Non-stick coatings after Nature's example

Biologically structured coatings offer properties that go far beyond the well-known lotus effect - i.e. self-cleaning in connection with water. As part of a funded joint project, new layer systems based on biological models are being developed. The focus is on the development of non-stick and corrosion protection layers.

For about ten years the term bionics, an artificial word from biology and technology, also known as "technical biology" or biomimetics, has been gaining in importance. The two most popular examples from basic research are the riblet film using the example of the skin of sharks and the lotus effect, based on the model of the top of the leaves of the lotus plant. Both developments are based on the discovery that nature prefers finely structured surfaces if engineers would use the smoothest possible.

Both in terms of optimum flow around air or water and to the lowest adhesion of particles, microstructured surfaces are smoothly superior, as the boundary layer of the flow is positively affected. Thus, braking turbulences are avoided and at the same time pollution is reduced, which is not only a matter of aesthetics since it results in increased weight and reduced streamline shape and is vital to organisms by reducing the pathogen infestation. at
technical developments, such as the construction of aircraft and ships, this aspect of the anti-pollution is becoming increasingly important, especially because of the current enormous increase in fuel and transport costs. Biological structures are mostly optimized for multiple problems and are often built with larger structure molecules and their ability to self-assemble, often resulting in structuring. Beyond self-cleaning in conjunction with water, the anti-adhesion property of a microstructured surface is of increasing industrial importance: as described by the example of the lotus effect, different size levels (hierarchies) of the leaf surface such as leaf veins, epidermis cells and wax crystals in their combination reduce the contact area by up to to 96%.

Together with the strong hydrophobicity of the wax layer, dirt and water are equally dissipated. This also applies to animals, such as dragonflies with their large rigid wings. Such a combination with properties beyond the self-cleaning technically to realize is useful for a variety of applications. Development requires cooperation between research and industry beyond pure basic research.

Close cooperation of research and industry
The company Rhenotherm has specialized in industrial coatings based on fluoropolymer for the application areas of non-stick, dry lubrication and corrosion protection for 30 years and is open to constant further development of coating systems to support varying customer requirements. For example, with Lotuflon, a micro-structured fluoropolymer coating with different structure levels was developed. The company is involved in a joint project funded by the Federal Ministry of Education and Research (VDI supervision, FKZ 13N8676) for the development of new and permanent anti-adhesive coatings. Partners from research and industry are involved: the company Nehlsen-BWB from Dresden (electroplating, aviation industry) is developing the structuring levels in µm and nm together with Rhenotherm. The Institute for Bioanalysis Technology in Heiligenstadt is working together with the University of Halle, Institute for Physiological Chemistry on the application and characterization of the new
Coating, the TU Dresden, Institute of Botany is working on the coordination and work on microscopic surveying.

Self-regenerating layers
Previous, artificially produced, microstructured non-stick coatings differ from the biological models in terms of their surface parameters, which are larger by a factor of 10 and more. Another problem can be the mechanical stability of the outermost layer, which up to now has not been self-regenerating, as is the case with many natural systems. The problem is circumvented by using a thicker top layer that can wear out during use. With the help of nature, new solutions are being worked on for both topics, because both the coating material and the decisive structural parameters are based on natural systems.

Tetraether lipids (TEL) are part of the shell of certain archaebacteria such as Thermoplasma or Sulfolo-bus. Archaebacteria live mostly in extremely inhospitable environments with high temperatures and low pH, for example in hot springs or the deep sea. Their shell must protect the cell interior from these hostile environmental influences. The TELs are, for example, 4 nm tiny cyclic molecules made of two alkane chains, which are each bridged at their ends with ether bonds and can be covalently bonded to the substrate via their head groups.

High chemical resistance
Because of their stability due to this structure, they are able to self-assemble into a closed layer of primarily perpendicularly parallel molecules whose properties can be derived from that of the bacterial envelope, such as temperature resistance up to 300 ° C, high chemical , UV and acid resistance.

Due to their molecular structure, TELs are unreactive, non-conductive and can be imagined as an insulator in multilayer stratification.

In contrast to the previously used anti-adhesive coatings made of fluoropolymers, the possibility of covalent attachment to almost any material offers high mechanical stability, since the layer not only rests on the substrate, but ultimately becomes one with it. The upward head group can in turn be functionalized in such a way that certain properties can be set - on the outside, for example, the coating is hydrophilic, while the TEL molecule itself remains hydrophobic.

Due to the small size and controllable, flexible functionality of the TEL, a wide range of applications is to be expected: One to two layers are suitable for sealing medical products such as fine cannulas or inner tubes and facilitating their cleaning, which is currently being used with other techniques difficult to reach. A covalently bound overlay with bactericidal colloidal silver is also possible. Applied to, for example, glass surfaces, tests have already successfully shown that optical sensors can be used virtually maintenance-free outdoors or in installations.

Main focus on non-stick and anti-corrosion properties
A completely non-breaking coating of facade glazing, which has not yet been technically possible, seems to be possible. If desired, the TEL can also be applied in thicker layers so that, for example, abrasion protection can be guaranteed in addition to the other desired properties such as chemical resistance, high non-stick properties or good traction thanks to a soft, rubber-like surface. In the possible applications, the main focus is on the coating of metal surfaces with regard to non-sticking and corrosion protection. Possible areas of application range from the coating of functional parts of, for example, tubs, rollers, demoulders, spray dryers and drums to the sealing of facade elements. The second bionic aspect plays an important role in many of the above-mentioned application possibilities, as a microstructuring that is adapted to the natural models reduces the contact surface for dust, water and even single-cell life forms such as algae and bacteria so much that very little can remain on these surfaces. With a corresponding polyfluorination, it is conceivable that even strong wetting agents and oils can no longer wet the surface - possibly for the first time a surface that completely spoils the mood for graphite sprayers.

But until then, you have to be patient. The project, which has a funding period of more than three years, started 2004 in November, and it will be several years before the first products are ready for use. A hitherto important hurdle is the limited availability of the coating material: TEL are, since they are obtained from the shells less prized single-celled organisms, in purified form so far only in small quantities in stock and therefore correspondingly expensive. However, 2 mg is sufficient for one-layer coverage of 1 m².