Mechanical behavior of injection molded beta-crystalline phase polypropylene

High purity and a large amount of beta-modification can be produced in polypropylene (PP) by adding a bicomponent beta-nucleator consisting of equal amounts of pimelic acid and calcium stearate. Wide angle X-ray diffraction (WAXD), scanning electron microscopy(SEM), static tensile tests, Izod impact tests, and instrumented drop weight impact testing were used to characterize the structure, morphology, and mechanical behavior of the beta-phase PP. The WAXD patterns show that the relative content of the beta-phase (K value) in the skin layer of the injection molded specimen is 0.54, whereas the K value in the core section is 0.92, SEM observations reveal that the beta-spherulite exhibits a sheaf-like structure and there are no clear boundaries between the spherulites. Static tensile tests indicate that the elongation at break of pure PP is enhanced by the addition of a beta-nucleator. The impact and drop weight tests also indicate that the impact resistance of PP is improved by adding the beta-nucleating agent. The improvement in impact toughness is associated with the formation of microcrazes within the PP.