Impact resistance of all-polypropylene composites composed of alpha and beta modifications

The impact behaviour of self-reinforced polypropylene (PP) composites was studied. α and β polymorphs of isotactic PP homopolymer and random copolymer (with ethylene) were used for matrix materials, whereas the reinforcement was a fabric woven from highly stretched split PP yarns. The composite sheets were produced by the film-stacking method and consolidated by hot pressing at 5 and 15 °C above the melting temperature (Tm) of the matrix-giving PP grade. The composite sheets were subjected to static tensile, dynamic falling weight impact and impact tensile tests at room temperature. Dynamic mechanical thermal analysis (DMTA) was also performed on the related composites and their constituents. The results indicated that the β-modification of the PP homopolymer is more straightforward than that of the PP copolymer. Stiffness and strength usually increased while the toughness (tensile impact strength, perforation impact energy) decreased with increasing temperature of consolidation. This was assigned to differences in the failure mode based on fractographic results.