Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2000-12-04
2002-04-30
Phasge, Arun S. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S523000, C204S633000, C204S634000
Reexamination Certificate
active
06379517
ABSTRACT:
This application is a 371 of PCT/DE99/03186 filed Sep. 27, 1999.
The invention relates to a method and a device for regenerating by electrodialysis an electroless metal deposition bath, especially an electroless nickel deposition bath.
The electroless metal-plating of workpieces has been known for a long time. For example, sanitary fittings made of plastics material are provided with metal layers in order to obtain a specific aesthetic appearance, or specific workpieces consisting of metal, in order to improve their serviceability, for example the wear-resistance or corrosion behaviour. Thus, in machine-building, parts which are mechanically heavily loaded receive resistant coatings comprising a largely amorphous nickel/phosphorus alloy layer in order to increase the resistance to abrasion, for example of bearing shells on moveable parts. In oil production, metal parts used in the off-shore domain are coated with a nickel/phosphorus layer of this type in order to improve the material resistance to chemical influences.
Electroless plating with metals is based on an autocatalytic process in which dissolved metal ions are reduced to metal by means of a reducing agent located in the deposition solution, and deposited on the workpiece to be coated. In this case, additional components are often incorporated into the metal layer, for example phosphorus. As well as nickel, copper can also be deposited by this method.
For the deposition of nickel/phosphorus layers, electrolytic and electroless methods can basically be used. Electrolytic methods are admittedly easier to handle; however they have the disadvantage that layers of uniform thickness can only be obtained if the parts to be coated have a simple geometry. The electrolytic metallisation of workpieces which have a complex geometry, for example curvatures, holes or undercuts, leads to an uneven layer thickness and thus in many cases to intolerable local fluctuations in the plating result. Moreover, the metal layers deposited in an electroless manner often have more advantageous mechanical properties than metal layers deposited by electrolytic means. For this reason, electroless methods are very frequently used for plating.
Electroless metal deposition is represented below in the example of electroless nickel deposition with simultaneous incorporation of phosphorus into the layer. In this process, a deposition solution is used, for example, which contains sodium hypophosphite as the reducing agent for nickel ions, as well as nickel ions, for example as nickel sulphate. The deposition reaction takes place according to the following reaction equation:
NiSO
4
+6 NaH
2
PO
2
±Ni+2 H
2
+2P+4NaH
2
PO
3
+Na
2
SO
4
Thus in this reaction, dissolved nickel and hypophosphite ions are constantly consumed, whilst the concentration of orthophosphite (H
3
PO
3
−
) increases as an oxidation product. Moreover the counterions of the nickel cations and hypophosphite anions accumulate in the form of Na
2
SO
4
.
Thus, methods of this type have the disadvantage that the process management is complicated in many cases and a large number of monitoring operations has to be carried out in order to achieve constant deposition conditions. In addition to this, the service life of the electroless deposition baths is limited. In metal deposition, the reducing agent and the metal ions are used up which have to be continually added as the method is carried out, in order to make available an approximately constant content of available reducing agent and available metal ions within a narrow band width. Since the reducing agent and the salts containing the metal ions leave behind, during the deposition reaction, products which accumulate in the deposition bath, the service life of the bath is inevitably limited. For example, the metal ions are added to the bath in the form of salts, such that disturbing anions, such as sulphate ions accumulate in the bath. The same is true for orthophosphite ions (H
2
PO
3
−
) which form in the bath through oxidation of hypophosphite ions.
The age of a bath is generally quoted in metal turnover (MTO). 1 MTO corresponds to the amount of deposited metal from the bath which corresponds to the initially used concentration of the metal ions in the bath, respectively in relation to the total volume of the bath. Generally, the degradation products in the bath reach such a high concentration after 6 to 10 MTO that the quality and deposition speed of the metal are no longer within tolerable ranges. Therefore baths of such an age are not used again. A new bath must be started and the spent one must be thrown away. What is disadvantageous is that the necessary disposal of the baths and the required new charging of fresh baths lead to high costs and considerable damage to the environment. For this reason, different methods have been proposed by means of which the service life of baths of this type can be extended.
In U.S. Pat. No. 5,221,328, a method for extending the service life of electroless nickel baths is described, by means of which method orthophosphite which has been produced in a nickel/phosphorus deposition bath can be precipitated as a metal salt and separated. Yttrium and lanthanides can be considered as precipitants. However, the necessary chemicals for this are extremely expensive. Moreover, the dissolved components of these additives, remaining in the bath, can impair the quality of the metal coatings.
In “Plating and Surface Finishing”, September 1995, pages 77 to 82, it is proposed by C. D. Iacovangelo that the disturbing precipitation of nickel orthophosphite be prevented through the addition of complexing agents. By this means, the concentration of dissolved free nickel ions is reduced.
In the ENVIRO CP-process of the company Martin Marietta, U.S.A., the disturbing components in the bath are separated by means of adsorption on ion-exchange resins. For the complete separation and regeneration of the deposition bath, a complex process is carried out in which a plurality of different ion-exchange columns and containers for diverse process liquids are needed.
Y. Kuboi and R. Takeshita describe a method of separating the undesired bath components by electrodialysis (Electroless Nickel Conference 1989, Proceedings, Prod. Finishing Magazine, 1989, pages 16-1 to 16-15). In this method, the electroless nickel bath is led as so-called diluate through an electrodialysis cell. The diluate compartment in the electrodialysis cell is, for this purpose, separated on the anode side by an anion-exchange membrane from the anode compartment which is in contact with the anode, and on the cathode side by a cation-exchange membrane from the cathode compartment which is in contact with the cathode. These two last-mentioned compartments are also referred to as concentrate compartments. The undesired sulphate and orthophosphite ions in the deposition bath are transported into the anode compartment and the undesired sodium ions, which come from the sodium hypophosphite used, are transferred into the cathode compartment. In laboratory tests, however, it has emerged that, in addition to the undesired sulphate, orthophosphite and sodium ions, the bath constituents important for the deposition process, namely the nickel and hypophosphite ions and the organic complexing agents (mostly carboxylic acids or anions thereof), are transported into the concentrate compartments.
In DE 43 10 366 C1, a method of regenerating electroless nickel/phosphorus baths by electrodialysis is described. The nickel/phosphorus bath to be regenerated is to this end led through a compartment in an electrodialysis cell which is separated from the adjacent compartments both on the cathode side and on the anode side by respectively one anion-exchange membrane (diluate compartment). Through the application of an electrical field, ortho- and hypophosphite ions are transferred into the concentrate compartment lying on the anode side of the diluate compartment. This solution is then transported into the cathode compartment which is in contact wi
Blaschke Manfred
Born Rainer
Heydecke Jens
Kraft Alexander
Richtering Werner
Atotech Deutschland GmbH
Paul & Paul
Phasge Arun S,.
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