Kevlar

Kevlar is a synthetic fiber known for its strength and heat resistance, belonging to the aramid family alongside Nomex and Technora. It was created by American chemist Stephanie Kwolek at DuPont in 1965, and its first commercial use came in the early 1970s, replacing steel in racing tires. The material is usually processed into ropes or fabric sheets, which can stand alone or be incorporated into composite materials.

The fiber’s high tensile strength-to-weight ratio—five times that of steel—makes it suitable for a wide range of products, from bicycle tires and racing sails to bulletproof vests. It is also used in modern marching drumheads that endure heavy impacts, as well as in mooring lines and other underwater applications. A chemically identical fiber called Twaron was developed by Akzo in the 1970s, with commercial production beginning in 1986 under Teijin Aramid. The name “Kevlar” is thought to have been created arbitrarily, with its exact origin unclear.

Kwolek’s work began in 1964, when her DuPont group sought a lightweight, strong fiber for tires in anticipation of a gasoline shortage. Her polymers, poly-p-phenylene-terephthalate and polybenzamide, formed liquid crystals in solution—unlike other polymers at the time. The solution appeared cloudy and opalescent when stirred, with low viscosity, and was typically discarded. Kwolek convinced technician Charles Smullen to test it on a spinneret, and the resulting fiber did not break, unlike nylon. Her supervisor and lab director recognized the breakthrough, sparking a new field of polymer chemistry. Modern Kevlar was introduced by 1971, though Kwolek was not heavily involved in developing its applications.

In 1971, Lester Shubin, director of Science and Technology for the National Institute for Law Enforcement and Criminal Justice, proposed using Kevlar to replace nylon in bulletproof vests. Earlier flak jackets made of nylon offered limited protection. Shubin recalled folding the material and shooting at it; the bullets did not penetrate. In tests, Kevlar was strapped onto anesthetized goats, which were shot in the heart, spinal cord, liver, and lungs. The animals’ heart rate and blood gas levels were monitored for lung injuries. After 24 hours, one goat died, while the others sustained non-life-threatening wounds. Shubin received a $5 million grant to research Kevlar for vests. A later variant, Kevlar 149, was invented by Jacob Lahijani at DuPont in the 1980s.

Kevlar is produced by a condensation reaction between 1,4-phenylene-diamine and terephthaloyl chloride, which yields hydrochloric acid as a byproduct. The resulting material has liquid-crystalline behavior, and mechanical drawing aligns the polymer chains along the fiber’s axis. Initially, hexamethylphosphoramide (HMPA) was used as the solvent, but DuPont replaced it with a mixture of N-methyl-pyrrolidone and calcium chloride for safety reasons. This process had already been patented by Akzo for Twaron, leading to a patent dispute. Production is costly because concentrated sulfuric acid is required to keep the water-insoluble polymer in solution during synthesis and spinning.

Several grades of Kevlar exist: K-29 for industrial uses like cables, asbestos replacement, tires, and brake linings; K49, a high-modulus version for cables and ropes; K100, a colored variant; K119, which offers higher elongation, flexibility, and fatigue resistance; K129, with higher tenacity for ballistic applications; K149, the highest tenacity for ballistic, armor, and aerospace uses; AP, with 15% greater tensile strength than K-29; XP, a lighter-weight resin combined with KM2 plus fiber; and KM2, designed for enhanced ballistic resistance in armor. Ultraviolet sunlight degrades Kevlar, so it is rarely used outdoors without protection.

When spun, Kevlar fiber has a tensile strength of about 3.6 GPa and a relative density of 1.44. Its strength comes from numerous inter-chain bonds, including hydrogen bonds between carbonyl groups and NH centers, as well as aromatic stacking interactions between adjacent strands. These interactions are more influential than the van der Waals forces and chain length that typically affect other synthetic polymers like ultra-high-molecular-weight polyethylene. Impurities such as salts or calcium can disrupt these bonds, so production avoids their inclusion. The fiber’s rigid molecules tend to form planar, sheet-like structures similar to silk protein. Kevlar maintains its strength and resilience down to cryogenic temperatures, where it is slightly stronger, while its tensile strength decreases at higher temperatures.