The Purdue University researchers who created the world’s whitest paint (earlier post) have developed a new formulation that is thinner and lighter—ideal for radiating heat away from cars, trains and airplanes.

The original world’s whitest paint used nanoparticles of barium sulfate to reflect 98.1% of sunlight, cooling outdoor surfaces more than 4.5°C below ambient temperature.

To achieve this level of radiative cooling below the ambient temperature, we had to apply a layer of paint at least 400 microns thick. That’s fine if you’re painting a robust stationary structure, like the roof of a building. But in applications that have precise size and weight requirements, the paint needs to be thinner and lighter.

—Xiulin Ruan, a Purdue professor of mechanical engineering and developer of the paint

Ruan’s team began experimenting with other materials, pushing the limit of materials’ capability to scatter sunlight. Their latest formulation is a nanoporous paint incorporating hexagonal boron nitride (hBN) 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.

Purdue University researchers have created a new formula for the world’s whitest paint, making it thinner and lighter. The previous iteration (left) required a layer 0.4 millimeters thick to achieve sub-ambient radiant cooling. The new formulation (right) can achieve similar cooling with a layer just 0.15 millimeters thick. This is thin and light enough for its radiant cooling effects to be applied to vehicles such as cars, trains and airplanes. (Purdue University photo/Andrea Felicelli)

An open-access paper on the research is published in Cell Reports Physical Science.

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.

—Felicelli et al.

Felicelli et al.

Computer simulations showed that the nanoplatelets are more effective in bouncing back the solar radiation than spherical nanoparticles used in previous cooling paints.

The paint also incorporates voids of air, which make it highly porous on a nanoscale. This lower density, together with the thinness, reduces weight. The newer paint weighs 80% less than barium sulfate paint yet achieves nearly identical solar reflectance.

This light weight opens the doors to all kinds of applications. Now this paint has the potential to cool the exteriors of airplanes, cars or trains. An airplane sitting on the tarmac on a hot summer day won’t have to run its air conditioning as hard to cool the inside, saving large amounts of energy. Spacecraft also have to be as light as possible, and this paint can be a part of that.

—George Chiu, a Purdue professor of mechanical engineering and an expert in inkjet printing

Patent applications for this paint formulation have been filed through the Purdue Research Foundation Office of Technology Commercialization.

This research was supported by the National Science Foundation with Award No. 2102645, and lead author Andrea Felicelli was supported by a National Science Foundation Graduate Research Fellowship. The research was performed at Purdue’s FLEX Lab and Ray W. Herrick Laboratories and the Birck Nanotechnology Center of Discovery Park District at Purdue.

Resources

• Andrea Felicelli, Ioanna Katsamba, Fernando Barrios, Yun Zhang, Ziqi Guo, Joseph Peoples, George Chiu, Xiulin Ruan (2022) “Thin layer lightweight and ultrawhite hexagonal boron nitride nanoporous paints for daytime radiative cooling,” Cell Reports Physical Science, doi: 10.1016/j.xcrp.2022.101058