Microwave curing of an epoxy-amine system: Effect of curing agent on the glass-transition temperature

Increasing demands on industrial applications in the microelectronics industries have led to the need for accelerated curing of high-performance structural adhesive systems like epoxies. The curing of such thermoset systems has become the bottleneck of the whole production process. Recently, microwave curing has been shown to be a viable alternative as an accelerated curing system. This paper investigates the effect of using different curing agents in microwave curing of an epoxy system on the final cured glass-transition temperature (T_g). Microwave radiation and thermal heating were performed on a diglycidyl ether of bisphenol-A (DGEBA). The three curing agents used were 4,4′-diaminodiphenylsulfone (DDS), 4,4′-diaminodiphenylmethane (DDM) and meta-phenylene diamine (mPDA). The use of different curing agents resulted in different glass-transition properties for the microwave curing of the three epoxy-amine systems. Whilst all three systems exibited a shorter curing time to reach the maximum percentage cure and T_g, the actual maximum values achieved for both percentage cure and T_g were significantly lower than for thermal curing. Evidently, during microwave curing, whilst a faster rate was obtained, the existence of the highly electron-attracting SO₂ functional group appeared to induce a delay in the reactivity of the amine functions sufficient to entrap them in the crosslink network, effectively inhibiting further curing. In contrast, both the DDM and the mPDA systems, having different electron-donating fuctional groups, were able to achieve maximum percentage cure and T_g values equal to those for thermal curing, at significantly shorter curing times, due principally to a greater reduction in the effective cure time than in the lag time. Microwave curing seemed to be more effective in reducing the overall cure time for the mPDA system. © 2001 Elsevier Science Ltd. All rights reserved.