Fracture toughness and failure mechanisms in unreinforced and long-glass-fibre-reinforced PA66/PP blends

In this paper unreinforced and long-glass-fibre-reinforced PA66/PP blends with different glass-fibre sizing were studied with respect to their fracture toughness determined by the typical K_c method. The fracture surfaces of these blends were studied by scanning electron microscopy in order to characterize the failure mechanisms. For the unreinforced blends a decrease in fracture toughness was observed when 25 wt% of polyamide (PA) was added to the polypropylene (PP) matrix, compared with the plain PP and PA matrices. On the other hand an increase in fracture toughness was observed when 25 wt% of PP was added to the PA matrix. This was explained by the differences in thermal expansion of PP and PA. The fracture toughness of the long-glass-fibre (LGF) composites were not affected by the glass-fibre sizing up to a PA/PP ratio of 50/50. After the phase inversion from a continuous PP to a continuous PA phase in the matrix (between PA/PP ratios of 50/50 and 75/25) the PA glass-fibre sized composite showed higher fracture toughness than the PP sized. This was explained by the change of the fibre-related failure mechanism from frequent fibre pull-out to fibre fracture. In addition the matrix affected the fracture toughness of the PA/PP75/25 blend with PA glass-fibre sizing in a positive way, resulting in the highest fracture toughness observed in this study.