Correlation of Decompression Time and Foaming Temperature on the Cell Density of Foamed Polystyrene

In this paper, the correlation between the foaming temperature and the decompression rate (decompression time) of the cell density that is the number of cells per unit volume remaining in the foamed plastic will be discussed. Foaming was carried out by the following method: the blowing agent was soaked into the resin as a solid state at high pressure under temperatures higher than the glass transition temperature of the resin. After the foaming agent reached its saturation state, cell nucleation and cell growth were accelerated by decompression. Finally, cell growth was halted by cooling. A device that can accurately control temperature and the decompression rate was designed, produced and verified for accuracy prior to this investigation. The polystyrene (PS) specimens were foamed under various foaming temperatures and the decompression rates using the above-mentioned method. The following results were obtained: 1. Cell density of foamed polystyrene shows time and temperature dependence as follows. The cell density increases when the decompression rate is quick, i.e. the decompression time is shortened under the condition of low foaming temperature, and cell density decreases when the decompression rate is slow, i.e. decompression time is lengthened under the condition of high foaming temperature, 2. Correlation is maintained between the temperature dependence and time dependence of the cell density of foamed PS, and it can be expressed by one master curve, 3. Based on this correlation, it is possible to predict the required foaming conditions of plastics having arbitrary cell densities.