Kevlar: Bullet-Proof Polymer
Plastic that is Five Times Stronger than Steel
Chemical structure and properties of the polymer which is used in body-armour and tennis rackets.
Of all the different polymers produced during the twentieth century, the one that has probably saved the most lives is known by the trade-name of Kevlar. Its range of properties have set it apart as one of the most useful long-chain compounds yet developed.
Polyamide
Kevlar is also known as poly(p-phenyleneterephtalamide). It is a polyamide, as is the equally famous polymer known as Nylon. A polyamide has recurring units joined by amide links which have a carbonyl group (C=O) and a amine group (N-H). In Kevlar the repeating groups between the amide links are benzene rings, flat hexagonal rings of six carbons each of which is bonded to a hydrogen atom.
Perfect Fibres
This structure makes this polymer form almost perfect fibres. This is for two reasons. First, the chains are always held straight because the bulk of the benzene rings prevent the amide links rotating and forming kinks in the chain, like they do in other nylons. Second, the benzene rings stack on top of each other very neatly and are held together by hydrogen bonds between the amide links, forming very regular, strong fibres.
Liquid Crystal
Although the polymer presented the possibility of very useful properties, for a long time there was a problem. The very properties which made it attractive also meant that it was totally unworkable; it would not melt and it did not dissolve in any solvents. Eventually, in 1965, DuPont research scientist, Stephanie Kwolek, devised a way of producing the polymer in a liquid crystal solution. Using the combination of a solvent called N-methyl pyrrolidone and calcium chloride, she produced the polymer suspended in solution from which they could be spun into fibres.
Properties
The properties of these fibres are truly amazing. Most amazing is its strength, it is five times as strong as steel, weight for weight. It is resistant to chemical attack and does not rust. It also withstands extremes of temperature, both high temperatures up to 400ºC and low temperatures as low as -196ºC.
Applications
These properties mean that this polymer can be used in a range of extreme applications. It is best known for its use in bullet-proof vests and other body armour used by the armed services, police officers and prison officers. It is also a constituent in many types of sports equipment such as tennis rackets and kayaks. It is used to make bicycle tyres puncture resistant, and to protect the engine compartments of passenger aircraft. It is also used in ropes and cables in suspension bridges, on ships and even in space.
This synthetic material, brought into our service by the ingenuity of a research chemist, provides a very useful material for so many different areas of life.
