Conventional water-repellent coatings used in products like raincoats and tents have been shown to persist in the environment and accumulate in people’s bodies, according to MIT. They’re likely to be phased out for safety reasons, and if so, that would leave an important need left vacant if researchers can’t find safe substitutes, MIT added.
But an MIT team has now come up with a promising solution in the form of a coating that adds water-repellency to natural fabrics like cotton and silk, and is more effective than the existing coatings. The new findings are described in the journal Advanced Functional Materials, in a paper by MIT professors Kripa Varanasi and Karen Gleason, former MIT postdoctoral student Dan Soto and two others.
“The challenge has been driven by the environmental regulators” because of the phaseout of the existing waterproofing chemicals, Varanasi said. But his team’s alternative actually outperforms the conventional materials, he said.
“Most fabrics that say ‘water-repellent’ are actually water-resistant,” said Varanasi, who is an associate professor of mechanical engineering. “If you’re standing out in the rain, eventually water will get through.” Ultimately, “the goal is to be repellent—to have the drops just bounce back,” and the new coating comes closer to that goal, he said.
Because of the way they accumulate in the environment and in body tissue, the Environmental Protection Agency (EPA) is in the process of revising regulations on the long-chain polymers that have been the industry standard for decades, Varanasi added.
“They’re everywhere and they don’t degrade easily,” he said.
The coatings currently used to make fabrics water repellent generally consist of long polymers with perfluorinated side-chains, MIT explained. But shorter-chain polymers that have been studied do not have as much of a water-repelling effect as the longer-chain versions. Another problem with existing coatings is that they are liquid-based, so the fabric has to be immersed in the liquid and then dried out, which generally leads to clogging the pores in the fabric, Varanasi said. That requires a second manufacturing step where air is blown through the fabric to reopen those pores, adding to the manufacturing cost and undoing some of the water protection.
So the MIT team combined a shorter-chain polymer that confers some hydrophobic properties and has been enhanced with some extra chemical processing, and a different coating process, called initiated chemical vapor deposition (iCVD), which was developed in recent years.
Using the iCVD coating process, which does not involve any liquids and can be done at low temperature, produces a thin, uniform coating that follows the contours of the fibers and does not lead to any clogging of the pores, which eliminates the need for the second processing stage to reopen the pores. An additional akin to sandblasting of the surface, can be added as an optional process to increase the water repellency.
The process works on many different kinds of fabrics, Varanasi noted, including cotton, nylon and linen, and even on materials such as paper, opening up a variety of potential applications. The system has been tested on different types of fabric, as well as on different weave patterns of those fabrics.
“Many fabrics can benefit from this technology,” he said. “There’s a lot of potential here.”
The coated fabrics have been subjected to a barrage of tests in the lab, including an industry standard rain test. The materials have been bombarded not only with water, but with various other liquids including coffee, ketchup, sodium hydroxide and various acids and bases, and have repelled all of those, too, according to MIT. The coated materials have also been subjected to repeated washings with no degradation of the coatings and have passed severe abrasion tests, with no damage to the coatings after 10,000 repetitions.
For now, the focus for the team at MIT will be to continue working on optimizing the chemical formula for the best possible water-repellency with the aim of licensing the patent-pending technology to existing fabric and clothing companies.