Spectra paint9/3/2023 ![]() ![]() The research was performed at Purdue’s FLEX Lab and Ray W. This research was supported by the Cooling Technologies Research Center at Purdue University and the Air Force Office of Scientific Research through the Defense University Research Instrumentation Program (Grant No.427 FA-0368). Patent applications for this paint formulation have been filed through the Purdue Research Foundation Office of Technology Commercialization. The technique that the researchers used to create the paint also is compatible with the commercial paint fabrication process. The researchers showed in their study that like commercial paint, their barium sulfate-based paint can potentially handle outdoor conditions. Their previous ultra-white paint was a formulation made of calcium carbonate, an earth-abundant compound commonly found in rocks and seashells. Ruan’s lab had considered over 100 different materials, narrowed them down to 10 and tested about 50 different formulations for each material. 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. ![]() The paint's solar reflectance is so effective, it even worked 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. It can also cool surfaces 8 degrees Fahrenheit below their surroundings under strong sunlight during noon hours. Using high-accuracy temperature reading equipment called thermocouples, the researchers demonstrated outdoors that the paint can keep surfaces 19 degrees Fahrenheit cooler than their ambient surroundings at night. The paint’s whiteness also means that the paint is the coolest on record. How the whitest paint is also the coolest The higher the concentration, the easier it is for the paint to break or peel off,” Li said. “Although a higher particle concentration is better for making something white, you can’t increase the concentration too much. There is a little bit of room to make the paint whiter, but not much without compromising the paint. “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. 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. The second feature is that the barium sulfate particles are all different sizes in the paint. “We found that using barium sulfate, you can theoretically make things really, really reflective, which means that they’re really, really 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. 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. Two features give the paint its extreme whiteness. The team’s research paper showing how the paint works publishes Thursday (April 15) as the cover of the journal ACS Applied Materials & Interfaces. Paints on the market that are designed to reject heat reflect only 80%-90% of sunlight and can’t make surfaces cooler than their surroundings. Typical commercial white paint gets warmer rather than cooler. 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. 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. An infrared camera shows how a sample of the whitest white paint (the dark purple square in the middle) actually cools the board below ambient temperature, something that not even commercial “heat rejecting” paints do. ![]()
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