Extruded Polystyrene Foam Board: Polystyrene Swollen with Methyl Chloride and Let Out below 130 °C

Extruded polystyrene foam is the blue insulation board. Dow's McIntire (US 2,450,436, 1948) found the way to big, even foam: soak polystyrene with methyl chloride under pressure, but let it out below 130 °C, well under methyl chloride's critical temperature of 143 °C. The gas boils off as the mass leaves the vessel, swelling it into small closed cells and chilling it fast enough to set. In his example the result was a stable log about 8 inches across, which is sawn into boards. Methyl chloride is a toxic, flammable gas: outdoors or with strong ventilation, respirator on. The picture: the patent's drawing.