As the temperatures outside grow hotter, people look for ways to keep their homes cooler. Air conditioning is expensive and energy intensive, but fans aren’t enough for many people. Interestingly, a new advance developed by Professor Xiulin Ruan and his team at Purdue University may have unlocked a much more cost-effective way to keep temperatures from soaring in the summer.

For the past nine years, Ruan, a professor of mechanical engineering at Purdue has been conducting research on creating the coolest white paint. The reason for his research was not to set a record or create the best white paint; the goal is sustainability and impacting global warming. The idea is that by keeping surfaces cool enough to reduce the need for internal air conditioning, the white paint will help reduce global warming.

“Breaking a record for the whitest paint wasn’t a goal for us – curbing global warming was,” Prof. Ruan said. “The idea was to create paint that would reflect sunlight away from a building. Making this paint really reflective, however, also made it really white.”

Prof. Ruan and his team keyed in on barium sulfate as well as its various particle sizes as the ingredient that would best reflect sunlight away from homes. According to Prof. Ruan’s research, using his white paint to cover a roof area of approximately 1,000 square feet could result in a cooling power of 10 kilowatts, more powerful than the air conditioners used by typical houses.

“Two features give the paint its extreme whiteness,” Prof. Ruan noted. “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 second feature is that the barium sulfate particles are all different sizes in the paint. What wavelength of light each particle scatters depends on its size, so a wider range of particle sizes allows the paint to scatter more of the light spectrum from the sun. The paint can cool outdoor surfaces more than 4.5°C below ambient temperature.”

In 2022, Prof. Ruan and his team developed a new formulation of his paint that is now thinner and lighter, making it ideal for radiating heat away from vehicles such as cars, trains and airplanes, among others. Simple models show that covering 1% of the Earth’s surface in their technology could mitigate the total effects of global warming; this has the team looking at possible coatings for roads and asphalt.

Prof. Ruan pointed out that the thinner paint formulation that the researchers made for applying to vehicles is a nanoporous paint incorporating hexagonal boron nitride as the pigment, a substance mostly used in lubricants.

“This new paint achieves nearly the same benchmark of solar reflectance (97.9%) with just a single 150-micron layer of paint,” he noted. “The paint also incorporates voids of air, which make it highly porous on a nanoscale. This lower density, together with the thinness, provides another huge benefit: reduced weight. The newer paint weighs 80% less than barium sulfate paint yet achieves nearly identical solar reflectance.”

Prof. Ruan’s white paint has caught the eye of countless people. The paint has been recognized as the whitest paint in Guinness World Records, has recently earned the prestigious 2023 Innovation Award in the sustainability category at the South by Southwest Conference & Festivals (SXSW), and was honored at the 2023 Gizmodo Science Fair.

At SXWS, Prof. Ruan used models of two barns with halogen lights directly above them to show the difference between his team’s paint and conventional paints on the market. One barn was painted with conventional paint, and the other with Purdue’s white paint. The barn painted in Purdue’s technology consistently held cooler internal temperatures by 8°F to 10°F and was also cooler to the touch.

“We are humbled to receive these amazing recognitions,” said Prof. Ruan. “They are impossible without the creative and hard work of our team members as well as great support from Purdue University, the School of Mechanical Engineering, colleagues in the field who made important contributions and inspired us, and sponsors at different stages of the work. We are pleased to see the tremendous public awareness and interest in mitigating climate crisis with innovative technologies, and these recognitions will motivate us to further develop and push our cooling paint technologies to market.”

Prof. Ruan and his team are currently working with the Purdue Research Foundation’s Office of Technology Commercialization to bring this paint to the market. He added that the timeline to commercialization has yet to be determined, although he also noted that the cost of the paint should be along the lines of conventional paint.

“We are in discussions right now to commercialize it. We have partnered with a company to scale up the paint and put it on the market,” Prof. Ruan reported. “Patent applications for this paint formulation have been filed through the Purdue Research Foundation Office of Technology Commercialization. There are still a few issues that need to be addressed, but progress is being made.”