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Bullet-resistant glass is usually constructed using polycarbonate, thermoplastic, and layers of laminated glass. 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 sandwiched between layers of regular glass.The ability of a glass itself to withstand shock is improved by the process of tempering. When treated with heating and cooling or with chemical processes, the glass becomes much stronger. 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 hammers, axes, clubs, and so forth. 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, (hopefully) 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 three inches thick.
Laminated glass layers are built from glass sheets bonded together with polyvinyl butyral, polyurethane or ethylene-vinyl acetate. This design has been in regular use on combat vehicles since World War II; it is typically thick and is usually extremely heavy.
| Sample thickness and areal densities for bullet-resistant glass materials | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| NIJ | XX | Glass Laminate | Poly | Acrylic | Glass-Clad Poly | ||||
| Level | Thickness | Density | Thickness | Density | Thickness | Density | Thickness | Density | |
| in. | lb/sq. ft. | in. | lb/sq. ft. | in. | lb/sq. ft. | in. | lb/sq. ft. | ||
| I | 1.185 | 15.25 | 0.75 | 4.6 | 1.25 | 7.7 | 0.818 | 8.99 | |
| II | 1.4 | 17.94 | 1.03 | 6.4 | 1.375 | 8.5 | 1.075 | 11.68 | |
| III | 1.59 | 20.94 | 1.25 | 7.7 | 1.288 | 14.23 | |||
| IV | 1.338 | 14.43 | |||||||
| V | 1.338 | 14.43 | |||||||
| VI | |||||||||
| VII | |||||||||
| VIII | 2.374 | 26.01 | |||||||
Bullet-resistant materials are usually tested by using a gun to fire a projectile from a set distance into the material in a set pattern. Levels of protection are based on the ability of the target to stop a specific type of projectile traveling at a specific speed. Experiments suggest that polycarbonate fails at lower velocities with regular shaped projectiles compared to irregular ones (like fragments), so that testing with regular shaped projectiles probably gives a conservative estimate of its resistance. When projectiles do not penetrate, the depth of the dent left by the impact can be measured and related to the projectile’s velocity and thickness of the material. Some researchers have developed mathematical models based on results of this kind of testing to help them design bulletproof glass to resist specific anticipated threats.
Well known standards for categorizing ballistic resistance include the following:
- Summary of Euronational (EN) 1063 test conditions in English
- Summary of Underwriter’s Laboratory (UL) ballistic resistance test conditions in English
- U.S. Department of Defense specifications for purchase of transparent armor – includes standards for bullet resistance (ATPD 2352P).
- U.S. National Insititute of Justice (NIJ) standard for ballistic resistant protective materials (NIJ Standard 0108.01).