Factors and processes determining the impact resistance of PP impact copolymers with multi-phase structure

The impact resistance of four polypropylene impact copolymers (ICPs) with multi-phase structures and widely differing characteristics was related to their structure. Blends were prepared from one of them and a high-density polyethylene (HDPE) to improve impact strength further. The structure of the materials was characterized by microscopy and dynamic mechanical thermal analysis. Mechanical properties were determined by tensile and impact testing, while local deformation processes were followed by volume strain measurements. The results obtained in the study proved that the shear-yielding of the matrix contributes the most among local processes to the increase of impact strength, while cavitation has a small effect on this latter property since its energy absorption is negligible. Both increasing elastomer content and decreasing particle size favor shear-yielding, thus improving impact strength. Considering the importance of elastomer content and elastomer particle size, a simple but very good model was created describing the dependence of the impact strength of ICPs on these latter two factors by using linear regression analysis. Although the addition of HDPE increases the fracture resistance of ICPs further, the extent of improvement is moderate, and the approach is economically disadvantageous.