WO1996036674A1

WO1996036674A1 - Thick-layer coil coatings and composition and process for preparing the same

Info

Publication number: WO1996036674A1
Application number: PCT/NL1996/000196
Authority: WO
Inventors: Albert Heyenk; Johannes Wilhelmus Besamusca
Original assignee: Dsm N.V.
Priority date: 1995-05-18
Filing date: 1996-05-08
Publication date: 1996-11-21
Also published as: AU5703896A; NL1000387C2; NL1000387A1

Abstract

The invention relates to the use of a composition comprising an acid-functional polyester having an acid number between 10 and 90 mg of KOH/gram of resin, a hydroxyl number lower than 5 mg of KOH/gram of resin and a molecular weight (Mn) between 1500 and 10,000, a crosslinking agent containing epoxy groups and an organic solvent in order to obtain a coated coil having a dry layer thickness of the coating of between 5 νm and 60 νm.

Description

THICK-LAYER COIL COATINGS AND COMPOSITION AND PROCESS FOR PREPARING THE SAME

The present invention relates to thick-layer coil coatings.

Conventionally, thick-layer coil coatings have been prepared from PVC plastisols. A description of PVC plastisols coil coatings can be found in ECCA CONFERENCE TRANSCRIPT by D. Kogler (ECCA GENERAL MEETINGS, Berlin, 15-18 May 1994, pp. 1-11), which is incorporated herein by reference. However, due to increasing concerns over the detrimental environmental impact of PVC plastisols, there recently have been greater efforts directed towards the discovery of alternatives for PVC plastisols for preparing coil coatings. One such alternative that has been considered is a polyester-based composition that includes melamine (derivatives) as crosslinking agents. In practice, this conventional coating has been applied to provide a maximum primer coating thicknesses of 20 μm. Another alternative composition that has been considered is a polyester based composition that includes crosslinking agents that contain isocyanate groups, which can provide a coating film having a maximum (dry) coat thickness of 35 μm. The drawback of these systems is that elimination products, such as methanol, butanol and blocking agent, are released, which may cause surface defects in the cured film.

It is therefore an object of the present invention to provide coated coils with a layer thicknesses of the dry coating of up to about 60 μm, and preferably in the range of about 20 μm to about 60 μm. The coating thickness is determined via ISO

2360,

It is another object of the present invention to provide a coating that exhibits excellent aesthetic quality and possesses high scratch resistance, hardness, flexibility, and anti-corrosion properties.

It is yet another object of the present invention to provide a coil coating that is essentially free of PVC plastisols so as to avoid the above- discussed environmental concerns.

It is a further object of the present invention to provide a composition that, when suitably cured, provides a coil coating possessing the aforementioned properties. These objects are provided by a composition which comprises an acid-functional polyester having an acid number in the range of about 10 to about 90 mg of KOH/gram of resin, a hydroxyl number lower than 5 mg of KOH/gram of resin and a molecular weight (Mn) in the range of about 1500 to 10,000, a crosslinking agent containing epoxy groups, and an organic solvent.

After curing of this composition coil coatings having a primer and/or top coat layer thickness between 5 μm and 60 μm can be obtained. This range can indicate the thickness of each individual layer.

Besides aesthetic improvement, the coatings obtained also exhibit high quality in terms of scratch resistance, hardness, flexibility and anti-corrosion properties.

After the coating composition is cured, the glass transition temperature of the resulting cured coil coating is generally at least 40°C. Consequently the desired hardness, scratch resistance, chemical resistance, and corrosion resistance of the coating are ensured.

Preferably, the hydroxyl number of the acid- functional polyester is about 0 mg of KOH/gram resin. Preferably, the acid number is in the range of about 30 to about 70 mg of KOH/gram resin.

Preferably, the molecular weight (Mn) is in the range of about 2000 to 7000.

Suitable compounds containing epoxy-groups for use as the crosslinker are, for instance, bisphenol-A epoxy resins (for instance Epikote 828™, Epikote 1001™ and Epikote 1004™ from Shell), hydrogenated bisphenol-A epoxy compounds, aliphatic epoxy compounds, epoxidized alkyd resins, epoxidized oils (for instance epoxidized linseed oil or soya bean oil), epoxidized borates and triglycidyl isocyanurate. Preferably, a bisphenol-A epoxy resin is selected as the crosslinking agent.

In addition, a suitable crosslinking agent can include a mixture of a compound containing epoxy- groups with another compound that contains melamine groups or isocyanate groups. The mixture of crosslinking agents preferably contains more than about 90 wt.% of the crosslinking agent containing epoxy- groups.

In general, the binder composition according to the invention has an equivalent ratio carboxyl : epoxy equivalent ratio in the range of about 0.85 : 1 to 1 : 0.85 and more preferably in the range of about 0.9 : 1 to 1 : 0.9.

The acid functional polyester resin can also contain a melt-incorporated solid catalyst. It is also possible to use a liquid catalyst or to mix in a catalyst solution in the paint formulation.

Suitable catalysts are described by Madac et al., in Kinetics and Mechanisms of Polyesterifications, Advances in Polymer Science, 182-198 (1985) which is incorporated herein by reference. Examples of suitable classes of catalysts selectable for the present invention include, N-dialkylamine pyridines, tertiary - A -

amines, imidazoles, guanidines, cyclic amines and latent amine catalysts or combinations thereof. If desired, the catalysts can be blocked. More specifically, exemplary catalysts include by way of example 2,4,6-tri(dimethylaminomethyl)phenol, dimethylbenzylamine, N-dimethylamino pyridine, benzotriazole, triethyl amine, triphenyl amine, 4,5- diphenyl imidazole, 1-ethyl imidazole, 2-methyl imidazole, 4-methyl imidazole, ethyl imidazole carboxylate, 5,6-dimethyl benzimidazole, 1-benzyl imidazole, imidazole, 1,1-carbonyl diimidazole, benzyl trimethyl ammonium chloride, choline chloride, triphenyl ethyl phosphonium bromide, tetramethyl guanidine (TMG) , isocyanate-TMG adducts (for instance isophorone diisocyanate-di-tetramethyl guanidine, tolonate-HDT-tetramethyl guanidine, or TMXDIdiTMG) , acetyl-TMG, 2-phenyl-l,1,3,3-tetramethyl guanidine, 1,5-diazabicyclo[4,3,0]non-5-ene and 1,5,7- triazabicyclo[4,4,0]dec-5-ene. Other suitable catalysts include tetraalkyl phosphonium bromide, tetrabutyl ammonium fluoride, cetyl triethyl ammonium bromide and benzothiazol.

A metal salt of a carboxylic acid or an alcohol or a metal hydroxide can also be used as the catalyst. Suitable salts include, for example, sodium stearate, lithium stearate, lithium acetate, sodium benzoate, sodium neodecanoate, lithium benzoate, sodium laurate, lithium laurate, lithium adipate, sodium octanoate, lithium octanoate, lithium neodecanoate and lithium versatate.

The weight percentage of the catalysts relative to the polyester generally is in the range of about 0.1 wt.% to about 5 wt.%.

The carboxyl-functional polyester preferably is prepared by reaction between a hydroxyl-functional polyester and an acid anhydride or a dicarboxylic acid. Suitable acid anhydrides include, for example, phthalic anhydride, tetrahydrophthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, pyromellitic anhydride, hexahydrophthalic anhydride, or a combination thereof. Preferably, trimellitic anhydride and hexahydrophthalic anhydride are used. Suitable dicarboxylic acids include, for example, adipic acid, isophthalic acid, hexahydroterephthalic acid, and sebacic acid. By preference an acid anhydride is used. The reaction between the hydroxyl-functional polyester and the acid anhydride or the dicarboxylic acid preferably takes place (in a second step) after synthesis of the hydroxyl-functional polyester is complete. In the reaction between the hydroxyl- functional polyester and the acid anhydride or the dicarboxylic acid, the molar ratio between the hydroxyl groups and the acid anhydride or dicarboxylic acid groups preferably is about equimolar. In this way, a polyester with a hydroxyl number lower than about 5, and more preferably about 0, can be obtained.

The carboxyl-functional polyester can also be obtained directly by means of reaction of polyalcohols with acids or acid anhydrides. In conducting such a reaction, the molar ratio between the polyalcohols and the acids or acid anhydrides is preferably in the range of about 1.0:1.2 to about 1.0:1.0 so as to obtain a polyester having a hydroxyl number lower than about 5, and more preferably about 0. Suitable polyalcohols for the preparation of the carboxyl-functional polyester include, by way of example, ethylene glycol, propylene glycol, diethylene glycol, butane diol (1,4), hexane diol (1,6), neopentyl glycol, 2-methyl-l,3-propane diol, 1,3-butane diol, 1,3-propane diol, 1,2-propane diol, 2-ethyl-2-butyl- 1,3-propane diol, trimethylpentane diol, hydroxypivalineopentylglycol ester, tricyclodecane dimethanol, cyclohexane dimethanol, biphenol-A- bishydroxyethyl ether, trimethylol propane, pentaerythritol, or a combination thereof.

Suitable acids for the preparation of the carboxyl-functional polyesters include, by way of example, isophthalic acid, terephthalic acid (dimethyl ester), adipic acid, sebacic acid, hexahydroterephthalic acid (CHDA), decane dicarboxylic acid, 5-6-butylisophthalic acid, dimerized fatty acids, or a combination thereof. Suitable acid anhydrides include, by way of example, phthalic anhydride, tetrahydrophthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, pyromellitic anhydride, hexahydrophthalic anhydride, or a combination thereof. Preferably, the acid is selected from the group consisting of isophthalic acid, terephthalic acid, adipic acid, or a combination thereof.

The esterification reaction preferably takes place under a nitrogen atmosphere at temperatures in the range of about 160°C to about 260°C. Catalysts such as, for example, dibutyl tin oxide, phosphorous acid, tin chloride, butylchlorotin dihydroxide (FASCAT™) , or tetrabutoxy titanate, and suitable antioxidants such as, for example, t inonylphenyl phosphite and triphenyl phosphite, can be used as additives. Water released during the esterification reaction preferably is distilled off and the desired esterification degree is obtained by applying an azeotropic distillation and/or a vacuum destination in the final stage.

The reaction yields a polyester which can be dissolved in an organic solvent or a mixture of organic solvents. The ratio of polyester to organic solvent is selected, or can be determined through routine experimentation, to provide a desired solids content. The solvent can be added directly after the polyester synthesis, or after synthesis and during the preparation of the composition. In general, the solids content of the polyester preferably is in the range of about 50% to about 100%.

Suitable solvents include, by way of example, aromatic hydrocarbon resins (for example, Solvesso types), N-methyl pyrrolidone, xylene, propylene glycolmonomethyl ether, methyl-propylene glycol acetate (PMA) , ethyl ethoxy propionate, ethylene-propylene glycol acetate, isophoron, dibasic ester (DBE) and butyl glycol. These solvents can be used alone or in any combination thereof. By preference, an aromatic hydrocarbon, PMA, DBE or a combination thereof is used.

Coil coatings can be obtained by generally known processes, such as described for instance in "Coil Coatings" by Joseph E. Gaske (Federation of

Societies for Coatings Technology, February 1987, pp. 7-19), which is incorporated herein by reference.

The curing conditions and additives can be chosen on the basis of the desired peak metal temperature (PMT), and the nature and the thickness of the substrate. In general, the curing time preferably is between about 20 seconds and about 70 seconds. The curing is preferably conducted at temperatures between about 250°C and about 400°C, which results in a PMT between about 204°C and about 249°C.

Suitable substrates include, for example, steel, tinned steel and aluminum.

The coatings prepared in accordance with the present invention are suitable as primer and as top coat for coils. The coatings can suitably be applied, by way of example, in part or in whole, to coils employed in household equipment and appliances, such as, for example, ref igerators, freezers, ovens, microwave ovens, and boilers. Further, the coatings of the present invention are suitable as coatings for caravans and as facade coating. In addition, the appearance of the coatings can be manipulated by the addition of pigments.

The monomers for preparing the polyester and the crosslinking agent of the composition of the present invention, and the curing conditions for curing the composition into a coil coating, are preferably selected with consideration of the particular intended application of the coil coating.

If desired, additives such as, for example, pigments, fillers, stabilizers, dispersing agents, flow promotion agents and de-foaming agents, can be included in the composition of the present invention.

The composition according to the invention is also suitable for other applications, especially can coatings. However, the desired layer thickness is in general lower and the curing conditions involved in the preparation of can coatings differ from those in coil coating manufacture.

A process for preparing coating compositions and thick-layer coil coatings is disclosed in Application No. NL 01000387, filed on May 18, 1995, the complete disclosure of which is incorporated herein by reference.

The present invention will now be illustrated by the following non-limiting examples, which serve to explain embodiments of the invention in more detail.

Example I a) Preparation of polyester resin

124 parts by weight of ethylene glycol, 677 parts by weight of neopentyl glycol, 236 parts by weight of hexane diol, 664 parts by weight of terephthalic acid, 2.3 parts by weight of trinonyl phenylphosphite and 1.16 part by weight of butylchlorotin dihydroxide (Fascat 4101™) were heated under a nitrogen atmosphere in a glass reaction flask with a mechanical stirrer, a thermometer and a still head with a Vigreux column. The esterification reaction started at 199°C? the reaction water formed was distilled off. The maximum reaction temperature was 235°C and the reaction continued until an acid number of 3 mg KOH/gram resin was reached. After cooling to 170°C, 996 parts by weight of isophthalic acid were added, followed by heating to 235°C. After 1 hour at 235°C azeotropic distillation was started and continued until an acid number of 3.4 was reached. Then the mixture was vacuum-distilled for 1.5 hours, followed by cooling to 225°C (acid number 2.0, OH number 18.3). After determination of the OH number (18.3 mg KOH/g resin) an equimolar quantity of trimellitic anhydride was added (145.8 g). After 1.5 hour at 225°C, the mixture was cooled to 195°C and 24.7 g of triphenyl ethyl phosphonium bromide was added. After 15 minutes at 195°C the resin obtained was diluted with a mixture consisting of the solvents dibasic ester (DBE™, Du Pont) and propylene glycol methyl ether acetate (in a weight ratio of 1:1) to obtain a solids content of 60% by weight.

The acid number of the solid resin was 35 mg KOH/gram resin and the hydroxyl number was 0 mg KOH/gram.

The viscosity, measured via a Physica Viscolab LC3 at 23°C, was 14.0 Pa.s (according to ISO 3219).

The molecular weight (Mn) amounted to 3760 (determined by means of gel permeation chromatography (GPC) on a polystyrene standard).

b) Preparation of binding agent composition

To 22.86 parts by weight of the resulting carboxyl-functional polyester resin were added 10.62 parts by weight of titanium dioxide (Kronos 2160™) , 3.48 parts by weight of an anti-corrosive pigment (Halox CW 491™), 3.48 parts by weight of a anticorrosive pigment (Zinc phosphate Z/PM) and 14.07 parts by weight of thinner (DBE/PMA 4:1). This mixture was ground to pigment paste.

During the preparation the temperature of the paste did not exceed 70°C. Next, after cooling to room temperature, 24.45 parts by weight of the polyester resin obtained in the foregoing and 3.59 parts by weight of epoxy resin (Epikote 828™, Shell Chemie B.V. ) were mixed. Then the mixture was diluted with DBE/PMA in a weight ratio of 4 : 1 to obtain a viscosity corresponding to an efflux time of 100-120 seconds (DIN Cup 4 at 23°C, DIN standard 53 211).

c) Coil coating

The binding agent composition thus obtained was applied onto hot-dipped galvanized steel (Bonder 1303) with a 150 μm wire coater . After curing in an oven with a stoving cycle of 44 seconds at 380°C, resulting in a PMT of 232-241°C, the following characteristics were determined: - layer thickness: 40 μm (determined in accordance with ISO 2360)

T-bend: T (determined in accordance with ASTM D 4145) solvent resistance (methyl ethyl ketone) : 100 dR - pencil hardness; H (determined in accordance with ASTM D 3363) - condensation water resistance: no blistering after 2000 hours (determined in accordance with DIN 50 017) - salt spray: after 1000 hours, a strip of 1-2 mm has come off along the scratch (determined in accordance with DIN 50 021).

Comparative Experiment A a) Preparation of polyester resin

87 parts by weight of ethylene glycol, 183 parts by weight of neopentyl glycol, 182 parts by weight of 1.6 hexane diol, 212 parts by weight of terephthalic acid, 23 parts by weight of adipic acid, 238 parts by weight of isophthalic acid, 202 parts by weight of phthalic acid anhydride, 1 part by weight of trinonyl phenylphosphite and 0.5 part by weight of dibutyl tin oxide were heated under a nitrogen atmosphere in a glass reaction flask with a mechanical stirrer, a thermometer and a still head with a Vigreux column. The esterification reaction started at 181°C; the reaction water formed was distilled off. The maximum reaction temperature was 235°C. When the reaction mixture became clear, azeotropic distillation with xylene was started and continued until an acid number of 3.8 was reached. After cooling to 175°C the resin obtained was diluted with a mixture consisting of the solvents PMA and DBE (in a weight ratio of 1:1) to obtain a solids content of 60%.

The acid number of the resin was 3.8 and the hydroxyl number was 30.

The viscosity, measured in a Physica Viscolab LC3 at 23°C, was 6.9 Pa.s (according to ISO 3219).

The molecular weight (Mn) amounted to 3280 (determined by means of GPC on a polystyrene standard).

b) Preparation of binding agent composition

To 17.0 parts by weight of the hydroxyl- functional polyester resin obtained in a) were added 10.5 parts by weight of zinc phosphate, 5.25 parts by weight of titanium dioxide (Kronos 2059™), 5.25 parts by weight of filler (Talc IT Extra™), 5.0 parts by weight of a flotation agent (Bentone 38™; 10% solid in xylene/ethanol 8/2) and 0.5 part by weight of a dispersion agent (Troykid Collidisperse™). This mixture was ground to pigment paste.

During the preparation the temperature of the paste did not exceed 70°C. Next, after cooling to room temperature, 33.0 parts by weight of the polyester resin obtained in the foregoing, 7.8 parts by weight of melamine crosslinker (Cymel 303™) and 0.2 part by weight of catalyst (Cycat 4040; 10% solid matter in ethylene glycol monoethyl ether acetate) were mixed. The mixture was then diluted with DBE and PMA (weight ratio 4 : 1) to obtain a viscosity corresponding to an efflux time of 100-120 seconds (DIN Cup 4 at 23°C, DIN standard 53 211) .

c) Coil coating

The composition thus obtained was applied onto hot dipped galvanized steel (Bonder 1303) with a 150 μm wire coater. After curing in an oven with a stoving cycle of 44 seconds at 380°C, resulting in a PMT of 232-241°C, the following characteristics were determined:

- layer thickness 30 μm: very irregular surface with a large amount of surface defects - pencil hardness; HB (determined in accordance with ASTM D 3363) solvent resistance (methyl ethyl ketone): 100 dR

From these tests it appears that a composition according to the invention provides a coil coating having a desirable layer thickness and a good appearance, whereas the composition according to the comparative experiment results in a coating having a large amount of surface defects due to the layer being too thick for hydroxyl-functional polyester-melamine systems.

Claims

C L A I M S

1. Use of a composition comprising an acid-functional polyester resin having an acid number in the range of 10 mg of KOH/gram of resin to 90 mg of KOH/gram of resin, a hydroxyl number lower than 5 mg of KOH/gram of resin and a molecular weight (Mn) in the range of 1500 to 10,000, a crosslinking agent containing epoxy groups, and an organic solvent in order to obtain coil coatings having a dry layer thickness of the coating of between 5 μm and 60 μm.

2. Use according to claim 1, characterized in that the hydroxyl number is about 0 mg of KOH/gram resin.

3. Use according to any one of claims 1-2, characterized in that the acid number is in the range of 30 to 70 mg of KOH/gram resin.

4. Use according to any one of claims 1-3, characterized in that the molecular weight (Mn) is in the range of 2000 to 7000.

5. Use according to any one of claims 1-4, characterized in that the crosslinking agent containing epoxy groups is bisphenol-A epoxide.

6. A coil coating having a dry layer thickness in the range of 5 μm to 60 μm obtained with a composition as described in any one of claims 1-5.

7. Use of an acid-functional polyester having an acid number in he range of 10 to 90 mg of KOH/gram of resin, a hydroxyl number lower than 5 mg of

KOH/gram of resin and a molecular weight (Mn) in the range of 1500 to 10,000 in the production of coil coatings having a dry layer thickness of between 5 μm and 60 μm.

8. A process for the production of a coil coating having a dry layer thickness of the coating in the range of 5 μm to 60 μm, by curing a composition comprising an acid-functional polyester having an acid number between 10 and 90 mg of KOH/gram of resin, a hydroxyl number lower than 5 mg of KOH/gram of resin and a molecular weight (Mn) between 1500 and 10,000, a crosslinking agent containing epoxy groups and an organic solvent and wherein the curing step is conducted for 20 to 70 seconds at temperatures between 250°C and 400°C.

9. A wholly or partly coated substrate comprising the coating according to claim 6.

10. Use of a composition as described in any one of claims 1-5 in the production of can coatings.