EFFECT OF THE CROSS-LINKING DEGREE ON CURING KINETICS OF AN EPOXY-ANHYDRIDE SYSTEM

The cure kinetics of a diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin with methyltetrahydrophthalic anhydride and an accelerator was studied by nonisothermal DSC data. The systems were uncured resin and partially cured with the following extents of cure measured by the residual heat method (alpha(DSC)): 0.37, 0.63, 0.81, and 0.90. The activation energy calculated by the Kissinger method increases from 63 kJ/mol for the uncured epoxy to 77 kJ/mol for the partially cured with alpha(DSC) = 0.90. Additionally, the activation energy calculated by the isoconversional method shows a dependence on the conversion degree ct. The activation energy tends to decreases initially with the conversion degree, possibly due to the autocatalytic effect; then, it passes through a minimum about alpha = 0.4 and, finally, increases slightly due to the increase of crosslinks which reduce the mobility of the unreacted groups. A simple, consistent method of kinetic analysis was applied. This method enables one to select the most convenient model and the calculation of kinetic parameters. A two-parameter (m, n) autocatalytic model (Sestak-Berggren equation) was found to be the most convenient model to study the curing of epoxy systems. The results show a dependence of the kinetic parameters on the initial degree of crosslinking of the partially cured epoxy. The exponent m tends to decrease with the extent of cure, while the exponent n remains practically invariable. These results show a change of the kinetic when the initial extent of cure of the epoxy system increases. The ln A data, A being the preexponential factor in the Arrhenius dependence of the temperature on the rate of conversion, increase with the extent of cure, showing a correlation with the calculated activation energy values. The nonisothermal DSC curves theoretically calculated show a very good agreement with the experimental data. The two-parameter (m, n) autocatalytic model gives a good description of the curing kinetics of epoxy resins with different extents of cure. (C) 1995 John Wiley & Sons, Inc.