Chemist Fine-tunes Functional Coatings
Functional coatings are at the heart of research trials at a Spanish university, where a team of scientists is striving to modify the surface properties of materials to obtain specific properties at a lower cost.
Chemist Alexander Santiago—a researcher in the Department of Polymer Science and Technology at the UPV/EHU University of the Basque Country’s Faculty of Chemistry—is leading the research, focused on three types of functional coatings: those that are resistant to microorganisms, those that have self-cleaning characteristics and those that possess anti-reflective properties.
Functional Coatings 101
Coatings or paints are applied to different surfaces for decorative and protective purposes, according to an announcement on the research. However, today the market for these materials is being subjected to increasingly tougher specifications and demands.
In addition to being decorative and protective, today's coatings must also have low microorganism-adherence, be easy to clean or be able to self-repair.
The development of materials of this type, known as "functional coatings," calls for the control of not only of their physical properties (mainly to do with their morphology) but also of the chemical ones of the surfaces produced, the researchers relate.
Thus, Santiago set out to make a contribution towards the development of functional coatings that are resistant to microorganisms (for example, for paints for marine vessels), and those that have self-cleaning properties (hydrophobic paints) and anti-reflecting properties (for coating mobile phone screens or spectacle lenses).
It took numerous trials and tests to obtain functional materials with the desired properties, the announcement said.
According to the university’s announcement, in his research of coatings resistant to microorganisms Santiago focused on 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.
Detail of the phase separation in the copolymer: When the siloxane (areas where yellow predominates) migrates to the surface, it generates greater roughness, and due to its hydrophobic characteristics, prevents the adhesion of organisms.
Through various measurements, the team confirmed that the hydrophobicity of the system depended to a greater extent on roughness than on the siloxane concentration on the surface, the university said.
Protein absorption measurements were used to determine the restriction of the adhesion capacity of the microorganisms on these films. These trials showed that the microorganisms stuck less to the films displaying phase separation.
To obtain materials that would display a self-cleaning effect (also hydrophobic coatings), inorganic nanoparticles of a hydrophobic nature were synthesized in advance and inserted into acrylic polymers using various methods, the researchers said. Specifically, they were silicon nanoparticles with an organic coating.
The best results were obtained by spraying these nanoparticles onto acrylic films, producing a super-hydrophobic surface and offering very good self-cleaning properties in addition to a high level of toughness, the team reported.
The method used "turned out to be a fast and relatively cost-effective one," Santiago added, "as we used silicon that is not as expensive as other substances used on the market."
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, according to the scientists.
However, the presence of the pores prevents the anti-reflecting surfaces from having suitable mechanical properties enabling them to be processed. Santiago studied the porosity/toughness relation with respect to the refractive index obtained and reported that the results were “promising.”
"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.”
Santiago noted that the team’s study of anti-reflective properties was in early research stages.