EP0106389A1 - Process for treating aluminium surfaces - Google Patents

Abstract

A conversion coating is formed on an aluminium surface by applying and drying onto the surface an aqueous composition having a pH of not more than 3.5 and containing fluoride, phosphate and zirconium and/or titanium and which is free of chromium, organic film-forming polymers and tannin.

Description

The invention relates to a method for applying conversion coatings on aluminum surfaces by means of solutions which contain titanium and / or zirconium, fluoride and phosphate ions, but are free from nitrate, nitrite and chromium ions and from organic film-forming polymers and tannin.

For chemical surface treatment of metals, for example as preparation for the application of paints, adhesives and plastics, processes are known in which the metal surface is cleaned in the first stage, rinsed with water in the second stage and finally with an aqueous one in the third stage , chemical conversion coatings forming solution and the liquid film is dried. As a result, a thin, non-metallic coating is formed on the metal, which can decisively improve the surface quality if the composition of the treatment liquid and the reaction conditions are selected accordingly. For example, Coatings made of lacquers, adhesives and plastics, if necessary in the form of foils, are characterized by much greater adhesion and considerably increased corrosion protection when applied to metal pretreated in this way.

Such methods work e.g. with an aqueous solution containing hexavalent chromium, trivalent chromium, alkali ions and silicon dioxide in certain proportions and coatings for electrical insulation, for corrosion protection and as a primer for paints and the like. (DE-AS 17 69 582).

Coatings can also be produced on metal surfaces with the aid of coating compositions which contain a compound of hexavalent chromium and a polymeric organic substance (so-called primer) and are then dried or baked on (AP-PS 197 164).

As a result of the presence of hexavalent chromium, the above-mentioned processes have the disadvantage that, in particular, precautionary measures are required when applying the coating agent and when handling the coated metal, and that the use of such coated metals as container material for food and beverages cannot affect the container contents .

In order to avoid the disadvantages associated with the use of hexavalent chromium treatment fluids, it is also known to clean the metal surface, in particular iron, zinc and aluminum, with an acidic aqueous solution, the chromium III ions, phosphate ions and finely divided silica , optionally also contains acetate, maleinate, zinc and / or manganese ions, to wet and dry the solution film (DE-OS 27 11 431). Although this method has considerable advantages over the abovementioned, it is disadvantageous that when the coated metals are used as the container material, a certain influence on foods and beverages as a result of the chromium III content of the layer cannot be ruled out completely, and that the treatment liquid is caused by the formation of poorly soluble chromium phosphate Instability tends.

Another category of processes for applying conversion coatings, especially for aluminum surfaces, provides immersion or spray treatment with compositions which contain certain amounts of zirconium and / or titanium as well as phosphate and active fluoride (US Pat. No. 4,148,670) or polyacrylic acid or ester and fluozirconic acid, Fluotitanic acid or fluosilicic acid (US Pat. No. 4,191,596) or tannin together with titanium and Fluorine ions (US Pat. No. 4,054,466) or complex fluorides of boron, titanium or zircon together with oxidizing agents, such as sodium metanitrobenzenesuronate (DE-AS 19 33 013).

The last-mentioned methods are disadvantageous in that, due to the water-soluble constituents of the coating agent, a rinsing treatment is required, which often leads to wastewater which is difficult to treat or only a comparatively complicated bath monitoring leads to usable results. Coating agents containing tannin sometimes lead to an undesirable discoloration of the treated metal surface. Because of the variety of known tannins, the coating agents are also difficult to reproduce in terms of their effect. Coating agents containing polymers tend to age and become unstable, particularly in the form of concentrates. In particular, a certain incompatibility with subsequently applied organic coatings is also not predictable.

The object of the invention is to provide a method for applying conversion coatings on aluminum surfaces which avoids the known, in particular the aforementioned disadvantages, and yet can be carried out simply and without additional effort.

• mindestens 1 g/1 Zirkonionen und/oder
• mindestens 0,5 g/l'Titanionen sowie
• mindestens 1,5 g/1 Phosphationen enthält, in der die Mol-Verhältnisse von Phosphationen zu Zirkon- und/oder Titanionen mindestens 0,5 und von Fluoridionen zu Zirkon- und/oder Titanionen mindestens 5 betragen, derart in Berührung bringt, daß beim nachfolgenden Auftrocknen des Flüssigkeitsfilms ein Schichtgewicht von 10 - 300 mg/m² resultiert.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the aluminum surface is coated with a solution which has a pH of not more than 3.5

• at least 1 g / 1 zirconium ions and / or
• at least 0.5 g / l titanions and
• contains at least 1.5 g / 1 phosphate ions in which the molar ratios of phosphate ions to zirconium and / or titanium ions are at least 0.5 and of fluoride ions to zirconium and / or titanium ions are at least 5, in such a way that in contact subsequent drying of the Liquid film a layer weight of 10-300 mg / m ² results.

The solution can be brought into contact with the aluminum surface in any conventional manner, such as by dipping and then draining, pouring over and spinning off, brushing, spraying using single or dual-substance nozzles, spraying or rolling on with flat or structured rollers in the same or in the opposite direction will. Roll-on processes are particularly suitable because of the comparatively precise dosing of the solution quantities. Applying a solution amount of approx. 3 - 20 ml / m ² aluminum surface is particularly advantageous. Because of the small amount of water to be evaporated, the amount of solution should be as low as possible.

In principle, the solution film applied to the aluminum surface can be dried at room temperature. In order to improve the formation of the conversion coating, however, higher temperatures, preferably from 50 to 120 ° C., given as the object temperature, should be aimed for.

According to a preferred development of the invention, the aluminum surface is brought into contact with a solution in which the molar ratio of phosphate ions to zirconium and / or titanium ions is at most 5. On the one hand, this ensures a perfect layer formation, on the other hand it prevents the sparingly soluble zirconium phosphate from precipitating.

Furthermore, it is advantageous to bring the aluminum surface into contact with a solution in which the molar ratio of fluoride to zirconium and / or titanium ions is at most 15. The increased fluoride supply accelerates the reaction taking place on the aluminum surface. In addition, a phosphate concentration which is advantageous for the aforementioned perfect layer formation can be stabilized perfectly.

With regard to a subsequent treatment by applying an organic coating, optimal results are achieved if, in a further advantageous embodiment of the invention, the aluminum surfaces are brought into contact with a solution in such a way that a layer weight of 20-200 mg / m ² results.

Before carrying out the method according to the invention, the aluminum surfaces must be cleaned thoroughly. The cleaning can be done with acidic cleaners based on phosphoric acid or sulfuric acid, which should also contain a surfactant, especially non-ionic type, if necessary also fluoride, or with strongly alkaline cleaners, e.g. with sodium hydroxide solution, condensed phosphate and surfactant. Then rinse thoroughly with water, in the last stage it is best to rinse with deionized water. Depending on the type of contamination, degreasing can also be carried out with or in addition with halogenated hydrocarbons.

The invention is illustrated by way of example and in more detail.

Examples

Aluminum sheets measuring 100 x 200 mm were cleaned in a strongly alkaline cleaner based on sodium hydroxide solution, condensed phosphate, non-ionic surfactant at 65 ° C. for 15 seconds, thoroughly rinsed with water and freed of excess rinsing water by squeezing rollers. The cleaned aluminum sheets were then immersed in the treatment solution at the reaction temperature for approx. 1 sec guided between grooved rollers or smooth rollers in such a way that 6 or 3 ml of solution film per m ^{2 of} aluminum surface remained.

The solution was dried at an object temperature of 80 ° C.

The solutions used, all of which had a pH in the range from 1.5 to 2.0, had the following table. The quantities given with regard to chemicals introduced or with respect to the titanium and zirconium content are given in g / 1.

For comparison, sheets were used which had been treated with a solution containing 5 g / 1 chromic acid and 5 g / 1 silica (in the form of Aerosil) (designated solution A) or served sheets which were only degreased and with deionized water had been rinsed (hereinafter referred to as B).

Following the application of the conversion coating, the aluminum sheets were subjected to several tests.

In a first test series, the aluminum sheets provided with the conversion coating were coated with an epoxy paint (Mobil SHX 2636) and the paint adhesion was tested according to the T-bending test.

A second series of aluminum sheets was provided with a gold lacquer and, after the boiling test, subjected to a test for blistering, discoloration and lacquer detachment.

In a third series of aluminum sheets, a hard PVC film was sealed using a PVC-based heat seal lacquer and the film's adhesion was determined after the peel test.

• Beim T=Biegetest werden lackierte Bleche um 180⁰ gebogen. Der Radius der Biegung ergibt sich aus der Wahl der wie ein Dorn wirkenden Zwischenlage. üblicherweise verwendet man ein oder mehrere Bleche der gleichen Dicke wie das Testblech. Bei der schwersten Beanspruchung wird ohne Zwischenlage gearbeitet und das Blech so weit gebogen, bis sich die unbehandelten Seiten berühren (sogen. T-0-Biegung). Die Biegung über ein Blech wird als T-1-Biegung, um zwei Bleche als T-2-Biegung bezeichnet. Nach der Biegung wird die Lackhaftung durch Aufbringen und Entfernen von Klebeband im Bereich der durch die Biegung beanspruchten Stelle ermittelt.

The test procedure is explained as follows:

• At T = bending test coated sheets are bent 180 ^0th The radius of the bend results from the choice of the intermediate layer acting like a mandrel. Usually one or more sheets of the same thickness as the test sheet are used. For the heaviest loads, work is carried out without an intermediate layer and the sheet is bent until the untreated sides touch (so-called T-0 bend). The bend over a sheet is referred to as a T-1 bend and around two sheets as a T-2 bend. After the bend, the paint adhesion is determined by applying and removing adhesive tape in the area of the point claimed by the bend.

The table below shows the test results with regard to the paint detachment for the T-bending test in percent - based on the bending point.

The boiling test consists of first boiling the painted aluminum sheets in deionized water for 3 hours and then assessing the formation of blisters and discoloration. In this case, some of the metal sheets are additionally subjected to mechanical stress before the cooking treatment in accordance with the aforementioned T-bending test or in accordance with the impact indentation.

In the case of stress according to the impact indentation, a falling body is allowed to hit the unpainted sheet metal side with a spherically shaped impact part (ball diameter 15.9 mm) shortly after the paint has hardened and at ambient temperature.

The peeling test is characterized in that sheet metal strips or sheet metal strips 200 mm long and 15 mm wide provided with a conversion coating are provided on one side with a sealing wax and then by means of heated press jaws at a contact pressure of 300 Newton / cm ² , a contact time of 4 sec and a temperature of the preheated press jaws 180 ^0C with a PVC-film are provided. After three days of storage in water, the loss of adhesion to a sheet not stored in water is determined by pulling off the film and is given in percent. The pull-off speed of the film is 30 mm / min.

The results of the above tests are tabulated below. It should be added that a minimal, just discernible discoloration was observed in the cooking test with the sheets provided with a conversion coating. In the case of the sheets mechanically stressed according to the impact indentation, a slight paint detachment was just noticeable. The sheets, which were only degreased and rinsed with fully demineralized water, showed complete detachment of the paint.

Claims (4)

1) Process for applying conversion coatings on aluminum surfaces by means of solutions which contain titanium and / or zirconium, fluoride and phosphate ions, but are free of nitrate, nitrite and chromium ions and of organic film-forming polymers and tannin, characterized in that the aluminum surfaces with a solution that has a maximum pH of 3.5 and which at least 1 g / zirconium ions and / or at least 0.5 g / 1 titanium ions as well contains at least 1.5 g / 1 phosphate ions, in which the molar ratios of phosphate ions to zirconium and / or titanium ions are at least 0.5 and of fluorine ions to zirconium and / or titanium ions are at least 5, in such a way that it contacts during subsequent drying of the liquid film results in a layer weight of 10 to 300 mg / m. 2) Method according to claim 1, characterized in that the aluminum surfaces are brought into contact with a solution in which the molar ratio of phosphate ions to zirconium and / or titanium ions is at most 5. 3) Method according to claim 1 or 2, characterized in that the aluminum surfaces in contact with a solution in which the molar ratio of fluoride to zirconium and / or titanium ions is at most 15. 4) Method according to claim 1, 2 or 3, characterized in that the aluminum surfaces are brought into contact with a solution such that a layer weight of 20 to 200 mg / m ² results.