1 00:00:02,040 --> 00:00:07,000 Speaker 1: Welcome to brain Stuff from how stuff works, Hey, brain Stuff, 2 00:00:07,080 --> 00:00:10,920 Speaker 1: Lorn vogelbom here. Bulletproof vests have saved plenty of lives, 3 00:00:11,039 --> 00:00:15,480 Speaker 1: but this gear isn't perfectly well bulletproof in all situations. 4 00:00:15,520 --> 00:00:18,880 Speaker 1: So exactly how well do they work? And do bulletproof 5 00:00:18,960 --> 00:00:22,759 Speaker 1: vests degrade over time? Can they fail? Let's look at 6 00:00:22,800 --> 00:00:25,680 Speaker 1: a bit of history. Early bulletproof vests were mainly made 7 00:00:25,720 --> 00:00:30,319 Speaker 1: from poly paraphenneline tera thalamide, better known as kevlar. It 8 00:00:30,400 --> 00:00:33,760 Speaker 1: was invented by Stephanique Falek, a chemist working for DuPont 9 00:00:33,800 --> 00:00:37,720 Speaker 1: in the early nineteen sixties. DuPont had already invented nylon 10 00:00:37,840 --> 00:00:39,800 Speaker 1: and spandex and was on the hunt for the next 11 00:00:39,800 --> 00:00:44,200 Speaker 1: generation of high performance fibers. Synthetic fibers are created with polymers, 12 00:00:44,440 --> 00:00:48,680 Speaker 1: large molecules composed of repeating subunits, effectively creating a linked 13 00:00:48,760 --> 00:00:50,800 Speaker 1: chain of molecules that are more than the sum of 14 00:00:50,800 --> 00:00:54,800 Speaker 1: their parts. What foalc discovered would eventually become kevlar and 15 00:00:54,800 --> 00:00:58,600 Speaker 1: would see application and tires, racing sales and bulletproof vests. 16 00:00:59,240 --> 00:01:03,200 Speaker 1: Bulletproof vests today are made from polyethylene fibers, a popular 17 00:01:03,240 --> 00:01:07,240 Speaker 1: plastic polymer seen in nearly everything we use, grocery bags, toys, 18 00:01:07,319 --> 00:01:11,399 Speaker 1: plastic trash bins, and so on. Polyethylene fibers, or pe 19 00:01:11,440 --> 00:01:14,040 Speaker 1: as it's called, are woven in layers to form the 20 00:01:14,080 --> 00:01:17,480 Speaker 1: guts of protective vests that are cheaper and stronger than kevlar. 21 00:01:18,080 --> 00:01:21,880 Speaker 1: In nine, a company called Allied Signal developed a competitor 22 00:01:21,920 --> 00:01:25,800 Speaker 1: for kevlar and called it Spectral Shield. Originally used for sailcloth, 23 00:01:26,000 --> 00:01:29,000 Speaker 1: this polyethylene fiber is now used to make lighter, yet 24 00:01:29,040 --> 00:01:33,480 Speaker 1: stronger material for using bulletproof vests alongside the traditional kevlar. 25 00:01:34,200 --> 00:01:37,840 Speaker 1: According to a manufacturer of bulletproof vests, the layers inside 26 00:01:37,840 --> 00:01:40,320 Speaker 1: a vest are designed to reduce the velocity of a 27 00:01:40,400 --> 00:01:43,040 Speaker 1: round to a point below the speed of sound. At 28 00:01:43,120 --> 00:01:46,040 Speaker 1: roughly this point, the fibers can have time to react 29 00:01:46,080 --> 00:01:49,080 Speaker 1: to the bullet and exert their toughness, their strength, and 30 00:01:49,080 --> 00:01:52,120 Speaker 1: how much they'll bend before they break. The more layers, 31 00:01:52,200 --> 00:01:54,680 Speaker 1: or the greater the density of the weaving of those layers, 32 00:01:54,880 --> 00:01:57,440 Speaker 1: the greater the toughness of the fibers, and the more 33 00:01:57,480 --> 00:02:01,279 Speaker 1: capable they are of stopping a bullet. Pistols fire bullets 34 00:02:01,280 --> 00:02:04,200 Speaker 1: at velocities that are just slightly above the speed of sound. 35 00:02:04,720 --> 00:02:07,320 Speaker 1: A bullet from a nine millimeter, for example, travels around 36 00:02:07,440 --> 00:02:09,720 Speaker 1: nine hundred and eighty feet per second. That's about three 37 00:02:09,800 --> 00:02:12,440 Speaker 1: hundred and eighty meters per second. The speed of sound 38 00:02:12,480 --> 00:02:14,760 Speaker 1: is around one thousand, one hundred and twenty feet or 39 00:02:14,840 --> 00:02:18,240 Speaker 1: three hundred and forty per second. Thus, a good vest 40 00:02:18,320 --> 00:02:20,160 Speaker 1: doesn't need to work too hard to bring a nine 41 00:02:20,200 --> 00:02:22,880 Speaker 1: millimeter bullets velocity down to a point where it can 42 00:02:22,919 --> 00:02:25,960 Speaker 1: make use of the toughness of its fibers, and slugs 43 00:02:25,960 --> 00:02:29,160 Speaker 1: from shotguns travel even slower, so they're relatively easy to 44 00:02:29,160 --> 00:02:33,000 Speaker 1: stop as well. A rifle bullet, however, is another matter, 45 00:02:33,240 --> 00:02:36,600 Speaker 1: entirely depending on the grain and quality of the propellant. 46 00:02:36,760 --> 00:02:39,359 Speaker 1: A bullet fired from an a R fifteen travels roughly 47 00:02:39,440 --> 00:02:42,040 Speaker 1: three thousand, three hundred feet per second. That's about a 48 00:02:42,080 --> 00:02:44,840 Speaker 1: thousand meters per second, which is nearly three times the 49 00:02:44,880 --> 00:02:47,760 Speaker 1: speed of sound. It'll take a far greater density of 50 00:02:47,760 --> 00:02:51,959 Speaker 1: ballistic fiber to stop that bullet. Most vests designed to 51 00:02:51,960 --> 00:02:54,760 Speaker 1: stop rifle bullets are backed up with high density insert 52 00:02:54,800 --> 00:02:58,079 Speaker 1: plates made from ceramics and polyethylene that break up a 53 00:02:58,120 --> 00:03:01,560 Speaker 1: bullet into pieces and capture it. Vests without those plates 54 00:03:01,560 --> 00:03:05,000 Speaker 1: would likely fail to capture a rifle bullet. The material 55 00:03:05,080 --> 00:03:07,640 Speaker 1: of the vests themselves is also prone to break down 56 00:03:07,680 --> 00:03:11,080 Speaker 1: for various reasons. Kevlar, for instance, degrades when exposed to 57 00:03:11,120 --> 00:03:14,080 Speaker 1: salt such as salt and human sweat. The longer a 58 00:03:14,120 --> 00:03:17,200 Speaker 1: person exerts themselves, while ena bulletproof vest made of kevlar 59 00:03:17,520 --> 00:03:20,640 Speaker 1: to the tune of years, the greater the likelihood of failure. 60 00:03:21,560 --> 00:03:24,320 Speaker 1: Vests made from polyethylene will also break down over time 61 00:03:24,360 --> 00:03:28,360 Speaker 1: when exposed to UV light. Manufacturers say that there's about 62 00:03:28,400 --> 00:03:31,400 Speaker 1: a five year lifespan on vests and caution against buying 63 00:03:31,480 --> 00:03:33,799 Speaker 1: used vests unless you know the manufactured date and the 64 00:03:33,919 --> 00:03:37,120 Speaker 1: materials used to make it. As with any product or 65 00:03:37,320 --> 00:03:41,040 Speaker 1: idea or anything, really, we recommend always doing your research, 66 00:03:41,240 --> 00:03:48,560 Speaker 1: especially if your life might be on the line. Today's 67 00:03:48,560 --> 00:03:51,280 Speaker 1: episode was written by Jared w Alexander and produced by 68 00:03:51,280 --> 00:03:53,560 Speaker 1: Tyler Klang. For more on this and lots of other 69 00:03:53,600 --> 00:04:07,840 Speaker 1: tough topics, visit our home planet, how Stuff Works dot com.