Towards low-cost, functional coatings

Researchers from Spain are working towards modifying the surface properties of materials to obtain specific properties at a lower cost.

The market is growing for coatings that are not only decorative and protective, but that also have additional properties, such as low microorganism adherence, ease of cleaning or self-repair properties. The development of these ‘functional coatings’ calls for the control of both their physical and chemical properties.

The research conducted by Alexander Santiago from UPV/EHU-University of the Basque Country focuses on the development of three types of functional coatings: ones that are resistant to microorganisms (eg, for paints for seagoing vessels), those which are self-cleaning (hydrophobic paints) and those with anti-reflecting properties (for coating mobile phone screens or spectacle lenses).

Microorganism resistance

The first of the hydrophobic coatings explored was a type of copolymer that gives rise to spontaneous phase separation between its components.

“One of the components of the copolymer (the polyurethane) gives the substrate adhesion and most of the other component (the siloxane) remains on the surface, which makes it rougher, and as it is also hydrophobic it stops organisms sticking to it,” explained Santiago.

Through various measurements it was confirmed that the hydrophobicity of the system depended to a greater extent on roughness than on the siloxane concentration on the surface. Protein absorption measurements were used to determine the restriction of the adhesion capacity of the microorganisms on these films. The trials showed that the microorganisms stuck less to the films displaying phase separation.

Self-cleaning

To obtain materials that would display a self-cleaning effect, inorganic nanoparticles of a hydrophobic nature were synthesised in advance and inserted into acrylic polymers using various methods. Specifically, they were silicon nanoparticles with an organic coating. The best results were obtained by spraying these nanoparticles onto acrylic films, and that way a super-hydrophobic surface was created offering very good self-cleaning properties in addition to a high level of toughness. The method used “turned out to be a fast and relatively cost-effective one,” said the researcher, “as we used silicon that is not as expensive as other substances used on the market.”

Anti-reflecting

To obtain anti-reflecting properties, the films need to have a refractive index lower than that of the substrate, which can be achieved by inserting porosity into them. But the presence of the pores prevents the anti-reflecting surfaces from having suitable mechanical properties enabling them to be processed. In this respect, the porosity/toughness relation was studied with respect to the refractive index obtained, with promising results.

“There is still work to be done because, for example, the nanoparticles do not take hold completely and because the final film with the nanoparticles is not as consistent as we would like,” explained Santiago.

Regarding the copolymers studied in relation to the biological anti-contamination properties, he said that “the early results were fairly favourable”, and added that “they could be extrapolated, but the problem is that we are working on a laboratory scale”.