40.2 VISCOSE RAYON
Polymer Chemistry : The raw material for the production of Viscose rayon is wood pulp. However, the manufacturing process emits toxic effluents which have adverse environmental effects. Because of its cellulosic base , it has some properties which are similar to those of cotton and any other natural cellulosic fibres. It has moderate resistance to acids and alkalis. However, it may be damaged by relatively weak acids. Moreover, due to the inelasticity of fibers, it has poor abrasion resistance.
Technology (Figure 40.2):
The raw materials used in this process are cellulose wood pulp sheets or cotton linters. These are placed in a steeping press with contact in aqueous NaOH Solution for a period of 2 – 4 hours at normal room temperature. A hydraulic ram presses out the excess alkali and the sheets are shredded to crumbs and aged for 2 – 3 days. The aging process has its direct consequence on the viscosity of the solution.
Later CS2 is added in a rotating drum mixer over a period of 3 hours. The orange cellulose xanthate which forms is transferred to a solubilizer , wherein it is mixed into dilute caustic . The mixing of cellulose xanthate and dilute caustic yields a orange colored viscous solution which contains 7 – 8 % cellulose and 6.5 – 7 % NaOH. It is digested at room temperature for 4-5 days. Thereafter , the solution is filtered and fed to spinning machines.
The spinning is carried out in extrusion spinnerettes , which are made up of platinum or gold alloys. The orifices of these spinnerets have a diameter of 0.1 – 0.2 mm. for continuous filament yarns and of diameter 0.05 – 0.1 mm for short fibre shapes. The solution extruded from the spinnerette is contacted with an acid bath which precipitates the filaments without causing them to break or stick together. The processing treatments of washing, desulphurizing, bleaching and conditioning takes place continuously and in order after the filaments are wound on a series of plastic rolls.
Figure 40.2 Manufacture of Viscose Rayon