A molecular trick borrowed from archery could help break the long-standing link between PFAS toxicity and liquid-repellent performance. Researchers at the University of Toronto have devised a “nanoscale fletching” approach – adding just seven ultrashort perfluorocarbons to the ends of silicone polymer chains – that matches the oil- and water-repelling power of long-chain PFAS coatings while drastically reducing fluorine content. Analytical techniques including X-ray photoelectron spectroscopy and total organic fluorine combustion ion chromatography confirmed minimal PFAS use without loss of performance, pointing to a possible transitional solution as regulators move to restrict the entire PFAS class.
Here, lead author Samuel Au, and co-author Kevin Golovin, Professor, who leads the DREAM Laboratory, discuss the inspiration behind the work, the analytical validation, and why even this safer coating is ultimately a stop-gap on the road to PFAS-free repellency.
What was the original spark behind this work?
PFAS remain the most effective class of chemicals for liquid-repellent coatings. However, among the various types of PFAS, there exists a trade-off between performance and toxicity. Longer-chain PFAS typically offer superior liquid repellency but are also more environmentally persistent (hence the term “forever chemicals”) and are associated with health risks such as liver toxicity, increased cancer risk, and developmental issues in infants. In contrast, ultrashort-chain PFAS are significantly less toxic and persistent in the environment. For instance, single perfluorocarbon groups have been widely used in pharmaceuticals to enhance drug potency, but their use in liquid-repellent coatings was always impractical due to the inferior performance. In our lab, we are developing a new coating strategy for liquid repellency by providing a solid surface with liquidlike properties. These liquidlike coatings repel liquids by utilizing flexible polymer chains to reduce the friction liquid droplets experience.
Considering this alternate mechanism for liquid repellent properties, we hypothesized that even single perfluorocarbons could provide superior liquid repellency (similar to the longer chain PFAS) by inserting them at the end of the flexible polymer chains (i.e. nanoscale fletching). This approach leverages the liquidlike properties of the coating to overcome the chain length dependence of PFAS coatings, effectively decoupling the toxicity of PFAS from their liquid repellency.
What makes your nanoscale fletching approach uniquely effective at minimizing PFAS content while preserving performance?
In a conventional long chain PFAS coating, only the single perfluorocarbon at the terminal of the PFAS chain directly contributes to the liquid repellent properties; the remainder of the fluorocarbons along the polymer backbone simply aid in densely packing the chains together into a solid crystal, enhancing liquid repellency. When we fletch a flexible PFAS-free polymer with single perfluorocarbons, we eliminate this fluorocarbon backbone while maintaining the high surface density of perfluorocarbons using the liquidlike nature of the flexible chain. This method allows us to minimize the PFAS content without compromising liquid repellency.
Can you give an overview of the role and importance of the analytical techniques used in this study?
To confirm that the fluorocarbon content was minimised, we utilized x-ray photoelectron spectroscopy (XPS) to characterize the relationship between the liquid repellency and the amount of single perfluorocarbons fletched to the polymer chain. XPS allowed us to quantify the amount of fluorine added to the surface of the coating and correlate it with the liquid-repellent properties, enabling us to determine the minimum amount of fluorocarbon needed for optimal liquid repellency. It turned out that seven fletched perfluorocarbons per chain proved to be the “goldilocks amount.”
Additionally, we applied total organic fluorine combustion ion chromatography (TOF-CIC): a specialised extension of liquid chromatography that quantifies the total amount of fluorine in the coating. TOF-CIC analysis showed that the amount of fluorine in our fletched polymer coating was significantly lower than other conventional PFAS coatings, supporting our hypothesis that the liquid repellency of PFAS could be decoupled from PFAS chain length. Overall, these two techniques were particularly important for this research, as they allowed us to verify our hypothesis and quantify the minimal fluorine used in our coating – while preserving liquid-repellent performance.
Were there any surprising results during testing?
In the initial plan, we only aimed to attach a single perfluorocarbon to each polymer chain, but coating performance fell short of expectations. Fortunately, we discovered – somewhat by chance – that short durations of oxygen plasma could controllably increase the amount of silanols along the polymer chain, which then allows for the attachment of multiple fletched perfluorocarbons. After optimization, we found that seven individual perfluorocarbons fletched to the end of each polymer chain resulted in the maximum liquid repellency with the minimum amount of fluorine.
How sure are you of the eventual degradation to TFA, and that TFA is significantly safer than long-chain PFAS?
The actual degradation pathway of PFAS is usually complicated and depends on multiple factors, but using single perfluorocarbons ensures that any coating degradation is very unlikely to release any fluorocarbon byproduct with a chain length greater than one. To evaluate the potential degradation product of the coating, we searched the literature for the degradation pathway of polymers similar to our coating. We found that, although degradation can lead to different intermediate compounds depending on environmental conditions, trifluoroacetic acid (TFA) is the most stable byproduct resulting from the various likely degradation mechanisms. Therefore, we believe that the eventual byproduct of our coating is most probably TFA.
TFA is significantly safer than long-chain PFAS. One key difference lies in its water solubility: TFA is highly water-soluble, whereas long-chain PFAS are virtually insoluble. As a result, long-chain PFAS tend to persist in human tissues (a process known as bioaccumulation), leading to long-term health effects once the concentration increases. In contrast, studies have shown that TFA is readily excreted in urine, resulting in much lower bioaccumulation. Furthermore, due to their high hydrophobicity, long-chain PFAS bind strongly to proteins via the hydrophobic effect, potentially disrupting normal biological functions. Conversely, since TFA is more polar and less hydrophobic, it exhibits much weaker protein binding and is therefore less likely to interfere with biological systems. In our work we also calculated that the total amount of TFA that could be released by our coating (for example, if deposited on a jacket and allowed to fully degrade) is negligible compared to other sources of environmental TFA, such as refrigerant leakage. So not only is TFA much safer than long-chain PFAS, the amount of TFA our concept would emit after eventual release (and, remember, they are covalently fletched to the polymer chain, not free to be released like a refrigerant) is insignificant.
Given the regulatory momentum against all PFAS, do you see this as a transitional solution – or could these coatings be viable long-term?
Considering the current EU plan for PFAS restrictions, all PFAS, including any substances containing at least one fully fluorinated carbon, are expected to be banned in the long term – including, unfortunately, our fletching molecule. Therefore, this coating can only serve as a transitional solution for liquid-repellent surfaces – though it has taken 70+ years to move away from long- and short-chain PFAS, so a transitional technology could be viable for decades realistically. However, to ensure long-term sustainability and regulatory compliance, there is an urgent need to develop high-performance, non-PFAS alternatives that can match the effectiveness of conventional PFAS coatings. This is an active area of research in our lab.
Do you think it likely we’ll find a substance that outperforms Teflon, but with no PFAS at all?
The exceptional liquid repellency of perfluorocarbons arises from the unique combination of their strong C-F bond, the low polarizability of fluorine atoms and the overall inertness of the fluorocarbons. These properties have not been found with other functional groups. It is very challenging to find alternatives that can outperform PFAS. However, the idea that chain length and liquid repellent properties were coupled persisted for many decades. Who knows what the next five years may bring?
What are your next steps for this work?
The primary challenge for making this coating practical is scalability. The oxygen plasma process we utilized is currently suitable only for small to medium-sized samples, making it impractical for treating large surface areas. Additionally, the existing process involves three separate steps, which must be streamlined into a continuous, integrated process to reduce costs and improve manufacturing efficiency. Further, our current polymer fabrication step relies on vapor deposition, which at the lab-scale consumes a large amount of reagent for each sample, particularly the single perfluorocarbon fletching compound. This alone would lead to unnecessary perfluorocarbon emissions if not addressed, contradicting the environmental motivation behind our work. Therefore, a more material-efficient fabrication approach is needed that maintains performance while minimizing reagent use. Finally, the coating’s abrasion resistance and overall durability require thorough evaluation to ensure suitability for real-world applications.
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