High-rigidity ethylene-propylene copolymer and process for producing same

Claim 1

A process for producing a high-rigidity, resistant to whitening upon stressing ethylene-propylene copolymer, which process comprises a sequence of three stages that includes: (1) in a first stage producing between 60 and 90% by weight of the final product by copolymerizing a monomer mixture of ethylene and propylene wherein ethylene constitutes 0.7-7.5% by weight of the mixture, (2) in a subsequent second stage producing between 5 and 17% by weight of the final product by polymerizing a monomer mixture of ethylene and propylene wherein ethylene constitutes 70-100% by weight of the mixture, and (3) in a subsequent third stage producing between 6 and 23% by weight of the final product by copolymerizing a monomer mixture of ethylene and propylene wherein the ethylene constitutes 40-70% by weight of the mixture, said three stages being carried out in the presence of hydrogen and a catalyst obtained by (a) reacting an organoaluminum compound (L) or a reaction product (P) of an organoaluminum compound (L) with an electron donor (A) with TiCl 4 (C), (b) reacting the resulting solid product (I) with an electron donor (A) and an electron acceptor (B), and (c) combining the resulting solid product (II) with an organoaluminum compound (L 2 ) and an aromatic carboxylic acid ester (R), (d) the molar ratio of the carboxylic acid ester (R) to the solid product (II) being between 0.2 and 10, and the partial pressure of said hydrogen being adjusted so that the ratio of the MFR of the copolymer fraction obtained in the first stage to the MFR of the product obtained in the second stage will be in the range of 0.1 to 100, and the MFR of the copolymer fraction obtained at the third stage will be in the range of 0.05 to 0.5.