The “blackest black” paint, famed for its thermal camouflage potential, has long absorbed 99.9 percent of public attention. Now it’s time to shed some light on the other end of the practical paint spectrum: the “whitest white.” Research shows that surfaces coated with a newly formulated white coloring reflect 98.1 percent of sunlight, creating a powerful cooling effect—any of the electricity required by commercial air conditioners.
This coating absorbs just 1.9 percent of sunlight compared with 10 to 20 percent for conventional white or “heat-reflective” paints, says Purdue University mechanical engineer Xiulin Ruan, co-author of a study on the substance in ACS Applied Materials & Interfaces. By reflecting so efficiently, the novel paint can actually help a coated building release the heat inside. The authors calculate that covering a 1,000-square-foot (more than 90-square-meter) roof with their paint could cool a building by about 10 kilowatts. This effect might not eliminate the need for energy-hungry air conditioners, but it could reduce their use. “Our model shows if you’re in, say, Reno or Phoenix, you can save up to 70 to 80 percent on air-conditioning in the summer,” Ruan says.
Scientists have been developing reflective paints for decades, but today’s commercial products still remain at or above the surrounding temperature. In the past 10 years researchers have found greater success with multilayered coatings that incorporate tiny particles of varying sizes, some on the nanoscale, to reflect many wavelengths of light. Teams at Stanford University and the University of Colorado Boulder have shown that such materials can cool a surface to below the ambient temperature. Unfortunately, manufacturing precise layers of multiple substances and applying them to a surface in a set order costs more—and requires a more intensive process—than simply slapping on some paint.
In 2014 Ruan decided to take a hybrid approach and create an ordinary paint that could easily be brushed or sprayed onto a surface but that would still incorporate a reflective nanomaterial. His team tested substances such as titanium dioxide and calcium carbonate and made a cooling but less reflective paint out of the latter last year. The researchers ultimately selected a relatively inexpensive compound called barium sulfate. Next they calibrated the necessary concentration to make the paint as reflective as possible, without reducing its ability to stick together. Finally, they made sure the barium sulfate particles came in a variety of sizes because each size scatters a different wavelength of light.
“I like this study. I think it’s promising,” says Yuan Yang, a Columbia University materials scientist, who was not involved in the research. “And I think there’s a potential to be commercialized.” Ruan says he hopes to bring a version of his paint to market in a year or two. “The price [of barium sulfate] is comparable to, or even slightly lower than, titanium dioxide that is used in commercial paints,” he adds. “Manufacturing doesn’t involve any expensive nanotechnology. Although it’s still a nanotech, it is a very affordable nanotech.”
There is plenty of competition from other researchers. And Yang notes that any new product like this will need to stand up to the real world, where grime coats surfaces over time. A manufacturer, he says, would have to grapple with “how to make sure that the paint stays white after 30 years of use.”
Ruan sees his work as a tool to fight the climate emergency. “President [Joe] Biden talked about cutting carbon emissions in half by the end of 2030,” he says. “Our paint can contribute to that goal because it lets us get cooling without using power.”
Editor’s Note (7/16/21): A version of this article with the title “Cool Color” was adapted for inclusion in the August 2021 issue of Scientific American. This text reflects that version, with the addition of some material that was abridged for print.