Reinigung superhydrophober Oberflächen

Abstract

The unique surface structure of the lotus leaf in combination with hydrophobic epicuticular wax crystalloids results in extreme water repellency and self-cleaning properties. In recent years biomimetic superhydrophobic surfaces have been fabricated by mimicking the structure of the lotus leaf. The biggest problem of the fine surface roughness is the sensitivity to oily contaminants and mechanical stress which limit the application of technical superhydrophobic surfaces. The destruction of the fine surface structure and the oily substances, which can migrate into the nanotextures, lead to an irreversible loss of the self- cleaning properties and an accumulation of soil in this area. Therefore, the aim of this thesis is to develop a detergent solution for superhydrophobic surfaces that already function at low concentrations specifically in the contaminated areas and that can be easily removed after cleaning by rinsing without damaging the surface.

The present thesis is part of the BMBF project “Biomimetische superhydrophobe Oberflächen: Funktionserhaltung durch Regeneration”. Scanning electron microscopy, contact angle and roll-off angle measurements were used to characterize the technical superhydrophobic glass and plastic surfaces and their wetting properties. Non-contact, optical profilometry was used to determine the differences in surface roughness and waviness between the two superhydrophobic surfaces by generating several surface parameters. Furthermore, dynamic contact angle measurements have been performed to study the wetting of Lotus-Effect® surfaces after the cleaning process and the resulting adsorption of surfactants at the surfaces. Compared to the glass sample, the super- hydrophobic plastic surface showed smaller roughness and higher waviness parameters. The values of the amplitude parameters Ra, Rq and Rz for the plastic surface are 0.920 µm, 1.304 µm and 8.526 µm. For glass we obtained 1.527 µm, 2.195 µm and 26.918 µm, respectively. Contact angle and advancing and receding contact angles on the Lotus-Effect® glass surface were about 156.1° ± 3.7°, 177° and 156°, respectively. For the Lotus-Effect® plastic surface we obtained a contact angle of 167.7° ± 1.5°. Furthermore, advancing and receding contact angles of 177° and 172° were measured. The dynamic contact angle measurements revealed that the glass surface was following the total wetting Wenzel regime whereas the plastic surface could be described by the laws of Cassie and Baxter.

The cleaning efficiency of 15 commercial surfactants of different surfactant nature and one basic cleaning formulation were examined using several optical methods. In addition, the influence of surfactant concentration, surfactant structure and type of contamination were investigated. In the case of pyrene as a standard contamination, video-enhanced contrast microscopy and fluorescence spectroscopy gave qualitative information about the cleaning efficiency.

TEGOTENS® AM VSF proved to be unsuitable for cleaning micro-und nanostructured Lotus- Effect® surfaces. UV/VIS-spectroscopy was best suited for a quantitative analysis of the cleaning efficiency. Four different surfactants were examined and the cationic surfactant TEGOTENS® DO of the decamine oxide type and the nonionic surfactant TEGOTENS® EC 11 of the end-capped fatty alcohol ethoxylate type proved to be most efficient on the nile-red contaminated surfaces. Further investigations were carried out to study the influence of the cleaning methods and the different structured Lotus-Effect® surfaces on the cleaning efficiency. Spray techniques have been demonstrated to be more effective in the cleaning of structured surfaces than dipping methods. Both Lotus-Effect® surfaces showed significant differences in their cleanability. In general the superhydrophobic plastic surface was easier to clean than the superhydrophobic glass surface. For the cleanability studies of the standard soil eleven additional surfactants were considered in order to investigate the effect of surfactant structure on the cleaning efficiency by using grayscale evaluations. In general, the cleanability of the anionic surfactants proved to be excellent. The best cleaning efficiency has been measured for Genapol® LRO (alkyl diglycol ether sulfate sodium salt, 78%) and REWOPOL® SB DO 75 (di-isooctyl sulfosuccinate, 87%).

Detergents are complex formulations containing not only single surfactants or surfactants mixtures but several different ingredients. Therefore, each of the eleven surfactants were combined with a typical basic cleaning formulation and the influence of the additives as a function of surfactant concentration on the cleaning efficiency were investigated. The anionic surfactants proved to be most efficient in the presence of the basic cleaning formulation. Especially Genapol® LRO (90%) and Hostapur® OS (83%) have been demonstrated to be very effective in removing the standard soil of the superhydrophobic surfaces at low concentrations. In cooperation with CAM-D Technologies GmbH these cleaning results were utilized to evaluate a relationship between the surfactant structure and the cleaning efficiency by using quantitative-structure-property-relationships (QSPR) models. Additionally, the QSPR models can be used to predict the cleaning efficiency of surfactants on Lotus-Effect® surfaces.

In conclusion, in cooperation with the industrial project partners a cleaning agent for micro- and nanostructured superhydrophobic surfaces was developed that is very effective in removing hydrophobic oily/greasy soils at low concentrations without damaging the surface.