Researchers at Purdue University in the US created the whitest paint yet. Reflecting 98% light, the paint may cool buildings enough to reduce the need for air conditioning.

In October, the researchers created an ultra-white paint made of calcium carbonate, an earth-abundant compound commonly found in rocks and seashells, which was the whitest up till then. The new paint is even whiter than that. In addition, it can surfaces cooler than the previous formulation.

“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,” said Xiulin Ruan, a Purdue professor of mechanical engineering.

The whitest white may be the closest equivalent of the blackest black, or Vantablack, which absorbs 99.9% of visible light. This white reflects 98.1%, compared to 95.5% for the previous paint the researchers developed, and 80-90% for commercial paints.

Two features give the paint its extreme whiteness. First, the paint contains a very high concentration of a chemical compound called barium sulfate, which is also used to make photo paper and cosmetics white. The compound ensures that the paint is very reflective. Second, the particles of the barium sulfate are all different sizes in the paint. How much each particle scatters light depends on its size, so a wider range of particle sizes allows the paint to scatter more of the light spectrum from the sun.

The paint can only get a little whiter, since increasing the particle concentration too much would make the paint easier to break or peek off.

The researchers demonstrated outdoors that the paint can keep surfaces 10 degrees Celsius (19 degrees Fahrenheit) cooler that their ambient surroundings at night, and 5 degrees Celsius (8 degrees Fahrenheit) in strong sunlight around noon.

The paint’s solar reflectance is effective enough to work in the middle of winter. During an outdoor test with an ambient temperature of 6 degrees Celsius (43 degrees Fahrenheit), the paint still managed to lower the sample temperature by 5 degrees Celsius (18 degrees Fahrenheit).

Photo: Purdue University / Jared Pike / Joseph Peoples