COMPOSITE COATING WITH IMPROVED CHIP RESISTANCE

Abstract: The invention provides a method of coating a substrate with first a layer of a chip resistant primer composition that has as a resinous portion a polyurethane polymer having a glass transition temperature of 0 DEG C or less and, optionally, a second component that has reactive functionality; and next with a layer of a thermosetting primer composition including a polyurethane polymer having a glass transition temperature of 0 DEG C or less, an acrylic polymer having a glass transition temperature that is at least about 20 DEG C higher than the glass transition temperature of said polyurethane polymer, and a crosslinking component that is reactive with at least one of the polyurethane polymer and the acrylic polymer; and finally with at least one layer of a topcoat composition. The reactive functionality of the second component is reactive with at least one polymer selected from the group consisting of the polyurethane polymer of the chip resistant primer composition, the polyurethane polymer of the thermosetting primer composition, the acrylic polymer of the thermosetting primer composition, and combinations thereof.

Description: COMPOSITE COATING WITH IMPROVED CHIP RESISTANCE Field cf the Invention The present invention relates to composite primer coatings that provide chip resistance and to aqueous primer compositions that provide such composite coatings. Background o the Invention Coating finishes, particularly exterior coating finishes in the automotive industry, are generally applied in two or more distinct lavers. One or more layers cf primer coating composition may be applied to the unpainted substrate first, followed by one or more topcoat layers. Each of the layers supplies important properties toward the durability and appearance of the composite coating finish. The primer coating layers may serve a number of purposes. First, the primer coating may be applied in order to promote adhesion between the substrate and the coating. Secondly, the primer coating may be applied in order tc improve physical properties of the coating system, such as corrosion resistance or impact strength, especially for improving resistance to gravel chipping. Third, the primer coating may be applied in order to improve the appearance cf the coating by providing a smooth layer upon which the topcoat layers may be applied. The topcoat layer or layers contribute other properties, such as color, appearance, and light stabilization. In the process of finishing the exterior of automotive vehicles today, metal substrates are usually first coated with an electrocoat primer. While the electrocoat primer provides excellent surface adhesion and corrosion protection, it is often desirable to apply a second primer layer. The second primer layer provides additional properties not available from the electrocoat primer. Resistance to gravel chipping is one of the critical properties provided by the second primer layer. The second primer layer may also enhance the corrosion protection of the finish and provide a smoother surface than the electrocoat primer. The second primer also serves to provide a barrier layer between the electrocoat primer layer, which usually contains aromatic moieties and other materials that can cause yellowing on exposure to sunlight, and the topcoat. Mitsuii et al, U.S. Patents 5,281,655, 5,227,422, and 4,948,829, all of which are incorporated herein by reference, disclose automotive basecoat coating compositions containing polyurethane resin emulsion, a second resin emulsion than can be an acrylic resm, and a crossiinking agent. In Mitsuji '829, the polyurethane resin is prepared by dispersing an isocyanate- functional prepolymer and having the water react with the isocyanate groups to chain-extend the prepolymer. The prepolymer is prepared using an aliphatic diisocyanate , a polyether or polyester diol, a low molecular weight polyol , and a dimethylolalkanoic acid. In Mitsuji v 655 and λ 422, the polyurethane resin is prepared by reacting an aliphatic poiyisccyanate , a high molecular weight polyol, a dimethylolalkanoic acid, and, optionally, a chain extender or terminator. Because the Mitsui 1 patents are directed to basecoat coatings, these patents provide no direction for preparing compositions that have the chip resistance and other properties required for primer coating layers. Hatch et al . , U.S. Patent 5,817,725, incorporated herein by reference, discloses an aqueous primer composition for golf balls that includes a polyurethane dispersion and an acrylic dispersion. The primer has a very low content of volatile organic solvent, which is important for minimizing regulated emissions from the coating process. The Hatch patent, however, does net disclose a curable (thermosetting) composition. More importantly, the golf bail primers cf the Hatch patent do not provide the properties, such as resistance to stone chipping and corrosion protection, that are required of an automotive primer. While the primer composition may be formulated to provide good resistance to gravel chipping fcr a vehicle body, some areas cf the vehicle are particularly prone to gravel chipping. These areas include the A pillars (pillars on either side of the windshield) , the front edge of the roof, the leading edge of the hood, and rocker panels. In these areas, it is advantageous to provide an additional layer of a chip-resistant primer before the primer that is applied to the rest of the venicle ooαy to ootam increased protection against stone chipping. In general, primer compositions applied for this purpose are solventborne , thermosetting compositions. While these chip-resistant layers nave worκed well with soiventoorne primer compositions, there remains a need for a chip-resistant primer composition compatible with aqueous primer compositions. Further improvements m chip resistance of the primer are also necessary. It would be desirable, therefore, to have a composite primer coating that includes an upper layer of an aqueous body primer composition that provides improved resistance to stone chipping and other properties that are important for an automotive primer and an under layer of a chip-resistant primer layer, compatible with the upper primer layer, particularly for wet-on-wet applications of the upper primer layer over the chip resistant primer layer, that provides additional chip resistance m particular areas of the vehicle body. In addition, for environmental and regulatory- considerations, it would be desirable to produce both the upper primer layer and the lower layer of chip resistant primer from compositions having a very low content of volatile organic solvent. Summary of the Invention The present invention provides a method of applying a composite coating to an automotive vehicle. In the method, a layer of a chip resistant primer composition is applied to at least one area of the vehicle and the applied primer composition forms a chip resistant primer layer. The chip resistant primer composition includes as the resinous portion a polyurethane polymer having a glass transition temperature of 0°C or less and, optionally, a second component that has reactive functionality. Then, a thermosetting primer composition is applied to the vehicle. The reactive functionality is reactive with either the polyurethane polymer of the chip resistant primer composition or with one of the components of the thermosetting primer composition. The thermosetting primer composition includes a polyurethane polymer, an acrylic polymer, and a crosslinking component that is reactive with at least one of the polyurethane polymer and the acrylic polymer. The polyurethane polymer has a glass transition temperature of 0°C or less. The acrylic polymer has a glass transition temperature that is at least about 20°C higher than the glass transition temperature of polyurethane resin. The polyurethane polymer of both primers and acrylic polymer are preferably dispersed or emulsified in an aqueous medium. As used herein, "emulsion" or "dispersion" will each be used to refer both to dispersions and emulsions. The invention further provides a composite coating having a first layer of a chip resistant primer, a second primer layer over the first layer of chip resistant primer, and a topcoat layer over the second primer layer. The first layer of chip resistant primer is formed from a composition including as the resinous portion a polyurethane polymer having a glass transition temperature of 0°C or less and, optionally, a second component that has reactive functionality. The reactive functionality is reactive with either the polyurethane polymer of the chip resistant primer composition or with one of the components of the primer composition forming the second primer layer. The second primer layer is the product of a primer composition including a polyurethane polymer has a glass transition temperature of 0°C or less, an acrylic polymer has a glass transition temperature that is at least about 20°C higher than the glass transition temperature of polyurethane resin, and a crosslinking component . Detailed Description of the Invention A layer of the chip resistant primer composition is applied to at least one area of the vehicle. In a preferred embodiment, the chip resistant primer composition is applied to one or more of the following vehicle areas: the A pillars (pillars on either side of the windshield) , the front edge of the roof, the leading edge of the hood, the front bumper, the rocker panels, and combinations of these. The chip resistant primer composition includes as the resinous portion polyurethane polymer having a glass transition temperature of 0°C or less and, optionally, a second component that has reactive functionality. The polyurethane polymer used has a glass transition temperature of about 0°C or less, preferably about -20°C or less, and more preferably about -30°C or less. The glass transition temperature of the polyurethane of the invention is in the range of from about -80°C to about 0°C, more preferably from about -65°C to about -10°C, still more preferably from about - 65°C to about -30°C, and even still more preferably from about -60°C to about -35°C. The weight average molecular weight of the polyurethane is preferably from about 15,000 to about 60,000, more preferably from about 15,000 to about 60,000, and even more preferably from about 20,000 to about 35,000. Polyurethanes are prepared by reaction cf at least one polyisocyanate and at least one polyol . The reactants used to prepare the polyurethane are selected and apportioned to provide the desired glass transition temperature. Suitable poiyisocyanates include, without limitation, aliphatic linear and cyclic poiyisocyanates, preferably having up to 18 carbon atoms, and substituted and unsubstituted aromatic poiyisocyanates. Illustrative examples include, without limitation, ethylene diisocyanate, 1 , 2 -diisocyanatcpropane , 1 , 3 -dnsocyanatopropane , 1,4-butylene diisocyanate, lysine diisocyanate, 1 , -methyiene bis (cyclohexyl isocyanate), isophorone diisocyanate, toluene diisocyanates (e.g., 2,4- toiuene diisocyanate ana 2,6-toluene diisocyanate) diphenyimethane , 4 ' -diisocyanate , metnyienebis -4 , 4 ' - lsocyanatocyciohexane , 1 , 6-hexamethylene diisocyanate, p- phenylene diisocyanate, tetramethyl xyiene diisocyanate, meta-xylene diisocyanate, 2 , 2 , 4-trimethyl-l , 6-hexamethylene diisocyanate, 1 , 12 -dodecamethylene diisocyanate, cyclohexane- 1,3- and -1 , 4-diisocyanate , l-isocyanato-2 -isocyanatomethyl cyclopentane , and combinations of two cr more of these. Biurets, allopnonates , isocyanurates , carbodiimides , and other such modifications of tnese isocyanates can also be used as the poiyisocyanates. In a preferred embodiment, the poiyisocyanates include methyienebιs-4 , 4 ' - lsocyanatocyciohexane , 1 , -hexamethylene diisocyanate, 1,12- dodecamethylene diisocyanate, and combinations thereof. It is particularly preferred to use at least one α, ω-alkyiene diisocyanate having four or more caroons, preferably 6 or more carbons, m the aikyiene group. Combinations of two or more poiyisocyanates m which one of the poiyisocyanates is 1 , 6-hexamethyiene diisocyanate are especially preferred. The polyol or polyols used to prepare the polyurethane polymer can oe selected from any of the polyols Known to be useful in preparing poiyurethanes , including, without limitation, 1 , 4 -outanedioi , 1 , 3 -outanedioi , 2 , 3 -outanedioi , 1 , 6 -hexanediol , neopentyl glycci , i , 3 -propaneαiol , 1,5- pentanediol , 1 , 6 -hexanediol , 1 , 9-nonanedιoi , etnyiene glycol, diethylene glycci, triethyiene glycol and tetraetnyiene glycol, propyiene glycol, dipropyiene glycol, giyceroi , cycionexanedimethanois , 2 -methyl -2 -etnyl- 1 , 3 -propanediol , 2- ethyl-1 , 3 -hexanediol , thiodigiycol , 2 , 2 , 4 - trιmethyl-1 , 3 - pentanediol , cyclohexanediols , tπmethylolpropane , trimethylolethane, and glycerin; polyester polyols such as the reaction products of any of the foregoing alcohols and combinations thereof with one or more polycarboxylic acids selected from maionic acid, maieic acid, succimc acid, giutaric aciα adipic aciα, azeiaic aciα, anhydrides thereof, and combinations thereof; polyether polyols, such as polyethylene giycols and polypropylene giycols; and comoinations of sucn polyols. Polyols having two nyαrcxyl groups are preferreα. The polyurethane is preferably prepareα using one or more polyester polyols. In a preferred emDodiment , the polyester polyol is the reaction proαuct of a mixture that comprises neopentyl glycol and adipic acid. While it is possible to prepare a nonionic dispersion of the polyurethane, the polyurethane dispersion is preferably anionic . Acid-functional polyurethanes that can be salted to form anionic dispersions or emulsions may be synthesized by including a monomer having acid functionality, such as, without limitation, diaikylpropionic aciαs including dimethylolpropionic acid, and alkali metal salts of ammo acids such as taurme, methyl taurme, 6 -ammo caproic acid, glycme, sulfanilic acid, diammo benzoic acid, ornithme, lysme and 1:1 adducts of sultones, such as propane sultone or butane sultone, with diammes, such as ethylene diarnme, hydrazme, or 1 , 6-hexamethylene diarnme . The hydroxyl groups react to form the urethane linkages while the acid group remains unreacted m the polyurethane polymerization. Suitable polyurethane polymers can be prepared by any of the known methods . In one method for preparing polyurethane polymers, the polyisocyanate component is reacted with an excess of equivalents of the polyol component to form a hydroxyl -functional polyurethane polymer. Alternatively, an excess of equivalents of the polyisocyanate component can be reacted with the polyol component to form an lsocyanate- functional prepolymer. The prepolymer can then be reacted further different ways. First, the prepolymer can be reacted with a mono- functional aiconol or amme to provide a non- functional polyurethane polymer. Examples of mono- functional alcohols and amines that may be used include polyethylene oxide compounds having one terminal hydroxyl group, lower mono-functional alcohols having up to 12 carbon atoms, amino alcohols such as dimethyiethanolamme , and secondary amines such as diethyiamme ana dimethyiamme . Secondly, the prepolymer can be reacted "with a polyfunctional polyol, poiyamme, or ammo alcohol compound to provide reactive hydrogen functionality. Examples of such polyfunctional compounds include, without limitation, the polyols already mentioned above, including triois such as trimethvioipropane ; poiyamines such as ethyienediamme , butyiamine, and propylamine ; and amino alcohols, such as diethanolamine . Finally, the prepolymer can be chain extended by the water during emulsification or dispersion of the prepolymer in the aqueous medi' m. The prepolymer is mixed with the water after or during neutralization. The polyurethane may be polymerized without solvent. Solvent may be included, however, if necessary, when the polyurethane or prepolymer product is of a high viscosity. If solvent is used, the solvent may be removed, partially or completely, by distillation, preferably after the polyurethane is dispersed the water. ' e polyurethane may have honionic hydrcphilic groups, such as polyethylene oxide groups, that serve to stabilize the dispersed polyurethane polymer. In a preferred embodiment, however, the polyurethane polymer is prepared with penαant acid groups as described above, and the acid groups are partially or fully salted with an alkali, such as sodium or potassium, or with a base, such as an amme , before or during dispersion of the polyurethane polymer or prepolymer in water. The chip resistant primer composition may also include a second component that has reactive functionality. The reactive functionality is reactive with either the polyurethane polymer of the chip resistant primer composition or with one cf the components of tne tnermosett g primer composition. When the chip resistant primer layer includes the second component, the composite coating has higher hardness, better cure and solvent resistance, and better mtercoat adhesion. In a preferred embodiment, the second component is a crosslmκer reactive with active hydrogen functionality on at least one of the polyurethane polymer of the chip resistant primer, the polyurethane polymer of tnermosettmg primer composition, and the acrylic polymer of the thermosetting primer composition. Examples of crosslmkers reactive with active hydrogen functionality include, without limitation, materials having active methyloi or metnyialkoxy groups, including ammoplast resins or pnenol/ formaldehyde adducts; blocked polyisocyanate curing agents; tπs(alkoxy carbonyiamino) triazir.es vava iabie from Cytec Industries under the tradename TACT); and combinations thereof. Suitable aminoplast resins are amine/aldehyde condensates, preferably at least partially etherified, and most preferably fully etherified. Melamme and urea are preferred amines, but other triazmes, triazoles, diaz es, guanidines, or guanammes may also Joe used to prepare the alkylated amine/aldehyde aminoplast resins crosslinking agents. The aminoplast resins are preferably amine/ formaldehyde condensates, although other aldehydes, such as acetaldehyde , crotonaldehyde , and benzaldehyde , may be used. Non-limiting examples of preferred aminoplast resins include monomeric or polymeric melamine formaldehyde resins, including melamine resins that are partially or fully alkylated using alcohols that preferably have one to six, more preferably one to four, carbon atoms, such as hexamethoxy methylated melam e; urea- formaldehyde resms including methylci ureas and siioxy ureas such as butyiated urea formaldehyde res , alkylated benzoguanimines , guanyi ureas, guanidines, biguanidines , poiyguanidines , and the like. Monomeric melamme formaldehyde resins are particularly preferred. The preferred alkylated melamme formaldehyde resms are water miscibie or water soluble. Examples of blocked poiyisocyanates include isocyanurates of toluene diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate blocked with a blocking agent such as an alcohol, an oxime, or a secondary amine such as pyrazole or substituted pyrazole. The crossl ker is preferably included m the resinous portion of the cnip resistant primer at from about 2% by weight to about 30% by weight, and more preferably from about 5% by weight to about 20% by weight, a particularly preferably about 5% to about 15% by weight . The thermosetting primer composition includes a polyurethane polymer, an acrylic polymer, and a crosslinking component that is reactive with at least one of the polyurethane polymer and the acrylic polymer. The polyurethane polymer has a glass transition temperature of 0°C or less. The polyurethane polymer may be any of those already described above for the chip resistant primer. In a preferred embodiment, the same polyurethane polymer is included m both the chip resistant primer and m the thermosetting primer. The acrylic polymer of the thermosetting primer composition has a glass transition temperature that is at least about 20°C higher than the glass transition temperature of polyurethane resin. The acrylic polymer is prepared according to usual methods, such as by bulk or solution polymerization followed by dispersion m an aqueous medium or, preferably, by emulsion polymerization m an aqueous medium. The acrylic polymer is polymerized from a monomer mixture that preferably includes an active hydrogen- functional monomer and preferably includes an acid-functional monomer. Examples cf active hydrogen- functional monomers include, witnout limitation, hydroxyl - functional monomers such as nydroxyethyl acrylate, hydroxyetnyl metnacrylate, hydroxypropyl acrylate, hydroxypropyl methacryiate , hydroxyoutyi acrylates, and hydroxyoutyl methacrylates; and carbamate- and urea- unctional monomers or monomers with functional groups tnat are converted to caroamate or urea groups after polymerization such as, without limitation, those disclosed m U.S. Patent 5,866,259, "Primer Coating Compositions Containing Carbamate-Functional Acrylic Polymers," the entire disclosure of which is incorporated herein by reference. Preferably, a sufficient amount of active hydrogen- functional monomer is included to produce an equivalent weight of 1000 or less grams per equivalent, more preferably 800 or less grams per equivalent, and even more preferaoly 600 or less grams per equivalent. It is preferred that the acrylic polymer is dispersed as an anicnic dispersion. Examples of suitaole acid-functional monomers include, without limitation, α, β-ecnylemcally unsaturated monocarcoxyiic acids containing 3 to 5 caroon atoms, α, β-ethyienicaliy unsaturated dicarcoxylic acids containing 4 to 6 carbon atoms and the anhydrides and monoesters of these. Examples include, without limitation, acrylic acid, methacrylic acid, crotonic acid, maleic acid or maleic anhydride, itaconic acid or itaconic anhydride, and so on. A sufficient amount of acid-functionai monomer is included to produce an acrylic polymer with an acio numoer of at least about 1, and preferably the acrylic polymer nas an acid number cf from aoout i to aoout 10. In addition to the ethylenically unsaturated monomer having acid functionality or used to generate acid functionality m the finished polymer, one cr more other ethylenically unsaturated monomers are employed as comonomers in forming the acrylic resms of the invention. Examples of such copolymeπzable monomers include, without limitation, derivatives of , β-ethylenically unsaturated monocarboxylic acids containing 3 to 5 carbon atoms, including esters, nitπles, or amides of those acids; diesters of α,β- etnylenicaliy msaturateα dicarboxyiic aciαs containing 4 to 6 carbon atoms; vinyl esters, vinyl etners , vinyl ketones, vinyl amides, and aromatic or heterocyciic aliphatic vinyl compounds. Representative examples of acrylic and methacrylic acids, amides and ammoaikyl amides include, without limitation, such compounds as acrylamide, N-(l,l- dιmethyi-3 -oxooutyl i -acrylamide , N-alκoxy amides such as methyiolamides ; N-aikoxy acrylamides such as n-butoxy acrylamide; N-ammoalkyl acrylamides or methacrylamides such as aminomethyiacryiamide , l-aminoethyl-2 -acrylamide , 1- aminopropyl -2 -acrylamide , 1-aminopropyi -2 -methacrylamide , N- 1- (N-butylamino) propyl - (3 ) -acrylamide and 1-aminohexyl- (6) - acrylamide and 1 - (N, N-dimethylammo) -ethyl- (2 ) - methacrylamide, 1- (N,N, -dimethylammo) -propyl- (3) -acrylamide and 1- (N, N-dimethylamino) -hexyi -( 6 ; -methacrylamide . Representative examples cf esters of acrylic, methacrylic, and crotonic acids include, without limitation, those esters from reaction with saturated aliphatic and cycloaliphatic alcohols containing 1 to 20 carbon atoms, such as methyl, ethyl, propyl, isopropyl, n-butyi, isobutyl, tert- butyl , 2-ethylhexyl , lauryi, stearyl, cyclohexyl, trimethylcyclohexyl, tetrahydrofurfuryl , stearyl, sulfoethyl, and isobornyl acrylates, methacrylates, and crotonates; and polyalkylene glycol acrylates and methacrylates. Representative examples of other ethylenically unsaturated polymerizabie monomers include, without limitation, such compounds as fumaπc, maleic, and itaconic anhydrides, monoesters, and diesters . Polyfunctional monomers may also be included to provide a partially crosslinked acrylic dispersion. Examples of polyfunctional compounds include, without limitation, ethylene glycol diacryiate, ethylene glycol dimethacrylate , triethyiene glycol diacryiate, tetraethylene glycol dimethacrylate, 1,6- hexanediol diacryiate, divmylbenzene , tπmethylolpropane tπacrylate, and so on. Representative examples of vmyl monomers that can be copolymerized include, without limitation, such compounds as vmyl acetate, vmyl propionate, vmyl ethers such as v yl etnyl ether, vmyl and vmylidene halides, and vmyl ethyl ketone . Representative examples of aromatic or heterocyclic aliphatic vmyl compounds include, without limitation, such compounds as styrene, α-methyl styrene, vmyl toluene, tert- butyl styrene, and 2 -vmyl pyrrolidone. After polymerization, the acid functionality is salted, preferably with an alkali or base, preferably an amme . Example of suitable salting materials include, without limitation, ammonia, monoethanolamme, ethylam e, dimethylamme, diethylamme, triethylamme, propylamme, dipropylamme , lsopropylam e , diisopropylamme , triethanolamme, butylamme, dibutylamme, 2 -ethylnexyiamme, ethylenediamme propylenediamme , ethylethanolamme , dimethylethanolamine , diethylethanolamme , 2-ammo-2- methylpropanol , and morpholme. Preferred salting materials include 2 -ammo-2 -methylpropanol and dimethylethanolamine. The acrylic polymers may be prepared as solutions m an organic solvent medium, preferably selected from water- soluble or water-miscible organic solvents, and then dispersed into water. After dispersion into water, the organic solvent can ce distilled from trie aqueous dispersion or emulsion. In a preferred method, the acrylic polymer is provided by emulsion polymerization. Preferably, a nonionic or an anionic surfactant is used for the emulsion polymerization. Suitaole surfactants include, without limitation, polyoxyethylenenonylpnenyl ethers , poiyoxyetnylenealkylallyl ether sulfuric acid esters, ammo and alkali salts of dodecvlbenzenesulfonic acid such as the dimethylethanolamine salt of dodecylbenzenesulfonic acid and sodium dodecvlbenzenesulfonic acid, and sodium dioctyisulfosuccmate . The polymerization typically proceeds by free radical polymerization. The free radical source is typically supplied by a redox initiator or by an organic peroxide or azo compound. Useful initiators include, without limitation, ammonium peroxydisulfate, potassium peroxydisuifate , sodium metabisulfite , hydrogen peroxide, t-outyi hydroperoxide , dilauryi peroxide, t-outyl peroxybenzoate , 2,2'- azobis (isobutyronitriie) , and redox initiators such as ammonium peroxydisulf te and sodium metabisulfite with ferrous ammonium sulfate. Optionally, a cnain transfer agent may be used. Typical chain transfer agents include mercaptans such as octyl mercaptan, n- or tert-dodecyl mercaptan, thiosalicylic acid, mercaptoacetic acid, and mercaptoethanoi ; halogenated compounds; and dimeric alpha- methyl styrene . Acrylic polymers prepared by emulsion polymerization can have weight average molecular weights of one million or more. The weight average molecular weight cf the acrylic dispersion is preferably from about 5,000 to about 5,000,000, more preferably from about 7500 to about 500,000, and even more preferably from about 10,000 to about 50,000. If prepared by solution polymerization and then dispersed in water, the acrylic polymer will generally have a number average molecular weight of from about 5000 to about 60,000. The molecular weight can be determined by gel permeation chromatography using a polystyrene standard or other known methods . The theoretical glass transition temperature of the acrylic polymer can be adjusted according to methods well- known in the art through selection and apportionment of the comonomers . The acrylic polymer has a glass transition temperature that is at least about 20°C higher than the glass transition temperature of polyurethane resin. Preferably, the acrylic polymer has a glass transition temperature that is at least about 40°C higher, more preferably about 50°C higher, than the glass transition temperature of polyurethane resin. In a preferred embodiment, the theoretical Tg of the acrylic polymer is between about -30°C and 80°C, more preferably between about -20°C and 40°C. The polyurethane polymer may be included in the thermosetting primer m an amount of at least about 40% by weight, preferably at least about 50% by weight, based on the comomed nonvolatile weights of the polyurethane polymer and the acrylic polymer. The polyurethane polymer may be included the primer m an amount of up to about 98% by weight, preferably up to about 80% by weight, based on the combined nonvolatile weights of the polyurethane polymer and the acrylic polymer. It is preferred to include from about 50% by weight to about 75% by weight, and even more preferred to include from about 65% by weight to about 75% by weight, of the polyurethane polymer, based on the combined nonvolatile weights of the polyurethane polymer and the acrylic polymer. The thermosetting primer composition also includes a crosslmker component. The crosslmker component includes one or more crosslmkers reactive with active hydrogen functionality, including any of those already described above as useful m the chip resistant primer composition. The crosslmker component preferably is from about 2% by weight to about 30% Oy weight, and more preferably from about 5% by weight to about 20% by weight, and particularly preferably about 5% to about 15% by weight of the combined nonvolatile weights of the polyurethane, the acrylic polymer, and the crosslinking component of the thermosetting primer composition. The chip resistant primer compositions and thermosetting primer compositions may include one or more catalysts. The type of catalyst depends upon the particular crosslmker component composition utilized. Useful catalysts include, without limitation, blocked acid catalysts, such as para- toluene sulfonic acid, dodecylbenzene suifonic acid, and dmonylnaphthylene disulfonic acid blocked with amines ; phenyl acid phosphate, monobutyl maieate, and butyl phosphate, hydroxy phosphate ester; Lewis acids, zmc salts, and tin salts, including dibutyl tin dilaurate and dibutyl tin oxide. The chip resistant primer coating compositions and thermosetting primer coating compositions according to the invention may further include pigments such as are commonly used m the art, including color pigments, corrosion inhibiting pigments, conductive pigments, and filler pigments. Illustrative examples of these are metal oxides, chromates, molybdates, phosphates, and silicates, carbon black, titanium dioxide, sulfates, and silicas. Other conventional materials, such as dyes, flow control or rheology control agents, and so on may be added to the compositions. m Tne chip resistant primer composition and the thermosetting primer composition may have a very low content of volatile of organic solvent. The polyurethane dispersion is preferably prepared as a solvent free or substantially solvent free dispersion. By "substantially solvent free" it is meant that tne dispersion has a volatile organic content of less than about 5% by weight of the primer composition. The acrylic dispersion is also preferably solvent free or substantially solvent free dispersion. The primer composition preferably has a volatile organic content of less than aoout 1.5, more preferably less than aoout 1.3, and even more preferably less than about 0.7. The volatile organic content of a coating composition is typically measured using ASTM D3960. The primer coating compositions of the present invention can be applied over many different substrates, including wood, metals, glass, cloth, plastic, foam, metals, and elastomers. They are particularly preferred as primers on automotive articles, such as metal or plastic automotive bodies or eiastomeric fascia. When the article is a metallic article, it is preferred to have a layer cf electrocoat primer oefore application of the primer coating composition of tne invention. The composite coating of the invention has, as adjacent layers, a first primer coating layer that is obtained by applying the cnip resistant primer composition of tne invention ano a second primer coating layer on top of the first primer coating layer that is obtained by applying the thermosetting primer coating composition. The composite coating nas a topcoat layer applied ever the primer coating layers. The topcoat layer may include a oasecoat coating layer applied over the primer coating _ayer and an outer, ciearcoat layer applied over the basecoat coating layer. Tne composite primer coating layers of tne invention is applied directly to tne substrate or over one or more other layers of primer, such as the electrocoat primer. The applied primer coating compositions are then baked and, at least m the case of the thermosetting primer composition, cured to form a primer coating layer. The electrocoat primer or other first layer of primer may be cured at the same time as the primer coating layers of the invention are baked m a process Known as "wet-cn-wet" coating. The composite primer coating layers formed from tne primer coating compositions cf tne invention are the outermost primer layers cf tne composite coating. A topcoat composition is applied over the primer coating layers and cύrea to form a topcoat layer. The suostrate at tnat point is then covered v.ιth a composite coating that has at least the two layers of primer coating derived from the inventive compositions and at least one layer of topcoat. In a preferred emnodiment , tne coating composition of tne present invention is overcoated with a topcoat applied as a color-plus-clear (basecoat-clearcoat) topcoat. In a basecoat -clearcoat topcoat, an underlayer of a pigmented coating, the basecoat, is covered with an outer layer of a transparent coating, tne clearcoat. Basecoat-clearcoat topcoats provide an attractive smooth and glossy finisn and generally improved performance. CrossimKing compositions are preferred as the topcoat layer or layers. Coatings of this type are well -Known in the art and include wateroorne compositions as well as soiventborne compositions. For example, the topcoat may be a clearcoat according to U.S. Pat. No. 5,474,811, applied wet- on-wet over a layer of a basecoat composition. Polymers known m the art to be useful m basecoat and clearcoat compositions include, without limitation, acrylics, vmyl, polyurethanes , poiycaroonates , polyesters, alkyds, ana pclysiioxanes . Acrylics and polyurethanes are preferred. Thermoset basecoat and clearcoat compositions are also preferred, and, to that end, preferred polymers comprise one or more kinds of crosslmκabie functional groups, such as caroamate, hydroxy, isocyanate, amine, epoxy, acrylate, vmyl, siiane, acetoacetate , ana so on. The polymer ray oe self-crosslinking, or, preferaoly, the composition may include a crosslinking agent such as a polyisocyanate or an aminoplast resin of tne kind described above. In one embodiment, waterborne basecoat compositions and/or clearcoat compositions having low volatile organic content are used. The waterborne basecoat and waterborne clearcoat compositions each preferably nas a volatile organic content of less than about 1.5, more preferably less than about 1.3, and even more preferably less than about 0.7. Each layer of the composite coatings of the invention can be applied to an article to be coated according to any of a number of tecnniques well-known m the art. These include, for example, spray coating, dip coating, roll coating, curtain coating, and the like. If an initial electrocoat primer layer is applied to a metallic substrate, the electrocoat primer is applied by electrodeposition. For automotive applications, the primer coating compositions of the invention and the topcoat layer or layers are preferably applied by spray coating, particularly electrostatic spray methods. Coating layers of about one mil or more are usually applied m two or more coats, separated by a time sufficient to allow some cf the solvent or aqueous medium to evaporate, or "flash," from the applied layer. The flash may be at amcient or elevated temperatures, for example, the flash may use radiant neat. The coats as applied can oe from 0.5 mil up to 3 mils dry, and a sufficient number of coats are applied to yield the desired final coating thickness. Tne chip resistant primer layer, whicn is formed from the chip resistant primer composition, may oe from about 0.5 mil to about 3 mils thick, preferably from about 0.8 mils to about 1.5 mils tmcK. Tne outermost primer layer, wnicπ is formed by reacting the tnermosettmg primer compositions cf the invention, may be cureα oy reaction of curing component with at least one the polyurethane res or tne acrylic resin, oefore the topcoat is applied. The cured primer layer may oe from about 0.5 mil to aoout 2 thιcκ, preferaoiy from about 0.8 mils to aooct 1.2 mils thicK. Color-plus-clear topcoats are usually applied wet-on- wet. The compositions are applied m coats separated by a flash, as described above, with a flash also between the last coat of the color composition and the first coat the clear. The two coating layers are then cured simultaneously. Preferaoiy, the cured basecoat layer is 0.5 to 1.5 mils thιcκ, and the cured clear coat layer is 1 tc 5 mi ^ s, more preferaoiy 1.6 to 2.2 mils, tnicK. Alternatively the primer layer is 1 of tne invention and the topcoat can oe applied "wet -on-wet . " For example, the chip resistant primer composition cf tne invention can oe applied, then tne app_ ^ ed layer flashed; then the topcoat car- be applied and flasned; the thermosetting primer composition of the invention can be applied, then the applied layer flashed; then the topcoat can be applied and flashed then the thermosetting primer, optionally the chip resistant primer (if it is thermosetting) and the topcoat can be cured at the same time. Again, the topcoat can include a basecoat layer and a clearcoat layer applied wet -on-wet. The thermosetting coating compositions described are preferably cured with heat . Curing temperatures are preferably from about 70°C to about I80°C, and particularly preferably from about 170°F to about 200°F for a composition including an unblocked acid catalyst, or from about 240°F to about 275°F for a composition including a blocked acid catalyst . Typical curing times at these temperatures range from 15 to 60 minutes, and preferably the temperature is chosen to allow a cure time of from about 15 to about 30 minutes. In a preferred embodiment, the coated article is an automotive body or part. The composite primer layers of the invention provide improved chip resistance as compared to previously known primers, while retaining the desirable properties of sandabiiity and corrosion resistance. Further, the primer compositions cf the invention can be formulated to have low volatile organic content and even no volatile organic content . The invention is further described in the following examples. The examples are merely illustrative and do not in any way limit the scope cf the invention as described and claimed. All parts are oy weight unless otherwise indicated. Examples Example 1. Preparation of a Pigment Paste A pigment paste was prepared by grinding a prem.ix of 3AYHYDR0L 140 AQ polyurethane dispersion aoout 40% nonvolatile , 59% water, and 1% toluene, glass transition temperature of about -45°C, pH of about 5.0 to about ".5, weight average molecular weight cf about 25,000, anionic Desmodur W/l , 6-hexamethylene diisccyanate/poiyester polyol- based polyurethane, available from Bayer Corporation, Pittsburgh, PA), titanium dioxide, barium sulfate extender, and carbon black on a horizontal mill to a fineness of 6 microns. The pigment paste was 63% by weight nonvolatile in water. The nonvo_atiles were 33.1% by weight of BAYHYDROL 140 AQ, 33.1% by weight of titanium dioxide, 33.1% by weight cf barium sulfate extender, and the balance carbon black. Example 2. Chip Resistant Area Primer Composition A chip resistant primer composition was prepared by mixing together 219.6 parts by weight of the Pigment Paste of Example 1, 212.4 parts by weight cf BAYHYDROL 140 AQ , 58.02 parts by weight of deionized water, and 2.45 parts by weight of a thickener material. The composition was adjusted tc 91 centipcise with the addition cf 22 grams cf water. Example 3. Chip Resistant Area Primer Composition A chip resistant primer composition was prepared by mixing together 219.6 parts by weight of the Pigment Paste of Example 1, 179.6 parts by weight of BAYHYDROL 140 AQ, 82.95 parts oy weignt cf deionized water, 14.4 parts by weight of RΞΞIMΞNE 747 a melamme formaldehyde resm available from Sclitia, St. Louis, MO, , 0.43 parts oy weight of ABΞX Ξ? 110 lanionic surfactant available from Rhodia , and 3.45 parts by weight of a thιcκener material. The composition was ad-usted tc 92 centipoise with tne addition of 22 grams of water. Example 4. Thermosetting Primer Composition A primer composition was prepared by first mixing together 17.51 parts by weight of BAYHYDROL 140 AQ polyurethane dispersion, 16.27 parts by weight of an emulsion of an acrylic polymer (glass transition temperature of 20 °C . , nonvolatile content of about 41% m water, acid number of about 8 mg KOH/g nonvolatile, hydroxyl equivalent weight of 511, salted with 2 -ammo-2 -methylpropanol to a pH cf aceit 6 tc 7 N , 20.9 parts deionized water, and 40.89 parts oy weight of the pigment paste of Example 1. To this mixture were added 2.71 parts oy weight cf RESIMENΞ 747 and 0.27 parts by weight cf ABΞX Ξ? 11C . A total cf 1.29 parts cy weignt cf an additive package defoamer, wetting agent, and fnιcκener was then added. Finally, the pH of the primer composition was adjusted to about 8.0 with 2 -ammo-2 -methylpropanol . The measured volatile organic content cf the primer composition is 0.24 pounds per gallon. The primer composition had a nonvolatile content of 42% by weight. The primer composition was adjusted before spray application with deionized water to a viscosity of "5 to 110 centipoise. The primer composition of Examples 2 and 3 was applied to electrocoat primed 4"xl2" steel panels. Before curing the first primer layer, the primer composition of Example 4 was applied over the first primer layer on each panel. Both primer layers were cured together according to the bake schedule shown m the table below to form a composite primer. Each of the primer layers was about 1.0 mil thick. The cured composite primer was then topcoated with commercial basecoat and clearcoat compositions. As comparative example, a panel was prepared by applying the primer composition of Example 4 directly to an electrocoat primed 4"xl2" steel panel. The primer layer was cured and topcoated with commercial basecoat and clearcoat compositions as before. As another comparative example, a panel was prepared by applying a layer cf a commercial chip resistant primer, U26AW415K and a layer of a commercial tnermosettmg primer, U28AW032, ooth available from BASF Corporation, Southfield, MI . Both primer layers were cured together according to the bake schedule shown m the table below to form a composite primer. Each cf tne primer layers was aoout 1.0 mil thιcκ. The cured composite primer was then topcoated with commercial basecoat and clearcoat compositions. The panels were then suoπectec to graveiometer testing according tc the test procedure cf 3AE J4G0, except that three tints if gravel were asec instead cf the one pint specified oy tne test method. Briefly, m the SAE J400 procedure, the panels are cooled to -20 centigrade fcr 1 hour prior tc the grave- test. The panel is positioned m a graveiometer ~acmne m an mπgnt position, 90 degrees from path cf gravel . One pint cf gravel is blown onto tne panel with an air pressure of 70 psi . [In testing the examples of the invention, three pints of gravel were used.] The panel is then warmed to room temperature, tape pulled with 3M 898 strapping tape, and rated according to chip rating standards on a scale of 0 to 9, with 0 corresponding to a standard having total celammation cf the coating and 9 corresponding tc a standard having almost no cnips . Tne grave_ometer ratings fcr tne panels obtained using the compositions of Examples 1 and 2 are shown m tne followmσ tao ^ e. SAE L.4G Crav -ometer rtatmαs, „s α 3 pints crave_ Primer layer s 30 Minutes at 2 "^ 5 ~ F 3a e 325°F 3aκe Example 2 /Exam le - -/ S - Example 3 /Example 4 - /3- 7τ/8- Example 4 only 7 - 6 | U26AW415K/U28AW032 6 5- The invention has been described m detail with reference to preferred embodiments thereof. It should be understood, however, that variations and modifications can be maoe withm the spirit and scope of the invention.

Claims: What is claimed is: 1. A method of coating a substrate, comprising steps of: (a) applying a layer of a chip resistant primer composition, wherein said chip resistant primer composition comprises as a resinous portion a polyurethane polymer navmg a glass transition temperature of 0°C or less ana, optionally, a second component that nas reactive functionality; (b applying over the layer of the chip resistant primer composition a layer of a thermosetting primer composition, wherein the thermosetting primer composition comprises a polyurethane polymer having a glass transition temperature of 0°C or less, an acrylic polymer having a glass transition temperature that is at least about 20°C higher than the glass transition temperature of said polyurethane polymer, and a crosslinking component that is reactive with at least one of the polyurethane polymer and tne acrylic po.ytner; and (c applying over the layer of the thermosetting primer composition at least one layer of a topcoat compos_tιon, wnerem the reactive functionality of tne second component is reactive with at least one polymer selected from the group consisting of the polyurethane polymer of the chip resistant primer composition, the polyurethane polymer of the thermosetting primer composition, tne acrylic polymer cf the thermosetting primer composition, and combinations thereof. 2. A method according to claim 1, wherein the chip resistant primer composition is net baκeo before the tnermosettmg primer composition is applied. 3. A method according to claim 1, wherein the chip resistant primer composition is baked before the thermosetting primer composition is applied. 4. A method according to claim 1, wherein the thermosetting primer composition is not cured before the topcoat composition is applied, and the thermosetting primer composition and topcoat composition are cured together. 5. A method according to claim 1, comprising a step of applying said chip resistant primer coating composition over a layer of an electrocoat primer. 6. A method according to claim 1, wherein tne topcoat coating composition comprises a basecoat coating composition and a clearcoat coatmc? composition. 7. A method according tc claim 1, wnere tne substrate is metal or plastic. S. A method according to claim 1, wnerem said substrate is an automotive vehicle oooy. 9. A method according tc claim S , wherein said chip resistant primer composition is applied to an area of said automotive vehicle oody selected from the group consisting of the A pillars, tne front edge cf tne rocf, tne leading edge of the nood, tne front bumper, tne rocκer panels, and combinations thereof. 10. A method according to claim 1, wherein the polyurethane of the chip resistant primer coating composition and the polyurethane of the thermosetting primer coating composition are the same . 11. A method according to claim 1, wherein tne chip resistant primer coating composition and the tnermosettmg primer coatmi composition are oetn aαueous . -. method ✓ X * d_ T •p o"' r- resistant primer coating composition includes tne seconc component . 13. A method according to claim 12, wherein the second component is an aminoplast resin. 14. A metnod according to ciaι~ 12 , wherein the aminoplast resm is a melamme formaldehyde resm. 15. A method according to claim 14, wherein the melamme formaldehyde resm is reactive with the acrylic resm of the thermosetting primer coating composition. 16. A method according to claim 10, wherein the polyurethane polymer has a glass transition temperature of about -20°C or less. 17. A method according to claim 10, wherein the poivurethane oolvmer has a αlass transition temperature of 18. A method according to claim 10, wherein the poiyuretnanε polymer has a glass transition temperature of about from aoout -30°C tc about :°C. 19. A method according to claim 10, wherein the polyurethane polymer is the reaction product of a polyester polyol and a polyisocyanate selected from the group consisting of methylene-bis-4 , 4 ' -isocyanatocyclohexane, 1 , 6-hexamethylene diisocyanate, 1 , 12-dodecamethylene diisocyanate, and combinations thereof. 20. A method according to claim 10, wherein the polyurethane polymer has a weight average molecular weight of from about 15,000 to about 60,000. 21. A method according to claim 10, wherein the polyurethane polymer is present in the aqueous coating composition as an anionic dispersion. 22. A method according to claim 1, wherein the acrylic polymer has a glass transition temperature of from about 20°C to about 40°C. 23. A method according to claim 15, wherein the acrylic polymer has a hydroxyl equivalent weight of 1000 or less. 24. A method according to claim 12, wherein the second component is included m the resinous portion of the chip resistant primer m an amount of from about 2% by weight tc about 30% by weight. 25. A method according to claim 1, wherein the polyurethane polymer of the thermosetting primer coating composition is from about 40% by weight to about 80% by weight of the combined nonvolatile weights of the polyurethane polymer and the acrylic poiymer of the tnermosettmg primer coating composition . 26. A method according to claim 1, wherein each of the primer compositions has a volatile organic content of less than acout 0. ^ pounds per gallon. 27. A composite coating produced according to the method of claim 1.

Inventors: GILBERT JOHN, MC NEIL ROCK, GILBERT, JOHN, MC NEIL, ROCK
Applicants: BASF CORP, GILBERT JOHN, MC NEIL ROCK, BASF CORPORATION, GILBERT, JOHN, MC NEIL, ROCK
Application year: 2000
Main classification: B05D7
IPC codes: B05D5, B05D7, C09D5, C09D133, C09D175