Kevlar



Kevlar is the DuPont Company's registered trademark for a very light, very strong synthetic fiber created in 1965 by Stephanie Kwolek and Herbert Blades. 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 body armor, because of its high strength-to-weight ratio, "...5 times stronger than steel on an equal weight basis...". Under water, Kevlar is less resistant to ballistic projectiles, although it is water resistant. 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 by bullets of different calibres 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 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 monomers 1,4-phenylene-diamine (para-phenylenediamine) and terephthaloyl chloride in condensation reaction yielding hydrochloric acid as a byproduct. The result is a 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.



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.

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 interactions between stacked 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 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 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.

At 450° C Kevlar sublimates.