the method of making polystyrene foam for foam products which includes utilizing atmospheric gases as the blowing agents or preferably mixing one or more of these gases and a blending agent which is non - volatile at the extrusion temperatures , introducing the mixture to a molten thermoplastic polystyrene resin containing nucleating agents such as sodium bicarbonate and citric acid and extruding the thermoplastic in the form of a foam . the resultant foam has substantially no residual blowing agent , is dimensionally stable at ambient temperatures and when used with a non - volatile blending agent can be made with small amounts of blowing agent permitting cooling of the melt without high loading on the extruder . the foam may be coextruded with one or more layers of thermoplastic films so that the resultant product is adpated for use for plates , cups , food containers or packaging and for labells for containers such as glass and plastic containers and for labels that are shrunk in place on a container .