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.

February 14, 2008

Yarn Dyeing

Yarn Dyeing:

In general wet processing of textiles, whether it is fiber, yarn or fabric dwells with the same chemistry. Only the application machinery is different.

Garment dyeing is one more divide in textile processing, where stitched garment pieces are dyed in the final stage.

The manufcturer of garment selects the method of processing according his special end use requirements. For example, if he wishes to produce a muti-colored stripped or checked shirting, he would opt for yarn dyeing and on the other hand if he wants to produce a solid shade for some trousering material, he prefers fabric processing after weaving are knitting.

According to his requirement,the manufacturer selects the phase of processing. In this blog we wish to focus on the various aspects of yarn dyeing.

We are well aware that yarn is available in the form of reeled hanks and wound cone/cheeses.

At this point the yarn dyeing industry itself divides in to two sectors. One as Hank yarn dyers and the other cheese dyeing units.

Hank Yarn Processing:

Any wet processing comprise of 3 parts. (1) Pretreatment, (2) Dyeing and (3) Finishing. Processing of yarn also comprise of the same above 3 parts. Let us have a look in to this.

Pretreatment of Yarn:

Pretreatment, as the name suggests is the first part in processing. According to the end use of yarn, the pretreatment starts. If the apparel manufacturer requires, a flaw less, smooth lusturouros yarn for his baby garments, then the process sequence is the one below.

1. Gas singeing
2. Grey Mercerising
3. Scouring and bleaching

Cheese Dyeing:

Special machineries are used to process yarn in cheese form. Cheese dyeing machineries are more sophisticated and automated when compared to the hanr dyeing machineries.

As said above, pretreatment of yarn in cheese form comprises of the following stages.

1) Gas Singeing and (2) grey mercerising has to carried out either from hank form or cone form. If the yarn is available in cone form, it is wound from one cone to another and in between the pasage length the gas singeing would be carried out.

2) Mercerisation is possible only if the yarn is in hank form.

3) Preparation of yarn in cheese form (Cheese Winding)ie., from hank or cone form it has to be convented in to cheese form using special winding machines. This is a very important stage in cheese processing. The package density of yarn over the cheese should be uniform throughout the length.

4) Scouring and bleaching.

Let us have detailed description of all stages one by one in my next posting. You can have a good reference of all textile processing in the "Textile Processing Guide - thesmarttime.com"