THE EFFECT OF AGING ON THE IMPACT STRENGTH OF (beta)-POLYPROPYLENES

This work focuses on the time stability of the (beta)-phase and the impact strength of (beta)-nucleated injection-molded polypropylenes. For the purposes of the study, various amounts of a specific (beta) nucleating agent based on N, Nʹ-dicyclohexyl-2,6-naphtalenedicarboxamide have been added to commercially available isotactic polypropylene. From the prepared blends standard impact testing specimens have been injection-molded. The structure has been characterized with the aid of wide-angle X-ray diffractometry, differential scanning calorimetry and polarized light microscopy, and toughness has been characterized by Charpy impact testing. At first, a measurement of impact strength and (beta)-phase content was carried out immediately after injection molding, and as a second step the same values were measured after 2 years. The results suggest that the (beta)-phase is time-stable, and that the notch impact strength of the samples does not significantly change with time. Furthermore, it was confirmed that the (beta)-phase has a positive influence on impact strength but it is not the only one important factor. There are other positive effects such as creation of structural heterogeneity, elongated chains in the skin layer, specific size of spherulites and their continuity with the amorphous phase.