GB609790A - Improvements in the production of cellulose esters - Google Patents

Abstract

Mixed cellulose esters are prepared by esterifying cellulose in the presence of an esterification catalyst with a mixture of the anhydrides of a saturated aliphatic acid and an unsaturated aliphatic acid, each of which acids contains at least four carbon atoms in the molecule. The conditions of esterification are such that the ester contains a relatively small proportion of unsaturated acid groups, about 0.2 to 0.3 per glucose residue. The most useful of these esters is cellulose butyrate crotonate, which may be moulded with or without a plasticiser at temperatures up to 200 DEG C., the moulded articles possessing both toughness and flexibility. Preferably, the catalyst is sulphuric acid, though phosphoric acid may be used, and only about 50-75 per cent of the catalyst should be present at the commencement of the reaction. The reaction temperature should not exceed 15 DEG C. The ester may be bleached prior to precipitation or following precipitation, in preference the bleaching being carried out both while the ester is in solution and again when it is in the fibrous form. The preferred bleaching agent is hydrogen peroxide, but sodium hypochlorite, bleaching powder and sodium sulphite may be employed. The ester may be stabilized by refluxing with alcohol and water containing 0.01-0.05 per cent sulphuric acid, by kneading with acetone or methyl ethyl ketone and adding magnesium carbonate until the pH of the supernatant liquid is between 6.9 and 7.1, or by heating with 6-10 times its volume of 0.02-0.05 per cent aqueous sulphuric acid under pressure at 120-130 DEG C. for 2-4 hours. In an example (all parts being by weight), 150 parts of cotton linters are agitated for one hour in a mixture of 450 parts butyric acid and 30 parts formic acid, and allowed to stand for about 58 hours. The cotton and pretreatment mixture are cooled to -10 DEG C. and added to 360 parts butyric anhydride and 180 parts crotonic anhydride to which is added 8.6 parts of sulphuric acid dissolved in 120 parts butyric anhydride. After 5-6 hours, the temperature rises to 15 DEG C. at which level it is maintained. 2.9 parts of sulphuric acid in 16 parts of butyric anhydride are added when the mixture clears. After 20 hours, water (25 per cent of cotton weight) is added, the sulphates hydrolysed out. The ester in solution is bleached by 750 parts glacial acetic acid containing 4.9 parts 30 per cent aqueous hydrogen peroxide, and then the acid catalyst neutralized with magnesium carbonate and the ester precipitated by excess water. If desired, it may be redissolved in glacial acetic acid and reprecipitated to obtain a fibrous compound. Stabilization is effected by heating under pressure with dilute sulphuric acid. The butyrate-crotonate so obtained is fully esterified and has a butyryl value of 71.6 per cent and a crotonyl value of 4.9 per cent, both calculated as the corresponding acids.