THE HETEROGENEOUS NUCLEATION OF MICROCELLULAR FOAMS ASSISTED BY THE SURVIVAL OF MICROVOIDS IN POLYMERS CONTAINING LOW GLASS-TRANSITION PARTICLES .1. MATHEMATICAL-MODELING AND NUMERICAL-SIMULATION

The existing models based on classical nucleation theory are not able to explain satisfactorily the nucleation phenomenon of microcellular f oams in thermoplastics. Here, we extend the analysis of Kweeder (24), who developed a new model that considers the presence of microvoids, r esulting from the thermal processing history of the polymer, as potent ial nucleation sites. The nucleation model ''concentrates'' on the str esses and thus void formations in the rubber particles. Since these ar e pre-existing microvoids, bubble nucleation depends on the survival o f these voids to grow rather than the formation of a new phase as mode led by classical nucleation theory. The population of viable microvoid s with a sufficiently large radius to survive and overcome surface and elastic forces has been modeled to yield the cell. density. A log-nor mal distribution, which relates to the rubber particle size, has been used to model the distribution of microvoids in the polymer composite material. The model depends on various process parameters such as satu ration pressure, foaming temperature, concentration of nucleating agen ts, solubility of the blowing agent in the polymer, and the modulus. H igh impact polystyrene (HIPS) was added to polystyrene to obtain polym ers with different concentrations of rubber gel particles, the nucleat ing agent, and used here for this study.