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Kevlar

Just after World War II ended, Stephanie Louise Kwolek tucked her new chemistry degree from the Carnegie Institute of Technology under her arm and—because she couldn’t afford medical school—took a research job at DuPont’s textile fibers department in Buffalo. Although she faced many challenges as one of the few women in chemical research, she liked the work so much that she soon dropped her plans to become a doctor. Two decades later she would invent Kevlar, one of the world’s most versatile materials, and along with it a new branch of polymer chemistry.

By the time Kwolek joined DuPont, the company had already enjoyed success with developing the synthetic fabric nylon. Other synthetics would soon come from the fibers department, including Dacron polyester and Lycra spandex. Kwolek became a seasoned researcher, eventually tasked with developing the next-generation high-performance fiber.

In 1964 a rumored gasoline shortage opened a potential market for DuPont. Kwolek and her colleagues developed a reinforcing fiber that could replace steel in radial tires and make vehicles lighter and more fuel efficient. Manufacturers put up great resistance to the innovation, however, because they used cheap steel wire for reinforcement. A changeover from steel to fiber would require new machinery at the tire plants. The idea went nowhere.

While not a commercial success, the project still yielded dividends. It had given Kwolek the chance to work with para-oriented aromatic polyamides, or durable polymers consisting of rodlike molecules joined with peptide bonds. Chemically, polymers are large molecules that consist of repeating units. They appear in nature in the form of proteins, nucleic acid, and the cellulose found in wood. Synthetic polymers form plastic, neoprene, polyethylene, and nylon. The stiff molecules of the polyamides that Kwolek worked with had quite different characteristics than the very flexible molecules of nylon that had proven ideal for stockings and other clothing. As she experimented with different kinds of these stiff polymers, she also looked for a solvent that would dissolve them into a solution and enable her to form fibers. In 1965 she found a solvent, but it created solutions that were cloudy, opalescent upon being stirred, and of low, almost waterlike, viscosity.

To create a fiber from a polymer solution, Kwolek needed to have the liquid forced through the tiny holes of a spinneret. Assuming that particulates caused the cloudiness, the man in charge of the spinneret at DuPont refused at first to spin the solution, protesting that it would plug up the holes. Kwolek insisted that the polymer solvent had bonded chemically and that the cloudiness had nothing to do with undissolved matter. They spun it without problems. She picked up a fiber and was surprised at how difficult it was to break.

Kwolek’s tests of the fiber’s tensile strength, elongation at break, and stiffness confirmed that she had created an extraordinary material, five times stronger than steel. Her supervisor and laboratory director immediately expanded her program. Today there are more than 200 applications for Kevlar, ranging from airplane parts and reinforced suspension bridge structures to flame-resistant mattresses, boots, and, famously, body armor. In 2006 DuPont recognized Cory Grogan of Atlanta as the 3,000th police officer saved by a Kevlar vest.

Ironically, the tire manufacturers who initially snubbed Kevlar’s cousin polymer have finally come around. In March 2009 Goodyear released its first Kevlar off-road tire model, the Wrangler MT/R, a stable alternative to traditional rubber and more resistant to torsion, tension, and heat.