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Originally intended to replace the steel belts in Tires , it is probably the most well known name in soft armor as Bulletproof Vest s. It is also used in Extreme Sports equipment, high-tension Drumhead applications, animal handling protection, Composite Aircraft construction, fire suits, yacht sails, as an Asbestos replacement, sometimes in loudspeaker cones, and recently, even in R/C model helicopter blades . When this polymer is Spun in the same way that a spider spins a web, the resulting commercial para-aramid fiber has tremendous strength, and is heat and cut resistant. Para-aramid fibers do not rust or corrode, and their strength is unaffected by immersion in water. When woven together, they form a good material for mooring lines and other underwater objects. However, unless specially waterproofed, para-aramid fiber’s ability to stop bullets and other projectiles is degraded when wet. PROPERTIES Kevlar is a processed Polyparaphenylene Terephthalamide that is a Aramid consisting of long Polymeric chains with a parallel orientation. 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 Interaction s between stacked strands. These interactions have a greater influence on Kevlar than Van Der Waals interactions and chain length that typicaly influence the propeties of other synthetic polymers and fibers like Dyneema . In addition the presence of Salt s 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. These properties result in its high mechanical strength and additionaly kevlar's remarkable heat resistance. The degree of Unsaturated carbons (the ratio of Carbon to Hydrogen atoms) is quite high, and decreases kevlar's Flammability . Kevlar molecules have Polar Groups accessible for Hydrogen Bond ing. 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 Composite material Fibre Reinforced Plastic . This same property also makes the fibers feel more natural and "sticky" compared to Nonpolar polymers like Polyethylene . In structural applications, Kevlar fibers can be bonded to one another or to other materials to form a Composite . Kevlar's main weaknesses are that it decomposes under Alkaline conditions or when exposed to Chlorine . While it can have a great Tensile Strength , sometimes in excess of 4.0 GPa, like all fibers it tends to Buckle in Compression . PRODUCTION Kevlar is Synthesized from the Monomer s 1,4- Phenyl -di Amine ( ''para''-phenylenediamine ) and Terephthaloyl Chloride . The result is a Polymer ic Aromatic amide ( Aramid ) with alternating Benzene Ring s 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 orientate in the direction of the fibre. Kevlar has a high price, 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 . SEE ALSO EXTERNAL LINKS
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