White paints typically reflect only about 80% of visible light, and they still absorb ultraviolet (UV) and near-infrared (near-IR) rays, which warm buildings. To do better, the new materials start by incorporating materials or structures that reflect nearly all the sun’s incoming rays, including near-IR heat and, in some cases, UV as well. They also contain polymers or other substances that, because of their chemical makeup, radiate away additional heat as mid-IR light, at wavelengths of 8 to 13 micrometres. The atmosphere does not block these wavelengths, effectively allowing the materials to shed excess heat into space without warming the surrounding air.
Yin and his colleague Ronggui Yang started last year with a plastic film embedded with tiny glass beads that cooled surfaces by up to 10C. However, applying such coatings to roofing and siding materials remains a sticking point. Highly reflective compounds can be integrated in traditional-looking shingles and clay tiles for new construction or renovation. But it’s been harder to come up with options for existing buildings. This is where the new passive cooling paint comes in. Researchers led by Yuan Yang and Nanfang Yu, applied physicists at Columbia University, had been experimenting with making highly reflective materials by adding air voids to plastics. One of their students, Jyotirmoy Mandal, stumbled on an easy method for creating a similar texture. Mandal was studying ways of curing a common polymer called polydimethylsiloxane from a liquid to a solid thin film when he noticed that under certain conditions, the material turned from transparent to white as it dried. Under the microscope he saw that an array of interconnected air voids had formed in the dried film, causing it to strongly reflect light.
The researchers found they could re-create the effect with other polymers. Eventually, they settled on a highly durable commercial polymer called PVDF-HFP. The polymer starts as a solution in acetone, to which the researchers add a small amount of water. When painted on a surface, the acetone quickly evaporates, and the polymer separates from the water, creating a network of water droplets. Finally, the water also evaporates, leaving a sponge-like arrangement of interconnected voids that reflect up to 99.6% of light, including IR, visible, and UV. The dried film also emits heat primarily in the desirable mid-IR range. Under the relentless midday sun of Phoenix, painted surfaces remained 6C cooler than the surrounding air, the researchers report in a paper published online in Science last week. They also showed that they could dye the paint, varying its appearance, although the coloured paint sacrificed some cooling. Though durable, PVDF-HFP is roughly five times as expensive as traditional acrylics used in paints, says Ronnen Levinson, a cool roof expert at Lawrence Berkeley National Laboratory in California. But the added cost carries a benefit of considerable savings in cooling costs.
