CA1081389A

CA1081389A - Heat-curable coating agent

Info

Publication number: CA1081389A
Application number: CA213,742A
Authority: CA
Inventors: Werner Andrejewski; Dieter Stoye
Original assignee: Chemische Werke Huels AG
Current assignee: Huels AG
Priority date: 1973-11-14
Filing date: 1974-11-14
Publication date: 1980-07-08
Also published as: JPS5080324A; IT1023123B; GB1481182A; JPS5242173B2; AU7526974A; BE822146A; DE2356768B1; BR7409477A; ZA747241B; SE414044B; AT334482B; SE7414191L; ATA904874A; NL169491C; FR2250808B1; NL169491B; SU618052A3; NL7414813A; FR2250808A1

Abstract

Abstract A heat-curable coating composition which contains as binder an am-inoplast and/or low-molecular precursors thereof mixed with a a saturated poly-ester, an oil-free or oil-modified alkyd resin and/or an acrylic resin contain-ing hydroxyl groups, wherein 0.1 to 20 per cent by weight, relative to the binder, of an adduct of an epoxide and a monofunctional sulphonic acid is present as a curing accelerator, the adduct containing 0.1 to 1.0 sulphonic acid group per epoxy group. The coating agents may be applied by customary process, such as, for example, brushing and spraying, and are also suitable for us in electrostatic lacquering processes. The coatings prepared therefrom have a reduced tendency to turn yellow.

Description

The present invention relates to a heat-curable coating agent based on a mixture of binders and proportions of solvents and also, optionally, on a solvent-free basis, which contains as binders aminoplasts and/or low-le-cular precursors thereof mixed with saturated polyesters, oil-free or oil-modi-fied alkyd resins and/or acrylic resins containing hydroxyl groups together with, optionally, customary auxiliaries.
Acid catalysts are frequently used in order to accelerate cross-linking in the heat-curable coating agents mentioned. Such catalysts can be inorganic acids, such as hydrochloric acid and phosphoric acid, and organic acids, such as toluenesulphonic acid, phenolsulphonic acid, maleic acid, fum-aric acid, itaconic acid, citric acid or salts thereof which can be split.
The addition of these accelerators has the disadvantage of increasing the viscosity on storage and greatly reducing the electrical resistance of the coating agent. Although some salts of the accelerators, particularly the morpholinium salts, effect a limited improvement in the storage stability, they also reduce the electrical resistance and, in addition, have the disadvan-tage of causing a greater yellowing of the heat-cured coatings. The electro-static lacquering processes, which on environmental grounds are distinctly favourable, are particularly affected in this way.
It has been the task of the invention to avoid the disadvantages, described above, of the state of the art. It has been solved by adding to the coating agents, as a curing accelerator, 0.1 to 20 percent by weight, relative to the binder, of an adduct of an epoxide and a monofunctional sulphonic acid, 0.1 to 1.0 sulphonic acid group per epoxy group being present in the adduct.
Accordingly, the invention provides a heat-curable coating compos-ition which contains as binder (1) an aminoplast, a low-molecular precursor `
thereof, or a mixture thereof, mixed with (2) a saturated polyester, an oil-free or oil-modified alkyd resin, an acrylic resin contai~ing hydroxyl groups, or a mixture of two or more thereof, wherein 0.1 to 20 percent by weight, ~ O~r~ f~G
L~ 30 relative to the binder, of an adduct of an epoxide and a monofunctional~sulph-onic acid is present as a curing accelerator, the adduct containing 0.1 to 1.0 sulphonic acid group per epoxy group.

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10~13~9 Aromatic, monofunctional sulphonic acids, for example benzenesul-phonic acid or derivatives thereof with aliphatic or aromatic substituents in the nucleus, such as o-, m- or p-toluenesulphonic acids, ethylbenzenesul-phonic acid, naphthylsulphonic acid or alkylated derivatives thereof, can be used as the sulphonic acids. Toluenesulphonic acids and alkylbenzenesulphonic acids are preferred.

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Aliphatic and/or aromatic epoxide resins - for example resins based on bisphenol A - or products based on Versatic~acid glycidol esters, glycidol ethers, epichlorohydrin, styrene epoxide, butadiene dioxide and/or epoxidised polybutadiene oils are employed as the second component of the adduct. Resins based on bisphenol A and Versatic acid glycidol ester are pre-ferably used.
The adduct is produced by preparing a solution of each of the two components of the adduct, which are then mixed with one another. The mixture is subsequently heated for 20 to 60 minutes to 20 - 150C, preferably to 60 -80C. The adduct solution can be used direct. In another possible process, the adduct is prepared by mixing or fusing the two components at 20 to 150C.
The adduct can be added to the binder mixture as such or in a dissolved form.
Aromatic hydrocarbons, alcohols, esters, ketones, glycol ethers or glycol ether-esters or corresponding mixtures of solvents are used as the solvent.
The adduct is employed in quantities of 0.1 to 20 per cent by weight, relative to the binder. It is a measure of the quantity of adduct necessary that, relative to the binder,0.02 to 2 per cent by weight, prefer-ably 0.3 to 0.8 per cent by weight, of sulphonic acid is advantageously con-tained in the binder.
The coating agents to be crosslinked contain a mixture of two components as the binder.
Aminoplasts and/or low-molecular precursors thereof are one compon-ent. Possible suitable aminoplasts are the known reaction products of alde-hydes, especially formaldehyde, with substances carrying several amino or amido groups, such as, for example, melamine,urea, dicyandiamide and benzoguanamine.
Mixtures of such products are also su1table. Aminoplasts modified by means of -; alcohols are particularly suitable.
As these resin-like products occasionally have a limited compatibi-lity with the second binder component to be used, it is preferable to employ the low-molecular aminoplasts~ which have a definite structure and are vir-~ ~ ~ f~erna~ k 2 -tually infinitely miscible with them. Examples of such aminoplasts with a definite structure are dimethylolurea, tetramethylolbenzoguanamine, trimethylol-melamine or hexamethylolmelamine, which can also be employed in a paTtially or completely etherified form, for example as dimethoxymethylurea, tetrakis-(methoxymethyl)-benzoguanamine, tetrakis-(ethoxymethyl)-benzoguanamine or poly-ethers of hexamethylolme~laminef, such as hexakis-(methoxymethyl)-melamine or hexakis-(butoxymethyl)-melamine. Hexamethylolmelamine derivatives which are etherified with alcohols having 1 to 4 carbon atoms and are liquid in an undiluted form at room temperature, are particularly preferred.
Saturated polyesters, oil-free or oil-modified alkyd resins and/or acrylic resins containing hydroxyl groups are used as the second binder com-ponent.
Possible suitable saturated polyesters are condensation products of diols and/or polyols with aromatic and/or cycloaliphatic dicarboxylic acids and aliphatic dicarboxylic acids.
Suitable diols are ethylene glycol, 1,2-propanediol, 1,3-propane-diol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,2-dimethylpropane-1,3-diol, 1,6-hexanediol, 2-ethylhexane-1,3-diol, 1,2-cyclohexanediol, 1,4-cyclo-h0xanediol, 1,2-bis-(hydroxymethyl)-cyclohexane, 1,3-bis-(hydroxymethyl)-cyclohexane, 1,4-bis-(hydroxymethyl)-cyclohexane, x,8-bis-(hydroxymethyl)-tri-cyclo-[5.2.1.02'6]-decane, x representing the numerical values 3, 4 or 5, diethylene glycol, triethylene glycol, dipropylene glycol or tripropylene glycol. Cycloaliphatic diols can be used in their cis- or trans-form or as a ; mixture of both forms.
Possible aromatic or cycloaliphatic dicarboxylic acids are phthalic acid, isophthalic acid, hexahydroterephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, hexahydroisophthalic acid an ~ /ene- or eY~
t~lcne-teYtra~ydrophthalic acid, hexachloro-endomethylene-tetrahydrophthalic acid or tetrabromophthalic acid, it being possible for the cycloaliphatic dicarboxylic acids to be employed in their trans- or cis-form or as a mixture , ~ .

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of both forms.
Suitable aliphatic dicarboxylic acids are succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, decanedicarboxylic acid or

2,2,4-trimethyladipic acid. However, it is also possible to employ unsaturat-ed dicarboxylic acids, such as, for example, maleic acid, fumaric acid, itaconic acid or citraconic acid.
Instead of the free dicarboxylic acids it is also possible to employ their esters with short-chain alkanols, for example dimethyl esters, diethyl esters or dipropyl esters. If the dicarboxylic acids form anhydrides, the latter can also be used, for example phthalic anhydride, hexahydrophthalic anhydride, tetrahydrophthalic anhydride, succinic anhydride, glutaric anhydride or maleic anhydride.
Polyesters with an average molecular weight of 240 to 4,000 are suitably employed.
Possible oil-modified alkyd resins are reaction products of diols and/or polyols with aromatic or cycloaliphatic dicarboxylic acids, aliphatic dicarboxylic acids and saturated or unsaturated fatty acids. Suitable saturat-ed ant unsaturated fatty acids are caproic acid, caprylic acid, pelargonic acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, eicosane-~ carboxylic acid, behenic acid, oleic acid, linoleic acid, lino-lenic acid and mixtures in the form of the linseed oil, castor oil, upgraded castor oil, tall oil, soya oil, groundnut oil or coconut oil fatty acids which are obtained from natural oils. Synthetic fatty acids, such as 2-ethylhexyl ' acid, ~-alkylcarboxylic acids or isononane acid, or mixtures of synthetic ; fatty acids having 7 to 9 carbon atoms can also be used. Instead of the free acids, the esters and natural oils are just as suitable.
Suitable acrylic resins are commercially obtainable acrylic acid ester polymerisation resins which are crosslinked by external agents and carry hydroxyl groups. They are copolymers prepared by known polymerisation pro- ¦~

cesses from acrylic acid esters, for example methacrylic acid methyl ester, .

methacrylic acid ethyl ester, acrylic acid methyl ester and higher alkyl esters of acrylic acid and methacrylic acid having, for example, 2 to 12 car-bon atoms in the a~cyl ester group. l~onoolefines or aromatic vinyl ccmpcunds are used as further ccmponents in the copolymers. The hydroxyl groups neces-sary for the crossli~ing are introduced by copolymerisation with, for ex-ample, acrylic acid hydroxyethyl esters or methacrylic acid hydroxyethyl esters, methacrylic acid hydroxypropyl esters or monoalkyl ethers or dialkyl ethers of diglycols or polyglycols. Acrylamide or methacrylamide or formal-dehyde ccndensation products thereof, acrylic acid or methacrylic acid, maleic acid or maleic acid half-esters, styrene or vinyltoluene can also be used for the copolymerisation.
m e binder components are commercial products. Depending on vis-cosity and miscibility, they can contain up to 60 per cent by weight of a customary solvent. mey are preferably processed free from solvent or with a low solvent content. In addition to liquid products, it is also possible to employ pulverulent products. In the latter case the accelerator is admixed as a powder.
In addition to pigments, the coating agents acc~rding to the in-vention can contain customary auxiliaries and additives, particularly flow control agents and optionally other binders in addition. The coating agents are applied by customary processes, such as, for example, brushing, spraying, dipping or rolling. m ey can also be employed in electrostatic lacquering processes. After being applied, they are stoved or cured at temperatures from lO0 to 250& .
The coating agents according to the invention possess a series of surprising advantages. ~hile the effectiveness of the accelerators is just as good as in the case of products known hitherto~ the viscosity remains stable even on prolonged storage; as a result, it is possible to add the accelerator at once when preparing the coating agent. Furthermore, the coatings prepared thereform have a smaller tendenc~ to turn yellow.

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10813#9 A particular advantage which must be singled out is that the use of the new accelerators makes it possible to employ, in electrostatic lacquer-ing processes, coating agents which are favourable to the environment, that is to say contain little or no solvent, since the electrical resistance remains virtually uninfluenced by these accelerators. This makes it possible fully to exploit the good environmental properties of solvent-free or low-solvent coat-ing agent systems which have only recently become known.
Examples Preparation of Polyester A
A mixture of 65.1 g of ethylene glycol (1.05 molç), 79.8 g of 1,2-propanediol (1.05 mols), 74 g of phthalic anhydride (0.5 mol) and 73 g of adipic acid ~0.5 mol) is heated according to the following time-temperature plan, while stirring and passing a gentle stream of nitrogen through the mix-ture: 2 hours at 140C, 2 hours at 160C, 4 hoursAat 180C, 4 hours at 190C
and 4 hours at 200C. During this time a total of 25 ml of water is separated off. Thb clear, colourless mixture of esters has an acid number of 2.5 mg KOH/g and a hydroxyl number of 454 mg KOH/g, which corresponds to an average molecular weight of 245.
Preparation of Polyester B
1,296 g of 1,4-bis-Chydroxymethyl)-cyclohexane (9 mols), 276 g of glycerol ~3 mols), 888 g of phthalic anhydride (6 mols), 730 g of adipic acid ~5 mols) and 200 g of xylene are heated at 200C for 8 hours under nitrogen and with continuous circulatory removal of water, The resulting polyester has an acid number of 4.5 mg KOH!g and a hydroxyl number of 91.4 mg KOH/g.
~; After cooling the melt to 140C, the polyester is dissolved in xylene to give a 60% strength solution.
Preparation of Catalyst 1 ~, 80 g of Versatic acid glycidol ester having an epoxide equivalent weight of 240-250 are treated with 20 g of p-toluenesulphonic acid monohydrate, whllst stirring. (Versatic acid glycidol ester can be obtained commercially -, ~
, 108~L3#9 ~ r ~ a from Deutsche Shell under the trade namc "*d~a E".) The mixture is stirred for 40 - 60 minutes. It is then allowed to cool and is diluted with 100 g of xylene.

The solution prepared in this way contains 10% of p-toluenesul-phonic acid (calculated as the free acid).
Preparation of Catalyst 2 50 g of a 10% strength solution of p-toluenesulphonic acid in Card4ra~
butylglycol are added to 50 g of Versatic acid glycidol ester (Calur~Y E) and the mixture is stirred at 50 - 60C for 60 minutes. It is allowed to cool and a catalyst solution containing 5% of p-toluenesulphonic acid (calculated as the free acid) is obtained.
Preparation of Catalyst 3 50 g of a 10% strength solution of p-toluenesulphonic acid in butylglycol are added to 50 g of an 80% strength solution of a bisphenol-A-epoxide resin having an epoxide equivalent weight of 225-280 (for example Epikote~ 834) and the mixture is stirred at 50-60C for 60 minutes. It is allowed to cool and a catalyst solution containing 5% of p-toluenesulphonic acid (calculated as the free acid) is obtained.
Preparation of Catalyst 4 30 g of p-toluenesulphonic acid monohydrate are added to 150 g of an 80% strength solution of a bisphenol-A-epoxide resin having an epoxide equivalent weight of 225-280. The temperature of the stirred mixture rises to 50-60C.
After the reaction has come to an end, which may be recognized by the reaction tc,mperature falling off after ap$roximately 60 minutes, the mix-ture is diluted with 120 g of xylene. The resulting catalyst solution con-tains 10% of p-toluenesulphonic acid (calculated as the free acid).
Preparation of Catalyst 5 100 g of a 10% strength solution of p-toluenesulphonic acid in butylglycol are added to 200 g of a 50% strength solution of a bisphenol-A-.

)813~9 epoxide resin having an epoxide equivalent weight of 450 - 500 ~for example Epikote ~ 1,001).
The~mixture is heated to 50 - 60C for approximately 60 minutes, while stirring. After cooling, a catalyst solution containing 3.3% of p-toluenesulphonic acid (calculated as the free acid) is obtained.
Preparation of a Gloss Paint Polyesters or a commercial alkyd resin were mixed with a commer-cially available hexakis-methoxy-methylmelamine derivative in the weight ratio of 7 : 3. If appropriate, both components are heated to 80C for 10 to 60 minutes in order to facilitate mixing. The mixture of binders thus obtained is pigmented with titanium dioxide in the weight ratio of 1 : 0.7.
The gloss paints of the experiments listed in the following table are catalysed as follows (per cent by weight in each case, relative to the binder):
A. No catalyst ; B. 0.5% of p-toluenesulphonic acid C. 2.35% of a 50% strength solution of the morpholinium salt of p-toluenesulphonic acid in ethylglycol 1. 0.5% of p-toluenesulphonic acid in the form of catalyst 2 according to the invention 2. 0.5% of p-toluenesulphonic acid in the form of catalyst 3 according to the invention.
The viscosity of the gloss paints was tested by means of a type D VT 180 Haake viscometer.
The electrical resistance of the lacquers was determined by means of the type 234-BA Randsburg~measuring instrument.
The measurements and the storage of the samples were carried out at 23C.
;

~ ~rc,cle~cl~ks 1~8~8g Table 1 Electrical resistance of the lacquers . . . .__ . .
Resin Experiment Electrical resistance A [kD~

h C 3 5 ~ ':

A >20,000 B 6,000 ~ _ . .. ___ ~ ~ t ~ 2D OOD ~
. . >20,000 ~0~138~

Table 2 Storage stability of the above lacquers under varying degrees of catalysis .
Visc Dsity [cP]
Resin component Experiment24 hours after 4 months after preparation preparation ., Polyester A C 500 600 2 300 350 _ A _ _ B 370 1,000 Polyester B C 690 3,500 . ~ 230 250 Commercially A 360 7,500 available alkyd B 250 gelled resin (contain-ing 40% of C 480 gelled coconut oil), 1 250 4,500 60% strength solution in 2 210 1,600 xylene _ . . .
Commercially A 410 1,700 available alkyd B 330 gelled i resin (contain-ing 40% of C 500 gelled oil), 60% 1 380 700 strength solu- 2 220 350 , tion in xylene .
. .

,.. .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A heat-curable coating composition which contains as binder (1) an aminoplast, a low-molecular precursor thereof, or a mixture thereof, mixed with (2) a saturated polyester, an oil-free or oil-modified alkyd resin, an acrylic resin containing hydroxyl groups, or a mixture of two or more thereof, wherein 0.1 to 20 percent by weight, relative to the binder, of an adduct of an epoxide and a monofunctional aromatic sulphonic acid is present as a curing accelerator, the adduct containing 0.1 to 1.0 sulphonic acid group per epoxy group.

2. A heat-curable coating composition according to claim 1, wherein the adduct of the epoxide ond the monofunctional aromatic sulphonic acid contains 0.4 to 0.5 sulphonic acid group per epoxy group.

3. A heat-curable coating composition according to claim 1 or 2 where-in the monofunctional aromatic sulphonic acid of the adduct is a toluene sul-phonic acid or alkylbenzenesulphonic acid.

4. A heat-curable coating composition according to claim 1 or 2 where-in the epoxide of the adduct is an epoxide resin based on bisphenol A or Ver-satic acid glycidol ester.

5. A heat-curable coating composition according to claim 1 or 2 where-in the adduct is present in an amount to provide from 0.3 to 0.8 percent by weight of sulphonic acid, relative to the binder.

6. A heat-curable coating composition according to claim 1 or 2 where-in one component of the binder is dimethylolurea, tetramethylolbenzoguanamine, trimethylolmelamine, hexamethylolmelamine or a partially or completely etheri-fied derivative thereof which has been etherified with an alkanol of 1 to 4 carbon atoms.

7. A heat-curable coating composition according to claim 1 or 2 where-in one component of the binder is a saturated polyester of molecular weight from 250 to 4000.

8. A heat-curable coating composition according to claim 1 or 2 which is solvent-free or contains a low proportion of solvent.

9. A heat-curable coating composition according to claim 1 or 2 which contains a flow control agent and one or more other auxiliaries or additives.

10. A process of coating a substrate which comprises applying to the substrate a heat-curable coating composition ac-cording to claim 1 and subsequently curing the composition by heating to a temperature of from 100 to 250°C.

11. A process according to claim 10 wherein the coating is applied by an electrostatic lacquering method.

12. A coated substrate obtained by a process according to claim 10 or 11.

13. A heat-curable composition which contains (a) an aminoplast or low molecular weight precursor thereof, as cross-linker, (b) a saturated polyester resin derived from (1) a dicarboxylic acid component, at least 50 mol% of which is terephthalic acid or isophthalic acid, the remainder being an-other aromatic dicarboxylic acid or up to about 10 mol % aliphatic dicarboxylic acid containing from 4-18 carbon atoms, (2) a glycol component, at least 50 mol % of which is neopentyl glycol, and (3) from about 2 to about 10 mol % of a polyol containing from 4-16 carbon atoms having at least three primary hydroxyl groups, and (c) from about 0.1 phr to about 3.0 phr by weight, based on 100 parts of (a) and (b), of the adduct reaction product of an epoxide and a monofunctional aromatic sulphonic acid, present as a curing accelerator and containing 0.1 to 1.0 sulphonic acid group per epoxy group.

14. The composition of claim 13 wherein the monofunctional sulphonic acid is toluene sulphonic acid.

15. The composition of claim 13 wherein the precursor is a hexaalkoxy methyl melamine wherein the alkoxy group contains from 1 to 4 carbon atoms.

16. The composition of claim 13 containing a low proportion of solvent.

17. The composition of claim 13 in the form of a free-flowing powder.

18. A process of coating a substrate which comprises applying the com-position of claim 13 to the substrate and subsequently curing the composition by heating.

19. A process of coating a substrate which comprises applying the com-position of claim 17 by electrostatic deposition.

20. A coated substrate obtained by a process according to claim 18 or 19.