Self-crosslinking resin

Abstract: A self-crosslinking resin comprising a resin containing a blocked isocyatate group, and a resin containing a hydroxyl group, and a carboxyl group or carboxylic anhydride group and attached to the resin by urethane bond.

Description: The present invention relates to novel self-crosslinking resins having within the molecule a blocked isocyanate group and a hydroxyl group which are reactive groups complementary to each other. The resins are useful especially for adhesive compositions and coating compositions of the aqueous type. Two-pack or single-pack coating compositions wherein acryl polyol or like hydroxyl-containing polymer is cured with an isocyanate crosslinking agent produce cured coatings which are excellent in chemical resistance, physical properties, weather resistance, etc. and are therefore in wide use, for example, as motor vehicle coating compositions. However, the two-pack composition has problems as to safety and hygiene when the isocyanate crosslinking agent is admixed with the other component or when the composition is applied, and further has the drawback that the coating preparation obtained by mixing the two components together has a short pot life before application, becomes viscous during coating operations and presents difficulty in cleaning the coating device. The single-pack composition has the drawback of being not always satisfactory in the compatibility of the crosslinking agent, i.e., blocked isocyanate, with the acryl polyol serving as the base resin, such that even if they are compatible, the cured coating formed differs in composition between the surface and the interior when observed microscopically and is low in curability, gloss and surface smoothness. To overcome these drawbacks, JP-A-186722/1988 discloses a self-crosslinking resin which is prepared by copolymerizing a polymerizable monomer containing an isocyanate group or such a monomer as blocked with a polymerizable monomer containing a hydroxyl group to introduce the complementary reactive groups into the molecule. Nevertheless, preparation of the disclosed resin involves problems. When a radically polymerizable monomer containing an unblocked isocyanate group is copolymerized with a hydroxyl-containing radically polymerizable monomer as disclosed in the publication, it is difficult to prevent the reaction between the isocyanate group and the hydroxyl group even if the copolymerization reaction is conducted at the lowest possible temperature, with the result that gelation is liable to occur during the reaction. The polymerization reaction has other problems with respect to initiators. Since the isocyanate-containing monomer is a derivative of α-methylstyrene according to the publication, azo polymerization initiators fail to achieve a sufficient polymerization conversion, whereas peroxide or carbonate initiators, if used, require a temperature of at least 100°C for the polymerization when a high conversion of polymerization is to be attained. On the other hand, gelation is less likely to occur during the polymerization reaction when blocked isocyanate-containing polymerizable monomers are used. However, especially when the polymerizable monomer used contains an isocyanate group blocked with a phenol or oxime blocking agent, there arises a need to conduct the reaction at a relatively high temperature (120 to 140°C), which entails the drawback of seriously coloring the product. If other blocking agents are used, a higher dissociation temperature will result, so that the self-crosslinkable resin obtained requires heating at a high temperature (at least 170°C) for curing. When heated at a low temperature (e.g. up to 120°C), the resin will not always be cured satisfactorily. As is well know, aqueous coating compositions have increasing importance from the viewpoint of air pollution control and savings in resources. However, water-soluble melamine resins, which are still chiefly used as crosslinking agents for aqueous coating compositions, have problems in curing properties and chemical resistance. Conventional blocked isocyanate crosslinking agents are difficult to use in aqueous composition because they require high-temperature heating and produce seriously colored coatings as described above. An object of the present invention is to provide a novel self-crosslinking resin which is free of the problems of the prior art described. Another object of the invention is to provide a self-crosslinking resin of the anionic type which has a blocked isocyanate group and a hydroxyl group in the molecule and is highly resistant to chemicals, curable even at low temperatures and especially useful for aqueous compositions and which can be prepared free of gelation and coloration without entailing a reduced conversion of polymerization. These and other objects of the invention will become apparent from the following description. The present invention provides a self-crosslinking resin which is a graft polymer having a weight average molecular weight of 1000 to 100,000, the graft polymer containing a first blocked isocyanate group containing polymer skeleton and a second polymer skeleton having a hydroxyl group and a carboxyl group or carboxylic anhydride group, wherein the first and the second polymer skeleton are linked by a urethane bond and wherein the first polymer skeleton is obtained by radical polymerization of a NCO-containing vinyl monomer optionally with other vinyl monomers and the second polymer skeleton is obtained by radical polymerization of a hydroxyl-containing vinyl monomer and a carboxyl- or a carboxylic anhydride group- containing vinyl monomer optionally with other vinyl monomers. The resin of the invention is self-crosslinkable owing to the presence of both blocked isocyanate and hydroxyl, i.e., complementary reactive groups, in the molecule, further has a carboxyl group or carboxylic anhydride group and can therefore be readily made into an aqueous composition. The self-crosslinking resin can be made curable at low temperatures. The resin of the present invention can be prepared preferably by one of the following processes (1) to (4) wherein a polymerization reaction is effected without entailing gelation, coloration and a reduction in the conversion of polymerization. More specifically, the self-crosslinking resin which comprises a resin containing a blocked isocyanate group, and a resin containing a hydroxyl group, and a carboxyl group or carboxylic anhydride group and attached to the resin by a urethane bond can be prepared from a vinyl polymer having at least two isocyanate groups in the molecule (hereinafter referred to as the "NCO-containing polymer") favorably, for example, by one of the following processes (1) to (4). (1) By reacting a hydroxyl-containing vinyl monomer with some of the isocyanate groups in the skeleton of the NCO-containing polymer to introduce a polymerizable double bond into the skeleton, then completely blocking the isocyanate groups remaining in the polymer skeleton with a blocking agent, and thereafter polymerizing an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer and a carboxyl-containing vinyl monomer with the introduced polymerizable double bond. (2) By blocking some of the isocyanate groups in the skeleton of the NCO-containing polymer with a blocking agent, then reacting a hydroxyl-containing vinyl monomer with the isocyanate groups remaining in the polymer skeleton to introduce a polymerizable double bond into the skeleton, and thereafter polymerizing an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer and a carboxyl-containing vinyl monomer with the introduced polymerizable double bond. (3) By reacting a hydroxyl-containing vinyl monomer with some of the isocyanate groups in the skeleton of the NCO-containing polymer to introduce a polymerizable double bond into the skeleton, then completely blocking the isocyanate groups remaining in the polymer skeleton with a blocking agent, and thereafter polymerizing an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer and a vinyl monomer containing a carboxylic anhydride group with the introduced polymerizable double bond. (4) By blocking some of the isocyanate groups in the skeleton of the NCO-containing polymer with a blocking agent, then reacting a hydroxyl-containing vinyl monomer with the isocyanate groups remaining in the polymer skeleton to introduce a polymerizable double bond into the skeleton, and thereafter polymerizing an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer and a vinyl monomer containing a carboxylic anhydride group with the introduced polymerizable double bond. The NCO-containing polymer to be used in the above processes is a polymer consisting essentially of an isocyanate-containing vinyl monomer (hereinafter referred to as the "NCO-containing monomer"), and further comprising other vinyl monomer when so required. The NCO-containing monomer is a compound having at least one unblocked isocyante group and at least one radically polymerizable double bond in the molecule. Examples of such compounds are methacryloyl isocyanate, 2-isocyanate ethyl methacrylate, m- or p-isopropenyl-α,α'-dimethylbenzyl isocyanate and addition product of a hydroxyl-containing vinyl monomer with a diisocyanate compound in the mole ratio of 1:1. One or at least two of these compounds are usable. The hydroxyl-containing vinyl monomer for use in preparing the NCO-containing monomer is a compound having at least one hydroxyl group and at least one radically polymerizable double bond in the molecule. Examples of such compounds are 2-hydroxyethyl (meth) acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl acrylate, 2,3-dihydroxypropyl acrylate, 2-hydroxy-3-phenoxyethyl acrylate, 2-hydroxy-3-ethoxyethyl acrylate and the like. Also usable is an equimolar adduct of acrylic acid or methacrylic acid with a glycol (having 2 to 20 carbon atoms). The diisocyanate compound to be used for preparing the NCO-containing monomer is a compound having two isocyanate groups in the molecules. Examples of such compounds are aromatic, alicyclic and aliphatic diisocyanate compounds including tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), methylcyclohexane-2,4(2,6)-diisocyanate, isophorone diisocyanate and trimethylhexane diisocyanate. Especially preferable among the above NCO-containing monomers are 2-isocyanate ethyl methacrylate, m-isopropenyl-α,α'-dimethylbenzyl isocyanate and equimolar adduct of 2-hydroxyethyl acrylate with isophorone diisocyanate. NCO-containing polymer is obtained by polymerizing such an NCO-containing monomer only, or copolymerizing the monomer with other vinyl monomer. The other vinyl monomer is preferably a compound having a radically polymerizable double bond in the molecule but not having active hydrogen reactive with the isocyanate group. Examples of such compounds are aromatic vinyl monomers such as styrene, α-methylstyrene and vinyltoluene; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate; fluorine-containing vinyl monomers such as VISCOSE-3F, -3MF, -8F and -8MF (all brand names for products of OSAKA ORGANIC CHEMICAL INDUSTRY LTD.), perfluorocyclohexyl (meth)acrylate, N-2-propylperfluorooctanesulfonic acid amide ethyl (meth)acrylate, vinyl fluoride and vinylidene fluoride; nitrogen-containing vinyl monomers such as N,N'-dimethylaminoethyl (meth)acrylate, N,N'-diethylaminoethyl (meth)acrylate, and N,N'-dibutylaminoethyl (meth)acrylate and N,N'-dimethylaminopropyl (meth)acrylamide; vinyl ether monomers such as vinyl ethyl ether and vinyl butyl ether; and glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, arylglycidyl ether, alkyletherified methylolacrylamide, (meth)acrylamide, (meth)acrylic acid chloride, vinyl chloride, vinylidene chloride, (meth) acrylonitrile, γ-methacryloxyalkyltrimethoxysilane, etc. These compounds are used singly, or at least two of them are used in admixture. The ratio of NCO-containing monomer/other vinyl monomer (by weight) is suitably 100/0 to 1/99, preferably 70/30 to 10/90, although not limited specifically insofar as the NCO-containing polymer has at least two free isocyanate groups in the molecule. The reaction to polymerize the NCO-containing monomer only or the NCO-containing monomer and other vinyl monomer in mixture is conducted preferably in an inert organic solvent free from active hydrogen. The organic solvent free from active hydrogen means a solvent free from active hydrogen which is reactive with the isocyanate group. Examples of useful solvents are aliphatic hydrocarbons such as hexane, heptane and octane, aromatic hydrocarbons such as benzene, toluene and xylene, ester solvents and ketone solvents, which are used singly or in admixture. When the self-crosslinking resin of the invention is to be used for giving aqueous compositions, the organic solvent for use in the polymerization is preferably a hydrophilic or water-soluble solvent free from active hydrogen. Examples of such solvents are diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, etc. Also usable are N-methyl-2-pyrrolidone, dimethylformamide and methyl cellosolve acetate. These hydrophilic solvents generally contain water, so that it is desirable to treat the solvent for dehydration or to apply a dehydrating agent to the solvent before use. When such a solvent is used for the polymerization, the self-crosslinking resin obtained can be diluted as it is with water. Even if a water-insoluble organic solvent is used for polymerization, on the other hand, an aqueous composition can be prepared from the resulting reaction mixture by removing the solvent from the mixture or concentrating the mixture in a vacuum or spray-drying the mixture to obtain a powder, and dissolving or dispersing the concentrate or powder in water or in a mixture of water and a hydrophilic solvent. Alternatively an aqueous composition can be obtained by converting the mixture to an aqueous liquid as by an azeotropic method. Examples of water-insoluble organic solvents usable in this case are hexane, heptane, octane, cyclohexane, benzene, toluene, xylene, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, methyl amyl ketone, diethyl ketone, ethyl butyl ketone, ethyl acetate, butyl acetate, isobutyl acetate, amyl acetate, cellosolve acetate, carbitol acetate, etc. Although the above-mentioned solvent having no active hydrogen is used in the step of polymerizing the NCO-containing monomer, solvents having active hydrogen are usable in a system wherein all or almost all the isocyanate groups of the NCO-containing polymer are reacted with a blocking agent and a hydroxyl-containing vinyl monomer and which has no free NCO groups therein. Examples of such solvents having active hydrogen are cellosolve, butyl cellosolve, propylene glycol monomethyl ether, methyl cellosolve, methanol, ethanol, propanol, butanol, benzyl alcohol, etc. These solvents are highly soluble in water and readily usable for giving aqueous composition. The polymerization reaction is conducted usually at a temperature of 50 to 180°C in the presence of a radical polymerization initiator. The molecular weight of the polymer to be obtained is adjustable, for example, by varying the concentration of the reaction system or the amount of initiator. The concentration of the reaction system is in the range of 20 to 80 wt.% calculated as the polymer. Examples of useful radical polymerization initiators are organic peroxides such as benzoyl peroxide, lauroyl peroxide, caproyl peroxide, tert-butyl peroctoate and diacetyl peroxide; azo catalysts such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile and dimethyl-2,2'-azoisobutyrate; dialkylperoxy dicarbonate such as diisopropyl-peroxy carbonate; and redox initiators. The concentration of the initiator, although variable over the range of 0.01 to 15 wt.% based on the whole amount of monomer(s), is preferably in the range of 0.1 to 10 wt.%. For polymerization, an electron beam, ultraviolet rays, etc. are usable instead of the initiator. Besides radical polymerization, ion polymerization or group transfer polymerization can be resorted to. In the case where m-isopropenyl-α,α'-dimethylbenzyl isocyanate is used as the NCO-containing monomer for polymerization, it is desired to conduct the reaction at a temperature of at least 100°C using a peroxide or carbonate initiator so as to achieve a higher polymerization conversion. It is more desirable to use the isocyanate in combination with an acrylate vinyl monomer, whereby a polymer of higher polymerization conversion can be obtained easily. The NCO-containing polymer thus prepared for use in the present invention is 500 to 50000, preferably 1000 to 30000, in weight average molecular weight. The NCO-containing polymer, which has at least two free isocyanate groups in the molecule, is preferably free from any active hydrogen. The polymer is preferably 10 to 200 g/1000 g resin, more preferably 20 to 150 g/1000 g resin, in isocyanate value. The hydroxyl-containing vinyl monomer to be reacted with the NCO-containing polymer in the foregoing processes (1) to (4) is preferably one of those exemplified for preparing the NCO-containing monomer. The reaction between the NCO-containing polymer and the hydroxyl-containing vinyl monomer is the urethanation reaction between the isocyanate group and the hydroxyl group. More specifically, the urethanation reaction is conducted by admixing the hydroxyl-containing vinyl monomer with an organic solution of the NCO-containing polymer and heating the mixture usually at a temperature of 40 to 100°C. When required, catalysts such as tin compounds may be used for this reaction. Examples of blocking agents for use in the processes (1) to (4) for blocking free isocyanate groups present in the NCO-containing polymer are phenols such as phenol, cresol, xylenol, p-ethylphenol, o-isopropylphenol, p-tert-butylphenol, p-tert-octylphenol, thymol, p-naphthol, P-nitrophenol and p-chlorophenol; alcohols such as methanol, ethanol, propanol, butanol, ethylene glycol, methyl cellosolve, butyl cellosolve, methyl carbitol, benzyl alcohol, phenyl cellosolve, furfuryl alcohol and cyclohexanol; active methylene compounds such as dimethyl malonate and ethyl acetoacetate; mercaptans such as butyl mercaptan, thiophenol and tert-dodecyl mercaptan; acid amides such as acetanilide, acetanisidide, acetamide and benzamide; imides such as succinimide and maleimide; amines such as diphenylamine, phenylnaphthylamine, aniline and carbazole; imidazoles such as imidazole and 2-ethylimidazole; ureas such as urea, thiourea and ethyleneurea; carbamic acid salts such as phenyl N-phenylcarbamate and 2-oxazolidone; imines such as ethyleneimine; oximes such as formaldoxime, acetaldoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime and cyclohexanoneoxime; sulfites such as sodium bisulfite and potassium bisulfite; lactames such as ε-caprolactam, δ-valerolactam, γ-butyrolactam and β-propiolactam. Especially preferable are phenol, lactam, alcohol and oxime blocking agents. For the reaction of the blocking agent with the NCO-containing polymer, the blocking agent is admixed with an organic solution of the NCO-containing polymer, and the mixture is reacted usually at a temperature of 20 to 100°C. A tin compound catalyst may be used when required. To introduce a polymerizable double bond into the NCO-containing polymer in the processes (1) and (3), the amount of the hydroxyl-containing vinyl monomer to be admixed with the polymer is such that the NCO-containing polymer, which is 500 to 50000 in weight average molecular weight, is given at least 0.1 polymerizable double bond per molecule. The number of polymerizable double bonds to be introduced into the polymer is preferably 0.5 to 1.5, most preferably 1, per molecule when the polymer has a weight average molecular weight of 1000 to 30000. As a rule, it is desired that the blocking agent be used in an amount required to react with all the free isocyanate groups remaining in the NCO-containing polymer having the polymerizable double bond introduced therein. In the processes (2) and (4), the blocking agent is admixed with the NCO-containing polymer in an amount required to allow free isocyanate groups to remain in the polymer in such an amount that the subsequent urethanation reaction of the hydroxyl-containing vinyl monomer will introduce generally the same amount of polymerizable double bonds into the polymer as in the processes (1) and (3). It is most desirable to use the blocking agent in an amount which is required to allow free isocyanate groups to remain in such an amount that one polymerizable double bond can be introduced into the NCO-containing polymer per molecule thereof and which is needed to completely block the other free isocyanate. After the reaction of the blocking agent in the processes (2) and (4), the hydroxyl-containing vinyl monomer is reacted with all the remaining free isocyanate groups. Subsequently in the processes (1) to (4), the blocked isocyanate-containing polymer thus obtained and having the polymerizable double bond is reacted in an organic solvent with an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer, and a carboxyl-containing vinyl monomer or vinyl monomer containing a carboxylic anhydride group to polymerize the unsaturated monomer component with the polymerizable double bond, whereby a self-crosslinking resin of the invention is obtained. This resin comprises a blocked isocyanate-containing resin, and a resin having hydroxyl and carboxyl (which may be a carboxylic anhydride group) and attached to the resin by a urethane bond. In other words, the self-crosslinking resin is a graft polymer comprising a blocked isocyanate-containing polymer and a polymer having hydroxyl and carboxyl (or acid anhydride group). Examples of useful organic solvents for the above polymerization are cellosolve, methyl cellosolve, butyl cellosolve, propylene glycol monomethyl ether, methanol, ethanol, propanol, isopropanol, butanol, benzyl alcohol and hexylene glycol. These solvents may be used singly, or at least two of them are usable in admixture. The unsaturated monomer component consists essentially of a hydroxyl-containing vinyl monomer, and a carboxyl-containing vinyl monomer or vinyl monomer containing a carboxylic anhydride group. When required, other vinyl monomer is usable additionally. The hydroxyl-containing vinyl monomer and other vinyl monomer are preferably those already exemplified. The carboxyl-containing vinyl monomer or vinyl monomer containing a carboxylic anhydride group is a compound having at least one carboxyl group or carboxylic anhydride group, and at least one radically polymerizable unsaturated bond in the molecule. Examples of such compounds are α, β-ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, maleic anhydride, itaconic acid, itaconic anhydride, fumaric acid and citraconic acid. The above-mentioned other vinyl monomer for use in the present reaction step may have active hydrogen reactive with the isocyanate group. The proportions of vinyl monomers constituting the unsaturated monomer component, although not limited specifically, are preferably 1 to 90 wt.%, more preferably 10 to 60 wt.%, of the hydroxyl-containing vinyl monomer, preferably 2 to 30 wt.%, more preferably 5 to 15 wt.%, of the carboxyl- or carboxylic anhydride-containing vinyl monomer, and preferably 0 to 99 wt.%, more preferably 25 to 85 wt.%, of the other vinyl monomer based on the whole weight of the unsaturated monomer. The unsaturated monomer component is polymerized with the blocked isocyanate-containing polymer having the polymerizable double bond usually in the presence of a radical polymerization initiator. Examples of radical polymerization initiators which can be used are organic peroxides such as benzoyl peroxide, lauroyl peroxide, caproyl peroxide, tert-butyl peroctoate and diacetyl peroxide; azo catalysts such as azobisisobutyronitrile, azobis-α,γ-dimethylvaleronitrile and dimethyl-α-α'-azoisobutyrate; dialkylperoxy dicarbonates such as diisopropylperoxy carbonate; redox initiators; etc. The concentration of the polymerization initiator is variable over the range of 0.01 to 15 wt.% based on the combined amount of the vinyl monomers and is preferably in the range of 0.1 to 10 wt.%. It is suitable that the proportion of the unsaturated monomer component to be polymerized with the blocked isocyanate-containing polymer having the polymerizable double bond be generally 5 to 500 parts by weight, preferably 10 to 350 parts by weight, per 100 parts by weight of the polymer although widely variable with the kind of resin to be obtained. The combined concentration of the polymer and the unsaturated monomer component present in the organic solvent is generally suitably 30 to 80 wt.%, preferably 40 to 70 wt.%. The polymerization can be effected by a known process. It is generally suitable that the polymerization temperature be in the range of 50 to 100°C, whereas if the reaction is conducted at a temperature higher than is needed, the blocking agent in the blocked isocyanate present in the polymer having the polymerizable double bond is likely to dissociate partly, permitting a reaction between the isocyanate and hydroxyl to make the reaction mixture more viscous or cause gelation. Preferably, therefore, the reaction is conducted at a temperature of up to 70°C. The self-crosslinking resin thus prepared according to the invention comprises a resin having at least one blocked isocyanate group, and a resin having at least one hydroxyl group and at least one carboxyl group or carboxylic anhydride group and attached to the former resin by a urethane bond. The self-crosslinking resin is 1000 to 100000, preferably 5000 to 50000, in weight average molecular weight. When required, the self-crosslinking resin of the invention is made into an anionic aqueous self-crosslinking resin by neutralizing the carboxyl in the resin or the carboxyl resulting from ring cleavage of the carboxylic anhydride group with a base, and admixing the resulting resin with water or a mixture of water and a hydrophilic organic liquid. The ring cleavage of the carboxylic anhydride group, which can be suitably effected by a known method, can be accomplished favorably, for example, at room temperature to 70°C in the presence of a basic catalyst such as a tertiary amine or an organotin compound and an alcohol. The neutralizing agent for the carboxyl group is not limited specifically insofar as it is a base. Examples of suitable neutralizing agents are primary, secondary and tertiary amines such as ammonia, methylamine, ethylamine, monoethanolamine, cyclohexylamine, dimethylamine, diethylamine, methylethanolamine, diethanolamine, ethylenediamine and triethylamine. Inorganic compounds such as caustic soda and caustic potash are also usable as neutralizing agents. The amount of such a base to be used, which varies with the proportion of carboxyl groups in the resin, is usually such as is sufficient to neutralize 20 to 100 mole % of the carboxylic groups. For example, water, hydrophilic organic liquid and other additive may be added to the resin to be neutralized, before or simultaneously with the addition of the agent. After the neutralization, the resin is admixed with an aqueous liquid with stirring, or the aqueous liquid is admixed with the resin with stirring, whereby an aqueous composition having an optional resin solids content is prepared. The resin solids content of the aqueous composition, although not limited specifically, is generally preferably in the range of 5 to 90 wt.%. The aqueous self-crosslinking composition thus obtained contains a resin having at least one blocked isocyanate group and at least one hydroxyl group, so that when the composition is heated, an isocyanate group is regenerated, which reacts with the hydroxyl group. The composition can therefore be provided as a single-pack composition which is self-crosslinkable. When the self-crosslinking resin of the invention is adapted to have equal amounts of blocked isocyanate and hydroxyl in the molecule, the resin affords self-crosslinking coatings having the highest degree of crosslinking. However, when the resin is to be used for coating compositions or adhesive compositions, it is desired that one of the functional groups be present in excess, for example, in view of adhesion to substrates and adjoining layers. To be suitable, the resin is 20 to 250 in hydroxyl value (mg KOH/g resin) and 15 to 250 in isocyanate value (g/1000 g resin). In preparing the NCO-containing polymer for use in the invention, the reaction system for polymerizing the NCO-containing monomer is free from any polymerizable monomer or solvent having active hydrogen which is reactive with isocyanate and therefore undergoes no gelation. According to the invention, the desired blocking agent to be reacted with the NCO-containing polymer needs to be heated only to an optimum temperature required for the reaction thereof with isocyanate groups, so that when the blocking agent used is, for example, of the phenol or oxime type which is susceptible to coloration at high temperatures, the agent need not be heated to a high temperature but is allowed to block the isocyanate groups at a temperature of not higher than about 100°C. This obviates any likelihood of coloration. Moreover, the blocking agent dissociates at a low temperature (up to 100°C) and is therefore advantageous for use in curing at low temperatures. Especially when low temperature curability is required, the most desirable NCO-containing monomer to be used is m-isopropenyl-α,α'- dimethylbenzyl isocyanate. Further even if the blocking agent used has a high dissociation temperature, the agent readily reacts with the NCO-containing polymer free of any gelation. Generally, the resin composition of the self-crosslinking resin is preferably 5 to 90 wt.% in resin solids content. The aqueous composition of the self-crosslinking resin containing both blocked isocyanate and hydroxyl and thus prepared is used, for example, as a coating composition or adhesive composition. For use as the coating composition or the like, the present composition has further incorporated therein various pigments (including metallic pigments), fillers, surface conditioning agents, deterioration preventing agents, and is usable as a single-pack composition. When required, other compounds or agents can be further incorporated into the composition which include polyols, reactive diluents and other crosslinking agents (such as melamine resin, blocked polyisocyanates, and epoxy resin, acid compounds and alkoxysilane compounds). Various curing catalysts are also usable. Examples of useful curing catalysts are metal catalysts such as tetraisopropyl titanate, tetrabutyl titanate, lithium acetate, iron(Ill) acetylacetonate, zinc 2-ethylhexonate, copper acetate, vanadium trichloride, tin octylate, dibutyltin diacetate, dibutyltin dioctoate, dibutyltin dilaurate and dibutyltin dimaleate. These metal catalysts may be used singly, or at least two of them are usable in admixture. Also preferably usable are tertiary amines such as trimethylamine, triethylamine, dimethylcyclohexylamine, N-tetramethylhexane-1,6-diamine, N-pentamethyldiethylenetriamine and 2-methyl-1,4-diazabicyclo [2,2,2] octane. Especially desirable to use are organotin compounds such as tin octylate and dibutyltin dilaurate. The amount of curing catalyst to be used, although different depending on the kind thereof, is generally 0.05 to 5 wt.% based on the self-crosslinking resin (solids). With the coating composition, adhesive composition or the like thus obtained, an isocyanate group is regenerated from the blocked isocyanate group when the composition is heated at 50 to 200°C, preferably 80 to 150°C, or owing to the presence of the curing catalyst, and the regenerated isocyanate group reacts with hydroxyl to form a urethane bond, giving a three-dimensional crosslinked structure. Further when the present composition is to be used, for example, as an electrophoretic coating composition, a voltage is applied for 1 to 10 minutes with the substrate to be coated placed in the bath and serving as a cathode, whereby the resin is deposited on the substrate. The coated substrate is then withdrawn from the bath, washed with water and thereafter baked at 100 to 150°C for 10 to 60 minutes, whereby an isocyanate group is regenerated from the blocked isocyanate group in the resin to react with hydroxyl, forming a urethane bond and giving a three-dimensionally crosslinked coating. Any substrate is useful insofar as it is electrically conductive. Especially preferable are those made of a metal such as iron, steel, aluminum or copper. The resin of the present invention can be easily prepared as by polymerization without entailing gelation or coloration, can be made curable at low temperatures and gives cured coatings which are outstanding in resistance to chemicals, physical properties, weather resistance, etc. The present invention will be described in greater detail with reference to the following examples, which nevertheless in no way limit the invention. The parts and percentages in Preparation Examples, Examples and Comparative Examples are all by weight. Example 1 Preparation of NCO-containing polymer Methyl isobutyl ketone (35 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer and maintained at 110°C with heating. A mixture of 23.1 parts of 2-isocyanateethyl methacrylate, 20 parts of styrene, 41.9 parts of n-butyl methacrylate, 15 parts of 2-ethylhexyl methacrylate and 2.5 parts of azobisiso-butyronitrile was added dropwise to the methyl isobutyl ketone over a period of 3 hours. Subsequently a mixture of 7.9 parts of methyl isobutyl ketone and 0.5 part of azobisisobutyronitrile was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 1 hour. (The reaction mixture obtained will be referred to as an "NCO-containing polymer solution a-1.") The resin solution obtained was a transparent liquid having a solids content of 70%. The solution was 62.6 g/1000 g resin in isocyanate value, O in Gardner viscosity (25°C) and about 9000 in weight average molecular weight. Introduction of polymerizable double bond The NCO-containing polymer solution a-1 (145.9 parts) was maintained at 80°C by heating, 1.3 parts of 2-hydroxyethyl acrylate was added to the solution with stirring, and the mixture was aged for 1 hour and thereafter cooled to 40°C. Methyl ethyl ketoxime (12.6 parts) was added dropwise to the cooled mixture over a period of 30 minutes, followed by aging for 3 hours. The resulting mixture was diluted to a solids content of 70% with isopropanol. (The reaction mixture obtained will be referred to as a "solution a-2 of NCO-containing polymer with double bond.") Preparation of self-crosslinking resin Isopropanol (40 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer and maintained at 70°C by heating. A mixture of 38 parts of n-butyl methacrylate, 20 parts of styrene, 15 parts of ethyl methacrylate, 15 parts of 2-hydroxyethyl acrylate, 12 parts of acrylic acid, 123.1 parts of the solution a-2 of NCO-containing polymer with double bond (70% in solids content) and 4 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the isopropanol over a period of 3 hours. Subsequently, a mixture of 10 parts of butyl cellosolve and 0.5 part of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the mixture over a period of 1 hour, followed by aging for 2 hour. After the completion of reaction, the mixture was cooled to 40°C, and a mixture of 14 parts of dimethyl ethanolamine and 14 parts of butyl cellosolve was added dropwise to the reaction mixture over a period of 1 hour at the same temperature, followed by a further reaction for 1 hour. The resin solution obtained was substantially colorless and transparent and was found to be 1 by a Hellige colorimeter. To the solution was added 319.5 parts of deionized water to obtain an aqueous resin solution. The resin prepared was 38.9 mg KOH/g resin in hydroxyl value, about 18000 in weight average molecular weight and 24.9 g/1000 g resin in isocyanate value. (The reaction mixture will be referred to as a "self-crosslinking resin solution A.") The resin solution A was about 30% in solids content and 80% in gel fraction ratio (Note 1). Note 1: Gel fraction ratio: The resin solution was applied to a tinplate to such a thickness that the coating would be 20 µm in thickness when cured, followed by heating at 160°C for 30 minutes for curing. The coating prepared was separated off by the mercury amalgam method, placed into a container made of 300-mesh stainless steel net and subjected to extraction at the reflux temperature for 7 hours using a Soxhlet extractor and acetone/methanol (1:1) solvent. The gel fraction ratio was calculated from the following equation. Gel fraction ratio (%) = Weight of coating after extraction Weight of coating before extraction X 100 Example 2 Preparation of NCO-containing polymer Methyl isobutyl ketone (35 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer and maintained at 118°C with heating. A mixture of 40 parts of m-isopropenyl-α,α'-dimethylbenzyl isocyanate, 10 parts of styrene, 40 parts of n-butyl acrylate, 10 parts of n-butyl methacrylate, and 4 parts of tert-butylperoxyisopropyl carbonate was added dropwise to the methyl isobutyl ketone over a period of 3 hours. Subsequently, a mixture of 7.9 parts of methyl isobutyl ketone and 1.0 parts of tert-butylperoxyisopropyl carbonate was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 1 hour. (The reaction mixture obtained will be referred to as an "NCO-containing polymer solution b-1.") The resin solution obtained was a transparent liquid having a solids content of 70%. The solution was 80.9 g/1000 g resin in isocyanate value, N in Gardner viscosity (25°C) and about 10000 in weight average molecular weight. Introduction of polymerizable double bond To 147.9 parts of the NCO-containing polymer solution b-1 was added dropwise 16.8 parts of methyl ethyl ketoxime at 40°C over a period of 30 minutes, the mixture was aged for 1 hours, and 12 part of 2-hydroxyethyl acrylate was then added to the mixture with stirring while maintaining the mixture at 70°C, followed by aging for 3 hour. The resulting mixture was thereafter diluted with isopropanol to a solids content of 70%. (This reaction mixture will be referred to as a "solution b-2 of NCO-containing polymer with double bond.") Preparation of self-crosslinking resin Isopropanol (40 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, and was maintained at 70°C with heating. A mixture of 34 parts of methyl methacrylate, 15 parts of n-butyl acrylate, 10 parts of styrene, 20 parts of ethyl methacrylate, 10 parts of 2-hydroxyethyl acrylate, 11 parts of acrylic acid, 79.2 parts of the 70% solution b-2 of NCO-containing polymer with double bond and 4 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the isopropanol over a period of 3 hours. Subsequently, a mixture of 10 parts of butyl cellosolve and 0.5 part of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 2 hour. After the completion of reaction, the reaction mixture was cooled to 40°C, and a mixture of 13.5 parts of dimethyl aminoethanol and 13.5 parts of butyl cellosolve was added dropwise to the reaction mixture over a period of 1 hour at the same temperature, followed by a further reaction for 1 hour. The resin solution obtained was substantially colorless and transparent and was found to be 1 by a Hellige colorimeter. To the solution was added 262.2 parts of deionized water to obtain an aqueous resin composition of the invention. The resin prepared was 31.1 mg KOH/g resin in hydroxyl value, 24.5 g/1000 g resin in isocyanate value and about 20000 in weight average molecular weight. (The reaction mixture will be referred to as a "self-crosslinking resin solution B.") The resin solution B was about 30% in solids content and 82% in gel fraction ratio (Note 1). Example 3 Preparation of NCO-containing polymer Methyl isobutyl ketone (35 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, and was maintained at 110°C with heating. A mixture of 35 parts of equimolar adduct of isophorone diisocyanate and 2-hydroxyethyl acrylate, 15 parts of styrene, 35 parts of n-butyl methacrylate, 15 parts of 2-ethylhexyl methacrylate and 2.5 parts of azobisisobutyronitile was added dropwise to the methyl isobutyl ketone over a period of 3 hours. Subsequently, a mixture of 7.5 parts of methyl isobutyl ketone and 0.5 part of azobisisobutyronitrile was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 1 hour. (The reaction mixture obtained will be referred to as an "NCO-containing polymer solution c-1.") The resin solution obtained was a transparent liquid having a solids content of 70%, and was N in Gardner viscosity (25°C), 43.4 g/1000 g resin in isocyanate value and about 10000 in weight average molecular weight. Introduction of polymerizable double bond The NCO-containing polymer solution c-1 (145.5 parts) was maintained at 80°C with heating, 1.2 part of 2-hydroxyethyl methacrylate was added to the solution with stirring, and the mixture was aged for 1 hour and thereafter cooled to 40°C. Subsequently, 10.9 parts of methyl isobutyl ketoxime was added dropwise to the mixture over a period of 30 minutes, followed by aging for 3 hours. The reaction mixture was then diluted with 5.2 parts of isopropanol to a solids content of 70%. (This reaction mixture will be referred to as a "solution c-2 of NCO-containing polymer with double bond.") Preparation of self-crosslinking resin Isopropanol (40 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, and was maintained at 70°C with heating. A mixture of 40 parts of n-butyl methacrylate, 20 parts of styrene, 15 parts of 2-ethylhexyl methacrylate, 10 parts of 2-hydroxyethyl methacrylate, 12 parts of acrylic acid, 142.9 parts of the solution c-2 of NCO-containing polymer with double bond and 4 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the isopropanol over a period of 3 hours. Subsequently, a mixture of 10 parts of butyl cellosolve and 0.5 part of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 1 hour. After the completion of reaction, the reaction mixture was cooled to 40°C, and a mixture of 11.2 parts of dimethyl aminoethanol and 14.8 parts of butyl cellosolve was added dropwise to the mixture over a period of 1 hour at the same temperature, followed by a further reaction for 2 hour. The resin solution obtained was substantially colorless and transparent and was found to be 1 by a Hellige colorimeter. To the solution was added 301.2 parts of deionized water to obtain an aqueous resin composition of the invention. The resin prepared was 29.1 mg KOH/g resin in hydroxyl value, 16 g/1000 g resin in isocyanate value and about 20000 in weight average molecular weight. (The reaction mixture will be referred to as a "self-crosslinking resin solution C.") The resin solution C was about 31% in solids content and 80% in gel fraction ratio (Note 1). Example 4 Preparation of NCO-containing polymer Methyl isobutyl ketone (35 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, and was maintained at 130°C with heating. A mixture of 40 parts of m-isopropenyl-α,α'-dimethylbenzyl isocyanate, 10 parts of styrene, 40 parts of n-butyl acrylate, 10 parts of 2-ethylhexyl acrylate and 4 parts of tert-butylperoxyisopropyl carbonate was added dropwise to the methyl isobutyl ketone over a period of 3 hours. Subsequently, a mixture of 7.9 parts of methyl isobutyl ketone and 1.0 part of tert-butylperoxyisopropyl carbonate was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 1 hour. (The reaction mixture obtained will be referred to as an "NCO-containing polymer solution d-1") The resin solution obtained was a transparent liquid having a solids content of 70%. The solution was 80.9 g/1000 g resin in isocyanate value, N in Gardner viscosity (25°C) and about 10000 in weight average molecular weight. Introduction of polymerizable double bond The NCO-containing polymer solution d-1 (147.9 parts) was maintained at 80°C with heating, 1.2 part of 2-hydroxyethyl acrylate was added to the solution with stirring, and the mixture was aged for 3 hours and thereafter cooled to 40°C. To the mixture was added dropwise 16.8 parts of methyl ethyl ketoxime over a period of 30 minutes, followed by aging for 3 hours at 60°C. The reaction mixture was then diluted with 7.1 parts of isopropanol to a solids content of 70%. (This reaction mixture will be referred to as a "solution d-2 of NCO-containing polymer with double bond".) Preparation of self-crosslinking resin Isopropanol (40 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, and was maintained at 70°C with heating. A mixture of 25 parts of methyl methacrylate, 24 parts of n-butyl methacrylate, 30 parts of styrene, 10 parts of 2-hydroxyethyl methacrylate, 11 parts of maleic anhydride, 79.2 parts of the solution d-2 of NCO-containing polymer with double bond (70% in solids content) and 5 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the isopropanol over a period of 3 hours. Subsequently, a mixture of 10 parts of butyl cellosolve and 0.5 part of 2,2'-azobis(2,4-dimethylvaleronitrile) was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 2 hour. After the completion of reaction, 30 parts of methanol was added to the reaction mixture, a mixture of 10 parts of dimethyl aminoethanol and 20 parts of butyl cellosolve was added dropwise to the mixture over a period of 1 hour while maintaining the resulting mixture at 60°C, and the mixture obtained was further reacted for 5 hours. The resin solution obtained was substantially colorless and transparent and was found to be 1 by a Hellige colorimeter. The solution was thereafter cooled to 30°C, and 229.1 parts of deionized water was added thereto to prepare an aqueous resin composition of the invention. The resin prepared was 31.1 mg KOH/g resin in hydroxyl value, 24 g/1000 g resin in isocyanate value and about 17000 in weight average molecular weight. (The reaction mixture will be referred to as a "self-crosslinking resin solution D.") The resin solution D was about 30% in solids content and 80% in gel fraction ratio (Note 1). Comparative Example 1 Butyl cellosolve (40 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, and was maintained at 100°C with heating. A mixture of 15 parts of 2-isocyanateethyl methacrylate blocked with methyl ethyl ketoxime, 30 parts of methyl methacrylate, 25 parts of n-butyl acrylate, 15 parts of 2-hydroxyethyl methacrylate, 10 parts of 2-ethylhexyl methacrylate, 5 parts of acrylic acid and 4 parts of azobisisobutyronitrile was added dropwise to the butyl cellosolve over a period of 3 hours. Subsequently, a mixture of 10 parts of butyl cellosolve and 1.0 part of azobisisobutyronitrile was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 1 hour. The resin solution obtained was a liquid having a solids content of 65% and colored yellowish brown. A mixture of 7 parts of triethylamine and 11.2 parts of isopropanol was added dropwise to 150 parts of the solution over a period of 1 hour, and the resulting mixture was further reacted for 1 hour, giving a resin solution which was yellowish brown and found to be 8 by a Hellige colorimeter. Deionized water (166.5 parts) was added to the solution to prepare an aqueous solution of resin (comparative resin solution 1). The resin obtained was 64.7 mg KOH/g resin in hydroxyl value and about 22000 in weight average molecular weight. The comparative resin solution 1 had a solids content of about 30% and a gel fraction ratio (Note 1) of 55%. Comparative Example 2 Xylene (35 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, with heating and was maintained at 80°C with heating. A mixture of 11 parts of m-isopropenyl-α,α'-dimethylbenzyl isocyanate, 20 parts of methyl methacrylate, 35 parts of n-butyl acrylate, 15 parts of 2-hydroxyethyl methacrylate, 15 parts of 2-ethylhexyl methacrylate, 4 parts of acrylic acid and 4 parts of α,α'-azobisisobutyronitrile was added dropwise to the xylene over a period of 3 hours in an attempt to prepare a polymer, whereas the reaction system gelled 1.5 hours after the start of the dropwise addition. Comparative Example 3 Butyl cellosolve (40 parts ) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, and was maintained at 100°C with heating. A mixture of 15 parts of m-isopropenyl-α,α'-dimethylbenzyl isocyanate blocked with methanol, 31 parts of methyl methacrylate, 25 parts of n-butyl acrylate, 15 parts of 2-hydroxyethyl methacrylate, 10 parts of 2-ethylhexyl methacrylate, 4 parts of acrylic acid and 4 parts of tert-butylperoxyisoproryl carbonate was added dropwise to the butyl cellosolve over a period of 3 hours. Subsequently, a mixture of 10 parts of butryl cellosolve and 1.0 part of tert-butylperoxyisopropyl carbonate was added dropwise to the resulting mixture over a period of 1 hour, followed by aging for 1 hour. This resin solution obtained was a liquid having a solids content of 66% and colored yellowish brown. A mixture of 5.6 parts of triethylamine and 11.2 parts of isopropanol was added dropwise to 150 parts of the solution over a period of 1 hour, and the resulting mixture was further reacted for 1 hour, giving a resin solution which was yellowish brown and found to be 8 by a Hellige colorimeter. Deionized water (166.5 parts) was added to the solution to prepare an aqueous resin solution (comparative resin solution 2). The comparative resin solution 2 was about 30% in solids content and 58% in gel fraction ratio (Note 1). Comparative Example 4 Xylene (35 parts) was placed into a reactor equipped with a stirrer, reflux condenser and thermometer, and was maintained at 80°C with heating. A mixture of 45 parts equimolar adduct of isophorone diisocyanate/2-hydroxyethyl acrylate blocked with methyl isobutyl ketoxime, 10 parts of styrene, 15 parts of 2-hydroxyethyl acrylate, 15 parts of n-butyl methacrylate, 10 parts of 2-ethylhexyl methacrylate, 5 parts of methacrylic acid and 2.5 parts of azobisisobutyronitrile was added dropwise to the xylene over a period of 3 hours. Subsequently, a mixture of 10 parts of butyl cellosove and 1.0 part of azobisisobutyronitrile was added dropwise to the resulting mixture, followed by aging for 1 hour. The resin solution obtained was a liquid having a solids content of 65% and colored reddish brown. A mixture of 5.8 parts of triethylamine and 11.2 parts of isopropanol was added dropwise to this solution over a period of 1 hour, followed by a further reaction for 1 hour. The resin solution obtained was reddish brown and was found to be 9 by a Hellige colorimeter. To the solution was added 166.5 parts of deionized water to prepare an aqueous resin solution (comparative resin solution 3). The resin obtained was 72.5 mg KOH/g resin in hydroxyl value and about 25000 in weight average molecular weight. The comparative resin solution 3 was about 30% in solids content and 58% in gel fraction ratio (Note 1). Example 5 Dibutyltin dilaurate was added to the self-crosslinking resin A in an amount of 1.0 part per 100 parts of the solids of the resin A. Example 6 Dibutyltin dilaurate was added to the self-crosslinking resin B in an amount of 0.5 part per 100 parts of the solids of the resin B. Example 7 Dibutyltin dilaurate was added to the self-crosslinking resin C in an amount of 1.0 part per 100 parts of the solids of the resin C. Example 8 Tin octylate was added to the self-crosslinking resin D in an amount of 1.0 part per 100 parts of the solids of the resin D. Comparative Example 5 Dibutyltin dilaurate was added to the comparative resin solution 1 in an amount of 1.0 part per 100 parts of the resin solids. Comparative Example 6 Dibutyltin dilaurate was added to the comparative resin solution 2 in an amount of 1.0 part per 100 parts of the resin solids. Comparative Example 7 Dibutyltin dilaurate was added to the comparative resin solution 3 in an amount of 1.0 part per 100 parts of the resin solids. Results of Property Tests Coatings were prepared from the compositions of Examples 5 to 8 and Comparative Examples 5 to 7 by different methods and then tested for properties. Table 1 shows the results. With reference to Table 1: (1) Coating method X: The composition was applied to a tinplate to such a thickness that the coating formed would be 20 µm thick on curing. Y: A cold rolled steel (dull) panel treated with zinc phosphate was coated with a cationic electrophoretic epoxy coating composition to a thickness which would be 20 µm on curing. The coating was baked at 170°C for 20 minutes, then polished over the surface with #400 sandpaper and thereafter wiped with gauze wetted with petroleum benzine for degreasing. An aminoalkyd intercoat composition for motor vehicles was further applied to the coating to a thickness of 30 µm when cured, baked at 140°C for 30 minutes. The resulting coating was polished with #600 sandpaper with application of water, drained, dried and cleaned with petroleum benzine to obtain a substrate. The composition to be tested was applied by spray coating to the substrate to a thickness of 20 µm when cured. Z: The composition was electrophoretically applied to a steel panel treated with zinc phosphate with application of a voltage of 250 V for 3 minutes at a bath temperature of 30°C. (2) Appearance of coating; The composition as applied was heated at 140°C for 30 minutes to form a cured coating, which was checked with the unaided eye for surface smoothness, coloration, etc. The coating was evaluated as "good" when having high surface smoothness and free of coloration. (3) Gel fraction ratio The composition was applied to a tinplate to a thickness of 20 µm when dried, and then baked at 120°C for 30 minutes, 140°C for 30 minutes or 160°C for 30 minutes. The coating prepared was separated off by the mercury amalgam method, placed into a container made of 300-mesh stainless steel net and subjected to extraction at the reflux temperature for 7 hours using a Soxhlet extractor and acetone/methanol (1:1) solvent. The gel fraction ratio was calculated from the following equation. Gel fraction ratio (%) = Weight of coating after extraction Weight of coating before extraction X 100 (4) Yellowing The clear composition free of any coloring agent was applied to the surface of a cured coating formed from an amino resin/acrylic resin-based white thermosetting coating composition to a thickness of 20 µm when cured, and baked at 160°C for 30 minutes for curing. On the other hand, the substrate itself was also baked at 160°C for curing as a blank. Subsequently, the coating formed by the composition and the blank were checked for a color difference (Δb) therebetween as an index of yellowing. (5) Gloss The cured coating was checked for 60° specular reflectivity according to JIS K-5400. (6) Pencil hardness The cured coating was checked for pencil hardness according to JIS K-5400. (7) Acid resistance A 0.5 ml quantity of 40% sulfuric acid was applied dropwise to the cured coating, which was then dried at 60°C for 15 minutes, thereafter washed with water and checked with the unaided eye. A: No change, B: Slight blushing, C: Many blisters. (8) Solvent resistance The cured coating was rubbed with gauze wet with xylol reciprocally 20 times over a stroke length of 10 cm while pressing the gauze against the coating with a great force. The coating surface was thereafter checked. The result was evaluated according to the following criteria. A: No change, B: Slight dulling, C: Marked dulling. (9) Water resistance A panel coated with the composition as cured was immersed in water at 40°C for 240 hours, and the coating was thereafter checked. Evaluation criteria are as follows. A: No change, B: Slight dulling, C: Dulling or blistering. (10) Gloss retentivity Determined by QUV accelerated weather test using an accelerated weathering tester, proudct of Q-Panel Company. The coating was irradiated with UV rays at 60° for 16 hours and then exposed to condensation water at 50°C for 9 hours as one cycle, and subjected to this cycle 40 times (1000 hours). The coating was thereafter checked for 60° specular reflectivity for comparison with the corresponding value before weathering. The gloss retentivity was calculated from the following equation. Gloss retentivity (%) = Reflectivity after weathering Reflectivity before weathering X 100 (11) Adhesion An adhesive tape was affixed to the coating formed with cross cuts (100 squares, 1 x 1 mm) and then forcibly peeled off. The number of remaining squares of coating was thereafter counted. A: None separated (100 squares remained) B: Some separated (up to 80 squares remained)

Claims: A self-crosslinking resin which is a graft polymer having a weight average molecular weight of 1000 to 100,000, the graft polymer containing a first blocked isocyanate group containing polymer skeleton and a second polymer skeleton having a hydroxyl group and a carboxyl group or carboxylic anhydride group, wherein the first and the second polymer skeleton are linked by a urethane bond and wherein the first polymer skeleton is obtained by radical polymerization of a NCO-containing vinyl monomer optionally with other vinyl monomers and the second polymer skeleton is obtained by radical polymerization of a hydroxyl-containing vinyl monomer and a carboxyl- or a carboxylic anhydride group- containing vinyl monomer optionally with other vinyl monomers. A self-crosslinking resin as defined in claim 1 which is a resin prepared from a vinyl polymer having at least two isocyanate groups in the molecule by reacting a hydroxyl-containing vinyl monomer with some of the isocyanate groups in the skeleton of the vinyl polymer to introduce a polymerizable double bond into the skeleton, then completely blocking the isocyanate groups remaining in the polymer skeleton with a blocking agent, and thereafter polymerizing an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer and a carboxyl-containing vinyl monomer with the introduced polymerizable double bond. A self-crosslinking resin as defined in claim 1 which is a resin prepared from a vinyl polymer having at least two isocyanate groups in the molecule by blocking some of the isocyanate groups in the skeleton of the vinyl polymer with a blocking agent, then reacting a hydroxyl-containing vinyl monomer with the isocyanate groups remaining in the polymer skeleton to introduce a polymerizable double bond into the skeleton, and thereafter polymerizing an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer and a carboxyl-containing vinyl monomer with the introduced polymerizable double bond. A self-crosslinking resin as defined in claim 1 which is a resin prepared from a vinyl polymer having at least two isocyanate groups in the molecule by reacting a hydroxyl-containing vinyl monomer with some of the isocyanate groups in the skeleton of the vinyl polymer to introduce a polymerizable double bond into the skeleton, then completely blocking the isocyanate groups remaining in the polymer skeleton with a blocking agent, and thereafter polymerizing an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer and a vinyl monomer containing a carboxylic anhydride group with the introduced polymerizable double bond. A self-crosslinking resin as defined in claim 1 which is a resin prepared from a vinyl polymer having at least two isocyanate groups in the molecule by blocking some of the isocyanate groups in the skeleton of the vinyl polymer with a blocking agent, then reacting a hydroxyl-containing vinyl monomer with the isocyanate groups remaining in the polymer skeleton to introduce a polymerizable double bond into the skeleton, and thereafter polymerizing an unsaturated monomer component consisting essentially of a hydroxyl-containing vinyl monomer and a vinyl monomer containing a carboxylic anhydride group with the introduced polymerizable double bond. A self-crosslinking resin as defined in at least one of the claims 1 - 5 which is 25 to 250 mg KOH/g resin in hydroxyl value and 15 to 250 g/1000 g resin in isocyanate value. An aqueous self-crosslinking resin characterized in that the resin is prepared from the self-crosslinking resin defined in at least one of the claims 1 - 6 by neutralizing the carboxyl group with a base thereof after subjecting the carboxylic anhydride group to ring cleavage when required, and subsequently dissolving or dispersing the resulting resin in water or a mixture of water and a hydrophilic organic liquid.

Inventors: NAKAO YASUSHI, YABUTA MOTOSHI, YUKAWA YOSHIYUKI, NAKAO, YASUSHI, YABUTA, MOTOSHI, YUKAWA, YOSHIYUKI
Applicants: KANSAI PAINT CO LTD, KANSAI PAINT CO. LTD.
Application year: 1992
Main classification: C08G18
IPC codes: C08G18