Glass is a wondrous material and, to many of us, a bit of a riddle. It is very easy to shatter, yet it is hard enough to protect us. It is totally transparent, yet it is made from opaque sand. It can be classified as a solid material, while others say it is more liquid, given that it is made from molten sand (sand that melts in temperatures as high as 1700°C (3090°F) and turns into a liquid). It dominates our lives; it is everywhere, from glass windows and mirrors to light bulbs and everything in between. Besides all the wonderful things we have created with glass to make our lives easier and more beautiful, some genius minds generated ideas to provide those that needed extra protection with a reliable type of glass that could even stop a bullet!
Bulletproof Glass: Background
Among the many uses of glass, is in the danger zone, when ducking bullets is required. Simply put, bulletproof glass is a variation of laminated safety glass and was invented by Édouard Bénédictus, a French chemist whose original idea was to use an early plastic called celluloid and have it sandwiched between two glass sheets, in 1909.
Over the course of the years, the need to protect oneself when caught in the combat zone became more and more intense. Those in the front line needed to have some sort of barrier in front of their body; a barrier strong enough to dissipate the energy of a bullet. Although there is no such thing as 100% bulletproof glass, as of now, at least, there is armoured glass that can withstand great force and power.
Why Ordinary Glass Shatters to Smithereens
It all has to do with Physics and the laws related to force and momentum. Take, for example, a baseball player. When trying to catch the baseball, he moves his hand backward. Wonder why? That movement allows him to reduce the energy of the fast-moving ball, which, in turns, reduces the force on his hand, and consequently saves himself from quite a lot of pain (if he caught the ball with his hand locked in one position, all that force and energy would cause a great deal of pain to the hand because the hand absorbs all that impact). Now, let’s go back to Physics. We were taught that the force an item exerts is equal to the rate at which its momentum changes. So, if we manage to gradually stop an item (the baseball, in our case), we slowly change its momentum and reduce the force we feel.
When it comes to glass, we know that it can’t bend or move so that it absorbs the energy gradually, unlike the hand. Therefore, if someone fires a bullet at a piece of glass, the bullet will pass right through it, with barely any loss of momentum, and shatter it. This adds an extra element of danger to the carrier of the glass pane. If they don’t get killed by the bullet itself, they might get seriously injured (to even killed) by the huge shards of the shattered glass. That is how, more or less, the idea of creating bulletproof glass came to being.
How Bulletproof Glass Works
As previously mentioned, no glass is actually totally bulletproof. This is why the glass used in armed forces, and not only, is called bullet-resistant and not bulletproof. That being said, bullet-resistant glass does shatter. However, what makes a big difference between ordinary glass and bulletproof glass is that the latter comes with layers of plastic sandwiched between the glass panes, which not only absorb but also dissipate the energy of the bullet. In other words, if you hold a piece of bulletproof glass in front of you and someone fires a bullet at you, the bullet will be significantly slowed down as it penetrates through the glass, causing much less damage (if any) than with any other type of glass. We may say that bullet-resistant glass is actually a bullet-energy-absorbing glass and below is why.
Compared to ordinary glass, bulletproof glass is made from multiple layers of tough glass. In between those layers, there are several layers of various plastics. This is called a laminate*. Sometimes, we may also find a final inner layer of a tough type of plastic called polycarbonate, which helps prevent the breaking of glass following the impact of a bullet to create dangerous shards of plastic or glass splinter. All these added layers of plastic make the glass 10 times thicker than a single glass pane.
Besides being heavier than a sheet of ordinary glass, it is also more bullet-resistant. Why? Because the energy from the bullet spreads out sideways through the various layers when it strikes the bullet-resistant glass. With each piece of glass and plastic it passes through, its energy is reduced, and, since it is spread over a large area, it is also absorbed much faster than when penetrating ordinary glass pane. The bullet eventually ends up with not enough energy to do much damage if it does manage to pierce through. As for the glass panes? They will most likely break, but not fly apart.
* Laminate is a particular kind of composite material in which two or more layers of different materials are bonded together with adhesives to provide added durability, strength or some other benefit.
How is Bulletproof Glass Made?
Until a few years ago, traditional bulletproof glass was made from alternating layers of plastic and glass. The glass was usually between 3-10mm (1/8-3/8 of an inch) thick and the plastic was a thin piece of PVB (polyvinyl butyral) whose thickness ranged between 1 and 3 mm (30-90mils). However, we have witnessed significant advances in the making of bulletproof glass. Newer kinds are much stronger. Although they also use a sandwich of glass and plastic, the plastic is made of ethylene vinyl acetate, acrylic glass, polycarbonate or ionoplast polymers; all known to be excellent shatter-resistant alternatives to glass. The thick layers of plastic and glass are also separated by thin layers of polyurethane, PVB or another plastic.
How is PVB-Based Bullet-Resistant Glass Made?
First, we need to make a laminate. To achieve that, a thin PVB film is sandwiched between the thicker glass. To cause it to bond to the glass, the PVB layer is heated, so the plastic melts, and compressed. This is a process often occurring in a vacuum, so air is not trapped between the layers, which would create a pain point to the glass (it will weaken the laminate)*. The unit is then put at high temperature, up to 150°C (300°F) and then “cooked” at high pressure (about 14X the atmospheric pressure) in a specially designed pressure cooker (created for industrial use) called the autoclave. At this point, it is very important to ensure that the pressure and heat of the autoclave will not change the optical properties of the plastic, which will make it difficult for someone to see through it.
* If air is trapped, the optical properties of the laminate are significantly affected. This means that the light which passes through will be distorted.
Uses of Bulletproof Glass
We find bulletproof glass in a wide variety of sizes and shapes, depending on the level of protection required per case. Among the most common uses of bullet-resistant glass is in banks (the drawers the teller uses to exchange money and paperwork with the customers, as well as the thick windows they sit behind, are usually bulletproof), and government buildings.
However, we also have armored vehicles reinforced to withstand attack and combat. They usually come with metal plates that help protect both the tyres (so that the car runs even when they are flat) and the car itself. Of course, they also feature bulletproof glass too. On armored vehicles, the bulletproof glass is usually one-way bullet-resistant. This means that a bullet will be resistant only on one side; the defensive one. The offensive side (the outer layer of the glass) is usually a brittle layer while the glass on the defensive side is much more flexible (hence, slows down the energy from a bullet almost instantly). This also means that if someone fires a bullet from the inside, it will easily pierce through the glass, allowing defenders to strike back at the offenders.
Finally, we have transparent armour gun shields for soldiers that operate machine guns mounted on armored vehicles. That particular shield protects the gunman, without blocking their visibility while shooting. Plus, there are also bulletproof eye glasses used by soldiers in combat.
At the moment, a new glass is being created as a means to provide military units with a lighter weight, defensive form of bullet-resistant glass. This new glass replaces the polymer layer with an aluminum oxynitride outer layer, which allows the creation of a more resistant and much lighter glass that can protect military aircraft and vehicles alike.
Basic bulletproof glass ranges from 3-4cm (1.185-1.59in). Generally speaking, the thicker the glass (and the more layers it has), the greater the protection it provides and the energy it can absorb. But, we have seen bulletproof glasses made twice this thick. The only issue with them is that they do tend to become heavier as they get thicker, which is a concern in the cases when one is trying to bulletproof vehicles, such as the Popemobile or the President’s car. Apart from that, the thicker the bulletproof glass, the more opaque it is made (light has to pass through many layers), which makes it difficult for a driver to have a clear view of the road.