Laminated glass is a type of safety glass that remains intact when shattered. In the event of breaking, it is held in place by an interlayer, usually made of polyvinyl butyral (PVB) that has been laminated between its two or more layers of glass. This interlayer keeps the layers of glass bonded even when broken. Other interlayers are also available in different thicknesses and hardness.
One of the most appreciated safety benefits is that its high strength prevents the glass from breaking up into large sharp pieces. It’s easily recognized by its characteristic “spider web” cracking pattern that results when an impact is powerful enough to crack the glass.
Laminated glass was invented in 1903 by a French chemist, Edouard Benedictus, who was inspired by a laboratory accident. A glass flask had become coated with the plastic cellulose nitrate. When it was dropped, it shattered but did not break into a multitude of hazardous pieces as expected. Realizing the potential safety benefits, Benedictus fabricated a glass-plastic composite to help reduce life-threatening bleeding injuries that often occurred in car accidents. However, it was not immediately adopted by automobile manufacturers. One of first practical industrial applications of laminated glass was in the manufacturing of reinforced eyepieces used in gas masks during World War I.
Today, laminated glass is produced by bonding two or more layers of ordinary annealed glass together with a plastic interlayer, usually Polyvinyl Butyral (PVB). The PVB is sandwiched by the glass which is passed through rollers to expel any air pockets. To form the initial bond, the glass is then heated to around 158 °F (70 °C) in a pressurized oil bath. A readily seen example of this is the tint you find at the top of some car windshields – it is in the PVB layer.
Multiple interlayers and thicker glass increases the overall strength and impact resistance of the laminated glass. Bulletproof glass, up to 100 mm thick, is often made of several layers of float glass, toughened glass and Perspex panels.
When aircraft windshields are manufactured, there are usually three sheets of 6mm toughened glass with thick PVB interlayers sandwiched between them.
Each manufacturing process may include glass layers of equal or unequal thickness.
It is NEVER recommended to attempt to cut both sides separately by pouring a flammable liquid (methylated spirits) into a crack in laminated glass, and then ignite it in order to try and melt the interlayer to separate the pieces. This is a dangerous and unsafe practice to be avoided.