BULLET RESISTANT GLASS
Bullet proof glass manufacturers are quick to point out that “bullet proof glass” is a little bit of a misnomer, since the material is not “bullet proof,” and only occasionally “glass”. The common “bullet proof window” is just one element in a bullet resistant system, but it is also the most visible element, synonymous with the idea of “bullet proof security”.
Even today, glass is still a part of many high-level bullet resistant systems. But the bulk of the bullet-stopping materials used to secure a bank, pharmacy, convenience store, or other building are going to be layers of acrylic, polycarbonate, polyurethane or other resins, and fiberglass. Although these materials are “bullet resistant,” in that each is rated in terms of the amount of firepower it can stop, that’s not the same as “bullet proof.” Even a Level 8 rated sheet of glass-clad polycarbonate–which can weather a shotgun blast, a burst from an AK-47, or round after round from a common 9mm pistol–will eventually succumb to a motivated attacker with sufficient ammunition.
THE ORIGIN OF BULLET RESISTANT GLASS
The earliest bullet resistant glass was deployed on the battlefields of World War II. By stacking sheets of tempered glass and laminating them to each other with layers of epoxy, Army engineers could craft bullet resistant glass to use in bunker and battle vehicle windows. But these early bullet resistant glass windows were very unwieldy: Usually about four inches thick, weighing more than 50 pounds per square foot, and limited to being cut into small rectangular windows.
BULLET RESISTANT GLASS COMES TO BANKS
After the war, structural engineers began to consider other applications for bullet resistant glass, such as bank security. These pioneers were greatly aided by advancements in another war-time material technology: transparent acrylic. Acrylic was first discovered in the late 1800s. Useful transparent acrylic was patented in 1933, and soon after marketed for use in early non-shattering “safety glass” windows. During the war clear acrylic was used for submarine and airplane windows and canopies. These were not bullet resistant by any means, because early acrylic was quite brittle. But the post-war discovery of advanced plastic additives and fillers resulted in acrylic that is suitable for fabricating bullet resistant glass. This acrylic bullet resistant glass is thinner than the WWII-era laminated bullet resistant glass–about an inch for comparable stopping power–and considerably lighter, just eight pounds per square foot. As an added bonus, since this transparent acrylic bullet resistant glass is a single sheet of thermoplastic, rather than many layers of tempered glass, it can be milled, routed, drilled, and flame polished, making it a remarkably flexible and attractive building material.
MANY OPTIONS IN BULLET RESISTANT GLASS
While acrylic, with its relatively low cost and flexible material qualities, is the preferred bullet resistant glass in many applications, it’s not the only option. Monolithic acrylic is great for stopping shots from a 9mm, .357 Magnum, .44, or shotgun, but those who need security from something heavier–an AK-47, M16, or hunting rifle–will want layered polycarbonate (another softer thermoplastic, capable of catching and holding a bullet), or even glass-clad polycarbonate, which is composed of laminated layers of tempered glass and polycarbonate, and can stop many shots from an assault rifle, and even withstand hurricane-force winds.