Process for preparing a micro open cell polystyrene foam

Disclosed is a process for making a micro open cell polystyrene foam having an average pore size less than 100 mum, which includes: compounding polystyrene, nucleating agents, fillers, and an exothermal chemical blowing agent; hot pressing the resulting compounded mixture into a green body without foaming; saturating the green body with a mixture of nitrogen and carbon dioxide at a supercritical condition in a batch mold; and foaming the PS green body by releasing the pressure after a period of saturating. Under such a process a PS foam has open cells and the strength of the PS foam is capable of enduring pressure greater than 1 kg/cm , which is suitable for the use as a core layer of vacuum insulation panel.