Electroless silvering ink

Abstract

An electroless waterless silver plating ink process for coating an article substrate with a bright metallic silver plate, wherein the silvering ink comprises a silver salt, an ammonium salt and dimethyl sulfoxide.

Claims

1. An electroless plating ink consisting of: a) a silver salt; b) an ammonium salt; and c) a polar aprotic solvent.

2. The plating ink of claim 1, wherein: a) the silver salt consists of silver nitrate b) the ammonium salt consists of ammonium bromide c) the polar aprotic solvent consists of dimethyl sulfoxide.

3. The plating ink of claim 1 wherein the polar aprotic solvent is not toxic.

Description

OBJECTIVES OF THE INVENTION

(1) An object of the invention is to provide a silvering ink for one step coating of a base metal, preferably copper, with bright silver.

(2) A further object of the invention is to provide a silver plated article using a silvering ink.

(3) A further object of the invention is to provide a silvering ink that does not need battery power to plate a base metal with silver.

(4) A further object of the invention is to provide a silvering ink that is fluid, stable, cyanide free, and relatively non-toxic.

(5) A further object of the invention is to provide a silvering ink that is amenable to use by immersion and spray plating means.

(6) A further object of the invention is to provide a silvering ink that is amenable to use by pen nib means at room temperature.

(7) Other objects, aspects and advantages of the invention will become apparent to those skilled in the art upon reviewing the following detailed description.

SUMMARY OF THE INVENTION

(8) Specifically, the presented invention is a silver plating ink useful in a one step process for plating an article substrate with bright metallic silver.

(9) Petitioner has surprisingly discovered that a non-aqueous ammonium complex of silver bromide dissolved in dimethyl sulfoxide is a viable ink for coating a copper substrate with a bright silver plate. A polar silver diammine ion, [Ag(NH.sub.3).sub.2].sup.+, is likely the complex produced in situ by reacting dry silver nitrate and dry ammonium bromide in dimethyl sulfoxide. Immersion of copper in such ink solution results in a one step process for coating red copper with bright silver. The ink solution is stable in storage for over six months despite exposure to air and light.

(10) Petitioner has also found that the waterless solution can be loaded in a marking pen as an ink. Application of the ink to the metal through a pen nib, writes a bright silver metal on a red copper surface.

DETAILS OF THE INVENTION

(11) Water is both a protic and polar solvent. Water is protic in that it contains hydrogen protons. Water is polar in that the hydrogen is ionic. Unfortunately, the charged hydrogen ions in water destabilize metal plating solutions.

(12) An aprotic substance is defined as a polar solvent without a protic hydrogen ion as present in water. Polar aprotic solvents such as dimethyl sulfoxide can dissolve polar solids. Petitioner has discovered that replacing water with a polar aprotic solvent allows an ink useful in a one step electroless process for plating an article with a bright metallic silver coat.

(13) Petitioner believes that combination of dry silver nitrate and dry ammonium bromide within waterless dimethyl sulfoxide forms a polar diammine complex of silver bromide, Ag(NH.sub.3).sub.2Br, which dissolves into the diammine ion of silver, [Ag(NH.sub.3).sub.2].sup.+. The parent silver salt to make the diammine ion includes the class of silver compounds soluble in dimethyl sulfoxide. Silver nitrate and silver acetate are preferable, but not limiting.

(14) To complex a parent silver salt into a silver diammine ion, an ammonium halide salt that is soluble in dimethyl sulfoxide is required. Ammonium bromide is preferred over ammonium chloride as it has been found to be more stable in solution and give a brighter and thicker silver coat.

(15) Other than dimethyl sulfoxide, common polar aprotic solvents are acetone, n-pyrrolidone, acetonitrile and methyl cellosolve. These are all much more volatile, toxic and flammable than dimethyl sulfoxide. Therefore, their usefulness is limited for purposes of this invention.

(16) Water up to about 10% can be tolerated in the silvering ink while still producing a bright silver coat on a copper substrate. At greater than 20% water, silver bromide begins to precipitates out of solution.

(17) Immersion of copper in the presented waterless and electroless plating solution results in a one step process for plating copper with a bright silver metallic coating. The waterless solution is clear and stable in storage for over six months despite exposure to air and light. Also, the plating solution works well at room temperature.

(18) Further the plating solution operates without need for an added reductant to produce elemental silver, as required in conventional electroless plating, and described, for example, in U.S. Pat. Nos. 63,875,442 and 5,322,553. Petitioner believes the silver diammine ion in the ink is self reducing on contact to the copper metal substrate.

(19) Pen plating, immersion plating, and spray plating means for applying the silvering ink to a substrate have been found to have optimal silver concentrations for a bright silver plate. An ink solution for pen plating requires the most concentrated silver content due to its desired contact time of a few minutes. Immersion plating requires the least concentrated silver content as the liquid can contact the base metal for days. A process room temperature of about 25.deg.C. is a satisfactory environment. Higher process temperatures tend to quicken plate formation.

(20) For plating a substrate through a pen nib, a preferable ink comprises 0.5 Molar as silver nitrate and 1 Molar as ammonium bromide in dimethyl sulfoxide. In a silvering solution for spray plating an article, 0.2M as silver nitrate and 0.4M as ammonium bromide in dimethyl sulfoxide comprises a preferable composition. For immersion plating an article, a preferable silvering composition comprises 0.1M as silver nitrate and 0.2 Molar as ammonium bromide in dimethyl sulfoxide.

(21) While preferred embodiments of the present invention have been described, it should be understood that various changes, adaptations and modifications may be made therein without departing from the spirit of the invention and its claims.