The curing reaction mechanism of acid anhydrides is divided into two forms according to the presence or absence of an accelerator.
1. In the absence of accelerant, the hydroxyl group in the epoxy resin reacts with the acid anhydride, the acid anhydride is opened, and then the addition polymerization reaction is performed, the sequence is as follows:
I Hydroxyl reacts with anhydride to form ester bonds and carboxylic acids;
II Addition of carboxylic acid to epoxy group to form hydroxyl group;
III The resulting hydroxyl groups continue to react with other anhydride groups.
In addition, in such a system, since it is in an acidic state, another epoxy group and a hydroxyl group react in parallel with the above reaction to form an ether bond. It can be seen from the above mechanism that the cured product contains two structures of ether bond and ester bond, and the reaction rate is dominated by the epoxy group concentration and the hydroxyl group concentration.
2. In the presence of accelerants, Lewis base was used to promote the curing reaction of anhydrides. The reaction of the promoter (usually a tertiary amine) to the attack of the anhydride began. The main reactions are:
I Accelerators attack acid anhydride to form carboxylate anions;
II A carboxylate anion reacts with an epoxy group to form an oxygen anion;
III Oxygen anions react with other anhydrides to form carboxylate anions again.
In this way, the acid anhydride and the epoxy group interact with each other, and the addition polymerization proceeds gradually. In the presence of the accelerator Lewis base, all the bonds formed were ester bonds, and no ether bonds were found as if no accelerator was present.
In the presence of the accelerator, the curing speed of the epoxy resin is also dominated by the hydroxyl concentration in the system. Therefore, the addition of accelerators is very effective for liquid epoxy resins, and the curing reaction can be completed at 120 ~ 150℃. However, for solid epoxy resin, full attention must be paid to its very short pot life. In the absence of an accelerator, in theory, one epoxy group should be used for one anhydride, but in fact only 80% to 90% of the theoretical amount of chemical is sufficient. In the presence of an accelerator, the amount of acid anhydride is the theoretical amount.