EP0081183B1 - Process for the electroless depositing of noble metal layers on the surfaces of non-noble metals - Google Patents

Abstract

1. Claims for the contracting states : BE, CH, DE, FR, GB, IT, LI, LU, NL, SE Process for the currentless deposition of silver or gold layers on the appropriate less electro-positive metals by contacting the object to be coated with a coating bath which contains a metal complex which is obtainable by reaction of a chloride of monovalent silver or gold with a base, which is capable of complex formation with the silver or gold, and with hydrochloric acid. 1. Claims for the contracting state : Austria Process for the currentless deposition of silver or gold layers on to appropriate less electro-positive metals by contacting the object to be coated with a coating bath which contains a metal complex which is obtainable by reaction of a chloride of monovalent silver or gold with a base, which is capable of complex formation with the silver or gold, and with hydrochloric acid.

Description

The invention relates to a method for the currentless deposition of silver or gold layers on surfaces of correspondingly less noble metals.

Metal objects coated with silver or gold layers play in numerous fields of technology, such as in electrical engineering, in electronics, in the construction of medical devices, in restoration technology, in corrosion protection, in the jewelry industry, finishing technology, space travel, in mechanics, but also increasingly in teaching.

The known methods for the electroless deposition of silver or gold layers on surfaces of correspondingly less noble metals have various disadvantages, some of which considerably restrict their use. Working with known commercially available coating baths requires relatively long dwell times of the workpieces in the coating baths, and coatings are obtained, the layer thicknesses of which sometimes do not meet the requirements. Another major disadvantage of the known coating baths is their toxicity, e.g. due to their cyanide content, which in particular causes problems in handling and waste disposal.

From US-A-3 294 578 a process for the electroless deposition of metal coatings on metallic surfaces is known, in which coating baths are used which contain a metal complex which is obtained by reacting a metal chloride with a nitrogen compound and hydrobromic acid or hydroiodic acid; this method is also unsatisfactory, particularly with regard to the speed and effectiveness of the metal deposition and the properties of the coatings.

GB-A-1411971 describes the electroless deposition of copper layers on steel by means of coating baths which contain a copper complex which is obtained by reacting copper (II) chloride with a nitrogenous base in the presence of HCl.

The object of the invention is therefore to provide a method for the electroless deposition of silver and gold layers, which avoids the disadvantages shown and which enables the production of well-adhering layers with sufficient layer thicknesses. This object is achieved with the present invention.

The invention relates to a method for the electroless deposition of silver or gold layers on correspondingly less noble metals by contacting the object to be coated with a coating bath which contains a metal complex which by reacting a chloride of monovalent silver or gold with a base which Complexation with which silver or gold is capable, and is available with hydrochloric acid.

In principle, all compounds which can be protonated by the hydrochloric acid used to prepare the coating bath are suitable as bases capable of forming complexes with the metal to be deposited (i.e. silver or gold). With regard to the stability of the complexes and the quality of the coating, preference is given to using bases which are easily protonated under the reaction conditions used.

• Ammoniumchlorid, Ammoniumbromid,
• Hydroxylamin-hydrochlorid,
• Hydrazin-dihydrochlorid,
• Methylammoniumchlorid,
• Benzylammoniumchlorid,
• Benzylammoniumbromid,
• 2-Aminopropan-hydrochlorid,
• Cyclohexylammoniumchlorid,
• 1-Amino-4-methyl-bicyclo[2.2.2]octanhydro- chlorid,
• 1-Aminoadamantan-hydrochlorid,
• Glycinmethylesterhydrochlorid oder
• Glycinäthylesterhydrochlorid;
• Carbonsäureamide, wie z. B.
• Formamid, N-Methylformamid,
• N-Isopropylformamid, N-Cyclohexylformamid,
• N-(2,4-Dimethyl-pentyl-3)-formamid,
• N,N-Dimethylformamid, N,N-Diäthylformamid,
• N-Methylacetamid, N-Äthylacetamid,
• N,N-Diäthylacetamid oder Propionamid;
• Harnstoffderivate, wie z.B.
• N,N'-Dimethylharnstoff oder
• N,N-Dimethylharnstoff:
  • basische Stickstoffheterocyclen, wie z.B.
  • Morpholin, N-Methylmorpholin,
  • N-Methyl-2-pyrrolidinon, N-Formyl-pyrrolidin,
  • 1-Aza-bicycio[2.2.2]octan-hydrochlorid,
• Pyridin oder Chinolin:
  • und basische Phosphorverbindungen, wie z.B.
  • Hexamethyl-phosphorsäure-triamid.

In general, basic nitrogen-containing compounds, such as, in particular, ammonia and amines, such as, for example, are particularly well suited for complex formation

• Ammonium chloride, ammonium bromide,
• Hydroxylamine hydrochloride,
• Hydrazine dihydrochloride,
• Methylammonium chloride,
• Benzylammonium chloride,
• Benzylammonium bromide,
• 2-aminopropane hydrochloride,
• Cyclohexylammonium chloride,
• 1-amino-4-methyl-bicyclo [2.2.2] octane hydrochloride,
• 1-aminoadamantane hydrochloride,
• Glycine methyl ester hydrochloride or
• Glycine ethyl ester hydrochloride;
• Carboxamides, such as. B.
• Formamide, N-methylformamide,
• N-isopropylformamide, N-cyclohexylformamide,
• N- (2,4-dimethylpentyl-3) formamide,
• N, N-dimethylformamide, N, N-diethylformamide,
• N-methylacetamide, N-ethylacetamide,
• N, N-diethylacetamide or propionamide;
• Urea derivatives, such as
• N, N'-dimethylurea or
• N, N-dimethyl urea:
  • basic nitrogen heterocycles, such as
  • Morpholine, N-methylmorpholine,
  • N-methyl-2-pyrrolidinone, N-formyl-pyrrolidine,
  • 1-aza-bicycio [2.2.2] octane hydrochloride,
• Pyridine or quinoline:
  • and basic phosphorus compounds, such as
  • Hexamethyl phosphoric acid triamide.

• Benzol, 1,2-Dichforbenzol, 1,2,3-Trichlorbenzol,
• Chlorbenzol oder Cyclohexan;
• von Alkoholen, wie z. B.
• Methanol, Äthanol, Propanol, 2-Propanol,
• 2-Methylpropanol, 1-Butanol, 2-Butanol,
• Diäthylenglykol, Triäthylenglykol, Glycerin,
• Cyclohexanol, äthylenglykolmonoäthyläther,
• Diäthylenglykolmonomethyläther oder
• Triäthylenglykoldimethyläther;
• von Äthern, wie z.B.
• Diisoamyläther, Diäthylenglykoldiäthyläther,
• Triäthylenglykoldimethyläther,
• Tetraäthylenglykoldimethyläther oder Dioxan;
• von Ketonen, wie z. B.
• Aceton, Acetylaceton, Methyl-isopropylketon,
• Diisopropylketon oder Cyclohexanon;
• von Carbonsäureestern, wie z.B.
• Essigsäuremethylester, Propionsäureäthylester,
• Acetessigester oder Phthalsäuredimethylester;
• von Carbonsäurenitrilen, wie z.B.
• Benzonitril, Benzylcyanid, Propionitril,
• Iso-butyronitril oder Acetonitril;
• oder auch von Schwefelverbindungen, wie z.B.
• Dimethylsulfoxid, Sulfolan, Thiosemicarbazid,
• Thiobenzamid oder N-Phenyl-thioharnstoff möglich.

In certain cases, the use of hydrocarbons and halogenated hydrocarbons, such as. B.

• Benzene, 1,2-dichlorobenzene, 1,2,3-trichlorobenzene,
• Chlorobenzene or cyclohexane;
• of alcohols such as B.
• Methanol, ethanol, propanol, 2-propanol,
• 2-methylpropanol, 1-butanol, 2-butanol,
• Diethylene glycol, triethylene glycol, glycerin,
• Cyclohexanol, ethylene glycol monoethyl ether,
• Diethylene glycol monomethyl ether or
• Triethylene glycol dimethyl ether;
• of ether, such as
• Diisoamyl ether, diethylene glycol diethyl ether,
• Triethylene glycol dimethyl ether,
• Tetraethylene glycol dimethyl ether or dioxane;
• of ketones, e.g. B.
• Acetone, acetylacetone, methyl isopropyl ketone,
• Diisopropyl ketone or cyclohexanone;
• of carboxylic acid esters, such as
• Methyl acetate, ethyl propionate,
• Acetoacetic ester or dimethyl phthalate;
• of carbonitriles, such as
• Benzonitrile, benzyl cyanide, propionitrile,
• Iso-butyronitrile or acetonitrile;
• or also of sulfur compounds, such as
• Dimethyl sulfoxide, sulfolane, thiosemicarbazide,
• Thiobenzamide or N-phenyl-thiourea possible.

In general, all metals which are less noble than the respective metal to be deposited can be used as the substrate for the metals to be deposited. With regard to the properties of the coatings (adhesion and thickness of the layer), particularly suitable substrate metals for silver are e.g. Zinc, iron, nickel, tin, lead and copper; and for gold e.g. Nickel, copper, silver, tin, zinc, lead, iron and platinum.

The reaction of the metal chloride (i.e. the silver or gold chloride) with the base and the hydrochloric acid can be done by simply mixing these components together. The reaction can be carried out without or in the presence of a solvent, and an excess of base can also serve as the solvent.

The molar ratio base / metal chloride / hydrochloric acid is chosen so that the entire amount of metal chloride is dissolved in the reaction. It is expediently in the range from 1 to 40/1/1, but the molar value of the base and the hydrochloric acid can also be substantially higher, for example twice as high. The most favorable molar ratio depends in particular on the type of implementation.

Suitable solvents are inert to the complex formation reaction, especially aprotic organic solvents, such as e.g. Carbon tetrachloride and especially acetone. The solvents have to be weaker basic than the base used. Under these conditions, a base, such as. As dimethylformamide, can be used as a solvent.

The reaction is carried out at room temperature or with heating. In the latter case, however, basic cleavage products result in particular in the case of bases sensitive to hydrolysis, which in turn produce hydrochloride with hydrochloric acid and complex with the noble metal chloride. This case occurs e.g. when formamids are reacted with hydrochloric acid and metal chloride in the heat. Cleavage occurs in formic acid and amine, the latter then immediately reacts to form the hydrochloride, which is the actually complexing agent. It may also be advantageous to carry out the reaction and the subsequent metal deposition under an inert gas atmosphere, e.g. under nitrogen.

The metal chloride is preferably added in finely powdered form; the hydrochloric acid can be added in liquid form or introduced in gaseous form.

• (a) Vorlegen der Base und Zugabe von Metallsalz und Säure, vorzugsweise unter Rühren. Das Molverhältnis Base/Metallsalz/Säure beträgt dabei > 30/1/1. Wenn die Base bei Raumtemperatur fest ist, wird zweckmässigerweise in Gegenwart eines Lösungsmittels, wie z. B. Aceton, gearbeitet.
• Das Metallsalz wird dann in feinpulverisierter Form zugegeben. Anschliessend wird die Chlorwasserstoffsäure bei Raumtemperatur zugetropft, wobei das Metallsalz in Lösung geht und eine farblose bis gelbliche Lösung resultiert. Wenn das Metallsalz nicht vollständig in Lösung geht, kann noch nachgesäuert werden.
• (b) Zugabe des Metallsalzes zu einer Mischung von Base und Säure unter Rühren. Geeignetes Molverhältnis wie unter (a).
• (c) Ein Mol Hydrochlorid der Base wird in einem aprotischen Lösungsmittel, das schwächer basisch ist als die eingesetzte Base, gelöst oder suspendiert. Dazu wird unter Rühren 1 Mol Metallsalz zugegeben, wobei entweder eine klare Lösung entsteht oder der Metallkomplex ausfällt.

The implementation is preferably carried out according to one of the following three process variants:

• (a) Submitting the base and adding metal salt and acid, preferably with stirring. The molar ratio base / metal salt / acid is> 30/1/1. If the base is solid at room temperature, it is conveniently in the presence of a solvent such as. B. acetone worked.
• The metal salt is then added in finely powdered form. The hydrochloric acid is then added dropwise at room temperature, the metal salt dissolving and resulting in a colorless to yellowish solution. If the metal salt does not completely dissolve, it can still be acidified.
• (b) Add the metal salt to a mixture of base and acid with stirring. Suitable molar ratio as in (a).
• (c) One mole of hydrochloride of the base is dissolved or suspended in an aprotic solvent which is less basic than the base used. For this purpose, 1 mol of metal salt is added with stirring, either a clear solution is formed or the metal complex fails.

The z. B. according to one of the process variants (a), (b) or (c) reaction solution obtained, optionally after dilution with a suitable solvent, can be used directly as a coating bath (metal deposition solution).

However, it may be expedient to dilute the solutions obtained with about ½ of the volume of a suitable solvent (an aprotic solvent which can be used for the reaction, such as, for example, acetone or carbon tetrachloride, or mixtures thereof) is suitable as the solvent to add three times the amount of hydrochloric acid required for the reaction. This enables a high stability of the solution, faster deposition, thicker and more uniform layer thicknesses and better utilization of the metal salt used.

The stability of the solutions obtained is generally very good. Solutions of silver complexes can e.g. B. can be kept almost unchanged over several years.

For space-saving storage and for transportation, however, it can also be expedient to isolate the metal complexes from their reaction solutions and to dissolve them only shortly before use. The metal complexes can by diluting the reaction solutions with a poorly dissolving solvent, such as. B. with acetone. The coating bath can then be prepared from these complexes if necessary by dissolving them in a suitable solvent, e.g. in dimethylformamide. The dissolution usually takes place with slight warming, e.g. at 60 ° C. To avoid decomposition of the complex and to maintain the separation quality and stability, overheating should be avoided.

The choice and amount of a complex-forming component (base, metal or hydrochloric acid) depends in particular on the type of the other complex-forming components, on the type of metal to be deposited, but also according to the type of metal substrate onto which it is deposited and the reaction conditions used, such as the type of solvent. It is also possible to use two or more bases. Gold / silver mixtures can also be deposited.

The selection, combination and quantitative ratio of the complex-forming components also depend on the desired deposition rate (reactivity) and selectivity of the coating bath. It was found that, as a rule, a decreasing base strength results in greater reactivity.

The metal layers are deposited on the substrate by the methods customary for electroless deposition from coating baths, in particular by immersing the objects to be coated in the deposition bath. The objects to be coated can generally have any shape, which is determined in particular by the later intended use.

For a perfect, well-adhering coating, it is necessary to clean the surface of the metals to be coated, paying particular attention to freedom from dust, grease, moisture and, in particular, oxide. After cleaning, the workpiece to be coated is then preferably immersed in the coating bath in the dry state. For a good and uniform coating, it is necessary to leave the object calm (motionless) in the stationary coating bath.

Instead of immersing the workpiece in the coating bath, the contacting can also be carried out by applying (brushing on, brushing) the coating solution (coating bath) onto the workpiece. With this coating method it is advisable to use coating baths that are as concentrated as possible. This process can be repeated any number of times until the desired layer thickness is reached. This method will be particularly preferable if only parts of an object are to be coated (this requires a partial covering with a layer that is later easy to remove again with the immersion method) or if immersion is not possible or is only possible with difficulty, e.g. in restoration technology.

The duration of the contact time depends primarily on the deposition speed and the desired layer thickness. The deposition process can be interrupted at any time (e.g. by removing the workpiece from the solution) and, after the coating has been assessed, it can be continued by contacting it again. This process can be repeated any number of times until the desired layer thickness is reached. After reaching the desired layer thickness, residues of the coating bath are washed with a suitable solvent, e.g. with methanol, ethanol or acetone, and the workpiece is dried, e.g. by wiping with a cloth.

The quality of the coating, especially its adhesive strength, depends to a large extent on the deposition rate. Deposition that is too rapid (reactivity that is too high) generally results in a less adherent <amorphic> coating than with a coating bath of lower reactivity. Favorable coating times are between one minute and one hour.

The deposition rate (reactivity) of the coating bath can be adjusted by suitable selection and combination of the complex-forming components. However, it is also dependent on the concentration of the metal complex in the coating bath and / or the acid concentration. As a rule, the rate of separation increases with increasing concentration of metal complex and acid. From very concentrated solutions, the deposition z. B. done in a few seconds.

By varying the complex-forming components, in particular base and hydrochloric acid, it is also possible to obtain coating solutions with which only certain metals are selectively coated. Selectivity is also related to reactivity. So z. B. the deposition rate for a particular metal can be regulated by varying the amount of acid. A change in the concentration of the metal complexes usually only affects the deposition rate.

The layer thicknesses that can be achieved are generally proportional to the metal complex concentration of the coating bath and the contact time. A suitable choice of the deposition conditions generally gives a layer thickness of 0.01 to 4 J.1m.

It is also possible to combine the method according to the invention for electroless metal deposition with a galvanic deposition method with the aid of current, the two deposition methods being able to take place simultaneously or in succession. As a rule, even thicker layer thicknesses can be achieved in this way.

It is advisable to use a coating bath only for substrates made of the same metal. The deposition (layer thickness) can be followed by potential measurement. For example, by measuring the potential on a copper sheet, the final value of the coating (maximum coating) is displayed after four days. In order to measure the potential with as little feedback as possible, an electrometer amplifier was used (input current <50 mA), and a silver wire was used as the reference potential. The initial potential was 100 mV and practically reached zero after the time specified above. The change in potential during the deposition process was recorded graphically with the aid of a recorder.

A dropwise addition of concentrated acid enables an almost quantitative utilization of the complexed metal for the deposition from those which appear to be <exhausted> Coating baths can be achieved. Too large an amount of acid is recognized by an immediate precipitation of the silver still in solution as a halide, in the case of gold as a metal.

The silver can be precipitated from the exhausted solutions by dilution with water as the halide or the gold by adding an aqueous iron (II) salt solution as the metal and sent to a recycling process. This makes it possible to keep the environmental impact low with the method according to the invention.

With the method according to the invention, a method is thus provided with which it is possible to produce easily adhering and corrosion-resistant coatings (e.g. gold-plating) in a very simple and rapid manner with layer thicknesses not previously achieved with currentless methods. The process can be carried out without great mechanical outlay and at room temperature, that is to say without much outlay in energy. Working at room temperature also makes it possible to coat objects where galvanic deposition or electroless coating with conventional baths was not possible due to their temperature sensitivity. Through simple recycling (separation of the metals from the <exhausted> coating baths, distillation of the solvents), the environmental impact is very low. In particular, the cyanide-free coating baths also avoid the problems with handling and waste disposal which occur with the known cyanide-containing coating baths. The use of non-toxic and difficultly volatile substances also enables the objects to be coated to be brought into safe contact by simply applying (brushing).

Finally, because of the large variation possibilities in the production of the coating baths shown, it is possible to obtain coating baths of different reactivity and selectivity, as a result of which the rate of deposition, the layer thickness, the selectivity for different metal substrates etc. can be regulated. In this way, e.g. also the possibility of specifically coating workpieces that are composed of different metals and that should not or cannot be dismantled.

The invention therefore also relates to coating baths for the electroless deposition of silver or gold layers on correspondingly less noble metals, containing a metal complex which, by reacting a chloride of monovalent silver or gold with a base which is capable of complexing with the metal, and with Hydrochloric acid is available.

The invention further relates to metal complexes which can be obtained by reacting a chloride of monovalent silver or gold with a base which is capable of forming a complex with the metal and with hydrochloric acid, and subsequent precipitation from the reaction mixture.

• H[Base]MenXn+

angenommen (n ist ganze Zahl, X ist Chlor, Me ist Ag oder Au).The general formula for the complexes according to the invention is based on mass-spectroscopic and X-ray structure analysis

• H [Base] MenXn +

assumed (n is an integer, X is chlorine, Me is Ag or Au).

Claims (26)

1. Process for the currentless deposition of silver or gold layers on to appropriate less electropositive metals by contacting the object to be coated with a coating bath which contains a metal complex which is obtainable by reaction of a chloride of monovalent silver or gold with a base, which is capable of complex formation with the silver or gold, and with hydrochloric acid.

2. Process according to claim 1, characterised in that a base capable of complex formation is used which can be protonised by the hydrochloric acid.

3. Process according to claim 1 or 2, characterised in that the base capable of complex formation is a basic nitrogen compound.

4. Process according to one of claims 1 to 3, characterised in that the mole ratio of base/silver or gold chloride/hydrochloric acid is 1 to 40/1/1.

5. Process according to one of claims 1 to 4, characterised in that one carries out the reaction of base, silver or gold chloride and hydrochloric acid in an aprotic organic solvent.

6. Process according to one of claims 1 to 5, characterised in that one carries out the reaction of base, silver or gold chloride and hydrochloric acid at room temperature.

7. Process according to one of claims 1 to 6, characterised in that one uses the reaction solution obtained by the reaction of the base, silver or gold chloride and hydrochloric acid directly as coating bath.

8. Process according to one of claims 1 to 6, characterised in that one adds to the reaction solution obtained by the reaction of the base, silver or gold chloride and hydrochloric acid about the three-fold amount of the hydrochloric acid used for the reaction and uses the solution thus obtained as coating bath.

9. Process according to one of claims 1 to 6, characterised in that one precipitates the metal complex from the reaction solution obtained by the reaction of the base, silver or gold chloride and hydrochloric acid, dissolves the precipitated metal complex in a suitable solvent and uses this solution as coating bath.

10. Process according to one of claims 1 to 9, characterised in that one dips the object to be coated into the coating bath.

11. Process according to one of claims 1 to 9, characterised in that one applies the coating bath to the object to be coated.

12. Process according to one of claims 1 to 11, characterised in that one carries out the deposition at room temperature.

13. Process according to one of claims 1 to 12, characterised in that one produces metal layers with layer thicknesses of 0.01 to 4 pm.

14. Process according to one of claims 1 to 13, characterised in that one combines it with a galvanic deposition process.

15. Process according to one of claims 1 to 14, characterised in that one deposits silver on to zinc, iron, nickel, tin, lead or cooper.

16. Process according to one of claims 1 to 14, characterised in that one deposits gold on to tin, zinc, lead, iron, platinum, nickel, copper, silver or their alloys.

17. Process for the production of a coating bath for the currentless deposition of silver or gold layers on to appropriate less electropositive metals, characterised in that one reacts a chloride of monovalent silver or gold with a base, which is capable of complex formation with the metal, and with hydrochloric acid and possibly adds an appropriate solvent.

18. Process according to claim 17, characterised in that there is used a base capable of complex formation which can be protonised by the hydrochloric acid used.

19. Process according to one of claims 17 or 18, characterised in that the base capable of complex formation is a basic nitrogen compound.

20. Process according to one of claims 17 or 18, characterised in that the mole ratio of base/silver or gold chloride/hydrochloric acid is 1 to 40/1/1.

21. Process according to one of claims 17 to 20, characterised in that one reacts base, silver or gold chloride and hydrochloric acid and uses the solution obtained as coating bath.

22. Process according to one of claims 17 to 20, characterised in that one reacts base, silver or gold chloride and hydrochloric acid and adds to the reaction solution obtained about the 3 fold amount of the hydrochloric acid used for the reaction.

23. Process for the production of a metal complex by reaction of a chloride of monovalent silver or gold with a base, which is capable of complex formation with the metal, and with hydrochloric acid and subsequent precipitation from the reaction mixture.

24. Process according to claim 23, characterised in that the base capable of complex formation is one which can be protonised by the hydrochloric acid.

25. Process according to claim 23 or 24, characterised in that the base capable of complex formation is a basic nitrogen compound.

26. Process according to one of claims 23 to 25, characterised in that the mole ratio of base/silver or gold chloride/hydrochloric acid is 1 to 80/1/5.