PHYSICAL AGING STUDIES IN EPOXY-RESINS .1. KINETICS OF THE ENTHALPY RELAXATION PROCESS IN A FULLY CURED EPOXY-RESIN

The physical aging of an epoxy resin based on diglycidyl ether of bisphenol-A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step at 130-degrees-C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9-degrees-C. Samples were aged at temperatures between 50 and 100-degrees-C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only at an aging temperature of T(g) - 10-degrees-C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time tau(eff). The rate of relaxation of the system given by 1/tau(eff) decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on tau, three characteristic parameters of the enthalpic relaxation process were obtained (In A = -333, E(H) = 1020 kJ/mol, C = 2.1 g/J). Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60-degrees-C, where sub-T(g) peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams-Watts function was applied. The values of the nonexponential parameter beta were slightly dependent on temperature, and the characteristic time was found to decrease with temperature. (C) 1994 John Wiley & Sons, Inc.