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Cotton
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- Spinning
- Further finishing processes
Quality of cotton lint may be hampered by the lack of an efficient ginning system. Once lint has been cleaned to remove trash (cotton cleaning), staple fibres are compacted by mechanical means into bales (bailing). Metallic ties are used to hold bales together, thus facilitating transport and storage.
Important developments have occurred in the crucially sensitive areas of cotton plant breeding and biotechnology (genetically modified (GM) cotton in particular). A gene conferring resistance to glyphosate (an active ingredient in herbicides) was transformed into cotton for the first time in 1987. Another milestone in genetic engineering of cotton occurred in 1989, when Monsanto developed the "Bt cotton" variety. Bt cotton is a pest- (rather than pesticide-) resistant variety. It contains a foreign gene obtained from bacillus thuringiensisa, which protects the plants from bollworm. In 1996 the Bt crop was first planted on a commercial scale in Australia and the USA. Since then, cotton crop varieties developed by genetic engineering (particularly Monsanto's Bt cotton variety) have been planted on more than one fifth of land under cultivation. Genetically modified cotton would cover 50% of cultivated land in Mexico and South Africa, compared to 80% in the USA and 66% in China. Argentina, Australia, India, and Indonesia also approved commercial planting of genetically engineered cotton in 2005.
According to estimates from CropLife International, cotton is the third largest GM commodity worldwide, behind soybeans and corn, with a market of approximately 430 million US dollars in 2002-2003.
In Francophone Africa, plans are being finalised to convert some cotton crops into transgenic cotton varieties over the next years. In Mali, for example, the government signed a research agreement with the US Agency for International Development (USAID), Monsanto, Syngenta and Dow Agrosciences in 2004 to develop and commercialise transgenic cotton by 2009. Field-testing of Bt cotton started in Burkina Faso in 2003. Field trials are now underway at research stations of the "Institut national de l'environnement et de recherche agricole" (Inera) in Farakoba (South-east) and Fada N'Gourma (East). The trials aim to assess the viability of Bt cotton and to develop varieties resistant to the caterpillars (which affects approximately half of the country annual cotton output).
For more information, please please refer to the following document: "Développement de la culture du coton génétiquement modifié au Mali" (Ministry of Agriculture, Livestock and Fisheries).
According to an article published in "Le Monde" on February 3, 2005 ( "Avec les OGM, réduire la fracture agricole mondiale", by Gilles Peltier), in 2005, 24% of world cotton under cultivation are estimated to be under GM varieties against 2% in 1997, corresponding to 34% of world cotton production and more than 30% of world exports.
According to the International Service for the Acquisition of Agri-biotech Applications (ISAAA), India increased its area of approved Bt cotton from approximately 100,000 hectares in 2003 to 500,000 hectares in 2004 when approximately 300,000 small farmers used Bt cotton. After suffering severe drought in 2002 and 2003, Australia increased its total cotton plantings to about 310,000 hectares of which 80%, equivalent to 250,000 hectares, were planted with biotech cotton in 2004. China increased its Bt cotton area for the seventh consecutive year; an increase of one-third from 2.8 million hectares in 2003 to 3.7 million hectares in 2004, equivalent to 66% of the total cotton area of 5.6 million hectares in 2004, the largest national cotton hectarage planted in China since the introduction of Bt cotton in 1997. Globally growth rate in commercialised biotech cotton from 2003 to 2004 was at 25%.
For more information the reader is referred to the website of the National Institute for Agricultural Research (INRA).
Spinning is the process of making yarn from unbundled fibres. It includes the following operations.
Upon arrival at the spinning mill, cotton bales are sampled according to lint quality and origin to ensure yarn homogeneity. They are then opened to make the lint fluffy by passage though bale-openers. The following important step in the spinning process is cleaning. Bale fibres are usually fed to air-jet (vortex) cleaners to remove extraneous matter from cotton lint (which may hamper further cotton processing and affect lint quality). At this stage loose fibres are not aligned and parallel in a single continuous strand. Carding is the process of straightening or paralleling the fibres.
Carding separates fibres from each other, straightens fibres, aligns and condenses them into a single continuous strand, and removes impurities. A sliver of approximately one-meter width is then obtained.
Cotton that has already been carded may be combed. Combining is an optional step in the ginning process. This process is only used to produce superior quality yarn and long- or extra long-staple fibres.
As a result of drawing (or doubling) the sliver is condensed into a thinner strand and becomes more uniform. The sliver is fed to several rubber rollers rotating at increasingly higher speed. Cotton bleaching (using either hypochlorite or peroxide) and dying often occur at this stage.
Eventually, several slivers are drawn and twisted together to form the final yarn. Twisting is made by two mechanical actions. First, a drawing frame condenses slivers into a thinner strand (slubbing) and winds it on a bobbin. A spinning frame then reduces roving to required size of single yarn (fine spinning). A suitable amount of twist is introduced according to the intended use of the fibre.
• Mercerization
Mercerization, a textile process named after its inventor, the English chemist John Mercer (1791-1866), was first developed in 1844. In 1889 Horace Lowe discovered the additional effect of enhancing the lustre by stretching the swollen materials while wet with caustic alkali and then washing off. Mercerization is a finishing process used to produce high quality fabrics, such as damasks. It consists essentially of impregnating stretched cotton with caustic soda (although other alkalis may be used). The treatment enhances the lustre of cotton (the fibres are swelled), making it similar to silk. It also increases the fibre strength and affinity for dyes. A related process (liquid ammonia treatment) produces some of the effects of mercerization. Double mercerization means both the yarn and the knitted fabric are mercerized.
For further information, please refer to the following
sites:
- Lectures
on Mercerization (Y. Matoba): textileinfo.com
- Glossary terms
- textiles: textile.org.uk
• Gassing
Gassing refers to passing a cotton thread at high speed through a flame to eliminate the fluff. Because 6-10% of the fibrous mass is lost in the process, the gassing of cotton yarns is a costly operation. A related process is singeing the fabric (rather than gassing the yarn), by passing cotton fabrics though a flame. However, singeing is less effective than gassing, in that only those hairs which protrude out of the fabric can be burned off.
• Sanforisation
A manufacturing process invented by Sanford Cluett in 1933, sanforisation is a treatment applied to fabric to reduce cloth shrinkage after washing. During the sanforising process, the fibres of the cotton fabric are stretched both in length and in width so that cloth that is washed and dried will not shrink much.
• Twisting
Many fine filaments or yarns are plied together (by inserting S or Z twists) to increase the diameter and tensile strength. In principle strands are twisted together in the opposite direction to the spinning twist.
