What Makes Kevlar So Strong, and How Can It Be So Light At The Same Time?

Air Date: 10/17/2005
Source:
Scientific American
Creator:
Vlodek Gabara
Air/Publish Date:
10/17/2005
Event Date:
10/17/2005
Resource Type:
Article
Copyright:
n/a
Copyright Date:
2005
Clip Length:
-

This 2005 Scientific American article explains how Kevlar, an aromatic polyamide (aramid) fabric, is strong enough to protect against ballistics and shrapnel, yet lightweight enough for soldiers to maneuver in and vehicles to carry without compromising speed or fuel-efficiency. Source: Scientific American, October 17, 2005

What Makes Kevlar So Strong, and How Can It Be So Light At The Same Time?

October 17, 2005

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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