The invention relates to cathodically depositable aqueous electropaints which contain cationic amine-modified epoxy resins and are pigmented with electroconductive carbon black and which, on deposition and crosslinking, provide electrophoretically overcoatable coatings and contain as electroconductive carbon black a carbon black having an iodine absorption of 870-930 mg/g, a specific surface area (BET/N.sub.2) of 850-1,000 m.sup.2 /g, a pore volume (DBP) of 330-390 ml/100 g and an average particle size of 25-35 nm in an amount of 1.5 to 5.1% by weight - based on total solids.

The present invention relates to cathodically depositable aqueous 
electropaints which contain cationic amine-modified epoxy resins and are 
pigmented with electroconductive carbon black and which, after deposition 
and crosslinking, provide electrophoretically overcoatable coatings. 
Cathodic electrocoating is a very frequently employed coating method, 
whereby water-dilutable synthetic resins with cathodic groups are applied 
to electroconductive bodies. 
Cationic amine-modified epoxy resins are particularly highly suitable for 
use as binders for aqueous cathodically depositable electropaints. 
In standard electrocoating, the coating deposited is electrically 
insulating and no longer electrophoretically overcoatable. 
To make it possible to use the technically very advantageous cathodic 
electrocoating process not just for base coating but for the production of 
multibuild coats as well, it has been tried to obtain electrophoretically 
overcoatable electropaint coats by depositing aqueous electropaints 
pigmented with conductive carbon blacks (GB No. 2,129,807). 
In said GB No. 2,129,807, 8-50% by weight of finely divided carbon--for 
example graphite or carbon black having an oil absorption of 45-115 ml/100 
g and a particle diameter of 15-85 .mu.m--are added to a cathodically 
depositable aqueous electropaint containing a cationic synthetic resin as 
binder. 
However, the use of carbon black in electropaints leads to flaws in the 
surfaces of the deposited coats (roughness, pores, holes . . . ), which 
reduce the protection against corrosion and adversely affect the 
appearance and the physico-chemical properties of the electrophoretically 
applied second coat. 
The surface flaws caused by the admixture of carbon black pigments decrease 
in intensity and frequency with decreasing carbon black concentration. 
A further very important point to be observed in the industrial application 
of carbon black pigmented electropaints is the reliability with which the 
carbon black pigment can be applied. Application reliability can be said 
to be high when electrophoretically overcoatable coatings having a 
tolerable degree of surface flaws ar obtained even in the event of major 
fluctuations in the concentration of carbon black. 
The invention has for its object to provide, for use in cathodically 
depositable aqueous electropaints which contain cationic amine-modified 
epoxy resins and are pigmented with electroconductive carbon black and 
which, after deposition and crosslinking, provide electrophoretically 
overcoatable coatings, carbon black pigments which not only can be 
employed in a very low concentration but also guarantee a high degree of 
application reliability. 
This object is achieved according to the invention by employing as the 
electroconductive carbon black a carbon black which has an iodine 
absorption of 870-930 mg/g, a specific surface area (BET/N.sub.2) of 
850-1,000 m.sup.2 /g, a pore volume(DBP) of 330-390 ml/100 g and an 
average particle size of 25-35 nm in an amount of 1.5-5.1% by 
weight--based on total solids. 
The invention also relates to a process for preparing a multibuild coating 
by applying to an electroconductive substrate by electrophoresis a 
cathodically depositable aqueous electropaint which contains a cationic 
amine-modified epoxy resin and is pigmented with electroconductive carbon 
black, crosslinking, and overcoating, again by electrophoresis, wherein 
the electropaint used for preparing the basecoat contains as 
electroconductive carbon black a carbon black in an amount of 1.5-5.1% by 
weight, based on total solids--which has the following analytical data: 
______________________________________ 
ASTM test 
method used 
______________________________________ 
Iodine absorption 
870-930 mg/g D 1510-79 
Specific surface area 
(BET/N.sub.2) 850-1000 m.sup.2 /g 
D 3027-78 
Pore volume (DBP) 
330-390 ml/100 g 
D 2414-79 
Average particle size 
25-35 nm 
______________________________________ 
The process is carried out as follows: The electroconductive substrate to 
be coated is dipped into the aqueous electrocoating bath and, after 
applying an electrical voltage between an anode and the substrate 
connected as cathode, is coated. 
On conclusion of deposition the substrate is removed from the bath, and the 
applied coat is rinsed off and baked in a known manner. This is followed, 
under the same conditions, by a further electrophoretic coating step and 
aftertreatment. 
Virtually any electroconductive substrates 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 carbon blacks used according to the invention are distinguished by an 
iodine absorption of 870-930 mg/g, a specific surface area (BET/N.sub.2) 
of 850-1,000 m.sup.2 /g, a pore volume (DBP) of 330-390 ml/100 g and an 
average particle size of 25-35 nm. 
A carbon black found to be particularly suitable has an iodine absorption 
of 900 mg/g, a specific surface area (BET/N.sub.2) of 950 m.sup.2 /g, a 
pore volume (DBP) of 360 ml/100 g and an average particle size of 30 nm. 
A carbon black of this type is marketed by Akzo Chemie under the trademark 
KETJENBLACK EC. In the brochure issued with this product it is pointed out 
that KETJENBLACK EC is usable for increasing the conductivity of polymer 
mixtures such as plastics and rubber mixtures and that to obtain a certain 
conductivity the amount of KETJENBLACK EC which is required is only a 
third or a quarter of that which would be required in the case of other 
electroconductive carbon blacks. 
However, the brochure provides no indication whatsoever that KETJENBLACK EC 
is usable in aqueous electropaints, and it is surprising that this carbon 
black :an be used in a very low concentration in cathodically depositable 
aqueous electropaints which contain cationic amine-modified epoxy resins 
and are pigmented with electroconductive carbon black and which, after 
deposition and crosslinking, are to provide electrophoretically 
overcoatable coats and at the same time guarantees a high degree of 
application reliability. 
The electropaints according to the invention provide coatings which are 
electrophoretically overcoatable not only by the anodic but also by the 
cathodic electrocoating process. 
If the electropaints pigmented according to the invention are used for the 
overcoats, it is also possible to build coatings which consist of more 
than two layers. 
Electropaints which contain less than 1.5% by weight--based on total 
solids--of the carbon blacks according to the invention provide coatings 
which can no longer be electrophoretically overcoated with a continuous 
second coat. 
If the carbon black concentration is above 5.1% by weight, the coatings 
obtained have no longer tolerable surface flaws (warts). 
The cationic amine-modified epoxy resins used as binders are reaction 
products formed from 
(A) polyepoxides 
(B) primary and/or secondary amines or salts thereof and/or salts of 
tertiary amines and optionally 
(C) polyfunctional alcohols, polycarboxylic acids, polyamines or 
polysulfides. 
Water-dispersible products are obtained after neutralization with an acid. 
Suitable for use as component A is any compound which contains two or more 
epoxy groups in the molecule. Preference is given to those compounds which 
contain two epoxy groups in the molecule and have a relatively low 
molecular weight of at most 750, preferably 400-500. 
Particularly preferred epoxy compounds are polyglycidyl ethers of 
polyphenols prepared from polyphenols and epihalohydrins. Suitable 
polyphenols are for example very particularly preferably bisphenol A and 
bisphenol F and particularly preferably 
1,1-bis-(4-hydroxyphenyl)-n-heptane. 
Also suitable are 4,4'-dihydroxybenzophenone, 
bis-(4-hydroxyphenyl)-1,1-ethane, bis-(4-hydroxyphenyl)-1,1-isobutane, 
bis-(4-hydroxy-tertiary-butylphenyl)-2,2-propane, 
bis-(2-hydroxynaphthyl)methane, 1,5-dihydroxynaphthanene and phenolic 
novolak resins. 
Preferred epoxy compounds also include polyglycidyl ethers of polyhydric 
alcohols, for example ethylene glycol, diethylene glycol, triethylene 
glycol, 1,2-propylene glycol, 1,4-propylene glycol, 1,5-pentanediol, 
1,2,6-hexanetriol, glycerol and bis-(4-hydroxycyclohexyl)-2,2-propane. 
It is also possible to use polyglycidyl esters of polycarboxylic acids, for 
example oxalic acid, succinic acid, glutaric acid, terephthalic acid, 
2,6-naphthalenedicarboxylic acid, dimerized linolenic acid. Typical 
examples are glycidyl adipate and glycidyl phthalate. 
It is also possible to use hydantoin epoxides, epoxidized polybutadiene and 
polyepoxy compounds which are obtained by epoxidizing an olefinically 
unsaturated alicyclic compound. 
Suitable for use as component B are primary and/or secondary amines and 
salts thereof and/or salts of tertiary amines, the secondary amines being 
particularly preferred components B. 
Preferably the amine should be a water-soluble compound. Examples of such 
amines are mono- and dialkylamines, such as methylamine, ethylamine, 
propylamine, butylamine, dimethylamine, diethylamine, dipropylamine, 
methylbutylamine and the like. Likewise suitable are alkanolamines such 
as, for example, methylethanolamine, diethanolamine and the like. It is 
further possible to use dialkylaminoalkylamines such as, for example, 
dimethylaminoethylamine, diethylaminopropylamine, dimethylaminopropylamine 
and the like. In most cases use is made of relatively low molecular weight 
amines, but it is also possible to use relatively high molecular weight 
monoamines. 
Polyamines having primary and secondary amino groups can be reacted with 
the epoxy groups in the form of their ketimines. The ketimines are 
prepared from the polyamines in a known manner. 
The amines can also contain other groups, but these groups should not 
interfere with the reaction of the amine with the epoxy group, nor cause 
the reaction mixture to gel. 
The charges required for water-dilutability and electrical deposition can 
be produced by protonation with water-soluble acids (for example boric 
acid, formic acid, lactic acid, propionic acid, butyric acid, hydrochloric 
acid, phosphoric acid, sulfuric acid, carbonic acid, preferably acetic 
acid) or, alternatively, by reacting the oxirane groups with salts of an 
amine. 
The amine salt used can be the salt of a tertiary amine. 
The amine part of the amine/acid salt is an amine which can be 
unsubstituted or substituted, as in the case of hydroxylamine, provided 
these substituents do not interfere with the reaction of the amine/acid 
salt with the polyepoxide or cause the reaction mixture to gel. Preferred 
amines are tertiary amines, such as dimethylethanolamine, triethylamine, 
trimethylamine, triisopropylamine and the like. Examples of other suitable 
amines are given in U.S. Pat. No. 3,839,252 in column 5, line 3 to column 
7, line 42. 
Suitable for use as component C are polyfunctional alcohols, polycarboxylic 
acids, polyamines or polysulfides or mixtures of compounds of these 
classes of substances. 
Suitable polyols include diols, triols and higher polymeric polyols, such 
as polyester polyols, polyether polyols. 
Polyalkylene ether polyols suitable for use as component C conform to the 
general formula: 
##STR1## 
n which R is hydrogen or a lower alkyl radical, with or without various 
substituents, n is 2 to 6 and m is 3 to 50 or even higher. Examples are 
poly(oxytetramethylene) glycols and poly(oxyethylene) glycols. 
The preferred polyalkylene ether polyols are poly(oxytetramethylene) 
glycols having a molecular weight within the range from 350 to 1,000. 
Polyester polyols can likewise be used as polymeric polyol component. The 
polyester polyols can be prepared by polyesterification of organic 
polycarbonate acids or anhydrides thereof with organic polyols which 
contain primary hydroxyl groups. Customarily the polycarboxylic acids and 
the polyols are aliphatic or aromatic dicarboxylic acids and diols. 
The diols used for preparing the polyesters include alkylene glycols such 
as ethylene glycol, butylene glycol, neopentylglycol and other glycols 
such as cyclohexanedimethanol. 
The acid component of polyester consists primarily of low molecular weight 
carboxylic acids or anhydrides thereof with 2 to 18 carbon atoms in the 
molecule. Suitable acids are for example phthalic acid, isophthalic acid, 
terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, adipic 
acid, azelaic acid, sebacic acid, maleic acid and glutaric acid. In place 
of these acids it is also possible to use their anhydrides, provided they 
exist. 
It is also possible to use as component (C) polyester polyols which are 
derived from lactones. These products are obtained by reacting an 
.epsilon.-caprolactone with a polyol. Products of this type are described 
in U.S. Pat. No. 3,169,945. 
The polylactone polyols which are obtained by this reaction are 
distinguished by the presence of a terminal hydroxyl group and by 
recurring polyester portions which are derived from the lactone. These 
recurring molecule fragments can conform to the formula 
##STR2## 
in which n is at least 4, preferably 4 to 6, and the substituent is 
hydrogen, an alkyl radical, a cycloalkyl radical or an alkoxy radical. 
Component C can also be an aliphatic and/or alicyclic polyfunctional 
alcohol or carboxylic acid having a molecular weight below 350. 
Advantageously these alcohols and carboxylic acids have a branched 
aliphatic chain, in particular a chain with at least one neo structure. 
Suitable compounds conform to the following general formula: 
##STR3## 
where 
Y=OH, COOH, 
X=(CH.sub.2).sub.n 
##STR4## 
R.sub.1, R.sub.2, R.sub.3 =H, alkyl radical having 1 to 5 carbon atoms, 
a=0;1, 
b=0;1, 
1=0-10, 
m,n=1-10. 
Specific examples are: diols, such as ethylene glycol, diglycol, 
dipropylene glycol, dibutylene glycol, triglycol, 1,2-propanediol, 
1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 
2,2-diethyl-1,3-propanediol, 2-methyl-2-ethyl-1,3-propanediol, 
2-methyl-2-propyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 
1,2-butanediol, 1,4-butanediol, 2,3-butanediol, 2-ethyl-1,4-butanediol, 
2,2-diethyl-1,3-butanediol, butene-2-diol-1,4, 1,2-pentanediol, 
1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 2,5-hexanediol, 
2-ethyl-1,3-hexanediol, 2,5-dimethyl-2,5-hexanediol, 1,3-octanediol, 
4,5-nonanediol, 2,10-decanediol, 2-hydroxyethyl hydroyacetate, 
2,2-dimethyl-3-hydroxypropyl 2,2-dimethylhydroxypropionate, 
2-methyl-2-propyl-3-hydroxypropyl 2-methyl-2-propylhydroxypropionate, 
4,4'-methylenebiscyclohexanol and 4,4'-isopropylidenebiscyclohexanol. Some 
preferred diols are 2,2-dimethyl-1,3-propanediol, 
3-methyl-1,5-pentanediol, 2,2-dimethyl-3-hydroxypropyl 
2,2-dimethylhydroxypropionate and 4,4'-isopropylidenebiscyclohexanol. 
The carboxylic acid used can be any one of a large number of dicarboxylic 
acids, such as oxalic acid, malonic acid, 2,2-dimethylmalonic acid, 
succinic acid, glutaric acid, adipic acid, hexahydrophthalic acid, maleic 
acid, fumaric acid, pimelic acid, suberic acid, azelaic acid, sebacic 
acid, itaconic acid, citraconic acid, mesaconic acid and glutaconic acid. 
Dicarboxylic acids which are preferably used are for example 
2,2-dimethylmalonic acid and hexahydrophthalic acid. 
It is also possible to use long-chain dicarboxylic acids as component C. 
Examples thereof are dimeric fatty acids, such as, for example, dimeric 
linoleic acid. 
Suitable polyamines for elastification can be prepared for example by 
reacting primary diamines and monoepoxides. The secondary, substituted 
diamines formed modify the epoxy resins in a suitable manner. 
Component C can also be a primary-tertiary diamine or an alkanolamine such 
as aminoethanol or aminopropanol. 
Suitable polyfunctional SH compounds are reaction products of organic 
dihalides with sodium polysulfide. Further SH compounds are for example 
reaction products of hydroxyl-containing linear polyesters, polyethers or 
polyurethanes with mercaptocarboxylic acids such as mercaptoacetic acid, 
2-mercaptopropionic acid, 3-mercaptopropionic acid, mercaptobutyric acid 
and the like. 
Polyphenols suitable for use as component C conform to the general formula 
(I) 
##STR5## 
in which X=alkylene, arylene, alkarylene, O,O-alkylene, O-arylene, 
O-alkarylene, S, S-alkylene, S-arylene, S-alkarylene, CO, CO-alkylene, 
CO-arylene, CO-alkarylene, NH, NH-alkylene, NH-arylene or NH-alkarylene, 
x=0 or 1, 
##STR6## 
Z=alkylene, alkylene radical based on polyesters, polyethers, polyamides, 
polycarbonates or polyurethanes and R=H, CH.sub.3, alkyl--, --O--CH.sub.3, 
--O--alkyl, --NO.sub.2, NR'.sub.2 ', --NR'R", --NHCOR'". 
Finally, component (C) can also comprise polyurethanes prepared by 
generally known methods. 
The binders used according to the invention can be conventionally 
crosslinked by addition of crosslinking agents or converted into 
self-crosslinking systems by chemical modification. A self-crosslinking 
system can be obtained for example by reacting the binder with a partially 
blocked polyisocyanate which has on average one free isocyanate group per 
molecule and whose blocked isocyanate groups only become unblocked at 
elevated temperatures. Suitable crosslinking agents are virtually all at 
least bifunctional compounds which react with oxirane groups, for example 
polyalcohols, polyphenols, polycarboxylic acids, polycarboxylic 
anhydrides, polycarboxamides, polyamines, polyisocyanates, phenolic 
resins. 
The crosslinking agents are generally used in an amount of 5 to 60, 
preferably 20 to 40, % by weight, based on the binder. 
Frequently employed methods for crosslinking binders are described for 
example in the following patent documents: GB No. 1,303,480, European 
Patent Application No. 12,463, U.S. Pat. No. 4,252,703 and GB No. 
1,557,516. 
Examples of suitable amino resin crosslinking agents are the hexamethyl 
ether of hexamethylolmelamine, the triethyltrimethyl ether of 
hexamethylolmelamine, the hexabutyl ether of hexamethylolmelamine and the 
hexamethyl ether of hexamethylolmelamine and polymeric butylated 
melamine-formaldehyde resins. Alkylated ureaformaldehyde resins are also 
usable. Preferred crosslinking agents are blocked polyisocyanates. With 
the binders used according to the invention it is possible to use any 
desired polyisocyanates 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, 
in general within the range from about 90.degree. to about 300.degree. C. 
In the preparation of blocked polyisocyanates, it is possible to use any 
desired organic polyisocyanate suitable for the crosslinking. Preference 
is given to isocyanates which contain about 3 to 36, in particular about 8 
to about 15 carbon atoms. Examples of suitable diisocyanates are 
trimethllene 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-isocyanatocyclohexyl)methane, bis(4-isocyanatophenyl)methane, 
4,4'-diisocyanatodiphenyl ether and 
2,3-bis(8-isocyanatooctyl)-4-octyl-5-hexylcyclohexane. 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-triisocyanatotoluene, 1,3,5-tris(6-isocyanatohexylbiuret), 
bis(2,5-diisocyanato-4-methylphenyl)methane and polymeric polyisocyanates, 
such as dimers and trimers of diisocyanatotoluene. It is further also 
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 or amines such as dibutylamine and diisopropylamine. The 
polyisocyanates and blocking agents mentioned, provided they are mixed in 
suitable ratios, can also be used for preparing the partially blocked 
polyisocyanates described above. 
Binders which are preferably usable for the present invention are disclosed 
in DEP No. 2,701,002, the contents of which are incorporated herein by 
reference. 
The aqueous coating composition according to the invention can in general 
contain customary additives such as, for example, coalescent solvents, 
pigments, surface-active agents, crosslinking catalysts, antioxidants, 
fillers and antifoams.

The invention is illustrated in more detail in the following examples. 
Parts and percentages are by weight, unless otherwise stated. 
Preparation of a polyurethane crosslinking agent for binder C (in 
accordance with Example 1 of DEP No. 2,701,002) 
A polyurethane crosslinking agent was prepared in a reactor by slowly 
adding 218 parts by weight of 2-ethylhexanol to 291 parts by weight of an 
80/20 isomer mixture of 2,4-/2,6-toluene diisocyanate with stirring in a 
nitrogen atmosphere, the reaction temperature being maintained below 
38.degree. C. by external cooling. The batch was then maintained at 
38.degree. C. for a further half hour and thereafter heated to 60.degree. 
C., whereafter 75 parts by weight of trimethylolpropane and then 0.08 part 
of dibutyltin dilaurate were added as catalyst. After an exothermic 
reaction at the start, the batch was maintained at 121.degree. C. for 1.5 
hours, until substantially all the isocyanate groups had been consumed, 
which could be seen on an infrared spectrum. The batch was then diluted 
with 249 parts of ethylene glycol monoethyl ether. 
Preparation of a binder concentrate of binder C (in accordance with Example 
6 of DEP No. 2,701,002) 
A suitable reactor was charged with 882.9 parts of a commercially available 
polyglycidyl ether of a polyphenol having an epoxy equivalent of 465, 
251.8 parts of poly(neopentylglycol adipate) having a molecular weight of 
530 and 69.9 parts of xylene. The batch was heated under reflux to the 
boil under a nitrogen atmosphere for about 20 minutes, during which the 
water was removed from the azeotropic mixture. After cooling to 
130.degree. C., 3.2 parts of benzyldimethylamine were added, and the batch 
was maintained at 130.degree. C. for 2 hours and 20 minutes. 862 parts of 
the polyurethane crosslinking agent were then added. The reaction 
temperature was reduced to 90.degree. C., and 73.8 parts of a solution 
containing 73% of non-volatile portions of the methyl isobutyl diketimine 
of diethylenetriamine in methyl isobutyl ketone were added. 56.1 parts of 
N-methylethanolamine were then added, and the batch was heated to 
110.degree. C. After that temperature had been maintained for about 1 
hour, 90.4 parts of ethylene glycol monohexyl ether were added. Methyl 
isobutyl ketone and ethylglycol were then added to dilute to a solids 
content of 65%. 
Catalyst paste for binder C (in accordance with Example 6 of DEP No. 
2,701,002) 
The catalyst paste used was prepared from the following starting materials: 
______________________________________ 
Constituents Parts by weight 
______________________________________ 
binder concentrate 
201.5 
deionized water 460.3 
dibutyltin oxide 
283.7 
______________________________________ 
These constituents were mixed and comminuted in a suitable mill to Hegman 
fineness No. 7. 
Carbon blacks used 
5 different carbon blacks were used as pigments. Their most important 
properties are given in the following table: 
R.sub.1 is a coloring black; the other 4 kinds of carbon black are 
pronounced conducting blacks. 
______________________________________ 
Particle BET surface 
DBP absorp- 
Carbon Manufac- diameter area tion 
black turer nm m.sup.2 g.sup.-1 
ml 100 g 
______________________________________ 
R.sub.1 
(Raven Columbian 70 28 70 
410) 
R.sub.2 
(Corax 
L) Degussa 23 150 123 
R.sub.3 
(Corax Degussa 18 265 136 
L6) 
R.sub.4 
(Conduc- 
Columbian 16 1075 228 
tex 
40-220) 
R.sub.5 
(Ketjen 
AKZO 30 950 360 
Black 
EC) 
______________________________________ 
Preparation of concentrates pigmented with carbon black 
The carbon blacks were incorporated into the binder by means of a 
dissolver. 
To this end, binder concentrate, carbon black and neutralizing agent were 
weighed out into a three liter metal can. By adding additional, 
concentrate-specific organic solvents the viscosity of the mixture was 
adjusted in such a way that the dissolver was able to agitate the mixture. 
Milling then takes place at 3,500 r.p.m. until no particles greater than 5 
.mu.m were present any longer (Hegman Grindometer). The carbon black 
pigmented and partially neutralized concentrates thus obtained are 
storable for some months and are then easily dilutable to electrocoating 
baths. 
Formulation of electrocoating baths 
The electrocoating baths were prepared by stirring together carbon black 
pigmented concentrate, additional binder, neutralizing agent and organic 
solvents using a V2A stainless steel blade stirrer and thereafter diluting 
the mixture with distilled water. 
The respective amounts were calculated in such a way that the coating baths 
within a carbon black concentration series always contained the same 
amounts of binder, solvent, neutralizing agent and water. They only 
differed in the carbon black they contained and hence also in the solids 
content. 
The composition of the baths is given in the table below for the customary 
3 l batch. The table also provides information about the degrees of 
neutralization .alpha. of the formulations. The solvent quantities relate 
to the original weight. The respective carbon black contents of the baths 
were obtained either by preparing a fresh mixture from the start or by 
weighing together appropriate amounts of high- and low-pigmented baths. 
The baths contain still larger amounts of organic solvent which were 
necessary for incorporating and wetting the carbon black but which 
interfere with the deposition. 
For that reason the baths were stirred for some time at elevated 
temperature before use. This stirring was accompanied by loss of solvent, 
which is replaced by distilled water, and an aging of the bath. 
__________________________________________________________________________ 
Types of carbon Carbon/% 
Binder 
black m.sub.BM /g 
m.sub.H.sbsb.2 O/g 
m.sub.Lsgm /g 
m.sub.R /g 
FK/% black in FK 
.alpha./% 
__________________________________________________________________________ 
A R.sub.1 213.3 
2285.1 
415.0 
0-68.8 
7.9-10.0 
0-22.9 143 
A R.sub.2 213.3 
2285.1 
415.0 
0-68.8 
7.9-10.0 
0-22.9 143 
A R.sub.3 213.3 
2285.1 
415.0 
0-68.8 
7.9-10.0 
0-22.9 143 
A R.sub.4 213.3 
2285.1 
415.0 
0-31.3 
7.9-8.9 
0-11.9 143 
A R.sub.5 213.3 
2285.1 
415.0 
0-31.3 
7.9-8.9 
0-11.9 143 
C R.sub.1 435.4 
2075.1 
297.2 
0-104.7 
15.0-18.0 
0-19.4 50 
C R.sub.2 435.4 
2075.1 
297.2 
0-104.7 
15.0-18.0 
0-19.4 50 
C R.sub.3 435.4 
2075.1 
297.2 
0-104.7 
15.0-18.0 
0-19.4 50 
C R.sub.4 435.4 
2055.0 
363.4 
0-58.6 
15.0-16.5 
0-11.9 50 
C R.sub.5 435.4 
2055.0 
363.4 
0-58.6 
15.0-16.5 
0-11.9 50 
__________________________________________________________________________ 
Composition of the baths 
BM = binder 
m = mass 
R = carbon black 
g = gram 
FK = solids content 
lsgm = solvent 
Depositions 
The depositions were customarily carried out on ST 1203 steel sheets 
measuring 130.times.45.times.1 mm. 
The steel sheets were pretreated as follows: 
(a) degreasing in 40.degree.-60.degree. C. petroleum ether 
(b) derusting with a steel brush 
(c) cleaning with an abrasive powder 
(d) rinsing off with distilled water 
(e) rinsing off with acetone 
(f) drying at 60.degree. C. 
Some coatings were carried out on phosphatized steel sheet (Bonder 120, 
Metallgesellschaft) of the same dimensions. 
The coating of the steel sheets was carried out at constant voltage; the 
entire voltage was applied across the system immediately on switching on. 
The deposition voltage was 300 V in the case of binder C and 150 V in the 
case of binder A. 
After the current was switched off, the coated samples were removed from 
the bath as quickly as possible and freed from adhering bath liquor by 
means of a concentrated jet of water. This is followed by rinsing with 
distilled water and blowing dry with air. 
The baking time was 20 minutes. The baking temperatures were 
170.degree.-180.degree. C. 
Binder A 
Binder A was the Luhydran E33 resin obtained from BASF AG, which is an 
anodically depositable polyacrylate resin which crosslinks via methylol 
ether groups (supply form: 70% strength in Isanol, acid number 37, 
neutralizing agent: dimethylethanolamine). 
Evaluation of the experimental results 
The following table shows for binder system C the minimum carbon black 
pigment concentration necessary for the deposition of a continuous second 
coat. 
For comparison, corresponding measurements were carried out on binder 
system A. 
______________________________________ 
Binder Pigment c.sub.GS 1% by weight 
______________________________________ 
C R.sub.1 &gt;19.4 
C R.sub.2 9.8 
C R.sub.3 7.5 
C R.sub.4 7.5 
C R.sub.5 1.5 
A R.sub.1 9.8 
A R.sub.2 4.4 
A R.sub.3 4.0 
A R.sub.4 4.0 
A R.sub.5 4.4 
______________________________________ 
C.sub.GS = concentration limit for depositing a continuous second coat 
The above table shows significant differences between the two binder 
systems C and A. Whereas system C, when filled with R.sub.5, provides 
electrophoretically overcoatable coatings even at extremely low carbon 
black concentrations, in system A the R.sub.5 -filled samples are not 
special; here it is possible--irrespective of the carbon black used--to 
overcoat at above a carbon black concentration of 4.0% by weight. 
On continuously increasing the carbon black concentration there comes a 
point where the surface flaws become intolerable. Wartlike structures 
appear. The formation of warts indicates that in these areas the wet film 
is already completely electroconductive. 
The following table gives the carbon black pigment concentrations (c.sub.W) 
from which on it was possible to identify warts in binder C. 
______________________________________ 
Binder Pigmentation 
c.sub.W /% be weight 
______________________________________ 
C R.sub.1 30.0 
C R.sub.2 17.8 
C R.sub.3 15.9 
C R.sub.4 11.9 
C R.sub.5 5.1 
______________________________________ 
C.sub.W = concentration limit of wart formation 
Since precise control of the carbon black pigment concentration in the 
electrocoating bath and in the deposited coat presents problems in 
industrial practice, the carbon black pigment used should offer a high 
degree of application reliability. 
If the minimum carbon black concentration c.sub.GS necessary for depositing 
a continuous second coat is divided by the carbon black pigment 
concentration from which on formation of warts is observable (c.sub.W), 
this produces a measure of application reliability: 
______________________________________ 
.sup.C GS 
Binder Pigmentation 
.sup.c W 
______________________________________ 
C R.sub.1 &gt;0.647 
C R.sub.2 0.551 
C R.sub.3 0.472 
C R.sub.4 0.630 
C R.sub.5 0.294 
______________________________________ 
The above table reveals that in the case of pigmentations with R.sub.5 no 
more than 29.4% of the amount of carbon black at which wart formation is 
observed is needed to obtain an electrophoretically overcoatable coating. 
The wide range between the concentration limits c.sub.GS and c.sub.W thus 
offers a high degree of application reliability, which is nowhere near 
reached by the other conducting carbon blacks R.sub.2, R.sub.3 and R.sub.4 
--which in addition need to be used in higher concentrations.