Science Fair Project Encyclopedia
Kevlar, also known as Twaron and poly-paraphenelyne 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 tyres, 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. de 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 aromatic-aromatic stacking interactions between the 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 has free groups accessible for hydrogen bonding on the outside. This results in the absorption of water and its good wet properties. This also makes the fibers feel more natural and "sticky" compared to common polymers like polyethylene. The free groups also makes its bonding to other types of polymers easier.
Kevlar's main weaknesses are that it decomposes under alkaline conditions or when exposed to chlorine. And while it can support great tensile stress, it is not strong under pressure (compressive stress). To compensate for this last problem, Kevlar is often used together with materials that are strong under compresive stress.
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.
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