Etching of plastic using acidic solutions containing trivalent manganese

Abstract

A method of preparing a solution capable of etching a platable plastic. The method comprises the steps of: (a) providing an electrolyte comprising a solution of manganese(II) in a solution of 9 to 15 molar sulfuric acid or phosphoric acid to an electrolytic cell; (b) applying a current to the electrolytic cell, wherein the electrolytic cell comprises an anode and a cathode; and (c) oxidizing the electrolyte to form manganese(III) ions, wherein the manganese(III) ions form a metastable sulfate complex. Thereafter, a platable plastic may be immersed in the metastable sulfate complex for a period of time to etch the platable substrate prior to subsequent plating steps.

Claims

1. A method of preparing a solution capable of etching a platable plastic, the method comprising the steps of: providing an electrolyte comprising manganese(II) ions in a solution of about 9 to 15 molar sulfuric acid or phosphoric acid to an electrolytic cell; applying a current to the electrolytic cell, wherein the electrolytic cell comprises an anode and a cathode; and oxidizing the electrolyte to form manganese(III) ions, wherein the manganese(III) ions form a metastable sulfate or phosphate complex, wherein the solution additionally comprises colloidal manganese dioxide.

2. The method according to claim 1, wherein the acid is sulfuric acid.

3. The method according to claim 2, wherein the sulfuric acid has a concentration of 12 to 13 molar.

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

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

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

7. The method according to claim 6, wherein the cathode is platinized titanium or platinum.

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

9. The method according to claim 8, wherein the area ratio of the anode to the cathode is at least about 10:1.

10. The method according to claim 1, wherein the anode current density is between about 0.1 to about 0.4 A/dm.sup.2.

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

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

13. A method of preparing a solution capable of etching a platable plastic, the method comprising the steps of: providing an electrolyte comprising manganese(II) ions in a solution of about 9 to 15 molar sulfuric acid or phosphoric acid to an electrolytic cell; applying a current to the electrolytic cell, wherein the electrolytic cell comprises an anode and a cathode; oxidizing the electrolyte to form manganese(III) ions, wherein the manganese(III) ions form a metastable sulfate or phosphate complex, and immersing the platable plastic in the metastable sulfate complex for a period of time to etch the platable substrate; wherein the platable plastic is immersed in the metastable sulfate complex for about 20 to about 30 minutes.

14. The method according to claim 13, wherein the platable plastic comprises acrylonitrile-butadiene-styrene or acrylonitrile-butadiene-styrene/polycarbonate.

15. An electrolyte capable of etching a platable plastic, the electrolyte comprising manganese(III) ions in a solution of 9 to 15 molar sulfuric acid or phosphoric acid, wherein the electrolyte additionally comprises colloidal manganese dioxide.

16. The electrolyte according to claim 15, wherein the acid is sulfuric acid.

17. The electrolyte according to claim 16, wherein the sulfuric acid has a concentration of 12 to 13 molar.

18. The electrolyte according to claim 15, wherein the concentration of the manganese(III) ions in the electrolyte is between about 0.005 molar and saturation.

19. The electrolyte according to claim 15, wherein the electrolyte does not contain any permanganate.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) The inventors of the present invention have found that trivalent manganese can readily he produced by electrolysis at low current density of divalent manganese ions in strong sulfuric acid, More particularly, the inventors of the present invention have discovered that a solution of trivalent manganese ions in strongly acidic solution is capable of etching ABS.

(2) Trivalent manganese is unstable and is highly oxidizing (standard redox potential of 1.51 versus normal hydrogen electrode). In solution, it very rapidly disproportionates to manganese dioxide and divalent manganese via the following reaction:
2Mn.sup.3++2H.sub.2O.fwdarw.MnO.sub.2+Mn.sup.2++4H+(3)

(3) However, in a strong sulfuric acid solution, the trivalent manganese ion becomes meta-stable and forms a cherry purple/red colored sulfate complex. The inventors have found that this sulfate complex is a suitable medium for the etching of ABS and has many advantages over chromium-free etches previously described.

(4) In one embodiment, the present invention relates generally to a method of preparing a solution capable of etching a plastic substrate, the method comprising the steps of;

(5) providing an electrolyte comprising manganese(II) ions in a solution of 9 to 15 molar sulfuric acid or phosphoric acid to an electrolytic cell;

(6) applying a current to the electrolytic cell, wherein the electrolytic cell comprises an anode and a cathode;

(7) oxidizing the electrolyte to form manganese(III) ions, wherein the manganese(III) ions form a metastable sulfate complex; and

(8) treating the plastic substrate with the electrolyte to etch the surface of the plastic substrate.

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

(10) While it is contemplated that both phosphoric acid and sulfuric acid would work in compositions of the present invention, in a preferred embodiment, the acid is sulfuric acid. The stability of manganese (III) ions in sulfuric and phosphoric acids has been studied. At ambient temperatures, the half life of the manganese (III) ions in 7M sulfuric acid is on of the order of 2 years. By comparison, the half life of similar concentrations of manganese (III) ions in 7M phosphoric acid was around 12 days. It is suggested that the much higher stability of the manganese (III) ions in sulfuric acid is due to the formation of mangano-sulfate complexes and the higher concentration of available hydrogen ion concentration in the sulfuric acid solution. A further problem with the use of phosphoric acid is the limited solubility of manganese (III) phosphate. Thus, although other inorganic acids such as phosphoric acid can be usable in the compositions of the present invention, it is generally preferred to use sulfuric acid.

(11) The remarkable stability of manganese (III) ions in strong sulfuric acid provides the following advantages in use: 1) Because the Mn(III) ions are formed at a low current density, the power requirements for the process are typically very low. 2) Because the anode operates at a very low current density, a small cathode in relationship to the anode area can be used to prevent cathodic reduction of the Mn(III) ions. This obviates the need for a divided cell and makes the engineering of an etchant regeneration cell simpler. 3) Because the process does not produce permanganate ions, there is no possibility of producing manganese heptoxide in the solution (this is a considerable safety hazard as it is violently explosive). 4) Because of the high stability of the Mn(III) ions in strong sulfuric acid, the etchant can be sold ready for use. In production, the etchant requires only a small regeneration cell at the side of the tank in order to maintain the Mn(III) content of the etch and prevent the build-up of Mn(II) ions. 5) Because other etch processes are based on permanganate, the result of the reaction of permanganate with Mn(II) ions causes rapid sludging with manganese dioxide and a very short lifetime of the etch. This should not be an issue with the Mn(II) based etch (although there may be some disproportionation over time). 6) The electrolytic production of Mn(III) in accordance with the present invention does not produce any toxic gases. Some hydrogen may be produced at the cathode, but owing to the low current requirements, this would be less than that produced by many plating processes.

(12) As described herein, in a preferred embodiment the acid is sulfuric acid. The concentration of sulfuric acid may be between about 9 and about 15 molar. The concentration of sulfuric acid is important in the process. Below a concentration of about 9 molar, the rate of etch becomes too slow to be of use and above about 14 molar, the solubility of manganese ions in the solution becomes too low to get a useful concentration of the manganese into solution. Additionally, very high concentrations of sulfuric acid tend to absorb moisture from the air and are hazardous to handle. Thus, in a preferred embodiment, the concentration of sulfuric acid is between about 12 and 13 molar. This concentration of sulfuric acid is dilute enough to allow the safe addition of water to the etch and strong enough to optimize the etch rate of the plastic. At this concentration of sulfuric acid, up to around 0.08M of manganese sulfate can be dissolved at the preferred operating temperature of the etch, For optimal etching, the concentration of manganese ions in solution should be as high as it is feasible to achieve.

(13) The manganese(II) ions are preferably selected from the group consisting of manganese sulfate, manganese carbonate and manganese hydroxide although other similar sources of manganese(II) ions known in the art would also be usable in the practice of the invention. The concentration of manganese(II) ions may be in the range of between about 0.005 molar and saturation. In one embodiment, the electrolyte also comprises colloidal manganese dioxide. This may form to some extent as a natural result of disproportionation of manganese (III) in solution, or may be added deliberately. Methods of preparing colloidal manganese dioxide are well known in the art.

(14) Manganese (III) ions can be conveniently generated by electrochemical means by the oxidation of manganese (II) ions. The methodology for efficient production of trivalent manganese ions was determined and it was found that with the use of a platinum or platinized titanium anode, manganese (II) could be efficiently oxidized to manganese (III) at a current density of between 0.1 and 0.4 A/dm.sup.2. At these current densities, the inventors of the present invention have found that the conversion efficiency of manganese (II) under these circumstances approaches 100%. Furthermore, at a current density of between 0.1 and 0.4 A/dm.sup.2 using a platinized titanium anode, the anode potential is below the oxygen discharge potential and manganese (III) ions are produced with high efficiency. The inventors have found that etching of ABS can be achieved by this method.

(15) The electrodes may comprise a material selected from the group consisting of platinum, platinized titanium, iridium oxide coated titanium, niobium and other suitable materials. The cathode may also be made of platinum, platinized titanium, niobium, iridium oxide coated titanium or any other suitable material and is preferably platinum or platinized titanium. The anode may be made of platinized titanium, platinum, iridium/tantalum oxide, niobium, or any other suitable material and is preferably platinum or platinized titanium. For efficient generation of manganese (III) ions, it is generally necessary to use an anode area which is large in comparison to the area of the cathode. Preferably, the area ratio of anode to cathode is at least about 10:1. By this means, the cathode can be immersed directly in the electrolyte and it is not necessary to have a divided cell (although the process would work with a divided cell arrangement, this would introduce unnecessary complexity and expense).

(16) In addition, it is generally preferable that the electrolyte not contain any permanganate ions.

(17) In another embodiment the present invention comprises immersing the platable plastic in the metastable sulfate complex for a period of time to etch the surface of the platable plastic. In one embodiment, the platable plastic is immersed in the solution at a temperature of between 30 and 80 C. The rate of etching increases with temperature and is very slow below 50 C. The upper limit of temperature is determined by the nature of the plastic being etched. ABS begins to distort above 70 C., thus in a preferred embodiment the temperature of the electrolyte is maintained between about 50 and about 70 C., especially when etching ABS materials. The time period of the immersion of the plastic in the electrolyte is preferably between about 20 to about 30 minutes.

(18) Articles etched in this manner may be subsequently electroplated using conventional pretreatment for plated plastics or the etched surface of the plastic could be used to enhance the adhesion of paint, lacquers or other surface coatings.

(19) As described in the examples that follow, the inventors of the present invention have determined by means of cyclic voltammetry that at the concentration of manganese (II) ions used in the etch of this invention, the oxidation is diffusion controlled so efficient agitation of the etch solution is necessary during the electrolytic oxidation process.

(20) In another embodiment, the present invention relates generally to an electrolyte capable of etching a platable plastic, the electrolyte comprising a solution of manganese(II) in a solution of 9 to 15 molar sulfuric acid or phosphoric acid. The electrolyte oxidizes to form manganese(III) ions, wherein the manganese(III) ions form a metastable sulfate complex where sulfuric acid is used.

(21) The invention will now be illustrated with the following non-limiting examples.

COMPARATIVE EXAMPLE 1

(22) A solution of 0.08 molar of manganese sulfate in12.5 molar sulfuric acid (500 ml) was heated to 70 C. and a piece of platable grade ABS was immersed in the solution. Even after an hour immersed in this solution, there was no discernible etching of the test panel and upon rinsing, the surface was not wetted and would not support an unbroken film of water.

EXAMPLE 1

(23) The solution of Comparative Example 1 was electrolyzed by immersing a platinized titanium anode of an area of 1 dm.sup.2 and a platinized titanium cathode of surface area 0.01 dm.sup.2 in the solution and applying a current of 200 mA for 5 hours.

(24) During this period of electrolysis, the solution was observed to change in color from almost colorless to a very deep purple/red color. It was confirmed that no permanganate ions were present.

(25) This solution was then heated to 70 C. and a piece of platable grade ABS was immersed in the solution. After 10 minutes of immersion, the test piece was fully wetted and would support an unbroken film of water after rinsing. After 20 minutes of immersion, the sample was rinsed in water, dried and examined using a scanning electron microscope (SEM). This examination revealed that the test piece was substantially etched and many etch pits were visible.

EXAMPLE 2

(26) A test piece of platable grade ABS was etched in a solution prepared as in Example 1 for 30 minutes at 70 C. The sample was then rinsed and plated using the following pretreatment sequence: 1) Treatment in a proprietary preparation for plating on plastic (M-neutralize, available from MacDermid, Inc) 2) Rinse 3) Pre-dip in 30% hydrochloric acid 4) Activate in a proprietary palladium colloid activator (D34 C, available from MacDermid, Inc.) 5) Rinse 6) Accelerate in a proprietary preparation (Macuplex Ultracel 9369, available from MacDermid, Inc.) 7) Rinse 8) Plate in electroless nickel process (Macuplex J64, available from MacDermid, Inc.) 9) Rinse 10) Plate in acid copper process to a thickness of 30 microns (CuMac Optima, available from MacDermid)

(27) In all cases, the process parameters were as recommended on the technical data sheet for each product.

(28) After the copper plating process was completed, the sample was dried and examined. The copper deposit was bright and clear with no evidence of blistering and showed good adhesion of the deposit to the substrate.

EXAMPLE 3

(29) A solution containing 12.5 M of sulfuric acid and 0.08 M manganese (ID sulfate was electrolyzed using a platinized titanium anode at a current density of 0.2 A/dm.sup.2. A platinized titanium cathode having an area of less than 1% of the anode area was used in order to prevent cathodic reduction of the Mn(III) ions produced at the anode. The electrolysis was performed for long enough for sufficient coulombs to be passed to oxidize all of the manganese (II) ions to manganese (III). The resulting solution was a deep cherry purple/red color. There were no permanganate ions generated during this step. This was also confirmed by visible spectroscopythe Mn(III) ions produced a completely different absorption spectrum from that of a solution of permanganate.

EXAMPLE 4

(30) The etching solution prepared as described above in Example 3 was heated to 65-70 C. on a magnetic stirrer/hotplate and test coupons of ABS were immersed in the solution for time periods of 20 and 30 minutes. Some of these test coupons were examined by SEM and some were processed in a normal plating on plastic pretreatment sequence (reduction in M-neutralize, predip, activate, accelerate, electroless nickel, copper plate to 25-30 microns). These test coupons were then annealed and subjected to peel strength testing using an Instron machine.

(31) Peel strength testing carried out on coupons plated for 30 minutes demonstrated peel strength varying between about 1.5 and 4 N/cm.

(32) Cyclic voltammograms were obtained from a solution containing 12.5M sulfuric acid and 0.08M manganese sulfate using a platinum rotating disk electrode (RDE) having a surface area of 0.196 cm.sup.2 at various rotation speeds. A model 263A potentiostat and a silver/silver chloride reference electrode were used in conjunction with the RDE.

(33) In all cases, the forward scan showed a peak at around 1.6V vs. Ag/AgCl followed by a plateau up to around 1.75V followed by and increase in current. The reverse scan produced a similar plateau (at a slightly lower current and a peak around 1.52V. The dependence of these results on the rate of electrode rotation indicates mass transport control is a primary factor in the mechanism. The plateau indicates the potential range over which Mn(III) ions are formed by electrochemical oxidation.

(34) A potentiostatic scan was performed at 1.7V. It was observed that the current initially dropped and then over a period of time increased. The current density at this potential varied between 0.15 and 0.4 A/dm.sup.2.

(35) Following this experiment, a galvanostatic measurement was taken at a constant current density of 0.3 A/dm.sup.2. Initially, the applied current density was achieved by a potential of about 1.5V but as the experiment progressed, after about 2400 seconds, and increase in potential to about 1.75V was observed.

(36) The results of these experiments demonstrate that manganese (III) ions can be generated by electrosynthesis at low current densities using a platinum or platinized titanium anode.

(37) After a period of etching for more than 10 minutes, it was observed that the surface of the ABS test coupons was fully wetted and would support an unbroken film of water alter rinsing. After a period of 20 or 30 minutes, the panels were noticeably etched.