purple fabric reflects sunlight and releases heat to stay cool in the sun

A PURPLE TEXTILE REFLECTS SUNLIGHT AND RELEASES HEAT

A new purple cooling fabric developed by researchers at Zhengzhou University in China and the University of Adelaide in Australia challenges the idea that the coolest option must be white. The experimental textile combines color with passive radiative cooling, using microscopic fibers containing a metal-organic framework and zinc oxide nanoparticles to reflect much of the sun’s energy while releasing heat. In outdoor tests, the material remained up to 6.2°C cooler than conventional purple cotton under direct sunlight.

The research addresses a basic contradiction in the design of cooling textiles. White fabrics are effective because they reflect sunlight, while darker and more saturated colors generally absorb more solar energy. That has left color largely at odds with thermal performance. Purple presents an especially difficult case. Its appearance depends on absorbing green light, one of the stronger regions of the solar spectrum. The researchers therefore set out to create a textile that could retain a visible purple hue without sacrificing its ability to manage heat. ‘We have traditionally had to make a choice between cooling performance and appearance,’ says Professor Jun Ma, a materials engineer at the University of Adelaide’s School of Chemical Engineering and a co-author on the paper. ‘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.’

image courtesy of Zhengzhou University in China and the University of Adelaide in Australia

ELECTROSPUN FIBERS COMBINE COLOR AND COOLING

The fabric is produced from microscopic electrospun fibers incorporating a purple metal-organic framework, or MOF, alongside zinc oxide nanoparticles, materials that together create the textile’s color while contributing to its optical properties. According to the researchers, the material reflects an average of 87.6% of solar radiation and emits 96.4% of mid-infrared radiation. The latter property allows heat to escape through the atmosphere, contributing to the passive cooling effect of the fabric.

The results suggest that color need not be treated as merely decorative in the development of performance textiles. Instead, the challenge becomes one of engineering the optical behavior of a material at a microscopic scale while maintaining the appearance expected of clothing.

image by Jonathan Borba

UP TO 6.2°C COOLER THAN PURPLE COTTON

During outdoor testing under direct sunlight, the experimental purple fabric was found to be 4.2°C cooler than commercial white cotton and 6.2°C cooler than commercially dyed purple cotton. Tests using simulated skin produced similarly pronounced results. When covered with the new fabric, the surface was up to 8°C cooler than uncovered simulated skin during the day. White cotton produced a maximum reduction of 4.6°C, while conventional purple cotton achieved 2.8°C.

For Yangzhe Hou, a PhD candidate at the University of Adelaide’s School of Chemical Engineering and a co-author on the research, the potential application extends beyond comfort. ‘For outdoor workers, athletes and anyone exposed to hot weather, staying cool is not just about comfort,’ Hou notes. ‘We are interested in developing fabrics that can help manage body temperature without consuming additional energy.’

Unlike many experimental cooling materials, the researchers say the fabric remains lightweight and flexible and allows water vapor, including perspiration, to pass through. It is also highly water-repellent and retained its cooling performance after 50 washing cycles. The material was additionally found to provide UV resistance and retained its optical performance after high-intensity UV aging equivalent to approximately 108 days outdoors.

The broader question, however, is whether the approach can move beyond purple. The researchers suggest that different metal-organic frameworks with different optical properties could eventually produce other colors.

‘The long-term opportunity is to move beyond the idea that cooling fabrics have to be white,’ Hou explains. ‘However, there is enormous potential to combine colour, comfort and cooling performance in the next generation of textiles.’ For now, the purple textile remains at the research and manufacturing stage, with the fabric still being produced in China.

image by Pixabay