WO2002072664A1

WO2002072664A1 - Coating composition comprising a phosphatized polyester

Info

Publication number: WO2002072664A1
Application number: PCT/NL2002/000173
Authority: WO
Inventors: Johannes Hendricus Groen; Wilhelmina Westerhof; Albertus Johannes Schiphorst
Original assignee: Dsm Ip Assets B.V.
Priority date: 2001-03-13
Filing date: 2002-03-13
Publication date: 2002-09-19

Abstract

The invention relates to a coating composition comprising a phosphatized polyester and a crosslinker, wherein the phosphatized polyester is a phosphatized aliphatic polyester with an acid number lower than 20 mg KOH/g polyester or a phosphatized aromatic polyester with an acid number lower than 10 mg KOH/g polyester and a number average molecular weight (Mn) ≥ 1700. The invention also relates to the use of such a coating composition and it further relates to the coating obtained by curing the coating composition and to a coated substrate. The invention further relates to a process for coating an aluminium substrate.

Description

COATING COMPOSITION COMPRISING A PHOSPHATIZED POLYESTER

The invention relates to a coating composition comprising a phosphatized polyester and a crosslinker. The invention also relates to the use of such a coating composition, to the coating obtained by curing the coating composition and to a coated substrate. The invention further relates to a process for coating an aluminium substrate. Such a coating composition is known from EP-A-0.101.838.

However the coating compositions described in EP-A-0.101.838 are not stable during storage. To overcome this disadvantage EP-A-0.101.838 has to add an amine to the composition for neutralization and stabilization. A further disadvantage of the coating composition is the fact that the coating properties decrease when increasing the curing temperature.

The purpose of the invention is to overcome these disadvantages. According to the invention this object is reached by providing a phosphatized aliphatic polyester with an acid number lower than 20 mg KOH/g polyester or a phosphatized aromatic polyester with an acid number lower than 10 mg KOH/g polyester and a number average molecular weight (M_n) ≥ 1700. Here and hereinafter with "phosphatized polyester" is meant a polyester containing chemically bound phosphorus. The phosphorus is thus part of the polyester architecture, either in the main chain or in side chains.

Further it has been found that the coating composition of the invention after curing results in a coating with improved adhesion to the substrate. Surprisingly the adhesion is even very good to not pre-treated aluminium, which normally requires one or more pre-treatment steps. Pre-treatment can have an inorganic (chromated) and/or organic (a primer based on an organic coating composition) character. The use of a coating composition according to the invention makes it possible to omit the pre-treatment step of the substrate, which is from an environmental and economic point of view very advantageous.

Further it has surprisingly been found that the coating composition according to the invention results in better curing behaviour at the same temperature than prior art coating compositions. Therefore the coating composition according to the invention makes it possible to cure at lower temperatures than prior art compositions while still reaching sufficient level of crosslinking. This phenomenon of low-temperature curing (LTC) systems is very desirable and therefore presently often looked for. The reason for the search for LTC-systems is that substrates that are sensitive to high(er) temperatures can now be coated at lower temperatures; secondly LTC-systems make the economics of the coating process more favourable. A further advantage of the coating composition according to the invention is the improved outdoor weathering durability compared to prior art coating systems.

The coating composition according to the invention can be used in all coating areas, for example in the fields generally known as coil coating, can coating and powder coating.

The aliphatic and aromatic phosphatized polyester can be prepared for example according to the method in US-A-5.859.154, by reacting at least one polyhydric alcohol with at least one polybasic carboxylic acid or anhydride or ester compound and at least one salt-forming phosphorus compound. Thus the salt-forming phosphorus compound is added to the reaction mixture in a very early stage. Generally no free, unreacted phosphorus is present in the polyester anymore.

With aliphatic polyester is here and hereinafter meant a polyester originating from only aliphatic monomers. Thus both the acid and the alcohol (or the equivalents thereof that are being used as the building blocks for the polyester) are aliphatic in nature. With aromatic polyester is here and hereinafter meant a polyester originating from at least one aromatic monomer. Thus either the alcohol or the acid (or the equivalents thereof that are being used as the building blocks for the polyester) or both are aromatic in nature. The man skilled in the art understands that where in this definition is written "an alcohol" or "an acid" it can be replaced by a mixture of alcohols or a mixture of acids.

The word "aliphatic" is here and hereinafter meant to include substituted and unsubstituted aliphatic compounds and substituted or unsubstituted cycloaliphatic compounds. The word aromatic is here and hereinafter meant to include both substituted and unsubstituted aromatic compounds. Substitution can also take place with groups containing hetero-atoms, for example hydroxyl-groups.

Suitable aliphatic polyhydric alcohols for preparing the aliphatic polyesters have a functionality of at least two and can contain from 2-24 carbon atoms. They include for example ethylene glycol, diethylene glycol, 1 ,4-butanediol, 1 ,6- hexanediol, neopentyl glycol, 2-methyl-1,3-propanediol, 1 ,3-butanediol, 1 ,3-propanediol, 1 ,2-propanediol, 2-ethyl-2-butyl-1 ,3-propanediol, trimethylpentanediol, hydroxypivalic neopentyl glycol ester, tricyclodecane dimethanol, cyclohexane dimethanol, hydrogenated diphenylol propane, trimethylolpropane and/or pentaerythritol.

Suitable aromatic polyhydric alcohols for preparing the aromatic polyesters have a functionality of at least two and can contain from 2-24 carbon atoms. They include for example bisphenol A bis(hydroxyethyl) ether.

Suitable aliphatic polybasic carboxylic acids for preparing the polyesters have a functionality of at least two and can contain from 2-36 carbon atoms. They can have a straight or branched chain. Preferably aliphatic polybasic carboxylic acids are used with 6-16 carbon atoms. They include for example adipic acid, sebacic acid, hexahydroterephthalic acid (CHDA), decane dicarboxylic acid and/or dimerised fatty acids or acid anhydrides for example tetrahydrophthalic anhydride, succinic anhydride, maleic anhydride and/or hexahydrophthalic anhydride.

Suitable aromatic polybasic carboxylic acids for preparing the polyesters have a functionality of at least two and can contain from 2-36 carbon atoms. Preferably aromatic polybasic carboxylic acids are used with 6-16 carbon atoms. They include for example phthalic acid, isophthalic acid and/or terephthalic acid, dimethyl terephthalate ester or acid anhydrides for example phthalic anhydride, trimellitic anhydride, 1 ,8- naphthalic anhydride and pyromellitic anhydride.

Suitable salt-forming phosphorus compounds include for example phosphoric acid, phosphorous acid, phosphinic acid, phosphinous acid, organic acid phosphate, phosphorous oxychloride, alkyl esters of phosphoric acid, anhydrides of phosphoric acid, hydrogen containing salts of phosphoric acid, hypo-phosphorous acid and mixtures thereof. Preferably phosphoric acid is used.

The polyester-forming reaction, the esterification, can take place in the presence of one or more catalysts. Examples of suitable catalysts are dibutyl tin oxide, tin chloride, butyl chlorotin dihydroxide (FASCAT®) or tetr abutyloxytitanate. The molecular weight (M_n) of the aliphatic phosphatized polyester that is used in the coating composition according to the invention is usually between 400 and 15000. The molecular weight (M_n) of the aromatic polyester that is used in the coating composition according to the invention is at least 1700. The higher the molecular weight in terms of M_n, the higher the flexibility of the final coating, therefore a higher M_n is preferred. However, the higher the molecular weight in terms of M„, the higher the viscosity of a solution of the polyester will be. A high viscosity is not desirable because for the final coating composition to stay within practical limits of viscosity a low solid content must then be accepted. However an as high as possible solid content is preferred. As a consequence of these two aspects an optimum has to be found. Therefore a preferred range for the M_n of the aliphatic phosphatized polyester is between 1000 and 8000, more preferred between 1700 and 6000 and even more preferred between 2000 and 5000. The preferred range for the M_n of the aromatic phosphatized polyester is between 1900 and 10000, more preferred between 2000 and 5000.

The acid number of the aliphatic or aromatic phosphatized polyester should not be too high because in that case the phosphatized polyester becomes instable and needs a neutralizing and/or a stabilizing agent. The acid number of the aliphatic phosphatized polyester is generally lower than 20 mg KOH/g polyester. Preferably the acid number is lower than 15 mg KOH/g polyester. More preferred is an acid number below 12 mg KOH/g polyester. The acid number of the aromatic phosphatized polyester is lower than 10 mg KOH/g polyester. Preferably the acid number is lower than 9 mg KOH/g polyester. A lower acid number is preferred, because the lower the acid number the better the stability of the coating composition will be. The hydroxy number of the phosphatized polyester is not particularly critical. It is however preferred to use a phosphatized polyester with a hydroxy number of at least 10 mg KOH/g polyester, as when the hydroxy number increases, generally the crosslink density will increase. A suitable upper limit is for example 300 mg KOH/g polyester, it is more preferred to use a phosphatized polyester with a hydroxy number of at most 100 mg KOH/g polyester, because the higher the hydroxy number is, the more brittle the final coating becomes.

A suitable amount of phosphorus in the phosphatized polyester lies in the range between 0.08-0.8 mol phosphorus/kg polyester. A preferred range is between 0.1- 0.3 mol phosphorus/kg polyester. The desired amount of phosphorus can be reached by using one phosphatized polyester with the desired amount, it can however also be reached by blending two or more phosphatized polyesters with different amounts of phosphorus in correct ratios. It is even possible to reach the desired amount of phosphorus by blending in correct amounts a phosphatized polyester (irrespective its nature) with a polyester that does not contain phosphorus at all (also irrespective its nature). It can therefore be envisaged to combine an aliphatic phosphatized polyester with an aromatic polyester that does not contain phosphorus or vice versa. With the "amount of phosphorus in the aliphatic or aromatic polyester" is here and hereinafter meant the total amount of built-in phosphorus based on the sum of all polyesters present. The amount of phosphorus in the aliphatic or aromatic phosphatized polyester can be obtained by adding during the polyester-forming reaction 0.8-8 w% phosphoric acid (based on the amount of synthesized polyester), or its equivalent amount when other phosphorus-containing sources are used. Examples of crosslinkers suitable for use in the coating composition according to the invention include amino resins, blocked or unblocked isocyanate resins, epoxy resins and combinations of any of them. The crosslinker can be selected depending on the desired use. Preferably use is made of an amino resin or blocked or unblocked isocyanate resin. More preferred is a melamine resin. Examples of suitable amino resin crosslinkers are benzoguanamine, melamine and urea-formaldehyde resins. The polyester : amino resin weight ratio is generally between 95:5 and 60:40 (based on solid resin).

Examples of suitable crosslinkers containing (blocked) isocyanate groups are hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophoron diisocyanate (IPDI), tetramethylxylene diisocyanate (TMXDI) and their dimers and trimers. Preferably these crosslinkers are blocked.

Examples of suitable compounds containing epoxy groups are bisphenol A epoxy resins (for example Epikote® 828, Epikote® 1001 and Epikote® 1004 from Shell), hydrogenated bisphenol A epoxy compounds, aliphatic epoxy compounds, epoxidised alkyd resins, epoxidised oils (for example epoxidised linseed oil or soybean oil), epoxidised borates and triglycidyl isocyanurate. Preferably a bisphenol A epoxy resin is used as an epoxy group containing crosslinker.

The reaction between the phosphatized polyester and the crosslinker can take place in the presence of a suitable external catalyst although it is not always necessary. Depending on the choice of crosslinker a suitable crosslink catalyst can be easily found by the man skilled in the art and added to the coating composition. Examples of suitable external catalysts for curing an OH-functional polyester and an amino resin as a crosslinker, include strong acids such as sulphonic acids, mono- and dialkyl acid phosphate, butyl phosphate and butyl maleate.

Suitable sulphonic acids include for example paratoluene sulphonic acid, methane sulphonic acid, nonyl benzene sulphonic acid, dinonyl naphthalene disulphonic acid and dodecyl sulphonic acid.

Suitable external catalysts for curing an OH-functional polyester and an isocyanate based crosslinker include, for example, dibutyl tin dilaureate and zinc octoate. When an external catalyst is present, it is generally present in an amount of between about 0.1 and about 5 wt.% (relative to the polyester). In the case that an amino resin crosslinker is used in connection with the phosphatized polyester according to the invention it is however preferred that an external catalyst for curing is absent, because in that case the gloss, haze, Q-UV durability and the outdoor weathering durability of the resulting coating are better. The coating composition according to the invention comprising the phosphatized polyester and the crosslinker, can be used in powder form. However the composition is also suitably used together with an organic solvent or a mixture of organic solvents or water. It is preferred to use an organic solvent or a mixture of organic solvents. The amount of solvent or solvents or water is chosen in such a way that the desired viscosity is obtained; the man skilled in the art can easily determine this. The solvent(s) or water can be added immediately after the polyester synthesis or in a later stage of the preparation of the coating composition. The solvent(s) or water is preferably added during the preparation of the coating composition. The coating composition according to the invention is preferably used for the preparation of a coil coating composition. The preferred solvent is an organic solvent. Suitable solvents include, for example, aromatic hydrocarbon resins (for example Solvesso® types (Exxon Chemicals)), N-methylpyrolidone, xylene, propylene glycol monomethylether, methylpropylene glycol acetate, dibasic ester, isophoron, ethylethoxypropionate, ethylene-propylene glycol acetate and/or butyl glycol. Preferably aromatic hydrocarbons and/or butyl glycol are used. The coating composition according to the invention can optionally comprise usual additives, for example pigments, fillers, stabilizers, dispersing agents, flow- promoting agents and de-foaming agents. The additives can be chosen depending on field of application. When the coating composition is used in the coil coating field the additives can be chosen depending on the desired peak metal temperature (PMT) and the nature of the substrate.

The invention further relates to the use of a coating composition according to the invention to coat a substrate and specifically to its use in a coil coating process. Coil coating processes are commonly known and are described for example in 'Coil Coatings' by Joseph E. Gaske (Federation of Societies for Coatings Technology, February 1987, pp. 7-19). The curing conditions and additives can be chosen to depend on the desired peak metal temperature (PMT) and the nature and thickness of the substrate. The curing time will generally be between 5 and 70, more preferred between 20 and 50 seconds at oven temperatures between 250°C and 400°C and a PMT between 204°C and 270°C. It has been found that the composition according to the invention can be used at much lower values of the PMT, without losing quality of the final cured coil coating. Preferably this coating composition for coil coating is therefore used at a PMT between 130°C and 210°C, more preferred between 150°C and 200°C. The processing window is thus extended. It ranges from for example 130°C, preferably 150°C, to approximately 270°C.

The invention further relates to the use of the coating composition in a powder coating process. Powder coating processes are well-known to the man skilled in the art and are described for example in "Powder Coatings, Chemistry and Technology" by T.A. Misev, John Wiley & Sons, 1991. Generally the curing conditions are dependent on the nature of the substrate and the curing mechanism. The curing time can vary for example between 5 and 40 minutes. The temperature at which curing takes place depends on the crosslinker chosen and can vary between 160 and 200°C.

The coating composition can be used to coat a desired substrate. Suitable substrates for coil coating include for example steel, tin-plated steel and aluminium. The use of the coating composition according to the invention in a coil coating process has appeared to be especially advantageous on a not-pretreated aluminium substrate. Prior art coating compositions made it necessary to carefully pretreat the surface to obtain a coating with sufficient adhesion to the substrate. The use of the coating composition according to the invention to coat a not-pretreated aluminium substrate makes the pretreatments superfluous, thereby reducing the effort and costs related to the manufacturing of a coated aluminium substrate.

Suitable substrates for can coating include for example steel, tin-plated steel (ETP, Electrolytic Tin Plate), chromium-plated steel (ECCS, Electrolytic Chromium- Chromium oxide Steel) and aluminium. Suitable substrates for powder coating include for example metal, wood and plastic.

The invention also relates to a coating obtained after curing the coating composition according to the invention. It specifically relates to a coil coating obtained after curing the composition according to the invention. The invention further relates to an entirely or partly coated substrate that is obtained by the use of a coating composition according to the invention.

In the coating areas of can and coil coating, the substrates to be coated are always pretreated by phosphating and chromating. This is done because the surface of the substrate normally will contain contaminants, which will interfere with adhesion of the coating composition to the substrate. With the coating composition according to the invention adhesion is improved, thereby making these pretreatments superfluous. The invention therefore also relates to a process for coating an aluminium substrate in which the phosphating and chromating pretreatment step is omitted and wherein a coating composition according to the invention is used. The coating composition according to the invention results after curing in improved adhesion between the coating and the substrate. The adhesion is improved in such an amount that the pre-treatment step of phosphating and chromating can be omitted. The invention will be elucidated with reference to the following, non- limiting examples.

Experiment I

Preparation of an aliphatic phosphatized polyester 1409 parts by weight of neopentyl glycol, 693 parts by weight of hydroxypivalyl hydroxypivalate, 174 parts by weight of trimethylolpropane, 2237 parts by weight of cis-1 ,2-cyclohexanedicarboxylic anhydride, 272 parts by weight of 1 ,4- cyclohexanedicarboxylic acid, 80 parts by weight of phosphoric acid (85%), 4.5 parts by weight of trinonylphenylphosphite and 2 parts by weight of butyl chlorotin dihydroxide (Fascat® 4101) were heated under a nitrogen atmosphere in a glass reaction flask with a mechanical stirrer, a thermometer and a still with a Vigreux column. The esterification reaction started at 170°C and the reaction water formed was removed through distillation. The maximum reaction temperature was 230°C. After one hour at 230°C this was changed to vacuum distillation until an acid number of 7 mg of KOH/gram was reached. After cooling to 170°C, the reaction mixture was thinned with Solvesso® 150 to 65% theory dry solids content. The dry solids content is determined by equable applying 0.2 gram phosphatized polyester solution on aluminiumfoil (15 x 20 cm). Next the foil with phosphatized polyester solution is dried during 15 min. in an oven at 150°C. The difference in weight before and after drying indicates the percentage of dry solids content. The determined dry solids content was 64.6%. The acid number of the solid phosphatized polyester was 7 mg of KOH/gram and the hydroxyl number was 35 mg of KOH/gram. The viscosity measured in a Physica Viscolab LC3 at 23°C was 38 dPa.s. The molecular weight (M_n) was 2650 gram/mol, (determined with the aid of gel permeation chromatography using a polystyrene standard). The glass transition temperature of the polyester was 20°C, (determined with a Mettler Toledo DSC821®; 5°C /min.).

Comparative Experiment A Preparation of an aliphatic polyester

1395 parts by weight of neopentyl glycol, 686 parts by weight of hydroxypivalyl hydroxypivalate, 172 parts by weight of trimethylolpropane, 2304 parts by weight of cis-1 ,2-cyclohexanedicarboxylic anhydride, 269 parts by weight of 1 ,4- cyclohexanedicarboxylic acid, 4 parts by weight of trinonylphenylphosphite and 2 parts by weight of butyl chlorotin dihydroxide (Fascat® 4101) were heated under a nitrogen atmosphere in a glass reaction flask with a mechanical stirrer, a thermometer and a still with a Vigreux column. The este fication reaction started at 168°C and the reaction water formed was removed through distillation. The maximum reaction temperature was 230°C. After one hour at 230°C this was changed to vacuum distillation until an acid number of 3 mg of KOH/gram was reached. After cooling to 170°C, the reaction mixture was thinned with Solvesso® 150 to 65% theory dry solids content. The dry solids content is determined as described above. The determined dry solids content was 64.8%. The acid number of the solid polyester was 3 mg of KOH/gram and the hydroxyl number was 47 mg of KOH/gram. The viscosity measured in a Physica Viscolab LC3 at 23°C was 29 dPa.s. The molecular weight (Mn) was 2550 gram/mol, (determined with the aid of gel permeation chromatography using a polystyrene standard). The glass transition temperature of the polyester was 21 °C, (determined with a Mettler Toledo DSC821 ®; 5°C /min). Experiment II

Preparation of an aromatic phosphatized polyester

2302 parts by weight of neopentyl glycol, 1257 parts by weight of adipic acid, 794 parts by weight of isophthalic acid, 530 parts by weight of phthalic anhydride, 160 parts by weight of phosphoric acid (85%) and 4.5 parts by weight of tnnonylphenylphosphite were heated under a nitrogen atmosphere in a glass reaction flask with a mechanical stirrer, a thermometer and a still with a Vigreux column. The esterification reaction started at 140^CC and the reaction water formed was removed through distillation. The maximum reaction temperature was 230°C. After two hours at 230°C, the mixture was cooled to 220°C and the reaction water formed was removed via vacuum distillation until an acid number of 8 mg of KOH/gram was reached. After cooling to 150°C, the reaction mixture was thinned to 70% theory solids with a 93:7 mixture of Solvesso® 100 and butyl glycol. The dry solids content is determined as described above. The determined dry solids content is 68.8%. The acid number of the solid phosphatized polyester was 8 mg of

KOH/gram and the hydroxyl number was 59 mg of KOH/gram. The viscosity measured in a Physica Viscolab LC3 at 23°C was 10 dPa.s. The molecular weight (Mn) was 2110 gram/mol, (determined with the aid of gel permeation chromatography using a polystyrene standard). The glass transition temperature of the polyester was -9°C, (determined with a Mettler Toledo DSC821®; 5°C /min.).

Comparative Experiment B Preparation of an aromatic polyester

2082 parts by weight of neopentyl glycol, 189 parts by weight of trimethylolpropane, 1391 parts by weight of adipic acid, 877 parts by weight of isophthalic acid, 587 parts by weight of phthalic anhydride and 4.5 parts by weight of tnnonylphenylphosphite were heated under a nitrogen atmosphere in a glass reaction flask with a mechanical stirrer, a thermometer and a still with a Vigreux column. The esterification reaction started at 140°C and the reaction water formed was removed through distillation. The maximum reaction temperature was 230°C. After two hours at 230°C, the mixture was cooled to 220°C and the reaction water formed was removed via vacuum distillation until an acid number of 8 mg of KOH/gram was reached. After cooling to 150°C, the reaction mixture was thinned to 70% theory solids with a 93:7 mixture of Solvesso® 100 and butyl glycol. The dry solids content is determined as described above. The determined dry solids content is 69.5%.

The acid number of the solid phosphatized polyester was 8 mg of KOH/gram and the hydroxyl number was 73 mg of KOH/gram. The viscosity measured in a Physica Viscolab LC3 at 23°C was 12 dPa.s. The molecular weight (Mn) was 2100 gram/mol, (determined with the aid of gel permeation chromatography using a polystyrene standard). The glass transition temperature of the polyester was -19°C, (determined with a Mettler Toledo DSC821®; 5°C /min.).

Example I

Preparation of a paint composition

To 36.9 parts by weight of the 64.6% solution of the phosphatized polyester described in Experiment I were added 4.1 parts by weight of pigment (Heliogren Grϋn L8730 ®), 1.5 parts by weight of pigment (Echtschwarz® 100), 0.5 parts by weight of antifoaming/flow-promoting agent (50% Disparlon L1984 ® in Solvesso 150 ®) and 5.2 parts by weight of a thinner (Solvesso 150 ® /butyl glycol 3:1). This mixture was then ground to a pigment paste. During the preparation the paste's temperature did not rise above 70°C.

After cooling to room temperature, 47.7 parts of the 64.6% solution of the phosphatized polyester described in Experiment I and 9.7 parts by weight of crosslinker (Cymel 303 ®) were subsequently added. The mixture was then diluted using a mixture of Solvesso 150 ® and butyl glycol in a 3:1 weight ratio until a viscosity of 90-110 seconds' flow time, DIN cup 4, at 23°C (DIN standard 53 211), was reached.

Comparative Example C

Preparation of a paint composition

To 36.9 parts by weight of the 64.8% solution of the polyester described in Comparative Experiment A were added 4.1 parts by weight of pigment (Heliogren Grϋn L8730 ®), 1.5 parts by weight of pigment (Echtschwarz 100®), 0.5 parts by weight of antifoaming/flow-promoting agent (50% Disparlon L1984 ® in Solvesso 150 ®) and 5.2 parts by weight of a thinner (Solvesso® 150/butyl glycol 3:1). This mixture was then ground to a pigment paste. During the preparation the paste's temperature did not rise above 70°C. After cooling to room temperature, 47.7 parts of the 64.8% solution of the polyester described in Comparative Experiment A, 1.05 parts of catalyst (Nacure 1419 ®) and 9.7 parts by weight of crosslinker (Cymel 303 ®) were subsequently added. The mixture was then diluted using a mixture of Solvesso 150 ® and butyl glycol in a 3:1 weight ratio until a viscosity of 90-110 seconds' flow time, DIN cup 4, at 23°C (DIN standard 53 211), was reached.

In the following examples the characteristics are determined as follows:

1) The solvent resistance test is done by counting the number of double rubs (forwards and back) necessary to remove the coating down to the metal. Rubbing is carried out with a piece of cottonwool soaked in a solvent (methyl ethyl ketone, MEK). The result is reported as a number of double rubs from 0-100 (numbers above 100 are all reported as > 100).

2) Gloss: ASTM-D-523

3) Layer thickness: ISO 2360 4) Adhesion after cross-cut tape test: DIN53151

5) A more critical adhesion test is the adhesion after immersion in water. This test is carried out as follows:

The coated panel is bend with a T bend of 1.5 (no cracks should be visible), according to ECCA T 7 or EN 13523-7 part 7. Than the panel is placed in water at 80°C for 48 hours. After this immersion the T bend is judged on cracks. Judgement:

1 = complete loss of adhesion

2 = big cracks

3 = moderate cracks

4 = small cracks 5 = no cracks

6) appearance: visually

7) flow: visually

8) T-bend flexibility: ECCA T 7 or EN 13523-7 part 7

9) T-bend adhesion, In this method the adhesion of the coating is tested with a adhesive tape. The panel is bended according to ECCA T 7 and after every bend the adhesion on the bend is measured with a tape. The bend were no coating damage is visible is noticed as T bend adhesion. Example II Coil coating

The paint composition according to Example I was applied on an untreated aluminium substrate using a 90 μm wire coater and was cured in an oven with a drying cycle of 40 seconds at 308°C, resulting in a peak metal temperature (PMT) of 240°C.

Comparative Example D Coil coating The method of Example II was repeated with the paint composition according to Comparative Example C instead of the paint composition according to Example I. The properties of the coatings are summarized in Table I:

TABLE I

Example III Coil coating

The paint composition according to Example I was applied on an untreated aluminium substrate using a 90 μm wire coater. After curing in an oven with a drying cycle of 25 seconds at 308°C, resulting in a peak metal temperature (PMT) of 190°C, the following characteristics were determined: solvent resistance (methyl ethyl ketone): >100 dR T bend 0.5 T

Gloss 20 60° = 73 / 86

Comparative Example E Coil coating

The method of Example III was repeated with the paint composition according to Comparative Example C instead of the paint composition according to Example I. The following characteristics were determined: solvent resistance (methyl ethyl ketone): 20 dR T bend 0.5 T

Gloss 20760° = 73 / 86

Example IV

Coil coating based on a phosphatized polyester without an amine as stabiliser The paint composition according to Example I was applied on an untreated aluminium substrate using a 90 μm wire coater. After curing in an oven with a drying cycle of 20 seconds at 308°C, resulting in a peak metal temperature (PMT) of

180°C, the following characteristics were determined: solvent resistance (methyl ethyl ketone): 50 dR T bend 0 T

Gloss 20760° = 73 / 86

Comparative Example F

Coil coating based on a phosphatized polyester with an amine as stabiliser The method of Example IV was repeated with the paint composition according to Example I with the addition of 0.6w% di-methyl-ethanol-amine (DMEA). The following characteristics were determined: solvent resistance (methyl ethyl ketone): 9 dR

T bend 0 T Gloss 20760° = 73 / 86

Example V

Preparation of a paint composition To 79.1 parts by weight of the 65% solution of the phosphatized polyester described in Experiment II were added 165.5 parts by weight of pigment (Kronos 21260 ®), 1.5 parts by weight of antifoaming/flow-promoting agent (50% URAD DD 27 ® in Solvesso 150 ®) and 30 parts by weight of a thinner (Solvesso 150 ® /butyl glycol 3:1). This mixture was then ground to a pigment paste. During the preparation the paste's temperature did not rise above 70°C.

After cooling to room temperature, 119.1 parts of the 65% solution of the phosphatized polyester described in Experiment II and 24.8 parts by weight of crosslinker (Cymel 303 ®) were subsequently added. The mixture was then diluted using a mixture of Solvesso 150 ® and butyl glycol in a 3:1 weight ratio until a viscosity of 90-110 seconds' flow time, DIN cup 4, at 23°C (DIN standard 53 211), was reached.

Comparative Example G Preparation of a paint composition To 79.1 parts by weight of the 65% solution of the polyester described in

Comparative experiment B were added 165.5 parts by weight of pigment (Kronos 21260 ®), 1.5 parts by weight of antifoaming/flow-promoting agent (50% URAD DD 27 ® in Solvesso 150 ®) and 30 parts by weight of a thinner (Solvesso 150 © /butyl glycol 3:1). This mixture was then ground to a pigment paste. During the preparation the paste's temperature did not rise above 70°C.

After cooling to room temperature, 119.1 parts of the 65% solution of the phosphatized polyester described in Comparative example B and 24.8 parts by weight of crosslinker (Cymel 303 ®) were subsequently added. The mixture was then diluted using a mixture of Solvesso 150 ® and butyl glycol in a 3:1 weight ratio until a viscosity of 90-110 seconds' flow time, DIN cup 4, at 23°C (DIN standard 53 211), was reached.

Example VI Coil coating

The paint composition according to Example V was applied on an untreated aluminium substrate using a 90 μm wire coater and was cured in an oven with a drying cycle of 40 seconds at 308°C, resulting in a peak metal temperature (PMT) of 240°C. Comparative Example H Coil coating

The method of Example VI was repeated with the paint composition according to Comparative Example G instead of the paint composition according to Example V.

The properties of the coatings are summarized in Table II.

TABLE II

Claims

1. Coating composition comprising a phosphatized polyester and a crosslinker, characterized in that the phosphatized polyester is a phosphatized aliphatic polyester with an acid number lower than 20 mg KOH/g polyester or a phosphatized aromatic polyester with an acid number lower than 10 mg KOH/g polyester and a number average molecular weight (M_n) ≥ 1700.

2. Coating composition according to claim 1 characterized in that the composition in addition comprises an organic solvent or a mixture of organic solvents.

3. Coating composition according to claim 1 or 2 characterized in that the amount of phosphorus in the phosphatized polyester lies in the range between 0.08-0.8 mol phosphorus/kg polyester.

4. Coating composition according to any one of claim 1-3 characterized in that the crosslinker is an amino resin and/or blocked or unblocked isocyanate resin.

5. Coating composition according to claim 4 characterized in that the amino resin is a melamine resin.

6. Coating composition according to claim 4 characterized in that in case an amino resin is used as the crosslinker an external catalyst for curing is absent.

7. Use of a coating composition according to any one of claim 1-6 to coat a substrate.

8. Use of a coating composition according to any one of claim 1-6 in a coil coating process.

9. Use according to claim 8 for coil coating a not-pretreated aluminium substrate.

10. Use according to any one of claim 7-9 characterized in that curing takes place in 5 to 70 seconds at an oven temperature between 250°C and 400°C, resulting in a peak metal temperature (PMT) of 150°C -270°C.

11. Use of a coating composition according to any one of claim 1-6 in a powder coating process.

12. Coating obtained by curing a coating composition according to any one of claim 1- 6.

13. Coating according to claim 12 characterized in that in case an amino resin crosslinker is used, curing of the composition takes place in the absence of an external catalyst for curing.

14. Entirely or partly coated substrate characterized in that a coating is obtained by the use of a coating composition according to any one of claim 1-6.

15. Process for coating an aluminium substrate characterized in that the phosphating and chromating pretreatment step is omitted and wherein a coating composition according to any one of claim 1-6 is used.