CN105209667B - Manganese (III) ion in strength sulfuric acid it is electrolytically generated - Google Patents

Abstract

The present application describes an electrolytic cell and a method for the electrochemical oxidation of manganese(II) ions to manganese(III) ions in the electrolytic cell. The electrolytic cell comprises (1) an electrolyte solution of manganese (II) ions in a solution of at least one acid; (2) a cathode immersed in the electrolyte solution; and (3) immersed in the electrolyte solution and in contact with the Anodes separated by cathodes. Various anode materials are described, including glassy carbon, reticulated glassy carbon, braided carbon fibers, lead and lead alloys. Once the electrolyte is oxidized to form a metastable complex of manganese (III) ions, the platable plastic can be contacted with the metastable complex to etch the platable plastic. In addition, pretreatment steps may also be performed on the platable plastic to condition the surface of the plastic prior to contacting the platable plastic with the metastable complex.

Description

Electrolytic Generation of Manganese(III) Ions in Strong Sulfuric Acid

Cross reference to related applications

This application is a continuation-in-part of currently pending application number 13/677,798 filed November 15, 2012 which is a continuation-in-part of currently pending application number 13/356,004 filed January 23, 2012 Continuing, their respective subject matter is hereby incorporated by reference in its entirety.

technical field

The present invention generally relates to an improved method for etching platable plastics such as ABS and ABS/PC.

Background technique

Plating non-conductive substrates (ie, plastics) with metals is known in the art for a variety of purposes. Plastic molding is relatively cheap to produce, and metallized plastics are used in many applications. For example, metallized plastics are used for decoration and for the manufacture of electronic devices. Examples of decorative uses include automotive parts, such as automotive interiors. Examples of electronic uses include printed circuit boards, where metal plated in selective patterns comprises the conductors of the printed circuit board, and metallized plastics 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 in printed circuit board manufacturing.

Plating on plastic surfaces is used in the production of various consumer items. Plastic molding is relatively cheap to produce, and plated plastics are used in many applications, including automotive interiors. Plating of plastics involves many stages. The first stage involves etching the plastic to provide mechanical adhesion of the subsequent metal coating and to provide a suitable surface for adsorption of the palladium catalyst typically used to catalyze the detachment of the initial metal layer from an autocatalytic nickel or copper plating process. deposition. Next, a deposited layer of copper, nickel and/or chromium may be applied.

The initial etching of plastic components is an important part of the overall process. However, only certain types of plastic components are suitable for electroplating. The most common type of plastic used for plating is acrylonitrile/butadiene/styrene (ABS) or a blend of ABS and polycarbonate (ABS/PC). ABS consists of two phases. The first phase is a relatively hard phase consisting of acrylonitrile/styrene copolymer and the second phase is a softer polybutadiene phase.

Currently, this material is almost always etched using a mixture of chromic acid and sulfuric acid, which is a very effective etchant for ABS and ABS/PC. The polybutadiene phase of the plastic contains double bonds in the backbone of the polymer, which are oxidized by chromic acid, thus causing complete fracture and dissolution of the polybutadiene phase exposed on the plastic surface, achieving effective etching of the plastic surface .

One problem with the traditional chromic acid etch step is that chromic acid is considered a carcinogen, and as regulations become more stringent, there is a need to replace chromic acid with safer alternatives wherever possible. The use of chromic acid etchants also has known serious disadvantages, including the toxicity of chromium compounds, which makes their disposal difficult, the chromic acid residues left on polymer surfaces which inhibit electroless deposition, and the removal of Difficulty rinsing off chromic acid residue. In addition, hot hexavalent chromium sulfuric acid solutions are a natural hazard for workers. Workers exposed to these chromium etching solutions on a daily basis often experience burns and upper respiratory bleeding. Therefore, the development of safer alternatives to acidic chromium etching solutions is highly desirable.

Early attempts to replace chromic acid for etching plastics generally focused on the use of permanganate ions as a substitute for chromic acid. The use of permanganates in combination with acids is described in US Patent No. 4,610,895 to Tubergen et al., which is incorporated herein by reference in its entirety. Later, US Patent Application Publication No. 2005/0199587 by Bengston suggested the use of permanganate in combination with an ionic palladium activation stage, which is hereby incorporated by reference in its entirety. US Patent Application Publication No. 2009/0092757 to Satou, which is incorporated herein by reference in its entirety, describes the use of acid permanganate solutions in combination with perhalide ions (eg, perchlorate or periodate). Finally, Enthone's International Publication No. WO 2009/023628 describes the use of permanganate ions in the absence of alkali metal or alkaline earth metal cations, which is hereby incorporated by reference in its entirety.

Permanganate solutions are also described in US Patent No. 3,625,758 to Stahl et al., which is hereby incorporated by reference in its entirety. Stahl suggested the suitability of chromium and sulfuric acid baths or permanganate solutions for preparing surfaces. In addition, U.S. Patent No. 4,948,630 to Courduvelis et al., incorporated herein 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 that of high Oxidation potential of manganate solutions. U.S. Patent No. 5,648,125 to Cane, incorporated herein 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. About 165℉ to 200℉.

It can be seen that many etching solutions have been proposed to replace chromic acid in the process of preparing non-conductive substrates for metallization. However, none of these processes have proven satisfactory for various economic, performance, and/or environmental reasons, and thus have not yet achieved commercial success or been accepted by the industry as suitable alternatives to chromic acid etching. In addition, the stability of these permanganate-based etching solutions may also be poor, leading to the formation of manganese dioxide sludge.

The present inventors have investigated the tendency of permanganate-based solutions to form sludge and undergo self-decomposition. In a strongly acidic environment, permanganate ions can react with hydrogen ions to produce manganese(II) ions and water according to the following reaction:

4MnO ₄ ^- +12H ⁺ → 4Mn ²⁺ +6H ₂ O+5O ₂ (1)

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

2MnO ₄ ^- +2H ₂ O+3Mn ²⁺ →5MnO ₂ +4H ⁺ (2)

Thus, formulations based on strongly acidic permanganate solutions are inherently unstable, whether permanganate ions added via the alkali metal salt of permanganate or permanganate ions generated electrochemically in situ. The poor chemical stability of acidic permanganates compared to currently used chromic acid etching makes them practically useless for large-scale commercial applications. Alkaline permanganate etch is more stable and is widely used in the printed circuit board industry for etching epoxy-based printed circuit boards, but alkaline permanganate is not effective for plastics such as ABS or ABS/PC of etchant. Consequently, manganese(VII) has not gained wide commercial acceptance as an etchant for these materials.

Attempts to etch ABS without the use of chromic acid have included the use of electrochemically generated silver(II) and cobalt(III). Certain metals can be anodized to a highly oxidized oxidation state. For example, cobalt can be oxidized from cobalt(II) to cobalt(III) and silver can be oxidized from silver(I) to silver(II).

However, there are currently no suitable commercially successful etchants for plastics based on permanganate (whether in acid or base form) or manganese in any other oxidation state or through the use of other acids or oxidizing agents.

Therefore, there is still a need in the art for an improved etchant that does not contain chromic acid and is commercially acceptable for preparing plastic substrates for subsequent electroplating.

Contents of the invention

It is an object of the present invention to provide a chromic acid-free etchant for plastic substrates.

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

Another object of the present invention is to provide a manganese ion based etchant for plastic substrates.

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

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

Yet another object of the present invention is to provide an improved pretreatment step for conditioning plastic substrates prior to etching.

In one embodiment, the present invention relates generally to an electrolytic cell comprising:

an electrolyte solution comprising manganese(III) ions in a solution of sulfuric acid and an additional acid selected from the group consisting of methanesulfonic acid, methanedisulfonic acid, and combinations thereof;

a cathode in contact with the electrolyte solution; and

an anode, which is in contact with the electrolyte solution.

In another embodiment, the present invention relates generally to an electrolytic cell comprising:

an electrolyte solution comprising manganese(III) ions in a solution of at least one 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 glassy carbon, reticulated glassy carbon, woven carbon fibers, lead, lead alloys, and combinations of one or more of the foregoing.

In another embodiment, the present invention relates generally to a method of preparing a solution for etching a plastic substrate, the method comprising the steps of:

providing an electrolyte in an electrolytic cell comprising a solution of manganese(II) ions in a solution of at least one acid, wherein the electrolytic cell comprises an anode and a cathode; and

applying electric current to the anode and cathode of the electrolytic cell; and

The electrolyte is oxidized to form manganese(III) ions, wherein the manganese(III) ions form metastable complexes.

In another embodiment, the present invention relates generally to electrodes suitable for the electrochemical oxidation of manganese(II) ions to manganese(III) ions in a strong acid solution.

In another embodiment, the present invention relates generally to a method of electrochemically oxidizing manganese(II) ions to manganese(III) ions comprising the steps of:

An electrolyte is provided in the electrolytic cell comprising a solution of manganese(II) ions in a solution of at least one acid, wherein the at least one acid comprises sulfuric acid and an additional acid selected from the group consisting of methanesulfonic acid, The group consisting of methanedisulfonic acid and combinations thereof, wherein the electrolytic cell comprises an anode and a cathode;

applying a current between the anode and the cathode; and

The electrolyte is oxidized to form manganese(III) ions, wherein the manganese(III) ions form metastable complexes.

In yet another embodiment, the present invention relates generally to a method of etching a plastic part comprising contacting the plastic part with a solution comprising manganese(III) ions and at least one acid.

detailed description

The inventors of the present invention have found that manganese ions can be readily produced by electrolysis at low current densities in strong acid solutions, preferably in strong sulfuric acid solutions, most preferably in at least 8M sulfuric acid solutions. More particularly, the inventors of the present invention have found that a solution of trivalent manganese ions in a strongly acidic solution is capable of etching ABS.

Trivalent manganese is unstable and highly oxidizing (standard redox potential of 1.51 versus standard hydrogen electrode). In solution, it disproportionates very rapidly to manganese dioxide and manganese via the following reaction:

2Mn ³⁺ +2H ₂ O→MnO ₂ +Mn ²⁺ +4H ⁺ (3)

However, in strong sulfuric acid solutions, manganese ions become meta-stable and form cherry purple/red colored sulfate complexes. The inventors have found that this sulfate complex is a suitable medium for etching ABS and has many advantages over prior art chrome free etching.

Accordingly, in one embodiment, the present invention generally relates to a method of preparing a solution capable of etching a plastic substrate, the method comprising the steps of:

providing an electrolyte in an electrolytic cell comprising a solution of manganese(II) ions in a solution of at least one acid, wherein the electrolytic cell includes an anode and a cathode; and

applying current to the anode and the cathode of the electrolytic cell; and

The electrolyte is oxidized to form manganese(III) ions, wherein the manganese(III) ions form metastable complexes.

In a preferred embodiment, the plastic substrate comprises ABS or ABS/PC.

While both phosphoric acid and sulfuric acid are considered suitable for the compositions of the present invention, in a preferred embodiment the acid is sulfuric acid. The half-life of manganese(III) ions in 7M sulfuric acid is about 2 years at room temperature. In contrast, the half-life of manganese(III) ions at the same concentration in 7M phosphoric acid is about 12 days. It is speculated that the higher stability of manganese(III) ions in sulfuric acid is due to the formation of manganese-sulfate complexes and the higher concentration of hydrogen ions available in sulfuric acid solutions. Another problem with the use of phosphoric acid is the limited solubility of manganese(III) phosphate. Thus, while other mineral acids such as phosphoric acid may be used in the compositions of the present invention, the use of sulfuric acid is generally preferred.

In use, the remarkable stability of manganese(III) ions in strong sulfuric acid provides the following advantages:

1) Since the manganese(III) ions are formed at low current densities, the power requirements of the process are usually very low.

2) Since the anode operates at very low current densities, it is possible to use a small cathode relative to the anode area to prevent cathodic reduction of manganese(III) ions. This eliminates the need for a separate tank and makes the engineering of the etchant regeneration tank simpler.

3) Since the process does not produce permanganate ions, it is impossible to produce manganese heptoxide (which is a safety hazard because it is highly explosive) in the solution.

4) Due to the high stability of manganese (III) ions in strong sulfuric acid, the etchant can be directly sold and used. In production, the etchant requires only a small regeneration tank on one side of the tank to maintain the manganese(III) content of the etchant and prevent the buildup of manganese(II) ions.

5) Since other etching processes are based on permanganate, the result of the reaction of permanganate with manganese(II) ions results in a rapid "sludge" of manganese dioxide and a very short lifetime of the etchant. This should not be a problem with manganese(III) based etchants (although some disproportionation may occur over time).

6) According to the present invention, the electrolytic generation of manganese(III) does not generate any toxic gas. While some hydrogen may be produced at the cathode, this is less than that produced by many electroplating processes due to the low current requirements.

As described herein, in preferred embodiments, the acid is sulfuric acid. The concentration of sulfuric acid is preferably at least 8 molar, more preferably from about 9 to about 15 molar. In this process, the concentration of sulfuric acid is important. At concentrations below about 9 molar, the etch rate becomes slower, and above 14 molar, the solubility of manganese ions in solution becomes lower. Additionally, very high concentrations of sulfuric acid tend to absorb moisture from the air and are hazardous to handle. Thus, in the most preferred embodiment, the concentration of sulfuric acid is about 12 to 13 molar, dilute enough to safely add water to the etch, and strong enough to optimize the etch rate of the plastic. At this concentration of sulfuric acid, up to about 0.08M manganese sulfate can be dissolved at the preferred operating temperature of the etch. For optimal etching, the concentration of manganese ions in the solution should be as high as it 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 may also be used in the practice of the present invention. group. The concentration of manganese(II) ions may range from about 0.005 molar to saturation. In one embodiment, the electrolyte also includes colloidal manganese dioxide. This may be to some extent a natural result of manganese(III) disproportionation in solution, or it may be added on purpose.

Manganese(III) ions can be conveniently generated by oxidation of manganese(II) ions by electrochemical means. Furthermore, it is generally preferred that the electrolyte does not contain any permanganate ions.

As described herein, in order to obtain a fast etch rate for ABS plastic, a high concentration of acid needs to be used. The presence of sulfate or bisulfate ions is required to form complexes with manganese ions, and a molar concentration of sulfuric acid of at least 8M is required for good stability of the etch. For good plastic etching, it was found that a sulfuric acid concentration of at least about 12M was required for rapid etching. This has the effect of reducing the solubility of manganese ions in the bath, and the maximum solubility of manganese ions in the bath is about 0.08M at operating temperature. Since the etch rate depends on the concentration of manganese(III) ions in the solution and the maximum conversion percentage for stability is about 50%, it is desirable to increase the amount of manganese that can be dissolved in the bath.

The inventors have found that the amount of manganese that can be dissolved in the bath can be increased by displacing a portion of the sulfuric acid with another acid, where the manganese ions can be more soluble.

The choice of suitable acids is limited. For example, hydrochloric acid produces chlorine at the anode, while nitric acid produces nitric oxide at the cathode. Perchloric acid and periodate are expected to produce permanganate ions, which decompose into manganese dioxide. Organic acids are generally rapidly oxidized by manganese(III) ions. Thus, acids with the required stability for oxidation and the ability to increase the solubility of manganese ions in the bath are methanesulfonic acid and methanedisulfonic acid. Since the solubility of manganese(II) is significantly better in methanesulfonic acid (and sulfuric acid) than in methanedisulfonic acid, the choice of the former yields better performance. Thus, methanesulfonic acid is the preferred additional acid and sulfuric acid is the preferred primary acid.

Based on the above, the present invention also generally relates to an electrolyte for etching ABS and ABS/PC plastics, comprising sulfuric acid, and using methanesulfonic acid or methanedisulfonic acid in combination to obtain a better solubility of manganese ions in the bath, wherein the electrolyte contains at least 8M sulfuric acid and contains from about 0M to about 6M methanesulfonic acid or methanedisulfonic acid, preferably from about 1M to about 6M methanesulfonic acid.

More particularly, the present invention relates generally to an electrolytic cell comprising:

an electrolyte solution comprising manganese(III) ions in a solution of sulfuric acid and an additional acid selected from the group consisting of methanesulfonic acid, methanedisulfonic acid, and combinations thereof;

a cathode in contact with the electrolyte solution; and

an anode, which is in contact with the electrolyte solution.

Furthermore, the present invention generally relates to an electrolytic cell comprising:

an electrolyte solution comprising manganese(III) ions in a solution of at least one 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 glassy carbon, reticulated glassy carbon, woven carbon fibers, lead, lead alloys, and combinations of one or more of the foregoing.

In addition, the present invention generally relates to a method for the electrochemical oxidation of manganese (II) ions to manganese (III) ions, comprising the steps of:

providing an electrolyte in an electrolytic cell, wherein the electrolyte comprises a solution of manganese(II) ions in a solution of at least one acid, wherein the electrolytic cell includes an anode and a cathode;

applying a current between the anode and the cathode; and

The electrolyte is oxidized to form manganese(III) ions, wherein the manganese(III) ions form metastable complexes.

Once the electrolyte has been oxidized to form a metastable complex, the platable plastic may be immersed in the metastable complex for a period of time to etch the surface of the platable plastic, in one embodiment, at The platable plastic is immersed in the metastable complex at a temperature of 30 to 80°C. The etch rate increases with temperature and is slow below 50 °C.

The upper temperature limit is determined by the nature of the plastic being etched. ABS begins to deform above 70°C, so in a preferred embodiment, especially when etching ABS material, the temperature of the electrolyte is maintained at about 50 to about 70°C. The time the plastic is immersed in the electrolyte is preferably from about 10 to about 30 minutes.

Objects etched in this manner may be subsequently plated using conventional pretreatments for the plastic being plated, or the etched surface of the plastic may be used to promote adhesion of paint, lacquer, or other surface coating.

The concentration of manganese(II) ions used in the etching of the present invention can be determined by voltammetric cycling. Oxidation is diffusion controlled, so efficient agitation of the etching solution is required during this electrolytic oxidation.

Anodes and cathodes that may be used in the electrolytic cells described herein may comprise a variety of materials. The cathode may comprise a material selected from the group consisting of platinum, platinized titanium, niobium, iridium oxide coated titanium, and lead. In a 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, boron doped diamond, or any other suitable material.

The inventors found that while the combination of manganese(III) ions and strong sulfuric acid (ie, 8-15 molar) can etch ABS plastic, this etchant is also very aggressive to the electrodes necessary to generate manganese(III) ions. In particular, anodes with titanium substrates are rapidly degraded by this etchant.

Therefore, in an attempt to identify more appropriate electrode materials, various other electrode materials were tested, including lead and graphite. Glassy carbon and reticulated glassy carbon are considered stronger and can generate manganese(III) ions when a current of preferably 0.1-0.4 A/dm ² (based on nominal surface area) is applied. In addition, since glassy carbon and reticulated glassy carbon are not cost-effective as electrodes in commercial applications, anodes can also be fabricated from woven carbon fibers.

Carbon fibers are manufactured from fibers of polyacrylonitrile (PAN). These fibers are subjected to an oxidation process at elevated temperatures, followed by a carbonization step at very high temperatures and under an inert atmosphere. The carbon fibers are then woven into sheets, which are often combined with various resin systems to produce high-strength components. Carbon fiber sheets also have good electrical conductivity, and the fibers typically have a turbostratic (ie, disordered layer) structure. Without wishing to be bound by theory, it is believed that this structure makes the carbon fibers effective as electrodes. The SP ² hybridized carbon atoms in the crystal lattice provide good electrical conductivity, while the SP ³ hybridized carbon atoms bond the graphitic layers together, fixing their positions and thus providing good chemical resistance.

Preferred materials for use in electrodes of the present invention include woven carbon fibers containing at least 95% carbon and not impregnated with any resin. To facilitate the handling and weaving process, carbon fibers are usually sized with epoxy resin, and this can account for up to 2% of the fiber weight. At this low percentage, the epoxy sizing is quickly removed by the high sulfuric acid content of the etch when used as an electrode. This can cause an initial slight discoloration of the etchant, but will not affect performance. After this initial "run-in" phase, the anode appears to be resistant to the electrolyte and effectively oxidizes manganese(II) ions to manganese(III).

The anode can be constructed by securing the woven carbon fiber material in a suitable frame provided with electrical contacts. Carbon fiber material can also be used as the cathode in the generation of manganese(III) ions, but it is more convenient to use lead, especially if the cathode is substantially smaller than the anode if an undivided cell is used.

The applied current density in the electrolytic cell is limited in part by the oxygen overpotential across the chosen anode material. For example, in the case of platinized titanium anodes, above a current density of about 0.4 ^A /dm2, the anode potential is high enough to release oxygen. At this point, the conversion efficiency of manganese(II) ions to manganese(III) ions decreases, thus wasting any further increase in current density. Furthermore, operating the anode at a higher overpotential required to generate higher current densities tends to produce manganese dioxide rather than manganese(III) ions at the anode surface.

It has surprisingly been found that lead anodes can be effectively used in the electrolytic cells described herein. Lead becomes passivated in strong acids due to the formation of a lead sulfate layer on the surface, which has very limited solubility in sulfuric acid. This deactivates the anode until a very high overpotential is reached (more than 2 V versus a standard hydrogen electrode). At potentials above this level, a mixture of oxygen and lead dioxide is produced. Although this high operating potential is expected to favor oxygen generation and the formation of permanganate ions rather than manganese(III) ions, experiments with lead anodes produced only manganese(III) ions and no permanganate. This can be confirmed by diluting the etchant with water, and the disproportionation of manganese(III) ions produces brown manganese dioxide and manganese(II) ions. Filtration of this solution produced a practically colorless solution characteristic of manganese(II) ions rather than the purple permanganate ions.

The inventors of the present invention have found that due to the very high efficiency of these anodes for the oxidation of manganese(II) ions, when using lead anodes, the rate of oxidation needs to be monitored. Thus, if the rate of oxidation is not monitored and controlled, too high a proportion of the manganese(II) ions are oxidized, leaving very low concentrations of manganese(II). In the absence of manganese(II) ions, the anode begins to oxidize manganese(III) ions to manganese(IV), which rapidly forms insoluble manganese dioxide.

Based on the above, it is important that no more than 50% of the original concentration, and preferably no more than 25%, of the manganese(II) ions are oxidized to manganese(III) ions in order to maintain the stability of the electrolyte. In the case of lead anodes, this involves monitoring the accumulation of manganese(III) ions by titration of the etching solution or using a redox electrode and stopping the electrolysis when the manganese(III) content reaches the desired amount. At a sulfuric acid concentration of 12.5M, manganese(III) ions are required to have a concentration in excess of 0.01M for efficient etching and maximum stability and not to exceed 0.04M based on the total manganese content of 0.08M.

The anode can comprise lead or a suitable lead alloy, and the type of alloy chosen can affect conversion efficiency. Pure lead or lead containing a small proportion of tin is particularly effective and yields a conversion efficiency of about 70%. It was also found that, with a reasonable degree of agitation, surprisingly high current densities could be applied and still maintain this switching rate.

After prolonged electrolysis using a lead anode, it was found that a film of manganese dioxide eventually formed. Once a significant amount of manganese dioxide has accumulated on the electrode surface, it tends to thicken very rapidly. However, manganese dioxide can be readily electrochemically reduced to manganese(II) ions. Therefore, manganese dioxide buildup can be mitigated or eliminated by periodically reversing the current in the cell. The time period between multiple current reversals is not critical, as long as sufficient Coulombic charge is applied during the reversal phase to reduce the amount of manganese dioxide that has deposited on the surface back to manganese(II) ions.

Based on the above, when using lead and lead alloy electrodes to generate manganese (III) ions in sulfuric acid solution to achieve the purpose of etching ABS or ABS/PC, the manganese (III) ions have reached a suitable working concentration, that is, based on A total manganese content of 0.08 M. A suitable working concentration may be 0.01 to 0.04 M, at which point the electrolysis procedure is preferably interrupted so that an effective amount of manganese(II) ions is left in solution so that the bath is stable and does not deposit excess di manganese oxide. Preferred electrode materials include, for example, pure lead, lead antimony containing about 4% antimony, lead tin anodes containing up to 5% tin, and lead/tin/calcium anodes. Other suitable lead alloys may also be used in the practice of the present invention. Furthermore, the use of periodically reversed currents prevents the accumulation of manganese dioxide film at the anode. This effectively maintains the conversion efficiency of the anode and reduces or eliminates the need to remove and clean the anode from the etch tank or regeneration tank.

Furthermore, in order to efficiently generate Mn(III) ions, it is generally necessary to use a larger anode area than the cathode area. Preferably, the area ratio of anode to cathode is at least about 10:1. Thereby, the cathode can be directly immersed in the electrolyte and there is no need to have a separate cell. While the process can work using a split cell configuration, this introduces unnecessary complexity and expense.

The invention will now be illustrated with reference to the following non-limiting examples:

Comparative example 1:

A solution of 0.08 molar manganese(II) sulfate in 12.5 molar sulfuric acid (500 ml) was heated to 70°C and a piece of platable grade ABS was dipped in the solution. Even after 1 hour of immersion in this solution, the test panel showed no discernible etching, and after cleaning, the surface did not "wet" and would not support an unbroken water film.

Example 1:

The solution was electrolyzed by immersing a platinum-coated titanium anode with an area of 1 dm ² and a platinum-coated titanium cathode with a surface area of 0.01 dm ² in the solution of Comparative Example 1 and applying a current of 200 mA for 5 hours.

During this electrolysis, the solution was found to change color from almost colorless to very dark purple/red. Confirm the absence of permanganate ions.

The solution was then heated to 70°C and a piece of platable grade ABS was dipped in the solution. After soaking for 10 minutes, the test piece wets out completely and supports an unbroken water film after rinsing. After soaking for 20 minutes, the samples were rinsed in water, dried and examined with a scanning electron microscope (SEM). This inspection showed that the test piece was substantially etched and many etch pits were visible.

Example 2:

A solution containing 12.5M sulfuric acid and 0.08M manganese(II) sulfate was electrolyzed using a platinum-plated titanium anode at a current density of 0.2A/dm ² . A platinized titanium cathode with an area of less than 1% of the anode area is used to prevent cathodic reduction of manganese(III) ions generated at the anode. Electrolysis is performed long enough to deliver a sufficient amount of coulombs to oxidize all manganese(II) ions to manganese(III). The resulting solution was a deep cherry purple/red color. No permanganate ions are generated during this step. This is also confirmed by the visible spectrum, the manganese(III) ion produces an absorption spectrum quite different from that of the permanganate solution.

Example 3:

The etching solution prepared as in Example 2 above was heated to 65-70° C. on a magnetic stirrer/hot plate, and a test piece of ABS was immersed in the solution for a period of 20-30 minutes. Some of these coupons were examined by SEM, and some coupons were processed in the normal electroplating plastic pretreatment sequence (reduction in M neutralization, pre-soaking, activation, acceleration, electroless nickel, copper plating to 25-30 microns) . These coupons were then annealed and tested for peel strength using an Instron machine.

A 30 minute peel strength test was performed on the plated coupons and showed that the peel strength varied between about 1.5 and 4 N/cm.

Voltammetric cycles were obtained from a solution containing 12.5M sulfuric acid and 0.08M manganese sulfate using a platinum rotating disk electrode (RDE) with a surface area of 0.196 ^cm2 at different rotational speeds. A model 263A potentiostat and a silver/silver chloride reference electrode were used in conjunction with RDE.

In all cases, the forward scan showed a peak at about 1.6 V vs. Ag/AgCl, followed by a plateau to around 1.75 V, followed by an increase in current. The reverse scan produces a similar plateau at slightly lower currents and a peak around 1.52V. The dependence of these results on the electrode rotation speed suggests that mass transport control is a major factor in this mechanism. The plateau indicates the potential range above which manganese(III) ions are formed by electrochemical oxidation.

A constant potentiometer sweep was performed at 1.7V. It is found that the current drops initially and increases after a while. The current density at this potential varied between 0.15 and 0.4 A/dm ² .

After this experiment, a galvanostatic amount test was performed at a fixed current density of 0.3 A/dm ² . Initially, the applied current density was reached by a potential of about 1.5V, but as the experiment progressed, after about 2400 seconds, the potential was found to increase to about 1.75V.

After etching for more than 10 minutes, the surface of the ABS coupon was found to be completely wet and supported an unbroken water film after cleaning. After a period of 20 or 30 minutes, the plate was visibly etched.

Example 4:

A solution was prepared comprising 10.5M sulfuric acid and 2M methanesulfonic acid. At a temperature of 68-70°C, 0.16M manganese sulfate can be easily dissolved, but only 0.08M can be dissolved in the case of dissolving manganese sulfate in a 12.5M sulfuric acid solution for comparison. The formulated solution was electrolyzed to produce manganese(III) ions at a manganese(III) concentration of 0.015M, which provided an etch rate comparable to that obtained from a 12.5M sulfuric acid solution having a manganese(III) concentration of 0.015M.

The electrolysis in the bath of Example 4 was continued until the manganese(III) content reached 0.04M and another plate was etched. An enhanced etch rate (about 25% higher than that obtained at a concentration of 0.015M) was obtained at this higher concentration of manganese(III) ions.

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 0.08M manganese sulfate in 12.5M sulfuric acid at a temperature of 65°C. ^The cathode in this cell is a lead sheet with a nominal measuring surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, resulting in a nominal anodic current density of 0.25 A/dm ² and a nominal cathodic current density of 2.5 A/dm ² .

It was found that the graphite anode cracked and degraded rapidly within less than 1 hour of electrolysis. Furthermore, no oxidation of manganese(II) ions to manganese(III) ions was found.

Comparative example 3:

An electrode comprising a titanium substrate coated with a mixed tantalum/iridium oxide coating (50% tantalum oxide, 50% iridium oxide) and having a nominal measurement surface area of ¹ dm2 was immersed in 500 ml of 12.5M sulfuric acid containing 0.08M manganese sulfate solution. ^The cathode in this cell is a lead sheet with a nominal measuring surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, resulting in a nominal anodic current density of 0.25 A/dm ² and a nominal cathodic current density of 2.5 A/dm ² .

It was found that manganese(III) formed rapidly in this solution and that the resulting solution could etch ABS plastic and could give good adhesion when subsequently plating the treated plastic. However, after a period of two weeks of operation (electrolyzing the solution for 8 hours/day), it was found that the coating layer was wrinkled from the titanium substrate, and the titanium substrate itself was dissolved in the solution.

Comparative example 4:

An electrode comprising a platinum-coated titanium substrate with a nominal measured surface area of ¹ dm2 was immersed in 500 ml of a solution containing 0.08M manganese sulfate in 12.5M sulfuric acid at a temperature of 65 °C. ^The cathode in this cell is a lead sheet with a nominal measuring surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, resulting in a nominal anodic current density of 0.25 A/dm ² and a nominal cathodic current density of 2.5 A/dm ² .

It was found that manganese(III) was formed rapidly in this solution and that the resulting solution could etch ABS plastic and could produce good adhesion when subsequently plating the treated plastic. However, after a period of two weeks of operation (electrolyzing the solution for 8 hours/day), it was found that the coating was wrinkled from the titanium substrate and the titanium substrate itself dissolved in the solution.

Example 5:

An electrode comprising glassy carbon and having a nominal measured surface area of 0.125 dm2 was immersed in 100 ml of a solution containing 0.08M manganese ^sulfate in 12.5M sulfuric acid at a temperature of 65°C. ^The cathode in this cell is a platinum wire sheet with a nominal gauge surface area of 0.0125 dm2. A current of 0.031 amps was applied to the cell, resulting in a nominal anodic current density of 0.25 A/dm ² and a nominal cathodic current density of 2.5 A/dm ² .

It was found that manganese(III) was formed rapidly in this solution and that the resulting solution could etch ABS plastic and could produce good adhesion when subsequently plating the treated plastic. The electrode was apparently not affected by prolonged electrolysis time.

Embodiment 6:

Electrodes comprising woven carbon fiber sheets (from the company Zoltek, Panex 3550K Tow with 1.5% epoxy sizing) were fixed in a plastic frame constructed of polyvinylidene fluoride (PVDF). An electrode with a nominal measuring surface area of ¹ dm2 was immersed in 500 ml of a solution containing 0.08M manganese sulfate in 12.5M sulfuric acid at a temperature of 65 °C. ^The cathode in this cell is a lead sheet with a nominal measuring surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, resulting in a nominal anodic current density of 0.25 A/dm ² and a nominal cathodic current density of 2.5 A/dm ² .

It was found that manganese(III) was rapidly formed in this solution and that the resulting solution etched ABS plastic and gave good adhesion when subsequently plating the treated plastic. The electrode was apparently not affected by prolonged electrolysis time. The electrode was used for electrolysis for more than two weeks and no degradation was detected. The low cost and rapid availability of this material make it suitable for many commercial applications.

Embodiment 7:

An anode consisting of lead having an effective surface area of 0.4 dm2 (ie not counting the back of the electrode) was immersed in a beaker containing ² liters of a solution containing 0.08M manganese sulfate in 12.5M sulfuric acid at a temperature of 68-70°C. The other electrode in the cell consisted of a lead cathode having a surface area of approximately 0.04 dm ² . The solution was stirred using a magnetic stirrer to obtain moderate agitation on the surface of the electrolyte. A current density of 0.4 A/dm ² was applied to the anode, and the rate of manganese(III) versus electrolysis time was determined. The amount of manganese(III) was determined by diluting a sample of the bath with phosphoric acid to prevent manganese(III) disproportionation and titrating with ferrous ammonium sulfate solution using diphenylamine dissolved in acid as indicator.

Experiments were repeated using current densities of 0.8 A/dm ² and 1.6 A/dm ² . Under the experimental hydrodynamic conditions (i.e., moderate agitation using a magnetic stirrer), since the conversion efficiency is the same (70%) as that obtained at 0.4 A/dm ² , it is evident that at a current density of 1.6 A/dm ² , Oxidation is not limited by mass transport. Further experiments were performed at 3.2 A/dm ² and it was found that the conversion efficiency had dropped to 42%, while the manganese(III) production rate was only 10% higher than that obtained at 1.6 A/dm ² . This indicates that the overall limiting current density for manganese production is about 1.6 A/dm ² under the agitation conditions used in this experiment. This corresponds to a switching rate about four times as high as that achieved by a platinized titanium anode.

The results of these experiments demonstrate that manganese(III) ions can be produced by electrolysis using manganese(II) ions in relatively high concentrations of sulfuric acid and operating at low current densities with platinum or Other anode materials can further improve the method, including glassy carbon, carbon fiber, lead and lead alloy anodes.

Furthermore, the slower etch rate of the manganese-based etch of the present invention compared to that obtained from chromic acid etch has demonstrated the need to provide a pretreatment step in order to produce higher adhesion values and to enable higher short etching time.

The purpose of this pretreatment step is to condition the surface of the plastic to be etched so that it etches more quickly and uniformly, resulting in shorter etching times and better adhesion.

The use of solvents to condition the surface of ABS plastic is known. However, recent regulations have severely limited the feasibility of using volatile solvents on plating lines because they are often flammable and have health and safety concerns (many are reproductively toxic and can cause liver damage). Therefore, the choice of solvents is limited.

Propylene carbonate is a relatively safe solvent with good water solubility, low toxicity and low flammability (flash point of 135°C), and is desirable from a health and safety standpoint. Gamma-butyrolactone is also available, but is more toxic and is a regulated drug in some countries due to its recreational use.

In the present invention, it was found that better results were obtained in combination with the manganese-based etching solutions described herein when the use of propylene carbonate was combined with organic hydroxy acids such as lactic acid, glycolic acid or gluconic acid. Propylene carbonate alone or with a wetting agent provides good adhesion and reduced etch times, but after etch, activation, and subsequent plating, has a poor appearance due to the tendency of ABS/PC blends to sink . The use of propylene carbonate in combination with these hydroxyacids during this pretreatment stage effectively avoids this problem.

Typically, the concentration of propylene carbonate is from about 100 to about 500 mL/L, and the concentration of the organic acid is from about 100 to about 500 mL/L. In addition, the operating temperature is generally about 20°C to 70°C, and the soaking time is about 2 to about 10 minutes.

The present invention therefore also generally relates to a pretreatment composition for platable plastic substrates comprising gamma-butyrolactone or propylene carbonate in combination with organic hydroxyacids such as lactic acid, glycolic acid or gluconic acid .

Comparative example 5:

The test piece composed of ABS/PC blend composed of 45% polycarbonate was immersed in the solution containing 150mL/L propylene carbonate, and the immersion time and temperature are shown in Table 1. Thereafter, the plate was cleaned and etched in a solution containing 12.5M sulfuric acid and 0.08M manganese, where 0.015M manganese ions had been electrolytically oxidized to manganese(III). Etching is performed at a temperature of 68-70° C. for 30 minutes. After this treatment, the board was cleaned and the board was activated using standard plating on top of the plastic pretreatment sequence (MacDermid D34 Palladium Activator, MacDermid Accelerator and MacDermid J64 Electroless Nickel as specified in the technical data), followed by electroplating in copper . The panel was inspected for decorative appearance and quantitative adhesion testing was performed by pulling the deposited layer from the substrate using an Instron tensile testing machine. The obtained adhesion values are shown in Table 1.

Table 1. Adhesion Values

The adhesion values were quite variable, and spots and pits were found on the plated part. The copper plating is also pitted.

Embodiment 8:

The experiment performed in Comparative Example 5 was repeated except that a preconditioner comprising 150 mL/L propylene carbonate and 250 mL/L 88% lactic acid solution was used. The results of these tests are shown in Table 2.

Table 2. Adhesion Values

With this pre-conditioner containing lactic acid, the sticky consistency is improved. After plating, the decorative appearance was found to be excellent and free from blemishes and dents.

Claims (52)

1. An electrolytic cell comprising: An electrolyte solution comprising manganese(III) ions and manganese(II) ions in a solution comprising sulfuric acid and an additional acid selected from the group consisting of methanesulfonic acid, methanedisulfonic acid, and combinations thereof The group, wherein said manganese (III) ion forms a metastable complex; a cathode in contact with the electrolyte solution; and an anode in contact with the electrolyte solution. 2. The electrolytic cell of claim 1, wherein the solution comprises at least 8M sulfuric acid. 3. The electrolytic cell of claim 2, wherein the solution comprises at least 12M sulfuric acid. 4. The electrolytic cell of claim 1, wherein the solution comprises 1M to 6M methanesulfonic acid or methanedisulfonic acid. 5. The electrolytic cell of claim 1, wherein the solution comprises 9 to 15 molar sulfuric acid, and 1M to 6M methanesulfonic acid. 6. The electrolytic cell of claim 1, wherein said anode comprises a material selected from the group consisting of glassy carbon, woven carbon fiber, lead, lead alloys, platinized titanium, platinum, iridium/tantalum oxide, niobium, boron-doped diamond, and Materials in the group consisting of one or more combinations of the foregoing. 7. The electrolytic cell of claim 6, wherein the anode comprises lead or a lead alloy. 8. The electrolytic cell of claim 6, wherein the anode comprises reticulated glassy carbon. 9. The electrolytic cell of claim 1, wherein the cathode comprises a material selected from the group consisting of platinum, platinized titanium, iridium/tantalum oxide, niobium, and lead. 10. The electrolytic cell of claim 9, wherein the cathode comprises lead. 11. The electrolytic cell of claim 7, wherein said electrolytic cell further comprises means for monitoring the concentration of manganese (II) ions in said solution. 12. The electrolytic cell of claim 1, wherein the anode has a larger area than the cathode. 13. A method for the electrochemical oxidation of manganese (II) ions to manganese (III) ions, comprising the steps of: An electrolyte is provided in an electrolytic cell comprising a solution of manganese(II) ions in a solution of at least one acid and an additional acid selected from the group consisting of methanesulfonic acid, methanedisulfonic acid, and combinations thereof the group, wherein said electrolytic cell comprises an anode and a cathode, wherein both said anode and said cathode are in contact with the same electrolyte; applying a current between the anode and the cathode; and The electrolyte is oxidized to form manganese(III) ions, wherein the manganese(III) ions form metastable complexes. 14. The method of claim 13, wherein the at least one acid comprises a sulfuric acid solution. 15. The method of claim 14, wherein the at least one acid comprises sulfuric acid at a concentration of at least 8M. 16. The method of claim 15, wherein the electrolyte comprises at least 8M sulfuric acid and 1M to 6M methanesulfonic acid. 17. The method of claim 13, wherein the anode comprises lead or a lead alloy. 18. The method of claim 17, including the step of monitoring the accumulation of manganese (III) ions in said solution. 19. The method of claim 18, wherein no more than 50% of the original concentration of manganese(II) ions is oxidized to manganese(III) ions. 20. The method of claim 19, wherein no more than 25% of the original concentration of manganese(II) ions is oxidized to manganese(III) ions. 21. The method of claim 18, wherein the accumulation of manganese (III) ions is monitored using a redox electrode, wherein electrolysis is stopped when the manganese (III) ion content reaches a desired amount. 22. The method of claim 18, wherein the accumulation of manganese (III) ions is monitored by titrating the solution, wherein electrolysis is stopped when the manganese (III) ion content reaches a desired amount. 23. The method of claim 13 including periodically reversing the electrical current in the electrolytic cell, thereby preventing accumulation of manganese dioxide on the anode. 24. The method of claim 13, further comprising the step of contacting a platable plastic with said metastable complex for a period of time to etch said platable plastic. 25. The method of claim 24, wherein the pladable plastic is contacted with a pretreatment composition to condition the surface of the platable plastic prior to contacting the platable plastic with the metastable complex condition, the pretreatment composition comprises a solvent selected from the group consisting of propylene carbonate, gamma-butyrolactone, and combinations thereof. 26. The method of claim 25, wherein the solvent comprises propylene carbonate. 27. The method of claim 25, wherein the pretreatment composition further comprises an organic hydroxy acid. 28. The method of claim 27, wherein the organic hydroxy acid is selected from the group consisting of lactic acid, glycolic acid, gluconic acid, and combinations of one or more of the foregoing. 29. The method of claim 18, wherein the method further comprises monitoring the concentration of manganese (II) ions in the solution. 30. The method of claim 27, wherein the pretreatment composition is maintained at a temperature of 20 to 70°C and the platable plastic is contacted with the pretreatment composition for 2 to 10 minutes. 31. The method of claim 13, wherein the manganese (II) ions are derived from a compound selected from the group consisting of manganese sulfate, manganese carbonate, and manganese hydroxide. 32. The method of claim 13, wherein the solution further comprises colloidal manganese dioxide. 33. The method of claim 13, wherein the concentration of manganese(II) ions in the electrolyte is from 0.005 molar to saturation. 34. The method of claim 13, wherein the cathode comprises a material selected from the group consisting of platinum, platinized titanium, iridium/tantalum oxide, niobium, and lead. 35. The method of claim 34, wherein the cathode comprises lead. 36. The method of claim 34, wherein the cathode comprises platinized titanium or platinum. 37. The method of claim 13, wherein the anode has a current density of 0.1 to 0.4 ^A /dm2. 38. The method of claim 13, wherein the temperature of the electrolyte is maintained at 30°C to 80°C. 39. The method of claim 13, wherein the electrolyte does not contain any permanganate. 40. The method of claim 24, wherein the platable plastic comprises acrylonitrile-butadiene-styrene or acrylonitrile-butadiene-styrene/polycarbonate. 41. A method of etching a plastic part, said method comprising contacting said plastic part with a solution comprising manganese(III) ions and at least one acid, wherein said manganese(III) ions form a metastable complex , and wherein the at least one acid comprises methanesulfonic acid or methanedisulfonic acid. 42. The method of claim 41, wherein the at least one acid further comprises sulfuric acid. 43. The method of claim 41, wherein the acid solution comprises at least 8M sulfuric acid and 1M to 6M methanesulfonic acid. 44. The method of claim 41, wherein the plastic part comprises acrylonitrile-butadiene-styrene. 45. The method of claim 41, wherein the manganese (III) ions are generated in the solution by electrolytic oxidation of manganese (II). 46. The method of claim 45, wherein the electrolytic oxidation occurs at an anode in the solution, and the anode comprises glassy carbon, woven carbon fibers, lead or a lead alloy. 47. The method of claim 46, wherein the anode comprises reticulated glassy carbon. 48. The method of claim 41 , wherein the plastic part is contacted with a pretreatment composition to condition the plastic part prior to contacting the plastic part with the solution comprising manganese(III) ions and at least one acid. Surface conditions of plastic parts, the pretreatment composition comprising a solvent selected from the group consisting of propylene carbonate, gamma-butyrolactone, and combinations thereof. 49. The method of claim 48, wherein the solvent comprises propylene carbonate. 50. The method of claim 48, wherein the pretreatment composition further comprises an organic hydroxy acid. 51. The method of claim 50, wherein the organic hydroxy acid is selected from the group consisting of lactic acid, glycolic acid, gluconic acid, and combinations of one or more of the foregoing. 52. The method of claim 50, wherein the pretreatment composition comprises 100 to 500 mL/L of propylene carbonate and 100 to 500 mL/L of the organic hydroxy acid.