WO1994014906A1

WO1994014906A1 - Binder composition for powder paints

Info

Publication number: WO1994014906A1
Application number: PCT/NL1993/000266
Authority: WO
Inventors: Dirk Armand Wim Stanssens; Wilhelmus Henricus Hubertus Antonius Van Den Elshout
Original assignee: Dsm N.V.
Priority date: 1992-12-23
Filing date: 1993-12-17
Publication date: 1994-07-07
Also published as: AU5720194A; BE1006473A3

Abstract

The invention relates to a binder composition for powder paints. The composition contains a polymer reactive with epoxy groups and a crosslinker having epoxy groups. The crosslinker comprises at least one cycloaliphatic group having at least one fused epoxide group. The crosslinker has a viscosity of below 1000 Pas (at 25 °C). The crosslinker preferably has an average functionality higher than 1.2 and lower than 6. The binder composition preferably contains a suitable catalyst.

Description

BINDER COMPOSITION FOR POWDER PAINTS

The invention relates to a binder composition for powder paints, which comprises a polymer that is capable of reacting with epoxy groups and a crosslinker having epoxy groups. The invention also relates to powder paints comprising said binder composition.

Thermosetting powder coatings have better hardness than thermoplastic powder coatings. As one consequence, historically there has been an intense effort to develop crosslinkers and polymers for thermosetting powder coating applications. This effort continues unabated. Indeed, polymers reactable with crosslinkers are still being sought to make binder compositions for thermosetting powder paints that have good flow, good storage stability and good reactivity as is evident from Merck, Powder Paints, Paintindia 47-52 (February 1992). The search is further complicated because the coating ultimately obtained from the powder paint must meet many and varying requirements, depending on the application. Various prior systems are known. Some systems release volatile components during curing. These systems suffer the drawbacks of forming coatings having blisters and/or of releasing undesirable emissions. In the latter regard, the volatile component, being of organic origin, can cause undesirable environmental or health concerns. In addition, it is found that not all desirable powder paint or coating properties are achieved. Other systems use polyesters and conventional crosslinkers containing an epoxy group. In general, in these systems no volatile components are released. However, the use of bisphenol-A epoxy resins in the so-called hybrid systems results in coatings that yellow and chalk relatively strongly on UV exposure, whereas the widely used triglycidyl isocyanurate (TGIC) crosslinker is toxicologically suspect. It is a purpose of the present invention to provide a non-toxic binder composition that will result in coatings having a appearance, weathering resistance and chemical resistance that are all of a good quality. Furthermore, it is an object to produce powder paints having good storage stability and good reactivity.

The invention is characterized in that the crosslinker comprises at least one cyclo-aliphatic group having at least one fused epoxide group.

The crosslinker has a viscosity of below 1000 Pas (at 25°C).

The binder composition comprising a liquid crosslinker results in powder paints and powder coating a good combination of properties. Surprisingly this epoxy functional crosslinker results in powder paints having good flow, good reactivity and good storage stability. Additionally, these powder paints result in powder coatings having good solvent resistance, good colour properties, good weathering resistance, good overbake resistance, good gloss, good chemical resistance and good impact resistance.

A diverse number of polymers reactable with epoxy groups can be used in the present invention. Exemplary polymers reactable with epoxy groups are characterized in general terms by the reactive functional groups involved. Suitable polymers include for example, a polymer with carboxyl groups, epoxy groups, anhydride groups, hydroxyl groups, acetoacetonate groups, phosphoric acid groups, phosphorous acid groups, thiol groups or combinations thereof. Preferably the polymer contains hydroxyl groups, anhydride groups, epoxy groups or carboxyl groups.

More preferably the polymer contains carboxyl groups. Preferably the polymer is substantially non- amino functional because alkyl-amino groups cause coatings with bad colour stability. In general, this means that the polymer contains less than 0.2 wt.%, preferably less than 0.1 wt.% of amino compounds as functional groups.

The polymer can, for example, be a polyester, a polyacrylate, a polyether (such as, for example, a bisphenol-based polyether or a phenol-aldehyde novolak), a polyurethane, a polycarbonate, a trifluoro ethylene copolymer or a pentafluoro propylene copolymer, a polybutadiene, a polystyrene or a styrene-maleic anhydride copolymer.

Among the suitable polymers, polyesters and polyacrylates are particularly preferred. The molecular weight (Mn) of the polymer is usually higher than 800, but is preferably higher than 1500. The polymer must flow well at temperatures between 100°C and 200°C and therefore has a molecular weight (Mn) below about 10,000, preferably below about 7,000. The polymer generally has a viscosity measured at 158°C lower than 8000 dPas. The viscosity will usually be greater than 100 dPas. The viscosity can advantageously range from about 300 to about 5,000 dPas. The viscosity is measured by the Emila method which is described in Misev, Powder Coatings; Chemistry and Technology, 287-288 (1991 Wiley and Sons). The temperature (158°C) is the temperature actually measured in the sample.

The Tg of the polymer is typically greater than about 20°C, preferably above 30°C and can be greater than 40°C, although, in particular, it is preferably greater than 60°C. The Tg of the polymer is usually lower than 120°C, otherwise preparation of the binder composition can become somewhat difficult. The Tg of the polymer can, as indicated hereinabove, be selected based on the target Tg for the binder composition.

If polymers are used that have only terminal groups reactive with an epoxy functionality, the polymer has an average functionality (reactable with epoxy groups) of higher than about 1.6 and preferably higher than 2. The polymer in general has an average functionality less than 5, preferably less than about 3. If polymers are used with pendant functional groups, such as polyacrylates, the average functionality will be higher than about 1.6, and preferably higher than 2. Such a polymer in general has an average functionality less than 8, preferably less than 4. The polymer contains functional groups that are reactable with epoxy groups. Such a polymer typically has a quantity of functional groups below about 2.7 meq/gram of resin (polymer). The quantity preferably is lower than 1.25 meq/gram of resin, and, in particular, it is preferably lower than about 0.90 meq/gram. The quantity of functional groups is generally greater than about 0.09 meq/gram polymer, but preferably higher than 0.18 meq/gram polymer.

The acid or hydroxyl number of polymers with respectively acid or hydroxyl functional groups can be calculated by multiplying the quantity given in meq/g by 56.1 (the molecular weight of KOH). Hence, a polymer with carboxyl reactive groups typically has an acid number below 150 mg KOH/gram of resin (polymer). The acid number preferably will be lower than 70 and, in particular is lower than 50. The acid number is generally greater than 5, but preferably higher than 10.

The equivalent ratio between the reactive groups in the polymer to epoxy groups (e.g. carboxyl groups in the polymer and epoxy groups in crosslinker) is usually between 1.6:1 and 0.5:1, and is preferably between 1:1 and 0.8:1. This ratio may be lower if the epoxy-functional crosslinker according to the invention is used in combination with other crosslinkers.

With the described polymers, various properties can be obtained in the binder and in the powder coating itself. Generally per 100 g of binder composition there is between about 1 and about 50 wt.% of epoxy functional cyclo-aliphatic groups, where the cyclo-aliphatic group has from 5 to 12 carbon atoms. Preferably, the composition contains between about 2 wt.% and 30 wt.% of epoxy functional cyclo¬ aliphatic groups.

Polyacrylates useful herein as the polymer reactable with epoxy groups can be based on (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, propyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, benzyl (meth)acrylate and hydroxyalkyl (meth)acrylates such as hydroxyethyl (meth)acrylate and hydroxypropyl

(meth)acrylate and/or glycidyl esters or glycidyl ethers of alkyl (meth)acrylates. By preference, the polyacrylates are substantially vinyl chloride-free. The polyacrylates can be obtained by known methods. In these methods, comonomers such as, for example, styrene, maleic acid/anhydride, as well as small amounts of ethylene, propylene and acrylonitrile, can be used. Other vinyl or alkyl monomers, such as, for example, octene, triallyl isocyanurate and diallyl phthalate can be added in small amounts.

A polyacrylate containing epoxy groups is obtained by using glycidyl (meth)acrylates in the synthesis of the polyacrylate.

A polyacrylate containing acid groups is usually obtained by copolymerization of the desired amount of acid, such as, for example, (meth)acrylic acid, maleic acid or fumaric acid.

A polyacrylate containing hydroxyl groups is obtained by copolymerization of the desired amount of monomers containing hydroxyl groups, such as, for example, hydroxyethyl (meth)acrylate and/or hydroxypropyl (meth)acrylate. A polyacrylate containing thiol groups can be obtained by copolymerization of a sufficient amount of a monomer containing a preferably blocked thiol group. Monomers containing a (blocked) thiol group include S- acetyl esters of thiol-ethyl (meth)acrylate, thiol- propyl(meth) acrylate, and combinations thereof. After polymerisation, the acetyl group can be deblocked by hydrolysis.

A polyacrylate containing acetylacetonate groups can be obtained by copolymerising the acetoacetonate ester of 2-hydroxy ethylacrylate.

The Tg of the polyacrylate is generally between about 30°C and about 120°C. Relatively greater amounts of crosslinker can be used in the binder composition when the Tg is at the higher end of the range. For optimum storage stability the Tg is preferably higher than 50°C. For polymer processing reasons the Tg is preferably lower than 100°C.

In general the viscosity of the polyacrylate is between 100 and 8000 dPas (measured at 158°C; Emila). Polyacrylates, such as epoxy, carboxy and hydroxy functional polyacrylates, are described in U.S. Patent No. 3,752,870, U.S. Patent No. 3,787,340, U.S. Patent No. 3,758,334, and G.B. Patent Specification 1,333,361, the disclosures of which are incorporated herein by reference.

A thermoset and cured powder coating according to the present invention in which a polyacrylate served as the polymer reactable with epoxy groups has a sufficient surface hardness. Substantial amounts of vinyl chloride are therefore undesired.

Polyurethanes useful as the polymer reactable with epoxy groups include those having a terminal acid group. These polyurethanes can be obtained by a number of methods. One method comprises allowing an isocyanate- terminated polyurethane to react with a hydroxy carboxylic acid such as, for example, hydroxy acetic acid, lactic acid, malic acid or hydroxy pivalic acid. Another method comprises allowing a hydroxy-terminated polyurethane to react with a dicarboxylic acid or an anhydride. Still another method comprises allowing an isocyanate-terminated polyurethane to react with amino acids such as, for example, ε-amino caproic acid.

In an analogous manner, urethanes that contain other functional groups can be obtained. Urethanes that contain epoxy groups can be obtained by allowing glycidol to react with a polyurethane containing isocyanate groups. Polyurethanes are described for example in Misev, "Powder Coatings" pp. 160-161.

Polyesters useful as the polymer reactable with epoxy groups are generally based on the residues of aliphatic polyalcohols and polycarboxylic acids.

The polycarboxylic acids generally are selected from the group consisting of aromatic and cycloaliphatic polycarboxylic acids because these acids tend to have a Tg increasing effect on the polyester. In particular two- basic acids are used. Examplary polycarboxylic acids are isophthalic acid, terephthalic acid, hexahydro terephthalic acid, 2,6-naphthalene dicarboxylic acid and 4,4-oxybisbenzoic acid and, in so far as available, their anhydrides, acid chlorides or lower alkyl esters such as e.g. the dimethylester of naphthalene dicarboxylic acid. Although not required, the carboxylic acid component usually comprises at least about 50 mol%, preferably at least about 70 mol%, isophthalic acid and/or terephthalic acid. Other suitable aromatic cycloaliphatic and/or acyclic polycarboxylic acids useful herein include, for example, 3,6-dichloro phthalic acid, tetrachloro phthalic acid, tetrahydro phthalic acid, hexahydro terephthalic acid, hexachloro endomethylene tetrahydro phthalic acid, phthalic acid, azelaic acid, sebacic acid, decane dicarboxylic acid, adipic acid, succinic acid, trimellitic acid and maleic acid. These other carboxylic acids can be used in amounts of up to at most 50 mol% of the total amount of carboxylic acids. These acids may be used as such, or, in so far as available at their anhydrides, acid chlorides or lower alkyl esters. Hydroxy carboxylic acids and/or optionally lactones can also be used, such as, for example, 12-hydroxy stearic acid, hydroxy pivalic acid and ε-caprolactone. Monocarboxylic acids, such as, for example, benzoic acid, tert.-butyl benzoic acid, hexahydro benzoic acid and saturated aliphatic monocarboxylic acids, can, if desired, be used in minor amounts. Useful polyalcohols, in particular diols, reactable with the carboxylic acids to obtain the polyester include aliphatic diols such as, for example, ethylene glycol, propane-1,2-diol, propane-1,3-diol, butane-1,2-diol, butane-1,4-diol, butane-1,3-diol, 2,2-dimethylpropanediol-l,3 (= neopentyl glycol), hexane-2,5-diol, hexane-1,6-diol, 2,2-bis-(4- hydroxy-cyclohexyl)-propane (hydrogenated bisphenol-A) , 1, -dimethylolcyclohexane, diethylene glycol, dipropylene glycol and 2,2-bis[4-(2-hydroxy ethoxy)-phenyl] propane, the hydroxy pivalic ester of neopentyl glycol.

Small amounts, such as less than about 4 wt.% but preferably less than 2 wt.%, of trifunctional alcohols or acids may be used in order to obtain branched polyesters. Examples of usefull polyols and polyacids are glycerol, hexanetriol, trimethylol ethane, trimethylol propane, tris-(2-hydroxyethyl)-isocyanurate and trimellitic acid. Tetrafunctional monomers generally are not preferred, because these may cause too much branching and gelling, although minute quantities can be used. Examples of useful polyfunctional alcohols and acids are sorbitol, pentaerithritol and pyromellitic acid. However, in order to synthesise branched polyesters, trifunctional monomers are preferred. Coating properties can e.g. be influenced by diol selection. For instance, if good weather resistance is desired, the alcohol component preferably contains at least 70 mol% neopentyl glycol, 1,4-dimethylolhexane and/or hydrogenated bisphenol-A. Caprolactone and hydroxypivalic acid are also useful if good weather resistance is desired.

It is also possible to copolymerize compounds carrying amine groups, such as, for example, hexane-1,6-diamine, butane-1, -diamine and ε-caprolactam. The amine group containing compound can replace at least part of the hydroxy group containing compound. The polyester having amide groups which is obtained generally exhibits an increased Tg, and the powder coating compositions obtained therefrom can have improved tribo- charging properties. These type of polyesters contain amide linkages and are not amino functional.

Compounds that are suitable for reaction with polycarboxylic acids to yield the desired polyesters are also monoepoxides such as, for example, ethylene oxide, propylene oxide, monocarboxylic acid glycidyl ester (for example Cardura E10™; Shell) or phenyl glycidyl ether.

The polyester preferably contains 5 wt.% to 30 wt.% of aliphatic acids and/or aliphatic alcohols. Examples of these compounds are adipic acid, cyclohexane dicarboxylic acid, succinic acid, cyclohexane dimethanol and hydrogenated bisphenol-A. The use of these monomers may improve the mechanical properties of the binder, a powder paint composition comprising said binder, or any powder coating prepared from the powder paint composition. The polyesters are prepared according to conventional procedures by esterification or transesterification, optionally in the presence of customary esterification catalysts such as, for instance, dibutyltin oxide or tetrabutyl titanate. Preparation conditions and the COOH/OH ratio can be selected so as to obtain end products that have an acid number and/or a hydroxyl number within the targeted range of values. A carboxylic acid functional polyester is preferably prepared in a series of steps. In the last step of which an aromatic or, preferably, aliphatic acid is esterified so as to obtain an acid-functional polyester. As known to those skilled in the art, in an initial step terephthalic acid is allowed to react in the presence of an excess of diol. Such reactions produce a mainly hydroxyl functional polyester. In a second or subsequent step, an acid functional polyester is obtained by allowing further acid to react with the product of the first step. A further acid includes, among others, isophthalic acid, adipic acid, succinic anhydride, 1,4-cyclohexane dicarboxylic acid and trimellitic anhydride. If trimellitic anhydride is used at a temperature of 170- 200°C, a polyester with a relatively high number of trimellitic acid end groups is obtained.

The polyester can be a crystalline polyester, although amorphous polyesters are preferred. Mixtures of crystalline and amorphous polyesters can be used. Amorphous polyesters have a viscosity generally within a range of between 100 and 8000 dPas (measured at 158°C, Emila). Crystalline polyesters usually have a lower viscosity in the range of about 2 to about 200 dPas. If the polyester contains carboxylic acid reactive groups, the acid number of the polyester is selected so that the desired amount of crosslinker can be used. The acid number preferably is higher than 10, and preferably higher than 15. The acid number is preferably lower than 50 and in a very preferred embodiment of the invention lower than 35.

Hydroxyl functional polyesters can be prepared in a manner known per se by the use of a sufficient excess of glycol (polyalcohol) in the polyester synthesis.

Epoxy functional polyesters can be prepared in a manner known per se such as, for example by reacting an acid polyester with an equivalent of diglycidyl terephthalate or epichlorohydrin per acid group. Suitable polyesters of this type are described in US-A-3576903. Phosphoric acid functional polyesters can be obtained by (trans)esterification of phosphoric acid (esters) with a hydroxy functional polyester. Another method for making phosphoric acid functional polyesters involves allowing P₂0₅ to react with a hydroxyl functional polyester.

The polyester preferably is substantially non- amino functional.

Suitable polyesters for use in powder coatings are for example described in U.S. Patent No. 4,147,737 and U.S. Patent No. 4,463,140, the disclosures of which are incorporated herein by reference.

The Tg of the polyester is selected to maintain the Tg of the polyester-crosslinker mixture high enough (preferably > 30°C) so that any powder paints or binders prepared therefrom are physically stable at room temperature. Polyester and crosslinker combinations with a lower Tg can, if desired, be employed in preparing a powder coating composition. However, to maintain powder stability such powders are kept under cooled conditions. The Tg of the polyester can be greater than 45°C, but preferably is greater than 60°C. The Tg is generally lower than 90°C.

The crosslinker according to the invention comprises at least one cyclo-aliphatic group having at least one fused epoxide group. The viscosity is lower than 1000 Pas (at 25°C) .

More preferably the crosslinkers according to the invention have a viscosity of between 0,2 and 200 Pas. The viscosity is measured by means of an 'ICI cone and plate' viscosimeter (see Misev, Powder Coatings; Chemistry and Technology, pp. 287-288; 1991).

Typically, the epoxy functional cyclo-aliphatic groups are obtained by epoxidation of unsaturated cyclo- aliphatic compounds. Ethylenically unsaturated compounds with carboxyl or hydroxyl functionality represent very suitable unsaturated cycloaliphatic compounds. Examples of ethylenically unsaturated cyclo¬ aliphatic compounds are cyclopentene-3-ol, cyclopentene-4- ol, tetrahydro phthalic anhydride, 4-methylol cyclohexene, 4-carboxy cyclohexene, cyclooctadiene and cyclooctatriene. In addition, unsaturated functionalized cyclic terpenes are very suitable.

A crosslinker according to the present invention usually contains several epoxy functional cyclo-aliphatic groups, per molecule although this is not a requirement. Polyunsaturated cyclic aliphatic compounds or alkyl esters thereof can be used. When an alkyl ester is used, the alkyl is, for example, methyl, ethyl, propyl, cyclohexyl or 2-ethylhexyl. Examples of polyunsaturated cyclic aliphatic compounds are 1,5-cyclooctadiene, 1,5,9- cyclododecatriene and 1,4,5,8,9,10-hexahydro naphthalene and derivatives from these.

The cyclo-aliphatic groups can be annellated, but generally they are covalently bond in the form of ester, ether or amide bonds, whether via a (cyclo)- aliphatic or an aromatic group.

The average functionality of crosslinkers according to the invention is preferably higher than 1.2 and more preferably higher than 1.7. The average functionality is preferably lower than 6, and more preferably lower than 4.

Examples of suitable crosslinkers include 3,4- epoxycyclohexyl-methyl-3,4-epoxycyclohexyl carboxylate (ERL-4221™; Union Carbide), bis(3,4- epoxycyclohexylmethyl)adipate (ERL-4299™; Union Carbide), 1,2-5,6-9,10-trisepoxy-cyclododecane, bis(l,2-5,6- bisepoxycyclododecane-9-ol)adipate.

Other suitable crosslinkers are disclosed in for example EP-A-481476.

Crosslinkers of the present invention can, for instance, be synthesized by formation of 4-(methyl- carboxy)-cyclohexene or 4-methyl (4-methylcarbox )- cyclohexene from methyl acrylate or methyl methacrylate with butadiene in a Diels-Alder reaction. In an analogous manner, a reaction of (meth)acrylic acid esters with cyclopentadiene or furan is possible. Additionally a reaction of butadiene with maleic anhydride can also be employed. This ester functional unsaturated compound can be transesterified with a polyalcohol or polyamine, such as, for example, trimethylol propane, pentaerythritol, 1,4-butane diamine, 4-aminoethyl-l,8-diamino octane, yielding an oligoester or oligoamide. The ethylenically unsaturated cyclo-aliphatic compound can be epoxidized with, for example, peracetic acid.

If desired the crosslinkers can be converted to epoxy functional oligomers with amine, carboxy or hydroxy functional components. Mixtures of the crosslinkers described hereinabove can be used, and can be combined at pre¬ selected ratios. The pre-selected ratio will depend on the desired application.

In order to function as a crosslinker herein, the epoxy-functional crosslinker, as such, reacts during curing to at least a substantial extent. Generally, the reaction should be such that mechanical and/or chemical resistance properties of the cured coating are obtained through the curing reaction of the epoxy-functional crosslinker and the polymer reactable therewith. In this respect the present epoxy-functional crosslinkers, are not serving as a flexibilizer and stabilizer.

Depending on the desired end-use application, crosslinkers described hereinabove can also be used in combination with still other crosslinkers. Crosslinkers containing epoxy groups, such as, for example, triglycidyl isocyanurate (TGIC) and polybisphenol-A-epoxides can be used in combination with crosslinkers described herein above. Another class of crosslinkers that can be used in such combinations are compounds containing (blocked) isocyanate groups, such as, for example, the caprolactam blocked isophorone diisocyanate trimer. A still further class of combinable crosslinkers contain β-hydroxyalkyl amide groups, such as, for example, Primid XL 522™ (Rohm and Haas). Polyfunctional oxazolines can also be used in combination with the present epoxy- functional crosslinkers.

A crosslinker comprising at least one C₅-C₂₆ linear or branched aliphatic chain and having an epoxy funtionality greater than 1, with the proviso that the epoxy groups are carried on the at least one aliphatic chain is also suitable for combination with the present crosslinkers. Suitable crosslinkers of this type include for example epoxidised oils such as for example linseed oil and soya bean oil.

Thus, relative to the amount of other crosslinkers, the amount of the epoxy-functional crosslinker is preferably such that more than 20% of crosslinking is obtained through that crosslinker. More preferably, it is desired that more than 35% of crosslinking, and in particular more than 50% of crosslinking, be obtained using the heretofore described crosslinker.

Although the crosslinker according to the invention can be used in combination with other crosslinkers, it is preferred to use the crosslinker according to the invention as the main crosslinker, and more preferably as the substantial sole crosslinker.

The present invention relates to the binder composition, as well as to a powder paint comprising the binder, as well as to a substrate, coated with the cured powder paint. A binder composition generally is defined as the resinous part of the powder paint.

The powder paint containing the binder composition according the invention preferably includes a small but effective amount of catalyst for the curing reaction between the polymer capable of reacting with epoxy groups and the crosslinker. The binder composition of the present invention can, if desired, be supplied as a single component system. In a single component system a large part or all of the polymer and substantially all of the crosslinker comprising an epoxidized aliphatic chain are supplied as a mixture, which mixture is preferably homogeneous. Such a homogenous single component mixture is advantageous, since no significant amounts of liquid components need to be processed while making a powder paint composition from such a mixture.

A single component binder system can be obtained by mixing the crosslinker with the polymer at a temperature above 70°C to form a homogenous mixture, followed by cooling, crushing and grinding the mixture to the desired particle size to obtain sufficiently chemically homogeneous powder particles. The crosslinker and polymer can be mixed in an extruder or a kneader.

Although the binder composition can be prepared according to various methods, it is preferably obtained by mixing the polymer and the crosslinker in a static mixer at elevated temperature for a short period of time. Elevated temperatures can be above 150°C and short periods of time can, for instance, be on the order of seconds, such as 20 seconds. The static mixer is preferred because low-viscosity materials, are facilely mixed with the polymer. The mixed product is thereafter cooled, crushed and ground to the desired particle size to obtain sufficiently chemically homogeneous powder particles.

As a general proposition, the grinding yields particle sizes on the order of 0.5 mm to about 15 mm. The particle sizes can fall in the range of 1 mm to 12 mm, and can average about 5 to 6mm. Usually about 80 % of the particles are larger than 1 mm, although it will be appreciated that the size is not critical. A powder paint composition can then be prepared by mixing the binder composition with a catalyst and, optionally, a pigment, customary fillers and other additives and optionally additional curing agents at a temperature above the melting point of the binder composition.

Instead of using the binder system as a single component, the various ingredients of the binder composition can also be mixed with the other ingredients of the powder paint during the preparation of the paint. In this embodiment the crosslinker - which is generally liquid at 20-40°C - can be added by means of a metering pump to an extruder as the polymer resin is being extruded. Mixing in general takes place above the melting point (or range) of the polymer. The crosslinker can also be incorporated in pigment or filler and subsequently added to the resin (polymer capable of reacting with epoxy groups) and mixed such as in an extruder. The catalyst and additives also can be added either to the polymer or to the crosslinker.

The catalyst and/or the curing agent can also be added by extrusion techniques during powder paint preparation, together with the pigments and the fillers. The catalyst and additives can, if desired, be applied as a masterbatch. Such a masterbatch can be a mixture of the polymer resin which is capable of reacting with epoxy groups that is also used for the binder composition or another - not reactive - resin with the catalyst and optionally all or a part of the additives. Subsequently, the various components can be mixed using an extruder or kneader at temperatures between, for example, about 70°C and about 150°C. In general, the mixing is conducted at temperatures above the melting point or within or above the melting range of the binder. Depending on the temperature used and the catalyst used, it may be necessary to conduct the mixing and cooling rapidly. The average residence time in the mixing apparatus is preferably less than half of the gel time of the system at the mixing temperature.

In a preferred embodiment of the invention, a two-component (or two-package) system for the preparation of a powder paint comprises a first component consisting essentially of all or a large part of .the polymer (i) and the crosslinker (ii), and a second component consisting essentially of a polymer (i) or another polymer and a catalyst for the curing reaction between the polymer (i) and the crosslinker (ii) (masterbatch). In case the polymer of the masterbatch is not the same as the one in the first component, it may be either a polymer that will react with the crosslinker or a substantially not-reactive polymer. One or both of the components may comprise customary additives as described below, in particular stabilizers or additional curing agents. The first component as defined here is essentially the "single component" as defined above.

It is also attractive to use a mixture of a crystalline polyester with an amount of 20-50 wt.% of crosslinker as a first component. This sort of masterbatch can be used in admixture with a further polymer that may constitute 30-70 wt.% of the binder composition. The further polymer may comprise the catalyst, or the catalyst may be added separately. If desired, the residence time during homogenization of a binder or powder paint composition can be selected such that there is some degree of reaction between the polymer and the crosslinker. A degree of pre- reaction between the polymer and crosslinker will shorten the reaction time needed to cure the powder paint composition and may increase the glass transition temperature of the powder paint.

The curing reaction between the polymer and the crosslinker to form the ultimate cured coating will generally occur in the presence of an effective amount of catalyst. In appropriate cases it is useful to apply an additional curing agent. With the binder composition according to the invention, the desired curing time can readily be selected by adjusting the amounts of and selection of the catalyst and/or curing agent. The importance of the heretofore described polymer-crosslinker ratio and of the amount of catalyst is elucidated in the already cited Misev, "Powder Coatings", pp. 174-223, the disclosure of which is incorporated herein by reference. Powder paints typically have particle sizes smaller than about 90 to 100 microns, and generally have particle sizes averaging about 50 microns, although particle sizes on the order of 20 microns can be used. The preparation of powder paints and the chemical curing reactions thereof to obtain cured coatings are generally described in, for example, the already cited Misev, "Powder Coatings", pp. 44-54, p. 148, and pp. 224- 226, the disclosure of which is incorporated herein by reference.

With a powder coating composition according to the invention it is possible to achieve a curing cycle of, for example, 150°C at 10 minutes. If desired, 20 minutes curing at 200°C is also possible. The amount of catalyst suitable for the reaction will be selected so that the desired curing and flow are obtained, such as, for example, in 20 to 30 minutes at 150°C, or in 10 to 15 minutes at 180°C, up to 5 to 10 minutes at 200°C.

Thus, the polymer capable of reacting with epoxy groups, the crosslinker, an amount of catalyst - if necessary - and an amount of additional curing agent - if any - will be selected so that the curing reaction is substantially complete within 30 min at 200°C.

Hence, the invention also relates to a process for preparing a wholly or partially coated substrate by applying a powder coating on a substrate wherein (a) the polymer capable of reacting with epoxy groups (i), the crosslinker (ii), optionally an amount of catalyst and optionally an amount of additional curing agent are selected so that the curing reaction is substantially complete within 30 min at 200°C, (b) curing the coating by subjecting it to heat for a sufficient time at a suitable temperature to obtain a cured coating (c) wherein the amount of epoxy functional crosslinker (ii) is such that more than 20% of the crosslinking is obtained through that crosslinker. Catalysts and curing agents known to those skilled in the art for epoxy-acid, epoxy-epoxy, epoxy-hydroxy and epoxy-anhydride reactions can be used with powder coating (paint) compositions based on the present binder composition. These catalysts generally contain tertiary amine groups or other basic nucleophilic groups.

For the epoxy-acid reaction, the relevant catalysts listed in Madec et al., Kinetics and Mechanisms of Polyesterifications, Advances in Polymer Science, 182- 198 (1985), the complete disclosure of which is incorporated herein by reference, can, in principle, be used.

Examples of suitable classes of catalysts are N- dialkylamine pyridines, tertiary amines, imidazole derivatives, guanidines and cyclic amine compounds. If desired, the catalysts may be blocked. Specific examples of catalysts include N-dimethylamino pyridine, benzotriazole, triethylamine or triphenylamine, 4,5-diphenyl imidazole, 1-ethyl imidazole, 2-methyl imidazole, 4-methyl imidazole, ethyl imidazole carboxylate, 5,6-dimethyl benzimidazole, 1-benzyl imidazole, imidazole or 1,1-carbonyl diimidazole, tetramethyl guanidine (TMG), isocyanate-TMG adducts (e.g., isophorone diisocyanate-di-tetramethyl guanidine, tolonate-HDT-tetramethyl guanidine, or TMXDIdiTMG) , acetyl-TMG, 2-phenyl-l,1,3,3-tetramethyl guanidine, l,5-diazabicyclo[4,3,0]non-5-ene and 1,5,7- triazabicyclo[4,4,0]dec-5-ene. Other catalysts include tetraalkyl phosphonium bromide, tetrabutyl ammonium fluoride, cetyl triethyl ammonium bromide, benzothiazole and lithium derivatives. Lithium derivatives suitably are lithium alkanolates such as lithium butanolate, lithiumtriazole, lithiumimidazole and lithium hydroxide. Preferably, tetramethyl guanidine comprising compounds, imidazole derivatives such as 1-benzyl imidazole or 4,5-diphenyl-imidazole and lithium derivatives are used as the catalyst because the coatings have good colour properties and good resistance to overbake.

For epoxy-anhydride reactions no catalyst is generally required. Nonetheless, it may still be advantageous to use a nitrogen-containing catalyst, which catalyst is as described above.

For epoxy-hydroxy reactions a strong Lewis acid can be used as the catalyst. Even so, an additional curing agent, such as, for example, a polyanhydride can be used. Anhydrides, such as trimellitic anhydride adducts, or styrene-maleic anhydride copolymers are quite suitable for that purpose. Powder paints comprising a binder consisting of e.g. an hydroxyl polyester, epoxidized oil and polyanhydride curing agent can simply be made by using a mixture of polyester and epoxidized oil as a single component. The anhydride curing agent can be added while making the powder paint.

For epoxy-epoxy reactions a strong Lewis acid can be used as a catalyst, although an additional curing agent is usually required. Known curing agents can be used as the additional curing agent. Known curing agents include, for example, the polyanhydrides, dicyano diamides, dicarboxylic acid, hydrazides and polyphenols. By preference, substituted dicyano diamides, substituted amines (such as, for example, methylene dianiline, 2-phenyl-2-imidazoline ester of pyromellitic or of trimellitic acid), polyphenols and anhydrides (preferably resinous anhydrides such as, for example, ethylene glycol bistrimellitate) are used.

The amount of catalyst is usually between 0.05 and 2 wt.%, preferably between 0.1 and 1.5 wt.% wherein the weight percents are with respect to the binder composition. The amounts of additional curing agent - if used - is in general between 1-15 wt.% with respect to the binder, preferably between 3 and 10 wt.%. Of course all customary additives can, if desired, be used in the powder coating systems according to the invention, such as, for example, pigments, fillers, deaerating agents, flow-promoting agents and stabilizers. Pigments include inorganic pigments, such as titanium dioxide, zinc sulphide, iron oxide and chromium oxide, as well as organic pigments such as azo compounds. Fillers comprise metal oxides, silicates, carbonates and sulphates. As additives, stabilisers such as primary and/or secondary antioxidants and UV stabilizers such as, for example, quinones, (sterically hindered) phenolic compounds, phosphonites, phosphites, thioethers and HALS compounds (Hindered Amine Light Stabilizers) can be used. In order to obtain powder coatings that have good stability during cure, the primary antioxidants appear important. Therefore, the powder paint preferably includes an effective amount of stabilizer, which in general is an amount of 0.1-2 wt.% with respect to the binder composition. Stabilizers are well known, and several of the useful ones are shown in the examples.

Deaerating agents are exemplified by benzoin or cyclohexane dimethanol bisbenzoate. Flow-promoting agents include, among others, polyalkyl acrylates, fluorohydrocarbons and silicon oils. Other additives include those which are used to improve tribo charging, such as, for example, sterically hindered tertiary amines. Powder paints according to the invention can be applied in the customary manner, for example by electrostatic spraying of the powder onto an earthed substrate, and curing the coating by subjecting it to heat for a sufficient time at a suitable temperature. The applied powder can be heated in, for example, a gas furnace, an electric furnace or by means of infrared radiation.

Industrial thermosetting coatings from powder paint (coating) compositions are further generally described in Misev, pp. 141-173 (1991). Compositions according to the present invention can be used in powder coatings for use on metal, wood and plastic substrates. Examples are general-purpose industrial coatings, coatings for machinery and for example for cans, domestic and other small equipment. Furthermore, the coatings are quite suitable in the automotive industry, to coat exterior and/or interior parts of vehicles such as cars.

EP-A-365428 and JP-A-61200177 disclose a powder coating composition comprising a polyester resin and a very specific alicyclic epoxy resin having ether bonds and three kinds of substituents. The epoxide group is attached but is not fused to the cycloaliphatic ring. This attachement can be illustrated as follows:

In contrast, the present invention is related to a crosslinker which comprises at least one cyclo-aliphatic group having at least one fused epoxide group. This can be illustrated as follows:

This difference in structure is very important and results in a different reaction mechanism and different reactivities towards functional groups and consequently in a difference with respect to mutagenecity. EP-A-435356 dicloses a solvent based coating composition based on a hydroxyl groups containing resin, an alicyclic polyepoxide crosslinking agent and a specific catalyst. In general solvent or water compositions are totally different from compositions for powder coatings. Powder coatings have been defined as coatings which are protective or decorative or both, formed by the application of a coating powder to a substrate and fused into, continuous films by the application of heat or radiant energy. Coating powders are finely divided particles of organic polymer which generally contain pigments, fillers, and additives which remain finely divided during storage under suitable conditions. The unique concept of powder coatings allows several performance advantages over liquid coatings, such as no solvent, excellent mechanically properties, and high material use efficiencies. The very source of this uniqueness, the powdered form, raises different formulating considerations than for liquid coatings. Raw materials must be designed and selected specifically for this application. Rheology of coating powders is completely different than for liquids. The solvent or water based coatings are unrelated to thermosetting powder paints because powder paints have to fullfil requirements amongst others relating to the melting point of the binder, rheologic properties reactivity and stability that are not applicable to wet coatings. US-A-3988288 describes powder coatings comprising an amino-functional polyester and a polyepoxide compound as a crosslinker. These powder coatings according to US-A-3988288, however, have a bad colour stability due to the alkylamines. The polyester according to the present invention is substantially non-amino functional.

The invention will be elucidated on the basis of the following non-restrictive experiment and example. The tests are described in the already cited Misev, pages 284- 303.

Experiment 1

Preparation of polyester resin

A 3-1 reactor vessel, equipped with a thermometer, a stirrer and a distillation set-up, was charged with 1.13 parts by weight of trimethylol propane, 15.44 parts by weight of terephthalic acid, 2.26 parts by weight of adipic acid, 18.76 parts by weight of neopentyl glycol, 0.02 wt.% dibutyltin oxide and 0.02 wt.% tris- nonyl phenyl phosphite.

While the reaction mixture was being stirred and a light nitrogen flow was passed over it, the temperature was then raised to about 170°C, and water was formed. The temperature was gradually raised further to a maximum of 245°C and the water was distilled off. The reaction was continued until the acid number of the polyester was lower than 12 mg KOH/g. Subsequently, in a second stage, 3.39 parts by weight of isophthalic acid were added and further esterification took place on a polyester having an acid number of 35 was obtained. The last part of the second step was carried out under reduced pressure. The characteristics of the resulting resin were:

- acid number: 34.7 mg KOH/gram;

- functionality: 3.23;

- viscosity: 1800 dPas (Emila 165°C);

- Tg: 56°C. The theoretical -COOH functionality is given, based on the amount of trifunctional monomer and the theoretical molecular weight.

Example I Preparation of powder paint

At 110°C 532 parts by weight of the polyester resin according to Experiment I were fed to a kneader (Werner & Pfleiderer ZSK30).

After the resin had melted completely, 100 parts by weight of titanium dioxide (KRONOS 2160™) was dispersed in the resin.

Then, 9 parts by weight of flow-promoting agent (Resiflow PV 5™; Worlee), 4.5 parts by weight of benzoin and 3 parts by weight of stabilizer (Irganox 1010™; Ciba Geigy) were added to the reaction mixture.

Next 66 parts by weight of bis[3,4- epoxycyclohexyljadipate ERL 4299™ from Union Carbide was added, and followed by the addition of 3 parts by weight of tetramethyl guanidine (TMG; Jansen Chimica) to the reaction mixture.

The resulting product was cooled, reduced in size, pulverized and screened to a maximum particle size of 90 μm.

The powder coating was applied electrostatically and cured during 8 minutes at 200°C. The characteristics of the resulting powder coating are shown in Table I:

TABLE I

Impact resistance¹' 160 ip

ESP^2> > 8

Adhesion^3> Gto

Gel time, 200°C⁴> 60 s

Tg 35°C

Acetone resistance⁵' > 100

Gloss, 20^o6) 82 60° 90

i) reverse impact test; ASTM-2794/69

2) Erichsen Slow Penetration; ISO 1520/DIN 53156 3) cross-hatch adhesion; ISO 2409/DIN 5315 4) DIN 55990; Part B S) ADR: acetone double rubs 6) ASTM D 523/70

The present invention provides a non-toxic binder composition resulting in powder paint and powder coating properties of a good quality.

Claims

C L A I M S

1. A binder composition for powder paints, comprising a polymer reactive with epoxy groups and a crosslinker having epoxy groups, characterized in that the crosslinker comprises at least one cyclo-aliphatic group having at least one fused epoxide group.

2. A binder composition according to claim 1, characterized in that the crosslinker has a viscosity of below 1000 Pas at 25°C.

3. Composition according to anyone of claim 1-2, characterized in that the polymer is substantially non-amino functional and contains hydroxyl groups, anhydride groups, epoxy groups or carboxyl groups.

4. Composition according to any one of claims 1-3, characterized in that the polymer is a polyester or a polyacrylate.

5. Composition according to any one of claims 1-4, characterized in that the composition further comprises a catalytically effective amount of a catalyst.

6. Composition according to any one of claims 1-5, characterized in that the crosslinker has an average functionality higher than 1.2 and lower than 6.

7. Composition according to claim 6, characterized in that the crosslinker has a viscosity of between 0,2 and 200 Pas (at 25°C) .

8. Process for the preparation of a binder composition according to any one of claims 1-7, comprising mixing a polymer reactive with epoxy groups and a crosslinker comprising at least one cyclo-aliphatic group having at least one fused epoxide group and having a viscosity of below 1000 Pas at 25°C.

9. Use in a powder paint of a crosslinker having epoxy groups, characterized in that the crosslinker comprises at least one cyclo-aliphatic group having at least one fused epoxide group and the crosslinker has a viscosity of below 1000 Pas (at 25°C).

10. A powder paint comprising a binder composition according to any one of claims 1-7, a pigment, and optionally customary fillers and additives.

11. Use of a binder composition according to any one of claims 1-7 in the preparation of powder paints.

12. Wholly or partially coated substrate, characterized in that as coating material use is made of a powder paint according to claim 10.

13. Composition, crosslinker, powder paint, process, use and substrate as substantially described and/or elucidated in the examples.