THERMAL-SPIKE-ENHANCED MOISTURE ABSORPTION BY POLYMER-MATRIX CARBON-FIBER COMPOSITES

The moisture absorption of a carbon-fibre-reinforced polymer composite , with a bismaleimide-modified epoxy resin matrix, was studied under c onstant hygrothermal conditions and under a series of thermal-spike co nditions, in an attempt to understand the so-called enhanced moisture absorption phenomenon. The spiking temperatures ranged from 100 degree s C to 160 degrees C, which are above and below the glass-transition t emperature (T-g) of the matrix in the 'wet' laminate. An enhancement t o the moisture absorption was observed which depended strongly on the spiking temperature. For the laminate conditioned under 96% relative h umidity (RH) and 45 degrees C, rapid enhancement occurred at approxima tely 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 i nduced by fibre configuration. It was observed that laminate configura tion had a profound effect in determining the moisture absorption char acteristics under thermal-spike conditions. The glass-transition tempe rature of the matrix was further reduced by the enhanced moisture abso rption. A higher spiking temperature was observed to cause a larger re duction in T-g, even though it may not induce a stronger enhancement i n moisture absorption. It was thus suggested that the reduction in T-g depended on the degree of plasticisation and hydrolysis of the polyme r matrix. The latter was probably more important when the laminates we re conditioned under high-humidity environments and spiked at high tem peratures. (C) 1997 Elsevier Science Limited.