Bullet-proof glass can be just extra thick, extra strong glass, but is more often a sandwich of different glasses and plastics. There is no specific recipe or formula: there are many different kinds of "bullet-proof glass".
Strength
Glasses where potassium is substituted for sodium tend to be harder due to higher melting temperatures. This does not necessarily mean stronger! Harder materials tend to break more easily due to less flexibility, when equal geometries are compared under equal conditions. The annealing also makes a difference. For "bulletproof" glass, a softer glass would be more useful in some places than a harder glass.
Thickness
The biggest trick to "bulletproof" glass is its thickness. Picture frame glass can be sufficiently strong at 1/16" thick. Window glass panes need to be thicker (to withstand wind gusts), on the order of 3/32", or even 1/8" for some areas. Door glass (needs to withstand the jolt from frequent closings) often runs 3/16" or 1/4", and may be cast (floated) with metal wire reinforcement. As you can see, the pattern shows strength to be more a function of thickness than of composition. Expect "bulletproof" glass to be 3/4" to 3" thick, depending on the application, and likely angled to help deflect bullets.
Composition
Bullet proof glass, and plastic analogs, are made of composite phases, not a single pure composition. The phases are sometimes laminated [sandwiched] or other times "globs" of one phase are suspended in the other. Two requirements are necessary:
- One phase is hard and the other is rubbery. This allows the impact energy of the projectile to be absorbed before stresses build up so high that the pane cracks.
- The second requirement is that the index of refraction of the two phase compositions must be nearly identical so that the composite remains visibly transparent.
Because the synthetic options in organic polymers is greater than inorganic glasses, most "bullet proof" glasses are indeed organic.
Source: Argonne National Laboratory
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