The production of cotton is considered complete when it has been picked up from the the plant, but it is not in a useable form. The seeds must be removed and the fibers packed in a bale before selling to the cotton mills. The picked cotton contains about 66% seeds, 33% cotton fibers and small amounts of leaf and dirt. The separation of seeds and fibers is carried out by a process called ginning. This is considered as the first mill processing of cotton as this is the first mechanical process to which raw cotton is put, if it is not picked by machine. The complete ginning process consists of preliminary cleaning and drying of cotton, separation of seeds from fibers, which are sometimes called cotton lint or linters, and pressing and wrapping into a bale of about 500 lbs.
Ginning-Cotton preliminary cleaned
Cotton - cultivation
Cotton is grown in more than sixty countries of the world, but United States, India, Russia, Brazil, Egypt and China are some of the largest producers of cotton. Cotton cultivation practices vary from country to country but the general practice is as follows :
The land is broken by a plough in winter or early spring to prepare a seed bed for planting. This preparation allows the land to hold the moisture falling on it. The planting season varies depending on the geographical location. In United States it is from March to middle of May, in Egypt, from early March to the end of April, in Russia, India and China from April to August. In tropical countries, the cotton plant assumes a tree like structure. Before the full height is reached (in about 40 days after the plant appears) the plant begins to form flower stalks. Flowering takes another 30 days. The opened flower is yellowish white on the first day when pollination occurs; it turns to pink on the next day when fertilisation takes place and the petals of the flowers fall on the third day. The immature seeds thus formed grow rapidly and the large cotton boll matures in 40-50 days.
When the cotton bolls are ripe, they burst, exposing a soft mass of cotton fibres. When the cotton fibers are exposed as a result of the bursting of the ripened boll, the cotton must be picked to prevent the fibers getting discoloured due to exposure to sunlight and air. In former times cotton harvesting was a hand operation, requiring considerable amount of hard labour. Now machine harvesting is also practised. In the picking method the cotton sections of the opened balls are removed from the burs leaving the burs on the stalk. In the pulling method, the open balls and burs are removed together. Sometimes leaves and other foreign matters also find their way in the cotton collected. Hence, pulled cotton is not as clean as picked cotton.
Cotton - Structure and properties
The word cotton is derived from the Arabic. Depending upon the arabian dialect, it is pronounced kutan, qutn, qutun etc. As soon as the cotton fiber is obtained from a plant it is classified as a natural, cellulose, seed, mono-cellular, staple fiber.
Macro structure of cotton
Under a microscope, a cotton fiber appears as a very fine, regular fiber. It ranges in length from about 10mm to 65 mm, depending upon the quality of the fiber. Cotton is a very fine fiber with little variation in fiber diameter; compared with wool for instance, its fiber diameter is not considered as critical a fiber dimension as its length. The fiber length to breadth ratio of cotton ranges from about 6000:1 for the longest and best types, to about 350:1 for the shortest and coarsest cotton types. The greater this ratio, the more readily can the cotton fibers be spun into yarn. Cotton fibers vary in colour from near white to light tan.
Polymer system of cotton
The cotton polymer is a linear, cellulose polymer. The repeating unit in the cotton polymer is cellobiose which consists of two glucose units. The cotton polymer system consists of about 5000 cellobiose units, that is its degree of polymerisation is about 5000. It is a very long, linear polymer, about 5000 nm in length and about 0.8 nm thick. Cotton is a crystalline fiber. Its polymer system is about 65 to 70 per cent crystalline and, correspondingly, about 35-30 per cent amorphous. Therefore, the cotton polymers are, in the main, well oriented and probably no further apart than 0.5 nm, in the crystalline regions.
Physical properties of cotton
(1) Tenacity - The strength of cotton fibers is attributed to the good alignment of its long polymers (that is its polymer system is about 70 per cent crystalline), the countless, regular, hydrogen bond formations between adjacent polymers, and the spiralling fibrils in the primary and secondary cell walls.It is one of the few fibers which gains strength when wet. It is thought this occurs because of a temporary improvement in polymer alignment in the amorphous regions of the polymer system. The improved alignment when wet results in an increase in the number of hydrogen bonds, with an approximate 5 per cent increase in fiber tenacity.
(2) Elastic plastic nature - The cotton fiber is relatively inelastic because of its crystalline polymer system, and for this reason cotton textiles wrinkle and crease readily. Only under considerable strain will cotton polymers give and slide past one another.
(3) Hygroscopic nature - The general crispness of dry cotton textile materials may be attributed to the rapidity with which the fibers can absorb moisture from the skin of the fingers. This rapid absorption imparts a sensation of dryness which, in association with the fibers inelasticity or stiffness, creates the sensation of crispness. The hygroscopic nature ordinarily prevents cotton textile materials from developing static electricity.
