In October, a team of researchers at Purdue University rolled out the world's whitest paint, touting its ability to reflect the sun's heat from rooftops, sending it back into outer space. Now, they're back with a whiter, more reflective paint that could potentially ease our reliance on air conditioning and reduce carbon dioxide emissions. 

A paper published Thursday in ACS Applied Materials & Interfaces demonstrates that the new formulation — based on very high concentrations of barium sulfate, a chemical used to make white paper and cosmetics — keeps surfaces cooler than the team's first ultrawhite paint, which was created with calcium carbonate and proved to be 95% reflective. Both formulations improved upon titanium dioxide, a common pigment in commercial white paints that reflects 90% to 92% of sunlight. The researchers found that their new barium sulfate-based compound is slightly more than 98% reflective.

Xiangyu Li, formerly affiliated with Purdue and now a postdoctoral associate at the Massachusetts Institute of Technology, was a lead author on the paper. He studies heat transfer on the nanoscale — specifically, how energy flows when metal particles are added to plastics to make them more conductive. That aligns with the basic concept of the ultrawhite paint, which has an acrylic polymer base, like most commercially sold paints. 

"Then, you put some particles in to reflect light," Li told The Academic Times. "What we're trying to do is make the prototype of a commercial paint for buildings and potentially cars and other surfaces facing the outdoors." He pictures the paint being applied to rooftops, automobiles, food-storage facilities at supermarkets, and data centers with immense cooling needs. The ultrawhite paint could also be used on heavy equipment associated with cell towers and 5G installations. 

Space cooling in general is responsible for much of our energy consumption, Li said. Relying less on air conditioners would reduce carbon dioxide emissions. "When you power on air conditioning, you're consuming electricity — using energy to move heat away," Li said. "At the end of the day, you're actually producing more heat."  

Air conditioning contributes to the urban island heat effect, in which cities are significantly hotter than their environs, Li said. Cities have fewer trees, as well as more exposed asphalt and building surfaces that store heat. Cities also have more air conditioning units pushing heat out of buildings. But coating a rooftop with super-reflective paint could get the same job done, without using energy, through a process known as radiative cooling. 

"It's a passive system," Li explained. "You don't need any energy input at all. There's always a thermal loss to the universe. When you make things whiter and reflect most of the sunlight, it's kind of like a passive air conditioner." 

To the naked eye, the ultrawhite paint looks pretty much the same as ordinary white paint, Li said. But using a base of barium sulfate is an improvement across the board: It is more reflective, absorbs less heat and is less expensive than titanium oxide. The size of particles within the paint also changes how it reflects light; for example, smaller particles reflect smaller wavelengths. And, as the researchers conclude in the paper, a mix of particle sizes reflects a broader spectrum of sunlight. "Previously, people used only one single particle size," Li said. "What we found is that having a wide range of particle sizes, each size particle reflects certain wavelengths, so overall you have a good reflection." 

In addition to measuring the cooling power of the ultrawhite paint, the researchers demonstrated its practical reliability by measuring how much mass it loses when exposed to the friction of turning wheels — replicating wear from the elements — and found that their paint is comparable to commercial options. Because it's acrylic-based and water-resistant like most store-bought paints, the paint also performed comparably when left outside for three weeks. 

Patent applications for the new paint formulation have been filed through the Purdue Research Foundation Office of Technology Commercialization. But it's unclear whether a product will be available to consumers anytime soon. "Trying to make a product and bring it to the market is a whole other story," Li said. 

Still, he believes the study shows that the ultrawhite paint has the potential to be more than a concept in a paper and can be produced on a large scale. "This is compatible with commercial paint fabrication," he explained. "If someone wants to make this [paint], you don't have to change much in your existing facility."   

And there's good news for homeowners who are interested in saving on energy costs but don't want an ultrawhite house: The team is looking at developing highly reflective paint in different colors, too. 

The study, "Ultrawhite BaSO4 paints and films for remarkable daytime subambient radiative cooling," published April 15 in ACS Applied Materials & Interfaces, was authored by Xiangyu Li, Joseph Peoples, Peiyan Yao and Xiulin Ruan, Purdue University.