# Patent

## Title
Electrolytic generation of manganese (III) ions in strong sulfuric acid using an improved anode

## Abstract

An electrolytic cell and a method of electrochemical oxidation of manganese (II) ions to manganese(III) ions in the electrolytic cell are described. The electrolytic cell comprises (1) an electrolyte solution of manganese(II) ions in a solution of 9 to 15 molar sulfuric acid; (2) a cathode immersed in the electrolyte solution; and (3) an anode immersed in the electrolyte solution and spaced apart from the cathode. Various anode materials are described including vitreous carbon, reticulated vitreous carbon, and woven carbon fibers.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No. 13/356,004, filed on Jan. 23, 2012, now pending, the subject matter of which is herein incorporated by reference in its entirety.


FIELD OF THE INVENTION
The present invention relates generally to a process of electrolytically generating manganese(III) ions in strong sulfuric acid using an improved anode.
BACKGROUND OF THE INVENTION
It is well known in the art to plate non-conductive substrates, (i.e. plastics) with metal for a variety of purposes. Plastic moldings are relatively inexpensive to produce and metal plated plastic is used for many applications. For example, metal plated plastics are used for decoration and for the fabrication of electronic devices. An example of a decorative use includes automobile parts such as trim. Examples of electronic uses include printed circuits, wherein metal plated in a selective pattern comprises the conductors of the printed circuit board, and metal plated plastics used for EMI shielding. ABS resins are the most commonly plated plastics for decorative purposes while phenolic and epoxy resins are the most commonly plated plastics for the fabrication of printed circuit boards.
Plating on plastic surfaces is used in the production of a variety of consumer items. Plastic moldings are relatively inexpensive to produce and plated plastic is used for many applications, including automotive trim. There are many stages involved in the plating of plastic. The first stage involves etching the plastic in order to provide mechanical adhesion of the subsequent metallic coatings and to provide a suitable surface for adsorption of the palladium catalyst which is typically applied in order to catalyze deposition of the initial metallic layer from an autocatalytic nickel or copper plating process. Following this, deposits of copper, nickel and/or chromium may be applied.
The initial etching of the plastic components is an essential part of the overall process. However, only certain types of plastic components are suitable for plating. The most common types of plastic for electroplating are acrylonitrile/butadiene/styrene (ABS) or a blend of ABS with polycarbonate (ABS/PC). ABS consists of two phases. The first phase is a relatively hard phase consisting of an acrylonitrile/styrene copolymer and the second phase is a softer polybutadiene phase.
Currently, this material is etched almost exclusively using a mixture of chromic and sulfuric acids, which is highly effective as an etchant for ABS and ABS/PC. The polybutadiene phase of the plastic contains double bonds in the polymer backbone, which are oxidized by the chromic acid, thus causing complete breakdown and dissolution of the polybutadiene phase exposed at the surface of the plastic which gives an effective etch to the surface of the plastic.
One problem with the traditional chromic acid etching step is that chromic acid is a recognized carcinogen and is increasingly regulated, insisting that wherever possible, the use of chromic acid is replaced with safer alternatives. The use of a chromic acid etchant also has well-known and serious drawbacks, including the toxicity of chromium compounds which makes their disposal difficult, chromic acid residues remaining on the polymer surface that inhibit electroless deposition, and the difficulty of rinsing chromic acid residues from the polymer surface following treatment. Additionally, hot hexavalent chromium sulfuric acid solutions are naturally hazardous to workers. Burns and upper respiratory bleeding are common in workers routinely involved with these chrome etch solutions. Thus, it is very desirable that safer alternatives to acidic chromium etching solutions be developed.
Early attempts to replace the use of chromic acid to etch plastic typically focused on the use of permanganate ions as an alternative to chromic acid. The use of permanganate in combination with acid is described in U.S. Pat. No. 4,610,895 to Tubergen et al., which is herein incorporated by reference in its entirety. Later, the use of permanganate in combination with an ionic palladium activation stage was suggested in U.S. Pat. Pub. No. 2005/019958 to Bengston, which is herein incorporated by reference in its entirety. The use of acid permanganate solutions in combination with perhalo ions (e.g., perchlorate or periodate) was described in U.S. Pat. Pub. No. 2009/0092757 to Satou, which is herein incorporated by reference in its entirety. Finally, the use of permanganate ions in the absence of alkali metal or alkaline earth metal cations was described in International Pub. No. WO 2009/023628 to Enthone, which is herein incorporated by reference in its entirety.
Permanganate solutions are also described in U.S. Pat. No. 3,625,758 to Stahl et al., which is herein incorporated by reference in its entirety. Stahl suggests the suitability of either a chrome and sulfuric acid bath or a permanganate solution for preparing the surface. In addition, U.S. Pat. No. 4,948,630 to Courduvelis et al., which is herein incorporated by reference in its entirety, describes a hot alkaline permanganate solution that also contains a material, such as sodium hypochlorite, that has an oxidation potential higher than the oxidation potential of the permanganate solution and is capable of oxidizing manganate ions to permanganate ions. U.S. Pat. No. 5,648,125 to Cane, which is herein incorporated by reference in its entirety, describes the use of an alkaline permanganate solution comprising potassium permanganate and sodium hydroxide, wherein the permanganate solution is maintained at an elevated temperature, i.e., between about 165° F. and 200° F. U.S. Pat. No. 4,042,729 to Polichette et al., which is herein incorporated by reference in its entirety, describes an etching solution that comprises water, permanganate ion, and manganate ion, wherein the molar ratio of manganate ion to permanganate ion is controlled and the pH of the solution is maintained at 11-13.
As is readily seen, many etching solutions have been suggested as a replacement for chromic acid in processes for preparing non-conductive substrates for metallization. However, none of these processes have proven satisfactory for various economic, performance and/or environmental reasons and thus none of these processes have achieved commercial success or been accepted by the industry as a suitable replacement for chromic acid etching. In addition, the stability of the etching solutions may also be poor, resulting in the formation of manganese dioxide sludge.
The tendency for permanganate based solutions to form sludge and undergo self-decomposition has been noted by the inventors here. Under strongly acidic conditions, permanganate ions can react with hydrogen ions to produce manganese (II) ions and water according to the following reaction:

4MnO4−+12-H+→4Mn2++6H2O+5O2  (1)

The manganese (II) ions formed by this reaction can then undergo further reaction with permanganate ions forming a sludge of manganese dioxide according to the following reaction:

2MnO4−+2H2O+3Mn2+→5MnO2+4H30  (2)

Thus formulations based on strongly acidic permanganate solutions are intrinsically unstable irrespective of whether the permanganate ion is added by alkali metal salts of permanganate or is electrochemically generated in situ. In comparison to the currently used chromic acid etches, the poor chemical stability of acidic permanganate renders it effectively useless for large scale commercial application. Alkaline permanganate etches are more stable, and are widely used in the printed circuit board industry for etching epoxy based printed circuit boards, but alkaline permanganate is not an effective etchant for plastics such as ABS or ABS/PC. Thus, manganese (VII) is unlikely to gain widespread commercial acceptance as an etchant for these materials.
Attempts to etch ABS without the use of chromic acid have include the use of electrochemically generated silver (II) or cobalt (III). Certain metals can be anodically oxidized to oxidation states which are highly oxidizing. For example, manganese (II) can be oxidized to permanganate (manganese VI), cobalt can be oxidized from cobalt (II) to cobalt (III) and silver can be oxidized from silver (I) to silver (II).
There is currently no suitable commercially successful etchant for plastics based on either permanganate (in either acid or alkaline form), on manganese in any other oxidation state or by using other acids or oxidants.
Thus, there remains a need in the art for an improved etchant for preparing plastic substrates for subsequent electroplating that does not contain chromic acid and that is commercially acceptable.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an etchant for plastic substrates that does not contain chromic acid.
It is another object of the present invention to provide an etchant for plastic substrates that is commercially acceptable.
It is another object of the present invention to provide an etchant for plastic substrates that is based on manganese ions.
It is still another object of the present invention to provide an electrode that is suitable for use in a strong acid oxidizing electrolyte but that is not degraded by the electrolyte.
It is still another object of the present invention to provide a suitable electrode for the generation of manganese(III) ions in strong sulfuric acid that is commercially acceptable.
To that end, the present invention relates generally to an electrode suitable for the electrochemical oxidation of manganese (II) ions to manganese (III) ions in a strong sulfuric acid solution.
In addition, the present invention relates generally to an electrolytic cell comprising:
an electrolyte solution comprising manganese(III) ions in a solution of acid, which is preferably from 9 to 15 molar sulfuric acid;
a cathode in contact with the electrolyte solution; and
an anode in contact with the electrolyte solution, wherein the anode comprises a material selected from the group consisting of vitreous carbon, reticulated vitreous carbon, woven carbon fibers, and combinations of one or more of the foregoing.
In another embodiment, the present invention relates generally to a method of electrochemical oxidation of manganese (II) ions to manganese (III) ions comprising the steps of:
providing an electrolyte comprising a solution of manganese (II) ions in a solution of acid, which is preferably from 9 to 15 molar sulfuric acid, in an electrolytic cell wherein the electrolytic cell comprises an anode and a cathode; and
applying a current to the anode and cathode of the electrolytic cell; and
oxidizing the electrolyte to form manganese(III) ions, wherein the manganese(III) ions form a metastable complex.


DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventors of the present invention have found that trivalent manganese can readily be produced by electrolysis at low current density of divalent manganese ions in strong sulfuric acid. More particularly, the inventors of the present invention have discovered that a solution of trivalent manganese ions in strongly acidic solution is capable of etching ABS.
Trivalent manganese is unstable and is highly oxidizing (standard redox potential of 1.51 versus normal hydrogen electrode). In solution, it very rapidly disproportionates to manganese dioxide and divalent manganese via the following reaction:

2Mn3++2H2O→MnO2+Mn2++4H+  (3)

However, in a strong sulfuric acid solution, the trivalent manganese ion becomes meta-stable and forms a cherry purple/red colored sulfate complex. The inventors have found that this sulfate complex is a suitable medium for the etching of ABS and has many advantages over chromium-free etches previously described.
In one embodiment, the present invention relates generally to a method of preparing a solution capable of etching a plastic substrate, the method comprising the steps of:
providing an electrolyte comprising a solution of manganese (II) ions in a solution of acid in an electrolytic cell, wherein the electrolytic cell comprises an anode and a cathode; and
applying a current to the anode and cathode of the electrolytic cell; and
oxidizing the electrolyte to form manganese(III) ions, wherein the manganese(III) ions form a metastable complex.
In a preferred embodiment, the plastic substrate comprises ABS or ABS/PC.
While it is contemplated that both phosphoric acid and sulfuric acid would be suitable for compositions of the present invention, in a preferred embodiment, the acid is sulfuric acid. At ambient temperatures, the half life of the manganese (III) ions in 7M sulfuric acid is on of the order of 2 years. By comparison, the half life of similar concentrations of manganese (III) ions in 7M phosphoric acid was around 12 days. It is suggested that the much higher stability of the manganese (III) ions in sulfuric acid is due to the formation of mangano-sulfate complexes and the higher concentration of available hydrogen ion concentration in the sulfuric acid solution. A further problem with the use of phosphoric acid is the limited solubility of manganese (III) phosphate. Thus, although other inorganic acids such as phosphoric acid can be usable in the compositions of the present invention, it is generally preferred to use sulfuric acid.
The remarkable stability of manganese (III) ions in strong sulfuric acid provides the following advantages in use:
1) Because the Mn(III) ions are formed at a low current density, the power requirements for the process are typically very low.
2) Because the anode operates at a very low current density, a small cathode in relationship to the anode area can be used to prevent cathodic reduction of the Mn(III) ions. This obviates the need for a divided cell and makes the engineering of an etchant regeneration cell simpler.
3) Because the process does not produce permanganate ions, there is no possibility of producing manganese heptoxide in the solution (this is a considerable safety hazard as it is violently explosive).
4) Because of the high stability of the Mn(III) ions in strong sulfuric acid, the etchant can be sold ready for use. In production, the etchant requires only a small regeneration cell at the side of the tank in order to maintain the Mn(III) content of the etch and prevent the build-up of Mn(II) ions.
5) Because other etch processes are based on permanganate, the result of the reaction of permanganate with Mn(II) ions causes rapid “sludging” with manganese dioxide and a very short lifetime of the etch. This should not be an issue with the Mn(III) based etch (although there may be some disproportionation over time).
6) The electrolytic production of Mn(III) in accordance with the present invention does not produce any toxic gases. While some hydrogen may be produced at the cathode, owing to the low current requirements, this would be less than that produced by many plating processes.
As described herein, in a preferred embodiment the acid is sulfuric acid. The concentration of sulfuric acid is preferably between about 9 and about 15 molar. The concentration of sulfuric acid is important in the process. Below a concentration of about 9 molar, the rate of etch becomes slow and above about 14 molar, the solubility of manganese ions in the solution becomes low. Additionally, very high concentrations of sulfuric acid tend to absorb moisture from the air and are hazardous to handle. Thus, in a most preferred embodiment, the concentration of sulfuric acid is between about 12 and 13 molar, which is dilute enough to allow the safe addition of water to the etch and strong enough to optimize the etch rate of the plastic. At this concentration of sulfuric acid, up to around 0.08M of manganese sulfate can be dissolved at the preferred operating temperature of the etch. For optimal etching, the concentration of manganese ions in solution should be as high as it is feasible to achieve.
The manganese(II) ions are preferably selected from the group consisting of manganese sulfate, manganese carbonate and manganese hydroxide although other similar sources of manganese(II) ions known in the art would also be usable in the practice of the invention. The concentration of manganese(II) ions may be in the range of between about 0.005 molar and saturation. In one embodiment, the electrolyte also comprises colloidal manganese dioxide. This may form to some extent as a natural result of disproportionation of manganese (III) in solution, or may be added deliberately.
Manganese (III) ions can be conveniently generated by electrochemical means by the oxidation of manganese (II) ions. In addition, it is generally preferable that the electrolyte not contain any permanganate ions.
In another embodiment the present invention comprises immersing the platable plastic in the metastable sulfate complex for a period of time to etch the surface of the platable plastic. In one embodiment, the platable plastic is immersed in the solution at a temperature of between 30 and 80° C. The rate of etching increases with temperature and is slow below 50° C. The upper limit of temperature is determined by the nature of the plastic being etched. ABS begins to distort above 70° C., thus in a preferred embodiment the temperature of the electrolyte is maintained between about 50 and about 70° C., especially when etching ABS materials. The time period of the immersion of the plastic in the electrolyte is preferably between about 20 to about 30 minutes.
Articles etched in this manner may be subsequently electroplated using conventional pretreatment for plated plastics or the etched surface of the plastic could be used to enhance the adhesion of paint, lacquers or other surface coatings.
As described in the examples that follow, the inventors of the present invention have determined by means of cyclic voltammetry that at the concentration of manganese (II) ions used in the etch of this invention, the oxidation is diffusion controlled so efficient agitation of the etch solution is necessary during the electrolytic oxidation process.
In another preferred embodiment, the present invention relates generally to an electrolyte capable of etching a platable plastic, the electrolyte comprising a solution of manganese(III) in an acid solution. The acid solution is preferably sulfuric acid.
The anodes and cathodes usable in the electrolytic cell described herein may comprise various materials. The cathode may comprise a material selected from the group consisting of platinum, platinized titanium, niobium, iridium oxide coated titanium, and lead. In one preferred embodiment, the cathode comprises platinum or platinized titanium. In another preferred embodiment, the cathode comprises lead. The anode may also comprise platinized titanium, platinum, iridium/tantalum oxide, niobium, or any other suitable material and is preferably platinum or platinized titanium.
In another preferred embodiment, the inventors of the present invention have found that the anode may comprise vitreous carbon and that the use of vitreous carbon anodes provides a commercially suitable electrode. The inventors discovered that while the combination of manganese (III) ions and strong sulfuric acid (i.e., 9-15 molar) can etch ABS plastic, the etchant is also very aggressive towards the electrodes necessary to produce the manganese (III) ions. In particular, anodes having a titanium substrate may be rapidly degraded by the etchant.
Therefore, in an attempt to determine a more suitable electrode material, various other electrode materials were examined, including lead and graphite. It was found that lead was rapidly attacked by the etchant when used as an anode (although it was determined to be suitable for use as a cathode) and that graphite anodes crumbled rapidly. However, vitreous carbon and reticulated vitreous carbon were determined to be more robust and could produce manganese (III) ions when an electrical current, preferably of between 0.1 and 0.4 A/dm2 (based on the nominal surface area), was applied. Thus, as described herein, anodes made of vitreous carbon may be used as an electrode. In addition, because vitreous carbon and reticulated vitreous carbon may not be cost-effective for use as the electrode in commercial applications, it was further determined that the anode may be manufactured from woven carbon fiber.
Carbon fiber is manufactured from fibers of polyacrylonitrile (PAN). These fibers go through a process of oxidation at increasing temperatures followed by a carbonization step at a very higher temperature in an inert atmosphere. The carbon fibers are then woven into a sheet which is typically used in combination with various resin systems to produce high strength components. Carbon fiber sheets also have good electrical conductivity and the fibers typically have a turbostratic (i.e., disordered layer) structure. Without wishing to be bound by theory, the inventors of the present invention believe that it is this structure which makes the carbon fibers so effective as an electrode. The SP2 hybridized carbon atoms in the lattice give good electrical conductivity while the SP3 hybridized carbon atoms link the graphitic layers together, locking them in place and thus providing good chemical resistance.
A preferred material for use in the electrodes of the invention comprises a woven carbon fiber containing at least 95% carbon and not impregnated with any resin. In order to facilitate the handling and the weaving process, carbon fibers are typically sized with an epoxy resin and this may comprise up to 2% of the fiber weight. At this low percentage, when used as an electrode, the epoxy sizing is rapidly removed by the high sulfuric acid content of the etch. This may cause an initial slight discoloration of the etch, but does not affect the performance. Following this initial “running in” stage, the anode appears to be resistant to the electrolyte and is effective at oxidizing manganese (II) ions to manganese (III).
Anodes can be constructed by mounting the woven carbon fiber material in a suitable frame with a provision made for electrical contact. It is also possible to use carbon fiber as a cathode in the generation of manganese (III) ions, but it is more convenient to use lead, particularly as the cathode is much smaller than the anode if an undivided cell is used.
In addition, for efficient generation of manganese (III) ions, it is generally necessary to use an anode area which is large in comparison to the area of the cathode. Preferably, the area ratio of anode to cathode is at least about 10:1. By this means, the cathode can be immersed directly in the electrolyte and it is not necessary to have a divided cell (although the process would work with a divided cell arrangement, this would introduce unnecessary complexity and expense).
In another preferred embodiment, the present invention also relates generally to an electrolytic cell comprising:
an electrolyte solution comprising manganese(III) ions in an acid solution;
a cathode in contact with the electrolyte solution; and
an anode in contact with the electrolyte solution, wherein the anode comprises a material selected from the group consisting of vitreous carbon, reticulated vitreous carbon, woven carbon fibers, and combinations of one or more of the foregoing.
The invention will now be illustrated with reference to the following non-limiting examples:
Comparative Example 1
A solution of 0.08 molar of manganese(II) sulfate in 12.5 molar sulfuric acid (500 ml) was heated to 70° C. and a piece of platable grade ABS was immersed in the solution. Even after an hour immersed in this solution, there was no discernible etching of the test panel and upon rinsing, the surface was not “wetted” and would not support an unbroken film of water.
Example 1
The solution of Comparative Example 1 was electrolyzed by immersing a platinized titanium anode of an area of 1 dm2 and a platinized titanium cathode of surface area 0.01 dm2 in the solution and applying a current of 200 mA for 5 hours.
During this period of electrolysis, the solution was observed to change in color from almost colorless to a very deep purple/red color. It was confirmed that no permanganate ions were present.
This solution was then heated to 70° C. and a piece of platable grade ABS was immersed in the solution. After 10 minutes of immersion, the test piece was fully wetted and would support an unbroken film of water after rinsing. After 20 minutes of immersion, the sample was rinsed in water, dried and examined using a scanning electron microscope (SEM). This examination revealed that the test piece was substantially etched and many etch pits were visible.
Example 2
A solution containing 12.5 M of sulfuric acid and 0.08 M manganese (II) sulfate was electrolyzed using a platinized titanium anode at a current density of 0.2 A/dm2. A platinized titanium cathode having an area of less than 1% of the anode area was used in order to prevent cathodic reduction of the Mn(III) ions produced at the anode. The electrolysis was performed for long enough for sufficient coulombs to be passed to oxidize all of the manganese (II) ions to manganese (III). The resulting solution was a deep cherry purple/red color. There were no permanganate ions generated during this step. This was also confirmed by visible spectroscopy—the Mn(III) ions produced a completely different absorption spectrum from that of a solution of permanganate.
Example 3
The etching solution prepared as described above in Example 3 was heated to 65-70° C. on a magnetic stirrer/hotplate and test coupons of ABS were immersed in the solution for time periods of 20 and 30 minutes. Some of these test coupons were examined by SEM and some were processed in a normal plating on plastic pretreatment sequence (reduction in M-neutralize, predip, activate, accelerate, electroless copper plate to 25-30 microns). These test coupons were then annealed and subjected to peel strength testing using an Instron machine.
Peel strength testing carried out on coupons plated for 30 minutes demonstrated peel strength varying between about 1.5 and 4 N/cm.
Cyclic voltammograms were obtained from a solution containing 12.5M sulfuric acid and 0.08M manganese sulfate using a platinum rotating disk electrode (RDE) having a surface area of 0.196 cm2 at various rotation speeds. A model 263A potentiostat and a silver/silver chloride reference electrode were used in conjunction with the RDE.
In all cases, the forward scan showed a peak at around 1.6V vs. Ag/AgCl followed by a plateau up to around 1.75V followed by and increase in current. The reverse scan produced a similar plateau (at a slightly lower current and a peak around 1.52V. The dependence of these results on the rate of electrode rotation indicates mass transport control is a primary factor in the mechanism. The plateau indicates the potential range over which Mn(III) ions are formed by electrochemical oxidation.
A potentiostatic scan was performed at 1.7V. It was observed that the current initially dropped and then over a period of time increased. The current density at this potential varied between 0.15 and 0.4 A/dm2.
Following this experiment, a galvanostatic measurement was taken at a constant current density of 0.3 A/dm2. Initially, the applied current density was achieved by a potential of about 1.5V but as the experiment progressed, after about 2400 seconds, and increase in potential to about 1.75V was observed.
After a period of etching for more than 10 minutes, it was observed that the surface of the ABS test coupons was fully wetted and would support an unbroken film of water after rinsing. After a period of 20 or 30 minutes, the panels were noticeably etched.
Comparative Example 2
An electrode comprising graphite and having a nominal measured surface area of 1 dm2 was immersed in 500 mL of a solution containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid at a temperature of 65° C. The cathode in this cell was a piece of lead having a nominal measured surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.
It was observed that the graphite anode rapidly crumbled and degraded within less than 1 hour of electrolysis. In addition, no oxidation of manganese (II) ions to manganese (III) was observed.
Comparative Example 3
An electrode comprising a titanium substrate coating with a mixed tantalum/iridium oxide coating (50% tantalum oxide, 50% iridium oxide) and having a nominal measured surface area of 1 dm2 was immersed in 500 mL of a solution containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid at a temperature of 65° C. The cathode in this cell was a piece of lead having a nominal measured surface are of 0.1 dm2. A current of 0.25 amps was applied to the cell giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.
It was observed that manganese (III) was rapidly formed in the solution and the resulting solution was capable of etching ABS plastic and producing good adhesion upon subsequent electroplating of the treated plastic. However, after a period of two weeks operation (electrolyzing the solution for 8 hours/day), it was observed that the coating was lifting from the titanium substrate and that the titanium substrate itself was dissolving in the solution.
Comparative Example 4
An electrode comprising a titanium substrate coated with platinum and having a nominal measured surface area of 1 dm2 was immersed in 500 mL of a solution containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid at a temperature of 65° C. The cathode in this cell was a piece of lead having a nominal measured surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.
It was observed that manganese (III) was rapidly formed in the solution and the resulting solution was capable of etching ABS plastic and producing good adhesion upon subsequent electroplating of the treated plastic. However, after a period of two weeks operation (electrolyzing the solution for 8 hours/day), it was observed that the coating was lifting from the titanium substrate and that the titanium substrate itself was dissolving in the solution.
Example 4
An electrode comprising vitreous carbon and having a nominal measured surface area of 0.125 dm2 was immersed in 100 mL of a solution containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid at a temperature of 65° C. The cathode in this cell was a piece of platinum wire having a nominal measured surface area of 0.0125 dm2. A current of 0.031 amps was applied to the cell giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.
It was observed that manganese (III) was rapidly formed in the solution and the resulting solution was capable of etching ABS plastic and producing good adhesion upon subsequently electroplating the treated plastic. The electrode appeared unaffected by periods of extended electrolysis.
Example 5
An electrode comprising a piece of woven carbon fiber (Panex 35 50K Tow with epoxy sizing at 1.5%, available from the Zoltek Corporation) was mounted in a plastic frame constructed of polyvinylidenefluoride (PVDF). The electrode, having a nominal measured area of 1 dm2, was immersed in 500 mL of a solution containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid at a temperature of 65° C. The cathode in this cell was a piece of lead having a nominal measured surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.
It was observed that manganese (III) was rapidly formed in the solution and the resulting solution was capable of etching ABS plastic and producing good adhesion upon subsequent electroplating of the treated plastic. The electrode appeared unaffected by periods of extended electrolysis. Electrolysis was carried out over two weeks using this electrode and no observable degradation could be detected. The low cost and ready availability of this material makes it suitable for many commercial applications.
The results of these experiments demonstrate that manganese (III) ions can be generated by electrosynthesis using manganese(II) ions in sulfuric acid at a relatively high concentration and operating at low current densities using a platinum or platinized titanium anode and that further improvements to the process can be realized by using a vitreous carbon or carbon fiber anode.

## Claims

1. A method of electrochemical oxidation of manganese (II) ions to manganese (III) ions comprising the steps of:
providing an electrolyte comprising a solution of manganese (II) ions in an acid solution in an electrolytic cell wherein the electrolytic cell comprises an anode and a cathode, and wherein the anode comprises a material selected from the group consisting of vitreous carbon, reticulated vitreous carbon and woven carbon fiber;
applying a current between the anode and the cathode; and
oxidizing the electrolyte to form manganese(III) ions, wherein the manganese(III) ions form a metastable complex.

2. A method according to claim 1 wherein the acid solution comprises sulfuric acid.

3. The method according to claim 2, wherein the platable plastic comprises acrylonitrile-butadiene-styrene or acrylonitrile-butadiene-styrene/polycarbonate.

4. A method according to claim 1 wherein the acid solution comprises 9 to 15 molar sulfuric acid solution.

5. The method according to claim 4, wherein the sulfuric acid has a concentration of 12 to 13 molar.

6. The method according to claim 1, further comprising the step of contacting platable plastic with the metastable complex for a period of time to etch the platable plastic.

7. The method according to claim 1, wherein the manganese(II) ions are derived from a compound selected from the group consisting of manganese sulfate, manganese carbonate and manganese hydroxide.

8. The method according to claim 1, wherein the solution additionally comprises colloidal manganese dioxide.

9. The method according to claim 1, wherein the concentration of the manganese(II) ions in the electrolyte is between about 0.005 molar and saturation.

10. The method according to claim 1, wherein the cathode comprises a material selected from the group consisting of platinum, platinized titanium, iridium/tantalum oxide, niobium and lead.

11. The method according to claim 10, wherein the cathode comprises lead.

12. The method according to claim 10, wherein the cathode comprises platinized titanium or platinum.

13. The method according to claim 1, wherein the anode comprises vitreous carbon.

14. The method according to claim 1, wherein the anode comprises woven carbon fibers.

15. The method according to claim 14, wherein the woven carbon fibers have a turbostratic structure.

16. The method according to claim 14, wherein the woven carbon fibers are produced from fibers of polyacrylonitrile.

17. The method according to claim 14, where the woven carbon fibers comprise at least 95% carbon and are impregnated with less than 2% resin, based on the weight of the carbon fibers.

18. The method according to claim 1, wherein an area of the anode is larger than an area of the cathode.

19. The method according to claim 1, wherein the anode current density is between about 0.1 to about 0.4 A/dm2.

20. The method according to claim 1, wherein the temperature of the electrolyte is maintained between about 30° C. and about 80° C.

21. The method according to claim 1, wherein the electrolyte does not contain any permanganate.