Kevlar®
by Chris Woodford. Last updated: December 7, 2009.
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, 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 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 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.
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 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 ("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 structure.
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).
