Thermosetting coating composition containing a blocked acid catalyst

A thermosetting coating composition contains an acid-curable amino resin and from 0.1 to 10 percent by weight of a blocked sulphonic acid calculated on the weight of amino resin. In order to reduce the electric conductivity of the composition after a prolonged pot-life, a sulphonic acid oximate is used as the blocked acid. The compositions are advantageous for coating a substrate by electrostatic spraying.

This invention relates to a thermosetting coating composition containing an 
acid-curable amino resin and such an amount of a blocked acid as 
corresponds to 0.1 to 10 percent by weight, calculated on the amino resin 
of potentially available suphonic acid. 
A thermosetting coating composition of the type indicated above is 
described in German Pat. Spec. No. 2,731,528. The blocked acids disclosed 
in the German specification are obtained from a sulphonic acid and 
epoxidized compounds (oxiranes), encompassing both the low-molecular 
weight epoxides such as ethylene oxide, propylene oxide and the like and 
epoxy resins. 
A disadvantage to the blocked acids disclosed in the German specification 
is that the conductivity of coating compositions containing these 
compounds will increase rapidly to a value which may give problems when 
these compositions are to be applied to a substrate by electrostatic 
spraying. 
It is therefore an object of this invention to provide a thermosetting 
coating composition containing an acid-curable amino resin which is devoid 
of the aforesaid disadvantage and is adapted for application on a 
substrate by electrostatic spraying. Another object of the invention is to 
provide an improved coating composition containing an acid curable amino 
resin and a blocked acid. Still another object of the invention is to 
provide articles having an improved coating of a thermosetting amino 
resin. 
The foregoing objects and others are accomplished in accordance with this 
invention, generally speaking, by providing a thermosetting coating 
composition containing an acid curable amino resin and blocked acid that 
is less likely to cause problems because of low electrical resistance. The 
thermosetting coating composition contains as a blocked acid a compound 
which corresponds to any one of the following formulae: 
##STR1## 
wherein, p=O or 1 and q=O or 1, and R.sub.1 is a substituted or 
unsubstituted alkyl group, alkenyl group, cycloalkyl group, cycloalkenyl 
group, aryl group, aralkyl group, aralkenyl group or heterocyclic group 
having not more than 20 carbon atoms; R.sub.2 has the same meansing as 
R.sub.1 or is an NH.sub.2, CN, a substituted or unsubstituted alkoxy 
carbonyl group, cycloalkoxy carbonyl group, aroxy carbonyl group or acyl 
group having not more than 20 carbon atoms, or the two groups R.sub.1 and 
R.sub.2 form part of a cyclic or heterocyclic group having not more than 
12 atoms in the ring; R.sub.3 is a substituted or unsubstituted alkyl 
group, alkenyl group, cycloalkyl group, cycloalkenyl group, aryl group or 
heterocyclic group having not more than 30 carbon atoms and R.sub.4 
represents a substituted or unsubstituted alkylene group, alkenylene 
group, cycloalkylene group, cycloalkenylene group, arylene group or 
bifunctional heterocyclic group having not more than 35 carbon atoms. 
Examples of suitable substituents in the groups R.sub.1, R.sub.2, R.sub.3 
and R.sub.4 of the above structural formula include the halogens fluorine, 
bromine or iodine; nitro groups, cyano groups and alkoxy or alkanoyl 
groups. Optionally, one or more ethylenically unsaturated groups may be 
present. This may be of particular importance if besides curing under the 
influence of an acid there will be further curing under the influence of 
radical reactions. It has been found that within the scope of the 
invention generally satisfactory results are obtained with coating 
compositions containing a blocked acid of any one of the above formulae 
wherein when p=O and q=1, R.sub.1 and R.sub.2 represent an aryl group and 
when p=1 and q=1, R.sub.1 represents an alkyl group or a cycloalkyl group 
and R.sub.2 represents an alkyl group, a cycloalkyl group or an aryl 
group. 
Partly because of the availability of the starting materials and the method 
of preparing the present compositions preference is given to compounds of 
any one of the above formulae, wherein p=O and q=1, R.sub.1 and R.sub.2 
represent a phenyl group, and when p=1 and q=1, R.sub.1 represents a lower 
alkyl group and R.sub.2 a lower alkyl group or phenyl group. Coating 
compositions of improved storage stability are also obtained when one or 
more of the following compounds 
##STR2## 
is used. 
Very favorable results are generally obtained when R.sub.3 represents a 
p-tolyl group or an alkyl group having 1 to 18 carbon atoms. 
Coatings possessing extraordinarily good properties are obtained when 
R.sub.4 is a dialkyl naphthalene group having not more than 35 carbon 
atoms. Most preferred compounds are those in which R.sub.4 is a dinonyl or 
a didodecyl naphthalene group. It is preferred that these nonyl groups or 
dodecyl groups should be highly branched. 
Examples of suitable blocked acids for the present coating compositions 
include: 
benzil monoxime tosylate; 
furil monoxime tosylate; 
acetone oxime cetyl sulphonic acid ester; 
cyclopentanone oxime oleyl sulphonic acid ester; 
p-nitro acetophenone oxime tosylate; 
2-hydroximinopropiophenone tosylate; 
.alpha.-oximinophenyl acetic acid ethyl ester tosylate; 
.alpha.-oximinocaproic acid ethyl ester tosylate; 
benzyl amido oxime tosylate; 
cyclopentanone oxime butyl sulphonic acid ester; 
cyclohexanone oximethyl sulphonic acid ester; 
p-chloro acetophenone oxime octyl sulphonic acid ester; 
di(2-naphthyl)-diketone monoxime ethyl sulphonic acid ester; 
propiophenone oxime tosylate; 
1-(2-furyl)-1-butanone oxime tosylate; 
the dinonyl naphthalene disulphonic acid diester of benzil monoxime 
.alpha.-oximinobenzoyl acetic acid ethyl ester tosylate, and the like. 
The blocked acids may be prepared by any known conventional process. In one 
attractive process, a monoketone or diketone is reacted with hydroxylamine 
or, preferably, with the hydrochloric acid salt thereof. 
After conversion of the salt of the hydroxylamine into the oxime by 
treating it with an aqueous sodium hydroxide solution, the oxime is 
dissolved in an organic solvent such as tetrahydrofuran, followed by 
successively adding an amine and the acid chloride of a sulphonic acid. 
The resulting sulphonic acid ester of the oxime is subsequently isolated. 
In another process an active methylene compound is first reacted with 
sodium alcoholate, followed by adding a solution of nitrite ester. After 
concentration by evaporation and further processing and extraction, the 
desired oxime is obtained. The oxime is reacted, in the presence of an 
amine, with the acid chloride of the required sulphonic acid. The 
resulting sulphonic acid ester of the oxime is subsequently isolated. 
The blocked acid to be used in the coating composition according to the 
invention may be employed in any suitable amount, preferably in an amount 
of 0.1 to 10 percent by weight, calculated on the weight of the amino 
resin of potentially avaiable sulphonic acid. Generally, there is no need 
to use more than 0.3 to 2% by weight of the blocked acid, calculated on 
the solid constituents of the coating composition. Any suitable 
acid-curable amino resins may be used in the coating composition of the 
invention. The amino resins are as a rule obtained by condensation of an 
aldehyde such as formaldehyde and a urea, melamine or guanamine and the 
lower alkyl ethers thereof. A particularly atttractive group of amino 
resins is the methylated melamine formaldehyde resins, including 
hexamethyoxy-methyl melamine and the mixed peralkoxy derivatives thereof. 
This hexamethoxymethy melamine may, for instance, be reacted with an alkyd 
resin under conditions such that a minimum degree of cross-linking occurs. 
The actual cross-linking will not occur until the acid is unblocked and 
released when the blocked acid-containing composition is heated. 
In addition to the acid-curable amino resin there may still be present 
another co-condensing polymer. Such a polymer should contain at least two 
hydroxyl, carboxyl, amine and/or amide groups. 
The co-condensable polymers may be addition or condensation polymers. 
Examples of suitable addition polymers include the homopolymers and the 
copolymers of acrylic and methacrylic acid and other unsaturated monomers 
or mixtures of monomers; the homopolymers and the copolymers of 
ethylenically unsaturated monomers such as styrene, substituted styrene; 
vinyl esters such as vinyl acetate, vinyl propionate and the like; 
.alpha.-olefins such as ethylene, propylene, 1-butene, 1-octene; vinyl 
chloride, vinylidene chloride, conjugated dienes such as 1,3-butadiene; 
fluorine-containing olefins such as vinyl fluoride and 
hexafluoropropylene; vinyl ether and allyl ether; and the mono- and the 
diesters of .alpha.,.beta.-ethylenically unsaturated dicarboxylic acids 
such as mono- and dialkyl maleates, mono- and dialkyl fumarates, mono- and 
dialkyl itaconates. 
The functional groups that are needed for effecting a condensation reaction 
with the acid-curable amino resin may be introduced during the preparation 
of the polymer or afterwards. 
Examples of suitable monomers include hydroxyethyl acrylate, hydroxypropyl 
acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, acrylic 
acid, methacrylic acid, itaconic acid, maleic acid, N-t-butyl-aminoethyl 
methacrylate, acrylamide and/or methacrylamide. 
One example of a postcondensation reaction is the hydroxy methylation of an 
amide-containing polymer. 
Examples of condensation polymers include the alkyd resins derived from a 
polyhydric alcohol and a polybasic acid. They include polyesters that may 
be modified or not with fatty acids and/or polyethers. The diols that are 
suitable for use in the preparation of the polyesters may be aliphatic or 
aromatic. Examples of suitable diols include ethylene glycol; propylene 
glycol-1,2; propylene glycol-1,3; butane diol-1,2; butane diol 1,3; butane 
diol-1,4; neopentyl glycol; 2,2,4-trimethylpentane-1,3-diol; decamethylene 
glycol; monoethyl ether of glycerol and/or the .alpha.-.beta. allyl ether 
of glycerol. The dicarboxylic acids suitable for use in the preparation of 
the polyesters may be aliphatic, cycloaliphatic or aromatic. 
Suitable dicarboxylic acids are malonic acid, succinic acid, glutaric acid, 
adipic acid, pimelic acid, suberic acid, azelaic acid, maleic acid, 
fumaric acid, tetrahydrophthalic acid, dilinoleic acid, diphenic acid, 
tetrachlorphthalic acid, isophthalic acid, terephthalic acid, o-phthalic 
acid, cyclohexane-1,2-dicarboxylic acid, the maphthalene dicarboxylic 
acids and/or trimethyl adipic acid. 
Also suitable for use in the coating compositions are natural polymers 
based on cellulose or derivatives thereof such as regenerated cellulose. 
The following examples are submitted for a better understanding of the 
invention. They are, of course, not to be construed as being limiting in 
any manner whatsoever. 
The test methods mentioned in the examples below were all carried out in 
accordance with ASTM and the DIN standards mentioned. The Persoz hardness 
was determined in conformity with the French NF-Standard T-30-016. 
In the examples the following ester diols were employed, use being made in 
all cases of hexamethoxymethyl melamine as cross-linking agent. 
Diester diol I: a mixture of equal parts by weight of (a) di(monodecanoic 
acid ester of trimethylol propane)-isophthalate and (b) a mixture of 1 
part of di(neopentyl glycol)-isophthalate and 2 parts of 
di(neopentylglycol)phthalate. 
Diester diol II: a 90% by weight solution in xylene of 1 part of 
di(neopentyl glycol)isophthalate, 1 part of di(neopentyl 
glycol)orthophthalate and 2 parts of dimer of di(3-methylpentane 
diol-1,5)isophthalate. 
Diester diol III: an alkyd resin made up of 28% by weight of fatty acids, 
47% by weight of phthalic anhydride and 25% by weight of polyalcohols.

EXAMPLE I 
Preparation of .beta.-benzil monoxime tosylate 
210 g of benzil were suspended in 200 ml of ethanol and cooled to 
-5.degree. C. To this suspension 70 g of hydroxylamine hydrochloride were 
gradually added in portions and at a rate such that the temperature of the 
reaction mixture did not exceed 0.degree. C. 
Subsequently, 120 g of NaOH in 600 ml of water were added with vigorous 
stirring at a temperature of -5.degree. to -3.degree. C., at which 
temperature stirring was continued for 11/2 hours. The reaction mixture 
was diluted with 1 liter of cold water and the non-converted benzil was 
filtered off. The filtrate was acidified with acetic acid and a white 
product precipitated. This product was washed with water and air dried. 
The yield was 209 g of an .alpha.-benzil monoxime, which was subsequently 
converted into the .beta.-isomer by dissolving it in 2 liters of benzene, 
to which 15 g of active carbon had been added, which suspension was boiled 
for 15 minutes with refluxing. After filtration of the carbon the filtrate 
was concentrated by evaporation under reduced pressure. 
The yield of .beta.-benzil monoxime was 152 g. Of this substance 67.5 g 
were dissolved in 150 ml of tetrahydrofuran and cooled to -5.degree. C. 
Next, there were added 45.5 g of triethylamine and subsequently, over a 
period of 45 minutes and at a temperature of -5.degree. to 0.degree. C. 
with vigorous stirring, 57 g of p-toluene sulphonyl chloride in 150 ml of 
tetrahydrofuran. After 30 minutes stirring the mixture was poured into 1 
liter of cold water and 40 ml of concentrated hydrochloric acid. The solid 
matter was filtered off, washed with cold water and dried over phosphorus 
pentoxide. Following recrystallization from a mixture of 3 parts of 
ethanol and 2 parts of acetone, 58.7 g of very pure benzil monoxime 
tosylate (melting point 113.3.degree. -115.5.degree. C.) were obtained. 
EXAMPLE II 
Preparation of p-nitroacetophenone oxime tosylate 
42 g (=0.6 mole) of hydroxylamine hydrochloride were successively dissolved 
in 90 ml of water and cooled in ice. Subsequently, 18 g of NaOH (0.3 mole) 
in 50 ml of water were added with proper stirring and cooling. To the 
resulting solution there were added 49.5 g of p-nitro-acetophenone in 200 
ml of ethanol followed by heating under reflux. Next, another 200 ml of 
ethanol were added, so that all solid matter went into solution. The 
solution was still boiled for 1 hour under reflux. After cooling in ice, a 
pale yellow product was obtained, which was filtered off, washed with 
water and dried. 
The yield of p-nitroacetophenone oxime was 51 g. 
In a following step 18 g of p-nitroacetophenone oxime in 300 ml of acetone 
were added to a solution cooled to 0.degree. C. of 4 g of NaOH (0.1 mole) 
in 50 ml of water. Next, 19.4 g (0.102 mole) of p-toluene sulphonyl 
chloride in 100 ml of acetone were added, with stirring and at a 
temperature of 5.degree. to 10.degree. C. 
After the solution had successively been stirred for 1 hour at 0.degree. to 
5.degree. C. and poured into 300 ml of benzene, it was washed four times 
with 125 ml of cold water and dried. 
After the benzene had been distilled off, a yellow residue remained, which 
after recrystallization from a mixture of benzene and hexane (1:1) had a 
melting point in the range of 123.4.degree. to 125.4.degree. C. 
EXAMPLE III 
Preparation of .alpha.-oximinocaproic acid ethyl ester tosylate 
21.6 g (0.1 mole) of n-butyl-diethyl malonate were cooled to -10.degree. C. 
followed by adding to it 15.6 g (0.133 moles of isoamyl nitrite. To this 
mixture a solution of 2.3 g of Na (0.1 mole) in 45 ml of ethanol 
(anhydrous) was added, with stirring, over a period of 11/2 hours at a 
temperature between -10.degree. and -8.degree. C. Stirring was continued 
for 16 hours at -10.degree. C. After evaporation and further treatment 
with cold water, extraction with ether, acidification and a second 
extraction with ether, 13.8 g of .alpha.-oximino-caproic acid ethyl ester 
were obtained. 
5.3 g (0.03 mole) thereof were dissolved in 25 ml of tetrahydrofuran, 
cooled to -10.degree. C., after which 4.55 g (0.045 mole) of triethylamine 
were added. To this solution there was added, over a period of 1 hour and 
at a temperature between -10.degree. and 0.degree. C., a solution of 5.7 g 
(0.03 mole) of p-toluene sulphonyl chloride in 25 ml of tetrahydrofuran. 
After 1 hour's stirring at 0.degree. C., the reaction mixture was stored 
for 16 hours at 4.degree. C. and subsequently poured into cold dilute 
hydrochloric acid and extracted three times with 100 ml of diethyl ether. 
After drying with magnesium sulphate, the ether was distilled off. The 
product was recrystallized from ethanol and had a melting point of 
43.2.degree. -46.8.degree. C. 
EXAMPLE IV 
A pigment dispersion was prepared by mixing 24 parts of titanium white with 
3 parts of an acrylate resin, 7.5 parts of hexamethoxymethyl melamine, 1 
part of xylene and 1 part of ethylglycol acetate. To the resulting mixture 
there was added diester diol I in a ratio of 3 parts of diester diol to 1 
part of hexamethoxymethyl melamine. The pigment-resin ratio was 0.67. Into 
this coating composition there was incorporated a 10%-solution in 
methylisobutyl ketone of syn-benziloxime tosylate in an amount which 
corresponds to 0.7% by weight of p-toluene sulphonic acid, calculated on 
the solid constituents. The paint composition was diluted with a mixture 
of xylene and ethylene glycol monoethyl ether acetate (1:1) until the 
viscosity obtained corresponded to an efflux time of 39 seconds in a G2 
Zahn cup. 
Part of the coating composition thus prepared was applied to a phosphated 
iron panel and cured for 25 minutes at 130.degree. C. The properties of 
the resulting paint film and the change in viscosity of the composition 
during storage at 50.degree. C. are summarized in Table I. 
TABLE I 
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thickness determined in accordance with 
ASTM D1186-53 40-45 mm 
gloss 20.degree. in accordance with ASTM D523 
9.5% 
Persoz hardness in accordance with NF T30-016 
298 seconds 
conical mandrel test in accordance with 
ASTM 522-60 (9 mm) 12 
Ford falling weight test (kg cm) 
ASTM D2794-69 
upper side 45 
under side 30 
Erichsen indentation test (mm) in conformity 
with DIN 53156 6.8 
change in viscosity with storage at 50.degree. C. 
(G2 Zahn cup, sec) 
initial viscosity 39.4 
after 4 weeks 77 
______________________________________ 
Another part of the coating composition was used to establish the influence 
of the presence of a blocked acid on the electric conductivity of 
resistance as a function of time. To that end four coating compositions 
were prepared, one of which (the blank) contained no acid and the others a 
blocked or non-blocked acid in such an amount as was needed to obtain a 
coating having the same mechanical properties after 25 minutes curing at 
130.degree. C. 
The resistance was measured immediately after preparation and again after 
storage times of 2 and 4 weeks at 35.degree. C. The results are summarized 
in the table below. Composition No. 1 contained as catalyst a commercially 
available acid blocked with an epoxy compound, according to German Pat. 
Specification No. 2,731,528, under the chemical name 
2,3-epoxypropyl-1,1-dimethyl heptane carboxylate. 
Composition No. 2 contained syn benzyl monoxime tosylate according to the 
invention. Composition No. 3 contained no catalyst and Composition No. 4 
contained non-blocked p-toluene sulphonic acid as catalyst. The catalyst 
concentrations given in the table are all in percent by weight p-toluene 
sulphonic acid, calculated on the solid constituents. 
TABLE II 
______________________________________ 
Resistance, k .OMEGA. 
Composi- 
Catalyst after 
tion concentration 
initially 
2 weeks 4 weeks 
______________________________________ 
1 1 3000 500 500 
2 0,7 3750 1650 1250 
3.sup.(1) 
-- 5500 6000 6000 
4 0,5 290 350 400 
______________________________________ 
.sup.(1) after curing for 25 minutes at 130.degree. C. the mechanical 
properties were deficient. 
The above results clearly show the great advantage obtained by the use of 
the catalyst of the present invention (in Composition No. 2). The amount 
in which it need to be added may be considerably smaller than that of the 
known catalyst in Composition No. 1, its electric resistance not only 
being satisfactory at the start, but even after storage times of 2 and 4 
weeks, which cannot be said of the resistance of Composition No. 1. 
EXAMPLE V 
2 parts of diester diol II were mixed with 1 part of hexamethoxymethyl 
melamine and such an amount of oxime blocked sulphonic acid as corresponds 
to 0,6% by weight of p-toluene sulphonic acid (calculated on the solid 
constituents). The resulting composition was subsequently diluted with a 
mixture of equal parts by weight of xylene and ethylene glycol monoethyl 
ether acetate until it was brought to spraying consistency. 
The paint was applied to a phosphated iron panel and cured for 30 minutes 
at 130.degree. C. The thickness of the coating applied was determined in 
accordance with ASTM D1186-53 and the Persoz hardness in accordance with 
NF T30-016. Also determined were both the initial viscosity and the 
viscosity after 4 weeks' storage at 40.degree. C. The viscosity was 
determined by meansuring the efflux time (in seconds) in a Ford cup No. 4. 
The results are given in Table III below. 
TABLE III 
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paint viscosity 
properties 
(Ford cup No. 4) 
thick- Persoz after 4 
ness hard- weeks at 
blocked acid (.mu.) ness initial 
40.degree. C. (sec) 
______________________________________ 
p-nitro acetophenone 
oxime tosylate 25 367 35 56 
syn benzil monoxime 
tosylate 25 365 31 71 
.alpha.-oximinocaproic acid 
ethyl ester tosylate 
25 363 33 62 
.alpha.-oximinophenyl acetic 
acid ethyl ester tosylate 
25 340 33 70 
benzylamidoxime tosylate 
25-30 336 31 90 
syn benzil monoxime 
mesylate 23 368 31 60 
syn benzil monoxime cetyl 
sulphonic acid ester 
28 332 34 40 
______________________________________ 
EXAMPLE VI 
31.1 g of diester diol III were mixed with 5.5 g of 
hexamethoxymethylmelamine and 0.44 g of benzil oxime tosylate. 
The resulting paint was subsequently diluted with xylene until it was at 
spraying consistency. The paint was applied to a phosphated iron panel and 
cured for 30 minutes at 130.degree. C. 
Repeating the procedure described hereinbefore, the thickness of the paint 
coating, the Persoz hardness and the storage stability at 40.degree. C. 
(change in viscosity in a Ford cup No. 4 over a period of 4 weeks) were 
determined. The results are given below: 
______________________________________ 
thickness of paint coating 25.mu. 
Persoz hardness (sec) 327 
the viscosity rose from 20 to 60 seconds. 
______________________________________ 
Although the invention has been described in detail for the purpose of 
illustration, it is to be understood that such detail is solely for that 
purpose and that variations can be made therein by those skilled in the 
art without departing from the spirit and scope of the invention except as 
it is limited by the claims.