US2211582A - Process of electrolytically coating nonconductive materials - Google Patents

Description

Patented Aug. 13, 1940 K 2,211,582

PROCESS OF ELECTROLYTIOAILY COATING NONCONDUCTIVE MATERIALS Samuel Ruben, New Rochelle, N. Y.

No Drawing. Application May 6, 1938.

Serial No. 206,372

I Claims. (Cl. 204-42) This invention relates to a process for producing sometimes results with consequent dislodging ing electrically conductive metal surfaces on nonof the plated metal film. conductive or high resistance materials, and to The art also discloses the application of metal products made thereby. coatings to nonconductive bases by applying fine- An object of the invention is the conversion 1y divided metal and a binder to the base, precipiof high resistance surface coatings to highly tating a metal layer on the coating as by immerconductlve coatings. sion in a silver or copper plating solution and Another object is to provide a method for thereafter electroplat g a yer of metal on the economically and simply producing thin highly precipitated metal surface. However, the deconductive metal surfaces upon insulating bases. posited conductive coating does not form a con- 10 A f th bje t i th provision of an imtinuous sheet, is not bonded to the base and may proved method of providing a conductive metal be readily rubbed off. surface on a high resistance base. -The present invention overcomes the difficulties Another object is the provision of th d inherent in the prior art methods and provides which f ilitates electroplating of metals, an improved material at lower cost. The coating 15 l A further object is the provision of an article prqduced by y P e e t method results in a low f manufacture comprising insulating base resistance surface, the conductivity of which and a conductive surface formed from metal parclosely p mates that of metal foil.

fl les joined together t g a low resistance In a endeavor to producea conductive sprayed l0 conductive path. metal surface upon an insulating or high reother objects m be apparent fr m the sistant basc'which would possess the advantages closure of a bonded unitary assembly and which would The invention eempflses the use f an be free of the limitations heretofore encountered lating or high resistance base such as mica, n the art, Ihave found the following: porcelain, paper, rubber, cellulose, plastics, etc" If an insulator base be coated with a layer of 25 a coating oi finely'divided copper alloy particles bronze POWder, Such as a copper nc 0% held on said base by a binder, the particles being mixtue and bonded base by ns of an substantially an metal but having high resistance organic binding material such as a natural or surface layers of superficial thickness practically synthetic resin lacquer etc-r Such a layer s a high insulating value of resistance along its 0 it?5L2 tfisi rifiilitfaei firffii tfi 123}, length. notwithstanding the mor p rticles a conductive state in situ on the base. apparently mp1ete1y metal and Contact with Heretofore several methods have been used to each I have found .that thls nonconducprovide electrically conductive coatings on insufive condmfm due to hlgh coritact resistance mung or high resistance bases By one method at the particle mteiifaces. This high contact re- 35 atomized metal is sprayed onto the insulator sur slstance which I have found to characteristic face by a gun By another method of fioccu1ent coppe base materials, 1s, I believe, vaporized metal is sputte'red upon the due to an invisible oxide layer of superficial thickmsul ness. The additive effect of this multiplicity of 40 surface. Such methods are unsmted for coating intebpartlcle resistance layers produces the high 40 ce tain types of Objects Such as thin nonmetallic resistance surface. Even though the binder be sheets. a e expensive inasmuch t require present in negligible quantities only, or even the use of hydro en, vacuum or Inert atmoswithout any binder at all, the high resistance or pheres, and t e 6034711188 Produced are in a great nonconductive quality of the bronze coatin still a y 68588 unsatisfactory due to rigidity and lack remains and rubbing or rolling will not eliminate 5' of uniform tythe inter-particle resistance; In the present in- Another e od, uc used. Particularly in vention I have eliminated the high resistance at the prepa on o ec o v e s o s ap the particle boundaries and rendered the coatthe nonconductive surface and electroplate a i du tiv over it length by exposing th metal over the graphite. While for a number of coating to the effects of a suitable acid, preferably 50 uses this method is satisfactory, the specific rehydrochloric acid in vapor form. The effect of sistance of the carbon or graphite is so high as this treatment as can be observed and measured, to preclude the initial use of high current densiis to eliminate the high contact resistance at the ties in the electroplating process. In addition, as particle interfaces and reduce the superficial 5 the graphite is not bonded to the insulator, peeloxide surfaces to metal, as a result of which the 56 resistance immediately drops to a very low value. For example, a strip of Ce1l0phane" one inch wide, six inches long, when sprayed with a bronze lacquer mixture will show a resistance in the order of 50 megohms. After exposure to the vapors from hydrochloric acid the resistance drops to 0.06 ohm and maintains this condition unless heated and exposed to oxygen. Hydrofluoric acid vapor may be used in place of hydrochloric acid vapor but its use in production presents dimculties.

As a base upon which additional layers of metal can be plated, this highly conductive layer allows application of higher current densities than could be applied to the commonly used graphitized surfaces and provides a continuous process for producing sheet material having a heavy electrodeposited metal surface bonded to the nonconductive base. Due to the fact that the resistance of the copper base layer is extremely low, somewhat in the order of copper or brass foil, it is possible to directly plate silver, gold, chromium, cadmium, nickel, etc., without the necessity of providing a preliminary copper coat. As the underlying metal layer is bonded to the base, the plating will not chip or peel.

While exposure to the vapor or gas of hydrochloric acid is the most rapid and preferred treatment, I have also found it possible to convert the bronze coating to a conductive condition by immersing the coated bronze article for several seconds in a suitable acid such as hydrochloric, hydrofiuoric, phosphoric, lactic or sulphuric acid, all of which bring about a reduction of the superficial oxide surfaces of the metal particles with resultant low resistance layers. This method requires more care than the vapor method and necessitates washing and drying toclean the surface of acid.

In practicing the invention, a copper bronze powder is sprayed on the insulator or high resistance member to a thickness of about mil, using a quick drying binder such as a solution of 2 ounces of cellulose in one gallon of amylacetate. This solution forms a porous skin between the particles and allows them to be treated. After the sprayed film is dried it is exposed for several seconds to the vapors of concentrated hydrochloric acid and thereafter heated to expel any surface-absorbed acid gas or vapor. The hydrochloric acid vapor reacts with the surfaces of the particles, reduces the cuprous and other oxides thereon, and in the process, some of the zinc which may be present in the bronze, is consumed.

The material as thus produced is suitable for many uses. However, in some cases it may be desirable to reinforce the sprayed surface with an electro-deposited metal. This may be readily accomplished at a higher .current density than possible in the electroplating of metals upon graphitized or carbonized surfaces, the low resistance of the sprayed surface making the use of high currents practicable. In part, this superiority to the usual graphite surfaces heretofore used is due to the more intimate bonding of the conductive particles to the insulator surface, the waterproof character of the bronze coating and its lower resistance. Any of the commonly used plating metals may be employed, for instance, copper chromium, tin, cadmium, lead, etcetera.

There are numerous applications in which the invention may be used such as, for example, in the production of electrostatic speaker diaphragms, unitary electrode-dielectric members for electrostatic condensers, the coating of insulated wires to afford a coaxial conductor external to the core, coating of end terminals on electrical resistance units such as carbon type variable resistors, coating by bronze spraying or dipping and subsequent electroplating of shingles, electrotypes, odd shaped bodies composed of insulating materials such as ornamental plastic objects, containers and a large number of other articles, providing electrostatic shields for glass radio tubes (as well as for other radio parts) by coating the glass with bronze lacquer and exposing the coating to hydrochloric acid vapor, etcetera.

The present invention allows the substitution of molded plastics for plated cast metal parts, such as door knobs and handles, etc., with the important advantage that the electroplated metal, such as nickel, silver or chromium has much greater durability due to the absence of corrosion which in most cases is brought about by galvanic action between the plated metal and the dissimilar metal base.

In some cases, as in the preparation of resistance units for radio volume controls, the bronze coating is treated only to the point where the desired resistance value is obtained, the interparticle contact resistance being only partially reduced.

In coating Cellophane, ethyl cellulose, cellulose acetate, paper or other sheet material, a continuous process is used, the Cellophane being coated with copper bronze lacquer as it is unrolled, being passed through an oven to dehydrate the Cellophane" and bake the coating, then exposed to the hydrochloric acid vapor, again heated, passed into electroplating bath, thereafter being washed, dried and rewound.

In general, the method described has been found exceedingly useful and superior to present available methods in providing a highly conductive layer on an insulating or high resistance surface.

While I have found a solution of 2 ounces of cellulose to one gallon of amylacetate to be a very satisfactory binder, other materials may be used such as urea formaldehyde resin, shellac, glyptal, meta methacrylate, etcetera. The amount of binder used should be limited so as not to form a complete physical insulating layer around the particles, but should be sumcient to'flrmly hold the metal particles upon the insulator base. In practice I have used a mixture consisting of 2 ounces of nitro cellulose to one gallon of amylacetate to which I add 20% by weight of finely divided bronze powder. When this mixture is sprayed the cellulose film formed around the bronze is porous and allows contact between the particles.

For some applications, as in the treatment of Cellophane" or other sheet materials, the bronze is applied to the sheet by a printing process instead of by spraying. In the printing process. the sheet is first coated with an adhesive such as urea resin and while still tacky it is coated with bronze powder which is brushed and rubbed on by rotary brushes. This coating is similar to the sprayed coating and is extremely high resistant until exposed to the hydrochloric acid vapor or other treatment above described.

This application is a continuation in part of my co-pending applications bearing Serial Number 168,114, filed October 9, 1937 and Serial Number 205,209, filed April 30, 1938.

Having described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. The method of providing a conductive metal coating upon a nonconductive object which comprises applying to said object a. coating of copper base particles and a binder, said particles having normally a high resistance oxide layer of superficial thickness at the surfaces thereof,

subjecting said coating to areducing acid vapor reacting with said particles and removing the oxide, and thereby converting said high resistance surfaces to a conductive state, thereafter electroplating a layer of metal over said coating.

2. The method described in claim 1 characterized in that the acid vapor is hydrochloric acid vapor.

3. The method of providing a conductive metal coating upon a nonconductive object which comprises applying to said object a coating of copper base particles and a binder, said particles having normally a high resistance oxide layer of superficial thickness at the surfaces thereof, and thereafter subjecting said coating to the action of a reducing acid chemically reacting with said particles and removing the oxide, and thereby converting the high resistance surfaces thereof to a conductive state, thereafter electroplating a layer of metal over said coating.

4. The method as described in claim 1 characterized in that the reducing acid is hydrochloric acid.

5. The method of providing a conductive metal coating upon a nonconductive basewhich comprises applying to said base a coating of copperzinc bronze particles and a binder, said bronze particles having normally a high resistance oxide layer of superficial thickness at the surfaces thereof, and thereafter subjecting said coating to the action of a zinc dissolving reducing acid capable of reacting with said bronze particles and removing the oxide and thereby converting the high resistance surfaces thereof to a conductive state, thereafter electroplating a layer of metal over said coating.

6. Continuous process for metal coating nonconductive sheet material of the class comprising Cellophane, paper and the like, which comprises applying to said sheet a coating of copper base particles and a binder, said particles havin normally a high resistance oxide layer of superficial thickness at the surfaces thereof, heatin said coated sheet, exposing said sheet to the action of a reducing acid vapor chemically reacting with said particles and removing the oxide and thereby converting the high resistance surfaces thereof to a conductive state, thereafter passing said sheet into an electroplating bath and plating a layer of metal upon said coating, thereafter washing and drying said sheet.

7. Process for the metal coating of shingles which comprises applying to a shingle a coating of copper base particles and a binder, said particles having normally a high resistance oxide layer of superficial thickness at the surfaces thereof, and thereafter subjecting said coating to the action of a reducing acid vapor chemically reacting with said particles and removing the oxide and thereby converting the highresistance surfaces thereof to a conductive state, thereafter electroplating a layer of metal over said coating.

SAMUEL RUBEN.