TEXTILE WET PROCESSING THE BASIC OPERATIONS We can consider the wet processing sector to have three distinct types of process included within it. These are represented along with their aims. 1-
PREPARATION PROCESS: Exists to ensure that the textile has the right physical and chemical properties to enable it to be coloured or finished. EXAMPLE: Desizing, Singeing, Scouring, Bleaching etc. 2-
COLOURATION PROCESSES: Exists to provide the textile with colour either for aesthetic reasons or for some functional purpose determined by the end-use of the product. EXAMPLE: Dyeing, Printing.
3-
FINISHING PROCESSES: Exists to provide the textile with the properties that the end-use demands and which have not already been provided by any earlier processes. EXAMPLE: Water Repellency, Flame Repellency, Antisoiling finish etc.
1
DEFINITION: Pretreatment means any treatment, which is done before actual (dyeing and printing) process. Textile pretreatment is the series of cleaning operations. All impurities which causes adverse effect during dyeing and printing is removed in pretreatment process.
AIM OF PRE-TREATMENT: • • •
Conversion of fabric from hydrophobic to hydrophilic state. To remove dust, dirt etc from the fabric. To achieve the degree of desire whiteness.
CONSTITUENTS OF COTTON: As discussed earlier pretreatment is the series of cleaning operations. In pretreatment all the impurities present in cotton are removed. Cotton fiber by nature contains: Cellulose 86.8% Natural Impurities: Oil and Waxes 0.7% Pectin’s 1.0% Carbohydrates 0.5% Proteins 1.2% Salt 1.0% Water 8.5% Colour Pigments: Others 2.0% Except cellulose and water all the impurities are removed in pretreatment.
IMPURITIES REMOVED DURING PRE-TREATMENT: Short Fibres Applied Impurities Artificial Impurities Natural Impurities Colour Pigments
⇒ ⇒ ⇒ ⇒ ⇒
(Size Material) (Oil, Strains, Dust, Dirt) (Oil, Wax, Pectin’s, Proteins) (Naturally present in cotton)
Singeing Desizing Scouring Scouring Bleaching 2
PRETREATMENT PROCESS OF COTTON FABRIC: 12345678-
Inspection of grey fabric. Marking of grey fabric. Stitching. Shearing / Cropping. Singeing. Desizing (only for woven fabric) Scouring. Bleaching. All process discuss in detailed one by one.
1-
INSPECTION OF GREY FABRIC:
After receiving the cloth from grey godown, it should be thoroughly checked and inspected before subjecting it to wet processing. The cloth is examined for the following:
FABRIC DEFECTS:
Oil, rust, stains, holes, damage selvedge and weaving faults like floats and read marks etc.
FABRIC SPECIFICATIONS:
Width, length, yarn count, ends and picks per inch and fabric composition.
2-
MARKING OF GREY FABRIC: Before carrying out any process, marks are made on the material for identification. For marking special inks are used capable of resisting all subsequent treatment which the material may have to go undergo.
3-
STITCHING:
4-
SHEARING / CROPPING:
5-
SINGEING:
6-
DESIZING:
Small pieces of fabric are stitched together so as to form a continuous length, which is workable in the subsequent processes. The stitching must be very secure so that the fabric may run smoothly on various machines.
Removal of fibres, tufts and loose threads from the surface of material by cutting them with blades on shearing or cropping machines. This process is carried out in open width. In this process tufts are not removed as closed as by singeing.
Singeing is the process in which protruding fibres present on the surface of the fabric are removed. In order to achieve the smooth surface which facilitate printing.
Desizing is the process in which size material is removed.
7-
SCOURING:
Scouring is the process in which natural as well as artificial impurities are removed. 3
8-
BLEACHING: Bleaching is the process in which we remove the colour pigments in order to achieve the degree of whiteness.
9-
MERCERIZATION: Mercerization is the optional process or in on the customer requirement. Mercerization is done to achieve the luster, strength, more absorbent etc.
PRETREATMENT OF WOOL FIBRES: (i) (ii) (iii)
Sequence of pretreatment of wool fibres a given below: Scouring. Carbonizing (remove the vegetable impurities) Bleaching.
PRETREATMENT OF MAN-MADE FIBRES: (i) (ii) (iii)
Desizing Scouring. Heating Setting.
(if size material is applied) (dimensional stability)
4
SINGEING
INTRODUCTION: This is usually the first stage in pre-treatment; consist of destroying by burning all the tufts, fibers and hairs protruding form the surface of the material (yarn and fabric). There by giving it smooth and cleaning face. Singeing is generally done o grey goods before any other treatment. The fabric is singed on one or both sides by passing either over plates or heated cylinder or by gas flame.
OBJECTIVES OF SINGEING: Surface hairs trape air in the fabric when it is immerged in water. This means that it takes longer time for water to enter the fabric, singeing therefore indirectly helps to increase the fabric wet ability. 5
To prominent the woven structure of the fabric. To create smooth surface for printing. To prevent firstly or cloudy after dyeing. To prevent or minimize the tending of landed fabrics composed of cellulose and synthetic fiber (mainly polyester) the form pills after abrasion during usage.
METHODS OF SINEING: Usually three types of singeing can be carried out: 1) Plate Singeing Machine. 2) Rotary Singing Machine. 3) Gas Singeing Machine.
1)
PLATE SINGEING MACHINE: Plate singeing essentially consist of two covered copper plates thick and slightly barreled which way be set in the fire clay and heated to bright redness by a furnace below or by a suitable heating arrangement. The cloth in open with passes and it contact with the hot plates at a speed varying between 135 to 225 meter/minute. The protruding fibers are burnt during the passage of the cloth through the machine. However, the fiber ends present in the interstices of warp and weft are not signed. Since they are not made assessable to the action of intense heat. Secondly since there is actual contact between the cloth and a hard and hot surface of the metal a certain amount of luster is produced in the cloth due to friction. The main objective to plate singeing is that it may not be possible to maintain the plates at a uniform temperature and this cause unnerve singeing.
2)
ROLLER SINGEING MACHINE:
Some of the defects of plate singeing machine are over come in roller singeing machine which way be considered as a modification of the formers. In this machine, the singeing surface is a rotating cylinder instead of stationary curved plate. It may be made of either copper or cast-iron and provided with the internal fairing system. A fresh not surface of the cylinder is presented into the coming fabric by the slow rotating of the cylinder. In this case, the surface temperature of the cylinder is more uniform at all places. Than in the case of hot plates. The reaction of the cylinder is in the opposite direction to fabric
PRE-REQUISITE Fabric should be completely dry, otherwise yellowish colour with heating take place on the fabric. POST-CARE Used water to extinguish the sparks. 6
movement which rises the naps-ensuring efficient singeing. As in plate singeing the fabric end in the interstices are not removed from the fabric and luster is imported to the fabric to some extent due to the contact with the hard and not-metal surface in roller singeing.
3)
GAS SINGEING MACHINE:
Gas Singeing is intended for single and double face singeing of fabric with passes through the guide roller. Then in brushing zone which rotate in the sense opposite to fabric to be singed. The brushes are placed in a special chamber from which dust and fluf removed form the fabric are exhausted by a fan.
Then the fabric passes over gas burner with ceramics nozzles where singing is covered out. Coming out of the gas singeing machine a box containing water. This bath may also contain desizing agent. In this way after singeing we can desize and batched the fabric.
7
DESIZING
INTRODUCTION: The major portion of the size consists of starch, wax and tallow. All these remain on the warp yarn even after weaving the cloth. It is necessary to remove the size from the cloth, otherwise the hydrophobic of the wax and tallow constraint the subsequent dyeing and printing process. Thus desizing is the term usually restricted to the process of removal of starch size can be performed efficiently and completely if are taken to degrade the starch in some way and to convert it to the product with a higher degree of water solubility. DESIZING Size has to be solubility and the very steps in the process are as given below: 1) Impregnation the fabric with desizing agent. 2) Allow the time for the agent to the absorbed into the size and degrade or solubilized it. 3) Wash out the degrade size material.
OBJECTIVES OF DESIZING: To prepare the fabric for dyeing and printing by removing the coating of size from the surface of the fabric. The chemical required for desizing depends upon the nature of the size applied.
8
1)
OXIDATIVE DESIZING: Some oxidizing agents, particularly hydrogen peroxide and some persulphate will degrade. Starch will out serious attack of the cellulose as long as reasonable care is taken with the process. The fabric is impregnated with hot solution of oxidizer and batched up for several hours or steamed for 20min at 100-105oC. Since hydrogen peroxide is used as bleach for cellulosic substrate as a bleaching process would normally follow desizing but H2O2 is used for desizing with size of PVA at 9PH. It is reasonable to suppose that the two processes might be combined into one. This is some time done, particularly where the amount of size to be removed is hot large. However, a more common practice is to economic on enzyme desizing process by using condition which are milder the removal of size is then completed action of the hydrogen per oxide bleach.
HYDROLYTIC DESIZING: Starch are the mixture of two carbohydrates. (a) Amylose ⇒ a long straight chain. (b) Amylopectin ⇒ a complex branch. The thickening power of starch is largely due to the amylopectin. These compound are in soluble in water but can be solubilized by hydrolysis of these polymers. Hydrolysis is generally brought about by using one of the following processes. (a) Rot or Bacteria Desizing. (b) Acid Desizing. (c) Enzymatic Desizing.
C
O
C
C O
C
C
C
O
CH 2
1:4 Amylose
C
C
C C
O
C O
C
C
O
Starch contain 19% amylose
and
81% amylopectin
C
C C
C
C
C C
C
H H
C
C C
C
O
C H 2OH
C
2-
C
9
1:4 & 1:6 Amylopectin
(a)
ROT DESIZING:
In this process, the grey cloth is dipped in a tank full of water for 24 hours and a give temperature range of 25-40oC. The bacteria present in water produce enzyme, which react with starch and converted them into glucose unit, which are soluble in water and on washing the fabric can easily be removed. This is slow but economic process.
(b) ACID DESIZING:
The process of acid steeping is also called as grey scouring. In this process grey cloth is placed in a weak solution(0.5-1%) of H2SO4 for 4-6hours at a maximum temperature of 40oC starch become soluble in water.
(c)
ENZYMATIC DESIZING:
Enzymes are biochemicals which a specific action on only one type of compound and which bring about a chemical change in it. They are contained with in all living system and carry out many functions. For Example: Their role in digestion, the modern agencies used for desizing into three groups.
ENZYMES (i) (ii) (iii)
Melt Preparation Pancreatic Amylose Bacterial Amylose
ORIGIN ⇒ ⇒ ⇒
Vegetable or cellulose enzyme Waste of slaughter house. Obtain from micro-organism.
The only amylose family of enzyme have an action on starch (Amylose and Amylopectin) and nothing else these amylose converted the starch mixture very rapidly to soluble dextrin and then more slowly convert this to reducing substances and sugar such as maltose. These water soluble product could be easily washed out in actual practice the grey cloth may first be passed through hot water and then through desizing solution, whose
Starch (Insoluble) ⇓ Dextrin (Insoluble) ⇓ Soluble Dextrin ⇓ Maltose (soluble) ⇓
Alpha glucose (soluble)
10
concentration, PH and temperature may be vary according to the type of enzyme.
ENZYMES
CONCENTRATION
TEMPERATURE
PH
Melt Extract
3-20gm/litre
50-60oC
6-7.5
Pancreatic
1-3gm/litre
50-60oC
6.5-7.5
Bacterial
0.5-1gm/litre
60-70oC
5.5-7.5
Enzymes suffer from one disadvantage, that if the condition of temperature and PH not favorable their desizing activity is destroyed. After passing fabric from desizing solution fabric is batched for 8-12hours. During this time starch is converted from insoluble starch to soluble α -glucose and than simple washing is done.
SCOURING INTRODUCTION: Cotton is based on 90% of cellulose and the remaining portion consists of impurities such as fats, waxes & organic compounds. The main impurities which must be removed are fats and waxes which are insoluble in water thus decreasing the water absorbency of fabric raw cotton contains: Cellulose 86.8%
Natural Impurities: Oil and Waxes Pectin’s Carbohydrates Proteins Salt
0.7% 1.0% 0.5% 1.2% 1.0%
Water
8.5%
Colour Pigments:
Others 2.0% These are three major processes in scouring to remove impurities. (i) Soaponofication. (ii) Emulsification. (iii) Solvent Extraction (Detergency). These processes are discussed in detail.
(i) SOAPONOFICATION: Soaponofication is the process in which fats are treated with caustic and lead the formation of hydrophilic soaps (sodium salts of fatty acids). CH 2 − OOCH | CH − OOCH | CH 2 − OOCH
35
C17
CH 2 OH |
35
C17 + NaOH
35
C17
→ CH − OH + 3 (C17 H 35 COONa ) | CH 2 OH
11
(ii) EMULSIFICATION
OR
SOAP
APPLICATION
IN
SCOURING: Waxes present in the fabric cannot be removed in saponification. These are esters of higher fatty alcohol & fatty acids similarly mineral oils, lubricants oil etc cannot be converted into water soluble product by boiling with NaOH solution. The process of emulsification is used in the scouring of cotton material containing non-saponifated oil such product can be removed by emulsifier. Thus the scouring solution should also contain an emulsifying agent in addition to Sodium hydroxide and wetting agent ordinary soap (washing) is good emulsifying agent.
(iii) DETERGENCY OR APPLICATION OF DETERGENT IN SCOURING: After removing the starch (by desizing) vegetable oil (by saponification) mineral oil and waxes (by emulsification), the remaining constituents of the size and other solid particles including dust and dirt particle have to be remove from the fabric. When the binding material such a starch, oil and waxes are removed by process described before the solid particle are loosened from the fabric and try to leave the fabric under proper condition where once they leave cloth and enter the scouring solution by dispersing in it. It is likely that get to prevent this redeposition. Detergent is added to scouring solution. The detergent keep the solid particles removed from the fabric dispersed or suspend in the solution and does allow them to settle on other parts of fabric. Soap is a good detergent. A good detergent is also good wetting agent.
PROCESS OF SCOURING: For this purpose, a variety of equipment is available to process the materials in different forms and by batch, semi-continuous and continuous processes. The ultimate aim of the scouring is to make the material uniformly and highly absorbent in a cost-effective manner so that there are no difficulties in the later processes of dyeing, printing and finishing. Some of the more common batch and continuous scouring machines are described below:
HIGH PRESSURE KIER: A kier is a cylindrical steel autoclave that is capable of withstanding high steam pressures and has a capacity varying between 1 to 3tons. To eliminate formation of rust marks kier is coated with a mixture of lime and sodium silicate that is periodically renewed. Composition of the scouring liquor, its temperature and duration of the treatment depend upon density of the fabric to be processed and ultimate degree of absorption required. A typical recipe, based on the dry 12
weight of the fabric and a liquor ratio of 4:1 is as under: Sodium Hydroxide 3.0% Sodium Silicate 0.5% Wetting Agent 0.1% At elevated temperatures sodium hydroxide completely breaks down proteins and pectates that are mainly present in the cuticle layer and converts these into water-soluble products. The oils and fats are converted into soap and this in turn emulsifies waxes that are removed by hot washing later.
However, the degraded substances are destroyed in the subsequent hypochlorite or peroxide bleach. Sodium silicate by virtue of its colloidal nature serves to keep the reacted impurities suspended and reduces their tendency to settle on the scoured fabric. The wetting agent that may be a mixture of anionic and non-ionic surfactants for having synergistic effect reduces surface tension of the scouring liquor and so helps in its quick penetration into the fibres. The liquor is rapidly raised to a temperature of 130oC and is kept circulating for 6 to 8 hours. When scouring is complete the brown colored liquor is discharged, preferably in stages with addition of hot water into the kier so that the impurities do not settle and stick to the fibres and are removed more completely. After washing fabric in the kier, the remaining impurities are removed in a rope machine.
CONTINUOUS SCOURING:
Kier scouring being a batch process is giving way to the continuous processes in spite of its producing the most absorbent goods. Continuous scouring of fabric is carried out both in rope and open-width forms but in form, the fabric especially the heavier weight and the wider-width types tend to develop erases or rope marks.
ROPE SCOURING: The most well known continuous rope scouring and bleaching machine is “J” box because its storage/steaming chamber has shape of the letter J. For scouring cloth is impregnated with caustic soda solution in a padder in concentrations of 30 to 60 g/l with 6 to 10g/l of a wetting agent at 120%pick up. The padded fabric is heated in a steaming tube to raise its temperature to about 95oC and then plaited into stainless steel heat-insulated J box. The fabrics pile moves downward under gravity and cloth is removed continuously after a dwell period of about one hour.
13
OPEN WIDTH SCOURING:
Rope scouring in the J box is highly productive and economical as cloth runs at a speed of 80 to 150 meters/min. However, it is not free from troubles and warp-wise rope marks are often encountered that cause unsatisfactory dyeing later. The heavy weight cotton and polyester/cotton blends are especially susceptible to formation of the rope marks. A J box was developed in which the fabric is fed, piled, stored and withdrawn in open width.
PAD-ROLL SYSTEM: The wet desized cloth with 70% pickup is padded in open width for a final pick up of 100% in the scouring liquor containing 4060g/l caustic soda and 10-15g/l of a wetting agent. Sometimes part of caustic soda is replaced with sodium silicate to obtain a comparatively better whiteness. The roll is gradually rotated for about 3 to 4 hours in the enclosed chamber at a temperature of 95-98oC before washing off the impurities in a washing range.
14
BLEACHING
INTRODUCTION: Cotton wax natural fats and added fatty matter and other impurities from the fabric are removed in desizing, and scouring process. But the natural colouring matter present in cotton is still present in the cloth. In order to obtain the white cloth, the normal practice is to decolorize the natural colouring material present in the fabric. Thus the final bleaching process is essential for obtaining a good white and this is done by bleaching in different ways. FOR EXAMPLE: By using dilute Sodium Hypochlorite (NaOCl) solution (Bleaching powder) at Room Temperature. By using Hydrogen peroxide (H2O2)solution at 80-85oC. By using Sodium Chlorite (NaClO2)solution at boil. By using some peroxy compounds like perac acid (CH3COOOH) Each of these characterized by certain advantages and disadvantages. An efficient bleaching process must ensure (i) A pure and permanent white. (ii) Level dyeing properties. Bleaching agents are mainly classified as follows: (i) Oxidative Bleaching Agent. (ii) Reducing Bleaching Agent.
SODIUM HYPOCHLORITE BLEACHING (NaOCl): Sodium hypochlorite the salt of hypochlorous (HOCl) does not exist in the solid form but an aqueous solution. The stability of this solution depends on the temperature, PH, concentration, presence of impurities, storage condition i.e. whither expose to light or not. The conditions of the bleaching agent varies according to the result required, the concentration depends upon the fabric quality, degree of whiteness 15
required, types of machine and next operation. General PH range is 10-11 or 10.511.5 during bleaching if PH reaches 9 then it is a danger level and at 7 PH the bleaching is worse and causes extreme damage to the cloth. The PH value is maintained by adding sod ash or by buffering agent. The suggested temperature for hypochlorite bleaching is 37-40oC. Generally reaction is accelerated with increases of temperature water for bleaching should be soft and even hard water can be used but should be free from Cu++ and Fe++. The time factor depends upon the following consideration. (a) Concentration. (b) PH value. (c) Degree of Whiteness. (d) The type of machine used in bleaching. Roughly for normal machine the time is 4-12hours. For liquor circulation it is 2-2.5 hours. If the concentration is increased the time is 1-2hours.
ADVANTAGES: 12-
Low chemical cost. Low energy input (heating cost)
DISADVANTAGES: 12345-
There is no rapid bleaching process possible. The danger of yellowing of the bleached fabric on storage. The danger of chemical damage to the cellulose (temperature & PH). The fibre must be prescoured before bleaching. Degrade most dyes and fluorescent Brighteners.
HYDROGEN PEROXIDE BLEACHING (H2O2): 12-
Hydrogen peroxide has achieved its dominant position as a bleaching agent because of two factors. It is environmental friendly. (potentially it can decompose into oxygen & water). It is versatile (it can used hot or cold batch wise and continuous) Hydrogen peroxide as supplied by the manufacturer is extremely stable typically, only a 3% loss of activity would result from storage at 40oC for 1 year. Hydrogen peroxide has a low dissociation constant and a weak acid in a alkaline solution. The peroxide anion is produced that is the active bleaching agent.
H2O2
⇒
H+
+ OOH-
H2O2
⇒
H2O + [O]
(Desirable) (Non-Desirable)
A high concentration of hydroxyl ions has an accelerating effect on the rate of reaction it is necessary to add a stabilizer. The common stabilizer is Sodium Silicate and it also gives protection against metal containments.
PROCESSES OF BLEACHING: Hydrogen peroxide bleaching can be done by Batchwise, Continuous & Semicontinuous method.
BATCHWISE PROCESS: 16
The bleaching chemicals are made up in a side tank. This about half filled with cold water then the surfactant or wetting agent, stabilizer and caustic soda are added with stirring. Stirring is therefore an absolute necessity any solid should be predissolved. Finally peroxide is added and if required the concentration of alkali and peroxide are determined by titration. The transfer pump is started and the contents of the tank transferred to the machine, before the temperature is raised to 85-95oC and hold at that temperature for 1-2hours. For jigger bleaching the chemicals would be added directly to the jig in the order given.
For Kier bleaching, the transfer pump supplies the water piler and when the kier is about half full, the steam to the kier heatest is opened to give a temperature of 40-50oC and loading is completed. The temperature is raised to 65-70oC over 30min and the kier is rested for 30min at this temperature to expel air. After raising to 80oC and resting again the is closed and the temperature raised and hold for 1-2hours at 95oC. NaOH is used in case of H2O2 bleaching. This is used to bring the PH upto 9-10 because H2O2 become active at this PH or oxidation is start at this PH. Stabilizers are used in bleaching for control the rate of oxidation of H2O2.
CONTINUOUS PROCESS:
Most woven and increasingly, fabrics are bleached with hydrogen peroxide by continuous method either in rope open width form. Most knitted fabrics scoured and bleached in a single stage spiral rope processing machines such as Kusters, Jemco but most woven fabrics are pad steam processed continuously in three stages desizing, scouring and bleaching. As the weaving stages of such a rate consume 75% of energy, much attention has been given combining process stages to reduce energy demand and minimize capital investment in the scope for combining stages depends upon the grey fabric quality. Grey cloth is first impregnated with a chemical solution and squeezed for white fabric 3-4% sodium hydroxide is used. The saturater has a set of squeeze on the saturation of the cloth. The cloth from saturated passes and into the steamer. After the cloth is uniformly heated then it goes into washer where it is thoroughly washed. This process is done at 100oC for 20-25min is steamer.
17
SEMI-CONTINUOUS PROCESS:
The pad batch process consists of padding the grey fabric with a strong solution of alkali and hydrogen peroxide and storing in batch form for 2-24hours depending on the storage temperature. In recent years interest in cold bleaching has increased as it provides a low energy preparation route with low capital investment, applicable to both woven and knitted fabrics. Cloth can be impregnated with the bleach solution on standard stainless steel pad mangles (or even jigs) but for regular production by this method an efficient open width saturated is recommended, simply because high pick is difficult to achieve on grey fabric. Cold pad batch bleaching requires: (i) Control of pad batch concentration and temperature preferably 25-35oC. (ii) Greater than 80% liquor pick up. (iii) Fabric storage without uneven drainage or surface drying. (iv) Washing off at a minimum temperature of 95oC.
MERITS OF HYDROGEN PEROXIDE BLEACHING: 12345678-
Environmentally acceptable. Decomposition products are oxygen and water. Excellent storage stability. Compatible with most dyes and FBAs. Give versatile processing (continuous, semi-continuous and batchwise) Produces a stable white fibre with and absorbency. Allows route shortening by combining stage (desize with scour, scour with bleach and desize with scour and bleach). No need for severe precleaning processes.
STABILIZERS & USES OF STABILIZERS: High PH and temperature lead to the decomposition of peroxide bleaching liquor and degradation of cellulose. The role of the stabilizer is simply to control or regulate these effects the act as buffers, sequestrates and in special cases, enhancing performance of the surfactant used in the bleach bath. The sequestering and inactivates metallic impurities which causes catalytic decomposition of hydrogen peroxide or precipitation of hydroxides or carbonates. These impurities the most common being calcium and iron are brought into the bleaching system by the fabric water supply or the other chemical used. The most common stabilizing agent have been used are Sodium Silicate. The stabilize were the sodium polysilicate (effective at lower), The sodium metasilicate (washed out readily) than the polysilicate also used sodium orthosilicate and potassium silicate. Substrates bleached with a silicate stabilizer must be washed off in soft neutral boiling water in order to prevent problem caused by insoluble silicates or silic acid. Although sodium silicate is both effective and economical, the problems associated with silicate deposits on fabric and machinery have made alternative non-silicate or organic stabilizers.
ADVANTAGES OF SODIUM SILICATE: 123-
Good degree of whiteness. Better bleaching of veg. matter and removal of husks. Masking of heavy metal ions.
DISADVANTAGES OF SODIUM SILICATE: 18
1234-
Sensitive to water hardness. Handle impairment. Reduction in absorbency. Blocking of pipes.
In order to avoid these disadvantages organic stabilizers together with magnesium salt 0.2g/litre.
MERCERIZATION
INTRODUCTION: Mercerization, the treatment off cotton with a strong caustic alkaline solution in order to improve the luster, hard and other properties, was names after its discoverer, John Mercer, and has been in use for sometime. It has been seeing an increase in application recently.
MERCERIZTION PROCESSING: If cotton is dipped into a strong alkaline solution such as lithium hydroxide, the fibres will swell and shrink. If the fibres are placed under tension while in this swollen state and then rinsed with water, the alkali will be removed and a permanent silk like luster will result.
EFFECT OF MERCERIZATION: Improve Luster. Increase ability to absorb dye. Improve reaction with a verity of chemicals. Improve stability of form. Improve strength/elongation. Improve smoothness. It has been shown that the increase in the luster occurs because of an effect. The cotton fiber do convoluted. The cross-sectional shape changes.
In mercerizing 4Ts are important. 1Twaddle. 2Temperature. 19
3-
Tension.
4-
Time.
(1) TWADDLE (Concentration of NaOH): If the concentration of NaOH is increased above 56oTw improvement in luster will be attained but if it is decrease below 48o Tw. The quality of luster will begin to be adversely affected.
(2) TEMPERATURE: High degree of luster is attained at temperature 18-20oC. As the temperature is increased the quality of luster is adversely affected but on lowering the temperature no improvement in the luster is obtained.
(3) TENSION: For acquiring better luster the material must be stretch to its original dimension (both in warp and weft direction during mercerization). If the material is allowed to shrink during mercerizing then quality of luster will be impaired on the other hand if the material is stretched more no improvement in luster is achieved.
(4) TIME: The optimum time for mercerizing is 30-60 seconds by increasing the duration of time no applicable improvement in the quality of mercerization can be achieved but if the time limit is less than 30 seconds in the quality of mercerization will be improved.
DIFFERENCE BETWEEN CONVENTIONAL & HOT MERCERIZATION: CONVENTIONAL MERCERIZATION
HOT MERCERIZATION
1- SWELLING Higher Fibre Swelling. Slow Swelling.
Less Fibre Swelling. Fast Swelling.
2- RELAXATION Slow relaxation.
Fast relaxation.
3- SHRINKAGE Higher residual shrinkage.
Less residual shrinkage.
4- SURFACE Surface swelling irregularity.
Surface swelling regularity.
5- FIBER PACKING 20
Tight Fibre packing harder handle NaOH Loose Fibre packing softer handle NaOH diffusion into yarn obstructed. diffusion into yarn upholder.
6- LUSTER A few strongly swollen (round) fibres in Significantly less strongly swollen fibre the surface of the yarn – interior less throughout the yarn cross-section interior lustrous. equally lustrous.
METHODS OF MERCERIZING: There are THREE main methods. 1Chain Mercerizing. 2Chain less Mercerizing. 3Slack Mercerizing.
1-
CHAIN MERCERIZING:
In order to make up for the shortenings of the roller mercerizing machine, a clip stenter is used for post-mercerization treatment, in which a widthwise tension is applied then most alkali is showered off the fabric kept by through alkali removal and neutralization using an openwidth washing machine.
2-
CHAINLESS MERCERIZATION:
This method of mercerization running fabric through a number of rollers without the use of a clip stenter is also called roller mercerization. The machine has a number of stainless rollers, or stainless and rubber rollers of a relatively large diameter tiered zigzag in close content to each other inside a long trough with the lower tier designed to submerge in alkaline solution for mercerization.
3-
SLACK MERCERIZING:
Slack mercerizing is done by exhaust. The fabric is slacked and treated with NaOH solution by this process of luster is obtained but absorption is increased. Slack mercerizing = Loose mercerizing or mercerizing without tension
EFFECT OF MERCERIZING ON DEAD COTTON: Immature fibres are finer than mature. They are more convoluted and contain less internal cellulose. It is their lack of internal structure that causes than to dye polar some dyes stuffs with exaggerate (more important really is) this specked appearance other minimize. The dead cotton problem can only be minimized by mercerizing the fabric by swelling (become bigger and rounder) the immature fibres and giving tem a rounder shape and a greater reflectivity. The swelling action opens up the internal structure of the fiber and makes it easier for dye to gain access and the increased reflectivity enables. The pale fibre to reflect their surrounding more mature darker & neighbours. The overall effect is that we no longer see than or at least not as. 21
OPTICAL BRIGHTENING AGENT (OBAs)
DEFINITION: The treatment of textile with fluorescent brightening agent is carried out to improve the quality of whiteness.
APPLICATION OF OBA: OBAs are available for application on to all types of substrate. There are anionic FBAs for application to cellulose in the presence of added salt. Anionic types for application to nylon or wool in the presence of acid disperse types for application to polyester and so on.
Most fluorescent brightening agent are applied to cellulosic fibre at temperature between 10-50oC in quantities varying from 0.05%-0.5% better exhaustion is obtained 5gm/l common salt are added these brightness applied to cotton in the manner of direct dyes. In case of animal fibres quantities of FBAs ranging from 0.25%-2.5% are applied at 40-50oC with 5% of formic acid(85%) or acetic acid (80%) and similar instruction would be appropriate for polyamide fibres. Polyester is brightened by exhaustion either by high temperature method at 125-130oC or at boil in the presence of carrier. Most of FBAs can also be applied by pad thermosole method at temperature upto 90oC. At temperature greater than this some brightness sublime. 22
PROPERTIES OF FBA: In most of the cases, the light fastness of fluorescent brightening agent in not good on protein and cellulosic fibre it is usually between 1 & 2 and reaches 3 in a few cases. Fastness to washing is also only moderate only polyamide fibre there are selected product with light fastness of 4, on polyester 7 and for acrylontrile 4-5. When Fluorescent Brightener on wool fade as a result of exposure to light a pronounced discolouration is frequently developed. It has been suggested that this is caused by decomposition product of the fluorescent compound. FBA are no substitute for bleaching but they improve the white and also allow reduction in the amount of bleaching agent required. Thus diminishing the risk of degradation of protein. They are frequently applied after bleaching.
FIBRE WHICH FLOURESCENT BRIGHTING APPLIED: (i) (ii) (iii) (iv) (v)
FIBRE FOR FBAs
Cotton Acetate & Tri-acetate Nylon Polyester Acrylic
CHEMICAL NATURE OF FBA ⇒ ⇒ ⇒ ⇒ ⇒
Diaminostibene diasalphonic acid. Diphexyl pyrazoline Diphexyl pyrazoline Pyrene derivatives & Nephthalimide Pyrazoline and Nephthalimide
MODE OF ACTION OF FBAs: All dye absorb light. Fluorescent dyes re-emit the absorbed energy as light of longer wave length. An FBAs is a fluorescent chemical that absorb in the ultraviolet region of the spectrum and emit blue violet light. Typical FBAs absorb light with a maximum absorption as a wave length between 340 and 380 mm and emit light with a maximum emission between 425 and 450mm.
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