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