Abstract:
A method of metallizing materials by coating a substrate material with a hydrophilic composite material; chemically plating said hydrophilic composite material with a metal to render the surface conductive; followed by electroplating a metal onto the conductive surface. The method can be, for example, utilized to produce metallized foams, embossing plates for reproduction of grains and textures, and decorative coatings for substrate materials.

Description:
This is a continuation of application Ser. No. 561,957 filed Mar. 25, 1975, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates, in general, to a method of metallizing materials, and particularly concerns a method of providing a substrate material with a coating of a hydrophilic composite material that can be chemically plated with a metal. 
     2. The Prior Art 
     Various applications for metallizing materials are known. Some examples of these applications are as follows: 
     Metallized foams having macroscopic inter-connected cells therein have been prepared in the prior art. A method of metallizing polyurethane foams, for example, is described in U.S. Pat. No. 3,549,505. 
     In the &#39;505 patent, a reticular, electrically nonconductive cellular structure of polyurethane having a self-supporting three-dimensional network of inter-connected annulets with macroscopic inter-connected cells therein is coated with a conductive material followed by electroplating. Pyrolysis following the electroplating step is optional. 
     One method of coating the polyurethane with a conductive material according to the &#39;505 patent is by electroless metal plating. When this method is utilized, the surface of the polyurethane is rendered hydrophilic by cleaning and etching processes. After the surface is rendered hydrophilic, the electroless metal is applied. 
     Other methods of producing metallized foams are known. Examples of such methods are described in U.S. Pat. Nos. 3,698,929 and 3,679,552. 
     A method of plating on plastics is described in U.S. Pat. No. 3,501,332. In that patent, organic polymer substrates are conditioned for electroless metal plating by impregnating the surface layer of said substrates with a metal diffused into the surface layer from a solution of a complex. The complex contains said metal in a zero-valent form dissolved in an organic solvent which has a dissolving or swelling action on the substrate. 
     Other methods of plating on plastics are known. Examples of such methods are described in U.S. Pat. Nos. 3,597,266; 3,632,388; 3,632,704; 3,661,538; and 3,716,394. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method is provided wherein the surface of a substrate material is coated with a hydrophilic composite material. The hydrophilic character of said composite material enables it to be chemically plated with a metal. Accordingly, this coating step eliminates the need for the cleaning and etching operations that are generally employed in the prior art. 
     One utility of the present invention is the production of metallized foams. The metallized foams are generally in the form of a substantially rigid, three-dimensional network having macroscopic, inter-connected cells therein. The metallized foams are produced by coating a cellular material, said material generally being in the form of a structure having a self-supporting, three-dimensional network of macroscopic, interconnected cells, with a hydrophilic composite material; chemically plating said hydrophilic composite material with a metal to render the surface conductive; followed by electroplating a metal onto and substantially throughout said structure, and, optionally, pyrolyzing the metallized structure to destroy the cellular material. 
     The metallized foam product can be compressed to any desired filling factor for use in heat exchangers, regenerators, catalyst supports, battery plates, electrodes and other applications. 
     Another utility of the present invention is in the reproduction of textured surfaces. The textured surface, for example a leather or other material with a surface grain, is metallized by rendering the surface conductive in accordance with the above-described method; followed by electroplating. The textured surface is then removed from the metal. The side of the metal that was in contact with the conductive textured surface is left with a negative impression of that surface. The metal can thus be used conventionally for an embossing roller or plate to artificially reproduce the textured surface in other materials, for example, plastics. 
     There are other utilities for the present invention, such as producing decorative metal coatings on substrate materials. 
     The substrate materials utilized in accordance with the method of the present invention, for any utility that might be envisioned, can be conductive or nonconductive. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Various substrate materials can be used in accordance with the present invention. These materials can be flexible or rigid. 
     Substrate materials utilized to make metallized foams, for example, can be comprised of various cellular materials. Thermally reticulated polyurethane foam is an excellent material as it is inexpensive and readily available. The composition of the cellular material utilized in accordance with this aspect of the present invention, however, is not critical as it is coated with the hydrophilic composite material prior to electroless plating. 
     Suitable cellular materials include, for example: polyurethane foam, carbon or graphite foams, silicate foams, styrene, aluminum foam, tungsten carbide foam and other organic and inorganic, open-cell foam materials. Also, cellular materials comprised of textiles such as cotton and other natural fibers, synthetic fibers, natural and synthetic fiber blends, wood products including paper and cardboard and any other material that can be coated with the hydrophilic composite material can be used. 
     In the reproduction of textured surfaces, the substrate material utilized can be comprised of any material having a textured surface. The material can, of course, be conductive or nonconductive. When conductive materials are used, the hydrophilic intermediate coating serves as a parting media for making embossing rollers or plates. 
     Exemplary materials include leather, textured polymeric materials, wood, natural or synthetic fibers and blends of natural and synthetic fibers and other organic and inorganic materials. Any other material that can be coated with the hydrophilic composite material also can be used. 
     It is clear from the foregoing that virtually any solid material can be utilized as a substrate and can be metallized in accordance with the present invention. Selection of a substrate material is therefore left to those skilled in the art who practice the present invention. 
     Coating the substrate material is effected by contacting it with a solution containing the components of the hydrophilic composite material by, for example, spraying, brushing or dipping. 
     It is critical that the composite material is hydrophilic. Also, it is critical that said hydrophilic composite material can be used to coat the substrate material by, for example, spraying, brushing, or dipping with the solution containing the components of said hydrophilic composite material. The solution containing the components of said hydrophilic composite material should dry as rapidly as possible; generally in less than about fifteen minutes at room temperature. 
     Suitable hydrophilic composite materials are derived from a solution comprising a two component blend in a solvent system (hereinafter referred to as &#34;composite material system&#34;) comprising at least one solvent. The blend must contain at least one film forming component and at least one hydrophilic component. Also, as a third component, at least one water-soluble polymeric component can be added to the composite material system to enhance wettability and platability. 
     The following theoretical discussion explains how the foregoing composite material system forms a hydrophilic surface. This discussion is theoretical only and is not intended to limit the scope of the present invention. 
     Each component of the composite material system has a particular function in forming the hydrophilic composite material coating. 
     The film forming component gives the hydrophilic composite material coating strength, thus eliminating the possibility of disintegration of said coating during subsequent treatments. That component also provides some adhesion of said coating to the substrate material during the subsequent treatment and plating steps. 
     The hydrophilic component renders the hydrophilic composite material coating wettable to aqueous plating solutions. This component should be insoluble in water. 
     The water-soluble polymeric component enhances wettability and platability of the hydrophilic composite material coating. That component is inherently hydrophilic due to its water solubility and it has the further effect of producing a microporous surface on said coating. This microporous surface is produced when the water-soluble polymeric component in the surface of said coating is partially dissolved on contacting water or water solutions of organic or inorganic salts. Said component is dispersed throughout said coating. Thus, when they dissolve, micropores are left in the surface of said coating. 
     The microporous surface facilitates sensitizing and activation of the hydrophilic composite material coating prior to electroless plating as the sensitizing and activation solutions are aqueous and the microporous, hydrophilic surface is thus easily wetted by said solutions. 
     Of course, if the water-soluble polymeric component is not added to the composite material system, the composite material surface will still be hydrophilic due to the hydrophilic component, and thus wettable but not microporous. In the art of electroless metal plating, sensitizing and activation solutions are used to facilitate the redox reaction that effects plating by reducing the ionized metal in solution (the metal to be plated) to the zero-valence state. 
     The solvents serve to dissolve all of the foregoing components into a homogeneous solution. Selection of the solvents is based on conventional know-how for preparing solvent-based coatings. 
     The components of the composite material system are preferably compatible so that when they are dissolved in a suitable solvent system, there is no phase separation and the composite material system is thus a clear solution. Some deviation from the preferred compatibility can, however, be tolerated. For example, a slightly cloudy solution and a solution wherein phase separation occurs after standing, can give excellent results within the scope of the present invention. 
     The operability of any composite material system can be determined by one skilled in the art by testing and by utilizing the literature and manufacturers&#39; tables on compatibility of the various components in various solvents and the compatibility of various solvents in other solvents. Useful literature for selecting solvents includes the following: The Technology of Solvents and Plasticizers, A. K. Doolittle (N.Y., 1954); Solvents Guide, Marsden and Mann (N.Y., 1963); Industrial Solvents, I. Mellan (N.Y., 1950); and Properties of Organic Solvents, J. E. Morgan (1962). 
     Information on selecting film forming components can be found in various publications including publications on paints. Exemplary literature includes the following: Synthetic Resins and Coatings, Noyes Data Corporation (1965) and Paint Additives, Noyes Data Corporation (1970). 
     In addition to the foregoing references, hydrophilic components and water-soluble polymeric components can be selected from references such as: Modern Plastics Encyclopedia, Vol. 51, No. 10A (McGraw Hill, October, 1974). 
     The trade literature of manufacturers also can be utilized to select the various components and solvents for the present invention. 
     The solids content of the components in the composite material system is generally from about one percent to about 40 percent by weight of the system. The weight ratio of the film forming component to the hydrophilic component is generally from about 3:10 to about 10:1. When a water-soluble polymeric component is utilized, the weight ratio of that component to the film forming component is generally from about 1:10 to about 2:1. 
     As the coated surface dries at room temperature, through evaporation of solvent, in a relatively short time, the cleaning and etching processes utilized in the prior art are replaced in the present invention, by, effectively, a one-step coating process. 
     The thickness of the hydrophilic composite material coating is generally from about 0.05 mils to about 5 mils. 
     The hydrophilic composite material coating is made conductive by electroless metal plating. Any conventional electroless metal plating method can be used. Suitable methods are described in U.S. Pat. No. 3,667,972 and Plating on Plastics With Metals, J. McDermott (Noyes Data Corp., 1974). 
     Electroless nickel, copper, silver and other electroless plated metals can be used. 
     Once the surface of the hydrophilic composite material coating is rendered conductive, any conventional electroplating technique can be used. Suitable electroplating methods are described in Modern Electroplating, F. A. Lowenheim (J. Wiley, 1942); Electroplating Engineering Handbook, A. K. Graham (Rheinhold, 1962); and Handbook Galvanotechnik, Karl Hanser Verlag (Munich, 1966). 
     The type of metal electroplated will generally be selected on the basis of economics and the end use of the metallized material. 
     If the substrate material is a cellular material, for example, the metallized foam product might be used in a heat regenerator. In this case, an electroplated metal having high heat conductivity and high heat capacity would be selected. Typical metals would then be nickel, copper or chromium. 
     Metallized foams made in accordance with the present invention can be modified by several subsequent procedures for particular applications. The following two operations exemplify such subsequent procedures and can be performed alone or in combination with others. These operations are as follows: 
     (1) Pyrolysis to destroy organic materials, and 
     (2) Compression to dimensions required for the desired filling factor. 
     Pyrolysis can only be carried out when the cellular material is organic. Methods of pyrolysis are described in the &#39;505 patent and other methods are well-known in the art. The need for pyrolysis depends on the end use for the metallized foam. For example, if that use is to be in a hot environment that could destroy the cellular organic material, pyrolysis prior to use would be desirable. 
     Under appropriate conditions, the pyrolysis step can be carried out simultaneously with annealing of the brittle electroplated metal. The parameters under which the electroplated metals can be annealed depend upon the type of metal. These parameters are well-known to those skilled in the art. Once annealing is carried out, the shape of the metallized foam can be changed by compression, bending, twisting or otherwise. 
     The annealed metallized foam can be compressed for applications where a low filling factor is needed. The filling factor is defined as follows: 
     
         Filling Factor = (V.sub.1 - V.sub.2)/V.sub.1 
    
     where: V 1  is the volume of a solid having the overall dimensions of the metallized foam and V 2  is the actual volume of the metallized foam, for example, the water displacement volume. Applications requiring a low filling factor are well-known in the art and can include, for example, uses of the metallized foam as a catalyst support or a heat exchanger. 
     It is advantageous to metallize a cellular material having a high filling factor. This is because the cells are larger and the screening effect of the cell walls during the electroplating is thereby minimized. Accordingly, more uniform electroplating can be achieved in a foam having a high filling factor. 
     If the substrate material is a textured material, electroplated metal having sufficient strength to be used as an embossing roller or plate would be selected. Metals such as nickel and copper, among others, are suitable for this purpose. As in the case of metallized foam, the brittle electroplated metal can be annealed to improve strength. This annealing step can be carried out simultaneously with pyrolysis of the textured material. The textured material also can be removed before or after annealing by separating it from the metal. This can be done by conventional means such as by mechanical or chemical means. 
    
    
     The following examples are submitted to illustrate but not to limit the present invention. 
     EXAMPLE I 
     A series of experiments was conducted to select composite material systems suitable for plating on glass substrates. Glass slides were utilized and viewed under a microscope to determine the quality of the composite material coating and the platability with electroless nickel. Electroless nickel plating was carried out in accordance with U.S. Pat. No. 3,667,972. 
     The following stock solutions were made up with 10% solids, by weight, in a cold solvent system: 
     I. 86-14% vinyl chloride--vinyl acetate 
     II. 87-13% vinyl chloride--vinyl acetate 
     III. polyvinylpyrrolidone 
     IV. polyvinylbutyral 
     The solvent system was: 
     
         ______________________________________      Parts by Volume                  Percent by volume______________________________________1,2 dichloroethane        100           44.5methyl ethyl ketone        100           44.5ethanol       25           11.0______________________________________ 
    
     The components of the composite material system will be designated in the following sub-examples by the roman numerals used to identify the foregoing stock solutions. 
     Ten milliliters of I. were mixed with 5 milliliters of III. A clear solution resulted. This composite material system was then brushed onto a glass slide and allowed to dry for 5 minutes at 70° F. The dry film was clear and smooth. Electroless nickel plating was then carried out. Ninety percent coverage with electroless nickel was achieved. 
     B 
     Five milliliters of cellulose acetate butyrate were added to the composite material system of A. The resulting solution had a pearl-like appearance and separated into layers on standing. After stirring the solution, the same procedure as in A was followed. A dry film that was grainy and cloudy resulted. Plating with electroless nickel gave ninety percent coverage with some blistering. 
     C 
     Twenty milliliters of I were mixed with 5 milliliters of III. A clear solution resulted. The same procedure as in A was followed. The dry film was clear and smooth. Plating with electroless nickel gave 95 percent coverage. 
     D 
     Five milliliters of cellulose acetate were added to the composite material system of A. The resulting solution was opalescent but did not separate into layers on standing. The same procedure as in A was followed. The dry film was cloudy but smooth. Ten percent coverage with electroless nickel was achieved. 
     E 
     Five milliliters of II were added to the composite material system of A. A clear solution resulted. The same procedure as in A was followed. The dry film was clear and smooth. One hundred percent coverage with electroless nickel was achieved. 
     F 
     Five milliliters of IV were added to the composite material system of A. The resulting solution was hazy but did not separate into layers on standing. The same procedure as in A was followed. The dry film was clear and smooth. One hundred percent coverage with electroless nickel was achieved. 
     G 
     Ten milliliters of I; 10 milliliters of II and 5 milliliters of IV were mixed. The resulting solution was opalescent. Ten milliliters of methyl isobutyl ketone was then added and the solution became clearer with very slight cloudiness. The same procedure as in A was followed. The dry film was smooth and clear. One hundred percent coverage with electroless nickel was achieved. 
     H 
     Fifteen milliliters of II and 5 milliliters of IV were added to the composite material system of A. The solution was opalescent. Ten milliliters of methyl isobutyl ketone was added but no improvement in solution compatibility was observed. Ten milliliters of isopropyl alcohol was then added. The solution was still opalescent, probably due to the insolubility of polyvinyl-pyrrolidone in methyl isobutyl ketone. The solution separated into two components on standing. After stirring the solution, the same procedure as in A was followed. The dry film was smooth and clear. One hundred percent coverage with electroless nickel was achieved. 
     I 
     Ten milliliters of I; 10 milliliters of II; 5 milliliters of IV; and 20 milliliters of methyl isobutyl ketone were mixed. The resulting solution was clear. The same procedure as in A was followed. The dry film was smooth but slightly cloudy. One hundred percent coverage with electroless nickel was achieved. 
     J 
     Ten milliliters of II and one milliliter of IV were added to the composite material system of A. An additional 20 milliliters of the above-defined solvent system also were added. The resulting solution was clear. The same procedure as in A was followed. The dry film was smooth but cloudy. One hundred percent coverage with electroless nickel was achieved. 
     K 
     An additional 10 milliliters of I were added to the composite material system of J. The same procedures as in J were followed and yielded the same results except that the dry film was clear. 
     L 
     Five milliliters of II and 5 milliliters of IV were added to the composite material system of C. An additional 20 milliliters of the above-defined solvent system also were added. The resulting solution was clear. The same procedure as in A was followed. The dry film was clear and smooth. One hundred percent coverage with electroless nickel was achieved. 
     M 
     Ten milliliters of I; 10 milliliters of II; 2 milliliters of III; 2 milliliters of IV and 20 milliliters of the above-described solvent system were mixed. The resulting solution was clear. The same procedure as in A was followed. The dry film was clear and smooth. One hundred percent coverage with electroless nickel was achieved. 
     The procedure of M was repeated except that the coating was peeled off prior to electroless plating. The thickness of the coating was 0.3 mil. 
     The procedure was again repeated except that two and three coatings were applied to the glass slide. These coatings had thicknesses of 0.6 mil and 0.9 mil, respectively. Plating on the double and triple coated slides gave one hundred percent coverage with electroless nickel. 
     EXAMPLE II 
     A thermally reticulated polyurethane foam was coated with about 0.3 mil thickness of a hydrophilic composite material. 
     The hydrophilic composite material contained 4 grams polyvinylpyrrolidone, 8 grams of a 90-10% mixture of vinyl chloride--vinyl acetate copolymer, 8 grams of a 87-13% mixture of vinyl chloride--vinyl acetate copolymer and 5 grams of cellulose acetate butyrate, dissolved in 50 milliliters of ethylene dichloride, 125 milliliters tetrahydrofuran and 50 milliliters of methyl ethyl ketone. This formulation is suitable for dipping or brushing but is adapted for spraying by diluting it in a 1:2 or 1:3 ratio with a solvent mixture of the following composition: ethylene dichloride--22%; tetrahydrofuran--56%; and methyl ethyl ketone--22%. In this example the polyurethane foam was sprayed with the composite material system to effect coating. 
     The polymer coating was allowed to dry for five minutes at room temperature (22° C.) and this was followed by electroless nickel plating as described in U.S. Pat. No. 3,667,972. This step was followed by conventional electroplating of copper to a thickness of about 10 mils. 
     A layer of nickel was then electroplated to a thickness of about 4 mils on the copper followed by pyrolysis at 760° C. under a hydrogen atmosphere for one hour. The resulting metallized foam was ductile. 
     EXAMPLE III 
     A swatch of leather was sprayed to a thickness of about 0.4 mils with the following composite material system: 
     4 grams polyvinylpyrrolidone 
     8 grams 90-10 vinyl chloride--vinyl acetate copolymer 
     5 grams cellulose acetate butyrate 
     100 milliliters ethylene dichloride 
     250 milliliters tetra-hydrofuran 
     100 milliliters methyl ethyl ketone 
     The coated leather was dried at about 23° C. for ten minutes. The coating was then plated with electroless nickel as described in U.S. Pat. No. 3,667,972. This step was followed by conventional electroplating of copper to a thickness of about 15 mils. The leather and the hydrophilic composite material layer were then peeled off leaving a high resolution copper negative of the textured leather surface. 
     Various modifications of the copper negative can be carried out. For example, a backing material such as a metal or rubber pad can be fastened to the back of the negative for added strength. Also, a thin coating of a harder metal such as chromium or nickel can be plated on the negative side for improved hardness without impairing the resolution of said negative. Other modifications apparent to those skilled in the art can be carried out. 
     Having set forth the general nature and some examples of the present invention, the scope is now particularly set forth in the appended claims.