Thermosetting Coating Composition - V

This invention relates to a novel, solvent-based, thermosetting epoxy ester coating composition. It relates also to such coating composition formulated, for example, as sprayable coating composition suitable for use as an automotive vehicle primer to make coatings which are highly resistant to corrosion, humidity and solvents.

Solvent-based coating compositions are known which can be applied, for example, to a substrate by spraying, and then cured by baking the coated substrate at an elevated temperature suitable to drive off the organic solvent and to promote crosslinking reaction. The resulting thermoset coating, if sufficiently humidity and solvent resistant, can provide aesthetic and functional advantages including corrosion protection for the underlying substrate.

It is an object of the present invention to provide solvent-based thermosetting coating compositions comprising hydroxy functional epoxy ester resins which are crosslinkable during cure on the surface of a substrate.

It is another object of the invention to provide a method of making a coating on a substrate, which coating provides advantageous physical properties including, for example, humidity and solvent resistance and corrosion protection for the underlying substrate. Additional aspects and advantages of the invention will be apparent from the following description thereof.

According to the present invention, a novel, organic solvent based coating composition, in addition to solvent and any pigments and additives such as, for example, catalyst, flow control agents and the like, comprises:

(A) crosslinkable hydroxy functional epoxy ester resin preferably having a number average molecular weight (Mn) between about 1,000 and about 4,000 and formed by chain terminating reaction of hydroxy functional secondary amine with the diepoxide reaction product of chain extension reaction of diepoxide with dicarboxylic acid; and

(B) polyfunctional, hydroxy-reactive crosslinking agent selected preferably from aminoplast crosslinking agent such as, for example, hexamethoxymethyl melamine, and blocked polyisocyanate crosslinking agent including, but not limited to, blocked trifunctional isocyanurate ring containing polyisocyanates and oligoester modified blocked isocyanates, or a compatible mixture of any of them.

Particularly preferred compositions of the invention are those adapted to be applied by spraying onto a substrate. Such compositions are especially useful as a primer coat on the bare, unpolished metal surface of an automotive vehicle body panel.

According to another aspect of the invention, a method of making a corrosion, solvent and humidity resistant coating on a substrate comprises applying to the substrate the solvent based thermosetting coating composition of the invention and subsequently subjecting the coating to an elevated temperature for a time period sufficient to substantially cure the coating layer. Typically, the novel coating compositions of the invention can be cured by heating to between about 240.degree. F. and about 400.degree. F., for a time period sufficient to yield a cured coating, for example for about 10 to about 60 minutes. According to preferred embodiments of the invention, the coating composition can be sufficiently cured for good coating properties by heating to about 240.degree. F. for about 20 minutes but, in addition, such preferred composition will tolerate curing at up to about 400.degree. F. for as much as about 60 minutes without substantial loss of such advantageous coating properties.

The coating compositions of the present invention provide cure-response advantages without sacrifice of advantageous physical properties in the cured coating. That is, when applied, for example, over a metallic substrate, such as when applied as an automotive vehicle primer coat over sheet steel, cured coatings according to the invention have been found to provide excellent adhesion to the substrate, excellent humidity resistance, and excellent corrosion resistance.

Other features and advantages of this invention will become more apparent from the following, detailed description thereof including the preferred embodiments and the best mode of carrying out this invention.

The hydroxy functional epoxy ester resin employed in the composition of the present invention preferably has a number average molecular weight (Mn) of between about 1,000 and about 5,000. More preferably, especially for use in sprayable coating compositions of higher solids content, the epoxy ester resin has a number average molecular weight (Mn) of between about 1000 and about 3000. The hydroxy functional epoxy ester resin is formed by chain terminating reaction of hydroxy functional secondary amine in approximately 1 to 1 equivalent ratio with the diepoxide reaction product of the chain extension reaction of diepoxide with dicarboxylic acid.

Thermosetting compositions of the invention comprise such epoxy ester resin and polyfunctional hydroxy-reactive crosslinking agent selected, preferably, from aminoplast crosslinking agent, blocked polyisocyanate crosslinking agent comprising at least two isocyanate groups blocked by reaction with an active hydrogen bearing blocking agent. The poly functional crosslinking agent is included in the composition in an amount sufficient to provide between about 0.5 and about 1.6 reactive groups per hydroxy functionality of the epoxy ester resin.

Each of the above major components of the compositions as well as optional components and other aspects of the invention are described hereinafter in greater detail.

A. Epoxy Ester Resin

As described above, the hydroxy functional epoxy ester resin is formed by reacting diepoxide, which has been chain extended with dicarboxylic acid, with hydroxy functional secondary amine in chain terminating reaction.

(i) Diepoxide Reactant

The diepoxide reactant suitable for preparing the hydroxy functional epoxy ester resin can be any of various diepoxides including many which are commercially available and which will be apparent to the skilled of the art in view of the present disclosure. While, ultimately, the choice of diepoxide reactant for preparing the hydroxy functional epoxy ester resin will depend to an extent upon the particular application intended for the final coating composition, terminal diepoxides, that is diepoxides bearing two terminal epoxide groups, are generally most preferred. These are generally more reactive and therefore require milder reaction conditions under which undesirable side reactions, for example, epoxy-epoxy reactions and gelation, can be more easily avoided.

Preferably, the diepoxide has a number average molecular weight (Mn) between about 100 and about 1000, and more preferably between about 100 and about 600.

Suitable diepoxide reactants include, for example, bisphenol-A epichlorohydrin epoxy resins such as the Epon (trademark) series, Shell Chemical Company, Houston, Texas, and the DER (trademark) series, Dow Chemical Company, Midland, Mich. These diglycidyl ether bisphenol-A resins and higher molecular weight analogs thereof, are preferred in view of their low cost and commercial availability.

Also suitable are cycloaliphatic diepoxy resins such as, for example, the Eponex (trademark) series, Shell Chemical Company, Houston, Tex.; hydantoin epoxy resins such as, for example, Resin XB2793 (trademark), Ciba-Geigy Corporation, Ardsley, N.Y.; and any of a wide variety of acyclic or cyclic aliphatic diepoxides such as, for example, 1,4-butanediol diglycidyl ether and 4-vinylcyclohexene dioxide and the like. Other suitable diepoxides are commercially available or are readily prepared by those skilled in the art and will be apparent to the skilled of the art in view of the present disclosure. Also, it will be understood from the foregoing that any mixture of compatible diepoxides may be used.

In addition to the diepoxide, a portion of the epoxy functionality can be provided by any compatible monoepoxy compound or polyepoxy compound or mixture of such compounds. Suitable polyepoxides include, for example, those of molecular weight about 200 to about 800. The polyepoxide can be any of the well known types such as polygylcidyl ethers of polyphenols. These can be produced by etherification of polyphenol with epihalohydrin in the presence of alkali. It will be recognized by the skilled of the art in view of the present disclosure, that in some instances, particularly where a coating composition of high solids content is less important, it may be desirable to incorporate polyepoxide of higher molecular weight. Preferably, any such polyepoxide contains free hydroxyl groups in addition to epoxide groups.

While polyglycidyl ethers of polyphenol can be employed, it may be desirable to react a portion of the reactive sites (hydroxyl or in some instances epoxy) with a modifying material to vary the film characteristics of the resin. The epoxy resin may be modified, for example, with isocyanate group containing organic materials or other reactive organic materials.

Other useful polyepoxides are the novolak resins including, for example, the novolak epoxy resins ECN 1235 (trademark) and ECN 1273 (trademark), Ciba-Geigy Corporation.

According to preferred embodiments of the present invention, epoxide compounds other than diepoxide compounds provide no more than about 15% and most preferably substantially none of the total epoxide functionality in the reactants used to form the epoxyester resin.

(ii) Dicarboxylic Acid Reactant

Dicarboxylic acids suitable for chain extending diepoxides discussed above preferably have a number average molecular weight of between about 145 and about 1000 and more preferably between about 500 and about 600. Suitable dicarboxylic acids include numerous commercially available materials, many of which will be readily apparent to the skilled of the art in view of the present disclosure. Suitable dicarboxylic acids include saturated and unsaturated, cyclic and acyclic, aliphatic and aromatic dicarboxylic acids and a compatible mixture thereof. Acyclic aliphatic dicarboxylic acids are generally preferred in view of the enhanced flexibility they provide to the cured coatings of the invention. Preferred dicarboxylic acids have the general formula (I):

wherein R"' is a divalent linking moiety substantially unreactive with the diepoxide resin. It will be apparent to the skilled of the art in view of the present disclosure, that R"' should be substantially unreactive also with the hydroxy functional secondary amine employed in preparation of the epoxy ester resin, and with hydroxy functionality generated in the chain-extension reaction. Preferably R"' is a divalent, organic, linking moiety. Particularly preferred are those dicarboxylic acids wherein R"' is selected from the group comprising a straight or branched alkylene or alkylidene moiety, preferably of about 4-42 carbons, for example (CH.sub.2).sub.n wherein n is preferably from about 4 to about 42, and the like or a mixture thereof. Dicarboxylic acids of this character have been found to provide good reactivity with the preferred diepoxides described above and to provide, ultimately, cured coatings of the invention having excellent physical properties, most notably excellent flexibility and corrosion protection.

Exemplary dicarboxylic acids include adipic acid, 3,3-dimethylpentanedioic acid, benzenedicarboxylic acid, phenylenediethanoic acid, napthalenedicarboxylic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and the like or a compatible mixture of any of them. The anhydrides of these acids, where the anhydrides exists, are, of course, embraced in the term "acid". While dicarboxylic acids according to formula (I) can be used, wherein R"' is an alkylene chain of less than 4 carbons, for example oxalic acid, malonic acid, succinic acid, glutaric acid and the like, these are less preferred in view of the somewhat lesser degree of flexibility provided thereby. Preferably the dicarboxylic acid provides two terminal carboxyl groups. Similarly, preferred aromatic dicarboxylic acids are those wherein the carboxylic groups are more spaced apart, for example, 1,4-benzene-dicarboxylic acid and 2,7-naphthalenedicarboxylic acid.

The most preferred discarboxylic acids are substantially saturated, acyclic, aliphatic dimer acids, which are well known to the skilled of the art and readily commercially available. These are typically the dimerization reaction products of fatty acids which have from 4 to 22 carbons and a terminal carboxyl group. Of these, dimer acid of 36 carbons is most preferred since it provides excellent reactivity with the preferred diepoxides described above, and provides, ultimately, cured coatings of the invention having excellent physical properties. In addition, dimer acid of 36 carbons is readily commercially available, for example, as Empol 1014 (trademark), Empol 1016 (trademark) and Empol 1018 (trademark), each available from Emery Industries, Inc., Cincinnati, Ohio. It should be recognized that most or all commercially available dimer acids contain some portion of trimer acid, typically, for example, about 5-10% but in some cases as much as 30% or more, and also contain a usually smaller portion of monocarboxylic acid. As used herein, the term "dimer acid" includes those containing such materials. Most useful in the present compositions are products that contain mostly dibasic acid and none or low amounts of tribasic and monobasic acids.

Aliphatic dicarboxylic acids are seen to provide additional advantages. In particular, while not wishing to be bound by theory, it is presently understood that epoxy ester resins derived therefrom wet the substrate surface better and provide enhanced adhesion between the substrate and the cured coating. They also flow better and, thus, provide an excellent smooth surface upon being cured. Also, the aliphatic units provide enhanced flexibility to the cured coating and this flexibility of the coating is seen to provide enhanced impact resistance.

Where corrosion protection for the substrate is particularly important, it may be preferred to employ dicarboxylic acid according to formula (I) above, wherein R"' is, at least in part, aromatic. It is believed that such aromatics in the coating composition of the invention, such as a primer composition for a metal substrate, are more resistant to hydrolysis than are aliphatics and, therefore, provide enhanced corrosion and moisture resistance. Of course, as also noted above, according to preferred embodiments of the epoxy ester resin, described above, the diepoxide reactant provides aromatic units to the resin and this would similarly contribute to corrosion and moisture resistance.

Other suitable dicarboxylic acids for the epoxy ester resin of the present invention will be apparent to the skilled of the art in view of the present disclosure.

(iii) Hydroxy Functional Secondary Amine Reactant

The hydroxy functional secondary amine which is reacted in chain terminating reaction with the above described diepoxide reaction product of diepoxide with dicarboxylic acid may be selected from a broad class of aliphatic, cycloaliphatic and aromatic hydroxy functional secondary amines.

Numerous such amines, which may bear mono- or dihydroxy functionality will be apparent to those skilled in the art. Exemplary of such amines are those having the formula (II): ##STR1## wherein Q and Q' each is selected from the group consisting of aliphatic, cycloaliphatic and aromatic divalent radicals which will not interfere with the chain termination reaction between the chain extended diepoxide and the N-hydrogen of the hydroxy functional secondary amine. Q and Q' in the above formula (II) may be the same or different but preferably are the same, and X is selected from hydrogen and hydroxyl radical. While the hydroxyl group on Q and/or Q' may be other than primary, primary hydroxyls are preferred since they are more reactive with blocked isocyanates or aminoplast crosslinking agents and, so, provide better cure response.

Examples of preferred radicals Q and Q' for the hydroxy functional amine of the above formula are:

Preferably Q and Q' are methylene, ethylene, or other lower alkylene group but, in general, each may be any noninterfering radical including, for example, benzyl, oxyalkylene, and the like.

Particularly preferred primary hydroxyl bearing secondary amines for use in preparing the hydroxy functional epoxy ester resin are diethanolamine, methylethanolamine, dipropanol amine, and methylpropanol amine. Other suitable hydroxy functional secondary amine reactants will be apparent to the skilled of the art in view of the present disclosure.

The hydroxy functional epoxy ester resin used in the composition of the present invention can be made according to techniques well known to the skilled of the art. The chain extension and chain termination reactions occur sequentially, with the chain extension of the diepoxide with dicarboxylic acid being carried out first. Diepoxide and dicarboxylic acid are charged into a suitable reactor and heated. The reactants are used in relative proportions to yield a chain extension reaction product bearing two unreacted epoxy groups and preferably substantially no unreacted carboxyl functionality. Suitable separation techniques are known to the skilled of the art for removal of unused reactants. It should be recognized that to assure rapid and/or more complete reaction of the diepoxide with carboxyl functionality, it is usually preferred to have a catalyst present. The use of catalyst is found to provide epoxy ester resin which yields coating compositions of advantageous physical properties. Suitable catalysts are commercially available and include any of the well known catalysts for epoxy-carboxyl reactions such as, for example, lithium neodecanoate which is preferred, and sodium carbonate, tertiary amines and other organometallic catalysts. Other suitable catalysts will be apparent to the skilled of the art in view of the present disclosure.

As noted above, after completion of the chain extension reaction of diepoxide with dicarboxylic acid, the diepoxide reaction product is reacted with hydroxy functional secondary amine in chain terminating reaction in approximately 1 to 1 equivalent ratio. This ratio is preferred, since excess epoxy could result in gelation of the reaction mixture.

B. Crosslinking Agent

The crosslinking agent employed in the novel solvent based coating compositions of the invention comprises polyfunctional, hydroxy-reactive crosslinking agent selected, preferably, from aminoplast crosslinking agents and blocked polyisocyanate crosslinking agents.

(a) Blocked Polyisocyanate Crosslinking Agent

Those embodiments of the novel solvent based coating compositions of the invention employing blocked polyisocyanate crosslinking agents exhibit exceptional shelf stability even when corrosion inhibiting pigments such as zinc chromate are used in high concentrations.

As used herein "blocked polyisocyanate" means an isocyanate compound containing two or more isocyanato groups, each of which has been reacted with a blocking agent which will prevent reaction of the isocyanate group at room temperature with compounds that conventionally react with such groups, and at least some of which will permit that reaction to occur at higher (cure) temperatures. In general, the blocked polyisocyanate may be prepared by reacting a sufficient quantity of an active hydrogen containing blocking agent with the polyisocyanate to insure that substantially no free isocyanato groups are present. The blocking agent may be represented by the formula BH and may be selected from numerous materials, hereinafter discussed, which bear an active hydrogen.

The proper proportion of blocked polyisocyanate crosslinking agent to hydroxy functional epoxy ester resin will depend, in part, upon the properties desired in the coating to be produced and, in part, upon the desired cure response of the coating composition (which will depend, in turn, upon the baking schedule to be used in curing the coating composition) and, in part, upon the desired storage stability of the composition, that is, upon the desired shelf life. Accordingly, the amounts of crosslinker that can be used with the epoxy ester resin varies considerably. Preferably the blocked polyisocyanate crosslinking agent is included in compositions of the invention in amounts such that upon deblocking of the blocked isocyanato groups at the cure temperature of the composition, the crosslinking agent will provide between about 0.5 and about 1.6, preferably between about 0.8 and about 1.3, reactive isocyanato groups per reactive group on the film forming resin of the coating composition as described above. Blocked polyisocyanates of numerous types may be employed in the compositions of the invention. Particularly suitable blocked polyisocyanates, which will be discussed further hereinafter, include blocked polymethylene polyphenol isocyanates, isocyanurate ring containing blocked polyisocyanates and certain oligoester modified blocked polyisocyanates.

In the preparation of the blocked polyisocyanate crosslinking agent, any suitable organic polyisocyanate may be used. Representative examples include the aliphatic compounds such as trimethylene, tetramethylene, pentamethylene, hexamethylene, 1,2-propylene, 1,2-butylene, 2,3-butylene, 1,3-butylene, ethylidene and butylidene diisocyanates; the cycloalkylene compounds such as 1,3-cyclopentane, 1,4-cyclohexane, and 1,2-cyclohexane diisocyanates; the aromatic compounds such as m-phenylene, p-phenylene, 4,4'-diphenyl, 1,5-naphthalene, and 1,4-naphthalene diisocyanates, the aliphatic-aromatic compounds such as 4,4'-diphenylene methane, 2,4- or 2,6-tolylene, or mixtures thereof, 4,4'-toluidine, and 1,4-xylylene diisocyanates; substituted aromatic compounds such as dianisidine diisocyanate, 4,4'-diphenylether diisocyanate and chlorodiphenylene diisocyanate; the triisocyanates such as triphenyl methane-4,4'4"-triisocyanate, 1,3,5-triisocyanate benzene and 2,4,6-triisocyanate toluene; the tetraisocyanates such as 4,4'-diphenyl-dimethyl methane-2,2',5,5'-tetraisocyanate; and the polymerized polyisocyanates such as tolylene diisocyanate dimers and trimers, and the like.

In addition, the organic polyisocyanate may be a prepolymer derived from a polyol including polyether polyol or polyester polyol, including polyethers which are reacted with excess polyisocyanates to form isocyanate-terminated prepolymers. The polyols may be simple polyols such as glycols, e.g., ethylene glycol and propylene glycol, as well as other polyols such as glycerol; tri-methylolpropane, pentaerythritol, and the like, as well as mono-ethers such as diethylene glycol, tripropylene glycol and the like and polyethers, i.e., alkylene oxide condensates of the above. Among the alkylene oxides that may be condensed with these polyols to form polyethers are ethylene oxide, propylene oxide, butylene oxide, styrene oxide and the like. These are generally called hydroxyl-terminated polyethers and can be linear or branched. Examples of polyethers include polyoxyethylene glycol, polyoxypropylene glycol, polyoxytetramethylene glycol, polyoxyhexamethylene glycol, polyoxynonamethylene glycol, polyoxydecamethylene glycol, polyoxydodecamethylene glycol and mixtures thereof. Other types of polyoxyalkylene glycol ethers can be used. Especially useful polyether polyols are those derived from reacting polyols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butylene glycol, 1,3-butylene glycol, 1,6-hexanediol, and their mixtures; glycerol, trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, pentaerythritol, dipentaerythritol, tripentaerythritol, polypentaerythritol, sorbitol, methyl glucosides, sucrose and the like with alkylene oxides such as ethylene oxide, propylene oxide, their mixtures, and the like.

A particular class of aromatic polyisocyanates which may be employed in the novel solvent based coating compositions of the invention are polymethylene polyphenol isocyanates having the formula: ##STR4## wherein x equals 1 to 3. The compounds, sold under the tradename "PAPI" by the Upjohn Chemical Company of Kalamazoo, Mich., are particularly useful in compositions of the invention, resulting in compositions exhibiting desirable toughness in the final cured coating.

The active hydrogen containing blocking agents which are reacted with the above described organic diisocyanates may be selected from numerous blocking agents which will be apparent to those skilled in this art. Preferred blocking agents include, for example, those selected from the group consisting of (i) aliphatic, cycloaliphatic and aromatic alkyl monoalcohols; (ii) hydroxyl amines; (iii) oximes; (iv) lactams; and (v) triazoles. Any suitable aliphatic, cycloaliphatic or aromatic alkyl monoalcohol may be used as a blocking agent in accordance with the present invention. For example, aliphatic alcohols, such as methyl, ethyl, chloroethyl, propyl, butyl, amyl, hexyl, heptyl, octyl, nonyl, 3,3,5-trimethylhexyl, decyl, and lauryl alcohols, and the like may be employed. Suitable cycloaliphatic alcohols include, for example, cyclopentanol, cyclohexanol and the like, while aromatic-alkyl alcohols include phenylcarbinol, methylphenylcarbinol, and the like. Minor amounts of even higher molecular weight relatively non-volatile monoalcohols may be used, if desired, to serve as plasticizers in the coatings provided by the invention. Examples of hydroxyl amines which may be employed as blocking agents include ethanol amine and propanol amine. Suitable oxime blocking agents include, for example, methylethylketone oxime, acetone oxime and cyclohexanone oxime. Examples of lactams which may be used as blocking agents are .epsilon.-caprolactam, .gamma.-butyrolactam and pyrrolidone, while suitable triazoles include compounds such as 1,2,4-triazole, 1,2,3-benzotriazole, 1,2,3-tolyl triazole and 4,5-diphenyl-1,2,3-triazole. Particularly preferred active hydrogen containing blocking agents are methylethyl ketoxime and 2-ethylhexanol.

(i) Isocyanurate Ring Containing Blocked Isocyanate Compounds

Within the scope of the above general class of blocked polyisocyanate crosslinking agents, a particular class or type of blocked polyisocyanate crosslinking agent which may be employed in the novel solvent based coating compositions of the invention comprises isocyanurate ring containing blocked isocyanate compounds. In general, these blocked polyisocyanates may be formed by blocking with the aforementioned blocking agents. These compounds may be formed by cyclotrimerization of difunctional isocyanates. Usually, the reaction does not stop in this stage and continues through the formation of polyfunctional oligomers or a mixture of such oligomers with a portion of the pure trifunctional polyisocyanate. Mixtures of trifunctional product and various polyfunctional oligomers are commercially available.

A particularly desirable blocked polyisocyanate crosslinking agent is the blocked form of the pure trifunctional isocyanurate represented by the following formula: ##STR5## wherein each L is selected independently from the group consisting of aliphatic, cycloaliphatic and aromatic groups and combinations thereof and B is the residue of an active hydrogen containing blocking agent. More specifically, this compound is disclosed in U.S. patent application Ser. No. 368,178 filed Apr. 14, 1982, the disclosure of which is hereby incorporated by reference.

(ii) Oligoester Modified Blocked Polyisocyanates

Still further particular blocked polyisocyanates useful as crosslinking agents in the novel solvent based coating compositions of this invention are oligoester modified blocked polyisocyanates prepared from a particular class of oligoester diols and triols. A first type of such oligoester modified blocked polyisocyanate is prepared from organic diisocyanates wherein one isocyanato group is more reactive than the other, with the more reactive isocyanato first being blocked with a blocking agent and the remaining isocyanato group then being reacted with hydroxyl functionality of an oligoester diol or triol as referred to above. The second type of oligoester modified blocked polyisocyanate may be prepared by reacting oligoester diols from the aforementioned class of oligoesters with an excess of organic diisocyanate so as to form an isocyanato terminated prepolymer followed by blocking of the terminal isocyanato groups of the prepolymer with an active hydrogen containing blocking agent. Each of these materials is particularly useful in the compositions of the invention and produces final cured coating compositions exhibiting outstanding flexibility.

Oligoesters of the type employed in the preparation of these crosslinking agents are described in U.S. Pat. No. 4,322,508 issued Mar. 30, 1982, the disclosure of which is hereby incorporated by reference. The hydroxy functional oligoesters within the useful class of materials (i) have a number average molecular weight (Mn) between about 150 and about 3000, preferably between about 230 and about 1000, (ii) bear 2 or 3 hydroxyl groups per molecule, and (iii) are formed by an esterification reaction between a carboxylic acid and an epoxide. The esterification reaction products are selected from the group consisting of:

(a) the esterification reaction product of polycarboxylic acid, i.e., carboxylic acid bearing 2 or more carboxyl groups, and monoepoxide;

(b) the esterification reaction product of polyepoxide, i.e., a compound having 2 or more epoxide groups, and monocarboxylic acid, preferably containing no ethylenic unsaturation, and bearing no hydroxy functionality;

(c) the esterification reaction product of hydroxy functional carboxylic acid and mono- or polyepoxide, preferably monoepoxide;

(d) the esterification reaction product of monocarboxylic acid and hydroxy functional mono- or polyepoxide, preferably monoepoxide; and

(e) mixtures of (a)-(d).

As noted above, the first type of oligoester modified blocked polyisocyanate crosslinking agent is prepared by (i) reacting organic diisocyanate bearing one isocyanato group which is more reactive than the other with a sufficient amount of an active hydrogen containing blocking agent to react substantially with all of the more reactive isocyanate groups, thus providing a half-blocked diisocyanate and (ii) reacting this half-blocked intermediate with the above discussed oligoester. The organic diisocyanates employed in this synthesis, as well as the active hydrogen containing blocking agents, are discussed above in connection with the preparation of the isocyanurate ring containing blocked isocyanate crosslinking agents useful in compositions of the invention. The organic polyisocyanate-blocking agent adduct intermediate is formed by reacting a sufficient quantity of the blocking agent with the organic diisocyanate to insure that one of the two --NCO groups on the diisocyanate is reacted. The reaction between the organic diisocyanate and the blocking agent is exothermic and the diisocyanate and the blocking agent are preferably admixed at temperatures no higher than about 80.degree. C., preferably below about 50.degree. C., to minimize the exothermic effect.

The diisocyanate/blocking agent intermediate is next reacted with the oligoester diol or triol described above so as to react substantially all free or unblocked isocyanato groups of the intermediate with hydroxyl groups of the oligoester. This reaction is carried out desirably at a temperature of about 80.degree.-120.degree. C.

As also discussed above, the second type of oligoester modified blocked polyisocyanate crosslinking agent useful in the novel solvent based coating compositions of the invention is prepared by reacting an excess of organic diisocyanate with an oligoester diol from the above described class of oligoesters followed by reaction of the terminal isocyanato groups formed on the resultant prepolymer with an active hydrogen containing blocking agent as described above so as to react with substantially all the isocyanato groups. The diisocyanate starting material is used in excess in amounts sufficient to insure that the intermediate is isocyanate terminated. Therefore, it is preferable that the organic diisocyanates and the dihydroxy functional oligoester be reacted in a molar ratio of from greater than 1:1 up to 2:1. Numerous diisocyanates of the type described hereinbefore may be employed in the preparation of this intermediate. While it is not necessary that one isocyanato group be more reactive than the other, the preparation of this type of crosslinking agent does not preclude the use of such material.

(b) Aminoplast Crosslinking Agent

According to alternative embodiments of the invention, the novel solvent based coating compositions employ hydroxy-reactive polyfunctional aminoplast crosslinking agents. It will be recognized that compatible combinations of suitable polyfunctional aminoplast crosslinking agent(s) and blocked polyisocyanate crosslinking agent(s) also can be used. Included within the aminoplast crosslinking agents suitable for use in the coating composition are numerous materials which are well known to the skilled of the art including, for example, alkylated melamine formaldehyde resins with one to about eight carbon atoms in the alkyl moiety. Other suitable aminoplast crosslinking agents will be apparent to the skilled of the art in view of the present disclosure. Many such crosslinking agents are readily commercially available including, for example, the Resimene (trademark) series. Monsanto Company, St. Louis, Mo., the most preferred being Resimene 717 (trademark), described as a low temperature cure methylated melamine-formaldehyde resin.

In addition, suitable polyfunctional aminoplast crosslinking agents can be prepared employing conventional techniques. Accordingly, for example, a lower alkanol such as methanol, ethanol, butanol, isobutanol, isopropanol, hexanol, 2-ethylhexanol or the like or a mixture of any of them is reacted with a melamine formaldehyde. Preferred crosslinking agents of this type include butylated melamine formaldehyde resin, methylated/butylated formaldehyde resin and polyalkyl hexamethoxymethyl melamine resin which is most preferred in view of its relatively lower cost, ready commercial availability, and its low reactivity with the epoxy ester resin of the invention at non-elevated temperatures. In this regard, preferred polyfunctional aminoplast crosslinking agent is substantially unreactive with the epoxy ester resin at or below about 60.degree. C. Other suitable aminoplast crosslinking agents will be apparent to the skilled of the art in view of the present disclosure.

The proper proportion of polyfunctional aminoplast crosslinking agent to epoxy ester resin will depend, in part, upon the properties desired in the coating to be produced and, in part, upon the desired cure response of the coating composition (which will depend, in turn, upon the baking schedule intended for curing the coating composition) and, in part, upon the desired storage stability of the coating composition, that is, upon the desired shelf life. Accordingly, the amounts of epoxy ester resin that can be blended with the crosslinker to form coating compositions of the invention may be varied considerably. Preferably, the crosslinking agent is used in amounts of about 5% to about 40% by weight of the total resin solids, more preferably about 20% to about 30%.

C. General Discussion-Other Aspects of the Invention and Other Components

The coating compositions of the invention provide a cured coating having the advantageous physical properties described above, over a wide range of cure temperatures and a wide range of solids levels. More specifically, the coating compositions according to preferred embodiments of the invention cure at temperatures from as low as about 120.degree. C. or less within about 15 minutes of less, and yet to cure and suffer no significant loss of advantageous physical properties at temperatures as high as about 200.degree. C. or more for periods up to about 60 minutes or more. Considered together with the good storage stability of the coating composition, it can be readily recognized that the present invention provides a significant advance in the coating composition art.

It will be within the skill of the art to determine the proper volatile organic content for a given coating composition of the invention and for a given application. Preferred solvents have relatively low volatility at temperatures appreciably below their boiling points such that solvent evaporation is low during storage and during application of the coating composition to the substrate. A suitable solvent system may include, for example, toluene, metyl ethyl ketone, isobutyl acetate, xylene, cellosolve acetate, acetone and a mixture of any of them. Other solvents which may be employed include terpenes, aliphatic and aromatic naphthas, and the like. Additional suitable solvents are commercially available and will be apparent to the skilled of the art in view of the present disclosure. Where the composition is to be formulated as a higher solids content primer composition employing aminoplast crosslinking agent, it generally is preferred to employ a portion of C-1 to C-8 alcohol solvent such as, for example, butanol, pentanol, hexanol, and the like or a mixture of any of them since these inhibit the crosslinking reaction of the polyfunctional aminoplast resin with the epoxy ester resin at room temperature and thereby improve storage stability. At elevated temperature during cure, the alcohol solvent evaporates and, hence, ceases to inhibit the crosslinking reaction. Preferred solvents also include, for example, methyl amyl ketone and the like, or a mixture thereof with C-1 to C-8 alcohol such as, for example, a 1:2 mixture by weight of butanol and methyl amyl ketone, respectively.

Any solvent allowed to remain in the cured coating should be inert so as to avoid adverse effect upon the cured coating or upon another coating layer used in conjunction with it during the curing process or thereafter. Preferrably, the cured coating is substantially free of solvent.

Sufficient solvent is used to reduce the viscosity of the coating composition to a level suitable for application to the substrate in the desired manner. It can be applied by any conventional method, including brushing, dipping, flow coating, spraying, etc. Spraying will generally be preferred, for example, for applying the compositions as an automotive primer.

Also preferably included in compositions of the invention employing aminoplast crosslinking agent is any of a variety of acid catalyst known to the skilled of the art to catalyse the aminoplast crosslinking reaction, for example, p-toluenesulfonic acid, phosphoric acid, phenyl acid phosphate, butyl phosphate, butyl maleate, and the like or a compatible mixture of any of them. Such acid catalyst is most useful for coating compositions intended for low temperature curing schedules and/or when highly etherified melamine resins are used such as hexa(methoxymethyl)melamine or the like. Such catalysts are used in amounts which depend, in part, upon the intended baking (curing) schedule. Typically, amounts of about 0.2% to about 3.% are used, more preferably about 0.4% to about 0.6% by weight of total resin solids. In addition to catalyst, flow control agent(s), for example, polybutyl acrylate; wetting agent(s), for example, silicone; pigments; pigment dispersants; corrosion inhibitors, for example, chromate pigments, numerous of all of which are known to the skilled of the art, may be employed in the coating compositions of the invention. In addition, suitable reactive additives can be used, including, for example, low molecular weight diol flow control agents and reactive diluents.

Compositions of the invention, and in particular primers of higher solids content, also may include antisettling or anti-sagging agents to control the thixotropic properties of the composition. Exemplary of available materials suitable for this purpose are Dislon (trademark) 6900-20X manufactured by Kusumoto Chemicals, Ltd., Tokyo, Japan and sold by King Industries, Norwalk, CT 06852; Bentone (trademark) 38, N. L. Industries, Highstown, N.J.; and Cab-O-Sil (trademark) M-5, Cabot Corporation.

Curing the coating composition requires baking for sufficient time at sufficiently elevated temperature to react the crosslinking agent with the hydroxyl functionality of the epoxy ester resin. The time and temperature required to cure the coating are interrelated and depend upon the particular epoxy ester resin, crosslinking agent, solvent and other materials, if any, and the amount of each comprising the coating composition. The coating compositions according to preferred embodiments of the invention, as described above, are found to provide best coating results when cured at temperature at about 300.degree. F. for about 20 minutes. It is a highly significant advantage of the invention, however, that these same coating compositions can withstand, for example, temperature as high as about 200.degree. C. (390.degree. F.) for periods of time as long as about 60 minutes. Accordingly, great flexibility is provided in both designing and implementing a curing schedule for parts coated with the coating compositions of the invention. Thus, in the assembly of automotive vehicles, for example, vehicles unavoidably held in a curing oven for long periods of time during unplanned assembly line shut-downs are recovered with cured and unharmed coatings.

Coating compositions according to the present invention, comprising the novel crosslinkable hydroxy functional epoxy ester resins of the invention and blocked polyisocyanate crosslinking agent, especially the preferred materials described above, are found to afford cured coatings with improved corrosion resistance, thus representing a significant advance in the art. A most preferred use of the coating composition of the invention is as a sprayable primer for use on a bare metal substrate such as a household or industrial appliance housing or an automotive vehicle body. Primer compositions typically are pigmented and any pigments commonly included in primer compositions for metal substrates and acrylic dispersion topcoats such as, for example, carbon black, iron oxide, lithopone, magnesium, silicate, silica, barium sulfate, TiO.sub.2, chrome yellow, calcium chromate, strontium chromate, zinc potassium chromate any the like may be used. The primer can be pigmented according to known methods including, for example, by grinding pigments in a portion of the curable resin and adding to the primer composition. The pigment-to-binder ratio of the primer may be as much as 4:1 by weight, respectively. It is preferred, however, to use a primer having a pigment-to-binder ratio of about 1:1 to 2:1 by weight, respectively.

No special expedients are necessary in formulating the primer compositions of this invention. For example, they may be prepared simply by incorporating the resinous components in a suitable solvent system. Thus, for example, by suitable mixing or agitation, each resinous component may be dissolved in a solvent and the resulting solutions combined to form finished primer compositions.

The solvent system may be any suitable combination of organic solvents as described above. For a sprayable automotive vehicle primer the solvent will comprise preferably about 25 to about 35 percent by weight of the total coating compositions although, of course, larger or smaller amounts may be utilized depending upon the solids content desired.

The primer is generally thinned to from about 65 to about 70 percent solids content for spraying purposes with conventional thinners such as aromatic hydrocarbons, commercial petroleum cuts which are essentially aromatic, and the like, and sprayed on to the metal base and cured. The primer is cured at elevated temperatures by any convenient means such as baking ovens or banks of infra-red heat lamps. Curing temperatures are preferably from about 135.degree. C. to about 165.degree. C., although curing temperatures from about 100.degree. C. to about 230.degree. C. may be employed, if desired.

The invention will be further understood by referring to the following detailed examples. It should be understood that the specific examples are presented by way of illustration and not by way of limitation. Unless otherwise specified, all references to "parts" are intended to means parts by weight.

In a suitable reactor are charged diepoxide, specifically 1240 parts of Epon 829 (trademark, Shell Chemical Co.) and dicarboxylic acid, specifically 1300 parts of Empol 1016 (trademark, Emery Industries, Inc. Cincinnati, Ohio). The temperature of the mixture is brought up to 177.degree. C. (350.degree. F.) at which point an exothermic reaction takes place that raises the temperature up to 204.degree. C. (400.degree. F.). The temperature is kept above 350.degree. F. while the acid number drops below 7. The batch is then cooled down to 250.degree. F. and 523 parts of diethanol amine and 500 parts of methyl amyl ketone are added. The mixture is then heated to 270.degree. F. and kept at this temperature for 1 hour. The batch is then thinned with 263 parts of methyl amyl ketone. The final resin has a viscosity of L.sub.3 at 80% solids.

Epoxy ester resins according to the invention are prepared in the manner generally of Example I. The components employed are shown in Table I, all amounts are shown in parts by weight.

A millbase, that is, a composition pigment paste is prepared by grinding in a ballmill the following mixture:

Coating compositions according to the invention are prepared, each being adapted for use as a sprayable, pigmented primer for application over bare, unpolished steel automotive vehicle body panels in an automobile vehicle assembly operation. The coating composition components are shown in Table II, below. Each coating composition is reduced with methyl amyl ketone to about 18-25 sec. #4 Ford Cup, 27.degree. C. (80.degree. F.) to provide a preferred spraying viscosity. It will be noted that use of a drier, as in Examples VII, IX and X, is optional, to catalyse reaction of fatty acid double bonds to provide additional crosslinking in the cured resin. In Table II, all amounts are expressed in parts by weight.

Additional coating compositions according to the invention, each adapted for use as a sprayable pigmented primer for application over, for example, bare, unpolished steel automobile vehicle body panels in an automotive vehicle assembly operation, are prepared as shown in Table III, below. It should be noted that use of a drier, as in the coating compositions illustrated in Table III, is optional. The epoxy ester resin employed in each coating composition is identified by reference to the Example according to which it was prepared. All amounts are expressed in parts by weight.

A blocked isocyanate crosslinker useful in compositions of the invention is prepared. In a suitable reactor, 417 parts of PAPI 580 (trademark). The Upjohn Company (Kalamazoo, Mich.) are charged under a nitrogen blanket. 261 parts of methyl amyl ketoxime are added dropwise in a period of 30 minutes keeping the temperature below 210.degree. F. by external cooling. After the addition is completed the temperature is maintained at 210.degree. F. for an additional hour at which time the complete reaction of the isocyanate is verified by I.R. The batch is then thinned with 226 parts of methyl amyl ketone. The resulting resin has a viscosity of Z.sub.6+ at 75% solids.

Blocked isocyanate crosslinking agents useful in compositions of the invention are prepared in the manner generally described in Example XV from the components shown in Table IV.

Coating compositions according to the invention are prepared, each being adapted for use as a high solids, sprayable, pigmented primer for application over bare, unpolished steel automotive vehicle assembly operation. The coating composition components are shown in Table V below. Each coating composition is reduced with methyl amyl ketone to about 18-25 sec. #4 Ford Cup, 27.degree. (80.degree. F.) to provide a preferred spraying viscosity. It will be noted that use of a drier is optional, to catalyse reaction of fatty acid double bonds to provide additional crosslinking in the cured resin. In Table V, all amounts are expressed in parts by weight.

In view of the foregoing disclosure, many modifications of this invention will be apparent to those skilled in the art. All such apparent modifications fall within the scope of this invention and are intended to be included within the terms of the appended claims.

It will be apparent from the foregoing that this invention has industrial applicability as a coating composition, especially as a coating composition for automotive vehicles, household appliances and the like, and other applications where excellent humidity and solvent resistance are needed to provide protection for a substrate, for example a metal substrate, against corrosion, wear and the like.