Yielding and impact behaviour of pp/sgf/epr ternary composites with controlled morphology

Abstract

Dependencies of the yield strength (σyc), yield strain (ɛyc) and Charpy notched impact strength (CNIS) of polypropylene (PP) reinforced with 30 wt% short glass fibres (SGF) and ethylene — propylene random copolymer (EPR) inclusions on EPR volume fraction (ve) were investigated within the interval of ve varying from 0–0.2. Only one limiting phase morphology has been attained reproducibly using a procedure based on chemical modification of PP. Adhesion enhancement between SGF and PP and complete separation between SGF and EPR was achieved by grafting PP with 2wt % maleic anhydride (MAH). Two regions existed on σyc versus ve curves in the case of complete separation of the reinforcement and elastomer. The observed increase of σyc with increasing ve within the interval 0<ve < 0.05 was attributed to the change in the mode of fracture from brittle to quasi-ductile. Such an explanation has been supported by a several fold increase in ɛyc. Above ve=0.05, a monotonic decrease of σyc with increasing ve was observed corresponding well with an explanation based on a reduction of matrix effective cross-section. In this interval of ve the concentration dependence of σyc was described quantitatively using existing composite models and satisfactory agreement between predictions and experimental data was obtained. The CNIS increased monotonically up to ve=0.1 for both homo- and copolymer based composites. Above ve=0.1, CNIS, measured at -20°C using 6 × 4 × 50 mm bars, notched accordingly ASTM D256 standard, increased for copolymer based composites while it remained constant for homopolymer based materials. Physical meaning of these data is, however, obscured by the inability to separate effects of ve from those of specimen geometry using only a single standard impact strength data.