Yarn Mercerisation - Part-2

Mercerisation:
The mercerising process introduces changes to cotton yarns e.g. increase in lustre, increase in tensile strength and increase in dye affinity.
If tension is applied, mercerisation generally causes increase in strength from 10% to 40% depending on the yarn construction.
The moisture content based on the dry weight of cotton increases with the concentration of caustic soda used. The percentage moisture present ranges from 6-12%.

Affinity for Dyestuffs:
The weight of dyestuff absorbed increases with increasing concentration of caustic soda up to 13.5% (30°TW) and thereafter the increase is less rapid. Tension applied and drying also have an effect on the affinity.. The effect of tension is to decrease the amount of dye absorbed when compared with yarn mercerised without tension. Drying a mercerised yarn decreases the affinity for dyestuff, this decrease being greater the higher the temperature of drying.

Due to this change in affinity for dyestuffs, air drying of mercerised cotton must be avoided after the yarn has been mercerised and before it is passed on to the next process. Precautions must be taken to keep such yarn wet, otherwise unlevelness in dyeing is likely to occur.

Wetting Agents in Mercerising:
Wetting agents are added to mercerising liquors on order to obtain quick penetration of the caustic soda solution. Several types of such agents are available. The most commonly used is Cresol which is relatively cheap in price. A small amount of a higher alcohol such as Butyl carbitol or butyl cellosolve is added to assist penetration and as an anti-foam.
To get the best use of a wetting agent it should be soluble and have good stability in caustic soda solution of mercerising strength (53-54° Tw). In some cases an added advantage is found if the wetting agent is soluble n strong caustic solution (76-80°Tw) used as feed liquor to the mercerising machine. By this means, the necessity for adding wetting agent to each of the machine tanks can be avoided, if facilities exist for providing a strong liquor feed line to each machine.
The quantity of wetting agent used is about 1to 2% by volume. This quantity should be sufficient to give a wetting time of 4-5 seconds under the standard conditions for wetting out.
Tests to check the wetting out properties of the mercerising liquor should be carried out every 4 hours or as necessity demands.
Nowadays use of cresylic wetting agents are prohibited by local authorities due to contamination drainage areas with phenolic compounds which exist in the wetting agent.