WATER TRANSPORT IN GRAPHITE EPOXY COMPOSITES

The diffusive and mechanical behavior of tetraglycidyl diaminodiphenyl methane (TGDDM) resin-based composites and diglycidyl ether of bisphenol-A (DGEBA) resin-based graphite/epoxy composites were investigated during water sorption at different temperatures. The water-absorption kinetics in both systems at 50, 70, 90, and 100-degrees-C were fitted by a Fickian diffusion model. However, a Langmuir-type, two-step sorption behavior was observed for water transport in DGEBA-based systems at 50 and 70-degrees-C. Using scanning electron microscopy, internal cracks due to water-absorption were found in the DGEBA-based samples after conditioning at 90 and 100-degrees-C in water, whereas no cracks were detected in TGDDM-based samples conditioned in water at 100-degrees-C. Ultrasonic testing did not show significant modulus or density change of the TGDDM-based samples conditioned in water at 100-degrees-C. No significant changes of dynamic modulus or damping factor were observed for the TGDDM-based samples redried after immersion in 100-degrees-C water, whereas slight changes were observed above 120-degrees-C for the samples containing absorbed water. However, both water-containing and redried DGEBA-based samples showed a decrease of dynamic modulus and an omega-transition around 120-degrees-C. A single-fiber fragment test revealed that the absorbed water at 80-degrees-C reduced significantly the interfacial shear strength of DGEBA/DDA resin-AS4 fiber samples and DGEBA/DDA resin-AU4 fiber samples.