Although people flip-flop interchangeably between the phrases “bulletproof glass” and “bullet-resistant glass,” they tend to be talking about the same thing: Bullet-resistant windows are primarily made of extremely durable, optically clear plastic. In the security industry, we say “bullet-resistant,” because there really is no such thing as truly “bulletproof” window materials. Given enough time and bullets of a large enough caliber, a sufficiently determined attacker will eventually be able to penetrate almost any commercially available window material.
What is Bulletproof Glass Made Of?
Broadly speaking, there are two types of solid (or “monolithic”) acrylic and laminated polycarbonate.
Monolithic acrylic is the most basic form of modern bulletproof glass. Just as its name suggests, it is a solid sheet of acrylic thermoplastic. These are either 1-1/4″ or 1-3/8″ thick, depending on the level of protection required.
Laminated polycarbonate is both more complex and more popular. Laminated polycarbonate is manufactured by layering materials together in order to leverage their different strengths. There are always one or more polycarbonate layers, and may also be layers of acrylic or glass. Between each layer is thin urethane or polyvinyl butyral (PVB) membrane. By carefully selecting which materials to layer in what way, glass manufacturers can make laminated bullet-resistant windows with a wide range of properties.
Laminated polycarbonate windows range in size: ¾”, 1″ or 1-1/4″ thick depending on the level of protection. When a window has glass “caps” as the surface material, with layers of thicker polycarbonate on the interior, it’s usually called “glass-clad polycarbonate.” Glass-clad polycarbonate laminated windows may be as thick as 2-1/2″ and able to resist sustained fire from tactical assault rifles and similar high-powered rifles.
Unique Bulletproof Glass Properties
The manufacturer starts with large sheets of 1/4″ to 1/2” glass, acrylic, or polycarbonate. They clean these, then layer them to specification in a sterile environment, alternating each layer with thin sheets of urethane or PVB membrane just a few thousandths of an inch thick. Without this flexible membrane interlayer, it would be impossible to bond together different types of glass and plastic. The manufacturer then processes this glass/acrylic/polycarbonate/polyurethane sandwich overnight under high temperature and high pressure, fusing the layers together.
Because manufacturers are already layering materials to create laminated polycarbonate, they can mix-n-match them to accentuate different properties. These different properties can be fairly mundane, resulting in tinted or mirrored windows. Or they can be fairly unusual. For example, one newer form of is one-way bulletproof glass, commonly used in armored vehicles.
How Does Bulletproof Glass Work?
A modern bullet is a very small, very light, very smooth object moving very fast. When it strikes a barrier, that barrier must absorb the bullet’s momentum, or the bullet will crack straight through.
While a bullet will pierce the exterior layer of a laminated security window, the inner layers of polycarbonate absorb the bullet’s energy. This stops the bullet from penetrating through the entire piece of glass. The “force absorbing” qualities of the polycarbonate also makes it extremely resistant to forced entry, explosive blasts, and extreme weather.
One-way bulletproof glass has two layers. The outside layer—on the threat side—is made of brittle glass and the inside layer is a flexible polycarbonate. A bullet that strikes the brittle external layer first causes the glass to break inward toward the polycarbonate layer. The glass breaking absorbs some of the bullet’s energy by spreading the force over a larger area and the flexible polycarbonate stops the bullet.
Bullets shot from the inside of an armored car can penetrate the bullet-resistant glass because they strike the softer polycarbonate layer first with more focused energy. The brittle glass layer then breaks outward, away from the bullet, allowing it to pass through with minor energy loss.
The following video shows three slow-motion examples of glass being shot. The first segment features standard plate glass (the glass used in most home windows). The second shows tempered glass (used in automobile passenger windows and as part of some bulletproof systems). And, finally, you’ll see a piece of bullet-resistant acrylic (which the host refers to as “bulletproof glass.”)
In terms of basic ballistics, firing a gun at a sheet of plate glass is very instructive. A 9mm bullet has a mass of around 7.5 grams (about the same as a freshly minted penny or nickel). These 7.5 grams of lead are moving at an average of 1,234 feet per second (i.e., 841 mph). And these aren’t just blobs or balls flying through space. They are smooth cones, designed to efficiently cut through a resisting fluid (in this case air), keeping a straight path and retaining momentum as they go. When it comes into contact with a brittle, rigid surface—like plate glass—it transfers very little of its momentum to the surface and instead pops through, like the little straw popping into a juice box.
Shooting Tempered Glass
Tempered glass is four or five times stronger than plate glass. The piece of tempered glass you see shot in the video clearly isn’t “bullet-resistant” in any way. But the earliest “bulletproof” windows were indeed made from stacks of laminated glass. Even today, manufacturers will occasionally use tempered glass in bullet-resistant systems.
As you’ll see at 1:15 in the video above (and at the reduced speed at 1:29), the tempered glass breaks very differently from plate glass. With plate glass, the light, fast bullet popped right through. With tempered glass, it utterly destroys the tempered glass, reducing it to many small cubes.
This is by design: Plate glass tends to break into large dagger-like shards. In a storm, accident, or explosion, large shards of glass can be more dangerous than whatever caused the window to break in the first place. Conversely, tempered glass is less likely to break to begin with, and when it does, the numerous little glass nuggets of shattered tempered glass are unlikely to cause loss of life or limb.
These two characteristics—strength and shattering to particles—are why this sort of glass is called “safety glass” in many industries. You’ll see it in public entryways, tabletops, display windows, exterior windows on tall buildings, and in the passenger windows of cars.
The Physics of Tempered Bullet-Resistant Glass
There is some neat physics behind this. The tempered glass begins its life as standard plate glass. Glass manufacturers cut it to size—it can’t be cut after tempering—and then precisely heat and cool it in a specialized annealing furnace. The surface of the glass naturally cools faster, compressing the still-molten interior. As the interior cools, it pulls against this hardened compressive envelope. This balance of compressive (squeezing) and tensile (pulling) forces makes the glass very strong. But when the surface is broken, these stresses are knocked out of balance, the crystalline structure collapses, and the glass crumbles. (For a little historical tempered-glass trivia, check out this earlier blog post about Prince Rupert’s Drops.)
These balanced stresses mean that a sheet of tempered glass eats up more of a bullet’s momentum upon impact. Early WWII-era bullet-resistant glass was made by laminating together layers of tempered glass; a bullet might shatter the first couple layers of this “glass sandwich,” but that shattering would slow the bullet enough to keep it from cracking through the barrier.
Shooting Bullet-Resistant Acrylic
At the 1:52 mark, we finally see a legitimate piece of bullet-resistant glass get shot. This is a sheet of monolithic acrylic (the shot is repeated in slow-motion at 2:16). You’ll note that this true bullet-resistant glass behaves very differently from either of the other kinds of glass. The thick, solid acrylic stops the bullet in its tracks.
Sharp-eyed viewers will see the acrylic jump back a little. Since the bullet slams into the acrylic—rather than popping through—the bullet-resistant glass is obliged to absorb all of the bullet’s forward momentum. And since the shot is from such close range and dead-on, the bullet doesn’t even ricochet. Instead, its energy is converted to heat by the impact, and the bullet melts. The boiling lead then spatters away as tiny droplets, which splash across the acrylic, marring its surface. (In this section of the one-million frame-per-second super-slow-mo footage you can see bullets actually boil and shatter as they hit lead blocks).
Shooting Bullet-Resistant Polycarbonate
The other popular bullet-resistant “glass”, vital to higher-level bulletproof systems, is polycarbonate. In this video, Adam Savage and Jamie Hyneman of Mythbusters explore common “bulletproof” myths (e.g., Can a deck of cards stop a .22? A lighter? A Bible?), including their own perception that the quarter-inch polycarbonate they use in their blast shields is bulletproof (SPOILER ALERT: it isn’t).
In the last half of the video, where they shoot at an actual Level 3 bullet-resistant polycarbonate box. This starts around the 5:00 mark. At 5:35 they’ve shot the box with a .357 Magnum. The polycarbonate wall of the box encases the bullet, rather than permitting it to ricochet. That’s because polycarbonate is significantly softer than acrylic. It allows the bullet to push underneath the surface. The polycarbonate behaves like a very thick fluid offering a tremendous amount of resistance, sapping the bullet of its energy.
Having stopped a .22, .357 Magnum, and .44 Magnum, the MythBusters pit their Level 3 bullet-resistant glass box against a .30-06 at 6:32. Level 3 bullet-resistant glass is rated to resist several shots from a 9mm, .357, or .44–and can often even stop bullets from an M16 or AK-47. But you need Level 4 rated glass to stop a high-powered rifle. And as you see in the video, that .30-06 bullet pops through the Level 3 polycarbonate just as easily as the .38 popped through the plate glass in the first video.
“[It] didn’t even jump” marvels host Jamie Hyneman. This indicates that almost none of the bullet’s momentum was transferred to the bullet-resistant box. Because the box couldn’t absorb the forward energy, the bullet cracked straight through.