New aspects of aging in epoxy networks. II. Hydrothermal aging

An amine cured epoxy is prepared in two different network states for hydrothermal aging. The "low-crosslinked" network has a considerable amount of residual reactive groups and a relatively high-molecular mobility. The low-crosslinked matrix contains high-crosslinked regions. In contrast, the "highly crosslinked" epoxy system has little reactive groups and a lower molecular mobility. Here, low-crosslinked regions are found in a high-crosslinked matrix. Hydrothermal loading for both networks is performed in demineralized water at temperatures below their glass transition. The water plasticizes both kinds of networks which remain in the glassy state, however. As a consequence, in the low-crosslinked epoxy, the increased molecular mobility promotes an ongoing curing reaction leading to the consumption of epoxy groups until an almost complete network has formed. As a new aging process, phase separation occurs in the highly crosslinked epoxy. The new phase is more mobile than the matrix because it has its own glass transition at a lower temperature. In addition, thermooxidative degradation is observed for both network states. Certainly, these chemical and structural changes in the epoxy networks should influence the performance of an adhesive joint, a coating, or a fiber-reinforced composite. (C) 2003 Wiley Periodicals, Inc.