DE102009024320A1 - Coating comprises corroded nanoparticular metal-/silicon oxide layers, in whose surface, fluorine compounds are bounded, or silica, silicon oxide compound, alumina and/or titania or mixed oxides of titania/silica - Google Patents

Abstract

The coating comprises corroded nanoparticular metal-/silicon oxide layers, in whose surface, fluorine compounds are bounded. The coating has a layer thickness of 400-800 nm and a roughness of 200-2000 nm. The coating consists of silica, silicon oxide compound (R-SiO3/2, where R is alkyl radical or fluoroalkyl radical with 1-18 carbon atoms), alumina and/or titania or mixed oxides of titania/silica that contain 70-80 mol% of titania. An independent claim is included for a method for the production of a coating.

Description

The The invention relates to a permanent ice-repellent and freezing point-lowering Equipment of material surfaces based on etched Metal oxide coatings. The coating becomes an antiadhesive Behavior against flowing media up to the temperature range of the liquid-solid phase transition scored into it.

The Icing of technical equipment and devices z. B. from Aircraft wings, power plants, floodgates, technical Means of transport in the maritime context, high voltage power lines is undesirable. Ice and hoarfrost formation lead to Lowering the efficiency up to the inability to function of technical equipment. For operators of outdoor facilities is therefore the increased demand for anti-adhesive Coatings. Specifically in wind turbines need rotor blades and towers are protected against icing. at icing of the rotor blades are potentially formed Kilo-heavy ice layers, which are released by the centrifugal force can. Fly in unfavorable conditions such ice sheets, projectiles, several hundred meters far. Wind turbines may therefore not in the appropriate weather operated, resulting in undesirable downtime of the equipment entails. Also the icing of power lines and telecommunication facilities can be a big economic problem. The necessary deicing is in any case costly and causes additional costs. Ice, water and dirt repellent Coatings can reduce the effectiveness of technical equipment and devices significantly improve.

The Development of ice-repellent coatings has for metallic Surfaces a special meaning. The thermal expansion coefficient of ice is slightly above that of the common one Metals, but far below the thermal expansion coefficient of many polymers. When internal tensions in ice due to high Subcooling rates can not be compensated this would be the case with strongly differing thermal expansion behavior for ice sheets to break and the ice sheets could crumble while on metals would adhere all the more firmly.

One concentrated much of the research and development work hitherto on constructive measures, the sticking to prevent ice and mechanisms for removing the ice. Only a small part of the work has been geared towards to change the surfaces of the materials so that over-freezing can be hindered or prevented.

Now are increasingly developing results of ice-repellent coatings known to those weakening intermolecular Forces at the interface of surface and Underlying ice. As the causes of ice adhesion become electrostatic attractions, hydrogen bonds as well as the van der Waals bonds discussed. Ice consists of polar water molecules, which interact strongly with any kind of surface whose Dielectric polarization deviates from that of the ice.

Thus, attempts have been made to minimize the adhesion of ice over its contribution to electrostatic attractions. About a constant electrical DC voltage, these forces can be influenced. Between electrically insulating coating and ice, a fine-meshed network is applied, which realizes the electrical contact / US 2003/0024726 A1 ; VF Petrenko, IA Ryzhkin, Surface states of Charge Carriers and Electrical Properties of the Surface Layer of Ice, J. Phys. Chem. B 1997, 101, pp. 6285-6289 /. As this mechanism presupposes a conductivity of the ice, it can only be used for aqueous solutions (eg NaCl dissolved) and is not applicable to distilled water.

It is known that water can bead off well on surfaces with reduced surface energy. According to a widespread thought model, these hydrophobic surfaces should also reduce the adhesion of the ice. Several papers refer to the ice-repellent properties of hydrophobic surfaces. A minimization of the surface energy can be z. B. with silicone reinforced surfaces. The surfaces then show minimal interaction with other substances. Such systems have been presented as ice adhesion reducing systems / WO 2004 078873 A1 /. Elsewhere it is shown that even proportions of a few percent of a tetrafluoroethylene in the coating system achieve an optimum hydrophobizing effect. JP 2000044863 A /.

So far, however, it is unclear whether a basic hydrophobicity of the coating system is necessary for its ice-repellent properties. The targeted influence on the surface roughness or the role of minimizing the surface roughness is also discussed controversially. Current and own investigations on the interaction between hydrophobicity and surface roughness show no clear tendency / Laforte, Caroline, J.-L. Laforte, J.-C. Laforte; The Tenth International Workshop on Atmospheric Icing of Structures, IWAIS 2002 /. Overall, layer systems with hydrophobizing properties can reduce the adhesion of the adhering ice and there facilitate with mechanical processes of ice removal. They can have a freeze-delay effect, but they can not prevent it.

Additional effects are expected from heterogeneous surface structuring of components with different hydrophobicity. For this purpose, two approaches are known, each of which uses the technology of sol-gel layer production. SiO _{2 base} layers usually showed a rather hydrophilic behavior, edge angle measurements yield a result of ~ 40 °. TiO ₂ layers also showed no pronounced hydrophobic behavior, edge angle measurements are ~ 60 °. Through a particle reinforcement of sol-gel layers, primarily with silicone particles, but also other, not specifically listed organic and inorganic admixtures, a modification of the hydrophilic-hydrophobic behavior is made / US 6,153,304 A /. The aim is to set a hydrophobic, anti-adhesive and ice-opaque character of the coating. Elsewhere, a biomimetic adaptation of a hydrophilic / hydrophobic membrane surface of ice nucleating bacteria is presented. Some tiny hydrophilic molecules in the layer allow localized ice formation at these sites. Spontaneous freezing of the surfaces should be avoided and the freezing process should be controlled in order to support the ice-repellent layer properties. WO 2005/075112 A1 /. In another work, a permanent polymeric coating is described which is intended to prevent icing of the surface. It is based on a polymer network with a subdivision of hydrophobic backbones and hydrophilic side chains in regular or random distribution. When the hydrophilic side chains come into contact with moist surroundings, they solvate. The ice-repelling effect is explained with an air or liquid film, which can then be formed between crystallized, solidified solution and the coated surface and is to be maintained to a certain hypothermia / WO 2008/071752 A1 /. Application examples of the two last-described inventions demonstrate an ice adhesion-reducing effect in such a way that once formed hoarfrost can be removed more easily mechanically on surfaces coated according to the invention than on uncoated surfaces.

It It is therefore an object of the invention to provide coatings with improved ice-repellent and freezing point-lowering properties to create. Reached that should not just be the complete icing is delayed, but freezing point depressing properties be achieved.

According to the invention the task by a coating with ice-repellent and freezing point lowering Properties solved from etched nanoparticulate Metal / silicon oxide layers exist in their surface Fluorine compounds are involved. This is the nanoparticulate Oxide coating later with a fluoride-containing solution etched, whereby the surface roughness of Layer advantageously changed and by the formation of fluorine-containing Surface structures promoted the ice repellency becomes.

Advantageous the coating according to the invention has a layer thickness 400-800 nm and a roughness of 200 to 2000 nm.

According to an advantageous embodiment of the invention, the coating consists of SiO ₂ , R-SiO _3/2 (R = alkyl radical or fluoroalkyl radical having 1-18 carbon atoms), Al ₂ O ₃ and / or TiO ₂ . Particularly preferred is a coating of TiO ₂ / SiO ₂ mixed oxides with a proportion of 70-80 mol% TiO ₂ .

To produce the nanoparticulate oxide coating according to the invention, sols prepared by acidic or alkaline hydrolysis of the corresponding silicon, aluminum or titanium alkoxides are used using the sol-gel technique, and stable layers of SiO ₂ , R-SiO after coating any material surfaces _3/2 (R = alkyl radical or fluoroalkyl radical having 1-18 carbon atoms), Al ₂ O ₃ and / or TiO ₂ form. The preparation of such nanoparticulate oxide layers can also be carried out from the gas phase (CVD, sputtering, thermal evaporation).

The coatings of the invention have a layer thickness of about 400-800 nm, preferably from 450-600 nm. These layers form a highly elastic, predominantly inorganic ceramic network, which also adapts to complicated underground structures. After drying and tempering to temperatures above 400 ° C, the layers are etched with a fluoride-containing aqueous or alcoholic solution. By the etching process and its temporal variation properties can be adjusted, which promote an anti-adhesive behavior against flowing media into the temperature range of the liquid-solid phase transition of aqueous solutions. This results in a surface roughness in the order of Ra = 300-450 nm, accompanied by a planar structuring of the surface in the order of 200-2000 nm. This surface design allows intervention in the occurring laminar and turbulent flow forms in the boundary layer region of the flow field of the flow around the invention coated element. The building wall shear stress is used to immediately dissolve any adhering ice crystal nuclei and by the flowing medium of the be stratified layer of the component. Due to the interaction with the circulating medium, the freezing or frosting of the coated components within a subcooling range of 5-2 Kelvin below the respective freezing point is avoided for a longer time or completely. In addition, the ice adhesion to the coatings according to the invention is fundamentally reduced.

to Invention also includes items such. B. Components, components, workpieces or bodies with the coating according to the invention.

According to the invention the coatings with ice-repellent and freezing point-lowering properties made as follows:

(1) Coating of a material surface by dipping, spraying, casting with a nanoparticulate oxide sol

The sol is prepared using the sol-gel technique by acidic or alkaline hydrolysis of the corresponding silicon, aluminum or titanium alkoxides and forms after coating any material surfaces stable nanoparticulate layers of SiO ₂ , R-SiO _3/2 (R = alkyl or Fluoroalkyl radical having 1-18 carbon atoms), Al ₂ O ₃ and / or TiO ₂ . The preparation of the coating solutions can also by dispersing nanoparticulate commercial TiO ₂ (Degussa P25 ^® eg.) In SiO ₂ -. Or Al ₂ O ₃ -Nanosolen or nanoparticulate SiO ₂ (eg Lydox ^{®, ®} Nyakol, Levasil® ^®) carried out _two -Nanosolen in TiO. Nanosols made of TiO ₂ / SiO ₂ mixed oxides containing 70-80 mol% of TiO ₂ are particularly advantageous. This nanosol contains particles up to a size of ~ 250 nm. The nanosol is applied to the surface of the material by dipping, spraying, pouring or other common coating techniques.

(2) drying and annealing the layer above 400 ° C

Of the applied thin film is first air-dried, is amorphous in the result and still contains organic Remainders. This is the reason for a heat treatment at. Annealing at temperatures above 400 ° C is used the purpose of crosslinking and partial crystallization of the applied film and pyrolysis, d. H. the removal of organic residues.

(3) Etch the layer through a fluoride-containing aqueous or alcoholic solution

After drying and tempering to temperatures above 400 ° C, the layers are etched with a fluoride-containing aqueous or alcoholic solution. For this purpose, a 0.1 to 10% aqueous hydrofluoric acid or a 1-10% aqueous solution of NH ₄ F or NH ₄ HF ₂ (NH ₄ F × HF) is used. On the one hand, chemically bound organic or inorganic fluorine-containing complexes are permanently introduced into the near-surface layer regions and, on the other hand, a defined surface roughness and surface structure are produced. The duration of the etching process is matched to component size and Ätzlösungsvolumen. It serves the purposeful functionalization and adjustment of the surface structuring on the order of 200-2000 nm.

The layer thicknesses of the sol-gel layers according to the invention are on average 400-800 nm. The small layer thickness is a prerequisite for the adhesion properties and necessary elasticity of the glassy layers. The set porosity is necessary in order to influence the flow process at the interface of the medium flowing around the coating system targeted and to prevent Eisanhaftungen. In this etching process complex fluorine-containing compounds are formed on the surface. Indications for this are provided by IR spectroscopic investigations of the etched and finished layer system. The fluorine-containing compounds are compounds in which the oxide oxygen has been partially replaced by fluorine, e.g. As SiOF ₂ , TiOF ₂ and their mixtures or hydrogen from organic molecular chains has been partially replaced by fluorine, z. B. CF ₃ .

The Hydrophobicity of the coating was determined by contact angle measurements rated. The layers show hydrophobic contact angles of 85 ° -90 ° Properties. Allow the so coated objects in the case of freezing, an easier lifting of the ice.

The coatings according to the invention from Grundsol and anchored fluorine-containing layer constituents have a freeze-retarding effect, because they achieve a reduction in surface tension. The evenly applied surface structuring leads to an influence on the flow conditions of the adjacent circulating medium (cooling water, Air masses). This mechanism causes the significant freezing point depression. The surface structuring changes the flow resistance in the near - surface area so that a shearing of the Ice crystallites can be used, posing as heterogeneous nuclei begin to form on the coated surface.

By coating with the layer system according to the invention is achieved that not only the complete icing is delayed, but freezing point lowering properties are achieved. The coatings according to the invention are suitable for the application area of cooling systems and heat exchangers: for inner tube linings in cooling technology, for the coating of Eisbreigeneratoren in cooling technology. In addition, use is possible wherever flow of a surrounding medium occurs. On the basis of the invention it becomes possible to achieve improvements with regard to the weather-influenced operation of technical installations. It is conceivable to use the coating system described for the coating of weather-affected rotor blades. The coating is also suitable for applications with antibacterial requirements or easy-to-clean requirements. The coating of metallic or ceramic or glassy surfaces is advantageous.

With attached 1 an embodiment of the invention will be explained in more detail. 1 shows an etched sol-gel coating on a substrate material.

One Test piece of an aluminum material, EN AW-Al 99.5, is by mechanical polishing for a sol-gel coating prepared and cleaned in an ultrasonic bath. In the dipping process becomes a sol-gel mixture of a 4% TEOT (Tetraethoxyorthotitanate / TEOS (Tetraethoxyorthosilicate) solution in the molar mixing ratio 75% / 25% at room temperature and dried at room temperature. The coating is annealed at 500 ° C. In the result This heat treatment is originally formed amorphous areas of the applied sol in partly crystalline areas around. At the same time, the degree of networking of the applied increases Sol-gel layer, d. H. the number of silanol bonds formed increases O-Si-O with further cleavage of OH groups. The coated test piece is etched in 10% hydrofluoric acid with panning for 5 min. Subsequently, the etching process under flowing Water stopped and the test piece for 20 min under rinsed in running water.

tested were the test pieces in an icing system, with their Help ice crystallization in flowing or flowing water Air can be realized. The experiments can be visual, be followed by microscope and by film recording. The core of this measurement setup is a compact icing system that the study of ice formation conditions in aquifer Facilities in very detailed form possible. The ice is in this chamber on the coated faces made of aluminum bodies (10 × 40 mm), the were mounted perpendicular to the flow channel. The sample temperature on the one hand by the temperature of the coolant Circulatory system is set and can also be through a separate, computer-controlled cooling system over Peltier elements are set.

tested the samples were taken from two points of view. Once will help a dynamic flow test (medium: distilled Water, flow velocity ≤ 1.5 m / s) the Temperature of the coated workpiece lowered (cooling rate 1 K / min) and the temperature is determined at which the Sample iced. That is, the moment is captured in located in the liquid or preferably at the interfaces from sample to medium flowing around first stable crystal nuclei can form. For the sake of comparability, all experiments were performed at the same cooling rate. With the invention coated and etched samples become subcooling rates of up to 5 Kelvin compared to uncoated test pieces reached.

To the Others were running tests under constant supercooling temperature carried out. Since the Eisbildungstemperatur always from depend on the subcooling rate / cooling rate will deliver these endurance tests under constant cooling other important information. Again, these tests were done by observing the samples at a constant temperature, and the time to failure was measured by freezing. The reference sample again was an uncoated aluminum sample used. The coating system described allows, compared with the uncoated substrate material, the aluminum alloy, a constant supercooling of 2 Kelvin.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• US 2003/0024726 A1 [0006]
• WO 2004078873 A1 [0007]
• - JP 2000044863A [0007]
• - US 6153304 A [0009]
• WO 2005/075112 A1 [0009]
• WO 2008/071752 A1 [0009]

Cited non-patent literature

• - VF Petrenko, IA Ryzhkin, Surface states of Charge Carriers and Electrical Properties of the Surface Layer of Ice, J. Phys. Chem. B 1997, 101, pp. 6285-6289 [0006]
• Laforte, Caroline, J.-L. Laforte, J.-C. Laforte; How a solid coating can reduce the adhesion of ice to a structure, The Tenth International Workshop on Atmospheric Icing of Structures, IWAIS 2002 [0008]

Claims (8)

Characterized in that the coating consists of etched nanoparticulate metal / silicon oxide layers in the surface of which fluorine compounds are incorporated. Coating according to claim 1, characterized that the coating has a layer thickness 400-800 nm and has a roughness of 200-2000 nm. Coating according to claim 1 or 2, characterized in that the coating consists of SiO ₂ , R-SiO _3/2 (R = alkyl radical or fluoroalkyl radical having 1-18 carbon atoms), Al ₂ O ₃ and / or TiO ₂ . Coating according to one of claims 1 to 3, characterized in that the coating consists of TiO ₂ / SiO ₂ mixed oxides containing 70-80 mol% of TiO ₂ . Process for producing a coating with ice-repellent and freezing point-lowering properties, characterized through the following steps: (1) Coating of a material surface by dipping, spraying, casting with a nanoparticulate oxide sol, (2) Dry and heat the layer above 400 ° C (3) etching the layer by a fluoride-containing aqueous or alcoholic Solution. Method according to claim 5, characterized in that that for the etching of the layer surface 0.1 to 10% aqueous hydrofluoric acid is used. A method according to claim 5 or 6, characterized in that for etching the layer surface, a 1-10% aqueous solution of NH ₄ F or NH ₄ HF ₂ (NH ₄ F × HF) is used. Use of coating with ice-repellent and Freezing point lowering properties according to claims 1-7 for Cooling systems and heat exchangers, for internal pipe linings in cooling technology, for the coating of Eisbreigeneratoren in cooling technology and for the coating of weather-influenced rotor wheels.