With the dog days of summer nearing an end, many are counting down the days until they can give the darker clothes in their wardrobe a bit more love. Though plenty of people flout seasonal rules and wear all-black year-round, light-colored fabrics are typically the better choice during  hotter months because they reflect sunlight and can help keep us cool. This heat-reflecting logic is also partly responsible for the old decree, “don’t wear white after Labor Day!” But what if fashion choices weren’t dictated by the weather? A team of researchers from China and Australia believes they may have cracked that code, and it starts with a rich purple fabric.

The researchers, hailing from Zhengzhou University and Adelaide University, set out to create a dark-colored material that was also effective at keeping people cool. They created an entirely new fabric made of a purple metal-organic framework and zinc oxide nanoparticles. The metal-organic framework which absorbs green light from the sun and gives the purple color, while the zinc oxide nanoparticles which reflect sunlight and radiate heat away. 

The material performed exceptionally well during testing. It reflected an average of nearly 88 percent of sunlight and stayed up to 11 degrees Fahrenheit cooler than conventional purple cotton outdoors. It was notably even 7.6 degrees cooler than white cotton. In a world where deadly hot summers are becoming more common, research suggests materials like this could provide outdoor workers with some much-needed relief. 

“We have traditionally had to make a choice between cooling performance and appearance,” Adelaide University materials engineer Jun Ma and study co-author said in a statement. “Our research shows that we can have both. We can make a fabric that looks purple, but at the same time reflects most of the sun’s energy and releases heat very efficiently.”

The team published their findings in the journal Small. 

Why do light colors feel cool?

Generally speaking, lighter colors are better bets for staying cool because they reflect more sunlight. White or unpigmented materials are the best at beating heat, whereas dark black material will feel the hottest.  This same idea applies to building and construction materials as well. Purple, while obviously dark, is also a unique choice for the research because it strongly absorbs green light, which is one of the strongest parts of the solar spectrum. 

Caveats are in order, though. Color isn’t the only factor in staying cool. Clothing material and fit are just as important, if not more so. Loose, breathable garments like cotton or linen allow far more airflow than tighter clothing. That’s why Bedouin nomads in the North African desert can wear mostly all black without collapsing from heat exhaustion. Still, all else being equal, lighter colors make for a cooler experience.

Making a cooling, purple fabric 

To create the fabric, the team combined a purple ZIF-67 metal-organic framework with zinc oxide nanoparticles. Using electrospinning, they spun the mixture into purple thermoplastic polyurethane fibers, which were then woven into fabric. They then cut the material into two-inch squares and exposed them to direct sunlight on a sunny day in Zhengzhou, China, alongside control squares of standard white and purple cotton.

The revised material reflected an average of 87.6 percent of solar radiation. In terms of overall cooling, at least in these tests, it was a resounding success. After a full day baking in the sun, the fabric remained 7.6 degrees cooler than white cotton and 11.2 degrees cooler than traditional purple cotton. The fabric also  achieved these thermal gains without sacrificing practical performance, and  was flexible, stretch-resistant, and highly breathable, allowing moisture to evaporate effectively rather than trapping sweat. It also maintained both its rich color and cooling properties after 50 wash cycles. Furthermore, UV accelerated-aging tests (simulating roughly 108 days of intense sunlight exposure) showed that the  cooling performance did not degrade over time.

Using science to stay cool in a warming world 

It’s still early in the process, but the team believes their new approach could make darker-colored materials less off-putting during warmer weather. While that’s potentially great news for all the goths out there, the researchers emphasize far more practical applications as well. These refined fabrics could be used to make workwear for construction workers, farmers, and others who endure long hours baking in the sun. For outdoor laborers, even modest cooling relief can make a life-saving difference.

“The long-term opportunity is to move beyond the idea that cooling fabrics have to be white,” Adelaide University Chemical Engineer PhD candidate and paper co-author Yangzhe Hou said in a statement.

This summer’s heat has moved from unpleasantly hot to deadly in many parts of the world, and it’s likely a sign of more to come. Both the fashion industry and the general public are already adapting to this brutal new reality. A 2022 New York Times report noted a marked rise in sales of shorts and sandals alongside a sharp decline in fleece and heavy outerwear in recent years, a shift attributed in part to rising global temperatures. Similarly, recent efforts to normalize wearing shorts in office settings and the gradual removal of neckties reflect a workplace culture adjusting to a hotter climate.

The new purple fabric research also joins a now lengthy list of other attempts to apply science and technology to make clothing cooler. An assortment of (often more expensive) cool-tech clothing brands with names Coolcore and Iso-Chill have gained popularity, promising to beat the heat. Kontoor Brands, which owns blue-jean giant Wrangler, even made an Insta-Cool shirt that used phase-change material initially used by NASA astronauts to stay cool. 

 

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