A new formula for white paint has given us the whitest white yet. It reflects a whopping 98.1 percent of all the light that hits it and stays significantly cooler than the ambient temperature, even when in full sun.
When used to coat buildings, the inventors say, the paint could aid in the fight against global warming by reducing our dependence on electrically powered air conditioning, a habit that is exacerbating the climate crisis.
‘If you were to use this paint to cover a roof area of about 1000 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.
“That’s more powerful than the central air conditioners used by most homes.”
The team’s work builds on the paint they developed last year, which then reached a record-breaking 95.5 percent reflectance rate. The new formula, the team said, brings it much closer to a true counterpart to Vantablack, the black pigment that absorbs up to 99.965 percent of visible light.
The image below, in optical light on the left and infrared on the right, shows how much cooler the painted surface is than the surface around it.
(Purdue University / Joseph Peoples)
Vantablack has its own applications, but engineers and materials scientists have been looking for an ultra-reflective white paint for a while for its cooling potential. Reflective cooling paints are already commercially available such as titanium dioxide paint, but they cannot reach temperatures lower than their environment.
To develop their new paint, the researchers looked for highly reflective white materials. Their previous paint was made from calcium carbonate particles – the chemical compound found in chalk, limestone, and marble – suspended in an acrylic paint medium.
For their new formula, they switched to barium sulfate, which occurs naturally as the mineral barite and is often used as a pigment in white paint.
“We looked at several commercial products, basically anything that is white,” said mechanical engineer Xiangyu Li of the Massachusetts Institute of Technology, formerly at Purdue.
“We found that with barium sulfate you can theoretically make things really reflective, which means they are really very white.”
The trick is in the size and concentration of the particles. A range of different particle sizes of barium sulfate allow the paint to scatter the maximum amount of light, and the more barium sulfate there is, the more light it can reflect. However, there is 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 percent barium sulfate in the acrylic medium.
Xiulin Ruan holds up a sample of the paint. (Purdue University / Jared Pike)
During field tests, the team found that their painted surface consistently managed to stay at least 4.5 degrees Celsius cooler than the ambient temperature, with an average cooling power of 117 watts per square meter. It maintained this even in the middle of winter.
In comparison, the team’s calcium carbonate paint had a surface temperature of more than 1.7 degrees Celsius below the ambient afternoon temperature and a cooling capacity of 37 watts per square meter – so the few extra percent reflectivity in the barium sulfate paint made a significant difference.
Due to the limitations of the materials, the barium sulfate paint probably can’t get much more reflective, but what the team has accomplished can actually change the world for the better.
Air conditioning injects heat into the Earth’s atmosphere in a variety of ways, including pushing hot air out of buildings, the heat from running the machines, and the electricity usually generated by fossil fuels that drives them and contributes to the carbon dioxide emissions.
Scientists have been looking for a method for passive radiation cooling since the 1970s. This barium sulfate paint works, is reliable, and can be commercially produced quite easily. The team has filed for a patent and hopes the paint will soon find its way to general use.
And then? Maybe we should use it for all artists except one.
The research is published in ACS applied materials and interfaces