TW202314050A - Method for electrodepositing a dark chromium layer, substrate comprising same, and electroplating bath thereof - Google Patents

Abstract

The present invention relates to a method for electrodepositing a dark chromium layer on a substrate, a respective electroplating bath for depositing such a dark chromium layer, and a respective substrate comprising said dark chromium layer. The electroplating bath comprises colloidal particles containing the chemical element aluminum. The substrate comprising said dark chromium layer is primarily suited for decorative purposes.

Description

Method for electrodepositing dark chrome layer, substrate containing it and electroplating bath

The present invention relates to a method of electrodepositing a dark chromium layer on a substrate, a respective electroplating bath for depositing the dark chromium layer, and a respective substrate comprising the dark chromium layer. The electroplating bath contains colloidal particles containing the chemical element aluminum. Substrates comprising the dark chromium layer are mainly suitable for decorative purposes.

Since the earliest days of chrome coatings, dark chrome coatings have observed a high level of interest due to their great appeal for decorative applications.

Even though it started with dark hexavalent chromium coatings, today the focus has shifted significantly to trivalent chromium coatings due to higher environmental acceptance. For some years, the demand for dark trivalent chrome coatings has been increasing, for example for decorative automotive parts. However, in some cases very dark shades can be considered too cool such that slight tint modifications are often required to create multiple shades for various decorative purposes. Typically, the degree of darkening varies significantly depending on deposition parameters and bath composition.

There is a continuing need to provide a dark chrome layer that allows fine tuning of slightly warmer or slightly darker tones. It is desirable to have a means of adjustment that allows this fine-tuning to be done in an easily handled manner.

In principle, dark trivalent chromium layers are known.

US 2011/155286 A1 refers to a composition for chemical conversion treatment, which includes trivalent chromium ions and optional inorganic silica sol or alumina sol.

JP 5890394 B2 refers to a chromium plating solution containing trivalent chromium and ceramic particles.

RU 2231581 C1 refers to a chromium electrolyte containing trivalent chromium ions and Al ₂ O ₃ powder.

US 2012/312694 A1 refers to an aqueous acidic trivalent chromium electrolyte containing trivalent chromium ions and colloidal silicon dioxide.

US 2015/354085 A1 refers to an apparatus for maintaining the efficiency of a trivalent chromium plating bath utilizing an ultraviolet (UV) radiation source which provides UV radiation to an aqueous electroplating bath such that the plating efficiency of the inhibiting bath is reduced.

US 2007/0227895 A1 refers to a process for the electrodeposition of crystalline-like chromium deposits on substrates. Purpose of the invention

It is an object of the present invention to provide a method for the electrodeposition of dark chromium layers which starts from a basic darkness but which, in addition, provides (I) a further reduced darkness and (II) a simultaneous high flexibility in the electroplating bath Measured to slightly vary the further reduced darkness to a specific target darkness by reducing or increasing the lightness L* according to the desired specification (according to the L*a*b* color space system). Furthermore, this flexibility preferably allows returning to the base shade without discarding the plating bath.

These objects are solved by a method of electrodepositing a dark chromium layer on a substrate, the method comprising the following steps (a) provide a substrate, wherein the substrate comprises a plastic substrate, (b) Provide an aqueous trivalent chromium plating bath comprising (i) trivalent chromium ions, (ii) for one or more complexing agents for such trivalent chromium ions, (iii) colloidal particles containing the chemical element aluminum, (iv) a first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less, and (v) Optionally, a second sulfur-containing compound different from (iv) having a sulfur atom with an oxidation number of +5 or less, (c) The substrate is brought into contact with the electroplating bath and an electric current is applied so that a layer of dark chromium is electrolytically deposited on the substrate.

Own experiments have shown (see examples below the text) that by means of colloidal particles containing the chemical element aluminum a further reduced darkness is obtained compared to a respective electroplating bath not containing these colloidal particles. The degree is mainly indicated by the further reduced L* value. This is especially the case if the aqueous trivalent chromium electroplating bath contains the first sulfur-containing compound, optimally methionine. However, own experiments have also shown that this effect does not necessarily occur in the case of colloidal particles (eg colloidal silicon dioxide) that do not contain aluminum.

Another great benefit of the method of the present invention is that the aqueous trivalent chromium electroplating bath comprises a combination of (iii) and (iv). Optimally, both act as darkening agents in the context of the present invention. However, they all vary significantly in their properties. Where (iii) is formed by particles, (IV) is preferably a soluble compound. This means that the concentration of (iii) can be adjusted by adding such colloidal particles or by partially (or completely) removing the colloidal particles from the plating bath. Optimally, such removal is achieved by physical/mechanical separation means, such as filtration. This generally allows fine tuning of the brightness L* depending on the amount present. This removal usually does not significantly affect the presence of (iv). In other words, the overall concentration of (iii) is preferably reversibly adjusted by physical means without affecting the overall concentration of (iv). This allows ideal utilization of a wide variety of shades of the same base plating bath.

Advantageously, the dark chrome layer is a decorative chrome layer. Typical applications are automotive parts, optimally automotive interiors. The electroplating bath used in the method of the invention is very suitable for obtaining such a dark chrome layer, optimally as defined throughout the text.

In the context of the present invention, the dark chrome layer is defined by the L*a*b* color system, preferably as introduced by the 1976 International Commission on Illumination (Commission Internationale de I'Eclairage), if not stated otherwise.

In step (a) of the method of the invention, a substrate is provided.

In the method of the present invention, the substrate includes a plastic substrate.

The method of the present invention is preferred, wherein the plastic substrate comprises acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene-polycarbonate (ABS-PC), polypropylene (PP), polyamide (PA), polyurethane (PU), polyepoxide (PE), polyacrylate, polyetherimide (PEI), polyetherketone (PEK), mixtures thereof and/or composites thereof; preferably propylene Nitrile butadiene styrene (ABS), acrylonitrile butadiene styrene-polycarbonate (ABS-PC), polyamide (PA), polyurethane (PU), polyepoxide (PE), polyacrylate , mixtures thereof and/or complexes thereof. Such plastic substrates are commonly used in mid-decoration applications, such as automotive parts, specifically ABS and ABS-PC.

The method of the present invention is preferred, wherein the plastic substrate comprises at least one metal layer (also most preferably). Preferably, at least one metal layer comprises a layer of copper or copper alloy and/or a layer of nickel or nickel alloy.

In some other cases, not in accordance with the present invention, the substrate is a metal substrate, preferably comprising iron, copper, nickel, aluminum, zinc, mixtures thereof and/or alloys thereof. An excellent metal substrate comprising iron is steel. Mixtures thereof preferably include complexes.

The method of the present invention is preferred, wherein after step (a) and before step (c), the method of the present invention comprises the following steps (a1) Pretreatment of substrates, optimal cleaning of substrates, optimal degreasing and/or rinsing of substrates.

Preferably, degreasing comprises electrolytic degreasing.

Preferably, rinsing comprises contact with an acid, preferably a mineral acid.

Water rinsing is preferred after step (a1).

In step (b) of the method of the present invention, the aqueous trivalent chromium electroplating bath is provided.

The electroplating bath contains water, preferably at least 55 vol.-% or more, more preferably 65 vol.-% or more, even more preferably 75 vol.-% or more, based on the total volume of the electroplating bath More, and even better 85 vol.-% or more, more preferably 90 vol.-% or more, most preferably 95 vol.-% or more is water. Optimally, water is the sole solvent.

The method of the present invention is preferred, wherein the water-based trivalent chromium electroplating bath is acidic, preferably having an acidity of 1.5 to 5.0, more preferably 2.1 to 4.6, even more preferably 2.4 to 4.2, and more preferably 2.7 to 4.0 3.8, optimally a pH in the range of 3.0 to 3.5. The pH is preferably adjusted by hydrochloric acid, sulfuric acid, ammonia, potassium hydroxide and/or sodium hydroxide.

The aqueous trivalent chromium electroplating bath comprises (i) trivalent chromium ions.

The method of the present invention is preferred, wherein in the electroplating bath, based on the total volume of the electroplating bath, trivalent chromium ions have a concentration of 5 g/L to 35 g/L, preferably 6 g/L to 32 g /L, more preferably 7 g/L to 29 g/L, even better 8 g/L to 26 g/L, even better 9 g/L to 23 g/L, most preferably 10 g /L to 22 g/L total concentration in the range.

Preferably, the trivalent chromium ions come from trivalent chromium salts, preferably from inorganic chromium salts and/or organic chromium salts, most preferably from inorganic chromium salts. Preferred inorganic chromium salts comprise chloride and/or sulfate anion, preferably sulfate anion. An excellent inorganic chromium salt is basic chromium sulfate. Preferred organochromium salts comprise carboxylic acid anions, preferably formate, acetate, malate and/or oxalate anions.

The method of the present invention is preferred, wherein in the aqueous trivalent chromium electroplating bath, based on the total volume of the aqueous trivalent chromium electroplating bath, trivalent chromium ions are selected together with iron ions as the case may be (about iron ions, in some cases Here, iron ions are optional but preferred, see text below) means 80 mol-% or more in the aqueous trivalent chromium electroplating bath, preferably 90 mol-% or more, more preferably 93 mol-% or more, even better 96 mol-% or more, most preferably 98 mol-% or more of all transition metal ions.

The aqueous trivalent chromium electroplating bath comprises (ii) one or more complexing agents for the trivalent chromium ions.

These compounds keep the trivalent chromium ions in solution. Preferably, the one or more complexing agents are not compounds of (iv), and (v) is thus preferably different from the compound of (iv).

The method of the present invention is preferred, wherein in the aqueous trivalent chromium electroplating bath, one or more complexing agents comprise organic acids and/or salts thereof, preferably organic carboxylic acids and/or salts thereof, most preferably comprise One, two or three carboxyl organic carboxylic acids and/or their salts.

Organic carboxylic acids and/or salts thereof (preferably also organic carboxylic acids and/or salts thereof comprising one, two or three carboxyl groups) are preferably substituted or unsubstituted. Preferred substituents include amino and/or hydroxyl. Preferably, the substituents do not contain SH moieties and/or SCN moieties. More preferably, one or more complexing agents for the trivalent chromium ions are not sulfur-containing compounds having sulfur atoms with an oxidation number of +5 or less.

More preferably, the organic carboxylic acid and/or its salt (preferably also an organic carboxylic acid and/or its salt comprising one, two or three carboxyl groups) comprises a carbamic acid (preferably an α-amino formic acid), hydroxyformic acid and/or its salts. Preferred (α-) carbamic acids include glycine, aspartic acid and/or salts thereof. Preferably, the carbamic acid (preferably respectively α-carbamic acid) is not a compound according to (iv), more preferably not a sulfur-containing carbamic acid (preferably respectively not a sulfur-containing α-amine methionine), most preferably not methionine. Especially preferably, one or more complexing agents are different from (iv) and (v).

The method of the present invention is better, wherein one or more than one complexing agent comprises formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, citric acid, glycine, aspartic acid and/or its salt, preferably formic acid, Acetic acid, oxalic acid, tartaric acid, malic acid, citric acid and/or their salts, more preferably formic acid, acetic acid, oxalic acid, tartaric acid, malic acid and/or their salts, even more preferably formic acid, acetic acid and/or their salts, most preferably Jiadi formic acid and/or its salt.

The method of the present invention is preferred, wherein based on the total volume of the aqueous trivalent chromium electroplating bath, one or more than one complexing agent has a range of 5 g/L to 200 g/L, preferably 8 g/L In the range of L to 150 g/L, more preferably in the range of 10 g/L to 100 g/L, even better in the range of 12 g/L to 75 g/L, and even better Total concentration in the range of 15 g/L to 50 g/L, optimally in the range of 20 g/L to 35 g/L.

The aqueous trivalent chromium electroplating bath comprises (iii) colloidal particles containing the chemical element aluminum.

The method of the present invention is preferred, wherein the aqueous trivalent chromium electroplating bath is a colloidal suspension, preferably a sol. This is due to the presence of the colloidal particles, which are preferably correspondingly smaller.

The method of the present invention is preferred, wherein the colloidal particles comprise nanoparticles, preferably nanoparticles. Preferably, the colloidal particles containing the chemical element aluminum have a particle size of less than 1000 nm, preferably less than 500 nm, more preferably at least 90% of the colloidal particles have a particle size of less than 500 nm, most preferably at least 90% of the colloidal particles The particle size of the particles is less than 150 nm.

The method of the present invention is preferred, wherein by volume, the average particle diameter D of these colloidal particles is ₁₀₀ nm or less, preferably 80 nm or less, more preferably 60 nm or less, Even more preferably 50 nm or less, most preferably 40 nm or less, excellently 30 nm or less, even most preferably 25 nm or less.

The method of the present invention is even better, wherein the colloidal particles at least include particles with a diameter of 100 nm or less, preferably 80 nm or less, more preferably 60 nm or less, even more preferably 50 nm Or smaller, preferably 40 nm or smaller, extremely preferably 30 nm or smaller, even most preferably 20 nm or smaller particles.

The method of the present invention is preferred, wherein based on the total volume of the aqueous trivalent chromium electroplating bath, the colloidal particles are in the range of 0.05 g/L to 15 g/L, preferably 0.1 g/L to 12 g/L L, more preferably 0.5 g/L to 10 g/L, even more preferably 1.0 g/L to 8 g/L, optimally 1.5 g/L to 6 g/L, even optimally 1.9 g/L present in total amounts ranging from 4 g/L to 4 g/L.

The method of the present invention is more preferably, wherein the colloidal particles comprise alumina.

The method _{of the} present invention is most preferably wherein the colloidal particles comprise _Al2O3 , preferably consist essentially of _Al2O3 . Optimally, these are the only colloidal particles in the aqueous trivalent chromium plating bath.

The aqueous trivalent chromium electroplating bath comprises (iv) a first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or lower. Preferably, its acids, salts, isomers and betaines are included. However, sulfate anions are not counted in (iv) and (v).

Optionally, the aqueous trivalent chromium electroplating bath comprises (v) a second sulfur-containing compound having a sulfur atom having an oxidation number of +5 or less other than (iv). Also, this preferably includes its acids, salts, isomers and betaines.

In some cases, the method of the present invention is preferred, wherein the aqueous trivalent chromium electroplating bath comprises (iv) and (v). Therefore, (v) is not optional in this case.

The process of the present invention is generally preferred wherein the first and optionally second sulfur-containing compounds having sulfur atoms having an oxidation number of +5 or less comprise divalent sulfur atoms. These compounds are preferably inorganic or organic.

In some cases, preferably, the first sulfur-containing compound comprises one or more than one organic sulfur-containing compound (preferably as described throughout the text), wherein preferably, the second sulfur-containing compound comprises one or more than one inorganic Sulfur-containing compounds (preferably as described throughout the text).

The process of the present invention is more generally preferred wherein the first and optionally second sulfur-containing compounds having a sulfur atom having an oxidation number of +5 or less are selected from the group consisting of (including salts thereof): (1) 2-(2-Hydroxy-ethylthio)-ethanol, (2) Thiazolidine-2-carboxylic acid, (3) Thiodiethylene glycol ethoxylate (4) 2-Amino-3-ethylthio-propionic acid, (5) 3-(3-Hydroxy-propylthio)-propan-1-ol (6) 2-Amino-3-carboxymethylthio-propionic acid, (7) 2-Amino-4-methylthio-butan-1-ol, (8) 2-Amino-4-methylthio-butyric acid, (9) 2-Amino-4-ethylthio-butyric acid, (10) 3-Formamidinothio-propane-1-sulfonic acid, (11) 3-Formamidinothio-propionic acid, (12) Thiothioline, (13) 2-[2-(2-Hydroxy-ethylthio)-ethylthio]-ethanol, (14) 4,5-dihydro-thiazol-2-ylamine, (15) Thiocyanic acid, (16) 2-Amino-4-methylsulfinyl-butyric acid, (17) 1,1-Dioxo-1,2-dihydro-1λ*6*-benzo[d]isothiazol-3-one (18) prop-2-yne-1-sulfonic acid, (19) Methanesulfinyl methane, and (20) 2-(1,1,3-trioxo-1,3-dihydro-1λ*6*-benzo[d]isothiazol-2-yl)-ethanesulfonic acid

The method of the present invention is also generally preferred, wherein (iv) and (v) together have a range of 16 mmol/L to 1150 mmol/L, preferably 45 mmol/L to 1000 mmol/L, based on the total volume of the electroplating bath. /L, more preferably 80 mmol/L to 900 mmol/L, even better 120 mmol/L to 800 mmol/L, even better 150 mmol/L to 700 mmol/L, most preferably 180 mmol /L to 650 mmol/L total concentration. This applies optimally to all sulfur-containing compounds (ie, the total concentration within all such compounds) in aqueous trivalent chromium plating baths having sulfur atoms with an oxidation number of +5 or less.

The method of the present invention is particularly preferred, wherein based on the total volume of the electroplating bath, the aqueous trivalent chromium electroplating bath is 15 mmol/L to 750 mmol/L, preferably 40 mmol/L to 650 mmol/L, more Preferably 70 mmol/L to 600 mmol/L, even better 100 mmol/L to 550 mmol/L, even better 120 mmol/L to 500 mmol/L, most preferably 140 mmol/L to 470 The total concentration in the range of mmol/L comprises (iv).

The method of the present invention is particularly preferred, wherein based on the total volume of the electroplating bath, the aqueous trivalent chromium electroplating bath is 1 mmol/L to 400 mmol/L, preferably 5 mmol/L to 350 mmol/L, more Preferably 10 mmol/L to 300 mmol/L, even better 20 mmol/L to 250 mmol/L, even better 30 mmol/L to 200 mmol/L, most preferably 40 mmol/L to 180 The total concentration in the range of mmol/L includes (v).

The method of the present invention is preferred, wherein the molar concentration of (iv) is higher than that of (v). In some cases, the method of the present invention is preferred, wherein in the aqueous trivalent chromium electroplating bath, the molar ratio of (iv) to (v) is greater than 1, preferably 1.1 or greater, most preferably 1.2 or larger. Optimally, the first sulfur-containing compound has the highest concentration of sulfur-containing compounds having sulfur atoms with an oxidation number of +5 or less.

In some cases, the method of the present invention is better, wherein in the aqueous trivalent chromium electroplating bath, the molar ratio of (iv) and (v) is in the range of 1.05 to 15, preferably 1.10 to 12, more preferably 1.15 to 10, even better 1.20 to 9, most preferably 1.25 to 8. Furthermore, preferably, the first sulfur-containing compound has the highest concentration of sulfur-containing compounds having sulfur atoms with an oxidation number of +5 or less.

In some cases, the method of the present invention is preferred, wherein the first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less comprises a nitrogen atom, more preferably an amine group, most preferably an amine group acid.

Preferably, the amino acid comprises an α-amino acid having a sulfur atom with an oxidation number of +5 or lower, most preferably a protein amino acid having a sulfur atom with an oxidation number of +5 or lower. Optimally, this includes methionine and cysteine.

In some cases, the method of the present invention is more preferred, wherein the first sulfur-containing compound comprises methionine. The aforementioned molar ratios apply optimally if the first sulfur-containing compound comprises methionine. In other cases, however, the method of the invention is preferred, wherein instead of the first sulfur-containing compound, the second sulfur-containing compound comprises methionine.

The method of the present invention is preferred, wherein based on the total volume of the electroplating bath, methionine has a concentration of 100 mmol/L to 500 mmol/L, preferably 110 mmol/L to 450 mmol/L, more preferably 120 mmol/L to 400 mmol/L, even better 130 mmol/L to 350 mmol/L, even better 140 mmol/L to 300 mmol/L, most preferably 150 mmol/L to 250 mmol/L The total concentration in the range of L. However, in some cases it is preferred that methionine has an even lower total concentration, preferably in the range of 15 mmol/L to 100 mmol/L, more preferably 20 mmol/L to 80 mmol/L Inside.

In some cases, the method of the present invention is preferred, wherein the second sulfur-containing compound comprises an inorganic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less, preferably comprising a thiocyanate anion. However, in some other cases, the method of the present invention is preferred, wherein instead of the second sulfur-containing compound, the first sulfur-containing compound comprises a thiocyanate anion. In the context of the present invention, the thiocyanate anion (ie, SCN ⁻ ) is considered to be inorganic, and the organic compound in which the thiocyanate moiety is contained is considered to be organic thiocyanate. Preferably, the thiocyanate anion is present via thiocyanate (eg potassium thiocyanate, sodium thiocyanate, ammonium thiocyanate) and/or via thiocyanic acid.

The method of the present invention is preferred, wherein based on the total volume of the electroplating bath, the thiocyanate anion has a concentration of 1 mmol/L to 400 mmol/L, preferably 3 mmol/L to 350 mmol/L, more preferably 5 mmol/L to 300 mmol/L, even better 8 mmol/L to 250 mmol/L, even better 12 mmol/L to 200 mmol/L, most preferably 15 mmol/L to 180 mmol/L The total concentration in the range of L.

Preferably, the aqueous trivalent chromium electroplating bath contains other compounds or preferably is free of specific compounds as outlined below.

The method of the present invention is preferred, wherein based on the total volume of the electroplating bath, the aqueous trivalent chromium electroplating bath is preferably at 0.1 mmol/L to 10 mmol/L, preferably 0.4 mmol/L to 8 mmol/L L, more preferably 0.8 mmol/L to 6 mmol/L, even more preferably 1.2 mmol/L to 5 mmol/L, preferably 1.5 mmol/L to 4.5 mmol/L, the concentration in the range further comprises Fe( II) Ions. In many cases, the Fe(II) ions positively affect plating performance. Also, in some cases, it is preferable that the dark chrome layer contains iron.

The method of the present invention is preferred, wherein in step (b), based on the total volume of the electroplating bath, the aqueous trivalent chromium electroplating bath is preferably at 0.2 mol/L to 1.3 mol/L, more preferably 0.3 mol/L to 1.1 mol/L, even more preferably 0.4 mol/L to 1.0 mol/L, even more preferably 0.5 mol/L to 0.9 mol/L, most preferably 0.6 mol/L to 0.8 mol/L The concentration in the range of L further includes sulfate anion. Preferably, sulfate ions are present due to a source of trivalent chromium ions, such as basic chromium sulfate. Sulfonate ions contribute extremely well to the conductivity of the electroplating bath.

The method of the present invention is preferred, wherein in step (b), the aqueous trivalent chromium electroplating bath further comprises a halogen anion, based on the total volume of the electroplating bath, preferably in the range of 0.1 mol/L to 6 mol/L The total concentration within the range, more preferably 0.5 mol/L to 5 mol/L, even more preferably 1 mol/L to 4.5 mol/L, even more preferably 1.5 mol/L to 4.2 mol/L, Optimally the total concentration of halide anions in the range of 2 mol/L to 3.9 mol/L.

The method of the present invention is more preferred, wherein based on the total volume of the electroplating bath, the halogen anion is preferably 0.5 mol/L to 5 mol/L, more preferably 0.8 mol/L to 4.7 mol/L, or even More preferably a total concentration in the range of 1.3 mol/L to 4.5 mol/L, even more preferably 1.8 mol/L to 4 mol/L, most preferably 2.3 mol/L to 3.7 mol/L comprises chloride anions. Chloride ions are preferably derived from chloride salts and/or hydrochloric acid, preferably from sodium chloride, potassium chloride, ammonium chloride, chromium chloride (at least as part of all chloride ions) and/or mixtures thereof. Typically, chloride ions exist as anions of conductive salts. Excellent conductivity salts are ammonium chloride, sodium chloride and potassium chloride, with ammonium chloride being the most preferred.

The process of the present invention is preferred wherein in step (b) the halide anion comprises, in some cases preferably in addition to the chloride anion, a bromide anion. Bromide ions generally avoid anodic formation of undesired hexavalent chromium species. Preferably, based on the total volume of the aqueous trivalent chromium electroplating bath, the bromide ion has a range of 3 g/L to 20 g/L, preferably 4 g/L to 18 g/L, More preferably in the range of 5 g/L to 16 g/L, even more preferably in the range of 6 g/L to 14 g/L, most preferably in the range of 7 g/L to 12 g/L concentration within. The bromide ions are preferably derived from bromide salts, preferably from sodium bromide, potassium bromide, ammonium bromide and/or mixtures thereof. Preferably, if sulfate ions are used in the aqueous trivalent chromium plating bath, bromide ions are also present.

The method of the present invention is preferred, wherein in step (b), the aqueous trivalent chromium electroplating bath further contains ammonium ions.

The method of the present invention is preferred, wherein in step (b), the aqueous trivalent chromium plating bath further comprises one or more than one pH buffer compound. Optimally, one or more than one pH buffer compound differs (ie, differs) from (ii). This preferably means that the one or more than one pH buffer compound does not comprise carboxylic acids, preferably no organic acids.

In many cases, the method of the present invention is preferred, wherein in the aqueous trivalent chromium plating bath, one or more than one pH buffer compound comprises boron-containing compounds, preferably boric acid and/or borate salts, most preferably boric acid. A preferred borate is sodium borate.

The method of the present invention is generally preferred, wherein in the aqueous trivalent chromium electroplating bath, one or more than one pH buffer compound has a pH value between 30 g/L and 250 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath. In the range of L, preferably in the range of 35 g/L to 200 g/L, more preferably in the range of 40 g/L to 150 g/L, even more preferably in the range of 45 g/L to 100 The total concentration in the range of g/L, optimally in the range of 50 g/L to 75 g/L. This applies even better to the boron-containing compound, and even better to the boric acid as well as to the borate, most preferably to the boric acid. Optimally, the one or more than one pH buffer compounds comprise boric acid but not borate. Therefore, the method of the present invention is the best, wherein based on the total volume of the aqueous trivalent chromium electroplating bath, the aqueous trivalent chromium electroplating bath is preferably at 35 g/L to 90 g/L, preferably 40 g Boronic acid is included in a total concentration in the range of 50 g/L to 80 g/L, more preferably 50 g/L to 70 g/L, most preferably 56 g/L to 66 g/L.

However, in some other cases, aqueous trivalent chromium plating baths do not explicitly contain different pH buffer compounds. Conversely, the one or more than one complexing agent for the trivalent chromium ions is present in such an amount and such that the one or more than one complexing agent not only acts as a complexing agent for the trivalent chromium ions, but additionally acts as a pH buffer compound way to choose. In the context of the present invention, this is not preferred, but possible.

The method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath is substantially free of ions and/or compounds comprising zinc, preferably free of ions and/or compounds comprising zinc. Preferably, the dark chromium layer is substantially free of zinc, preferably contains no zinc.

The method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath is not a conversion treatment composition. In other words, aqueous trivalent chromium electroplating baths are not suitable for conversion coatings and/or for application over zinc or zinc alloy layers. In other words, the method of the present invention is not a conversion coating method.

The method of the present invention is preferred, wherein the substrate is substantially free of zinc and zinc alloy layers, preferably free of zinc and zinc alloy layers.

The method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably free of, fluoride ions. Preferably, the dark chrome layer is substantially fluorine-free, preferably fluorine-free.

The method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath is substantially free, preferably free of phosphate anions, more preferably substantially free, preferably free of phosphorus containing compounds. Preferably, the dark chromium layer is substantially free of phosphorus, preferably contains no phosphorus.

The method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath is substantially free of sulfite anions, preferably free of sulfite anions.

The method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath is substantially free of compounds comprising chromium with an oxidation number of +6, preferably free of compounds comprising chromium with an oxidation number of +6. Thus, the electroplating bath is substantially free of hexavalent chromium, preferably free of hexavalent chromium. This means in particular that at least no addition of hexavalent chromium to the aqueous trivalent chromium electroplating bath is intended.

In some cases, the method of the present invention is preferred, wherein the aqueous trivalent chromium electroplating bath is substantially free of cobalt-containing ions and/or compounds, preferably free of cobalt-containing ions and/or compounds. Preferably, the dark chrome layer is substantially cobalt free, preferably cobalt free. In other cases, however, the method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath contains ions and/or compounds comprising cobalt. Preferably, the dark chromium layer contains cobalt. Although cobalt is environmentally questionable, in some cases cobalt provides an additional darkening effect.

The method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath is substantially free of soluble aluminum compounds (including salts thereof), preferably free of soluble aluminum compounds (including salts thereof), preferably free of Dissolve aluminum ions.

The method of the present invention is preferred wherein the aqueous trivalent chromium electroplating bath is substantially free of nickel ions, preferably free of nickel ions. In some cases typical Ni contamination of up to 150 ppm was observed, which is essentially acceptable and thus considered substantially free of nickel ions. Therefore, in some cases, preferably based on the total weight of the aqueous trivalent chromium electroplating bath, nickel ions have a concentration of 0 ppm to 200 ppm, preferably 1 ppm to 150 ppm, and most preferably 2 ppm to 100 ppm. concentration within the range. Optimally, however, the aqueous trivalent chromium plating bath does not contain nickel. Preferably, the dark chrome layer is substantially nickel-free, preferably nickel-free.

It is generally preferred to avoid environmentally questionable nickel and cobalt species. This usually results in uncomplicated wastewater treatment and bath disposal. Additionally, neither nickel nor cobalt is generally required to obtain dark shades.

The method of the present invention is preferred, wherein the aqueous trivalent chromium electroplating bath is substantially free of sulfamic acid and its salts, preferably free of sulfamic acid and its salts.

In step (c) of the method of the present invention, the substrate is brought into contact with an aqueous trivalent chromium electroplating bath and an electric current is applied so that a dark chromium layer is electrolytically deposited on the substrate.

The method of the present invention is preferred, wherein the current in step (c) is preferably 3 A/dm ² to 30 A/dm ² , more preferably 4 A/dm ² to 25 A/dm ² , or even More preferably direct current in the range of 5 A/dm ² to 20 A/dm ² , most preferably 6 A/dm ² to 18 A/dm ² .

The method of the present invention is preferred, wherein in step (c) at least one anode is utilized. The at least one anode is preferably selected from the group consisting of graphite anodes, noble metal anodes and mixed metal oxide anodes (MMO).

Preferred noble metal anodes include platinized titanium anodes and/or platinum anodes.

Preferred mixed metal oxide anodes include platinum oxide coated titanium anodes and/or iridium oxide coated titanium anodes.

The method of the present invention is preferred, wherein the dark chromium layer electrolytically deposited in step (c) has a thickness of 0.05 µm to 1 µm, preferably 0.1 µm to 0.8 µm, more preferably 0.125 µm to 0.6 µm, the best Layer thickness in the range of 0.15 µm to 0.5 µm.

The method of the present invention is preferred, wherein in step (c), contacting is carried out for 1 minute to 20 minutes, preferably 2 minutes to 15 minutes, more preferably 3 minutes to 10 minutes.

The method of the present invention is preferred wherein in step (c) the contacting is carried out at a temperature in the range of 20°C to 60°C, preferably 25°C to 52°C, more preferably 30°C to 45°C.

In step (c) of the method of the present invention, the dark chrome layer is preferably 60 or less according to the L*a*b* color space system, more preferably 58 or less, even more preferably 56 or less A lightness value L* of small, even better 53 or less, most preferably 51 or less is electrodeposited (ie it is an electroplated chrome layer) on the substrate.

The method of the present invention is more preferred, wherein in step (c), according to the L*a*b color system, the L* value of the dark chrome layer is between 45 and 59, preferably between 47 and 55, and most preferably between 49 and 53 range. Optimally, the dark chrome layer contains the chemical element aluminium. This means that the colloidal particles are preferably incorporated into the dark chrome layer.

The method of the present invention is preferred, wherein in step (c), according to the L*a*b color system, the a* value of the dark chrome layer is between -0.5 and +3.0, preferably 0 to 2.5, more preferably +0.3 to +2, optimally in the range of +0.5 to 1.5.

The method of the present invention is preferred, wherein according to the L*a*b color system, the b* value of the dark chrome layer is from +3.1 to +7, preferably from 3.5 to 6.5, more preferably from +4 to +6, most preferably Good place in the range of +4.5 to 5.5.

It is preferred that the method of the present invention further comprises, prior to step (c), at least one metallization step of depositing at least one metal or metal alloy layer, preferably at least one plating step of depositing at least one nickel or nickel alloy layer nickel step. In many cases, two or even three such metallization steps are preferably involved.

Optimally, the at least one layer of nickel or nickel alloy comprises at least one layer of bright nickel and/or (preferably or) at least one layer of satin nickel, most preferably at least one layer of bright nickel.

The method of the present invention is more preferred, wherein in addition to the at least one bright nickel layer and/or the at least one satin nickel layer, at least one nickel or nickel alloy layer comprises at least one semi-bright nickel layer, preferably at least one semi-bright nickel layer. Bright nickel finish. At least one layer of semi-bright nickel is preferably optional. Optimally, if applied, at least one semi-bright nickel layer is deposited before the at least one bright nickel layer and/or the at least one satin nickel layer.

The method of the invention is also preferred, wherein at least one nickel or nickel alloy layer comprises at least one MPS nickel layer, preferably at least one MPS in addition to the at least one bright nickel layer and/or the at least one satin nickel layer A nickel layer, preferably at least one MPS nickel layer in addition to the at least one bright nickel layer and/or the at least one satin nickel layer, and further in addition to the at least one semi-bright nickel layer. In the context of the present invention, MPS means that the MPS nickel layer contains non-conductive micro-particles, which generate micropores in the subsequent chromium layer, preferably in the dark chromium layer. At least one MPS nickel layer is preferably optional.

In some cases, the method of the present invention is preferred where the MPS nickel layer is adjacent to the dark chromium layer.

In other cases, the method of the present invention is preferred, wherein the dark chromium layer is adjacent to at least one bright nickel layer and/or at least one satin nickel layer, which in most cases is preferred, optimally with at least one bright nickel layer. Nickel layer combination.

Preferably, the dark chrome layer is part of the layer stack.

The present invention further relates to an aqueous trivalent chromium electroplating bath comprising (i) trivalent chromium ions, (ii) for one or more than one complexing agent for such trivalent chromium ions, (iii) colloidal particles containing the chemical element aluminum, (iv) the first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less, and (v) Optionally, a second sulfur-containing compound different from (iv) having a sulfur atom with an oxidation number of +5 or less.

Preferably, with regard to the method of the present invention, in particular, the foregoing statements regarding the aqueous trivalent chromium electroplating bath used in the method of the present invention are equally applicable to the aqueous trivalent chromium electroplating bath of the present invention, and are best described as especially good features.

Furthermore, the present invention also relates to a substrate comprising a dark chromium layer, wherein the dark chromium layer comprises the chemical element aluminum and has a color of 60 or less, preferably 58 or less, more preferably according to the L*a*b color system An L* value of 56 or less, even more preferably 53 or less, most preferably 51 or less, wherein the substrate comprises a plastic substrate.

Preferably, what has been said previously regarding the method of the invention, in particular about the dark chrome layer obtained by the method of the invention, also applies to the substrate of the invention, is best described as a particularly preferred feature. This applies optimally to the appearance and composition of the dark chrome layer mentioned above explicitly or implicitly in relation to the method of the invention.

The substrate of the invention is excellent, wherein the dark chrome layer comprises the chemical element aluminum and has a color in the range of 45 to 59, preferably 47 to 55, most preferably 49 to 53 according to the L*a*b color system. L*value.

The substrate of the present invention is particularly preferred, wherein the a* value of the dark chromium layer is in the range of -0.5 to +3.0, preferably 0 to 2.5, more preferably +0.3 to +2, and most preferably +0.5 to 1.5 .

The substrate of the present invention is particularly preferred, wherein the b* value of the dark chromium layer is in the range of +3.1 to +7, preferably 3.5 to 6.5, more preferably +4 to +6, most preferably +4.5 to 5.5 .

Substrates of the present invention are preferred wherein the dark chrome layer is substantially cobalt free, preferably cobalt free. In contrast, other cases where the dark chrome layer contains cobalt are preferred.

Most preferred are substrates of the invention wherein the dark chrome layer is substantially nickel-free, preferably nickel-free.

Preferably, the dark chrome layer is not the only metal layer between the dark chrome layer and the substrate.

The substrate of the present invention is preferred, wherein the substrate comprises at least one nickel layer or nickel alloy layer below the dark chromium layer.

The substrate of the present invention is preferred, wherein the at least one nickel layer or nickel alloy layer comprises at least one bright nickel layer or at least one satin nickel layer. In many cases, this is optimal.

The substrate of the present invention is preferred, wherein the at least one nickel layer or nickel alloy layer comprises at least one semi-bright nickel layer, preferably at least one layer other than the at least one bright nickel layer and/or the at least one satin nickel layer A layer of semi-bright nickel. At least one layer of semi-bright nickel is preferably optional. Optimally, at least one semi-bright nickel layer is the nickel layer (of all nickel layers) closest to the substrate.

The substrate of the present invention is preferred, wherein the at least one nickel layer or nickel alloy layer comprises at least one MPS nickel layer, preferably at least one other than the at least one bright nickel layer and/or the at least one satin nickel layer The MPS nickel layer, preferably at least one MPS nickel layer in addition to the at least one bright nickel layer and/or the at least one satin nickel layer, and further in addition to the at least one semi-bright nickel layer. In the context of the present invention, MPS denotes micropores.

The substrate of the present invention is preferred, wherein the at least one MPS nickel layer faces the side where the dark chrome layer is located and faces the other side where the at least one bright nickel layer or the at least one satin nickel layer is located.

The substrate of the present invention is preferred, wherein the at least one semi-bright nickel layer is located below the at least one bright nickel layer or the at least one satin nickel layer.

Even more preferred is a substrate of the invention wherein the dark chromium layer is part of a layer stack comprising (adjacent or not) in the direction from the dark chromium layer to the substrate: (i) dark chrome layer, (ii) as the case may be, at least one layer of MPS nickel, (iii) at least one layer of bright nickel and/or (preferably or) at least one layer of satin nickel, and (iv) At least one layer of semi-bright nickel, as appropriate.

If not stated otherwise, the aforementioned specific features with respect to the substrate of the invention preferably also apply to the method of the invention.

In some cases, the layer stack preferably includes a sealant layer and/or an anti-fingerprint layer, most preferably on a dark chrome layer. If both are applied, the sealant layer is preferably applied first, followed by the anti-fingerprint layer, which preferably forms the very outermost layer.

The spirit of the present invention is further illustrated in the following examples without limiting the scope of the present invention as defined in the patent claims herein. Example (a) provides the substrate:

For the following examples, a copper plate (99 mm x 70 mm) was used as the substrate, mainly for simulating a plastic substrate on which a copper layer is deposited.

In the first step, the substrate was cleaned by electrolytic degreasing with Uniclean® 279 (product of Atotech), 100 g/L at room temperature (RT). Then, the copper plate was dipped with 10 vol% H ₂ SO ₄ and rinsed with water.

In a second step, the cleaned and rinsed substrate was nickel-plated to obtain a bright nickel layer on top of the copper plate (parameters: 10 min at 4 A/dm ² ; Makrolux® NF electrolyte; product of Atotech). (b) Provision of aqueous trivalent chromium plating baths

Use the following aqueous trivalent chromium alkaline plating bath solutions: 128 g/L basic chromium sulfate 46 g/L formic acid 60 g/L boric acid 12 g/L ammonium bromide 100 g/L ammonium chloride 110 g/L Potassium chloride 25-45 g/L Methionine 0.1 g/L Sodium Dipentyl Sulfosuccinate

Other ingredients and amounts are summarized in Tables 1 and 2 below. In Table 1, "E" refers to an example according to the present invention, wherein in Table 2, "CE" refers to a comparative example not according to the present invention.

The final pH was about 3.

In all examples according to the invention (ie E1 to E11 ), alumina nanoparticles (Nanobyk-3603; BYK-Chemie GmbH) with a particle size _D50 of 25 nm were used. For comparative examples, no particles were used at all or SiO ₂ nanoparticles (ThermoFisher; 43110; at least 40% concentration) with a particle size of 20 nm were used. (c) Bringing the substrate into contact with the plating bath

Electroplating was performed in a Hull Cell with a graphite anode and a mounted substrate as cathode. A current of 5 A was applied to the aqueous trivalent chromium electroplating bath for 3 minutes, the temperature of the bath being about 35°C for all examples (including comparative examples).

Accordingly, a respective layer of dark chromium was deposited on top of the nickel-plated copper plate. Subsequently, the substrate with the dark chrome layer was rinsed with water.

The color according to the L*a*b* color space system was determined by a colorimeter (Konica Minolta CM-700D spectrophotometer). Calibration is performed by black and white standards. Color determination is made at the area in the center of the substrate. The measurement area is positioned 1 cm from the left edge and 2 cm from the lower edge (left edge pointing towards the anode). Table 1: Summary of compositions and results according to the invention serial number Al-nanoparticles [g/L] Fe(II) [mmol/L] Methionine [g/L] L*; a*; b* E1 1 2 30** 49.9; +1.0; +5.6 E2 1.7 2 30** 49.5; +0.9; +5.6 E3 2.3 2 30** 48.9; +0.9; +5.7 E4 1 2 25 56.0; +0.3; +2.6 E5 1.7 2 25 55.3; +0.4; +2.9 E6 2.3 2 25 55.2; +0.3; +3.1 E7 1 4 25 57.6; +0.1; +1.9 E8 1.7 4 25 56.6; +0.2; +2.3 E9 2.3 4 25 55.7; +0.3; +2.8 E10 1 4 43* 53.9; +0.5; +4.0 E11 2.3 4 43* 52.1; +0.7; +4.7 *additionally contains about up to 20 g/L thioethylene glycol **additionally contains about up to 20 g/L thioethylene glycol and thiocyanate anion Table 2: Summary of Composition and Results of Comparative Examples serial number SiO ₂ -nanoparticles [ml/L] Fe(II) [mmol/L] Methionine [g/L] L*; a*; b* CE1 0 2 30** 55.8; +0.4; +3.2 CE2 2 2 30** 55.2; +0.4; +3.3 CE3 3 2 30** 55.3; +0.4; +3.3 CE4 4 2 30** 56.2; +0.4; +3.2 CE5 0 2 25 61.7;-0.3;0 CE6 0 4 25 61.6; -0.1; +0.4 CE7 0 4 43* 58.9; +0.2; +1.3 *additionally contains up to about 20 g/L thioethylene glycol **additionally contains up to about 20 g/L thioethylene glycol and thiocyanate anion

Comparative Examples CE1 and CE5 to CE7 show conditions that do not have any particles but contain only an organic colorant such as methionine. Interestingly, the presence of silica nanoparticles does not significantly reduce the L* value (compare CE1 with CE2 to CE4); at least not for the plating baths containing methionine. In these cases, the L* value remained substantially at 55. Nevertheless, all chrome layers obtained were already dark.

However, after addition of colloidal particles containing the chemical element aluminum, in particular alumina nanoparticles, a significant further reduction in the L* value (ie further darkening) was observed. In detail: CE1 (L* 55.8) => E1 to E3 (L* 48.9 to 49.9; at least about 10% reduction); CE5 (L* 61.7) => E4 to E6 (L* 55.2 to 56.0; at least about 9% smaller); CE6 (L* 61.6) => E7 to E9 (L* 55.7 to 57.6; at least about 6% smaller); and CE7 (L* 58.9) => E10 and E11 (L* 52.1 to 53.9 ; by at least about 8%).

In each context, ie in each comparative example and its corresponding inventive example, the L* value is reduced by between about 6% and 10%.

As also shown in Tables 1 and 2, the a* values remained substantially constant in all examples. However, the addition of colloidal particles containing the chemical element aluminum significantly affects the b* value, which essentially always increases. In detail: CE1 (b* +3.2) => E1 to E3 (b* about +5.6); CE5 (b* 0) => E4 to E6 (b* +2.6 to +3.1); CE6 (b* + 0.4) => E7 to E9 (b* +1.9 to +2.8); and CE7 (b* +1.3) => E10 and E11 (b* +4.0 to +4.7). Thus, in the case of colloidal particles containing the chemical element aluminum, a warmer beige shade is obtained.

The dark chrome layers obtained by means of the invention are glossy.

Claims (15)

A method for electrodepositing a dark chromium layer on a substrate, the method comprising the following steps (a) provide the substrate, wherein the substrate comprises a plastic substrate, (b) Provide an aqueous trivalent chromium plating bath comprising (i) trivalent chromium ions, (ii) for one or more complexing agents for such trivalent chromium ions, (iii) colloidal particles containing the chemical element aluminum, (iv) a first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less, and (v) Optionally, a second sulfur-containing compound different from (iv) having a sulfur atom with an oxidation number of +5 or less, (c) contacting the substrate with the electroplating bath and applying an electrical current so that the dark chrome layer is electrolytically deposited on the substrate. The method of claim 1, wherein the aqueous trivalent chromium electroplating bath is a colloidal suspension, preferably a sol. The method according to claim 1 or 2, wherein the colloidal particles comprise nanoparticles, preferably nanoparticles. The method according to any one of the preceding claims, wherein the colloidal particles have an average particle diameter _D50 of 100 nm or less, preferably 80 nm or less, more preferably 60 nm or more by volume Small, even more preferably 50 nm or less, most preferably 40 nm or less, extremely preferably 30 nm or less, even most preferably 25 nm or less. The method according to any one of the preceding claims, wherein the colloidal particles at least include a particle size of 100 nm or less, preferably 80 nm or less, more preferably 60 nm or less, even more preferably Particles of 50 nm or less, most preferably 40 nm or less, excellently 30 nm or less, even most preferably 20 nm or less. The method according to any one of the preceding claims, wherein based on the total volume of the aqueous trivalent chromium electroplating bath, the colloidal particles are in the range of 0.05 g/L to 15 g/L, preferably 0.1 g/L to 12 g/L, more preferably 0.5 g/L to 10 g/L, even more preferably 1.0 g/L to 8 g/L, optimally 1.5 g/L to 6 g/L, even optimally 1.9 present in total amounts ranging from g/L to 4 g/L. The method of any one of the preceding claims, wherein the colloidal particles comprise alumina. The method _of any one of the preceding claims, _wherein the colloidal particles comprise _Al2O3 , preferably consist essentially of _Al2O3 . The method according to any one of the preceding claims, wherein the first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or lower comprises nitrogen atoms, more preferably amine groups, most preferably amino acids. The method of any one of the preceding claims, wherein the first sulfur-containing compound comprises methionine. The method of any one of the preceding claims, wherein the second sulfur-containing compound comprises an inorganic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less, preferably comprising a thiocyanate anion. An aqueous trivalent chromium electroplating bath comprising (i) trivalent chromium ions, (ii) for one or more complexing agents for such trivalent chromium ions, (iii) colloidal particles containing the chemical element aluminum, (iv) a first sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less, and (v) Optionally, a second sulfur-containing compound different from (iv) having a sulfur atom with an oxidation number of +5 or less. A substrate comprising a dark chromium layer, wherein the dark chromium layer comprises the chemical element aluminum and has a color of 60 or less, preferably 58 or less, more preferably 56 or less according to the L*a*b color system, Even better L* value of 53 or less, most preferably 51 or less, Wherein the substrate includes a plastic substrate. The substrate according to claim 13, wherein the a* value of the dark chrome layer is in the range of -0.5 to +3.0, preferably 0 to 2.5, more preferably +0.3 to +2, most preferably +0.5 to 1.5. The substrate according to claim 14, wherein the b* value of the dark chromium layer is in the range of +3.1 to +7, preferably 3.5 to 6.5, more preferably +4 to +6, most preferably +4.5 to 5.5.