Safety glass is a required option in many glass applications because of
its physical characteristics that reduce the likelihood of injuries
resulting from accidentally broken glass. Unlike regular glass, safety
glass does not shatter into sharp, jagged and often small pieces when
broken. This important characteristic makes it the preferred choice of
many glass buyers.
There are two main classes of safety glass: tempered glass and laminated glass. Tempered glass, which has been heat-treated to increase its strength and resistance to shattering, breaks into small, rounded pieces when broken. Laminated safety glass has several internal and external layers of polymer films which keep the glass inside its window panel even after it has been broken. If under extreme force, the glass will still crack and need replacing, but the layers of polymer film keep the pieces of glass together. The automotive industry uses safety glass almost exclusively in car and truck windows; nearly all side and rear windows are tempered, and all front windows are laminated to protect passengers from dangerous glass shards in the case of an accident. Security buildings, shower doors, greenhouses, glass cutting boards and many offices use safety glass to prevent severe injury in case of accidental breakage and splintering. Safety glass is especially important for buildings which are more prone to glass breaking such as those buildings at risk of earthquakes, tornadoes or terrorist attack.
Most glass fabricators create laminated and safety glass with various processes in order to achieve different properties. The laminating and tempering processes may be combined to make safety glass stand up under extremely harsh circumstances. Laminated glass is formed by "sandwiching" thin clear polyvinyl butyral (PVB) film between two or more sheets of glass. The PVB film, which is flexible and elastic, also coats the outer layers of the glass "sandwich", increasing the entire sheet's strength as well as flexibility. When shattered, laminated glass does not implode or explode from its frame, but remains inside its laminated coatings even after shattering. Bullet-proof glass is a classic example of this, although bullet-proof glass is very thick, containing many layers of glass and laminate. Tempered glass is strengthened through a heat treating process which heats and quickly cools sheets of glass, often multiple times, to harden the glass and to change its crystalline structure so that when the sheet breaks, it collapses into small, rounded pieces rather than shattering. Tempered and laminated glass serves different functions; laminated glass is designed to prevent breakage, while tempered glass is designed primarily to break safely if an accident occurs. Both tempered and laminated glass have improved strength, however, and many high-impact applications use safety glass that has been both laminated and tempered.