Kevlar®

Last updated: May 15, 2008.

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 certainly 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 why it works.

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, 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 needed.

Kevlar is simply a super-strong plastic. If that sounds less impressive, 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 chemical structure (how the atoms in its molecules are arranged) and partly due to the way it's made into fibers that are knitted tightly together.

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. It starts to make more sense if you consider it 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 not unlike 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 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 syructure.

Like Nomex, Kevlar is a distant relative of nylon, the first commercially successful "superpolyamide", developed by DuPont in the 1930s. Kevlar was introduced much more recently (only in 1971).

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 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 or 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 or -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.

Source: All information from Kevlar Technical Guide, published by DuPont. Please note: This file is in PDF format and about 4MB, so it can take a while to download.

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.

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.

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.

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 finished material we know as Kevlar.

What's Kevlar used for?

Kevlar can be used by itself or as part of a composite material (one material combined with other materials) 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, for boatbuilding, in the strings of archery bows, and in car, boat, and even aircraft bodies. It's even used as a tough, durable building material. DuPont's Kevlar website has lots more information about Kevlar's applications.