The whitest paint is here – and it’s the coolest. Literally.

In an effort to stop global warming, Purdue University engineers have created the whitest paint to date. Covering buildings with this paint could one day cool them enough to reduce the need for air conditioning, the researchers say.

In October, the team created an ultra-white paint that pushed the limits of white paint. Now they have overcome that. The newer paint is not only whiter, but can also keep the surfaces cooler than the formulation the researchers had previously demonstrated.

“If you were to use this paint to cover a roof area of ​​about 1,000 square meters, we estimate that you could get a cooling power of 10 kilowatts. It is stronger than the central air conditioners used by most homes.” said Xiulin Ruan, a Purdue professor of mechanical engineering.

Researchers believe that this white may be the closest equivalent to the darkest black, “Vantablack,” which absorbs up to 99.9% of visible light. The new whitest formulation reflects up to 98.1% of sunlight – compared to 95.5% of sunlight reflected by researchers’ previous ultra-white paint – and sends infrared heat away from a surface at the same time. .

Typical commercial white paint becomes warmer than cold. Paints on the market that are designed to repel heat reflect only 80% -90% of sunlight and cannot make surfaces colder than their surroundings.

The research paper of the team that shows how the paint works is published on Thursday (April 15) as the cover of the magazine Materials and interfaces applied ACS.

Which makes the whitest paint so white

Two characteristics give the paint its extreme white. One of these is the very high concentration of paint in a chemical compound called barium sulfate, which is also used to make white paper and cosmetics.

“We looked at various commercial products, practically everything is white,” said Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology who worked on the project as a Purdue Ph.D. student in Ruan’s lab. “I found that using barium sulfate, theoretically you can make things really, really reflective, which means they’re really, really white.”

The second feature is that barium sulfate particles have different sizes in the paint. How much particle scatters light depends on its size, so a wider range of particle sizes allows the paint to scatter more of the spectrum of sunlight.

“A high concentration of particles, which also have different sizes, gives the paint the widest spectral spread, which contributes to the greatest reflectance,” said Joseph Peoples, a Purdue PhD. mechanical engineering student.

There is a little room to make the paint whiter, but not much without compromising the paint.

“Although a higher concentration of particles 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. .

As the whitest paint is also the coolest

The white of the paint also means that the paint is the strongest recorded. Using high-precision temperature reading equipment called thermocouples, the researchers demonstrated outdoors that paint can keep surfaces 19 degrees Fahrenheit cooler than the environment at night. It can also cool surfaces to 8 degrees Fahrenheit under the environment in bright sunlight during lunch hours.

The sun’s reflective paint is so effective that it worked even in the middle of winter. During an outdoor test with an ambient temperature of 43 degrees Fahrenheit, the paint still managed to lower the sample temperature by 18 degrees Fahrenheit.

This white paint is the result of six years of research based on attempts that began in the 1970s to develop radiant cooling paint as a viable alternative to traditional air conditioners.

Ruan’s lab analyzed over 100 different materials, reduced them to 10, and tested about 50 different formulations for each material. Their previous whitest paint was a formulation made of calcium carbonate, a compound abundant in the earth that is commonly found in rocks and shells.

The researchers showed in their study that, like commercial paint, their barium sulfate paint can withstand external conditions. The technique used by researchers to create the paint is also compatible with the commercial process of making the paint.

Patent applications for this paint formulation have been filed through the Purdue Research Foundation’s Technology Marketing Office.