Morphological behaviour and instrumented dart impact properties of beta-crystalline-phase

Specimens consisting of high-purity beta-phase polypropylene were prepared by adding a bicomponent beta-nucleator consisting of equal amounts of pimelic acid and calcium stearate. Scanning electron microscopy (SEM), differential scanning calorimetry (d.s.c.), dynamic mechanical analysis and instrumented drop weight impact tests were used to characterize the morphology, thermal behaviour and the impact properties of the beta-phase polypropylene. SEM examinations show that the beta-spherulites exhibit a sheaf-like structure with no clear boundaries between them. This distinct spherulitic morphology results in a substantial improvement in the falling weight impact resistance. Fractographic analysis reveals that microfibrils and voids were formed in the fracture induction area of the beta-form specimen. The greater impact strength observed in the beta-form material is due to the larger energy dissipation which is associated with the formation of microfibrils. Furthermore, d.s.c. analysis showed that there is no beta --> alpha phase conversion during the impact test. (C) 1996 Elsevier Science Ltd.