Water damage in glass fibre/resin composites

Abstract

Thin films of polyester resin containing glass fibres of several different compositions have been exposed to water at three different temperatures (20, 60, 100°C) and examined by means of optical and scanning electron microscopy. The incidence and development of fibre debonding has been studied by using the optical anisotropy arising from resin shrinkage on to the fibres during cure. To aid interpretation of these experiments, measurements have been made of resin dimensional changes produced by water immersions. At each temperature, the first response to diffused water is resin swelling and, in hot water, this is superseded by shrinkage, the magnitude of which becomes considerable after prolonged immersion, e. g. 8% linear shrinkage after 2000 h in boiling water. Evidence is reported suggesting that this shrinkage is mainly due to leaching of low molecular weight material from the resin. The interfacial bond between resin and clean glass fibres is rapidly destroyed by diffused water at all three temperatures. However, the use of a coupling agent produces vast improvements in bond life. In fact, debonding in the presence of a coupling agent has been observed only for hot water immersions; even then, the bond withstands the interfacial tension present during early resin swelling and is destroyed very much later when the interfacial stress is compressive owing to resin shrinkage. The mechanism by which this debonding is eventually initiated depends on glass composition. With ‘E’ and ‘C’ glass fibres, bond fracture is due to osmotic pressure generated at the interface by water soluble constituents leached from the fibre, and is often accompanied by the growth of cracks into the resin from the fibre surface. With fused silica fibres, which contain negligible amounts of impurities, small regions of debonded interface appear at fibre ends after much longer immersion times and are attributed to high interfacial shear stresses caused by resin shrinkage. Debonding facilitates relative longitudinal movement between fibre and resin, enabling each fibre end to act as a rigid indentor pushing into the adjacent resin. In hot water, resin indentation cracks result and subsequent resin shrinkage and gross plastic deformation lead to their displacement along the fibres, followed by the successive nucleation and displacement of further indentation cracks.