Cool upgrade to Purdue's whitest paint

In an effort to curb global warming, Purdue University researchers developed a cool solution – paint. But not just any paint, with what they believe to be the closest opposite of the blackest black, this ultra-white paint can not only repel heat but cool down the interior of the surface it is painted on.

In September 2021, the Purdue team created an ultra-white paint that earned the Guinness World Record for an unintended side effect to reducing global warming, creating the whitest paint ever.

Now, its newest rendition of the whitest white paint has gotten an upgrade.

"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, the head of the team and professor of mechanical engineering at Purdue. "That's more powerful than the central air conditioners used by most houses."

The researchers believe 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 when exposed to sunlight, and paints on the market that are designed to reject heat reflect only 80% to 90% of sunlight and do not make their surfaces cooler than the surroundings.

However, with nearly 100% reflective capabilities, Purdue's whitest white does just that.

What makes it so white?

Two features give this ultra-white paint its extreme whiteness. One is a very high concentration of a chemical compound called barium sulfate, often 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 Ph.D. 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, with each particle scattering light depending on its size – so wider particle sizes allow 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 Ph.D. student in mechanical engineering.

While there is still a bit of room to make the paint even whiter, the scientists have found this to be difficult without compromising its already impressive product.

"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.

Cool paint

The whiteness of the paint also means that it is the coolest on record. Using high-accuracy temperature reading equipment called thermocouples, the team demonstrated that the paint could keep surfaces roughly 19 degrees Fahrenheit (seven degrees Celsius) cooler than their ambient surroundings at night.

In addition to this, the paint can also cool the surface by roughly eight degrees Fahrenheit (13 degrees Celsius) below their surroundings under strong sunlight during peak hours.

Because the paint's solar reflectance is so effective, it was even capable of working in the middle of winter.

During an outdoor test with an ambient temperature of about 43 degrees Fahrenheit (6 degrees Celsius), the paint still managed to lower the sample temperature by 18 degrees Fahrenheit (8 degrees Celsius).

The product of seven years of development, building on attempts that go back to the 1970s to develop radiative cooling paint as a feasible alternative to traditional air conditioners, Ruan's lab considered over 100 different materials and narrowed it down to 10, all while testing nearly 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 limestone and seashells.

The researchers showed in their study that, like commercial paint, their barium sulfate-based paint could potentially handle inclement conditions. Furthermore, the technique used to synthesize the pain is also compatible with the commercial fabrication process.

To date, patent applications have been filed for the paint's formula, but no further information as of yet has come out on whether the Purdue team will employ a start-up to begin selling its whitest paint.