United States. In an effort to curb global warming, engineers at Purdue University have created the whitest paint yet with which they will be able to coat buildings and cool them sufficiently.
In October, the team created an ultra-white paint that pushed the boundaries of what white paint can look like. Now they have overcome it. Not only is the newer paint whiter, but it can also keep surfaces cooler than the formulation the researchers had previously demonstrated.
"If you used this paint to cover a roof area of about 1,000 square feet, we estimated you could get a cooling power of 10 kilowatts. That's more powerful than the central air conditioners used by most homes," said Xiulin Ruan, a professor of mechanical engineering at Purdue.
The researchers believe that this white may be the closest equivalent to the blacker black, "Vantablack", which absorbs up to 99.9% of visible light. The new whiter paint formula reflects up to 98.1% of sunlight, compared to 95.5% of sunlight reflected by the researchers' previous ultra-white paint, and sends infrared heat away from a surface at the same time.
Typical commercial white paint becomes warmer rather than colder. Paints on the market that are designed to reject heat reflect only 80% to 90% of sunlight and cannot make surfaces cooler than their surroundings.
What makes whiter paint so white?
Two characteristics give the painting its extreme whiteness. One is the very high concentration in paint of a chemical compound called barium sulfate, which is also used to make photo paper and cosmetics white.
"We looked at various commercial products, basically anything that's white," said Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology, who worked on this project as a Purdue Ph.D. student in Ruan's lab. "We found that by using barium sulfate, you can theoretically make things really, really reflective, which means they're really, really white."
The second feature is that barium sulfate particles have different sizes in the paint. The amount of light each particle scatters depends on its size, so a wider range of particle sizes allows the paint to scatter more of the sunlight spectrum.
"A high concentration of particles that are also of different sizes gives paint the widest spectral scattering, which contributes to the greatest reflectance," said Joseph Peoples, a Perdue Ph.D. student of mechanical engineering.
There's a little room to make the paint whiter, but not much without compromising the paint.
"Although a higher concentration of particles is better for making something white, you can't increase the concentration too much. The higher the concentration, the easier it will be for the paint to break or peel off," Li said.
How the whitest paint is also the freshest
The whiteness of the paint also means that the paint is the coldest on record. Using high-precision temperature reading equipment called thermocouples, the researchers demonstrated outdoors that paint can keep surfaces 19 degrees Fahrenheit cooler than their ambient environment at night. It can also cool surfaces to 8 degrees Fahrenheit below their surroundings in intense sunlight during the midday hours.
The solar reflectance of the paint is so effective that it even worked in the middle of winter. During an outdoor test with an ambient temperature of 43 degrees Fahrenheit, the painting managed to reduce the temperature of the sample by 18 degrees Fahrenheit.
Source: Perdue University.
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