This paper deals with the effect of hot water immersion on tensile and fatigue properties of long glass fibre-reinforced polypropylene (LGF/PP). Tensile tests and pulsating-tension fatigue tests have been carried out using smooth specimens of two grades of the material, with and without an acid modified polypropylene (APP). Specimens were immersed in distilled water at 80°C up to 6666h. In both materials, the weight increased rapidly and reached the maximum value, then decreased gradually with immersion time. The material without APP showed initially larger increase in weight, but over the immersion time of 2500h more remarkable decrease was observed. Since the weight gain of the resin itself was negligible, water entered into the interface between the fibres and the resin. Therefore, the weight loss was attributed to leaching of surface treatment agents and the dissolusion of glass fibres by hot water. The variation in weight with immersion time depended strongly on water temperature. The weight increased gradually with time in distilled water at ambient temperature and the material without APP showed larger weight gain than the material with APP, indicating that the addition of APP was effective in improving water absorption characteristics in LGF/PP. Tensile and fatigue strengths were considerably decreased after hot water immersion, but the material with APP still exhibited higher strengths than the material without APP. The length of fibres appeared on fracture surfaces tended to be shorter in immersed specimens, suggesting the degradation of the fibres.
