Researchers at Purdue University (West Lafayette, Indiana, USA) have developed a new formulation of its ultra-white paint that is thinner and lighter than the previous iteration, thereby making it ideal for radiating heat away from cars, trains, and airplanes. Their research has been published inCell Reports Physical Science.
In an effort to curb global warming and reduce energy consumption, the Purdue research team initially created the formulation dubbed the “world’s whitest paint” in October 2020. Since that time, the paint has been featured in this year’s edition of “Guinness World Records” and “The Late Show With Stephen Colbert.”
“I’ve been contacted by everyone from spacecraft manufacturers to architects to companies that make clothes and shoes,” says Xiulin Ruan, a Purdue professor of mechanical engineering and developer of the paint. “They mostly had two questions: Where can I buy it, and can you make it thinner?”
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 (40.1 °F) below ambient temperature. While covering a roof in this paint could essentially cool a home with much less air conditioning, the researchers noticed a problem.
“To achieve this level of radiative cooling below the ambient temperature, we had to apply a layer of paint at least 400 microns [15.7 mils] thick,” Ruan says. “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.”
For this reason, 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, a substance mostly used in lubricants, as the pigment. This new paint achieves nearly the same benchmark of solar reflectance (97.9%) with just a single 150-micron (6.0-mil) layer of paint.
“Hexagonal boron nitride has a high refractive index, which leads to strong scattering of sunlight,” says Andrea Felicelli, a Purdue PhD student in mechanical engineering who worked on the project. “The particles of this material also have a unique morphology, which we call nanoplatelets.”
Ioanna Katsamba, another PhD student in mechanical engineering at Purdue, ran computer simulations to understand if the nanoplatelet morphology offers any benefits.
“The models showed us that the nanoplatelets are more effective in bouncing back the solar radiation than spherical nanoparticles used in previous cooling paints,” Katsamba says.
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 achievers nearly identical solar reflectance.
“This light weight opens the doors to all kinds of applications,” says George Chiu, a Purdue professor of mechanical engineering and an expert in inkjet printing. “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 money. Spacecraft also have to be as light as possible, and this paint can be a part of it.”
As to where one can buy the paint, Ruan offers the following explanation: “We are in discussions right now to commercialize it. There are still a few issues that need to be addressed, but progress is being made.”
“Using this paint will help cool surfaces and greatly reduce the need for air conditioning,” Ruan adds. “This not only saves money, but it reduces energy usage, which in turn reduces greenhouse gas emissions. And unlike other cooling methods, this paint radiates all the heat into deep space, which also directly cools down our planet. It’s pretty amazing that a paint can do all that.”
Source: Purdue University, www.purdue.edu.