PFAS-free coating for sealing technology

The abbreviation PFAS is “doing the rounds” and possible restrictions or bans on perfluorinated and polyfluorinated alkyl compounds are also affecting the coating sector. This is because PTFE belongs to the PFAS group and high-performance PTFE coatings have become almost indispensable in sealing technology. This is why a coating specialist has developed a PFAS-free bonded coating and is offering an adequate replacement for premium PTFE coating systems on seals even before any restrictions have been decided.

When it comes to the demands of sealing technology, PTFE coatings have until now been regarded as the epitome of performance. They help elastomer seals to be fitted without damage and with little effort and prevent the small components from sticking to each other or even to the housing. In addition, premium PTFE bonded coatings provide long-term low coefficients of friction, reduce rubber abrasion and thus extend the service life of seals in dynamic use. Polytetrafluoroethylene is therefore not only an important sealing material, but also makes a valuable contribution in functional coatings. Now, however, a general REACh restriction proposal published by the ECHA on February 7, 2023, perfluorinated and polyfluorinated alkyl compounds, or PFAS for short, are under discussion. The outcry in the coatings industry is correspondingly strong. Only five years after the co-solvent N-methyl pyrrolidone (NMP) was banned from water-based coatings due to REACh specifications, the use of fluorine-containing PTFE is now also being called into question. And with it, most of the recently developed NMP-free coating systems.

Better to act than to wait

At this point in time, nobody knows which of the fluorinated alkyl substances will ultimately be affected by restrictions and to what extent. The industry has taken a stand and decisions are still pending. Nevertheless, after initial whispers about PFAS restrictions, a North Rhine-Westphalian coating service provider, in association with its coating manufacturer, is already developing a high-performance replacement for conventional PTFE coating systems – and can already provide a solution for sealing technology.

The coating base must be right

Functional coatings are subject to high stresses, especially in the sealing sector. In addition to various application parameters, such as media and temperatures, there are two details that are particularly challenging for a bonded coating. Firstly, elastomers have a low surface energy and are therefore difficult to coat. This is why the polymer surfaces are cleaned and pre-treated in plasma to improve wettability and coating adhesion. At the same time, the coating must also be suitable for rubber and must not detach from the surface even when stretched. This is because elastomer seals are very flexible and are often stretched considerably, particularly during assembly. The development team therefore decided to expand on the basis of their latest, NMP-free high-performance bonded coating. This has a broad chemical resistance, can be optimally processed and adheres excellently to elastomers. In addition, the underlying coating system has impressive elasticity.

Rubber friction is complex and PTFE is not easy to replace

The selected basis for the coating system therefore offers the best possible conditions for a PTFE-free high-performance coating. Nevertheless, the way forward remains challenging, as it is not in fact possible to replace PTFE one-to-one with another solid lubricant without adapting the entire coating system. One reason for this is the complexity of rubber friction, which is significantly influenced by the viscoelastic material properties of elastomers. In relation to rubber, classic friction laws lose their validity, whereas adhesion and deformation forces play a greater role. The functional layer must be correspondingly adaptable and the Alsdorf team has set its sights on several components of the coating mixture in addition to PTFE. With corresponding success, as initial stress tests show.

Application-related tests reveal the quality of the coating

The team uses coated FKM and EPDM O-rings as the basis for all material and parts tests. In order to test the bonded coating as meaningfully as possible, the team in Alsdorf always coats under series conditions and attaches great value to test conditions that match the application and are standard for sealing technology. Once the new PFAS-free bonded coating has passed initial internal tests, the coating service provider sends the parts to a renowned institute for further analysis. The materials experts there test the new product in accordance with the relevant test standards for elastomers and compare it with the base bonded coating and three of the latest generation of competitor coatings. All comparative coatings are also NMP-free, but contain PTFE as a solid lubricant. As is usual for elastomers in the sealing sector, the coated test specimens undergo a hot air ageing and cold folding test, depending on the sealing material, and are stored in distilled water or in reference oils and fuel (Fig. 4). The subsequent expansion allows the quality of the functional layers to be assessed. Finally, the test engineers simulate the external sealing assembly of the O-rings using a specially constructed device and record the maximum press-in forces.

Reliable results

The new PFAS-free coating passes all tests with flying colours. When new, it can be stretched by 100 percent without concern, for example during assembly, without suffering any damage. Just like the PTFE-containing comparison coatings, the new coating neither cracks nor separates from the component surfaces even after exposure to media or hot air up to 150 °C (Fig. 3). And although the Alsdorf PTFE reference coating so far remains unmatched in the assembly tests, the PFAS-free bonded coating is also impressive in its first stage of development. For example, it halves the maximum press-in forces in multiple assembly of the O-rings and therefore already outperforms two of the current PTFE-containing competitor coatings (Fig. 2). It is therefore clear that the newly developed, PFAS-free bonded coating will already take its place in sealing technology. Nevertheless, the development team remains on the ball and continues to optimize for even better performance.


APO GmbH Massenkleinteilbeschichtung

  • Konrad-Zuse-Straße 2b,
    D-52477 Alsdorf
  • +49 (0)2404-5998-0


Contact person

Antonio Pozo, Manager