Effects of molecular weight on the perforation impact behavior of injection-molded plaques of α- and β-phase isotactic polypropylenes

This study was devoted to the instrumented falling weight impact (IFWI) behavior of injection-molded α- and β-phase isotactic polypropylene (iPP) homopolymers. The perforation impact response of iPP with various melt flow indices (MFIs), and thus molecular weight (MW) characteristics, was studied at two different temperatures (T = 23 °C and T = -40 °C) and incident impact speeds (vinc = 5 and 10 m/s). The impact resistance of β-iPP was superior to the α- modification. The absolute resistance to perforation increased with increasing MW or decreasing MFI, whereas the relative toughness improvement between the β-and α-iPPs followed an opposite tendency. The molding-induced skin-core morphology did not affect practically the out-of-plane response of the impacted plaques. Changes in the fractograms (viz. force-time curves) under various experimental conditions were traced to variations in the failure mode, showing a competition between radial and circumferential cracking with respect to the clamping ring. In the case of the more ductile α-iPP, circumferential cracking was favored.