Polystyrene (PS) is a thermoplastic polymer obtained from the polymerization of monomeric styrene. There are four main types: crystal PS or GPPS (General Purpose Polystyrene), which is transparent, rigid and brittle; high impact polystyrene or HIPS (from English: High Impact Polystyrene), is impact resistant and opaque whitish; expanded polystyrene or EPS (English: Expandable Polystyrene; PSE in French), very light; and extruded polystyrene, similar to expanded polystyrene but denser and more impermeable. The main applications of shock-resistant PS and glass PS are the manufacture of containers by extrusion-thermoforming, and of various objects by injection moulding. The expanded and extruded forms are mainly used as thermal insulation in construction and to form protective shells in the packaging of fragile objects to protect them. EPS is also used for the production of fish boxes or refrigerators for the transport of vaccines, due to its insulating capacity.