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. Combinations of preferred embodiments with other preferred embodiments are within the scope of the present invention.

Electroplating is widely used in industry to improve the surface qualities of objects. For example the resistance to abrasion and corrosion, lubricity, reflectivity, electrical conductivity, or appearance of objects can be improved.

Typically, during electroplating a metal coating is made on a substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts is usually the cathode of an electrolytic cell filled with an electroplating bath. The electroplating bath is a typically solution of a salt of the metal to be coated. The anode is usually either a block of that metal, or of some inert conductive material.

The electroplating bath usually contains complexing agents which keep the metal salts in solution. Especially amine-containing complexing agents, such as diethylenetriaamine (DETA), are important additives widely used in electroplating industry. However, such amine- containing complexing agents form cyanides and byproducts during undesired anoxic oxidation. Cyanides are not only toxic, but also too strong complexing agents which reduce the concentration of metal salts can be electrodeposited.

Beside reducing the cyanide formation in the electroplating bath during electroplating further objects of the inventions were to achieve a high quality optical appearance, homogenous composition of the metals in the layer, linear gradient of the layer thickness of the deposited metals according to the gradient of the current density. The objects were achieved by 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.

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 electroplating bath may comprise from 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 alkali, earth alkali, ammonium, organic salts, or mixtures thereof. Preferred salts of the diacid are alkali salts, such as sodium or potassium salts. The diacid may be present in acid form, salt form and mixtures thereof, e.g. in aqueous solution depending on the pH value.

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

The current usually flows between an anode and a cathode through the electroplating bath. The anode is can be made of zinc, manganese, iron, stainless steel, nickel, carbon, or a corrosion resistant metal such as platinum-plated titanium or palladium-tin alloy.

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

The cathode current density can 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 cathode current can be applied for 1 min to 5 hours, or 5 min to 2 hours, or 10 min to 1 hour. The electroplating can be carried out as barrel electroplating or rack electroplating. The electroplating can be carried out with or without injection of air, and with or without agitation of the substrate.

The anode or cathode regions may be separated or membrane anodes may be used. Preferably, the electroplating is performed without separating the anode and cathode from another by a membrane or by separators.

The electroplating can be performed at a temperature from 5 to 50 °C, or 10 to 40 °C.

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

The electroplating bath usually comprises a metal ion source. Suitable metal ion sources are any materials capable of providing free metal cations when in an aqueous solution. The metal ion source may include salts of the metals, or elemental metal. Suitable metal ion sources are ion sources of zinc, nickel, cobalt, manganese or iron, wherein zinc and nickel are preferred. The metal ion source may comprise a single or a mixture of different metal ion sources.

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

Suitable salts of the metal, such as zinc or nickel, are inorganic metal salts, such as halides, carbonates, hydroxides, ammonium sulfates, sulfamates, acetates, formates, nitrates, and sulfates, as well as hydrates thereof.

Preferred zinc ion sources are zinc chloride, zinc sulfate, zinc oxide, zinc hydroxide, zinc acetate, zinc carbonate, zinc tartrate, and zinc sulfamate, wherein zinc chloride and zinc sulfate are in particular preferred. Preferred nickel ion sources are nickel chloride, nickel sulfate, nickel hydroxide, nickel acetate, ammonium nickel sulfate, nickel carbonate, nickel formate, and nickel sulfamate, wherein nickel chloride and nickel sulfate are in particular preferred.

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

The concentration of the metal ions, e.g. of the zinc ions or of the nickel ions, can be in the range from 0.01 to 100 g/l, or 0.1 to 50 g/l, or 0.2 to 20 g/l.

The electroplating bath may comprise further electroplating additives in addition to the diacid, such as brighteners, leveling agents, complexing agents, water softening agent, or anti-foaming agents.

Suitable brighteners are alkyl naphthalenes, benzene sulfonic acids, benzene disulfonic acids, benzene trisulfonic acids, naphthalene disulfonic acids, naphthalene trisulfonic acids, benzene sulfonamides, naphthalene sulfonamides, benzene sulfonimides, naphthalene sulfonimides, vinyl sulfonamides, allyl sulfonamides, salts thereof, and combinations thereof.

Further examples of brighteners are the condensation product of piperazine, guanidine, formalin, and epichlorohydrin; pyridinium propyl sulfonate; N-benzyl-3-carboxy pyridinium chloride; trigonelline; sodium propargyl sulphonate; propargyl alcohol; ethyleneglycolpropargylalcohol ether; ethoxylated butyne diol; sodium diamylsulfosuccinate; N,N'-bis[3-(dimethylamino)propyl]urea, polymer with 1,3-dichloropropane; carboxyethylisothiuronium betaine; ethyl hexyl sulfate; benzothiazole; N-benzyl nicotinate; benzyl-2-methylimidiazole; formaldehyde condensate of 2-naphthalene sulfonate; methyl naphthyl ketone; benzalacetone; cumene sulfonate; sodium vinyl sulfonate; benzothiazolium- 2-[4-(dimethylamino)phenyl]-3,6-dimethyl chloride; N,N-dimethyl-dithiocarbamyl propyl sulfonic acid sodium salt; 3-mercapto-1-proanesulfonic acid, sodium salt; O- ethyldithiocarbonato-S-(3-sulfopropyl)-ester, potassium salt; bis-(3-sulfopropyl)-disulfide, disodium salt; 3-S-iosthiouronium propyl sulfonate; 3-(benzothiazolyl-2-mercapto)-propyl- sulfonic acid, sodium salt; N-(polyacrylamide); safranin; crystal violet and derivatives thereof; phenazonium dyes and derivatives thereof; thiodiglycol ethoxylate; sodium lauryl sulfate; 1- hydroxyethylen-1,1-diphosphonic acid; ethoxylated beta naphthol; sodium salt of a sulphonated alkylphenol ethoxylate; sulfurized benzene sulfonic acid; butynediol dihydroxypropyl sulfonate; sodium saccharin; 3-mercapto-1 -propanesulfonic acid, sodium salt; the formaldehyde condensate of 1 -naphthalene sulfonic acid; benzotriazole; sodium benzoate; the aqueous reaction product of 2-aminopyridine with epichlorohydrin; ureylene quaternary ammonium polymer; the aqueous reaction product of imidazole and epichlorohydrin; vanillin; anisaldehyde; piperonal; thiourea; polyvinyl alcohol; reduced polyvinyl alcohol; o-chlorobenzaldehyde; a-napthaldehyde; condensed naphthalene sulfonate; niacin; pyridine; 3-hydroxypropane sulfonate; allyl pyridinium chloride; dibenzenesulfonamide; pyridinium butane sulfonate; sodium allyl sulfonate; sodium vinyl sulfonate; naphthalene trisulfonic acid; cumene sulfonate; carboxymethylpyridinium chloride; propargyl hydroxypropyl ether sulfonate; o-sulfobenzaldehyde; aqueous reaction product of imidazole and epichlorohydrin; mercaptothio ether; polyvinylpyrrolidone; sodium adipate; chloral hydrate; sodium gluconate; sodium salicylate; manganese sulfate; cadmium sulfate; sodium tellurite; and glycine. The concentration of brightener in the electroplating bath can range from 0.01 g/l to 10 g/l.

Suitable leveling agents are a sulfur compounds, for example 3-mercapto-1 ,2,4-triazole or thiourea. The concentration of leveling agent in the electroplating bath can range from 0.01 g/l to 1 g/l.

Suitable complexing agents are alkyleneamine compounds such as ethylenediamine, triethylenetetramine, and tetraethylenepentamine; ethylene oxide or propylene oxide adducts of the above-described alkyleneamines; amino alcohols such as N-(2-aminoethyl)- ethanolamine and 2-hydroxyethylaminopropylamine; poly(hydroxyalkyl)alkylenediamines such as N-2(-hydroxyethyl)-N,N',N'-triethylethylenediamine, N,N'-di(2-hydroxyethyl)-N,N'- diethylethylenediamine, N,N,N',N'-tetrakis(2-hydroxyethyl)propylenediamine, and N,N,N',N'- tetrakis(2-hydroxypropyl)ethylenediamine; poly(alkyleneimines) obtained from ethyleneimine, 1 ,2-propyleneimine, and the like; poly(alkyleneamines) and poly(amino alcohols) 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 invention also relates to the aqueous alkaline electroplating bath which comprises the diacid of the formula (I) or salts thereof, where n is 2, 3 or 4, the metal ion source, and optionally the further additives selected from brighteners, leveling agents, complexing agents, water softening agent, or anti-foaming agents.

The invention also relates to the aqueous alkaline electroplating bath which comprises the diacid of the formula (I) or salts thereof, where n is 2, 3 or 4,

- the metal ion source, which is a zinc ion source, and optionally a further metal ion source, and optionally the further additives selected from brighteners, leveling agents, complexing agents, water softening agent, or anti-foaming agents.

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

The invention also relates to the diacid of the formula (I) where n is 2, 3 or 4, or salts thereof. Preferably, n is 3. In another preferred form of the diacid of the formula (I) n is 3 or 4. Suitable salts of the diacid are alkali, earth alkali, ammonium, organic salts, or mixtures thereof. Preferred salts of the diacid are alkali salts, such as sodium or potassium salts. The diacid may be present in acid form, salt form and mixtures thereof, e.g. in aqueous solution depending on the pH value.

The invention also relates to a method for preparing a diacid of the formula (I) or salts thereof, where n is 2, 3 or 4, comprising a hydrolysis of a dinitrile of the formula (II) The dinitrile is usually hydrolyzed in the presence of a base, e.g. in aqueous solution. The base is preferably a hydroxide of an alkali metal, such as sodium hydroxide, potassium hydroxide or lithium hydroxide. The molar ratio of the dinitrile to the hydroxide of the alkali metal can be from 1:1 to 1 :4, or 1 :1.5 to 1:3, or 1:1.7 to 1 :2.3. The dinitrile is usually hydrolyzed at temperatures between 5 to 100 °C, or 50 to 100 °C, or 70 to 100 °C. An excess of water can be removed from the resulting diacid by heat or vacuum.

The invention also relates to a method for preparing a dintrile of the formula (II) where n is 2, 3 or 4, comprising an addition of acrylonitrile to a diamine of the formula (III)

The acrylonitrile and the diamine are usually present in a molar ratio from 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 usually takes place at a temperature from 5 to 100 °C, or 10 to 80 °C, or 15 to 60 °C. During the addition the reactants can be cooled to maintain the temperature. The addition of the acrylonitrile to the diamine is also known as Michael addition. The addition can take place in an aqueous solution, e.g. where the diamine is present in water and the acrylonitrile is added to the aqueous solution.

The diamine of the formula (III) can be used as high purity grade (e.g. at least 90, 95, or 98 wt% purity) or as technical grade (e.g. with a purity of 50 to 90 wt%, or of 60 to 80 wt%). The diamine of the formula (III) in technical grade may comprise amine-containing byproducts, such as a branched amine or a cyclic amine comprising a piperazine unit. Technical grade triethylenetetraamine (TETA) may contain tris-aminoethylamine (also known as “branched TETA”), N,N’-bisaminoethylpiperazine (also known as “Bis AEP”) and/or N-[(2-aminoethyl) 2- aminoethyl]piperazine (also known as “PEEDA”).

When adding acrylonitrile to the diamine of the formula (III) the amine-containing byproducts may react with the acrylonitrile to prepare corresponding dinitriles and optionally corresponding higher nitriles. The method for preparing the diacid of the formula (I) comprises the hydrolysis of the dinitrile of the formula (II), which may contain the dinitriles and optionally higher nitriles resulting from the addition of the acrylonitrile to the amine-containing byproducts. The diacid of the formula (I) may contain corresponding diacids and optionally higher acids resulting from the hydrolysis of the dinitriles and optionally higher nitriles, which result from the addition of the acrylonitrile to the amine-containing byproducts.

The invention also relates to a use of the diacid of the formula (I) or salts thereof, where n is 2, 3 or 4 (preferably where n is 3 or 4), for reducing the cyanide formation during electroplating. The cyanide formation can be reduced to a achieve a concentration of cyanide (CN ^_ ) concentration in the electroplating bath of below 100 mg/l, or below 80 mg/l, or below 50 mg/l. The cyanide concentration can be determined by photometry, e.g. after an electric charge of 25 Ah.

Examples

Example 1 - Michael Addition of Acrylonitrile to linear TETA

694,68g (4,751 mol) of linear triethylenetetramine (TETA, purity 99%) was cooled and 140,39g water were added. 529,34g (9,976mol) of acrylonitrile (ACN) were added at 40 °C After the ACN dosage a post reaction time of about one hour was maintained before the vessel was emptied. The yields was 95 wt% according to NMR.

The amine number and the HNMR confirmed the structure of the dinitrile of the following formula:

Example 2 - Hydrolysis of Dinitril 491 ,73g (1 ,947mol) of the dinitrile made in Example 1 were heated and 417,62g water was added. At 85°C NaOH dosage was started and a total amount of 623,16g NaOH 25% (155,79g NaOH 100%, 3,895mol)were added. Shortly after the dosage was initiated gas development was visible. After the NaOH dosage was completed a post reaction was maintained at 90°C until no further gas development was visible.

The content was cooled to a temperature of about 70°C and the vessel was slowly evacuated and a slow nitrogen flow while heating was applied. Reflux was maintained until stable conditions were reached. Following distillation was started. After a duration of 30 minutes the vessel was aerated with nitrogen and cooled before the distilled amount of water was compensated and the vessel emptied.

The yield was 95 wt% at the purity 90 %. The amine number and the H-NMR confirmed the structure of the diacid of the following formula:

Example 3 - Michael Addition of Acrylonitrile to technical TETA

528,42g (3,61 mol) of technical grade triethylenetetraamine (TETA, purity 70 %, contains about 20% N,N’-bisaminoethylpiperazine and about 10% Tris-aminoethylamin) was cooled and 106,68g water were added. 421,36g (7,941 mol) of acrylonitrile were added at 40 °C. After the ACN dosage a post reaction time of about one hour was maintained before the vessel was emptied. The yield was 95 wt% based on all primary amino groups in the technical TETA.

Example 4 - Hydrolysis of Dinitril

517,12g (2,048mol) of the dinitrile made in Example 3 were heated and 417,62g water were added. At 85°C NaOH dosage was started to achieve a total amount of 655,33g NaOH 25% (175,36g NaOH 100%, 4,096mol)Shortly after the dosage was initiated gas development was visible. After the NaOH dosage was completed a post reaction was maintained at 90°C until no further gas development was visible. The content was cooled to a temperature of about 70°C and the vessel was slowly evacuated and slow nitrogen flow while heating was applied. Reflux was maintained until stable conditions were reached. Following distillation was started. After a duration of 30 minutes the vessel was aerated with nitrogen and cooled before the distilled amount of water was compensated and the vessel emptied.

The yield was 80 wt% at the purity 80 %. The amine number and the H-NMR confirmed the structure as in Example 2.

Example 5 - Electroplating in standard Hull cell

The electroplating application tests were performed in a standard Hull cell with a rectifier and each repeated two or three times. The Hull cell was a trapezoidal container filled with 250 ml of the electroplating bath. This shape allowed to place the test steel panel on an angle to the stainless-steel anode. An electric current of 1 A was applied for 25 min at 38 °C. At the end the panel was rinsed with water and dried with pressured air.

The aqueous alkaline electroplating bath contained the following components as shown in Table 1 :

- Lutron® Q 75 is an aqueous solution of N,N,N',N'-tetrakis-(2-hydroxypropyl)-ethylene- diamine, pH 10-12, about 25 % water content, commercially available from BASF SE.

- Lugalvan® IZE is an aqueous solution of an addition product of imidazole and epichlorohydrin, pH 8-10, about 55 % water content, commercially available from BASF SE.

- Lugalvan® BPC 48 is an aqueous solution of benzylpyridine-3-carboxylate, pH about 6, about 52 % water content, commercially available from BASF SE.

- DETA is diethylenetriamine, commercially available from BASF SE.

Table 1. Composition of electroplating bath (amount in g/l)

a) comparative

The comparative electroplating bath A included DETA instead of the Diacid of Example 1 or 2. DETA is the standard complexing agent used in electroplating industry.

The comparative electroplating bath B included TETA instead of the Diacid of Example 1 or 2. TETA used was the starting material in Example 1.

The test panels prepared in Example 5 A, B, C and D were further analyzed in Examples 6-8.

Example 6 - Optical appearance

The optical appearance of the Zn/Ni deposition layer on the test panels was similar when using electroplating baths A, C and D. A homogenously grey and dull surface was observed. This results demonstrated that the Diacids of Example 1 and 2 give a at least similar optical appearance like the industry standard DETA.

The electroplating bath B resulted in unsuitable depositions: a very low layer thickness, and a bad and inhomogeneous alloy composition. TETA contains only one ethylene amine unit more than DETA and is the base unit for the preparation of the Diacids in Example 1 and 2. However, TETA was clearly not suitable for achieving the required optical appearance.

Example 7 - Layer thickness

The Hull cell was a trapezoidal container. This shape allowed to place the test steel panel on an angle to the stainless-steel anode, so one side of the panel is closer to the anode (which results in a higher current density) and the other side of the panel is more far away (which results in a lower current density).

Since there is a uniformly linear gradient of the current density also a linear gradient of the layer thickness of the deposited metals are expected.

The layer thickness [pm] on the electroplated panels from Examples 5 A, C and D was analyzed by Fischerscope® X-Ray XDal from Helmut Fischer GmbH (Germany) on nine spots with 1 cm distance and starting 1 cm from the left edge. The results are summarized in Table 2 and in Figure 1.

Figure 1 shows the deposited layer thickness in pm as determined in Example 7 on the panels at nine spots with 1 cm distance and starting 1 cm from the left edge after electroplating according to Example 5. The good linear gradient of the layer thickness can be best seen in Figure 1b) (Example 5D) and Figure 1c) (Example 50). For comparison, the comparative electrobath Example 5A in Figure 1a) is not linear, but rather curved.

Table 2. Layer Thickness [pm] a) comparative

Example 8 - Alloy composition

As explained above, one side of the panel is closer to the anode (which results in a higher current density) and the other side of the panel is more far away (which results in a lower current density). During the electroplating Zn and Ni are deposited simultaneously. Goal was to have the same Zinc-Nickel alloy composition independent of the distance from the substrate to the anode.

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

The data in Table 3 showed that the new diacid additives allow the similar good uniformity of alloy composition independent of the distance from the substrate to the anode (and thus independent of the current density).

Table 3. Alloy Composition a) comparative

Example 9 - Cyanide Formation

The state-of-the-art electroplating additives like DETA are slowly oxidized at the anode to form cyanide (CN ^_ ) byproduct. This is very problematic, because cyanide is not only toxic, but also a strong complexing agent. Thus the electroplating bath has to be exchanged often and laboriously. Long term stability tests of the electroplating baths were performed in a 1 liter right-angled cell (15x10x6 cm) including 750 ml electroplating bath, which had the same composition as in Example 5. Steel plating panels with an area of 1 dm ² were used, and two opposite electrodes with 1 dm ² . The applied current density at 35 °C was 5 A/dm ² for 30 min.

Additives are usually consumed during electroplating and thus some additives were added as following:

- After every plated panel 100 mg/L Lugalvan® BPC 48.

- After every two plated panels: 3,25 g/L Zn (as ZnO); 24,25 g/L NaOH; 33,3 mg/L Ni (as NiSCL) complexed with 3,33 g/L diacid or the comparative compound as shown in Table 4; and 1,5 g/L Lugalvan® IZE.

- After every four plated 10 g/L Lutron® Q 75; and 2 g/L triethanolamine

After an electric charge of each 5 Ah (ampere hours) a sample of the electroplating bath was taken and analyzed on cyanide via photometry as follows: a) Sample preparation

The sample was weighed into the reaction apparatus and mixed with 100 ml water. After adding 10 ml of copper (II) sulfate solution and 300 mg of tin (II) chloride, the apparatus was be closed with the reflux condenser and the absorption vessel, which was filled with 10 ml of sodium hydroxide solution. 10 ml of hydrochloric acid was added and heated up to the boiling point. A nitrogen flow transferred the developing hydrogen cyanide in the absorption vessel. The transfer was completed after 1 hour of reaction time. The content of the absorption vessel was transferred with pure water into a 50 ml volumetric flask and the volumetric flask was filled up. b) Measurement

An aliquot of the prepared sample was placed in a 100 ml separating funnel and, if necessary, supplemented with ultrapure water to a volume of 25 ml. 10 ml of phosphate buffer solution was added, the solution is checked whether the pH value was around 7. After adding 0.5 ml of chloramine T the solution was shaken. 15 ml of pyridine - pyrazolone reagent (3-methyl-1-phenyl-2-pyrazolin-5-one and 4,4'-bis(3- methyl-1-phenyle-5-pyrazolone) was added and allowed to stand for 30 min. Then exactly 20 ml of n-butanol was added and, and the aqueous phase discarded. The butanol phase was filtered through a dry pleated filter into a dry 25 ml volumetric flask. At 632 nm in a 10 mm cuvette the solution was measured against butanol (as zero value), on the photometer. The calibration points were applied from a KCN reference material and measured with the corresponding aliquots.

The results are summarized in Table 4. It was demonstrated that the diacids used in the electroplating baths of Example 5 C and 5 D resulted in a significant lower cyanide formation. Additionally, in the comparative electroplating bath of Example 5 A a sediment was observed after 20 Ah, which was not seen when the electroplating baths of Example 5 C and 5 D were used.

Table 4. Cyanide Concentration [mg per kg electroplating bath] a) comparative