Cool paint shows white is the new black

In an effort to curb global warming, engineers at Purdue University have created the whitest paint yet. Coating buildings with this paint may one day cool them off enough to reduce the need for air conditioning, the researchers say.

In October 2020, the team created an ultra-white paint that pushed limits on how white paint can be – now they've outdone that. The newer paint is not only whiter but can also keep surfaces cooler than the formulation previously demonstrated by the researchers. They report this whitest paint in a paper in ACS Applied Materials & Interfaces.

"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 kilowatts. That's more powerful than the central air conditioners used by most houses," said Xiulin Ruan, a professor of mechanical engineering at Purdue University.

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 80–90% of sunlight and can't make surfaces cooler than their surroundings.

Two features give this paint its extreme whiteness. One is the paint's very high concentration of a material 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 this project as a 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," explained Joseph Peoples, a PhD student in mechanical engineering at Purdue University.

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.

The paint's whiteness also means it is the coolest on record. Using high-accuracy temperature reading equipment called thermocouples, the researchers demonstrated that the paint can keep outdoor 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, it even worked in the middle of winter. During an outdoor test at 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 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 also compatible with the commercial paint fabrication process.

This story is adapted from material from Purdue University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.