Just How White Can White Paint Get?

Scientist holding a bottle of white paint.  Purdue University researchers have created a coating that reflects 98% of light.

Professor Xiulin Ruan with white boron nitride paint.
picture: Photo: Purdue University/Jared Pike Graphics: Vicki Letta

It makes sense: When it’s hot outside, don a dark shirt—wear white instead. A team of researchers at Purdue University put this shared knowledge to work to find ways to do so Brilliant buildingsAnd planes and even cars With the whitest paints in the world.

The concept of using white paint to deflect heat is not new. Since the 1970s, scientists and manufacturers have been developing whiter, whiter paints to help reflect sunlight and keep buildings and other surfaces cooler. However, the whitest paints available on the market today reflect 90% of the light – a figure that seems high but means that they still absorb 10% of the sunlight and therefore cannot cool the surface below ambient temperature.

In 2014, a group from Stanford Paper books On the use of layers of reflective surfaces of inorganic compounds to reflect sunlight and cool surfaces below the ambient temperature during the day, a process that, although effective, was expensive and not scalable. Purdue engineering professor Xiulin Ruan, inspired by Stanford’s work, began to wonder about more practical applications of nanotechnology in working with paints, specifically to combat climate change.

“We weren’t really thinking about developing whiter Rowan said. “My background is as a civil engineer and researcher, so I’m interested in that heat. I’m starting to wonder, can we get scalable nanotechnology…so we can revisit the paints to make them.” [cooling] a job?”

Ruan and his team are winners of the 2023 Gizmodo Science Fair, attempted to improve a pre-existing commercial coating that used titanium dioxide, but realized that titanium dioxide absorbs UV light. “It basically limits the performance,” Rowan said. His team began experimenting with different formulations of white paint that included different compounds that did not absorb UV light, playing with particle size and particle concentration to achieve higher and higher reflectance.

“We spent many years doing formulations and simulations, and it took many postdocs and PhD students to figure out the trick,” said Rowan. “We really had to combine all the factors and push them to the limits to achieve this super performance.”

In the end, Rowan’s team came up with a coating that used high concentrations of barium sulfate particles of various sizes, which were able to reflect 95% of the sunlight. It was so effective that it quickly surpassed its own record, and in 2021 it created paint that reflects 98.1% of sunlight. It is currently the whitest color on the planet.

After the success of 2021, Ruan began receiving inquiries from people in industries such as aerospace and aviation, who were interested in how they could use paint to cool their products. “They asked us two questions: Can you make this thinner, and can you make it lighter?” He said.

Although the barium sulfate coating is highly reflective, it requires a layer of at least 0.4 mm thick to be effective—something unsuccessful in industries seeking to make their products as light as possible. The paint was also not very sticky: the barium sulfate paint was formulated as a gel for maximum effectiveness, making it difficult to apply to surfaces other than buildings or structures.

“It’s not easy to make paint paintable, sprayable, or crushable,” Rowan said.

He tapped fellow Purdue University student George Chiu, who has a background in photonics and digital printing, as well as two doctoral students, Andrea Felicelli and Ioana Katsamba, to figure out a way to preserve the light-reflecting properties of barium sulfate without the extra baggage. Adds the complex.

Rowan, Felicili, Katsamba, and Chiu look at paint samples in the lab.

Rowan, Felicili, Katsamba, and Chiu look at paint samples in the lab.
picture: Purdue University / Jared Pike

“Barium sulfate has reflective properties that allow us to stay below ambient temperature,” said Felicelli, whose research focuses on biologically inspired nanocomposites. “But it also contributes a lot to the weight of the paint.”

They began to search for different materials to use in a new formulation of paint. They eventually settled on hexagonal boron nitride, an inorganic compound mostly used in lubricants. Not much literature has been written about how particle shape helps reflect sunlight, and Katsamba was tasked with modeling how boron nitride helps make coatings reflective—a topic that had not been addressed before. Boron nitride has a unique shape called nanoplatelets, which has been shown to help it deflect sunlight more effectively than the spherical particles in other compounds.

“The simulations show that it’s not only the properties of boron nitride that differ from barium sulfate, but also the shape that really helps achieve the high reflectivity that we have now,” Katsamba said.

Even after settling on a compound that models showed would be great at reflecting sunlight, making the paint itself was still a challenge—particularly given that the morphology of boron nitride made some paints thicker and less viscous.

“We definitely had some trouble getting the particles to fully incorporate into the paint mixture,” Felicelli said. “But over time, we’ve been able to work around different curing techniques. And we’ve been able to achieve an application that’s very similar to your usual commercial white paint.”

The resulting coating with boron nitride is almost as effective as the first formula with barium sulfate: it can reflect up to 97.9% of sunlight. What’s more, it only requires a 0.15-millimeter layer to be effective, and thanks to the compound’s porous structure that holds a lot of air, it weighs 80% less than barium sulfate paint.

While you can’t go out and buy this ultra-white paint right now, Rowan said the team is in talks with manufacturers to start the process of making it and selling it as a product. The paint is also enhanced and tested to ensure it can resist dirt and can last as long under UV light as commercial white paints. Rowan said he’s also interested in creating reflective paint in different colors, as well as paints that can transform from reflective to absorbent, depending on the season — like a building-sized version of drinkware that changes color when you pour it hot. or cold water in it.

“Right now, paint is for cooling, so it’s great for hot climates,” he said. But in Indiana, for example, we need heating in the winter. It would be nice if you could switch from the whiter paint in the summer to one that can absorb the sun’s rays. And that’s a challenge to make it cost-effective, to make it scalable. We’re investigating, we can’t reveal all the details right now, but we can learn from nature to help us understand how to make these kinds of technologies possible.”

Read more: The whitest paint in the world is literally the coolest


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