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

What changes mercerising bring on cotton? 

• Improved luster
• Increased ability to absorb dye
• Improved reactions with a variety of chemicals
• Improved stability of form
• Improved strength/elongation
• Improved smoothness
• Improved hand

Reaction of Cotton with alkaline solution:

     If cotton is dipped into a strong alkaline solution such as lithium hydroxide, caustic soda, or potassium hydroxide, the fibers will swell and shrink. One can get following  two effects:

•       Under Tension: Yarn/Fabric are placed under tension while in this swollen state and then rinsed with water, the alkali will be removed and a permanent silk-like luster will result.
•        Without Tension:  Alternatively, after swelling, if the alkali is rinsed off when the fiber is in its shrunk state, an increase in luster may not be obtained, but the fibers will fix in that shrunk state, thus giving good elasticity to external stress.  

       Behaviour of Cotton fibre with different concentration of Sodium hydroxide: If a cotton hair is dipped in different alkaline solutions, no change in appearance will be visible up to 10°Bé (6.58%), but above 11°Bé (7.3%), the hair slowly loses its natural twist (this usually being in the order of 150-300 times per inch). 

       Above 13°Bé, untwisting and shrinkage in the longitudinal direction now gradually starts to increase, and as the concentration nears 16°Bé, untwisting and shrinkage advance rapidly. Between 18 (12.69%) and 22°Bé (16%), shrinkage of the length reaches its maximum and untwisting for the most part ceases. 

Relationship between density, Baume (Bé°) and Concentration (C%)

Mercerization:

Treatment of cotton yarn or fabric with concentrated sodium hydroxide for a minute or less is called mercerization. The process of mercerization is named after the John Mercer, who was the first to discover in 1844, that the treatment of cotton with concentrated sodium hydroxide solution brought about permanent changes in the fibre's properties. He claimed that the treatment resulted in significant  increase in the tensile strength, water absorbency and dyeability of the fibres. It was also observed by him that treated material underwent considerable shrinkage. In the case of yarn, contracting in length was observed while in the case of fabric, shrinkage in both the warp and weft direction were observed.The excessive shrinkage in the cotton yarn and fabric make this process nonviable in the industry thus it was not commercialised. 

It took more than forty years until it was found by H.A.Lowe that shrinkage could be prevented if the yarns or fabrics were kept under tension during mercerization. When mercerizing is done under tension, the fibre also acquire a silk-like luster.

3. Bleaching with Sodium Chlorite (NaClO₂):

Bleaching process with Sodium Chlorite is carried out in acidic conditions. During the process yellow-brownish ClO₂ gas is released which is toxic in nature. The bleaching action is supposed to be carried out by this gas. When a solution of NaClO₂ is acidified, ClO₂, hypochlrous acid (HClO₂), sodium chlorate (NaClO₃) and NaCl are formed. ClO₂ and HClO₂ are used as bleaching species. 

The optimum pH for bleaching with sodium chlorite is 3 to 5. Which can be achieved using formic, acetic or phosphoric acid.  Below pH 3, there may be severe damage to the cotton fibre. Bleaching rate increases if the temperature is around 90^oC. Below 50 ^oC, there is a negligible bleaching effect.

This bleaching agent can be used for acrylic, polyester and rayon. It removes all kitties, wax etc from cotton and also the feel of the fabric is very soft. It gives better whiteness than hydrogen peroxide. 

There are some disadvantages also associated with this bleaching agent. It creates pollution as ClO₂ liberate during process. It also corrodes equipment. This bleaching agent cannot be used for continuous process as H₂O₂. 

Effect of pH on Hydrogen Peroxide bleaching:

Hydrogen peroxide is applied under alkaline conditions, and the optimum pH for bleaching is between 10 and 11. At a pH below 10 hydrogen peroxide is quite stable, and oxidation will occur too slowly. When pH of the bleaching bath is above 11, hydrogen peroxide decomposes too fast and may cause damage to the fibres. Beyond pH 11.8, all the hydrogen peroxide converted to HOO and reaction is out of control.

Effect of Time duration and Temperature:

The ideal bleaching temperature for hydrogen peroxide is 95 to 100 degree C. Below 80 degree C, the evaluation of perhydroxyl ions are slow. Due to this rate of bleaching is also slow.  The time duration of bleaching depends on the temperature, type of fibre and machines. 

Concentration of Hydrogen peroxide:

The concentration of hydrogen peroxide also depends on type of machinery being used. In the case of Batch process (Jigger etc), 2 to 4 % Hydrogen peroxide (o.w.f) is taken with MLR of 1:10 to 1:20. While in the continuous process 1-2% (o.w.f) [resultant concentration as in the continuous process it is used in g/l) will be sufficient as higher concentration may damage the fibres. 

Stabilizer: 

Beside hydrogen peroxide and a base (Sodium hydroxide or sodium carbonate), the bleaching bath also contains stabilizing agent. Sodium silicate (Na2SiO3.5H2O) has been found to be the most suitable stabilizer. It is a base that acts as a buffer at the proper pH of 10.5 to 11.5, and in addition, it prevents the catalytic decomposition of hydrogen peroxide caused by heavy metals. As we have already discussed that the catalytic effect of transition metal ions (e.g Fe+++ or Cu++) on hydrogen peroxide is accelerating its decomposition may cause great damage to the fibres. Sodium silicate also act as a sequestering agent and with transition metals forms soluble complexes. Mg++ ions, however, have been found to posses a stabilizing effect on hydrogen peroxide. Accordingly, when using soft water a small amount of a magnesium salt is added to the bleaching bath.

Now a days organic stabilizer also being used also known as silicate free or non silicate stabilizers. 

Beside stabilizer,  sequestering agents and wetting agents also used during bleaching process. Most common sequestering agents are EDTA and Sodium hexa phosphate.

Advantage and uses:

Hydrogen peroxide is also known as universal bleaching agent as it is most widely used for bleaching fibre like cotton, rayon, wool and silk, environment friendly and economical viable also. Unlike hypochlorites bleach, it does not require full scouring. It gives permanent whitening effect and less damage to the fibres.

 Bleaching recipe for guidance:

The process wise recipes of bleaching of cotton for guidance are given below: 

i) Batch process:

a) Formulation:
Hydrogen peroxide (35%)=4 to 8% (o.w.f)
Wetting agent=0.1 to 0.5 % (o.w.f)
NaOH=0.5 to 1.0% (o.w.f)
Sodium Silicate=2 to 4% (o.w.f)
b) Process parameters:
Temperature=95 to 100 degree C
Time= 60 to 90  Minutes 

After the above process, drop the bath and material rinse well with slightly basic solution to avoid the formation of insoluble silicates. Finally it may be neutralized using acetic acid or green acid. 

ii) Continuous process:
 In a typical continuous process in open bath the padding bath contain the following:

Hydrogen peroxide (50%)=1.5 to 2.5% 
Wetting agent=0.1 to 0.2 % 
NaOH=0.2 to 0.6% 
Sodium Silicate=1.0 to 1.5% 

After padding, the fabric is passed through a steamer. Steaming time may be vary from as little as a few minutes to one hours or more, depending on the type of steam and steamer used.

Combined scouring and bleaching process:

For saving time, energy, labour etc, combine scouring and bleaching is very popular.

In oxidative desizing, where scouring takes places as well, the cotton fabric is padded with a solution containing sodium hydroxide, hydrogen peroxide, a stabilizer, and wetting agent. After padding, the fabric is steamed for the proper amount of time depending on the type of steamer and the concentration of chemical used. After steaming the fabric is rinsed with hot water several times to remove the polyvinyl alcohol and other emulsified. During the tratment, sizing chemical is oxidized and become water soluble. 

The combine scouring and bleaching process is very popular in the cotton hosiery processing.   

Bleaching with Hydrogen per oxide (H2O2):

Hydrogen peroxide (H2O2) is a simplest peroxide (compound with oxygen-oxygen single bond). The pure hydrogen peroxide is a colourless liquid, slightly viscous than water. It is strong oxidizing agent and used as a bleaching agent and disinfectant. It was first used as bleaching agent in 1920's. Today, it is estimated that 90 to 95% of cotton and cotton/synthetic blends are bleached with hydrogen peroxide. For commercial use the the hydrogen peroxide is available 35 to 70% concentration. It is ecologically acceptable and economically feasible.  

Mechanism of Hydrogen per Oxide:

Hydrogen peroxide is weak acid and in water it is ionize to form a hydrogen ion and a perhydroxyl  ion   as per the given equation:

H2O2  +  H2O----------------H+         +       HOO- (Perhydroxyl ion) 

This perhydroxy ion is the active hydrogen peroxide and unstable and break down as per the following equation: 

HOO(-) ---------------  OH(-)    +   O (Active and Nascent oxygen)

The bleaching, most probably takes place via the reaction of HOO(-) or O with coloured material (Like grey cotton) where by they oxidized into colourless compounds. 

Since the amount of HOO(-) at pH 7 is extremely small, neutral solutions of hydrogen peroxide are quite stable.The catalytic effect of transition metal ions (e.g. Cu++, Fe+++) on hydrogen peroxide, accelerating its decomposition and may cause great damage to the fibre.

In order to activate hydrogen peroxide for the bleaching reaction to take place, a base is added to the bleaching bath. The base will shift the equilibrium dissociation to the right:

NaOH    +   H2O2 ----Equilibrium-----------------Na (+)      +   HOO (-)   + H2O

Effect of processing parameters on sodium hypochlorite bleach:

It is very important to maintain processing parameters like pH, time and temperature during hypochlorite bleaching process. The effect of various parameters on bleaching action given below:

i) Effect of pH: pH plays an important role for getting good bleaching effect without damaging cotton fibres.

If pH is less than 10, there will be almost no bleaching take place.

If pH is from 5.0 to 8.5, mainly HOCl is formed. Due to this the bleaching action will be very fast and this may lead to rapid degradation of fibre.

If pH is less than 5.0, chlorine gas is librated and there will be almost no bleaching action.

Therefore for hypochlrite bleaching process the optimum pH should be maintain in between 9 and 10. To maintain this pH, Sodium Carbonate is used.

ii) Effect of time and Temperature: Best results can be obtained if bleaching can be done at 35-40 degree C for 45 to 60 minutes.

Important note: Hypochlorite bleaching should be carried out in stainless steel. If the pot is made of copper or iron, it is possible that iron and copper catalyze the oxidation of cellulose by hypochlorite, which ultimately damage cellulose fibre. It is also advisable that the water used in the bleaching process should be free from iron and copper.

Bleaching process:

Following recipe may be used for bleaching of cotton using sodium hypochlorite:

NaOCl                    : 2.5% active bleach

Sodium Carbonate:  to maintain pH 9 to 10

Prepare the bath with the above ingredients and run the material from 45 to 60 minutes. The temperature should be maintained from 35 to 40 degree C. After the bleaching process, drain the bath and wash the material in cold water and then treat it with antichlor.

What is Antichlor? Fabrics bleached with hypochlorite will develop a distinctive chlorine odor. This odor is due to presence of residual chlorine in the cellulose material. This odor can easily be removed with an after treatment consisting of sodium bisulfite and acetic acid