Environmentally harmless stoving lacquer systems based on alkyd resins containing condensed monohydric alcohols

A water-insoluble condensation product of a polyalcohol, a dicarboxylic acid, from 5 to 35% by weight of a saturated monohydric alcohol having from 1 to 3 carbon atoms and, optionally, a monocarboxylic acid, said condensation product being soluble in lacquer solvents and useful for producing stoving lacquers.

This invention relates to binders for lacquer systems which are harmless to 
the environment and which can still be worked up as liquids while having a 
high solids content. 
Lacquer systems which contain little or no solvent have been known for a 
long time. They include, among others, the powder lacquers. These 
pulverulent lacquer mixtures have an advantage in that they do not emit 
any harmless solvents when hardened by heat. However, their application 
entails considerable technical difficulties because they are liable to 
cake together when subjected to heat and because certain colour tones can 
be obtained only by melting the powders to form uniform lacquer melts 
which must then be size-reduced again when cold (G-DR patent specification 
No. 55,820). 
The polyurethane coatings which can be obtained from diisocyanates and 
liquid polymers which contain hydroxyl groups may be regarded as 
solvent-free liquid systems. These systems have, however, the disadvantage 
that mixtures of diisocyanate components with polymers which contain 
hydroxyl groups are stable only for a limited length of time (German 
Offenlegungsschrift No. 2,105,062). 
On the other hand, stoving lacquers with a high solvent content, based on 
fatty acid modified alkyd resins to which melamine resins are added have 
been in wide use in the field of stoving lacquers for many years. There 
has been no lack of attempts to render these lacquer systems less harmful 
to the environment and considerable efforts have been made to replace the 
organic solvents with water. Systems which can be diluted with water have, 
however, the disadvantage that in addition to the harmful amines required 
for neutralization they also require substantial quantities of harmful 
solvents for dilution. In addition, large quantities of energy are 
required to evaporate the solvents of these systems in the stoving 
process. 
Lastly, the electrophoretic lacquering process should also be mentioned 
here. This system, however, has the disadvantage that it can only be used 
for lacquering parts which are conductive. 
It has already been attempted to concentrate the technically valuable 
solvent-containing stoving lacquer systems, and in German 
Offenlegungsschrift No. 2,019,282 there have been disclosed binder systems 
which, in the form of clear lacquers, contain about 30% by weight of 
solvent. These systems, however, have the disadvantage that they are 
liable to form bubbles and it is therefore recommended to apply the 
lacquers at temperatures of about 60.degree. C. 
A further disadvantage is that no unsaturated fatty acids may be used to 
achieve these objects and not only is the choice of polyalcohol pairs and 
dicarboxylic acid pairs restricted, but, in addition these components must 
be so chosen that they are adapted to each other. 
The binders have only a limited compatibility with aminoplasts and often 
only after a preliminary heat treatment. 
It has also been proposed to convert alkyd resins into liquid lacquer raw 
materials by a precondensation with methylol melamines carried out with 
heating, in order to obtain a liquid stoving lacquer system which contains 
little or no solvent. This choice of method has, however, the serious 
disadvantage that methylol melamines and alkyd resins are incompatible 
with each other before they are combined. To render them compatible with 
each other, heat treatment is necessary, but this results in a loss of 
valuable methylol groups which are required for the crosslinking reaction. 
The result is that the finished lacquers will only harden at very high 
temperatures, e.g. 180.degree. C. (German Offenlegungsschriften Nos. 
2,036,289, 2,036,714 and 2,055,107). 
Low-solvent lacquer systems based on oligomeric ricinene alkyd resins have 
been described in Luxembourg Patent Specification No. 66506. Ricinene 
alkyd resins, however, generally have the disadvantage that they wrinkle, 
which, for many purposes, is undesirable. 
Luxembourg Patent Specification No. 66506 describes stoving lacquer systems 
which are harmless to the environment and which are based on mixtures of 
high molecular weight, polyfunctional hydroxyl compounds and 
monomolecular, polyfunctional hydroxyl compounds with melamine resin. The 
examples show that even the clear lacquers obtained in this way are 
extremely viscous and therefore can only be applied with heating. 
It is an object of this invention to provide binders for stoving lacquers 
which can be applied in the cold, based on alkyd resins which have already 
been widely used for decades. 
This invention relates to stoving lacquer systems based on alkyd resins of 
polyalcohols and polycarboxylic acids having molecular weights of from 500 
to 2000, preferably from 600 to 1300, which alkyd resins contain hydroxyl 
groups and carboxyl groups and are optionally modified with monocarboxylic 
acids, and in addition other compounds incorporated by condensation, which 
other condensed compounds are monohydric alcohols. 
The incorporation of monohydric alcohols in alkyd resins is by no means new 
(Joh. Scheiber, Chemie und Technologie der Kunstlichen Harze, Page 654 et. 
seq.). Alkyd resins modified in this way have been used in various high 
solvent lacquer systems but have failed to achieve any position of 
importance on account of their properties. 
It was therefore surprising and not foreseeable that the incorporation of 
monohydric alcohols would be able to give rise to an extremely highly 
soluble alkyd resin with a very wide spectrum of compatibility, which 
could be used as a binder in low-solvent lacquer systems, i.e. systems 
containing less than 30% by weight of solvent, which systems have 
excellent lacquer technical properties and can be applied when cold. It 
has also been found that the excellent lacquer technical properties are 
not lost if the lacquer systems containing more than 30% by weight of 
solvent and preferably 30 - 60% by weight are used. Such high solvent 
lacquer systems may be used, for example, in plants which are equipped 
with a solvent combustion plant, especially if the stoving ovens are 
heated by the heat supplied by the combustion plant. Preferably, however, 
the stoving lacquer systems according to the invention are used 
solvent-free or with a low solvent content. 
The lacquer systems based on the alkyd resins according to the invention 
can even be applied in thick layers to a vertical surface without 
dripping, which is in contradiction to the existing teaching that 
low-molecular weight alkyd resin stoving lacquers tend to drip in the 
stoving process (Luxembourg Patent Specification No. 66,506). The lacquer 
coats obtained are brilliant white with a high gloss and an even surface. 
They are elastic and have a unique hardness substantially in excess of 
that which would have been expected and give excellent results in the 
corrosion and weather tests. 
The coatings have no tendency to form bubbles, even when applied in thicker 
layers and may therefore be used, for example, for spraying motorcars 
without first preparing the surface with fillers. This not only 
rationalises the spraying operation but also has an important advantage 
from the point of view of preventing environmental pollution since the 
high solvent spray filler normally required can now be replaced by the 
low-solvent binder system. 
The acid numbers are within the normal range for alkyd resins used as 
stoving lacquers. With acid numbers of from 3 to 20, lacquers which are 
exceptionally stable in storage can be obtained, whereas with acid numbers 
of from 20 to 40 highly reactive lacquers can be obtained, i.e. lacquers 
which are crosslinked at a lower temperature. The OH-number should be 
between 40 and 300 and is preferably between 60 and 150. It can be 
determined by suitable choice of the quantity of polyalcohol used in 
relation to the quantity of dicarboxylic acid, monocarboxylic acid and 
monohydric alcohol. The molecular weight of the polyester can also be 
determined by a suitable choice of the components. 
The polyalcohols used may be divalent aliphatic or cycloaliphatic alcohols 
with 2 to 15 carbon atoms such as ethylene glycol, propylene glycols, 
diethylene glycol, dipropylene glycols, butanediols, isobutenediol, 
neopentyl glycol, dimethylolpropane, hexanediols, perhydro-bisphenol and 
dimethyl cyclohexanes, or trihydric alcohols such as glycerol, trimethylol 
ethane, trimethylol propane or trimethylol hexane. Alcohols with a higher 
valency may also be used, such as pentaerythritol, dipentaerythritol or 
sorbitol or mixtures of higher valent alcohols. Partially etherified 
polyalcohols such as trimethylolpropane monoallyl ether may also be 
incorporated. 
The dicarboxylic acids used may be aromatic, cycloaliphatic or aliphatic 
dicarboxylic acids or derivatives thereof, for example phthalic acid, 
isophthalic acid, terephthalic acid, hexahydrophthalic acid, 
hexahydroisophthalic acid, hexahydroterephthalic acid, tetrahydrophthalic 
acid, methyl-tetrahydrophthalic acid, endomethylene tetrahydrophthalic 
acid, endoethylene tetrahydrophthalic acid, adipic acid, succinic acid, 
glutaric acid, maleic acid, fumaric acid or suberic acid as well as 
dimeric fatty acid; phthalic acid and adipic acid are preferred. 
Suitable monocarboxylic acids, which should be present in an amount of from 
0 to 40% by weight, include fatty acids such as 2-ethyl-hexanoic acid-(1), 
fatty acids obtained in the first runnings of distillation, coconut-oil 
fatty acid, ricinoleic acid, ricinene-fatty acid, soya-oil fatty acid, 
conjugated soya oil fatty acid, safflower-fatty acid, linseed-oil fatty 
acid, wood-oil fatty acid, ground-nut oil fatty acid and tall-oil fatty 
acid; these may be used as fatty acids or in the form of their oils. 
Preferably from 80 - 100% by weight of the monocarboxylic acid component 
consists of soya oil fatty acid and from 20 to 0% by weight consists of 
the other monocarboxylic acids mentioned above because exceptionally 
smoothly spreading coatings are then obtained. Monocarboxylic acids such 
as benzoic acid, butyl benzoic acid, hexahydrobenzoic acid, 
p-tetrabutyl-hexahydrobenzoic acid and acrylic acid as well as other 
aliphatic cycloaliphatic or aromatic monocarboxylic acids may also be 
used. 
Suitable monohydric alcohols are those which contain 1 to 3 carbon atoms 
per molecule, such as methanol, ethanol, propanols. 
The monoalcohol content may be from 5 to 35% by weight and the sum of 
monoalcohol content and monocarboxylic acid content should be from 5 to 
52% by weight. 
Exceptionally good results are obtained if the quantity of monoalcohol is 
from 5 to 12% by weight when the monocarboxylic acid content is from 22 to 
35% by weight or if the quantity of monoalcohol is from 20 to 35% by 
weight when the monocarboxylic acid content is from 0 to 5% by weight. The 
hardness and elasticity can be adjusted in conventional manner by a 
suitable choice of the composition of the dicarboxylic acids, 
polyalcohols, monocarboxylic acid and monoalcohols. Thus, exceptionally 
hard coatings are obtained when using phthalic acid, propanediol, 
trimethylolpropane, benzoic acid and methanol whereas highly elastic 
coatings are obtained when using adipic acid, hexanediol, 
trimethylolpropane, 2-ethyl-hexanoic acid-(1) and propanol. 
The alkyd resins may be prepared by condensation of the monomers by the 
usual methods, the polyesters being condensed until the desired acid 
number is obtained. Alternatively, however, a polycondensate with a lower 
acid number may first be prepared and this may then be acidified to the 
required acid number by the addition of a dicarboxylic acid anhydride, 
which is accompanied by semiester formation. 
An azeotropic method of procedure is recommended in order to avoid any loss 
of monomers in the process of esterification. 
Alkyd resins which have no monoalcohols built into them are obtained if a 
semiester is first prepared from a dicarboxylic acid anhydride and 
monoalcohol, preferably monomethyl phthalate, and this semiester is then 
reacted with the remainder of the monomers to form the polycondensate. 
In one process which is particularly preferred, an alkyd resin with an acid 
number of below 5 is first prepared from polyalcohol, dicarboxylic acid 
and monocarboxylic acid, using a molar ratio of dicarboxylic acid to 
polyols of 0.4 - 0.9, preferably 0.5 - 0.80, and this alkyd resin is then 
reacted in a second stage of the process with a semiester of dicarboxylic 
acid and monoalcohol, preferably monomethyl phthalate, to form a 
polycondensate still containing free hydroxyl groups. This method is 
particularly advantageous because it prevents the formation of 
dicarboxylic acid diethers from dicarboxylic acid and methanol. If 
desired, the acidity of the reaction product may subsequently be increased 
by adding dicarboxylic acid anhydride. 
The polycondensates may, of course, contain small quantities of 
unesterified semiesters of monoalcohol and dicarboxylic acid, the quantity 
of semiester depending on the esterification equilibrium. 
Suitable crosslinking components for oven drying two-component lacquers 
are, for example, aminoplasts such as urea formaldehyde resin, triazine 
resins such as formoguanamine resin, acetoguanamine resin, benzoguanamine 
resins or their definite preliminary stages, in which the methylol groups 
may be partly or completely etherified with monohydric alcohols containing 
1 to 4 carbon atoms. 
The alkyd resins can be used to produce solvent-free, low-solvent or 
high-solvent stoving lacquers which can be applied when cold. The 
solvents, if used, may be conventional lacquer solvents such as 
hydrocarbons, alcohols, esters or ketones. The usual auxiliary agents such 
as stabilisers, pigments and fillers, may be used in the high-solvent 
lacquers. This advantage in the method of application does not, of course, 
exclude the possibility of applying the lacquers when hot. It is therefore 
possible to prepare high solid stoving lacquer systems which can be 
applied when either hot or cold. It is, of course, also possible although 
not necessarily advisable to add excess quantities of solvent to the 
lacquer systems and apply them with a low solids content. 
Stoving lacquer systems which are completely free from solvent can be 
prepared by replacing all the solvent with a reactive diluent, for example 
a polyalcohol, which is capable of crosslinking with the melamine resin in 
the stoving process. Reactive diluents of this kind are, for example, 
glycerol, 2-ethyl hexane-1,3-diol and castor oil. 
The following examples serve to explain the object of the invention without 
restricting it. The parts given are parts by weight, the percentages are 
percentages by weight.

EXAMPLE 1 
1340 parts of trimethylolpropane, 888 parts of phthalic acid anhydride and 
1112 parts of soya oil fatty acid are esterified to an acid number 3 at 
220.degree. C. in a nitrogen atmosphere. 
This precondensate has a molar ratio of condensed dicarboxylic acid to 
condensed polyalcohols of 0.6. 
3160 parts of the precondensate are reacted with 1116 parts of 
cyclohexanol-phthalic acid semiester to an acid number of 5 and viscosity 
corresponding to a time of outflow of 100 seconds (determined according to 
DIN 53 211 as 80% xylene solution) based on the sum of xylene and the 
alkyd resin. 4204 parts of the resulting product are then reacted with 296 
parts of phthalic acid anhydride under conditions required for semiester 
formation to produce a polyester having an acid number of 20 and a 
viscosity corresponding to a time of outflow of 145 seconds (80% in 
xylene). 
The alkyd resin according to the invention contains about 9% of 
cyclohexanol condensed in it, with a fatty acid content of about 25%, an 
OH-number of 96 and a molecular weight of 1050. 
A low-viscosity, 80% white lacquer can be prepared from 125 parts of the 
80% solution, 100 parts of titanium dioxide and 35.7 parts of a 70% 
solution of a highly reactive melamine resin in butanol (Maprenal 
experimental product 5527 of Cassella Farbwerke Mainkur A.G., Frankfurt). 
This lacquer produces high gloss, extremely firmly adhering, nail-hard and 
elastic lacquer coats. 
The very wide compatibility spectrum of the alkyd resins described in the 
invention is demonstrated by the following clear lacquer test: if the 
alkyd resin and melamine resin are compatible with each other, clear 
lacquer coatings can be produced from them. If the binder and melamine 
resin are incompatible with each other, the coatings are opaque. 
The binder/melamine resin ratio used in the clear lacquer test is 4:1 and 
stoving is carried out at 30.degree.- 120.degree. C. 
______________________________________ 
Methanol- 
etherified highly 
melamine resin reac- 
(Cymal 301 of Cyanamid 
tive 
Comp. representatives 
melamine 
Cyanamid GmbH, Munich) 
resin 
______________________________________ 
Alkyd resin obtained 
according to the example 
of the invention 
clear clear 
Alkyd resin according 
to the example in 
Luxembourg Patent 
Specification No. 66506 
clear opaque 
______________________________________ 
In contrast to the alkyd resin obtained according to the example in the 
Luxembourg Patent Specification, the alkyd resins according to this 
invention are compatible with the highly reactive melamine resin. 
EXAMPLE 2 
1340 parts of trimethylolpropane, 888 parts of phthalic acid anhydride and 
1112 parts of soya oil fatty acid are esterified to an acid number 3 at 
220.degree. C. in a nitrogen atmosphere. 
This precondensate has a molar ratio of condensed dicarboxylic acid to 
condensed polyalcohols of 0.6. 
3160 parts of the precondensate are reacted with 1116 parts of 
cyclohexanol-phthalic acid semiester to an acid number of 5 and a 
viscosity corresponding to a time of outflow of 100 seconds (80% in 
xylene, determined according to DIN 53 211). 4204 parts of the resulting 
product are then reacted with 296 parts of phthalic acid anhydride under 
conditions required for semiester formation at 220.degree. C. to form a 
polyester with an acid number of 15.5 and a viscosity corresponding to a 
time of outflow of 95 seconds (50% in xylene). 
The alkyd resin according to the invention contains about 9% of 
cyclohexanol condensed in it and has a fatty acid content of about 25%, an 
OH-number of 90 and a molecular weight of 1600. 
A 48% white lacquer can be prepared from 200 parts of the 50% solution, 100 
parts of titanium dioxide and 35.7 parts of a 70% solution of the highly 
reactive melamine resin in butanol as described in Example 1. 
The very wide compatibility spectrum of the alkyd resins described in the 
invention is demonstrated by the following clear lacquer test: if the 
alkyd resin and melamine resin are compatible with each other, clear 
coatings can be produced. If the binder and melamine resin are 
incompatible, the coatings obtained are opaque. 
The binder/melamine resin ratio used in the clear lacquer test is 4:1 and 
stoving is carried out at 30.degree. to 120.degree. C. 
______________________________________ 
Methanol- highly reactive 
etherified 
melamine 
melamine resin 
resin 
______________________________________ 
Alkyd resin according to 
the example of the 
invention clear clear 
Alkyd resin according to 
the example in Luxembourg 
Patent Specification No. 
clear opaque 
66506 
______________________________________ 
In contrast to the alkyd resin obtained according to the example in the 
Luxembourg Patent Specification, the alkyd resins according to the present 
invention are compatible with the highly reactive melamine resin.