The whitest paint is here – and it's the coolest. Literally.

Xiulin Ruan, a Purdue University professor of mechanical engineering, holds up his lab&#;s sample of the whitest paint on record. (Purdue University/Jared Pike)

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New paint formulation could help buildings rely less on air conditioning

WEST LAFAYETTE, Ind. &#; In an effort to curb global warming, Purdue University engineers have created the whitest paint yet. Coating buildings with this paint may one day cool them off enough to reduce the need for air conditioning, the researchers say.

In October, the team created an ultra-white paint that pushed limits on how white paint can be. Now they&#;ve outdone that. The newer paint not only is whiter but also can keep surfaces cooler than the formulation that the researchers had previously demonstrated.

&#;If you were to use this paint to cover a roof area of about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts. That&#;s more powerful than the central air conditioners used by most houses,&#; said Xiulin Ruan, a Purdue professor of mechanical engineering.

The researchers believe that this white may be the closest equivalent of the blackest black, &#;Vantablack,&#; which absorbs up to 99.9% of visible light. The new whitest paint formulation reflects up to 98.1% of sunlight &#; compared with the 95.5% of sunlight reflected by the researchers&#; previous ultra-white paint &#; and sends infrared heat away from a surface at the same time.

Typical commercial white paint gets warmer rather than cooler. Paints on the market that are designed to reject heat reflect only 80%-90% of sunlight and can&#;t make surfaces cooler than their surroundings.

The team&#;s research paper showing how the paint works publishes Thursday (April 15) as the cover of the journal ACS Applied Materials & Interfaces.

What makes the whitest paint so white

Two features give the paint its extreme whiteness. One is the paint&#;s very high concentration of a chemical compound called barium sulfate, which is also used to make photo paper and cosmetics white.

&#;We looked at various commercial products, basically anything that&#;s white,&#; said Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology, who worked on this project as a Purdue Ph.D. student in Ruan&#;s lab. &#;We found that using barium sulfate, you can theoretically make things really, really reflective, which means that they&#;re really, really white.&#;

The second feature is that the barium sulfate particles are all different sizes in the paint. How much each particle scatters light depends on its size, so a wider range of particle sizes allows the paint to scatter more of the light spectrum from the sun.

&#;A high concentration of particles that are also different sizes gives the paint the broadest spectral scattering, which contributes to the highest reflectance,&#; said Joseph Peoples, a Purdue Ph.D. student in mechanical engineering.

There is a little bit of room to make the paint whiter, but not much without compromising the paint.

&#;Although a higher particle concentration is better for making something white, you can&#;t increase the concentration too much. The higher the concentration, the easier it is for the paint to break or peel off,&#; Li said.

How the whitest paint is also the coolest

The paint&#;s whiteness also means that the paint is the coolest on record. Using high-accuracy temperature reading equipment called thermocouples, the researchers demonstrated outdoors that the paint can keep surfaces 19 degrees Fahrenheit cooler than their ambient surroundings at night. It can also cool surfaces 8 degrees Fahrenheit below their surroundings under strong sunlight during noon hours.

The paint's solar reflectance is so effective, it even worked in the middle of winter. During an outdoor test with an ambient temperature of 43 degrees Fahrenheit, the paint still managed to lower the sample temperature by 18 degrees Fahrenheit.

This white paint is the result of six years of research building on attempts going back to the s to develop radiative cooling paint as a feasible alternative to traditional air conditioners.

Ruan&#;s lab had considered over 100 different materials, narrowed them down to 10 and tested about 50 different formulations for each material. Their previous ultra-white paint was a formulation made of calcium carbonate, an earth-abundant compound commonly found in rocks and seashells.

The researchers showed in their study that like commercial paint, their barium sulfate-based paint can potentially handle outdoor conditions. The technique that the researchers used to create the paint also is compatible with the commercial paint fabrication process.

Patent applications for this paint formulation have been filed through the Purdue Research Foundation Office of Technology Commercialization. This research was supported by the Cooling Technologies Research Center at Purdue University and the Air Force Office of Scientific Research through the Defense University Research Instrumentation Program (Grant No.427 FA-17-1-). The research was performed at Purdue&#;s FLEX Lab and Ray W. Herrick Laboratories and the Birck Nanotechnology Center of Purdue&#;s Discovery Park.

About Discovery Park

Discovery Park is a place where Purdue researchers move beyond traditional boundaries, collaborating across disciplines and with policymakers and business leaders to create solutions for a better world. Grand challenges of global health, global conflict and security, and those that lie at the nexus of sustainable energy, world food supply, water and the environment are the focus of researchers in Discovery Park. The translation of discovery to impact is integrated into the fabric of Discovery Park through entrepreneurship programs and partnerships.

About Purdue University

Purdue University is a top public research institution developing practical solutions to today&#;s toughest challenges. Ranked the No. 5 Most Innovative University in the United States by U.S. News & World Report, Purdue delivers world-changing research and out-of-this-world discovery. Committed to hands-on and online, real-world learning, Purdue offers a transformative education to all. Committed to affordability and accessibility, Purdue has frozen tuition and most fees at -13 levels, enabling more students than ever to graduate debt-free. See how Purdue never stops in the persistent pursuit of the next giant leap at https://purdue.edu/.

Writer, Media contact: Kayla Wiles, 765-494-,

Source: Xiulin Ruan,

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Ultra-white BaSO4 Paints and Films for Remarkable Daytime Subambient Radiative Cooling

Xiangyu Li, Joseph Peoples, Peiyan Yao  and Xiulin Ruan

DOI: 10./acsami.1c

Radiative cooling is a passive cooling technology that offers great promises to reduce space cooling cost, combat the urban island effect and alleviate the global warming. To achieve passive daytime radiative cooling, current state-of-the-art solutions often utilize complicated multilayer structures or a reflective metal layer, limiting their applications in many fields. Attempts have been made to achieve passive daytime radiative cooling with single-layer paints, but they often require a thick coating or show partial daytime cooling. In this work, we experimentally demonstrate remarkable full daytime sub-ambient cooling performance with both BaSO4 nanoparticle films and BaSO4 nanocomposite paints. BaSO4 has a high electron bandgap for low solar absorptance and phonon resonance at 9 µm for high sky window emissivity. With an appropriate particle size and a broad particle size distribution, BaSO4 nanoparticle film reaches an ultra-high solar reflectance of 97.6% and high sky window emissivity of 0.96. During field tests, BaSO4 film stays more than 4.5°C below ambient temperature or achieves average cooling power of 117 W/m2. BaSO4-acrylic paint is developed with 60% volume concentration to enhance the reliability in outdoor applications, achieving solar reflectance of 98.1% and sky window emissivity of 0.95. Field tests indicate similar cooling performance to the BaSO4 films. Overall, our BaSO4-acrylic paint shows standard figure of merit of 0.77 which is among the highest of radiative cooling solutions while providing great reliability, the convenient paint form, ease of use and the compatibility with commercial paint fabrication process.

Barium sulfate is an odd term for the majority of the population. We can say that barium sulfate is ubiquitous in everyday life. It appears as the end product of barium sulfate. Look at your home, for example, the walls might have been coated with barium sulfate. Barium sulfate paint, as one of the pigments, has been recognized as an improvement in thickness, abrasion, water-resistant, and impact-resistant properties.

Furthermore, barium sulfate has a high reflectivity between 300-400 micrometers, which can prevent light-aging. Therefore, it is a kind of high-efficiency, low-cost, white inorganic light-stabilizing agent. As we can tell, barium sulfate paint is very popular in the paint industry. In this paper, we will discuss these causes.

Three forms of barium sulfate used in the coating industry

1. Barite powder is prepared by washing, crushing, and drying of natural barium sulfate. It has a lot of impurities. The quality of the ore depends largely on its own, and the cost is relatively low. It is often used as a white pigment, low-grade paint, and ink industry filling.

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2. Precipitated barium sulfate is manufactured artificially. Unlike barite powder, precipitated barium contains few impurities. It is slightly soluble in water and insoluble in acids. The deposit of barium sulfate is used as a filling material in middle and high-grade paint and printing ink. It's more expensive than the barite powder.

 3. Nano-precipitation barium sulfate is a kind of improved and deposited barium sulfate. Due to the complexity of the manufacturing process, the production cost of nano-precipitated barium sulfate is very expensive. Moreover, it has an excellent isolating effect, which can make the pigments keep some distance from each other, thereby increasing the efficiency of the pigment. Nano-precipitation BaSO4 is widely applied in high-grade paints and coatings.

Advantages of physical characteristics

One of the reasons why barium sulfate paint is popular is that barium sulfate's special properties can improve the performance of the paint. The 4 main characteristics and merits of it are as follows.

1. Gravitational dispersant

Due to its high specific gravity, barium sulfate can be used as a gravity disperser for maintaining the dispersion of the pigments. As the pigment starts to deposit, barium sulfate particles will be attracted to scatter the deposited pigment in the coating. This prevents granules from forming and maintains uniform coating.

2. Improve fluidity and coherence

Barium sulfate is an evenly distributed grain that can be filled in between granules to enhance the size of the coating, thus improving its fluidity and uniformity.

3. Enhance hardness

Barium sulfate granules are very tough and can be used as a solid grain in the coating. This kind of grain structure can improve the hardness and abrasion resistance of the paint efficiently, and thus improve its wear-resisting and weatherability.

4. Settlement and segregation prevention

Because of its high density and non-soluble nature, barium sulfate is highly dispersed, stable, and can avoid the sedimentation and separation of pigment and other constituents. Thus, it is possible to prolong the lifetime of paint and maintain its quality and stability.

Extensive variety of applications 

Another explanation is that barium sulfate paint can be applied to numerous sorts of items.

1. Modern field: it has been applied broadly to a wide range of enterprises, like the petrochemical business, steel, non-ferrous metal purifying, and so forth. Here, barium sulfate paint can safeguard modern gear from erosion and high temperatures.

2. Building field: it is likewise a typical structure covering material, for outside and inside wall covering, waterproof covering, and beautiful covering. Because of its exceptional water opposition and embellishment execution, it can keep the development from being hit by the outside and improve its tasteful quality.

3. Plastic field: because of its incredible bond, barium sulfate coating is generally applied to plastic items, for example, cars, domestic devices, and furniture.

4. Electronic field: barium sulfate covering material is able to successfully protect electronic hardware from high temperatures and electric shock as a result of its brilliant high-temperature opposition and protection execution.

Conclusion

With everything taken into account, it is feasible to presume that barium sulfate has different benefits in working on the exhibition and nature of paint. Magnificent execution and many uses of barium sulfate paint permit it to be more practical. In the interim, offering strong support for the improvement of the coating industry is reasonable.

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