EP0587525A1 - Pretreatment process for metal or alloy articles - Google Patents

Abstract

Before lacquering, gluing or plastic-cladding, materials such as strips or foils (16) made of metals such as aluminium or aluminium alloys are pretreated by electrochemical oxidation in an electrolyte bath (12) using alternating or three-phase current. Said strips or foils are passed through an aqueous electrolyte which is heated to 15-95 DEG and adjusted to a pH of -1 - 15. The electrolyte contains 0.01 - 20 % by weight of an organic compound which forms a complex with the metal, such as aluminium. An amount of current of 5 - 500 coulombs/dm<2> is supplied to the working surface of the strips or foils (16). <IMAGE>

Description

The invention relates to a process for the pretreatment of materials made of metals or metal alloys before painting, gluing or plastic lamination by electrochemical oxidation with alternating current or three-phase current in an electrolyte bath.

For example, aluminum strips or foils refined by lacquering or lamination are used, in particular for the production of packaging for foodstuffs, the aluminum being protected from corrosion by the lacquer layer. For many types of packaging, it is sufficient if the varnish or adhesive is applied to untreated aluminum. For demanding applications, on the other hand, such as the production of deep-drawn containers for pasteurisable or sterilisable filling goods, the tapes or foils must be pretreated for sufficient adhesion. Pretreatment by continuous degreasing followed by chemical conversion to form a conversion layer is common. Most of these processes are based on a reaction of chromic acid, hydrofluoric acid and phosphoric acid with aluminum, the conversion layers formed containing chromium-VI compounds which are increasingly undesirable for food packaging or are even considered to be of concern. Baths for the chemical formation of chromium VI-free convection layers have already been described, but the quality of these layers with regard to adhesion and corrosion resistance is still insufficient, especially for sterilizable packaging.

From DE-PS 17 71 057 it is known to achieve the continuous pretreatment of an aluminum strip before applying the varnish by passing the strip through an aqueous, sulfuric acid bath at a temperature between 50 ° C and the boiling point of the bath, in which electrodes are located , guided and is electrochemically oxidized with alternating current. In this process, an oxide layer is formed on the aluminum strip, following the periods of the alternating current, if the aluminum is the anode, and largely dissolved again if the Aluminum forms the cathode. The dissolution does not take place uniformly, but in a punctiform manner with formation of craters in the very thin (approx. 0.05 µm) oxide layer. The fact that the layer is fissured with the craters is one of the reasons for the good adhesion of coating materials, especially varnishes.

A further improvement in paint adhesion is achieved with the method described in DE-OS 33 25 802, according to which the oxidation of an aluminum strip with 3-phase alternating current (three-phase current), one phase of which is rectified, is carried out in such a way that the strip has three electrodes happened one after the other. At least one of these electrodes, preferably the middle one, is connected to the rectified phase, the others to the two remaining three-phase phases.

The disadvantage of the known oxidation processes in an acidic environment is the relatively high concentration of sulfuric acid of 15-20% by weight and the process temperature of at least 80 ° C., falling below this results in a significant reduction in adhesion. With oxidation in basic electrolytes, e.g. Hydroxides, carbonates and phosphates, in concentrations below 1% by weight and temperatures below 50 ° C, can be used to produce oxide layers with good adhesion. However, these basic processes require exact adherence to the pH value, the temperature and the substance concentration, which is associated with major problems when the content of dissolved aluminum increases. Even small deviations from the target value can jump from good to poor adhesion to the oxide layers.

From US 4,976,827 it is known to improve paint adhesion by using an aqueous electrolyte with a pH of 4.2 to 12.5 and containing an organic compound carrying an amino group, the amino group having to carry at least one carboxyl group on the nitrogen atom.

The inventor has set himself the task of creating a process for the pretreatment of materials made of metals or metal alloys before painting, gluing or plastic lamination by electrochemical oxidation in an electrolyte bath, which has the same or better adhesion guaranteed and that it is less pH dependent. Furthermore, non-toxic components should be used, which do not pollute the environment with decomposition gases or wastewater contaminants and which do not leave any harmful substances in the conversion layer formed.

wobei n eine Zahl von 1 bis 20 darstellt, oder Co-Oligomere dieser Verbindungen untereinander oder Co-Oligomere dieser Verbindungen mit 3-Butensäure, Maleinsäure oder Fumarsäure, enthält, geleitet werden, und der Arbeitsfläche der Werkstoffe eine Strommenge von 5-500 Coulomb/dm² zugeführt wird.The object is achieved according to the invention in that the metals or metal alloys are heated in an aqueous electrolyte heated to 15-95 ° C. and adjusted to a pH of -1 to 15,
the 0.01-20% by weight of at least one metal compound reacting organic compound of the general formula (a)

R-COOH,


where R has the meaning of -H; an alkyl group with 1 to 8 carbon atoms; an alkyl group with 1 to 3 carbon atoms which is substituted with cycloalkyl with 5 to 10 carbon atoms; a cycloalkyl group with 5 to 10 carbon atoms; has an alkenyl group with 2 to 8 C atoms or an aryl group with 6 to 12 C atoms;
and or
of the general formula (b)

R¹-SO₃H,


where R 'is an alkyl group having 1 to 8 carbon atoms; has an alkyl group with 1 to 3 C atoms which is substituted by cycloalkyl with 5 to 10 C atoms, or an aryl group with 6 to 12 C atoms;
and or
of the general formula (c)

HOOC-CR '' = CH-COOH,


where R ″ has the meaning of -H or an alkyl group having 1 to 3 carbon atoms,
and or
the general formulas (d),

where n represents a number from 1 to 20, or contains co-oligomers of these compounds with one another or co-oligomers of these compounds with 3-butenoic acid, maleic acid or fumaric acid, and the working surface of the materials has an amount of current of 5-500 coulombs / dm² is supplied.

In the compounds of the general formula (a) R and in the compounds of the formula (b) R 'can have the meaning of an alkyl group having 1 to 8 carbon atoms. Examples include methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, t-butyl, pentyl, hexyl, heptyl, octyl or R-ethyl-hexyl.

Examples of cycloaklyl groups with 5 to 10 carbon atoms for R in compounds of the formula (a) are e.g. Cyclopentyl, cyclohexyl, 2- or 4-methylcyclohexyl, dimethylcyclohexyl, trimethylcyclohexyl, t-butylcyclohexyl or cycloctyl.

Examples of alkyl groups with 1 to 3 carbon atoms, which are substituted by a cycloalkyl group with 5 to 10 carbon atoms, such as those mentioned in compounds of the formula (a) and for R 'in compounds of the formula (b) Methyl, ethyl, propyl or isopropyl groups with, for example, cyclopentyl, cyclohexyl, 2- or 4-methylcyclohexyl, dimethylcyclohexyl, trimethylcyclohexyl, t-butylcyclohexyl or cycloctyl.

Examples of alkenyl groups with 2 to 8 carbon atoms for R in compounds of the formula (a) are e.g. Vinyl, propenyl, allyl, butenyl, methylallyl or hexenyl.

Examples of aryl groups with 6 to 12 carbon atoms for R or R 'in compounds of the formula (a) or. (b) are phenyl or substituted phenyl such as methylphenyl, dimethylphenyl or trimethylphenyl.

R '' in compounds of formula (c) can be an alkyl group such as methyl, ethyl, propyl or isopropyl.

Preferred connections are: R '''- COOH, R '''- CH = C (CH₃) -COOH HOOC-CR '''= CH-COOH R "" - CH = CH-COOH R '''- CH = CH-CH₂-COOH R '''- CH₂-CH₂-COOH, R '''- CH₂-SO₃ R '''- CH = CH-CH₂-COOH, R '''- C₆H₄-SO₃H
where R '''has the meaning of -H, methyl or ethyl, preferably of -H.

Preferred compounds are allylic acid, acetic acid, propionic acid and particularly preferably acrylic acid.

Preferred compounds are the oligomers of acrylic acid, the oligomers of methacrylic acid, co-oligomers of acrylic acid and methacrylic acid, co-oligomers of acrylic acid and / or methacrylic acid with 3-butenic acid, maleic acid or fumaric acid, or mixtures of the compounds mentioned.

The aqueous electrolyte can also contain an inorganic acid and / or an alkali metal or alkaline earth metal hydroxide.

The inorganic acid can be, for example, sulfuric acid, phosphoric acid, hydrochloric acid or be nitric acid. The amount of acid can be up to 10% by weight, expediently from 0.01 to 10% by weight, calculated as 100% acid, based on the electrolyte. Sulfuric acid is preferred, used in an amount of, for example, 0.1 to 2.2% by weight, advantageously 0.4 to 2% by weight and preferably 0.5% by weight, calculated as 100% acid. based on the electrolyte.

The alkali or alkaline earth metal hydroxide can e.g. LiOH, NaOH, KOH, Ca (OH) ₂, Mg (OH) ₂, etc.; NaOH, KOH and Ca (OH) ₂ are preferred. The amount of alkali metal hydroxides and alkaline earth metal hydroxides can be up to 10% by weight, expediently from 0.01 to 10% by weight, based on the electrolyte.

Aqueous electrolytes containing 1 to 7% by weight and in particular 5% by weight of acrylic acid and 0.1 to 2% by weight and in particular 0.5% by weight of H₂SO₄ are preferred.

The materials can e.g. Plates, sheets, strips, thin strips, foils, fabrics, knitted fabrics, felts or fibers made of metals or metal alloys or which are plated or coated with metals or metal alloys.

As metals e.g. Ferrous or non-ferrous metals, respectively. their alloys e.g. Iron, steel, copper, zinc, galvanized iron, tin, bronze, non-ferrous metals, aluminum etc. can be used. Aluminum and aluminum alloys are preferred. Examples of aluminum alloys are AlFeSi, AlMn0.5Mg0.5 and AlMg2.5.

In industrial application practice of the method according to the invention, it has proven to be advantageous to use the organic compounds reacting with metals or metal alloys preferably in a concentration of 0.1 to 20% by weight, in particular 1 to 10% by weight. With electrolytes containing 5% by weight of the organic compound reacting with metals or metal alloys, oxide layers with an optimal paint adhesion of, for example, more than 30 N / 15 mm (peeling force value) can be produced, in particular if the required conductivity and the pH value with sulfuric acid can be set. Especially Processes in which aqueous electrolytes containing 5% by weight of acrylic acid and 0.5% by weight of sulfuric acid are used are advantageous. With these electrolytes, peeling force values of, for example, 40-85 N / 15 mm are achieved. The peeling force values relate, for example, to epoxy-phenol paints.

According to the invention, the temperature of the electrolyte is kept at 15 to 95 ° C. and preferably at a value between 40 and 65 ° C. and in particular at 50 ° C. to 60 ° C.

The complexing agents are saturated with the metal dissolved during the electrochemical oxidation, which is associated with a reduction in the oxidation ability of the electrolyte. A higher concentration of the complexing agent down to 10% by weight delays this saturation accordingly. This is associated with a reduction in the pH value and the conductivity of the electrolyte. By preferably continuously adding z. B. alkalis, especially in the form of sodium hydroxide or potassium hydroxide, the initial pH value, the conductivity and thus also the oxidation ability of the electrolyte can be kept constant. After the concentration of the metal in the electrolyte has reached a certain limit, it begins to precipitate out as metal hydroxide, which can be filtered off. This largely stabilizes the bath composition, so that only the amount of electrolyte carried out with the belt has to be replaced and the conductance may need to be corrected.

To avoid fogging of devices, electrodes and wells with the precipitated metal hydroxide, a stabilizer, preferably a polysaccharide, such as e.g. Dextran or starch, preferably in an amount of approximately 0.05% by weight, can be used.

The quality of the oxide layer on the materials is determined by the surface area (= work surface) supplied amount of electricity per unit area, the best results being achieved in the range of 20-100 coulombs / dm².

The materials are advantageously passed through the electrolyte at such a speed that the current density between 3 and 100 A / dm², expediently 6-25 A / dm² and advantageously 12 A / dm². The electrolysis mass is dimensioned such that the oxidation time, ie the throughput time of the materials in the electrolyte in the region of the electrodes or the electrochemical reaction time is 0.5 to 15 sec, preferably 2 to 7 sec, and in particular 3 to 5 sec.

The voltage can be 20 V, for example.

The pretreated materials are e.g. 1 to 4 times, preferably 3 or 4 times, rinsed with water. The rinsed materials can be left to dry at ambient conditions or can be dried at 90 to 140 ° C for about 20 to 30 seconds.

The type and form of alternating current used for the oxidation is not critical per se; it can have a sinusoidal, rectangular or triangular shape or be present as a direct current superimposed with alternating or pulsed current. It also has no significant influence on the oxide layer formation, whether the oxidation takes place with two-phase alternating current or with three-phase current.

Even better results are achieved when using three-phase current with a rectified phase via a diode, three electrodes preferably supplying the current to the materials, such as, for. B. tapes or foils, and the middle electrode is particularly rectified. The connection of the minus potential of the other phases to the central electrode is practically equally advantageous. The slightly reinforced oxide layers result in better corrosion resistance.

The power supply to the electrodes and z. B. the tape or film do not matter. There are practically the same result if, for. B. the tape or the film, contacted directly, forms an electrode, or if the current is connected to two or more separate electrodes, under which the tape coupled over the electrolyte represents a current-carrying connections, as in DE-OS 33 25 802 is described.

Compared to alternating current, oxide layers produced with direct current have poorer paint adhesion, and pretreatment with direct current is also less and less advantageous for gluing or plastic lamination.

The pretreated materials can e.g. painted, laminated or pasted. It was found that painting one day after a pretreatment according to the invention and painting a month after the pretreatment according to the invention were applied to each sample gave equally good results and the pretreated surface did not change in 4 weeks.

• Fig. 1 einen Vertikalschnitt durch eine Elektrolysezelle mit Durchleitung beispielsweise eines Bandes,
• Fig. 2 eine symbolisch dargestellte Anordnung mit zweiphasigem Wechselstrom und dem Band als Elektrode,
• Fig. 3 eine symbolisch dargestellte Anordnung für die Drehstromoxidation, und
• Fig. 4 eine Variante von Fig. 3 mit einer gleichgerichteten Phase.

The invention is explained in more detail by way of example with reference to the drawings. They show schematically:

• 1 shows a vertical section through an electrolysis cell with passage of, for example, a strip,
• 2 is a symbolically illustrated arrangement with two-phase alternating current and the band as an electrode,
• Fig. 3 shows a symbolically illustrated arrangement for three-phase oxidation, and
• Fig. 4 shows a variant of Fig. 3 with a rectified phase.

The electrolysis cell shown in FIG. 1 comprises an oxidation container 10 with an electrolyte 12. A metal strip, and here an aluminum strip 16, is unwound from a roll 14, guided horizontally through the electrolyte 12 via deflection rollers 18 in the region of two electrodes 20a, 20b and onto it the other side of the cell wound on a roll 22. The electrodes 20a, 20b are embedded in a holder 24 made of electrically insulating, electrolyte-resistant material such that only the electrode surfaces facing the aluminum strip, which form the working surface, are not insulated. The vertical walls 26 of the holder 24 directed onto the aluminum strip 16 protrude the electrodes 20a, 20b and are guided up to the vicinity of the aluminum strip 16.

The single-phase transformer 30 connected to the alternating current source 28 supplies the electrodes 20a, 20b via the conductors 32 with two-phase sinusoidal alternating current with a frequency of 50-60 Hz. A simple transformer with a mains connection is the cheapest solution. Laboratory tests have found that other types of alternating current, e.g. Rectangular, triangular, pulse currents from lower or higher frequencies do not bring any significant improvement in the quality of the oxide layer.

The symbolic arrangement shown in FIG. 2 shows a variant according to which the conductor 32a is connected directly to the aluminum strip 16. The secondary circuit is closed via the electrolyte (not shown), the single electrode 20 and the conductor 32b.

The symbolic arrangement for the three-phase oxidation shown in FIG. 3 comprises a three-phase transformer 34, of which only the secondary windings 36a, 36b and 36c are shown. These secondary windings are connected to the electrodes 20a, 20b and 20c via the conductors 32a, 32b and 32c.

The arrangement of FIG. 4 differs from FIG. 3 only in that a diode 38 is installed in the middle conductor 32b, this phase is therefore rectified.

According to a variant shown in dashed lines in FIGS. 3 and 4, the aluminum strip 16 can be connected directly to the secondary power source via a conductor 40.

Claims (12)

Process for the pretreatment of materials made of metals or metal alloys before painting, gluing or plastic lamination by electrochemical oxidation with alternating current or three-phase current in an electrolyte bath,
characterized in that
the metals or metal alloys (16) by means of an aqueous electrolyte (12) heated to 15 to 95 ° C. and adjusted to a pH value of -1 to 15,
the 0.01 to 20% by weight of at least one organic compound of the general formulas (a) which reacts with the metal or the metal alloy

R-COOH,

wherein R has the meaning of -H, an alkyl group with 1 to 8 C atoms, a cycloalkyl group with 5 to 10 C atoms, an alkenyl group with 2 to 8 C atoms or an aryl group with 6 to 12 C atoms;
and or
of the general formula (b)

R'-SO₃H,

where R 'is the meaning of alkyl having 1 to 8 carbon atoms or the meaning of an alkyl group having 1 to 3 carbon atoms which is substituted by a cycloalkyl group having 5 to 10 carbon atoms or an aryl group having 6 to 12 carbon atoms Has;
and or
of the general formula (c)

HOOC-CR '' = CH-COOH,

where R ″ has the meaning of -H or an alkyl group having 1 to 3 carbon atoms,
and or
of the formulas (d),

where n represents a number from 1 to 20, or co-oligomers of these compounds with one another or co-oligomers of these compounds with 3-butenoic acid, maleic acid or fumaric acid, and the working surface of the materials an amount of current of 5 to 500 coulombs / dm 2 is fed. A method according to claim 1, characterized in that as the organic compounds reacting with metals or metal alloys, the oligomers of acrylic acid, the oligomers of methacrylic acid, co-oligomers of acrylic acid and / or methacrylic acid with 3-butenic acid, maleic acid or fumaric acid, or mixtures of said compounds are used will. Process according to Claim 1, characterized in that the organic compounds which react with metal or metal alloys are used in a concentration of 0.1 to 20% by weight, preferably 1 to 10% by weight. A method according to claim 1, characterized in that the aqueous electrolyte contains an inorganic acid, wherein the amount of acid is up to 10 wt .-%, calculated as 100% acid, based on the electrolyte. A method according to claim 1, characterized in that the aqueous electrolyte contains an alkali metal hydroxide or alkaline earth metal hydroxide, the amount of alkali metal hydroxides and alkaline earth metal hydroxides being up to 10% by weight, based on the electrolyte. A method according to claim 1, characterized in that the aqueous electrolyte 1 to 7 wt .-% acrylic acid, and in particular 5 wt .-% acrylic acid, and 0.1 to 2 wt .-% H₂SO₄ and in particular 0.5 wt .-% H₂SO₄ contains. A method according to claim 1, characterized in that a temperature of the electrolyte of 40 to 65 ° C and in particular 50 ° C to 60 ° C is set. A method according to claim 1, characterized in that the oxide layer is generated with a current of 5 to 100 coulombs / dm². A method according to claim 1, characterized in that the oxidation is carried out with a current density of 3-100 A / dm 2, preferably 6-25 A / dm 2 and advantageously 12 A / dm 2. A method according to claim 1, characterized in that the oxidation is carried out with a cycle time in the region of the electrodes (20, 20a, 20b, 20c) of 0.5 to 15, preferably 2 to 7 sec, and in particular 3 to 5 sec. A method according to claim 1, characterized in that three-phase current is used, wherein three electrodes (20a, 20b, 20c) are arranged and the middle electrode (20b) is preferably fed with rectified current. A method according to claim 1, characterized in that aluminum or aluminum alloys are used as metals or metal alloys.

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