Cure behavior of an epoxy-anhydride-imidazole system

The cure behavior of an epoxy-anhydride system with one of imidazole derivatives as an accelerator was investigated for both stoichiometric and epoxy-rich formulations. The mixtures were isothermally cured for different times at various temperatures, and the glass transition temperature (T-g) was determined by differential scanning calorimetry (DSC). There was a considerable increase in the T-g during curing, and master curves were formed by shifting and superposing the T-g vs. cure time curves. The activation energies obtained by the shift were 15.2 kcal mol(-1) and 16.5 kcal mol(-1) for the stoichiometric and epoxy-rich systems, respectively. For the epoxy-rich one, plateaus were observed on the T-g vs. cure time curves, and two exothermic peaks appeared on DSC curves in the early stage of cure and became one thereafter. This phenomenon is related to the competitive reactions between esterification and etherification reactions. We found that the maximum T(g)s were 80 degrees C for esterification and 125 degrees C for etherification in the excess epoxy system.