“Cool paint” made from plastic pollution beats the heat

A worker spraying a coat of cool paint on the roof of a building at the test-site in Singapore. Photo Credit: Nanyang Technological University

You may not know the Urban Heat Island (UHI) effect by name, but you’ve probably felt it – uncomfortably so – many times: it’s the phenomenon in which urban areas experience warmer temperatures than outlying rural surroundings, caused by (a) the modification of urban land surfaces, and (b) the waste heat generated by energy usage.

Rising temperatures in cities around the world are increasing demand for energy-intensive air conditioning systems, but relying heavily on these technologies isn’t feasible if countries want to hit their climate targets. Another strategy against UHI involves using so-called “cool paint” coatings, which contain additives that reflect the sun’s heat to reduce surface heat absorption and emission. To date, however, most studies of cool paint coatings have been either simulation-based or tested in scaled-down models, and understanding of its application in real-world scenarios is limited. But now, researchers from Nanyang Technological University (NTU) in Singapore have developed and tested a new cooling paint that mixes recycled plastics – including acrylic, old PVC pipes, and polystyrene foam – with barium sulphate, with the added twist of making the recycled plastics more porous by creating tiny air-filled holes in them during the production process, the better to scatter the sun’s rays. And then they carefully documented the results.

To carry out their real-world experiments, the NTU researchers selected four rectangular buildings that created two parallel “street canyons” – narrow streets flanked by buildings – in an industrial estate west of Singapore. One canyon, called a “cool canyon,” was coated with cool paints on the roofs, walls, and road pavement, while the other canyon was left uncoated. Using environmental sensors, the NTU team then monitored the conditions in the two canyons over two months, including air movement, surface and air temperature, humidity, and radiation. The researchers say they found that during a 24-hour cycle, the cool canyon saw up to a 30 per cent reduction in heat released from the built-up surfaces, resulting in the air temperature in the cool canyon being cooler than the conventional canyon by up to 2°C during the hottest time of the day, at around 4 p.m. Also, compared to conventional roofs, the roofs with the cool paint coating reportedly reflected 50 per cent more sunlight and absorbed up to 40 per cent less heat as a result, during the hottest time of a sunny day. The coated walls also prevented most of the heat from entering the industrial buildings, the team said.

The new paint – which, as noted, used a mix of unsorted plastic waste – also outperformed cool paints developed using only a single type of plastic waste, which often can’t achieve temperatures lower than the surrounding air. This suggests that the NTU team’s approach can reduce the need for sorting different types of plastic, thereby helping tackle another problem: plastic pollution.

Going forward, the NTU team plans to focus on how the cool paint coating holds up over time in the same experiment location.