EP0492487B1 - Means of temporary protection of bare silver and copper surfaces against tarnishing and method of using it - Google Patents

Description

The invention is based on an agent for protecting bare silver surfaces against tarnishing, which contains, as a hydrophobic inhibitor, a long-chain aliphatic mercapto compound with at least 12 carbon atoms in the chain and water.

Silverware and silver-plated parts tarnish in an atmosphere containing hydrogen sulfide and turn brown to black due to the formation of silver sulfide. In order to prevent tarnishing, it is known to protect silver surfaces with an inorganic cover layer, for example with rhodium or with silver chromate, which are applied by electrolytic processes. It is also known to protect silver surfaces by means of transparent lacquers or waxes, which are applied by dipping, spraying or vapor deposition. A major disadvantage of these coatings is that they do change the external appearance of the silver, which is particularly disadvantageous for decorative objects. In applications in electrical engineering, especially in electrical contacts, such passivating coatings are not desirable because they greatly increase the electrical contact resistance. In addition, such passivation layers make soft soldering of a silver surface difficult. For this reason, means are also known with which silver surfaces are only temporarily protected from tarnishing, namely from the time of manufacture to the time of further processing or use. It is known from DE-OS 1 621 459 to dissolve long-chain aliphatic compounds which contain an SH group (mercapto compounds) in volatile organic solvents, namely in chlorinated hydrocarbons, with the solution to treat the silver surfaces to be protected and then drying by draining the solution and allowing the remaining solution still adhering to the surface to evaporate. In the case of sufficiently long-chain aliphatic mercapto compounds as inhibitors, a transparent, colorless, ideally monomolecular protective layer remains on the silver surface, which at least adheres to the goods until they are processed further or reach the consumer. The disadvantage here is that the inhibitors are dissolved in halogenated hydrocarbons which are harmful to the environment.

To avoid halogenated hydrocarbons as solvents, it is known from DE-OS 39 05 850, the inhibitors, which in turn are long-chain aliphatic compounds with SH group, namely mercapto compounds with 12 to 25 carbon atoms, in organic solvents such as glycols and / or Solve glycol ethers with a total of 4 or 5 carbon atoms and treat the silver surfaces to be protected with this solution. Glycols and glycol ethers have the advantage of being less harmful to the environment than halogenated hydrocarbons. However, they have the disadvantage that they are flammable and that the vapors escaping from the treatment solution can form an explosive mixture, so that special precautions must be taken when working with such solutions. In order to counteract the development of flammable or explosive gas mixtures, it is already known from DE-OS 39 05 850 not to dry the treated surfaces by letting the solvent evaporate. Rather, the solvent is removed from the treated surfaces by rinsing with aqueous solvents or with warm water with detergents. In order to reduce the evaporation rates, it is also known from DE-OS 39 05 850 to add water to the solution of the inhibitor in glycol or glycol ether. The glycol and the glycol ether dissolve in the water and are thereby diluted. However, care is taken to ensure that with this dilution the solubility limit of the inhibitor in the glycol or glycol ether is not exceeded so that there is no clouding or precipitation. The addition of water reduces the dangerousness of the glycol or the glycol ether, but does not eliminate it.

From DE-AS 1 279 428 it is already known to protect silverware to protect against sulfur tarnishing with the aid of aliphatic thiocarboxylic acids, which are used in an aqueous emulsion and are said to adhere better to the silver surface. The aqueous emulsion does not require an emulsifier because the thiocarboxylic acids have a hydrophilic carboxyl group, which makes them partially water-soluble. Aqueous emulsions of thiocarboxylic acids have only a short shelf life and are difficult to handle. They have therefore found no entry into practice for the dip treatment or spray treatment of silverware, rather the thiocarboxylic acids have been used as an additive to polishing pastes (DE-AS 1 279 428, claim 5). Another, particularly significant disadvantage of thiocarboxylic acids is that water can attach to their hydrophilic carboxyl group, which ultimately enables the hydrogen sulfide from which the silver surface is to be protected to attack the silver surface, so that there is no protective layer which is impermeable to hydrogen sulfide receives. On decorative silverware, the agent, which is supposed to protect against tarnishing of the surface, should be applied as a film so uniform that the gloss of the surface is retained. This is hardly possible with an aqueous emulsion of thiocarboxylic acids, because it is not enough to remove the excess of the emulsion from the Letting the surface run off, rather one would have to rinse the surface with water; however, a considerable part of the thiocarboxylic acid would be rinsed off again because it has hydrophilic groups.

A precious metal cleaning and polishing bath is known from CH-A-478 925, which contains long-chain thiols as tarnishing protection, which are brought into solution by a primary alcohol as solubilizer. In combination with a detergent substance, an alkylaryl polyglycol ether can also be present as an emulsifier.

From FR-A-1 347 705 a paste for cleaning and polishing tarnished silver is known, which among other things. contains a thiol and a non-ionic surfactant.

The present invention has for its object to provide a means for the temporary protection of bare silver surfaces, also in the field of electrical engineering, in particular in the case of electrical contacts, which can be handled easily and safely, even in immersion baths, and is environmentally compatible. This object is achieved by an agent with the composition specified in claim 1. A particularly suitable method for using this agent is the subject of claim 5. Advantageous developments of the invention are the subject of the subclaims.

The agent according to the invention contains, as organic inhibitor substance, a hydrophobic aliphatic thio compound with at least 12 carbon atoms in the chain from the group of thio alcohols and their esterification products without hydrophilic carboxyl groups, the thio alcohols being particularly preferred. However, they are not dissolved in an organic solvent, but used in the form of an aqueous emulsion. Surprisingly, it has been shown that with these hydrophobic compounds , wafer-thin, but dense and therefore reliably passivating cover layers can also be formed on silver surfaces or silver alloy surfaces if the inhibitor substances are not used in organic solution, but in the form of an aqueous emulsion, although they are present in the emulsion in the form of much larger particles than in a real solution.

In particular, it has been shown that the emulsifier, which is inevitably required to bring the selected inhibitors into an emulsion, is only incorporated in a negligible amount in the inhibitor film which forms on the silver surface which has been treated with the emulsion. After treating the silver surface with the emulsion, the emulsifier can be easily washed off by rinsing with water, while the passivating film of the inhibitor substance remains on the treated silver surface.

The silver surface is treated in the emulsion at a temperature which is above the melting point of the inhibitor substance. In this case, the inhibitor substance is particularly readily distributed evenly thin on the surface to be passivated. The subsequent rinsing of the surface is then carried out at a temperature below the melting point of the inhibitor substance, which has the advantage that it then adheres particularly firmly to the passivated surface.

The agent according to the invention is easy to handle. It contains neither an environmentally hazardous nor a flammable solvent, but only water as the carrier substance of the emulsion. The sub-substances contained in the water, the inhibitor and the emulsifier, are biodegradable.

The agent according to the invention is easy to use. It can be marketed as a concentrate of the inhibitor and the emulsifier and can be made ready for use by the user by stirring it into water. It can be applied by dipping or spraying. The goods to be treated are preferably immersed in the emulsion because the treatment times are then the shortest. For the treatment of the goods by immersion in the emulsion, rinsing and drying, conventional devices such as in electroplating technology can be used, with the difference that there is no need for electrodes and that there are no problems with the disposal of the baths and rinsing water.

The agent according to the invention contains 2 to 20 g / l of the inhibitor.

The emulsifier is contained in the agent in an amount of 2 to 20 g / l.

To assist wetting, up to 2 g / l, preferably 0.05 to 1 g / l, of an anionic or nonionic surfactant can be added to the agent. Cationic surfactants are unsuitable.

According to the invention, inhibitors are selected which have at least 12 and preferably not more than 20 C atoms in the chain. With less than 12 carbon atoms, it is sometimes not possible to achieve a well-adhering protective coating, and in part there are strong odor nuisances. With more than 20 carbon atoms, the inhibitors become too solid in consistency and it becomes more difficult to produce a satisfactory emulsion.

It has been shown that the inhibitors can be emulsified very finely and with long-term stability and can be applied to the surfaces to be protected in a thin but dense layer. Particularly suitable emulsifiers for this purpose are branched fatty alcohols having 9 to 20 carbon atoms, in particular having 10 to 15 carbon atoms, the branched fatty alcohols preferably being alkoxylated, in particular ethoxylated, with a degree of alkoxylation of 2 to 10.

Surprisingly, it has been shown that when the passivating agent is adjusted to a pH of 1-10, better 1-8, preferably 2-4, the emulsifier can be washed out of the deposited inhibitor layer practically completely with water, which is important for reliable surface protection is. At high pH, less inhibitor is washed out, which increases the protective effect and the lubricating effect; however, stains can appear on the surface, which can disturb the appearance of decorative surfaces. Decorative surfaces are therefore preferably passivated with an agent whose pH is lower, preferably between 2 and 4, with sufficient inhibitor substance remaining even after washing out to form a tight protective layer. If the pH is above 10 there is a risk of the emulsion splitting.

It has also been shown, unexpectedly, that the agent according to the invention is also suitable for passivating surfaces made of copper and copper alloys. The same applies accordingly to bimetal parts made of a silver material on the one hand and a copper material on the other hand, for example for bimetal contact rivets. The water-emulsified inhibitor also absorbs very well on copper and copper-containing materials and protects the surface against the attack of sulfur compounds and against oxidation. The good solderability and the electrical properties are not impaired.

Examples of baths with which opaque and well-adhering coatings can be achieved on surfaces of silver and copper materials:
1. Dissolved in one liter of deionized water: 2 - 20 g Inhibitor: Octadecanethiol 2 - 20 g Emulsifier: Branched fatty alcohol ethoxide with 8 to 16 carbon atoms (C₈ - C₁₆) and a degree of ethoxylation of 2 to 10 0.00 - 1 g Surfactant: Sodium lauryl ether phosphate pH 2 adjusted with a mineral acid
Particularly suitable for silver and silver alloys.
2. Dissolved in one liter of deionized water: 2 - 20 g Inhibitor: Octadecanethiol 2 - 20 g Emulsifier: Branched fatty alcohol ethoxylates with 8 to 16 carbon atoms (C₈ - C₁₆) and a degree of ethoxylation of 2 to 10 0.00 - 1 g Surfactant: Sodium lauryl sulfate pH 10 adjusted with NaOH.
Particularly suitable for parts made of copper or copper alloys.

These baths are preferably used in such a way that the parts to be passivated are immersed in the approx. 40 ° C to 50 ° C warm baths. After 2 to 3 minutes, the surface of the parts is even and dense with the inhibitor wetted. The parts are then rinsed with cold water and then dried, preferably with hot air.

The effect of the tarnish protection layer on silver can be easily demonstrated with the help of a two percent sodium sulfide solution. Untreated silver parts tarnish immediately, treated parts remain bare over a longer period. The effect of the protective layer on copper was checked by passivating copper parts for several hours in a hydrogen sulfide atmosphere with 1 ppm H₂S; the copper parts did not change color.

Claims (7)

1. Means for temporarily protecting bare silver surfaces and silver alloy surfaces against tarnishing, which, as a hydrophobic inhibitor, contains a long-chained aliphatic thio-compound with at least one SH group and at least 12 C atoms in the chain from the thio-alcohol group and whose ester products contain water and one water-soluble emulsifier, with the aid of which the inhibitor is emulsified in the water and its pH-value adjusted to between 1 and 10, characterised in that it contains

   2 to 20 g/l of inhibitor,

   2 to 20 g/l of emulsifier,

   0 to 2 g/l of an anionic or non-ionic surfactant,

   the remainder being water, and that the emulsifer is selected from the group of the branched, alkoxylated - preferably ethoxylated - fatty alcohols with 4 to 20 C atoms and an alkoxylation level of 2 to 10.
2. Means as claimed in claim 1, characterised in that it is adjusted to a pH-value smaller than 7, preferably within the range of 2 to 4.
3. Means as claimed in claim 1, characterised in that it contains 0.05 to 1 g/l of the surfactant.
4. Means as claimed in one of the previous claims, characterised in that it contains no inhibitors with more than 20 C atoms in the chain.
5. Method of stabilising bare silver and silver alloy surfaces or copper and copper alloy surfaces with an emulsion of a composition as set out in claims 1 to 4, characterised in that the surfaces are treated with the emulsion at a temperature above the melting point of the inhibitor, and that the surfaces are thereafter rinsed at a temperature below the melting point of the inhibitor.
6. Method as claimed in claim 5, characterised in that the surfaces are rinsed with water.
7. Method as claimed in claim 5 or 6, characterised in that the rinsed surfaces are dried with hot air.