When it comes to keeping cool on hot days, it's not so much a matter of wearing less clothing as it is a matter of wearing the right clothing. A new fabric coating could help in that regard, and it's essentially made of chalk.
In natural, unfiltered sunlight, it's the near-infrared and ultraviolet wavelengths that deliver heat to our bodies.
With that fact in mind, scientists have previously developed experimental fabrics that incorporate compounds such as titanium dioxide, which provide a cooling effect by reflecting those wavelengths away from the wearer's body. The complex techniques that are currently required for applying those substances to textile fibers, however, make the technology difficult to scale up for commercial production.
Other groups have explored the use of light-reflecting organic polymers such as polyvinylidene difluoride. Although these substances are sometimes easier to work with, their production requires the use of PFAS (aka "forever chemicals") which persist for a very long time in the environment, and have been linked to numerous health problems.
Seeking an alternative that is both easy and eco-friendly, a team at the University of Massachusetts Amherst took inspiration from the reflective limestone-based exterior plasters that are used to keep homes cool in hot climates. Calcium carbonate is the main ingredient in limestone, and chalk is a type of limestone.
Utilizing a technique known as chemical vapor deposition, the scientists started by applying a 5-micrometer-thick adhesive layer of poly(2-hydroxyethyl acrylate) to small squares of various commercially available fabrics.
Those squares were then repeatedly dipped in solutions containing calcium or barium ions, and in solutions containing carbonate or sulfate ions. Doing so caused uniformly sized calcium carbonate crystals to form on the fabric, resulting in a reflective matte white finish.
The finished samples were subsequently tested outdoors in the sunlight, at air temperatures of over 90 ºF (32 ºC). It was found that the temperature beneath the squares was an average of 8 ºF (4 ºC) cooler than the ambient temperature.
Additionally, the difference jumped to a total of 15 ºF (8 ºC) when temperatures beneath samples of the treated fabrics were compared to temps below untreated samples of those same fabrics. That's significantly more cooling than would be produced simply via a shading effect.
And importantly, tests showed that repeated washings didn't cause the coating to come off of the underlying materials.
“What makes our technique unique is that we can do this on nearly any commercially available fabric and turn it into something that can keep people cool," says Evan D. Patamia, a grad student who led the study along with Prof. Trisha L. Andrew and undergrad Megan K. Yee. "Without any power input, we’re able to reduce how hot a person feels, which could be a valuable resource where people are struggling to stay cool in extremely hot environments."
The technology is now being commercialized via a spinoff company. Patamia will be presenting the team's findings at the fall meeting of the American Chemical Society.
