A new ultrawhite paint is the most reflective yet (Cell Rep. Phys. Sci. 2022, DOI: 10.1016/j.xcrp.2022.101058). A typical 150 µm thick coat of the paint reflects 97.9% of sunlight and cools the surface it’s on by 5–6 °C in direct sun. It could help cool airplanes, cars, and trucks without using any power and help reduce the need for air conditioning.
Commercial white paints made of titanium dioxide nanoparticles reflect 80–90% of sunlight but absorb some heat, which reduces their benefits during daytime. Last year, Xiulin Ruan of Purdue University and colleagues made a cooling paint using barium sulfate nanoparticles that reflects most of the sun’s rays, thus radiating more infrared heat than it absorbs (ACS Appl. Mater. Interfaces 2021, DOI: 10.1021/acsami.1c02368). But the paint works best in a coat 400 µm thick—much thicker than the typical 120 µm coat on vehicles.
The new paint is a mix of hexagonal boron nitride (hBN) nanoplatelets and an acrylic binder and reflects more light at a reduced thickness. Two-dimensional hBN flakes also have a very high refractive index, making them good at scattering light. One coat is all that’s needed to get effective cooling.
Although less hBN paint is required, it is relatively expensive, “so this material is more suitable for applications where weight and thickness matter, like transportation,” Ruan says.