A family of hard, rigid, transparent thermoplastics which emit a characteristic metallic ring when dropped. Styrenes are free from odor and taste and burn with a sooty flame. Polystyrene has outstanding electrical properties, good thermal and dimensional stability, and is resistant to staining. However, it is somewhat brittle, and is often copolymerized or blended with other materials to obtain desired properties. High-impact grades are produced by adding rubber or butadiene copolymers. Heat resistance is improved by including some alpha- or methyl styrene copolymers. Copolymerization with methyl methacrylate improves light stability, and copolymerization with acrylonitrile increases resistance to chemicals. Styrene polymers and copolymers possess good flow properties at temperatures safely below degradation ranges, and can be easily extruded, injection-molded or compression molded. Polystyrene was widely used after World War II for injection molded toys and a wide variety of household items. Today it is used for yogurt containers, refrigerator linings, vending cups, bathroom cabinets, toilet seats and tanks, closures, instrument control knobs and a host of other products.