Several years ago, during a conversation with a police officer, a salesman took a gun and fired it against himself, to prove the quality of the DuPont Kevlar¹ vests he was selling. He flew about 2 metres backwards, but wasn't injured. This example shows the value of Kevlar's unusually high strength.

Discovery

The research leading to Kevlar was undertaken mainly by Stephanie Louise Kwolek of DuPont. Stephanie joined DuPont in 1946 with a chemistry degree from Carnegie Institute of Technology. She was especially interested in polymer chemistry, and in 1964 she began experimenting with poly-p-phenylene-terephthalate (PPD-T) and polybenzamide (PBA). Stephanie was a creative researcher and was intent on breaking new ground in the then male-dominated world of chemical engineering.

She became the first to prepare pure monomers that could be used to synthesise PBA. She found a solvent and identified the low-temperature polymerisation conditions, and this produced an unusual polymer solution that was fluid and cloudy, rather than clear and viscous. She found that the tough fibres which formed had startling properties. They were stronger and stiffer than any known synthetic fibre. Eventually, from these findings, Kevlar was born in 1971.

Chemical Properties

Poly-para-phenylene terephthalamide is a polymer. It's a polymer that contains recurring amide groups (R-CO-NH-R) as integral parts of the main polymer chain. It's made by a condensation reaction between monomers, in which the molecules are linked through the formation of the amide groups. The most important amide polymers are the nylons, a versatile class of material that is an indispensable fibre and plastic.

Specifically, Kevlar is an aramid, an aromatic nylon. It's prepared by condensation of a diamine and terephthalic acid, which is a carboxylic acid that contains a hexagonal benzene ring in its molecules. The close packing of the aromatic polymer chains, along with the orientation of the molecules produces an exceptionally strong fibre.

The polymer used for Kevlar is wet-spun from a solution of concentrated sulphuric acid. Due to the rod-like structure of the aramids, a 'liquid-crystalline' solution is obtained. This orients the molecules in a specific direction even before they are spun, creating fibres of ultrahigh strength and stiffness. Kevlar is lighter than nylon, has a tensile strength about five times that of steel and is more resistant to abrasion than other high-strength fibres. It has no melting point but decomposes at temperatures above 400° Celsius.

Applications

Kevlar is a versatile material which is strong, tough, stiff, high-melting and well suited for uses such as radial tyres, heat- or flame-resistant fabrics, and bullet-proof clothing². Other commercial applications include cables, aircraft panels and boat hulls, sports equipment such as golf club shafts and lightweight bicycles, and as asbestos replacements in clutches and brakes.

It is available in such forms as yarns, unwoven mats and chopped fibres. Because of its toughness, it requires special cutting and machine techniques to be worked. Recently, it has started to be used in composite materials applications, like strengthening carbon fibre structures.

As has been said, it is extremely strong. One university used to have a demonstration set up in their engineering workshop where they had a 250lb engine suspended from the joists by a piece of Kevlar thread the thickness of sewing thread.

Kevlar is revolutionary and its design innovations make it one of the toughest commercial materials in existence today. One day, it could save your life, one way or the other.

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¹ Kevlar, the trade name for poly-para-phenylene terephthalamide, is a registered trademark of DuPont.
² Herbert Frood invented the first (non-Kevlar) bullet proof clothing in 1916.