THE NUCLEATION OF MICROCELLULAR FOAMS IN SEMI CRYSTALLINE THERMOPLASTICS∗

Abstract

The nucleation of microcellular foams in amorphous thermoplastics has been performed by supersaturation with gas at an elevated temperature. Pressure and temperature are then carefully reduced in the vicinity of the glass transition temperature of the material. The result is a foam structure with cells on the order of 10 microns. This material exhibits greatly increased impact strength, as well as thermal and electrical insulation properties as compared to conventional foams. A new process has been developed to produce microcellular foams in semi-crystalline polymers. The process operates in the vicinity of the polymer's melting point, as opposed to the glass transition point. This is due the low solubility of the gases in and the rigidity of the crystalline phase. Microcellular foams have been produced successfully in polypropylene. The effects of various additives have been investigated experimentally. The theory developed for the amorphous materials has been compared to these new experimental results and again qualitatively agrees.