Wednesday, October 01, 2008

Homemade Bulletproof Glass

There are many approaches to building a clear material which can stop bullets. The simplest is just to use a tough material like polycarbonate plastic, and just make it thick enough to do the job. I've seen handgun bullets stopped by such material, very easily, and with little if any deformation to the sheet. However, the material must be incredibly thick to stop rifle bullets effectively, since it is tough but soft.  For rifle bullets, the sheets must be several inches or more thick. Polycarbonate is an expensive plastic.

A cheaper method is to layer sheets of clear materials to form a laminate with the polycarbonate. This cuts down on how much polycarbonate must be used. In this case we are trying to adopt the useful properties of each material into a final product. For example, something that is hard and tough.

Examples of laminate configurations that could work to stop a bullet:

An acrylic / polycarbonate laminate has the advantage of being lightweight. However, acrylic is not typically UV stable and scratches relatively easily. There are coatings available that can help prevent scratches; I have not tried any of them.

A glass / plastic laminate has the advantage of being very scratch resistance and UV stable. Glass is also much harder than any plastic, and much harder than the lead/copper that bullets are constructed from. It does, however weigh a considerable amount because glass is heavy compared to plastic.

I decided to build some test panels using acrylic and polycarbonate. 

Acrylic is a hard, relatively brittle material that tends to shatter. Polycarbonate is a relatively soft, pliable material that tends to deform. Gluing these two together will create a hard surface that is supported by the softer, tougher backing. When struck with a bullet, the acrylic will crack but remain together and the bullet's energy will be absorbed over a larger area by the polycarbonate. The same thickness of either material by itself will not work nearly as well, as the acrylic will shatter and disintegrate, whereas the polycarbonate will just just deform to let the bullet pass. 

I've glued two acrylic sheets together and then to a polycarbonate one as well. As you can see not all the clear Liquid Nails glue squeezed out toward the edges. Oh well, good for a first try. The result is a 7" x 5.5" x .26" laminate sheet of ballistic "glass". I'm pretty sure this will stop the .22 LR round fired from my pistol and rifle (100% sure actually), as well as a .38 special from a 2-inch barrel revolver. I built a second sheet identical to the one above but with better glue distribution. Liquid Nails adhesive is a little too fast drying and thick to work really well for making ballistic sheets. Clear polyester casting resin should work MUCH better.

I third sheet I built is only 3.5" x 2.75" but over 1/2" thick. It has four layers of plexiglas and two of lexan. The glue distribution is almost perfect, with only a small bubble here and there. I discovered that smaller sheets are WAY easier to manufacture without flaws. Many thin sheets makes sense for the acrylic portion, since more surface area is glued, and less material will be lost with each "ballistic incident". However a solid, thick peice of polycarbonate would be more ideal than multiple sheets in the laminate. Unfortunately, thicker lexan isn't sold at Home Depot.

Test and Results Analysis:

When fired at the threat side (acrylic first) the panel defeated one shot from a .380 Auto and one from a .38 special. Both were fired from around 10 feet using FMJ (ball) ammo. The bullets were not recovered. The acrylic spalled off from the front of the panel where struck, but otherwise held up. The panel has a radial stress fracture towards the middle of the sheet. But I suspect the panel could have taken more abuse. As you can see the liquid nails was not fully cured, there are little bubbles created by the impact where it separated the layers slightly. 

The small and thick panel failed because of poor adhesion, which ripped the polycarbonate layer off. I actually shot it with an AK round (7.62X39mm), curiously it did not penetrate the last layer, I think it just pushed it out of the way, there is no way something transparent and that thin can stop a round from an AK-47.

What I hadn't realized is that one of the sheets was backwards when I tested it, and I successfully made one-way bullet resistant glass (bullets will pass from polycarbonate side but will be defeated from the acrylic side. ) Unfortunately liquid nails does not cure without air, and was still liquid in between the layers on this panel, so the sheets delaminated when hit from non-threat side and thus were compromised (it subsequently failed from threat side), since there was a gap between the poly and acrylic at that point.That was hit four times, twice from non-threat side, and twice from threat side, with .22LR from a 3.5" barrel from about 10 feet. Clearly, the materials will NOT perform if they do not remain laminated.

What I learned was the adhesive was crucial - you have to use something that cures completely, like a thermoset (polyester or epoxy resin would work) but it also must be clear. Clear solvent glue made specifically for acrylic and lexan that may also work well, its made by 3M.

I think window glass would actually perform better in place of the acrylic, becuase it is much harder. It is also much cheaper. From what I have seen of using simple glass cutting tools such as a tungsten glass cutter and nibblers, it may also be easier to work with. Although the spall off the front would be much more dangerous; a spray of razor-sharp and relatively heavy glass shards is worse than particles of lightweight plastic. But it IS the threat side, so maybe it doesn't matter much. The only issue I could see with this is it might be difficult to glue to window glass, although you may not need to get a really strong bond. Half-cured liquid nails did the trick for the panel that worked.

More testing in the future, with a more suitable adhesive.


Anonymous said...

for bullet resistant glass try laminating window glass between two plastics? it would make it so you can glue plastic to plastic and possibly make a 2 way bullet resistant glass.

Jennifer Lawrence said...

One Way Bullet Proof Glass can resist physical attacks, ballistic impact and forced entry. It combines the scratch, heat and chemical resistance of a strengthened glass surface and the impact resistance of a polycarbonate core.

Anonymous said...

I do not understand because you do not use these materials to make your armor, because using fiberglass, the acrylic and polycarbonate are easily molded with heat to make armor.

scrallet smith said...

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Gunner Jacky said...

Nice information, you people can also take help from the other safety devices to make things really safer for you. You can also take help from the MA Gun License to get the guns of your choice and with proper legalities to make it more secure and keep yourself and your loved ones safer. But one thing must be kept in mind that they are mere safety devices not the killing machines.

Anonymous said...

I am really excited to see that bulletproof glass can be home made.

Unknown said...

I did not knew that bulletproof glass could be homemade but now I do thanks!

Iman said...

The problem of manufacturing larger panels is the amount of expansion and contraction of the Polycarbonate vs the glass.
As obvious the thermal expansion rate of glass and polycarbonate is different and when you intend to laminate them (in a commercial grade, where this is done by using autoclaves and heat ovens) the polycarbonate expands and then contracts and causes the full panel (glass/polycarbonate sandwich) to either crack or bend and curve.
Im still trying to find a cure for this, but any thoughts are welcome.