At noon on a sunny summer day, the temperature of a conventional dark-colored flat roof can reach 150 degrees Fahrenheit (65 degrees Celsius), according to the U.S. Department of Energy. That heat will warm the inside of a building or a house as well, making it necessary to use air conditioning — an energy expenditure that in turn often requires burning fossil fuels at electrical power plants, whose emissions contribute to the progression of climate change. It's a bedeviling problem that might be easily solved, if we only had roofs that reflected solar energy back into the sky, instead of absorbing it.
The Whitest Paint Ever Created
That's why researchers' development of the whitest paint ever created — an "ultra-white" formulation that reflects up to 98.1 percent of sunlight — could turn out to be a really big deal in the effort to reduce energy consumption and combat the warming of our planet. The breakthrough is detailed in this article, published April 15, 2021, in Applied Materials & Interfaces, an American Chemical Society (ACS) journal.
"This paint not only reflects the majority of the sunlight to avoid overheating, but also cools itself down colder than the air surrounding it, even under direct sunlight," Xiangyu Li, the paper's first author, says via email. He's a postdoctoral researcher at Massachusetts Institute of Technology who worked on the project while in graduate school at Purdue. (His coauthors include Joseph Peoples, Peiyan Yao and corresponding author Xiulin Ruan, a professor of mechanical engineering at Purdue, who leads the university's Nanoscale Energy Transport and Conversion Laboratory.)
"In some way, it is effectively an air conditioner without consuming any electricity," Li says.
Not only does the paint absorb very little energy from sunlight, but it also emits thermal energy in the infrared range back into the atmosphere, so that it travels out and away from Earth. "Both effects help reduce electricity or cooling needs," Li says.
Researchers have been trying to make paints that could be used in passive cooling since the 1970s, but most of them have absorbed too much sunlight, so that they don't provide any cooling effect in the daytime, Li explains. In recent years, scientists have also been exploring exotic technologies for creating light-reflecting surfaces, such as multi-layered nanonstructured films, but those solutions tend to be expensive and not easily scalable to buildings, according to Li.
That brought the researchers back to thinking that the best approach "is in the form of particle-polymer composite, which is similar to commercial paint," Li explains.
Ordinary white paint isn't quite white enough to help much with passive cooling — it only reflects 80 to 90 percent of sunlight, and actually gets warmer from absorbing ultraviolet light. So, the task became finding a way to make white paint even whiter. And that turned out to be a challenge.
"We first tried multiple materials with high electron band gap to avoid solar absorption," Li says. "However, the paints often appeared semi-transparent at low thickness due to the lack of ability to reflect sunlight. During our previous studies on commercial paints and theoretical modeling, we identified that both high concentration and broad particle size distribution were beneficial. As we implemented these two approaches, we saw the paint solution appear whiter compared to previous attempts."
As Li explained in a Purdue news release, the researchers looked at "various commercial products, basically anything that's white." Eventually, "[w]e found that using barium sulfate, you can theoretically make things really, really reflective, which means that they're really, really white."
You may have heard of barium sulfate before. It's the stuff that medical patients take in drink or tablet form to coat the esophagus, stomach and intestines when they're getting an X-ray exam or CT scan, so that doctors can see diseased or damaged areas more clearly, according to MedlinePlus. As a paint ingredient, Li notes, it's already widely available and safe for humans and the environment.
The researchers used a variety of sizes of barium sulfate particles in the paint, which enabled the paint to scatter more of the spectrum.
They also had to make sure that the paint would last long enough outside to be suitable for painting rooftops. "We tested the abrasion resistance, three-week outdoor weathering and a water rinse test," Li explains. "All three showed promising results as initial durability tests. We also tested the viscosity, and the paints can be brushed and dried very similar to commercial paint. It can also be fabricated in a similar way. In this work, we not only achieved a paint that is superior to the commercial paint, but also do not need specialized equipment or materials to fabricate, transport or apply the paint."
Ultra-white paint looks pretty similar to regular white paint, because the difference in reflectivity, while significant in terms of cooling, is imperceptible to human eyes. "In the paper, we have a photo of both paints, which look almost identical to each other," Li says.
The next step in the research is to optimize the paint for long-term durability under different sorts of weather conditions. There isn't yet a target date for it to be market-ready, so don't search for it at the local hardware store just yet.