Investigation of the isothermal curing and glass transition behaviors of epoxy-imidazole systems

The isothermal curing behavior of an epoxy-imidazole system was investigated through thermal, mechanical, and compositional analyses. Precise control over temperature and time afforded the monitoring of the progression of the curing reaction and enthalpy relaxation via fast scanning calorimetry. The real-time tracking of the glass transition temperature (Tg) during isothermal curing revealed a temperature-dependent relationship between Tg and thermally estimated conversion, distinct from epoxy-amine systems. Unlike previous research suggesting plasticization through cyclic-compound formation in epoxy-imidazole systems, such compounds were not detected in the early curing stages in this study. Furthermore, an endothermic reaction from imidazole regeneration was suggested to overlap with the main exothermic reaction of polymerization. Therefore, assessing thermally estimated conversion in the intermediate curing stage through differential scanning calorimetry was deemed practically impossible for epoxy-imidazole systems. The Tg values of resins with different thermal histories were equal after complete curing under high-temperature/long-duration conditions.