PFAS-free coatings: silicone, plasma, SLIPS alternatives

Why PFAS are so difficult to replace in repellent coatings

PFAS achieve unmatched water and oil repellency because the carbon–fluorine bond has extremely low polarizability, producing surfaces with surface free energy low enough to repel even oils and other low-surface-tension liquids. No currently available non-fluorinated chemistry replicates this at the same level, which is why the transition away from PFAS is an engineering challenge — not merely a regulatory compliance exercise.

The governing physics is described by Young’s equation: cos(θ) = (γ_sv − γ_sl) / γ_lv, which relates the equilibrium contact angle (θ) to the interfacial tensions between the solid–vapor, solid–liquid, and liquid–vapor phases. A lower solid surface free energy pushes the contact angle higher. PFAS compounds sit at the extreme low end of surface free energy, which is why alternatives — even good ones — must compensate through surface architecture as well as chemistry.

Regulations enforced by bodies including ECHA under EU REACH and the US EPA are accelerating the phase-out of persistent, bioaccumulative, and toxic PFAS compounds across textiles, food packaging, automotive finishes, and electronics. The result is that R&D teams must now deliver coatings that are both PFAS-free and functionally equivalent — a target that no single drop-in replacement currently meets across all applications.

Alternative chemistries: silicones, hydrocarbons, and bio-polymers

Three classes of non-fluorinated chemistry form the foundation of PFAS-free repellent coatings, each with a distinct performance profile and scalability ceiling. Silicone and hydrocarbon polymers are the most mature options for water repellency, while bio-based polymers open a separate path for food-contact applications.

Silicone polymers (PDMS)

Polydimethylsiloxane (PDMS) is a common first choice for PFAS replacement. Its methyl (–CH₃) groups present a low-energy surface that resists water wetting, and it can be applied via standard spray or dip-coating processes. Daikin Industries has developed fluorine-free water-repellent compositions based on long-chain hydrocarbon polymers and aqueous dispersions using emulsifiers with a specific HLB range of 7.5–13.5 to ensure stable formulations. The University of Toronto has also explored “nanoscale fletching” techniques that use minimal fluorination on PDMS to achieve high repellency, reducing — though not eliminating — fluorine content.

Hydrocarbon-based polymers

Long-chain acrylic polymers — specifically (meth)acrylic acid esters with C18–30 hydrocarbon groups — provide water repellency through the same methyl-group mechanism as silicones. These are commercially available and can be formulated into aqueous dispersions for textile finishing. The critical limitation is shared with silicones: neither chemistry produces surface energy low enough to repel oils. As North Carolina State University research on PFAS-free firefighter gear has highlighted, the loss of oil repellency can introduce new hazards, including increased flammability risk when gear is exposed to fuels.

Bio-polymer coatings for food packaging

For paper-based food packaging, bio-based polymers including starch and zein offer a sustainable and food-safe path to both oil and water resistance. Patented by the US Department of Agriculture, aqueous coatings using these materials can be applied on standard paper coating equipment, are inexpensive, and are recyclable. This approach is well-suited to single-use food packaging but is not transferable to high-performance textile or industrial applications.

Advanced surface engineering for combined water and oil repellency

When chemistry alone cannot close the performance gap left by PFAS, surface engineering — either physical structuring or vacuum-phase deposition — provides a route to combined hydro- and oleophobicity. Three approaches have demonstrated the strongest results: halogen-free plasma polymerization, mussel-inspired slippery surfaces (SLIPS), and templated microstructuring.

Halogen-free plasma polymerization (PECVD)

Sefar AG has patented a method using low-pressure plasma-enhanced chemical vapor deposition (PECVD) with halogen-free precursors — organosilane, siloxane, and/or hydrocarbon monomers — to deposit a highly cross-linked repellent thin film directly onto polymeric fabrics. The process creates surfaces that repel both water and oils, with durability stemming from the covalently bonded, cross-linked film structure. The trade-off is significant: PECVD requires vacuum processing, representing a capital-intensive investment that limits scalability compared to wet-chemical approaches.

Mussel-inspired slippery surfaces (SLIPS)

Slippery Liquid-Infused Porous Surfaces (SLIPS) take a fundamentally different approach: rather than creating a dry repellent surface, they infuse a porous or textured substrate with a lubricating liquid that is immiscible with the liquids being repelled. A published approach uses polydopamine (PDA) as a substrate-independent adhesion layer, combined with monoaminopropyl-polydimethylsiloxane and infused silicone oil, to create a stable, molecularly smooth liquid–liquid interface. This interface repels a wide range of liquids including oils, and the process is applicable via simple spray or dip methods to virtually any substrate. The primary durability challenge is lubricant depletion under shear or repeated washing — the lubricant must be stably locked into the surface structure to maintain long-term performance.

Templated microstructuring and the lotus effect

3M Innovative Properties Company has patented a templating method in which particles are embedded in a surface and then removed, leaving behind micro-structured cavities with re-entrant (tapered) geometry. These structures trap air in the Cassie-Baxter state, minimising liquid–solid contact and amplifying the repellency of whatever base chemistry is used. The Cassie-Baxter equation — cos(θ*) = f_s(cos(θ) + 1) − 1 — predicts how the solid fraction in contact with the liquid (f_s) determines the apparent contact angle on such structured surfaces. The limitation is mechanical: fine surface features are vulnerable to abrasion, and high liquid pressure can collapse the trapped air layer, transitioning the surface to the Wenzel state and causing liquid pinning.

“The stability of the trapped air layer is critical. High pressure or impact can force the liquid into the texture — the Wenzel state — leading to pinning and a complete loss of repellency.”

UV-cured super-lubricating coatings

Fuzhou University has patented a fluorine-free, transparent super-lubricating coating formulated from a sulfhydryl compound, styrene copolymer, a low surface energy component, and a photoinitiator, cured under UV light. The formulation creates a durable, oil-proof surface with excellent adhesion resistance against various organic liquids. The UV curing step adds process complexity but delivers a self-contained, solvent-free route to oleophobicity that does not require vacuum equipment.

Performance validation: beyond the static contact angle

Validating PFAS-free coatings requires a multi-stage measurement protocol — static contact angles alone are insufficient predictors of real-world repellency. A comprehensive approach covers quantitative wettability analysis followed by accelerated aging and durability testing.

Quantitative wettability analysis (S7)

The primary instrument is an optical tensiometer, which measures static and dynamic contact angles for water, oil (e.g., decane, diesel), and other application-relevant liquids. Static contact angles above 90° indicate hydrophobicity; above 70° indicates oleophobicity. For super-repellency, the target is above 150°. Critically, dynamic measurements — advancing angle, receding angle, contact angle hysteresis, and roll-off (sliding) angle — are better predictors of self-cleaning performance than static readings alone. A roll-off angle below 10° indicates that droplets will slide freely, carrying contaminants with them. High contact angle hysteresis, even with a high static angle, signals strong droplet adhesion and poor self-cleaning behaviour.

Accelerated aging and durability testing (S8)

Durability testing must be tailored to the target application. For textiles, repeated laundering cycles in industrial washing machines are the standard stress test. For high-touch or outdoor surfaces, mechanical abrasion testing and weatherometer exposure (combined UV, heat, and moisture cycling) simulate years of real-world degradation. The goal is to identify performance trade-offs and degradation mechanisms before a product reaches market — not after. For SLIPS-type coatings, lubricant retention under shear is the critical durability variable. For micro/nano-structured surfaces, resistance to abrasion-induced structural collapse is the primary concern.

Characterisation beyond contact angle measurement — including scanning electron microscopy (SEM) and atomic force microscopy (AFM) — is used to assess surface morphology and confirm that the micro/nano-scale structures responsible for the Cassie-Baxter state are intact after aging. These tools, alongside the optical tensiometer, form the core analytical toolkit for PFAS-free coating validation, as referenced in the materials science literature published by Nature and standards bodies including ISO.

Application-specific recommendations and trade-offs

There is no universal PFAS-free drop-in replacement. The correct strategy depends on the application’s performance requirements, process infrastructure, and cost constraints. The primary trade-off in every case is sacrificing the exceptional oil repellency of fluorochemistry for improved environmental and regulatory standing.

Water repellency only: silicone and hydrocarbon polymers

For applications where only water repellency is critical — such as certain outdoor textiles or architectural coatings — silicone- and hydrocarbon-based polymer coatings offer a mature, scalable, and cost-effective solution. They are applied via standard spray or dip-coating lines, require no specialised capital equipment, and are commercially available at scale. The limitation is absolute: they do not repel oils.

High-performance durable applications: plasma PECVD and UV-cured coatings

For applications demanding both hydro- and oleophobicity with high durability — including industrial filtration fabrics, protective gear, and technical textiles — halogen-free plasma polymerization (PECVD) and UV-cured super-lubricating coatings are the most promising solutions. Both deliver robust repellency and good durability, but require specialised capital equipment: PECVD reactors for plasma processing and UV curing lines for photopolymerization. These are best suited for high-value products where performance cannot be compromised on cost grounds.

Scalable substrate-independent applications: SLIPS

Mussel-inspired SLIPS coatings offer a highly versatile one-step process applicable to a wide range of substrates via simple spray or dip methods, without vacuum equipment. The polydopamine adhesion layer makes the process substrate-independent. Long-term durability — specifically lubricant retention under shear or repeated washing — must be rigorously validated for each specific use case before commercial deployment.

Sustainable food-contact applications: bio-polymers

For single-use food packaging, bio-polymer coatings based on starch and zein are the optimal choice. They are inexpensive, recyclable, and food-safe, providing a clear path to sustainability in this sector. They are not appropriate for high-performance or durable applications. The PatSnap Materials Science intelligence platform tracks emerging bio-polymer patent activity in this space across the US Department of Agriculture and academic institutions globally.

Across all application categories, R&D professionals reformulating PFAS-free coatings should consult patent intelligence to avoid freedom-to-operate conflicts with existing filings from organisations including Daikin Industries, Sefar AG, 3M Innovative Properties Company, and Fuzhou University. The PatSnap Insights blog covers emerging patent trends in functional coatings and surface engineering. Regulatory guidance is also available directly from WIPO on international patent classification for surface treatment technologies.