Bullet-resistant glass
Bulletproof glass (also known as ballistic glass, transparent armor, or bullet-resistant glass) is a type of strong but optically transparent material that is particularly resistant to being penetrated when struck. Like any material, however, it is not completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft. The softer layer makes the glass more elastic, so it can flex instead of shatter. The index of refraction for both of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear, undistorted view through the glass. Bulletproof glass varies in thickness from 3⁄4 to 3 1⁄2 inches (19 to 89 mm).
Bulletproof glass is used in windows of buildings that require security, such as jewelry stores and embassies, as well as military and private vehicles.
Bullet-resistant glass is constructed using layers of laminated glass. The more layers the higher the protection. When a weight reduction is needed 3mm of polycarbonate (a thermoplastic) is laminated onto the safe side. This polycarbonate stops the spall. The aim is to make a material with the appearance and clarity of standard glass but with effective protection from small arms. Polycarbonate designs usually consist of products such as Armormax, Makroclear, Cyrolon, Lexan or Tuffak, which are often laminated as the final layer (Glass clad Polycarbonate).
The polycarbonate usually has one of two types of coating to resist abrasion: a soft coating that heals after being scratched (such as elastomeric carbon-based polymers) or a hard coating that prevents scratching (such as silicon-based polymers).
The plastic in laminate designs also provides resistance to impact from physical assault from blunt and sharp objects. The plastic provides little in the way of bullet-resistance. The glass, which is much harder than plastic, flattens the bullet, and the plastic deforms, with the aim of absorbing the rest of the energy and preventing penetration. The ability of the polycarbonate layer to stop projectiles with varying energy is directly proportional to its thickness, and bulletproof glass of this design may be up to 3.5 inches thick.
Laminated glass layers are built from glass sheets bonded together with polyvinyl butyral, polyurethane, Sentryglas or ethylene-vinyl acetate. When treated with chemical processes, the glass becomes much stronger. This design has been in regular use on combat vehicles since World War II. It is typically thick and is usually extremely heavy.
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