EP0648863B2 - Enamellable oxide layer - Google Patents

Abstract

Aluminium-containing substrate for enamel coatings, comprising, at least on the areas intended for enamel coating, a surface layer of aluminium or an aluminium alloy and, on this surface layer, an oxide layer which constitutes an isotropic barrier layer with a thickness of from 0.01 to 1 mu m and a dielectric constant of between 5 and 10. Adjacent to this barrier layer there may also be a porous oxide layer with a thickness of from 0.1 to 10 mu m. An enamel coating having a thickness of, for example, from 50 to 200 mu m is applied to the substrate. The enamelled substrates can be employed as exterior panelling on vehicles or exterior facing on buildings, the enamel-coated substrates being outstandingly protected against external influences.

Description

The invention relates to the use of an aluminum-containing substrate for enamel coatings containing at least on the areas intended for an enamel coating a surface layer of aluminum or one Aluminum alloy and on this surface layer an oxide layer.

Aluminum materials, such as foils, Strips or sheets are used, for example, for facade and vehicle construction used as cover plates. Such cover plates are usually anodized to to be resistant to environmental influences. In the Magazine "Aluminum" 54th year, 1978, number 8, pages 527 and 528, W. Grosskopf, "Overview of application and carrying out the enamelling of aluminum ", is the enamelling of pure aluminum strips up to 0.3 mm thick. It was found, that not every aluminum material can be enamelled, and especially magnesium-containing aluminum alloys are unsuitable for enamelling.

In the USSR patent 1 120 034 A a Process for enamelling aluminum alloys described, wherein the alloy is anodized and the anodized article with a pattern of an enamel powder coated, dried and the enamel baked is to a decorative surface resp. a decorative Get 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, describes that on the one hand the worst results from the application an enamel layer on an anodized aluminum would be achieved, on the other hand anodized aluminum again a much better bond and Show resistance to thermal shock. Until then was therefore the problem of a long time on one aluminum-containing substrate adhering enamel layer unsolved. There is always fear that Such a layer peels off soon after the order or flakes off.

On August 24, 1994 under number 0611 834 A1 published European patent application of the same Inventor with priority date from 02/18/1993 a document in accordance with Article 54 (3) EPC and describes an aluminum-containing substrate for enamel coatings, which on the side to be coated with email has a pore-containing oxide layer.

The object of the present invention is a substrate to make that available for the application with an enamel layer.

According to the invention the task is accomplished by Claim 1 solved. Advantageous embodiments of the use of the product are in claims 2 to 10 claims.

Aluminum-containing substrates are, for example, sheet-like Substrates made of aluminum or its alloys. Examples are foils, strips, sheets or Profiles. The substrates can also be any body Build with an aluminum surface or be aluminum-containing surface. Other substrates are composite materials that have at least one top layer made of aluminum or an aluminum alloy consist. Foils, tapes are preferred or sheets of aluminum or aluminum alloys applied. The isotropic barrier layer can overlap the whole area of aluminum or aluminum alloy extend or can at least at the points be provided on which the enamel coating to be applied.

The isotropic barrier layer provides a non-porous one Oxide layer and has a high temperature resistance as well as one versus aluminum or aluminum alloys increased chemical resistance. Enamel layers generally show good results on oxides Liability. Through time and temperature dependent diffusion processes can on the one hand be part of the Enamel layer and on the other hand also components of the aluminum-containing substrate, such as metallic Alloy components or impurities, get into the oxide layer and form a layer, which does not adhere well to the enamel layer Guaranteed aluminum-containing substrate. Such components (Detention inhibitors) that reduce adhesion and thus, for example, spalling problems of the Enamel layer can, for example Copper or magnesium. The diffusion of such adhesion inhibitors into the oxide layer, the high baking temperature because of, essentially during Firing process of the enamel coating.

With the aluminum-containing substrates according to the present Invention is due to their isotropic barrier layer prevents diffusion of adhesion inhibitors or at least diminished such that good adhesion one deposited on the substrate according to the invention Enamel layer is guaranteed.

The density of the isotropic barrier layer is advantageously between 2.5 and 3.8 g / cm ³ and preferably has a dielectric constant between 8.5 and 10.

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

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

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

Wrought aluminum alloys can, for example contain: up to 1.5% by weight silicon, up to 1.0% by weight Iron, up to 4.0% by weight copper, up to 1.5% by weight Manganese, up to 6.0% by weight magnesium, up to 7.0 % By weight zinc, up to 0.2% by weight titanium and up to 1.6 % By weight of other elements, balance aluminum. Especially substrates made of an aluminum alloy are preferred, containing 0.25 to 1.5 wt .-% silicon, up to 0.3% by weight iron, up to 0.25% by weight copper, 0.1 to 0.8% by weight of manganese, 2.7 - 5.0% by weight of magnesium, to 1% by weight zinc, 0.01 to 0.2% by weight titanium, up to 0.2 Wt% chromium, and up to 1.5 wt% other elements, Rest 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, AlMgSi-CuMg, AlMgSi and AlZnMg. Of these types are in turn particularly preferably aluminum casting alloys containing: Up to 11.0% by weight silicon, up to 1.0% by weight Iron, up to 5.2% by weight 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, balance aluminum.

The substrates can only use these aluminum or aluminum-containing surface layer and on top of it Surface layer, at least partially, according to the invention Oxide layer, or e.g. in the sense of a composite or a laminate layers containing aluminum or layers of others Materials arranged on the back of the surface layer be while the oxide layer according to the invention is arranged on the front. The aluminum or aluminum-containing surface layer must be analogous are on the surface so that the inventive Oxide layer on it appropriately resp. can be attached to it. The inventive The oxide layer then forms the. outside exposed layer for further coating with the enamel layer.

The manufacturing process is carried out such that the oxide layer at least on the enamel coating Areas by means of anodic oxidation (anodizing) in an electrolyte with a pH in the range between 5 and 7 in the temperature range from 30 to 80 ° C is formed, the anodic oxidation Surface layer made of aluminum or an aluminum alloy or at least that for the enamel coating intended area in an electrolyte is given, and between the surface layer and a second one located in the same electrolyte A voltage is applied to the electrode.

Can carry out the anodic oxidation the aluminum-containing substrate or at least for the enamel coating in a designated area Given electrolytes and as a positive electrode (anode) be switched. As a negative electrode (cathode) then another serves in the same electrolyte given electrode made of, for example, stainless steel Steel or lead. If the electric is an electrical Voltage is applied to the cathode Hydrogen gas, at the anode or in the electrolyte submerged aluminum-containing substrate oxygen gas. This oxygen reacts with the aluminum to alumina, which is on the surface layer forms an oxide layer.

The production process used in practice the oxide layer essentially runs such that the substrate and especially the one with enamel Coating surface made of aluminum or one Pretreated aluminum alloy the surface is degreased first, then rinsed and finally pickling, the pickling for example with a sodium hydroxide solution in one Concentration from 50 to 200 g / l at 40 to 60 ° during one to ten minutes can be done. Subsequently can be rinsed and cleaned with a Acid such as Nitric acid, especially a concentration from 25 to 35% by weight in the room temperature range typically 20 - 25 ° C during 20 - Neutralized for 60 s, rinsed again and if necessary be dried.

The properties of the oxide layer formed depend largely from the electrolysis conditions such as for example electrolyte composition, pH value, Electrolyte temperature, applied voltage and the Electrolysis current. When using an acidic Electrolyte forms an oxide layer, which is essentially a non-porous base or barrier layer and one contains porous outer layer. During the anodic Oxidation in acidic electrolytes forms on the A non-porous base or barrier layer and at the same time that during the anodic Oxidation formed oxide layer on the outside partly chemical due to field-induced redissolving dissolved again. This creates on the surface an oxide layer with fine pores, for example stand perpendicular to the surface and against the Surface are open. The thickness of the oxide layer reached their upper limit when there is growth and solution balance what of the electrolyte composition, depends on the current density and the temperature.

In the pH-neutral or approximately pH-neutral electrolyte, the oxide is not or only very slightly redissolved during the electrolysis process and no pores are formed. In the present text, solutions which have a pH in the range between 5 and 7 are referred to as pH-neutral or approximately pH-neutral electrolytes. Typical examples of such electrolytes are boric acid (H ₃ BO ₃ ) or aqueous solutions of ammonium salts with borates, phosphates, tartrates, citrates, vanadates or molybdate and mixtures thereof. In practice, for example, electrolytes such as aqueous solutions with 1% by weight NH ₄ H ₂ PO ₄ (9% phosphate), 10% by weight H ₃ BO ₃ (7% borate), 5% by weight NH ₄ molybdate (2.5% Mo oxide) or 2% by weight NH ₄ vanadate (2% vanadium oxide), with the information in parentheses the compounds typical of the individual electrolytes in atomic percentages which are in the oxide layer and in particular to be built into their outer surface.

The surfaces to be treated with the Electrolytes brought into contact and by means of direct current, Pulse current, alternating current or asymmetrical AC anodized. Under direct current are also practically similar currents, for example by full wave rectification of a single phase alternating current or by rectifying one Three-phase alternating current are generated, understood. As asymmetrical AC types can for example sinusoidal alternating current with a Voltage / time curve with unevenly high Amplitudes in the positive and negative part, more rectangular AC with a voltage / time curve with equally high amplitudes and unequal lengths of time of the positive and negative part, more rectangular AC with a voltage / time curve with unequal amplitudes in the positive and negative Area or rectangular alternating current with a Voltage / time curve with unequal amplitudes and unequal time shares of positive and negative Partly applied. Furthermore, sinusoidal AC curves with leading edge in positive and negative part and also other asymmetrical Alternating currents with interrupted current flow are used, e.g. with triangular alternating current.

The anodic oxidation in pH-neutral or approximately pH-neutral electrolytes is expediently carried out with a voltage up to 600 V, preferably up to 500 V, and a current density up to 120 A / m ² , preferably up to 100 A / m ² .

The anodic oxidation in pH-neutral or approximately pH-neutral electrolytes can be carried out, for example, by continuously increasing the applied voltage up to the maximum value such that the current density is kept constant at this level after an initially continuous increase up to the predetermined value. After reaching the maximum voltage, the current density then decreases due to the increasingly thick oxide layer and reaches a residual current density after a certain time. The method according to the invention is preferably carried out until the current density has fallen to a value between 1 and 10 A / m ² after the maximum voltage has been applied.

The thickness of the oxide layer obtained depends on the voltage and is in the range between 10 and 16 Å / V and especially between 11 and 15 Å / V. The Oxide layer can have a slight on its free surface Contain concentration of ions. This ion concentration is essentially due to the electrolyte is determined and is therefore on the outer surface area the oxide layer limited. The inventive The oxide layer is particularly poor in Magnesium and prevents during and after baking the enamel coating a further diffusion of magnesium from the surface layer.

After the anodizing process, the substrate or the treated surface of further treatments, such as. Rinsing or impregnation.

Such post-treatment without rinsing represents, for example, the impregnation of the oxide surface with a flux. Such fluxes can Compounds or ions contained in the enamel are very easily soluble and therefore, for example, one cause better enamel anchoring on the oxide surface. The oxide surfaces can be such compounds or ions, such as vanadium oxide, ammonium vanadate, Molybdenum oxide, ammonium molybdate, Ammonium borate, ammonium phosphate etc. already contained by the anodic oxidation. In this case their effect is brought about by the application of a corresponding Flux supported. You can also Flux increase the wettability of the oxide surface and / or lower the melting point of the enamel frit.

The substrate is according to the present invention for enamel coatings with a melting point of 480 ° C to the melting point or near the melting point of the substrate used. With will be close to the melting point for example temperatures between 20 or 10 ° C below the melting point. Appropriately becomes the product as a substrate for enamel coatings based on alkali-silico-titanates, if necessary with baking temperature-reducing additives, e.g. the compounds, such as oxides, lithium, barium, Antimons, cadmiums, bismuths or vanadiums applied. In a preferred embodiment relates the present invention 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 the 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 of sodium from 18 to 22% by weight is preferred 20 to 22% by weight of aluminum from 0.5 to 3.2% by weight, preferably 2.8 to 3.2% by weight of the lithium of 3.5 to 4.2% by weight, preferably 3.8 to 4.2% by weight, of the boron from 5 to 8% by weight, preferably 6.5 to 8% by weight, of the Zircons 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 of the Antimons from 0 to 2.6% by weight, the strontium from 0 to 1.5% by weight and the phosphorus from 0 to 2.5% by weight.

Enamel coatings, which are used as frit, are preferred applied to the substrate with additives and through Heat treatment or baking in an enamel coating a thickness of 50 to 200 microns, preferred from 50 to 120 µm and in particular 70 to 100 µm, be transferred. The enamel coating on your part can, for example, from a mixture of oxides in the specified proportions are generated. The oxides are usually fritted, i.e. as Mixture that was ground. This frit can in turn with processing aids such as Boric acid, sodium metasilicate, potassium hydroxide, titanium dioxide and pigments are added. Typical Example of a frit composition contains: 100 parts Frit, about 4 parts boric acid, one part with sodium metasilicate, a portion of potassium hydroxide, five to fifteen Parts of titanium dioxide and one to seven parts of pigment. According to the requirements for the color design Coloring pigments can be used in the enamelling be applied. The enamelling can, for example comprise a layer, one layer a frit composition on the substrate surface is applied and burned in one burning process, i.e. is 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 applied in multiple firing processes become. Other methods of application of enamel layers consists of applying two or further frit layers or frit compositions with just one burn. The frit can for example an average grain size of less than 74 µm and suitably less than 44 µm. The frit can be sprinkled, sprayed, dipped or slurries are applied. More options are electrostatic spraying or electrophoresis. In some cases, the frit, provided that it is Suspension aids such as water applied was dried. After drying can the coated substrate is placed in an oven, the combustion process being continuous or gradual can be done. Typical burning times are in the Range between 3 and 10 minutes, with burning times between 3 and 6 minutes are preferred. Typical Firing temperatures are between 480 and 560 ° C. All procedures can be gradual or continuous be performed.

The surface layer and the oxide layer arranged above constitutes a substrate which is particularly suitable for an enamel coating is suitable because the wetting by the components the enamel coating during the baking process is particularly pronounced and thereby the use of fries with lower Melting point, i.e. with a melting point of, for example up to 20 ° C below the usual range, is made possible.

The substrates according to the present invention with the Enamel coatings have an extremely smooth surface on. The substrate is through the enamel coating against mechanical, physical, chemical and actinic influences and, for example, against environmental influences largely protected. The surface is smooth, shiny and of great hardness. Because of the smooth Surface can, for example, dirt, dyes, solvent-based dyes or in a carrier medium Any dyes found do not penetrate pores and change or deface the appearance of the surface. The great hardness of the surface protects against Abrasion and other mechanical influences.

Existing substrates with an enamel coating are suitable, for example, in facade construction as facade panels for indoor and outdoor applications, as an outer layer on composite panels for facades or for the Interior, 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. Also the substrates coated with enamel are suitable according to the present invention for furniture in public Areas such as poster pillars, post boxes, vending machines and the like, e.g. through vandalism are exposed to an increased risk. The Enamel coatings on the inventive For example, substrates can be so smooth Have surface that just by weathering a strong self-cleaning of the email-coated Substrates.

Claims (10)

1. Use of an aluminium-containing substrate for enamel coatings containing a surface layer of aluminium or an aluminium alloy at least on the surfaces provided for an enamel coating, and a pore-free oxide layer which is an isotropic barrier layer of a thickness of 0.01 to 0.5 µm having a dielectric constant between 5 and 10, wherein the barrier layer is the externally exposed layer as the substrate for enamel coatings having a melting point from 480°C to the melting point of the substrate.
2. Use according to claim 1 as a substrate for enamel coatings which is applied to the substrate as a frit with addititives and is converted to an enamel coating of a thickness of 50 to 200 µm, preferably 50 to 120 µm, and in particular 70 to 100 µm, by heat treatment.
3. Use according to claim 1, wherein the thickness of the isotropic barrier layer is between 2.5 and 3.8 g/cm³.
4. Use according to claim 1, wherein the isotropic barrier layer has a dielectric constant between 8.5 and 10.
5. Use according to claim 1, wherein the surface layer is an aluminium wrought alloy.
6. Use according to claim 5, wherein the aluminium wrought alloy contains up to 1.5 wt.% silicon, up to 1.0 wt.% iron, up to 4.0 wt.% copper, up to 1.5 wt.% manganese, up to 6.0 wt. magnesium, up to 7.0 wt.% zinc, up to 0.2 wt.% titanium, and up to 1.6 wt.% of other elements, remainder aluminium.
7. Use according to claim 5, wherein the surface layer is an aluminium wrought 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 wt.% chromium and up to 1.5 wt.% of other elements, remainder aluminium.
8. Use according to claim 1, wherein the surface layer is an aluminium cast alloy.
9. Use according to claim 8, wherein the aluminium alloys contain up to 11.0 wt.% silicon, up to 1.0 wt.% iron, up to 5.2 wt.% copper, up to 0.5 wt.% manganese, up to 7.5 wt.% magnesium, up to 10 wt.% zinc, up to 0.3 wt.% titanium, up to 1.2 wt.% nickel, up to 0.03 wt.% lead, up to 0.03 wt.% tin and up to 0.05 wt.% of other elements, remainder aluminium.
10. Use according to claim 1, wherein the enamel coatings contain the oxides of silicon in quantities from 27 to 33 wt.%, preferably 30 wt.%, of potassium from 9 to 12 wt.%, preferably 9.5 to 11.5 wt.%, of titanium from 18 to 22 wt.%, preferably 20 to 22 wt.%, of sodium from 18 to 22 wt.%, preferably 20 to 22 wt.%, of aluminium from 0.5 to 3.2 wt.%, preferably 2.8 to 3.2 wt.%, of lithium from 3.5 to 4.2 wt.%, preferably 3.8 to 4.2 wt.%, of boron from 5 to 8 wt.%, preferably 6.5 to 8 wt.%, of zirconium from 0.05 to 3 wt.%, preferably from 2.3 to 3 wt.%, of zinc from 0.8 to 2.0 wt.%, preferably 0.8 to 1.5 wt.%, and optionally one or more of the oxides of magnesium from 1 to 1.5 wt.%, of cadmium from 0 to 5 wt.%, of antimony from 0 to 2.8 wt.%, of strontium from 0 to 1.5 wt.% and of phosphorus from 0 to 2.5 wt.%.