New aspects of aging in epoxy networks. I. Thermal aging

The thermal aging of an amine-cured epoxy in the glassy state is studied for two network states by using DSC and attenuated total reflection-infrared (IR-ATR). The "low-crosslinked" network possesses a relatively high molecular mobility and a considerable amount of residual reactive groups. In the low crosslinked matrix, the presence of high crosslinked regions is revealed. In contrast, the "highly crosslinked" epoxy system has a reduced molecular mobility and only small reactive groups. The high crosslinked matrix contains low crosslinked regions. Thermal loading for both networks is performed below their glass transition. During thermal aging, an ongoing curing reaction takes place in the low-crosslinked epoxy. Thermooxidative degradation and the disintegration of short-range ordering are observed as well. The highly crosslinked epoxy system undergoes a phase separation of relatively mobile segments in the low mobile matrix, which is a reversible process on heating. Thermooxidative degradation is also detected for this kind of network. In summary, for the "low" and the "highly" crosslinked epoxy, significant chemical and structural changes take place during thermal aging even though the networks are vitrified. It is convincing that these changes in the cured epoxy should exert an influence on the mechanical properties of a bonded structure. (C) 2003 Wiley Periodicals, Inc.