The resistance of glass reinforced thermosetting polymers to thermohumid conditions

Advanced composite materials based mainly on epoxy resins are being used in increasing numbers of components in the aerospace industry. Such components have to survive in a range of moisture and temperature environments in different parts of the world at varying altitudes. It is important therefore to have sufficient information about the behaviour of composite components to predict what effect these environments will have on their properties. The aim of the work reported in this thesis was to provide such information not only for epoxy based systems but to make comparisons with polyester and vinyl ester based materials. Five glass fibre reinforced resin systems were used. Two vinyl esters, one polyester, one straight epoxy and one epoxy prepreg mixture. The effect of immersion in distilled water and exposure to humid air at 60% and 95% relative humidity was investigated at temperatures ranging from 25°c to 70oc, for periods of sixteen and forty days. During these periods moisture uptake for both unidirectional and bidirectional materials was recorded on a daily basis, and variations in glass transition temperature were determined. At the end of each period the ultimate tensile stress, tensile modulus, tensile strain to failure, interlaminar shear strength and interplanar shear strength of each material was determined. The water absorption results for the vinyl ester resins, polyester and straight epoxy resin initially showed Fickian diffusion characteristics. In the epoxy prepreg material a two stage diffusion process was observed. No equilibrium water absorption plateau was obtained over forty days at 60% relative humidity for any of the materials, at any temperature. All the mechanical properties dropped under these conditions and this was shown to be as a result of degradation at the glass-resin interface. At 95% relative humidity the fall in mechanical properties was greater and not recoverable. Under these conditions plasticization of the matrix had occurred. All the material samples which were subject to hot water under- went pronounced degradation. The degradation process was shown to be due to penetration of water at the glass-resin interface, followed by attack on the coupling agent and glass fibre surface. This degradation process was confirmed.by micro-observations of the fracture surfaces.