Page 110 - Basic Principles of Textile Coloration

VISCOSE FIBRE 99

uniformity as continuous filaments and large scale production of staple fibre gives
better economics. For staple fibres, combination of the output of several spinnerets
forms a band or tow of filaments, which can then be cut into short lengths. Before
cutting, the filaments may be crimped to improve the adhesion of the short fibres
for yarn spinning. Crimping may be achieved mechanically or by coagulating under
conditions that produce an asymmetric cross-section, with more skin on one side
of the filament than the other. When wet, the asymmetric fibres swell more on the
thin-skinned side and tend to curl. Bicomponent fibres of two different kinds of
viscose behave similarly.

6.2.3 Improved forms of viscose

As we have seen, the cellulose skin in regular viscose has better orientation and
strength. So, to produce high tenacity viscose, less acid and a high concentration
of zinc sulphate in the spinning bath are used. Then, all the cellulose is
regenerated slowly and the filament is all skin. These filaments have no core, do
not shrink during regeneration of the cellulose, and have a uniform cross-section.
Alternatively, regeneration of the cellulose is delayed by using a very dilute acid
bath. This allows time for better stretching and improved orientation.

High tenacity viscose fibres are used for tyre belts, hoses, and industrial
applications. They are rarely dyed. There are various ways of controlling the
coagulation process with bath additives. A higher degree of stretching produces
filaments with greater orientation and crystallinity. This gives filaments that may
be up to three times stronger than regular viscose, but still with acceptable
extensibility. High tenacity viscose has lower water absorption and swelling but
higher wet strength.

A variety of other high-strength viscose filaments are also produced whose
mechanical properties are much closer to those of cotton. These are called high
wet modulus (HWM), modal and polynosic viscoses but there are various
definitions of these names. The word ‘polynosic’ is presumably derived from
‘polymer’ and ‘cellulosic’ and refers to viscoses with a fine micro-fibrillar structure
resistant to cold 8% aqueous NaOH solution. The generic name ‘modal’ is used to
describe viscoses having high tenacity and a high wet modulus; ‘polynosic’ usually
refers to the type with the highest values of wet modulus. The elastic modulus is
the stress that must be applied to produce a unit elongation. The higher the value,
the more rigid the fibre. All the HWM viscoses have higher wet strength than
regular viscose, increased resistance to swelling by alkali solution, a higher DP and