A self-healing, water-repellent, spray-on coating developed at the University of Michigan (UM) is hundreds of times more durable than its counterparts. This could enable waterproofing for which current treatments are too fragile. It could also lower the resistance of ship hulls, a step that would reduce fuel use.
Current water-repellent finishes are available, but they are typically not strong enough for applications like clothing or ship hulls.
“Thousands of superhydrophobic surfaces have been looked at over the past 20 or 30 years, but nobody has been able to figure out how to systematically design one that’s durable,” said Anish Tuteja, University of Michigan associate professor of materials science and engineering. “I think that’s what we’ve really accomplished here, and it’s going to open the door for other researchers to create cheaper, perhaps even better superhydrophobic coatings.”
The coating is made of a mix of a material called ‘fluorinated polyurethane elastomer’ and a specialised water-repellent molecule known as F-POSS. It can be easily sprayed onto virtually any surface and has a slightly rubbery texture that makes it more resilient than its predecessors.
If it is damaged, the coating can heal itself hundreds of times. It can bounce back “even after being abraded, scratched, burned, plasma-cleaned, flattened, sonicated and chemically attacked”, the researchers wrote in a paper recently published in ACS Applied Materials & Interfaces.
In addition to recovering physically, the coating can heal itself chemically. If water-repellent F-POSS molecules are scraped from the surface, new molecules will naturally migrate there to replace them. That’s how the coating can renew itself hundreds of times. Its healing ability is limited only by its thickness.
The discovery is being commercialised by HygraTek, a company founded by Tuteja with assistance from U-M Tech Transfer.
The project’s researchers discovered that even more important than durability is a property called ‘partial miscibility’, or the ability of two substances to partially mix together. The other key variable the team discovered is the stability of the water-repellent surface.
Most water-repellent coatings work because their surface has a very specific geometry, often microscopic pillars. Water droplets perch on the tips of these pillars, creating air pockets underneath that deny the water a solid place to rest and cause it to roll off easily. But such surfaces tend to be fragile – slight abrasion or even the pressure of the water itself can damage them.
The team’s research found that a surface that’s slightly pliable can escape this pitfall – even though it seems less durable, its pliable properties enable it to bounce back from damage.
The paper is titled ‘Designing self-healing superhydrophobic surfaces with exceptional mechanical durability’. - April 2017