Referate und Hausarbeiten : Biologie
Viscose rayon is made from wood pulp. The chemical process of dissolving wood pulp was first discovered in 1840 by F.G. Keller, a noted German weaver. The viscose method of using wood pulp to manufacture rayon was developed in 1892, more than fifty years later, by C.F. Cross and E.J. Bevan, both British scientists. The first viscose manufacturing plant was in 1905, the first in the United States in 1910.
· Dissolution:The cellulosic raw materials for rayon are wood chips (usually from spruce or pine) or cotton linters. These are treated to produce sheets of purified cellulose containing 87-98% cellulose. They are then bleached with sodium hypochloride (NaOCl) to remove natural colour. These cellulose sheets are then soaked in 18% caustic soda for 1 to 2 hours producing sheets of alkali cellulose. Any excess alkali is pressed out. The substance is broken up into flakes or grains called cellulose crumbs, which are aged for two or three days under controlled temperature and humidity. Liquid carbon disulfide is added to the crumbs to change the cellulose into cellulose xanthate, a light orange substance that is still in crumb form. These crumbs are dissolved in a weak solution of caustic soda and transformed into a viscous solution called "viscose", honey-like in colour and consistency.
To produce the rayon filament, the viscose solution is aged, filtered, then vacuum-treated to remove any air bubbles that could weaken the filament and cause it to break. It is then pumped through spinnerets into a bath of sulfuric acid, which coagulates the cellulose xanthate to form regenerated filaments of 100% cellulose. The many variations and different properties of viscose such as luster, strength, softness and affinity for dyes, are influenced here by varying the technique and by the addition of external materials.
Once extruded, the freshly formed viscose must be purified and strengthened. It...