| 摘要 |
Cellulose acetates and other organic esters of cellulose are prepared from celluloses of low viscosity, namely, a cuprammonium p viscosity of 90--100 centipoises, obtained by subjecting celluloses of relatively high viscosity to the action of acid media or neutral oxidizing agents. By the use of such celluloses, cheaper and more plentiful cellulosic materials such as wood pulps and low grade cotton linters, become available for esterification; there are obtained cellulose esters of greater uniformity and improved solubility, and their solutions are of greater clarity; the esterification may be carried out at lower temperatures, with smaller proportions of catalyst and of acetic acid etc.; and the solutions of the cellulose esters spin more easily with fewer broken filaments. Esterification may be carried out in solution or in suspension; the catalyst used may be sulphuric acid, sulphuryl chloride or other sulphur containing catalyst, or zinc chloride or other metal halides with or without halogen acids, for example, as described in Specifications 308,322, 308,323, 309,201, 312,095, 312,096, and 312,098, [all in Class 2 (ii)], and the cellulose ester produced may be subjected to ripening until a desired solubility is developed. For effecting the desired reduction in viscosity of the cellulose, the following acid media and oxidizing agents are instanced: acetic acid solutions of sulphuric acid and sulphuryl chloride, aqueous solutions of mineral acids, and solutions of bleaching powder and hydrogen peroxide. The low viscosity cellulose may be treated for the removal of soda-soluble constituents before use. Before the actual esterification, the cellulose may be pretreated with formic, acetic &c. acid in liquid or vapour form as described in Specifications 263,938 and 312,098, [both in Class 2 (ii)]. In examples, cotton linters for conversion to cellulose acetate are subjected to a pretreatment for the purpose of lowering their viscosity with boiling 0.5 per cent sulphuric acid or centinormal hydrochloric acid, or with a mixture of sulphuryl chloride and acetic acid, or with the vapours of hydrogen peroxide. |
