As a thermoplastic polymer, polystyrene is in a solid (glassy) state at room temperature but flows if heated above about 100 °C, itsglass transition temperature. It becomes rigid again when cooled. This temperature behavior is exploited for extrusion, and also formolding and vacuum forming, since it can be cast into molds with fine detail.It is very slow to biodegrade and therefore a focus of controversy, since it is often abundant as a form of litter in the outdoorenvironment, particularly along shores and waterways especially in its foam form.