INTERNAL-STRESS IN A CURED EPOXY-RESIN SYSTEM

An epoxy resin system based on a diglycidyl ether of bisphenol-A type difunctional epoxy resin containing different amounts of a tetrafunctional curing agent, namely metaphenylene diamine, was used to prepare two types of specimen. First, an aluminium strip was coated with a thin layer of the epoxy-amine mixture and then subjected to two curing-cooling cycles. Second, bulk specimens were cast in the form of sheets which were cut into rectangles and their dynamic torsional spectra obtained from 100 to 180-degrees-C. In the resin-coated metal plate, the internal stress was measured by the bending beam technique and was found to be highest in samples with high glass transition temperatures (T(g)s). This is apparently because the internal stress arises due to the difference in the thermal contraction coefficients between the resin and the metal and develops as the sample cools from the T(g) to room temperature. The rate of stress relaxation in these samples is also high, apparently because they are the farthest from their equilibrium state and so have higher free volume and a more open structure. In the case of cast sheets, the internal stresses are again the highest in samples with high T(g) but arise from non-uniform cooling. An intermediate relaxation peak, the alpha' peak, observed in these samples has been shown to be related to the frozen-in internal stresses which relax on sub-T(g) ageing due to free volume collapse.