View source for Kevlar

[[Image:Kevlar chemical structure H-bonds.png|thumb|400px|Kevlar's molecular structure; BOLD: monomer unit; DASHED: hydrogen bonds.]]

'''Kevlar''' is the [[DuPont|DuPont Company]]'s registered [[trademark]] for a very light, very strong [[synthetic fiber]] created in 1965 by [[Stephanie Kwolek]] and Herbert Blades.<ref name=whatiskevlar>{{cite web
|url=http://www.dupont.com/kevlar/whatiskevlar.html
|title=What is Kevlar
|publisher=DuPont
|accessdate=2007-03-28
}}</ref> As a material, Kevlar was first commercially used in the early 1970s; it is spun into ropes or fabric sheets that can be used as such or as an ingredient in [[composite]] material components. Currently, kevlar has many applications, ranging from bicycles to [[Bulletproof vest|body armor]], because of its high strength-to-weight ratio, ''"...5 times stronger than steel on an equal weight basis..."''.<ref name=whatiskevlar/> Under water, Kevlar is less resistant to ballistic projectiles, although it is water resistant. <ref>[http://www.lbl.gov/MicroWorlds/Kevlar/index.html What is Kevlar]</ref> It is a member of the [[Aramid]] family of synthetic fibres and a competitor of the material [[Twaron]], manufactured by [[Teijin]].

==History==
In the 1970s, one of the most significant achievements in body armour development was the DuPont chemical company's invention of Kevlar ballistic fabric, a material originally meant to replace steel belting in vehicle tires. The [[US]] [[National Institute of Justice]]'s development of Kevlar body armour for police use was a four-phase testing project.

The first test phase determined whether or not Kevlar fabric could stop a lead bullet. The second phase determined the number of fabric layers needed to prevent [[penetration (weapons)|penetration]] by [[bullets]] of different [[calibre]]s and velocities, and the development of a [[prototype]] bullet-resistant [[vest]] most protective against the more common calibre bullets ([[.38 Special]] and [[.22 long rifle]]) encountered in street policing.

In 1973 the [[U.S. Army]] researchers at the [[Edgewood Arsenal]] responsible for designing the bullet-proof vest had developed a seven-layer vest for field trials. They discovered that water, [[ultraviolet radiation]] from [[sunlight]] or other sources, [[dry-cleaning]] [[chemicals]],[[ chlorine]] [[bleach]], and repeated washings reduced its penetration resistance. To protect against these problems, the Kevlar bullet-proof vest was water-proofed and covered with sun- and chemical-resistant fabric.

==Properties==

When Kevlar is [[spinneret|spun]], the resulting fibre has great [[tensile strength]] (ca. 3 000 MPa), a [[relative density]] of 1.44, and does not  [[rust]]. When used as a woven material, it is suitable for mooring lines and other underwater application objects. 

There are three grades of Kevlar: (i) Kevlar, (ii) Kevlar 29, and (iii) Kevlar 49. Typically, Kevlar is used as reinforcement in tires and rubber mechanical goods. Kevlar 29's industrial applications are as cables, in [[asbestos]] replacement, brake linings, and body armour. Kevlar 49 has the greatest tensile strength of all the aramids, and is used in plastic reinforcement for boat hulls, aeroplanes, and bicycles. The [[ultraviolet light]] component of sunlight degrades and decomposes Kevlar, hence it is rarely used outdoors without protection against sunlight.

== Production ==
Kevlar is [[synthesised]] from the [[monomer]]s 1,4-[[phenylene]]-di[[amine]] ([[P-Phenylenediamine|''para''-phenylenediamine]]) and [[terephthaloyl chloride]] in [[Condensation polymer|condensation reaction]] yielding [[hydrochloric acid]] as a byproduct. The result is a [[liquid crystal|liquid-crystalline]] behaviour and mechanical drawing orienting the polymer chains in the fibre's direction. [[Hexamethylphosphoramide]] (HMPA) was the polymerization solvent first used, but toxicology tests demonstrated it provoked tumors in the noses of rats, so DuPont replaced it by a N-methyl-pyrolidone and calcium chloride as the solvent. 

[[Image:Kevlar chemical synthesis.png|center|566px|The reaction of 1,4-phenylene-diamine (''para''-phenylenediamine) with terephthaloyl chloride yielding kevlar]]

Kevlar production is expensive because of the difficulties arising from using [[toxic]] concentrated [[sulfuric acid]], needed to keep the water-insoluble polymer in solution during its synthesis and [[spinning (textiles)|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 part of its high 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 the [[Van der Waals bonding|van der Waals]] interactions and chain length that typically influence the properties of other synthetic polymers and fibers such as [[Dyneema]]. 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 rather like [[silk]] protein.

==Thermal properties==
For a polymer Kevlar has very good resistance to high temperatures, and maintains its strength and resilience down to cryogenic temperatures (-196° C); indeed, it is slightly stronger at low temperatures.

At higher temperatures the tensile strength is immediately reduced by about 10-20%, and after some hours the strength progressively reduces further. For example at 160° C about 10% reduction in strength occurs after 500 hours. At 260° C 50% reduction occurs after 70 hours.<ref>[http://www2.dupont.com/Kevlar/en_US/assets/downloads/KEVLAR_Technical_Guide.pdf KEVLAR Technical Guide]</ref>

At 450° C Kevlar [[Sublimation (chemistry)|sublimates]].

== See also ==
* [[Aramid]]
* [[Bulletproof vest]]
* [[DuPont]] (Company that invented and manufactures Kevlar)
* [[Interceptor body armor]]
* [[Nomex]]
* [[Personnel Armor System for Ground Troops]]
* [[Spider silk]]
* [[Twaron]]
* [[Ultra high molecular weight polyethylene]]

==Notes==
<div class="references-small">
{{reflist|2}}
</div>

== References ==
* Kadolph, Sara J. Anna L. Langford. Textiles, Ninth Edition. Pearson Education, Inc 2002. Upper Sadddle River, NJ
*{{cite journal
 | author = D. Tanner, J. A. Fitzgerald, B. R. Phillips 
 | year = 1989
 | title = The Kevlar Story - an Advanced Materials Case Study
 | journal = [[Angewandte Chemie International Edition in English]]
 | volume = 28
 | issue = 5
 | pages = 649 - 654
 | doi = 10.1002/anie.198906491
 }}
*{{cite journal
 | author = E. E. Magat
 | year = 1980
 | title = Fibres from Extended Chain Aromatic Polyamides, New Fibres and Their Composites
 | journal = Philosophical Transactions of the Royal Society of London Series A
 | volume = 294
 | issue = 1411
 | pages = 463-472
 | url = http://links.jstor.org/sici?sici=0080-4614%2819800121%29294%3A1411%3C463%3AFFECAP%3E2.0.CO%3B2-P 
 }}

== External links ==
{{commons|Aramid|Kevlar}}

* [http://www.kevlar.com/ Kevlar Home Page]
* http://www.pslc.ws/macrog/aramid.htm
*[http://www.designdictionary.co.uk/en/kevlar.htm Kevlar - Design Dictionary] Illustrated article about Kevlar
* [http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDUKEV29 Matweb material properties of Kevlar]
* {{US patent|5565264}}
* http://www.lbl.gov/MicroWorlds/Kevlar/index.html

* http://web.umr.edu/~wlf/Synthesis/kevlar.html

{{DuPont}}
{{Fibers}}
[[Category:Organic polymers]]
[[Category:Personal armour]]
[[Category:DuPont]]
[[Category:Synthetic fibers]]
[[Category:Brand name materials]]

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