How bulletproof glass works
"Bulletproof" glass is very different to ordinary glass. More correctly called bullet-resistant glass (because no glass is totally bulletproof), it's made from multiple layers of tough glass with "interlayers" of various plastics. Sometimes, there's a final inner layer of polycarbonate (a tough type of plastic) or plastic film to prevent "spalling" (where dangerous shards of glass or plastic splinter off following the impact of a bullet). This sandwich of layers is called a laminate. It can be up to ten times thicker than a single pane of ordinary glass and it's usually very heavy.
When a bullet strikes bulletproof glass, its energy spreads out sideways through the layers. Because the energy is divided between a number of different pieces of glass and plastic, and spread over a large area, it is quickly absorbed. The bullet slows down so much that it no longer has enough energy to pierce through—or to do much damage if it does so. Although the glass panes do break, the plastic layers stop them flying apart. Think of bulletproof glass as "energy-absorbing" glass and you'll have a good idea how it works.
How do you make bulletproof glass?
Traditional bulletproof glass is made from alternating layers of glass (typically 3–10mm or ⅛–⅜in) and plastic, where the plastic is simply a thin film of polyvinyl butyral (PVB) (around 1–3mm or 30–90 mils thick). Newer, stronger kinds of bulletproof glass use a sandwich of glass and plastic made of acrylic glass, ionoplast polymers (such as SentryGlas®), ethylene vinyl acetate, or polycarbonate, with the thick glass and plastic layers separated by thinner films of various plastics, such as PVB or polyurethane.
To make simple PVB-based bulletproof glass, the thin PVB film is sandwiched between the thicker glass to make a laminate, which is heated and compressed so the plastic melts and begins to bond to the glass. Often this process happens in a vacuum to prevent air becoming trapped between the layers, which makes the laminate weaker and affects its optical properties (distorting the light that passes through). The unit is then fully "cooked" at a much higher temperature (up to about 150°C or 300°F) and pressure (up to about 13–14 times normal atmospheric pressure) in an autoclave (a kind of industrial pressure cooker). The main difficulty with the process is ensuring that the plastic and glass layers stick together properly with no air trapped between them, and ensuring the autoclave's heat and pressure doesn't distort the plastic so it becomes difficult to see through. (You can read more about the manufacturing process in US Patent: 5,445,890, listed in full in the references below.)