GB736964A - Improvements in or relating to the treatment of cellulose esters - Google Patents

Abstract

The thermal colour-stability of water-insoluble cellulose esters of carboxylic acids (including mixed esters and ether-esters in which the number of ester groups exceeds the number of ether groups in the molecule), prepared in the presence of sulphuric acid as catalyst in the form of an aqueous slurry having a pH below 7, is improved by bleaching the ester with a water-soluble inorganic chlorite, the pH being adjusted with acetic acid or a phosphoric acid, or a derivative of phosphoric acid whose aqueous solutions have a pH less than 7. The concentration of the cellulose ester in granular or flake form in the slurry may be 7-15 per cent by weight and the pH of the slurry may be adjusted to 3-6, preferably 4-5. The mixture may be heated to 75-130 DEG C. under a pressure sufficient to maintain the water in the liquid phase. An aqueous 3 to 5 per cent solution of the chlorite may then be added and the bleaching effected with agitation in 0.5-6 hours, preferably 1-3 hours. The bleached ester is then drained and washed with water. The proportion of chlorite used may be 0.001 to 0.015 part per part of cellulose ester. The chlorite may be a water-soluble alkali-metal or alkaline-earth metal chlorite or ammonium chlorite. The bleaching of the cellulose ester may also be effected by passing chlorine dioxide gas into an aqueous slurry of the ester having a pH above 7 but not over 9, the chlorine dioxide being absorbed by the alkali present with the formation of a chlorite and the mixture being subsequently acidified with one of the prescribed acidifying agents to the desired pH below 7. The bleaching may be carried out at any point in the production of the cellulose ester after the precipitation of the ester by the addition of water to the esterification reaction mixture. Thus the bleaching may be effected in a slurry formed by preliminary washing of the ester with water after its precipitation, sufficient acetic or other acid being retained by the precipitated ester particles to render the addition of further acid to adjust the pH unnecessary. In an example 3000 parts by weight of cellulose acetate (prepared with sulphuric acid as catalyst) in the form of a 14 per cent slurry in water are bleached for one hour at 95 DEG C. with 6.5 parts of sodium chlorite, drained and washed in the usual manner. In further examples aqueous slurries of cellulose acetate which have been adjusted to pH 4 with orthophosphoric acid are bleached at 90-95 DEG C. in a stainless steel vessel with 350, 700 and 1400 parts of sodium chlorite per million of slurry. No corrosion of the stainless steel occurs. Specification 693,699 is referred to.