Kevlar is a fibre so strong that layers of its cloth stop bullets. Like nylon, it is a polyamide, its monomers joined by amide links, and its strength comes from the form of its chains.
Kevlar is made from benzene-1,4-diamine, a benzene ring with an -NH₂ group on two opposite corners, and benzene-1,4-dicarboxylic acid, the same acid that makes PET. Each amine group joins an acid group through an amide link, so the chain is benzene rings and amide links taking turns. Factories use a faster-reacting form of the acid, with each -OH swapped for a chlorine atom, so each new link gives off hydrogen chloride, HCl.
Drag the model to turn it. Pinch to zoom, or use the buttons.
Drag the model to turn it. Pinch to zoom, or use the buttons.
Straight, stiff rods
Every part of Kevlar's chain is flat and stiff. A benzene ring is flat, the atoms of an amide link lie flat in one plane, and each ring joins the chain at opposite corners, so the chain runs straight, like a rod. Nylon's chains have runs of CH₂ groups that twist about their single bonds, so they bend and fold.
Stiff rods line up almost perfectly. Side by side, Kevlar's chains make flat sheets, with each N–H on one chain hydrogen-bonded to a C=O on the next, and the sheets stack on top of each other. The polymer is spun from a solution in concentrated sulfuric acid, and the rods line up as they flow through the spinneret, so the fibre comes out with its chains already in line.
Strong and heat-proof
To break a Kevlar fibre, a pull has to overcome a great many hydrogen bonds at once, along chains that all point the same way. DuPont, which makes it, puts Kevlar at about five times as strong as steel, weight for weight. It has no melting point: heated, it starts to break down at about 450 °C. Layers of woven Kevlar in body armour catch a bullet and spread its energy over many fibres, and Kevlar also goes into cut-resistant gloves, bicycle tyres, sails and firefighters' clothing.