Viscose is a semi-synthetic fiber. "Viscose" can mean:
- A viscous solution of cellulose, which can be made into rayon or cellophane
- A synonym for rayon
- A specific term for viscose rayon—rayon made using the viscose (cellulose xanthate) process
The viscose process dissolves pulp with aqueous sodium hydroxide in the presence of carbon disulfide. This viscous solution bears the name viscose. The cellulose solution is used to spin the viscose rayon fiber, which may also be called viscose. Viscose rayon fiber is a soft fiber commonly used in dresses, linings, shirts, shorts, coats, jackets, and other outerwear. It is also used in industrial yarns (tyre cord), upholstery and carpets, and in the casting of cellophane.
Viscose rayon is a fiber of regenerated cellulose; it is structurally similar to cotton but may be produced from a variety of plants such as soy, bamboo, and sugar cane. Cellulose is a linear polymer of β-D-glucose units with the empirical formula (C6H10O5)n. To prepare viscose, dissolving pulp is treated with aqueous sodium hydroxide (typically 16-19% w/w) to form "alkali cellulose," which has the approximate formula [C6H9O4-ONa]n. The alkali cellulose is then treated with carbon disulfide to form sodium cellulose xanthate.
- [C6H9O4-ONa]n + nCS2 → [C6H9O4-OCS2Na]n
The higher the ratio of cellulose to combined sulfur, the lower the solubility of the cellulose xanthate. The xanthate is dissolved in aqueous sodium hydroxide (typically 2-5% w/w) and allowed to depolymerize to a desired extent, indicated by the solution's viscosity. The rate of depolymerization (ripening or maturing) depends on temperature and is affected by the presence of various inorganic and organic additives, such as metal oxides and hydroxides. Air also affects the ripening process since oxygen causes depolymerization.
Rayon fiber is produced from the ripened solutions by treatment with a mineral acid, such as sulfuric acid. In this step, the xanthate groups are hydrolyzed to regenerate cellulose and release dithiocarbonic acid that later decomposes to carbon disulfide and water:
- [C6H9O4-OCS2Na]2n + nH2SO4 → [C6H9O4-OH]2n +2nCS2 + nNa2SO4
- H2COS2 → H2O + CS2
Aside from regenerated cellulose, acidification gives hydrogen sulfide, sulfur, and carbon disulfide. The thread made from the regenerated cellulose is washed to remove residual acid. The sulfur is then removed by the addition of sodium sulfide solution and impurities are oxidized by bleaching with sodium hypochlorite solution.
French scientist and industrialist Hilaire de Chardonnet (1838–1924)—who invented the first artificial textile fiber, artificial silk—created viscose. British scientists Charles Frederick Cross and Edward John Bevan took out British patent no. 8,700, "Improvements in Dissolving Cellulose and Allied Compounds" in May, 1892. In 1893 they formed the Viscose Syndicate to grant licences, and in 1896 formed the British Viscoid Co. Ltd. to exploit the process.
The use of viscose is declining. Instead, rayon may be manufactured using the Lyocell process, which uses N-methylmorpholine N-oxide as the solvent and produces little waste product, making it relatively eco-friendly.
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