Viscose is a type of rayon fiber that is made from cellulose that are regenerated as cellulose fiber. The molecular structure of natural cellulose is preserved in the process. The many types and grades of viscose fibers can imitate the feel and texture of natural fibers such as silk, wool, cotton, and linen. The types that resemble silk are often called artificial silk. The fibre is used to make textiles for clothing and other purposes.
Viscose can mean:
- A viscous solution of cellulose
- A synonym of rayon
- A specific term for viscose rayon — rayon made using the viscose (cellulose xanthate) process
The molecular structure of rayon is that of cellulose, i.e., a linear polymer of β-D-glucose units with the empirical formula (C6H10O5)n. To prepare viscose, pulp is treated with aqueous sodium hydroxide (typically 16-19% w/w) to form "alkali cellulose", which has the approximate formula [C6H9O4-ONa]n. This material is allowed to depolymerize to an extent. The rate of depolymerization (ripening or maturing) depends on temperature and is affected by the presence of various inorganic additives, such as metal oxides and hydroxides. Air also affects the ripening process since oxygen causes depolymerization. The alkali cellulose is then treated with carbon disulfide to form sodium cellulose xanthate.
- [C6H5(OH)4-ONa]n + nCS2 → [C6H5(OH)4-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). The solution's viscosity is determined by the extent of depolymerization of the alkali cellulose.
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 carbon disulfide.
- [C6H5(OH)4-OCS2Na]2n + nH2SO4 → [C6H5(OH)4-OH]2n +2nCS2 + nNa2SO4
Aside from regenerated cellulose, acidification gives hydrogen sulfide (H2S), 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 or hydrogen peroxide solution.
Because carbon disulfide is highly toxic, its use on a large scale demands care. Historically, however, several incidents have resulted in many poisonings. With production facilities often located in developing countries, concerns for worker safety continue. Newer control technologies have enabled improved collection of carbon disulfide and reuse of it, resulting in a lower emissions of carbon disulfide.
Viscose or rayon may also be manufactured using the Lyocell process, which uses N-methylmorpholine N-oxide (NMO) as the solvent to dissolve the cellulose. NMO is more expensive that carbon disulfide, such that losses during processing must be minimized.
French scientist and industrialist Hilaire de Chardonnet (1838–1924), inventor of 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.
Products made from viscose
- Krässig, Hans; Schurz, Josef; Steadman, Robert G.; Schliefer, Karl; Albrecht, Wilhelm; Mohring, Marc; Schlosser, Harald (2002). "Cellulose". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a05_375.pub2.
- Wheeler, Edward (1928). The Manufacture of Artificial Silk With Special Reference to the Viscose Process. New York: D. Van Nostrand company.
- Paul David Blanc (2016). Fake Silk The Lethal History of Viscose Rayon. Yale University Press. p. 325. ISBN 9780300204667.
- Woodings, Calvin R. "A Brief History of Regenerated Cellulosic Fibres". Woodings Consulting Ltd. Retrieved 26 May 2012.
- Day, Lance; Ian McNeil (1998). Biographical Dictionary of the History of Technology. Taylor & Francis. p. 113. ISBN 0-415-19399-0.