Polypropylene compositions having improved impact strength properties at low temperatures and process for preparing same

Claim 2

Polypropylene compositions having a high impact strength at low temperatures comprising the following essential components: - 100 parts by weight of polypropylene having an isotac- .ticity index higher than 90; - 8-25 parts by weight of a copolymeric fraction (1) soluble in xylene at 23°C consisting of an amorphous ethylene-propylene copolymer containing from 20 to 80% by weight of ethylene; - 2-10 parts by weight of a fraction (2) consisting of a crystalline ethylene-propylene copolymer exhibiting a polyethylenic crystallinity, containing from 50 to 90% by weight of ethylene; and in which the total content of polymerized ethylene, referred to the weight of polypropylene and of fractions (1) and (2), ranges from 4 to 20%, the weight ratio of total polymerized ethylene to fraction (1) in said compositions being lower than 1 and preferably ranging from 0.2 to 0.8.

Claim 3

A process for preparing the compositions as claimed in claim 1, by means of a process for polymerizing propylene and ethylene-propylene mixtures carried out either continuously or discontinuously in consecutive steps comprising at least a step of stereoregular homopolymerization of propylene and a step of copolymerization of mixtures of ethylene and propylene, said process being characterized in that the polymerization is accomplished in the presence of stereospecific coordination catalysts comprising a titanium compound carried on a magnesium halide in active form having a surface area larger than 3 m 2 /g and/or having a X-ray spectrum in which the line of maximum intensity appearing in the spectrum of nonactivated magnesium halide is broadened or substituted by a halo, by operating under conditions in which in the homopolymerization step a polypropylene having an isotacticity index higher than 90 is obtained in an amount ranging from 65 to 90% by weight of the final product, in the copolymerization step an amorphous ethylene- propylene copolymer extractable with xylene at 23°C is obtained in such amounts that the weight ratio of the total polymerized ethylene to such fraction is lower than 1, and the total content of polymerized ethylene in the final product ranges from 4 to 20% by weight.