Prey: window. Predator: a careless newspaper toss, a drive-by shooting, a Romeo throwing rocks. Whatever the reason, many a homeowner has reason to ask, “What is a shatterproof window? And how can I get one?”
The Physics of Shattered Windows
When punctured, the average glass window shatters like an Independence Day firework. Glass comes in two forms: tempered and untempered. Tempered glass is rapidly cooled during production to generate internal stresses. These stresses pull the surfaces inwards, creating compression, but allow internal fractures to propagate like viruses. Tempered glass doesn’t crack; it disintegrates like so many frozen raindrops. Untempered glass, on the other hand, breaks piecemeal into large, dangerous shards.
Sandwich Safety Glass
Shatterproof windows usually use laminated safety glass, invented in 1903 by a French chemist thanks to a provincial laboratory accident. Laminated safety glass is built like a sandwich: two panes of glass on both sides and a sticky interlayer between. The interlayer, often made from polyvinyl butyral, keeps the glass bonded even when broken, resulting in a characteristic “spider web” cracking pattern. Laminated glass is standard fare in automobile windshields, office buildings, skylights and hurricane windows. Depending on its performance, it may resist bullets, bomb lasts, crowbars, fire or bomb blasts, according to the Glass and Glazing Federation.
Alternatives to Laminated Safety Glass
In recent years, shatterproof windows have taken on two new definitions. Manufacturers have developed window films that act much like the laminated glass interlayer, only they are applied on the exterior. Some can alter the reflectivity and emissivity of the window as well.
Related Resource: Egress Window
Shatterproof “Glass” of the Future
Shatterproof “glass” might, in fact, not even be true glass. Hammerglass and Lexan, for instance, manufacture polycarbonate plastic sheets hundreds of times stronger than typical soda-lime glass, and researchers at the California Institute of Technology have created a metallic glass that bends like soft plastic when pushed. It may not be long before wayward footballs, rocks, even bullets, will merely ricochet rather than shatter.
