Fig.1 Xiulin Ruan, a Purdue University professor of mechanical engineering,

and his students have created the whitest paint on record.

Purdue University mechanical engineering Professor Xiulin Ruan and his students have made it into the Guinness Book of World Records for the whitest paint ever recorded.

Fig.2 Xiulin Ruan holds up his lab's sample of the whitest paint on record.

White is a color that contains all the colors of the spectrum of light. It is the brightest one we come into contact with in our daily life. It is also a special color. It can not be reconciled with others. It is the most expensive and most cherished one in the art raw pigment box. It is also the most expensive one in automobile spray painting and industrial coatings.

There are many reasons to raise the "value" of white. Take the white paint commonly used in industry areas as an example, compared with other coatings, its color is affected by impurities more obviously, so its purification process will be more complex. At the same time, white paint is easy to be oxidized and yellow, then how to choose, how to stabilize, these are also difficult in the whole process.

In short, the whiter the paint, the harder it is to make. But scientists have never stopped striving for the ultimate, as humans have already created the blackest Black, "Vanta Black," which can absorb 99.9% of visible light. On the other hand, one man is also dedicated to the ultimate white, and after more than six years of hard work by him and his team, he finally achieved it. Xiulin Ruan, a professor at Purdue University in the United States, recently led a team to create the whitest white paint in human history. The ultra-white paint reflects up to 98.1 percent of visible light and also radiates infrared heat away from the painted surface, providing excellent radiative cooling properties.

Fig.3 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.

Professor Xiulin Ruan has been working on energy and thermophysics engineering since he joined Purdue University in 2008. After recently publishing his work on ultra white paint, Ruan gave several interviews and said that if he applied the paint to a roof covering with about 1,000 square inches, the interior of the house can get 10 kilowatts of cooling power, even cooler than most houses with central air conditioning.

Before this, it was very difficult to achieve efficient passive (radiant) refrigeration. One of the most advanced solutions is to use complex multilayer structures or reflective metal layers, but this limits their application in many areas. The other is to achieve passive cooling through paint, but most of the white coatings used today are still in the range of 80-90% reflectivity, low reflectivity makes it difficult to keep the surface cool. This requires painting a layer of very thick paint, which not only waste but is also very easy to board, and its scope of application is narrow, and can only play a role in some specific weather conditions.

To solve this problem, the team went from creating a white coating based on calcium carbonate with a 95.5% reflectivity to creating a new nano-composite ultra-white coating based on barium sulfate, which is sure to push the development of passive refrigeration even further. Their paper was published as the cover article in the American Chemical Society journal Applied Materials and Interfaces.

The secret of "whitening”

This super white coating has two whitening "secret", one is the coating substrate mixed with a high concentration of barium sulfate. This compound has a high electron band gap, low sunlight absorption, and high reflectivity. It is often used as a filler to change the consistency of some white paints. It can also be used as a brightening agent to be added to photographic paper and cosmetics.

The researchers tested a wide range of commercially available white objects and found that adding barium sulfate did significantly increase the object's reflectivity, or make it look whiter; the second is that the size of the barium sulfate particles inside the coating is different, which has a large size fluctuation range. The extent to which each particle scatters light fluctuates with its particle size, so a larger particle size range allows the paint to scatter a larger range of visible light.

The combination of high concentration and different particle sizes gives the new coatings an extremely high reflectivity. Of course, the concentration of barium sulfate should not be too high, although the high concentration will help to make the paint whiter, too high concentration will make the paint more likely to crack or flake after drying, greatly affecting the user experience.

Instead of air conditioning

White light is known to be a complex color, and an opaque object absorbs different colors of light and reflects the same color, so white objects have a higher reflectivity. When white paint is used on the roof, it can largely reflect the solar radiation heat, effectively reducing the radiation heat transfer and convection heat transfer.

According to a 2010 study in Geophysical Research Letters, computer simulations found that if roofs in cities were painted with ideal white paint, they could reduce the heat island effect by 33%, lowering the overall temperature of the city by 0.4 ° C and significantly lowering indoor temperatures, thereby reducing the use of air conditioning and reducing emissions.

Fig.4 Microstructure of barium sulfate - acrylic acid composite coatings with volume concentration of 60%

Purdue's paint has a much higher reflectivity than normal commercial paint, which means it cools better. The cooling performance of coatings was tested outdoors using a high-precision thermocouple temperature detection device. In a demonstration, the paint can be applied to a variety of environments, allowing a painted surface to be as much as 11 ° C cooler than its surroundings at night and as much as 5 ° C cooler in the midday sun. In further experiments, the researchers found that even in a cold winter environment (the test temperature was about 6 ° C), it could still reduce the surface of the sample by 10 ° C, demonstrating the coating's excellent reflective cooling properties.

Fig.5 Purdue University researchers Xiulin Ruan and Joseph Peoples use an infrared camera to

compare the cooling performance of white paint samples on a rooftop.

In fact, the new ultra-white paint originally came from the brainchild of a team of researchers who wanted it to replace traditional air conditioners for cooling homes. After that, they made many improvements on the basis of the radiation cooling coating developed in the 1970s and conducted more than six years of matching research and tests. For the selection of substrate materials for coatings, they initially considered more than 100 different materials, and after a lot of experiments, gradually narrowed the range down to 10. They then cross-tested each of the 10 groups with 50 different formulations.

They initially obtained a white coating based on calcium carbonate, but calcium carbonate may react in extreme weather such as acid rain, and lead to the deterioration of the coating. Therefore, they introduced the barium sulfate based coating and optimized the stability of the coating specifically for the complex outdoor environment. Finally, they chose the composite coating formula of barium sulfate and acrylic acid with a volume concentration of 60%. This also shows that calcium carbonate is not as good as barium sulfate in acid and alkali resistance.

According to Professor Xiulin Ruan, their new coating has better reflectivity, cooling performance, and stability than other commercial white coatings currently in use. Meanwhile, the manufacturing process is compatible with that of ordinary commercial coatings and the cost is controllable. Therefore, this kind of coating has a broad application prospect in reducing the cost of space cooling, combating the urban island effect, and mitigating global warming in the future. In an interview with the BBC, he added: "We have done a very rough calculation that it would take just 1% of the Earth's surface (maybe some rocks in no man's land) to have some effect on climate change."

Pay attention to the precipitated barium sulfate of Hongzhi New Materials

Yunfu Hongzhi New materials has the ability to produce a variety of precipitated barium sulfate products with an average particle size of 30 nm to 10 microns, and focuses on providing customized products and services. What's more, it is worth mentioning that we can carry out different surface treatments for products according to customer needs, which also means that our products can be equivalent to Blanc Fixe in the world.

Based on the research of Professor Xiulin Ruan's team, barium sulfate creates the whitest coating in the world, which indicates that our precipitated barium sulfate plays an important role as a functional additive in coating applications. Barium sulfate has high whiteness, acid and alkali resistance and other properties, it is produced by different particle size powder combinations, so that the coating has the widest spectral scattering, helping to obtain the highest reflectivity, called heat reflective coating.

Engineers from Hongzhi also said that the combination of high concentration barium sulfate and different particle sizes of barium sulfate can reduce the gap between particles and improve reflection. Since barium sulfate has a low oil absorption value, more can be added. The particle size distribution of our precipitated barium sulfate is narrow and the particles are more uniform. Through the combination of different particle sizes, the effect will be better. Our M series precipitated barium sulfate is very popular in paints requiring high gloss, and the SG series also has unique features in matte and semi-matte paints.

According to the research of Professor Xiulin Ruan's team, not only for architectural coatings, but also for the spray paint of automobiles, aircraft and spacecraft, etc., barium sulfate can increase solid content and reduce VOC, which can help reduce the consumption of air conditioning, and since these air conditioning systems consume fuel, it can also help save gasoline. In coating applications, the addition of a functional material, precipitated barium sulfate, combats global warming by keeping the surface cool enough to reduce the need for internal air conditioning.

Explain that the above pictures are from Purdue University in the United States.