Kevlar is very strong and very light, weight for weight about five times as strong as steel. This is due to its chemical structure, as well as the way the polymer has been processed before leaving the factory. Kevlar is a polymeric aromatic amide, an aramid. This means it is a polymer containing a benzene ring, linked together through amide groups. Because the ratio of carbon to hydrogen atoms is quite high it requires strong concentrations of oxygen before it starts to burn--leading to a low flammability. The planar aromatic rings polymerize in rigid chains, and hydrogen bonding between the hydrogen atoms of one chain, and oxygen of another leads to a strong planar sheet structure. When the material is made into fibres, the flat sheets are spun 360 degrees around the fibre axis, forming the cylindrical fibre shape.
Kevlar has a high price at least partly because of the difficulties caused by the use of concentrated acid in its manufacture. Kevlar is very insoluble and during polymerization it starts to precipitate out of the solution before chains of sufficient length have formed. The acid is necessary to keep the polymer in solution long enough for polymerisation to occur properly.
An early use for Kevlar was to replace steel cords in car tyres. It is now commonly used in bullet-proof vests, other types of light armor and extreme sports equipment such as bike saddles and plastrons for fencing.