How Does Bulletproof Glass Work?

Are you interested in learning the mechanism behind bulletproof glass? Is it actually bulletproof?

Bulletproof glass, or more precisely bullet-resistant glass, is a strong, transparent material designed to withstand specific levels of impact, depending on its composition and thickness. It consists of multiple layers of glass and polycarbonate materials laminated together. Upon impact, the hard outer layer deforms the bullet, while the softer inner layers absorb and disperse the kinetic energy, preventing penetration.

Bullet-resistant glass is used in law enforcement agencies, the military, and various buildings requiring higher security.

In this article, we’ll cover a brief history of bulletproof glass, its use cases, how it works, what it is made from, and a few limitations.

The Short History of Bulletproof Glass

In 1903, French chemist Édouard Bénédictus accidentally knocked a glass jar to the floor, and while it cracked, it didn’t shatter into pieces. Later, he realized that the jar had previously contained a liquid plastic that had left a thin, transparent layer of plastic inside. This layer held the cracked glass together, preventing it from breaking apart. He experimented with layering glass sheets bonded with a plastic interlayer, which led to the invention of laminated safety glass, which he patented in 1909.

The applications of laminated safety glass were seen during World War I, used in gas mask lenses and vehicle windshields to protect soldiers from shrapnel and debris.

During World War II, the demand for more protective materials led to further advancements. Thicker and more durable laminated glass was developed and integrated into military vehicles and bunkers. This period also saw the introduction of polyvinyl butyral (PVB) as an interlayer, which offered improved clarity and adhesion over earlier materials. Today, bulletproof glass is integral to various sectors beyond the military.

Use Cases of Bulletproof Glass Today

Bulletproof glass serves as a critical security measure in various sectors, protecting against ballistic threats. Below are some examples:

• Financial institutions — Banks and financial institutions use bulletproof glass at counters and drive-through windows to safeguard employees from potential armed robberies.
• Government buildings — Government facilities, including courthouses and embassies, incorporate bulletproof glass in entry points, reception areas, and offices.
• Military — Military vehicles are equipped with bulletproof glass to shield soldiers from gunfire and explosive devices.
• Retail — Retail stores and malls install bulletproof glass in storefronts and display cases to prevent smash-and-grab thefts.
• Personal vehicles for high-profile individuals — Bulletproof glass is often installed in the personal vehicles of high-profile individuals, such as politicians, celebrities, etc.

The Working Principle of Bulletproof Glass

Bulletproof glass is designed to protect against ballistic threats by absorbing and dispersing the kinetic energy of incoming bullets. This is achieved through a multi-layered structure that combines materials with varying properties to neutralize the force of a bullet upon impact.

In the following sections, we’ll break down the process.

Hard Layer

When a bullet hits the bulletproof glass, the energy is transferred through both hard and soft layers.

During the initial impact, the bullet first contacts the hard outer layer, usually composed of materials such as tempered glass or acrylic, which can resist penetration, causing the bullet to flatten upon impact. This deformation increases the bullet's surface area, reducing its velocity and dispersing its kinetic energy over a broader area.

Softer Layer

After the deformation, the bullet encounters inner layers made of softer, more elastic materials such as polycarbonate or specialized plastics. These softer layers flex upon impact, absorbing the remaining kinetic energy of the bullet.

The flexibility of the inner layers allows them to dissipate the energy effectively, preventing the bullet from fully penetrating the glass structure.

This combination of hard and soft layers in bulletproof glass ensures that the energy from a ballistic impact is systematically managed — the hard outer layer slows and deforms the bullet, while the softer inner layers absorb and neutralize the remaining energy, preventing penetration of the barrier.

What Is Bulletproof Glass Made Of?

The specific composition depends on the desired level of protection, but it involves layering different materials to get both strength and transparency.

These are the materials used in bulletproof glass:

• Laminated glass — Multiple layers of glass bonded together with interlayers of polyvinyl butyral (PVB) or ethylene-vinyl acetate (EVA). The glass layers provide a hard surface that deforms incoming bullets, while the interlayers hold the glass together upon impact, preventing shattering.
• Polycarbonate — This thermoplastic material is durable and transparent, and it is known for its high impact resistance. It’s often used as an inner layer as polycarbonate absorbs and disperses the bullet's energy.
• Glass-Clad Polycarbonate — It’s a hybrid structure combining layers of glass and polycarbonate. This composition leverages the hardness of glass and the flexibility of polycarbonate to provide robust protection against ballistic threats.
• Acrylic — Acrylic is a transparent plastic material known as polymethyl methacrylate (PMMA). It’s used in some bullet-resistant applications as acrylic is lighter than glass and can stop certain types of bullets. However, it may not offer the same level of protection as laminated glass or polycarbonate composites.

These materials are tested and certified according to industry standards such as the Underwriters Laboratories (UL) UL 752 rating system.

What Does the Manufacturing Process of Bulletproof Glass Look Like?

The production of bullet-resistant glass involves a strict lamination process:

5. Layer preparation — Glass and polycarbonate layers are cut to the required dimensions, while interlayer materials (PVB or EVA) are prepared to match the size of the glass and polycarbonate layers.
6. Assembly — The layers are stacked in a specific sequence, starting with a glass layer, followed by an interlayer, and then a polycarbonate or additional glass layer, depending on the design specifications.
7. Lamination —The assembled materials are subjected to heat and pressure in an autoclave. The lamination process bonds the layers together, eliminating air pockets and ensuring optical clarity.
8. Cooling and inspection — The laminated glass is gradually cooled to room temperature. It undergoes inspections for any defects, such as bubbles or delamination.

Limitations of Bulletproof Glass

While commonly referred to as "bulletproof," no glass is entirely bulletproof to ballistic threats. The correct term is "bullet-resistant" because such glass is designed to withstand specific threat levels based on factors such as firearm type, bullet caliber, and impact velocity.

Two factors can affect the performance of bullet-resistant glass:

7. Thickness and weight — Higher protection levels require thicker and heavier glass, which can pose a challenge in terms of structural support and installation.
8. Decreased visibility — As bullet-resistant glass becomes thicker to offer higher protection levels, it can lead to decreased optical clarity.

Bulletproof Glass Final Thoughts

Bulletproof glass or ballistic glass, or "bullet-resistant glass," is created by layering materials such as tempered glass, polycarbonate, and PVB.

Upon impact, the hard outer layer deforms the bullet, increasing its surface area and reducing its velocity. Meanwhile, the softer inner layers absorb and dissipate the remaining kinetic energy, preventing penetration.

There are several use cases of bullet-resistant glass, from financial institutions, government buildings, and military vehicles to retail stores and personal vehicles.

It's important to note that no glass is entirely bulletproof; its effectiveness depends on factors such as thickness, weight, and the specific threat level it is designed to resist. Thicker glass offers higher protection but can lead to increased weight and decreased visibility.