HistoryPoly-paraphenylene terephthalamide – branded Kevlar – was invented by Polish-American chemist Stephanie Kwolek while working for DuPont, in anticipation of a gasoline shortage. In 1964, her group began searching for a new lightweight strong fibre to use for light but strong tires. The polymers she had been working with at the 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 away. However, Kwolek persuaded the technician, Charles Smullen, who ran the "spinneret", to test her solution, and was amazed to find that the fibre did not break, unlike nylon. Her supervisor and her laboratory director understood the significance of her discovery and a new field of polymer chemistry quickly arose. By 1971, modern Kevlar was introduced. However, Kwolek was not very involved in developing the applications of Kevlar.
ProductionKevlar is synthesised in solution from the monomers 1,4 phenylene-diamine (para-phenylenediamine) and terephthaloyl chloride in a condensation reaction yielding hydrochloric acid as a byproduct. The result has liquid-crystalline behaviour, and mechanical drawing orients the polymer chains in the fibre's direction. Hexamethylphosphoramide (HMPA) was the solvent initially used for the polymerisation, but for safety reasons, DuPont replaced it by a solution of N-methyl-pyrrolidone and calcium chloride. As this process was patented by Akzo in the production of Twaron, a patent war ensued. The reaction of 1,4-phenylene-diamine (para-phenylenediamine) with terephthaloyl chloride yielding kevlar. Kevlar production is very expensive because of the difficulties arising from using concentrated sulfuric acid, needed to keep the water-insoluble polymer in solution during its synthesis and spinning. Several grades of Kevlar are available:
- Kevlar K-29 – in industrial applications, such as cables, asbestos replacement, brake linings, and body/vehicle armour.
- Kevlar K49 – high modulus used in cable and rope products.
- Kevlar K100 – coloured version of Kevlar
- Kevlar K119 – higher-elongation, flexible and more fatigue resistant.
- Kevlar K129 – higher tenacity for ballistic applications.
- Kevlar AP – has 15% higher tensile strength than K-29.
- Kevlar XP – lighter weight resin and KM2 plus fibre combination.
- Kevlar KM2 – enhanced ballistic resistance for armour applications
Structure & PropertiesWhen Kevlar is spun, the resulting fibre has a tensile strength of about 3,620 MPa, and a relative density of 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 from aromatic stacking interactions between adjacent strands. These interactions have a greater influence on Kevlar than the van der Waals interactions and chain length that typically influence the properties of other synthetic polymers and fibres 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-like structures rather like silk protein.
Thermal PropertiesKevlar maintains its strength and resilience down to cryogenic temperatures (−196 °C); 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 at 160 °C (320 °F) about 10% reduction in strength occurs after 500 hours. At 260 °C (500 °F) 50% strength reduction occurs after 70 hours.
ApplicationsCryogenics Kevlar is often used in the field of cryogenics for its low thermal conductivity and high strength relative to other materials for suspension purposes. Most often used to suspend a paramagnetic salt enclosure from a superconducting magnet mandrel in order to minimise any heat leaks to the paramagnetic material. It is also used a thermal standoff or structural support where low heat leaks are desired. Armour Kevlar is a well-known component of personal armor such as combat helmets, ballistic face masks, and ballistic vests. The PASGT helmet and vest used by United States military forces since the 1980s both have Kevlar as a key component, as do their replacements. Other military uses include bulletproof facemasks used by sentries and spall liners used to protect the crews of armoured fighting vehicles. Even Nimitz-class aircraft carriers include Kevlar armor around vital spaces. 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 and security. Personal Protection Kevlar is used to manufacture gloves, sleeves, jackets, chaps and other articles of clothing 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. Sports Equipment It is used as an inner lining for some bicycle tires to prevent punctures. In table tennis, plies of Kevlar are added to custom ply blades, or paddles, in order to increase bounce and reduce weight. It is used for motorcycle safety clothing, especially in the areas featuring padding such as shoulders and elbows. In Kyudo or Japanese archery, it may be used as an alternative to more expensive hemp for bow strings. It is one of the main materials used for paraglider suspension lines. In Fencing it is used in the protective jackets, breeches, plastrons and the bib of the masks. Tennis racquets are often strung with Kevlar. It is even used in sails for high performance racing boats. It is increasingly being used in the "peto", the padded covering which protects the picadors' horses in the bullring. Shoes With advancements in technology, Nike used Kevlar in shoes for the first time. It launched the Elite II Series, with enhancements to its earlier version of basketball shoes by using Kevlar in the anterior as well as the shoe laces. This was done to decrease the elasticity of the tip of the shoe in contrast to nylon used conventionally 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. Audio Equipment Kevlar has also been found to have useful acoustic properties for loudspeaker cones, specifically for bass and midrange drive units. Additionally, Kevlar has been used as a strength member in fibre optic cables such as the ones used for audio data transmissions. Building Construction A retractable roof of over 60,000 square feet (5,575 square metres) of Kevlar was a key part of the design of Montreal's Olympic stadium for 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. Smartphones The Motorola Droid RAZR has a kevlar backplate, chosen over other materials such as carbon fibre due to its resilience and lack of interference with signal transmission
Composite MaterialsAramid fibres are widely used for reinforcing composite materials, often in combination with carbon fibre and glass fibre. The matrix for high performance composites is usually epoxy resin. Typical applications include monocoque bodies for F1 racing cars, helicopter rotor blades, tennis, table tennis, badminton and squash rackets, kayaks, cricket bats, and field hockey, ice hockey and lacrosse sticks.
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