MIT researchers develop better waterproof coatings

WASHINGTON - A research team at the Massachusetts Institute of Technology (MIT) developed a coating that could make natural fabrics water-repellent and was more effective than the existing ones.

The materials, described in a study published in the latest journal Advanced Functional Materials, have been bombarded in the lab with not only water but various other liquids including coffee, ketchup, sodium hydroxide, and various acids and bases, and have repelled all of them well.

The coated materials have been subjected to repeated washings with no degradation of the coatings and also have passed severe abrasion tests, with no damage to the coatings after 10,000 repetitions, according to the study.

"Most fabrics that say 'water-repellent' are actually water-resistant," says Kripa Varanasi, an associate professor of mechanical engineering at MIT and the paper's co-author.

"If you're standing out in the rain, eventually water will get through," Varanasi said. "The goal is to be repellent, to have the drops just bounce back." The new coating comes closer to that goal, Varanasi said.

The coatings currently used to make fabrics water repellent generally consist of long polymers with perfluorinated side-chains, since shorter-chain polymers tend not to have as much of a water-repelling effect as the longer-chain versions.

However, conventional longer-chain polymers have been shown to accumulate in the environment and in body tissue.

MIT researchers combined a different coating process called initiated chemical vapor deposition (iCVD) with a shorter-chain polymer that, by itself, confers some hydrophobic properties and has been enhanced with some extra chemical processing.

"The biggest challenge was finding the sweet spot where performance, durability, and iCVD compatibility could work together and deliver the best performance," said Dan Soto, a former MIT postdoc and the paper's co-author.

Another problem with conventional coatings is that they are liquid-based, so the fabric has to be immersed in the liquid and then dried out. This tends to clog all the pores in the fabric, so it requires a second manufacturing step in which air is blown through the fabric to reopen those pores, adding to the manufacturing cost and undoing some of the water protection.

Using the iCVD coating process, which can be done at low temperature and does not involve liquids, the researchers produced a very thin, uniform coating that follows the contours of the fibers and did not lead to any clogging of the pores, sparing the 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, according to the study.

It turned out that the process worked 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.