THERMAL-OXIDATION OF ANHYDRIDE CURED EPOXIES .3. EFFECT ON MECHANICAL-PROPERTIES

Three samples based on a flexibilized anhydride cured epoxy network and respectively unfilled (M), silica filled (S) and hydrated alumina filled (A), were submitted to thermal ageing in air, at temperatures ranging between 125 and 200-degrees-C, and their flexural properties were periodically measured. It appeared that for all the samples, the stress-deformation behaviour remains unchanged except for the rupture stage. The ultimate elongation decreases continuously as a result of the formation of a brittle oxidized superficial layer, whereas the stress variation depends upon the shape of the initial stress-elongation curve. The ductile-brittle transition induced by ageing, and the stress decrease versus time in the brittle state, can be interpreted in terms of fracture mechanics. Some consequences of these results on the methodology of lifetime prediction are discussed.