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 (T g) during isothermal curing revealed a temperature-dependent relationship between T g 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 T g values of resins with different thermal histories were equal after complete curing under high-temperature/long-duration conditions. [Display omitted] • The glass transition temperature – conversion curve depends on curing temperature. • An endotherm from imidazole regeneration overlap with the main exothermic polymerization. • During imidazole regeneration, cyclic compounds derived from N-dealkylation was not detected. • The conversion of intermediate stage of curing cannot determined by conventional DSC. • The glass transition temperatures were finally equal after complete curing. [ABSTRACT FROM AUTHOR]