# Patent

## Title
Latent amino groups and isocyanate groups comprising polyurethane

## Abstract

This application is a division of application Ser. No. 08/265,369, filed Jun. 24 , 1994 allowed.

## Background
This invention relates generally to aqueous polyurethane dispersions, and more particularly, to aqueous polyurethane dispersions that contain polyisocyanate dimer and trimer and are useful as coating compositions.

Aqueous polyurethane dispersions are well known and are used in the production of a variety of useful polyurethane products, including, for example, coatings, films, adhesives, sealants, and the like, collectively referred to as so-called "ACES". Such dispersions are typically produced by dispersing a water-dispersible, isocyanate-terminated polyurethane prepolymer in an aqueous medium together with an active hydrogen containing chain extender, such as a diamine.

The prepolymers used in the preparation of the dispersions are generally substantially linear, that is to say difunctional, and are typically obtained by reacting an excess of a diisocyanate with an isocyanate-reactive component comprising a polymeric diol in the presence of a reactive compound, for example a diol or diisocyanate, containing an ionic or nonionic hydrophilic group or site.

Attempts to introduce higher functionality into the dispersed polyurethanes, in an effort to enhance the physical properties of the resulting ACES or shorten the curing time for these products, have not been entirely successful. One method of increasing the functionality is by incorporating a higher functionality polyol, such as a triol or tetrol, into the prepolymer, and the use of such polyfunctional active hydrogen compounds in the preparation of ionic polyurethane dispersions has been described in U.S. Pat. No. 4,554,308. The amount of polyfunctionality that can be introduced in this way is limited by the tendency of the more highly crosslinked prepolymers to gel and to form large micelles when dispersed, resulting in poor film-forming properties for this dispersion.

Another method of introducing higher functionality employs a linear prepolymer in conjunction with a trifunctional chain extender, such as diethylene triamine. This approach has been described in U.S. Pat. Nos. 4,203,883 and 4,408,008. One problem associated with these triamine cross-linked urethane dispersions is that they tend to form gels prior to use and they tend to provide poor coalescence of the films on substrates during use.

Yet another method of introducing higher functionality uses a triisocyanate in the prepolymer preparation, but this methodology causes similar problems to those that result when using a triol or tetrol in the preparation of the prepolymer, including gelation and poor dispersion of the prepolymer due to large micelle formation.

A different method of incorporating triisocyanates has been described in U.S. Pat. No. 4,507,431 which describes a process for preparing aqueous dispersions of cross-linked polyurethane ionomers. These dispersions are made by mixing an isocyanate containing prepolymer dissolved in a water-miscible organic solvent having a boiling point of from 20 to 100.degree. C. with a polyfunctional polyisocyanate cross-linking compound having an isocyanate functionality of about 2.2 to 4. The isocyanate-containing prepolymer is prepared from a linear polyhydroxy compound having a molecular weight of from 800 to 5000, and the prepolymer has exclusively aliphatic or cycloaliphatic terminal isocyanate groups in amounts from 0.1 to 2% by weight incorporated therein by employing an aliphatic diisocyante and/or cycloaliphatic diisocyanate compound alone as the diisocyanate reactant with said polyhyroxy compound or in conjunction with a non-aliphatic or non-cycloaliphatic diisocyanate reactant, and containing salt groups in amounts of from 0.02 to 1% by weight; so that there are from 0.1 to 1.5 isocyanate groups of said polyisocyanate per isocyanate group of said prepolymer; dispersing the resulting solution in from 40 to 80% by weight, based on the polyurethane prepolymer and the polyisocyanate, of water and evaporating the organic solvent. Unfortunately, evaporation of the solvent in the production of the aqueous dispersion in accordance with the teachings of the '431 patent is undesirable from an environmental standpoint.

Still another method of preparing aqueous polyurethane dispersions is disclosed in U.S. Pat. No 5,169,895. The '895 patent discloses an aqueous dispersion of a water-dispersible polyurethane, wherein the polyurethane represents the reaction product of (a) a water-dispersible isocyanate-terminated polyurethane prepolymer having an NCO content of 2.1 to 10% by weight, (b) an organic polyisocyanate having an average isocyanate functionality of 2.1 to 4.0, and (c) an active hydrogen-containing chain extender, but with the proviso that the polyurethane is prepared in the absence of the steps recited to prepare the polyurethane dispersion in the above-described '431 patent. Examples of the organic polyisocyanate of component (b) of the '895 patent include the trimers of hexamethylene diisocyanate (so-called "HDI"), isophorone diisocyanate (so-called "IPDI"), and 2,4-toluene diisocyanate, biuret-modified HDI, and the adduct of 2,4-toluene diisocyanate and trimethylolpropane, as recited at column 5, lines 27-31 of the '895 patent.

New aqueous dispersions of polyurethanes providing improved physical properties for ACES applications, relative to the above-described prior art compositions such as those described in the '895 patent, would be highly desired by the ACES community. The present invention provides one such improved composition.

In one aspect, the present invention relates to a composition being an aqueous dispersion of polyurethane in water, said composition comprising water and the reaction product of: having an NCO content of between about 1.5 and 10% by weight, trimer (also referred to herein as "dimer/trimer") and having an average isocyanate functionality of between 2.1 and 4.0, and

In another aspect, the present invention relates to a process for preparing an aqueous dispersion of polyurethane which comprises reacting, in an aqueous medium, a reaction mixture comprising: having an NCO content of between about 1.5 and 10% by weight, trimer and having an average isocyanate functionality of between 2.1 and 4.0, and

In yet another aspect, the present invention relates to a method for coating a substrate which comprises contacting the substrate with a coating composition comprising a dispersion in water of the reaction product of: having an NCO content of between about 1.5 and 10% by weight, trimer and having an average isocyanate functionality of between 2.1 and 4.0, and

Preferably, the water-dispersible isocyante-terminated polyurethane prepolymer (i.e., component (a) described in the above-mentioned composition, process and method claims) comprises the reaction product of: weight in the range from 250 to 5000, and isocyanate or isocyanate-reactive groups.

In other aspects of the present invention, components (a) and (b) employed in the above described composition, process and method, are suitably combined, to provide a polyurethane prepolymer having at least a portion of the dimer and the trimer incorporated into the prepolymer.

These and other aspects will become apparent upon reading the following detailed description of the invention.

It has now been surprisingly found by the present inventors in accordance with the present invention that the use of a polyisocyanate adduct mixture comprising an isocyanate dimer and an isocyanate trimer, together with a water-dispersible isocyanate-terminated polyurethane prepolymer, in the preparation of aqueous polyurethane dispersions, provides a polyurethane that, in use for example in ACES applications, exhibits enhanced physical properties, including increased tensile strength and elongation, relative to the properties provided by prior art water-dispersible polyurethanes. Further, increasing the trimer content of the polyisocyante adduct mixture, relative to the dimer content, enables the production of ACES coatings having a high initial modulus without being brittle.

The polyisocyanate used in making the isocyanate-terminated prepolymer employed in the present invention may be aliphatic, cycloaliphatic, araliphatic or aromatic polyisocyanate. Examples of suitable polyisocyanates include ethylene diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane-1,4-diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, m- and p-tetramethylxylene diisocyanates, p-xylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanates, 5-naphthalene diisocyanate. Mixtures of polyisocyanates can be used and also polyisocyanates which have been modified by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocyanurate residues.

The polyol employed to make the isocyanate-terminated prepolymer is suitably a high molecular weight polyol, preferably having a number average molecular weight (M.sub.n) of between about 400 and about 10,000, preferably between 400 and 3,000. Examples of the high molecular weight compounds include: dihydric alcohols to which trihydric alcohols may be added and polybasic, preferably dibasic carboxylic acids. Instead of these polycarboxylic acids the corresponding carboxylic acid anhydrides or poly carboxylic 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 unsaturated and/or substituted, e.g. by halogen atoms. Examples of these acids include succinic acid, adipic acid, suberic acid, azelaic acid, sebacid 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-glycol terephthalate. Suitable polyhydric alcohols include ethylene glycol, 1,2- and 1,3 propylene glycol, 1,3- and 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, neopentyl glycol, diethylene glycol, 2-methyl-1,3-propylene glycol, 2,2-dimethyl-1,3-propylene glycol, the various isomeric bis-hydroxymethylcyclohexanes, glycerine and trimethylolpropane. of e-caprolactone initiated with the above-mentioned polyhydric alcohols. from reaction of the polyhydric alcohols previously set forth for preparing the polyhydroxy polyesters (preferably dihydric alcohols such as 1,3-propanediol, 1,4-butanediol, 1,4-dimethylol cyclohexane, 1,6-hexanediol, diethylene glycol, triethylene glycol or tetraethylene glycol) with phosgene, diaryl carbonates such as diphenyl carbonate or cyclic carbonates such as ethylene or propylene carbonate. Also suitable are polyester carbonates obtained by the reaction of lower molecular weight oligomers of the above-mentioned polyesters or polylactones with phosgene,diaryl carbonates or cyclic carbonates. compounds which contain reactive hydrogen atoms with alkylene oxides such as propylene oxide, butylene oxide, styrene oxide, tetrahydrofuran, epichlorohydrin or mixtures of these alkylene oxides. Certain proportions of ethylene oxide may also be included, provided the polyether does not contain more than 10% by weight of ethylene oxide; however, polyethers which do not contain ethylene oxide are preferably used. Suitable starting compounds containing at least one reactive hydrogen atom include the polyols set forth as suitable for preparing the polyhydroxy polyesters and, in addition, water, methanol, ethanol, 1,2,6-hexanetriol, 1,2,4-butanetriol, trimethylol ethane, pentaerythritol, mannitol, sorbitol, methyl glycoside, sucrose, phenol, isononyl phenol, resorcinol, hydroquinone and 1,1,1- or 1,1,2-tris(hydroxylphenyl)ethane. Polyethers which have been obtained by the reaction of starting compounds containing amino groups can also be used, but are less preferred for use in the present invention. Suitable amine starting compounds include ethylene diamine, diethylene triamine, triethylene tetraamine, 1,6-hexanediamine, piperazine, 2,5-dimethylpiperazine, 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, bis(4-aminocyclohexyl) methane, bis(4-amino-3-methylcyclohexyl)methane, 1,4-cyclohexanediamine, 1,2-propanediamine, hydrazine, aminoacid hydrazides, hydrazides of semicarbazido carboxylic acids, bis-hydrazides and bis-semicarbazides, ammonia, methylamine, tetramethylenediamine, ethanolamine diethanolamine, triethanolamine, aniline, phenylenediamine, 2,4- and 2,6-toluylenediamine, polyphenylene polymethylene polyamines of the kind obtained by the aniline/formaldehyde condensation reaction and mixtures thereof. Resinous materials such as phenol and cresol resins may also be used as the starting materials. The preferred starting compounds for the polyethers are those compounds which exclusively contain hydroxyl groups, while compounds containing tertiary amine groups are less preferred and compounds containing isocyanate-reactive-NH groups are much less preferred.

In order to render the polyurethanes water-dispersible to enable formation of the dispersions of the present invention, it is necessary to chemically incorporate hydrophilic groups, i.e., anionic groups, potential anionic groups or nonionic hydrophilic groups, into the isocyanate-terminated prepolymer component. Suitable hydrophilic components contain at least one (preferably at least two) isocyanate or isocyanate-reactive group and at least one hydrophilic group or potential hydrophilic group. Examples of compounds which may be used to incorporate potential ionic groups include aliphatic hydroxy carboxylic acids, aliphatic or aromatic aminocarboxylic acids with primary or secondary amino groups, aliphatic hydroxy sulfonic acids and aliphatic or aromatic aminosulfonic acids with primary or secondary amino groups. These acids preferably have molecular weights below 400. It should be emphasized that the carboxylic acid groups are not considered to be isocyanate-reactive groups due to their sluggish reactivity with isocyanates.

The preferred anionic groups for incorporation into the polyurethanes in accordance with the present invention are carboxylate groups and these groups may introduced by using hydroxy-carboxylic acids of the general formula:

wherein Q represents a straight or branched, hydrocarbon radical containing 1 to 12 carbon atoms, and x and y represent values from 1 to 3. Examples of these hydroxy-carboxylic acids include citric acid and tartaric acid.

The preferred acids are those within the above-mentioned formula wherein x=2 and y=1. These dihydroxy alkanoic acids are described in U.S. Pat. No. 3,412,054, herein incorporated by reference. The preferred group of dihydroxy alkanoic acids are the .alpha.,.alpha.-dimethylol alkanoic acids represented by the structural formula

wherein Q' is hydrogen or an alkyl group containing 1 to 8 carbon atoms. The most preferred compound is .alpha.,.alpha.-dimethylol propionic acid, i.e., wherein Q' is methyl in the above formula.

The acid groups may be converted into hydrophilic anionic groups by treatment with a neutralizing agent such as an alkali metal salt, ammonia or primary, secondary or preferably tertiary amine in an amount sufficient to render the hydroxy functional polyurethanes water dispersible. Suitable alkali metal salts include sodium hydroxide, potassium hydroxide, sodium hydride, potassium hydride, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate. The use of alkali metal salts as neutralizing agents is less preferred than the use of volatile organic compounds such as volatile amines since they lead to reduced resistance to water swell in the coatings or other ACES product produced from the water dispersible compositions of the present invention. Therefore, less than 50%, preferably less than 20% and most preferably none of the acid groups should be neutralized with alkali metals.

In addition, the use of tertiary amines that are relatively volatile is preferred in order to facilitate easy removal of the tertiary amine component during curing of the polyurethane composition. The preferred volatile amines for neutralizing the acid groups are the tertiary amines, while ammonia and the primary and secondary amines are less preferred. Examples of suitable amines include trimethyl amine, triethylamine, trisopropylamine, tributyl amine, N,N-dimethylcyclohexyl amine, N,N-dimethylstearyl amine, N,N-dimethylaniline, N-methylmorpholine, N-ethylmorpholine, N-methylpiperazine, N-methylpyrrolidine, N-methylpiperidine, N,N-dimethylethanolamine, N,N-diethylethanolamine, triethanolamine, N-methyldiethanolamine, dimethylamionpropanol, 2-methoxyethyldimethylamine, N-hydroxyethylpiperazine, 2-(2-dimethylaminoethoxy)ethanol and 5-diethylamino-2-pentanone. The most preferred tertiary amines are those which do not contain isocyanate-reactive groups as determined by the Zerewitinoff test since they are particularly capable of reacting with isocyanate groups during the curing of the compositions of the present invention on the desired substrate.

The acid groups on component (iii) of the isocyanate-terminated prepolymer may be converted into hydrophilic anionic groups by treatment with the alkali metal or preferably the volatile amine either before, during or after their incorporation into the prepolymer. However, it is preferred to neutralize the acid groups after their incorporation.

The polyisocyanate adduct useful in the present invention is suitably a mixture, blend or separately added combination of polyisocyanate trimer and polyisocyante dimer. Suitable trimers include the isocyanurate group-containing polyisocyanates prepared from the previously described aliphatic and/or cycloaliphatic diisocyanates. Particularly preferred are isocyanato-isocyanurates based on 1,6-diisocyanatohexane and/or 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate or IPDI). The production of these isocyanurate group-containing polyisocyanates is described, for example, in DE-PS 2,616,416, EP-OS 3,765, EP-OS 10,589, EP-OS47,452, U.S. Pat. No. 4,288,586 and U.S. Pat. No. 4,324,879. The isocyanato-isocyanurates generally have an average NCO functionality of 3 to 3.5 and an NCO content of 5 to 30%, preferably 10 to 25% and most preferably 15 to 25% by weight.

Suitable dimers include uretdione diisocyanates prepared from the previously described aliphatic and/or cycloaliphatic diisocyanates. The uretdione diisocyanates are preferably prepared from hexamethylene diisocyanate ("HDI") and/or of IPDI. Although, in theory, the uretdione diisocyanates can be used as the sole component of the polyisocyanate adduct, the present invention requires that at least some amount of trimer also be present in the polyurethane dispersions of the present invention.

In addition to the combination of a dimer and trimer, urethane and/or allophanate group-containing polyisocyanates prepared from the previously described aliphatic and/or cycloaliphatic diisocyanates, preferably hexamethylene diisocyanate or IPDI, by reacting excess quantities of the diisocyanates with the previously described low molecular weight polyols, preferably trimethylol propane, glycerine, 1,2-dihydroxy propane or mixtures thereof, are also suitably employed in fabricating the polyisocyanate adducts, if desired. If used, the urethane and/or allophanate group-containing polyisocyanates have a most preferred NCO content of 12 to 20% by weight and an (average) NCO functionality of 2.5 to 3. Likewise, oxadiazinetrione groups can be employed, if desired, in the preparation of the polyisocyanate adducts useful in the present invention, such as, for example, those prepared from HDI.

A chain extending agent is also employed in accordance with the present invention.Examples of useful chain extenders or chain-lengthening agents are low-molecular-weight polyols and polyamines. Examples of useful low-molecular-weight polyols are glycols exemplified as materials for the aforementioned polyester polyols and alkeylene oxide low-mole adducts thereof (less than 500 in molecular weight); alkylene oxide low-mole adducts (less than 500 in molecular weight) of bisphenol; trihydric alcohols such as glycerin, trimethylolethane, trimethylolpropane and alkylene oxide low-mole adducts (less than 500 in molecular weight) of such alcohols; and mixtures of at least two of these polyols. Examples of useful polyamines are aliphatic polyamines such as ethylenediamine, N-hydroxyethylethylenediamine, tetramethylenediamine, hexamethylenediamine and diethylenetriamine; alicyclic polyamines such as 4,4'-diaminodicyclohexylmethane, 1,4-diaminocyclohexane and isophoronediamine; aliphatic polyamines having an aromatic ring, such as xylylenediamine and tetramethylxylylenediamine; aromatic polyamines such as 4,4'-diaminodiphenylmethane, tolylenediamine, benzidine and phenylenediamine; and inorganic diamines such as hydrazine; and mixtures of at least two of these polyamines. The amount of the chain lengthening agent is usually 0.3 to 30 wt. %, preferably 0.5 to 20 wt. %, based on the high-molecular-weight polyol in preparing the prepolymer.

In order to reduce the viscosity of the water dispersible polyisocyanates an organic solvent may be added to the water dispersible polyisocyanate.

The aqueous coating compositions according to the present invention may be applied to substrates using any of the various techniques known in the art, and the coating compositions are particularly suitable for ACES applications. In addition, the aqueous compositions may be blended with other types of resins optionally containing isocyanate-reactive groups or with amine- or phenol-formaldehyde condensates known in the art. They can also contain pigments, levelling agents, catalysts, and other auxiliaries known in the art. Examples of the application techniques, resins and auxiliaries are set forth in U.S. Pat. No. 4,408,008, which is herein incorporated by reference.

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

Patent Literature Control Experiment--Coating Composition Prepared using an IPDI-Terminated Prepared in Accordance with Example 1 of U.S. Pat. No. 5,169,895

Into a 2000 ml resins reactor equipped with stirrer, heating mantle, thermometer and nitrogen are charged 375 g (0.3790 eq.) on an ethylene adipate diol, 28.05 g (0.4183 eq.) of dimethylol propionic acid (DMPA), 165.9 g (1.265 eq.) dicyclohexylmethane-4,4'-diisocyanate (H.sub.12 MDI), 142.5 g of N-methyl-2-pyrrolidinone (NMP) and 0.6 g of dibutyl tin dilaurate (DBTL). The system is stirred and heated to 80.degree. C. for 2.5 hrs. The reactor is cooled to 60.degree. C. and maintained for 1.5 hrs and 103.75 g (0.5337 eq.) Des N-100 (biuret of 1,6-hexane diisocyanate) and 21.15 g (0.2090 eq.) of triethylamine (TEA) is added and maintained at 60.degree. C. for 1.5 hrs after which time 250 g of the pre-polymer reaction mixture is poured into 431.9 g water containing 2.5 g of Triton X-350 surfactant. The mixture was mixed well on a drill press. After five minutes 7.37 g of 64% hydrazine is added and the mixture mixed well on a drill press. The final product is poured into a quart jar and capped with nitrogen.

Test specimens for physical property determination were made by drawing thin films on tin coated plates and on cold rolled steel plates coated with an ECOAT and a primer.

Coating Composition Made Using 1,6-hexane diisocyanate dimer/trimer Versus Control Experiment

This experiment was run under the same conditions and with the same ratio of reactants as the "Patent Literature Control Experiment" of Example 1 above except the absolute amounts were scaled back to those indicated.

Into a 1000 ml resins reactor equipped with stirrer, heating mantle, thermometer and nitrogen are charged 158.02 g (0.157 eq.) on an ethylene adipate diol, 11.82 g (0.1762 eq.) of dimethylol propionic acid (DMPA), 70.65 g (0.533 eq.) dicyclohexylmethane-4,4'-diisocyanate (H.sub.12 MDI), 60.04 g of N-methyl-2-pyrrolidinone (NMP) and 0.25 g of dibutyl tin dilaurate (DBTL). The system is stirred and heated to 80.degree. C. for 2.5 hrs. The reactor is cooled to 60.degree. C. and maintained for 1.5 hrs and 44.42 g (0.241 eq.) of 1,6-hexane diisocyanate dimer/trimer and 8.92 g (0.0881 eq.) of triethylamine (TEA) is added and maintained at 60.degree. C. for 1.5 hrs after which time 316 g of the pre-polymer reaction mixture is poured into 542.82 g water containing 3.16 g of Triton X-301 surfactant. The mixture was mixed well on a drill press. After five minutes 9.31 g of 64% hydrazine is added and the mixture mixed well on a drill press. The final product is poured into a quart jar and capped with nitrogen.

Isophorone Diisocyanate Water Dispersed Urethanes 1,6-hexane diisocyanate dimer/trimer

Into a 1000 ml resins reactor equipped with stirrer, heating mantle, thermometer and nitrogen are charged 158.02 g (0.1597 eq.) on an ethylene adipate diol, 11.82 g (0.1762 eq.) of dimethylol propionic acid (DMPA), 59.24 g of dibutyl tin dilaurate (DBTL). The system is stirred and heated to 80.degree. C. for two hours. The reactor is cooled to 60.degree. C. and 44.42 g (0.241 eq.) 1,6-hexane diisocyanate dimer/trimer and 8.91 g (0.0881 eq.) of triethylamine (TEA) is added and maintained at 60.degree. C. for one hour after which time 316 g of the pre-polymer reaction mixture is poured into 542.82 g water containing 3.16 g of Triton X-301 surfactant. Five minutes after the addition is complete 9.31 g of 64% hydrazine is added. The final product is poured into a quart jar and capped with nitrogen.

1,6-hexane diisocyanate dimer/trimer with added dimer

Example #3 was repeated under identical conditions and using identical reactants, except that the isocyanate was formulated to have a 25% higher dimer content.

Isophorone diisocyanate with no additional isocyanate added

Example #3 was repeated, but with the isophorone diisocyanate being the only isocyanate present. The polyol used was a 1000 MW hexanediol adipic acid polyester diol.

Isophorone diisocyanate with added 1,6-hexane diisocyanate dimer/trimer

Example #3 was repeated with a blend of isophorone diisocyanate and 1,6-hexane diisocyanate dimer/trimer as the isocyanate portion. The polyol used was a 1000 MW hexanediol adipic acid polyester diol.

As the results presented in Tables 1-6 below demonstrate, the addition of dimer/trimer provided films on a coated substrated having higher modulus and lower elongation, relative to films made using coating compositions not containing dimer/trimer. Abbreviations employed herein: DMPA         Dimethylol propionic acid H.sub.12 MDI Dicyclohexylmethane-4,4'-diisocyanate IPDI Isophorone diisocyanate HDI D/T 1,6-hexane diisocyanate dimer/trimer NMP N-methyl-2-pyrrolidinone TEA Triethylamine DBTBL Dibutyltindilaurate QUV YI A yellowness index measurement of Q ultraviolet light using ASTM E-313 ______________________________________ ______________________________________ Comparison of Physical Properties of Coatings Made from HDI/Biuret versus HDI/Dimer Trimer Property         Example #1 Example #2 ______________________________________ Tensile Modulus psi at 100% 2980 2590 200% NA 3860 Tensile strength psi 3120 5800 Elongation  95  290 ______________________________________ "NA" denotes not applicable. ______________________________________ Comparison of Mechanical Properties of Coatings Made from HDI/Biuret versus HDI/Dimer Trimer Test #      Property       Example #1 Example #2 ______________________________________ Thickness, mils 2         2 ASTM D-523-89 Gloss, 20.degree. 83.2 82.3 60.degree. 90.4 90.2 ASTM D-3359 Adhesion Pass (5) Pass (3) ASTM D-3363 Hardness, lead# 1 1 ASTM D-522 Conical Bend Pass Pass Dry time, min 20 20 ASTM D-2794 Impact Direct, ambient temp. 160+ 160+ Reverse, ambient temp. 160+ 160+ Direct, -17.degree. C. 60 80 Reverse, -17.degree. C. 5 5 ASTM E-313 QUV, Y.I., 0 hrs. -1.57 -1.52 QUV, Y.I., 2000 hrs. 0.33 0.36 ______________________________________ ______________________________________ Effect of Additional Dimer Example #3 Example #4 ______________________________________ Polyol          158.02    158.02 DMPA 11.82 11.82 IPDI 59.24 59.24 DBTL 0.25 0.25 NMP 60.04 60.24 HDI D/T 44.42 34.17 Additional HDI Dimer 0 10.25 TEA 8.91 8.91 Prepolymer used 316 284.4 64% Hydrazine 9.31 7.26 Water 542.82 488.54 ______________________________________ ______________________________________ Physical Properties of Coatings Made from HDI/Dimer versus HDI/Dimer with Added Dimer Test Method Property      Example #3 Example #4 ______________________________________ ASTM D-412 Tensile Modulus psi at 100% 2220 2345 200% 3510 3350 300% NA 5415 ASTM D-412 Tensile strength psi 5110 7120 ASTM D-412 Elongation, %  270  357 ______________________________________ "NA" denotes not applicable. ______________________________________ Effect of 1,6-hexane Diisocyanate Dimer/Timer Example #5 Example #6 ______________________________________ Polyol        100.59    100.59 DMPA 14.16 14.16 IPDI 63.74 63.74 DBTL 0.30 0.30 NMP 71.94 71.94 HDI D/T 0 48.51 TEA 9.69 10.68 Prepolymer used 227.52 284.4 64% Hydrazine 3.74 8.38 Water 390.83 488.54 ______________________________________ ______________________________________ Physical Properties of Coatings Made from IPDI versus IPDI with Added HDI/Dimer Property        Example #5 Example #6 ______________________________________ Tensile Modulus psi at 100% 2105 4260 200% 3650 NA 300% 5300 NA Tensile Strength psi 5650 4720 Elongation, %  295  127 ______________________________________ "NA" denotes not applicable.

Incorporation of Dimer/Trimer directly into the prepolymer.

Into a 1000 ml resins reactor equipped with stirrer, heating mantle thermometer and nitrogen are charged 103.52 g (0.213 eq.) on an ethylene adipate diol, 15.79 g (0.2354 eq.) of dimethylol propionic acid (DMPA), 71.07 g (0.639 eq.) isophorone diisocyanate, 54.08 g (0.289 eq.) 1,6-hexane diisocyanate dimer/trimer, 80.21 g of N-methyl-2-pyrrolidinone (NMP) and 0.33 g of dibutyl tin dilaurate (DBTL). The system is stirred and heated to 80.degree. C. for 4.0 hrs. The cooled reaction mixture had an NCO content of 6.1%.

The pre-polymer reaction mixture, 284.4 g, is poured into 450.54 g water containing 2.84 g of Triton X-301 surfactant. The mixture was mixed well on a drill press. After five minutes 9.82 g of 64% hydrazine is added followed by a 30 g water rinse. The mixture is mixed well on a drill press. The final product is poured into a quart jar and capped with nitrogen.

Test specimens for physical property determination were made by drawing thin films on tin coated plates and on cold rolled steel plates coated with an ECOAT and a primer. ______________________________________ Mechanical Property Data Test #       Property       Example #7 ______________________________________ Thickness, mils 1.2 ASTM D-523-89 Gloss, 20.degree. 95.5 60.degree. 112.7 ASTM D-3359 Adhesion Pass (5) ASTM D-3363 Pencil Hardness, lead # 4 ASTM D-522 Conical Bend Pass Dry Time, min 20 ASTM D-2794 Impact Direct, ambient 110 Reverse, ambient 160 Direct, -17.degree. C. 40 Reverse, -17.degree. C. 0 ASTM E-313 QUV, Y.I., 0 hrs. -2.63 QUV. Y.I., 1056 hrs. 0.22 ASTM D-412 Tensile Strength, pst 464.5 ASTM D-412 Elongation, % 62 ______________________________________

While the invention has been described above with reference to specific embodiments thereof, it is apparent that many changes, modifications and variations can be made without departing from the inventive concept disclosed herein. Accordingly, it is intended to embrace all such changes, modifications and variations that fall within the spirit and broad scope of the appended claims. All patent applications, patents and other publications cited herein are incorporated by reference in their entirety.

## Claims

1. A method for coating a substrate which comprises contacting the substrate with a coating composition comprising a dispersion in water of the reaction product of: having an NCO content of between about 1.5 and 10% by weight, said prepolymer having incorporated therein uretdione moieties and isocyanurate moieties, and

2. The method of claim 1 wherein the molar ratio of said uretdione moieties to said isocyanurate moieties is between about 1:10 and about 10:1.

3. The method of claim 1 wherein said isocyanurate moiety is selected from the group consisting of isocyanato-isocyanurates based on 1,6-diisocyanatohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane, and combinations thereof.

4. The method of claim 1 wherein said uretdione moiety is selected from the group consisting of isocyanato uretdiones of hexamethylene diisocyanate, isophorone diisocyanate, and combinations thereof.

5. The method of claim 1 wherein said water-dispersible isocyante-terminated polyurethane prepolymer comprises the reaction product of: weight in the range from 250 to 5000, and isocyanate or isocyanate-reactive groups.

6. The method of claim 5 wherein the compound of component (iii) is dihydroxyalkanoic acid.