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

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 degrees C to 160 degrees 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 degrees C, rapid enhancement occurred at approximately 110 degrees C, and for the laminates conditioned at 75% RH and 45 degrees C, it occurred at approximately 120 degrees 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 glass-transition 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 high-humidity environments and spiked at high temperatures. (C) 1997 Elsevier Science Limited.