GB681865A - Preparation of cellulose esters - Google Patents

Abstract

Cellulose is esterified with a mixture of an organic acid anhydride, sulphuric acid as catalyst, and a solvent for the cellulose ester which is formed, and the cellulose ester is then ripened in the esterification solution in two stages, the first at a temperature of 50-60 DEG C. for 20-40 minutes in presence of 1.5-2.5 per cent of sulphuric acid (calculated on the weight of the cellulose starting material), and the second at 82-87 DEG C. in presence of 0.7-0.9 per cent of sulphuric acid for a time sufficient to reduce the acyl content of the cellulose ester to the desired value. The total ripening time may be 300-400 minutes. At the end of the esterification, unreacted anhydride may be destroyed by adding water, and the sulphuric acid may be partly or completely neutralized, e.g. by means of a compound of a metal which forms a sulphate which is insoluble in the esterification solution. Magnesium acetate or carbonate, calcium acetate or carbonate, zinc acetate or oxide, may be used. If the sulphuric acid used in the esterification has been completely neutralized, the required amount of fresh sulphuric acid is added shortly before the ripening step. Water is also added. The total amount of water at the beginning of the first ripening stage may be 20-60 per cent calculated on the cellulose. Before the second stage of ripening a further 50-125 per cent may be added. The temperature of the ripening solution may be raised to the desired degree by injecting steam. Ripened cellulose esters of high viscosity characteristics may be obtained, e.g. a cellulose acetate having in 6 per cent acetone solution a viscosity which, measured by a flow method, exceeds that of glycerine at the same temperature. The process may be applied to the production of cellulose propionate. In an example, cotton linters are esterified with a mixture of acetic acid, acetic anhydride, and sulphuric acid. At the end of the esterification, water is added in the form of dilute acetic acid to destroy the unreacted anhydride and to provide 38 per cent of water based on the weight of cotton. The water also contains in solution sufficient magnesium acetate to reduce the free sulphuric acid to 1.5 per cent of the cotton linters. The temperature is raised to 60 DEG C. by injecting steam and is maintained for 30 minutes. A further 57 per cent of water with sufficient magnesium acetate to reduce the sulphuric acid to 0.85 per cent is then added The temperature is raised to 85 DEG C. and kept at this value for 270 minutes. The cellulose acetate is then precipitated by means of water.