Kevlar and Nomex differ by where one chemical link sits on a carbon ring
The fibre in bulletproof vests and the fabric in firefighters' suits are close chemical cousins. Both are aramids, chains of six-carbon rings joined by amide links. Attach those links on opposite sides of the ring and you get Kevlar's strength; attach them two atoms apart and you get Nomex's fireproofing.
Aramid is short for aromatic polyamide, a label coined in 1972 and accepted by the US Federal Trade Commission in 1974 as a fibre category separate from nylon. The dividing line is precise: at least 85 percent of the amide links, each a CO group joined to an NH group, must connect two aromatic rings. Below that the material counts as nylon. Because the molecules line up along the fibre axis, more of the chemical bonds bear load, and the result melts only above 500 °C.
Geometry decides personality. In para-aramids the links sit at positions 1 and 4 on the ring, directly opposite each other; in meta-aramids they sit at positions 1 and 3. DuPont commercialised the meta form first in the early 1960s, calling it HT-1 and then Nomex, a fibre that handles like ordinary textile yet neither melts nor catches fire in normal air. It went into protective clothing, insulation, filters and asbestos replacements.
The stiffer, stronger para form followed, building on research by Monsanto and Bayer. DuPont launched it as Kevlar in 1973, and Akzo brought out the nearly identical Twaron in 1978, triggering a patent fight through the 1980s partly over the solvent system invented at Akzo by Leo Vollbracht, which uses calcium chloride and an organic solvent to dissolve the stubborn polymer. The fibre is then spun from pure anhydrous sulfuric acid. Global para-aramid capacity was about 41,000 tonnes a year in 2002.
Uses run from body armour, where it competes with polyethylene fibres like Dyneema, to brake linings, tyres, sailcloth, tennis strings and climbing prusiks that might otherwise melt from rope friction. Health questions arose in the 1990s, when lab tests showed some asbestos-like effects on cells. A 2009 review concluded inhaled fibrils are shortened and cleared quickly, though it was funded by DuPont and Teijin Aramid.
Source: Aramid
