DYEING PREPARATORY PROCESSING

FOR

COTTON AND WOOL

Submitted To: - Submitted By :- Mr. Krishnaraj G Anjali

M F tech-Ist sem Apparel Production

Roll No. 05

CONTENTS

Processing of Cotton 1) Introduction 42) Impurities in grey fabric 53) Standard Process Sequence 64) Grey Checking 75) Stitching 8 6) Shearing and Cropping 97) Singeing 10 8) Desizing 149) Scouring 19

10) Bleaching 2411) Mercerization 29

Processing of Wool1) Introduction 352) Cropping 36 3) Scouring 374) Crabbing 395) Carbonizing 406) Milling 417) Bleaching 42

Bibliography 44

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PROCESSING

OF

COTTON

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INTRODUCTION

After the grey fabric comes out of the loom shed, for a layman it is a ready cloth but it still contains heap of impurities, defect and need to be given some physical and chemical treatment so as to make wearable. It is here that the work of textile chemist begins. This processing of grey fabric into a wearable cloth is mainly chemical in nature and termed as “wet processing”. The word wet is used because the processing involved in the conversion of the grey fabric into the finished cloth is generally done with wet treatment as in dry state it is troublesome. So the work of processor is a vital and key piece in getting the cloth from the loom to market and finally on the body.

A generalized component in the cotton fabric is: -

COMPONENT PROPORTION Cellulose 80-85% Waxes and fatty acid 0.4-1% Proteins 0.8-1.8% Pectins 0.3-1% Mineral matter 1-1.8% Moisture 6-8% Others 1-4% (Gums and lignous substance)

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IMPURITIES IN GREY FABRIC

There are three types of impurities present in the grey fabric. They are as follows: -

1) Natural impurities : - The various types of natural impurities present in grey fabric are: -

a) Waxesb) Pectinsc) Pigmentsd) Colloid substancese) Color

These mask the absorbency of the fabric.

2) Added impurities : - The various added impurities are deliberately added for improvement of certain properties during weaving operations. Some are: -

a) Gumsb) Poly vinyl alcoholc) Fatty compound

3) In process impurities : - These are produced in the grey fabric due to the development of the cellulose material during process. These are: -

a) Broken cotton seeds known as kitties (black spots on the fabric)

b) Leafy materialc) Oil stainsd) Lubricants

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SEQUENCE PROCESS STANDARD

A generalized sequence that can be followed for processing is: -

Grey Checking ↓ Stitching ↓ Shearing and Cropping ↓ Singeing ↓ Desizing ↓ Scouring ↓ Bleaching ↓ Mercerization

The end use of the textile will determine whether all of those stages are necessary. In some cases the stages may occur in a different order or two stages may be combined into one. Above sequence is only generalized one.

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GREY CHECKING

When a grey fabric enters a process house then the first process that is carried out is grey checking and mend able defects are removed. The whole process is carried out on a fabric inspection table.

Various parts of a fabric inspection table are: -1) Beam stand : - Manually or automatic which holds the fabric beam on

it2) Fabric beam : - It is roll of fabric from loom3) Guide rollers : - They help in fabric movement and are negatively

driven by the surface contact of the fabric4) Inclined plate : - This is the main part and is made of wood or metal

on the two ends and of glass in the middle, as illuminator works better on the glass. It is adjustable and generally set at an angle of 45.

5) Rollers : -They grab the fabric coming and pull it and are positively driven by the motor

6) Meter Reading device : - This device works for noting the fabric length

7) Illuminator : - These are light sources either below or above the glass plate

Whenever some defect is seen on the fabric then the flow of the fabric is stopped by pressing down the handle at the lower end of the plate and after mending the defect the machine is started by lifting the handle up.

The two main defects are mended at the fabric inspection table: -1) Loose ends : - The grey fabric contains several loose ends along

the selvedge of the fabric which are removed by cutting them at the inclined plate

2) Stains : - The grey fabric may have some stains on it, which can be removed by using stain removers of two types.

The two types are: -a) Water based : - In this water itself is used as solventb) Solvent based: - In this some solvents are used to emulsify

the stains out. They are efficient but the particular portion of which the stain is removed becomes highly absorbent which creates problem in dying as it gets darker than rest of fabric

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STITCHING

After grey checking the next process in sequence is “stitching”. In order to maintain the continuity of the process long length of the fabric is required and the several length of the fabric of same type are stitched using stitching machine.

Different types of stitching are done. These are: -1) Loop stitch : - This is the most common type of stitching used for

ordinary fabrics. The machine is stationary and the fabric moves.2) Flag stitch : - This is used for delicate fabrics; in this a small piece of

fabric is folded and put in between the two ends of the two fabrics. It provides strength so that fabric withstands the processing tension.

3) Narrow stitch : - In this type the fabric is stationary and the stitching machine moves. This is done for export quality fabrics as stitching is absolutely straight and crease free.

SORT NUMBER: - The different length of either same or different quality fabrics are stitched and are marked with a number for identification by yellow marker on both ends of a length of the fabric known as sort no.

LOT NUMBER: - The total length of fabric of same quality that is processed in single day or shift is given lot no. in order to identify the fabric and is written in yellow marker which is not removable in further processing.

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SHEARING AND CROPPING

The fabric after grey checking and stitching still have certain defects which are to be removed mechanically. There are certain loose fibers and loose yarns over the fabrics. The maximum length of the loose yarns that are removed here is about 8cm. Shearing means cutting off and Cropping means cutting the loose yarns as short as possible.

The process of shearing and cropping is done in a sequence. The fabric in its full width is subjected to the curved blade roller cutter and before this the fabric is passed over brush roller so that: -

a) Any small loose fibers which can be taken up are taken up by the brush roller

b) The loose yarns are all raised up by the brushes

After passed through a pair of brush roller and a cutter the fabric is again passed through another pair of the brush rollers and cutter. This helps in shearing both sides of the fabrics.A problem arises if the stitched part passes over the cutter with the same speed, hence a seam detector is placed about 3-4m before the cutter. When a seam is detected than cutter is being stopped and it move back so that the stitch portion passes unsheared. More the no. of seams less is the efficiency of the cutter.

Cropping efficiency (%)= A-B ×100 A

A is no. of yarns originally present. B is no. of uncut yarns.

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SINGEING

After having done the shearing and cropping still very small fibers are left out on the surface of the fabric. These are termed as Protruding fibers. This process of removing these short fibers by means of heat or burning is termed as SINGEING. Cotton and cellulosic fibers behave ideally on singeing as they form light pale dusty ash on singeing as they can be easily removed. In case of blend fabrics composed of cellulosic and synthetic fibers, the pills are formed due to abrasion and singeing can minimize this. The protruding fibers if left in fabric may cause the following problems: -1) They may cause capillary action during dying and hence they will

look darker than the rest of the fabric2) In case of roller printing when the blade remove the extra color than

these fibers may cause the slippage of the blade and hence uneven printing

3) They give harsh feel to the finished fabric4) They also cause problem in printing of fabric.

There are some operations that must be performed before singeing. They are: -1) The fabric should be completely dried before singeing by passing

over drying cylinders.2) The anti mildew agents should be removed from the fabric otherwise

at high temperature gets converted into HCl and degrades the fabric.

Singeing is continuous process carried out on dry open width of the fabric. Speed of 150-400 meters per minute is typical to avoid the scorching of the fabric. The aim is to burn down the protruding fibers and not the fibers locked inside the yarn, which are not protruding. The exact running speed is largely determined by the amount of hairiness and construction of yarn and fabric.

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There are mainly three types of singeing: -

1) PLATE SINGEING This machine consists of a pair of curved copper plate on which the fabric is passed in full width. The plates are heated from bottom by oil or gas oven. When the fabric comes in contact with the red hot plates the protruding parts of fibers burns off. The transverse motion of the fabric is arranged so that the fabric remains in contact with the plate but at different parts so that local cooling of the plate doesn’t take place.

The main drawback of this process is that protruding fibers lying in the interstices are not singed. It also causes uneven singeing due to uneven heating.

2) ROLLER SINGEING In this machine roller or cylinder made up of copper or cast iron, which are internally heated, are used. The fabric moves over the heated cylinder in direction opposite to the direction of the cylinder. Here effect of local cooling is less. But here also fibers in the interstices are not singed and fabric also gets some lusture. Also due to the passage of same width of the fabric for long time, groove is formed over that region so when wider fabric is singed only selvedges get singed and this creates problem of uneven dyeing.

3) GAS SINGEING MACHINE Now a days most widely used singeing machine is gas-singeing machine. Here the fabric is just passed over a burning flame this singe off all the protruding fibers in the interstices. No unwanted lusture is obtained and both sides are uniformly singed under control flame.

The advanced modern singeing machine is mostly preferred industrially and uses following sequence:-

Feeding unitIt is concerned with giving the tight and continuous slackness supply of the fabric to the machine. For this Guide rollers, Pneumatic rollers and Tension device are used.Guide rollers are circular rollers, which guide the fabric over its width.Pneumatic rollers are used to keep the selvedge of the fabric slackness and uncurled, for this a flat plate with rollers in between is used.

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Tension devices are provided at the entry of the singeing unit and this arrangement consist of a flat plate in between two rollers. The arrangement can be inclined to increase the path length.

Brushing unitThis unit is used to clean up the loose fibers and yarns over the surface of the fabric. For this there is simple arrangement with an adjustable roller and a brush roller. Also the protruding fibers get raised up and singeing becomes much more efficient.

Singeing unitThis unit is used to singe off the short protruding fibers from the fabric surface. For getting the protruding fiber burn off a burner pair is used with a direct flame. The fuel used is mostly LPG. The gap between the flame and the fabric and the angle of flame depends on the fabric and the extent of the singeing required. During the process the fabric gets a high temperature and to avoid the chances of catching fire there is cooling unitand exhaust unit. In cooling unit the fabric is passed over metal roller in which flows cold water and bring temperature down whereas the exhaust fans take off the exhaust hot air out of department.

Quenching unitAfter singeing the fabric is exposed to the “live steam arrangement” which just aims at putting off any fire if present on the fabric purpose. In this arrangement a long cylindrical pipe with a very small slit is used. The fabric is at lower temperature than the steam hence when steam comes in contact with the fabric it condenses and brings down its temperature.

Desizing unitAfter quenching we go for the removal of the size known as desizing. The process is carried out in desizing bath, which can be open or closed. The temperature of bath is about 60 ºC

Nipping and BatchingNipping is the process of squeezing away the extra liquid from the fabric, for this the fabric is passed between the two rollers, which are arranged such that the squeezed liquid goes back to the desized bath.

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Batching is done after nipping, as fabric is to be stored for 6-8 hours for proper desizing to take place. For this the fabric is wound over roller made of rubber or ebonite.

fig 1. Modern Gas Singeing Machine

Safety Precautions during singeing1) It should be separate from other units2) It should have fire proof doors which may be closed in case of fire to

prevent the spreading3) Burners should be put off carefully. Now a days automatically burner

goes off if the machine stops4) Fire extinguisher should be there at hand

As there is always fear of fire in singeing unit so above precautions should be taken carefully and seriously.

DESIZING

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Now once the fabric is singed off we have to remove the size that is deliberately added on the yarns during the pre weaving processes so as to increase the strength of yarn, as yarn has to under go great stress and strain on the loom. Size is nothing but a mixture of starch, PVA, softeners, gums etc. This process of desizing is necessary otherwise the fabric will give varying and uneven results in the dying printing etc. Desizing is nothing but the removal of the starch by hydrolyzing it into small water-soluble compounds and to get the fabric starch free.

Methods Of Desizing

It can be done in no. of ways. Some of the methods are: -

Rot SteepingThis is an outdated method of desizing. The fabric to be desized is kept impregnated in warm water and then squeezed and stored in large pits for about 24 hours. The fabric is given hot wash followed by cold wash. During storage there is development of microorganism, which secretes some starch hydrolyzing substances.The advantage of this process is that it is very safe as no chemical is used but the demerit includes the amount of large time and space for pits required.

Acid DesizingIn this process the low concentrated mineral acids are used to hydrolyze the starch, generally concentration of 0.5% is used. The fabric is padded in the acid solution and then squeezed and stored for about 60 minutes. After this the fabric is given a thorough hot wash followed by the cold wash.The main merit is that the process is very fast, efficient and economical. But there is possibility of tendering of the fabric if acidity of solution is high, also if any part of the fabric is exposed to the air during the process then local drying of that area take place and increases the concentration of acid at that part band degrade that part.

Alkali Desizing

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Starch can also be hydrolyzed using 2-3% solution of an alkali like NaOH. The fabric is padded in the alkali solution then squeezed and stored for about 4-6 hours. It is then given a thorough hot wash followed by cold wash. The main advantage is that some scouring action also takes place during this process. But the process is expensive and there is risk of degradation of the fabric due to local drying.

Enzymatic DesizingIt is one of the most commonly used methods in the industry as it is absolutely safe process. The desize bath contains bacterial enzymes, common salt, wetting agent and acetic acid.The fabric is passed through this desize bath maintained at a specific temperature and pH range, after this the fabric is squeezed and then batched and stored for about 8 hours followed by hot and cold wash. While carrying out this process temperature limit is to be maintained, as enzymes are active within a narrow limit of temperature. They work best at 60 ºC Thermal stabilizers are also used. Enzymes need a slightly alkaline solution to work out their best; hence range of 5.5-6.5 is needed. The pH is adjusted using acetic acid.The wetting agent used should be compatible with the fabric enzymes and other chemical used. Enzymes are nothing but biocatalyst. They work for rapid hydrolyzation of the starch into smaller compounds. They are also termed as Amylase as they work for removing starch Amylose. These are highly sensitive to catalyst and work within the narrow range of temperature, ph and other chemicals. If conditions are not adequate either they don’t activate or get deactivated. Enzyme Temp. pH Malt extract 50-60 ºC 6-7.5

(vegetable origin) Pancreatic 50-60 ºC 6.5-7.5 (animal origin) Bacterial 60-70 ºC 5.5-7.5 (vegetable origin)

The amylase is further of two types: -

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a) ά-Amylaseb) β-Amylase

In case of the Pancreatic and Bacterial enzyme there is 95-99% of the ά-amylase but in case of malt extract the two types are in ratio of 1:5 to 1:6 respectively.

Action Of Enzymes On The Starch: - The work of the enzymes is to help in rapid hydrolysis of the starch. Starch itself is a combination of two types of polysaccharides namely Amylose and Amylopectin.

fig.2 Structure of amylose

fig 3. Structure of amylopectin Amylose is a straight chain polymer and Amylopectin is branched polymer. Both of the Amylose and Amylopectin are water insoluble, hence their conversion into water soluble compound is required in order to remove them. In case of Amylose the degree of polymerization is 100-120 and in case of Amylopectin it is 500-1000. The hydrolysis of the starch takes place in a very progressive way.

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Starch (Insoluble) ↓ Dextrins (Insoluble) ↓ Dextrins (Soluble)

↓ Maltose (Soluble) ↓ ά-Glucose (Soluble)

Commercially we don’t go up to last step shown but just restrict ourselves to Dextrin (Soluble) just to avoid chemicals used and time as well. Amylose is attacked by the ά-amylase in a very random way. Similarly it attacks the amylopectin. The β-amylase gets attached to one of the end of the broken simple chain and keep on removing the maltose units that are formed. ά-amylase is largely preferred for use than the other.

Oxidative Desizing In this process the desizing is carried out by the oxidation of the oxidizing agents like: -

1) Sodium Hypo chlorite2) Hydrogen Peroxide3) Sodium Bromite The oxidizing agents under optimum chemical and temperature conditions liberate nascent oxygen, which help in the oxidation of the starch into water-soluble form.

1) Using Sodium Hypo chlorite This is also known as Grey Chemicking because NaOCl is also used as bleaching agent. For this process the fabric is padded in a solution of NaOCl (2gpl) and stored for 1 to2 hour followed by hot and cold wash. The chemical concentration, temperature and pH are to be maintained.

2) Using Hydrogen Peroxide

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This chemical is very strong oxidizing as well as a bleaching agent. Hence it can carry out both the process in one go. The Hydrogen Peroxide (1%) acts as an oxidative desizing agent, the sodium silicate (1%) acts as a stabilizer and soda ash (0.5%) is just used to adjust the ph of the solution. For the process the fabric is padded in solution containing the above chemicals in right proportion and then kept in closed vessel after batching where it is steamed at 80-85 ºC. At this temperature the chemicals are active and little scouring also take place. This is not preferred as it is quite expensive.

3) Using Sodium Bromite In this process solution of sodium bromite (0.1%-0. 3%) is made and fabric is padded in it at 60 ºC for about 1 hour. After this the fabric is scoured or given hot wash followed by cold wash.

The oxidative desizing combines the action of scouring and desizing and help in increasing the basic whiteness of the fabric but is costly and not suitable for light and delicate fabrics.

Tests For Desizing Efficiency We can check how far our aim is fulfilled both quantitatively and qualitatively.a) Quantitative method % Wt loss = wt of sized f/c – wt of desized f/c ×100 Wt of sized f/cb) Qualitative method For this method we take the desized fabric and put a drop of iodine on it, and check the results.

1) If the fabric turns blue or violet it confirms the presence of starch. Hence desizing has not been done properly and redesizing is required.

2) If the spot becomes brownish it confirms that no more starch is present on the fabric and hence sizing is perfect and we can proceed to further processes.

SCOURING

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After the fabric is desized it still contains certain fats and oil and other impurities, which are mainly hydrophobic in nature and reduce the absorbency of the fabric and cause problem in subsequent processes. This process of removing off the hydrophobic impurities from a desized fabric is known as scouring. Circulating a boiling mixture of alkali through it at a high temperature carries this out.

A series of reaction takes place in the fabric. These are: -1) NaOH converts all all the saponifiable oils into soluble soap and

soluble glycerine.2) Proteins are all degraded into soluble amino acid3) All types of mineral matters is dissolved4) All dust dirt particles are removed by the detergents5) Waxes are emulsified by the soluble soaps.

Chemicals Used For Scouring Process1) Alkali like NaOH2) Wetting agent like soluble soap3) Sequestering agent which help in keeping metal ions away4) pH stabilizer like soda ash5) Reducing agent

The main actions, which take place during scouring, are1) Saponification2) Emulsification3) Detergency

Saponification: -This is a process of conversion of insoluble and water immiscible oil into water-soluble product. The oil present in the size are all water insoluble. When these are treated with an alkali say NaOH at high temperature the outcome is not sufficient, this is because of the high surface tension of water and avoids the wetting of the hydrophobic surface. For this problem we uses wetting agent which have tendency to reduce the surface tension of the water e.g. soapIn the presence of this the higher fatty acid are converted into fatty acid and glycerine, glycerine is water soluble and fatty acid formed is further reacted with alkali giving the soluble salt of alkali.

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Emulsification: -When we saponify the fabric we get the oils removed but mineral oils are still present and are removed by this method.Emulsion is a stable mixture of two otherwise immiscible liquids. We use an emulsifying agent, which keeps the emulsion formed for a longer time. When the mineral oil or waxes are emulsified they can be easily removed by washing.

Detergency: -The dust and dirt particles are removed by using a good detergent. The detergent keeps the dirt and dust particles in a stable suspension in water and does not allow them to settle again.

The scouring can be done in following ways: -1) Kier and J-Box when scouring is to be done in rope form2) Open width pad roll system when open width treatment is required

A) Kier is the most often used machine for the scouring process in the industry. They are horizontal and vertical cylindrical vessel in which the fabric is piled up in the rope form and then hot alkaline scouring liquor is circulated through the fabric. They are generally made up of iron and are available in various capacities; generally 2-ton kier is used. The kier can be horizontal or vertical and open or pressure kier. The process of arrangement of the fabric in a particular way is known as piling. It can be done automatically with the help of vertical trunk that keep moving backward and forward to ensure even piling. Along with this scouring liquid is also passed so that rapid impregnation fabric with chemicals and water takes of place. This is termed as mechanical piling. Another is done manually in which a worker is required but this is not uniform and also scouring liquor cannot be poured at this time. Hence it can also lead to seepage of scouring liquor and improper impregnation and hence improper scouring. The process of getting the fabric piled up, the liquor circulating inside the kier is known as kiering. It consists of following steps. First the fabric is being piled up and then the scouring recipe is prepared. After pouring the recipe with the help of the ring spray as it help in evenly spraying of the liquor, heating and circulation of the liquor is done. Removal of air is being done, as air must be avoided. It is done to prevent the tendering of the fabric. This is done with the help of the air vent valve. Set the pressure to optimum level.

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The overloading of the fabric is being avoided as it causes improper scouring of the fabric. The temperature should be kept 130˚C and pressure should be 1.8kg/cm for about 8-10 hours. The M.L.R should be about 1:4 to 1:5. A safety valve is provided over the lid of the pressure kier to allow the excess pressure to go off in the form of the steam if the pressure exceeds the desired value. The process of unloading the fabric is known as dekiering. It is done by pressure reduction with the help of safety air vent valve. Draining of the excess liquor and then wash is given to the fabric at least two times and finally unload the fabric.

The Open Kier differs from a pressure kier in not having a lid for closing of kier that is there is no pressure in an open kier. The rest of conditions are same except the temperature is 80-90˚C B) J-Box is another important machinery for scouring and is termed as this because the shape of machine resembles to the letter J. The fabric is treated with the alkali solution before it enters the J-Box. The fabric is passed in the rope form and evenly piled up and then treated with steam for about 60-90 minutes. The scouring time being less the concentration of the alkali solution is as high as 4-5%. The process of the J-Box scouring can be made continuous by taking out the scoured fabric from one end and at the same time feeding up of the unscoured fabric from the other end. The fabric after being squeezed off from the rollers passes through pre heater where the cold fabric is being conditioned to the temperature of the machine as this help in good scouring. In the saturator the fabric is impregnated in the scouring solution. The fabric is piled up evenly in the U shape structure of the J-Box and exposed to stem for about 60-90 minutes at about 95-97˚C. The steam being condensed is drained off.

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C) Open width or pad roll process is another method but here the fabric passes in full width and hence prevents the rope markings on the fabric. The time required for the processing is 90-150 minutes and the temperature is 97-99ºC but the concentration of the alkali is 5% on the wt of the fabric. The fabric in its full width is passed through a trough, which has a large number of the bottom and top rollers and is full of scouring solution, after squeezing the fabric is moved to the reaction chamber. The reaction chamber has two types of steam pipes. One is used for direct heating other is used for indirect heating. The direct heating is used for rise of temperature and indirect heating is used for maintaining the temperature.

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BLEACHING

The process of desizing and scouring make the fabric more absorbent. But it still has the pale appearance due to the presence of the natural color material like pigments etc. These pigments cannot be removed. The only way to tackle these pigments is to decolorize those using suitable oxidizing agents. This process is known as Bleaching. The process of bleaching gives a sparkling whiteness to the fabric and hence makes it suitable for further processing like dyeing.

A good bleaching agent should have following properties: -1) It should ensure a pure and permanent whiteness to the fabric.2) It should give level dyeing properties.3) It should not cause any tendering of the fabric, which cause the loss in tensile strength of the fabric.

Types Of Bleaching: - Different methods and different chemicals are available to carry out the process of bleaching. The various chemicals used are: -

1) By using dilute Sodium Hypo chlorite solution at room temperature

2) By using Hydrogen Peroxide solution at 80-85ºC.3) By using Sodium Chlorite solution at boil.4) By using certain compounds like per acetic acid

A) Sodium Hypochlorite Bleaching: - Sodium Hypo chlorite bleaching is done by using Sodium Hypo chlorite as a bleaching agent. This process is also known as Chemicking Sodium Hypo chlorite, NaOCl is a salt of Hypochlorous acid HOCl. NaOCl is a highly unstable compound at normal conditions of temperature and pH. It does not exist in the solid form and is present in aqueous solution. It have tendency to undergo self-decomposition. 3NaOCl → 2NaCl + NaClO3

At the same time one other reaction also proceeds that is 2NaOCl→2NaCl +O2

This oxygen helps in the oxidation, which helps in the bleaching action.

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Hence in order to keep the NaOCl solution in a stable form we require the following condition: -

1) Temperature should be 0-5ºC2) Concentration should be 2N3) Store in a cool and dark place.

The pH of the solution highly influences the action of this bleaching agent. Take small amount of the NaOCl solution and titrate it against 1N HCl solution and observe the change in the pH. It is observed that at 8.4 pH even by the small addition of HCl there is sudden drop in the pH. Again the pH becomes regularly moving until it gets a similar change at pH 4.6. The two points of Inflexion are due to following reason: -At pH 8.4 the NaOH present gets completely neutralized by the HCl and the HOCl is formed by the action of the NaOCl NaOCl + HCl →HOCl +NaClHence the absence of the alkali causes the drop in that pH. Between 8.4 to 4.6 the NaOCl gets titrated against HCl and gives HOCl At pH 4.6 NaOCl gets exhausted and HOCl gives Cl2 on reaction with HCl Hence the absence of any alkali at pH 4.6 causes a steep drop in the pH on the addition of even small amount of the HCl. Now we can divide the whole NaOCl bleaching in three zones:-1) pH 12 to 8.4 (NaOH, NaOCl, NaCl)2) pH 8.4 to 4.6 (NaOCl, NaCl, HOCl)3) pH 4.6 to 1 (NaCl, HOCl, Cl2)

The NaOCl is having a very strong bleaching action at pH 7, which causes the tendering of the fabric. At pH below 4.6 the free chlorine is liberated during bleaching action and may cause the undesirable effects. At pH more than 8.4 there is not such problem and hence generally the bleaching is carried out at 10-11 pH.

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Taking the bleached fabric and treating it with N/25 NaOCl solution and the time in which the concentration of the NaOCl becomes half the original that is known as time of half change can also determine the effect of the pH on the action. For the pH where NaOCl has strong oxidative action the time of half change is less and vice a versa. This indicates that the action of the Sodium Hypo chlorite is very strong between pH 8.4 to 4.6 and is strongest at pH 7.

Conditions For Hypo chlorite Bleachinga) Concentration of NaOCl : -

The concentration used should be just enough to give 2-3 gm/l of the available chlorine

b) Temperature : -The bleaching should be carried out at Room Temperature. The NaOCl bleaching done at elevated temperature causes a rapid oxidation, which may even, causes tendering of the fabric itself.

c) pH : -The pH maintained is generally between 10-11. Preparation Of NaOCl

Commercially the NaOCl is prepared by passing free chlorine gas through a beaker containing NaOH solution. The reaction between the two gives NaOCl. 2NaOH +Cl2 →NaCl +NaOCl +H2O This reaction is highly exothermic in nature and in order to prevent any accident the temperature of the set up should be maintained as low as 5-10ºC

Main Features Of NaOCl Bleaching: -1) It gives after yellowing that is it gives temporary whiteness2) It is suitable for cellulosic fibers but cannot be used for wool, silk

etc.3) It requires more water for washing4) It is relatively unsafe5) It cannot be used for bleaching the colored material as it may even

bleach the color6) It doesn’t have any scouring action7) It is relatively cheap process

B) Hydrogen Peroxide Bleaching: -

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The bleaching carried out by NaOCl is known as Half Bleach. Now using a further strong oxidizing agent that is H2O2 can also carry out the process of bleaching. This bleaching is known as Full Bleach. The use of H2O2 is preferred because it gives permanent and more whiteness to the material. The reaction involved in the preparation of the H2O2 is: - BaO2 + 2HCl →BaCl2 +H2O2

Properties of H2O2: -1) It is colorless and syrupy liquid2) It is absolutely stable under acidic conditions3) It is sensitive to sunlight4) It decomposes if allowed to react with heavy metals5) It i8s highly unstable under alkali condition

The decomposition of the H2O2 is given by the following reaction: – 2H2O2→2H2O + O2

Normally the concentration of the Hydrogen Peroxide is expressed in volume. It is the volume of oxygen in ml liberated by 1 ml of the H2O2

at NTP on complete decomposition.

Conditions for Hydrogen Peroxide Bleaching: -1) pH Since it is known that this chemical is stable under acidic conditions, hence we need alkaline pH for its decomposition. With increase in the pH the action of this becomes more prominent.2) Temperature The temperature is leading factor in this bleaching. The optimum temperature used is 80-85ºC. If the temperature is less than the optimum than the action of the H2O2 is very less known as Under Bleaching. If the temperature is above the mentioned one than the action of H2O2 becomes very strong known as Over Bleaching. So the temperature should be controlled efficiently. For this purpose we use Stabilizer, which works for the two purposes: -1) It helps in the controlled oxidation of the Hydrogen Peroxide2) It keeps the metal ions away thereby hindering the rapid

decomposition of H2O2

The commonly used stabilizer is Sodium Silicate. Now its use is avoided, as its removal from the fabric after the process is difficult. Dyeing carried out on a fabric containing the traces of this chemical

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shows the light spots on the dyed fabric. So now a day Non Silicate stabilizer is used such as Stabilizer NS. The bleaching is carried out mainly in the following types of the machines: -

1) Kiers2) J-Box3) Open width Pad Roll system

Bleaching Recipe: - H2O2 0.5-0.8% owf Sodium Silicate 0.5-1% owf Na2CO3 0.5-1% owf Sequestering agent 0.03-0.05% owf pH 9.5-10.5 Temperature 80-85ºC

Action of H2O2: - When we bleach a fabric in presence of some alkali like NaOH than a Per hydroxyl ion is released. It is this Per Hydroxyl ion that acts as an oxidizing agent. Main Features of Peroxide Bleach: -

1) It give permanent whiteness and gives scouring action2) It is universal bleaching agent that is can be used for all kinds of

fabric3) It requires less water for washing 4) It is a safer and costlier process5) It can be used for colored material as doesn’t bleach the color.

C) Sodium Chlorite Bleaching Besides the above two methods of the bleaching we can also use Sodium Chlorite as bleaching agent. It is stable under alkaline and neutral

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condition but under acidic condition it rapidly decomposes giving out Chlorine dioxide, which acts as a good oxidizing agent This type of bleaching is not used commonly because NaClO2 have a corrosive action on the equipment and the ClO2 liberated have a very obnoxious smell.

Treatment Conditions Temperature 95-100ºC Time 1-2 hours pH 4-5

SOURINGAfter bleaching is over we usually go for an intermediate process before dyeing this is known as Souring. After bleaching the fabric contains certain water insoluble salts, which are not removed by washing. These if not removed gives a harsh feel to the fabric and also gives an uneven dyeing. Hence we go for a souring in which the bleached fabric us treated with a small amount of very dilute acid likes HCl. This treatment removes the water insoluble salts; the chlorine present and even removes the alkali traces. No washing is done after scouring.

MERCERIZING

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Mercerization is an optional process in the sequence of the wet processing. Also it is applicable to some specific fibers like cotton etc. It can be done any time prior to dyeing but mostly it is done after bleaching. John Mercer developed the process of mercerization. It involves the treatment of the fabric with the highly concentrated alkaline solution. Generally the material is treated with 25% of NaOH for relatively short time says 40-45 seconds. This is because the longer duration may even cause the degradation of the fabric. After the treatment the fabric is thoroughly washed to remove the traces of the alkali. Caustization is the process of treating the cotton with 17% of NaOH solution. This process increases the dye ability of the fabric but doesn’t add to its lusture. Stages of the Mercerization: -1) Grey stage Here the mercerization is not optimum as grey fabric lacks in proper absorbency. So some wetting agent is added but they are also not stable under mercerization conditions. Also the recovery of the caustic is not easy as grey fabric is full of impurities and they go into the NaOH solution. But at the same time some desizing and scouring may also takes place.2) Desized stage This stage of mercerization also have same problem as above but here the recovery of caustic is easier as level of impurities is less and the residual alkali can directly be used for scouring.3) Scouring / Bleaching stage This stage gives very good lusture property and mercerization action as the degree of absorbency is high and is generally preferred.

The cotton fabric is generally mercerized in the under given forms: - 1) Yarn form

2) Fabric form

Effects of Mercerization 1) Swelling

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During this process there occurs the lateral swelling of the fiber. This occurs due to preferential uptake of the NaOH. The cross-section of the fiber before mercerize action is elliptical and after the process becomes relatively circular.

2) Lusture Since the fiber cross-section becomes circular, the overall surface becomes more even and hence gives better reflection of light and so the lusture increases.3) Tensile Strength The lumen or the hollow part of the fiber cross-section comes close to such extent that sometimes the lumen may even disappear due to the mercerization process. This is due to thickening of the fiber lumen and hence the tensile strength increases.

fig 4. Change in crossection of cotton

Change in the Cellulose Structure: - By the X ray studies it was found that the dimension of the unit cell structure are altered in the mercerization process. The nature of cellulose structure is rhombus. So during the mercerization there is slight movement in the crystal lattice due to change in the angle. This movement exposes the –OH groups of the cellulose to the NaOH and hence the reactivity of the material increases and the Cellulose I is converted into Cellulose II. The complete conversion of the Cellulose I to Cellulose II is possible only under slack condition, when done under tension only partial conversion takes place.

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Tests for Efficiency of Mercerization1) Barium activity Number2) Lusture3) Axial ratio4) Deconvolution count

Barium Activity Number depends upon the amount of Barium Hydroxide absorbed by the mercerized fabric. The absorption of the mercerized fabric is more than the unmercerized one. Making the light incident on the mercerized sample at the 45º and 90º and then the amount of the light reflected is then checked tests Lusture. The angle of reflection will be 45º if the surface is smooth and the sample is properly mercerized.

Axial Ratio decreases when the cotton fabric is mercerized and is checked by the microscope. This is so because the cotton fiber has elliptical cross section and it becomes relatively circular on mercerization.

Deconvolution Count is the counting of the fibers, which don’t have any convolution. Convolution is the fold and twist, which are present in the cotton fiber. The number of the convolution free fibers per 100 fibers is termed as deconvolution count.

fig 5. Graph showing the increase in volume of cotton fiber

Ways of Mercerization

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1) Treating the material with NaOH and then stretching but this process doesn’t gives lusture and require more force for stretching

2) Stretching the material during treatment with NaOH and this requires less force in stretching and gives better lusture.

Woven Fabric Mercerization Mostly two methods are followed in the woven fabric mercerization. These are: -1) The Chain Mercerization2) The Chainless Mercerization

The Chain Mercerization The fabric is dipped twice and squeezed in the padding mangle and then passed through air passage over the cylinder, which ensures the good absorption of the caustic. The fabric is maintained under tension by the series of clips and then the fabric passes under water spray to wash the alkali out again. The fabric may shrink if width of the selvedges were not gripped.

The Chainless Mercerization In this the mangle is replaced with impregnation tanks through which the cloth passes in contact with he bowed rollers and these rollers are also used in the rinsing stage to maintain the tension. The curvature of the rollers is such that they pull the fabric in the direction of the selvedge but at the same time the cloth attempt to shrink in width as a result of the swelling action of the alkali. These outward and inward pull results in the tension in the fabric.

fig. 6 Diagram showing Chainless Mercerization

Knitted Fabric Mercerization

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Since the mercerization take place in tension and this can be easily done in case of woven fabric but in the knitted ones it is very difficult to apply the tension as it can distort the fabric structure in serious and irrecoverable way. So the process would be carried out with the fabric in tubular form on machinery specially designed for the purpose and Sodium Hydroxide is used as above.

Yarn Mercerization Using the Hank mercerization and Cone-to-Cone mercerization can do it. In the Hank mercerization hanks are loaded on to the pairs of the tension rollers and then the rollers move in the vertical direction to apply tension on the hanks and at the same time the spray release the NaOH on the yarn and the squeeze rollers start squeezing the yarn and assist the absorption of the alkali. The tension rollers also for the even impregnation of the alkali and in washing the wash water and the acid. The sprayer than sprays the hot water and then he cold water in order to remove the caustic and finally as tension is released then he acid is sprayed to neutralize the fabric.

In the Continuous Yarn Mercerization the continuous package is processed. Hank mercerization is batch operation and is the continuous one. The stretch wheel is a device which causes the yarn to be held back slightly and hence the take up package is therefore winding on the yarn somewhat faster than it is leaving the let off package. This creates the stretch that in turn provides the tension to the fabric. This process is more uniform and hence more economical also.

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PROCESSING

OF

WOOL

INTRODUCTION

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Wool is animal protein fiber mainly obtained from sheep. The wool obtained from sheep is not pure and hence we go for purification. The pure wool is about 50-60% of raw wool taken and this quantity differs from breed to breed. The impurities in raw wool vary from 30-75%. The impurities mean Pectins, dirt, wax and mineral matter etc.

COMPOSITION OF WOOL

COMPONENT %AGE Keratin 33 Dirt 26 Suint 28 Wax 12 Mineral matter 1

Among the impurities the two components that are removed chemically are suint and wax. Suint is the water-soluble impurity present in the wool. It is formed due to the drying up of the sweat secreted by the sweat glands. Also the photochemical and atmospheric action that wool undergoes also helps in the production of the suint. The suint mainly contains the potassium salts of the fatty acids like oleic acid, stearic acid. It also contains the organic acid like acetic acid, lactic acid. Suint also contains amino acids. The inorganic matter present in suint is generally 56% and acids are 34%.

Wool wax is the non-saponifiable impurity present in the wool and is often called as wool fat. Wool wax is made up of the fatty acid and cholesterol. For its removal the wool is treated with KOH solution for prolonged period of time at high temperature and the wool wax dissociate into salts of fatty acid and cholesterol. The alkaline conditions are maintained carefully during this process so as to prevent the degradation of the wool.

Dirt is mainly deposited on the wool due to the greasy matter present on the skin of the sheep. Along with the removal of the wax the dirt is also gets removed.

CROPPING

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Wool and wool rich blends are not normally singed to remove the surface hair. The problem with protein fibers is that when they burn they form a hard black residue, which is much difficult to remove from the fabric surface than the pale dusty ash that is formed in case of the cellulosic fibers. The alternate method for removing surface hair is to take it off by some mechanical technique known as cropping. The cropping machine is fitted with series of helical blades, which rotate at very high speed. The fabric passes beneath these blades and the hair is removed by the cutting action created between the rotary blade and the stationary blade beneath the fabric.

Several sets of blade are used so that if hairs are not removed in single pass than other ones remove them causing the efficient cropping. The process is slow but at the same time can be controlled easily. This process can be performed again in the finishing also.

SCOURING

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The scouring of wool is done in order to get rid of the wool wax present on the wool as this may cause the trouble in the further processing. The scouring of wool is different from cotton scouring in following way: -

1) In the cotton the waxy material is less than 0.5% but in the wool it is about 30-70%

2) In the case of cotton high temperature and high alkaline condition are there but in case of wool the strong alkali cannot be used as wool is sensitive to alkali and mild alkali is used to avoid fabric damage.

The wool can be scoured in many ways that is either in loose fiber form, yarn form or in fabric form. The different machines are used depending upon the state in which we are scouring.

Harrow machine This machine is used for the open fiber wool scouring. There are rakes that reciprocate and allow wool to move. During this movement the wool fiber gets opened up leading to the removal of dirt. The trough is filled up with a mixture of alkali and soap. The soap causes the detergency action on the wool. The heavy material gets settled at the bottom. The wool is then squeezed to remove the excess liquor. There are four bowls in sequence for efficient scouring. The concentration of the soap and alkali in these bowls is different. There is counter flow process that involves the movement of water from 1st bowl to 2nd and then to 3rd and 4th.This can be achieved by having different bowls at different height. This mechanism helps in saving water and in proper scouring. The temperature involved in the open fiber scouring is 50 deg Celsius. This is most important process as most of impurities get removed in this process.

Tape scouring machineThis machine is so called because it carries out the process of scouring by the motion of two felts, which are in continuous motion. This machine is used for the yarn scouring of the wool. This carry out the scouring at relatively milder alkalinity. The hanks of the yarn are placed between the two moving felts.

Fabric scouring machineThe wool can also be scoured in its fabric form by using this machine. The machine consists of large trough containing the scouring solution in

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which the fabric lies in the rope form. Above the trough there is pair of squeezing rollers through which the fabric in rope form is passed with the help of guide rollers. The fabric is in endless form by stitching the two ends together to increase the efficiency. The scouring liquor is made up of Soda Ash and soap solution in which the fabric remains dip for most of the time and then is taken out by squeezing the excess liquor which flow back to box

Fig 7. Diagram of wool fabric scouring machine

CRABBING

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The main problem that rises due to the scouring of the wool is that the woolen fabric becomes distorted and gave a crumble effect. To avoid this we go for a pre scouring process called as Crabbing.

The crabbing is nothing but a setting process in which the various possible distortion of the fabric is all eliminated. Wool during its pre processes undergoes various types of strain which when fabric is relaxed then these strain develops distortions in the fabric. Shrinkage also takes place, to avoid all the above problems we go for crabbing.

In this process the wool fabric in open width is passed through hot water and is then batched within the hot water and kept for 10 minutes. This process sets the fabric in a particular shape and hence avoids distortion. Small amount of the wetting agents may also be added to the water to activate the process. The pH should be 7 but the efficiency of the process increased at slightly alkaline conditions provided that it should be done with utmost care as alkaline solution may degrade the fabric. Crabbing action at acidic pH is not efficient. Moreover the alkali crabbing also gives some scouring action and hence we generally go for alkaline scouring at 9.2 pH.

The amount of liquor, temperature and pressure of squeezing should be taken care of for efficient process. In order to get even treatment the batched fabric is wounded back to initial roller.

CARBONISING

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This is the subsidiary process followed along with the scouring process. There are several types of the cellulosic impurities like vegetable matter present in the wool. These if not removed may cause problem during spinning, weaving and even during dyeing. The process of the removal of the cellulosic impurities is known as carbonizing. Treating the wool with strong mineral acid does this removal, which hydrolyze and then solublize the impurities giving the pure wool. There is dry and wet carbonizing. In the dry carbonizing the material is exposed to the fumes of HCl acid in a closed chamber at a temp of 90-94 deg. Celsius. Generally we go for wet carbonizing in case of wool fiber form. We take 8% of the mineral acid and treat the material in acid solution, squeeze it off and dry at a temp of 80-90 deg. Celsius. The dry material may be taken into crusher to crush the small cellulosic impurities. Then the fabric is thoroughly washed and gives mild alkali treatment to neutralize but this is not necessary as dyeing of wool is good under acidic conditions. Mostly we go for batch carbonization.

fig.8 Diagram showing Carbonizing of wool

MILLING

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This is neither a preparatory nor a scouring process but more of a finishing process. This process makes the woolen fabric more full and compact. This process mostly done in fabric form and is fully mechanical process carried out by the pressure factor. We can also combine milling with scouring if the impurities present are high.

There are two types of milling machine that are available. These are: -1) Rotary machine2) Fulling’s stocks machine

Rotary machine: - In this machine fabric is batched on a roller and the fabric is then beated up continuously with mechanical hammer. It is relatively older machine and is not used now a day.

Fulling’s stock machine: - A relatively better machine and is similar to the rope scouring machine. The fabric is passed through mouthpiece, then through pair of squeezing rollers and then through a spout and back to the solution. The fabric is passed as this causes compactness and sufficient scouring action also.

There are two types of milling, acid and alkali milling. In the acid we uses 0.2-0.5% of sulphuric acid. The best milling action is at pH 10 and mild alkali like Soda Ash is used.

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BLEACHING

Unlike cotton wool cannot be bleached with oxidizing agents like NaOCl. This is because these cause an after yellowing of the woolen fabric and they also cause the degradation of the wool. But we can use the universal bleaching agent like Hydrogen Peroxide. Sulphur dioxide can also be used.

Bleaching with Hydrogen Peroxide: - This can be used under following conditions: - MLR 1:8 to 1:10 pH 8-9 Temperature 50-55 deg Celsius EDTA 2gm/l Liq.Ammonia Traces Hydrogen Peroxide 0.3-0.5% owf

During bleaching the pH is kept between 8-9 because higher alkalinity may degrade the fabric. EDTA is sequestering agent, which keep away the metal ions as they may catalyze the action of bleaching agent. The treatment time is about overnight but we can reduce this to 3-5 hours by using the machine. In the bath bleaching we can use the liquor again after removal of fabric by adding the required amount of chemicals again. If the wool material is at acidic pH liquid ammonia is used to increase the pH of the wool. Sometime at low temperature and pH the stabilizer is not used, as action Hydrogen Peroxide is very slow.

For the dyed woolen material the bleaching is done in acidic conditions. The dyed yarns get decolorized in the alkaline condition but under acidic pH the decomposition of Hydrogen Peroxide is very slow. The treatment time is about 24 hours and dull whiteness is obtained.

Bleaching with Sulphur Dioxide: - This is relatively old method. Here this is reducing agent used for wool bleaching. This is also known as Stoving because treatment is given on

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the stove, which is enclosed chamber on which the woolen material is kept. Below we had the provision for keeping 2.3-3.2 kg of sulphur and it was then burnt. This released Sulphur dioxide, which does the bleaching action. The material is exposed for about 8 hours. This type is not preferred because Wool does not get a durable whiteness and color fades on exposure to air, light etc. The material also gives bad smell. Also if any trace of Sulphur Dioxide is left in material it may get oxidize to form Sulphuric acid, which cause irritative feel to the skin of the wearer.

Bleaching with Sodium Dithionate It is white powder highly reductive in nature. It is prepared by passing Sulphur Dioxide over Zinc dust and then NaOH treatment. This is commonly known as Sodium Hydrosulphite, which is readily soluble in water. This is not used generally as gives harsh feel to fabric and unstable during use. This is used in Stripping.

BIBLOGRAPHY

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1) Technology of bleaching and mercerization V A Shenai2) Textile scouring and bleaching E R Trotman3) Textile chemistry R H Peters4) Introduction to Textile mercerizing J T Marsh5) The Motivate Series Andrea Wynne6) Understanding Textiles Tortora7) Introduction to textile processing machinery

R S Bhagwat8) Bleaching Mercerization and Dyeing of cotton

Prayag 9) http://www.thesmarttime.com/Desizing.machines.html ( fig. 1)

10) http://www.scientificpsychic.com/fitness/carbohydrates1.html ( fig. 2,3)

11) http://www.textileinfo.com/en/tech/mercerize/Page02.html ( fig. 4)

12) http://www.textileinfo.com/en/tex/mercerize/Page03.html ( fig. 5)

13) http://aida.ineris.fr/bref/bref_text/bref/angalais/bref/BREF_tex-fig.2.11 (fig. 6)

14) Fig. 10.4 http://aida.ineris.fr/bref/bref_text/breftext/anglais/bref/BREF_tex_gb55.html (fig. 7)

15) http://aida.ineris.fr/bref/bref_text/bref/angalais/bref/BREF_tex-fig.2.13 (fig. 8)

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