Speaker 1: Welcome to brain Stuff from how stuff works, Hey, brain Stuff,
Speaker 1: Lorn vogelbom here. Bulletproof vests have saved plenty of lives,
Speaker 1: but this gear isn't perfectly well bulletproof in all situations.
Speaker 1: So exactly how well do they work? And do bulletproof
Speaker 1: vests degrade over time? Can they fail? Let's look at
Speaker 1: a bit of history. Early bulletproof vests were mainly made
Speaker 1: from poly paraphenneline tera thalamide, better known as kevlar. It
Speaker 1: was invented by Stephanique Falek, a chemist working for DuPont
Speaker 1: in the early nineteen sixties. DuPont had already invented nylon
Speaker 1: and spandex and was on the hunt for the next
Speaker 1: generation of high performance fibers. Synthetic fibers are created with polymers,
Speaker 1: large molecules composed of repeating subunits, effectively creating a linked
Speaker 1: chain of molecules that are more than the sum of
Speaker 1: their parts. What foalc discovered would eventually become kevlar and
Speaker 1: would see application and tires, racing sales and bulletproof vests.
Speaker 1: Bulletproof vests today are made from polyethylene fibers, a popular
Speaker 1: plastic polymer seen in nearly everything we use, grocery bags, toys,
Speaker 1: plastic trash bins, and so on. Polyethylene fibers, or pe
Speaker 1: as it's called, are woven in layers to form the
Speaker 1: guts of protective vests that are cheaper and stronger than kevlar.
Speaker 1: In nine, a company called Allied Signal developed a competitor
Speaker 1: for kevlar and called it Spectral Shield. Originally used for sailcloth,
Speaker 1: this polyethylene fiber is now used to make lighter, yet
Speaker 1: stronger material for using bulletproof vests alongside the traditional kevlar.
Speaker 1: According to a manufacturer of bulletproof vests, the layers inside
Speaker 1: a vest are designed to reduce the velocity of a
Speaker 1: round to a point below the speed of sound. At
Speaker 1: roughly this point, the fibers can have time to react
Speaker 1: to the bullet and exert their toughness, their strength, and
Speaker 1: how much they'll bend before they break. The more layers,
Speaker 1: or the greater the density of the weaving of those layers,
Speaker 1: the greater the toughness of the fibers, and the more
Speaker 1: capable they are of stopping a bullet. Pistols fire bullets
Speaker 1: at velocities that are just slightly above the speed of sound.
Speaker 1: A bullet from a nine millimeter, for example, travels around
Speaker 1: nine hundred and eighty feet per second. That's about three
Speaker 1: hundred and eighty meters per second. The speed of sound
Speaker 1: is around one thousand, one hundred and twenty feet or
Speaker 1: three hundred and forty per second. Thus, a good vest
Speaker 1: doesn't need to work too hard to bring a nine
Speaker 1: millimeter bullets velocity down to a point where it can
Speaker 1: make use of the toughness of its fibers, and slugs
Speaker 1: from shotguns travel even slower, so they're relatively easy to
Speaker 1: stop as well. A rifle bullet, however, is another matter,
Speaker 1: entirely depending on the grain and quality of the propellant.
Speaker 1: A bullet fired from an a R fifteen travels roughly
Speaker 1: three thousand, three hundred feet per second. That's about a
Speaker 1: thousand meters per second, which is nearly three times the
Speaker 1: speed of sound. It'll take a far greater density of
Speaker 1: ballistic fiber to stop that bullet. Most vests designed to
Speaker 1: stop rifle bullets are backed up with high density insert
Speaker 1: plates made from ceramics and polyethylene that break up a
Speaker 1: bullet into pieces and capture it. Vests without those plates
Speaker 1: would likely fail to capture a rifle bullet. The material
Speaker 1: of the vests themselves is also prone to break down
Speaker 1: for various reasons. Kevlar, for instance, degrades when exposed to
Speaker 1: salt such as salt and human sweat. The longer a
Speaker 1: person exerts themselves, while ena bulletproof vest made of kevlar
Speaker 1: to the tune of years, the greater the likelihood of failure.
Speaker 1: Vests made from polyethylene will also break down over time
Speaker 1: when exposed to UV light. Manufacturers say that there's about
Speaker 1: a five year lifespan on vests and caution against buying
Speaker 1: used vests unless you know the manufactured date and the
Speaker 1: materials used to make it. As with any product or
Speaker 1: idea or anything, really, we recommend always doing your research,
Speaker 1: especially if your life might be on the line. Today's
Speaker 1: episode was written by Jared w Alexander and produced by
Speaker 1: Tyler Klang. For more on this and lots of other
Speaker 1: tough topics, visit our home planet, how Stuff Works dot com.