Coating processes – Immersion or partial immersion – Metal base
Reexamination Certificate
1998-08-26
2001-06-12
Talbot, Brian K. (Department: 1762)
Coating processes
Immersion or partial immersion
Metal base
C427S098300, C427S305000, C427S443100, C210S668000, C210S688000
Reexamination Certificate
active
06245389
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for circulating an electroless nickel plating solution in which an aged plating solution can be reused.
BACKGROUND ART
An electroless nickel plating solution has hitherto been generally composed of nickel sulfate as a nickel source in combination with sodium hypophosphite as a reducing agent. Sodium hydroxide or ammonium hydroxide has also been used to maintain the resulting plating solution at a predetermined pH level. When nickel is electrolessly plated using a plating solution of such composition, sodium sulfate is formed and, over time, accumulates in the plating solution. The sodium sulfate results from a reaction between a sodium phosphite derived from an oxidation of the sodium hypophosphite and a sulfate group derived from the nickel sulfate for use as the nickel source. This accumulation causes the plating solution to age, eventually inviting phenomena such as reduced plating speed, extraordinary deposition, deteriorated film quality and the like. Thus, after being used for a certain period of time the plating solution is replenished, for continuous use, with fresh liquid chemicals to thereby compensate for those components that have been used up. When in a state wherein both the sodium sulfate and the sodium phosphite are contained in high concentrations, the plating solution is not recycled and ultimately disposed in the ocean or the like as a waste liquid.
From the point of view of global environmental protection, however, the above mentioned plating solution waste has been banned from oceanic disposal since 1995 by the provisions of the London Dumping Treaty. Consequently, a demand has been voiced for the development of an electroless plating solution that affords prolonged service life or permits disposal with minimal environmental pollution.
Various attempts have been made to remove sodium phosphite and sodium sulfate which are built up during electroless nickel plating as discussed hereinbefore, but none of them are being practiced on a commercial basis. The methods which have been proposed for the removal of the sodium phosphite and sodium sulfate materials include, for instance, a method in which an electrolysis membrane is employed, a method in which an aluminum sheet or an aluminum foil is placed in an aged alkaline plating solution in such a method that nickel is deposited, followed by recovering the resultant nickel with nitric acid and then allowing a chelate resin to adsorb a portion of the remaining undeposited nickel in the mother liquor (Japanese Patent Laid-Open No. 51-6136); and a method in which nickel ions and sodium ions are separated from a plating solution in advance by an ion exchange resin such that sodium and nickel are separated from each other by adsorption into the ion exchange resin, the nickel alone being put back into the plating solution for subsequent reuse.
Still another method has been adopted in which part of an aged plating solution is disposed of and replaced with fresh liquid chemicals so as to extend the life time of the plating solution.
These, however, though likely to be regarded as rational processes in some respects, have too many economic and technological drawbacks to be suitable for practical application. Moreover, the method of partially substituting fresh liquid chemicals for an aged plating solution cannot be said to offer a fundamental solution to the problem.
Accordingly, the object of the present invention is to provide an industrially advantageous method which enables reuse of an aged electroless nickel plating solution. More particularly, the invention provides a method for circulating an electroless nickel plating solution which is as non-polluting as possible to the global environment.
DISCLOSURE OF THE INVENTION
In view of the prior art, the present inventors have done intensive research, and found that an aged plating solution can be reusable through which specific treatment processes carried out in a specific order, and completed the present invention.
That is to say, the present invention provides a method for circulating an electroless nickel plating solution, characterized in that the system comprises the following process steps effected in the following order:
(A) a step of electrolessly plating nickel using a plating solution containing nickel hypophosphite as a chief chemical material for supplying a plating metal ion Ni
2+
and a diphosphorous acid ion H
2
PO
2
−
acting as a reducing agent;
(B) a step of removing HPO
3
2−
from a plating solution that has aged in the step (A);
(C) a step of desalting the mother liquor separated in the step (B); and
(D) a step of adjusting the components of the treated plating solution and then circulating same back into the step (A) of electroless nickel plating.
Additionally, the present invention provides a system of circulating an electroless nickel plating solution, which further includes, prior to the step (B), a step wherein the plating solution having aged in the step (A) is treated with a chelate resin to thereby remove the Ni
2+
.
Moreover, the present invention provides a system of circulating an electroless nickel plating solution, wherein the plating solution used in the step (A) further contains at least one member selected from the group consisting of a chelating agent, a buffering agent, a pH adjustor and a stabilizer.
Furthermore, the present invention provides a method of circulating an electroless nickel plating solution, wherein in the step (B) the pH of the plating solution having aged in the step (A) is adjusted to 6-9 with a calcium salt to thereby cause calcium phosphite to precipitate, and the precipitate is thereafter separated and removed.
The present invention also provides a method of circulating an electroless nickel plating solution wherein, in the step (C) the mother liquor derived from the step (B) is allowed to pass through an electrodialysis cell, a cation exchange resin, or a reverse osmosis membrane.
The present invention is directed to a method of circulating an electroless nickel plating solution wherein the system is essentially constituted by steps (A), (B), (C) and (D) performed in that order. Each of these steps is described hereinafter.
REFERENCES:
patent: 4303704 (1981-12-01), Courduvelis et al.
patent: 5112392 (1992-05-01), Anderson et al.
patent: 5609767 (1997-03-01), Eisenmann
Horikawa Ken
Mita Muneo
Nakao Hidehiro
Tashiro Katsuhiro
Nippon Chemical Industrial Co. Ltd.
Talbot Brian K.
Wenderoth Lind & Ponack LLP
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