Fiber Definitions — textile fibers, yarn fibers and fiber properties from Apparel Search
Kevlar, also known as Twaron and poly-paraphenylene terephthalamide, is a synthetic fiber that is five times stronger than steel, weight for weight. Kevlar is very heat resistant and decomposes above 400 degrees C without melting. It is usually used in bulletproof vests, in extreme sports equipment, and for composite aircraft construction, and also 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, and 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 fiber reinforced plastic. This same property also makes the fibers feel more natural and "sticky" compared to non-polar polymers like polyethylene.
solution during synthesis and spinning.
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
The chemical synthesis of kevlar from 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride
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Kevlar's heat behaviour is unusual for a polymer: it decomposes above 400 degrees C without melting, and because it is highly unsaturated, with a high ratio of carbon to hydrogen atoms, it has a low flammability. Those properties are what put it into bulletproof vests, fire suits, extreme sports equipment and composite aircraft construction. The same molecular structure gives it a second, practical advantage: Kevlar molecules have polar groups accessible for hydrogen bonding, so water at the surface gives good wetting properties, which is important for bonding the fibers to other types of polymer to form a fiber reinforced plastic. That bonding character also makes the fibers feel more natural and sticky compared with non-polar polymers like polyethylene. In structural applications, Kevlar fibers can be bonded to one another or to other materials to form a composite, which is how a fiber that buckles in compression on its own becomes a rigid, load-bearing structure in a composite panel, a tire cord or an aircraft part.
Kevlar has a high price at least partly because of the difficulties caused by the use of concentrated sulfuric acid in its manufacture; the harsh conditions are needed to keep the highly insoluble polymer in solution during synthesis and spinning. That cost is the price of the parallel orientation itself: the polymer strands are produced randomly aligned, and only dissolving and spinning them orients the chains in the direction of the fiber, which is what turns a polymer into a fiber five times stronger than steel, weight for weight. The trade-off is characteristic of high-performance fibers generally: the more the fiber's properties depend on molecular order, the more demanding and expensive the process that creates that order, and the more the end use must justify it. Kevlar justifies it in protection, where weight, heat and tensile strength are the whole product.
Kevlar is a type of aramid consisting of long polymeric chains with a parallel orientation. Its strength comes from intra-molecular hydrogen bonds and phenyl stacking interactions between aromatic groups in neighbouring strands, which are much stronger than the van der Waals interaction found in other synthetic polymers and fibers such as dyneema. Its relatively rigid molecules form a planar sheet-like structure similar to silk protein.
Kevlar's main weaknesses are that it decomposes under alkaline conditions or when exposed to chlorine, and while it can support great tensile stress, like all fibers it tends to buckle in compression. Water entering the fiber interior can take the place of bonding between molecules and reduce the material's strength, and salts and certain impurities, especially calcium, interfere with strand interactions and must be avoided in production.
Kevlar is synthesized from the monomers 1,4-phenyl-diamine, para-phenylenediamine, and terephthaloyl chloride, giving a polymeric aromatic amide, an aramid, with alternating benzene rings and amide groups. The polymer strands are dissolved and spun, which orients the polymer chains in the direction of the fiber. Concentrated sulfuric acid is needed in manufacture to keep the highly insoluble polymer in solution, which contributes to its high price.