Cotton
Cotton � Cotton fibres are the seed hairs of the plant
Gossypium. They are usually off-white in colour although some
varieties have been bred to incorporate a natural colour. Each fibre is
formed by the elongation of a single cell from the surface of the seed.
The word cotton is derived from it�s Arabic name pronounced kutan, qutn
or qutan depending on the dialect.
Under a microscope , a cotton fibre appears as a very fine, regular
fibre, looking like a twisted ribbon or a collapsed and twisted tube.
These twists are called convolutions.
Almost half of the world�s requirements for textile fibres are met by
cotton. It is grown in many parts of the world where a hot dry climate
is to be found, the main producers being USA, the former USSR, China,
India, Egypt, Africa and South America.
Cotton consists typically of between 88 to 96% cellulose with the rest
being protein, pectic substances (congealed gum-like carbohydrates), ash
and wax. After scouring and bleaching, cotton is then about 99%
cellulose. The fibres are weakened and destroyed by acids but are
resistant to alkalis.
The fibre length varies with the type and quality, within the range 10
to 65mm; the fibre diameter ranges from 11 to 22 �m. Cotton is a
relatively strong fibre with a strength of 25 to 35 cN/tex and a
breaking elongation of 7 to 9%. It is stronger when wet. Cotton also
absorbs moisture readily, which makes cotton clothes comfortable to wear
in warm weather (water retention of 50%, moisture regain of 7%).
Cotton fibre burns readily and is not inherently resistant to oxidising
agents, and biodegradation, as well as acids. Despite these shortcomings
cotton has a good wear life. Its properties can also be readily modified
by chemical finishes which provide enhanced performance, e.g. crease
resistance and flame resistance.
It
is used in both 100% form and in blends with other fibres for household
textiles and apparel. Synthetic fibres have largely replaced its use in
industrial textiles.
The environmental impact of the cotton manufacturing process starts in
the field with the fertilisers and insecticides used in the growing of
the cotton plant. These have a direct impact on the land and the local
flora and fauna. The US alone spends about 500 million dollars a year on
pesticides for protection against the bollworm and other harmful bugs.
These pesticides also have an effect on people living in the viscinity
of cotton fields. Nausea, diarrhoea and throat irritation being the most
common complaints. It is also interesting to note that the gas leak at
Bhopal , India which killed over 3,000 people came from a pesticide used
in the spraying of cotton crops.
The ginning or separation of the seed from the fibre is a dusty process
that has serious health implications on cotton workers in developing
countries, where the separation is carried out using traditional
methods. The fibre is then packaged and transported across the globe.
The production of natural fibres is conducted in many separate
locations, transportation of the cotton bales has therefore a
significant impact on the environment.
The spinning process introduces another set of significant impacts,
producing more dust, noise and waste fibre and is also a relatively
large consumer of electricity (as much as 2� tonnes of oil per tonne of
yarn). Relative humidity in the processing plant must also be controlled
to minimise breakages, another consumer of energy. Weaving creates a
similar set of impacts plus the added effect of using size and biocides
has to be considered on the aqueous environment. Sizes are either
natural, like starches or synthetic such as polyvinyl alcohol. The new
sizes make weaving on the faster modern machinery a lot easier, but they
have the drawback of being harder to biodegrade from the waste water.
Cotton preparation, that is singeing, desizing, scouring, bleaching and
mercerising impacts on both the air and water. Singeing produces a dusty
odorous emission, whilst the other preparative processes are the major
contributors to BOD/COD in a textile effluent. There is also the problem
of pentachlorophenol (pcp) on imported fabric which is washed out during
preparation. This is an eco-toxic rot-proofing chemical with a very low
discharge consent (maximum allowable concentration).
Cotton is by far the most popular fibre in use today, at least in terms
of volume of production. The most widely used class of dyestuff on
cotton are reactive dyes, which unfortunately are also the most poorly
exhausted, producing a more coloured effluent. Colour consents on
discharges have therefore been enforced in certain areas where there are
significant numbers of cotton dyers.
The main pollution problem associated with cotton finishing processes is
that of formaldehyde emissions from resin and other finishes. Low
formaldehyde formulations have improved over the past few years but not
far enough to eliminate the need for abatement on many stenters.
Attempts have been made to reduce the impact of cotton growing by
eliminating pesticide use and by growing coloured strains of cotton so
that the preparation and dyeing of the cotton is minimised. The bollworm
can be eliminated by imposing a three month fallow period at the end of
the growing season, and certain short fibre coloured cottons used by
Indians in Central America have been cross bred with long fibre strains
by a company in the US called Foxfibre. However, the amount of organic
cotton grown in the US is still only a tiny fraction of the global
output ( by the mid-1990�s, 80 cotton growing countries were producing
about 85 million bales of cotton).
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