Measured and Calculated Expansion of Polystyrene Beads Comprising Four Blowing Agents in Hot Silicone Bath and in Water Vapor as well as in Extrusion for Boards

The published models were sophisticated and described the expansion in dependence on time only in the first stage. The object was to explain the discrepancy between foaming under pressure release XPS and foaming by heat supply EPS by model calculations. The rate of expansion of small samples comprising blowing agent and polystyrene was measured by buoyancy in a silicone bath at 110°C and that of extrusion on photographs of the volume increase after the nozzle. A viscosity model and a diffusion model were established, and experimental data were compared with calculated data. The expansion rate in the silicone bath was about 100 times slower than that in extrusion at the same nozzle temperature. The velocity of foaming in the bath by heat supply was observed to be dominated by viscosity and that of foaming under pressure release in extrusion to be stirred by diffusion. Calculations according to the viscosity model allowed the description of foaming in silicone, and the diffusion model reproduced the data of extrusion. The common feature of both models was their simplicity. According to the models, the efficiency of blowing agents was only dependent on the molecular weight and on the solubility. The time determining influence on foaming was diffusion in extrusion of XPS and viscosity for expansion of EPS in silicone bath and water vapor.