A self-healing, water-repellant coating that's ultra durable

These are tests showing the durability of the repellent coating under just two of various conditions that it can weather. 

A self-healing, water-repellent spray-on coating developed at the University of Michigan is hundreds of times more durable than its counterparts and could enable waterproofing of vehicles, clothing, rooftops and countless other surfaces for which current waterproofing treatments are too fragile. It could also lower the resistance of ship hulls, reducing the fuel consumption of the massive ships that transport ninety percent of the world’s cargo. 

The developers say the new concoction is a major breakthrough in a field where decades of research have failed to produce a durable coating. While water-repellent coatings are available at present, they tend to be far too fragile for applications like clothing or ship hulls. This discovery changes that.

“Thousands of superhydrophobic surfaces have been looked at over the past twenty or thirty years, but nobody has been able to figure out how to systematically design one that’s durable,” explains Anish Tuteja, an associate professor of materials science and engineering at U-M. “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 specialized 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.

How do you test your research? Try to break it. #SuperhydrophobicCoating

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 note in a paper to be published March 29 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 to the surface 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 commercialized by HygraTek, a company founded by University of Michigan materials science and engineering associate professor Anish Tuteja with assistance from the U-M Office for Technology Transfer.

Mathew Boban, Macromolecular Science & Engineering PhD Student, and Kevin Golovin, Materials Science & Engineering PhD Student, test a superhydrophobic surface by burning it as part of its resistance testing against water. 

Beyond the coatings detailed in the paper, the project produced what amounts to a recipe that researchers can use to optimize future coatings to a specific application’s requirements for cost, durability and other factors. So, while the coating detailed in the study is costly to produce, the team says their research should enable other makers to easily tweak the formula, for example to produce a version that’s only slightly less effective but far less costly.As lead author and U-M doctoral student Kevin Golovin explains, the team used a process that was radically different from previous research in the field.

“Most materials science researchers have focused on identifying one specific chemical system that’s as durable and water-repellent as possible. We approached the problem differently, by measuring and mapping out the basic chemical properties that make a water-repellent coating durable. And some of the results surprised us.”

For example, most hydrophobic coatings are made of two main ingredients: an active molecule that provides the water-repellency and a binder. Generally, researchers have assumed that using more durable ingredients would make a more durable coating. But Tuteja’s team found that that’s not necessarily the case.

They discovered that even more important than durability is a property called “partial miscibility,” or the ability of two substances to partially mix together. Chemicals that are more compatible with each other make a much more durable product, even if they’re less durable individually.

A water droplet bounces off the hydrophobic surface. 

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 importance of this property came as another surprise to the research team.

Tuteja estimates that the coatings will be available for use before the end of 2017 for applications including water-repellent fabrics and spray-on coatings that can be purchased directly by consumers.