Quasi-static and low-velocity impact responses of polypropylene random copolymer composites with adjustable crystalline structures

Abstract

Currently, the effect of cooling rates on mechanical properties of continuous glass fiber-reinforced isotactic polypropylene (CGF/iPP) composites cannot be well revealed, due to the imperfect impregnation containing voids, limited selection of crystalline structures (i.e., boundary defects and lamellae thickness) and impact energy. Moreover, compared to the polypropylene random copolymer (PPR) with toughness-rigidity balance, the brittleness of iPP and incompatibility of iPP/elastomer system also limit the further improvement of impact resistance. Furthermore, to date, the influence of cooling rates on mechanical properties of CGF/PPR composites has not been reported. Herein, we reveal the influence mechanism of multiple crystalline structures on quasi-static, and on low-velocity impact behavior of CGF/PPR composites based on perfect impregnation. Results show that the tensile strength and impact resistance under perforation threshold both increase first and then decrease with the increased cooling rate, while the impact resistance at non-perforation threshold gradually improves. Severe boundary defects mask the contribution of other crystalline structures, resulting in the lowest tensile strength. After overcoming boundary defects, the type of lamella is the most important factor for the effect of stress transfer. In the case of non-perforation damage, laminate with the smallest crystallinity and the widest size distribution only suffers from subcritical damage. The stiffness of amorphous area generated by the density of tie chains and chain entanglements becomes the decisive factor for improving impact resistance of laminate above perforation threshold. This work may pave the way for improving mechanical properties of thermoplastic composites via generating specific crystalline structures.