Patent Application
20070278105
Optimization of natural convection through the interstices of a foam matrix results in a more efficient electroplating process and a metal foam having more uniform deposition of metal throughout its structure. Accordingly, the techniques disclosed herein advantageously allow increased strength of the finished material, as well as more uniform surface and interior structure, increased tensile strength, dimensional stability, wear resistance, and corrosion resistance.
Natural convection of electrolyte solution through the interstices of a foam matrix is optimized during electroplating by inclining or tilting the foam cathode in a plater.
The angle required for inducing net flow across the thickness of the foam may range from about 1 to about 45 degrees, e.g., from about 2 to about 30 degrees and preferably ranges between about 10 to about 20 degrees. The angle is advantageously closer to vertical since the depleted, lower density electrolyte solution forms a more laminar flow upwards, creating a better pressure differential and flow rate across the foam than a more horizontal angle, which leads to a more turbulent flow of the depleted electrolyte. Turbulent flow results in more rapid mixing and dissipation of the low-density electrolyte emerging from a more horizontally positioned foam (e.g., greater than about 45 degrees) and actually results in diminished driving force for the flow across the foam compared to an electrode positioned closer to vertical. Other advantages of the present invention are that the simplicity and serviceability of a vertical plater are retained compared to a horizontal plater and productivity per unit of plant area is better than either a vertical or a horizontal plating apparatus.
In another aspect, an inclined foam plating system optionally incorporates techniques for redistributing current density from the top of the plating zone to areas below. In this manner, local excess current densities are avoided and a more uniform product is obtained. Foams plated at high current densities tend to have a non-uniform thickness profile, e.g., a high DTR. In a typical vertical foam plater, e.g., see
In one embodiment, the electrolyte gap is increased at the top of the plater relative to the bottom. This produces a higher electrolyte voltage (IR) drop in the upper zone to reduce current density there while increasing current density in the lower zone with a narrower electrolyte gap and a smaller IR drop. The electrolyte gap is increased by increasing the distance between the cathode and the anode near and at the top of the plater relative to the bottom. The tapered electrolyte gap can be obtained by supporting one or more anodes in an inclined position relative to the cathode or by making one or more anodes wider at one end than at the other.
In another embodiment for increasing electrolyte resistance at the top of the plater where current is supplied, a current reducing mask is positioned between a foam cathode and the anode(s) in the top plating zone of a plater. The mask is preferably a non-conductive porous sheet which permits electrolyte to pass through, but slows the rate of plating.
Suitable open cell foams for use herein are well known. Those which may be employed include any natural or synthetic polymeric foams such as cellulose, hydroxypropyl cellulose, polyurethanes, including a polyether-polyurethane foam or a polyester polyurethane foam; polyesters, olefin polymers, such as a polypropylene or polyethlyene; vinyl and styrene polymers, polyphenol, polyvinyl chloride and polyamides. These foam substrates may have an average number of pores per inch within a wide range, typically within a range of about 5 to about 100 pores per inch (ppi.). In preferred embodiments the natural or synthetic foam is capable of being vaporized after deposition of the desired metal so that only metal is left at the end of production. In order to electroplate the foam, it must be made at least partially electrically conductive. The foam can be made conductive by any technique known to those skilled in the art, e.g., coating with a latex graphite; electroless plating with a metal such as copper or nickel; coating with an electrically conductive paint or ink containing carbon powder, or a metal powder such as silver powder or copper powder; and vacuum deposition of a metal. It will be understood that non-foam materials may also be employed as substrate materials. Filaments, including fibers or threads, may also serve as a substrate for the deposition of an electroconductive metal. The foam starting material can, however, also be formed from organic materials having electrical conductivity or consist of metal fibers. In the last-mentioned cases the application of an electrically conducting surface layer is not necessary and can be dispensed with. For convenience, all the above materials described in this paragraph will be referred to herein as “foam”.
In general, and by way of example, a plating apparatus for use in accordance with the present disclosure may include a plating tank provided with a means of supply and removal of electrolyte bath; guides to guide pre-plated continuous foam down into the tank and then upward between anodes, e.g., baskets, towards the electrical contacts; a device for transporting foam located above the bath; a device(s) for supplying electrical current to the anode(s) and foam contacts; wherein the foam moving past the anode (or between 2 or more anodes) is inclined from vertical to allow depleted, lower density electrolyte inside the foam to rise and establish a natural convection driven diagonal flow of electrolyte through the foam. In a preferred embodiment, the anodes are positioned around the foam strip to substantially equalize current density distribution as described above, e.g., the electrolyte (foam to anode) gap increases from bottom zone to the top zone or through utilization of a current density reducing mask. In another preferred embodiment, the anodes are positioned such that the gap between the anode facing the upper face of the foam is smaller than the gap relative to the anode facing the lower side of the foam. This increases current density at the upper face of the foam where the electrolyte is more depleted and current efficiency is reduced.
Referring to the example shown in
The strip of conductive foam 12 is directed into the bath 26 downwardly and makes a turn around a lower immersed idler roll 28. The idler roll 28 may be made of any material inert to the electroplate bath, e.g., plastic. Suitable plastic materials include nylon, polyvinyl chloride, polyethylene and polypropylene. The strip 12 then travels upward from the idler roll 28 to a metal cathode pinch-roller assembly 30, electrically connected to a power source, e.g., by means of a conventional slip ring (not shown). The anodes 16, 18 can be consumable or non-consumable. The cathode foam portion 14 of the strip 12 is passed between the anodes at an angle described above to provide diagonal convection through the cathode foam portion 14. Thus, the cathode foam portion 14 of the strip 12 is plated on both sides and exits the container 22 as plated foam 15. It should be understood that in alternative embodiments, only one anode may be present which would tend to limit plating to one side of the strip 12. In other alternative embodiments, the anodes are maintained at uneven distances from the anode, e.g., closer to one side of the foam than the other, to cause a thicker plated coat on the side of the foam closest to the anode. In this manner, foam strips can be produced that are made to easily coil in the direction of the more lightly plated side.
Referring to the example shown in
The strip 102 of conductive foam is directed into the bath 116 downwardly and makes a turn around a lower immersed idler roll 111. The foam cathode portion of the strip 104 then travels upward from the idler roll 111 to a metal cathode pinch-roller assembly 118, electrically connected to a power source, e.g., by means of a conventional slip ring (not shown). As above, the anodes 106 and 108 can be consumable or non-consumable. The cathode foam portion 104 of the strip 102 is passed between the anodes at an angle described above to provide diagonal convection through the portion 104.
Referring to the example shown in
Where a preferred porous metal article is made and electroplating of an open-cell foam is involved, the plating is often nickel plating and the resulting porous nickel sheet may generally have a weight within the range, e.g., of from about 300 grams per square meter, up to about 5,000 grams per square meter, of a major face of the article. More typically, this will be a sheet weight within the range of from about 400 to about 2,000 grams per square meter. For very openly porous material, the nickel plating weight will generally be, e.g., between about 1,000 and about 2,000 grams per square meter of the article. In certain embodiments, the anode baskets, for use with the above-described bath, may be filled with consumable nickel chips (not shown).
If desired, the method can also be supplemented by a heat treatment step, following metal deposition, the purpose of which is to remove the polymeric foam substrate material internally present, for example by means of pyrolysis. For example, after the completion of the plating, the resulting metallized article can be washed, dried, and may be thermally treated, e.g., to decompose a polymer core substance. In some instances, the article may be annealed, such as in a reducing or inert atmosphere. Such treatments are well-known in the art. See, e.g., U.S. Pat. No. 4,978,431 the entire contents of which are hereby incorporated by reference. When metal is plated, thermal decomposition may be conducted at a temperature ranging, e.g., from about 500° C. to about 800° C. for up to about 3 hours depending on the plastic foam (polymer) used. Annealing can be carried out by any known methods. For example, in the case of nickel, it may be carried out, e.g., in a hydrogen atmosphere at a temperature ranging from about 800° C. to about 1200° C. for up to about 30 minutes. The heat treatment conditions can also be chosen such that sintering of the deposited metal takes place, so that the structure is even more mechanically strengthened.
In accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention. Various modifications may be made to the examples and embodiments set forth herein without departing from the scope and spirit of the invention which is defined by the appended claims. For example, numerous plating zones can be incorporated by adding additional anodes which are passed by an inclined foam cathode strip. Those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.
