Viscose rayon reaction involved

The natural fibers obtained from cotton, wood, flax, hemp, and jute all are cellulose fibers and serve as raw materials for the textile and paper industries. In addition to its use as a natural fiber and in those industries that depend on wood as a construction material, cellulose is used to make cellulose acetate (for making rayon acetate yarn, photographic film, and cellulose acetate butyrate plastics), nitric acid esters (gun cotton and celluloid7), and cellulose xanthate (for making viscose rayon fibers). The process by which viscose rayon is manufactured involves converting wood pulp or cotton Iinters into cellulose xanthate by reaction with carbon disulfide and sodium hydroxide ... 

Regenerated cellulose, cellulose acetate, acrylic and some other fibres are spun from solution, either by dry spinning, with evaporation of solvent, or by wet spinning into a coagulation bath. In viscose rayon, the solute is sodium cellulose xanthate, which is a chemical derivative of cellulose soluble in caustic soda, so that a chemical reaction is involved in fibre formation. 

Wet spinning also involves pumping a solution to the spinneret. Now, however, the polymer is precipitated in an immiscible liquid. Polyacrylonitrile in dimethyl formamide, for example, can be precipitated by passing a jet of the solution through a bath of water, which is miscible with the solvent but causes the polymer to coagulate. Cellulose triacetate can be wet-spun from a methylene chloride-alcohol mixture into a toluene bath, where it precipitates. In other fibers, the precipitation can involve a chemical reaction. Viscose rayon is made by regenerating cellulose from a solution of cellulose xanthate. 

Viscose is a solution of cellulose xanthate in caustic soda. This solution can be processed into rayon or cellophane. The production of viscose consists of two steps. First, certain forms of cellulose, mainly from sulfite pulp and cotton linters, are reacted with pure caustic soda. This reaction converts the -OH groups on the cellulose unit to -ONa. The caustic soda must be pure and is obtained from mercury cells. Caustic soda from diaphragm cells contains sodium chloride which makes it unusable without purification. This solution must be aged for 2-3 days. Aging reduces the length of the cellulose chain which makes it easier to dissolve into caustic soda. The second step involves the reaction of the alkali cellulose and carbon disulfide in excess caustic soda. The net reaction of viscose is ... 

In spinning, the alkaline viscose solution is exposed to an acidic spin-bath under carefully controlled conditions to form filaments of rayon. The process as described by Vroom involves a complex series of chemical and physical reactions that take place almost simultaneously [170]. Initially, the action of the add or salt spin-bath causes the viscose to coagulate and forms a skin around the filament. Then, as the acid penetrates into the viscose filament, neutralization of the alkali occurs. At the same time, cellulose xanthate is decomposed regenerating cellulose and carbon disulfide from which it was made (Equation 10.12). 

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