A new polymer coating will cool buildings and cars: here's how

Scientists, including one of Indian origin, invented a polymer coating that serves as a spontaneous air cooler and can be applied as paint on roofs, buildings, water tanks, vehicles and even spacecraft.

Researchers at Columbia University in the United States used a solution phase inversion technique that confers a high-performance outdoor passive radiative cooling (PDRC) polymer with a porous foam-like structure. PDRC is a phenomenon in which a surface spontaneously cools by reflecting sunlight and emitting heat to a colder atmosphere.

Developing practical designs for the PDRC has been difficult: many recent design proposals are complex or expensive and can not be widely applied or applied to roofs and buildings of different shapes and textures, according to Jyotirmoy Mandal. Until now, white paints, inexpensive and easy to apply, were the reference for the PDRC, they said.

However, white paints usually contain pigments that absorb ultraviolet (UV) light and do not reflect very well the long wavelengths of the sun; their performance is therefore modest. The air voids in the new porous polymer, described in the journal Science, disperse and reflect sunlight, due to the difference in refractive index between the air voids and the surrounding polymer .

The polymer becomes white and thus avoids solar heating, while its intrinsic emittance effectively makes it lose heat in the sky. The team relied on previous work that demonstrated that simple plastics and polymers, including acrylic, silicone and PET, were excellent heat radiators and could be used for PDRC treatment.

The challenges were how to ensure that these normally transparent polymers reflect sunlight without using silver mirrors as reflectors and make them easily deployable. The researchers decided to use phase inversion as it is a simple, solution-based method for creating voids in the dispersion of light in polymers. .

Polymers and solvents are already used in paints, and the new method essentially replaces the pigments of white paint with air spaces reflecting all the wavelengths of sunlight, from UV to infrared. "This simple but fundamental modification produces exceptional eflectance and thermal emittance that equal or surpass those of state-of-the-art PDRC designs, but with paint-like comfort," said Mandal.

"The researchers found that the high solar reflectance and high thermal emittance of their polymeric coating kept it significantly cooler than its environment under very different skies, for example six degrees Celsius in Arizona's arid desert and three degrees Celsius. Bangladesh, "said Yuan Yang, an assistant professor at Columbia.

"The fact that cooling is achieved in both desert and tropical climates, with no thermal protection or shielding, demonstrates the utility of our design wherever cooling is required," Yang said. The team also created colored polymer coatings with cooling capabilities by adding colorants.

"Achieving a superior balance between color and cooling performance compared to current paints is one of the most important aspects of our work," said Nanfang Yu, an associate professor at Columbia. "For exterior cladding, the choice of color is often subjective, and paint manufacturers have been trying to make colored coatings, such as those used for roofs, for decades," Yu said.