Kevlar

Meera Senthilingam

This week, a strong compound which revolutionised our modern materials and has saved many lives. And its discovery was all down to a particularly dedicated chemist, as David Lindsay explains:

David Lindsay

When American chemistry major Stephanie Kwolek went looking for a job to fund her way through medical school in the 1940s, she could hardly have imagined that the temporary job she'd pick up would last over 40 years. Although she'd never make it to med school, she still saved lives, and thousands of them, by discovering a new material that would revolutionise almost every aspect of the modern world.

In 1946, 23 year old Stephanie Kwolek graduated with a major in chemistry from what is now Carnegie-Mellon University. She was keen to enrol in medical school, and decided to find a job to fund her studies. So she started work at DuPont, the company which gave us Nylon, Teflon and countless other useful inventions, and whose slogan at the time was 'Better things for better living through chemistry'. Kwolek would be so successful at DuPont, and find her chemical research so rewarding, that she stayed at DuPont until her retirement, 40 years later, in 1986. In the mid-1960s, a petrol shortage was forecast in the United States, and one of DuPont's ideas to deal with this was to make car tyres more durable, and hence more efficient. Kwolek picked up the project and began making new, polyamide-based materials. Unlike another polyamide, Nylon, where the amide groups are linked by a flexible line of six carbon atoms, Kwolek's polymers featured amide groups joined linearly through a benzene - six carbon atoms in a planar, rigid, hexagon-shaped ring. Benzene is from a class of compounds called aromatics, and these polymers were polyaromatic amides, which became known as aramids. These new polymers behaved strangely in solution, giving cloudy, thin solutions, as opposed to the clear viscous solutions polymers normally give. It turned out that these aramids displayed liquid crystalline properties, and they were not viewed as promising candidates for the tyre strengthening project. However, Kwolek was undeterred. She managed to persuade her colleagues to spin her new materials into fibres. The fibres turned out to have incredible properties, with a tensile strength five times as much as steel, in a material that was as light as fibreglass. DuPont capitalised rapidly on the discovery of this wonder material, and by the 1970s the fibre was being sold under the name of Kevlar, and being used in a host of applications.

So what gives Kevlar its remarkable strength? Each polymer molecule is a long, linear chain, and these chains bundle into fibres, held together by strong intermolecular forces of hydrogen bonding between the amide groups on different chains, and through a powerful association of the benzene rings known as pi-stacking. Concentrated solutions of Kevlar are spun into extremely tough fibres, in a process which is crucial to producing material with high strength. The result of these molecular forces and the processing of the polymer is an incredibly strong bulk material, which has found applications in countless areas of our life.

These applications of Kevlar are all around us. For a start, and despite those initial doubts, Kevlar has indeed proved to be useful in toughening up both car and bicycle tyres, as was originally intended. Kevlar is also used in the composites of carbon fibre bicycle frames, and in the composites used in boats and even in boat sails. Kevlar reinforces high pressure hoses and undersea cables, and is used as a protective sheath for delicate fibre optic cables. Its relative stability at high temperature means it finds use in personal protective equipment for firefighters, as well as in fire-resistant mattresses. It's even possible to have a hurricane resistant 'storm room' built into your home, which uses Kevlar barriers to protect the occupants. However, although Kevlar is physically very strong, it does carry a chemical weakness: as with many polymers, ultraviolet radiation causes the polymer chains to degrade and so Kevlar must be protected from sunlight when used outdoors.

Its combination of strength and lightness leads to perhaps the most famous use of Kevlar, in the protective equipment used by military and law enforcement personnel: bulletproof vests and helmets; a modern day suit of armour. This application of Kevlar has saved thousands of lives, and indeed there is a story of one American policeman who visited Stephanie Kwolek and asked her to autograph the bulletproof vest that saved his life. Kwolek also received more formal honours for her research, including induction into the National Inventor's and National Women's Halls of Fame, the National Medal of Technology and the American Chemical Society's Perkin Medal.

When Stephanie Kwolek began her research into reinforcing car tyres, nobody could have known the material that would emerge from her work would affect so much of our lives, and would save so many when used in bulletproof vests. Kwolek's work has turned out to be research with real impact.

Meera Senthilingam 

And that's thanks to Kevlar's protection against external imapct. That was Glasgow University's David Lindsay with the chemistry of Kevlar. Now, next week: more life saving, but on a biological level.

Simon Cotton

10 million soldiers died in the first world war, about two thirds of them on the field of battle. The rest of the deaths were due to diseases post-conflict, and many of those were caused by the bacterial infection of wounds. In October 1914, just two months after the outbreak of war, the director general of the British Army Medical Service said:  'We have in this war gone straight back to all the septic infections of the Middle Ages'. Many doctors and scientists who survived the battlefields remembered this and it inspired their subsequent researches. One of these was a Scot who served in the Royal Army Medical Corps of the British Army. His name was Alexander Fleming.

Meera Senthilingam

And to find out how Fleming used this inspiration to discover a compound to fight these infections - penicillin - join Simon Cotton in next week's Chemistry in it's element. Until then, thank you for listening. I'm Meera Senthilingam.