Polystyrene is made by polymerising styrene, which is produced by combining benzene and ethylene. The polystyrene thus produced is one of the thermoplastic materials sometimes called linear polymers. These are materials that soften on the application of heat and harden as they cool.
The expanded foam (known as EPS) is produced as small beads containing a pentane blowing agent which is expanded by treatment with steam to form pre-foam of the required density. The pre-foam is then fed into moulds and more heat (steam) is applied. This heat fuses the beads together to produce items in a variety of shapes and sizes or to make blocks for cutting into sheets and other products.