Ultrawhite paint cools buildings

Engineers from Purdue University have created what is claimed to be the whitest paint yet, in an effort to curb global warming. The researchers say that coating buildings with the new paint formulation may one day cool them off enough to reduce the need for air conditioning.

“If you were to use this paint to cover a roof area of about 1000 square feet, we estimate that you could get a cooling power of 10 kW. That’s more powerful than the central air conditioners used by most houses,” Purdue Professor of Mechanical Engineering Xiulin Ruan said.

The researchers believe that the white they have developed may be the closest equivalent of the blackest black — ‘Vantablack’ — which absorbs up to 99.9% of visible light. The new white paint formulation reflects up to 98.1% of sunlight and sends infrared heat away from a surface at the same time.

Typical commercial white paint gets warmer rather than cooler. Paints on the market that are designed to reject heat reflect only 80–90% of sunlight and can’t make surfaces cooler than their surroundings.

The team’s research paper, showing how the paint works, is published in ACS Applied Materials & Interfaces.

What makes the whitest paint so white?

Two features give the paint its extreme whiteness. One is the paint’s very high concentration of a chemical compound called barium sulfate, which is also used to make photo paper and cosmetics white.

“We looked at various commercial products, basically anything that’s white,” said Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology, who worked on the project as a Purdue PhD student in Ruan’s lab. “We found that using barium sulfate you can theoretically make things really, really reflective, which means that they’re really, really white.”

The second feature is that the barium sulfate particles in the paint are all different sizes. How much each particle scatters light depends on its size, so a wider range of particle sizes allows the paint to scatter more of the light spectrum from the sun.

“A high concentration of particles that are also different sizes gives the paint the broadest spectral scattering, which contributes to the highest reflectance,” Purdue PhD Mechanical Engineering student Joseph Peoples said.

There is a little bit of room to make the paint whiter, but not much without compromising the paint.

“Although a higher particle concentration is better for making something white, you can't increase the concentration too much. The higher the concentration, the easier it is for the paint to break or peel off,” Li said.

How the whitest paint is also the coolest

The paint’s whiteness also means that the paint is the coolest on record. Using high-accuracy temperature reading equipment called thermocouples, the researchers demonstrated that the paint can keep surfaces 19°F cooler than their ambient surroundings at night. The paint can also cool surfaces 8°F below their surroundings under strong sunlight during noon hours.

The paint’s solar reflectance is so effective that it even worked in the middle of winter. During an outdoor test with an ambient temperature of 43°F, the paint still managed to lower the sample temperature by 18°F.

The paint is the result of six years of research, building on attempts going back to the 1970s, to develop radiative cooling paint as a feasible alternative to traditional air conditioners.

Ruan’s lab considered more than 100 different materials, narrowed them down to 10 and tested about 50 different formulations for each material. Their previous whitest paint was a formulation made of calcium carbonate, an earth-abundant compound commonly found in rocks and seashells.

The researchers showed in their study that, like commercial paint, their barium-sulfate-based paint can potentially handle outdoor conditions. The technique that the researchers used to create the paint is compatible with the commercial paint fabrication process.

Image caption: Purdue University Professor of Mechanical Engineering Xiulin Ruan holds up his lab’s sample of the whitest paint on record. Image credit: Purdue University/Jared Pike.