Bob Ruckle, Seung-Tong Cheung and Yanjun Luo

Siltech Corporation, 225 Wicksteed Avenue, Toronto, ON, Canada

Introduction

Perfluoroalkyl substances (PFAS) have long been under scrutiny by regulatory agencies in the US and EU. Being chemically and thermally stable, there are limited chemical degradation pathways available for PFAS. Being synthetic there is no natural biochemical pathway for nature to digest these materials. Therefore, they tend to build up in nature.

The argument against removing all PFAS compounds is that not all PFAS are equal in terms of exposure or possible toxicity. Some PFAS are low risk and highly irreplaceable.
The American Chemical Society managed website is a great resource for gaining more detail in this debate.

Many end users are currently searching for alternatives. This is not trivial as PFAS offer unique properties and performances. PFAS materials provide surface tension reduction, coefficient of friction reduction, hydrophobicity, oleophobicity, and chemical stability. Previously, we reported that silicones are an excellent option to consider for applications which rely on any first three of these properties.

What remains is oleophobicity, perhaps the most difficult property of PFAS compounds to emulate. Simple organosilicone treatments are usually relatively ineffective at providing this oleophobicity.

PFAS-free oleophobic materials are being developed by many teams. To date, the solutions are generally more complicated and not as effective as using PFAS. High exposure applications were the first to react to PFAS regulations and market pull. As an example, the fast-food packaging industry has removed PFAS from nearly all their packaging. The new approaches rely largely on paper treated with various high molecular weight (often biobased) polymers.
The paper companies also use proprietary techniques to improve their fiber performance.
The less impervious paper products provide sufficient oleophobicity for that direct consumer impact application.

Another important application that was also addressed early was the non-stick cookware industry. The PFAS-free solution in this case is ceramic cookware coatings. These use the metal surface of the cookware to their advantage and react it with sol-gels of silica and organosilanes at high temperature. These coatings provide the non-stick properties but are not always as durable.

Superhydrobicity and oleophobicity have been studied extensively in the academic community. A good review article can be found here. Meso-structuring of a low surface energy pendant chain is important to achieve this ultimate repelling behavior. Mono-functional PFAS or silicone species are commonly used as the low surface energy pendant with each giving similar results.

We have had some success instilling oleophobicity with our technologies. The general approach we are taking is using silicon Q resins and/or monofunctional reactive silicones to “structure” in situ.

Approach and Early Results

We published that linear organosilicone materials with chemical anchors on the organic appendages offer some oleophobicity as gauged by their stain resistance. While the results are not at the level of the best PFAS, there is significant improvement, comparable to or better than our perfluoro ~C4F9 based fluoroalkyl silicone products.

We have seen oleophobicity with our film forming emulsions, which are chemically very similar to the T resin ceramic approach taken by the cookware industry. These involve silicones reacted with organo trialkoxysilanes resulting in organic groups that are structured within the matrix. We theorize that this structuring is necessary for oleophobicity.

We have also seen oleophobicity with our trimethoxysilane functional silicone polymers that can react at both ends of the polymer chain and provide organization and spacing to the film.

We noted oleophobicity with our MDTQ resins (Figure 1). These resins have silicon atoms with four oxygen bonds (Q groups) that provide a strong matrix. They also involve trialkoxy (T) and dialkoxy (D) silicone groups which can space and anchor pendant groups providing the structuring of the pendant chains.

Figure 1. Vegetable Oil Beading with DTQ Resin Treated Cardboard

Following this hypothesis, we continued to pursue unique chemistries that can mimic this structuring in the matrix. A recent approach results in a strong oleophobic effect uses a proprietary branched silicone with a single reactive point. The idea is to provide structure by adhering the reactive site into the matrix and leaving the slightly branched silicone pendant material to provide low surface energy.

In Figure 2, a UV reactive substance coded DT10 is combined at 5% with an acrylate functional UV resin and cured onto a Q panel. White mineral oil applied to the surfaces of the control and treated panels show beading of the oil with the DT10 treated coating.

Figure 2. Beading of WMO on UV panels. Control is on the left.

We extended this approach into di-functional reactive analogues. These should behave like the TMS materials described above with the two reactive sites providing sufficient structure.

Table 1 shows the products evaluated in this study. These include the monofunctional branched silicone, three difunctional branched silicones, three difunctional linear silicones, and two C4F9 – PFAS/ silicone materials as controls.

Table 1. Structures Evaluated

Referring to Table 2, the branched structures designated with T in its name did significantly better than the others and the PFAS containing materials.

Table 2. Screening Results

We evaluated dose response with TDi100. Looking at the OCA in Table 3, one sees use level increases as OCA increases. The 9% formulation achieves the highest result of 77° OCA with. The lowest use level evaluated, 3%, gave an OCA of 61° that is much higher than no additive and and slightly higher than the PFAS control at 5%.

Table 3. Round 2 Results

Very recent work has included a monofunctional silicone locked into Siltech T35, a Q resin matrix. This shows exciting improvements in the receding contact angles of mineral oil as well as very good marker release over a fluorosilicone control.

Future work will continue to improve on oleophobicity by fine-tuning this assembly approach.

References

1. https://www.americanchemistry.com/chemistry-in-america/chemistries/fluorotechnology-per-and-polyfluoroalkyl-substances-pfas?gclid=EAIaIQobChMI3LmbifbcgAMVw9TjBx1iKwDyEAMYASAAEgJKDfD_BwE
2. B. Ruckle, S.-T. Cheung and Y. Luo, Replacing PFAS with Modern Silicon Cross-Linking Moieties to Confer Water and Oil Repellency, Release and Protection Properties, Proceedings of the 51st Waterborne Symposium, New Orleans, LA, 2024.
3. https://www.fda.gov/food/hfp-constituent-updates/fda-announces-pfas-used-grease-proofing-agents-food-packaging-no-longer-being-sold-us
4. https://www.foodengineeringmag.com/articles/101782-get-the-pfas-out-of-food-packaging-materials-asap
5. https://cen.acs.org/materials/coatings/PFAS-paper-food-packaging/99/i36.
6. B. Y. Jeon and S. M. Kim, US Patent 7,879,449, Non-stick Ceramic Coating Composition and Process, 2011.
7. G. Guerrero-Vacas, F. Comino and O. Rodríguez-Alabanda, Evaluation of the Effectiveness and Durability of Commercial Non-stick Coatings, J Food Eng 2024, 370, 111959.
8. Y. Jiale, C. Feng, Y. Qing, H. Jinglan and H. Xun, Superoleophobic Surfaces, Chem Soc Rev 2017, 46, 4168 – 4217.
9. B. Ruckle, S.-T. Cheung Ruckle and Y. Luo, Novel Organosilicone Fluoro-free Anti-graffiti Agents, Proceedings of the 43rd Waterborne Symposium, New Orleans, LA, 2016.
10. B. Ruckle, S.-T. Cheung Ruckle and Y. Luo, Fluoro-Free Anti-Graffiti Properties from a Novel OrganoSilicone, European Coatings Show, 2017.

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