February 15, 2008

Hank Yarn Processing

Processing of Hank Yarn:

Weaving and dyeing are two eyes of a single face. From age old time, dyeing was carried using so many natuarally available resources. Experimentation to form different colors had been going on still then. During early days of these experimentations, yarn was available only in hank forms.

Hank yarn has been dyed manually from tubs and vats, throughout the world until today. Right from scouring to bleaching and dyeing, are being done in small vats. This trade is still running as cottage industry in India, Sri Lanka, Bangladesh and some interior parts of Africa.

At this stage this Hank Yarn dyeing industry divides in to two parts.

1. Manual dyeing of Hank yarns
2. Machine dyeing of Hank yarns.

Manual dyeing of Hank yarns :

Though the chemistry of processing does not change in any manual or machine dyeing, there are interesting and skilled workmanship in manual rather than machine processing. The dyeing industry was started only as a cottage level and still in most part of the world we can see the manual processing.

For manual processing the requirments of tools to do the work are very few. A plastic, wooden or stainless steel vessel according to the volume of material to be processed, a holding device for the material such as a stick, pole or stainless steel rods, a stirring device to either stir the water bath or the material according to the possibilities and convenience, the dyestuffs and chemicals and a weighing balance.

The processing sequence would be - Wetting or soaking the material, scouring, bleaching, neutralization, dyeing, finishing and drying.

Wetting : The greige cotton yarn does not have the tendency to absorb water, a natual coating of wax and oil over the surface of cotton prevents it from being wetted out. To wet the yarn uniformly, in a tub of water, a good wetting agent of OT type is added (0.5 to 1 gram/liter). The greige goods are soaked in this bath over night. Next day the wet yarn from the bath is taken out and dressed on poles or sticks, for bleaching.

Bleaching: Usually a hypochlorite bleach at room temperature would be carried out in wooden vats or tubs. The hank yarn that is hanging from poles are being turned upside down every 5 to 10 mintutes cycle. The bath would contain 10 to 30 g/l of 35% Calcium Hypochlorite. After the bleaching process is over (after 30 to 90 minutes), the yarn is taken out from the poles and washed thouroughly in tub containing running water. The washing would be followed by neutralization of pH and removal of hypochlorite, simultaneously by treating the wet yarn with 1 to 2 g/l of Sulphuric acid for a few turns in the washing tub itself.

Dyeing of Reactive Cold Brand Dyes: Since these manual units does not have any heating facility to heat the water bath, only processing at ambient temperature is possible. Cold brand reactive dyes are most suitable for this.

The water bath is prepared with 0.5 g/l of acetic acid to make the pH slightly acidic - 5.5 to 6. The wet bleached yarn squeezed and spread evenly on poles. Then the properly dressed yarn is entered in to the water bath containing only acetic acid. The hank yarn is worked in this bath by continuous turnings for about 15 minutes. Then the dissolved dyestuff is added in to the bath in two portions. After the addition of dyestuff, the turns are continued for 30 minutes. Adequate quantity of common salt is weighed, dissolved, filtered and added in to the dyebath in two portions. The dyeing is continued for another 45 minutes. Soda ash is weighed for the required amount and dissoved. It is also added in to the bath in two portions. Then the dyeing would be continued for 45 to 60 minutes depending up on the depth of the shade. After the dyeing is over, the yarn is taken out from the poles, washed in running water, soaped at cold with excess soaping agents, again washed and finally fixed with a cationic dye fixing agent.

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