POLYPROPYLENE COMPOSITIONS HAVING IMPROVED IMPACT STRENGTH PROPERTIES AT LOW TEMPERATURES AND PROCESS FOR PREPARING SAME

摘要

Polypropylene compositions having high impact strength at low temperatures comprising as essential components: 100 parts by weight of polypropylene having an isotacticity index >90, 8-25 parts by weight of an amorphous copolymeric fraction (1) of ethylene-propylene soluble in xylene at 23 DEG C., containing 20-80% by weight of ethylene, 2-10 parts by weight of an ethylene-propylene crystalline copolymeric fraction (2) having a crystallinity of the polyethylene type containing 50-90% by weight of ethylene; and in which the weight ratio between the total polymerized ethylene and fraction (1) is <1. The compositions are prepared by polymerization processes comprising at least a step of homopolymerization of propylene to stereoregular polymer and a subsequent step of polymerization of ethylene-propylene mixtures in the presence of catalysts comprising a titanium compound carried on a magnesium halide in active form.