EP3332050B1 - Abrasion resistant sol gel coating on aluminium and a method for producing such a coating - Google Patents

Description

The present invention relates to an abrasion-resistant and adherent sol-gel coating on aluminum surfaces and to a process for its production. Moreover, the present invention relates to a process for the surface treatment of aluminum in order to improve the adhesion of subsequently applied sol-gel coatings.

PROBLEM:

Sol-gel layers impart high corrosion resistance and temperature resistance to metal surfaces. In addition, they improve the cleaning ability and protect the surfaces from fingerprints and dirt. Therefore, sol-gel coatings offer significant economic and qualitative advantages.

Sol-gel coatings on surfaces of stainless steel, steel, copper and copper alloys generally have a sufficiently high adhesive strength that allows deformation of coated surfaces, for example by bending or edges without peeling of the coating and prevents detachment by rubbing or wiping.

In contrast, sol-gel coatings on surfaces of aluminum and aluminum alloys show a significantly lower adhesive strength. Sol-gel coatings applied to aluminum surfaces directly and without any special pre-treatment of the surfaces, even with slight deformation of the surfaces by bending, even with large bending radii, cracked and detached. They can be partially rubbed off with a thumb or fingernail.

The low adhesive strength is due to the properties of the natural oxide layers, which are formed by the influence of oxygen from the environment, and protect the aluminum from corrosion.

STATE OF THE ART:

Surfaces on aluminum and aluminum alloys to which serviceable and adherent sol-gel coatings are to be applied are pretreated by anodizing in the prior art prior to the sol-gel coating (Anodizing) subjected. The natural oxide layer is replaced by a modified oxide layer with suitable suitability as a primer for sol-gel coatings. In this case, the workpieces to be treated are immersed in a special electrolyte and switched using DC as an anode in the circuit, wherein on the surfaces of a layer of aluminum oxide is applied (eg DE 10 2012 019 969 A1 ). These oxide layers are suitable as primer for the subsequent sol-gel coating.

This electrochemical pretreatment by means of anodizing is complex and costly. It is limited in use by the shape and size of the workpieces to be treated and requires special equipment similar to galvanic systems with all the associated requirements relating to environmental protection and wastewater treatment.

Methods of providing a primer for subsequent sol-gel coating on an aluminum surface are variously mentioned in the art. However, the aluminum surface is usually freed from naturally existing or originally existing oxide layer on a regular basis. For example, the WO 01/59179 A1 a process for the surface treatment of, inter alia, aluminum in a two-stage chemical process associated with mechanical abrasion. Initially, the surface is treated with sulfuric acid in such a way that a fresh, ie oxide-free, metal surface is produced. That is, following the treatment of the surface with an acid, fresh oxide layers are formed on the metal in an oxidative process. The US 5,356,492 A describes a non-toxic process for the treatment of aluminum surfaces in which the corrosion resistance can be improved by subsequent treatment with an alkaline solution containing molybdate, nitrite and metasilicate ions. The layer thus obtained can then be further improved in terms of its corrosion resistance by applying a sol-gel layer. This document can not be specified whether For example, treatment with, for example, hydrogen peroxide in a multi-step process can improve the adhesion of the sol-gel coating to the aluminum surface, especially if the sol-gel coating is applied immediately after treatment with the oxidizing agent.

The EP 1 457 267 A1 describes a method for producing formed aluminum sheet metal parts having a decorative surface. In the method described as coil coating method, the steps are in succession: providing a strip of aluminum, optionally continuous degreasing of the strip, optionally electrochemical, chemical or mechanical glazing of the possibly defatted strip, continuous pretreatment of the possibly degreased and / or shined strip Producing a pretreatment layer suitable as a primer for a lacquer layer. Also in this process, an optional oxide layer on the metal surface is completely removed before the process is carried out (compare page 3, line 14). In the described oxidative process anodic oxidation of the aluminum is regularly carried out. This process ensures the provision of a porous, anodically produced oxide layer. Aqueous oxidants wherein the aqueous oxidizer has a pH in the range 4 to 8 are not suggested in this document.

The US 2004/0177898 A1 describes a method of treating metallic surfaces, such as aluminum surfaces. In this method, an oxidizing agent is used together with a sulfate providing agent to obtain a passivated surface, which is to consist essentially of manganese, oxygen, sulfur and carbon. Whether the surfaces obtained in the process described are suitable as primer for a subsequent sol-gel coating, can not be found in this document. According to the embodiments of this document (see in particular Example 1) but it is provided that in carrying out the process, first a pretreatment takes place, in which the optionally present oxides are completely removed.

The present invention is based on the finding that aluminum surfaces, which already have a naturally formed oxide layer, can be pretreated with an aqueous, oxidizing solution such that the adhesion of sol-gel coatings on pretreated aluminum surfaces compared to non-pretreated surfaces is significantly improved.

The invention is explained in more detail in the claims.

The invention relates to a method for producing a sol-gel-coated aluminum surface, which is obtained by pretreating the surface already provided with an original or naturally formed oxide layer in a first step with an oxidizing aqueous solution drying and subsequent application and curing of a sol-gel layer.

Under the original or natural oxide layer in the case of the present invention, such an oxide layer is understood, which usually forms directly on aluminum surfaces, once their surface has been treated by mechanical or other means so that a bare aluminum layer is obtained. On such a bare aluminum layer, a so-called passive layer is formed, which consists essentially of alumina. Such a natural aluminum oxide layer is regularly insufficient as a primer (see also the examples in the experimental part of this application).

According to the invention, abrasion-resistant and adherent sol-gel coatings are obtained on aluminum surfaces, which usually have a layer thickness of 0.5 to 5.0 μm. These coatings can also be colored. For this purpose, inorganic color pigments are very particularly suitable.

Insofar as aluminum surfaces are used in the present application, they also address aluminum alloys whose chemical behavior is comparable to that of an aluminum surface.

The aqueous oxidizing agents contain conventional oxidizing agents such as peroxides, persulfates, perchlorates, perborates in an appropriate concentration. In addition, oxidizing agents such as potassium persulfate or hydrogen peroxide are also suitable.

Other ingredients are not necessary in the aqueous oxidizing agent, but also not generally mandatory exclude.

The aqueous oxidizing agents have a pH in the range of 3 to 8, or 4 to 8. The concentration of, for example, hydrogen peroxide is usually in a range of 1 to 30% by weight. Solid oxidizing agents are usually added in an amount of 5 to 50 g / l (for example, 5 to 50 g / l potassium persulfate).

The treatment of the surfaces is usually carried out at room temperature by dipping, spraying or wiping for a period of at least 2 minutes, preferably from 5 to 10 minutes.

However, it has been shown that a longer duration of treatment is not necessarily disadvantageous.

For example, following the treatment of the aluminum surface with the aqueous oxidizing agent, the surfaces are rinsed with water and then dried.

Following drying of the surfaces, the sol-gel coating is applied.

Sol-gel coatings are usually obtained from two reaction components, which are mixed shortly before processing in a fixed ratio to each other. This mixture is last added as a third component, a dilution, usually an alcohol. Dilution sets the concentration of the reaction mixture and the viscosity of the final batch.

It will be understood by those skilled in the art that the sol-gel is first applied in the form of a liquid sol having colloidal particles suspended therein, which subsequently converts to a gel and, after thermal curing, finally forms a solid, hard topcoat. So if the "application of the sol-gel coating" or the "thermal curing of the sol-gel coating" is the expert, in which state the sol-gel system is.

The sol-gel is preferably a silica sol based on silanes which are dissolved in solvents, wherein the silica sol preferably also contains one or more further sol-forming elements, preferably one or more elements from the group consisting of Al, Ti, Zr, Mg, Ca and Zn, these elements replacing the Si atoms in the colloidal structures. Preferred sol-gel coatings / sol-gel coatings are in EP2145980 described. Reference is made in particular to the in EP2145980 described sol-gel coatings and the method for their use.

The starting compounds for forming the preferred sols and finally the sol-gel coating are preferably hydrolyzable silanes of the formula SiR ₄ , where the 4 radicals R 2-4 comprise hydrolyzable radicals OR 'and 0-2 comprise nonhydrolyzable radicals R " Thus, silanes can also be represented as Si (OR ') _4-n R " _n where n = 0,1 or 2. If additional sol-forming elements as just described are used, appropriate compounds should be selected according to the valences of the elements as starting compounds, such as AlR ₃ , etc.

The hydrolyzable radicals OR 'are hydroxy, alkoxy and / or cycloalkoxy radicals. Suitable examples thereof include, for example, hydroxy, methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, i-butoxy, t-butoxy, pentoxy, hexoxy, cyclopentyloxy, cyclohexyloxy, in particular Ethoxy, n-propoxy and isopropoxy are preferred. The hydrolyzable radicals OR 'may be identical or different from one another.

The non-hydrolyzable radicals R ", if present, are alkyl and / or cycloalkyl radicals, suitable examples of which include, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, i-butyl, t-butyl, pentyl, hexyl, cyclopentyl, cyclohexyl radicals, with particular preference being given to methyl, ethyl, n-propyl and isopropyl radicals The nonhydrolyzable radicals R "may likewise be identical or different from one another.

The starting compounds of the preferred sols can consist of a single type of silane, but often they are mixtures of several silanes (and optionally additional sol-forming starting compounds of other elements). include. It is preferred that at least one of the components of the starting compounds is a silane of the formula Si (OR ') _4-n R " _n where n = 0, ie Si (OR') ₄ For example, a preferred sol-gel lacquer may be the starting materials TEOS (tetraethoxyorthosilane) and MTES (methyltriethoxysilane) and / or DMDES (dimethyldiethoxysilane).

In addition, it is of course also possible to use other additives customary in the field of sol-gel systems, for example additional network formers, such as acryloxypropyltrimethoxysilane or methacryloxypropyltrimethoxysilane, which can provide further organic crosslinks, especially if a not inconsiderable proportion of the starting compounds are so-called network-converting compounds of the formula Si (OR ') _4-n R " _n where n = 1 or 2.

In the sol, the starting compounds are partially hydrolyzed to the corresponding hydroxy compounds (such as orthosilicic acid, trihydroxyalkylsilane, etc.), which can be promoted by the addition of a catalyst such as acid. Due to the high tendency for condensation of these hydroxy compounds, these can now condense with elimination of water to form smaller siloxane networks. The sol already contains colloidal particles containing siloxane bonds. Siloxane bonds are bonds of the form ≡Si-O-Si≡, where "≡" symbolizes any three independent bonds with other elements, in particular OH, OR 'and R ", thus forming a three-dimensional crosslinked structure in the colloidal particles Where OR 'and R "have the same meaning as above.

The sol-gel coating usually has a stoving temperature of less than 300 ° C, preferably from 200 ° C to 250 ° C, on.

The sol-gel coating can also be colored, for example by adding inorganic color pigments. By the degree of coverage with inorganic color pigments, i. by the proportion by weight of the inorganic color pigments in the sol-gel, color intensity and depth can be adjusted.

The viscosity of the sol-gel varnish can be adjusted by a person skilled in the art. It is known that the sol, with a correspondingly high dilution in its solvent, is sufficiently low-viscosity to be applied by spraying, spraying, rolling or brushing.

Suitable solvents for the sol are water and especially alcohols such as methanol, ethanol, n-propanol or isopropanol, with ethanol and isopropanol being preferred because of their physical properties and the low toxicity of their vapors.

The sol-gel may include inorganic color pigments, e.g. SICOCER® Black 10901, SICOCER® Blue 2502, or SICOCER® Red 2355 from BASF.

The sol-gel coating is preferably applied by spraying or rolling, spraying or brushing are also possible. Preferably, however, it is done by spraying, since this allows precise control of the amount applied per unit area.

After coating, the surfaces can be dried until the solvent has evaporated. The dried surfaces are then thermally cured. Preferably, there is no discoloration of the coating during curing. The thermal curing in step (iii) is preferably carried out at a temperature of less than 300 ° C, preferably in a range of 200 ° C to 300 ° C. Preferably, the curing takes place for a period of about 20 to 60, preferably 30 minutes at temperatures in the range of 160 ° C to 280 ° C, preferably 200 ° C to 250 ° C in air.

The glass-ceramic coating preferably has a thickness of 0.5-5.0 μm, preferably 1.0-5.0 μm, or 0.5-3.0 μm, and most preferably 1.0-3.0 μm. The glass-ceramic coating preferably has a uniform thickness with fluctuations of preferably less than 10% of the layer thickness.

According to the invention, an aluminum surface with an abrasion-resistant and adherent sol-gel coating is achieved which is distinguished by improved abrasion resistance and adhesion. A particular advantage of the invention lies in the fact that an abrasion-resistant and adherent surface is obtained in a particularly simple, cost-effective and environmentally friendly manner. A further advantage is that, compared to the prior art in the method according to the invention, restrictions with regard to the shape and the size of the workpieces to be treated are hardly to be considered.

EXAMPLES:

In the following examples, various AlMg 4.5 bare aluminum surfaces were first degreased, rinsed, and dried. A natural oxide layer had previously formed on these sheets before the surface was further treated or provided with a sol-gel coating.

Example 1:

Two AlMg 4.5 aluminum sheets with a 240 grit ground surface were degreased, rinsed, dried and treated as follows:
A sol-gel coating was applied by spraying onto a sheet without further pretreatment and then baked in air at 240 ° C. for a period of 30 minutes. After cooling, the sheet was bent 90 ° by a radius of 10d. The sol-gel layer cracked and broke off in the area of the bend.

The second sheet was weighed into a 10% aqueous solution for 10 minutes. Hydrogen peroxide immersed, then rinsed with water and dried. Subsequently, an identical sol-gel coating was applied by spraying and baked at a temperature of 240 ° C for 30 minutes in air. After cooling, the sheet was bent by 180 ° with a radius of 2d. The coating remained free of cracks in the area of the bend, homogeneous and firmly adhering.

Example 2:

Two AlMg 4.5 bare-surface aluminum sheets were degreased, rinsed, dried and treated as follows:
A sol-gel coating was applied by spraying onto a sheet without further pretreatment and then baked in air at 240 ° C. for a period of 30 minutes. After cooling, the sheet was bent at a radius of 10d by an angle of 90 °. The sol-gel coating cracked in the area of the bend and came off.

The second sheet was weighed for 10 minutes with a 10% aqueous solution. sprayed, then rinsed and dried. Subsequently, an identical sol-gel coating was applied by spraying and baked at a temperature of 240 ° C in air. After cooling, the sheet was bent at a radius of 2d by an angle of 180 °. The coating in the area of the bend remained free of cracks, was homogeneous and firmly adhering.

Example 3:

Two aluminum sheets of AlMg 4.5 with a smooth surface were degreased, rinsed and dried.

A sol-gel coating was applied by spraying onto a sheet without further pretreatment and then baked in air at 240 ° C. for a period of 30 minutes. After cooling, the sheet was bent at a radius of 10d by an angle of 90 °. The sol-gel coating cracked in the area of the bend and came off.

The second plate was sprayed with an aqueous solution containing 40 g / l potassium persulfate at pH 4 for 5 minutes, then rinsed and dried. Subsequently, an identical sol-gel coating was applied and baked at a temperature of 240 ° C in air. After cooling, the sheet was bent at a radius of 2d by an angle of 180 °. The coating in the area of the bend remained free of cracks, was homogeneous and firmly adhering.

Claims (5)

1. A method for producing an abrasion-resistant aluminum surface, characterized in that

   a) an aluminum surface with a natural oxide layer is first provided with a base which is obtained by treating the aluminum surface with an aqueous oxidizing agent, selected from the group consisting of peroxides, persulfates, perchlorates and perborates, with a pH value of the aqueous oxidizing agent in a range of 3 to 8

   b) the surface pretreated in such a way is optionally rinsed and then dried if necessary, and immediately subsequently

   c) an application and a hardening of a sol-gel coat takes place.
2. The method according to claim 1,
   wherein the treatment with the aqueous oxidizing agent is carried out at ambient temperature for a period of at least 2 minutes.
3. The method according to claim 2,
   wherein the ambient temperature is 15°C to 25°C.
4. The method according to any one of the preceding claims,
   wherein the treatment with the aqueous oxidizing agent is carried out over a period of 5 to 10 minutes.
5. The method according to any one of the preceding claims,
   characterized in that the sol-gel coating on the aluminum surface has a coat thickness of 0.5 to 5.0 µm.