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Whitest-ever paint holds promise of cooler buildings

 Whitest-ever paint holds promise of cooler buildings

The paint has been developed at Purdue University in an effort to curb global warming. Coating buildings with this paint may one day cool them off enough to reduce the need for air conditioning, the researchers say.

In October, the team had created an ultra-white paint that they believed pushed limits on how white paint can be. They have now outdone that with a newer paint that they say is not only is whiter but  can also keep surfaces cooler than the previous formulation.

“If you were to use this paint to cover a roof area of about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts,” said Xiulin Ruan, a Purdue professor of mechanical engineering. “That’s more powerful than the central air conditioners used by most houses.”

The researchers believe that this white may be the closest equivalent of the blackest black, ‘Vantablack’, which absorbs up to 99.9% of visible light. The new whitest paint formulation reflects up to 98.1% of sunlight – compared with the 95.5% of sunlight reflected by the researchers’ previous ultra-white paint – 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 was published yesterday (15 April) as the cover of the journal ACS Applied Materials & Interfaces. The paper is called Ultra-white BaSO4 Paints and Films for Remarkable Daytime Subambient Radiative Cooling.

Two features give the paint its extreme whiteness. One is the paint’s very high concentration of a chemical compound called barium sulphate, 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 this project as a Purdue PhD student in Ruan’s lab. “We found that using barium sulphate, you can theoretically make things really, really reflective, which means that they’re really, really white.”

The second feature is that the barium sulphate particles are all different sizes in the paint. 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,” said Joseph Peoples, a Purdue PhD student in mechanical engineering.

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,” Li said. “The higher the concentration, the easier it is for the paint to break or peel off.”

The paint’s whiteness also means that the paint is, said the research team, the coolest on record. The researchers demonstrated outdoors that the paint can keep surfaces 19°F cooler than their ambient surroundings at night. It 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.

This white 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 had considered over 100 different materials, narrowed them down to 10 and tested about 50 different formulations for each material. Their previous ultra-white 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 sulphate-based paint can potentially handle outdoor conditions. The technique that the researchers used to create the paint also is compatible with the commercial paint fabrication process, they said.

Patent applications for this paint formulation have been filed through the Purdue Research Foundation Office of Technology Commercialization. This research was supported by the Cooling Technologies Research Center at Purdue University and the Air Force Office of Scientific Research through the Defense University Research Instrumentation Program. The research was performed at Purdue’s Flex Lab and Ray W Herrick Laboratories and the Birck Nanotechnology Center of Purdue’s Discovery Park.