Process for preparing carboxyalkyl cellulose esters

A process for preparing C.sub.2 -C.sub.4 esters of carboxy(C.sub.1 -C.sub.3 alkyl) cellulose having an inherent viscosity of about 0.2 to 0.70 dL/g, as measured in a 60/40 (wt./wt.) solution of phenol/tetrachloroethane at 25.degree. C., and having a degree of substitution per anhydroglucose unit (DS/AGU) of carboxy(C.sub.1 -C.sub.3 alkyl) of about 0.20 to 0.75, which comprises:

• (a) slurrying water wet carboxy(C.sub.1 -C.sub.3 alkyl) cellulose (acid form) in a solvent selected from the group consisting of acetic acid, propionic acid, and butyric acid, and mixtures thereof, thereby dewatering said carboxy(C.sub.1 -C.sub.3 alkyl) cellulose to form a mixture;
• followed by
• (b) treating said mixture with a compound selected from the group consisting of acetic anhydride, propionic anhydride, and butyric anhydride, and mixtures thereof, in the presence of a strong acid catalyst;
• followed by
• (c) heating at a temperature of about 40.degree. to 55.degree. C. until the reaction is complete;
• followed by
• (d) adding slowly a mixture of water, an alkanoic acid, and optionally an amount of a C.sub.2 -C.sub.5 alkanoic acid salt of an alkali or alkaline earth metal insufficient to totally neutralize the strong acid catalyst;
• (e) heating said solution at a temperature of about 55.degree. to 85.degree. C. for about 1 to 15 hours, which effects partial hydrolysis of the carboxy(C.sub.1 -C.sub.3 alkyl) cellulose alkanoic ester;
• (f) treating said solution with an equimolar amount, based on the amount of strong acid catalyst, of a C.sub.2 -C.sub.5 alkanoic salt of an alkali or alkaline earth metal dissolved in water and an alkanoic acid.