EP0611834A1 - Aluminium based substrate for enamelling - Google Patents

Abstract

Aluminium-containing substrate for enamel coatings, containing an oxide layer at least on the sides of the enamel coatings, the substrate containing a surface layer of aluminium or an aluminium alloy and carrying on this layer a porous oxide layer more than 0.2 mu m thick, with a pore diameter of from 0.03 to 0.15 mu m and a pore depth/diameter ratio of from 2 to 60. An enamel coating is applied to the substrate in a thickness of, for example, from 50 to 200 mu m. The enamelled substrates can be used as exterior trim or cladding on vehicles or buildings, the enamel-coated substrates having outstanding protection against external influences.

Description

The invention relates to an aluminum-containing substrate for enamel coatings, comprising an oxide layer at least on the sides of the enamel coating, a method for producing the oxide layer and the use of the aluminum-containing substrates.

Aluminum materials such as foils, tapes or sheets are used as cover sheets for facade and vehicle construction, for example. Such cover plates are usually anodized in order to be resistant to environmental influences. In the magazine "Aluminum" 54th year, 1978, volume 8, pages 527 and 528, W. Grosskopf, "Overview of the application and implementation of the enamelling of aluminum" describes the enamelling of pure aluminum strips up to 0.3 mm thick. It has been found that not every aluminum material can be enamelled, and aluminum alloys containing magnesium in particular are unsuitable for enamelling.

In the USSR patent 1 120 034 A a method for enamelling aluminum alloys is described, wherein the alloy is anodized and the anodized article is coated with a pattern of an enamel powder, dried and the enamel is baked to give a decorative surface or. to get a decorative pattern. In "The Surface Treatment and Finishing of Aluminum and its Alloys", Finishing Publication Ltd., Teddington, Middlesex, England, 5th edition, volume 2, 1987, page 1162, it is described that, on the one hand, the worst results are obtained by using an enamel layer anodized aluminum would be achieved, on the other hand anodized aluminum would in turn show a much better bond and resistance to thermal shock. Thus far, the problem of an enamel layer adhering to an aluminum-containing substrate for a long time had not been solved. It is always to be feared that such a layer will peel off or flake off soon after the application.

The object of the present invention is to avoid these disadvantages and to provide a substrate that is particularly suitable for the application of an enamel layer and a method for Production of the substrate and to propose a use for the substrate.

According to the invention, the object was achieved in that the substrate contains a surface layer made of aluminum or an aluminum alloy and there is a pore-containing oxide layer on this layer, the oxide layer having a thickness of more than 0.2 μm, the pore diameter being 0.03 to 0 , 15 microns and the ratio of pore depth to pore diameter is from 2 to 60.

Aluminum-containing substrates are, for example, sheet-like substrates made of aluminum or its alloys. Examples are foils, strips, sheets or profiles. However, the substrates can also be bodies of any shape with an aluminum surface or an aluminum-containing surface. Other substrates are composite materials that consist of at least one cover layer made of aluminum or an aluminum alloy. Foils, strips or sheets made of aluminum or aluminum alloys are preferably used. The oxide layer can extend over the entire surface of the aluminum or the aluminum alloy or can at least be provided at the points at which the enamel coating is to be applied.

The oxide layer contains pores. In particular, pores are provided which are open towards the surface. The pore distribution over the surface is advantageously uniform. The aluminum-containing substrate advantageously has an oxide layer with a thickness of 0.2 to 2 μm. The aluminum-containing substrate advantageously has an oxide layer with a pore diameter of 0.04 to 0.1 μm.

Substrates with a surface layer made of pure aluminum, essentially containing aluminum and the commercially available impurities or made of aluminum alloys, can be used as the aluminum-containing substrate.

Aluminum substrates can, for example, be an aluminum with a purity of 98.5% by weight and higher, preferably 99.0% by weight and higher and in particular 99.5% by weight and higher, and the rest of commercially available impurities contain.

Wrought aluminum alloys are preferred for the substrates. These alloys include e.g. the types AlMg, AlMgSi, AlCuMg, AlZnMg and AlZnMgCu.

Wrought aluminum alloys can contain, for example: up to 1.5% by weight of silicon, up to 1.0% by weight of iron, up to 6.0% by weight of copper, up to 1.5% by weight of manganese, up to 6.0% by weight of magnesium, up to 7.0% by weight of zinc, up to 0.2% by weight of titanium and up to 1.6% by weight of other elements, the rest being aluminum. Substrates made of an aluminum alloy are particularly preferred, containing 0.25 to 1.5% by weight of silicon, up to 0.3% by weight of iron, up to 0.25% by weight of copper, 0.1 to 0, 8% by weight manganese, 2.7-5.0% by weight magnesium, up to 1% by weight zinc, 0.01 to 0.2% by weight titanium, up to 0.2% by weight Chromium, and up to 1.5% by weight of other elements, balance aluminum.

Examples from the practice of substrates are aluminum alloys AlMg3, AlMg3Si, AlMg5, AlMg5Si and AlMg10.

Aluminum casting alloys are also preferred for the substrates. These alloys include e.g. the types: AlSi, AlSiMg, AlSiCuMg, AlMgSiCuMg, AlMgSi and AlZnMg. Of these types, in particular aluminum casting alloys are particularly preferred containing: up to 11.0% by weight of silicon, up to 1.0% by weight of iron, up to 5.2% by weight of copper, up to 0.5% by weight. % Manganese, up to 7.5% by weight magnesium, up to 10% by weight zinc, up to 0.3% by weight titanium, up to 1.2% by weight nickel, up to 0, 03% by weight of lead, up to 0.03% by weight of tin and up to 0.05% by weight of other elements, the rest being aluminum.

The substrates can only have this aluminum or aluminum-containing surface layer and, at least partially, the oxide layer above this surface layer, or further aluminum-containing layers or layers of other materials can be arranged on one side of the surface layer, for example in the sense of a composite or a laminate, while the oxide layer is on the other side can be arranged. The aluminum or aluminum-containing surface layer must be located on the surface, so that the oxide layer is properly attached to it resp. can be attached to it. The oxide layer then forms the layer now exposed on the outside for further coating with the enamel layer.

The present invention also relates to a method for producing the aluminum or aluminum-containing substrate for enamel coatings. According to the invention, the method is carried out in such a way that an anodizing process with an acidic electrolyte is carried out on the substrate, at least on the sides of the enamel coatings provided, at temperatures of 30 to 80 ° C. and a voltage of 30 to 120 volts for 1 to 60 seconds.

The anodizing process can be carried out, for example, in such a way that the surface of the substrate to be anodized is brought into contact with the acidic electrolyte. The acidic electrolyte can contain, for example, a strong acid or a mixture of strong acids. Typical examples of such acids are sulfuric acid (H₂SO₄), or phosphoric acid (H₃PO₄). Other acids that can be used include oxalic acid or sulfosalicylic acid. Mixtures of the acids mentioned can also be used. In practice, for example, sulfuric acid is used in amounts of 40 to 200 g / l and preferably 50 to 160 g / l (sulfuric acid based on 100% acid). Phosphoric acid can also be used as an electrolyte in an amount of 60 to 100 g / l and in particular 85 g / l, the amount of acid being based on 100% pure acid. Another preferred electrolyte is sulfuric acid in a mixture with oxalic acid, in particular an amount of 150 to 200 g / l sulfuric acid being mixed with, for example, 30 to 50 g / l oxalic acid. Another electrolyte contains, for example, 130 to 170 g / l sulfosalicylic acid mixed with 6 to 10 g / l sulfuric acid. The surfaces to be treated are brought into contact with the electrolyte and carried out with direct current, pulse current, alternating current or asymmetrical alternating current. Direct current is also understood to mean practically identical currents, for example those generated by full-wave rectification of a single-phase alternating current or by rectification of a three-phase alternating current. As asymmetrical types of alternating current, for example, sinusoidal alternating current with a voltage / time curve with unequal high amplitudes in the positive and negative part, rectangular alternating current with a voltage / time curve with equally high amplitudes and unevenly long time portions of the positive and negative part, rectangular alternating current with a voltage / time curve with unevenly high amplitudes in the positive and negative range, or rectangular alternating current with a Voltage / time curve with unequal amplitudes and unequal time portions of the positive and negative part can be applied. In addition, sinusoidal alternating current curves with phase gating in the positive and negative part and also other asymmetrical alternating currents with interrupted current flow can be used, such as with triangular alternating current.

In practice, the anodizing process is carried out at electrolyte temperatures of 40 to 60 ° C.

Particularly good results are achieved when the anodizing process is carried out at a voltage of 30 to 90 volts. It is advantageous to carry out the anodizing process for 5 to 30 seconds. The process of producing the oxide layer used in practice essentially takes place in such a way that the substrate and in particular the surface to be coated with enamel made of aluminum or aluminum alloy is subjected to a pretreatment, the surface first being degreased, then rinsed and finally pickled, wherein the pickling can be carried out for example with a sodium hydroxide solution in a concentration of 50 to 200 g / l at 40 to 60 ° for one to ten minutes. The surface can then be rinsed and cleaned with an acid such as Nitric acid, in particular a concentration of 25 to 35% by weight in the room temperature range of typically 20-25 ° C., is neutralized for 20-60 seconds, rinsed again and optionally dried.

The optionally pretreated substrate surface, respectively. the substrate can be anodically switched in the electrolytic solution, whereupon an electrical current passes between the metal acting as an anode and a cathode likewise immersed in the solution. After the anodizing process, the substrate or the treated surface can be subjected to further treatments, such as rinsing or impregnation, such as with a flux.

The present invention also relates to the use of the product according to the invention as a substrate for enamel coatings. In particular, the substrate according to the present invention can be used for enamel coatings with a melting point of 480 ° C. up to or near the melting point of the substrate. Temperatures between 20 or 10 ° C. below the melting point are described as being near the melting point. The product is expediently used as a substrate for enamel coatings based on alkali-silico-titanates, optionally with additives which lower the baking temperature, e.g. the compounds such as oxides, lithium, barium, antimony, cadmium, bismuth or vanadium are used. In a preferred embodiment, the present invention relates to the use of the product as a substrate for enamel coatings from a frit, containing the oxides of silicon in amounts of 27 to 33% by weight, preferably 30% by weight, of potassium in 9 to 12% by weight. %, preferably 9.5 to 11.5% by weight, of the titanium from 18 to 22% by weight, preferably 20 to 22% by weight, of the sodium from 18 to 22% by weight, preferably 20 to 22% by weight, of aluminum from 0.5 to 3.2% by weight, preferably 2.8 to 3.2% by weight, of lithium from 3.5 to 4.2% by weight, preferred 3.8 to 4.2% by weight, boron from 5 to 8% by weight, preferably 6.5 to 8% by weight, zircon from 0.05 to 3% by weight, preferably from 2 , 3 to 3% by weight of the zinc from 0.8 to 2.0% by weight, preferably 0.8 to 1.5% by weight of the magnesium from 1 to 1.5% by weight of the Cadmium from 0 to 5% by weight, the antimony from 0 to 2.8% by weight, the strontium from 0 to 1.5% by weight and the phosphorus from 0 to 2.5% by weight.

Preference is given to enamel coatings which are applied to the substrate as frits with additives and converted into an enamel coating with a thickness of 50 to 200 μm, preferably 50 to 120 μm and in particular 70 to 100 μm, by heat treatment or baking. The enamel coating in turn can be produced, for example, from a mixture of oxides in the specified proportions. The oxides are usually present as a frit, ie as a mixture that has been ground. Processing fritters such as boric acid, sodium metasilicate, potassium hydroxide, titanium dioxide and pigments can in turn be added to this frit. Typical example of a frit composition contains: 100 parts of frit, about 4 parts of boric acid, one part with sodium metasilicate, one part of potassium hydroxide, five to fifteen parts of titanium dioxide and one to seven parts of pigment. Coloring pigments can be used in accordance with the requirements for the color design of the enamelling. The enamelling can comprise, for example, a layer, a layer of a frit composition being applied to the substrate surface and baked in one firing process, ie being transferred into the enamel coating. Further methods are also within the scope of the invention, according to which two layers in two firing processes, three layers in three firing processes, respectively. multiple layers can be applied in multiple firing processes. Other methods of applying enamel layers consist of applying two or more frit layers or frit compositions with only one firing process. The frit can, for example, have an average grain size of less than 74 μm and expediently less than 44 μm. The frit can be applied by sprinkling, spraying, dipping or slurrying. Other options are electrostatic spraying or electrophoresis. If necessary, the frit, if applied with a suspension aid such as water, must be dried. After drying, the coated substrate can be placed in an oven, and the firing process can be carried out continuously or in stages. Typical burning times are in the range between 3 and 10 minutes, with burning times between 3 and 6 minutes being preferred. Typical firing temperatures are between 480 and 560 ° C. All procedures can be carried out step by step or continuously.

The surface layer according to the invention and the oxide layer arranged above it represent a substrate which is particularly suitable for an enamel coating, since the wetting due to the constituents of the enamel coating is particularly pronounced during the stoving process and therefore the use of frits with a lower melting point, i.e. with a Melting point of up to 20 ° C below the usual range, for example. The oxide layer according to the invention is particularly low in magnesium and copper and prevents further diffusion of magnesium from the surface layer during and after the enamel coating has been baked. A negative influence copper is switched off because the oxide layer is poor in copper due to the redissolving during anodization.

The substrates according to the present invention with the enamel coatings have an extremely smooth surface. Thanks to the enamel coating, the substrate is largely protected against mechanical, physical, chemical and actinic influences and, for example, against environmental influences. The surface is smooth, shiny and extremely hard. Due to the smooth surface, for example, dirt, dyes, solvent-based dyes or dyes in a carrier medium cannot penetrate into pores and change or deface the appearance of the surface. The high hardness of the surface protects against abrasion and other mechanical influences.

Existing substrates with an enamel coating are suitable, for example, in building construction as facade panels for indoor and outdoor applications, as an outer layer on composite panels for facades or for interior construction, as cladding panels or body parts for vehicles such as railroad cars, buses and other road and rail vehicles and for applications in Areas of corrosive atmosphere. The substrates coated with enamel according to the present invention are also suitable for furnishing in public areas such as poster columns, letter boxes, vending machine boxes and the like, which e.g. are at increased risk from vandalism. The enamel coatings on the substrates according to the invention can, for example, have such a smooth surface that only the weathering results in a strong self-cleaning of the enamel-coated substrates.

All parts or percentages are by weight unless otherwise stated.

Examples

Pretreatment:

• a) Entfetten,
• b) spülen,
• c) beizen in einer Natronlauge, enthaltend 100 g/l NAOH, bei 50 °C während 3 Minuten,
• d) spülen,
• e) neutralisieren in 30 Gew.-% wässriger HNO₃, bei 20 °C während 30 Sekunden,
• f) spülen,
• g) trocknen.

The substrates described below are pretreated as follows:

• a) degreasing,
• b) rinse,
• c) pickling in a sodium hydroxide solution containing 100 g / l NAOH at 50 ° C. for 3 minutes,
• d) rinse,
• e) neutralize in 30 wt .-% aqueous HNO₃, at 20 ° C for 30 seconds,
• f) rinse,
• g) dry.

The frits used are based on alkali silicate titanate with a composition: % By weight Frit 1 Frit 2 SiO₂ 27-33 30th K₂O 9.5 - 11.5 10th TiO₂ 20-22 18th Na₂O 20-22 18th Al₂O₃ 2.8 - 3.2 0.5 Li₂O 3.8 - 4.2 3.5 B₂O₃ 6.5 - 8 5 ZrO₂ 2,3 - 3 0.05 MgO 1 - 1.5 - ZnO 0.8 - 1.5 2nd CdO₂ - 5 Sb₂O₅ - 2.8 SrO - 1.5 P₂O₅ - 2.5 Baking temperature: 500 - 520 ° C 540 - 560 ° C

The frits are ground with aids, so that a mixture of approximately the following composition results: Frit Share 100 Boric acid 4th Na metasilicate 1 KoH 1 TiO₂ 5 - 15 pigment 1 - 7

Example 1, comparison:

A substrate made of an aluminum-containing alloy AlMg₃, for example a sheet with a thickness of 2 mm, is prepared according to the pretreatment described above. A low-melting enamel frit according to composition 1 mentioned above is baked in one layer and one baking process at 500 ° C. for 10 minutes. The result is an enamel-coated substrate, but the enamel coating shows no adhesion and peels off. Underneath, the substrate made of aluminum alloy has a gray surface.

Example 2:

A substrate made of an aluminum alloy AlMg₃ is prepared and anodized according to the pretreatment steps a) to f) above. The electrolyte contains 50 g / l H₂SO₄ and the anodization is carried out at 55 to 60 ° C with a current density of 15 A / dm² at 30 volts for 12 seconds. After the anodization was complete, a layer thickness of 9000 Å (0.9 µm) and a pore diameter of 310 Å (0.031 µm) were measured. The anodized substrate, which now carries an oxide layer, is enamelled with a low-melting enamel frit of the above composition 1 in one layer and a firing process at 500 ° C. and for 10 minutes. The resulting product has perfect adhesion and a perfect appearance of the enamel layer on the substrate. After forming the sheet and an impact resistance test in accordance with DEZ leaflet F6.2, no enamel flaking can be found.

Example 3:

The experiment according to Example 2 is repeated, but the substrate is an AlMg₁SiCu alloy. The process is carried out in accordance with Example 2 and the equally good results in Example 2 are obtained accordingly.

Example 4:

An aluminum alloy-containing substrate AlFe1.5Mn is divided into two samples, a sample a) undergoes the same pretreatment as in comparative experiment 1 and a sample b) undergoes the same treatment as sample 2. The sample a) which has not been anodized and the anodized sample b ) are provided with enamelling, using a low-melting frit according to composition 1. An enamel layer is applied and burned in at 490 ° C for 8 minutes. The comparison sample a) results in a rough surface of the enamel coating and a matt aspect, while the sample b) according to the invention has a glossy surface with high smoothness.

Example 5:

A substrate in the form of an AlMg₃-aluminum alloy is subjected to the pretreatment as described above steps a) to f) and then anodized. The electrolyte contains 160 g / l H₂SO₄ and 40 g / l oxalic acid. The temperature during the anodization is set to 40 ° C, the current density is 10 A / dm² at a voltage of 30 volts. Anodization takes place over 27 seconds. The substrate has an oxide layer thickness of 14,000 Å (1.4 µm) and a pore diameter of 420 Å (0.042 µm).

The same material is exposed to another electrolyte during anodization, this time containing 150 g / l sulfosalicylic acid and 8 g / l H₂SO₄. The temperature during the anodization is 40 ° C, the current density is 5 A / dm² and the voltage is 41 volts. The anodizing time is 18 seconds and an oxide layer results in a thickness of 4500 Å (0.45 µm) and a pore diameter of 500 Å (0.05 µm).

With the same substrate, a third variant of anodization is carried out, an electrolyte containing 85 g / l H₃PO₄ is applied and anodized at 60 ° anodizing temperature, at a current density of 15 A / dm² and 88 volt voltage for 6 seconds. The result is an oxide layer with a thickness of 3800 Å (0.38 µm) with a pore diameter of 1000 Å (0.1 µm).

The enamelling according to Example 1 is repeated with all three samples. All samples show a good appearance and show perfect adhesion of the enamel layer on the substrate. In all cases, the layer thickness of the enamelling is 70 - 90 µm.

Claims (16)

Aluminum-containing substrate for enamel coatings, containing an oxide layer at least on the sides of the enamel coatings,
characterized in that
the substrate contains a surface layer made of aluminum or an aluminum alloy and on this layer there is a pore-containing oxide layer, the oxide layer having a thickness of more than 0.2 μm, the pore diameter being 0.03 to 0.15 μm and the ratio of pore depth to pore diameter is from 2 to 60. Aluminum-containing substrate according to claim 1, characterized in that the oxide layer has a thickness of 0.2 to 2 µm. Aluminum-containing substrate according to claim 1, characterized in that the oxide layer has a pore diameter of 0.04 to 0.1 µm. Aluminum-containing substrate according to claim 1, characterized in that the surface layer is a wrought aluminum alloy. Aluminum-containing substrate according to claim 4, characterized in that the wrought aluminum alloy up to 1.5 wt .-% silicon, up to 1.0 wt .-% iron, up to 6.0 wt .-% copper, up to 1.5 % By weight of manganese, up to 6.0% by weight of magnesium, up to 7.0% by weight of zinc, up to 0.2% by weight of titanium and up to 1.6% by weight of other elements , Balance aluminum, contains. Aluminum-containing substrate according to claim 4, characterized in that the surface layer is a wrought aluminum alloy containing 0.25 to 1.5 wt .-% silicon, up to 0.3 wt .-% iron, up to 0.25 wt .-% Copper, 0.1 to 0.8 wt% manganese, 2.7 to 5.0 wt% magnesium, up to 1.0 wt% zinc, 0.01 to 0.2 wt% Titanium, up to 0.2% by weight of chromium and up to 1.5% by weight of other elements, the rest being aluminum. Aluminum-containing substrate according to claim 1, characterized in that the surface layer is a cast aluminum alloy. Aluminum-containing substrate according to claim 7, characterized in that the cast aluminum alloys up to 11.0 wt .-% silicon, up to 1.0 wt .-% iron, up to 5.2 wt .-% copper, up to 0.5 % By weight of manganese, up to 7.5% by weight of magnesium, up to 10% by weight of zinc, up to 0.3% by weight of titanium, up to 1.2% by weight of nickel, up to 0.03% by weight of lead, up to 0.03% by weight of tin and up to 0.05% by weight of other elements, the rest being aluminum. A method for producing an aluminum-containing substrate for enamel coatings according to claim 1, characterized in that at least on the areas of the enamel coatings an anodizing process in an acidic electrolyte, at temperatures of 30 to 80 ° C and a voltage of 30 to 120 volts for 1 to 60 seconds is carried out. A process for producing an aluminum-containing substrate for enamel coatings according to claim 9, characterized in that the anodizing process is carried out at electrolyte temperatures of 40 to 60 ° C. A process for producing an aluminum-containing substrate for enamel coatings according to claim 9, characterized in that the anodizing process is carried out at a voltage of 30 to 90 volts. A method for producing an aluminum-containing substrate for enamel coatings according to claim 9, characterized in that the anodizing process is carried out for 5 to 30 seconds. Use of the product according to claim 1 as a substrate for enamel coatings. Use of the product according to claim 13 as a substrate for enamel coatings with a melting point of 480 ° C to the melting point of the substrate. Use of the product according to claim 13 as a substrate for enamel coatings, containing the oxides of silicon in amounts of 27 to 33% by weight, preferably 30% by weight, of potassium from 9 to 12% by weight, preferably 9.5 to 11.5% by weight of the titanium from 18 to 22% by weight, preferably 20 to 22% by weight, the sodium from 18 to 22% by weight, preferably 20 to 22% by weight, of the aluminum from 0.5 to 3.2% by weight, preferably 2.8 to 3.2% by weight, of the lithium from 3.5 to 4.2% by weight, preferably 3.8 to 4.2% by weight .-%, of boron from 5 to 8 wt .-%, preferably 6.5 to 8 wt .-%, of zircon from 0.05 to 3 wt .-%, preferably from 2.3 to 3 wt .-% , the zinc from 0.8 to 2.0% by weight, preferably 0.8 to 1.5% by weight, and optionally one or more of the oxides of magnesium from 1 to 1.5% by weight, the Cadmium from 0 to 5% by weight, the antimony from 0 to 2.8% by weight, the strontium from 0 to 1.5% by weight and the phosphorus from 0 to 2.5% by weight. Use of the product according to claim 13 as a substrate for enamel coatings, which is applied to the substrate as a frit with additives and is converted by heat treatment into an enamel coating with a thickness of 50 to 200 µm, preferably 50 to 120 µm, and in particular 70 to 100 µm.