It’s a designer’s dilemma: The color white screams sleek, contemporary design, but keeping it clean—especially for buildings located in bustling cities—can be a costly and trying chore.
Richard Meier, FAIA, has faced that challenge in many of his buildings including the 2006 Jubilee Church project, built in Rome to celebrate the 2,000th anniversary of Christianity. In an attempt to create a coating to keep Meier’s stark white forms clean with little maintenance, Italcementi Group, the project’s lead technical adviser, turned to titanium dioxide—a white pigment often used in products such as toothpaste and sunscreen. Titanium dioxide also has significant photocatalytic properties. The coating works, in essence, with sunlight to set off a chemical reaction that accelerates the natural decomposition of organic matter, allowing the building to slough off environmental staining.
The coating did its job, but in the process Italcementi Group learned just how capable titanium dioxide itself was to clean the air around it—by “eating” nitrogen oxides, the main offender found in air pollution caused by burning fossil fuels.
The discovery is fortuitous, as air quality remains a major concern in the United States. In its “State of the Air 2013” report, the American Lung Association found that, despite an overall uptick in the quality of the nation’s air, 42 percent of the population lives where pollution levels are too often dangerous to breathe.
Titanium dioxide continues to pop up in a variety of building materials as scientists and manufacturers look to chip away at air pollution in the built environment. In 2011, Alcoa introduced EcoClean, a titanium dioxide coating that can be applied to the company’s aluminum architectural panels. And the honeycomb exterior of Mexico City’s Torre de Especialidades hospital, featuring a German product called Prosolve370e, handles the smog of an estimated 1,000 cars per day—all while looking good.
TiO2 and You
Common sense dictates that it’s wise to avoid creating the pollutant in the first place, but research is showing that applying titanium dioxide coatings to paving surfaces could help curb smog near its point of origin.
Earlier this year, researchers at the Netherlands’ Eindhoven University of Technology laid a city-block’s-worth of pavers that were coated with titanium dioxide. And in 2010 Marwa Hassan, an assistant professor in the Louisiana State University’s Department of Construction Management, installed the United States’ first photocatalyic asphalt and concrete pavements on that university’s campus.
Both sets of researchers saw significant reduction in nitrogen oxides. At Eindhoven, there was a 19 percent reduction each day during the year-long study. Hassan’s laboratory findings, published in the American Society of Civil Engineers’ Journal of Materials in Civil Engineering, saw reductions of 31 to 55 percent. In the U.S., 12.5 billion square feet of asphalt shingles are produced every year, and four out of five homes are covered with them, according to the Asphalt Roofing Manufacturers Association. Could titanium dioxide turn the ubiquitous asphalt roofing of the average American home into a passive pollution scrubber?
Adem Chich, executive director for asphaltic low slope product development for New Jersey–based commercial and residential roofing manufacturer GAF, thinks it might be possible.
That is, of course, if it can be employed without significant threat to the bottom line.
“This is a technology that has been around for a while and can be deployed in several forms. Companies in Europe, Japan, and the United States are marketing [roofing] membranes with granules that are coated with anatase titanium dioxide, which absorbs the ultraviolet component of sunlight, acts as a catalyst to form hydroxyl radicals, which then oxidize and destroy most volatile organic compounds and nitrous oxide pollutants,” Chich says.
Siplast claims that 500 squares of its Eco-Activ membrane will consume the nitrogen oxide emissions released by 23 passenger cars or light trucks driven 12,000 miles each. In addition, Siplast says the titanium dioxide continues to work through the life of the roof membrane.
“There are still questions as to how cost-effective this approach is,” Chich continues. “There are, however, meteorological simulations going on at national laboratories to model pollutant generation and transport, as well as how photocatalysis can be deployed more effectively. It’s potentially a great marketing feature, especially if the incremental cost becomes small.”