MORPHOLOGICAL BEHAVIOR AND INSTRUMENTED DART IMPACT PROPERTIES OF BETA-CRYSTALLINE-PHASE

Specimens consisting of high-purity beta-phase polypropylene were prep ared by adding a bicomponent beta-nucleator consisting of equal amount s of pimelic acid and calcium stearate. Scanning electron microscopy ( SEM), differential scanning calorimetry (d.s.c.), dynamic mechanical a nalysis and instrumented drop weight impact tests were used to charact erize the morphology, thermal behaviour and the impact properties of t he beta-phase polypropylene. SEM examinations show that the beta-spher ulites exhibit a sheaf-like structure with no clear boundaries between them. This distinct spherulitic morphology results in a substantial i mprovement in the falling weight impact resistance. Fractographic anal ysis reveals that microfibrils and voids were formed in the fracture i nduction area of the beta-form specimen. The greater impact strength o bserved in the beta-form material is due to the larger energy dissipat ion which is associated with the formation of microfibrils. Furthermor e, d.s.c. analysis showed that there is no beta --> alpha phase conver sion during the impact test. (C) 1996 Elsevier Science Ltd.