POLYPROPYLENIC COMPOSITIONS HAVING IMPROVED IMPACT STRENGTH CHARACTERISTICS AT LOW TEMPERATURES AND PROCESS FOR PREPARING SAME

摘要

POLYPROPYLENE COMPOSITIONS HAVING IMPROVED IMPACT STRENGTH PROPERTIES AT LOW TEMPERATURES AND PROCESS FOR PREPARING SAME 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.degree.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 ac tive form.