This coating not only adds water-repellency to natural fabrics such as cotton and silk, but is also more effective than the existing coatings, according to a study published in the journal Advanced Functional Materials.
"The challenge has been driven by the environmental regulators" because of the phase-out of the existing waterproofing chemicals, explained Kripa Varanasi, Associate Professor at the Massachusetts Institute of Technology (MIT).
But it turns out his team's alternative actually outperforms the conventional materials.
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.
Fabrics that resist water are essential for everything from rainwear to military tents, but conventional water-repellent coatings have been shown to persist in the environment and accumulate in our bodies.
Because of the way they accumulate in the environment and in body tissue, the US Environmental Protection Agency (EPA) is in the process of revising regulations on the long-chain polymers that have been the industry standard for decades.
What this MIT team did, Varanasi explained, is to combine two things: a shorter-chain polymer that, by itself, confers some hydrophobic properties and has been enhanced with some extra chemical processing; and a different coating process, called initiated chemical vapour deposition (iCVD).
Using the iCVD coating process, which does not involve any liquids and can be done at low temperature, produces a very thin, uniform coating that follows the contours of the fibres and does not lead to any clogging of the pores, thus eliminating the need for a second processing stage to reopen the pores.
Then, an additional step, a kind of sandblasting of the surface, can be added as an optional process to increase the water repellency even more, the study said.
The process works on many different kinds of fabrics, including cotton, nylon, and linen, and even on non-fabric materials such as paper, opening up a variety of potential applications, Varanasi said.
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