ES2704672T3 - Electrolytic generation of manganese (III) ions in strong sulfuric acid - Google Patents

Abstract

Electrolytic cell comprising: an electrolytic 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, crosslinked vitreous carbon, woven carbon fibers and combinations of one or more of the foregoing.

Description

DESCRIPTION

Electrolytic generation of manganese (III) ions in strong sulfuric acid

Cross reference to related applications

This application is a continuation in part of the application with serial number 13 / 356,004, filed on January 23, 2012, currently in process.

Field of the invention

The present invention relates generally to a process for 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 coat non-conductive substrates (ie plastics) with metal for a variety of purposes. Plastic moldings can be produced relatively cheaply and metallized plastics are used for many applications. For example, metallized plastics are used for decoration and for the manufacture of electronic devices. An example of a decorative use includes auto parts, such as the trim. Examples of electronic uses include printed circuit boards, wherein the metallization in a selective pattern comprises the conductors of a printed circuit board, and the metallized plastics used for EMI shielding. ABS resins are the most commonly metallized plastics for decorative purposes, while phenolic and epoxy resins are the most commonly metallized plastics for the fabrication of printed circuit boards.

The metallization of plastic surfaces is used in the production of a variety of consumer goods. Plastic moldings can be produced relatively cheaply and metallized plastic is used for many applications, including automotive moldings. There are many stages involved in the plating of plastics. The first stage consists of etching the plastic to provide the mechanical adhesion of the subsequent metallic coatings and to provide a suitable surface for the palladium catalyst adsorption which is generally applied to catalyze the deposition of the initial metallic layer of a metallized autocatalytic process with copper or nickel. Next, deposits of copper, nickel and / or chromium can be applied.

The initial engraving of the plastic components is an essential part of the overall procedure. However, only certain types of plastic components are suitable for metallization. The most common types of plastic for electrometallisation are acrylonitrile / butadiene / styrene (ABS) or a mixture of ABS with polycarbonate (ABS / PC). ABS consists of two phases. The first is a relatively hard phase consisting of an acrylonitrile / styrene copolymer and the second phase is a softer polybutadiene phase.

At present, the etching of this material is carried out using almost exclusively 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's main chain, which is oxidized with chromic acid, thus causing a complete decomposition and dissolution of the polybutadiene phase exposed on the surface of the plastic that gives an effective etching to the surface of plastic.

A problem with the stage of engraving with traditional chromic acid is that chromic acid is a recognized carcinogen and is increasingly regulated, insisting regulations that, whenever possible, the use of chromic acid is replaced by safer alternatives. The use of a chromic acid etching agent also has known and serious drawbacks, such as the toxicity of chromium compounds, which make it difficult to remove them, as chromic acid residues remain on the surface of the polymer that inhibit chemical deposition, as well as the difficulty of rinsing the chromic acid residues from the surface of the polymer after treatment. In addition, solutions of hot hexavalent chromium sulfuric acid are by nature hazardous to workers. Upper respiratory tract burns and hemorrhages are common among workers who routinely handle these chromium etching solutions. Therefore, it is highly desirable that safer alternatives to acid chrome etching solutions be developed.

The first attempts to replace the use of chromic acid for the etching of plastics have generally 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. Patent No. 4,610,895 to Tubergen et al. Later, the use of permanganate in combination with an ion palladium activation step was suggested in U.S. Patent Publication No. 2005/019958 to Bengston. The use of acid permanganate solutions in combination with perhalo ions (e.g., perchlorate or periodate) was described in U.S. Patent Publication No. 2009/0092757 to Satou. Finally, the use of permanganate ions in the absence of alkali metal or alkaline earth metal cations was described in International Patent Publication No. WO 2009/023628 of Enthone

Permanganate solutions are also disclosed in U.S. Patent No. 3,625,758 to Stahl et al. Stahl suggests the suitability of a bath of chromium and sulfuric acid or a solution of permanganate for the preparation of the surface. In addition, U.S. Patent No. 4,948,630 to Courduvelis et al., Discloses a hot alkaline permanganate solution that also contains a material, such as sodium hypochlorite, which has a higher oxidation potential than the oxidation of the permanganate solution and is able to oxidize ions of manganate to permanganate ions. U.S. Patent No. 5,648,125 to Cane, discloses the use of an alkaline permanganate solution comprising potassium permanganate and sodium hydroxide, in which the permanganate solution is maintained at an elevated temperature, i.e. between approximately 74 ° C and 93 ° C (165 ° F and 200 ° F). United States Patent No. 4,042,729 to Polichette et al. Discloses an etching solution comprising water, permanganate ion and manganate ion, in which the molar ratio between the manganate ion and the ion is controlled. of permanganate and the pH of the solution is maintained at 11-13.

As already indicated, many etching solutions have been suggested as substitutes for chromic acid in the preparation processes of non-conductive substrates for metallization. However, none of these procedures has proven to be satisfactory for various economic, performance and / or environmental reasons and, therefore, none of these procedures has been commercially successful nor has it been accepted by the industry as a suitable substitute for etching with chromic acid. In addition, the stability of the etching solutions can also be low, resulting in the formation of manganese dioxide sludge.

The present inventors have observed the tendency of permanganate-based solutions to form sludge and experience self-decomposition. Under strongly acidic conditions, the 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 undergo an additional reaction with permanganate ions to form a manganese dioxide sludge according to the following reaction:

2MnO4- 2H2O 3Mn2 + ^ 5MnO2 4H + (2)

Therefore, formulations based on strongly acidic permanganate solutions are intrinsically unstable, regardless of whether the permanganate ion is added by alkali metal salts of permanganate or generated electrochemically in situ. Compared to the currently used chromic acid etching agents, the low chemical stability of acid permanganate makes it effectively useless for large scale commercial applications. The alkaline permanganate etching agents are more stable and their use is widespread in the printed circuit board industry for the etching of epoxy-based printed circuit boards, but the alkaline permanganate is not an effective etching agent for plastics such as ABS or ABS / PC. Therefore, manganese (VII) is unlikely to obtain wide commercial acceptance as an etching agent for these materials.

Attempts to burn ABS without the use of chromic acid have included the use of silver (II) or cobalt (III) generated electrochemically. Certain metals can be oxidized anodically to oxidation states that 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).

At present there is no commercially successful etching agent suitable for plastics based on permanganate (in acid or alkaline form), on manganese in any other oxidation state or by the use of other acids or oxidants.

Therefore, there remains a need in the art for an improved etching agent to prepare plastic substrates for subsequent electrometallization which does not contain chromic acid and which is commercially acceptable.

Summary of the invention

It is an object of the invention to provide an etching agent for plastic substrates that does not contain chromic acid.

Another object of the present invention is to provide an etchant for plastic substrates that is commercially acceptable.

Another object of the present invention is to provide an etching agent for plastic substrates based on manganese ions.

Still another object of the present invention is to provide an electrode which is suitable for use in a strong acid oxidizing electrolyte but which is not degraded by the electrolyte.

Still another object of the present invention is to provide a suitable electrode for the generation of manganese (III) ions in strong sulfuric acid that is commercially acceptable.

For this purpose, the present invention relates in general to an electrode suitable for the electrochemical oxidation of manganese (II) ions to manganese (III) ions in a strong sulfuric acid solution.

The present invention provides an electrolytic cell according to claim 1, comprising:

an electrolytic solution comprising manganese (III) ions in an acid solution, which is preferably 9 to 15 molar sulfuric acid;

a cathode in contact with the electrolytic solution; Y

an anode in contact with the electrolytic solution, wherein the anode comprises a material selected from the group consisting of vitreous carbon, crosslinked vitreous carbon, woven carbon fibers and combinations of one or more of the foregoing.

The present invention also provides a method according to claim 10, 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 sulfuric acid or a solution of phosphoric acid in an electrolytic cell in which the electrolytic cell comprises an anode and a cathode, and wherein the anode comprises a material selected from the group consisting of vitreous carbon, crosslinked vitreous carbon and woven carbon fiber;

applying a current between the anode and the cathode, the current density of the anode being between 0.1 and 0.4 A / dm2; Y

oxidize the electrolyte to form manganese (III) ions, the manganese (III) ions forming a metastable complex.

The present invention also provides a method according to claim 28 for engraving a plastic part.

The preferred features are defined in the dependent claims.

Detailed description of the preferred embodiments

The inventors of the present invention have discovered that trivalent manganese can be easily 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 highly oxidative (standard redox potential of 1.51 against the normal hydrogen electrode). In solution, it is rapidly disproportionated into manganese dioxide and divalent manganese through the following reaction:

2Mn3 + 2H2O ^ MnO2 Mn2 + 4H + (3)

However, in a strong sulfuric acid solution, the trivalent manganese ion becomes metastable and forms a purple / cherry red color sulfate complex. The inventors have discovered that this sulfate complex is a suitable medium for ABS etching and has many advantages over the chromium-free etching agents described above.

A process for preparing a solution capable of etching a plastic substrate is also described herein, the method 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; Y

apply a current to the anode and the cathode of the electrolytic cell; Y

oxidize the electrolyte to form manganese (III) ions, the manganese (III) ions forming 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 the compositions of the present invention, in a preferred embodiment, the acid is sulfuric acid. At room temperature, life Average of manganese (III) ions in 7M sulfuric acid is of the order of 2 years. In 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 manganese (III) ions in sulfuric acid is due to the formation of mangano-sulfate complexes and the higher concentration of available hydrogen ions in the sulfuric acid solution. An additional problem with the use of phosphoric acid is the limited solubility of manganese (III) phosphate. Therefore, although it is possible to use other inorganic acids, such as phosphoric acid, in the compositions of the present invention, the use of sulfuric acid is generally preferred.

The remarkable stability of manganese (III) ions in strong sulfuric acid provides the following advantages during its use:

1) Because the Mn (III) ions are formed at a low current density, the energy requirements for the process are generally very low.

2) Because the anode operates at a very low current density, a small cathode can be used in relation to the anode area to avoid the cathodic reduction of Mn (III) ions. This avoids the need for a split cell and simplifies the engineering of the regeneration cell of the etching agent.

3) Because the process does not produce permanganate ions, there is no possibility of producing manganese heptoxide in the solution (this represents a considerable safety risk, since it produces violent explosions).

4) Due to the high stability of Mn (III) ions in strong sulfuric acid, the etchant can be sold ready to use. In production, the etching agent requires only a small regeneration cell on the side of the tank to maintain the Mn (III) content of the etch and prevent the accumulation of Mn (II) ions.

5) Because other etching procedures are based on permanganate, the result of the reaction of permanganate with Mn (II) ions causes a rapid "sludge formation" with manganese dioxide and a very short life of the engraving. This should not be a problem with engraving based on Mn (III) (although there may be some disproportion with time).

6) The electrolytic production of Mn (III) according to the present invention does not produce toxic gases. While some hydrogen may be produced at the cathode, due to the low current requirements, this would be less than that produced in many metallizing 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 procedure. Below a concentration of approximately 9 molar, the etching rate becomes slow and above about 14 molar, the solubility of the manganese ions in the solution becomes low. In addition, very high concentrations of sulfuric acid tend to absorb moisture from the air and their handling is dangerous. Therefore, in a more preferred embodiment, the concentration of sulfuric acid is between about 12 and 13 molar, and sufficiently dilute to allow safe addition of water to the etchant and strong enough to optimize the etching rate of the etchant. plastic. At this concentration of sulfuric acid, up to about 0.08 M of manganese sulfate can be dissolved at the preferred operating temperature of the etching. For optimal etching, the concentration of manganese ions in the solution should be as high as can be achieved.

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 could also be used in the practice of the invention. The concentration of manganese (II) ions can range from about 0.005 molar to saturation. In one embodiment, the electrolyte also comprises colloidal manganese dioxide. This can be formed to some extent as a natural result of the disproportion of manganese (III) in the solution, or it can be added deliberately.

The manganese (III) ions can be conveniently generated by electrochemical means by oxidation of the manganese (II) ions. In addition, it is generally preferable that the electrolyte does not contain permanganate ions.

In another embodiment, the present invention comprises immersing the metallizable plastic in a metastable sulfate complex for a period of time to etch the surface of the metallizable plastic. In one embodiment, the metallizable plastic is immersed in the solution at a temperature of between 30 and 80 ° C. The engraving speed increases with temperature and is slow below 50 ° C. The upper limit of temperature is determined according to the nature of the plastic to be engraved. The ABS starts to distort above 70 ° C, so in In a preferred embodiment, the electrolyte temperature is maintained between about 50 ° C and about 70 ° C, especially when recording ABS materials. The period of time of immersion of the plastic in the electrolyte is preferably between about 20 and about 30 minutes.

The articles engraved in this way can be subsequently electrometallized using a conventional pretreatment for metallized plastics or the engraved surface of the plastic can be used to improve 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 etching of the present invention, oxidation is controlled by diffusion of such that efficient agitation of the etching solution is necessary during the electrolytic oxidation process.

In another preferred embodiment, the present invention relates generally to an electrolyte capable of etching a metallizable plastic, said electrolyte comprising a solution of manganese (III) in an acid solution. The acid solution is preferably sulfuric acid.

The 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, titanium coated with iridium oxide and lead. In a preferred embodiment, the cathode comprises platinized platinum or titanium. In another preferred embodiment, the cathode comprises lead.

The inventors of the present invention have discovered 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 (ie, 9-15 molar) can burn ABS plastics, the etchant is also very aggressive towards the electrodes needed to produce the ions of manganese (III). In particular, the anodes having a titanium substrate can be rapidly degraded by the etchant.

Therefore, in an attempt to determine a more suitable electrode material, various other electrode materials, including lead and graphite, were examined. It was found that the etching agent rapidly attacked the lead when it was used as an anode (although it was determined to be suitable for use as a cathode) and that the graphite anodes disintegrated rapidly. However, it was determined that the vitreous carbon and the vitreous carbon crosslinked were more robust and could produce manganese (III) ions if an electric current of between 0.1 and 0.4 A / dm2 was applied (depending on the surface area) nominal). Therefore, as described herein, anodes made of vitreous carbon can be used as electrodes. In addition, because the vitreous carbon and the crosslinked vitreous carbon may not be cost effective for use as electrodes in commercial applications, it was further determined that the anode could be fabricated from woven carbon fiber.

Carbon fiber is manufactured from polyacrylonitrile fibers (PAN). These fibers go through an oxidation process at increasing temperatures followed by a carbonization step at a very high temperature in an inert atmosphere. The carbon fibers are then woven into a sheet that is normally used in combination with various resin systems to produce high strength components. The carbon fiber sheets also have good electrical conductivity and the fibers generally 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 that makes the carbon fibers as effective as an electrode. The SP2 hybridized carbon atoms in the network provide good electrical conductivity, while the SP3 hybridized carbon atoms connect the graphite layers together, securing 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. To facilitate the handling and the weaving process, the carbon fibers are generally sized with an epoxy resin and this can comprise up to 2% of the weight of the fiber. At this low percentage, when used as an electrode, the size of the epoxy is quickly eliminated by the high content of sulfuric acid in the engraving. This may cause a slight initial discoloration of the engraving, but does not affect performance. After this initial "preparation" stage, the anode appears to be resistant to the electrolyte and is effective in the oxidation of manganese (II) to manganese ions

The anodes can be constructed by mounting the woven carbon fiber material in a suitable frame making a provision 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, especially because the cathode is much smaller than the anode if an undivided cell is used.

[0047] Further, for the efficient generation of manganese (III) ions, it is generally necessary to use an anode area that is large in comparison with the cathode area. Preferably, the anode to cathode area ratio is at least about 10: 1. In this way, the cathode can be immersed directly in the electrolyte and it is not necessary to have a divided cell (although the procedure would work with a split cell arrangement, this would introduce unnecessary complexity and expense).

The present invention relates generally to an electrolytic cell comprising:

an electrolytic solution comprising manganese (III) ions in an acid solution;

a cathode in contact with the electrolytic solution; Y

an anode in contact with the electrolytic solution, wherein the anode comprises a material selected from the group consisting of vitreous carbon, crosslinked vitreous carbon, woven carbon fibers and combinations of one or more of the foregoing.

Next, the invention will be illustrated with reference to the following non-limiting examples:

Comparative example 1:

A 0.08 molar solution of manganese sulfate (II) in 12.5 molar sulfuric acid (500 ml) was heated to 70 ° C and a piece of metallizable type ABS was immersed in the solution. Even after one hour submerged in this solution, there was no perceptible etching on the test panel and, after rinsing, the surface did not "moisten" and could not stand any film of uninterrupted 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 of 0.01 dm2 in the solution and applying a current of 200 mA for 5 hours.

During this period of electrolysis, it was observed that the solution changed color from practically colorless to a very intense purple / red color. It was confirmed that there were no permanganate ions present.

This solution was then heated to 70 ° C and a piece of metallizable type ABS was immersed in the solution. After 10 minutes of immersion, the test piece was completely wetted and could withstand a film of uninterrupted water after rinsing. After 20 minutes of immersion, the sample was rinsed with water, dried and examined using a scanning electron microscope (SEM). This examination revealed that the test piece was substantially etched and that many engraving holes were visible.

Example 2:

A solution containing 12.5 M sulfuric acid and 0.08 M manganese sulfate (II) was electrolyzed using a platinized titanium anode at a current density of 0.2 A / dm2. A platinized titanium cathode with an area of less than 1% of the anode area was used to prevent the cathodic reduction of the Mn (III) ions produced at the anode. The electrolysis was carried out long enough for enough coulombs to pass to oxidize all the manganese (II) to manganese (III) ions. The resulting solution was purple / deep cherry red. No permanganate ions were generated during this stage. This was also confirmed by visible spectroscopy: the Mn (III) ions produced an absorption spectrum completely different from that of a permanganate solution.

Example 3:

The etching solution prepared as described above in Example 3 was heated to 65-70 ° C on a magnetic stirrer / hot plate and ABS test pieces were immersed in the solution for periods of time of 20 and 30 minutes. Some of these test pieces were examined by SEM and some were processed in a normal metallization in a plastic pretreatment sequence (reduction in M-neutralize, pre-immersion, activation, acceleration, chemical nickel, copper metallization at 25-30 microns ). Next, these test pieces were annealed and subjected to a peel strength test using an Instron machine.

The peel strength test performed on the metallized pieces for 30 minutes showed that the peel strength varied between about 1.5 and 4 N / cm.

Cyclic voltammograms were obtained from a solution containing 12.5 M sulfuric acid and 0.08 M 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 together with the RDE.

In all cases, direct exploration presented a peak around 1.6 V versus Ag / AgCl followed by stable levels around 1.75 V and followed by an increase in current. The inverse exploration produced a similar stable level (at a slightly lower current and a peak around 1.52 V. The dependence of these results on the The rotation speed of the electrode indicates that the mass transport control is a main factor of the mechanism. The stable level indicates the potential interval in which the Mn (III) ions are formed by electrochemical oxidation. A potentiostatic exploration at 1.7V was performed. It was observed that the current was initially reduced and then increased after a period of time. The current density at this potential varied between 0.15 and 0.4 A / dm2. After this experiment, a galvanostatic measurement was taken at a constant current density of 0.3 A / dm2. Initially, the applied current density with a potential of approximately 1.5V was achieved, but, as the experiment progressed, after approximately 2,400 seconds, an increase in potential was observed at approximately 1.75V.

After an engraving period for more than 10 minutes, it was observed that the surface of the ABS test pieces was completely wetted and could withstand a film of uninterrupted water after rinsing. After a period of 20 or 30 minutes, the panels were recorded remarkably.

Comparative example 2:

An electrode comprising graphite was immersed and had a nominal surface area measured of 1 dm2 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 surface area measured of 0.1 dm2. A current of 0.25 amperes was applied to the cell, which gave a nominal current density of the anode of 0.25 A / dm 2 and a rated current density of the cathode of 2.5 A / dm 2.

It was observed that the graphite anode disintegrated and degraded rapidly in less than 1 hour of electrolysis. In addition, oxidation of manganese (II) to manganese (III) ions was not observed.

Comparative example 3:

An electrode comprising a titanium substrate coating with a mixed coating of tantalum / iridium oxide (50% tantalum oxide, 50% iridium oxide) and having a nominal surface area measured of 1 dm2 was immersed in 500 ml of a solution containing 0.08 M of manganese sulphate 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 surface area measured of 0.1 dm2. A current of 0.25 amperes was applied to the cell, which gave a nominal current density of the anode of 0.25 A / dm 2 and a rated current density of the cathode of 2.5 A / dm 2.

It was observed that the manganese (III) formed rapidly in the solution and the resulting solution was able to etch the ABS plastic and produce good adhesion in the electrometalized back of the treated plastic. However, after a period of two weeks of operation (electrolyzing the solution for 8 hours / day), it was observed that the coating was rising from the titanium substrate and that the titanium substrate was dissolving in the solution.

Comparative example 4:

An electrode comprising a titanium substrate coated with platinum and having a nominal surface area measured of 1 dm2 in 500 ml of a solution containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid was immersed. a temperature of 65 ° C. The cathode in this cell was a piece of lead having a nominal surface area measured of 0.1 dm2. A current of 0.25 amperes was applied to the cell, which gave a nominal current density of the anode of 0.25 A / dm 2 and a rated current density of the cathode of 2.5 A / dm 2.

It was observed that manganese (III) formed rapidly in the solution and the resulting solution was able to etch the plastic to Bs and produce good adhesion in the electrometalized back of the treated plastic. However, after a period of two weeks of operation (electrolyzing the solution for 8 hours / day), it was observed that the coating was rising from the titanium substrate and that the titanium substrate was dissolving in the solution.

Example 4:

An electrode comprising vitreous carbon and having a nominal surface area measured of 0.125 dm2 in 100 ml of a solution containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid at a temperature of 65 ° was immersed. C. The cathode in this cell was a piece of platinum wire having a nominal surface area measured of 0.0125 dm2. A current of 0.031 amperes was applied to the cell, which gave a nominal current density of the anode of 0.25 A / dm 2 and a rated current density of the cathode of 2.5 A / dm 2.

It was observed that manganese (III) formed rapidly in the solution and the resulting solution was able to etch the plastic to Bs and produce good adhesion in the electrometalized back of the treated plastic. The electrode it was not affected by periods of extended electrolysis.

Example 5:

An electrode comprising a piece of woven carbon fiber (Panex 3550K Tow with 1.5% epoxy size, marketed by Zoltek Corporation) was mounted on a plastic frame constructed of polyvinylidene fluoride (PVDF). The electrode, which had a measured nominal area of 1 dm2, was immersed in 500 ml of a solution containing 0.08 M 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 surface area measured of 0.1 dm2. A current of 0.25 amperes was applied to the cell, which gave a nominal current density of the anode of 0.25 A / dm 2 and a rated current density of the cathode of 2.5 A / dm 2.

It was observed that the manganese (III) formed rapidly in the solution and the resulting solution was able to etch the ABS plastic and produce good adhesion in the electrometalized back of the treated plastic. The electrode was not affected by periods of extended electrolysis. The electrolysis was carried out for two weeks using this electrode and an observable degradation could not be detected. The low cost and immediate availability of this material make 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 titanium anode platinized and that further improvements can be made in the process using a carbon fiber or glassy carbon anode.

Claims (31)

1. Electrolytic cell comprising: an electrolytic solution comprising manganese (III) ions in an acid solution; a cathode in contact with the electrolytic solution; Y an anode in contact with the electrolytic solution, wherein the anode comprises a material selected from the group consisting of vitreous carbon, crosslinked vitreous carbon, woven carbon fibers and combinations of one or more of the foregoing. 2. The electrolytic cell according to claim 1, wherein the anode comprises vitreous carbon. 3. The electrolytic cell according to claim 1, wherein the anode comprises woven carbon fibers. 4. The electrolytic cell according to claim 3, wherein the woven carbon fibers have a turbostratic structure. 5. The electrolytic cell according to claim 3, wherein the woven carbon fibers comprise at least 95% carbon. The electrolytic cell according to claim 1, wherein the acid solution comprises a solution of sulfuric acid. 7. The electrolytic cell according to claim 6, wherein the acid solution comprises sulfuric acid of 9 to 15 molar. The electrolytic cell 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. 9. The electrolytic cell according to claim 8, wherein the cathode comprises lead. 10. 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 sulfuric acid or a solution of phosphoric acid in an electrolytic cell in which the electrolytic cell comprises an anode and a cathode, and wherein the anode comprises a material selected from the group consisting of vitreous carbon, crosslinked vitreous carbon and woven carbon fiber; applying a current between the anode and the cathode, the current density of the anode being between 0.1 and 0.4 A / dm2; Y oxidize the electrolyte to form manganese (III) ions, the manganese (III) ions forming a metastable complex. 11. A process according to claim 10, wherein the acid solution comprises sulfuric acid. 12. A process according to claim 10, wherein the acid solution comprises a sulfuric acid solution of 9 to 15 molar. 13. A process according to claim 12, wherein the sulfuric acid has a concentration of 12 to 13 molar. The method according to claim 10, further comprising the step of contacting the metallizable plastic with the metastable complex for a period of time to etch the metallizable plastic. 15. The process according to claim 10, wherein the manganese (II) ions are derived from a compound selected from the group consisting of manganese sulfate, manganese carbonate and manganese hydroxide. The process according to claim 14, wherein the metallizable plastic comprises acrylonitrilobutadiene-styrene or acrylonitrile-butadiene-styrene / polycarbonate. The process according to claim 10, wherein the solution additionally comprises colloidal manganese dioxide, or wherein the concentration of manganese (II) ions in the electrolyte is between about 0.005 molar and saturation. 18. The method according to claim 10, wherein the cathode comprises a material selected from the group consisting of platinum, platinized titanium, iridium / tantalum oxide, niobium and lead. 19. The process according to claim 18, wherein the cathode comprises lead or wherein the cathode comprises titanium or platinized platinum. 20. The process according to claim 10, wherein the anode comprises vitreous carbon. 21. The process according to claim 10, wherein the anode comprises woven carbon fibers. 22. The process according to claim 21, wherein the woven carbon fibers have a turbostratic structure. 23. The process according to claim 21, wherein the woven carbon fibers are produced from polyacrylonitrile fibers. 24. The process according to claim 21, wherein 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. 25. The method according to claim 10, wherein an area of the anode is larger than a cathode area. 26. The process according to claim 10, wherein the electrolyte temperature is maintained between 30 ° C and about 80 ° C. 27. The process according to claim 10, wherein the electrolyte does not contain any permanganate. 28. A method for etching a plastic part, said method comprising contacting the plastic part with a solution comprising manganese (III) ions and acid. wherein the manganese (III) is generated in the solution by electrolytic oxidation of the manganese (II) and in which the electrolytic oxidation occurs at an anode in the solution and such an anode comprises vitreous carbon, cross-linked vitreous carbon or woven carbon fiber . 29. The process according to claim 28, wherein the acid comprises sulfuric acid. 30. The process according to claim 29, wherein the acid comprises sulfuric acid of 9 to 15 molar. 31. The process according to any one of claims 28 to 30, wherein the plastic part comprises acrylonitrile-butadiene-styrene (ABS).