What Causes a Car Window to Shatter?by Rebecca Cioffi
Two Types of Safety Glass
The glass in car windows is not a simple glass like you find in home windows or in drinking glasses; it is a glass created to keep jagged, sharp pieces from injuring someone in a car crash. This is commonly referred to as safety glass.
There are two types of safety glass used in cars today: tempered and laminated. Tempered is very strong and is created by heating the glass to an extremely high temperature and then cooling it down very quickly. This produces a very strong glass. It is known for being used on car side and back windows, as well home uses like shower doors, coffee pots and the glass doors on ovens.
The other type of safety glass is laminated glass. This type has three layers. The outside layers are conventional glass, but the center layer is a thin core of sheet vinyl that is bonded by heat to the other two glass layers. This is the kind you find in the front windshield of a car.
How Tempered Safety Glass Shatters
Tempered glass uses the extremely high heat to cure the glass, but what is happening is that the core of the glass, the inner layer, becomes compressed much more quickly than the outer layers. This happens over and over again during the tempering process, and an ultra fine series of stress lines develop. They are not visible to the naked eye, but break the glass, and it shatters along these many and tiny stress lines into small, safer pieces. These pieces are fairly uniform in size, and the edges are smooth.
How Laminated Safety Glass Shatters
Laminated glass allows the two outside layers of glass to crack, but the inside vinyl layer holds all the pieces together. This vinyl layer remains intact, and the broken glass remains adhered to the vinyl layer. In fact, in accidents where the front windshield is broken, many times it can actually be peeled away out of the window intact. The pieces hold and come away as one piece. Even in severe crashes, this laminated glass might break into large pieces, but it will not break into small or jagged, dangerous pieces.