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

Increasing demands on industrial applications in the microelectronics indus tries have led to the need for accelerated curing of high-performance struc tural adhesive systems like epoxies. The curing of such thermoset systems h as become the bottleneck of the whole production process. Recently, microwa ve curing has been shown to be a viable alternative as an accelerated curin g system. This paper investigates the effect of using different curing agen ts in microwave curing of an epoxy system on the final cured glass-transiti on temperature (T-g). Microwave radiation and thermal heating were performe d on a diglycidyl ether of bisphenol-A (DGEBA). The three curing agents use d 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 short er curing time to reach the maximum percentage cure and T-g, the actual max imum values achieved for both percentage cure and T-g were significantly lo wer than for thermal curing. Evidently, during microwave curing, whilst a f aster rate was obtained, the existence of the highly electron-attracting SO 2 functional group appeared to induce a delay in the reactivity of the amin e 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, a t significantly shorter curing times, due principally to a greater reductio n in the effective cure time than in the lag time. Microwave curing seemed to be snore effective in reducing the overall cure time for the mPDA system . (C) 2001 Elsevier Science Ltd. All rights reserved.