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What makes Kevlar¿ so strong? And how can it be so light at the same time?

kevlar vest



DUPONT
Chemist Vlodek Gabara, a DuPont Fellow, explains.

Kevlar¿ is an organic fiber in the aromatic polyamide (aramid) family that combines high strength with light weight, and comfort with protection. Kevlar¿ is five times stronger than steel on an equal weight basis and provides reliable performance and solid strength. This unique combination of attributes ensures that members of law enforcement, corrections personnel and the military will be safe from harm that can come in many forms, including bullets, knives, switchblades and shrapnel. In fact, Kevlar¿ garments have so far saved the lives of nearly 3,000 law enforcement officials.

DuPont discovered Kevlar¿ in 1965. Before then, scientists knew that chemical bonds between atoms were very strong, but researchers were unable to arrange these molecules into large structures (relative to the size of a molecule) to capitalize on this strength. Using organic polymers based on "light elements"--such as carbon, nitrogen, hydrogen and oxygen--rather than "heavy elements" such as iron, gives the advantage of low-weight structures. For example, Kevlar¿ fiber has a density of 1.4 grams per cubic centimeter compared with iron's 7.9 grams per cubic centimeter. To achieve both the strength and stiffness of Kevlar¿, the molecular chains within the organic fiber needed to be fully extended and perfectly aligned to make them strong, stiff and tough. Such a high degree of alignment was not easy to achieve. Kevlar¿ fibers are based on poly-paraphenylene terephthalamide, a rigid molecule that makes it easier to realize a fully extended, or straight, chain configuration.

Also, these rigid molecules will even arrange in solutions. Such solutions are called liquid crystalline, which underscores their good organization. Poly-paraphenylene terephthalamide molecules behave like uncooked spaghetti, whereas other, less rigid molecules behave more like cooked strands of spaghetti. Thus, the nature of the molecule makes it easier to achieve the desired aligned structure. In addition, poly-paraphenylene terephthalamide strongly resists high temperatures and flames. Offering strength under heat, Kevlar¿ protects against thermal hazards up to 800 degrees Farenheit.

This combination of unusual properties makes Kevlar¿ useful for a broad range of applications, such as ballistic vests, cut-resistant gloves and blast and flame barriers. Kevlar¿ has also boosted sports gear performance. Applications in that vein include bicycle tires that are virtually flat-free and puncture-resistant; running shoes that maximize the energy output of runners; boats that are lighter and more damage-tolerant; and durable lightweight sails that tolerate high winds and saltwater.

Originally published on September 30, 2002.

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