Water-Repellent Coating Better, Safer

Water-repellent fabrics are used in a wide range of applications, but the conventional coatings used to create them are likely to be phased out in the wake of studies showing their dangers: They have been shown to persist in the environment, and to accumulate in human body tissue. Environmental regulators are currently in the process of revising regulations on the long-chain polymers that have been the industry standard for decades.

A team at MIT, however, has hit upon a promising solution that uses a shorter-chain polymer to add water-repellency that, despite studies showing less of a hydrophobic effect for such polymers, is actually more effective than conventional coatings. The shorter-chain structure also makes the polymer significantly less likely to persist and bioaccumulate.

An important difference between the new coating and existing ones is the process by which it is applied. Traditionally, this is done by immersing a fabric in a liquid-based coating, then allowing it to dry out – which clogs fabric pores and prevents the fabric from “breathing.” By contrast, the new coating is applied with a liquid-free process called initiated chemical vapor deposition (iCVD). The process produces a very thin, uniform coating that follows the fiber contours of the fabric, eliminating clogged pores. An optional “sandblasting” step can also be added to further increase water repellency.

"Most fabrics that say 'water-repellent' are actually water-resistant," said Kripa Varanasi, an associate professor of mechanical engineering. "If you're standing out in the rain, eventually water will get through." The goal, he says, is to be repellent – to have the drops just bounce back – and the new coating comes closer to that goal.

The process works not only on fabrics such as cotton, nylon and linen, but also on non-fabric materials such as paper. The coated materials have also been tested for repellency not only to water, but to various other liquids including coffee, ketchup, sodium hydroxide and various acids and bases. They have also been subjected to repeated washings and severe abrasion tests.

Results have been promising. Under severe abrasion, Varanasi noted, the fiber will eventually be damaged – but the coating will not.

The research team plans to continue working on optimizing their formula for repellency, and hopes to license the patent-pending technology to existing fabric and clothing companies. Their research appears in the journal Advanced Functional Materials.