Ask A Scientist© Chemistry Archive

name         Andrew
status       student
age          16

Question -   Ok, completed one quarter of chemistry in highschool, the
last thing that we covered was glass making. Glass made with Na breaks
easily, where as glass made with K is much stronger because of the whole
larger atoms and what not, but my questions is, what is bullet proof
glass made out of?
------------------------------------------------
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: 1. 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. 2. 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.

Vince Calder
=========================================================
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.  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.
Tim Spry
=========================================================