Walk through fire and bullets

It's as flame-proof as asbestos and stronger than steel. John Emsley on the miracle plastic, Kevlar

Earlier this month, the Metropolitan Police announced that all officers requiring one would be supplied with a bullet- and stab-proof vest. It marked another small triumph for a material whose properties put most other plastics to shame. Besides warding off bullets, Kevlar now adds strength to tennis rackets, Formula One cars, space suits, military armour, safety gloves, oil tanker hawsers and optical fibres.

Kevlar was discovered in 1965 by Stephanie Kwolek, who was working for the US chemical giant Du Pont on a project to design a fibre that had the heat resistance of asbestos and the stiffness of glass fibre. Problems arose from the nature of the polymer and the special solvent needed to make it, which was discovered to be carcinogenic.

These delayed its launch until 1982, by which time it had cost $500m to develop. It was hailed as "a miracle in search of a market". It is still searching for the elusive mass market, despite its unique properties. The original hope was that it would replace the rayon fibre and steel wire reinforcement in tyres. Du Pont refuses to say how much Kevlar is produced at its plants in the US, Japan and Maydown in Northern Ireland.

The polymer consists of long strands of benzene rings interconnected with amide groups, very like those in protein. Kevlar forms when a benzene with two amine groups reacts with another benzene with two acid chloride groups. What gives the polymer its remarkable strength is its regularity of structure. In most fibres the strands of polymer are a random, tangled mass, but in Kevlar the attractive forces between strands are so strong that they line up into parallel rows, flat sheets, and pack as rigid layers on top of one another.

This regularity creates processing problems because it makes the polymer insoluble, although it will dissolve in pure sulphuric acid, from which it can be extracted unharmed. This is one way in which Kevlar can be processed. In addition to being almost immune to chemical attack, Kevlar is also fire-resistant, flexible and lightweight. Spun into fibres and heat-treated, the fibres get even stronger - leading to their use in so many pieces of equipment.

At a presentation on the role of chemistry in sport at the House of Commons last month, Professor John Holloway of Leicester University extolled its virtues. "Five times stronger than steel and more elastic than carbon fibre," he said, claiming that it had pushed the performance limits of sports equipment well beyond that of traditional materials.

When Kevlar fails, it does so progressively rather than catastrophically, thereby providing another margin of safety. Thus Formula One racing cars use it to protect the drivers, though its limitations do show up. Brian O'Rourke, chief structural engineer of Williams Grand Prix Engineering at Wantage, Oxfordshire, said: "While Kevlar may have high tensile strength it has poor compression strength, and is difficult to paint. Even so, because it has good rigidity-to-weight ratio it is incorporated into the laminate used to reinforce the driver's survival cell, where it provides superb puncture resistance."

Few, if any, plastics offer the package of benefits that Kevlar provides. It is flame-resistant and self-extinguishing, and gives off little smoke, so it is preferred for conveyor belts, especially in mines, and for hoses used in the chemical industry and engines. Oil tankers use Kevlar rather than steel mooring ropes.

Finally, it can stand extremes that defeat many other plastics: it does not become brittle at low temperatures, even as low as -70C, so optical fibres are coated with it if they are to be exposed to the severity of mountain conditions. It is unaffected by long exposure to weathering or the sea, and three years immersed in either boiling water or in hydrocarbon solvent leave it unchanged.

Besides bullet-proofing, Kevlar's life-saving properties could even stretch further. It is already used to line the engine compartments of planes, to limit damage if a turbine blade flies off. Its strength and lightness have led to its use in the framework of the Boeing 757.

It could work for bomb-proofing, too. The present theory for the cause of the crash of TWA 800 in the sea near Long Island last month, which killed 229 passengers and crew, is a bomb in the aircraft's hold. A bomb also killed the 270 people on PanAm flight 103, which exploded over Lockerbie in 1988. Both aircraft might have survived had their baggage holds been sealed with Kevlar panels, according to the Defence Evaluation Research Agency.

Dr John Emsley is science writer in residence at Imperial College, London.