Summer heat will be greeting us soon. Cities get especially hot this time of year, thanks to concrete absorbing more sunlight than vegetated surfaces. Many businesses partially offset this in their buildings by painting the roofs white. The tactic is effective, but commercial white paint still gets hotter than the surrounding air. A new, ultrawhite paint may change this. A team of engineers, led by Xiulin Ruan, has designed white paint that stays more than 8 degrees Fahrenheit lower than ambient air even under direct sunlight. The findings were published this spring in Applied Materials & Interfaces and patented in April 2020. The innovative new paint could significantly reduce cooling costs in buildings worldwide, lowering global carbon dioxide emissions in the process.

 To get the ideal white paint for cooling off, you have to consider two things: how much sunlight it can reflect, and how much heat it can effectively radiate out. Reflecting sunlight is a straightforward concept. If you make a substance that reflects more sunlight when painted onto a roof, then the paint heats up less (and so does the roof it’s coating). The authors’ creation, an acrylic paint with nanoscale clumps of barium sulfate suspended in it, reflects 98% of sunlight. That’s better than all commercial white paints.

Their barium sulfate paint is also great at emitting heat. The nuance here is how that heat is radiated. When people say ‘heat radiation,’ they’re really referring to things emitting infrared radiation, which is light at wavelengths that we can’t see. Each chemical compound emits a unique set of wavelengths; the authors’ newly invented paint emits much of its infrared radiation at wavelengths that our atmosphere doesn’t absorb due to its carefully crafted composition. This, combined with the paint reflecting almost all sunlight, is why the paint is actually 8 degrees F lower than the surrounding air even when the Sun is shining directly on it. The authors demonstrated similar success with a barium sulfate film that can be placed over a rooftop.

The paint and film are currently just proofs of concept, not commercially viable products. Still, businesses who want to lower cooling costs and environmental advocates who lower carbon emissions both have an appetite for ultrawhite paint like this.

Xiulin Ruan is a professor in the School of Mechanical Engineering at Purdue University.

Managing Correspondent: Jordan Wilkerson

Original Science Article: Ultrawhite BaSO4 Paints and Films for Remarkable Daytime Subambient Radiative Cooling

Image Caption/Credit: White rooftop of Walmart store in Las Vegas, Nev. (provided by Walmart Corporate on flickr)

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