How do bulletproof vests work? - Max G. Levy
By 1975, Richard Davis had been shot
in the chest at close range 192 times.
But not only was he completely healthy,
each of these bullets had been shot
by Davis himself
as part of a demonstration
to sell his new product:
the bulletproof vest.
Playing with firearms
is always a bad idea,
but after testing his design
on empty vests,
Davis became convinced that taking
a bullet himself was the only way
to prove the vest’s efficacy.
And when people saw Davis walk away
with just some stinging pain
a minor cut,
they may have stopped
questioning his sanity
and started wondering how such
a light, flexible piece of clothing
could stop a bullet.
The secret was in the material:
a synthetic fiber invented
a decade earlier
by a material chemist
named Stephanie Kwolek.
Her employers at DuPont had found
huge success with nylon,
the world's first synthetic fiber,
and they wanted Kwolek to create
something even stronger
they could use to mass produce
durable, lightweight tires.
Like all synthetic fibers,
nylon is a polymer:
a long chain of repeating molecules,
or monomers.
While some polymers repeat
the same monomer over and over,
others chain multiple monomers
in a steady pattern.
It’s these two variables—
which molecules are involved
and how they bond to one another—
that give each polymer
its unique properties.
So, seeking to build on the
strengths of nylon,
Kwolek began a lengthy process
of trial and error,
combining various monomers in novel ways.
And one of these resulting polymers was
immediately very weird.
Named Kevlar, this alternating blend
of 1,4-phenylene-diamine
and terephthaloyl chloride combine
at the molecular level
to form a series of parallel chains.
At rest, these chains align
in strict rows,
giving the polymer order
and crystalline strength.
But when pressure is applied,
the chains wriggle around,
allowing the material to flow
like a liquid.
This so-called liquid crystal polymer
was unprecedented,
and when Kwolek’s team spun
the viscous fluid into a fiber,
the results were better than
they could have hoped.
Not only were the fibers flexible
and resistant to heat, acid,
and various chemicals,
when woven together,
they were also stronger than steel.
Metals are incredibly sturdy because
of their unique atomic bonds.
Where non-metal molecules are typically
held together by the attraction
between a nucleus and a set number
of electrons,
metal nuclei are surrounded
by a sea of shared electrons.
It takes a ton of energy to overcome
the strength and resiliency
of these countless bonds.
So when a bullet hits a steel plate,
the material can usually absorb
all the impact’s energy
before the metal is pierced.
Compare this to a bullet hitting wood.
The bonds holding wood together require
much less energy to break,
which is why bullets can travel much
further through wood than metal.
Kevlar’s atomic bonds are also weaker
than metal’s.
But it compensates with a huge number
of hydrogen bonds.
While not as strong as the atomic bonds
within molecules,
the attraction of hydrogen atoms
and oxygen atoms between molecules
also requires a huge amount of energy
to overcome.
And when threads of Kevlar’s polymer
chains are woven into fabric,
this strength is multiplied.
When a bullet hits Kevlar, the mesh
of highly aligned, liquid-like chains
absorb huge amounts of energy,
wiggling wildly while still clinging
to their neighboring chains
via hydrogen bonds.
And even if a bullet does have enough
energy to penetrate the Kevlar,
it would be moving considerably slower
with much less destructive force.
Of course, Kevlar is not
immune to everything.
Strong forces can still be felt
through the fabric,
and its fibers gradually lose strength
under ultraviolet light.
Additionally, new liquid crystal fibers
hold up better against acid.
But Kwolek’s invention remains
one of the most versatile
and widely used materials on Earth.
Today, companies rely on Kevlar’s
lightweight impact resistance
and durability in helmets, kayaks,
spacecraft, and automobiles.
Speakers sometimes use Kevlar because
it can push air efficiently
and quickly come to a dead stop
when you pause your music.
And yes, it also makes excellent tires.
