Influence of Water Exposure on the Relaxation Behaviour of Short Fibre Reinforced Polycarbonate

An approach based on a multi-relaxation (MR) test, in conjunction with a spring-dashpot model for data analysis, was employed to evaluate the influence of hydrothermal treatment at 65 °C on the viscous and quasi-static stress responses of polycarbonate (pure PC) and its short glass fibre composite (GF-PC) during the relaxation stages. The findings indicate that while this hydrothermal treatment did not affect the ductility of pure PC, it resulted in a significant ductility decrease in GF-PC. Furthermore, results from the modelling indicated that a parallel three-branch model was sufficient to simulate the stress decay during the relaxation stages of pure PC, but an additional branch (designated as the M-branch) was required for GF-PC. The study also found that for GF-PC, the hydrothermal treatment led to a distinct increase in the viscous stress response of the M-branch, but not in the other two viscous branches. This phenomenon is believed to stem from the coexistence of two types of PC matrix in GF-PC: one influenced by the presence of fibres and the other unaffected. Without the hydrothermal treatment, both types of PC matrix exhibit a similar deformation-dependence (based on stroke of the test machine) in their stress response; however, because the hydrothermal treatment affects the strength at the fibre–matrix interface, the stress responses of the two matrix types differ during relaxation. Consequently, the M-branch is believed to capture the altered stress response of the fibre-influenced PC matrix. The study concludes that under the investigated hydrothermal conditions, the reinforcing effect of the short fibre on the mechanical strength of GF-PC could be lost. Therefore, the addition of short glass fibre does not inherently guarantee mechanical reinforcement of PC.