Tag Archives: white paint

Researchers create the “whitest paint” — it reflects 98% of sunlight

What if paint could cool off a building enough to not need air conditioning? A group of US researchers has produced just that: the whitest ever paint. It cooled surfaces by 4.5ºC below the ambient temperature and reflected 98% of sunlight.

Image credit: Flickr / Marco Verch

Cooling represents a significant sector of energy consumption in both residential and commercial applications. This is set to become a growing problem as the climate heats up more and more, as more people buy air conditioning equipment to cope with rising temperatures. Urban areas are especially problematic: cities act as “hot islands” as vegetation is replaced with concrete and asphalt, infrastructure that’s darker and absorbs more heat. 

White-painted roofs have long been considered a solution to cool buildings. They are currently used all around the world, from New York to India — but there’s a problem. The reflective white paints currently on the market only reflects 80-90% of sunlight and absorb UV light. This means that oftentimes, they can’t cool surfaces below ambient temperature.

Last year, researchers from Purdue University in the United States created an ultra-white paint that pushed limits on how white paint can be. But now they have outdone their work, creating a newer paint that isn’t just whiter but can also keep surfaces cooler than the formulation that the researchers had previously demonstrated.

“If you were to use this paint to cover a roof area of about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts. That’s more powerful than the central air conditioners used by most houses,” Xiulin Ruan, a Purdue professor of mechanical engineering, said in a statement. 

The researchers argue this new white is probably the closest equivalent of the blackest black, known as “Vantablack,” which absorbs up to 99.9% of visible light. The paint formulation reflects up to 98.1% of sunlight – compared with the 95.5% of the white they developed in 2020 – and sends infrared heat away from the surface as well. 

The new paint was revealed in a paper in the journal ACS Applied Materials & Interfaces, in which the researchers explained the reasons behind the paint’s cooling performance. They used barium sulphate as the pigment at a 60% concentration, which, unlike conventional titanium dioxide pigment, doesn’t absorb UV light. 

At the same time, the researchers used another trick: pigment particles of different sizes. The amount of light dispersed by a particle depends on its size, so using a range distributes more of the light spectrum from the sun. The team tried out over 100 different materials and tested around 50 formulations for each of the most promising. 

“We looked at various commercial products, basically anything that’s white,” Xiangyu Li, a postdoctoral researcher at MIT, who worked on this project as a Purdue Ph.D. student, said in a statement. “We found that using barium sulfate, you can theoretically make things really, really reflective, which means that they’re really, really white.”

The new paint is the result of six years of research, building on attempts going back to the 1970s to develop radiative cooling paint. As it uses a standard acrylic solvent, the paint could be manufactured like any other, the researchers argued. It would also be similar in price, they argued, as barium sulphate is actually cheaper than titanium dioxide. 

The university and the research team have already filed a patent for the paint and are now working with large companies towards commercialization. They believe that the paint will be widely available in the market in one or two years. This could help address cooling’s growing emissions and at the same time bring down the heat in big cities.

This white paint cools rooftops below the surrounding temperature, even under direct sunlight

We’ve all experienced it at some point: white things stay cooler in the sunlight, while black things get hotter as they absorb more sunlight. This is more than just a physics curiosity, it has significant impacts on our world.

This is the reason, for instance, why the houses around the arid areas of the Mediterranean are often painted white — to reflect more sunlight and keep the buildings a bit cooler. Now, a team of researchers at Purdue University are taking things to the next level: they’ve developed a special white paint that reflects almost all the sunlight cast upon it, helping to keep the surface cool.

Professor Xiulin Ruan (left) and PhD student Joseph People (right) studying the Purdue radiative cooling paint and the commercial paint samples placed side by side. Image credits: Jared Pike of the School of Mechanical Engineering at Purdue University.

The problem of urban temperature is more complex than it seems. It’s not just climate heating that makes cities hot, there’s also the urban heat island effect.

Cities act as ‘hot islands’ due to several reasons. For starters, there’s the land modification, the clearing of vegetation and replacing it with infrastructure — infrastructure that’s darker and capable of absorbing more heat. There’s also the changing of water and wind flow, and the extra heat generated by cars, electricity production and ironically, the air conditioners that keep us cool.

Scientists have worked to develop radiative cooling paints for decades, but their efficiency and cost-effectiveness has remained challenging. This new paint might change things.

“It is a persistent task to develop a below-ambient radiative cooling solution that offers a convenient single-layer particle-matrix paint form and high reliability,” says Xiulin Ruan, a professor at the School of Mechanical Engineering at Purdue University in Indiana and an author of the study. “This is critical to the wide application of radiative cooling and to alleviate the global warming effect.”

This infrared image shows that the P pattern pained with the Purdue radiative cooling paint is much cooler than the background painted with commercial paint. Image credits: Xiangyu Li, PhD student of the School of Mechanical Engineering at Purdue University.

The paint developed by Ruan and colleagues used calcium carbonate fillers, a relatively cheap and abundant compound, as opposed to the usual titanium dioxide particles used in this type of paint. The calcium carbonate substances minimize the amount of ultraviolet light the paint absorbs, and the particle concentration and structure boosts scattering to dissipate as much energy as possible.

When the paint sample was put to the test in Indiana, over a two-day period, it remained 10 degrees Celsius below the ambient temperature at night and at least 1.7 degrees below the ambient temperature when the sun was at its zenith. Even when compared to another commercial white paint of the same thickness, the new paint was able to maintain a significantly lower temperature.

The paint could be used in a great nuber of applications, from housing to electrical equipment or cars.

“This paint may even be used to combat climate change since it rejects sunlight and radiates heat into space,” says Ruan.

However, researchers still need to ensure that the paint is cost-effective and resilient enough under realistic conditions (i.e. its resistance to water, dust, or detergent). So far, the team is confident.

“Our paint is compatible with the manufacturing process of commercial paint, and the cost may be comparable or even lower,” says Ruan. “The key is to ensure the reliability of the paint so that it is viable in long-term outdoor applications.”

Journal Reference: Li et al.: “Full Daytime Sub-ambient Radiative Cooling in Commercial-like Paints with High Figure of Merit”, Cell Reports Physical Science https://www.cell.com/cell-reports-physical-science/fulltext/S2666-3864(20)30236-8