Most buildings in urban areas today employ substantial mechanical ventilation systems to regulate their indoor environments. Consequently, these structures consume massive amounts of energy generated by fossil fuels, which further exacerbates rising outdoor air temperatures and global warming. After developing an ultra-white paint using calcium carbonate in October 2020, researchers at Purdue University, USA have outdone themselves and created the 'whitest white' paint on record yet. But what makes it so special?

They describe it as an analogue to Vantablack (the darkest shade of black), capable of absorbing 99.9% of visible light. The researchers also predict that this ultra-white paint's high reflectivity could, in all probability, reduce a structure's reliance on indoor air conditioning.

The project is a result of over six years of dedicated research, built on foundational attempts to develop radiative cooling paints as an alternative to air conditioners. Conducted at FLEX Lab, Ray W. Herrick Laboratories, and the Birck Nanotechnology Center of Purdue's Discovery Park, the research was supported by Cooling Technologies Research Center at Purdue University and the Air Force Office of Scientific Research through the Defense University Research Instrumentation Program, USA. 

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“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 is more powerful than the central air conditioners used by most houses,” explains Xiulin Ruan, a professor of mechanical engineering at Purdue University, in an official press release shared by the institution. The new formulation could reflect up to 98.1% of incident light, a significant improvement compared to the 95.1% reflected by the team's previous ultra-white paint. Moreover, the paint also impresses by deflecting harmful infrared radiation away from the surface it is applied on. In contrast, commercially manufactured, thermally reflective paints predominantly heat their surroundings and are often only capable of reflecting 80 - 90 per cent of radiation.

Having initially reviewed over a hundred unique materials, the researchers narrowed their options down to the ten most feasible ones. They then tested approximately 50 different formulations for each material before arriving at their result. “We found that using barium sulphate, you can theoretically make things really, really reflective, which means that they are really, really white,” says Xiangyu Li, a researcher at the Massachusetts Institute of Technology, who was involved as a Purdue PhD student in Ruan’s lab.

In fact, the paint's whiteness and unrivalled reflectivity stem from high concentrations of barium sulphate nanoparticle films as well as nanocomposites. Barium sulphate is used in the manufacture of white-coloured cosmetics and photo paper, and the particles used in the paint vary broadly in size, which allows the paint to scatter more light over the solar spectrum. “A high concentration of particles that are also different sizes gives the paint the broadest spectral scattering, which contributes to the highest reflectance,” shares Joseph Peoples, a PhD student in mechanical engineering at Purdue.

With the aid of sensitive temperature recording instruments known as thermocouples, the researchers ascertained that their ultra-white paint cooled outdoor surfaces by up to 19 degrees Fahrenheit below the surrounding ambient air temperature at night. During strong sunlight at noon, the applied paint cools surfaces by eight degrees Fahrenheit. It also displayed exemplary performance in an outdoor test during winter by cooling surfaces by 18 degrees below the contextual air temperature of 43 degrees Fahrenheit.

According to the research team, there is little leeway above the current 60 per cent volume concentration to make the paint whiter - however, pushing this may compromise its viability in large-scale applications. “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 explains.

The researchers have filed patent applications for the paint formulation through the Purdue Research Foundation Office of Technology Commercialization. Having established its potential feasibility in outdoor applications, the team also devised its manufacturing technique to ensure compatibility with conventional paint production methods. Their results chart an exciting new horizon which showcases the potential of exterior finishes that employ radiative cooling, to tackle exorbitant building air conditioning systems and their adverse effects on climate change.

(Text by Jerry Joe Elengical, intern at STIRworld.com)