Structure and Thermodynamic Aspects of Phase Segregation of Commercial Impact Polypropylene Copolymers

Abstract

Commercial isotactic polypropylene homopolymer is widely employed in applications for thermoplastics. However, it is limited in its applications by its relatively low impact strength and high brittleness temperature. Its properties can be greatly enhanced by blending a minor fraction of a rubber—like material. This is commercially practiced by synthesizing reactor blends of isotactic polypropylene and ethylene—propylene rubbers (EPR) in—situ. Commercial blends of isotactic polypropylene and EPR are called impact polypropylene and constitute a significant segment of the polypropylene market. The effect of the added EPR is to toughen the brittle, low impact strength isotactic polypropylene matrix. The details of the toughening of a high Tg, hard polymeric material with a low Tg, rubberlike material are available elsewhere.(1) The rubber—like phase is known to exist as a phase-segregated discrete particle in a continuous matrix of the hard phase. The rubber—like particles are typically added at low concentration and, thus, form a discrete phase in the continuous matrix of the hard phase. An example of a toughened polymer is impact polypropylene(2). Polypropylene itself has poor impact resistance and is very brittle at low temperatures. However, when about 10% – 20% of an ethylene/propylene copolymer (EPR) containing about 50% ethylene is added to polypropylene, the impact increases dramatically and the brittleness temperature is decreased markedly. The EPR is known to exist as very small, discrete particles in the polypropylene matrix(2). These particles toughen the matrix against crack propagation by dissipating large amounts of energy in the matrix material around the particle, thereby blunting the crack and inhibiting crack propagation.(1)