vertically upwards whilst being inflated-by compressed air (which may be chilled for a better cooling effect) fed in through the centre of the die-so that it forms an elongated bubble closed at the top by the nip of a pair of rollers. Cooling air is also blown through an air ring onto the outside of the tube above the die. For a given die orifice size and extrusion rate, the final film thickness is determined by the rate of take-off, and the amount of radial expansion of the bubble (controlled by a sizing frame). These two parameters also govern the degree of biaxial orientation undergone by the film. The bubble is collapsed (folded flat) by a guide frame operating in * EKK Kleinewefers, Bochum, West Germany.
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B. J. Lanham and W. V. Titow
conjunction with the above-mentioned rolls at the top of the take-off tower. The resulting flattened tube of film may be slit longitudinally (commonly to produce two sheets separately wound up) or it may be wound up directly as 'lay-flat tubing'. An extrusion line for PVC sheet is shown schematically in Fig. 13.47 of Chapter 13. The essential elements of a set-up of this kind are-from the extruder out-a sheet die, cooling and polishing rolls (commonly a vertical stack of three, but up to five may be used), a conveyor and haul-off unit (normally a relatively simple two-roll nip arrangement) usually incorporating an edge trimmer for the sheeting, and sheet collection equipment: for thick sheeting (say upwards of about 1·5 mm) the collection equipment would normally be that shown in the figure, i.e. sheet cutting and stacking units; thinner sheeting would usually be reeled up on a winder. The sheet die, connected to the extruder via an adapter, usually has a manifold of the 'coat-hanger' type* and adjustable lips for control of sheet thickness. For coextruded sheet the die is preceded by a coextrusion block to introduce the input from the satellite extruder(s) supplying the additionallayer(s). Note: In the case of one kind of uPVC sheet (0,46 mm thick) 'capped', by coextrusion, with an acrylic layer (0,076 mm) for decorative and protective effects, it was calculated49 that this method of production could offer a material cost saving of up to about 36% in comparison with extrusion lamination to acrylic film.
Extrusion lines for blown film and for extrusion-cast sheeting are available from several equipment manufacturers, e.g. inter alia, Barmag (blown film, and sheeting, including coextruded productsEurope and USA), Francis Shaw (blown film, and sheeting-UK and Europe), Kestermann (sheeting-Europe), and Windmoler and HOlscher (blown film-Europe and USA). Apart from conventional thickness gauges (radiation, and other types) long established in industrial use, advanced equipment for continuous, on-line monitoring and-in some cases-automatic control of product quality is available for use on film and sheet extrusion trains. Examples include the following. The 'NC-P' thickness control
* The geometry of a coat-hanger die design with uniform flow rate and residence time across the die width is discussed in a paper by Matsubara. 5o
19 Rigid PVC: Main Products-Production, Properties and Applications
893
system (Windmoler and Holscher, West Germany and USA) for blown film,51 utilising IR transmission as the property monitored, and automatic adjustment of individual cooling sectors in the ring to effect corrections. The Sussex pneumatic system (Sussex Instruments Ltd, UK) for continuous monitoring of film or sheet thickness 52 with a feedback control loop operating on the extruder or line speed controls. The 'Intec 500' laser-based inspection system (Intec Corp., USA)53 for detection of defects (fish-eyes, gels, wrinkles, streaks, holes, contaminants, and surface imperfections). Sheet or film profile control systems,54,55 which-like that forming part of the 'Profitmaster 5510' extruder control system (LFE Corp., USA)-eouple detection of irregularities in profile with automatic correction through adjustment of the sheet die lip. Microprocessor-based systems for web width control are also available, e.g. the 'Sigmat 770' system (TMS Systems Ltd, UK), originally offered for blown film of width up to about 10 m. 19.5.3 Applications and Properties
The main uses of thick uPVC sheet are mentioned in Chapter 20. The applications as cladding may be considered to encompass both sheet and profile, in that some cladding systems involve the use of panels (which may be of the pressed, and/or thermoformed56 variety) and others of extrusions narrow enough to be classed as profiles. The main service performance requirements applicable to external cladding are similar to those that window-frames have to meet, including good resistance to impact and weathering. Similar formulating approaches are used (d. Section 19.4.3 above), with acrylic and chlorinated polyethylene impact modifiers popular in formulations. Note: Particularly good performance has been claimed57 for a type of modified acrylic impact modifier commercially represented by Durastrength 200 (M & T Chemicals Inc., USA). Some aspects of the service behaviour of external cladding are related to the effects of processing. Apart from any detrimental effects of incomplete gelation, shrinkage and surface distortion can take place-in consequence of recovery of production-induced stresses-at elevated temperatures resulting from solar heating. The surface distortion (local rippling and bulging) is sometimes referred to as 'oil canning'. uPVC materials with high enough heat distortion temperatures (say about 87°C) tlre less prone to this effect. The stress
894
B. J. Lanham and W. V. Titow
relaxation behaviour can be studied and assessed by thermomechanical analysis: the results can have practical relevance to selection of cooling conditions in processing, or suitable annealing temperatures if such treatment is being considered. 58 uPVC foils and films find their principal outlets in various areas of packaging. Foils are used mainly in the production of thermoformed packaging elements, e.g. support containers and trays for chocolates and confectionery, blister (bubble) packs, boxes, etc. Films of, typically, semi-rigid compositions are employed for shrink-wrapping of a wide range of articles and goods. Shrink-wrap films are stretched in production to effect various combinations of longitudinal (machine direction) and transverse stretch, suited to the requirements of the intended use. The objects to be shrink-wrapped are first overwrapped or sleeve-wrapped in the film: this is normally done on semi or fully automatic equipment. The wrapping is heat-sealed, to form a closed bag, and the package is passed through a heating tunnel ('shrink tunnel') where the stretch imparted in production is caused to retract as the film reaches the appropriate temperature (roughly corresponding to that at which it was stretched), with the result that the film shrinks onto the contents. The degrees of retraction built into the film are said to be 'balanced' if they are the same in both directions. Fairly typically, shrink-wrapping films may range in thickness between about 12 and 30 .urn. The temperatures in the heating tunnel may be up to about 180°C depending on film type and thickness and the residence time: higher temperature settings are needed with shorter times and/or thicker films. Some typical property data for commercial shrink-wrap films are given, by way of illustration, in Table 19.11. The formulations for uPVC sheeting and film compositions are legion, varying with the nature, production method and end-use of the product. The following two basic types are mentioned purely by way of general, illustrative examples. uPVC foil for deep-draw thermoforming: VCNA copolymer resin (low K value, 10% vinyl acetate) Stabilisers: Ba/CdlZn epoxidised soyabean oil Lubricant: stearic acid
100 3phr 3phr
0·5 phr
BS 2782:1970-515B BS 2782: 1970-515A
30/30
100-160
90 1
4500
40
39-58 150-200 230
Food items b (including ones for deep-freeze)
40/20 30/30 10/10
100-160
90 1
1700
25
54-64 150-200 100
Non-food articlesC
10/10
40/20 30/30
100-160
90 1
4600
50
49-69 175-250 300
Heavier or sharp-pointed objects d
Typical wrapping application area
• Based on data from technical and sales literature for Vinophane films (BCL Films Division, Bridgwater, UK). b e.g. tray-borne or unsupported fruit, fish, meats (fresh or cooked), cheeses, bakery products, sandwiches. C e.g. toys, games, cards, table mat or coaster sets, small textile items. d e.g. cutlery, sharp-cornered toys, tools, stationery, clothing, household textiles. e 25-llm film. f 19-1lm film. g 12·5-llm film.
Examples of available shrink characteristics: LongitudinaVtransverse shrinkage (%)
Heat-sealing temperature range Cc)
BS 2782:1970-514A
Optical properties: Gloss (%) Hazeo(%)
BS 3177
ASTM D 882-67 ASTM D 882-67 BS 2782:1970-306F
Test method
Barrier properties: g Water vapour transmission (at 25°C, 75% RH) (g m- 2 (24 h)-I) O 2 transmission (at 21°C, 44% RH) (cm 3 m- 2 (24 h)-I atm- I)
Physical properties: Tensile strength (MPa)' Elongation at break (%)e Impact resistance at O°C (g(
Properties and units
TABLE 19.11 Some Typical Properties of Commercial Shrink-wrap PVC Films (Relative Density 1·3; Thickness 12-25 pm)Q
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American-type cladding (siding) composition: Homopolymer PVC resin (K value 66) Stabiliser: a butyltin mercaptide Impact modifier: acrylic type (or chlorinated polyethylene) Lubricants: calcium stearate paraffin wax (MP 74°C) fatty acid ester Pigment (UV screen): TiO z
100 1·6 phr 6phr 1·9 phr 0·8 phr 0·9 phr 12phr
A basic formulation for extruded (blown) uPVC film is given in Section 4.6.1(c) of Chapter 4. Apart from the use of VCNA copolymers as the resin components in some thermoformable sheet compositions, two copolymers of vinyl chloride find their way into certain packaging films. Vinyl chloride/ propylene copolymers have been used (for their greater ease of processing-d. also Chapter 1, Table 1.2) in films for food wrapping (especially for fresh meat) and in over-wrap films for cigarette packets. 59 Vinyl chloride/vinylidene chloride copolymers are used as components of some packaging films where very low permeability is a requirement (see also Chapter 1, Section 1.5.2). Laminated sheets of polyvinyl fluoride and PVC have been used in the production of vacuum-formed containers for some packaging applications where the superior barrier properties and stability of PVF were of particular benefit. 59 Many national and international standards relating to uPVC sheet and film are listed in Section 5 of Appendix 1. An ISO standard for the designation and classification of PVC film and sheeting is in preparation. * 19.6 GRAMOPHONE RECORDS
Gramophone records are one of the only two uPVC products of industrial significance to be manufactured by compression moulding (the other being pressed sheets). The dominant requirements for record compositions are reasonable * ISO draft proposal 6236: Plastics-PVC film and sheeting-Part 1: Designation.
19 Rigid PVC: Main Products-Production, Properties and Applications
897
rigidity and high compositional homogeneity of the product, combined with ease of flow in production. The latter two requirements are dictated by the need for the best possible replication of the intricate groove pattern of the record mould, necessary for fidelity and good quality of sound reproduction. For the same reasons it is important that the polymer should be free from fish-eyes and particulate impurities, and the carbon black used as the pigment (as well as all other formulation components) should be well dispersed with no coarse aggregates of particles, lumps, or other inhomogeneities, for maximum freedom from surface defects. The frictional characteristics of the composition must also be such as to ensure freedom from 'hiss' in playing, and smooth running of the stylus generally-this, as well as the processing characteristics, are factors in the choice of lubricant. To meet this combination of requirements, record compound formulations are based on a high-purity grade vinyl chloride/vinyl acetate copolymer of a low K value and vinyl acetate content of about 15%-say Breon AS 60/40 (K value 47-50), or Corvic R46/88-with the minimum number and amounts of additives. An example of a basic formulation is given in Chapter 4, Section 4.6.5: this example shows the use of a lead stabiliser-other stabiliser systems are also employed (liquid ones for best melt flow and completeness of dispersion), e.g. calcium stearate/antimony mercaptide (see Chapter 9, Section 9.4.3(b)). Melt compounding is invariably used. The original processstill practised in some places-involves an internal mixer~ mill~ extruder sequence to produce pelleted compound from the formulation components; a weighed amount of the pellets is fluxed to form a coherent, hot slug, which is fed to the mould and pressed into a record with little or no flash; the moulding is briefly pre-cooled in the mould, and cooling completed (in an air stream) after removal. A typical slug temperature would be 165-170°C, and mould temperature about 95°C. Flow behaviour of the compound is checked with a plastometer to ensure uniform quality. In more modern versions of the process a dry blend is melt-compounded in a compounding extruder, and metered portions of the melt fed to the mould.
19.7 INJECTION·MOULDED uPVC ARTICLES These are mainly fittings for pipe systems of various kinds. Some other mouldings are mentioned in the summary review of PVC products in
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B. J. Lanham and W. V. Titow
Chapter 26. Injection moulding of uPVC is discussed in Chapter 15. An example of a basic formulation for injection-moulded pipe fittings is given in Section 4.6.7(a) of Chapter 4. The fine structure of uPVC injection mouldings is influenced by the moulding conditions in a way generally similar to that observed with other materials. For example, a study by Copsey and Gooding60 indicated a 'skin-and-core' structure, (with some molecular orientation in the skin and little in the core), increasing density of the mouldings with higher melt temperatures and injection rates, and a correlation between the processing conditions and shear moduli of the mouldings (see also Section 15.3 of Chapter 15, and Section 1.5.1 of Chapter 1).
REFERENCES 1. Jacobson, U. (1959). Brit. Plast., 32(4), 152. 2. Corvic Vinyl Chloride Polymers (1982). Technical publication of AECI Chlor-Alkali and Plastics Ltd, Johannesburg, RSA. 3. Howie, J. A. (1971). Chem. Brit., 7(10), 428-33. 4. Mining: Why PVC Now? Technical booklet; AECI Ltd, Plastics Division, Johannesburg, RSA. 5. Properties of Plastics (1976). Technical booklet, Shell Chemicals UK Ltd, London, England. 6. Titow, W. V. and Lanham, B. J. (1975). Reinforced Thermoplastics, Applied Science Publishers, London. 7. Plastics: The Principles of Injection Moulding. ICI Technical Service Note G103 (2nd Edn), ICI Plastics Division, Welwyn Garden City, Herts, England. 8. Press, J. B. and Trebucq, D. A. (1977). In Developments in PVC Production and Processing-i, (Eds A. Wheelan and J. Craft), Applied Science Publishers, London, Ch. 9. 9. Handbook of Plastics Test Methods (1981). (Ed. E. Brown), George Godwin Ltd and The Plastics and Rubber Institute, London, 2nd Edn, p.189. 10. ISO 899-1981. Plastics-Determination of tensile creep. 11. ASTM E 6-81. Standard definitions of terms relating to methods of mechanical testing. 12. BS 4618. Recommendations for the presentation of plastics design data. Part 1: Mechanical properties. Section 1.1: 1970: Creep. 13. ASTM D 2990-77. Tensile, compressive, and flexural creep and creep rupture of plastics. 14. Turner, S. (1964). Brit~ Plast., 37(6), 322-4, and (12), 682-5. 15. Ogorkiewicz, R. M. and Bowyer, M. P. (1969). Brit. Plast.. 42(9), 125-8. 16. Bucknall, c., Gotham, K. V. and Vincent, P. I. (1972). In Polymer Science, (Ed. A. D. Jenkins), North Holland, Amsterdam, Vol. 1, Ch. 10.
19 Rigid PVC: Main Products-Production, Properties and Applications
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17. ASTM E 206-72 (Reapproved 1979). Standard definitions of terms relating to fatigue testing and statistical analysis of fatigue data. 18. Andrews, E. H. (1968). In Testing of Polymers, (Ed. W. E. Brown), Interscience, New York, Ch. 6. 19. Andrews, E. H. (1973). In The Physics of Glassy Polymers, (Ed. R N. Haward), Applied Science Publishers, London, Ch. 7. 20. 'Spotlight on fatigue' (7 brief papers by different authors), ASTM Standardisation News (1980), 8(2), 8-27. 21. Gotham, K. V. (1974). In Thermoplastics Properties and Design, (Ed. R. M. Ogorkiewicz), John Wiley and Sons, London, Ch. 4. 22. Gotham, K. V. and Hitch, M. J. (1975). Pipes Pipelines Int., February, pp.10-17. 23. Gotham, K. V. and Turner, S. (1973). Polym. Engng. Sci., 13(3),113. 24. Moore, D. R, Gotham, K. V. and Hitch, M. J. (1978). 'The mechanical properties of PVC and how these are influenced by changes in processing, formulation and polymer type', paper presented at PRI International Conference on PVC Processing, Egham Hill, Surrey, England, fr-7 April, 1978. 25. Benton, J. L. and Brighton, C. A. (1965). IGE J., March, pp. 185-202. 26. Benjamin, P. (1979). Plast. Rubb. Int., 4(5), 269-73. 27. Swiss, M. (1976). Plast. Rubb. Wkly, 17th September, pp. 33-4. 28. Stephenson, D. (1979). Pipes Pipelines Int., December, pp. 9-17. 29. Cist, J. D. and Smith, J. G. (1978). 36th ANTEC SPE Proceedings, pp. 548-50. 30. Smith, M. (1976). Plast. Rubb. Wkly: 24th September, pp. 25-7; 1st October, pp. 1fr-17; 8th October, p. 29. 31. Anon. (1981). Mod. Plast. Int., 11(11), 35. 32. Smoluk, G. R (1982). Mod. Plast. Int., 12(11),44-7. 33. Zechinati, J., Harvey, B. and Despain, C. R (1978). 36th ANTEC SPE Proceedings, pp. 740-4. 34. Marshall, G. P. and Birch, M. W. (1982). Plast. Rubb.: Process. and Appln, 2(4), 369-79. 35. Hucks, R. T. (Jr) (1981). AWWA J., July, pp. 384-<:i. 36. Janson, L.-A. and Valimaa, P. (1973). Pipes Pipelines Int., January and February. 37. Gaube, E., Diedrich, G. and Miiller, W. (1976). Kunststof!e, 66(1),2-8. 3£:. Szamborski, E. C. and Marcelli, R A. (1976). 34th ANTEC SPE Proceedings, pp. 324-6. 39. Anon. (1976). Mod. Plast. Int., 6(10), 22; and Plast. Rubb. Wkly, 16th July, p. 8. 40. Anon. (1979). Prospect (AECI Quarterly), 18(3), 2-6; and private communication from K. R Hart, Research Director, Duropenta (Pty) Ltd. 41. Anon. (1976). Brit. Plast. Rubb., November, p. 70. 42. Davies, B. (1975). Plast. Rubb. Wkly, 3rd October, pp. 20-1. 43. Harris, W. D. (1976). 34th ANTEC SPE Proceedings, pp. 154-<:i1. 44. Anon. (1982). Plast. Technol., 28(4), 19. 45. 'Special Window Frame Feature', Plast. Rubb. Wkly, 6th June, 1981.
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B. J. Lanham and W. V. Titow
46. Anon. (1982). Pfost. Rubb. Wkfy, 13th November, pp. 8-9, and 20th November, pp. 8-11. 47. Neudell, D. H. (1982). Pfost. Techno!., 28(8), 31-5. 48. Anon. (1981). Mod. Pfost. Int., 11(9), 8 and 69. 49. Johnson, J. E. (1978). 36th ANTEC SPE Proceedings, pp. 36-40. 50. Matsubara, Y. (1979). Pofym. Engng. Sci., 19(3), 169-72. 51. Anon. (1981). Mod. Pfost. Int., 11(11), 11. 52. Anon. (1979). Pfost. Rubb. Int., 4(3), 115. 53. Anon. (1981). Pfost. Technof., 27(2), 15-17. 54. Rudd, N. E. (1978). 36th ANTEC SPE Proceedings, pp. 565-8. 55. Anon. (1981). Mod. Pfast. Int., 11(7), 10. 56. Acton, J. (1976). Pfost. Rubb. Wkfy, 23rd April, pp. 17-19. 57. Stoloff, A. (1979). Pfost. Engng, 35(7),29-31. 58. Changfoot, J., Dickson, A. G., Noel, F. and Stark, W. M. (1976). 34th ANTEC SPE Proceedings, pp. 42-4. 59. Briston, J. (1976). Packag. Rev., 96(3),71-4. 60. Copsey, C. J. and Gooding, V. J. (1977). 'Effects of processing on the morphology and properties of injection moulded PVC', paper presented at the Institute of Physics Conference on Processing, Structure, Properties and Performance of Polymers, University of Nottingham, July 1977.
CHAPTER 20
PVC Sheet and its Fabrication W. V. Trrow
20.1 INTRODUCTION Very few people nowadays can be unfamiliar with plastics sheet or film. However, it may be of some interest to consider briefly the actual meanings of these terms, and the significance of the distinction between the products they designate. The main English-language standards of plastics nomenclature l - 3 all base their definitions of sheet on the obvious cardinal feature, viz. that this laminar product is thin in relation to its length and width. Some2 ,3 also relate the term to a single piece of such product? which may have been individually produced or cut from sheeting, the latter term being defined as sheet in continous form (e.g. a long length supplied as a roll 1,2). One of the standards 1 recognises synonymous use of 'sheet' and 'sheeting'. On the above basis a film is clearly a kind of sheet (or sheeting). Indeed the only distinction between the two-both in standard terminology and in common usage-is purely a matter of thickness, and not actually precisely defined in industry. Thus whilst the general sequence (in order of decreasing thickness) sheet~ thin sheet~ film is universally recognised, the actual demarcatory thickness value between thin sheet and film (or, for that matter, thin and thick sheet) is not uniformly quantified and can vary in practice depending on the material, the field of application and even the country concerned. However, in an attempt at standardisation, the general thickness limit below which a plastics sheet should be referred to as a film is set by the three above-mentioned standards at 250.um (0·010 in). The arbitrary 901
902
W. V. Titow
nature of this figure is actually pointed out in the ISO standard, l and illustrated by a note in the BS specification to the effect that in the case of some materials the term 'film' should be reserved for still thinner sheet, and that specifically for unsupported rigid PVC film the upper thickness limit should be 0·003 in (about 76,um). The term 'foil' is also sometimes applied in certain practical contexts to plastics film and thin sheet materials, although it is not defined in the nomenclature standards, only one of which2 mentions it as a (discouraged) alternative to 'film'. Thus the thin films (plastics or metal) used in the so-called hot-foil stamping method of surface decoration (see Section 20.3.5(a) below) are commonly referred to as foils: this is quite logical in view of the long-standing (and standardised) relevant terminology for thin metal films* as well as common usage in the packaging field where such films (employed e.g. for wrapping chocolate) are known as foils. Less obviously logical (and vis-a.-vis the accepted meanings just mentioned even somewhat contradictory) but none the less fairly widely encountered in commercial and industrial contexts, is the use of 'foil' for thick plastics film or thin sheet (with PVC roughly 0·003-0·03 in) for such packaging applications as thermoformed insert trays, blister packs, etc. Finally, the word foil occasionally crops up in contexts where it is clearly employed as a loose synonym for plastics film in general. Note: It appears possible that the use of 'foil' in English in the latter two connotations may owe something to the influence of relevant plastics terminology in other languages (ct. Foliethe correct German term for 'film', and French feuille for 'film' or 'sheet', in the plastics context).
PVC sheeting is commonly produced by calendering or extrusion. Sheet for certain purposes (vacuum forming, special decorative surface effects), and thick sheets (panels) are also made by compression moulding: uPVC sheet up to about 25 mm thick can be produced in * The four definitions of 'foil' in the Compilation of ASTM Standard Definitions (3rd Edn: 1976) are, respectively, 'a thin sheet of a material, usually a metal, not exceeding 0·005 in in thickness'; 'a rolled product, rectangular in cross section, of thickness less than 0·006 in (0'15 mm)'; 'any strip of sheet less than 0·005 in (0·13 mm) in thickness'; and 'a term often applied to thin sheet or strip usually 0·005 in (0,13 mm) or less in thickness'. This is in substantial agreement with the definitions of aluminium foil, and copper and zinc alloy foil, in BS 4342: 1968.
20 PVC Sheet and its Fabrication
903
this way (by heat-laminating a number of thinner sheets under pressure): the thick sheets are used for such applications as, for example, the construction of fume scrubbers, chemical plant, covers and linings for chemical storage or processing tanks, special trunking and ducting resistant to chemical corrosion, and the like. Pressed sheet reinforced with a long-fibre grade of chrysotile asbestos (e.g. Duraform-TBA, UK) is used in some of these applications, as well as for external and internal wall cladding of some industrial premises (including flush wall lining of food factories), motorway signs, etc. The nominal thickness range of rigid PVC sheet (produced by any of the three main methods) covered by BS 3757: 1978 is 0·25-24·0 mm. In the plastics industry the term 'fabrication' is usually understood to cover a range of techniques by which such intermediate products as sheet or rod are processed into finished products. It is a characteristic feature of many fabrication techniques that, despite the wide extent of automation, they often involve a certain amount of more or less direct manipulation by an operative. Some fabrication techniques find their main application where the dimensions of the finished product are too large for production by other methods and/or where final operations have to be carried out on site. Examples are production, by welding, of chemical ducting from rigid PVC sheet; lining of chemical tanks (metal or GRP) with PVC sheeting (which may have to be cut to shape, and then joined by welding); and welding of PVC flooring or reservoir linings (of flexible PVC sheeting). Although such operations as cutting and welding (as well as printing) are widely performed on PVC pipes, the greatest variety of fabrication and surface decoration techniques is used in the processing of PVC sheet materials in their various forms (unsupported sheet and film, rigid and flexible; fabric-supported sheeting; sheet laminates, flooring, etc.). It is for this reason that a discussion of the main fabrication methods is coupled with a brief consideration of PVC sheet in this chapter. Among sources of information relevant to fabrication, an early review by Zade,4 and the Plastics Institute Monograph by Estevez and Powell, 5 are still of some interest in addition to newer publications on such topics as solvent welding6 and machining7 of plastics (including laser machining8 ). The nature, properties and applications of unsupported PVC sheet and film are briefly dealt with in the next section, with reference to some commercial products. The manufacture of sheet and film is discussed in Chapters 18 (calendering) and 19 (extrusion); film casting
904
W. V. Titow
from solution is covered in Chapter 24. PVC layers (coatings) on fabric support are dealt with in Chapters 22 and 25.
20.2 UNSUPPORTED PVC SHEET MATERIALS There are many primary manufacturers of PVC sheet and film for use in a variety of direct applications (e.g. tank lining, wall cladding, glazing, roofing, with sheet; article and pallet stretch-wrapping with film) or for further conversion (e.g. production of ducting and chemical plant from rigid sheet; bags, blister packages, table cloths, decorative self-adhesive surface coverings from film). The main general types of product represented in the manufacturers' ranges are: -rigid PVC homopolymer sheet and film; -rigid PVC copolymer sheet and film; -plasticised PVC sheet and film; -plasticisedlrigid PVC sheet laminates;* -PVC sheet laminates with other materials (see e.g. Chapter 16, Section 16.2). Most types of sheet and film are available in clear and pigmented grades, and many in various embossed finishes. Special products, such as corrugated or wire-laminated sheeting, and uPVC sheeting laminated-or 'capped' by coextrusion-with relatively thin decorative or protective layers (e.g. of acrylic polymers, for weathering resistance) are also on the market. Some characteristics and applications of commercial PVC sheet materials are illustrated in Table 20.1 by reference to a number of ICIt Darvic and Flovic products. Although the data quoted are not the
* This type of laminate was specially developed for lining metal tanks for heavy chemical duty. Bonding large areas of rigid PVC sheet to mild steel presents serious problems. However, when the laminate is bonded (by means of a special adhesive) with its plasticised layer to the metal surface, and inter-sheet joints are sealed by welding with a uPVC welding rod, satisfactory adhesion is achieved, coupled with the overall effect of an all-uPVC internal tank surface. 10 t Currently manufactured by a subsidiary, Weston Hyde Products Ltd, formed by the recent amalgamation of Wallington Weston and the ICI PVC sheet product manufacturing operation at Hyde.
20 PVC Sheet and its Fabrication
905
latest for these ranges,9 they provide a valid general indication of the kind of materials concerned and their uses. The following may also be mentioned by way of further examples. Roofing and wall-cladding sheet: Sintaclad (ICI)-rigid corrugated sheet for roofing applications* and for cladding of buildings exposed to corrosive chemical atmospheres: good retention of light transmission (several years in moderate climates) is claimed for the transparent version (tinted in 'solar bronze' to reduce heat build-up). Both the transparent and opaque versions have Class 1 ratings in the spread-of-flame test of BS 476: Part 7. Biaxially oriented uPVC sheeting (clear or translucent) developed by Solvay & Cie., Belgiumt (based on their Bellvic emulsion resin grades claimed particularly suitable for outdoor applications) offers improved impact resistance imparted by the orientation, coupled with retention of this property down to very low temperatures (-40°C is claimedll). Cobex 1350 (Storey Bros,:j: UK)-sheet for interior wall cladding in food factories, bottling plants and the like (meets the requirements of BS 3757, Part 1, and BPFIBIBRA Code for food-contact materials). Linings for chemical or food-storage tanks and reactors: Vybak (Storey Bros, UK)-pressed PVC sheet (service temperatures up to 95°C claimed for GRP-clad installations). Armodur (Societe la Cellophane, France)-rigid PVC sheet made by pressing from calendered sheeting based on a mass-polymerised resin grade. Telcovin TL (Telcon Plastics, UK)-flexible PVC sheet. Safety doors: Astraglass (Dynamit Nobel, West Germany and UK)-flexible PVC sheet. Sheet and film for thermoforming: Kydex (Rohm and Haas, USA: European Licensee Mazzucchelli Celluloide, Italy)-acrylic-modified PVC sheet. Fromopack (Wallington Weston,§ UK)-range of thermo-
* A summary of general data on the span of corrugated PVC sheeting in roof construction in relation to maximum safe stress and deflection, and to modulus reduction with temperature, has been published by Gamski. 12 t cf. also Oriex, biaxially stretched PVC sheeting of Tenneco Chemicals Inc., USA. :j: Now part of the Wardle Storeys group. § See the earlier footnote on Darvic sheet products.
Limited number of colours. Scratched-plate surface finish Grey green
0·5-19·0mm (0·02o.l in)
0·8-25·4mm (&-1 in)
1·6-4·8mm (rl,-rl,in)
CF standard grade
Industrial grade
Unplasticised/ plasticised grade (laminate)
Nature and/or colour' Various colours; two types of surface finish (matt or polished) Various colours; two types of surface finish (matt or polished)
Thickness range
0·5-12,7 mm (0·020-~ in)
Darvic sheet Standard grade
Type and/or grade
Main applications
More flexible than the industrial grade
Tank lining
Contains no known tox- Interior lining (including vehicles) and exterior ic ingredients cladding High resistance to weathering. Not recommended for foodcontact use Not normally recomChemical plant mended for foodcontact use (but a special, suitable grade can be supplied)
Special features
TABLE 20.1 Some Properties and Applications of 'Darvic' and 'Flovic' PV-c Sheeting
~
C
:::J
:<:::
~
~
All standard grade colours
Up to 0'76mm (0,003 in)
0·75-4·5 mm (0·030-0·180 in)
Darvicfilm
Flovic sheet (vinyl chloride/vinyl acetate copolymer)
a
Various transparent col- High impact strength. ours Good weathering. High clarity. Not for food-contact use. Incorporates wire mesh reinforcement
1·0-6·5 mm (0·040-0·260 in)
Clear wire laminate
Glazing (including safety glazing), lighting, machine guards, windscreens, illuminated signs Glazing (including roofs)
Multi-colour, and opaque-clear laminates are available.
Stronger in longitudinal Packaging direction. Suitable for shaping, but not vacuum forming Opaque-white and some Contains no known tox- Vacuum forming food colours ic ingredients packaging
Various transparent col- High impact strength. ours Good weathering. High clarity. Not for food-contact use
0·5-6·5mm (0.020-0.260 in)
Clear grade
s
;:,
6"
l:>
F;.
0-
~
i;'
;:, I:l..
l:>
'"~
v, ;:r-
~
."
~
I\,)
20 PVC Sheet and its Fabrication
909
formable packaging sheeting in clear, rigid PVC, for blister packs, skin packs, etc. Vinophane (British Cellophane, UK)-clear PVC film for various shrink-wrap packaging applications.
Road Signs: Pacton SQ (ICI)-impact-resistant rigid PVC sheet. Interior wall coverings for buses and aircarft: Storeytrim (Storey Bros, UK)-self-adhesive PVC sheeting. Kydex (some versions). Bondene (Storey Bros, UK)-aluminiumJPVC sheet laminate. Miscellaneous: PVC-Glas (Simona GmbH, West Germany)high-transparency natural and coloured sheets (in 1-20 mm thicknesses). Simona-PVC-EL antistatic sheet (surface resistivity down to 106 Q). Information on manufacturers and suppliers of PVC sheet and film for their numerous applications is available from various sources (some mentioned in other connections in this book), including the following:
International: The current editions of: (i) International Plastics Directory (Handbuch der Internationalen Kunststoffindustrie), Verlag fur Internationale Wirtschaftsliteratur, Zurich. (ii) European Plastics Buyers' Guide, IPC Industrial Press, London. UK:
(i)
British Plastics Federation Buyers' Guides.
Plate N Applications of rigid PVC sheet. (Top) Processing-solution tanks in an automatic colour film processor. Tanks fabricated from Darvic by Refrema NS, Roskilde, Denmark. Darvic was chosen because it is unaffected by the colour-processing chemicals and because it can be shaped, machined and welded accurately to make strong, durable tanks which fit precisely into place in the processors. (Courtesy of ICI Plastics and Petrochemicals Division and Weston Hyde Products Ltd.) (Bottom) Instrumentation panel: complete wall cladding of Duraform reinforced sheet with decorative (unreinforced) surface, chosen-as a replacement for steel sheets-because of its combination of good load-bearing and non-conductive properties. (Phillips Petroleum Company's North Sea Gas Terminal at Bacton, Norfolk-courtesy of TBA Industrial Products Ltd.)
910
W. V. Titow
(ii) The current edition of Packaging Review, IPC Business Press, London. (iii) The current edition of Buyers' Guide to Plastics Products used in Agriculture and Horticulture, British Agricultural and HOHicultural Plastics Association, London. USA:
(i) The current edition of Modern Plastics Encyclopedia, McGrawHill, New York. (ii) Films, Sheets and Laminates: Desk-top Data Bank, International Plastics Selector Inc., San Diego, California.
20.3 MAIN FABRICATION TECHNIQUES APPLICABLE TO PVC SHEET MATERIALS AND PARTS
20.3.1 VVelding The international standard for plastics nomenclature l defines welding as 'the process of uniting softened surfaces of materials, generally with the aid of heat', and points out that in some countries the term 'sealing' (rather than 'welding') tends to be applied to processes in which the surfaces of films are united by the application of heat and pressure. All the main techniques employed in the welding (as distinct from bonding or cementing-see Section 20.3.2) of PVC involve the application of heat to, or its generation in, the material at the site of the joint. Hence the main factors instrumental in weld formation are the welding temperature, amount of heat, duration of heating, and the pressure applied. As in all processing of PVC, it is important to minimise the risk of direct thermal degradation (which in this case would be largely localised in the area of the welded joint) and generally to ensure the minimum practicable increase of the heat history of the material in this area. The recommended general temperature ranges for welding rigid and flexible PVC are, respectively, 180-210°C13 and 250-300°c. l4 Below the lower limits of the two ranges incomplete or faulty welds are likely, whilst exceeding the upper limits can cause unacceptable degradation of the polymer.
20 PVC Sheet and its Fabrication
911
Hot-gas welding: In the main embodiment of this method the edges of sheets to be joined are softened by a stream of hot air or other gas, and a similarly heated filler rod (of the same or similar material) is laid into the gap to complete the joint. In a variant of the technique, known as hot-gas overlap welding, the welding nozzle is passed between the faces of overlapping sheet pieces to be joined: no filler rod is used, but the joint is made, between the heated surfaces in the overlap area, under pressure applied manually or mechanically. A development of the hot-gas method, called extrusion welding, employs a heavy-duty hot-gas gun to pre-heat the work pieces, in conjunction with a miniature extruder (comprising a screw and barrel, a reduction gear box and an electric motor drive developed from an industrial hand drill) which lays a bead of hot filler material into the gap of the joint as the device is drawn along it. The ultimate strength of joints made manually by any variant of hot-gas welding is cardinally dependent on the operator's skill. Hot-gas welding is used where fully mechanised or automated techniques are not suitable because of the size and/or complexity of the workpieces, or where the number of joints required is small and simple enough to favour a manual method. Examples are seam welding of PVC flooring, liners for refrigerated vans, chemical or food-storage tank linings, wall linings in hospital operating theatres, industrial clean rooms, food halls, etc. Heated-tool (hot-plate) welding: In this method those surfaces of the parts to be joined which are to form the joint interface are heated by contact with the hot surfaces of a metal tool (often a relatively simple platten with built-in electrical resistance heaters), and subsequently pressed together to form the joint. Indirect heated-element welding: In this method, which is normally confined to the heat welding (heat sealing) of thermoplastic films and thin sheeting, insulated* heating elements are applied (with simultaneous application of suitable pressure, either via the elements themselves or by some other means) to the outer surfaces of the workpieces to be joined, so that the corresponding inner surfaces
* The insulation (commonly PTFE) serves as a parting layer preventing disruption (through sticking) or marking of the plastics surface by the hot element.
912
W. V. Titow
participating in joint formation are heated indirectly, through the thickness of the material. The elements may be heated only during the bonding operation (thermal impulse welding) or may be constantly maintained at the appropriate temperature (as in some continuous film-sealinG operations). Friction welding: The two variants of this method are spin welding and vibration welding. In both the surfaces to be joined are softened by heat generated when they are rubbed together, but in spin welding the rubbing is actuated by complete rotation, and in vibration welding by reciprocating, lateral displacement, linear or angular 15 (commonly at about 100 cycles per second). In both variants the parts are ultimately joined under pressure. Vibration welding is more versatile than the spin variant which is normally restricted to parts of which at least one (and preferably both) must be symmetrical and rotatable. Although in principle friction welding is applicable to both rigid and flexible PVC, in practice difficulties can arise with the latter, and the technique is largely used for suitable uPVC parts. High-frequency welding: In this technique, also known as dielectric welding or radio-frequency (RF) welding, the bond areas of the parts to be joined are subjected to an electric field of high frequency: this induces molecular vibration causing internal heat generation within the material. The contacting surfaces of the parts so heated are joined under pressure (relatively light). It has been estimated 16 that over a half of all the bonding in industrial conversion of flexible PVC film and sheeting into products (see Section 20.3.1(c)) is carried out by the high-frequency method, and that around 90% of all plastics products manufactured with the aid of this technique are made of PVC. Note: To be suitable for high-frequency welding, a thermoplastic material should have a loss tangent (dissipation factor) of at least 0·01 (at 1 MHz and 20°e) to ensure sufficient heating in a high-frequency field. Hence, among PVC materials, the method is primarily applicable to those based on flexible compositions (cf. loss tangent values in Appendix 3). It may thus be regarded as mutually complementary with ultrasonic welding, which is normally suitable only for rigid compositions (see below).
20 PVC Sheet and its Fabrication
913
Ultrasonic welding: With regard to the bond-formation mechanism this may be considered a special case of frictional welding. Intense mechanical vibration of one of the surfaces to be bonded against the other is caused by placing the material in contact with the 'sonotrode' or 'horn' of an ultrasonic generator (often vibrating at 20 kHz, but in general up to about 40 kHz). The vibration at the interface, which is of relatively low amplitude, produces frictional heat, and softens the surfaces which merge under applied pressure (normally maintained throughout the welding operation). Possibly the greatest single advantage of ultrasonic welding is the short bonding time required (typically 0-1-1 s). The main limiting factors are the sound transmission characteristics of the material (which should be good, with the least possible energy loss), its softening point, modulus, and coefficient of friction. The technique is more suitable for rigid than flexible PVC (in general, the softer the thermoplastic material the more difficult it is to weld). Ultrasonic welding equipment is supplied by some manufacturers of high-frequency welders, and also by companies specialising in this line, e.g. Ultrasonics Ltd (UK); Branson Sonic Power Co., their USA associate; Dukane Corp. (USA); or Telsonic (Switzerland and UK).
Good summaries of most welding methods applicable to PVC are given in Refs 13 and 14. Some of the techniques are also described in a booklet published by the Institute of Welding in the UK (Data on the Welding of Thermoplastics). Two DIN specifications list (with drawings and brief explanations) respectively, the processes for the welding of plastics (named in German, English and French)17 and their technical principles. 18 The following standards are also of some peripheral interest. ISO 2553-1974. Welds-Symbolic representation on drawings. BS 499. Welding terms and symbols. Part 1: 1965. Welding, brazing and thermal cutting glossary. Part 2: 1980. Specification for symbols for welding. BS 2759: 1956. Glossary of terms used in industrial high-frequency induction and dielectric heating. Published reviews of welding methods for thermoplastics of interest here include a relatively early one by Streese,19 and a more recent one already referred to. 16 An excellent account of the use of welding
914
W. V. Titow
techniques (thermal-impulse, heated-tool, ultrasonic, and highfrequency welding) for the production of welded seams in coated fabrics has been published by Koehler. 20 Analyses of some welding processes for thermoplastics in terms of their principal parameters are given in papers by Frankel and Wang21 (ultrasonic welding), and Potente and Reinke 22 (heated-tool, ultrasonic, and friction welding). Some of the welding methods and their applications in the fabrication of PVC products are discussed further in the following sections.
(a) Hot-gas Welding* Mass production of articles from PVC often involves welding of complicated shapes. This is usually best done by high-frequency welding machines. Plastics film and thin sheeting (PVC, polyethylene, polypropylene and others) are also commonly welded by the indirect heated-element method. Hot-gas (in practice normally air or nitrogen) welding remains a favourite method for batch production (especially where the products are large and/or complicated), for many on-site operations (particularly where those involve the jointing of sheet materials to form continuous large-area layers, as, for example, with PVC flooring), and for butt-welding of relatively thick sheets (as, for example, in wall lining, or the production of tank linings). The welding of pipes by the hot-gas technique in the construction of pipelines is also practised. These, and many other applications stem from the general versatility of the method and portability of the equipment. Inter alia, it is of considerable practical significance that the rate, extent and degree of heating of the material in the course of the welding operation can be readily varied, for best results, in line with the operator's knowledge and experience of what is required with a particular material in particular circumstances. The factors directly relevant in this connection are the temperature and rate of flow of the hot gas, the distance between the nozzle of the welding torch and the material, the rate of formation of the joint (speed of the welding operation), and the magnitude and method of application of pressure. * This section is based in part on an earlier version-here extensively edited, revised and supplemented-contributed to the previous edition by Mr R. W. Berman of Welwyn Tool Co. Ltd, UK.
20 PVC Sheet and its Fabrication
915
WELDING TORCHES
A welding torch is basically a simple instrument, but a large range of designs, developed for different purposes, is available. * There are two general types: 'self-blowing' torches, and lightweight torches which require an external gas supply. Self-blowing torches, often called 'welding guns', are self-contained in that they incorporate a motor and a small turbine, housed in the handle together with a heating element. A typical modern version of the latter is a ceramic block, pierced by a large number of channels through which the gas (normally air) passes on its way to the nozzle, and where it is heated by contact with resistance wires threaded through the channels. The heating element is encased in an element holder of special heat-resisting steel which can withstand temperatures in excess of 600°C and become red-hot without distortion. The steel holder terminates in an air outlet nozzle. The nozzles, or nozzle/holder assemblies, are normally exchangeable: designs include wide-slot and multiple outlet varieties (the latter enabling a skilled operator to work at considerably increased speed). It is usual to have a temperature control (e.g. a ring switch) on the handle, whereby the temperature can be varied (in some designs stepwise by activating consecutive sections of the heating element). The motor-actuated turbine of the welding gun delivers a steady air stream. In normal models the air flow and pressure cannot be altered. However the flow is sufficient for all common operations. It may be noted that it is the flow which is the directly important factor. The operating pressure is normally relatively low, say about 1-2Ibfin- z. Torches of the other general type ('non-self-blowing') are frequently referred to as 'welding pistols'. Their heating elements, element holders and gas outlets are similar to those of self-blowing torches, and in many cases these parts are interchangeable between the two types. The welding pistol can be connected either to an air blower or to a gas (air or nitrogen) bottle. The advantages this tool offers in some of its industrial uses include the ability to employ an inert gas (nitrogen) where oxidative degradation of the plastic undergoing heating is to be minimised, and reducibility of the gas flow to levels below those normal for the welding gun; most models incorporate a reduction valve (in its simplest form a flow-restricting screw mounted in the handle) * In the UK, for example, from the Welwyn Tool Co. Ltd, Rediweld Ltd, or Goodburn Plastics Ltd.
916
W. V. Titow
and/or a gas flow meter. The gas should be free from oil, water and other contaminants. WELDING TECHNIQUE
The basic technique is similar to that of arc welding of metals, in that the operator uses the hand-held gun to heat a welding rod and the workpiece simultaneously, with the rod being drawn along the line of the weld and part of the hot gas stream softening the plastics material in front of the weld area. The operation thus involves suitable filling of the gap between two workpieces (sheets) butted together. A strip of the material concerned may be used as the welding rod, but special extruded rods are available, and preferable for most purposes. In contrast with metal welding, there is relatively little risk of fire, explosion, or injury to the operator. The welding rods used for most purposes are unfilled, but should otherwise be as close as possible in composition to the material to be welded: the right grades are normally obtainable from the material producer or from suppliers recommended by him. In either case-and particularly the latter-it is necessary to identify clearly (preferably by sample) the material for which the rod is to be used. Rods for industrial welding purposes (as distinct from floor welding) are supplied in various cross-sectional shapes. The advantage of this is that the groove (see further on) can be filled with one line of suitably shaped (say triangular or trefoil) welding rod, where several lines of cylindrical welding rod might otherwise have been necessary. Floor-welding rods are normally plasticised and hence flexible. It is for this reason that they are often referred to as 'welding cables'. The materials (sheets, floor tiles, etc.) to be welded must be prepared. With sheets of kin thickness or more, a single 'V' shape groove of 60° angle is made, either by a hand tool or-for industrial unplasticised material-with the aid of a grinding wheel and inclined table. For plasticised floor tiles the groove can be produced by a motor-driven chamfering tool. One version runs on four rollers, two of which engage in the gap between the two sheets (and in the groove as soon as the machine starts chamfering). This self-guiding arrangement makes the use of a straight edge unnecessary. With thick sheets, or if convenient generally, a double groove can be used. Before welding, the sheet edges and the welding rod should be wiped with methyl ethyl ketone or another suitable solvent. As to the welding operation itself, a distinction may be made between industrial welding and floor welding.
20 PVC Sheet and its Fabrication
917
INDUSTRIAL WELDING
Before the welding operation proper, it is advisable to tack the material (normally relatively thick sheets or sections) together so as to create a bottom to the groove. This is done with the aid of a tacking jet which can be pushed onto the nozzle of the welding gun or pistol. In the early welding method the welding rod was held in one hand and the welding tool (pistol or gun) in the other, the rod being pressed into the groove as the immediately concerned sections of both became sufficiently softened by the hot gas. The method is known as the 'pendulum' method of welding because the hot-gas outlet of the tool is swung up and down during the operation; it is still used for welding difficult corners. In all other cases the welding is facilitated and speeded up by the use of speed-welding nozzles (see Fig. 20.1). A speed-welding nozzle is an attachment, push-fitting the end of the welding tool, consisting of a sleeve ending in a tongue and a hot-gas outlet. The sleeve holds the welding rod so that it is pre-heated, and then pressed by the tongue into the heated groove. The use of this attachment not only speeds up the welding, but in many cases enables it to be performed with one hand. Speed-welding nozzles are available for welding rods of various diameters and cross-sectional shapes (e.g. triangular, trefoil).
Fig. 20.1 Speed-welding nozzles.
For plastics films and thin sheets, where the material is too thin for grooving, overlap welding is used. This operation can be considerably mechanised for many applications (e.g. production of tarpaulins, in situ joining of films used as seepage barriers, etc.) and the mechanisation adapted to so-called 'tape welding' in which the film edges to be joined are butted together and sealed with special thermoplastic tape which overlaps the joint on both sides. The lining of a metal tank with PVC sheeting (which may be the uPVClpPVC laminate previously mentioned) can be examplified by the following sequence of operations. (i)
Hammering out, grinding out, or filling-as appropriate-of any dents, bulges, high points (e.g. welds) or depressions in the metal.
918
W. V. Titow
(ii) Shot- or sand-blasting the surface, followed by de-greasing (e.g. with trichloroethane). (iii) Preparation of the sheet (including pre-shrinking by heating if recommended by the supplier) and cutting to size. (iv) Application of an appropriate adhesive (most manufacturers' ranges-e.g. Bostic, 3M-include suitable compounds) to the tank walls (in some cases also to the sheet) strictly in accordance with the adhesive supplier's instructions. (v) Attaching the pre-cut sheet to the walls: the usual order of covering is sides, floor, and lip edges. (vi) Hot-gas welding of all joints (butt and corner). The completed joints of a lining should be checked for freedom from flaws, cracks and other leak-promoting faults. This is usually done with the aid of an electrical spark tester, obtainable from most suppliers of hot-gas welding equipment and some general sources of industrial test equipment. For the production of PVC linings for reinforced plastics tanks, a British Standard covering the construction of such vessels23 recommends a minimum PVC sheet thickness of 3 mm, and specifies that the tensile strength across a weld should be not less than 90% of that of the sheet itself. A corresponding figure, given in DIN 1693013 as the general minimum 'valence ratio' for hot-gas welded joints between rigid PVC sheets is 0·6 (i.e. tensile strength across the weld at least 60% that of the sheet), whilst ASTM D 178924 stipulates a general minimum of 75%. The latter specification lists, and illustrates, the main causes of poor welds. FLOOR WELDING
This is carried out in premises with PVC tiled floors, especially where considerations of hygiene make a continuous surface particularly desirable, e.g. in hospitals. In essence, the operation consists in grooving the joint lines between the tiles and sealing by welding in the normal way. In manual operation the welding rod (welding cable) may be fed and laid in with the aid of a feed roller, with the handle hollowed out to take the cable, and a grooved wheel to apply it to the groove. However, here too the process can be considerably speeded up by the use of a speed-welding nozzle (suitable for the cable diameter employed). A still faster rate of working is made possible by floor seam-welding machines, which run automatically, following a groove,
20 PVC Sheet and its Fabrication
919
laying a weld, and shutting off upon reaching a wall, so that the operator's function is reduced to moving the equipment over into consecutive grooves, and general supervision. Welding rates of 30 it min- 1 are possible with this kind of automatic operation, if the grooves are properly made, preferably by a mechanised chamfering tool. Felt- or foam-backed PVC flooring was originally welded from the back, after chamfering away the backing layer. The modern method permits the welding of such flooring in position, i.e. when stuck down in the normal way. Exact chamfering with a motorised tool engaging in the gap between the sheets enables a groove to be produced which is accurately centred and of a depth so pre-adjusted that the felt or foam layer remains virtually undisturbed. The welding can then be carried out manually or with an automatic machine. In all floor welding, the seam (original welding cable) will remain protruding above the tile or sheet surface on completion of the welding operation. These protrusions must be removed, to make the floor surface flat and even. This can be done with a sharp knife ('spatula') or another hand tool (e.g. a spatula fitted with a cable-cutting slide), but is best accomplished with a trimming plane whose knives can be set to cut no deeper than is necessary to ensure that the seam does not 'sink in' afterwards. (b) Extrusion Welding This modification of the hot-gas welding method was developed in Germany. The main general features of the extruder component of a typical extrusion welder have already been mentioned. The second main component of this tool is a heavy-duty hot-air gun (usually mounted on the side of the extruder): this pre-heats the material in the area of the joint to be formed as the tool is moved along the groove and just before the nozzle of the extruder head lays-in a line of melt. In comparison with conventional hot-gas welding, extrusion welding enables welds to be produced in one operation where otherwise the sheet thickness and width of groove would necessitate the consecutive laying-in of several welding rods, and can give much higher weld strengths. Heat degradation of the material can also be reduced with this technique.
(c) High-frequency Welding The principle of this technique has already been mentioned: Some of
920
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Plate 0 Autodex MK V automatic feed and RF welding unit, with standard 6 kW generator: 30 x 20 in (760 x 508 mm) overall working area; high-speed automatic production, with manual option; platen pressure load variable between 200 and 1500 lb (91-680 kg); indexing accuracy ±0·5 mm. (Courtesy Radyne Ltd, Wokingham, Berks., England.)
its features and applications are reviewed in Ref. 16. The appropriate sections of the parts to be joined are subjected to an alternating high-frequency field by positioning them suitably between two electrodes. The induced internal molecular vibration generates heat within the material which becomes softened to the point where it can be welded under pressure. The electrodes may be flat or threedimensionally shaped, depending on the configuration of joint required, and should be temperature-controlled so that they do not become overheated (by heat transfer from the material) during welding. In commercial equipment they also serve as the means of applying pressure to the joint. Various arrangements and electrode designs are used. For more efficient material heating (through reduction of heat loss to the comparatively cooler metal of the electrodes) thermal barriers are normally interposed between the
20 PVC Sheet and its Fabrication
921
electrodes and the work. Originally the material commonly used for this purpose was a hard paper known as 'Pertinax', but a variety of others have proved more successful, e.g. silicone-impregnated glass fabric, PTFE sheet, etc. The internationally recognised standard operating frequency for the welding equipment is 27·12 MHz, with a tolerance limit of only ±0·6%. The narrow margin this leaves for variation necessitates a high degree of built-in frequency control (which is reflected in equipment cost), inter alia, to ensure (commonly through the use of auto-tuners) that any change in capacity caused by reduction in the thickness of material between the electrodes as fusion takes place in the course of welding, does not result in frequency drift beyond the prescribed limits. Purely from the technical standpoint, production may in fact be improved by operating at frequencies higher than the standard (cf., for example, the 50 MHz machines originally available from StanelcoThermatron Ltd*). The optimum power for welding depends very much on the sheet thickness. In general, the thinner the material the higher the power required to compensate for the proportionately greater heat loss from the plastic to the metal of the electrodes (which are colder than the plastic heated up in the course of welding). It is because of this that at very low material thicknesses welding may not be possible at all. The already mentioned use of thermal barriers is aimed at reducing the heat loss and thus redressing the balance to some extent. As an example, for film 0·003 in thick, tOO MHz is necessary, and the length of heating time makes little difference, although it does, of course, above 0·010 in. As a general, approximate guide, 1 s may be regarded as about right for the heating time of films up to 0·005 in. For sheets about 0·020 in thick (for example) a time of 6 s should be aimed at, although less would be preferable if the power is available. The best seam is obtained by using a short time cycle (excess will cause plasticiser loss and embrittlement), a light pressure, a reasonable heat dosage, and electrodes with suitably radiused edges. Sharp edges will cause a field concentration and overheating. The nature of the compound influences the weld obtained. Metallic particles and furnace blacks can give conducting paths and serious troubles on thin sheets. Really first-class dispersion should, therefore, be obtained if these types of fillers are to be employed. Generally, the lower the filler content (including pigment) the better the result. * Member of the International Group Wilcox and Gibbs Inc.
922
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Excess material can make welding more difficult and weak seams may result. Highly filled cheap products should therefore be used with caution, particularly with inflatable goods where good seams are vital.
Note: Arcing-a potential source of serious damage to the electrodes and danger to the operator-was a real possibility with early versions of welding equipment, especially when processing thin films. In modern machines the hazard has been virtually eliminated by the incorporation of arc-sensing units. High-frequency welding machines are available from a number of manufacturers. Stanelco-Thermatron (with its parent group) and Radyne Ltd are among those whose equipment represents wide ranges of size and power rating, and includes complete production lines, in many cases incorporating also printing and other operations. The products welded on the equipment typically include the following: Blood-transfusion and other PVC medical equipment; liquid packaging; clothing (including protective garments and rainwear); footwear; stationery covers, folders and binders; book covers; fancy goods; handbags; luggage; garden and other furniture covers; upholstery; inflatables; baby wear; pram covers; quilted goods; tarpaulins; large welded sheets; car trim. The welding press platen sizes range from, say, about 25 x 3 mm (for welding blood-transfusion equipment) to 3 x 2 m (for inflatable rafts), with generator power ratings of the order of 100 W at the lower end of the range up to 100 kW at the higher end. (d) Heated-tool Welding As has been mentioned, in this method the surfaces to be welded are heated by direct contact with an electrically heated element which may be quite intricately shaped to give the type of weld required. In practice the surfaces are actually pressed against the element (pre-heated to the appropriate temperature), allowed to remain in contact for the appropriate time, the heating element is withdrawn and the surfaces are brought together under the appropriate pressure to form a weld. It is claimed25 that heated-tool welding is one of the quickest and most economical methods of heat-welding thermoplastics. Apart from the properties of the material itself, the following points are of crucial importance to the quality of the weld: the tool temperature and surface condition, pressure during pre-heating, time
20 PVC Sheet and its Fabrication
923
of heating and tool withdrawal time ('change-over time'), welding pressure, and the cooling rate and time. Thermostatic control of the heating element is important for good results. A popular form of heating tool is the heating mirror type supplied, for example, in the UK by Bielomatik London Ltd, or the Welwyn Tool Co. Ltd. A good early outline of the heated-tool welding method has been provided by Neubert 26 of Bielomatik Leuze and Co. (West Germany) who, through their subsidiary Bielomatik London Ltd, supply what is probably the biggest heated-tool machine range in the UK, with a number of machines for the hot-welding of plastics sheeting, pipe and the like. Automatic welding machines specifically for the welding of PVC window-frames are available (e.g. Urban AKS 3313-Elu Machinery).
20.3.2 Bonding
In this chapter a distinction is made between the welding of plastics parts, sheets, etc., in which heat is the principal agent promoting the union between the two parts of a joint, and bonding, wherein the union is brought about with the aid of solvents, solvent cements or adhesives. Note: The state of attachment achieved between the two joint surfaces when the union is completed is referred to as the 'bond' irrespective of the method employed in making the joint. This nomenclature, convenient for the present purpose, is broadly in line (but not in full conformity) with the relevant IS0 27 and ASTM* standard definitions. (a) Solvent Bonding This operation (sometimes also referred to as 'solvent welding') may be broadly defined as the formation of a joint wherein a self-bond between two polymeric components is promoted by the presence of a solvent in temporarily high concentration. The type of joint best suited to solvent bonding is a lap joint. The * Source referred to in the footnote on p. 902.
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method is not normally used for butt joints where these are likely to experience significant stresses (especially bending stresses) in service. The essential features of bond formation in this method are the interdiffusion of polymer molecules across the original interface between the two components of the joint, and the promotion and acceleration of this process by the swelling (increase of the free volume content) of the polymer of the joint components at the interface, caused by sorption of the solvent. The solvent concentration in the polymer, originally high, is ultimately reduced to a low constant value, mainly by further penetration inwards (away from the joint line), and-to a minor extent-also by evaporation from the 'edges' of the bond zone. 6 As normally practised, the solvent bonding operation involves the following main stages: (i)
The surfaces to be joined are pre-cleaned by wiping with a cleaning solvent (not a strong swelling agent for the polymer). Methyl ethyl ketone or carbon tetrachloride may be used with PVC. (ii) A suitable solvent or mixture of solvents is applied to the surfaces. With PVC parts brushing-on is a common method, but dipping may be possible in some cases (especially if the parts are small). In solvent bonding of PVC the solvent usually contains some dissolved PVC polymer or compound (commonly up to about 10%, but up to 30% in some special 'strong' cements). Such compositions are known as solvent cements (also sometimes called 'bodied cements' or 'solvent dopes'). In comparison with neat solvents, the use of solvent cements for bonding offers the advantages of reduced evaporation loss at the time of application, better manipulative control and less tendency to spread outside the intended contact area.
Note: It is sometimes thought that, by virtue of its polymer content, a solvent cement is gap-filling. This is not the case with solvent cements for PVC (and indeed not with most such cements formulated for other plastics) mainly because the amount of polymer they contain is proportionately low. It is therefore essential for good results that the surfaces of parts to be joined should fit closely together with no gaps. (iii) The joint is left 'open' for a short time. In this period the
20 PVC Sheet and its Fabrication
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solvent is sorbed into the surfaces, producing a softened, swollen polymer layer. In some cases a second application of cement may be made, followed by another short 'open' period. (iv) The components are brought together to close the joint. Where practicable this is usually done under positive pressure, which may be maintained for some time.
Note: The main function of the pressure is to promote intimate surface contact between the components. Where-as, for example, in the cylindrical joints betwen pipes, or pipes and their fittings-application of pressure perpendicular to the joint interface is not readily practicable, it is especially important that the initial fit between the surfaces concerned should be gap-free. The assembled joint must be left for a suitable time (normally at least 24 h *) as the strength develops gradually, in step with the progress of interdiffusion of molecular chains across the original interface, and progressive reduction of solvent concentration in the bond zone. Where practicable-e.g. with an essentially planar joint between parts which can be held in a pressure jig-elosing pressure may usefully be maintained during this period. Suitable heating can accelerate the development of full strength. The ultimate strength of a properly made solvent joint may equal that of the bulk material of the components, but the initial 'green' strength is normally quite low (cf. Table 20.2) A discussion of the mechanism of bond formation in solvent-bonded joints has been published. 6 Some practical aspects of solvent bonding of plastics are featured in papers by Mittrop,28 and Trauernicht. 29 Detailed instructions for bonding procedures are also normally provided by suppliers of solvent cements, and-in some cases-by manufacturers of products (especially pipes and pipe fittings) bonded with such compositions. The main advantages of solvent bonding are the relative simplicity of the method which-in its common variants-does not require special * For pressure-pipe joints (pipe-to-pipe or titting-to-pipe), made with good solvent cement, and assembled and aged at room temperature, it is a reasonable 'rule of thumb' to allow at least H hours' ageing for each atmosphere of service pressure (within the rated limit for the pipe).
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W. V. Titow
TABLE 20.2 Strength Development in Joints Between Rigid PVC Parts Bonded with Solvent Cement" Time from assembling (h)
0·5 1 4 8 16 32 48 96
Breaking load (in shear) (lbf(kN»
113 (0·503)
120 220 290 400 495 505 580
(0·534) (0·979) (1·290) (1·779) (2·202) (2·246) (2·580)
a PVC
resin (Breon AS 70142), 5%; tetrahydrofuran, 70%; cyclohexanone, 25%.
equipment or power and heat sources, and the good ultimate bond strengths obtainable. It is for these reasons that the method has been widely used in the bonding of rigid PVC pipes and fittings. However, it should be noted that the quality of joints depends critically on the skill and conscientious working of the operator, as well as on good fit (absence of gaps) between the surfaces to be joined. Other limitations are the volatility, somewhat toxic nature, and flammability of the solvents used (which may become particularly significant when operating in confined spaces and/or near potential sources of ignition). The already mentioned fact that a solvent-bonded joint cannot be subjected to normal service stresses for some time after assembly is also a consideration. In general, solvent bonding may be regarded as more suitable for rigid than for flexible PVC parts, because of the possibility of local effects of the solvent(s) on the plasticiser content and distribution in the area of the joint, which may lead to hardening and other undesirable consequences. However, where this possibility has been properly taken into account (and-preferably-the level of any adverse effects shown to be acceptably low by suitable tests) flexible PVC products may be successfully joined by the solvent method. An interesting, if minor, example is the solvent bonding of flexible PVC tubing and fittings, stitched into certain types of high-altitude flying suits to serve as conduit for a liquid heat-exchange medium.
20 PVC Sheet and its Fabrication
927
Many commercial solvent cements have been marketed for a large number of years. The original Tensol Cement No. 53 (ICI, UK) and Tangit (Henkel International, West Germany) may be mentioned by way of example. In addition to BS 4346: Part 3: 1982 and ASTM D 2564-80, listed in Appendix 1 (Sections 6.1 and 6.3 respectively), the following standards are of interest in connection with solvent cements for PVC products and their application. ASTM D 2855-81. Making solvent-cemented joints with poly(vinyl chloride) (PVC) pipe and fittings. ASTM F 402-80. Safe handling of solvent cements used for joining thermoplastic pipe and fittings. ASTM F 656-80. Primers for use in solvent cement joints of poly(vinyl chloride) (PVC) plastic pipe and fittings. ASTM F 545-80. PVC and ABS injected solvent cemented plastic pipe joints. ASTM D 2846-82. Chlorinated poly(vinyl chloride)(CPVC) plastic hot- and cold-water distribution systems. (b) Adhesive Bonding
In general, adhesive bonding of uPVC-whether to itself or other substrates-is easier than that of pPVc. In the latter case plasticiser on or at the surface may act as a parting layer preventing the formation of a bond by some adhesives, or the formed bond may be weakened by subsequent migration of plasticiser into the adhesive layer (which can be appreciably softened thereby) whilst the local loss of plasticiser may leave the PVC surface less uniform and hence more prone to disruption under the influence of temperature changes and some other environmental effects. Some of the solvents used in solvent-based adhesives can also have an adverse effect on the PVC surface (of both rigid and plasticised compositions). However, a number of adhesives are available, based on certain suitable polymer systems represented in the ranges of most major suppliers. Table 20.3 gives a general indication of the main types of adhesive systems, with a few examples of their commercial versions and references to some applications. Advice is available-and should be sought-from adhesive manufacturers on the applicability of adhesives for any specific purpose, and on the suitability of any given adhesive for any stated use and method of application.
General applicability
Both uPVC and pPVC Polyurethanea can be bonded with (single- or twosuitable versions component systems: i.e. resin, or resin and hardener) Both uPVC and pPVC Cyanoacrylate can be bonded with suitable versions (but with pPVC bonds are weaker and time to handling strength longer)
Type
Loctite IS 414} CyanolitC 811 Eastmand 910
b
Bostik PA 779
Bostik 3250
Adhesive
Various rapid bond applications
Footwear (especially soles to uppers) Film laminates
Some typical uses
Examples of commercial products
TABLE 20.3 Main Types of Adhesives for PVC
Cyanocrylates have fast setting times (of the order of 10 s) and give strong bonds
Basically two-pack systems, but the resin component can be used alone
Remarks
tv
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S
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:-::::
~
00
For uPVC and pPVC bonding
For uPVC and pPVC bonding
Nitrile rubber
Hot-melt (several polyesterbased)
InstawelcV
Selected members of the Permabond e C and F ranges Bostik 4141 and 4142 (water-based: emulsion types) Bostik M890 (a modified'two-part acrylic system) Bonding pPVC to polyurethane foam (e.g. in motor car upholstery), Flooring adhesives Mainly uPVC
Bonding uPVC (various products) to itself and certain substrates
Some suppliers: a Bostik companies in most Western countries. Hager & Kassner-West Germany. b Loctite-USA and UK. C Industrial Science Ltd-UK. d Eastman Kodak, and associated companies in most Western countries. e Permabond Adhesives-UK, and associate companies. f National Adhesives and Resins Ltd-UK.
Predominantly uPVC
Acrylic (acrylate copolymers) Acrylic adhesives are used in some PVC pressure-sensitive products (e.g. tapes)
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930
W. V. Titow
20.3.3 Machining
Although ordinary engineering workshop equipment can be successfully used for machining plastics, including PVC, certain tool characteristics, speeds and settings may be used to advantage. A useful review of the subject has been made by Korani. 30 Special tools, including portable equipment, are available for machining plastics sheet, etc. In the UK a range of such equipment is supplied, for example, by Interwood Ltd. This includes, inter alia, routers, trimmers, edgeclipping trimmers, and shears. Some general pointers concerning the most common operations are briefly summarised below: most of these relate primarily to typical rigid PVC sheet. Wherever possible, asbestos-filled uPVC sheeting should be cut to the required size(s) by the suppliers, most of whom offer such service. CUTTING
Ordinary scissors or shears can be used for cutting thin material. Electrically operated nibbling shears are particularly suitable for sheets up to about 0·080 in thick. Guillotining can be carried out quite successfully, although commercial power-operated machines should be slowed down. The bevel angles should be 60-80°, the higher figure being for blades which are not often sharpened. For normal thicknesses cutting temperatures should be 2G-30°C, but for sheets of, say, 0·25 in a temperature of 50°C is of great assistance. DRILLING
For drilling the included point angle should be 140° and a quick spiral should be employed. Without lubricant a peripheral speed of 100 ft min -1 is satisfactory but this can be increased to 200 ft min- 1 with lubricants. For holes over 0·5 in the drills and feeds used with steel can be employed. ENGRAVING
Spindle speeds of about 5000 r min -1 with a low rate of feed should be employed. The included angle of the cutter should be 45-60° tending towards the higher figure. A clearance angle of about 40° or larger is to be preferred and the material should be chilled before work begins. Apart from soluble oil, a suitable silicone emulsion may be employed. If a coolant is not used the swarf may be prevented from sticking to the sheet (by static) by wiping with an antistatic agent.
20 PVC Sheet and its Fabrication
931
PLANING AND POLISHING
The surfaces cannot be planed, only buffed. Ordinary woodworking planes can be used along edges. For polishing normal calico buffing mops may be employed with suitable compounds. Speeds below 5000 ft min -1 are usual. ROUTERING
The following are the recommended conditions: the sheet temperature should be greater than 20°C; use a high-speed steel cutter (not tungsten carbide), Z-shaped, preferably l-iin in diameter; a smooth feed and maximum machine speed are desirable; machine vibration should be avoided. A fixed head router is preferable. SAWING
Sawing presents no special difficulties, but hollow ground blades should be used for sheets thicker than rl; in. Lubricant is not normally necessary, but vibration must be avoided otherwise chipping may occur. Speeds of 5000-10000 ft min- 1 should be used with circular saws and with sheets over rl; in thick the blade should not be less than kin thick. There should be a slight negative rake with no front or top bevel and there may be six to nine teeth per inch. A typical feed rate is 4ftmin- 1 . SCREW-THREAD CUTTING
Threads can be cut on screw-cutting lathes or by using standard taps and dies. When cutting a male thread, chamfer the end of the rod so that the thread does not continue to the end. When cutting a female thread, countersink the top of the hole. Turpentine substitute is an effective lubricant. TURNING AND MILLING
The simplest turning tool is a profile cutter, and increasing the front rake angle reduces the amount of heat generated. With a straight turning tool a 10° top rake and 20° front clearance give good results. The following are general rules: the included cutting angle should be about 60°; the tool tip must have a radius of 0·025 in for a side cutter and more for a rou,nd-nosed cutter; the tool faces must have a high polish; the tool must be sha~ilicon carbide can be used to advantage; the speed of the work should be about 500 ft min -1. If troubles are experienced, it is because the correct conditions are not being used. For example, chipping and shattering are chiefly
932
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caused by low top rake; overheating is caused by low front rake; chipping and rough surfaces are caused by too fast a feed. Milling is similar to turning and a 10° positive front rake and 20° top rake is suitable.
20.3.4 Conversion and Manipulation of PVC Film and Sheeting for Packaging There is a variety of techniques and machines for these purposes. The subject is, however, within the scope of packaging rather than PVC technology. A relatively early review published by Bippus and Ackermann (Mechanical Processing of pvc Foils in the Packaging Sector)31 is still of interest. Other sources of information include books by Briston and Katan,32 Oswin,33 and Bruins,34 some relevant RAPRA Abstracts, and the current edition of Modern Packaging Encyclopedia (McGraw-Hill).
20.3.5 Surface Decoration, Marking, and Other Surface Processing of PVC Materials and Products
In terms of surface area processed, films and sheeting-both unsupported and fabric-supported (Le. where they constitute coating on textiles or paper)-are probably the most important single group among the PVC products to which the various surface threatments are applied. It is for this reason that the present section, which is intended to give a brief, general outline of the main processes involved, has been placed in this chapter. However, as should be evident from the contents, the processes are no less significant in their application to many other products made from PVC compositions. (a) Surface Decoration PRINTING
The rotogravure process-popular for continuous sheet materials ('webs' in printers' parlance) in both plastics and textiles-is particularly widely used for printing PVC sheeting, coated papers (e.g. vinyl wall-coverings) and fabrics, on machines usually specially designed for these products. Solvent-based inks are commonly
20 PVC Sheet and its Fabrication
933
employed: in some cases these are prepared on the premises. The solvents are, typically, ketones: often methyl ethyl ketone is a major constituent of the solvent system, which usually also contains a keying-solvent component, still more aggressive towards PVC (e.g. tetrahydrofuran, isophorone). The binder resins of the inks may be PVC polymers or copolymers in combination with others (e.g. acrylic polymers). Note: Some commercial inks of the emulsion type (i.e. water-based) are also used for PVC sheet printing (cf., for example, the crosslinkable Vinaprint GV inks (Claremont Polychemical Corp., USA) for PVC-coated fabrics and wall-coverings).
Some PVC articles and mouldings (especially bottles) are printed by the silk-screen and offset techniques. Pad-printinj35 has also been coming into use: this can be particularly useful for very thin-walled parts, and for convex, concave, textured or irregular surfaces. Various specialised versions of the basic equipment are available. An interesting example is a rotary pad-printer for in-line application of wood-grain surface print to extruded profile. 35 Inks for pad-printing on PVC are commonly based on PVC resins and acrylic binders. HEAT TRANSFERS
In this method, a pre-printed image (which may be multi-coloured) carried on a special release-coated support (usually a paper or plastics strip) is applied to the surface of the article to be decorated under heat and pressure, whereupon the image is transferred to the surface and fused-on (the inks of the original print having been formulated to be compatible and fusible with the surface material).36,37 The carrier strip is moved out of the way (usually by winding up) after the image has been fixed. The method is advantageous for the decoration of articles (especially blown bottles and containers) in that it allows multi-colour images to be deposited in one pass, at relatively low cost. However, not every commercial heat-transfer process is suitable for every type of plastics article. Examples of those processes applicable to PVC bottles are the Therimage process (Dennison Manufacturing Co., USA), the D/-NACAL process (Diamond International Corp., USA), and the DR/-CAL process (Dri-Print Foils Inc., USA). In general the heat transfer method of surface decoration is more economical for large volumes of production.
934
W. V. Titow
HOT-FOIL STAMPING
This method is also sometimes known as 'hot-foil marking', 'hot stamping', 'gold blocking', 'gold stamping' or 'dry printing'. It is similar in some respects to the heat-transfer method, in that the image-forming material is carried on a release-coated tape (usually a polyester film 0·020-0·030 in thick) and is eventually fused (after separation of the material from the carrier tape in the image area) onto the surface of the plastics article, under pressure and heat. In most cases the image is actually also impressed to some extent into the surface, somewhat after the manner of an inlay-emboss print. The image-forming material is a thin foil (hence the common name for the process) of appropriate plastic, suitably coloured with heat-resistant and generally fast colourants (for application to PVC surfaces the foil is usually a film of vinyl chloride polymer or copolymer composition), or-for metallic effects-a metal foil (normally aluminium). With metal foils a hot-melt adhesive layer must be used, appropriate for the article's surface. The make-up of a tape for hot-foil stamping is shown schematically in Fig. 20.2. The basic difference between the heat-transfer process and hot-foil stamping is that in the latter there is no pre-printed image: the image-forming material extends over the whole surface of the carrier, and an image is formed-in the shape of a die (of metal or silicone rubber) which also actuates its transfer to the article-only at the actual time of transfer. Thus, for multi-colour effects, several passes are necessary--one for each colour-and this increases the cost of the operation. The technique is, therefore, usually employed for one- or two-colour designs. Other limitations are that no half-tones are
A
F
S R C
Fig. 20.2 Two basic structural variants of hot-foil stamping tape: schematic representation. C, carrier base; R, release layer (e.g. wax, silicone); S, protective surface layer for image (polymeric); F, metal foil; A, hot-melt adhesive; P, coloured polymer composition layer.
20 PVC Sheet and its Fabrication
935
possible in the image, and that the article's wall thickness (at least in the image area) must be uniform and not too low, to withstand the heat and pressure of the transfer operation. The main advantages of hot-foil stamping are that it is the only method of true metallic printing on plastics, that suitably embossed dies can produce inlay-emboss print effects (on smooth or irregular surfaces) and that high quality prints are obtainable without any after-treatment. EMBOSSING
The thermoplasticity and general nature of PVC compositions make them suitable for embossing. Depending on the form and thickness of the material and the process conditions, the operation may affect only the surface (thick sheets, thick-walled products) or result in overall texturing (thin sheets and films). Sheet products are normally embossed in the hot state (fairly typically at 140°C or higher), by a passage through the nip of a pair of rollers (commonly an engraved metal roll and a rubber backing roll). For calendered sheeting the operation may be run in-line (see Chapter 18). Separate embossing is also practised, with extruded sheeting, or with calendered sheeting where only short embossing runs are involved, or where very deep emboss and/or especially high temperatures are called for. Other factors being equal, a low-gloss embossed finish is obtained if the embossing roller is heated, and a high-gloss one with a cool roller. Local surface embossing of PVC products (e.g. for marking purposes) may be carried out with special versions of high-frequency heating equipment incorporating suitably surface-shaped dies. The so-called 'chemical' embossing of cellular PVC flooring, wall coverings and the like is dealt with in Chapter 25. (b) Surface Marking This is normally carried out for the purposes of product coding (e.g. date and batch marks), identification (e.g. marking with the manufacturer's name and/or trademark), and quality marking (e.g. the 'kite' mark of the British Standards Institution). Suitable direct printing techniques are widely used (as, for example, in the in-line marking and coding of PVC pipes, electrical conduit, cable covering, etc). Local embossing with a heating die is also practised, as is spray-painting (through a stencil). Etching-in of code marks on articles (e.g. PVC bottles) by means of a pulsed laser beam impinging on the cut-out of a suitable stencil forms
936
W. V. Titow
the basis of a modern marking system. * The advantages of this method are relative operational simplicity, cleanliness, and insensitivity to any surface contamination on the article being marked. Marks of area 0·010-1 in 2 can be produced. Apart from the references already cited in the text, articles by the following authors are relevant to various methods of surface decoration of plastics: Kiihne 39 (an early review of processes), Jolley40 (hot-foil stamping), Clayton41 (gravure printing on PVC sheeting). (c) Surface Processing The kind of processing fa:Iling under this heading consists of treatments carried out to protect the PVC surface, or to upgrade some of its properties by chemical modification. Surface coating (typically with lacquers based on acrylic, epoxy, or polyurethane resins) of PVC layers on wall-coverings, upholstery, etc., is one example. This application is further discussed in Chapter 22. Reference has also already been made (in Chapter 9) to the incorporation of UV stabilisers in surface-coating lacquers for PVC materials for outdoor service, as a means of protection against weathering, alternative to the inclusion of such agents in the PVC composition itself. Another kind of surface treatment is exposure to an electron beam, or contact with plasma. In the former case, controlled irradiation in an electron accelerator (several industrial versions of this equipment are now operational) can bring about a predetermined degree of cross-linking of the PVC polymer either in the immediate surface layer, or to a considerable depth (roughly up to about 1 cm with many compositions), improving resistance to heat and weathering, hardness and modulus, and reducing permeability. Examples of PVC products benefiting from such treatment are films for some packaging or medical applications, roofing sheets, and electrical cable insulation. Similar effects (but confined to a relatively thin 'skin') are produced by plasma treatment.
REFERENCES 1. ISO 472-1979. Plastics-Vocabulary. 2. BS 1755. Glossary of terms Used in the plastics industry. Part 1: 1967. Polymerization and plastics materials.
* The 'Lasermark' system-Lumonics Inc., Kanata, Ontario, Canada. 38
20 PVC Sheet and its Fabrication
937
3. ASTM D 883-78a. Standard definitions of terms relating to plastics. 4. Zade, H. P. (1959). Plastics, 24, 136. 5. Estevez, J. M. J. and Powell D. C. (1960). Manipulation of Thermoplastic Sheet, Rod and Tube, Iliffe and Sons Ltd, London. 6. Titow, W. V. (1978). In Adhesion 2, (Ed. K. W. Allen), Applied Science Publishers, London, Ch. 12. 7. Kobayashi A. (1967). Machining of Plastics, McGraw-Hill, New York. 8. Machining with Lasers (1973/74). In Modern Plastics Encyclopedia, McGraw-Hill, New York, pp. 504-5. 9. ICI Technical Service Note D106 (Second Edn): Darvic Price List, March, 1970. 10. Bateman G. T. (1970). Brit. Plast., 43(5), 96-9. 11. Anon. (1981). Mod. Plast. Int., 11(3), 44. 12. Gamski, K. M. (1978). 36th ANTEC SPE Proceedings, pp. 20-2. 13. DIN 16930: 1964. Welding of rigid PVC (rigid polyvinyl chloride); Directions. 14. DIN 16931: 1959. Welding of flexible PVC (flexible polyvinyl chloride); Directions. 15. Mengason, J. (1976). 34th ANTEC SPE Proceedings, pp. 594-7. 16. Anon. (1979). Eur. Plast. News, 6(3), 11-28. 17~ DIN 191~Part 3: 1977. Welding: Welding of plastics: Processes. 18. DIN 16 96~Sheet 1: 1974. Welding of thermoplastics: Principles. 19. Streese, G. (1969). Kunststoffe, 59(11), 679-84. 20. Koehler, T. J. (1970). Mod. Plast., 174-7. 21. Frankel, E. J. and Wang, K. K. (1978). 36th ANTEC SPE Proceedings, 57-60. 22. Potente, H. and Reinke, M. (1981). Plast. Rubb.: Process. Appln, 1(2), 149-60. 23. BS 4994: 1973. Vessels and tanks in reinforced plastics. 24. ASTM D 1789-65 (Reapproved 1977). Welding performance of poly(vinyl chloride) structures. 25. Bielomatik London Ltd, Technical Publication HV-310. 26. Neubert, W. Welding Machines for Assembling Plastic Parts, Technical Publication, Bielomatik Leuze (Bielomatic London Ltd). 27. ISO 6354-1982 (ElF). Adhesives-Vocabulary. 28. Mittrop, F. (1969). Kunststoffe, 59(10),685-7. 29. Trauernicht, J. O'R. (1970). Plast. Techno!., 16(8),43-9. 30. Korani, R. (1969). Kunststoffe, 59(10), 688-9. 31. Bippus, W. and Ackermann, H. (1968). Kunststoffe, 58(3), 197-206. 32. Briston, J. H. and Katan, L. L. (1974). Plastic Films, Butterworth, London. 33. aswin, c. R. (1975). Plastic Films and Packaging, Applied Science Publishers, London. 34. Bruins, P. F. (1975). Packaging with Plastics, Gordon and Breach, New York. 35. Rogers, M. (1979). Plast. Technol., 25(10), 73-7. 36. Titow, W. V. (1966). Plast. Technol., 12(8), 38-40. 37. Frazier, J. F. (1978). 36th ANTEC SPE Proceedings, pp. 212-4. 38. Anon. (1981). Mod. Plast. Int., 11(10), 21.
938
w.
V. Titow
39. Kuhne, G. (1969). Kunststoffe, 59(10), 697-9. 40. Jolley, R. A. (1978). Plast. Rubb. Int., 3(6), 255-7. 41. Clayton, F. R. (1978). 'Printing techniques relating to the gravure process', Paper presented at the PRJ International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978.
CHAPTER 21
PVC Pastes: Properties and Formulation W. V.
TITOW
21.1 INTRODUCTION The processing of PVC in paste form constitutes an important, long-established area of the technology of this versatile polymer. Pastes are made with emulsion-type PVC resins (although suspension resins are sometimes added for special effects-see Sections 21.2.2 and 21.3.1 below). In the early days such resins were selected for the purpose from among the standard grades: subsequent availability of special paste-making emulsion polymers did much to further the advance of the technology and applications of PVC pastes. In the PVC context, the word 'paste' may be regarded-and is used in this book-as the broadest generic term for a suspension comprising particulate vinyl chloride polymer or copolymer as the principal, or the only suspensoid, and liquid plasticiser(s) as the main, or the sole constituent of the suspending medium. The suspension will normally contain stabiliser(s), and may also incorporate various other relatively minor constituents (e.g. colourants, fillers, etc.). * A PVC paste is also often called a plastisol, the two terms being treated as completely synonymous. * In view of this very common usage, the question whether they are properly interchangeable is largely academic, although it should be noted that pastes incorporating
* Although the term 'paste' is quite common in the UK, it is not included in the main relevant English-language nomenclature standards (ISO 472-1979; BS 1755: Part 1: 1967; ASTM D 883-80) all of which define 'plastisol' and 'organosol' . 939
940
W. V. Titow
organic solvents as constituents of the liquid dispersion medium additional to the plasticiser(s) are widely known as 'organosols', whilst pastes thickened (by the addition of thickening agents) to the point where they are no longer free-flowing are occasionally referred to as 'plastigels'. Organosols are useful in applications where low initial viscosity but a hard final product with a low plasticiser content are required: this ultimate condition is achieved by evaporation of the solvent in the final stages of processing. However, the presence of a solvent in the original composition, as well as the need for its eventual removal, can complicate production operations, and thus constitute a restriction on the use of organosols. Plastigels, which may vary in consistency from that of butter to putty, and may be highly thixotropic, are used in applications where such rheological properties are desirable (e.g. in some spread-coating operations). Yet another variant of PVC paste is a rigisol. * The term was coined to describe plastisols (i.e. solvent-free pastes) so formulated that they are free-flowing at plasticiser content levels which would normally result in high viscosity.l,2 This means that a rigisol can be processed like an ordinary plastisol giving, however, much harder ultimate products: inter alia, this can afford cost savings as thinner sections may be produced to meet particular hardness and stiffness requirements. As with solid PVC compounds (cf. Chapter 4), the main considerations in formulating a paste are the effects of composition upon the service properties of the ultimate product and on the behaviour of the paste in processing. The end-use aspects of paste formulation are substantially the same as those of the solid compounds, but differences arise in the processing context, principally as a result of the liquid state of the paste. Factors important in this connection include paste viscosity, viscosity stability over reasonable periods of time (resistance to ageing-see Section 21.2 below), and viscosity changes under shear (in stirring, spreading, etc.-see Section 21.2.2(iii) below). 21.2 PVC PASTES: RHEOLOGICAL PROPERTIES AND THEORY 'Paste' is a technical term, much less precisely definitive than such scientific terms as, say, 'emulsion' or 'solution'. The definition varies, * Not to be confused with Rigidsol, the original trade name for a brand of PVC pastes of the Watson-Standard Co. USA.
21
PVC Pastes: Properties and Formulation
941
as does the nature of the paste, according to the field of application. For example, wallpaper-hanging paste may be an aqueous colloidal solution of a cellulose derivative or starch, whereas toothpaste is essentially a suspension of particulate solid(s) in water, containing certain soluble additives. Most pastes are in fact basically suspensions, and many are characterised by a comparatively high concentration of the suspensoid. As has already been indicated, PVC pastes belong to this category, consisting as they do essentially of a comparatively high proportion of PVC resin particles suspended in a plasticiser or plasticiser mixture. The unique suitability of PVC pastes in many applications, and hence their technical importance, rest essentially on two general characteristics: the fact that pastes are liquid (even the plastigels will flow or spread under moderate shear) and can readily be processed in this state in a number of applications, and the fact that on heating (which can be applied at the appropriate stage of the technical process) they are irreversibly converted (fused) to very highly viscous solutions of polymer in plasticiser. On cooling these solutions solidify; the resulting product is plasticised PVC. It should be clear, even from the above brief summary, that the knowledge and control of rheology of PVC pastes is of great importance. The essential practical requirements are: (i) that -as prepared-the paste should have the right rheological properties for application; (ii) that these properties should be constant for reasonable periods of time in storage, and should either remain so through processing to the time where heat is applied to gel the paste or should change as required at the appropriate point; (iii) that the fusion (and the gelation which precedes it) should proceed at the right rate and generally be controllable. Unfortunately, the rheological factors associated with the above points are not simple, and are influenced in a complex way by the nature and composition of the paste system. 21.2.1 Viscosity of a Simple Suspension
In general, the viscosity of a suspension might be a priori expected to be a function primarily of the amount of suspensoid present and the viscosity of the suspending liquid. In the simplest case of dilute,
942
W. V. Titow
substantially monodisperse suspensions in which there is no appreciable interaction between the suspensoid and the medium, those are indeed the two governing factors, and such systems closely obey Einstein's expression: 'TJs 'TJo
= 1 +2.5cp
(1)
where 115 and 110 are, respectively, the viscosities of the suspension and the suspending liquid and cp is the volume fraction of the suspensoid. The ratio 1151110 is the relative viscosity (11r). At suspensoid volume fractions higher than about 0·025 the relative viscosity predicted by the above relationship rapidly becomes much lower than that actually observed. Other more complicated expressions of a similar nature, e.g. the modified Einstein equation: 'TJs - 'TJo 'TJo
= 2.5cp + 14.1cp2
(2)
and the Mooney equation:* 11r =
(l+0·5cp)1/2 1·25cp 1 exp - -cp 1 -cp
(3)
have been shown by Johnston and Brower'! to give a better fit with actual experimental results, but even with those the viscosity values predicted for suspensoid volume fractions over about 0·2 decreased sharply and progressively in comparison with actual experimental figures. 3 21.2.2 Main Compositional Factors Influencing the Apparent Viscosityt of PVC Pastes At this point it will be useful to list the important compositional characteristics of PVC pastes which distinguish them from the simple suspension system so far considered, and complicate their theoretical treatment as well as the rheological properties in practice. (i) In PVC pastes the suspensoid volume fraction can be quite high (considerably in excess of 0·2); it will also be particularly high in rigisols, and in plastisols to which 'extender' polymers have been added. The latter are non-paste-making PVC polymers of * Originally derived for the viscosity of rubber latices.
t i.e.
viscosity measured at a single, constant rate of shear.
21
PVC Pastes: Properties and Formulation
943
large particle size; when used to replace part of the paste polymer they usually lower the viscosity of the paste (ct. Section 21.3.1 below). (ii) PVC pastes are not simple, monodisperse suspensions. When prepared by the usual methods (ct. Chapter 13, Sections 13.4.3 and 13.4.4(f); and Chapter 22, Section 22.1.1) which give a dispersion of the polymer particles in the plasticiser(s) good by ordinary industrial standards (but which do not employ special, powerful means-such as, for example, ultrasonic vibration-to secure complete particle separation), the pastes will normally contain some particle aggregates ('secondary particles') in addition to completely separated primary particles. The primary particles of paste-making PVC resins range in size between 0·1 f-lm and 2 f-lm: aggregate sizes of 1·8-70 f-lm have been found in pastes prepared from typical representatives of such polymers. 4 Whilst some commercial paste resins have particle size distributions so narrow that the resulting pastes may be regarded as practically monodisperse, a broad particle size range (polydispersity) is a feature of others; substantially bimodal primary particle size distributions are also encountered4 ,5 (whereas pastes containing added extender polymer, of particle size normally in the range 8~140 f-lm, are certainly grossly bimodal systems). The particle size and size distribution of the paste polymer strongly influence the viscosity and flow behaviour of the paste (see Section 21.3.1 below). (iii) Paste polymer particles are resistant to, but not completely immune from, attack (swelling or even solution) by plasticiser at room temperature. The interaction, though limited and slow, can result in a rise of the paste viscosity through an effective increase in the volume fraction of the solid (swelling of polymer particles) combined with increase in the viscosity of the plasticiser brought about by dissolution of some of the polymer (the finest particles). Such effects are normally collectively termed 'ageing'. 21.2.3
Expressions for the Apparent Viscosity of Pastes
As can be seen, the ageing of pastes is an extra complicating factor in the theoretical prediction of their rheological behaviour, as well as in their application and quality control.
w.
944
V. Titow
All the above factors must be taken into account in a theoretical treatment of paste viscosity. The treatments outlined here all relate to apparent viscosity at a given, constant (and normally low) rate of shear. The reason for this is explained in the following section. According to the findings of Ram and Schneidero certain modified forms of the equations of Mooney7 and Eilers8 with some empirical corrections, describe well the relationship between the apparent viscosity and the suspensoid volume fraction for certain plastisols at constant shear rates, even when ageing effects are taken into consideration. Ram and Schneider also proposed the following simplified empirical relationship: TJ
=A
;WJ
B exp [1_
(4)
where 1/ is the apparent viscosity of the plastisol, A and B are empirical constants, W is the polymer weight fraction and Wp the 'critical' weight fraction (weight fraction at gelation, when the viscosity becomes extremely high). The concept of critical weight (strictly: critical volume) was also independently advanced by Johnson and Brower,3 who used it to develop what is one of the best expressions available for the relationship between the apparent viscosity of a plastisol and its suspensoid fraction; the equation was demonstrated to hold for several resin/plasticiser systems (including plastisols containing extender resins) and thus not to be invalidated by considerable variations in particle size and size distribution of the suspensoid. The equation was also shown to be equally applicable to suspensions of glass beads in plasticisers; in conjunction with the other data this demonstrates that the equation is not invalidated by ageing effects within reasonable limits. Moreover, the authors suggested a simple way of estimatingwith the aid of the critical volume concept-the extent of the volume increase of polymer particles in plastisols as a result of plasticiser absorption. Their estimates indicate volume increases from about 0-13% for stable resins to 1· 76% for less stable ones. The equation of Johnston and Brower3 is: 10gloTJr=
(1-33-0-84!t)(~)
(5)
where ({Jc is the critical volume fraction, and other symbols are as defined above.
21
PVC Pastes: Properties and Formulation
945
The critical volume fraction is defined as the volume fraction of polymer particles when those have absorbed the plasticiser to the point where it no longer fills completely the interstices between the particles and consequently the viscosity becomes extremely high. Assuming that the equation and the master curve which can be plotted from it (lOglO l1r versus the 'relative volume fraction' cp/CPc) do not change, the critical volume for a particular system can be calculated from viscosity measurements at a known volume fraction, and the critical volume fraction thus determined. This provides ready means of characterising and/or predicting the apparent viscosity, at least at low shear rates, for plastisols based on a given resin, over a wide range of compositions and with various plasticisers. Ageing properties of plastisols, and the effects of extender resins can also be similarly characterised. 21.2.4 Variation of Paste Viscosity with Rate of Shear, or with Time at Constant Shear Rate The preceding section dealt with the ways in which the main compositional properties of PVC pastes influence their viscosity even when that is determined at a single, constant rate of shear. It is because of this simplifying limitation that the viscosity was described as 'apparent'. The qualification is necessary, because-and this is a very important rheological characteristic of PVC pastes-their viscosity also varies with the rate of shear; both the phenomenon and the manner of the variation (which is complex) are of course also functions of the nature and composition of the paste. The subject is extensive, and a full discussion would be beyond the scope of this book. However, a few important points of particular significance to technological practice may be briefly mentioned. In order to do this it is useful to define a few essential concepts. Consider a thin layer of liquid of thickness d.x, between two parallel plates, each of area A (Fig. 21.1). If the bottom plate is held and the top one moved in the direction of the arrow with a force F, the liquid
~A
dV/-A-7----=' 1
Fig. 21.1
~
F
946
w. V. Titow
layer will be subjected to shear. The shearing force will be opposed by the internal cohesion or viscosity of the liquid. Let the force F impart a velocity V to the top plate. Then the thin lamina of the liquid in immediate contact with the surface of the plate will move with the same velocity; a similar lamina adjacent to the surface of the stationary bottom plate will be at rest, and any lamina of liquid in a position intermediate between the two will be moving with a velocity which will be greater than zero but smaller than V. In laminar (Le. not turbulent) flow, that velocity will also be proportional to the distance of the lamina from the bottom plate. In other words a velocity gradient dVldx will be set up. For the purpose of the present consideration the simplifying assumption may be made that the velocity gradient is equal to the 'shear rate' (i.e. the change of shape of the whole liquid layer under the action of the shearing force); this equality does in fact exist in the conditions of simple shear. In mathematical terms the relationship is dVldx = y, where y is the shear rate. As the force F is acting over the area A the shear stress may be defined as: F T=(6) A It might reasonably be expected that the extent of deformation of the liquid layer (the shear rate) should be proportional to the magnitude of the shearing force per unit area (the shear stress), i.e.
ret: y or r = y times a constant
(7)
This is indeed so for many liquids; such liquids are called 'Newtonian'. Their behaviour conforms to eqn (7) in which the proportionality constant (usually given the symbol 'YJ) is called the Newtonian viscosity; Le. for Newtonian liquids: T
-=7j
"Y
(7a)
and the graph of r versus y is a straight line of slope 'YJ, passing through the origin (Fig. 21.2). Liquids for which the shear rate is not directly proportional to shear stress (Le. the ratio r/y is not a constant) are called non-Newtonian. In such liquids the ratio of the shear stress to shear rate varies in some
21
PVC Pastes: Properties and Formulation
947
Shear stress (T)
Shear rate
("y)
Fig. 21.2 Viscosity relationship for Newtonian liquids.
manner with the shear rate, i.e. T
- == "'('Y) 'Y
(8)
or with time at constant shear rate, i.e. r:ly == 1jJ(t). PVC pastes belong to this group. It is worth noting in passing that the value of the ratio determined for single, specified values of r: and y is called 'apparent viscosity' and this is the sense in which this term was used in the preceding section. Unlike Newtonian viscosity which is a constant, apparent viscosity values will depend on the shear rate. In practical terms this means that a PVC paste will flow (or spread, or drain) differently in processing according to the amount of shear it is experiencing. The relationship between r:lyand yor time (i.e. the nature of 1jJ(y) and 1jJ(t) , or the manner of variation of apparent viscosity with shear rate and time) in non-Newtonian fluids can assume various more or less complex forms, corresponding to certain known types of rheological behaviour. Those most important or common in PVC pastes will now be briefly mentioned. 'BINGHAM BODY' BEHAVIOUR
This is the case where a certain minimum stress is necessary before appreciable flow will start, but-once this point (known as the 'yield point' or 'yield value') has been reached-the behaviour becomes Newtonian. The mathematical statement of this is: r: == 77Y + K
(9)
948
W. V. Titow
Shear stres/s (T)
Yield value
Shear rate
(1')
Fig. 21.3 Bingham body behaviour. where K is a constant associated with the yield value (cf. for example Ref. 8). The relationship is illustrated in Fig. 21.3. The yield value is important in the application of PVC pastes to vertical surfaces (e.g. in spraying or dipping) for which the paste should be formulated for Bingham body behaviour, with a yield point most suited to the particular conditions. 9 DILATANCY AND PSEUDOPLASTICITY
Where the apparent viscosity rJa( = -r/y) changes with the rate of shear, the function (-r/y) = 'ljJ( y) (cf. eqn (8) above) is a power function which may be written in the form: 9 rJa = Kyn-l
(10)
where the value of n in the power index reflects the nature of the rheological behaviour. As can be seen, if n = 1 the apparent viscosity will be constant and therefore the behaviour Newtonian. With n > 1 the shear stress and the apparent viscosity will increase with increasing shear rate (see Fig. 21.4(a) and (b), respectively). Such behaviour is known as shear thickening or dilatancy. With 1 > n > 0 the shear stress and the apparent viscosity will decrease with increasing shear rate (see Fig. 21.5(a) and (b), respectively). This behaviour, which is the opposite of dilatancy, is known as shear thinning or pseudoplasticity. Pastes with high polymer: plasticiser ratios tend to be dilatant, especially in process conditions involving high rates of shear. This may, for example, make them difficult to spread in a coating process, but will also counteract
21
949
PVC Pastes: Properties and Formulation
Apparent viscosity
Shear stress
(77 8 )
(T)
Shear rate (')')
Shear rate (')')
Fig. 21.4 Dilatant behaviour in which increasing shear rate is accompanied by an increase in shear stress (a) and in the apparent viscosity (b).
Shear stress
Apparent viscosity
(T)
(77a)
Shear rate
("y)
Shear rate
("y)
Fig. 21.5 Pseudoplasticity behaviour (the reverse of dilatancy): increasing shear rate is accompanied by a decrease in shear stress (a) and in the apparent viscosity (b).
excessive penetration into the substrate. 9 A study by Cawthra et ai. 1O has shown that Breon P 13011-TXP plastisols tend to be dilatant. Pseudoplastic pastes give good spreadability and surface finish in certain coating operations involving high shear rates. THIXOTROPY AND RHEOPEXY
If the apparent viscosity varies with time at constant rate of shear (TJa = r:/y = 1jJ(t» two principal types of behaviour may be exhibited. The apparent viscosity (and the shear stress) may decrease with time, or it may increase with time. These two cases are known respectively
W. V. Titow
950
Shear stress
(T)
or Apparent viscosity
(71a)
Time Fig. 21.6 Thixotropic behaviour: the shear stress and apparent viscosity decrease with time. as thixotropy and rheopexy (also sometimes termed negative thixotropy or antithixotropy): they are illustrated in Figs 21.6 and 21.7. An example of thixotropic behaviour is 'non-drip' paint which will spread and flow on stirring and/or brushing, although it is gel-like when not worked. The viscosity of thixotropic PVC pastes (which may appear to thicken excessively on storage) may similarly be reduced by stirring. Pastes thickened as a result of true ageing cannot, of course, be restored in this way. Rheopectic behaviour is also sometimes observed in PVC pastes. Figures 21.3 to 21.7 are schematic illustrations of the main general types of behaviour mentioned here. Explanations and discussions of these phenomena (which are not of course exclusively associated with PVC pastes) abound in the literature, textbooks and reference books; see, for example, accounts by Mendelson,11 Lodge,12 Reiner 13 and
Shear stress
(T)
or Apparent viscosity Time
Fig. 21.7 Rheopectic behaviour: shear stress and apparent viscosity increase with time.
21
PVC Pastes: Properties and Formulation
951
Fredrickson. 14 Actual plots from experiments on pastes have been published by Cawthra et al. 10 In the type of suspension exemplified by a basic form of PVC plastisol (dispersion of resin in plasticiser(s)), non-Newtonian flow behaviour is attributable to modes of interlocking of the particles (flocculation mechanisms) which impart various degrees of 'structure' to the generally liquid system. For the purposes of a systematic treatment the particle interactions may be regarded as weak mechanical linkages, increased or reduced under various rates and durations of shear. 9 In such terms, dilatancy of a plastisol will arise as a result of a build-up of structure, and pseudoplasticity will be the result of breakdown of structure already present,15 when shear is applied. In Bingham body behaviour the breakdown begins only at a certain, sufficiently high shear stress (the yield value). As would be expected from these considerations (and as demonstrated in investigations and actual processing), dilatancy, pseudoplasticity, thixotropy, rheopexy and Bingham body behaviour of PVC pastes are reversible phenomena. For a suspension containing a high proportion of suspensoid particles closely packed, a special kind of ordering of the packing array may be necessary to allow the cooperative movement of particles required for flow: otherwise the particles may be forced together in a way which will hinder flOW. 16 ,17 As pointed out by Rangnes and Palmgren,18 the need for cooperative particle movement to facilitate or enable flow is greatest in monodisperse systems, and hence-other factors being equal-such systems have a greater tendency to dilatancy than polydisperse systems in which the smaller particles can be accommodated, and can move, in the interstices among the larger particles. Apart from the amount of PVC resin particles present, their size and size distribution, the rheology of a paste is also influenced by the amounts and characteristics of the other constituents. These effects are discussed in further sections of this chapter. It should also be noted that, under different conditions, one and the same paste may exhibit more than one type of rheological behaviour. 21.2.5
Gelation and Fusion of PVC Pastes
A PVC paste is transformed into the solid substance of the ultimate paste-derived product by heating at an elevated temperature. As the temperature of the paste rises, the plasticiser penetrates into the
952
W. V. Titow
polymer particles which swell and merge, first loosely and then progressively more fully, until-if the temperature is sufficiently high and the time adequate-the process culminates in complete mutual solution of polymer and plasticiser with the formation of a homogeneous plasticised PVC melt: on cooling this solidifies into the familiar plasticised PVC material. Overall, the transformation may be regarded as a phase inversion: a change from a suspension of particulate polymer in the continuous phase of liquid plasticiser, to a dispersion (solution) of plasticiser in a continuous polymer matrix, solid at room temperature. . It is usual to distinguish two main, consecutive stages in the transformation-gelation and fusion of the paste. Note: Either of the two terms (and particularly 'gelation'), has sometimes been used to denote the whole transformation process. However, with better understanding of the phenomena concerned, usage has progressively tended to conform with definitions which may be formulated on the following lines. Gelation (or gelling) of a PVC paste is the process whereby absorption of the plasticiser(s) by the polymer particles, which is the first major consequence of heat treatment (or sometimes the result of drastic ageing), brings about the formation of a relatively weak gel. The state attained by the material with the formation of the gel is also sometimes referred to as 'gelation'. This state may be considered to exist until further heating results in fusion. Note: ASTM D 833-80 defines 'gel' in this context as 'a state between liquid and solid that occurs in the initial stages of heating, or upon prolonged storage'. Fusion is the process whereby (or the state attained when), as a result of heating, the polymer particles of a PVC paste, permeated by the plasticiser(s) which they have entirely absorbed, become fully merged to form a physically homogeneous plasticised PVC material with mechanical and other properties developed to the full. Thus the gelation and fusion of a PVC paste are marked by morphological changes, which are paralleled by changes in properties. From the practical standpoint of industrial processing and use of pastes and their products, the properties are of more immediate significance than the morphology. Even for the purposes of more fundamental
21
PVC Pastes: Properties and Formulation
953
study of the gelation/fusion process, following the changes in some selected, relevant property offers the advantages of continuous, rapid, relatively simple monitoring, none of which would be readily attainable in direct examination of changes in the morphology of the composition processed. In the present state of knowledge, major changes in certain relevant properties can, in any case, be related to important stages of modification of the fine structure of the paste material. Of particular interest in investigations involving continuous examination of changes in a selected property of the paste is the identification of the fusion temperature or fusion point (still sometimes referred to as 'gelation temperature'), i.e. that temperature (reached in processing after a period dependent on the rate of heating in the particular conditions employed) at which fusion of the material is achieved. It may be noted that the actual value of the fusion point depends to some extent on the method of determination, which should, therefore, always be stated when such figures are quoted. Fusion points determined in instrumental measurements of such properties as viscosity, modulus, and others (see below) are often several degrees lower than the temperatures to which the particular pastes must be heated to attain the highest tensile strength (as measured after cooling to room temperature). In general, complete fusion (and full strength development) will not normally be achieved in paste products heated to temperatures below 160°C, irrespective of the heating rate and time. The paste property systematically examined in the greatest number of studies of the gelation/fusion process has been the viscosity, often measured indirectly in terms of torque in a torque rheometer (most frequently a Brabender Plastograph or Plasti-Corder).19-24 In investigations of this kind a constant rate of heating is usually employed (with the paste undergoing shear mixing under controlled conditions) and the torque is plotted as a function of temperature. The main peak in the curve is usually taken to indicate the fusion point (sometimes called the 'relative fusion point'), although the material may not yet be completely fused at this stage (see Chapter 5, Section 5.6.1). The magnitude (torque value) of this peak depends on paste composition, and in particular the compatibility of the plasticiser(s) used: the higher the compatibility the lower the torque-cf. Chapter 5, Section 5.6.1. Some plastograph curves obtained by Cayrol et ai. 25 with plastisols* * Based on Pevikon PVC resins of the PE and PS series (Kema Nord, Sweden).
954
W. V. Titow
containing between 50 and 100 phr DOP are shown in Fig. 21.8 by way of general illustration. The same investigators also used a balance rheometer (the Contraves instrument) to follow changes in the viscosity, shear modulus and loss angle of their plastisols. The viscosity effects are illustrated in Fig. 21.8 and shear modulus changes in Fig. 21.9: the fusion points as indicated by the curve peaks for each of the two properties are fairly close for one and the same plastisol. The authors noted that the temperature position of the fusion peaks did not appear to be dependent on the nature of stabilisers or other additives in the plastisols. Other equipment used to investigate changes in viscosity of pastes undergoing gelation/fusion on heating included the Brookfield viscometer, 26-28 and the Bendix Ultra Viscoson viscometer. 29 An oscillatory, parallel-plate rheometer (Rheometrics
5
4
..'" Q.
1'0
";e
Ui
..-
.o u
.;
::>
CI'
L
~
'" o 3
.J
0·5
2
50
100 T~mp~ratur~
150
, 'C
Fig. 21.8 Viscosity (Contraves balance rheometer) and torque (Brabender Plastograph) versus temperature for three plastisols. Heating (or cooling) rate: 3°C per minute (not linear for the Plastograph). One sample (PE 709) cooled to 120°C after fusion and then re-heated. For the torque curves the times of fusion are shown. • , Plastisol based on Pevikon PE 709 emulsion resin; f:::" plastisol based on Pevikon PE 712 emulsion resin; • plastisol based on Pevikon PS 690 suspension resin. (Reproduced from Ref. 25 by courtesy of the editor of Polymer Engineering and Science.)
21
PVC Pastes: Properties and Formulation
955
'l'e u
c
'"
.
"
5
2
::I
"Eo 01
o
...J
4
50
100
150
Tqmpqraturq. ·C
200
Fig. 21.9 Shear modulus versus temperature for the three plastisols of Fig. 21.8 during gelation and fusion in a Contraves balance rheometer. Conditions and coding as for Fig. 21.8. One sample (PE 709) cooled to 120°C after fusion and then reheated. (Reproduced from Ref. 25 by courtesy of the editor of Polymer Engineering and Science.) Mechanical Spectrometer)* was employed by Nakajima et al. 30 to follow the effects of heating not only on the viscosity but also on the elastic and loss moduli of a plastisol, in the temperature range 55-195°C. Some of their results are shown in Fig. 21.10. The rate of the morphological transformations involved in the gelation and fusion of a PVC paste, as well as the actual temperatures at which distinguishable phases of the overall process will occur in a particular case, depend principally on the nature of the PVC polymer and plasticiser(s) and the resultant compatibility between them (often simply referred to as plasticiser compatibility). A general outline of plasticiser/polymer interactions in processing is given in Chapter 5 * As is pointed out in the second paper of Ref. 30, viscoelastic measurements carried out with this apparatus on gelling and fusing paste do not suffer from two shortcomings to which torque rheometer measurements are subject. Thus, although the torque rheometer provides a good 'fingerprint' of the gelation/fusion process, it subjects the material to mechanical working (not experienced in most cases of industrial processing), whilst the torque values alone give no indication whether the material is in a rubbery or a more fluid state.
W. V. Titow
956
W : 0·628 rad
s·l
...
Q.
;,
"..c
~
G'
o
2
4
6
8
10
12
TimG,min
14
16
18
20
Fig. 21.10 Viscoelastic properties of a PVC plastisol on gelation and fusion. G', elastic modulus; Gil, loss modulus; "'I, viscosity; w, angular frequency. (Reproduced from Ref. 30 (first paper), by courtesy of the editor of Polymer Engineering and Science.)
(Section 5.5): this certainly applies also to the gelation/fusion of pastes, but the process may usefully be considered in more detail here. The first effect of heat treatment applied to a PVC paste to gel and fuse it is a short-term reduction of paste viscosity. This is attributed to a drop, with increasing temperature, in the viscosity of the plasticiser before the polymer particles become sufficiently swollen (and the plasticiser 'bodied-up' by any dissolved polymer) to counteract this effect by increasing the viscosity of the paste as a whole. Other factors being equal, the magnitude of the initial viscosity drop will thus depend on the nature of the plasticiser. This is illustrated by the curves of Fig. 21.11. The same figure also demonstrates that the paste viscosity soon begins to rise steeply with rising temperature as heating proceeds: the ascending portions of the curves in fact correspond to the
21
PVC rnin 100 PI..licill' 15
PVC Pastes: Properties and Formulation
957
E
•
·.•
= I:
:0
to-
Fig. 21.11 Variation of gel rate curves with plasticiser choice. 31 A, Dipropylene glycol dibenzoate; B, butyl 2-ethylhexyl phthalate; C, di-2ethylhexyl phthalate; D, di-2-ethylhexyl hexahydrophthalate; E, tri-2ethylhexyl phosphate; F, diisodecyl phthalate; G, di-2-ethylhexyl adipate; H, diisodecyl adipate. (Reprinted from SPE J., 17(2), 1961.) initial, near-vertical parts of the viscosity, torque, and modulus plots in Figs 21.8 and 21.9). The sharp increase of the viscosity and modulus of the paste reflected in these mutually corresponding features is caused by progressive absorption of the plasticiser into the polymer particles (with some, relatively minor, contribution from viscosity increase of temporarily remaining 'free' plasticiser, due to dissolution of some polymer). These phenomena are salient elements of the gelation process: the temperature at which the sharp viscosity rise occurs (or, more accurately, the mean of the temperature range-admittedly very narrow in most cases in practice-over which it takes place) is usually called the 'setting' (or 'set') temperature, or sometimes the gelation temperature. Its actual value will be governed principally by the compatibility of the plasticiser and resin, in which the chemical nature of the plasticiser is a cardinal factor (cf. Fig. 21.11 where the relatively good compatibility of phthalate and phosphate plasticisers in comparison with the adipates is reflected in the positions and shapes of the rising portions of the corresponding curves). In general, the higher the compatibility of the plasticiser(s) with the polymer in a paste the easier and faster the gelation (and the ultimate fusion): the best solvating plasticisers (BBP, triaryl phosphates) lead in this respect, followed by some of the widely used phthalates. In practice this affords scope for
958
w.
V. Titow
lowering the gelation and fusion temperatures for the same duration of heat treatment, or reducing treatment times at the same temperatures, by substituting some or all of the plasticiser in the formulation with one of higher compatibility (e.g. a general-purpose phthalate with a triaryl phosphate) if other processing requirements (e.g. paste rheology, which is also affected by plasticiser compatibility-see Section 21.3.2), cost considerations, and ultimate service performance requirements do not preclude such substitution. The end of the steep rise of the viscosity, torque and modulus curves of Figs 21.8 and 21.9, marked by a small peak in most of these plots, is regarded by some authors25 as corresponding to the 'dry point' in the gelation process, at which all the plasticiser is assumed to have been absorbed by the polymer but the strength of the resulting solid material is still very low. However, some comparative curves published by Greenhoe,26 who has distinguished and named a number of sub-stages in the gelation/fusion process, suggest that it is more likely to be the 'gel point', or even possibly the 'haze point' or 'elastomeric point' (in Greenhoe's nomenclature-cf. Refs 26 and 32, and Chapter 5, Section 5.5)-states reached consecutively at temperatures progressively higher than the dry-point temperature (but still considerably lower than the fusion-point temperature). The next point of interest-and major importance-in the progress of the gelation/fusion process is the fusion temperature. As already mentioned, this is the temperature corresponding to the main, final peaks of the curves of Figs 21.8 and 21.9. The drop in the curves at the still higher temperatures to the right of this peak has been attributed to the disruption (melting) of centres of ordered structure (microcrystallites) believed to form in the plasticised PVC polymer (d. Chapter 5, Section 5.4), and thought-when present-to act in the same way as microcrystalline regions in naturally crystalline polymers, i.e. as quasi cross-linking points stiffening the structure. However, as has also been mentioned, the fusion point indicated by the main peaks of the viscosity or modulus versus temperature curves does not necessarily correspond to the development of the highest room-temperature tensile strength appropriate to the composition. * In many cases that
* In material which has lost a significant amount of plasticiser in the course of heat treatment such loss may (if no substantial polymer degradation has yet occurred) produce an increase of tensile strength beyond the value normal for the properly fused composition. However, in such a case there will also be an appreciable reduction of extensibility and increase of stiffness (ct. Chapter 12, Section 12.3).
21
PVC Pastes: Properties and Formulation
959
ultimate strength level is attained only by heating to a somewhat higher temperature for an appropriate time. A method originated by McKenna33 and subsequently employed by Greenhoe 32 (cf. Chapter 5, Section 5.5) to investigate the course of gelation and fusion of PVC pastes (in the form of cast films) provided the basis for a practically orientated laboratory test used in industry as an aid in the formulation and evaluation of paste compositions, and-in some cases-production control. The method is variously known, in its main versions, as the 'hot bench' test, the 'gel block' test or the 'temperature-gradient bar' test: some literature references relevant to these tests and their results are given in Section 5.5 of Chapter 5. All the tests employ a metal element (bar, block, etc.) whose surface is differentially heated to produce several zones kept at increasingly higher, closely controlled temperatures. A thin layer of paste is placed on the surface so that it spans the heating zones. After an appropriate time (which may be one ofthe variables investigated) the PVC layer-now solidified in varying degrees along its length-is removed and the degree of gelation/fusion attained by the consecutive sections determined, by a tensile strength test or in other ways. Note: A plot of tensile strength (from the point where that becomes high enough to be measured) versus temperature is similar in shape to the curves of Figs 21.8 and 21.9.
Because of its usefulness as an indicator of completeness of fusion and its relevance, as a material property, in many service situations, tensile strength is widely employed as the criterion in assessing the results of heat treatment of PVC pastes in tests and actual processing. Note: The appearance of the surface of a paste-derived product can provide a useful general clue to the completeness of fusion. A dull, matt surface is a strong, positive sign that fusion has not been complete.
Other tests of interest in the practical context include the solvent immersion test (several versions), and the 'clear point' test (also several variants). Solvent immersion tests for paste-derived PVC products are discussed in Chapter 22 (Section 22.1.2). As originally conceived, this kind of test was a pass-or-fail check to determine whether the material of such products (very often the PVC coating on a fabric) had been completely fused in processing, the criterion of complete fusion being absence of any sign of disruption by the solvent used as test medium.
960
W. V. Titow
However, some solvent immersion tests for paste products can also give an indication of the extent to which the state of the material falls short of complete fusion (as reflected in the time of onset and severity of solvent attack).34 The clear point test-discussed in Chapter 5, Section 5.6.1-gives a relative index of the compatibility of a plasticiser with the PVC polymer: this can be used as an indication of the relative ease (and to some extent, especially in the light of relevant experience and background data, the likely temperature) of gelation and fusion of a paste based on the particular polymer/plasticiser system. The results of appropriate tests, carried out and interpreted in the right context, can provide guidance relevant in the formulation and processing of pastes, but no laboratory test can be a full substitute for practical production trials. 21.2.6 The Measurement of Viscosity of PVC Pastes
Reference has already been made in the preceding section to the role of viscosity measurement as an important technique in the study of the gelation and fusion of PVC pastes, and to the equipment employed for this purpose. Another important application of viscometry in PVC paste technology is its use in the determination of rheological properties relevant to such aspects of industrial production as paste formulation and quality control (including evaluation of ageing characteristics), applicational suitability, and trouble-shooting. Two types of viscometer are of particular interest in this connection-the Brookfield rotational viscometer (for determination of the apparent viscosity of pastes at relatively low rates of shear) and the Castor-Severs viscometer (an extrusion rheometer used for apparent-viscosity determinations at high shear rates). The Brookfield instrument measures viscosity in terms of the arresting torque on a spindle rotating in the composition under test. The models used with PVC pastes (RV and LV) are available in a number of variants (e.g. RVF, RVT, RVT-lOO, LVF)35,36 offering different (though overlapping) spindle speed ranges: those recommended by the relevant ASTM standard36 are models RVF and LVF. The combination of spindle size (designated by a number) and spindle speed-which should be chosen in accordance with the viscosity characteristics of the material and the desired precision of measurement-should be stated when quoting the 'Brookfield viscosity' of a
21
PVC Pastes: Properties and Formulation
961
paste: this is expressed in conventional units (Pa s; P; cP), the value being calculated from a formula involving factors representing spindle speed (for a particular spindle size) and the dial reading on the instrument. Cone-and-plate rotational viscometers (e.g. the Haake-Rotovisco instrument)37 are also sometimes used with PVC pastes. Useful comments on the determination of viscosity of polymer dispersions, emulsions and solutions with a rotational viscometer are given in ISO 3219. The Castor-Severs viscometer is a rheometer in which the paste is extruded by gas (normally nitrogen) pressure under standard conditions, through a die of standard dimensions. Several determinations are made, each at a different pressure value (in the relevant IS0 38 and ASTM39 standards, respectively, within the ranges 100-2500 kPa and 69-690 kPa), and the apparent viscosity ('YJa) is calculated-in appropriate units (Pa s; P; cP)-for each pressure (and hence shear stress) level as the ratio of shear stress (r or a) to shear rate (y or e), the values of these two factors being given by the relationships: r = Prl2l and
y = 4WI!r?pt
where P is the pressure used in a determination, r is the die channel radius, l is the die channel length, W is the weight of paste extruded in time t, and p is the density of the paste (all in appropriate units). The results may be represented as a graph of r versus y, or 'YJa versus y (d. Section 21.2.4). For the monitoring of paste viscosity in fully routinised industrial operations, simpler equipment-giving viscosity values in relative terms-may be adequate in some cases. Two examples are the Gardner viscometer and the Ford Cup viscometer. In the former, the time is measured for a plunger to fall, under a standard load, through a cylinder containing the paste; modified versions (e.g. with perforated plungers) have also been used. With the Ford Cup the time which the paste takes to run out of a standard-size cup through an orifice of standard dimensions is measured. It need hardly be pointed out that in tests of this kind the conditions and method of operation must be very strictly standardised if reproducible and comparable results are to be obtained. In practice this means, inter alia, that the paste should always be prepared in the same way, stored in the same conditions, and viscosity measurements should be taken each time after the same storage time and by eactly the same procedure.
962
W. V. Titow
21.3 PASTE COMPONENTS AND FORMULATION Pastes are PVC compounds, and therefore all those normal constituents of a PVC compound necessary for its intended end-use are found in pastes. These ingredients are discussed in Chapters 4-11. They are the polymer, plasticiser(s), stabiliser(s), colourant(s), filler(s); external lubricants are also occasionally included, and speciality additives required in processing (e.g. blowing agents) or end-use (e.g. antistatic agents). The principal differences between pastes and solid PVC compounds are, of course, in their physical state and in processing. Therefore those ingredients which are added because of special requirements arising in connection with these two aspects will be different in the two types of compound. Thus polymeric modifiers are not used in pastes and neither are lubricants with exclusively internal-lubricant action. On the other hand pastes will contain viscosity modifiers, diluents or thickeners which are absent from solid compounds, because their functions are specifically associated with the liquid character of the pastes. This factor also influences in a marked way the choice of some of the other 'major' components. The individual components of a paste formulation will now be briefly discussed.
21.3.1 The Polymer (a) Paste Polymers As has already been mentioned these must both: (i) be resistant at room temperature to the plasticisers used (this makes for good paste stability), and (ii) have good affinity for the plasticisers in order to rapidly fuse with (dissolve in) them at the appropriate elevated temperature, for good gelation and fusion. These are conflicting requirements, but modern paste-making polymers achieve a reasonably successful compromise, assisted by the high sphericity of the particles (which mades for relatively low surface-to-volume ratio) and their fairly dense surface (an obstacle to plasticiser penetration at room temperature). The particle size and size distribution characteristics of different paste polymer grades are also suited to different rheological require-
21
PVC Pastes: Properties and Formulation
963
ments. Thus resins recommended for high-viscosity pastes tend to have comparatively small particles and narrow particle size distributions verging on monodispersity: such distributions, with means between about 0·1 f.lm and 0·6 f.lm, have been reported for some resins,S and means in the range 0·23-0·46 f.lm for others. 4 Medium-viscosity paste resins are more typically polydisperse: broad size distributions of both primary particles and aggregates (which were also present) have been reported for some commercial products, with primary particle size medians in the range 0·8-1·5 f.lm, i.e. somewhat larger than in some typical high-viscosity paste resins. Bimodal distribution of primary particle sizes is a feature of some paste resins,6 notably certain commercial polymers recommended for low-viscosity pastes (characterised also by a secondary-particle content relatively higher than that of typical high- and medium-viscosity paste resins).4 (b) Extender Polymers Reference has already been made to these in Section 21.2.2. They are homopolymers or copolymers made by the suspension method, with particle sizes generally in the range 80-140 f.lm. Extender polymers are usually included in pastes, replacing part of the paste polymer, to lower paste viscosity whilst preserving the overall polymer content. Homopolymers with a non-absorbent particle surface, and copolymers with low co-monomer contents are used for this purpose. The viscosity-reduction effect is illustrated in Fig. 21.12. It arises, and is at maximum at about 50% replacement, because the paste now behaves as a system of the coarse extender-resin particles, suspended in a 'liquid' consisting of the dispersion of the paste resin in the plasticisers. 40 ,41 The viscosity effect is reasonably permanent and the ageing properties of the paste may even be improved in some cases, as shown b."/ Fig. 21.13. Occasionally suspension homopolymers with absorbent surfaces, or copolymers with high co-monomer contents, are used as extender polymers. In such cases the viscosity of the paste increases sharply, and ageing properties deteriorate; both effects are due to the absorption of some of the plasticiser into the extender-polymer particles. Viscosity-lowering extender polymers are employed in pastes for coating, spraying and rotational casting applications (ct. Chapter 22). They are also utilised in rigisols. The selection and use of extender polymers are discussed by Park. 42
W. V. Titow
964 100000
Polym~r
/ plasticisar
80000 Q.
u
.
,., 40000
'iii
.. o
u
':>
20000
o
10
20
30 ./.
40
50
60
70
60
90
100
past~ polym~r r~plac~d
Fig. 21.12 Effect of replacing a paste polymer (Breon P130/1) by a suspension-type VCNnC copolymer (Breon CS 100/30).
...
A.
,
.
>-
'w
.. o
10000
.!!
>
5000
a
5
Ag. of
10 IIlStl,
15
days
Fig.21.13 Ageing characteristics of Breon P130/l :CS100/30 blends (polymer: plasticiser, 60: 40).
21
PVC Pastes: Properties and Formulation
965
21.3.2 Plasticisers In addition to the properties important from the point of view of end-use, which are discussed in Chapter 7, the choice of plasticiser for pastes is governed by the paste viscosity and general rheological characteristics which the plasticisers will impart; this includes also the gelation and fusion properties, as has been illustrated in Section 21.2.5. In addition to the studies of these effects already mentioned in that section, there are several earlier publications on the effect of plasticisers on the rheology of pastes. 43--47 Other factors being equal, the initial viscosity of the paste is significantly influenced by the bulk viscosity of the plasticiser(s), but this may be overshadowed by the effect of plasticiser affinity (solvating power). As would be expected in the light of the relevant considerations already discussed, highly solvating plasticisers will normally tend to produce higher paste viscosities. In those normal pastes which are subject to appreciable ageing effects, the main viscosity increase will commonly take place within a few hours from the completion of mixing; it is therefore reasonable to measure paste viscosities for routine control purposes 12-24 h after preparation. As has been mentioned in Chapter 7, a plasticiser mixture will normally impart to the compound physical properties and end-use characteristics intermediate between those imparted by its individual components. This is so also in pastes where, moreover, the principle extends to such properties of the liquid pastes as flow properties and ageing characteristics. An illustration of this is provided by the examples in Table 21.1. The effect of some individual plasticisers on the viscosity of PVC pastes is illustrated by Table 21.2. Solvating power of plasticisers as a factor in paste ageing is discussed in a paper by Bigg and Hill. 49 The following further comments may be made on the effects of various plasticisers in pastes. In general, the phthalate plasticisers give medium-viscosity pastes, with low to medium setting (gelation) temperatures. Most phthalates, and especially general-purpose Cs phthalates, impart some thixotropy to the paste even when used alone in a relatively low concentration (down to about 50 phr) so that the paste is very viscous. This feature is useful where pastes are required which will not flow easily without being worked (i.e. without shear), but which must spread readily under
w. V. Titow
966
TABLE 21.1 Viscosity of Pastesa with Mixed Plasticisers48 Plasticiser composition
Relative apparent viscosity at indicated shear stress (DOP 18 h viscosity = 1(0) 0·159Ibfin- 2
DOP-l00% DOP- 90%: Polyester (Flexol R2Hy DOP- 80%: Polyester (Flexol R2H) DOP- 70%: Polyester (Flexol R2H) DOP- 90%: 1TP DOP- 80%: 1TP DOP- 90%: Chlorinated paraffin (Halowax 4004Y DOP- 80%: Chlorinated paraffin (Halo wax 4(04) DOP- 70%: Epoxy compound (Paraplex G60)d
30days b
2·23Ibfin- 2 30days b 18h 30days~
18h
30 days
l8""h
10% 20% 30% 10% 20% 10%
100 141 246 259 123 133 158
175 196 351 351 196 186 214
1·75 1-40 1·43 1·36 1·60 1·37 1·35
100 174 344 309 217 213 173
145 281 354 398 295 301 202
1·45 1·61 1·03 1·29 1·36 1·41 1·17
20%
158
196
1·24
186
217
1-17
30%
175
205
1·16
149
186
1·25
• PVC resin Bakelite QYNV 100 pbw, total plasticiser 60 pbw. b This ratio is an index of the viscosity stability. C Union Carbide. d Rohm and Haas.
TABLE 21.2 Effect of Plastieisers on Ageing Characteristics of Pastes (ICI 'Come' Polymer) Plasticiser
Viscosity (Gardner) 1 day 28 days
DBP DAP DOP Bisoflex 791 a DNP
TIP
TXP TOP
DAS HexapLas PPL b HexapLas PPA
a
b
B.P. Chemicals. ICI.
Too viscous 72 120 80 124 68 110 69 69 410 1200 320 480 13 48 6 4 1030 300 Too viscous
21
PVC Pastes: Properties and Formulation
967
shear, as, for example, in coating operations. However, it should be noted that some plasticisers thixotropic in their effect at moderate rates of shear (e.g. DOP, DOS) may promote dilatant behaviour when the paste is subjected to high shear rates. As would be expected, plasticisers whose own viscosity is high tend to make viscous pastes. Paste viscosity is also promoted by plasticisers with good solvating power (e.g. DBP-see Table 21.2). Moreover, such plasticisers usually tend to promote dilatancy, which may be pronounced at low rates of shear: triaryl phosphates, and some aromatic esters of glycols (e. g. diethylene glycol dibenzoate) are examples of plasticisers with this kind of action. These general considerations are relevant in paste formulation, as neither high viscosity (unrelieved by thixotropy) nor dilatancy is normally desirable in a paste. Note: Some pastes for the production of flexible PVC foams
constitute an exception here: a relatively high viscosity, either natural to the formulation or-as may be preferableresulting from dilatant behaviour in processing, can promote uniformity and stability of the foam cells during their formation, and help to maintain these features until the structure is fixed by gelation. Promotion of a measure of dilatancy to these ends is one of the advantages of inclusion of suitable, rapidly solvating plasticisers (usually BBP; in some cases DBP as part of the plasticiser system-d. Chapter 25) in pastes for foaming, although rapidity of gelation and fusion to solidify the foam quickly may be regarded as their main function. Viscous and/or dilatant pastes are more difficult to stir, de-aerate and transfer in production and directly prior to application; they do not flow or spread easily. This can make for problems in casting and coating operations, and cause difficulties where smooth, even coatings (especially thin ones) are required. Both high viscosity and dilatancy can be counteracted by incorporating in the paste a suitable plasticiser (usually as part of the plasticiser system); some aliphatic diester ('low-temperature') plasticisers are particularly effective for this purpose and selected phthalates can also be useful, e.g. DINP (which, in addition to lowering paste viscosity, also offers lower volatility in comparison with Cs phthalates). Other phthalate plasticisers noteworthy from the point of view of advantageous paste rheology and
968
W. V. Titow
viscosity stability in storage are dicapryl phthalate (DCP) and Hexaplas
OPN. * Where relatively high temperatures and long times of setting
and fusion may be required, such higher phthalates as DDP and DLDP should be considered: their comparatively low solvating power also confers good ageing stability on the paste. The strong solvating ability of triaryl phosphate plasticisers accounts for the fact that their inclusion in a paste formulation makes for increased viscosity and, in most cases, dilatant behaviour. At the same time they promote rapidity and ease of gelation and fusion, and their tolerance of extenders is good. The 'low-temperature' (aliphatic diester) plasticisers lower the paste viscosity, very considerably in some cases (e.g. DIDA; some 'nylon acid' esters-d. Section 21.4.2), and impart thixotropic properties (some dialkyl sebacates and adipates are particularly effective). The ability of several members of this group to promote a large drop in paste viscosity with increasing temperature makes them useful in formulations for rotational casting. Pastes containing aliphatic diesters (normally-in view of their secondary-plasticiser character-as one component of the plasticiser system) tend to have good ageing resistance and relatively high setting temperatures. Epoxy plasticiser/stabilisers are employed in pastes for the same end-use effects as in solid compounds. Some (epoxidised soyabean oil) will generally tend to increase paste viscosity, whilst others (epoxy alkyl esters) normally have the opposite effect. Polymeric plasticisers tend to give fairly high viscosity and may promote dilatancy. However, the ageing properties imparted are good. These plasticisers are used in pastes essentially for the permanence of properties in end-use which they confer. Crosslinkable and polymerisable plasticisers have a special application in some pastes, where their use permits a relatively low initial viscosity to be combined with conversion to a semi-rigid or even fully rigid ultimate product. However, their incorporation in paste formulations gives rise to special considerations. Unless radiation can be used as a means of effecting polymerisation or cross-linking of the plasticiser in the fused product (and this additional treatment, usually expensive in any case, may not be practicable) an initiator or cross-linking agent, commonly a peroxide, will usually be included, and this may affect the stabiliser system or have some effect on properties. These specialist * Proprietary product of ICI: see Chapter 6, Section 6.5.5.
21
PVC Pastes: Properties and Formulation
969
plasticisers and their uses are mentioned in Section 6.10.4 of Chapter 6. Examples of such constituents of PVC compositions include diallyl phthalate (e.g. Bisomer DALP-BP Chemicals International Ltd), certain glycol dimethacrylates,50 trimethylol propane trimethacrylate, Santoset (a proprietary product of Monsanto), and the Sartomer plasticisers (Ancomer Ltd, UK). Some other paste-relevant properties and effects of various plasticisers and extenders are pointed out in the discussion of these materials in Sections 6.5 and 6.10 of Chapter 6.
21.3.3 Stabilisers In this respect the service-suitability aspect of paste formulation is very similar to that of solid compositions, whilst the effects of stabilisers on paste rheology are associated largely (though not exclusively) with their physical state: liquid stabilisers and stabiliser systems are used where the least effect on the flow properties (minimal viscosity increase) is desired. A comparison of some common stabilisers in a paste compound (Breon P13011, 100 parts; TIP, 33 phr; DOP, 33 phr; stabiliser, 1·67 phr) on the basis of the Congo Red test is given below. Dibasic lead phosphite Basic lead carbonate Calcium stearate Barium ricinoleate Lead stearate Cadmium stearate Control (no stabiliser)
Time to breakdown (min) 101 83
39 27 19 14
4
Stabiliser systems based on combinations of Ba, Cd and Zn compounds (see Chapter 9) are more widely used in PVC pastes than any other stabiliser type. Liquid Ca/Zn and Mg/Zn systems are useful in pastes where risk of sulphur staining in end-use is a consideration: they are also advantageous to paste rheology. Some are approved for non-toxic applications. Some Ca/Zn and Cd/Zn as well as certain lead stabilisers function also as 'kickers' for the blowing agent in flexible foam production from pastes. Liquid organotin stabilisers (especially the sulphur-free systems) are of interest where maintenance of
970
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V. Titow
reasonably low paste viscosity, or viscosity reduction, is an important consideration. Good heat and light stability can be obtained in paste products stabilised with basic lead carbonate and dibasic lead phosphite; the former stabiliser has been useful for opaque formulations in the absence of non-toxicity requirements. Calcium or lead silicate have been used in translucent formulations. Calcium stearate functions as stabiliser/lubricant for transparent compositions. However, in formulating with stearates it should be remembered that they tend to increase initial paste viscosity and can intensify viscosity increase on ageing.
21.3.4 Fillers Fillers are included in pastes for the same end-use purposes as in solid compounds, but their effects on paste rheology and ageing characteristics must be considered, as most can increase the viscosity and reduce the shelf life of the paste, or modify the response to shear. Films and coatings produced from filled pastes are usually less tacky and harder than those made from unfilled ones, but other physical properties may be inferior. In some cases the optical effects of a filler in the finished product are particularly important, e.g. the achievement .of the correct degree of translucency to impart a life-like appearance to pastemoulded PVC dolls. Other factors being equal, paste viscosity tends to rise with increasing filler loading and with decreasing particle size. The former factor increases viscosity partly as a result of the increase in the particulate phase content (and modification of its size distribution) in the paste (cf. Section 21.2.4), and partly because more particles are available to adsorb some plasticiser onto their surface. Particle size reduction also increases plasticiser adsorption by the filler because the specific surface (surface-to-volume ratio) goes up. The surface adsorption effects can be considerably modified if the filler particles are coated (see Chapter 8, Section 8.3.2). The amount, distribution and chemical nature of the coating all play their role, the most common overall effect being a reduction of plasticiser adsorption-and hence of viscosity increase-by the presence of the coating. Some of the effects of organic titanate coatings on fillers upon the viscosity and other properties of filled PVC pastes are discussed in a paper by Monte and Sugerman. 51
21
971
PVC Pastes: Properties and Formulation
TABLE 21.3
on Absorption of Fillers Filler
Oil absorption number 16 30 33 36
Barytes Slate powder Dolomite Whiting China clay (medium fine) China clay (very fine) Silica Water-ground mica
35 55
42
79
An indication of the likely magnitude of the effect of a filler on paste viscosity is given by its oil absorption avalue (cf. Chapter 8, Section 8.3.3), which is influenced by the factors just mentioned. A few oil absorption values of common fillers are shown, by way of example, in Table 21.3, and others in Table 21.4 where some effects of fillers on the viscosity of a paste and on the mechanical properties of the ultimate product are also illustrated. The table indicates, inter alia, something of the paste viscosity response to the incorporation of precipitated calcium carbonate fillers of, respectively, low and high oil TABLE 21.4 Effect of Some Fillers on a PVC Pastea and its Ultimate Product Filler Nature
None Calcined clay Blanc fixe Ppted CaCO/ Ppted CaC0 3c Barytes Diatomaceous earth Lithopone
Viscosity Tensile Modulus Elongation Oil (cP: 1 day) strength at 100% at break, absorption (lbfin- 2 ) extension phr (%) (No.) (lbfin- 2 )
0 20 31 21 20 34 18 33
10000 17000 15000 18000 40000 7000 40000 40000
2400 2050 2250 2100 2250 2220 1900 1750
1000 1125 1100 1000 1100
950 1175 1000
350 340 320 350 370 420 270 320
66
38 36 45 16 148 37
a Breon P1301l, 100 parts; DOp, 66·7 phr; filler as shown (each weight represents a volume equivalent to that of 20 phr calcined clay). b Low oil absorption grade. c High oil absorption grade.
W. V. Titow
972 1500
Sturcal L _ - - - - - - - - 3 3 phr
_ - - - - - - - - - - C a I O f i l A.4 33phr
ll. >, .... "iii
ou
III
:>
__
---------Sturcal L 8phr ~_ _- - - - - - - - - - - - C a l o f i l A.4 8phr
__------------NO filhzr
o
5
10 TimlZ, days
15
Fig. 21.14 Effects of two CaC03 fillers on viscosity of a PVC paste. Sturcal L (John E. Sturge Ltd), medium oil absorption filler; Calofil A4 (John E. Sturge Ltd), low oil absorption, resin-coated filler. Paste formulation: PVC resin (Breon P130/1) DOP
TIP
Filler Stabiliser (white lead paste)
lOOpbw 33 phr 33 phr as shown 1.7 phr
absorption. The rheological effects of two commercial grades of CaC03 filler are also shown in Fig. 21.14; the main features demonstrated by the curves of this figure are fairly typical, viz. higher paste viscosity with higher loadings of both fillers, and generally lower viscosity with the coated filler (except for the slight initial reversal at the higher level of filler loading, where the early viscosity of paste containing the uncoated filler is somewhat lower, because of the slower rate of wetting out). Filler grades specially recommended for pastes are available from many suppliers. As with other major components of pastes (e.g. plasticisers) the effect of fillers on viscosity stability is of interest to the formulator and processor: this is often expressed in
21
PVC Pastes: Properties and Formulation
973
terms of the ageing index (sometimes called the viscosity stability index--d. Table 21.1) calculated as a ratio VL: V S , where VL is the viscosity measured after an appropriate, long period (say 15 days) and Vs is the viscosity value obtained in an earlier measurement (say the 24 h or 48 h viscosity). 21.3.5 Thickening Agents (for Thixotropic Plastisols and Plastigels) Thixotropic plastisols are used in various dipping processes, where one-dip coatings are required or where no-drip coatings are essential, and in spreading processes to reduce penetration and mobility. Normal pastes can be made thixotropic or converted into plastigels by the addition of various thickening agents. These include certain grades of precipitated or fumed silica, e.g. Gasil 23 and Neosyl (Joseph Crosfield and Sons Ltd), Cab-o-Sil (Cabot Corp.), Aerosil 200 (Bush, Beach and Segner Bailey), special bentonites, some grades of aluminium stearate, e.g. Higel No.1 (Albright and Wilson Ltd), and Sylodex (W. R. Grace (UK) Ltd)---ehrysotile asbestos fibres in the form of 'crumb' produced by wetting out with plasticiser (2 parts DIBP to 1 part of asbestos). Some fillers can also have similar effects, e.g. very fine whitings and carbon blacks. The normal method of working is to pre-gel the plasticiser with the agent and use the resultant product for preparing the paste. The process is very simple and consists of dispersing the agent in the plasticiser and warming until a clear solution is obtained (with soluble thickening agents). A marked increase in viscosity also takes place. After cooling the plasticiser is ready for use. In the case of aluminilJm stearate (Higel No.1), a 25% solution (pre-gel) is made in the plasticiser and the amount on the plastisol is 2-10%, a good average being 5%. DOP and DOS can readily be pre-gelled in this way but TIP cannot be. The sort of increase in viscosity which can be achieved (Breon P130/1 100; DOP 66·7 phr; white lead paste 1 phr and 5% pre-gel Higel No. 1 in DOP) is from 10 000 cP to 106 000 cPo Aluminium stearates G and 2027 (Durham Raw Materials Ltd) behave in a similar way. The latter will gel at a lower temperature, the actual gelation temperature depending on the type of plasticiser used. The recommended use levels for both grades are 1% upwards. Aerosil is a useful material for plastisols. It increases the degree of thixotropy by about 60%, it can be easily dispersed (by stirring into the
974
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V. Titow
plastisol or dispersing in the plasticiser) and it gives a stable product. Sedimentation of filler is also prevented and the addition of 0'5-1 % is required. Bentone 27 (F. W. Berk and Co. Ltd) is another suitable material. Sometimes plastigels of putty-like or even modelling consistency are required. Suitable materials here are Santocel C (Monsanto Co., USA), a fine diatomaceous silica, and Neosyl C, used to the extent of 5-15%. For modelling purposes 30-50% of fillers such as French chalk or kieselguhr are employed. Again 10-30% of a finely divided china clay or calcium carbonate may be used or titanium dioxide in plasticiser. Some recipes are (in phr): Corvic P65150 Dialkyl 70 phthalate or similar phthalate Tritolyl phosphate Aluminium stearate Santocel Cor 54 Lead carbonate Pigment
A 100 40 40 4 6
4
B
C
100
100
80
55
4 6
4 As required
2·5 2·5 4
As indicated by the above formulations, in general the higher the plasticiser content, the more thickening agent is required. Mixing procedure is as follows. As aluminium stearate does not dissolve in tritolyl phosphate, the soap must first be dispersed in a phthalate plasticiser. This is done by making a paste with the soap and some of the plasticiser, adding the remaining plasticiser to form a slurry, then heating this to about 120-140°C (248-284°F) with slow stirring. When this temperature is reached, a clear mobile solution is formed, which on cooling changes to a soft jelly. To this jelly is added any additional plasticiser necessary, the Corvic P65150, heat stabilisers and pigments, mixing being carried out in a dough mixer which can be water-cooled. About 10-15 min mixing is required and the temperature of the mix should not be allowed to exceed 35-37°C (95-98°F). In the later stages of mixing, Santocel C should be added, the result being a firm putty-like mass. The mixed compound should be stored for 24 h or so at room temperature before use, to allow full thixotropic properties to be developed. Within this period marked thickening occurs, but thereafter changes are slow. Over a period of months plastigels may harden slightly but they may be used safely some months after manufacture.
21
PVC Pastes: Properties and Formulation
975
The gelled material has the same good ageing properties as normal PVC compounds. 21.3.6 MisceUaneous Paste Components These include the following. (a) Viscosity Depressants These are used when it is essential, for end-use properties, to employ a particular plasticiser system and plasticiser: polymer ratio which at the same time give unduly high paste viscosities. Viscosity depressants are usually surface-active agents of various kinds. The effect of a particular depressant in a particular formulation cannot be entirely predicted and therefore tests are necessary. Most viscosity depressants also retard the ageing of the paste. These compounds are added in low quantities (of the order of 1%) in order not to upset the formulation, and to avoid exudation. Polyethylene glycols and Lubrol MOA (non-ionic surfaceactive agent: fatty alcohol/ethylene oxide condensate-ICI) are examples. (b) Diluents These are usually organic solvents producing a thinning effect on the paste, and are mainly used in organosols (see Section 21.4.1).
(c) Other Minor Additives Occasionally Included These include colourants, blowing agents, fungicides and bactericides, tire-resisting agents, odourants and deodorants. 21.4 PASTES FOR RIGID PRODUCTS: ORGANOSOLS AND RIGISOLS Apart from the use of cross-linking plasticisers in plastisols, the requirements of initial low viscosity and semi-rigid tinal products can be met by organosols and rigisols. Brief reference has been made to both in this chapter; a few more details may now be given. 21.4.1
Organosols
These are essentially plastisols containing additional diluents. The diluents may be aromatic hydrocarbons (e.g. xylene and toluene), aliphatic hydrocarbons (e.g. white spirit), or certain ketones (e.g.
976
W. V. Titow
methyl ethyl ketone, methyl isobutyl ketone); alcohols, glycol ethers and chlorinated hydrocarbons have also been used. The diluents are employed in circumstances somewhat similar to those in which viscosity depressants are used, but, unlike the depressants, they are volatilised orf in the course of processing. Accounts of their use were first given in early literature. 52-54 Organosols have their place where the process demands a paste combining low viscosity with high solids content, to yield a relatively rigid final product. For example, sprayable compositions can be formulated, with the aid of suitable volatile thinners, containing as much as 60% non-volatile matter, with up to 150 phr filler, and very low plasticiser content (down to a few per cent if necessary).
21.4.2 Rigisols As mentioned in Section 21.1, these are plastisols specially formulated to achieve low viscosity for processing, but combining this with high polymer content and hence rigidity of the ultimate products. The factors which are adjusted and controlled to achieve this result are the following.! (i)
Selection (and if necessary blending) of paste-grade PVC polymers with particle size and size characteristics best suited to give the kind of particle packing system in the paste that will promote the lowest practicable viscosity combined with highest polymer content. Selected suspension-grade homopolymer resins may be included, or some vinyl chloride/vinylidene chloride copolymers, as extenders for the paste-grade resin(s) to reduce the viscosity of the system still further. (ii) Selection of plasticisers which promote low viscosity and thixotropy in a paste, so that the plasticiser content can be reduced to a minimum. Several aliphatic ester 'lowtemperature' plasticisers are especially suitable, in particular the AGS ('nylonate') esters. Note: It has been suggested! that the packing of the polymer particles achieved by suitable blending is a close multimodal packing with the smaller particles almost filling the interstitial spaces among the larger ones, and that the amount of plasticiser should be that which then fills the remaining inter-particle voids, just preventing the particles from touching. It is further suggested that such a 'lubricated' system
21
PVC Pastes: Properties and Formulation
977
is considerably different from an ordinary plastisol in which the polymer particles may be regarded as floating in the plasticiser: it obviously contains proportionately less plasticiser. ! (iii) Careful attention to the choice of stabilisers and fillers. The stabilisers should preferably be liquid with no thickening effect on the plasticiser, and the filler loading should not exceed 15 phr. (iv) Use of viscosity depressants: polyethylene glycol (400) monolaurate has been recommended as particularly effective.! (v) Use of diluents. It is an academic point whether the inclusion of a solvent in a paste otherwise formulated as a rigisol makes it into an organosol. In practice, white spirits and aliphatic naphtha (free from aromatics) are diluents useful in lowering the viscosity of a rigisol. It has also been stated! that the speed of mixing in the course of preparation is a factor in successful production of rigisol pastes. High-speed mixing is preferable in the first ('dry' or 'thick' stage-see Chapter 22, Section 22.1.1) to ensure that any aggregates that may tend to form are thoroughly broken up, but once the composition has been homogenised the mixing speed should be reduced. Useful advice on basic rigisol formulations for specific purposes is available from manufacturers of paste resins and plasticisers. One 2 Parts by weight example is given below: PVC resins: paste-grade emulsion polymer, low viscosity (K value* 65) 50 50 100 suspension polymer (K value* 55) 50 50 Plasticisers (AGS esters): Reomol MDt 25 30 Reomol MNt 25 Stabilisers: BalCd liquid system 2 2 2 epoxy co-stabiliser 2 2 2 2 2 2 Viscosity depressant: Lubrol MOA Apparent' viscosity of paste (at low shear rate) (P) 18 22 42 after 1 day after 7 days 18 20 98 BS softness No. of ultimate product 6 7 9 * 0·5 g polymer in 100 ml dichloroethane at 25°C.
t Ciba-Geigy.
978
W. V. Titow
REFERENCES 1. Goodier, K. (1960). Proceedings of the International Congress on the Technology of Plastics Processing, Amsterdam 17th-19th October, N.C. V't Raedthuys, Amsterdam. 2. Ciba-Geigy Technical Service Bulletin PL 3.3 (1975). 3. Johnston, C. W. and Brower, C. H. (1970). SPE J., 26(9), 31-5. 4. Underdal, L., Lange, S., Palmgren, O. and Thorshaug, N. P., (1978). 'PVC paste technology and polymer characteristics', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 5. Bjerke, O. (1966). 'Relation between the distribution of size of the primary particles and the rheological properties of PVC plastisols', SCI Monograph No. 26, pp. 370-80. 6. Ram, A. and Schneider, Z. (1970). Ind. Eng. Chem., Prod. Res. Develop., 9(3), 286-91. 7. Mooney, M. (1951). J. Colloid Sci., 6, 162. 8. Eilers, H. (1941). Kolloid Z., 97,313. 9. Breon Pl30II Paste Resin. (1969). BP Chemicals International Ltd., Technical Manual No.2, pp. 5-10. 10. Cawthra, c., Pearson, G. P. and Moore, W. R. (1965). J. Plast. Inst., 33, 39-44. 11. Mendelson, R. A. (1968). Encyclopedia of Polymer Science and Technology, Vol. 8, (Eds H. F. Mark et al.), Interscience, New York, pp. 588-90. 12. Lodge, A. S. (1964). Elastic Liquids, Academic Press, New York, pp. 228, 246-8. 13. Reiner, M. (1960). Lectures on Theoretical Rheology, North Holland Publishing Co., Amsterdam. 14. Fredrickson, A. G. (1964). Principles and Applications of Rheology, Prentice-Hall Inc., New York, pp. 27, 142-4. 15. Gillespie, T. (1966). J. Colloid Interface Sci., 22,554-9. 16. Hoffman, R. L. (1974). J. Colloid Interface Sci., 46,491-7. 17. Strivens, T. A. (1976). J. Colloid Interface Sci., 57,476-80. 18. Rangnes, P. and Palmgren, O. (1971). J. Polym. Sci.-Part C, 33, 181-7. 19. Thinius, K., Reicherdt, W. and Hosselbarth, B. (1963). Plaste u. Kaut., 10,339-41. 20. Goodrich, J. E. and Porter, R. S. (1967). Polym. Engng. Sci., 7, 45-8. 21. Lee, G. C. N. and Purdom, J. R. (1969). Polym. Engng. Sci., 9, 360-;;. 22. Schreiber, H. P. (1970). Polym. Engng. Sci., 10, 13-15. 23. Shah, P. L. and Allen, V. R. (1970). SPE J., 26,56-60. 24. Buckley, C. D. (1971). J. Cell. Plast., 7,23. 25. Cayrol, B., Klason, C. and Kubat, J. (1974). Polym. Engng. Sci., 14(12), 868-72. 26. Greenhoe, J. A. (1961). Plast. Technol., 7(2), 35-8. 27. Hoy, K. L. (1966). J. Appl. Polymer Sci., 10(12), 1871-93. 28. Johnston, C. W. and Brower, C. H. (1966). SPE J, 22(11),45-52. 29. Alter, H. (1959).1. Appl. Polym. Sci., 2(6),312-17.
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30. Nakajima, N., Ward, D. W. and Collins, E. A. (1979). Polym. Engng. Sci., 19(3), 210-14; and Nakajima, N. and Ward, D. W. (1983). J. Appl. Polym. Sci., 28(2), 807-22. 31. Bauer, W. H. (1961). SPE J., 17(2), 174--7. 32. Greenhoe, J. A. (1960). Plast. Technol., 6(10),43-7. 33. McKenna, L. A. (1958). Mod. Plast., June, pp. 142-5. 34. Titow, W. V. Unpublished work. 35. ISO 2555-1974. Resins in the liquid state or as emulsions or dispersionsDetermination of Brookfield RV viscosity. 36. ASTM D 1824-66 (Reapproved 1980). Apparent viscosity of plastisols and organosols at low shear rates by Brookfield viscometer. 37. Ciba-Geigy Technical Service Bulletin PL 9.1 (1977). 38. ISO 4575-1978. Plastics-Polyvinyl chloride pastes-Determination of apparent viscosity using a Severs rheometer. 39. ASTM D 1823-66 (Reapproved 1979). Apparent viscosity of plastisols and organosols at high shear rates by Castor-Severs viscometer. 40. Lewis, T. B. and Nielson, L. E. (1968). Trans. Soc. Rheol., U(3), 421-4. 41. Farris, R. J. (1968). Trans. Soc. Rheol., U(2), 281-93. 42. Park, R. A. (1975). Plast. World, 33(1), 48. 43. Severs, E. T. and Austin, J. M. (1954). Ind. Eng. Chem., 46(2), 369. 44. Darby, J. R. and Graham, R. R. (1955). Mod. Plast., 32, 148. 45. Newton, D. S. and Cronin, J. A. (1958). Brit. Plast., 31, 426. 46. Werner, A. C. (1957). Mod. Plast., 34, 137. 47. Todd, W. D., Esarove, D., and Smith, W. M. (1956). Mod. Plast., 34, 159. 48. 'Bakelite Vinyl Dispersion Resin QYNV' (1956). Union Carbide International Co., Technical Release No. 14. 49. Bigg, D. C. H. and Hill, R. J. (1976). J. Appl. Polym. Sci., 20(2),565-8. 50. British Patent No. 694444, Union Carbide and Carbon Corp. 51. Monte, S. J. and Sugerman, G. (1978). 36th ANTEC SPE Proceedings, pp.781-4. 52. Werner, A. C. (1959). Mod. Plast., 36, 126. 53. Nielson, E. R. (1950). Mod. Plast., 27, 97. 54. Powell, G. M., Quarles, R. W., Spessard, C. I., McKnight, W. H. and Mullen, I. E. (1951). Mod. Plast., 28, 129.
CHAPTER 22
Preparation, Processing and Applications ofPastes W. V. TITaw
22.1 INTRODUCTION 22.1.1 Preparation In the early days, with old-type resins,. grinding was essential to produce satisfactory pastes. With modern paste polymers 'stir-in' techniques in good mixing equipment are effective. The method and equipment used in the industry for paste preparation are discussed in considerable detail in Chapter 13 (Sections 13.4.3 and 13.4.4(f)). The use of an effective mixer and proper technique should ensure that the paste is free from lumps, with all constituents uniformly distributed, and not excessively aerated (to facilitate the postpreparative de-aeration which will usually be carried out). Of the two possible general modes of operation, continuous mixing is in practice confined to industrial processes: batch mixing is practised on both industrial and laboratory scale. Modern equipment (e.g. a planetary mixer) makes it possible to prepare a satisfactory paste batch in a one-step operation, and versions of this kind of technique are laid down in standard specifications (ISO 4612-1979; DIN 54800-1979) for the preparation of standard pastes to be used in the control and evaluation of formulation factors (especially the nature of the PVC resin) and the rheological properties of pastes. However, this does not detract from the practical value of a good two-stage method of paste preparation, especially for low-viscosity pastes, both in the laboratory and for production purposes. The following is a brief, general outline of one such method. 981
982
w.
V. Titow
All dry components (pigments, fillers, solid stabilisers, etc.) are thoroughly pre-dispersed in a small quantity of the plasticiser (say between 1 and 3 pbw plasticiser to 1 pbw dry materials). This is best done on mill-type equipment (say three-roll or five-roll mills, cone mills) or by ball milling. The polymer is then placed in the mixer with the addition of plasticiser in quantity just sufficient to form a paste, and into this the pre-dispersed dry components are added. This is the first stage of mixing known as the 'thick stage' and the aim is to have just enough plasticiser to ensure mixing at maximum shear. The shearing action will produce heat, and care should be taken that the temperature of the paste should not exceed about 30°C, otherwise the plasticiser will start to attack the polymer and the ultimate viscosity of the paste will be higher than it should. The mixer can be water-cooled if necessary. The 'thick-stage' stirring should be continued for some time (roughly 10-30 min depending on conditions). At the end of this period the gradual addition of the rest of the plasticiser (and any diluents that may be employed) is commenced. This marks the beginning of the 'thin stage' of the mixing process. The plasticiser addition may conveniently be made continuous from, say, a suitable funnel arrangement. The thin stage mixing may take very roughly about 1 h. The stirring action traps air in the paste, which is undesirable for most processes, and it must be removed. One of the best ways of effecting this is to mix under vacuum. If this is not possible the finished paste is subjected to vacuum or, for the thinner types, allowed to stand for 24 h. Storage is important and should receive careful consideration. If the pastes are prepared and formulated correctly they will store satisfactorily for long periods. They must, of course, be stored in a cool place. The material of the container is also important. Iron or zinc should be avoided unless the former is protected by a lacquer which is not attacked by the paste. Glass, aluminium and tinplate are satisfactory. The above method is suitable for both laboratory and work-scale preparation of pastes. For very small quantities the whole operation can be effected on a mill mixer (say a triple-roll mill) by a suitable number of passages. 22.1.2 Conversion to Products
As discussed in Section 21.2.5 of the preceding chapter, the conversion of the liquid paste to a solid PVC product involves gelation and fusion, actuated by heat. The initial drop in viscosity which precedes the
22
Preparation, Processing and Applications of Pastes
983
'setting' of the paste can also be significant in some processes. The state reached by a paste which has set to a considerable extent but has not fused, is sometimes called 'semi-gel' or 'pre-gel'. These essentially practical terms are not very precise, and in fact relate to what is a fairly advanced degree of gelation, in which the paste may be sufficiently cohesive to withstand some handling: this is sometimes utilised in, say, multiple dipping or coating techniques, where each consecutive paste layer is 'pre-gelled' after application, and then the whole coating is finally fused at an appropriate, higher temperature. As has been mentioned, the temperature necessary for complete fusion cannot be lower than 160°C (and will be appreciably higher for many paste compositions). Heating above the appropriate lowest temperature of complete fusion will effect fusion in a shorter time. If the temperature is too high, some plasticiser loss will first occur, and then decomposition of the polymer may follow if heating is prolonged. It is important for the purposes of process and quality control to test the completeness of fusion of paste-derived products. Determination of relevant physical properties (tensile strength, hardness, etc.) is one obvious way of doing this, but it can be cumbersome as a routine check. Solvent tests have been proposed from time to time, particularly for use with PVC coatings on fabrics, and some of those are convenient to use and quite reliable. The tests include the ethyl acetate immersion methods of Kling! and Schimke? and the acetone test widely used with PVC-coated gloves. The general principle of such tests is that a test strip, or a complete article (e.g. a PVC-coated glove), is immersed in the solvent either in a strained condition (e.g. a strip bent into a loop) or freely suspended, and the PVC is watched for signs of flaking, i.e. rising of small fragments of the solvent-swollen material away from (i.e. above) the surface (in the case of a coated fabric, sometimes also away from the fabric surface). In some versions of the test the flaking site is predetermined by a nick made in the specimen, but spontaneous flaking will normally occur in material which is incompletely fused. If they form, the flakes are clearly visible and unmistakable in appearance: severe flaking may result in the flakes becoming completely detached: in the case of under-fused PVC coatings partial stripping from the supporting layer may occur. In solvent tests of this kind it is usual to regard the PVC material as satisfactorily fused if no flaking is observed in a specified period of time: this may be anything from about 5 to 30 min, depending on the aggressiveness of the solvent used and the presence or absence of imposed strain in the specimen.
984
W. V. Titow
In the course of a study3 of gelation and fusion effects in the coating of fabric-backed PVC gloves produced by a dip-coating process, the author showed that an acetone immersion test* normally used to check~n a pass or fail basis-whether satisfactory fusion has been achieved in an industrial operation, could also provide data on the degree of fusion (where fusion complete in terms of the test had not been attained), and an indication of the minimum temperature necessary for complete fusion of the particular paste composition under the heating conditions employed. Substantially identical fabric glove liners were dip-coated (on production formers, in a standard way) with the same PVC paste (a standard production formulation). Groups of three of the paste-coated liners withdrawn at random from the batch so produced were subjected to heat treatment, each group at a different temperature/time combination. After the heat treatment, followed by a cooling and conditioning period, the acetone test was carried out on two gloves from each group (the third being retained for reference). Where flaking occurred, the time for the first flake to appear was noted (these times were in excellent mutual agreement for the duplicate determinations on gloves from the same group). The test results were plotted as shown in Fig. 22.1. In the figure each point represents the result of one test (duplicate determination) on gloves from the group which had undergone heating at the temperature and for the time given, respectively, by the relevant abscissa and ordinate values. Points marked with crosses correspond to tests in which flaking occurred: the time (minutes) of appearance of the first flake is shown by each point. The circled points represent those tests in which no flaking took place during an immersion period of 24 h: i.e. where the PVC coating could be regarded as properly fused in terms of the test. The regions of the plot occupied by the circles and the crosses correspond to heat treatment conditions producing, respectively, complete and incomplete * The specimen used in this test is a complete glove with a circular hole of 2·5 cm diameter cut in the middle of the back, about 2 cm above the finger base line (to admit the solvent to the interior of the glove). The specimen is suspended, fingers downwards, in a beaker, and acetone is quickly poured in until it surrounds and fills the glove, with the surface level standing about 2·5 cm above the edge of the hole. Timing is commenced from the moment this level is reached, the surface of the glove being closely watched for signs of flaking. In a popular industrial version of this test the fusion of the PVC coating is considered complete if no flakes appear within 30 min.
985
22 Preparation, Processing and Applications of Pastes 45 40
x
n·3
35
.••..
30
•..
25
.!
•
20
»
15
'j
-.•.
0
10 5
o ---- \
)(
H
x 4-5
x
2·5
x
2·6
x
x HI
1"4 160
170
I
180
190
x
x 2-3
)(
2·2
200
OVln tllllperltlill
)(
3·3
3·2 I
210
I
220
230
I
240
(·C l
Fig. 22.1 Effects of heating time and temperature on the completeness of fusion of PVC coating on dip-coated gloves. Each point on the plot relates to an acetone immersion test on gloves 'cured' under the conditions (time/ temperature combination) given by the appropriate ordinate and absicissa values. x, Tests on gloves 'cured' under these conditions resulted in flaking after the number of minutes indicated by the numericals (e.g. x3.:r-first flake appeared after 3·3 minutes); 0, gloves 'cured' under these conditions did not flake during 24 h continuous immersion.
fusion. The curve constituting the boundary between the two regions is thus the locus of the minimum time/temperature conditions for complete fusion in the particular process. The steeply rising portion of this curve may be regarded as asymptotic to the 172°C temperature line: this indicates that 172°C is the minimum temperature for complete fusion of the paste (and in the conditions) used. Figure 22.1 also demonstrates that the test is sensitive enough to distinguish between degrees of fusion where fusion had not been complete. This is shown by the fact that the times to flaking increase with increasing oven temperature and with increasing oven time.
986
W. V. Titow
22.2 APPLICATIONS 22.2.1 Rotational Casting This is the main process for producing hollow moulded articles (dolls' heads and body parts, hollow 'squeaky' toys, playing balls, etc.) from PVC paste, offering several advantages over slush moulding which is the alternative method (see the next section). The development of rotational casting began around 1950, and the first descriptions of machinery4-6 are of that vintage. The principles and early practice of this process were described by Meazey.7 Modern rotational casting equipment, of various sizes, numbers of moulds and capacities, is available from several manufacturers in the UK, USA and Europe, most of whom will be found in the current editions of the relevant directories. 8 Rotational casting (also known as rotary casting, roto-casting, or rotational moulding) has gained popularity for the following reasons, most of which represent advantages over slush moulding. Rotationally moulded hollow articles can be produced with uniform wall thickness and fine surface detail, from a pre-metered amount of paste. Virtual absence of flash minimises trimming operations, and the amount of material waste is also minimal. The equipment and operation are automated to a large extent, whilst the capital expenditure on machinery and moulds is moderate by present-day standards. The principle and basic process of rotational casting are simple, although in their modern embodiments the equipment and technique have attained considerable sophistication and versatility. Typically, multiple moulds-earried on one or more arms indexed to move between operational stations-are each charged with the appropriate, metered amount of paste at the filling station, and closed. Transfer to the heating/cooling station follows, the moulds being spun in one plane and simultaneously rotated at right angles to the spin direction at the outset and throughout the heating period. The heat treatment takes place in an oven powered by electricity, gas or oil, employing hot air or IR radiation as the heating medium. The complex two-directional rotation of the moulds spreads the paste evenly over their interior surfaces, where it is gelled and fused by the heat. The rotary movement is continued during cooling (by an air stream and/or water spray) in a cooling chamber, after which the moulds are returned to the filling station where they are unloaded prior to the commencement
22 Preparation, Processing and Applications of Pastes
987
of the filling step of the next cycle. A typical cycle time may be about 15 min. Spinning rates of 5-16 r min- 1 are fairly representative, with the ratio of these to the rotation rates between about 1: 1 and 1 :4. For a given product, the best rates and their combinations are established on the basis of trials. Simple moulds can be quite satisfactory, made by the electrodeposition technique (e.g. nickel on copper for good release properties and thermal conductivity), or cast or machined in aluminium. Some rotational mouldings (e.g. playing balls) are inflated after manufacture with air introduced through a hypodermic needle, the resulting hole being heat-sealed. Rotationally cast products may also be filled with foam generated from a polyurethane composition poured in through a hole foamed in situ whilst the hollow moulding is supported in a retaining mould. This method has been widely used in the production of car arm-rests with outer 'skins' rotationally moulded from PVC paste. However, in that particular application, it is being strongly challenged by essentially one-step moulding of the arm-rests in structural polyurethane foam. The following basic formulations illustrate the kind of pastes used for some rotationally cast products. Rotational casting formulation for soft dolls or playing balls (paste viscosity about 12P):
PVC resin: Corvic P65/54* Plasticisers: DAP DNP Stabiliser: Cd/Ba liquid system
l00pbw 40phr 50phr 3phr
Rotational casting formulation for harder objects:
PVC resins: Corvic P65/54 Corvic D55/3t Plasticiser: Pliabrac 810t. Stabiliser: CdlBa liquid system Viscosity depressant: Lubrol MOA§
50pbw 50pbw 25 phr 3phr 2phr
* Medium-viscosity, paste-grade resin (ICI).
t Suspension resin
(leI) here used as extender polymer.
:j: Proprietary plasticiser of Albright and Wilson (a dialkyl phthalate, prepared
from a blend of straight-chain octyl and decyl alcohols). § Non-ionic surface-active agent (ICI).
988
W. V. Titow
Rotational casting formulation for car arm-rest 'skins':
PVC resins: Corvic P65/54 Corvic D55/3 Plasticisers: DNP Reomol MN* Paraplex G62t Stabiliser: Irgastab TI50* Viscosity depressant: Lubrol MOA
70pbw 30pbw 50phr lOphr 2phr 1·5 phr 2phr
Rotational mouldings are sometimes surface-finished by spraying with a lacquer to provide a decorative, tack-reducing finish which also improves resistance to soiling. The lacquers used are the same as those applied, for similar purposes, to PVC coatings on fabrics (see Section 22.2.6). 22.2.2 Slush Moulding This technique may be regarded as the precursor or rotational casting, to which it has yielded considerable ground over the years. In comparison with rotational casting, slush moulding is more labour-intensive, whilst repeated handling of the same paste, which also experiences warming in the process (see below), can cause aeration and adversely affect the rheological stability. These and other points of comparison will be apparent from the following brief ol,ltline of the process. The principle of slush moulding is that the paste is poured into a lightweight, open mould, and then poured out leaving a layer on the inside surface of the mould. The thickness of the retained layer may be governed entirely by paste rheology, or it may be increased by heating to effect a degree of gelation. In either case the gelation and fusion of the retained material (which becomes the hollow slush moulding) is completed by a final heat treatment. The mould is cooled sufficiently to enable the moulding to be removed without damage or permanent distortion, but not to a temperature so low that removal may become
* An AGS ester 'low temperature' plasticiser (Ciba-Geigy).
t Epoxidised soyabean oil (Rohm and Haas).
:j: A modified, sulphur-free tin carboxylate stabiliser (Ciba-Geigy).
22
Preparation, Processing and Applications of Pastes
989
difficult: in practice a temperature of about 35°C will usually be suitable. The moulding is most often blown out of the mould by a jet of compressed air. With shapes which make removal more difficult, vacuum can be applied to the opening of the mould to collapse the moudling first. The main practical variants of the basic method are outlined below. In one, sometimes referred to as the 'one-pour' method, the moulds are filled with paste and passed on a conveyor through an oven or heating bath in such a way that only the body of each mould is heated (not the paste surface exposed in the open top). The object is to gel the desired thickness of paste adjacent to the mould wall, and the heating time (dwell period) and temperature will be adjusted to this end in accordance with the gelling properties of the paste and the relevant thermal characteristics of the mould. After the heat treatment the moulds are emptied of the ungelled paste (this may be by inversion on the conveyor, so that the paste runs into a catchment trough underneath) and conveyed to a second heating station-normally an oven-operated at a temperature required to complete, in the course of a convenient dwell period, the gelation and fusion of the material remaining in the mould. The arrangement should be such that both the mould and the material inside it (accessible to heat through the mould opening) are uniformly and effectively heated: slow rotation of the moulds may assist in this, but should not be necessary with an efficient heating set-up. The temperatures of treatment will vary considerably depending on the paste composition and conditions, but will normally be within the range 17D-230°C. After leaving the oven the moulds are cooled (by water or air) in a cooling chamber to a temperature most appropriate to easy removal of the mouldings (see above), which is then effected. In another variant the moulds are pre-heated at around 170°C for a time depending on their size and thickness. They are then filled with paste and allowed to stand (this may be on a moving conveyor) for 1-2 min. After this the moulds are inverted so that the paste, other than that of the layer gelled on the mould walls, drains out. The gelled layer, which may be up to 3-4 mm thick, is then fused by heating the moulds in a fusion oven. Cooling and de-moulding follow as before. A method combining elements of both above variants, known as the 'two-pour' method, is also used. In this, cold moulds are filled with paste, and inverted immediately after filling so that whilst some paste remains on the interior wall surface the layer is thin. The draining may
990
W. V. Titow
take place on a conveyor, into a catchment trough, as described previously. The moulds are placed in (passed through) an oven where the paste layer is pre-gelled. In practice this treatment is short (a few minutes) at a temperature of 17~220°C. On leaving the oven, the moulds are re-filled with paste and then emptied almost directly, but-because they are hot-a paste layer of fairly substantial thickness is gelled onto the previous, thin coating. The actual thickness of the second layer will be determined by the characteristics of the paste, its residence time in the hot mould and the temperature of the mould. The combined paste layer inside each mould is then fused by heating in a second oven, and the moulds cooled down and emptied as before. In slush moulding, moderate variation in various paste and process factors is not absolutely critical, but once suitable conditions have been established they should be adhered to as closely as possible. Attention must also be paid to the condition of the paste which is being re-used, particularly with regard to air bubbles and adventitious contamination that may have been introduced, and any gelled particles, as well as to the prevention of any water from the cooling process entering the paste (as this can cause blistering in the mouldings). The re-circulated paste should be passed through a fine-mesh screen on its way into the holding tank, where it should be de-aerated, preferably under vacuum. The following formulation exemplifies a basic composition for the production of PVC overboots by slush moulding. PVC resins: Corvic P65/54 Corvic D55/3
Plasticisers: DIOP Pliabrac 810
Stabiliser: Colourant:
Cd/Ba liquid system Blue Masterbatch C
70pbw 30pbw 55phr 15phr 3phr 0·75 phr
Blue Masterbatch C
PVC resin: Plasticiser: Stabiliser: Colourants:
Corvic P65/54
DNP Calcium stearate Reckitt's: ultramarine blue M 6925 blue 8651
100pbw 60phr 4phr 1 phr 2phr
22 Preparation, Processing and Applications of Pastes
991
22.2.3 Paste Casting This is a relatively straightforward technique, employing comparatively simple moulds; however a large number are needed for mass operation. The main types of product manufactured by paste casting are shock-absorbent pads moulded onto the ends of air and oil filters for vehicle engines, printing rollers, and various kinds of mats (table mats, doilies, anti-slip under-mats for rugs). The end-pads for filters are made in open dish moulds, by pouring the required amount of paste into the mould in which the filter is standing on one of its ends, gelling and fusing the paste around the filter 'insert' by placing the mould with its contents on a hot plate or in an oven, and finally cooling and removing the filter with the moulded-on pad. The whole operation is then re-run to mould a pad onto the other end of the filter. In the production of printing rollers, a metal core is first coated with a thin priming layer (typically an acrylic and/or epoxy resin composition) to promote positive interfacial adhesion to the ultimate paste-produced pPVC roller body. The core is placed axially in a cylindrical mould, the paste is poured in, and the assembly transferred to an oven where the paste is gelled and fused, and then removed for cooling and de-moulding. The heating conditions must ensure complete fusion throughout the PVC body of the roller, to develop maximum structural uniformity and durability: these, together with the correct degree of softness and resilience (which, for properly fused material, are determined by the paste formulation), are important in this application. The roller (in particular the ends) may require final machining for correct size: this may necessitate freezing the PVC if it is too soft. Like the filter mounting pads, mats and like products are produced by open casting, usually into a shallow dish mould. Excess paste may be removed by passing a doctor blade across the top of the mould, and the moulds subjected to a vacuum if the filling operation has introduced air bubbles. Gelation and fusion are carried out in an oven (or sometimes on a suitable hotplate or heated conveyor) as for the filter mounting pads. In the heating operation, a longer treatment at a lower temperature (providing that this is not below the minimum temperature for complete fusion) will normally be preferable to a shorter period at a higher temperature, to avoid under-fusing the inside with possible overheating of the outside of the products. PVC films are cast or spread from paste onto a suitable base (e.g. release
992
W. V. Titow
paper) in fabric coating operations: this application is discussed in Section 22.2.6.
22.2.4 Dip Coating and Moulding (a) Hot-dip Coating This is a useful method of producing ppve coatings on metal objects suitable for dipping in paste. In essence, the article to be coated is heated, dipped into the paste, allowed to drain for an appropriate time, and heated to complete the gelation and fusion of the coating layer. Articles of thickness substantially below 3 mm (e.g. thin wire) will not normally be suitable for treatment by this process, because low heat capacity hampers satisfactory fusion. For similar reasons it is difficult to obtain a good coating on sharp edges. The two essential items of equipment are an oven (with air circulation or other means of ensuring maintenance of uniform temperature distribution) and a dipping tank equipped with a slow-speed stirring arrangement. Oven temperature of 175-200o e will usually be satisfactory. Stirring the contents of the tank, whilst necessary to maintain general paste uniformity, cannot prevent some lumping and viscosity rise brought on by the continual dipping of hot objects: the state of the paste must therefore be monitored. The objects to be coated must be thoroughly cleaned and degreased before dipping. This gives some adhesion but of relatively low degree: if positive interfacial bonding is required the surface of the object must be primed by application of a priming coat of the kind mentioned in Section 22.2.3, deposited from solution in a solvent. Two commercial examples are Deckor Primer (Scott Bader, UK) and Vinatex Adhesive MP7A (Vinatex Ltd, UK). The primed article to be coated is first heated in an oven to a temperature between 900 e and 130oe: depending on the thickness this may take 5-10 min. It is then dipped in the paste and left long enough to build-up the desired (or maximum practicable) thickness of coating: the time required for this is normally quite short-typically 1-2 min. On wire, coatings 0·5-0·8 mm can usually be obtained in one dip. The temperature of the paste may be between about 17°e and 300 e depending on the composition. The speeds of entry into the paste and of withdrawal are factors in the process. If the object is withdrawn too quickly a poor coating will
22 Preparation, Processing and Applications of Pastes
993
result-ideally the rate should be the same as that at which the paste drains off. Not much heat should be left in the object: the final fusion is effected in a separate step. 'Dip marks' (paste drips) usually formed at the bottom of the article may be brushed off before the heat treatment for fusion. This will be appropriate to the paste composition, coating thickness and other relevant process factors. An under-fused coating which may appear matt and be weak or even crumbly, may be returned to the oven for further treatment, providing that the fault is not due to the oven temperature being generally below the minimum necessary for complete fusion. Articles PVC-coated by the hot-dip process include fence posts and fittings, handles, thick wire baskets and trays, mounting brackets and the like. (b) Hot-dip Moulding This process is closely similar to hot-dip coating, except that the object being dipped is a suitably shaped metal former (which may be treated with a release agent), and the coating becomes a hollow moulding when it is stripped from the former after fusion and cooling. Two procedural variants are in use. In one, the formers are pre-heated in an oven and dipped into the paste in the dipping tank at a controlled rate: this is usually quite fast (say about 10 s to submerge the former to the required depth) to avoid differential cooling with consequent uneven paste layer build-up. The immersed formers are left for the time necessary for the coating layer thickness required: this residence period may be quite short. They are then withdrawn at a uniform rate (normally slower than the dipping rate) so regulated as to avoid excess paste being dragged out of the bath through viscosity effects. Once out of the bath, the formers are inverted to let any drips merge back into the coating layers, and transferred to a second oven to fuse the coatings. This treatment is followed by cooling (which may be forced), stripping, and any trimming operations that may be necessary. The second variant involves the same dipping and withdrawal procedures, but the pre-heating of the formers and gelation/fusion of the paste coatings picked up are both done by immersing the formers in a tank of a liquid heating medium. More effective heating for shorter periods (e.g. completion of gelation and fusion about 2-3 min in many cases) are claimed as the main advantages over the method involving oven heating, with consequent faster cycles and reduced
994
W. V. Titow
tendency to the formation of 'drips' and 'runs' of paste on the articles directly before and in the early stages of the gelation/fusion treatment. Disposable, unsupported PVC gloves (typically produced from a basic plastisol composition containing about 100 phr DOP), tool and handlebar grips, golf club covers, and covers for cable terminals, are examples of mouldings produced by the hot-dip process.
(c) Cold-dip Coating In this process the object undergoing dipping is cold. The advantage of this is that the viscosity and general condition of the paste in the dipping tank remain stable, and there is no accumulation of partly gelled lumps and particles. Although on some metal objects the finish obtainable may not be as good as that from hot dipping, cold dipping can be useful in some cases where the object is of irregular thickness: in hot dipping the thicker parts, having greater heat capacity, will tend to build up a thicker coat. Cold-dip coating is important as the method of production of fabric-lined PVC work gloves. These are made by drawing knitted fabric gloves (the 'liners' for the ultimate composite articles) onto hand-shaped formers (usually metal, but sometimes also ceramic), cold dipping the liners on their formers fingers-downwards into the PVC paste, withdrawing, allowing to drain, inverting to let any drip marks at the ends of fingers and thumbs flow back into the coating, and then gelling and fusing the paste layer by passage through an oven under suitable conditions of oven temperature and dwell time. Operation in modern plants is continuous and highly automated. The thickness of PVC coating and degree of its penetration into the fabric of the liner are influenced by the fabric's construction, the rheological properties of the paste, the rates of dipping and withdrawal of the formers, the length of the draining period, and the gelation/fusion conditions. An appreciable degree of penetration is desirable for good union between coating and liner, but a layer of free fabric should remain on the inside of the glove to fulfil the moisture absorption and cushioning functions important to the wearer's comfort. For these reasons any extensive 'strike through' of the paste to the inside of the glove is a fault. A glove-dipping plant is shown in Plate P. Other fibrous products cold-dip coated with PVC paste, in which a degree of mechanical keying through partial penetration of the coating into the surface of the substrate contributes to the strength of union
22
Preparation, Processing and Applications of Pastes
995
Plate P Fully automatic dipping plant for PVC-coated industrial gloves. (Comasec SA, Paris, France-Courtesy Mr A. Charnaud.)
between the two, are household clothes lines and some types of cords and ropes. Cold-dip coating of metal objects-where there is no surface anchorage effect of the kind afforded by a fibrous substrate-is generally more difficult to operate and control than the hot-dip process. A low-viscosity, non-dilatant paste may be used to build up the required coating thickness, by multiple dipping, from a number of thin layers, each pre-gelled before the application of the next one. In this kind of procedure enough heat must be applied at each gelling step to soften the previous coat sufficiently to ensure good merging with the one undergoing gelation. The combined layer is ultimately fused as a whole in a final heat treatment. In some cases a one-dip coat may be given with a viscous paste formulated to be strongly thixotropic at the relatively low shear rates involved in the dipping process: such a paste will 'set-up' in the form of a fairly thick coating, with very little draining, directly after withdrawal from the tank.
996
W. V. Titow
An example of a basic formulation for a low-viscosity, cold-dip paste is given below: PVC resin: Breon P 130/1 Plasticiser: DOP DOS Stabiliser: White lead paste
100pbw 65phr 16phr 2phr
22.2.5 Spray Coating The area of application of this method is similar to that of dip coating. It is, however, particularly useful for objects which are either too large to be easily manipulated in dipping, or of intricate shape. Plastisols for spraying should have low viscosity and be non-dilatant. A definite yield point for flow (Bingham body behaviour) is also desirable as it restricts flow after deposition, although it also makes levelling more difficult: this is, in any case, not normally as easy as with paints for spray application where the solvent vehicle promotes the necessary degree of mobility. Raising the temperature of application can assist the levelling of the paste coating by reducing its viscosity. The incorporation of a small proportion (say about 10 phr) of solvent (e.g. white spirit) can also be helpful in this connection. However, if too much solvent is used a two-stage heat treatment may be necessary, to remove the solvent and then to gel and fuse the coating. Limiting the proportion of solvent added to well below the level usual for a true organosol also enables a thicker coating to be achieved in one operation: as an example, say 0·25-1·25 mm depending on the paste formulation and application conditions, as against 0·03-0·06 mm with some true organosols. For thick organosol-applied coatings, multiple application may be necessary, with solvent evaporation after the deposition of each layer to avoid blistering or orange-peel effects in the final product. The spraying method and equipment may be of the air-spray or airless variety. In the former, the spray gun used should be of the external mix type. A commercial example is the De Vilbiss JGA gun fitted with a No. 306 air cap and size D fluid tip, and connected to a pressure feed container for the paste. Pressure feed is normally more efficient than gravity feed, although the latter type can be used satisfactorily in relatively small-scale operations for spraying organosols of suitably low viscosity. A typical capacity range for pressure feed containers would be 0·5-40 gal. The airless method uses a high hydraulic pressure, in the region of
22
Preparation, Processing and Applications of Pastes
997
2000 lbf in- z, to force the paste through a small spray orifice (typically O·OB-in bore) and thereby obtain the right atomisation. Electrostatic spraying of PVC paste may be carried out by either of the above two general methods, with appropriate arrangements for charge generation. This mode of operation, which is particularly useful with metal objects, can offer economies in paste consumption and coating time: uniformity of coating of intricately shaped surfaces is another advantage of a properly run electrostatic spray-coating process. Problems which may be experienced in spray coating with pastes are generally similar to those encountered in paint spraying. Some of the more common ones are: a 'pebble' finish caused by excessively high line pressure or the gun being held too far from the work; runs which may form if the gun is too close or the paste too fluid; wrinkles or sags on vertical surfaces where the coating has been applied too thickly. After spraying, the coating is gelled and fused by a heat treatment. This may be preceded by drying if an organosol has been used. The drying temperature should be kept to a reasonable minimum to prevent bubbling by the departing solvent, and to keep down the polymer's heat history. The gelation/fusion of coatings on metal substrates may take about 5-10 min at an oven temperature of around 180aC. Higher temperatures may be used to shorten the time, but caution is necessary, as hot spots can develop (especially in a large oven) which can lead to local overheating. The following are two examples of basic formulations for spraycoating pastes: Plastisol PVC resin: Plasticisers:
Stabiliser:
Corvic P 65/54A DAP DNP Paraplex G62 Stanclere80(AKZO, Chemie, Sarl, France)
100pbw 40phr 50phr 2phr 0·75 phr
Breon P 130/1 DOA Mellite131 (Albright and Wilson Ltd, UK) MIBK white spirit TiO z paste (1 : 1 in DOP)
100pbw 25phr 2phr 15phr 15 phr 5phr
Organosol
PVC resin: Plasticiser: Stabiliser: Solvents (diluents) : Pigment paste:
998
W. V. Titow
22.2.6 Coating of Sheet Materials (Fabrics and Paper) PVC pastes find a very important commercial outlet in the coating of fabrics and papers in the manufacture of such products as tarpaulins, tent fabrics, awnings, upholstery materials, wall-coverings, bookbinding fabrics and papers, leathercloth for travel and fancy goods and garments (protective and fashion), floor coverings, conveyor and drive belts, and adhesive tapes. The coating procedures fall into two general groups-direct coating and transfer coating (also known as reverse coating*). In procedures of the first kind the paste is applied directly to the substrate to be coated (textile or non-woven fabric, paper), whilst the essential features of a transfer method are that the coating is first deposited on a carrier material with an easy-release surface, the substrate proper is laminated to the deposited layer (which may be composite), and the carrier removed. Extrusion coating, and lamination to a preformed PVC film, are alternatives to paste coating as methods of manufacture of some of the above mentioned products (as well as others-e.g. decorative surface coverings such as imitation veneers and the like). In the film lamination method, a calendered film may be laminated to the appropriate substrate in-line, i.e. as the hot material leaves the calender (see Chapter 18). A finished film (manufactured by calendering or extrusion) can also be applied to a substrate in an entirely separate operation. In this case the film will be heated or treated with an adhesive for bonding to the substrate: the latter may itself be carrying a tie coat to facilitate bonding, or a coating (say a foamable layer) directly applied, so that the operation becomes a combination of direct coating and film lamination techniques (see also further on in this section). The common basic arrangements for applying a coat of PVC paste to a continuous sheet material are schematically illustrated in Figs 22.2 and 22.4. Their practical embodiments form the paste-application sections of various industrial coating units and lines. In the laboratory, paste layers can be hand-cast with a simple film-coating frame, with a fixed or adjustable gap under the spreading edge. * Not to be confused with reverse roller coating which is one of the ways whereby paste is applied to a sheet material by means of an arrangement of rollers-see Fig. 22.4 and the associated text.
22
Preparation, Processing and Applications of Pastes
999
The suitability of each basic type of industrial coating arrangement in a particular situation will depend on several mutually interacting factors, especially the nature of the substrate (with particular reference to its permeability, and extensibility under the tension experienced in processing), paste rheology, thickness of coating and degree of penetration required. Note: In some cases (e.g. certain tarpaulins and ground sheets, conveyor belts, some types of rainwear and protectivegarment fabrics) complete impregnation of the substrate may be required. With most true coatings on fabrics a depth of penetration of 1/3-1/2 of the fabric thickness will normally be aimed at to combine good 'keying' of the coating with the preservation of satisfactory flexibility and 'handle' of the finished product. For a given application system and paste rheology, the thickness and degree of penetration of the coat (and, to some extent, the rate of coating) are influenced by the size and configuration of the gap between the substrate and the coating element (doctor blade or roller) as governed by the setting and (especially with a doctor knife) the profile of the latter, the tension applied to the substrate, and the nature and positioning of any support under the substrate at the coating point. Some of the ways in which the factors just mentioned will affect the casting operation and its results are fairly obvious. Thus a thick, heavily filled, non-thixotropic paste will tend to produce thick coatings with relatively limited penetration, and will make for comparatively slow rates of application. A light substrate with an open structure and substantial extensibility will be more prone to penetration by a given paste in a given coating process than a dense, heavy, stiff one. A doctor knife with large radius of the leading edge, set at an angle to the substrate, will tend to produce a heavier coating than a vertically set, finely radiused blade. Such general considerations provide useful broad guidelines, and a few more are mentioned below among further comments on the knife and roller coating arrangements. However, the individual factors do not operate in isolation: they invariably interact, and the effects of the interaction must be taken into account (and the results confirmed by practical trials) in any given case.
1000
w. v.
Titow
(a) Paste Coating (Spreading) by Doctor Knife The basic variants of this method are represented schematically in Fig. 22.2. In coating with a doctor knife the blade is set over the substrate and the paste is poured or pumped from a reservoir so that it forms a constantly replenished uniform 'roll' or 'coil' in front of the blade across the width of the substrate 'web'. Depending on the properties of the latter, and the coating characteristics required, the consistency of the paste may range from that of a fairly free-flowing liquid to a thick dough: the knife blade profile and setting (i.e. whether vertical or inclined), and the effective spreading gap will also be chosen accordingly. Some typical profiles are illustrated in Fig. 22.3, with an indication of their applications. The coating speed is subject to the factors just mentioned: depending on the conditions, the speeds for a fairly representative range of operations may vary between about 10 and 100 it min-I. In the knife-over-roller and knife-over-plate arrangements, the support roller may be of steel or rubber, and the plate is usually steel.
Fig. 22.2 Knife coating (spreading) arrangements: schematic representation. A, Knife over roller; B, air knife; C, knife over plate; D, knife over blanket.
B
c
D
l. I E
(
Fabrics: over rubber roll or blanket
Fabrics: as D
D
F
C
B
Fabrics: as air knife, or over rubber roll or blanket Heavy fabric: over blanket Paper: over steel roll Paper: over metal roll
A
Substrate, and coating arrangement
Typical uses
May be angled (up to about 4°) AsD
Vertical, or angled up to 3° Usually vertical
Vertical
Usual knife setting
Heavy (suitable for thick, heavy pastes)
Suitable for range of coating weights (with range of paste viscosities) Heavy and/or penetrating
Thin (including antiqueing on embosssee text) Thin (especially priming or tie coats)
Thickness and/or weight of coating
Good surface finish; typical products: floor coverings, brattice cloth
Appropriate for highspeed coating; good surface finish
Remarks
Fig. 22.3 Some common doctor blade profiles: schematic representation. Substrate movement from left to right. Other factors being equal the amount of coating will increase with increasing radius of the leading edge of the blade.
A
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W. V. Titow
In both these arrangements there is a fairly direct relationship between the setting of the doctor blade and the coating thickness: this is a help in setting up and controlling a coating operation. With a knife-overblanket system the situation is more complicated, as the tension of the rubber belt and the speed of its movement also have a considerable effect. Similar considerations apply in connection with the tension in the fabric and its rate of advance in the air knife arrangement where, however, absence of support under the knife creates further complications: the tension generated in the fabric by the pressure of the knife is comparatively high, and more variable than with the knife-overblanket system. With relatively light and extensible fabrics this can lead to excessive weight and/or penetration of coating, as well as variability in these respects. Hence, in practice the arrangement is best suited to the coating of fairly substantial substrates of reasonable strength and low extensibility, with pastes of medium or high viscosity which are not greatly thixotropic or pseudoplastic. The general effects of incorrect selection or setting in the matter of such basic process parameters as knife profile and positioning, fabric tension and rate of advance, and paste rheology, are broadly predictable; and conversely, their manifestations should be fairly readily traceable back to these causes. Faults originating in this way will most commonly take the form of deviations from the desired weight per unit area (or thickness) of coating and/or the extent of its penetration into the substrate. The magnitude of such defects in a given case will depend on the degree of departure from the optimum processing arrangements and the relative importance-in the paticular operation-of the factor(s) responsible. Thus, for example, a highly dilatant paste may (especially with an unsuitable knife profile and setting) flow so poorly that areas of the substrate will remain uncoated (a fault sometimes known as 'missing'). Other faults, whose origins in particular cases may not be attributable to basic process parameters, are also encountered. Those usually known as 'streaking' and 'blobbing' are possibly the most characteristic. Streaking takes the form of lines or streaks caused by the drag of particles under the doctor blade. The particles may be present for various reasons: they may be relatively coarse particles (or aggregates) of pigment or filler (or even polymer) persisting from the original preparation of the paste; they may be polymer agglomerates formed in the high-shear region under the blade (especially in high-viscosity pastes at excessive spreading rates); in some composi-
22
Preparation, Processing and Applications of Pastes
1003
tions the compatibility of constituents may be affected by the high-shear conditions under the blade to the point where separation and agglomeration can occur-this particular problem can be prevented by suitable formulation in the light of relevant experience. Otherwise the first practical remedy to be considered when streaking occurs is milling the paste on an efficient mill. In general, use of good quality components (especially polymer(s), pigment(s) and filler(s» and proper compounding of the paste to ensure thorough dispersion and homogenisation are purposeful preventive measures. Blobbing is the presence of blobs on or in the otherwise smooth and level surface of the coating. The immediate cause of this fault is local build-up of paste at the back of the doctor blade and periodic dropping off of the accumulations, but the reasons for the build-up are not entirely clear: it is believed to be associated in some way with the high pressure experienced by the paste under the blade, but the differences between conditions, respectively normal (i.e. resulting in fault-free operation) and abnormal in this regard have not been fully elucidated. In some cases too-high paste viscosity and/or insufficient thixotropy are definitely instrumental in the origin of the fault, in that it may be remedied by appropriate adjustments of paste rheology: milling the paste or changing the profile of the doctor blade may also be helpful on occasion. Turbulence in the paste under the blade may occasionally arise, and cause air trapping. Two other well-known faults, which can occur with any coating method, are pinholing and blistering. A common cause is excessive evaporation of a volatile substance when heat is applied to the paste-coated material in processing to gel/fuse the coating, causing blowing of expandable layers, etc. The culprit may be moisture present in the substrate, or originally present in a formulation constituent (e.g. polymer, filler) and introduced with it into the paste, or (less commonly) acquired by the paste during or after preparation. It may also be a volatile impurity or low-molecular-weight residue in a paste component. In all such cases the fault is frequently batch-related. In organosols, pinholing or blistering can sometimes be caused by untimely or too rapid evaporation of solvent. Occasionally the appearance of pinholes may not be traceable to any of the above factors. In such cases, and if the fault is not of catastrophic proportions, it may be helpful to use two coating stations in series (Le. apply the coating in two consecutive layers) so that the number of pinholes penetrating the total coating thickness is reduced in
1004
W. V. Titow
proportion to the chance that a pinhole in the first layer will coincide with one in the second.
(b) Paste Coating by Roller In roller coating, a uniform layer of paste is first formed on the surface of a roller (the transfer roll) whence it is transferred to the substrate as the latter passes through the nip of the transfer roll and a backing roll (commonly of rubber, or rubber-sleeved). In the simplest practical arrangement (not much used in PVC paste coating) the transfer roller picks up the coating material directly from a trough in which it is partly immersed, excess is removed by a suitably positioned doctor knife, and the resulting fairly uniform layer then transferred to the substrate. The most popular roller sytem for applying PVC paste to continuous sheet substrates is known as reverse roller coating. Its main advantage is good control of the weight (and thickness) and uniformity of coating applied, by virtue of the fact that a layer of the coating material is accurately pre-formed before transfer to the substrate: the amount actually transferred can also be further adjusted by the setting of the nip between the transfer and backing rollers (see Fig. 22.4). The nature of the substrate is also less critical than in knife spreading as a factor in the process and the characteristics of the resultant coating. A nip-fed version of a reverse roller coating station is schematically shown in Fig. 22.4.
Fig. 22.4 A reverse-roller coating arrangement (nip feed): schematic representation; only basic components shown.
22
Preparation, Processing and Applications of Pastes
1005
In modern roller-coating equipment additional control over the thickness uniformity of coating across the width of the substrate is afforded by provision for roll crossing as a means of adjustments in this regard. Such means of controlling the profile of a plastics sheet material undergoing processing by rollers are a feature of some calenders, and are discussed in Chapter 18. (c) Direct-coating Process A typical basic direct-coating unit will comprise a paste application section (of either knife- or roller-coating type) and a gelation/fusion oven. This assembly, with appropriate unwinding and wind-up arrangements for the substrate, and paste reservoir and feed, may constitute the entire set-up for siinple single-coat application. However, a modern coating line can comprise a number of coating units (including double-sided coaters in some cases, e.g. for the production of completely sealed or impregnated fabrics), with the appropriate gelling ovens and cooling stations (cooled drums or chambers). Embossing and over-lacquering units may sometimes be included in a line (with the coating being embossed directly after fusion, while still hot enough to take the impression), but such surface decoration and finishing is often done in a separate operation (see below). Tensioning control and drive systems for the substrate, incorporating unwinding, compensating, rewinding and wind-up gear, are typical features of good industrial equipment, as are measuring devices (e.g. ~ray gauges) for the monitoring of coating thickness and its running adjustment via a link-up to the coating station(s). Substrate widths of over 1 m (e.g. for wall-coverings or heavy-duty fabrics) to about 4 m (e.g. for floor coverings) are processed. Coating speeds, say with some wall-coverings, may be over 600 ft min-I. Oven temperatures can, and should, be controlled closely with regard to both level and distribution, for uniform gelation/fusion throughout the coating and across its full width. Temperature zoning should be available to regulate the temperature rise in the coating (and the operation of the blowing agent in expanded coatings) for best results. With organosols, pre-heating in a relatively low temperature zone is practised to prevent skinning of the paste layer which can lead to blistering as the temporarily trapped volatiles eventually escape. Some ovens are provided with integrated exhaust incineration to prevent air pollution, and with heat recovery systems.
1006
W. V. Titow
(d) Transfer(Reverse )-coating Process
The modern versions of this process are capable of close control, producing good-quality uniform, multi-layer coatings. The process is also particularly useful where such coating is required on a weak or easily distorted substrate (e.g. a relatively light knitted fabric). The principle of the transfer-coating method is to produce-by a direct application method-a layer (which may be composite) of paste material(s) on the smooth surface of a continuous support (which may incorporate or carry a release agent), laminate the substrate to be coated to the paste layer, and remove the laminate. The temporary support may be a stainless steel band or, very commonly, a strong paper coated with a silicone release agent. An excellent surface finish which is usually obtained is an additional advantage in transfer coating. A procedure for the production of a typicalleathercloth9 ,10 (for, for example, fancy goods, upholstery or garments) provides an example of transfer coating where the finished product will be a fabric carrying a coating comprising an expanded (foamed) intermediate layer, and a 'skin' (wear-resistant layer) with or without a thin bonding layer between the foam layer and the fabric. Note: The three-layer construction is also common for fabric-based
PVC flooring (normally produced by reverse-roller coating, not the transfer method).
In this kind of operation, the paste formulated for the skin layer is spread ('cast') on the surface of strong kraft paper treated with a silicone release agent. The layer may be gelled before the next operation, or it may be left ungelled. The next step is to cast a second layer of suitable foamable paste composition onto the skin layer: if the latter has been gelled in the meantime, this second step will constitute a 'wet on dry' application; if not, then the application will be 'wet on wet'. The second coat (or both if the application has been wet on wet) is then usually pre-gelled at a temperature low enough not to cause foaming or excessive hardening. The base fabric is applied to the 'free' surface of the second coat and fixed in place by a heat treatment which will gel and fuse the PVC and produce expansion of the intermediate layer (by activating the blowing agent-see Chapter 25). In another variant, gelation/fusion and blowing may be accomplished before the application of the fabric, and a third thin bonding coat of paste
(without blowing agent) may then be applied and the fabric laminated to this to be finally fixed by heating. In either case the material is then
22
Preparation, Processing and Applications of Pastes
1007
stripped from the paper (with the original first PVC/paper interface becoming the top surface of the coated fabric). Where only simple, single-station coating equipment is available, the wet-on-dry procedure will be followed, each coating step being effected in a separate passage. In modern large-scale coating practice the whole process can be carried out on a single line with multiple coating, heating and cooling stations, and usually incorporating the facilities and refinements mentioned above in connection with direct coating. The final stripping of the substrate/coating laminate from the carrier may also be carried out in-line, or on a separate machine. In the latter mode of operation faster coating speeds may be possible (say up to about 500 ft min-I) as stripping is, in general, slower than coating. Cellular leathercloth may also be produced by a variant of transfer coating involving a film lamination step. In this method the foamable paste is first cast onto the carrier (release paper) and the fabric applied to this layer (which may be pre-gelled). The laminate is passed through an oven to foam and fuse the PVC, then through a cooling station, and the carrier is stripped off. Next a thin layer of paste is applied to the coating's surface; this paste is formulated (often on the basis of a vinyl chloride copolymer) to fuse at a relatively low temperature at which the foamed layer will not collapse. A thin calendered film, formulated to function as the skin layer of the composite coating, is then laminated to the assembly and fixed on by a heat treatment which fuses the thin adhesive layer. (e) Promotion of Adhesion between Coating and Substrate The use for this purpose of a tie (bonding) or priming coat has already been mentioned. Such coats may be applied to a substrate (fabric, paper, metal) to be subsequently coated by a direct or transfer method, or to the 'free' surface of a transfer coat deposited (if composite, assembled) on a carrier prior to the final lamination to the substrate, or to a film in the film-lamination coating method. In some cases, especially with direct coating, a priming coat can serve as both a bonding layer and a barrier restricting penetration into a porous substrate of the material of the main coat. Such priming costs on paper or fabric substrates may be thin latex-deposited PVC layers (see Chapter 23.) Certain adhesion promoters may be incorporated in the coating paste. For instance, special liquid isocyanurates (e.g. Vulcabond
1008
W. V. Titow
VP-ICI) have been used as such additives in PVC pastes for coating nylon and polyester fabrics in the production of tarpaulin and protective clothing materials.
(f) Surface Decoration and Finishing of pvc Paste Coatings The surface of many products manufactured by paste coating of continuous substrates is structured by embossing for such decorative effects as, for example, grain and leathercloth, or various raised patterns on wall-coverings (so-called 'chemical emboss' achieved by selective control of foaming is discussed in Chapter 25). Sometimes the surface is printed (commonly by a gravure printing process)l1 either as the only decorative treatment or prior to embossing. The printing and embossing may be carried out as a separate operation after coating. With some products (e.g. flooring, decorative surfacing materials) the intermediate layer of the composite coating may be printed, so that the print is protected by a clear skin layer which is applied after printing. A thin top coat of a lacquer is commonly applied to paste-produced PVC coatings, especially those of high plasticiser contents (say roughly above 50 phr). The lacquer layer may have (or combine) any of the following functions: prevention of plasticiser migration (and the resultant surface tackiness), increasing resistance to abrasion and soiling, imparting or reducing surface gloss, modifying surface colour, imparting special decorative effects (especially in conjunction with embossing-see below). Many commercial lacquers are dilute solvent solutions of acrylic resins often in conjunction with a PVC copolymer, e.g. some of the Deckor lacquers (Scott Bader, UK), or Argutop LG (Argus Chemical Corp., USA; Argus Chemical GmbH, West Germany). Some processors make up their own lacquers: a fairly representative example of a basic formulation would be: *
Copolymer (VCNA) resins: Breon 202 Breon 425 Polymethyl methacrylate resin: Solvents: toluene methyl ethyl ketone
2pbw 3pbw 5pbw 40pbw 40pbw
Heat and light stabilisers may be incorporated to protect the resin (especially the PVC) components of the lacquer itself and also-where required-the PVC coating underneath. In the latter case, say where * See also Chapter 24, Section 24.4.
22
Preparation, Processing and Applications of Pastes
1009
exposure to sunlight will occur in service, the lacquer may serve, inter alia, as a vehicle for an extra light stabiliser interposed in this way between the incident radiation and the PVC coating. Colourants are also common additives. Matting agents are incorporated in lacquers for matt finishes. The resin components of a lacquer may also be (or include) polymers other than those already mentioned, such as polyurethanes (e.g. Argutop LV), polyamide (e.g. Argutop LN 1060-a high gloss commercial lacquer), or cellulose acetate butyrate (for surface slip and anti-blocking). Keying solvents (e.g. cyclohexanone, isophorone, tetrahydrofuran) may be included in the lacquer formulation for better ultimate adhesion to the PVC surface, but care must be taken that their presence does not disfigure the emboss during heat treatment. Whilst the lacquers have traditionally been solvent-based, aqueous polyurethane and acrylate systems (colloidal dispersions) have also made their appearance among the commercial products. Two examples are Neo-Rez R-900 (Polyvinyl Chemical Industries Inc., USA) and Laqua WB 240 (Bee Chemical Co., USA). Lacquers are usually applied by an engraved (gravure) roller or doctor knife. Special effects may be achieved on embossed surfaces if the colour of the lacquer is different from that of the coating. Thus, most emboss patterns may be 'topped' by applying a coloured lacquer by roller in light contact with the surface so that only the protruding portions are covered and thereby made to contrast with the depressions. The speed of passage of the coated, embossed material under the roll, and its presentation to the roll surface, can be arranged to deposit the lacquer predominantly on the 'slopes' to one side of the tops of the raised portions of the emboss. This effect is known as 'shading'. With suitable heavy emboss patterns a coloured lacquer can also be applied (usually by a doctor blade suitably profiled and set-ef. the caption of Figure 22.3) so that it is deposited in the depressions, whilst the tops of the raised elements of the embossed pattern are wiped substantially clean and thus retain their original colour. This effect, produced on some leathercloths used for decorative purposes, is referred to as 'antiqueing' or 'antique finish'. (g) Testing of Coated Materials Completeness of fusion of coating may be determined by a solvent test. In some cases a test of coating-strength properties may be practicable (e.g. a bursting-strength test on the coated material, or a tensile
1010
W. V. Titow
strength and elongation test on the coating removed from the substrate with the aid of a leather-splitting machine). Most of the other tests and determinations commonly carried out on coated materials, including conveyor belting, will be found in the standard specifications listed in Section 7.1 of Appendix 1. The properties of most frequent interest include weight of coating per unit area, breaking strength (bursting strength with some coated knitted fabrics), resistance to flexural fatigue, adhesion of coating, abrasion resistance, tear strength, low-temperature properties of coating (cold-crack, cold-bend, etc.-see Chapter 12, Section 12.2), water and air permeability, volatile matter content (cf. also ISO 176-1976; BS 2782: 1970, Method 107F; AS 1441.8: 1973; Canadian Standard 4-GP-149: 1972), the effects of weathering and ageing, and various performance tests directly relevant to end-use. An early summary of some test techniques and equipment was published by Dalton,12 and a useful test for soiling resistance (carried out on an abrasion tester) by Kratschmann. 13 Note: Resistance of flooring materials to staining by contact with rubber (shoe soles, ferrules) or shoe polish, and by other agencies, is assessed in various ways, some used as standard tests, e.g. by plasticiser manufacturers. 14
Test data on some properties of PVC-coated nylon and polyester fabrics, with special reference to creep resistance, are contained in a paper by Blumberg et al. 15 22.2.7 Miscellaneous Paste Processing Methods of Minor Significance The methods mentioned in this section have never been of primary industrial importance. In some cases their interest has become more historical than current as a result of increasing applicational scope of modern solid flexible PVC compounds for injection moulding and extrusion.
(a) Low-pressure Injection Moulding Split moulds (usually of aluminium or chromium-plated steel), gated on the parting line (and clamped during the moulding and heating operations) may be employed, the paste being injected by air pressure (the simplest and cheapest way in most cases), ram, or screw. The greatest single application of the process has been in the moulding of
22 Preparation, Processing and Applications of Pastes
1011
soft PVC soles onto the uppers of certain types of cheap footwear. Fairly typical conditions would be: injection of paste under about 50 lbf in -2 pressure into a mould pre-heated to about 170°C and thereafter maintained at that temperature for the time required to complete the gelation/fusion of the moulding (normally a few minutes). Soft sealing gaskets for tin box lids have also been produced by low-pressure injection moulding. Direct injection of a paste into the gap of a joint between metal components (inter alia, in vehicle construction) to act as a caulking medium (after gelation/fusion by external application of heat) is another example of the application of this kind of process.
(b) Compression Moulding This is not a paste-processing method of any substantial practical significance. Whilst it has been used to make-in a controlled, reproducible way-moulded sheets serving as a source of specimens for evaluating and monitoring the properties of paste products, it is common practice to make sheets for the same purpose by casting (and fusing) a layer of paste on a metal plate (in some cases on a 'hot bench' or 'gel block'-see Chapter 21, Section 21.2.5). A typical compressionmoulded sheet of this kind would be a disc of a size and thickness appropriate to the dimensions and number of test specimens required, moulded in a flash mould. In outline, a suitable moulding procedure would comprise pre-heating the mould to about 80°C, filling with paste, closing, heating to about 170°C (or such higher temperature as may be necessary for complete fusion) and keeping at that temperature for about 10 min under a pressure of about 40 tons, then cooling under a somewhat higher pressure (say 45-50 tons) to counteract shrinkage. (c) Extrusion Extrusion of plastisols-with the extruder barrel, screw and die heated to effect gelation/fusion of the paste in the equipment-was explored relatively early in the development of paste processing, but it cannot be regarded as a technique significant in the present-day industrial context. The position is substantially the same with regard to the cold extrusion of plastigels which, by virtue of their consistency, can retain the shape imparted by the die without gelation, so that the extrudate can be gelled and fused subsequently, by passage through an oven or a hot liquid (e.g. oil, glycerol) bath.
1012
W. V. Titow
Note: The shape retention ability of a plastigel is the property utilised in the application of specially formulated pastes of this kind as modelling compounds. Some early applications of plastigels are mentioned in Ref. 5.
REFERENCES 1. Kling, A. (1951). Kunststoffe, 41(8),240-2. 2. Schimke, F. (1956). Kunststoffe, 46(11), 537-9. 3. Titow, W. V. (1958). Investigation into the validity of the claims of British Patent 624 795 (James North and Sons Ltd). Unpublished. 4. Anon. (1949). Plast. Ind., 7, 11. 5. Anon. (1951). Mod. Plast., 29,87. 6. US Patents 2629131,2629 134 and 2 681 472. 7. Meazey, A. C. (1959). Brit. Plast., 32,55. 8. European Plastics Buyer's Guide, IPC Business Press Ltd, London. Buyers Guide for Plastics Processing Machinery and Equipment, British Plastics Federation, (Machinery and Engineers Group), 5, Belgrave Square, London SWIX 8PH, UK. Plastics Manufacturing Handbook and Buyers' Guide: Plastics Technology Magazine, Rubber/Automotive Division of Hartman Communications Inc. (a subsidiary of Bill Communications Inc.), 633 Third Ave., NY, NY 10017, USA. 9. Meazey, A. C. (1968). Brit. Plast., 41, 133-40. 10. Schmidt, P., and Polte, A. (1967). Kunststoffe, 57(1),25. 11. Clayton, F. R. (1978). 'Printing techniques relating to the gravure process,' paper presented at the PRI International Conference on PVC Processing. Egham Hill, Surrey, England, 6-7, April, 1978. 12. Dalton, W. K. (1960). Plastics, 25(268), 71-81. 13. Kratschmann, F. (1960). Kunststoffe, 50(9), 534-5. 14. Ciba-Geigy Technical Service Bulletin PL 1.1.1, 1976. 15. Blumberg, H., Krummheuer, W. and Nebe, J. (1976). Kunststoffe, 66(2), 97-103.
CHAPTER 23
PVC Latices Revised and edited by W. V. TlTaw
23.1 INTRODUCTION In the previous edition the nature and properties of PVC latices were discussed in this chapter principally by reference to one commercial range of products of this type-the Breon vinyl latices. With the permission of the suppliers, BP Chemicals International Ltd, the text was based to a substantial extent on the appropriate part of their relevant publication (Technical Manual No.5). This was done because its technical scope and depth of treatment of the subject were considered to make that part of the manual useful as a good account of the technical and applicational aspects of PVC latices generally, even though actual data quoted by way of illustration did relate to members of the Breon range. These considerations, and the relevance of the basic technical information presented, remain valid irrespective of the changes consequent upon the take-over of BP's PVC production operations by ICI in the early 1980s. The chapter is, therefore, still essentially based on the former text, although some editorial changes and additions have been made. The original references to Breon latices have been retained in their former role of illustrative examples: whilst other good commercial PVC latices, e.g. those of the Geon (B. F. Goodrich) or Lutofan 200D (BASF) ranges, would provide illustrations more current in the purely commercial sense, it will be understood that corresponding grades of latices from different sources, and the finished products of their conversion, all differ among themselves, but none fundamentally. 1013
1014
W. V. Titow
As in the case of the Breon latices, references to other commercial materials (ingredients of latex compositions, and others) are by way of example only. In this connection it may be noted that not all material trade names or sources of supply mentioned may be current: specific up-to-date information on compounding ingredients for PVC latex compositions, and other relevant materials, can be obtained from manufacturers of PVC latices, or suppliers of the types of additive concerned. A latex may be defined as a stable dispersion of fine polymer particles in water, containing also some non-polymeric constituents (emulsifiers and others-see following sections of this chapter). * Of the three general types of liquid PVC composition-solution, paste and latex-the last two are both dispersion systems. However, they differ in several important respects, the most fundamental difference being the nature of the liquid dispersion medium-water in the latex, and plasticiser (or plasticiser/solvent mixture) in a paste. This difference is a cardinal factor in the respective processing and uses of the two types of system. Although in both cases the ultimate product is a solid PVC material, with a latex this is obtained by evaporation of the dispersion medium, whilst a paste undergoes a phase inversion without substantial loss of plasticiser which remains as an important component of the final product.· In broad terms, the polymer contents of latices and pastes are comparable, but latices have lower viscosities which may be varied by dilution; their compounding versatility is also greater in some respects. In comparison with solutions, PVC latices have much higher solids contents in the viscosity range employed in processing. Unlike that of a solution, the viscosity of a latex is independent of the molecular weight of the polymer component (so that higher molecular weights may be used for better mechanical properties of the final product), whilst its liquid component is relatively cheap and innocuous. However, generally speaking, latices are less stable and more variable in processing behaviour than solutions, with greater variability in the ease of wetting of some substrates. PVC latices are produced by emulsion polymerisation. The particle size of the solid (polymer) phase is typically around 0·1-0·25 ,um. The total amount of PVC processed in the form of latices is relatively small: the bulk is worked-for the major outlets-by the * The ISO definition is 'a colloidal, aqueous dispersion of a polymeric material' (ISO 472-1979).
23
1015
PVC Latices
main processes (extrusion, calendering and moulding of solid compositions; paste coating). All industrially significant uses of PVC latices involve their application to some kind of substrate, whereupon a deposit of solid polymer is ultimately formed after the water has been removed by evaporation (with or without some intermediate absorption into the substrate). The general mechanism of deposit formation is virtually the same for all polymer latices 1,2 (including PVC ones-d. also Section 23.4.2 below): it may be regarded as comprising several stages, each characterised by the corresponding state of the latex layer on the substrate and the associated changes in the rate of evaporation of the aqueous phase. 2 The layer of latex as originally deposited on the substrate (cf. Fig. 23.1(A)) loses water relatively rapidly by evaporation at a rate comparable with that for pure water. This reduces the water content comparatively quickly to the point at which the layer consists of polymer particles in fairly loose mutual contact, with the interstitial spaces containing residual water (Fig. 23.1(B)). In this state the interfacial area between the water and ambient air is substantially B
A
c
Fig. 23.1
D
Formation of a polymer film from a latex layer: schematic representation (after Vanderhoff et at. 2). A, Latex layer as applied to substrate; B, particulate deposit in loose packing array: advanced stage of drying; C, substantially dry, consolidated deposit: particles deformed, close packing; D, polymer film formed after full coalescence of particles.
1016
W. V. Titow
reduced, resulting in a decrease in the rate of evaporation. Further loss of water eventually leads to very close packing of the particles with extensive inter-surface contact (the closer the softer and more easily deformable the polymer), amounting to the formation of a granular polymer layer with little free liquid water remaining (Fig. 23.1(C)). At this stage the rate of loss of residual water becomes relatively very slow, being governed by that of its movement through capillary channels between deformed polymer particles, or diffusion through the polymer substance. In most cases the particles of the granular layer finally coalesce fully into a continuous film of essentially uniform structure (Fig. 23.1(D)). Depending on the nature of the polymer (and the presence or absence of plasticisers) the coalescence may have to be accelerated by the application of heat (heat fusion), occasionally also aided by pressure. Whilst the closest, most complete mutual contact among the polymer particles of the granular layer is a prerequisite to effective film formation, the basic mechanism of particle coalescence is that responsible for all self-bonding of polymers generally, viz. the diffusion of polymer chains (or their parts) across the original boundaries in areas of surface contact. In practice this does not take place to a sufficient extent below the glass-transition temperature of the polymer, whilst above the Tg the process is accelerated by heating and enhanced by pressure. 3 Most applications of PVC latices involve heating to accelerate the removal of the aqueous phase and subsequent fusion of the particles. 23.2 TYPES OF PVC LATICES
PVC latices may be classified according to the nature of the polymer of the particulate phase, i.e. vinyl chloride homopolymer or copolymer latices, and the presence or absence of external plasticiser(s), i.e. plasticised or unplasticised latices. In common with any other form of vinyl chloride copolymer material or product, the particles of a copolymer latex may be regarded as 'internally' plasticised by the presence of comonomer units in their molecular chains (see Chapter 1, Section 1.2; and Chapter 5, Section 5.2). In the practical context of latex technology the main effects of this chemical modification are similar to those of inclusion of external plasticiser(s) in a homopolymer latex, in that the temperature needed for particle fusion is lowered, and the ultimate product (fused film) becomes softer and more extensible. External plasticisation-necessary for satisfactory film formation by vinyl chloride homopolymer
1017
23 PVC Latices
latices-is also not uncommon with copolymer latices in which it increases still further the effects of the internal plasticisation. Plasticised PVC latices, whether homopolymer or copolymer, are either supplied with the plasticiser(s) already incorporated, or prepared by the user from the unplasticised versions (see further on in this section and Section 23.4.2.) Some properties of PVC latices are shown in Table 23.1. TABLE 23.1 Some Properties of PVC Latices Breon latex General nature Minimum pH value % tolal solids Brookfield viscosity" (cP) Specific gravity a
151 Homopolymer
351 352 Unplasticised copolymer
8·0 52-55
8·0 54-58
8·0 55-59
12-28 1·17-1·19
20-40 1·17-1·19
30-42 1·17-1·19
576 599 Plasticised copolymer 8·0 55-59
8·0 54-58
35-55 30-40 1·12-1-14 1·12-1·14
652 Unplasticised copolymer 7·0 49--53 7-12 1·23--1·25
No.1 spindle, 60 r min-I; 25°C.
23.2.1 Homopolymer Latices (e.g. 'Breon 151')
These are not normally film-forming in the absence of plasticisers, tending to give rather powdery deposits on drying even at elevated temperatures. External plasticisation promotes particle coalescence after deposition in processing, but temperatures of about 160°C are needed for satisfactory fusion, assisted by pressure in some cases. 23.2.2 Unplasticised Copolymer Latices
Depending on the nature and proportions of co-monomer present, particle fusion may require elevated temperatures (e.g. 13o-140°C for Breon 351; 14o-150°C for Breon 352), or the Tg may be low enough for coalescence, with satisfactory film formation at room temperature. The fusion properties of a copolymer latex of the latter kind (e.g. Breon 652) are, therefore, not improved by external plasticisation, although for applications where extra softness is required in the ultimate film a plasticiser may be incorporated. Neither do the film properties of a room-temperature fusing latex apparently benefit from processing at elevated temperatures, as indicated by the data of Table 23.2.
w.
1018
V. Titow
TABLE 23.2 Physical Properties of Films from Plasticised 'Breon 352' and 'Breon 652' Heating temperature (oc) for 2 min
25 90-100 150
23.2.3
Plasticised Breon 352 Tensile Elongation strength at break (%) (lbfin- 2 )
1052 1673 3823
180 270 480
Breon 652 Tensile Elongation strength at break (%) (lbfin- 2 )
2445 2255 2600
760 620 750
Plasticised Copolymer Latices
Some vinyl chloride copolymer latices supplied in plasticised form are claimed to offer greater efficiency of plasticisation than that obtainable by post-plasticising an otherwise similar originally unplasticised version (even if plasticisation is effected by addition of plasticiser preemulsified in water-d. Section 23.4.2). This is the case with Breons 576 and 599, the plasticised counterparts of, respectively, Breon 351 and 352, each incorporating about 35 phr of a phthalate plasticiser. Blending an unplasticised latex with the corresponding plasticised version enables the stiffness of the final product to be varied within limits represented by that obtainable with each of the two components used alone. Further plasticiser (pre-emulsified in water-d. Section 23.4.2) may be added to a plasticised latex if required. The plasticised Breon copolymer latices (576 and 599) were developed to supplement the range of applications covered by their unplasticised counterparts, especially where lower fusion temperature, good colour and resistance to yellowing of the final product under the influence of heat or UV light are of primary importance. However, these improvements are gained at a slight sacrifice of strength and rigidity in comparison with products of Breon 352. 23.3 SOME PROPERTIES OF POLYMERIC PRODUCTS FROM PVC LATICES 23.3.1 Mechanical Properties PVC polymers deposited from latices requiring heat treatment (with pressure in some cases) for effective particie fusion, attain the
1019
23 PVC Latices
maximum strength only after the appropriate treatment of this kind. Films should be fused under conditions as near the ideal as possible. In collecting laboratory data, cast film testing is subject to many errors. To obtain a true comparison of properties and to overcome these difficulties Breon vinyl latices were dried out at 23°C and the polymer moulded in the form of a disc 6 in in diameter, 0·03 in thick. Pressing conditions were 5 min at 165°C under 1 ton in- 2 pressure. Breon 151, 351 and 352 powdered before pressing, but the plasticised grades formed continuous film. Table 23.3 indicates the results obtained when dumbbell samples cut from the moulded films were pulled at 5 in min- 1 on an Instron tensile machine. Optimum strength may be approached simply by heating but unplasticised latices normally require between 10 and 20 parts of added plasticiser to form unpressed films. TABLE 23.3
Physical Properties of Pressed Films from 'Breon' Vinyl Latices Breon latex
151 351 576 352 599 652
Tensile strength (lbfin- 2 )
7075 8030 2140 7575 2115 1360
Elongation at break (%)
Modulus (lbfin- 2 ) at elongation 100%
300%
230
1825
860 380
1850 760
500%
10
15 330 20 240 570
1200
23.3.2 Toxicity Considerations Where PVC latices or their products are required to come into contact with foodstuffs, clearance (and general advice) should be obtained from the manufacturers.
23.4 COMPOUNDING There are two categories of compounding ingredients for vinyl latices: (i) stabilisers, wetting agents, thickeners, anti-foaming agents and pH modifiers affecting latex properties;
1020
w.
V. Titow
(ii) heat stabilisers, plasticisers, fillers, pigments and anti-blocking agents affecting polymer end properties. Latex stability must be maintained when compounding ingredients are added, and the manufacturers' recommendations on the preparation of compounding additives should be followed. Attention must be given to the physical form and pH of the additive and its compatibility with the basic latex. The mixing process must give thorough intermingling of the components under conditions of shear below the critical level. The slow, even speed paddle or turbine mixers are recommended, but care must be taken to avoid 'vortexing' or 'cavitation' which introduce excessive air. Careful preparation of additives reduces the need for excessive mechanical agitation and the dangers of flocculation. 23.4.1 Latex Property Modifiers
(a) Latex Stability Vinyl latices are treated with stabilisers to ensure adequate chemical and mechanical stability for normal handling, compounding and processing. Stabilisers are carefully chosen to cover a wide range of applications and quantities kept to the minimum practical level. Certain factors which adversely affect the stability of PVC latices should be avoided. These are: (i) (ii) (iii) (iv) (v) (vi) (vii)
high concentrations of monovalent ionic salts; small amounts of polyvalent cations; strong inorganic and organic acids; water-soluble solvents such as alcohols and ketones; prolonged exposure to freezing conditions; excessively high temperatures; conditions of very high shear, such as pumping via closeclearance gear pumps.
Materials improving stability include protective colloids and certain surface-active agents. When choosing a suitable stabiliser, wetting properties of the latex compound, water sensitivity of the dried film and the nature of likely destabilising influences must be borne in mind. Chemical stability of PVC anionic latices is improved by anionic surface-active agents. Surface tension drops in proportion to the amount of surface-active
23
PVC Latices
1021
agent added, thereby increasing the wetting power of the system. This factor can be advantageous for covering impermeable surfaces and penetrating fibrous substrates. However, reduced surface tension gives rise to increased foaming and the higher the level of added surfactant the greater the sensitivity of the dried film to water. Non-ionic surfactants, such as ethylene-oxide/fatty-alcohol condensation products, offer the greatest protection against chemical destabilisation and a degree of mechanical stability. Protective colloids such as casein or Lytron 820 (Monsanto Co., USA) improve the stability of the latex under conditions where mechanical shear can cause breakdown of the emulsion system. Table 23.1 shows that the minimum recommended pH for Breon vinyl latices is 7-8. It is important to maintain this level during processing and ingredients added to the latex must be adjusted to a minimum pH 8 before addition. (b) Wetting Agents Anionic and non-ionic surfactants which efficiently reduce tension improve wetting characteristics. Those recommended include alkyl sulphosuccinates and polyoxyethylene alkyl phenol derivatives.
(c) Thickeners Several types of polymeric substances which form viscous solutions in water when dissolved in relatively low concentrations, are of interest as latex thickeners. Among those successfully used with Breon PVC latices have been cellulose derivatives (methyl cellulose, sodium carboxymethyl cellulose), polyacrylates (including some sodium and ammonium polyacrylate salts), and colloidal solutions of casein and sodium alginate. Different types of thickener, and even individual thickeners within the same type, can differ considerably in their effects on a given PVC latex, or on different latices: this is illustrated, by way of examples, by the data of Figs 23.2-23.7. Some latices can be adversely affected (destabilised) by certain thickeners. Specific advice on thickening agents most suitable for use in a particular latex (and its given application) is available from latex manufacturers. In general, polyacrylates have been said4 to be particularly useful with Breon PVC latices, and less prone to causing sedimentation than cellulose derivatives.
100000
so 000
I
--_....... -.- --
.......... I
p........- .... ~--.
.
to 000 e
-. -.. i ...... -..
5000
..
~
", I
_..I - ._.-..
,~
I
.
I
I
I
I __ iiiiiiiit_ _
o
7;
1-,-
1-_-1-
i
e-
N
~,
~:...-
I
I
",-.-
_.
.
-
. I
MC (2-5) MC (1-5) $CMC (Z·S) MC (0'15) SCMC '1-5)
SCMC {0'75}
1000
50 0
10 0
I
o
Fig. 23.2 Cellulose thickeners in Breon 151. Figures in brackets indicate dry parts per 100 parts dry polymer by weight. Me, Methyl cellulose; SCMC, sodium carboxymethyl cel!~lose.
100 000 ~
50 000
...
~
"""Iiii
--...:.:.: PA (z.s)
/ 10000
-
c::::;;,;;liiii
e
~
o....
I
I
~
5000
--
'A (1·25)
PA (0·15)
;
,
~
~
·i•
I
. u
.;
...
..!•• .;
i
1000
I I
.
-1 I
500
I
I
I
I
I I II I
I
.1
100
o
I
I J Mltllring lilll. (dlYs II '00
I I
I
I
--
I
1
IIr.1pllllllr. ZJ' C)
Fig.23.3 Polyacrylate thickeners in Breon 151. (NB Sodium and ammonium polyacrylates adversely affected Breon 151.) Figures in brackets indicate dry parts per 100 parts dry polymer by weight.
100 000
50000
-_.-....
10000
~.:.":
o
N
. ...•.. ii
5000 1---
L
~
1--
... -;
--.--',--.--._.- --_.--.....------- .-. -.
-- - .. ~_.
~.- ~.-
--...-
~-- ~--
!
MC (2·5) MC (1·5) MC (0·15) SCMC (2'5)
--
SCMC (1-5)
---
SCMC(O'15)
::0
.a•
1000
500
100
o
3
7
Fig. 23.4 Cellulose thickeners in Breon 351. Figures in brackets indicate dry parts per 100 parts dry polymer by weight.
100000
--
50000
"""Iii
10000
--.-...
-
E
...o
~
•..
5000
-.r:;..
IL
.-..
- -
-
10"-
~
-- =::: ~..:: .. ~j
..-.J--
......
I
~
.;
•
..
I
~
APA (H) SPA (H)
=- -
j
l--
PA (N) APA (1'U) SPA (1-25) SPA (0'15) PA (1'25) APA (0·75)
-
PA (O·lS)
•
oM
.;•
1000
500 I
100
o
J
7
Fig. 23.5 Polyacrylate thickeners in Breon 351. Figures in brackets indicate dry parts per 100 parts dry polymer by weight. APA, Ammonium polyacrylate; SPA, sodium polyacrylate.
tOO 000
50000
10000
oN
5000
iii
... u
._-"'-- --.--,.-- -- .....
_.- .--...---- .. -- ---.---"""--- '-_.-- ----'"----.,--._-- -.....=w
~~
I
1000
-
--
SCMC (2'5) SCMC (1-5)
MC (z.s~ SCMC (0· 5) MC (t 5)
I
.....
-. --
I I
MC (0·15)
I
500
!
I
100
o
l
Matyri"_ t,ma (4arl at '"
tamparatura 23'C I
Fig. 23.6 Cellulose thickeners in Breon 576. Figures in brackets indicate dry parts per 100 parts polymer by weight.
100000
50000
' ..... .. -' ....... .....
~.
...
••1---
I'
_-..-
-- --
E
e-
o N
5000
II. u ~
.
.; o
--- --
-
---- ••
r-••
10000
;
..-
~;
~_
APA (2-5)
.
PA (z.s) APA (1· 25) SPA (1-25)
~
~--'
PA (1-15) APA (0·15) .sPA (0· 7S)
=:=.~ j
··
SPA U·S)
I
.. u
.;:
,,
... D
~ ::Ii
PA (0·15)
I
I
· I
1000
500
i I I
100
o
J
I
I
,
I
1
MIIUlinlllill1 1~IYs II loom Ilmperlluil 2J"C)
Fig. 23.7 Polyacrylate thickeners in Breon 576. Figures in brackets indicate dry parts per 100 parts dry polymer by weight.
1028
W. V. Titow
Note: Sedimentation of PVC latices is analogous to creaming ot rubber latices: both are manifestations of destabilisation, but the relatively heavy PVC particles (SG > 1) settle downwards through the aqueous medium, whereas the particles of a rubber latex, which are of relatively low SG, rise to the surface to form a 'cream'.
The basic rules for introducing a thickener into a PVC latex are the same as for other latices. The best dispersion and maximum efficiency are obtained when the latex is added slowly, under constant, slow agitation, to a solution of the thickener. An alternative method is to 'masterbatch' the thickener. This involves thoroughly mixing the thickener solution for a large compound batch with a small amount of the latex mix in an efficient small mixing vessel. This added to the main batch will disperse when stirred to give maximum efficiency. When very large batches of highly filled latex are prepared it is sometimes possible to add the thickener solution directly. Effectiveness depends here largely on the agitator efficiency. (d) Anti-foaming Agents Both silicone and non-silicone based anti-foams have been used effectively with Breon vinyl latices. It is important that the most suitable type is chosen with each latex for maximum efficiency. Silicone Antifoam A and RD have good general-purpose efficiency. The Nopco and Bevaloid grades most suitable for each Breon latex are given in Table 23.4. TABLE 23.4 Anti-foaming Agents for 'Breon' Latices a
Breon latex
151 351 352 576 599 652 a
b
Bevaloid Type 566 or 942 6250 6250 6250 60 6250
Amount (% on latex) 0·3-0·5 0·2-D·3 0·1-D·2 0·1-D·2 0·05-D·1 0·1-D·2 O·2-D·3
Bevaloid Ltd (in the UK). Diamond Shamrock Chemicals (UK) Ltd.
Nopco b Foamaster Type at 0·25% on total latex NDW or 8034 DNH-1 or 8034-E NX2 or NDW DNH-1 or 8034-E DNH-1 or NXZ DNH-l or NXZ
23
PVC Latices
1029
(e) pH Modifiers and Buffers The pH of dispersions, emulsions or solutions of additives to be compounded with PVC latices must be adjusted to a value of at least 7, or 7-8. Alternatively the pH of the base latex should be raised and buffered to prevent destabilisation by neutral or slightly acid additives. Strongly acid substances must not be added to anionic latices in normal circumstances. The pH modifier most often used is ammonium hydroxide. This has the advantage of not causing flocculation if added in concentrated solutions, and of evaporating on drying. Alternatives are 'fixed' alkalis, such as sodium or potassium hydroxide. However, whilst these do raise the pH effectively, their use in concentrations above 5% (2% in some cases) causes flocculation: less concentrated solutions may dilute the latex mix to an undesirable extent. Moreover, fixed alkalis remain in the latex film, impairing its water resistance and increasing moisture transmission. The alkali phosphates or sodium bicarbonate also act as pH modifiers. Common buffering agents are phosphates such as trisodium phosphate or sodium hexametaphosphate (Calgon-Albright and Wilson). Phosphate addition to a latex sytem partially counteracts the effects of sharp pH changes, aids dispersion of solid ingredients and, to a certain extent, sequesters the harmful cations of calcium, zinc, and magnesium. A more powerful sequestering agent is ethylene diamine tetra acetic acid. This material does not have the economic or buffering advantages of phosphate, but used carefully in small quantities it helps prevent the insolubilisation of stabiliser soaps by calcium, zinc and magnesium.
23.4.2 Polymer Property Modifiers
The compounding ingredients discussed in the preceding section are used primarily to balance the latex properties for processing, with relatively little effect on those of the solid end product. Additives incorporated into PVC latices to modify the properties of the polymer in the end product (and, in the case of plasticisers, also to facilitate its production in continuous film form) serve essentially the same purposes as they do in PVC compositions processed in solid or paste form. The main ones are heat stabilisers, plasticisers, fillers, and opacifying pigments and colourants.
1030
W. V. Titow
For some applications PVC latices may be used as supplied, without further modification. (a) Heat Stabilisers The particulate (solid) phase of a PVC latex, consisting as it does of a vinyl chloride polymer or copolymer, is prone to heat degradation on exposure to elevated temperatures. In practice this may occur in the course of formation of the solid end product (dried film, binder deposit in bonding applications) or thereafter. The manifestations and likely mechanisms of degradation of PVC are discussed in Chapter 9. With the exception of those which are film-forming at room temperature (ct. Breon 652), solid deposits from PVC latices develop their maximum strength only after fusion at an elevated temperature. The detrimental effects of heating on the polymer should be minimised by initially drying off the water from a latex layer on a substrate (ct. Fig. 23.I(A» at around 100°C and then ensuring a rapid rise to fusion temperature, followed by a cooling cycle. In many processes PVC latices are applied in very thin layers which dry readily and require fusion times of only a few seconds: heat stabilisation is usually unnecessary in such cases. In general it will be required where very high temperatures or prolonged treatment times at about IS0°C or over are to be used. The choice of stabiliser will be made-from the wide range available (see Chapters 9 and lO)-in the light of the processing and service conditions, as well as suitability of the stabiliser(s) for conversion to a fine dispersion in water-the form in which it should be added to the latex. A solid stabiliser must, therefore, be capable of being comminuted (commonly by ball-milling) to a fine aqueous suspension, and a liquid one of being emulsified in water. Thorough dispersion of a stabiliser in the aqueous phase of a latex may not necessarily lead to a significant improvement of the heat stability of the PVC polymer: it is essential for the stabiliser to be intimately associated with the polymer. The main points to be considered when choosing heat stabilisers for PVC latices are: (i) effectivity of heat-stabilising action; (ii) ease of dispersion or emulsification in water and stability of the dispersion formed (including stability after mixing with the latex); (iii) compatibility with the latex system (including absence of
1031
23 PVC Latices
coagulant effects and non-interference with other compounding ingredients) . Not many stabilisers satisfy all these conditions, but organotin compounds as a class are acceptable, especially when used in conjunction with an epoxy co-stabiliser. Heat stabilisation of PVC latex products does not follow exactly that of dry compositions, and a stabiliser effective in the latter will not necessarily be so in a latex system. Reasons advanced for this relate to the influence of water, as well as emulsifiers and dispersing agents introduced in processing. Two heat stabiliser systems have been found 4 to give excellent results with Breon PVC latices. These are based, respectively, on a liquid dibutyltin mercaptide (Mellite 39) and dibutyltin dilaurate, with the addition of an epoxy plasticiser (Lankroflex ED3):
Components Mellite 39 (Albright and Wilson Ltd, UK) Lankroflex ED3 (Lankro Chemicals Ltd, UK) Tween* 20 (Honeywill-Atlas Ltd, UK) Spant 20 (Honeywill-Atlas Ltd, UK) Dibutyltin dilaurate Morpholine oleate Water
System 1 System 2 (pbw, wet) 28·60
The emulsified heat stabiliser system should be added to the latex early in the compounding cycle; under constant slow stirring. The Mellite 39 system has proved slightly more effective than the DBTD system, but in either case optimum results can be expected at approximately 4 parts of mixed organotin/epoxy system per 100 parts of PVC polymer (dry).
(b) Plasticisers PVC homopolymer latices dry out to hard, normally discontinuous deposits. Softness and flexibility can be achieved only by the use of copolymer (internally plasticised) latices, or by external plasticisation, * Polyoxyethylene sorbitan monolaurate (emulsifier). t A sorbitan fatty ester (emulsifier).
1032
W. V. Titaw
or both. In the Breon PVC latex series, deposit softness increases in the sequence Breon 151 < 352 < 351 < 599 < 576 < 652: in comparison with deposits from Breon 351 and 352, those from Breon 599 and 576-which are their plasticised versions (cf. Section 23.2)-demonstrate the further softening and flexibilisation brought about by the presence of external plasticiser. Incorporation of about 25 phr DOP in a PVC homopolymer latex typically gives a hard and stiff (but generally cohesive) film after drying and heat fusion of the deposit (at about 150-160°C); 50 phr DOP imparts appreciable flexibility to the fused film; 75 phr produces a highly flexible film; and with 100 phr DOP the fused film is very soft, flexible, and slightly tacky. Note: In an externally plasticised PVC latex, the plasticiser is not
normally absorbed to any substantial extent by the polymer particles with which it is co-suspended in the aqueous phase. Plasticisation is only effected when the water has been driven off and the polymer deposit heated to the appropriate temperature. Differences in softness between particles of different PVC latices are sometimes demonstrated by electron micrographs, in which some copolymer particles (e.g. of Breon 352) may be seen to have been deformed from the original spherical shape by the impact of the electron beam, while the harder homopolymer particles (of Breon 151) maintain their original sphericity. 4 In most latex applications the end product is a continuous polymer film, obtained after the removal of water from the system and coalescence of the polymer particles. A latex will normally contain 30-50% of polymer particles by volume. Film formation is believed to proceed as follows. As the water evaporates to the extent roughly corresponding to that represented by Fig. 23.1(B), the particle content increases to about 70% (for the normal latex particle size range of 0·1-0·25 llm). The particles pack closely together, but intimate surface contact is hampered, for a time, by forces of repulsion set up as a result of interaction of electrical surface-charge layers. 2 ,4,5 Further evaporation of the water results in the contraction ('necking-in') of the water/air interface between the packed particles (commencing in the surface layer): this creates forces (sometimes referred to as the 'capillary forces'6) drawing the particles closer together. The capillary forces eventually exceed the electrostatic repulsion and the particles
23 PVC Latices
1033
move together into close mutual surface contact. At this stage the capillary forces are supplemented in their action on the particles7 by forces arising from the polymer/water interfacial tension ('surface tension forces'S). If the particles are sufficiently soft, they deform as they pack tightly under the influence of these forces (as, for example, in the case of rubber and some PVC copolymer latex particles), achieving intimate, extensive surface contact. The quality of softness which makes this possible is also usually associated with relative freedom of movement of the polymer chains. The combination of good surface contact and chain mobility plays a cardinal part in the ease of coalescence of the particles (cf. Section 23.1) through the basic mechanism of inter-diffusion of polymer chains across particle boundaries. Coalescence (and the attendant development of cohesion in the polymer layer) will-in favourable circumstances-proceed until the layer is transformed into a homogeneous film. The term 'autohesion' has been coined for this kind of effect. 9 In the context of the morphology of latex deposits, autohesive cohesion may be .contrasted with the much weaker one produced in a layer of closely packed but essentially discrete polymer particles by capillary and surface tension forces developed on drying. With hard particles, coalescence sufficient for the ultimate formation of a continuous film may occur only at suitably elevated temperatures (heat fusion). The very hard particles of a PVC homopolymer latex (e.g. Breon 151) require the addition of plasticiser for satisfactory fusion even at high temperatures. External pressure is also employed in some cases. The presence of a plasticiser in the polymer deposit facilitates fusion, as its penetration into the polymer increases the free volume and reduces the effective Tg , thus effectively increasing chain mobility (as well as particle softness). The likely performance of a plasticiser in the ultimate latex film may be estimated from a knowledge of its effects in solid PVC compositions. However, with base polymers of the same type it may be necessary to reduce the amount of plasticiser used in a latex formulation, because of the greater inherent flexibility of thin films. The main factors to be taken into account in selecting plasticisers for latex formulations include all those relevant in the formulation of solid compositions (cf. Chapters 4 and 7), with-additionally-ease of emulsification for blending with the latex. Where nitrile rubber is to be used as a plasticising modifier of high permanence in the ultimate film (cf. Chapter 11, Section 11.2.2) this is introduced by addition to the
W. V. Titow
1034
TABLE 23.5 Physical Properties of Films from 'Breon' Vinyl Latices after the Addition of Plasticiser
Latex Breon 151
Breon 352
Breon 599
a
Plasticiser (35 phr)
Tensile strength (lbfin- 2 )
Elongation at break (%)
DAP TIP Breon 1562a Acetyl tributyl citrate (ATBC) DAP TIP Breon 1562a ATBC Phthalate-preplasticised grade
1800 3200 2780
150 150 150
2000 1850 2400 2700 1750
210 300 210 100 250
1700
350
Nitrile rubber latex.
PVC latex of the appropriate nitrile rubber latex (e.g. Breon 1561 high-nitrile latex, or 1562 medium-nitrile latex which imparts somewhat better low-temperature flexibility to the ultimate film). It is essential in latex working to obtain an even, stable plasticiser/ latex mixture. Pre-emulsification is the most common method used for ester types. Sometimes it is possible to emulsify the plasticiser directly into the latex compound. This involves adding a carefully controlled type and amount of emulsifier to the latex and agitating the oil in slowly. Type and amount of emulsifier vary according to the plasticiser and the base latex used. Optimum association of polymer and plasticiser is not often possible by this method because of limitations on the shear rate which can safely be applied to emulsion latex systems. Other ingredients such as fillers and thickeners assist in maintaining an even mixture and preventing phase separation. The addition of pre-emulsified plasticiser to Breon vinyl latices at 35 parts per 100 parts of dry polymer affects the physical properties of the dried, fused polymer film according to the type of plasticiser chosen. The data in Table 23.5 were obtained on thick films cast on glass plates, dried at 23°C and fused at 160°C. The higher plasticising efficiency obtained by pre- as against post-plasticisation is demonstrated by the higher elongation and lower tensile strength of film from Breon 599 vis-a-vis that from Breon 352 post-plasticised with 35 phr
23 PVC Latices
1035
DAP (as the nature and contents of the polymer and plasticiser of the two latices are closely comparable). In general, for a given base polymer, the higher the plasticiser content the lower the tensile strength and the higher the elongation of the resultant film. As with solid compositions, epoxidised oils are not commonly used as primary plasticisers in PVC latices, but they may be included for improved heat stability. The hard PVC latices, Breon 151,351 and 352, require plasticisation and/or pressure, in addition to heat fusion, to form films. The minimum amount of general-purpose phthalate plasticiser which has to be added to render Breon 151 film-forming varies, according to processing conditions, from 15 phr to 20 phr; it is slightly less for Breon 351 and 352. Where-as is sometimes necessary-a very hard film has to be formed of lower plasticiser content (and without use of pressure), a temporary, 'fugitive' plasticiser may be used. An example is polypropylene glycol, which evaporates from PVC at fusion temperatures of about 160°C but assists film formation at about lOO°C in the earlier stages of processing. (c) Fillers Fillers are added to PVC latices for one or more of the following effects on the final product: cost reduction; reduction ('flattening') of gloss; increase of hardness and stiffness; reduction of tack. The fillers most commonly used are calcium carbonate, clay, barytes, and some types of particulate silica. Relatively small amounts of filler (up to about 20 phr) modify latex-produced PVC films without too drastic an effect on their physical properties. The actual amount of filler to be used in given circumstances will depend on its type and particle size, as well as the nature of the polymer and the plasticiser level. However, there is a volume concentration of filler (or pigment) for a PVC latex, as there is for a surface-coating emulsion in the paint industry, above which a sharp drop occurs in a number of physical properties of the ultimate film. Experimental evidence indicates that at this 'critical pigment volume concentration' (cpvc) the filler becomes the major structural component of the film, with the role of the polymer changing from that of a continuous matrix to one of a binder holding the filler particles together without enveloping them entirely. Whilst the binding power of PVC in such a system is quite good, the resulting cohesion is inferior to that of a continuous film. 4 The cpvc may be determined for the latex-produced filled (or pigmented) films of most polymers by
1036
W. V. Titow
measuring a physical property, such as tensile strength or elongation at break, at various filler loadings, * and plotting the results. Although the actual property values will depend on several factors (and especially the nature and binding power of the polymer in the particular system) the general form of the plot is fairly universal: it is typified by the curve of Fig. 23.8, in which the cpvc is marked by the change of slope. A water sorption method, described by Liberti and Pierre Humbert,1O may also be used to detect the cpvc. This indicates that while 'free' polymer is present in the system afer the surface area of the filler has been covered, a coherent film retaining most of the polymer properties is maintained. But at the point where all the polymer is adsorbed onto the filler surface the film loses coherence as its structure becomes discontinuous. Therefore the cpvc may be defined as a function of the particle surface area of the filler, its ability to adsorb polymer, and the amount and particle size of the polymer. 700 600 500 ;: 400 ".
~ 300
.c
I
I
to
I
c:
I
o
~200
I
iii
I I
'"oc:
I I I I
I
100~2---:3~-4~~5~-:-6~7-:8~9;;-:'0~---+1-:2i::-0---:!::---;J40:---:50=Plgmcmt conc..ntratlon l vol. ". Critical plgm"nt volum .. conc..ntration (cpvc)
Fig. 23.8 Elongation at break of a latex-produced, pigmented PVC film versus pigment loading.
* The % volume concentration of filler in the dry film is given by: weight of filler weight of film
x SG of filler x 100 x SG of film
23
PVC Latices
1037
In many applications small amounts of filler below the cpvc level are used, to cheapen the mix, modify flow characteristics, or reduce the gloss of the final product. The relevant points made in Chapter 8 concerning the properties and effects of fillers apply here. Thus, for economy, whitings followed by clays are the most widely available and attractively priced fillers in most countries. Whitings are available in many different particle size forms according to the method of manufacture and mineral source: the cost varies depending on particle size and size distribution, chemical purity, and presence or absence of surface treatment. Similar considerations apply to clays which, however, are more expensive grade for grade. Mica fillers, also occasionally used, are the most expensive. In general, whitings are the softest fillers and cause the least stiffening, whilst clays impart intermediate hardness-and micas high hardness-at the same loading levels. Barytes is sometimes incorporated in PVC latices to increase the density and weight per unit area of the ultimate coatings, as only relatively small loadings of this high-density filler with good covering power (see Chapter 8, Section 8.2.2) are needed to produce significant increases. Where barytes is used, the latex viscosity must be adjusted to prevent the filler settling out and forming a compact, hard-todisperse layer at the bottom of the container. Gloss reduction may be brought about by the incorporation of small amounts of fine-particle-size silica. The water absorption and soap demand of these finely divided particles must be satisfied before addition to the latex in order to prevent flocculation. Bentonite clay may be used to modify flow characteristics of latex mixes. This clay has a layered plate-like structure which readily absorbs water, imparting 'body' to the compound. The yield value developed can be usefully employed in coating mixes where restricted flow, low penetration and good dimensional stability in the wet state are required. Initial difficulty in wetting-out bentonite may be overcome by treating it with a small amount of alcohol before water dispersion. Small amounts of filler used well below cpvc must be in fine particle form, well dispersed in water to minimise depression in polymer strength. Coarse aggregates or large particles form points of weakness in the structure which start or propagate physical breakdown. It is normal to predisperse these fillers in water with an added dispersing agent and pass the resultant slurry through a cone or high-speed colloid mill to de-aggregate the particles. When very small amounts of extra
1038
W. V. Titow
fine dispersions are needed, ball mills may be used to achieve fine filler slurries but this process is too slow and uneconomic for large-scale production. Near to the cpvc point when the polymer becomes a binder, choice of filler becomes more critical. Fine particles have large surface area and therefore a greater binder demand but give increased covering power. They may be tolerated where binding power is the prime requirement in a surface coating, but where economy is the main consideration coarse fillers may be added in greater amounts before the film cracks and fails physically. China clay is normally preferred for dense, high-hardness coatings, and coarse whitings for maximum filler levels and economy. (d) Pigments Unfilled and unpigmented vinyl latices produce transparent colourless films, and for some applications it is necessary to pigment the polymer or improve the appearance of filled compositions with opacifying pigments or colours. Choice of pigments is restricted as certain metallic ions such as iron and zinc under some circumstances catalyse the decomposition of PVC. Reaction depends on the chemical form of the ion and service temperature. Many grades of metal oxide pigments may be safely used with PVC but care is needed in the choice of grade. Where there is a danger of small proportions of latent metal ions existing in the pigment or being generated during its preparation for addition to the latex, oxides of iron, zinc or copper are best avoided. Titanium dioxide is the preferred pigment for a white base but Iithopone and blanc fixe could be used. It is advisable to incorporate a white base in filled and unfilled stocks required in pastel shades. A wide range of colours is possible with vinyl latex base, as the polymer itself is clear and does not discolour under normal circumstances. Some care is needed in the use of Breon 652 which does discolour more readily during service exposure to UV light or elevated temperatures. The normal range of organic pigments is usually satisfactory for coloured compositions, especially aqueous pastes prepared for use in latex systems. Pigments and dyes, like many compounding ingredients, are prepared in aqueous dispersion prior to addition for the greatest efficiency.
23
PVC Latices
1039
23.5 ANTI-BLOCKING TECHNIQUES
Blocking is unintended surface adhesion between plastics films (or sheets) or between such films (or sheets) and other surfaces. The films or sheets may be either the regular, unsupported kind, or coating layers on substrates. * Blocking occurs most commonly between continuous, flat surfaces of soft, tacky polymers forced into close contact under pressure. The magnitude of the effect (which may be measured by determining the force required to separate the surfaces 1Z) is a function of the pressure and the degree of tackiness. Films produced from latices of soft PVC copolymers, or ones containing high proportions of external plasticiser(s), are prone to blocking in circumstances favourable to this effect. The tendency is best counteracted by the addition to the original latex of one producing a harder film (e.g. Breon 151 or 351) providing that the extra hardness and stiffness thus imparted to the resulting film can be tolerated. In this way clarity is unaffected and gloss retained. Blocking may also be reduced by breaking up the flatness of the film surface through embossing. Its thermoplastic nature makes the latex-produced PVC layer suitable for this treatment which, in the simplest form, may consist of subjecting the layer (usually a coating on a substrate) to radiant (IR) heat and then passing the material through the nip of cold embossing rolls (a hard rubber backing roll nipping under pressure on a suitably engraved steel roll). These two methods do not involve basic compounding modifications (except for latex blending in the first) but the problem may also be tackled by suitable additives. Most fillers reduce surface tack, and blocking is seldom encountered in filled formulations. The most efficient anti-blocking filler is finely divided silica which, in small proportions up to 20 parts per 100 parts of polymer by weight flattens the surface gloss and reduces tack with the minimum effect on transparency. Another method is to incorporate a wax emulsion into the latex formulation: e.g. paraffin or amide waxes have limited solubility in PVC and reduce surface tack by blooming. Because of low compatibility, physical properties of the polymer are unaffected as long
* This definition is closely in line with those given in relevant standards ll ,12
(some of which are somewhat less comprehensive).
1040
W. V. Titow
as the wax concentration is below 10%. Blocking may also be reduced by increasing the fusion temperature above the minimum required. 23.6 APPLICATIONS
pvc
latices provide an advantageous alternative to the other two liquid PVC systems-pastes and solutions-for a number of applications. In many they are the only type of liquid system suitable in practice. The main use areas are the bonding and coating of certain textiles, some paper treatments, leather finishes, and heat-activated adhesive applications.
23.6.1 Textile Applications (a) As Bonding Agents in Non-woven Fabrics In this capacity PVC latices can provide good bonding strength, chemical resistance, and colour. Copolymer latices, usually with external plasticiser (e.g. Breon 576), are normally favoured for ease of fusion and high flexibility of the product, but a plasticised homopolymer latex (or unplasticised copolymer one) may be used where greater product stiffness is required and higher fusion temperatures are admissible. Where plasticiser permanence is of particular importance, a nitrile rubber latex may be blended into the PVC latex in place of a conventional external plasticiser. The type of basic formulation concerned is illustrated by the following examples: 4 Formulation 1 Formulation 2 (pbw, dry solids)
Plasticised vinyl chloride copolymer 100 latex (Breon 576) Unplasticised vinyl chloride copolymer latex (Breon 351) 60 Nitrile rubber latex (medium acrylonitrile-Breon 1562) 40 Wetting agent 0·25 0·25 Antifoaming agent 0·10 0·10 The latex compositions for bonding non-woven fabrics are normally diluted with water to a degree governed by the method of application and the required properties of the finished product. These factors also determine the admissibility of fillers and pigments in the formulation. Various application methods are employed,13-15 depending on the
1041
23 PVC Latices
nature and intended use of the textile, including full saturation of fibre mat by immersion (followed by removal of excess latex by squeezing or suction), impregnation with foamed binder latex, spraying, printing, and others. A review of the properties, applications, and structural effects of vinyl chloride copolymer latices used as binders for non-woven fabrics has been published by Schlauch and Caimi. 16 (b) For Coating or Impregnation of Fabrics Applications of PVC latices in this field include backings for carpets (tufted and woven), flame-retardant finishes, water-proofing finishes (e.g. for tarpaulins and tent fabrics), priming (keying, bridging or tie) coats on fabrics to be coated with PVC pastes and/or made into belting or other composite structures (wherein the priming coat reduces penetration of the main coating composition into the fabric and improves its adhesion), and impregnation coatings for shoe-lining and book-cover fabrics (to impart heat-bonding properties, or/and moisture resistance, or/and stiffness). The following basic formulations are illustrative of some of the types used (figures are pbw dry solids): (i) High-quality, stiff backing for woven carpet
Vinyl chloride copolymer latex (Breon 351) Nitrile rubber latex (carboxylated, high acrylonitrile-Breon 1571) Sodium carboxymethyl cellulose (added as 5% solution) Water
40 60
2
Amount required to make total solids in composition 30%
(ii) Flame retardant fabric finish Vinyl chloride copolymer latex (Breon 351) Stabiliser (dibasic lead phoSphite) Plasticiser(s) Whiting Antimony trioxide COlourant} Thickener
100 5 50 100
25 As required
1042
W. V. Titow
Compositions based on formulation (ii) may be used to impregnate or coat a fabric, and the viscosity is adjusted accordingly. Heating at 140°C will normally be required to fuse the PVC properly (for full development of mechanical properties and wash resistance). A latex of a room-temperature fusing copolymer (Breon 652) may be used instead of Breon 351 in this formulation for 'on site' applications or generally where no heating equipment suitable for fusion is available.
Note: Coatings produced with Breon 652 may yellow on outdoor exposure. Suitable dark-coloured pigments should therefore preferably be incorporated to mask the effect where appropriate. 23.6.2 Paper Treatments Paper may be treated with PVC latices-alone or in conjunction with nitrile rubber or certain acrylic polymer latices-in various ways, for a number of purposes. Thus a suitable latex composition may be added to the pulped fibres before sheet formation ('wet-end' addition on the paper-making machine) ultimately to impart to the paper (after appropriate fusion treatment, say press finish at about IS0°C) improved strength, stiffness, water resistance, flame retardance, and thermoformability. Finished paper may also be impregnated or coated for similar effects, as well as to provide protection against fats, greases, moisture and abrasion, e.g. for such products as drinking cups, ice-cream cartons, book covers and wall-coverings. In the latter two applications, smooth surface finish promoted by the latex treatment may be a desirable decorative effect, or alternatively, an embossed finish may be produced by a suitable treatment utilising the thermoformability imparted by the polymer deposited from the latex. Latex coatings on paper can introduce heat-bonding capability (see Section 23.6.4 below). Where applied as priming coats they playa role analogous to that of such coatings on textile fabrics. Improvements in paper strength which can be produced in some cases by suitable latex treatment are illustrated by the figures of Table 23.6. The following formulations have been recommended4 for the base coat of a scrub-resistant vinyl wallpaper (to be applied by the air-knife
23
1043
PVC Latices
TABLE 23.6
Some Strength Characteristics of Paper Before and After Impregnation with a Blenda of PVC and Soft Acrylic Polymer Latices (Data from Ref. 4) Material
Tensile strength (kgfcm- 2 )
Bursting strength (kgfcm- 2 )
Tear strength (kgcm- I )
Bending length (cm)
155
10·5
58
11·0
223
12·1
90
52
2·0
33
58
2·1
37
1. Paper impregnated with
latex blend and dried at 90°C for 10 min 2. Same paper, treated as under 1, and then pressed for 3 min at 170°C under 12·4 kgf cm- 2 3. Original paper as used for 1 and 2, without latex but
A. Heat-treated as under 1 B. Heat-treated and pressed as under 2
6·0
Breon 352, 6 parts; Breon 2671 £2, 4 parts. Solids content of impregnation bath: 40%. Pick-up (% of dry fibre weight): 66·6 ± 1·7.
a
technique and top-coated with uncompounded Breon 576):
Breon 576 Breon 151 65% Dialphanyl phthalate emulsion Coalescing solvent (Bisol DPS-BP Chemicals) 40% Calcium stearate Pre-ground pigment dispersion * Water
Formulation 1 Formulation 2 (pbw, wet) 100 65 22
0·7
5 5 100 100 Amount. required for a viscosity rating of 20s with No.4 Ford cup
* A 50% aqueous pigment 'grind' produced by stirring together the following components (in the order given), and colloid-milling prior to use (pbw): Water Polyacrylate dispersant (Dispex 115-Allied Colloids) Sodium hexametaphosphate (Calgon-Albright and Wilson) Clay (water dispersible grade) Titanium dioxide (Tioxide R-HD-Laporte Chemicals)
46.6 0·2 0·2 30 20
1044
w.
V. Titow
23.6.3 Leather Finishes In some of these the polymeric (binder) component may be provided by a blend of a PVC latex with a nitrile rubber one, as exemplified by the following formulation (intended to be combined 1: 1 with a suitable aqueous pigment dispersion):4
pbw, wet (dry) Breon 1562 Breon 652 Triethanolamine oleate (latex stabiliser) 10% aqueous tetra sodium pyrophosphate (buffer against residual acidity in leather) Water
135
(55)
90 10
(45)
to pH 10 10
On application of the finish the main functions of binder are to bond the pigment(s) to the leather, whilst providing some surface protection against moisture and abrasion, and enhancing appearance (increasing gloss).
23.6.4 Adhesive Applications Because of their thermoplasticity and polar nature, PVC films deposited from suitably formulated latex compositions on such substrates as paper, cardboard, fibreboard, some textile fabrics and certain wood products can serve as effective heat-activated adhesives in applications where bonding through a paste coating or via pre-formed sheet lamination is unattractive or impracticable. Simple radiofrequency (rt) welding of substrates precoated with a latex-produced film is a useful route to some products for the automotive industry, packaging, and upholstery applications. Thus, for example, PVCcoated boards may be rf-welded to PVC foam and film to form a range of soft decorative finishes for automotive and domestic purposes. Their ability to retain the padding and durable PVC leathercloth finishes in a rigid construction makes the products attractive, whilst the rf-welding production route is cheap to operate. Breon 151 is claimed4 to be a versatile, cheap basic component for a range of adhesive formulations for work with porous, polar substrates: in such adhesives it may be used alone, or in blends with other PVC, nitrile rubber, or acrylic latices. A starting formulation for a latex
23
PVC Latices
1045
coating to enable PVC sheeting to be rf-welded onto fibreboard or wood for applications in vehicle production or furniture, is illustrated by the following example4 (figures are pbw (dry)):
65 30
Breon 576 Breon 151 Breon 1562 Antifoaming agent Non-ionic stabiliser Pigment Thickener (ammonium polyacrylate)
10 0·1 0·5 10 0·5
Blends of Breon 351 or 352 with a nitrile latex may be used as heat-sealable coatings on paper, or-with addition of suitable tackifying resins-as water-based adhesives with applications in the building industry.
REFERENCES 1. Blackley, D. C. (1966). High Polymer Latices, Vol. 1, Applied Science Publishers, London, Ch. 1. 2. Vanderhoff, J. W., Bradford, E. B. and Carrington, W. K. (1973). J. Polym. Sci., Part C (Polymer Symposia), No. 41, 155-74. 3. Titow, W. V. (1978). In Adhesion 2 (Ed. K. W. Allen), Applied Science Publishers, London, Ch. 12. 4. Breon Latices: Technical Manual No.5. BP Chemicals (UK) Ltd, London, 1969. 5. Glasstone, S. (1948). Textbook of Physical Chemistry, 2nd Edn, Macmillan and Co. Ltd, London, p. 1220. 6. Brown, G. L. (1956). J. Polym. Sci., 22,423. 7. Vanderhoff, J. W., Tarkowski, H. L., Jenkins, M. C. and Bradford, E. B. (1966). J. Macromol. Chem. 1, 361. 8. Dillon, R. E., Matheson, L. A. and Bradford, E. B. (1951). J. Colloid Sci. 6, 108. 9. Voyutskii, S. S. (1963). Autohesion and Adhesion of High Polymers, Polymer Reviews, Vol. 4, Wiley-Interscience, New York. 10. Humbert, L. and Humbert, P. (1959). Official Digest, 181,413, 736. 11. ISO 472-1979. Plastics-Vocabulary. ASTM D 883-78a. Standard definitions of terms relating to plastics. 12. ASTM D 1893-67 (Reapproved 1978). Blocking of plastic film. ASTM D 3354-74 (Reapproved 1979). Blocking load of plastic film by the parallel plate method. BS 2782: 1970: Method 31OA. Blocking offlexible PVC sheet.
1046
W. V. Titow
13. Krcma, R. (1971). Manual of Nonwovens, Textile Trade Press, Manchester, England, and W. R. C. Smith Publishing Co., Atlanta, Ga., USA. 14. Gillies, M. T. (1979). Nonwoven Materials, Noyes Data Corp., NJ, USA. 15. Whitehead, D. A. (1980). Tex. Ind. SA, February, pp. 5-11. 16. Schlauch, W. F. and Caimi, R. J. (1974). Text. Chern. Color., 6(10), 223-9.
CHAPTER 24
PVC Solutions and their Applications W. V. TITaw
24.1 INTRODUCTION The applications of vinyl chloride polymer and copolymer solutions are less extensive than those of solid compounds or pastes. However, the solutions-and the special resins from which most are made-are well established in the application areas where they are utilised, and the advantages they offer are recognised. The principal uses are in surface coatings (including certain paints, corrosion-protective coatings, and overlacquers for PVC), some printing inks, adhesives (solvent cements) for PVC, the production of PVC fibres, and solution-casting of PVC films. These applications are discussed in more detail in the following sections. Although standard suspension-, emulsion- and mass-polymerised PVC resins dissolve in the appropriate solvents, they are not too well suited for much of the solution work where special processing and end-use requirements have to be met: the common ones include ready solubility in mixed-solvent systems (up to high solids content levels for some applications), tolerance for diluents (especially aromatic hydrocarbons), good compatability with other polymeric solutes, solution viscosities suitable for particular uses, good adhesion of solutiondeposited films to substrates (especially metals), and high strength and toughness of the films and other end products. Resins-mainly copolymers of vinyl chloride with other monomers (especially vinyl chloride/acetate copolymers)-are available to meet these and other special requirements. These polymers are 'solution' resins in two senses: they are produced for solution applications, and most are made 1047
w. v.
1048
Titow
by solution polymerisation which gives uniform molecular weight (narrow molecular weight distribution-d. Chapter 1, Section 1.5.1) controllable at the desired level (low, medium or high), and high purity of the polymer-all features particularly desirable for solution use.
24.2 COMPONENTS OF PVC SOLUTIONS By definition, the two essential components of a PVC solution must be PVC polymer(s) and solvent(s). The solutions used for some practical applications (e.g. some solvent cements, film-casting solutions) are just such simple systems. Solutions for other purposes can be multicomponent compositions (see below and Section 24.4).
24.2.1 The PVC Polymer Depending on the application, the polymer component of a PVC solution may be a vinyl chloride homopolymer, a copolymer, or a terpolymer produced either directly from the appropriate monomers or by a post-modification of a copolymer (e.g. hydrolysis of a proportion of the acetyl groups of polyvinyl chloride/acetate to hydroxyls), or a mixture of any of these. As well as the polymer type, the molecular weight will also be chosen in the light of the intended use: it will normally represent a compromise between ease of solution (and achievement of a high solids content, where required) combined with manageable viscosity (all promoted by low molecular weight) and good mechanical properties of the final product which normally improve with increasing molecular weight. (a) Homopolymers
In broad terms, these offer better service properties (including strength and heat-softening characteristics) than the copolymers, but have more restricted solubility and, in products, lower flexibility, with poorer adhesion to substrates (usually no air-dried adhesion to metals). Hence, in the absence of special considerations, homopolymers are preferable for film-casting solutions and copolymers or terpolymers for most other applications, especially coatings. (b) Copolymers Vinyl chloride/acetate copolymers are widely used in solutions for surface coating applications. Some information relevant in this context
24 PVC Solutions and their Applications
1049
is given in Table 24.1 by reference to members of a well-known commercial range. A copolymer of vinyl chloride with triftuorochloroethylene, as well as copolymers with vinyl isobutyl ethers, are also available for solution-coating applications. Both types have good solubility characteristics and the former particularly good service properties (but poor adhesion to metal unless stoved after application at a high temperature). The air-dried adhesion of the VIBE copolymer to metal is good. Solution-applied coatings of vinyl chloride/vinylidene chloride copolymers of high VDC content serve as barrier layers on packaging materials.
(c) Terpolymers The terpolymers in general use as solution resins are vinyl chloride/ vinyl acetate copolymers containing either maleic acid or vinyl alcohol residues as the third component. The maleic acid is introduced by using maleic anhydride as the third comonomer in polymerisation; the vinyl alcohol groups are produced in situ in the usual way, by hydrolysis of some of the acetyls of a VCNA copolymer. Some information on commercial representatives of both kinds of terpolymer is given in Table 24.1. In both, useful properties result from the presence of the additional functional groups. The hydroxyl groups provide reactive sites through which cross-linking can be effected by reaction with amino resins (urea- or melamine-formaldehyde) or isocyanates. 1 The -OH groups also improve compatibility with other polymers (e.g. nitrocellulose and alkyd resins in surface-coating formulations l ). The carboxyl groups in acid-modified VCNA copolymers improve solubility in relatively weak solvent systems and impart good air-dry adhesion for a number of substrates (especially metals and paper), important in coating and adhesive applications (ct. Table 24.7). 24.2.2 Solvents and Diluents
The solubility characteristics of PVC solution resins vary with their chemical type (i.e. whether homopolyer, copolymer or terpolymer, and the nature and amounts of the co-monomers where present), as do tolerance for diluents, and solution viscosity at a particular concentration in a given solvent or solvent system. These aspects of solubility are also affected by the molecular weight: in general, for a given chemical make-up of the polymer, the ease of solution, diluent
-
-
-
13-16
12-13
Approx. 10
11·8-14·2 0·8-1·2
84-87
87-88
Approx. 90
85-88
VYHD
VYLF
VYNS
VMCH
maleic acid
-
Other
14-15
Vinyl acetate
85-86
Vinyl chloride
% Composition
VYHH
Desig· nation
60
Best solvents include some ketones, especially MEK and MIBK; relatively low tolerance for aromatic hydrocarbons Similar to VYHH
82
Medium-high
Medium
Soluble in some ketones and esters
Soluble in some ketones, esters, chlorinated hydrocarbons, alone or in combination with aromatic hydrocarbons As VYHH (higher solids content solutions possible)
Solubility characteristics
16
36
57
Solution viscosityb (cP)
Medium-low
Medium (lower than VYHH)
Medium
Molecular weight
TABLE 24.1 Ucar" Solution Resins
General coating uses (alone or in combination with other resins)
General coating uses (where highest toughness and durability not essential) Blending with VYHH to increase solids content, gloss and 'build' in paints Overiacquers for PVC coatings; strippable coatings; protective coatings
General coating uses (especially where high toughness and durability required)
Some typical uses
Good air-dry adhesion of coatings to metal, paper and other substrates; tough, durable coatings.
Poor air-dry adhesion of coatings to unprimed metal surfaces (improved by baking)
Acceptable in some food-contact applications
Acceptable in several food-contact applications
Remarks
15·5-18·9
2·~5·3
79-82
89·5-91·5
89.5-91.5
79-83
VMCA
VAGH
VAGD
VROH
Medium
Medium (lower than VAGH) Low
5.2--6.5 vinyl alcoholc
Approx. 5 vinyl alcohol
Similar to VMCC
H-2·5 maleic acid
5·2--6·5 vinyl alcoholc
Medium (lower than VMCH)
0·8-1·1 maleic acid
17
43
87
20
37
Acceptable in several Similar to VMCH, Similar to VMCH food-contact but higher tolerance (but where applications for aromatic toughness and hydrocarbons and durability higher solids content requirements lower) solutions possible Good solubility in Coating and adhesive Recommended for ketone/aromatic use in conjunction applications solvent mixtures with cross-linking systems (esp. VERRd ) Soluble in some Coatings (alone or in Acceptable in several ketones, esters and food-contact combination with applications; chlorinated other resins) hydrocarbons; crosslinkable by virtue of hydroxyl tolerance for content alcohols in the solvent systems Acceptable in several Similar to VAGI:>. Similar to VAGH, food-contact applicabut higher solubility tions; crosslinkable and maximum solids by virtue of hydroxyl contents in solutions content As VAGH, but lower Similar to VAGH Similar to VAGD, costs in coatings as but still easier and higher applied solids greater solubility and cheaper solvent systems can be used
a
The former. Bakelite (Vinylite) range of vinyl chloride copolymer resins for solution applications (Union Carbide Corp. in the USA, and associate companies elsewhere). Table based on data from Union Carbide technical literature. b 15% resin in 1: 1 MEK: toluene at 25°C. C Formed by hydrolysing part of the vinyl acetate component. d See Note on pp. 1058-9.
Approx. 12-16
2.~5.3
14·4-17·7
81·5-84·5
VMCC
w.
1052
V. Titow
tolerance and maximum achievable concentration, all increase, and solution viscosity decreases, with decreasing molecular weight. Although all PVC polymers are susceptible, in varying degrees, to many ketone, chlorinated hydrocarbon, and ester solvents, as well as to some aromatic hydrocarbons (cf. Chapter 12, Section 12.8), the homopolymer will give solutions of reasonable concentrations and workable viscosities in only relatively few solvents. Of those, tetrahydrofuran and cyclohexanone are the most important in the technological context. Methylcyclohexanone and isophorone are also relevant, especially for mixed solvent systems, and dimethylformamide is an effective 'booster' for solutions of PVC polymers of high molecular weight. Methyl isobutyl ketone is a useful co-solvent in some systems. For copolymers used in solution applications, the range of effective solvents increases, as does general ease of solution, with increasing proportion of co-monomer(s) present and activity of chemical groups brought thereby into the polymer chain. Solvents effective at room temperature include many ketones, some aliphatic esters, chlorinated hydrocarbons, and nitro compounds. Certain other compounds also act as solvents or co-solvents, e.g. the cellosolves (ethylene glycol monoalkyl ethers), and some aromatic hydrocarbons (benzene, toluene, xylene) can have a marked swelling action. Some solubility data for commercial copolymers are given in Tables 24.2 and 24.3. Tolerance for diluents in solvent systems also becomes greater in the main (although it varies from one copolymer to another): for example, considerable proportions of toluene or xylene can be included in some TABLE 24.2 Saturation Concentrations of a Commercial Vinyl Chloride/Acetate Copolymer (Breon AS 70/42) in Various Solvents Solvent
Weight (g/lOO g solution)
Solvent
Weight (g/lOO g solution)
Acetone Methyl ethyl ketone Methyl isobutyl ketone Cyclohexanone Methylcyclohexanone Mesityl oxide
25 30 25 30 27 32
Isophorone Benzene Toluene Xylene 50/50 acetone/xylene 50/50 MEK/xylene
28 1 1 1
28 28
24 PVC Solutions and their Applications
1053
TABLE 24.3 Solubility (in 10% Concentration) of a Commercial Vinyl Chloride/Acetate Copolymer for Solution use (Vinylitea VYHH) Solubility
Solvent
Acetone Butyl acetate Carbon tetrachloride Cellosolve solvent Cellosolve acetate Dibutyl phthalate Dichloroethyl ether Dimethyl phthalate Dioxane Ethanol Ethyl acetate Ethylene dichloride Ethylene glycol Ethyl ether Dioctyl phthalate Isophorone
at 25°C
at 95°C
S S I I S S S S S I S S I I I S
S S I I S S S S S I S S I I PS S
Solvent
Solubility at at 25°C 95°C
Isopropanol (anhydrous) Isopropyl acetate Mesityl oxide Methanol Methyl acetate Methyl ethyl ketone Methyl cellosolve acetate Methyl isobutyl ketone Propylene dichloride Propylene oxide Tricresyl phosphate Hydrogenated naphtha (diluent) Xylene (diluent) Toluene
I S S I S-CI S S S S S I SO
I S S I S-CI S S S S S PS PS
SO SW
PS S
Key: S, soluble; I, insoluble; PS, partially soluble; CI, cloudy solution; SO, softens. a Materials of this range now marketed under the trade name Ucar-see footnote to Table 24.1. surface-coating formulations. All these points are illustrated by the examples of compositions given in Section 24.4, which also demonstrate that the choice of solvent, or solvent/diluent, system in any given case is determined by the nature, mode of application and end-use of the composition. Aliphatic hydrocarbons, alcohols, and water have precipitant action on PVC solution copolymers, but alcohol diluents can be tolerated in moderate amounts by hydroxyl-containing polymers (especially when dissolved in good solvent(s) in relatively low concentrations) . In general, the solubility of PVC solution resins tends to increase with rising temperature. Highly concentrated solutions may become thixotropic or gel permanently (especially when prepared at an elevated temperature and then cooled down to room temperature). Viscosity increases with the solute content (cf. Fig. 24.1), and-for
1054
W. V. Titow 10000
l
I
.i
:
I
0..1000 u
/
'iii 0
u
l/l
;;: c
(f)
/
. /
I .:/ :/
/
-:
.: / /
~
I I.
I'
1/ I •• •
.• ••
:1
,. '" . ' " . • !/ ,. •
//
100
:
,
,
....Q (5
!
,
....» ::J
:
..
<
",,7
/
....-
.... ..-7 ....-
/
10 0
5
10
15
20
010 Polymer
25
30
35
Fig. 24.1 Solution viscosity (Brookfield) versus concentration for some commercial copolymers. Ucar VYNSIMIBK; _ . Ucar VYNSIMEK; Ucar VAGHIMIBK; + + + + Ucar VAGDlMIBK; ----Breon AS 70142 in MEK:xylene, 1: 1.
many systems, but not invariably-with iQcreasing percentage of diluent at the same solute concentration. The requirements applicable to solvent systems are those for surface-coating or film-casting solutions generally, embodying such considerations as applicational functionality (including evaporation characteristics), health and fire hazards, and cost. 24.2.3
Other Solution Constituents
Like any other PVC composition, a solution may incorporate several of the additives commonly included in PVC formulations. However, the nature and applications of PVC solutions modify considerably the need for, or relative importance of, some of the usual formulation constituents. Thus heat stabilisers, essential in heat-processed compounds, may be omitted from many solution formulations, except
24 PVC Solutions and their Applications
1055
those for stoving finishes or where the product may experience significant heat in service. In these cases the choice of stabiliser should be made in the light of advice from suppliers (of both stabilisers and solution resins) and its suitability verified by tests. With polyvinyl chloride/acetate copolymer, or modified copolymer, solution resins the following general points are relevant. Selected tin or BalCd stabiliser systems can be particularly suitable, preferably used in conjunction with an epoxy co-stabiliser. However, BalCd stabilisers are not recommended for solutions of hydroxyl-modified copolymers,2 and metal soap stabilisers generally can impair adhesion in surfacecoating and adhesive compositions; blooming can also be a problem in some cases, and possible opacifying effects should be borne in mind where transparent final products (e.g. coatings) are required. Basic lead carbonate, satisfactory for some applications (in the absence of nontoxicity requirements) is not suitable for clear formulations: a further consideration with this kind of s.tabiliser is possible development of alkalinity which may be troublesome particularly with acid-containing terpolymer resins. Urea- or melamine-formaldehyde resins can have a stabilising action in clear baking finishes, when present at the level of about 3 phr. A tin mercaptide in conjunction with an epoxy resin (e.g. Bakelite Resin ERL 2774-Union Carbide) has also been recommended2 as a typical stabiliser system for a baked surface coating (in the respective proportions of 1 phr and 5 phr). With polyvinyl chloride/acetate copolymer resins, including hydroxyl-modified grades, zinc stabilisers can cause rapid colour development on heating, especially in unplasticised compositions, or those of low plasticiser content. Stabilisation against light is normally effected by suitable pigmentation, in particular with titanium dioxide used in substantial proportions in many coating formulations, or with carbon black (up to about 6 phr) where a black colour is acceptable. In the absence of protective pigments the resistance of copolymer and modified copolymer solution resins to photochemical degradation is not sufficient for prolonged outdoor service, although a clear film or coating may withstand limited exposure if an effective light stabiliser system is included in the formulation, say about 1 phr of a good UV absorber (e.g. Cyasorb UV-24*-see Chapter 9, Section 9.5) in conjunction with 3-5 phr of a suitable epoxy co-stabiliser (e.g. Bakelite Resin ERL 4221-Union Carbide). * American Cyanamid Co., USA, and associated companies elsewhere.
1056
W. V. Titow
Plasticisers may be incorporated in PVC solutions, for increased flexibility of the ultimate products. The compatability of the usual PVC plasticisers both with the solutions and the PVC polymers or copolymers in the final, solid products is adequate for the purposes of the established applications. In practice, therefore, the plasticiser(s) will, as usual, be chosen for the final properties required (in the light of cost considerations), the main technical limitation on the amount incorporated being development of tackiness in films and coatings and impairment of the adhesion of such products to substrates (especially metals). To the extent to which a meaningful generalisation is possible, about 40 phr may be regarded as the top plasticiser content limit for tack-free coatings based on vinyl chloride/acetate copolymers. The amounts used in solution-deposited coatings based on hydroxylmodified copolymers may typically range between about 15 phr and 30 phr. In some cases, plasticised films and coatings may be improved by a short heat treatment at about 70°C. The already mentioned UV-protective action of pigments (best with carbon black and titanium dioxide, but exerted by all otherwise suitable pigments at sufficiently high loading levels) is one of the reasons for their inclusion in PVC solution-coating formulations, additional to the functions of colouring the end product and imparting covering power to the coatings. Many of the pigments discussed in Chapter 11 (Section 11.3.5) are used, several in relatively high loadings. A few examples of pigment contents of surface coating solutions based on vinyl chloride/acetate copolymers (including hydroxyl-modified grades) are given below. Selected soluble colourants can be used in solutions for clear coatings or films, in rather lower concentrations, as their role is confined to imparting colour.
Pigment Titanium dioxide Carbon black Lead chromate Lead sulphate (white lead) Chrome orange Chrome green (chromic oxide) Iron oxide brown Phthalocyanine green Phthalocyanine blue Aluminium powder
Typical loading range (phr) 50-90 4-6 75-125 75-125 75-100 50-75 40-75 10-20 10-20 30-50
24 PVC Solutions and their Applications
1057
For applications involving heating (e.g. baked finishes), pigments containing zinc should be avoided, as should-preferably-also iron pigments, because of possible promotion of polymer degradation mentioned above in connection with zinc stabilisers. Similarly, alkaline pigments can cause problems in compositions based on acid-modified vinyl chloride/acetate copolymers. Conversely, some lead pigments can have a stabilising action (e.g. white lead). At the high loading levels at which many pigments are used in some PVC solution-coating compositions they may, in some respects, also be regarded as fillers, in that their modifying effects on the properties of the resin in the final product will be broadly in line with those of particulate fillers, as discussed in Chapter 8. In view of the rather specialist applications of PVC solution compositions, fillers with a purely extender (cheapening) function are used only to a relatively limited extent (e.g. clay in some coating formulations) although there are no general technical reasons precluding their incorporation in that role. The properties of PVC solutions and their ultimate products are also influenced by polymeric materials, other than the PVC solution resins, that may be included in some formulations (e.g. for many surface coatings), which may incorporate, for example, epoxy resins, certain cellulose derivatives, or alkyd resins (see also Section 24.4). Such formulations are, however, more properly in the province of surfacecoating technology rather than that of PVC. Lubricants are not included in PVC solution formulations since the processing of this special type of PVC composition does not involve the kind of manipulation of the polymer under heat and shear in which these additives are beneficial (see Chapter 11, Section 11.1.1). Indeed, the presence of a lubricant in, say, a solution-coating composition could impair the ultimate adhesion to substrates. 24.3 PREPARATION OF PVC SOLUTIONS AND SOLUTION COMPOSITIONS FOR PARTICULAR APPLICATIONS
The viscosities of simple PVC solutions used directly (e.g. for film casting) or in the preparation of composite surface coatings are, in most cases, sufficiently low for paddle or impeller mixers to be employed in their preparation, both on the laboratory and industrial
1058
W. V. Titow
scale. The mixer should preferably be covered to reduce solvent loss, and a reflux facility is desirable where heating is applied to assist dissolution of the polymer and keep solution viscosity down for ease of stirring (although heating for these purposes will normally be mild-say at about 35°C). Where the solution is to be heated to assist solvent removal after application, and especially if a deposited film will be post-treated by heating (as with baked finishes), consideration must be given to inclusion of a heat stabiliser in the formulation; in any case, the temperature and duration of heating should not exceed the lowest values necessary for good results. The generally lower heat stability of copolymer resins in comparison with homopolymers should be borne in mind in these connections. Since thermal degradation of PVC resins can be promoted by the presence of iron, the working surfaces of mixing equipment or those of storage containers should not be of mild steel: stainless steel, glass or enamel are the preferred materials. In simple solution preparation involving only the PVC resin and solvent(s), with no other solid constituents (other polymers, pigments) to be incorporated, a useful technique is to add the resin portionwise into the vortex produced in the solvent(s) by the rotation of the stirrer, each portion being allowed to dissolve before the next is added, to avoid lumping. If the solvent system includes also a diluent, the resin may be wetted out with this first, before being stirred into the solvent(s). Alternatively, the diluent may be added slowly, with vigorous stirring, to the solution of resin in the solvent(s); but the pre-wetting method can be particularly useful in preventing lump and gel formation. Where the amounts of solvent(s) and diluent called for by the formulation permit it, the resin should first be dispersed in the diluent, and the solvent added gradually, with vigorous stirring to the resulting suspension, the stirring being continued until solution is complete. As with any polymer solution for clear products, or products of low thickness-e.g. films, fibres-where the presence of even small particulate impurities, gels, or bubbles can seriously affect the properties and appearance, PVC solutions for film casting and clear coatings should be filtered and de-aerated. Note: Ready-made solutions of some vinyl chloride copolymers are commercially available. Two examples are Ucar* vinyl resin solution VYNC (a clear, 40% solution of a hydroxyl-modified
* Formerly Bakelite.
24 PVC Solutions and their Applications
1059
vinyl chloride/acetate copolymer in isopropyl acetate) and Ucar vinyl resin solution VERR-4(j3 (a 40% solution of an epoxy-modified vinyl chloride/acetate copolymer of low molecular weight in MEK: toluene). The VYN C solution is recommended for use in coatings incorporating nitrocellulose or nitrocellulose/alkyd blends, as well as coatings modified by reaction (via the hydroxyl groups) with amino resins or isocyanates. The VERR-40 solution, in combination with an acid-modified VCNA copolymer (e.g. Ucar YMCA) can be used for high-solid baked coatings with very good adhesion to metal, hardness and gloss. Examples of another kind of commercial PVC solutions are proprietary solvent cements for bonding pipes and fittings (see Chapter 20, Section 20.3.2). Because of the nature of the solvents and diluents used, the precautions to be taken in the preparation, storage, transport and handling of PVC solutions are substantially the same as those called for with flammable liquids generally. The preparation of pigmented PVC solution compositions (for use as paints and the like), which may also contain other constituents, e.g. plasticisers (cf. Section 24.4), may be carried out in more than one way, depending on the equipment available. Advice is readily obtainable from the resin or pigment suppliers in particular cases. A useful general approach, which can give compositions producing coatings of good gloss, is to prepare first a solution of the PVC resin(s) in the solvent(s) as outlined above. The pigment (and stabiliser, if used) is then pre-dispersed in the diluent and plasticiser(s), withpreferably-some grinding aid, in the ball mill. The resin solution is finally added to the mill and grinding continued for the requisite time. For coatings of maximum gloss, a suitable pigment concentrate should be used. This may be a solid masterbatch (highly pigmented chips of the appropriate resin) or the appropriate pigment concentrate paste. The paste need only be diluted with the resin solution in the required proportion to make the complete coating composition (if plasticisers are to be included, these can be stirred into the resin solution before blending with the pigment concentrate). Masterbatch chips must be dissolved in solvent (part of the total amount called for by the formulation): the quantity of solvent used should be the minimum necessary to form a thick, pasty (but complete) solution. This solution is then blended with the resin solution (by stirring-in the latter). Where
1060
W. V. Titow
good gloss of the final coating is not a primary requirement, the plasticiser(s), pigment, and grinding aid (and stabiliser if used) may be pre-dispersed directly in the resin solution (in a high-speed disperser, two-roll mill, or other suitable equipment), and the dispersion ground in a paint or ball mill in the usual way. Details of the preparation of paints in which PVC resins are modified by, or serve as modifiers for, other polymeric binders, and oils, belong to the field of surface coatings, and are thus outside the scope of this book. It may be mentioned, however, that the following types of paint binder materials are among those with which vinyl chloride/acetate copolymer and terpolymer resins are compatible (in varying degrees, but generally adequate for paint formulation): urea-formaldehyde resins, phenolic resins, alkyd resins, epoxy resins, polyketone resins, castor oil and other oils, urethane prepolymers. Note: The urethane prepolymers react with the -OH groups of hydroxyl-modified VCNA copolymers, giving tough coatings with very good adhesion. The reaction can proceed at a significant rate at toom temperature, so that the storage life of mixtures is limited to 8-24 h,z For this reason, surface-coating compositions based on urethane prepolymers and the modified copolymers (e.g. Ucar VAGH or VAGD) are formulated as two-component systems, to be mixed directly before use.
24.4 APPLICATIONS Solutions of vinyl chloride/acetate copolymer resins find a number of applications as surface coatings, of which the following are typical: overlacquers for PVC coatings on fabric and paper in such products as leathercloth, floor-coverings and vinyl wallpapers (see also Chapter 22, Section 22.2.6); strippable coatings (sprayed or dipped) for temporary protection of metal surfaces and various products during transport and storage, cocooning compositions (applied by spraying) for equipment and article protection in similar circumstances; protective coating (resistant to chemicals and moisture) for concrete, wood and metal in the building industry. Some relevant starting formulations are given, by way of example, in Table 24.4. Solutions of the modified copolymers (whose carboxyl or hydroxyl groups variously improve adhesion to substrates and reactivity with
1061
24 PVC Solutions and their Applications
TABLE 24.4 Vinyl Chloride/Acetate Copolymer Solutions for Some Coating Applications: Examples of Basic Formulations (Based on data from the technical literature of Union Carbide Corp.)
Component
Formulation (Pbw) Overiacquer for vinyl coatings
Ucar VYNS Ucar VYHH Methyl methacrylate resin Cellulose acetate-butyrate resin' Plasticiser Aluminium powder Methyl ethyl ketone (MEK) Methyl isobutyl ketone (MIBK) Toluene
5·0-7·5 a 2·5-5·0 0·2-0·4
40d 60d
Strippable Cocooning coating solution solution 10 5
16b
4
8 6 70
27 27 27
a Toughness,
flexibility and adhesion improve with increasing resin content. For bridging large gaps (say about 40 cm) 2 parts of the resin should be replaced with polyvinylidene chloride resin of high molecular weight. 'May be omitted if surface slip and blocking resistance not required, or replaced with amorphous silica if the attendant matting effect acceptable. d The amount of the solvent system used may be varied according to viscosity required for application and the desired solids content (but the ratio of 4: 6 MEK:MIBK should be preserved). b
other resins) alone or in combination with one another and/or with the unmodified copolymers, are used in a variety of coating applications where moisture and chemical resistance, toughness and gloss are of interest. The applications include primers and top coats for wood and fibre or particle board (in which the copolymers-especially the hydroxyl-modified resins-are usually combined with other resins, e.g. alkyd, nitrocellulose, urea-formaldehyde, urethane prepolymer), and various paper coating uses, notably coatings for food-packaging papers in which the hydroxyl-modified copolymers (offering particularly good adhesion) may be used alone or in blends with vinyl acetate and other polymers. The same copolymers are also especially useful in decorative/protective coatings for paper labels in view of their excellent adhesion to a variety of print types, and the gloss obtainable after drying at about 105-120°C (enhanced further by a short baking treatment). 2
W. V. Titow
1062
Industrial maintenance and marine paints for steel and other substrates constitute a long-established, important area of application of solution systems based on the vinyl chloride/acetate copolymers and terpolymers. 4 Such systems, nowadays normally of the high-build variety, offer some adyantage over each of the rival types of coating (alkyd, epoxy, or chlorinated rubber) in handling, performance (especially corrosion and weathering resistance) or costs. Something of the nature of the formulations, and the suitability of different resins for particular kinds of these finishes, is indicated by the examples given in Table 24.5. Baked finishes (solution-applied by roller or spray) for tinplate or sheet iron, based on hydroxyl-modified VeNA copolymer in conjunction with the previously mentioned co-reactive resins (UF, MF, epoxy, TABLE 24.5
High-build Vinyl Marine and Maintenance Paints: Examples of Basic Formulations (Based on data from Ref 4)
Formulation (Pbw)
Component
Cellosolve acetate Methyl butyl ketone Toluene Xylene VM & P naphtha Ucar resin VAGH Ucar resin VYHH Ucar resin YMCA Ucar resin VYHD Tricresyl phosphate Didecyl phthalate Thixotropic agent Red lead Rosin Cuprous oxide Dispersant Titanium dioxide Clay extender Zinc phosphate
Red lead primer
Shipbottom antifouling
White anticorrosive primer
White topcoat
44·45 6·35 6·98 4·44 1·27 13·18
13·15 1·88 2·07 1·32 0·38
36·76 5·25 5·78 3·67 1·05
38·20 5·46 6·00 3·82 1·09
2·49
1·36
2·27
1·96 20·01
1-39
21·90
15·66
3·92 0·92
3·90 1·05
11·53
0·27 10·95 13·60
9·75 65·30
9·22
24 PVC Solutions and their Applications
1063
urethane prepolymer) offer excellent protective properties combined with adhesion good enough to withstand drawing, stamping and forming operations involved in the formation into containers, closures and the like. 2 Solutions of vinyl chloride homopolymer of relatively high molecular weight (for good mechanical properties) are used (at about 30% solids concentration) for the production-on a limited scale (because of the relatively high cost in comparison with calendering or extrusion)-of high clarity PVC film for special packaging applications. The solution is cast onto a stainless steel band in the way originally developed for casting cellulose acetate film. 5 ,6 Lack of molecular orientation, and hence freedom from mechanical and optical anisotropy (which, inter alia, virtually eliminates retraction on heating and affects optical properties) is a special feature of cast polymer films. The polymeric components of PVC solutions used as solvent cements for bonding pipes, pipe fittings, and certain other PVC products may be homopolymers, copolymers, standard PVC compounds,7 or the actual compositions of the mouldings, etc., to be bonded. Solvent cements for PVC are discussed in Section 20.3.2 of Chapter 20, where attention is drawn to some relevant standard specifications. Two further ones, not mentioned either in that section or in Appendix 1, are: ASTM D 3138-80 Solvent cements for transition joints between acrylonitrile-butadiene-styrene (ABS) and poly (vinyl chloride) (PVC) non-pressure piping components. ASTM F 493-80 Solvent cements for chlorinated poly (vinyl chloride) (PVC) plastic pipe and fittings. PVC fibres are spun from solvent solutions of suitable copolymers. 8 24.5 ADHESION OF SOLUTION·APPLIED PVC COATINGS TO SUBSTRATES
In general, the adhesion of vinyl chloride homopolymers and vinyl chloride/acetate copolymers to smooth substrates (especially metal, but also others) is poor, although it can be improved in some cases by a baking treatment (cf. Table 24.6). The presence of carboxyl or hydroxyl groups in the modified copolymers improves the adhesive properties of these resins substantially, as indicated in Table 24.7.
1064
W. V. Titow
TABLE 24.6 Adhesion of Vinyl Chloride/Acetate Copolymer Resin (Breon AS 70/42) Film Solution -deposited onto Aluminium Sheet Q
Stoving conditions
Solution solids
Resin (20 g)
None 30 min. at 50°C 15 min. at 150°C
Resin (15 g) + maleic acid (5 g)
Resin (10 g)
None } 30 min. at 50°C 30 min. at 150°C
+ maleic acid (10 g)
None 30 min. at 50°C } 30 min. at 1500C
Q
Adhesion
Air-dried film peeled easily Slight adhesion Adequate adhesion Air-dried film peeled easily Adequate adhesion Air-dried film peeled easily Adequate adhesion
Solution: solids, 20 g; acetone, 40 g; toluene, 40 g; phosphoric acid, 0·5 g.
TABLE 24.7 General Adhesion Characteristics of Commercial Vinyl Copolymer Resins for Solution Use (Based on data from Ref. 2) Resin type
Surface
Metal (clean, smooth) Phosphated metal Wood Glass Paper Cloth PVC Polyvinyl butyral Phenolic resins Urea-formaldehyde resins Methacrylates and acrylates Q
Vinyl chloride acetate (Ucar VYHHor VYHD)
Carboxylated VClVA (Ucar YMCA, VMCC or VMCH)
Hydroxylated VCIVA (Ucar VAGH or VAGD)
P P P P P P E P P P
E E
P F
F E G F-E E F F F
F F G G E E G G
E
E
E
1065
24 PVC Solutions and their Applications
TABLE 24.7-eontd.
Surface
Resin type Vinyl chloride acetate (Ucar VYHHor VYHD)
Carboxylated VCIVA (Ucar VMCA, VMCC or VMCH)
Hydroxylated VCIVA (Ucar VAGH or VAGD)
P P F P
F F F P
P E F
P
G
G
E
G G
Nitrocellulose Alkyd resins Chlorinated rubber Oleoresinsa Shellac Concretea Plastera Key: E
P
G
E
F-E
= excellent; G = good; F = fair; P = poor.
a Adhesion
can vary with type.
Notably, the air-dried adhesion of carboxyl-modified copolymer is particularly good, e.g. Ucar VMCH adheres well, on air-drying, to iron, steel, cadmium and brass, although on zinc a phosphate pretreatment or a wash primer is desirable (wash primers are useful in many other cases). Admixing with the VMCH resin can improve the adhesion of other vinyl chloride/acetate copolymer coatings.
REFERENCES 1. Bakelite Vinyl Resin Solution VYNC, Union Carbide Corp. Technical Bulletin, 1971. 2. Bakelite Hydroxylated Vinyl Resins VAGH and VAGD, Union Carbide Corp. Technical Bulletin, 1971. 3. Bakelite Vinyl Chloride-Acetate Resin Solution VERR-40, Union Carbide Corp. Technical leaflet, 1978. 4. High-build Vinyl Maintenance Paints, Union Carbide Corp. Technical Bulletin, 1973. 5. Yarsley, V. E. and Flavell, W. (1956). Cellulosic Plastics: Part 1: Cellulose Acetate, Cellulose Ethers, and Regenerated Cellulose, Plastics Monograph No. C6, Plastics Institute, London. 6. Couzens, E. G. and Yarsley, V. E. (1968). Plastics in the Modern World, Penguin Books Ltd, Harmondsworth, Middlesex, England. 7. ASTM D 2564-80: Specification for solvent cements for poly (vinyl chloride) (PVC) plastic pipe and fittings. 8. Cook, J. G. (1964). Handbook of Textile Fibres, Merrow Publishing Co., Watford, England.
CHAPTER 25
Cellular PVC Materials and Products w.
V. TITaw
25.1 INTRODUCTION Neither the terminology nor its usage in the field of cellular plastics is as yet fully standardised. The principal nomenclature standards* offer definitions of some basic terms, but these are by no means mutually identical. Moreover, none of the definitions can be said to be fully comprehensive, and they are not adhered to strictly in industrial usage. For the purpose of the present chapter the most important relevant terms are defined as follows. Cellular PVC: A PVC material or product whose apparent density is significantly lower than that of its parent PVC composition by virtue of the presence of numerous cells (voids) dispersed throughout its mass.
This definition conforms closely with those of 'cellular plastic' given in the ISO and ASTM standards. * Cellular vinyl:t A cellular PVC whose solid material is a flexible PVC composition. PVC foam = expanded PVC: A cellular PVC in which the cells have
* ISO 472-1979; ASTM D 883-80; BS 1755: Part 1: 1967.
t As
mentioned in Chapter 1, the use of 'vinyl' for 'PVC', especially 'flexible PVC', is widespread, although it might be questioned on strict terminological grounds (see Chapter 1, Section 1.1). In the context of cellular PVC the 'flexible' connotation of 'vinyl' is particularly strong. 1067
1068
W. V. Titow
been formed by a gas evolved in, or introduced into, the parent PVC composition in the course of production or processing. Vinyl foam = expanded vinyl: A PVC foam whose solid material is a flexible PVC composition. Blown PVC: A PVC foam in which the cell-producing gas has been generated or expanded within the parent PVC composition (and not, say, mechanically admixed at atmospheric pressure). Blown vinyl: A blown PVC whose solid material is a flexible PVC composition.
In both the commercial and the technical sense PVC foams form the most important group among cellular PVC products. They are also significant among plastics foams generally, with several applications where their processability and/or properties offer special advantages over alternative cellular materials including those (like, for example, polyurethane foams) whose volume of use is much greater in many areas. Two characteristic examples are the foam layers of composite coatings on vinylleathercloth (d. Chapter 22), and rigid foam cores of GRP or other sandwich panels used in the construction of vehicle bodies (e.g. refrigerated vans, modern high-speed train cabs l ). In the former application, processability in paste form-unique to pPVCand good bonding to the skin and anchor layers of the coating are cardinal advantages; as are the strength of rigid PVC foam (greater than that of polyurethane or polystyrene foam) and its low flammability in the constructional sandwich panels. Note: The floor in the cab of the British Rail high-speed train is a GRP/PVC foam panel. In the prototype a more complicated sandwich was used, with a composite PVC/polyurethane foam core layer in which the PVC foam (of density about 40 kg m-3) was the strength-imparting component.
Like other expanded plastics PVC foams are usually classified by reference to type, i.e. flexible or rigid, and density. The size and nature (open or closed) of the cells is also a significant characteristic, influencing such properties as, for example, the resilience and compression behaviour of flexible foam or the heat conductivity and moisture permeability of rigid foam insulation. The description of a plastics foam as low, medium, or high
1069
25 Cellular PVC Materials and Products
density, very common in practice, is not exact, as the density ranges corresponding to each of these terms are not strictly defined. The following figures can, however, serve as a rough, general guide. Low density range Medium density range High density range
kg m-3
lb /t- 3
10-50 50-350 350-900
0·6-3 3-21 21-54
Expanded PVC can be produced in a variety of densities. Most of the commercial rigid foams which find application in vehicle and marine construction, cold-store insulation and some aircraft interior fitments, are of low to medium density, say, in general terms, 30-400 kg m -3, with densities of 80-120 kg m -3 fairly typical for the PVC foam cores of constructional sandwich panels for vehicle bodies. Many types of flexible foam fall within the same general density range, but higher densities, up to about 800 kg m -3, are also practicable. Such high densities are fairly typical for (and may even be exceeded in) flexible injection-moulded microcellular shoe soles, or certain types of permeable coatings on leathercloth (cf. about 1250 kg m- 3 for solid flexible PVC). Some rigid structural foam products (e.g. extruded cellular PVC profiles, injection-moulded parts) are also in the high density range. The production of vinyl foam from plastisols provides the basis of important industrial processes and such large-tonnage commercial products as coated fabrics, flooring and foam-backed carpets. These processes and outlets are dealt with in the following sections, as are other applications and production methods of cellular PVc. The production of expanded vinyl coatings on fabrics is also mentioned in Chapter 22. Microporous PVC sheets with intercommunicating cell structure, which are used as battery separators and filter media, may be mentioned as an example of a cellular PVC material manufactured by methods other than foaming (see Section 25.2.2 below).
25.2 PRODUCTION METHODS AND PROCESSES 25.2.1 Foams (a) Dispersed-gas Blowing The general principle of this method is the incorporation of an inert gas, usually a halogenated hydrocarbon or CO2 , into a PVC plastisol at
1070
w.
V. Titow
low temperature and high pressure (the gas usually dissolves in the plasticiser(s) under these conditions), followed by pressure release and heating, carefully controlled to bring about synchronised expansion and solidification (fusion) of the composition, which is finally cooled. The principle has been utilised in several commercial processes, including the Elastomer process2 ,3 (originated in the USA, but also used in Europe), the Dennis process4 (sometimes known as the 'Fay Foamer' process) and the German Trovipor process of the Dynamit Nobel company. 5 The PVC foam manufactured by processes of this kind is flexible, mainly open-cell (about 90% of the cells intercommunicating), and of low to medium density (typically 60-270 kg m- 3). It is normally produced in the form of continuous sheet (slab) several inches thick, which is usually slit into thin layers (e.g. for use in upholstery). The sequence of operations in the Trovipor process is schematically shown in Fig. 25.1. Machines have been available since the mid-1960s capable of slitting the foam slab into layers of thickness down to 1·5 mm (0,050 in) with a tolerance of about 0·25 mm (0,010 in).6 The layers may be bonded to textile fabrics with adhesives, for use, for example, in the proquction of foam-backed clothing~ However, such techniques will not produce cellular PVC leathercloth, partly because of the comparatively coarse nature of the foam, and partly because the thickness of coating on many types of leathercloth is only about 0·040 in or even less. It is also very difficult (and costly) to produce cellular layers of this thickness by direct deposition of 'wet' dispersed-gas-blown foam onto a fabric. Cellular leathercloth is not, therefore, made from this type of foam, although it is a major field of application for foams produced by other processes, as described below. The most important application of foams made by dispersed-gas blowing was for many years in the automobile industry as padding and cushioning material, especially in car seat upholstery, where the open-cell nature (and hence 'breathability') of the foam, its good resilience, and-perhaps most importantly-its suitability for highfrequency welding to PVC sheet materials used for seat surfacing, were important advantages. However, over the past ten years the use of vinyl seating and trim in motor cars has declined drastically in favour of fabrics, now widely used for this purpose (in conjunction with polyurethane foam cushioning). Vinylleathercloth is currently installed in only a few per cent of European cars (the percentage is substantially higher in cars for export to, or made in, some overseas countries, notably in Africa). Vinyl foam produced by dispersed-gas blowing still
25
1071
Cellular PVC Materials and Products
(B)
m
I
____ --.l...-
~I ()
Inlrt gu
(C)
0
!
p
I
I
_
P s
,q
r
I
()
Fig. 25.1 Schematic representation of the Trovipor process. * A. Paste preparation: a, PVC; b, plasticiser; c, additives; d, mixer. B. Gasification plant: e, gas circulation pump; f, cooler; g, autoclave; h, paste; i, gas stream; k, perforated plate; I, paste feed pump; m, paste feed container. C. Spray and fusion plant: n, spray tower; 0, h.f. heating; p, after-heating; q, cooling tunnel; r, to transverse or longitudinal cutters; s, conveyor belt.
features prominently in such major end-uses as travel and fancy goods, certain types of foam-backed clothing, embossed quilting, and some types of furniture upholstery (especially where low flammability is of particular importance). (b) 'Chemical' Blowing This method is used industrially in the manufacture of many cellular PVC products (see Table 25.1), but it is also convenient for small-scale preparation of PVC foams, e.g. in laboratory operations. A blowing • H. V. Finkmann et al. (1969). The foaming of plastics, Kunststoffe, 59(9), 695; reproduced by kind permission of the publishers, Carl Hanser Verlag.
Free ~ blowing
The sheet may be laminated with a fabric and topped with a dense skin. Sheet or
Calendered sheet (produced by compounding and pro-
Sheet blowing
Solid (gelled) skin usually formed first by rotational casting from ordinary plastisol. Mould (vented to prevent pressure build-up during blowing) then partially filled with foaming composition (amount adjusted to give foam filling mould but with atmospheric pressure within cells), heated in hotair oven, and finally cooled
Plastisol
Deposition of plastisol layer on sheet support (e.g. release paper or conveyor band) for subsequent lamination with a fabric, or direct onto a fabric; heating to expand and gel (hot-air or IR oven); cooling. Posttreatments: embossing, lacquering, printing
Cavity filling
Q
Principal production operations or process stages
Plastisol
Form of composition (starting material)
Direct expansion
Method type
Carpet and flooring underlays, embossed wallpapers, cellular leathercloth, upholstery, cushioning
Typical products
Largely closed cell at medium densities, largely open-cell at low densities
Foam flooring, cellular leathercloth
Soft: low to medium density, Foam filling for arm rests, mainly open-cell structure, handlebar grips, dolls and but mixed in some products soft toys
Largely closed cell at medium densities, largely open-cell at low densities (could be described as mixed cell structure)
Characteristics of foam produced
TABLE 25.1 Main Production Methods Involving Chemical Blowing
~~
~
:<::
0
-.l IV
-
pounds
E"ru,;on
r
Plastisol (incorporating polymerising component, e.g. methyl methacrylate)
Substantially the same as in general extrusion of PVC compounds, but preferably with finer screen packs and land-less dies (for good surface finish). Temperature
Porous irrigation tubing, filtration Electrical cable sheathing, pipe, weatherstrip and other foamed profiles
Medium to high density, mainly closed cell
Buoyancy blocks, fishing floats, structural laminates insulation (thermal and acoustic), shock-absorbent materials
Shock-absorbent material for crash pads and helmets, life jackets, packaging
Medium density, mainly open cell
Rigid, medium density, closed cell
Rigid, low density, closed cell
Semi-rigid or soft, low to medium density, mainly closed cell
A strong mould is filled completely ('105%') with plastisol. It is then placed in a press (with a covering gasket) and pressure applied sufficient to keep mould closed during blowing. Platens are heated at the required temperature for the required time, then cooled to cool the moulding to below 30°C (surface temperature) before removal. The pressures, temperatures and times are established in pointer trials. Expansion of the moulding is completed by heating in boiling water or circulating-air oven (at about 100°C)
Plastisol
Plastisol (incorpor. ating volatile or Pressure blowmg { cross-linking plasticiser)
l laminate then conveyed with (could be described as mixed positive support through cell structure) heating tunnel (hot air) at the temperature necessary to fuse the material and effect blowing
cessing the composition at temperatures below the decomposition point of the blowing agent)
w
-.J
...... 0
~
l::>I::
~ "
;:,
'"I::>-
l:;"
;:l.
"'"
~
C"'l
-.:::
~
...
:::::: I:: ;:;-
Q
~
Moulding compoundsb
Form of composition (starting material)
Q
Characteristics of foam produced
Essentially as in normal inNormally medium to high jection moulding, but exdensity, mixed cell structure pandable-cavity moulds or short shots usually employed. The following factors are also important: careful temperature control, injection time (preferably short), mould gate size
(and hence rate of decomposition of blowing agent) and cooling and take-off rate should be particularly carefully controlled. The hot extrudate emerging from the die is weaker than in extrusion of 'solid' PVC
Principal production operations or process stages
Shoe and slipper soles; rigid structural foam mouldings
Typical products
b
Q
In each case containing the appropriate quantity of blowing agent, properly dispersed (d. also Section 25.3 below). These may be purchased as special blowing grades, or produced in-plant either by a complete compounding operation (from individual formulation components) or by compounding the blowing agent(s), or blowing agent masterbatch, into an appropriate PVC composition. In all cases the temperatures should be so controlled that the blowing agents are not activated. In compounding dry blends the blowing agent(s) may be added either pre-dispersed (if solid, by milling) in the plasticiser(s), or in powder form with the drying agent (flow promoter). In the latter case the addition may be made after the cooling of the mix, as a further safeguard against activating the blowing agent prematurely; however, the resulting degree of dispersion may not be as good as when the blowing agent is pre-milled in plasticiser. A good solvating plasticiser (e.g. BBP) should be included in the plasticiser system of any plasticised composition for blowing.
Blowing in extrusion and injection moulding
Method type
TABLE 25.1-eontd.
o'l:
:::'1
~
:-::
~
......
25
Cellular PVC Materials and Products
1075
agent (a chemical which decomposes on heating to generate a gas) is intimately dispersed or dissolved in a PVC composition. Subsequent heating causes evolution of gas with consequent cell formation in, and expansion of, the material. The products are described as free blown if foamed unconstrained at atmospheric pressure, or pressure blown if in a mould at a higher pressure. Blowing in the course of extrusion (e.g. in the production of foamed profiles or pipes-ef. also Chapters 14 and 19) may be regarded as intermediate between these two modes: although the blowing agent decomposes in the hot stock inside the extruder barrel, the gas evolved is believed to remain in solution in the melt under the prevailing high pressure, and blowing takes place only when the pressure is released as the melt leaves the die. 7 Similarly, in injection moulding of cellular articles (e.g. microcellular PVC shoe soles) the blowing agent decomposes in the melt, but-although blowing is effected in the mould-expansion takes place only when pressure is reduced either because a reduced shot volume is used (initially underfilling the cavity) or if the mould incorporates a movable base plate8 or other means of expanding the cavity. These are the two most common arrangements for injection moulding of foamed PVC parts. The material of both injection-moulded and extruded cellular PVC products is high-density foam encased in an integral skin. Such foam is known as 'structural foam'. The general characteristics of structural foams, and methods of their production by extrusion and injection moulding are summarised in Refs 9 and 10 (the latter being particularly concerned with injection moulding of fibre-reinforced thermoplastic foams). Successful operation of the two processes for foam production, and the quality of the products, are cardinally dependent upon proper control over cell formation: the key factors instrumental in this are the chemical nature, physical state, and degree of dispersion of the blowing agent; the correct relationship between its decomposition characteristics (temperature range, rate of gas evolution and total amount generated) and the processing temperature(s) of the plastics host material;ll the melt viscosity of the plastics material;1l,12 and other factors influencing cell stability during formation, e.g. the presence of cell control agents.
Note: Some commercial additives of this kind are effective, at low incorporation levels (about 1 phr) , in promoting cell uniformity and eliminating 'overblow' even in foaming compositions heavily filled with mineral fillers (cf., for example, product VS-I03-Air Products and Chemicals Inc., USA).
1076
W. V. Titow
There are several commercial systems for the continuous production of chemically blown structural foam injection mouldings, based on consecutive presentation to the injection unit of moulds carried on a rotary table (cf., for example, the Schloeman-Siemag equipment originated in Europe,13 or the rotary version of the Hettinga set-up l4 developed in the USA *), or a multi-station line (as, for example, in another version of the Hettinga equipment I4). A method claimed to produce-on largely conventional equipmentparticularly smooth-surfaced extruded sections in rigid PVC structural foam (of densities 500-1200 kg m3) was patented in the mid-1970s in France by CDF Chemie. 15 The causes and mechanisms of formation of the well-known 'swirl' patterns in the surface of structural foam products are summarised in a paper by Gross and Angell. 16 Table 25.1 gives an indication of the physical form of the PVC composition appropriate to the various modes of 'chemical' blowing, together with the main operations or process steps, the types of foam produced, and typical applications. Many applications overlap in some areas with those of PVC foams produced by the other methods discussed in this chapter. It will be self-evident that all the compositions in column 2 of the table will contain blowing agents, normally introduced at the compounding stage (and nowadays very commonly in masterbatch form: see also Section 25.3 below). For the sake of convenience, dry blends for foam production have been mentioned in conjunction with extrusion and moulding compounds (in the footnotes to Table 25.1). However, two points should be noted. The preparation of dry blend powders for foam production is very similar to that of 'ordinary', non-foaming versions (say, for use as extrusion feed stock), except for the addition of blowing agent(s). At the same time, the foamable dry blends may be processed in various ways, notably by doctor-knife spreading as in the production of carpet backings, which have been a significant outlet for such compositions. Other application methods have included ftuidised-bed coating or spraying (e.g. for expanded protective coatings on metals). A good account of foams from dry blends has been given by Hartman et al. 17 (c) Gas Entrainment ('Mechanical Frothing') In this method a PVC plastisol is expanded into a stable 'wet' foam (froth) by vigorous mechanical agitation or whipping to entrain a gas,
* The reM ('thermoplastic cellular moulding') system, marketed by the Foam Moulding Corporation, USA.
25 Cellular PVC Materials and Products
1077
usually air. The foam is fused by heating. The plastisol must contain a surface-active agent to facilitate foaming and stabilise the froth, which is usually spread on a support (frequently a fabric, to form a laminate, or release paper for subsequent lamination to fabric-...cf. Chapter 22). Fusion is effected in a hot-air, IR or rf oven. The method normally produces soft foams, with largely open-cell structure; the cells can be very fine. A wide range of densities, roughly 300-800 kg m- 3 or even higher, may be achieved by varying the plastisol formulation, and adjusting the production procedure. The foam may be embossed and surface-coated (e.g. with a lacquer). The main applications are similar to those of PVC foam produced by the direct expansion of a plastisol in the free-blowing technique; they include cellular leathercloth, carpet backing and garment padding. The Vanderbilt process3,11l and its variant developed by Scott Bader and Co., Ltd,6 exemplify industrial processes utilising the gasentrainment method. (d) In-situ Gas Evolution and Cross-linking This is not a general principle on a par with, say, 'chemical' blowing or mechanical frothing, but the cardinal feature of a process* for the production of rigid cross-linked PVC foam (Vinylcel). The superior thermal and dimensional stability, chemical resistance, mechanical strength and stiffness claimed for this material in comparison with conventional rigid PVC foams are attributable to the cross-linked structure. Preparation of the foam is said to start with a composition comprising a PVC resin, vinyl monomer, maleic anhydride, a diisocyanate, and a free-radical catalyst, and to involve, as its main phases, copolymerisation of the monomer with the anhydride, evolution (on heating) of CO2 (with consequent blowing) concurrent with hydrolysis of the anhydride and its reaction with the diisocyanate which gives rise to the cross-linked structure. Foams in the density range 32-64 kg m-3 (2-4lb ft-3) produced by this process found applications in sandwich panels and thermal insulation.
(e) Solvent (Monomer) Blowing This is not an established process, but a technique once devised 19 for the production of expanded beads from vinyl chloride/ethylene * Originally operated by Kleber Colombes SA in France, and Johns Manville Corp. and B. F. Goodrich Chemical Co. in the USA.
1078
W. V. Titow
copolymers. The beads may be consolidated into cellular sheet or moulded articles by surface 'welding' under heat and pressure: they have also been proposed for use in the production of syntactic foams through bonding with resins or other suitable media. The beads are made from particles of the suspension-polymerised copolymer swollen in the monomer mixture and expanded by rapid volatilisation of the permeant on heating under reduced pressure, i.e. in a way somewhat similar to the solvent blowing of polystyrene beads. The method cannot be used with PVC homopolymer as, quite apart from the health hazard presented by vinyl chloride monomer (see Chapter 1, Section 1.2; and Chapter 12, Section 12.9.1), the homopolymer is not dissolved or appreciably swollen by it. 25.2.2
Other Cellular PVC Materials
The commercial applications of materials produced by the methods mentioned in this section are much less extensive and numerous than those of PVC foams.
(a) The 'Lost Filler' Method This is one of the oldest methods of producing cellular plastics materials. Its essential features are that a particulate additive, capable of being subsequently removed by dissolution, is intimately and uniformly dispersed in a polymeric composition, the filled composition is formed into the desired product (e.g. sheet, moulding, etc.), and the additive is dissolved or leached out. In practice the level of loading is such that there is substantial mutual contact between the particles of the soluble filler to enable effective dissolution on immersion in the appropriate liquid medium: the finished cellular product thus has an intercommunicating pore structure, the cell size being influenced by the degree of dispersion and particle size of the soluble filler. The earliest fillers used in practice were finely comminuted mineral salts, dissolved out by immersion of the filled material in water. In the PVC context an early practical embodiment of the method was the original Porvic process20 for the production of porous sheeting. The method was also used in the manufacture of PVC battery separators21 (although nowadays these are normally made by sintering of powder-see below). In both cases the removable filler was starch: this was leached out of the formed products by hydrolysis and dissolution with dilute sulphuric acid.
25 Cellular PVC Materials and Products
1079
An interesting variant of the general method-with finely dispersed droplets of water constituting the 'filler'-has been proposed22 for the production of porous PVC film from a plastisol. The paste may contain up to 50 phr of water, the voids being formed as this evaporates when heat is applied to gel and fuse the material. The process is claimed to be controllable so that the vaporisation causes comparatively little expansion, and the main proportion of the voids arise simply as a result of the disappearance of the dispersed water from the composition. (b) Sintering of Powder The principle of this long-known method, by which porous sheets and mouldings can be produced from plastics powders, is surface fusion of the particles (appropriately size graded for good processing and pore size of the product) at an elevated temperature, and sometimes also under pressure. Solvents or plasticisers may be present to facilitate the bonding. With PVC and other plastics powders to which variants of the method are applicable,2°,23 intercommunicating cell structures are usually obtained. The only industrially significant application of powder sintering in modern PVC technology is the production of battery separators, where the method has been used for a long time. 21 The production is continuous, the powder being preformed on a moving belt-typically co-acting with another element, e.g. a roller (as in the Jungfer roll-coating process)-and sintered in the familiar ribbed form of the separator plate. A special grade of emulsion homopolymer is used, with particle size range and distribution suited to good dry flow and giving a high angle of repose to maintain sharp rib definition during sintering. The molecular weight is relatively high, for good product strength and stiffness, whilst the generally small size and uniformity of the particles characteristic of emulsion-type resins make for fine, uniform ultimate porosity. As heat stability is not a service requirement, stabilisers are not normally used: in their absence the heating during sintering can turn the product a fawn or light brown colour, but it does not degrade the relevant properties significantly. Apart from the moisture content (a few per cent, conditioned into the powder as it helps smooth fusion) the only additive present is a small amount of a surface-active agent which promotes wetting of the separator plate by the aqueous acid solution in the battery.
1080
w.
V. Titow
25.3 FORMULATION AND PROCESS FACTORS IN FOAM PRODUCTION The properties of all PVC products are governed by the formulation and, in varying degrees, also by the processing method and conditions. The effects of these factors are particularly pronounced in the case of cellular PVC, and especially foams. 25.3.1 Effects of Formulation and Processing Variables on Foam Properties Other factors being equal, increasing the molecular weight (K value) of the polymer will improve the mechanical properties of a cellular PVC product by upgrading those of the solid substance of the cell walls. The effect is greatest in rigid materials and at high densities, and relatively less marked in flexible materials especially at low densities. However, the processing characteristics of a PVC composition (e.g. melt viscosity of a rigid compound, gelation/fusion properties of a plastisol) are also influenced by the K value of the polymer. This influence, where sufficiently pronounced (and not counteracted by adjustments in the formulation and/or processing conditions) may be ultimately reflected in such characteristics of the final product as its cell size and structure: these characteristics are also factors in the mechanical properties of the cellular material as a whole, and their effect may modify the direct one of molecular weight of the polymer on the properties of the solid walls of the cells. In flexible foams produced from pastes by chemical blowing or mechanical frothing, the main polymer component will be an emulsion-type PVC resin: its grade (as determined by the particle size and size distribution) will influence paste viscosity and general rheological behaviour in processing (see Chapter 21, Section 21.3.1), including cell formation in foaming (which, in turn, affects the finished foam properties). The presence of any extender polymer will also have an effect in these regards. It has long been known, as a matter of practical experience, that the foaming behaviour of a paste of one and the same formulation can be significantly affected by the make (Le. source of supply) of the emulsion polymer used. There is good evidence24 that such effects are attributable to differences in the nature and content of emulsifier residues present in vinyl chloride polymers and copolymers produced
25
Cellular PVC Materials and Products
1081
by the emulsion method by different manufacturers: the differences are reflected in the degree and extent to which the residual emulsifiers interfere with the action of the surface-active agents used as cell stabilisers in paste formulations for foaming (particularly important in foam production by the mechanical frothing method-see below). In general, any factor affecting paste rheology can influence cell formation in processing and hence the properties of the ultimate foam product. The amount of plasticiser(s) is a cardinal factor. In general, softer foam, with decreasing modulus and strength, is obtained with increasing plasticiser content. The choice of plasticisers is also important: for low-density foams the presence of a good solvating plasticiser is essential, to promote rapid setting of the cell structure at the appropriate, early, point of the heat treatment, and good gelation/fusion characteristics generally. Such plasticisers are also beneficial, for the same reasons, in higher density foam compositions. BBP or the triaryl phosphates, as well as some other strongly solvating plasticisers (e.g. propylene glycol dibenzoate), are included in foam formulations for this reason, commonly in conjunction with cheaper phthalate plasticisers like DOP, DAP, etc. Fillers also affect foaming behaviour and the ultimate foam properties. Whilst cell stabilisers are significant as processing additives in PVC compositions for foam production by chemical blowing or dispersed-gas blowing (ct. Section 25.2.1 above), they are cardinally important components of paste formulations foamed by mechanical frothing. In such formulations the special surface-active agents used as cell stabilisers also promote the initial mixing-in of the air. To discharge both functions properly, the agent must have the right chemical structure to combine the requisite degree of solubility in the plasticiser system with correct alignment of its molecules at the air/paste interface of the cell (bubble) walls, in a way optimal for effectivity in three relevant actions characterised by Acton and Debal25 as: (i)
lowering the interfacial tension between paste and air (to reduce the energy required for frothing, and to promote stability in the bubbles formed); (ii) increasing surface elasticity; (iii) promoting surface plasticity. The effect of (ii) is seen by these authors as reduction of the
1082
W. V. Titow
tendency of paste forming a cell wall to flow, under gravity, towards the bottom of the cell and thus cause local attenuation at the top which may culminate in rupture and collapse. That of (iii) is increased resistance to similar but longer-range draining of paste down the liquid columns formed by the interconnecting walls of vertical 'stacks' of cells. In view of the complexity of the mechanism of their actions, it is not surprising that the surface-active agents used as frothing aids/cell stabilisers in PVC pastes can differ in their effectivity in different formulations. In the absence of relevant practical experience, suitable selection will be aided by advice from suppliers (of surfactants for use in plastics compositions, * or of PVC polymers or plasticisers) and finalised on the basis of trials. Examples of commercial additives of this kind are Atmer 152 (a mixture of surface-active agents in a phthalate plasticiser carrier, supplied by Atlas Chemical Industries); the range of silicone-based surface-active agents marketed by the Dow group of companies; and products in the Dekor Cellset range (Scott Bader & Co. Ltd). The main processing factors governing cell formation-and hence the quality and properties of the ultimate foam product-in the manufacture of structural foam by chemical blowing have been mentioned in Section 25.2.1. Closely similar considerations apply to foams chemically blown from pastes (ct. Table 25.1) where, however, the gelation/fusion properties of the paste constitute an additional factor of primary importance. This is so also in the foaming of pastes by dispersed-gas and mechanical frothing methods where, in analogy with the operation of a chemical blowing agent, the dispersion of the gaseous blowing medium should be as thorough, initimate and uniform as possible. In mechanical frothing of pastes the air is normally entrained by means of a mixing head into which it is fed, under pressure, simultaneously with the paste. The air pressure at the head, and the back-pressure of the discharging air/paste stream are parameters influencing the ultimate foam density, 2S ,26 with the air-to-paste feed ratio as another important factor. 2S The foam density can also vary with the thickness of the 'wet' froth layer before gelation and fusion: with froth deposits on a fabric backing, Acton and Debal2s
* See, for example, the British Plastics Federation's Buyers Guide for Plastics Additives (section on antistatic, viscosity control, depression and wetting agents), or the Plastics Technology Buyers' Guide.
25
Cellular PVC Materials and Products
1083
found density increases after fusion which were most pronounced for the thinnest layers. The relationship between the formulation and processing conditions on one hand and properties of chemically blown flexible foam on the other has been receiving a considerable amount of attention over the years. A study by Renshaw et al. 27 demonstrated that increasing concentrations of plasticiser, filler and blowing agent all reduce the tensile strength of the foam and that the plasticiser concentration is the principal factor determining foam elongation (the latter being directly proportional to the former). Elongation was also found to increase with processing temperature within the investigated limits but, as might be expected, to decrease with the concentrations of filler and blowing agent. Other findings were that the compression deflection is inversely proportional to the blowing agent concentration, that the compression set values are governed mainly by the plasticiser and filler content (increasing with the former and decreasing with the latter), that the foam density is a function almost exclusively of the blowing agent concentration, and that the use of a good solvating plasticiser is necessary for good quality foam of the type studied. Similar work by Deanin et al. 28 indicated that for chemically blown vinyl foams with open-cell contents in the range 0-90%, the open: closed cell ratio correlated directly with low-temperature flexibility and volume resistivity, and inversely with modulus, strength, elongation, permanent set, resilience and abrasion resistance. Modulus and strength also varied inversely with the plasticiser content, whilst decreasing foam density produced decreasing modulus, strength and elongation. In an extension of this work,29 the effects of plasticiser content and foam density were found to be generally similar also in substantially open-cell foams produced by mechanical frothing of plastisols of a simple basic formulation. However, at comparable densities (in the range 19-751b ft- 3 ), the mechanical properties of the latter foams were considerably inferior to those of their chemically blown, partly-open-cell counterparts. In his investigation of the effect of formulation and processing on the properties of foams blown with azodicarbonamide (Genitron A C/2 -Fisons, UK) Visnovsky30 has evaluated such parameters as the nature of the PVC resin, the activator, plasticiser, and ambient temperature during expansion. His results show how variations in all these parameters can affect foam properties, and illustrate particularly well the effect of molecular weight (K value) of the resin.
w.
1084
V. Titow
Molecular weight is also an important factor in cellular PVC produced by the 'lost filler' method, in which high molecular weight polymers should be employed for best results. The effects of formulation variables on the properties of expanded vinyl flooring were the subject of a thorough study by Jones and Lavender. 31 Modern vinyl flooring-a product of very considerable commercial importance-is usually a two- or multi-layer material incorporating an expanded foam layer produced by chemical blowing. The two-layer construction consists of a vinyl wear layer, which may include a transparent vinyl coating for print protection, backed by a fairly thick foam layer; the multi-layer construction comprises essentially a clear wear layer, a thinner foam layer and a fabric layer with, in some cases, an additional barrier layer between foam and fabric. The fabric layer may serve as a backing layer to the foam or it may be contained between the foam and the wear layers. 31 ,32 The main service requirement to be met by vinyl flooring is rapid recovery after compression; other service properties which, although of primary importance, are not unique to vinyl flooring, include composition stability (no migration or evaporation of plasticiser), dimensional stability, good wear and soiling resistance. The above-mentioned study was primarily concerned with the effects of formulation variables on recovery properties, but the work and its results are a good general illustration of the way in which the formulation parameters can influence the properties of a foam. Some of the conclusions of Dietz33 are also of interest in connection with the compression and other properties of plastisol foam (blown with azodicarbonamide) as functions of the formulation variables. The main results indicate that-all other things being equal-better compression recovery properties are shown by foams with: (i)
lower plasticiser contents;
(ii) higher densities; and (iii) higher filler loadings (over the minimum of 20 phr). Dietz also found that a maximum of 30% of blending (i.e. non-paste-forming) resin could be used in plastisol formulation without affecting the compression properties, lower foam densities resulting at high blending resin levels. This latter effect is attributable to the reduction in plastisol viscosity and lower melt viscosity permitting better utilisation of gas. Finally, higher molecular weight resins were considered more difficult to process into good foam products.
25 Cellular PVC Materials and Products
25.3.2
1085
Chemical Blowing Agents-Nature and Operation
The blowing agent is a cardinal component of PVC formulations for the production of foams by 'chemical' blowing. Blowing agents are compounds which decompose on heating within a defined temperature range, to generate a large volume of gas. They can be solid or liquid, and inorganic or organic in nature. In general terms, the principal requirements to be met in sufficient degree by a blowing agent for any plastics composition may be listed as follows: (i) stability at ordinary temperatures (good shelf life); (ii) compatability with the plastics composition and good dispersibility therein (down to solution where appropriate); (iii) gas evolution at a high but controllable rate in a suitable, preferably narrow, temperature interval within the normal processing temperature range of the plastics material concerned; (iv) innocuous nature of the gas(es) evolved; (v) no excessive temperature rise (exotherm) in decomposition; (vi) non-toxicity of the blowing agent itself, combined with maximum volatilisation of decomposition products and innocuous (non-toxic and non-corrosive) nature of any residues left behind in the expanded material: such residues should also be permanently retained (no exudation or 'blooming'), have no adverse effect on the properties of the foam, and impart no colour or odour. Just as in the production of flexible foam by the dispersed-gas process it is important to synchronise properly the expansion of the gas with the temperature and rate of gelation of the PVC paste, so in chemical blowing the temperature and rate of gas evolution by the blowing agent should be similarly synchronised. The requirement that the blowing agent must generate the gas(es) at the temperature and rate suited to the expansion of the plastics material concerned at the appropriate stage in processing, is in fact a general one, applicable to any given composition. Inorganic blowing agents are not much used in PVc. The most common one, sodium bicarbonate, albeit a cheap material, is rather difficult to disperse finely and intimately in PVC compositions, whilst its temperature range for gas evolution is low as well as relatively
1086
W. V. Titow
broad-a combination of features disadvantageous from the point of view of what is desirable in the processing of PVC (including the gelation and fusion of pastes). The use of sodium borohydride has been explored, but not implemented on a significant scale. Thus the chemical blowing agents in industrial use for the production of PVC foams are organic ones. Their main qualifications for this application are that their operating temperature ranges fit in with those of the relevant PVC compositions (either directly, or when modified by the presence of activators-see below) and that their compatibility with these compositions is good. Their other advantages include availability of the solid ones in uniform particle sizes (including very fine ones in some grades) for control of final cell size and uniformity, and relatively narrow decomposition ranges (see Fig. 25.2). The organic chemical blowing agents also tend to be self-nucleating during decomposition, which makes for cell uniformity. The blowing agent most widely used with PVC compositions, including pastes, is azodicarbonamide (ADA), sometimes called azobisformamide (ABFA). It comes close to meeting all the principal general requirements mentioned above and, in addition, is not flammable in ordinary conditions. The general operational temperature range (i.e. the range of plastics stock temperature in processing) for which unmodified ADA is suitable is about 20o-250°C (d. its decomposition range in Fig. 25.2). This would be rather high for PVC processing (especially pastes), but the range can readily be extended downwards to about 165°C (or even lower in certain special cases) by the presence of activators, introduced either as formulation components external to the blowing agent, or as modifying additives incorporated in the commercial version. Thus 165-230°C can be regarded as the typical operational range in PVC of most commercial ADA blowing agents (see Table 25.2). The use of activators (sometimes referred to as 'kickers') in conjunction with ADA (as well as some other blowing agents-see below) is an important feature of plastics foam production by chemical blowing. The principal effect of an activator is to lower the decomposition (gas evolution) temperature of the blowing agent: many also increase the decomposition rate. Lead, zinc, and cadmium compounds, including several used as PVC stabilisers or components of stabiliser systems, act as activators for ADA. This long-known fact 34 is widely utilised in formulating compositions for chemically blown PVC foams: as little as 0·5 phr of a kicker can be sufficient for
25 250
1087
Cellular PVC Materials and Products
A
200 I
en
E 150
X
XX
x
x
'0
....IIIto
I
l...
/
..:-'-x
/
x x
:
I I
~1oo to
en
x
;
I
CIl
;
I
III
X
x
I
50
I ;" /
/
:
I
xX
x xX
xx
o 250 B
200 'en
E 150 '0 to
....
'"to ~
. ··f · :
215·C
185'C
-----
l...
en
170·C ..............................
100
170·C
CIl
III
50
o Fig. 25.2 Volume of gas generated by some blowing agents, as a function of temperature (A), and time at a constant temperature (B). - - Azodicarbonamide (ADA); x x x ADA non-plate-out grade; ... ADA activated with a liquid CdlBalZn stabiliser system; - - . - - ADA activated with a liquid Pb stabiliser system; ------ a commercial hydrazide-based blowing agent; - - 4, 4'-oxybis(benzene sulphonylhydrazide) (OBBS).
EF (profile,! sheeting; EF (profile,! sheeting; EF (profile,d wire and
165-230 165-180 165-230 From 145 160-200
AZNP130
AZ760
AZ3990
AZ754
AZRV
Modified: Non plate-out
Non plate-out (coml?ounding grade) Flow-treated
Activated
Activated
Kempore FF
Kempore 125N
Genitron (or Ficel) EP Kempore MC
Genitron (or Ficel) AC Azocel Porofor ADC Kempore
Materials
Fisons Industrial Chemicals, UK (Genitron) Haake Inc., USA (Ficel) Fairmount Chemical Co., USA (Azocel) Bayer, West Germany and Mobay Chemical, USA (Porofor) Stepan Chemical Co., USA (Kempore)
Sources
Examples of other similar commercial products b
b
a
Based in part on data from Uniroyal Chemical technical literature. Not necessarily directly equivalent. CSome applicatIOn, with sUitable activation. d A main application. • A main application, with suitable activation. fS ome application. Key: ER, rigid extrusions; EF, flexible extrusions; SR, rigid structural foam mouldings; SF, flexible structural foam mouldings; R, rotational mouldings; C, calendered sheeting; P, paste products.
SF· d R" p .• CC E~ (fiJm' and sheeting; wire and cable) ERd (profile, pipe, sheeting); SRQ
cabl~;
wire and cable')
wire and cable')
ERe.. (profile, pipe, sheeting) EF (sheeting,C wire and cabled) SR;c C;< P"
165-230
AZ130
Regular
Application areas
Approximate operational range Cc)
Celogen grade
General type
~
:::l S
:0::::
~
00
TABLE 25.2 Operational Temperatures and Applications of Some Commercial Azodicarbonamide Blowing Agents, with Special .<=> Reference to the 'Celogen' Range (Uniroyal Chemical)Q 00
25 Cellular PVC Materials and Products
1089
activation (i.e. much less than required for stabilisation, in normal circumstances, with Pb, Zn and Cd stabilisers). Selected liquid or paste-form stabilisers containing the above metals can be particularly effective as activators, and are featured in the ranges offered by all major suppliers (d., for example, Irgastab L600, ABC 2, or CZ ll-Ciba-Geigy). Certain organotin stabilisers also have an activating effect (e.g. Mark 1915-Argus Chemical, USA). Note: In formulating a PVC composition for blowing with ADA (or another blowing agent sensitive to activation) the relevant considerations will thus include, inter alia, the following points: whether the blowing agent is internally activated; if not, the best stabiliser system to give the required activation; whether any other formulation components can interfere by spurious activation (e.g. lubricants like lead or zinc stearate, pigments based on Pb, Zn or Cd, or even certain carbon blacks)-in case of doubt practical trials should be carried out.
The effect of le.ad activator* concentration on the density of extruded PVC foam is illustrated in a paper by Wells. 35 The use of isocyanates as retarders (i.e. the opposite of activators) in certain foams blown with borohydrides was patented in the USA some time ago,36 although isocyanates have also been considered as indirect blowing agents, generating carbon dioxide through reaction with water in processing. The principal gaseous products generated-in a high yield (cf. Fig. 25.2)-by ADA at the decomposition temperature are nitrogen, carbon dioxide and carbon monoxide. A small amount of ammonia is also evolved, and this should be borne in mind where the processing surfaces of equipment may be susceptible to corrosion by this reagent (e.g. beryllium copper moulds). Although azodicarbonamide is a solid, insoluble in plasticisers, it can be satisfactorily dispersed in PVC compositions, being available as powder or as dispersion (paste) in plasticisers. Both forms are supplied in several particle size grades (e.g. Fison's Genitron ACI2, AC/3 and AC/4 are powder grades of ADA of progressively finer particle size), and the pastes in different plasticisers. In general, the rate of decomposition in processing increases with decreasing particle size: this behaviour is particularly marked in the * Dibasic lead phthalate (Dythal-National Lead Co.).
1090
W. V. Titow
presence of activators. Hence the coarser grades afford better control over blowing (although, with good control, smaller cell sizes can be obtained with finer grades). Examples of commercial versions of azodicarbonamide are given in Table 25.2. Apart from modification to the decomposition temperature and decomposition rate of ADA by incorporation of suitable activators in some commercial versions (as well as through differences in particle size as just mentioned), the other principal variations available are represented by plate-out resistant and easy-flow grades. Note: As with some lubricants and stabilisers, plate-out is deposition of residues on the processing surfaces of equipment: in the case of compositions blown with ADA the deposit consists essentially of cyanuric acid or its compounds, appearing as a whitish powder or sticky layer. Cyanuric acid is a residual solid product of decomposition of ADA. Inclusion of finely divided silica in the blowing agent is a common way of counteracting the formation and deposition of this substance without materially affecting the generation of gaseous decomposition products, except for nucleating action which the fine particulate additive can exert.
The following blowing agents are also of interest in the production of PVC foam. 4,4'-Oxybis(benzene sulphonylhydrazide) (OBBS or OBSH): The general operational temperature range of this agent is 13o-170°C, the blowing gases generated being mainly nitrogen and water vapour. Its main industrial applications in PVC are in the production of flexible foams (extruded profiles and sheeting, injection-moulded structural foam, and paste-derived foam products). Examples of commercial versions are Genitron OB (Fisons, UK), Celogen OT (Uniroyal, USA), Nitropore OBSH (Stepan Chemical, USA), Sulfocel (Fairmount Chemical, USA). Azobisisobutyronitrile (AZDN): This blowing agent decomposes between about 80°C and HO°C in PVC compositions (in which it is not normally responsive to activation), evolving principally nitrogen, but also generating some toxic volatiles. It can be used in the production of foam from pastes. Some of its commercial versions are Genitron
25 Cellular PVC Materials and Products
1091
AZDN (Fisons, UK), Porolor N (Bayer, UK and Europe), Poly-Zole AZDN (Stepan Chemical, USA). Blow~ng agents with decomposition temperatures low enough to coincide with the gelation temperature range of PVC pastes are of particular interest for the chemical blowing of paste-produced foams. A commercial example (with an operational temperature range of about lOS-BOaC) is Celogen TSH (Uniroyal Chemical, USA). Another blowing agent in this general category, originally marketed by Du Pont under the name Nitrosan, is N,N'-dimethyl-N,N' -dinitrosoterephthalamide (DMDNT). This compound decomposes at about lOO°C, with an exceptionally low exotherm, which makes it particularly interesting for the production of thick-section foam from paste, where excessive heat build-up can affect the polymer. At the same time, the fact that the decomposition temperature is low also restricts the main use of DMDNT primarily to pastes. This is because there it can be incorporated by dispersion at room temperature, whereas where heating is necessary with other types of composition the DMDNT may begin to decompose at temperatures in excess of 50°C. A technique has also been developed 36 for foam production by room-temperature blowing of PVC paste with a liquid blowing agent of the azo type whose decomposition is initiated by an organic acid. A polyfunctional monomer (e.g. trimethylolpropane trimethacrylate) is also included in the composition (together with a peroxide catalyst) to help stabilise the foam by polymerising to form a three-dimensional network at the appropriate stage in the process before the final fusion of the PVC composition. The liquid state of the blowing agent makes for ease and homogeneity of dispersion in the paste. An example of a blowing agent of this type is 2-t-butylazo-2-hydroxybutane which generates nitrogen as the main blowing gas (up to about 140 ml g-l): this reagent is one of the commercial Lucel range (Pennwalt Corp., USA). Dispersibility in plasticised PVC compositions amounting to solution has been emphasised as a feature of a group of liquid blowing agents which are esters of azodicarboxylic acids,37 soluble in many plasticisers. Decomposition into nitrogen gas and entirely liquid residues which are colourless, non-toxic, non-staining, non-blooming and odourless are further features of interest.37 Incorporation levels of blowing agents range fairly widely, from about 0·5-0·8 phr in structural foam compositions for injection
1092
W. V. Titow
moulding, to 2-10 phr in foamable paste coatings on substrates for flooring with 'chemical' emboss effects (see Section 25.4). The form in which the blowing agent is introduced, and the method of incorporation depends on the nature of the PVC composition and general operational and cost considerations. Most commercial blowing agents in large-scale industrial use are solids, available as powders (in various particle sizes), concentrates, and masterbatches. Liquid dispersions (in plasticisers or oils) are also supplied. The incorporation method used (e.g. tumbling of powder with injection-moulding compound followed by subsequent dispersion in the melt in the barrel of the injection machine, or stirring or milling into paste) must result in dispersion of the blowing agent in the composition which is sufficiently intimate and uniform to avoid uneven blowing with consequent variability in cell size, or formation of holes. 25.4 SOME SURFACE TREATMENTS-EMBOSSING AND LACQUER COATING OF FLEXIBLE CELLULAR SHEET MATERIALS
Embossing is a major decorating technique for such commercially important vinyl sheet products incorporating foam layers as floor and wall coverings ('cushioned' by virtue of the presence of such layers) and certain leathercloths. The processes used are of two general kinds: mechanical embossing and the so-called chemical embossing. An emboss-like effect can also be obtained by printing a pattern in foamable paste and then expanding and fusing the printed deposits. 25.4.1 Mechanical Embossing
Relatively coarse embossing and/or quilting by application of heat and pressure came into practice in the manufacture of PVC upholstery linings: 5 ,6 however, in that kind of application the main object was to structure the material in a particular way, rather than to decorate or improve its surface in the sense in which this is done by embossing PVC flooring, wall-coverings, and leathercloth for certain fancy goods. Cellular leathercloth is embossed by the general process mentioned in Chapter 22 (Section 22.2.6), but preferably with a fixed gap between the pattern roller and pressure roller, and with both rollers watercooled, the surface layer of the leathercloth being softened by the
25 Cellular PVC Materials and Products
1093
application of heat before the material enters the nip of the rolls. 38 Other methods include 'direct' embossing as the material emerges from the fusion oven (this is mainly applicable to cellular leathercloth with a woven fabric base), and the use of embossed release paper on which the foam layer is cast (cf. Table 25.1).38 In all cases care must be taken that heat applied to the foam, and/or roll pressure, do not cause uncontrolled collapse of the cell structure.
25.4.2 Chemical Emboss This is effected by printing on the surface of a foamable PVC layer (commonly a pre-gelled layer of paste on an appropriate backing) a composition incorporating a reagent which modifies locally the extent of expansion when heat is applied. The modifier may be an activator for the blowing agent, in which case the expansion will be greater in the areas of the printed pattern, and these areas will form the raised portion of the resulting emboss effect. Alternatively, it may be an inhibitor either for the activating action of a kicker present in the layer (but not for its heat-stabilising action if the kicker is also the stabiliser), or for the blowing agent itself, in the sense of reducing its susceptibility to activation by the kicker: in either case the printed pattern areas will form the 'valleys' of the emboss effect resulting from differential expansion, on heating, of the foamable layer. Where the blowing agent is ADA and-as is normal-a suitable kicker is also present, external deactivators for the kicker may be, for example, trimellitic anhydride or fumaric acid. In the alternative variant the ADA itself may be locally desensitised against activation by the kicker if the printed pattern contains a suitable reagent, say thiourea, which can react with the azo group to form a stable compound. The modifying agents are usually incorporated in coloured printing inks, so that the two modes of surface decoration (coloured printed pattern and emboss) are combined in the finished product, the decorative value of the combination being enhanced by the excellent mutual register inherent in the process. The inks are usually applied by rotogravure. The height of the raised portions and depths of the depressions of the emboss can be varied selectivity, to produce multi-level effects, by appropriate variation of the depth of engraving on the printing roller. The amount of activator or retardant in the ink (which may vary within the range of about 5-20%), and the permeability of the PVC layer (affected-for a given composition-by
1094
w.
V. Titow
the degree of pre-gelation), are also factors influencing the emboss pattern depth. Clear, non-foaming wear layers are normally applied to flooring over the printed pattern before the final heat treatment which effects foaming and fusion. Variants of the chemical emboss method, which has been in industrial use for a considerable time, have been patented by Congoleum-Nairn Inc. 39 and Fisons Industrial Chemicals. 4o
25.4.3 Emboss Effects by Screen Printing of Paste In this method, a foamable paste is printed onto the appropriate base layer (fabric, paper, etc.) with the aid of a seamless cylindrical printing screen of the kind originally developed for applying coloured printing compositions to textiles. * The principle of the method is an adaptation of the flat screen printing technique: the screen cylinder contains the paste which is forced out through the pattern areas by an internally mounted squeegee as the cylinder rotates in contact with the moving web. An unpatterned screen cylinder can be used to deposit a continuous coating. A fairly common procedure in the production of, say, a floorcovering by the printing method is to deposit a layer of foamable paste as a base coat on the substrate with a plain screen cylinder, then print the pattern also with an expandable paste composition which may be suitably coloured (where appropriate the pattern may be built up by a number of printing stations). The printing may be wet-on-wet, or wet-on-dry (with pre-gelling before each new step). A wear layer may finally be printed on, and blowing and fusion (preceded by gelation if printing had been wet-on-wet) effected by a heat treatment.
25.4.4 Lacquer Coating A coat of lacquer may be applied over the wear layer of sheet materials with PVC foam coatings, or-in some cases (e.g. certain wall-coverings or leathercloths where a wear layer is not required)directly over the expanded layer (with or without emboss effects). The role and functions of the lacquer coat, and some typical compositions, have been mentioned in Chapter 22, Section 22.2.6. It may be noted * The Stork rotary screen system. originated by M. de Vries: Stork Brabant BV, The Netherlands: Stork Inter America Corp., USA.
25
Cellular PVC Materials and Products
1095
that the 'breathability' of cellular leathercloths which are required to be permeable may be impaired by the lacquer layer. 25.5 EXAMPLES OF BASIC FORMULATIONS
The best formulations for pastes used in the production of flexible PVC foam by the dispersed-gas blowing method-which is the domain of big manufacturers operating under patents and licences-are rarely disclosed. They are in some cases the results of extensive development work involving, inter alia, gelation/fusion and viscosity studies of the kind mentioned in Chapter 21, Sections 21.2.5 and 21.2.6. Such pastes normally contain fairly high proportions of plasticisers and include a good solvating plasticiser: they are thus similar in general type to pastes used for the production of flexible foam by chemical blowing. Basic formulations for chemically blown foams are exemplified by those shown in Table 25.3. Some examples relating to foams produced by gas entrainment are given in Table 25.4. Basic formulations for the individual layers of a composite coating of a cellular leathercloth are included among the examples at the end of Chapter 4. 25.6 EVALUATION AND TESTING
Apart from such general tests as determinations of heat stability, viscosity and gelation behaviour of pastes, etc., some evaluation and test methods are of special relevance or sole applicability to compositions for the production of cellular PVC, and the resulting products. Thus, gas evolution by a blowing agent (and the effects of activators in this connection) may be examined by heating the agent (with the appropriate proportion of activator, where relevant) in dispersion in a plasticiser (say 1: 10 solids in DOP) at temperatures relevant to processing, and collecting the volatiles formed in a gas burette. Thermogravimetric analysis-over the appropriate temperature interval-of a sample of an actual foaming composition can yield similarly relevant data on foaming behaviour, as can differential thermal analysis and thermomechanical analysis. The latter can also be applicable where the foaming and gelation effects in foamable pastes
PVC paste-forming resin PVC calendering resin Dibutyl phthalate Dioctyl phthalate Butyl benzyl phthalate Dialphanyl phthalate Chlorinated paraffin (plasticiser extender) Whiting Azodicarbonamide (Genitron AC/2) Genitron CR CdlZn stabiliser White lead paste (7:1 in DBP)
Formulation (Pbw)
Method and technique
4
2·5
-
-
-
4
2·5
-
20 5
60
-
100 -
Calendered sheet (foam layer)
Free blowing
5
100 65 20 -
Coated plastisol (foam layer)
-
TABLE 25.3
5
5
5
2·0
0·5
18 4
-
5·0
50
100
-
0·5 1·0
-
2·0
55
100
-
Blowing in injection moulding
0·5
5
-
100
Extrusion blowing
-
120
-
100 -
Pressure blowing
100
-
145
-
-
100
Cavity fillings b
Specimen Formulations for Chemically Blown FoamsD
~
0
:::'j
~
~
~
.....
Coated fabric and unsupported sheet; leathercloth; flooring; wall-covering
Soft 12-40 Ib ft- 3 Mixed
Laminated and unsupported sheet; leathrcloth; flooring
Medium/soft 20-40 lb ft- 3 Mixed
0·25
Soft toy filling
Soft Variable Mixed, fine
a
Kindly supplied by Fisons Industrial Chemicals. b This application is included as an illustration, but its commercial significance is very limited.
End application
Type Density Cell structure
Foam characteristics
Stearic acid Zinc oxide (active) Epoxidised oil Ca stearate Pb stearate Pb silicate
Cushioning
Soft SIb ft- 3 Mainly closed Profiles
Medium/soft 37lb ft- 3 Closed
Profiles/pipe
Rigid 56lbfc 3 Closed
0·5 0·5 3·0
Shoe sole mouldings
Medium/soft 43lb ft- J Closed
2·0
0·5
§
a
12-
~
~
s::.
1:;"
~.
~
r:s
."
f
t;:
b
a
8
8
50
30 55
30 55
100 (Breon P130/I)
100 (Breon P130/l)
Recommended for cellular leathercloth. Recommended for permeable cellular leathercloth.
Dialphanyl phthalate Butyl benzyl phthalate Nylonate plasticiser (Monoplas 230) Foam promoter (Deckor Cellset 5) Paris whiting
PVC paste-forming resin
20 lb [t- 3 and higher
10-30 lb [t- 3
8
45 40
100
30-50 lb [t- 3
(Norvinyl PlO)
a
6
10
20 30
100 (Breon PI30/I)
50 lb [t- 3 and highe~
Formulation (phr) to give resultant foam density or
TABLE 25.4 Specimen Formulations for Foams Produced by Gas Entrainment26
is <:
:<:: :::1
;:e
00
...... ~
25
CelluLar PVC Materials and Products
1099
are to be followed. Some suggestions regarding the use of thermal analysis techniques for the determination of such formulation components as stabilisers, fillers, impact modifiers and others in foam compositions (not specifically PVC) have been put forward by Breakey and Cassel. 41 The effects of formulation variables on the foaming characteristics of a paste composition and the resultant cellular product can be examined by casting a film of about I-mm thickness on a metal or glass plate and heating at constant temperatures for different times, and/or similarly casting onto a gel block and differentially heating for suitable constant times: in each case the results are evaluated in terms of the extent of foaming (foamed layer thickness) in conjunction with cell size and uniformity obtained. As in the case of solid PVC products, most of the tests normally carried out on cellular ones are property tests relevant to product comparison and service performance. National and international standard specifications are, collectively, the most relevant source of applicable test methods. Although the tests contained in the four main groups of standards (ISO, BS, ASTM and DIN) listed in Section 8 of Appendix 1 do overlap to some extent in nature and scope, they are also complementary in many directions: that section should therefore be considered and consulted when a particular test is required. A few salient or additional points are mentioned below, as well as some references not listed in Appendix 1. The properties of rigid PVC foam relevant to its applicationsalready mentioned-in building, vehicle, and marine construction, include density, compressive strength, cross-breaking strength, dimensional stability, heat conductivity (the K factor, indicative of insulation value), permeability to moisture, and flammability. Many of the appropriate British standard test methods are contained in BS 4370, the general test specification for rigid plastics cellular materials. BS 3869 is specifically directed to rigid PVC foams for use in thermal insulation and building applications. Friability of rigid cellular insulation materials may be measured by a tumbling test (ASTM C 421) which has been claimed42 to correlate well with abrasion resistance determined on a Taber Abraser. Standard methods are also available for determining the resistance of such materials to damage by dropping (ASTM C 487), their impact resistance (ASTM C 589) and indentation hardness (ASTM C 569). Two relevant water vapour transmission methods (ISO 1663 and BS 4370: Part 2: 1973: Method 3) are included in Table 12.1 (Chapter 12).
1100
W. V. Titow
The test methods of BS 4443 are useful for the determination of many service-relevant properties of flexible plastics foams. Although not directed specifically to PVC foams, these methods may be used for determining such properties of these materials as tensile strength and elongation, compression set and stress/strain characteristics in compression: the latter two are particularly relevant to upholstery applications, although it has been argued 43 that compression set, as measured in many standard tests, is a doubtful criterion for characteristing flexible, closed-cell foams. Flexible, cellular PVC sheeting is specifically covered by BS 4023. Determination of heat shrinkage of cellular soling material is given in BS 5131: Part 2. The completeness of fusion is an important factor in properties of products made from PVC pastes, including paste-produced foams. The use of solvent tests for the completeness of fusion of solid products is discussed in Chapter 22 (Section 22.1.2). A similar approach was adopted by Carey and Rogers 44 in their tests on flexible PVC foam, in which the foam was treated with ethyl acetate, and its tensile strength then determined to provide an indication of the 'degree of fusion'. It is occasionally of interest to know the total volume of voids and/or the respective volume percentages of open and closed cells in a cellular material. The determination methods given in standard specifications differ in the form of apparatus and in the shape and size of the specimens. However, they are all based on the same principle, which is an application of Boyle's Law: when a specimen of cellular material is placed in the chamber of the test apparatus, the closed cells and the solid material (cell walls) displace their own volume of air, causing a proportionate increase in pressure: this is measured, and the value used to calculate the percentage contents of closed cells, open cells, and solid material in the specimen. The methods include those of Remington and Pariser45 (for a time the subject of ASTM Standard D 1940-62T, now withdrawn) and Payne and Stephenson46 (sometimes known as the 'ICI method'), the air pycnometer method of ASTM D 2856-70 (Reapproved 1976), and the method of BS 4730: Part 2: 1973 (Method 10). The last two methods are technically similar to those of ISO 4950. All the above standard methods are intended for rigid cellular materials. With porous PVC sheets or mouldings used as battery separators or filters, the properties commonly determined by tests are water and/or
gas permeability,21 the pore size (an average value may be determined according to BS 1752 or, for air filters, BS 2577), total pore volume
25 Cellular PVC Materials and Products
1101
(determined by the absorption of a suitable liquid21 ) and mechanical properties (in particular tensile and flexural strengths, determined by the standard test methods). For battery separators, electrical resistance and chemical resistance are also of interest. 21
REFERENCES 1. Gotch, T. M. (1979). Plast. Rubb. Int., 4(3), 119-24. 2. Elastomer Chemical Corporation US Patent 2666036; British Patents 767465; 770237. 3. Ward, D. W. (1969-70). Modern Plastics Encyclopedia, pp. 256-9. 4. Dennis, I., US Patent 2763475. 5. Fuchs, 0., Heckalt, F., and Herz, A. (1965). Kunststoffe, 55(9), 717-23. 6. Anon. (1967). Brit. Plast., 40(8), 64-9. 7. Dilley, E. R. (1966). Trans. J. Plast. Inst., Feb., pp. 17-21. 8. Challenger, P. (1970). Polym. Age, 1,9-12. 9. Harris, W. D. (1976). 34th ANTEC SPE Proceedings, pp. 154-61. 10. Titow, W. V. and Lanham, B. J. (1975). Reinforced Thermoplastics, Applied Science Publishers, London. Ch. 10. 11. Shea, F. C. (1978). 36th ANTEC SPE Proceedings, pp. 205-7. 12. Nakajima, N., Ward, D. W. and Collins, E. A. (1976). J. Appl. Polym. Sci., 20(4), 1187-98. 13. Anon. (1976). Plast. Rubb. Wkly, 27th August, p. 10. 14. Anon. (1982). Plast. Rubb. News, October, pp. 45-7. 15. Anon. (1975). Plast. Rubb. Wkly, 22nd August, p. 12. 16. Gross, H. L. and Angell, R. G. (1976). 34th ANTEC SPE Proceedings, pp.162-5. 17. Hartman, J. V., Kozlowski, R. R. and Podnar, T. (1966). J. Cell. Plast., 2(4),214-17. 18. R. T. Vanderbilt Co. Inc., US Patent 3288729. 19. Bartl, H., and Wingler, F. (1970). Kunststoffe, 60(1), 19-22. 20. Titow, W. (1961). J. Plast. Inst., Dec., pp. 186-93. 21. Collins, D. H. (Ed.) (1963). Batteries, McMillan, New York, p. 16-17. 22. Greenhoe, J. A. (to Monsanto Chemical Co.), US Patent 2864777. 23. Laskowski, W. and Skowronski, T. (1969). Polim. Tworz. Wielk. 14(8), 389-94. 24. Simonik, J. (1978). 36th ANTEC SPE Proceedings, pp. 773-fJ. 25. Acton, J. and Debal, F. (1976). Plast. Rubb. Wkly, 18th June, pp. 22-3. 26. Deckor Cellset 5, Vinyl Foam Stabiliser, Scott Bader & Co. Ltd, Technical Booklet. 27. Renshaw, J. T., Cannon, J. A. and Minchen, J. D. (1970). SPE J., 26, 47-50. 28. Deanin, R. D., Kapasi, V. C., Georgacopoulos, C. N. and Picard, R. J. (1974). Polym. Engng. Sci., 14(3), 193.
1102
W. V. Titow
29. Deanin, R. D., Calcuttawala, A. H. and Kapasi, V. C. (1976). 34th ANTEC SPE Proceedings, pp. 501-2. 30. Visnovsky, L. (1970). Brit. Plast., September, pp. 90-3; October, pp. 109-12. 31. Jones, P. W. and Lavender, C. M. (1969). Expended Vinyl Flooring: A Study GJ ' Variables in Foam Formulation, Published by Fisons Industrial Chemicals, March, (ref. GIB/2). 32. Conger, R. P. (1968). SPE J., 24(3), 43-5. 33. Dietz, G. R. (1968). Ibid. 24(9), 49-52. 34. Nass, L. I. (1963). Mod. Plast., 40(7), 151-65. 35. Wells, J. N. (1968). J. Cell. Plast., 4(3), 109-12. 36. Gallagher, R. B., Kamens, E. R., Lanse, F. W. and Kamath, V. R. (1978). 36th ANTEC SPE Proceedings, pp. 777-9. 37. Sheppard, C. S., Schack, H. N. and Mageli, O. L. (1966). J. Cell. Plast., 2(2), 97-111. 38. Cellular Leathercloth, Fisons Technical News. GIB/9, 1969. 39. Congoleum-Nairn Inc., US PateHts 3293094 and 3293108. 40. Fisons Industrial Chemicals, US Patent 3464934. 41. Breakey, D. and Cassel, B. (1979). Plast. Technol., 25(12), 75-8. 42. Hilado, C. J. (1969). J. Cell. Plast., 5(1), 5fr8. 43. Baumann, H. (1977). Kunststoffe, 67(9), 509-13. 44. Carey, R. H. and Rogers, E. A. (1956). Mod. Plast., 33(12), 139-46 and 22fr9. 45. Remington, W. J. and Pariser, R. (1958). Rubb. World, 138,261-4. 46. Payne, N. and Stephenson, C. E. (1969). Brit. Plast., 42, July, 135-6.
CHAPTER 26
Applications of pvc w.
V. TlTaw
Although PVC runs second to the polyolefins (considered collectively) in terms of total tonnage utilised, it has a much wider spectrum of uses. Some of its numerous and diverse applications are exclusive to it, whilst in many others it competes successfully not only with other plastics but also with older materials. The wide and varied applicability and usage of PVC are due to a combination of relatively moderate cost with a number of technical factors which include: its useful general properties as a plastics material (strength, chemical resistance, low flammability, suitability for foodcontact and medical applications, good electrical properties, and others); (ii) the fact that the properties can be widely varied, and the applicational scope of the material thus greatly increased, by suitable formulation, and in particular incorporation of plasticisers; (iii) the processing versatility: whilst PVC can be processed-and converted into important products (e.g. pipes, sheeting, floor-coverings, wire and cable insulation, etc)-by the standard plastics processing techniques of extrusion, injection moulding, blow moulding, and thermoforming, it is also particularly suited to calendering and finds important applications because of its processability in such special forms as paste (which is unique to PVC) and latex.
(i)
The preceding chapters will-it is hoped-have demonstrated to the 1103
W. V. Titow
1104
reader something of the significance, role, interaction and effects of these and many other relevant factors operative in the technology and utilisation of PVc. The purpose of the present chapter is to summarise in brief outline the main applications of PVC materials and products, as well as many of its miscellaneous uses including some ranging from the less common to the unusual. Numerous references to, or descriptions of, the forms and ways in which PVC is used will also be found throughout the book in the sections on subjects to which they are relevant. The most substantial single outlet for PVC is in rigid pipes and fittings. Other big application areas are sheeting and film (rigid and flexible), and-for plasticised compositions-electrical wire and cable coverings, and coated fabrics. An extensive list of applications of PVC (as well as other plastics) in building has been published by JCL I The subject is also discussed in an article by Fischer2 and the main applications of PVC in general have been briefly reviewed by Smith. 3 The classification scheme adopted for the purpose of the summary review contained in the following sections is somewhat arbitrary; it is, however, not highly unconventional; it is also considered reasonably logical, as is the designation as 'primary' of those products which-like PVC sheeting, profile or pipe-are manufactured in continuous form, and are either used directly, or converted, fabricated, or assembled into secondary, derivative products. 26.1 MAIN APPLICATIONS OF PRIMARY PVC PRODUCTS Some of the secondary products manufactured from the primary ones are also mentioned under the various sub-headings of this section: others are included-as appropriate-in further sections. 26.1.1 Pipes and Tubing (a) Rigid (uPVC) Pipes Rigid pipes (with corresponding ranges of appropriate fittings usually produced by injection moulding) are widely used in the following application areas (see also Chapter 19, Section 19.3). Building (including some ship and aircraft applications): Sanitary
26 Applications of pvc
1105
pipework (soil, waste water and ventilation pIpmg, waste traps for basins, baths, showers and sinks); potable water supply pipes; rainwater systems (guttering and down-pipes); electrical conduit and trunking. Note: Chlorinated PVC (CPVC) is among the rigid plastics pipe materials fulfilling the main conditions for use in domestic hot-water systems. 5 The other materials are polybutylene6 and cross-linked polyethylene. Water distribution systems: Pressure pipes for potable water distribution (below and above ground), for cold-water services (including cooling water in mines), and for irrigation systems. Sewage and waste-water drains: uPVC pipes have been particularly successful in underground sewage systems. Competitive pipe materials include pitch-impregnated fibre ('pitch fibre'), glazed clay, concrete (including asbestos-reinforced cement) and high-density polyethylene. Drainage gullies have been moulded in uPVc. * Aeration systems in sewage-treatment installations Land (sub-soil) drainage Pipes for compressed air and vacuum lines:t For example, in mines and industrial premises. ABS is a competitor in this application, among common non-metal pipe materials. Gas supply pipelines: Here the use of uPVC pipes (even of the preferred impact-resistant 'ductile' compositions) is limited to some countries (and generally tends to be restricted to pressures not much above 1 bar in practice, although the nominal ratings can be considerably higher). Possibility of environmental stress-cracking is a technical consideration (see Chapter 12, Section 12.5). Mediumdensity polyethylene is a competitive plastics material in this application.
* A multi-component moulded uPVC drainage gully won a British Design Council award in 1976. 4 t For some high-pressure applications the uPVC pipe may be strengthened by an outer casing of GRP. 3
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W. V. Titow
Pipes for chemical plant and other industrial installations: This is another successful application area for uPVC. Competitive thermoplastics are high-density polyethylene, polypropylene, and-at the expensive end-polyvinylidene fluoride. (b) Flexible Tubing The use of extruded flexible PVC tubing as garden hose is well known and widespread. Heavy-duty versions, reinforced with polyester or nylon braid and overcoated with another pPVC layer formulated for abrasion resistance, find applications as pressure hose in mining (some types have bursting strengths of up to about 7 MPa), irrigation or industrial lines, on building sites, etc. Flexible PVC tubing with a 'concertina' structure is used as conduit in many applications. This type of tubing is manufactured by simultaneous extrusion, through a special die, of a flexible compound and a strip of harder composition which becomes embedded as a reinforcing helix in the wall of the resulting flexible tube. 7 Varieties of this tubing are marketed, under the trade name Heliflex, by A. G. Petzetakis of Athens (and associates outside Greece): the company, founded by the patentee of the original process, claims to be the world's largest producer of PVC hose (over 30000 tonnes said to have been manufactured in 1981). Vacuum-cleaner hose is another application of flexible PVC tubing. Transparent PVC tubing is used in general laboratory and plant applications. Tubing for medical applications is mentioned in Section 26.4.6 below. 26.1.2 Extruded Profiles and Channels (see also Chapter 19, Section 19.4). Rigid extruded profiles used for external cladding of buildings (siding) represent a substantial outlet in North America: some are 'capped' by coextrusion with a layer of acrylic polymer for colour effects and improved weathering and wear resistance. uPVC window frames and complete window systems, produced from special compositions formulated for resistance to weathering and impact, have made great gains-originally in Europe (especially West Germany and Austria), and more recently in the UK and USA-in capturing this application
26 Applications of pvc
1107
from the main traditional materials (wood and aluminium). In West Germany, where uPVC is currently the leading window construction material, the PVC polymer consumption for this application exceeded 100000 tonnes already in 1979. However, uPVC windows are not yet suitable for long service in hot countries with high intensity of sunlight (Australia, central South Africa), although continuing improvement in formulations may alter the position in a few years' time. In most uPVC window systems the profile sections (assembled by welding) are reinforced with metal (commonly aluminium): GRP rod reinforcement is also featured (e.g. in the Thermassive system-Schock GmbH, West Germany8), and uPVC-covered wood profiles have been used. Door and window frames for internal use in buildings (where the service requirements are less exacting than for external windows, and profiles of simpler cross-sectional shape can be used) are also made from uPVC, as are internal window ledges. Other uses of rigid profiles include framing and trim for uPVC cladding panels, and expanded beading, mouldings, picture frames, and furniture trim. Solar collector panel housings have also been made from uPVC profile, for service in moderate climates. 9 Flexible PVC profiles find a major application as waterstops (waterbars) for the leak-proofing of joints in concrete structures (dams, reservoirs, sewage works, basements, retaining walls, etc.): quite intricate cross-sectional shapes, including ribbed, fluted and hollow sections are available. Other uses include edge trim, treads and nosings for stairs, draft excluders and the like, as well as application in vehicle fittings (seat welting; decorative and protective trim strips). 26.1.3
Unsupported Sheeting and Film
(a) Rigid Sheet (see also Section 19.5 of Chapter 19, and Chapter 20) Rigid PVC sheeting is produced by extrusion or calendering: thick sheets are made by compression laminating layers of either kind of sheet. The following are the main application areas.
Building: Roofing; corrugated and wire-laminated sheeting is used, as well as plain; many transparent versions. Note: Among the many examples of uPVC-roofed structures is the pool section of the Leisure Centre in Sunderland (UK)
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W. V. Titow
covered by a 2025 m2 barrel-vaulted roof, glazed in Sindlon Stratum ICI transparent plain uPVC sheeting. Cladding, external and internal (including partitioning): in the UK as well as in other countries these applications are subject to the requirements of the relevant Building Acts. Safety doors and glazing (clear sheeting); light fittings (mainly translucent); fabrication into chutes and ducting. Chemical and other plant: Lining of tanks for chemical or food storage; absorption towers; casings for high-speed air washers in air-conditioning plant; fabrication into chutes, ducting and extraction hoods. Other uses: Interior wall covering and lining of vehicles, aircraft and boats-an interesting example is the interior cabin trim in Jetstream light aircraft (British Aerospace): the cabin side walls and upper side walls are lined with Fromoplas (Wallington Weston, UK) toughened uPVC sheet. 10 Thermoforming into parts and containers (VCNA copolymer sheet, or acrylic-modified sheets used in this application); thermoformed cabin fittings for aircraft are produced from ABS sheet/PVC foam laminates (see Chapter 16, Section 16.2). Road signs and sign boards.
(b) Flexible Sheet Sheeting used as lining for water reservoirs, tanks, effluent lagoons, and other large-scale seepage barrier applications, * swimming pool linings; damp-proof coursing and roof lining* for buildings. Fabricated products include: safety swing doors (usually transparent) for factory and storage premises (typically about 5 mm thick or over); suspendedstrip curtain doors (strips typically about 4 mm thick and 200-300 mm wide) for air-conditioned premises, cold rooms, and the like; products fabricated from thin sheeting-shower curtains; table covers; some types of rainwear and inflatables; mattress covers for general use (camping, hostels) and medical applications (water-beds used for patients suffering from burns or bed sores). Note: The mattress and water-bed cover sheet formulations contain biocide additives.
* Blends of PVC with chlorinated polyethylene (acting as a solid, permanent plasticiser) are being increasingly used for these applications.
26 Applications of pvc
1109
Self-adhesive decorative surface coverings, and wall-coverings of the unsupported kind (paper- or fabric-supported PVC wall-coverings are mentioned in Section 26.2.3 below). (c) Foil and Film RIGID FILM AND FOIL
The main applications of these materials are in packaging. PVC foil is thermoformed into blister (bubble) packs, nesting trays for confectionery and biscuits, nesting packs for medicinal tablets, containers, etc. PLASTICISED FILM
Semi-rigid film is used for shrink-packaging a variety of articles (see Chapter 19, Section 19.5.3), including food items. Special plasticised PVC compositions are among the common plastics materials of 'clingwrap' films familiar in their household and catering applications (the competitive materials are LDPE and EVA). pPVC films are also formulated for stretch-wrapping applications. The principle of stretch-wrapping may be regarded as in a sense the reverse of shrink-wrapping, in that the film is stretched under the pull exerted in the wrapping operation: the resulting tight grip is actuated by the material's elasticity, which also promotes good holding performance with contents which may be subject to shifting, settling or shrinking (in this respect PVC is superior to its main competitors in this application, EVA and LDPE). The film is wrapped around the contents several times: it is fastened either by virtue of the self-cling of the overlapping layers, or in some cases-depending on the material and wrapping system used-by means of a heat seal or mechanical crimping. The holding force obtainable with stretch-wrapping is normally greater than that in shrink-wrapping. Another advantage is the absence of a heat treatment: this is particularly significant with large loads or goods that are sensitive to heat. On the other hand a shrink-wrap conforms better to the shape of the contents, provides a greater degree of protection from the environment, and is less sensitive to load irregularities (sharp corners, etc.). The biggest single outlet for stretch-wrapping film is the packaging of goods-loaded pallets (e.g. palleted textile yarn packages, bagged materials, etc.); another use is in bundling (wrapping groups of products to 'unitise' them for wholesale distribution). In some application areas the uses of shrinkand stretch-wrapping overlap (notably in pallet wrapping). PVC film
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W. V. Titow
has been used in stretch-wrapping since the mid-1970s. As formulated for this application, the material offers clarity, good strength and abrasion resistance, and a considerable degree of self-cling. However, in wrapping, it starts at a lower effective stress than LDPE, and its percentage stress retention with time is also lower, as illustrated by the following typical figures:
Initial stress (MPa) LDPE PVC
13·7 7·4
Stress (MPa) retained after time shown 2 min 5 min IOmin
Ih
24h
11·2 4·9
9·8 3·2
9·6 3·2
10·4 3·9
10·0 3·4
This makes LDPE a better choice for wrapping heavy loads, but the lower initial stress of the PVC film means that it is easier to wrap (less force required). Apart from LDPE film (used for stretch-wrapping since about 1970), and the recently introduced linear LDPE, PVC's other competitor in this field is EVA: such copolymer films of low vinyl acetate content came into use contemporaneously with PVC, and the tougher high-VA copolymer films towards the end of the 1970s. The lower density of all these olefinic films in comparison with PVC is a general advantage; some also have higher elongation. The biggest suppliers of PVC films for stretch-wrapping are the Resinite Division of Borden Chemical, and Goodyear (Films Division) in the USA, with their subsidiaries abroad. A useful, brief review of stretch-wrapping (with special reference to its comparison with shrink-wrapping, and to the properties of the films used) was published recently by Johnson and Langford. 11
26.1.4 Foam (a) Rigid Foam Used as the cellular layer of some sandwich and multi-layer panels for the construction of refrigerated vans, other transport vehicles (see Chapter 25, Section 25.1) al)d some boat hulls.
26 Applications of pvc
1111
Note: The hull construction of the yacht Eclipse, top individual points scorer in the 1979 Admirals Cup series, was: resin gel coat/aramid fibre (Kevlar 49) cloth (0·2 mm thick)/continuousfilament glass cloth (0·7 mm thick)/the Kevlar 49 cloth/uPVC foam layer (25 mm thick)/the> Kevlar 49 cloth/the glass filament cloth/the Kevlar 49 cloth/gel coat. 12
Materials of some types of buoys and floats; cellular profiles and certain types of pipe (see Chapter 19, Sections 19.3.4 and 19.4.1). (b) Flexible Foam Once widely used as the cushioning material in car upholstery: this use has declined drastically in the last ten years. Still used widely as the foam layer in coated-fabric flooring and cellular leathercloth (see Section 26.2.1 below). Material of injection-moulded microcellular soles, certain types of carpet-backing, crash pads, and some toy fillings.
26.2 COMPOSITE PRODUCTS (COATED, LAMINATED, OR FILLED) 26.2.1 Coated Fabrics (see also Chapters 22 and 25) This is a big outlet for flexible PVC. The main types of product (mostly produced by paste coating) are: leathercloth of all kinds (both solid and cellular coatings) used in the production of upholstery, protective and foul-weather clothing, travel and fancy goods; (ii) tarpaulins (usually with a synthetic-fibre base, commonly nylon or polyester), inflatable buildings, * other inflatables, t brattice cloth; (iii) some types of water barriers, hovercraft skirts, life rafts, bulk liquid freight bags (some produced by hot lamination of PVC/nitrile rubber blend sheeting to nylon fabric 13);
(i)
* Very large air-inflatable structures (usually from PVC-coated nylon) may be delivered to the site in sections, and finally fabricated by welding before erection. t e.g. large inflatable air bags for breaking the fall in rescue situations-an example is the bag developed (originally for use by a film stuntman) by Greengate Polymer Coatings Ltd in the UK14 (similar products are also available in continental Europe and the USA).
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(iv) PVC backing on carpets (see Chapter 23); (v) continuous fabric- or felt-backed flooring; (vi) fabric-backed PVC adhesive tapes for electrical insulation, medical and other uses; (vii) PVC-impregnated sectional blinds for picture windows; (viii) PVC-coated gloves; (ix) PVC-coated glass-fibre fabric, used, for example, in the building industry for ducting (wrapped over flexible wire helical frames). 15
26.2.2 Conveyor Belting This might be classified as a kind of coated fabric, but is in fact much more substantial and heavy than coated fabrics for all other uses. The main application of PVC conveyor belting is in mines, although some grades are also used in manufacturing processes. The two main structural types of conveyor belting are known as multi-ply and solid-core belts. Multi-ply belting is produced from layers of fabric (cotton duck, and/or synthetic fibre) individually coated on both sides with flexible PVC (applied in paste form) and laminated together: the resulting multi-layer band is over-coated on both sides with an outer coat (typically about 1 mm thick) formulated for toughness, abrasion resistance, and low electrical resistivity (antistatic property). The outer coverings may also be calendered sheets heat-laminated to the interply assembly: the latter may typically contain 3 piles (common with synthetic fibre fabrics) or 5 plies (with cotton duck fabric). To obtain the necessary shape and smoothness of edge contour, an extruded flexible profile may be applied under heating, or a paste of suitable composition moulded on. A common continuous method of producing multi-ply belts is the Rotocure process,16 in which the pre-coated plies (with the coating pre-gelled by passage through an oven) are combined, under pressure, on the surface of a slowly rotating, heated drum of large diameter, where the fusion of the paste layers in the resulting laminate is completed. Stepwise lamination in a daylight press may also be used, but this is generally slower than the continuous method. Solid-core belting consists of a fabric layer encapsulated by coating/impregnation in paste-applied flexible PVC, over-coated with a harder surfacing composition. In some versions the yarns of the core fabric may be pre-impregnated with PVC. 16
26 Applications of pvc
1113
26.2.3 Sheet-type PVC Interior Wan-Coverings The salient features of this kind of wall-covering have long been known: the main ones are-in combination-distinctive appearance with many possible decorative effects, durability, and washability (in all but a few cases of special composite effects, like a flocked or metallised finish). The majority of PVC wall-coverings consist of clear vinyl coatings on the more traditional materials such as paper, or cotton fabric, but laminates of PVC with woven acrylics, hessian, and even silk, or metallised layers, are also represented. Further variants include wall-coverings with embossed PVC layers, which mayor may not be cellular. Note: Non-coated, special wall-coverings are also made; these are panels thermoformed in rigid PVC, and might thus be classified as wall cladding for interior use. The panels are made-in three-dimensional effects-to resemble brickwork or stone walls. A commercial example is Stone Decor (Triplastic Gesellschaft, Niederheim, West Germany). In general, the wall-coverings fall into two categories: supported and unsupported products. In the supported products the base material carrying the polyvinyl chloride layer may be paper (of varying thickness and substance) or fabric of varying weave and consisting of various types of fibre (synthetic fibres, cotton, jute or silk). The PVC layers of the supported products, and the 'body' of the unsupported products, are essentially the same in nature, appearance and physical characteristics. They consist of plasticised PVC compositions and may be either solid or cellular (expanded). The PVC composition may be bulk-coloured, or natural, or white. The surface of the PVC layer, or of the PVC sheet in the unsupported wall-coverings, may be plain or embossed. It may also be printed with a design. The wall-covering, whether supported or unsupported, may carry a separate surface layer which may be a thin coating of metal or a protective plastics film or a lacquer coating. The commercial products represent many combinations of the above principal structural features. The following may be quoted as illustrative examples: Mural Mousse Somvyl Decor 5106 (Sommer Exploitation, Neuilly,
1114
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V. Titow
France): An unsupported, cellular, bulk-coloured material with a printed embossed surface, produced by paste-coating on a carrier (cf. Chapter 22).
Suwide Placo 180-182 (Helmonde Textile Co., Helmond, The Netherlands): Fabric-supported solid PVC layer, bulk-coloured, embossed, surface-protected by Tedlar (Du Pont) fluoropolymer layer. The Ferralon range (Chamberlain Plastics, Rushden, Northants, UK): Paper-supported, thin, solid, colourless PVC layer with metallised surface (plain or embossed). The Ferralon range is not recommended for large-scale wall covering, but is fully illustrative of the similar (but cotton scrim-backed) Metalon range which is. The Galan range (Galon AB, V. Frolunda, Sweden): Fabric-based PVC wall-covering produced by extrusion of a plasticised sheet and integral bonding to the fabric support.
26.2.4 PVC Coatings and Coverings on Metal Substrates (a) Wire and Cable Insulation and Coverings Some aspects of this subject are also covered in Chapters 12 (Section 12.3), 13 (Section 13.4.2.4), 14 (Section 14.8), and mentioned elsewhere in the book. Suitably formulated PVC is a good electrical insulator: * this fact coupled with the relative ease of application (by extrusion), good mechanical properties, and low flammability, have been instrumental in making wire insulation and cable covering into major applications of pPVC compositions. The relatively low heat resistance and maximum service temperature of PVC is a limitation, but these can be substantially increased by cross-linking: specially formulated compositions containing radiation sensitisers are applied by normal extrusioncoating procedures, and the coatings are subsequently cross-linked by exposure to high-energy radiation (an electron beam, or emission from a cobalt-60 source). Some cross-linked coatings suffer no cut-through after application for 5 min of a weighted soldering iron at 350°C. 18 Other property improvements include increases in toughness, abrasion * The factors affecting the volume resistivity of a PVC composition have been discussed in some detail in a paper by Wingrave. 17
26 Applications of pvc
1115
resistance, tensile strength at elevated temperatures (e.g. about 3 MPa at 150a C for a Type 5 hard insulation to BS 6747 18), and resistance to cracking under stress, as well as a reduction in creep and improved solvent resistance. Examples of some applications of wire with cross-linked PVC insulation are: flexible cord for hot domestic electrical appliances, jumper wires in telephone exchanges (specified, for example, by the UK Post Office); under-the-bonnet wiring in some quality motor vehicles (including, for example, Jaguar cars).18 A useful brief review of the service requirements for modern cables, mentioning several applications of PVC in this area, has been published by Nye. 19 In certain types of power cables (10 and 20 kV) corrugated polyester film is used as a binder and thermal insulation in the form of a helical wrapping between the inner PVC covering of the core and the outer PVC sheathing. 2o Note: Wire for the production of chain-link fencing is also extrusion-coated with PVC compositions.
(b) PVC/Metal Sheet Laminates As with any other PVC coatings on metal where positive adhesion is required, a primer is first applied to the metal: typically, the primers are epoxy- and/or acrylic-based. The main functions of the PVC layer are decorative and protective. With suitable priming the metal sheet may be formed without dislodging the PVC layer. Steel plate and aluminium sheeting are available with PVC coating (e.g., respectively, Hishi-metal-Mitsubishi Plastics Industries Ltd, Japan,21 and Bondene22-Storey Brothers and Co. Ltd*): applications of the former include electronic equipment casings, electric cookers, refrigerators, lighting apparatus and show cases: 21 the PVC/aluminium sheeting is also used for electronic equipment casings, interior and exterior signs and display panels, and road-tunnellinings. 22 26.2.5 Laminates of PVC with Non-metallic Materials (a) Sandwich Panels PVC-sheet-faced panels with insulant filling (non-PVC foam or other porous material) are used for some constructional and partitioning
* See footnote :j: on p. 905.
1116
W. V. Titow
applications (for use of PVC foam cores in composite panels see Section 26.1.4(a) above). (b) PVC/Polystyrene Sheet Laminate This type of laminate, comprising a decorative PVC film on a polystyrene sheet backing carrying a pressure-sensitive adhesive layer is manufactured* for use as surface finish (wood and other effects), e.g. in the radio and television industries, furniture and interior fitting applications.
(c) PVc/Polyacetal Laminated Sheeting has also been produced. 23
26.2.6 Unsupported PVC Flooring and Floor Tiles This is a substantial application. The flooring (continuous, or cut into tiles) is produced by calendering from compositions based on VCNA copolymers and filled with asbestos fibres (see also Chapter 4, Section 4.6.2; Chapter 8, Section 8.2.1; Chapter 9, Section 9.4.3(b); and Chapter 18). Note: These PVC flooring materials can occasionally suffer embrittlement, shrinkage and cracking under conditions of heavy use in industrial applications (some of these effects have been attributed to the use of solvent-gel cleaners and strong detergents). However they are widely used in hospitals and schools, and domestic kitchens and bathrooms, where they offer wide choice of colours and patterns, ease of cleaning, good cushioning, insulation and reasonable price (the last three features represent advantages over linoleum).
A special type of dust-capturing flooring with a permanently tacky surface (achieved by deliberate formulating for limited plasticiser migration) is commercially availablet for use in premises where dust-free atmosphere is important-e.g. production premises for pharmaceuticals or electronic components, operating theatres and intensive-care units in hospitals. The material is also offered as an alternative to chemically impregnated rugs in offices, with the claim
* In the UK, for example, by Matcon Ltd, Mayland, Essex.
t In the UK from
Dycem Ltd, Bristol.
26 Applications of pvc
1117
that the latter require shampooing, on the average every 6-8 weeks, whereas the special flooring should perform satisfactorily if cleaned about every 20 months. 24
26.3 PVC FIBRES AND FIBRE PRODUCTS Apart from the vinyl chloride copolymer fibres mentioned in Chapter 1 (Section 1.5.2) homopolymer fibres are also produced (by solution spinning). There are many commercial products. Some examples are: Rhovyl (continuous filament and staple-originally produced by Societe Rhovyl SA, France) and other fibres from the same makers (e.g. Thermovyl, Fibravyl, Retractyl); Movil (Montecatini, Italy) and PeCe (Agfa Wolfen, West Germany) fibre. PVC fibres have been used in the production of filter cloths, wadding and braiding for use in the chemical industry. Other applications include protective clothing (in which the low flammability and chemical resistance of the fibres are utilised), tarpaulins, fishing nets, and awnings. Blends with other fibres are also used: one example is the use of shrinkable Retractyl fibre yarns in cloque fabrics, where retraction under heating of this PVC yarn produces special effects. The use of fibres of VCNA copolymer modified with maleic acid has also been proposed 25 in heat-sealable thermoplastic papers to improve fibrillation, sheet formation and sealing.
26.4 MISCELLANEOUS PRODUCTS AND APPLICATIONS 26.4.1 Gramophone Records These are discussed in Section 19.6 of Chapter 19. An example of a modern record-production plant-daimed to be the most modern in the EEC-is that operated by CBS Records at Aylesbury, Bucks, UK. The automatic compounding line, utilising a Buss KoKneader KG 20-25 compounder and other Buss AG equipment (supplied by Buss-Hamilton Ltd, Cheadle Hulme, UK) has a capacity of 1625 kg h- 1 for free-flowing granulate: output (in mid-1982) was 100000 per day of both 7-in and 12-in discs (produced by 66 presses).26
1118
W. V. Titow
26.4.2 Blown Bottles and Containers (see Chapter 17)
The development and growth of this application was given impetus first by the development of suitable uPVC moulding compositions combining stability in processing and service with good clarity, strength, and suitability for food-contact applications (cf. Chapters 9, 10, 11 and 17), and then by developments in processing (cf. Chapter 17). 26.4.3 Footwear (see also Chapter 11, Section 11.2.2, and Chapter 25)
Microcellular moulded pPVC, and PVC blend soles have been in use for a considerable time, as have pPVC uppers in some types of footwear. The main synthetic materials competing with PVC in the former application are polyurethane, some thermoplastic rubbers, and EVA (as well as natural rubber), and in the latter mainly polyurethane. 27 ,28 Complete units (boots, shoes and sandals) are produced from PVC by injection moulding,29 including all-weather golf shoes (from a PVC/nitrile rubber blend30). 26.4.4 Battery Separators (see Chapter 25, Section 25.2.2)
The main competitive materials in this application are resin-bonded papers and porous polyethylene. 26.4.5
Powder-coated Products and Mouldings Produced by Powder-coating Methods
Along with other thermoplastics, PVC is suitable for powder-coating applications. The two main techniques by which PVC powders are applied are dip-coating (in fluidised beds )31-33 and electrostatic deposition (electrostatic spraying) ,31,33,34 which started to be practised more recently, with the advent of compositions of suitable particle size and high resistivity. * The formulational versatility of PVC is a factor in its applicability by the powder techniques. Plasticised compositions are
* e.g. in the UK, Vyflex PC 80 ES-Plascoat Systems, (formerly Plastic Coating Systems). Typically sprayed (with object earthed) at a negative potential of 60-100 kV, followed by oven treatment at 220-260°C to fuse the powder deposit.
26 Applications of pvc
1119
normally used, commonly in the form of dry blends, but meltcompounded compositions-eomminuted by freeze-grinding-have been employed: these offer the usual advantages of this type of compound but the cost differential is even higher than between dry blend and pellet feedstocks, because of the expense of the grinding operation. Because of the lack of adhesion between PVC and metal surfaces priming is necessary in powder coating as in other coating techniques. The main considerations in the two techniques, brought out well in Newton's summary,31 may be listed as follows: Electrostatic deposition Typical coating thickness (mm)
Preferred particle size of powder (Ilffi)
Pre-heating of part to be coated
Automation
0·075-0·2 (tends to be selflimiting) 30-80 (oversize particles tend to accumulate on recirculation) Not normally necessary (powder coating held by electrostatic attraction until sintered) Readily effected with suitable equipment
Fluidised bed dipping 0·25-0·75 (not self-limiting) Over 100 (some oversize particles acceptable) Required
Possible, but needs careful design
The useful features of powder coating in comparison with other methods are the 'dry' nature of the process (absence of liquid media like solvents or plasticisers), feasibility of required thickness build-up in a single pass, and suitability for coating difficult shapes (especially sharp corners and edges-particularly with electrostatic spray). Articles powder-coated with PVC include wirework (special coating formulations are available with the requisite resistance to hot detergent solutions, for caoting dishwasher baskets), metal furniture and vehicle seat frames,32 pipework,35 road signs, railings and posts of balustrading. * An interesting application is a clear coating on
* Including ones for use in heavy weathering conditions, e.g. the safety balustrading and fog detector hood of the Bull Point lighthouse at Woolacoombe, Devon, UK have a PVC powder-applied protective coating. 36
w.
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V. Titow
laboratory glassware l4 to retain the pieces in the event of breakage: the coating also increases the resistance of the object to impact (e.g. in accidental dropping) and provides some first-line protection against abrasion. Compositions can be formulated for the production of cellular coatings by the fluidised-bed technique. Mouldings can be produced with PVC powders by methods analogous to those used with PVC pastes (see Chapter 22, Sections 22.2.1, 22.2.2 and 22.2.4), i.e. rotational moulding, slush moulding (usually referred to as 'static moulding' when a powder is used), and dip moulding (sometimes also called 'fusion moulding'3? when practised with a powder composition) employing a fluidised bed. 26.4.6 Medical Applications
Plasticised PVC compositions are the materials of many kinds of tubing (drainage, endotracheal, infusion), blood-collection and transfusion sets, stopcocks, aprons and sheeting. Note: In one case of blood-storage containers made of DOP-
plasticised composition (with a Ca/Zn stabiliser) some plasticiser was leached out, and even detected in patients' bodies after repeated, large-scale transfusions (no adverse effects were found attributable to its presence). 38 Special extrusion lines are available for the production of medical-grade PVC tubing. * Bags fabricated from clear, flexible PVC sheeting have been used for enclosing wounded limbs to provide an environment controlled in terms of bacteria content, humidity, temperature and pressure, to secure suitable conditions for healing without ordinary dressing (whilst the transparent nature of the bag allows the wound to be observed).t Thermoformed PVC trays, designed specially to accommodate the * e.g. Model 116 line of Betol Machinery Ltd, Luton, Beds, UK designed for the production of soft, radio-opaque PVC tubing for surgical use: the tubing, with an inside diameter of 1·2mm and wall thickness of O'12mm, has an internal web, off-set so that it divides the interior into two unequal compartments. 39 t Sterishield bag, developed in the UK by the Biomedical Research and Development Unit of the Department of Health and Social Security.40,41
26 Applications of pvc
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contents (which may be sealed-in with a transparent PVC film) are available with single-use medical devices-e.g. anaesthetic handventilation circuit assemblies for chest operations. Rigid, transparent compositions based on vinyl chloride/propylene copolymers are used for moulding and extrusion of components for medical applications.3° Flexible PVC film tapes with a pressure-sensitive adhesive layer are widely used as water-resistant sticking plasters. 26.4.7 Applications in Motor Cars
Apart from its virtually universal use as insulation on electrical wiring, the usage of PVC in motor cars tends to vary somewhat from country to country, and also to a certain extent from model to model. * However it is one of the three materials which-on the averagejointly account for about 75% of the plastics content of a car (the other two are polyurethane and ABS). With the decline in popularity of PVC leathercloth upholstery (ct. Chapter 25, Section 25.2.1)although it is still used in some parts of the world-the main interior applications of PVC in a typical western motor car are door panels (in some cases a 2-mm PVC foam-backed sheeting adhesive-bonded to a filled polypropylene materia!), rear parcel-shelf covers, head-linings (usually with a polyurethane foam backing), crash pads (also foam-backed), seat trim, and-in some cases-arm rests (ct. Chapter 22, Section 22.2.1). In some models the steering wheel and gear-lever knob may also be PVC-covered. PVC paste compositions are used as underseals and for corrosion protection in wheel arches and other vulnerable areas. 26.4.8 Tubular·frame Furniture and Related Applications
Interest has been growing on both sides of the Atlantic,42 and also overseas (e.g. in Australia and South Africa)43 in furniture-mainly for outdoor use-playground equipment, balustrades, and the like, made from rigid PVC pipe specially formulated for impact and weathering resistance. The pipe may be plain or externally patterned-in some cases to resemble bamboo, cane, reed or wicker. t Special 'furniture * e.g. the total PVC content (including electrical insulation) in the current model of the VW Golf motor car is about 15 kg. t e.g. the Ramboo tubing of Plastirama, Mexico.
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W. V. Titow
grade' uPVC compounds have come onto the market formulated on lines similar to those of window-frame profile compositions.
Note: For example, Geon 80 x 3 (B. F. Goodrich) is said to be42 an acrylic-modified composition with about 7 phr Ti0 2 . Another example is Conoco RPlOO (Conoco Chemicals).
In fabricating the pipe into the products it is cut into lengths and suitably shaped (after pre-heating). Shaped surface effects, e.g. bamboo, etc., can be produced by expanding the hot pipe inside a mould by air pressure. The products are then assembled with the aid of solvent bonding or special couplings. Good weathering performance with respect to appearance and strength retention over two years' exposures in Florida and Arizona has been reported. 42 26.5 SOME SPECIAL, UNUSUAL, OR MINOR PRODUCTS AND APPLICATIONS Baby pants: These are made from thin pPVC sheeting (about 0·1 mm thick), and usually incorporate some form of foam (polyurethane) padding, fabric-covered elastic band, and fastening studs. The sheeting is formulated for resistance to hardening by loss of plasticiser through washing and wear in use, tear and staining resistance, and good weldability. Typically, DIDP is used as the plasticiser together with Cd/Ba/Zn stabiliser systems with very low susceptibility to sulphide staining. A tear strength (Elmendorf test, e.g. to BS 1763) of 180 g per 0·1 mm of thickness is a reasonable practical minimum value. Fishing lures: PVC lures moulded in the shapes of sand eel, lugworm, squid and pilchard have been reported to be highly successful as aids to commercial fishing in Cornwall. 44 The lures are produced from metal-pigmented compositions, in moulds made from highly accurate carved acrylic models of the animals. Coupled with the paint-spray finish this results in very realistic appearance. Hooks larger than practicable with live bait can be used. Simulated skin: A PVC composition developed by the UK Ministry of Defence (Stores and Clothing Research and Development Establishment) has been successfully used-in the form of a sheet about 0·15 mm thick (produced by Storey Bros, Lancaster, UK)-as a
26 Applications of pvc
1123
simulant for human skin in the evaluation of clothing for protection against molten metal. 45 The material has a weight per unit area of 230 g m- 2 . The formulation is given as: PVC resin Di-alphanaphthyl phthalate
TIP
Chlorinated paraffin White pigment Lead tartrate Lead stearate Calcium stearate
100 26phr 13·75 phr 12·50 phr 10phr 1·75 phr 1·00 phr 0·75 phr
pPVC selective membranes: Films of PVC plasticised with tributyl, dibutylcresyl, and dicresylbutyl phosphates have been found to have selective permeability for uranyl nitrite, with the diffusion rate of the compound strongly dependent on the chemical nature of the plasticiser. 46 ,47 PVC-based plastic from waste products: Bags, biogas (methane) generators, pond linings, moulded solar collector panels and other sheet and moulded products have been produced from a composition* combining scrap PVC with two waste products-'red mud' (a residue from the processing of bauxite ore in the production of aluminium) and used engine oil. The 'red mud' is claimed to provide considerable reinforcement and act as stabiliser for the resin (by virtue of its metal salt and oxide content).48 PVC cricket pitch surface: Green pvc sheeting (Ruberoid, UK), laid between the stumps in two 33 ft lengths, each 6 ft wide, on a smooth concrete base, has been shown-in extensive use 49-to provide a robust, wear-resistant surface with playing characteristics similar to a medium to fast grass pitch. Industrial duckboard: Embossed or smooth heavy pPVC duckboard is commercially availablet for use in industrial premises, milking sheds, trucks for animal transport, and the like. *Developed at the Taiwanese Industrial Technology Research Institute. t e.g. in the UK the Heron duckboard manufactured by Plastic Extruders Ltd: a softer, hollow-section version is also made for use on worktops where a cushioning effect is required.
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Rug and mat underlay: Two special PVC products are noteworthy. A rayon mesh coated with PVC fibre flock which provides a surface grip (Ako-Stop, developed in West Germany50), and a highly plasticised PVC material in the form of sheet or netting for use as dust-capturing floor mat or non-slip underlay (Protect-A-Mafl-Dycem Plastics Ltd, Bristol, UK). Ball valves: These have been produced in PVC (for use in food and chemical industries)52 and PVC/polyacetal combinations (for use with water and other non-corrosive liquids).53 PVC bricks: Several designs have been proposed in the UK, Germany and Sweden, most with channels to take cement (so that the brick structure ultimately becomes a framework strengthened by a cement core). The British-developed Inca-Brick (Inca Construction Company Ltd) aroused particular interest. Despite claims that costs compare favourably with those of conventional brickwork, with building times cut by a factor of 10 or more,54 the bricks have not come into widespread use. Protective sheath for hydrophones: Specially formulated PVC tubing has been used as protective sheathing for lines of hydrophones (about 100 m long) towed by geological survey vessels engaged in mapping the sea-bed by echo-sounding techniques.55 The equipment, sealed inside the tubes, is surrounded by paraffin which acts as a protective medium: the tube material must thus be resistant to paraffin and sea water, and retain its properties at near-zero temperatures (for use in the North Sea and other cold waters). Various minor applications and products: Reflective, self-adhesive vinyl tape for clothing, walls, etc., for use in mines and other sites and work situations. Hazard-warning labels (self-adhesive, PVC-film backed) bearing symbols for radio-activity, poison, inflammable material, and other hazards. Ear-protection pads covered with flexible PVc.56 Decoy dummy aircraft fabricated in PVC (some equipped with radar reflectors for correct response).57 A low-density core material in sheet form, for decorative and some constructional applications has been made by a novel sheet-drawing process, from various thermpolastics, including PVC (Nor-Core, Norfield Corp., Conn., USA).58 Heat-shrinkable
26 Applications of pvc
1125
end-caps and harness clips for cables (e.g. the Heatshrink fittings range, Thomas Ness Ltd, UK).59 Cellular PVC monofilament has been used as the bristles in rough-duty brushes. 60 Safety goggles, moulded in high-clarity PVC compound (We/vic X16/909-ICI). Composting bins for garden-use, made of interlocking, perforated rigid PVC profiles. 61 Extruded, fluted PVC curtain rods covered (by an in-line operation during production) with vacuum-metallised polyethylene terephthalate sheet. 62
REFERENCES 1. Plastics in Building, ICI Plastics Division, Technical Publication G.25 (revised periodically). 2. Fischer, P. (1968). Kunststoffe, 58(1), 21-5. 3. Smith, P. I. (1969). Appl. Plast., 12(7), 17-19. 4. Anon. (1976). Plast. Rubb. Wkly, 2nd April, p. 10; 25th June, p. 11. 5. ASTM D 2846-81. Chlorinated poly(vinyl chloride) (CPVC) plastic hotand cold-water distribution systems. 6. ASTM D 3309-81b. Polybutylene (PB) plastic hot- and cold-water distribution systems. 7. Petzetakis, A. G. British Patent 984247, and corresponding patents in other countries. 8. Anon. (1981). Eur. Plast. News, 8(9), 109. 9. Anon. (1976). Plast. Rubb. Wkly, 20th August, p. 7. 10. Anon. (1981). Plast. Rubb. Wkly, 29th August, p. 5. 11. Johnson, F. M. and Langford, A. J. (1982). Plast. Rubb. Int., 7(6), 217-20. 12. Anon. (1980). Plast. Rubb. Int., 5(1), 11. 13. Anon. (1976). Plast. Rubb. Wkly, 17th December, pp. 12-13. 14. Anon. (1982). Eur. Plast. News, 9(7), 29. 15. Anon. (1982). Plast. Rubb. Int., 7(6), 205. 16. Breon P. 130/1 Paste Resin: Technical Manual No.2, B.P. Chemicals (UK) Ltd, 1969, p. 61. 17. Wingrave, J. A. (1978). 36th ANTEC SPE Proceedings, pp. 580---5. 18. Matheson, A. F. (1981). Electrical Times, 5th June. 19. Nye, H. F. (1976). Plast. Rubb., 1(2), 87-90. 20. Anon. (1976). Plast. Rubb. Wkly, 5th November, p. 26. 21. Anon. (1976). Plast. Rubb. Wkly, 10th September, p. 9. 22. Anon. (1976). Plast. Rubb. Wkly, 16th April, p. 11. 23. Kubitzki, C. and Schulz, G. (1965). Kunststoffe, 55(9), 727-8. 24. Anon. (1982). Mod. Plast. Int., 12(11), 6. 25. US Patent 2899351; Morse, E. A. (Assignor to Personal Products Corp.). 26. Anon. (1982). Eur. Plast. News, 9(8), 6.
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27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62.
W. V. Titow
Anon. (1977). Plast. Rubb. Wkly, 20th May, p. 18. Pettit, D. (1981). Plast. Rubb. Int., 6(5), 205-10. PVC for Footwear, ICI Technical Service Note W.109. Anon. (1976). Plast. Rubb. Wkly, 17th December, p. 10. Newton, D. S. (1978). Plast. Rubb. Int., 3(5), 203-6. Anon. (1976). Plast. Rubb. Wkly, 16th April, p. 9. Anon. (1979). Mod. Plast. Int., 9(2), 8-11. Cross, U. (1981). Chern. Britain, 17(1), 24-6. Anon. (1976). Plast. Rubb. Wkly, 5th November, p. 36. Anon. (1976). Plast. Rubb. Wkly, 30th April, p. 1. Anon. (1979). Plast. Technol., 25(5), 135. McHattie, G. V. 'General aspects of use ot polymers in biomedical applications', Paper presented at the Symposium on Polymers in Biomedical Applications, BruneI University, 2nd May, 1974. Anon. (1982). Eur. Plast. News, 9(12), 41. Anon. (1977). Plast. Rubb. Wkly, 28th January, p. 1. Anon. (1977). Plast. Rubb. Wkly, 21st January, p. 14. Anon. (1982). Mod. Plast. Int., U(12), 50-1. Anon. (1982). Plast. Rubb. News, November, p. 24. Anon. (1975). Plast. Rubb. Wkly, 3rd October, p. 19. Metha, P. N. and Willerton, K. (1977). Text. Inst. Ind., 15(10), 334-7. Bloch, R., Finkelstein, A., Kedem, O. and Vofsi, D. (1967). Ind. Engng. Chern.: Process Design Develop., 6,231. Vofsi, D., Kedem, 0., Bloch, R. and Marian, S. (1969). J. Inorg. Nucl. Chern., 31,2631-4. Hao, L. c., Tang, H. S. and Hsu, W. W. (1978). 36th ANTEC SPE Proceedings, p. 768. Anon. (1976). Plast. Rubb. Wkly, 19th November, p. 4; and Anon. (1979). Plast. Rubb. Int., 4(6), 248. Anon. (1982). Plast. Rubb. News, November, p. 43. Anon. (1975). Plast. Rubb. Wkly, 27th June, p. 14. Anon. (1975). Plast. Rubb. Wkly, 5th September, p. 18. Anon. (1976). Plast. Rubb. Wkly, 23rd July, p. 11. Anon. (1968). Australian Plast. Rubb. J., 23(2), 12. Anon. (1975). Plast. Rubb. Wkly, 17th October, p. 21. Anon. (1979). Plast. Rubb. Wkly, 27th July, p. 1. Anon. (1976). Plast. Rubb. Wkly, 17th September, p. 35. Anon. (1975). Plast. Rubb. Wkly, 5th September, p. 16. Anon. (1976). Plast. Rubb. Wkly, 26th November, p. 16. Anon. (1976). Plast. Rubb. Wkly, 23rd January, p. 12. Anon. (1975). Plast. Rubb. Wkly, 16th May, p. 16. Anon. (1975). Plast. Rubb. Wkly, 10th January, p. 12.
APPENDIX 1
Standards Relevant to PVC Materials and Products Compiled by N. HERBERT and W. V. TITOW
With the exception of Sections 1, 7 and 8 (and a few individual entries in other sections) the standards listed in this appendix are those relating specifically to PVC (and-in some cases-major constituents, e.g. plasticisers). Many general 'plastics' standards which are used in the testing of PVC are mentioned in the relevant chapters and (some) in Appendix 3. Copies of ISO standards and national standards of countries foreign to one's own are available from (and in fact must be ordered through) the local national ISO member body which acts as sales agent in that country for ISO and for all other ISO member bodies. These bodies are normally the individual countries' own national standardising bodies. As new standards are being reviewed, revised or amended, standardising bodies provide the means to maintain up-to-date records. The four bodies regarded as the most important (cf. Chapter 1, Section 1.7) are given below together with some observations on certain general aspects of the standards listed in this appendix.
International Organization for Standardization (ISO) Headquarters: 1, Rue de Varembe, Case Postale 56, CH-1211 Geneve 20, Switzerland. The ISO publishes a Catalogue of all its standards annually. The ISO Bulletin is published monthly and includes a list of newly published 1127
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standards and draft international standards. The ISO technical committees dealing with plastics are TC 61 Plastics and TC 138 Plastics pipes, fittings and valves for the transport of fluids. Plasticisers are found under TC 47 Chemistry.
United Kingdom British Standards Institution (BSI), Linford Wood, Milton Keynes MK14 6LE. The annual British Standards Yearbook gives a list of all British standards with a short summary of each one, and an alphabetical index. The BSI News is published monthly and may be used for manual updating of the Yearbook. In addition to lists of all new and revised standards and amendments to standards published during the month, draft standards and 'new work started' are also given. A comprehensive list of standards received from all parts of the world is contained in BSl's Worldwide List of Published Standards which is issued monthly. A large number of standard specifications is contained in BS 2782:1970 Methods of testing plastics, which is currently (1984) under revision. Many of the specifications in this collection have already been published in the revised form, and some entirely new ones have been added. Work is in progress on others. The relevant revised and new specifications have been included in this appendix, as have those originally published in the 1970 edition which have not yet been revised. Note: In the body of this appendix the degree of equivalence of a British Standard to an ISO standard is indicated where possible according to the coding used in the BSI Yearbook, viz.
= identical in every detail;
1- technically equivalent though the wording and presentation may differ quite extensively; ± a related standard; covers subject matter similar to that covered by a corresponding international standard.
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United States of America
ASTM
American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103.
Until recently, the Annual Book of ASTM Standards consisted of 48 parts, each part containing the standards covering a specific main subject or an aspect of that subject. The division has now been restructured into 16 sections subdivided into 66 volumes. Section 8 contains the volumes directly relevant to plastics:
Volume
~::~:} 08.03
08.04
Subjects covered
Plastics-General test methods, nomenclature Plastics-Materials, films, reinforced and cellular plastics; high modulus fibres and composites Plastic pipes
Corresponding parts in former classification
Part 35 Part 36 Part 34
Other volumes of interest in connection with some PVC materials and products, and the Index volume, are:
::::: } 10.03
00.01
Electrical insulation-Test methods: solids and solidifying fluids Electrical insulation-Specifications: solids, liquids, and gases; protective equipment Index-Subject index; alphanumeric list
Part 39 Part 40 Part 48
The parts and the individual standards are obtainable from the ASTM. Note: Where an ASTM designation is followed by a number in brackets, this indicates the year of the latest reapproval by the committee concerned. ANSI: The national standardising body in the United States of
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N. Herbert and W. V. Titow
America is the: American National Standards Institute (ANSI), 1430 Broadway, New York, New York 10018. A great many individual standards issued by the ASTM have been adopted by the ANSI as national standards. These adopted standards may be purchased through a local national ISO member body. West Germany DIN Deutsches Institut fUr Normung, Burggrafenstrasse 4-10, 1000 Berlin 30. The DIN-Katalog is published annually, most of the German titles being given in English as well. The catalogue may be kept up to date by Ergiinzungen (Supplements) which are published monthly and are cumulative. Further information on standards is given in DINMitteilungen (DIN-reports), which is also a monthly publication. The latter two are in German only. DIN standards on plastics which have been translated into English are available in ring binders in the following collections: Sales no. 10053 Plastics 1. Test standards for mechanical, thermal and electrical properties 10056 Plastics. Duroplast resins and duroplast moulding materials 10705 Plastics 2. Test standards on chemical, optical, usability and processing properties 10789 Plastics. Semi-finished products of thermoplastic plastics 10790 Plastics. Pipes, pipeline components and pipe joints of thermoplastic plastics The collections of DIN standards in German are reduced in size and bound into Taschenbiicher (Pocketbooks) in A5 size, and cover the same classification of the collections translated into English, viz. TAB 18 Kunststoffe 1. Prtifnormen tiber mechanische, thermische und elektrische Eigenschaften
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TAB 21
Kunststoffe 3. Duroplast-Kunstharze und DuroplastFormmassen TAB 48 Kunststoffe 2. Prtifnormen tiber chemische, optische Gebrauchs- und Verarbeitungs-Eigenschaften TAB 51 Kunststoffe 8. Halbzeuge aus thermoplastischen Kunststoffen TAB 52 Kunststoffe 5. Rohre, Rohrleitungsteile und Rohrverbindungen aus thermoplastischen Kunststoffen
Japan Japanese Industrial Standards Committee, c/o Standards Department, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, 1-3-1 Kasumigaseki, Chiyodaku, Tokyo 100. In view of Japan's standing as a major producer and consumer of PVC, the existence of many relevant Japanese Standards should not be overlooked (e.g. JIS K 6720 (1977), Polyvinyl chloride resins; JIS K 6741 (1975), Unplasticised polyvinyl chloride pipes. It has not been practicable to include them in this appendix. However, from the technological standpoint, they do not cover any important specifications or methods or materials not dealt with by standards from one or more of the four sources represented here. It is useful to remember that Japanese Standards are regularly included in the already mentioned BSI Worldwide List of Published Standards and that they are available (generally in English translation) from the BSI.
1 PLASTICS TERMINOLOGY, PROPERTIES AND TESTING: GENERAL 1.1 Terminology (a) General ISO 472-1979 Plastics-Vocabulary BS 1755 Glossary of terms used in the plastics industry
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N. Herbert and W. V. Titow
Part 1: 1982 (=t= ISO 472) Polymer and plastics technology Part 2: 1974 Manufacturing processes BS 3558: 1980 Glossary of rubber terms BS 4815: 1972 Glossary of generic names for man-made fibres BS 5168: 1975 Glossary of rheological terms ASTM C 168-80 Definitions of terms relating to thermal insulating materials ASTM C 274-68 (1980) Definitions of terms relating to structural sandwich construction ASTM D 16-80 Definitions of terms relating to paint, varnish, lacquer and related products ASTM D 883-80 Definitions of terms relating to plastics ASTM D 907-82 Definitions of terms relating to adhesives ASTM D 1418-81 Recommended practice for rubber and rubber latices-Nomenclature ASTM D 1566-82 Definitions of terms relating to rubber ASTM E 6-81 Definitions of terms relating to methods of mechanical testing ASTM E 12-70 (1981) Definitions of terms relating to density and specific gravity of solids, liquids and gases ASTM E 41-79 Definitions of terms relating to conditioning ASTM E 206-72 (1979) Definitions of terms relating to fatigue testing and the statistical analysis of fatigue data ASTM E 284-81 Definitions of terms relating to appearance of materials
ASTM F 17-76 Definitions of terms relating to flexible barrier materials
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ASTM F 141-79 Definitions of terms relating to resilient floor coverings ASTM F 412-81 Definitions of terms relating to plastic piping systems DIN 7732 Part 1 (1963) Standardized terms and definitions relating to plastics; summary (b) Common Names and Abbreviations ISO 1043-1978 Plastics-Symbols
BS 3502 Schedule of common names and abbreviations for plastics and rubbers Part 1: 1978 Principal commercial plastics Part 3: 1978 Rubber and rubber lattices
BS 4589: 1970 Abbreviations for rubber and plastics compounding materials ASTM D 1600-83 Abbreviations of terms relating to plastics DIN 7723 (1971) Abbreviations for plasticizers DIN 7728 Part 1 (1978) Symbols for terms relating to homopolymers, copolymers and polymer compounds Part 2 (1980) Symbols for reinforced plastics (c) Equivalent Terms in Various Languages ISO 194-1981 Plastics-List of equivalent terms DIN 7730 Part 1 (1965) Plastics, equivalent terms in German, English, French and Russian following ISO/R 194-1961
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1.2 General Test Conditions and Methods (a) Conditioning and Testing Conditions ISO 291-1977 Plastics-Standard atmospheres for conditioning and testing ISO/R 483-1966 Plastics-Methods for maintaining constant relative humidity in small enclosures by means of aqueous solutions ISO 554-1976 Standard atmospheres for conditioning and/or testing-Specifications ISO 558-1980 Conditioning and testing-Standard atmospheres-Definitions ASTM D 618-61 (1981) Conditioning plastics and electrical insulating materials for testing DIN 50013 (1979) Climates and their technical application; preferred temperatures DIN 50014 (1975) Atmospheres and their technical application; standard atmospheres DIN 50015 (1975) Atmospheres and their technical application; constant test atmospheres DIN 50016 (1962) Testing of materials, structural components and equipment; method of test in damp alternating atmosphere DIN 50017 (1982) Climates and their technical application; stress in condensed water containing climates DIN 50018 (1978) Testing of corrosion; methods of test in condensation water alternating atmosphere containing sulphur dioxide DIN 50019 Part 1 (1979) Climates and their technical application; climates with regard to technology, sign and cartographical graph of the open-air climates Part 2 (1963) Testing of materials, structural components and equipment; open air climates; data on climates
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Part 3 (1979) Climates and their technical application; climates with regard to technology; climatic patterns based on statistics Supplement to Part 3 (1979) Climates and their technical application; climates with regard to technology, geographical survey for open-air climate patterns based on statistics (b) Some General Test Methods ISO 62-1980 Plastics-Determination of water absorption ISO 171-1980 Plastics-Determination of bulk factor of moulding materials ISO 3451/1-1981 Plastics-Determination of ash-General methods 2 VINYL POLYMERS AND COPOLYMERS (Chapter 2) 2.1 General (Designation, Coding, Characterisation Tests)
ISO 1060/1-1982 Plastics-Homopolymer and copolymer resins of vinyl chloride. Part 1: Designation ISO 1060/2-1978 Plastics-Homopolymer and copolymer resins of vinyl chloride. Part II: Determination of properties ISO 1163/1-1980 Plastics-Unplasticized compounds of homo- and copolymers of vinyl chloride. Part 1: Designation ISO 6186-1980 Plastics-Determination of pourability ASTM D 1755-81 Specification for poly(vinyl chloride) resins Note: This specification gives test methods referred to in some of the ASTM standards listed in the sections that follow, e.g. Sections 2.2 to 2.9, etc.
ASTM D 2474-81 Specification for vinyl chloride copolymer resins
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N. Herbert and W. V. Titow
ASTM D 2873-70 (1982) Test for interior porosity of poly(vinyl chloride) (PVC) resins by mercury intrusion porosimetry ASTM D 3591-77 Recommended practice for determining logarithmic viscosity number of poly(vinyl chloride) (PVC) in formulated compounds DIN 7746 Part 1 (1979) Vinyl chloride (VC) polymers; homopolymers; classification and designation Part 2 (1979) Vinyl chloride (VC) polymers; homo- and copolymers, determination of properties DIN 7747 (1979) Vinyl chloride (VC) polymers; copolymers, classification and designation
2.2 Viscosity ISO 174-1974 Plastics-Determination of viscosity number of PVC resins in dilute solution ISO/R 1628-1970 Plastics-Directives for the standardisation of methods for the determination of the dilute solution viscosity of polymers ISO 3219-1977 Plastics-Polymers in the liquid, emulsified or dispersed stateDetermination of viscosity with a rotational viscometer working at defined shear rate BS 2782 Part 7 Rheological properties Method 730A: 1979 (± ISO/R 1628) Determination of reduced viscosity (viscosity number) and intrinsic viscosity of plastics in dilute solution Method 730B: 1978 (= ISO 3219) Determination of the viscosity of polymers in the liquid, emulsified or dispersed state using a rotational viscometer working at a defined shear rate ASTM D 1243-79 Test for dilute solution viscosity of vinyl chloride polymers
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DIN 53726 (1983) Testing of plastics; determination of viscosity number and K-value of vinyl chloride (VC) polymers 2.3 Chlorine Content ISO 1158-1978 Plastics-Vinyl chloride homopolymers and copolymers-Determination of chlorine ASTM D 1303-55 (1979) Test for total chlorine in vinyl chloride polymers and copolymers DIN 53474 (1976) Testing of plastics, rubber and elastomers; determination of the chlorine content 2.4 Vinyl Acetate Content in VCIVA Copolymers ISO 1159-1978 Plastics-Vinyl chloride-vinyl acetate copolymers-Determination of vinyl acetate 2.5 Ash and/or Sulphated Ash Content ISO 1270-1975 Plastics-PVC resins-Determination of ash and sulphated ash BS 2782: Part 4 Method 454A: 1978 Determination of ash Method 454B : 1978 (= ISO 1270) Determination of sulphated ash 2.6 Volatile Matter (Including Water) ISO 1269-1980 Plastics-Homopolymer and copolymer resins of vinyl chlorideDetermination of volatile matter (including water) BS 2782: Part 4 Method 454D: 1978 (= ISO 1269) Determination of volatile matter (including water) of PVC resins ASTM D 3030-79 Test for volatile matter (including water) of vinyl chloride resins
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2.7 Impurities and Foreign Matter
ISO 1265-1979 Plastics-PVC resins-Determination of number of impurities and foreign particles ASTM D 2222-66 (1978) Test for methanol extract of vinyl chloride resins 2.8 Bulk Density
ISO 60-1977 Plastics-Determination of apparent density of material that can be poured from a specified funnel ISO 61-1976 Plastics-Determination of apparent density of moulding material that cannot be poured from a specified funnel ISO 1068-1975 Plastics-PVC resins-Determination of compacted apparent bulk density BS 2782: Part 6 Method 621A: 1978 (= ISO 60) Determination of apparent density of moulding material that can be poured from a funnel Method 621B : 1978 (= ISO 61) Determination of apparent density of moulding material which cannot be poured from a funnel Method 621D : 1978 (= ISO 1068) Determination of compacted apparent bulk density of PVC resins ASTM D 1895-69 (1979) Tests for apparent density, bulk factor, and pourability of plastic materials NB Method A equivalent to ISO 60 Method C equivalent to ISO 61 2.9 Particle Size
ISO 1624-1978 Plastics-Vinyl chloride homopolymer and copolymer resinsSieve analysis in water
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ISO 4576-1978 Plastics-Aqueous dispersions of homopolymers and copolymers-Determination of gross particle content by sieve analysis ISO 4610-1977 Plastics-Vinyl chloride homopolymer and copolymer resinsSieve analysis using air-jet sieve apparatus BS 2782: Part 4 Method 454F: 1978 (= ISO 4610) Sieve analysis of vinyl chloride homopolymer and copolymer resins using air-jet sieve apparatus ASTM D 1705-61 (1980) Particle size analysis of powdered polymers and copolymers of vinyl chloride 2.10 Bromine Number
ISO 3499-1976 Plastics-Aqueous dispersions of homopolymers and copolymers of vinyl acetate-Determination of bromine number 2.11 pH of Aqueous Extract
ISO 1264-1980 Plastics-Homopolymer and copolymer resins of vinyl chlorideDetermination of pH of aqueous extract BS 2782: Part 4 Method 454C: 1978 (= ISO 1264) Determination of pH of aqueous extract of PVC resins 2.U Miscellaneous Properties Relevant to Processing
ASTM D 2396-79 Recommended practice for powder-mix test of poly(vinyl chloride) (PVC) resins using a torque rheometer ASTM D 2538-79 Recommended practice for fusion test of poly(vinyl chloride) (PVC) resins using a torque rheometer ASTM D 2873-70 (1982) Test for interior porosity of poly(vinyl chloride) (PVC) resins by mercury intrusion porosimetry
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ASTM D 3367-75 (1980) Test for plasticizer sorption of poly(vinyl chloride) resins under applied centrifugal force ASTM D 3596-77 Recommended practice for determination of gels (fish eyes) in general purpose poly(vinyl chloride) (PVC) resins 2.13 Methanol Extract ASTM D 2222-66 (1978) Test for methanol extract of vinyl chloride resins 2.14 VCM Content DIN E 53743 (1979) Draft Testing of plastics; gas chromatographical determination of vinyl chloride (VC) in polyvinyl chloride (PVC)
3 VINYL COMPOUNDS (Chapter 3) 3.1 General (Designation, Coding, Characterisation Tests) (a) Rigid Compounds ISO 1163/1-1980 Plastics-Unplasticized compounds of homopolymers and copolymers of vinyl chloride. Part 1: Designation ASTM D 1784-81 Specification for rigid poly(vinyl chloride) (PVC) compounds and chlorinated poly(vinyl chloride) (CPVC) compounds ASTM D 2124-70 (1979) Analysis of components in poly(vinyl chloride) compounds using an infrared spectrophotometric technique ASTM D 3010-71 (1981) Recommended practice for preparing compression-moulded test sample plaques of rigid poly(vinyl chloride) compounds DIN 7748 Part 1 (1979) Plastic moulding materials; unplasticized PVC moulding materials; classification and designation Part 2 (1979) Plastic moulding materials; unplasticized PVC moulding materials; determination of properties
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(b) Flexible Compounds ISO 289811-1980 Plastics-Plasticized compounds of homopolymers and copolymers of vinyl chloride. Part 1: Designation ISO 2898/2-1980 Plastics-Plasticized compounds of homopolymers and copolymers of vinyl chloride. Part 2: Determination of properties BS 2571 : 1963 Flexible PVC compounds ASTM D 2124-70 (1979) Analysis of components in poly(vinyl chloride) compounds using an infrared spectrophotometric technique ASTM D 2287-81 Specification for non-rigid vinyl chloride polymer and copolymer molding and extrusion compounds DIN 7749 Part 1 (1979) Plastic moulding materials; plasticized polyvinyl chloride (PVC) moulding materials; classification and designation Part 2 (1979) Plastic moulding materials; plasticized polyvinyl chloride (PVC) moulding materials; preparation of specimens and determination of their properties (c) Pastes ISO 4612-1979 Plastics-PVC paste resins-Preparation of a paste DIN 54800 (1979) Testing of plastics; preparation of polyvinyl chloride (PVC) paste for testing purposes DIN 54801 (1979) Testing of plastics; determination of apparent viscosity at high rates of shear of polyvinyl chloride (PVC) paste by Severs capillary viscometer
(d) Miscellaneous ASTM D 729-81 Specification fOr vinylidene chloride molding compounds ASTM D 3364-74 (1979) Test method for flow rates for poly(vinyl chloride) and rheologically unstable thermoplastics
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3.2 Properties and Tests (a) Bulk Density and Pourability
I
ISO 60-1977 ISO 61-1976 BS 2782: Part 6: Method 621A: 1978 BS 2782: Part 6: Method 621B: 1978 ASTM D 1895-69 (1979)
For titles see Section 2.8 above
DIN 53466 (1960) Testing plastics. Determination of bulk factor of moulding materials DIN 53467 (1960) Testing plastics. Determination of apparent density of moulding material that cannot be poured from a specified funnel DIN 53468 (1974) Testing plastics. Determination of apparent density of moulding material that can be poured from a specified funnel (b) Water Absorption BS 2782: Part 5 Method 502e: 1970 Water absorption and water soluble matter of polyvinyl chloride extrusion compound See also DIN 7748: Part 2 (1979) in Section 3.1(a) of this Appendix. (c) Temperature Effects (i) PHYSICAL BS 2782: Part 1 Method 122A: 1976 Determination of deformation under heat of flexible polyvinyl chloride compound Method 150B: 1976 Determination of cold flex temperature of flexible polyvinyl compound Method 150C: 1983 Determination of low temperature extensibility of flexible polyvinyl chloride sheet ASTM D 746-79 Test for brittleness temperature of plastics and elastomers by impact
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ASTM D 1043-72 (1981) Stiffness properties of plastics as a function of temperature by means of a torsion test ASTM D 1593-81 Specification for non-rigid vinyl chloride plastic sheeting
(ii)
CHEMICAL
ISO/R 182-1970 Plastics-Determination of the thermal stability of polyvinyl chloride and related copolymers and their compounds by splitting off of hydrogen chloride ISO 305-1976 Plastics-Determination of thermal stability of polyvinyl chloride, related chlorine-containing polymers and copolymers, and their compounds-Discoloration method BS 2782: Part 1 Method BOA: 1976 (=f:. ISO/R 182) Determination of the thermal stability of polyvinyl chloride by the Congo red method Method BOB: 1976 (=1= ISO/R 182) Determination of the thermal stability of polyvinyl chloride by the pH method ASTM D 793-49 (1976) Test for short-time stability at elevated temperatures of plastics containing chlorine ASTM D 2115-67 (1980) Recommended practice for oven heat stability of poly(vinyl chloride) compositions DIN 53381 Testing of plastics; determination of the thermal stability of polyvinyl chloride and related copolymers and their compounds; Part 1 (1971) Congo red method Part 2 (1975) Discoloration method Part 3 (1971) pH method (d) Mechanical Properties BS 2782: Part 3: Method 365A: 1976 Determination of softness number of flexible plastics
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(e) Miscellaneous Properties and Analysis ASTM D 2124-70 (1979) Analysis of components in poly(vinyl chloride) compounds using an infrared spectrophotometric technique ASTM D 2151-68 (1977) Test for staining of poly(vinyl chloride) compositions by rubber compounding ingredients ASTM D 2538-79 Recommended practice for fusion test of poly(vinyl chloride) (PVC) resins using a torque rheometer ASTM D 3421-75 Extraction and analysis of plasticiser mixtures from vinyl chloride plastics ASTM D 3596-77 Recommended practice for determination of gels (fish eyes) in general purpose poly(vinyl chloride) (PVC) resins
4 PLASTICISERS (Chapters 5-7)
4.1 Bulk Properties Specifications for individual plasticisers (as chemical materials) may be found in the various catalogues of standards under the appropriate compound names. Some of these are given here, as are some general tests on plasticisers and some specifications covering groups of compounds used as plasticisers.
ISO 1385/1-1977 Phthalate esters for industrial use-Methods of test-Part 1: General ISO 1385/2-1977 Phthalate esters for industrial use-Methods of test-Part II: Measurement of colour after heat treatment (Diallyl phthalate excluded) ISO 1385/3-1977 Phthalate esters for industrial use-Methods of test-Part III: Determination of ash
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ISO 1385/4-1977 Phthalate esters for industrial use-Methods of test-Part IV: Determination of acidity to phenolphthalein-Titrimetric method ISO 1385/5-1977 Phthalate esters for industrial use-Methods of test-Part V: Determination of ester content-Titrimetric method after saponification ISO 2520-1974 Tritolyl phosphate for industrial use-List of methods of test ISO 2521-1974 Tritolyl phosphate for industrial use-Determination of acidity to phenol red-Volumetric method ISO 2522-1974 Tritolyl phosphate for industrial use-Determination of apparent free phenols content-Volumetric method ISO 2523-1974 Adipate esters for industrial use-List of methods of test ISO 2524-1974 Adipate esters for industrial use-Measurement of colour after heat treatment ISO 2525-1974 Adipate esters for industrial use-Determination of acidity to phenolphthalein-Volumetric method ISO 2526-1974 Adipate esters for industrial use-Determination of ash-Gravimetric method ISO 2527-1974 Adipate esters for industrial use-Determination of ester content-Volumetric method BS 573, 574, 1995, 1996, 2535, 2536, 3647: 1973 Plasticizer esters Comprises: BS 573 Dibutyl phthalate BS 574 Diethyl phthalate BS 1995 Di-(2-ethylhexyl) phthalate BS 1996 Dimethyl phthalate BS 2535 Dibutyl sebacate BS 2536 Di-(2-ethylphenyl) sebacate BS 3647 Dimethoxyethyl phthalate BS 1998: 1970 Triphenyl phosphate
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BS 1999: 1964 (± ISO 2520/2) Tritolyl phosphate BS 4835: 1973 (± ISO 1385, ISO 252417) Methods of test for plasticizer esters BS 4968-70: 1973 Di-isobutyl phthalate, di-isooctyl phthalate and di-isooctyl sebacate ASTM D 1045-80 Sampling and testing plasticizers used in plastics ASTM D 1249-81 Specification for octyl ortho-phthalate ester plasticizers ASTM D 2288-69 (1980) Test for weight loss of plasticizers on heating DIN 53400 (1970) Testing of plasticizers; determination of density, refractive index, flash point and viscosity DIN 53401 (1975) Determination of saponification value DIN 53402 (1973) Determination of acid value DIN 53404 (1952) Testing of plasticizers; determination of saponification rate DIN 53409 (1967) Testing of plasticizers and solvents; determination of Hazen colour (platinum cobalt colour, APHA method) 4.2
Properties in Association with PVC (Compatibility, Volatility, Migration)
ISO 176-1976 Plastics-Determination of loss of plasticizers-Activated carbon method ISO 177-1976 Plastics-Determination of migration of plasticizers BS 2782: Part 4 Method 465A and 465B: 1979 (= ISO 176) Determination of loss of plasticizers (activated carbon method) BS 2782: Part 5 Method 51lA: 1970 Effect of polyvinyl chloride compound on the loss tangent of polythene
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ASTM D 1203-67 (1981) Tests for loss of plasticizer from plastics (activated carbon methods) ASTM D 2134-66 (1980) Test for softening of organic coatings by plastic compositions ASTM D 2383-69 (1981) Recommended practice for testing plasticizer compatibility in poly(vinyl chloride) (PVC) compounds under humid conditions ASTM D 3291-74 (1980) Test for compatibility of plasticizers in poly(vinyl chloride) plastics under compression ASTM D 3421-75 Extraction and analysis of plasticizer mixtures from vinyl chloride plastics DIN 53405 (1981) Testing plasticizers; determination of migration of plasticizers DIN 53407 (1971) Testing plastics; determination of loss in weight of plasticized plastics by the activated carbon method DIN 53408 (1967) Testing plastics; determination of solubility temperature of polyvinyl chloride in plasticizers
4.3 Effects on PVC ISO 4574-1978 Plastics-PVC resins for general use-Determination of hot plasticizer absorption ISO 4608-1977 Plastics-PVC resins for general use-Determination of plasticizer absorption at room temperature BS 2782: Part 4 Method 454E: 1978 (= ISO 4608) Determination of plasticizer absorption at room temperature of PVC resins for general use ASTM D 2396-79 Recommended practice for powder-mix test of poly(vinyl chloride) (PVC) resins using a torque rheometer ASTM D 3367-75 (1980) Test for plasticizer sorption of poly(vinyl chloride) resins under applied centrifugal force
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DIN 53408 (1967) Testing of plastics; determination of solubility temperature of polyvinyl chloride in plasticizers 5 PVC SHEETING AND FILMS (Chapter 20) 5.1 Rigid BS 3757: 1978 Rigid PVC sheet BS 4203: 1980 Extruded rigid PVC corrugated sheeting ASTM D 1927-81 Specification for rigid poly(vinyl chloride) plastic sheet ASTM D 2123-81 Specification for rigid poly(vinyl chloride-vinyl acetate) plastic sheet DIN 16927 Part 1 (1977) Sheets of rigid polyvinyl chloride (rigid PVC), normal impact strength; technical delivery specifications Part 2 (1977) Sheets of rigid polyvinyl chloride (rigid PVC), raised impact strength; technical delivery specifications DIN 16929 (1965) Tubes and sheets of rigid PVC (rigid polyvinyl chloride); resistance to chemicals; recommended practice 5.2 Flexible BS 1763: 1975 Thin PVC sheeting (calendered, flexible, unsupported) BS 2739: 1975 Thick PVC sheeting (calendered, flexible, unsupported) BS 2782: Part 6 Method 643A: 1976 Shrinkage on heating of film intended for shrink wrapping applications BS 3878: 1982 Flexible PVC sheeting for hospital use
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ASTM D 1239-55 (1971) Test for resistance of plastic films to extraction by chemicals. Discontinued in 1980 ASTM D 1593-81 Specification for non-rigid vinyl chloride plastic sheeting ASTM D 1893-67 (1978) Test for blocking of plastic film ASTM D 3083-76 Flexible poly(vinyl chloride) plastic sheeting for pond, canal and reservoir lining ASTM D 3354-74 (1979) Test for blocking load of plastic film by the parallel plate method DIN 16937 (1971) Sheets of bitumen-resistant nonrigid PVC (nonrigid polyvinyl chloride) for waterproofing of buildings; requirements, testing DIN 16938 (1971) Sheets of non-bitumen-resistant non-rigid PVC (non-rigid polyvinyl chloride) for damp-proofing DIN 53372 (1970) Testing of plastics films; determination of break at low temperature of films of nonrigid polyvinyl chloride The following list gives further DIN specifications on the testing of plastics sheet and film (short titles): DIN 53353 DIN 53365 DIN 53366 DIN 53369 DIN 53370 DIN 53374 DIN 53375 DIN 53377 DIN 53378 DIN 53380 DIN 53488
(1971): Tear test (1974): Mass per unit area (1975) Draft: Blocking (1976): Shrinking stress (1976): Thickness (1959): Flexure (1972): Coefficients of friction (1969): Dimensional stability (1965): Colour fastness to hydrogen sulphide (1969): Gas transmission rate (1963): Hole test
5.3 Sheet and Film Fabrication and Products BS 1776: 1951 Fabrication of lightweight articles (other than rainwear) from polyvinyl chloride sheeting
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BS 3501: 1962 Dinghy buoyancy equipment ASTM D 1789-65 (1977) Test for welding performance of poly(vinyl chloride) structures DIN 1910 Part 3 (1977) Welding; welding of plastics, processes DIN 16930 (1964) Welding of rigid PVC (rigid polyvinyl chloride); recommended practice DIN 16931 (1959) Welding of nonrigid PVC (nonrigid polyvinyl chloride); recommended practice DIN 16995 (1976) Packaging materials; plastics films, main properties, special properties, test methods Note: Some properties of PVC films and sheeting are covered by general plastics film and sheeting standards. Such standards are mentioned in the appropriate chapters and in Appendix 3. Some examples are water vapour permeability, gas permeability, electrical properties and flammability. BS 1133 Packaging Code; Section 21: 1976 Regenerated cellulose film, aluminium foil and flexible laminates is also relevant to plastics films for packaging (cf. DIN 16995 above).
6 PVC PIPES, TUBING AND PIPE FITTINGS 6.1
Rigid Pipes and Fittings, Including Pressure Pipes
ISO DATA 7-1979 Unplasticized polyvinyl chloride pipes and fittings-Chemical resistance with respect to fluids ISO 580-1973 Moulded fittings in unplasticized polyvinyl chloride (PVC) for use under pressure-Oven test ISO 727-1979
Unplasticized polyvinyl chloride (PVC) fittings with plain sockets for pipes under pressure-Dimensions of sockets-Metric series
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ISO 2035-1974 Unplasticized polyvinyl chloride (PVC) moulded fittings for elastic sealing ring type joints for use under pressure-Pressure resistance test ISO 2043-1974 Unplasticized polyvinyl chloride (PVC) moulded fittings for elastic sealing ring type joints for use under pressure-Oven test ISO 2044-1974 Unplasticized polyvinyl chloride (PVC) injection-moulded solventwelded socket fittings for use with pressure pipe-Hydraulic internal pressure test ISO 2045-1973 Single sockets for unplasticized polyvinyl chloride (PVC) pressure pipes with elastic sealing ring type joints-Minimum depths of engagement ISO 2048-1973 Double socket fittings for unplasticized polyvinyl chloride (PVC) pressure pipes with elastic sealing ring type joints-Minimum depths of engagement ISO 2505-1981 Unplasticized polyvinyl chloride (PVC) pipes-Longitudinal reversion-Test methods and specification ISO 2507-1982 Unplasticized polyvinyl chloride (PVC) pipes and fittings-Vicat softening temperature-Test method and specification ISO 2508-1981 Unplasticized polyvinyl chloride (PVC) pipes-Water absorption-Determination and specification ISO 2536-1974 Unplasticized polyvinyl chloride (PVC) pressure pipes and fittings, metric series-Dimensions of flanges ISO 2703-1973 Buried unplasticized polyvinyl chloride (PVC) pipes for the supply of gaseous fuels-Metric series-Specification ISO 3114-1977 Unplasticized polyvinyl chloride (PVC) pipes for potable water supply-Extractability of lead and tin-Test method ISO 3460-1975 Unplasticized polyvinyl chloride (PVC) pressure pipes-Metric series-Dimensions of adapter for backing flange
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ISO 3472-1975 Unplasticized polyvinyl chloride (PVC) pipes-Specification and determination to resistance to acetone ISO 3473-1977 Unplasticized polyvinyl chloride (PVC) pipes-Effect of sulphuric acid-Requirement and test method ISO 3474-1976 Unplasticized polyvinyl chloride (PVC) pipes-Specification and measurement of opacity ISO 3603-1977 Fittings for unplasticized polyvinyl chloride (PVC) pressure pipes with elastic sealing ring type joints-Pressure test for leakproofness ISO 3604-1976 Fittings for unplasticized polyvinyl chloride (PVC) pressure pipes with elastic sealing ring type joints-Pressure test for leakproofness under conditions of external hydraulic pressure ISO 3606-1976 Unplasticized polyvinyl chloride (PVC) pipes-Tolerances on outside diameters and wall thicknesses ISO 4434-1977 Unplasticized polyvinyl chloride (PVC) adapter fittings for pipes under pressure-Laying length and size of threads-Metric series ISO 4439-1979 Unplasticized polyvinyl chloride (PVC) pipes and fittingsDetermination and specification of density BS 3505: 1968 (1982) (oF ISO 2505, ISO 3114, ISO 3472-3, ± ISO 3474) Unplasticized PVC pipe for cold water services BS 3506: 1969 Unplasticized PVC pipe for industrial purposes BS 3943: 1979 Plastics waste traps BS 4346 (± ISO 2035, ISO 2043-5, ISO 2048) Joints and fittings for use with unplasticized PVC pressure pipes Part 1: 1969 Injection moulded PVC fittings for solvent welding for use with pressure pipes, including potable water supply Part 2: 1970 Mechanical joints and fittings principally of unplasticized PVC
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Part 3: 1982 Solvent cement BS 4514: 1983 Unplasticized PVC soil and ventilating pipe, fittings and accessories BS 4576 Unplasticized PVC rainwater goods Part 1: 1982 Half-round gutters and circular pipe BS4607 Non-metallic conduits and fittings for electrical installations Part 1: 1970 Rigid PVC conduits and conduit fittings. Metric units Part 2: 1970 Rigid PVC conduits and conduit fittings. Imperial units Part 3: 1971 Pliable corrugated, plain and reinforced conduits of selfextinguishing plastics material Part 5: 1982 Rigid conduits, fittings and components of insulating materials BS 4660: 1973 Unplasticized PVC underground drain pipe and fittings BS 5481 : 1977 Specification for unplasticized PVC pipe and fittings for gravity sewers BS CP 312 Plastics pipework (thermoplastics materials) Part 2: 1973 Unplasticized PVC pipework for the conveyance of liquids under pressure ASTM D 876-71 Testing non rigid vinyl chloride polymer tubing used for electrical insulation ASTM D 1785-76 Specification for poly(vinyl chloride) (PVC) plastic pipe, schedules 40, 80 and 120 ASTM D 2152-80 Degree of fusion of extruded poly(vinyl chloride) (PVC) pipe and molded fittings by acetone immersion ASTM D 2241-80 Specification for poly(vinyl chloride) (PVC) plastic pipe (SDR-PR)
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ASTM D 2464-76 Specification for threaded poly(vinyl chloride) (PVC) plastic pipe fittings, schedule 80 ASTM D 2466-78 Specification for poly(vinyl chloride) (PVC) plastic pipe fittings, schedule 40 ASTM D 2467-76a Specification for socket-type poly(vinyl chloride) (PVC) plastic pipe fittings, schedule 80 ASTM D 2665-81 Specification for poly(vinyl chloride) (PVC) plastic drain, waste and vent pipe and fittings ASTM D 2672-80 Specification for bell-end poly(vinyl chloride) (PVC) pipe ASTM D 2729-80 Specification for poly(vinyl chloride) (PVC) sewer pipe and fittings ASTM D 2740-80 Specification for poly(vinyl chloride) (PVC) plastic tubing ASTM D 2846-81 Specification for chlorinated poly(vinyl chloride) (CPVC) plastic hot- and cold-water distribution systems ASTM D 2949-78 Specification for 3·25-in outside diameter poly(vinyl chloride) (PVC) plastic drain, waste, and vent pipe and fittings ASTM D 3033-81 Specification for type PSP poly(vinyl chloride) (PVC) sewer pipe and fittings ASTM D 3034-81 Specification for type PSM poly(vinyl chloride) (PVC) sewer pipes and fittings DIN 1187 (1982) Drain pipes of unplasticized polyvinyl chloride; dimensions, general requirements, test methods DIN 3441 Valves of unplasticized polyvinyl chloride Part 1 (1982): requirements and tests Part 2 (1977): ball valves, dimensions Part 3 (1977): diaphragm valves, dimensions Part 4 (1978): oblique pattern valves, dimensions
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DIN 3543 Part 3 (1978) Tapping valves for plastics pipes, dimensions DIN 4279 Part 7 (1975) Internal pressure test of pressure pipelines for water, pressure pipes of unplasticized PVC DIN 6660 (1980) Pneumatic tube systems; conveyor tubes of rigid PVC DIN 6661 (1979) Pneumatic tube systems; sleeves of rigid PVC DIN 6664 (1980) Pneumatic tube systems, conveyor tube bends 900 of rigid PVC DIN 8061 Part 1 (1974) Pipes of rigid PVC; general quality requirements, test methods Part 2 (1971) Pipes of high impact PVC; general quality requirements, test methods DIN 8062 (1974) Pipes of rigid PVC; dimensions DIN 8063 Parts 1-11 Pipe joints and their elements for pipes of unplasticized PVC under pressure DIN 8079 (1974) (Preliminary Standard) Pipes of PVC-C (chlorinated polyvinyl chloride) dimensions DIN 8080 (1974) (Preliminary Standard) Pipes of PVC-C; general quality requirements and test methods DIN 16450 (1975) Socket-fittings for pressure main lines of rigid polyvinyl chloride; symbols DIN 16451 Parts 1-7 Socket-fittings of cast iron with lamellar graphite for pressure main lines of rigid polyvinyl chloride DIN 16929 (1965) Tubes and sheets of rigid PVC; resistance to chemicals, recommended practice DIN 19531 (1980) Pipes and fittings of unplasticized PVC, with rubber ring sockets,
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for waste and soil installation inside buildings; dimensions, technical specifications for delivery DIN 19532 (1979) Pipe-lines of unplasticized PVC for drinking water supply; pipes, pipe connections, fittings for pipe lines DIN 19534 Part 1 (1979) Pipes and fittings of unplasticized PVC (polyvinyl chloride) with sockets for elastic sealing ring joints for sewage; dimensions Part 2 (1979) Pipes and fittings of unplasticized PVC (polyvinyl chloride) with sockets for elastic sealing ring joints for sewage; technical delivery specifications DIN 19538 (1980) Draft Pipes and fittings of chlorinated polyvinyl chloride (PVC-C) with rubber ring sealed sockets for hot water resistant waste and soil installations inside buildings; dimensions, technical specifications for delivery DIN 86012 (1978) Pipe lines of unplasticized polyvinyl chloride (unplasticized PVC) on ships, cemented type joints; summary of components DIN 86013 (1978) Pipes of unplasticized polyvinyl chloride (unplasticized PVC) on ships with pipe fittings for solvent cement joints DIN 86015 (1976) (Preliminary standard) Pipe lines of unplasticized polyvinyl chloride on ships, cemented type joints; application, processing, laying 6.2 Flexible Tubing BS 1882: 1976 Specification for flexible polymeric tubing and drainage sheeting (radio-opaque) for medical use BS 3746: 1964 (1983) PVC garden hose BS 4607 Non-metallic conduits and fittings for electrical installations Part 3: 1971 Pliable corrugated, plain and reinforced conduits of selfextinguishing plastics material
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ASTM D 876-80 Testing nonrigid vinyl chloride polymer tubing used for electrical insulation ASTM D 922-80 Specification for nonrigid vinyl chloride polymer tubing ASTM D 3150-81 Specification for crosslinked and noncrosslinked poly(vinyl chloride) heat shrinkable tubing for electrical insulation DIN 16940 (1964) Extruded hoses of nonrigid PVC (nonrigid polyvinyl chloride); permissable deviations for dimensions for which tolerances are not indicated DIN 16942 (1966) Water-hoses of nonrigid PVC (nonrigid polyvinyl chloride); dimensions 6.3
Miscellaneous Standards Relevant to Pipes
ISO 3514-1976 Chlorinated polyvinyl chloride (CPVC) pipes and fittingsSpecification and determination of density ISO 3608-1976 Chlorinated polyvinyl chloride (CPVC) pipes-Tolerances on outside diameters and wall thicknesses ISO 4065-1978 Thermoplastic pipes-Universal wall thickness table BS 4962: 1982 Pipes for use as light sub-soil drains BS 5556: 1978 (± ISO 161/1) Specification for general requirements for dimensions and pressure ratings for pipe of thermoplastics materials (metric series) BS CP 312 Plastics pipework (thermoplastics materials) Part 1: 1973 General principles and choice of material ASTM D 2152-80 Test for degree of fusion of extruded poly(vinyl chloride) (PVC) pipe and molded fittings by acetone immersion ASTM D 2564-80 Specification for solvent cements for poly(vinyl chloride) (PVC) plastic pipe and fittings
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ASTM D 2855-81 Recommended practice for making solvent-cemented joints with poly(vinyl chloride) (PVC) pipe and fittings ASTM D 3036-73 Specification for socket-type poly(vinyl chloride) (PVC) plastic line couplings ASTM F 409-81 Specification for thermoplastic accessible and replaceable plastic tube and tubular fittings ASTM F 412-81 Definitions of terms relating to plastic piping systems DIN 16929 (1965) Tubes and sheets of rigid PVC (rigid polyvinyl chloride); resistance to chemicals, recommended practice
7 PVC-COATED MATERIALS AND PRODUCTS Several standards covering plastics-coated fabrics generally are relevant to PVC-coated fabrics, and have been included in this list. 7.1
Coated Fabrics, Including Conveyor and Transmission Belting
ISO 1419-1977 Fabrics coated with rubber or plastics-Accelerated ageing and simulated service tests ISO 1420-1978 Rubber and plastics coated fabric-Determination of resistance to penetration by water ISO 1421-1977 Fabrics coated with rubber or plastics-Determination of breaking strength and elongation at break BS 351: 1976 (=1= ISO 22) Specification for rubber, balata or plastics flat transmission belting of textile construction for general use BS 490 Conveyor and elevator belting BSO 490 Part 1: 1972 (=f:. ISO 282, ISO 283, ISO 703, ISO 1121) Rubber and plastics conveyor belting of textile construction for
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general use (of either multi-ply, single-ply or solid woven construction) Part 2: 1975 (± ISO 251, ISO 252, ISO 282, ISO 283, ISO 432, ISO 433) Rubber and plastics belting of textile construction for use on bucket elevators BS 3424 Parts 0-7: 1982 Testing coated fabrics BS 3546 Parts 1 and 2: 1981 Coated fabrics for water resistant clothing BS 5790 Coated fabrics for upholstery Part 1: 1979 Specification for PVC coated knitted fabrics Part 2: 1979 Specification for PVC coated woven fabrics ASTM D 751-79 Testing coated fabrics ASTM D 2136-66 (1978) Testing coated fabrics-Low-temperature bend test ASTM D 2137-75 Test for rubber property-Brittleness point of flexible polymers and coated fabrics DIN 16922 (1981) Flexible sheet materials, manufactured using plastics; technological classification. DIN specifications relating to coated fabrics generally-not specifically PVC-coated-(short titles): DIN 53352 (1982): DIN 53353 (1971): DIN 53354 (1981): DIN 53356 (1982): DIN 53357 (1982): DIN 53358(1971): DIN 53359 (1957): DIN 53360 (1982): DIN 53361 (1982): DIN 53362 (1980):
Mass per unit area Thickness Tensile test Tear growth test ('trouser-leg' specimen) Adhesion Mass per unit area of coating Cracking on repeated flexing Elongation Cold creasing Determination of stiffness in bending
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7.2 Other Coated Materials and Products (including chain-link fencing, powder-coated wire and other products) BS 1651: 1966 Industrial gloves BS 4102: 1971 Steel wire for fences DIN 3036 Part 1 (1978) Plastic coated steel wires Part 2 (1978) Plastic coated steel wires 8 CELLULAR VINYLS (Chapter 25) Standards relating specifically to cellular vinyls are still comparatively few, and those covering cellular plastics generally are frequently used. For this reason most of the latter standards have been listed here. Specifications relating to fabrics coated with cellular vinyls are given in Section 7 of this Appendix.
8.1 Rigid CeDular Materials ISO 844-1978 Cellular plastics-Compression test of rigid materials ISO 845-1977 Cellular rubbers and plastics-Determination of apparent density ISO 1209-1976 Rigid cellular plastics-Bending test ISO 1922-1981 Cellular plastics-Determination of shear strength of rigid material ISO 1923-1981 Cellular plastics and rubbers-Determination of linear dimensions ISO 1926-1979 Cellular plastics-Determination of tensile properties of rigid materials ISO 2581-1975 Plastics-Rigid cellular materials-Determination of 'apparent' thermal conductivity by means of a heat-flow meter
Al Standards Relevant to PVC Materials and Products
1161
ISO 2796-1980 Cellular plastics-Test for dimensional stability of rigid materials ISOrrR 2799-1978 Cellular plastics-Determination of the temperature at which fixed permanent deformation of rigid materials occurs under compressive load ISO 2896-1974 Rigid cellular plastics-Determination of water absorption BS 3869: 1965 Rigid expanded polyvinyl chloride for thermal insulation purposes and building applications BS 4370 Methods of test for rigid cellular materials Part 1: 1968 (=1= ISO 844, 845, 1923) Methods 1-5 Part 2: 1973 (=1= ISO 1922; ± ISO 1926) Methods 6-10 Part 3: 1974 Methods 11-13 ASTM D 1621-73 (1979) Test for compressive properties of rigid cellular plastics ASTM D 1622-63 (1975) Test for apparent density of rigid cellular plastics ASTM D 1623-78 Test for tensile properties of rigid cellular plastics ASTM D 2126-75 Test for response of rigid cellular plastics to thermal and humid aging ASTM D 2842-69 (1975) Test for water absorption of rigid cellular plastics DIN 53421 (1971) Testing of rigid cellular plastics; compression test DIN 53423 (1975) Testing of rigid cellular plastics; bending test DIN 53424 (1978) Testing of rigid cellular plastics; determination of dimensional stability under heat in the case of flexural load and compression load DIN 53425 (1965) Testing of rigid foams; creep-depending-on-time compression test under heat
1162
N. Herbert and W. V. Titaw
DIN 53427 (1976) Testing of rigid cellular plastics; determination of shear strength of rigid cellular plastics in the form of a sandwich between metal plates DIN 53430 (1975) Testing of rigid cellular plastics; tensile test DIN 53432 (1977) Selfskinning rigid foams; test methods DIN 53433 (1979) Testing of rigid cellular plastics; determination of water absorption by dipping in water 8.2 Flexible Cellular Materials
BS 4023: 1975 Flexible cellular PVC sheeting BS 4443 (~ ISO 485, 1794, 1798, 1856,3386) Methods of test for flexible cellular materials Part 1: 1979 Methods 1 to 6 Part 2: 1972 (= ISO 2439) Method 7-Indentation hardness test Part 3: 1975 Method 8--Determination of creep Method 9-Determination of dynamic cushioning performance Part 4: 1976 (;zf ISO 2440) Methods 10-12 Part 5: 1980 Test for dynamic fatigue by constant load pounding Part 6: 1980 Methods 14-16 ASTM D 1565-81 Specification for flexible cellular materials-vinyl chloride polymers and copolymers (open-cell foam) ASTM D 1667-76 (1981) Specification for flexible cellular materials-vinyl chloride polymers and copolymers (closed-cell vinyl) DIN 53570 (1981) Testing cellular materials; determination of linear dimensions DIN 53571 (1978) Testing of flexible cellular materials; test of the tensile strength
Ai Standards Relevant to PVC Materials and Products
1163
DIN 53572 (1979) Testing of flexible cellular materials; determination of compression set after constant deformation DIN 53574 (1977) Flexible cellular materials; test for dynamic fatigue by constant load pounding DIN 53576 (1976) Testing of flexible cellular materials; hardness testing by indentation techniques DIN 53577 (1976) Testing of flexible cellular materials; determination of compression stress value and compression stress-strain characteristics DIN 53578 (1974) Testing of flexible cellular materials; testing of ageing DIN 53579 Part 1 (1980) Testing of flexible cellular materials; hardness test on finished parts, compression test on shaped parts Part 2 (Draft) (1977) Testing of flexible cellular materials; hardness test on finished parts, compression test on profiles
8.3 MisceUaneous Standards (including those relevant generally to cellular plastics materials and products) (a) Definition and Classification DIN 7726 Part 1 (1966) Cellular materials; definitions of terms, classification (b) Physical Properties-General DIN 53420 (1978) Testing of cellular materials; determination of apparent density
(c) Thermal Properties-General ASTM C 177-76 Test for steady-state thermal transmission properties by means of the guarded hot plate ASTM D 696-79 Test for coefficient of linear thermal expansion of plastics
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N. Herbert and W. V. Titow
(d) Flammability and Burning ASTM E 84-81 Test for surface burning characteristics of building materials ASTM E 162-81 Test for surface flammability of materials using a radiant heat energy source
(e) Chemical Resistance and Permeability ASTM E 96-80 Tests for water vapor transmission of materials DIN 53428 (1967) Testing of foams; determination of the resistance to liquids, vapours, gases and solid materials (f) Insulation Materials (i) THERMAL ASTM C 351-61 (1973) Test for mean specific heat of thermal insulation ASTM C 421-77 Test for tumbling friability of preformed block-type thermal insulation DIN 18164 Part 1 (1979) Foamed plastics as insulating building materials; insulating materials for thermal insulation Part 2 (1979) Foamed plastics as insulating building materials; insulating materials for impact sound insulation (ii) ELECfRICAL ASTM D 149-81 Tests for dielectric breakdown voltage and dielectric strength of electrical insulating materials at commercial power frequencies ASTM D 257-78 Tests for D-C resistance or conductance of insulating materials ASTM D 1673-79 Tests for relative permittivity and dissipation factor of expanded cellular plastics used for electrical insulation
Al
Standards Relevant to PVC Materials and Products
1165
(iii) ACOUSTICAL ASTM C 384-77 (1981) Test for impedance and absorption of acoustical materials by the impedance tube method ASTM C 423-81 Test for sound absorption and sound absorption coefficient by the reverberation room method (g) Cushioning Materials ASTM D 1596-78 Test for shock absorbing characteristics of package cushioning materials ASTM D 2221-68 (1979) Test for creep properties of package cushioning materials (h) Sandwich Structures ASTM C 273-61 (1980) Shear test in flatwise plane of flat sandwich constructions or sandwich cores ASTM C 393-62 (1980) Flexure test of flat sandwich constructions
9 PVC WIRE AND CABLE INSULATION, CABLE SHEATHING AND JACKETING BS 6004: 1975 PVC-insulated cables (non-armoured) for electric power and lighting BS 6231: 1981 Specification for PVC-insulated cables for switchgear and controlgear wiring BS 6346: 1969 (1977) PVC-insulated cables for electricity supply BS 6485: 1971 (1977) PVC-covered conductors for overhead power lines BS 6746: 1976 PVC insulation and sheath of electrical cables
1166
N. Herbert and W. V. Titow
BS 6746C: 1969 (1979) Colour chart for PVC insulation and sheath of electric cables ASTM D 876-80 Testing nonrigid vinyl chloride polymer tubing used for electrical insulation ASTM D 922-80 Specification for nonrigid vinyl chloride polymer tubing ASTM D 1047-79 Specification for poly(vinyl chloride) jacket for wire and cable ASTM D 1755-81 Specification for poly(vinyl chloride) resins Section 13: Electrical conductivity of water extract (This test distinguishes between electrical and non-electrical grades of unprocessed PVC resin) ASTM D 2219-81 Specification for poly(vinyl chloride) insulation for wire and cable, 60°C operation ASTM D 2220-80 Specification for poly(vinyl chloride) insulation for wire and cable, 75°C operation ASTM D 2405-81 Specification for general-purpose acrylonitrile-butadiene/poly(vinyl chloride) (NBRlPVC) jacket for wire and cable ASTM D 2432-81 Specification for heavy-duty acrylonitrile-butadiene/poly(vinyl chloride) (NBRlPVC) jacket for wire and cable ASTM D 2633-82 Testing thermoplastic insulated and jacketed wire and cable ASTM D 2708-81 Specification for extra-heavy-duty acrylonitrile-butadiene/poly(vinyl chloride) (NBRlPVC) jacket for wire and cable
General: Test methods for electrical properties of plastics insulation materials are given in the following collections of standards: BS 2782: Part 2. Electrical properties ASTM Book of Standards: Volumes 08·02, 10·01-10·03 DIN Handbook Plastics 1. Mechanical, thermal and electrical properties
Al
Standards Relevant to PVC Materials and Products
1167
10 PVC FLOORING BS 3260: 1969 PVC (vinyl) asbestos floor tiles BS 3261 Unbacked flexible PVC flooring Part 1: 1973 Homogeneous flooring BS 4902: 1976 Specification for sheet and tile flooring colours for building purposes BS 5085 Backed flexible PVC flooring Part 2: 1976 Cellular PVC backing DIN 16950 (1977) Flooring materials; vinyl-asbestos-tiles, requirements, test methods DIN 16951 (1977) Floor coverings; polyvinyl chloride (PVC) flooring without backing, requirements, test methods
11 VARIOUS PRODUCT STANDARDS AND TESTS 11.1 Colour Bleeding and Staining ISO 183-1976 Plastics-Qualitative evaluation of the bleeding of colorants BS 2782: Part 5 Method 542A: 1979 (= ISO 183) Qualitative evaluation of bleeding of colourants ASTM D 2151-68 (1977) Test for staining of poly(vinyl chloride) compositions by rubber compounding ingredients
11.2 Miscellaneous ISO 580-1973 Moulded fittings in unplasticized polyvinyl chloride (PVC) for use under pressure-Oven test
1168
N. Herbert and W. V. Titow
BS 3887: 1965 Regenerated cellulose and unplasticized PVC pressure-sensitive closing and sealing tapes ASTM D 2301-73 (1979) Specification for vinyl chloride plastic pressure-sensitive electrical insulating tape ASTM D 3005-73 (1979) Specification for low-temperature resistant vinyl chloride plastic pressure-sensitive electrical insulating tape DIN 16941 (1964) Extruded profiles of nonrigid PVC (nonrigid polyvinyl chloride); permissable deviations for dimensions for which tolerances are not indicated DIN 53419 (1977) Extruded profiles of unplasticized polyvinyl chloride (unplasticized PVC); test method of the behaviour against methylene chloride
APPENDIX 2
Quantities and Units: The SI System: Unit Conversion Tables Compiled by W. V. TITOW
The measurement and expression of the properties of the great variety of PVC materials and products in existence today, as well as the description of their performance in their numerous applications, involve the use of various units from a number of scientific disciplines and technical fields. The conversion tables and other information provided in this appendix are offered as relevant and-it is hopedpotentially useful in these connections. Sources of further information include the following standards and publications: ISO 31* Covering the general principles concerning quantities, units and symbols (ISO 31/0-1974), the quantities and units of space and time (ISO 31/1-1978), periodic and related phenomena (ISO 31/2-1978), mechanics (ISO 31/3-1978), heat (ISO 31/4-1978), electricity and magnetism (ISO 31/5-1979), light and related electromagnetic radiations (ISO 31/6-1973), acoustics (ISO 31/7-1978), physical chemistry and molecular physics (ISO 31/8-1973), atomic and nuclear physics (ISO 31/9-1973), nuclear reactions and ionising radiations (ISO 31/10-1973), and solid state physics (ISO 31/13), as well as mathematical signs and symbols for use in the physical sciences and technology (ISO 31/11-1978), and dimensionless parameters (ISO 31/12-1975).
* ISO Standard Handbook 2: 'Units of Measurement', contains the texts of all relevant ISO standards. 1169
1170
W. V. Titow
ISO 1000-1973. * 'SI Units and recommendations for the use of their multiples and of certain other units'. BS 350. 'Conversion factors and Tables' (Part 1 :1974, Part 2: 1962 with Supplement No.1: 1967). BS 1991. 'Letter symbols, signs and abbreviations' Covering general aspects (Part 1: 1976), chemical engineering, nuclear science and applied chemistry (Part 2: 1961), fluid mechanics (Part 3: 1961), structures, materials and soil mechanics (Part 4: 1961), applied thermodynamics (Part 5: 1961), electrical science and engineering (Part 6: 1975) with a list of subscripts for electrical technology (Supplement No.1: 1973 to Part 6). BS 3763: 1976. 'The International System of Units (SI)' ASTM Metric Practice Guide (1976). US National Bureau of Standards. Changing to the Metric System. (1969). Anderton, P. and Bigg, P. H., HMSO. The International System of Units (1973). HMSO. The Use of SI Units (1969). British Standards Institution. Quantities, Units and Symbols (1975). The Royal Society. A Dictionary of Scientific Units. (1964). lerrard, H. G. and McNeill, D. B., Chapman and Hall.
Standardisation of units, with emphasis on the use of those of the SI system, has-for some time now-been strongly promoted in the science and technology of plastics. The SI units are, therefore, given prominence in this section. The SI system is based on seven so-called base units. It also contains two supplementary units, and a number of derived units with special names. Multiples and sub-multiples of all SI units are, of course, also within the system.
* ISO Standard Handbook 2: 'Units of Measurement', contains the texts of all relevant ISO standards.
A2
Quantities and Units: The Sf System: Unit Conversion Tables
1171
Named Units of the SI System Quantity (usual symbol(s) in brackets)
Unit Name
Common Equivalent in equivalent base (and Symbol in Sl supplementary) units Sl units
Length (L, I) Mass (M, m) Time (T, t) Electric current (I) Thermodynamic temperature (e, T) Luminous intensity (J, I) Amount of substance (N, n)
metre kilogram second ampere
m kg s A
kelvin candela
cd
mole
mol
Supplementary units
Plane angle (cr, {3, y, 0, Solid angle (Q, w)
radian steradian
Tad sr
Derived units with special names
Absorbed dose (ionising radiation) Electric capacitance (C) Electric conductance (G) Electric potential, tension (V, U) Electric resistance (R) Energy (E), Work (W, w), Quantity of heat (Q, q) Force (F) Frequency (v, f) illuminance (E) Inductance (L, M) Luminous flux (4)) Magnetic flux (4)) Magnetic flux density (B) Power (P), Energy flux (E",) Pressure (P), Stress (a, r) Quantity of electricity, Electric charge (Q, q)
gray farad siemens
Gy F S
J kg-I Cy-l Ay-l
m2 s- 2 s' A 2 m- 2 kg-I S3 A 2 m- 2 kg- 1
volt ohm
Y
WA- 1 YA- 1
m2 kgs- 3 A- 1 m2 kgs- 3 A- 2
joule newton hertz lux henry lumen weber tesla watt pascal
J N Hz Ix
Nm
m2 kg S-2 m kgs- 2 S-I
H
WbA- 1
1m Wb T W Pa
Ys Wbm- 2 J S-l Nm- 2
coulomb
C
Base units
!p,
etc.)
K
Q
cd srm- 2 m2 kgs- 2 A- 2 cd sr m2 kgs- 2 A- 1 kgs- 2 A- 1 m2 kgs- 3 kg m- I S-2 As
Decimal multiples and sub-multiples of units in the SI and other systems are denoted by adding directly (i.e. without a space or hyphen) the appropriate prefix to the name of the unit, or the appropriate symbol to the unit symbol. The use of multiples and sub-multiples which are not powers of 1000 (indicated by parentheses in the list below) is discouraged in the SI system.
1172
W. V. Titow
Multiple Prefix Symbol
x 1018 exa E
x 1015 pela P
Sub-multiple Prefix Symbol
x 10- 1 x 10- 2 (deci) (cenli) d c
1012 lera
X
T
10- 3 milli m
X
109 giga G
X
10- 6 micro
X
/.l
6 X 10 mega M
10- 9 nano n
X
x 1cP kilo k
x lOZ (hecla) h
10- 12 pica
X
X
P
10- 15 femlo f
xlO (deca) da
X
10- 18 atto a
Unit Conversions: Time Other common units
SI unit s 1 3·155 6926 x 10' 8·640 x 1Q4 3600 60 a
b
year 3·169 x 1 2·738 x 1·141 x 1·901 X
10- 8 10- 3 10-4 10- 6
d· (solar day)b
h· (hour)
min· (minute)
1·157 X 10- 5 365·24 1 4·167 X 10- 2 6·944 X 10- 4
2·778 X 10- 4 8·766 x 103 24 1 1·667 X 10- 2
1·667 X 10- 2 5·259 x lOS 1440
60 1
Units recognised for use with the International System. 1 sidereal day = 86164·090 6 seconds; 1 year = 366·25 sidereal days.
Unit Conversions: Electric Current Only the SI unit (ampere) or its multiples or sub-multiples in use. Unit Conversions: Luminous Intensity Only the SI unit (candela) or its multiples or sub-multiples in common use. Unit Conversions: Amount of Substance Only the SI unit (mole) now in common use (mainly in calculations in chemistry and physics). The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in exactly 0-012 kg of 12c. The entities (which must be specified in each particular case) may be atoms, ions, molecules, electrons, or groups of such units.
1000 10 1 1 X 10- 3 1 X 10- 7 914·4 304·8 25·4
100 1 0·1 1 X 10- 4 1 X 10- 8 91·44 30·48 2·54
1 0·01 1 X 10- 3 1 X 10- 6 1 X 10- 10 0·9144 0·304 8 0·0254 2·54 X
10
4
1 X 106 1 X 104 1 X 103 1 1 X 10- 4
jimQ
1X 1X 1X 1X
1010 108 107 104 1
(angstrom)
A
QThe name 'micron' for this unit (micrometre) is discouraged.
mm
cm
m
Sf units
Unit Conversions: Length
3·28084 0·032808 3·2808 X 10- 3 3 1 0·083333
1 0·333333 0·027778
ft (foot)
1·093610 0·010 936 1·093 6 X 10- 3
yd (yard)
Other common units
36 12 1
39·3701 0·393701 0·039370
in (inch)
?3
~
-.I
........ '"
if
c::r-
;:;'J
;:l
'"5't::l
-<:
;:l
~.
~
~
~
~ too>
~
'"
;;l
~.
~
~
~:
§
K:l
1 1 X lO-4 1 X lO-6 0·8361 0·09290 6·452 X lO-4
m
2
mm 1 X 1()6 100 1 8· 361 27 X lOS 9·2903 X 10' 645-16
1 X 10" 1 0·01 8·361 X 10' 929 6·4516
2
cm 2
SI units
Unit Conversions: Area
1·196 1·196xlO- 4 1·196 X 10- 6 1 0·1111 7·716 X 10- 4
yd
2
10·764 1·076 X 10- 3 1·076 X 10- 5 9 1 6·944 X lO-3
it>
Other common units
1·550 X lO3 0·1550 1·550 X lO-3 1·296 X 10' 144 1
in 2
~
0
~
:0::::
~
~
...... ......
a
cm3 UK gal (UK gallon) 220·0 2-2 X 10- 4 2·2 X 10-7 1 0·8327 0-2200 6-229 3·605 X 10- 3
mm 3
1 X 109 1 X 103 1 4-546 X 106 3-785 X 106 1 X 106 2-832 X 107 1·6387 x 104
Unit recognised for use with the International System.
1 x 106 1 6 1 X 101 1 X 10-3 1 x 10- 9 4·546 X 10-3 4-546 X 103 3-785 x 10-3 3-785 X 103 1 x 10- 3 1 X 103 0-02832 2·832 x 104 1-6387 x 10- 5 16-39
m3
Sf units
Unit Conversions: Volume
264-2 2-642 X 10- 4 2·642 X 10-7 1·201 1 0·264 2 7-4805 4·329 X 10- 3
US gal (US gallon) 1 x 103 1 X 10- 3 1 X 10- 6 4-546 3·785 1 28·32 0·01639
(litre)a
I
fr3
35-32 3-532 X 10-5 3·532 X 10- 8 0-1605 0·1337 0-03532 1 5·787 x 10- 4
Other common units
~
6-1024 x 104 0·061024 6·1024 x 10- 5 277·4 231·0 61·024 1·728 x 103 1
-.l V1
........
1r
<::>-
~
~
o· ;:s
~
~
~ ;,..
~
""
~
'"~
~.
~
in 3
~
~
S S. :::to
to
~
b
a
3·43775 x 1cP 60 1 0·0166667 54 5·4 X 103
57·2958 1 0·0166667 2·77778 x 10- 4 0·9 90
1 0·0174533 2·908 88 x 10- 4 4·84814 X 10- 6 0·0157080 1·57080
2·06265 x 3·6 X 60 1 3·240 x 3·240 X
5
10 105
3
10 103
" (secondt
Other units g
63·6620 1·11111 0·0185185 3·08642 x 10- 4 1 100
(grade)
Units recognised for use with the International System. The degree and the second may be subdivided decimally, e.g. 57.2958° = 57°17'44.8".
(minuteY
° (degree)ab
rad (radian)
S1 unit
Unit Conversions: Plane Angle
0·636620 0·011111 1·851 85 x 10- 4 3·08642 x 10- 6 0·01 1
(right angle)
L
~
~
:-=:::
~
a-
--.I
--
a
1·23457 x 10- 4 1
1 0·81
The solid angle of a sphere (=4n).
4·052 85 x 10
3·28281 x 1<>3
0·079577 5 1
1 12·5664 3·046 17 x 10- 4 2·46740 x 10- 4
3
(square grade)
OK
00 (square degree)
sp (spat)a
Other units
sr (steradian)
SI unit
Unit Conversions: Solid Angle
-J -J
--
ff
0-
~
;:
0"
'"C::l"
;:
~
~ ::::
~
~
~
'"
;:J
~
S. ~
l:l
s:~
§
10
~ N
1 x lQ3 1 1 X 106
1 1 x 10- 3 1 x 103 1·01605 x 103 907·185 50·8023 0·453592 0·028350
4
f'
1 x 10- 3 1 X 106 1 1·01605 0·907185 0·05080 4·5359 x 10- 4
(tonne) 1·102 3 x 10- 3 1·102310 1·12 1 0·056 5·00 X 10-4
0·984207 1 0·892857 0·05 4·4643 x 10- 4
US tone
9·842 1 x 10- 4
UK ton b
19·6841 20 17·8571 1 8·9286 x 10- 3
0·019684
cwt (hundredweight)
Other common units lb
2·20462 2·20462 x 10- 3 2·20462 x 103 2·24 x 103 2 x 103 112 1 0·0625
(pound)
35·2740 0·035274 3·52739 x 104 3·584 X 104 3·2 X 104 1·792 x 103 16 1
oz (ounce)
b
a
Also known as 'metric ton' or 'metric tonne'. Unit recognised for use with the International System. 1 t = 1 Mg (which is an SI unit). The 'long' ton. e The 'short' ton.
5·08023 x 10 453·592 28·3495
g (gram)
kg
Sf units
Unit Conversions: Mass
~
0
:::1
~
~
-..l 00
..... .....
A2 Quantities and Units: The Sf System: Unit Conversion Tables
1179
Unit Conversions: Density 51 units
Other common units
kgm- 3
gcm- 3
kgl- 1
Ibin- 3
Ibft- 3
lb UKgal- 1
1 1 X 1
1 X 10- 3 1 1 27-6799 0·0160185 0·099776
I X 10- 3 I I 27·6799 0·0160185 0·099776
3·6127 X 10- 5 0·036127 0·036127 I 5·7870 X 10-' 3-604 6 X 10- 3
0·062428 62-4280 62-4280 1·728XI
0·010022 10·0224 10·0224 277-419 0·160544 I
Unit Conversions: Mass Flux SI unit
Other common units
1 2·777 6 x 10- 4 0,16666 1·3561 x 10- 3
6
3600
1
600·00 4·88243
1·6666 x 10- 3 1 8·1370 x 10-3
737·34 0·20481 122·89 1
Unit Conversions: Force 51 unit
Other common units
N
kg! (kilogram-force)"
dyn (dyne)
UK tonf (UK ton-force)
lbf (pound-force)
pdl (poundal)
I 9·807 I x 10- 5 9·964 x I
0·1020 I 1-02 X 10- 6 1·016 x 1
I X lOS
1·004 X 10-' 9·842 x 10-'
0·2248 2·2046 2·248 X 10- 6 2·240 x I
7·233 70·93 7·233 X 10- 5 7·207 x 10' 32·174 I
• Called 'kilopond' (kp) in Germany.
1 4·464 x 10-' 1·390 x 10- 5
bar
1 X 10- 5 1·013 1 0·9807 0·06895 2-491 X 10- 3 1·333 X 10- 3 1 X 10- 6
atm (standard atmosphere)b
9·869 x 10- 6 1 0·9869 0·96708 0·06805 2·458 X 10- 3 1·316 X 10- 3 9·869 X 10- 7
b
° 1 Pa
1·450 X 10- 4 14·70 14·50 14·22 1 0·03613 0·01934 1·450 X 10- 5
clbfin- z
Other common units
foo atm.
1·020 X 10- 5 1·033b 1·020 1 0·07031 2·540 X 10- 3 1·360 X 10- 3 1·020 X 10- 6
kgfcm- z
= 1 N m- z ; 1 MPa = 1 MN m- z = 1 N mm- z . 1 atm = 1·033228 technical atmosphere (at); 1 at = 1 kgf cm- z. C Often denoted by 'psi'. dOften referred to as 'inches water gauge' (in w.g.). C 1 mm Hg = 1 torr (to within one part in seven million); 1 torr =
1·013 X 105 1 X 105 9·807 X 104 6·895 X 103 249·1 133·3 0·100
1
PaO
SI unit
Unit Conversions: Pressure, Stress
4·015 X 10- 3 406·8 401·5 393·7 27·68 1 0·5352 4·015 X 10- 4
in HZO d (inches of water)
7·501 X 10- 3 760 750·1 735·6 51·71 1·868 1 7·501 X 10- 4
mmHgC (millimetres of mercury)
10 1·013 x 1x 9·807 x 6·895 x 2-491 x 1·333 X 1
106 106 105 104 103 103
dyn cm- z
~
~
~ :0:::
~
-
1
X
10-7
2·724 X 10- 6 1·163 X 10- 6 29·307 2·931 x 10- 4 3·766 X 10-7
2·778
kWh (kilowatt hour) 0·1020 3·671 x 105 1 0·4270 1·0759 x 107 107·59 0·1383
kgfm
0·2388 8·5985 x 105 2·3420 1 2·5200 x 107 252·00 0·3238
cal (calorieY
1 X 10- 5 1·285 X 10- 8
1
9·478 0·03412 9·294 x 10- 8 3·968 X 10- 8
X 10- 9
therm
Other common units
1·285
X 10- 3
1
9·478 X 103·4121 X 103 9·294 X 10- 3 3·968 X 10- 3 1 X 105
4
Btu (British thermal unity
0·7376 2·6552 x 106 7·2330 3·0880 7·7817 x 107 778·17 1
ftlbf
The The The The The
a
4°C calorie, cal4 == 4·2045 J. I5°C calorie, calIS == 4·1855 J. mean (0°-100°C) calorie, cal (mean) == 4·1897 J. thermochemical calorie, cal (thermochem) == 4·1840 J. mean (32°-2I2°P) Btu, Btu (mean) == 1055·79 J.
In this table these units have their International Table values, now in general use (1 cal == 4·1868 J; 1 Btu == 1055·06 J. These values are always implied when the unit is written without a specifying subscript (i.e. simply 'cal' or 'Btu') but the notation 'cain' and 'BtuIT' is still sometimes employed. Other values occasionally used are:
1 3·600 X 106 9·8066 4·1868 1·055 06 x 108 1·055 06 x 103 1·3558
J
SI unit
Unit Conversions: Energy, Work, Heat
~
--
~
~
:-:l
" ~ ~.
§
~
§'
~
'"
~
~
'" ~
~.
§'
~
§
~
;:0
s:
§
lC
::t>.
""
W. V. Titow
1182
Unit Conversions: Power Sf unit
W 1 735·5 9·807 4·187 745·7 0·29307
Other common units metric horsepower'
kgfms- I
cal S-I
hpb (horsepower)
Btuh- 1
x 10- 3
0·10197 75·00 1 0·4268 76·04 0·02988
0·2388 175·7 2·343 1 178·1 0·06999
1·341 x 10- 3 0·98632 0·01315 5·613 x 10- 3 1 3·929 x 10- 4
3·4128 2·510 x 103 33-47 14·29 2·545 x 1Q3 1
1·360
1 0·013 33 5·692 x 10- 3 1·0139 3·984 x 10- 4
No symbol for this unit in English-speaking countries (in France ch or CV; in Germany PS). Unit now regarded as obsolescent. b Unit traditionally used in the UK and USA. Now regarded as obsolescent.
a
Unit Conversions: Frequency-Units in Common Use Quantity
Sf unit
hertz (Hz) reciprocal second (s-I)
Periodic frequency Rotational frequency (i.e. number of rotations in unit time) Angular frequency
Other units cycle per second (c s-I)a revolutions per second; minute, etc. (i.e. r S-I, rOlin-I; etc.)b
reciprocal second (s-I)
Equivalent to Hz but discouraged (both equivalent to S-I). bThe notation 'rev' for 'revolution' (as in rev/min) is now discouraged.
a
Unit Conversions: Temperature
As a unit of temperature (temperature interval) I K a = IOC = l·goR = l·goP equivalent to A temperature reading of
(kelvin)
(degrees Celsius)
OR (degrees Rankine)
yKa xOC uOR zOF
y x + 273·15 ul1·8
y - 273·15 x (ul1'8) - 273·15 (z - 32)/1'8
l'8y l'8(x + 273·15) u z + 459·67
is
Ka
(z + 459·67)/1·8
o~
The SI unit of temperature. b Unit recognised for use with the International System.
a
of
(degrees Fahrenheit)
1·8y-459·67 1·8x + 32
u - 459·67 z
2·388 X 10-3 0·2388 1 2·778 x 10- 3 4·134 x 10- 3 3·445 X 10- 4
1 X 10- 2 1 4·187 0·01163 0·01731 1·442 x 10- 3
1 0·8598 85·98 360·0 1 1·488 0·1240
kcal h- 1 m- 1 K- 1
Other common units
0·5778 57·78 241·9 0·6720 1 0·08333
aBtu h- 1 [t- 1 °F- 1
6·933 693·3 2·903 x 103 8·064 12·00 1
Btu in h- 1 [t- 2 °F- 1
a
Where they form part of the units in this table, the calorie and Btu have their international table values (see the Energy Unit Conversion Table).
100 418·7 1·163 1·731 0·1442
acal S-l cm- 1 K- 1
Wcm- 1 K- 1
Wm- 1 K- 1
51 unit
Unit Conversions: Thermal Conductivity
~
00
.....
...... ......
~ ~
;;l
§'
~
~
.~
~ [
~ to>
'" :::z
~
~.
~
s::...
I:>
~
.
::>. ::>.
§
lC
tv
W. V. Titow
1184
Unit Conversions: Thermal Resistivity This is the reciprocal of thermal conductivity, and the unit conversions can be found by using the reciprocals of the appropriate units and values in the thermal conductivity table, e.g. for the thermal conductivity of 1W m- 1 K- 1 the corresponding thermal resistivity is 1 K m W- 1 == 1I6·933°Fft2 h in- 1 Btu- 1
= 0·14423 of ft2 h in- 1 Btu- 1 Unit Conversions: Thermal Conductance (Heat Transfer Coefficient) Sf unit
Other common units
0·85984 1 3·6001 x 104 4·88240
1
1-16301 4·1868 x 104 5·67827
2·38846 x 10- 5 2· 7777 x 10- 5
0·17611 0·204 81 7·372 9 x 103 1
1
1·3562 x 10- 4
a Where they form part of the units in this table, the calorie and Btu have their international table values (see the Energy Unit Conversion Table).
Unit Conversions: Thermal Resistance This is the reciprocal of thermal conductance, and the unit conversions can be found by using the reciprocals of the appropriate units and values in the thermal conductance table; e.g. for the thermal conductance of 1 W m- 2 K- 1 the corresponding thermal resistance is 1 K m 2 W- 1 == 110·17611 OF ft2 h Btu- 1
= 5·678 2rF ft2 h Btu- 1 Unit Conversions: Viscosity Kinematic viscosity
Dynamic viscosity Sf unit
1
0·1 1 x 10- 3
Other common units
Sf unit
pa (poise)
cP
m 2 s- 1
(centipoise)
10 1 0·01
1 X 103 100 1
1 1 X 10- 4 1 X 10- 6
Other common units (stokes)
sf>
cSt (centistokes)
1 X 104 1 0·01
1 X 106 100 1
APPENDIX 3
Some Material Properties of pvc Products and Compounds Compiled by W. V.
TITOW
Nm- 2 (=Pa); lbf in- 2 (psi); kgf cm- 2 ; For fibres: g per denier, or g per tex
Maximum stress which the material will withstand before failure (yield or break) NB Comments on common types of failure in strength tests are given in BS 4618: Section 1.3: 1975
Tensile strength
arb
kgm- 3 gcm- 3 Ibin- 3 lb ft- 3
Common units
Mass of a unit volume. For practical purposes numerically equal to relative density (also sometimes called specific gravity), which is the ratio of the density of a material to that of water at the same temperature.
Brief definition
Density p
Property arid usual symbol
M; S: 1·301·45 g cm- 3 M(G): approx 1·5gcm- 3 F: approx 1·4 gcm- 3 PVC homopolymer: 1·4 gcm- 3
Rigid PVC
M: 1·10-1·35 g cm- 3
Flexible PVC
Typical values or value ranges for pVC'
M: 31-60 MPa M (and other compounds): 10-25 (BS 2782 or BS 2782: Part 3: ASTM D 638)" MPa Methods 320 A to F: 1976 M(G): approx (BS 2782) Methods 326 A to C: 1977 (for 110 MPa S: 15-21 MPa films); (ASTM D (ASTMD 882) ASTM D 638-82; 638) ASTM D 759-66 (1976) (at low S: 38-45 MPa and elevated temperatures); F: 2·7-3·0 g per ASTM D 882-81 (for thin sheet denier and films); ( = 330-370 ASTM D 1708-79 (microtensile MPa approx) specimens) ; DIN 53 455-1981
ISO/R 527-1966; ISO/R 1184-1970 (for films);
BS 2782: Part 6: Methods 620A-D: 1980; BS 4618: Section 5.1: 1970; ASTM D 792-66 (1979) (Displacement method); ASTM D 1505-68 (1979) (Density gradient tube); DIN 53479-1976
ISO/R 1183-1970;
Some standards relevant to determination in plastics
ISO 6383-1983; BS 1763: 1975 (thin sheeting: Elmendorf test); BS 2739: 1975 (thick sheeting); BS 2782: Part 3: Method 36OB: 1980 (trouser-tear test);
Very low values for normally plasticised PVC materials
M: 62-100 MPa (ASTMD 790) M(G): approx 138MPa (ASTMD 790)
S: 20-100 N mm- 1
M: 150-400% S: 120-250%
M: 2-40% M(G): 1-2% S: 5-35% F: 10-20%
b.d
a
(continued)
M = mouldings; M(G) = mouldings with 25% glass fibre; S = sheeting and/or film; F = fibre. Symbol not in wide general use. c Specification numbers in brackets after numerical property values indicate the source of standard test(s) by which the values were obtained, not that the values are numerical requirements laid down in the particular specification(s).
N; kgf; lbf; or g; oz; lb; or Nmm-\ kgf mm- 1
Force or load (stated directly or per unit specimen thickness) required to initiate tearing orland propagate a tear in specified test conditions.
b
Tear resistance (tear strength) of sheeting and film
OF
ISO 178-1975; BS 2782: Part 3: Method 335A: 1978; ASTM D 790-81; DIN 53452-1977
Maximum stress in the outer As for tensile strength fibre of a specimen at the moment of break in bending (in specified conditions). NB Flexural strength at yield is the analogous stress at yield (calculated from a formula appropriate to the test method).
As for tensile strength (measured in the same tests)
Flexural strength
Normally % of the original length; occasionally units of length
The increase in ~he length of a test specimen caused by the tensile stress at the moment of rupture (or yield)
Elongation at break (or yield) eTd
Impact resistance (Impact strength)
Tear resistance (contd)
Property and usual symbol
Energy required to break a standard test piece by impact in a standard test. NB A useful outline of impact behaviour of plastics is given in BS 4618: Section 1.2: 1972 (with special reference to design data)
Brief definition
Nm (=J); kgfcm; lbfft; per unit length of notch (for notched specimens) or per unit area of the cross-section of the unnotched part, or (for unnotched specimens) per unit specimen width or crosssectional area.
Common units
ISO 179-1982 (Charpy test); ISO 180-1982 (Izod test); BS 2782: 1970: Method 306A (Izod test); BS 2782: Part 3: Method 351A: 1977 (Charpy test); BS 2782: 1970: Methods 306B and C (falling weight test); ASTM D 256-81 Methods A and C (Izod test);! Method B (Charpy test); ASTM D 1822-79 (tensile impact); ASTM D 3029-78 (falling weight test); DIN 53 443-1975 (falling weight test);
ASTM D 1004-66 (1981); ASTM D 1922-67 (1978) (Elmendorf test); ASTM D 1938-67 (1978); ASTM D 2582-67 (1978) (snag resistance) ; DIN 53363-1979
Some standards relevant to determination in plastics
Flexible PVC
M: 0·5-25 lbf With small ft in- I amounts of plasticiser impact (ASTMD strength can be 256) M(G): approx very low «0·5 llbfft in- I lbfft in-I) the embrittlement (ASTMD 256) being caused by the so-called 'antiplasticisation' effect. Very high at normal plasticiser contents
Rigid PVC
Typical values or value ranges for PVC'
~
:::'l 0
:0:::
~
00 00
-
h
g
f
Ability of material to resist indentation under rigidly specified conditions (including particular, specified combinations of indentor and stress). NB Determination of hardness by scratch resistance is not relevant to PVC. Completely arbitrary, related to arbitrary hardness scales used in particular test methods (e.g. Shore, Rockwell, Barcol specified scale units). ISO 868-1978 (Durometer hardness); ISO 2039-1973 (Ball indentation); ISO 2039/2-1981 (Rockwell hardness); BS 2782 Method 365A: 1976 (BS softness numberS); Method 365D: 1978 (ball indentation hardness); Method 1001: 1977 (Barcol hardness h ) ; BS 2719: 1975 (pocket-type meters: Shore Durometer); ASTM D 785-65 (1981) (Rockwell hardness); ASTM D 2240-81 (Durometer hardness); ASTM D 2583-81 (Barcol hardnessh ); DIN 53456-1973 (Ball indentation hardness); DIN 53 505-1973 (Durometer hardness)
ASTM test essentially similar to ISO 180, but differs (in root radius of notch) from that of BS 2782. Applicable mainly to pPVc. Relevant to glass-reinforced plastics.
Hardness
DIN 53448-1977 (tensile impact) DIN 53453-1975 (Charpy test) M (and other compounds): Durometer (Shore D scale): 65-85 Rockwell (R scale): 110-120 Ball Indentation (DIN 53 456): 75155
(continued)
M (and other compounds): BS softness No. 15-90 Durometer (Shore A scale): 50-95 Rockwell (R scale) 5-80
~
-
t;-
§
~c
g
~
I::l
t
~
"tl
.s;,
~ ::to ~
.g
~
~ E:.:
I::l
~
~
v,
).. ...,
As for tensile (or shear) strength
As for tensile (or flexural) strength
'Modulus of rigidity'. Ratio of shear stress to shear strain in reversible conditions. NB In practice usually determined by torsional methods
Ratio of stress to corresponding strain in bending, in reversible conditions
(b) in shear G
(c) in flexure EB
Common units
As for tensile strength
Brief definition
Ratio of tensile stress to tensile strain in reversible conditions. NB Secant modulus: The ratio of stress to corresponding strain at any given point on the stress-strain curve
Elastic moduli: (a) in tension; E
Property and usual symbol
ISO 178-1975; ISOITR 4137-1978 (Alternating flexure method); BS 2782: Part 3: Method 332A: 1976 (stiffness of film); Method 335A: 1978; ASTM D 790-81; DIN 53457-1968
ISO 537-1980 (Torsion pendulum method); ASTM D 1043-72 (1981); ASTM D 2236-81 (Torsion pendulum method); DIN 53447-1981 (Torsion pulley method)
ISO/R 527-1966; BS 2782: Part 3: Methods 320A to F: 1976; ASTM D 638-82; ASTM D 882-81 (for thin sheet and films); DIN 53457-1968
Some standards relevant to determination in plastics Flexible PVC
Very low at norM: 2·0-3·5 GPa mal plasticiser (ASTMD contents 790) M(G): approx 8·5 GPa (ASTMD 790) S: 2·0-3·0 GPa
Very low at norM (and other commal plasticiser pounds): contents 1·0-1·8 GPa
Very low at norM (and other commal plasticiser pounds): contents 2·5-3·5 GPa (ISO, BS, ASTMor DIN methods)
Rigid PVC
Typical values or value ranges for pVC'
~
:::l S
~
~
>-' >-'
:g
i
Also known as Young's modulus, elastic modulus, or modulus of elasticity.
(ASTMC 177)
Wm-1K- 1
;:
~
it
l:>
<"> 1;;
(continued)
~
-
f}
;:s
(ASTM C 177) ]
Wm- 1 K- 1
M (and other compounds): 0·14-0·17
M (and other compounds): 0·14-0·28
BS 874: 1973 (1980); BS 4618: Section 3.3: 1973; ASTM C 177-76; DIN 52612-1979
See relevant conversion table in Appendix 2
The quantity of heat which passes in unit time through unit area of a slab of uniform material of infinite extent and unit thickness, when unit difference of temperature is established between its parallel faces
Thermal conductivity
k;)"
M (and other ~ compounds): ~ General-purpose It plasticised com- ~ pound typically ~ about 40°, but i:!-. the range is wide ~ (ASTM D 1525 .Q., does not recom- ~ mend the test for ~ pPVC for that ~ reason) ~ M (and other compounds): 65-100°C (ISO 306:5 kg load)
ISO 306-1974; BS 2782 : Part 1: Methods 120A to E: 1976; ASTM D 1525-76; DIN 53460-1976
~
~
Vicat softening point
"'"
:t.
°Cor OF
Low at normal plasticiser contents
The temperature at which a flat-ended needle of specified dimensions penetrates a specimen to a prescribed depth under a prescribed load (usually 1 kg or 5 kg), in standard test conditions
M: 2·2-3·5 GPa (ASTMD 695)
ISO 604-1973; ASTM D 695-80; DIN 53457-1968
As for tensile (or compressive) strength
'Bulk modulus'. Ratio of change in external pressure to fractional change in volume, in reversible conditions
(d) in compression K
(BS 2782: Method 335A)
BS 4618: Section 3.1: 1970; ASTM D 696-79
K- 1 ; 0C- 1 ; °F- t
°C; of
%
Change in length per unit original (reference) length per degree temperature change; i.e. a= 6.LI(L o . 6.T)
The temperature at which, in specified test conditions, a test specimen (bar of prescribed dimensions) undergoes a specified deflection under a flexural load causing a maximum fibre stress in the specimen of either 1·82 MPa (2541bf in -Z) or 0·455 MPa (66lbf in- Z)
Percentage deformation of a sheet specimen of prescribed dimensions by a specified load at 70°C, under stated test conditions
Coefficient of linear thermal expansion
Deflection temperature under load
Deformation under heat of flexible PVC compounds k
a
BS 2782 :Part 1: Method 122A:1976
ISO 75-1974; BS 2782: Methods 121A and B: 1976; ASTM D 648-72 (1978); DIN 53461-1969
Some standards relevant to determination in plastics
Common units
Brief definition
Property and usual symbol
Very low at normal plasticiser contents (pPVC not normally tested for this property)
M (and other ·compounds) : 15-65% (BS 2782)
-
M (and other compounds) : 10 x 10- 5_ 25 x 10- 5 K- 1 (ASTMD 696)
Flexible PVC
M (and other compounds): 6O-80°Ci (ISO, BS, ASTMor DIN) M(G): 7085°Ci (ISO, BS, ASTMor DIN)
M (and other compounds): 5 x 10- 5_ 15 x 10- 5 K- 1 (ASTMD 696) M(G): 2·7 x 10- 5 (ASTMD 696)
Rigid PVC
Typical values or value ranges for pvc'
'l:
:::'l S
~
:<::;
to.>
'0
...... ......
Ratio of the capacitance (Cx ) of a given configuration of electrodes with the particular material as the dielectric, to the capacitance (C v ) of the same electrode configuration with vacuum (or air) as dielectric; Le. k' = C)Cv
The ratio of true power dissipated to the apparent power absorbed during the passage of an alternating current through a dielectric
Permittivity'" (dielectric constant) k'; £'
Loss tangentn (dissipation factorO) tan 0 None (a ratio)
None (a ratio)
J g-IK- 1 ; Ical g-l 0C-\ IBtu Ib- 1 °F- 1
BS 2782: 1970: Method 207A; BS 4618: Section 2.2: 1970; ASTM D 150-81; DIN 53 483-1969, 1970
BS 2782: 1970: Method 207A; BS 4618: Section 2.1 : 1970; ASTM D 150-81; DIN 53 483-1969, 1970; (NB ISO 1325-1973 PlasticsDetermination of electrical properties of thin sheet and film)
BS 4618: Section 3.2: 1973; ASTM C 351-61 (1973)
M (and other compounds): 4·5-8·5 at 50 Hz 3·5-4·5 at 1 MHz (ASTMor DIN)
M (and other compounds): 1.0--2.0 J g-l K- 1
j
~.
~ ~ ::t '"
§:
..,<;;
~
'"
~ 3
~
W
..... ..... \D
~
.,
~o
~
M (and other M (and other -s;, comcompounds) : ." pounds): 0·OS-O·15 at ~ 60Hz 0·007-0·017 0·04-0·14 at ~ at 60Hz 0·006-0·019 1 MHz at 1 MHz (ASTM D 150) § .,." (ASTMD 150) ~
M (and other compounds): 3·3-3·6 at 50Hz 2·9-3·1 at 1 MHz (ASTM or DIN)
M (and other compounds): 0·S-O·9 J g-l K-1
At 254Ibfin- 2 . k The temperature of heat distortion (extension or shrinkage), under tensile load, of 0·0025-1·5 mm thick, relatively stiff plastics sheeting (room-temperature elastic modulus >69 MPa) may be determined by the method of ASTM D 1637-61 (reapproved 1976). I Numerically identical. m Strictly, relative permittivity (see, e.g. BS 4618 or ASTM D 150). n Term favoured in the UK. o Term favoured in the USA and Europe. (continued)
Amount of heat required to raise the temperature of a unit mass of material by one degree (within a specified temperature range)
Specific heat c
Resistivity p;s
Property and usual symbol
BS 4618: Section 2.3: 1975; BS 2782: 1970: Methods 202A andB; ASTM D 257-78; DIN 53482-1967
BS 4618: Section 2.4: 1975; BS 2782: 1970: Method 203A; ASTM D 257-78; DIN 53482-1967
Qm Qcm
Q
Volume resistivity: The electrical resistance between opposite faces of a unit-side cube of the material. NB May also be defined in terms of potential gradient and current density (see, e.g. ASTM D 257 or BS 4618 Section 2.3)
Surface resistivity: The resistance between surfacemounted electrodes of unit width, at unit spacing. NB May also be defined in terms of potential gradient and current per unit width of surface (see, e.g. ASTM D 257 or BS 4618 Section 2.4)
Some standards relevant to determination in plastics
Common units
Brief definition
M (and other compounds) : 1012_10 15 Q cm at 60% RH; room temperature (ASTM;DIN)
Flexible PVC
M (and other M (and other comcompounds): 1011_10 12 Q pounds): 1013_10 14 Q at 60% RH; at 60% room RH; temperature room (DIN) temperature (DIN)
room temperature (ASTM; DIN)
RH',
M (and other compounds): About 1016 Qcm or higher at 60%
Rigid PVC
Typical values or value ranges for PVC"
~
is
:::;j
~
:-:::
.j:o.
\0
--
The amount of water absorbed by a standard specimen in prescribed test conditions
Water absorption g; mg; mass % (volume % for cellular plastics)
Vm- I ; Vcm-I; Vmm- I ; V mil-I (1 mil = 0·001 in)
P
(continued)
M (and other M (and other compounds) : compounds): 3O-150mg (48 h) 8-50mg (BS 2782: (48 h) Method (BS 2782: 502e) Method 0·15-1·0%' 502e) 0·07-0·40%' (ASTMD 57024h (ASTMD57024 h specimen 0·125 in specimen thick) 0·125 in thick)
ISO 62-198()"·q (cold water absorption; boiling water absorption) ; BS 2782: 1970 Method 502C (absorption by PVC compounds) Methods 502F and G P Methods 503B and C q ASTM D 570-81;p,q ASTM D 2842-69 (1975) (for cellular plastics); DIN 53471-1976;q DIN 53473-1979 (absorption from humid atmosphere); DIN 53 495-1973P
M (and other compounds: 25D-400V mil-I (ASTMSpecimen 125 mil thick)
M (and other compounds): 400-500 V mil-I (ASTMSpecimen 125 mil thick)
BS 2782: 1970: Methods 201; ASTM D 149-81; DIN 53 481-1974
Methods closely similar technically (cold water absorption). q Methods closely similar (absorption on immersion in boiling water). , Highest absorption is exhibited by some filled compounds.
Field strength (ratio of applied voltage to thickness) required to produce breakdown of the material under specified test conditions
Dielectric strength
...... ......
\0 VI
1}
l:: ;>
~o
~
~
!i
a
f2
~
~
.s;,
~
'" i;.
.g
~
~ §.:
~
~
~
.......
)..
Fo
required to initiate sliding; = minimum force required to maintain it (at a particular speed); and L = the force (usually gravitational) acting normally to the surfaces to maintain their contact
/is
= FsIL Ilo = FoIL where Fs = the minimum force
BS 2782: 1970: Method 311A; BS 4618: Section 5.6: 1975; ASTM D 1894-78 (/is and Ilo of plastics film and sheeting); ASTM D 3028-72 (1978) (Ilo of plastics solids and sheeting)
None (a ratio)
The two coefficients of friction, static' (/is) and dynamic' (Ilo) are defined by the expressions:
Coefficients of friction
Il
ASTM D 1044-78 (Taber abraser); ASTM D 1242-56 (1981); ASTM D 673-70 (1982) (Mar resistance) ; DItol,53 754-1977
No conventional units. Measured in terms of mass loss by the specimen, or visual effects, e.g. marring of surface, loss of transparency.
Some standards relevant to determination in plastics
Common units
Resistance to surface damage or wear caused by rubbing by prescribed abrasives in strictly specified conditions. NB Abrasion resistance (adhesion) of print on thin PVC sheeting prescribed in BS 1763: 1975 (measured according to BS 2782 : 1970: Method 310B)
Brief definition
Abrasion resistance
Property and usual symbol
Flexible PVC
:::J
~
Abrasion resistance of PVC materials and products varies widely depending, inter alia, on formulation and test conditions. As an example, loss by calendered sheeting in a version of the Taber test may be 10-80 mg per 1000 ~ cycles under a 1 kg load (with a :0::: CS-10 wheel).
Rigid PVC
Typical values or value ranges for pVC'
~
-
t
S
Ratio of the velocity of light in
particular material
vacuo to the velocity in the
None (a ratio)
Also known as the starting coefficient of friction. Also known as the kinetic or sliding coefficient of friction.
/-l;n
Refractive index ISO/R 489-1966; BS 4618: Section 5.3: 1972; ASTM D 542-50 (1977); DIN 53 491-1955 PVC resin: approx 1·55 M: 1·52-1·55
~
-
!:;-
;:s
s::
~o
~
~
§
a
f2-
~
~
."
~
~
.g ~.
~
~
The refractive index is often lowered by plasticisation, but the effect (like com- ~ pound clarity) v... depends on the ~ plasticiser (and 3 the formulation '" ~ generally) .
INDEX 1
General
Acrylic impact modifiers, 99,107,391,395, 396,482,887 lacquers, 1008-9, 1061 light-protective coatings, 482 processing aids, 98, 108, 372-5, 395,396,795 thickeners for PVC latices, 1021, 1023, 1025, 1027 Acrylonitrile butadiene styrene (ABS) copolymers blends with PVC, 99, 393,395 impact modifiers, as, 99,113,114, 390,391,393-5,482 processing aids, as, 373 sheet laminates with PVC, 745 Activators ('kickers'), see under Blowing agents Adhesion promoters for PVC coatings, 62, 174, 1007-8, 1119 solution-deposited copolymer films, of, 1047, 1049-51, 1061, 1063-5 Adhesive bonding of PVC, see under Bonding of PVC Ageing of pastes, 943-5, 963,964, 966 AGS acid ester plasticisers (see also under Aliphatic diester piasticisers), 149, 165, 166,976, 977
Alcohols for plasticiser production, 156 Aliphatic diester plasticisers, 163-6 adipates, possible health hazard, 211 PVC compounds, performance in, 166 Alumina trihydrate effects in PVC, on electrical properties, 248 hardness, 248 tensile properties, 247 Vicat softening point, 248 flame and smoke retardant, as, 215, 247,248,431,433 properties of, 222-3 Anti-blocking (see also Blocking) agents, 251 techniques, 1039 Antimony stabilisers, 275-7, 348 calcium stearate, synergism with, 276,286 organotin, comparison with, 277 Antimony trioxide, 215, 247,429, 430,432 Antioxidants, 255, 292-4 Antiplasticisation, 142-3, 849-50, 1188 'Antique finish' ('Antiqueing'), 1001, 1009 Antistatic agents, 83, 419-25 nature and use, 422-5
1199
1200
Index I General
Antistatic agents-contd. thermal stability, effect on, 106, 260,423-4 A p / Po ratio (of plasticisers), 130-1, 140-1 physical properties of PVC, effect on, 140-1 Asbestos filler, as, 216, 217, 242, 243 surface treatments for, 242 flooring, in, 109-10, 217, 233 hazards of, 242 heat stability of PVC, effect on, 217 properties, 243 sheet reinforced with, 217, 903, 908-9 . thixotropic additive, as, 242 AZDN,1090-1 Azobisformamide (ABFA), see Azodicarbonamide Azodicarbonamide (ADA), 108(r90 Baby pants, 1122 Barium stabilisers, 91, 109, 113--15,275, 280,303-4,338,341,345, 347-54,356 sulphate, 219-20, 222-3, 282-3, 285 Barrier properties of blown bottles, 458, 462-3, 798-800 PVC compositions, see Permeability shrink-wrap PVC films, 895 Barytes, 219, 222 Battery separators passim, 1069 polymer for, 854 production, 1078, 1079 property tests, 1100, 1101 PVC's competitor materials, 1118 Biaxial stretching effects on impact resistance, 19, 27, 761, 774-5, 794 permeability, 19,29,464,774-5 'Bingham body' behaviour, 947-8, 951,996
Biodegradation, see Microbiological attack Blanc fixe, 220, 222 Bleeding (colourants), 404 Blends of PVC with ABS polymers, 375, 393--5 nitrile rubber, 397-401, 1118 other polymers, 21, 24 polyurethane, 170, 197,401 SAN polymers, 374 styrene/maleic anhydride copolymers, 24 Blocking definition, 1039 determination of, 1039, 1045, 1149 films and coatings, 1039 standard tests, 1039, 1045, 1149 Blooming colourants, of, 404 surface tack reduction by, 1039 Blow moulding basic elements, 764 blowing arrangements in, 778-80 bottom-blow, 779 dip (displacement) process, 764, 766, 773-4, 778, 780, 789 equipment and process arrangements, 775-80 extrusion process, 764, 76(r72, 788 equipment, 784-7, 789 methods, 775-7 parison control in, 769, 771-2 waste trimming in, 780-1 formulation aspects, 112-13, 793--7 general principle, 763 historical development, 763-4, 769, 773 injection process, 766, 772-3, 777, 784, 787, 789 equipment, 787, 789 methods, 778 'lost blowhead' technique in, 779 main process features, 765-84 materials, 764-5, 784, 797 multicavity moulds for, 780, 785 multi-station, 775, 778, 789 product cooling, 781, 784
Index I
Blow moulding-eontd.
product-eontd.
removal systems, 781 PVC compositions for end-use aspect, 792 formulation aspect, 793-5, 796 processing aspect 789-91 single-stage processes, 775-8 stretch-blow methods 766-7, 769, 770, 772, 774-5, 787, 789 extrusion, 766-7, 769, 770 injection, 765-66, 772, 778 product properties, effects on, 767,774-5,797,800 terminology, 764, 767, 769, 776, 778-9 top-blow, 778-9 two-stage processes, 775, 776-8 Blow mouldings applications, 764, 795-7 composite walls, with, 765 properties, 792, 797-800 tests for, 789-99 Blowing agents, 1071, 1074, 1075, 1085-92 activators ('kickers') for, 115,270, 279,286,350,353 commercial azodicarbonamide, 1088 decomposition of products, 1085, 1089-90 rates, 1075, 1085, 1087 temperatures, 1085, 1087, 108891 incorporation levels, 1091-2 operational temperature ranges of, 1085-8, 1090-1 performance requirements for, 1085 retarders for, 1089 test for gas evolution by, 1095 volumes of gas generated by, 1087 Bonding of PVC adhesive, 918, 927-9 distinction from welding, 923 solvent and solvent cement, 923-7 compositions, 927, 1063 mechanism, 924-5
General
1201
Bonding of PVC-contd. solvent and solvent cement-eontd. procedures, 924-5 specifications relevant to, 927, 1063 Brittle-ductile transition, 386-8, 866 Brittle fracture, 384-5, 866, 868 Brittle point, 386, 388, 440 Brittle temperature, 386-8 Brittleness temperature, 386, 440,441 Bronzing, 405 BS softness fillers, effect of, 218 measurement, 65, 184-5 number scale, 184, 186 plasticisers, effect of, 162, 172, 185, 187 rubber hardness, and, 184, 186 Shore hardness, and, 184,186 temperature, effect of, 189,450 values of pPVC, 72-7, 1189 Bulk (mass) polymer particle characteristics, 46, 47,690-1 polymerisation, 40 Burning (see also Flammability) plastics, of, 424, 426-7, 428, 501-2, 508-9 PVC, of effect of plasticisers, 161,204-6, 503,508 products of 499-500 role of chlorine in, 161,427,429, 502-3,508 Cable and wire insulation and coverings (see also under Compounding) application (by extrusion), 717-19 compounds, 72-3,111,346,581 cable design, and, 602 cross-linkable, 72, 449-50, 1114 formulation, 111, 202, 346 production, see under Compounding PVC/nitrile rubber, 400,1166 reclaimed PVC in, 601-2
1202
Index 1 General
Cable and wire insulation and coverings-contd. compounds-contd. stabilisation of, 90, 266-7,269, 337,343,349 heat tests for, 449,1115,1165-6 properties and applications, 401, 1114-15 Cadmium compounds pigments, as, 409, 414-15, 418 stabilisers, as, 91-2, 278-80, 282-3, 384-5,831 stabilising action of, 302-4 toxic hazards of, 280, 342, 384,409, 831 UVabsorbers, interaction with, 297 Calcium carbonate fillers, 100-1, 221-40,252,832-3 applications, 100, 232-40 cost considerations, 232 dispersion characteristics of, 231 loading, effects of, 233, 238 plastisols, in, 233, 234, 237 precipitated, 220-2, 225, 241,252 properties, 228-31 surface-treated, 220, 222, 225-8, 234-6,238,241,833 types, 224-5 uPVC, in, 235, 236, 239-41, 246 Calcium resinate, 327 Calcium stearate stabilisersnubricants, 93, 104, 114, 275,276, 277,278,286,305,349, 364-6 synergism with antimony mercaptide, 276, 364 Calcium sulphate, 220-1 Calcium/zinc stabilisers (see also Zinc-containing stabilisers), 283-4 Calender, 589, 591, 804-8, 815, 822, 828 compounding and feed for, 589-99, 810-15 main components of, 804-5 roll (bowl), 804-8 arrangements, 591, 805, 806, 828 bending, 807
Calender---eontd. roll (bowl)---eontd. crossing, 807-8 crowning, 807 deflection, 806-7 number in stack, 804-5, 828 numbering, 805-6 stripper rolls, 820-1, 822 suppliers, 804 Calendered sheet applications, 847, 1107-9 defects and faults in, 809, 823-4, 833-7 flooring, for, 828 lamination of, 829-30, 837-8, 839-
40
low-temperature strain in, 823-4, 827 morphology of, 809 press-finished, 837 properties and tests, 809, 84Q-6 surface treatments applied to, 8389 thickness gravimetric, 825 stretch-down effect, 821 typical range, 803-4 Calendering, 589-91, 597, 803-48 advantages over extrusion, 804 broad definition, 803 Calandrette system, by, 891 formulation aspects, 830-3 general features, 803, 808-9 historical development, 803, 812 lines automated, 597 flooring, 828-9 general purpose, 589,809-28 small-scale, 813 material temperature in, 590, 820 materials processed by, 803 operation outline, 808-9, 816-17, 820 roll speed (friction) ratios in, 806, 817,829 temperature settings in, 590, 817-20
Index I
Calendering-eontd.
scrap reprocessing, in, 596, 826-7 sheet path in, 817 sheet production aspects in cooling, 824-5 edgetrimrning, 596,826-7 embossing, 821-3 lamination, 829-30 stretch-down, 821 thickness control, 825-6 wind-up, 827-8 techniques (general), 590 Carbon black colourant, as, 102, 248-9, 414, 417-18 compounding into PVC, 587 conductive filler, as, 249,424 light stabiliser, as, 105,249,297, 418,482,1056 types, 248, 417-8 Cell stabilisers (for foam production), 1075, 1081-2 Cellular PVC (see also Foams and Blowing agents), 1067-102, 1110-11 applications, 1068-74, 1076-9, 1084, 1092-4, 1096-101, 1110-11 evaluation and testing, 1095, 1099101, 1160-5 rnicroporoussheet, 1069, 1078-9 production methods blowing and foaming techniques, see under Foams 'lost filler' technique, 1078-9 sintering of powder, 1079 rigid, properties of, 1099 terminology, 1067-8 Chalking, 405 Chemical blowing agents, see Blowing agents Chemical blowing of foams, 1071-6, 1085-92 Chemical embossing, 1093-4 Chemical resistance of PVC, 48795 data, 488-93 tests for, 494-5
General
1203
Chlorinated compounds paraffins degradation of, 255-6 flame retardants, as, see under Flame retardants plasticiser extenders, as, 171-3 polyethylene (CPE) calendering of, 803 chemical structure of, 396 degradation of, 255 impact modifier, as, 396-7, 887, 888 plasticiser, as 397, 832 polyvinyl chloride (CPVC), 24-9 applications, 5, 25, 28, 851,868, 1105 chemical structure of, 25-7 commercial compounds of, 25, 27,28,832 fibres from, 5,25 production of, 5, 24-5 properties of, 26-9, 858, 868 solvent cements for, 1063 thermoformability of, 745, 761 Chromium pigments, 409, 415,418, 1056 Citrate plasticisers, 174 Cladding, 452, 893-4 Clay filler, as, 100, 101,216,217-19, 220,223,971,1037 oil absorption of, 971 thickener for pastes, as, 973-4 Clear point temperature, 127-9,959 commercial plasticisers, of, 129 Coated fabrics applications and products, 1111-12, 1113--14 production, 829, 998-1009, 1072, 1077,1094 properties and tests, 844-5, 100910 Coating, see under Lacquers, Pastes, and Surface Coextrusion, 482, 685, 765, 884, 892 Cold bend temperature, 193,442 Cold crack temperature offilm, 193, 441
1204
Index 1 General
Cold-dip coating, 994-6 Cold flex (Clash and Berg) temperature definition, 442 determination, 193,442 effect of 'Antistat A', 425 plasticisers, 166, 169, 173, 192--4, 195,205 phthalate-plasticised PVC, of, 156 Colourants, 82, 102-3, 401-19, 10567 choice considerations, 407-10 commercial,412-15 dispersion, test for, 404 fluorescent, 403--4 formulating with, 102-3 general types,403 health and safety considerations, 409 migration of, 102-3,404 nacreous, 403 pearlescent, 403 physical forms, 103,402,405,410 PVC stability, effects on, 103,260, 404,482,1056-7 service properties, 410-11 stability, 408 Compounding, 513-671, 810-14, 867, 1019-38, 1057-60 cable compounds, of, 586-8,601-2 continuous Buss Turbine, by, 549, 605, 657-9 paste dissolvers, by, 606-7, 656-7 powder blends, of, 549-5 de-gassed melt in, 594-5 downstream' equipment associated with, 513-15, 660-71 extruders use in, 599, 609-51 ingredients (PVC formulation components), 81-3, 86-103, 51419 conveyance of, 525-30, 531 metering and weighing of, 53247,604,606 mixing of, 550, 556, 558, 574-6 sequence of addition, 372, 574 storage of, 516-17, 519-25, 530-2
Compounding~ontd.
in-line calendering, for, 589-97, 81014 extrusion, for, 549, 867 latex compositions, of, 1019-38 masterbatches, of, 587 melt, 577-603 equipment for, 609-51 pellet production and handling in, 526, 530, 557, 582-6, 587,588 mixing aspects of, 549-51 operations, general nature and objectives of, 79, 513-14 pastes, of, 603-9, 981-2 continuous, 605, 657-9 equipment for, 652-9 laboratory-scale, 981-2 standards for, 981 powder blends, of, 547-77 cold,548 continuous, 549, 867 effects of fillers, 519 hot, 548-9,551,574-6 mixers used, 554 high-speed (intensive), 557-62, 564-71,576-7 hot/cold combination, 562--4, 572-3,577 re-cycled material, of, 599-603 schematic outline, 514, 515 solution compositions, of, 105760 typical line, 516-17 'upstream' equipment associated with, 513, 515, 519--47 Compounds (see also PVC compositions) commercial,59-77 costing of, 83-5 definitions, 1--4, 79 injection moulding, 726-7 pellet (granulate), melt compounded designation of, 64 feedstocks, as, 518, 686-7,7367, 789-90, 851-2
Index I
Compounds-contd. pellet (granulate), melt compounded-contd. production of, see Compounding, . Melt physical forms, 80, 514 powder, see Powder compounds production of, see Compounding Compression moulding gramophone records, of, 896-7 pastes, of, 1011 thick sheets, of, 837-8, 891, 902-3 Congo Red test, 320, 322, 1143 Conveyor belting, 421-2, 1112 Cooling of PVC mouldings, 781-4 Copolymers abbreviations for, 4 ABS blends with PVC, in, 375 impact modifiers, as, 99, 367, 373,390,39>'-5 processing aids, as, 373 butadiene/acrylic ester, 20 butadiene/acrylonitrile, see Nitrile rubber, and under Impact modifiers butadiene/fumaric ester, 391 butadiene/methylisopropenyl ketone, 391 butadiene/2-vinyl pyridine, 391 copolyester elastomer (Hytrel 3495), 391 EVA, 96, 99,390,395 MBS, 99, 367, 390, 394-5 SAN, 373, 374 styrene/maleic anhydride, 24 vinyl chloride, of, 19-21,22-3,832, 855 commercial, 5, 41 emulsions (latices) of, 1016-18 graft, 20, 41, 887 morphology of, 21 properties of (general), 20 solution type, 1048-9, 1050-1, 1052, 1053, 1054 VC/acrylic ester, 23, 887 VC/acrylonitrile (fibres), 20, 22 VC/ethylene, 22, 41
1205
General
Copolymers-contd. vinyl chloride, of,-eontd. VCIEVAt 20,23, 99, 367,392,
394-~,887
VClfumaric ester, 23 VC/itaconic ester, 23 VC/maleic ester, 23 VClmaleinimide (N-substituted), 20 VClpropylene, 22, 41, 795,851, 896 VCltrifluorochloroethylene (TFCE), 23, 1049 VC/vinyl acetate, 5, 19,22,39, 41,42,50,832,851,894, 897,904,907,1008,1048-9, 1050-1, 1052, 1053, 1060-2 1064-5 VC/vinyl alkyl ether, 23,795 VClvinylidene chloride, 19,20, 21, 22, 41, 832, 851, 896, 964 Co-precipitates, 266, 286, 340 nature, 286 Sr/Zn laurate, 286 Co-stabilisers epoxy compounds, see under Epoxy stabilisers organic, 278, 283, 285, 292 phosphites, 282-3, 290-2, 304-5 polyols, 305 CPVC (chlorinated polyvinyl chloride) see under full name cpvc (critical pigment volume concentration), 1035-6 Crack initiation (see also Fracture) nibs and impurities, by, 381 notches, by, 381,387,390 role in impact resistance, 380-1 Creep, 859-63 coated fabrics, resistance to, 1010 data for uPVC, 451, 861-3, 864 definitions, 860, 862 modulus, 862 PVC pipe, of, 863, 866, 879 rate, 860 rupture, 862 strain, 859, 862
1206
Index I
Creep-contd. temperature, effect of, 450, 451, 759 tests on thermoforming sheet, 75960 Cricket pitch surface, 1123 'Critical solution temperature' (in plate-out), 688 Cross-staining, see under Sulphide staining Crystallinity plasticised PVC, in, 12~1, 139, 142 PVC polymers, in, see under Polyvinyl chloride VCIVDC copolymers, in, 21 Degradation of PVC, see under Photochemical degradation and Thermal degradation Dilatancy, 948-9, 951, 967 Dip coating, 992-6 Dry blends (see also Powder compounds, and powder blends under Compounding) commercial, 61, 64, 70, 795 conveyance of, 562, 564 feeds, as, 518 blow moulding, in, 789-90, 795 extrusion, in, 686, 691,867 foamed product production, in, 1074,1076 nature,79,80,548-9,564,574-6 passim, 1,513,514 production of, 372, 547-53 standard designation of, 63, 64,65 uPVC processing, in, 851, 852, 867 Dry colours, 405 Dry point, 122, 123 Duckboard, 1123 Electrical insulation, see Cable and wire insulation Electrical properties of PVC, 73, 90, 101, 107, 206, 207, 859,1114-15,1165-6, 1193-5
General
Embossing, 821-3, 935, 1008-9, 1092-4 calendering, in, 821-3, 838 cellular sheet materials, of, 1092-4 'chemical' 1093-4 direct, 1092 PVC paste coatings, of, 1005, 1008-9 screen printing, by, 1094 Emulsion polymerisation, 39, 40, 41 polymers applications, 54, 86, 87, 107, 854 particle characteristics of, 46-8, 943,962-3 purity of, 87,259-60 PVC pastes, in, 962-3 latices, in, 1014 Environmental stress cracking and crazing, 466-9 cracking agents for PVC, 469, 868 definition, 467 mechanism, 468-9 PVC pipes, of, 469, 868 resistance, tests for, 468 Epoxy plasticisers, 17~1, 290 Epoxy stabilisers, 93, 107,278,282, 287,288-90,347 formulations, in, 109, 110, 112, 113,115,289,347,348,351, 354,355 interaction with unsaturated additives, 290 mode of action, 304 Estertin stabilisers, 271, 273, 347 Extenders fillers, as, 215, 231,232 plasticisers, for, 94, 96-7 chlorinated paraffin, 171-3 hydrocarbon, 174 Extruded film applications, 894-5, 1063, 1109-10 production of, 599, 891-2, 1063 Extruded sheet applications, 893, 904-9, 1107-8 production of, 598-9, 891-2 Extruders blow moulding, use in, 784-7
Index I
Extruders-contd. cascade, 583, 598, 599, 600,610, 621,623,628-30,706 commercial, 609-51, 698, 703-10 compounding, 609-51 heating and cooling, 579-80, 612, 623,627,634,642,676,677, 678,681-2 LID ratio, 673, 677 main components of, 674-82 manufacturers of, 704-5 output factors affecting, 680 PVC compositions, 599, 630, 632-3,635,637,639,645, 682-3, 707, 708 planetary 610, 639-42, 644-5, 813 single screw, 673, 677, 698-9 energy efficiency with PVC, 683 screw configurations, 698-9 twin screw, 675, 699-703, 706-8 mode of operation: differences from single-screw, 700-1 screw arrangements, 675, 702 venting of, 677, 683-4 Extrusion, 673-721 ancillary equipment, 710, 713, 718-19,870-1,874-6,885-6, 891,892 blow moulding, 766-72, 784-7 compounding, see under Compounding and Extruders (compounding) computers, use of, 686, 698 dies, 680-1 blown film, 712, 891 cross-head, 717, 785 pipe, 870-1, 872-4 sheet, 892 variable-thickness (for blow moulding), 769, 771 feedstocks for, 514, 518,686-7, 789-90,851-2 feedstocks pre-heating, for, 682, 687 formulation aspects, 274, 687, 853, 876-8,886-9,894,896
General
1207
Extrusion-contd. fusion and gelation in completeness, 688, 694-8, 878 phenomenology, 692-4 general nature of process, 673 high-speed, 714-7 historical development, 6, 7, 673, 699 lines, 704-5, 706 cable and wire coating, 718 pipe, 869-76 PVC hose (braid-reinforced), 719 sheet, 598, 600,892 window frame profiles, 706, 708, 885 normal, of pPVC, 713-14 pastes, of, 1011 pipes, of, 869-78 products, common faults in, 688-9 profile, of, 884-9 sheet, of (see also under Extruded sheet) advantages over calendering, 890-1 equipment, 598, 600, 892 melt temperature profiles, 597
Fatigue, 863-6 data for uPVC, 865-6 definition, 863, 864 dynamic, 863 life, 864 resistance of uPVC, 864-866 temperature, effect of, 450 Feedstocks for PVC processing, see under Compounds and Powder compounds Fillers (see also under individual names and types), 99-102, 215-54 conductive, 248-50 cost considerations, 100,215,216, 221,231-2 definition, 215 effects in PVC, on BS softness No., 218 density, 100, 220, 233
1208
Index I
Fillers---contd. effects in PVC, on-eontd. paste rheology, 228, 229, 237, 251,970-3 tensile properties, 219, 247 fibrous, 217,240,242-6 flame retardant (see also under Flame retardants), 842-3 functional, 215, 240-51 glass, 243, 245, 246 latices, in, 1035-8 loading, effects of, 220 microfibre, 242-4 mineral, 216-23 oil (plasticiser) absorption by, 104, 221,222-3,226,230-1,971-2 pastes, in, 970-3 properties (general), 222-3 reinforcing, 240-6 silicaceous, 216-19, 251 smoke suppressant, see under Smoke starch, 250-1 suppliers, 251-3 surface treated, 104, 225, 228, 242, 833, 970, 971, 972 trade names, 221, 222-3, 226,227, 252,253 wood flour, 250 Film (see also under uPVC) applications of, 893-6, 1109-10 conversion and manipulation of (for packaging), 932 definition, 890, 901-2 extruded, see Extruded film latex, from, 1015-16, 1018, 1019, 1034 properties, 860, 893-6, 1064, 11867, 1190, 1197 shrink-wrap, 894-5 solvent-cast, 890, 1058, 1063 stretch-wrap, 199, 1109--10 thickness measurement and control, 892-3 normal range, 890,901-2 'Fish-eyes' count, specification for, 89, 1140, 1144
General
'Fish-eyes'-eontd. melt-processed compositions, in, 49, 689, 793, 835 nature, 49, 89 passim, 51 standard tests for, 89, 1140, 1144 Fishing lures, 1122 Flame retardants, 83, 424-34, 508 chlorinated paraffins as, 204-6, 432 fillers, as, 206, 242-3, 247-8,431, 433 modes of action, 429--31, 433-4 phosphate plasticisers, as, 160-3, 204-6,430-2 special additives, as, 427, 429--34 Flammability (see also Burning) definition, 502 PVC, of (see also under Burning), 161,204-6,427,429,502-3 terminology, 424, 426, 501-2 test specifications, 426, 434, 502, 505-7, 509 Flooring (see also under Asbestos and Calendered sheet) antistatic, 422 asbestos-filled, stabilisers for, 10910,349,352 calendered,828-9 coated, 1005, 1006, 1010, 1084, 1094 dust-capturing, 1116 embossed, 1092, 1094 expanded, 1084, 1092 fillers in, 109-10, 217,232-4 formulations, 109--10 linoleum, advantages of PVC over, 1116 stain resistance of, 210, 1010 unsupported, 1084, 1116-17 Flow moulding, 737-8 Flow promoters dry (powders), 251 melt, definition, 373 Foams (see also Cellular PVC, and under Pastes), 1069-78, 1080-92 cross-linked, 1077 definition, 1067-8
Index I
Foams---eontd.
density general ranges, 1069 values for PVC, 1068, 1069, 1070, 1076, 1077,1083, 1097, 1098 evaluation and test methods, see under Cellular PVC formulations for chemically blown, 1097 mechanically frothed, 1098 microcellular, 1069, 1075 production methods 'chemical' blowing, 1071-6, 1080, 1085-92 dispersed-gas blowing, 1069-71 mechanical frothing (gas entrainment), 1076-7, 1081-2 products, 1068-77, 1079, 1084, 1088, 1094, 1096-7, 1099, 1100,1110-11 properties of blowing agent, effect of. 1083 cell stabilisers, effect of, 10812 factors affecting, 1080-4 plasticisers, effect of, 1081, 1083, 1084 PVC polymer, effect of, 1083-4 structural, 1075 'Fogging', 157, 202-4, 446 leatherc!oth, role in, 202 tests, 202-4 Foil applications, 894, 1109 definition, 890, 902 stamping, see Hot-foil stamping thermoforming, for, 894 Footwear,76-7,89, 401, 581,586, 1011, 1069, 1074, 1096-7, 1118 Formulation, 79-115 components kinds, 81-3 mutual effects, 103-6,265,3347,339 nature and functions (general), 86-103
General
1209
Formulation---eontd. components---eontd. side effects, 105-6,296-7,305-7, 308-9,364-7 costing, 83-5 examples blow mouldings, 112-13,355, 374, 796 flooring, 109-10 foam compositions, 115, 1096-8 gramophone records, 112 injection mouldings, 113,346 miscellaneous, 348, 351, 1008, 1123 pastes, 114-15,972,974,977, 987, 988, 990, 996, 997 pipes and tubing, 110,345,355, 877 profiles, 114, 346, 354 sheet and film, 107-9, 199, 203, 348,355,818-9,894-6 solutions, 1061-2 wire and cable coverings, 111, 346 general considerations, 80-81 Fracture impact, on, 375, 376, 377, 380, 384 modes of, 380, 384-6 uPVC, of, 387, 388-90, 866 Fusion point (pastes), 853, 854 'Gassing' calcium sulphate, by, 221 lead stabilisers, by, 266 Gear pumps, 684-5 'Gel block', 122, 959,1011,1099 Gelation/fusion melt processing, in, 132, 688, 689-98 pastes, of, 122,694, 951-60,982-5, 1095 stages in, 951-2, 956-9 solvent tests for degree of, 696, 697,878-9 foams, in, 1100 paste products, in, 959-60, 983-5 pipes and profiles, in, 697,878-9, 889
1210
Index 1 General
Gels, 89, 689, 793 Glass transition temperature, see Tg Global radiation, 476, 477 Gloves dip coating of, 994-5 solvent tests on, 983-5 Gramophone records colourants for, 404 compounds for, 586 formulation, 112, 897 polymers for, 853, 854 production, 896-7, 1117 stabilisers for, 112,270,276,286, 348,897 Gravimetric thickness, 825 'Grow moulding', 883 Hardness (see also BS softness) data for PVC, 1189 Durometer (Shore), 65, 184, 186 international rubber scale, 184 plasticisers, effect of, 188 tests, 184, 1183 Heat (see also under Stability) ageing, 443 degradation, see Thermal degradation of PVC history, 80, 313-15, 578, 851 life, 313-15 non-degradative effects of, 450-2 resistance, 443-52 transfers, see Transfers Heat stabilisers (see also under
individual and type names)
activators for blowing agents, as, see under Blowing agents blow mouldings, in, 291, 793, 796 calendering, in, 831 chelator, 275, 282, 285, 287, 291-2, 352,354 commercial, 335-57 applications, 342-56 cadmium-free systems, 284-5, 351,353 suppliers of, 336, 342-56 trade names of, 280, 281, 282, 283-7,292,343-6
Heat stabilisers-eontd. complexes, 275, 337 composite metal, 91-3, 281-6, 335, 337-8,345,347-56 cross-staining by, 272, 278, 309 detection and analysis of, 330 evaluation of, 311-28 hazard and hygiene considerations, 265-6,280,341-2 latices, in, 1030-1 lubricants, and or as, 95-8, 104-5, 265,269,274,278,281,282, 283,286,306,339-40,364-6 metal soaps and salts as, 275, 337-8 mode of action, 291, 299-305 non-metallic, 93, 286-92, 348 non-toxic, 92, 106,272,278,279, 281,283-4,287,291,348 one-pack systems, in, 335, 340-1, 348,351 organic, 286-92 pastes, in, 969-70 physical forms, 266, 273,275, 282-3,340-1,343,345,347, 348-51,353-4,356 polymeric, 271 selection criteria, 262-3, 335, 337 single-metal compounds as, 275-81, 343,348,349 types, 89-93, 264 window-frame formulations, in, 887 Hose, see Tubing 'Hot bench', see Gel block Hot-dip coating and moulding, 992-4 Hot-foil stamping, 934-5 Hydrocarbons, plasticiser extenders as, 174 Impact modifiers (see also under
individual type names) 98-9,
371,390-401,793-4 evaluation of effects, 381 fillers as, 226, 236, 241, 246, 391, 419 mode of action, 391 polymeric, 99,105,390-401,853-4 side effects of, 392-3
Index I
Impact modifers-eontd. window-frame compositions, in, 20, 482,88~
Impact resistance (impact strength), 375-90 biaxial stretching, effect of, 19,27, 761, 774, 793-4 blow mouldings, of, 774, 793-4, 798-9 definition, 375-6, 1188 factors affecting, 377 mode offailure, and, 384-7 notches, effect of, 380-3, 389,390 PVC polymer, effect of (in compositions), 855 temperature, effect of, 855 tests, 375-6, 378-81, 382-3, 799, 1188 units, 382-3, 384, 1188 uPVC, of, 387-90, 855, 1188 Injection moulding, 723-41 compounds, 726-7, 736-7 commercial, 61, 62, 64, 68,70, 76-7,726,852 'easy flow', 726 formulation, 726 melt flow test for, 727 time-temperature behaviour of, 725 types, 733 equipment, 729-35 features relevant to PVC, 730-3, 735 general considerations, 740 selection, 738 faults, 734, 739 feedstocks, 514, 518, 737, 852 melt properties important in, 724-6 process factor effects on products, 728-9 stages and operation, 723-4, 731-5 temperature effects, 724-6 settings, 732-4 thermal input in, 724 trouble shooting in, 738-40 uPVC, of, 723, 731-5, 738-40
General
1211
Injection mouldings, 62, 68, 70, 76-7, 897-8, 1118, 1121, 1122 faults in, 734, 739 formulations for, 113 morphology of, 19,729,898 orientation in, 19,728-9 property data, 1185-95 quenching stresses in, 728 shrinkage of, 727 Intensive mixers, see under Mixers Internal mixers, 596, 810-12, 818 International Rubber Hardness (IRH),184 Kvalue cellular PVC, effects in, 1080, 1083 concept, 14,43 effect on properties and processing, 86,793,799,853,855,1080, 1083 fatigue resistance, and, 864 Fikentscher, 15, 43, 44,854 gelation/fusion of plastisols, effect in, 1080 ICI,86-7 molecular weight, and, 15,44-5,86 specific viscosity, and, 14,44-5 viscosity number, and, 14,44-5 'Kickers' , see under Blowing agents (activators for) Lacquer coating, 298, 1008-9, 1094-5 compositions, 298, 1008-9, 1050, 1060,1061 methods, 1009 objectives of, 298, 1008 rotational mouldings, of, 988 Latices, 1013-46 acrylic, 1044 definition, 1014 film formation by, 1015-16 nitrile, 1040, 1041, 1042, 1043, 1044,1045 vinyl (PVC) anti-foaming agents for, 1028 applications, 1040-5
1212
Latices-eontd. vinyl (PVC)-contd.
Index I
compounding ingredients and techniques for, 1019-38 products from, 1019, 1034, 1036 sedimentation in, 1028 stability of, 1020-1 types, 1016-18 Lead stabilisers, 90-1, 265-70, 337, 343-6,349,350 applications, 265-70 chemical nature, 265-70 electrical insulation, suitability for, 90 hazards, 265-6, 337, 341-2 mode of action, 299 physical forms, 266, 340-1, 343 Leathercloth, 998, 1006, 1007,1008, 1009, 1069, 1070, 1072, 1092,1094,1095,1096-7, 1098 applications, 1111 'fogging', role in, see under Fogging Light stability, see under Stability Limiting oxygen index, see Oxygen index Lithium stabilisers, 280-1, 349 Loop test, 131, 135 Low-temperature plasticisers, 97, 163, 165, 166, 193 Low-temperature properties of PVC, 192-5,388-90,393,394, 400,439-42 'Lubricant value', 369 Lubricants, 82, 95, 98, 359-71 blow mouldings, for, 794-5 calendering, role in, 95, 817, 830-1 chemical structure, 362-3 commercial sources, 371 compatibility with PVC, 360-2 compounds used as, 95, 104, 105, 362-3 concentrates, 370 formulating with, 95, 98 functions and effects, 95, 98, 359-63 mutual effects with fillers and pigments, 367
General
Lubricants-eontd. mutual effects with-eontd.
plasticisers, 366 polymeric modifiers, 366-7 stabilisers, 95, 104, 364-5 one-pack systems, in, 98, 339, 340, 370 stabilising effects of, 95, 105,339, 364-5 Lubrication effects of plasticisers, 82, 360, 363, 366 polymeric modifiers, 367 stabilisers, 269, 274, 278, 281, 282,283,284,286,366 evaluation of effects, 361-2, 367-70 external, 82, 95, 359-63 internal, 82, 95 rheological effects of, 360-2, 368, 369 Machining of rigid PVC, 930-2 Magnesium carbonate, 247, 433 Mass polymerisation and polymers, see under Bulk Masterbatches colour, 406 compounding of, 587 Maximum swelling temperature, 123 Maximum torque temperature, 132 MBS impact modifiers, 99, 390, 391, 392,394,395 Microbiological attack, 208-9, 483-6 definition, 48~ manifestations and mechanism, 483 protective additives, 208, 484-5 resistance to plasticisers, of, 208-9, 483-4 PVC, of, 208, 483, 484 vinyl chloride polymers, of, 483 testing of resistance, 208, 485 Migration of colourants, 404, 411 plasticisers, 135, 175, 199-200, 1146-7 Mills ball, 652, 654 roll, 656
Index I
Mixers (see also under Compounding), 554, 556, 558, 603, 605 continuous, 549, 605, 657-9, 867 dissolver, 656-7 general types, 554-7 horseshoe, 652, 653 hot/cold, 562-4 intensive, 557-62, 564-71, 576-7 paddle, 557, 655 planetary, 652, 653 pneumatic, 666-71 ribbon blender, 557, 653-5 rotating, 556, 558 vertical screw, 666, 667 Modacrylic fibres, 20 Molecular weight definitions, 13-14 distribution, 15-16 polymeric plasticisers, of, 165, 167, 168 PVC, of K value, and, 15,43-6 p. value, and, 125 polymerisation temperature, and, 16,46 processing and properties, effect on (see also under Kvalue), 86 Molybdenum trioxide, 247, 430, 4334 Mould shrinkage other polymers, of, 782 PVC, of, 727, 782 Moulding blow, see Blow moulding and Blow mouldings compression pastes, of, 1011 solid PVC, of, 837-8, 891, 896-7, 902 dip, 993-4 injection pastes, of, 1010-11 solid PVC, of, 723-41 rotational,986-8 slush,988-90 p. Value, 125-6
General
1213
Newtonian viscosity, 946-7 'Nibs', 49, 689, 793, 835 Nitriding, 678, 679, 735 Nitrile rubber blends with PVC, 170,397-401 cable compounds, in, 400, 1166 commercial, 399-400 footwear, in, 1118 latices, in, see under Latices (nitrile) processing of, 399-400 modifier, as, 99, 397-401 plasticiser, as, 96, 367, 398 Non-toxic additives for food contact applications, 106 Non-toxic colourants, 409 Non-toxic plasticisers, 210-11 Non-toxic stabilisers, 92-3, 272,2834,287-92 'Nylonate' (nylon acid) plasticisers, see AGS acid ester plasticisers Oil absorption, see under Fillers 'Oil canning', 893 'One-pack' additive systems, 98, 265, 266,273,298,339,340,348, 351,370,406-7 Organosols, 940, 975-6, 996, 997, 1005 Organotin stabilisers, 92-3, 270-4, 300-1,338,346-8,353,354, 355 antimony, comparison with, 277 characteristics and applications, 272-4 chemical nature, 270-4 effects, 272, 306 mode of action, 300-1 polymeric, 271 'reverse ester', 274 single-pack systems, in, 273, 340 types, 270-1 'Oxo' process, 152, 156 Oxygen index, 502 plasticisers, effect of, 161,205,508 plastics, values for, 503 test methods, 505, 506
1214
Index I
Pastes (see also Organosols, Plastigels, Plastisols, and Rigisols) car body undersealing, for, 229, 1121 casting of, 991-2 coating of sheet materials with, 998-1010 common product faults, 1002-4 techniques, 998-1002, 1004-7 commercial, 60, 62, 69 components and formulation of, 962-75 compression moulding of, 1011 definition, 939-41 dip coating with, 992-3, 994-6 dip moulding of, 993-4 extrusion of, 1011-12 foam production from, 115,967 formulations, 1095-8 methods and effects, 1069-77, 1080-4, 1091 foamable coating layers, 115, 1006 gelation/fusion of, see under Gelation/fusion low-pressure injection moulding of, 1010-11 preparation, processing and applications, 981-1012 production,603-9,981-2 products, 114-15,284,987-8,990, 991,993,994-5,999,1006, 1007, 1010, 1011 properties and formulation, 939-79 rheological properties of, 228-9, 237,945-51 room-temperature blowing of, 1091 rotational casting of, 986-8 slush moulding of, 988-90 spray coating with, 996-7 standard specifications for preparation, 51, 981, 1141 viscosity determination, 51, 9601,1141 terminology, 939-40 viscosity of, see under Viscosity Pellets, see under Compounding and Compounds Permachor, 462-3
General
Permeability, 452-66 biaxial orientation, effect of, 19, 27, 761, 774, 799 blow mouldings, of, 774, 798,799 coefficient definitions, 453, 455, 461 units, 459 definition, 452, 453, 455, 460 determination containers, on, 462, 798 plastics, on, 462 standard methods, 456-8 porous materials, of, 460-2 PVC, data for, 451, 463-6 units, 453-4, 456-9, 460 Phosphate plasticisers, 94, 96-7, 15963 flammability, effect on, 160-3, 204-6,430-1,432,503 halogenated alkyl phosphates, 163, 431 mixed alkyl aryl phosphates 161 PVC compounds, performance in, 160, 162 thermal degradation, effect on, 260 trialkyl phosphates, 160 triaryl phosphates, 159, 160 Phosphite stabilisers, see under Costabilisers Photochemical degradation, 260-1 impurities, effect of, 261, 482 mechanism, 261 protective additives, see UV stabilisers, Antioxodants, Carbon black, and Titanium dioxide relevant tests, 328-30, 476-80 weathering, in, 472, 474-5 Phthalate plasticisers, 93-7,152-9 flammability, effect on, 161,205, 508 general-purpose, 153 higher, 156-8 linear, 153, 156 lower, 152-3 miscellaneous, 158-9 modified, 158-9 possible health hazard, 210 PVC compounds, performance in, 154-5
Index I
General
1215
Pigments (see also Colourants) Plasticisers-contd. chemical nature of, 119-20, 136-9 latices, in, 1036, 1038 behaviour in PVC, and, 136-9 solutions, in, 1056-7, 1059 citrate, 174 weathering, effect on (see also classification of, 147-9 Carbon black and Titanium commercial, 147-80 dioxide), 482, 1056 compatibility Pipes, see under uPVC expressions, 125-32 Plasticisation external materials, with, 175 external,5-6,117-18 high humidity, at, 204 internal, 5, 20, 118, 1016 mechanisms of, 120, 121, 122, 124 tests, 123, 127, 131-2 TXP isomers, of, 128 theories of, 120-2 cross-linkable, 174, 968-9, 1073 vinyl latices, of, 1016-17, 1018, definition, 117 1031-5 desirability function, 183 Plasticised PVC (pPVC) diffusion in PVC, 135-7 ageing of, 138-9 effectivity (efficiency), 132-4, 185, chemical resistance of, 487, 488-9 189, 192 definition, 2-4, 590 (calendered effects in PVC compositions, on sheet) (see also under individual filler loading, effects of, 233, 238-9 flammability of, 161, 204-6, 427, properties) 430-1,432,433,503,508 BS softness No., 187-9,450, products, 986-98,1072-4,1106, 1189 electrical properties, 156,206, 1108, 1109-10, 1111, 1112, 1114-15, 1118, 1120, 1121, 207,1193-4 1122-3, 1124 elongation at break, 191,850, properties of (see also under indi1187 flammability, 204-6, 508 vidual property names), 65, 67,69,72-7,118,154-5, impact strength, 388, 441-2, 850, 157,158,160, 161, 162,164, 1188 166,173,187-212,441-2, low-temperature performance, 447-9,464,465,469,472-3, 94,192-5,205,439-42 483-4,487,488-9 modulus, 193, 1190 stain resistance of, 209-10,1010 softness, 184-5, 187-9, 1189 weathering of, 261, 472-3, 482 tensile properties, 190-1,450, Plasticiser absorption 1186-7 fillers, by, see under Plasticiser efficiency factor, 134, 189 demand of fillers extraction, 135, 196-8 PVC polymer particles, by, 47, 48, exudation, tests for, 131 123, 125, 127 flexible seals, in contact with, 175 Plasticiser demand of fillers (see also foams, effects in, 1081, 1083-4 under Fillers (oil absorpfood, contact considerations, 106, tion)), 226, 230-1, 970-2 210-11 Plasticisers (see also under individual formulating with, 93-7,105,149, or type names) 181-4 bulk properties of, 139, 160, 164, fusing, rapid, see solvating (below) 166,167,172, 1144-6 glycol ester, 173-4 cellular PVC, effects in, 1081, handling and storage of, 175, 179, 530-2,539-40 1083-:4
1216
Index I
Plasticisers-contd. health and safety aspects, 211-12 history of, 5-6 interaction, parameter (Flory-J{uggins), 128, 130 PVC polymer, with, 122-32 'low temperature' , see aliphatic diester plasticisers migration, 135, 199-200 tests for, 200 miscellaneous, 170--5 monoester, 173 monomeric, definition, 119 names and abbreviations, 148-51 pastes, in, 153, 159, 265-9,976-7 permanence, 195-204 polymeric, 96-7,165-70 compositions, in, 199 definition, 119-20 nature, 165-70 solid, 169-70, 197,394-401 polymerisable, 174,968-9,1073 primary, definition of, 118 producers of, 177-9 properties in PVC, 118, 140-1, 154-5, 158, 160-2, 164, 166, 169, 173 rapid fusing, see solvating (below) resistance to insect and rodent attack, 209, 486 microbiological attack, 208-9, 483-6 secondary, definition of, 118 selection of, 96-7,124-5 solubility parameters of, 127 solvating, 94, 97,115,158,174, 1074,1081 synonyms, 148-52 types, 119, 149 usage features (general), 96-7 vinyl latices, in, 1031-5 viscosity-temperature relationship, 176 volatilisation, 135, 157,200-2, 212, 446-8 Plastigels, 973-5
General
Plastisols (see also Pastes), 945, 953-5,973,997,1069,10723, 1076-7, 1079, 1084, 1096-
8
basic form of, 951 nature, 939 synonymity with pastes, 939 Plastometers, see under rheometers Plate-out, 91, 251, 305, 307-8,687-8, 780, 835, 1090 blow moulding, in, 780 blowing (foam) compositions, with, 1090 calendering, in, 835 deposits, composition of, 305,688, 1090 extrusion, in, 687-8 test for, 307-8 Polymeric modifiers, see under Impact modifiers and Processing aids Polyurethane lacquers, 1009 modifiers, 168-70, 197,401 Polyvinyl chloride (homopolymer) chemical structure, 13-18 crystallinity, 18-19, 121,694-5 fine structure (morphology), 18-19 glass transition temperature effect of blending with nitrile rubber,398 typical values, 18 molecular orientation, 19 syndiotactic, 18, 121 Porous materials permeability of, 460-1 production methods, 250, 1078, 1079 Powder coating and moulding, 61, 111820 compounds, 514,518,852 as feedstocks in blow moulding, 789-90 calendering, 808 extrusion, 686-7, 851, 885 injection moulding, 737, 852 gelation of, 691-2
Index I
Powder---eontd. compounds---eontd. production of (see also under
Compounding), 372, 547-9, 867 sintering of, 1079 types, 548, 686-7 pPVC, see Plasticised PVC Printing, see Surface decoration of PVC Processing aids, 98, 372-5, 392-3 blow mouldings, in, 795 effects in PVC, 373-4 impact modifiers, action as, 373, 392-3 mode of action, 374 polymers used as, 372-3 Pseudoplasticity, 948--9 Purging extruders, of, 685-6 injection machines, of, 734 materials for, 685-6, 734 PVC applications (see also under relevant
individual headings) 1103-26
ball valves, 1124 bricks, 1124 cellular, see Cellular PVC compositions (see also under Compounding and Compounds) commercial,59-77 melt compounded, 79, 80 production of, see Compounding definitions, 1-4 expanded, see under Cellular PVC fibres and fibre products, 5, 20, 22, 1117 flexible (see also Plasticisation and Plasticised PVC) definition, 2, 3 products (see also under Plasticised PVC (products) and Pastes (products», 1106, 1108, 1109, 1110, 1111 foams, see Cellular PVC and Foams general definition, 1 health hazards, 495-501
General
PVC---eontd.
1217
historical development, 4-7 IR spectrum of, 34-5 laminates with other materials, 745, 909,1111-16 medical applications, 1120--1 motor cars, use in, 1121 plasticised, see Plasticised PVC polymer particles morphology,46,48,51,689-91 size, 46, 47, 51, 690, 943, 962-3 polymers (resins) characterisation tests, 50--2 characteristics, 41-9, 86-9 commercial producers and trade names, 55-7 properties, 54, 854 high purity, 89, 853 history, 4-5 kinds, 38-41,53,86-8,1047-8, 1050--1 production processes, 37-41, 47 properties and tests, 47, 50-4, 852, 1135-68 quality, 89 standard designation, 49, 53 'red-mud' filled, 1123 reprocessing, 9-10, 310, 599-603 rigid, see Rigid PVC sheet, see Sheet solutions, 1047-65 applications of, 1050--1, 1060--3 cocooning, for, 1061 film casting, for, 1063 miscellaneous components of, 1054-7 pigments, in, 1056, 1057, 1059, 1060 polymers used in adhesion to substrates, 1049, 1059, 1061-5 nature, 1048--9, 1050--1, 1052, 1053 preparation of, 1057-60 ready-made, 1058--9 solvent-cements, as, 924-7, 1063 solvents used in, 1049-54
1218
Index I
General
PVC-contd. solutions-contd.
Roll test, 131, 135 Rotational casting, 986-8
Re-cycling compounding, and, 599-603 general problems, 9, 10 sulphide staining in, 310 'Red mud' (as reinforcing filler), 1123 Relative melting temperature, 123 Rheometers balance (Contraves), 954, 955 capillary, 327-8, 369, 580,581 oscillatory (parallel plate), 954-5 plastometers Macklow-Smith, 65 piston (ASTM), 369, 727 review (passim), 790 torque evaluation of lubricants, use for, 362,368-9 extrusion characteristics, determination in, 369 fusion time and rate, determination in, 362, 368, 790-1, 954 Plasti-Corder, see under Brabender in Index 3 Plastograph, see under Brabender in Index 3 RAPRA, see Index 3 stability tests, use for, 316, 319, 326-8 Rheometry, reference to review of, 790 Rheopexy, 949-50 Rigid PVC (see also uPVC) chemical.resistance of, 487,488-9, 491-4 definition, 2, 3, 849 feedstocks, 851-2 typical property value ranges, 1185-97 Rigisols, 940, 976-7 Rodent attack, resistance of PVC to, 209,486
Screeners, see UV stabilisers Selective membranes, 1123 Semi-rigid PVC definition, 2, 590 (sheet), 849 properties, 849-50 Sheet applications of, 847, 893-Q, 903-9, 922,1107 calendered, see Calendered sheet conversion and manipulation of (for packaging), 932 definition, 803-4, 890, 901-2 embossing, 821-4, 838-9,935 extruded, see Extruded sheet fabrication of, 910-36 'planishing' (press finishing) of, 837 press-laminated, 837-8, 891, 902-3 property data, 819, 841, 842-3, 846, 1185-95 tank lining with, 917-18 thermoforming, see Thermoforming of sheet thickness measurement and control, 82~, 892-3 values and ranges, 804, 890-1, 901-3 types 904, 906-907 Shore (Durometer) hardness BS softness, and, 184, 186, 188 determination of, 184, 1189 typical value ranges for PVC, 1189 Siding, see Cladding Simulated skin, 1122-3 'Single-pack' additive systems, see 'One-pack' additive systems Single-screw extruders, 673, 674, 677, 679-80,698-9,704-5,709 Slush moulding, 988-90 Smoke density, 427, 434 evolution enhancement by some additives, 431
surface coatings, as, 1059-63 viscosity of, 1054 usage statistics, 8, 10-11 Pyrolysis, 424, 426, 427, 428
Index I
Smoke-contd. evolution-contd. plastics, by, 427, 502 tests for, 434, 505-7, 509 nature, 427 PVC, from burning, 499, 502, 509 suppressants, 83, 242, 247-8, 42934 Softness (see also BS softness) measurement, 184-5 plasticisers, effects of, 185,187-9, 450,1189 Solid-gel transition temperature, 123, 127,131 Solubility parameter, 125-7 plasticisers, of, 126, 127 PVC polymer, of, 127 Solution polymerisation, 39, 42, 1048 Solution polymers, 1048-9, 1050-1 Solutions, see PVC solutions Solvation point, 123 Solvent cements, see under Bonding of PVC, Chlorinated polyvinyl chloride, and PVC (solutions) Stabilisation (see also Heat stabilisers, UV stabilisers and Antioxidants) blow moulding compositions, of, 793 (see also Non-toxic stabilisers) calendering compositions, of, 831 effects, evaluation of, 312-30 external,264 'in kettle', 264 internal, 264 mechanisms, 264, 291, 299-305 Stabilisers, see under functional types (e.g. Heat stabilisers, UV stabilisers, Antioxidants) Stability heat, to adverse effects of formulation components, 260 concept, 311-15 induction time, and, 312 polymer nature, effect of, 259-60 stability time, and, 259, 312-15
General
1219
Stability-contd. heat, to-contd. testing, 315-28 light, to testing, 328-30 Stain resistance of pPVC, 209-10, 1010 Standards institutions, 29, 1127-31 test specifications, 29-30, 1134-68, 1185-95 Static electricity charges, 420-2 electrical resistivity, and, 420-2 measurement, 421-2 Stress concentrating features, 377, 381, 386,695 cracking, see Environmental stress cracking rupture strength, 862 whitening filled compositions, in, 102, 106, 230 impact-modified compositions, in, 392, 794 mechanism, 392 Strontium stabilisers, 249, 281 Sulphide staining cross-staining, 91, 105,272,278, 309,310 occurrence, 91, 106,276,308-11 tests for, 310-11 Surface decoration of PVC, 838-9, 932-5, 1005, 1008-9, 1092-5 Surface marking of PVC, 935-6 Surface processing of PVC, 936 Surface treatment of fillers, see under Fillers and Calcium carbonate fillers PVC cellular coatings, 1092-5 paste coatings, 1005, 1008-9 sheeting, 838-9, 932-6 Suspension polymer particle characteristics, 46, 47,690-1 polymerisation, 39-40, 41 Syndiotactic PVC polymer, 18, 121
1220
Index 1 General
Synergism co-stabilisation, in, 91, 104,276, 283,340 lubricant/stabiliser, 104,274,364-5 calcium stearate/antimony mercaptide, 276, 364 glycerol esters/thiotins, 364
Tg
PVC polymer and compounds, of, 18,361,398,460 role in film formation from latex, 1016, 1017 mode of fracture, 388 Talc, 217, 223 Tank lining with PVC sheet, 917-18 Temperature gradient bar, see Gel block Temperature index (UL), 445 Tensile strength paste products, of, 959 plasticisers, effects of, see under Plasticisers and Effects in PVC temperature, effect of, 388, 450 typical values for PVC, 1186 Termite attack, 209, 486 Thermal decomposition products plastics, of (general), 426-7,428 PVC, of, 256-8, 259, 499-500 Thermal degradation of PVC, 256-60 activation energy for, 259 and specific energy for melt generation, 311-12 different PVC polymers, 259-60 mechanisms, 256-9 nature and measurement, 313, 326, 327 time/temperature effects in melt processing, 578, 725 Thermal diffusivity, 781-2 Thermoforming of sheet, 743-62 advantages, 744 basic methods, 743 combination processes for, 743 CPVC properties relevant to, 745, 761
Thermoforming of sheet--eontd. creep tests relevant to, 757, 75960 equipment for integrated lines, 757 suppliers, 757, 758 materials used assessment of, 757, 759--60 nature of, 745 products distortion of, 756-7 finishing of, 754-~ quenching stresses in, 744 related processes, and, 746 techniques matched-mould and related, 755-
6
pressure, 756 vacuum, 745-54 temperature draw ratio, and--effects on product quality, 759--60 role in process, 751-2, 756-7, 760 Thickening agents latices, in, 1021-8 pastes, in, 251, 973--5 Thixotropy, 949-50, 973, 976 Time/temperature effects in melt processing, see under Thermal degradation Tin stabilisers, see Organotin stabilisers Titanium dioxide clay, in, 218 light stabiliser, as, 108,297,419, 482,1056 types, 418 white pigment, as, 102,418--19, 1062 Toners, 411 Torque rheometer, see under Rheometers Toughness, 376, 399, 440 Transfers (surface decoration), 933 Trialkyl phosphate plasticisers, 149, 160, 161, 162 Triaryl phosphate plasticisers, 149, 159-60, 161, 162
Index I
Trimellitate plasticisers, 96-7, 149, 163, 164 PVC compounds, performance in, 164 Tubing (flexible), 674, 719, 1106, 1120 Underlays (rugs and mats, for), 1124 uPVC chemical resistance of, 487, 488-93, 494 definition, 2, 3 fillers in, 235, 236, 239-40,241, 242-6,250 film (see also Extruded film), 890-2,894,895,1109 injection moulded articles, see Injection mouldings pipes applications, 867-9, 882, 883, 1104-6, 1121-2 formulation aspects, 853, 876-8 production of, 869-78 properties and tests, 378-80, 697, 878-9,880-1 special forms of, 879,882-3 types, 867-9 water flow, through, 869 profiles applications,883-4,1106-7 cellular, 884, 885 formulation aspects, 886-9 production of, 884-6 specifications and tests, 889 types, 883 window frame, 20, 482, 883, 884, 885,886-9,1106-7 properties (see also under indi-
vidual property names), 65,
66,68,71,463-4,466, 469,489-94,849,850,85566,878-9,899,893-5118595 sheet, see under Calendering, Calendered sheet, Extruded sheet, Extrusion, and Sheet UV stabilisers, 255, 292-9
1221
General
Vacuum forming, 745-54 Venting of extruders, 612, 621, 623, 627,629,635,636,638,677, 683-4 Vinyl chloride copolymers, see under Copolymers homopolymers, see under Polyvinyl chloride Vinyl chloride monomer (VCM) detection and determination, 496 explosive limits, 38, 500 hazards of, 9, 38, 496-7, 500 polymerisation of, 38-41 production, 7, 37-8 properties, 38 regulatory measures, 9, 38,497 'stripping' from PVC polymer, 39, 41,497 Viscometers (see also Rheometers), 954,960-1 Bendix Ultra Viscoson, 954 Brookfield, 954, 960 Castor-Severs, 961 cone-and-plate (Haake-Rotovisco), 961 Contraves balance, 954, 955 Ford cup, 961 Gardner, 961 Viscosity depressants for pastes, 975-7 expressions and terms, 14, 44-5, 52
(passim),960-1
melts, of, 369, 581, 790 number, see under K value pastes, of, 941-51, 953-8 ageing index, see stability index (below) extender polymers, effect of, 943,963,964 fillers, effect of, 970-3 measurement of, 51, 953-5,9601,1141 plasticisers, effect of, 957, 965-8 thickening agents, effect of, 9735 solutions, of, 14-15,44-5,50, 1050-1, 1054 specific (see also under K value)
1222
Index I
Viscosity-eontd. stability index (pastes), 966, 973 standards and definitions, 14,44-5, 50,52,1136-7 Volatile loss, 200-2, 447-8
Wall coverings (interior), 998, 1005, 1042-3, 1113-14 Weathering (see also under Photochemical degradation), 206-8, 469-82 definition, 469 effects, 206-8,261,471-3,476,480 factors instrumental in, 470, 472-3, 474,475-6, 481-2 plasticisers, effects of, 207-8, 482 resistance assessment, 471, 474-7, 480 formulating for, 107, 480-2, 886-8 processing, effect of, 482 solvent residues, effect of, 482 tests methods, 470-1, 474-5, 477, 480, 1134 radiation sources in, 476-9 WeldIng of PVC (see also Bonding) definition, 910, 923 flooring, 914,916,918-19
General
Welding of PVC-eontd. recommended temperature ranges, 910 specifications relevant to, 913, 918 techniques dielectric welding, 912 extrusion welding, 911, 919 friction welding, 912 heated tool welding, 911, 922-3 high frequency welding, 912, 919-22 hot gas welding, 911, 914-19 indirect heated element welding, 911 radio-frequency (RF) welding, 912 spin welding, 912 thermal impulse welding, 912 ultrasonic welding, 913 vibration welding, 912 White lead, 266-7 Whiting, see Calcium carbonate fillers (types) Zinc-containing stabilisers, 91, 93, 106, 109-10, 112, 113, 115, 283-4,286,302-3,309,325, 338,345,347-53,355,356, 364,793,831,969,1086, 1087,1096
INDEX 2
Material and Product Trade Names
Beetle PVC resins, 23, 57 Bennite, 252 Bentone, 974 Benvic PVC resins, 905 Benzoftex plasticisers, 174, 179 Bevaloid defoaming agents, 1028 bioMET, 208, 209 Bisoftex plasticisers, 35, 177, 966 Bisol DPS, 1043 Bisomer DALP, 969 Blendex modifiers, 203, 346, 355, 394,395 Blue B, 404 Bondene,909,1115 Borden PVC resins, 56 Bostik adhesives, 918,928-9 BQ stabilisers, 343, 344,346 Breon latices, 1013, 1014, 1017-19, 10218, 1030-2, 1034, 1035, 1038-45 PVC compounds, 399, 400, 857, 859,861 PVC resins, 57, 203, 424, 465, 796, 832,854,855,897,926,964,969, 971,972,973,996,997,1008, 1052, 1054, 1064, 1098 Britomya, 222, 227,234-5,236,252, Baco FRF, 252 Bakelite PVC resins (see also Ucar) , 346 56,966,1051,1055 Brux alloy, 708 Ballotini,223 Burgess clays, 252 Baropan stabilisers, 369, 371 BZ 329 stabiliser, 285 1223
Acryloid modifiers equivalence to Paraloid, 375,396 formulations, in, 108, 395, 887 Adimoll plasticisers, 178 Admex plasticisers, 179 Aerosil, 252, 973 Airco PVC resins, 22, 56 Ako-Stop, 1124 Alaiftex plasticisers, 177 Alfol alcohols, 151, 156, 164 Alphanol 79 alcohols, 150, 156, 164 Amaplast Yellow RRT, 404 Amoco Resin 18, 373 Antifoam A, 1028 Antistat A, 424, 425 Araclor plasticiser extenders, 179 Arbeftex plasticisers, 177 Argutop lacquers, 1008, 1009 Armodur, 905 Artevyl PVC resins, 56 Astraglass, 905 Atmer 152, 1082 Azocel blowing agents, 1088
1224
Index 2
Material and Product Trade Names
Cab-o-Sil, 973 Cadmium colourants, 414, 415 Calgon, 1029, 1043 Calibrite, 233, 234, 237, 252 Calmote, 252 Calofil, 252, 972 Calofort, 252, 972 Calopake,252 Carina PVC resins, 56 Carstab 700, 295 Celanese (Courtaulds) plasticisers, 163,177 Celcon, 735 Cellosolve, 1052, 1053, 1062 Celogen blowing agents, 1088, 1090 Cereclor plasticiser extenders, 127, 128, 130, 171, 172, 173, 177, 178, 432 Ceroxin lubricants, 371 Chemigum N8, 203 ChemPurge, 685 Chlorafin plasticiser extenders, 179 Chlorez plasticiser extenders, 178 Chlorowax plasticiser extenders, 178 Chrome Green DC 3593, 409 Cinquasia Violet RT 795 D, 407 Citroflex plasticisers, 174, 179 Clarechem CLA-1500, 430 Clorafin, 179 Cobex 1350, 905 Conductex 975, 424 Conoco PVC compounds, 63, 1122 PVC resins, 56 Contrastat, 178 Corvic PVC resins, 34, 35, 52, 54, 56, 410,796,819,854,897,966,974, 987,988,990,997 Crestapol plasticisers, 129, 177 Cromophtal colourants, 408, 413 Cyanolit 811, 928 Cyasorb UV stabilisers, 294, 1055 Dacovin PVC resins, 57 Darvic,451,904,906-7,909 Dawsonite, 242, 243, 244, 248, 433 Day/Cal, 234
Day-Glo pigments, 403 Deckor Cellset, 1082, 1098 lacquers, 1008 primer, 992 Dekadur,28 Delrin, 735 Denkavinyl PVC resins, 56 Diamond 450, 832 Dieldrin, 209 Diepox plasticisers, 178 Diolpate plasticisers, 129, 168, 177, 196 Diplast plasticisers, 178 Disflamoll plasticisers, 178 Dispex 115, 1043 Dobane plasticisers, 177 Drapex plasticisers, 177 Duraform, 217, 903, 909 Durastrength 200, 893 Durelast 100, 170, 197,401 Durostabe stabilisers, 280, 348, 349, 351 Dutrex plasticiser extenders, 174, 177 Dynel fibre, 20, 22 Dythal, 1089 Eastman 910, 928 Edenol plasticisers, 171, 178 Ekalit M, 401 Ekavyl PVC resins, 56, 57 Electrofine plasticiser extenders, 174, 177 Elvaloy, 170, 395, 452, 887 Emerwax lubricants, 371 Emery plasticisers, 178 Emiblend, 424 Enerfiex plasticiser extenders, 174, 177 Epivyl PVC resins, 56 Epoxol plasticisers, 179 Estabex ABF, 208, 484 Estabex plasticisers, 178 Estaflex plasticisers, 178 Estol 294, 796 Ethoquad C12, 423 Ethyl PVC compounds, 63, 726, 732, 795,857
Index 2 Material and Product Trade Names
Etinox PVC resins, 57 Exon PVC resins, 57 Ex-Static, 423 Extrudex pipe, 869 Ferralon wall coverings, 1114 Ferro-Check stabilisers, 284 Fibravylfibre, 1117 Ficel blowing agents, 1088 Flexol plasticisers, 179, 966 Flovic, 904, 907 Fordcal, 252 FPC PVC resins, 57 Franklin Fiber, 221 Fromopack sheet, 905 Fromoplas sheet, 1108 FW Rubine Toner BOS, 412 Fybex,242,243 Fyroflex flame and smoke retardant plasticisers, 431 Gafstat AE 610, 423 Galon wall coverings, 1114 Gama-Sperse, 227 Garbeflex plasticisers, 177 Garbefos plasticisers, 177 Garotalc, 223 GasH, 252, 307, 973 Gedeflex plasticisers, 177 Genitron blowing agents, 1083, 1088, 1089, 1090, 1096 Geon compounds, 28,53, 726,1122 CPVC resins, 25, 28 PVC latices, 1013 PVC resins, 56 flakuenka, 234, 252 flalowax 4004, 966 flalvic PVC resins, 57 fleatshrink fittings, 1125 fleliflex tubing, 1106 flercoflex plasticisers, 179 flercolyn plasticisers, 179 fleron duckboard, 1123
1225
flexaplas plasticisers, 158, 177, 178, 201,966,968 fligel,973 flishi-metal, 1115 flostaform, 735 flostalit PVC compounds, 63 flostalit PVC resins, 20, 56, 395 flostalit Z, 887 flostavinyl colour concentrates, 406 flydral, 252 flydrocarb, 222, 235 flytrel3495, 391 Igelit PC, 25 Inca-Brick, 1124 Instaweld adhesives, 929 Interstab lubricants, 371 stabilisers, 271 Ionac PE 100, 423 Ionox antioxidant, 293 Irgalite colourants, 412, 414 Irganox antioxidants, 293, 294, 347 Irgasan DP 300, 208,485 Irgastab stabilisers, 108, 109, 113, 114,115,203,271,273,277,286, 287,292,308,347,348,351,374, 796,988, 1089 Irgastat antistatic agents, 424 Irgawax lubricants, 108, 110, 113, 203,308,355,371,794,796 Irgazin colourants, 413 Iriodin pigments, 403 Irvinil PVC resins, 57 Jayflex plasticisers, 177, 178 Kane Ace B 12, 796 Kaneka PVC resins, 56 Kematal, 735 Kem-Gard 911A, 432 Kempore blowing agents, 1088 Kenplast plasticiser extenders, 174 Ken-React, 227 Kesscoflex plasticisers, 178 Kevlar,1111
1226
Index 2 Material and Product Trade Names
Kodaflex plasticisers, 174, 178, 210 Kohinor PVC resins, 57 Kombipur, 401 Kosmos 70, 408 Kronitex plasticisers, 159, 178 Kronos Ti0 2 , 308 Kydex,745,905,909 Lacqvyl PVC resins, 56 Landex modifiers, 401 Lankroflex plasticisers, 171, 177, 351, 355,1031 Lankromark stabilisers, 286, 348, 352-4,355,371 Lankromet stabilisers, 286, 353 Lankroplast lubricants, 355, 371 Lankrostat antistatic agents, 115,422, 423 Laqua WB 240, 1009 Levapren modifiers, 395, 887 LF, LH, LL, stabilisers 345 Linevol alcohols, 156, 167 plasticisers, 110, 150, 164, 177,205 Lipinol plasticiser extenders, 174, 178 Loctite IS 414, 928 Lonzavyl PVC resins, 57 Loxiollubricants, 110, 354, 371 Lubraplas lubricants, 371 Lubriollubricants, 371 Lubrol, 422, 975, 977, 987, 988 Lucalor CPVC compounds, 28, 63 Lucel blowing agents, 1091 Lucovyl PVC resins, 56 Lutofan PVC resins, 56, 1013 Lytron 820, 1021 Manomet stabilisers, 351 Mark stabilisers, 277, 282, 285, 877, 1089 Marvinol PVC resins, 57 Mearlin Luster pigments, 403 Mellite stabilisers, 351, 796, 997, 1031 Mesamoll plasticisers, 130, 173, 178 Metaglide lubricants, 371 Metasap lubricants, 371 Metawax lubricants, 371
Microcarb,252 Microdol, 252 Microlith colour concentrates, 406 Microspin colour concentrates, 406 Mikro-Chek 12, 484 Millicarb, 234, 235, 236,252 Mirvyl PVC resins, 57 Mobisol plasticiser extenders, 174 Modifier PIM 101, 397 Mollan plasticisers, 177 Moly flame and smoke retardants, 430 Monastral Fast Blue BGS, 414 Monolite Fast Black LS, 414 Monolube lubricants, 371 Monoplas 230, 1098 Monoplex plasticisers, 171, 179 Morden R, 236 Morflex plasticisers, 129, 179 Movil fibre, 1117 Mural Mousse Somvyl Decor wall coverings, 1113-14 Neo-Rez R900, 1009 Neosyl, 973, 974 Neralit PVC resins, 56 Nipolit PVC resins, 56 Nitropore blowing agents, 1090 Nitrosan, 1091 Nopco Foamaster Defoaming agents, 1028 Nopcowax lubricants, 371 Nor-Core 1124 Nordxyl, 250 Norvinyl PVC resins, 56, 1098 Nuoplaz plasticisers, 175, 179,210 Nuostabe stabilisers, 286, 348, 350, 351 Nycoflex plasticisers, 177 Omya, 100,227,234,237,238,252 Omyalite, 227, 235, 236, 252, 345, 346,354,355 Oncor flame retardants, 430 Ongard flame retardants, 430, 432, 434 Oriex, 905
Index 2 Material and Product Trade Names
Pacton SQ, 909 Palamoll plasticisers, 129, 177 Palatinol plasticisers, 177, 178, 203 Pantaprene L, 23 Paraloid modifiers equivalence to Acryloid, 375, 396 formulations, in, 345, 346, 354,355 Paraplex plasticisers, 133, 171,179, 796,966,988,997 Pe-ee fibre, 5, 1117 Pekevic PVC resins, 57 Permabond adhesives, 929 Peroxidol plasticisers, 179 Pertinax, 921 Pevikon PVC resins, 57, 850, 953, 954 Pfinyl 402, 236 Phosclere stabilisers, 292 Phosftex plasticisers, 179 Phosgard LSV, 433 Plaskon PVC resins, 57 Plasthall DIDG, 165 Plastifiant K, 177 Plastolein plasticisers, 129, 168, 178 Plastomoll plasticisers, 177, 178 Pliabrac plasticisers, 159, 177,424, 987,990 Pliovic PVC resins, 23, 57 PMF (slag fibre), 242, 243 Polyblend 500 (Breon), 399,400 Polycarb, 226, 252 Poly-Check stabilisers, 282 Polycizer plasticisers, 179 Polydon PVC compounds, 63 Polyvin, 686 Poly-Zole blowing agents, 1091 Porofor blowing agents, 1088, 1091 Protect-A-Mat, 1124 PVC-Glas, 909 PV colourants, 412-13, 415 PX plasticisers, 179 Q-ce1l300, 223 Querton antistatic agents, 423 Quirvyl PVC resins, 57 Ramboo tubing, 1121 RapidPurge,685
1227
Ravinil PVC resins, 57 Ravolen plasticiser extenders, 174 RedHHR,404 Remafin colour concentrates, 406 Reofos plasticisers, 129, 159,160, 162,176,177,187,188,190,191, 194,203, 205,210 ReoP1ol plasticisers, 110, 129, 163, 165,176,177,203,410,977,988 Reoplast plasticisers, 171, 176, 177, 308,347,348,351 Reoplex plasticisers, 110, 129, 168, 176,177,199,211 Reproxal plasticisers, 178 Resin QX-2611, 423 Retractyl fibre, 1117 Reynosal PVC resins, 57 Rhenoftex, 25 Rhodamine B, 404 Rhodiastab 50, 283 Rhovyl filament, 1117 Ribstruct pipe, 883 Ricatyl plasticisers, 178 Ricon PVC resins, 57 Rigidsol, 940 Royalite, 745 Rubine Toner 2BRS, 406 Ruco PVC resins, 57 Rucodur PVC compounds, 396 Salol,295 Sanduvor VSU, 295 Santicizer plasticisers, 129, 161,162, 177,179,210,433 Santicizer 711, 156, 832 Santocel,974 Santoset plasticiser, 969 Sapchim lubricants, 374 Saran fibre, 22 Sartomer plasticisers, 969 SB 739 stabiliser, 286 Scadoplast, 178 Scandinol plasticisers, 177 SCC PVC resins, 56 Scon PVC resins, 57 Shinetsu PVC resins, 56 Shintech PVC resins, 56
1228
Index 2 Material and Product Trade Names
Sico Red WRC, 308 Sicol plasticisers, 178 Sicotan colourants, 408 Sicron PVC compounds, 63 Sicron PVC resins, 56 Simona-PVC-EL, 909 Sintaclad,905 Sintilon Stratum sheet, 1108 Sioplas, 588 Siscoversal colour concentrates, 410 Snow white, 221 Snowcal,252 Solemite, 227 Solvic PVC resins, 56, 203,308 Span 20, 1031 SPC PVC resins, 56 Spreaflex plasticisers, 178 SQ stabilisers, 343, 344, 346 Staflex plasticisers, 178, 179 Stanclere stabilisers, 271, 796, 997 Stenollubricants, 371 Sterishield, 1120 Stone Decor panels, 1113 Storeytrim, 909 Strandex, 343, 345 Sturcal, 259, 972 Sulfocel blowing agent, 1090 Superlon, 252 Super-Pflex 200, 227, 235 Superpolyroc pipe, 887 Supra colourants, 415 Suwide Placo wall coverings, 1114 Swada pigments, 403 Sylodex, 242, 973 Syloid,252 Synpron stabilisers, 277, 285 Syntewax lubricants, 371 Tangit solvent cement, 927 Telcovin TL, 905 Tenneco PVC resins, 56 Tensol solvent cement, 927 Texanol plasticisers, 151, 174,175, 178 Therm Check stabilisers, 277 Thermolite stabilisers, 271 Thermovyl fibre, 1117
Timonox, 253 Tinuvin light stabilisers, 108, 109, 295,347,351 Tioxide Ti0 2 , 346, 354, 355, 1043 Trihyde, 222, 252 Trocal Colour profile, 884 Trovidur HT, 25 Trovipor PVC resins, 57 TV-2 biostat, 484 Tween 20, 1031
Ucar PVC resins, 1050-1, 1054, 1058-61, 1062, 1064-5 Ultramoll plasticisers, 129, 170, 178 Ultramoll PU, 170,401 Unem plasticisers, 178 Uniflex plasticisers, 179 Unimoll plasticisers, 178 Uraplast plasticisers, 168, 178 UV-Chek AM 541A, 294 Uvinullight stabilisers, 294, 295
Vacupiast, 482 Varian PVC resins, 56 Vestinol plasticisers, 178 Vestolit PVC resins, 23, 56,887 Vinamold hot-melt compounds, 63 Vinaprint GV inks, 933 Vinatex Adhesive MP 7A, 992 Vinco stabilisers, 356 Vinidur PVC resins, 56 Vinnol PVC resins, 23, 56, 203, 877 Vinoflex PVC resins, 56 Vinophane, 895,909 Vinychlon PVC resins, 56 Vinylcel, 1077 Vinylite PVC resins, 1051, 1053 Vinyon HH, 22 Vipla, Viplavil, PVC resins, 56 Viton, 175 Vixir PVC resins, 57 VS-103 (foam cell stabiliser), 1075 Vulcabond VP, 1007 Vulcafor colourants, 412, 414 Vybak,905
Index 2 Material and Product Trade Names
Vyflex powder-coating compounds, 63,1118 Vygen PVC resins, 57 Vynamon colourants, 412-15
Winnofil, 252 Witaclor plasticisers, 178 Witamol plasticisers, 178 Wolflex plasticisers, 177, 200
Wacker plasticisers, 178 Wavihol pipe, 882 WaxE,355 Welvic PVC compounds, 63,70-7, 856,1125
XO 2243 (CPE), 832 XQ stabilisers, 343-6 Z-core alloy, 678
1229
INDEX 3
Named Equipment and Processes
ACS 500 colour analyser, 408 Automa Speed blow-moulding equipment, 781, 787 Banbury internal mixer, 811 Battenfeld extrusion and blowmoulding equipment, 704, 787, 788 Bekum blow-moulding machines, 771, 775, 786, 788 Bell blow-moulding equipment, 786 Bendix Ultra Viscoson viscometer, 954 Berstorff calendering and extrusion equipment, 704, 804, 813 Beta gauge, 825 Betol Model 116 extrusion line, 1120 Bitruder, 610, 637-9, 706, 707,708 BKMI calendering equipment, 804 BLT 1000 leak finder, 798 Brabender Plasti-Corder, 326, 327, 328, 368, 791,953 Plastograph, 132, 368, 953, 954 Brookfield viscometer, 954, 960-1, 1017 BuW controller, 872 Biihler-Miag pellet cooler, 663-4 Buss-Kneader, 581, 584, 594, 610, 614-23,624-5,626,627,647,649 650,810,811,813,1117
Buss mixing turbine, 549, 605,606-7, 657-9 Butler chimney apparatus, 505
Calandrette system, 891 Cascade extruder, see under Extruders (Index 1) Castor-Severs viscometer, 960, 961 Cincinnati Milacron extrusion and blow-moulding equipment, 704, 785,871 Clash and Berg method, 193, 194,442 Coexcel process, 884 7842 Color Analyzer II, 408 Contraves rheometer, 954, 955 Corex D light filter, 478,479 Corpoplast process, 776-8 Cuvar extrusion die, 873 Davis-Standard extrusion equipment, 699, 704 DCEIMSG compounder, 867 'Dead Load' hardness tester, 184 Dennis process, 1070 De Vilbiss JGA spray gun, 996 DI-NA-CAL process, 933 Dow cell, 462 DRI-CAL process, 933 1231
1232
Index 3 Named Equipment and Processes
Durometer (Shore) hardness tester, 184, 186, 1189 Dynatup impact tester, 380
Elastomer process, 1070 EMMA apparatus, 474 EMMAQUA apparatus, 474 EMS Industrie extrusion equipment, 704 Fade-Ometer, 478 Farrel group calendering equipment, 804 Fay Foamer process, 1070 FCM compounder, 610, 629, 633-5 Fischer blow-moulding equipment, 787, 788, 789 Ford cup, 961 Francis Shaw extrusion equipment, 705,892 Gardner colorimeter XL 805, 408 impact tester, 380, 381 mixer, 6 viscometer, 961, 966 Haake-Rotovisco viscometer, 961 Henschel mixers, 549 HPM Corporation extrusion equipment 699, 704 Hunkar control systems, 771 Hunkar ILC process, 783
ICI method for foam cell content
determination, 1100 Intec 500 inspection system, 893 'Interval blowing' cooling system for blow mouldings, 783 Jungfer (sintering) process, 1079
Kaufman extrusion equipment, 705 Kautex blow-moulding machines, 764 Kombiplast, 581, 584, 610, 623, 626-7,629,631,632-3 Kraus Maffei extrusion equipment, 682 Krupp--Kautex KEB blow-moulding machines, 769, 781, 787, 788 Lasermark system, 936 Leesona blow-moulding machine, 773 Lesieur test bottle, 799 Luvitherm process, 590 Macklow-Smith plastometer, 65 Maco VI control system, 771 Maillefer extrusion equipment, 719 Maplan extrusion equipment, 682, 705,706-7 Mapre extrustion equipment, 702, 705, 706 Marrick process, 776-7 Match-Mate 3000 colour analyser, 408 Maximat blow-moulding machine, 787 Measurex 2001/25 control system, 826 Melville Plastics Engineering Extruder, 708-10 Mesco process, 10 Microscal apparatus, 479 Moog control systems, 771 MPCN compounder, 584-5, 610, 635,636-7 NBS smoke chamber, 434 NC-P thickness control system, 892 Oswag extrusion equipment, 705 OXO process, 152, 156 Planetary extruder, see under Extruders (Index 1)
Index 3 Named Equipment and Processes
Plasti-Corder, see under Brabender Plastifikator, 581, 609, 610, 611, 61213 Plastimac blow-moulding equipment, 786,789 Plastograph, see under Brabender Porvic process, 1078 Profitmaster 5510 control system, 893 QUV apparatus, 476, 479 RAPRA torque rheometer, 369 Reifenhauser extrusion equipment, 705, 707 Rheometries 'Mechanical Spectrometer', 954 Rollex calender line, 813 Rota-Sonic scanner, 872 Rotocure process, 1112 Saum Systems blow-moulding equipment, 773 Schloemann-Siemag blow-moulding machine 773 Schramm-Zebrowski method (in flammability tests), 504 Shell higher olefin process (SHOP), 156 Shelley 'Linear Series' thermoforming line, 757 SIDEL blow-moulding machines, 768, 769, 787, 788 Sigmat 770 control system, 893 Speed blow-moulding machines, see Automa Speed
1233
Statigun, 421 Stork system, 1094 Sussex thickness control system, 893 Taber Abraser, 1196 TCM foam moulding system, 1076 Technoform PPZ process, 875 Therimage process, 933 Thermassive window frame system, 1107 Thermorex gear pumps, 684 Topformer process, 743 TQ-150 mercury lamp, 329 Tri-Delta blow-moulding machines, 789 Trovipor process, 1070, 1071 Turbosphere mixer, 559, 560 Ultra Viscoson viscometer, see Bendix Ultra Viscoson Underwriters tunnel furnace test, 506 Uniloy 300 blow-moulding line, 781 Urban AKS 3313 welding equipment, 923 Vanderbilt process, 1077 Vibrochrom FFR 2 colorimeter, 408 Weather-Ometer, 207, 475, 478 Werner Pfleiderer ZSK extruder, 623, 626, 632, 677 Xenotest apparatus, 207, 330, 478 XP-2 smoke chamber, 505
