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, and so are likely to be phased out for safety reasons. That leaves a big gap to be filled if safe substitutes can be found. The goal is for the materials to be repellent — for the drops to bounce back.
A coating was developed that not only adds water-repellency to natural fabrics such as cotton and silk, but is also more effective than the existing coatings. The challenge has been driven by environmental regulators because of the phaseout of the existing waterproofing chemicals. Because of the way they accumulate in the environment and in body tissue, the EPA 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. 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. This tends to clog all the pores in the fabric 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.
Polymers with fewer than eight perfluorinated carbon groups do not persist and bioaccumulate nearly as much as those with eight or more — the ones most in use. For the new process, two things were combined: 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 vapor deposition (iCVD), which was developed in recent years.
Using the iCVD coating process that does not involve any liquids and can be done at low temperature produces a very thin, uniform coating that follows the contours of the fibers and does not lead to any clogging of the pores, thus eliminating the need for 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 further.
The process works on many different kinds of fabrics including cotton, nylon, and linen, and even on nonfabric 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.
The coated fabrics have been subjected to 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 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.