Composition for electrodepositing multiple coatings onto a conductive substrate

The present invention relates to an electrodepositable conductive coating composition which contains at least one cationic acrylic polymer, a crosslinking agent and conductive carbon black having an average particle size of about 20 nm or less, a surface area (BET) of about 240-300 m.sup.2 /g and an oil absorption (DBP) of about 100 to 150 ml/100 g. After curing, the conductive coating can be electrocoated with an electrodepositable topcoat having a defect-free appearance and a high degree of gloss.

The present invention relates to an electrodepositable conductive primer 
coating which can be electrocoated with an electrodepositable topcoat. 
BACKGROUND OF THE INVENTION 
The coating of conductive substrates by electrodeposition is a well known 
and important industrial process. Electrodeposition is widely used in the 
automotive and related off-road vehicle industries to apply primers and 
topcoats onto conductive substrates. 
Generally, there are two types of electrodeposition processes--anodic 
electrodeposition and cathodic electrodeposition. Both methods are 
performed by inducing an electrical current within a coating cell 
containing a coating compound. Cathodic electrodeposition is accomplished 
by first immersing the part to be coated into an electrolytic solution 
containing the coating composition. A negative charge is imparted onto the 
conductive substrate to be coated. The positively charged ionic species of 
the coating composition then move through the electrolyte medium via means 
of the electrophoretic phenomenon so as to coat the substrate. Anodic 
electrodeposition brings about similar results by reversing the electrical 
polarity so that the substrate to be coated acts as an anode and attracts 
negatively charged ions of the coating composition. 
Cathodic electrodeposition generally provides better gloss and color 
retention characteristics than anodic electrodeposition, as well as 
superior coating thickness capabilities. Typical commercial cathodic 
electrodeposition films are not conductive after cure. However, having a 
conductive electrodeposited film can be advantageous for a number of 
reasons. For instance, a conductive film permits the electrodeposition of 
another layer over the initial electrodeposited layer. Multiple 
electrodeposited layers can be beneficial since most commercial cathodic 
electrodeposition systems have certain practical limits to the amount of 
film build obtainable, and during the cure there is a tendency for the 
film to pull away from sharp edges thereby reducing edge corrosion 
protection. One way of increasing the film build and improving edge 
coverage is by electrodepositing another layer over the initial cured 
film. But in order to electrodeposit an additional coating, it is 
necessary that the initial coating have sufficient conductivity to allow 
the electrodeposition of another layer to occur. 
The present invention is directed to a conductive electrodepositable 
coating composition containing conductive carbon black. The use of carbon 
black in electrodepositable coatings is generally known. However, the use 
of carbon black in such coatings is also known to result in surface flaws 
of the deposited coatings. Surface flaws such as roughness, pin holes, 
craters and crawling reduce the protection against corrosion and adversely 
affect the appearance of the electrodeposited top coat. Nevertheless, it 
has been discovered that a coating composition containing a certain 
conductive carbon black results in a smooth coating, over which a 
relatively thick top coat having a defect-free appearance and a high 
degree of gloss may be electrodeposited. 
The present invention involves the sequential steps of cathodically 
electrodepositing a first conductive composition layer onto a metallic 
substrate, thermally curing the cathodically coated substrate, and then 
optionally cathodically electrodepositing a second composition layer onto 
the conductive coated substrate and thermally curing the second layer. 
SUMMARY OF THE INVENTION 
The present invention is directed to an electrodepositable aqueous coating 
composition comprising (a) at least one cationic acrylic polymer; (b) a 
crosslinking agent; and (c) conductive carbon black having an average 
particle size of about 20 nm or less, a surface area (BET) of about 240 to 
300 m.sup.2 /g and an oil absorption (DBP) of about 100 to 150 ml/100 g; 
wherein the conductive carbon black is in an amount of 4.5 to 6.5% by 
weight based on total solids content of the coating composition. After the 
electrodeposition of this coating composition onto a conductive substrate, 
the coating is thermally cured and may be overcoated with an additional 
electrodepositable top coat.

DETAILED DESCRIPTION OF THE INVENTION 
The conductive coating composition of the present invention is useful as a 
primer coating wherein additional coatings are electrodeposited onto the 
conductive primer coated substrate. The first coat and those applied 
subsequently can be distinguished by their pigmentation. In addition to 
functioning as a primer coating, the coating composition of the present 
invention may function as the sole pigmented coating on the substrate. 
The electrodepositable coating composition of the present invention 
generally contains at least one binder resin, a crosslinking agent and a 
conductive pigment. Acrylic resins are preferred as the binder resin, 
since the coating applied onto the substrate must be decorative and 
because it may be directly exposed to weathering and sunlight. The 
coatings applied on top of the first conductive coat may contain the same 
binders as the first coat, i.e., acrylic resins. The top coat and primer 
compositions need not contain identical binders, but must be compatible to 
have acceptable intercoat adhesion. 
Binder Resins 
The binder resin of the present invention comprises at least one cationic 
acrylic polymer. These acrylic polymers are prepared by the addition 
polymerization of ethylenically unsaturated monomers such as alkyl 
acrylates and methacrylates including methyl methacrylate, butyl 
methacrylate, hexyl methacrylate, octyl methacrylate, isodecyl 
methacrylate, stearyl methacrylate, methyl acrylate, ethyl acrylate, butyl 
acrylate, hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, decyl 
acrylate and dodecyl acrylate; hydroxyalkyl esters such as hydroxyethyl 
and hydroxypropyl acrylate and methacrylate; and amides of acrylic acid 
and methacrylic acid such as acrylamide and methacrylamide and their 
N-alkoxymethyl derivatives thereof such as N-ethoxy and N-butoxy 
acrylamide and methacrylamide. 
The acrylic functional monomers may be copolymerized with other 
ethylenically functional monomers such as, for example, vinyl aromatic 
compounds including styrene, alpha-methyl styrene, alpha-chloro styrene 
and vinyl toluene; and aminoalkyl esters of acrylic acid and methacrylic 
acid including aminomethyl, aminoethyl, aminopropyl, aminobutyl and 
aminohexyl esters, and N,N-dimethylamino ethyl ester, 
(N-methyl-N-butylamino)-ethyl ester and (N,N-dimethylamino)-hexyl ester. 
The acrylic polymers can be prepared by conventional free radical initiated 
polymerization techniques in which the polymerizable monomers are 
polymerized in the presence of a free radical initiator until conversion 
is complete. Examples of free radical initiators are those which are 
soluble in the mixture of monomers such as azobisisobutyronitrile, 
azobis(alpha, gamma-dimethylvalcronitrile), tertiary-butyl perbenzoate, 
tertiary-butyl peroctoate, benzoyl peroxide and ditertiary-butyl peroxide. 
The preferred cationic acrylic resin comprises the reaction product of (a) 
at least one alkyl ester of acrylic acid or methacrylic acid; (b) at least 
one vinyl aromatic compound; (c) at least one hydroxyalkyl ester of 
acrylic acid or methacrylic acid; and (d) at least one aminoalkyl ester of 
acrylic acid or methacrylic acid. A particularly preferred cationic 
acrylic resin useful in the coating composition of the present invention 
comprises: 
______________________________________ 
Wt. % 
______________________________________ 
butyl acrylate 5-40 
methyl methacrylate 0-15 
styrene monomer 15-40 
dimethylamino ethyl methacrylate 
5-10 
hydroxyethyl acrylate 5-20 
azobisisobutyronitrile 0.5-4 
butyl cellosolve 0-15 
______________________________________ 
Crosslinking Agent 
Preferred crosslinking agents are blocked polyisocyanates. With the 
binders, it is possible to use any desired polyisocyanate where the 
isocyanate groups have been reacted with a compound, so that the blocked 
polyisocyanate formed is resistant to hydroxyl groups at room temperature, 
but reacts at elevated temperatures within the range from about 
195.degree. to about 570.degree. F. In the preparation of blocked 
polyisocyanates, preference is given to isocyanates which contain about 3 
to 36, and in particular about 8 to about 15 carbon atoms. Examples of 
suitable diisocyanates are trimethylene diisocyanate, tetramethylene 
diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, 
propylene diisocyanate, ethylethylene diisocyanate, 2,3-dimethylethylene 
diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-cyclopentylene 
diisocyanate, 1,4-cyclohexylene diisocyanate, 1,2-cyclohexylene 
diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 
2,4-toluylene diisocyanate, 2,6-toluylene diisocyanate, 4,4-diphenylene 
diisocyanate, 1,5-naphthylene diisocyanate, 1,4-naphthylene diisocyanate, 
1-isocyanatomethyl-5-isocyanato-1,3,3-trimethylcyclohexane, 
bis(4-isocyantocyclohexyl)methane, bis(4-isocyantophenyl)methane, 
4,4'-diisocyanatodiphenyl ether and 
2,3-bis(8-isocyanatooctyl)-4-octyl-5-hexylcylcohexane. 
It is also possible to use polyisocyanates of higher isocyanate 
functionality. Examples thereof are tris(4-isocyanatophenyl)methane, 
1,3,5-triisocyanatobenzene, 2,4,6-triisocyantotoluene, 
1,3,5-tri(6-isocyanatohexylbiuret), 
bis(2,5-diisocyanato-4-methylphenyl)methane and polymeric polyisocyanates, 
such as dimers and trimers. It is further possible to use mixtures of 
polyisocyanates. The organic polyisocyanates which come into consideration 
for use as crosslinking agents in the invention can also be prepolymers 
which are derived for example from a polyol, including a polyether polyol 
or a polyester polyol. 
To block the polyisocyanates it is possible to use any desired suitable 
aliphatic, cycloaliphatic or aromatic alkyl monoalcohols. Examples thereof 
are aliphatic alcohols, such as methyl, ethyl, chloroethyl, propyl, butyl, 
amyl, hexyl, heptyl, octyl, nonyl, 3,3,5-trimethylhexyl, decyl or lauryl 
alcohol, cycloaliphatic alcohols such as cyclopentanol and cyclohexanol, 
aromatic alkyl alcohols, such as phenylcarbinol and methylphenylcarbinol. 
Other suitable blocking agents are hydroxylamines such as ethanolamine, 
oximes such as methyl ethyl ketone oxime, acetone oxime and cyclohexanone 
oxime, amines such as dibutylamine and diisopropylamine, or caprolactam. 
Conductive Pigment 
The electroconductive pigment of the invention is carbon black having an 
average particle size of 20 nm or less, a surface area (BET) of 240-300 
m2/g, and an oil absorption (DBP) of 100-150 ml/100 g. A conductive carbon 
black found to be particularly useful has an average particle size of 18 
nm, a surface area (BET) of 265 m.sup.2 /g, and an oil absorption (DBP) of 
120 ml/100 g (according to ASTM D2414). Such a carbon black is available 
in bead form from Degussa under the name PRINTEX L6. 
The coating composition of the present invention generally comprises about 
4.5-6.5% by weight of conductive carbon black, based on the solids content 
of the coating composition. Preferably, the coating composition comprises 
about 5% by weight of conductive carbon black based on the solids content 
of the composition. A carbon black content greater than about 6.5% by 
weight based on the solids content of the coating composition generally 
produces cured coatings having unacceptable surface defects. 
Supplemental pigments and extenders can also be used in conjunction with 
the conductive pigment to decrease the gloss of the coating and enhance 
corrosion protection. The supplemental pigments which can be employed 
include silica, clay and the like. 
Electrodeposition Process 
The electrodepositable coating compositions of the present invention are 
dispersed in aqueous medium. Besides water, the aqueous medium may also 
contain a coalescing solvent. Useful coalescing solvents include 
hydrocarbons, alcohols, esters, ethers and ketones. Specific coalescing 
solvents include isopropanol, butanol, 2-ethylhexanol, isophorone, 
4-methoxy-pentaone, ethylene and propylene glycol and the monoethyl, 
monobutyl and monohexyl ethers of ethylene glycol. 
Virtually any conductive substrate can be coated by the process according 
to the invention. Customarily they are metal substrates, for example iron, 
steel, copper, zinc, brass, tin, nickel, chromium or aluminum, which can 
be phosphatized, chromatized or otherwise pretreated. 
The conditions under which electrodeposition is carried out are, in 
general, similar to those used in electrodeposition of other types of 
coatings. The temperature of the electrodeposition bath is generally 
between 75.degree. and 95.degree. F. The applied voltage may be varied 
greatly and can be, for example, as low as one volt or as high as several 
thousand volts, although typically between about 50 volts and 500 volts 
are employed. Preferably, the applied voltage is between about 75 and 200 
volts and the conductive composition is electrodeposited for about 1.5-3 
minutes. 
After deposition, the conductive coating is dried or cured at elevated 
temperatures by any convenient method, such as by baking in an oven. 
Curing temperatures depend principally on the curing agent employed. When 
the curing agent is a blocked isocyanate such as described above, curing 
is usually accomplished by baking in an oven at a temperature of between 
about 300.degree. and 400.degree. F. for about 5-30 minutes. The thickness 
of the coating deposited on the article is a function of the bath 
characteristics, the electrical operating characteristics, the immersion 
time and the like. 
The conductivity of the cured conductive primer coating may be measured 
using the Ransburg meter which is calibrated in "Ransburg Units" (RU) on a 
scale of 65-165. A minimum reading of 130 RU is generally needed to 
cathodically electrodeposit a high quality topcoat layer onto the 
conductive primer layer. Preferably, the conductivity of the conductive 
primer is about 140 RU. 
The top coat is electrodeposited over the conductive primer coating on the 
substrate using the same electrocoating process described above to apply 
the conductive primer layer and baked under the same conditions to form a 
cured finish having excellent appearance and good physical properties. The 
dry film thickness of the conductive primer coating is about 0.8 to 1.8 
mils. The dry film thickness of topcoat is about 1.0 to 2.4 mils. The 
20.degree. gloss of the cured top coat is about 65 to 80%. 
The distinctness of image (DOI) of the cured topcoat is at least 60. The 
DOI was measured by the projection of various size broken rings, known as 
Landolt rings, onto the cured topcoat surface. A value of 100 is assigned 
to the smallest set of rings and incrementally smaller values to the 
uniformly increasing larger rings. The image of the reflected rings on the 
topcoat surface was observed at a specified distance and the films 
assigned a DOI value of the number corresponding to the smallest set of 
rings in which the break is discernible. The DOI is directly related to 
the smoothness of the electrodeposited coating. The smoother the coating, 
the higher the DOI. 
The following examples illustrate the invention. All parts and percentages 
are on a weight basis unless otherwise indicated. 
Example A 
Preparation of Cationic Acrylic Resin 
A cationic acrylic resin was prepared by conventional polymerization 
reaction at 230.degree. F. The ingredients used are given below. The 
resulting acrylic resin had an NVM (weight percentage of nonvolatile 
materials) of 71.5%. 
______________________________________ 
Wt. % 
______________________________________ 
2-butoxy ethanol butyl cellosolve 
24.7 
butyl acrylate 32.9 
dimethylamino ethyl methacrylate 
7.3 
styrene monomer 18.3 
hydroxyethyl acrylate 
14.6 
azobisisobutyronitrile 
2.2 
______________________________________ 
Example B 
Preparation of Cationic Acrylic Resin 
A cationic acrylic resin was prepared by conventional polymerization 
reaction at 230.degree. F. The ingredients used are given below. The 
resulting acrylic resin had an NVM (weight percentage of nonvolatile 
materials) of 60.5%. 
______________________________________ 
Wt. % 
______________________________________ 
2-butoxy ethanol butyl cellosolve 
35.5 
butyl acrylate 6.2 
styrene monomer 29.2 
dimethylaminoethyl methacrylate 
5.9 
hydroxyethyl acrylate 
10.0 
methylmethacrylate 5.9 
azobisisobutyronitrile 
1.4 
hydroxypropyl acrylate 
2.0 
2-butoxyethyl acetate 
3.8 
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Example C 
Preparation of Crosslinking Agent 
A blocked isocyanate crosslinking agent was prepared by reacting the 
following in propylene glycol monomethyl ether acetate: 
______________________________________ 
Wt. % 
______________________________________ 
caprolactam 30.9 
Desmodur N-751 69.1 
______________________________________ 
Example D 
Preparation of Conductive Primer Electrodeposition Bath 
A conductive primer electrodeposition bath was prepared by first preparing 
a make-up bath solution containing Cationic Acrylic Resins A and B, 
crosslinking agent, conductive pigment, coalescing solvent, additional 
pigments and fillers, and neutralizing agent, and thereafter diluting the 
make-up solution with deionized water. The composition of the make-up 
solution was as follows: 
______________________________________ 
Wt. % 
______________________________________ 
Crosslinking Agent C 
24.8 
Cationic Acrylic Polymer B 
18.7 
Butyl Cellosolve 8.7 
Printex L6 3.4 
Pine Oil 0.5 
Clay 1.9 
Syloid (silica) 2.9 
Cationic Acrylic Polymer A 
38.1 
Phosphoric Acid 0.4 
Lactic Acid 0.4 
______________________________________ 
To one part make-up solution was added three parts (by volume) of deionized 
water to reduce the NVM of the diluted bath to about 13.0 to 13.5%. The 
diluted bath was then stirred in an open vessel for about 24 hours to 
allow the electrodeposition bath to become a homogeneous solution and 
reach equilibrium prior to electrocoating. 
The temperature of the bath was about 80.degree. F., and the prepared 
phosphated steel panels were cathodically electrocoated using 50-225 volts 
for 2 minutes and rinsed. The panels were baked for about 20 minutes at 
380.degree. F. to give a coating having a dry film thickness of 1.0-1.5 
mils. 
Example E 
Preparation of Topcoat Electrodeposition Bath 
A topcoat electrodeposition bath was prepared by first preparing a make-up 
bath solution containing Cationic Acrylic Resin A, crosslinking agent, 
pigments and fillers, coalescing solvent, and neutralizing agent, and 
thereafter diluting the concentrated bath solution with deionized water. 
The composition of the make-up solution was as follows: 
______________________________________ 
Wt. % 
______________________________________ 
Crosslinking Agent A 
28.3 
Butyl Cellosolve 3.2 
Pine Oil 1.2 
BYK VP320 (organosiloxane) 
1.1 
Cationic Acrylic Polymer A 
59.6 
Pigment 5.4 
Phosphoric Acid 0.6 
Lactic Acid 0.6 
______________________________________ 
To one part of make-up solution was added five parts (by volume) of 
deionized water to reduce the NVM of the diluted bath to 10.0-10.5%. The 
diluted bath was then stirred in an open vessel for about 24 hours to 
allow the electrodeposition bath to become a homogeneous solution and 
reach equilibrium prior to electrocoating. 
The zinc phosphated cold rolled steel panels that had been coated, rinsed 
and baked in Example D with film builds of 1.0-1.5 mils were immersed in 
the topcoat electrodeposition bath of Example E and cathodically 
re-electrocoated by applying 100-225 volts for about 2 to 3 minutes. After 
rinsing, these panels were thermally cured by placing the panels in an 
oven at 380.degree. F. for about 20 minutes, to give a total dry film 
build of about 2.5 to 3.6 mils. The resulting cured coating had a 
20.degree. gloss of 74%. 
Comparative Examples 
Several conductive carbon blacks were used to make conductive coating 
compositions. Table I contains the properties of these carbon blacks. 
TABLE I 
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BET 
DBP absorp- 
Particle 
surface 
Ash 
Carbon tion Size area Content 
Black Supplier (ml/100 g) 
(nm) (m.sup.2 /g) 
(%) 
______________________________________ 
XE2 Degussa 400 35 1000 0.7 
L6 Degussa 120 18 265 0.2 
Conductex 975 
Columbian 170 21 270 1.0 
Ultra 
Conductex SC 
Columbian 115 20 220 1.5 
Ultra 
Black Pearls 
Cabot 330 12 1500 1.21 
2000 
Vulcan XC72 
Cabot 192 30 254 0.03 
Vulcan 0 
Cabot 116 17 140 0.01 
Acethylene 
Chevron 390 42 80 0.001 
Black 
______________________________________ 
Each of these carbon blacks was incorporated into the make-up solution for 
the conductive primer electrodeposition bath which was used to 
electrodeposit a conductive layer onto prepared phosphated steel panels in 
accordance with the process of Example D. Table II gives the results of 
the evaluation of the conductive coatings containing each of the 
conductive carbon blacks. 
TABLE II 
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Carbon Acceptability of 
Black Primer Coating 
Reason 
______________________________________ 
XE2 Unacceptable 
Too textured 
L6 Acceptable Smooth, Topcoat has 
excellent appearance, passed 
corrosion testing.sup.1, passes 
hydrolytic stability testing.sup.2 
Conductex 975 Ultra 
Unacceptable 
Fails hydrolytic stability 
testing.sup.2 
Conductex SC Ultra 
Unacceptable 
Too textured 
Black Pearls 2000 
Unacceptable 
Failed corrosion testing.sup.1 
Vulcan XC72 Unacceptable 
Too textured 
Vulcan 0 Unacceptable 
Too textured 
Acethylene Black 
Unacceptable 
Unacceptable plating 
characteristics, i.e., non- 
uniform film build, 
non-continuous film 
______________________________________ 
.sup.1 To pass corrosion testing, the coating must have &lt;3 mm creep/scrib 
after 192 hours of salt spray according to ASTM B117. 
.sup.2 To pass hydrolytic stability testing, the electrodeposition bath 
must be substantially free of pigment settling or separation after a 
minimum of 4 weeks of static pumping and produce no substantial decrease 
in the appearance and performance characteristics of the deposited 
coating. 
Of the conductive blacks evaluated, only the conductive carbon black having 
an average particle size of about 20 nm or less, a surface area (BET) of 
about 240 to 300 m.sup.2 /g and an oil absorption (DBP) of about 100 to 
150 ml/100 g produced acceptable conductive primer coatings.