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White paint is coolest yet

Purdue University professor of mechanical engineering, Xiulin Ruan, holds up his lab’s sample of the whitest paint on record (Photo: Jared Pike, Purdue University)

USA: Engineers at Purdue University say they have created a white paint that reflects up to 98.1% of sunlight and could cool surfaces 4.5ºC below ambient even under the noon-day sun.

The new paint improves on the ultra-white paint the team at Purdue created last year. The newer paint is not only whiter but can also keep surfaces cooler than the previous formulation.It is said to be able to 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.

“If you were to use this paint to cover a roof area of about 1,000ft2 [93m2], we estimate that you could get a cooling power of 10kW. That’s more powerful than the central air conditioners used by most houses,” said Xiulin Ruan, a Purdue professor of mechanical engineering.

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 researchers claim.

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. The previous ultra-white paint was a formulation made of calcium carbonate, an earth-abundant compound commonly found in rocks and seashells.

An infrared camera shows how a sample of the whitest white paint (the dark purple square in the middle) actually cools the board below ambient temperature (Photo Joseph Peoples, Purdue University) 

“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 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 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.

The paint’s whiteness also means that the paint is the coolest on record. Using thermocouples, the researchers demonstrated outdoors that the paint can keep surfaces 19ºF (10.5ºC) cooler than their ambient surroundings at night. It can also cool surfaces 8ºF (4.5ºC) below their surroundings under strong sunlight during noon hours.

The paint’s solar reflectance is so effective, it also said to have even worked in the middle of winter. During an outdoor test with an ambient temperature of 43ºF (6.1ºC), the paint still managed to lower the sample temperature by 18ºF (10ºC).

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Radiative cooling paint could reduce the need for air conditioning22 October 2020
USA: Engineers at Purdue University have created a white paint that is said to be able to keep surfaces up to 18ºF (10ºC) cooler than ambient. Read more…

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