April 03, 2008

Liquid Ammonia Treatment of textile yarn

Liquid ammonia can be regarded simply as a new medium for tailoring the dimensions and properties of cellulosic materials to shrink, swell, stretch and relax and can therefore be used to obtain a variety of effects on many materials.
The economics of cotton yarn manufacture hinge on the price of raw material comprising it, e.g. more than 25% of the cost of a cotton sewing thread is accounted for by the raw cotton price. An accepted yardstick of a cotton is the strength it produces in yarn and thread forms and it was to this end that much of the development work of the liquid ammonia process was designed.

Properties of Liquid Ammonia Treated yarn:
The following properties have been established for liquid ammonia treated yarns:

  1. Tensile strength significantly increases.
  2. The elongation at break is only about 2/3 that of untreated yarn.
  3. Loop strength and knot strength increases slightly.
  4. Abrasion resistance is reduced but this decrease is less than caustic soda mercerising.
  5. After bleaching or dyeing, treated yarns have virtually zero shrinkage when treated in boiling water.
  6. A pleasing lustre is imparted to the treated yarns albeit slightly less than for caustic mercerising.
  7. Dye affinity is increased by 3/4 of the amount attained by caustic soda mercerising.
  8. Moisture absorption is increased but again to some what lesser degree than for caustic mercerising.
  9. The heat resistance is substantially increased.

Liquid ammonia treated yarns are significantly cheaper in price than caustic mercerised yarns.

The elimination of hank winding is possible, due to the high speed reaction in liquid ammonia which permits package to package processing.

Maximum strength increases, require maximum stretch in the ammonia moving zone but this is difficult to apply without breakage to yarns. However, if the stretch is reduced and more modest strength increases accepted ( of the order of 20% - 30%) is readily possible to liquid ammonia treat singles yarn. This is a sharp contrast to the difficulties in processing singles yarn by mercerising.

It is therefore possible to produce this means a lustrous singles yarn for use in weaving and knitting applications.

From ecological view point also, ammonia is more readily and cheaply recoverable than caustic soda mercerising liquors which produce effluent and which has to be disposed of. The problem of caustic liquor discharge to rivers is so acute in some countries that permission to erect mercerising plants is difficult to obtain.

Early difficulties of dye affinity variations between packages of liquor ammonia treated yarns have now been eliminated by improved control of the treatment process.

The technological difficulties of converting pressurised liquid ammonia and recovering pressurised liquid ammonia from the gas evolved during the process, have been successfully overcome.

Yarn Mercerisation - Part-2

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.

April 01, 2008

Fungus formation in yarn before and after dyeing

Fungus formation is the common problem all dyers would have come across. If the yarn is kept stored under moist conditions for more than 24 hours, there is every possibility of it getting afflicted by natural fungus formation.

These fungus would appear at the beginning like yellow patches and spots and slowly turn in to green or blue and end up in black color. These are both aerobic and anaerobic. The portions affected by fungus, if left unnoticed may become weak and tender. If it is a ready for dyeing fabric, then that portion would be dyed lighter or darker according to the nature of dye.

The best solution to avoid the formation of fungus on cotton yarn or fabric, is to keep either completely dry before storage or it it needs to be kept under wet condition, it should be kept in slightly alkaline condition - pH 8 to 9, because all fungus has a tendency to grow in acidic medium only.