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Wet processing


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Wet processing

  1. 1. Textile Chemical ProcessingWhat is the need of Textile Chemical Processing? In Textile Chemical Processing the chemical treatments are given to the fabric afterbeing manufactured. Actually the fabric coming from the loom is not having properties likeabsorbency, softness etc and the most important is that the appearance of the fabric is dirty orpale yellow; we cannot use it directly for making apparels or clothing. So, it is necessary togo for chemical processing of the material to make it wearable.The general sequence of processes carried out on gray clothGray Inspection Stitching Mechanical Cleaning Singeing DesizingScouring Bleaching Mercerising Dyeing/Printing FinishingGray Inspection It is checked whether the grey fabrics are in conformity with standards, and all weaving faults are marked out. Fabric inspection involves three possible steps: perching, burling and mending. Perching is a visual inspection and the name derives from the frame, called a perch, of frosted glass with lights behind and above it. The fabric passes through the perch and is inspected. Flaws, stains or spots, yam knots and other imperfections are marked. Burling is the removalof yam knots or other imperfections from the fabric. The faults are then mended and any knots in the material are then pushed to the back. Mending isobviously, the actual repair of imperfections. Knotting should be done carefully andthoroughly so that the repair or holes is not visible.Stitching After the goods have been inspected and checked they are classed in the grey room,according to quality and stamped. Goods of similar weight, width and construction and thegoods which will receive a similar treatment are sewn together, end to end, by sewingmachines especially constructed for this purpose and each batch is given a number called lot number. The fabrics are usually sewn on circular machine. Stitching should be done in such a manner that the creases in fabric at the time of stitching should be avoided. The use of proper stitching thread is necessary to avoid stitch marks during colour padding. For heavy fabrics intended for mercerizing and continuous operations, the seam should be wider (15 mm) and stronger. The pre-cleaning of grey fabrics may be carried out in a separate unit just before cropping and shearing operations. The purpose of brushing is to
  2. 2. remove the short and loose fibres from the surface of the cloth. It also removes husk particlesclinging to the cloth. Brushing is mainly done to fabrics of staple fibre content, as filamentyams usually do not have loose fibre ends. Cylinders covered with fine bristles rotate overthe fabric, pick up loose fibres, and pull them away by either gravity or vacuum. The raisedfibre ends are cut off during shearing operation. Brushing before cropping minimizes pilling. Shearing is an operation consists of cutting the loose strands of fibres from either surfaceof a fabric with a sharp edged razor or scissors. By manipulating the shearing it is alsopossible to cut designs into pile fabrics. Good cropping is perhaps, the simplest way ofreducing the tendency of blended fabrics to pill. In the case of cotton fabrics, in particular,care should be taken to see that the shearing blades do not scratch the surface of the fabric,which otherwise can cause dyeing defects during subsequent dyeing.SINGEING The objective of singeing is to remove projecting fibres or protruding fibres, which givesit a fuzzy appearance, from the surface of the fabric so as to give it a smoother, cleanerappearance. The reason for which singeing is necessary:(i) Singeing improves the end use and wearing properties of textiles.(ii) The burning-off of protruding fibres results in a clean surface which allows the structure of the fabric more clear.(iii) Singeing reduces the fogginess caused by differing reflection of light by the projecting fibre and the dyed fabrics appear brighter.(iv) Singeing is an effective means of reducing pilling in blended fabrics containing synthetic fibres.(v) Unsinged fabrics soil more easily than singed fabrics.(vi) A closely singed fabric is essential for printing fine intricate patterns.(vii) Singeing process facilitates and speeds up desizing, if the fabric is impregnated with desizing liquor immediately after singeing.On the other hand there are singeing faults which are not visible and once occurred can nolonger be repaired. They are:(i) Uneven singeing effect can cause streaks when the fabric is dyed, or bubbles when the fabric is finished.(ii) In the cotton system singeing is done on the grey cloth, but for blended fabrics containing synthetic fibres grey state singeing is not advisable because small globules of melted synthetic fibres absorb dye preferentially, giving cloth a speckled appearance.(iii) There is a possibility of thermal damage to temperature sensitive fibres, for instance polyester.(iv) Stop-offs can cause heat bars on fabrics. Creasing produces streaks which are magnified when dyed. Generally, singeing is done on both sides of the fabric. No chemical change occurs inthe fabric during singeing and the reaction is basically one of oxidation. Singeing anddesizing can be frequently combined by passing the singed cloth through the water bath
  3. 3. which includes enzymes. The enzymes digest the various sizing agents, making it easy toremove them during the scouring operation.Singeing Machinery: Singeing machineries are mainly based on direct and indirect singeing systems. Thedirect singeing may be done either on a hot plate, or on a rotary cylinder, or on a gas singeingmachine or on a machine combining plates and gas burners. The special features of indirectsingeing systems are no flame contact, uniform singeing, heat retention zone and singeing bymeans of heat radiations. The indirect system produces fabrics which have a softer touch ascompared to other methods. Gas singeing is more convenient, more economical and moreeffective than other methods and is well accepted commercially. The plate singeing and rollersingeing machines are now out of date. 1. Plate Singeing Machine Figure - A Plate singeing Machine with alternating arrangement In plate singeing machine there are two curved Copper plates, 1-2 inches thick. TheseCopper plates are set in the fire clay and heated to bright redness by the furnace below or bysuitable heating arrangement. The cloth in open width passes over and in contact with the hotplates, at a speed of 135 – 225 m/min. Automatic transverse motion is fitted to the machine,which constantly changes the part of the hot plate in contact with the fibre, to avoid the localcooling and non-uniform heating of the plates. The protruding fibres are burnt during the passage of the cloth through the machine.However the fibres present in the interstices of the warp and weft are not singed. In this case,as there is actual contact of the fabric with the hard hot surface of the metal, a certain amountof lustre is impregnated in the cloth due to friction. The drawback of this machine is it maynot be possible to maintain plates at uniform temperature and this causes uneven singeingwhich gives rise to faint streaks when the cloth is dyed.
  4. 4. 2. Roller Singeing Machine Figure- A Roller Singeing Machine The fabric is made to pass over, in contact with rotating cylinders. These cylinders aremade up of cast iron or copper and are heated by internal firing system. In this case, surfacetemperature is more uniform than in the case of hot plates. However, the problem of localcooling of the cylinder by contact with the cloth causes uneven singeing. The rotation of thecylinder is in the opposite direction to the fabric movement, which erases the neps on fibreensuring an efficient singeing. In this case also, the fibre present in the interstices of warp and weft are not removedfrom the surface and the lustre is impregnated to the fabric due to the contact of the hot andhard metal surface. 3. Gas Singeing Machine In this process, the fabric is passed rapidly over a row of gas flames at a high speedand then immediately into the quench bath to extinguish the sparks. And cool the fabric. Themachine consists of one or more gas burners giving continuous flat or vertical flames. Herethe flame is produced by compressed air and coal gas. The flame is adjusted with respect tothe width of the fabric. The angle of the burners and the height of the flame can also beadjusted. When the cloth is drawn over flame at a high speed, the protruding fibres burnwithout damaging the cloth. The speed of transmission of the cloth through the singeingmachine has to be adjusted to suit the amount of singeing required without the risk ofburning the cloth. The position of the burner must be taken into consideration according to the fabric tobe singed (Fig. 2-11). There are three different methods of applying flame to the material bychanging the position of the flame to offer:
  5. 5. Figure- Gas Singeing Machine1. Tangential singeing Fig. 2-11 (a)2. Singeing onto water cooled rollers Fig. 2-11 (b)3. Singeing into the fabric Fig. 2-11 (c)
  6. 6. Singeing of Different Fibres materials: In singeing the short fibres are burnt off from the surface of the fabric by direct orindirect heating systems without damage to the cloth by scorching or burning. The thermalbehavior of different kinds of fibres is different and singeing at higher temperature isnaturally associated with greater hazards on excessive contact period and may cause thermaldegradation of the fibre. In case of vegetable fibres, grey singeing is necessary as it leads to slight yellowingwhich needs subsequent bleaching to get high degree of whiteness. Grey singeing is alsoeconomical as singeing at any other stages of processing requires additional washing anddrying. Vegetable and regenerated fibres fabrics can be singed very strongly with maximumburner intensity to obtain good results. Regenerated fibres normally bum to a little less easilythan natural fibres. Wool has poor combustion properties and are very sensitive totemperatures and hence woollen materials are not subjected to intense flame like cotton. Inwoolen fabric flame is not generally allowed to penetrate the material and this can beobtained by blowing air through the fabric from the opposite side of the flame so that theflame will be restricted only on the surface of the fabric. Alternatively, the fabric can beguided to water cooled guide rollers allowing the flame to heat the cloth. When the flamestrikes the fabric it is reflected by air/steam cushion created within the material. Amongst the synthetic fibres polyester has the greatest significance. It melts at280°-290°C but does not burn till about 500°C. Reflector or refractory singeing machinesproduce smears of fused polymer on the surface of the polyester cloth and thereforeunsuitable for polyester material. Thus flame singeing machine with a powerful flame isneeded and also helps in overcoming the problems of oligomers i.e. the small chain polymersthat come to the surface. High temperature singeing process may sometimes change theglass-transition temperature (Tg) of synthetic fibres that lead to uneven dyeing. For blended fibre fabrics singeing conditions are to be selected depending on thesensitiveness of the kinds of fibres to heat, blend composition, weight of fabric and fabricgeometry. For example, singeing should be carefully conducted to avoid heat damage of theacetate component of the acetate/viscose blended fibre fabrics. Though singeing improves theresistance to pilling of the polyester/wool blended fabrics, but should not be carried out onlow weight fabrics because of risk of damage. If singeing is carried out after dyeing thesublimation fastness of disperse dyes used must be adequate to withstand the singeingconditions.
  7. 7. Desizing The grey cotton fabric contains natural as well as those added to the fabric such as sizeto facilitate weaving. Size normally contains an adhesive (film former) and a lubricant. For cotton fabricsthe film former is usually starch or a starch derivative. All starches are, by their very nature,either water-insoluble or only sparingly soluble. For viscose the most important sizing agentis the cellulose derivative carboxymethylcellulose (CMC), which has good water solubility.In addition to natural products, such as starch and starch derivatives, synthetic sizes based onstyrenemaleic acid copolymers, polyvinyl alcohol, polyacrylates or polyacrylamides are usedon polyester/cotton or polyester/viscose, as well as mixtures of starch and polyvinyl alcohol.The synthetic polymer sizes and carboxymethylcellulose are also used on continuous-filament warps made from acetate, triacetate, nylon or polyester. The lubricant in a sizeformulation is usually tallow, but spermaceti, paraffin wax and mineral oils are sometimesemployed. These lubricants impart good smoothness and low frictional properties to the yarnand are therefore beneficial for weaving, but they are insoluble in water and difficult toremove from the fibre surface, which can lead to severe problems in desizing. Severalchemical manufacturers offer wax like products that are water-soluble for addition to sizeformulations to improve suppleness and smoothness of the yarn; being water-soluble theseare relatively easily removed during desizing.Concept of Desizing It is necessary to remove the size from the cloth; otherwise the hydrophobicity of thewax and the tallow hinder subsequent dyeing and printing. Waxes and tallow are removed inthe later (scouring), while the starch is removed during desizing. Desizing is the term usuallyrestricted to the removal of starch from the fabric. Chemically starch is poly-α-glucopyranose in which straight chain (amylase) andbranched chain (amylopectin) polymers are present. These are shown in Fig.
  8. 8. Both of these components of starch are insoluble in water, but they can be solubilisedby hydrolysis of these long chain compounds to shorter ones. The hydrolysis of starch takesplace in following stages.Starch (Insoluble) Dextrin (Insoluble) Soluble dextrin (soluble)Maltose (soluble) α-glucose (soluble) For the desizing purpose the hydrolysis is carried out up to the stage of soluble dextrinonly not to further to α-glucose, because our aim of desizing is to make the size materialsoluble.Classification of Desizing MethodsThe desizing methods can be classified as inHydrolytic Desizing1. Rot steeping
  9. 9. This is the oldest and cheapest method of desizing. The main feature of this process is that no special chemicals are required. The cloth is first impregnated with warm water (at 40°C) through a padding mangle and then squeezed to about 100% expression. One of padding mangle used is shown in figure. The cloth is then allowed to stand for days in pits or cemented tanks. The micro- organisms, naturally present in water multiply and secrete starch liquefying (hydrolysing) enzymes which solubilise the starch present in the size on fabric. The cloth is finally washed with water to remove the starch. The main disadvantage of this process is that it is a slower process and requires an enormous floor space for storing water impregnating cloth.2. Acid Desizing The cloth from the singeing machine is impregnated through a solution of dilute sulphuric acid or dilute hydrochloric acid (0.25% owf), followed by batching for about 8 -12 h. At first sight the use of acid could appear to be dangerous, since conditions, which degrade starch, are those on which cellulose is also liable to be attacked. Since the hydrolysis of the starch is exothermic reaction, the temperature of the desizing bath may rise to higher side, even to 50°C, but at this temperature dilute acid solution doesn’t attack or hydrolyze cellulose. But if the cloth impregnated with dilute mineral acid solution is exposed to the air during the storage period, localized drying due to evaporation cause increase in the concentration of acid at that portion and if the concentration is sufficiently high cellulose is hydrolysed. To avoid this, moistened jute cloth is placed over impregnated fabric. A subsequent washing after acid treatment completely removes the starch. Water and aqueous acid extract significant quantities of the impurities. They not onlydegrade starch-based product but also offer the advantage of removing Ca and Mg salts fromthe cotton fabric. The concentration of the acid can be as high as 2 % for short times and aslow as 0.2 % for overnight stripping. Enzymatic Desizing The most effective way of removing starch from fabrics is by the use of extractscontaining appropriate enzymes. An outstanding feature of enzyme desizing is the specificnature of the enzyme action. Enzymes are biocatalysts; they differ from normal chemicalcatalysts because they are very specific in their action, are thermo-labile, have relatively lowenergies of activation and are usually active only over a narrow range of pH. They areclassified by the names of the substrate they decompose, e.g. amylases degrade amylose,proteases to proteins, and cellulases to cellulose.Some properties of enzymes are as follow:
  10. 10. 1) Enzymes are complex and high molecular weight protein molecules. Chemically enzymes are proteins of high molecular weight e.g. molecular weight of the enzyme α-amylase is about 100,000. 2) Enzymes are susceptible to high temperature and pH values outside their optimum ranges because they can be denatured. Still, most enzymes function best at or near the neutral point at temperatures between 40-60°C; above these temperatures, the activity is reduced. One exception is α-amylase from bacteria, which may under some circumstances, be used at temperatures in excess of this. The different preparations are applied under different conditions. 3) Enzymes react at specific parts of the substrate molecule because these are specific to their action. Desizing enzymes may be classified based on the source from which they are obtained as in fig. Desizing Enzymes Animal Based Vegetable Based Examples Viveral WASTE PANCREAS Malt Extracted Bacterial Degomma Slaughter house Examples ExamplesClotted blood etc Diastofar Biolase. Oxidative Desizing1. Chlorine desizing The active agent in case of chlorine desizing is gaseous chlorine. For the Cl2 desizing, open width cloth is impregnated with water and squeezed at required percentage expression. The squeezed fabric is passed through a chamber, which is provided with a false bottom, through which Cl2 gas is passed. In this case Cl2 reacts with water present in the cloth producing nascent oxygen and this nascent oxygen attacks starch, there by solubilizing it. Cl2 + H2O → 2HCl + [O] Since cellulose is difficult to oxidize than starch, the oxidation of cellulose isprevented or minimized by controlling the quantity of Cl2 gas passed and time of contact.The Cl2 gas may be replaced by dilute hypochlorite solution of 1-2 g/l available Cl 2. For thissodium hypochlorite (NaOCl) or bleaching powder (CaOCl2) is used. For this method thecloth is impregnated with bleaching solution at 30°C (room temperature), squeezed andallowed to stand for one hour at room temperature. It is then washed and afterwards antichlorwith HCl.
  11. 11. 2. Sodium chlorite desizing In this method Sodium chlorite (NaClO2) is used under acidic condition for oxidizing the starch present in the grey cloth. Sodium chlorite in the presence of ammonium sulphate gives good desizing efficiency. Sometimes pH of desizing bath may be adjusted between 4 - 4.5 with the help buffer of Sodium acetate and acetic acid. Cotton fabric is padded at room temperature with an expression of 100% with an aqueous solution of 15g/l Sodium chlorite and 10 g/l ammonium sulphate and 1g/l of wetting agent. Then fabric is heated to 80-90 °C for 1 hour. Then it is washed and neutralized.3. Bromite Desizing Sodium bromite, is used for the desizing is a salt of bromous acid, HBrO2 (like sodiumchlorite, the salt of chlorous acid, HClO2). This has a powerful oxidising action on the starch.This is due to the combined effect of bromous acid, HBrO2 and hypobromous acid, HOBr.This is accompanied by the conversion of bromine dioxide into oxygen and bromine.Hydrolysis of this bromine produces more hypobromous acid and the nascent oxygengenerated is responsible for the oxidation of starch. There are different methods of oxidation, but the most likely one is the breaking ofmost stable ether linkage of the glucose ring by sodium bromite. As shown above by the oxidation the ether group converted to aldehyde and thenfurther to carboxylic acid which lead to reduction in D.P of starch and convert to watersoluble products. In another mode of oxidation of starch, the opening of the glucose ring by rupture ofC2-C3 link takes place that lead to formation of a dialdehyde. The dialdehyde formed is notsoluble in water, but soluble in hot alkaline solution.
  12. 12. Hence the sodium bromite treatment should be followed by a hot alkaline treatmentwith or without an intermediate rinsing operation. On the other hand sodium bromitetreatment also reduces the natural impurities present in the material mainly oxidise the someof the natural colouring pigments present which lead to the reduction in the chemicalrequirement for bleaching. The general process with sodium bromite involve the padding with liquor contains,0.3 % sodium bromite, a wetting agent and stabiliser at room temperature. The long paddingtime is required for 6-20 min. The pH is the most important factor in this case, the pH shouldbe around 10, and below pH 9 the decomposition of bromite is rapid. While above pH 11 theoxidation of starch is very slow. Temperature more than 40°C leads to degradation ofcellulose. Then after storage for desired treatment time fabric is washed and treated with hotsodium hydroxide solution to remove the converted starch completely.Test for Desizing Efficiency After the desizing by any of above methods, we now go for checking how far ourpurpose is achieved or not. We can check the desizing efficiency both quantitatively asqualitatively. The quantitative method to check the desizing efficiency is the weight loss.Scouring The desizing process is actually a destarching process because in this the starch presenton warp yarns is liquefied by either hydrolytic or oxidative reaction and removed insubsequent washing step. But after desizing the fabric still contains fats and waxes (bothnatural as well as added), which adversely affect the absorbency of the fabric. Theseimpurities are removed from the fabric by the process of “Scouring”. Thus the main purpose of scouring cotton fabric is to remove the natural as well as addedimpurities of essentially of hydrophobic nature as completely as possible and leave the fabricin highly absorptive state without undergoing a significant chemical or physical damage. The scouring process is done by boiling the fabric in an alkali solution. The mainprocesses occur during scouring are:
  13. 13. 1. Saponification of oils present in the fibre. 2. Waxes and unsaponifiable material is removed by emulsification of the same. 3. Pectins are changed into their soluble salts of pectic acid. 4. Mineral matters are dissolved. 5. Proteins are hydrolysed into degradation soluble products. 6. Dirt or dust is removed and held in a stable suspension by the detergents present in the scouring bath.1) Vegetables oils, animal fats and mineral oils are not soluble in water. Thus when grey cotton fabric immersed into water, the oil present in cotton will not allow the water to spread on the fibres. These vegetable oils are glycerides of fatty acids like stearic acid, palmitic acid, and oleic acid. When such oils are heated with NaOH the oil splits into its constituents fatty acids and glycerine, out of which glycerine is water soluble. The fatty acid again react with NaOH to form its sodium salt i.e. soap which is also soluble in water. That’s why this reaction is called saponification. Thus the saponification reaction converts the insoluble and water immiscible oil is converted to water-soluble products. CH2 – OOC - C17H35 CH2 – OH CH – OOC - C17H35 CH – OH + 3C17H35 - COOH CH2 – OOC - C17H35 CH2 – OH Tristearin (Glyceride of stearic acid) Glycerine stearic acid 2) The waxes present in the cotton as well as in size formulations cannot be removed by saponification. Waxes are esters of high molecular weight fatty acids and alcohols. The waxes and lubricating oils are not converted into their soluble products. They are therefore removed by emulsification. 3) The Pectin substances are present in the cotton in the form of insoluble salts of Calcium, Magnesium and Iron. These bivalent metal salts are solubilized in alkaline solution. Pectic acid is a compound of high molecular weight containing carboxylic group for every 6 Carbon atom. It is insoluble in water but soluble in alkaline solution. 4) The quantity of inorganic matters is in the range of 0.7- 0.6% by the weight of anhydrous cotton. The main constituents are Na2C03, K2O, Na2O, K2CO3, CaO, and CaCO3. 85% of these materials can be removed by simply boiling with water. Phosphorous present in the form of organic and inorganic compounds which are mostly soluble in hot water and which can become insoluble in the presence of alkali earth metals. Therefore use of hard water during scouring can precipitate alkali earth metal phosphate, which can get deposited on the surface of the fibre instead of getting eliminating from it. The scouring process requires use of soft water because the use of hard water would cause precipitation or insolubalizaion of soap. It must be considered that cotton fibre contains Ca and Mg salts (pectin salts) which are freed during alkali treatment and can also contribute to the insolubility of soaps and at the same time remains
  14. 14. attached to the fabric in the form of hydroxides. Thus they might disturb subsequent operations such as bleaching, dyeing and printing. It is therefore necessary to add sequestering agents (chelating agents or metal complexing agents e.g. Nitrilo Triacetic Acid (NTA), EDTA, gluconic acid. Sequestering agents also help in the elimination of iron, which can give problems during subsequent bleaching with hydrogen peroxide. 5) Cotton proteins consist of protoplasmic residues. Proteins are mainly concentrated in the primary wall. The known colour, which appears during scouring, could be due to the reaction between proteins and carbohydrates in the alkaline medium. 6) During scouring some dust, dirt and solid particles are loosened from the fabric. These particles leave the fabric and enter into the scouring bath but again get deposited on other parts of fabric/fibres. So to remove these particles and to keep this in suspension or dispersion form, detergents are added in scouring bath. A detergent is a good wetting agent. If the detergent is used in scouring bath another wetting agent need not be added to the scouring bath. Therefore, there are three components in a cotton scouring bath: caustic, to swell anddissolve the motes and to saponify oils and waxes, surfactant, to lower the baths surfacetension so it can wet-out the fabric faster and to emulsify oils and waxes and chelating agent,to form water dispersible complexes with heavy metals.General Recipe for Scouring:Bleaching
  15. 15. After the removal of the waxes and other hydrophobic type of impurities from greyfabric by the desizing and scouring the fabric is now in a more absorbent state. But still havethe pale appearance due to the presence of natural colouring materials like pigments etc.these pigments can not be removed the only way to tackle these pigments is to decolourisethem using suitable oxidising agents. This will make the fabric in a super white form. Thisprocess of decolouration of natural pigments is called the bleaching. The process ofbleaching gives a sparkling whiteness to the fabric and hence makes it suitable for furtherprocessing.Methods of BleachingThe various chemicals employed to carry out bleaching process are: 1. Dilute hypochlorite solution preferably Sodium Hypochlorite (NaOCl) 2. Hydrogen Peroxide (H2O2) solution 3. Sodium Chlorite (NaClO3) solution 4. Certain peroxy compounds like peracetic acid 5. Potassium Permanganate (KMnO4) solution Now we discuss the effect of different bleaching agents under various conditions ofconcentration, pH, temperature, time and activators. So that we can choose optimumconditions of bleaching with a particular bleaching agent.Sodium Hypochlorite Bleaching: It is done by using sodium hypochlorite (NaOCl) as a bleaching agent. This process isalso known as chemicking. NaOCl is a highly unstable compound at normal conditions oftemperature and pH. It doesn’t exist as solid form. As it is highly unstable so it undergoesself decomposition by the following reactions: 2NaOCl NaCl + NaClO2 3NaOCl 2NaCl + NaClO3 2NaOCl 2NaCl + O2The bleaching mechanism of sodium hypochlorite consists of the following reaction: NaOCl NaCl + (O) The bleaching action of sodium hypochlorite depends on several factors the mostimportant of which is the pH. The pH of solution highly influences the action of bleachingagent. Because the hypochlorite ionize differently under different pH conditions and activecomponent can be affective in these different stages are: 1. At pH greater than 8.5 the hypochlorite is present as NaOCl. 2. Between pH 5 and 8.5 the solution consists of predominantly the hypochlorus acid (HOCl). NaOCl + H+ HOCl + Na+ 3. As the pH falls below 5 the liberation of chlorine takes place and when the pH falls below 3 the whole HOCl converted into chlorine.
  16. 16. In the region of pH -7 hypochlorous acid and hypochlorite are present in approximate same concentrations the rate of attack on cellulose is greatly enhanced. As the pH falls below 5 the liberation of chlorine takes place which pollutes the environment and corrode the container. In the pH region above 8.5 (in between 9 to 11) very little changes occur in cellulose i.e it is the normal working range for hypochloride solution. Thus the suitable pH range for sodium hypochloride bleaching is the 10-11. Now the other factor which affects the efficient bleaching after pH is the concentration. Generally the concentration of NaOCl which give 2-3 g/l available chlorine isenough. The variation of rate of oxidation with change in concentration of NaOCl shows thatthere is a continuous steep increase in the bleaching action until the concentration is 5 g/lavailable chlorine. The concentration giving more than 5 g/l available chlorine has negligiblechange in the rate of oxidation. Commercially the concentration should be such that theavailable chlorine should be 2-3 g/l. The other important factor considered for efficient bleaching is the temperature. TheNaOCl bleaching should always be carried out at room temperature. The elevatedtemperature causes a rapid oxidation which may cause even tendering of the fabric. Also thevariation of rate of oxidation with temperature is more prominent at higher concentration ofNaOCl. Longer the treatment time larger is the oxidation of material and our aim to achievegood whiteness with minimum damage to material. Generally a solution containing 2g/lavailable chlorine requires 2 hours for good whiteness; the same effect can be obtained in80-90 mins by using solution of 4g/l available chlorine. General Recipe: Concentration - 2-3 g/l available chlorine pH - 10-11 Temperature - Room temperature Treatment time - 2 hoursHydrogen Peroxide (H2O2) Bleaching:
  17. 17. The bleaching process carried out by NaOCl is known as Half bleach because thewhiteness obtained is temporary not permanent. So, the hypochlorite bleaching shouldfollowed by another bleaching treatment. Now instead of NaOCl, strong oxidising agentH2O2 is used to carryout bleaching. This bleaching with H2O2 called as full bleach becausethe whiteness obtained is permanent. Properties of H2O2 • It is a colourless syrupy liquid • It is absolutely stable under acidic conditions • It is sensitive to sunlight. • It decompose if allow to react with heavy metals. • It is highly unstable to alkali like NaOH, Na2CO3, rapidly decomposition takes place. 2 H2O2 2 H2O + O2 Concentration of H2O2 the concentration of H2O2 is expressed in terms pf Volumes.This volume means the volume of O2 gas in ml liberated by complete decomposition of 1 mlof H2O2 solution at NTP. 2 H2O2 2 H2 O + O2 2×34 gm 32 gm at NTP 2×34 gm 22.4 ltr at NTP 34 gm 11.2 ltr at NTP 34% H2O2 solution 100ml 11.2 ltr 34% H2O2 solution 1ml 112 ml of O2 at NTPMechanism of peroxide bleaching Though hydrogen peroxide is stable in acidic medium, but bleaching occurs by theaddition of alkali or by increased temperature. Hydrogen peroxide liberates perhydroxyl ion(HO2-) in aqueous medium and chemically behaves like a weak dibasic acid. The perhydroxylis highly unstable and in the presence of oxidisable substance (coloured impurities in cotton),it is decomposed and thus bleaching action takes place. Sodium hydroxide activateshydrogen peroxide because H+ ion is neutralized by alkali which is favorable for liberation ofHO2-. H2O2 + NaOH H2O + HO2- However, at higher pH (above 10.8) the liberation of HO2- ion is so rapid that itbecomes unstable with the formation of oxygen gas which has no bleaching property. If therate of decomposition is very high, the unutilized HO2- may damage the fibre. A safe andoptimum pH for cotton bleaching lies between 10.5 to 10.8 where the rate of evolution ofperhydroxyl ion is equal to the rate of consumption (for bleaching). At higher pH, hydrogenperoxide is not stable and hence a stabiliser is frequently added in the bleaching bath. The process of regulation or control of perhydroxyl ion to prevent rapiddecomposition of bleach and to minimize fibre degradation is described as stabilisation.Stabilisers for peroxide normally function by controlling the formation of free radicals. Theseare complex blends of a selection of materials serving a number of functions. They couldinclude any of the following: • Alkali, e.g. caustic soda/carbonate/silicate.
  18. 18. • Dispersant, e.g. acrylates/phosphonates. • Sequestrants, e.g. EDTA/TPA/gluconates. • Inorganics, e.g. magnesium salts. • Colloid stabilisers, e.g. acrylic polymers. The selection of alkali to be used in peroxide bleaching is dependent on the fibres orblends being bleached. Sodium silicate is the most conventional, easily available and widelyused stabiliser. This sodium silicate act as a multifunctional agent: • Act as a stabilizer • Keeps the metal ions away i.e. act as sequestering agent • It also acts as a buffer and maintains the alkaline pH. Actually sodium silicate itself not acts as a stabilizer instead it reacts with MgCl2 presentin water thereby giving MgSiO2 which act as a stabilizer. Though sodium silicate works as multifunctional agent but now days avoided to use asa stabilizer. This is due to the following disadvantages associated with the use of silicate: • The removal of silicate after bleaching is difficult. • There are several compounds are formed by action of sodium silicate which are also difficult to remove. • During dyeing some light spots are formed on fabric containing traces of sodium silicate. This is due to the presence of sodium silicate.So, instead of sodium silicate we prefer to use newly developed organic stabilizers known as(Stabilizer-NS) Non silicate stabilizer. Commonly used organic stabilizers are: • Amino-polycarbonic acid • Aliphatic acids • Aldoamines etc.General Recipe of H2O2 Bleaching: H2O2 Concentration - 2-4% Sodium silicate - 0.5-1% NaOH/Na2CO3 - 0.5-1% Sequestering agent - 0.1-0.3% pH - 9.5-10.5 Temperature - 80-85°C Time - 90 minsComparison of hypochlorite and Peroxide bleaching: H2O2 Bleaching Sodium Hypochlorite BleachingPeroxide is universal bleaching agent can be It is mainly used for cellulosic fibres not foremployed to wool, silk as well as cotton. protein fibres like wool, silk.Peroxide is milder agent so degrading affect Hypochlorite is having degrading affect onon cellulose is less. cellulose.
  19. 19. Peroxide also gives mild scouring action so It doesn’t give any scouring action.simultaneous scouring and bleaching ispossible in continuous process.It doesn’t affect the coloured material so it It can’t be used over coloured material.can be used for coloured materials.With H2O2 there is no need of danger of There is a problem of corrosion andequipment corrosion and no unpleasant unpleasant odours.odours.Only rinsing after bleaching is sufficient Hypochlorite bleaching needs an antichlor treatment.Bleaching with peroxide is a costlier than Relatively it is less costly.hypochlorite.Hydrogen peroxide bleaching requires No such need of stabilizers in hypochloritestabilization usually with silicates which bleaching.have problem of stains on subsequent dyeing.Sodium Chlorite (NaClO2) Bleaching: Besides NaOCl and H2O2 we may se sodium chlorite (NaClO2) as a bleaching agent.NaClO2 is a stable compound under alkaline or neutral conditions but it rapidly decomposesunder acidic conditions giving the chlorine dioxide ClO2 which act as oxidising agent.ThisClO2 act as a bleaching agent. But we often avoid using sodium chlorite bleachingbecause: NaClO2 have a corrosive action on the equipment and the ClO 2 liberated have aobnoxious smell. 5 NaClO2 + 4HCl 4 ClO2 + 5 NaCl + 2 H2O In the past time the NaClO2 was used for bleaching only synthetic materials likepolyester, nylon etc. due to which this bleaching process also known as Synthobleach.Treatment conditions for sodium chlorite bleaching are: Temperature - 95°-100° C pH - 10-11 Treatment time - 1-2 hoursBleaching of Wool and Silk: Scoured Wool varies in shade from the light cream of wools considered to have goodcolor to discolored The main problem is that the whiteness of wool attained during bleachingis not permanent. Wool tends to yellow over a period of atmospheric exposure ofapproximately six months. Amongst the oxidising agents hydrogen peroxide is most
  20. 20. commonly used because sodium hypochlorite gives a deep rust color and sodium chloritedevelops pink coloration on wool. Traditionally wool is bleached by oxidative processeseither in the presence of alkaline stabiliser or under acidic condition of hydrogen peroxide In the alkaline condition, wool is treated at pH 8-10 with a 1.5-3 volume solution ofhydrogen peroxide containing 2-3 g/1 stabilizer, which may be sodium silicate or sodiumpyrophosphate. Bleaching may be carried out at 50°C for 3 to 5 h and then rinsed, treatedwith dilute acetic acid and rinsed again. The level of whiteness can be controlled byconcentration of hydrogen peroxide, length of treatment time, pH and temperature oftreatment bath.Bleaching of Viscose: Filament viscose rayon may not require bleaching since this is normally carried outduring manufacture. However, viscose in staple form requires bleaching as it may notnecessarily include a bleaching treatment during its manufacture. The same reagents as thoseused for bleaching linen and cotton fabrics are useful for these fibres. For very goodwhiteness, rayon may be bleached on a jigger with alkaline hypochlorite or combined scourand bleach using hydrogen peroxide (up to 1 vol. strength) containing sodium silicate andalkaline detergents-at a temperature of about 70°C.Bleaching of Blended Fibre Fabrics:Polyester/Cellulosic Blends: Polyester fibre in blends with cellulosic fibres in the ratios of 65/35 and 50/50 arecommon construction. When cellulose portion is rayon, the blends rarely require bleaching,but when cotton is present bleaching is usually necessary. Bleaching treatments of suchblends are normally required to remove the natural colours of cotton, sighting colours and ifthe polyester portion is turned yellow at the time of heat-setting operation. Chlorinebleaching, peroxide bleaching and chlorite bleaching are employed widely. If the polyesterportion requires bleaching, then chlorite bleaching is used, as this bleaching agent bleachesboth polyester and cellulose. If the polyester portion does not need bleaching, then peroxidebleaching is more convenient. Alkaline hydrogen peroxide bleaching is the most preferredsystem for polyester/cotton blends.Polyester/Wool Blends: In general, blends containing wool and polyester fibres can be bleached with hydrogenpeorxide either in acid or alkaline medium without risks of damage. In acid medium, thefabric is treated with a solution containing 30-40 ml/l H2O2 (35%), 2-4 g/1 organic stabilizer,0.25 g/1 wetting agent and 0.25 g/1 detergent at pH 5.5-6 (acetic acid) for 40-60 min at 80°Cor 2-2.5 h at 65°C. The treated fabrics are then given warm and cold rinse. In alkalinemedium, the bath comprises of (35%), 30-40 ml/l H2O2; sodium pyrophosphate, 2-4 g/l;ammonia to maintain the pH 8.5-9. The bath is set at 40°C and the goods are treated for 2-4h, and rinsed well in warm and cold water.Nylon/Cellulosic Blends: Blends of nylon and cellulosic fibres may be bleached with either H2O2 or NaClO2H2O2 does not bleach nylon and normal methods of bleaching degrade nylon. Blends
  21. 21. containing 30% or less of nylon may be bleached by the continuous H2O2 method, and insuch cases cotton will absorb the peroxide preferentially and so protect the nylon fromdamage. The goods are entered into a bath containing 2-3 volume H2O2, 1 g/1 sodiumhydroxide flake, 0.2 g/1 peroxide stabilizer, 0.25 g/1 sequestering agent at 40°C thetemperature is raised to 85°C and then the treatment continued for 1 h. The treated goods arethen cooled and rinsed thoroughly. Hypochlorite does not damage nylon but it has got nobleaching action on it. Sodium chlorite causes no degradation of either cellulosic orpolyamide and is a better bleaching agent the fabric is treated with a solution containingsodium chlorite (2-5 g/l) at pH 3 to 4 at 90°C for 1 1/2 to 2 h. This is followed by a treatmentin a 2 g/1 solution of sodium carbonate at 40-50°C and finally hot and cold rinses are givenin water.Nylon/Wool Blends: It is difficult to bleach this blends since the method normally used for nylon degradewool. Alkaline H2O2 bleaching always damages the polyamide fibres to some extent. Normalalkaline H2O2 bleaching process may be used with safety on blends containing up to 25%polyamide, but acid bleach must be used when proportion exceeds this figure. The fabric canbe bleached with a solution containing 12-15 ml/1 H2O2 (35%); 2 g/1 tetrasodiumpyrophosphate, 1 g/1 EDTA (30%) at 60-65°C for 45-60 min and then rinsed well in water.Determine the bleaching Efficiency:Absorbency Test: The simple test of measuring the absorbency of sample consists of allowing a drop ofwater to fall from a fixed height (2.5 cm) on to the conditioned fabric sample, which ismounted on an embroidery frame of about 6 inches diameter. A stop watch is started as soonas the drop falls on the fabric and stopped as the water drop is completely absorbed by thefabric. This complete absorption of drop is ensure by appearance of a dull spot on fabric i.e.the reflected light disappear from the edge of drop. This time is termed as the absorbencytime. Yet another method for absorbency test is the measurement of the time required forthe sample of about 1 inch size to sink in water, termed as sinking time. A drop absorbencyor sinking time of about 5 sec is generally considered satisfactory for well prepared fabric. In case of synthetic fabrics or their blends the above test methods are not applicableand for such materials strip test is used. In this we measure the height of water raised bycapillary action in the fabric. In his test method a 5cm wide strip is cut across the fillingdirection. Then numbers of these strips are immersed 1mm deep in 1% aqueous solution ofC.I. Direct Blue 86 for 2 sec and then immediately placed on wire screen. After drying, thecapillary rise of dye solution is measured.Whiteness:
  22. 22. Whiteness is related to the luminosity as well freedom from yellowness. It is measuredby measuring the reflectance of the specimen against a standard white (magnesium oxide/ceramic) tile which represents a whiteness value of 100.Cuprammonium Fluidity: This test indirectly measures the chemical degradation of cotton cellulose during pre-treatment process. The principal of this method is based on the fact that the damagedcellulose has lower M.W due to less DP. So the solution of damaged cellulose will be lessviscose and have more fluidity as compare to undamaged one. In this test, the conditionedcotton sample is exactly weighted and dissolved in cuprammonium hydroxide solution. Theflow time of this solution between two fixed mars on a calibrated viscometer (fluidity tube) ismeasured at a specific temperature. The fluidity value, F is calculated from F= C/t, where“C” is viscometer constant and “t” is flow time. The results are expressed as Rhes (poise-1),which is reciprocal of unit of viscosity. Fluidity value of 5-8 is considered satisfactory fornormal bleached cotton fabric.SilkA. Degumming of Raw Silk Degumming is at the heart of wet processing of raw silk. The main purpose of thedegumming process are to remove the Sericin from the fibre, to remove some impurities (e.g.waxes, fats, mineral salts) affecting both the dyeing and printing processes, to make the fibrehighly absorbent for dyes and chemicals and to reveal the lustre of fibroin and to improve theappearance of the fibre. The fact that the raw silk contains two components fibroin andSericin, which covers the filaments. Sericin contains some impurities, for example, waxes,fats, mineral salts and pigments. Sericin has the same amino acid residues, as fibroin but theproportions contained in both components are quite different. As a result of this, thedegumming process must be carefully carried out on silk in the appropriate conditionsotherwise the fibroin may be damaged. The pH range from 4 to 8 is normally safe for fibroinand it has been found that alkaline conditions are less harmful to fibroin than acid conditions.In contrast to fibroin, the solubility of Sericin is very high at pH values between 1.5 and 2and between 9.5 and 10.5. The Sericin is removed from the fibre but the fibroin must not bedamagedTable - Composition of raw silk Fibroin 70-80% Sericin 20-30% Carbohydrates 0.7% Wax materials 0.4-0.8% Inorganic matter 0.6%
  23. 23. Natural pigments 0.2%There are 5 ways of Degumming silk 1. Degumming with water under pressure at 115°CWater at room temperature does not dissolve silk but silk is highly susceptible to dissolutionin boiling water. For complete removal of Sericin, in case of cultivated varieties of silk, it isnecessary to extract the silk yarn with water at 120°C for 4 hours. For this reason, thisprocess gives a risk of fibroin being damaged when the time of treatment is prolonged. Thisprocess needs large autoclaves to treat the fibre in silk industry. A further disadvantage isthat this process gives incomplete degumming and sometimes soap or synthetic detergentmust be added to improve the degumming effect. Therefore this process is very difficult tocontrol and now it is not used in silk industry in order to remove Sericin from silk. 2. Degumming with soap (at 98°C)Different soaps like olive oil, palm oil can be used for degumming. Marseilles soap, an oliveoil soap, is an outstanding soap for degumming because of its high degree of hydrolysiswhich gives better lustre. For example, this process may be carried out using 10 – 20 g/l soapat 92 – 98 °C for 2- 4 hours adjusted pH to 10.2 – 10.5 in order to react effectively upon thesericin.The degumming action of the soap is due to alkali formed, which forms a chemical bondwith Sericin and produce soda salt, on the hydrolysis of the soap. The Sericin, in the form ofsoda salt, is separated by soap and dissolved in water due to the emulsification action ofsoap. The quantity and type of soap required degumming generally depends upon the natureand type of silk.Disadvantage of soap degumming areThe process requires soft water. The metallic ions such as Ca and Mg combine with soap andproduce insoluble metallic soap, which deposits on fibre and reduces the lustre of fabric.Combination of soap and alkali accelerate the process.As a result of the high temperature, this process tends to attack both sericin and fibroinbecause of the sensitive nature of fibroin itself and chemical similarity of fibroin and sericin. 3. Alkali DegummingAlkalis hydrolyse protein by attacking the peptide bonds and are said to have severedamaging effect on proteins. Hence, this process has to be carried out under controlledcondition, so as not to result in over degumming. For this process, pH should be maintained
  24. 24. between 9.5-10.5. Below pH 9.5, rate of degumming is too slow causing prolonged exposureand hence mechanical damage. Above pH 10.5 there is a danger of fibroin being attacked.Alkalis used for degumming are caustic (NaOH), caustic soda (Na2CO3), sodium bicarbonate(NaHCO3), K2CO3, Na2SiO3, trisodium phosphate, sodium phosphate, borax and ammonia.Among these caustic soda is the most preferred alkali. Alkali is rarely used alone, since itleaves the silk rather harsh in handle and it is recommended to use buffer system. Hencecaustic soda and sodium bicarbonate is the widely used buffer.The optimum concentrations are: Na 2CO3 – 1.06% NaHCO3 – 0.84% Non-ionic surfactant – 0.3%Degumming can be carried out at 100°C for 2 hours with MLR 1:40. 4. Acid degummingIt is comparatively safe method, as the action of organic acids was reported to be much lesspronounced on silk than that of mineral acids. Different acids used for degumming are Lacticacid, Tartaric acid, Oxalic acid, Succinic acid, Citric acid. Degumming is carried out with0.05 moles/L acid and 3g/l non-ionic surfactant at 100°C for 60 min. considering the weightloss and tenacity the best result are obtained with succinic acid and monochloro acetic acid. 5. Enzymatic degummingEnzymes are proteins, catalysing a specific chemical reaction, which are known as ‘bio-catalysts’. They work at atmospheric pressure and in mild conditions (e.g. at 40°C, pH 8.0).Trypsin, papain and bacterial enzymes are the main types of enzymes for silk degumming.These enzymes are called ‘proteases’ because they degrade and their degradation productsare polypeptides, peptides and other substances by hydrolysis of the –CO-NH- linkage. Proteolytic enzyme action on protein
  25. 25. Enzymatic degumming has the following advantage over the conventional degumming withalkaline soap. • It has a specific reaction thereby it may give a minimum damage to fibroin. • It has a lesser risk of over degumming than alkaline soap degumming, • Weight loss can be easily modified by adjusting the concentration of enzyme, the reaction time and the use of optimum pH and temperature. • With the enzyme method, silk is treated at low temperature (e.g. at 40°C) not only reducing energy costs but also preventing fibre weakness. • Enzyme treatment is an environmentally friendly process because enzymes are readily biodegrade in nature. • There is no soap required in enzyme degumming process. Therefore, uneven dyeing problem caused by metallic soap can be avoided.Enzymatic degumming also has some economic disadvantages as: • It needs some pre-treatment processes, since the gum must be swollen before the enzyme bath. • It is very slow reaction compared to alkaline soap degumming.Degumming of the silk is carried out in form of hanks as well as fabric. B. Silk dyeing C. Finishing of Silk 1. Weighting of SilkThe process of increasing the weight of silk material is known as “Silk Weighting”.Silk approximately losses 25% weight after degumming. This loss in weight leads to greaterloss of money because silk is a very expensive material. To compensate this loss someweighting material is artificially added to the silk material by chemical means.Major objectives of weighting: • To increase the weight • To reduce the lmpiness • To impart bulky effect. • To control the scroopy feel.
  26. 26. • To give body to the silk material.Weighting methods:Generally the metallic Tin salts are used for weighting the silk. There are 3 methods ofweighting of silk. 1. Silk material is soaked/dipped in the Stannic chloride (SnCl4) solution, followed by fixation with Sodium carbonate (Na2CO3) and then washed the material. By this process there is a slight increase in weight. But it also reduces the strength of the silk. 2. The material is soaked in Stannic chloride (SnCl4) solution followed by fixation with sodium phosphate. Stannic Chloride + Sodium Phosphate Stannic Phosphate It is then washed and acidified with small quantity of H2SO4 & again washed the material. Increase in weight is sufficient/ considerable in this method but it also reduces the strength. 3. Material is soaked in Stannic chloride (SnCl4) solution followed by fixation with Sodium silicate. It will give sufficient weight & not reduce the strength of silk. Stannic Chloride + Sodium Silicate Tri silicate of TinA better process is the combination of these 3 processes: 1. Picking/Soaking with Stannic chloride (SnCl4) solution. 2. Phosphating: Followed by the fixation with Sodium phosphate. 3. Washing 4. Acidifying: Treatment with small quantity of H2SO4. 5. Washing 6. This process is repeated several times until we get required weight after this the material treated with sodium silicate. 2. Scroopy feel of Silk
  27. 27. A typical type of sound in the silk material is called the scroopy feel i.e a crisp sound of silkcalled the scroopy feel. This feel is largely accepted & appreciated by the market. This feel isgiven by the treatment with 2 – 3% of acetic acid or tartaric acid or citric acid in coldconditions i.e at room temperature and dried. This process is given at the end of wetprocessing. This feel is also obtained by the treatment of very dilute mineral acid. 3. Anticrease TreatmentSilk is by nature crease resistant but to give body to the silk material treatment of some gummaterial like gum Arabic, gum tragacanth or dextrin (a type of gum having viscosity) isgiven. Kandari is a special vegetable product which has no smell and it appears like onion.The thin solution of this kandari is applied for finishing of silk sarees. The solution isprepared by boiling this vegetable product with water for several hours & then filters it withmuslin cloth. This solution is used for finishing of silk sarees. This solution is applied bybrush or by spray.Silk sarees tightly & fully stretched on a hexagonal roller and then this chemical solution isapplied by brush or sprayed over the saree. This hexagonl roller provides tension to fabricand no crease is there on the fabric.After dyeing the fabric is wound over the rollers.