Impact polypropylene copolymers: fractionation and structural characterization

Abstract

Impact polypropylene copolymers may be produced in a two-reactor system to yield a blend of homopolymer with an ethylene-propylene rubber (EPR). The polypropylene homopolymer, which is itself brittle and has low impact strength, is markedly toughened by the presence of the EPR. The rubber-like EPR exists as a phase-segregated discrete particle in a continuous matrix of the hard phase. The molecular structure analysis of the resulting complex mixture is a formidable task. The purpose of this paper is to describe a preparative and an analytical temperature-rising elution fractionation (t.r.e.f.) technique as the primary tools to separate and characterize a commercial impact copolymer. These techniques permit the isolation and subsequent characterization of the components of the impact copolymer by ancillary techniques, primarily 13C nuclear magnetic resonance and differential scanning calorimetry. These techniques were applied to the structural analysis of a commercial impact-grade polypropylene with a melt flow rate of 6. It was found that this impact copolymer was composed of about 75 wt% of isotactic polypropylene, about 17 wt% of a highly non-crystalline EPR and about 8 wt% of semicrystalline ethylene-propylene copolymers. A major component of the semicrystalline ethylene-propylene copolymers was an ethylene-rich copolymer containing between 0 and 8 wt% of propylene comonomer. The separate characterization of the components of the impact copolymer gives insight into the chemical synthesis process used to produce these copolymers. Further, it permits one to gain a better understanding of the location and function of each of the components in the complex mixture.