Curing of off-stoichiometric amine–epoxy thermosets

Abstract

The kinetics of the epoxy–amine polycondensation and the epoxy homopolymerization in off-stoichiometric epoxy/amine formulations with excess of epoxy groups, and in the presence of an anionic initiator have been investigated. Diglycidyl ether of bisphenol A (DGEBA) and diethylenetriamine (DETA) have been used as epoxy and amine reagents, respectively, and 2-methylimidazole (2MI) has been used as anionic initiator. This study has been carried out using a differential scanning calorimeter (DSC). The thermal–mechanical properties of the partially cured and fully cured materials with and without initiator have been determined by DSC and dynamic-mechanical analysis. First, off-stoichiometric DGEBA/DETA mixtures with excess of DGEBA, with and without 2MI, have been reacted isothermally at low temperatures, where only the epoxy–amine condensation takes place, because the epoxy homopolymerization has a very low curing rate. Afterward, samples containing 2MI have been heated at different heating rates to study the homopolymerization process of the epoxy excess. The kinetics of both processes have been analyzed with an isoconversional method to determine the activation energy, and the Sestak–Berggren equation has been applied to determine the frequency factor and the orders of reaction. In the isothermal curing, amine–epoxy condensation, the activation energy and the frequency factor decrease with increasing degree of conversion, but in the homopolymerization process, both magnitudes increase with the degree of conversion. Results show that the dual-curing character of off-stoichiometric DGEBA/DETA thermosets with 2MI as anionic initiator renders them suitable for multistage curing processes in which the degree of cure and material properties in the intermediate stage can be controlled easily and final material properties can be enhanced.