Functional coatings: “An extremely exciting field to protect the environment

What is the definition of a functional coating?

Dr. Gesa Patzelt: The definition differs only slightly when different experts are asked. Ultimately, the general properties of a coating, such as corrosion protection or decoration, should be supplemented by other functions. These are properties that span a very wide range and can also be defined differently for a wide variety of applications.

In the aviation industry, for example, these include anti-contamination, anti-ice, erosion stability, but also reduced flow resistance on the wings. Ultimately, all the technologies that lead to fuel savings. In principle, many developments are triggered by the topic of environmental protection, for example, because one wants to reduce CO₂ emissions. That is why functional coatings are also an extremely exciting field for making a contribution to protecting the environment.

Dr Gesa Patzelt works as a project leader at Fraunhofer IFAM in Germany. She will host a seminar on Functional Coatings in October. (Picture: Jan Gesthuizen)

This is certainly not only the case in aviation?

Patzelt: That’ s right, there are also anti-fouling coatings in this area that are intended to prevent microorganisms from growing on ships. Currently, biocides are mainly used for this purpose, but some of them are already banned due to their toxicity. We are also developing functional concepts as alternatives for this. Here we are helping, so to speak, to make the oceans less polluted. On the one hand, by using less fuel such as heavy oil and, on the other hand, by using little or no biocides at all.

Are there concrete examples of functional coatings that are already available on the market?

Patzelt: Yes, that’s the self-healing wood varnish that we developed together with Adler-Werk Lackfabrik. This coating contains microcapsules. If damage is caused by hail, a self-healing agent consisting of polymers, emerges from the capsules and protects the wood substrate because the cracks formed are partially filled.

There are also some coatings that are on the verge of commercialisation. For example, we have developed a lacquer with a microstructure that is modelled on shark skin. This is used on the threshold in aviation and wind energy, where it is intended to reduce wall friction resistance.

What about the classic functional coating, the easy-to-clean coating?

Patzelt: There are some things here that have been in use for a long time. Everyone knows Lotusan, for example. In principle, the durability of Easy-to-Clean coatings is not yet quite optimal. This is a general problem with functional coatings.

What is the reason for this?

Patzelt: That is different for each case. In order to understand this, you have to know how functionalities are created. The easiest way is to achieve functionality using additives. These are mobile in the polymer matrix and migrate to the surface in order to develop the functionality there. However, they can also disappear  from the coating in this way. Of course, the reservoir of additives will eventually be exhausted.

Coatings that copy the structure of shark skins can reduce flow resistance and increase the efficiency of ships, airplanes or wind turbines. (Picture: wildestanimal -stock.adobe.com)

You also have to see how functional additives fit into the formulation and whether all other additives and fillers and pigments used in the formulation are compatible with these additives. It can also happen that incompatibilities are created that lead to lower resistance. In general, it is therefore better to develop the polymer matrix in line with this and possibly crosslink the active substances with it. In this case, the stability is much higher.

But it also sounds more complex.

Patzelt: Yes, it is a bit more complex. But it is also possible to couple additional functional groups to additives, which then interact with the polymer matrix. For example, an isocyanate could interact with OH groups of the additive and crosslink. In this way, these substances are firmly integrated and do not migrate to the surface, where they are directly anchored and available for longer.

These are now two strategies, one to add additives in the traditional way and the other to incorporate them in a fixed way. Are there any other strategies?

Patzelt: Of course, there are the microcapsules that I mentioned earlier. Here, the chemical structure of the substances to be encapsulated is decisive. Different substances can be encapsulated, depending on whether they are in an aqueous or solvent-containing environment. This would again be an example of a reservoir which is present in a coating but which in this case is only released again through damage.

Thanks to microcapsules there are already wood coatings that have some degree of self-healing after hail damage. (Picture: encierro – stock.adobe.com)

This is not necessarily just about self-healing. For example, fluorescent dyes are also encapsulated, which then act as an indicator when a coating is damaged. This is useful for materials that have a certain tendency to crack which is not necessarily clearly visible.

In addition, we also have a concept in which we expose the top layer of evenly distributed fillers and then functionalise them, so that this works later on. Or you already have a functionalisation of the particles exposed with plasma. The prerequisite is that the functionalisation is not altered by the plasma. These microcapsules ensure that the protective function of self-healing coatings is maintained.

Are there any concepts that also optically restore the initial situation?

Patzelt: Yes, but they are much further away from the application. This is the area of intrinsic self-healing, where self-healing originates from the polymer. Either with metal complexes present in the polymer matrix, which first separate when damaged but then reorient themselves, or via hydrogen bonds, which then reorient themselves after the damage and completely heal the coating again.

The Fraunhofer IFAM also had a PhD thesis that showed that the self-healing of the polymer investigated here only works with a certain humidity. This means that there is always some kind of trigger that enables self-healing, which means that such coatings cannot be used all over the world.

So far, we have mainly talked about individual functionalities. Is it also possible to create two or three different functionalities?

Patzelt: There are certainly functionalities that can be combined with each other. For example, elastomer coatings that have very good erosion stability can also be made hydrophobic so that they also have easy-to-clean or dirt-repellent properties.

But the functionalities can sometimes have a negative effect on each other. For example, the structure of the coating can suffer. Of course, it would be great to have one coating for all functionalities, but unfortunately that won’t work.

Structured surfaces are interesting when one wants to combine hydrophobic and olephobic characteristics. (Picture: ag visuell)

If we look further into the future, what will become important in the area of functional coatings?

Patzelt: In general, the topic of structured surfaces is interesting when it comes to different phobias, i.e. the combination of hydrophobic and olephobic surfaces. This is an approach to repel dirt or insects, which consist of many different components.

But there is still a lot to develop. In particular, we need to understand the interactions between micro- and nanostructures, their combination and the ideal appearance of the structures. 

We have not been able to predict in advance how certain structures will affect an ice test or an anti-contamination test, for example. If we understood this exactly, we would be able to move more in the direction of a toolbox in the future.

You will soon be giving a seminar on functional coatings. What can participants expect there?

Patzelt: There will be a complete overview of the functionalities developed so far. I will show concrete results from many projects on functional coatings in many different applications with examples. The seminar will also provide some help in the direction of a toolbox that shows what possibilities there are for functionalising a coating.

The interview was conducted by Jan Gesthuizen