Xiulin ruan holds up a sample of the whitest paint

Whitest, coolest paint ever made

A team of US engineers has developed the whitest – and therefore coolest – paint in the world, able to reflect 98.1% of the sunlight that hits its surface.

The researchers from Purdue University in the US say that the paint could improve energy efficiency by reducing reliance on air conditioning.

A description of the paint is published in the journal ACS Applied Materials & Interfaces.

Things appear white when they reflect all wavelengths of visible light. Similarly, objects that are dark or black absorb all visible light. This makes it relatively easy to rate the ‘whiteness’ or ‘blackness’ of objects – simply measure how much light is being reflected.

In 2014, a team of UK researchers unveiled ‘Vantablack’, a compound that uses nanostructures to absorb 99.965% of all light that interacts with it. This new paint uses similar logic to achieve the opposite result: it uses nanoscale structures to reflect as much light as possible.

While the paint can’t yet reflect as much light as Vantablack absorbs black, it reflects significantly more than the previous ‘whitest’ paint: a compound announced by the same team of Purdue researchers in October 2020, which reflected 95.5% of sunlight.

The new paint is mostly composed of barium sulphate, which is used as a commercial whitener in cosmetics and photo paper.

 “We looked at various commercial products, basically anything that’s white,” says Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology who worked on this project as part of his PhD at Purdue. “We found that using barium sulphate, you can theoretically make things really, really reflective, which means that they’re really, really white.”

An infrared camera shows how a sample of the whitest white paint (the dark purple square in the middle) actually cools the board below ambient temperature, something that not even commercial “heat rejecting” paints do. Credit: Purdue University/Joseph Peoples

The barium sulphate particles are a range of different sizes, which allows them to scatter more rays of sunlight.

“A high concentration of particles that are also different sizes gives the paint the broadest spectral scattering, which contributes to the highest reflectance,” says Joseph Peoples, a PhD student at Purdue.

While the colour could be slightly whiter, this would make it a less effective paint.

“Although a higher particle concentration is better for making something white, you can’t increase the concentration too much,” says Li. “The higher the concentration, the easier it is for the paint to break or peel off.”

As it stands, the research shows that the paint can handle outdoor conditions and could be made commercially.

The high reflectivity of the paint means it can keep surfaces much cooler than their surroundings, something that commercial white paints struggle to do. The engineers found that the paint could make surfaces 11°C cooler than their surroundings at night, and 4°C cooler in the middle of the day.

“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,” says Xiulin Ruan, a professor of mechanical engineering at Purdue and leader of the research team. “That’s more powerful than the central air conditioners used by most houses.”

The team has filed a patent for their cool, white paint.

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