Acid acceleration of epoxide condensations

Abstract

The addition of carboxylic acid to Epon 834‐phthalic anhydride resins accelerates the rate of cure. The maximum thermal yield point corresponds to reaction of all the epoxide groups in the ratio of two anhydride carbonyls to one epoxide oxygen or of one acid carbonyl to one epoxide oxygen. The initial presence of some secondary alcohol groups appears to be essential for initiation of reaction as suggested by Fisch and Hofmann. Carboxylic acids react directly with epoxide groups to produce secondary alcohols which in turn react with phthalic anhydride, regenerating carboxylic acid groups. Succinic acid gives a rapid cure (although not as fast as amines) to a resin with only slightly lowered thermal yield point, while benzoic acid depresses the yield point by about 15° because it acts as a chain‐stopper.