ENVIRONMENTAL AGING OF EPOXY-RESINS - SYNERGISTIC EFFECT OF SORBED MOISTURE, TEMPERATURE, AND APPLIED STRESS

The exposure to a humid environment at relatively high temperatures produces a reduction of the physical properties of the epoxies, due both to a moisture-induced plasticization and microcavities formation. Equilibrium moisture sorption levels are found to be represented, for the same polymer and in the same temperature and humidity conditions, both by linear and upward isotherms depending upon the humidity history to which the system has been subjected. Such humidity history dependence is progressively lost as the temperature of the experiment is decreased. Sorption data are analyzed in the light of a transport model in which part of diffusion molecules are completely immobilized in the newly formed microcavities. The nature of the hypothesized damaging process is in agreement with the diffusion coefficient depression and with the solubility increase experimentally found and theoretically predicted by a previously presented model.