Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROLYTIC GENERATION OF MANGANESE (III) IONS IN STRONG SULFURIC ACID
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of application Serial No.
13/356,004,
filed on January 23, 2012, now pending.
FIELD OF THE INVENTION
The present invention relates generally to a process of electrolytically
generating
manganese(III) ions in strong sulfuric acid using an improved anode.
BACKGROUND OF THE INVENTION
It is well known in the art to plate non-conductive substrates, (i.e.
plastics) with
metal for a variety of purposes. Plastic moldings are relatively inexpensive
to produce
and metal plated plastic is used for many applications. For example, metal
plated plastics
are used for decoration and for the fabrication of electronic devices. An
example of a
decorative use includes automobile parts such as trim. Examples of electronic
uses
include printed circuits, wherein metal plated in a selective pattern
comprises the
conductors of the printed circuit board, and metal plated plastics used for
EMI shielding.
ABS resins are the most commonly plated plastics for decorative purposes while
phenolic
and epoxy resins are the most commonly plated plastics for the fabrication of
printed
circuit boards.
Plating on plastic surfaces is used in the production of a variety of consumer
items. Plastic moldings are relatively inexpensive to produce and plated
plastic is used
for many applications, including automotive trim. There are many stages
involved in the
plating of plastic. The first stage involves etching the plastic in order to
provide
mechanical adhesion of the subsequent metallic coatings and to provide a
suitable surface
for adsorption of the palladium catalyst which is typically applied in order
to catalyze
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deposition of the initial metallic layer from an autocatalytic nickel or
copper plating
process. Following this, deposits of copper, nickel and/or chromium may be
applied.
The initial etching of the plastic components is an essential part of the
overall
process. However, only certain types of plastic components are suitable for
plating. The
most common types of plastic for electroplating are
acrylonitrileibutadiene/styrene (ABS)
or a blend of ABS with polycarbonate (ABS/PC). ABS consists of two phases. The
first
phase is a relatively hard phase consisting of an acrylonitrile/styrene
copolymer and the
second phase is a softer polybutadiene phase.
Currently, this material is etched almost exclusively using a mixture of
chromic
and sulfuric acids, which is highly effective as an etchant for ABS and
ABS/PC. The
polybutadiene phase of the plastic contains double bonds in the polymer
backbone, which
are oxidized by the chromic acid, thus causing complete breakdown and
dissolution of
the polybutadiene phase exposed at the surface of the plastic which gives an
effective
etch to the surface of the plastic.
One problem with the traditional chromic acid etching step is that chromic
acid is
a recognized carcinogen and is increasingly regulated, insisting that wherever
possible,
the use of chromic acid is replaced with safer alternatives. The use of a
chromic acid
etchant also has well-known and serious drawbacks, including the toxicity of
chromium
compounds which makes their disposal difficult, chromic acid residues
remaining on the
polymer surface that inhibit electroless deposition, and the difficulty of
rinsing chromic
acid residues from the polymer surface following treatment. Additionally, hot
hexavalent
chromium sulfuric acid solutions are naturally hazardous to workers. Burns and
upper
respiratory bleeding are common in workers routinely involved with these
chrome etch
solutions. Thus, it is very desirable that safer alternatives to acidic
chromium etching
solutions be developed.
Early attempts to replace the use of chromic acid to etch plastic typically
focused
on the use of permanganate ions as an alternative to chromic acid. The use of
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permanganate in combination with acid is described in U.S. Patent No.
4,610,895 to
Tubergen et al. Later, the use of permanganate in combination with an ionic
palladium
activation stage was suggested in U.S. Pat. Pub. No. 2005/019958 to Bengston.
The
use of acid permanganate solutions in combination with perhalo ions (e.g.,
perchlorate
or periodate) was described in U.S. Pat. Pub. No. 2009/0092757 to Satou.
Finally, the
use of peimanganate ions in the absence of alkali metal or alkaline earth
metal cations
was described in International Pub. No. WO 2009/023628 to Enthone.
Permanganate solutions are also described in U.S. Pat. No. 3,625,758 to Stahl
et al. Stahl suggests the suitability of either a chrome and sulfuric acid
bath or a
permanganate solution for preparing the surface. In addition, U.S. Pat. No.
4,948,630
to Courduvelis et al., describes a hot alkaline pen ianganate solution that
also contains
a material, such as sodium hypochlorite, that has an oxidation potential
higher than
the oxidation potential of the permanganate solution and is capable of
oxidizing
manganate ions to permanganate ions. U.S. Pat. No. 5,648,125 to Cane,
describes
the use of an alkaline permanganate solution comprising potassium permanganate
and sodium hydroxide, wherein the permanganate solution is maintained at an
elevated temperature, i.e., between about 165 F and 200 F. U.S. Pat. No.
4,042,729
to Polichette et al., describes an etching solution that comprises water,
permanganate
ion, and manganate ion, wherein the molar ratio of manganate ion to
permanganate
ion is controlled and the pH of the solution is maintained at 11-13.
As is readily seen, many etching solutions have been suggested as a
replacement
for chromic acid in processes for preparing non-conductive substrates for
metallization.
However, none of these processes have proven satisfactory for various
economic,
performance and/or environmental reasons and thus none of these processes have
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achieved commercial success or been accepted by the industry as a suitable
replacement
for chromic acid etching. In addition, the stability of the etching solutions
may also be
poor, resulting in the foi 'nation of manganese dioxide sludge.
The tendency for permanganate based solutions to form sludge and undergo self-
decomposition has been noted by the inventors here. Under strongly acidic
conditions,
permanganate ions can react with hydrogen ions to produce manganese (II) ions
and
water according to the following reaction:
4Mn04- + 12-H 4Mn2+ + 6H20 + 502 (1)
The manganese (II) ions formed by this reaction can then undergo further
reaction
with permanganate ions forming a sludge of manganese dioxide according to the
following reaction:
2Mn04- + 2H20 + 3Mn 2+ 5MnO7 + 41{ (2)
Thus foimulations based on strongly acidic permanganate solutions are
intrinsically unstable irrespective of whether the permanganate ion is added
by alkali
metal salts of permanganate or is electrochemically generated in situ. In
comparison to
the currently used chromic acid etches, the poor chemical stability of acidic
permanganate renders it effectively useless for large scale commercial
application.
Alkaline permanganate etches are more stable, and are widely used in the
printed circuit
board industry for etching epoxy based printed circuit boards, but alkaline
permanganate
is not an effective etchant for plastics such as ABS or ABS/PC. Thus,
manganese (VII) is
unlikely to gain widespread commercial acceptance as an etchant for these
materials.
Attempts to etch ABS without the use of chromic acid have include the use of
electrochemically generated silver (II) or cobalt (III). Certain metals can be
anodically
oxidized to oxidation states which are highly oxidizing. For example,
manganese (II) can
be oxidized to permanganate (manganese VI), cobalt can be oxidized from cobalt
(II) to
cobalt (III) and silver can be oxidized from silver (I) to silver (II).
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There is currently no suitable commercially successful etchant for plastics
based
on either permanganate (in either acid or alkaline fowl), on manganese in any
other
oxidation state or by using other acids or oxidants.
Thus, there remains a need in the art for an improved etchant for preparing
plastic
substrates for subsequent electroplating that does not contain chromic acid
and that is
commercially acceptable.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an etchant for plastic substrates
that does
not contain chromic acid.
It is another object of the present invention to provide an etchant for
plastic
substrates that is commercially acceptable.
It is another object of the present invention to provide an etchant for
plastic
substrates that is based on manganese ions.
It is still another object of the present invention to provide an electrode
that is
suitable for use in a strong acid oxidizing electrolyte but that is not
degraded by the
electrolyte.
It is still another object of the present invention to provide a suitable
electrode for
the generation of manganese(III) ions in strong sulfuric acid that is
commercially
acceptable.
To that end, the present invention relates generally to an electrode suitable
for the
electrochemical oxidation of manganese (II) ions to manganese (III) ions in a
strong
sulfuric acid solution.
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In addition, the present invention relates generally to an electrolytic cell
comprising:
an electrolyte solution comprising manganese(III) ions in a solution of acid,
which is preferably from 9 to 15 molar sulfuric acid;
a cathode in contact with the electrolyte solution; and
an anode in contact with the electrolyte solution, wherein the anode comprises
a
material selected from the group consisting of vitreous carbon, reticulated
vitreous
carbon, woven carbon fibers, and combinations of one or more of the foregoing.
In another embodiment, the present invention relates generally to a method of
electrochemical oxidation of manganese (II) ions to manganese (III) ions
comprising the
steps of:
providing an electrolyte comprising a solution of manganese (II) ions in a
solution
of acid, which is preferably from 9 to 15 molar sulfuric acid, in an
electrolytic cell
wherein the electrolytic cell comprises an anode and a cathode; and
applying a current to the anode and cathode of the electrolytic cell; and
oxidizing the electrolyte to foul' manganese(III) ions, wherein the
manganese(III)
ions form a metastable complex.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventors of the present invention have found that trivalent manganese can
readily be produced by electrolysis at low current density of divalent
manganese ions in
strong sulfuric acid. More particularly, the inventors of the present
invention have
discovered that a solution of trivalent manganese ions in strongly acidic
solution is
capable of etching ABS.
Trivalent manganese is unstable and is highly oxidizing (standard redox
potential
of 1.51 versus nonnal hydrogen electrode). In solution, it very rapidly
disproportionates
to manganese dioxide and divalent manganese via the following reaction:
2Mn3+ + 2H20 Mn02 + Mn2+ + 4H+ (3)
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However, in a strong sulfuric acid solution, the trivalent manganese ion
becomes
meta-stable and forms a cherry purple/red colored sulfate complex. The
inventors have
found that this sulfate complex is a suitable medium for the etching of ABS
and has many
advantages over chromium-free etches previously described.
In one embodiment, the present invention relates generally to a method of
preparing a solution capable of etching a plastic substrate, the method
comprising the
steps of:
providing an electrolyte comprising a solution of manganese (II) ions in a
solution
of acid in an electrolytic cell, wherein the electrolytic cell comprises an
anode and a
cathode; and
applying a current to the anode and cathode of the electrolytic cell; and
oxidizing the electrolyte to form manganese(III) ions, wherein the
manganese(III)
ions form a metastable complex.
In a preferred embodiment, the plastic substrate comprises ABS or ABS/PC.
While it is contemplated that both phosphoric acid and sulfuric acid would be
suitable for compositions of the present invention, in a preferred embodiment,
the acid is
sulfuric acid. At ambient temperatures, the half life of the manganese (III)
ions in 7M
sulfuric acid is on of the order of 2 years. By comparison, the half life of
similar
concentrations of manganese (III) ions in 7M phosphoric acid was around 12
days. It is
suggested that the much higher stability of the manganese (III) ions in
sulfuric acid is due
to the formation of mangano-sulfate complexes and the higher concentration of
available
hydrogen ion concentration in the sulfuric acid solution. A further problem
with the use
of phosphoric acid is the limited solubility of manganese (III) phosphate.
Thus, although
other inorganic acids such as phosphoric acid can be usable in the
compositions of the
present invention, it is generally preferred to use sulfuric acid.
The remarkable stability of manganese (III) ions in strong sulfuric acid
provides
the following advantages in use:
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1) Because the Mn(III) ions are formed at a low current density, the power
requirements for the process are typically very low.
2) Because the anode operates at a very low current density, a small cathode
in
relationship to the anode area can be used to prevent cathodic reduction of
the
Mn(III) ions. This obviates the need for a divided cell and makes the
engineering of an etchant regeneration cell simpler.
3) Because the process does not produce permanganate ions, there is no
possibility of producing manganese heptoxide in the solution (this is a
considerable safety hazard as it is violently explosive).
4) Because of the high stability of the Mn(III) ions in strong sulfuric acid,
the
etchant can be sold ready for use. In production, the etchant requires only a
small regeneration cell at the side of the tank in order to maintain the
Mn(III)
content of the etch and prevent the build-up of Mn(II) ions.
5) Because other etch processes are based on permanganate, the result of the
reaction of permanganate with Mn(II) ions causes rapid "sludging" with
manganese dioxide and a very short lifetime of the etch. This should not be an
issue with the Mn(III) based etch (although there may be some
disproportionation over time).
6) The electrolytic production of Mn(III) in accordance with the present
invention does not produce any toxic gases. While some hydrogen may be
produced at the cathode, owing to the low current requirements, this would be
less than that produced by many plating processes.
As described herein, in a preferred embodiment the acid is sulfuric acid. The
concentration of sulfuric acid is preferably between about 9 and about 15
molar. The
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concentration of sulfuric acid is important in the process. Below a
concentration of about
9 molar, the rate of etch becomes slow and above about 14 molar, the
solubility of
manganese ions in the solution becomes low. Additionally, very high
concentrations of
sulfuric acid tend to absorb moisture from the air and are hazardous to
handle. Thus, in a
most preferred embodiment, the concentration of sulfuric acid is between about
12 and 13
molar, which is dilute enough to allow the safe addition of water to the etch
and strong
enough to optimize the etch rate of the plastic. At this concentration of
sulfuric acid, up to
around 0.08M of manganese sulfate can be dissolved at the preferred operating
temperature of the etch. For optimal etching, the concentration of manganese
ions in
solution should be as high as it is feasible to achieve.
The manganese(II) ions are preferably selected from the group consisting of
manganese sulfate, manganese carbonate and manganese hydroxide although other
similar sources of manganese(II) ions known in the art would also be usable in
the
practice of the invention. The concentration of manganese(II) ions may be in
the range of
between about 0.005 molar and saturation. In one embodiment, the electrolyte
also
comprises colloidal manganese dioxide. This may form to some extent as a
natural result
of disproportionation of manganese (III) in solution, or may be added
deliberately.
Manganese (III) ions can be conveniently generated by electrochemical means by
the oxidation of manganese (II) ions. In addition, it is generally preferable
that the
electrolyte not contain any permanganate ions.
In another embodiment the present invention comprises immersing the platable
plastic in the metastable sulfate complex for a period of time to etch the
surface of the
platable plastic. In one embodiment, the platable plastic is immersed in the
solution at a
temperature of between 30 and 80 C. The rate of etching increases with
temperature and
is slow below 50 C. The upper limit of temperature is determined by the nature
of the
plastic being etched. ABS begins to distort above 70 C, thus in a preferred
embodiment
the temperature of the electrolyte is maintained between about 50 and about 70
C,
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especially when etching ABS materials. The time period of the immersion of the
plastic
in the electrolyte is preferably between about 20 to about 30 minutes.
Articles etched in this manner may be subsequently electroplated using
conventional pretreatment for plated plastics or the etched surface of the
plastic could be
used to enhance the adhesion of paint, lacquers or other surface coatings.
As described in the examples that follow, the inventors of the present
invention
have determined by means of cyclic voltammetry that at the concentration of
manganese
(II) ions used in the etch of this invention, the oxidation is diffusion
controlled so
efficient agitation of the etch solution is necessary during the electrolytic
oxidation
process.
In another preferred embodiment, the present invention relates generally to an
electrolyte capable of etching a platable plastic, the electrolyte comprising
a solution of
manganese(III) in an acid solution. The acid solution is preferably sulfuric
acid.
The anodes and cathodes usable in the electrolytic cell described herein may
comprise various materials. The cathode may comprise a material selected from
the
group consisting of platinum, platinized titanium, niobium, iridium oxide
coated titanium,
and lead. In one preferred embodiment, the cathode comprises platinum or
platinized
titanium. In another preferred embodiment, the cathode comprises lead. The
anode may
also comprise platinized titanium, platinum, iridium/tantalum oxide, niobium,
or any
other suitable material and is preferably platinum or platinized titanium.
In another preferred embodiment, the inventors of the present invention have
found that the anode may comprise vitreous carbon and that the use of vitreous
carbon
anodes provides a commercially suitable electrode. The inventors discovered
that while
the combination of manganese (III) ions and strong sulfuric acid (i.e., 9-15
molar) can
etch ABS plastic, the etchant is also very aggressive towards the electrodes
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produce the manganese (III) ions. In particular, anodes having a titanium
substrate may
be rapidly degraded by the etchant.
Therefore, in an attempt to detei __________________________________________
mine a more suitable electrode material, various
other electrode materials were examined, including lead and graphite. It was
found that
lead was rapidly attacked by the etchant when used as an anode (although it
was
determined to be suitable for use as a cathode) and that graphite anodes
crumbled rapidly.
However, vitreous carbon and reticulated vitreous carbon were determined to be
more
robust and could produce manganese (III) ions when an electrical current,
preferably of
between 0.1 and 0.4 A/dm2 (based on the nominal surface area), was applied.
Thus, as
described herein, anodes made of vitreous carbon may be used as an electrode.
In
addition, because vitreous carbon and reticulated vitreous carbon may not be
cost-
effective for use as the electrode in commercial applications, it was further
determined
that the anode may be manufactured from woven carbon fiber.
Carbon fiber is manufactured from fibers of polyacrylonitrile (PAN). These
fibers go through a process of oxidation at increasing temperatures followed
by a
carbonization step at a very higher temperature in an inert atmosphere. The
carbon fibers
are then woven into a sheet which is typically used in combination with
various resin
systems to produce high strength components. Carbon fiber sheets also have
good
electrical conductivity and the fibers typically have a turbostratic (i.e.,
disordered layer)
structure. Without wishing to be bound by theory, the inventors of the present
invention
believe that it is this structure which makes the carbon fibers so effective
as an electrode.
The SP2 hybridized carbon atoms in the lattice give good electrical
conductivity while the
SP3 hybridized carbon atoms link the graphitic layers together, locking them
in place and
thus providing good chemical resistance.
A preferred material for use in the electrodes of the invention comprises a
woven
carbon fiber containing at least 95% carbon and not impregnated with any
resin. In order
to facilitate the handling and the weaving process, carbon fibers are
typically sized with
an epoxy resin and this may comprise up to 2% of the fiber weight. At this low
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percentage, when used as an electrode, the epoxy sizing is rapidly removed by
the high
sulfuric acid content of the etch. This may cause an initial slight
discoloration of the
etch, but does not affect the performance. Following this initial "running in"
stage, the
anode appears to be resistant to the electrolyte and is effective at oxidizing
manganese
(II) ions to manganese (III).
Anodes can be constructed by mounting the woven carbon fiber material in a
suitable frame with a provision made for electrical contact. It is also
possible to use
carbon fiber as a cathode in the generation of manganese (III) ions, but it is
more
.. convenient to use lead, particularly as the cathode is much smaller than
the anode if an
undivided cell is used.
In addition, for efficient generation of manganese (III) ions, it is generally
necessary to use an anode area which is large in comparison to the area of the
cathode.
Preferably, the area ratio of anode to cathode is at least about 10:1. By this
means, the
cathode can be immersed directly in the electrolyte and it is not necessary to
have a
divided cell (although the process would work with a divided cell arrangement,
this
would introduce unnecessary complexity and expense).
In another preferred embodiment, the present invention also relates generally
to
an electrolytic cell comprising:
an electrolyte solution comprising manganese(III) ions in an acid solution;
a cathode in contact with the electrolyte solution; and
an anode in contact with the electrolyte solution, wherein the anode comprises
a
material selected from the group consisting of vitreous carbon, reticulated
vitreous
carbon, woven carbon fibers, and combinations of one or more of the foregoing.
The invention will now be illustrated with reference to the following non-
limiting
examples:
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Comparative Example 1:
A solution of 0,08 molar of manganese(II) 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:
The solution of Comparative Example 1 was electrolyzed by immersing a
platinized titanium anode of an area of 1 dm2 and a platinized titanium
cathode of surface
area 0.01 dm2 in the solution and applying a current of 200 mA for 5 hours.
During this period of electrolysis, the solution was observed to change in
color
from almost colorless to a very deep purple/red color. It was confirmed that
no
peinianganate ions were present.
This solution was then heated to 70 C and a piece of platable grade ABS was
immersed in the solution. After 10 minutes of immersion, the test piece was
fully wetted
and would support an unbroken film of water after rinsing. After 20 minutes of
immersion, the sample was rinsed in water, dried and examined using a scanning
electron
microscope (SEM). This examination revealed that the test piece was
substantially etched
and many etch pits were visible.
Example 2:
75 A
solution containing 12.5 M of sulfuric acid and 0.08 M manganese (II) sulfate
was electrolyzed using a platinized titanium anode at a current density of 0.2
A/dm2. A
platinized titanium cathode having an area of less than 1% of the anode area
was used in
order to prevent cathodic reduction of the Mn(III) ions produced at the anode.
The
electrolysis was performed for long enough for sufficient coulombs to be
passed to
oxidize all of the manganese (II) ions to manganese (III). The resulting
solution was a
deep cherry purple/red color. There were no pelinanganate ions generated
during this
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step. This was also confirmed by visible spectroscopy - the Mn(III) ions
produced a
completely different absorption spectrum from that of a solution of
permanganate.
Example 3:
The etching solution prepared as described above in Example 3 was heated to 65-
70 C on a magnetic stirrer/hotplate and test coupons of ABS were immersed in
the
solution for time periods of 20 and 30 minutes. Some of these test coupons
were
examined by SEM and some were processed in a normal plating on plastic
pretreatment
sequence (reduction in M-neutralize, predip, activate, accelerate, electroless
nickel,
copper plate to 25- 30 microns). These test coupons were then annealed and
subjected to
peel strength testing using an Instron machine.
Peel strength testing carried out on coupons plated for 30 minutes
demonstrated
peel strength varying between about 1.5 and 4 N/cm.
Cyclic voltammograms were obtained from a solution containing 12.5M sulfuric
acid and 0.08M manganese sulfate using a platinum rotating disk electrode
(RDE) having
a surface area of 0.196 cm2 at various rotation speeds. A model 263A
potentiostat and a
silver/silver chloride reference electrode were used in conjunction with the
RDE.
In all cases, the forward scan showed a peak at around 1.6V vs. Ag/AgC1
followed by a plateau up to around 1.75V followed by and increase in current.
The
reverse scan produced a similar plateau (at a slightly lower current and a
peak around
1.52V. The dependence of these results on the rate of electrode rotation
indicates mass
transport control is a primary factor in the mechanism. The plateau indicates
the potential
range over which Mn(III) ions are formed by electrochemical oxidation.
A potentiostatic scan was performed at 1.7V. It was observed that the current
initially dropped and then over a period of time increased. The current
density at this
potential varied between 0.15 and 0.4 A/dm2.
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Following this experiment, a galvanostatic measurement was taken at a constant
current density of 0.3 A/dm2. Initially, the applied current density was
achieved by a
potential of about 1.5V but as the experiment progressed, after about 2400
seconds, and
increase in potential to about 1.75V was observed.
After a period of etching for more than 10 minutes, it was observed that the
surface of the ABS test coupons was fully wetted and would support an unbroken
film of
water after rinsing. After a period of 20 or 30 minutes, the panels were
noticeably etched.
Comparative Example 2:
An electrode comprising graphite and having a nominal measured surface area of
1 dm2 was immersed in 500 mL of a solution containing 0.08 M of manganese
sulfate in
12.5 M sulfuric acid at a temperature of 65 C. The cathode in this cell was a
piece of
lead having a nominal measured surface area of 0.1 dm2. A current of 0.25 amps
was
applied to the cell, giving a nominal anode current density of 0.25 A/dm2 and
a nominal
cathode current density of 2.5 A/dm2.
It was observed that the graphite anode rapidly crumbled and degraded within
less
than 1 hour of electrolysis. In addition, no oxidation of manganese (H) ions
to
manganese (III) was observed.
Comparative Example 3:
An electrode comprising a titanium substrate coating with a mixed
tantalum/iridium oxide coating (50% tantalum oxide, 50% iridium oxide) and
having a
nominal measured surface area of 1 dm2 was immersed in 500 mL of a solution
containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid at a
temperature of 65 C.
The cathode in this cell was a piece of lead having a nominal measured surface
are of 0.1
dm2. A current of 0.25 amps was applied to the cell giving a nominal anode
current
density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.
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It was observed that manganese (III) was rapidly foimed in the solution and
the
resulting solution was capable of etching ABS plastic and producing good
adhesion upon
subsequent electroplating of the treated plastic. However, after a period of
two weeks
operation (electrolyzing the solution for 8 hours/day), it was observed that
the coating
was lifting from the titanium substrate and that the titanium substrate itself
was
dissolving in the solution.
Comparative Example 4:
An electrode comprising a titanium substrate coated with platinum and having a
nominal measured surface area of 1 dm2 was immersed in 500 mL of a solution
containing 0.08 M of manganese sulfate in 12.5 M sulfuric acid at a
temperature of 65 C.
The cathode in this cell was a piece of lead having a nominal measured surface
area of
0.1 dm2. A current of 0.25 amps was applied to the cell giving a nominal anode
current
density of 0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.
It was observed that manganese (III) was rapidly formed in the solution and
the
resulting solution was capable of etching ABS plastic and producing good
adhesion upon
subsequent electroplating of the treated plastic. However, after a period of
two weeks
operation (electrolyzing the solution for 8 hours/day), it was observed that
the coating
was lifting from the titanium substrate and that the titanium substrate itself
was
dissolving in the solution.
Example 4:
An electrode comprising vitreous carbon and having a nominal measured surface
area of 0.125 dm2 was immersed in 100 mL of a solution containing 0.08 M of
manganese sulfate in 12.5 M sulfuric acid at a temperature of 65 C. The
cathode in this
cell was a piece of platinum wire having a nominal measured surface area of
0.0125 dm2.
A current of 0.031 amps was applied to the cell giving a nominal anode current
density of
0.25 A/dm2 and a nominal cathode current density of 2.5 A/dm2.
16
CA 02889342 2015-04-23
WO 2014/077957
PCT/US2013/061860
It was observed that manganese (III) was rapidly formed in the solution and
the
resulting solution was capable of etching ABS plastic and producing good
adhesion upon
subsequently electroplating the treated plastic. The electrode appeared
unaffected by
periods of extended electrolysis.
Example 5:
An electrode comprising a piece of woven carbon fiber (Panex 35 50K Tow with
epoxy sizing at 1.5%, available from the Zoltek Corporation) was mounted in a
plastic
frame constructed of polyvinylidenefluoride (PVDF). The electrode, having a
nominal
measured area of 1 drn2, was immersed in 500 mL of a solution containing 0.08
M of
manganese sulfate in 12.5 M sulfuric acid at a temperature of 65 C. The
cathode in this
cell was a piece of lead having a nominal measured surface area of 0.1 dm2. A
current of
0.25 amps was applied to the cell, giving a nominal anode current density of
0.25 A/dm2
and a nominal cathode current density of 2.5 A/dm2.
It was observed that manganese (III) was rapidly formed in the solution and
the
resulting solution was capable of etching ABS plastic and producing good
adhesion upon
subsequent electroplating of the treated plastic. The electrode appeared
unaffected by
periods of extended electrolysis. Electrolysis was carried out over two weeks
using this
electrode and no observable degradation could be detected. The low cost and
ready
availability of this material makes it suitable for many commercial
applications.
The results of these experiments demonstrate that manganese (III) ions can be
generated by electrosynthesis using manganese(II) ions in sulfuric acid at a
relatively
high concentration and operating at low current densities using a platinum or
platinized
titanium anode and that further improvements to the process can be realized by
using a
vitreous carbon or carbon fiber anode.
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