World’s ‘whitest’ self-cooling paint is thinner than ever, now ideal for vehicles

WEST LAFAYETTE, Ind. — Already featured in this year’s edition of Guinness World Records and “The Late Show With Stephen Colbert,” the world’s whitest paint just got an upgrade. This incredible pigment was already capable of keeping surfaces cool, thus reducing the need for air conditioning. Now, scientists have created an even thinner and lighter version of the paint. A team at Purdue University says this latest formulation is perfect for radiating heat away from cars, trains, and airplanes.

“I’ve been contacted by everyone from spacecraft manufacturers to architects to companies that make clothes and shoes,” says Xiulin Ruan, a Purdue professor of mechanical engineering and developer of the paint, in a university release. “They mostly had two questions: Where can I buy it, and can you make it thinner?”

The initial version of the world’s whitest paint created by Xiulin Ruan used nanoparticles of barium sulfate to reflect 98.1 percent of sunlight away, subsequently cooling outdoor surfaces more than 8.1°F below ambient temperature. That means if you covered a home’s roof in this paint, it would need much less air conditioning, but there was a problem.

“To achieve this level of radiative cooling below the ambient temperature, we had to apply a layer of paint at least 400 microns thick,” Prof. Ruan explains. “That’s fine if you’re painting a robust stationary structure, like the roof of a building. But in applications that have precise size and weight requirements, the paint needs to be thinner and lighter.”

'Whiter than white' paint
Xiulin Ruan, a Purdue University professor of mechanical engineering, holds up his lab’s sample of the whitest paint on record.

How did scientists thin out this paint?

Prof. Ruan and his team began working with other materials, experimenting with the limit of materials’ capability to scatter sunlight. The resulting version of the paint is nanoporous and incorporates hexagonal boron nitride as the pigment, which is a substance common in lubricants. This new paint achieves the same level of solar reflectance (97.9%) as the old version with just a single 150-micron layer of paint.

“Hexagonal boron nitride has a high refractive index, which leads to strong scattering of sunlight,” explains Andrea Felicelli, a Purdue PhD student in mechanical engineering who worked on the project. “The particles of this material also have a unique morphology, which we call nanoplatelets.”

“The models showed us that the nanoplatelets are more effective in bouncing back the solar radiation than spherical nanoparticles used in previous cooling paints,” comments Ioanna Katsamba, another PhD student in mechanical engineering at Purdue who ran computer simulations assessing if the nanoplatelet morphology offers any benefits.

Can you buy this paint stores?

The new thin paint also includes voids of air, making it highly porous on a nanoscale. This lower density, in combination with the thinness, provides yet another huge benefit: a much lighter weight. The new paint weighs 80 percent less than barium sulfate paint yet achieves nearly identical solar reflectance.

“This light weight opens the doors to all kinds of applications,” notes George Chiu, a Purdue professor of mechanical engineering and an expert in inkjet printing. “Now this paint has the potential to cool the exteriors of airplanes, cars or trains. An airplane sitting on the tarmac on a hot summer day won’t have to run its air conditioning as hard to cool the inside, saving large amounts of energy. Spacecraft also have to be as light as possible, and this paint can be a part of that.”

If you’re thinking about heading over to your local hardware store for a few buckets, study authors say the paint isn’t commercially available right now – but that may soon change.

“We are in discussions right now to commercialize it,” Ruan says. “There are still a few issues that need to be addressed, but progress is being made.”

“Using this paint will help cool surfaces and greatly reduce the need for air conditioning,” the researcher concludes. “This not only saves money, but it reduces energy usage, which in turn reduces greenhouse gas emissions. And unlike other cooling methods, this paint radiates all the heat into deep space, which also directly cools down our planet. It’s pretty amazing that a paint can do all that.”

The study is published in Cell Reports Physical Science.

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