Wool
Wool � The most important of the fibres of animal origin is
wool, the hair of the sheep (Ovis ales). The characteristics of
wool fibres differ depending on the breed and general condition of the
sheep.
Wool, the hair of various breeds of sheep, is a warm springy fibre. It�s
scaly structure led in the past to unwanted felting (matting of fibres)
and shrinkage in laundering but modern finishing treatments have now
overcome these difficulties; similarly, attack by the clothes moth is no
longer a problem.
Wool has a natural crimpiness, which gives it good elasticity, bulk,
crease-resistance and durability. Its strength when wet is less than
when it is dry. Important properties of wool are a dry strength of 11.5
to 13 cN/tex, a breaking elongation of 30 to 40% and a moisture regain
of 16 to 18%.
Technologically, the diameter, or fineness, of the wool fibre is
probably the most important property. It is determined to a large extent
by the grade assigned to a lot, or batch, of wool. Diameters range from
16 microns in the finest Merinos to over 40 microns in the coarsest
long-wool types. Staple length generally increases with increasing fibre
diameter, ranging from four to eight centimetres for fine Merino up to
35 centimetres for coarse Cotswold wool. Staple length is lower than
mean fibre length because of the crimp that results in a helical
configuration in an individual fibre and an uni-planar wave in the
staple.
Wool fibres are hygroscopic, absorbing water from a moist atmosphere in
a reversible way. Because most fibres are dependent on moisture content,
this characteristic is extremely important, especially in numerous
processing steps, including carding, combing, spinning and weaving.
Relative humidity in the processing plants must be controlled for
optimum processing conditions. Upon uptake of water , the fibre swells
anisotropically (unequally), increasing up to 16% in diameter while the
change in length is only about 1.2 to 1.5%. The ability of wool fibres
to absorb and release moisture contributes to wearing comfort in
garments made of wool.
In
the UK alone there are 34 main breeds of sheep and a further 43 rare
breeds and hybrids. When wool fibres have been processed it is not
possible to say with confidence from what breed of sheep or country they
have originated because wool fibres from several breeds have the same
microscopical appearance. There are four main types of wool fibre: fine,
coarse, outercoat and kemps. Each type has a characteristic scale
pattern so that it is possible to distinguish between types, although it
is not possible to assign a type to a breed of sheep.
Wool fibres are generally white, with the finer fibres having a circular
cross section and the coarse fibres being more irregular in contour.
Kemp fibres are usually chalky white in appearance, very coarse and
brittle, with a long tapering tip and a taper over a short distance to
the root end.
An
important aspect of the environmental impact of wool processing is the
use of pesticides on sheep. Although the problem has been minimised over
recent years with the introduction of tighter restrictions or even bans
on the use of some formulations, this is still a major concern since
these toxic chemicals make their way into rivers.
Another impact at this early stage which is maybe less obvious, is the
methane generated globally by the many hundreds of millions of sheep.
Methane is a powerful greenhouse gas and contributes to the effect of
global warming.
Again, as with cotton there is also the effect of transporting huge
quantities of textiles across the globe with the associated fuel usage
and fume generation problems.
The scouring of the raw wool, the next stage, is a process which
produces a liquid effluent with a very high organic load and solids
content and varying levels of pesticide contamination. The highly
polluting nature of this waste and the strict discharge consents
associated with the process means that scourers have to treat their
effluent on-site.
Another environmental problem is that many of the dyestuffs available
for obtaining good dark shades on woollen fabric or yarn are mordant
dyes containing chromium. The discharge of chromium is strictly
controlled and so companies are being forced into using alternatives or
consider on-site effluent treatment.
In
the carpet industry permethrin, another pesticide, is added to the yarn
that goes into making contract carpets, this is a quality requirement to
protect the wool from moth attack. The effluent from this process must
therefore be treated and continually monitored.
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