Electrodepositable preparations having low organic solvent content, and processes for the preparation thereof

Electrodepositable stable coating preparation essentially comprising a synthetic resin (A), which contains amino groups and, if appropriate, hydroxyl groups, a hardener (B) which is capable of transesterification and/or transamidation, water as diluent (C), if appropriate, organic solvents (D), and also the conventional coating additives and, if appropriate, hardening catalysts (E), wherein the organic solvents content is a maximum of 10% by weight, relative to the solids content. The invention furthermore relates to a process for the preparation of this coating preparation, which is distinguished by a low organic solvents content with good storage stability.

Processes for the preparation of cathodically depositable coating 
preparations which contain binders which cross-link at low temperatures 
are described, for example, in German Patent Application No. P 3,602,981.5 
(title: "Hardening components for synthetic resins which contain groups 
which are capable of forming amides or esters with carboxylic acids"), 
filed on the same day, Austrian Patent Application No. 1602/85 (title: 
"Process for the preparation of cross-linking components for coating 
binders"), and German Offenlegungsschriften No. 3,315,469 and No. 
3,417,441. These binders consists of amino group-containing synthetic 
resins which, if appropriate, contain additional OH groups, and of the 
hardener for these synthetic resins. The amino groups may be primary, 
secondary or tertiary. In order to prepare the coating preparations, the 
synthetic resins and hardeners, present in organic solvents, are mixed and 
their amino groups are entirely or partially neutralized by reaction with 
a water-soluble acid. The mixture is subsequently diluted with water. 
The organic solvents in which the synthetic resins are present are 
water-soluble and generally have a boiling point of about 100.degree. C., 
for example diethylene glycol monomethyl or dimethyl ether or propylene 
gycol monomethyl or dimethyl ether. Their presence is necessary during the 
preparation of the synthetic resins. Thus for, example, polymers of 
.alpha.,.beta.-unsaturated monomers can only be prepared in solution, it 
only being possible to achieve low molecular weights by polymerization at 
elevated temperatures. The preparation of suitable polymers by 
polymer-analogous reaction also only succeeds in solvents at elevated 
temperature. The epoxide resins, which are solid at room temperature, must 
be dissolved in organic solvents before reaction with compounds containing 
amino groups and, if appropriate, also acid groups. 
A disadvantage of the process used hitherto is the relatively high 
proportion of organic solvents in the cathodically depositable coating 
preparations prepared therefrom. It is generally between 3 and 10% by 
weight at a solids content of the bath of 15 to 20% by weight, i.e., the 
organic solvents content, relative to the solids content, may be up to 50 
to 60% by weight. 
There is great interest in reducing the proportion of organic solvents so 
that the electrodepositable coating preparation is virtually free of 
solvents, possibly apart from small proportions which have a favorable 
effect on the reduction of the film-formation temperature. Although the 
removal of the solvents by distillation from the synthetic resin and the 
hardener in separate operations and the preparation of aqueous dispersions 
which are stable over a relatively long period have succeeded, 
sedimentation or floating, or both effects simultaneously, occur, however, 
after a short time on mixing the synthetic resin and the hardener 
dispersions. However, the synthetic resin and hardener can also not be 
mixed before the dilution with water since the synthetic resins are solid 
or very viscous at room temperature. Liquefaction by increasing the 
temperature is likewise ruled out since the cross-linking reaction starts 
above 40.degree. to 50.degree. C.

The invention therefore has the object of providing an electrodepositable 
coating preparation, based on the reactive components mentioned above, 
which has a particularly low organic solvents content, in which no 
reaction has yet occurred between the components, and which also exhibits 
no sedimentation and/or floating on storage for a number of weeks. 
The invention therefore relates to an electrodepositable, stable coating 
preparation essentially comprising a synthetic resin (A) which contains 
amino groups and, if appropriate, hydroxyl groups, a hardener (B) which is 
capable of transesterification and/or transamidation, water as diluent 
(C), if appropriate, organic solvents (D), and also the conventional 
coating additives and, if appropriate, hardening catalysts (E), wherein 
the organic solvents content is a maximum of 10% by weight, preferably a 
maximum of 7.5% by weight, and particularly 2.0 to 7.0% by weight, 
relative to the total solids content. This term shall include pigments and 
other solid additives, if any; it is normally determined at 180.degree. 
C./0.5 h according to DIN 52316. 
The invention furthermore relates to a process for the preparation of these 
electrodepositable paint preparations with the feature that the resin (A) 
and, if appropriate, also the hardener (B) are initially, separately, 
substantially freed of solvent, the residue remaining is diluted with a 
water-soluble solvent having a boiling point below 100.degree. C., and 
subsequently the resin (A) and hardener (B) are mixed at a batch 
temperature at which the components do not react, whereupon the amino 
groups present are partially or completely neutralized using a 
water-soluble acid, the batch is additionally diluted with water, and the 
organic solvents are removed from the aqueous dispersion under reduced 
pressure at slightly elevated temperature. 
Cationic resins such as have already been described in great number in the 
literature, are employed as compounds (A). The requirement for their 
possible use is a number of basic groups, such as primary, secondary or 
tertiary amino groups, which is sufficient to ensure perfect dilutability 
with water. If these resins (A) contain primary and/or secondary amine 
groups then they may or may not contain also hydroxyl groups and 
preferably they do. If only tertiary amino groups are present in (A), then 
(A) must contain them in order to enable cross-linking by the hardener (B) 
via transesterification. The amino equivalent weight is expediently 150 to 
3000, preferably 500 to 2000. The hydroxyl equivalent weight of the 
resins, if they have OH groups, is generally between 150 and 1000, 
preferably 200 to 500. In addition, the resins may contain C.dbd.C double 
bonds, the C.dbd.C equivalent weight preferably being 500 to 1500. 
The molecular weight (mean weight) of these synthetic resins (A) is usually 
in the range from about 300 to about 50,000, preferably about 5000 to 
about 20,000. 
Examples of such synthetic resins (A) are described in the Journal of 
Coatings Technology, Vol. 54, No. 686, (1982), p. 33 to 41 ("Polymer 
Compositions for Cationic Electrodepositable Coatings"), to which 
reference is made here. Polymers of .alpha.,.beta.-olefinically 
unsaturated monomers which contain hydroxyl and/or amino groups may be 
mentioned here. The hydroxyl or amino groups may be introduced using 
appropriate monomers in the copolymerization, for example by means of 
hydroxyl or amino esters of .alpha.,.beta.-olefinically unsaturated 
carboxylic acids, such as hydroxyalkyl (meth)-acrylates or aminoalkyl 
(meth)acrylates, or by polymeranalogous reaction with diamines or 
polyamines, for example with N,N-dimethylaminopropylamine, with formation 
of amide, amino or urethane groups. The polyaminopolyamides, which can be 
obtained from dimerized fatty acids and polyamines, are a further group. 
Aminopolyether polyols, which are accessible by reaction of primary or 
secondary amines with a polyglycidyl ether, are particularly suited for 
this. Sufficient epoxide groups to convert all amino groups into tertiary 
amino groups should expediently be present here. The preferred 
polyglycidyl ethers are polyglycidyl ethers of bisphenol A and similar 
polyphenols. They can be prepared, for example by etherifying a polyphenol 
using an epihalohydrin, such as epichlorohydrin, in the presence of 
alkali. 
The polyglycidyl ethers of the polyphenols may be reacted as such with the 
amines, but it is frequently advantageous to react some of the reactive 
epoxide groups with a modified material in order to improve the film 
properties. The reaction of the epoxide groups with a polyol or a 
polycarboxylic acid is particularly preferred. The following may be used 
here as polyols: 
Polyether polyols, which are prepared by addition polymerization of 
alkylene oxides (for example ethylene oxide, propylene oxide, 
tetrahydrofuran) with low-molecular-weight polyols having 2 to 8 carbon 
atoms and a molecular weight of about 50 to 300 (for example ethylene 
glycol, diethylene glycol, propylene glycol, dipropylene glycols, 
glycerol, trimethylolpropane, 1,2,6-hexanetriol, pentaerythrite). If 
ethylene oxide is used alone or in combination with other alkylene oxides 
as alkylene oxide components, the water-solubility of the synthetic resin 
(A) is improved; 
polyester polyols, which are prepared by reaction of the above mentioned 
low-molecular weight polyols or epoxy compounds, for example fatty acid 
glycidyl esters, with polycarboxylic acid (for example adipic acid, 
succinic acid, maleic acid, phthalic acid, or terephthalic acid), or 
derivatives thereof; 
polyester polyols which are prepared by ring-opening polymerization of a 
cyclic ester, such as caprolactone or butyrolactone; 
urethane-modified polyls which are obtained by reaction of an excess of the 
abovementioned polyether polyols or polyester polyols with an organic 
polyisocyanate. 
The abovementioned polycarboxylic acids are obtained by reaction of the 
polyols described above with an excess of polycarboxylic acids or, 
preferably, the anhydrides thereof. They can likewise be obtained by 
esterification of polycarboxylic acids, or anhydrides thereof, using 
low-molecular weight polyols, such as ethylene glycol, propylene glycol, 
etc. Low-molecular weight polyether polyamines or polyamines, such as, for 
example, hexamethylenediamine, may also be employed in place of the 
low-molecular weight polyols. 
The modification of the aminopolyether polyols using polyols or 
polycarboxylic acids is preferably carried out before the reaction of the 
polyglycidyl ethers with the primary or secondary amaines. However, it is 
also possible to select the ratio of the polyglycidyl ether used as 
starting material to the amines in such a fashion that an excess of epoxy 
groups is present. The epoxy groups may then be reacted with the 
polycarboxylic acids or polyols. It is furthermore possible to further 
modify, the final product, which no longer contains epoxide groups, by 
reaction of the hydroxyl groups with glycidyl ethers. 
The terminal and/or pending ester groups which the hardener (B) contains 
are substantially stable in neutral aqueous media, but in the alkaline 
medium of the deposited film they react with the primary/secondary amine 
groups and/or the hydroxyl groups of resin (A) under the curing 
conditions, i.e., normally temperatures above 120.degree. C., preferably 
130.degree. to 180.degree. C. and times longer than 0.3 h, preferably 0.5 
to 1 h, via transamidation and/or transesterification. The reactivity of 
the ester groups can be increased by using lower ester alcohols and/or by 
increasing the electrophilic activity of the carboxyl group by suitable 
substituents. 
As hardener (B), Michael-addition products may be employed, for example, 
such as are described in German Offenlegungsschriften No. 3,315,469; No. 
3,417,441 and No. 3,602,981. Other hardeners also applicable in the 
coating compositions of the invention are disclosed in German 
Offenlegungsschriften No. 3,103,642 and No. 3,315,469 as well as in 
European Offenlegungsschriften Nos. 12,463 and 82,201. Compounds which 
contain the structural element of the formula 
##STR1## 
in which X.sub.1 and X.sub.2 represent CO.sub.2 R, CN or 
##STR2## 
and R denotes an alkyl radical preferably having 1 to 8 carbon atoms, are 
likewise suitable. 
Such compounds may be prepared, for example, by reaction of a diisocyanate 
with a CH-acidic carbonyl compound of the formula CH.sub.2 X.sub.2 and 
with polyols, polyaminoalcohols or polyamines, and are published, for 
example, in Austrian Patent Application No. 1602/85. With this, reference 
is made to the above-mentioned literature, including the preferred 
embodiments described therein. The hardeners (B) are generally present as 
solutions containing 60 to 80% by weight of solids. The solvent component 
can be removed from them by the same method as in the case of the 
synthetic resin (A). Depending on the solids content and concentration of 
(B) in the coating preparation, the removal of the solvent may be omitted, 
if appropriate. 
The synthetic resin (A) and the hardener (B) are generally mixed in amounts 
such that the ratio of the sum of the groups which are capable of ester 
and/or amide formation in the synthetic resin to the sum of the groups 
which are capable of transesterification and/or transamidation and the 
double bonds optionally present in the hardener is 10:1 to 1:10, 
preferably 3:1 to 1:3. The amounts here should be selected so that an 
adequate cross-linking density of the coatings results and the latter have 
a good solvent resistance and high flexibility. In general, this is 
achieved at a ratio of parts by weight of synthetic resin (A) and hardener 
(B) (in each case solid) in the range from 90:10 to 30:70, preferably 
50:50 to 80:20. 
The coating preparations according to the invention generally also contain 
the known additives, such as pigments, pigment pastes, antioxidants, 
surfactants, solvents, leveling and thickening agents, reactive thinners, 
etc, and also, if appropriate, hardening catalysts. Such additives are 
known and are conventionally employed in the coatings industry. Suitable 
cataysts for the preparation are, for example, metal salts of organic 
acids, particularly zinc, lead, iron or chromium octoate or naphthenate. 
The catalysts also accelerate the cross-linking reactions of any double 
bonds which may be present in the hardener molecule. The amount of these 
catalysts is expediently between 0 and 10% by weight, preferably between 
0.1 and 2.0% by weight, calculated as the quantity by weight of metal and 
relative to the total weight of synthetic resin and hardener. 
In the process according to the invention, the organic solvent is 
substantially removed from the resin (A) and, if appropriate, from the 
hardener (B), preferably under reduced pressure in the range from 20 to 
130 mbar and at the lowest possible temperature, preferably below 
100.degree. C. In general, it is sufficient here to remove the solvent to 
a solids content of 90 to 95% by weight. Dilution is then effected by 
addition of a water-soluble solvent having a boiling point of less than 
100.degree. C., and the resin (A) is mixed with the hardener (B) at a 
temperature of preferably below 40.degree. C. After the neutralization of 
the amino groups, if appropriate only partially, using a water-soluble 
acid and the additional dilution with water, the organic solvent is 
removed from the aqueous dispersion under reduced pressure at a maximum 
temperature of preferably 40.degree. C. Dilution with water can 
subsequently again be effected. In this fashion, aqueous dispersions are 
obtained which are stable for several months, to be precise at least 6 
months, without addition of further substances, for example emulsifiers. 
The dilution of the concentrated resin solution or hardener solution with 
the water-dilutable solvents having a boiling point of below 100.degree. 
C., preferably below 80.degree. C., is generally carried out by adding the 
solvent slowly to the still hot, highly-concentrated resin solution, and 
cooling the latter to the boiling point of the respective solvent 
employed. The batch is then stirred at this temperature until a 
homogeneous mixture is produced. Solvents which may be employed are lower 
alcohols and ketones, for example methanol, ethanol, isopropanol, acetone 
and methyl ethyl ketone. In general, a solids content of the solution of 
more than 50, preferably more than 60%, by weight is produced. The 
resultant mixture is mixed with the hardener (B) at a temperature at which 
the components do not react, for example below 40.degree. C. 
Partial or complete neutralization of the amino groups which are present in 
the case of the cathodically depositable coating may be carried out using 
water-soluble acids before dilution with water. However, the water-soluble 
acid may alternatively be mixed completely or partially before the 
dilution with the water required, so that the neutralization is not 
carried out until during the dilution. In general, only sufficient acid is 
added so that the bath has the stability necessary and precipitations do 
not occur. Suitable acids are, for example, formic acid, acetic acid, 
lactic acid and phosphoric acid. 
In general, sufficient water is added so that the dispersion has a solids 
content of more than 20, preferably more than 30% by weight. 
The removal of the organic solvent from the aqueous dispersion is then 
preferably carried out under reduced pressure at 20 to 70 mbar, and at 
temperatures such that the components cannot react. In general, this 
process is carried out at a maximum temperature of 40.degree. C., 
preferably 30.degree. C. A water-solvent mixture, from whose composition 
the residual contents of organic solvents in the aqueous dispersion is 
determined, is removed here by distillation. 
The electrodeposition of the coating preparations according to the 
invention is carried out by known processes, reference being made to these 
here. The deposition may be carried out on all electrically conducting 
substrates, for example metal, such as steel, copper, aluminum and the 
like. Electro-dip coatings may here be present as clear coatings or as 
pigmented preparations. 
In the following excamaples, P and % always denote parts by weight and 
percent by weight respectively. The solids content was determined at 
180.degree. C./0.5 H. 
EXAMPLE 1 
(a) Preparation of the synthetic resin (A) 
An aminopolyether polyol was prepared by known methods from 65.5% of a 
bisphenol A epoxy resin having an epoxy equivalent weight of 480, 18.8% of 
a polyester of trimethylolpropane, adipic acid, isononanoic acid and 
tetrahydrophthalic anhydride having an acid index of 65 mg of KOH/g and a 
hydroxyl index of 310 mg of KOH/g, 6.1% of diethanolamine, 4.4% of 
2-ethylhexylamine and 5.2% of diethylaminopropylamine. The reaction of the 
epoxy resin with the polyester was carried out at 130.degree. C. until an 
epoxy equivalent weight of about 620 (or a solid resin) was attained. For 
the further reaction, the amines were initially introduced and the product 
of the reaction of epoxy resin and polyester was added at 80.degree. C. 
The reaction was ended when an epoxide index of approximately 0 was 
attained. The product existed as a 65% solution-in propylene glycol 
monoethyl ether and had an amine index of 96 mg of KOH/g. 
(b) Preparation of the hardener (B) 
2160 P of hydroxyethyl acrylate having an acid index of 1 and 3.8 P of zinc 
acetylacetonate were placed in a reactor and heated to 60.degree. C., 1636 
P of toluylene diisocyanate were slowly added dropwise, and the mixture 
was kept at 60.degree. C. until the content of --N.dbd.C.dbd.O groups was 
less than 0.2%. 15 P of hydroquinone and 844 P of ethylene glycol 
monohexyl ether were subsequently added. An 80% strength clear resin 
solution containing 9.5% of double bonds were obtained. 
1016 P of this precursor and 10 P of KOH, 30% strength in methanol, were 
placed in a reactor and heated to 80.degree. C. and 132 P of dimethyl 
malonate were slowly added dropwise at such a rate that 80.degree. C. was 
not exceeded. The mixture was kept at this temperature until the content 
of --C.dbd.C-- double bonds had fallen to 4.2%. The reaction mixture was 
then diluted with ethylene glycol monohexyl ether to a solids content of 
80%, and 2 P of acetic acid were added. A yellowish resin solution was 
obtained. 
(c) Aqueous synthetic resin/hardener dispersion 
From 3528 P of the solution of the above aminopolyether polyol, 1036 P of 
propylene glycol monomethyl ether were removed by distillation under 
reduced pressure at 90.degree. to 110.degree. C., and 784 P of ethanol 
were slowly added at 90.degree. C. The mixture cooled to 78.degree. C. 
during this, and was stirred at this temperature until a homogeneous 
mixture was produced. The mixture was subsequently cooled to 30.degree. 
C., and 1229 P of the hardener solution and 106 P of lead octoate were 
added successively. After the mixture had become homogeneous, it was 
transferred into a solution of 59 P of formic acid (85% strength) in 6610 
P of deionized water. 2227 P of a mixture of water, ethanol and a little 
propylene glycol monomethyl ether and ethylene glycol monohexyl ether were 
removed from the approximately 31% strength aqueous dispersion by 
distillation under reduced pressure of about 40 mbar at 35.degree. to 
40.degree. C. 9053 P of a 37% strength aqueous dispersion were obtained 
which, according to analysis by gas chromatography, still contained 0.6% 
of ethanol, 0.4% of propylene glycol monoethyl ether and 1.4% of ethylene 
glycol monohexyl ether. 
(d) Electro-dip coating and application testing 
13.7 P of TiO.sub.2, 1.2 P of lead silicate and 0.1 P of carbon black were 
added to 43 P of the 37% strength aqueous binder/hardener dispersion, the 
mixture was comminuted in a bead mill, and a further 79 P of the 37% 
strength aqueous dispersion were subsequently added to the batch. The 
batch was then diluted with deionized water using a stirrer at high speed 
until the solids content was 20%. The coating bath was stirred for 24 
hours and then had the following characteristics: Organic solvents 
content, relative to the solids content: 4.8% by weight; pH 5.8; 
conductivity 1740 .mu.Scm.sup.-1 ; meq value 40. It is stable for at least 
6 months. After deposition for 2 minutes at 300 V at a bath temperature of 
28.degree. C. and hardening (30 minutes, 163.degree. C.) on a phosphated 
steel sheet, connected as the cathode, a smooth coating having a film 
thickness of 20 .mu. m, a solvent resistance of more than 100 double 
strokes with acetone and a value of 80 cm in the Niessen reverse impact 
test is obtained.