Cool color? Purple fabric reflects solar radiation

By Stephen Beech

A new purple fabric could beat soaring heat, say scientists.

Experiments suggest the cooling material, developed by Australian and Chinese researchers, may help people stay cooler during scorching summer days — without having to wear white.

Wearing purple is usually associated with the late pop icon Prince or comic character the Incredible Hulk who donned frayed purple pants.

But it could become more common after the new high-tech fabric was shown to stay up to 6.2°C cooler than conventional purple cotton when exposed to direct sunlight.

The research team says their findings, published in the journal Small, potentially open the door to a new generation of "cool" clothing designed to keep people comfortable during increasingly common heatwaves.

The breakthrough came when researchers from Zhengzhou University in China and Adelaide University, Australia, looked at making colored clothing that actually cools people down.

Most passive cooling fabrics are white because they work by reflecting sunlight away from the body.

Darker colors absorb more of the sun's energy, making them much harder to cool effectively.

The researchers say purple is particularly challenging because the color comes from absorbing green light — one of the strongest parts of the solar spectrum.

Study co-author Jun Ma said the researchers wanted to prove that effective cooling did not have to mean giving up color.

Ma, a materials engineer from Adelaide University's School of Chemical Engineering, said: "We have traditionally had to make a choice between cooling performance and appearance.

"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 researchers created the fabric using microscopic fibers containing a purple material called a metal-organic framework (MOF) and zinc oxide nanoparticles.

Together, the materials give the fabric its distinctive purple color while helping it perform like a highly effective cooling material.

The fabric reflects an average 87.6% of solar radiation, while also emitting 96.4% of mid-infrared radiation — allowing heat from the fabric to escape through the atmosphere toward outer space.

In outdoor testing under direct sunlight, the purple fabric was 4.2°C cooler than commercial white cotton and 6.2°C cooler than commercially dyed purple cotton.

The testing also showed that simulated skin covered with the new fabric was up to 8°C cooler than uncovered simulated skin during the day.

That compared with a 4.6°C maximum reduction for white cotton and just 2.8°C for conventional purple cotton.

Co-author Yangzhe Hou says the technology could eventually benefit people who spend long periods outdoors.

He said: "For outdoor workers, athletes and anyone exposed to hot weather, staying cool is not just about comfort.

"We are interested in developing fabrics that can help manage body temperature without consuming additional energy."

Unlike many experimental cooling materials, the new fabric is also lightweight and flexible, can withstand significant stretching, and allows water vapor — including perspiration — to pass through.

It was also found to be highly water-repellent and retained its cooling performance after 50 washing cycles.

The fabric provided strong ultraviolet (UV) resistance and retained its optical performance after high-intensity UV age testing equivalent to about 108 days outdoors.

Hou, a PhD candidate in Adelaide University's School of Chemical Engineering, says the research could ultimately pave the way for cooling clothing in a much wider range of colors.

He added: "The long-term opportunity is to move beyond the idea that cooling fabrics have to be white.

"However, there is enormous potential to combine color, comfort and cooling performance in the next generation of textiles."

The researchers say the approach could potentially be adapted to other colors by using different metal-organic frameworks with different optical properties, creating opportunities for a broader range of aesthetically appealing cooling fabrics.