In Mediterranean countries many houses are painted in light colors. It looks cheerful, but is above all useful: light colors heat up less in the sun. A dark color radiates heat more easily, in the form of deep infrared light. You hardly notice this during the day, because dark paint heats up very quickly in the sunlight, but it cools down much faster at night. A white paint that can also radiate heat well is special and can even provide passive cooling. The idea is that as much radiant energy as possible is sent back into space. If there is more energy going out than going in, the surface remains cooler than the environment.

Most white paint contains the pigment titanium dioxide. That reflects up to 90 percent of visible light, but it does absorb other solar radiation, such as UV light. Surfaces painted with this still heat up. Other approaches to passive cooling include applying multiple layers, with each layer reflecting a different part of the light. In addition to paint, we also look at metals, gels or wood from which the substances that absorb UV light have been removed.

The Purdue University researchers looked at a wide range of substances that may be suitable for reflecting a wider part of the light spectrum in relatively cheap paint than just visible light. Last October they published a paint containing calcium carbonate, which had 95 percent reflectivity.

The paint they are now publishing about contains barium sulphate. This fabric absorbs virtually no sunlight, also no UV light and virtually no infrared. In addition to the substance itself, the volume of particles in the paint and the varying size of the particles play a role in the reflectivity. They arrived at a composition in which 60 percent of the volume consists of barium sulphate particles, in sizes from 300 to 500 nanometers. For the desired effect, the paint must be applied in a layer of 400 micrometers, almost half a millimeter.

The fabric itself isn’t that white at all

Rolf van Benthem Eindhoven University of Technology

Two field tests were performed. At West Lafayette, Indiana, where the university is located, they saw that the painted surface remained 4.5 to 10 degrees Celsius cooler than its surroundings during the day. They compared it to commercially available white paint, where the surface temperature rose by 6.8 degrees during the day. The second test took place in Reno, Nevada, where the humidity is lower and heat radiation could in principle escape more easily. The measurements were about the same. It delivered a record passive cooling capacity of 117 watts per square meter – higher than some air conditioning installations, the researchers write.

“It is nice that they have achieved this result with barium sulphate, because that substance is not so white by itself. It is often used in paint, but then as a filler that gives it weight and flow,” says Rolf van Benthem, professor of coating technology at Eindhoven University of Technology and researcher at chemical company DSM. „Titanium dioxide, the working horse in this industry, is much whiter. Much fewer particles are needed to be opaque.”

The paint is porous

The high volume of barium sulphate particles required in this paint is immediately a major disadvantage, says Van Benthem. “It makes the paint porous, there will be small cavities in it. This makes the paint less qualitative as a protective layer. The researchers do write that they achieve comparable resistance to other paints, but the accompanying graph does not show such a positive picture at all. The layer does wear out faster. Rain is also more likely to reach the underlying surface, which can lead to rust or rot.”

But protection is therefore not the most important goal of this research, he nuances himself. “This is mainly about optical power. And more than 100 watts per square meter of cooling capacity is really something, I don’t want to dismiss that.”

“This paint also makes the most of what we call atmospheric transparency. Light of a certain wavelength, between 8 and 13 micrometers, can return to the top unhindered and this paint radiates that considerably.” But if there is a lot of moisture or carbon dioxide in the air, the heat cannot escape very well. “In a desert, where it is warm and there is little moisture in the air, this paint will work best. Above a smog city, or somewhere where there is a lot of moisture in the air, the cooling effect will be less.”

The high cooling values ​​are therefore not representative for the Netherlands. “On a sunny day it will also have a good effect here, on a muggy day with veil clouds less,” says Van Benthem. “But if you have half of the cooling capacity left over, that is also attractive.”

Because barium sulphate is already used in paint, it will not be difficult for paint manufacturers to add it to paint in larger volumes, Van Benthum suspects. “I can imagine that in a year or two there will also be paints with more barium sulphate on the Gamma shelves. The composition must then be adapted, so that it is also sustainable in the Dutch weather.”