First I need to start by disputing the name, as bulletproof glass is neither bulletproof nor is it a glass. Technically it should be called ‘bullet resistant laminate composite’, however that doesn’t sound anywhere near as cool.
Why is it not bulletproof? Well with enough time, enough effort and a big enough caliber bullet, the bulletproof glass can eventually be penetrated and will break, hence is not really bulletproof.
The strength and the durability of the bullet-resistant glass will depend on how it is made and the thickness of the laminate glass itself (how many layers there are).
Normal glass will shatter when one bullet is fired at it. This is because glass can’t bend (elastically deform) and absorb the energy of the bullet, so the bullet just passes through, losing a very small amount of energy as it breaks the glass.
Bulletproof glass is made up of layers of glass and a plastic (polymer) layer, usually polycarbonate. Polycarbonate is a transparent (see-through), soft, ductile plastic with very high strength and is a sixth of the weight of glass. Because the polycarbonate is soft, it is able to deform and stretch by taking the energy from the moving bullet. If the first layer does not stop the bullet, it will pass through to the next polycarbonate layer and maybe even the next layer, but eventually enough of the energy from the bullet will be absorbed by the polycarbonate causing the bullet to slow down so much that it doesn’t have enough energy to pierce through the final piece of glass.
If the glass is too thin, and the bullet does make it through, it is likely that enough energy has been absorbed from the bullet that it won’t do much damage on the other side of the glass.
The strength of bulletproof glass depends on how many layers of glass and polycarbonate there are, with more layers resulting in a thicker piece of bulletproof glass. A shot from a rifle would produce more force than a handgun so the glass would need to be thicker if it needed to resist rifle fire.
Typically bulletproof glass ranges from 7mm to 75mm in thickness, however as the 75mm piece of glass will be very expensive and very heavy, it’s only used in extreme cases. Also the thicker the bulletproof glass the more opaque (less see-through) it is, which can cause problems if the glass is used for the windshield of a car.
Although the layers of glass within bulletproof glass can break, the polycarbonate layers which are bonded to the glass prevent them from shattering and flying apart, and so the whole structure remains intact, unlike normal glass where the pieces fly everywhere.
So where can you find bulletproof glass? Well the most common place would be in banks or high end jewellery stores, and if you are in New Zealand, then check out the glass on the lower floors of Wellington Central Police Station, or on the front of the traffic cameras in Auckland city.