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The reaction of acid anhydride curing agent epoxy resin requires a small amount of alcohol and water, free acid and other accelerators in the resin / anhydride system, which can be slowly cured by heating. Therefore, the acid anhydride does not directly react with the epoxy group, and the ring of the acid anhydride must be opened.

1. The effect of active hydrogen on the ring opening of anhydrides. Bisphenol A epoxy contains hydroxyl group, which can open the anhydrides. A hydroxyl group produces a carboxyl group, and polyols can connect the two anhydrides and play a cross-linking role. The addition of hydroxy compounds such as ethylene glycol, glycerin and low molecular polyether with hydroxyl can accelerate the ring opening reaction. Water can generate two carboxyl groups in the anhydride, so humidity has an effect on the curing of the anhydride.

Esterification reaction: This is the main reaction of curing epoxy resin with anhydride. The carboxyl group and epoxy group are added to form an ester group. The carboxyl groups formed by the esterification reaction further ring-open the acid anhydride and react with the epoxy group to form a three-dimensional structure. At high temperatures, some carboxyl groups can catalyze the ring opening of epoxy groups to form structures dominated by ether bonds.

2. The effect of tertiary amines on the ring opening of anhydrides. The tertiary amine forms an ion pair with the acid anhydride, and the epoxy group is inserted into this ion pair, then the carboxyl negative ion opens the epoxy group, forming an ester bond, and generating a new anion. For example, 2-Ethyl-4-methylimidazole and 2,4,6 - (N, N-Dimethylaminomethyl)- phenol, that is K-54# (known abroad as dmp-30 #).
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