Effect of Number Density of Epoxy Functional Groups on Reaction Kinetics for Epoxy Resin

Improving the heat resistance of epoxy resins remains an important and challenging issue across a wide variety of applications. One strategy to address this challenge is the design of cured resins based on multifunctional epoxy. Compared to conventional difunctional epoxy resins, multifunctional epoxy resins are expected to easily form a highly cross-linking structure, which is anticipated to contribute to enhanced heat resistance. However, there is a lack of information about the detailed mechanisms of the formation of such cross-linking structures and their effects on the physical properties. We herein tracked the kinetics of curing reactions of difunctional and trifunctional epoxies with an amine hardener. Despite the identical reactivity of epoxy groups in both base monomers, the trifunctional epoxy system cured faster. In addition, in the difunctional epoxy system, gelation was followed by vitrification, while in the trifunctional epoxy system, gelation and vitrification occurred simultaneously. The accelerated curing reaction observed in the trifunctional epoxy system could be explained in terms of the localized temperature increase from the reaction heat, which subsequently accelerated the following reactions. The resulting post-cured trifunctional epoxy resin did not exhibit a clear glass transition. This, so-called Tg-less behavior was due to the glass transition temperature of the trifunctional epoxy unit being higher than its decomposition temperature. This mechanism for the Tg-less behavior, combined with the accelerated curing reactions, provides valuable insights for the design of thermosetting resins with enhanced thermal stability.