Wall, Clean Thyself
Self-cleaning paints and coatings that use photocatalytic titanium dioxide (TiO2) and are widely used in Europe to combat bacteria, smog, and dirt could soon be available in the U.S. With the help of a little UV light, tiny molecules of titanium dioxide can use their strong oxidation power to kill bacteria attempting to attach to a wall or remove smog and odor in outdoor areas.
These experimental coatings use natural processes, such as mimicking the natural form of the lotus plant, and sophisticated chemical reactions to use the air and the sun to fight pollution.
Daylight fights smog
Millennium Chemicals of Lincolnshire, U.K., has been selling its EcoPaint, a patented paint formulation that uses TiO2 to convert foul nitrous oxide (NOx) gases into harmless by-products, since 2004. The paint's base is polysiloxane, a silicon-based polymer. Embedded in the base are spherical nanoparticles of titanium dioxide and calcium carbonate. When NOx gases seep into the paint on a building or other surface and adhere to the TiO2, the particles use the sun's energy to break down the gases into nitric acid, which is then washed away by rain or neutralized by the paint's calcium-carbonate particles to create carbon dioxide, water, and calcium nitrate, another soluble acid.
The company says a 0.3 mm layer of EcoPaint could effectively neutralize noxious nitrogen oxide gases for up to five years in a heavily polluted city. EcoPaint is being lab tested as part of the European Union's Photocatalytic Innovative Coverings Applications for Depollution Assessment program (PICADA). Millennium Chemicals said it wants to expand the product to the U.S. by 2010.
Using TiO2 as a natural repellent
Researchers at the Georgia Institute of Technology in Atlanta are using titanium dioxide as part of a solution that mimics one of nature's best non-stick surfaces, the Asian lotus plant, to help create more reliable electronic transmission systems and photovoltaic arrays that retain their efficiency.
The plant's ability to repel water and dirt results from an unusual combination of a superhydrophobic (water-repelling) surface and a combination of micron-scale hills and valleys and nanometer-scale waxy bumps that create rough microscopic surfaces that don't give water or dirt a chance to adhere.
“Because of the combination of nano-scale and micron-scale structures, water droplets can only contact about 3% of the surface,” said Dr. C.P. Wong, a professor in Georgia Tech's school of materials science and engineering. “They're just not touching very much of the lotus surface as compared to a smooth surface.”
To create a coating capable of insulating applications including high-voltage power lines and PV panels, Georgia Tech researchers attempted to duplicate the two-tier lotus surface using a variety of materials, including polybutadiene (synthetic rubber), which deteriorates under UV radiation.
Wong and his team have developed a lotus surface able to withstand UV rays, using a combination of silicone, fluorocarbons, and titanium dioxide. Their prototype coating has shown excellent durability in long-term testing.
“It's not easy to get dust and dirt off a smooth surface,” he said. “Though it seems counterintuitive, the roughness actually helps the cleaning process. We believe this lotus surface will have many potential applications.”