Thermal-spike-enhanced moisture absorption by polymer-matrix carbon-fibre composites

Abstract

The moisture absorption of a carbon-fibre-reinforced polymer composite, with a bismaleimide-modified epoxy resin matrix, was studied under constant hygrothermal conditions and under a series of thermal-spike conditions, in an attempt to understand the so-called enhanced moisture absorption phenomenon. The spiking temperatures ranged from 100 °C to 160 °C, which are above and below the glass-transition temperature (T g) of the matrix in the ‘wet’ laminate. An enhancement to the moisture absorption was observed which depended strongly on the spiking temperature. For the laminate conditioned under 96% relative humidity (RH) and 45 °C, rapid enhancement occurred at approximately 110 °C, and for the laminates conditioned at 75% RH and 45 °C, it occurred at approximately 120 °C. Laminates with different configurations were also studied, which allowed assessment of the effects of thermal residual stresses and stress distributions induced by fibre configuration. It was observed that laminate configuration had a profound effect in determining the moisture absorption characteristics under thermal-spike conditions. The glasstransition temperature of the matrix was further reduced by the enhanced moisture absorption. A higher spiking temperature was observed to cause a larger reduction in T g, even though it may not induce a stronger enhancement in moisture absorption. It was thus suggested that the reduction in T g depended on the degree of plasticisation and hydrolysis of the polymer matrix. The latter was probably more important when the laminates were conditioned under highhumidity environments and spiked at high temperatures.