Bulletproof glass, also known as ballistic glass, is an extremely strong type of glass that is designed to withstand the impact of bullets and other projectiles. The thickness required for bulletproof glass depends on the caliber of weapon it needs to protect against. Generally, the thicker and more layers in the glass, the higher level of protection it provides.
What is bulletproof glass made of?
Bulletproof glass is constructed using multiple layers of glass and plastic laminates. The core of the glass is often regular annealed glass. This is then sandwiched between layers of plastic such as polycarbonate or acrylic.
The plastic interlayers absorb and dissipate the energy from a bullet impact, spreading the force through multiple layers of glass and preventing the bullet from fully penetrating the glass. The thicker each layer and the more total layers, the more energy absorption and bullet resistance the glass provides.
Common bulletproof glass construction
Some typical bulletproof glass constructions from lowest to highest protection level include:
| 2 layers of annealed glass laminated together with 1 plastic interlayer |
| 2 layers of annealed or tempered glass laminated with 2 or more plastic interlayers |
| 3 or more layers of glass laminated with multiple plastic interlayers |
| 3 or more layers of glass bonded together with polycarbonate or acrylic |
| Ballistic glass-clad polycarbonate sheet – thin glass bonded to thick polycarbonate |
The most basic bulletproof glass uses just 2 pieces of regular glass laminated together with a thin plastic layer in between. Higher protection levels stack more layers of glass and plastic.
How is bulletproof glass tested?
Bulletproof glass is tested by shooting it with projectiles from various weapons under controlled conditions. The glass must stop the bullets from fully penetrating to pass the test.
There are two main standards for rating bulletproof glass – UL 752 and NIJ Ballistic Resistance. These standards test the glass using ammunition from different types of firearms shot from set distances.
UL 752 Ratings
The UL 752 standard rates bulletproof glass on a 8-level scale from Level 1 (lowest) to Level 8 (highest). It tests with common handgun calibers:
| Level 1 – 9mm |
| Level 2 – .357 Magnum |
| Level 3 – .44 Magnum |
| Level 4 – .30 cal rifle |
| Level 5 – 7.62mm rifle |
| Level 6 – 9.3mm rifle |
| Level 7 – 5.56mm rifle |
| Level 8 – 7.62mm armor piercing rifle |
Higher levels are tested with higher caliber, higher velocity rifles at close range. Level 8 resists .30-06 armor piercing military ammunition.
NIJ Ballistic Ratings
The NIJ (National Institute of Justice) standard provides another widely used ballistics rating scale from NIJ Level I to IV. It tests with higher caliber rifles than the UL 752 standard:
| Level I – 9mm and .357 Magnum |
| Level IIA – 9mm and .357 Magnum |
| Level II – .357 Magnum and .44 Magnum |
| Level IIIA – .357 SIG and .44 Magnum |
| Level III – 7.62mm rifle |
| Level IV – .30 cal armor piercing rifle |
NIJ Level IV glass can withstand multiple shots from military-grade .30-06 caliber armor piercing rounds. This represents the highest practically useful small arms protection level for glass.
How thick does bulletproof glass need to be?
The thickness of ballistic glass required depends on the level of bullet resistance needed. Higher protection levels require thicker glass with more layers. Some general thickness guidelines:
| Handguns (NIJ Level I to IIIA): 0.5 to 1 inches thick |
| High powered rifles (NIJ Level III and IV): 1 to 3 inches thick |
| Military rifles (NIJ Level IV): 2 to 3 inches thick |
Minimum thickness examples
Here are some minimum thicknesses required to stop different caliber threats:
| 9mm handgun – 0.5 inches (13mm) |
| .357 Magnum revolver – 0.6 inches (15mm) |
| .44 Magnum revolver – 0.75 inches (19mm) |
| 7.62mm high-powered rifle – 1 inch (25mm) |
| .30-06 armor piercing rifle – 2.25 inches (57mm) |
These thickness examples assume high quality bulletproof glass using hardened glass layers and multiple plastic interlayers. More layers and thickness provides redundancy and margin of safety.
How many layers are in bulletproof glass?
The number of glass layers in bulletproof glass depends on the required protection level and overall thickness. More layers enhance bullet resistance.
Typically bulletproof glass will have 2 to 5 layers of glass laminated together with plastic interlayers:
| Basic protection: 2 to 3 glass layers |
| High protection: 3 to 5 glass layers |
| Military armor: 4 to 10 glass layers |
The interlayer material also affects bullet resistance. About 0.060 to 0.250 inches of viscoelastic plastic laminate between each layer of glass is common. Thinner interlayers save weight and cost while thicker ones enhance protection.
Example 5 layer bulletproof glass construction
– 0.250″ glass layer
– 0.125″ polycarbonate laminate
– 0.250″ glass layer
– 0.060″ polycarbonate laminate
– 0.250″ glass layer
– 0.125″ polycarbonate laminate
– 0.250″ glass layer
– 0.060″ polycarbonate laminate
– 0.250″ glass layer
This example 5 layer glass would provide roughly NIJ Level III protection capable of stopping high powered rifle rounds. The total thickness is around 1.5 inches.
What types of glass are used?
Bulletproof glass uses extremely hard, strong types of glass engineered to be bullet-resistant. Two main types are used:
– Tempered glass – Heat-treated for 3-5 times more strength than annealed glass. When shattered it breaks into small rounded cubes instead of sharp shards.
– Polycarbonate thermoplastic – Shatter-resistant with superior toughness and tear strength. Up to 250 times stronger than annealed glass.
Tempered glass provides hardness to resist penetration while polycarbonate plastic absorbs impact energy without breaking. Different constructions use different combinations of glass and polycarbonate layers bonded together.
Glass layer choices
Common glass layer options in bulletproof glass include:
| – Annealed glass |
| – Heat strengthened glass |
| – Tempered glass |
| – Chemically strengthened glass |
| – Borosilicate glass |
| – Polycarbonate |
| – Acrylic (PMMA) |
Hardened tempered and chemically strengthened glass provides the most bullet resistance. Polycarbonate and acrylic layers also add impact absorption.
How is bulletproof glass made?
Manufacturing bulletproof glass involves carefully laminating together glass and plastic layers at high pressures and temperatures. The main steps include:
1. Cut sheets of glass and plastic interlayers to size. Annealed, tempered, chemically strengthened or polycarbonate sheets are used.
2. Clean and polish glass surfaces so they bond together.
3. Apply a PVB or polyurethane interlayer film to the glass sheets.
4. Stack together the glass and plastic layers in a hydraulic press.
5. Heat under pressure to initially bond the layers.
6. Autoclave at high pressure and temperature to fully fuse the laminate.
7. Inspect the finished bulletproof glass sheets.
8. Frame or encase the glass as needed for the final application.
The glass and interlayers fuse together into an optically clear composite optimized for bullet resistance. Higher temperature and pressure produces better adhesion between the laminate layers.
Where is bulletproof glass used?
Bulletproof glass has many security applications including:
– Cashier booths at banks, gas stations, retail stores
– Airport luggage screening points and boarding gates
– Bank teller windows
– Jewelry store display cases
– Ticket booths at events
– Executive limousines and armored vehicles
– Military and police vehicles
– Riot shields carried by police
It provides transparent protection from potential gunfire attacks in public locations. Bulletproof glass is also popular in private homes among security conscious individuals. Specially designed bulletproof doors, windows and walls help keep intruders out.
Vehicles
Many armored cars and military vehicles have windows made of thick ballistic glass or acrylic. This protects the occupants from high caliber gunfire while maintaining visibility.
Banks
Tellers at banks are kept safe from armed robbers by bulletproof glass partitions. The bandit barriers provide clear face-to-face interaction while stopping potential bullets from handguns and shotguns.
Businesses
Convenience stores, gas stations, and other retail businesses use bulletproof glass enclosures to protect cashiers from violent attacks. The cashier can safely conduct transactions without being exposed.
Bulletproof glass thickness examples
Here are some example bulletproof glass thicknesses for different applications:
Store front windows and doors
– Withstand medium caliber handgun bullets
– 0.55 to 0.75 inches thick
– 2 to 3 layers of glass and plastic
Bank teller windows
– Withstand magnum handgun and shotgun bullets
– 0.8 to 1 inches thick
– 3 layers of glass laminated together
Armored limousines
– Stop high powered rifle rounds
– 1.25 to 1.5 inches thick
– At least 4 layers of glass and polycarbonate
Military vehicles
– Stop armor piercing rifle bullets
– 2 to 6 inches thick
– Multiple bonded layers of ballistic glass-clad polycarbonate
The actual thickness and construction depends on the desired protection level. More layers and thickness provides redundancy and greater bullet stopping power.
How much does bulletproof glass cost?
Bulletproof glass costs between $25 and $100 per square foot on average. The exact price depends on:
– Protection level – Higher rating costs more
– Thickness – From 0.5 to 6 inches thick
– Type – Glass, acrylic, polycarbonate layers
– Layering – More layers increases cost
– Size – Bulk discounts on large orders
– Frame – Basic or custom framing
Basic NIJ Level II glass starts around $30 per square foot. High protection NIJ Level IV glass costs $70 to $100 per square foot. Installation and custom framing also adds to the total project cost.
Bulletproof glass pricing factors
– Raw materials – Specialty glass and plastics
– Energy costs – High pressure and temperature production
– Labor – Skilled technicians hand assemble
– Volume discounts – Bulk orders are cheaper per square foot
– Customization – Unique shapes, tinting, patterning
– Shipping – Heavy, fragile material
Economies of scale make bulletproof glass more affordable. Small batches are very expensive. The exact cost depends on the manufacturer and reseller margins as well.
Conclusion
The thickness of bulletproof glass required depends primarily on the level of ballistic protection needed. Handgun resistance requires glass 0.5 to 1 inch thick. High powered rifle protection needs 1 to 3 inch thick glass. And stopping military-grade armor piercing rounds requires extremely thick laminated glass composites 2 to 6 inches thick. More layers of glass and plastic also increase bullet resistance. Through advanced manufacturing techniques, bulletproof glass can be engineered to stop almost any caliber of bullet while allowing visibility. This life-saving technology protects people against potential attacks in many public and private settings.
