Powder paint and a polyester resin for powder paint

Abstract: The invention relates to a powder paint, comprising a polyepoxy compound, a polyester resin and customary additives, the polyester resin having an acid number ranging from 15 through 70 and a hydroxyl number of 10 or below and being substantially based on units of dicarboxylic acids, glycols and monomers of functionality three or higher, as dicarboxylic acids use is being made of at least 15 mole % of isophthalic acid and at least 5 mole % of 1,4-cyclohexane dicarboxylic acid, as the glycols use is being made of at least 50 mole % of branched aliphatic glycols with 5-11 carbon atoms, which may optionally comprise an ester group, and at most 50 mole % of aliphatic glycol with 2-6 carbon atoms, and wherein the monomers have a functionality three or higher in an amount of at most 8 mole % of dicarboxylic acids plus glycols.

Description: The invention relates to a thermosetting powder paint comprising an epoxy resin, a polyester resin and customary additives. The invention also relates to a polyester resin which is suitable for powder paint composition. A polyester resin for a powder paint that is cured with epoxy resin generally has an acid number ranging from 15 through 70 mg KOH/gram polyester, a hydroxyl number of 10 mg KOH/gram polyester or below and is substantially based on units of aromatic dicarboxylic acids, such as terephthalic acid and isophthalic acid, and aliphatic glycols, such as neopentylglycol and the ester of neopentylglycol and hydroxypivalic acid, besides a minor amount of trifunctional monomer. Such polyester resins are decribed in, for instance, US-A-4379895. Although the specification of US-A-4379895 refers to isophthalic acid as a possible monomer for the preparation of a polyester resin, it was found that powder paints with polyester resins based on isophthalic acid did not yield a paint coat with good impact resistance. Further, DSM Resins (Netherlands) marketed a polyester resin for powder paint, which resin consisted roughly of equimolar quantities of isophtalic acid and terephtalic acid as dicarboxylic acids and neopentylglycol as aliphatic glycol. Although such a resin based on isophthalic acid combined with an epoxy resin such as triglycidyl isocyanurate (TGIC), yields a powder paint which, after curing, gives relatively good weathering resistance in comparison with powder paints based on polyester resins composed of at least 70 mole % of terephthalic acid and 30 mole % or less of isophthalic acid. The drawback of such a resin, however, is that its mechanical properties are insufficient for a number of applications, particularly when a relatively short curing time is used. The object of the invention is to provide a powder paint, and a polyester resin for a powder paint, which, as cured paint, yields good UV resistance in combination with good mechanical properties, the good mechanical properties being reached after a relatively short curing time. According to the invention this is accomplished by means of a powder paint comprising a polyepoxy resin, a polyester resin and customary additives, the polyester resin having an acid number ranging from between 15 through 70 and a hydroxyl number of 10 or below and being substantially based on units of dicarboxylic acids, glycols and monomers of functionality three or higher, as dicarboxylic acids use being made of at least 75 mole % isophthalic acid, and at least 5 mole % of 1,4-cyclohexane dicarboxylic acid, as glycols use being made of at least 50 mole % of branched aliphatic glycols with 5-11 carbon atoms, which may optionally contain an ester group, and at most 50 mole % of aliphatic glycols with 2-6 carbon atoms, and wherein the monomers have a functionality three or higher in an amount of at most 8 mole % of dicarboxylic acids plus glycols. The essence of the invention is that 1,4-cyclohexane dicarboxylic acid has an unexpected influence on the impact resistance of the coating. Previous attempts to improve the impact resistance of a polymer based on isophthalic acid, while retaining the other properties, using a monomer known to improve impact resistance, in general, such as adipic acid, had failed, as is also apparent from the comparative experiments. Very good results have been obtained with an acid polyester resin based on 75-90 mole % of isophthalic acid and 10-25 mole % of 1,4-cyclohexane dicarboxylic acid. By preference use is made of at least 90 mole % of the above-mentioned dicarboxylic acids (isophtalic acid and 1,4-cyclohexane dicarboxylic acid) taken together. Other polycarboxylic acids can, in addition, be applied in amounts of at most 20 mole %, by preference up to maximum 10 mole % of the total of carboxylic acids. These are, for instance, terephthalic acid, tetra-hydrophthalic acid, hexahydroendomethylene tetrahydrophthalic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, dimer fatty acid, adipic acid, succinic acid, maleic acid, benzene-1,2,4-tricarboxylic acid, pyromellitic acid, trimesic acid, 3,6-dichlorophthalic acid and tetrachlorophthalic acid. Hydroxy carboxylic acids and/or optionally lactones, too, can be applied, for instance 12-hydroxystearic acid or epsilon-caprolactone. In minor amounts, at most 5 mole %, also mono-carboxylic acids such as benzoic acid, tert. butylbenzoic acid, hexahydrobenzoic acid and saturated aliphatic monocarboxylic acids can be added during the preparation. The amount of branched aliphatic glycol with 5-11 carbon atoms preferably 70 mole % or more, and more in particular 90 mole % or more since such an amount yields better weatherproof products. Within the above-mentioned limits, as aliphatic diols use may be made, besides the above-mentioned glycols, of for instance ethylene glycol, propane-1,2-diol, propane-1,3-diol, hexane-2,5-diol, hexane-1,6-diol, 2,2-[bis (4-hydroxy-cyclohexyl)]-propane 1,4-dimethylolcyclohexane, and 2,2,4-trimethyl pentane diol-1,3 and smaller amounts of polyols, such as glycerol, hexane triol, pentaerythritol, sorbitol, trimethylol ethane, trimethylol propane and tris-(2-hydroxyethyl)-isocyanurate. It is also possible to use epoxy compounds instead of diols, and the same holds for polyols. The polyester resins are prepared by esterification or interesterification, optionally in the presence of customary catalysts such as, for instance, di-butyltin oxide or tetrabutyl titanate. When the processing conditions and COOH/OH ratio are suitably chosen, end products with an acid number ranging from 15 through 70, are obtained. In case use is being made of more than 75 mole% of isophtalic acid as the dicarboxylic acid end products with an acid number ranging from 45 through 55 are obtained. The polyester resins preferably have almost no free hydroxyl groups left, i.e. the hydroxyl number is 10 or lower, preferably 5 mg KOH/g or lower. The polyester resins preferably have a glass transition temperature (Tg) between 40°C and 80°C, more in particular between 50°C and 70°C. This is important for the storage stability of the powder paint. The amount of monomer of functionality three or higher is preferably in the range of 0.2 mole % through 6 mole % relative to the other monomers, in particular in the range of 0.5 mole % through 4.5 mole %. The molecular weight of the polyester preferably ranges from 2,500 through 7,000, more in particular from 3,000 through 5,000. By preference, the molecular weight combined with the amount of branching is chosen such that the functionality of the polyester ranges from 2.1 through 3.5. As crosslinking agent use is made of an epoxy resin with a functionality of 2 or more. Preferably epoxy compounds are used that do not comprise a phenolic carbon-oxygen bond, this bond being light-sensitive. As such, preferably trisglycidylisocyanurate (TGIC), diglycidyl terephthalate or the hexahydro derivative are eligible. The amount of epoxy compound used in the powder paint depends on the acid number of the polyester resin with which it is combined, and ranges from 0.8 through 1.2, preferably from 0.9 through 1.1 epoxy equivalents per carboxyl equivalent. This amounts to an epoxy resin : polyester resin weight ratio between 4:96 and 15:85. The polyester resin and the epoxy resin are homogeneously mixed, preferably in the melt, using an extruder. As a rule the customary additives are mixed in before extrusion. This is especially the case with flow agents, curing catalysts and optionally pigments and/or fillers. The extrudate is subsequently cooled ground and screened, and the fraction with a particle size 90 micron or below is applied to a substrate by means of an electrostatic spraying device and stoved for 10 to 30 minutes at a temperature ranging from 160 through 200°C. The paint coat of the wholly or partly coated substrate thus obtained exhibits an excellent combination of gloss, flow and mechanical properties, as well as exceptionally good UV resistance, as is illustrated also by the following examples. EP-A-24680 describes a powder lacquer binder comprising a carboxylated polymer and triglycidyltriazolidine. The carboxylated polymer can be chosen from many acids. There is nowhere an indication to use the very specific carboxylfunctional polyester according to the present invention which comprises 1,4-cyclohexane dicarboxylic acid. EP-A-25478 describes a solvent based thermosetting coating based on a mixture of a binder based on aminoplasts and hydroxyfunctional polyesters and organic solvents. This application does not relate to powder coatings. Furthermore, the present invention relates to a very specific carboxylfunctional polyester. Examples I-VI Preparation of a polyester resin A 3-litre reactor vessel fitted with a thermometer, a stirrer and a distillation device, was filled with the monomers listed (in moles) in Table 1. Stirring was then applied and a light nitrogen flow was passed over the reaction mixture while the temperature was being raised to 200°C, with water formation. The temperature was gradually raised further to a maximum of 250°C, and the water was distilled off. The reaction was continued until the acid number of the polyester was below 10 mg KOH/g. Subsequently, the monomer for the second step was added and esterification was continued to an acid number (AN) as shown in Table 2. The latter part of this process was carried out under reduced pressure. Table 2 presents the acid number as well as the viscosity (η), measured according to Emila rheometer (in dPa.s, D = 17.6s-1, 165°C) and the glass transition temperature )Tg, Mettler TA-3000 system, 5°C/min). I II III IV V VI Theoretical Mn 3000 3540 3000 3000 3000 3000 Functionality 2.75 3.25 3.00 3.00 3.00 3.00 AN 54 56 56 54 51 55 η 245 650 575 490 533 405 T g 56 62 62 61 59 57 Example VII Powder paint preparation 540 g of each of the granulated polyester resins according to the Examples I-VI was dry-blended with 60 g triglycidyl isocyanurate, 300 g titanium dioxide pigment (ex-Kronos, Type C1 310), 9 g Resiflow PV-5 (a polyacrylate flow agent, ex-Worle ), 4.5 g benzoin, and optionally a small amount of catalyst for setting of the gelling time, and then placed in an extruder unit (make Buss, Type PR 46 or a Werner and Pfleiderer, ZSK 30). The extrudate was cooled, ground and screened, the screen fraction < 90 micrometer being collected for use as powder paint. This powder paint was electrostatically sprayed onto steel panels that had previously been degreased with trichloroethane. Use was made of electrostatical spraying equipment of make Gema, Type HP 720. After spraying with powder paint, the panels were stoved at a temperature of 200°C, the residence time being as specified in Table 3. Then the following properties were assessed: gloss (Gardner 20°C and 60°C, ASTM-D-523-67) mechanical properties (Reverse Impact, ASTM-D-2794-69): at 11 kg/cm2 = higher than 160 psi flow (visual assessment) QUV (ASTM-G-53, lamp: UVB-313). The results are presented in Table 3. With the powder paints according to the invention cured paint coats are obtained which, after 10 to 15 minutes' curing at 200°C, have a QUV (60°C) higher than 1000 and a good reverse impact. Comparative Experiment A In the same way as in the examples described above, an experiment was conducted using a monomer that generally has a flexibility improving effect: adipic acid (AA). The monomers are specified in Table 4, the properties of the polyester in Table 5, and the results of the paint in Table 6. Monomers (in moles) 1st step IPA 10.09 A A 0.5 NPG 10.84 TMP 0.75 2nd step IPA 2.75 Polyester properties Theoretical Mn 3000 Functionality 2.75 AN 52 η 270 T g 56 In this experiment no satisfactory reverse impact was obtained.

Claims: Claims for the following Contracting States : AT, BE, CH, DE, DK, FR, GB, GR, IT, LI, NL, SE Powder paint, comprising a polyester resin, triglycidylisocyanurate and customary additives, the polyester resin having an acid number ranging from 15 through 70 and a hydroxyl numer of 10 or below and being substantially based on units of dicarboxylic acids, glycols and monomers of functionality three or higher, as dicarboxylic acids use is being made of at least 75 mole % of isophthalic acid and at least 5 mole % of 1,4-cyclohexane dicarboxylic acid, as the glycols use is being made of at least 50 mole % of branched aliphatic glycols with 5-11 carbon atoms, which may optionally comprise an ester group, and at most 50 mole % of aliphatic glycol with 2-6 carbon atoms, and wherein the monomers have a functionality three or higher in an amount of at most 8 mole % of dicarboxylic acids plus glycols. Powder paint according to claim 1, characterized in that as dicarboxylic acid use is being made of 75-90 mole % of isophtalic acid and 10-25 mole % of 1,4-cyclohexane dicarboxylic acid. Powder paint according to any one of claims 1-2, characterized in that the weight ratio of triglycidylisocyanurate to polyester resin is between 4 : 96 and 15 : 85. Polyester resin having an acid number in the range of 15 through 70 and a hydroxyl number of 10 or below and being substantially based on units of dicaboxylic acids, glycols and monomers of functionality three or higher, wherein the dicarboxylic acids are comprised of at least 75 mole % of isophtalic acid and at least 5 mole % of 1,4-cyclohexane dicarboxylic acid, as glycols use is being made of at least 50 mole % of branched aliphatic glycols with 5-11 carbon atoms, which may optionally comprise an ester group, and at most 50 mole % of aliphatic glycols with 2-6 carbon atoms, and wherein the monomers have a functionality three or higher in an amount of at most 8 mole % of dicarboxylic acids plus glycols. Polyester resin according to claim 4, characterized in that as dicarboxylic acid use is made of substantially 75-90 mole % isophtalic acid and 10-25 mole % 1,4-cyclohexane dicarboxylic acid. Use of a polyester resin according to any one of claims 4-5 in the preparation of a powder paint composition. Wholly or partly coated substrate, wherein the coating material is comprised of a powder paint according to any one of claims 1-3. Claims for the following Contracting State : ES Process for the preparation of a powder paint, comprising a polyester resin, triglycidylisocyanurate and customary additives, the polyester resin having an acid number ranging from 15 through 70 and a hydroxyl numer of 10 or below and being substantially based on units of dicarboxylic acids, glycols and monomers of functionality three or higher, as dicarboxylic acids use is being made of at least 75 mole % of isophthalic acid and at least 5 mole % of 1,4-cyclohexane dicarboxylic acid, as the glycols use is being made of at least 50 mole % of branched aliphatic glycols with 5-11 carbon atoms, which may optionally comprise an ester group, and at most 50 mole % of aliphatic glycol with 2-6 carbon atoms, and wherein the monomers have a functionality three or higher in an amount of at most 8 mole % of dicarboxylic acids plus glycols by homogeneously mixing of the polyester and the trisglycidylisocyanurate in an extruder. Process for the preparation of a powder paint according to claim 1, characterized in that as dicarboxylic acid use is being made of 75-90 mole % of isophtalic acid and 10-25 mole % of 1,4-cyclohexane dicarboxylic acid. Process for the preparation of a powder paint according to any one of claims 1-2, characterized in that the weight ratio of triglycidylisocyanurate to polyester resin is between 4 : 96 and 15 85. Process for the preparation of a polyester resin having an acid number in the range of 15 through 70 and a hydroxyl number of 10 or below and being substantially based on units of dicaboxylic acids, glycols and monomers of functionality three or higher, wherein the dicarboxylic acids are comprised of at least 75 mole % of isophtalic acid and at least 5 mole % of 1,4-cyclohexane dicarboxylic acid, as glycols use is being made of at least 50 mole % of branched aliphatic glycols with 5-11 carbon atoms, which may optionally comprise an ester group, and at most 50 mole % of aliphatic glycols with 2-6 carbon atoms, and wherein the monomers have a functionality three or higher in an amount of at most 8 mole % of dicarboxylic acids plus glycols by esterification of interesterification. Process according to claim 4, characterized in that as dicarboxylic acid use is made of substantially 75-90 mole % isophtalic acid and 10-25 mole % 1,4-cyclohexane dicarboxylic acid. Use of a polyester resin obtained according to any one of claims 4-5 in the preparation of a powder paint composition. Wholly or partly coated substrate, wherein the coating material is comprised of a powder paint obtained according to any one of claims 1-3.

Inventors: BELDER EIMBERT GERRIT, HOFKAMP ALBERTUS RONALD, HOUWELING MARTEN, KOLDIJK FOKELTJE AKKE, MISEV TOSKO ALEXANDER, VISSER FRITS, BELDER, EIMBERT GERRIT, HOFKAMP, ALBERTUS RONALD, HOUWELING, MARTEN, KOLDIJK, FOKELTJE AKKE, MISEV, TOSKO ALEXANDER, VISSER, FRITS
Applicants: DSM NV, DSM N.V.
Application year: 1990
Main classification: C08G63
IPC codes: C08G63, C09D167, C09D163, C09D5