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Item Open Access Wetting, de-icing and anti-icing behavior of microstructured and plasma-coated polyurethane films(2019) Grimmer, Philipp E. S.; Hirth, Thomas (Prof. Dr. rer. nat.)Ice build-up on surfaces, for example on wings of airplanes or on rotor blades of wind turbines, impairs the functionality of transportation vehicles or technical systems and reduces their safety. Therefore, functional anti-ice surfaces are being researched and developed, which shall enable an easy removal or reduce the amount of ice on the surfaces at risk. The starting hypothesis for this work is that superhydrophobic polyurethane (PU) films with microstructure base diameters of 35 µm or more reduce the wetting by water, show a low ice adhesion for easy removal of ice and reduce or delay icing. Superhydrophobic PU films for passive anti- and de-icing were created by hot embossing and plasma enhanced chemical vapor deposition (PECVD). The hot embossing process as well as the plasma coating and etching processes were analyzed for the dependence of the surface characteristics on different process parameters. The functionalized PU films were characterized for their surface topography, surface chemistry, stability against erosion, wettability, ice adhesion and icing behavior. For comparison, the ice adhesion and icing behavior were examined on relevant technical materials (aluminum, titanium, copper, glass, epoxy resin of carbon fiber reinforced polymer and other fluoropolymers) and on some commercial anti-ice coatings. The PU films were chemically analyzed by IR spectroscopy. As the first process step for functionalization, microstructures of cylindrical, elliptical or linear shape were imprinted in PU films by a hot embossing technique with different ns-pulsed laser-drilled stamps and characterized by several microscopy methods. The microstructures had heights of 15 µm to 140 µm, diameters or widths of 35 µm to 300 µm and distances (pitch values) of 50 µm to 500 µm. The embossing process was analyzed and optimized in terms of the process parameters temperature, pressure, time, PU film release temperature and reproducibility of the microstructures. In a second functionalization step (PECVD) the microstructured surfaces were coated with thin, hydrophobic plasma polymers using different fluorocarbon precursors (CHF3, C3F6 and C4F8) or hexamethyldisiloxane (HMDSO). Different process parameters for plasma coating and etching (Ar or O2 plasmas) were used in order to create various nanoscale roughness values. Electron spectroscopy for chemical analysis (ESCA), spectroscopic ellipsometry and atomic force microscopy (AFM) were used for analysis of the chemical composition, the thickness and the nanoroughness of the plasma polymers. The functionalizations, especially the plasma coatings, were completely worn off by a UV/water weathering test (1000 h, X1a CAM 180 Test, SAE J-2527), but showed sufficient stability against sand erosion (DIN 52348), in a long-term outdoor test for 13.5 months and against fivefold repeated pull-off of ice. The silicone-like plasma coatings were more stable than the fluorocarbon plasma coatings. The wetting behavior of water was determined by static, advancing and receding contact angle measurements. Static contact angle measurements with diiodomethane (DIM) were made for determination of the surface free energies of the relevant surfaces. Advancing contact angles of over 150° and very low contact angle hysteresis values below 10° were reached on some of the cylindrically and elliptically structured PU samples with microstructure base diameters in the range of 35 µm to 50 µm. The measured water advancing contact angles did not reach the theoretical values of the Cassie-Baxter state. Starting from a mixed wetting state near Cassie-Baxter in case of the superhydrophobic PU surfaces, they approached the Wenzel state with an increasing pitch/diameter (P/d) factor. Fluorescence laser scanning microscopy images were taken of some microstructured, uncoated or plasma coated samples during the wetting by a water drop containing a fluorescent dye. These images show the Wenzel state or a mixed wetting state by visualization of the interface between the water droplet and the surface. A new icing test chamber and a test setup were developed for characterization of the ice adhesion and the icing behavior. The tensile ice adhesion was measured at -20 °C by pull-off of ice cylinders (highly purified water, (<0.056 µS/cm, diameter of 4 mm, similar to the diameter of large raindrops) and compared to the theoretical values and the wetting behavior. The technical material surfaces measured for comparison showed a high ice adhesion, which led to cohesive fractures especially on the metal surfaces, whereas some of the commercial anti-ice coatings showed lower ice adhesion values. The flat, plasma coated PU surfaces showed adhesive fractures with a reduced ice adhesion compared to the technical material surfaces and uncoated PU and revealed a good correlation of the ice adhesion with the wetting behavior of water (work of adhesion). On the other hand, the microstructured PU surfaces showed a greatly increased ice adhesion in comparison to the flat PU and technical material surfaces which was enhanced even further by the plasma coatings and did not correlate with the wetting behavior. The reason for this is the wetting transition from the Cassie-Baxter to the Wenzel state during the cooling or freezing process, leading to an increased ice-surface contact area and mechanical interlocking of the ice with the micro- and nanostructures. The freezing of water drops was examined in thermodynamic equilibrium (static experiment) and under quasi-steady conditions (dynamic experiment). In the static experiment, 15 µl water drops (corresponding to medium to large raindrops) at room temperature were dispensed onto a cold surface at a constant temperature of -20 °C. The freezing delay times, the crystallization times and the total freezing times were measured and compared to calculated expected values. On the flat samples, the freezing delay times could be extended by the plasma treatments. On the microstructured samples, the freezing (nucleation) could sometimes be delayed even further, but not always reproducible because of an unstable Cassie-Baxter state. In the dynamic experiment, 25 µl water drops (corresponding to large raindrops) were cooled down in quasi-steady conditions with the surface and the surrounding atmosphere by a constant, low cooling rate of 1 K/min while the water drop temperature was measured by an IR camera for determination of the surface-specific nucleation temperature and crystallization time. A lower nucleation temperature could be measured on the flat, plasma coated PU surfaces compared to uncoated PU and the hydrophilic glass and metal surfaces. The superhydrophobic PU surfaces did not show a further reduction of the nucleation temperature because of an unstable Cassie-Baxter state. The resulting measured nucleation temperatures were compared to the expected values calculated with an enhanced nucleation theory including a quasi-liquid interfacial layer of the ice nucleus and a Poisson process. Overall, it is shown that hot embossing and PECVD are useful processes for creating superhydrophobic PU surfaces with regard to a roll-to-roll process. The flat, plasma coated PU films show a reduced ice adhesion and lowered nucleation temperature compared to the relevant technical material surfaces. The microstructured, plasma coated PU films are far more water repellent than the flat, plasma coated PU surfaces or the other technical materials. However, the microstructures with base diameters of 35 µm or more and the nanoroughness of the plasma coatings cannot stabilize the Cassie-Baxter state of a freezing water drop enough for a low ice adhesion or a significant decrease of the nucleation temperature. These superhydrophobic PU films are therefore not more icephobic than the flat, plasma coated PU films. In the outlook, the reduction of the geometrical parameters of the microstructures (diameter D, distance P) and nanostructures (curvature radius R) of the surface functionalizations for lower ice adhesion values and nucleation temperatures is proposed.Item Open Access Barrier properties and analysis of defects of plasma polymerized hexamethyldisilazane-based films(2019) Troia, Mariagrazia; Hirth, Thomas (Prof. Dr.)A great variety of commercially available goods, e. g. food products, require a degree of protection against gases and vapors. Electronic devices whose active layers are based on organic materials in particular demand extremely low oxygen transmission rates in order to attain adequate lifetimes. In order to do so, an encapsulation of the device by means of a barrier becomes necessary. In case of flexible devices, such as organic light emitting devices (OLEDs), conventional encapsulation methods relying on stiff glass lids cannot be employed. Plasma-enhanced chemical vapor deposition (PECVD) methods on the other hand have been proven to be successful in obtaining thin films (in the range of tens or hundreds of nanometers) which combine good barrier performances with flexibility and other favorable mechanical properties. In the current work, thin silica-like (SiOx) films have been deposited on polyethylene terephthalate (PET) through a low-pressure microwave plasma and a gaseous feed consisting of hexa-methyldisilazane (HMDSN) and oxygen, with the aim of providing flexible oxygen barrier layers with additional properties as transparency, colorlessness, good adhesion to the substrate and resilience. Operational parameters such as the gas feed composition, microwave power and deposition time have been investigated and optimized, thus obtaining inorganic barriers with an optimal thickness in the 50 to 100 nm range and with a barrier improvement, when compared to the uncoated substrates, up to a factor of 100. The defects in the barriers have been investigated by means of a concurrently developed non-destructive method for their localization and identification, based on the precipitation of calcium carbonate crystallites on top of them, which allows the defect to be later retrieved and investigated by means of microscopy methods. Further analyses of the transmission rates have been carried out at different temperatures in order to investigate the permeation mechanisms through the bulk and the defects. The films, when compared to barriers deposited via the common precursor hexamethyldisiloxane (HMDSO), obtained in the same experimental setup, showed consistently better properties in a wider range of conditions, proving HMDSN to be a better precursor for thin films with barrier applications. Multilayer systems, based on the combination of SiOx films and an intermediate organic layer optimized in parallel to the barriers, have been developed, tested and used successfully for the encapsulation of flexible Organic Light Emitting Device (OLED) prototypes printed on polymers.