New coatings make natural fabrics waterproof

Washington: MIT scientists have developed a novel coating that can make natural fabrics such as cotton and silk water repellant. Fabrics that resist water are essential for everything from rainwear to military tents, but conventional waterrepellent 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. “The challenge has been driven by the environmental regulators” because of the phaseout of the existing waterproofing chemicals, said Kripa Varanasi, an associate professor at Massachusetts Institute of Technology ( MIT) in the US. However, it turns out his team's alternative actually outperforms the conventional materials. “Most fabrics that say ‘ water- repellent’ are actually water- resistant. If you're standing out in the rain, eventually water will get through,” said Varanasi. Ultimately, the goal is to be repellent — to have the drops just bounce back. The new coating comes closer to that goal, he said. The coatings currently used to make fabrics water repellent generally consist of long polymers with perfluorinated side- chains. The trouble is, 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, Varanasi said, 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.