Scientists shed light on stain protection
You can't make something clean without making something else dirty. Wash your clothes and you dirty the water — and often pollute streams, rivers and lakes. Make clothes resist dirt, however, and there's less to clean up. That logic underpins products like ScotchGuard, a chemical designed to prevent dirt from soaking in and staining materials. It saves cleaning and sometimes saves the clothes. But ScotchGuard is gone from the marketplace — not because of any proven deficiency or danger but from forward-thinking concerns that adding such chemicals to the world around us probably isn't a good thing. Michael Kelley, an applied research program manager working at the Department of Energy's Thomas Jefferson National Accelerator Facility in Newport News, Va., has found a way to get the benefits of fabric protection without the environmental concerns. The same technology also promises plastic wrap that kills germs on contact without affecting food as well as shower curtains that play keep-away with mildew. Kelley works his magic with light, specifically deep ultraviolet light. Not the stuff of suntans, deep ultraviolet is a high-energy form of light far beyond the visible spectrum. At exactly the right wavelength, it interacts with the molecules of polyester and makes them uniquely chemically reactive. They want to grab hold of anything. "UV activates the surface chemically," said Kelley. "Once it is activated it will react with whatever we have in the chamber with it that's in the gas phase." In a special chamber, Kelley provides fluorocarbon molecules — slippery stuff related to Teflon — dispersed as gas molecules that permanently attach themselves to the polyester. "It gives us good repellency to things that are oil-based," said Kelley. "How many times have we been eating dinner when something bound for our mouth ends up elsewhere? What we have been making lets you blot up the spill so that it will come off."
Compared to the traditional way of adding soil repellency — soaking the fabric with a liquid chemical like ScotchGuard — Kelley's UV treatment is friendly both to the pocketbook and the environment. Traditional treatments glop on in liquid form and form coatings on fibers dozens (or more) molecules thick.
The UV treatment coats the fiber exactly one molecule thick. Nothing more is needed because only the surface of the fiber reacts with dirt. That means less chemical to treat each pair of slacks (or whatever needs to be made soil resistant).
In addition, liquid coatings wash off — which means they eventually end up in the environment — and must be refreshed or replaced. The UV-treated fibers hold tenaciously to the coating.
"After your send your slacks to the dry cleaner, the polyester (would) still resist soiling," said Kelley.
Although Kelley and his graduate student Zhengmao Zhu have proven the process works, so far they have made coatings only on films, not actual fibers. The films are easier to work with and make the results easier to measure, but Kelley sees little difficulty in translating the process to fibers and cloth.
"You could treat filaments as they are spun, you could treat the yarn or you could treat the final fabric. Typically, you treat the fabric," said Kelley. "If you are clever, you can get almost as much benefit from treating the fabric as from individual fibers, and it's a lot easier to do."
Wrapping it up
On the other hand, Kelley believes UV-mediated coatings on films have their own promise. One area he's looking at is antimicrobial films, coatings that kill any bacteria that stray upon the plastic surface.
"We can make the surface take up trace amounts of silver, and the silver makes it antimicrobial," he notes.
Treated with silver, ordinary food packaging could kill germs, helping preserve food and prevent disease. Moreover, the silver surface doesn't come off, so there's no danger to people or animals from the treatment.
Despite using a precious metal, the UV-mediated silver coating is surprisingly cost-effective. According to Kelley, treating a square yard of material would require a smaller fleck of silver than could be seen with the unaided eye.
The same silver treatment could be applied to cloth as well. Kelley sees potential in furnishings for hospital rooms that won't harbor microorganisms as well as a treatment for clothing such as hospital gowns and scrubs. The same treatment would prevent mildew growth on shower curtains.
Kelley believes that his research into UV-mediated soil resistant coatings is far enough along the way that that it could yield new products quickly.
"If someone wanted to do this commercially, it would not take more than a couple years before we could have garments in people's hands," said Kelley. "But I believe the first applications will come in food packaging because you can add a lot more value there."