Purple Fabric Rivals White for Keeping Cool in the Sun

Researchers from Zhengzhou University, Adelaide University, the University of South Australia, and Central China Normal University have developed a purple-hued fabric that stays cool in the sun, challenging the assumption that only white materials can do that job. Published in the journal Small, the work combines a metal-organic framework (MOF) called ZIF-67, a cobalt-based material, with zinc oxide nanoparticles.

"We have traditionally had to make a choice between cooling performance and appearance," said Adelaide University materials engineer Jun Ma. "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." ZIF-67 absorbs light only in the optical region of the spectrum, with high reflectivity in the near-infrared, and emits notably in the mid-infrared range, which helps the material shed heat rather than let it build up.

To turn the MOF into a wearable material, the team used a single-step method of electrospinning thermoplastic polyurethane fibers. The resulting fabric reflects nearly 88% of incident sunlight while emitting just over 96% of its light in the mid-infrared range. Adding zinc oxide nanoparticles let the fabric also absorb near-ultraviolet light.

In outdoor tests on a sunny day in Zhengzhou, China, 5-centimeter-square swatches of the purple fabric cut temperatures by 5.6°C, compared with only 3.1°C for white polyurethane swatches. At night, the purple fabric was 3.4°C cooler, versus 1.2°C for the white material. The fabric also repelled water and kept its cooling performance after 50 washes.

"The long-term opportunity is to move beyond the idea that cooling fabrics have to be white," said Yangzhe Hou, a PhD candidate at Adelaide University's School of Chemical Engineering.