Polycarbonates are strong, stiff, hard, tough, transparent engineering thermoplastics that can maintain rigidity at up to 140°C and toughness at down to -20°C or even lower for special grades. The material is amorphous (thereby displaying excellent mechanical properties and high dimensional stability), is thermally resistant up to 135°C and rated as slow burning. Special flame retardant grades exist which pass several severe flammability tests.