Influence of the beta-crystalline phase on the mechanical properties of unfilled and calcium carbonate-filled polypropylene: Ductile cracking and impact behavior

The beta -crystalline form of isotactic poly(propylene) (PP) has been long recognized to have a greater mechanical absorption capacity than the alpha -crystalline form. This is of major importance for improving impact properties and crack resistance of injection-molding parts. Unfilled PIP samples together with calcium carbonate-filled PP samples having various beta/alpha -phase ratios, with nearly constant morphological parameters, have been investigated from the standpoint of ductile crack propagation and impact behavior. The presence of the beta -crystalline phase turned out to improve both properties. The beta spherulites are notably more prone to craze initiation than alpha spherulites that display a propensity for cracking. Subsequent crack propagation appears to be faster in the latter ones. The plastic zone ahead from the crack tip broadens, and the specific plastic energy increases with increasing beta -phase content. The lower elastic limit of the beta phase is likely to promote the early crazing. However, the suspected higher density of tie molecules in beta spherulites provides more numerous and stiffer microfibrils. The impact strength of PP is also improved by the presence of beta crystals as a result of greater energy-absorption capabilities. However, filled samples turned out insensitive to the beta phase. A discussion is made about the origins of the beta -phase-induced improvement of the mechanical properties. The possible role of the beta --> alpha transition is also explained. (C) 2001 John Wiley & Sons, Inc.