PROCESS FOR PREPARING PLASTIC ELECTROCOATED EMI/RFI SHIELDING

Abstract: A process for electrocoating plastic substrates comprises mounting the plastic substrate onto a plating rack and then electrolessly plating the substrate. The electrolessly plated substrate is then mounted on a second rack and immersed in a cathodic electrocoating solution wherein a non-conductive paint coating deposits onto the substrate. The non-conductive paint coating is then cured by baking at a temperature sufficient to cure the coating, yet insufficient to cause the substrate to deform.

Description: PROCESS FOR PREPARING PLASTIC ELECTROCOATED E I/RFI SHIELDING FIELD OF THE INVENTION This invention relates to EMI/RFI shielding in general and, more particularly, to decorative, EMI/RFI shielded electrocoated plastic components for electronic systems. BACKGROUND OF THE INVENTION EMI/RFI shielding is required by many electronic systems to absorb or reflect electromagnetic and radio frequency emissions. Such shielding is generally in the form of metal enclosures around electronic assemblies or subassemblies. For aesthetic reasons, it is often desirable to paint such enclosures. Electrocoating is one method which is used for applying a decorative coat of paint onto such metal enclosures. In electrocoating processes, a metal substrate is immersed in an electrocoating solution and an organic resin, i.e., paint, is electrolytically deposited onto the metal substrate. Electrocoating solutions are typically aφ? β -ous emulsions of an electrocoating paint. The paint comprises a particular resin system, such as an epoxy, acrylic, or polyester system, along with cross-linking agents, such as melamine or blocked isocyanates, and solvents, pigments, and fillers. A solubilizing agent is present which combines with the resin particles to impart an electrical charge on the surface of the resin particle. The solubilizing agent acts as a dispersant to disperse the particles in the water and thereby form a stable emulsion. The metal substrate is immersed in the electrocoating composition, and a potential is established between the substrate and another electrode immersed in the composition. The substrate may be made either anodic or cathodic to the other electrode. As a result of the potential, the charged colloidal paint particles migrate toward and deposit on the metal substrate. The colloidal paint particles deposit and form a nonconductive film over the surface of the substrate. Because the film is nonconductive, the plating rat slows as the film thickness builds up and, when particular thickness is achieved, no further depositio occurs at that location. Deposition continues, -however, at other locations until the same thickness is achieved. Accordingly, a uniform thickness is eventually achieve over the entire substrate. Following deposition of the electrocoating paint, the substrate is rinsed and then baked in an oven a high temperatures to cure the paint. Such curin typically .requires a temperature of at least abou 300 β F. SUMMARY OF THE INVENTION The present invention provides a process for makin plastic EMI/RFI shielded electrocoated components fo electronic systems and the like. The process comprise first electrolessly plating a plastic substrate t establish a conductive surface over the substrate Electroless deposition is followed by cathodicall electrocoating the plated substrate. The electrocoate • substrate is then subjected to a curing step to cure th electrocoated paint. In a preferred embodiment of the invention, a injection-molded, three-dimensional, plastic substrat . is processed through an electroless plating proces wherein the substrate is first mounted on a platin rack, then immersed in a chemical etchant solution, the a catalyst solution, and finally an electroless platin solution. In the electroless plating solution, conductive layer of metal, such as copper or nickel, chemically deposits over the surface of the substrate. The electrolessly plated substrate is then mounte on a electrolytic plating rack such that the meta contacts of the plating rack are in electrical contac with the electrolessly plated conductive layer. Th mounted substrate is then immersed in an electrocoatin solution. A current is established between th substrate and an anode immersed in the bath, an electroating paint is electrolytically deposited to uniform thickness over the entire electrolessly plate surfac . The electrocoated substrate is then heated to temperature and for a time sufficient to cure th electrocoated paint without thermally deforming th plastic substrate or causing a loss of adhesion betwee the electrolessly deposited conductive layer and th substrate surface. 1. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a process particularly applicable to the manufacture of lightweight, decorative EMI/RFI shielded, three- 5 dimensional, plastic components for electronic systems. Such shielding components typically form enclosures around electronic assemblies or subassembl es- ~ > and -reflect and/or absorb electromagnetic and radio frequency emissions, whether generated by the electronic 0 assembly or subassembly, or by some outside source. The process comprises first providing a three- dimensional plastic substrate in the desired shape. The substrate is preferably formed by injection-molding and is made of a platable plastic material. The plastic 5 material may be any suitable plastic which is thermally stable at the temperature at which the applied electrocoated paint is cured. Preferred platable plastics include nylons, polyesters, polysul ones, and ABS-polysulfone alloys, polycarbonates and polycarbonate 0 alloys. The formed platable plastic substrate is mounted on a suitable plating rack and then electrolessly plated, preferably by conventional techniques. For nylon substrates, the electroless plating processes described 5 in U.S. Patent Nos. 4,335,164; 4,309,462; and 4,315,045 to Dillard, et.al. and assigned to Crown City Plating Company of El Monte, California, are preferably employed. In such processes, the nylon substrate is first 0 preconditioned by immersion in an aqueous alkali metal hydroxide solution having a pH of at least about 10 and a temperature of about 150 β F, and preferably about 185°F. The substrate is then etched by immersion in a solution containing an organic acid, preferably tri- 5 chloroacetic acid. After rinsing, the substrate is seeded with a metal electroless plating catalyst, suc as that described in U.S. Patent No. 3,011,920 to C.R. Shipley, Jr. If required, the substrate is then • immersed in an activating solution to activate the catalyst, and then immersed in an electroless plating solution wherein metal, preferably copper or nickel, deposits onto the catalyzed surface of the substrate. For polyester substrates, the electroless plating process described in U.S. Patent No. 4,325,991 to Donovan et al. , also assigned to Crown City Plating Company of El Monte, California, is employed. In such a process, the polyester substrate is optionally first contacted with a detergent rinse, followed by immersion in a hydrolyzer solution containing, for example, 350 grams per liter of chromic acid and 140 millimeters per liter of sulfuric acid, and maintained at a temperature of about 100°F to about 185°F. The polyester substrate is then conditioned by contact with an alkaline conditioner having a pH of at least 8 and maintained a a temperature of at least 135°F. The substrate is then contacted with " an aqueous acid fluoride etch having a pH less than abou 5 and a fluoride concentration of at least about 1 mole per liter. The fluoride etch is preferably maintained at a temperature from about 125°F to about 150 β F. The etched polyester substrate is then catalyzed and electrolessly plated generally as described above. For polysulfone substrates and ABS-polysulfone alloyed substrates, an electroless plating process as described in U.S. Patent No. 4,125,649 to Donovan et al., assigned to Crown City Plating Company, is preferably employed. In such a process, the substrate is first contacted with a chromic acid/sulfuric acid hydrolyzer solution as described above, followed by immersion in a pre-etch conditioner containing chlorinated compounds, such as dichloropropanol. The conditioner is maintained at a temperature of from about 130° to about 150"F. The substrate is then immersed in - an etchant, preferably a chromic acid etchant as described in U.S. Patent No. 3,668,130 to Kadison et al., assigned to Crown City Plating Company. The etchantrcontains- frσm:__aboutr8v5.-.to about-10. * 5__-pounds of •chromic, acid per gallon of etchant. Following etching, the substrate " is rinsed, immersed in an " " alkaline cleanser, and then catalyzed and electrolessly plated as described above. Depending on the applicable curing temperature of the electrocoated paint, other platable plastics, such as acrylonitrile butadiene styrene (ABS) and polycarbonates, may be used. Such plastics are electrolessly plated, for example, according to the processes described in U.S. Patent Nos. 3,668,130 to Kadison et al. and 4,325,992 to Donovan et al., both assigned to Crown City Plating Company. In the electroless process, a thin conductive layer of metal deposits onto the surface of the substrate in contact with the electroless solution. After the plastic substrate has been electrolessly plated, it is rinsed thoroughly and dried. If desired, the substrate is then removed from the plating rack and mounted on another plating rack, preferably a steel noncoated electrolytic plating rack. The substrate is mounted so that the metal contacts of the rack are in electrical contact with the conductive metal layer electrolessly deposited over the surface of the plastic substrate. Re-racking the substrate onto a plating rack is generally not preferred because of the added handling of the substrate which increases the rejection rate due to accidental marring, fingerprints, and the like. Also, the increased labor and the need for an additiona plating rack increases the cost of processing. However, re-racking is generally required when th • points of contact between the first plating rack and th substrate are in recesses which entrap air when immerse in the various treating solutions, including th electroless plating solution. Such recesses do not ge electrolessly plated. Without re-racking, the meta contacts of the rack would not be in electrical contac with the conductive electroless plate which deposit over other areas of the substrate. Such electrica contact is required in the electrocoating step. In other situations, re-racking may simply b desirable. For example, many electroless plating rack are uncoated and therefore plate during the electroles plating process. Such racks, are coated during th electrocoating process. This, of course, waste electrocoating material and makes the racks difficult to strip. Even if a coated plating rack is used, i.e. one coated with * a nonconductive material except for the contacts, metal often- ' deposits on the coating material during the electroless plating process. If the deposited metal is in electrical communication with the metal contacts of the rack, the deposited metal will be electrocoated. Again, this wastes electrocoating material and makes the racks difficult to strip. Once the electrolessly plated substrate is suitably racked, it is then immersed in an electrocoating solution. A voltage is established between the substrate and one or more electrodes also immersed in the solution. The voltage is sufficient to cause the deposition of charged organic resin components onto the substrate. After a select time, preferably from about 10 to about 90 seconds, the electrocoated substrate is removed, rinsed with water and dried, preferably with an air gun. The electrocoated substrate is then subjected to a curing step, preferably baking at a temperature and for a time sufficient to cure the electrocoated paint, yet insufficient to warp the plastic substrate or cause any loss of adhes±on- between, the substrate " ''and - the -electrolessly deposited metal layer, or between the metal layer and the electrocoated layer. For nylon substrates, preferred baking temperatures do not exceed about 300"F. For polyester substrates, preferred baking temperatures do not exceed about 300"F. For polysulfone substrates, a baking temperature of not more than 290"F is preferred. ABS-polysulfone alloys are preferably baked at not more than 280 β F. For most polycarbonates, it is preferred not to exceed 280"F, and for ABS, it is preferred not to exceed about 185°F. .. The electrocoating solution may be based on any suitable system, including acrylic, polyester, epoxy, and urethane systems. Currently, it is preferred to use an epoxy-based system as curing temperatures, for epoxy-based systems tend to be lower than for of other systems. The presently preferred electrocoating solution is manufactured by PPG Industries under the trade name PowerCron R 650. Such an electrocoating solution is an epoxy-based solution which requires a curing temperature of about 250°F. When such a solution is used, it is preferred to maintain the bath at about 76° to 78"F and utilize a voltage of from about 75 to 100 volts. Deposition is allowed to occur for about one minute. Curing is then accomplished by baking the electrocoated substrate at about 250"F for about 20 minutes. While an electrocoating solution based on a variety of resin systems may be used, it is preferred that a cathodic electrocoating solution be employed. Cathodic electrocoating solutions avoid electrolytic dissolution of the electrolessly deposited metal on the surface of the substrate. That is, if an anodic electrocoating solution is used when a voltage is established to make the substrate anodic, electrolessly deposited metal on the surface of the substrate will dissolve as the charged electrocoating particles deposit. Because the electroless layer is thin, such metal dissolution could cause a loss of conductivity which, in turn, could prevent an adequate buildup in the thickness of the electrocoated paint and possibly prevent deposition of the electrocoating paint entirely. If desired, the electrolessly plated substrate may be electrolytically plated prior to electrocoating. Such an additional step in the process may be desirable in certain cases. For example, if the use of an anodic electrocoating solution were desired, the electrolytic deposition of metal over the electroless layer would allow the overall thickness of the metal to buildup sufficiently to avoid any problems created by the use of an anodic electrocoating solution. In such a situation, the electrolytically deposited metal may be any suitable metal or combination of metals. Copper or a layer of copper followed by a layer of nickel is presently pref rred. In certain applications, it may be desirable to use a clear electrocoated paint over a decoratively plated plastic substrate. Such a decorative plate typically comprises one or more layers of electrolytically deposited metal over the electrolessly deposited layer. The final electrolytic layer, e.g. gold, brass, chrome, etc. , imparts the desired appearance to the part, and the underlying layers, if used, impart desired physical properties, such as corrosion resistance, smoothness or leveling, ductility, and the like. While the presently preferred method of curing is by baking at a suitable temperature, it is also understood that other methods may be used. The suitability of other methods depend upon many factors, including-ithe -particular-*"electrocoating--solution -used, the size and shape of the substrate, and the availability of equipment. For example, ultraviolet light, electron beam, or microwave curing may be suitable in some applications. If a polyurethane system, involving the reaction of a polyol and an isocyanate is used, curing may be accomplished by contact of the electrocoated substrate with a gaseous tertiary-amine catalyst, as described, for example, in U.S. Patent Nos. 4,343,924; 4,366,193; 4,434,893; 4,365,039; 4,374,167; 4,374,181; and 4,368,222, all of which are incorporated herein by reference. .Contact occurs in a suitably enclosed curing chamber. In such process, a scrubber or other suitable equipment would be used to prevent any escape of the amine gas into the atmosphere. * It is apparent that the present invention may be used to prepare decorative plastic components for applications other than EMI/RFI shielding. In fact, the present invention may be used to substitute electrocoated plastic parts for conventional electrocoated metal parts. Electrocoated plastic substrates offer numerous advantages over conventional electrocated metal substrates. Plastic substrates are much lighter, and the material tends to be less expensive than corresponding metal substrates. Moreover, complex shapes are often easier to produce by injection-molding than by conventional casting or other metal fabrication techniques. Because electroless processes involve chemical deposition rather than electrolytic deposition, complex shapes typically do not create problems with respect to coverage of the electrolessly deposited metal. That is, metal deposition over all areas of the substrate in contact with the electroless plating solution occurs whether those areas are high- current density or low-current density areas. This enables electrocoating over the entire surface of the substrate plated in the electroless process. Moreover, the deposited electroless layer provides sufficient conductivity so that the electrocoated substrates can be used as EMI/RFI shielding, if so desired. It is further apparent that, if the deposited electrocoating is platable, i.e. can be electrolessly plated, the process of first applying a conductive electroless layer followed by a nonconductive electrocoated layer may be repeated, if desired, one or more times to yield a laminate or sandwich effect. EXAMPLE 1 A hub cap made ?of Capron 8260 resin, a mineral- filled nylon manufactured and sold by Allied Corporation, was immersed in a sodium hydroxide solution containing about 150g/l sodium hydroxide at about 185 β F for about 5 minutes. The substrate was rinsed and immersed in an etch solution containing about 15% trichloracetic acid at ambient temperature. The substrate was then rinsed and immersed in the second sodium hydroxide solution containing 150g/l sodium hydroxide and maintained at 150"F for about 1 minute. The article was then immersed in an acid solution containing 4% fluoboric acid at about 125DF for about 1 minute. The substrate was then catalyzed by immersion for about 60 seconds in a colloidal palladium catalyst manufactured by The Shipley Company under the trade name Cataposit PM 958. The catalyst solution was maintained at about 120 β F. The substrate was then immersed in an acid accelerator to expose the palladium metal. The accelerator" contained 4% fluoboric acid ..and .was maintained at about 120"F. Contact time was for about 120 seconds. The substrate then was electrolessly plated by immersion in a nickel electroless plating solution sold by The Shipley Company under the trade name Niposit PM 980. Immersion time was about 420 seconds, and the temperature of the solution was about 80*F. The substrate was then rinsed and dried. After electroless plating, the substrate was removed from the electroless plating rack and mounted on an electrolytic plating rack. The substrate was then cleaned by immersion in an alkaline soak cleaner. The cleaned substrate was then .electrolytically plated with copper for twenty minutes at about 2 volts. The plated substrate was then rinsed and dried. The electrolytically plated substrate was then racked onto a steel non-coated rack and immersed in a cathodic acrylic electrocoating solution sold under the trade name PowerCron s 650 by PPG Industries. The electrocoating solution was maintained at a temperature of 76" to 78°F, and a voltage was established of 75 to 100 volts. Electrocoating continued for one minute. The electrocoated part was then rinsed, blown dry wit an air gun, and baked for twenty minutes at 250"F to cure the electrocoated paint. EXAMPLE 2 A hub cap made of Capron 8260 resin, a mineral filled nylon manufactured and sold by Allied Corp., wa electrolessly plated by the process described in Exampl 1. The electrolessly plated substrate was then racke onto a steel non-coated rack and immersed in the sam electrocoating solution as described in Example 1, i.e., a cathodic acrylic electrocoating solution sold unde the trade name PowerCron 650 by PPG Industries. The electrocoating solution was maintained at a temperature of about 80" A F, and a voltage was established of 125 volts for 30 seconds and was then increased to 150 volts for an additional 15 seconds. The electrocoated part was then rinsed, blown dry with an air gun, and baked for twenty minutes at 270° A F to " cure the electrocoated paint. EXAMPLE 3 A window regulator made of Capron 8260 resin, a mineral-filled nylon manufactured and sold by Allied Corp., was electrolessly plated by the process described in Example 1. The electrolessly plated substrate was then electrolytically plated with copper, nickel and bright brass. The electrolytiφally plated substrate was then racked onto a steel non-coated rack and immersed in a cathodic acrylic electrocoating solution sold under the trade name PowerCron 650 by PPG Industries. The electrocoating solution was maintained at a temperature of about 85DF, and a voltage was established of about 150 volts. Electrocoating continued for one minute. The electrocoated part was then rinsed, blown dry with an air gun, and baked for twenty minutes at 270DF to cure the electrocoated paint. The results of Examples 1, 2 and 3 showed an adherent uniform paint coating over the surface of the plastic substrates.

Claims: WHAT IS CLAIMED IS; 1. A process for applying a coating of electrocoated paint onto a non-conductive plastic substrate comprising: mounting the plastic substrate on a plating rack; ■ immersing rthe plastic * substrate—in . -an etching solution for a time sufficient to etch the surface of the plastic substrate; immersing the etched plastic substrate in a catalyst solution for a time sufficient to seed the surface of the etched plastic substrate with catalyst; immersing the seeded plastic substrate in an electroless plating solution for a time sufficient for a conductive coating to deposit over at least a portion of the surface of the plastic substrate; immersing the electrolessly plated plastic substrate and a second electrode in an electrocoating solution and establishing a current between the second electrode and the electrolessly plated plastic substrate such that a non-conductive coating deposits on he electrolessly plated plastic substrate; and • curing the non-conductive coating deposited on the plastic substrate. 2. A process as claimed in claim 1, wherein the non-conductive coating is cured by heating the substrate to a temperature and for a time sufficient to cure the non-conductive coating, yet insufficient to deform the plastic substrate. 3. A process as claimed in claim 1, wherein the plastic substrate is made of a material selected from the group consisting of nylon, polycarbonate, polyester, polysulfone, ABS and ABS-polysulfone alloy resins. 4. A process as claimed in claim 3, wherein the plastic substrate is made of nylon and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 300 β F. 5. A process as claimed in claim 3, wherein the plastic substrate is made of polyester and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 300 β F. 6. A process as claimed in claim 3, wherein the plastic substrate is made . of polycarbonate and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 280 β F. 7. A process as claimed in claim 3, wherein the plastic substrate is made of polysulfone and the electrocoated non-conductive coating is cured by heating at a temperature φf not more than about 290"F. 8. A process as claimed in claim 3, wherein the plastic substrate is made of ABS-polysulfone alloy and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 280"F. 9. A process as claimed in claim 3, wherein the plastic substrate is made of ABS and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 185"F. 10. A process as claimed in claim 3, wherein the plastic substrate is removed from the plating rack after electroless plating and mounted on a second plating rack before immersing into the electrocoating solution. 11. A process as claimed in claim 1, wherein the non-conductive coating deposited on the plastic substrate in the electrocoating solution is curable by contact ^ -with;a . gaseous tertiary'.-amine * and is contacted with a gaseous tertiary amine in a concentration and for a time sufficient to cure the non-conductive coating. 12. A process for applying a coating of electrocoated paint onto a non-conductive plastic substrate comprising, mounting the plastic substrate on a plating rack; immersing the plastic substrate in an etching solution for a time sufficient to etch the surface of the plastic substrate; immersing the etched plastic substrate in a catalyst solution for a time sufficient to seed the surface of the etched plastic substrate with catalyst; immersing the seeded plastic substrate in an electroless plating solution for a time sufficient to deposit a first conductive layer of metal over at least a portion of the surface of the plastic substrate; immersing the electrolessly plated substrate in an electrolytic plating solution and electrolytically depositing at least one second conductive layer of metal over the first conductive layer; - immersing the electrolessly plated plastic substrate in an electrocoating solution and electrolytically depositing a non-conductive coating over the second conductive layer of metal; and curing the non-conductive coatin deposited on the second conductive layer of metal. 13. A process as claimed in claim 12, wherein the non-conductive coating is cured by heating the substrate to a temperature and for a time sufficient to cure the non-conductive coating, yet insufficient to deform the plastic substrate. 14. A process as claimed in claim 12, wherein the plastic substrate is made of a material selected from the group consisting of nylon, polycarbonate, polyester, polysulfone, ABS and ABS-polysulfone alloy resins. 15. A process as claimed in claim 14, wherein the plastic substrate is made of nylon and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 300"F. 16. A process as claimed in claim 14, wherein the plastic substrate is made of polyester and the electrocoated no^-condμctive coating is cured by heating at a temperature of not more than about 300"F. 17. A process as claimed in claim 14, wherein the plastic substrate is made of polycarbonate and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 280 β F. 18. A process as claimed in claim 14, wherein the plastic substrate is made of polysulfone and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 290 β F. 19. A process as claimed in claim 14, wherein the plastic substrate is made of ABS-polysulfone alloy and the electrocoated non-conductive coating is cured by heating at a temperature of not more than about 280*F. 20. A process as claimed in claim 14, wherein the plastic substrate is .. made of ABS -and the electrocoated non-.conductive coating is cured by heating at a temperature of not more than about 185'F. 21. A process as claimed in claim 14, wherein the plastic substrate is removed from the plating rack after electroless plating and mounted on a second plating rack before immersing into the electrocoating solution. 22. A process as claimed in claim 12, wherein the non-conductive coating deposited on the plastic substrate in the electrocoating solution is curable by contact with a gaseous tertiary a ine and is contacted with a gaseous tertiary amine in a concentration and for a time sufficient to cure the non-conductive coating. t v 23. A process as claimed in claim 12, wherein the non-conductive coating is clear.

Inventors: COOMBES ROBERT L JR, COOMBES, ROBERT, L., JR.
Applicants: CROWN CITY PLATING CO, CROWN CITY PLATING COMPANY
Application year: 1988
Main classification: B01D57
IPC codes: B05D3, C25D13