MASSACHUSETTS – Researchers from Massachusetts Institute of Technology (MIT) have developed patent-pending technology which applies short-chain polymer water-repellents using a pioneering application process which does not involve any liquids and can be carried out at low temperature. The technology, which will be marketed to fabric and apparel business, potentially offers a new, cost effective – and green – way to imbue clothing with water repellent properties. It also addresses issues around conventional water-repellent coatings using long chain fluorinated chemistry techniques which have been shown to persist in the environment and which are bioaccumulative. These are likely to be phased out for safety reasons in the US and Europe in the next few years.
The solution from the team at MIT has come up with is a coating which not only adds water-repellency to natural fabrics such as cotton and silk, but is also – according to tests – more effective than the existing coatings. The new findings are described in the Journal of Advanced Functional Materials, in a paper by MIT professors Kripa Varanasi and Karen Gleason, former MIT postdoc Dan Soto, and two others.
“The challenge has been driven by the environmental regulators,” because of the phaseout of the existing waterproofing chemicals, Varanasi told Apparel Insider.
“Most fabrics that say ‘water-repellent’ are actually water-resistant,” added 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. The new coating comes closer to that goal.”
Because of the way they accumulate in the environment and in body tissue, the US Environmental Protection Agency is in the process of revising regulations on the long-chain polymers that have been the industry standard for decades.
The coatings currently used to make fabrics water repellent generally consist of long polymers with perfluorinated side-chains. Up until now, shorter-chain polymers that have been studied do not have as much of a water-repelling – or hydrophobic – 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. This tends to clog all the pores in the fabric, Varanasi told us, so the fabrics no longer can breathe as they otherwise would. That 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.
To address these issues, the team at MIT combined a shorter-chain polymer that, by itself, confers some hydrophobic properties and which has been enhanced with some extra chemical processing. They also used 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, consequently eliminating the need for the second processing stage to reopen the pores.
Then, in an additional step, a kind of sandblasting of the surface, can be added as an optional process to increase the level water repellency even more.
The process works on many different kinds of fabrics, Varanasi told us, including cotton, nylon, and linen, and even on non-fabric 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 wide range of tests in the lab, including a standard rain test used by industry. 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 them well.
The team hopes to license the patent-pending technology to existing fabric and clothing companies.
