Kevlar was developed in the 1960s in response to a demand for a ‘super fibre’. It certainly met those demands.
Kevlar is five times as strong as steel on a weight for weight basis, yet at the same time is lightweight, flexible and comfortable, hence one of its most longstanding uses – in bullet proof clothing.
Look at the structure of Kevlar shown below. Sections of three chains are shown.
© NASA/Carla Thomas
The chains are similar to those in nylon in that they contain the repeating amide group. So, like nylon chains, they can attract each other and line up via hydrogen bonding. This accounts in part for the substance’s strength. However, unlike nylon, there is no twisting possible along the direction of the chain, so a flat, very ordered sheet is formed.
These sheets can align around the fibre axis leading to an incredibly strong product. The only atoms involved are carbon, hydrogen, oxygen and nitrogen, hence its lightweight properties.
© Science Museum/Science and Society Picture Library
How does the bulletproof vest work? The fibres are cross woven together creating a web. The bullet is caught in this web. The fibres absorb some of the energy of the bullet and disperse it to other fibres in the weave.
New uses are being found for Kevlar all the time. The ropes that secured the airbags in the Mars Pathfinder landing apparatus were made from Kevlar. It is used to make kayaks because it provides excellent impact resistance for little weight. It is also used in run-flat tires because the rim isn’t damaged when driving to the nearest assistance.
The illustration below shows just how many parts of modern cars are made from modern materials, including polymers.
Mars Pathfinder solar-powered research aircraft, test flight, November, 1996. The ropes that secured the airbags for Pathfinder’s landing apparatus were reinforced with Kevlar.
Kevlar: tough enough to stop shots going through this UK policewoman's bullet- and stab-resistant vest, 1996.