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Author: Rebecca Trager2021-04-20T13:30:00+01:00

Source: © Purdue University/Jared Pike

Ruan Xiulin and his laboratory’s whitest paint sample ever recorded

The whitest white paint in the world was made by improving the formulation of an ultra-white paint developed by researchers at Purdue University in Indiana in October. This formula can provide an inexpensive way to cool the building by simply painting the roof.

The extreme whiteness of the new paint is due to the very high content of barium sulfate, which is also a key ingredient in photographic paper and cosmetics. It seems to be the closest white counterpart to Vantablack, which is one of the darkest known substances and was developed in 2014 by Surrey Nanosystems in the United Kingdom. The barium sulfate particles in the product vary in size, which gives it the broadest light scattering spectrum and contributes to its high reflectivity.

The new white paint can reflect up to 98.1% of sunlight, which is a significant improvement from the 95.5% reflectivity the team created last year. The paint that the team made before was based on a calcium carbonate formula. In contrast, Vantablack (a type of ultra-black coating) can absorb up to 99.9% of visible light.

The new white paint also makes the surface cooler than the previous version. Scientists say that coating residential and commercial buildings with this product will reduce the need for air conditioning.

Corresponding author Ruan Xiulin, a professor of mechanical engineering at Purdue University, said that this new technology may be most attractive to hot and poor areas where air conditioning is not widely available, such as India and Africa. For example, he estimated that if a new coating formulation was applied to a roof of approximately 90 square meters, it would result in a typical 10 kilowatt cooling. This is more powerful than a typical household central air conditioner.

Source: © Purdue University/Joseph Peoples

The infrared camera shows how the whitest white paint sample (the dark purple square in the middle) actually cools the circuit board to below ambient temperature, even commercial "heat-dissipating" paint cannot do this

In a field test in the spring of Indiana, Purdue's team found that its coating kept the surface temperature 10.5°C lower than the surrounding environment at night and kept the surface temperature at 4.5°C at noon. In contrast, commercial white paint increases the surface temperature by 6.8°C.

However, the cooling effect of the coating also works in winter. In an outdoor test with an ambient temperature of 6°C, the researchers proved that the coating can reduce the sample temperature by as much as 10°C. 

Most white paints on the market use titanium oxide as a filler and reflect 80-90% of sunlight. "This means that these products absorb 10-20% of the sunlight," Ruan explained. "We looked at the available white commercial coatings to see what we can do differently and hope to introduce better nanotechnology." The final product is the result of six years of work creating radiant cooling coatings.

A patent application for this coating formulation has been submitted, Ruan said that further testing is needed, but it may be on the market within a year or two. He pointed out that barium sulfate is slightly cheaper than titanium oxide, so the cost should be similar to traditional white paint.

The Purdue team has not yet created a new paint. Mas Subramanian, a chemist and professor of materials science at Oregon State University, emphasized that the chemical properties of barium sulfate as a white pigment have been understood since the 18th century, and he was not involved in this research. "Barium sulfate has been around for centuries, but they adjusted the particle size and created a microstructure that can better reflect heat," he told Chemical World. "This is not necessarily a breakthrough, but it is a major advancement in reflective coating technology."

David Darling, vice president of health, safety and environmental affairs of the American Coatings Association, also pointed out that before introducing such coatings to the market, it is necessary to consider the environmental, health and safety issues related to such coatings.

X Li et al., ACS Appl. alma mater. Interface, 2021, DOI: 10.1021/acsami.1c02368

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