Formulating epoxy resin systems with anhydrides can offer great advantages such as long pot life, low viscosity, excellent thermal stability, high Tg’s and low shrinkage. However, curing with anhydrides can also be very challenging as they require significant heat input to initiate the polymer reaction. In addition, there are typically competing reactions that can have an impact on physical and/or mechanical properties.
For example, the opening of the anhydride ring with an alcohol hydroxyl group can form a monoester which can then react with emerging carboxylic acid groups to form a diester. This is the esterification reaction.
Another competing reaction involves an epoxy group reacting with existing or emerging hydroxyl groups to form ether linkage. This is the etherfication reaction and according to literature at low elevated temperatures this reaction proceeds at about the same rate as esterification.
Literature on this subject cites that at higher temperatures esterification is favored over etherfication. Furthermore, literature suggests that more esterification leads to diminished high temperature performance in the final polymer. In systems where the resin is gelled at high temperature esterification will predominate and there is little opportunity for etherfication to take place which again can compromise thermal stability. Add to that, absent a catalyst the epoxy/anhydride reaction requires very high temperatures (>200oC) to fully initiate. Therefore, the use of the right catalyst is going to have a significant impact on both processing and performance.
Hycat™ 3100S, Hycat™ AC-4 and our recently developed Hycat™ XP-1 are all excellent anhydride catalysts designed to significantly reduce curing temperatures. When compared to other catalysts such as imidazoles or BDMA, Hycat™ Catalysts can lower onset temperatures by 30o – 40oC. This translates into lower temperature gel cycles that will promote more etherfication and thereby enhance thermal stability characteristics in the final cure. But what truly sets Hycat™ Catalysts apart is what we term a “relaxed exothermic profile.” Reactions catalyzed by Hycat™ have a broad exothermic distribution with no sudden spikes in energy release. This allows curing to continue smoothly at lower temperatures and further promote even more etherfication. In addition, this reaction characteristic that is unique to Hycat™ Catalysts also provides for greater control over heat dissipation leading to exceptional quality polymer cures.