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Military Mixes "Smart" Coatings

The U.S. Army wants self-healing, corrosion-resistant polymers to camouflage tanks. Researchers think nanotubes will help them get there.

Convoys of United States Army tanks are rumbling across Kuwait this month-ready again to play a key role in war with Iraq. But before they fire a single shot, those tanks are already locked in battle with old foes: whipping desert sands, blistering sun, and even the air itself.

Each year, the axis of corrosion costs the U.S. Army $10 billion dollars-$2 billion for painting and scraping alone, labor-intensive work that’s hazardous to people and the environment. So last fall, the Army’s Tank-automotive and Armaments Command, Armament Research, Development and Engineering Center (TACOM-ARDEC) in Picatinny Arsenal, NJ asked a coalition of researchers at the New Jersey Institute of Technology, Clemson University, and the University of Illinois to do something about it. The command awarded them $838,000-and promised up to $1.5 million more-to find materials that combine self-healing characteristics with the ability to change color and sense structural damage or environmental changes. Dan Watts, the man leading NJIT’s program, says their research has been pushed along by unexpectedly rapid advances in self-healing polymers and electronics made from carbon nanotubes that go “beyond the realm of interesting academic speculation and approach economic practicality.”

Finding the Right Mix

For several years, materials researchers have pursued self-healing polymers that can be applied to a surface-such as industrial machinery-and that would resist the deterioration that occurs over time (see “Nano Biomaterials,” TR, November 2002). But no one has yet found a material that can repair itself repeatedly, work as a thin coating, be stored for years, and-for military applications-resist chemical agents. To that end, the Picatinny group sought out Nancy Sottos, a professor of mechanics at the University of Illinois at Urbana-Champaign. Sottos’s lab engineered an epoxy with micron-scale capsules that burst when cracks form, quickly sealing them. (Click for animation.) “The key to making it a product is the shelf life,” Sottos says. “Right now, we mix them and break them up right away.”

In addition to self-repairing properties, the Army’s ideal smart coating must also incorporate nanoscale devices that detect corrosion at it happens-perhaps, Watts says, by sensing movement in the material.

To sense its environment, receive commands, and propagate color changes from one molecule to the next, the coatings will need wiring. At Clemson, researchers think carbon nanotubes may serve; they fill the tubes with iron to create rudimentary circuits, although it’s still unclear whether this low-power approach can create the range of colors the Army needs. Back at the New Jersey Institute of Technology, their collaborators are working to control the nanotubes with electricity, light, and laser, says Joseph Argento, deputy of the Army’s Industrial Ecology Center at Picatinny.

The collaboration is investigating micro-electromechanical systems (MEMS), under study elsewhere at Picatinny, though there’s doubt the microscopic machines will provide the right mix for smart coatings. “That’s still a sexy technology,” says Laura Battista, an environmental engineer at Picatinny who works on smart coatings. “But there’s nothing off the shelf right now.” MEMS could be useful, however, in “screens” that make vehicles invisible to satellites, say these researchers.

The ultimate goal, Watts says, is to display an image of the vehicle’s surroundings on its surface. But here’s one problem: getting the false image to blend in convincingly might require mounting the camera near the observer-the enemy, in this case. Watts says the Army might settle for a second-best “chameleon” effect that rapidly changes from one pattern to another.

Flip a Switch, Paint Your House

Smart coatings will find applications beyond the military, researchers say. Watts envisions corrosion-resistant cars that can be customized on the spot, programmable billboards, and color-changing fabrics. The exteriors of buildings could be redesigned with the flick of a switch. Car companies are in the process of developing self-healing bumpers, Argento says. “You can just imagine what this coating could do for buildings and bridges that you don’t have to worry about corroding,” he adds.

Despite the formidable hurdles, the researchers aim to produce a prototype as early as 2005 (although possibly as late as 2009) if the money lasts. Components-such as self-healing adhesives that aren’t exposed to the elements-could arrive sooner. “We’re building a good, solid technical basis that suggests this should be possible,” says Watts. “What is still unknown is how it all will be integrated.”

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