Xiulin Ruan, a Purdue University professor of mechanical engineering, holds up his lab’s sample of the whitest paint on record. Photo by Jared Pike, courtesy of Purdue University.
Xiulin Ruan, a Purdue University professor of mechanical engineering, holds up his lab’s sample of the whitest paint on record. Photo by Jared Pike, courtesy of Purdue University.

In 2020, scientists at Purdue University developed a new super-white acrylic paint that reflects 95.5 percent of sunlight. Since then, they’ve made it even whiter, with a new formula that has increased sunlight reflection to 98.1 percent, which sets the whitest paint record with Guinness World Records.

“We weren’t really trying to develop the world’s whitest paint,” Xiulin Ruan, a Purdue professor of mechanical engineering who oversaw the research, told the New York Times. “We wanted to help with climate change, and now it’s more of a crisis, and getting worse. We wanted to see if it was possible to help save energy while cooling down the Earth.”

To the naked eye, it might not be apparent that you’re looking at the whitest white. Because the paint scatters sunlight as it reflects it, there’s no blinding effect. But while ultra-white may look similar to paint shades you might buy at Home Depot, the newly developed paint is way more effective as a cooling tool than its more light-absorbent counterparts.

What does this mean in practice? Purdue’s ultra-white paint keeps surfaces cool to the touch even in the blazing heat. Compared to the air temperature at mid-afternoon, a surface painted with the whitest white can be eight degrees Fahrenheit cooler. At night, the difference is even more pronounced, at up to 19 degrees.

Purdue researchers Xiulin Ruan (left) and Joseph Peoples compare the cooling performance of white paint samples on a rooftop. Photo by Jared Pike/Purdue University.

Painting buildings the whitest white could help offset the urban heat island effect, caused because most manmade buildings absorb more heat than natural landscapes, sending temperatures rising.

The Purdue team created their initial ultra-white paint with calcium carbonate particles. The new version—the result of six years of research testing more than 100 different materials—is made with barium sulphate. (Vantablack, the much-vaunted “blackest black” that absorbs 99.965 percent of light, is made from carbon nanotubes, as is an even blacker black developed by MIT scientists with artist Diemut Strebe captures 99.995 percent of light.)

“We found that using barium sulfate, you can theoretically make things really, really reflective, which means that they’re really, really white,” Xiangyu Li, who was a student in Ruan’s lab, said in a statement, noting that a higher concentration of barium sulfate particles increases reflectivity, but that adding too much makes the paint prone to cracking.

An ultra-white paint sample shown in infrared light. The dark purple square where the paint was applied is colder than the ambient temperature. Photo by Joseph Peoples courtesy of Purdue University.

White paint has long been used in hot climates on rooftops and building exteriors to try to keep homes cool, but because traditional paints only reflect 80 percent to 90 percent of sunlight, the heat continues to rise.

With this whitest white, it’s almost as if Ruan has created an air conditioner that runs on zero energy—and without the downside of emitting heat outside even as the A/C cools interiors. The air conditioning needs of a building coated with barium sulphate paint are expected to decrease by up to 40 percent.

While the whitest white paint is an effective reflector, you would need a lot of it to make a noticeable impact on Earth’s temperature.

Xiulin Ruan, a Purdue University professor of mechanical engineering, with the whitest white paint. Photo by John Underwood, courtesy of Purdue University.

In 2019, a study in the journal Joule found that to stop global temperatures from rising, you would need to cover one to two percent of the Earth’s surface with a reflective material like the whitest white paint. That’s just over half the area of the Sahara Desert.

Purdue developed the whitest white for use on rooftops, but interest in the paint has been widespread.

“I’ve been contacted by everyone from spacecraft manufacturers to architects to companies that make clothes and shoes,” Ruan said in a statement. “They mostly had two questions: Where can I buy it, and can you make it thinner?”

The world’s whitest paint previously required a layer 0.4 millimeters thick to achieve sub-ambient radiant cooling. A new thinner, lighter formulation can work just as well at just 0.15 millimeters thick. Photo by Andrea Felicelli, courtesy of Purdue University.

In response, Ruan’s team tweaked the formula to make a more lightweight version for cars.

A commercial version of whitest white won’t be available for at least a year. In the meantime Ruan’s lab is working with a yet-to-be-announced paint manufacturing company to make it more durable and dirt resistant, as well as to develop other colors that use ultra-white as a base, improving their reflective properties as well.