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