Image Courtesy: Adelaide University
A new purple cooling fabric developed by researchers in China and Australia could help people stay cooler in extreme heat without relying on the traditional approach of wearing white clothing.
In outdoor tests, the experimental material remained up to 6.2 degrees Celsius, or 11.2 degrees Fahrenheit, cooler than conventional purple cotton under direct sunlight. The researchers say the technology could eventually enable cooling clothing in colors that have traditionally been considered unsuitable for passive cooling.
Most passive cooling fabrics are white because they reflect much of the sunlight that would otherwise heat the material. Darker colors generally absorb more solar energy, making them considerably harder to cool. Purple presents a particular challenge because its color is produced by absorbing green light, which makes up a significant portion of solar radiation.
The new material tackles the problem using microscopic fibers containing a purple metal-organic framework, or MOF, combined with zinc oxide nanoparticles. Together, the materials give the fabric its color while allowing it to reflect sunlight and radiate heat efficiently.
The fabric reflected an average of 87.6% of incoming solar radiation and emitted 96.4% of mid-infrared radiation. This allows heat absorbed by the material to escape through the atmosphere rather than remaining trapped in the fabric.
During outdoor testing, the new material was 4.2 degrees Celsius cooler than commercial white cotton and 6.2 degrees cooler than commercially dyed purple cotton. A simulated skin surface covered with the fabric was up to 8 degrees Celsius cooler than uncovered simulated skin. White cotton produced a maximum reduction of 4.6 degrees Celsius, while conventional purple cotton achieved only 2.8 degrees.
Professor Jun Ma, a materials engineer at Adelaide University and co-author of the research, said the results demonstrate that cooling performance and appearance do not necessarily have to be traded against each other.
The material is also lightweight, flexible and stretchable, while allowing water vapor from perspiration to pass through. Researchers found that it remained water-repellent and retained its cooling performance after 50 washing cycles. It also maintained its optical properties after accelerated UV testing equivalent to roughly 108 days outdoors.
The researchers believe the same approach could eventually be adapted to other colors by using different MOFs with tailored optical properties. That could lead to clothing designed to combine color, comfort and passive cooling without requiring additional energy.
