Polystyrene packaging material. Expanded polystyrene chips and block. Polystyrene is a polymer made from styrene, a hydrocarbon liquid manufactured from petroleum. It is melted through heating for moulding or extrusion purposes and returns to a solid state when cooled. Expanded polystyrene is produced from a mixture of polystyrene and a gaseous blowing agent (usually pentane or carbon dioxide). The solid plastic expands into a foam when it is heated by steam.