Page 106 - Basic Principles of Textile Coloration

VISCOSE FIBRE 95

is not always necessary. Thorough washing finally eliminates all the residual
chemicals. The cakes or bobbins may also be dyed at this stage, before drying.

Manipulation of numerous cakes and bobbins is time-consuming and these
batch operations limit the rate of production. Fully continuous aftertreatment and
washing, immediately after spinning, is much more productive. For each
treatment, the extruded and stretched filaments wind around, and advance along,
a pair of rollers with convergent axes while being sprayed with the appropriate
washing solution.

Wood pulp, and the other chemicals used, are relatively cheap and viscose has
become the predominant regenerated cellulose fibre. However, viscose production
involves an extremely lengthy series of operations requiring strict control. Large
volumes of water are necessary and the alkali, soluble carbohydrates and sulphur
compounds in untreated effluent pose an environmental problem. A newer type of
regenerated cellulose (called lyocell) is now being manufactured by a process
generating less water pollution (Section 6.2.4).

6.2.2 The morphology, properties and uses of viscose

The cellulose in cotton has a DP of around 2000 and a degree of crystallinity of up
to 70%. For regular viscose, the DP (250–400) and crystallinity (25–30%) are
much lower. The crystallites in viscose are also 4–5 times smaller than in cotton
and have a lower degree of orientation. It is therefore not surprising that viscose is
a much weaker fibre than cotton. Crystalline zones in a fibre are responsible for
strength and rigidity, whereas the amorphous regions provide accessibility,
flexibility and extensibility. Wet spinning involves extrusion of a solution of free,
individual cellulose molecules. These tend to orient themselves along the filament
axis during extrusion and this continues during stretching after coagulation. The
molecules are not, however, aligned to the same extent as in cotton.

Regular viscose fibres are weaker than those of cotton, particularly when wet
(Figure 6.1). They are much less rigid and much more plastic. Elongation under a
stress of more than 2% can cause displacement of the cellulose molecules and
therefore permanent elongation. The strength of viscose filaments is improved by
stretching them after extrusion, while they are still plastic. Even so, the strength of
wet viscose may be as much as 50% lower than when dry. Because the filaments
have low crystallinity and high accessibility, water absorption causes fibre swelling
and increases the extensibility by 20%. Swelling of viscose filaments can be a