Researchers at the University of Massachusetts Amherst, USA, developed a chalk-based coating that cools the air underneath treated fabric.

In the sunlight, your body and clothing absorb ultraviolet (UV) and near-infrared (near-IR) light, warming you up. In addition to your own body heat, this can lead to uncomfortably warm temperatures fast.

One way of staying cooler is using radiative cooling, which is a way of deflecting the sun’s rays and push out natural body heat. To do so, scientists have developed textiles combined with materials with light-refracting particles. These are often either synthetic particles, like titanium oxide or aluminium oxide, or organic polymers. However, the latter require PFAS or forever chemicals in their production process.

Since neither of these options are sustainable to scale up, the researchers of Massachusetts Amherst tried to find a natural or at least environmentally benign solution. They first developed a method to apply durable polymer coatings on fabric, called chemical vapour deposition (CVD). This method uses less steps and has less environmental impact that other ways to attach coatings.

Then, the researchers found inspiration in crushed limestone-based plasters, which have historically been used to keep houses cool in extremely sunny places. They innovated a process to integrate calcium carbonate, the main component in limestone and chalk, as well as bio-compatible barium sulphate onto the polymer applied by CVD. Small particles of calcium carbonate are good at reflecting visible and near-IR wavelengths, and barium sulphate particles reflect UV light.

Depending on the thickness of the layers of calcium and barium, the air temperatures underneath treated fabric was 4 degrees Celsius (8 degrees Fahrenheit) cooler than the ambient temperature of 32 degrees Celsius (90 degrees Fahrenheit) in the middle of the afternoon, and even 8 degrees Celsius (15 degrees Fahrenheit) lower than the air underneath untreated fabric.

Photo: University of Massachusetts Amherst