Bullet Resistant Glass Explanation:

Bulletproof glass, ballistic glass, transparent armor, or bullet-resistant glass is a strong and optically transparent material that is particularly resistant to penetration by projectiles. Like any other material, 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 that it can flex instead of shatter. The index of refraction for all 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 such security, such as jewelry stores and embassies, and of military and private vehicles.

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Construction:

A rough visualization of bulletproof glass, composed of layers of plastic sheeting (grey) and layers of glass (blue)

Bullet-resistant glass is constructed using layers of laminated glass. The more layers there are, the more protection the glass offers. When a weight reduction is needed, polycarbonate (a thermoplastic) is laminated onto the safe side to stop 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: 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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Environmental effects:

The properties of bullet-resistant glass can be affected by temperature and by exposure to solvents or UV radiation, usually from sunlight. If the polycarbonate layer is below a glass layer, it has some protection from UV radiation due to the glass and bonding layer. Over time the polycarbonate becomes more brittle because it is an amorphous polymer (which is necessary for it to be transparent) that moves toward thermodynamic equilibrium.

An impact on polycarbonate by a projectile at temperatures below −7 °C sometimes creates spall, pieces of polycarbonate that are broken off and become projectiles themselves. Experiments have demonstrated that the size of the spall is related to the thickness of the laminate rather than the size of the projectile. The spall starts in surface flaws caused by bending of the inner, polycarbonate layer and the cracks move “backwards” through to the impact surface. It has been suggested that a second inner layer of polycarbonate may effectively resist penetration by the spall.

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