US 3513015 A
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United States Patent 3,513,015 PREVENTION OF SKIP PLATING IN AN ELECTROLESS NICKEL BATH James E. Fitzpatrick, New Castle, Del., and Wassily Poppe, Springfield, Pa. (both Avisun Corporation, P.O. Box 426, Marcus Hook, Pa. 19061) No Drawing. Filed May 3, 1967, Ser. No. 635,694 Int. Cl. C23c 3/02 US. Cl. 117-47 8 Claims ABSTRACT OF THE DISCLOSURE In the art of nickel plating non-conductive articles, skip plating with an electroless nickel plating solution is minimized by contacting the article, after sensitizing with an oxidizable metal salt and nucleating with a noble metal salt, with a hydrogen peroxide solution prior to contact with the electroless nickel plating solution.
This invention relates to an improvement in a method for electroplating non-conducting substrates, and more particularly to an improvement in applying a continuous coating to such articles prior to an electroplating step.
Methods for electroplating articles fabricated from nonconducting materials such as plastics are well known to the art. In such processes it is usual to first clean the article with a mild caustic wash, followed by neutralization; then treating with a conditioning solution consisting of a solution of chromium trioxide in a strong oxidizing mineral acid such as concentrated sulfuric acid or phosphoric acid or mixtures thereof; then sensitizing the surface with a solution of a readily oxid'izable salt such as stannous chloride, which causes the salt to be adsorbed on the surface; followed by a nucleating treatment with an aqueous solution of a noble metal salt such as palladium chloride, which is reduced by the oxidizable salt and forms a metallic film at discrete activated sites.
From this point the plating process may proceed in two ways. In one process the nucleated article is immersed in a treating solution of a copper salt containing a reducing agent such as formaldehyde, trioxymethylene, and the like. Such treatment deposits a thin continuous coating of copper on the surface of the article capable of conducting electricity. The article may then be electroplated with copper by conventional means, followed, if desired by electroplating with nickel and chromium. No difiiculty is usually experienced in depositing a continuous film of electroless copper on the object to be plated. For a general description of the plating process see Modern Plastic Encyclopedia Issue, 1967, September 1966, pp. 1019- 1024.
In certain applications, however, the presenece of copper in the plating is undesirable. For example, plated automobile grilles are tested by exposing them to a salt spray. Plated parts in which the base coat is copper corroded badly when subjected to this test. For such applications it is necessary to substitute an electroless nickel coating for the electroless copper coating. Deposition of such electrodes nickel coatings are described in US. Pat. 2,532,283.
When using electroless nickel, a phenomenon known as skip plating sometimes occurs, in which certain areas of the object to be plated fail to accept the electroless nickel coating, leading to non-adherent electrodeposition of metal on those spots on the surface which were incompletely covered with electroless nickel during subsequent electroplating, or, in extreme cases, the coating is so discontinuous that no plating whatever takes place on such areas. This problem is found in electroplating ABS, but it becomes particularly serious when electroplating polyolefins, such as crystalline polypropylene and high density polyice ethylene. The reason why this phenomenon takes place is not known, but one of many possible theoretical explanations is that Ni++ and H+ ions are competing with each other to plate out or to form a layer on the previously activated sites of the substrate. If the competition is won byNi ions they plate out and form the basis for further nickel plating. If the competition is won by H+ ions they may build up atomic or ionic monolayers, thus interrupting the nickel plating. Whether or not skip plating has or has not taken place can be visually observed when the articles are withdrawn from the electro less nickel bath. If the surface of the article is a uniform shade of dull gray, skip plating has not taken place. If, however, the surface has a mottled appearance, or if shiny areas appear, skip plating has occurred. In the present state of the art, in order to salvage the articles it is necessary to strip the previous coatings from the article, as by dissolution with acid, and to return the article to the first step of the plating process.
It is the object of this invention to prevent skip plating in an electroless nickel bath, and, if such should occur, to provide a method for conditioning the article for return to the electroless nickel bath without the necessity of removing the material previously deposited on the article and recycling it to the first step of the plating article.
We have found that the foregoing object can be realized by immersing the articles in a weak aqueous solution of hydrogen peroxide for a time of thirty seconds to one minute after the nucleation step but before the electroless nickel step. Alternatively, the articles may be passed directly ot the electroless nickel step from the nucleation step, and only those articles showing skip plating after the electroless nickel step can be contacted with the hydrogen peroxide solution and returned directly to the electroless nickel step. The aqueous hydrogen peroxide solution should contain from about 1% to about 10% by weight of hydrogen peroxide, preferably about 2% to about 5%. Higher concentrations may be used, but the use of stronger solutions only adds to the cost of the process without any improvement in results over the use of the less concentrated solutions.
In order that those skilled in the art may more fully appreciate the nature of the invention and of the manner of carrying it out, the following control and examples are given.
CONTROL A conditioning solution was made up in the following manner. 18 g. of a composition consisting of 64% by weight of chromium trioxide and 36% of sodium hydrogen sulfate was added to 500 m1. of an inorganic acid composition consisting of 40% by weight of sulfuric acid (96% concentration), 39.5% of phosphoric acid (85% concentration), 3% chromium trioxide, and 17.5% water. The acid solution was heated to C. and 3% by volume of a commercially available mineral spirit having a boil ing point of 159 C.193 C. and consisting predominantly of C C cycloparaffins was added slowly to the acid solution. An exotherm occurred which raised the temperature to C. After about two hours no excess mineral spirit was observed, and the evolution of gas ceased, indicating complete conversion of the mineral spirit into oxidation products. During the reaction about 9% by weight of water was volatilized.
A number of plaques were molded from a commercial crystalline polypropylene containing, in addition to the usual stabilizers, 0.5% by weight of Triton X-100, a commercially available detergent of isooctylphenyl polyethoxyethanol. Some of these plaques were immersed in the Conditioning solution described above for ten minutes at 85 C. rinsed in deionized water; dipped in a stannous chloride sensitizing solution containing, per liter of solution, g. of SnCl and 40 ml. of HCl at ambient temperatures for two minutes; again rinsed with water; dipped in a nucleating solution containing, per gallon, 1 g. of palladium chloride an 10 ml. of HCl for two minutes at ambient temperature, and again rinsed with water.
The plaques are then dipped into an electroless nickel solution containing, per liter, 30 g. of NiCl 50 g. of sodium glycollate, and 10 g. of sodium hypophosphite for minutes at 190 F. The pH of the solution is adjusted to about 5 with caustic soda. After this treatment the plaques are washed and inspected. It is found that extensive skip plating has taken place on some of the plaques.
Example 1 The plaques from the control which exhibited skip plating were dipped in a solution made up by mixing one part of a hydrogen peroxide solution with twelve parts of water for one minute at ambient temperatures. They were then returned to the electroless nickel bath and treated for 15 minutes at 190 F. Upon removal from the bath they were found to be uniformly coated with electroless nickel and exhibited no signs of skip plating.
Example 2 The procedure of the control was followed using the remainder of the plaques, except that after the nucleation step the plaques were contacted with the hydrogen peroxide solution of claim 1 for one minute prior to immer- U sion in the electroless nickel bath. Upon removal from the electroless nickel bath no skip plating could be observed.
The plaques from Examples 1 and 2 were then electroplated with nickel in the conventional manner to deposit a 1 mil film of nickel on the plaque. The bond strength of the plating was 18 lb./in. Bond strength is measured by the pull test in which two parallel cuts are made into the plated metal coating one half inch apart, an additional vertical cut is made to form a tab, one end of the resulting tab is raised suificiently to allow gripping by a tensile testing machine. The specimen is then placed in a tensile tester machine. The specimen is then placed in a tensile tester and the tab is pulled vertically from the surface. The force required to pull the tab is measured as the bond strength.
What is claimed is:
1. In a process for electroplating non-conductive articles wherein the article is successively contacted with a conditioning solution comprising a strong mineral acid; a sensitizing solution comprising an oxidizable metal salt; a nucleating solution comprising a noble metal salt; and an electroless nickel plating solution comprising a reducible nickel salt; followed by electroplating with nickel, the improvement which consist in contacting the article with an aqueous solution containing at least 1 weight percent of hydrogen peroxide after contact with the nucleating solution and before contacting with the electroless nickel plating solution.
2. The process of claim 1 in which the non-conductive article is formed of polypropylene.
3. The process of claim 1 in which the non-conductive article is formed of polyethylene.
4. The process of claim 1 in which the non-conductive article is formed of an acrylonitrile-butadiene-styrene copolymer.
5. In a process for electroplating non-conductive articles wherein the article is successively contacted with a conditioning solution comprising a strong mineral acid; a sensitizing solution comprising a reducible tin salt; a nucleating solution comprising a noble metal salt; and an electroless nickel plating solution comprising a reducible nickel salt; followed by electroplating with nickel, the improvement which consists in removing from the plating cycle, at the end of the electroless nickel plating step, those articles which show evidence of skip plating, contacting the removed articles with an aqueous hydrogen peroxide solution containing at least 1 weight percent of hydrogen peroxide, and reintroducing the removed articles to the plating cycle at the electroless nickel plating step.
6. The process of claim 5 in which the non-conductive article is polypropylene.
7. The process of claim 5 in which the non-conductive article is polyethylene.
8. The process of claim 5 in which the non-conductive article is an acrylonitrile-butadiene-styrene copolymer.
References Cited UNITED STATES PATENTS 2,690,402 9/ 1954 Crehan. 3,212,918 10/1965 Tsu et a1. 3,265,511 8/1966 Sallo. 3,305,460 2/1967 Lacy. 3,370,974 2/1968 Hepfer.
MURRAY KATZ, Primary Examiner US. Cl. X.R.
831 UNITED STATES PATENT OFFlQi-Q Invontofls) James E. Fitzpatrick and easily Poppe It is car rifled that error av ears in the above-identified atom t and that said Letters Patent are hereby corrected as shorsn below:
I Column 1, line 60, the word "electrodes" should be --electrolesa--. I
Column 2, line 32, the word "at" should be -to--.
Column 3, line 6, the word "an should be --and--. Column Claim 5, line 19, "a reducible" should be --a.n oxidizable--.
smqgg QSEAL) Attcst:
WILLIAM E. sum. Edward M. Fletcher, In c i sioner 01 Patnts Attesting Officer