WO1996007707A1

WO1996007707A1 - Epoxy resin powder coating composition

Info

Publication number: WO1996007707A1
Application number: PCT/EP1995/003503
Authority: WO
Inventors: Christian Jean Charles De Cock; Marc Jozef Rans; Marianne Angele Walravens
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1994-09-05
Filing date: 1995-09-04
Publication date: 1996-03-14
Anticipated expiration: 1997-03-05
Also published as: AU3522595A

Abstract

A thermosetting epoxy resin powder coating composition comprising: (a) at least one epoxy resin having on average at least one 1,2-epoxy group per molecule, (b) at least one optionally substituted, saturated or unsaturated heterocyclic organic compound being a 1 ring system of either 5, 6 or 8 members that contains at least one carboxamido group represented by general formula (I) or by its isomeric structure (II) wherein the carbon atom and nitrogen atom that make up each carboxamido group are members of said ring system, and (c) at least one quaternary ammonium or -phosphonium salt, wherein compounds (a) preferably cyanuric or isocyanuric acid, and (b) are present in such amounts that the ratio of equivalents of epoxy groups provided by compound (a) to moles of compound (b) (ratio 1) is lower than 3.0.

Description

EPOXY RESIN POWDER COATING COMPOSITION

The present invention relates to a thermosetting epoxy resin powder coating composition, to the use thereof for coating a substrate, to a substrate coated therewith and to a process for its preparation. Powder coatings are increasingly of interest to the coatings industry in view of the distinct advantages from the ecological and economical point of view, as compared to solvent- and even water-based coatings. Ecological advantages are for example no air pollution, reduced fire hazards and reduced health hazards. Economical advantages are for example reduced application costs, easy to handle, easy to control film build up and relatively easy to recycle/re-use. Thermosetting powder coating compositions are particulate compositions usually consisting of the following components: a thermosetting compound (binder) , a cross linking (or curing) agent and pigments, extenders, flow control agents and other additives.

The thermosetting compound is usually an epoxy resin, an acrylic resin, a polyurethane resin, a polyester resin or a suitable combination of the aforementioned resins. Thermosetting powder coating compositions comprising an epoxy resin as the thermosetting compound (hereafter referred to as epoxy resin powder coating compositions) provide good spraying characteristics and in particular good chemical- and corrosion resistance, good hardness and good adhesion. The curing agents generally applied in epoxy resin powder coating compositions are acid functional polyesters, amine derivatives, anhydrides (or adducts thereof) , dicyandiamide (if desired accelerated or substituted) and compounds. Application of epoxy resin powder coatings can be protective or decorative. Mainly protective applications of powder coatings are for example the coating of pipes, containers, machinery and concrete reinforcing rebars for which usually thick films are required. Mainly decorative applications of powder coatings are for example in domestic equipment (e.g. refrigerators, freezers and washing machines), metal furniture, building components and fittings usually require thin films i.e. in the range of from 15 to 60 μm. The resin/curing agent systems currently applied for decorative applications by the powder coating industry are mainly epoxy/polyester- or epoxy/anhydride systems. Both of said systems however have drawbacks. The epoxy/polyester systems often have worse flow as compared to pure epoxy based systems, due to the presence therein of the polyester. But, more importantly the whiteness stability of epoxy resin/polyester systems is often unsatisfactory. The epoxy resin/anhydride systems may have better whiteness stability as compared to epoxy resin/polyester systems, but these systems often have a relatively low storage stability and some of the anhydrides are relatively toxic. In view of the problems associated with the use of said epoxy resin curing agent systems, it is an object of the present invention to provide an epoxy resin powder coating composition, primarily intended for decorative applications, that has an excellent combination of low toxiticity of the curing compound, good reactivity and good flow of the compo- sition and a high level of initial whiteness and an excellent whiteness stability (low tendency of yellowing in the course of time) of the coating, whilst it retains the favourable properties connected with the use of an epoxy resin compound as the thermosetting binder such as for example good hardness of the coating. The present invention thus relates to a thermosetting epoxy resin powder coating composition comprising: (a) at least one epoxy resin having on average at least one 1,2-epoxy group per molecule, (b) at least one optionally substituted-, saturated- or unsaturated heterocyclic organic compound being a 1 ring system of either 5, 6, or 8 members that contains at least one carboxamido group represented by the following general formula I or by its isomeric structure of the formula II

-C—N- (I) -C=N- (II)

II I I

O H OH wherein the carbon atom and nitrogen atom that make up each carboxamido group are members of said ring system, and (c) at least one quaternary ammonium or - phosphonium salt, wherein compounds (a) and (b) are present in such amounts that the ratio of equivalents of epoxy groups provided by compound (a) to moles of compound (b) (ratio 1) lower than 3.0.

The epoxy resin powder coating compositions of the present invention in particular have the desired level of reactivity and flow of the coating composition and provide coatings that have a relatively high initial whiteness and excellent whiteness stability, whilst the hardness of the coating is at a high level as well. Epoxy resin powder coating compositions comprising cyanuric acid or its iso er isocyanuric acid (a well known compound that falls within the definition given above for compound (b) ) are known in the art. European patent application publication number 517,333 relates to thermosetting epoxy resin powder coatings comprising an epoxy resin and a blend of isocyanuric acid and 2- phenyli idazoline. These powder coatings in particular are capable of producing a matt finish. US 3,676,397 discloses modification of epoxy resins with cyanuric acid to form an oxazolidinone modified epoxy resin. In the modification process the molar ratio of polyglycidylether to cyanuric acid ranges of from 3:1 to 9:1 (this is a ratio of equivalents epoxy groups to moles cyanuric acid (ratio 1) in the range of from 6.2 to 18), the temper¬ ature ranges from 130-180 °C and only quaternary ammonium or phosphonium compounds that do not have more than one phenyl or aromatic group in their cation portion can be used as catalyst for the reaction between the poly¬ glycidylether and the cyanuric acid compound. These oxazolidinone modified epoxy resins are used in moulding powder formulations that show improved impact strength. US patent No. 5,112,926 relates to epoxy resin composi- tions comprising a triazine modified epoxy resin, a brominated epoxy resin, a multifunctional epoxy resin and a curing agent. The triazine can be cyanuric acid and the preferred ratio of equivalent weight of active hydrogen to that of the epoxide group is in the range of from 0.25 to 0.75. Since cyanuric acid has three active hydrogens per molecule said ratio is equivalent to a ratio of equivalents of epoxide to mols cyanuric acid (ratio 1) in the range of from 4.0 to 12.0. Quaternary ammonium or phosphonium compounds can be used as catalysts for the chain extending reaction between the epoxy resin and the triazine. The triazine modified epoxy resin is used for the preparation of a varnish, not for an epoxy powder coating, that is curable with conventional epoxy resin curing agents such as for example amines, amides and anhydrides. Said varnish is specifically intended for use in printed circuit boards and in particular provide high Tg and good toughness.

None of the prior art documents discussed above disclose the use of a physical blend of compound (b) and compound (c) as a curing agent/catalyst combination in epoxy resin powder coatings. In particular the finding that the use of this curing agent/catalyst combination provides coatings with good whiteness and excellent whiteness stability is totally unexpected in view of said prior art documents. When using such amounts of compounds (a) and (b) that ratio 1 is 3.0 or higher, the epoxy resin powder coating composition does in particular not show the desired level of reactivity. Said desired level of reactivity is suitably indicated by a gel time in the range of from about 30 seconds to about 200 seconds when determined according to the method ISO/DIN 8130-6 as indicated in the accompanying examples.

Suitable compounds (a) for use in the epoxy resin powder coating composition according to the present invention are epoxy resins known in the art suitable for use in powder coating compositions. Particularly preferred are 1,2-epoxy compounds having melting ranges starting above 40 °C, including higher molecular weight compounds, the so called solid resins. The epoxy resins may be saturated or unsaturated and they may be aliphatic, cycloaliphatic, aromatic or heterocyclic. They may further contain substituents which under the conditions of mixture or upon cure do not cause any undesired side reactions such as alkyl or aryl substituents hydroxyl groups and ether groups. Solid resins having an Epoxy Group Content (EGO between 1000 and 2000 mmol/kg, in particular between 1000 and 1600 mmol/kg, are preferred. Blends of solid resins may also be used. Examples of solid resins are the poly- glycidylpolyethers of 2,2-bis (4-hydroxyphenyl)propane (bisphenol-A or BPA) obtainable by reacting BPA with epichlorohydrin in a molar ratio of 1: 1.9 to 1.2 in the presence of an alkalihydroxide in an aqueous medium. Polyglycidylpolyethers can also be obtained by reacting a polyglycidylether of BPA with less than the equimolar amount of bivalent phenol preferably in the presence of a catalyst such as a tertiary amine, a tertiary phosphine or a quaternary phosphonium salt. The epoxy resin may also be a solid epoxidised polyester which has been obtained for example through the reaction of a polyvalent alcohol and/or a polybasic carboxylic acid or its anhydride with a low molecular weight polyepoxide. Examples of such low molecular weight polyepoxides are the liquid diglycidylether of BPA, diglycidylphthalate, diglycidyladipate, diglycidyltetrahydrophthalate, diglycidylhexahydrophthalate, diglycidylmaleate and the methyl ester of 3, 4-epoxycyclohexylmethylcarboxylic acid. Other suitable, commercially available, solid epoxy resins manufactured from BPA and epichlorohydrin that have been developed especially for powder coatings are the EPIKOTE grades EPIKOTE 3002, EPIKOTE 3003, EPIKOTE 3004, EPIKOTE 3007, EPIKOTE 1002, EPIKOTE 1004, EPIKOTE 1055, EPIKOTE 1007, EPIKOTE 517 and EPIKOTE 2017, all available from Shell Chemicals Europe (EPIKOTE is a trademark) .

Other suitable epoxy resins are the ones described under X) Y) and Z) below.

X) The advanced resins products disclosed in European patent application publication number 518,408 obtainable by reacting a) a diglycidyl ester of an alpha,alpha'-dibranched dicarboxylic acid of the general formula I

with b) an alpha,alpha'-dibranched dicarboxylic acid of the general formula II

wherein n is 0 or 1, wherein R is independently selected from the group consisting of straight and branched chain alkyl, cycloalkyl, arylalkyl and aryl, or both R's may form part of a substituted or unsubstituted cycloaliphatic ring system comprising 5, 6 or 8 carbon atoms, in which case n should be 0; wherein R^ is independently a hydrogen atom or a methyl group, wherein R2 is a single bond, substituted or unsubstituted alkylene or arylene or a radical of the formula III

⁽R3-⁾ _m]_ X <R3>m₂ ⁽HI⁾ wherein m^ and m.2 are independently 0 or 1, wherein R3 is independently selected from the group consisting of substituted or unsubstituted alkylene, and X is a single bond or one of the following bridging groups 1 to VI

(I) (II) (III) (IV) (V) (VI)

wherein R4 is independently selected from the group consisting of C^ to C4 alkyl; with the proviso that when X is bridging group (V) ,m^ is 0 and when X is bridging group (VI),rri2 = 0 and when n is 0, R2 is a single bond'. Examples of the aforesaid compounds a) are the diglycidylesters of diethylmalonic acid (DGEDEMA), of 2, 2 , 5, 5,-tetramethyladipic acid (DGETMAA) , of sulphodipivalic acid and of tetrapropyl- adipic acid whereas examples of the aforesaid compounds b) are diethyl alonic acid (DEMA) , 2,2, 5, 5,-tetrarnethyl- adipic acid (TMAA) , sulphodipivalic acid and tetra- propyladipic acid. Thus suitable examples of advanced resin products are for example: the reaction product obtainable by reacting 4 moles of DGEDEMA with 3 moles of DEMA for 2.5 hours at 120 °C so that the final epoxy group content is 1/25 meq/1 and the acid value is

0.05 meq/g; and the reaction product obtainable by reacting 4 moles of DGETMAA and 3 moles of DEMA for 2.5 hours at 120 °C so that the final epoxy group content is 1.15 meq/g and the final acid value is < 0.05 meq/g. Y) The outdoor durable semi crystalline polyglycidyl- esters as disclosed in European patent application number 94200776.6 obtainable by glycidation of the acid groups of a semi crystalline acid functional polyester the latter being a reaction product of at least compound(s) A and compound(s) B, wherein compound A is a straight chain aliphatic alpha, omega dicarboxylic acid having at least 6 carbon atoms and wherein compound B is a straight chain aliphatic alpha, omega-primary diol having at least 4 carbon atoms, said reaction product having an acid content of from > 1.6 meq/g to < 4.0 meq/g a number average molecular weight of at least 500 , a melting temperature (Tm) by Differential Scanning Calorimetry of > 25 °C an average acid functionality of 2 or more and essentially no reactive groups other than carboxylic groups. Examples of the aforesaid compounds A are: adipic-, pimelic-, suberic-, azelaic-, sebacic- or 1, 10-decanedicarboxylic acid; and examples of the aforesaid compounds B are: 1, 4-butanediol, 1,5- pentanediol, 1, 6-hexanediol, 1, 7-heptanediol, 1,8- octanediol, 1, 9-nonanediol , 1, 10-decaneciol or 1,12- dodecanediol . Thus suitable semi crystalline poly- glycidylesters are for example: crystalline poly- glycidylesters prepared by esterification of 4 moles of 1, 10-decanedicarboxylic acid or alternatively 4 moles of adipic acid with 3 moles of 1, 6-hexanediol in the presence of 5% by weight of xylene at elevated temperature followed by glycidation of the resulting acid functional polyester.

Z) The outdoor durable polyglycidylesters as disclosed in European patent application No. 93202081 obtainable by glycidating a linear tertiary aliphatic carboxylic functional polyester resin, the latter obtainable by reacting a) at least one compound A' comprising one mono- functional primary- or secondary hydroxyl group and/or at least one compound A" comprising one primary- or secondary hydroxyl group and one tertiary aliphatic carboxyl group and b) at least one aromatic or cycloaliphatic dicarboxylic acid compound B comprising two aromatic- or secondary aliphatic carboxyl groups or the anhydride thereof and c) at least one diol compound C comprising two aliphatic hydroxyl groups which may each independently be a primary or a secondary hydroxyl group and d) at least one dihydroxy onocarboxylic acid compound D comprising a tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which may each independently be primary or secondary hydroxyl, the molar ratio of compounds A' : A" : B : C : D being M : N : X+Y+l : X : Y, wherein M + N = 2,

X ranges from 2 to 8, and

Y ranges from 2-N to 8, at a temperature of from 100 to 240 °C, until essentially all the non-tertiary carboxyl groups as initially present in the reaction mixture have been reacted.

Examples of the aforesaid compounds A' are: aliphatic and cycloaliphatic alcohols having one primary or one secondary hydroxyl group and having of from 1 to 6 carbon atoms such as for example methanol, isopropanol, neopentanol, 2-butanol and cyclohexanol. Examples of the aforesaid compounds A" are aliphatic and cycloaliphatic alcohols having one primary or one secondary hydroxyl group and having from 1 to 6 carbon atoms and having in addition one tertiary aliphatic carboxyl group such as for example cyclohexane-l-methyl-4-hydroxyl carboxylic acid, the 1:1 (molar ratio) reaction product of dimethylolpropionic acid and cyclohexanecarboxylic acid and hydroxypivalic acid. Examples of the aforesaid compounds B are for example phthalic acid, tetrahydro- phthalic acid, hexahydrophthalic acid, methylhexa- hydrophthalic acid, terephthalic acid, isophthalic acid, endomethylenetetrahydrophthalic acid, methylendo- methylenetetrahydrophthalic acid, 1, 4-cyclohexanedi- carboxylic acid and 1, 3-cyclohexanedicarboxylic acid or combinations of the these compounds B. Examples of the aforesaid compounds C include branched aliphatic, cycloaliphatic or araliphatic compounds containing two aliphatic hydroxyl groups each individually being either a primary or a secondary hydroxyl group such as for example propylene glycol, neopentylglycol, hydrogenated diphenylolpropane, hydrogenated 4, 4'-dihydroxydiphenyl, 1, 4-cyclohexanedimethylol, 1, 4-dihydroxylcyclohexane, hydroxypivalylhydroxypivalate and 2-butyl-2-ethyl-l, 3- propanediol or combinations of these compounds C A typical example of a suitable compound D is dimethylolpropionic acid. Such a polyglycidylester resin is for example obtained by reacting 1 mole of hydroxypivalic acid, 9 moles of hexahydrophthalic acid, 2 moles of dimethylolpropionic acid and 6 moles of hydrogenated diphenylolpropane in the presence of 5% by weight of xylene at 210 °C until the theoretical acid value is reached, followed by glycidation of the acid groups of the linear acid functional polyester thus obtained with epichlorohydrin.

Suitable compounds (b) for use in the epoxy resin powder coating composition according to the present invention can be represented by the following general formula A and/or its isomeric structure according to the formula B:

wherein Z is an organic structure that provides either 3, 4 or 6 optionally substituted atoms that are members of the ring. Preferred compounds (b) have 5 or 6 members in the ring systems. Compounds of the general formula (e) A and/or B comprising at least two carboxamido groups that are members of the ring are preferred, those compounds comprising three carboxamido groups that are members of the ring being particularly preferred. The ring member atoms other than carbon and nitrogen from the carboxamido group (s) may be substituted with one or two organic atoms or -groups such as for example saturated or unsaturated, linear or branched, substituted or unsubstituted alkyl groups; carbocyclic, or heterocyclic groups that are optionally aromatic; halogen atoms; alkylether groups; carbonyl groups and mercapto groups. Preferred ring member atoms other than the ones from the carboxamido group (s) are carbon atoms that are preferably unsubstituted i.e. having at least one hydrogen radical attached. Preferred compounds (b) are represented by the below general formulae 1 or 2:

2 , wherein at least one of the groups R^D in the above formula 1^' is a hydrogen atom and wherein at least one of the groups R^ in the above general formula 2 is a hydroxyl group, whilst the other groups R^ and R^ and groups or atoms attached to the single bonds in the compounds of the general formulae 1 and 2 may be hydrogen, hydroxyl or any one of the substituents referred to above.

Particular preferred are compounds represented by the general formula 1 which are known under the generic name of triazine derivatives. Particularly preferred are compounds of the general formula 1 wherein at least two of the groups R^ are hydrogen. Most preferred is the compound of the general formula 1 wherein all three groups R^$ are hydrogen, also known as isocyanuric acid, or its isomer represented by a compound of the general formula 2 wherein all substituents R^ are hydroxyl groups, known as cyanuric acid or 2,4, 6-triazinetriol. The use of (iso)cyanuric acid as compound (b) in the thermosetting epoxy resin powder coating composition according to the present invention was found to be particularly preferred in view of its low toxicity and low cost and in view of the fact that the powder coating compositions according to the present invention co - prising (iso)cyanuric acid show an excellent combination of the desired level of reactivity and flow of the coating composition on the one hand and good whiteness and excellent whiteness stability of the cured coating on the other hand. When using (iso) cyanuric acid as compound (b) it was found that the best combination of good whiteness, good whiteness stability, acceptable gel times and good flow is obtained when ratio 1 is in the range of from 0.5 to 2.9, more preferably in the range of from 0.5 to 2.75 and when the ratio of moles of compound (b) to moles of compound (c) (hereafter referred to as ratio 2) is in the range of from 5 to 95, more preferably in the range of 'from 5 to 60.

Examples of other suitable commercially available compounds (b) are: (2,4,6(1H, 3H, 5H-)pyrimidinetrione (barbituric acid), 2,4(1H, 3H-)pyrimidinetrione (uracil), urazole, 2, 4-dihydroxy-5-methyl-pyrimidine (thymine) , 2, -imidazolinedione (hydantion) , 4, 6-dihydroxy-2- methylpyrimidine (2-methyl-4, 6-pyrimidinediol) , 2,4- dihydroxy-2-mercaptopyrimidine and 2,4-dihyroxy-6- methylpyrimidine (6-methyluracil) .

Suitable compounds (c) are: quaternary ammonium compounds such as: tetra-n-butylammoniumbromide, benzyltrimethylammonium chloride, tetraethyl- ammoniumchloride, tetramethylammnoniumchloride, benzyltrimethylammonium hydroxide, benzyl- dodecyldimethylammonium chloride; and quaternary phosphonium salts such as: ethyltriphenylphosphonium iodide, methyltriphenylphosphonium iodide, triphenyl (2, 5- dihydroxyphenyl)phosphonium hydroxide, ethyltri- phenylphosphonium acetate, ethyltriphenylphosphonium bromide, ethyltriphenylphosphonium chloride, benzyltriphenylphosphonium bromide, butyltriphenyl- phosphonium bromide and pentyltriphenylphosphonium bromide, or a blend of any one of the aforesaid compounds. The quaternary ammonium and - phosphonium salts may be used as such when they are solid at room temperature preferably having a melting point of higher than 30 °C more preferably higher than 40 °C, or alternatively, when liquid at said temperature they may be used when deposited on a solid carrier such as aluminosilicates, in particular the crystalline aluminosilicates known as Zeolites. The pigments, extenders, flow control agents and other additives may be used in the powder coating compositions of the present invention in a wide range of proportions with respect to the 1,2-epoxy compounds. Most suitable white pigments are the rutile titanium dioxides such as for example the commercially available grades Kronos 2220 and 2310; Tiofine R-81 and R-85; and Tioxide RTC-4U and RCR-2. Suitable coloured pigments are most types of inorganic pigments, with the exception of iron oxides, hydrated chromium oxides, black iron oxides and ultramarin blues all of which have insufficient thermal stability. Suitable extender pigments or fillers are blanc fixe or calcium carbonate. Most of the powder coating compositions require a flow control agent which modifies the surface tension of the film in the melt stage preventing the formation of craters in the film and improving the wetting of the substrate. Most widely used materials for this purpose are special acrylate polymers such as those known under the tradenames Modaflow and Acronal. Other flow control agents are certain polyvinyl- butyrals, silicone fluids and fluorocarbon compounds. For obtaining optimal dispersion of the flow control agent in the powder coating composition use is often made of masterbatches of epoxy resins comprising a relatively high amount of flow control agents. Such epoxy resin masterbatches are for example the commercially available grades EPIKOTE 3003-FCA-l having an epoxy group content of 1330 mmol/kg and containing 1% by weight based on the total weight of the composition of Acronal LR 8820, EPIKOTE 3003-FCA-2 having an epoxy group content of 1315 mmol/kg and containing 2% by weight of Acronal LR 8820, and EPIKOTE 3003-FCA-10 having an epoxy group content of 1215 mmol/kg and containing 10% by weight of Acronal LR 8820. Other types of additives may be used in the powder coating composition e.g. to modify the rheology of the coating. To increase flow and minimise any orange-peel small quantities of epoxidised oils can be incorporated as a masterbatch in resin. To impart thixotropy small quantities of a finely divided silica such as A^'erosil 380 can be incorporated.

There are a number of feasible methods for the manufacture of epoxy resin powder coating compositions but two methods are commercially significant, i.e. extruder compounding and Z-blade mixing. Extruder compounding is the method generally preferred. Extruder compounding usually consists of the following steps: masterbatches of flow control agents or other liquid additives are being prepared in a Z-blade mixer or an other suitable heated mixing device; the particle size of the resin and masterbatches is then reduced by crushing or coarse grinding, the thus obtained particles of the ingredients are premixed in a dry blender, e.g. a ribbon blender or other suitable device, the blend is melt mixed in an extruder, the extrudate is cooled e.g. on a band cooler, coarse grinded and fine grinded and the thus obtained particles are packed. For the preparation of the powder coating composition of the present invention it is preferred to prepared a preblend of compounds (b) and (c) before blending in the other components. The physical blend of compounds (b) and (c) can suitably be prepared by blending both components in the desired ratio at room temperature for several minutes using a standard type dry blender followed by grinding to reduce the particle size, preferably to less than 15 μm. This blend is stable and may be stored before use, usually for a period of at least 6 months. For obtaining the desired flow and sprayability the particle size of the powder coating should lie in the range of from 10 to 150 μm, preferably between 40 and 80 μm. The powder coating can be applied to the substrate according to methods known in the art, such as electrostatic or triboelectric spraying, fluidized bed sintering and electrostatic fluidized bed sintering. After the powder coating is applied to the substrate or object to be coated, the latter is heated preferably to a temperature in the range of from 160 to 240 °C more preferably from 175 to 220 °C for curing. Curing may take place in one or more cycles. The powder coating compositions of the present invention show an excellent combination of the desired level of reactivity, good flow of the coating compositions and high impact strength and above all good whiteness and excellent whiteness stability of the coating and are particularly suitable for decorative applications. The present invention is illustrated by means of the following examples. EXAMPLES Chemical compounds (ingredients) used in the powder coating compositions

EPIKOTE 3003 is a solid epoxy resin having on average 2 epoxy groups per molecule produced from bisphenol A and epichlorohydrin having an epoxy group content of 1370 mmol/kg available from Shell Chemicals Europe. EPIKOTE 3003 FCA-10 is a solid epoxy resin having an epoxy group content of 1215 mmol/kg and containing 90% by weight of EPIKOTE 3003 and 10% by weight of Acronal LR 8820, available from Shell Chemicals Europe. DSM P2127 is an acid functional polyester available from DSM Resins B.V.

Cy Acid is cyanuric acid obtained from Sigma (Molecular weight (MW) = 129) .

Ti02 is titanium dioxide. ETPPI is ethyltriphenylphosphonium iodide (Mw = 418) .

ETPPBr is ethyltriphenylphosphonium bromide (Mw = 371) .

BenTPPCl is benzyltriphenylphosphonium chloride (Mw = 389) . ButTPPBr is butyl triphenylphosphonium bromide

(Mw = 399) .

PenTPPBr is pentyltriphenylphosphonium bromide (Mw = 413) . tBABr is tetrabutylammonium bromide (Mw = 322) . 2MI is 2-methylimidazole.

ETPPI, ETPPBR, BenTPPCl, ButTPPBr, PenTPPBr, and tBABr are all available from Technochemie GmbH. Preparation of the powder coating compositions

The powder coating compositions were prepared using conventional extruder compounding and were applied to QD panels. The amounts of the ingredients used in the powder coating compositions are shown in tables 1 and 2. The layer thickness of the cured coatings is was 60 micron. Evaluation of the powder coating compositions The powder coating compositions were evaluated according to the following test methods whereof the results are presented in tables 1 and 2. Gel time

The gel times (in seconds) of the powder coating compositions was determined according to ISO/DIN 8130-6 at 180 °C using a Coesfeld geltimer in which a hotplate (cup) and a timer are combined, according to ISO/DIN 8130-6. Determination of coating flow

The flow of the coating was determined essentially in accordance with DIN 554945 and DIN 53230, although the rating scale differs from the one proposed in the latter German Standard. In the present test a rating scale was established by eight reference panels which were classified according to their surface roughness measured by a Perthen Pethometer M4P stylus instrument. The actual evaluation is done by comparing each panel with the set of reference panels preferably under oblique daylight. Each of the panels is then compared with the set of reference panels and receives a ranking between 1 and 8. Panels with equal rankings are subsequently compared to one another, which may result in a more diversified classification. The notation is in ranking number, optionally followed by +, ++, - or —. Measurement of whiteness of powder coatings

The whiteness of the coatings was determined after cure for 17 minutes at 180 °C and also after an additional 60 minutes at 210 °C (so called overbake) using a Colorquest apparatus available from Hunter Associates Laboratory Inc., USA. A whiteness index according to ASTM E313 can be easily measured using this apparatus. High index values correspond to high whiteness.

The difference in whiteness of the cured coating before and after the overbake procedure (indicated in table 1 as Delta W) is a measure of the whiteness stability of the coating i.e. the lower the delta W the greater the whiteness stability. Measurement of the resistance to cracking/rapid deformation The impact resistance of the coating compositions was determined using the impact test according to a combination of the following standard methods: ASTM D 2794-84/DIN 55669 and ISO TR 6272-1979. In this test a standard weight is dropped from variable height onto a coated panel in order to asses the resistance of the coating to cracking and/or stripping from the metallic substrate which is subjected to the deformation.

Test panels have been conditioned for at least sixteen hours in standard atmosphere (23 °C +/- 2 °C; 50 +/- 5% relative humidity) prior to testing and the test itself is performed in said standard atmosphere. The test apparatus^' used is an Erichsen Variable Impact Tester model 304 complying with the American Standard (indenter diameter 15.9 mm) however weight and indenter are in one piece which is in agreement with the ISO and DIN specifications. The impact resistance is reported as the product of the drop height and the mass of the weight in inch . lb. The test is performed in direct and reverse mode. In direct mode the indentation is made on the coated side of the test panel whilst in the reverse mode the indenter hits the uncoated side of the panel. The results are presented as Direct Impact and Reverse Impact. Results From the data presented in tables 1 and 2 the following can be concluded. The powder coating compositions according to the present invention in examples according to the invention 1 to 12 show an excellent combination of good reactivity and good flow of the uncured composition and good impact resistance and good whiteness stability of the coating, whereas powder coating compositions according to comparative examples A and B comprising a compound (b) as curing compound however not comprising a compound (c) as accelerator but 2-methyl imidazole as accelerator show a significantly worse performance of the coating as to its whiteness stability.

Moreover comparative example C, an epoxy resin powder coating composition comprising a 50:50 weight percent blend of an epoxy resin and an acid functional polyester, an example of an epoxy resin/acid functional polyester powder coating composition currently used by the powder coating industry for decorative applications, not only shows a significantly worse whiteness stability, but also a bad flow.

Finally, comparative example D shows that when ratio 1 is 3.0 the gel time of the composition is excessively high, which does not allow the use of the composition as a powder coating.

TABLE I

* ingredients are in grammes.

** ratio 1 is the ratio of equivalents of epoxy groups provided by compound (a) to moles of compound (b) *** ratio 2 is the ratio of moles of compound (b) to moles of compound (c)

TABLE I I

* ingredients are in grammes.

** ratio 1 is the ratio of equivalents of epoxy groups provided by compound (a) to moles of compound (b)

*** ratio 2 is the ratio of moles of compound (b) to moles of compound (c)

**** in view of the high gel time of the composition it can not be used as a powder coating.

Claims

C L A I M S

1. A thermosetting epoxy resin powder coating composition comprising:

(a) at least one epoxy resin having on average at least one 1,2-epoxy group per molecule, (b) at least one optionally substituted-, saturated- or unsaturated heterocyclic organic compound being a 1 ring system of either 5,6, or 8 members that contains at least one carboxamido group represented by the following general formula I or by its isomeric structure of the formula II

-C—N- (I) -C=N- (II)

II ^I I

O H OH wherein the carbon atom and nitrogen atom that make up each carboxamido group are members of said ring system, and (c) at least one quaternary ammonium or -phosphonium salt, wherein compounds (a) and (b) are present in such amounts that the ratio of equivalents of epoxy groups provided by compound (a) to moles of compound (b) (ratio 1) is lower than 3.0.

2. A thermosetting epoxy resin powder coating composition as claimed in claim 1 wherein compound a) is a solid epoxy resin produced from bisphenol A and epichlorohydrin having an EGC between 1000 and 1600 mmol/kg.

3. A thermosetting epoxy resin powder coating composi¬ tion as claimed in claim 1 or 2 wherein compound b) has 5 or 6 members in the ring system.

4. A thermosetting epoxy resin powder coating composition as claimed in anyone of the claims 1 to 3 wherein compound b) comprises at least two carboxamido groups.

5. A thermosetting epoxy resin powder coating composition as claimed in claim 4 wherein compound b) is a compound that can be represented by the following general formulae 1 or 2

2

wherein at least one of the groups R^ in each compound of the general formula 1 is a hydrogen atom and wherein at least one of the groups R^ in each compound represented by the above formula 2, is a hydroxyl group, whilst the remainder of the groups R^ and R^ and those atoms or groups attached to the single bonds in the compounds of the general formulae 1 or 2 are hydrogen, hydroxyl or any other suitable organic atom or group such as: saturated- or unsaturated-, linear- or branched-, substituted- or unsubstituted alkyl groups; carboxylic- or heterocylic groups that are optionally aromatic; halogen atoms; alkylether groups; carbonyl groups; and mercapto groups.

6. A thermosetting epoxy resin powder coating composition as claimed in claim 5 wherein compound b) is a compound according to the general formula 1 wherein all three groups R^ are hydrogen and/or the isomer of this compound which is: a compound according to the general formula 2 wherein all three groups R^ are hydroxyl.

7. A thermosetting epoxy resin powder coating composition as claimed in claim 6 wherein ratio 1 is in the range of from 0.5 to 2.75 and wherein the ratio of moles of compound (b) to moles of compound (c) (ratio 2) is in the range of from 5 to 60.

8. A thermosetting epoxy resin powder coating composition as claimed in any one of the claims 1 to 7 wherein compound (c) is tetra-n-butylammoniumbromide, benzyltrimethylammonium chloride, tetraethyl- ammoniumchloride, tetramethylammonium chloride, benzyltrimethylammonium hydroxide, benzyldodecyl- dimethylammoniu chloride, ethyltriphenylphosphonium iodide, methyltriphenylphosphonium iodide, triphenyl (2, 5- dihydroxyphenyl)phosphonium hydroxide, ethyltri¬ phenylphosphonium acetate, ethyltriphenylphosphonium bromide, ethyltriphenylphosphonium chloride, benzyl- triphenylphosphonium bromide, butyltriphenylphosphonium bromide or pentyltriphenylphosphonium bromide or a blend of one of more of the aforesaid compounds.

9. A process for the preparation of a thermosetting epoxy resin powder coating composition as claimed in any one of the claims 1 to 8 consisting of at least the following steps: i) premixing the compounds (a), (b) and (c) with conventional additives, ii) melt mixing the blend in an extruder, iii) cooling the extrudate and iv) grinding the cooled extrudate.

10. Use of a thermosetting epoxy resin powder coating composition as claimed in any one of the claims 1 to 8 for coating a substrate.

11. A substrate coated with a thermosetting epoxy resin powder coating composition as claimed in any one of the claims 1 to 8.