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| Names | |
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| IUPAC name Poly(azanediyl-1,4-phenyleneazanediylterephthaloyl)[1] | |
| Identifiers | |
| ChemSpider |
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| Properties | |
| [-CO-C6H4-CO-NH-C6H4-NH-]n | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Kevlar (para-aramid)[2] is a strong, heat-resistantsynthetic fiber, related to otheraramids such asNomex andTechnora. Developed byStephanie Kwolek atDuPont in 1965,[3][2][4] the high-strength material was first used commercially in the early 1970s as a replacement for steel in racing tires. It is typically spun into ropes orfabric sheets that can be used as such, or as an ingredient incomposite material components.
Kevlar has many applications, ranging from bicycletires andracing sails tobulletproof vests, due to its hightensile strength-to-weight ratio; by this measure it is five times stronger than steel.[2] It is also used to make modern marchingdrumheads that withstand high impact, and formooring lines and other underwater applications.
A similar fiber,Twaron, with the same chemical structure was developed byAkzo in the 1970s. Commercial production started in 1986, and Twaron is manufactured byTeijin Aramid.[5][6]
Poly-paraphenylene terephthalamide (K29) – branded Kevlar – was invented by the Polish-American chemistStephanie Kwolek while working for DuPont, in anticipation of a gasoline shortage. In 1964, her group began searching for a new lightweight strong fiber to use for light, but strong, tires.[7] The polymers she had been working with, poly-p-phenylene-terephthalate and polybenzamide,[8] formedliquid crystals in solution, something unique to polymers at the time.[7]
The solution was "cloudy,opalescent upon being stirred, and of lowviscosity" and usually was thrown away. However, Kwolek persuaded the technician, Charles Smullen, who ran thespinneret, to test her solution, and was amazed to find that the fiber did not break, unlikenylon. Her supervisor and her laboratory director understood the significance of her discovery and a new field ofpolymer chemistry quickly arose. By 1971, modern Kevlar was introduced.[7] However, Kwolek was not very involved in developing the applications of Kevlar.[9]
In 1971,Lester Shubin, who was then the Director of Science and Technology for the National Institute for Law Enforcement and Criminal Justice,[10] suggested using Kevlar to replace nylon in bullet-proof vests.[11] Prior to the introduction of Kevlar,flak jackets made of nylon had provided much more limited protection to users. Shubin later recalled how the idea developed: "We folded it over a couple of times and shot at it. The bullets didn't go through." In tests, they strapped Kevlar onto anesthetized goats and shot at their hearts, spinal cords, livers and lungs. They monitored the goats' heart rate and blood gas levels to check for lung injuries. After 24 hours, one goat died and the others had wounds that were not life threatening.[12][13][verification needed] Shubin received a $5 million grant to research the use of the fabric in bullet-proof vests.
Kevlar 149 was invented byJacob Lahijani of Dupont in the 1980s.[14]
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Kevlar issynthesized in solution from the monomers 1,4-phenylene-diamine (para-phenylenediamine) andterephthaloyl chloride in acondensation reaction yieldinghydrochloric acid as a byproduct. The result hasliquid-crystalline behavior, and mechanical drawing orients the polymer chains in the fiber's direction.Hexamethylphosphoramide (HMPA) was the solvent initially used for thepolymerization, but for safety reasons, DuPont replaced it by a solution ofN-methyl-pyrrolidone and calcium chloride. As this process had been patented by Akzo (see above) in the production ofTwaron, apatent war ensued.[15]
Kevlar production is expensive because of the difficulties arising from using concentratedsulfuric acid, needed to keep the water-insoluble polymer in solution during its synthesis andspinning.[16]
Several grades of Kevlar are available:
Theultraviolet component of sunlight degrades and decomposes Kevlar, a problem known asUV degradation, and so it is rarely used outdoors without protection against sunlight.[22]
When Kevlar isspun, the resulting fiber has atensile strength of about3,620 MPa (525,000 psi),[23] and arelative density of 1.44 (0.052 lb/in3). The polymer owes its high strength to the many inter-chain bonds. These inter-molecularhydrogen bonds form between the carbonyl groups and NH centers. Additional strength is derived fromaromatic stacking interactions between adjacent strands. These interactions have a greater influence on Kevlar than thevan der Waals interactions and chain length that typically influence the properties of other synthetic polymers and fibers such asultra-high-molecular-weight polyethylene. The presence ofsalts and certain other impurities, especiallycalcium, could interfere with the strand interactions and care is taken to avoid inclusion in its production. Kevlar's structure consists of relatively rigid molecules which tend to form mostly planar sheet-like structures rather likesilk protein.[24]
Kevlar maintains its strength and resilience down to cryogenic temperatures (−196 °C (−320.8 °F)): in fact, 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: enduring 160 °C (320 °F) for 500 hours, its strength is reduced by about 10%; and enduring 260 °C (500 °F) for 70 hours, its strength is reduced by about 50%.[25]
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Kevlar is often used in the field ofcryogenics for its lowthermal conductivity and high strength relative to other materials forsuspension purposes. It is most often used to suspend aparamagnetic salt enclosure from asuperconducting magnet mandrel in order to minimize any heat leaks to the paramagnetic material. It is also used as a thermal standoff or structural support where low heat leaks are desired.
A thin Kevlar window has been used by theNA48 experiment atCERN to separate a vacuum vessel from a vessel at nearly atmospheric pressure, both 192 cm (76 in) in diameter. The window has provided vacuum tightness combined with reasonably small amount of material (only 0.3% to 0.4% ofradiation length).[citation needed]
Kevlar is a well-known component ofpersonal armor such ascombat helmets,ballistic face masks, andballistic vests. ThePASGT helmet and vest that were used byUnited States military forces used Kevlar as a key component in their construction. Other military uses include bulletproof face masks andspall liners used to protect the crews ofarmoured fighting vehicles.Nimitz-class aircraft carriers use Kevlar reinforcement in vital areas. Civilian applications include: high heat resistance uniforms worn by firefighters, body armour worn by police officers, security, and police tactical teams such asSWAT.[26]
Kevlar is used to manufacture gloves, sleeves, jackets,chaps and other articles of clothing[27] designed to protect users from cuts, abrasions and heat. Kevlar-based protective gear is often considerably lighter and thinner than equivalent gear made of more traditional materials.[26]
It is used formotorcycle safety clothing, especially in the areas featuring padding such as the shoulders and elbows. In the sport offencing it is used in the protective jackets, breeches, plastrons and the bib of the masks. It is increasingly being used in thepeto, the padded covering which protects thepicadors' horses in the bullring.Speed skaters also frequently wear an under-layer of Kevlar fabric to prevent potential wounds from skates in the event of a fall or collision.
Inkyudo, or Japanesearchery, it may be used forbow strings, as an alternative to the more expensive[28]hemp. It is one of the main materials used forparaglider suspension lines.[29] It is used as an inner lining for somebicycle tires to prevent punctures. Intable tennis, plies of Kevlar are added to custom ply blades, or paddles, in order to increase bounce and reduce weight.Tennis racquets are sometimes strung with Kevlar. It is used in sails for high performance racing boats.
In 2013, with advancements in technology,Nike used Kevlar in shoes for the first time. It launched the Elite II Series,[30] with enhancements to its earlier version ofbasketball shoes by using Kevlar in theanterior as well as theshoe laces. This was done to decrease the elasticity of the tip of the shoe in contrast to the nylon conventionally used, as Kevlar expanded by about 1% against nylon which expanded by about 30%. Shoes in this range included LeBron, HyperDunk and Zoom Kobe VII. However these shoes were launched at a price range much higher than average cost of basketball shoes. It was also used in the laces for theAdidas F50 adiZero Prime football boot.
Several companies, includingContinental AG, manufacture cycle tires with Kevlar to protect against punctures.[31]
Folding-bead bicycle tires, introduced to cycling byTom Ritchey in 1984,[32][circular reference] use Kevlar as a bead in place of steel for weight reduction and strength. A side effect of the folding bead is a reduction in shelf and floor space needed to display cycle tires in a retail environment, as they are folded and placed in small boxes.
Kevlar has also been found to have useful acoustic properties forloudspeaker cones, specifically for bass and mid range drive units.[33] Additionally, Kevlar has been used as astrength member in fiber optic cables such as the ones used for audio data transmissions.[34]
Kevlar can be used as an acoustic core on bows forstring instruments.[35] Kevlar's physical properties provide strength, flexibility, and stability for the bow's user. To date, the only manufacturer of this type of bow isCodaBow.[36]
Kevlar is also presently used as a material for tailcords (a.k.a. tailpiece adjusters), which connect thetailpiece to theendpin of bowed string instruments.[37]
Kevlar is sometimes used as a material on marching snare drums. It allows for an extremely high amount of tension, resulting in a cleaner sound. There is usually a resin poured onto the Kevlar to make the head airtight, and a nylon top layer to provide a flat striking surface. This is one of the primary types of marching snare drum heads.Remo's Falam Slam patch is made with Kevlar and is used to reinforce bass drum heads where the beater strikes.[38]
Kevlar is used in thewoodwind reeds of Fibracell. The material of these reeds is a composite of aerospace materials designed to duplicate the way nature constructs cane reed. Very stiff but sound absorbing Kevlar fibers are suspended in a lightweight resin formulation.[39]
Kevlar is sometimes used in structural components of cars, especially high-value performance cars such as theFerrari F40.[40]
The chopped fiber has been used as a replacement for asbestos inbrake pads.[41] Aramids such as Kevlar release less airborne fibres thanasbestos brakes and do not have the carcinogenic properties associated with asbestos.[42][43]
Wicks forfire dancing props are made of composite materials with Kevlar in them. Kevlar by itself does not absorb fuel very well, so it is blended with other materials such asfiberglass orcotton. Kevlar's high heat resistance allows the wicks to be reused many times.
Kevlar is sometimes used as a substitute forTeflon in some non-stick frying pans.[44]
Kevlar fiber is used in rope and in cable, where the fibers are kept parallel within apolyethylene sleeve. The cables have been used insuspension bridges such as the bridge atAberfeldy, Scotland. They have also been used to stabilize cracking concrete cooling towers by circumferential application followed by tensioning to close the cracks. Kevlar is widely used as a protective outer sheath foroptical fiber cable, as its strength protects the cable from damage and kinking. When used in this application it is commonly known by the trademarked name Parafil.[45]
Kevlar was used by scientists atGeorgia Institute of Technology as a base textile for an experiment in electricity-producing clothing. This was done by weavingzinc oxidenanowires into the fabric. If successful, the new fabric will generate about 80 milliwatts per square meter.[46]
A retractable roof of over 60,000 sq ft (5,600 m2) of Kevlar was a key part of the design of theOlympic Stadium, Montreal for the1976 Summer Olympics. It was spectacularly unsuccessful, as it was completed 10 years late and replaced just 10 years later in May 1998 after a series of problems.[47][48]
Kevlar can be found as a reinforcing layer inrubberbellowsexpansion joints and rubberhoses, for use in high temperature applications, and for its high strength. It is also found as a braid layer used on the outside of hose assemblies, to add protection against sharp objects.[49][50][51]
Somecellphones (including theMotorola RAZR Family, theMotorola Droid Maxx,OnePlus 2 andPocophone F1) have a Kevlar backplate, chosen over other materials such as carbon fiber due to its resilience and lack of interference with signal transmission.[52]
The Kevlar fiber/epoxy matrix composite materials can be used in marine current turbines (MCT) or wind turbines due to their high specific strength and light weight compared to other fibers.[53]
Aramid fibers are widely used for reinforcing composite materials, often in combination withcarbon fiber andglass fiber. The matrix for high performance composites is usuallyepoxy resin. Typical applications includemonocoque bodies forFormula 1 cars, helicopter rotor blades,tennis,table tennis,badminton andsquashrackets,kayaks,cricket bats, andfield hockey,ice hockey andlacrosse sticks.[54][55][56][57]
Kevlar 149, the strongest fiber and most crystalline in structure, is an alternative in certain parts of aircraft construction.[58] The wing leading edge is one application, Kevlar being less prone than carbon or glass fiber to break in bird collisions.
