A highly rigid composition of 'propylene-ethylene block copolymer' and a process for its manufacture

摘要

A propylene.ethylene block copolymer composition having a high stiffness and high-impact properties and its production process are provided, which composition is produced by using a specified Ziegler-Natta type catalyst; carrying out polymerization step (I) wherein homopolymerization of propylene is carried out by means of two or more polymerization vessels so as to afford 60 to 95% by weight based upon the total weight, and a specified range of the maximum value and the minimum value of the melt flow rates of the polymers obtained in the respective vessels; and carrying out polymerization step (II) wherein propylene-ethylene random copolymerization is carried out by means of one or more polymerization vessels so as to afford 5 to 40% by weight based upon the total quantity, the ratio of the melt flow rate at the polymerization step (I) to the melt flow rate at the polymerization step (II) being made to fall within a specified range, whereby the resulting ethylene.propylene block copolymer composition is far superior in the collective strength to known ethylene.propylene block copolymer compositions.