by Chris Woodford. Last updated: May 18, 2014.
Nature has given us
some amazing materials. There's wood: a material so strong and
versatile you can use it for everything from making paper to building
houses. There's also wool, with insulation so effective it lets sheep
stand outside in the snow all winter. Or how about skin: a material
that will repair itself automatically and often completely invisibly
in only a matter of days? Truly incredible though these materials
are, they're far from perfect for every application,
especially in the modern world where the challenges we face are ones
nature could never have anticipated. That's why we now rely on
synthetic materials such as Kevlar®. It's a plastic strong enough to
stop bullets and knives—often described as being "five times
stronger than steel on an equal weight basis." It has many
other uses too, from making boats and bowstrings to reinforcing tires and
brake pads. Let's take a closer look at how it's made and what makes
it so tough!
Photo: A piece of Kevlar after being
hit by a projectile. You can see a dent (coming up toward the camera)—but you can't see a hole.
You might be bruised by this impact (or suffer what's called a "blunt trauma" injury), but you wouldn't die.
Picture courtesy of US Army.
What exactly is Kevlar?
Kevlar is one of those magic modern materials people talk about all the time without ever
really explaining any further. "It's made of Kevlar", they say,
with a knowing nod, as though that were all the explanation you
Kevlar is simply a super-strong plastic. If that sounds
unimpressive, remember that there are plastics—and there are plastics.
There are literally hundreds of synthetic plastics made by
polymerization (joining together long chain molecules) and they have
widely different properties. Kevlar's amazing properties are partly due to its internal structure (how its molecules
are naturally arranged in regular, parallel lines) and partly due to the way it's made into fibers that are
knitted tightly together.
Photo: Kevlar textiles get their properties partly from the inherent strength
of the polymer from which the fibers are made and partly from the way the fibers are knitted tightly together, as shown here
in a NASA ballistics test. Picture courtesy of NASA Glenn Research Center (NASA-GRC).
Kevlar is not like cotton—it's not something anyone can make from the right raw
materials. It's a proprietary material made only by the DuPont™
chemical company and it comes in two main varieties called Kevlar 29
and Kevlar 49 (other varieties are made for special applications). In
its chemical structure, it's very similar to another versatile
protective material called Nomex.
Kevlar and Nomex are examples of
chemicals called synthetic aromatic polyamides or aramids for short.
Calling Kevlar a synthetic aromatic polyamide polymer makes it sound unnecessarily complex.
Things start to make more sense if you consider that description one word at a time:
- Synthetic materials are made in a chemical laboratory
(unlike natural textiles such as cotton, which grows on plants, and
wool, which comes from animals).
- Aromatic means Kevlar's molecules have a strong, ring-like
structure like that of benzene.
- Polyamide means the ring-like aromatic molecules connect
together to form long chains. These run inside (and parallel to) the
fibers of Kevlar a bit
like the steel bars ("rebar") in reinforced concrete.
- Polymer means that Kevlar is made from many identical
molecules bonded together (each one of which is called a monomer). Plastics are the most familiar polymers
in our world. As we've seen,
the monomers in Kevlar are based on a modified, benzene-like ring
Like Nomex, Kevlar is a distant relative of nylon, the first commercially successful
"superpolyamide", developed by DuPont in the 1930s. Kevlar was introduced in 1971,
having been discovered in the early 1960s by chemist Stephanie Kwolek, who earned a patent for her invention with Paul Morgan in 1966.
What's so good about Kevlar?
Photo: Super-strong Kevlar is best known for its use in body armor.
Picture by Lcpl Joseph A. Stephens, USMC, courtesy of Defense Imagery.
These are some of Kevlar's properties:
- It's strong but relatively light. The specific tensile
strength (stretching or pulling strength) of both Kevlar 29 and Kevlar 49 is over eight times greater
than that of steel wire.
- Unlike most plastics it does not melt: it's
reasonably good at withstanding temperatures and decomposes only at
about 450°C (850°F).
- Unlike its sister material, Nomex, Kevlar can be ignited but burning
usually stops when the heat source is removed.
- Very low temperatures
have no effect on Kevlar: DuPont found "no embrittlement or
degradation" down to −196°C (−320°F).
- Like other plastics, long
exposure to ultraviolet light (in sunlight, for example) causes
discoloration and some degradation of the fibers in Kevlar.
- Kevlar can resist attacks from many different chemicals, though long exposure to strong
acids or bases will degrade it over time.
- In DuPont's tests, Kevlar remained "virtually unchanged" after exposure to hot
water for more than 200 days and its super-strong properties are
"virtually unaffected" by moisture.
And what's bad?
It's worth noting that Kevlar also has its drawbacks. In particular, although it has
very high tensile (pulling) strength, it has very poor compressive strength (resistance to
squashing or squeezing). That's why Kevlar isn't used instead of steel as a primary
building material in things like buildings,
bridges, and other structures where compressive forces are common.
How is Kevlar made?
There are two main stages involved in making Kevlar. First you have to produce the basic
plastic from which Kevlar is made (a chemical called poly-para-phenylene
terephthalamide—no wonder they call it Kevlar). Second, you have to
turn it into strong fibers. So the first step is all about chemistry;
the second one is about turning your chemical product into a more
useful, practical material.
Polyamides like Kevlar are polymers (huge molecules made of many identical parts joined
together in long chains) made by repeating amides over and over
again. Amides are simply chemical compounds in which part of an
organic (carbon-based) acid replaces one of the hydrogen atoms in
ammonia (NH3). So the basic way of making a polyamide is to take an
ammonia-like chemical and react it with an organic acid. This is an
example of what chemists call a condensation reaction because two substances fuse
together into one.
Artwork: Kevlar's monomer: C=carbon, H=hydrogen, O=oyxgen, N=nitrogen, — is a single chemical bond, and = is a double bond. This basic building block is repeated over and over again in the very long chains that make up the Kevlar polymer. Source: "US Patent: 3287323: Process for the production of a highly orientable, crystallizable, filament" by Stephanie Kwolek et al.
Kevlar's chemical structure naturally makes it form in tiny straight rods that pack
closely together, like lots of stiff new pencils stuffed tightly into
a box (only without the box). These rods form extra bonds between one another (known as hydrogen bonds)
giving extra strength—as though you'd glued the pencils together as well.
This bonded rod structure is essentially what gives Kevlar its amazing properties.
(More technically speaking, we can say the Kevlar rods are showing what's called
nematic behavior (lining up in the same direction), which is also
what happens in the liquid crystals used in LCDs (liquid crystal displays).)
You probably know that natural materials such as wool and cotton have to be spun into fibers
before they can turned into useful textile products—and the same is
true of artificial fibers such as nylon, Kevlar, and Nomex.
The basic aramid is turned into fibers by a process called wet spinning, which involves forcing a hot, concentrated, and
very viscous solution of poly-para-phenylene terephthalamide through a spinneret (a metal former a bit like a sieve) to make long, thin,
strong, and stiff fibers that are wound onto drums. The fibers are then cut to length and woven into a tough mat to make the
super-strong, super-stiff finished material we know as Kevlar.
Artwork: How Kevlar is made. 1) The rodlike Kevlar molecules start off in dilute solution. 2) Increasing the concentration increases the number of molecules but doesn't make them align. At this stage, the molecules are still tangled up and not extended into straight, parallel chains. 3) The wet-spinning process causes the rods to straighten out fully and align so they're all oriented in the same direction—forming what's called a nematic structure—and this is what gives Kevlar its exceptionally high strength. Image based on an original artwork from DuPont's Kevlar Technical Guide (see references below).
What's Kevlar used for?
Kevlar can be used by itself or as part of a composite material (one material combined with
others) to give added strength. It's probably best known for
its use in bulletproof vests and knifeproof body armor, but it has
dozens of other applications as well. It's used as reinforcement in
car tires, in car brakes, in the strings of archery
bows, and in car, boat, and even aircraft bodies. It's even used in buildings
and structures, although not (because of its relatively low compressive strength) as the primary structural material.
What makes Kevlar such a good antiballistic material?
If you've read our article on bullets, you'll know that they
damage things—and people—because they travel at high speeds with huge
amounts of kinetic energy. Although there's no such thing as completely "bulletproof," materials like
bulletproof glass do a good job at protecting us by absorbing (soaking up)
and dissipating (spreading out) the energy of a bullet.
Kevlar is an excellent antiballistic (bullet- and knife-resistant) material because
it takes a great deal of energy to make a knife or a bullet pass through it. The tightly woven
fibers of highly oriented (lined-up) polymer molecules are extremely hard to move apart: it takes energy to separate them. A bullet
(or a knife pushed hard by an attacker) has its energy "stolen" from it as it tries to fight its way through.
If it does manage to penetrate the material, it's considerably slowed down and does far less damage.
Although Kevlar is stronger than steel, it's about 5.5 times less dense (the density of Kevlar
is about 1.44 grams per cubic centimeter, compared to steel, which is round about 7.8–8 grams
per cubic centimeter). That means a certain volume of Kevlar will weigh 5–6 times less than the same volume of steel.
Think back to medieval knights with their cumbersome suits of armor: in theory, modern Kevlar gives just as much protection—but it's light and flexible enough to wear for much longer periods.
Photo: Think of Kevlar as a lightweight modern alternative to heavy, cumbersome, medieval suits of armor! Photo by Staff Sgt. Nate Hauser courtesy of US Marine Corps.
More layers = more protection
If you think of Kevlar "soaking up" the energy of a bullet, it's fairly obvious
that a greater thickness of Kevlar—more layers of the material bonded
together—will give more protection. As you can see from this chart,
the more layers you have, the faster you need to fire a bullet to get it
to penetrate through Kevlar armor. In other words, if you want to protect
soldiers against high-velocity rifle bullets, you're going to need much thicker
armor than if you simply want to protect police officers against handgun bullets,
which have lower velocity and less kinetic energy.
You can see this clearly in the official US National Institute of Justice Body Armor Classification,
which ranks bulletproof vests and other body protection (made of Kevlar and other materials)
on a scale from I to IV for its ability to protect against bullets fired from weapons of different power. At the low end of the scale,
type IIA armor has to protect against smaller handgun bullets (typically 9mm full metal jacketed
bullets weighing 8.0g or 0.3 oz and fired at about 373 m/s or 834 mph); you need at least 16 layers of Kevlar for that.
Higher up the scale, type IIIA armor has to resist more powerful handheld bullets
(such as .44 Magnum bullets weighing 15.6 g or 0.6 oz and fired at 436 m/s or 975 mph); that needs twice as much Kelvar—at least 30 layers. It's important to note that even Kevlar has its limits. For protection against rifle bullets (ordinary ones or armor-piercing ones), which travel much faster (850–900 m/s or 1900–2000 mph) with considerably higher kinetic energy, Kevlar isn't enough: you need body armor made from steel or ceramic plates (classified as type III and IV).
Chart: You need a greater thickness of Kevlar body armor to stop higher-speed bullets. If you fire a bullet
faster than the penetration speed, it will pierce through the armor but exit with less speed than it entered, because
the armor will absorb some of its kinetic energy. Please note that this chart is simply a very rough illustration; it's not
based on any particular bullet type.
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"Nomex", "Kevlar", and "DuPont" are trademarks or registered trademarks of E. I. Du Pont de Nemours and Company.
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