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 e poxy 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 ac tivation energy calculated by the Kissinger method increases from 63 k J/mol for the uncured epoxy to 77 kJ/mol for the partially cured with alpha(DSC) = 0.90. Additionally, the activation energy calculated by t he isoconversional method shows a dependence on the conversion degree ct. The activation energy tends to decreases initially with the conver sion 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 unr eacted groups. A simple, consistent method of kinetic analysis was app lied. This method enables one to select the most convenient model and the calculation of kinetic parameters. A two-parameter (m, n) autocata lytic model (Sestak-Berggren equation) was found to be the most conven ient model to study the curing of epoxy systems. The results show a de pendence of the kinetic parameters on the initial degree of crosslinki ng of the partially cured epoxy. The exponent m tends to decrease with the extent of cure, while the exponent n remains practically invariab le. 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 pre exponential factor in the Arrhenius dependence of the temperature on t he rate of conversion, increase with the extent of cure, showing a cor relation with the calculated activation energy values. The nonisotherm al DSC curves theoretically calculated show a very good agreement with the experimental data. The two-parameter (m, n) autocatalytic model g ives a good description of the curing kinetics of epoxy resins with di fferent extents of cure. (C) 1995 John Wiley & Sons, Inc.