Kevlar (para-aramid) is a strong, heat-resistant synthetic fiber, related to other aramids such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, 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 or fabric sheets that can be used as such, or as an ingredient in composite material components. Kevlar has many applications, ranging from bicycle tires and racing sails to bulletproof vests, due to its high tensile strength-to-weight ratio; by this measure it is five times stronger than steel. It is also used to make modern marching drumheads that withstand high impact, and for mooring lines and other underwater applications. A similar fiber, Twaron, with the same chemical structure was developed by Akzo in the 1970s. Commercial production started in 1986, and Twaron is manufactured by Teijin Aramid. The term Kevlar is of obscure origin and likely created arbitrarily.
History
Poly-paraphenylene terephthalamide (K29) – branded Kevlar – was invented by the 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 fiber to use for light, but strong, tires. The polymers she had been working with, poly-p-phenylene-terephthalate and polybenzamide, formed liquid crystals in solution, unlike other 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 fiber 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. In 1971, Lester Shubin, who was then the director of Science and Technology for the National Institute for Law Enforcement and Criminal Justice, suggested using Kevlar to replace nylon in bullet-proof vests. 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. Shubin received a $5 million grant to research the use of the fabric in bullet-proof vests. Kevlar 149 was invented by Jacob Lahijani of Dupont in the 1980s.
Production
Kevlar is synthesized 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 behavior, and mechanical drawing orients the polymer chains in the fiber's direction. Hexamethylphosphoramide (HMPA) was the solvent initially used for the polymerization, but for safety reasons, DuPont replaced it by a solution of N-methyl-pyrrolidone and calcium chloride. As this process had been patented by Akzo (see above) in the production of Twaron, a patent war ensued. Kevlar production is 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, tires, and brake linings. Kevlar K49 – high modulus used in cable and rope products. Kevlar K100 – colored version of Kevlar Kevlar K119 – higher-elongation, flexible and more fatigue resistant Kevlar K129 – higher tenacity for ballistic applications Kevlar K149 – highest tenacity for ballistic, armor, and aerospace applications Kevlar AP – 15% higher tensile strength than K-29 Kevlar XP – lighter weight resin and KM2 plus fiber combination Kevlar KM2 – enhanced ballistic resistance for armor applications The ultraviolet component of sunlight degrades and decomposes Kevlar, a problem known as UV degradation, 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 of about 3,000 MPa (440,000 psi), and a relative density of 1.44 (0.052 lb/in3). 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 fibers such as ultra-high-molecular-weight polyethylene. The presence of salts and certain other impurities, especially calcium, 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 like silk protein.
Thermal properties 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%.
Applications
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