CA2036556A1 - Process for the production of pigmented powder coatings - Google Patents

Abstract

Abstract of the disclosure:
Process for the preparation of pigmented powder coatings The invention relates to a process for the preparation of pigmented crosslinkable powder coatings composed of a) the binder component, b) the hardener component, which can react with the binder component a) at elevated temperature, c) pigments and d) if appropriate customary powder coating additives, which comprises first dispersing component c) and if appropriate component d) in all or some of binder com-ponent a) or all or some of hardener component b) with the aid of dispersing machines employed in the prepara-tion of solvent-containing coatings at a temperature above the melting range of component a) or b), and then mixing the dispersion with the remaining individual components in the customary manner, and extruding and grinding the mixture.

The pigmented powder coatings obtainable by this process allow coatings of improved surface quality, in particular improved flow and reduced orange peel effect, to be obtained.

Description

2036~

HOECHST AKTIENGESELLSCHAFT HOE 90/F 079 Dr. ZR/AP

Description Process for the preparation of pigmented powder coatings Powder coatings are particularly environment-friendly and economical coating substances. This is because solvents can be dispensed with and only very slight losses occur during use. Systems which can be crosslinked by means of heat and harden during stoving due to a chemical reaction and give a thermoset film are at present predominantly employed as powder coatings.

~hese powder coatings are currently prepared almost exclusively by the extrusion method, i.e. the individual components (resin, hardener, pigments, additives) are first mixed, and then homogenised in extruders or kneaders. The extrusion is carried out at such a low temperature that still no reaction, or only a minimal reaction, take~ place between the resin and hardener. ~n the other hand, the temperature should be high enough for the resin t~ be present as a highly viscous liquid and for good wetting of the pigment to be achieved. ~ com-promise between good wetting of the pigment on the one hand and the start of the crosslinking reaction on the other hand must also be chosen in respect of the resi-dence time in the extruder.

This method for the preparation of pi~mented thermoset-ting powder coatings has serious disadvantages: th~
surfaces of the coatings obtained from the powder coat-ings produced in this way is often poorer than that from coatings containing solvent, and the films exhibit a pronounced so-called orange peel effect. This effect occurs to a particularly high degree with acrylate powder coatings which, inter alia, are therefore used only rarely, although their use would be desirable because of their very good resistance to weathering. Further~ore, - 2 - 2036~ri~
different batches can be produced with a good reproduci-bility, for example in respect of color, only by means of a precise process procedure.

Preparation processes for pigmented powder coatings in which a pigmented resin solution and a pigmented resin/-hardener solution are first prepared have also been described. The solvent is then removed in a vacuum extruder (US Patent 3,993,849, German Offenlegungsschrift 2,005,691) or by vacuum thin film evaporation (German Offenlegungsschrift 2,005,691). Another possibility is spray-drying (British Patent 1,364,244) of the pigmented resin/hardener dispersion or coagulation of pigmented resin solutions in water (German Offenlegungsschrift 2,008,711). In these preparation processes, the disper-sion operation corresponds to the prior art for the preparation of coatings containing solvent, and is therefore particularly easy to carry out. However, the problems lie in the drying. Even small residues of solvent have an adverse effect on the storage stability of the powders. Drying of pigmented resin/hardener dispersions must moreover be carried out at the lowest possible temperatures in order to avoid undesirable crosslinking or precrosslinking.

A simple process for the preparation of pigmented powder coatings which does not have these disadvantages and provides pigmented powder coatings from which coatings of a significantly better surface quality, i.e. good flow and little orange peel effect, can be produced has now been found. Another advantage of this process is that the coatings have a high covering power, i.e. that either the pigment concentration in the coating can be reduced or the layer thickness can be reduced. Both possibilities offer economic advantages.

The patent relates to a process for the preparation of pigmented, crosslinkable powder coatings composed of 3 203~
a) the binder (resin) component, b) ~he hardener component, which can react with the binder component a) at elevated temperature, c) pigments and d) if appropriate customary powder coating additives, which comprises first dispersing component c) and if appropriate component d) in all or some of binder component a) or all or some of hardener component b) with the aid of dispersing machines which are usually employed in the preparation of solvent-containing coatings, at a temperature above the melting range of component a) or b), and then mixing the dispersion with the remaining individual com-ponents in the customary manner, extruding the mixture and grinding the extrudate.

Binder (resin) components a) which can be employed are in principle all the binders tresins~ known for this purpose in the powder coatings field. Such resins are described, inter alia, in H. Kittel, ~Lehrbuch der Lacke und Beschichtungen tTextbook of Paints and Coatings]", volume VIII, part 2, page 11 et seq.; Colomb Verlag, 1980, ~merican Paint and Coatings Journal, August 1988, pages 40-50 and Journal of Coatings Technology, 59 (750), 1987, pages 39-47. Examples which may be mentioned here are:
5 1) Acrylate resins containing OH, C02H and~or epoxy groups, 2) polyester resins containing OH and/or C02H groups and 3) epoxy resins, and mixtures of such resins.
The acrylate resins 1) can be composed of, for example, _ 4 _ 203~
a) acrylic or methacrylic acid esters of monohydric alcohols having 1-18 carbon atoms, such as, for example, methyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexylacrylate, lauryl meth-acrylate, isobornyl acrylate and i~obornyl meth-acrylate;

b) monomers containing hydroxyl groups, such as acrylic or methacrylic acid monoe~ters of polyfunctional alcohols, for example hydroxyethyl acrylate, hyd-roxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate or hydroxybutyl methacrylate, or such as vinylglycol;

c) monomers containing epoxide groups, such as glycidyl methacrylate or glycidyl acrylate;

d) monomers containing carboxyl groups, such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, fumaric acid, maleic acid and mono-alkyl esters of unsaturated dicarboxylic acids. The monomer containing acid groups is preferably em-ployed in the polymer in an amount of 3 - 25~ by weight;

e) other copolymerized monomers, such as vinylaro-matics, such as styrene, alpha-methylstyrene and vinyltoluene, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, N-substitution products of these amides, N-vinylpyridine, maleic acid esters and the like.

The resins can also be polyester-modified, for example by grafting acrylate monomers and other copolymeri~able monomers on to low molecular weight polyesters.

Suitable acrylate resins are known, for example, from U.S. Patents 3,932,367, 3,939,127, 3,991,132, 3,991,133, 3,993,849, 3,998,768, 4,009,131, 4,091,048, 4,37~,954, 4,402,983 and 4,727,111, European Published Specifica-tions 209,377, 214,448, 220,637, 240,984, 256,369, 275,051, 284,996, 299,420 and 314,447, and German Offen-legungsschriften 2,353,040, 2,441,624, 2,441,753, 2,509,410, 2,516,978 and 3,545,061.
These acrylate resins can be prepared by the customary polymerization processes, such as solution, precipita-tion, bulk, bead or emulsion polymerization, also using customary polymerization initiators and molecular weight regulators. Bulk polymerization processes such as are decribed, for example, in European Published Specifica-tion 56,971 and German Offenlegungsschrift 2,642,327 are particularly preferred.
The polyester resins 2) are prepared from polycarboxylix acids or derivatives thereof, such as phthalic acid, phthalic terphthalate, trimellitic anhydride, pyro-mellitic anhydride or adipic acid, and polyhydroxy compounds, such as ethylglycol, propylene glycol, butane-diol, hexanediol, hydroxypivalic acid, neopentyl glycol, dimethylolcyclohexane, trimethylolethane or trimethylol-propaneglycerol. The preparation methods are known. The prepartion is in general carried out at temperature up to 220°C in the melt. Suitable polyester resins are described, for example, in U.S. Patent 4,801,680, European Published Specifications 85,913, 242,714, 244,823, 322,827, 322,834 and 312,331, and German Offen-legungsschriften 3,525,110, 3,618,355, 3,711,374, 3,740,932, 3,908,031 and 3,936,973.
The epoxy resins 3) which can be employed contain on average more than one epoxide group/molecule. The epoxy resins can be saturated or unsaturated, and aliphatic, cycloaliphatic, araliphatic or heterocyclic. Epoxy resins based on bisphenol A are particularly suitable. Epoxy novolaks can also be employed. Another class of epoxy resins comprises compounds containing more than one - 6 - 203~
epoxide group/molecule, for example triglycidyl i80-cyanurate, triglycidylurazole, glycidyl ethers, fiuch as those based on bisphenol A, and glycidyl esters, such as those of phthalic acid or tetrahydro- or hexahydro-phthalic acid. Suitable epoxy resins are known, forexample, from the handbook "Epoxidverbindungen und Epoxidharze [Epoxy Compounds and Epoxy Resins]" by A.M.
Paquin, Springer Verlag, Berlin 1958, Chapter IV, from Lee, Neville ~Handbook of Epoxy Resins~, 1967, Chapter 2 and from Wagner/Sarx, ~Lackkunstharze [Synthetic Resins for Coatings]", Carl Hanser Verlag (1971), page 174 et seg., European Published Specification 292,771 and German Offenlegungsschriften 3,624,078 and 3,624,117.

The hardening agents b) employed according to the inven-tion are likewise known from the literature. Possiblecrosslinking agents for OH-containing polyesters or OH-containing acrylate resins are, in particular, the following known classes of substances: polycarboxylic anhydrides, polycarboxylic acids, amino resins, masked polyisocyanates and polyoxazolones.

Polycarboxylic anhydrides are, for example, poly(adipic acid) anhydride, poly(azelaic acid) anhydride, poly-( ebacic acid) anhydride, poly(dodecanedioic acid) anhydride or reaction products thereof with polyhydric alcohols, and other polymeric anhydrides.

The polycarboxylic acid can be a CO2H-containing polyester or a CO2H-containing acrylate resin, such as have already been described above. However, a dicarboxylic acid, for example an aliphatic or aromatic dicarboxylic acid, such as succinic acid, glutaric acid, adipic acid, dodecane-dioic acid, azelaic acid and the like, can also be employed.

The amino resins are based on addition products of formaldehyde with substances containing amino or amido groups. Reaction products of alcohol and formaldehyde - 7 - 2 036 ~ i ~
with melamine, urea or benzoguanamine are the most common. These condensation products can be monomeric or polymeric. The amino resins contain methylol or similar alkylol groups, and usually at least some of these alkylol groups are etherified with alcohol. ~ethanol or butanol is preferably employed as the alcohol.

Possible masked polyisocyanates are polyisocyanates in which the i~ocyanate groups are completely or partly blocked, for example with caprolactam, alcohols, ketox-imes, malonic esters, acetoacetic esters and the like. It is also possible to employ acylurethanes, such as are described, for example, in German Offenlegungsschriften 3,421,293 and 3,421,294. Polyisocyanate derivatives which contain uretdione and urethane groups and are free from blocking agents are also employed.

Polyoxazolones which contain at least 2 oxazolone radi-cals in the molecule are disclosed, for example, in German Offenlegungsschrift 2,516,978.

The following substances can be employed in particular as crosslinking agents for CO2H-containing polyesters or COzH-containing acrylate resins: polyepoxides, polyoxazo-lines, OH-containing polymers and betahydxoxyalkylamides.

Suitable polyepoxides or OH-polymers have already been described above ~in this context, see also German Offen-legungsschrift 3,618,355). Polyoxazolines are to be found, for example, in German Offenlegungsschrift 2,901,157 or German Auslegeschrift 1,050,540, and beta-hydroxyalkylamides are to be found in German Offen-legungsschrift 2,509,237.

Possible crosslinking agents for epoxy resins are above all polycarboxylic acids, polyanhydrides and amino resins, such as have already been described abave.

It is also possible in each csse to employ mixtures of - 8 - '~036~
the corresponding hardeners.

In order to give the powder coating the desired color, the customary pigments are added as component c). The pigments are in general employed in an amount of 1 to 50%
by weight, preferably 2 to 40% by weight, based on the total coating, the amount depending on the nature of the pigments and also on the layer thickness. In the case of carbon black, an amount of about 1~ by weight, preferably 1 to 3% by weight, will often already be adequate, while the amount of titanium dioxide is specifically ~0 to 40%
by weight. The other pigments are preferably employed in amounts of 5 to 40~ by weight, and the pigments can in each case also be mixtures.

It is possible to employ inorganic or organic pigments such as, for example, iron oxides, such as iron oxide red, yellow or black, lead oxides and chromates, carbon black, titanium dioxide, talc, barium sulfate, chromium yellow, phthalocyanine blue, phthalocyanine green, chromium green, zinc chromate, chromium-titanium yellow, nickel-titanium yellow, cobalt blue, cobalt green, molybdenum orange, molybdenum red, cadmium orange, cadmium red, azo pigments, triphenylmethane pigments, metallic nuclear pigments, carbonyl pigments, dioxazine pigments and the like.

Various additives such as are usually contained in powder coatings can be used as component d), in particular degassing agents, such as, for example, benzoin, which is in general employed in amounts of 0.1 - 3.0% by weight, based on the total coating. Flow control agents, for example oligomeric polyacrylates, such as, for example, polylauryl (meth)acrylate, polybutyl (meth)acrylate and poly-2-ethylhexyl (meth)acrylate, or fluorinated polymers or polysiloxanes, can furthermore be used. The known W
absorbers and antioxidants can be added to improve the resistance to weathering.

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To prepare the pigmented powder coatings, components c) and if appropriate component d) are first dispersed in one of components a) or b) or portions thereof. This dispersion is carried out in the customary dispersing machines which are otherwise employed in the preparation of coatings containing solvent, for example toothed colloid machines, dissolvers (double-walled temperature-controlled vessels with dispersing disks) or stirred mills. It must be possible to heat the dispersing machines, because the dispersion operation is carried out at relatively high temperatures, i.e. above the melting point of the particular component a) or b) or correspond-ing mixtures of a) and b). The temperature should be chosen here so that the visco~ity of the pure component a) or b) is advantageously below 3000 mPa-s, preferably between 500 and 2500 mPa s. Temperatures of 100C to 200C are in general possible. In most cases, the tem-perature will probably be 160-180C.

The materials can be mixed in the dry form before the dispersion process. It is usually advantageous first to melt component a) or b) and then to introduce component c) and if appropriate d) successively, while stirring.
The ratio of component c) to resin component a) or hardener component b) depends on the nature, particle size and surface treatment of the pigment and the nature of the binder or hardener, and can vary within wide limits. The dispersed mixture is then mixed in the dry state with the component a) or b) still lacking, and the mixture i~ then extruded in the customary manner at temperatures of 80C to 120C. The extrudate is cooled and comminuted and then ground on a mill, average par-ticle sizes of about 40 to 90 ~m, preferably 50 to 70 ~m, being sought, depending on the intended use. Any coarse particle content present can be sieved off.

The powder coatings obtainable according to the invention can be applied to any desired substrates, such as metal, glass or plastic, by the customary methods. Electrostatic 1 o 2 0 ~ 6 3 A3 b spraying is particularly suitable. Hardening is carried out at temperatures between 100C and 250C, depending on the crosslinking mechanism.

It was surprising that powder coatings prepared by this process produce very smooth surfaces. This particularly applies to pigmented acrylic powder coatings, which have a very rough surface if they are prepared in an extruder by conventional processes. Our own experiments have shown that even by changing the residence time or the tempera-ture during extrusion or by several extrusions, wetting o~ the pigment - especially in acrylic resins - and therefore the quality of the surface can be improved only insignificantly. This improvement was also therefore not to be expected, becau~e according to statements in the literature perfect pigment distribution cannot be achieved by preparation of pigment concentrate melts (see Kittel, ~Lehrbuch der Lacke und Beschichtungen [Textbook of Paints and Coatings]", volume XIII, part 2, 1980). It was also not foreseeable that the pigmented powder coatings which are prepared by the process according to the invention have a relatively high covering power.

Example l 437 g of an acrylic resin a) containing hydroxyl groups (copolymer of 29.9~ by weight of styrene, 17.4% by weight 2S of hydroxyethyl methacrylate, 10% by weight of n-butyl methacrylate, 12.4% by weight of n-butyl acrylate, 30%
by weight of methyl methacrylate and 0.3% by weight of methacrylic acid) and 5 g of benzoin were heated to 170C
in a double-walled temperature-controlled vessel (1 l capacity, 10 cm diameter). 300 g of titanium dioxide were then introduced and were finely dispersed with the aid of - a dispersing disk of 4 cm diameter at a peripheral speed of about 13 m/second. After the resin/pigment mixture had cooled, it was mixed with 258 g of a masked aliphatic polyisocyanate (~Additol XL 432, Hoechst AG~ and the mixture was extruded on a twin-screw extruder at 80-100C

203~

and 300 revolutions/minute. After cooling and precom-minution to a powder of Hverage particle size 80 ~m, the extrudate wa~ ground. The coar~e particle~ having par-ticle sizes above 125 ~m were sieved off, and the powder wa~ sprayed on to degreased, earthed iron sheets, using an electrostatic powder spraying unit at 60 kV, such that a layer thickness of 45 ~m resulted after stoving at 200C/20 minutes.

Example 2 Analogously to Example 1, 291 g of the acrylate resin a), 300 g of titanium dioxide and 5 g of benzoin were fir~t dispersed, a further 146 g of the acrylate resin a) and 258 g of Additol XL 432 were then added and the mixture was extruded.

Example 3 Analogously to Example 1, 117 g of the acrylate resin a), 300 g of titanium dioxide and 5 g of benzoin were first dispersed, 330 g of the acrylate resin a) and 258 g of Additol XL 432 were then added and the mixture was extruded.

Comparison Experiment 4 437 g of the acrylate resin a) containing hydroxyl qroups, 5 g of benzoin and 300 g of titanium dioxide were mixed in the dry state and the mixture was then extruded at 120C and 150 revolutions/minute. 258 g of Additol XL
432 were then added and the mixture was further processed as described in Example 1.

Comparsion Experiment 5 Analogously to Comparison Experiment 4, 291 g of the acrylate resin a), 300 g of titanium dioxide and 5 g of benzoin were first extruded, a further 146 g of the - 12 - 2o3~rj~;
acrylate re~in a) and 258 g of Additol XL 432 were then added and the mixture was further proces~ed a~ descrLbed in Ex~mple l.

Comparison Experiment 6 Analogously to Comparison Experiment 4, 177 g of the acrylate xesin a), 300 g of titanium dioxide and 5 g of benzoin were first extruded, a further 330 g of the acrylate resin a) and 258 g of Additol XL 432 were then added and the mixture was further processed as described in Example 1.

Comparison Experiment 7 437 g of the acrylate resin containing hydroxyl groups, 258 g of Additol XL 432, 300 g of titanium dioxide and 5 g of benzoin were mixed in the dry state and then extruded at 80-100C and 300 revolutions/minute, the extrudate was ground and applied and the coating was stoved.

Examples C~arison Examples - - -Gloss (%) 92 93 93 84 86 86 72 60 lens F1CM* 2 2 2 4 4 4 3 * evaluated with ratings from 1 to 5;
l is very good -;
The powder coatings prepared by the process according to the invention exhibit a considerably better flow and . 30 better gloss than the powder coating according to Com-parison Experiment 7 prepared by the known process.
Although varying the known process by predispersion in the extruder (see Comparison Experiments 4 - 6) gives 2036a.jb better ~108B value~ than Comparison Experiment 7, the flow i8 even poorer.

Claims (11)

1. A process for the preparation of a pigmented cross-linkable powder coating comprised of a) a binder component, b) a hardener component which can react with the binder component a) at elevated temperature, c) pigments and d) optionally customary powder coating additives, com-prising first dispersing component c) and optionally component d) in at least some of binder component a) or at least some of hardener component b) with the aid of dispersing machines which are employed in the pre-paration of solvent-containing coatings at a tempera-ture above the melting range of component a) or b), and then mixing the dispersion with the remaining individual components, extruding the mixture and grinding the extrudate.

2. The process of claim 1 wherein the binder component a) is a resin which can be cross-linked with the hardener component b) selected from the group consisting of acrylate resin, polyester resin and epoxy resin.

3. The process of claim 1 wherein component a) is an acrylate resin.

4. The process of claim 3 wherein the hardener component b) is a compound selected from the group consisting of polycarboxylic anhydrides, polycarboxylic acids, amino resins, masked polyiso-cyanates, uretdiones, polyepoxides and .beta.-hydroxyalkylamides.

5. The process of claim 1 wherein the binder component a) is an acrylate resin containing hydroxy groups or a polyester modified acrylate resin containing hydroxyl groups and the hardener com-ponent b) is a masked polyisocyanate or a uretdione.

6. The process of claim 1 wherein the binder component a) is an acrylate resin containing epoxide groups or an acrylate resin containing epoxide and hydroxyl groups and the hardener component b) is a polycarboxylic acid or a polycarboxylic anhydride.

7. The process of claim 1 wherein the binder component a) is an acrylate resin containing carboxyl groups and the hardener com-ponent b) is a .beta.-hydroxyalkylamide.

8. The process of claim 1 wherein component a) is a polyester resin and the hardener component b) is a compound selected from the group consisting of polyepoxides, .beta.-hydroxyalkylmides, polycar-boxylic anhydrides, polycarboxylic acids, amino resins, masked polyisocyanates and uretdiones.

9. The process of claim 1 wherein the dispersion of com-ponents c) and optionally d) in component a) or b) is carried out at a temperature of 100°C to 200°C, preferably 150°C to 200°C.

10. The process of claim 9 wherein component c) and option-ally d) are added to component a).

11. The process of claim 1 wherein the dispersion is carried out by means of a toothed colloid machine, a stirred mill or a dissolver.