Page 105 - Basic Principles of Textile Coloration

94 ARTIFICIALLY MADE FIBRES BASED ON CELLULOSE

its sodium salt, called alkali- or soda-cellulose (Scheme 6.2(i)). Pressing out the
excess NaOH solution removes some soluble carbohydrates, and the moist, solid
product is next broken up into crumbs and allowed to age. During this period,
oxidation in the presence of air causes some depolymerisation. The crumbs of
alkali-cellulose are then churned with about 10% of their weight of carbon
disulphide, forming sodium cellulose xanthate (Scheme 6.2(ii)). Addition of dilute
aqueous NaOH gives a viscous, brown solution, from which the name ‘viscose’
was derived.

The viscose solution is then left to ripen. The viscosity gradually increases as
some of the xanthate ester groups hydrolyse back to hydroxyl groups (Scheme 6.2
(iii)). The degree of substitution (DS) of the hydroxyl groups of cellulose
represents the number of hydroxyl groups per glucose unit that have undergone
chemical substitution. The value of the DS varies from 0, for cellulose, up to a
maximum of 3.0. The initially formed sodium cellulose xanthate has a DS of just
less than 1.0 but during ripening this falls to about 0.33. Too large a decrease in
the DS results in precipitation of cellulose.

During ripening, the average DP also decreases further. The depolymerisation of
the cellulose in the ageing and ripening stages of viscose production determines
the viscosity of the final spinning solution. This is important for controlling the
filament integrity on initial extrusion into the coagulating bath. The low DP of the
final cellulose is, however, the major reason for the weakness of this fibre.

The ripe viscose solution is finally degassed and filtered in preparation for wet
spinning. Extrusion of the solution into a bath containing acid and salts first
coagulates the cellulose xanthate, which then hydrolyses to reform cellulose
(Scheme 6.2(iii)). The still plastic filaments pass around and are stretched
between rotating pulleys. Stretching produces finer filaments, but also orients the
cellulose molecules along the filament axis so that they begin to interact with each
other by hydrogen bonding. Without such orientation, viscose filaments would be
too weak for textile use. The drawn filaments may be collected in a Topham box (a
rotating cylindrical container in which the filaments press against the walls as a
result of centripetal force), or wound up on perforated tubes to form bobbins.

The filaments produced require a number of aftertreatments such as
desulphurising, bleaching if necessary, washing, drying and oiling. The
aftertreatment and washing processes are carried out on batches of cakes or
bobbins. Washing with Na2S solution removes any precipitated elemental sulphur
from the filaments, which tends to cause a yellow coloration. Like bleaching, this