Structure-property relationships in glass-reinforced polyamide, Part 3: Effects of hydrolysis ageing on the dimensional stability and performance of short glass-fiber-reinforced polyamide 66

We present results on an in-depth study of the effects of hydrolysis testing on the mechanical performance, weight change, and dimensional stability of injection molded glass-fiber-reinforced polyamide 66 (PA66) based on two chopped fiber products with different sizing formulations. Composite and resin samples have been characterized both dry as molded and after conditioning at either 120 or 150 degrees C for a range of times up to 1,000 h. The results reveal that hydrothermal ageing in water-glycol mixtures results in significant changes in the mechanical performance, weight, and dimensions of these materials. The negative effects of conditioning could be mitigated to some degree by the appropriate choice of the glass-fiber sizing; however, the sizing effect diminished with increasing conditioning time. All materials showed a weight increase due to conditioning at 120 degrees C that was typical of a single Fickian diffusion process and there was clear evidence of multiple processes involved when conditioning at 150 degrees C. It was not apparent that the glass-fiber sizing affected the dimensional stability of the composites. We show that there is a strong correlation between the swelling of these samples and the level of fluid adsorption. Although the PA66 resin showed reasonably homogeneous swelling, the composites exhibited different levels of swelling depending on direction. These effects were well in line with the known effects of fibers on restriction of the matrix deformation (mechanical, thermal, or moisture swelling) in the fiber direction. These differences correlate well with the average fiber orientation with respect to the various direction axes. Composite tensile strength and unnotched impact resistance appeared to scale inversely with the level of swelling of the material.