The world's whitest paint is now thinner, with almost the same solar reflectance

Last year, researchers at Purdue University made a Guinness World Record when their white paint was declared the whitest ever. In little over a year, the researchers have returned by turning it into a thinner version than ever, making it ideal for use in cars, spaceships, and airplanes, Phys.org has reported.

Interesting Engineering had reported that the world's whitest paint was seven years in the making and was designed to help fight climate change and save energy. While these were noble causes and the paint did deliver 98.1 percent reflection of sunlight, the next challenge was to ensure that the discovery could be made practical as well.

Researchers at Purdue were also inundated with requests to make the paint thinner so that it could be applied for a wide range of applications, including shoes and clothes.

Making it thinner

For the original paint to be effective, researchers needed to paint at a thickness of 400 microns. While this might not seem much and works well for robust structures such as the roof of a building, there are multiple applications that have precise size and weight requirements.

To address these challenges, the researchers moved away from barium sulfate, the original component used to make the whitest paint, which has a rich history of being used to make white products such as cosmetics and photo paper. They found an alternative in boron nitride, a hexagonal pigment that is used in lubricants.

With a layer of paint just 150 microns thick, the researchers were able to achieve a solar reflection of 97.9 percent, nearly equal to what was achieved with a 400-micron thick layer.

How boron nitride works

Boron nitride has a high refractive index which means that the pigment can scatter most of the sunlight it receives. Moreover, the researchers put to use the material's unique morphology to deliver high solar reflectance.

Researchers in the mechanical engineering department at Purdue used computer simulations to understand how this new morphology termed nanoplatelets helps. They found that when compared to spherical nanoparticles that are used in other cooling paints, nanoplatelets can bounce back solar radiation more effectively.

Additionally, paint made using this material trapped air at nanoscales which made it highly porous. This offers a significant advantage since the painted surface has a lower density of paint and hence lower weight too. When compared to paint made using barium sulfate, the new paint weighs nearly 80 percent less without compromising on the solar reflectance.

This opens up the potential applications of the paint to areas of aerospace as well. Using this paint on aircraft can help keep its interiors cooler by up to 4.5 degrees Celsius, reducing the dependence on air conditioning. Using the paint for spacecraft can help in the reduction of the weight that needs to be carried to orbit. "This not only saves money, but it reduces energy usage, which in turn reduces greenhouse gas emissions," said Xiulin Ruan, a professor at Purdue University, who developed the paint.

Interestingly, the paint also radiates the heat into deep space, thereby directly cooling the planet too. This is in stark difference from other cooling technologies that rely on heat transfer to cool buildings, causing a build-up of heat in the atmosphere.

The research findings were published in the journal Cell Reports Physical Science.

Abstract

Thin and lightweight radiative cooling paints are needed for many weight-sensitive applications. However, it is difficult to achieve high solar reflectance with thin layers. This work develops ultrawhite hBN-acrylic paints that achieve solar reflectance of 97.9% and sky window emissivity of 0.83 with only 150 μm thickness and 0.029 g/cm2 weight, representing significant reductions from previous radiative cooling paints. The high refractive index and nanoplatelet morphology of hBN enable a unique combination of Mie scattering-like high scattering coefficient and Rayleigh scattering-like strong backscattering, and a porosity of 44.3% offers high refractive index contrast between hBN and air; all contribute to achieve high solar reflectance with a thin coating. Field tests show full daytime cooling under direct sunlight, reaching 5–6°C below ambient temperature on average during daylight hours. Our hBN-acrylic paint demonstrates comparable cooling performance with recent best technologies, and the thinness and light weight reduce barriers toward many practical applications.