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.

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