Safety glass is designed to shatter differently from other types of glass, to help prevent injuries from flying shards. There are two types of safety glass: tempered glass and laminated glass. Tempered glass is exceptionally strong and heat resistant and gets its strength from a tempering process in which the glass is heated to extremely high temperatures and cooled down quickly. Common applications for tempered glass include rear and side windows on automobiles, glass shower doors, glass coffee carafes and windows in oven doors.
Laminated glass has three layers. The outer layers are made of ordinary glass; the middle layer is a clear, thin vinyl sheet that has been bonded to the glass on each side. Common applications for laminated glass include auto windshields, skylight windows and windows designed to resist hurricane-strength winds.
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How Tempered Glass Shatters
During the tempering process, the inner layers of a pane of glass become compressed at a higher rate than the outer layers. This process is repeated several times to create a complex web of stress lines that is invisible to the naked eye.
When something causes tempered glass to shatter, it shatters along all of these stress lines. The glass breaks into many small bits that are rounded or cuboid in shape, often with relatively smooth edges. The bits are often about the size of the eraser on a pencil. In cases of auto accidents where tempered glass windows shatter and the shards spray across an occupant's body, the bits typically cause only superficial injuries, if any.
Because the network of stress lines covers the entire pane of each piece of tempered glass, it will all shatter at once when it fails. Ordinary glass, on the other hand, may break in one area while remaining intact elsewhere.
How Laminated Glass Shatters
The thin vinyl layer bonded in the middle of laminated glass is designed to hold the entire panel together. All modern car windshields are made from laminated glass, and when a car is involved in a serious accident, the entire windshield can often be peeled off of the car, much like a foil lid on a container of yoghurt. This is because the glass panes on both sides of the vinyl are completely shattered, leaving the windshield none of its former rigidity, while the inner vinyl layer is completely flexible, like a sheet of cling film.
Light impact to laminated glass may crack just one layer of glass, leaving the other intact. Severe impact may puncture or sever the vinyl layer, but even in these cases, laminated glass will cling together in several large pieces.