Reinigung superhydrophober Oberflächen
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Date
2011-10-27
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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.
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Keywords
Reinigung, Superhydrophobe Oberflächen, Tenside