پنبه

seed-hair fibre of a variety of plants of the genus Gossypium, belonging to the Malvaceae family, native to most subtropical countries. The shrubby plant, growing up to 6 m (20 feet) high in the tropics, characteristically ranges from 1 to 2 m in height when it is under cultivation. It produces creamy-white flowers, which soon turn deep pink and fall off, leaving the small green seedpods, known as cotton bolls, which contain the seeds. Seed hairs, or fibres, growing from the outer skin of the seeds, become tightly packed within the boll, which bursts open upon maturity, revealing soft masses of the fibres. These are white to yellowish white in colour, ranging from about 2 to 4 cm (0.75 to 1.5 inches) in length. They are composed of about 87 to 90 percent cellulose (a carbohydrate plant substance), 5 to 8 percent water, and 4 to 6 percent natural impurities.

Cotton fibres may be classified roughly into three large groups, based on staple length (average length of the fibres making up a sample or bale of cotton) and appearance. The first group includes the fine, lustrous fibres with staple length ranging from about 2.5 to 6.5 cm and includes types of the highest quality—such as Sea Island, Egyptian, and pima cottons. Least plentiful and most difficult to grow, long-staple cottons are costly and are used mainly for fine fabrics, yarns, and hosiery. The second group contains the standard medium-staple cotton, such as American Upland, with staple length from about 1.3 to 3.3 cm. The third group includes the short-staple, coarse cottons, ranging from about 1 to 2.5 cm in length, used to make carpets and blankets, coarse and inexpensive fabrics, and blends with other fibres.

Cotton is harvested when the bolls open. If it is to be mechanically picked, the leaves are usually chemically removed, encouraging uniform opening of the bolls. Most of the seeds (cottonseed; q.v.) are separated from the fibres by a mechanical process called ginning.

Ginned cotton is shipped in bales to a textile mill for yarn manufacturing. A traditional yet common processing method is ring spinning, by which the mass of cotton may be subjected to opening and cleaning, picking, carding, combing, drawing, roving, and spinning. The cotton bale is opened and its fibres are raked mechanically to remove foreign matter (e.g., soil and seeds). A picker (picking machine) then wraps the fibres into a lap. A card (carding) machine brushes the loose fibres into rows that are joined as a soft sheet, or web, and forms them into loose untwisted rope known as card sliver. For higher quality yarn, card sliver is put through a combing machine, which straightens the staple to a finer degree and removes unwanted short lengths, or noils. In the drawing (drafting) stage, a series of variable-speed rollers attenuates and reduces the sliver to firm uniform strands of usable size. Thinner strands are produced by the roving (slubbing) process, in which the sliver is converted to roving by being pulled and slightly twisted. Finally, the roving is transferred to a spinning frame, where it is drawn further, twisted on a ring spinner, and wound on a bobbin as yarn.

Faster production methods include rotor spinning (a type of open-end spinning), in which fibres are detached from card sliver and twisted, within a rotor, as they are joined to the end of the yarn. For the production of cotton blends, air-jet spinning may be used; in this high-speed method, air currents wrap loose fibres around a straight sliver core.

Blends (composites) are made during yarn processing by joining drawn cotton with other staple fibres, such as polyester or casein.

The procedure for weaving cotton yarn into fabric is similar to that for other fibres. Cotton looms interlace the tense lengthwise yarns, called warp, with crosswise yarns called weft, or filling. Warp yarns often are treated chemically to prevent breaking during weaving.

Cotton, one of the world's leading agricultural crops, is plentiful and economically produced, making cotton products relatively inexpensive. The fibres can be made into a wide variety of fabrics ranging from lightweight voiles and laces to heavy sailcloths and thick-piled velveteens, suitable for a great variety of wearing apparel, home furnishings, and industrial uses. Cotton fabrics can be extremely durable and resistant to abrasion. Cotton accepts many dyes, is usually washable, and can be ironed at relatively high temperatures. It is comfortable to wear because it absorbs and releases moisture quickly. When warmth is desired, it can be napped, a process giving the fabric a downy surface. Various finishing processes have been developed to make cotton resistant to stains, water, and mildew; to increase resistance to wrinkling, thus reducing or eliminating the need for ironing; and to reduce shrinkage in laundering to not more than 1 percent. Nonwoven cotton, made by fusing or bonding the fibres together, is useful for making disposable products to be used as towels, polishing cloths, tea bags, tablecloths, bandages, and disposable uniforms and sheets for hospital and other medical uses. The world's leading producers of cotton are China, the United States, India, Pakistan, Uzbekistan, Brazil, Turkey, Australia, Turkmenistan, and Egypt.

+ نوشته شده توسط ROLIN در چهارشنبه چهاردهم آذر 1386 و ساعت 22:19 |
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