Kevlar
Kevlar (poly-paraphenylene terephthalamide) is the DuPont Company’s brand name for a synthetic material constructed of para-aramid fibers that the company claims is five times stronger than the same weight of steel, while being lightweight, flexible and comfortable. It is also very heat resistant and decomposes above 400 °C without melting. It was invented by Stephanie Kwolek of DuPont from research into high performance polymers, and patented by her in 1966. Kevlar is a registered trademark of E.I. du Pont de Nemours and Company.
Originally intended to replace the steel belts in tires, it is probably the most well known name in soft armor (bulletproof vests). It is also used in extreme sports equipment, composite aircraft construction, fire suits and as an asbestos replacement.
When this polymer is spun in the same way that a spider spins a web, the resulting commercial para-aramid fiber has tremendous strength, and is heat and cut resistant. Para-aramid fibers do not rust or corrode, and their strength is unaffected by immersion in water. When woven together, they form a good material for mooring lines and other underwater objects. However, unless specially waterproofed, para-aramid fiber’s ability to stop bullets and other projectiles is degraded when wet.
Properties
Kevlar is a type of aramid that consists of long polymeric chains with a parallel orientation. Kevlar derives its strength from inter-molecular hydrogen bonds and aromatic stacking interactions between aromatic groups in neighboring strands. These interactions are much stronger than the van der Waals interaction found in other synthetic polymers and fibers like dyneema. The presence of salts and certain other impurities, especially calcium, would interfere with the strand interactions and has to be avoided in the production process. Kevlar consists of relatively rigid molecules,which form a planar sheet-like structure similar to silk protein.
These properties result in its high mechanical strength and its remarkable heat resistance. Because it is highly unsaturated, i.e. the ratio of carbon to hydrogen atoms is quite high, it has a low flammability.
Kevlar molecules have polar groups accessible for hydrogen bonding. Water that enters the interior of the fiber can take the place of bonding between molecules and reduce the material's strength, while the available groups at the surface lead to good wetting properties. This is important for bonding the fibers to other types of polymer, forming a fibre reinforced plastic. This same property also makes the fibers feel more natural and "sticky" compared to nonpolar polymers like polyethylene.
In structural applications, Kevlar fibers can be bonded to one another or to other materials to form a composite.
Kevlar's main weaknesses are that it decomposes under alkaline conditions or when exposed to chlorine. While it can have a great tensile strength, sometimes in excess of 4.0 GPa, like all fibers it tends to buckle in compression.
Production
Kevlar is synthesized from the monomers 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride. The result is a polymeric aromatic amide (aramid) with alternating benzene rings and amide groups. When they are produced, these polymer strands are aligned randomly. To make Kevlar, they are dissolved and spun, causing the polymer chains to orient in the direction of the fiber.
Kevlar has a high price at least partly because of the difficulties caused by the use of concentrated sulfuric acid in its manufacture. These harsh conditions are needed to keep the highly insoluble polymer in solution during synthesis and spinning.
