Blocked Polyisocyanates With Improved Storage Stability

1. Field of the Invention

The present invention is directed to increasing the storage stability of blocked polyisocyanates by adding a stabilizing amount of a compound having monofunctional reactivity toward isocyanates.

2. Description of the Prior Art

Coating compositions based on a blocked polyisocyanate component and a component containing isocyanate-reactive hydrogens are known. The purpose of the blocking agent is to prevent the polyisocyanate from reacting with the isocyanate-reactive component at ambient temperature conditions and thus allows the two components to be mixed and stored prior to their actual use. When the composition is baked at an elevated temperature, the blocking agent is released and the reaction of the two components commences. When using common blocking agents such as .epsilon.-caprolactam, unblocking temperatures in excess of 170.degree. C. are needed to provide acceptable rates of unblocking.

The high temperatures needed for unblocking are unfavorable for two reasons. First, the high temperatures can cause yellowing of the reacted polyurethane compositions. Second, the energy requirements are much higher when compared to competitive systems based on aminoplast resins and polyhydroxyl compounds which can be baked at temperatures of as low as about 125.degree. C. using acid catalysis. Since the unblocking temperatures of the conventional polyisocyanate-based systems are much higher, the manufacturing facilities designed for the competitive aminoplast systems cannot accommodate the conventional blocked polyisocyanate systems. Accordingly, even though the compositions based on blocked polyisocyanates and compounds containing isocyanate-reactive hydrogens yield products with a superior combination of hardness and elasticity when compared to the competitive systems, the need exists for a polyisocyanate-based system which is stable under ambient conditions and wherein the blocked polyisocyanate component may be reacted at lower temperatures within an acceptable period of time.

While it is known from U.S. Pat. Nos. 2,801,990; 3,779,794; 4,007,215; 4,087,392; 4,101,530; 4,132,843 and 4,332,965; British Pat. Nos. 1,442,024 and 1,523,103; German Offenlegungsschrift No. 2,623,081 and German Auslegeschrift No. 2,639,491 that polyisocyanates blocked with C--H acidic compounds such as malonic acid esters and acetoacetic acid esters can be reacted at lower temperatures, it has been found that when combined with suitable co-reactants, these systems do not remain stable. When these systems are stored, the viscosity gradually increases until the systems gel. The higher the storage temperature, the faster gelation occurs.

The addition of stabilizers having monofunctional reactivity towards isocyanate groups to improve the storage stability of these systems forms the basis of U.S. application Ser. No. 498,225, filed May 26, 1983, now U.S. Pat. No. 4,437,593. It was also proposed to improve the storage stability of these systems by separately storing the two components. However, it was unexpectedly found that polyisocyanates blocked with C--H acidic compounds do not remain stable even in the absence of a suitable co-reactant.

Accordingly, it is an object of the present invention to improve the storage stability of polyisocyanates blocked with malonate-based blocking agents. It is an additional object to provide polyisocyanates blocked with malonate-based blocking agents which when mixed with suitable co-reactants can be reacted at lower temperatures than conventional blocked polyisocyanate systems and result in polyurethanes possessing properties which are superior to the competitive, low temperature systems.

These and other objects may be achieved by proceeding in accordance with the present invention as described below.

The present invention is directed to a process for improving the storage stability of a composition which contains

(a) a blocked polyisocyanate prepared by blocking the isocyanate groups of an organic polyisocyanate with a blocking agent comprising a di-C.sub.1 -C.sub.12 -alkyl and/or -alkoxyalkyl malonate and

(b) is free from compounds containing at least two isocyanate-reactive hydrogens, which comprises incorporating a stabilizing amount of a compound having monofunctional reactivity towards isocyanates into the composition.

The present invention is also directed to the storage stable composition produced in accordance with the above process.

The blocked polyisocyanates used in the compositions of the present invention preferably contain an average of about 2-6, preferably about 2-4, blocked isocyanate groups per molecule and may be prepared from virtually any organic polyisocyanate, preferably from polyisocyanates containing 2-4 isocyanate groups. Preferred are polyisocyanates having aromatically-, aliphatically- or cycloaliphatically-bound isocyanate groups, or mixtures thereof.

The polyisocyanates used for preparing the blocked polyisocyanates may be monomeric in nature or adducts prepared from organic diisocyanates and containing biuret, allophanate, urea, urethane or carbodiimide groups or isocyanurate rings. Suitable polyisocyanates which may be used as such or as intermediates for preparing polyisocyanate adducts include ethylene diisocyanate, 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,4,4-trimethyl-1,6-hexamethylene diisocyanate, 1,12-dodecane diisocyanate, cyclobutane-1,3-diisocyanate, cyclohexane-1,3- and 1,4-diisocyanate and mixtures of these isomers, 1-isocyanato-2-isocyanatomethyl cyclopentane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (isophorone diisocyanate or IPDI), 2,4- and 2,6-hexahydro tolylene diisocyanate and mixtures of these isomers, 2,4'- and/or 4,4'-dicyclohexyl methane diisocyanate, 1,3- and 1,4-phenylene diisocyanate, 2,4- and 2,6-tolylene diisocyanate and mixtures of these isomers, diphenyl methane-2,4'- and/or -4,4'-diisocyanate, naphthalene-1,5-diisocyanate, triphenyl methane-4,4',4"-triisocyanate, polyphenyl polymethylene polyisocyanates of the type obtained by condensing aniline with formaldehyde followed by phosgenation, and mixtures of the above-mentioned polyisocyanates.

Polyisocyanate adducts containing biuret groups may be prepared from the previously mentioned diisocyanates according to the processes disclosed in U.S. Pat. Nos. 3,124,605: 3,358,010: 3,644,490; 3,862,973: 3,903,126; 3,903,127; 4,051,165; 4,147,714 or 4,220,749 by using coreactants such as water, tertiary alcohols, primary and secondary monoamines, and primary and/or secondary diamines. The preferred diisocyanate to be used in these processes is 1,6-diisocyanatohexane.

Polyisocyanate adducts containing allophanate groups may be prepared by reacting the previously mentioned diisocyanates according to the processes disclosed in U.S. Pat. Nos. 3,769,318 and 4,160,080, British Pat. No. 994,890 and German Offenlegungsschrift No. 2,040,645.

Polyisocyanate adducts containing isocyanurate groups may be prepared by trimerizing the previously mentioned diisocyanates in accordance with the processes disclosed in U.S. Pat. Nos. 3,487,080; 3,919,218; 4,040,992; 4,288,586; and 4,324,879; German Auslegeschrift No. 1,150,080; German Offenlegungsschrift No. 2,325,826; and British Pat. No. 1,465,812. The preferred diisocyanates to be used are 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, mixtures of these isomers, 1,6-diisocyanatohexane, isophorone diisocyanate and mixtures of the latter two diisocyanates.

Polyisocyanate adducts containing urea and/or urethane groups and based on the reaction product of the previously mentioned diisocyanates and compounds containing 2 or more isocyanate-reactive hydrogens may be prepared according to the process disclosed in U.S. Pat. No. 3,183,112. In preparing polyisocyanate adducts the average isocyanate functionality is determined from the functionality of the compounds containing isocyanate-reactive hydrogens. For example, theoretically when an excess of a diisocyanate is reacted with a diol, a polyisocyanate with a functionality of approximately 2 will be produced, while a triol co-reactant will result in a polyisocyanate functionality of at least 3. By using mixtures of compounds containing isocyanate-reactive hydrogens, various functionalities can be obtained. Suitable compounds containing 2 or more isocyanate-reactive hydrogens are those with molecular weights of up to 400 as set forth hereinafter, while the preferred diisocyanates are 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, mixtures of these isomers, 1,6-diisocyanatohexane and isophorone diisocyanate.

Prior to their use in accordance with the present invention, the polyisocyanates are blocked with C--H acidic compounds such as a di-C.sub.1 -C.sub.12 -alkyl and/or -alkoxyalkyl, preferably a C.sub.1 -C.sub.4 -dialkyl malonate. The most preferred blocking agent is diethyl malonate. Preferably, these blocking agents are used as the sole blocking component for reaction with the polyisocyanates. However, it is possible to use up to about 20 mole %, preferably up to about 10 mole %, of other known blocking agents, e.g. secondary or tertiary alcohols such as isopropanol or t-butanol; oximes such as formaldoxime, acetaldoxime, butanone oxime, cyclohexanone oxime, acetophenone oxime, benzophenone oxime or diethyl glyoxime lactams such as .epsilon.-caprolactam or .delta.-valerolactam; phenols such as phenol or cresol; N-alkyl amides such as N-methyl acetamide; imides such as phthalimide; imidazole; or alkali metal bisulfites. While polyisocyanates blocked with these other known blocking agents will react normally with isocyanate-reactive compounds when using sufficiently elevated temperatures, they will not react significantly at the preferred low temperature baking conditions which may be employed for curing compositions containing polyisocyanates blocked with the malonate-based blocking agents. Accordingly, polyisocyanates blocked with these other known blocking agents should only be used in the amounts specified when low temperature baking conditions are employed. To compensate for the low reactivity of these blocked polyisocyanates the amount of the isocyanate-reactive component to be used in combination with the compositions of the present invention may be correspondingly reduced. The unreacted blocked polyisocyanates will remain in the cured coating and provide a softening effect.

It is also possible to replace up to about 60 mole %, preferably up to about 50 mole %, of the malonate-based blocking agents with acetoacetic acid C.sub.1 -C.sub.12 -, preferably C.sub.1 -C.sub.4 -alkyl or -alkoxyalkyl esters such as ethylacetoacetate or ethoxyethylacetoacetate. It has been found that when acetoacetic acid esters are exclusively used as the blocking agent, the reactivity of blocked polyisocyanate towards isocyanate-reactive compounds is reduced in the presence of the monofunctional stabilizer resulting in coatings which are tacky and incompletely cured. However, when equimolar mixtures of the dialkyl malonate and acetoacetic acid esters are used as the blocking agent, fully cured films are obtained from the stabilized compositions of the present invention and isocyanate-reactive compounds.

The reaction between the polyisocyanates and the blocking agent is generally conducted at above about 50.degree. C., preferably from about 60.degree. to 100.degree. C., optionally in the presence of a basic catalyst such as diazabicyclooctane, triethyl amine, alkali metal alcoholates such as sodium methoxide or alkali metal phenolates such as sodium phenolate.

In addition to using the previously described polyisocyanates or polyisocyanate adducts for preparing the blocked polyisocyanate component of the present invention, it is also possible to prepare the blocked polyisocyanate component from isocyanate-terminated prepolymers. These prepolymers are formed by reacting an excess of the previously described polyisocyanates with high molecular weight isocyanate-reactive compounds, and optionally low molecular weight isocyanate-reactive compounds. Prepolymers prepared exclusively from polyisocyanates and low molecular weight isocyanate-reactive compounds are referred to as polyisocyanate adducts containing urea and/or urethane groups and have previously been discussed. A sufficient excess of the polyisocyanate should be used to ensure that the prepolymers are terminated with isocyanate groups.

It should also be ensured that the isocyanate-terminated prepolymers remain soluble in the commonly used polyurethane solvents and do not gel. Gelation may result when sufficiently cross-linked, isocyanate-terminated prepolymers are prepared from polyisocyanates or isocyanate-reactive compounds containing more than two reactive groups. Minimal amounts of cross-linking do not lead to gelation; however, once a sufficient cross-linked density is achieved, gelation occurs. The critical cross-link density, commonly referred to as the gel point, may be calculated by known methods or readily determined by simply reacting the desired components and observing whether gel particles form. In order to avoid gelation, it is preferred to prepare the isocyanate-terminated prepolymers from the polyisocyanates described as suitable for use in preparing the polyisocyanate adducts rather than using the polyisocyanate adducts themselves. It is additionally preferred to prepare the isocyanate-terminated prepolymers from high molecular weight isocyanate-reactive compounds which do not contain excessive amounts of branching in order to further reduce the possibility that gelation will occur. Finally, it is preferred to prepare the isocyanate-terminated prepolymers by adding the isocyanate-reactive compound to the polyisocyanate since this helps to maintain an excess of isocyanate throughout the formation of the prepolymer. Following the formation of the isocyanate-terminated prepolymers, the prepolymers are blocked with the C--H acidic compounds in the previously described manner.

The high molecular weight compounds to be used with the previously described polyisocyanates for preparing the isocyanate-terminated prepolymers are selected from the known compounds containing isocyanate-reactive groups, preferably hydroxyl groups, which are at least difunctional in the sense of the isocyanate-addition reaction. These compounds generally have an average functionality of about 2 to 8, preferably about 2 to 4. The compounds containing at least two isocyanate-reactive hydrogen atoms generally have a molecular weight of from 400 to about 10,000, preferably from 400 to about 8,000.

Preferred high molecular weight compounds containing isocyanate-reactive hydrogen atoms are the known polyester polyols, polyether polyols, polyhydroxy polyacrylates and polycarbonates containing hydroxyl groups. In addition to these preferred polyhydroxyl compounds, it is also possible in accordance with the present invention to use polyhydroxy polyacetals, polyhydroxy polyester amides, polythioethers containing terminal hydroxyl groups or sulphydryl groups or at least difunctional compounds containing amino groups, thiol groups or carboxyl groups. Mixtures of the compounds containing isocyanate-reactive hydrogen atoms may also be used

High molecular weight polyester polyols which are suitable include, e.g. reaction products of polyhydric, preferably dihydric alcohols to which trihydric alcohols may be added and polybasic, preferably dibasic carboxylic acids. Instead of free polycarboxylic acids, the corresponding polycarboxylic acid anhydrides or polycarboxylic acid esters of lower alcohols or mixtures thereof may be used for preparing the polyesters. The polycarboxylic acids may be aliphatic, cycloaliphatic, aromatic and/or heterocyclic and they may be substituted, e.g. by halogen atoms, and/or unsaturated. The following are mentioned as examples: succinic acid, adipic acid, suberic acid; azelaic acid; sebacic acid, phthalic acid; isophthalic acid; trimellitic acid; phthalic acid anhydride; tetrahydrophthalic acid anhydride; hexahydrophthalic acid anhydride; tetrachlorophthalic acid anhydride; endomethylene tetrahydrophthalic acid anhydride; glutaric acid anhydride; maleic acid; maleic acid anhydride; fumaric acid; dimeric and trimeric fatty acids such as oleic acid; which may be mixed with monomeric fatty acids; dimethyl terephthalate and bis-glycolterephthalate. Suitable polyhydric alcohols include, e.g. ethylene glycol propylene glycol-(1,2) and -(1,3); butylene glycol-(1,4) and -(1,3); hexanediol-(1,6); octanediol-(1,8); neopentyl glycol cyclohexanedimethanol (1,4-bis-hydroxymethylcyclohexane); 2-methyl-1,3-propanediol glycerol; trimethylolpropane; hexanetriol-(1,2,6); butanetriol-(1,2,4); trimethylolethane; triethylene glycol; tetraethylene glycol; polyethylene glycol; dipropylene glycol; polypropylene glycol; dibutylene glycol and polybutylene glycol. The polyesters may also contain a proportion of carboxyl end groups. Polyesters of lactones, e.g. .epsilon.-caprolactone, or hydroxycarboxylic acids, e.g. .omega.-hydroxycaproic acid, may also be used.

The high molecular weight polyethers which are preferably used according to the invention are obtained in known manner by the reaction of starting compounds which contain reactive hydrogen atoms with alkylene oxides such as ethylene oxide; propylene oxide; butylene oxide; styrene oxide; tetrahydrofuran or epichlorohydrin or with any mixtures of these alkylene oxides.

Suitable starting compounds containing reactive hydrogen atoms include, e.g. water; methanol; ethanol; ethylene glycol; propylene glycol-(1,2) or -(1,3); butylene glycol-(1,4) or -(1,3); hexanediol-(1,6); octanediol-(1,8); neopentyl glycol; 1,4-bis-hydroxymethylcyclohexane; 2-methyl-1,3-propanediol; glycerol; trimethylolpropane; hexanetriol-(1,2,6); butanetriol-(1,2,4); trimethylolethane; pentaerythritol; mannitol; sorbitol; methyl glycoside; sucrose; phenol; isononylphenol; resorcinol; hydroquinone; 1,2,2- or 1,1,3-tris-(hydroxyphenyl)-ethane; ammonia; methylamine; ethylene diamine; tetra- or hexamethylene diamine; diethylenetriamine; ethanolamine; diethanolamine; triethanolamine; aniline; phenylenediamine; 2,4- and 2,6-diaminotoluene and polyphenylpolymethylene polyamines of the kind obtained by aniline-formaldehyde condensation optionally containing alkyl substituents such as bis-(4-amino-3-methyl-phenyl)-methane. Resinous materials such as phenol and resol resins may also be used as the starting materials.

Polyethers modified by vinyl polymers are also suitable for the preparation of the isocyanate-terminated prepolymers. Products of this kind may be obtained by polymerizing, e.g. styrene and acrylonitrile in the presence of polyethers (U.S. Pat. Nos. 3,383,351; 3,304,273; 3,523,095; 3,110,695 and German Patent No. 1,152,536).

Among the polythioethers which should be particularly mentioned are the condensation products obtained from thiodiglycol on its own and/or with other glycols, dicarboxylic acids, formaldehyde, aminocarboxylic acids or amino alcohols. The products obtained are either polythio-mixed ethers, polythioether esters or polythioether ester amides, depending on the co-components.

Suitable polyacetals include the compounds which can be prepared from glycols such as diethylene glycol; triethylene glycol; 4,4'-dioxethoxy-diphenyldimethylene; hexanediol and formaldehyde. Polyacetals suitable for the purpose of the invention may also be prepared by the polymerization of cyclic acetals.

Polycarbonates containing hydroxyl groups include those known per se such as the products obtained from the reaction of diols such as propanediol-(1,3), butanediol-(1,4) and/or hexanediol-(1,6), diethylene glycol, triethylene glycol or tetraethylene glycol with diarylcarbonates, e.g. diphenylcarbonate, or phosgene.

Suitable polyhydroxy polyester amides and polyamides are, for example, the predominantly linear condensates obtained from polybasic saturated and unsaturated carboxylic acids or their anhydrides and polyvalent saturated or unsaturated aminoalcohols, diamines, polyamines and mixtures thereof.

Suitable monomers for producing hydroxy-functional polyacrylates include acrylic acid, methacrylic acid, crotonic acid, maleic anhydride, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, glycidyl acrylate, glycidyl methacrylate, 2-isocyanatoethyl acrylate, and 2-isocyanatoethyl methacrylate.

The preferred high molecular weight polyol components are the polyester, polyacrylate, polycarbonate and polyether polyols or mixtures thereof.

In addition to the high molecular weight compounds, the isocyanate-terminated prepolymers may also optionally be prepared from low molecular weight isocyanate-reactive compounds having an average molecular weight of up to about 400. The low molecular weight isocyanate-reactive compounds should have an average functionality of about 2 to 8, preferably from about 2 to 6 and most preferably from about 2 to 4, and may also contain ether, thioether, ester, urethane and/or urea bonds.

Examples of low molecular weight compounds include the polyamines and diols or triols used as chain lengthening agents or cross-linking agents in polyurethane chemistry such as those listed as suitable for preparing the polyester and polyether polyols. Suitable diols and triols include propylene glycol-(1,2) and -(1,3), butylene glycol-(1,4) and -(1,3); hexanediol-(1,6); octane diol-(1,8); neopentyl glycol, cyclohexane dimethanol (1,4-bis-hydroxymethylcyclohexane); 2-methyl-1,3-propanediol; glycerol; trimethylolpropane; hexane triol-(1,2,6); butanetriol-(1,2,4) or trimethylolethane, and also glycols such as ethylene glycol, diethyleneglycol, triethylene glycol, tetraethylene glycol and polyethylene glycols having a molecular weight of up to 400. In addition compounds such as dipropylene glycol, polypropylene glycols having a molecular weight of up to 400, dibutylene glycol, polybutylene glycols having a molecular weight of up to 400, thiodiglycol and castor oil may also be used according to the invention.

Suitable polyamines are essentially hydrocarbon polyamines which have isocyanate-reactive hydrogens according to the Zerewitinoff test, e.g., primary or secondary amine groups. The polyamines are generally aromatic, aliphatic or alicyclic amines containing between about 1 to 30 carbon atoms, preferably about 2 to 15 carbon atoms, and most preferably about 2 to 10 carbon atoms. Examples of suitable polyamines include diaminoethane, 1,6-diaminohexane, piperazine, 2,5-dimethylpiperazine, 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, bis-(4-aminocyclohexyl)-methane, bis-(4-amino-3-methyl-cyclohexyl)-methane, 1,4-diaminocyclohexane, 1,2-propylenediamine, hydrazine, amino acid hydrazides, hydrazides of semicarbazidocarboxylic acids, bis-hydrazides, bis-semicarbazides, diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, N,N,N-tris-(2-aminoethyl)amine, N-(2-piperazinoethyl)ethylene diamine, N,N'-bis-(2-aminoethyl)-piperazine, N,N,N'-tris-(2-aminoethyl)-ethylene diamine, N-[N-(2-aminoethyl)-2-aminoethyl]-N'-(2-aminoethyl)piperazine, N-(2-aminoethyl)-N'-(2-piperazinoethyl)-ethylene diamine, N,N-bis-(2-aminoethyl)-N-(2-piperazinoethyl)-amine, N,N-bis-(2-piperazinoethyl)-amine, polyethylene imines, iminobispropylamine, guanidine, melamine, N-(2-aminoethyl)-1,3-propane diamine, 3,3'-diaminobenzidine, 2,4,6-triaminopyrimidine, polyoxypropylene amines, tetrapropylenepentamine, tripropylenetetramine, N,N-bis-(6-aminohexyl)amine, N,N'-bis-(3-aminopropyl)-ethylene diamine and 2,4-bis-(4'-aminobenzyl)-aniline.

Also suitable are ester diols of the general formulae

and

in which

R represents an alkylene or arylene group having from 1 to 10, preferably 2 to 6 carbon atoms,

x=2 to 6 and

y=3 to 5,

e.g. .delta.-hydroxybutyl-.epsilon.-hydroxycaproic acid ester; .omega.-hydroxyhexyl-.delta.-hydroxybutyric acid ester; adipic acid-bis-(.beta.-hydroxyethyl)-ester and terephthalic acid-bis-(.beta.-hydroxyethyl)-ester; as well as diol urethanes of the general formula

in which

R' represents an alkylene, cycloalkylene or arylene group having from 2 to 15, preferably from 2 to 9 carbons and

x represents an integer of from 2 to 6,

e.g. 4,4'-dicyclohexyl-methane-bis-(.beta.-hydroxyethylurethane) or 4,4'-dicyclohexyl-methane-bis-(.beta.-hydroxybutylurethane).

Also suitable are diol ureas of the general formula ##STR1## in which

R" represents an alkylene, cycloalkylene or arylene group having from 2 to 15, preferably from 2 to 9 carbons and

R"' represents hydrogen or a methyl group and

x=2 or 3,

e.g. 4,4'-dicyclohexyl-methane-bis-(.beta.-hydroxyethylurea).

Also suitable as low molecular weight isocyanate-reactive components are the amino alcohols, especially those defined according to the following formula, ##STR2## wherein

R"' represents hydrogen or a methyl group, and

x=2 or 3.

The storage stability of the blocked polyisocyanates is improved in accordance with the present invention by incorporating a stabilizing amount of a compound having monofunctional reactivity toward isocyanate groups. The stabilizing compounds include primary, secondary or tertiary monoalcohols and primary or secondary monoamines having molecular weights of up to about 400. Suitable monofunctional compounds include methanol, ethanol, propanol, isopropanol, 1-butanol, 2-butanol, t-butanol, methyl amine, ethyl amine, propyl amine, 2-aminopropane, butyl amine, 2-aminobutane, t-butylamine, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether and propylene glycol monomethyl ether. Additional examples of suitable monofunctional alcohols are contained in U.S. Pat. No. 4,355,138, herein incorporated by reference.

The preferred stabilizing compounds are the highly volatile, low molecular weight monoalcohols and monoamines, especially the monoalcohols, since at the baking temperatures necessary for curing the compositions of the present invention in combination with an isocyanate-reactive component, these monofunctional compounds are volatilized from the coating compositions and do not form a part of the cured coating to any substantial degree. However, when it is desired to retain the stabilizer in the cured coating, it is preferred to use monoalcohols having a vaporization point higher than the baking temperature. The retained stabilizers have a softening effect on the cured coating. The stabilizers are added in amounts greater than about 0.5%, preferably greater than about 1.0%, and most preferably greater than about 2% by weight based on the weight of the blocked polyisocyanates.

The upper limit of the stabilizers may exceed about 50%, but is preferably about 40%, and most preferably about 20%, based on the weight of the blocked polyisocyanates. Even though amounts greater than about 20% by weight do not normally further improve the stability, amounts greater than 20% may be used when it is desired to also use monoalcohols as solvents for the compositions of the present invention.

As mentioned above, a solvent or solvent mixture may be used during the production of the blocked polyisocyanates. When a solvent is employed, the solvent or solvent mixture preferably remains in the composition until it is used. However, it is of course also possible to use a solvent simply to promote thorough mixing of the compounds used for preparing the blocked polyisocyanates and subsequently to distill off this solvent (in vacuo) leaving a ready-to-use mixture in solvent-free form which may be redissolved in solvents at any later stage.

Suitable solvents include the known polyurethane solvents, for example, toluene, xylene, butyl acetate, ethylacetate, ethylene glycol monoethyl ether acetate (EGA), ethylene glycol monomethyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol monomethyl ether acetate, diethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, methyl ethyl ketone or methyl isobutyl ketone, hydrocarbon solvents such as hexane and heptane, aromatic solvents and also mixtures of the above solvents.

In the compositions prepared according to the present invention, the use of solvents is not always necessary, the solvent being used primarily to reduce the viscosity of the compositions to a workable range. Generally the solids content of the composition is greater than 20% and may be as high as 100%, based on the weight of the blocked polyisocyanate and excluding the weight of the stabilizer.

Additives, such as catalysts, pigments, dyes and levelling aids, may be added as required to the compositions of the present invention.

The compositions produced according to the present invention may be stored as such for prolonged periods at room temperature without gel formation or any other undesirable changes occurring. When subsequently mixed with an isocyanate-reactive component (such as those disclosed as suitable for preparing the isocyanate-terminated prepolymers) to form a coating composition, they may be diluted as required to a suitable concentration and applied by the conventional methods, for example spraying or spread coating, and heated, generally to temperatures in excess of about 100.degree. C., preferably from about 100.degree. to 150.degree. C., more preferably from about 120.degree. to 130.degree. C., in order to cure the coating.

The coating compositions may be used as coating agents for primer, intermediate or surface coatings for a variety of different substrates. The resulting coatings possess excellent adhesion to substrates, are uniform and exhibit excellent mechanical and chemical properties and water and solvent resistance, especially hardness, impact resistance and elasticity.

The invention is further illustrated, but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.

The following components were used in the following examples as indicated.

2484 parts of polypropylene glycol (MW 4000) were added to and reacted with 1000 parts of a 70/30 mixture of 4,4'- and 2,4'-diphenylmethane diisocyanate at a temperature of 60.degree.-70.degree. C. until an NCO content of 7.8% was obtained.

2880 parts of this isocyanate-terminated prepolymer were mixed with 995 parts of diethyl malonate and 20 parts of a 25% solution of sodium methoxide in methanol and heated to 60.degree.-70.degree. C. for several hours until the NCO content was essentially zero. 1900 parts of the diethyl malonate blocked, isocyanate-terminated prepolymer were then mixed with 1200 parts of ethylene glycol monoethyl ether acetate.

A dry reaction vessel was charged with 564 parts of a polymeric isocyanate prepared by the phosgenation of an aniline-formaldehyde condensate (NCO content--31.5%, viscosity at 25.degree. C.--200 cps) and containing less than 0.1% 2,2'-, 47-48% 4,4'-, and 2-3% 2,4'-diphenylmethane diisocyanate and 50% of higher functional polymeric isocyanates. With continuous stirring 200 parts each of polypropylene glycol (average MW 2000, OH no. 56) and polypropylene tetraol (average MW 3600, OH no. 62, prepared by the propoxylation of ethylene diamine) were added to the reaction vessel at room temperature. The mixture was blanketed with dry nitrogen and heated to a temperature of 80.degree.-88.degree. C. The mixture was kept within this temperature range until the measured NCO content was at or slightly below the theoretical value of approximately 16% (2-4 hours).

780 parts of this isocyanate-terminated prepolymer were mixed with 528 parts of diethyl malonate and 6.6 parts of a 25% solution of sodium methoxide in methanol. The mixture was heated to 60.degree.-70.degree. C. and maintained within that temperature range for several hours until the NCO content was essentially zero.

528 parts of a polymeric isocyanate prepared by the phosgenation of an aniline-formaldehyde condensate (NCO content--31.9%, viscosity at 25.degree. C.--80 cps) and containing 3% 2,2'-, 39% 4,4'- and 23% 2,4'-diphenylmethane diisocyanate and 35% of higher functional polymeric isocyanates were mixed with 704.7 parts of diethyl malonate and 6.2 parts of a 25% solution of sodium methoxide in methanol. The mixture was heated to 60.degree.-70.degree. C. and maintained within that temperature range for several hours until the NCO content was about 2%.

The theoretical reaction product of 3 moles of 2,4-diisocyanatotoluene and 1 mole of trimethylolpropane was prepared by reacting an excess of the diisocyanate with the triol and subsequently removing the excess diisocyanate by distillation. The polyisocyanate adduct containing urethane groups was diluted to a solids content of 75% with ethyl acetate. 1000 parts of the 75% polyisocyanate adduct solution were mixed with 590 parts of diethyl malonate and 6.7 parts of a 25% solution of sodium methoxide in methanol. The mixture was heated to 60.degree.-70.degree. C. and maintained within that temperature range for several hours until the NCO content was essentially zero.

14.1 parts of diethylene glycol, 35.4 parts of trimethylolpropane and 132.5 parts of a polypropylene glycol (MW 1000) were mixed with 221.4 parts of ethylene glycol monoethyl ether acetate and 147.6 parts of xylene, charged to a reaction vessel and heated to 60.degree. C. 196 parts of a diisocyanate mixture of 80% 2,4- and 20% 2,6-diisocyanatotoluene were then added and the temperature was increased to 70.degree.-80.degree. C. for 2 hours. The temperature was then raised to 90.degree.-100.degree. C. until the isocyanate content was reduced to 5.22%. The mixture was cooled to 80.degree. C. and 176.3 parts of diethyl malonate were added. After the mixture had cooled to 70.degree. C., 1.7 parts of a 25% solution of sodium methoxide in methanol were added. The mixture was maintained at 60.degree.-70.degree. C. until the NCO content was essentially zero.

233.2 parts of a polypropylene glycol (MW 2000), 179.3 parts of a glycerine initiated, polypropylene oxide/polyethylene oxide-tipped triol (MW 4800, PO/EO wt. ratio 83/17), 121 parts of a glycerine initiated polypropylene triol (MW 3000), 0.04 parts of trimethylolpropane and 125 parts of ethylene glycol monoethyl ether acetate were charged to a reaction vessel and heated to 40.degree. C. 83.2 parts of a diisocyanate mixture of 80% 2,4- and 20% 2,6-diisocyanatotoluene were then added and the temperature was increased to 70.degree.-80.degree. C. for 2 hours. The temperature was then raised to 90.degree.-100.degree. C. until the isocyanate content was reduced to 2.70% which is slightly below the theoretical value. The mixture was cooled to 90.degree. C. and 94 parts of diethyl malonate were added. After the mixture had cooled to 70.degree. C., 0.9 parts of a 25% solution of sodium methoxide in methanol were added. The mixture was maintained at 60.degree.-70.degree. C. until the NCO content was essentially zero.

987.5 parts of a diisocyanate mixture of 80% 2,4- and 20% 2,6-diisocyanatotoluene were charged to a reaction vessel followed by the successive addition of 634.5 parts of ethylene glycol monoethyl ether acetate, 634.5 parts of xylene, 71.3 parts of diethylene glycol, 178.5 parts of trimethylolpropane and 670 parts of polypropylene glycol (MW 1000). The reaction temperature was maintained at 70.degree.-80.degree. C. and the NCO content was reduced to the theoretical value or slightly below. After cooling to room temperature 1000 parts of the isocyanate-terminated prepolymer and 274.6 parts of diethyl malonate were charged to a reaction vessel followed by the addition of 4.4 parts of a 25% solution of sodium methoxide in methanol. The reaction mixture was then heated to 60.degree.-70.degree. C. and maintained within that temperature range until the NCO content was essentially zero.

600 parts of a 90% solution in ethylene glycol monoethyl ether acetate of a polyisocyanate containing isocyanurate groups and prepared from 1,6-diisocyanatohexane and 492 parts of diethyl malonate were charged to a reaction vessel followed by the addition of 5.5 parts of a 25% solution of sodium methoxide in methanol. The reaction mixture was then heated to 60.degree.-70.degree. C. and maintained within this temperature range until the NCO content was essentially zero.

The invention is further illustrated, but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.

The preceding Polyisocyanate Components either in the presence or absence of stabilizers were stored at 50.degree. C. and the viscosities were determined periodically. The Polyisocyanate Components, stabilizers and viscosities are set forth in the following Tables. The initial viscosities, if reported, were measured at the time the compositions were prepared. In determining the solids content, the weight of the blocking agent, including any excess, was counted as solids; however, the stabilizers and solvents were not included as solids.

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.