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What's Kevlar? |
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Kevlar is the DuPont Company 's brand name for the particularly light but very strong synthetic fibre . Created in DuPont's labs in 1965 by Stephanie Kwolek , Herbert Blades , and Phil Thier, Kevlar was first used commercially in the early 1970s. It can be spun into ropes or sheets of fabric that can either be used as-is, or used in the construction of composite components. Kevlar is now used in a wide range of applications, from bicycles to body armor , due to its high strength-to-weight ratio (see Tensile strength ), "...5 times stronger than steel on an equal weight basis..." . [1] It is a member of the Aramid family of synthetic fibres and similar to Twaron from Teijin .
History
In the 1970s, one of its most significant achievements in the development of body armor was the invention of DuPont's Kevlar ballistic fabric. Ironically, the fabric was originally intended to replace steel belting in vehicle tires. The development of Kevlar body armor by NIJ was a four-phase effort that took place over several years.
The first phase involved testing Kevlar fabric to determine whether it could stop a lead bullet. The second phase involved determining the number of layers of material necessary to prevent penetration by bullets of varying speeds and calibers and developing a prototype vest that would protect officers against the most common threats: the 38 Special and the 22 Long Rifle bullets.
By 1973, researchers at the Army's Edgewood Arsenal responsible for the bullet proof vest design had developed a garment made of seven layers of Kevlar fabric for use in field trials. It was determined that the penetration resistance of Kevlar was degraded when wet.
The bullet resistant properties of the fabric also diminished upon exposure to ultraviolet light, including sunlight. Dry-cleaning agents and bleach also had a negative effect on the antiballistic properties of the fabric, as did repeated washing. To protect against these problems, the vest was designed with waterproofing, as well as with fabric coverings to prevent exposure to sunlight and other degrading agents. Kevlar was first used commercially in the early 1970s. It can be spun into ropes or sheets of fabric that can either be used as-is, or used in the construction of composite components. Kevlar is now used in a wide range of applications, from bicycles to body armor
Properties When Kevlar is spun in the same way that a spider spins a web, the resulting fiber has tremendous strength, and is heat- and cut-resistant. The fibers do not rust or corrode. When woven together, they form a good material for mooring lines and other underwater objects.
There are three common grades of Kevlar: Kevlar, Kevlar 29, and Kevlar 49. Kevlar is typically used as reinforcements in tires and other rubber mechanical goods. Kevlar 29 is used in industrial applications such as cables, asbestos replacement, brake linings, and body armor. Kevlar 49 is considered to have the greatest tensile strength of all the aramids, and is used in applications such as plastic reinforcement for boat hulls, airplanes, and bikes.
Kevlar is susceptible to breakdown from ultraviolet light (such as sunlight) and hence is almost never used unprotected or unsheathed.
Production
Kevlar is synthesised from the monomers 1,4- phenyl -diamine ( para -phenylenediamine ) and terephthaloyl chloride in condensation reaction giving hydrochloric acid as byproduct. The result is a liquid-crystalline behaviour and mechanical drawing causing the polymer chains to orient in the direction of the fiber. The carcinogenic Hexamethylphosphoramide (HMPA) was originally used as the solvent for the polymerisation, but the precautions necessary made production highly expensive. A new solvent, a mixture of N-methyl-pyrolidon and calcium chloride, is now used.
Kevlar is expensive, in part, due to the difficulties arising from 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 .
Chemical properties
Fibers of Kevlar consist of long molecular chains produced from poly-paraphenylene terephthalamide. There are many inter-chain bonds making the material extremely strong. Kevlar derives a portion of its improved strength from inter-molecular hydrogen bonds formed between the carbonyl groups and protons on neighboring polymer chains and the partial pi stacking of the benzenoid aromatic stacking interactions between stacked strands. These interactions have a greater influence on Kevlar than van der Waals interactions and chain length that typically influence the properties of other synthetic polymers and fibers like Dyneema . The presence of salts and certain other impurities, especially calcium , could interfere with the strand interactions and caution is used to avoid inclusion in its production. Kevlar's structure consists of relatively rigid molecules, which tend to form mostly planar sheet-like structures that have similarities to silk protein.
This Article was cited from http://en.wikipedia.org/wiki/Kevlar |
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