CA2049855A1

CA2049855A1 - Powder coating compositions

Info

Publication number: CA2049855A1
Application number: CA 2049855
Authority: CA
Inventors: Christian Wamprecht; Hans-Joachim Kreuder
Original assignee: Individual
Current assignee: Bayer AG
Priority date: 1990-08-30
Filing date: 1991-08-26
Publication date: 1992-03-01
Also published as: JPH04298576A; EP0473033A1; DE4027372A1

Abstract

Mo-3516 LeA 27,831 POWDER COATING COMPOSITIONS

ABSTRACT OF THE DISCLOSURE
The present invention relates to powder coating compositions which are solid below 30°C and contain A) a copolymer component which is solid below 30°C and contains at least one copolymer having both carboxylic anhydride and hydroxyl groups, B) an epoxide component which contains at least one polyepoxide having at least two epoxide groups per molecule, C) an isocyanate component which contains at least one polyisocyanate having at least two optionally blocked isocyanate groups per molecule.

Description

Mo-3616 - LeA 27,831 WDER COATING COMPOSITIONS
BACKGROUND OF THE INVENTION
Field o~ the Invention The present invention relates to powder coating compositions whlch are solid below 30'C and contain a mixture of a copolymer component containing incorporated anhydride and hydroxyl groups, a polyepoxide component containing at least two epoxide groups per ~olecule and a polyisocyanate component containing at least two optionally blocked isocyanate groups per molecule.
DesçriPtion of the Prior Art The use of compositions containing polyacrylates having at least two carboxyl groups per molecule and polyepoxide compounds, which may be based on acrylates, is known. Systems such as these are described, for example, in Japanese Patent Application 73-29 319 (publication number: 49 116 134; C.A. 82 (24): 157 882 e), Japanese Patent Application 75-146 705 (publication number: 52 069 936; C.A. 88 (12): 75 374 a), DE-OS

2,347,680, US-PS 3,991,132 and US-PS 3,991,133.
Compositions containing polyacrylates having at least two hydroxyl groups per molecule and polyisocyanate compounds, which may be based on acrylates and wherein the isocyanate groups may optionally being blocked, are also known. Systems of this type are described in DE-OS 1,965,7409 DE-OS 2,105,777, DE-OS 2,236,514, DE-DS 2,542,191, DE-OS 2,735,497, DE-OS

3,328,133, EPA 254 152 or EPA 286 799.
Systems based on multicamponent mixtures are also known.
Thus, DE OS 3,232,463 desoribes a co~plioated three-component ~ixture of an ~socyanate component containing carboxyl ~roups, a polyol component and an epoxide component. -Le A 27 831-., . - ~ :
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-It has now been found that copolymers which are prepared from olefinically unsaturated compounds and contain both ~ncorporated hydroxyl groups and incorporated intramolecular carboxylic anhydride groups in the same molecule, in combi-. 5 nation with a polyepox;de component and a polyisocyanate component containing optionally blocked isocyanate groups represent excellent binders for po~der coating compositions, provided $hat they possess the necessary softening point or softening range.
SUMMARY OF IHE INVENTION
The present invention relates to powder coating --compositions which are solid below 30-C and contain a A) a ~ ~ copolymer component whioh is solid below 30'C and contains at least one copolymer having both carboxylic anhydride and hydroxyl groups, B) an ~poxide component which contains at least one polyepoxide having at least two epoxide groups per molecule, C) an isocyanate component which contains at least one polyisocyanate having at least two optionally blocked isocyanate groups per molecule.
~ETAILED DESCRIPTION OF l~E INVENTION
Preferably the powder coating compositions acc~rding to the present invention are solid below 30-C and contain A) 20 to 9B parts by ~eight o~ a copol~mer comp~nent containing at least one copol~mer which has a weight average molecular weight of 1,500 to 75,000, is prepared from olefinically unsaturated compounds and contains ~3 0.1 to 6.0~ by weight of fre~ hydroxyl groups and il) 1 to 30% by weight of carboxyl k anhydride grsups (cak ulated as C4H203) in che~ically bound form, Mo3616 -2 ~

B) 1 to 79 parts by weight of an epoxide component containing at least one organic polyepoxide which has at least two epoxide groups per molecule and C) 1 to 79 parts by weight of an isocyanate component containing at least one organic polyisocyanate which has at least two optionally blocked lsocyanate groups per molecule, wherein the parts by weight ~ add up to 100, and provided that for every anhydride group of co~ponent A) there are 0.2 to 8 epoxide groups sf component B~ and a total of 0.2 to 8 free and/or blocked isocyanate groups of component C).
The powder coa~ng compositions aocording to the inven~on may comp~ise components A), B), C) and optionally D) (as explained in detail hereinafter) each in powder-form, i.e. each powder par~cle consis~ng of A), B), C) or D).
Preferably, however, the powder coadng compositions compnse "mixed powders" ABC or ABCD i.e. powders in which each powder par~cle comprises A), B) and C) resp. A), B), C) and D). Such "mi~ed powders" are obtained if the coa~ngs are manufactured in exlruders or hleadeIs as explained in more detail hereinafter.

I Copolymer component AJ contains at least one copolymer ¦ having both chem kally incorporated free hydroxyl groups in a quanti$y of 0.1 to 6.0~o by weight, preferably 0.2 to 4.0% by weight, and chemically incorporated intramslecular cyclic carboxylic anhydride groups corresponding to the formulas / \ "
-CH CH- CH2 C=0 C C and/or C ~ 0 O O O

- ' ' ` ' , 2 ~
- 3 a -in a quantity of I to 30% by we;ght, preferably 5 to 25% by weight. The copolymers have a weight average molecular weight (Mw, as determined by gel permeation chromatography using polystyrene as standard) of 1,500 to 75,000, preferably 2,000 to 60,000 and more preferably 3,000 to 40,000. The copolymers are preferably based on monoo1efinically unsaturated monomers.
In a particularly preferred embodiment, the copolymers contain 0.2 to 8, preferably 0.4 ~o 4 and more preferably 0.8 to 2 hydroxyl groups per anhydride group.

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Three groups of ulefinically unsaturated monomers are used for the preparation of the copolymers, i.e., a) olefinically unsaturated monomers containing hydroxyl groups, b) olefinically unsaturated monomers containing intramolecular anhydride groups and c) non-functional, olefinically unsaturated monomers free from hydroxyl and anhydride groups.
Monomers a) are used in a amount of 0.~ to 50 parts by weight, preferably 5 to 40 parts by weight; ~onomærs b) are used ~n an amount of 1 to 30 parts by weight, preferably 5 to 25 parts by weight; and monomers c) are used ~n a quantity of 20 to 98.5 parts by weight, preferably 35 to 90 parts by weight, wherein the parts by weight of monomers a), b) and c) preferably add up to 104. Another factor to be taken into consideration is that the percentages of the individu~l monomers in the mixture to be copolymerized are selected such that the above-mentioned amounts of chemically incorporated hydroxyl and anhydride groups are present in the copolymers.
The content of these groups in the copolymers corresponds to the content of these groups in the monomer mixture because it may be assumed that the chemical composition of the copolymers corresponds to the chemical composition of the monomer mixture.
The monomers a) are monoolefinically unsaturated alcohols which preferably have a molecular weight of 58 to 500 and preferably have aliphatically bound hydroxyl groups. Suitable monomers include hydroxyalkyl esters of acryl;c and ~ethacrylic acid such dS hydroxyethyl (meth)acrylate ~(meth)acrylate refers to both acrylate and methacrylate groups], hydroxypropyl tmeth)acryl ate and 4-hydroxybutyl ~meth)acryl ate; hydroxyal kyl 3Q vinyl ethers such as hydroxyethyl vinyl ether and hydroxybutyl vinyl ether; allyl alcohol; hydroxy derivatives of ~meth)-acrylamide such as N-(3-hydroxy-2,2-dimethylpropyl)-(meth~-acrylamide; react~on products of glyc~dyl (meth~acrylate with monocarboxylic acids; react1On products of (meth)acrylic acid Mo3616 . . . , ,, :
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with monoepoxide compounds; and reaction products of the prev;ously ~entioned OH-functional, olefinically unsaturated compounds with ~-caprolactone or butyrolactone.
Suitable examples of monomers b~ include itaconic 5 anhydride or ~aleic anhydride; maleic anhydride ~s preferred.
Monomers c) have a molecular weight cf 86 to 400 and include those which are used as monomers in the previously described prior ~rt processes. Monomers c) are preferably monoolefinically unsaturated monomers which are free from hydroxyl and anhydride groups.
Examples include esters of acrylic and methacrylic acid such as ~ethyl acrylate, ethyl acrylate, n-buty1 acrylate, isobutyl acrylate, tert. butyl acrylate, 2-ethylhexyl acrylate, cyclohexyl methacrylate, methyl methacrylate, n-butyl 15 methacrylate, isobutyl methacrylate and 2-ethylhexyl methacrylate; aromatic vinyl compounds suoh as styrene, vinyl toluene, ~-methyl styrene, ~-ethyl styrene and nucleus-substituted diethyl styrenes, isopropyl styrenes, butyl styrenes and methoxystyrenes which may optionally be present as 20 isomer mixtures; vinyl ethers such as ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-bu$yl vinyl ether and isobutyl vinyl ether; and vinyl esters such as vinyl acetate, vinyl propionate and vinyl butyrate.
Mixtures of the monomers a) to c) mentioned by way of 25 . ~xample may of course be used to carry out the eopolymerization reaction, provided that the monomers used in the preparation of the copolymers are selested so that the resulting copolymers are solid below 30~C, preferably below 40-C, and are liquids at temperatures above 150-C, preferably above 120-C. In ~ther words, the copolymers ~ust have a softening point or softening range, as dete~mined by differential thermoanalysis (DTA)9 of 30 to 150-C, pref~rably 40 to 120C. This requirement, which ~s crucial to the suitability of the copolymers for use in accordance w~th the ~nvention, is satisfied when a suitable ratio of Wsoftening" monomers to "hardening" monomers is used Mo3616 .

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in the preparat;on of the copolymers. The "softening~ monomers lead to a reduction in the softening temperature of the oopolymers, while the hardening" ~onomers lead to an increase in the softening temperature of the copolymers.
~Softening~ monomers include alkyl esters of acrylic ac;d ~ethyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate and ethyl diglycol acrylate; vinyl esters such as vinyl propionate; and vinyl ethers such as vinyl ~ethyl ether, vinyl ethyl ether, o v~nyl propyl ether, vinyl ~sobutyl ether, vinyl octadecyl ether and vinyl cyclohexyl ether.
~Hardening~ ~onomers include alkyl esters of methacrylie acid such as methyl ~ethacrylate, ethyl methacrylate, isobutyl methaerylate, cyclohexyl methacrylate, 2-phenyl ethyl meth-aorylate, tetrahydrofurfuryl ~ethacrylate, isopropyl methacrylate, tert. butyl methacylate, neopentyl methacrylate, ~sobornyl methacrylate and benzyl methacrylate; aromatic vinyl compounds such as styrene, vinyl toluene, ~-methyl styrene and ~-ethyl styrene; and heterocyclic vinyl compounds such as vinyl pyrrolidone, vinyl caprolactam and vinyl carbazole.
The copolymers may be prepared by the copolymerization of monomers a) to c) usin3 standard radical polymerization processes such as bulk or solution polymerization. In this case, the monomers are oopolymerized at ~empera~ures of 60 to 140C, preferably 80 to 120-C in the presence of radical formers and, optionally, molecular weight regulators.
The preparation of the copolymers is preferably carried out in inert solvents. Suitable solvents include aromatic hydrocarbons such as benzenes toluene and xylene; esters such as ethyl aeetate, butyl acetate, hexyl aeetate~ heptyl acetate, methyl glycol acetate, ethyl glycol acetate and ~ethoxypropyl aoetate; ethers such as tetrahydrofuran, dioxane and diethylene glycol dlmethyl ether; ketones such as acetone, methylethyl ketone, ~ethyl isobutyl ketone, methyl n~amyl ketone ~nd methyl is~amyl ketone; and mixtures of such solvents.
Mo3616 , , :
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The preparation of the copolymers may be continuous or discontinuous. Normally, the monomer mixture is introduced uniformly and continuously into a polymerization reactor, the initiator is added and, at the same time, the corresponding s quantity of polymer is continuously removed. Substantially chemically uniform copolymers may advantageously be produced in this way. Substantlally chemically uniform copolymers may also be obtained by adding the react1On mixture to a stirred tank reactor at a constant rate without removing the pclymer.
It ls also possible initially to introduce part of the ~onomers, for example in the previously described solvents, and to subsequently add the remaining monomers and aux;liaries separately or together at the reaction temperature.
In general, the polymerization takes place under atmospheric pressure, although it may also be carried out at pressures of up to 20 bar.
The initiators are used in quantities of 0.05 to 15% by weight, based on the total quantity of monomers. Suitable initiators are known and inolude aliphatic azo compounds such as azodiisobutyronitrile, azo-bis-2-methylvaleronitrile, 1,1'-azo-bis-1-cyclohexane nitrile and 2,2'-azo-bis-isobutyric acid alkyl ester; symmetrical diacyl peroxides such as acetyl, propionyl and butyryl peroxide, bromine-, nitro-, ~ethyl- or methoxy-subst1tuted benzoyl peroxides and lauryl peroxides;
symmetrical peroxydicarbonates such as diethyl, diisopropyl, dicyclohexyl and dibenzoyl peroxydicarbonate; tert. butyl peroxy-2-ethyl hexanoate; tert. butyl perbenzoate;
hydroperoxides such as tert. butyl hydroperoxide and eumene hydroperoxide; and dialkyl peroxides such as d kumyl peroxide, tert. butyl cu~yl perox~de and di-tert. butyl peroxide.
To regulate the molecular weight of the copolymers, typ~cal regulaturs may be used during their preparation.
Examples of suitable molecular weight regulators are tert.
dodecyl mercaptan, n-dodecyl mercaptan and diisopropyl xanthogene disulfide. The regulators may be used in quantities Ms3616 , ., J~ 3 of 0.1 to 10% by weight, based on the total quantity of ~onomers.
The solutions of the copolymers which are obtained during the copolymerization may be subjected w~thout further working Up to the evaporation or degassing process in which the solvent is removed, for exa~ple in an evaporation extruder at approximately 120 to 160-C under a vacuum of 10 to 400 mbar, preferably 150 to 300 mbar. The copolymers to be used in accord~nce with the invention are obtained.
o Component B3 contains a~ least one organic compound having at least two epoxide groups per molecule which has a so~er~ing point or range, as determined by dif~erential thermoanalysis (DTA) of from 0 to 150C, preferably of ~om 30 ~ 150C and most preferably of from 40 to 120C. C:omponents B) which have a so~ening point or range of belo.v 30C or which are even liquid at room temperature may be used in combina~on with components A) having a ~elatively high softening point or range within the range disclosed hereinbefore, provided the pIefe~ed method for prepa~ng the powder coating compositions in kneaders or ex~ruders is used ln which case the combina~ions are solids below 30C even if such components B) (and/or C)) are used. The prefe~Ted compounds of component B) include those which contain 2 to 50, pre~erably 2 to 10 epoxide groups per molecule.

Typical examples of suitable or preferred polyepoxides include tr~glycidyl isocyanurate, terephthalic acid diglysidyl ester or those based on glycidyl ethers of bisphenol A and marketed, for example9 by Shell under the names -Epikote 1001 (epox~de group content approx. 2 moles/kg3, Epikote 1002 (epox~de group content approx. l.S moles/kg), Epikote 1055 (epoxide group content approx. 1.2 moles/kg), Epikote 1007 (epoxide group content approx. 0.55 moles/kg3, and Epikote 3003 ~epoxide group content approx. 1.3 moles/ kg3. Also suitable are glycidyl ethers o~ heterocyclic sompounds such as ' , - 8 a ~
triglycidyl urazole and epoxide-functional copolymers of olefinically unsaturated compounds haYing a glass transition temperature of 230-C and prepared, for example, by the co-use of glycidyl (meth~acryla~e as comonomer.
Component C) contains at least one organic compound having at least two optionally blocked isocyanate groups per moleeule and a softening point or range, as determined by differential thermoanalysis (DT~) of from 0 to 150C, preferably of from 30 to 150C and most preferably of from 40 to 120C. Components C) which have a softening point or range of below 30C or which are even liquid at room temperature may be used in combina~on with components A) having a relatively high softenin~ point or range within the range disclosed hereinbefore, provided the polyisocyanates haveonly blocked isocyanate groups and ~e preferred method for preparing the powder coating composi~ons in kneaders or exlruders is used in which case ~e combina~on are sollds below 30C even if component C) (and/cr B)) was liquid at ~his temperature.

.
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The preferred components C) include those which contain (on a statistical average~
2 to 10, preferably 2 to 4 op~onally blocked isocyanate groups per molecule. -Part~cularly suitable polyisocyanates having optionally blocked 1socyanate groups are derivatives of ~iisocyanates such as 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (IPDI), 1,~-di~socyanatocyclohexane, 4,4'-diisocyanatodicyclohexyl ~ethan2, 1,3- or 1,4-b~s-(2-isocyanatoprop-2-yl)-benzene and 1-1socyanato-1-methyl-4(3)-isocyanatomethyl cyclohexane.
It ~s particularly preferred to use the der1vatiYes of ~ -cycloaliphatic diisocyanates, more particularly IPDI, which are completely or partly blocked, generally with ~-caprolactam.
Unmodified diisocyanates of the type mentioned by way of example blocked with ~-caprolactam may also be used, provided that they satisfy the requirements regarding melting point or melting range. Other suitable dlisocyanates include ~-caprolactam-blocked IPDI (DE-OS 2 105 777); ~-oapro-lactam-blorlted reaction products of IPDI with substoichio-mBtric quantlties of polyhydric alcohols (DE-OS 2 542 191, : DE-OS 3 004 876); at least partly blocked trimers of IPDI: (DE-OS 2 842 641 and DE-OS 2 735 497); hydraz~ne-modified, ~-caprolactam-blocked paint polyisocyanates according to DE-OS
3 039 824; uretdione-modified derivatives o~ IPDI accordlng to EP-A~O 045 994, EP-A-O 045 995 and EP-A-O 045 998; and der1vatives of 1,3- or l,4-bis-(2-1sosyanatoprop-2-yl)-benzene or 1-~socyanato- 1-methyl -4(3) -~ socyanatomethyl cyclohexane containing fr~e ~socyanate groups, as described for example in EP-A-O 254 152.

Mo3616 '3'~
- 9 a -Although it is theoretically possible to use components B) and/or C) which have melting resp.
soft~ning points or ranges of below 30C ~e preferred components B) and C) are solid below 30C, preferably below 40C and liquid above 150C, preferably above 120C.

, : , ~: -. ,, 3~

Component D) is selected from the known auxiliaries and additives from powder coating terhnology, including p;gments such as titanium dioxide and flow control agents such as the s~licone compounds typically used for this purpose.
Although the powder coatings may be stoved without accelerators, catalysts may be added which accelerate the reaction between the hydroxyl and carboxylic anhydride groups or hydroxyl and isocyanate groups and the epoxide and oarboxylic ac~d groups. These catalysts reduce the stoving time and/or the stoving temperature.
Such catalysts ~nclude compounds containing tertiary amino groups such as l,4-diazabicyclo[~.2.2]octane, 1,8-diaza-bicyclo[5.4.0]undec-7-ene9 1,5-diazabicyclor4.3.0~non-5-ene, 1,2-dimorphol~noe~hane and 1,3,5- ~icyclohexyDhe~ahydko-1.3,5-triazine; sal~s based on compounds containing tertiary amino groups and quaternary ammonium groups such as (2-hydroxy-ethyl)-trimethyl ammonium chloride, triethylamine hydro-chloride, tetrabutyl ammonium chloride, tetraethyl ammonium bromide, tetrahexyl ammon~um chloride and tetramethyl ammonium chloride; organotin compounds such as tin dioctoate, dibutyl tin dilaurate, d1butyl tin diacetate and dibutyl tin dichloride; and phosphorus compounds such as triphenyl phosphine.
To prepare the ready-to-use powder coating ccmpositions, the solventless copolymers A), polyepoxide compounds B), polyisocyanate compounds C) and optionally additives D) are preferably homogenized in the melt in extruders or kneaders, preferably at temperatures of 100 to 120-C. After the melt has solidified, the solid is ground and freed by sieving from particles larger ~han the requ~d s~,forex~unple,above 0~ n. Ihe coa~ng powde~ may, ofcou~e, ~so be pnepared bynnuongpowde~ A),B),C)andop~onaUy D)previously p~paIedrna ~p ~ ~ process.
The quantities in which components A~t B) and C) are used are generally selected so that, for ~very anhydride group of component A3, there are 0.2 to 8, preferably 0.4 to 4 and ~ore preferably 0.8 to 1.5 epoxide ~roups of component B) and 0.2 to ;

2 f3 ~

8, preferably 0.4 to 4 and more preferably 0.8 to 1.5 (blocked) isocyanate groups o~ component C).
In the stoving process, the hydroxyl groups of component A) presumably react first with the anhydride groups of component A) ~ith opening of the anhydride r~ng and formation of semiester structures. The free carboxylic acid groups of these semiester structures may then react with the epoxide groups of component B) with formation of hydroxyl groups.
F~nally, these hydroxyl groups may react with the ~soeyanate groups of component C) to form urethane groups after the blocking agent, if any, has been removed. Due to the complex crosslinking reactions which take place during stoving of the powder coat~ng compositions acoording to the invention~ the coatings obtained are highly crosslinked and show extremely high resistance tu solvents. High-gloss or matt coatings are obtained, depending upon the composition of and mixing ratios between components A) to S~ so that the powder coating compositions according to the inventicn may be used for various potential applications.
The powder coating compositions according to the invention may be ~pplied to heat-resistant substrates by standard powder applicat~on processes such as eleotrostatic powder spraying or fluidized bed coating. The coatings may be hardened by heating to temperatures of 130 to 220-C, preferably 150 to 200-C.
Hard, glossy or ~att, solvent-resistant coatings are obtained which combine excellent corrosion-inhibiting properties with high color stability under heat.
S1nce no free carboxyl groups are initially present in the powder coating compositions, they show high stability in storage at room temperature or slightly elevated temperatures belcw the softening temperature so that, in contrast to prior art systems, no so-called creeping crosslinking reactions take place.

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The powder coa~ng composi~ons acrording to ~e inven~ion aré suitable coa~ng heat-resissant materials such as, for example, metal or glass substrates.
In the following examples~ all percentages are by weight, unless otherwise stated.
I 6eneral procedure ~or the preparation of hydroxyl- and anhydride-functional copolymers A1 to A7 Part I was initially ~ntroduced into ~ 25 liter, stainless steel pressure reactor equipped with a stirrer and with a heat~ng and cooling system and heated to the reaction temperature. Part II and part III were then added beginning at the same t~me over total periods of 3 hours and 3.5 hours, respectively. The mixture was then stirred for 1 hour at the indicated temperature.
The polymer solutions obtained were completely freed from the solvent in a commercially available evaporation extruder over a period of about 2 minutes at a temperature of approximately 150~C and under a vacuum of approximately ~00 mbar. The copolymers were subsequently cooled and granulated.
The reaction temperatures and the composition of parts I
to III are shown in Table I together with the characteristic data of the copolymers o~tained.

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2t~!198 II. Production of the coating powders according to the invention Copolymers Al to A7 were melted and homogenized in an extruder with polyepoxide B), polyisocyanate C) and optionally other auxiliaries and additives D). After the discharged melt had solidif;ed, the product was ground, applied to 0etal test plates by electrostatic spraying and then hardened for 15 minutes at 170-C.
The solvent resistance of the approximately 55 ~m thick o paint films was tested by a rubbing test using a cotton wool plug soaked in acetone. The results of this test are expressed as the number of double rubs which the paint film was able to withstand without visibly changing. No film was subjected to more than 50 double rubs.
The particular formulations in % by weight and the solvent resistance as the degree of crosslinking are shown in 7able II.

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Although the invention has been described in detail in the foregoing for the purpose of ~llustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

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Claims

1. A powder coating composition which is solid below 30°C and comprises A) a copolymer component which is solid below 30°C comprises at least one copolymer having both carboxylic anhydride and hydroxyl groups, B) a polyepoxide component which comprises at least one polyepoxide having at least two epoxide groups per molecule and C) a powder-form polyisocyanate component which comprises at least one polyisocyanate having at least two isocyanate groups per molecule.

2. The powder coating composition of Claim 1 wherein component A) is present in an amount of 20 to 98 parts by weight and comprises at least one copolymer which has a weight average molecular weight of 1,500 to 75,000, is prepared from olefinically unsaturated compounds and contains i) 0.1 to 6.0% by weight of free hydroxyl groups and ii) 1 to 30% by weight of carboxylic anhydride groups (calculated as C4H2O3) in chemically bound form, component B) is present in an amount of 1 to 79 parts by weight and component C) is present in an amount of 1 to 79 parts by weight, provided that for every anhydride group of component A) there are 0.2 to 8 epoxide groups of component B) and a total of 0.2 to 8 free and/or blocked isocyanate groups of component C.

3. The powder coating composition of Claim 1 wherein component A) comprises a copolymer obtained by the radical-initiated copolymerization of a) 0.5 to 53 parts by weight of olefinically unsaturated monomers containing hydroxyl groups, b) 1 to 30 parts by weight of olefinically unsaturated monomers containing anhydride groups and c) 20 to 98.5 parts by weight non-functional olefinically unsaturated monomers which are free from hydroxyl and Mo-3616 anhydride groups.

4. The powder coating composition of Claim 2 wherein component A) comprises a copolymer obtained by the radical-initiated copolymerization of a) 0.5 to 50 parts by weight of olefinically unsaturated monomers containing hydroxyl groups, b) 1 to 30 parts by weight of olefinically unsaturated monomers containing anhydride groups and c) 20 to 98.5 parts by weight non-functional olefinically unsaturated monomers which are free from hydroxyl and anhydride groups.

5. The powder coating composition of Claim 3 wherein monomer b) comprises maleic anhydride and/or itaconic anhydride.

6. The powder coating composition of Claim 4 wherein monomer b) comprises maleic anhydride and/or itaconic anhydride.

7. The powder coating composition of Claim 3 wherein monomer a) comprises a hydroxyalkyl ester of acrylic and/or methacrylic acid.

8. The powder coating composition of Claim 4 wherein monomer a) comprises a hydroxyalkyl ester of acrylic and/or methacrylic acid.

9. The powder coating composition of Claim 5 wherein monomer a) comprises a hydroxyalkyl ester of acrylic and/or methacrylic acid.

10. The powder coating composition of Claim 6 wherein monomer as comprises a hydroxyalkyl ester of acrylic and/or methacrylic acid.

11. The powder coating composition of Claim 1 wherein component B) comprises a member selected from the group consisting of triglycidyl isocyanurate, terephthalic acid diglycidyl ester and glycidly ethers of bisphenol A.

Mo-3616

12. The powder coating composition of Claim 2 wherein component B) comprises a member selected from the group consisting of triglycidyl isocyanurate, terephthalic acid diglycidyl ester and glycidyl ethers of bisphenol A.

13. The powder coating composition of Claim 4 wherein component B) comprises a member selected from the group consisting of triglycidyl isocyanurate, terephthalic acid diglycidyl ester and glycidyl ethers of bisphenol A.

14. The powder coating composition of Claim 10 wherein component B) comprises a member selected from the group consisting of triglycidyl isocyanurate, terephthalic acid diglycidyl ester and glycidyl ethers of bisphenol A.

15. The powder coating composition of Claim 1 wherein component C) comprises a modified polyisocyanate wherein the isocyanate groups may be blocked with blocking agents for isocyanate groups, said modified polyisocyanate being based on an organic diisocyanates comprising a member selected from the group consisting of 1-isocyanato-3,3,5-trimethyl-5-isocyanato-methyl cyclohexane, 1,4-diisocyanato-cyclohexane, 4,4'-diisocyanatodicyclohexyl methane, 1,3-or 1,4-bis-(2-isocyanatoprop-2-yl)-benzene and 1-isocyanato-1-methyl-4(3-isocyanatomethyl)-cyclohexane.

16. The powder coating composition of Claim 2 wherein component C) comprises a modified polyisocyanate wherein the isocyanate groups may be blocked with blocking agents for isocyanate groups, said modified polyisocyanate being based on an organic diisocyanates comprising a member selected from the group consisting of 1-isocyanato-3,3,5-trimethyl-5-isocyanato-methyl cyclohexane, 1,4-diisocyanato-cyclohexane, 4,4'-diisocyanatodicyclohexyl methane, 1,3-or 1,4-bis-(2-isocyanatoprop-2-yl)-benzene and 1-isocyanato-1-methyl-4(3-isosyanatomethyl)-cyclohexane.

Mo-3616

17. The powder coating composition of Claim 4 wherein component C) comprises a modified polyisocyanate wherein the isocyanate groups may be blocked with blocking agents for isocyanate groups, said modified polyisocyanate being based on an organic diisocyanates comprising a member selected from the group consisting of 1-isocyanato-3,3,5-trimethyl-5-isocyanato-methyl cyclohexane, 1,4-diisocyanato-cyclohexane, 4,4'-diisocyanatodicyclohexyl methane, 1,3-or 1,4-bis-(2-isocyanatoprop-2-yl)-benzene and 1-isocyanato-1-methyl-4(3-isocyanatomethyl)-cyclohexane.

18. The powder coating composition of Claim 10 wherein component C) comprises a modified polyisocyanate wherein the isocyanate groups may be blocked with blocking agents for isocyanate groups, said modified polyisocyanate being based on an organic diisocyanates comprising a member selected from the group consisting of 1-isocyanato-3,3,5-trimethyl-5-isocyanato-methyl cyclohexane, 1,4-diisocyanato-cyclohexane, 4,4'-diisocyanatodicyclohexyl methane, 1,3-or 1,4-bis-(2-isocyanatoprop-2-yl)-benzene and 1-isocyanato-1-methyl-4(3-isocyanatomethyl)-cyclohexane.

19. The powder coating composition of Claim 14 wherein component C) comprises a modified polyisocyanate wherein the isocyanate groups may be blocked with blocking agents for isocyanate groups, said modified polyisocyanate being based on an organ k diisocyanates comprising a member selected from the group consisting of 1-isocyanato-3,3,5-trimethyl-5-isscyanato-methyl cyclohexane, 1,4-diisocyanato-cyclohexane, 4,4'-diisocyanatodicyclohexyl methane, 1,3-or 1,4-bis-(2-isocyanatoprop-2-yl)-benzene and 1-isocyanato-1-methyl-4(3-isocyanatomethyl)-cyclohexane.

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