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 and 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 that 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.