Bulletproof Glass

One of the most important developments in armor was that of transparent armor (TA). To the public, transparent armor is known as "bullet proof glass". Even among those who work with it the most common name is "bullet resistant glass". Manufacturers don't want to misrepresent their product as proofed against all bullets so they use the word resistant instead of the word proof.

The irony is that it is rarely glass-only and sometimes there is no glass in it. In fact TA most often takes the form of a laminate consisting of a sheet of polycarbonate with a layer of glass on each side. Think of this symetrical laminated armor like a hamburger where the meat is a plastic and the bun halves are glass. The laminates are bonded with types of glues.

Polycarbonate alone is usably bullet resistant. A half inch thick sheet of polycarbonate can easily stop .22LR bullets. The reason for adding the glass laminates is two-fold. First, the glass serves as a hard, scratch resistant surface. Second, the glass adds to the bullet resistance. Glass is harder than plastic, but is brittle and shatters easily. The plastic offers structural support, holding the bonded glass even after it shatters. Holding the glass in the path of the bullet forces the bullet to expend kinetic energy (energy of motion) breaking the bonds of the glass molecules. Adding more layers of alternating glass and plastic increases the resistance to penetration.

Analyses of injuries by armor penetration has revealed the distinct danger posed by spalling. Spalling is a type of armor impact damage whereby fragments of armor are hurled off of the backside towards the intended victim. These fragments can cause serious bodily harm. To avoid this, TA may be laminated on the backside with a final layer of plastic, possibly as a thin film.