An ultra-white paint incorporating barium sulfate (BaSO₄) particles that is up to 98.1% reflective, developed by engineers at Purdue University, has earned a Guinness World Records title for the whitest paint. The record appears in the 2022 edition of Guinness World Records.

The 98.1% reflectivity compares to a reflectivity of 80–90% for other white paints designed to reflect sunlight that are currently available. Such paints are considered to be a potential technology for keeping cities cooler and reducing electricity use; buildings with a coating of this would need to rely far less on energy-hungry air conditioning. The Purdue team published its latest findings in ACS Applied Materials & Interfaces earlier this year.

An infrared camera (right panel) 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. Credit: Purdue University/Joseph Peoples

Xiulin Ruan, a professor of mechanical engineering at Purdue, invented the paint with his graduate students. The idea was to create paint that would reflect sunlight away from a building. Making this paint really reflective, however, also made it really white. Because the paint absorbs less heat from the sun than it emits, a surface coated with this paint is cooled below the surrounding temperature without consuming power.

Using this new paint formulation to cover a roof area of about 1,000 square feet could result in a cooling power of 10 kilowatts, the Purdue researchers showed. Scaling that up, the scientists estimated that it would only require 0.5–1% of Earth’s surface to be coated in this paint (e.g., by painting roofs) to reverse global warming to date.

This white paint is the result of research building on attempts going back to the 1970s to develop radiative cooling paint as a feasible alternative to traditional air conditioners. Ruan’s lab had considered more than 100 different materials, narrowed them down to 10 and tested about 50 different formulations for each material.

Two features make this paint ultra-white: a very high concentration of barium sulfate and different particle sizes of barium sulfate in the paint. What wavelength of sunlight each particle scatters depends on its size, so a wider range of particle sizes allows the paint to scatter more of the light spectrum from the sun.

The researchers have partnered with a company to scale up the paint and put it on the market. 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 FA9550-17-1-0368). The research was performed at Purdue’s FLEX Lab and Ray W. Herrick Laboratories and the Birck Nanotechnology Center of Purdue’s Discovery Park.

Resources

• Xiangyu Li, Joseph Peoples, Peiyan Yao, and Xiulin Ruan (2021) “Ultrawhite BaSO4 Paints and Films for Remarkable Daytime Subambient Radiative Cooling” ACS Applied Materials & Interfaces 13 (18), 21733-21739 doi: 10.1021/acsami.1c02368

• Xiangyu Li, Joseph Peoples, Zhifeng Huang, Zixuan Zhao, Jun Qiu, Xiulin Ruan (2020) “Full Daytime Sub-ambient Radiative Cooling in Commercial-like Paints with High Figure of Merit,” Cell Reports Physical Science, Volume 1, Issue 10 doi: 10.1016/j.xcrp.2020.100221 (Open access)