Waterproof fabric coating

Alternative waterproofing technique proves safer and more effective

Image credit: Varanasi and Gleason research groups

A new process developed by researchers at Massachusetts Institute of Technology (MIT) could be adopted as an environmentally friendly and non-toxic alternative to conventional waterproof coatings for natural fabrics.

While natural materials such as cotton, linen and silk look attractive and are mercifully breathable in hot weather, they are less comfortable when caught in the rain. Waterproof fabrics – which block out water while allowing for water vapour (such as that in sweat) to pass through – are necessary not just for raincoats, but also for tents and inflatable boats.

Despite the obvious benefits of waterproof fabrics, these fabrics tend to be waterproofed with the addition of coatings that are toxic, accumulate in our bodies and take an extremely long time to break down in the environment. For these reasons, conventional waterproofing is likely to be phased out in some countries, including the US.

A team of researchers at MIT have now proposed a solution to this problem: a process that renders natural fabrics water-repellent while not toxic.

“The challenge has been driven by the environmental regulators,” said Professor Kripa Varanasi, one of the MIT engineers behind the solution. According to Varanasi - who is best known for inventing water-repellent ketchup bottles that leave no sauce behind - as well as being a more health- and environmentally-friendly approach to waterproofing, this approach actually results in more effective waterproofing, with water droplets bouncing right off the coating.

“Most fabrics that say water-repellent are actually water-resistant,” he said. “If you’re standing out in the rain, eventually water will get through.”

These conventional coatings consist of long-chain polymers with perfluorinated side chains; these long polymers have been demonstrated to have a stronger water-repelling effect than shorter-chain versions. These coatings are normally liquid-based, meaning that to apply the coating, the fabric is immersed in the liquid before being dried out, clogging pores in the fabric and rendering it stuffy and unbreathable. Changing this requires a second step in the process to blow air through the fabric to reopen these pores.

Varanasi and his colleagues combined a shorter-chain polymer with some enhanced water-repellent properties with an alternative coating process: initiated chemical vapour deposition. This process can be performed at low temperature and leaves behind a very fine and uniform coating on the fabric which does not clog its pores. A second step – which effectively sandblasts the surface – could be added in order to improve water repellency further.

According to the MIT engineers, the process works on a range of materials; cotton, nylon, linen and even paper. The coated fabrics were tested in the lab – not just with the industrial standard (the rain test) – but being bombarded with coffee, ketchup and various acids and bases; the coating protected the fabric from every liquid and showed no sign of damage even after 10,000 repetitions.

“[Under severe abrasion] the fibre will be damaged but the coating won’t,” said Varanasi. The engineers plan to continue optimising their coating and to eventually license the technology to existing clothing companies.

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