Bulletproof glasses; It is used in the windows of buildings that require extra level of security such as jewelers, banks, embassies, military and private vehicles. In movies and TV shows, bulletproof glass is often portrayed as indestructible. No matter what weapon is used or how many bullets are fired, these glasses remain intact and unchanged. The only problem is that in real life bulletproof glass is neither truly “bulletproof” nor is it entirely “glass”.
The correct term for bulletproof glass is “bullet resistant”; because with enough time and effort, or with a sufficiently large caliber bullet, it is possible to prevail over the so-called “bulletproof” of glass. The strength and durability of bullet resistant glass depends on how it is made and the thickness of the final product.
Why Do Normal Glasses Break?
If you’ve ever caught a fast-moving baseball or soccer ball, you know the trick is to pull your hand back and slowly lower the energy of the ball, thus reducing its energy very slowly. This reduces the force applied to your hand, so catching it is less painful. More scientifically speaking, the force the ball exerts on your hand is equal to the rate at which the momentum of the ball changes. So if you change its acceleration gradually, that is, if you stop it gradually, the force you feel decreases.
Let’s say you stop the ball in half a second – almost instantly. In this method, you feel a heavy force while your hand absorbs the blow. Now, let’s say you replay the moment and take two seconds instead of half a second to stop the ball. This time, when the ball takes four times as long to stop, your hand will only feel a quarter of the force as before, so the ball will do much less damage to you.
Unlike your hand, a piece of glass cannot move. If a bullet hits ordinary glass, the glass cannot bend and absorb energy very gradually. So, simply, it shatters. The bullet continues on its way with almost no loss of momentum. Therefore, ordinary glass provides no protection as it is completely ineffective at slowing bullets and absorbing their energy.
How to Make Glass Bullet Resistant?
There are three main types of lead-resistant glass:
Acrylic is a hard, clear plastic that looks like glass. A single piece of acrylic greater than 2.5 cm thick is considered bullet resistant. The advantage of acrylic is that it is stronger than glass, more impact resistant and %50 lighter than glass. Although acrylic can be used to create bulletproof glass, the final product does not contain real glass.
Polycarbonate is also a type of plastic, but differs from acrylic in many ways. Polycarbonate is a very durable soft plastic. It is one-third the weight of acrylic and one-sixth the weight of glass, making it easy to work with, especially when dealing with thickness. Polycarbonate is combined in layers to create a bullet resistant product. Acrylic repels bullets, while polycarbonate catches the bullet and absorbs its energy, preventing it from exiting the other side. Polycarbonate is more expensive than other types of materials, including glass and acrylic, so it is often used in conjunction with other materials for bullet resistant glass.
3. Bullet Resistant Glass Made of Glass Lined Polycarbonate
This type of glass uses a combination of materials to achieve the desired result. The lamination technique is used when producing glass-coated polycarbonate bullet resistant glass. A piece of polycarbonate material is laminated or sandwiched between sheets of ordinary glass and then subjected to a heating and cooling process to mold the materials into one piece. The result is a product similar to glass but thicker and more durable.
Thickness plays a big role in a product’s ability to resist bullets. Bullet resistant glass is designed to remain intact when exposed to one or more (a certain number of) bullets.
Depending on the strength of the bullet fired and what type of weapon is used, a thicker piece of glass may be required to stop a bullet. For example, a shot from a rifle is much more powerful than a shot from a 9mm pistol. Therefore, the glass thickness required for a 9mm pistol is less than for a rifle. The final thickness of bullet resistant glass usually ranges from about 1 cm to 8 cm.
The newest and greatest design for bullet resistant glass is unidirectional glass. Unidirectional bullet resistant glass consists of two layers: fragile glass and a flexible material like the polycarbonate plastic material we just mentioned. When a bullet first hits the fragile glass sheet, the glass breaks against the plastic, which absorbs some of the bullet’s energy and spreads it over a larger area, so the polycarbonate material prevents the bullet from escaping.
When a fired bullet hits the polycarbonate material first, most of the force is concentrated in a small area that prevents energy from being absorbed. Because the glass breaks out of the polycarbonate, the bullet retains enough energy to break the glass and fly towards its target. For this reason, a bullet resistant glass with glass at the front and polycarbonate material at the back is seen as the most ideal option for armored vehicles.
Aside from not being truly bulletproof as mentioned earlier, another problem with bulletproof glasses is that they are heavy. Its thick and heavy composition makes it difficult to apply to automobiles, although it is OK in applications such as windows or cabins inside buildings. In automobile design, thickness and weight are not only an engineering constraint, but also a major issue for visibility. The harder the glass, the less transparent it will be. This makes them impossible to use as windshields as they can affect the driver’s vision while driving.
Despite these limitations and cost, bullet resistant glass has saved and continues to save the lives of many people from past to present.