June 14, 2004

Clothes launder own fabric

Catalytic cotton chows down on dirt.

by Mark Peplow
Nature News

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In the classic 1951 film, The Man in the White Suit, Alec Guinness played a scientist who invents a fabric that never gets dirty or wears out. A chemist's pipe dream perhaps, but the prospect of self-cleaning clothes might be getting closer.

Scientists have invented an efficient way to coat cotton cloth with tiny particles of titanium dioxide. These nanoparticles are catalysts that help to break down carbon-based molecules, and require only sunlight to trigger the reaction. The inventors believe that these fabrics could be made into self-cleaning clothes that tackle dirt, environmental pollutants and harmful microorganisms.

The titanium dioxide particles covering the cloth are just 20 nanometres across, about 2,500 times smaller than the width of a human hair. The researchers' key breakthrough was to ensure that these particles had exactly the right arrangement of atoms, called an 'anatase' crystal structure, which has previously been difficult to achieve in such tiny grains. This arrangement boosts the particles' catalytic power.

The researchers, Walid Daoud and John Xin from the Hong Kong Polytechnic University, Kowloon, dipped cotton patches into a liquid slurry of titanium dioxide for just half a minute before removing them, padding them dry, and heating them to 97 °C in an oven for 15 minutes. Three hours in boiling water completed the coating process, which they describe in the latest edition of the Journal of the American Ceramic Society1.

Tough on grime

"Titanium dioxide, in the presence of ultraviolet light, will oxidize a wide range of organic materials," explains Bob Skelton, a chemical engineer from Cambridge University, Britain. When light hits the catalyst it frees up electrons within the crystal and these react with oxygen from the air.

This generates free-radical oxygen, a powerful oxidizing agent that can break down grime into smaller particles such as carbon dioxide and water. Because the catalyst does not get used up, it can keep on working as long as it is exposed to sunlight.

Self-cleaning materials are particularly popular in the Far East, and a quarter of all toilet bowls coming on to the market in Taiwan, for example, now come with self-cleaning nano-coatings. So clothes may be the next step.

"I know a number of garment manufacturers that are interested in self-cleaning fabrics," says Andy Garland of the Institute of Nanotechnology in Stirling, Scotland, an umbrella body that forges links between academic institutions and industry.

Although the self-cleaning shirt is still a long way from reaching the high street, the Hong Kong scientists might want to take careful note of the plot of The Man in the White Suit. Fearing that his invention might threaten their livelihoods, textile manufacturers took Guinness prisoner to keep his remarkable fabric under wraps.

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