Note: Descriptions are shown in the official language in which they were submitted.
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CASE 5863
1 This invention generally relates to an improved
method of electrodepositing a water-soluble or water-
dispersible coating resin onto a conductive surface, and,
in particular, is directed to an alkaline pretreatment to
improve both the cured and uncured resin adhesion and the
corrosion resistance of the cured product.
The electrodeposition of water-based coatings,
commonly termed electrocoatin~, is a widely used process
which has many advantages over other methods of coating,
such as spraying, dipping, rolling and the like. The advan-
tages of electrocoating are well known. The process
deposits a film of uniform thickness on essentially any
conductive surface, even those which have sharp points and
edges. The electrocoated film when applied is relatively
water-free, and, thus, will not run or drip when taken out
~` of the bath. Because little or no organic solvents are
used in the resin system, the process is essentially fume-
less and requires no extensive fume collection and incin-
eration equipment. This latter point is important in view
of the increased concern over environmental pollution. An
additional aavantage is the fact that a second or top coat
can be applied over the electrocoated film without curing
the electrocoated film and then both coats can be cured in -
one baking operationO By eliminating the necessi~y of two
~5 furnaces, the cost of a two~coat process can be consider-
ably reduced.
The electxocoating process generally comprises
immersing the article ~o be coated into the electrocoating
bath, usually as an anode, and passing a current through
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1 the bath between the article and el.ectrode. ~he process
usually is self-arresti.ng in that as the thickness of the
coating increases, the resistance thereof also increases,
thereby limiting the amount of coating which is electro-
deposited.
The overall anodic electrocoating processinvolves four separate processes, namel.y, electrophoresis,
electrocoagulation, electroendosmosis and electrolysis.
Electrophoresis involves driving negatively charged resin
particles to the positively charged anode which is the
article to be coated. In electrocoagulation, the resin .
particle loses a negative charge in the close vicinity of
the anode or in contact therewith which causes the resin
particles to lose their stability and coagulate on or about
15 the anode. Electroendosmosis occurs during and after ~.:
electrocoagulation and involves driving water out of the
`~ coagulated resin, thus, in effect, drying out the electro-
deposited coating. Electrolysis also occurs causing
evolution of hydrogen at the cathode and oxygen at the
anode. With aluminium and other reactive metals, anodic
oxidation usually occurs at least initially. Most com-
meraial electrocoating systems are anodic in that the
. article to be coated is the anode in the electrocoating : .
cell as described above. However, in certain situations,
cathodic deposition, wherein the article is the cathode
and the coating resin carries a positive charge, has been
found useful. -
The coating resin can be water-soluble in that :-
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.~ ~ it disassociates into macro-ions to form a true solution ~ -
or it can be water-dispersible or w~ter emulsifiable.
1 Usually all three are present in commerciaLly available
resin systems. At any rate, in the anodic electrocoating
process, the resin particles are negatively charged and
under the application of electrical field are driven toward
t~e anode. Because of the negative charge, the particles
tend to repel one another and thereby form a stable dis-
persed phase or solution. However/ upon losing the nega-
tive charge at the anode, the resin particles coalesce and
deposit as the insoluble acid forming a tenacious film on
the conductive surfaceO It is not presently well known
how this discharge occurs because after the lnitial forma-
tion of the resin layer, which is essentially nonconduct-
ing, no direct contact with the metal surface for elec-
trical discharge can occur. It is probable that after the
initial layer of resin is deposited, charge transfer
occurs by ion migration. Specific anodic reactions can
and probably do vary from resin system to resin system.
Most commercially available resins for anodic --
electrocoating generally are polycarboxylic acid-based
resins and frequently are acrylic or methacrylic acid-
based resins. To solubilize the resins, they are usually
completely or nearly completely neutralized by a base,
such as an amine or KO~. With cathodic electrocoating,
, the resin generally is a basic polymer xesin which has
been neutralized wi~h a soluble acid. During anodic
e1ectrocoating, the amine t~akes on a hydrogen ion and is
driven to the cathode where H2 is liberated. The amine
or other neufralizing agent is not deposited in the coat-
ing and will stay in the bath except, for small amounts
30 which are lost through dragout. ~o maintain a relatively
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1 constant level of amine, it is preferred to tr~at the bath
in an ultrafilter or other suitable device to remove amines
and other low molecular weight contaminants from the bath.
For an excellent discussion on the use of ultrafilters in
purifying electrocoating baths, see the article "Ultrafil-
tration of Electrocoating Systems", in Non~lluting Coatings
and Coating Processes, Plenum Press (1973) edited by J. L.
Gardon and J. W. Prane. Coupling agents which assist in
solubilizing the paint resin are frequently added. The
resin can be pigmented or clear as desired.
Conventional coating of metal products generally
comprises first, thoroughly cleaning and degreasing the
metal strip or sheet such as by treatment with a cleaning
solution, usually an inhibited aqueous alkaline cleaning
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solution (or alternately etching in a highly alkaline solu-
tion), rinsing with water, and then treating in an acidic
chromate and/or phosphate containing solution to form a
chemical conversion coating on the metal sheet or strip.
After conversion coating, the sheet or strip is rinsed
thoroughly, surface dried and then passed to subsequPnt
coating operations. The purpose of a conversion coat is
i to provide an improved base for the application of the ~-
coating resin so as to more firmly adhere the resin to the
metal surace. Typical conversion coatings for ferrous
products are phosphate-type coatings. Typical conversion
coatings for aluminium products inc.lude chrome-phosphate-
type or ferricyanide accelerated chromic chromate-type
coatings. The coating composition for the former is
believed to be A12O3 2CrPO4 8H2O, whereas, the latter is
believed to be CrFe(CN~6-6Cr(OH)3-FI2CrO4-4A12O3-8H2O. The
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1 ferricyanide accelerated conversion coating is probably
the most frequently used with the aluminium products,
particularly in continuous coil coating processes, because
it provides a more corrosion-resistant coating and because
the coating weight is much higher than other coatings at
equivalent treatment times due to the accelerated coating
formation. With chromate containing coatings, frequently
the last rinse will be a dilute solution of chromic acid
or a mixture of chromic acid and phosphoric acid to improve
corrosion resistance. The temperature of the conversion
coating bath can be maintained from about room temperature
to boiling point of the solution, but generally is main- ;-
tained between about 80 and 120F.
Although the electrocoating process offers many
advantages over the conventional application of coating
resins or paints, frequently, the quality of electrocoated
- products, such as coating adhesion (both cured and uncured)
and corrosion resistance, axe considerably less than prod- ;
ucts which have been coated in a conventional manner. The
~; 20 low quality of electrocoated products is particularly
; noticeable in the continuous coil coating of sheet or
strip wherein the treatment times are very short and where
the strip or sheet must be handled frequently.
.
Against this background, the present invention
~; 25 was developed.
This invention generally relates to the electro-
coating of metal products and is particularly directed to
the prepaxation of the su~face of the metal product for
subsequent electrocoating so as to improve the coating
adhesion and the corrosion resistance of the electrocoated
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1 product.
According to the invention there is provide~l a
method of electrocoatiny a metal surace ~herein the
metal surface is treated so as to form thereon a chemical
conversion coating, the metal surface is immersed as an
electrocle in an aqueous bath containing an electro-
depositable polyelectrolyte resin and an electrical current
is passed between said surface and an anode in said bath
so as to electrodeposit said resin onto said surface, the
improvement comprising treating the metal surface having
a chemical conversion coating thereon with an aqueous
alkaline solution having a p~-I greater than 8, and then :~
surface drying said treated surface prior to immersing
the surface as an electrode into the resin containing
a~ueous bath.
In accordance with the present invention, after
cleaning of the strip or sheet and the formation thereon
of a chemical conversion coating, the strip is treated
with an aqueous alkaline solution having a pH of at least
8, preferably from about 9 to 12. Suitable alkaline
solutions include a~ueous solutions of alkali metal hydroxides,
such as lithium h~droxide, potassium hydroxide and sodium
hydroxide, and also ammonium hydroxide and the like.
It is presently not clear how this alkaline
treatment of the conversion coating provides the improved
wet and dry adhesion of the electrocoated film and the
improved corrosion resistance of the final coa-ted productO
It is believed that the alkaline treatment is dehydrating
the surface of the conversion coating. The dehydration
of the conversion coating surface apparently prevents the
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1 water in the conversion coa-ting from being driven into the
electrocoa-ted film by elec-troendosmo~is durinq electro-
coating. This latter point is believed to ~e a lar~e
Eactor in the low quality of electrocoated products. It
is also possible that the alkaline rinse may he accelerating
the anodic reactions.
The overall process of the present invention
yenerally will comprise
(1) cleaning and degreasing such as treatment in
an inhibited alkaline cleaner or etching in a highly
alkaline etching solution;
(2) rinsing with water;
(3) treating in an aeidic aqueous solution eon-
taining chromates and/or phosphates to form a chemieal
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1 conversion coating;
(4j rinsing with water;
(5) treating with an aqueous alkaline solution
having a pH of at least 8, preferably 9 to 12-;
(6) suxface drying the metal product; and
(7) electrodepositing a resinous film on the
metal surface.
After electrocoating, the metal product can be passed to
an oven for curing or to other coating facilities for the
application of a top coat. The process provides for sub-
stantially impro~ed product qualities, such as cured coat-
ing adhesion and corrosion resistance, and also improves
the wet adhesion of the uncured electrocoated film which
minimizes the traffic damage which frequently is charac-
l$ teristic of electrocoating operations. Generally, no
rinsing of the metal surface is required after the alkaline
treatment. After the alkaline treatment, the strip need
only be dried and then passed to the electrocoating opera-
tion.
20 ~ The cleaning solution can be any conventional
cleaning solutlon. Concentrations between about 0.5 and
10 ounces per gallon of water o~ commercially available
cleaners, such as Ridoline 35 and 72, sold by Amchem Corp.,
and Okite 27, sold by the Oakite Corp., have been found
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suitable. After cleaning, the metal surfaces are rinsed
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with water and then passed to the conversion coating
treatment facility. Most con~entional conversion coatings
on aluminium products are either a chromic phosphate type
coating ox a *erric~anide accelexat~od chromate coating.
The chromi~ phosphate type of coating is formed by reacting
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l the aluminium surface with an acid solution containing
phosphates, hexavalent chromium and a source of fluoride
I(e.g., ~lodine 401-45 sold by Amchem Corp.), whereas the
!chromate-type coating is formed by treatment with a solu-
I5 tion con~aining hexavalent chromiuml a source of fluoride
jand ferrlcyanide as an accelerator (e.g.,*Alodine 1200S).
IThe coatings are quite complex and at least initially are
in the form of an acidic gel. Treatment temperatures for
the conversion coating bath range from about 80 to 120F
After the acidic conversion coating treatment, the metal
surface should be rinsed well to prevent any carryover of
the chromates, phosphates and fluorides and the like into
subsequent treatment tanks~ After the aforesaid rinsing,
the metal surface is yiven a treatment with an aqueous
alkaline solution having a pH gre~ter than 8, preferably
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about 9 to 12. Suitable alkaline solutions include solu-
tions of sodium hydroxide, potassium hydroxide, ammonium
hydroxide and the like. Treatment times need not bé exten- ;
slve. Usually from about 0.5 to lO seconds is adequate in
preparing the surface for electrocoating. Extensive treat-
ment times are not practical on a coil coating line with
continuous lengths of metal sheet or strip, but they do
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not detrimentally affect the quality of the electrocoated
product. The temperature can be as low as 60F, but pref-
erahly the alkaline treatment is at an elevated temperaturebetween about 90 to 180F to assist in the surface drying
of the metal surface after the alkaline treatment. The
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thus-treated metal product is surface dried and then passed
to an electrocoating operation for the application of a
polyelectrolyte resin. The electrodeposition can be either
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1 anodic or catllodic depending upon t~e type of resin
employed.
According to a fur-ther feature o~ the invention
we provide a method of electrocoating continuous lengths
of sheet or strip wherein the sheet or strip is cleaned,
treated so as to form on the surface thereof a chemical
conversion coating, the sheet or strip is immersed as an
electrode in an aqueous bath containin~ an electro-
depositable polyelectrolyte resin and an electrical current
is passed between said surface and an electrode in said
bath so as to elec-trodeposit said resin onto the surface
of said sheet or strip, the improvement comprising treat-
ing the surface of the said sheet or strip having a
chemical conversion coating thereon with an aqueous alkaline
solution having a pH greater than 8, and then surfacedrying said treated sheet or strip prior to immersing the
sheet or strip as an electrode into the resin containing
aqueous bath.
~ ~ Reference is made to the drawing which represents - -.
; 20 an embodiment of the present invention wherein continuous
.~ lengths of sheet or strip are electrocoated. The strip
or sheet is first cleaned and degreased (or etched) at 10,
rinsed at 11 and then transferred to tank 12 or the forma- -
; tion of the conversion coat. After the conversion coating,
25 Wle strip is rinsed at station 13 and treated at station 14 `~:.
` with an aqueous alkaline solution with a pH over 8 and then
air dried at 15. Following drying, the strip is directed
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over electrical contact roll 16 into electrocoating tank :~
17 containing an ~queous dispersion or solution of electro-
depositable polyelèctrolyte resin. The sheet or strip
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passes in close proximity to electrodes 18 where most of
-the electrodeposition occurs and then ouk o:E the tank for
surface drying and suhsequent treatments, such as curin~
or the application of a top coat and then curiny.
The alkaline rinse of the present invention is
particularly advantageous in continuous coil coating lines
because in addition to an improved coating quality, the
alkaline rinse provides an improved adh0sion of the uncured
resin to the metallic substrate which minimizes the removal
of the uncured coating (commonly termed "pick-off") by
subsequent strip or sheet handling equipment, such as
rollers and the like.
To illustrate the advantages of the present
invention, the following examples are given.
E~ le 1
Panel= of 3105 aluminum alloy (~luminum
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1 Association designation) were cleaned in an inhibited alka-
line cleaner, rinsed with water and pretreated in a solu-
tion of ~lodine 401-45 to produce a conversion coating
thereon having a coating weight of approximately 30 milli-
grams per square foot. After rinsing with water, thepanels were treated in an aqueous solution of NaOH having
a pH of 10Ø Panels were then air dried and electrocoated
(135 volts for 5 seconds) in a 10% (by weiyht) bath of
*Lectropon, an electrodepositable resin sold by DeSoto, Inc.,
to provide a uniform film thickness of 0.23 mil. Electro-
coated panel was squeegee wiped to remove excess paint, air ~ -
dried, roll coated with ~uracron, an acrylic solvent-
based resin sold by PPG, Inc., to a total coating thickness
oE 1.0 mil and baked at 520F for 45 seconds. The coated
panel had excellent appearance and adhesion between the
coating and the alumlnium substxate. Subsequent acceler-
ated corrosion tests indicated superior corrosion resist-
ance.
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Example 2
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Panels of 3105 aluminium alloy (Aluminum Associ-
ation designation~ were cleaned in an inhibited alkaline
cleaner, rinsed with water and then treated in a solution
of Alodine 1200S to produce a conversion coating havlng a
coating weight o about 30 milligrams per square foot.
After rinsing with water, the panels were treated in an
aqueous alkaline solution of NaOH having a pH o 10Ø
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The panels were air dried and electrocoated (150 volts
for 5 seconds) in a 10% (by weight~ bath of Lectropon, an
~ : electrodepositable resin sold by DeSoto, Inc., to a coat-
;~ 30 ing~thickness of 0.3 mil. The electrocoated panel was
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1 squeegee wiped to remove any excess coating, air dried,
roll coated with*Duracron 100, an acrylic solvent-based
resin sold by PPG, Inc., to a total coating thickness of
1.0 mil and baked at 520F for 45 seconds. The coated
panels exhibited excellent appearance and adhesion between
the coating and the metal substrate and when tested in
accelerated corrosion tests, indicated superior corrision
resistance~
Example 3
In a continuous coil coating line wherein
aluminium alloy sheet was first cleaned in an i~hibited
alkaline cleanerj rinsed, treated in an acidic chromate
containing solution to form a conversion coating thereon,
electrocoated, roll coated to put on a top coat and then
both coatings were cured in a single baking operation,
tests were conducted wherein the sheet was treated in an
; aqueous alkaline solution of NaOH at a pH 10 after the
conversion coating and wherein the strip was merely rinsed
with deionized water after the conversion coating. Cured
samples of the tests were evaluated by determining the
minimum bend radius with no cracking and no removal of
coating from bend area by Cellophane tape (the smaller the
radius the stronger the bond between the resin and sub- -
strate). The samples which were treated with the alkaline
2S solution after the conversion coating had a minimum bend
radius of OT (the thickness of the sheet~, whereas ~he
samples which had been rinsed in deionized water had a
minimum bend radius of lT. During the electrocoating
testsS when the sheet was treated with a deionized water - -
rinse after the conversion coating, pick-off was consider-
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1 able, whereas when treated with an alkaline rinse in
accordance with the present invention, pick-off was no
problem. The corrosion resistance of the alkaline rinsed
material was far superior to the deionized water rinsed
material.
Example 4
Two steel panels were cleaned and then given a -
treatment in a solution of ~onderite 40 (Parker Chemical
Co.) to form a zinc phospha-te coating on the panels. One
of the panels was treated in an aqueous alkaline solution
of NaOH (pH 10) and the other was treated in an acidic
rinse solution at a pH of 5 and then both panels were
electrocoated in a 10~ (by weight) bath of Lectropon to
form a primer coat of 0.2 mil, squeegee wiped, air dried ''d~ '
and then both were roll coated with*Duracron 100 to form
a total coating thickness of 1 mil~ After curing, the
panels were tested for coating adhesion by subjecting both
panels to a 100-inch pound impact of a 5/8" diameter
steel mandrel. The panel which was given an acidic rinse
exhibited a 40~ removal (by No. 6 Scotch brand*Cellophane
tape) of the coating on the convex portion of the panel,
whereas the panel given an alkaline rinse exhibited only
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a 1`0% removal of the coating.
It is obvious that various modifications and
25 improvements can be made to the present invention without
departing from the spirit thereof and the scope of the
appended claims.
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