DE102009024320B4 - Coatings with ice-repellent and freezing point-lowering properties, process for their preparation and use - Google Patents

Abstract

Coating with ice-repellent and freezing-point-lowering properties, characterized in that the coating consists of etched nanoparticulate SiO 2, R-SiO 3/2, with R = alkyl radical or fluoroalkyl radical having 1-18 carbon atoms, Al 2 O 3 and / or TiO 2, in the surface of which fluorine compounds are incorporated.

Description

The invention relates to a permanent ice-repellent and freezing-point lowering finishing of material surfaces based on etched metal oxide coatings. The coating achieves an anti-adhesive behavior with respect to the surrounding media up to the temperature range of the liquid-solid phase transition.

The icing of technical equipment and devices z. As of aircraft wings, power plants, floodgates, technical means of transport in the maritime context, high voltage power lines is undesirable. Ice and hoarfrosting lead to a reduction in efficiency up to the inability of technical systems. Operators of outdoor facilities therefore have an increased demand for anti-adhesive coatings. Specifically in wind turbines rotor blades and towers must be protected against icing. With icing of the rotor blades, potentially heavy ice layers are formed, which can be released by the centrifugal force. Under unfavorable conditions such ice layers fly, projectiles equal, several hundred meters far. Wind turbines may therefore not be operated in the appropriate weather conditions, resulting in undesirable downtime of the plants result. The icing of power lines and telecommunications equipment can also pose a major economic problem. The necessary deicing is in any case costly and causes additional costs. Ice-, water- and dirt-repellent coatings can considerably improve the effectiveness of technical systems and equipment.

The development of ice-repellent coatings has a special significance for metallic surfaces. The coefficient of thermal expansion of ice is slightly above that of common metals, but far below the coefficient of thermal expansion of many polymers. If internal tensions in the ice can not be balanced due to high subcooling rates, this would lead to breakage with widely differing thermal expansion behavior for ice sheets, and the ice sheets could crumble while adhering to metals all the more.

Much of the research and development has so far focused on constructive measures to prevent the adhesion of ice and mechanisms for removing the ice. Only a small part of the work has so far been aimed at changing the surfaces of the materials so that over-freezing can be hindered or prevented.

Developmental results of ice-repellent coatings are increasingly becoming known, based on mechanisms for weakening intermolecular forces at the interface of surface and ice. As the causes of ice adhesion, electrostatic attractions, hydrogen bonds, and van der Waals bonds are discussed. Ice consists of polar water molecules that strongly interact with any type of surface whose dielectric polarization differs 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 /. It is shown elsewhere 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 /. Altogether, layer systems with hydrophobizing Properties reduce the adhesion of adhering ice and thus facilitate mechanical processes of ice removal. They can have a freeze-delay effect, but they can not prevent it.

Coatings with improved reflection properties are known. So teaches US 4,535,026 B1 Glass body such. B. windows or lenses to be provided with an anti-reflective coating, which is to reduce the light reflection. For this, the glass body is coated with SiO ₂ and etched with hydrofluoric acid (aqueous HF solution). This should reduce the light reflection. Subject of US 5 268 196 B1 are methods of making an antireflecting layer. The coating consists of SiO ₂ and a light metal oxide of the alkali and alkaline earth metals. This coating is treated with fluorine gas to replace at least a portion of the oxygen atoms of the light metal oxides with fluorine. However, these coatings do not affect freezing point or ice repellency.

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

According to the invention the object is achieved by a coating with ice-repellent and freezing point-lowering properties and by a process for their preparation and their use according to the features of claims 1, 4 and 7. Further embodiments include the features of the subordinate claims 2 and 3 as well as 5 and 6.

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

According to an advantageous embodiment of the invention, the coating consists 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. Through the etching process and its temporal variation, it is possible to set properties which have an anti-adhesive behavior with respect to the surrounding media up to the temperature range of the liquid phase. Promote solid phase transition of aqueous solutions. This results in a surface roughness in the order of Ra = 300-450 nm, accompanied by a surface 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 build-up wall shear stress is used to immediately dissolve any adhering ice crystal nuclei and tear away from the coated surface of the component by the flowing medium. 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.

The invention also includes items such. As components, components, workpieces or body with the coating of the invention.

According to the invention, the coatings having ice-repellent and freezing-point-lowering properties are prepared as follows:

(1) Coating 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

The applied thin film is first air-dried, is in the result amorphous and still contains organic residues. This is followed by a heat treatment. The annealing at temperatures above 400 ° C serves the purpose of crosslinking and partial crystallization of the applied film and the 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 bonded organic or inorganic fluorine-containing complexes are permanently introduced into the near-surface layer areas and on the other hand generates a defined surface roughness and surface structure. The duration of the etching process is matched to component size and Ätzlösungsvolumen. It serves the purposeful functionalization and adjustment of surface structuring in 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 evaluated by the contact angle measurements. The layers show hydrophobic properties with a contact angle of 85 ° -90 °. The objects coated in this way allow the ice layer to lift off more easily in the event of over-frosting.

The coatings according to the invention of base sol and anchored fluorine-containing layer constituents have a freeze-delay effect, since they achieve a reduction in the surface tension. The evenly introduced surface structuring leads to an influence on the flow conditions of the adjacent flowing medium (cooling water, air masses). This mechanism causes the significant freezing point depression. The surface structuring changes the flow resistance in the near-surface region so that a shearing of the ice crystallites can begin, which begin to form as heterogeneous crystallization nuclei 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.

A test piece of an aluminum material, EN AW-A1 99.5, is prepared by mechanical polishing for a sol-gel coating and cleaned in an ultrasonic bath. In the dipping method, a sol-gel mixture of a 4% TEOT (tetraethoxyorthotitanate / TEOS (tetraethoxyorthosilicate) solution in a molar mixing ratio of 75% / 25% is coated at room temperature and dried at room temperature The coating is tempered at 500 ° C. As a result At the same time, the degree of crosslinking of the applied sol-gel layer increases, ie the number of silanol bonds O-Si-O formed with further cleavage of OH groups increases The coated test piece is etched with stirring in 10% hydrofluoric acid for 5 minutes, then the etching process is stopped under running water and the test piece is rinsed for 20 minutes under running water.

The test pieces were tested in an icing system, with the help of which ice crystallization in flowing water or flowing air can be realized. The experiments can be followed visually, by microscope and by film recording. The core of this measurement set-up is a compact icing system that enables the study of ice formation conditions in water-bearing systems in very detailed form. The ice is formed in this chamber on the coated end faces of aluminum bodies (10 x 40 mm), which were mounted perpendicular to the flow channel. The sample temperature is adjusted on the one hand by the temperature of the coolant of the circulatory system and can also be adjusted by a separate, computer-controlled cooling system via Peltier elements.

The samples were tested under two aspects. Once, using a dynamic flow test (medium: distilled water, flow rate ≤ 1.5 m / s), the temperature of the coated workpiece is lowered (cooling rate 1 K / min) and the temperature at which the sample is frozen is determined. That is, the moment is detected in which first stable crystal nuclei can form in the liquid or preferably at the boundary surfaces of the sample to be flowed around the medium. For the sake of comparability, all experiments were carried out at the same cooling rate. With the inventively coated and etched samples supercooling rates of up to 5 Kelvin are achieved in comparison to uncoated test pieces.

On the other hand, long-term tests were carried out under a constant supercooling temperature. Since the ice formation temperature will always depend on the supercooling rate / cooling rate, these endurance tests under constant cooling provide further important information. These tests, in turn, were performed by observing the samples at a constant temperature and measuring the time to failure by freezing. The reference sample used again was an uncoated aluminum sample. The described coating system allows for a constant supercooling of 2 Kelvin compared to the uncoated substrate material, the aluminum alloy.

Claims (7)

Coating with ice-repellent and freezing-point-lowering properties, characterized in that the coating consists of etched nanoparticulate SiO ₂ , R-SiO _3/2 , with R = alkyl radical or fluoroalkyl radical having 1-18 carbon atoms, Al ₂ O ₃ and / or TiO ₂ , in whose surface fluorine compounds are involved. Coating according to claim 1, characterized in that the coating has a layer thickness of 400-800 nm, preferably 450-600 nm, a Roughness of 300-450 nm and a surface structuring of the surface of the order of 200-2000 nm. Coating according to one of claims 1 or 2, characterized in that the coating TiO ₂ / SiO ₂ mixed oxides containing 70-80 mol% TiO ₂ . A process for producing a coating with ice-repellent and freezing-point-lowering properties according to claim 1, characterized by the following steps: a. Coating of a material surface by dipping, spraying or casting with a nanoparticulate, SiO ₂ , R-SiO _3/2 with R = alkyl radical or fluoroalkyl radical having 1-18 carbon atoms, oxide sol containing Al ₂ O ₃ and / or TiO ₂ , b. Dry and heat the layer above 400 ° C c. Etching the layer through a fluoride-containing aqueous or alcoholic solution. A method according to claim 4, characterized in that 0.1 to 10% aqueous hydrofluoric acid is used to etch the layer surface. A method according to claim 4 or 5, 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 coatings with ice-repellent and freezing point-lowering properties according to claims 1-6 for cooling systems and heat exchangers, for interior pipe linings in cooling technology, for the coating of Eisbreigeneratoren in cooling technology and for the coating of weather-influenced rotor wheels.

Images