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a concentrated solution of cellulose xanthate in a dilute solution of caustic soda. Viscose is used to produce viscose fibers and films (cellophane), artificial coverings (tarpaulin), and so on. The main source of viscose is wood cellulose. The cellulose is first treated with an aqueous solution of caustic soda (mercerization) until the formation of so-called alkaline cellulose, which is then crushed to increase its reactivity and is subjected to oxidative breakdown in air to decrease its molecular weight. The cellulose is then treated with carbon disulfide, resulting in the formation of cellulose xanthate, which is further dissolved in an aqueous solution of caustic soda at a temperature of 10°-12° C. All these processes take place in one or more constant-operation and periodic-operation devices. In the USSR a method for producing viscose in one piece of equipment (the VA apparatus) was developed in the late 1940’s and is widely used. The viscose is put through double or triple filtration to eliminate contaminants and is thoroughly freed of air bubbles (usually under vacuum), which would hinder its final processing.
Viscose is a highly viscous solution, with a density of 1.12 g/cm3; it is bright orange, but in the presence of admixtures it is brown or greenish. The viscose used for the formation of fibers and films usually contains 6.5-9.0 percent cellulose (in the form of its xanthate), 6.5-7.5 percent caustic soda, and approximately 2.0-2.5 percent bound sulfur; the remainder is made up of water and a small quantity of admixtures (sulfur compounds of sodium, carbonates, calcium salts, ferrous sulfide, and dissolved hemicellulose). The viscosity of solutions of viscose varies from 3 to 10 newtons · sec/m2 (30 to 100 poises) and increases sharply with an increase in the cellulose content; in addition, the viscosity value is directly proportional to the degree of polymerization (to the molecular weight of cellulose) and to the amount of caustic soda in the viscose. Free carbon disulfide in the viscose gradually bonds with the caustic soda and is transformed into thiocarbonate and other sulfurous admixtures. The so-called maturation of the viscose—that is, the partial hydrolysis of the cellulose xanthate—takes place because of this process. As a result of hydrolysis, the degree of esterification of the cellulose xanthate decreases, with a corresponding decrease in its sulfur and sodium content.
The ripeness of the viscose increases continuously with an increase in the time of maturation. Temperature exerts a strong influence on the speed of maturation. The maturation time for the production of viscose fibers is 35-45 hours, and for the production of films (cellophane) it is 80-90 hours.
In some cases various substances—for example, matting agents (to eliminate glassy sheen in the finished fiber), dyes (to color the finished articles evenly and permanently), and modifiers and surface-active substances (in the production of high-strength and structurally uniform fibers)—are added to the viscose.