BR112015021067B1 - electrolytic cell and method of electrochemical oxidation of manganese ions (ii) to manganese ions (iii) - Google Patents

Abstract

ELECTROLYTIC GENERATION OF MANGANESE IONS (III) IN STRONG SULFURIC ACID. An electrolytic cell and a method of electrochemical oxidation of manganese(II) ions to manganese(III) ions in the electrolytic cell are described. The electrolytic cell comprises (1) an electrolyte solution of manganese(II) ions in a solution of at least one acid; (2) a cathode immersed in the electrolyte solution, and (3) an anode immersed in the electrolyte solution and separated from the cathode. Various anode materials are described, including glassy carbon, crosslinked glassy carbon, crosslinked carbon fibers, lead and lead alloy. Once the electrolyte has been oxidized to form a metastable manganese(III) ion complex, a galvanizable plastic can be contacted with the metastable complex to causticize the galvanizable plastic. Furthermore, a pre-treatment step can also be carried out on the galvanizable plastic, before contacting the galvanizable plastic with the metastable complex, to condition the plastic's surface.

Description

CROSS REFERENCE TO RELATED ORDERS

[001]This application is a continuation in part of application Serial No. 13/677,798, filed November 15, 2012, now pending, which is a continuation in part of application Serial No. 13/356,004, filed 23 of January 2012, now pending, the subject matter of which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

[002] The present invention relates, in general, to an improved process for etching galvanizable plastics, such as ABS and ABS/PC.

BACKGROUND OF THE INVENTION

[003] It is well known in the art to galvanize non-conductive substrates (i.e., plastics) with metal, for a variety of purposes. Plastic moldings are relatively inexpensive to produce and metal-galvanized plastic is used for many applications. For example, plastics galvanized with metals are used for decoration and for manufacturing electronic devices. An example of a decorative use includes auto parts, such as the finish. Examples of electronic uses include printed circuits, where metal galvanized in a selective pattern comprises the conductors of the printed circuit, and plastics galvanized with metals used for shielding against EMI. ABS resins are the most commonly galvanized plastics for decorative purposes, while phenolic and epoxy resins are the most commonly galvanized plastics for the manufacture of printed circuit boards.

[004] Galvanizing over plastic surfaces is used in the production of a variety of consumer items. Plastic moldings are relatively inexpensive to produce and galvanized plastic is used for many applications, including automotive finishing. There are many stages involved in plating plastic. The first stage involves causticizing the plastic to provide mechanical adhesion of subsequent metallic coatings and to provide a suitable surface for adsorption of the palladium catalyst, which is typically applied to catalyze the deposition of the initial metallic layer from a nickel or nickel plating process autocatalytic copper. After that, copper, nickel and/or chromium deposits can be applied.

[005] The initial causticizing of plastic components is an essential part of the overall process. However, only certain types of plastic components are suitable for galvanizing. The most common types of plastic for plating are acrylonitrile/butadiene/styrene (ABS) or a mixture of ABS and polycarbonate (ABS/PC). ABS consists of two phases. The first phase is a relatively hard phase consisting of an acrylonitrile/styrene copolymer and the second phase is a softer polybutadiene phase.

[006]Currently, this material is almost exclusively causticized using a mixture of chromic and sulfuric acids, which is highly effective as a caustic for ABS and ABS/PC. The polybutadiene phase of the plastic contains double bonds in the polymer backbone, which are oxidized by chromic acid, thereby causing the complete decomposition and dissolution of the polybutadiene phase exposed on the plastic surface, which gives an effective causticization of the plastic surface. .

[007] A problem with the traditional chromic acid causticization step is that chromic acid is a recognized carcinogen and is increasingly regulated, requiring that, whenever possible, the use of chromic acid be replaced by safer alternatives. The use of a chromic acid caustic also has well-known and serious drawbacks, including the toxicity of chromium compounds, which makes it difficult to eliminate, chromic acid residues remaining on the polymer surface that inhibit deposition without current, and the difficulty of rinsing chromic acid residues from the polymer surface after treatment. Additionally, hot hexavalent chromium sulfuric acid solutions are naturally hazardous to workers. Burns and upper respiratory bleeding are common in workers routinely involved with these chromium etching solutions. Thus, it is highly desirable that safer alternatives to acidic chromium causticizing solutions be developed.

[008] Initial attempts to replace the use of chromic acid to causticize plastics typically focused on the use of permanganate ions as an alternative to chromic acid. The use of permanganate in combination with acid is described in U.S. Patent No. 4,610,895 to Tubergen et al., which is incorporated herein by reference in its entirety. Later, the use of permanganate in combination with an ionic palladium activation stage was suggested in Pat. No. 2005/0199587 to Bengston, which is hereby incorporated by reference in its entirety. The use of acidic permanganate solutions in combination with perhalo ions (eg, perchlorate or periodate) has been described in the Pub. U.S. No. 2009/0092757 to Satou, which is incorporated herein by reference in its entirety. Finally, the use of permanganate ions in the absence of alkali metal and alkaline earth methyl cations has been described in International Pub. No. WO 2009/023628 to Enthone, which is incorporated herein by reference in its entirety.

[009] Permanganate solutions are also described in U.S. Pat. No. 3,625,758 to Stahl et al, which is incorporated herein by reference in its entirety. Stahl suggests the suitability of a bath of chromium and sulfuric acid or a permanganate solution to prepare the surface. In addition, Pat. US No. 4,948,630 to Courduvelis et al., which is incorporated herein by reference in its entirety, describes a hot alkaline permanganate solution that also contains a material, such as sodium hypochlorite, that has a greater oxidation potential. than the oxidation potential of the permannate solution. Pat. US No. 5,648,125 to Cane, which is incorporated herein by reference in its entirety, describes the use of an alkaline permanganate solution comprising potassium permanganate and sodium hydroxide, where the permanganate solution is maintained at a temperature high, ie, between about 73.9°C (165°F) and 93.3°C (200°F).

[010]As is readily seen, many causticizing solutions have been suggested as a replacement for chromic acid in processes for preparing non-conductive substrates for metallization. However, none of these processes have proven satisfactory for a variety of economic, performance and/or environmental reasons, and thus none of these processes have achieved commercial success or been accepted by the industry as a suitable replacement for chromic acid causticization. Furthermore, the stability of these permanganate-based causticizing solutions may also be unsatisfactory, resulting in the formation of manganese dioxide sludge.

[011] The tendency for permanganate-based solutions to form sediment and undergo self-decomposition has been investigated by the inventors here. Under strongly acidic conditions, permanganate ions can react with hydrogen ions to produce manganese (II) ions and water, according to the following reaction:

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

[013] Thus, 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 is electrochemically generated in situ. Compared to currently used chromic acid caustics, the poor chemical stability of acid permanganate makes it effectively worthless for large scale commercial application. Alkaline permanganate caustics are more stable, and are widely used in the printed circuit industry to causticize epoxy-based printed circuit boards, but alkaline permanganate is not an effective caustic for plastics such as ABS or ABS/ PRAÇA. Thus, manganese (VII) is unlikely to gain widespread commercial acceptance as a caustic for these materials.

[014]Attempts to causticize ABS without the use of chromic acid have included the use of electrochemically generated silver(II) or cobalt(III). Certain metals can be anodically oxidized to oxidation states that are highly oxidizing. For example, cobalt can be oxidized from cobalt(II) to cobalt(III) and silver can be oxidized from silver(I) to silver(II).

[015] However, there is currently no commercially successful caustic suitable for plastics based on permanganate (in acidic or alkaline form), or manganese in any other oxidation state or by use of other acids or oxidants.

[016]Therefore, there remains a need in the art for an improved caustic, to prepare plastic substrates, for subsequent plating, that does not contain chromic acid and that is commercially acceptable.

SUMMARY OF THE INVENTION

[017] It is an object of the invention to provide a caustic for plastic substrates that does not contain chromic acid.

[018] It is another objective of the present invention to provide a caustic for plastic substrates that is commercially acceptable.

[019] It is another objective of the present invention to provide a caustic for plastic substrates that is based on manganese ions.

[020] It is yet another objective of the present invention to provide an electrode that is suitable for use in a strong acid oxidizing electrolyte, but that is not degraded by the electrolyte.

[021] It is yet another objective of the present invention to provide an electrode suitable for the generation of manganese (III) ions in strong sulfuric acid, which is commercially acceptable.

[022] It is yet another objective of the present invention to provide an improved pre-treatment step, to condition the plastic substrate before causticizing.

[023] In one embodiment, the present invention relates, in general, 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 in methane sulfonic acid, methane disulfonic acid and combinations thereof; a cathode in contact with the electrolyte solution; and an anode in contact with the electrolyte solution.

[024] In another embodiment, the present invention relates, in general, 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, where the anode comprises a material selected from the group consisting of vitreous carbon, reticulated vitreous carbon, entangled carbon fibers, lead, lead alloy, and combinations of one or more of the preceding ones.

[025] In another embodiment, the present invention relates, in general, to a method of preparing a solution capable of etching a plastic substrate, the method comprising the steps of: providing an electrolyte comprising a solution of manganese (II) ions in a solution of at least one acid in an electrolytic cell, where the electrolytic cell comprises an anode and a cathode; and applying a current to the anode and cathode of the electrolytic cell; and oxidize the electrolyte to form manganese(III) ions, where manganese(III) ions form a metastable complex.

[026] In another embodiment, the present invention relates, in general, to electrodes that are suitable for the electrochemical oxidation of manganese (II) ions to manganese (III) ions in a strong acid solution.

[027] In another embodiment, the present invention relates, in general, to a method of electrochemical oxidation of manganese (II) ions to manganese (III) ions, comprising the steps of: providing an electrolyte comprising a solution of manganese ions (II) in a solution of at least one acid, where the at least one acid comprising sulfuric acid and an additional acid selected from the group consisting of methane sulfonic acid, methane disulfonic acid and combinations thereof, in an electrolytic cell, where the electrolytic cell comprises an anode and a cathode; apply a current between the anode and the cathode; and oxidize the electrolyte to form manganese(III) ions, where manganese(III) ions form a metastable complex.

[028] Still in another embodiment, the present invention relates, in general, to a method of etching a plastic part, the method comprising contacting the plastic part with a solution comprising manganese (III) ions and at least one acid.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES

[029] The inventors of the present invention have found that trivalent manganese can readily be produced by electrolysis, at low current density of divalent manganese ions, in a strong acid solution, preferably a strong sulfuric acid solution, more preferably a solution of sulfuric acid at least 8M. 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 causticizing ABS.

[030]Trivalent manganese is unstable and is highly oxidizing (standard redox potential 1.51 versus normal hydrogen electrode). In solution, it disproportionate very quickly to manganese dioxide and divalent manganese, through the following reaction:

[031]However, in a strong sulfuric acid solution, the trivalent manganese ion becomes metastable and forms a red-cherry purple/red colored sulfate complex. The inventors have found that this sulfate complex is a suitable medium for causticizing ABS and has many advantages over prior art chromium-free caustics.

[032] Thus, in one embodiment, the present invention relates, in general, to a method of preparing a solution capable of etching a plastic substrate, the method comprising the steps of: providing an electrolyte comprising a solution of manganese ions (II) in a solution of at least one acid in an electrolytic cell, where the electrolytic cell comprises an anode and a cathode; and applying a current to the anode and cathode of the electrolytic cell; and oxidize the electrolyte to form manganese(III) ions, where manganese(111) ions form a metastable complex.

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

[034] While it is contemplated that both phosphoric acid and acid would be suitable for the compositions of the present invention, in a preferred embodiment, the acid is sulfuric acid. At ambient temperatures, the half-life of manganese(III) ions in 7M sulfuric acid is on the order of 2 years. By comparison, the half-life of similar concentrations of manganese(III) ions in 7M phosphoric acid was around 12 days. It is suggested that the much greater 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 the sulfuric acid solution. An additional problem with the use of phosphoric acid is the limited solubility of manganese(III) phosphate. Thus, although other inorganic acids, such as phosphoric acid, can be used in the compositions of the present invention, it is generally preferred to use sulfuric acid.

[035] The incredible stability of manganese(III) ions in strong sulfuric acid provides the following advantages in use: 1) Because Mn(III) ions are formed at a low current density, the power requirements for the process are typically very low. 2) Because the anode operates at a very low current density, a small cathode in connection with the anode area can be used to prevent cathodic reduction of Mn(III) ions. This avoids the need for a split cell and makes engineering a caustic regeneration cell simpler. 3) Because the process does not produce permanganate ions, there is no possibility of producing manganese heptoxide in the solution (this is a considerable safety hazard as it is violently explosive). 4) Because of the high stability of Mn(III) ions in strong sulfuric acid, the caustic can be sold ready to use. In production, the caustic only requires a small regeneration cell next to the tank to maintain the Mn(III) content of the caustic and prevent the accumulation of Mn(II) ions. 5) Because the other causticizing processes are based on permanganate, the result of the reaction of the permanganate with the Mn(II) ions causes rapid "sediment formation" with manganese dioxide and a very short duration of the caustic. This would not be a problem with Mn(III)-based caustic (although there may be some disproportionate over time). 6) The electrolytic production of Mn(III) according to the present invention does not produce any toxic gas. Although some hydrogen can be produced at the cathode, due to low current requirements, this would be less than that produced by many galvanizing processes.

[036] As described in this document, in a preferred embodiment, the acid is sulfuric acid. The concentration of sulfuric acid is preferably at least 8 molar, more preferably between about 9 and about 15 molar. The concentration of sulfuric acid is important in the process. Below a concentration of about 9 molar the causticizing rate becomes slow and above about 14 molar the solubility of manganese ions in the solution becomes low. Additionally, very high concentrations of sulfuric acid tend to absorb moisture from the air and are dangerous to control. Thus, in a more preferred embodiment, the sulfuric acid concentration is between about 12 and 13 molar, which is diluted enough to allow safe addition of water to the caustic and strong enough to optimize the plastic's caustic rate. At this concentration of sulfuric acid, up to approximately 0.08M of manganese sulfate can be dissolved at the preferred operating temperature of the caustic. For optimal causticization, the concentration of manganese ions in solution would be as high as is feasible to achieve.

[037] Manganese(II) ions are preferably selected from the group consisting of manganese sulfate, manganese carbonate and manganese hydroxide, although other similar sources of manganese(II) ions, known in the art, would also be usable in the practice of the invention. The concentration of manganese(II) ions 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 degree as a natural result of the disproportionation of manganese(III) in solution, or it can be added deliberately.

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

[039]As described in this document, to obtain fast caustic rates on ABS plastic, it is necessary to use a high concentration of acid. The presence of sulfate or bisulfate ions is required to form a complex with the manganese ions and a molar concentration of sulfuric acid of at least 8M is required to obtain good caustic stability. For good plastic causticization, it has been found that a sulfuric acid concentration of at least about 12M is required for rapid causticization. This has the effect of reducing the solubility of the manganese ions in the bath and the maximum solubility of the manganese ions in the bath at the operating temperature is about 0.08M. Because the causticizing rate depends on the concentration of manganese(III) ions in solution and the maximum conversion percentage to maintain stability is about 50%, it would be desirable to increase the amount of manganese that can be dissolved in the bath.

[040]The inventors discovered that it is possible to increase the amount of manganese that can be dissolved in the bath, replacing a part of the sulfuric acid with another acid in which the manganese ions can be more soluble.

[041]The choice of acids that may be suitable is limited. For example, hydrochloric acid would produce chlorine at the anodes and nitric acid would produce nitric oxides at the cathode. Perchloric and periodic acids would be expected to generate permanganate ions which would decompose to manganese dioxide. Organic acids would generally be rapidly oxidized by manganese(III) ions. Thus, the acids that would have both the necessary stability to oxidation and the ability to increase the solubility of manganese ions in the bath are methanesulfonic acid and methane disulfonic acid. Since manganese(II) solubility is much better in methane sulfonic acid (and sulfuric acid) than it is in methane disulfonic acid, first choices yield better performance. Thus, methane sulfonic acid is the preferred additional acid and sulfuric acid is the preferred primary acid.

[042] Based on this, the present invention also refers, in general, to an electrolyte for causticizing ABS and ABS/PC plastics, comprising sulfuric acid in combination with methane sulfonic acid or methane disulfonic acid, to obtain better solubility of manganese ions in the bath, where the electrolyte contains at least 8M sulfuric acid and contains about 0M to about 6M methanesulfonic acid or methane disulfonic acid, preferably from about 1M to about 6M of methane sulfonic acid.

[043] More particularly, the present invention relates, in general, 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 methane sulfonic acid, methane disulfonic acid and combinations thereof; a cathode in contact with the electrolyte solution; and an anode in contact with the electrolyte solution.

[044] In addition, the present invention also relates, in general, 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, where the anode comprises a material selected from the group consisting of glassy carbon, crosslinked glassy carbon, entangled carbon fibers, lead, lead alloy, and combinations of one or more of the foregoing .

[045] In addition, the present invention also relates, in general, to a method of electrochemical oxidation of manganese (II) ions to manganese (III) ions, comprising the steps of: providing an electrolyte comprising a solution of manganese ions ( II) in at least one acid in an electrolytic cell, where the electrolytic cell comprises an anode and a cathode; apply a current between the anode and the cathode; and oxidize the electrolyte to form manganese(III) ions, where manganese(III) ions form a metastable complex.

[046]Once the electrolyte has been oxidized to form a metastable complex, the galvanizable plastic can be immersed in the metastable complex for a period of time to causticize the surface of the galvanizable plastic. In one embodiment, the galvanizable plastic is immersed in the metastable complex at a temperature of between 30 and 80°C. The caustic rate increases with temperature and is slow below 50°C

[047]The upper temperature limit is determined by the nature of the plastic being etched. ABS starts to deform above 70°C, so, in a preferred embodiment, the electrolyte temperature is maintained between about 50 and about 70°C, especially when causticizing ABS materials. The time period for immersing the plastic in the electrolyte is preferably between about 10 to about 30 minutes.

[048] Articles etched in this mode can be subsequently galvanized, using conventional pretreatment for galvanized plastics, or the etched surface of the plastic can be used to enhance the adhesion of paint, varnishes or other surface coatings.

[049]The concentration of manganese (II) ions used in the caustic of this invention can be determined by means of cyclic voltammetry. Oxidation is diffusion controlled, so efficient stirring of the caustic solution is required during the electrolytic oxidation process.

[050]The anode and cathode usable in the electrolytic cell described in this document can comprise several materials. The cathode may comprise a material selected from the group consisting of platinum, platinum titanium, niobium, iridium oxide coated titanium, and lead. In a preferred embodiment, the cathode comprises platinum or platinum titanium. In another preferred embodiment, the cathode comprises lead. The anode may also comprise platinum titanium, platinum, iridium/tantalum oxide, niobium, boron doped diamond, or any other suitable material.

[051] The inventors have found that while the combination of manganese(III) ions and strong sulfuric acid (ie, 8-15 molar) can causticize ABS plastic, caustic is also very aggressive towards the electrodes needed to produce the ions manganese (III). In particular, anodes having a titanium substrate can be rapidly degraded by the caustic.

[052] Therefore, in an attempt to determine a more suitable electrode material, several other electrode materials were examined, including lead and graphite. It was determined that the glassy carbon and the crosslinked glassy carbon were stronger and capable of producing manganese(III) ions when an electric current, preferably between 0.1 and 0.4 A/dm2 was applied (based on the nominal surface area). Furthermore, because vitreous carbon and crosslinked vitreous carbon may not be economical to use as the electrode in commercial applications, the anode can also be manufactured from woven carbon fiber.

[053]Carbon fiber is manufactured from polyacrylonitrile (PAN) fibers. These fibers undergo an oxidation process at increasing temperatures, followed by a carbonization step at a much higher temperature, in an inert atmosphere. The carbon fibers are then woven into a sheet, which is typically used in combination with various resin systems to produce high strength components. Carbon fiber sheets also have good electrical conductivity and the fibers typically have a turbostratic structure (i.e., disordered layer). Without wishing to be bound by theory, it is believed that it is this structure that makes carbon fibers effective as an electrode. The SP2 hybridized carbon atoms in the lattice give good electrical conductivity, while the SP3 hybridized carbon atoms bind the graphite layers together, fixing them in place and thus providing good chemical resistance.

[054] 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 control and the interweaving process, carbon fibers are typically treated with an epoxy resin and this can comprise up to 2% of the fiber weight. At this low percentage, when used as an electrode, the epoxy treatment is quickly removed by the high sulfuric acid content of the caustic. This can cause a slight initial discoloration of the caustic, but it does not affect performance. After this initial "insertion" stage, the anode appears to be resistant to the electrolyte and is effective in oxidizing manganese(II) ions to manganese(III).

[055] Anodes can be constructed by mounting the woven carbon fiber material in a suitable structure, with conditions made for electrical contact. It is also possible to use carbon fiber as a cathode in the generation of manganese(III) ions, but it is more convenient to use lead, particularly since the cathode is much smaller than the anode if an undivided cell is used.

[056]The current density that can be applied to the electrolytic cell is limited, in part, by the overvoltage of oxygen over the chosen anode material. As an example, in the case of platinum titanium anodes, above a current density of approximately 0.4 A/dm2, the anode potential is high enough to release oxygen. At this point, the efficiency of converting manganese(III) ions to manganese(II) ions drops and thus any further increase in current density is wasted. Furthermore, operating the anodes at the highest overvoltage required to produce the highest current density tends to produce manganese dioxide on the surface of the anode, rather than manganese (III) ions.

[057] It has surprisingly been found that lead anodes can be effectively used in the electrolytic cell described in this document. Lead becomes passive in strong sulfuric acid due to the formation of a layer of lead sulfate on the surface, which has very limited solubility in sulfuric acid. This makes the anode passive until a very high overvoltage is reached (more than 2V versus a standard hydrogen electrode). At potentials above this level, a mixture of oxygen and lead dioxide is produced. Although such high operating potential was expected to favor oxygen production and the formation of permanganate ions over manganese(III) ions, experiments using a lead anode produced only manganese(III) ions and no permanganate. This can be verified by diluting the caustic with water - the manganese(III) ions disproportionate, producing brown manganese dioxide and manganese(II) ions. Filtering the solution produces a virtually colorless solution, characteristic of manganese(II) ions, rather than the purple color of permanganate ions.

[058] The inventors of the present invention found that monitoring the oxidation rate is necessary when lead anodes are used, because of the very high efficiency of these anodes for the oxidation of manganese (II) ions. Thus, if the oxidation rate is not monitored and controlled, a very high proportion of manganese(II) ions are oxidized, leaving a very low concentration of manganese(II). In the absence of manganese(II) ions, the anode begins to oxidize manganese(III) ions to manganese(IV), which quickly forms insoluble manganese dioxide.

[059] On this basis, it is important that no more than 50%, and preferably no more than 25%, of the original concentration of manganese (II) ions are oxidized to manganese (III) ions, 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 caustic solution or using a redox electrode and stopping the electrolysis when the manganese(III) content reaches the desired level. At a sulfuric acid concentration of 12.5M, it is necessary to have a concentration of more than 0.01M manganese(III) ions for effective causticization and maximum stability, and no more than 0.04M, based on a total manganese content of 0.08M.

[060]Anodes can comprise lead or a suitable lead alloy, and the type of alloy chosen can affect the conversion efficiency. Pure lead or lead containing a small percentage of tin is particularly effective and produces conversion efficiencies of approximately 70%. It has also been found that with a reasonable degree of agitation, surprisingly high current densities can be applied and still maintain this conversion rate.

[061]With prolonged electrolysis using a lead anode, it was observed that, in the end, a manganese dioxide film was formed. Once a significant amount of manganese dioxide has nucleated on the electrode surface, it tends to thicken very quickly. However, manganese dioxide is readily electrochemically reduced back to manganese(II) ions. In this way, manganese dioxide buildup can be lessened or eliminated by the process of reversing the cell current periodically. The time period between current inversions is not critical, provided sufficient coulombic charge is applied during the inversion phase to reduce the amount of manganese dioxide that has deposited on the surface back to manganese(II) ions.

[062] Based on this, when using lead and lead alloy electrodes for the purpose of generating manganese (III) ions in sulfuric acid solutions, for the purpose of causticizing ABS or ABS/PC, the process of electrolysis is preferably stopped when the manganese(III) ions have reached a suitable working concentration, which can be between 0.01 and 0.04M, based on the total manganese content of 0.08M, such that an amount The effective amount of manganese(II) ions remains in solution so that the bath is stable and does not precipitate excessive amounts of manganese dioxide. Preferred electrode materials include, for example, pure lead, antimony lead containing about 4% antimony, tin lead anodes containing up to 5% tin, and lead/tin/calcium anodes. Other suitable lead alloys can also be used in the practice of the invention. In addition, the use of reverse current periodically prevents manganese dioxide films from accumulating on the anode. This is useful for maintaining anode conversion efficiency and reducing or eliminating the need to remove and clean anodes from the caustic tank or regeneration cell.

[063] In addition, for the efficient generation of manganese (III) ions, it is, in general, necessary to use an area of the anode that is large compared to the area of the cathode. Preferably, the anode to cathode area ratio is at least about 10:1. By this means, the cathode can be immersed directly into the electrolyte and it is not necessary to have a split cell. Although the process would work with a split-cell arrangement, this would introduce unnecessary complexity and cost.

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

COMPARATIVE EXAMPLE 1:

[065] A solution of 0.08 molar manganese(II) sulphate in 12.5 molar sulfuric acid (500 ml) was heated to 70°C and a piece of galvanizable grade ABS was immersed in the solution. Even after an hour immersed in this solution, there was no noticeable causticization of the test panel and, with rinsing, the surface was not "wet" and would not withstand a continuous film of water.

EXAMPLE 1:

[066] The solution of Comparative Example 1 was electrolyzed by immersing a platinum titanium anode of an area of 1 dm2 and a platinum titanium cathode of a surface area of 0.01 dm2 in the solution and applying a current of 200 mA for 5 hours.

[067]During this period of electrolysis, the solution was observed to change in color from almost colorless to a very intense purple/red color. It was confirmed that no permanganate ions were present.

[068]This solution was then heated to 70°C and a piece of galvanizable grade ABS was immersed in the solution. After 10 minutes of immersion, the test piece was completely wet and would support a continuous film of water, after rinsing. After 20 minutes of immersion, the sample was rinsed in water, dried and examined using a scanning electron microscope (SEM). This examination revealed that the test piece was substantially etched and that many etched holes were visible.

EXAMPLE 2:

[069] A solution containing 12.5 M sulfuric acid and 0.08 M manganese (II) sulfate was electrolyzed using a platinum titanium anode at a current density of 0.2 A/dm2. A platinum titanium cathode, having an area of less than 1% of the anode area, was used to prevent cathodic reduction of the Mn(III) ions produced at the anode. Electrolysis was carried out for a period sufficient for enough coulombs to be passed through to oxidize all manganese(II) ions to manganese(III). The resulting solution was a purple/red cherry red color. There were no permanganate ions generated during this step. 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:

[070] The caustic solution prepared as described above in Example 2 was heated to 65-70°C on a magnetic stirrer/hot plate and the ABS test samples were immersed in the solution for periods of time of 20 and 30 minutes. Some of these test samples were examined by SEM and some were processed in a normal plating, following plastic pretreatment (reduction in M-neutralize, pre-dip, activate, accelerate, current-free nickel, copper plate to 25 -30 microns). These test samples were then annealed and subjected to peel strength testing using an Instron machine.

[071] The peel strength test performed on the test samples, galvanized for 30 minutes, demonstrated a peel strength ranging between about 1.5 and 4 N/cm.

[072] Cyclic voltammograms were obtained from a solution containing 12.5M sulfuric acid and 0.08M manganese sulfate, using a platinum rotating disk (RDE) electrode, having a surface area of 0.196 cm2 in different rotation speeds. A Model 263A potentiostat and a silver/silver chloride reference electrode were used in combination with the RDE.

[073] In all cases, the advanced scan showed a peak around 1.6V vs. Ag/AgCl, followed by a plateau to about 1.75V, followed by an increase in current. The reverse scan produced a similar plateau (at a slightly lower current and peak around 1.52V). The dependence of these results on the electrode rotation rate indicates that mass transport control is a major factor in the mechanism. The threshold indicates the range of potentials above which Mn(III) ions are formed by electrochemical oxidation.

[074]The potentiostatic scan was performed at 1.7V. It was observed that the current initially dropped and then, over a period of time, increased. The current density at this potential ranged between 0.15 and 0.4 A/dm2.

[075]After this experiment, a galvanostatic measurement was obtained at a constant current density of 0.3 A/dm2. Initially, the applied current density was obtained by a potential of about 1.5V, however, as the experiment progressed, after about 2400 seconds, an increase in potential to about 1.75V was observed.

[076]After a causticizing period of more than 10 minutes, it was observed that the surface of the ABS test samples was completely wet and would support a continuous film of water after rinsing. After a period of 20 or 30 minutes, the panels were visibly etched.

EXAMPLE 4:

[077]A solution was formulated comprising 10.5M sulfuric acid and 2M methane sulfonic acid. At a temperature of 68-70°C, it was possible to dissolve 0.16M of manganous sulphate easily, whereas in the comparative case of dissolving the manganous sulphate in a solution of 12.5M of sulfuric acid, it was only possible to dissolve 0.08M. The formulated solution was electrolyzed to produce a manganese(III) concentration of 0.015M manganese(III) ions, which gave a caustic rate comparable to that obtained from a 12.5M sulfuric acid solution having a manganese concentration ( III) of 0.015M.

[078] Electrolysis was continued in the bath of Example 4 until the manganese (III) content reached 0.04M and another panel was causticized. An increased causticizing rate was obtained at this higher concentration of manganese(III) ions (approximately 25% greater than that obtained at a concentration of 0.015M).

COMPARATIVE EXAMPLE 2:

[079] An electrolyte comprising graphite and having a nominal measured surface 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° Ç. The cathode in this cell was a piece of lead having a nominal measured surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.

[080]It was observed that the graphite anode quickly crumbled and degraded within less than 1 hour of electrolysis. Furthermore, no oxidation of manganese(II) ions to manganese(III) was observed.

COMPARATIVE EXAMPLE 3:

[081] An electrode comprising a titanium substrate coating with a tantalum oxide/mixed iridium coating (50% tantalum oxide, 50% iridium oxide), and having a nominal measured surface area of 1 dm2, was immersed in 500 ml of a solution containing 0.08 M of manganese sulfate in 12.5 M of sulfuric acid, at a temperature of 65°C. The cathode in this cell was a piece of lead having a nominal measured surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.

[082] It was observed that manganese(III) was quickly formed in the solution and the resulting solution was able to causticize the ABS plastic and produce good adhesion with the subsequent galvanizing 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 suspending from the titanium substrate and that the titanium substrate itself was dissolving into the solution.

COMPARATIVE EXAMPLE 4:

[083] An electrode comprising a platinum-coated titanium substrate and having a nominal measured surface area of 1 dm2 was immersed in 500 mL of a solution containing 0.08 M manganese sulfate in 12.5 M sulfuric acid. rich, at a temperature of 65°C. The cathode in this cell was a piece of lead having a nominal measured surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.

[084] It was observed that manganese(III) was quickly formed in the solution and the resulting solution was able to causticize the ABS plastic and produce good adhesion with the subsequent galvanizing 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 suspending from the titanium substrate and that the titanium substrate itself was dissolving into the solution.

EXAMPLE 5:

[085] 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 0.08 M manganese sulfate in 12.5 M sulfuric acid, at a temperature of 65°C. The cathode in this cell was a piece of platinum wire having a nominal measured surface area of 0.0125 dm2. A current of 0.031 amps was applied to the cell, giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.

[086] It was observed that manganese(III) was quickly formed in the solution and the resulting solution was able to causticize the ABS plastic and produce good adhesion by subsequently galvanizing the treated plastic. The electrode appeared unaffected by periods of prolonged electrolysis.

EXAMPLE 6:

[087]An electrode comprising a piece of braided carbon fiber (Panex 35 50K Tow, 1.5% epoxy treated, available from Zoltek Corporation) was mounted on a plastic frame constructed of poly(vinylidene fluoride) (PVDF). The electrode, having a nominal measured area of 1 dm2, was immersed in 500 mL of a solution containing 0.08 M of manganese sulfate in 12.5 M of sulfuric acid, at a temperature of 65°C. The cathode in this cell was a piece of lead having a nominal measured surface area of 0.1 dm2. A current of 0.25 amps was applied to the cell, giving a nominal anode current density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.

[088] It was observed that manganese(III) was quickly formed in the solution and the resulting solution was able to causticize the ABS plastic and produce good adhesion with the subsequent galvanizing of the treated plastic. The electrode appeared unaffected by periods of prolonged electrolysis. Electrolysis was carried out for two weeks using this electrode and no observable degradation could be detected. The low cost and ready availability of this material make it suitable for many commercial applications.

EXAMPLE 7:

[089] An anode consisting of lead, having an effective surface area (ie, not counting the back side of the electrode) of 0.4 dm2, was immersed in a beaker containing 2 liters of a solution comprising 0.08M of 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 dm2. The solution was stirred using a magnetic stirrer to obtain moderate stirring on the electrolyte surface. A current density of 0.4 A/dm2 was applied to the anode and the manganese (III) ratio was determined versus the electrolysis time. The amount of manganese (III) was determined by diluting a sample from the bath with phosphoric acid, to prevent disproportionation of the manganese (III), and titrating with a solution of ferrous ammonium sulfate, using diphenylamine dissolved in acid as a indicator.

[090]The experiment was repeated using a current density of 0.8 A/dm2 and 1.6 A/dm2. Under the hydrodynamic conditions of the experiment (ie, moderate agitation using a magnetic stirrer), the oxidation did not appear to be limited by mass transport at a current density of 1.6 A/dm2, since the conversion efficiency was the same as that obtained at 0.4 A/dm2 (70%). An additional experiment was conducted at 3.2 A/dm2 and it was found that the conversion efficiency had dropped to 42% and the manganese(III) generation rate was only about 10% higher than that obtained at 1, 6 A/dm2. This indicates that, under the agitation conditions used in the experiment, the global limiting current density for manganese generation was about 1.6 A/dm2. This corresponds to a conversion rate approximately four times greater than what can be obtained from a platinum titanium anode.

[091] 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 an anode platinum or platinum titanium, and that further process improvements can be achieved using a variety of other anode materials, including glassy carbon, carbon fiber, lead, and lead alloy anodes.

[092] Furthermore, the slower caustic rate of the manganese-based caustic of the invention, compared to the caustic rate obtained from a chromic acid caustic, demonstrated a need to provide a pretreatment step to produce higher values adhesion and allow for shorter causticizing times.

[093] The purpose of the pretreatment step is to condition the surface of the plastic to be etched so that it is etched more quickly and evenly, leading to shorter etched times and better adhesion.

[094]The use of solvents to condition the surface of ABS plastics is known. However, recent regulations severely limit the feasibility of using volatile solvents over a galvanizing line, as they are often flammable and have health and safety concerns (many are reprotoxic and can also cause liver damage). Therefore, the choice of solvents is limited.

[095]Propylene carbonate is a relatively safe solvent, having good solubility in water, low toxicity and low flammability (the flash point is 135°C) and is ideal from a health and safety standpoint. Gamma butyrolactone also works well, but it is more toxic and, in some countries, it is a controlled drug because of its use for pleasure.

[096] In the present invention it has been found that better results can be obtained in combination with the manganese-based caustic solutions described herein when the use of propylene carbonate is combined with an organic hydroxy acid such as acid lactic acid, glycolic acid or glycolic acid. The use of propylene carbonate alone or with a wetting agent gives good adhesion and reduced caustic times, but the cosmetic appearance of ABS/PC blends is unsatisfactory after causticizing, activation and subsequent galvanizing because it is subject to formation of holes. The combination of propylene carbonate with these hydroxy acids, in the pre-treatment stage, is effective in preventing this problem.

[097] Typically, the concentration of propylene carbonate is between about 100 and about 500 mL/L and the concentration of organic acid is between about 100 and about 500 mL/L. Furthermore, the operating temperature is typically between about 20° and 70°C and the immersion time is between about 2 and about 10 minutes.

[098] Thus, the present invention also relates, in general, to a pretreatment composition for the galvanizable plastic substrate, comprising the Gamma butyrolactone or the propylene carbonate in combination with an organic acid hydroxy, such as lactic acid, glycolic acid or glycolic acid.

COMPARATIVE EXAMPLE 5:

[099] A test sample composed of an ABS/PC blend consisting of 45% polycarbonate was immersed in a solution containing 150 mL/L of propylene carbonates, for the times and temperature shown in Table 1. Thereafter, the panel was rinsed and causticized in a solution containing 12.5M sulfuric acid and 0.08M manganese, where 0.015M of the manganese ions had been electrolytically oxidized to manganese(III). Caustication was carried out for 30 minutes at a temperature of 68-70°C. After this treatment, the panel was rinsed, activated using a standard galvanizing, following pre-treatment of plastics (palladium activator MacDermid D37, MacDermid accelerator and MacDermid J64 uncurrent nickel, according to the technical data sheets), and then galvanized to copper. The cosmetic appearance of the panels was examined and a quantitative adhesion test was carried out by removing the deposit from the substrate using an Instron tensile testing machine. The adhesion values obtained are shown in Table 1.

[0100]The adhesion values are quite variable and it was observed that stains and holes were observed on the galvanized parts. The copper cladding was also pitted.

EXAMPLE 8:

[0101] The experiments performed in Comparative Example 5 were repeated, but using a preconditioner comprising 150 mL/L of propylene carbonate and 250 mL/L of 88% lactic acid solution. These results of these tests are shown in Table 2.

[0102] The use of this pre-conditioner, which included lactic acid, gave improvements in the consistency of adhesion. After galvanizing, it was observed that the cosmetic appearance was excellent, being free from stains and the formation of holes.

Claims (11)

1. Electrolytic cell CHARACTERIZED by the fact that it comprises: 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 methane sulfonic acid, acid disulfonic methane and its combinations; wherein the solution comprises at least 8M sulfuric acid; a cathode in contact with the electrolyte solution; and an anode in contact with the electrolyte solution. 2. Electrolytic cell according to claim 1, CHARACTERIZED by the fact that the solution optionally comprises sulfuric acid of at least 12M, and/or in which the solution comprises between 1M and 6M of methane sulphonic acid or acid disulfonic methane, and/or wherein the solution comprises from 9 to 15 molar sulfuric acid and between 1M and 6M methanesulfonic acid. 3. Electrolytic cell according to claim 1, CHARACTERIZED by the fact that an area of the anode is larger than an area of the cathode and/or that the anode comprises a material selected from the group consisting of vitreous carbon, crosslinked vitreous carbon, crosslinked carbon fibers, lead, lead alloy, platinum titanium, platinum, iridium/tantalum oxide, niobium, boron doped diamond and combinations of one or more of the foregoing, with lead or lead alloy being preferred; wherein the anode also comprises lead or lead alloy, the electrolytic cell optionally also comprising a means for monitoring the concentration of Mn(II) in the solution. 4. Electrolytic cell, according to claim 1, CHARACTERIZED by the fact that the cathode comprises a material selected from the group consisting of platinum, platinum titanium, iridium/tantalum oxide, niobium and lead, with lead being preferable; wherein the cathode comprises lead or wherein the cathode comprises platinum or platinum titanium. 5. Method of electrochemical oxidation of manganese (II) ions to manganese (III) ions CHARACTERIZED by the fact that it comprises the steps of: providing an electrolyte comprising a solution of manganese (II) ions in a solution of sulfuric acid in fur concentration minus 8M, and an additional acid selected from the group consisting of methane sulphonic acid, methane disulfonic acid and their combinations in an electrolytic cell, where the electrolytic cell comprises an anode and a cathode; apply a current between the anode and the cathode; and oxidize the electrolyte to form manganese(III) ions, where manganese(III) ions form a metastable complex. 6. Method according to claim 5, CHARACTERIZED by the fact that the electrolyte comprises between 1M and 6M of methane sulfonic acid. 7. Method according to claim 5, CHARACTERIZED by the fact that the anode comprises lead or lead alloy and the method optionally further comprises the step of monitoring the accumulation of manganese (III) ions in the solution, in which further optionally when the method comprises the step of monitoring the accumulation of manganese(III) ions in the solution, optionally, no more than 50% of the original concentration of manganese(II) ions are oxidized to manganese(III) ions, preferably no more than that 25% of the original concentration of manganese(II) ions is oxidized to manganese(III) ions; and/or even more optionally, when the method comprises the step of monitoring the accumulation of manganese(III) ions in the solution, the accumulation of manganese(III) ions is optionally monitored using a redox electrode, in which the electrolysis is stopped when the manganese(III) content reaches the desired level; and/or even more optionally, when the method comprises the step of monitoring the accumulation of manganese (III) ions in the solution, the accumulation of manganese (III) ions is optionally monitored by titration of the caustic solution, in which electrolysis is stopped when the manganese(III) content reaches the desired level; and/or optionally additionally, when the method comprises the step of monitoring the accumulation of manganese(III) ions in the solution, the method further comprises monitoring the concentration of manganese(II) ions in the solution. 8. Method according to claim 5, CHARACTERIZED by the fact that it comprises the step of periodically reversing the current in the electrolytic cell, so that the accumulation of manganese dioxide on the anode is prevented. 9. Method according to claim 5, CHARACTERIZED in that it further comprises the step of contacting a galvanizable plastic with the metastable complex for a period of time to causticize the galvanizable plastic, wherein optionally the galvanizable plastic comprises acrylonitrile-butadiene - styrene or acrylonitrile-butadiene-styrene/polycarbonate, and/or in which, before contacting the galvanizable plastic with the metastable complex, the galvanizable plastic is contacted with a pre-treatment composition to condition the surface of the galvanizable plastic, the pretreatment composition comprising a solvent selected from the group consisting of propylene carbonate, gamma butyrolactone, and combinations thereof, preferably being propylene carbonate as solvent, optionally wherein the pretreatment solution further comprises an acid hydroxy organic, preferably wherein the hydroxy organic acid is selected from the group containing it consists of lactic acid, glycolic acid, glycolic acid, and combinations of one or more of the foregoing; additionally optionally wherein when the pretreatment solution comprises an organic hydroxy acid, the pretreatment composition is maintained at a temperature between 20 to 70°C and the galvanized plastic is contacted with the pretreatment composition by 2 to 10 minutes. 10. Method according to claim 5, CHARACTERIZED by the fact that the cathode comprises a material selected from the group consisting of platinum, platinum titanium, iridium/tantalum oxide, niobium and lead, optionally wherein the cathode comprises lead and/or the cathode comprises platinum titanium or platinum. 11. Method according to claim 5, CHARACTERIZED by the fact that the anode current density is between 0.1 and 0.4 A/dm2 and/or in which the electrolyte temperature is maintained between 30°C and 80°C.