Barium sulfate creates the world's whitest paint

Xiulin Ruan, a Purdue University professor of mechanical engineering, and his students have created the whitest paint on record

 

Professor Xiulin Ruan of Mechanical Engineering at Purdue University and his students once created the whitest paint ever recorded, which has now been included in the Guinness Book of World Records. This ultra-white paint can reflect up to 98.1% of visible light, and because of its strong reflectivity, it can even emit infrared heat from the painted surface, and has excellent radiation cooling performance.

 

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

 

White is a color that contains all the colors in the spectrum. It is the color with the highest brightness that we come into contact with in our daily lives, and it is also a special color. It cannot be mixed with other colors. It is the most expensive and cherished existence in the paint box of art students. It is also a relatively expensive one among car spray paint and industrial coatings.

 

There are many reasons for raising the "value" of white. Take the white paint commonly used in the industry as an example. Compared with other paints, its color is more obviously affected by impurities, so its purification process will be more complicated. At the same time, white paint is easily oxidized and turns yellow. What kind of materials to choose and how to stabilize its matrix are also technical difficulties.

 

In short, the whiter the paint, the harder it is to make. But scientists have never stopped pursuing the ultimate, just like humans have created the blackest black "Vanta Black", which can absorb 99.9% of visible light. In contrast, there is a man who is also obsessed with pursuing the ultimate white. After more than six years of hard work by him and his team, he finally did it recently. He is Professor Ruan Xiulin from Purdue University in the United States. Recently, he led his team to create the whitest white paint in human history. This ultra-white paint can reflect up to 98.1% of visible light, while also emitting infrared heat from the painted surface, and has excellent radiation cooling performance.

 

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.

 

Since joining Purdue University in 2008, Professor Xiulin Ruan has been committed to research in energy and thermophysics engineering. After recently publishing the research results of super white paint, he accepted interviews from many media and talked about how if this paint is applied to about 1,000 square inches of roof, 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 (radiative) cooling. At present, a relatively advanced solution is to use complex multi-layer structures or reflective metal layers, but this limits their application in many fields. Another is to achieve passive cooling through paint, but the reflectivity of most white paints currently used is still in the range of 80%-90%, and the low reflectivity makes it difficult to keep the surface cool. This requires painting a very thick layer of paint, which is not only wasteful but also very easy to harden, and its application range is narrow and can only work under certain specific weather conditions.

 

To solve this problem, the research team invented a calcium carbonate-based white coating with a reflectivity of 95.5%. Now, the new nanocomposite ultra-white coating based on barium sulfate they invented has undoubtedly promoted the development of passive cooling technology to a greater extent. Their paper was published as a cover article in the journal "American Chemical Society Applied Materials & Interfaces".

 

"Whitening" secrets

This super-white paint has two whitening "secrets". One is the high concentration of barium sulfate added to the paint matrix. This compound has a high electronic band gap, low sunlight absorption and high reflectivity. It is often used as a filler to change the consistency and added to some white paints. It can also be used as a whitening agent and added to photo paper and cosmetics.

The researchers tested a large number of white items on the market and found that adding barium sulfate can indeed greatly increase the reflectivity of the object, that is, it will make it look whiter; the second is that the barium sulfate particles inside the paint are of different sizes and have a large particle size fluctuation range. The degree to which each particle scatters light fluctuates with its particle size, so the larger the particle size range distribution of the particles inside the paint, the more visible light the paint can scatter.

The superposition effect of these two "secrets" of high concentration and different particle sizes makes this new paint have extremely high reflectivity. Of course, the concentration of barium sulfate cannot be too high. Although high concentration helps to make the paint whiter, too high a concentration will also make the paint more likely to crack or peel after drying, greatly affecting the user experience.

 

Replace air conditioner

As we all know, white light is a complex color light, and an opaque object will absorb different colors of light and reflect the same color of light, so white objects have a higher reflectivity. When white paint is used to paint the roof, it can reflect the sun's radiant heat to a large extent, effectively reducing radiation heat transfer and convection heat transfer.

According to a study in Geophysical Research Letters in 2010, if the roofs of houses in the city are painted with ideal white paint, computer simulation found that this can reduce the heat island effect by 33%, reduce the overall temperature of the city by 0.4℃, and significantly reduce the indoor temperature, thereby reducing the use rate of air conditioners and achieving the purpose of energy conservation and emission reduction.

 

Microstructure of barium sulfate-acrylic acid composite coating with a volume concentration of 60%

 

The reflectivity of the paint developed by Purdue University is much higher than that of ordinary commercial paint, which means that it has a better cooling effect. The researchers used a thermocouple high-precision temperature detection device to test the cooling performance of the paint outdoors. In the demonstration experiment, this paint can be used in a variety of environments. At night, it can make the painted surface 11°C lower than the surrounding temperature, and in the strong sunlight at noon, it can also cool the surface to 5°C lower than the surrounding environment. In further experiments, the researchers found that even in the cold environment of winter (the test temperature was about 6°C), it could still reduce the surface of the sample by 10°C, which proved that the paint has excellent reflective cooling performance.

 

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, this new type of ultra-white paint originally came from a whim of the research team. They originally wanted to use ultra-white paint to replace traditional air conditioners for house cooling. After that, they made many improvements based on the radiation cooling paint developed in the 1970s, and conducted more than six years of ratio research and experiments. For the selection of the base material of the paint, they initially considered more than 100 different materials. After a lot of experiments, they gradually narrowed the range to 10. After that, they conducted cross-tests of 50 different formulas for each of these 10 groups of materials.

 

They initially obtained a calcium carbonate-based white paint, but calcium carbonate may react in extreme weather such as acid rain and cause the paint to deteriorate, so they launched a barium sulfate-based paint, and specifically optimized the stability of the paint for complex outdoor environments, and finally selected a barium sulfate-acrylic acid composite paint formula with a volume concentration of 60%. This also shows that calcium carbonate is not as good as barium sulfate in acid and alkali resistance.

 

Professor Xiulin Ruan said that the new type of paint they invented is superior to other commercial white paints currently used in terms of reflectivity, cooling performance and stability. At the same time, its manufacturing process is compatible with ordinary commercial paint processes and the cost is controllable. Therefore, this paint has broad application prospects in reducing space cooling costs, combating urban island effects, and alleviating global warming in the future. In an interview with the BBC, he also said: "We made a very rough calculation that only 1% of the earth's surface (perhaps some rocks in uninhabited areas) needs to be coated with this paint to play a role in combating climate change to a certain extent."

 

Pay attention to the precipitated barium sulfate of Yunfu Hongzhi New Materials Co., Ltd.

Yunfu Hongzhi New Materials Co., Ltd. has the ability to produce various precipitated barium sulfate products with an average particle size of 30 nanometers to 10 microns, focusing on providing customized products and services. It is worth mentioning that we can perform different surface treatments on products according to customer needs.

 

Based on the research of Professor Xiulin Ruan's team, barium sulfate creates the whitest paint in the world, which shows that the precipitated barium sulfate of Yunfu Hongzhi New Materials Co., Ltd. plays an important role as a functional additive in coating applications. Barium sulfate has high whiteness and acid and alkali resistance. It is produced by combining powders of different particle sizes, which can make the coating have the widest spectral scattering and help to obtain the highest reflectivity. It is called heat reflective coating.

 

Yunfu Hongzhi New Materials Co., Ltd. engineers also said that by adding high concentrations of barium sulfate and combinations of different particle sizes of barium sulfate to reduce the gaps between particles and improve reflection, more can be added due to the low oil absorption value of barium sulfate. Our precipitated barium sulfate has a narrow particle size distribution and more uniform particles. The effect will be better through the combination of different particle sizes. Our Ultra White Barium Sulfate is very popular in coatings that require high gloss, and Extinction Barium Sulfate also has its unique features in matte and semi-matte coatings.

 

 

Professor Xiulin Ruan's team has made great achievements not only in architectural coatings, but also in spray paints for cars, airplanes and spacecraft. Barium sulfate can increase solid content and reduce VOC, which can help reduce air conditioning consumption, and since these air conditioning systems consume fuel, it can also help save gasoline. In coating applications, adding precipitated barium sulfate, a functional material, can combat global warming by keeping the surface cool enough to reduce the need for internal air conditioning.