Effect of Talc Content on the Volume Expansion Ratio of Extruded PP Foams

Abstract

This paper elucidates the effects of cell density on the volume expansion behavior of polypropylene (PP) foams blown with butane in extrusion. The cell density was controlled by varying the talc content, and foam expansion was observed at a fixed blowing agent content while varying the melt and die temperatures. As observed in our previous studies, the curve of the final expansion ratio of PP foam versus temperature showed a typical mountain shape for each talc content, confirming that the two governing expansion mechanisms were gas loss and stiffening of melt. As the talc content increased, the expansion curve skewed towards the lower temperature, which showed that the expanded foams with a high talc content were more susceptible to gas loss at elevated temperatures. This indicates that the processing temperature should be decreased to have a large expansion ratio from the extruded PP foams at a high talc content. In other words, the optimum temperature to maximize expansion decreased at a higher talc content. It is believed that the shift of the expansion curves was caused by the promoted expansion rate of extruded foams with a greater talc content because of the reduced diffusion distance of gas molecules to the nearest stabilized nucleus. On the other hand, the increased cell density at a high talc content increased the number of cell layers in the cross section of the extruded foam, and thereby the gas loss was localized to the cells on the surface which acted favorable for the final expansion ratio to a certain degree.