WO2002024798A2

WO2002024798A2 - Aminoplast-based crosslinkers and powder coating compositions containing such crosslinkers

Info

Publication number: WO2002024798A2
Application number: PCT/US2001/028253
Authority: WO
Inventors: Ronald R. Ambrose; Anthony M. Chasser; Shengkui Hu; John R. Schneider; Jackie L. Smith
Original assignee: Ppg Industries Ohio, Inc.
Priority date: 2000-09-21
Filing date: 2001-09-10
Publication date: 2002-03-28
Also published as: AU2001290716A1; WO2002024798A3

Abstract

A crosslinking agent which is the ungelled reaction product of (a) an aldehyde condensate of glycoluril and (b) a polyol is provided. The crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 15°C. A method for preparing the crosslinking agent is also provided. Further provided is a curable powder coating composition including a solid particulate, film-forming mixture of (A) a polymer containing reactive functional groups having a glass transition temperature of at least 30°C and (B) the previously described crosslinking agent. Additionally provided is a multilayer composite coating composition comprising a base coat deposited from a film-forming composition and a top coat over the base coat deposited from the previously described curable powder coating composition. Coated substrates are also provided.

Description

A INOPLAST-BASED CROSSLINKERS AND POWDER COATING COMPOSITIONS CONTAINING SUCH CROSSLINKERS

FIELD OF THE INVENTION The present invention relates to crosslinking agents based on glycoluril derivatives and to curable powder coating compositions containing such crosslinking agents.

BACKGROUND OF THE INVENTION In recent years, powder coatings have become increasingly popular because these coatings are inherently low in volatile organic content ("VOC"), which significantly reduces emissions of volatile organic compounds into the atmosphere during application and curing processes.

Hydroxyl, carbamate and/or epoxy functional resins, such as acrylic and polyester resins having relatively high glass transition temperatures

("Tg"), are commonly used as main film-forming polymers for these coatings. The relatively high Tg of such acrylic polymer systems provides powder coatings having good storage stability. However when exposed to the extreme temperatures which can be encountered during shipping and/or storage in many geographic areas, even better powder coating stability is desired.

Aminoplast resins are well known in the art as low cost crosslinking agents for hydroxyl, carboxyl and/or carbamate functional polymers in conventional liquid coating compositions. Common aminoplast resins are based on condensation products of formaldehyde with an amino- or amido- group carrying substance. Condensation products obtained from the reaction of alcohols and formaldehyde with melamine, urea or benzoguanamine are most commonly used in liquid coating compositions where they provide enhanced coating properties such as durability, chemical resistance and mar resistance. Such aminoplast resins typically are in liquid form and, as such, generally are not suitable for use in curable powder coating compositions. The alkoxylated aldehyde condensates of glycoluril, which are solid products, are the aminoplast resins most commonly employed as crosslinking agents in curable powder coating compositions. Although crystalline in form, these materials nonetheless can depress the Tg of the curable powder coating composition significantly, even when combined with high Tg film- forming polymers such as the acrylic polymers described above. Such a depression in Tg can result in poor powder stability.

In view of the foregoing, there remains a need for an aminoplast crosslinking agent suitable for use in curable powder coating compositions which provides a storage stable powder composition having the desirable coating properties usually associated with aminoplast-based liquid coatings.

SUMMARY OF THE INVENTION

In accordance with the present invention, provided is a crosslinking agent comprising the ungelled reaction product of (a) an alkoxylated aldehyde condensate of glycoluril; and (b) a polyol. The crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 15°C.

The present invention is also directed to a method for preparing the powder crosslinking agent. The method comprises the steps of (1) combining reactants (a) an alkoxylated aldehyde condensate of glycoluril, and (b) a reactive polyhydric compound in a molar ratio of reactant (a) to reactant (b) ranging from 1.5 to 3.0:1 , to form a reaction admixture; (2) heating the reaction admixture to a temperarture ranging from 90° to 135°C; and (3) maintaining that temperature for a time sufficient to produce an ungelled reaction product having a glass transition temperature of at least 15°C which is essentially free of hydroxyl functionality as determined by infrared spectroscopy.

Further provided is a curable powder coating composition comprising a solid particulate, film-forming mixture of (1) a polymer containing reactive functional groups having a glass transition temperature of at least 30°C, and (2) the crosslinking agent described above.

The present invention is also directed to a multi-layer composite coating composition comprising a basecoat deposited from a film-forming composition and a topcoat over the basecoat. The topcoat is deposited from the aforedescribed curable powder coating composition. Coated substrates are also provided.

DETAILED DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of "1 to 10" is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

As mentioned above, the crosslinking agent of the present invention comprises the ungelled reaction product of (a) an alkoxylated aldehyde condensate of glycoluril; and (b) a reactive polyhydric compound, wherein the crosslinking agent is essentially free of hydroxyl functionality and has a Tg of at least 15°C.

By "ungelled" is meant that the reaction product has an intrinsic viscosity when dissolved in a suitable solvent. The intrinsic viscosity of the reaction product is an indication of its molecular weight. A gelled reaction product, on the other hand, since it is of essentially infinitely high molecular weight, will have an intrinsic viscosity too high to measure. Moreover, the reaction product can be melted, solidified and remelted.

The alkoxylated aldehyde condensates of glycoluril suitable for use as component (a) can be prepared by reacting glycoluril, or acetylene diurea, with an aldehyde, preferably formaldehyde, to form tetra-alkylol glycoluril. In a preferred embodiment, the alkylol groups are etherified with a mono-alcohol, preferably a Ci to C₆ mono-alcohol, to form tetra-alkoxy alkyl glycoluril. A suitable, nonlimiting example of such a tetra-alkoxyalkyl glycoluril is tetra- methoxy methyl glycoluril which is commercially available under the tradename POWDERLINK^® 1174 from Cytec Industries, Inc. Also suitable for use in preparing the crosslinking agent of the present invention is cyclohexanol etherified tetra-methylol glycoluril.

The reactive polyhydric compound (b) can be any of a variety of materials having two or more groups which are reactive towards the glycouril, for example, hydroxyl groups. In a preferred embodiment, the polyhydric compound (b) comprises a diol, a triol, or a mixture of the two. Examples of suitable diols include cycloaliphatic diols such as those selected from hydrogenated Bisphenol A, cyclohexane dimethanol, cyclohexane diol and mixtures thereof. Cyclohexane dimethanol and hydrogenated Bisphenol A are the preferred diols. Examples of suitable triols include those selected from trimethylol propane, tris(hydroxyethyl) isocyanurate, and mixtures thereof.

As mentioned above, the present invention is also directed to a method for preparing the previously described crosslinking agent. The alkoxylated aldehyde condensate of glycoluril (a) and the reactive polyhydric compound (b) are combined in a suitably equipped reaction vessel, preferably with a suitable solvent and an appropriate strong acid as catalyst. Any suitable solvent can be used with aromatic solvents being preferred. Non-limiting examples of suitable aromatic solvents include xylene, toluene, and mixtures thereof. Suitable examples of strong acids suitable as a catalyst include, but are not limited to, para-toluene sulfonic acid and dodecyl benzene sulfonic acid. Normal condensation techniques as are well known in the art can be used. The reaction admixture is heated to a temperature ranging from 90° to 135°C and the reaction is terminated when the end point (i.e., the disappearance of the OH signal) is detected by infrared spectroscopy. The molar ratio of the aldehyde condensate of glycoluril (a) to the polyhydric compound (b) typically ranges from 1.5 to 3.0:1 , preferably 1.8 to 2.4:1 , and more preferably 1.9 to 2.2:1. An excess of the glycoluril component (a) relative to the polyhydric component (b) is preferred in order to ensure that the crosslinking agent is essentially free of hydroxyl functionality. The reaction is monitored for the disappearance of hydroxyl functionality relative to an internal standard (i.e., the signal of a constant structure which will remain essentially unchanged during the reaction, for example, the carbonyl signal) via infrared spectroscopy.

As mentioned above, the crosslinking agent typically has a T_g of at least 15°C. The T_g of the crosslinking agent can be measured experimentally using differential scanning calorimetry (rate of heating 10°C per minute, T_g taken at the first inflection point). Unless otherwise indicated, the stated T_g as used herein refers to the measured T_g.

The present invention also relates to a curable powder coating composition comprising a solid particulate, film-forming mixture of (A) a polymer containing reactive functional groups having a glass transition temperature of at least 30°C, and (B) the crosslinking agent described above.

Curable powder coatings are particulate compositions that are solid and free flowing at ambient room temperature. As mentioned above, the curable powder coating compositions of the present invention comprise, as a first component (A), at least one reactive functional group-containing polymer having a glass transition temperature of at least 30°C, such as a hydroxyl and/or epoxide functional polymer and as a second component (B), the crosslinking agent described above. The components (A) and (B) of the curable powder coating composition may each independently comprise one or more functional species, and are each present in amounts sufficient to provide cured coatings having a desirable combination of physical properties, e.g., smoothness, optical clarity, scratch resistance, solvent resistance and hardness.

As used herein, the term "reactive" refers to a functional group that forms a covalent bond with another functional group under suitable reaction conditions.

As used herein, the term "cure" as used in connection with a composition, e.g., "a curable composition," shall mean that any crosslinkable components of the composition are at least partially crosslinked. In certain embodiments of the present invention, the crosslink density of the crosslinkable components, i.e., the degree of crosslinking, ranges from 5% to 100% of complete crosslinking. In other embodiments, the crosslink density ranges from 35% to 85% of full crosslinking. In other embodiments, the crosslink density ranges from 50% to 85% of full crosslinking. One skilled in the art will understand that the presence and degree of crosslinking, i.e., the crosslink density, can be determined by a variety of methods, such as dynamic mechanical thermal analysis (DMTA) using a Polymer Laboratories MK III DMTA analyzer conducted under nitrogen. This method determines the glass transition temperature and crosslink density of free films of coatings or polymers. These physical properties of a cured material are related to the structure of the crosslinked network.

According to this method, the length, width, and thickness of a sample to be analyzed are first measured, the sample is tightly mounted to the Polymer Laboratories MK III apparatus, and the dimensional measurements are entered into the apparatus. A thermal scan is run at a heating rate of 3°C/min, a frequency of 1 Hz, a strain of 120%, and a static force of 0.01 N, and sample measurements occur every two seconds. The mode of deformation, glass transition temperature, and crosslink density of the sample can be determined according to this method. Higher crosslink density values indicate a higher degree of crosslinking in the coating.

The polymer (A) can be any of a variety of polymers having aminoplast-reactive functional groups as are well known in the art, so long as the T_g of the polymer is sufficiently high to permit the formation of a stable, solid particulate composition. The T_g of the polymer (A) typically is at least 30°C, preferably at least 40°C, more preferably at least 50°C. The T_g of the polymer (A) also typically is less than 130°C, preferably less than 100°C, more preferably less than 80°C. The T_g of the functional group-containing, polymer (A) can range between any combination of these values inclusive of the recited values.

Also, as used herein, the term "polymer" is meant to refer to oligomers and both homopolymers and copolymers. Unless stated otherwise, as used in the specification and the claims, molecular weights are number average molecular weights for polymeric materials indicated as "Mn" and obtained by gel permeation chromatography using a polystyrene standard in an art- recognized manner.

Non-limiting examples of polymers having reactive functional groups useful in the curable powder coating compositions of the invention as the polymer (A) include those selected from the group consisting of acrylic, polyester, polyurethane, polyepoxide and polyether polymers. The polymer (A) preferably comprises reactive functional groups selected from hydroxyl, epoxy, carboxyl and/or carbamate functional groups. In one embodiment of the present invention, the polymer (1) comprises hydroxyl and/or carbamate functional groups. Hydroxyl and/or carbamate functional group-containing acrylic polymers and/or polyester polymers are preferred. In another embodiment of the invention, the polymer (A) comprises epoxy and/or hydroxyl functional groups.

Suitable functional group-containing acrylic polymers include copolymers prepared from one or more alkyl esters of acrylic acid or methacrylic acid and, optionally, one or more other polymerizable ethylenically unsaturated monomers. Suitable alkyl esters of acrylic or methacrylic acid include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. As used herein, by "(meth)acrylate" and like terms is meant both methacrylates and acrylates. Suitable other polymerizable ethylenically unsaturated monomers include vinyl aromatic compounds, such as styrene and vinyl toluene; nitriles, such as acrylonitrile and methacrylonitrile; vinyl and viπylidene halides, such as vinyl chloride and vinylidene fluoride and vinyl esters, such as vinyl acetate; epoxy functional acrylates such as glycidyl (meth)acrylate.

In one embodiment, the acrylic polymers contain hydroxyl functionality which can be incorporated into the acrylic polymer through the use of hydroxyl functional monomers such as hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate which may be copolymerized with the other acrylic monomers.

The acrylic polymer can be prepared from ethylenically unsaturated, beta-hydroxy ester functional monomers. Such monomers are derived from the reaction of an ethylenically unsaturated acid functional monomer, such as monocarboxylic acids, for example, acrylic acid, and an epoxy compound which does not participate in the free radical initiated polymerization with the unsaturated acid monomer. Examples of such epoxy compounds are glycidyl ethers and esters. Suitable glycidyl ethers include glycidyl ethers of alcohols and phenols, such as butyl glycidyl ether, octyl glycidyl ether, phenyl glycidyl ether and the like. Suitable glycidyl esters include those which are commercially available from Shell Chemical Company under the tradename CARDURA E; and from Exxon Chemical Company under the tradename GLYDEXX-10. Alternatively, the beta-hydroxy ester functional monomers are prepared from an ethylenically unsaturated, epoxy functional monomer, for example glycidyl (meth)acrylate and allyl glycidyl ether, and a saturated carboxylic acid, such as a saturated monocarboxylic acid, for example, isostearic acid. The hydroxyl group-containing acrylic polymers useful in the compositions of the present invention typically have a hydroxyl value ranging from 5 to 150, preferably from 10 to 100, and more preferably from 20 to 50. The acrylic polymer is typically prepared by solution polymerization techniques in the presence of suitable initiators such as organic peroxides or azo compounds, for example, benzoyl peroxide or N,N-azobis (isobutyronitrile). The polymerization can be carried out in an organic solution in which the monomers are soluble by techniques conventional in the art. Pendent and/or terminal carbamate functional groups can be incorporated into the acrylic polymer by copolymerizing the acrylic monomer with a carbamate functional vinyl monomer, such as a carbamate functional alkyl ester of methacrylic acid. These carbamate functional alkyl esters are prepared by reacting, for example, a hydroxyalkyl carbamate, such as the reaction product of ammonia and ethylene carbonate or propylene carbonate, with methacrylic anhydride. Other carbamate functional vinyl monomers may be used, such as the reaction product of isocyanic acid (HNCO) with a hydroxyl functional acrylic or methacrylic monomer such as hydroxyethyl acrylate, and those carbamate functional vinyl monomers described in U.S. Patent No. 3,479,328.

As is preferred, carbamate groups can also be incorporated into the acrylic polymer by a "transcarbamoylation" reaction in which a hydroxyl functional acrylic polymer is reacted with a low molecular weight carbamate derived from an alcohol or a glycol ether. The carbamate groups exchange with the hydroxyl groups yielding the carbamate functional acrylic polymer and the original alcohol or glycol ether.

The low molecular weight carbamate functional material derived from an alcohol or glycol ether is first prepared by reacting the alcohol or glycol ether with urea in the presence of a catalyst such as butyl stannoic acid.

Suitable alcohols include lower molecular weight aliphatic, cycloaliphatic and aromatic alcohols, such as methanol, ethanol, propanol, butanol, cyclohexanol, 2-ethylhexanol and 3-methylbutanol. Suitable glycol ethers include ethylene glycol methyl ether and propylene glycol methyl ether. Propylene glycol methyl ether is preferred.

Also, hydroxyl functional acrylic polymers can be reacted with isocyanic acid yielding pendent carbamate groups. Note that the production of isocyanic acid is disclosed in U.S. Patent No. 4,364,913. Likewise, hydroxyl functional acrylic polymers can be reacted with urea to give an acrylic polymer with pendent carbamate groups.

Epoxide functional acrylic polymers are typically prepared by polymerizing one or more epoxide functional ethylenically unsaturated monomers, e.g., glycidyl (meth)acrylate, with one or more ethylenically unsaturated monomers that are free of epoxide functionality, e.g., methyl (meth)acrylate, isobomyl (meth)acrylate, butyl (meth)acrylate and styrene. Examples of epoxide functional ethylenically unsaturated monomers that may be used in the preparation of epoxide functional acrylic polymers include, but are not limited to, glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 2-(3,4-epoxycyclohexyl)ethyl (meth)acrylate and allyl glycidyl ether. Examples of ethylenically unsaturated monomers that are free of epoxide functionality include those described above as well as those described in U.S. Patent No. 5,407,707 at column 2, lines 17 through 56, which disclosure is incorporated herein by reference.

In a preferred embodiment of the present invention, the epoxide functional acrylic polymer is prepared from a majority of (meth)acrylate monomers and is referred to herein as an "epoxide functional (meth)acrylic polymer."

The functional group-containing acrylic polymer (A) typically has an Mn ranging from 500 to 30,000 and preferably from 1000 to 5000. If carbamate functional, the acrylic polymer typically has a calculated carbamate equivalent weight typically within the range of 15 to 150, and preferably less than 50, based on equivalents of reactive carbamate groups. Non-limiting examples of functional group-containing polyester polymers suitable for use as the polymer (A) in the curable powder coating compositions of the present invention include linear or branched polyesters having hydroxyl and/or carbamate functionality. Such polyester polymers are generally prepared by the polyesterification of a polycarboxylic acid or anhydride thereof with polyols and/or an epoxide using techniques known to those skilled in the art. Usually, the polycarboxylic acids and polyols are aliphatic or aromatic dibasic acids and diols. Transesterification of polycarboxylic acid esters using conventional techniques is also possible.

The polyols which usually are employed in making the polyester (or the polyurethane polymer, as described below) include alkylene glycols, such as ethylene glycol and other diols, such as neopentyl glycol, hydrogenated

Bisphenol A, cyclohexanediol, butyl ethyl propane diol, trimethyl pentane diol, cyclohexanedimethanol, caprolactonediol, for example, the reaction product of epsilon-caprolactone and ethylene glycol, hydroxy-alkylated bisphenols, polyether glycols, for example, poly(oxytetramethylene) glycol and the like. Polyols of higher functionality may also be used. Examples include trimethylolpropane, trimethylolethane, pentaerythritol, tris- hydroxyethylisocyanurate and the like. Branched polyols, such as trimethylolpropane and tris-hydroxyethylisocyanurate, are preferred for use in the preparation of the polyester. The acid component used to prepare the polyester polymer can include, primarily, monomeric carboxylic acids or anhydrides thereof having 2 to 18 carbon atoms per molecule. Among the acids which are useful are cycloaliphatic acids and anhydrides, such as phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, methylhexahydrophthalic acid, 1 ,3-cyclohexane dicarboxylic acid and 1 ,4- cyclohexane dicarboxylic acid. Other suitable acids include adipic acid, azelaic acid, sebacic acid, maleic acid, glutaric acid, decanoic diacid, dodecanoic diacid and other dicarboxylic acids of various types. The polyester may include minor amounts of monobasic acids such as benzoic acid, stearic acid, acetic acid and oleic acid. Also, there may be employed higher carboxylic acids, such as trimellitic acid and tricarballylic acid. Where acids are referred to above, it is understood that anhydrides thereof which exist may be used in place of the acid. Also, lower alkyl esters of diacids such as dimethyl glutarate and dimethyl terephthalate can be used. Because it is readily available and low in cost, terephthalic acid is preferred.

Pendent and/or terminal carbamate functional groups may be incorporated into the polyester by first forming a hydroxyalkyl carbamate

-n- which can be reacted with the polyacids and polyols used in forming the polyester. The hydroxyalkyl carbamate is condensed with acid functionality on the polyester yielding carbamate functionality. Carbamate functional groups may also be incorporated into the polyester by reacting a hydroxyl functional polyester with a low molecular weight carbamate functional material via a transcarbamoylation process similar to the one described above in connection with the incorporation of carbamate groups into the acrylic polymers or by reacting isocyanic acid with a hydroxyl functional polyester. Epoxide functional polyesters can be prepared by art-recognized methods, which typically include first preparing a hydroxy functional polyester that is then reacted with epichlorohydrin. Polyesters having hydroxy functionality may be prepared by art-recognized methods, which include reacting carboxylic acids (and/or esters thereof) having acid (or ester) functionalities of at least 2, and polyols having hydroxy functionalities of at least 2. As is known to those of ordinary skill in the art, the molar equivalents ratio of carboxylic acid groups to hydroxy groups of the reactants is selected such that the resulting polyester has hydroxy functionality and the desired molecular weight.

The functional group-containing polyester polymer typically has an Mn ranging from 500 to 30,000, preferably about 1000 to 5000. If carbamate functional, the polyester polymer typically has a calculated carbamate equivalent weight within the range of 15 to 150, preferably 20 to 75, based on equivalents of reactive pendent or terminal carbamate groups.

Non-limiting examples of suitable polyurethane polymers having pendent and/or terminal hydroxyl and/or carbamate functional groups include the polymeric reaction products of polyols, which are prepared by reacting the polyester polyols or acrylic polyols, such as those mentioned above, with a polyisocyanate such that the OH/NCO equivalent ratio is greater than 1 :1 such that free hydroxyl groups are present in the product. Such reactions employ typical conditions for urethane formation, for example, temperatures of 60°C to 90°C and up to ambient pressure, as known to those skilled in the art. The organic polyisocyanates which can be used to prepare the functional group-containing polyurethane polymer include aliphatic or aromatic polyisocyanates or a mixture of the two. Diisocyanates are preferred, although higher polyisocyanates can be used in place of or in combination with diisocyanates.

Examples of suitable aromatic diisocyanates include 4,4'- diphenylmethane diisocyanate and toluene diisocyanate. Examples of suitable aliphatic diisocyanates include aliphatic diisocyanates, such as 1 ,6- hexamethylene diisocyanate and trimethyl hexamethylene diisocyanate. Also, cycloaliphatic diisocyanates can be employed. Examples include isophorone diisocyanate, tetramethylxylenediisocyanate and 4,4'-methylene-bis- (cyclohexyl isocyanate). Examples of suitable higher polyisocyanates include 1 ,2,4-benzene triisocyanate and polymethylene polyphenyl isocyanate. Terminal and/or pendent carbamate functional groups can be incorporated into the polyurethane by reacting a polyisocyanate with a polyester polyol containing the terminal/pendent carbamate groups. Alternatively, carbamate functional groups can be incorporated into the polyurethane by reacting a polyisocyanate with a polyester polyol and a hydroxyalkyl carbamate or isocyanic acid as separate reactants. Carbamate functional groups can also be incorporated into the polyurethane by reacting a hydroxyl functional polyurethane with a low molecular weight carbamate functional material via a transcarbamoylation process similar to the one described above in connection with the incorporation of carbamate groups into the acrylic polymer. The hydroxyl and/or carbamate functional group-containing polyurethane polymers typically have an Mn ranging from 500 to 20,000, preferably from 1000 to 5000. If carbamate functional, the polyurethane polymer typically has a carbamate equivalent weight within the range of 15 to 150, preferably 20 to 75, based on equivalents of reactive pendent or terminal carbamate groups.

Although generally not preferred, for some applications it may be desirable to employ a functional group-containing polyether polymer in the curable powder coating compositions of the present invention. Suitable hydroxyl and/or carbamate functional polyether polymers can be prepared by reacting a polyether polyol with urea under reaction conditions well known to those skilled in the art. More preferably, the polyether polymer is prepared by a transcarbamoylation reaction similar to the reaction described above in connection with the incorporation of carbamate groups into the acrylic polymers.

Examples of polyether polyols are polyalkylene ether polyols which include those having the following structural formulae (II) and (III):

(ID or

(III) where the substituent Ri is hydrogen or lower alkyl containing from 1 to 5 carbon atoms including mixed substituents, n is typically from 2 to 6, and m is from 8 to 100 or higher. Note that the hydroxyl groups, as shown in structures (II) and (111) above, are terminal to the molecules. Included are poly(oxytetramethylene) glycols, poly(oxytetraethylene) glycols, poly(oxy-1 ,2- propylene) glycols and poly(oxy-1 ,2-butylene) glycols.

Also useful are polyether polyols formed from oxyalkylation of various polyols, for example, diols, such as ethylene glycol, 1 ,6-hexanediol, Bisphenol A and the like, or other higher polyols, such as trimethylolpropane, pentaerythritol and the like. Polyols of higher functionality which can be utilized as indicated can be made, for instance, by oxyalkylation of compounds, such as sucrose or sorbitol. One commonly utilized oxyalkylation method is reaction of a polyol with an alkylene oxide, for example, propylene or ethylene oxide, in the presence of a conventional acidic or basic catalyst as known to those skilled in the art. Typical oxyalkylation reaction conditions may be employed. Preferred polyethers include those sold under the names TERATHANE and TERACOL, available from E. I. Du Pont de Nemours and Company, Inc. and POLYMEG, available from Q O Chemicals, Inc., a subsidiary of Great Lakes Chemical Corp.

Epoxide functional polyethers can be prepared from a hydroxy functional monomer, e.g., a diol, and an epoxide functional monomer, and/or a monomer having both hydroxy and epoxide functionality. Suitable epoxide functional polyethers include, but are not limited to, those based on 4,4'- isopropylidenediphenol (Bisphenol A), a specific example of which is EPON® RESIN 2002 available commercially from Shell Chemicals.

Suitable functional group-containing polyether polymers typically have a number average molecular weight (Mn) ranging from 500 to 30,000 and preferably from 1000 to 5000. If carbamate functional, the polyether polymers have a carbamate equivalent weight of within the range of 15 to 150, preferably 25 to 75, based on equivalents of reactive pendent and/or terminal carbamate groups and the solids of the polyether polymer.

As aforementioned, the preferred carbamate functional group- containing polymers typically also contain residual hydroxyl functional groups which provide additional crosslinking sites. Preferably, the hydroxyl/carbamate functional group-containing polymer (1) has a residual hydroxyl value ranging from 1 to 10, more preferably from 0.2 to 10; and even more preferably from 0.5 to 10 (mg of KOH per gram). The functional group-containing polymer (A) typically is present in the curable powder coating compositions of the present invention in an amount ranging from at least 5 percent by weight, preferably at least 20 percent by weight, more preferably at least 30 percent by weight, and even more preferably at least 40 percent by weight based on the total weight of the film- forming composition. The functional group-containing polymer (A) also typically is present in the curable powder coating compositions of the present invention in an amount less than 90 percent by weight, preferably less than 85 percent by weight, more preferably less than 80 percent by weight, and even more preferably less than 70 percent by weight based on the total weight of the curable powder coating composition. The amount of the functional group- containing polymer (A) present in the curable powder coating compositions of the present invention can range between any combination of these values inclusive of the recited values.

As mentioned above, the powder coating compositions of the present invention further comprise, as component (B), the crosslinking agent described above. The crosslinking agent (B) typically is present in the powder coating compositions of the present invention in an amount ranging from at least 5 percent by weight, preferably at least 10 percent by weight, more preferably at least 15 percent by weight, and even more preferably at least 20 percent by weight based on the total weight of the powder coating composition. The crosslinking agent (B) also typically is present in the powder coating compositions of the present invention in an amount less than 90 percent by weight, preferably less than 70 percent by weight, more preferably less than 50 percent by weight, and even more preferably less than 25 percent by weight based on the total weight of the powder coating composition. The amount of the crosslinking agent (B) present in the powder coating compositions of the present invention can range between any combination of these values inclusive of the recited values.

If desired, the curable powder coating compositions of the present invention also can include an adjuvant curing agent different from the crosslinking agent (B). The adjuvant curing agent can be any compound having functional groups reactive with the functional groups of the polymer (A). Non-limiting examples of suitable adjuvant curing agents include, for example, blocked isocyanates, triazine compounds, glycoluril resins, and mixtures thereof.

The blocked isocyanates suitable for use as the adjuvant curing agent in the curable powder coating compositions of the invention are known compounds and can be obtained from commercial sources or may be prepared according to published procedures. Upon being heated to cure the curable powder coating compositions, the isocyanates are unblocked and the isocyanate groups become available to react with the functional groups of the polymer (A).

Any suitable aliphatic, cycloaliphatic or aromatic alkyl monoalcohol known to those skilled in the art can be used as a blocking agent for the isocyanate. Other suitable blocking agents include oximes and lactams. Non-limiting examples of suitable blocked isocyanate curing agents include those based on isophorone diisocyanate blocked with e-caprolactam; toluene diisocyanate blocked with e-caprolactam; or phenol-blocked hexamethylene diisocyanate. The blocked isocyanates mentioned immediately above are described in detail in U.S. Patent No. 4,988,793 at column 3, lines 1 to 36. Preferred blocked isocyanate curing agents include BF 1530, which is the reaction product of epsilon-caprolactam blocked T1890, a trimerized isophorone diisocyanate ("IPDI") with an isocyanate equivalent weight of 280, and BF 1540, a uretidione of IPDI with an isocyanate equivalent weight of 280, all of which are available from Creanova of Somerset, New Jersey.

Conventional aminoplast crosslinkers can be used as the adjuvant curing agent provided that the Tg of the coating is not lowered to an undesirable extent. A particularly preferred class of aminoplast resins include aldehyde condensates of glycoluril, such as those described above. Glycoluril resins suitable for use as the adjuvant curing agent in the powder coating compositions of the invention include POWDER LINK^® 1174 commercially available from Cytec Industries, Inc. of Stamford, Connecticut.

When employed, the adjuvant curing agent typically is present in the curable powder coating compositions of the present invention in an amount ranging from 1 to20 percent by weight, preferably from 1 to 15 percent by weight based on the total weight of the curable powder coating composition.

Also suitable for use as an adjuvant curing agent in the curable powder coating compositions of the present invention are triazine compounds, such as the tricarbamoyl triazine compounds described in detail in U.S. Patent No. 5,084,541. When used, the triazine curing agent is typically present in the curable powder coating composition of the present invention in an amount ranging up to about 20 percent by weight, and preferably from about 1 to 20 percent by weight, percent by weight based on the total weight of the curable powder coating composition.

Mixtures of the above-described curing agents also can be used advantageously.

Also, it should be understood that for purposes of the present invention, the curable powder coating compositions which contain epoxy group-containing polymers typically also include an epoxide-reactive curing (i.e., crosslinking) agent, preferably an acid functional curing agent, in addition to the aminoplast-based crosslinking agent (B). A secondary hydroxyl group is generated upon reaction of each epoxy functional group with an acid group of the epoxide-reactive curing agent. These secondary hydroxyl groups are then available for further reaction with the aminoplast-based crosslinking agent (B). Epoxide-reactive curing agents which can be used in curable powder coating compositions comprising epoxide functional polymer may have functional groups selected from the group consisting of hydroxyl, thiol, primary amines, secondary amines, acid (e.g. carboxylic acid) and mixtures thereof. Useful epoxide reactive curing agents having amine functionality include, for example, dicyandiamide and substituted dicyandiamides. Preferably, the epoxide reactive curing agent has carboxylic acid groups.

In one embodiment of the present invention, the epoxide reactive crosslinking agent has carboxylic acid functionality and is substantially crystalline. By "crystalline" is meant that the co-reactant contains at least some crystalline domains, and correspondingly may contain some amorphous domains. While not necessary, it is preferred that the epoxide reactive curing agent have a melt viscosity less than that of the epoxy functional polymer (at the same temperature). As used herein and in the claims, by "epoxide reactive crosslinking agent" is meant that the epoxide reactive crosslinking agent has at least one functional group that is reactive with epoxide functionality. Preferably, the epoxide reactive curing agent is a carboxylic acid functional curing agent, which contains from 4 to 20 carbon atoms. Examples of carboxylic acid functional crosslinking agents useful in the present invention include, but are not limited to, dodecanedioic acid, azelaic acid, adipic acid, 1,6-hexanedioic acid, succinic acid, pimelic acid, sebasic acid, maleic acid, citric acid, itaconic acid, aconitic acid and mixtures thereof.

Other suitable carboxylic acid functional curing agents include those represented by the following general formula IV, (IV)

In general formula IV, R is the residue of a polyol, E is a divalent linking group having from 1 to 10 carbon atoms, and n is an integer of from 2 to 10. Examples of polyols from which R of general formula IV may be derived include, but are not limited to, ethylene glycol, di(ethylene glycol), trimethylolethane, trimethylolpropane, pentaerythritol, di-trimethylolpropane, di-pentaerythritol and mixtures thereof. Divalent linking groups from which E may be selected include, but are not limited to, methylene, ethylene, propylene, isopropylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, cyclohexylene, e.g., 1 ,2-cyclohexylene, substituted cyclohexylene, e.g., 4-methyl-1,2-cyclohexylene, phenylene, e.g., 1 ,2- phenylene, and substituted phenylene, e.g., 4-methyl-1 ,2-phenylene and 4- carboxylic acid-1 ,2-phenylene. The divalent linking group E is preferably aliphatic.

The curing agent represented by general formula IV is typically prepared from a polyol and a dibasic acid or cyclic anhydride. For example, trimethylol propane and hexahydro-4-methylphthalic anhydride are reacted together in a molar ratio of 1:3 respectively, to form a carboxylic acid functional curing agent. This particular curing agent can be described with reference to general formula IV as follows, R is the residue of trimethylol propane, E is the divalent linking group 4-methyl-1 ,2-cyclohexylene, and n is 3. Carboxylic acid functional curing agents described herein with reference to general formula I are meant to include also any unreacted starting materials and/or co-products, e.g., oligomeric species, resulting from their preparation and contained therein.

Curable powder coating compositions comprising an epoxide functional polymer and an epoxide reactive curing agent can also include one or more cure catalysts for catalyzing the reaction between the reactive functional groups of the crosslinking agent and the epoxide groups of the polymer. Examples of cure catalysts for use with acid functional crosslinking agents include tertiary amines, e.g., methyl dicocoamine, and tin compounds, e.g., triphenyl tin hydroxide. When employed, the curing catalyst is typically present in the curable powder coating composition in an amount of less than 5 percent by weight, e.g., from 0.25 percent by weight to 2.0 percent by weight, based on total weight of the composition.

Curable powder coating compositions comprising epoxide functional polymers and epoxide reactive curing agents typically have present therein epoxide functional polymer in an amount ranging from 2 percent to 50 percent by weight, based on total weight of the composition, e.g., from 70 percent to 85 percent by weight, based on total weight of the composition. The epoxide reactive curing agent is typically present in the curable powder coating composition in an amount corresponding to the balance of these recited ranges, i.e., 5 to 40, particularly 15 to 30, percent by weight. The equivalent ratio of epoxide equivalents in the epoxide functional polymer to the equivalents of reactive functional groups in the epoxide-reactive curing agent is typically from 0.5:1 to 2:1 , e.g., from 0.8: 1 to 1.5:1. Curable powder coating compositions comprising epoxide functional polymers and epoxide reactive curing agents typically contain the crosslinking agent (B) in an amount ranging from 2 to 50 weight percent, preferably from 3 to 40 weight percent, more preferably from 4 to 30 weight percent and even more preferably from 5 to 20 weight percent based on total weight of the powder coating composition.

The curable powder coating compositions of the present invention can further include additives as are commonly known in the art. Typical additives include benzoin, used to reduce entrapped air or volatiles; flow aids or flow control agents which aid in the formation of a smooth and/or glossy surface, for example, MODAFLOW® available from Monsanto Chemical Co., waxes such as MICROWAX® C available from Hoechst, fillers such as calcium carbonate, barium sulfate and the like; carbon black or Shepard Black pigments and dyes; UV light stabilizers such as TINUVIN® 123 or TINUVIN® 900 available from Cytec Industries, Inc. and catalysts to promote the various crosslinking reactions.

Such additives are typically present in the powder coating compositions of the present invention in an amount ranging from 5 to 50 weight percent based on total weight of the powder coating composition.

The curable powder coating compositions of the invention are typically prepared by blending the functional group-containing polymer (A) and the crosslinking agent (B), along with any adjuvants, additives and catalyst, if employed, for approximately 1 minute in a Henschel blade blender. The powder is then extruded through a Baker-Perkins twin screw extruder at a temperature ranging from 70°F to 130°F (21.1°C to 54.4°C). The finished powder then can be classified to an appropriate particle size, typically between 20 and 200 microns, in a cyclone grinder/sifter. The curable powder coating compositions of the invention can be applied to a variety of substrates including metallic substrates, for example, aluminum and steel substrates, and non-metallic substrates, for example, thermoplastic or thermoset composite substrates. The curable powder coating compositions are typically applied by spraying, and in the case of a metal substrate, by electrostatic spraying which is preferred, or by the use of a fluidized bed. The powder coating can be applied in a single sweep or in several passes to provide a film having a thickness after cure of from about 1 to 10 mils (25 to 250 micrometers), usually about 2 to 4 mils (50 to 100 micrometers). Generally, after application of the powder coating composition, the powder coated substrate is heated to a temperature sufficient to cure the coating, typically to a temperature ranging from 250°F to 500 ° F (121.1°C to 260.0°C) for 1 to 60 minutes, and preferably from 300° F to 400° F (148.9°C to 204.4°C) for 15 to 30 minutes.

The curable powder coating composition can be applied as a primer or primer surfacer, or as a topcoat, for example, a "monocoat". The curable powder coating composition of the invention also can be advantageously employed as a topcoat in a multi-component composite coating composition. Such a multi-component composite coating composition generally comprises a pigmented basecoat deposited from a pigmented film-forming composition and a top coat applied over the basecoat, the top coat being deposited from the powder coating composition as described above. In a preferred embodiment, the multi-component composite coating composition is a color- plus-clear system where the top coat is deposited from a curable powder coating composition which is substantially pigment-free top coat, i.e., a clear coat. The film-forming composition from which the basecoat is deposited can be any of the compositions useful in coatings applications, for example, in automotive applications where color-plus-clear systems are most often used. A film-forming composition conventionally comprises a resinous binder and a pigment to serve as a colorant. Particularly useful resinous binders include acrylic polymers, polyesters including alkyds, and polyurethanes.

The resinous binders for the basecoat can be organic solvent-based materials, such as those described in U.S. Patent No. 4,220,679. Water- based coating compositions, such as those described in U.S. Patent Nos. 4,403,003; 4,147,679; and 5,071 ,904, also can be used as the basecoat composition.

As mentioned above, the basecoat compositions also contain pigments of various types as colorants. Suitable metallic pigments include aluminum flake, bronze flake, copper flake and the like. Other examples of suitable pigments include mica, iron oxides, lead oxides, carbon black, titanium dioxide, talc, as well as a variety of color pigments.

Optional ingredients for the basecoat film-forming compositions include those which are well known in the art of surface coatings and include surfactants, flow control agents, thixotropic agents, fillers, anti-gassing agents, organic co-solvents, catalysts and other suitable adjuvants.

The basecoat film-forming compositions can be applied to the substrate by any of the conventional coating techniques, such as brushing, spraying, dipping or flowing, but they are most often spray-applied. The usual spray techniques and equipment for air spraying, airless spraying and electrostatic spraying can be used.

The basecoat film-forming compositions are typically applied to the substrate such that a cured basecoat having a film thickness ranging from 0.5 to 4 mils (12.5 to 100 micrometers) is formed thereon.

After forming a film of the basecoat on the substrate, the basecoat can be cured or alternatively given a drying step in which solvent, i.e., organic solvent and/or water, is driven off by heating or an air drying step before application of the clear coat. Suitable drying conditions will depend on the particular basecoat film-forming composition and on the ambient humidity with certain water-based compositions. In general, a drying time ranging from 1 to

15 minutes at a temperature of 75°F to 200°F (21 °C to 93°C) is adequate. The substantially pigment-free curable powder coating composition is applied to the basecoat by any of the methods of application described above. As discussed above, the clear coat can be applied to a cured or a dried basecoat before the basecoat has been cured. In the latter case, the clear coat and the basecoat are cured simultaneously.

Illustrating the invention are the following examples which are not to be considered as limiting the invention to their details. Unless otherwise indicated, all parts and percentages in the following examples, as well as throughout the specification, are by weight.

EXAMPLES

Example AA describes the preparation of a crosslinking agent in accordance with the present invention. Examples A through D describe the preparation of curable powder coating compositions which employ the crosslinking agent of Example AA. Examples A and B describe the preparation of powder coating compositions which include various levels of catalyst, and Examples C and D describe the preparation of analogous compositions which further include a uretidione crosslinker. Comparative Examples E to H describe the preparation of curable powder coating compositions comprising a conventional glycoluril crosslinking agent which are analogous to the compositions of Examples A to D, respectively.

EXAMPLE AA

This example describes the preparation of a crosslinking agent of the present invention. The crosslinking agent was prepared as follows.

Added to a three-liter, four-necked reaction vessel equipped with a thermometer, stirrer, nitrogen inlet, and means for removing the reaction byproduct (i.e., methanol) were 660.0 parts by weight of POWDERLINK^® 1174, 144.0 parts by weight of cyclohexanedimethanol, 384.0 parts by weight of xylene, and 2.0 parts by weight of p-toluenesulfonic acid. The admixture was heated to a temperature of 110 °C over a 30 minute period, and the temperature was maintained as the methanol by-product was removed from the system. The reaction was monitored by infrared spectroscopy for the disappearance of OH_functionality (3200-3600cm^'1) and was terminated when this end point was detected. Thereafter, the reaction product was concentrated in vacua at a temperature of 100° to 130 °C and a pressure of 3 to 50 mm Hg to remove the xylene solvent. The reaction product thus obtained was a pale yellow solid.

EXAMPLES A THROUGH D

² MTSI , methyl tolyl sulfonimide available from Cytec Industries, Inc.

³ Acrylic flow additive dispersed on silica, commercially available from Estron Chemical.

⁴ Benzoin, a degassing additive available from Mitsubishi Chemical Corp.

⁵ Wax additive available from Clariant Additives.

⁶ Hydroxyl functional polyester resin commercially available from Reichold Chemicals, Inc.

⁷ R706, a titanium dioxide pigment available from E.l. duPont de Nemours and Co.

⁸ Pigment blend of Irgazin Blue, Ultramarine Blue, Black Iron Oxide, Ultramarine Violet, and AEROSIL^® 200 / CAB-O-SIL^® -5 (handling agents).

COMPARATIVE EXAMPLES E THROUGH H

Crystalline glycoluril resin commercially available from Cytec Industries, Inc

PREPARTION OF POWDER COATING COMPOSITIONS

For each of the powder coating compositions of Examples A to D and Comparative Examples E-H, all of the listed components were blended for 10 seconds at 3500 rpm in a PRISM blender. The powders were then fed through a 19 millimeter, twin screw extruder available from b&p Process Equipment and Systems, by way of an ACCU-RATE auger feeder. The extruder zones were set at 52°C in zone one and 110°C in the following three zones. The resulting chip was classified to a median particle size of approximately 40 microns.

Each of the powder coating compositions thus prepared were applied by electrostatic spray using a Nordson Versa-Spray II, corona-type spray gun to B1000 P60 DIW steel test panels (available from ACT Laboratories, Inc.) to a targeted cured film thickness of 2.0 to 3.0 mils (50 to 75 micrometers).

TEST PROCEDURES: The powder storage stability of each powder coating composition was evaluated by storing a 20g sample of each powder coating composition at a temperature of 40°C for a 24 hour period. The stability of the powder was determined upon visual inspection.

Film hardness was determined by measuring pencil hardness in accordance with ASTM-D-3363.

Adhesion of the cured coating to the substrate was tested in accordance with ASTM-D-3359.

Flexibility/impact resistance (both direct and reverse impact) was evaluated using a Gardner Impact Tester in accordance with ASTM-D-2794.

Gloss was evaluated at 20 degrees using a BYK Glossgard II glossmeter available from BYK Chemie.

Test results for each of the powder coating compositions are reported below in the following Table 1.

TABLE 1

* Comparative examples.

The data presented in Table 1 above illustrate that the powder coating compositions of Examples A through D which contain the crosslinking agents of the present invention have powder storage stability properties which are superior to those of the analogous compositions which contain a conventional glycoluril crosslinkers. As can be seen other properties were not adversely affected by the use of the crosslinking agent of the present invention as a direct replacement for the conventional glycoluril crosslinking agent.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications which are within the spirit and scope of the invention, as defined by the appended claims.

Claims

THEREFORE WE CLAIM:

1. A crosslinking agent comprising the ungelled reaction product of the following reactants: (A) an alkoxylated aldehyde condensate of glycoluril; and

(B) a reactive polyhydric compound, wherein the crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 15°C.

2. The crosslinking agent of claim 1 , wherein the alkoxylated aldehyde condensate of glycoluril comprises tetramethoxy methylglycoluril.

3. The crosslinking agent of claim 1 , wherein the polyhydric compound (B) comprises a polyol.

4. The crosslinking agent of claim 3, wherein the polyol comprises a cycloaliphatic diol.

5. The crosslinking agent of claim 4, wherein the polyol compound (B) is selected from hydrogenated Bisphenol A, cyclohexane dimethanol and mixtures thereof.

6. The crosslinking agent of claim 1 , wherein the polyhydric compound Bb) comprises a triol selected from trimethylol propane, tris(hydroxyethyl) isocyanurate and mixtures thereof.

7. The crosslinking agent of claim 1 , wherein the molar ratio of reactant (A) to reactant (B) ranges from 1.5 to 3.0:1.

8. A crosslinking agent comprising an ungelled reaction product of the following reactants:

(A) tetramethoxy methyl glycoluril, and (B) a polyol selected from hydrogenated Bisphenol A, 'cyclohexane dimethanol and mixtures thereof, present in a molar ratio of reactant (a) to reactant (b) of 1.5 to 3.0:1 , whrein the crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 15°C.

9. A method for preparing a powder crosslinking agent, said method comprising the following steps:

(1) combining the following reactants: (a) an alkoxylated aldehyde condensate of glycoluril, and

(b) a reactive polyhydric compound in a molar ratio of reactant (a) to reactant (b) ranging from 1.5 to 3.0:1 to form a reaction admixture;

(2) heating the reaction admixture to a temperarture ranging from 90° to 135°C;

(3) maintaining the temperature achieved in step (2) for a time sufficient to produce an ungelled reaction product having a glass transition temperature of at least 15°C which is essentially free of hydroxyl functionality as determined by infrared spectroscopy.

10. The method of claim 9, wherein the alkoxylated aldehyde condensate of glycoluril (a) comprises tetramethoxy methylglycoluril.

11. The method of claim 9, wherein the reactive polyhydric compound (b) comprises a polyol.

12. The method of claim 11 , wherein the reactive polyhydric compound (b) comprises a cycloaliphatic diol.

13. The method of claim 12, wherein the polyhydric compound (b) comprises a cycloaliphatic diol selected from hydrigenated Bisphenol A, cyclohexane dimethanol and mixtures thereof.

14. The method of claim 11 wherein the polyhydric compound (b) comprises a triol selected from trimethylol propane, tris(hydroxyehtyl)isocyanurate and mixtures thereof.

15. The method of claim 9, wherein the reactants (a) and (b) of step (1) are combined in a molar ratio of reactant (a) to reactant (b) ranging from 1.5 to 3.0:1.

16. The method of claim 9, wherein further combined with reactants (a) and (b) of step (1) is (c) an acid catalyst.

17. The method of claim 16, wherein the acid catalyst (c) is selected from p-toluene sulfonic acid, dodecanebenzene sulfonic acid, dodecanenaphthalene sulfonic acid and mixtures thereof.

18. A curable powder coating composition comprising a solid particulate, film-forming mixture of the following components:

(A) a polymer containing reactive functional groups, said polymer having a glass transition temperature of at least 30°C; and (B) a crosslinking agent having functional groups reactive with the functional groups of the polymer (A), said crosslinking agent comprising the ungelled reaction product of the following reactants:

(1) an alkoxylated aldehyde condensate of glycoluril; and

(2) a reactive polyhydric compound, wherein the crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 15°C.

19. The curable powder coating composition of claim 18, wherein the polymer (a) is selected from acrylic, polyurethane, polyether, polyepoxide and polyester polymers and mixtures thereof.

20. The curable powder coating composition of claim 19, wherein the polymer (a) comprises an acrylic polymer.

21. The curable powder coating composition of claim 19, wherein the polymer (a) comprises a polyester polymer.

22. The curable powder coating composition of claim 19, wherein the polymer (a) comprises a polyurethane polymer.

23. The curable powder coating composition of claim 19, wherein the polymer (a) comprises a polyepoxide polymer.

24. The curable powder coating composition of claim 18, wherein the polymer (a) comprises functional groups selected from hydroxyl, epoxy and/or carbamate functional groups.

25. The curable powder coating composition of claim 24, wherein the polymer (a) comprises hydroxyl and/or carbamate functional groups.

26. The curable powder coating composition of claim 24, wherein the polymer (a) comprises epoxy and/or hydroxyl groups.

27. The curable powder coating composition of claim 18, wherein the polymer (a) has a glass transition temperature ranging from 30°C to 110°C.

28. The curable powder coating composition of claim 18, wherein the polymer (A) is present in an amount ranging from 10 to 90 weight percent based on total weight of the composition.

29. The curable powder coating composition of claim 18, wherein the polyhydric compound (2) comprises a cycloaliphatic diol selected from hydrogenated Bisphenol A, cyclohexane dimethanol and mixtures thereof.

30. The curable powder coating composition of claim 18, wherein the polyol (2) comprises a triol selected from trimethylol propane, tris(hydroxyethyl) isocyanurate and mixtures thereof.

31. The curable powder coating composition of claim 18, wherein the molar ratio of reactant (1) to reactant (2) ranges from 1.5 to 3.0:1.

32. The curable powder coating composition of claim 18, wherein the crosslinking agent (b) is present in an amount ranging from 3 to 40 weight percent based on total weight of the composition.

33. The curable powder coating composition of claim 18, further comprising at least one pigment.

34. The curable powder coating composition of claim 26 wherein the powder coating composition further comprises an acid functional crosslinking agent.

35. The curable powder coating composition of claim 34, wherein the acid functional crosslinking agent comprises a carboxylic acid functional curing agent containing 4 to 20 carbon atoms.

36. The curable powder coating composition of claim 34, wherein the crosslinking agent (b) is present in an amount ranging from 5 to 90 weight percent based on total weight of the composition.

37. The curable powder coating composition of claim 18, comprising a solid particulate, film-forming mixture of the following components:

(A) an acrylic polymer containing hydroxyl and/or carbamate reactive functional groups, said polymer having a glass transition temperature of at least 30°C; and (B) a crosslinking agent having functional groups reactive with the functional groups of the polymer (a), said crosslinking agent comprising the ungelled reaction product of the following reactants:

(1) tetramethoxymethyl glycoluril; and (2) a reactive polyhydric compound selected from hydrogenated Bisphenol A, cyclohexane dimethanol and mixtures thereof, wherein the crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 15°C.

38. The curable powder coating composition of claim 18, comprising a solid particulate, film-forming mixture of the following components:

(A) an acrylic polymer containing epoxy functional groups, said polymer having a glass transition temperature of at least 30°C; and

(B) a crosslinking agent having carboxylic acid functional groups reactive with the epoxy functional groups of (A); and

(C) a crosslinking agent comprising the ungelled reaction product of the following reactants: (1) tetramethoxymethyl glycoluril; and

(2) a reactive polyhydric compound selected from hydrogenated Bisphenol A, cyclohexane dimethanol and mixtures thereof, wherein the crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least

15°C.

39. A multilayer composite coating composition comprising a basecoat deposited from a film-forming coating composition and a top coat over the basecoat deposited from a powder top coating composition comprising a solid particulate, film-forming mixture of the following components: (A) a polymer containing reactive functional groups, said polymer having a glass transition temperature of at least 30°C; and

(B) a crosslinking agent having functional groups reactive with the functional groups of the polymer (a), said crosslinking agent comprising the ungelled reaction product of the following reactants:

(1) an aldehyde condensate of glycoluril; and

40. The multilayer composite coating composition of claim 39, wherein the polymer (a) is selected from the group consisting of acrylic, polyurethane, polyether, polyepoxide and polyester polymers and mixtures thereof.

41. The multilayer composite coating composition of claim 39, wherein the polymer (a) comprises hydroxyl, epoxy and/or carbamate functional groups.

42. The multilayer composite coating composition of claim 41 , wherein the polymer (a) comprises hydroxyl and/or carbamate functional groups.

43. The multilayer composite coating composition of claim 41 , wherein the polymer (a) comprises epoxy and/or hydroxyl functional groups.

44. The multilayer composite coating composition of claim 39, wherein the polymer (a) has a glass transition temperature ranging from 30°C to 80°C.

45. The multilayer composite coating composition of claim 39, wherein the polymer (a) is present in the powder top coating composition an amount ranging from 10 to 90 weight percent based on total weight of the top coating composition.

46. The multilayer composite coating composition of claim 39, wherein the alkoxylated aldehyde condensate of glycoluril (1) comprises tetramethoxy methyl glycoluril.

47. The multilayer composite coating composition of claim 39, wherein the reactive polyhydric compound (2) comprises a cycloaliphatic diol selected from hydrogenated Bisphenol A, cyclohexane dimethanol and mixtures thereof.

48. The multilayer composite coating composition of claim 39, wherein the polyhydric compound (2) comprises a triol selected from trimethylol propane, tris(hydroxyethyl) isocyanurate and mixtures thereof.

49. The multilayer composite coating composition of claim 39 wherein the molar ratio of reactant (1 ) to reactant (2) ranges from 1.5 to 3.0:1.

50. The multilayer composite coating composition of claim 39, wherein the crosslinking agent (b) is present in the powder top coating composition in an amount ranging from 5 to 90 weight percent based on total weight of the top coating composition.

51. The multilayer composite coating composition of claim 39, wherein the powder top coating composition is substantially pigment-free.

52. The multilayer composite coating composition of claim 43, wherein the powder top coating composition further comprises an acid functional crosslinking agent.

53. The multilayer composite coating composition of claim 52, wherein the acid functional crosslinking agent comprises a carboxylic acid functional curing agent containing 4 to 20 carbon atoms.

54. The multilayer composite coating composition of claim 52, wherein the crosslinking agent (b) is present in the powder top coating composition in an amount ranging from 3 to 40 weight percent based on total weight of the powder top coating composition.

55. The multilayer composite coating composition of claim 39, wherein the powder top coating composition comprises a solid particulate, film-forming mixture of the following components:

(A) an acrylic polymer containing hydroxyl and/or carbamate reactive functional groups, said polymer having a glass transition temperature of at least 30°C; and

(B) a crosslinking agent having functional groups reactive with the functional groups of the polymer (A), said crosslinking agent comprising the ungelled reaction product of the following reactants: (1) tetramethoxymethyl glycoluril; and

15°C.

56. The multilayer composite coating composition of claim 39, wherein the powder top coating composition comprises a solid particulate, film-forming mixture of the following components:

(B) a crosslinking agent comprising the ungelled reaction product of the following reactants: (1) tetramethoxymethyl glycoluril; and (2) a reactive polyhydric compound selected from hydrogenated Bisphenol A, cyclohexane dimethanol and mixtures thereof, wherein the crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least

15°C; and

(C) a crosslinking agent having carboxylic acid functional groups reactive with the epoxy functional groups of (A).

57. A substrate coated with the curable powder coating composition of claim 18.

58. A substrate coated with the multilayer composite coating composition of claim 39.