The new substance repels both water and grease about as well as standard non-stick coatings — but it contains much lower amounts of per- and polyfluoroalkyl substances (PFAS), a family of chemicals that have raised environmental and health concerns.

“The research community has been trying to develop safer alternatives to PFAS for a long time,” says Professor Kevin Golovin (MIE), who heads the Durable Repellent Engineered Advanced Materials (DREAM) Laboratory at U of T Engineering.

“The challenge is that while it’s easy to create a substance that will repel water, it’s hard to make one that will also repel oil and grease to the same degree. Scientists had hit an upper limit to the performance of these alternative materials.”

Since its invention in the late 1930s, Teflon — also known as polytetrafluoroethylene or PTFE — has become famous for its ability to repel water, oil and grease alike. Teflon is part of a larger family of substances known as per- and polyfluoroalkyl substances (PFAS).

PFAS molecules are made of chains of carbon atoms, each of which is bonded to several fluorine atoms. The inertness of carbon-fluorine bonds is responsible for the non-stick properties of PFAS.

However, this chemical inertness also causes PFAS to resist the normal processes that would break down other organic molecules over time. For this reason, they are sometimes called “forever chemicals.”

In addition to their persistence, PFAS are known to accumulate in biological tissues, and their concentrations can become amplified as they travel up the food chain.

Various studies have linked exposure to high levels of PFAS to certain types of cancer, birth defects and other health problems, with the longer chain PFAS generally considered more harmful than the shorter ones.

Despite the risks, the lack of alternatives means that PFAS remain ubiquitous in consumer products: they are widely used not only in cookware, but also in rain-resistant fabrics, food packaging and even in makeup.

“The material we’ve been working with as an alternative to PFAS is called polydimethylsiloxane or PDMS,” says Golovin.

“PDMS is often sold under the name silicone, and depending on how it’s formulated, it can be very biocompatible — in fact it’s often used in devices that are meant to be implanted into the body. But until now, we couldn’t get PDMS to perform quite as well as PFAS.”

To overcome this problem, MIE PhD student Samuel Au developed a new chemistry technique that the team is calling nanoscale fletching. The technique is described in a paper published in Nature Communications.

“Unlike typical silicone, we bond short chains of PDMS to a base material — you can think of them like bristles on a brush,” says Au.

“To improve their ability to repel oil, we have now added in the shortest possible PFAS molecule, consisting of a single carbon with three fluorines on it. We were able to bond about seven of those to the end of each PDMS bristle.

“If you were able to shrink down to the nanometre scale, it would look a bit like the feathers that you see around the back end of an arrow, where it notches to the bow. That’s called fletching, so this is nanoscale fletching.”

Au and the team coated their new material on a piece of fabric, then placed drops of various oils on it to see how well it could repel them. On a scale developed by the American Association of Textile Chemists and Colorists, the new coating achieved a grade of 6, placing it on par with many standard PFAS-based coatings.

“While we did use a PFAS molecule in this process, it is the shortest possible one and therefore does not bioaccumulate,” says Golovin.

“What we’ve seen in the literature, and even in the regulations, is that it’s the longest-chain PFAS that are getting banned first, with the shorter ones considered much less harmful. Our hybrid material provides the same performance as what had been achieved with long-chain PFAS, but with greatly reduced risk.”

Golovin says that the team is open to collaborating with manufacturers of non-stick coatings who might wish to scale up and commercialize the process. In the meantime, they will continue working on even more alternatives.

“The holy grail of this field would be a substance that outperforms Teflon, but with no PFAS at all,” says Golovin.

“We’re not quite there yet, but this is an important step in the right direction.” 

This article was originally published by the University of Toronto and can be read here.

For more on sustainable innovations in coatings, visit PCI’s Sustainable topic page.