Diblocked Diisocyanate Diurea Oligomers and Coating Compositions Comprising Same

This invention relates to novel resin systems and to high solids, solvent based coating compositions comprising same. A first component comprises any of certain novel chain-extendable, crosslinkable diblocked diisocyanate diurea oligomers. The second comprises polyepoxide. The resin components provide chain-extension polymerization during cure at elevated temperature, in situ, on the surface of a substrate. The resin system is self-crosslinking, that is, no additional crosslinking agent is required. The cured coatings of the invention are highly humidity and solvent resistant and provide exceptional corrosion resistance.

Solvent based coating compositions are known which employ high molecular weight (e.g. 2,000 to 10,000) polymer resins having crosslinking functionality, and a suitable crosslinking agent. Typically, such coating compositions are applied to a substrate, for example, by spraying, and are then cured by baking the coated substrate at an elevated temperature suitable to drive off the organic solvent and to promote the crosslinking reaction. The resulting thermoset coating, if sufficiently humidity, solvent and corrosion resistant, can provide aesthetic and functional advantages including corrosion protection for the underlying substrate.

Coating compositions comprising such high molecular weight polymer resins typically comprise only 25% to 50% solids so as to be sprayable or otherwise conveniently applicable to a substrate. The viscosity of coating compositions of higher solids content is typically too high for this purpose. Conventional epoxy ester based automotive vehicle spray primers, for example, typically have a volatile organic content ("VOC") of approximately 540 g/l.

Elimination of the volatile organic solvent portion during curing of these conventional low-solids coating compositions presents toxicity and in some cases flammability hazards. Furthermore, bulk volume of these coating compositions is relatively large and therefore presents undesirable material handling difficulties, and added expense. Furthermore, excessive solvent losses and/or solvent recovery equipment add considerable expense to the coating operation. Recently, governmental regulations on hydocarbon emissions, particularly applicable to automotive coating operations, mandate a significant reduction in volatile organic content for coating compositions. Thus, for example, governmental guidelines for 1982 presently require that emissions of volatile organics from automotive vehicle primer coating operations be reduced to that equivalent to using coating compositions of no greater than 350 g/l (2.9. lb./gal.) VOC. To meet that guideline, coating compositions of VOC greater than 350 g/l can be employed in conjunction with emissions treatment equipment to achieve the specified emissions limit. Such treatment equipment presents significant additional expense, however, and thus there is a great need to provide coating compositions of VOC reduced near to, or preferably even lower than, the 350 g/l governmental limit.

In response to these concerns, high solids coating compositions have been suggested which, typically, employ low molecular weight multi-functional adducts or copolymers in combination with multi-functional crosslinking agents. These high solids coating compositions are less viscous and, therefore, can be applied by spraying, for example, with far lower VOC than was possible with conventional epoxy ester based coating compositions or other conventional coating compositions comprising high molecular weight polymer resins. After application to the substrate, high solids coating compositions are cured by baking at a cure temperature, that is, at an elevated temperature suitable to drive off the volatile organic content and to promote polymerization and crosslinking of the multi-functional low molecular weight component(s).

Typically, high solids coating compositions yield cured coatings having polymeric networks that differ significantly in structure and morphology from the polymeric networks provided by conventional, low solids coating compositions comprising high molecular weight polymers. Consequently, the physical properties of the coatings provided by such high solids coatings compositions can differ significantly from those of the cured coatings provided by the conventional, low solids coating compositions. In particular, the cured coatings obtained from known high solids coating compositions can be inferior in that they can be less flexible, less solvent resistant, less adherent to the substrate and/or for other reasons provide less corrosion inhibition for the underlying substrates. Accordingly, it would be highly desirable to provide a coating composition comprising low molecular weight materials suitable for use in high solids, solvent based coating compositions and yet which, upon curing, form coatings having physical properties, particularly corrosion resistance, comparable to or better than the physical properties of coatings obtained from conventional low solids solvent based coating compositions.

Accordingly, it is an object of the present invention to provide novel resin compositions suitable for use in high solids, solvent-based coating compositions. In this regard, it is a particular object of the invention to provide novel coating compositions which are curable by chain-extension and crosslinking during cure, in situ, on the surface of a substrate to form polymeric coatings similar in properties to those obtainable through use of conventional low solids, solvent-based coating compositions.

It is a particular object of the invention to provide a coating composition of sufficiently low VOC to facilitate compliance with governmental guidelines. It is also an object of the invention to provide such a coating composition which can be applied to a substrate by spraying or other known method.

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

According to the present invention, low molecular weight chain-extendable, crosslinkable diblocked diisocyanate diurea oligomers are provided which are suitable for use in high solids, organic solvent based coating compositions. The novel oligomers of the invention are the reaction product of half-blocked diisocyanate with suitable diamine and have a number average molecular weight of preferably about 300 to about 5000, more preferably about 300 to about 1500. The half-blocked diisocyanate is the reaction product of organic diisocyanate with monofunctional blocking agent.

According to another aspect of the invention, a novel solvent-based resin composition comprises the novel chain-extendable, crosslinkable diblocked diisocyanate diurea oligomer of the invention, polyepoxide bearing preferably about 2 to 10, more preferably about 2 to 4 epoxide groups, and having molecular weight of about 100 to 1000, more preferably about 300 to 700, and suitable organic solvent. The resin composition comprises latent chain-extension polymerization functionality and undergoes both chain-extension polymerization and crosslinking reaction, in situ, on the surface of the substrate during cure of the coating to form high molecular weight polymers. More specifically, the blocked isocyanate groups of the novel oligomers of the invention undergo thermal de-blocking at elevated temperature between about 120.degree. C. and 250.degree. C., in more preferred embodiments between about 150.degree. C. and 220.degree. C., and then react with the epoxy functionality of the polyepoxide forming a polymer bearing oxazolidone linkages. The resin composition further comprises latent self-crosslinking functionality. That is, the de-blocked isocyanate functionality is also reactive with the N-hydrogen functionality of the urea moieties of the chain-extended polymerization product which is formed during cure of the coating composition. That is, during cure of the coating composition, in situ, on the surface of the substrate, each blocked isocyanate group is de-blocked and will undergo chain-extension reaction with an epoxy group of the polyepoxide component of the coating composition or will undergo crosslinking reaction with a urea moiety of the chain-extended polymerization reaction product forming during cure. If the polyepoxide is a diepoxide, then the epoxy isocyanate reactions will provide substantially linear chain-extension polymerization. If the polyepoxide bears three or more epoxide groups, then epoxy/isocyanate reaction will provide not only chain-extension polymerization but also an additional mode of crosslinking in the cured coating. Thus, the degree of crosslinking in the cured coating can be controlled, in part, by the selection of the polyepoxide employed in the coating composition. Accordingly, the novel resin composition of the invention is curable to form a coating, in situ, on the surface of a substrate, employing both a chain-extension polymerization reaction and separate and distinct crosslinking reactions.

The resin composition of the invention can be formulated into high solids coating compositions having a viscosity as low as about 25-40 seconds, #4 Ford Cup or less, at 27.degree. C. at calculated VOC of 350 to 400 g/l or less.

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 novel resin composition of the invention and heating the resin composition to between about 120.degree. C. and about 250.degree. C. and preferably to between about 150.degree. C. and about 220.degree. C. for a period sufficient to yield a cured coating. The cured coating, which is yet another aspect of the invention, is solvent and humidity resistant and has been found to provide exceptionally good corrosion resistance.

As used herein, a high solids coating composition is one comprising polymerizable resin in which a volatile organic solvent content of about 400 g/l (3.4 lb./gal.) or less yields a viscosity of less than approximately 40 sec. #4 Ford Cup at 27.degree. C. (80.degree. F.). Thus, such high solids coating composition could be applied, for example, by spray techniques to a substrate.

The novel crosslinkable, chain-extendable diblocked diisocyanate diurea oligomer of the invention preferably has a number average molecular weight about 300 to about 5000, more preferably about 300 to about 1500. These oligomers are provided as the reaction product of a suitable diamine with suitable half-blocked diisocyanate. The two amine groups of the diamine each react with the free isocyanate functionality of a different half-blocked diisocyanate molecule, each forming a urea linkage. The reaction product comprises two blocked isocyanate groups, one from each of the two half-blocked diisocyanate molecules which reacted with the diamine. The diamine and half-blocked diisocyanate are reacted together according to methods which are well known to the skilled of the art. It will be within the skill of the art in view of the present disclosure to select a suitable diamine, of which many, including aromatic and aliphatic diamines, are readily commercially available. Preferred aliphatic diamines are of molecular weight about 50 to about 700, more preferably about 50 to about 300 and include, for example, alkylenediamines, wherein the alkylene moiety is straight or branched chain and has, preferably about from 2 to about 20, more preferably about 3 to about 12 carbons, and of which terminal alkylenediamines, that is, alkylenediamines bearing two terminal amine functionality, are most preferred, for example, 1,6-hexanediamine, 1,5-pentanediamine, 1,4-butanediamine and a mixture of any of them. Also preferred are cycloaliphatic diamines of about 4 to about 20 carbons, for example, isophorone diamine, and aromatic diamines wherein each amine is substituted on the same benzene ring or on different benzene rings linked through a covalent bond or through one or more carbons of a one to seven carbon aliphatic moiety, for example, toluene diamine and 4,4'-methylenedianiline.

Lower molecular weight aliphatic diamines, those of one to six carbons, for example, ethylenediamine and hexanediamine, are preferred over aromatic diamines if providing a very high solids content (that is, low VOC) coating composition is of primary concern. The lowest molecular weight aromatic diamines, for example, toluene diamine, have seven or more carbons and require a greater VOC for any desired viscosity.

Suitable half-blocked diisocyanates comprise the reaction product of a suitable organic diisocyanate in approximately 1:1 molar ratio with suitable monofunctional blocking agent, and include those of number average molecular weight about 120 to about 2000, preferably about 120 to about 600. Suitable organic diisocyanates are readily commercially available and will be apparent to the skilled of the art in view of the present disclosure. Suitable diisocyanates include aromatic diisocyanates, for example, phenylene diisocyanates and toluene diisocyanates, and aliphatic diisocyanates, for example, isophorone diisocyanates and diisocyanatoalkane wherein the alkyl moiety has preferably from about three to ten carbons, for example, 1,4-diisocyanatobutane and 1,6-diisocyanatohexane and the like or a compatible mixture of any of them. Most preferably, the organic diisocyanate has molecular weight less than about 250. If corrosion resistance is of primary concern in the cured coating, for example, in the case of an automotive vehicle primer or topcoat, it may be preferred to use an aliphatic diisocyanate, for example, isophorone diisocyanate and 1,1-hexane diisocyanate. Aromatic diisocyanates provide suitable coatings, however, and may be preferred in view of their lower cost.

Suitable half-blocked diisocyanate is prepared by reaction of any suitable organic diisocyanate, as described above, with sufficient monofunctional blocking agent to block approximately one half of the isocyanate functionality. Accordingly, monofunctional blocking agent is reacted with organic diisocyanate in approximately 1:1 molar equivalent ratio. Suitable techniques well known to the skilled of the art can be employed to maximize the yield of half-blocked diisocyanate, such as, for example, adding the blocking agent slowly to the organic diisocyanate under reaction conditions. The half-blocked diisocyanate is then reacted with the previously described diamine in molar ratio of about 2:1, respectively, to produce the chain-extendable, diblocked diisocyanate diurea oligomer of the invention.

Suitable readily commercially available monofunctional blocking agents are well known to the skilled of the art. The blocking agent is selected such that the blocked isocyanate group will remain blocked for long periods of time at normal storage temperatures, but will be substantially totally "de-blocked" at elevated "cure" temperature. In addition, since the blocking agent will be released when the coating composition is cured by baking, it is preferred that the blocking agent have high volatility near its de-blocking temperature and so will diffuse rapidly through the coating composition and evaporate completely therefrom during the baking step. Any blocking agent allowed to remain in the cured coating should be inert to the cured coating and to the substrate and to any other coatings to be used in conjunction with it. It is within the skill of those skilled in the art to select a suitable blocking agent to provide a de-blocking temperature meeting the requirements of each particular application intended for a coating composition of the invention. It will typically be preferred that the blocked isocyanate functionality be de-blocked (i.e., that the coating composition be curable) at a temperature within the range of about 150.degree. to 220.degree. C. Accordingly, preferred monofunctional blocking agents are selected from the group comprising suitable amides, for example caprolactam, phenols, ketoximes and lower alcohols, for example, primary and secondary alkanol of one to about eight carbons, for example methanol, ethanol, any propanol, any butanol, any pentanol, including cyclopentanol, and the like, or a mixture of any of them.

The novel solvent based resin compositions of the invention comprise the novel chain-extendable crosslinkable diblocked diisocyanate diurea oligomers of the invention and suitable polyepoxide. Preferred polyepoxides have from 2 to about 10 epoxide functionality, more preferably from 2 to about 4. Preferably the polyepoxide (or each of them) has a number average molecular weight between about 100 and 1000, and more preferably between about 300 and 700. Numerous suitable polyepoxides are readily commercially available and will be apparent to the skilled of the art in view of the present disclosure. Preferred polyepoxides include, for example, any of a wide variety of acyclic or cyclic aliphatic polyepoxides such as, for example, 1,4-butane diol diglycidyl ether, vinylcyclohexene diepoxide and Araldite Cy179 (trademark, Ciba-Geigy Corporation, Ardsley, N.Y.), and aromatic polyepoxides such as, for example, Bisphenol A epichlorohydrin epoxy resins and the like or a compatible mixture of any of them. Preferred polyepoxides include terminal polyepoxides, that is, polyepoxides bearing two terminal epoxide groups, since these are generally more reactive and therefore provide coating compositions which cure faster and/or at lower temperature. Preferred polyepoxides include diepoxides. Most preferred in view of their commercial availability are, for example, for example, acyclic or cyclic aliphatic diepoxides such as, for example, 1,4-butanediol diglycidyl ether and 4-vinylcyclohexene dioxide and aromatic diepoxides such as, for example, Bisphenol A epichlorohydrin epoxy resins, for example, Epon 828 (trademark) and other members of the Epon (trademark) series, Shell Chemical Company, Houston, Tex., and DER 331 (trademark) and other members of the DER (trademark) series, Dow Chemical Company, Midland, Mich. Also preferred are cycloaliphatic diepoxy resins, for example, the Eponex (trademark) series of Shell Chemical Company, Houston, Tex., and hydantoin epoxy resins, for example, Resin XB2793 (trademark), Ciba-Geigy Corporation, Ardsley, N.Y. and epoxy novalak resins, for example, Epon 152 (trademark) and Epon 154 (trademark), Shell Chemical Company.

In general, of the above described diepoxides, the lower molecular weight diepoxides are preferred, for example, Epon 828 (trademark), since a resin composition of correspondingly lower viscosity (or lower VOC) is provided. Other, higher molecular weight diepoxides, for example, higher molecular weight members of the Epon (trademark) series, are suitable for coating compositions of somewhat higher viscosity (or lower solids content). The higher molecular weight members of the Epon series, for example Epon 1001 and Epon 1004, and the like may be somewhat less preferred, since they afford coating compositions of higher VOC (or lower solids content) and with de-blocked isocyanate functionality. Also, however, improved properties, for example, improved corrosion resistance have been achieved with coating compositions comprising polyols prepared using such materials and the choice of suitable polyepoxide will depend upon the particular application intended for the coating composition.

Polyepoxide can be used in approximately stoichiometric amount in the coating composition. That is, the coating composition can comprise equal molar equivalents of polyepoxide and diblocked diisocyanate diurea oligomer. Such compositions provide efficient chain-extension polymerization during cure of the coating composition. While there can be an excess of oligomer, there should not be an excess of polyepoxide, since unreacted epoxy functionality can adversely effect the humidity, solvent and corrosion resistance of the cured coating. Preferably the oligomer and polyepoxide are used in a weight ratio of 1:1 to about 10:1 equivalent weights, respectively, and more preferably in a ratio of 1:1 to about 2:1.

While not wishing to be bound by theory, it is presently understood that, upon curing the coating composition, the polyepoxide and the de-blocked diisocyanate diurea oligomer undergo chain-extension reaction to form a polymer bearing oxazolidone ring linking moieties. The polyoxazolidone so formed is believed to crosslink through reaction of N-H functionality of the urea moieties of the polymer with de-blocked isocyanate functionality of the oligomers to form ureylene linkages. The degree of crosslinking in the cured coating provided through this mode of crosslinking can be controlled by selection of the ratio of epoxide functionality to isocyanate functionality in the resin composition. The greater the excess of isocyanate functionality, the higher the crosslink density in the cured coating.

In addition to crosslinking by de-blocked isocyanate/urea N-hydrogen reaction, the polyepoxide can provide crosslinking in the cured coating. While a diepoxide which has undergone chain-extension reaction with a de-blocked isocyanate functionality of each of two oligomer molecules provides no additional epoxy functionality, a polyepoxide of three or more epoxide groups which has undergone chain-extension reaction with two de-blocked oligomer molecules can react with the de-blocked isocyanate of one or more additional oligomers and thereby provide an additional mode of crosslinking and thus a more highly crosslinked polymeric structure in the cured coating. Accordingly, in general, diepoxides are preferred over polyepoxides of 3 or more epoxide groups if greater flexibility is desired in the cured coating. It should be recognized, however, that higher homologs of the epoxy resins useful in the present invention often comprise hydroxy functionality in addition to the epoxy functionality. It is believed that de-blocked isocyanate funcitionality of the oligomer will react with such hydroxy functionality. This aspect of the invention provides significant advantage where more efficient crosslinking is desired.

It will be appreciated from the above, that network crosslink density can be controlled, and therefore the flexibility and other properties of the cured coating can to a large extent be controlled by selection of the polyepoxide and of the diblocked diisocyanate diurea oligomer used in the coating composition. Crosslink density increases and flexibility decreases as the epoxide equivalent weight of the polyepoxide is reduced and/or as the isocyanate equivalent weight of the diblocked diisocyanate diurea oligomer is reduced. Thus, it will be apparent to the skilled of the art in view of the present disclosure that the selection of the polyepoxide and the selection of the organic diisocyanate, blocking agent and alkanolamine reactants for preparing the diblocked diisocyanate diurea oligomer provides substantial control of the crosslink density in the cured coating. Thus, for example, if the polyepoxide used is diepoxide, there will be a lower crosslink density in the cured coating than if the polyepoxide used is of similar molecular weight but has three or more epoxy groups per molecule.

Sufficient solvent is used in the coating composition of the invention to reduce the viscosity of the coating composition to a level suitable for application to the substrate in the desired manner. The molecular weight of the polyepoxide and of the diblocked diisocyanate diurea oligomer will affect the volatile organic content of the coating composition at a desired viscosity. Where a high-solids coating composition is desired, preferably lower molecular weight components are employed, since this has been found to provide high-solids coating compositions which can be applied easily to a substrate by spray or other means in a coating composition having a calculated volatile organic content of about 350 g/l (2.9 lb./gal.) or less. More specifically, while conventional epoxy ester-type automotive spray-applied primer coating compositions are known to require a volatile organic content of about 540 g/l, the novel coating compositions of the present invention require as little as about 350 g/l to 400 g/l (2.9 lb./gal. to 3.4 lb./gal.) or less VOC (calculated) to provide a viscosity of less than about 40 sec., #4 Ford Cup at 27.degree. C. (80.degree. F.). Of course, the coating compositions of the invention need not be formulated as a "high solids" compositions, but rather can have a higher VOC to provide a lower viscosity. It is generally preferred in automotive vehicle spray coating applications and the like, for example, that sufficient solvent be used to provide a viscosity of about 15 to 40 sec., #4 Ford Cup at 27.degree. C. (80.degree. F.).

It will be within the skill of the art to determine the proper volatile organic content for a given coating composition of the invention for a given application. In general, suitable solvents include, for example, Cellosolve (trademark), Butyl Cellosolve (trademark), Butyl Cellosolve Acetate (trademark), Hexyl Cellosolve (trademark), Hexyl Cellosolve Acetate (trademark), Proposol P (trademark), Proposol B (trademark), Proposol M (trademark), all of Union Carbide Corporation, New York, N.Y., butanol, methyl ethyl ketone, methyl amyl ketone and the like, or a compatible mixture of any of them. Additional suitable solvents will be apparent to the skilled of the art in view of the present disclosure.

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 used in conjunction with it during the curing process or thereafter. Preferably the cured coating is completely free of solvent. The preferred solvents, in addition, have relatively low volatility at temperatures appreciably below their boiling point such that solvent evaporation is low during storage and/or application of the coating composition to the substrate.

Also preferably included in the coating composition of the invention is any of a variety of commercially available catalysts which, in view of the present disclosure, will be apparent to the skilled of the art to be suitable to catalyze the chain-extension polymerization reaction between epoxide and de-blocked isocyanate functionality. Suitable catalysts include, for example, alkoxy bases, for example, sodium phenoxide, sodium methoxide, sodium butoxide, and tertiary amines, for example, hexamethylene triamine, and the like or a compatible mixture of any of them. Some catalyst may however reduce the stability of the coating composition. In addition, any of a variety of catalysts for the isocyanate de-blocking reaction can also be included in the coating composition, for example, dibutyl tin dilaurate. Also suitable as catalyst are Anchor 1040 (trademark) and Anchor 1115 (trademark) available from PVO International Corporation, San Francisco, Calif., which comprise amine blocked boron trifluoride and which have been found to catalyse the isocyanate de-blocking reaction. However, it should be recognized that amine catalyst should generally not be employed together with acid catalysts.

In addition, flow control agent, for example, polybutyl acrylate; wetting agent, for example, silicone; pigments; pigment dispersents, 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.

According to another aspect of the invention, a coating on a substrate is provided, which coating comprises the chain-extended, crosslinked polymer product following cure of a coating comprising the resin system of the invention. The coating composition can be a low solids composition, that is, it can have a high VOC, but generally a high solids composition, that is, one having a low VOC, is preferred for the reasons given above. 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 composition as an automotive vehicle body primer or topcoat. In such spraying applications, the coating compositions of the invention are especially advantageous for use as high solids compositions.

Curing the coating composition requires baking for sufficient time at sufficiently elevated temperature to de-block the blocked isocyanate functionality of the diblocked diisocyanate diurea oligomers and to promote the subsequent chain-extension and crosslinking reactions. The time and temperature required to cure the coating are interrelated and depend, in part, upon the particular diblocked diisocyanate diurea oligomer, polyepoxide, sont and other materials, if any, used in the coating composition and upon the relative proportion of each. Employing a volatile organic content of about 360 g/l and selecting preferred components as described above, the required bake time and temperature is typically about 20 to 30 minutes at about 180.degree. C. The temperature required for cure can be reduced to about 150.degree. C. for 20 to 30 minutes by addition of suitable catalyst such as any of those described above or otherwise known to the skilled of the art, for example, dibutyl tin dilaurate and tertiary amine.

It is a significant advantage of the coating composition of the invention that it does not require the addition of crosslinking agent. Known high solids coating compositions employing melamine crosslinking agent, for example, are disadvantaged by the cost of same and are further disadvantaged by the problems associated with the possible evolution of formaldehyde during cure of such coating compositions. It is another advantage of the invention, notwithstanding that a crosslinking agent is not added to the coating composition of the invention, that the crosslink density can be easily controlled to a large extent. As indicated above, the molecular weight and carbon chain lengths of the polyepoxide and of the reactants for preparation of the diblocked diisocyanate diurea oligomers of the invention can be selected to provide the desired crosslink density in the cured coating. In addition, the number of epoxy groups per polyepoxide molecule affords control of the crosslink density in the cured coatings. In addition, the relative proportion of polyepoxide to oligomer can be selected to control crosslink density, since excess oligomer can provide crosslinking via the active hydrogen of the urea moieties of the polymer product of the chain-extension reaction.

High solids coating compositions according to the present invention, have been found to afford cured coatings with good humidity and solvent resistance and with exceptionally good corrosion resistance. The corrosion resistance has been found to be comparable and even better than that of conventional epoxy ester based, low solids sprayable coating compositions. The significant reduction in volatile organic content provided by the high solids coating composition of the invention can be seen to present, therefore, a highly advantageous advance in the art. Thus, for example, cured coatings according to the invention have been found to provide excellent corrosion resistance when applied over a metallic substrate such as, for example, when applied as an automotive vehicle primer coat over bare sheet steel. While not wishing to bound by theory, the exceptional corrosion inhibition provided by the invention is believed to stem, in part, from the presence of C-N bonds both in the urea moieties of the chain-extension reaction product and in the ureylene crosslinking moieties. In addition, the absence of ester linkages in the cured coating is believed to improve corrosion resistance. Ester linkages are known to be attacked by hydroxide, a product of the metal corrosion process. In contrast, the carbon-nitrogen bonds of the cured coating of the invention are believed to be highly alkali resistant and thus highly resistant to degradation processes involving hydrolysis by cathodicly generated hydroxide, including hydroxide generated by the corrosion of a metal substrate. Accordingly, the coating composition of the invention provide improved corrosion resistance in comparison to known coating compositions, for example, those comprising ester or even urethane linkages. Thus, for example, automotive vehicle primers comprising the coating composition of the invention and having a calculated volatile organic content of 350-390 g/l, have been discovered to provide corrosion resistance far superior to that provided by typical low solids, e.g., greater than 500 g/l volatile organic content, epoxy ester-based primers.

As presently understood, chain-extension reaction by each of the two isocyanate functionality of a diblocked diisocyanate diurea oligomer, which are de-blocked during cure of the coating composition, provides substantially linear chain-extension, in situ, on the surface of the substrate. De-blocked isocyanate functionality not undergoing chain-extension reaction is available for crosslinking reaction with active N-hydrogen at the urea functionality of the polyurea polymer forming by chain-extension reaction during curing. Accordingly, it is preferred that the blocked isocyanate groups and two of the epoxy of the polyepoxide can be an end group. Reactions between such epoxy end groups and de-blocked isocyanate end groups are believed to provide most efficient chain-extension during cure.

Ethanol, 9.07 g, was added to isophorone diisocyanate, 43.6 g, in methyl amyl ketone, 13.2 g with 0.2 g dibutyl tin dilaurate. The mixture was heated at 60.degree.-80.degree. C. for 2 hours. The resulting half-blocked aliphatic diisocyanate was cooled to room temperature and stored. The product was characterized by its infrared spectrum (after solvent evaporation) which showed a reduction of the isocyanate absorption at 2250 cm.sup.-1 and the presence of an intense carbonyl absorption at 1710 cm.sup.-1.

The diblocked diisocyanate diurea oligomer is prepared by adding 37. g of the alcohol half-blocked aliphatic diisocyanate prepared in Example I to 5.8 g of 1,6-hexanediamine in 40. g of Hexyl Cellosolve Acetate. The rate of addition is controlled to maintain a gentle reflux. The reaction is stirred after completion of the addition for approximately 2 hours at 60.degree. C. and then cooled to room temperature and stored. Infrared spectroscopy shows no absorption for isocyanate by the product.

An automotive vehicle primer comprising a coating composition according to the invention is prepared by mixing a pigment package and a resin package, the following:

The pigment package is thoroughly dispersed into the binder package. The resulting composition is ready for use as an automotive primer. It has a calculated volatile organic content of about 370 g/l, and a viscosity of less than 40 sec., #4 Ford Cup, at 27.degree. C.

The coating composition of Example III is filtered, sprayed on Parker cold rolled bare unpolished steel panels and baked at 180.degree. C. for 30 minutes. The resulting cured coating has excellent solvent and humidity resistance and shows little or no adhesion loss after 24 hours salt spray exposure according to ASTM test method B117 using a Singleton SCCH Corrosion test cabinet operated at 35.degree..+-.2.degree. C.

Alcohol half-blocked aliphatic diisocyanate is prepared according to the procedure of Example I.

The diblocked diisocyanate diurea oligomer is prepared by adding 37.0 g of the alcohol half-blocked aliphatic diisocyanate prepared in Part A to 99. g of 4,4'-methylenedianiline in 60. g methyl ethyl ketone. The ratio of addition is controlled to maintain a gentle reflux. The reaction is stirred after completion of the addition for approximately 2 hours at 60.degree. C. and then cooled to room temperature and stored. Infrared spectroscopy shows no absorption for isocyanate by the product.

An automotive vehicle primer comprising a coating composition of the invention is prepared by mixing a pigment package and a resin package as follows:

The pigment package is thoroughly dispersed into the binder package. The resulting composition is ready for use as an automotive primer.

The coating composition of Example VI is sprayed on bare, unpolished steel panels and baked at 180.degree. C. for 30 minutes. The resulting cured coating has excellent solvent and humidity resistance and shows little or no adhesion loss after 24 hours salt spray exposure according to ASTM test method B117 using a Singleton SCCH Corrosion test cabinet operated at 35.degree..+-.2.degree. C.

Particular embodiments of the present invention described above are illustrative only and do not limit the scope of the invention. It will be apparent to the skilled of the art in view of the foregoing disclosure that modifications and substitutions can be made without departing from the scope of the invention.