The curing reaction mechanism of anhydrides can be divided into two forms according to the presence or absence of accelerators.
1. In the absence of accelerants, the hydroxyl group in epoxy resin reacts with anhydride first, and the anhydride is turned on, then the addition polymerization is carried out. The order is as follows:
Hydroxyl reacts with anhydride to form ester bond and carboxylic acid.
Carboxylic acid is added to epoxy group to form hydroxyl group.
The hydroxyl groups formed continue to react with other anhydrides.
The reaction proceeds repeatedly to form bulk polymers. In addition, in this system, due to the acidic state, the reaction parallel to the above reaction is the reaction of other epoxy groups with hydroxyl groups, resulting in the formation of ether bonds. From the above mechanism, it can be seen that there are two kinds of structures in the cured product: ether bond and ester bond, and the reaction rate is controlled by the concentration of epoxy group and hydroxyl group.
2. In the presence of accelerants, the acid anhydride curing reaction is promoted by Lewis base. Accelerators (usually tertiary amines) initiate an attack on anhydride. The main reactions are as follows:
accelerator attacked anhydride to form carboxylate anion.
carboxylate anion reacts with epoxy group to form oxygen anion.
oxygen anion reacts with other anhydrides to form carboxylate anion again.
In this way, acid anhydride reacts with epoxy group, and additive polymerization is carried out step by step. In the presence of promoter Lewis base, all the bonds formed are ester bonds, and no ether bond was found as in the absence of promoter.
In the presence of accelerants, the curing rate of epoxy resin is also controlled by the concentration of hydroxyl groups in the acceptor system. Therefore, the addition of accelerators is very effective for liquid epoxy resin, and the curing reaction can be completed at the temperature of the 120～150℃. But for solid epoxy resin, we should pay attention to the problem of very short application period. In the absence of accelerants, theoretically, an epoxy group should be used for an anhydride, but in fact, only 80%-90% of the theoretical chemical quantity is enough. In the presence of accelerants, the amount of anhydride used is the chemical theoretical quantity.