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 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 structure.
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 US Patent 3,287,323 for her invention, with Paul Morgan, in 1966.
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.
Woodford, Chris. (2008/2012) Kevlar. Retrieved from https://www.explainthatstuff.com/kevlar.html. 2019