The nucleation of microcellular thermoplastic foam with additives: Part II: Experimental results and discussion

Experiments were performed to validate the model for the nucleation of microcellular foams in amorphous thermoplastic polymers. The polystyrene-zinc stearate system was chosen as the model system. Other additives such as stearic acid and carbon black were also investigated. Molecular weight and orientation effects were studied. Nitrogen and carbon dioxide were used to produce the microcellular bubbles. Results show that amounts of soluble additives at levels just below their solubility limit and high gas saturation pressures yield the most acceptable foams—ones with a large number of uniform small bubbles. In this region, the bubble number is sensitive to both the gas saturation pressure and the concentration of solutes. Increasing the concentration of soluble additives above the solubility limit has little effect on bubble number and almost eliminates the dependence on saturation pressure. Molecular weight and orientation had no effect on the number of bubbles produced. Similarly, carbon black, which is insoluble in and which bonds well to polystyrene, produced no effect on bubble numbers. The agreement between theoretical predictions and experimental results is reasonably good.