Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Description
Method Of Cationic Electrodeposition
Using Dissolution Resistant Anodes
Technical Field
The presen~ invention relates to electrodeposition, and more
particularly, relates to cationic electrodeposition of aqueous
dispersions of cationic resinous compositions.
Background Art
Cationic electrodeposition has been used industrially since
10 l972. The early cationic electrodeposition compositions comprised
quaternary ammonium salt group-containing resins in combination with
aminoplast curing agents. In 1976, cationic compositions comprising
amine salt group-containing resins in combination with blocked
isocyanate curing agents were introduced for priming automobile
15 bodies. Today, over 90 percent of the automobile bodies are primed by
cationic electrodeposition and practically all of the cationic
compositions use the amine salt-blocked isocyanate resins.
In cationic electrodeposition, the part being coated is of
course the cathode. The counter-electrode or anode is usually made of
20 a corroslon-resistant material such as stainless steel since most
cationic electrodeposition baths are acidic in nature. Because of the
electrochemical reactions which occur at the anode, the stainless
steel electrode slowly dissolves during the cationic electrodeposition
process. The rate of dissolution depends prlncipally on the current
25 density, temperature of the electrodeposition bath to which the anode
is exposed; the greater the current density and the higher the tempera-
ture, the faster the rate of ion dissolution. Also, the composition
to which the electrode is exposed can affect the rate of dissolution.
The presence of chloride ion greatly accelerates dissolution, and
30 other unknown constituents of the electrodeposition bath can also
affect dissolution. It has been found, for example, that
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electrodeposition baths in one location may be relatively passive to
the stainless steel anodes, whereas electrodeposition baths in another
location employing the-same cationic paint may be very aggressive
towards the stainless steel anode. The dissolution of the anode
5 results in low film builds and poor appearance. Eventually, if the
dissolution is great enough, the anode must be replaced resulting in a
time-consuming and expensive shut down o~ the electrodeposition
process.
It is an object of the present invention to overcome the
above problems and to provide for a method of cationic electrodeposi-
tion with an anode which is resistant to deterioration and dissolution
in all cationic electrodeposition environments. Practicing cationic
electrodeposition in this manner would insure consistent results in
terms of coating quality and would also result in considerable savings
15 from not having to replace the anodes because of dissolution.
Disclosure of Invention
In accordance with the present invention, a method of
electrocoating an electrically conductive surface serving as a cathode
in an electrical circuit comprising said cathode and an anode immersed
20 in an aqueous dispersion of a cationic resinous composition is pro-
vided. The method comprises passing electric current between the
cathode and the anode to cause a coating to deposit on the cathode.
The anode consists of a substrate of a self-supporting material to
which is adhered a coating of a conductive material selected from the
25 group consisting of platinum, palladium, rhodium, ruthenium, osmium,
iridium, gold, oxides thereof and mixtures thereof.
The electrode does not dissolve nor deteriorate in the
cationic electrodeposition environment, provides for consistent
quality coatings, and provides for considerable maintenance savings
3~ associated with not having to replace the dissolved stainless steel
electrodes because of dissolution.
In the process of cationic electrodeposition, an aqueous
electrodeposition bath containing an electrodepositable paint is
placed in contact with an electrically conductive anode and an electri-
35cally conductive cathode and upon passage of an electric current,
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usually direct current, between the anode and cathode whi]e immersed
in the electrodeposition bath, an adherent film of paint is deposited
on the cathode. The electrodeposition of the paint occurs at a con-
stant voltage, typically between 50 and 500 volts, and at a current
density of about 0.5 to 10 amperes per square foot, with higher cur-
rent densities being used during the initial stages of the
electrodeposition and the current density gradually decreasing as the
deposited coating insulates the cathode.
Usually the cathode, such as a series of automobile bodies,
10 are introduced into the electrodeposition bath or tank sequentially
and continuously. The cathode passes through the bath where it passes
a series of anodes arranged from the beginning to the end. The anodes
first in line or towards the entrance end of the tank are subjected to
the greatest current flows, and in the case of the stainless steel
15 electrodes, dissolve the fastest. It is these anodes which are prefer-
ably replaced with the anodes of the present invention. Although all
the stainless steel anodes may be replaced with the electrodes of the
present invention, it may not be necessary to replace the stainless
steel anodes which are positioned more towards the exit end of the
20 tank since these electrodes may not have that great a current flow
(due to the insulating effect of the deposited coating) and may not
significantly dissolve in the bath. Therefore, the electrodes in the
bath towards the entrance end of the tank should be those of the
invention, whereas the other electrodes more towards the exit end of
25 the tank may be of the conventional stainless steel type.
The anodes may be exposed directly to the electrodeposition
paint or as is more usually the case, they may be part of an electro-
dialysis cell positioned within the electrodeposition bath, in which
instance, the anodes are separated from the electrodeposition paint by
30 semi-permeable membranes which are permeable to ionic materials such
as acid anion and water-soluble anionic impurities such as chloride
ion but impermeable to resin and pigment of the paint. The ionic
materials which are attracted to the anode and pass through the mem-
brane can then be removed from the bath by periodically flushing the
35 anode area with water. In an electrodialysis cell, the anode area is
commonly referred to as the anolyte cell and the liquid in which the
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anode is in contact the anolyte solution. Using the anodes in this
manner is particularly desirable when the buildup of excess acid from
the cationic electrodeposition resin is a particular problem.
The electrodeposition paints which are used in the process
5 of electrodeposition comprise cationic resins, pigments, crosslinkers
and adjuvant materials such as flow control agents, inhibitors,
organic co-solvents and of course the dispersing medium, water.
Specific examples of cationic electrodeposition composltions are those
based on cationic resins which contain active hydrogens and include
10 amine salt groups, for example, the acid-solubilized reaction products
of epoxy resins and primary or secondary amines in combination with
capped isocyanate curing agents. Cationic electrodeposition paints
employing these resinous ingredients are described in U.S. Patent No.
4,031,050 to Jerabek. Specially modified cationic resins such as
15 those containing primary amine groups formed from reacting poly-
epoxides with diketimines containing at least one secondary amine
group, for example, the methyl isobutyl diketimine of diethylene
triamine, are also well known electrodeposition resins and cationic
paints employing these resinous ingredients are described in U.S.
20 Patent No. 4,017,438 to Jerabek et al. Modified cationic resins such
as those obtained by chain extending the polyepoxide to increase its
molecular weight can also be used in the method of the invention.
Such resins are described in U.S. Patent No. 4,148,772 to Jerabek et
al in which the polyepoxide is chain extended with a polyester polyol
25 and in U.S. Patent No. 4,468,307 to Wismer et al in which the poly-
epoxide is chain extended with a particular polyether polyol. Also,
chain extension such as described in Canadian Patent 1,179,443 can be
used.
The cationic electrodeposition paints preferably contain
30 capped isocyanate curing agents because these curing agents provide
for low temperature cure and the development of optimum cured coating
properties. However, cationic electrodeposition paints based on epoxy
resins and capped polyisocyanates are often contaminated with chloride
ion which is a by-product of the method of preparation of the epoxy
35 resins and capped polyisocyanates. Many epoxy resins are made from
epichlorohydrin and certain polyisocyanates are made from phosgene.
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Chloride has a very adverse effect on the dissolution of the conven-
tional stainless steel electrodes. It is therefore with cationic
paints containing chloride ion that the invention is particularly
useful. Such paints typically have a chloride ion concentration of at
least 10, usually 10 to 200 parts per million (ppm) based on total
weight of the aqueous dispersion.
The anodes which are useful in the process of the invention
comprise a substrate of a self-supporting material which is chemically
resistant and to which the coating of the specific metals and metal
oxides described below will adhere. The substrate can be a metal but
preferably is a valve metal. The term "valve metal" defines a metal
which under anionic conditions oxidizes to form a chemically resistant
oxide on the surface and is resistant to the passage of current. By
chemically resistant is meant the substrate is resistant to the sur-
15 rounding electrolyte, that isD the electrodeposition paint or theanolyte solution, and is not sub~ect to an appreciable extent to
erosion, deterioration or to electrolyte attack.
Examples of suitable valve metals include titanium, tanta-
lum, niobium and alloys of these metals such as titanium with 1 to 15
20 percent by weight molybdenum. Because of its excellent corrosion
resistance, cost, availability, and adhesion to the metal or metal
oxide coating, titanium is the preferred valve metal.
It is not essential that the entire substrate be of the
valve metal. Rather, a core of metal such as copper or aluminum may
25 be cladded or coated with the valve metal.
To the self-supporting substrate is adhered a coating or a
layer of a material which is electrically conductive and which func-
tions as an anode in an electrical circuit. Also, the material should
be chemically resistant under anionic conditions to the surrounding
30 electrolyte. Examples of suitable materials are the metals platinum,
palladium, rhodium, ruthenium, osmium, iridium, gold, and alloys of
two or more of these metals. Also, oxides of these metals such as
ruthenium oxide and iridium oxide and mixtures of two or more oxides
can be used. Also, mixtures of metals and metal oxides can be used.
35 Because of cost and performance in an electrodeposition environment,
ruthenium oxide and iridium oxide are preferred with ruthenium oxide
being the most preferred.
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The thickness of the substrate and the outer layer of the
metal or metal oxide is not critical. It only is necessary that the
thickness of the substrate furnish a self-supporting structure and the
metal or metal oxide layer be present in an amount sufficient to
function as an anode, that is, to be able to combine current density
requirements with corrosion resistance.
Typically, the substrate is from about 50 to 500 mils in
thickness and the metal or metal oxide layer is from 0.01 to lO mils
in thickness. The coating of the metal or metal oxide layer can be on
both sides of the substrate or on one side, that is, the side facing
the cathode. Preferably, the substrate is entirely covered with a
metal or metal oxide layer.
The configurations of the anodes are not particularly
critical but for use in electrodeposition tanks, they are usually
square or rectangular. Typically, for use in industrial electrodepo-
sition tanks, electrodes having an area of from about lO to 50 square
feet are used, and as mentioned above, usually a series of electrodes
are positioned in the tank extending from the entrance to the exit end
of the tank.
The procedure for making the electrodes is generally a
proprietary process with the manufacturers. In general, the metal or
metal oxide can be applied by evaporative techniques, thermal decompo-
sition of suitable metal or metal oxides in organic medium, and by
electroplating. In most of the application methods, a valve metal is
first etched and then coated with the metal in the liquid phase. In
the instance the oxide is desired, the oxide is precipitated by
chemical, thermal or electrical means. Oxides of the group of metals
can also be applied directly to the valve metal support in a molten
bath of the oxide.
30 Examples
; In the following examples, the corrosive effects of typical
cationic electrodeposition paints towards a stainless steel anode and
ruthenium oxide-coated titanium and iridium oxide-coated titanium
anodes were evaluated. One cationic electrodeposition paint was based
35 on an acid-solubilized epichlorohydrin-bisphenol A type epoxy resin-
amine reaction product and a capped isocyanate curing ag~nt. The
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epoxy resin was an epichlorohydrin-bisphenol A type. The paint was
available from PPG Industries, Inc. under the trademark UNI-PRIME.
The second paint was a cationic acrylic prepared from glycidyl meth-
acrylate and contained a capped polyisocyanate curing agent. The
paint was available from PPG as ED-4000. Samples of anolyte solutions
from the paints were collected and used for testing. The anodes being
tested were 6 inches by 1 inch and were made part of an electrical
circuit inserted between two 6 inch by 1 inch steel cathodes. The
electrode spacing was about 2 inches and the electrodes were immersed
10 to a 2-inch depth in the anolyte solutions. The effects of tempera-
ture, amperage and time on the loss of weight of the electrodes was
measured and is reported in Table I below.
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