EP2327482A1 - Method for producing a structured surface - Google Patents

Abstract

The method for producing a structured surface on a carrier material (12), comprises coating the carrier material with a silicate-based sol-gel-lacquer, partially hardening the coating, structuring the partially-hardened coating through stamping by a matrix (16), and completely hardening the stamped coating (14). The matrix for producing the structured surface comprises a stamping structure (18) with three dimensional stamping elements (20) with dimensions in a range of 30 nm to 30 mu m. The partial hardening and complete hardening of the coating are continuously carried out at 140-160[deg] C. The method for producing a structured surface on a carrier material (12), comprises coating the carrier material with a silicate-based sol-gel-lacquer, partially hardening the coating, structuring the partially-hardened coating through stamping by a matrix (16), and completely hardening the stamped coating (14). The matrix for producing the structured surface comprises a stamping structure (18) with three dimensional stamping elements (20) with dimensions in a range of 30 nm to 30 mu m. The partial hardening and complete hardening of the coating are continuously carried out at 140-160[deg] C in a continuous furnace. The stamping is continuously carried out with a stamping roller as matrix or with a heating matrix or in a heating chamber. The partial hardening of the coating and the structuring are partially carried out in single process. The carrier material consists of aluminum or aluminum alloy. The sol-gel-lacquer is polysiloxane produced by acid-catalyzed hydrolysis and condensation of a mixture of a first solution containing alkoxy silane and a second solution containing a colloidal silicic acid. The first solution is a solution with a first and second alkoxy silane and a solvent mixed with the first solution. The first alkoxy silane is tetraethoxysilane and the second alkoxy silane is methyl triethoxysilane. The solvent of the first solution is a polar solvent such as alcohol or ethylacetate. The alcoholic solvent consists of glycol or glycol compound and is ethylene glycol or buthoxyethanol.

Description

The invention relates to a method for producing a structured surface on a carrier material.

The properties of a surface, such. As hydrophilic or hydrophobic behavior or optical effect can be influenced by the topography of the surface. Many observed in nature, wetting phenomena such. For example, the self-cleaning effect on the leaves of the lotus plant is due to the unique micro- and nanostructures on the surfaces. The leaves of these plants always stay clean because water droplets easily roll off the ultrahydrophobic leaf surface, collecting and removing dirt particles. The self-cleaning effect of water droplets is therefore often referred to as the "lotus effect".

Investigations on surfaces of different materials with surface structures in the micro- and nanoscale have shown that the topography or the roughness of the surfaces compared to the chemical composition generally has a stronger influence on the wettability.

In the article " Facile preparation of superhydrophobic coatings by sol-gel processes "by R. Taurino et al., Journal of Colloids and Interface Science 325 (2008) 149-156 For example, the wettability of multilayer glass surfaces coated with TiO ₂ / ZrO ₂ / SiO ₂ sol-gel paints is investigated. The inorganic-containing layers provide, with each additional layer, a fractal surface with high surface roughness parameter values. Wetting tests with water show that with increasing roughness values, the contact angle becomes larger and thus the hydrophobic character of the surface becomes more pronounced.

In the article " Wetting phenomena on micro-grooved aluminum surfaces and modeling of the critical droplet size "by AD Sommers et al., In Journal of Colloid and Interface Science 328 (2008) 402-411 investigates the behavior of water droplets on aluminum surfaces with parallel microgrooves produced by reactive ion etching, with a width and depth of 5 to 30 μm. Water droplets applied to the grooved aluminum surface with a microinjection syringe show significantly higher contact angles to a smooth surface without grooves, and the droplet volume observed at slip start is reduced by more than 50%. The hydrophobicity is achieved primarily by the anisotropic surface topography.

The item " Large-Area Fabrication of a Nanostructure-Induced Hydrophobic Surface from a Hydrophilic Polymer "by Chaowei Guo et al., In ChemPhys Chem2004, 5, 750-753 describes an embossing process for the production of surfaces with a regular array of polycarbonate columns in the nanometer range. The embossing plate or the embossing roller is made of aluminum with anodized oxidized, porous surface with a regular array of hexagonal pores with a uniform diameter of z. B. about 30 nm (nanometers) and an intermediate distance of about 120 nm. The embossing of the polycarbonate takes place in a temperature range of 150-170 ° C at a pressure of about 3 x 10 ⁵ Pa. The resulting in pressing the polycarbonate mass into the pores of the stamping columns with substantially the pore diameter corresponding column diameter. Water wettability tests indicate that the contact angle is 85 ° for a smooth surface polycarbonate film and over 140 ° for the nanoscale structure surfaces.

Out EP-B-0 772 514 For example, self-cleaning surfaces of objects having a raised and recessed surface structure are known. The elevations are in the range of 5-200 microns, the wells in the range of 5-100 microns. At least the elevations are made of hydrophobic Polymer materials, such as. B. PTFE. The surface structures can be produced by heated embossing pieces or rolls.

It is well known to use tapes in glazing materials, e.g. Pure aluminum, high purity aluminum or AlMg alloys based on aluminum with a purity of 99,5% and greater, such as e.g. 99.8%, which produce diffuse or directed light reflection depending on the application, and to coat these with a so-called sol-gel varnish. Suitable sol-gel coatings form a transparent protective layer, which protects the underlying reflective surface from mechanical or chemical influences and thereby ensures a long life of the reflector at high reflection values. Particularly suitable sol-gel coatings are also weather-resistant and are therefore well suited for outdoor use. The transparent protective layer is in particular a clear, colorless, transparent protective layer.

The WO 2001/86327 describes such a weatherproof and corrosion-resistant reflector of a reflector body made of aluminum or an aluminum alloy with a reflective surface, wherein the reflector has an outer, final transparent protective layer of a sol-gel lacquer with a thickness of more than 1 micron. The sol-gel lacquer consists of a polysiloxane, which is prepared from an alcoholic silane solution and an aqueous colloidal silica solution.

The EP-A-1 791 001 discloses a reflector with an outermost, final transparent protective layer of a sol-gel lacquer of a polysiloxane, wherein the polysiloxane is prepared by acid-catalyzed hydrolysis and condensation of a mixture of a solution A and an aqueous solution B, wherein the solution A is a solution with a first and second alkoxysilane and a solvent which is miscible with the solution A, and the first alkoxysilane is a tetraethoxysilane (TEOS) and the second alkoxysilane is represented by the formula X _n Si (OC ₂ H ₅ ) _4-n , where X is a organic Rest and n is a number from 0 to 3. The paint system is characterized by the fact that, on the one hand, it contains as few harmful components as possible and liberates it during its processing and, on the other hand, has a flash point of> 21 ° C. during each processing step.

Solar reflectors are exposed to the weather and thus normally laden with a dust and dirt particles atmosphere. To maintain a high reflectivity, it is necessary to maintain the reflectors and to clean them from time to time. It would be desirable to have a simple cleaning option with which the maintenance intervals for reflectors can be extended.

The invention has for its object to provide a method of the type mentioned above, which allows the production of a structured surface in the micrometer to Submikrometerbereich in a simple manner. The structured surface should also be environmentally friendly, scratch-resistant and corrosion-resistant. Another object of the invention is to provide a simple cleaning facility for metal surfaces, in particular of aluminum or an aluminum alloy, as used for example in solar reflectors.

• Beschichten des Trägermaterials mit einem silikatbasierten Sol-Gel-Lack,
• Teilaushärten der Beschichtung,
• Strukturieren der teilausgehärteten Beschichtung durch Prägen mittels einer Matrize, und
• Vollaushärten der geprägten Beschichtung.

For solving the problem according to the invention, a method characterized by the following steps to be carried out successively:

• Coating the support material with a silicate-based sol-gel lacquer,
• Partial hardening of the coating,
• Structuring the partially cured coating by embossing using a die, and
• Fully hardening the embossed coating.

In order to produce the structured surface, the die preferably has an embossing structure with three-dimensional embossing elements with dimensions in one Range between about 30 nm (nanometers) and 30 microns (microns) on. Such embossed structures can be produced by methods known from the prior art.

The partial curing and full curing of the coating is preferably carried out in a temperature range of about 130 ° C to 200 ° C, in particular 140 ° C to 160 ° C. The curing preferably takes place continuously in a continuous furnace.

The embossing step can be performed with a preferably heated stamping plate. However, in particular for a continuous production of coil-wound strip material, it is advantageous to carry out the embossing with a preferably heated embossing roll.

The embossing step can also be carried out in a heated chamber.

Partial curing of the coating and patterning may be performed at least in part in a single operation.

To a particular extent, the process according to the invention is suitable for the treatment of support materials made of metal, in particular of aluminum or an aluminum alloy.

The textured surface shows a pronounced hydrophobic behavior compared to a smooth surface and consequently a self-cleaning effect. With the inventive method thus surfaces of support materials, such as. As the reflective layer of solar reflectors, with one equipped with "easy-to-clean" effect.

A preferred sol-gel lacquer is a polysiloxane prepared by acid-catalyzed hydrolysis and condensation of a mixture of an alkoxysilane-containing solution A and a solution B containing colloidal silica. wherein the solution A is in particular a solution with a first and second alkoxysilane and a solvent miscible with the solution A, and the first alkoxysilane is a tetraethoxysilane (TEOS) and the second alkoxysilane by the formula X _n Si (OC ₂ H ₅ ) _{4 in} which X denotes an organic radical and n is a number from 0 to 3. The second alkoxysilane is preferably a methyltriethoxysilane (MTEOS).

The solvent of the solution A is preferably a polar solvent, such as alcohol or ethyl acetate, wherein the alcoholic solvent particularly contains or consists of a glycol or a glycol compound, and is in particular an ethylene glycol or buthoxyethanol.

The coating which is applied directly to the carrier material is a sol-gel lacquer which has developed from a sol-gel system applied directly to the aluminum or the aluminum alloy. Gels are dimensionally stable, easily deformable, fluid-rich disperse systems consisting of a solid, irregular, three-dimensional network and a liquid. A sol-gel system is to be understood as a sol-gel lacquer prepared by the sol-gel technology which, after product-appropriate application and curing, forms a hardened protective layer firmly bonded to the substrate, in this case a carrier material. The layer is preferably a transparent hardening sol-gel lacquer, which reveals the color tone of the substrate. By transparent hardening sol-gel lacquer is to be understood in particular a clear, colorless, transparent layer. The layer applied to the cleaned surface of the substrate is preferably a sol-gel lacquer, in particular a sol-gel lacquer of a polysiloxane and advantageously a sol-gel lacquer of an alcoholic silane solution, in particular an alkoxysilane solution and a aqueous polysiloxane solution prepared polysiloxane. Polysiloxane is the term for polymers of crosslinked siloxanes. The polysiloxane is in particular by a condensation reaction between hydrolyzed and crosslinkable silanes, especially alkoxysilanes, and colloidal silicic acid.

The condensation reaction between hydrolyzed silanes, in particular alkoxysilanes, with one another and hydrolyzed silanes, in particular alkoxysilanes, and colloidal silica leads to the formation of an inorganic network of polysiloxanes. At the same time, organic groups, in particular alkyl groups or simple alkyl groups, are incorporated into the inorganic network via carbon bonds. However, the organic groups, or the alkyl groups, do not directly participate in the polymerization or crosslinking of the siloxanes, i. they do not serve to form an organic polymer system but merely for functionalization. The function is that the organic groups, in particular the alkyl groups, are attached to the outer sides of the polysiloxanes during the sol-gel process and thereby form a water-repellent layer which imparts a pronounced hydrophobic property to the sol-gel lacquer.

The sol-gel process described leads, as mentioned, by targeted hydrolysis and condensation of alkoxides of silicon and silica to a sol-gel lacquer of an inorganic network with incorporated alkyl groups. The polysiloxanes obtained thereby are more likely to be assigned to the inorganic polymers.

In the preparation of a preferred embodiment of a sol-gel lacquer as a protective layer, it is expedient to use two base solutions A and B.

The solution A is an alcoholic solution of one or more different alkoxysilanes, wherein the alkoxysilanes are present in an anhydrous medium in non-hydrolyzed form. The solvent is conveniently an alcohol, such as methyl, ethyl, propyl or butyl alcohol and preferably isopropyl alcohol.

The alkoxysilanes are described by the general formula X _n Si (OR) _4-n in which "R" is a simple alkyl, preferably from the group comprising methyl, ethyl, propyl and butyl. "X" is suitably also an alkyl, preferably from the group comprising methyl, ethyl, propyl and butyl. Suitable alkoxysilanes are, for example, tetramethoxysilanes (TMOS) and preferably tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMOS) and further alkoxysilanes.

In a particularly preferred embodiment, the solution A is prepared from tetraethoxysilane (TEOS) and / or methyltrimethoxysilane (MTMOS) with a methyl, ethyl or propyl alcohol and in particular with an isopropyl alcohol as solvent. The solution A may e.g. 25-35 wt .-% (wt .-%), in particular 30 wt .-%, TEOS and 15 - 25 wt .-%, in particular 20 wt .-%, containing MTMOS, both dissolved in 40 - 60 wt .-% , in particular 50 wt .-%, isopropyl alcohol.

Solution B contains colloidal silica dissolved in water. In an expedient embodiment, the solution B is adjusted by means of acid, preferably by means of nitric acid (HNO ₃ ) to a pH of between 2.0 and 4.0, preferably between 2.5 and 3.0 and in particular of 2.7.

The silicic acid used is expediently a silicic acid stabilized in an acid medium, the pH of the silicic acid advantageously being 2 -4. The silicic acid is advantageously as low in alkali as possible. The alkali content (eg Na ₂ O) of the silica is preferably below 0.04% by weight.

The solution B contains, for example, 70-80% by weight, in particular 75% by weight, of water as solvent and 20-30% by weight, in particular 25% by weight, of colloidal silica. The solution B is expediently by means of nitric acid (HNO ₃ ) to a pH between 2.0 to 3.5, preferably between 2.5 to 3.0 and in particular of 2.7.

The combining and mixing of the two base solutions A and B in the presence of nitric acid leads to a hydrolysis reaction between the water contained in solution B and the alkoxysilanes contained in solution A.

Hydrolysis reaction: Si (OR) _n + nH 2 O → Si (OH) _n + nR (OH)

At the same time, a condensation reaction takes place in which, with the elimination of water, a siloxane bond (Si-O-Si) is built up from two Si-OH groups in each case. Progressive polymerization results in a network of polysiloxanes to which alkyl groups are attached. The new mixed solution is in a gel state.

The two solutions A and B are preferably mixed in a weight ratio of 7: 3 parts.

The sol-gel lacquer is expediently applied in gel form to the support material or to the corresponding surface, or deposited and then dried or cured. The drying process consists in expelling the water and alcohols remaining in the sol-gel paint, which hardens the sol-gel paint and creates a corrosion-resistant and weather-resistant protective layer on the substrate surface.

The coating is carried out, for example by application, centrifuging or spraying, expediently in a continuous process which is suitable for the treatment of strip-form carrier material. Particularly preferred coating methods are spraying, spraying, dipping or dip coating.

In particular, sol-gel systems, which are available, for example, under the trade name CERAPAINT from Akzo Nobel, are suitable for the formation of the corrosion protection layer.

Corrosion protection layers (sol-gel coatings), which are created by applying a sol-gel system, ie the coating of the actual support material with a sol-gel system, require for their preparation a curing or drying process in which the Sol-gel system is converted into the resistant sol-gel varnish.

The coated with the sol-gel lacquer carrier material is expediently by means of radiation, such as UV radiation, electron radiation, laser radiation, or by thermal radiation, such as infrared radiation (infrared), or by convection heating or a combination of the aforementioned drying or curing, dried or hardened.

Convection heating can conveniently be effected by exposure to heated gases, such as air, nitrogen, noble gases or mixtures thereof. The sol-gel lacquer layer is preferably dried or hardened in a continuous oven.

Fig. 1

die zur Durchführung der Prägeversuche eingesetzte Anordnung;

Fig. 2

die Draufsicht auf die Prägeseite einer Matrize mit einer ersten Prägestruktur;

Fig. 3

den Querschnitt durch die Prägestruktur von Fig. 2 in vergrösserter Darstellung;

Fig. 4

die Draufsicht auf die Prägeseite einer Matrize mit einer zweiten Prägestruktur;

Fig. 5

den Querschnitt durch die Prägestruktur von Fig. 4 in vergrösserter Darstellung.

Further advantages, features and details of the invention will become apparent from the following description of stamping tests and with reference to the drawing; this shows schematically in

Fig. 1

the arrangement used to carry out the embossing tests;

Fig. 2

the top view of the embossing side of a die having a first embossing structure;

Fig. 3

the cross section through the embossed structure of Fig. 2 in an enlarged view;

Fig. 4

the top view of the embossing side of a die with a second embossing structure;

Fig. 5

the cross section through the embossed structure of Fig. 4 in an enlarged view.

An in Fig. 1 illustrated arrangement for producing a structured surface on a substrate shows an embossing chamber 10 with adjustable internal temperature. In the embossing chamber 10 are provided with a coating 14 of a sol-gel lacquer carrier material 12 and a die die 16 with an embossed structure 18. For structuring the surface of the provided with the sol-gel lacquer coating 14 substrate 12 is the Matrizenstempel 16 moves in the arrow direction and pressed with a sufficiently large contact pressure against the coated substrate 12, so that the embossed structure 18 is transferred to the sol-gel coating 14. As with any embossing process, the surface topographies of the die stamp 16 and embossed thereon substrate 12 and the sol-gel paint coating 14 are complementary to each other, ie peaks or elevations on the embossing surface of Matrizenstempels 16 become valleys or depressions in the surface the carrier material 12 and vice versa.

• h = 4,0 µm b = 5,5 µm c = 5,1 µm d = 5,0 µm

The in the FIGS. 2 and 3 Embossed structure 18 shown consists of a regular juxtaposition of stamping elements 20 in the form of truncated pyramids. In the Fig. 3 indicated dimensions of the embossed structure 20 are as follows:

• h = 4.0 μm b = 5.5 μm c = 5.1 μm d = 5.0 μm

• h = 4,0 µm b = 1,5 µm e = 10 µm

The in the 4 and 5 Embossed structure 18 shown consists of a regular juxtaposition of embossing elements 20 in the form of vertically projecting, parallel strips of rectangular cross-section. In the Fig. 5 indicated dimensions of the embossed structure 20 are as follows:

• h = 4.0 μm b = 1.5 μm e = 10 μm

In the in Fig. 1 In the arrangement shown, embossing tests were carried out on aluminum sheets of purity Al 99.85 provided with a sol-gel lacquer coating.

• Lösung A:
  • 200 g Isopropylalkohol
  • 150 g Tetraethoxysilan (TEOS)
  • 458 g Methyltrimethoxysilan (MTES)
• Lösung B:
  • 335 g 26 %-ige kolloidale Kieselsäure enthaltend 248 g Wasser und 87 g SiO₂

The sol-gel lacquer used for the coatings was obtained by mixing the following two solutions A and B in a ratio of 7: 3:

• Solution A:
  • 200 g isopropyl alcohol
  • 150 g of tetraethoxysilane (TEOS)
  • 458 g of methyltrimethoxysilane (MTES)
• Solution B:
  • 335 g of 26% colloidal silica containing 248 g of water and 87 g of SiO ₂

The pH of solution B was adjusted to about 2.5 with addition of nitric acid (HNO ₃ ).

The mixed solution was sprayed onto the aluminum sheet samples.

The sol-gel lacquer coatings were cured to varying degrees at a temperature of 150 ° C. for three different curing times of 3, 5 and 30 minutes corresponding to a curing state "soft", "medium" and "hard". Coating prior to embossing a partial cure 150 ° C were subjected to 30 minutes, could no longer emboss under the selected pressure conditions.

The results of the embossing tests on coatings subjected to the embossing of a partial curing at 150 ° C for 3 or 5 minutes and subsequently with the in the FIGS. 2 and 3 or in the 4 and 5 embossed structures shown are summarized in Table 1. Table 1: Results of embossing tests embossed pattern Conditions during partial curing before embossing temp. / Duration Conditions during embossing temperature / duration / pressure Evaluation of the quality of the embossed structure after full cure at 150 ° C / 30 min. Line pattern according to Fig. 2/3 150 ° / 5 min. 150 ° / 10 min. / 2.5 kN / cm ² Structure clearly visible Line pattern according to Fig. 4/5 150 ° / 3 min. 25 ° / 10 min. / 2.5 kN / cm ² no visible structure Truncated pyramid pattern according to Fig. 4/5 150 ° / 3 min. 150 ° / 10 min. / 0.63 kN / cm ² Structure clearly visible

Claims (13)

Process for the production of a structured surface on a carrier material (12), characterized by the steps Coating the carrier material (12) with a silicate-based sol-gel lacquer, Partial hardening of the coating (14), - structuring the partially cured coating (14) by embossing by means of a die (16), and Fully hardening the embossed coating (14). A method according to claim 1, characterized in that the matrix (16) for producing the structured surface has an embossing structure (18) with three-dimensional embossing elements (20) with dimensions in a range between about 30 nm (nanometers) and 30 microns (microns). A method according to claim 1 or 2, characterized in that the partial curing and full curing of the coating (14) in a temperature range of about 130 ° C to 200 ° C, in particular 140 ° C to 160 ° C, is performed. A method according to claim 3, characterized in that the partial hardening and full curing of the coating (14) is carried out continuously in a continuous furnace. Method according to one of claims 1 to 4, characterized in that the embossing step is carried out with a stamping plate as a die (16). Method according to one of claims 1 to 4, characterized in that the embossing step is carried out continuously with an embossing roll as a die (16). A method according to claim 5 or 6, characterized in that the embossing step is carried out with a heated die (16). Method according to one of claims 5 to 7, characterized in that the embossing step in a heated chamber (10) is performed. Method according to one of claims 1 to 8, characterized in that the partial hardening of the coating (14) and the structuring is carried out at least partially in a single operation. Method according to one of claims 1 to 9, characterized in that the carrier material (12) consists of metal, in particular of aluminum or an aluminum alloy. Process according to one of Claims 1 to 10, characterized in that the sol-gel lacquer is a polysiloxane prepared by acid-catalyzed hydrolysis and condensation of a mixture of an alkoxysilane-containing solution A and a solution B containing colloidal silica, the solution A being in particular a Solution with a first and second alkoxysilane and a solvent which is miscible with the solution A, and the first alkoxysilane is a tetraethoxysilane (TEOS) and the second alkoxysilane is represented by the formula X _n Si (OC ₂ H ₅ ) _4-n , where X describes an organic radical and n is a number from 0 to 3. A method according to claim 11, characterized in that the second alkoxysilane is a methyltriethoxysilane (MTEOS). A method according to claim 11, characterized in that the solvent of the solution A is a polar solvent, such as alcohol or ethyl acetate, in particular, that the alcoholic solvent contains or consists of a glycol or a glycol compound, and in particular is an ethylene glycol or buthoxyethanol.

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