Curable coating composition

Abstract: A curable coating composition is described comprising two components that are reactive with one another upon curing to form urethane linkages. The composition further comprises a third component that includes one or more epoxide groups. Coatings prepared with this coating composition can be cured and coated with additional coating(s), providing good intercoat adhesion to the subsequent coating.

Description: Field of the Invention This invention relates to curable coating compositions, and in particular to compositions to produce urethane-crosslinked coatings for use in multilayer coatings. Background of the Invention Coating compositions are often divided into thermoplastic and thermoset. Thermoplastic coating compositions utilize high molecular weight polymers dispersed in an organic or aqueous solvent. After the coating composition is applied to a substrate, the solvent is removed and the polymers set up to form a film. Thermoset or curable coating compositions utilize two components that are reactive with one another under certain cure conditions. The reactive groups on these components are referred to as 'functional groups'. After the composition containing these components is applied, the coated substrate is subjected to cure conditions, causing the functional groups to react and form a cured film of a crosslinked matrix. Various combinations of functional groups have been used in the art for curable coating compositions. One widely used combination utilizes OH-functional groups and the alkylol groups on aminoplast resins. These functional groups react to form ether bridges in the crosslinked coating. Another combination utilizes acid functional groups and epoxy functional groups, which react to form ester bridges in the crosslinked coating. Coating compositions that comprise two components that are reactive with one another and a third component are known for example from the JP 61,258,869, wherein acrylic polymers, epoxy resins and polyisocyanates are reacted with each other, or the US 4,169,825, wherein hydroxy-containing polyesters, epoxy resin and crosslinker are reacted with each other. However these compositions do not feature a polymeric component that utilises carbamate functionality. The compositions according to JP 61,258,869 do not feature an improved intercoat adhesion either. Further the compositions according to US 4,169,825 use carbamate compounds that are conventional blocked isocyanates, which means that the cured polymemetwork has a higher density and a lower branching value than that which is obtainable with polymeric carbamate-functional compounds. Curable coating compositions that cure via the formation of urethane bridges in the crosslinked coating have also been used in the art. Urethane bonds are often desirable for their durability, resistance to attack by various agents in the environment, impact resistance, and other physical properties such as stress release. Urethane bridges can be formed by various combinations of functional groups, such as OH functional groups and isocyanate functional groups, carbamate groups and aminoplast resins, or cyclic carbonate groups and amino groups. In many coating applications, it is often necessary to apply an additional coating on top of an already-cured coating. This may be done to achieve certain visual or physical properties, or it may be necessary to repair areas where the cured coating has been damaged or where coating defects exist. In such cases, it is important that the coating applied on top of the cured coating have adequate adhesion to the cured coating. Even when the cured coating is sanded prior to application of the additional coating, the adhesion factor is still a concern with regard to overspray areas onto unsanded coating surfaces. Intercoat adhesion can be of particular concern with regard to cured urethane coatings. Accordingly, the present invention is directed toward such a curable coating composition having good intercoat adhesion properties. Summary of the Invention The present invention provides a curable coating composition comprising two components that are reactive with one another upon curing to form urethane linkages, wherein the two components that are reactive with one another are: (A) a component comprising a plurality of primary carbamate groups, and (B) a component comprising a plurality of groups that are reactive with the carbamate groups, and said composition further comprising a third component that includes one or more epoxide groups. Coatings prepared with this coating composition can be cured and coated with additional coating(s), providing good intercoat adhesion to the subsequent coating. Description of the Preferred Embodiments Various combinations of components can be used as the two components that are reactive with each other to form urethane linkages according to the present invention. As used herein, the term "urethane linkage" refers to a linkage in the matrix of a cured coating having the formula: where R is H, substituted or unsubstituted alkyl preferably of 1 to 6 carbon atoms, or substituted or unsubstituted cycloalkyl, preferably up to 6 ring carbon atoms. The present invention utilizes a carbamate-functional material having a plurality of primary carbamate groups as one of the components. A variety of carbamate-functional materials may be used. These include materials described in WO 94/10211 and US 5,356,669, Following will be described general procedures for the manufacturing of carbamate-functional compounds, it being understood however, that according to the instant invention only compounds having a plurality of primary carbamate groups are to be used. A carbamate-functional polymer may be used as the carbamate-functional material in the practice of the present invention. Carbamate-functional polymer components used in the composition of the invention can be prepared in a variety of ways. One way to prepare such polymers is to prepare an acrylic monomer having a carbamate functionality in the ester portion of the monomer. Such monomers are well-known in the art and are described, for example in U.S. Patents 3,479,328, 3,674,838, 4,126,747, 4,279,833, and 4,340,497. One method of synthesis involves reaction of a hydroxy ester with urea to form the carbamyloxy carboxylate (i.e., carbamate-modified acrylic). Another method of synthesis reacts an α,β-unsaturated acid ester with a hydroxy carbamate ester to form the carbamyloxy carboxylate. Yet another technique involves formation of a hydroxyalkyl carbamate by reacting a primary or secondary amine or diamine with a cyclic carbonate such as ethylene carbonate. The hydroxyl group on the hydroxyalkyl carbamate is then esterified by reaction with acrylic or methacrylic acid to form the monomer. Other methods of preparing carbamate-modified acrylic monomers are described in the art, and can be utilized as well. The acrylic monomer can then be polymerized along with other ethylenically-unsaturated monomers, if desired, by techniques well-known in the art. An alternative route for preparing carbamate-functional polymers is to react an already-formed polymer such as an acrylic polymer with another component to form a carbamate-functional group appended to the polymer backbone, as described in U.S. Patent 4,758,632. One technique for preparing carbamate-functional polymers involves thermally decomposing urea (to give off ammonia and HNCO) in the presence of a hydroxy-functional acrylic polymer to form a carbamate-functional acrylic polymer. Another technique involves reacting the hydroxyl group of a hydroxyalkyl carbamate with the isocyanate group of an isocyanate-functional acrylic or vinyl monomer to form the carbamate-functional acrylic. Isocyanate-functional acrylics are known in the art and are described, for example in U.S. Patent 4,301,257. Isocyanate vinyl monomers are well-known in the art and include unsaturated m -tetramethyl xylene isocyanate (sold by American Cyanamid as TMI®). Yet another technique is to react the cyclic carbonate group on a cyclic carbonate-functional acrylic with ammonia in order to form the carbamate-functional acrylic. Cyclic carbonate-functional acrylic polymers are known in the art and are described, for example, in U.S. Patent 2,979,514. A preferred approach is a transcarbamylation or transesterification reaction of a hydroxy-functional polymer with an alkyl carbamate or hydroxyalkyl carbamate. A more difficult, but feasible way of preparing the polymer would be to transesterify an acrylate polymer with a hydroxyalkyl carbamate. Other polymers may be used as well. For example, a carbamate-functional polyurethane can be prepared as is described in U.S. patent application Serial No. 08/098,169. A carbamate-functional polyester can be prepared as described in JP 51/4124. Carbamate-functional polymers can have a molecular weight of 2000-20,000, and preferably from 4000-6000. Molecular weight as used herein means weight average molecular weight, and can be determined by the GPC method using a polystyrene standard. The carbamate content of the polymer, on a molecular weight per equivalent of carbamate functionality, will generally be between 200 and 1500, and preferably between 300 and 500. One class of carbamate-functional polymer component can be represented by randomly repeating units according to the following formula: In the above formula, R 1 represents H or CH 3 . R 2 represents H. The values x and y represent weight percentages, with x being 10 to 90 % and preferably 40 to 60 %, and y being 90 to 10 % and preferably 60 to 40 %. In the above formula, A represents repeat units derived from one or more ethylenically unsaturated monomers. Such monomers for copolymerization with acrylic monomers are known in the art. They include alkyl esters of acrylic or methacrylic acid, e.g., ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, butyl methacrylate, isodecyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate and vinyl monomers such as unsaturated m -tetramethyl xylene isocyanate (sold by American Cyanamid as TMI®), styrene, vinyl toluene, L represents a divalent linking group, preferably an aliphatic of 1 to 8 carbon atoms, cycloaliphatic, or aromatic linking group of 6 to 10 carbon atoms. Examples of L include -(CH 2 )-, -(CH 2 ) 2 -, -(CH 2 ) 4 -, In one preferred embodiment, -L- is represented by -COO-L'- where L' is a divalent linking group. Thus, in a preferred embodiment of the invention, the carbamate-functional polymer component is represented by randomly repeating units according to the following formula: In this formula, R 1 , R 2 , A, x, and y are as defined above. L' may be a divalent aliphatic linking group, preferably of 1 to 8 carbon atoms, e.g., -(CH 2 )-, -(CH 2 ) 2 -, -(CH 2 ) 4 - or a divalent cycloaliphatic linking group, preferably up to 8 carbon atoms, e.g., cyclohexyl, However, other divalent linking groups can be used, depending on the technique used to prepare the polymer. For example, if a hydroxyalkyl carbamate is adducted onto an isocyanate-functional acrylic polymer, the linking group L' would include an -NHCOO- urethane linkage as a residue of the isocyanate group. Lower molecular weight carbamate-functional materials, such as oligomeric or non-polymeric materials may also be used in the practice of the of the present invention, as long as they have a plurality of primary carbamate groups. Such compounds can be prepared in a variety of ways. One way to prepare such carbamate-functional is to react an alcohol ('alcohol' is defined herein as having more than one OH group) with a urea to form a compound with carbamate groups. This reaction is accomplished by heating a mixture of the alcohol and urea. Another technique is the reaction of an alcohol with cyanic acid to form a compound with primary carbamate groups(s) (i.e., unsubstituted carbamates). This reaction is also performed under heat, preferably in the presence of a catalyst as is known in the art. Carbamates may also be prepared by reaction of an alcohol with phosgene and then ammonia to form a compound having primary carbamate group(s). Another approach is to react an isocyanate (e.g., HDI, IPDI) with a compound such as hydroxypropyl carbamate to form a carbamate-capped isocyanate derivative. Finally, carbamates can be prepared by a transcarbamylation approach where an alcohol or hydroxyalkyl carbamate is reacted with an alkyl carbamate (e.g., methyl carbamate, ethyl carbamate, butyl carbamate) to form a primary carbamate group-containing compound. This reaction is performed under heat, preferably in the presence of a catalyst such as an organometallic catalyst (e.g., dibutyltin dilaurate). Other techniques for preparing carbamates are also known in the art and are described, for example, in P. Adams & F. Baron, "Esters of Carbamic Acid", Chemical Review , v. 65, 1965. Various alcohols can be used in the preparation of carbamate compounds useful in the practice of the invention. They generally have from 1 to 160 carbon atoms, preferably 1-60 carbon atoms, and may be monofunctional or polyfunctional (preferably a functionality of 2 to 3), aliphatic, aromatic, or cycloaliphatic. They may contain just OH groups, or they may contain OH groups plus heteroatoms such as O, S, Si, N, P, and other groups such as ester groups, ether groups, amino groups, or unsaturated sites. Examples of useful alcohols include 1,6-hexanediol,1,2-hexanediol, 2-ethyl-1,3-hexanediol, ethyl-propyl-1,5-pentanediol, 2-methyl-2,4-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 2,4,7,9-tetramethyl-5-decyn-4,7-diol, 1,3-dihydroxyacetone dimer, 2-butene-1,4-diol, pantothenol, dimethyltartrate, pentaethylene glycol, dimethyl silyl dipropanol, and 2,2'-thiodiethanol. groups. Another method of synthesis is to first react the isocyanate groups on a polyisocyanate with a compound having a group that is reactive with isocyanate and also a non-NCO functional group. This adduct is then reacted with a compound comprising at least one carbamate group or group that can be converted to carbamate and at least one group reactive with the non-NCO functional groups. Examples of non-NCO functional groups include carboxyl, epoxy, hydroxyl, amino. For example, an OH-functional adduct (which can be formed by reacting a polyisocyanate with an amino alcohol) can be reacted with the oxygen of a COO portion of the carbamate group or an alkyl carbamate or with the methylol group of methylol acrylamide (HO-CH 2 -NH-CO-CH=CH 2 ). In the case of the COO group on an alkyl carbamate, the hydroxyl group on the polyurethane undergoes a transesterification with the Coo group, resulting in the carbamate group being appended to the polyurethane. In the case of methylol acrylamide, the unsaturated double bond is then reacted with peroxide to form an epoxy group. The epoxy groups are then reacted with CO 2 to form cyclic carbonate groups, which are converted to carbamate groups by reaction with ammonia. Alternatively, an acid-functional polyisocyanate (which can be formed by reaction of a polyisocyanate with a hydroxy-functional carboxylic acid) can be reacted with acetic anhydride to generate an anhydride-functional triisocyanurate, which can then be reacted with an hydroxyalkylcarbamate. The above-described polyisocyanates are adducted with compounds containing a carbamate group or group that can be converted to carbamate and a group that is reactive with the NCO- or non-NCO-functional group on the polyisocyanate. Carbamate-containing compounds that can be adducted onto the NCO groups of a diisocyanate or an isocyanurate are preferably active hydrogen-containing carbamates such as hydroxyalkyl carbamates (e.g., hydroxypropyl carbamate or hydroxyethyl carbamate). Compounds containing groups that can be converted to carbamate and groups that are reactive with NCO include active hydrogen-containing cyclic carbonate compounds convertible to carbamate by reaction with ammonia (e.g., the reaction product of glycidol and CO 2 ), monoglycidyl ethers (e.g., Cardura E®) convertible to carbamate by reaction with CO 2 and then ammonia, and monoglycidyl esters (e.g., the reaction product of a carboxylic acid and epichlorohydrin) convertible to carbamate by reaction with CO 2 and then ammonia, allyl alcohols where the alcohol group is reactive with NCO and the double bond can be converted to carbamate by reaction with peroxide, and vinyl esters where the ester group is reactive with NCO and the vinyl group can be converted to carbamate by reaction with peroxide, then CO 2 , and then ammonia. Non-polymeric or oligomeric carbamate-functional compounds will generally have a molecular weight of 75-2000, and preferably from 75-1500. As used herein, molecular weight means weight average molecular weight. Molecular weight can be determined by the GPC method. A number of materials can be used as the component to react with carbamate to form a urethane linkage as defined above. These include melamine formaldehyde resin (including monomeric or polymeric melamine resin and partially or fully alkylated melamine resin), urea resins (e.g., methylol ureas such as urea formaldehyde resin, alkoxy ureas such as butylated urea formaldehyde resin), polyanhydrides (e.g., polysuccinic anhydride), phenol/formaldehyde adducts, and polysiloxanes (e.g., trimethoxy siloxane). Aminoplast resin such as melamine formaldehyde resin or urea formaldehyde resin are especially preferred. Even more preferred are aminoplast resins where one or more of the amino nitrogens is substituted with a carbamate group for use in a process with a curing temperature below 150°C, as described in U.S. patent 5,300,328. The composition of the present invention also comprises a component that includes one or more epoxide groups. Epoxides are well-known in the art. The epoxide may be of the general formula: where R1, R2, R3, and R4 are each independently H (with the proviso that at least one of R1-R4 is other than H), an organic radical, which may be polymeric or non-polymeric and may contain unsaturation and/or heteroatoms, or one of R1 or R2 together with one of R3 or R4 may form a cyclic ring, which may contain unsaturation and/or heteroatoms. Although essentially any epoxide can be used in the practice of the present invention, the epoxide is preferably substantially free of groups that are reactive with either of the two components that are reactive with one another to form urethane linkages. By 'substantially free' of such groups, it is meant that the degree of reaction between either of the two components that are reactive to form urethane linkages and any reactive component on the epoxide is sufficiently low so as to avoid any undesired adverse impact on the intercoat adhesion properties of the coating. Useful epoxides can be prepared from alcohols, e.g., butanol, trimethylol propane, by reaction with an epihalohydrin (e.g., epichlorohydrin), or by reactio of an allyl group with peroxide. Oligomeric or polymeric polyepoxides, such as acrylic polymers or oligomers containing glycidyl methacrylate or epoxy-terminated polyglycidyl ethers such as the diglycidyl ether of bisphenol A (DGEBPA), can also be used. Epoxidized polyurethane resins or polyester resins can be prepared by reacting OH group-containing polyurethanes or polyesters, as are known in the art, with an epihalohydrin. Epoxides can also be prepared by reacting an isocyanate-terminated component such as a monomeric polyisocyanate or polymer or oligomer with glycidol. Other known polyepoxides, e.g., epoxy-novolacs, may also be used. In one preferred embodiment, the epoxide is an acrylic-containing polymer or oligomer, preferably deriving its epoxy groups from glycidyl methacrylate monomer, glycidyl acrylate, allyl glycidyl ether, cyclohexyl monoepoxyy methacrylate, the epoxide of the dimer of cylopentadiene methacrylate, or epoxidized butadiene, more preferably glycidyl methacrylate. In another preferred embodiment, both the epoxy-containing component and one of the components that reacts to form urethane linkages are acrylic polymers or oligomers. The epoxide is preferably present in the coating composition in an amount of from 0.0001 to 0.05 equivalents of epoxy per 100 g of resin. In a preferred embodiment, the composition of the present invention also includes a component, which may be the same as or different from any of the other components, comprising one or more acid groups. Any type of acid may be used, including Bronsted or Lewis acids. Acids may be inorganic acids (e.g., phosphoric), but organic acids are preferred. Various types of organic acids may be used, such as phenolics, cresylics, or hydroxy acids (e.g., citric acid, phenol, cresol, tartaric acid, amino acids), or carboxylic acids, with carboxylic acids being preferred. Organic acids may be monofunctional or polyfunctional. In one embodiment, the acid is monofunctional. Such monofunctional acids include octanoic acid, benzoic acid, acetic acid, hexanoic acid, or benzylic acid. Polyfunctional organic acid components may also be used. The organic acid component can be a monomeric polyacid or an adduct thereof, or it can be a polymeric or oligomeric polyacid. For monomeric polyacids, usually liquid polyacids are employed. Non-limiting examples of these acids are succinic acid, glutaric acid, adipic acid, azelaic acid, oxalic acid, phthalic acid, isophthalic acid, hexahydrophthalic acid, methylhexahydrophthalic acid, maleic acid chlorendic acid. Polyacids of higher acid functionality, e.g., trimellitic acid, tricaballylic acid, aconitic acid , can also be employed. Higher molecular weight polyacid-containing adducts can also be used. Examples of useful polyacid-containing adducts are acid-containing polyesters, acid-containing polyurethanes, acid-containing acrylics. An example of the acid-containing polyesters can be prepared by reacting an excess of a monomeric polyacid as described above with a polyol. Alternatively, in a preferred embodiment, a cyclic anhydride (e.g., a 1,2-acid anhydride such as hexahydrophthalic anhydride and alkylhexahydrophthalic anhydride) can be reacted with a polyol, such as 1,6-hexanediol, trimethylol propane and polycaprolactone triol to form a half-ester polyacid. Illustrative examples of acid containing acrylics are copolymers of an ethylenically unsaturated monomer containing an acid group. The copolymers may be prepared by using conventional techniques such as free radical polymerization or anionic polymerization in, for example, a batch or semi-batch process. One or more other ethylenically unsaturated monomers that do not contain an acid group can be incorporated into the acid-containing polymer. Examples of the ethylenically unsaturated monomers containing an acid group can be acrylic acid, methacrylic acid, itaconic acid, and maleic acid. Other copolymerizable monomers can be alkyl ester of acrylic or methacrylic acid, e.g., ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, butyl methacrylate, isodecyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate ; vinyl monomers such as styrene, vinyl toluene. Copolymerizable monomers containing groups which will react with the acid groups under the addition reaction conditions chosen should be avoided so as to produce an ungelled product. Useful anhydrides include monomeric anhydrides such as alkyl hexahydrophthalic anhydride wherein the alkyl group has up to 7 carbon atoms, e.g., methylhexahydrophthalic anhydride, succinic anhydride, methylsuccinic anhydride, dodecenylsuccinic anhydride, octadecenylsuccinic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, tetrachlorophthalic anhydride, endomethylene tetrahydrophthalic anhydride, chlorendic anhydride, itaconic anhydride, citraconic anhydride, and maleic anhydride. The anhydride may also be polymeric, such as copolymers of maleic anhydride with other ethylenically unsaturated monomers. Such copolymers are preferably formed in the same manner as the acid-containing copolymers previously discussed. The acid component is preferably present in an amount of from 0 to 0.5 equivalents of acid per 100 g of resin solids, and more preferably 0.00008 to 0.008 equivalents of acid per 100 g of resin solids. In one preferred embodiment of the invention, one of the components that are curable to form urethane linkages is an acrylic resin, and the component that includes one or more epoxy groups is an epoxy acrylic resin. Such an epoxy acrylic resin preferably includes one or more of the components lauryl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, or butyl acrylate. The epoxy equivalent weight, molecular weight, and glass transition temperature of the epoxy acrylic resin are adjusted by varying the monomer lineup to optimize the performance in the particular coating composition by techniques known in the art. A solvent may optionally be utilized in the coating composition used in the practice of the present invention. Although the composition used according to the present invention may be utilized, for example, in the form of substantially solid powder, or a dispersion, it is.often desirable that the composition is in a substantially liquid state, which can be accomplished with the use of a solvent. This solvent should act as a solvent with respect to all of the components in the composition. In general, depending on the solubility characteristics of various components, the solvent can be any organic solvent and/or water. In one preferred embodiment, the solvent is a polar organic solvent. More preferably, the solvent is a polar aliphatic solvents or polar aromatic solvents. Still more preferably, the solvent is a ketone, ester, acetate, aprotic amide, aprotic sulfoxide, or aprotic amine. Examples of useful solvents include methyl ethyl ketone, methyl isobutyl ketone, m-amyl acetate, ethylene glycol butyl ether-acetate, propylene glycol monomethyl ether acetate, xylene, N-methylpyrrolidone, or blends of aromatic hydrocarbons. In another preferred embodiment, the solvent is water or a mixture of water with small amounts of co-solvents. The coating composition used in the practice of the invention may include, independent of the acid component described above, a catalyst such as an acid catalyst to enhance the cure reaction. For example, when a polycarbamate and aminoplast compounds are used as the components to form urethane linkages, a strong acid catalyst may be utilized to enhance the cure reaction. Such catalysts are well-known in the art and include, for example, p -toluenesulfonic acid, dinonylnaphthalene disulfonic acid, dodecylbenzenesulfonic acid, phenyl acid phosphate, monobutyl maleate, butyl phosphate, and hydroxy phosphate ester. Strong acid catalysts are often blocked, e.g. with an amine. Other catalysts that may be useful in the composition of the invention include Lewis acids, zinc salts, and aluminum salts. In a preferred embodiment of the invention, the solvent is present in the coating composition in an amount of from about 0.01 weight percent to about 99 weight percent, preferably from about 10 weight percent to about 60 weight percent, and more preferably from about 30 weight percent to about 50 weight percent. Coating compositions can be coated on the article by any of a number of techniques well-known in the art. These include, for example, spray coating, dip coating, roll coating, curtain coating. For automotive body panels, spray coating is preferred. Any additional agent used, for example, surfactants, fillers, stabilizers, wetting agents, dispersing agents, adhesion promoters, UV absorbers, HALS, etc. may be incorporated into the coating composition. While the agents are well-known in the prior art, the amount used must be controlled to avoid adversely affecting the coating characteristics. The coating composition of the invention may be clear or it may be a pigmented paint coating. When it is pigmented, the pigment may be any organic or inorganic compounds or colored materials, fillers, metallic or other inorganic flake materials such as mica or aluminum flake, and other materials of kind that the art normally names as pigments. Pigments are usually used in the composition in an amount of 1% to 100%, based on the total solid weight of components in the coating composition (i.e., a P:B ratio of 0.1 to 1). In one preferred embodiment, the coating composition according to the invention is used as the clearcoat of a composite color-plus-clear coating. Pigmented basecoat compositions useful therewith include any of a number of types well-known in the art, and does not require explanation in detail herein. Polymers known in the art to be useful in basecoat compositions include acrylics, vinyls, polyurethanes, polycarbonates, polyesters, alkyds, and polysiloxanes. Preferred polymers include acrylics and polyurethanes. In one preferred embodiment of the invention, the basecoat composition also utilizes a carbamate-functional acrylic polymer. Basecoat polymers may be thermoplastic, but are are preferably crosslinkable and comprise one or more type of cross-linkable functional groups. Such groups include, for example, hydroxy, isocyanate, amine, epoxy, acrylate, vinyl, silane, and acetoacetate groups. These groups may be masked or blocked in such a way so that they are unblocked and available for the cross-linking reaction under the desired curing conditions, generally elevated temperatures. Useful cross-linkable functional groups include hydroxy, epoxy, acid, anhydride, silane, and acetoacetate groups. Preferred cross-linkable functional groups include hydroxy functional groups and amino functional groups. Basecoat polymers may be self-cross-linkable, or may require a separate cross-linking agent that is reactive with the functional groups of the polymer. When the polymer comprises hydroxy functional groups, for example, the cross-linking agent may be an aminoplast resin, isocyanate and blocked isocyanates (including isocyanurates), and acid or anhydride functional cross-linking agents. The coating compositions described herein are preferably subjected to conditions so as to cure the coating layers. Although various methods of curing may be used, heat-curing is preferred. Generally, heat curing is effected by exposing the coated article to elevated temperatures provided primarily by radiative heat sources. Curing temperatures will vary depending on the particular blocking groups used in the cross-linking agents, however they generally range between 82°C and 177°C. The curing time will vary depending on the particular components used, and physical parameters such as the thickness of the layers, however, typical curing times range from 15 to 60 minutes. The invention is further described in the following example. Preparation 1 A coating composition was prepared having the following formulation: Preparation 2 A coating composition was prepared having the identical composition as Preparation 1 with the addition of 1.65 parts by weight of an epoxy-functional acrylic polymer having a weight average molecular weight of about 20,000 with a monomer lineup (by weight) of 38.5% glycidyl methacrylate, 59.5% 2-ethylhexyl acrylate, 1% styrene, and 1% methyl methacrylate. Preparation 3 A coating composition was prepared having the identical composition as Preparation 2 with the addition of 0.29 parts by weight of octanoic acid. Example 1 The coating compositions of Preparation 1, Preparation 2, and Preparation 3 were coated onto a primed steel panel as the clearcoat of a basecoat/clearcoat composite coating having a black pigmented basecoat that utilized a hydroxy-functional acrylic resin and a melamine resin crosslinker. The panels were cured as described below for each of the intercoat adhesion tests. High-Bake Repair (HBR) Test The coated panels were cured for 20 minutes at 132°C and allowed to cool. The panels were then coated with a wet-on-wet basecoat/clearcoat coating having a clearcoat that was identical to Preparation 1 and a basecoat composition having the following composition: Component Parts by Weight OH-functional acrylic resin dispersion 23.66 acrylic microgel dispersion 26.11 Resimene® 755 melamine resin 17.18 Black pigment grind paste 21.82 Acrylic polymer flow additive 0.15 N-methyl pyrrolidone 1.40 UV and HALS solution 2.88 Nacure® 5225 blocked acid catalyst 1.62 ethanol 1.90 n-butyl acetate 3.27 The panels were cured for 20 minutes at 132°C and cooled. The panels were then subjected to an adhesion test as described in ASTM 3359 involving scribing a portion of the coating with a cross-cut pattern and applying and removing pressure-sensitive adhesive tape. The panels were evaluated for pass/fail with a pass representing 10% or less of the second base/clear coating being removed during the test and a fail being greater than 10% of the second base/clear coating being removed during the test. Intercoat Adhesion (ICA) III Test The panels were processed and tested as described for the HBR test, except the first base/clear coating was cured for 30 minutes at 146°C and the second base/clear coating was cured for 20 minutes at 127°C. Intercoat Adhesion (ICA) IV Test The panels were processed and tested as described for the HBR test, except the first base/clear coating was cured for 60 minutes at 146°C and the second base/clear coating was cured for 20 minutes at 127°C. The results are described in Table I below: Preparation HBR ICA III ICA IV 1 (comparison) Fail Fail Fail 2 (invention) Pass Pass Fail 3 (invention) Pass Pass Pass

Claims: A curable coating composition comprising two components that are reactive with one another upon curing to form urethane linkages, wherein the two components that are reactive with one another are: (A) a component comprising a plurality of primary carbamate groups, and (B) a component comprising a plurality of groups that are reactive with the carbamate groups, and said composition further comprising a third component that includes one or more epoxide groups. A curable coating composition according to claim 1 wherein said component comprising one or more epoxy groups is substantially free of groups that are reactive with either of said two components that are reactive with one another to form urethane linkages. A curable coating composition according to claim 1 wherein the epoxide groups on the third component are derived from glycidyl methacrylate, glycidyl acrylate, allyl glycidyl ether, cyclohexyl monoepoxy methacrylate, the epoxide of the dimer of cyclopentadiene methacrylate, and epoxidized butadiene. A curable coating composition according to claim 1 wherein the epoxide groups on the third component are derived from glycidyl methacrylate. A curable coating composition according to claim 1 wherein said third component is selected from the group consisting of epoxy group-containing acrylic resins, epoxy-terminated DGEBPA resins, epoxy novolac resins, epoxy group-containing polyurethane resins, epoxy group-containing polyester resins. A curable coating composition according to claim 1 wherein component (B) is an aminoplast resin. A curable coating composition according to claim 6, wherein component (B) is a melamine resin. A curable coating composition according to claim 1 wherein component (A) is a primary carbamate-functional acrylic resin. A curable coating composition according to claim 8 wherein said third component is an acrylic resin. A curable coating composition according to claim 9 wherein the epoxy groups on said third component are derived from glycidyl methacrylate, glycidyl acrylate, allyl glycidyl ether, cyclohexyl monoepoxy methacrylate, the epoxide of the dimer of cyclopentadiene methacrylate, and epoxidized butadiene. A curable coating composition according to claim 10 wherein said epoxy groups are derived from glycidyl methacrylate. A curable coating composition according to any of claims 1 to 11, including a component, which may be the same as or different from any of the other components, comprising one or more add groups. A curable coating composition according to claim 12 wherein the component comprising one or more add groups is different from components (A), (B), or the said third component A curable coating composition according to claim 13 wherein the add component is an organic acid. A curable coating composition according to claim 14 wherein the organic add is a monofunctional organic add. A curable coating composition according to claim 14 wherein the organic acid is a carboxylic acid.

Inventors: BRIGGS RODNEY L, MENOVCIK GREGORY G, OHRBOM WALTER H, REHFUSS JOHN W, BRIGGS, RODNEY L., MENOVCIK, GREGORY G., OHRBOM, WALTER H., REHFUSS, JOHN W.
Applicants: BASF CORP, BASF CORPORATION
Application year: 1996
Main classification: C08G18
IPC codes: C08G71, C09D201, C09D175, C08G18