ABSTRACT: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.

An amine cured epoxy is prepared in two different network states for hydrothermal aging. The "lowcrosslinked" 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 highcrosslinked 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.

ABSTRACT:The thermal and hydro-thermal aging of a hot-cured epoxy system (diglycidylether of bisphenol A (DGEBA) ϩ dicyandiamide (DDA)) in the glassy state is revisited using DSC and IR attenuated total reflection spectroscopy. Because of the diffusion of DDA from the solid particles into the liquid DGEBA matrix, curing produces a highly crosslinked amorphous matrix that contains low crosslinked amorphous regions. After full curing, the network possesses a relatively low molecular mobility and no residual reactive groups. Thermal and hydro-thermal loading is performed at 60°C, well below the principal glass transition temperature (T g1 ϭ 171°C). Both aging regimes cause significant chemical and structural changes to the glassy epoxy. It undergoes a phase separation of relatively mobile segments inside the low mobile matrix, providing a second glass transition that shifts from T g2 ϭ 86 -114°C within 108 days of aging. This phase separation is reversible on heating into the viscoelastic state. Hydro-thermal aging leads to a reversible and a nonreversible plasticizing effect as well. On thermal aging, no chemical changes are observed but hydro-thermal aging causes significant chemical modifications in the epoxy system. These modifications are identified as a partial degradation of crosslinks produced by the cyano groups of the DDA and correspond to the nonreversible plasticitation. These changes in the cured epoxy should exert an influence on the mechanical properties of an adhesive bond.

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