Kevlar, also known as Twaron and poly-paraphenylene terephthalamide, is a synthetic fibre that is five times stronger than steel, weight for weight. Kevlar is very heat resistant and decomposes above 400 °C without melting. It is usually used in bulletproof vests, in extreme sports equipment, and for composite aircraft construction. It is also used as a replacement for steel cords in car tires, in fire suits and as an asbestos replacement. Kevlar was invented by the DuPont corporation in the early 1960s, following the work of Stephanie Kwolek. Kevlar is a registered trademark of E.I. du Pont de Nemours and Company.
Kevlar is a type of aramid that consists of long polymeric chains with a parallel orientation. Kevlar derives its strength from intra-molecular hydrogen bonds and phenyl stacking interactions between aromatic groups in neighboring strands. These interactions are much stronger than the van der Waals interaction found in other synthetic polymers and fibers like dyneema. The presence of salts and certain other impurities, especially calcium, would interfere with the strand interactions and has to be avoided in the production process. Kevlar consists of relatively rigid molecules, which form a planar sheet-like structure similar to silk protein.
These properties result in its high mechanical strength and its remarkable heat resistance. Because it is highly unsaturated, i.e. the ratio of carbon to hydrogen atoms is quite high, it has a low flammability.
Kevlar molecules have polar groups accessible for hydrogen bonding. Water that enters the interior of the fiber can take the place of bonding between molecules and reduce the material's strength, while the available groups at the surface lead to good wetting properties. This is important for bonding the fibers to other types of polymer, forming a fibre reinforced plastic. This same property also makes the fibers feel more natural and "sticky" compared to non-polar polymers like polyethylene.
Kevlar's main weaknesses are that it decomposes under alkaline conditions or when exposed to chlorine. While it can support great tensile stress, like all fibers it tends to buckle in compression. In structural applications, Kevlar fibers can be bonded to one another or to other materials to form a composite.
Kevlar is synthesized from the monomers 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride. The result is a polymeric aromatic amide (aramid) with alternating benzene rings and amide groups. When they are produced, these polymer strands are aligned randomly. To make Kevlar, they are dissolved and spun, causing the polymer chains to orient in the direction of the fiber.
Kevlar has a high price at least partly because of the difficulties caused by the use of concentrated sulfuric acid in its manufacture. These harsh conditions are needed to keep the highly insoluble polymer in solution during synthesis and spinning.