The dimensional stability of glass–fibre reinforced polyamide 66 during hydrolysis conditioning

Injection moulded glass–fibre reinforced polyamide 66 composites based on two glass fibre products with different sizing formulations and unreinforced polymer samples have been characterised both dry as moulded and during conditioning in a water–glycol mixture at 70 °C for a range of times up to 400 h. The results reveal that hydrothermal ageing in water–glycol mixtures causes significant changes in the weight and dimensions of these materials. All conditioned materials showed a time dependent weight increase which could be characterised as pseudo-Fickian. The weight change could be well modelled by a Fickian diffusion process with a time dependent diffusion coefficient. It was not apparent that changing the glass fibre sizing affected the dimensional stability of the composites. There was a strong correlation between the swelling of these samples and the level of fluid absorption. The composites exhibited different levels of swelling depending on direction. These effects were well in line with the influence of fibres on restriction of the matrix deformation in the fibre direction. These differences correlated well with the average fibre orientation with respect to the various direction axes.