EP0555244A1

EP0555244A1 - Electrolyte additive for a colorant bath for colouring aluminium and process for colouring aluminium.

Info

Publication number: EP0555244A1
Application number: EP91918011A
Authority: EP
Inventors: Riese-Meyer Loert De; Volker Sander; Juergen Lindener
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 1990-10-29
Filing date: 1991-10-21
Publication date: 1993-08-18
Anticipated expiration: 2011-10-21
Also published as: ZA918569B; EP0555244B1; AU646508B2; ATE121145T1; CN1066496C; KR930702556A; JPH06502217A; CA2095247A1; YU170691A; AU8743991A; MX9101817A; PT99342B; CA2095247C; KR0185157B1; DE59105203D1; US5409592A; ES2070514T3; PT99342A; AR245786A1; WO1992007976A1

Abstract

Un nouvel additif de l'électrolyte d'un bain colorant d'acide sulfurique contenant de l'étain et servant à colorer, sous un courant alternatif, des surfaces anodisées en aluminium se compose d'un mélange synergique d'au moins un anti-oxydant défini par une des formules générales (I à IV) et au moins un agent améliorateur de la dispersion de la formule générale (V). L'invention concerne également un procédé de teinture sous un courant alternatif de surfaces anodisées en aluminium avec l'additif d'électrolyte décrit.A novel additive for the electrolyte of a tin-containing sulfuric acid dye bath for coloring anodized aluminum surfaces under alternating current consists of a synergistic mixture of at least one anti- oxidant defined by one of the general formulas (I to IV) and at least one agent for improving the dispersion of the general formula (V). The invention also relates to a method of dyeing under alternating current anodized aluminum surfaces with the disclosed electrolyte additive.

Description

"Electrolyte additive for a coloring bath for aluminum coloring and process for coloring aluminum"

The invention describes a new electrolyte additive for a sulfuric, tin (II) -containing dye bath for alternating current coloring of anodized aluminum surfaces, which consists of a synergistic mixture of at least one antioxidant of one of the general formulas I to IV and at least one scattering improver of the general formula V and there is a method for alternating current coloring of anodized aluminum surfaces using the electrolyte additives according to the invention.

Because of its base character, aluminum is known to be coated with a natural oxide layer, the layer thickness of which is generally less than 0.1 μm (Wemick, Pinner, Zurbrügg, Weiner;

"The surface treatment of aluminum", 2nd edition, Eugen Leuze Verlag, Saulgau / Württ., 1977).

Significantly thicker oxide layers can be obtained if aluminum is oxidized electrolytically. This process is known as anodizing, in older parlance also anodizing. As Electrolyte is preferably used as sulfuric acid, chromic acid or phosphoric acid. Organic acids such as oxalic, maleic, phthalic, salicylic, sulfosalicylic, sulfophthalic, tartaric or citric acid are also used in some processes.

However, sulfuric acid is most commonly used. Depending on the anodizing conditions, this process can achieve layer thicknesses of up to 150 μm. For outdoor applications such as Facade cladding or window frames, however, suffice for layer thicknesses of 20 to 25 μm.

The anodization is generally carried out in 10 to 20% sulfuric acid with a current density of 1.5 A / mm and a temperature of 18 to 22 ° C. within 15 to 60 minutes, depending on the desired layer thickness and intended use.

The oxide layers produced in this way have a high absorption capacity for a large number of organic and inorganic substances or dyes.

Electrolytic dyeing processes have been known since the mid-1930s, in which anodized aluminum can be colored in heavy metal salt solutions by treatment with alternating current. The elements of the first transition row, such as Cr, Mn, Fe, Co, Ni, Cu and in particular Sn, are used above all. The heavy eta11salze are mostly used as sulfates, whereby a pH of 0.1 to 2.0 is adjusted with sulfuric acid. One works at a voltage of about 10 to 25 V and the resulting current density. The counter electrode can either consist of graphite or stainless steel or of the same material which is dissolved in the electrolyte. In this process, the heavy metal pigment is deposited in the pores of the anodic oxide layer in the half period of the alternating current, in which aluminum is the cathode, while in the second half period the aluminum oxide layer is further strengthened by anodic oxidation. The heavy metal is deposited on the bottom of the pores, causing the oxide layer to color.

A problem with the coloring in tin electrolytes, however, is the easy oxidizability of the tin, which leads to precipitations of basic tin (-IV) oxide hydrates (tin acid) when used and in some cases even when the Sn solutions are stored. Aqueous tin (II) sulfate solutions are known to be oxidized to tin (IV) compounds by the action of atmospheric oxygen or by reaction at the electrodes under current load. This is when staining in anodized tin electrolytes. Aluminum is very undesirable, since on the one hand it interferes with the process (frequent renewal or replenishment of the solutions which are unusable due to the formation of precipitation) and on the other hand it leads to considerable additional costs due to the tin (IV) compounds which cannot be used for coloring. A number of processes have therefore been developed which differ in particular by the type of stabilization of the mostly sulfuric acid tin (II) sulfate solutions for the electrolytic aluminum coloring.

By far the most common are phenol-like compounds such as phenolsulfonic acid, cresolsulfonic acid or sulfosalicylic acid (see for example in SA Pozzoli, F. Tegiacchi; Korros. Korro¬ sionsschutz Alum., Event. Eur. Foed. Korros., Vortr. 88th 1976. 139 -45 or in Japanese Patent Laid-Open JP-A-7813583, 78 18483, 77 135841, 76 147436, 7431614, 73 101331, 7120568, 75 26066, 76 122637, 54 097545, 56 081598 and in GB-C-1482390). Also polyfunctional phenols such as the diphenols hydroquinone, pyrocatechol and resorcinol (see in Japanese laid-open documents JP-A-58 113391, 57 200221 and in FR-C-23 84 037) and the triphenols phloroglucin (JP-A-58113391 ), Pyrogallol (SA Pozzoli, F. Tegiacchi; Corros. Corrosion Protection Alum., Veranstst. Eur. Foed. Korros., Vortr. 88th 1976. 139-45 or in Japanese published documents JP-A-58 113391 and 57200221) or gallic acid (JP-A-53 13583) have already been described in this connection.

Another important problem with electrolytic coloring is the so-called scattering ability (deep scattering), which is the product property of coloring anodized aluminum parts that are at different distances from the counterelectrode with a uniform color. Good spreadability is particularly important if the aluminum parts used have a complicated shape (coloring of the depressions), if the aluminum parts are very large and if, for economic reasons, many aluminum parts are to be colored at the same time and medium shades are to be achieved. In application, therefore, a high spreadability is very desirable, since incorrect production is avoided and the optical quality of the colored aluminum parts is generally better. The process is more economical due to its good spreadability, since more parts can be colored in one operation.

The term spreadability is not identical to the term uniformity and must be strictly differentiated from this. The uniformity concerns a coloring with the least possible local disturbances in the color (spotty coloring). Poor uniformity is mostly due to impurities such as nitrate or process errors in the anodization. A good color Under no circumstances should beelectrolyte impair the uniformity of the coloring.

A dyeing process can achieve good uniformity and still have poor spreading power; the reverse is also possible. The uniformity is generally only influenced by the chemical composition of the electrolyte, while the scattering ability also depends on electrical and geometric parameters, such as the shape of the workpiece or its position and size.

DE-A-26 09 146 describes a process for coloring in tin electrolytes, in which the scatterability is set by the special circuit and voltage arrangement.

DE-A-2428635 describes the use of a combination of tin (II) and zinc salts with the addition of sulfuric acid and additional boric acid as well as aromatic carboxylic and sulfonic acids (sulfophthalic acid or sulfosalicylic acid) in the electrolytic gray coloring of anodically oxidized Objects made of aluminum. In particular, excellent scattering of the dyeing effect should be achieved when the pH is between 1 and 1.5. The setting of the pH to 1 to 1.5 is a basic prerequisite for good electrolytic coloring. It is not described whether the added organic acids have an effect on the spreadability. The spreadability achieved is also not quantified.

DE-C-32 46 704 describes a process for electrolytic coloring in which good scattering capacity is ensured by using a special geometry in the dye bath. In addition, cresol and phenol sulfonic acid, organic substances such as dextran trin and / or thiourea and / or gelatin ensure an even coloration. The disadvantage of this method is the high investment that is required for the creation of the mechanical devices. The addition of deposition inhibitors such as dextrin, thiourea and gelatin has only a slight influence on the scatterability, since the deposition process in electrolytic dyeing differs significantly from that in galvanic tinning. A possibility of measuring the improvements in the spreadability is also not given here.

From the European patent application EP-A-354 365 of the applicant, a method for electrolytic metal salt coloring of anodized aluminum surfaces is also known, in which the antioxidants of the general formulas I to IV (see in the claims) together with the scattering improvers p- Toluenesulfonic acid and / or naphthalenesulfonic acid can be used. However, the scatter improvers mentioned in this document lead to malodorous decomposition products during electrolysis, so that further use of these scatter improvers is avoided.

The object of the present invention was to provide a new electrolyte additive for a sulfuric acid, tin (II) -containing dye bath for AC coloring of anodized aluminum surfaces, which provides the problems known from the above-mentioned prior art, how to ensure lasting dye bath stability, avoidance of Sn (II) oxidation and at the same time guarantee good spreading power.

The present invention accordingly relates to an electrolyte additive for a sulfuric acid, tin (II) -containing dye bath for AC coloring of anodized aluminum surfaces. Chen, containing at least one antioxidant and at least one litter improver, characterized in that the electrolyte additive a) as antioxidant, at least one compound of one of the general formulas I to IV,

(I) (II) (III

in which Rl and R2 represent hydrogen, alkyl, aryl, alkylaryl, alkylarylsulfonic acid, alkylsulfonic acid each having 1 to 22 carbon atoms and their alkali metal salts, and in which R3 represents one or more hydrogen and / or alkyl, aryl, Alkylaryl radicals having 1 to 22 carbon atoms are at least one of the radicals R, R2 and R3 being a radical not equal to hydrogen, and b) at least one aromatic carboxylic acid of the general formula V as a scattering improver

(V)

in which R * to R5 represent hydrogen, hydroxyl, carboxyl and / or sulfonic acid residues.

Another object of the present invention is a process for AC coloring of anodized aluminum surfaces in a sulfuric acid, tin (II) -containing dye bath, characterized in that an electrolyte additive according to the above definition is used at a pH of 0. 1 to 2.0, at a temperature of 10 to 30 ° C and at an alternating voltage with a frequency of 50 to 60 Hz and a terminal voltage of 10 to 25 V for electrolytic coloring in the sulfuric acid, tin (II) containing dye bath.

An essential advantage of the electrolyte additive according to the invention is the use of oxidation-stable, water-soluble scatter improvers. Especially in the case of longer operating times, p-toluenesulfonic acid, known from the teaching of EP-A-354365, develops malodorous vapors by oxidizing the methyl group, which makes long dyebath use intolerable. According to the invention, it is therefore particularly important to equip the scatter improver with oxidation-stable, functional groups, such as carboxyl, hydroxyl and / or sulfonic acid groups. The functional groups mentioned also ensure the required water solubility.

In a preferred embodiment of the present invention, the electrolyte additive contains at least one of the compounds of the general formulas I to IV in an amount of 0.01 to 2 g / l as an antioxidant and at least one of the compounds of the general formula V in an amount of 0.1 to 30 g / 1 - each based on the dye bath - as a litter improver. For the purposes of the present invention, the antioxidants of the general formulas I to IV in the above concentrations are in particular 2-tert-butyl-1,4-dihydroxybenzene (tert-butylhydroquinone), methylhydroquinone, trimethylhydroquinone, 4-hydroxy-2, 7-naphthalene disulfonic acid and / or p-hydroxyanisole used.

In the context of the present invention, 5-sulfosalicylic acid, 4-sulfophthalic acid, 2-sulfobenzoic acid, benzoic acid and / or benzene hexacarboxylic acid are used in particular as a scatter improver of the general formula V. The combination of 5-sulfosalicylic acid and 4-sulfophthalic acid has proven to be particularly effective in the sense of a synergistic effect.

In a preferred embodiment of the present invention, the electrolyte additive according to the invention accordingly contains, in each case based on the total volume of the dyebath:

a) as an antioxidant t-butyl hydroquinone in an amount of 0.01 to 2 g / 1 and

b) 5-sulfosalicylic acid in an amount of 0.5 to 6 g / 1 and 4-sulfophthalic acid in an amount of 5 to 20 g / 1 as a litter improver.

In a particularly preferred embodiment, the electrolyte additive according to the invention - in each case based on the total volume of the dyebath - contains in particular: a) as an antioxidant t-butylhydroquinone in an amount of 0.1 to 0.5, preferably 0.2 to 0, 3 g / 1 and b) as a scattering improver 5-sulfosalicylic acid in an amount of 1 to 3 g / 1, preferably 1.5 to 2.5 g / 1 and 4-sulfophthalic acid in an amount of 8 to 12 g / 1, preferably 10 g / 1.

The dyeing is usually carried out with the aid of a tin (II) sulfate solution which contains about 3 to 20 g, preferably 7 to 16 g, of tin (II) per liter. Coloring is preferably carried out at a pH of 0.1 to 2.0, corresponding to 16 to 22 g of sulfuric acid per liter, at a temperature of about 14 to 30 ° C. The alternating voltage or alternating current superimposed on the direct current (50 to 60 Hz) is preferably set at 10 to 25 V, preferably 15 to 18 V, with an optimum of approximately 17 V + 3 V.

In the sense of the present invention, the term alternating current coloring means either the coloring with pure alternating current or the coloring with "direct current superimposed alternating current" or "alternating current superimposed direct current". The value of the terminal voltage is given in each case. The coloring begins at a current density of usually about 1 A / dm ² , which then drops to a constant value of 0.2 to 0.5 A / dm ² . Depending on the tension, the metal concentration in the dyebath and the dipping times, different tones are obtained, which can vary between champagne-colored and various bronze tones to black.

According to a further embodiment, the method of the present invention is characterized in that the electrolyte contains further heavy metal salts in addition to tin, for example nickel, cobalt, copper and / or zinc (see Wernick et al, loc. Cit.).

The electrolyte additive according to the invention will be explained in more detail with reference to the following examples: Examples

Test methods

a) Quick test to assess the storage stability of the baths (test 1)

An aqueous electrolyte was prepared, each containing 20 g / 1 sulfuric acid and 10 g / 1 Sn (II) ions, as well as corresponding amounts of an electrolyte additive. 1-1- solutions were stirred vigorously at room temperature with a magnetic stirrer and gassed with 12 l / h of pure oxygen over a glass frit. The content of Sn (II) ions after 4 hours was recorded iodometrically. The percentage decrease in Sn (II) concentration was recorded.

b) Assessment of the antioxidant effect under current load (test 2)

In each case an aqueous electrolyte was produced which contained 20 g / 1 sulfuric acid, 10 g / 1 Sn (II) ions and corresponding amounts of an electrolyte additive. The permanent electrolysis (alternating current 50 Hz, voltage: 12 V) was carried out with stainless steel electrodes. The flowing amount of electricity was registered with an Ah counter. The characteristic behavior of the oxide layer to be colored was simulated by corresponding sinusoidal distortion of the alternating current at high capacitive loads. The

The amount of Sn (II) ions oxidized by electrode reactions was determined by continuous iodometric titration of the electrolyte and by gravi etric determination of the reductively deposited Sn and the difference from the suirane of these two values to the initial amount of dissolved Sn (II). As a measure of that The Ah value was chosen to have an antioxidant effect, at which a reduction in the Sn (II) concentration by 5 g / l can no longer be prevented.

c) Assessment of the spreadability (test 3)

Test sheets measuring 50 mm x 460 mm x 1 mm made of the DIN material AI 99.5 were conventionally pretreated and then electrolytically colored in a dye bath with a suitable geometry (electrode at a distance of 1 to 5 cm from the counter electrodes). In addition to 10 g / 1 Sn (II) and 20 g / 1 sulfuric acid, the dyebath also contained different amounts of the test substances (see examples and comparative examples). The standard dyeing was 16 V (alternating current 50 Hz) for 5 minutes. The color result was recorded numerically as follows: First, the tin distribution on the test sheet was determined at 10 different points in the longitudinal direction (ie every 5 cm) by measurement with a scattered light reflectometer against the white standard titanium dioxide (= 99%). The "mean coloring" results from the individual measured values. The scatterability is determined from this as a measure of the correspondence of each measuring point with the mean value and is given as a process value. The Streufähig¬ mean speed of 100% _r is that the test sheet over the entire length ein¬ uniformly colored. The closer the values come to 0%, the more differently the sheet ends are colored.

Electro coloring

Test sheets made of the DIN material AI 99.5 (No. 3.0255) were conventionally pretreated (degreased, pickled, decapitated) and after the GS process (200 g / 1 sulfuric acid, 10 g / 1 Al (III), air flow rate, 1 , 5 A / dm ² , 18 ° C) anodized for 60 minutes. This resulted in a layer structure of approximately 20 μm. The one so pretreated As described in the following examples, sheets were electrolytically colored with alternating current (50 Hz). The results are summarized in Table 1.

example 1

Electrolyte: 10.0 g / 1 Sn (II)

20.0 g / 1 sulfuric acid 0.2 g / 1 t-butylhydroquinone 2.0 g / 1 5-sulfosalicylic acid

10.0 g / 1 4-sulfophthalic acid

Coloring parameters: 16 V, 5 minutes

Example 2

10.0 g / 1 Sn (II)

Coloring parameters: 16 V, 5 minutes

Example 3

10.0 g / 1 Sn (II) 20.0 g / 1 sulfuric acid 0.2 g / 1 t-butylhydroquinone 10.0 g / 14-sulfophthalic acid

Dyeing parameters: 16 V, 5 minutes Example 4

10.0 g / 1 Sn (II)

20.0 g / 1 sulfuric acid 0.2 g / 1 methylhydroquinone 2.0 g / 1 5-sulfosalicylic acid

10.0 g / 1 4-sulfophthalic acid

Coloring parameters: 16 V, 5 minutes

Example 5

10.0 g / 1 Sn (II) 20.0 g / 1 sulfuric acid 0.2 g / 1 trimethylhydroquinone 2.0 g / 1 5-sulfosalicylic acid 10.0 g / 1 4-sulfophthalic acid

Coloring parameters: 16 V, 5 minutes

Example 6

10.0 g / 1 Sn (II) 20.0 g / 1 sulfuric acid 0.2 g / 1 t-butylhydroquinone 10.0 g / 1 benzene hexacarboxylic acid

Coloring parameters: 16 V, 5 minutes

Example 7

10.0 g / 1 Sn (II) 15

20.0 g / 1 sulfuric acid

0.2 g / 1 trimethylhydroquinone 20.0 g / 1 2-sulfobenzoic acid

Coloring parameters: 16 V, 5 minutes

Comparative example 1

Electrolyte: 10.0 g / 1 Sn (II)

20.0 g / 1 sulfuric acid

Coloring parameters: 16 V, 5 minutes

Comparative example 2

Electrolyte: 10.0 g / 1 Sn (II)

20.0 g / 1 sulfuric acid 0.2 g / 1 t-butylhydroquinone

Coloring parameters: 16 V, 5 minutes

Comparative example 3

Electrolyte: 10.0 g / 1 Sn (lT)

20.0 g / 1 sulfuric acid 2.0 g / 1 5-sulfosalicylic acid

Coloring parameters: 16 V, 5 minutes Comparative example 4

Electrolyte: 10.0 g / 1 Sn (II)

20.0 g / 1 sulfuric acid 10.0 g / 1 4-sulfophthalic acid

Coloring parameters: 16 V, 5 minutes

Comparative Example 5:

Electrolyte: 10.0 g / 1 Sn (II)

20.0 g / 1 sulfuric acid

0.2 g / 1 t-butyl hydroquinone

20.0 g / 1 p-toluenesulfonic acid

Coloring parameters: 16 V, 5 minutes

The results in Table 1 clearly show that an electrolyte additive containing a mixture of an antioxidant of one of the general formulas I to IV and a scattering improver of the general formula V (Examples 1 to 7) has the coloring properties of

Tin (II) salt electrolytes, such as storage stability, antioxidant activity and scatterability, were significantly improved compared to Comparative Examples 1 to 4. In Comparative Example 5, an intensifying, malodorous odor occurs after only 15 minutes.

Claims

P a t e n t a n s r u c h e

Electrolyte additive for a sulfuric acid, tin (II) -containing dye bath for AC coloring of anodized aluminum surfaces, containing at least one antioxidant and at least one scattering improver, characterized in that the electrolyte additive a) as antioxidant has at least one compound of one of the general formulas I to IV,

in which R * and R ^{2 are} hydrogen, alkyl, aryl, alkylaryl, alkylarylsulfonic acid, alkylsulfonic acid each having 1 to 2 carbon atoms and their alkali metal salts, and in which R ^ is one or more hydrogen and / or alkyl, aryl -, Alkylaryl radicals having 1 to 22 carbon atoms, at least one of the radicals R ¹ , R ² and R ^{3 being} a radical un equal to hydrogen, and b) at least one aromatic carboxylic acid of the general formula V as a scattering improver

in which R ¹ to R5 represent hydrogen, hydroxyl, carboxyl and / or sulfonic acid residues.

2. Composition according to claim 1, characterized in that the electrolyte additive as antioxidant at least one of the compounds of one of the general formulas I to IV in an amount of 0.01 to 2 g / 1 and as a scattering improver at least one of the compounds of general formula V in an amount of 0.1 to 30 g / 1 - each based on the total volume of

Dye bath - contains.

3. Composition according to claim 1 and 2, characterized in that t-butyl hydroquinone, methyl hydroquinone, tri-methyl hydroquinone, p-hydroxyanisole and / or 2,7-disulfo-4-hydroxy-naphthalene are used as antioxidants.

4. Means according to claim 1 and 2, characterized in that 5-sulfosalicylic acid, 4-sulfophthalic acid, 2-sulfobenzoic acid, benzoic acid and / or benzene hexacarboxylic acid are used as scattering improvers.

5. Composition according to claim 1 to 4, characterized in that it a) as an antioxidant t-butylhydroquinone in an amount of 0.01 to 2 g / 1 and b) 5-sulfosalicylic acid in an amount of 0.5 to 6 g / 1 and 4-sulfophthalic acid in an amount of 5 to 20 g / 1 - in each case based on the total volume of the dyebath - as a scatter improver.

6. Composition according to claim 5, characterized in that it a) as an antioxidant t-butylhydroquinone in an amount of 0.1 to 0.5, preferably 0.2 to 0.3 g / 1 and b) 5-sulfosalicylic acid as a spreading improver in an amount of 1 to 3 g / 1, preferably i-, 5 to 2.5 g / 1 and 4-sulfophthalic acid in an amount of 8 to 12 g / 1, preferably 10 g / 1 - in each case based on the Total volume of the dye bath - contains.

7. A process for Wechselströmeinfärbuπg anodized aluminum surfaces in a sulfuric acid, zinπ (II) containing Färbe¬ bath, characterized in that an electrolyte additive according to claim 1 to 6 at a pH of 0.1 to 2, at egg a temperature of 10 to 30 ° C and at an alternating voltage with a frequency of 50 to 60 Hz and a clamping voltage of 10 to 25 V for electrolytic coloring in the sulfuric acid, tin (II) containing dye bath.