New Paint Formulation Keeps Surfaces Cooler than Previous Ultra-White Paints

In an attempt to mitigate global warming, engineers from Purdue University have produced the world’s whitest paint.

According to the team, using this paint to coat buildings may someday cool them off sufficiently to decrease the requirement for air conditioning.

In October 2021, the researchers produced an ultra-white paint that pushed the limitations of the facet of white paints. Now, the team has exceeded those limitations. Not only is the newer paint whiter but it can also keep the surfaces cooler than the formulation previously demonstrated by the team.

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. That’s more powerful than the central air conditioners used by most houses.

Xiulin Ruan, Professor of Mechanical Engineering, Purdue University

According to the team, this white may be similar to the blackest black, 'Vantablack,' which absorbs visible light of up to 99.9%.

When compared to the researchers’ previous ultra-white paint that reflects 95.5% of sunlight, the formulation of the new whitest paint reflects up to 98.1% of sunlight and, at the same time, diverts infrared heat away from a surface.

Normal commercial white paint gets warmer instead of cooler. Commercially available paints that are developed to reject heat can reflect just 80% to 90% of sunlight and are incapable of making surfaces cooler than their environments.

The new research article that explains the functions of the white paint appeared on the cover page of the ACS Applied Materials & Interfaces journal on April 15^th, 2021.

What Makes the Whitest Paint so White

A couple of features add extreme whiteness to the paint. One feature of the paint is its high concentration of a chemical compound, known as barium sulfate, which is also used for making cosmetics and photo paper white.

“We looked at various commercial products, basically anything that’s white,” stated Xiangyu Li, a postdoctoral researcher from the Massachusetts Institute of Technology, who worked on the new study as a Purdue University PhD student in Ruan’s laboratory. “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 of the paint is that all the particles of barium sulfate have different sizes in the paint.

The amount of light scattered by each particle relies on its size, and hence, a broader range of particle sizes enables the paint to disperse more amounts 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.

Joseph Peoples, PhD Student in Mechanical Engineering, Purdue University

There is a slight possibility to make the paint whiter, but not much without affecting 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.

Xiangyu Li, Postdoctoral Researcher Massachusetts Institute of Technology

How the Whitest Paint is Also the Coolest

The whiteness of the paint also implies that the paint is the coolest available to date. With the help of high precision temperature reading equipment, known as thermocouples, the team demonstrated in outdoor settings that the new paint can keep surfaces 19°F cooler than their ambient surroundings at nighttime.

The paint can even cool the surfaces 8°F below their ambient temperatures under intense sunlight at the time of noon hours.

The solar reflectance of the paint is so effective that it even functioned in the middle of winter. During an outdoor experiment with an ambient temperature of 43°F, the paint was able to reduce the sample temperature by as much as 18°F.

The white paint is the outcome of six years of studies based on attempts dating back to the 1970s to produce radiative cooling paint as a viable option to conventional air conditioners.

Ruan’s laboratory had considered more than 100 different materials, reduced them down to 10, and tested approximately 50 different formulations for every material. The team’s previous ultra-white paint was a formulation composed of calcium carbonate, an abundant compound available on earth and generally found in seashells and rocks.

In their study, the researchers demonstrated that similar to commercial paint, the new barium sulfate-based paint can possibly manage outdoor conditions. The method used by the team to produce the paint can also be used with the commercial paint fabrication procedure.

The researchers have filed patent applications for this paint formulation via the Purdue Research Foundation Office of Technology Commercialization.

The study was funded by the Cooling Technologies Research Center at Purdue University and the Air Force Office of Scientific Research via the Defense University Research Instrumentation Program (Grant No.427 FA9550-17-1-0368). The study was conducted at Purdue’s FLEX Lab and Ray W. Herrick Laboratories and the Birck Nanotechnology Center of Purdue’s Discovery Park.

Journal Reference:

Li, X., et al. (2021) Ultrawhite BaSO₄ Paints and Films for Remarkable Daytime Subambient Radiative Cooling. ACS Applied Materials & Interfaces. doi.org/10.1021/acsami.1c02368.