Swiss Researchers Develop Color-Changing Fiber Coating

The coated polyester filament before (right) and after the heat test at 150 degrees (left). The color change from blue to white is clearly visible and the safety of the product is no longer guaranteed. Image courtesy of Empa.

Researchers with the Swiss Federal Laboratories for Materials Science and Technology (Empa) (Dübendorf, Switzerland) and ETH Zürich (Zürich, Switzerland) have developed a coating that changes color when exposed to high temperatures through friction or fire. Such an ability would aid those who use safety ropes, such as fire brigades, or suspension ropes for heavy loads, such as construction crews.

According to researchers, high-performance fibers that have been exposed to high temperatures typically lose their mechanical properties undetected—and in the worst-case scenario, they can tear when lives depend on them most. To that end, Empa and ETH Zurich researchers first began developing a coating system in 2018 as part of a Master’s thesis, which the Empa team was subsequently able to develop into a technology that could be applied to fibers.

“It was a process involving several steps,” says Dirk Hegemann from Empa's Advances Fibers lab. At first, the coatings only worked on smooth surfaces, which caused researchers to modify their method so that it would also work on curved surfaces. Ultimately, Hegemann and his team used a process they had already developed for electrically conductive4 fibers and applied this so-called “sputtering process” to their latest coating.

Three layers are required to ensure that the fiber changes color when heater: The first layer is silver, which consists of polyethylene terephthalate (PET), a resin in the polyester family, and Vectran a high-tech fiber. The second intermediate layer is titanium nitrogen oxide, which ensures that the silver remains stable. The third and final layer is the amorphous layer of germanium-antimony tellurium (GST) that causes the color change.

When this 20-nm thick GST layer is exposed to elevated temperatures, it crystalizes and changes from blue to white. This color change is caused by a physical phenomenon called interference in which two different waves of light meet and either amplify or weaken one another. This color change can be adjusted to a temperature range between 100 and 400 degrees, depending on the chemical composition of the temperature-sensitive layer.

Because the possible applications of this technology are open, Hegemann and his team are currently looking for possible project partners. This will enable the researchers to refine certain limitations of the fibers, such as their inability to be stored for long periods of time without losing functionality. “Unfortunately, the phase-change materials oxidize over the course of a few months,” he says.

With that said, however, he still believes that the basic principle works and durability can be a topic for future research. “As soon as the first partners from industry register their interest in our own products, the fibers can be further optimized according to their needs,” says Hegemann.

Source: Empa,