THE HETEROGENEOUS NUCLEATION OF MICROCELLULAR FOAMS ASSISTED BY THE SURVIVAL OF MICROVOIDS IN POLYMERS CONTAINING LOW GLASS-TRANSITION PARTICLES .2. EXPERIMENTAL RESULTS AND DISCUSSION

The experimental data obtained for the nucleation of microcellular foams are compared with the theoretical model developed in the first part of this paper. Polystyrene (PS) with rubber particles as nucleation sites is used as an exploratory system. Nitrogen is used as a physical blowing agent to nucleate the bubbles. The influence of process variables, such as saturation pressure, foaming temperature, and concentration and size of rubber particles, is discussed. Results indicate that all these variables play important roles during the nucleation process. A nucleation mechanism based on the survival of microvoids against the resisting surface and elastic forces has been modeled to obtain the cell nucleation density. Increase in saturation pressure increases the cell density to a critical pressure. Beyond this critical pressure, there is no increase in bubble number, indicating that all microvoids are activated. The effect of temperature is more complex than the effect of pressure. Increase in concentration of the rubber particles increases the nucleation cell density. In general, the experimental data are well described by the nucleation model presented in Part I.