There
are two other man-made regenerated fibres, namely cuprammonium and modal
(polynosic). Standard-type
viscose fibres have moderate strength and are relatively stiff compared
with fibres such as cellulose acetate and nylon. Strength is reduced in
wet conditions, so that heavy duty end uses would not be expected. Viscose
is a limp handling fibre because its polymer system is so very amorphous.
Its polymers are not sufficiently long for a more satisfactory alignment,
and so do not allow the formation of more hydrogen bonds which would result
in a more rigid polymer system and thus a crisper handle to the fibre
and textile. The
very amorphous polymer system of viscose, as well as its polar polymers,
make viscose the most absorbent fibre in common use. The slightly more
crystalline polynosic rayon is somewhat less absorbent than viscose. As
would be expected, viscose and the other regenerated cellulosics have
somewhat similar thermal and chemical properties to cotton. However, the
shorter polymers and the very amorphous nature of regenerated fibres are
responsible for their much greater sensitivity to acids, alkalis, bleaches,
sunlight and the weather. Whereas
all fibre making processes are governed largely by their inherent costs
and the utility of the product, three additional factors assume great
importance; raw material supply, energy requirements, and environmental
impact of the production process. The use of raw material, wood pulp,
in the viscose process means that a renewable resource is involved, and
this is produced from softwoods growing in northern latitudes on land
unusable for other forms of agriculture, or from hardwoods growing at
a phenomenal rate in sub-tropical areas. The production time of 10 to
20 years for wood compares with 100 million years for the non-renewable
fossil fuels such as oil used as feedstock for synthetic polymer production.
Further, the use of wood pulp for the viscose process is only a small
fraction of total pulp usage, and in general pulp production keeps up
with demand. Other materials required for the viscose process are caustic
soda, sulphur chemicals and salts, and these are in abundant supply. The
considerable pollution of air, watercourses, and land associated with
older viscose plants, and current health fears over carbon disulphide
air pollution, have been significant problems. Technological advances
are however, making possible complete recovery and recycling of many chemical
materials used in the process. In
general, the status and prospects for continuous filament viscose have
declined and there were many plant closures in the 1970’s. However, the
fact that the source of raw material is renewable, and the desirable properties
of the product in which comfort and moisture are prominent, point to a
more promising future. Problems with relatively high energy costs and
high manpower requirements are still being investigated to ensure investment
in viscose plant.
• Viscose – Viscose fibres together with acetate fibres represent
man-made fibres made from cellulosic (wood pulp) sources. These fibres
were developed around 1900.
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