A new formula for white paint has given us the whitest so far. It reflects 98.1% of all the light that hits it, remaining much colder than the ambient temperature, even when we are in full sun.
If used to cover buildings, say its inventors, paint could help fight global warming by reducing our dependence on electric air conditioning, a habit that exacerbates the climate crisis.
“If you were to use this paint to cover a roof area of about 1,000 square meters [92.9 square meters]”We estimate that you could get a cooling capacity of 10 kilowatts,” said mechanical engineer Xiulin Ruan of Purdue University.
“It’s stronger than the central air conditioners used by most homes.”
The team’s work is based on the paint it developed last year, which reached a reflection rate of 95.5%. The new formula, the team said, brings it much closer to being a true counterpart of Vantablack, the black pigment that absorbs up to 99.965% of visible light.
The image below, in optical light on the left and infrared on the right, shows how cold the painted surface is than the surface around it.
(Purdue University / Joseph Peoples)
Vantablack has its own applications, but materials engineers and scientists have been pursuing ultra-reflective white paint for some time for its potential cooling capabilities. Reflective cooling paints are already commercially available, such as titanium dioxide paint, but cannot reach lower temperatures than the environment.
To develop their new paint, the researchers looked for white materials with high reflection. Their previous paint was made of calcium carbonate particles – the chemical compound found in chalk, limestone and marble – suspended in an acrylic paint medium.
For their new formula, they resorted to barium sulfate, which occurs naturally as a mineral barite and is commonly used as a pigment in white paint.
“We looked at various commercial products, practically everything is white,” said mechanical engineer Xiangyu Li of the Massachusetts Institute of Technology, who was in Purdue.
“I found that using barium sulfate, theoretically you can make things really, really reflective, which means they’re really, really white.”
The trick is the size and concentration of the particles. A range of different sizes of barium sulphate particles allows the paint to scatter the maximum amount of light and, of course, the more barium sulphate it is, the more light it can reflect. There is, however, a point where too much barium sulfate can compromise the integrity of the paint, making it brittle and flaky when it dries.
The sweet spot, the researchers found, is a concentration of about 60% barium sulfate in the acrylic medium.
Xiulin Ruan picking up a sample of paint. (Purdue University / Jared Pike)
During field tests, the team found that their painted surface consistently managed to stay cooler than the ambient temperature by at least 4.5 degrees Celsius, achieving an average cooling power of 117 watts per square meter. He maintained this even in the middle of winter.
For comparison, the team’s calcium carbonate paint had a surface temperature of more than 1.7 degrees Celsius below ambient temperature at noon and a cooling power of 37 watts per square meter – so the reflectivity of a few percent more of the paint with barium sulfate made a significant difference.
Due to material limitations, barium sulfate paint probably can’t get much More reflective, but what the team accomplished could change the world for the better.
Air conditioning injects heat into the Earth’s atmosphere in several ways, including pushing hot air out of buildings, the heat of machine operation, and the electricity usually generated by running fossil fuels, which contributes to carbon dioxide emissions.
Scientists have been looking for a method of passive radiative cooling since the 1970s. This barium sulphate paint works, is reliable and can be produced commercially quite easily. The team has filed a patent and hopes the paint could soon find a place in common use.
And then? Maybe we should license all artists except one.
The research was published in Materials and interfaces applied ACS.