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Kevlar (also produced by the Teijin Twaron company under the trade name Twaron) is a type of high strength synthetic fiber first produced by the DuPont corporation in the early 1960s, following the work of Stephanie Kwolek. It is a registered trademark of E.I. du Pont de Nemours and Company. Kevlar is very heat resistant and decomposes above 400 °C without melting.
Kevlar is very strong and very light, weight for weight about five times as strong as steel. This is due to its chemical structure, as well as the way the polymer has been processed before leaving the factory. Kevlar is synthesized from alternating monomers of 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride. The result is a polymeric aromatic amide (aramid) with alternating benzene rings and amide groups. The chains are relatively rigid and form hydrogen bonds between the strands. This leads to a planar sheet structure, similar to silk protein, resulting in its high mechanical strength and its remarkable heat resistance. When the material is spun, the polymer chains are oriented in direction of the fiber. Because it is highly unsaturated (i.e., the ratio of carbon to hydrogen atoms is quite high), it has a low flammability.
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 growing, highly insoluble polymer in solution.
An early use for Kevlar was to replace steel cords in car tires. It is now commonly used in bulletproof vests, as an asbestos replacement, in extreme sports equipment such as bike saddles, as wicks for juggling torches, plastrons for fencing, and for composite aircraft construction.
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