US3592744A - Method of preventing rack plating in continuous plating cycle for nonconductive articles - Google Patents

Abstract

A METHOD IS DISCLOSED FOR TREATING PLASTIC SHEATHED METAL JIGS OR RACKS CONVENTIONALLY USED IN SUPPORTING AND TRANSPORTING NONCONDUCTIVE ARTICLES THROUGH CHEMICAL AND ELECTROPLATING SOLUTIONS IN THE COURSE OF A CONTINUOUS PLATING CYCLE FOR METALLIZING THE ARTICESS, WHEREBY TO PREVENT OR REDUCE DEPOSIT OF METAL ON THE RACKS THEMSELVES. O THIS END, THE RACKS ARE TREATED TO CAUSE ADSORPTION OF HEXAVALENT CHROMIUM IONS AT THE SURFACE OF THEIR PLASTIC SHEATHED PROTIONS.

Description

United States Patent 3,592,744 METHOD OF PREVENTING RACK PLATING IN CONTINUOUS PLATING CYCLE FOR NON- CONDUCTIVE ARTICLES John J. Grunwald, New Haven, Eugene D. DOttavio,

Thomaston, and Frank L. Durso, Cheshire, Conn, assignors to MacDermid Incorporated, Waterbury, Conn. N0 Drawing. Filed Dec 2, 1968, Ser. No. 780,567 Int. Cl. C2311 /60; B44d 1/092; B231) 27/00 US. Cl. 204-20 5 Claims ABSTRACT OF THE DISCLOSURE A method is disclosed for treating plastic sheathed metal jigs or racks conventionally used in supporting and trans porting nonconductive articles through chemical and electroplating solutions in the course of a continuous plating cycle for metallizing the articles, whereby to prevent or reduce deposit of metal on the racks themselves. To this end, the racks are treated to cause adsorption of hexavalent chromium ions at the surface of their plastic sheathed portions.

The art of metallizing plastics both for decorative and functional applications is undergoing rapid growth. The general method employed consists of chemically depositing on the plastic object a preliminary or initial conductive metallic coating, Whereafter it can be electroplated by known electro-chemical techniques as if the objects were made of metal. The successful application of the initial conductive metallic layer is of crucial importance for the subsequent successful electroplating of one or more layers of the same or different metal to build up a total metallic deposit of desired thickness and finish characteristics. Copper, nickel, chromium and sometimes cobalt are the metals most commonly applied to commercial articles in the automotive appliance, plumbing and related industries today.

There are several commercial methods for metallizing plastics. All consist of immersing the object to be plated first in a number of aqueous or nonaqueous systems comprising an electroless or chemical plating cycle, each of such systems contributing to a specific alteration of the polymer surface of the object to be plated to cause it to accept a thin adherent initial metallic layer on which heavier depositscan subsequently be deposited electrochemically, i.e. by conventional electroplating procedures. In the foregoing operations, the articles to be plated are racked, that is suspended from plating jigs or racks adapted to support and transport the articles during their passage from one plating treatment solution to another as well as during immersion in each of such plating solutions. It is also a function of such racks to make electrical contact between the objects and the cathode bar of the electroplating tank or tanks while the articles are undergoing treatment in that portion of the overall cycle of operations. However, it is a practical necessity to provide some means of preventing deposit of metal plate on the conductive racks themselves, since this is not only wasteful of plating metal and plating current but is also disruptive of proper, uniform plating of the articles. The racks are accordingly coated with a sheath of polymeric electrically insulating material, except for the terminal clips and wires where electrical contact must be established with the parts to be electroplated. Difiiculty arises, however, even though the racks are thus insulated, if they are used in supporting and transporting the articles through the initial electro- 3,592,744 Patented July 13, 1971 less plating portion of the metallizing operation, since the plastic sheathing on the racks will normally react in much the same way as the plastic articles themselves to receive and retain an initial metallic film which of course will then accept additional metal plate in the subsequent electroplating portion of the cycle. Where this occurs, the racks must be periodically stripped of deposited metal by mechanical or chemical means. The process is time-consuming and damaging to the racks, resulting in accelerated obsolescence of them. It is the avoidance of this problem to which the present invention is primarily directed.

In order to afford a better understanding of the problem, a description of a typical method used in industry for metallizing plastic substrates will be helpful for purposes of reference. The operations in a conventional plastic article metallizing cycle comprise the steps enclosed in solid lines in the flow sequence shown below, While the steps enclosed in dotted lines represent alternatves in the procedure in accordance with this invention:

. I E Step A i i Anti-plate treatment of i plating rack by immersion in aqueous solution of hexavalent chromium ions. I I

Step 1 Hang plastic articles to be plated on conveyor-carried plating racks and immerse articles and racks in organic solvent pretreatment 0 ution.

S 2 Water rinse.

Step A l Same as Step A. i

Step 3 i Immerse racked articles in chromic-sulfuric acid etch solution.

Step 4 Immerse racked articles in water and/or chrome reducing rinse.

Step 5 Immerse racked articles in stannous chloridehydroehloric acid sensitizing solution.

Step 6 Water rinse.

Step 7 Immerse racked articles in palladium-ehloritie-hydrochloric acid activating solution.

(Continued on following page) (Continued from preceding page) Step 8 Water rinse.

Step 9 Immerse activated plastic articles while still on rack in chemical plating solution containing a reducible salt of the metal to be initially deposited on the articles.

Step Water rinse.

Step 11, etc. Immerse chemically plated articles while still on same racks in conventional electroplating solutions to electrodeposit one or more layers of the same or different plating metals to provide the final desired total thickness and external finish.

Water rinse, remove finished articles from racks and return latter to start of cycle.

Most of the compositions employed in the prior art cycle of operations are well known in the industry, including the chromic-sulfuric acid etch of Step 3, and the sensitizing, activating and chemical plating solutions of Steps 5, 7 and 9. See for example, US. Pat Nos. 2,430,- 581; 2,702,253; 2,766,138; 2,871,142 and 3,075,855. Further details of the organic solvent pretreatment, Step 1, are disclosed in copending U.S. appln. Ser. Nos. 654,901 now Pat. No. 3,486,361 and 717,006. Step 4 involves the removal of excess hexavalent chromium from the parts, following the etch treatment in Step 3, and depending on the plastic being plated and particulars of the subsequent plating procedures selected, may be simply a thorough water rinse or a chrome-kill solution of phosphoric acid or a surfactant solution of the type disclosed in copending appln. Ser. No. 758,589. In some instances the separate sensitizing and activating operations of Steps 5 and 7 are replaced by a colloidal palladium-tin chloride activator solution of the type disclosed in copending US.

appln. Ser. No. 654,307 (now Pat. No. 3,532,518, issued Oct. 6, 1970) and this is followed generally by acceleration in a fluoroboric acid solution.

The foregoing sequence of steps has enjoyed considerable commercial success and is used in many practical operations. However there are numerous variations and deviations in one or more of the steps which in some cases provide improved operation depending on the nature of the polymer from which the article to be plated is formed. For example, while immersion of an article in Step 3 of the foregoing procedure is essential for complete coverage and good metal-to-polymer adhesion in the case of plastic substrates of ABS (acrylonitrile-butadiene-styrene), it may be deleterious for successful coverage of polyamide substrate materials, such as nylon, for example, and is generally omitted from the plating sequence for such materials. On the other hand, nylon and polypropylene substrates generally require preplating treatment in an organic solvent solution, Step 1, while such step is unnecessary in the treatment of ABS substrates. Thus it requires specific adaption of Steps 1 through 9 in the foregoing outline to accommodate the nature of a given polymer undergoing metallizing. Once the substrate has been metallizcd in the chemical plating portion of the procedure, all substrates behave identically thereafter and can be electroplated under essentially the same conditions.

It will be apparent from the foregoing that if the same plating racks are employed in transporting the articles through the various steps of the chemical plating portion of the cycle, i.e. Steps 19, as are used for continuing the treatment of the articles in the electroplating portion of the cycle, i.e. Step 11 etc., the plastic coating or sheathing of the plating racks may also become metallized, making the entire rack electrically conductive and causing it to electroplate in all solutions following Step 10. In addition to the obvious problem of wastefully plating out substantial amounts of metal on the rack, the more serious effect of rack plating is that it wastefully consumes electrical current thereby robbing the parts to be plated of the electrical energy needed for adequate build-up of the required plate composite. Plated deposits on the racks themselves introduce a shielding eficct in respect to the articles supported by them, reducing the current density on the parts to be plated during the electroplating process, thus requiring greater total plating time for a desired plate thickness. Even more seriously, the effect of rack plating and the resulting reduction of plating current density on the parts results in misplating or incomplete coverage, especially in the chromium plating bath where below a given current density satisfactory plating cannot be accomplished.

Attempts to obviate the ditficulties have usually been of three general types. In the first, parts to be plated are mounted on a different rack, or are re-racked, following Step 9 or 10 in the foregoing outline. This results in additional manpower requirements and an increase in labor cost. Another procedure employed has been to eliminate all racking of the articles prior to Step 11 since the operations involved in the chemical plating portion of the overall cycle, namely Steps l-lO, require no external electrical current to be supplied to the parts and they can be immersed in the various steps in bulk, i.e. in baskets, plating barrels or the like. Thereafter, the parts are mounted on a suitable plating rack for processing through the electroplating steps. The obvious drawback of this method arises from increased contamination of the various solutions by carry-over and drag-in during transfer of the parts in bulk from one solution to the next. Furthermore there is substantial opportunity for damaging the parts through rubbing and scratching against each other since they are simply jumble-packed in the basket or barrel employed in transporting them through the several processing steps. This is especially prevalent if the parts are large and heavy. Finally, attempts have been made to reduce the disadvantages of rack plating while still utilizing the racks to afford a continuous processing cycle without re-racking and partially successful efforts have resulted from so controlling the nature and composition of the materials used in Steps 4 through 9 as to provide sutlicient driving force to effect chemical plating of the parts but not of the plastic sheathed or coated portion of the racks. There is, however, great practical difficulty in commercial use of this method because it narrows the latitude of the operating conditions, i.e. concentration, temperature and permissible processing time in the various steps, and accordingly there is a constant struggle between incomplete part coverage and partial rack coverage.

It is therefore the principal object of this invention to provide a method of so treating the racks that the plastic sheathed portion does not become plated, yet plating of the articles themselves is in no Way adversely affected. A further object of the invention is to provide a method which may be incorporated directly in the normal plating cycle and be compatible with such cycle. Thus the invention here disclosed provides a method that permits a continuous plating operation to be employed for both chemical and electrodeposition portions of the overall plating cycle without reracking of the articles at any time throughout the cycle. Such continuous uninterpreted sequence of process operations, from the initial step of mounting the parts on the rack to the final step of unloading the finished plated parts from the rack, is especially important in high speed automated plating operations.

It has been discovered that inclusion of hexavalent chromium ions, -i.e. Cr, in the superficial layer of plastic sheathing on the rack results in substantial reduction or complete elimination of rack plating. While the inclusion of chromium ions in the plastic surface may be accomplished in numerous ways, this can be simply accomplished by soaking the racks in an acidic aqueous solution rich in chromic oxide, thereby diifusing hexavalent chromium from solution into the plastic sheathing of the rack. Such an operation can be readily incorporated directly in the sequence of steps through which a rack is advanced by the conventional conveyor in traversing a complete cycle of operations in the plating system. Notwitstanding the fact that the plastic sheathing of the racks is subjected in each cycle to the reducing action of the chrome-kill solution at Step 4 in the cycle described above, sufiicient selectivity exists between the plastic parts themselves and the plastic sheathing such that the chromium ion level in the sheathing can be and is maintained high enough in accordance with the procedure here outlined to prevent rack plating while allowing normal plating of the parts supported on the racks to proceed. The following examples will serve to illustrate the nature of this invention.

EXAMPLE 1 Nylon parts are mounted on a vinyl plastic coated rack which has been previously soaked for 5 minutes in a solution containing 6 pounds of chromic acid per gallon equivalent to 45% by weight at 140 F. using a plating cycle as hereinabove described, with the exception of eliminating Step 3 and inserting the antiplate treatment of the racks, Step A, ahead of Step 1. Parts are successfully plated by first depositing an initial layer of copper or nickel in the electroless or chemical plating portion of the cycle, followed by nickel and/or chromium plating in the electroplating portion of the cycle, all without re-racking.

EXAMPLE 2 ABS parts are mounted on a vinyl plastic coated rack and plated in a cycle similar to the foregoing, except that Steps 1 and 2 are eliminated so that the sequence of steps then becomes Step A, 3, etc. In this case, Step A comprises immersing the rack and supported articles in an aqueous solution containing 4 pounds of chromic acid per gallon (approximately 33% by weight) for a period of 30 seconds at 120 F. No plating of the plastic coating of the racks occurs, while complete coverage of the ABS parts is accomplished.

EXAMPLE 3 Polypropylene parts are mounted on a vinyl plastic coated rack and plated through the previously described conventional plating cycle commencing with Steps 1 and 2 but the procedure is then modified to insert Step A directly ahead of Step 3. Step A involves the same type of anti-plate composition as Step A but in this particular example the solution contains 1.5 pounds of chromic acid per gallon (approximately 16% by weight), and the plastic coated rack and parts are immersed in it for 2 minutes at 110 F. Following this, the process is resumed at Step 3 and continues with the balance of the procedure as outlined. This results in complete electroplate coverage of the polypropylene parts with no plating on the plastic coated rack.

EXAMPLE 4 The plastic coating employed commercially is commonly a PVC (polyvinyl chloride) base resin material which can be applied by dipping the racks in a solution of such material and either evaporating a solvent carrier if such is used or otherwise curing the polymer material to form an adherent film or coating on the rack. However, the method of the present invention is likewise applicable to plating racks coated with polypropylene sheathing, provided the articles to be plated are not also polypropylene, and when treated in accordance with the invention, such sheathing will not acquire a metal plate in traversing through the metallizing operations described. For example, a polypropylene sheathed rack used in supporting nylon parts following the procedure of Example 1, results in complete plating of the parts with no plating of the rack.

' EXAMPLE 5 Similarly ABS parts are mounted on polypropylene sheathed racks and processed as in Example 2. Again complete metal coverage of the parts is obtained with no deposit of metal on the racks.

EXAMPLE 6 Polystyrene parts supported on vinyl plastic coated racks, following the procedure in Example 3, also give complete article coverage without encountering rack plating.

EXAMPLE 7 Polysulfone parts supported on vinyl plastic coated racks are processed in the same manner as in Example 3 above, using the same chromic acid concentration, time and temperature conditions as in the aforesaid example. Complete plating of the parts is obtained with no rack plating.

In each of the foregoing examples, the treatment produces a distinctive green-brown coloration of the rack sheathing due apparently to adsorption of hexavalent chromium, and it is readily observed that the freedom from rack plating is strongly dependent on the degree of coloration.

EXAMPLE 8 ABS parts supported on plastic coated racks are processed in accordance with Example 2 above, but in this case Step A comprises immersing the rack and supported articles in a mixed chromic-sulfuric acid solution having a high Baum in which the hexavalent chromium ion content is only about 1.4% by weight while the sulfuric acid is about Even when using an excessive processing time in Step A of several minutes and a temperature of around F., rack plating readily occurs. It is also noted that the characteristic deep green-brown coloration of the plastic coating, which results in each of the previous examples after processing in Step A or Step A, is not obtained in this case.

EXAMPLE 9 A further modification of the procedure outlined in Example 8, in which the hexavalent chromium ion concentration in Step A is raised to 12-14% by weight, while the sulfate (sulfuric acid) concentration is reduced to 40-50%, produces a noticable decrease in rack plating and may in some cases provide acceptable operating conditions where the plating activator employed subsequently is reasonably selective, and/or where the plastic material of the parts being plated is of the relatively more easily platable type. Here again the characteristic green-brown coloration of the plastic rack coating is observable, though substantially lighter in color than that obtained in Examples 1 through 6.

It is apparent from the foregoing illustrative examples and from other research conducted in connection with this invention that the hexavalent chromium ion must be present in the anti-rack plating solution at a minimum concentration of about (by weight), but at this level, it will generally be necessary to employ longer treatment times and higher solution temperatures. Optimally it is in the range of 16%, as seen from Example 3, but may go as high as (Example 1) or even to saturation (about 52% by weight in straight aqueous solution). The straight aqueous chromic acid solution gives best results and is preferred in most instances, but sulfate ion can be present provided its concentration is not in excess of by weight and preferably this should be kept below 40%, as seen by Example 9.

What is claimed is:

1. In the process of plating non-conductive articles with a metal in a continuous plating cycle wherein the articles are suspended from plating racks and advanced without re-racking through a complete operating cycle comprising first chemically plating an initial metal deposit on the articles, including the steps of subjecting the racked articles to organic solvent and/or acid etch pretreatment, sensitizing, activating and electroless deposition of metal, and then electrodepositing a further metal late on the chemically plated articles to build up a final desired total thickness and type of metal finish thereon, and wherein said racks are metal but all portions subjected to immersion in the plating solutions during the cycle of plating operations are encased in a plastic sheath except the rack tips from which the articles are suspended: the improvement to reduce the deposit of plating metal on said racks which comprises subjecting at least said sheathed portion thereof to an anti-plating treatment comprising immersing the racks in an aqueous acid solution containing hexavalet chromium ions in adequate concentration and for sufiicient length of time to effect adsorption of hexavalent chromium in the surface of said plastic sheath.

2. The process as defined in claim 1, wherein said rack anti-plating solution contains hexavalent chromium ions at a concentration of at least 10% by Weight.

3. The process of continuous plating of non-conductive articles supported on plastic sheathed racks as defined in claim 1, wherein the articles are formed of acrylonitrile-butadiene-styrene resin and the sheathed portions of the racks are of polyvinyl chloride or polypropylene, which comprises the steps of racking the articles and then immersing them while in racked condition in an aqueous solution of chromic acid to produce adsorption of hexavalent chromium in the sheathed portions of the racks equivalent to that resulting from immersing them in a solution containing four pounds of chromic acid per gallon for a period of 30 seconds at F.; and thereafter advancing said racked articles directly to the usual acid etch pretreatment step and proceeding with the remainder of said chemical and electroplating operations.

4. The process of continuous plating of non-conductive articles supported on plastic sheathed racks as defined in claim 1, wherein the articles are formed of nylon and the sheathed portions of the racks are polyvinyl chloride or polypropylene, which comprises first immersing the racks in an aqueous solution of chromic acid to produce adsorption of hexavalent chromium in the sheathed portions equivalent to that resulting from immersing them in a solution containing six pounds of chromic acid per gallon for a period of 5 minutes at F.; thereafter racking the articles on the pretreated racks and progressing them sequentially in racked condition through the said plating operations but omitting said acid etch pretreatment step.

5. The process of continuous plating of non-conductive articles supported on plastic sheathed plating racks as defined in claim 1, wherein the articles are formed of polypropylene or polystyrene and the sheathed portions of the racks are polyvinyl chloride, which comprises the steps of racking the articles and then immersing them in such racked condition, after said organic solvent but before said acid etch pretreatment steps, in an aqueous solution of chromic acid to produce adsorption of hexavalent chromium in the sheathed portions of the racks equivalent to that resulting from immersing them in a a solution containing two pounds of chromic acid per gallon for a period of two minutes at 100 F.

References Cited UNITED STATES PATENTS 3,370,974 2/1968 Hepfer 117-47 3,142,581 7/1964 Leland 204-30 3,471,376 10/1969 Sambestre et al. 20420 OTHER REFERENCES Plating on Plastics by H. Narcus Plating, August 1968, pp. 816-820.

JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner US. Cl. X.R.

ll7-47A, 138.8R; 20430