RELAXATIONS IN THERMOSETS .25. CALORIMETRIC STUDIES OF THE CURING KINETICS OF PURE AND RUBBER-CONTAINING EPOXY-BASED THERMOSETS

The curing kinetics of diglycidyl ether of bisphenol-A, with ethylene diamine, propylene diamine, and hexamethylene diamine, with 60 wt % amine-terminated butadiene acrylonitrile (ATBN) and without, have been measured under both isothermal and ramp-curing conditions. The phase separation of ATBN is undetectable by calorimetry, but has a significant effect on both the rate of curing and the extent of cure. In all cases, its presence decreases the extent of cure and lowers the T(g) of the thermoset formed. The total heat of reaction for the curing is 445 +/- 15 J / g and remains unaffected by both the presence of an elastomer and the increase in the molecular size of the curing agent. None of the formalisms available for the curing kinetics of epoxy-based thermosets is consistent with the results for the neat thermosets. The average activation energy for the reactions increases from 107 to 150 kJ/mol when ethylene diamine is replaced by hexamethylene diamine, but curing reaches near completion in a shorter time with the latter than with the former. The T(g) of the latter thermoset is lower than that of the former and follows a decreasing trend with increase in the molecular size of the curing agent-an effect that is related to the increase in the length of cross-links in the network structure.