21 July 2004�Someday, drivers will be able to change the color of their cars with the flip of a switch, and paint jobs will perpetually look the way they did the car left the showroom. The basic science underlying these technologies is being worked on today by researchers who have been called on to keep the paint on military vehicles looking brand new.
Their aim, as part of a so-called smart-coating project begun in September 2002 at the U.S. Army's behest, is to produce a polymer-based prototype paint incorporating self-healing by 2005. Participating in the project are the New Jersey Institute of Technology in Newark, the Army's Armament Research, Development, and Engineering Center at Picatinny Arsenal in northwest New Jersey, the Army Construction Engineering Research Laboratory (CERL) in Champaign, Ill., and Clemson University in South Carolina. The researchers are focusing on a way to suspend microcapsules in a primer coat that is applied to a vehicle's surface before it is covered with a protective topcoat. The capsules will remain at the ready, and when the paint is scratched, their contents will spill out, resealing the scratch automatically.
The Army has good reasons for taking its paint jobs seriously. It spends around US $10 billion a year to keep its vehicles corrosion free. The cost of its current maintenance procedure, which entails periodically pulling its war machines out of service in order to scrape off old paint and apply fresh coats, isn't the only problem. Logistics are affected because the service always has to purchase more vehicles than it needs for combat. Also, the harsh chemical solvents required to remove the special corrosion resistant paint it currently uses pose an environmental problem.
To make self-healing paint a reality , the researchers working on the $2.5 million project had to come up with a reliable way to store fresh paint in the midst of paint that has been cured. The microcapsules they developed--which vary in size from 60 � to 150 micrometers in diameter--were made of urea formaldehyde. The material can contain the self-healing solution and other functional compounds, remain stable over the long term, and yet break easily when the coating is scratched.
In a punishing head-to-head test against ordinary paint, separate samples of special paint--one doped with capsules containing a film-forming agent and another laced with the healing agent and corrosion-fighting chemicals--convinced researchers at CERL that the Army is on the right track. Each formulation was applied to a strip of metal, allowed to cure for two weeks, scribed with a razor blade, and then left for more than 2000 hours in an accelerated weather testing facility that replicates the assault on a vehicle's finish by salt, moisture, and ultraviolet rays. The infiltration of corrosion on the metal covered by one of the microcapsule formulations--with the healing agent and an anticorrosion chemical called Irgacor 287--was 81 percent lower than that for the control.
Fully aware that technologies developed for the military find use in important civilian applications, the researchers have also been testing microcapsules for use in other areas. One project tested the effectiveness of capsules that yield calcium carbonate when the paint is scratched. Coatings containing this substance have been shown to reduce the amount of lead dust on floors and other surfaces resulting from the deterioration of lead-based paint. This could lessen the incidence of lead poisoning in children who live in buildings where lead paint was used before it was outlawed in the United States in 1979.
The microcapsules are actually filled with calcium hydroxide Ca(OH)2. When the coating is damaged, releasing the compound, it reacts with carbon dioxide (CO2) in the air, creating a thin film of calcium carbonate (CaCO3).
Another interesting research area involves the addition of material to the microcapsules' contents that fluoresce or luminesce with the turn of a switch. When a vehicle is brought in for evaluation and placed under a UV light, sections where this material has been exposed because of scratches will tell repair personnel exactly where to focus their efforts.
What's next? After the group at the New Jersey Institute of Technology presents the first self-healing paint prototype, the goal will be to include the results of several other research tracks into the system, says Laura Battista, an environmental engineer at Picatinny Arsenal's Industrial Ecology Center. The team at Picatinny wants to improve self-healing so that the microcapsules are capable of resealing themselves after use. (The ones currently being tested are strictly single use.)
One strong candidate for use as a resealable capsule is the carbon nanotube. But, says Frank Misurelli, a public affairs officer at Picatinny Arsenal, nanotubes are expensive to produce, making the experimental coatings far too costly for use outside the lab. He noted that refined, so-called single-walled carbon nanotubes currently cost about $1.4 million per kilogram ($650 000 a pound).
To make smart coatings worthwhile, the cost of applying them would have to be near the cost of today's military paint. Though it is likely that in the future--through economies of scale and further technical advances--the price will dip below $22 000 per kilogram ($10 000 per pound), the researchers are intent on using as few nanotube capsules as possible. "The difference," said Misurelli, "will be in the level of sensitivity." The concentration of nanotubes will determine whether damage is detected on an atomic, molecular, or larger-scale level.
Representatives of the U.S. Army Research Development and Engineering Command, based at Picatinny, told IEEE Spectrum that future smart coating systems will benefit from their efforts at "putting vein-like etchings into the coating that reproduce the functionality of the human vascular system." Misurelli added that the team has been working in preparation for the development of nanomachines that will someday move the capsules around this network of conduits. And Daniel J. Watts, who leads the research effort for the New Jersey group, says nanotubes can be designed to move around through the paint and, upon reaching an area where the coating has been breached, dilate to allow just enough of their contents to escape to repair the damage.
Even further into the future is technology that will allow vehicle coatings to diagnose themselves. When the extent of the damage overwhelms the paint's self-healing capability, its smart-sensing capability will kick in. Nanomachines incorporated in the coating will send signals to the driver or to a remote station notifying someone that it requires attention.
There has also been a great deal of buzz about the possibility of changing a vehicle's color on the fly by introducing a current that affects the properties of chemicals in the coating. If those chemicals absorb all the frequencies of visible light except, say, red, that is how the car will appear: all red.
In the case of the Army, the color of a smart-sensing tank moving from a jungle-based battle theater could be changed to make it suited to, say, the desert--without having to be scraped and repainted. Misurelli and the researchers on the Army project declined to discuss this aspect of their work, saying they were "constrained not to talk about any aspect of camouflage."