PVC PLASTICIZERS

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PVC Handbook Charles E. Wilkes, Charles A. Daniels, James W. Summers ISBN 3-446-22714-8

Vorwort

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18.08.2005

Preface

In this single handbook the editors aim to give a diverse audience of readers a complete account of all aspects of PVC – from monomer manufacture to polymerization; the gamut of such additives as stabilizers, lubricants, plasticizers, impact modifiers, fillers and reinforcing agents; blends and alloys; compounding and processing; characterization; combustion resistance and weatherability; product engineering design; applications; environmental and safety; and finally the PVC industry dynamics. Jim Summers’ Introduction presents a good historical background on PVC and several of the individual chapters give a historical perspective to the technologies therein. The handbook contains both practical formulation information as well as a mechanistic view of why PVC behaves as it does. The authors are from both industry and academia. Not surprisingly, many of the industry authors are from the former BF Goodrich laboratories, where much of the industry’s technology was developed. Overall, however, about ten PVC and chemical- supplier companies are represented by the authors. When I joined the BF Goodrich laboratories from graduate school in the mid-sixties, PVC research was one of the many challenges there. I had the great privilege of many conversations with Dr. Waldo Semon, who was still roaming our halls at that time. One of his quotes that will always stick with me was: “Chuck, you’ll find that PVC is perhaps the most inert chlorine compound in existence.” His words preceded by decades the health and safety concerns with chlorinated materials in general. But they are true today. PVC is a very safe material when used and disposed of properly. I probably wouldn’t have agreed to take on this daunting task if it weren’t for tremendous contribution of my co-editors, Jim Summers and Chuck Daniels. With the addition of their broad network of PVC experts, we were able to organize this sterling group of authors. And, they made many detailed technical enhancements to the diverse chapters. Thanks also to Christine Strohm and coworkers at Carl Hanser Publishers for soliciting outside recommendations regarding the make-up of the handbook and for many enhancements in the chapters’ readability. Many of the fundamental technology discoveries related to PVC were made in the 1930’s through the 1950’s. Since those times, continuous improvement, broadening of applications, and process improvements for cost reduction and safety have been the mainstay of PVC research. The billions of pounds of PVC made today still use the free radical catalysts discovered in the ‘30’s. On the innovation front, some readers will be familiar with the PVC Technology Consortium organized by the Edison Polymer Innovation Corporation and carried out over the period 1998 to 2005. I have been privileged to direct this consortium of twenty one sponsor companies from fourteen countries who funded 12 research projects in six universities. I’d like to point out a few highlights of results from that consortium. Rich Jordan University of Chicago), Bill Brittain (University of Akron) and Tony Rappe (University of Colorado) and coworkers gained great understanding toward the metallocene polymerization of vinyl chloride, but in the end were unsuccessful. Their very excellent work has been published and will hopefully stimulate future success in some laboratory. On the other hand, Virgil Percec (University of Pennsylvania) and coworkers have succeeded in living radical polymerization

XII

Preface

of vinyl chloride. This technology has produced narrow molecular weight distribution homopolymers as well as a range of block copolymers (both high Tg and low Tg). Several publications and patents have resulted from this landmark work. Bill Starnes (College of William & Mary) and coworkers have discovered a family of new non-metal PVC stabilizers. Chapter 4 herein and several publications and patents describe the results. Jim White and coworkers (University of Akron) have published and patented new high performance alloy technology, including new block-copolymer compatibilizers. Joe Kennedy and coworkers (University of Akron) have published and patented some novel modifications of PVC via anionic polymerization. Kyonsuku Min and coworkers (University of Akron) published on the preparation of PVC-polyurethane alloys by their reactive formation in a twin screw extruder. Eric Baer and Anne Hiltner (Case Western Reserve University) published excellent fundamental studies of toughness, creep and fatigue resistance in PVCs. Jerry Lando and Morty Litt (Case Western Reserve University) attempted to modify PVC stereostructure by polymerization additives. And, Miko Cakmak (University of Akron) published on the characterization of chemical and morphological changes in PVC compounds during extrusion processing by online measurements. I personally hope that this body of published and patented knowledge will result in a renaissance for PVC – resulting in many new applications and continued good growth. Charles E. Wilkes Akron, Ohio April 2005

Charles Daniels

Charles Wilkes

James Summers

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PVC Handbook Charles E. Wilkes, Charles A. Daniels, James W. Summers ISBN 3-446-22714-8 Leseprobe 1

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18.08.2005

5

Plasticizers

LEONARD G. KRAUSKOPF, ALLEN GODWIN

5.1

Introduction

In 1951, the International Union of Pure and Applied Chemistry (IUPAC) developed a universally accepted definition for a plasticizer as a substance or material incorporated in a material (usually a plastic or an elastomer) to increase its flexibility, workability, or distensibility. A plasticizer may reduce the melt viscosity, lower the temperature of a second-order transition, or lower the elastic modulus of the product. In 2003, the worldwide market for plasticizers was more than 4.6 million metric tonnes (10 billion pounds), with approximately 90% applied as plasticizers for PVC. In North America, plasticizer consumption was about one million metric tonnes (2.2 billion pounds), with ExxonMobil Chemical, BASF, Sunoco, and Eastman Chemical Company as the major producers. The plasticizer market in Europe is about 1.3 million metric tonnes (2.8 billion pounds), with the three largest producers being ExxonMobil Chemical, Oxeno, and BASF. The region of the world with the largest plasticizer production is the Far East, with approximately 2.2 million metric tonnes (5 billion pounds) produced annually. There are numerous plasticizer producers in that region, the major producers being Nan Ya Plastics, Union Petrochemical Corp., Dahin Co., Aekyung Industrial Co, and LG Chemical. Throughout the period from 1970 to 1995, the worldwide plasticizer markets grew at rates above the various GNPs; however this trend has started to decrease in North America and in Europe. In recent years, the average growth rate in those regions has ranged between 2 and 3%, with projected growth rates of only 1–2%. The Far East is not only the largest market for plasticizers but continues to show the highest growth rates, with the Chinese plasticizer market reported to have grown in excess of 12% in 2002. This rapid growth in China has also contributed to the decline in growth rates in many other parts of the world, as Chinese imports have displaced locally produced materials.

5.2

Historical Developments

Several authors have documented the historical developments of plasticizers and their use in PVC. Sears and Darby [1] provide an extensive review, including citations of the use of water and other liquids as “quasi-plasticizers” in non-polymeric materials. The use of plasticizers in PVC and other polymers originated as extensions from low volatility solvents. Weinberg [2]

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[References on Page 198]

points out that Waldo Semon, of B. F. Goodrich, originated the use of plasticized PVC using plastisols (dispersion of PVC particles in plasticizers). Semon’s objective was to apply corrosionresistant linings to metal storage tanks, which he accomplished via fused plastisol coatings on wire mesh secured to tank interiors. Krauskopf [3] reviewed plasticizers used in polymers, beginning with the use of camphor in nitrocellulose (1868) by the Hyatt brothers and up to Gresham’s patented use of DOP (di-2-ethylhexyl phthalate) in PVC [4] in the early 1940s. The use of DOP prevailed as the preferred general-purpose plasticizer for PVC until the late 1970s. In 1968, more than 550 different materials were listed as commercial plasticizers, available from over 75 suppliers in the USA [5]. Changes in costs and availability of raw materials that serve as plasticizer feedstock have caused a significant reduction in the number of plasticizer suppliers and plasticizer products in use. Although there are still approximately 70 different plasticizers available, about 80% of the worldwide consumption is comprised of three plasticizers, di-2-ethylhexyl phthalate (DOP), diisononyl phthalate (DINP), and diisodecyl phthalate (DIDP).

5.3

Mechanisms of Plasticization

For a plasticizer to be effective, it must be thoroughly mixed and incorporated into the PVC polymer matrix. This is typically obtained by heating and mixing until either the resin dissolves in the plasticizer or the plasticizer dissolves in the resin. The plasticized material is then molded or shaped into the useful product and cooled. Different plasticizers will exhibit different characteristics in both the ease with which they form the plasticized material and in the resulting mechanical and physical properties of the flexible product. Several theories have been developed to account for the observed characteristics of the plasticization process. A significant review of the theoretical treatment of plasticization is described by Sears and Darby [6]. In this treatment, plasticization is described by three primary theories, with some modifications. According to the Lubricating Theory of plasticization, as the system is heated, the plasticizer molecules diffuse into the polymer and weaken the polymer-polymer interactions (van der Waals’ forces). Here, the plasticizer molecules act as shields to reduce polymer-polymer interactive forces and prevent the formation of a rigid network. This lowers the PVC Tg and allows the polymer chains to move rapidly, resulting in increased flexibility, softness, and elongation. The Gel Theory considers the plasticized polymer to be neither solid nor liquid but an intermediate state, loosely held together by a three-dimensional network of weak secondary bonding forces. These bonding forces acting between plasticizer and polymer are easily overcome by applied external stresses allowing the plasticized polymer to flex, elongate, or compress. Free Volume is a measure of the internal space available within a polymer. As free volume is increased, more space or free volume is provided for molecular or polymer chain movement. A polymer in the glassy state has its molecules packed closely but is not perfectly packed.

5.3 Mechanisms of Plasticization

175

The free volume is low and the molecules cannot move past each other very easily. This makes the polymer rigid and hard. When the polymer is heated to above the glass transition temperature, Tg, the thermal energy and molecular vibrations create additional free volume which allows the polymer molecules to move past each other rapidly. This has the effect of making the polymer system more flexible and rubbery. Free volume can be increased through modifying the polymer backbone, such as by adding more side chains or end groups. When small molecules such as plasticizers are added, this also lowers the Tg by separating the PVC molecules, adding free volume and making the PVC soft and rubbery. Molecules of PVC can then rapidly move past each other. If the plasticizer uniformly went into the PVC, it would behave similarly to an uncured rubber, with lots of creep and high compression set. For example, uncured tires do not hold their shape; they require a crosslinking cure to give them dimensional stability. Likewise, a thermoplastic elastomer such as PVC requires physical crosslinks which are meltable to make them thermoplastic. These meltable crosslinks are the PVC crystallites which give PVC a physical cure. Therefore, the plasticizer must not be a powerful solvent for all the PVC parts, but must be selective in enterring the amorphous PVC part and must not enter and destroy the crystalline part of PVC. The mechanistic explanation of plasticization considers the interactions of the plasticizer with the PVC resin macromolecules. It assumes that the plasticizer molecules are not permanently bound to the PVC resin molecules but are free to self-associate and to associate with the polymer molecules at certain sites such as amorphous sites. As these interactions are weak, there is a dynamic exchange process whereby, as one plasticizer molecule becomes attached at a site or center, it is readily dislodged and replaced by another. Different plasticizers yield different plasticization effects because of the differences in the strengths of the plasticizerpolymer and plasticizer-plasticizer interactions. At low plasticizer levels, the plasticizer-PVC interactions are the dominant interactions, while at high plasticizer concentrations plasticizerplasticizer interactions can become more significant. This can explain the observation of “anti plasticization”, wherein low plasticizer levels (< 15 phr) increase rigidity in PVC, as measured by modulus, tensile strength, elongation and low temperature properties. For a plasticizer to be effective and useful in PVC, it must contain two types of structural components, polar and apolar. The polar portion of the molecule must be able to bind reversibly with the PVC polymer, thus softening the PVC, while the non-polar portion of the molecule allows the PVC interaction to be controlled so it is not so powerful a solvator as to destroy the PVC crystallinity. It also adds free volume, contributes shielding effects, and provides lubricity. Examples of polar components would be the carbonyl group of carboxylic ester functionality or, to a lesser extent, an aromatic ring; the non-polar portion could be the aliphatic side chain of an ester. The balance between the polar and non-polar portions of the molecule is critical to control its solubilizing effect; if a plasticizer is too polar, it can destroy PVC crystallites; if it is too non-polar, compatibility problems can arise. Useful tools in estimating plasticizer compatibility are the Apolar/Polar Ratio method developed by Van Veersen and Meulenberg [7] and the solubility parameter methods [8–11].

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5.4

5 Plasticizers

[References on Page 198]

Types of Plasticizers

Plasticization is achieved by incorporating a plasticizer into the PVC matrix through mixing and heat. Plasticizers may be classified as either monomeric or polymeric plasticizers, depending on their synthesis steps, which relates in part to their molecular weight. It is preferred to categorize plasticizers on the basis of their chemical structure and associated performance when employed in PVC. The IUPAC definition of a plasticizer is entirely focused on performance characteristics when combined with a polymer; there is no implication of chemical structure or physical properties of the plasticizer per se. Early technical publications, therefore, presented rather vague categorizations based on observed performance properties. Attempts to correlate neat plasticizer properties with performance characteristics were unsuccessful; generalizations became possible only after development of large, coherent databases of properties measured on flexible PVC as a function of a broad range of plasticizer levels (i.e., 20–90 phr) for many commercial and experimental plasticizers [12]. The key performance properties are influenced by plasticizer level (phr) as well as the chemical type. In addition, variations in isomeric structure and homologues within any given chemical family contribute performance variations that have been measured in flexible PVC compositions. Table 5.1 shows the major chemical families of PVC plasticizers vs. key performance criteria. An orderly comparison of plasticzers is facilitated by separating all plasticizers types into three subgroups relating to their performance characteristics in PVC:
General Purpose (GP): plasticizers providing the desired flexibility to PVC along with an overall balance of optimum properties at the lowest cost. These are dialkyl phthalates ranging from diisoheptyl (DIHP) to diisodecyl (DIDP), along with low cost oils called “extenders”.
Performance Plasticizers (PP): contribute secondary performance properties desired in flexible PVC beyond the GP type, while imposing somewhat higher costs. Table 5.1 identifies these key performance criteria as “Strong solvaters”, “Low temperature” and “Low volatility”. These include specific phthalates and other types of plasticizers. Strong solvaters have higher polarity and/or aromaticity. Conversely, low temperature types, such as aliphatic dibasic esters, are less solvating and have higher diffusivity. Low volatility requires high molecular weight plasticizers, such as trimellitates and polyesters (polymeric).
Specialty Plasticizers (SP): provide properties beyond those typically associated with flexible PVC designed for general purpose or specialty characteristics. These exceptional characteristics are typically a function of specific chemical plasticizer families and may vary as a function of isomeric structure and/or homologues. Such properties are shown in Table 5.1 as “Low diffusivity”, “Stability”, and “Flame resistance”. Few phthalates meet these special requirements. Polyester plasticizers provide low volatility and low diffusivity, along with low smoke (in the absence of aromaticity) under fire conditions. Epoxy plasticizers provide adjuvant thermal stability to PVC; phosphates and halogenated plasticizers provide fire retardant properties. Specialty plasticizers impose even higher costs than PP grade plasticizers.

177

5.4 Types of Plasticizers

Table 5.1

Plasticizer Family/Performance Grid

Family

General purpose

Performance plasticizers

Specialty plasticizers

Strong solvent

Low temp

√

√

√

√

Trimellitates

√

X

√

Aliphatic dibasic esters

X X

X

Phthalates

X

Polyesters √

Epoxides √

Phosphates Extenders

Low Low Stability Flame volatility diffusion resistance

√

√

X

√

X

X

Miscellaneous

X

X

X

X = Primary performance function √ = Secondary performance function

Table 5.1 indicates the primary performance characteristics associated with each chemical family by “X”, while “√” denotes secondary functions associated with products in that class of plasticizers. Formulating refinements in plasticizer performance and cost constitute the selection of preferred isomers and homologues of any given chemical family, or combinations thereof. Phthalates are the most widely used class of plasticizers in PVC. As shown, they contribute the most complete array of required performance properties in flexible PVC. In addition, their cost and availability supports their preference. While historically DOP – di(2-ethylhexyl) – phthalate has been the product of choice, the current market for GP plasticizers includes dialkyl phthalates that are slightly different homologues of DOP, such as diisoheptyl (C7), diisooctyl (C8), diisononyl (C9) and diisodecyl (C10) phthalates; their combined usage totals more than 80% of the worldwide plasticizer market. Note that the family of phthalate plasticizers show an offering in all of the performance categories, as indicated by the “√”. Performance comparisons of these materials are reviewed in Sections 5.5 through 5.9.
Phthalate esters: prepared by the esterification of two moles of a monohydric alcohol with one mole of phthalic anhydride. Although phthalate esters can be prepared from many different alcohols, the range of alcohols used to make plasticizers for PVC applications is generally limited from C4 to C13 alcohols. Phthalate esters prepared from alcohols below C4 are too volatile, while phthalate esters prepared from alcohols greater than C13 have limited compatibility. Many commercial grade phthalates are prepared using a mixture of monomeric alcohols, such as butanol with 2-ethylhexanol, or blends of linear heptanol, nonanol, and undecanol, and so forth. Di-2-ethylhexyl phthalate (DOP), which is prepared from 2-ethyl hexanol, establishes the standard against which other plasticizers may be compared.
Extenders: shown in the general purpose plasticizer category because they are most commonly employed with phthalates to reduce costs in general purpose flexible PVC.

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[References on Page 198]

These low cost oils have limited compatibility in PVC; for example, naphthenic hydrocarbons may be used up to 35 weight% in dialkyl phthalate plasticizers, while aliphatic hydrocarbons are limited to less than 10%. Higher molecular weight phthalates are less tolerant of extender levels due to their reduced compatibility in PVC. Chlorinated paraffin extenders are not widely used in the U.S., but are commonly employed as secondary plasticizers worldwide. Chlorinated paraffins are produced by chlorination of hydrocarbons up to a chlorine content in the range of 30–70%. These secondary plasticizers are used to reduce cost and to improve fire resistance. The plasticizers with lower chlorine content have lower specific gravity, viscosity, and color, while higher chlorine content imparts increased fire resistance. “Performance Plasticizers” have three subgroups:
“PP-SS”, strong solvator;
“PP-LT”, low temperature, and
“PP-LV”, low volatility. In addition to selected phthalate candidates, other chemical structures contribute desired performance attributes. “PP-SS”s, strong solvaters, are a result of increased polarity and/or aromaticity. Thus, lower molecular weight phthalates such as dihexyl (DHP) and butyl, octyl (BOP), as well as butylbenzyl (BBP) phthalate fall into this category; these plasticizers also contribute to volatile fuming during processing and volatilization in end use applications. In addition, there are non-phthalate plasticizers of high aromaticity that serve as strong solvators. Such materials are benzoate esters and tri(cresyl) phosphate. “PP-LT”s are low temperature phthalates made with normal or “linear” alcohols. These lessbranched alkyl groups contribute improved low temperature properties in all the chemical families of plasticizers. The entire family of aliphatic dibasic esters contributes exceptional low temperature properties. They are prepared by the esterification of one mole of dibasic carboxylic acid, such as adipic or azelaic acid, with two moles of monohydric alcohols. Lower molecular weight alcohols are used with higher molecular weight acids, and vice versa, such that the total carbon content per molecule ranges between C18 and C26. This maintains the apolar/polar ratio required to provide PVC compatibility along with low temperature properties. Di-2-ethylhexyl adipate (DOA) is the standard and most widely used plasticizer in this class. Di-2-ethylhexyl azelate (DOZ), di-2-ethylhexyl sebacate (DOS), and diisononyl adipate (DINA) are used for low temperature applications requiring lower plasticizer volatility. Plasticizer structural relationships with low temperature performance will be reviewed in Sections 5.5 through 5.9. “PP-LV”s are low volatility plasticizers primarily because of their high molecular weight, which is also reflected in low vapor pressure. High molecular weight phthalates that serve as “LV” plasticizers include those having molecular weights greater than DIDP (446). Increasing the molecular weight of phthalates increases the ratio of apolar/polar functionality until loss of PVC compatibility occurs at molecular weights greater than that of DTDP (530). High molecular weight phthalates having low volatility and compatibility with PVC include DIUP, UDP, DTDP, 911P, and DUP, all shown in Table 5.2. Two chemical families are noted for their use as low volatility plasticizers – trimellitates and polyesters (also referred to as polymerics).

5.4 Types of Plasticizers

Table 5.2

Plasticizer Acronyms, Chemical Compositions, and Substitution Factors

Acronym

Chemical structure

Molecular weight

Subst.* factor

Phthalates BBP

butyl, benzyl

ca. 312

0.94

BOP

butyl, 2-ethylhexyl

ca. 365

0.94

DHP

di(isohexyl)

334

0.96

DIHP

di(isoheptyl)

362

0.97

DOP

di(2-ethylhexyl)

390

1.00

DIOP

di(isooctyl)

390

1.01

DCP

di(2-normal-octyl) (aka capryl)

390

NA

DINP

di(isononyl)

418

1.06

DIDP

di(isodecyl)

446

1.10

DIUP

di(isoundecyl)

474

1.16

UDP

di(iso C11, C12, C13)

ca. 502

1.21

DTDP

di(isotridecyl)

530

1.27

Linear phthalates DBP

di(n-butyl)

278

0.86

79P

di(linear C7, C9)

ca. 390

1.00

NHDP(610P)

di(n-C6, C8, C10)

ca. 418

0.99

DNNP

di(n-nonyl)

418

0.94

L9P

di(linear nonyl)

418

0.99

7911P

di(linear C7, C9, C11)

ca. 418

1.00

911P

di(linear C9, C11)

ca. 446

1.05

DUP

di(linear C11)

474

1.14

NODTM

tri(n-C8, C10)

ca. 592

1.12

TOTM

tri(2-ethylhexyl)

546

1.17

TIOTM

tri(isooctyl)

546

1.19

TINTM

tri(isononyl)

588

1.27

Trimellitates

Adipates 79A

di(linear C7, C9)

ca. 370

0.90

DOA

di(2-ethylhexyl)

370

0.93

DIOA

di(isooctyl)

370

0.94

DINA

di(isononyl)

398

0.98

179

180

5 Plasticizers

Table 5.2

[References on Page 198]

(continued)

Acronym

Chemical structure

Molecular weight

Subst.* factor

Phosphates DDP

isodecyl, diphenyl

390

0.96

TOF

tri(2-ethylhexyl)

435

1.00

TCP

tricresyl

368

1.31

Epoxides OET

2-ethylhexyl epoxy tallate

ca. 410

0.96

ESO

epoxidized soybean oil

ca. 1,000

1.10

Others

*

DOTP

di(2-ethylhexyl)terephthalate

390

1.03

DINCH

di(isononyl) cyclohexane-1,2-dicarboxylate

422

NA

Substitution factor = PHR required for 80A Durometer hardness at room temperature vs. required DOP level (52.9 phr).

Trimellitates are the product of three moles of monohydric alcohols and trimellitic anhydride (TMA). The third alkyl group, compared to phthalates, contributes higher molecular weight; the third ester group contributes sufficient polarity to maintain PVC compatibility. “Specialty Plasticizers” are also divided into three subgroups:
“SP-LD” for low diffusion;
“SP-Stab” for stabilizing function, and
“SP-FR” for fire resistance in PVC. Low diffusivity is contributed by high molecular weight and highly branched isomeric structures. Diisodecyl phthalate (DIDP) and diisotridecyl phthalate (DTDP) impart improved resistance to diffusion-controlled plasticizer losses, and are sometimes used in combination with more costly diffusion-resistant plasticizers. But the polyester family is noted for its outstanding performance in this category. Polymeric plasticizers are typically polyesters, with a molecular weight range from 1,000 to 8,000. Polyethylene copolymers (EVA’s, VAE’s, etc.) and terpolymers can range up to > 500,000. Polyesters are prepared by the esterification of propylene glycol or butylene glycol with aliphatic dibasic acids. The greater the plasticizer viscosity, or molecular weight, the greater its permanence. Polymeric plasticizers composed of branched structures are more resistant to diffusivity losses than those based on linear isomeric structures; on the other hand they are more susceptible to oxidative attack. The polarity, or the oxygen-to-carbon ratio, also impacts extraction resistance of the polymerics. Lower polarity materials exhibit better extraction resistance towards polar extraction fluids such as soapy water. Glutarate polymerics reportedly have a proven history of providing good weathering resistance [13].

5.4 Types of Plasticizers

181

Interestingly, the trimellitate plasticizers demonstrate improved resistance to diffusivitycontrolled losses only under certain conditions. Trimellitates in combination with polyester plasticizers control migration from PVC refrigerator gaskets, which can cause crazing of the ABS door liner. However, trimellitates fail to provide reduction of plasticizer diffusivity under oil immersion tests. Pentaerythritol esters are a type of “miscellaneous” plasticizers that impart both low volatility and diffusivity. Pentaerythritol and dipentaerythritol are tetra and hexa alcohols, respectively; they are esterified with a stream of straight chain fatty acids to make plasticizers. Hercoflex® 600 is the pentaerythritol tetraester and 707 is a mixture of tetra and hexa esters, using a mixture of pentaerythritol and dipentaerythritol. Their molecular weights are approximately 600 and 750, respectively, which contributes to both low volatility and diffusivity. Epoxy plasticizers enhance thermal and UV stability of PVC. They are the only class of plasticizers that undergo a chemical grafting onto the PVC polymer at the site of labile chlorides in the presence of mixed metal stabilizers [14]. This chemical family is composed of essentially two types of epoxidized natural products. Epoxidized oils, such as soybean oil (ESO) and linseed oil (ELSO) are prepared by the use of peracetic acid, which adds the oxirane structure at unsaturated (double bond) sites. These oils have molecular weights of approx. 1,000, causing them to perform as low volatility plasticizers. The other group of epoxy plasticizers is represented by octyl epoxy tallate (OET). This product results from the epoxidation of tall oil esters, which are the esterified product of tall oil acids. The OET has a molecular weight of approx. 410, and is a monoester. This causes it to have more limited compatibility in PVC, and to contribute toward lower plastisol viscosity and low temperature properties. The primary performance attributes of epoxy plasticizers are their role in PVC stabilization, which is accomplished at less than 10 phr levels. Therefore, while they contribute to the plasticization in PVC, the secondary plasticizer effects are minimized. A commercial curiosity of the “epoxidized phthalate-type” structure was found to contribute beneficially to thermal stability, while otherwise imparting the expected properties of the dialkyl phthalate counterpart [15]. Flame resistant plasticizers include halogenated (preferably brominated) phthalates and the phosphate family. Brominated phthalate esters are produced by the esterification of tetrabromophthalic anhydride with various alcohols, most typically 2-ethylhexanol. Phosphate plasticizers which may be considered as “inorganic esters” are prepared by the slow addition of phosphorous oxychloride to alcohol or phenol. The highly aromatic tricresyl phosphate (TCP) is the most effective fire retardant, but generates high smoke under fire conditions. Trialkyl phosphates (like TOF) are less efficient in fire resistant properties. Commercial phosphate plasticizers use combinations of aryl and C8 and C10 alkyl groups to offer a balance of fire reduction, volatility, and efficiency. A combination of phosphate plasticizers, antimony trioxide, and zinc borate yields a superior flame retardant grade of PVC for demanding applications such as plenum cable jacketing and electrical insulation [16]. Phosphate plasticizers may be combined with phthalates to reduce formulating costs. Miscellaneous plasticizers include “phthalate-like” esters, benzoates, sulfonates, pentaerythritol esters, citrates, and similar materials. “Phthalate-like” esters include DOIP [di (2-ethylhexyl) meta (called “iso”) phthalate], and DOTP [di (2-ethylhexyl) para (called “tere”) phthalate], which are isomeric structures of DOP. DOTP is commercially available at similar costs to DOP; Section 5.8 reviews the performance

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characteristics of DOIP and DOTP. Hexamoll® DINCH is di (isononyl) cyclohexane-1,2-dicarboxylate [17], the hydrogenated product of the corresponding di C9 phthalate ester(DINP). As indicated [15], its performance characteristics in PVC are expected to be similar to the phthalate counterpart, except for having less solvency for PVC. DINCH has recently been introduced by BASF as a candidate for applications with sensitivity for peculiar health and environmental concerns. These sensitivities are also addressed with the recent introduction of a novel triester plasticizer completely devoid of carbon ring configurations; it is claimed for use in medical applications and “low smoke” grade PVC electrical insulations [18]. Benzoates are the esterification products of benzoic acid and selected glycols, usually diols. Preferred glycols are dipropylene glycol and butane diols. One commonly used benzoate is dipropylene glycol dibenzoate (DPGDP, commercially Benzoflex® 9-88). Its preferred use is in PVC flooring products, owing to its strong solvating strength, and it reportedly controls plasticizer bleeding into asphalt adhesives. Benzoflex® 1046 is a mixed ester of benzoate ester of Texanol®. Texanol is an “ester-ol” resulting from the Aldol and Tischenko condensation reactions of three moles of isobutyraldehyde. Esterification with benzoic acid yields the mixed ester. Its preferred use is in vinyl sheet flooring, where the benzoate enhances processing, while the low molecular weight contributes a hardened, stain resistant surface, due to volatilization. Sulfonates also exhibit strong solvency for PVC. Mesamoll® is a product of Bayer; it is described as the phenyl cresyl esters of pentadecyl sulfonic acid. It is reportedly resistant to hydrolyses and diffusion controlled plasticizer losses. Citrates are promoted for plasticized PVC applications facing exceptional toxicological and/or environmental constraints. However, neat citric acid does not meet PVC compatibility requirements. Therefore, citrate plasticizers are tetraesters, resulting from the reaction of one mole of an organic acid (with the single alcohol group) and three moles of alcohol, which esterify the three acid groups.

5.5

Plasticizer Performance

The previous section summarized key plasticizer families and associated performance characteristics. Within each family, there are a variety of isomeric structures and homologues that contribute measurable variations in performance. The phthalate family includes a great variety of isomers and homologues that are useful as plasticizers for PVC. This, then, is an appropriate plasticizer family for the evaluation of the effects of chemical structures in PVC. Generalizations derived from phthalate structures appear to translate into other chemical families as well. Performance plasticizers (PP) contributing low volatility include those having molecular weights greater than that of DIDP (446) and those of less branching (oxidative resistance). Linear alkyl structures contribute low temperature properties as well as lower volatility. All aliphatic dibasic esters, such as the adipates, contribute exceptional low temperature properties. The chemical structures that contribute improved low temperature properties typically impart lower plastisol viscosities, due to their own lower viscosity; likewise, their reduced tendency for solvation of PVC resin contributes to improved viscosity stability

5.5 Plasticizer Performance

183

under storage conditions. On the negative side, the low temperature type plasticizers impair compatibility in PVC and diffusion-controlled plasticizer loss in end use applications. Octyl epoxy tallate (OET) and tri(2-ethylhexyl) phosphate demonstrate performance characteristics similar to those of low temperature plasticizers. Performance plasticizers having strong solvating characteristics include phthalates having higher polarity and aromaticity. These structural features also contribute increased volatility due to smaller non-polar tails. Non-phthalate plasticizers having strong solvency are highly aromatic (benzoates, TCP) or other more polar structures such as sulfonates. Epoxy plasticizers contribute the unique feature of enhancing thermal stability, and also contribute plasticization properties consistent with their molecular structure. That is, epoxidized oils (about 1,000 molecular weight) enhance low volatility, while monoester OET enhances low temperature performance. Disciplined studies of carefully controlled model formulations, sample preparation, and conditioning have shown little correlation between neat plasticizer properties and their performance in PVC [19–22]. Key physical properties appear to correlate with plasticizer performance in PVC only when the comparisons are restricted to homologues of a given chemical family. Further correlation of physical properties of the neat plasticizer with performance in PVC is confounding; a summary of key physical and performance properties of plasticizers for PVC verifies the point [15]. The only mild chemical interaction between plasticizers and PVC polymers allows for the calculation of predicted specific gravity of the plasticized PVC compositions. Yet this chemical association precludes quantitative prediction of plasticizer losses due to volatilization (i.e., using vapor pressure), or diffusion-controlled migration [19, 23–25]. Oxidative or hydrolytic degradation, on the other hand, have the opposite effect when attempting to predict plasticizer transience from PVC. The color of plasticized PVC compositions is typically not altered by the plasticizer. This is because most commercial grade plasticizers are near “water-white” in color. Highly colored (amber–brown) plasticizers would, of course, impart undesired color to flexible PVC compositions. The effects of a wide variation in plasticizer level on the mechanical properties of PVC compounds are listed in Table 5.3. This example shows typical properties for general purpose PVC containing DINP levels ranging from zero (rigid vinyl) to about 600 phr (parts per hundred resin, by weight). The average consumption of plasticizers in flexible PVC stands at 50 phr. Useful commercial products typically range from about 20 to 100 phr; fishing lures, at about 600 phr, are an exceptional product. Reliable generalizations of plasticizer structure/performance relationships require extensive evaluations using disciplined model formulations, raw materials, sample preparation, and conditioning in the measurement and cataloging of the data. Such procedures have been described [12]. Other publications have utilized these data to provide analyses and comparisons of various commercial and experimental plasticizers [15, 23, 26–28]. The translation of this basic information to specific property requirements of flexible PVC products is enhanced by an awareness of some generalizations relevant to properties influenced by plasticizer chemical structure as well as level (phr) in the PVC, as follows:

184

5 Plasticizers

Table 5.3

[References on Page 198]

Typical Properties of General Purpose Vinyl Plastic Products

Rigid

Semirigid

Flexible

Very flexible

Extremely flexible

DINP, phr

0

34

50

80

600

Wt% of composition

0

25

33

44

86

Specific gravity, 20/20 °C

1.40

1.26

1.22

1.17

1.02

Hardness Durometer A, 15 s

–

94

84

66

< 10

> 900 > 130,000

69 10,000

12 1,700

3.4 500

– –

> 41 > 6,000

31 4,500

21 3,100

14 2,000

– –

< 15

225

295

400

–

> 23 > 73

–16 +3

–32 –26

–47 –53

– –

Typical properties

a

Flexural stiffness at 23 °C MPa psi b

Tensile strength MPa psi

b

Elongation (%) Brittleness °C °F

c

Examples

a b c

Bottles, Shades, pipe, siding, shoe heels, records thin films, produce wrap

Wall-cover- Boots, ing, book- gloves, binders, water beds upholstery, garden hose

Fishing lures

ASTM D 747 ASTM D 882 ASTM D 746

Source: Krauskopf, L. G., in Encyclopedia of PVC, 2nd ed., Nass, N. L. and Heiberger, C. A. (Eds.), Marcel Dekker (1988), p. 149 (reprinted with permission)


Hardness (softness) is significantly influenced by plasticizer level, as well as type of plasticizer, which controls plasticizer “efficiency”.
Tensile strength and ultimate elongation (% extension at failure) are influenced by plasticizer level, but these properties are not significantly altered as function of plasticizer type with PVC formulated to specified room temperature hardness.
Modulus (stiffness, flexibility) may be measured under tensile stress (ASTM D 882) at a specified strain level, or under flexural stress (ASTM D 747), or under torsional stress (ASTM D 1043). Modulus values vary significantly as a function of plasticizer level, and are somewhat influenced by plasticizer type (efficiency) when measured at room temperature and formulated to specified hardness. The three different techniques for determination of modulus at room temperature result in significantly different absolute values, due to variations in test methodology forces of extension, compression, and shear.

5.5 Plasticizer Performance

185


Low temperature properties, both low temperature modulus and brittleness, are significantly influenced by plasticizer level (phr) and type. Low temperature modulus values are determined by ASTM D 1043 (Clash-Berg, Tf), while brittleness temperature is determined by ASTM D 746 (TB). NOTE: Reliable measurements of failure properties require that test specimens be completely free of surface imperfections (nicks/cuts), completely fused and conditioned at room temperature (ASTM conditions preferred) for at least 24 hours following preparation of specimens. Variations in isomeric structure are primarily imparted by the nature of the alkyl moiety. These configurations are a function of the starting materials and processes used to produce the plasticizer-grade alcohols. Plasticizers made with more linear (less branched) molecular structures are more efficient, and impart improved low temperature properties and volatilization and oxidative resistance [29, 30]. Krauskopf [31] differentiated plasticizer performance effects as a function of five different degrees of branching using commercial grade di(C8),di(C9), and di(C10) phthalates; these were grouped as:
Normal: 100% unbranched; primarily an academic product, except for a limited amount of commercial products based on mixtures of normal C6, C8, and C10 alcohols.
Linear: a mixture of normal and monomethyl branched alcohols. These “linear” alcohols are produced by the hydroformylation (Oxo process) of normal alpha olefins. The resultant alcohol is approximately a 70/30 molar ratio of normal/2-methyl branched isomers.
Slightly branched (SLBR): primarily a mixture of monomethyl and dimethyl branched alcohols. Commercially, the “slightly branched” alcohols are the hydroformylation products of octenes that are dimerized normal butenes. The resultant alcohol (nonanol) is a random mixture of monomethyl octanols and dimethyl heptanols.
Moderately branched (MODBR): primarily dimethyl or monoethyl (i.e., 2-ethylhexyl) branched; these serve as the major type in “General Purpose” plasticizer category. “Moderately branched” nonanol is the hydroformylation product of mixed olefins generated by the dimerization of a mixture of propylene and normal butene feeds. The resultant olefin is a mixture of hexenes, heptenes, and octenes, which are separated by distillation. The octenes are hydroformylated to give nonanol mixtures primarily composed of dimethyl substituted C7 backbones.
Highly branched (HIBR): triple methyl branched; a specific product available in Europe. The “highly branched” nonanols are the hydroformylation product of trimethyl branched pentenes which are a product of dimerized isobutene; the resultant alcohol is primarily 3,5,5-trimethyl hexanol. These are notably susceptible to oxidative attack, while showing increased resistance to diffusivity. These variations in degrees of branching demonstrate measurable effects on PVC properties with respect to plasticizing efficiency, low temperature properties, diffusivity, volatility, and stability to thermal and oxidative degradation. The normal C9 phthalate has a substitution factor (S.F.) of 0.94, while the plasticizing efficiency of the “linear” nonyl phthalate is equivalent to that of DOP (S.F. = 1.00). When compared at equivalent hardness in PVC, the “linear” phthalate is only slightly deficient to the normal nonyl phthalate with respect to low temperature

186

5 Plasticizers

[References on Page 198]

properties and volatility. The “moderately branched” (MODBR) DINP shows less plasticizing efficiency (S.F. = 1.06) and a deficiency in low temperature properties and volatility vs. the normal nonyl phthalate. Compared to DOP performance at specified room temperature hardness, the DINP (MODBR) provides equivalent low temperature properties with significantly lower volatility. The performance of the “slightly branched” (SLBR) DINP is essentially equivalent to a 50/50 mixture of di (normal nonyl) phthalate and moderately branched DINP (MODBR). Wadey studied a series of designed isomeric variations of DINP and the effects on plasticizer performance in PVC [30]. The conclusions are consistent with the generalizations cited above.

5.6

Plasticizer Efficiency

Plasticizer “efficiency” may be quantified as a function of PVC Durometer hardness. Similar comparisons may be made for other mechanical properties, but hardness test reliability and the common practice of a designated room temperature hardness value supports its use to quantify plasticizing efficiency. Figure 5.1 graphically portrays quantitative determination of plasticizer efficiency, expressed as “Substitution Factor” (SF); in this example, the hardness values are compared for DINP (MOD-BR) to DOP [di-2-ethylhexyl phthalate] plasticized PVC. 95

Shore A Durometer hardness (15 s)

90 85 80 75 70 65 DINP DOP

60 55

52.9

56.2

50 20

30

40

50

60

70

80

90

100

Plasticizer phr Figure 5.1 Durometer A hardness of DINP vs. DOP (Source: Krauskopf, L. G., in Handbook of PVC Formulating, Wickson, E. J. (Ed.) (1993) Wiley, New York, p. 171, courtesy John Wiley, reprinted with permission)

5.7 Low Temperature

187

It is shown that 80 Durometer A hardness is provided by 52.9 phr DOP, while 56.2 phr DINP is required to provide the same hardness. Thus, the substitution factor (SF) for DINP vs. DOP is 1.06, as shown in Equation 5.1: ⎛ phr plasticizer at Durometer 80 ⎞ ⎛ 56.2 phr DINP ⎞ Substitution Factor (SF) = ⎜ ⎟=⎜ ⎟ 52.9 ⎠ ⎝ phr DOP at Durometer 80 ⎠ ⎝

(5.1)

The “SF” indicates that DINP-MODBR is 6% less efficient than the plasticizing efficiency of DOP. In other words, DINP needs to be added at a level 6% higher than the DOP level, in order to achieve the same hardness or softness. It has been found that this ratio (substitution factor) is consistent over plasticizer levels ranging from about 20 to 90 phr. The question of acceptability, then, rests on comparative formulating costs and other critical properties provided at the specified room temperature hardness. In general, it is found that when compared at equivalent hardness, the DINP-MODBR plasticized PVC will have preferred low temperature properties as well as significantly less plasticizer loss due to volatilization and diffusivity. Most commercial grades, and many experimental plasticizers, have been evaluated in PVC over a wide range of levels (phr). The disciplined cataloguing of the performance properties allows for easy comparisons of cost effective formulating options at specified hardness values as well as at specified plasticizer levels. A computer program includes optimizations as a function of filler content as well as plasticizer selection and predicted properties in PVC [22]. Table 5.2 lists relative plasticizing efficiency values determined for commercial grade plasticizers, along with acronyms, chemical compositions, and molecular weights.

5.7

Low Temperature

Tables 5.4 and 5.5 show low temperature flex (Tf) and brittleness (TB) for PVC using different plasticizers. Table 5.4 compares low temperature properties when formulated to equivalent room temperature hardness (80A Durometer); Table 5.5 shows low temperature properties at equivalent plasticizer levels (50 phr, by weight), in a recipe commonly used for “screening” plasticizer performance. It is shown that for given alkyl structures, the trimellitate family provides similar or only slightly improved low temperature properties vs. the phthalate counterparts, because of the need for higher plasticizer levels to provide the target room temperature hardness, due to the lower plasticizing efficiency of trimellitates versus phthalates. Adipate plasticizers, on the other hand, impart significantly improved low temperature properties (by about –25 to –35 °C) versus their phthalate counterparts, in spite of the fact that they are more efficient (substitution factors of < 1.00) in providing target room temperature hardness. The more linear alkyl structures in the plasticizer contribute improved low temperature properties (by about –5 to –7 °C) vs. the branched isomers. They also have lower substitution factors (higher plasticizing efficiency) to meet room temperature hardness. It should be noted that end-use products require low temperature tests to be conducted on whatever form, or shape, the product has.

188

5 Plasticizers

Table 5.4

[References on Page 198]

Low Temperature Properties of Unfilled, General Purpose PVC, Formulated to Meet 80 A Durometer Hardness at Room Temperature a

b

Tf (°C)

TB (°C)

BBP

–10.7

–12.1

BOP

–25.8

–31.1

DIHP

–24.6

–32.9

DOP

–27.7

–34.9

DIOP

–27.5

–32.8

DOTP

–31.9

–36.5

DINP

–29.2

–35.8

DIDP

–31.6

–37.8

DIUP

–32.2

–37.8

UDP

–32.8

–41.8

DTDP

–39.3

–42.9

610P

–36.1

–45.5

79P

–35.7

–40.2

7911P

–34.9

–42.2

L9P

–37.4

–44.4

911P

–39.8

–47.6

DUP

–43.0

–53.7

TOTM

–29.4

–39.0

TIOTM

–27.5

–37.8

TINTM

–31.4

–38.8

DOA

–50.9

–61.7

DIOA

–49.3

–63.2

DINA

–51.6

–64.4

79A

–52.5

–66.1

Phthalates

Trimellitates

Adipates

PVC formula by weight: PVC-100, plasticizer (concentration adjusted to yield a shore A of 80), Liquid Ba/Cd/Zn stabilizer – 2.0, stearic acid 0.25. a ASTM D1043 b ASTM D746

5.7 Low Temperature

Table 5.5

Low Temperature Properties of Unfilled, General Purpose PVC at 50 PHR Plasticizer a

b

Tf (°C)

TB (°C)

BBP

–11.0

–12.3

BOP

–26.9

–31.9

DIHP

–23.6

–32.3

DOP

–24.9

–32.9

DIOP

–23.8

–30.0

DOTP

–27.7

–33.4

DINP

–23.6

–31.8

DIDP

–23.6

–31.8

DIUP

–22.4

–30.5

UDP

–20.9

–32.9

DTDP

–24.4

–31.5

610P

–33.5

–43.5

79P

–32.7

–38.0

7911P

–32.0

–40.2

L9P

–34.3

–42.1

911P

–34.3

–43.4

DUP

–33.3

–45.9

TOTM

–19.2

–31.8

TIOTM

–15.9

–29.1

TINTM

–17.3

–27.9

DOA

–52.8

–62.7

DIOA

–49.8

–63.5

DINA

–50.5

–63.8

79A

–55.3

–67.6

Phthalates

Trimellitates

Adipates

PVC formula by weight: PVC-100, plasticizer – 50 Liquid Ba/Cd/Zn stabilizer – 2.0, stearic acid 0.25. a ASTM D1043 b ASTM D746

189

190

5 Plasticizers

[References on Page 198]

Thus, performance test results measured on finished commercial PVC plastics may be influenced by factors other than the formulating predictions. For example, incompletely fused or improperly conditioned, or otherwise damaged, specimens may experience undue failure in mechanical property testing. Likewise, the translation of catalogued values may require experience. For example, properly prepared PVC insulation for electrical conductors typically meets low temperature mandrel bend testing at temperatures approximately –15 °C lower than the predicted brittleness (TB by ASTM D 746) values of formulated PVC.

5.8

Permanence (Transience) of Plasticizers

Plasticizers have a strong affinity for PVC polymers, but do not undergo a chemical reaction that causes bonding, or grafting, to the polymer. Note, however, that epoxy plasticizers are an exception, in that they undergo chemical grafting onto PVC in their role as stabilizers, replacing labile chlorides [14] in addition to their role of acid absorption. Other functional additives are known to graft and/or polymerize in the PVC matrix, but these are generally not considered as traditional “external” plasticizers. Copolymers for example, can lower PVC’s Tg as plasticizers do. But at the same time, any significant level of co-monomer will disrupt the syndiotactic PVC structure and disrupt the ability to form crystallites. The crystallites are the physical cross-links that hold the structure together as a thermoplastic elastomer. Thus with copolymers, creep increases, compression set increases, and long-term elasticity is lost. Thus, the plasticizers, when not grafted or copolymerized, may be separated from the PVC matrix due to extraction by solvents, oils, water, surface rubbing, volatility, migration into adjacent media, or degradation mechanisms. Investigations of plasticizer transience found that quantitative predictions were confounded by “compatibility”, which was difficult to quantify [19, 20, 24, 25]. However, Quackenboss determined that two controlling mechanisms (other than effects of degradation) are at play under conditions that contribute to loss of plasticizer. These are the rate of loss that occurs at the surface of the specimen vs. the rate at which the plasticizer diffuses to the surface; the slowest rate is the controlling factor. For example, most plasticizers have extremely low solubility in water, and therefore exhibit surface-controlled loss rates under aqueous environments. Plasticizer losses due to extraction by oily media (in which plasticizers are highly soluble) are controlled by diffusivity rates. Volatile losses of plasticizer are influenced by vapor pressure, solvency strength for the polymer and oxidative degradation, as well as the ambient airflow rate in the test chamber. It is known that test chamber atmospheres saturated with plasticizer vapor allow for reversed absorption of plasticizer into the test specimens. Polymeric plasticizers of high molecular weight (≈1,000 to > 500,000) and of bulky molecular structures show excellent permanence because of low diffusivity. However, this family of plasticizers is also noted for sensitivity to hydrolytic degradation in aqueous atmospheres. Dialkyl phthalate plasticizers range in molecular weight from 278 (dibutyl) to about 530 (ditridecyl). Commercial experience has shown that dibutyl is unacceptably volatile (except in some adhesive applications), while ditridecyl phthalate is useful for PVC applications under

5.9 Solvency, Miscibility, or Compatibility

191

high temperatures for extended periods; trimellitates and polyesters are typically even lower in volatility. Preferred “General Purpose” plasticizers range in molecular weight from 362 (DIHP) to 418 (DINP), while DOP has an intermediate molecular weight at 390. The volatility of DINP is significantly less than that of DOP, while DIHP is significantly more volatile than DOP in most applications. This characteristic often dictates the preferred choice of the general purpose plasticizer for given applications. Di(linear alkyl) phthalates impart lower volatility and improved oxidative resistance vs. their branched counterparts. All phthalate, trimellitate, and aliphatic dicarboxylic diesters show excellent resistance to hydrolytic attack under exposure to aqueous environments. Diffusion-controlled transience is poor for the aliphatic dicarboxylic diesters, but good for branched phthalates; linear dialkyl phthalates are measurably less resistant than the branched phthalates, but significantly better than the aliphatic dicarboxylic diesters. Oil extraction resistance of trialkyl trimellitates is not good compared to that of phthalates. This is apparently due to their lower plasticizing efficiency as well as to the increased proportion of alkyl moieties in the molecular structure. Two isomers of DOP have been found to have novel resistance to migration into F2 nitrocellulose lacquer finishes [32, 33]. These are known as DOIP [di 2-ethylhexyl meta (called “iso”) phthalate] and DOTP [di 2-ethylhexyl para (called “tere”) phthalate]. Their overall performance in PVC Is similar to that of DOP, except for a slight indication of being less compatible. The “mar resistant” feature may also be imparted to flexible PVC by alternate practices, such as top coating technology and/or the use of polyester plasticizers. However, this novel performance trait of DOIP and DOTP remains largely a technical anomaly.

5.9

Solvency, Miscibility, or Compatibility

These terms are essentially interchangeable with respect to liquids and other lower molecular weight reagents added to PVC. Whether rigid or flexible, the systems behave as solid solutions, and abide by the three-dimensional solubility parameter concept of Hansen [10, 34]. Bench scale methods designed to measure the solvency, miscibility, or compatibility of plasticizers – and other reagents – in PVC are confounded by the simultaneous effect of diffusibility. This interfering mechanism is present when attempting to measure plasticizer take-up, gelation temperatures, or compatibility (phase separation). While the solubility forces are extremely small (units are (cal/cm3)1/2), their presence is responsible for the energy required to molecularly combine plasticizers with PVC resin (up-take or swelling) and to hold them together (compatibility for duration of the application). A statistical analysis of independent plasticizer variables versus take-up rates in PVC [11, 35] showed the following relationships. Equation 5.2 expresses dry blend time as a function of plasticizer viscosity and specific gravity for phthalates, trimellitates, and aliphatic dicarboxylic diesters that are used as plasticizers in PVC. Dry Blend Times @ 88 °C = 10.05 + 0.218 · (Viscosity) – 10.08 · (Specific Gravity)

(5.2)

192

5 Plasticizers

[References on Page 198]

Where: Dry Blend Times are minutes using ASTM D 2396, viscosity is plasticizer viscosity at 88 °C, centistokes (cS), specific gravity is specific gravity of the plasticizer at 20 °C. That study found that the statistical confidence for evaluating plasticizer take-up is significantly improved by limiting the analyses to the eleven commercial grade phthalates tested. Equation 5.3 expresses dry blend time as a function of plasticizer viscosity (distinctly) for dialkyl phthalates: Dry Blend Times @ 88 °C = –0.067 + 0.282 · (Viscosity) – 0.012 (Viscosity – 8)2

(5.3)

Similar relationships were developed for gelation temperatures. Equation 5.4 shows initial gelation temperatures are a function of plasticizer molecular weight and solvency strength for dialkyl phthalates, while Equation 5.5 shows that dialkyl phthalate plasticizers influence final gelation temperature exclusively as a function of solvating strength. Initial Gelation Temperature = –8.35 + 0.118 · (MW) + 0.001 · (MW – 450)2 + 21.4 · (δ) (5.4) Final Gelation Temperature, °C = 71.48 + 39.30 · (δ)

(5.5)

Where: Initial gelation temperature for plastisols, °C, ref. [36], final gelation temperature for plastisols, °C, ref. [36], MW is molecular weight of the plasticizer, δ is Hansen’s Interaction Radius (HIR). HIR is the distance between PVC resin and the plasticizer on Hansen’s three-dimensional solubility parameter grid. Smaller values of δ indicate stronger interaction forces. The investigation that developed plastisol gelation values [36] indicated that the ultimate fusion temperature is a function of the PVC resin, to the exclusion of plasticizer solubility parameters.

5.10

Processability

The ease with which various processes combine the liquid plasticizer with PVC polymer is a function of the physical and chemical properties of the plasticizer as well as the polymer characteristics. The mixing, fluxing, fusing, and shaping of the vinyl involves the application of elevated temperatures, up to about 160 °C to 170 °C. Thus, plasticizer fuming (volatility) is of concern. In addition, the rheology of plastisols (dispersions of PVC in plasticizer) critically impacts the ease of shaping and controlling thickness of end products; further, initial and final gelation temperatures (influenced by plasticizer selection) influence processing of plastisols. As shown in Section 5.9, plasticizer selection influences dry blend rates. Melt viscosity, during “hot compounding” processes, is influenced by plasticizer characteristics, including solvency

5.11 Plasticizer Markets

193

for the PVC resin. Many investigators have studied these characteristics. Commercial practice includes the use of up to 10–20% of the plasticizer system as “strong solvating” type plasticizers, such as aryl-alkyl phthalates, benzoates, sulfonates, and so forth. Volatility is typically the limiting factor on use levels of strong solvating plasticizers. Higher molecular weight plasticizers typically offset volatility while imposing constraints on ease of processing.

5.11

Plasticizer Markets

Plasticizers are used to produce flexible PVC products for many different end uses or market segments. Figure 5.2 depicts a worldwide analysis of plasticizer consumption by PVC market segments. The largest market segment is film, sheeting, and coated substrates. In this segment, the majority of plasticizer consumed is for products produced in calendering operations. The primary factors in plasticizer selection are low cost and ease of processing, with DOP meeting this requirement in most parts of the world. In North America and Europe, DINP is the preferred plasticizer choice based on the above criteria and factoring in regulatory issues regarding the use of DOP. If greater permanence is required, plasticizers such as DIDP, L9P, 911P, and DUP may be used. For coated substrates prepared through a coating process, plastisol viscosity, gelation or fusion behavior, and emissions are all-important concerns. DINP is found to give a more consistent, stable viscosity than DOP while reducing emissions. For products that need a slight reduction in gelation or fusion temperature, DIHP (diisoheptyl phthalate) or BBP can be used to replace a small portion of the primary plasticizer. Vinyl flooring is another major market for plasticized PVC. There are basically three types of products: vinyl tile, resilient vinyl sheet flooring, and vinyl backed carpeting or carpet squares. Floor tiles are comprised of about 80% calcium carbonate held together by the fused flexible PVC binder. The most commonly used plasticizers in floor tiles are DOP and DINP, while Miscellaneous 14% Adhesives, Sealants, Coatings 7%

W ire & Cable 19%

Flooring 8% Food & Regulated Uses 5% Extruded & Molded 12%

Figure 5.2 End use markets for plasticized PVC

Film & Coated Fabrics 35%

Produktinformation

Seite 1 von 1

PVC Handbook Charles E. Wilkes, Charles A. Daniels, James W. Summers ISBN 3-446-22714-8 Leseprobe 2

Weitere Informationen oder Bestellungen unter http://www.hanser.de/3-446-22714-8 sowie im Buchhandel

http://www.hanser.de/deckblatt/deckblatt1.asp?isbn=3-446-22714-8&style=Vorwort

18.08.2005

10

Flexible PVC

WILLIAM COAKER

10.1

Origins

Polyvinyl chloride (PVC) homopolymer is a semi-crystalline polymer with a relatively high room temperature tensile modulus of 2400 to 4140 MPa (3.5–6.0 · 105 psi), depending on formulation) that can be lowered by plasticizing entities to produce semi-rigid and flexible items. Many of these have turned out to be commercially useful and cost competitive. Use of other additives, in addition to plasticizers, is essential to making successful flexible PVC products. These include stabilizers, pigments, fillers, lubricants, and many specialty additives such as fire retardants, anti-microbials, UV-screeners, and antistats for particular applications. To put the development of flexible PVC in perspective, some historical facts are of interest. Of the seminal discoveries leading to successful flexible PVC products, some were empirical and others the results of classical theory-driven research. Space allows only a few of these milestones to be described here. In Germany prior to World War I, the development of electric lighting resulted in a large excess of calcium carbide as acetylene lamps were phased out. Fritz Klatte, working for Chemische Fabrik Griesheim-Elektron, found that acetylene can react with HCl to form vinyl chloride monomer (VCM), which in turn, can be polymerized to PVC using free-radical initiators. Klatte was then commissioned to find uses for the hard, horny intractable PVC resin. He obtained several patents [1, 2], but Griesheim-Elektron let them expire in 1926. Klatte had shown promising leads but his team had not identified good enough plasticizers, heat stabilizers and process aids to make superior PVC replacements for celluloid, lacquers, oil cloth, coatings generally, fibers, and so forth. Other German companies then took up the challenge and had developed commercial PVC formulations by 1939. Shortages of conventional materials during World War II led to practical uses for both flexible and rigid PVC in wartime Germany. Post-war allied studies of German industry publicized these world-wide, e.g., [3]. Between 1926 and 1933, Waldo Semon at BFGoodrich in the United States discovered that tricresyl phosphate (TCP) and dibutyl phthalate (DBP) were effective plasticizers for PVC. In addition, he determined that basic silicate of white lead was an adequate stabilizer to make processable flexible PVC formulations plasticized with TCP or DBP. Goodrich then patented some of these formulations and commercialized coated fabrics and films made from them and trade-named them Koroseal® [4, 5]. Supplementing the antecedent German work, which sought volume uses for acetylene, Carbide and Carbon in the United States sought outlets for their large excess of ethylene dichloride

316

10 Flexible PVC

[References on Page 361]

(EDC), which was a by-product of their ethylene chlorhydrin manufacture. In 1930, they found that they could produce pure VCM by treating EDC with caustic soda. They made PVC homo- and co-polymers from the VCM and supplied samples to Waldo Semon and others. General Electric developed plasticized PVC insulation and jacketing for electric wires and cables. They trade-marked these Flamenol®, reflecting their resistance to ignition compared to the then-standard flexible insulations and sheathings made from natural rubber. In 1933, a patent covering the PVC plasticizer di-2-ethyl hexyl phthalate (known as DEHP or more often DOP) was issued to Lucas Kyrides of Monsanto Chemical Company [6]. Monsanto sold the patent to Union Carbide, because at the time Monsanto was more interested in developing plasticized polyvinyl butyral interlayers for automobile windshields and windows than in plasticized PVC. Ironically, it was Monsanto’s air-oxidation process for making inexpensive phthalic anhydride, which later made low cost DOP a reality in the United States [7]. T. L. Gresham at BFGoodrich ran extensive tests and identified DOP as the best available plasticizer for PVC homopolymers used for making flexible PVC. Just as in Germany, World War II stimulated the development of uses for flexible and semirigid PVC in the United States, Great Britain, and other allied countries. This was driven by shortages of rubber, leather, and other naturally derived raw materials. After World War II, it became obvious that many of the substituted plasticized PVC products had performance properties and costs superior to those of the natural items they had replaced. Many became established as accepted items of commerce.

10.2

Types of PVC Resins Used in Flexible Applications

The largest volume PVC resin type used in flexible and semi-rigid applications is aqueous suspension PVC homopolymer, made to have sufficient porosity in the particles to absorb enough plasticizer to meet the desired flexibility and hardness specifications of the intended end-product. For most flexible uses, the resins range from medium to high molecular weight. By normally accepted conventions this means from approx. 30,000 to approx. 60,000 number average molecular weight. In terms of commonly used tests based on dilute solution viscosity, this means from approx. 0.57 to 1.10 inherent viscosity, or from approx. 51 to 71 Fikentscher K [8]. Using a mercury intrusion porosimeter to measure resin porosity, it is common to require about 0.30ml/g or more for resins used for moderately flexible items made with monomeric plasticizers; and about 0.40ml/g or more for highly flexible items made with monomeric plasticizers and for items flexibilized with polymeric plasticizers. In addition to pore volume, pore diameter may be measured for PVC resins used in flexible products. Larger pore diameters contribute to faster uptake of plasticizers. Specialty soft plasticized PVC compounds requiring excellent compression-set properties call for the use of ultra-high molecular weight PVC resins with high porosity. These may exhibit molecular weights as high as 150,000 number average.

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317

Flexible PVC products injection-molded or extruded at very high shear rates may be made with low molecular weight PVCs having less than 30,000 number average molecular weight. However, using PVC resins with such low molecular weights sacrifices physical properties, including compression set, elongation at failure, tensile strength, tear strength, and fatigue resistance. It is difficult to make ultra-low molecular weight PVC resins having high particle porosities. Very low molecular weight resins are not used for highly flexible items, because items made with them are too weak. When formulating flexible PVC compounds for specific exacting uses, many compromises are often involved in selecting the best resin. For instance, other things being equal, the higher the molecular weight of the chosen resin, the higher the processing temperature needs to be during fabrication in order to achieve optimal fusion. Also, for a given processing temperature, use of higher molecular weight PVCs tends to give lower gloss on a finished product. If low gloss is required on an extruded product,which otherwise calls for use of a medium molecular weight PVC, it is necessary to specify a specialty “low gloss” resin offered by a few PVC manufacturers. The particle shapes, size limits, size distribution, and internal porosities of the PVC grains formed during suspension or mass polymerization processes are functions of many factors dealt with in other chapters of this book (see, e.g., Chapters 3 and 11). For flexible applications, particle sphericity tends to provide good bulk flow behavior along with efficient particle packing and higher bulk density in dry blends. Irregular, knobby resin particles, broad particle size distributions, and high particle porosity tend to give low bulk density and poor bulk flow behavior. For flexible applications, a relatively narrow PVC particle size distribution is desirable and is mandated by many users’ specifications. In the United States, a typical specification for general purpose PVC aimed at flexible markets is ≥ 99.8% particles must pass through a U.S. 40 mesh sieve, whose nominal openings are 420 microns (16.5 mils) square; 10% maximum by weight retained on a U.S. 60 mesh sieve, whose openings are 250 microns (9.8 mils) square; and 2% maximum through a U.S. 200 mesh sieve whose openings are 74 microns (2.9 mils) square. The aim point for average particle size (APS) is normally between 100 and 80 mesh. That is between 149 and 177 microns (5.9 and 7.0 mils). For special purposes, suspension and mass PVC resins may have their APSs skewed to finer or coarser numbers. The differences between mixing and processing methods for general-purpose flexible PVC and plastisol techniques for making flexible PVC products are described in Section 10.4 and in Chapter 9. The distinctions between PVC resins made for use in plastisols and general purpose PVC resins are addressed here. Plastisol resins are sometimes called dispersion resins, or, mostly in Europe, paste resins. They are made by emulsion, microsuspension, or special proprietary processes. Typically, they contain spherical, solid particles ranging in size from 0.1 to 1.1 microns in diameter. In bulk resin, these resin particles provide large surface area due to their small diameters. They are readily wetted by plasticizers due to their surfactant content and absorb some plasticizer during mixing. Because the particles are solid PVC, they absorb plasticizers quite slowly at typical ambient temperatures. The plastisol resins made by emulsion polymerization may be dewatered by spray drying or a combination of coagulation and conventional drying. Weak agglomerates of the basic emulsion resin particles are formed during drying. These are normally broken up to a substantial degree in grinding procedures before use of the

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resins. Most PVC plastisol resins are fluffy, have relatively low bulk densities compared to general purpose PVCs, and poor bulk flow properties. In most cases, they have to be given antistatic treatments to get them to flow satisfactorily in bulk handling systems. Consequently, plastisol resins are often bagged by the manufacturer before shipment to the customer. The systems used for bulk handling of plastisol resins are specially designed for this purpose and usually are dedicated to particular plastisol resins, and only used in large volume applications. Microsuspension PVC resins used in plastisols differ from their emulsion-polymerized counterparts both in particle size distribution and emulsifier content. Their emulsifier content is lower than that of most emulsion resins and this contributes to higher clarity in clear formulations, low moisture sensitivity of the end products, and little or no contribution to fogging in automobile interiors. Another class of PVC resins made for use in plastisols is called plastisol-extender resins. These are solid particle resins made by suspension polymerization with particles finer than general purpose resins, but coarser than microsuspension plastisol resins. The rationale for the use of such resins in plastisols is at least threefold. First, they are less expensive to make than true plastisol resins. Second, due to their coarser particle size on average, they contribute to more efficient particle packing in a plastisol when used in conjunction with a standard plastisol resin. This allows higher PVC content in a plastisol of given viscosity. Third, use of extender resin in a plastisol usually gives a rougher surface finish with lower gloss when this is desirable. This is important in sheet flooring constructions and plastisol-coated metal siding, for instance. Typical screen analyses for plastisol extender resins are: > 99.6% through 140 mesh, 5% maximum on 200 mesh and > 65% through 325 mesh, using U.S. standard screens. Their APS is thus equal to or less than 44 microns (1.7 mils), which is the size of the 325 mesh screen openings. Specialty PVC resins are made for producing flexible PVC items by rotational molding from powders and powder-coating of pre-heated parts. Some of them are specialty copolymers designed for blending with specialty homopolymers aimed to achieve particular performance results. Graft copolymers constitute another type of resin used for making vinyl TPEs and unplasticized flexible vinyl articles or lightly plasticized low extractibles flexible PVC. An example is Vinnol VK 801 listed as having 50% EVA content, and offered by Vinnolit (which was formed by the merger of Hoechst and Wacker’s PVC operations). Solution vinyl resins are a specialty dealt with in dedicated texts [9].

10.3

Particulate Architecture of PVC Resins Used in Flexible Products

This section deals with general purpose PVC resins made for use in flexible products. The particle architectures of other PVC resins are covered in Chapters 3, 9, and 11 of this book. In the early years of the PVC industry, a common problem in the manufacture of flexible PVC items by calendering and extrusion was the occurrence of gel particles, often referred to as

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“fisheyes”. These are relatively solid PVC particles, which do not absorb plasticizer as readily as the other resin particles in the formulation and which do not fuse readily at the processing temperatures used. Large fisheyes are the most objectionable. As the technology of polymerizing VCM has improved, the occurrence of fisheyes in flexible products has become a less frequent or severe problem. But it still exists, because the most frequent cause of fisheyes in PVC is inadequate cleaning of polymerization reactors between batches. PVC particles that go through two or more polymerization batches tend to have reduced porosity for absorption of plasticizers and to contain enough cross-linked, insoluble PVC to interfere with normal fusion. PVC manufacturers, who try to save money by reducing the frequency and diligence of reactor cleaning, produce resins with unacceptable content of gels. A critically important part of PVC resin morphology is its semi-crystalline nature. This is responsible for the resistance of flexible PVC to heat distortion, creep, and compression set. When plasticized PVC started replacing rubbers in insulation and sheathing for electrical wire and cable, rubber technologists asserted that no thermoplastic composition flexible and soft enough to satisfy wire and cable flexibility requirements could meet the required specifications for resistance to thermal distortion, cut-through, compression distortion, compression-set, agglomeration of multiple insulations within a cable, abrasion resistance, and so forth. They believed that only cross-linked rubbers could be soft and flexible enough and still maintain their integrity under harsh conditions of use and testing. They asserted that no thermoplastic such as plasticized PVC would replace cross-linked rubbers. But based on its physical and flammability properties, plasticized PVC took over most of the indoor wiring insulation and jacketing market in developed countries. It was after this happened that many of the reasons why plasticized PVC fulfilled these requirements were elucidated. Numerous thermoplastic elastomers (TPEs) have since been developed, which are both thermoplastic and resistant to compression-set, creep, abrasion, and thermal distortion. The seminal discoveries of the micro-morphology of rigid and flexible PVC are dealt with in detail in Chapters 3, 9, and 12 of this book. However, a terminology rationalizing the behavior of plasticized PVC is presented here. Rational terminology describing the morphology of PVC suspension- and mass-polymerized resin particles was proposed by Geil [10]. The lower end of Geil’s size hierarchy also applies to emulsion- and microsuspension-polymerized PVC resins. Table 10.1 is adapted from Geil. The crystalline microdomains in plasticized PVC, which are interspersed between amorphous regions, act like cross-links in rubbers in resisting creep, compression-set, and heat distortion, except that they are thermally reversible. Most of these crystallites melt at processing temperatures and re-form during cooling of fused plasticized PVC melts. The melting and recrystallization behavior, however, is complex because PVC contains several different kinds of crystallites, which melt and reform over a range of temperatures. Their melting and reformation is affected by the kind of plasticizer present. For instance, “fast fusing” plasticizers, of which butyl benzyl phthalate (BBP) is an example, depress the melting temperatures of the crystallites, thus promoting fusion at lower processing temperatures. “Low temperature” plasticizers, of which dioctyl adipate (DOA) is an example, primarily plasticize the amorphous regions of the PVC and have little effect on crystallite melting temperature, thus requiring higher processing temperatures than fast fusion or general purpose plasticizers.

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Table 10.1

[References on Page 361]

Size Hierarchy of PVC Particulate Phenomena

Term

Size

Description

Grain

70–420 µm diameter

Free-flowing powder produced in suspension or mass polymerizations

Agglomerates of primary particles

3–10 µm diameter

Formed during polymerization by coalescing of primary particles

Primary particles

1 µm diameter

Formed at single polymerization sites by precipitation of newly-formed polymer into discrete molecular aggregates

Domains

0.1 µm diameter

Observed after certain types of mechanical working of the PVC

Microdomains

0.01 µm spacing

Crystallites capable of holding portions of tie molecules, which maintain integrity of primary particles and span between primary particles in well fused PVC

Secondary crystallinity

0.01 µm spacing

Crystallites re-formed during cooling of a fused PVC melt

Antiplasticization by plasticizers is a phenomenon that PVC formulators must take into account. This is the stiffening effect of low levels of some plasticizers, which takes place after fusing and cooling a formulation. It occurs most often with fast fusing plasticizers, to a moderate extent with general purpose plasticizers, and much less with low temperature plasticizers. It is observed at plasticizer concentrations of approx. 1 to 15 parts per hundred resin (phr). But this range varies with plasticizer efficiency. Antiplasticization is explained as being mainly caused by the promotion of crystallite formation by low levels of plasticizers without an offsetting plasticization of the amorphous regions, which occurs at higher plasticizer levels. It must be noted that these extra crystallites are of the low melting type and do not raise the heat distortion temperature of PVC, which is reduced by as little as 1 phr of most plasticizers or plasticizing stabilizers, such as the thiotins. Other factors discussed in the literature as contributing to antiplasticization effects are hydrogen bonding, Van der Waals forces, steric hindrance, small, localized increases in molecular order, and decreased free volume [11, 12]. Many authors have contributed to understanding the relationships between PVC particle morphology, fusion, and the processing rheology of PVC. Some of these are Collins and Krier [13], Berens and Folt [14], Singleton and Isner [15], Pezzin [16], Collins and Daniels [17], Lyngaae-Jorgensen [18], Summers [19, 20], and Rosenthal [21]. Their original findings relate to both rigid and flexible PVC.

10.4 Favored Processing Methods for Flexible PVC

10.4

321

Favored Processing Methods for Flexible PVC

The natures of the PVC resins and other additives used in flexible PVC compounds have conditioned the selection of efficient material handling procedures for making the primary mixes. The most common processing type is “dry blending”, also known as “powder mixing”. More than 85% of suspension and bulk PVC resins are initially processed by dry blending. Historically, large volume, jacketed ribbon blenders and various other kinds of available mixing equipment were used. For flexible PVC formulations using liquid plasticizers, resins with sufficient porosity to absorb the liquids are intermixed with the other ingredients to get them mutually well dispersed down to the particulate level. The desired resulting product is a freeflowing powder. For most formulations, it is necessary to heat them during mixing to facilitate absorption of the plasticizers into the resin and to avoid loss of product through formation of undesirable lumps of wetted pigments and fillers. The main drawbacks to the large ribbon blenders were that cycle times were surprisingly long to achieve good blend uniformity and it required considerable technical effort of a largely empirical nature to determine the optimal order and timing for addition of individual ingredients. It was not unusual to produce a fair amount of off-grade product during the process of mixing cycle optimization when new formulations were introduced. Much theoretical work has been done on dry blending of flexible PVC formulations. Diffusion of liquid plasticizers into PVC resin particles is a kinetic process whose rate varies inversely with the viscosity of the plasticizer [22]. During dry blending, liquid plasticizers penetrate the amorphous matrix surrounding the crystallites in the PVC resin particles. The crystallites remain intact because of the thermodynamic barrier which prevents fusion until a certain temperature is reached. This depends on the particular plasticizer or plasticizer mixture being used and the molecular weight of the PVC resin. While studying the Flory-Huggins interaction parameter, χ, for various plasticizers with PVC, Anagnostopoulos et al. [23], developed a microscope hot-stage fusion test, whereby the temperatures at which PVC resins of different molecular weights fuse in particular plasticizers are readily measured. These fusion temperatures are higher than dry blending drop temperatures. A dry blend is a non-fused mixture of PVC, plasticizer, and other ingredients. Park published a method for running the pressure stain test for PVC dry blends [24], which determines the point in a commercial dry blending cycle at which the blend no longer stains brown paper or cigarette paper sheets between which it is pressed. This correlates with the small-scale laboratory dry time test ASTM D2396. From a practical viewpoint, a dry blend cycle is complete when the dry blend, after cooling, flows well through a funnel or an extruder hopper and does not cake during storage. The pressure stain test is a way of predicting when this state has been reached while the blend is still hot and in the blender. Commercial blenders are stopped during a cycle and a “thief” is used to take small samples for testing. Dry blends of plasticized PVC formulations are over-dried if they are dusty and fuse with more difficulty in subsequent processing operations, such as extrusion, than a blend made with a shorter dry blending cycle, which is slightly damper and less dusty, but still has good bulk flow and non-caking behavior.

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Low speed dry blending mixers are generally of the ribbon mixer type. These are bulky semicylindrical horizontal mixers supplied with a jacket surrounding the side and bottom surfaces through which heat or cooling can be applied. For mixing, radial blades are mounted on a horizontal rotating shaft, which may turn at 40 to 70 rpm. Intermediate speed dry blenders generally consist of a horizontal cylindrical jacketed chamber with axially mounted blades, which can be run fast enough to achieve fluidization of the PVC during blending. Where the investment can be justified, batch high speed, high intensity, dry blending mixers are used for flexible PVC. Typically, these consist of a vertical drum with a dish bottom through the center of which a drive shaft penetrates. One or more sets of mixing blades are attached to the drive shaft. A baffle penetrates the top of the mixer, which can be tightly sealed to the drum. By running the drive shaft at speeds appropriate for the particular formulations being mixed, mixing occurs with the generation of sufficient frictional heat to raise the batch temperature to a desired drop temperature, such as 105 °C in 4 to 8 minutes. These intensive mixers all have Froude numbers* much greater than 1. Typical shaft speeds are 500 to 1200 rpm with blade tip speeds between 10 and 50 m/s (33 and 164 ft/s). Typical batch capacities are 80 to 200 kg (176 to 440 lb). Some of these mixers are designed to sustain high vacuum or air purge during mixing for removing traces of unwanted volatiles such as moisture or residual vinyl chloride monomer [25]. The PVC and other ingredients are fluidized during blending in these mixers. The high speed mixers generally dump their batches into lower speed jacketed rotary coolers having Froude numbers less than 1. The coolers introduce little frictional heat and cool by conduction into their cold water jackets. To avoid adventitious condensation of moisture from the ambient air, freezing brines are not used in these cooler jackets. Specialty dedicated blending procedures are used for flexible PVC compounds made with solid plastifiers, polyolefin elastomers (POEs), and compatibilizers. * Froude number = R ω2 g where R = blade radius ω = angular velocity, radians/s g = gravitational acceleration in consistent units Trade names for typical intensive mixers are Henschel, Papenmeier, Welex, and Littleford. To shorten mixing cycles, high viscosity plasticizers (polymeric liquids) are normally preheated before addition to the mixer. In many factories the cooled dry blend is air conveyed to interior storage before being fed to fabrication equipment or intermediate processing units. It must not “cake” during storage, and it must flow readily and uniformly in the feed hoppers of equipment such as extruders and proportioning devices, for example, those used for color control operations. From interim storage, dry blends are conveyed to fluxing devices, such as compounding extruders, Banbury mixers, Farrel Continuous Mixers, Buss Ko-Kneaders, and fusion systems made by Coperion, Krauss-Maffei, Leistritz, Reifenhauser, and many others. The hot, fluxed

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323

PVC may then go to pelletizing units and be sold as compound pellets. Alternatively, it may be fed directly to fabricating equipment such as extruders, calenders, or injection molding presses. Another type of mixing for preparing flexible PVC goods involves compounding of liquid plastisols. This is done in liquids mixing equipment using plastisol grade PVC resins, suitable plasticizers, and other additives such as stabilizers, fillers, pigments, and other ingredients. These may include viscosity modifiers, air release agents and thinners, which are dedicated for use in plastisols, modified plastisols, and organosols, discussed later in Section 10.5 and Chapter 11. Inverted conical Nauta mixers, pony mixers, high speed Cowles Dissolvers, medium speed Ross Power Mixers, and three-roll mills are typical of the mixers used for manufacture of plastisols. In most cases, plastisols are de-aerated after mixing and stored at controlled temperatures, preferably at or below 23 °C (73 °F), to prevent heat-induced viscosity build-up and other changes in their desired rheology caused by aging. Plastisols are fabricated into end-use items by liquids-forming procedures followed by gelling and fusion in ovens. Plastisol-derived sheet flooring is the largest volume use for plastisols. Many of these flooring constructions are sophisticated, involving a flexible backing covered with a foamable plastisol on which a design is printed with some inks containing a foaming inhibitor and overlaid by a clear plastisol wear layer.

10.5

Designing Flexible PVC Compounds

10.5.1

Formulation Development

There are two general approaches to formulating flexible vinyl materials. When a project involves a novel untried concept for which the technical requirements of the product are unknown, a set of tentative needs is “guesstimated”. Trial formulations, whose properties bracket the tentative needs, are developed and parts or items submitted to field trials. The process is iterated until a satisfactory product is developed or the project is abandoned as impractical or too costly. When the technical and economic requirements for the new product are known and considered feasible using flexible PVC, these are listed and used as guidelines. They involve physical and optical properties, stability to heat and light, decorative, electrical and toxicological requirements, density, odor, allowable cost, and so forth. Specifications and necessary qualification tests must be defined, including needs to run field trials at customers or evaluations at outside testing services, such as Underwriters’ Laboratories or suppliers’ or customers’ laboratories. The total cost of the development program and the potential profitability of the new product need to be estimated to justify proceeding with development. In developed countries, the existing markets for flexible PVC are defined and competitive. In some cases, several plastics and plastic alloys, whose economics are close together, are competing and flexible PVC is simply defending or increasing its market share.

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In undeveloped countries, flexible PVC has many opportunities to improve the standard of living of the people by satisfying unfulfilled needs and replacing natural materials, over which flexible PVC has some clear-cut advantages. When the technical requirements for a new product cannot be satisfied by flexible PVC, it is industry practice to refrain from submitting a defective vinyl part for trial. There should be at least a 50% probability of success to justify submittal of trial items.

10.5.2

General Problems in Formulation Development

In the flexible PVC industry, the term compounding has meanings specific to whether it is applied to solid or liquid systems. For solids, the steps generally are mixing a dry blend from solid and liquid ingredients, or mixing a wet blend, then fluxing the dry or wet blend until it will flow properly in forming equipment, shaping the melt into an end product, and cooling the hot product before it loses its desirable shape. In the case of plastisols and organosols, compounding means mixing the solid resin and other solid ingredients uniformly into liquid plasticizers and other liquid ingredients so as to achieve a targeted rheology suitable for follow-up liquids-forming operations (reverse roll coating, spread-coating, rotational molding, dip coating, spray coating, strand coating, injection molding, etc.), followed by fusion in a suitable oven or microwave treatment, and finally succeeded by cooling before the product loses its desirable form and shape. For PVC latexes, ingredients are normally added in waterborne solution, emulsion, or dispersion and mixed into the base latex under gentle agitation, so that it does not coagulate the latex or mixture of latexes. The compounded latex is then applied to the substrate to be treated by common latex application methods, such as dipping, impregnation, coagulation, reverse roll coating, spraying, and so forth. The water is then evaporated and, if necessary, the product is fused and cooled. When more than one company is involved in developing a product, the importance of accurate and complete inter-company communications cannot be over-emphasized. An example of a product failure, which was corrected, was the manufacture of a large number of electric alarm clocks with high-impact polystyrene (HIPS) cases around which the vinyl-jacketed cords were tightly wound before packaging them. After some of the clocks were sold it was found that the HIPS cases had all been marred by migration of plasticizer from the cord jackets. The maker of the cords had not been told that their jackets were going to be in direct contact with HIPS. The problem was solved at a modest increase in cost of the cords by replacing the commodity phthalate originally used with a blend of higher molecular weight phthalate and trimellitate and modifying the packaging procedure. The vinyl industry has sometimes been victimized by callous formulators, who make items from the cheapest formulation that satisfies initial requirements without providing suitable in-use and aging behavior. An example with vinyl-coated bookbinders was one in which the vinyl coating was plasticized with di-hexyl phthalate. When placed in contact with photocopy inks this binder “lifted” the inks and ruined the copy’s appearance. A second example was a “non-migratory” binder plasticized with epoxidized soy bean oil (ESO) as sole plasticizer at about 50 phr. Initially, this binder exhibited exemplary non-migratory behavior. But after

10.5 Designing Flexible PVC Compounds

325

about a year of exposure to fluorescent light and some sunlight, the binders with this composition were ruined by tacky exudates which gave them a surface like fly paper. The problems were solved at some increase in formulation cost by using a combination of medium molecular weight polymeric plasticizer and a high molecular weight phthalate for the nonmigratory binder and just a high molecular weight phthalate for the regular binder. Non-availability of optimal raw materials at competitive cost is a problem frequently faced by vinyl compounders. For instance, compounder A may have bulk storage for PVC resins J, K, and L along with plasticizers P, Q, and R. Compounder B has more extensive storage facilities and greater purchasing power for getting special deals on resins, plasticizers, and other raw materials. Compounder A, due to his lower overhead costs, has an advantage as long as he confines himself to his niche markets. But compounder B’s products have better cost/ performance in other markets due to his superior raw materials situation and more versatile compounding equipment. In competitive situations, compounders may need to modify their mixing and fluxing procedures or upgrade their equipment. For instance, slow dry blending may be causing a production bottleneck caused by the use of a polymeric plasticizer or a slow dry blending monomeric plasticizer, such as DTDP. One approach is to reformulate, but the bottleneck may be more readily eliminated by pre-heating the plasticizers before adding them to the mixer and/or use of a faster blending, more porous PVC resin. It is a luxury to work in a plant where the mixing, fluxing, and forming equipment are all optimized for the products being manufactured. In older plants, it is common practice to optimize throughput by selecting raw materials. For these situations, compounders use the lowest molecular weight PVC resin that satisfies end-use requirements, along with as much process aid that economics allow, and as fast-fusing a plasticizer system that is consistent with end product requirements. In calendering plants producing thin gauge films, it is normal to pass the fluxed feed through an extruder-strainer to eliminate adventitious metal contamination, which could cause very expensive damage to the finishing rolls of the calender. These strainers need to be designed for the rheology and throughput of the stocks run on the calender. A strainer designed for highly flexible formulations tends to overheat rigid stocks at typical desired throughputs. This may limit the flexibility of calender lines with regard to switching back and forth from flexible to rigid stocks. Similarly, continuous compounding mixers need to be designed for the rheology and the throughput of the stocks they are handling. Equipment designated to the manufacture of diced or pelletized compounds also needs to be suited to the rheology of the range of compounds being manufactured in order to achieve optimal through-puts. Versatility can be achieved by stocking a range of parts for compounders, pelletizers, and dicers. End-product performance failures in terms of resistance to compression-set, retention of elongation after oven aging, fatigue after repeated flexing, abrasion resistance, plasticizer extraction by oils or fats, and environmental stress-cracking of rigid plastics in contact with a flexible PVC part often can be corrected by switching to higher molecular weight PVC resins in the rigid PVC, higher molecular weight plasticizers, and/or introducing specialty additives such as process aids or “plastifiers” in place of part of the plasticizer system in the flexible PVC. To maintain throughput in spite of higher melt viscosity, the formulator usually has to adjust the stabilizer and lubricant systems to accommodate higher stock temperatures.

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For composite products, such as coated fabrics or carpets backed with flexible vinyl materials, either by plastisol coating or laminating, the fusion temperature of the vinyl must not exceed the tolerance of the fiber in the fabric or carpet. This is critical with some polyolefin fibers. Suitable low fusion temperatures can often be achieved in the vinyl composition by using copolymer resins and fast-fusing plasticizers such as BBP or DHP. For vinyl automobile undercoats and sealants, paint oven temperatures dictate selection of low-fusing resins and plasticizers. Fully fused vinyl undercoats survive considerable abuse and prolong the life of a vehicle, particularly in regions where roads are heavily salted in winter. Poorly fused undercoats fail prematurely. In flexible PVC products which are foamed during processing, whether they use hot melt or plastisol technology, the fusion characteristics of the resin-plasticizer system must be matched to the behavior of the blowing agent-kicker-stabilizer in order to achieve manufacture of good quality uniform cell-size closed-cell foams. For consistently satisfactory results, excellent control of the rheology and time-temperature profiles is essential. Extrusion, calendering, injection molding, thermoforming, and compounding processes are sometimes marred or shut down by a phenomenon known as plate-out. This comprises the formation of sticky deposits on the hot surfaces of processing machinery, including mill rolls, calender rolls, extruder screws, dies, molds, and so forth. The surface appearance of finished goods deteriorates in the early stages of a plate-out problem. If this is not recognized and corrected, it may lead to catastrophe such as “shipwreck” on a fast-running calender. This occurs when the calender web stops releasing from the final calender roll and folds back into the calender nip. This often causes severe degradation before the calender can be shut down. An expensive and labor-intensive clean-up always follows a calender “shipwreck”. Plate-out is caused by precipitation and transfer of oxidation and/or hydrolysis products from components of the formulation (usually the stabilizer and lubricant system). The initial plateout, if not observed and eliminated, then proceeds to build up by occluding solids from the formulation such as colorants, fillers, and smoke- and flame-retardants. If ignored, the plateout eventually stops the plastic product from releasing from the hot/coated metal or releases a badly marred product from a coated mold, calender roll, or die. Products marred by plateout usually exhibit poor printability. Sometimes, plate-out can be eliminated “on the run” by temporarily increasing the amount of an abrasive, such as talc, or lubricant such as stearic acid, in the formulation. Plate-out is prevented in some formulations by including a small amount of a scouring agent such as talc or some grades of silica. Often, an operation plagued by plate-out has to be shut down and the affected metal surface manually or operationally cleaned. Raw materials suppliers should be consulted about persistent plate-out problems. Lippoldt [26] published an extensive study of plate-out. Pollution regulations differ so widely from country to country and region to region that general rules for coping with them are meaningless. Processors need to dispose of solid, liquid, and gaseous wastes and vapors generated in the processing of flexible PVC in compliance with local regulations (see Chapter 18).

10.5 Designing Flexible PVC Compounds

10.5.3

327

Properties Often Specified for Semi-Rigid and Flexible PVC Products

Tensile strength and elongation at failure (ASTM D638) depend primarily on the level and type of plasticizer or other flexibilizer in the formulation, but also on resin molecular weight. Higher molecular weight resins in fully fused formulations give higher tensile strength and elongation at failure. 100% Modulus, defined as tensile stress at 100% elongation, is a useful measure of the stiffness of plasticized PVC, because it is relatively easy to measure accurately and reproducibly. For historical reasons, DOP is generally recognized as the benchmark plasticizer for PVC. With a medium-high molecular weight PVC, at 23 °C, DOP at 25 phr gives a 100% modulus of about 22.8 MPa (3300 psi), which is classified as semi-rigid. Between 35 phr DOP and approx. 85 phr DOP (100% modulus 4.48 MPa or 650 psi), PVC is considered flexible Above 85 phr DOP, PVC is called highly flexible. When comparing the efficiencies of different plasticizers, substitution factors (SFs) compared to DOP are generally used. However, most authors calculate these from Shore hardness measurements, which do not correlate exactly with 100% modulus, see Chapter 5 for more details. The brittleness temperature of flexible PVC is generally measured by ASTM D-746, which is a cold impact test run on specimens punched from standard test sheets 1.9 ± 0.25 mm (75 ± 10 mils) thick. However, on calendered PVC films, some people prefer to use the Masland Impact Test (ASTM D1790). This test is run on films 10 mils (0.25 mm) or less in thickness under specified impact conditions. In this test, the results are sensitive to the direction of sampling and the direction of fold due to the molecular orientation effects of calendering. Outside the United States, local testing procedures may be preferred. In commercial laboratories the low temperature properties of flexible PVC are often estimated from stiffness measurements run by ASTM D 1043, which measures apparent modulus of rigidity, G, at different temperatures. The way D1043 is run, the angular deflection may extend beyond the elastic limits of the plastic at lower temperatures, so that the result is “apparent” rather than an actual modulus of elasticity, E, as measured by ASTM D 747. To convert G to E, the simplifying assumption is made that E = 3 G, which is only true if Poisson’s Ratio for the material under test conditions is 0.5. The temperature at which E = 931 MPa (135,000 psi) is reported as Tf, the flex temperature, which is the temperature at which the material is considered to have lost most of its elastomeric properties. Sometimes T4, (E = 6.90 MPa or 10,000 psi), regarded as the upper end of a material’s useful temperature range, is also reported. Academic laboratories generally use more precise methods of measuring moduli as functions of temperatures. Abrasion resistance of flexible PVC is often measured by the Taber Abrasion Test (ASTM D 4060). Results are reported as weight loss per 1000 cycles under conditions agreed to between the interested parties. Results are important for automobile undercoatings, boot and shoe soles, floor coverings, mine belts, and electrical cords for use under harsh conditions. The hardness of flexible PVC materials is commonly measured by Shore Hardness (ASTM D 2240) using the A scale. Sometimes the D scale is used on semi-rigid compounds with plasticizer levels at or below 40 phr DOP equivalent. Conditioning of test specimens at the test temperature is critical. Aging after processing is also very important. This is explained as being due to the slowness with which PVC crystallites reform after processing. For accurate

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results, at least one week of aging at 23 °C (73 °F) is recommended. Hardness readings increase with age after molding or other processing. Note also that Shore hardness readings drop rapidly during the first several seconds after specimen contact. For vinyl plastics, ASTM specifies taking Shore A hardness readings after 15 seconds. However, many commercial laboratories use 10 second Shore hardness. Test sample thickness is critical. ASTM specifies using specimens molded to 0.64 cm (0.25 in) thickness. On calendered or extruded films and sheets, many laboratories stack several thicknesses, but this is less precise than using specimens molded to 0.64 cm thickness. In commercial quality control testing, operators never wait until Shore hardness has stabilized before testing. They specify definite aging and conditioning periods of usually less than a day. In the United Kingdom, British Standard Softness (B.S. 2782:32A) is generally specified. This test correlates well with 15 s Shore A hardness in the sense that a plot of Shore A hardness against British Standard softness is a straight line. The same considerations regarding aging and conditioning of flexible PVC specimens apply as for Shore Hardness testing. For flexible PVC compounds used as primary insulation on electrical wires and for electrical tapes, electrical properties are critical. Tests commonly used include dielectric constant (ASTM D150), dielectric strength (ASTM D149), and volume and surface resistivity (ASTM D 257). The Underwriters’ Laboratories Insulation Resistance Test is specified for insulation compounds to be used on wires slated for use in wet locations. The fire resistance of most flexible PVCs is less than that of rigid PVC. However, formulations can be devised to meet stringent flammability requirements, such as those for plenum cables. These require enough flexibility for installation in confined spaces and must also pass the UL-910 (NFPA 262) test [27]. Other flammability tests often used on flexible PVC products include: the UL-VW-1 Vertical Wire Flame Test; the Oxygen Index Test (ASTM D2863); the DOT 302 MVSS Test for materials used in automobile interiors; the UL-94 Test run in the horizontal or vertical modes; the UL Vertical Tray Flame Test (UL 1581 for tray cables); and the UL-1666 Test for riser cables. The Cone Calorimeter Test, ASTM E1354, can be used to rank small samples of flexible vinyl materials for rate of heat release after ignition, ease of ignition, and emission of obscurational smoke. The test is versatile, because the heat flux to which samples are exposed can be varied from about 10 to 100 kW/m2. Rate of heat release, sample mass loss rate, and smoke are measured or calculated from measured parameters. For smoke evolution, the NBS Smoke Chamber Test (ASTM E662) is still used, because many laboratories have the equipment. Cone calorimeter results are acknowledged to be more meaningful. Measuring the toxicity of smoke from burning PVC is complex and has been controversial. It is discussed in Chapter 13, along with general flammability testing issues. Other tests used on flexible PVC products include retention of elongation after oven aging, resistance to extraction of plasticizers by chemicals, weatherability, stain resistance, and effects on taste and odor of foods packaged in flexible PVC. For niche products, many other specialized tests are used. Physical and electrical testing of PVC are discussed in Chapter 12.

10.6 Additives Used in Flexible PVC Compounds

10.6

Additives Used in Flexible PVC Compounds

10.6.1

Liquid Plasticizers and Solid Flexibilizers

329

Primary plasticizers are the principal additives responsible for flexibilizing PVC. These are classified as monomeric, polymeric, epoxy, and specialty flame-retardant plasticizers. They are low volatility liquids whose polarity and other characteristics are such that they are sufficiently compatible with PVC not to be readily squeezed out of plasticized PVC by moderate pressure [28]. Secondary plasticizers are low volatility liquids whose compatibility with PVC is such that they can be used along with primary plasticizers as part of the plasticizer system, but which exude if used as sole plasticizer. Chlorinated paraffins are common examples of secondary plasticizers for PVC, used because they are low in cost and less flammable than most primary plasticizers. There are several types of solid flexibilizers for PVC, which include compatible nitrile rubbers, compatible polyurethanes, compatible polyesters, ethylene-carbon monoxide-vinyl acetate terpolymers, and some poly-acrylates. Many people refer to these materials as PVC “plastifiers” to distinguish them from liquid plasticizers. These solid materials are chiefly used in PVC thermoplastic elastomer (TPE) compounds and specialty PVC materials, some with low flammability and low smoke evolution, for use in applications such as plenum cables. The volume cost of these plastifiers is higher than that of most plasticizers. When used as sole flexibilizer for PVC, plastifiers give compounds with higher melt viscosity than corresponding plasticized compounds of equivalent hardness and flexibility. Plasticizers for PVC and theories of plasticization are discussed in detail in Chapter 5. A few additional practical comments will be offered in the following. The definition of plasticizers adopted by IUPAC in 1951 is still generally accepted: a substance or material incorporated in a material (usually a plastic or elastomer) to increase its flexibility, workability, or distensibility. A plasticizer may reduce the melt viscosity, lower the temperature of a second order transition, or lower the elastic modulus of a product. In comparison, a process aid improves processability without necessarily reducing melt viscosity or the stiffness of the product. Practical requirements for a successful plasticizer for PVC are that it be cost-effective, stable, low in color, compatible with PVC, readily dispersible in PVC, low in volatility, low in odor, low in toxicity, have good permanence, and must not interact unfavorably with other needed formulating ingredients or otherwise compromise the end-use properties of the product in which it is employed. Plasticization theory works reasonably well in quantifying the behavior of single plasticizers in PVC. When mixtures of plasticizers of different chemical families are used, the correlations between pragmatic performance parameters and scientific measurements on idealized systems become too loose to maintain the latter as standards for predicting the performance of plasticized PVC in the marketplace. However, on individual new plasticizer candidates, calculated or measured entities such as hydrogen bonding parameters, Flory-Huggins interaction parameters, dielectric constants, dipole moments, and solubility parameters can be used to predict compatibility with PVC.

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Primary plasticizers used in PVC fall into the followingchemical families: dialkyl orthophthalates, alkyl benzyl phthalates, dialkyl tere-phthalates, epoxides, aliphatic carboxylic diesters, polyester-type polymerics, phosphate esters, trimellitate esters, benzoate and dibenzoate esters, alkyl sulphonic esters of phenol and cresol, and miscellaneous types. The 2003 Modern Plastics World Encyclopedia lists 402 plasticizers of which 279 are indicated to be compatible with PVC. Forty suppliers are listed. Dialkyl ortho-phthalate esters are the most frequently used plasticizers in PVC applications. The alcohols range from hexyl (C6) to tridecyl (C13), and may be linear or branched. Increasing the degree of branching in the alcohol gives a plasticizer with higher volatility, greater susceptibility to oxidation, poorer low temperature brittleness in PVC, and higher volume resistivity in formulated PVC. Di-2-ethyl hexyl phthalate, also known as DEHP or DOP, is the industry standard general purpose (GP) plasticizer against which other dialkyl phthalates and PVC plasticizers generally are compared via efficiency factors (EF). In addition to making recommendations on how to use plasticizers based on their experience, several plasticizer suppliers calculate the exact concentrations of their plasticizers required with a standard PVC resin to produce a desired set of physical properties, if it is attained in PVC plasticized with their products. The selection of the best phthalate plasticizer to use for a particular application is guided by economics, toxicological regulation (if required), ease of processing, and performance in end-use. Aliphatic carboxylic diesters, such as the phthalates, are generally identified by acronyms. They are based on aliphatic dibasic acids esterified by alcohols ranging from C7 to C10. The dibasic acids have carbon numbers varying from C5 (glutaric) to C10 (sebasic). Di-2-ethylhexyl adipate is known as DOA. The azelates and the adipates do not lower the melting points of PVC crystallites as much as the corresponding phthaltates do, but they flexibilize the amorphous regions of the PVC more efficiently, and they are lower in molecular weight and specific gravity. Hence, they impart higher flexibility weight-for-weight and better low temperature properties. DOA is less compatible with PVC than DOP and is considerably more volatile. DOA is regulated by FDA for use in produce-wrap and meat-wrap films. Most polyester-type polymeric plasticizers are condensation products of glycols with dibasic organic acids. 1,3 buylene glycol and adipic acid are the most often used starting materials. C8 or C10 alcohols are commonly used for terminating the polymerizations at average molecular weights between 1,000 and 8,000. Acid-terminated polymeric plasticizers are less environmentally stable than their alcohol-terminated analogs. The chief advantage of polymeric plasticizers over general purpose monomeric plasticizers is greater permanence. The chief disadvantages are higher cost, lower plasticizing efficiency, poorer low temperature properties, and reduced environmental stability of end products exposed to combinations of warmth, humidity, UV light, and/or active microbial cultures. Practical formulations often contain mixtures of polymeric and monomeric plasticizers. Trimellitate ester plasticizers are made by reacting trimellitic anhydride with plasticizer-grade alcohols. Tri-2-ethyl hexyl trimellitate is known as TOTM. These esters represent the state-ofthe-art in low volatility monomeric plasticizers. Their principal uses are in 90 °C- and 105 °Crated electrical wire insulations and jackets and other applications requiring plasticizers volatility lower than is attainable with higher molecular weight phthalates. Adams reviewed the status of trimellitate plasticizer use in the United States [29].

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331

Epoxy plasticizers have oxirane oxygen groups in their molecules formed by the epoxidation of olefinic double bonds in their starting raw materials: O O O // / \ // R–CH–CH– + CH3–C–OH R–CH = CH– + CH3–C–O–OH ⎯⎯→ cat. They are used as co-stabilizers along with suitable mixed metal stabilizers and some of the newer types of stabilizers. Epoxidized soy bean oil (ESO) and epoxidized linseed oil (ELO) are the most widely used epoxides. They have the disadvantage of being food nutrients for molds, some bacteria, and fungi. Sound formulators use epoxides at low levels because the oxirane oxygen group has a strong compatibilizing action with PVC. Use of higher levels of ESO or ELO risks formation of tacky “spew” resulting when the oxirane oxygen is photo-oxidized or hydrolyzed. To get the stabilizing action of oxirane oxygen without the risk of exudation or microbial attack, some formulators use epoxy resins even though these cost more than ESO or ELO. Phosphate ester plasticizers made from phosphorus oxychlorides have the general structure: (R1O)(R2O)(R3O)P = O Where, R1, R2, and R3 are alkyl or aryl moieties. Numerous triaryl and alkylaryl phosphate plasticizers are available. They are more expensive than phthalate esters, have excellent compatibility with PVC, and burn with lower heat release than phthalates. The principal use of phosphate esters is in flame-retarded and smoke-suppressed formulations. Dipropylene glycol dibenzoate exemplifies the benzoate ester plasticizers, which are used mostly in stain-resistant flooring. Several miscellaneous plasticizers are used enough to be worth mentioning. Some citrate esters, such as acetyl tri-n-hexyl citrate and butyryl tri-n-hexyl citrate, find specialty uses in some blood bags and food wraps. Citrates are also used in toys produced by the plastisol process, where the toy is intended for use by young children. Polymerizable plasticizers are available for specialty applications such as insulation on electrical wires, which have to be connected by soldering and where retraction of the insulation due to heat must not occur. Alkyl sulfonate esters of phenol are sold in Europe under the trade name Mesamoll®. Texanol Isobutyrate® (TXIB) is used as a volatile, viscosity-reducing plasticizer/diluent in plastisols for flooring sheet-goods and coil coatings. Specialty flame-retardant plasticizers are exemplified by Great Lakes DP-45, which is a tetrabromophthalate ester with outstanding fire-retardance and low plasticizing efficiency due to its high molecular weight and high specific gravity. Secondary plasticizers, extenders and diluents include chlorinated paraffins, naphthenic hydrocarbons, alkylated aromatics, and some linear paraffins.

10.6.2

Lead-Based Stabilizers

Stabilizers have been used in flexible PVC compositions to prevent degradation during processing and forming into finished shapes. Mainly due to pressures from environmentalists,

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but also partly due to the results of fundamental research, there have been more changes in stabilizers during the last 20 years than in any other aspect of PVC technology. Historically, lead-based stabilizer systems were the first commercially successful ones for PVC. They are generally fine particle size basic solids, which disperse readily in flexible PVC compositions so that there are no significant unstabilized volume elements. Atomic chlorine and HCl released from degrading PVC, readily form basic lead chlorides which do not promote further degradation of PVC. Related theory is covered in Chapter 4. A simple way to generalize the action of heat stabilizers in flexible PVC is the following: thermal degradation of PVC molecules starts at defect structures which may take several forms but all involve labile chlorine atoms. Unless an active stabilizer molecule is close to the site from which labile chlorine releases from PVC, a progressive “unzippering” of successive HCl molecules from the PVC is initiated. Stabilizers prevent this as follows: | + —C–ClL | labile chlorine on PVC

M–S— stabilizer

→

| —C–S— | stabilized PVC

+

M–Cl spent stabilizer

Desired features for stabilizers used in flexible PVC include that they should preferably be colorless, odorless, nontoxic, tasteless, non-staining, non-volatile, nonconductive, nonextractible, non-migrating, non-plasticizing, non-plating, resistant to oxidation and hydrolysis, non-exuding, non-chalking, and non-lubricating or only weakly lubricating. They should also be low in cost, shelf stable, readily available, easily dispersed in PVC, compatible with PVC and other additives, homogeneous, heat stable, light stable, environmentally acceptable, chemically stable, easy processing, and efficient in stabilizing action. Even though finely powdered litharge (PbO) was a fairly effective stabilizer for flexible PVC, Waldo Semon abandoned it early, because of its color, in favor of basic carbonate of white lead (BCWL). Over the years, this has been replaced by tribasic lead sulfate (TBLS), dibasic lead phthalate, and dibasic lead phosphite, all manufactured as fine white powders. TBLS has the lowest cost of these three, but is sufficiently basic to hydrolyse some polymeric plasticizers. Dibasic lead phosphite is the most expensive of the three, but is favored in some applications because it has more light-stabilizing action than TBLS or dibasic lead phthalate. All these lead stabilizers sulfur-stain on contact with mercaptides or hydrogen sulfide. They have to be handled carefully due to their tendency to “dust”. When breathed or ingested by humans, they are slightly toxic, but only slightly so due to their low solubilities in water or saliva. They have refractive indices between 2.0 and 2.25, which are high enough to make them unusable in transparent or translucent applications due to their pigmenting action. They are among the most cost-effective stabilizers for plasticized PVC, but are generally being phased-out due to pressure from environmentalists on the PVC industry to stop using leadcontaining stabilizers, pigments, or lubricants. In the United States, problems of worker exposure to lead have been overcome by handling the powdered lead stabilizers in closed bulk air pallet systems, in pre-weighed batch charges (each in its own PVC bag), or in prilled stabilizer-lubricant one-packs. In the United States, the permissible exposure limit (PEL) for airborne lead is 0.05 mg/m3 [30].

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333

For building wire insulations for use in damp or wet locations, lead stabilizers perform the best. Many suppliers have qualified compounds, which pass long-term insulation-resistance testing. This requires immersion in water at 75 or 90 °C for twenty-six (26) or more weeks without significant loss of dielectric properties. Lead-replacement and low-lead systems continue to be actively evaluated for these uses. TBLS and dibasic lead phthalate and dibasic lead phosphite have low solubilities in water at pH 6 to 8 (neutral). But, due to the amphoteric nature of lead, they are more soluble if the extractant is buffered to be acidic or alkaline. When lead-stabilized vinyl insulation or sheathing materials are ground to a fine particle size and subjected to EPA’s Toxic Characteristic Leaching Procedure (TCLP), which is run under acidic conditions and allows a maximum lead concentration in the leachate of 5mg/l, marginal or failing results may be experienced. Therefore, lead-stabilized wire and cable PVC scrap is either recovered or sent to secure landfills, which are expensive. By contrast with lead, 100 mg/l of barium is allowed in TCLP leachates. Calcium and zinc are not regulated in this test. Grossman has described low extractable lead stabilizers [31].

10.6.3

Mixed Metal Stabilizers

For many years, the most popular mixed metal stabilizers for flexible PVC were based on barium and cadmium or barium-cadmium-zinc combinations, along with various phosphites and epoxy plasticizers or resins. Cadmium has been phased out, because it is considered to be a toxicity hazard. But cadmium is present in much old flexible PVC rework. Today, many mixed metal stabilizers for flexible PVC use zinc compounds, which exchange their anions for labile chorine atoms on PVC molecules. The zinc chloride formed in these exchanges is a potent Lewis acid capable of catalyzing catastrophic dehydrochlorination of PVC. Therefore, zinc is backed up by barium or calcium in the stabilizer at a higher level than the zinc. The barium and calcium compounds do not react with the labile chlorine atoms on PVC as actively as the zinc compounds do. Then, by anion exchange, barium or calcium chlorides are formed in the mixed metal system, and the zinc ceases to be part of a strong Lewis acid. The barium and calcium chlorides are weak Lewis acids and promote PVC degradation much less than zinc chloride does. In 1993, Baker and Grossman presented work on cadmium-free mixed metal stabilizers [32]. Today, use of cadmium has been phased out. The barium-zinc and calcium-zinc stabilizers may be either solids or liquids. The workhorse solids consist of barium or calcium stearate, plus some zinc stearate, together with various synergists. Mixed fatty acid salts, including palmitates and laureates, are also often used. In liquid systems, barium alkyl phenates and zinc octoate may be used together, with high boiling solvents compatible with PVC. Other synergistic ingredients include epoxides and phosphite antioxidants, whose solubility parameters are close to those of PVC and other ingredients such as plasticizers in the formulation. Mixed metal stabilizers have been used for years in clear flexible PVC formulations. Alkyl aryl phophites improve clarity and help maintain “good, early color”. Pentaerythritol was found empirically to be beneficial. Phenolic antioxidants such as butylated hydroxytoluene (BHT) and Bisphenol A are included in many formulations. It is necessary to protect liquid mixed metal stabilizers from exposure to humid air by handling them in closed bulk or semi-bulk systems. Quite small amounts of water in many mixed

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metal stabilizers are sufficient to cause phase separation and serious loss of properties by hydrolyzing some of the phosphite and adding to a portion of the epoxide. Numerous calcium-zinc mixed metal stabilizers are sanctioned by FDA for use in flexible PVC food contact films. Regulated phosphites and polyols are used as synergists in these stabilizers, some of which are sold as one-pack systems. New calcium-zinc stabilizers were described by Bacaloglu [33]. The compositions of most lead-replacement stabilizers are proprietary because of unresolved patent and technical issues. They are reported to contain combinations of primary and secondary metals, metallic chloride deactivators, inorganic acid acceptors, metal coordinators, and antioxidants. Some of these use hydrotalcites similar to the well-known antacid Maalox®, which has aluminum, magnesium, hydroxyl, and carbonate functionalities. β-diketones, such as Rhodiastab 83® or Rhodiastab 50®, are recommended to prevent early discoloration in some lead-replacement stabilizer systems. A novel approach using “latent mercaptides” was described by Conroy [34]. Promising early work on the stabilization of PVC by “plasticizer thiols” was described by Starnes [35]. Stabilizer technology is covered in Chapter 4. Organotin stabilizers are very successful in the United States in rigid PVC, but are only used in specialty flexible applications. When foaming flexible PVC with azodicarbonamide blowing agents, it is advisable to use a stabilizer recommended by the blowing agent manufacturer. For satisfactory foaming, the stabilizer needs to be matched to the desired temperature range for foam formation. For instance, some lead stabilizers are good “kickers” for blowing in the range 160 to 180 °C (320 to 356 °F). Some zinc-containing stabilizers are effective kickers for blowing above 180 °C.

10.6.4

Fillers

Generically, filler may be any low cost solid, liquid, or gas which occupies volume in a part and reduces its volume-cost. The flexible PVC industry uses the term “fillers” to refer to inert particulate solids incorporated into formulations for various reasons, including hardening, stiffening, and reduction of volume-cost. Functional fillers are added to improve specific properties. Examples are calcined clays added to wire insulation formulas to raise electrical volume resistivity, fumed silica or bentonite clay added to plastisols to increase their yield value, and hollow microspheres used to lower specific gravity while achieving other desired filler effects. Particulate solids called fillers must not dissolve in the flexible PVC matrix. Since many flexible vinyl products are sold by volume rather than weight, their volume-cost is the dominant economic parameter. For use in volume-cost calculations, the specific gravity of calcite is 2.71; that of true dolomite is 2.85 and that of aragonite is 2.95 The most widely used fillers in flexible and semi-rigid PVC are grades of dry-ground, wetground, or precipitated calcium carbonate derived from limestone or marble, which are predominantly calcite. This is the stable crystal structure of CaCO3 at ordinary temperatures and pressures. Marble consists of small, interlocking crystals of calcite. Calcite is soft, having a Mohs hardness of 3. Therefore, pure calcium carbonate fillers are low in abrasivity to processing equipment. Grades which contain significant fractions of hard silicates are much more abrasive. Recent work carried out in a PE carrier resin confirms this long-accepted fact and shows that coarser grades are more abrasive than fine particle size fillers [36].

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335

Considerations in selecting a particular grade of calcium carbonate filler include the purity of the original ore, whether it has been dry-ground or wet-ground or precipitated, the average particle size and size distribution, and whether the particles have had a surface treatment. The “packing fraction” (PF) is a measure of how efficiently finer particles fill the voids between coarser particles. Presence of iron oxides such as Fe2O3 in the filler tends to color a compound yellow-brown and will compromise its heat stability unless it is stabilized to withstand the presence of the iron oxide. The average size of filler particles is usually defined in terms of an equivalent spherical diameter (esd). The ratio of the average lengths of the major to minor axes of filler particles is called the “aspect ratio”. The most used fillers have aspect ratios of less than 4 : 1. Reinforcements such as glass or metal fibers generally have aspect ratios in excess of 10 : 1. Vinyl floor tile made by calendering tolerates filler particle sizes up to 99% through a U.S. Standard 50 mesh screen having 297 micron openings (11.7 mils). Typical electrical insulations and cable jackets, which are extruded, require fillers with an average esd of 3 microns or less and coarsest particles of 12 microns diameter (0.47 mils). Cable jackets designed to give low HCl emission on burning generally use precipitated calcium carbonates having 0.6 micron esd. The best filler particle sizes for most flexible PVC applications are determined by experience, in optimizing end-use properties and minimizing cost. The softest non-carbonate filler used in flexible PVC is talc represented as 3 MgO · 4 SiO2 · H2O. Zero or very low content of asbestos-related minerals is specified for talcs used with PVC. Talc is often added to calendering formulations to reduce plate-out on the rolls and to extrusion formulations to reduce plate-out on screws and dies. Talc may also be dusted at 0.1 to 0.25% onto PVC compound cubes or pellets to improve flow in bulk handling systems and hopper cars. Mica is added to PVC compounds to impart a non-blocking surface and to provide stiffening when that is also desired. Typical grades used in non-blocking calendered films are fine-ground so that > 99% passes a 325 mesh screen (with openings of 1.7 mils or 44 microns). Diatomite (amorphous silica) is added to PVC plastisols to increase viscosity and yield value and to reduce surface gloss after fusion. Fumed silica may be added to hot-processed compounds as a scrubbing agent and to plastisols to increase viscosity and yield value. The refractive index (RI) of flexible PVC matrices usually ranges between 1.51 and 1.53 because the RI of PVC is 1.55 and that of typical phthalate plasticizers ranges between 1.48 and 1.50. TiO2, with an RI of 2.76 for rutile, is a strong pigment, which contributes a high degree of opacity. Calcium carbonate (calcite), with an RI of 1.65, is a weak pigment as well as a filler for flexible PVC. Barium sulfate (Barytes), with a slightly lower RI (1.6) than calcite, may be used in translucent flexible vinyl compounds, but allowance must be made for its high specific gravity (4.5). The high gravity is an advantage for use in sound-absorbing and visco-elastic damping compounds. Clear vinyl compounds are generally unfilled. The principal advantages of inorganic fillers in flexible PVC include cost reduction, stiffening, reducing coefficients of thermal expansion, and contributing to better flammability behavior. Specific heats per unit volume are comparable for most fillers and many polymers. The disadvantage of using high levels of fillers in flexible PVC is the reduction of tensile and tear strength, elongation at failure, toughness at low temperatures, abrasion resistance, and resistance to attack by moisture and chemicals. High filler levels also compromise processability by increasing melt viscosity.