GB597295A - Improvements in the production of organic acid esters of cellulose - Google Patents

Abstract

A process for ripening organic acid esters of cellulose comprises the injection of a hot vapour such as steam into the solution of the primary ester so that the conversion to the secondary ester takes place at about 60 DEG C. The ester may be that of any of the lower fatty acids such as acetic, propionic or butyric, or it may be a mixed ester such as cellulose acetate-propionate, or acetate-butyrate. The catalyst for the esterification may be sulphuric or phosphoric or a mixture of these acids. The process is preferentially carried out in stages and the acid present as a catalyst in the esterification should be neutralised partly before the ripening is commenced. Suitable neutralising agents are the acetates of calcium, magnesium and zinc, the carbonates of calcium, magnesium, aluminium, barium, copper, lithium and strontium, the oxides and hyroxides of aluminium, barium, copper, lithium and strontium and also zinc oxide. A mixture of magnesium acetate and calcium acetate is used preferentially. The cellulosic material (e.g. cotton, cotton linters, wood pulp or material from straw and hulls) may be pretreated with acetic acid in accordance with known methods before esterification. By the present process the total time of preparing the cellulose ester is cut down and no substantial reduction in the degree of polymerization is effected. In an example, 100 parts of cotton linters are pretreated with 35 parts of acetic acid, 0.5 parts of sulphuric acid and 0.5 parts of water for 3 hours at 22 DEG C. (All parts are by weight.) It is then transferred to an acetylizer containing 240 parts of acetic anhydride, 330 parts of acetic acid and 13.25 parts of sulphuric acid, and acetylated at a temperature below 34 DEG C. When the reaction is complete enough water to react with the excess acetic anhydride is added, 25 per cent of the sulphuric acid is neutralized and water added in an amount equal to 15 per cent of the cellulose. The charge is ripened at 30 DEG C. for 30 minutes. An additional 42 per cent of water is now added, a further 35 per cent of the sulphuric acid neutralised and the temperature is raised to 60 DEG C. in a period of 40 minutes by the direct injection of steam. During this stage the mass is agitated. The third stage is carried out at 60 DEG C. and 33 per cent of water per hour is added until the cellulose acetate has an acetyl value of 54.4 per cent (as acetic acid). It is then precipitated by the addition of a large excess of water and treated further with solvents as may be required.