TW202409351A - Stable alkaline electroplating bath with a diacid - Google Patents

Abstract

The present invention relates to a method for electroplating a substrate comprising a step of applying a current through an aqueous electroplating bath which comprises the substrate and a diacid of the formula (I) or salts thereof, where n is 2, 3 or 4. The invention also relates to an aqueous alkaline electroplating bath which comprises the diacid of the formula (I) or salts thereof, a metal ion source, and optionally further additives selected from brighteners, leveling agents, complexing agents, water softening agent, or anti-foaming agents. The invention also relates to the diacid of the formula (I) or salts thereof, and to a method for preparing the diacid of the formula (I) or salts thereof, comprising a hydrolysis of a dinitrile of the formula (II); and to a method for preparing the dinitrile of the formula (II) where n is 2, 3 or 4, comprising an addition of acrylonitrile to a diamine of the formula (III); and to a use of the diacid of the formula (I) or salts thereof for reducing the cyanide formation during electroplating.

Description

Stable alkaline plating bath containing diacid

The present invention relates to a method for electroplating a substrate, comprising the step of applying a current through an aqueous electroplating bath. The present invention also relates to an aqueous alkaline electroplating bath. The present invention also relates to a diacid of formula (I) or a salt thereof, and a method for preparing a diacid of formula (I) or a salt thereof; and a use of a diacid of formula (I) or a salt thereof to reduce the formation of cyanide during electroplating. The combination of preferred embodiments with other preferred embodiments is within the scope of the present invention.

Electroplating is widely used in industry to improve the surface quality of objects. For example, it can improve the wear and corrosion resistance, lubricity, reflectivity, conductivity or appearance of objects.

Typically, during electroplating, a metallic coating is formed on a substrate by reducing the cations of the metal with the aid of direct current. The part to be coated is usually used as the cathode of an electrolytic cell filled with an electroplating bath. The electroplating bath is typically a solution of the salt of the metal to be coated. The anode is usually a piece of this metal or some inert conductive material.

Electroplating baths usually contain ligands to keep the metal salt in solution. Amine-containing ligands, such as diethylenetriaamine (DETA), are important additives widely used in the electroplating industry. However, these amine-containing ligands form cyanide and byproducts during the undesirable anaerobic oxidation. Cyanide is not only toxic, but also too strong a ligand, which reduces the concentration of metal salt that can be electrodeposited.

Besides reducing the formation of cyanide in the plating bath during electroplating, other objects of the invention are to achieve high quality optical appearance, uniform composition of the metal in the layer, linear gradient of the layer thickness of the deposited metal according to the current density gradient.

The present invention relates to a method for electroplating a substrate, which includes the step of applying electric current through an aqueous electroplating bath. The aqueous electroplating bath includes a substrate and a diacid of formula (I) or a salt thereof, wherein n is 2, 3 or 4. The present invention also relates to an aqueous alkaline electroplating bath, which contains a diacid of formula (I) or a salt thereof, a metal ion source, and optionally a brightener, a leveling agent, a complexing agent, a water softener or a defoaming agent. Other additives of the agent. The present invention also relates to a diacid of formula (I) or a salt thereof, and a method for preparing the diacid of formula (I) or a salt thereof, which comprises hydrolyzing the dinitrile of formula (II); and a method of preparing the diacid of formula (II) A process for dinitriles, wherein n is 2, 3 or 4, which comprises adding acrylonitrile to a diamine of formula (III); and a diacid of formula (I) or a salt thereof to reduce cyanide during electroplating The purpose of its formation.

These objects are achieved by a method for electroplating a substrate, the method comprising the step of applying a current through an aqueous electroplating bath, the aqueous electroplating bath comprising the substrate and a diacid of formula (I) (I) or a salt thereof, wherein n is 2, 3 or 4.

Preferably, n is 3. In another form, n is 2. In another form, n is 4. In another preferred form, n is 3 or 4.

The plating bath may contain 0.1 to 200 g/l, 1 to 100 g/l, 2 to 50 g/l or 3 to 30 g/l of the diacid.

Suitable salts of the diacid are alkaline metal salts, alkaline earth metal salts, ammonium salts, organic salts or mixtures thereof. Preferred salts of the diacid are alkaline metal salts, such as sodium salts or potassium salts. Depending on the pH value, the diacid may be present in acid form, salt form and mixtures thereof, for example in aqueous solution.

The electroplating bath usually contains an alkaline component , such as an alkaline metal hydroxide. Suitable alkaline components are sodium hydroxide, potassium hydroxide or lithium hydroxide. The concentration of the alkaline component may be 50 to 250 g/l, or 100 to 200 g/l. The electroplating bath may have a pH > 10, or preferably a pH > 11.

Electrical current usually flows between the anode and cathode through a plating bath. Anodes can be made from zinc, manganese, iron, stainless steel, nickel, carbon or corrosion-resistant metals such as platinum-coated titanium or palladium-tin alloys.

The cathode can be the substrate to be electroplated. The substrate may be made of metal or alloys such as iron, nickel and copper, alloys thereof, or aluminum zincate. The substrate may have any shape, such as plate, cuboid, solid cylinder, hollow cylinder, sphere.

The cathodic current density may be 0.1 to 20 A/dm ² , or 0.2 to 10 A/dm ² , or 0.5 to 6 A/dm ² . The cathodic current may be applied for 1 minute to 5 hours, or 5 minutes to 2 hours, or 10 minutes to 1 hour.

Plating can be done by roller plating or hanging plating. Plating can be done with or without air injection and with or without stirring the substrate.

The anode region or cathode region may be separate or a membrane anode may be used. Preferably, electroplating is performed without separating the anode and cathode from each other by a membrane or separator.

Electroplating can be performed at a temperature of 5 to 50°C or 10 to 40°C.

The electroplating bath is an aqueous electroplating bath, in which the water content may be at least 500, 600, 700 or 800 g/l.

Electroplating baths typically contain a source of metal ions . A suitable source of metal ions is any material capable of providing free metal cations when in aqueous solution. The source of metal ions may include salts of metals or elemental metals. Suitable metal ion sources are zinc, nickel, cobalt, manganese or iron ion sources, with zinc and nickel being preferred. The metal ion source may include a single metal ion source or a mixture of different metal ion sources.

Preferably, the electroplating bath contains a zinc ion source, and optionally other metal ion sources, such as a nickel ion source. The source of zinc ions or nickel ions may include salts of these metals, or the source may be any material that provides at least some free zinc ions and nickel ions, such as elemental zinc and elemental nickel. The zinc and nickel sources may further include other metal alloys, zinc-containing or nickel-containing compounds.

Suitable salts of metals such as zinc or nickel are inorganic metal salts such as halides, carbonates, hydroxides, ammonium sulfates, sulfamides, acetates, formates, nitrates and sulfates, and its hydrate.

Preferred zinc ion sources are zinc chloride, zinc sulfate, zinc oxide, zinc hydroxide, zinc acetate, zinc carbonate, zinc tartrate and zinc sulfamate, among which zinc chloride and zinc sulfate are particularly preferred.

Preferred nickel ion sources are nickel chloride, nickel sulfate, nickel hydroxide, nickel acetate, nickel ammonium sulfate, nickel carbonate, nickel formate and nickel sulfamate, among which nickel chloride and nickel sulfate are particularly preferred.

The concentration of the metal ion source, for example, the concentration of the zinc ion source or the nickel ion source may be in the range of 0.1 to 100 g/l, or 0.5 to 50 g/l, or 1 to 30 g/l.

The concentration of metal ions, such as zinc ions or nickel ions, may range from 0.01 to 100 g/l, or from 0.1 to 50 g/l, or from 0.2 to 20 g/l.

In addition to the diacids, the electroplating bath may contain other electroplating additives such as brighteners, levelers, complexing agents, water softeners or defoamers.

Suitable brighteners are alkyl naphthalenes, benzenesulfonic acid, benzene disulfonic acid, benzene trisulfonic acid, naphthalene disulfonic acid, naphthalene trisulfonic acid, benzene sulfonamide, naphthalene sulfonamide, benzenesulfonimide, naphthalene Sulfonimide, vinylsulfonamide, allylsulfonamide, salts thereof, and combinations thereof.

Other examples of brighteners are piperine. , guanidine, formalin and epichlorohydrin condensation products; pyridinyl propane sulfonate; N-benzyl-3-carboxypyridinium chloride; cucurbitacin; sodium propargyl sulfonate; propargyl alcohol; ethylene glycol propargyl alcohol ether; ethoxylated butynediol; sodium diamyl sulfosuccinate; N,N'-bis[3-(dimethylamino)propyl]urea, polymer with 1,3-dichloropropane; carboxyethyl isothiourea betaine; ethylhexyl sulfate; benzothiazole; N-benzyl nicotinate; benzyl-2-methylimidazole; formaldehyde condensate of 2-naphthalenesulfonate; methyl naphthalene ketone; benzylideneacetone; sodium cumenesulfonate; vinylsulfone sodium; benzothiazolium-2-[4-(dimethylamino)phenyl]-3,6-dimethyl chloride; N,N-dimethyldithiocarbamoylpropyl sulfonate sodium salt; 3-butyl-1-propanesulfonic acid, sodium salt; O-ethyldithiocarbonate-S-(3-sulfopropyl)-ester, potassium salt; bis-(3-sulfopropyl)-disulfide, disodium salt; 3-S-isothioureapropyl sulfonate; 3-(Benzothiazolyl-2-butyl)-propyl-sulfonic acid, sodium salt; N-(polyacrylamide); safranin; crystal violet and its derivatives; phenazonium ) dyes and their derivatives; thiodiglycol ethoxylate; sodium lauryl sulfate; 1-hydroxyethylene-1,1-diphosphonic acid; ethoxylated beta-naphthol; sodium salt of sulfonated alkylphenol ethoxylate; sulfonated benzenesulfonic acid; butynediol dihydroxypropyl sulfonate; sodium saccharin; 3-butyl-1-propanesulfonic acid, sodium salt; formaldehyde condensate of 1-naphthalenesulfonic acid; benzotriazole; sodium benzoate; aqueous reaction products of 2-aminopyridine and epichlorohydrin; ureidoquaternary ammonium polymer; aqueous reaction products of imidazole and epichlorohydrin; vanilla extract; anisaldehyde; piperonal; thiourea; polyvinyl alcohol; reduced polyvinyl alcohol; o-chlorobenzene Formaldehyde; a-naphthaldehyde; condensed naphthalene sulfonate; nicotinic acid; pyridine; 3-hydroxypropane sulfonate; allylpyridinium chloride; diphenylsulfonamide; pyridine butane sulfonate; sodium allyl sulfonate; sodium vinyl sulfonate; naphthalenetrisulfonic acid; sodium cumene sulfonate; carboxymethylpyridinium chloride; propargyl hydroxypropyl ether sulfonate; o-sulfobenzaldehyde; aqueous reaction products of imidazole and epichlorohydrin; phenyl sulfide; polyvinylpyrrolidone; sodium adipate; chloral hydrate; sodium gluconate; sodium salicylate; manganese sulfate; cadmium sulfate; sodium tellurite; and glycine. The concentration of brightener in the plating bath can range from 0.01 g/l to 10 g/l.

Suitable leveling agents are sulfur compounds, such as 3-hydroxy-1,2,4-triazole or thiourea. The concentration of the leveling agent in the plating bath may be in the range of 0.01 g/l to 1 g/l.

Suitable ligands are alkyl enamine compounds such as ethylenediamine, triethylenetetramine and tetraethylenepentamine; ethylene oxide or propylene oxide adducts of the above alkyl enamines; amine alcohols such as N-(2-aminoethyl)-ethanolamine and 2-hydroxyethylaminopropylamine; poly(hydroxyalkyl) alkylene diamines such as N-2(-hydroxyethyl)-N,N',N'-triethylethylenediamine, N,N'-di( N,N,N',N'-tetra(2-hydroxyethyl)-N,N'-diethylethylenediamine, N,N,N',N'-tetra(2-hydroxypropyl)ethylenediamine; poly(alkylene imine) obtained from ethyleneimine, 1,2-propyleneimine, etc.; poly(alkyleneamine) and poly(amino alcohol) obtained from ethylenediamine, triethylenetetramine, ethanolamine, diethanolamine; or mixtures thereof. The concentration of the complexing agent may be 5 to 200 g/l, or 30 to 100 g/l.

The present invention also relates to an aqueous alkaline electroplating bath containing a diacid of formula (I) (I) or a salt thereof, wherein n is 2, 3 or 4, a source of metal ions, and other additives selected from the group consisting of brighteners, leveling agents, complexing agents, water softeners or defoaming agents as necessary.

The present invention also relates to an aqueous alkaline electroplating bath containing a diacid of formula (I) (I) or its salt, wherein n is 2, 3 or 4, a metal ion source, which is a zinc ion source, and other metal ion sources selected as needed, and selected from the group consisting of brighteners, leveling agents, Other additives such as complexing agents, water softeners or defoaming agents.

The aqueous alkaline electroplating bath preferably comprises a diacid of formula (I) wherein n is 3 or 4.

The present invention also relates to a di-acid of formula (I) (I) Where n is 2, 3 or 4, or a salt thereof. Preferably, n is 3. In another preferred form of the diacid of formula (I), n is 3 or 4. Suitable salts of diacids are alkali metal salts, alkaline earth metal salts, ammonium salts, organic salts or mixtures thereof. Preferred diacid salts are alkali metal salts, such as sodium or potassium salts. Depending on the pH value, diacids can exist in acid form, salt form and mixtures thereof, for example in aqueous solutions.

The present invention also relates to a method for preparing the diacid of formula (I) or a salt thereof (I) wherein n is 2, 3 or 4, which contains dinitrile of hydrolyzable formula (II) (II).

Dinitriles are usually hydrolyzed in the presence of a base, for example in aqueous solution. The base is preferably an alkali metal hydroxide, such as sodium hydroxide, potassium hydroxide or lithium hydroxide. The molar ratio of dinitrile to alkali metal hydroxide can be 1:1 to 1:4, or 1:1.5 to 1:3, or 1:1.7 to 1:2.3. Dinitriles are typically hydrolyzed at temperatures between 5 and 100°C, or between 50 and 100°C, or between 70 and 100°C. Excess water can be removed from the resulting diacid by heat or vacuum.

The present invention also relates to a method for preparing a dinitrile of formula (II) (II) wherein n is 2, 3 or 4, comprising adding acrylonitrile to a diamine of formula (III) (III).

Acrylonitrile and diamine are usually present in a molar ratio of 1.9:1 to 2.5:1, or 2.0:1 to 2.4:1, or 2.0:1 to 2.3:1. The addition is usually carried out at a temperature of 5 to 100°C, or 10 to 80°C, or 15 to 60°C. During the addition, the reactants can be cooled to maintain temperature. The addition of acrylonitrile and diamine is also called Michael addition. The addition can be carried out in an aqueous solution, for example where the diamine is present in the water and acrylonitrile is added to the aqueous solution.

The diamine of formula (III) can be used as a high purity grade (e.g., at least 90, 95 or 98 wt% purity) or as an industrial grade (e.g., purity of 50 to 90 wt%, or 60 to 80 wt%). The industrial grade diamine of formula (III) may contain amine-containing by-products, such as branched amines or piperidines. Industrial grade triethylenetetraamine (TETA) may contain 3-aminoethylamine (also known as "branched TETA"), N,N'-bis(aminoethyl)piperidin (also known as "bis-AEP") and/or N-[(2-aminoethyl)2-aminoethyl]piperidin (Also known as "PEEDA").

When acrylonitrile is added to the diamine of formula (III), the amine-containing by-product can react with acrylonitrile to prepare the corresponding dinitrile and optionally the corresponding higher nitrile.

The method for preparing the diacid of formula (I) comprises hydrolyzing the dinitrile of formula (II), which may contain a dinitrile and optionally a higher nitrile produced by the addition of acrylonitrile to an amine-containing byproduct. The diacid of formula (I) may contain the corresponding diacid and optionally a higher acid produced by hydrolyzing the dinitrile and optionally a higher nitrile, which are produced by the addition of acrylonitrile to an amine-containing byproduct.

The present invention also relates to the use of a diacid of formula (I) or a salt thereof (I) wherein n is 2, 3 or 4 (preferably wherein n is 3 or 4), which is used to reduce the formation of cyanide during electroplating. The formation of cyanide can be reduced to achieve a cyanide (CN ^- ) concentration in the electroplating bath of less than 100 mg/l, or less than 80 mg/l, or less than 50 mg/l. The cyanide concentration can be determined photometrically, for example after charging for 25 Ah.

EXAMPLES Example 1 - Macroaddition of acrylonitrile to linear TETA

694.68 g (4751 mol) of linear triethylenetetramine (TETA, purity 99%) were cooled and 140.39 g of water were added. 529.34 g (9.976 mol) of acrylonitrile (ACN) were added at 40 °C. After the ACN was dosed, the reaction time was kept for about one hour before the vessel was emptied. The yield was 95 wt% according to NMR.

The amine value and HNMR confirmed the structure of the dinitrile of the following formula: . Example 2 - Hydrolysis of dinitrile

491.73 g (1947 mol) of the dinitrile prepared in Example 1 were heated and 417.62 g of water were added. Start dosing NaOH at 85°C, and the total amount added is 623.16 g NaOH 25% (155.79 g NaOH 100%, 3895 mol). Gas production was seen shortly after dosing began. After dosing of NaOH was complete, the post-reaction was held at 90°C until no further gas evolution was seen.

The contents were cooled to a temperature of about 70°C and the container was slowly evacuated and a slow flow of nitrogen was applied while heating. Maintain reflux until stable conditions are reached. Then distillation begins. After 30 minutes, the container was purged with nitrogen and cooled, then the amount of distilled water was compensated and the container was emptied.

At a purity of 90%, the yield was 95wt%. The amine value and H-NMR confirmed the structure of the diacid of the following formula: Example 3 - Addition of maltose to industrial grade TETA

Cool 528.42 g (3.61 mol) of industrial grade triethylenetetramine (TETA, purity 70%, containing about 20% N,N'-bis(aminoethyl)piperidin and about 10% tris-aminoethylamine) and 106.68 g of water were added. 421.36 g (7.941 mol) of acrylonitrile were added at 40°C. After the ACN was dosed, the reaction time was maintained for about one hour before the container was emptied. The yield was 95 wt % based on all primary amine groups in technical grade TETA. Example 4 - Hydrolysis of dinitrile

517.12 g (2048 mol) of the dinitrile prepared in Example 3 were heated and 417.62 g of water were added. Start dosing NaOH at 85°C to reach a total of 655.33 g NaOH 25% (175.36 g NaOH 100%, 4096 mol). Gas production was seen shortly after dosing began. After dosing of NaOH was complete, the post-reaction was maintained at 90°C until no further gas evolution was seen.

The contents were cooled to a temperature of about 70°C and the container was slowly evacuated and a slow flow of nitrogen was applied while heating. Maintain reflux until stable conditions are reached. Then distillation begins. After 30 minutes, the container was purged with nitrogen and cooled, then the amount of distilled water was compensated and the container was emptied.

At a purity of 80%, the yield was 80 wt%. The amine value and H-NMR confirmed the same structure as Example 2. Example 5 - Electroplating in a standard Hull cell

The electroplating application tests are carried out in a standard Hull cell with a rectifier, with 2 or 3 repetitions each. The Hull cell is a trapezoidal container containing 250 ml of electroplating bath. This shape allows the test steel plate to be placed at an angle to the stainless steel anode. A current of 1 A is applied for 25 minutes at 38°C. Finally the plate is rinsed with water and dried with compressed air.

The aqueous alkaline plating bath contains the components shown in Table 1 below: -   Lutron® Q 75 is an aqueous solution of N,N,N',N'-tetra-(2-hydroxypropyl)-ethylenediamine, pH 10-12, about 25% water content, available from BASF SE. -   Lugalvan® IZE is an aqueous solution of the addition product of imidazole and epichlorohydrin, pH 8-10, about 55% water content, available from BASF SE. -   Lugalvan® BPC 48 is an aqueous solution of 3-benzylpyridine carboxylate, pH about 6, about 52% water content, available from BASF SE. -   DETA is diethylenetriamine, available from BASF SE.

Table 1. Composition of electroplating bath (quantity in g/l) A ⁾ B ^a) C D Zinc oxide 8.09 (6.5 Zn) 8.09 (6.5 Zn) 8.09 (6.5 Zn) 8.09 (6.5 Zn) Nickel(II) sulfate x 6 H ₂ O 2.24 (0.5 Ni) 2.24 (0.5 Ni) 2.24 (0.5 Ni) 2.24 (0.5 Ni) NaOH 120 120 120 120 Triethanolamine (coordinating agent) 8 8 8 8 Lutron® Q 75 (coalescent agent) 40 40 40 40 Lugalvan® IZE (brightener) 2 2 2 2 Lugalvan® BPC 48 (brightener) 0.05 0.05 0.05 0.05 DETA 10 - - - TETA - 10 - - Example 1-Acid - - 10 - Example 2-acid - - - 10 a) Comparison

Comparative plating bath A included DETA instead of the diacid of Example 1 or 2. DETA is a standard complexing agent used in the electroplating industry.

Comparative plating bath B included TETA instead of the diacid of Example 1 or 2. TETA used was the starting material in Example 1.

In Examples 6-8, the test panels prepared in Examples 5A, B, C, and D are further analyzed. Example 6 - Optical Appearance

The optical appearance of the Zn/Ni deposits on the test panels was similar when using plating baths A, C and D. A uniform grey and dull surface was observed. The results show that the diacids of Examples 1 and 2 gave an optical appearance at least similar to that of the industry standard DETA.

Plating bath B resulted in inappropriate deposition: very low layer thickness, and poor and uneven alloy composition. TETA has only one more ethylamine unit than DETA and is the basic unit of the diacids in Preparation Examples 1 and 2. However, TETA was clearly not suitable to achieve the desired optical appearance. Example 7 - Layer thickness

The Hull cell is a trapezoidal container. This shape allows the test steel plate to be placed at an angle to the stainless steel anode so that one side of the plate is closer to the anode (which results in a higher current density) and the other side of the plate is further away from the anode (which results in a lower current density).

Since there is a uniform linear gradient in the current density, a linear gradient in the thickness of the deposited metal layer is also expected.

The layer thickness [μm] of the electroplated plates from Examples 5A, C and D was analyzed by Fischerscope® X-Ray XDal from Helmut Fischer GmbH (Germany) at nine points at a distance of 1 cm and 1 cm from the left edge ]. The results are summarized in Table 2 and Figure 1.

Figure 1 shows the thickness of the deposited layer in µm, measured as in Example 7 on the plate at nine points 1 cm from the left edge and at a distance of 1 cm after electroplating according to Example 5. Good linear gradients in layer thickness can be seen in Figure 1b) (Example 5D) and Figure 1c) (Example 5C). In contrast, comparative electroplating bath Example 5A in Figure 1a) is not linear but curved.

Table 2. Layer thickness [µm] Measuring point from left edge [cm] Plate from Example 5A ^a) From the board of Example 5C Plate from Example 5D 1 7.63 4.97 4.17 2 6.17 4.37 3.67 3 4.83 3.77 3.23 4 3.93 3.17 2.93 5 3.33 2.67 2.50 6 2.83 2.10 2.13 7 2.40 1.63 1.70 8 1.73 1.27 1.33 9 1.40 0.80 0.87 a) Comparative Example 8 - Alloy Composition

As mentioned above, one side of the plate is closer to the anode (which results in a higher current density), while the other side of the plate is further away from the anode (which results in a lower current density). During electroplating, Zn and Ni are deposited simultaneously. The goal is to have the same zinc-nickel alloy composition independent of the distance from the substrate to the anode.

The alloy composition of the electroplated plates from Examples 5A, C and D was analyzed using a Fischerscope® X-Ray XDal from Helmut Fischer GmbH (Germany) at nine points at a distance of 1 cm from the left edge. The results are summarized in Table 3.

The data in Table 3 show that the new diacid addition allows similarly good uniformity of alloy composition independent of the substrate-to-anode distance (and therefore independent of current density).

Table 3. Alloy composition Measuring point from left edge [cm] Plate from Example 5A ^a) From the plate of Example 5B ^a) From the board of Example 5C Plate from Example 5D Ni [%] Zn [%] Ni [%] Zn [%] Ni [%] Zn [%] Ni [%] Zn [%] 1 13.6 86.4 14.4 85.6 13.1 86.9 12.6 87.4 2 13.3 86.7 14.4 85.6 12.6 87.4 12.2 87.8 3 12.8 87.2 13.4 86.6 12.8 87.2 12.2 87.8 4 12.6 87.4 16.2 83.8 11.9 88.1 12.0 88.0 5 12.6 87.4 23.7 76.3 12.6 87.4 11.4 88.6 6 12.7 87.3 29.1 71.9 11.8 88.2 11.3 88.7 7 12.4 87.6 35.6 64.4 11.5 88.5 11.2 88.8 8 11.4 88.6 41.4 59.6 11.6 88.4 11.5 88.5 9 10.6 89.4 52.4 47.6 12.4 87.6 11.7 88.3 a) Comparative Example 9 - Formation of Cyanide

State-of-the-art plating additives such as DETA slowly oxidize at the anode to form cyanide (CN ^- ) as a by-product. This is very problematic because cyanide is not only toxic but also a strong chelating agent. As a result, the plating bath must be changed frequently and laboriously.

Long-term stability tests of the electroplating bath were carried out in a 1-liter right-angle tank (15x10x6 cm) containing 750 ml of electroplating bath, which had the same composition as in Example 5. Use a steel plate with an area of 1 ^dm2 and two opposite electrodes of 1 _dm2 . A current density of 5 A/dm ² was applied at 35°C for 30 minutes.

Additives are usually consumed during electroplating, so the following additives are added: - After each plate: 100 mg/l Lugalvan® BPC 48. - After every second plate: 3.25 g/l Zn (as ZnO); 24.25 g/l NaOH; 33.3 mg/l Ni (as NiSO ₄ ) in combination with 3.33 g/l of the diacid or comparative compound shown in Table 4; and 1.5 g/l Lugalvan® IZE. - After every fourth coat: 10 g/l Lutron® Q 75; and 2 g/l triethanolamine

After every 5 Ah (ampere hours) of charge, a sample of the plating bath was removed and analyzed photometrically for cyanide as follows: a) Sample preparation The sample was weighed into the reaction device and mixed with 100 ml of water. After adding 10 ml of copper (II) sulfate solution and 300 mg of tin (II) chloride, the equipment was closed with a reflux condenser and an absorption container containing 10 ml of sodium hydroxide solution. Add 10 ml hydrochloric acid and heat to boiling point. The nitrogen flow diverts the hydrogen cyanide produced in the absorption vessel. The transfer was complete after a reaction time of 1 hour. Transfer the contents of the absorption container to a 50 ml volumetric flask with pure water and fill the volumetric flask. b) Measurement Place an aliquot of the prepared sample in a 100 ml separatory funnel and, if necessary, add ultrapure water to bring the volume to 25 ml. Add 10 ml of phosphate buffer solution and check that the solution pH is approximately 7. After adding 0.5 ml of Chloramine T, shake the solution. Add 15 ml of pyridine-pyrazolium reagent (3-methyl-1-phenyl-2-pyrazolin-5-one and 4,4'-bis(3-methyl-1-phenyl-5-pyridine) azole)) and let it sit for 30 minutes. Then exactly 20 ml of n-butanol are added and the aqueous phase is discarded. The butanol phase was filtered through a dry pleated filter into a dry 25 ml volumetric flask. The solution was measured on a photometer against butanol (as zero value) at 632 nm in a 10 mm colorimetric tube. Calibration points are derived from the KCN reference material and measurements are made using corresponding aliquots.

The results are summarized in Table 4. It was shown that the diacid used in the electroplating baths of Examples 5C and 5D resulted in significantly lower cyanide formation.

Furthermore, in the comparative plating bath of Example 5A, deposits were observed after 20 Ah, which were not observed when using the plating baths of Examples 5C and 5D.

Table 4. Cyanide concentration [mg/kg electroplating bath] Charge [Ah] The bath composition in Example 5A The bath composition in Example 5B The bath composition in Example 5C was The bath composition in Example 5D was Diacid or comparative compound DETA ^a) TETA ^a) Example 1-Acid Example 2-acid 5 41 17 4 5 10 87 42 10 11 15 130 62 19 18 20 144 70 28 26 25 141 85 34 35 a) Comparison

without

[Figure 1] shows the thickness of the deposited layer in μm.

Claims (16)

A method of electroplating a substrate, which includes the step of applying current through an aqueous electroplating bath, the aqueous electroplating bath containing the substrate and the diacid of formula (I) (I) or a salt thereof, wherein n is 2, 3 or 4. The method of claim 1, wherein the electroplating bath comprises 0.1 to 200 g/l of diacid. The method of claim 1 or 2, wherein the electroplating bath contains a metal ion source, preferably a zinc ion source, and other metal ion sources as needed. The method of any one of claims 1 to 3, wherein the electroplating bath comprises a zinc ion source and a nickel ion source. The method of any one of claims 1 to 4, wherein the electroplating bath has a pH >10. A method as claimed in any one of claims 1 to 5, wherein the electroplating bath comprises other electroplating additives selected from brighteners, leveling agents, astringents, water softeners or defoamers. A method as claimed in any one of claims 1 to 6, wherein n is 3. An aqueous alkaline electroplating bath comprising a diacid of formula (I) (I) or a salt thereof, wherein n is 2, 3 or 4, a metal ion source which is a zinc ion source, and other metal ion sources as required, and other additives selected from brighteners, leveling agents, agglomerating agents, water softeners or defoaming agents as required. An aqueous alkaline electroplating bath as claimed in claim 8, wherein the electroplating bath contains 0.1 to 200 g/l of diacid. An aqueous alkaline electroplating bath as claimed in claim 8 or 9, wherein n is 3 or 4. A diacid of formula (I) or a salt thereof (I), where n is 3 or 4. A method for preparing a diacid or a salt thereof of formula (I) (I), wherein n is 3 or 4, comprising a dinitrile of the hydrolyzed formula (II) (II). The method of claim 12, wherein the dinitrile is hydrolyzed in the presence of a base, preferably in the presence of an alkaline metal hydroxide. A method for preparing dinitrile of formula (II) (II), wherein n is 3 or 4, comprising the addition of acrylonitrile to the diamine of formula (III) (III). The method of claim 14, wherein the molar ratio of the acrylonitrile and the diamine is 1.9:1 to 2.5:1. Use of a di-acid of formula (I) or a salt thereof (I), where n is 2, 3 or 4, is used to reduce cyanide formation during electroplating.