Bleaching with Ozone:

Ozone is a very powerful oxidizing agent and having very good bleaching agent. It is very common for bleaching of  wood pulp. Following are some facts and properties of Ozone:

i)             Ozone is prepared by passing electric discharge through cold and dry oxygen and is highly unstable, thus liberates [O]. The nascent oxygen is a powerful oxidizing agent. It oxidizes organic matter and kills bacteria. Ozone acts as a sterilizer. When 2—3  of ozone is injected into the water, the water gets sterilized within 10-15 min. The only disadvantage of this method is that it is very expensive.

                                              Ozone sterilizer

ii)            Ozone is a pale blue gas, slightly soluble in water and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, where it forms a blue solution. Most people can detect about 0.01 ppm of ozone in air where it has a very specific sharp odor.

iii)           Ozone is one of the strongest oxidizing agents known, exceeded in electronegative oxidation potential by F₂ and the oxygen atom, far stronger than O₂ .

iv)           Ozone is formed from oxygen in a strongly  endothermic reaction and decomposes easily into molecular and atomic oxygen, with a half-life of about half an hour in atmospheric conditions. Ozone can be generated from oxygen, air or from other N2/O2 mixtures. The first step towards ozone formation in gas discharges is the dissociation of O2 molecules by electron impact

In the commercial production, ozone generator, in which cylindrical discharge tubes are used.  Large ozone generators use several hundred discharge tubes.  Large ozone generating facilities produce several hundred kg ozone per hour at a power consumption of several megawatts. The capacity of a big ozone generator is up to 100 kg/h. The main applications are in water treatment and in pulp bleaching

Denim Jean bleaching using Ozone:

Reduce chemical cost. Does not damage denim Short process time Reduce the operational cost  Does not leave any chemical residue on the surface.

Hydrogen peroxide Killer:

It has been observed that some time ( after bleaching with hydrogenperoxide), the presence of residual peroxide in the fabric, effect the dyability of the fabric especially for the dyes which are senstive to oxidation. The presence of residual peroxide may cause shade variation in the dyed fabric. 

In the conventional process, to remove this residual peroxide either fabric is washed several times or treat the fabric with some inorganic reducing agent. Addition of reducing agent is not a right choice because it is difficult to select optimum dosage of reducing agent and if the dosage is high, it may further affect the shade adversely. The several time washing is found to be more useful but it require  a lot of water to wash out residual peroxide. 

To solve the problem of residual hydrogen peroxide, now a days enzyme is used. This enzyme is called peroxidates or catalases. This enzyme act as a reducing agent. The application of this enzyme is ecologically friendly as only oxygen and water are produced. Only small dosage of this enzyme sufficient to reduce residual peroxide.As this process remove/kill peroxide, it is also called peroxide killer. Following process may be used in a jigger:

• Take bleach fabric after application of hydrogen peroxide 
• Wash thoroughly
• Treat this fabric with enzyme (1-2 ml/ltrs) for 5-20 mins at room temperature at neutral pH

Limitations: Following are the limitations:

• At high temperature and pH there is a poor stability of this enzyme
• To prevent the deactivation of enzyme, the bleach bath should be cooled before the addition of enzyme and alkali
• Sometimes cause unacceptable changes in the dyed fabrics

Desizing using Low-Temperature Plasma treatment: Recent development in plasma treatment of textile material has revealed that it has enormous application as an alternative to textile processing in terms if saving in energy, water etc.

(Figure source from Indian Journal of Fibre and Textile Research, Vol 36, March 2011, p42) 

 Plasma is considered as fourth state  of matter (Three states :Solid, liquid and gas). Plasma treatment is a ecological viable desizing process. In this treatment special type of equipment is used to generate Plasma. The textile material is exposed to the Plasma in an evacuated chamber. The removal of size depends upon radio frequency power, dwell time and plasma gas composition.

In this process, starch sized cotton textiles are subjected to 100-200w oxygen plasma and exposed for 8-10 min. This process convert 65% of all organic sizing materials to gaseous compounds which are evacuated. Subsequent rinsing in water at room temperature removes 30-35% of left out size. There is one draw back associated with this process. Like solvent treatment size recovery and recycling are not possible in this treatment.  

Bio-Polishing: Bio polishing is a process that uses cellulase enzymes to remove fuzz or loose fibers and dead cotton from the cotton fabric.or yarn. This treatment is designed to improve fabric quality and provide following advantages:

•         Improve pilling resistance

•         A clear, lint and fuzz-free surface structure

•         Improved drape and softness

•         The effect are durable

Removal of protruding fibres from garment surface using cellulase enzymes is called bio-polishing. These enzymes are proteins and capable of hydrolysing cellulose (cotton). In bio-polishing they act upon the short fibres protruding from fabric surface and make the fibres weak which are easily removed during washing. This process imparts soft and smooth feel and reduces fuzz or pilling tendency. This process is applicable to garment or fabric made of cotton and its blends. 

Most of the cases it is considered as a finishing process e.g. in the case of Denim washing to create fading effect. But in some of the cases it is used in the before dyeing such as in knitted cotton fabric to imitate outcome of the singeing process.

Cellulases are derived from both fungal and bacterial sources. They find extensive application on cellulosic materials and about 1 0% of the finishing of these materials is estimated to be performed by these enzymes to achieve various effects. They also find application in food, pharma and paper industries.

Cellulases have a protein like structure are susceptible to degradation due to temperature, ionizing radiation, light, acids, alkali, and biological effect factors. Cellulases are capable of breaking the 1,4-B-glucoside bond of cellulose (cotton) randomly under optimum conditions. It resulted smooth fabric surface with the loss of surface fibres and the hand becomes soft. There is also loss in strength proportional to the amount of weight reduction. 

There are mainly three types of cellulases:

1) Acid stable or acid enzyme (more effective in pH range of 4.5 - 5)

2) Neutral stable or neutral enzyme (effective at pH 7)

3) Alkaline or alkaline enzyme stable (not used widely) 

Following recipe may be used for biopolishing of cotton fabric using acid enzyme:

•         Enzyme dosage 1-2% owf

•         pH 4.5-5.5

•         Temperature 40-55^oC

•         Time 30-60 min.

To prevent any damage of the cotton fabric after this operation it is very essential that the reaction be terminated at the end of treatment. The process is called deactivation or enzyme inactivation. If the enzyme is not inactivated entirely then at the end of the reaction fibres get damaged and even extreme cases total destruction of the material may be resulted. The enzyme inactivation is therefore of great importance from the technical point of view. 

There are two distinct process of termination of enzyme:

1) Hot treatment at 80 0C for 20 minutes.

2) By raising the pH to 11-12.

Advantages: Following are important advantages

1. Hairiness, fluffs and pills are removed and thus reduce pilling problem.
2. Improve fabric hand
3. Smooth surface and Improved lustre.
4. Improved sewability.
5. Environment friendly as no harsh chemical is used

Disadvantages: Following are disadvantages out of this treatment. Both are linked together:

1. Loss in weight
2. Loss in strength

Some Important processes in Fabric preparation

Bio-scouring: Scouring yarns and fabric is key to ensuring wettability/absorbency of the textile and ensure consistent dyeing. Conventional scouring requires high temperatures and high consumption of chemicals such as sodium hydroxide, sodium carbonate and hydrogen peroxide. The conventional scouring process involving the harsh environment is slowly being replaced with environment friendly approach using enzymes. These enzymes remove the non cellulosic impurities present in the fabric. Such a process enhance the absorbency of the fabric without appreciable strength loss and also d help in the proper dyeing and finishing of the fabric. With enzymes, the process is called bioscouring, and it saves water, chemicals, and energy.

Enzyme treatments are safe to use because they only degrade one type of impurities, and do not harm the fibre. Pectinase degrade pectin, protease degrade protein, lipaces degrade all types of waxes, oils and fats. For bioscouring, combination of these enzymes are used. 

Advantages:

• Bioscoured cotton generally softer than conventionally scoured cotton
• No significant difference in colour depth or hue of the cotton fibre after scouring conventionally or bioscouring and there is no change in colour fastness properties 
• Bioscouring is less expensive than conventional scouring process
• Enzymes are bio-degradable, therefore bioscouring is eco-friendly
• Less water consumption in bioscouring, therefore lower effluent
• Saving in water, energy and time in the bioscouring process 
• Bioscouring can be used in batch and continuous process
• Better whiteness in bioscouring than conventional scouring

How to know the fabric is properly mercerized or not?

There are various methods to to test the quality of mercerization. These are:

i) Lustre, %

ii) Microscopic examination

iii) Deconvolution count

iv) Barium activity number

i) Lustre, %: The mercerized sample is illuminated at an angle of incidence of 45 degree with the help of filament lamp. Light reflected at 45 degree and 90 degree is measured by photo electric cell. Lustre is defind by the contrast ratio of specular to diffuse reflectance. The specular reflectance measurement method is considered as an accurate evaluation of lustre. 

Lustre = contrast ratio of specular to diffuse reflectance

Lustre= Light reflected at 45 degree/ light reflected at 90 degree

ii) Microscopic examination:The test involves the assessment of degree of mercerization by counting the number of twisted and untwisted fibres (deconvolution count) while viewing through the microscope. If the sample under investigation is immersed in a solution containing 20 g iodine in 100 ml potassium iodide for 3 seconds and then wash off well, mercerized cotton will be stained bluish black and unmercerised cotton will be white.

iii) Deconvolution count: Under this mercerised fibres are cut down and examine under microscope and fibre with and without convolution are counted. The numbers of fibres without convolution per 100 fibres is called "Deconvolution Count" Generally percent unconvoluted fibres are measured. Generally the unconvoluted fibre(%) in the unmercerized cotton is around 5 to 15 %, while in fully mercerized it become upto 80%.

iv Barium Activity Number: The degree of mercerization can be measured by this test. In increase in absorption properties are shown by alkali's, the ratio of soda absorbed by mercerized to that by the unmercerized cotton may be used as a parameter of the degree to which the cellulose has been swollen. From the practical point of view, Barium hydroxide number is very easy to estimate, and the ratio of uptakes for this reagent is referred to as the barium activity number. In the test, the mercerized and unmerserized samples weighing 1 g were treated with 30 ml of 0.25 N Barium hydroxide solution for 2 hours. Then 10 ml of solution (with few drops of phenolphthalein indicator) from that was titrated against 0.1 N hydrochloric acid (pink colour to colourless). A blank was also run without any fibre sample. The barium activity number is measured by using following formula:

Barium activity number = {(B-M)X100}/ (B-U)

Where B is the titration reading for blank, M for mercerised and U for unmercerized. While unmercerized cotton gives a barium activity number of 100, it ranges between 115 to 130 for mercerized fabric and 150-160 for mercerised yarns.  

ii) Hot mercerization: Due to the rapid and extensive swelling of cotton fibres during impregnation with caustic soda solution, the structure is compacted at the surface and further penetration of sodium hydroxide is almost impossible, resulting in non uniform mercerization. Mercerizing at elevated temperature lowers the level of swelling and facilitates more even penetration of liquor into the inner parts of the yarn. In the process, the fabric is saturated with caustic soda solution at 100 degree Celsius, followed by controlled hot stretching, controlled cooling and finally traditional tension controlled washing and rinsing. Through better penetration of liquor is achieved, the complete conversion of crystalline structure from cellulose I to Cellulose II is retarded at high temperature. The principle of hot mercerizing is shown in the adjacent diagram.

The process sequence of hot mercerization is given below:

Step-1:Saturation of cotton with sodium hydroxide solution (240 to 280 gpl for full mercerizing effect and 110 to 135 gpl for semi mercerizing effect) preferably under relaxed condition at temperature 80 to 100 degree Celsius for 5 to 10 seconds

Step-2: Controlled hot stretching 

Step-3: Cooling the stretched material to a temperature less than 25 degree Celsius

Step-4: Tension controlled washing up to the caustic concentration in the fabric reaches 60 gpl (Stabilisation)

Step-5: Final washing under normal conditions

Advantages:

1. The degree of mercerizatrion is more uniform

2. Due to the better penetration of alkali solution, the desired mercerising effect is achieved more rapidly

3. The period of contact between material and caustic solution can be reduced by 50-80% (5 to 10 Seconds) resulting in greater productivity.

4. More uniform colouration than cold mercerised cotton

5. Improved lustre, tensile strength, dimensional stability, easy care properties and better receptivity to subsequent scouring and bleaching.

Two important mercerizing processes, these are:

i) Slack Mercerization

ii) Hot Mercerization

i) Slack Mercerization:  The slack mercerizing is a process in which good moisture absorption, good dye uptake and stretchability (ability to extend and recover after extension) is required but strength and lustre is not so important. The slack mercerizing can be carried out for the products like bandages, casual wear, skirts, boat covers etc.  In this process, cotton fabrics are allowed to shrink without tension in a solution of caustic soda (25 to 30%) at around 30 degree Celsius for 60 seconds. The fibres during then, shrinks to a level of 15 to 19% accompanied by considerable lateral swelling of fibres in the yarn. Following three changes take place in the slack mercerizing:

a) An increase in diameter due to permanent swelling

b) A decrease in length i.e shrinkage

c) An increase in the atmospheres volume at the loss of crystalline region.

How do you carry out Slack Mercerizing:

If slack mercerization is to be carried out continously then J-Box can be used. The J-Box consists of caustic soda liquid (23%) within which the fabric is plated and remains for 5 minutes. It then passes through a nip followed by a series of was boxes (Open Soaper) and then cylinder or stenter for drying purpose. Nipping is done very near to the places where the fabric just comes out of the J-Box in order to reduce the tension. Open soaper are used for washing and souring in which the bottom rollers are over driven to reduce the tension. 

2. Chain Mercerizer:

In order to make up for the shortcomings of the roller mercerizing machine, a clip stenter is used for post-mercerization treatment, in which a widthwise tension is applied then most alkali is showered off the fabric kept on the stenter, followed by thorough alkali removal and neutralization using an open-width washing machine.In this machine a heavy padding mangle is used for the application of alkaline solution in the 2 dip/2 nip method, with sufficient time allowed for penetration and swelling of the fabric in a timing cylinder, instead of undergoing an operation using so many rollers and so much solution as in roller mercerization, to ensure reduced use of the alkali. 

It is to be noted that all the fabric mercerizing process involve 4 steps:

i) Impregnation with caustic soda solution

ii) Stabilisation (first rinsing stage under tension)

iii) Washing in open width washing range

iv) Neutralising 

Recipe for mercerizing fabric:

Caustic soda: 20-23% (48-52 ^oTw) or (28-30 ^oBe)

Temperature: approx 20^oC

Time: 30-45 seconds

Machine Feature:

In a chain mercerizer, the cotton fabric is padded with two number of 3 bowl padding mangle, the troughs of which containing the required concentration of caustic soda and wetting agent. Between the two impregnation, the fabric goes around a set of dummy rolls (for timing and swelling), which are driven by fabric movement. The two squeezing nips of the padding mangle run at different speeds, the second one running fastest than the first one, in order to maintain the required tension in the warp way. 

After impregnation, the fabric is fed into the clips of a stenter, where it is stretched in the weft direction to the required dimensions. The alkali in the fabric is rinsed out with sprays situated over the stenter, initially with hot water and then with cold water. Thoroughly and even washing out to the order of less than 60 gpl of caustic soda remaining in the fabric. Until then the tension should not be released. Then the fabric is washed thoroughly in an open width washing range and neutralised. Here a counter current washing system is employed, wherein fresh water is introduced at the delivery end of the machine and the first wash liquor that the cloth meets in the stenter is slightly alkali concentration. 

2. Chain Mercerizer:

In order to make up for the shortcomings of the roller mercerizing machine, a clip stenter is used for post-mercerization treatment, in which a width wise tension is applied then most alkali is showered off the fabric kept on the stenter, followed by thorough alkali removal and neutralization using an open-width washing machine.In this machine a heavy padding mangle is used for the application of alkaline solution in the 2 dip/2 nip method, with sufficient time allowed for penetration and swelling of the fabric in a timing cylinder, instead of undergoing an operation using so many rollers and so much solution as in roller mercerization, to ensure reduced use of the alkali. 

It is to be noted that all the fabric mercerizing process involve 4 steps:

i) Impregnation with caustic soda solution

ii) Stabilisation (first rinsing stage under tension)

iii) Washing in open width washing range

iv) Neutralising 

Recipe for mercerizing fabric:

Caustic soda: 20-23% (48-52 ^oTw) or (28-30 ^oBe)

Temperature: approx 20^oC

Time: 30-45 seconds

Machine Feature:

In a chain mercerizer, the cotton fabric is padded with two number of 3 bowl padding mangle, the troughs of which containing the required concentration of caustic soda and wetting agent. Between the two impregnation, the fabric goes around a set of dummy rolls (for timing and swelling), which are driven by fabric movement. The two squeezing nips of the padding mangle run at different speeds, the second one running fastest than the first one, in order to maintain the required tension in the warp way. 

After impregnation, the fabric is fed into the clips of a stenter, where it is strtched in the weft direction to the required dimensions. The alkali in the fabric is rinsed out with sprays situated over the stenter, initially with hot water and then with cold water. Thoroughly and even washing out to the order of less than 60 gpl of caustic soda remaining in the fabric. Until then the tension should not be released. Then the fabric is washed thoroughly in an open width washing range and neutralised. Here a counter current washing system is employed, wherein fresh water is introduced at the delivery end of the machine and the first wash liquor that the cloth meets in the stenter is slightly alkali concentration. 

b) Knit Mercerization 

1). Open Mercerization 

To carry out this types of mercerization on circular knits, it is required to first open the tubular knits using tubular slitting device as shown in the figure.

For mercerization purpose the open knits are glued to make artificial selvage just like woven fabric. If opened knits are not glued, it is possible it would curl. Due to curling mercerization would be difficult.  After gluing (making artificial selvage) the mercerization is carried out similar to woven cotton fabric.

It is important to control the change in dimension of cotton knits. To control dimensions, it is needed to keep residual shrinkage below 2%. Another important factor in knits is that during processing there should not be any deformation in the loops. Therefore, while mercerizing, one has to take care of these factors. Beside this mercerization should bring good absorbency, softness and luster in the knits.

                 

2). Closed Mercerization (Tubular Mercerization) 

As its name indicates, in such cases there is no need to open knits. The mercerizing is carried out in the tubular form. In this type of mercerization two tubular fabrics in flat form is fed in to the machine side by side.

During mercerization, strong pressure/tension is applied on longitudinally direction to the folds of the two edges of the long sides of the fabric. Due to this tension some time it causes differences in yarn density on both faces of the knits, which results edge mark on the inside of the fabric. This can be avoided by controlling pressure/tension and adjustment of nip. Other methods that prevent the formation of edge marks include introducing air to the inside of the tube at strategic spots due to which the knit expands to a cylindrical shape, setting several round plastic blades or rings with alterable circumferences at intervals inside the tubular knit, or inserting a bar-shaped frame inside, thus preventing the fixing of the edges during the treatment, and also expanding the width of the fabric. 

4. Tow mercerization 

In this type of mercerizing a rope of 400 or more yarns are wrapped around the beam. This type of beam is called balls. 8 to 10 of such balls are put on a stand and the each rope is un-wrapped from the each balls simultaneously. These 8 to 10 ropes of yarns are mercerized continuously in a manner similar to that of roller mercerizing of fabric. 

It is very important to appropriate twist the yarns forming a rope. High twist of yarn may lead to uneven mercerising. 

The instrument used in this mercerizing similar to the Indigo Rope dyeing machine or row of soapers.  

The tension on the yarn is controlled, and the shrinkage due to swelling during absorption of the alkali and the level of strain after this can be adjusted freely. 

Advantage: This type of mercerizing provide large amount of yarn with good quality. This mercerized yarn may be used for knitting purpose. However, in addition to producing a soft hand in the final product, this type of mercerization is very even, and so it is the best method for attaining level dyeing. 

Disadvantages: However, when treating several thousand threads of yarn at once at a rate of around 20m/min, it is difficult to ensure that each yarn is sufficiently mercerized in comparison to the single-end method.

The other problem associated with the equipment used is the separation of the yarns in the rope after it is dried at the conclusion of the mercerization process, and the way in which the yarns are unwound is very important for ensuring the smoothness of the operation. 

5. Warp mercerization 

While tow mercerization involves the treatment of a lot of yarn lined up in rope-form, in warp mercerization yarn is wound onto a beam and fed into a machine with the same system as in a slasher-sizing machine. Mercerization takes place with sheets of separate threads, and the machinery used can be exactly the same as that in tow mercerization. 

Thus, in the warp-beam method and the tow method, only the handling of yarn is different. In the tow mercerizing, yarns are handled in the rope form (having 400 or more yarn in one rope), while in the warp mercerizing the yarn are handled in sheet form. The penetration of alkali is more in the sheet form than tow form.

Disadvantage: The main problem associated with warp mercerising  is that during treatment the breakage of a single thread can lead to major difficulties, and so if the yarn is not of very good even quality, industrial implementation of this method is difficult. 

2. Cheese mercerization 

Cheese mercerising is not as popular as Hank mercerizing. As in in this type, mercerization is carried in cheese form, full mercerization is not possible. 

Another problem associated with this type of mercerization is that there is no control of shrinkage inside and outside of the yarn wrapped on cheese.

3. Single-end mercerization 

In this case mercerizing is carried out on yarn in continuous process. The mercerizing may be cone to cone or cheese to cheese. Following process flow is adopted for mercerizing:

• Yarn (cone or cheese form) is passes through the alkali solution
• Rinsing with hot and cold water
• Neutralisation
• Rinsing
• Drying
• Mercerized yarn taken on Cheese or cone

Disadvantages: As in such type of mercerizing only single end of yarn is mercerized at around  450m/min, the productivity per machine is low. The machine is  costlier. 

Advantages: As the process sequence is automated in this machine, it only requires a very small number of staff. A problem in this form of mercerization is the relative difficulty of controlling the tension on the yarn as it is introduced, and differences in the level of tension between machines and between cheeses or cones can occur easily and lead to patchy dyeing. Other problems relate to yarn breakage, yarn overlap, and yarn skewing. Yarn count also influence the performance of this type of mercerizing. It is difficult to mercerized single fine count yarn. 

Changes in the cotton material in mercerizing:

Mercerizing brings various types of changes in the cotton materials. Some of them discussed below:

1. Changes in fibre: Following changes take place:
a) Swelling of fibre
b) Cross section morphology changes from bean type shape to oval shape
c) Fibre length decreases due to shrinkage

2. Changes in molecule:
a) Due to swelling there is a adjustment of hydrogen bonds
b) Orientation of molecular chains i.e parallelization of chain in amorphous region as well as crystalline region along the direction of fibre length
c) Increase crystallinity
3. Chemical changes:
a) Increased rate of reaction on hydrolysis and oxidation
b) Liberation of heat during the caustic treatment (heat of sorption and heat of reaction)
c) Increase in the alkali and iodine absorption


Let us discuss various types of mercerizing:

a) Yarn Mercerization:

Yarn mercerising can be caried out in two types of yarns:

• Grey yarn mercerizing
• Wet mercerizing (after wet treatment such as scouring)

1. Hank mercerization :

Hank mercerization is most common mercerizing technique among the yarn mercerizing techniques. In the hank mercerizing yarn is wrapped on two adjustable rollers  so yarn will look like a hank. The yarn is moved by the turning of the rollers, with penetration of the alkali, application of tension and rinsing occurring automatically. 

In this, one cycle takes about three to five minutes, and four to eight kilo grams can be treated at one time. In the latest machines, all operations are automated, including control of the alkaline solution's concentration and temperature and the addition and recovery of the alkali, along with application of tension on the yarn and rinsing. 

Advantage: An advantage of hank mercerization is that during the yarn's absorption of the alkali, treatment can be carried out without tension, and so the alkali solution is able to penetrate the inside of the yarn sufficiently, and after the fibers have swelled sufficiently, any level of tension can be applied and the yarn rinsed for removal of the alkali. This allows the production of goods with satisfactory mercerization effects. 

Disadvantage: However, if the winding or the handling of the hank is inappropriate, disarrangement of the lengths of yarn in one loop can result in different tensions, leading to uneven mercerization, which can often result in patchy dyeing. 

Another problem in hank mercerization is that conventional rinsing after mercerization is insufficient, and without neutralization through separate rinsing with hot water, the remaining alkali can cause problems. 

Steps for Hank mercerizing:

Following steps are taken for hank mercerizing: 

Loading yarn on the rollers-----------> Lowering of rollers-----------> Caustic tray movement --------> Pre tensioning-------------> Free shrinkage----------> Lye tensioning--------> Squeezing---------. Washing tray movement---------->First wash and tensioning----------> Second wash-------> Third wash------->Final wash-------> Squeezing-------> Lifting of rollers------> Unloading yarns

Changes in cross section view of cotton with strong alkali:

When cotton fibre treated with strong alkali, it swell. The cross section view of cotton fibre changes from bean type (squashed circular pipe) to oval shaped when reacted with strong alkali like sodium hydroxide  during mercerization. This change in cross section brings luster in the fibre. Following figures shows the changes occur in cross section of cotton fibre during mercerization:

The swelling which occur in the cotton fibre with strong alkali treatment is relative to the longitudinal shrinkage in the cotton fibres. It means that with the increase in swelling of the fibre, length of the fibre decreases. 

Occurring Physical Changes in cotton during mercerization:

Mercerization brings following physical changes in the cotton:

1. Swelling of fibre in the amorphous and intra crystalline regions
2. Swelling brings breaking of old bonds
3. Opening of fibre structure
4. After washing sodium hydroxide is removed and new bonds are formed
5. Increase in amorphous region and Increase in number of OH groups, due to these two factors the cotton has higher moisture content which resulted higher dye uptake and reactivity
6. Slack mercerization has more amorphous region than tension mercerization

Effect on strength of cotton by mercerization: 

Mercerization process removes convolutions which also remove weak spots at the point of reversal. The caustic treatment align fibres in the new configuration. More parallel and compact fibres due to removal of convolutions. The  more parallel and compact fibres obtain when tension is applied. Due to these changes higher strength is obtained in the mercerized fabric/yarn than unmercerized fabric/yarn. The strength is found to be more in the tension mercerization than slack merceisation. 

Types of Mercerization 

Following are the important Mercerization methods:

a) Yarn mercerization 

Batch : 

• Hank mercerization
• Cheese mercerization

Continuous : 

• Single end mercerization
• Tow mercerization
• Warp mercerization

b) Knit fabric mercerization

• Open mercerization
• Closed mercerization (Round mercerization, tubular knit mercerization)

c) Woven fabric mercerization

• Chainless mercerization (Roller mercerization)
• Chain mercerization (Stenter mercerization)
• Batch-up mercerization