Researchers from the Massachusetts Institute of Technology have developed a new process that enables manufactures to make a water-repellent coating on natural fabrics like cotton and silk, without needing to use harmful chemicals.

Conventional water-repellent coatings, commonly used in materials like rainwear and military tents, have shown to persist in the environment and accumulate in human tissues.

The coatings used to make fabrics water repellent usually consist of long polymers with perfluorinated side-chains. While past research has shown that polymers with less than eight perfluorinated carbon groups do not persist and bioaccumulate as much as those with eight or more, shorter-chain polymers do not have as much of a hydrophobic effect as the longer-chain versions.

The current coatings are also liquid-based, which means that the fabrics have to be immersed in the liquid and then dried out, ultimately clogging all the pores in the fabric.

To make the fabrics more breathable, manufacturers must blow air through the fabric to reopen the pores, adding to the overall cost of the product, while also reducing some of the original water protections.

The researchers were able to combine a shorter-chain polymer that confers some hydrophobic properties that was enhanced with extra chemical processing with a different coating process called initiated chemical vapor deposition (iCVD).

ICVD produces a very thin, uniform coating that follows the contours of the fibers and does not clog the pores. The process does not involve any liquids and can be done at very low temperatures.

 The researchers used a kind of sandblasting on the surface, which can be added as an optional process to increase the water repellency even further.

“The biggest challenge was finding the sweet spot where performance, durability, and iCVD compatibility could work together and deliver the best performance,” former MIT postdoc Dan Soto said in a statement.

The researchers conducted a number of different tests on the coatings, including standard rain test used by industry. The materials also successfully repelled a variety of liquids including, water, coffee, ketchup, sodium hydroxide and various acids and bases.

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.

“Most fabrics that say ‘water-repellent’ are actually water-resistant,” Kripa Varanasi, an associate professor of mechanical engineering, said in a statement. “If you’re standing out in the rain, eventually water will get through. The goal is to be repellent — to have the drops just bounce back.”

According to the researchers, the process works on several different types of fabrics, including cotton, nylon and linen. The process can also work on nonfabric materials like paper.  

The U.S. Environmental Protection Agency is currently in the process of revising regulations on long-chain polymers.