抄録
In this paper, the effects of the equivalency of foaming time and temperature on the cell size and density, as well as the number of cells per unit volume remaining in foamed polystyrene (PS) plastics, are investigated. The foaming was carried out by the following procedure. First, a solid resin was saturated with a blowing agent under high pressure at a temperature lower than the glass transition temperature of the resin in a pressure vessel. After the blowing agent reached its saturation state, the pressure was released at a constant rate. The specimen was then taken out of the pressure vessel and heated in a hot bath to allow foaming under various foaming temperatures and times. Finally, the specimen was cooled in a water bath in order to halt cell growth. The following results were obtained.
(1) Cell size becomes small when the foaming time is shortened at a low foaming temperature and becomes large when the foaming time is lengthened at a high foaming temperature.
(2) Cell density increases when the foaming time is shortened at a low foaming temperature and decreases when the foaming time is lengthened at a high foaming temperature.
(3) The cell density of foamed PS shows an equivalency between foaming time and temperature, i. e., a master curve can be obtained. This suggests that a time-temperature equivalency law holds for cell density.
(4) The time-temperature shift factors obtained from constructing master curves gives two different Arrhenius activation energies, which are similar to the those describing the viscoelastic behavior of materials.
(5) Based on this equivalency of time and temperature, it is possible to predict the required foaming conditions of plastics having arbitrary cell densities.
