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Speaker 1: Welcome to brain Stuff, a production of iHeartRadio. Hey brain Stuff,

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Speaker 1: Lauren vogelbomb here with a classic episode from our archives.

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Speaker 1: Today's gets into the science and history behind bulletproof vests,

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Speaker 1: how they work and when they don't work so well,

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Speaker 1: Hey brain Stuff, Lauren Vogel bomb here. Bulletproof veests have

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Speaker 1: saved plenty of lives, but this gear isn't perfectly well

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Speaker 1: bulletproof in all situations.

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Speaker 2: So exactly how well do they work? And do bulletproof

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Speaker 2: vests degrade over time? Can they fail? Let's look at

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Speaker 2: a bit of history. Early bulletproof vests were mainly made

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Speaker 2: from polyperiphenline tarphalamide, better known as kevlar. It was invented

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Speaker 2: by Stephanique Follack, a chemist working for DuPont in the

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Speaker 2: early nineteen sixties. DuPont had already invented nylon and spandex,

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Speaker 2: and was on the hunt for the next generation of

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Speaker 2: high performance fibers. Synthetic fibers are created with polymers molecules

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Speaker 2: composed of repeating subunits, effectively creating a linked chain of

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Speaker 2: molecules that are more than the sum of their parts.

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Speaker 2: What Kolk discovered would eventually become kevlar and would see

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Speaker 2: application in tires, racing sales, and bulletproof vests. Bulletproof vests

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Speaker 2: today are made from polyethylene fibers, a popular plastic polymer

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Speaker 2: seen in nearly everything we use, grocery bags, toys, plastic

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Speaker 2: trash bins, and so on. Polyethylene fibers, or pe as

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Speaker 2: it's called, are woven in layers to form the guts

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Speaker 2: of protective vests that are cheaper and stronger than kevlar.

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Speaker 2: In nineteen eighty nine, a company called Allied Signal developed

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Speaker 2: a competitor for kevlar and called it Spectra Shield. Originally

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Speaker 2: used for sailcloth, this polyethylene fiber is now used to

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Speaker 2: make lighter, yet stronger material for use in bulletproof vests

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Speaker 2: alongside the traditional kevlar. According to a manufacturer of bulletproof vests,

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Speaker 2: the layers inside a vest are designed to reduce the

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Speaker 2: velocity of a round to a point below the speed

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Speaker 2: of sound. At roughly this point, the fibers can have

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Speaker 2: time to react to the bullet and exert their toughness,

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Speaker 2: of their strength, and how much they'll bend before they break.

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Speaker 2: The more layers, or the greater the density of the

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Speaker 2: weaving of those layers, the greater the toughness of the fibers,

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Speaker 2: and the more capable they are of stopping a bullet.

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Speaker 2: Most pistols fire bullets at velocities that are just slightly

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Speaker 2: above the speed of sound. A bullet from a nine millimeter,

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Speaker 2: for example, travels at around nine hundred and eighty feet

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Speaker 2: per second. That's about three hundred and eighty meters per second.

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Speaker 2: The speed of sound is around one thousand, one hundred

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Speaker 2: and twenty feet or three hundred and forty meters per second. Thus,

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Speaker 2: a good vest doesn't need to work too hard to

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Speaker 2: bring a nine millimeter bullets velocity down to a point

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Speaker 2: where it can make use of the toughness of its fibers,

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Speaker 2: and slugs from shotguns travel even slower, so they are

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Speaker 2: relatively easy to stop as well. A rifle bullet, however,

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Speaker 2: is another matter, entirely depending on the grain and quality

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Speaker 2: of the propellant. A bullet fired from an AR fifteen

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Speaker 2: travels roughly three thousand, three hundred feet per second. That's

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Speaker 2: about one thousand meters per second, which is nearly three

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Speaker 2: times the speed of sound. It'll take a far greater

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Speaker 2: density of ballistic fiber to stop that bullet. Most vests

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Speaker 2: designed to stop rifle bullets are backed up with high

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Speaker 2: density insert plates made from ceramics and polyethylene that break

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Speaker 2: up a bullet into pieces and capture it. Vests without

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Speaker 2: those plates would likely fail to capture a rifle bullet.

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Speaker 2: The material of the vests themselves is also prone to

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Speaker 2: break down for various reasons. Kevlar, for instance, degrades when

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Speaker 2: exposed to salt such as the salt and human sweat.

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Speaker 2: The longer a person exerts themselves while in a bullet

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Speaker 2: proof vest made of kevlar to the tune of years,

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Speaker 2: the greater the likelihood of failure. Vests made from polyethylene

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Speaker 2: will also break down over time when exposed to UV light.

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Speaker 2: Manufacturers say that there's about a five year lifespan on

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Speaker 2: vests and caution against buying used vests unless you know

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Speaker 2: the manufactured date and the materials used to make it.

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Speaker 2: As with any product or idea or anything, really, we

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Speaker 2: recommend always doing your research, especially if your life might

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Speaker 2: be on the line. Today's episode is based on the

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Speaker 2: article how bulletproof are bulletproof vests? On how stuffworks dot com,

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Speaker 2: written by Jared w Alexander. Brain Stuff is production of

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Speaker 2: iHeartRadio in partnership with how stuffworks dot Com and is

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Speaker 2: produced by Tyler Klang. Four more podcasts my heart Radio,

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Speaker 2: visit the iHeartRadio app, Apple Podcasts, or wherever you listen

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Speaker 2: to your favorite shows.