Abstract:
A relatively inexpensive process for making plastic components having textured chrome finishes that may have fine, crisp, clean lines and intricate details includes steps of electroplating a layer of etchable metal on a surface of a plastic article, typically preserving the surface of the etchable metal, etching a desired relief pattern on the layer of etchable metal, typically cleaning and activating the surface of the relief patterned etched metal layer, and electroplating a layer of decorative metal on the relief patterned etched metal layer.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to decorative finishes and more particularly to a process for creating a plated plastic article having a decorative relief pattern.  
       BACKGROUND OF THE INVENTION  
       [0002]     Decorative chrome finishes on plastic components have long been popular in automotive, appliances, teletronics, and household applications. While chrome plated plastic articles having decorative relief patterns are known and desired, it is expected that the demand for more delicately textured or patterned chrome finishes will increase in the near future. A textured chrome finish on a plastic component can be achieved by applying a chrome plating over a pre-textured molded plastic component. However, this practice is not economically acceptable in the initial marketing phase of potential products, and is unacceptable for certain low volume or specialty products, because pre-textured molds are relatively expensive. Further, it is extremely difficult to consistently produce delicate and/or intricate textured patterns in a plastic molding process. In addition, because of the thicknesses of the multiple layers required for chrome plating a plastic substrate, much of the original detail in the molded relief pattern may be lost during the plating process.  
         [0003]     U.S. Pat. Nos. 3,843,763 and 3,869,535 disclose methods for modifying embossing rollers, plates or dies by subjecting machine engraved or electroformed roller to a combination of plating steps. The embossed roller is first subjected to a bright metal plating step with a metal characterized by a high or medium leveling effect to produce a combined brightening and smoothing effect on the roller. This step is followed by a second plating step used as a finishing coating for the roller. The second plating is suitably achieved using a metal having a relatively lower leveling effect such as chromium or nickel. The second plating step provides a hard coating surface which is resistant to corrosion and erosion while at the same time enhancing the textured pattern of the end desired product. The disclosed process starts with a substrate having the desired relief pattern.  
         [0004]     U.S. Pat. No. 4,278,739 discloses a method of metallizing materials by coating a substrate material with a hydrophilic composite material, electrolessly metal plating the hydrophilic composite material with a metal to render the surface conductive, and electroplating a metal onto the conductive surface. The method is said to be useful for producing metallized forms, embossing plates for reproduction of grains and textures, and decorative coatings for substrate materials.  
         [0005]     U.S. Pat. No. 4,600,480 discloses a method for plating selected surfaces of a plastic substrate without plating other selected surfaces. The disclosed method involves electrolessly plating the substrate to provide an electroless metal layer over all of the first selected surfaces and at least a portion of second selected surfaces, mounting the substrate on an electroplating rack so that the current density at the second selected surfaces is lower during electroplating than at the first selected surfaces, whereby the substrate is electroplated to provide intermediate metal layers which extend over at least all of the first selected surfaces, and then electroplating with a final metal different from the metals of the electroless and intermediate layers at a voltage whereby the final metal deposits over the first selected surfaces but not over the second selected surfaces. The electroplated substrate is then immersed in a stripping solution which dissolves the electroless and intermediate metals but not the final metal.  
         [0006]     U.S. Pat. No. 4,820,553 discloses a process for conditioning surfaces of a polyester or polyamide material for electroless plating. The method comprises exposing surfaces of the polyester and/or polyamide material to a composition which comprises a solvent system containing water, a water-soluble organic solvent and solvated hydroxyl ions to etch the surface of the substrate to improve adhesion of a metal coating with the substrate.  
         [0007]     U.S. Pat. No. 6,489,034 discloses a method of applying a metal onto a copper layer by steps of stabilizing a surface of a copper layer by applying an oxide layer to the copper layer, and vapor depositing a metal, such as chromium, directly onto the oxide surface of the copper layer.  
         [0008]     There remains a need for improved processes for preparing textured decorative plastic components, especially techniques that allow preparation of delicately, textured patterned finishes which have fine, crisp, clean lines and intricate details. Especially needed is a process of this type which is relatively inexpensive and/or provides greater flexibility in the preparation of low cost plastic articles having a textured finish.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention provides an economically acceptable process for making plastic components having delicately textured or patterned finishes, and provides a flexible technology for producing decorative patterns having fine, crisp, clean lines and intricate details on a variety of plastic parts having a metal layer. The process includes steps of electroplating a layer of etchable metal on a surface of a plastic article; typically preventing the etchable metal surface from tarnishing; etching a desired relief pattern on the layer of etchable metal; typically cleaning and activating the surface of the relief patterned etched metal layer; and depositing a finish layer on the relief patterned etched metal layer.  
         [0010]     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification and claims. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0011]     In accordance with an embodiment of the invention, a process for preparing a plastic article having a desired relief pattern in a plated surface generally involves depositing a relatively thick layer of etchable metal on a surface of the article; forming the desired relief pattern on an exposed surface of the etchable metal layer; and depositing a finish layer over the relief pattern.  
         [0012]     Generally, there are several preparation steps prior to the step of electroplating a layer of etchable metal on the surface of the article. Typically, an electrically conductive electroless coating is provided prior to electroplating of the layer of etchable metal. Electroless coating generally involves steps of cleaning and etching the substrate, neutralizing the etched surface, catalyzing the neutralized surface (e.g. in a solution that contains palladium chloride, stannous chloride and hydrochloric acid), followed by immersion in an accelerator solution (which is either an acid or a base), and forming a metallic coating on the activated substrate. The surface of the substrate is typically conditioned by cleaning with a detergent solution and etched by dipping the substrate in an etchant (e.g., a mixed solution of chromic acid and sulfuric acid). The metallic coating may be deposited on the activated substrate by immersing the substrate in a chemical plating bath containing nickel or copper ions and deposing the metal thereon from the bath by means of the chemical reduction of the metallic ions. The resulting metallic coating is useful for subsequent electroplating because of its electrical conductivity. It is also conventional to wash the substrate with water after each of the above steps. Other suitable techniques for pretreating a plastic substrate to provide an electrically conductive coating to render the substrate receptive to electroplating operations are well known in the art, and may be employed prior to electroplating a layer of etchable metal on a surface of the article in accordance with the principles of this invention.  
         [0013]     Typical plastic materials that have been rendered receptive to electroplating, and which may be subsequently electroplated include acrylonitrile-butadiene styrene (ABS) resins, polyesters, polyethers, polyacrylics, polycarbonates, polyamides, polyolefins, polyvinylchloride, polycarbonate (PC) resins/ABS alloy polymers and phenol-formaldehyde polymers. The process of this invention may be applied to these and other plastics.  
         [0014]     After electroless plating or coating of the substrate with an electrically conductive material, the layer of etchable metal is electroplated onto the article. In order to provide a visually perceivable and aesthetically acceptable relief pattern, a relatively thick layer of etchable metal is electroplated onto the treated surface of the plastic article. A suitable thickness of the etchable metal layer is typically from about 50 to about 500 micrometers. A thinner layer may be utilized, although this may limit the ability to provide a visually discernible, aesthetically acceptable relief pattern. Greater thicknesses may also be used, but are generally unnecessary to achieve an aesthetically acceptable textured or graphical relief pattern. While various metals may be used for electroplating a suitable etchable metal layer, including nickel, tin, zinc, cobalt, etc., a copper electroplate is preferred because of its relatively low cost and excellent etchability properties. Specifically, it is possible to chemically, mechanically (e.g., such as with sand blasting), or ablatively (e.g., such as with a laser) etch a desired relief pattern into a copper surface to produce relatively sharp, precise relief structures.  
         [0015]     While it is not necessary, the layer of etchable metal that is subsequently etched to form the desired relief pattern may be electroplated over any number of previous layers. Accordingly, as used herein, electroplating a layer of etchable metal on the article may be achieved by either electroplating directly on the plastic article treated with an electroless coating, or on one or more layers of material deposited on the surface of the plastic article.  
         [0016]     Before etching the relief pattern into the layer of etchable metal, it is desirable to first activate the exposed surface of the etchable metal layer in order to form a thin layer of preservative film on the etchable metal surface. This may be achieved by contacting the exposed surface of the etchable metal layer with an activating solution comprising from about 1% to about 10% by weight hydrogen peroxide (H 2 O 2 ) and from about 5 to about 20% by volume of sulfuric acid (H 2 SO 4 ). A suitable contact time is from about 5 seconds to about 60 seconds, and contacting of the exposed surface of the etchable metal layer with the activating solution may be performed at ambient conditions (e.g., normal atmosphere pressure and normal facility or room temperatures). Before etching the etchable metal layer, it may be desirable to contact the exposed surface of the etchable metal layer with an aqueous corrosion inhibiting solution to prevent the etchable metal surface from tarnishing. The expression “contacting” as used herein, unless otherwise indicated, refers to immersion, spraying or any other treatment that provides appropriate contact of the surface with a liquid treatment composition. A suitable contact time of the exposed surface of the etchable metal layer with the corrosion inhibitor solution is from about 1 second to about 10 minutes. Examples of corrosion inhibiting agents that may be employed in an aqueous corrosion inhibitor include benzotriazole, 2-mercaptobenzimidazole, 2-mercapatobenzothiazole, 2-mercaptobenzoxazole, their derivatives, or a combination of these corrosion inhibiting agents. Such agents are typically present in the aqueous corrosion inhibitor solution at a concentration of at least 10 mg/L.  
         [0017]     Any of various etching techniques that are well known may be employed for etching the desired relief pattern into the surface of the etchable metal layer. In the case of chemical etching, the exposed surface of the etchable metal layer is coated with a material (typically a synthetic polymer material) that is either chemically resistant to the etchant used for etching the metal, or which can be rendered chemically resistant, such as by cross-linking. Any of various masking techniques known in the art may be employed, including wax masking, etchable film masking, brush-coat masking, spray-coat masking, immersion-coat masking, UV cure photoresist masking, etc. After the coating has been applied to the exposed surface of the etchable metal layer, sections of the coating are removed to expose sections of the underlying etchable metal layer. The patterned coating or mask and the exposed sections of the surface of the etchable metal layer are contacted with a chemical etchant that removes (e.g., dissolves) metal at the surface of the etchable metal layer, thereby forming a relief pattern into the metal that corresponds with the mask pattern. Conventional photoresist materials may be used for preparing the patterned coatings or masks. Typically, precise patterns can be formed into the photoresist materials by selective exposure of the photoresist material to ultraviolet radiation. Typically ultraviolet radiation is used to either decompose the exposed areas of the photoresist resin coating or to cross link the exposed areas of the photoresist resin coating. In the case where the ultraviolet radiation decomposes sections of the photoresist coating, the decomposed areas are removed, typically by contacting these areas with a solvent, and the remaining portions, which constitute the mask, are chemically resistant to the etchant. In the case where the ultraviolet radiation cross links the exposed sections of the photoresist material, the cross-linked sections are rendered chemically resistant to the etchant, and the unexposed sections are removed (such as with a solvent) to form the mask.  
         [0018]     Suitable etchants for etching the surface of a metal layer, such as copper, include various acid solutions. Specific examples of metal (e.g., copper) etchants include a solution comprising from 50 g/L to 500 g/L ferric chloride (FeCl 3 ); a solution comprising from about 5% to about 20% by weight of hydrogen peroxide plus from about 15% to about 30% by volume of sulfuric acid (H 2 S 4  ); a solution comprising from about 15% to about 25% by weight of ammonium persulfate plus from about 15% to about 30% by volume of sulfuric acid; a solution comprising from 250 g/L to 380 g/L nitric acid, and a solution comprising from about 150 to about 350 g/L CrO 3  plus from about 200 to about 400 g/L of sulfuric acid (H 2 SO 4 ). Typically, contacting of the exposed surface of the etchable metal layer with the etchant solution is performed at ambient temperature (e.g., from about 50° F. to about 90° F.) for a period of from about 30 seconds to about 2 minutes to produce an etch depth of from about 20 to about 400 micrometers. After the exposed areas of the etchable metal layer have been contacted with an etchant for a suitable period of time to achieve a desired etch depth, the etch material is immediately contacted with water to remove the etchant and stop the etching process.  
         [0019]     The etching process can cause stains at the etch surfaces. In order to remove such stains the etched surfaces may be exposed to a hydrochloric acid solution (e.g., 25-50% HCl by volume) at ambient temperature for a relatively short period of time (e.g., from about 1 second to 60 seconds) to remove the stains. Immediately thereafter, the surfaces are rinsed with water and dried (e.g., with compressed oil-free air). Thereafter, the remainder of the photoresist material (mask) is removed with an organic stripping solvent, such as denatured alcohol, heptane, etc. The chemical etching process may be repeated a plurality of times using different mask patterns, different depths, etc. to prepare various textured patterns, graphical designs, text, etc.  
         [0020]     As an alternative to chemical etching, sand blasting or other mechanical techniques may be employed, or laser or other ablative techniques may be employed.  
         [0021]     Because the etching process may leave invisible masking material and etching agent residue on surfaces of the etched metal layer, it is usually necessary to further clean and activate the surfaces of the etched metal layer for subsequent electroplating. Cleaning solutions useful for removing residues from the surface of the etched metal layer generally contain at least one organic solvent and at least one surfactant. An example of a preferred cleaning solution contains 20-35% by weight ethyl ethanol, 10-40% by weight heptane, 2-10% by weight triethanolamine, 1-3% by weight non-ionic surfactant (such as butyl or hexyl cellosolve), 10-50% by weight p-mentha-1,8-diene, and up to 10% by weight isopropanol. The cleaning process is typically carried out at a temperature of from about 70° F. to about 110° F. Contacting of the etched metal layer with a cleaning solution may be accompanied with mechanical agitation or ultrasonic agitation for at least about  1  minute. Soft brushing may also be helpful to eliminate heavy contaminants on the surface of the etched metal layer. After cleaning masking and etchant residues from the surface with the cleaning solution, the cleaning solution is rinsed from the etched surface with a water-soluble solvent, such as ethanol, 1-propanol, 2-propanol, or mixtures of these solvents. Thereafter, the water-soluble solvents are immediately rinsed from the surface of the etched metal layer with water. In some cases, it may be desirable to further remove inorganic contaminants from the textured (etched) metal layer by immersion in or spraying with a 5-10% commercial alkaline cleaner, such as Polyprep Cleaner 2202 (Henkel), Gardoclean S 5206 (Oakite), etc. at a temperature of from about 140° F. to about 180° F. for at least 30 seconds, and immediately thereafter rinsing with water. After the cleaning and rinsing steps, it may be necessary to remove corrosion or tarnish stains by immersion in an acid solution, such as a 20-50% by volume hydrochloric acid solution, or a 15-30% by volume sulfuric acid solution, etc., at ambient temperature for a period of from about 10 seconds to about 120 seconds, and immediately thereafter rinsing the acid from the textured (etched) metal layer with demineralized water, and drying (such as with compressed, oil-free air).  
         [0022]     The surface of the textured (etched) metal layer may be activated by contact with an activating solution prior to subsequent electroplating. For example, a suitable activating solution for subsequent acid copper electroplating is a solution comprising from about 1% to about 15% by weight hydrogen peroxide (H 2 O 2 ) and from about 10% to about 30% by volume sulfuric acid (H 2 SO 4 ). A suitable contact time with the activation solution is about 5 seconds to about 60 seconds at room temperature, followed by rinsing with water.  
         [0023]     Before the finish layer is deposited on the surface of the plastic component, it may be desirable to electroplate one or more layers over the textured metal layer having the desired relief pattern. Specifically, it may be desirable to utilize a conventional acid copper electroplating process to level or fill light scratches left on the etched surface during prior texturing and cleaning operations. It may also be desirable to electroplate one or more layers of other metals, particularly nickel, on the relief patterned layer before depositing a finish layer on the relief patterned layer. For example, a semi-bright nickel layer may be electroplated onto the textured metal layer prior to electroplating chrome onto the component. In addition, or alternatively, a bright nickel layer may also be electroplated onto the textured metal layer prior to electroplating a chrome finish layer. In addition, or alternatively, a microporous nickel layer may be electroplated onto the plastic article between the textured metal layer and the finish layer in order to retard corrosion. Accordingly, the expression “depositing a finish layer over the patterned metal layer” refers to either depositing a finish layer directly on the etched or patterned metal layer, or depositing a finish layer on one or more layers previously applied to the etched metal layer.  
         [0024]     The finish layer may comprise a relatively thin chrome or other metal (e.g., nickel) layer deposited over the etched metal layer using known electroplating techniques. Other suitable finish layers include metals such as rhodium, gold, palladium, platinum, silver, black nickel, nickel, or other metals deposited over etched metal layer using any of various metal plating techniques, including vacuum deposition, physical vapor deposition, chemical vapor deposition, etc. A non-metallic material may be used as the finish layer, or may be applied over a metal finish layer. Examples of non-metallic finish or overcoat layers include clear or tinted organic (e.g., polymer) coating compositions, electrophoretic coatings, opaque paints, etc.  
         [0025]     To facilitate different processing steps at different facilities, it may be desirable to apply a corrosion inhibitor to the surface of the relief patterned metal layer after cleaning the surface of the relief patterned layer. This facilitates storage and/or transportation of the article to another facility for subsequent processing. The corrosion inhibitor may be removed, such as with cleaning solvents or the like, prior to subsequent processing.  
         [0026]     The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.