Page 111 - Basic Principles of Textile Coloration

100 ARTIFICIALLY MADE FIBRES BASED ON CELLULOSE

a more fibrillar internal structure similar to cotton. Their properties are therefore
much closer to those of cotton. Compared to regular viscose, they are more
crystalline and oriented, imbibe less water and swell less.

HWM viscose is used as staple for blending with other fibres using the cotton
carding system. The fibres consist of cellulose fibrils and have no skin. The
average DP is 500–600, and the fibres are 50–60% crystalline with larger
crystallites than in cotton. Because of this, and the higher orientation than in
regular viscose, HWM viscose fibres are less likely to deform under wet conditions.
On water absorption, the swelling is intermediate between that of regular viscose
and cotton. For many materials, HWM viscose competes with cotton, but, unlike
cotton, it has a soft silky handle, with an attractive subdued lustre. It can be used
for materials where regular viscose would be inappropriate because of its poor wet
strength. It is blended with cotton to a large extent and also with polyester staple.

Polynosic fibres have a higher DP of around 500–700, compared to the value of
250–400 for regular viscose. The higher values of the DP for HWM and polynosic
fibres are achieved by curtailing the ageing of the soda-cellulose and the ripening
of the viscose solution to avoid depolymerisation. The viscose solution is prepared
under much milder, neutral conditions. Regeneration is also slowed down by
spinning into a dilute sulphuric acid solution without salts. Other additives are
also present to retard coagulation. The filaments exhibit a high degree of internal
fibrillation and their properties are much closer to those of cotton.

6.2.4 Lyocell fibres [3]

The lyocell fibres developed by Courtaulds and Lenzing appear to have solved the
two major problems of viscose production: the excessive environmental pollution
and the poor wet strength of the viscose filaments. The generic name ‘lyocell’ is
from the Greek verb lyein meaning ‘to dissolve’, and is used for fibres of
regenerated cellulose produced directly from a solution of cellulose rather than
from a cellulose derivative. Courtaulds’ product is called Tencel.

Few details of the manufacture of this type of fibre have been divulged but the
following steps seem likely. Firstly, an aqueous suspension of ground cellulose
(13%) in water (20%) and N-methylmorpholine-N-oxide (67%) is prepared. The
solvent is obtained by oxidation of N-methylmorpholine with hydrogen peroxide
(Scheme 6.4). Thin film evaporation of the water then increases the cellulose
solubility and a true solution is obtained in the amine oxide solvent. This solution
is stabilised by an anti-oxidant such as propyl gallate (propyl-3,4,5-