Waterborne coatings and binder systems for use therein

Abstract: A binder system is provided which is suitable for use as a waterborne coating, particularly as a topcoat over hydro-basecoats. The binder in accordance with the present invention comprises a water-soluble acrylic resin which has been modified with a monoisocyanate functional compound. The waterborne coating comprises this binder system in an aqueous base. The resulting waterborne coating exhibits high gloss, high solids content with low cosolvent content, good application characteristics, and good stone-chip and weathering resistance. The present waterborne coating also finds use, for example, as an electrocoat, primer/surfacer and/or solid color topcoat.

Description: Background of the Invention The present invention relates generally to waterborne coatings. More particularly, the present invention relates to acrylic-based binder systems for waterborne coatings and the waterborne coatings produced from such binder systems. The emphasis on environmental protection together with increasingly restrictive legislation has led to the increased use of coatings based upon water rather than organic solvents. Although waterborne primer/surfacers may now be considered normal state-of-the art coatings in the automobile industry, color topcoats continue to represent a major source of organic solvent pollution. When it is considered that current, conventional, solid-color topcoats contain 50% or more organic solvents and that some 5 kg of paint are required to coat a compact car, it is clear that the total solvent emission from this source is substantial. The increasingly popular clear-over-base metallic finishes exacerbate this problem, with the basecoat alone presently accounting for 50% of total emissions from the paint line. Such basecoats may contain up to 88% solvent and require a final clear coating which itself may have 50% or more organic solvent content. Considerable work has, therefore, been undertaken to replace these solvent-based coatings with higher solids and waterborne systems, and major efforts have been concentrated on the metallic basecoats. Various systems have been proposed and examples are described in EP-A-228 142, EP-A-238 222, EP-A-242 731, EP-A-251 921 and US 4 213 886, all of which are incorporated by reference. Conventional clearcoats or two-component clearcoats with higher solid contents (60-65%) are proposed as final coatings for these basecoats. The use of conventional organic solvent-based topcoats with the hydro-basecoats, however, leads to problems. In the "wet-in-wet" application of the solvent-based topcoat to the hydro-basecoat, for example, appearance defects due to "strike-in" phenomena may be produced. To minimize this, an expensive and time consuming pre-dry step must be utilized for the basecoat. Additionally, organic solvents utilized in the conventional topcoats remain a source of pollution. It would, therefore, be highly desirable to produce a waterborne coating which finds utility, in part, as a topcoat for hydro-basecoats. Additionally, such waterborne coating should desirably have physical and mechanical properties comparable to conventional coating materials, should be easy to apply (i.e., using the wet-in-wet method), and should not exhibit "strike-in" phenomena when so applied. It should also possess good pigment/extender wetting and dispersion properties for use in pigmented forms such as surfacers, solid-color topcoats and the like. Waterborne coatings produced from water-soluble acrylic and/or polyester resin binder systems have been described in the literature and utilized commercially, but only with limited success as topcoats for hydro-basecoats. As topcoats, they generally exhibit unfavorable rheology, and require the use of large amounts of amines or cosolvents to lower viscosity sufficiently for reliable application and to ensure stability of the system. This, however, results in poor appearance (solvent "strike-in") and also in a low solids content which makes it difficult to reach the required film thickness with normal application methods. More recently, waterborne topcoats have been produced from "hybrid" binder systems comprising combinations of water-soluble resins with aqueous dispersion resins. An example is the combination of Synthacryl 6483 (a commercial acrylic dispersion) with Synthacryl 6484 (a commercial water soluble acrylic resin), which is recommended by the Hoechst AG as a water-based clearcoat for two-coat metallic finishes. Other examples are described in U.S. Patent Nos. 3 919 154, 3 953 644, 4 518 724 and 4 624 973, all of which are hereby incorporated by reference. The resulting waterborne coatings display somewhat improved properties, but rheology and low solids content problems remain. Also, the waterborne coatings utilizing these hybrid binder systems normally still require large amounts (20-25%) of cosolvents, resulting in a topcoat with less than ideal appearance characteristics, sagging on application and a tendency to solvent-popping on drying. Summary of the Invention The present invention provides a waterborne coating especially well suited for use as a topcoat over hydro-basecoats, and a process for producing such waterborne coatings. In its overall concept, the waterborne coating of the present invention comprises an acrylic-based binder system and a curing agent therefor in an aqueous base. The binder system is characterized in that it comprises a water-soluble acrylic resin which has been modified with a monofunctional isocyanate compound. The water-soluble acrylic resin may be modified with the monofunctional isocyanate compound, for example, by (1) reacting the resin directly with the monofunctional isocyanate compound, or (2) reacting an hydroxyacrylic monomer with the monofunctional isocyanate compound, then (co)polymerizing the resulting urethane modified monomer, as further discussed below. It should be noted that, as utilized herein, the term "monofunctional isocyanate compound" is not intended to include compounds also containing a "blocked" isocyanate group, which becomes unblocked during normal curing conditions of the binder (i.e., at temperatures below 160°C). In other words, for example, compounds containing a "free" isocyanate group and a "blocked" isocyanate group are, for the purposes of the present invention, to be considered polyfunctional isocyanate compounds. As examples of such "polyfunctional" isocyanate compounds are those utilized in US3773729 to modify a variety of resins in the production of waterborne coatings. Specifically disclosed in this reference are self-crosslinking binders which cure at elevated temperatures by the unblocking of the blocked isocyanate groups then subsequent reaction with counterreactive functionality (e.g, hydroxyl groups) also on the binder. To increase the solids content and/or modify the rheology of the binder system and ultimate waterborne coating, an aqueous dispersion resin, preferably an aqueous acrylic dispersion resin, may be utilized as an optional component of the binder system. A waterborne coating may be produced from the binder system by any one of a number of well-known techniques. The resulting waterborne coating in accordance with the present invention displays high gloss, high solids content with low cosolvent content, good application characteristics, and good stone-chip and weathering resistance, making it especially well suited for use as a topcoat over a hydro-basecoat. The waterborne coating may also find use in pigmented form as an electrocoat, primer/surfacer and/or solid color topcoat. These and other features and advantages of the present invention will be more readily understood by one skilled in the art from a reading of the following detailed description. Detailed Description of the Preferred Embodiments As previously mentioned, the waterborne coatings of the present invention comprise an acrylic-based binder system and a curing agent therefor in an aqueous base. The binder system of the present invention is characterized in that it comprises, in its overall concept, a water-soluble acrylic resin which has been modified with a monofunctional isocyanate compound. Suitable water-soluble acrylic resins are those produced from the polymerization of one or more hydroxyacrylic monomers. Copolymers of such hydroxyacrylic monomers with one or more other olefinically unsaturated monomers, as further described below, are especially suitable. The water-soluble arylic resins are preferably anionic in character with an average molecular weight (M n ) of between 500 to 20 000, an hydroxyl number within the range of 10 to 400 (mg KOH/g resin) and an acid value of from 5 to 150 (mg KOH/g resin). After at least partial neutralization of the carboxyl groups with, for example, an amine, these water-soluble acrylic resins give clear, stable solutions in water. In preferred embodiments, such water-soluble acrylic resins are produced by the free-radical polymerization of from 10 to 100, more preferably 20 to 80, parts by weight of hydroxyacrylic monomer with 0 to 90, more preferably 20 to 80, parts by weight other copolymerizable monomer. Parts by weight here is based upon 100 parts by weight total monomer mixture. Suitable hydroxyacrylic monomers comprise the hydroxy-substituted esters of acrylic and methacrylic acid, more preferably the C₁-C₁₀ alkyl esters. Particularly preferred include, for example, hydroxypropyl methacrylates, hydroxyethyl acrylates and hydroxyethyl methacrylates. Suitable other olefinically unsaturated comonomers include those normally utilized in producing water-soluble acrylic resins such as, for example, acrylic and methacrylic acid and their C₁-C₁₀ alkyl esters; glycidyl acrylate and methacrylate; other acrylic and methacrylic acid derivatives such as acrylamide, acrylonitrile and acrolein; polyethyleneglycol monoacrylate and methacrylate; vinyl aromatics such as styrene and vinyl toluene; maleic and fumaric acid; and the like. Particularly preferred are the α,β-ethylenically unsaturated monomers, especially butyl methacrylate, butylacrylate, 2-ethylhexyl acrylate, methacrylic acid and methyl methacrylate. If self-crosslinking of the binder is desired, it is also particularly preferred to include acrylamide derivatives such as N-methylolacrylamide and N-methoxymethylacrylamide. The binder system of the present invention is produced by modifying these water-soluble acrylic resins with a monofunctional isocyanate-compound. The monofunctional isocyanate compounds are those selected from: (a) monoisocyanate compounds, and (b) adducts of polyisocyanates with active hydrogen group-containing (single primary reactive group) compounds. As mentioned above, the term "monofunctional isocyanate compound" is not intended to include compounds containing a "blocked" isocyanate group, which becomes unblocked during normal curing conditions of the binder. As examples of suitable monoisocyanate compounds may be mentioned those of the general formula (I): R - NCO (I) wherein R is a C₈-C₂₅ alkyl group, preferably a linear alkyl group, with stearyl isocyanate (C₁₈) being particularly preferred. As examples of suitable polyisocyanates may be mentioned aliphatic, cycloaliphatic and aromatic di, tri or tetra isocyanates (isocyanate functionality 2 to 4) such as 1,2-propylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, 2,3-butylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, dodecamethylene diisocynate, ω,ω'-dipropyl ether diisocyanate, 1,3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate, 1,4-cyclohexane diisocyanate, isophorone diisocyanate, 4-methyl-1,3-diisocyanatocyclohexane, trans-vinylidene diisocyanate, dicyclohexyl methane-4,4'-diisocyanate, 3,3'-dimethyl-dicyclohexyl methane-4,4'-diisocyanate, a toluene diisocyanate, 1,3-bis(isocyanatomethyl)benzene, a xylylene diisocyanate, 1,5-dimethyl-2,4-bis(isocyanatomethyl)benzene, 1,5-dimethyl-2,4-bis(2-isocyanatoethyl)benzene, 1,3,5-triethyl-2,4-bis(isocyanatomethyl)benzene, 4,4-diisocyanato-diphenyl, 3,3'-dichloro-4,4'-diisocyanatodiphenyl, 3,3'-diphenyl-4,4'-diisocyanatodiphenyl, 3,3'-dimethoxy-4,4'-diisocyanatodiphenyl, 4,4'-diisocyanatodiphenyl methane, 3,3'-dimethyl-4,4'-diisocyanatodiphenyl methane, a diisocyanatonaphthalene, the adduct of 2 molecules of a diisocynate (for instance hexamethylene diisocyante or isophorone diisocyanate) and a diol such as ethylene glycol, the adduct of 3 molecules of hexamethylene diisocyanate and 1 molecule of water (available under the trademark Desmodur N of Bayer), the adduct of 1 molecule of trimethylol propane and 3 molecules of toluene diisocyanate (available under the trademark Desmodur L of Bayer), the adduct of 1 molecule of trimethylol propane and 3 molecules of isophorone diisocyanate, compounds such as 1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatoluene, isocyanurate group-containing isocyanate compounds, and the adduct of 1 molecule of pentaerythritol and 4 molecules of toluene diisocyanate, or mixtures of 2 or more of the above-envisaged isocyanate compounds. It is especially preferred that use should be made of a diisocyanate or triisocyanate containing 8-36 carbon atoms. Particularly preferred polyisocyanates include hexamethylene diisocyanate, tetramethylxylene diisocyanate, trimethylhexane diisocyanate, toluene diisocyanate, cyclohexyl diisocyanate, diphenylmethane diisocyanate and their cyclic trimeric forms as isocyanurates; and aliphatic di- and tri-isocyanates such as trimer hexamethylene diisocyanate (Desmodur N) and isophorone diisocyanate. As suitable active hydrogen group-containing compounds may be mentioned monohydroxy functional compounds, monoamine functional compounds and monothiol functional compounds. Preferred are the monohydroxy and monoamine functional compounds. It should be noted that the active hydrogen-group containing compounds may also contain more than one type of active hydrogen group, as exemplified further below. The term "active hydrogen group-containing" compound, however, is not intended to cover those compounds having more than a single primary reactive active hydrogen group. As examples of preferred monohydroxy functional compounds may be mentioned one or more of the following: (1) a monohydroxy substituted carboxylic compound of the general formula (II): HO - R¹ - COOR² (II) wherein R¹ is a hydrocarbon group having from 1 to 25 carbon atoms, and R² is hydrogen or a hydrocarbon group having from 1 to 18 carbon atoms. More preferred are the group of hydroxy fatty acids and their esters where R¹ is an aliphatic hydrocarbon group having from 4 to 24 carbon atoms, and R² is hydrogen or a C₁-C₁₈ alkyl group. Especially preferred are those hydroxy fatty acids where R¹ is an aliphatic, linear, saturated hydrocarbon group having from 12 to 20 carbon atoms and R² is hydrogen, whereby hydroxy stearic acid is particularly preferred. (2) a monoalcohol of the general formula (III): HO - R³ (III) wherein R³ is a hydrocarbon group having from 1 to 25 carbon atoms. More preferred are those alcohols where R³ is a C₁-C₁₈ alkyl group, with ethanol, butanol, lauryl alcohol and stearyl alcohol being particularly preferred. (3) a polyoxyalkylene glycol monoalkylether of the general formula (IV): HO - [(C₃H₆O) n /(C₂H₄O) n1 ] - R⁴ (IV) wherein R⁴ is a C₁-C₄ alkyl group, n is 0 or a whole number from 1 to 25, and n1 is 0 or a whole number from 2 to 40, with the proviso that 1 ≦ n + n1 ≦ 50 . The combination of n being 0, n1 being from 10 to 12 and R⁴ being a methyl group is especially preferred. (4) a monohydroxy substituted carboxamide compound of the general formula (V): HO - R¹ - CON(R⁵)₂ (V) wherein R¹ is as defined above, and each R⁵ may independently be hydrogen or a hydrocarbon group having from 1 to 18 carbon atoms. As examples of preferred monoamine functional compounds may be mentioned one or more of the following: (5) a monoamine of the general formula (VI): R⁶ - HN - R³ (VI) wherein R³ is as defined above, and R⁶ is hydrogen or a hydrocarbon group having 1 to 25 carbon atoms, with the proviso that the monoamine (5) should not contain more than 26 carbon atoms in total. The combination of R³ being a C₁-C₁₈ alkyl group and R⁶ being hydrogen is especially preferred, with ethylamine, butylamine, laurylamine and stearylamine being particularly preferred. (6) a polyoxyalkyleneamine monoalkylether of the general formula (VII): R⁷ - HN - [(C₃H₆O) n /(C₂H₄O) n1 ] - R⁴ (VII) wherein R⁴, n and n1 are as defined above, and R⁷ is hydrogen, a C₁-C₂₀ alkyl group or a polyoxyalkylene group of the formula [(C₃H₆O) n /(C₂H₄O) n1 ], wherein n and n1 are as defined above. The combination of R⁷ being hydrogen, R⁴ being a methyl group, n being from 2 to 4 and n1 being from 16 to 20 is especially preferred. As examples of preferred monothiol functional compounds may be mentioned one or more of the following: (7) monothiol substituted carboxylic acid and esters of the general formula (VIII): HS - R⁸ - COOR² (VIII) wherein R² is as defined above and R⁸ is a hydrocarbon group having 1 or 2 carbon atoms. (8) monothiols of the general formula (IX): HS-R⁹ (IX) wherein R⁹ is a hydrocarbon group having from 2 to 12 carbon atoms. The monofunctional isocyanate adduct of the polyisocyanate and active hydrogen group-containing compound is produced by reacting the components, by any well-known manner, in an amount whereby one mole of polyisocyanate is reacted with X-1 moles of active hydrogen group-containing compound, wherein X is the number of isocyanate groups (as indicated before, preferably 2 to 4) per polyisocyanate molecule. In other words, if a diisocyanate is utilized, the mole ratio of diisocyanate to active hydrogen group-containing compound will be 1:1; if a triisocyanate is utilized, the mole ratio will be 1:2; a tetraisocyanate 1:3; and so on. The resulting adduct, therefore, will have an average of one isocyanate group per molecule (monofunctional isocyanate). As previously mentioned, the water-soluble acrylic resin is reacted with the monofunctional isocyanate compound to produce the binder systems of the present invention. From 1 to 40, more preferably from 1 to about 20, parts by weight of the monofunctional isocyanate is reacted with from 60 to 99, more preferably from about 80 to about 99, parts by weight of the acrylic resin, the combination comprising 100 parts by weight. Various reaction schemes for the water-soluble acrylic resins and monofunctional isocyanate compounds can be envisioned. In a first preferred reaction scheme, the water-soluble acrylic resin is reacted directly with the monofunctional isocyanate compound, in the amounts as described above, to produce the binders of the present invention. In an alternative preferred reaction scheme, at least one mole of the hydroxyacrylic monomer is reacted per mole of the monofunctional isocyanate compound to produce a urethane modified acrylic monomer. This monomer is then polymerized, and/or copolymerized with additional hydroxyacrylic monomer and/or other olefinically unsaturated monomers, in the amounts and of the types as described above, by well-known free-radical initiated polymerization methods to produce the binders of the present invention. To increase the solids content and/or modify the rheology of the binder system, an aqueous dispersion resin may be added to, or produced in situ within, the modified water-soluble acrylic resin. In preferred embodiments, the binder system may comprise an additional 100 parts by weight or less, more preferably an additional 10 to 70 parts by weight, of the aqueous dispersion resin. Aqueous dispersion resins suitable for use with the binders of the present invention include non-ionic or anionic dispersions of acrylic, polyurethane and other synthetic resins, preferred use being made of aqueous acrylic dispersion resins. Such dispersions also include internally cross-linked dispersions ("microgels") as described in, for example, EP-A-38 127, EP-A-238 108 and EP-A-273 530, all of which are incorporated by reference. A wide variety of dispersion resins may be utilized based upon the desired end-use of the resulting coating, and one skilled in the art can easily choose the type and characteristics of the dispersion resins utilized for a particular desired end-use. As pre-prepared components, such aqueous dispersion resins are well-known in the art and generally commercially available. For example, for a clearcoat having good weather resistance, film hardness, solvent resistance and like characteristics, an aqueous acrylic dispersion typified by Synthacryl VSW 6483 (Hoechst AG) or Neocryl XK62 (Polyvinyl Chemie) may be used. For a surfacer requiring somewhat more elasticity, improved stone-chip resistance and sandability, a polyurethane typified by Neorez R 974 (Polyvinyl Chemie) may be used. When used as a pre-prepared component, the aqueous dispersion resin is added to the modified acrylic resin by simple blending of the two components. When made in situ, such aqueous dispersion resin is formed within the modified acrylic resin by well-known procedures, for example, as taught in U.S. Patent No. 3 919 154, previously incorporated by reference. The resulting binder system in accordance with the present invention should be characterized by an acid value of from about 5 to about 150 (mg KOH/g resin), more preferably from about 10 to about 70, and still more preferably from about 15 to about 35, and an hydroxyl number of about 20 to about 250 (mg KOH/g resin), more preferably from about 40 to about 150. The aforedescribed binder systems are utilized to produce waterborne coatings by blending with other suitable components in accordance with normal paint formulation techniques. Typically, the modified water-soluble acrylic resin is at least partially neutralized with, for example, an amine, then charged into a vessel with the other components being sequentially blended using normal stirring techniques. The viscosity of the mixture is adjusted finally with demineralized water to obtain the desired application properties. If a pigmented system is desired, a mill-base can be prepared comprising the major part of the pigment dispersed in at least a portion of the water-soluble resin by means of sand-milling, ball-milling etc. The mill-base is subsequently "let-down" by blending with the remaining binder and other components, then adjusted to the desired viscosity. It is an essential feature of the invention that the so-produced coatings also contain a curing agent for the binder. Such curing agents include, for example, N-methylol groups and/or N-methylol ether groups-containing amino resins obtained by reacting an aldehyde, such as formaldehyde, with a compound containing amino groups or amide groups, such as melamine, urea, N,N'-ethylene diurea, dicyandiamide and benzoguanamine (for the preparation of such compounds see Houben-Weyl, Methoden der organischen Chemie, Band 14/2, pp. 319-371 (1963)). It is preferred that the afore-described compounds should entirely or partly be etherified with alcohols having 1 to 6 carbon atoms, for instance with methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, amyl alcohol, hexanol or mixtures of the alcohols referred to above. Particularly, use is made of a methylol melamine containing 4 to 6 methylol groups per molecule of melamine, at least 3 methylol groups being etherified with methanol, ethanol, a propanol or a butanol, or a butanol-etherified condensation product of formaldehyde and N,N'-ethylene diurea. More particularly, use is made of a hexaalkoxy-methyl melamine whose alkoxy group contains 1 to 4 carbon atoms. Other well-known additives such as, for example, antioxidants, ultraviolet absorbers, cosolvents, leveling agents, rheology control agents, pigments, colorants and the like, may be utilized in the waterborne coating of the present invention without unduly affecting the properties thereof. In fact, without a pigment or colorant the waterborne coating as described above may be utilized as a clearcoat. With the pigmentation, the waterborne coating may be utilized as a colored topcoat. These waterborne coatings also find use as electrocoats and primer/surfacers. The coatings according to the invention may be applied to a substrate in any desirable manner, such as by roller coating, spraying, brushing, sprinkling, flow coating, dipping, electrostatic spraying or electrophoresis. Curing may be carried out at ambient temperature or at elevated temperature to reduce the curing time. Preferably, the coating is baked at higher temperatures in the range of, for instance, 60°C to 160°C, in a baking oven over a period of 10 to 60 minutes. The foregoing more general discussion of this invention will be further exemplified by the following specific examples offered by way of illustration and not limitation of the above-described invention. Examples Preparation of Binders Method A : Urethane Modified Monomer Modified Monomer 1 (MM1) 0,75 moles Desmodur N, 1,5 moles 12-hydroxy stearic acid, 975 g diethylene glycol dimethylether and 1% (based on the weight of Desmodur N) of dibutyltin dilaurate were placed in a 2 litre, four-necked round-bottom flask fitted with a stirrer, thermometer and gas inlet tube, and stirred together for 4 hours at 60°C. 0,75 moles 2-hydroxypropyl methacrylate and 0,5% dibutyltin dilaurate (based on the weight of the 2-hydroxypropyl methacrylate) were subsequently added and the mixture stirred until the IR-spectrum of a sample showed no isocyanate groups to be present. After completion of the reaction, 100 ppm of the monomethylether of hydroquinone were added as a stabiliser. The solids content of the solution was about 50%. Modified Monomer 2 (MM2) The procedure of MM1 was repeated with 1,5 moles of Breox MPEG 550 (a methoxy polyethyleneglycol of mean molecular weight 550 from BP Chemicals) being used in place of the 12-hydroxy stearic acid. The solids content of the solution was adjusted to about 50% by adjustment of the solvent (diethylene glycol dimethylether). Modified Monomer 3 (MM3) 1,0 mole IPDI (isophorone diisocyanate from Veba-Chemie AG), 1,0 mole 12-hydroxy stearic acid, 666 g diethylene glycol dimethylether and 0,1% (based on the weight of IPDI) of dibutyltin dilaurate were placed in a 2 litre, four-necked round-bottom flask fitted with a stirrer, thermometer and gas inlet tube, and stirred together for 4 hours at 50°C. 1,0 mole 2-hydroxypropyl methacrylate and 1,0% dibutyltin dilaurate (based on the weight of the 2-hydroxypropyl methacrylate) were subsequently added and the mixture stirred until the IR-spectrum of a sample showed no isocyanate groups to be present. After completion of the reaction, 100 ppm of the monomethylether of hydroquinone were added as a stabiliser. The solid content of the solution was about 50%. Modified Monomer 4 (MM4) The procedure of MM3 was repeated with ethanol being used in place of the 12-hydroxy stearic acid. The solids content of the solution was adjusted to about 50% by adjustment of the solvent (diethylene glycol dimethylether). Modified Monomer 5 (MM5) The procedure of MM1 was repeated with 1,5 moles of Polyglykol B01/20 (a polypropyleneglycol monobutyl ether of mean molecular weight 700 from Hoechst) being used in place of the 12-hydroxy stearic acid. The solids content of the solution was adjusted to about 50% by adjustment of the solvent (diethylene glycol dimethylether). Modified Monomer 6 (MM6) The procedure of MM3 was repeated with 1,0 moles of Jeffamine M 1000 (a polyoxyalkylene monoamine of mean molecular weight 1000 from Texaco Chemical Company) being used in place of the 12-hydroxy stearic acid. The solids content of the solution was adjusted to about 50% by adjustment of the solvent (diethylene glycol dimethylether). Binder 1 (B1) A 2 litre, four-necked round-bottom flask fitted with a stirrer, thermometer and gas inlet tube was charged under nitrogen with 300 parts by weight diethylene glycol monobutylether, 46,7 parts by weight Trigonox K 80 (cumene hydroperoxide based initiator from Akzo Chemie), and 10% of weight of the total of a mixture of acrylic monomers consisting of 526 parts by weight butyl acrylate, 235 parts by weight methyl methacrylate, 170 parts by weight 2-hydroxypropyl methacrylate, 50 parts by weight methacrylic acid and 100 parts by weight of MM1. The charge was heated to 140°C and the remainder of the acrylic monomer mixture was added over a period of 3,5 hours, being allowed to polymerise over a further 3 hours. The final solution of acrylic resin had a solids content of 74,5% and the resin had an acid value of 33,5 and an hydroxyl value of 65. Binder 2 (B2 ) The procedure of B1 was repeated using 100 parts by weight of MM2. The resin solution had a solids content of 73,5% and the resin had an acid value of 30,4 and an hydroxyl value of 65. Binder 3 (B3) The procedure of B1 was repeated using 100 parts by weight of MM3. The resin solution had a solids content of 74,3% and the resin had an acid value of 32,3 and an hydroxyl value of 65. Binder 4 (B4) The procedure of B1 was repeated using an acrylic monomer-mixture comprising 415 parts by weight butyl acrylate, 186 parts by weight methyl methacrylate, 17 parts by weight 2-hydroxypropyl methacrylate, 50 parts by weight methacrylic acid acid and 400 parts by weight of MM4. The resin solution had a solids content of 74,1% and the resin had an acid value of 33,7 and an hydroxyl value of 65. Binder 5 (B5) The procedure of B1 was repeated using 100 parts by weight of MM5. The resin solution had a solids content of 75,3% and the resin had an acid value of 35,0 and an hydroxyl value of 65. Binder 6 (B6) The procedure of B1 was repeated using 100 parts by weight of MM6. The resin solution had a solids content of 75,6% and the resin had an acid value of 44,6 and an hydroxyl value of 65. Method B : Urethane Modified Resin Urethane Component 1 (UC1) 0,75 moles Desmodur N, 1,5 moles 1 2-hydroxy stearic acid, 866 g diethylene glycol dimethylether and 0,5% (based on the weight of Desmodur N) of dibutyltin dilaurate were placed in a 2 litre, four- necked round-bottom flask fitted with a stirrer, thermometer and gas inlet tube, and stirred together for 6 hours at 60°C. The resulting solution had a solids content of about 50% and an isocyanate functionary of 1. Urethane Component 2 (UC2) 1,0 mole IPDI, 1,0 mole 12-hydroxy stearic acid, 522 g diethylene glycol dimethylether and 0,1% (based on the weight of IPDI) of dibutyltin dilaurate were placed in a 2 litre, four-necked round-bottom flask fitted with a stirrer, thermometer and gas inlet tube, and stirred together for 6 hours at 50°C. The resulting solution had a solids content of about 50% and an isocyanate functionality of 1. Binder 7 (B7) A 2 litre, four-necked, round-bottom flask fitted with a stirrer, thermometer and gas inlet tube was charged under nitrogen with 300 parts by weight diethylene glycol dimethylether and heated to 145°C. A mixture of acrylic monomers and initiator comprising 526 parts by weight butyl acrylate, 175 parts by weight 2-hydroxypropyl methacrylate, 235 parts by weight methyl methacrylate, 50 parts by weight methacrylic acid and 35 parts by weight Trigonox 42S (t-butylperoxy-3.5.5. trimethyl hexanoate from Akzo Chemie) was subsequently added over a period of 3,5 hours and allowed to polymerise for a further 1 hour. After cooling to 70°C, 100 parts by weight of UC1 was added and the mixture stirred until the IR-spectrum of a sample showed no isocyanate groups to be present. The resin solution had a solids content of 74,8% and the resin had an acid value of 32,7 and an hydroxyl value of 65. Binder 8 (B8) The procedure of B7 was repeated using 100 parts by weight of UC2. The resin solution had a solids content of 74,3% and the resin had an acid value of 33,2 and an hydroxyl value of 65. It should be noted that B7 and B8, produced by the alternative route of Method B, were found to be indistinguishable in properties such as viscosity, rheology, stability, etc. from B1 and B3, respectively, produced via Method A. Comparative Binder 1 (CB1) For comparison purposes B7 was repeated without any urethane component being employed. The resulting acrylic resin solution had a solids content of 74,5% and the resin had an acid value of 34,3 and an hydroxyl value of 67. Preparation of Coating Examples Comparative Example 1 (CE1) - Hybrid System A waterborne clearcoat with the following composition was prepared from commercially available materials: Synthacryl VSW 6484 67,6 parts by weight Maprenal MF 915 (methylated melamine-formaldehyde resin, Hoechst AG) 26,9 parts by weight Maprenal MF 927 (methylated melamine-formaldehyde resin, Hoechst AG) 3,5 parts by weight Additol XW 329 (levelling agent, Hoechst AG) 0,1 parts by weight Demineralized water 43,0 parts by weight Synthacryl VSW 6483 48,4 parts by weight Sandovur 3212 (UV-absorber and light stabilizing agent, Sandoz AG) 3,2 parts by weight Demineralized water 22,3 parts by weight The materials were blended in the order given and the resulting clearcoat was allowed to stand overnight before being adjusted to an application viscosity of 60 s (DIN 53211) with demineralized water. The clearcoat had a solids content of 38% and contained 19% organic cosolvent. Comparative Example 2 (CE2) - No urethane component A waterborne clearcoat was prepared with the following composition: Comparative Binder 1 (CB1) 56,4 parts by weight Dimethylethanolamine 1,1 parts by weight Cymel 327 (methylated melamine-formaldehyde resin, American Cyanamid Co.) 33,3 parts by weight Butyl diglycol 12,0 parts by weight Paint Additive 11 (mar resistance agent, Dow Corning GmbH) 0,5 parts by weight Additol XW 395 (levelling and wetting agent, Hoechst AG) 0,2 parts by weight Tinuvins 1130 + 292 (3:2) (UV absorber + light stabilizer, Ciba-Geigy AG) 2,5 parts by weight Demineralized water 60,0 parts by weight Synthacryl 6483 57,1 parts by weight The materials were blended in the order given and the resulting clearcoat was allowed to stand overnight before being adjusted to an application viscosity of 23 s (DIN 53211) with demineralized water. The clearcoat had a solids content of about 46% and contained about 13% organic cosolvent. Comparative Example 3 (CE3) A conventional acrylic/melamine clearcoat, as used in the automobile industry, was used as a comparative example. The material is commercially available from Akzo Coatings GmbH under the designation "05-10077 AY-Klarlack". Example (E1) CE2 was repeated using 56,4 parts of B1 in place of CB1. The resulting clearcoat had a solids content of about 46% and contained about 13% organic cosolvent. Example 2 (E2) CE2 was repeated using 57,1 parts of B2 in place of CB1 and 1,0 parts of dimethylethanolamine. The resulting clearcoat had a solids content of about 46% and contained about 13% organic cosolvent. Example 3 (E3) CE2 was repeated using 56,5 parts of B3 in place of CB1. The resulting clearcoat had a solids content of about 46% and contained about 13% organic cosolvent. Example 4 (E4) CE2 was repeated using 56,7 parts of B4 in place of CB1. The resulting clearcoat had a solid content of about 46% and contained about 13% organic cosolvent. Example 5 (E5) CE2 was repeated using 55,8 parts of B5 in place of CB1 and 1,2 parts of dimethylethanolamine. The resulting clearcoat had a solids content of about 42% and contained about 13% organic cosolvent. Example 6 (E6) CE2 was repeated using 55,6 parts of B6 in place of CB1 and 1,0 parts of dimethylethanolamine. The resulting clearcoat had a solids content of about 44% and contained about 13% organic cosolvent. The mechanical properties of the above coatings were investigated by applying a 35-40 µm dry film thickness layer to an untreated, bare steel panel by pneumatic spraying and baking for 15 minutes at 60°C followed by 25 minutes at 130°C. Each clearcoat was also applied over a pre-dried (10 mins/60°C) waterborne basecoat (a metallic silver product based on an aqueous acrylic dispersion resin and available from Akzo Coatings GmbH under the designation "40-11390 Hydro Basecoat") and baked as above. The resulting film was then evaluated as to gloss and visible appearance (body, levelling, strike-in, etc.) In addition, each clearcoat was applied over the above hydro-basecoat which had not been pre-dried but merely allowed to flash-off at room temperature (about 23°C) for 10 minutes, thereby simulating the wet-in-wet method of application generally used in the automobile industry for conventional base and clearcoats. The base and clearcoat were subsequently baked together as above and the resulting film was evaluated as to gloss and visible appearance. The results are presented in Table 1. The results show a much improved hardness in comparison to the known hybrid system (CE 1) while retaining good flexibility. Cross-linking is also better, as indicated by the xylene resistance. In addition, the higher solids content (46% as against 38%) and lower level of cosolvent (13% against 19%) are particularly advantageous. With regard to the conventional solvent-borne system (CE 3), it may be seen that the mechanical values are comparable but that improved flexibility has been attained. The major differences, and advantages, are apparent from the visual appearance of the final coatings, where examples 1-6 show much better body and levelling properties. The known hybrid system (CE 1) also exhibits very severe strike-in, even when the basecoat is pre-dried, and the conventional system (CE 3), although producing acceptable results on pre-dried basecoat, cannot be satisfactorily applied wet-in-wet as the resulting film exhibits severe levelling and other surface defects. The CE 2 example, although not exhibiting severe strike-in, produces films lacking in body. Many modifications and variations besides the embodiments specifically mentioned may be made in the compositions and methods described herein without substantially departing from the concept of the present invention.

Claims: A waterborne coating composition comprising an aqueous base and a binder system, characterized in that the waterborne coating further comprises a curing agent for the binder system and that the binder system comprises a water-soluble acrylic resin which has been modified with a monoisocyanate compound by (i) reacting the water-soluble acrylic resin directly with the monofunctional isocyanate compound, /or (ii) reacting an hydroxyacrylic monomer with the monofunctional isocyanate compound to produce a urethane modified acrylic monomer, then (co)polymerizing the urethane modified monomer, wherein the monofunctional isocyanate compound is selected from: (a) a monoisocyanate compound, and (b) an adduct of a polyisocyanate compound with an active hydrogen group-containing compound which will not become unblocked during curing temperatures below 160°C, wherein from 60 to 99 parts by weight of the water-soluble acrylic resin is modified with from 1 to 40 parts by weight of the monofunctional isocyanate, the combination comprising 100 parts by weight. The waterborne coating composition according to claim 1, characterized in that the monofunctional isocyanate compound comprises a monoisocyanate compound of the general formula (I) R - NCO (I) wherein R is a C₈-C₂₅ alkyl group. The waterborne coating composition according to claim 2, characterized in that the monoisocyanate compound comprises stearyl isocyanate. The waterborne coating composition according to claim 1, characterized in that the monofunctional isocyanate compound comprises the adduct of a polyisocyanate compound with an active hydrogen group-containing compound comprisisng at least one of a monohydroxyfunctional compound, a monoamine functional compound or a monothiol functional compound. The waterborne coating composition according to claim 4, wherein the active hydrogen group-containing compound comprises one or more of: (1) a monohydroxy substituted carboxylic compound of the general formula (II) HO - R¹ - COOR² (II) wherein R¹ is a hydrocarbon group having from 1 to 25 carbon atoms, and R² is hydrogen or a hydrocarbon group having from 1 to 18 carbon atoms; (2) a monoalcohol of the general formula (III) HO - R³ (III) wherein R³ is a hydrocarbon group having from 1 to 25 carbon atoms; (3) a polyoxyalkylene glycol monoalkylether of the general formula (IV) HO - [(C₃H₆O) n /(C₂H₄O) n1 ] - R⁴ (IV) wherein R⁴ is a C₁-C₄ alkyl group, n is 0 or a whole number from 1 to 25, and n1 is 0 or a whole number from 2 to 40, with the proviso that 1 ≦ n+n1 ≦ 50 ; and (4) a monohydroxy substituted carboxamide compound of the general formula (V) HO - R¹ - CON(R⁵)₂ (V) wherein R¹ is a hydrocarbon group having from 1 to 25 carbon atoms, and each R⁵ may independently be hydrogen or a hydrocarbon group having from 1 to 18 carbon atoms. The waterborne coating composition according to claim 5, characterized in that the active hydrogen group-containing compound comprises the monohydroxy substituted carboxylic compound of the general formula (II), wherein R¹ is an aliphatic hydrocarbon group having from 4 to 24 carbon atoms, and R² is hydrogen or a C₁-C₁₈ alkyl group. The waterborne coating composition according to claim 5, characterized in that the active hydrogen group-containing compound comprises the monoalcohol of the general formula (III), wherein R³ is a C₁-C₁₈ alkyl group. The waterborne coating composition according to claim 7, characterized in that the monoalcohol comprises one or more of ethanol, butanol, lauryl alcohol and stearyl alcohol. The waterborne coating composition according to claim 4, characterized in that the active hydrogen group-containing compound comprises one or more of: (5) a monoamine of the general formula (VI) R⁶ - NH - R³ (VI) wherein R³ is a hydrocarbon group having from 1 to 25 carbon atoms, and R⁶ is hydrogen or a hydrocarbon group having from 1 to 25 carbon atoms, with the provisio that the monoamine should not contain more than 26 carbon atoms in total; and (6) a polyoxyalkyleneamine monoalkylether of the general formula (VII) R⁷ - HN - [(C₃H₆O) n /(C₂H₄O) n1 ] - R⁴ (VII) wherein R⁴ is a C₁-C₄ alkyl group, n is 0 or a whole number from 1 to 25, and n1 is 0 or a whole number from 2 to 40, with the proviso that 1 ≦ n+n1 ≦ 50 , and R⁷ is hydrogen, a C₁-C₂₀ alkyl group or a polyoxyalkylene group of the formula [(C₃H₆O) n /(C₂H₄O) n1 ]. The waterborne coating composition according to claim 9, characterized in that the active hydrogen group-containing compound comprises a monoamine of the general formula (VI), wherein R³ is a C₁-C₁₈ alkyl group and R⁶ is hydrogen. The waterborne coating composition according to claim 10, characterized in that the monoamine comprises one or more of ethylamine, butylamine, laurylamine and stearylamine. The waterborne coating composition according to claim 5 or claim 10, characterized in that the active hydrogen group-containing compound is selected from the monohydroxy substituted carboxylic compound of the general formula (II), the polyoxyalkylene glycol monoalkylether of the general formula (IV) and the polyoxyalkyleneamine monoalkylether of the general formula (VII). The waterborne coating composition according to claim 12, characterized in that the active hydrogen group-containing compound comprises the monohydroxy substituted carboxyl compound of the general formula (II), wherein R¹ is an aliphatic, linear, saturated hydrocarbon group having from 12 to 20 carbons atoms, and R² is hydrogen. The waterborne coating composition according to claim 12, characterized in that the active hydrogen group-containing compound comprises the polyoxyalkylene glycol monoalkylether of the general formula (IV), wherein n is 0, n1 is from 10 to 12 and R⁴ is a methyl group. The waterborne coating composition according to claim 12, characterized in that the active hydrogen group-containing compound comprises the polyoxyalkyleneamine monoalkylether of the general formula (VII), wherein n is from 2 to 4, n1 is from 10 to 12, R⁴ is a methyl group and R⁷ is hydrogen. The waterborne coating composition according to any one of claims 1-15, characterized in that from 60 to 99 parts by weight of the water-soluble acrylic resin is modified with from 1 to 40 parts by weight of the monofunctional isocyanate compound, the combination comprising 100 parts by weight. The waterborne coating composition according to any one of claims 1-16, characterized in that the binder system is at least partially neutralized and has, prior to neutralization, an hydroxyl number of from 20 to 250 (mg KOH/g resin) and an acid value of from 5 to 150 (mg KOH/g resin). The waterborne coating composition according to any one of claims 1-17, characterized in that the binder system comprises an additional 100 parts by weight of an aqueous dispersion resin. The waterborne coating composition according to any one of claims 1-18, characterized in that the coating composition further comprises a pigment. A clear topcoat comprising the waterborne coating composition according to any one of claims 1-18. A method of coating a substrate by applying a basecoat to the substrate, applying a topcoat onto such basecoat and curing the so-applied coatings, characterized in that as topcoat a coating composition according to any one of claims 1-18 or 20 is utilized. The method according to claim 21, characterized in that the topcoat is applied wet-in-wet onto the basecoat. The method according to claim 22 or 22, characterized in that curing is carried out at a temperature of 160°C or less. The method according to any one of claims 21-23, characterized in that the basecoat comprises a metallic basecoat. The method according to any one of claims 21-24, characterized in that the basecoat comprises a hydro-basecoat. A substrate coated with the waterborne coating composition of any one of claims 1-20, or by the method according to any one of claims 21-25.

Inventors: FREESE KARL-HEINZ, HAUEFLER HARTMUT, METZGER CARL WALTER, MUENCH JUERGEN, ORTH ULRICH, FREESE, KARL-HEINZ, HAUEFLER, HARTMUT, METZGER, CARL WALTER, MUENCH, JUERGEN, ORTH, ULRICH, Münch, Jürgen
Applicants: AKZO NV, AKZO N.V.
Application year: 1989
Main classification: C09D133
IPC codes: C08F20, C08L75, C08F8, C08L33, C09D133, C08G18, C09D175