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KEVLAR – BY ABHISHEK JAGUESSAR

Kevlar is the registered trademarkfor a para-aramid synthetic fiber , related to other aramids such as Nomexand Technora. Developed at DuPontin 1965, this high strength material was first commercially used in the early 1970s as a replacement for steel in racing tires. Typically it is spun into ropes or  fabricsheets that can be used as such or as an ingredient in composite materialcomponents.

Currently, Kevlar has many applications, ranging from bicycle tires and racing sails to body armor because of its high tensile strength-to-weight ratio; by this measure it is 5 times stronger than steel on an equal weight basis. When used

as a woven material, it is suitable for mooring lines and other underwater applications.

A similar fiber called Twaronwith roughly the same chemical structure was developed by Akzo in the 1970s; commercial productionstarted in 1986, and Twaron is now manufactured by Teijin.

History

Poly-paraphenylene terephthalamide - branded Kevlar - was invented by Stephanie Kwolekwhile working for DuPont. In anticipation of a gas shortage, in 1964 her group began searching for anew lightweight strong fiber to use for light but strong tires. The polymers she had been working with atthe time, poly-p-Phenylene-terephthalate and polybenzamide, formed liquid crystal while in solution,

something unique to those polymers at the time.

The solution was "cloudy, opalescent upon being stirred, and of low viscosity" and usually was thrown

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away. However, Kwolek persuaded the technician, Charles Smullen, who ran the "spinneret" to test her solution, and was amazed to find that the fiber did not break, unlike nylon. Both her supervisor andlaboratory director understood the significance of her discovery and a new field of  polymer chemistryquickly arose. By 1971, modern Kevlar was introduced. However, Kwolek was not very involved indeveloping the applications of Kevlar.

ProductionKevlar is synthesizedin solution from the monomers 1,4-phenylene-diamine ( para-phenylenediamine) and terephthaloyl chloridein a condensation reactionyielding hydrochloric acidas a byproduct. The result has liquid-crystallinebehavior, and mechanical drawing orients the polymer chains in the fiber's direction.Hexamethylphosphoramide (HMPA) was the solvent initially used for the polymerization, but for safetyreasons, DuPont replaced it by a solution of N -methyl-pyrrolidone and calcium chloride. As this processwas patented by Akzo (see above) in the production of Twaron, a patent war ensued.

Kevlar (poly paraphenylene terephthalamide) production is expensive because of the difficulties arisingfrom using concentrated sulfuric acid, needed to keep the water-insoluble polymer in solution during its synthesis and spinning

.

Several grades of Kevlar are available:

1. Kevlar K-29 – in industrial applications, such as cables, asbestos

replacement, brake linings, and body/vehicle armor.

2. Kevlar K49 – high modulus used in cable and rope products.

3. Kevlar K100 – colored version of Kevlar 

4. Kevlar K119 – higher-elongation, flexible and more fatique resistant.

5. Kevlar K129 – higher tenacity for ballistic applications.

6. Kevlar AP – has 15% higher tenacity than K-29.

7. Kevlar XP – lighter weight resin and KM2 plus fiber combination.

8. Kevlar KM2 

 – enhanced ballistic resistance for armor applications

The ultraviolet component of sunlight degrades and decomposes Kevlar, a problem known as UVdegradation, and so it is rarely used outdoors without protection against sunlight.

Structure and properties

When Kevlar is spun, the resulting fiber has a tensile strength

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of about 3,620 MPa, and a relative densityof 1.44. The polymer owes its high strength to the many inter-chain bonds. These inter-molecular hydrogen 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 onKevlar than the van der Waals interactions and chain length that typically influence the properties of other synthetic polymers and fibers such as 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-likestructures rather like silkprotein.

Thermal properties

Kevlar maintains its strength and resilience down to cryogenic temperatures (−196 °C); in fact, it isslightly stronger at low temperatures. At higher temperatures the tensile strength is immediately reducedby about 10–20%, and after some hours the strength progressively reduces further. For example at 160°C (320 °F) about 10% reduction in strength occurs after 500 hours. At 260 °C (500 °F) 50% strengthreduction occurs after 70 hours.

Armor 

Kevlar is a well-known component of  personal armor such as combat helmets, Ballistic face masks, and Ballistic vests. The PASGT helmet and vestused by United Statesmilitary forces from the 1980s into 2005 both have Kevlar as a key component, as do their replacements. Other military uses include bulletproof facemasks used by sentries and spall liners usedto protect the crews of armoured fighting vehicles. Related civilian applications include Emergency Service's protection gear if it involves high heat (e.g.,tackling a fire), and Kevlar body armor such as vests for police officers, security, and SWAT.

Personal protection

Kevlar is used to manufacture gloves, sleeves, jackets, chaps and other articles of clothing designed toprotect users from cuts, abrasions and heat. Kevlar based protective gear is often considerably lighter and thinner than equivalent gear made of more traditional materials.

Sports equipment

It is used as an inner lining for some bicycle tires to prevent punctures, and due to its excellent heat resistance, is used for fire poi wicks. In table tennis,plies of Kevlar are added to custom ply blades, or paddles, in order to increase bounce and reduceweight. It is used for  motorcycle safety clothing, especially in the areas featuring padding such as shoulders and elbows. It was also used as speed

control patches for certain Soap Shoesmodels.

In Kyudoor Japanese archery, it may be used as an alternative to more expensive hempfor  bow strings. It is one of the main materials used for paraglider suspension lines.

It is also used in the laces for the adidas F50adizero Prime football boot.

It is even used in sails for high performance racing boats.

Audio equipment

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Kevlar has also been found to have useful acoustic properties for loudspeaker cones, specifically for bass and midrange drive units.

Drumheads

Kevlar is sometimes used as a material on marching snare drums. It allows for an extremely highamount of tension, resulting in a cleaner sound. There is usually some sort of resin poured onto thekevlar to make the head airtight, and a nylon top layer to provide a flat striking surface. This is one of theprimary 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.

Woodwind reeds

Kevlar is used in the woodwind 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 soundabsorbing Kevlar fibers are suspended in a lightweight resin formulation.

Frying pans

Kevlar is sometimes used as a substitute for teflon in some non-stick frying pans.

Rope, cable, sheath

The fiber is used in woven rope and in cable, where the fibers are kept parallel within a  polyethylenesleeve. The cables have been used in suspension bridges such as the bridge at Aberfeldyin Scotland. They have also been used to stabilise cracking concrete cooling towers by circumferential applicationfollowed by tensioning to close the cracks. Kevlar is widely used as a protective outer sheath for opticalfiber cable, as its strength protects the cable from damage and kinking. When used in this application it iscommonly known by the trademarked name parafil.

Electricity generation

Kevlar was used by scientists at Georgia Institute of Technologyas a base textile for an experiment in electricity-producing clothing. This was done by weaving zincoxide nanowires into the fabric. If successful, the new fabric would generate about 80 milliwatts per square meter.

Building construction

A retractable roof of over 60,000 square feet (5,575 square metres) of Kevlar was a key part of thedesign of Montreal's Olympic stadiumfor the 1976 Summer Olympics

. It was spectacularly unsuccessful, as it was completed ten years late and replaced just ten years later in May 1998 after a series of problems.

Brakes

The chopped fiber has been used as a replacement for asbestos in brake pads. Dust produced fromasbestos brakes is toxic, while aramids are a benign substitute.

Expansion joints and hoses

Kevlar can be found as a reinforcing layer in rubber  bellowsexpansion joints and rubber  hose

s, 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.

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Particle physics experiment

A thin kevlar window has been used by the NA48 experiment at CERNto separate a vacuum vessel from a vessel at nearly atmospheric pressure, both 192 cm in diameter.The window has provided vacuum tightness combined with reasonably small amount of material (only0.3% to 0.4% of radiation length).

Composite materials

Aramid fibers are widely used for reinforcing composite materials, often in combination with carbon fiber and glass fiber. The matrix for high performance composites is usually epoxy resin. Typical applicationsinclude monocoquebodies for F1 racing cars, helicopter rotor blades, tennis, table tennis, badmintonand squash rackets, kayak

s, cricketbats, and field hockey, ice hockeyand lacrossesticks.