Effects of Water on a Graphite/Epoxy Composite

The effects of water-induced physical and chemical property modification on a graphite/epoxy (GrEp) laminate were investigated. The laminate was consolidated using unidirectional 1300/934 graphite/epoxy prepreg. Specimens were conditioned, hydrothermally, by immersion in distilled water at 45, 60, 75 and 90'C for more than 9000 hours. The composite properties were characterized by evaluating the specimen's weight change due to water absorption and desorption, by hygrothermally induced expansion, and by water-induced change of the glass transition temperature (T,). Optical microscopy and scanning electron microscopy (SEM) were conducted to assess surface modification associated with resin cracking and resin mass loss. Water sorption in the Gr/Ep material exhibited both Fickian and non-Fickian diffusion behavior. Diffusion data showed that the time for the onset of non-Fickian behavior was inversely related to the increase in exposure temperature. Anomalous (i.e., non-Fickian) behavior in the composite resulted from both chemical modification and physical damage to the epoxy resin. Moisture-induced cracks, voids, and surface resin peeling were revealed by SEM and optical microscopy. Surface cracking and resin mass loss phenomena were used to describe water sorption behavior in composites. Thermomechanical analysis (TMA), water desorption, and infrared (IR) spectral results suggest that water retention in the composite can be characterized: (1) by water trapped at defects such as crack tips, interfaces, and voids; (2) by water interaction associated with weak hydrogen bonding within the resin; and (3) by water bonded to hydrophilic functional groups of the polymer chains.