JP2023531317A - Method for electrodepositing a dark chromium layer on a substrate and substrate completely covered with a dark chromium layer on at least one side - Google Patents

Abstract

The present invention relates to a method for electrodepositing a dark chromium layer on a substrate and a substrate completely covered on at least one side with a dark chromium layer. The method includes utilizing an aqueous trivalent chromium electroplating bath containing colloidal particles and includes treating the substrate with a rinse solution having a temperature of 50° C. or higher.

Description

The present invention relates to a method for electrodepositing a dark chromium layer on a substrate and a substrate completely covered on at least one side with a dark chromium layer. The method includes utilizing an aqueous trivalent chromium electroplating bath containing colloidal particles and includes treating the substrate with a rinse solution having a temperature of 50° C. or higher.

BACKGROUND OF THE INVENTION From the inception of chrome layers, there has been interest in dark chrome layers. Starting with dark hexavalent chromium layers, today there is a significant shift in focus to trivalent chromium layers due to increased environmental acceptance.

Typically, each chrome layer, and in particular a dark chrome layer, is characterized by color values with reference to the L ^* a ^* b ^* color space system. The value L ^* defines the luminance (or sometimes called lightness), while the values a ^* and b ^* define the color of the respective chrome layer. An L ^* value of 100 defines a diffuse white color, while an L ^* value of 0 is a deep black color. The a ^* and b ^* values can be positive or negative, with a ^* values representing green (negative) and red (positive) and b ^* values representing blue (negative) and yellow (positive). A combination of a ^* and b ^* of 0 represents a neutral gray, while lower L ^* values (eg, 50 or less) turn to a darker neutral black.

Various L ^* a ^* b ^* values can be produced depending on the chemical composition of each aqueous trivalent chromium electroplating bath and/or its deposition parameters. The more pronounced these variations are, the more pronounced the optical difference between the two electroplated chromium layers.

Recently, it has been observed that a certain color instability is seen over time, especially in dark chromium layers. That is, the dark chromium layer immediately after deposition has a certain L ^* a ^* b ^* value, which then shifts at least slightly over time in days and weeks. The result is an undesirable color change or at least perception. In most cases this is unacceptable for many decorative applications.

It has also been observed that this change ceases after a sufficiently long time. In other words, the color stabilizes after a sufficient amount of time, which can last for weeks or even months, and no further noticeable changes occur. Unfortunately, from a commercial standpoint, it is unacceptable to store each item for such an extended period of time prior to final use. Accelerated aging or artificial aging has therefore been tested.

US Patent Application Publication No. 2020/094526 relates to a black plated resin component and method for manufacturing the same. Various aging tests are disclosed involving relatively short immersion in hot water at 80°C.

While such accelerated aging provides acceptable results and a certain color stability in a relatively short time, independent experiments have shown that the uniformity/distribution of color over the area is often not satisfactory, especially when different current densities are locally required in the deposition process. As a result, uniform color perception can be compromised, which is particularly unacceptable for decorative applications.

Accordingly, there is a continuing need to provide improved methods and electroplating baths for providing improved dark electroplated chromium layers.

OBJECT OF THE INVENTION The object of the present invention was also to provide a method for electrodepositing dark chromium layers on substrates which overcomes the disadvantages mentioned above and in particular allows a more uniform color perception and thus an increased color uniformity without impairing relatively rapid accelerated aging.

It was also an object of the present invention to provide respective substrates having such properties.

SUMMARY OF THE INVENTION These problems are addressed by the present invention, a method for electrodepositing a dark chromium layer on a substrate, comprising:
(a) providing a substrate;
(b) the following (i) trivalent chromium ions;
(ii) one or more complexing agents for the trivalent chromium ions;
(iii) providing an aqueous trivalent chromium electroplating bath comprising colloidal particles and (iv) one or more sulfur-containing compounds having a sulfur atom with an oxidation number of +5 or less;
(c) contacting the substrate with the electroplating bath so that a dark chromium layer is electrolytically deposited on the substrate and an electric current is applied to completely cover at least one side of the substrate;
(d) treating the substrate with the electrolytically deposited dark chromium layer with a rinsing liquid, wherein the rinsing liquid has a temperature of 50° C. or higher and no current is applied during step (d).

Emphasis is placed on decorative applications in the context of the present invention. Therefore, in the context of the present invention, a dark chrome layer preferably refers to a decorative dark chrome layer. Accordingly, the method of the present invention focuses on substrates having a dark chromium layer completely covering at least one side of said substrate. In other words, the dark chrome layer at least completely covers the first side of the substrate. Most preferably, this is the front side, and in the context of the present invention this side is the visually/optically perceptible side of the substrate. In most cases, this is preferably a decorative substrate designed to be visually perceived by the end user. Preferably, optionally also at least one side is completely covered by a dark chromium layer. If more than one face is completely covered by the dark chromium layer (i.e. the second, third face, etc.), most preferably in the context of the present invention the focus is on the face with the larger surface area (preferably the face perceived visually by the end user). This preferably also applies to the substrate according to the invention.

As shown in the Examples section below, substrates completely covered by a dark chromium layer on at least one side show improved color uniformity (ie reduced ΔE), thereby reducing risk.

DETAILED DESCRIPTION OF THE INVENTION In step (a) of the method of the invention, a substrate is provided.

Preferred is the method of the invention wherein the substrate comprises a non-metallic substrate, preferably a plastic substrate.

Non-metallic substrates, preferably plastic substrates, are acrylonitrile-butadiene-styrene (ABS), acrylonitrile-butadiene-styrene-polycarbonate (ABS-PC), polypropylene (PP), polyamides (PA), polyurethanes (PU), polyepoxides (PE), polyacrylates, polyetherimides (PEI), polyetherketones (PEK), mixtures thereof and/or composites thereof, preferably acrylonitrile-butadiene-styrene (ABS), acrylonitrile Preference is given to processes according to the invention comprising butadiene-styrene-polycarbonate (ABS-PC), polyamides (PA), polyurethanes (PU), polyepoxides (PE), polyacrylates, mixtures thereof and/or composites thereof. Such plastic substrates, especially ABS and ABS-PC, are typically used for decorative applications such as automotive parts.

Preferred is the method of the invention, wherein the substrate, preferably a non-metallic substrate, more preferably a plastic substrate, is provided with (most preferably additionally) at least one metal layer. Preferably, the at least one metal layer is selected from the group consisting of copper layers, copper alloy layers, nickel layers, nickel alloy layers and combinations thereof.

After step (a) and before step (c),
(a1) Pre-treating the substrate, preferably cleaning the substrate, most preferably degreasing and/or pickling the substrate is preferred.

Preferably, the degreasing includes electrolytic degreasing.

Preferably, pickling comprises contacting with an acid, preferably an inorganic acid.

Water washing is preferably carried out following step (a1).

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

The electroplating bath comprises water, preferably at least 55 vol.% or more, more preferably 65 vol.% or more, even more preferably 75 vol.% or more, even more preferably 85 vol.% or more, even more preferably 90 vol.% or more, most preferably 95 vol.% or more, based on the total volume of the electroplating bath. Most preferably, water is the only solvent.

Preferred is the process of the present invention wherein the aqueous trivalent chromium electroplating bath is acidic, preferably having a pH in the range of 1.5 to 5.0, more preferably 2.1 to 4.6, even more preferably 2.4 to 4.2, even more preferably 2.7 to 4, most preferably 3.0 to 3.8. The pH is preferably adjusted with hydrochloric acid, sulfuric acid, ammonia, potassium hydroxide and/or sodium hydroxide.

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

In the above electroplating baths, preferred is the method of the present invention wherein trivalent chromium ions have a total concentration in the range 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 more preferably 8 g/L to 26 g/L, even more preferably 9 g/L to 23 g/L, most preferably 10 g/L to 22 g/L, based on the total volume of the electroplating bath.

Preferably, the trivalent chromium ions are derived from trivalent chromium salts, preferably inorganic chromium salts and/or organic chromium salts, most preferably inorganic chromium salts. Preferred inorganic chromium salts contain chloride anions and/or sulfate anions, preferably sulfate anions. A highly preferred inorganic chromium salt is basic chromium sulfate. Preferred organic chromium salts contain carboxylate anions, preferably formate, acetate, malate and/or oxalate anions.

In the aqueous trivalent chromium electroplating bath, preferred is the method of the present invention wherein the trivalent chromium ions together with any iron ions (for optional but sometimes preferred iron ions see below) account for 80 mol% or more, preferably 90 mol% or more, more preferably 93 mol% or more, even more preferably 96 mol% or more, most preferably 98 mol% or more of the total transition metal ions in the aqueous trivalent chromium electroplating bath, based on the total volume of the aqueous trivalent chromium electroplating bath.

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

Such compounds retain trivalent chromium ions in solution. Preferably, the one or more complexing agents are not compounds of (iv) and are therefore preferably different from (iv).

In aqueous trivalent chromium electroplating baths, the method of the present invention is preferred wherein the one or more complexing agents comprise organic acids and/or salts thereof, preferably organic carboxylic acids and/or salts thereof, most preferably organic carboxylic acids and/or salts thereof containing 1, 2 or 3 carboxylic acid groups.

The organic carboxylic acids and/or salts thereof (preferably also organic carboxylic acids and/or salts containing said 1, 2 or 3 carboxylic acid groups) are preferably substituted with substituents or unsubstituted. Preferred substituents include amino and/or hydroxyl groups. Preferably, the substituents do not contain SH and/or SCN moieties.

More preferably, the organic carboxylic acid and/or its salt (preferably, furthermore the organic carboxylic acid and/or its salt containing one, two or three carboxylic acid groups) comprises an aminocarboxylic acid (preferably an α-aminocarboxylic acid), a hydroxylcarboxylic acid and/or a salt thereof. Preferred (α-)aminocarboxylic acids include glycine, aspartic acid and/or salts thereof. Preferably, the aminocarboxylic acid (preferably each α-aminocarboxylic acid) is not a compound according to (iv), more preferably not a sulphur-containing aminocarboxylic acid (preferably not each sulphur-containing α-aminocarboxylic acid), most preferably not methionine. It is particularly preferred that the one or more complexing agents are distinct from (iv).

The present invention wherein the one or more complexing agents comprise formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, citric acid, glycine, aspartic acid and/or salts thereof, preferably formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, citric acid and/or salts thereof, more preferably formic acid, acetic acid, oxalic acid, tartaric acid, malic acid and/or salts thereof, even more preferably formic acid, acetic acid and/or salts thereof, most preferably formic acid and/or salts thereof. is more preferred.

The one or more complexing agents are in the range of 5 g/L to 200 g/L, preferably 8 g/L to 150 g/L, more preferably 10 g/L to 100 g/L, even more preferably 12 g/L to 75 g/L, even more preferably 15 g/L to 50 g/L, most preferably 20 g/L to 35 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath. Methods of the invention having total concentrations in the range of L are preferred.

The aqueous trivalent chromium electroplating bath contains (iii), colloidal particles.

Preferred is the method of the invention in which the aqueous trivalent chromium electroplating bath is a colloidal suspension. This is due to the presence of the colloidal particles. However, very dilute colloidal suspensions are preferred.

Preferred is the method of the invention, wherein the colloidal particles contain one or more chemical elements selected from the group consisting of silicon, aluminum and carbon, preferably silicon and aluminum, most preferably the colloidal particles contain the chemical element aluminum.

Preferred is the method of the invention, wherein the colloidal particles comprise nanoparticles, preferably nanoparticles. Preferably, the colloidal particles 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 have a particle size of less than 150 nm.

Preferred is the process of the invention wherein said colloidal particles have an average particle size _D50 by volume of 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, very most preferably 30 nm or less, even most preferably 25 nm or less.

More preferred is the process of the present invention wherein said colloidal particles comprise at least particles having a particle size of 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, very most preferably 30 nm or less, even most preferably 20 nm or less. Most preferably it does not exceed a particle size of 100 nm.

Preferred is the method of the invention wherein the colloidal particles containing the chemical element silicon comprise silica, preferably silica colloidal particles.

Preferred is the method of the invention, wherein the colloidal particles comprising the chemical element aluminum comprise aluminum oxide, preferably aluminum oxide colloidal particles.

Most preferred is the process of the invention wherein said colloidal particles comprise aluminum oxide, preferably aluminum oxide colloidal particles.

Preferred is the method of the present invention, wherein the colloidal particles containing the chemical element carbon comprise nanodiamonds, preferably nanodiamond colloidal particles.

In step (b), the colloidal particles are in an amount of 0.05 g/L to 100 g/L, preferably 0.1 g/L to 80 g/L, more preferably 0.25 g/L to 60 g/L, even more preferably 0.5 g/L to 45 g/L, most preferably 0.75 g/L to 35 g/L, and most preferably 1 g/L to 20 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath. Preferred are methods of the invention present in the total amount of the range.

The aqueous trivalent chromium electroplating bath includes (iv) one or more sulfur-containing compounds having sulfur atoms with an oxidation number of +5 or less. Preferably, its acids, salts, isoforms and betaines are included. However, sulfate anions are not included in (iv).

In some cases, the process of the invention is preferred, wherein said one or more sulfur-containing compounds comprise a divalent sulfur atom.

In step (b) the aqueous trivalent chromium electroplating bath comprises (iv) at a total concentration ranging from 1 mmol/L to 1150 mmol/L, preferably from 16 mmol/L to 900 mmol/L, more preferably from 30 mmol/L to 800 mmol/L, even more preferably from 70 mmol/L to 700 mmol/L, most preferably from 110 mmol/L to 595 mmol/L, based on the total volume of the aqueous trivalent chromium electroplating bath. is generally preferred.

Optionally preferred is a process of the invention wherein (iv) comprises at least one inorganic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less. In other cases, however, the process of the invention is preferred in which (iv) comprises at least one organic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less. In yet other cases, the process of the present invention is preferred, wherein (iv) comprises at least one inorganic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less and at least one organic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less.

Preferred is the process of the invention wherein (iv) comprises thiocyanate anion, preferably at least thiocyanate anion. In the context of the present invention, the thiocyanate anion (ie SCN ⁻ ) is considered inorganic and organic compounds containing a thiocyanate moiety as a functional group are considered organic thiocyanates. Preferably, the inorganic thiocyanate anion is present via thiocyanate (eg ammonium, potassium, sodium thiocyanate) or thiocyanic acid.

In the aqueous trivalent chromium electroplating bath, the thiocyanate anion has a total concentration in the range 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 more preferably 8 mmol/L to 250 mmol/L, even more preferably 12 mmol/L to 200 mmol/L, most preferably 15 mmol/L to 180 mmol/L, based on the total volume of the electroplating bath. The method of the invention is particularly preferred.

(iv) is
- comprising an organic sulfur-containing compound having at least a sulfur atom with an oxidation number of +5 or less and further having a nitrogen atom, preferably an amino group,
- more preferably, (iv) comprises an amino acid having at least a sulfur atom with an oxidation number of +5 or less,
- most preferably (iv) comprises at least methionine,
The method of the invention is preferred.

In some cases, this is preferably applied in addition to the inorganic sulfur-containing compounds mentioned above, which preferably contain thiocyanate anions. In other cases, however, it is preferably applied instead of the aforementioned inorganic sulfur-containing compounds, which preferably contain thiocyanate anions. In the latter case, (iv) contains only at least one of the above organic sulfur-containing compounds.

Preferably, amino acids having a sulfur atom with an oxidation number of +5 or less include α-amino acids having a sulfur atom with an oxidation number of +5 or less, most preferably protein-constituting amino acids having a sulfur atom with an oxidation number of +5 or less. Most preferably it contains methionine and/or cysteine.

In the aqueous trivalent chromium electroplating bath, the organic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less and further having a nitrogen atom (preferably an amino acid having at least a sulfur atom with an oxidation number of +5 or less, more preferably the α-amino acid) is 10 mmol/L to 550 mmol/L, preferably 30 mmol/L to 480 mmol/L, more preferably 60 mmol/L to 410 mmol/L, and even more preferably 80 mmol/L, based on the total volume of the aqueous trivalent chromium electroplating bath. Preferred are methods of the invention having total concentrations ranging from L to 350 mmol/L, even more preferably from 100 mmol/L to 280 mmol/L, most preferably from 130 mmol/L to 200 mmol/L.

In the aqueous trivalent chromium electroplating bath, (iv) is
- at least one inorganic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less, preferably a thiocyanate anion,
and/or (preferably and)
Particularly preferred is the method of the invention comprising an organic sulfur-containing compound having at least a sulfur atom with an oxidation number of +5 or less and further comprising a nitrogen atom, preferably an amino group, more preferably an amino acid having a sulfur atom with an oxidation number of +5 or less, most preferably methionine.

For this reason, the process of the invention is optionally preferred, wherein the aqueous trivalent chromium electroplating bath contains two or more compounds of (iv), most preferably thiocyanate anions, methionine, acids and/or salts thereof.

In some cases, even more preferred is the method of the invention, wherein the aqueous trivalent chromium electroplating bath contains one or more of the following compounds (including salts thereof):
(1) 2-(2-hydroxy-ethylsulfanyl)-ethanol,
(2) thiazolidine-2-carboxylic acid,
(3) thiodiglycol ethoxylate,
(4) 2-amino-3-ethylsulfanyl-propionic acid,
(5) 3-(3-hydroxy-propylsulfanyl)-propan-1-ol,
(6) 2-amino-3-carboxylmethylsulfanyl-propionic acid,
(7) 2-amino-4-methylsulfanyl-butan-1-ol,
(8) 2-amino-4-methylsulfanyl-butyric acid,
(9) 2-amino-4-ethylsulfanyl-butyric acid,
(10) 3-carbamimidoylsulfanyl-propane-1-sulfonic acid,
(11) 3-carbamimidoylsulfanyl-propionic acid,
(12) thiomorpholine,
(13) 2-[2-(2-hydroxy-ethylsulfanyl)-ethylsulfanyl]-ethanol,
(14) 4,5-dihydro-thiazol-2-ylamine,
(15) thiocyanic acid,
(16) 2-amino-4-methanesulfinyl-butyric acid,
(17) 1,1-dioxo-1,2-dihydro-1λ ^* 6 ^* -benzo[d]isothiazol-3-one,
(18) prop-2-yn-1-sulfonic acid,
(19) methanesulfinylmethane,
(20) 2-(1,1,3-trioxo-1,3-dihydro-1λ ^* 6 ^* -benzo[d]isothiazol-2-yl)-ethanesulfonic acid

Preferably, the above compounds refer to (iv), most preferably compounds (1)-(16) and (19) in an aqueous trivalent chromium electroplating bath.

Preferably, the aqueous trivalent chromium electroplating bath contains further compounds or, preferably, does not contain certain compounds as outlined below.

In step (b), the process of the present invention wherein the aqueous trivalent chromium electroplating bath further comprises Fe(II) ions, preferably in a concentration ranging from 0.1 mmol/L to 10 mmol/L, preferably from 0.4 mmol/L to 8 mmol/L, more preferably from 0.8 mmol/L to 6 mmol/L, even more preferably from 1.2 mmol/L to 4 mmol/L, most preferably from 1.5 mmol/L to 2.5 mmol/L, based on the total volume of the aqueous trivalent chromium electroplating bath. is preferred.

In many cases, the Fe(II) ions have a positive impact on electroplating performance. Furthermore, in some cases it is preferred that the chromium layer contains iron.

In step (b), the aqueous trivalent chromium electroplating bath preferably contains sulfuric acid in a concentration ranging from 0.2 mol/L to 1.3 mol/L, more preferably from 0.3 mol/L to 1.1 mol/L, even more preferably from 0.4 mol/L to 1.0 mol/L, even more preferably from 0.5 mol/L to 0.9 mol/L, most preferably from 0.6 mol/L to 0.8 mol/L, based on the total volume of the aqueous trivalent chromium electroplating bath. Preferred are methods of the invention which further comprise an anion. Preferably, the sulphate ions are present by means of a source of trivalent chromium ions, such as basic chromium sulphate. Sulfate ions contribute favorably to the conductivity of the electroplating bath.

In step (b), the aqueous trivalent chromium electroplating bath contains halogen anions, preferably halogens in a total concentration ranging from 0.1 mol/L to 6 mol/L, more preferably from 0.5 mol/L to 5 mol/L, even more preferably from 1 mol/L to 4.5 mol/L, even more preferably from 1.5 mol/L to 4.2 mol/L, most preferably from 2 mol/L to 3.9 mol/L, based on the total volume of the aqueous trivalent chromium electroplating bath. Preferred are methods of the invention which further comprise an anion.

More preferred is the method of the present invention wherein the halogen anions comprise chloride anions in a total concentration preferably in the range of 0.5 mol/L to 5 mol/L, more preferably 0.8 mol/L to 4.7 mol/L, even more preferably 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, based on the total volume of the aqueous trivalent chromium electroplating bath. The chloride ions are preferably derived from chloride salts and/or hydrochloric acid, preferably sodium chloride, potassium chloride, ammonium chloride, chromium chloride (at least as part of the total chloride ions) and/or mixtures thereof. Chloride ions are usually present as the anion of the conducting salt. Highly preferred conductive salts are ammonium chloride, sodium chloride and potassium chloride, with ammonium chloride being most preferred.

Preference is given to processes according to the invention in which in step (b) the halogen anions preferably comprise bromide anions in addition to or instead of chloride ions. Bromide ions generally avoid anodic formation of undesirable hexavalent chromium species. Preferably, bromide ions have a concentration in the range of 3 g/L to 20 g/L, preferably in the range of 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, based on the total volume of the aqueous trivalent chromium electroplating bath. Bromide ions are preferably derived from bromide salts, preferably sodium bromide, potassium bromide, ammonium bromide and/or mixtures thereof. Preferably, bromide ions are also present when sulfate ions are utilized in the aqueous trivalent chromium electroplating bath.

Preferred is the process of the invention, wherein in step (b) the aqueous trivalent chromium electroplating bath further comprises ammonium ions.

Preferred is the method of the present invention wherein in step (b) the aqueous trivalent chromium electroplating bath further comprises one or more pH buffering compounds. Most preferably, the one or more pH buffering compounds are separate (ie different) from (ii) and (iv). That is, preferably the one or more pH buffering compounds are free of carboxylic acids, preferably free of organic acids.

In many cases, the method of the invention is preferred in which the one or more pH buffering compounds comprise a boron-containing compound, preferably boric acid and/or borate salts, most preferably boric acid, in the aqueous trivalent chromium electroplating bath. A preferred borate is sodium borate.

In the aqueous trivalent chromium electroplating bath, the one or more pH buffering compounds, based on the total volume of the aqueous trivalent chromium electroplating bath, is in the range of 30 g/L to 250 g/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, still more preferably in the range of 45 g/L to 100 g/L, most preferably in the range of 50 g/L to 75 g/L. Methods of the invention having concentrations are generally preferred. This applies even more preferably to said boron-containing compound, even more preferably said boric acid in addition to said borate salt, most preferably boric acid. Most preferably, the one or more pH buffering compounds comprise boric acid, but no borate. Thus, most preferred is the process of the present invention wherein the aqueous trivalent chromium electroplating bath preferably comprises boric acid in a total concentration ranging from 35 g/L to 90 g/L, preferably from 40 g/L to 80 g/L, more preferably from 50 g/L to 70 g/L, most preferably from 56 g/L to 66 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath.

However, in some other cases, the aqueous trivalent chromium electroplating bath does not explicitly contain a separate pH buffering compound. Rather, the one or more complexing agents for trivalent chromium ions are present and selected in amounts such that they not only act as complexing agents for trivalent chromium ions, but also act as pH buffering compounds. In the context of the present invention this is less preferred but possible.

Preferred is the method of the present invention wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably completely free of, ions and/or compounds containing zinc. Preferably, the dark chromium layer is substantially free of zinc, preferably completely free of zinc.

Preferred is the method of the invention in which the aqueous trivalent chromium electroplating bath is not a conversion composition. In other words, aqueous trivalent chromium electroplating baths are not suitable for application over conversion coatings and/or zinc or zinc alloy layers. Still further stated, the method of the present invention is not a conversion coating method.

Preferred is the method of the invention wherein the substrate is substantially free of zinc and zinc alloy layers, preferably completely free.

Preferred is the method of the present invention wherein the aqueous trivalent chromium electroplating bath is substantially free of fluoride ions, preferably completely free. Preferably, the dark chromium layer is substantially free of fluorine, preferably completely free of fluorine.

Preferred is the method of the present invention wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably no phosphate anions, more preferably substantially free of, preferably no phosphorus-containing compounds. Preferably, the chromium layer is substantially free of phosphorus, preferably completely free of phosphorus.

Preferred is the method of the present invention wherein the aqueous trivalent chromium electroplating bath is substantially free of, and preferably completely free of, sulfite anions.

Preferred is the method of the present invention wherein the aqueous trivalent chromium electroplating bath is substantially free of, and preferably completely free of, compounds containing chromium with an oxidation number of +6. For this reason, the electroplating bath is substantially free of hexavalent chromium, preferably completely free of hexavalent chromium. This means in particular that no hexavalent chromium is at least intentionally added to the aqueous trivalent chromium electroplating bath. However, this does not exclude hexavalent chromium, which inevitably forms in trace amounts at the anode.

In some cases, the method of the present invention is preferred in which the aqueous trivalent chromium electroplating bath is substantially free of, and preferably completely free of, ions and/or compounds containing cobalt. Preferably, the chromium layer is substantially free of cobalt, preferably completely free of cobalt. In other cases, however, the method of the invention is preferred in which the aqueous trivalent chromium electroplating bath contains ions and/or compounds containing cobalt. Preferably, the chromium layer contains cobalt in such cases. Cobalt is environmentally problematic, but in some cases provides a darkening effect, preferably an additional darkening effect.

Preferred is the process of the present invention wherein the aqueous trivalent chromium electroplating bath is substantially free of soluble aluminum compounds (including salts thereof), preferably completely free, and preferably free of dissolved aluminum ions.

Preferred is the method of the present invention wherein the aqueous trivalent chromium electroplating bath is substantially free of nickel ions, preferably completely free. In some cases, a typical Ni contamination of up to 150 ppm is observed, which is basically acceptable and therefore considered substantially free of nickel ions. For this reason, it is sometimes preferred that nickel ions have a concentration in the range of 0 ppm to 200 ppm, preferably 1 ppm to 150 ppm, most preferably 2 ppm to 100 ppm, based on the total weight of the aqueous trivalent chromium electroplating bath. Most preferably, however, the aqueous trivalent chromium electroplating bath does not contain nickel.

It is generally preferred to avoid environmentally problematic nickel and cobalt species. This generally makes wastewater treatment and disposal of the bath less complicated. In addition, neither nickel nor cobalt are necessarily required to obtain a dark chromium layer.

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

In step (c) of the method of the present invention, the substrate is contacted with an aqueous trivalent chromium electroplating bath, a dark chromium layer is electrolytically deposited on the substrate, and an electric current is passed to completely cover at least one side of the substrate. Preferably, the dark chromium layer comprises a dark chromium alloy layer. In other words, a dark chromium layer in the context of the present invention preferably likewise refers to a dark chromium alloy layer. Preferably, the features throughout this document preferably apply to the dark chromium alloy layer as well.

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

Preferred is the method of the invention, wherein in step (c) at least one anode is provided and 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.

Preferred is the process of the invention wherein the chromium layer electrolytically deposited in step (c) has a layer thickness in the range 0.05 μm to 1 μm, preferably 0.1 μm to 0.8 μm, more preferably 0.125 μm to 0.6 μm, most preferably 0.15 μm to 0.5 μm.

Preferred is the process of the present invention wherein the contact in step (c) is from 1 minute to 20 minutes, preferably from 2 minutes to 15 minutes, more preferably from 3 minutes to 10 minutes.

Preferred is the process of the invention wherein the contact is carried out in step (c) at a temperature in the range 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 invention, a dark chromium layer having a lightness value L ^* according to the L ^* a ^* b ^* color space system and a ^* and b ^* values is electrolytically deposited onto the substrate, in particular a dark chromium layer completely covering at least one side of the substrate (i.e. this is an electroplated dark chromium layer). Typically, the L ^* , a ^* and b ^* values vary slightly to have the highest and lowest L ^* values (L2 ^* and L1 ^* respectively), the highest and lowest a ^* values (a2 ^* and a1 ^* respectively), and the highest and lowest b ^* values (b2 ^* and b1 ^* respectively) on said at least one side completely covered by the dark chromium layer. The closer the respective highest and lowest values are to each other, the more uniform the overall color perception.

Preferred is the method according to the invention in which the dark chromium layer is preferably black. That is, it is preferably an electroplated black chrome layer. This preferably also applies to the substrates of the invention (see below).

The process of the invention is preferred, wherein said dark chromium layer completely covering at least one side ^has an L ^* value in the range of 30-55, preferably 33-53, more preferably 35-51, even more preferably 37-50, even more preferably 39-49, most preferably 40-47, and even most preferably 41-45 according to the L*a ^* b color system.

More preferred is the process of ^the invention wherein said dark chromium layer completely covering at least one side has a maximum L ^* value L2* of 55 or less, preferably 53 or less, more preferably 51 or less, even more preferably 50 or less, even more preferably 49 or less, most preferably 47 or less, and even most preferably 45 or less according to the L ^* a ^* b color system.

Preferred is the process of the invention wherein said dark chromium layer completely covering at least one side has an a ^* value in the range -1.5 to +3, preferably -1 to +2.5, most preferably -0.5 to +2.

More preferred is the method of the invention wherein said dark chromium layer completely covering at least one side has a highest a2 ^* value of +3 or less, preferably +2.5 or less, more preferably +2 or less, most preferably +1.5 or less. Most preferably, the highest a2 ^* value is at least positive.

Preferred is the method of the present invention wherein said dark chromium layer completely covering at least one side has a b ^* value in the range of -1.5 to +6, preferably -1.2 to +5, more preferably -0.9 to +4, even more preferably -0.7 to +3, even more preferably -0.5 to +2.7, most preferably -0.3 to +2.5, and even most preferably 0 to +2.2.

More preferred is the process of the invention, wherein said dark ^chromium layer completely covering at least one side has a highest b ^* value b2* of +6 or less, preferably +5 or less, more preferably +4 or less, even more preferably +3 or less, most preferably +2 or less according to the L ^* a ^* b color system.

Preferred is the method of the invention further comprising, prior to step (c), at least one metal plating step depositing at least one metal or metal alloy layer on the substrate, most preferably at least one nickel plating step depositing at least one nickel and/or nickel alloy layer. In many cases, two or even three such metal plating steps are preferably included.

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

More preferred is the method of the invention, wherein the at least one nickel and/or nickel alloy layer comprises at least one semi-bright nickel layer, preferably at least one semi-bright nickel layer in addition to said at least one bright nickel layer and/or said at least one satin nickel layer. At least one semi-bright nickel layer is preferably optional. Most preferably (if applicable), at least one semi-bright nickel layer is deposited before said at least one bright nickel layer and/or said at least one satin nickel layer.

Also preferred is the method of the present invention wherein the at least one nickel and/or nickel alloy layer comprises at least one MPS nickel layer (i.e., a non-conductive particle-containing nickel layer), preferably at least one MPS nickel layer in addition to said at least one bright nickel layer and/or said at least one satin nickel layer, most preferably said at least one bright nickel layer and/or said at least one satin nickel layer, and even at least one MPS nickel layer in addition to said at least one semi-bright nickel layer. In the context of the present invention, MPS means that the MPS nickel layer contains non-conductive fine particles that create micropores in the dark chromium layer. At least one MPS nickel layer is preferably optional.

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

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

Preferably, the dark chromium layer is part of a layer stack.

In step (d) of the method of the present invention, the substrate obtained from step (c) is treated with a rinsing liquid, the rinsing liquid having a temperature of 50° C. or higher, and no electrical current is applied during step (d). Therefore, step (d) is an electroless step, most preferably a simple rinse step. In other words, preferably no external current is applied and preferably no chemical reducing agent is required for this step. Moreover, steps (c) and (d) are separate steps, ie individual steps, followed by step (c) first followed by step (d).

In the context of the present invention step (d) is a very important step as it allows rapid and direct formation of the desired dark shade of the dark chromium layer and is considered an accelerated aging step. The temperature applied in step (d) is not the temperature used in step (c), but preferably the temperature used in step (d) is higher than the temperature used in step (c).

Preferred is a process of the invention wherein the treatment in step (d) is carried out at a temperature in the range 55°C to 99°C, preferably 60°C to 95°C, more preferably 63°C to 91°C, even more preferably 66°C to 88°C, even more preferably 69°C to 85°C, most preferably 72°C to 83°C.

Preferred is a process according to the invention, wherein in step (d) the rinsing liquid comprises water, ie is preferably aqueous, preferably substantially only water. Most preferably, the rinse consists of water.

More preferred is the process of the invention, wherein in step (d) the treatment with the rinse is immersion in the rinse. However, in some cases, the process of the invention is preferred, wherein in step (d) the treatment with rinsing liquid comprises contact with a stream of rinsing liquid, preferably a constant stream of rinsing liquid.

Preferred are processes of the invention wherein the treatment in step (d) is carried out for 2 minutes to 90 minutes, preferably 3 minutes to 75 minutes, more preferably 4 minutes to 60 minutes, even more preferably 6 minutes to 50 minutes, most preferably 8 minutes to 40 minutes, even most preferably 10 minutes to 35 minutes.

The method of the invention preferably does not exclude further steps, such as additional rinsing, washing, pre-treatment and/or post-treatment steps. Preferably, the steps defined in the examples below apply equally to the general methods described throughout this document. Preferred post-treatment steps preferably include sealing with an inorganic and/or organic sealant and/or contacting with an anti-fingerprint composition.

によると、ΔＥは、上記少なくとも片面を完全に覆う上記暗色クロム層を基準にして０．０００１～３．５、好ましくは０．０００１～３、より好ましくは０．０００１～２．５、さらにより好ましくは０．０００１～２．３、最も好ましくは０．０００１～２の範囲であり、
－ Ｌ２^＊は５０以下である、
基板に関する。 The invention further relates to a specifically defined substrate comprising a dark chromium layer at least completely covering one side of the substrate. The invention particularly relates to a substrate completely covered on at least one side with a dark chromium layer, said dark chromium layer completely covering said at least one side comprising:
- the lowest L ^* value L1 ^* and the highest L ^* value L2 ^* ,
- the lowest a ^* value a1 ^* and the highest a ^* value a2 ^* , and - the lowest b ^* value b1 ^* and the highest b ^* value b2 ^*.
, where

ΔE is in the range 0.0001 to 3.5, preferably 0.0001 to 3, more preferably 0.0001 to 2.5, even more preferably 0.0001 to 2.3, most preferably 0.0001 to 2, based on said dark chromium layer completely covering said at least one side, according to
- L2 ^* is 50 or less,
Regarding substrates.

What has been said above with respect to the method of the invention preferably likewise applies to the specifically defined substrates of the invention (where technically applicable). This most preferably applies to the L ^* a ^* b ^* values and layer thicknesses defined for the method of the invention. However, the following features of the substrate of the invention preferably also apply to the method of the invention (if not already mentioned herein above).

Substrates of the present invention having an L2 ^* of 49 or less, preferably 48 or less, more preferably 47 or less, even more preferably 46 or less, even more preferably 45 or less, most preferably 44 or less, and even most preferably 43 or less are preferred.

Substrates of the invention are preferred wherein b2 ^* is no greater than +7, preferably no greater than +6, more preferably no greater than +5, even more preferably no greater than +4, most preferably no greater than +3.

Preference is given to substrates according to the invention in which the dark chromium layer contains aluminum and/or silicon, preferably aluminum.

More preferred are substrates of the invention in which the dark chromium layer comprises particles, preferably particles containing aluminum and/or silicon, most preferably particles containing aluminum.

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

Substrates according to the invention comprising, in addition to the dark chromium layer, at least one nickel and/or nickel alloy layer underneath the dark chromium layer are preferred.

Substrates of the invention are preferred, wherein said at least one nickel and/or nickel alloy layer comprises at least one bright nickel layer or at least one satin nickel layer. This is most preferred.

Preferred is a substrate of the invention wherein said at least one nickel and/or nickel alloy layer comprises at least one semi-bright nickel layer, preferably comprising at least one semi-bright nickel layer in addition to said at least one bright nickel layer and/or said at least one satin nickel layer. At least one semi-bright nickel layer is preferably optional. Most preferably, the at least one semi-bright nickel layer is the respective nickel and nickel alloy layer of all the nickel and nickel alloy layers closest to the substrate.

Preferred are substrates of the invention wherein said at least one nickel and/or nickel alloy layer comprises at least one MPS nickel layer, preferably said at least one bright nickel layer and/or said at least one satin nickel layer plus at least one MPS nickel layer, most preferably said at least one bright nickel layer and/or said at least one satin nickel layer, and even said at least one semi-bright nickel layer plus at least one MPS nickel layer. In the context of the present invention, MPS stands for microporous.

Preferred are substrates of the invention wherein said at least one MPS nickel layer faces a dark chromium layer on one side and said at least one bright nickel layer or said at least one satin nickel layer on the other side.

Substrates of the invention in which said at least one semi-bright nickel layer underlies said at least one bright nickel layer or said at least one satin nickel layer are preferred (defined when viewed from above towards the substrate).

the dark chrome layer is part of a layer stack, the layer stack being defined (adjacent or non-adjacent) when viewed from above towards the substrate;
(i) a dark chrome layer;
(ii) optionally at least one MPS nickel layer;
(iii) at least one bright nickel layer and/or (preferably or) at least one satin nickel layer;
(iv) optionally with at least one semi-bright nickel layer.

Optionally, the layer stack preferably comprises a sealant layer and/or an anti-fingerprint layer, most preferably on the dark chromium layer. When both are applied, preferably the sealant layer is applied first, followed by the anti-fingerprint layer, which preferably forms the outermost layer.

Without limiting the scope of the invention, which is defined in the claims, the spirit of the invention is further illustrated in the following examples.

Example (a) Substrate preparation:
For the following examples, copper panels (100 mm x 70 mm) were used as substrates, primarily to mimic plastic substrates with a copper layer on top.

In the first step, the substrate was cleaned by electrolytic degreasing with 100 g/L Uniclean® 279 (a product of Atotech) at room temperature (RT). The copper panel was then pickled with 10% by volume H ₂ SO ₄ followed by a water rinse.

In a second step, the cleaned and rinsed substrate was subjected to nickel plating to obtain a bright nickel layer on the copper panel (parameters: 10 min at 4 A/ ^dm2 ; UniBrite 2002, product of Atotech).

(b) Preparation of aqueous trivalent chromium electroplating bath The following basic aqueous trivalent chromium electroplating bath was used:
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 30 g/L methionine 15 g/L potassium thiocyanate 2 mmol/L Fe(II) ions

The final pH value was 3.5.

The added amount of colloidal particles and their types are summarized in Table 1 below. "CE" stands for comparative example and "E" stands for example according to the invention.

(c) Contact between the substrate and the electroplating bath Electroplating was performed in a Hull cell with a graphite anode and the substrate as the cathode. A current of 5 A was passed through an aqueous trivalent chromium electroplating bath having a temperature of about 35° C. for 3 minutes. Agitation was achieved by air agitation. As a result, each dark electroplated chromium layer was completely deposited on a specifically defined optical area of a nickel-plated copper panel, mimicking a typical plastic substrate that would otherwise be completely covered on its front surface.

(d) Treatment of Substrate with Rinse Liquid The substrate with the dark chromium layer was then rinsed with hot water (80° C.) in a rinsing step for 30 minutes by immersing the substrate in a beaker containing said water.

After the rinsing step, the substrates were stored for 15 days to achieve constant color balance. L ^* a ^* b ^* values in accordance with the L ^* a ^* b ^* color space system were then determined at various positions of the optical surface (Konica Minolta CM-700 D spectrophotometer; CIE standard illuminator D65 and 10° standard observer). The spectrophotometer was calibrated using black and white standards.

The L ^* a ^* b ^* values were determined in the range from 3.5 cm from the left edge of the substrate (and 2 cm from the bottom edge of the substrate; the left edge faces the anode side) to 1 cm from the left edge. This region typically determines the highest and lowest L ^* a ^* b ^* values across the optical surface. For this, the highest and lowest values, respectively, were determined and summarized in Table 2 below.

Table 2 shows that for all the examples according to the invention (ie E1-E5) the ΔE is reduced compared to their respective comparative examples (ie CE1 and CE2 respectively). Hence, the overall color impression defined by L ^* , a ^* and b ^* is more uniform.

E4 in particular exhibits a highly desirable b ^* value of less than 3 combined with a relatively low ΔE.

Furthermore, Examples E4 and E5 show that relatively high concentrations of thiocyanate anions generally lead to lower b ^* values compared to Examples E1-E3, regardless of the type of particles used. This also preferably applies to the luminance L ^* .

In addition, Example 4 is the only example that reaches a highly desirable b ^* value of less than +3, which is very closely related to neutral black tones.

Generally, ΔEs above 3.5 (see CE2) are rarely desired.

Claims (15)

A method of electrodepositing a dark chromium layer on a substrate, comprising:
(a) providing a substrate;
(b) the following (i) trivalent chromium ions;
(ii) one or more complexing agents for said trivalent chromium ions;
(iii) colloidal particles;
(iv) providing an aqueous trivalent chromium electroplating bath comprising one or more sulfur-containing compounds having a sulfur atom with an oxidation number of +5 or less;
(c) contacting the substrate with the electroplating bath, wherein the dark chromium layer is electrolytically deposited on the substrate and current is applied to completely cover at least one side of the substrate;
(d) treating the substrate with the electrolytically deposited dark chromium layer with a rinsing liquid, the rinsing liquid having a temperature of 50° C. or higher and no electrical current being applied during step (d). 2. The method of claim 1, wherein said colloidal particles comprise one or more chemical elements selected from the group consisting of silicon, aluminum and carbon, preferably silicon and aluminum, most preferably said colloidal particles comprise the chemical element aluminum. 3. The method of claim 1 or 2, wherein said colloidal particles comprise nanoparticles, preferably nanoparticles. 4. The method of any one of claims 1 to 3, wherein the colloidal particles comprise at least particles having a particle size of 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, very most preferably 30 nm or less, even most preferably 20 nm or less. 5. A method according to any one of claims 2 to 4, wherein said colloidal particles containing the chemical element silicon comprise silica, preferably silica colloidal particles. 5. A method according to any one of claims 2 to 4, wherein said colloidal particles containing the chemical element aluminum comprise aluminum oxide, preferably aluminum oxide colloidal particles. 5. A method according to any one of claims 2 to 4, wherein said colloidal particles containing the chemical element carbon comprise nanodiamonds, preferably nanodiamond colloidal particles. In step (b), the colloidal particles are in an amount of 0.05 g/L to 100 g/L, preferably 0.1 g/L to 80 g/L, more preferably 0.25 g/L to 60 g/L, even more preferably 0.5 g/L to 45 g/L, most preferably 0.75 g/L to 35 g/L, even most preferably 1 g/L to 20 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath. 8. A method according to any one of claims 1 to 7, wherein is present in a total amount in the range of 9. The method of any one of claims 1-8, wherein (iv) comprises a thiocyanate anion. (iv) is
- at least an organic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or less and additionally a nitrogen atom, preferably an amino group,
- more preferably, (iv) comprises an amino acid having at least a sulfur atom with an oxidation number of +5 or less,
- most preferably (iv) comprises at least methionine,
A method according to any one of claims 1 to 9. 11. The method of any one of claims 1 to 10, wherein the dark chrome layer completely covering at least one side has a maximum L ^* value L2 ^* of 55 or less, preferably 53 or less, more preferably 51 or less, even more preferably 50 or less, even more preferably 49 or less, most preferably 47 or less, and even most preferably 45 or less according to the L ^* a ^* b color system. 12. The method of any one of claims 1 to 11, wherein the dark chrome layer completely covering at least one side has a highest b ^* value b2 ^* of +6 or less, preferably +5 or less, more preferably +4 or less, even more preferably +3 or less, most preferably +2 or less according to the L ^* a ^* b color system. A substrate completely covered on at least one side with a dark chrome layer, said dark chrome layer completely covering said at least one side, comprising:
- the lowest L ^* value L1 ^* and the highest L ^* value L2 ^* ,
- the lowest a ^* value a1 ^* and the highest a ^* value a2 ^* , and - the lowest b ^* value b1 ^* and the highest b ^* value b2 ^* ,
, where

ΔE is in the range 0.0001 to 3.5, preferably 0.0001 to 3, more preferably 0.0001 to 2.5, even more preferably 0.0001 to 2.3, most preferably 0.0001 to 2, based on said dark chromium layer completely covering said at least one side, according to
- L2 ^* is 50 or less,
substrate. 14. The substrate of claim 13, wherein b2 ^* is +7 or less, preferably +6 or less, more preferably +5 or less, even more preferably +4 or less, most preferably +3 or less. 15. Substrate according to claim 13 or 14, wherein the dark chromium layer comprises aluminum and/or silicon, preferably aluminum.