Acid anhydride curing agent is another kind of epoxy resin curing agent after amine. The position of organic acid anhydride curing agent is very important which is characterized by it has at least one anhydride group in each molecule. Acid anhydride curing agent mainly includes fatty anhydride, alicyclic anhydride, aromatic anhydride, unsaturated anhydride and mixed anhydride. Compared with amine curing agent, acid phenol curing agent can endow epoxy resin with many new properties, such as higher reaction equivalent, smaller reaction heat, longer application period, lower curing shrinkage, better heat resistance and better electrical insulation. However, they are easy to being dump and hydrolyze; at higher temperature or under the catalysis of tertiary amines can form polymers which will result in increased viscosity and poor process performance.
Because of the weak reacting activity with epoxy resin, anhydride can cure epoxy resin well at a higher temperature, then the intermediate and epoxy groups of epoxy resin may be esterified with anhydride. The addition of a light amount of tertiary amines and phenols in the epoxy monohydride system can shorten the curing time, reduce the reaction temperature and inhibit the vinegar reaction. The reaction mechanism of catalyzing of tertiary amine anhydride cured epoxy resin is still controversial and many scholars support the proposed mechanism. First, tertiary amines form a zwitterionic pair with anhydride, in which the oxygen anion of the search group attacks the epoxy group to form the vinegar bond and a new alkoxy anion while it turn binds to the anhydride molecule to form a new ion pair to continue the open of the epoxy group. An epoxy-anhydride crosslinking network can be finally formed by the continuous reaction, chain expansion, branching and cross-linking as the above description.