disclosed is a method for making polystyrene foam which utilizes one or more atmospheric gases , particularly co 2 , as the blowing agent in combination with a polymer processing aid , typically an ester that is relatively non - volatile at the extrusion temperature range . the blowing agent and the ppa may both be introduced into the molten thermoplastic polystyrene resin or the ppa may be incorporated in the solid source polystyrene resins . the resulting foam will be substantially free of residual blowing agent and dimensionally stable at ambient temperatures .