Patent Publication Number: US-6713182-B2

Title: Aminoplast-based crosslinkers and powder coating compositions containing such crosslinkers

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This patent application is a divisional of U.S. patent application Ser. No. 09,666,575, filed Sep. 21, 2000, now U.S. Pat. No. 6,451,928. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a crosslinking agent based on aminotriazine compounds and to powder coating compositions containing these crosslinkers. 
     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, carboxyl, 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 polymers 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. Examples of these aminoplast resins include the methylol and alkoxymethyl derivatives of ureas, melamines and benzoguanamines which are most commonly used in liquid coating compositions. Such aminoplast resins provide enhanced coating properties such as exterior durability, chemical resistance and mar resistance. 
     Attempts to produce powder coating compositions based on conventional aminoplast resins which exhibit these desirable properties heretofore have been unsatisfactory because these materials are typically in liquid form and, as such, cause poor powder stability. 
     The methoxylated aldehyde condensates of glycoluril, which are solid products, are the aminoplast resins most commonly employed as crosslinking agents in powder coating compositions. Although crystalline in form, these materials nonetheless can depress the Tg of the 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 also can result in poor powder stability. 
     Moreover, the use of conventional aminoplast resins in powder coating compositions can result in the phenomenon commonly referred to as “gassing”. “Gassing” occurs as a result of vaporization of the alcohol generated in the thermally induced aminoplast crosslinking reaction. The alcohol vapor is driven off through the coating film upon heating and, as the viscosity of the coating increases during the curing process, pinholes or craters are formed as the gas escapes through coating surface. 
     U.S. Pat. No. 3,759,854 discloses heat-fusible powder coating compositions prepared by pre-reacting a thermosetting polyester resin and a suitable crosslinking resin such as a condensation product of an aldehyde with melamine, urea or benzoguanamine. The reaction product typically is prepared from 1 to 50 weight percent of the amine-aldehyde material and the reaction is carried to an extent such that the resulting powder has a softening point of at least 65° C. 
     U.S. Pat. No. 5,302,462 discloses a process for preparing a partially cured, but non-gelled, powder coating composition. The powder coating composition is prepared by reacting a less than stoichiometric amount of methoxymethyl aminotriazine with a linear, hydroxyl-terminated polyester. The ratio of polyester to triazine ranges from 97:3 to 70:30 by weight. Methanol is stripped from the reaction mixture under reduced pressure. The powder coating composition provides a cured film free of surface defects caused by outgassing of methanol which volatilizes during the curing process. 
     U.S. Pat. No. 3,980,732 discloses a process for preparing a curable powder resin composition having a sintering temperature above 40° C. The method comprises condensing a methylolamino compound with an aliphatic alcohol and an aliphatic diamide to produce an aminoplast condensate with a T g  ranging from −10° C. to 100° C. and blending the aminoplast condensate with an acrylic or polyester resin having a glass transition temperature ranging from 60° C. to 100° C. The methylolamino compound is selected from methylolureas and methylolaminotriazines and contains no more than one unmethylolated NH bond per molecule. At least half of the methylol groups of the methylolamino compound have been condensed with the aliphatic alcohol or the aliphatic diamide. The molar ratio of the aliphatic alcohol to the methylolamino compound is at least 2; and the molar ratio of the methylolamino compound to the aliphatic diamide ranges from 1:0.5 to 1:1.5. 
     U.S. Pat. No. 4,185,045 discloses a powder coating composition comprising a solid crosslinking agent having a softening point ranging from 50° C. to 120° C. and prepared by heating 40 to 75% by weight of an acrylic polyol and 60 to 25% by weight of an alkoxyaminotriazine at 500 to 1200, and a base resin having a softening point ranging from 60° C. to 130° C. The alkoxyaminotriazine has less than 1 non-methylolated NH bond per triazine ring and at least 80% of the methylol groups have been etherified with an aliphatic or alicyclic alcohol or ethylene glycol monoalkyl ether. 
     U.S. Pat. No. 4,230,829 discloses a solid crosslinking agent having a softening point of 50° C. to 120° C. and prepared by heating 40 to 70% by weight of a polyester polyol and 60 to 30% by weight of an alkoxyaminotriazine. The alkoxyaminotriazine has one or less non-methylolated NH bond per triazine ring and at least 80% of the methylol groups have been alkoxylated with an alcohol. Powder coating compositions are prepared from a mixture of the crosslinking agent and a base resin having a softening point of 60° C. to 130° C. 
     While the above-described prior art aminoplast-based crosslinkers for powder coating compositions provide some improvement in “gassing” and powder stability over their liquid aminoplast counterparts, the powder coating compositions containing these crosslinkers can, nonetheless, exhibit some of the aforementioned deficiencies. In addition, many of the crosslinkers disclosed in the prior art are high molecular weight, high viscosity, partially cured, and, thereby, unstable mixtures. Thus, there remains a need for an aminoplast crosslinking agent suitable for use in powder coating compositions which provides a storage stable powder composition having the desirable coating properties usually associated with aminoplast-based liquid coatings without causing coating surface defects due to “gassing”. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, provided is a crosslinking agent comprising the ungelled reaction product of (a) a polyester polyol and (b) an aminotriazine compound having one or less non-alkylated NH bond per triazine ring. The polyester polyol (a) comprises the reaction product of (i) a cycloaliphatic polyol and (ii) a cyclic polycarboxylic acid or anhydride. The crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 10° C. 
     Also provided is a method for preparing the aforementioned crosslinking agent. The method comprises the steps of (I) combining the previously described polyester polyol and an aminotriazine compound having one or less non-alkylated NH bond per triazine ring to form a reaction admixture; (II) heating the reaction admixture to a temperature ranging from 90° C. to 135° C.; and (III) maintaining that temperature for a time sufficient to obtain an ungelled reaction proudct 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 (A) a polymer containing reactive functional groups and having a T g  of at least 30° C. and (B) the crosslinking agent described above. 
     Also provided is a multi-layer composite coating composition comprising a basecoat deposited from a film forming composition and a topcoat over at least a portion of 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 values, 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 aforementioned, the crosslinking agent of the present invention comprises the ungelled reaction product of (a) a polyester polyol and (b) an aminotriazine compound. The crosslinking agent is essentially free of hydroxyl functionality and has a Tg of at least 10° 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 polyester polyol (a) comprises the condensation reaction product of (i) a cycloaliphatic polyol and (ii) a cyclic polycarboxylic acid or anhydride. 
     The cycloaliphatic polyol (i) can be any of a variety of polyhydric cycloaliphatic compounds well known in the art. Suitable examples of cycloaliphatic polyols include those selected from the group consisting of hydrogenated Bisphenol A, hydrogenated Bisphenol F, hydrogenated Bisphenol E, M, P, Z, and the like cyclohexyl dimethanol, cyclohexane diol and mixtures thereof. Hydrogenated Bisphenol A is preferred. 
     The cyclic polycarboxylic acid or anhydride (ii) can be any cyclic compound having two or more carboxylic acid groups per molecule. Preferably, the cyclic polycarboxylic acid/anhydride (ii) is selected from the group consisting of hexahydrophthalic acid; phthalic acid, isophthalic acid, terephthalic acid, anhydrides thereof, and mixtures thereof. Hexahydrophthalic anhydride is preferred. 
     The polyester polyol (a) can be prepared by general condensation reaction techniques well-known in the art so long as the ratio of reactants (i) and (ii) and reaction conditions are such that the resulting condensation reaction product comprises two or more reactive hydroxyl groups per molecule. For purposes of the present invention, the molar ratio of the cycloaliphatic polyol (i) to the cyclic polycarboxylic acid or anhydride (ii) typically ranges from 1.0 to 2.5:1, preferably from 1.5 to 2.2:1, and more preferably from 1.8 to 2.2:1. 
     The polyester polyol (a) is prepared generally as follows. The acid/anhydride and alcohol(s) are reacted using condensation techniques well-known to the art while collecting distillate. The reaction is monitored by periodic sampling for acid value and hydroxyl number measurements. The reaction is terminated when these parameters meet the set requirements. 
     The polyester polyol (a) typically has a Mn ranging from 400 to 2000, preferably from 400 to 1500, and more preferably from 400 to 1000. Also, the hydroxyl value of the polyester polyol (a) typically is greater than 130, and preferably ranges from 130 to 190, and more preferably from 140 to 175. 
     Further, the polyester polyol (a) typically has a glass transition temperature of at least 30° C., preferably at least 35° C., and more preferably at least 40° C. Also, the polyester polyol (a) typically has a glass transition temperature less than 120° C., preferably less than 110° C., and more preferably less than 100° C. The T g  of the polyester polyol (a) can range between any combination of these values inclusive of the recited values. 
     The T g  of the polyol 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 . 
     As discussed above, the crosslinking agent of the present invention comprises the ungelled reaction product of the polyester polyol (a) described in detail above and an aminotriazine compound (b) which has one or less non-alkylated NH bond per triazine ring. 
     The aminotriazine compounds having one or less non-alkylated NH bond per triazine ring which are useful as component (b) in the preparation of the crosslinking agent of the present invention include aminoplast resins such as alkoxyalkyl derivatives of melamine, benzoguanimine, acetoguanamine, formoguanamine, spiroguanamine and the like. Typically, the aminotriazine component (b) comprises an (alkoxyalkyl) aminotriazine, with methoxymethyl aminotriazines being preferred. 
     Aminoplast resins are based on the 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 common and preferred herein. However, condensation products of other amines and amides can also be employed, for example, aldehyde condensates of triazines, diazines, triazoles, guanadines, guanamines and alkyl- and aryl-substituted derivatives of such compounds, including alkyl- and aryl-substituted ureas and alkyl- and aryl-substituted melamines. Some examples of such compounds are N,N′-dimethyl urea, benzourea, dicyandiamide, formaguanamine, acetoguanamine, glycoluril, ammeline, 2-chloro-4,6-diamino-1,3,5-triazine, 6-methyl-2,4-diamino-1,3,5-triazine, 3,5-diaminotriazole, triaminopyrimidine, 2-mercapto-4,6-diaminopyrimidine and 3,4,6-tris(ethylamino)-1,3,5 triazine. 
     While the aldehyde employed is most often formaldehyde, other similar condensation products can be made from other aldehydes, such as acetaldehyde, crotonaldehyde, acrolein, benzaldehyde, furfural and glyoxal. 
     The aminoplast resins preferably contain methylol or other alkylol groups, and in most instances, at least a portion of these alkylol groups are etherified by a reaction with an alcohol. Any monohydric alcohol can be employed for this purpose, including such alcohols as methanol, ethanol, propanol, butanol, pentanol, hexanol, cyclohexanol, heptanol and others, as well as, benzyl alcohol and other aromatic alcohols, cyclic alcohols such as cyclohexanol, monoethers of glycols, and halogen-substituted or other substituted alcohols, such as 3-chloropropanol and butoxyethanol. Commonly employed aminoplast resins are substantially alkylated with methanol or butanol. 
     Preferred aminoplast resins for use as the aminotriazine compound (b) in the preparation of the crosslinking agent of the present invention are the highly alkylated, low imino aminoplast resins which have a degree of polymerization (“DP”) of less than 1.5. Generally, the number average degree of polymerization is defined as the average number of structural units per polymer chain (see George Odian,  Principles of Polymerization , John Wiley &amp; Sons (1991)). For purposes of the present invention, a DP of 1.0 would indicate a completely monomeric triazine structure, while a DP of 2.0 indicates two triazine rings joined by a methylene or methylene-oxy bridge. It should be understood that the DP values reported herein and in the claims represent average DP values as determined by gel permeation chromatography data. 
     Preferred aminotriazine compounds include (methoxymethyl)melamine-formaldehyde resin, for example CYMEL® 300; ethylated-methylated benzoguanimine-formaldehyde resin, for example CYMEL® 1123; and methylated-butylated melamine-formaldehyde resin, for example CYMEL® 1135, all of which are commercially available from Cytec Industries, Inc. The present invention is also directed to a method for preparing the above-described crosslinking agent. The polyester polyol (a) and the aminotriazine compound (b) are combined in a suitably equipped reaction vessel, typically 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. Non-limiting examples of strong acids suitable for use as a catalyst include, but are not limited to, para-toluene sulfonic acid, dodecyl benzene sulfonic acid anddodecyl benzene disulfonic acid. Normal condensation techniques as are well-known in the art can be used. The reaction admixture is heated to a temperature ranging from 900 to 135° C., preferably from 100° C. to 130° C., and held at that temperature for a period sufficient to obtain an ungelled product having a Tg of at least 10° C. The reaction is terminated when the end point (i.e., the disappearance of the OH signal) is detected by infrared spectroscopy. 
     In the preparation of the crosslinking agent of the present invention, the polyester polyol (a) and the aminotriazine compound (b) are combined in a weight ratio such that the aminotriazine compound (b) is in excess. This results in a stable crosslinking agent which 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 unchanged during the reaction, for example, the carbonyl signal) via infrared spectroscopy. 
     Typically, the polyester polyol (a) constitutes 45 weight percent or less, preferably 40 weight percent or less, and more preferably 35 weight percent or less of the reaction mixture, based on the total combined weight of the polyester polyol component (a) and the aminotriazine compound (b). Also, the polyester polyol (a) typically constitutes at least 10 weight percent, preferably at least 15 weight percent, and more preferably at least 20 weight percent of the reaction mixture, based on the total combined weight of the polyester polyol component (a) and the aminotriazine compound (b). The percent by weight of the polyester polyol (a) present in the reaction mixture can range between any combination of these values inclusive of the recited values. 
     Typically the aminotriazine compound (b) constitutes 90 weight percent or less, preferably 85 weight percent or less, and more preferably 80 weight percent or less of the reaction mixture, based on the total combined weight of the polyester polyol component (a) and the aminotriazine compound (b). Also, the aminotriazine compound (b) typically constitutes at least 55 weight percent, preferably at least 60 weight percent, and more preferably at least 65 weight percent of the reaction mixture, based on the total combined weight of the polyester polyol component (a) and the aminotriazine compound (b). The percent by weight of the aminotriazine compound (b) present in the reaction mixture can range between any combination of these values inclusive of the recited values. 
     The crosslinking agent of the present invention typically has a glass transition temperature of at least 10° C., preferably at least 15° C., more preferably at least 20° C., and even more preferably at least 25° C. Also, the crosslinking agent typically has a glass transition temperature less than 85° C., preferably less than 80° C., more preferably less than 75° C., and even more preferably less than 70° C. The glass transition temperature of the crosslinking agent can range between any combination of these values, inclusive of the recited values. 
     As mentioned above, 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, and (B) the crosslinking agent described in detail 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 reactant (A), at least one reactive functional group-containing polymer having a glass transition temperature of at least 30° C., e.g., a hydroxyl and/or an epoxide functional acrylic polymer, and as a second reactant (B), the crosslinking agent described above. The reactants (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 valves 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 or a combination thereof. In one embodiment of the present invention, the polymer (A) 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. Ethylenically unsaturated carboxylic acid functional monomers, for example acrylic acid and/or methacrylic acid, can also be used when a carboxylic acid functional acrylic polymer is desired. Non-limiting examples of suitable other polymerizable ethylenically unsaturated monomers include vinyl aromatic compounds, such as styrene and vinyl toluene; nitriles, such as acrylonitrile and methacrylonitrile; vinyl and vinylidene halides, such as vinyl chloride and vinylidene fluoride and vinyl esters, such as vinyl acetate. 
     In one embodiment of the present invention, the acrylic polymers contain hydroxyl functionality which can be incorporated into the acrylic polymer through the use of hydroxyl functional monomers such as hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate and hydroxypropyl methacrylate which may be copolymerized with the other acrylic monomers mentioned above. 
     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 methacrylate and allyl glycidyl ether, and a saturated carboxylic acid, such as a saturated monocarboxylic acid, for example, isostearic acid. 
     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(isobutylronitrile). 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 can include the reaction product of hydroxyethyl methacrylate, isophorone diisocyanate and hydroxypropyl carbamate. Still 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. Pat. 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. Pat. 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, isobornyl (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. Pat. No. 5,407,707 at column 2, lines 17 through 56, which disclosure is incorporated herein by reference. In one embodiment of the present invention, the epoxide functional acrylic polymer is prepared from a majority of (meth)acrylate monomers. 
     The functional group-containing acrylic polymer typically has a 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 can include linear or branched polyesters having hydroxyl, carboxyl 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, trishydroxyethylisocyanurate and the like. Branched polyols, such as trimethylolpropane, are preferred 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 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 a 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 straight chain aliphatic diisocyanates, such as 1,6-hexamethylene diisocyanate. Also, cycloaliphatic diisocyanates can be employed. Examples include isophorone diisocyanate 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 a 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 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 (I) and (II):                    
     or 
     where the substituent R 1  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 (III) 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 0 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 preferably have a number average molecular weight (Mn) ranging from 500 to 30,000 and more 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. 
     It should be understood that the preferred carbamate functional group-containing polymers typically contain residual hydroxyl functional groups which provide additional crosslinking sites. Preferably, the functional group-containing polymer (A) has a hydroxyl value ranging from 10 to 150, more preferably from 10 to 100; and even more preferably from 10 to 60 (mg 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 resin solids in the film-forming composition. The functional group-containing polymer (A) 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 80 percent by weight, more preferably less than 75 percent by weight, and even more preferably less than 70 percent by weight based on the total weight of the powder coating composition. The amount of the functional group-containing polymer (A) present in the 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 powder coating compositions of the present invention can also 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) described above. Non-limiting examples of suitable adjuvant curing agents include blocked isocyanates, triazine compounds, glycoluril resins, and mixtures thereof. 
     The blocked isocyanates suitable for use as the adjuvant curing agent in the 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 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 ε-caprolactam; toluene 2,4-toluene diisocyanate blocked with ε-caprolactam; or phenol-blocked hexamethylene diisocyanate. The blocked isocyanates mentioned immediately above are described in detail in U.S. Pat. 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, N.J. 
     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 POWDERLINK® 1174 commercially available from Cytec Industries, Inc. of Stamford, Conn. 
     When employed, the adjuvant curing agent typically is present in the powder coating compositions of the present invention in an amount ranging from 5 to 10 percent by weight, preferably from 5 to 20 percent by weight, more preferably from 5 to 30 percent by weight, and even more preferably from 5 to 50 percent by weight based on the total weight of the powder coating composition. 
     Also suitable for use as an adjuvant curing agent in the powder coating compositions of the present invention are triazine compounds, such as the tricarbamoyl triazine compounds described in detail in U.S. Pat. No. 5,084,541. When used, the triazine curing agent is typically present in the 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 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 can be generated upon reaction of each epoxy functional group with a functional 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 an 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 crosslinking 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 two functional groups that are reactive with epoxide functionality. 
     Preferably, the epoxide reactive crosslinking 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,                    
     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 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 of the present invention comprising an epoxide functional polymer as reactant (A) and an epoxide reactive curing agent typically contain the crosslinking agent (B) in an amount ranging from 1 to 60 weight percent, preferably from 2 to 40 weight percent and more preferably from 5 to 20 weight percent based on total weight of the powder coating composition. 
     The 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; pigments and dyes as colorants; UV light stabilizers such as TINUVIN® 123 or TINUVIN® 900 available from Ciba Specialty Chemicals Corporation 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 powder coating compositions of the invention are typically prepared by blending the functional group-containing polymer (A) and the crosslinking agent (b) for approximately 1 minute in a Henschel blade blender. The powder is then usually catalyzed and 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 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 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”. In one embodiment, the present invention is directed to a curable powder coating composition which is advantageously employed as a topcoat in a multi-layer composite coating composition. Such a multi-layer composite coating composition generally comprises a film-forming basecoat which is typically deposited from a pigmented film-forming composition and a topcoat applied over the basecoat, the topcoat being deposited from the curable powder coating composition of the present invention as described above. In a preferred embodiment, the multi-component composite coating composition is a color-plus-clear system where the topcoat is deposited from a powder coating composition which is substantially pigment-free, 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. Pat. No. 4,220,679. Water-based coating compositions, such as those described in U.S. Pat. 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 can 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 curable powder topcoating composition can be applied over at least a portion of the basecoat by any of the methods of application described above. As discussed above, the curable powder topcoating composition can be applied to a cured or a dried basecoat before the basecoat has been cured. In the latter case, the topcoat 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 A describes the two-step preparation of a crosslinking agent of the present invention. Comparative Example 1 describes the preparation of a conventional powder coating composition based on an epoxy functional acrylic resin and an acid functional polyester crosslinking agent. Comparative Example 2 describes the preparation of an analogous powder coating composition which further includes a conventional glycoluril crosslinking agent (for crosslinking with the secondary hydroxyl groups generated upon reaction of the epoxy and acid groups). Example 3 describes the preparation of powder coating composition of the present invention which includes the crosslinking agent of Example A as a replacement for the glycoluril crosslinking agent in Comparative Example 2. Comparative Example 4 describes the preparation of a powder coating composition based upon a hydroxyl functional polyester resin and a conventional glycoluril crosslinking agent. Example 5 describes the preparation of the analogous powder coating composition of the present invention containing the crosslinking agent of Example A as a replacement for the glycoluril crosslinking agent in Comparative Example 4. 
     Example A 
     This example describes the preparation of a crosslinking agent of the present invention. The crosslinking agent was prepared in two steps as described below. 
     Example A-1 
     Preparation of the Polyester Polyol 
     A polyester polyol was prepared as follows. Added to a two-liter, four-necked reaction vessel equipped with a thermometer, stirrer, nitrogen inlet, and a separator were 955.0 parts of hydrogenated Bisphenol-A, 308.0 parts of hexahydrophthalic anhydride, 1.3 parts of di-butyl tin oxide, and 1.3 parts of triisodecylphosphite. The reaction mixture was melted by heating, then further heated to a temperature of 230° C. while water resultin from the condensation reaction was removed through the separator. When an acid value of 2 was achieved, the reaction was terminated. The polyester polyol thus prepared had a hydroxyl number of 160 and Tg of 65° C. 
     Example A-2 
     Preparation of the Crosslinker 
     The crosslinker was prepared as follows. Into a three-liter, four-necked reaction vessel equipped with a thermometer, stirrer, nitrogen inlet, and means for removing the reaction by-product (methanol) were added 768.0 parts of CYMEL® 300, 426.9 parts of the polyester polyol of Example A-1 above, 512.2 parts of xylene, and 0.34 part of p-toluenesulfonic acid. The admixture was heated to 80° C. and held at that temperature for a period of 20 minutes at which time the reaction mixture became homogenous. The reaction mixture was then further heated to a temperature of 118° C. and held at that temperature as the methanol by-product was removed from the system. The reaction was monitored by infrared spectroscopy for the disappearance of hydroxyl signal (3200-3600 cm −1 ) terminated when this end point was detected. The mixture was subsequently cooled to a temperature of 100° C. at which time 0.94 part by weight of sodium carbonate was added. The reaction mixture was further stirred at a temperature of 100° C. for one hour before being filtered through a 5 micron filtration bag. Thereafter, the mixture was concentrated in vacuo at a temperature of 100° to 130° C. at a pressure of 3 to 50 mm Hg to remove the xylene solvent. The reaction product thus obtained was a pale yellow solid with a softening temperature of 40° C. 
     Powder Coating Compostions 
     Examples 1 through 5 
     Comparative Example 1 describes the preparation of a conventional powder coating composition based on an epoxy functional acrylic resin and an acid functional polyester crosslinking agent. Comparative Example 2 describes the preparation of an analogous powder coating composition which further includes a conventional glycoluril crosslinking agent (for crosslinking with the secondary hydroxyl groups generated upon reaction of the epoxy and acid groups). Example 3 describes the preparation of powder coating composition of the present invention which includes the crosslinking agent of Example A as a replacement for the glycoluril crosslinking agent in Comparative Example 2. Comparative Example 4 describes the preparation of a powder coating composition based upon a hydroxyl functional polyester resin and a conventional glycoluril crosslinking agent. Example 5 describes the preparation of the analogous powder coating composition of the present invention containing the crosslinking agent of Example A as a replacement for the glycoluril crosslinking agent in Comparative Example 4. The powder coating compositions were prepared from a mixture of the following ingredients: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                 EXAMPLE 5 
                 EXAMPLE 6 
                   
                 EXAMPLE 8 
                   
               
               
                   
                 (Comparative) 
                 (Comparative) 
                 EXAMPLE 7 
                 (Comparative) 
                 EXAMPLE 9 
               
               
                   
                 (weight in 
                 (weight in 
                 (weight in 
                 (weight in 
                 (weight in 
               
               
                 INGREDIENTS 
                 grams) 
                 grams) 
                 grams) 
                 grams) 
                 grams) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 CRYLCOAT 291 1   
                 — 
                 — 
                 — 
                 270 
                 270 
               
               
                 ALBESTER 
                 270 
                 270 
                 270 
                 — 
                 — 
               
               
                 5060 2   
               
               
                 Epoxy Functional 
                 50 
                 50 
                 50 
                 — 
                 — 
               
               
                 Acrylic Resin 3   
               
               
                 POWDERLINK 
                 — 
                 17 
                 — 
                 15 
                 — 
               
               
                 1174 
               
               
                 Crosslinking 
                 — 
                 — 
                 56 
                 — 
                 47 
               
               
                 agent of 
               
               
                 Example A 
               
               
                 MODAFLOW PL 
                 4 
                 4 
                 4 
                 4 
                 4 
               
               
                 200 4   
               
               
                 Benzoin 
                 3.5 
                 3.5 
                 3.5 
                 3.5 
                 3.5 
               
               
                 MICROWAX C 
                 3.0 
                 3.0 
                 3.0 
                 3.0 
                 3.0 
               
               
                 Titanium dioxide 6   
                 170 
                 170 
                 170 
                 170 
                 170 
               
               
                   
               
               
                   1 A hydroxyl functional polyester resin commercially available from UCB.  
               
               
                   2 An acid functional polyester resin having an acid number of approximately 30, commercially available from McWhorther Technologies, Inc.  
               
               
                   3 Epoxy functional acrylic resin prepared from 50% glycidyl methacrylate, 35% methylmethacrylate, 10% butyl methacrylate, 5% styrene, using t-amyl peracetate as initiator.  
               
               
                   4 Acrylic flow additive available from Monsanto Co.  
               
               
                   5 R706 commercially available from E.I. DuPont de Nemours and Company.  
               
            
           
         
       
     
     Each of the above-described powder coating compositions was prepared as follows. For each of the powder coating compositions of Examples 1 through 4, 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&amp;p Process Equipment and Systems, by way of an ACCU-RATE auger feeder. 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). Two panel sets were prepared wherein the coating compositions were cured at two different cure temperatures. One panel set was cured at 320° F. (160° C.) for 20 minutes, and another set was cured at 380° F. (193.3° C.) for 20 minutes. 
     Testing Procedures: 
     The powder storage stability of each powder coating composition was evaluated by storing a 20 g 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. Powder stability results are reported from best to worst as follows: excellent, good, slightly cakey, cakey, slightly clumpy, clumpy, fused and sintered. 
     The propensity of the coating composition to “gas” upon curing was tested by increasing the cured film thickness of the powder coating on a test panel until surface defects (i.e., pinholes) formed due to the escape of gases through the coating surface during the cure process. Values reported represent the maximum film thickness achieved just prior to the development of the pinholes in the coating surface. 
     Chemical resistance and extent of cure was evaluated by double rubs using methyl ethyl ketone. Results reported are the extent of film surface marring or softening in the area contacted with the methyl ethyl ketone after 200 double rubs. 
     Flexibility and impact resistance (both direct and reverse impact) was evaluated using a Gardner Impact Tester in accordance with ASTM-D-2794. 
     Two sets of test panels were prepared. Test results are reported in the following Table 1 for the set of test panels coated with each of the powder coating compositions of Examples 1 through 5 which were cured at 380° F. (193.3° C.) for 30 minutes. Test results for an analogous set of coated test panels which were cured at 320° F. (160° C.) for 30 minutes are reported below in the following Table 2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 (380° F. (193.3° C.) for 30 minutes) 
               
            
           
           
               
               
               
               
               
               
            
               
                 TEST 
                 Example 1 
                 Example 2 
                   
                 Example 4 
                   
               
               
                 PERFORMED 
                 (Comparative) 
                 (Comparative) 
                 Example 3 
                 (Comparative) 
                 Example 5 
               
               
                   
               
               
                 MEK double 
                 slight mar 
                 slight mar 
                 no mar 
                 5 double rubs 
                 no mar 
               
               
                 rubs 
                   
                   
                   
                 coating 
               
               
                   
                   
                   
                   
                 removed 
               
               
                 Gassing 
                 &gt;8 mils 
                 &gt;8 mils 
                 &gt;8 mils 
                 &gt;8 mils 
                 &gt;8 mils 
               
               
                   
                 (&gt;200 
                 (&gt;200 
                 (&gt;200 
                 (&gt;200 
                 (&gt;200 
               
               
                   
                 micrometers) 
                 micrometers) 
                 micrometers) 
                 micrometers) 
                 micrometers) 
               
               
                 Powder 
                 slightly cakey 
                 sintered 
                 Clumpy 
                 sintered 
                 clumpy 
               
               
                 stability 
               
               
                 Direct impact 
                   30 
                 30 
                 &gt;160 
                 &lt;10 
                 &gt;160 
               
               
                 (in-lbs) 
               
               
                 Reverse 
                 &lt;20 
                 30 
                 &gt;160 
                 &lt;10 
                 &gt;160 
               
               
                 impact 
               
               
                 (in-lbs) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 (320° F. (160° C.) for 30 minutes) 
               
            
           
           
               
               
               
               
               
               
            
               
                 TEST 
                 Example 1 
                 Example 2 
                   
                 Example 4 
                   
               
               
                 PERFORMED 
                 (Comparative) 
                 (Comparative) 
                 Example 3 
                 (Comparative) 
                 Example 5 
               
               
                   
               
               
                 MEK double 
                 slight mar 
                 slight mar 
                 no mar 
                 5 double rubs 
                 slight mar 
               
               
                 rubs 
                   
                   
                   
                 coating 
               
               
                   
                   
                   
                   
                 removed 
               
               
                 Gassing 
                 &gt;8 mils 
                 &gt;8 mils 
                 &gt;8 mils 
                 &gt;8 mils 
                 &gt;8 mils 
               
               
                   
                 (&gt;200 
                 (&gt;200 
                 (&gt;200 
                 (&gt;200 
                 (&gt;200 
               
               
                   
                 micrometers) 
                 micrometers) 
                 micrometers) 
                 micrometers) 
                 micrometers) 
               
               
                 Powder 
                 slightly cakey 
                 sintered 
                 Clumpy 
                 sintered 
                 clumpy 
               
               
                 stability 
               
               
                 Direct impact 
                   30 
                 30 
                 &gt;160 
                 &lt;10 
                 &gt;160 
               
               
                 (in-lbs) 
               
               
                 Reverse 
                 &lt;20 
                 30 
                 &gt;160 
                 &lt;10 
                 &gt;160 
               
               
                 impact 
               
               
                 (in-lbs) 
               
               
                   
               
            
           
         
       
     
     The data presented in Tables 1 and 2 above illustrate that the crosslinking agents of the present invention provide powder coating compositions having improved impact resistance and powder stability over those compositions containing a 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.