A Critical Evaluation of Polypropylene Impact Copolymer Morphology

Abstract

In order to improve Polypropylene (PP) impact strength, the process of copolymerizing propylene with ethylene is being used. Polypropylene copolymer is essentially a rubber-toughened material consisting of polypropylene homo-polymer matrix in which ethylene-propylene rubber (EPR) is distributed. The overall impact behavior of the copolymer is solely controlled by the rubber morphology and its distribution in polypropylene matrix, which plays a critical role on crystallinity. Even a similar impact grade polypropylene can sometimes show variation in impact behavior depending on the EPR phase size and distribution. To understand the root cause, a detail morphology study of two different lots of same impact polypropylene grade was carried out. A significant difference in impact strength was found. An effort was made to understand the irregular impact failure pattern by analyzing the crystallization phenomena and crystal growth morphology. Inconsistency in rubber phase dispersion was found to contribute towards final impact properties. The difference in crystallization behavior of the polypropylene lots was also found to be affected by the distribution and domain size of rubber phase.  When the EPR particles are of proper size of about 0.4 µm, they initiate many sub critical crazes that absorbs significant energy during the stressing of PP and also act to interrupt the crazes before fracture occurs - ultimately increases the impact property. Rubber phase size & distribution also affecting the overall crystallization process. Material having higher impact strength shows less dominant crystal growth as crystallization process is hindered by the presence of uniform distribution of rubber phase.

Keywords: Polypropylene, impact copolymer, elastomer, phase morphology, crystal growth