METHOD FOR FORMING A PAINT FILM, AND PAINTED OBJECT

Abstract: To offer a method for forming paint films of outstanding acid resistance, finished appearance, water resistance and adhesion, etc., even when a water-based base finishing coat and solvent-based top finishing coat are painted wet-on-wet. A method for forming a paint film by a method for forming a hardened paint film in which a water-based thermosetting base finishing coat is painted onto the surface of an object to be painted, a thermosetting top coat is painted on top of this wet-on-wet, and the paint films are hardened at the same time, in which the aforementioned top coat which contains as a primary resin a copolymer which is a copolymer of a vinyl monomer which has a ring structure and other vinyl monomer(s) in which the proportion of vinyl monomer having a ring structure in the total quantity of monomers is 20-50 wt.%, and the proportion of other vinyl monomer(s) is 80-50 wt.%, and which has a glass transition temperature of 0-60 DEG C, a solubility parameter of 9.0-11.0 and a weight-average molecular weight of 4000-30,000.

Description: Method for forming a paint film, and painted object Technical field of the invention The present invention relates to a method for coating a base finishing coat and a top finishing coat wet-on-wet to form a thermosetting paint film, and to painted objects obtained by this means. More specifically, the present invention relates to a method for forming thermosetting paint films of outstanding acid resistance, finished appearance, water-resistance and adhesion, etc. , suitable for finishing coat paint films, and painted objects obtained by this means. Prior art Thermosetting paints which form a hardened paint film by applying a base finishing coat and top finishing coat wet-on-wet and stoving the resulting paint films at the same time are widely used as finishing coats for painting the outer skins of automobiles. However, the fact that top finishing coat paint films are prone to water staining by acid rain of low pH due to atmospheric pollution, which lowers their appearance, has become a problem in recent years. In order to solve this problem, paint compositions with raised water resistance due to crosslinking of the principal resin of the top finishing coat with a hardener to form a composite, etc., have been investigated: for example, in Japanese Unexamined Patent 3-275780 paint compositions are proposed in which the principal components are an acrylic resin containing hydroxyl groups and carboxyl groups, an acrylic resin containing hydroxyl groups and epoxy groups, an a ino resin and a quaternary phosphonium salt. - a - However, if the concentration of carboxyl groups and epoxy groups in such paint compositions are increased in order to obtain a high degree of acid resistance, the finished appearance of the paint film is lowered; and raising the glass transition temperature of the hardened paint film invites decreases in recoat adhesion and bending resistance. In this connection, in the painting above large quantities of solvent are employed when painting the base finishing coat, and therefore water-based paints have come to be predominantly used in order to improve the working environment. Solvent-based paints, on the other hand, are predominantly used for top finishing coats because of the importance of the finished appearance. However, water-based base finishing coats and solvent-based top finishing coat are not readily compatible because of the difference in the solvent, and it is extremely difficult to obtain finishing coat paint films which have a high level of balance between water-resistance and finished appearance and also have outstanding adhesion. Problem which the invention is intended to solve The purpose of the present invention is to solve this problem by offering a method for forming a paint film which can form hardened paint films with outstanding acid resistance, finished appearance, water resistance and adhesion, etc., even when a water-based base finishing coat and a solvent-based top finishing coat are painted wet-on-wet, and offering painted objects obtained with this method. Means for solving the problem The present invention is the following method for forming a paint film, and painted objects. (1) A method for forming a paint film, which is a method for forming a hardened paint film in which a water-based thermosetting base finishing coat is painted onto the surface of an object to be painted, a thermosetting top finishing coat is painted on top of this wet-on-wet, and the resulting paint films are hardened at the same time, characterized in that the aforementioned top finishing coat contains as a primary resin a copolymer which is a copolymer of a vinyl monomer which has a ring structure and other vinyl monomer(s) in which the proportion in the total quantity of monomers of vinyl monomer having a ring structure is 20-50 wt%, and the proportion of other vinyl monomer is 80-50 wt%, and has a glass transition temperature of 0-60°C, a solubility parameter of 9.0-11.0 and a weight-average molecular weight of 4000-30,000. (2) Painted objects obtained by the method of (1) above. The glass transition temperatures (Tg) in the present invention are values found by Equation (1) below, of G. Fox (Bull. Am. Phys. Soc. No. 3, 123 (1956)). 1/Tg = Σ (wn/Tgn) (1) (Where n indicates a natural number representing the number of monomers, wn indicates the wt% of the nth monomer, and Tgn indicates the Tg of a homopolymer of the nth monomer. ) The solubility parameters in the present invention are values found by means of Equation (2) below, by Fedors' method (Polymer Engineering and Science 14 (2) - 4 - ( Feb . 1974 ) ) Σ ( Δ e , ) S p ( 2 ) ( Δ , ) (Where Δei is the cohesive energy per unit functional group, and Δvi molecular value per unit functional group.) The weight-average molecular weights (Mw) in the present invention are values determined by gel permeation chromatography against a polystyrene standard. The base finishing coat used in the present invention is a water-based thermosetting paint. There are no restrictions as to this water-based thermo¬ setting paint, and prior water-based base finishing coats can be used as they stand; however, water-based paints in which the principal components are a water- soluble thermosetting resin or water-dispersable thermosetting resin, a metallic pigment and/or colored pigment, an organic solvent which can dissolve in water and deionized water, etc., together with optional body pigments, viscosity adjusting agents and paint surface modifiers, etc., are preferred. Any paint can be employed which contains a principal resin which is a water-soluble or water- dispersable thermosetting resin above, selected from alkyd resins, polyester resins, acrylic resins and cellulosic resins, for example, in a water-soluble or water-dispersible form, together with a crosslinking agent. Examples of crosslinking agents which can be employed include water-soluble or water-dispersible amino resins obtained by condensation or co- condensation of melamine, benzoguanamine or urea etc., with formaldehyde, or water-soluble or water- dispersible blocked isocyanates. The principal resin and crosslinking agent employed can in themselves be already known. Similarly, there are no particular restrictions regarding metallic pigments above, and those used in ordinary paints can be employed; examples include surface-treated aluminum, copper, brass, bronze and stainless steel, etc., micaceous iron oxide, leafing aluminum powder and mica flakes coated with titanium oxide or iron oxide, etc. Likewise, there are no particular restrictions regarding colored pigments above, and those employed in ordinary paints can be employed; examples include inorganic pigments such as titanium oxide, bengala, yellow iron oxide and carbon black, etc. , and organic pigments such as phthalocyanine blues, phthalocyanine greens, quinacridone red pigments and isoindolinone yellow pigments, etc. The principal resin of the top finishing coat employed in the present invention is a copolymer of a vinyl monomer having a ring structure and other vinyl monomers, and is ordinarily obtained by free radical copolymerization of these monomers. Vinyl monomers having a ring structure are monomers which have a mononuclear or polynuclear aromatic, alicyclic and/or heterocyclic ring structure and a vinyl group. The ring can be substituted with substituent groups. Other vinyl monomers are monomers which have a vinyl group but do not have a ring structure above. -__,, - - _ - - 97/22421 Examples of monomers which have a ring structure above include styrene, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, tetracyclo[4.4.0.l 2,9 .i 7 ' 10 ] - dodecyl 3-(meth)acrylate, adamantyl (meth)acrylate, t- butylcyclohexyl (meth)acrylate, and 2- (meth)acryloyloxyethylhexahydrophthalic acid, etc. These can be employed singly or they can be employed in combinations of 2 or more. Examples of other vinyl monomers which copolymerize with vinyl monomers which have a ring structure include (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, stearyl (meth)acrylate and dodecyl (meth)acrylate, etc., vinyl monomers which include a carboxyl group, such as acrylic acid, methacrylic acid, ethacrylic acid, propylacrylic acid, isopropylacrylic acid, crotonic acid, aleic acid, itaconic acid, maleic acid monoesters, itaconic acid onoesters and acrylic acid di er, etc., lactone-modified vinyl monomers, such as acrylic acid ε-caprolactone addition products, methacrylic acid ε-caprolactone addition products and 2- hydroxyethyl (meth)acrylate ε-caprolactone addition products, etc. , monoesters of vinyl groups which include a hydroxyl group, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3- hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)- acrylate and glycerol (meth)acrylate, etc., and acid anhydrides such as succinic anhydride, phthalic anhydride, hexahydrophthalic anhydride and methylated hexahydrophthalic anhydride, etc. , vinyl monomers which include a hydroxyl group, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3- - 7 - hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)- acrylate and butane-1, -diol mono(meth)acrylate, etc., monoethers of hydroxyl-group-containing vinyl monomers above with polyether polyols such as polyethylene glycol and polypropylene glycol, etc., and vinyl monomers which include an epoxy group, such as glycidyl (meth)acrylate and 3,4-epoxycyclohexylmethyl (meth)- acrylate, etc. These can be employed singly or they can be employed in combinations of 2 or more. The proportion of vinyl monomer having a ring structure in the total quantity of monomers which are copolymerized is 20-50 wt%, and preferably 25-45 wt%, and the proportion of other monomers is 80-50 wt%, and preferably 75-55 wt%. With a proportion of less than 20 wt% of monomer having a cyclic structure the acid resistance of the resulting paint films is inadequate, and more than 50 wt% is undesirable because adhesion between the base finishing coat and top finishing coat is lowered. Polymerization initiators used in the afore¬ mentioned copoly erization include t-butyl peroxybenzoate and t-butyl peroxy-2-ethylhexanoate , etc.; the quantity employed is 0.01-4 parts by weight, and preferably 0.2-2.7 parts by weight, to 100 parts by weight of total monomer. The aforementioned copolymerization is performed at a temperature ordinarily of 90-170°C, and preferably 100-150°C, and the reaction time is ordinarily 3-8 hours, and preferably 4-6 hours. As a reaction medium an organic solvent such as xylene, hexane or toluene, etc., can be employed. The transition temperature of the principal resin of the top finishing coat is 0-60°C, and preferably 5- +50 and more preferably 5-55°C. With less than 0°C the acid resistance of the resulting paint films is inadequate, and more than 60°C is undesirable because finished appearance is lowered due to poor smoothness. Similarly, the solubility parameter of the principal resin of the top finishing coat is 9.0-11.0, and preferably 9.5-10.5, and more preferably 9.8-10.5. With less than 9.0 the gasoline resistance of the resulting films is inadequate, and more than 11.0 is undesirable because finished appearance and water resistance are lowered. Moreover, the weight-average molecular weight of the principal resin of the top finishing coat is 4000- 30,000, and preferably 5000-25,000, and more preferably 7000-22,000. With less than 4000 weather resistance is inadequate, and more than 30,000 is undesirable because the paint is not sufficiently fine, and the finished appearance of the paint films is lowered. There are no specific restrictions as to the mode of curing top finishing coats which include a principal resin described above, i.e. the combination of principal resin and hardener, and any system can be used which can be employed for painting automobiles, such as a hydroxyl-group-containing resin/melamine resin system, hydroxyl-group-containing resin/- isocyanate system, hydroxyl-group-containing resin/- blocked isocyanate system, carboxyl-group-containing resin/epoxy system, epoxy-group-containing resin/- carboxyl-group-containing resin, or silicone cross- linking, etc., for example. These modes can be used on their own, or in combinations of 2 or more. Hydroxyl-group-containing acrylic resins obtained by copolymerizing at least a vinyl monomer having a ring structure as already described, such as styrene, etc. , for example, and a hydroxyl-group-containing monomer such as 2-hydroxyethyl (meth)acrylate, etc., for example, as essential monomers, with other vinyl monomers, can be employed as aforementioned hydroxyl- group-containing principal resins. It should be noted that the vinyl monomer having a ring structure, the hydroxyl-group-containing vinyl monomer and the other vinyl monomers can each be single monomers or combinations of 2 or more monomers. Carboxyl-group-containing resins obtained by copolymerizing at least a vinyl monomer having a ring structure as already described, such as styrene, etc., for example, and a carboxyl-group-containing monomer such as acrylic acid, etc. , for example, as essential monomers, with other vinyl monomers, can be employed as aforementioned carboxyl-group-containing principal resins. It should be noted that the vinyl monomer having a ring structure, the carboxyl-group-containing vinyl monomer and the other vinyl monomers can each be single monomers or combinations of 2 or more monomers. Epoxy-group-containing resins obtained by copolymerizing at least a vinyl monomer having a ring structure, such as styrene, etc., for example, and a epoxy-group-containing monomer such as glycidyl ( eth)- acrylate, etc., for example, as essential monomers, with other vinyl monomers, can be employed as aforementioned epoxy-group-containing principal resins. It should be noted that the vinyl monomer having a ring structure, the epoxy-group-containing vinyl monomer and the other vinyl monomers can each be single monomers or combinations of 2 or more monomers. Melamine resins as aforementioned hardeners include butylated melamine resins, methylated melamine resins and mixed butylated/methylated ether melamine resins, etc. , obtained by methylolation of aminotriazine, and alkylating with cyclohexanol or a Cl-6 alkanol. Concrete examples of butylated melamine resins include Yuban (trade name Mitsui Toatsu Chemical) and Superbeckamin (trade name Dainippon Ink & Chemicals) ; concrete examples of methylated melamine resins and butylated/methylated melamine resins include Cymel (trade name Mitsui Cyamid) and Nikalac (trade name Sanwa Chemical) , etc. Examples of isocyanates or blocked isocyanates which can be employed as aforementioned hardeners include polyisocyanate compounds such as hexa ethylene diisocyanate, trimethylhexamethylene diisocyanate, isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate and hydrogenated xylylene diisocyanate, etc., and adduct forms, biurets and polyisocyanurates thereof, for example, and these compounds blocked with a blocking agent such as a compound containing active hydrogen, including phenol and phenols such as m- cresol, xylenol and thiophenol, etc., alcohols such as methanol, ethanol, butanol, 2-ethylhexanol and cyclohexanol, etc., caprolactam, methyl ethyl ketone oxime, ethyl acetoacetate and diethyl malonate, etc. Examples of epoxy compounds which can be employed as aforementioned hardeners include compounds containing epoxy groups, such as bisphenolic epoxy resins, alicyclic epoxy resins, homopolymers and copoly ers of glycidyl (meth)acrylate or 3-4- epoxycyclohexylmethyl (meth)acrylate, etc., and poly- glycidyl compounds obtained by reacting a polycarboxylic acid or polyol with epichlorhydrin, etc. Examples of carboxyl-group-containing resins which can be employed as aforementioned hardeners include (1) products of partial esterification of a polyol having at least 2, and preferably 2-50, hydroxyl groups per molecule and an acid anhydride, (2) addition compounds of a polyisocyanate having at least 2, and preferably 2-50, isocyanate groups per molecule with a hydroxycarboxylic acid or amino acid, (3) homopolymers of carboxyl-group-containing α,β-unsaturated monomers and copolymers with other α,β-unsaturated monomer(s) , and (4) polyester resins having a terminal carboxyl group, etc. For aforementioned silicone crosslinking vinyl polymers which contain silyl groups, in which the main chain comprises substantially a vinyl polymer, with at least 1 silicon atom per molecule bound terminally or in a side chain to a hydrolysable moiety can be employed; the Kanegafuchi Chemical Industry product Zemlac (trade name), etc., can be given as an example. Polymers such as acrylic polymers, polyester polymers, urethane polymers, non-aqueous dispersions of acrylic polymers and crosslinked polymer particles, etc., and additives employed in prior paints, including colorings such as pigments and dyes, etc., pigment dispersants, viscosity adjusting agents, sag preventing agents, levelling agents, hardening catalysts, gel- preventing agents, ultraviolet absorbers and/or free radical scavengers, etc., can also optionally be added to the top finishing coat used in the present invention, within ranges that do not detract from the purpose of the present invention. In the majority of cases the top finishing coat used in the present invention will be used as a clear coat; however, it can also be used in the form of a semi-transparent or opaque paint by adding a large quantity of coloring. An organic solvent can also optionally be used in top finishing coat paints used in the present invention; an aromatic hydrocarbon solvent, ester solvent, ketone solvent or alcohol solvent, etc. , can be employed as this organic solvent. In the present invention an aforementioned water- based base finishing coat and top finishing coat are painted wet-on-wet, i.e. the top finishing coat is painted on the unhardened base finishing coat paint film, and hardened by heating simultaneously in 2 coats/1 bake. In a specific painting method, an aforementioned water-based base finishing coat paint is first coated onto the surface of the object to be painted. For this it is desirable to paint using a spray coater, such as an air spray coater, an airless spray coater or a Ransburg No. 1 or Ransburg No. 2 type electrostatic spray coater, etc., for example to give a dry film thickness of the order of 10-30 μm, and preferably 15-20 μm. After coating with a water-based base finishing coat paint in this way, the object is left for about 1- 20 minutes at a temperature of 50-100°C, and then a top finishing coat paint is coated onto the painted surface using a conventional spray coater such as an air spray coater, an airless spray coater or a Ransburg No. l or Ransburg No. 2 type electrostatic spray coater, etc., for example to give a dry film thickness of the order of 20-50 μm, and preferably 25-45 μm. Both of the paint films above are then heated at a temperature of 100-180°C, and preferably 120-160°C, for 10-120 minutes, and preferably 30-60 minutes, using a hot air oven, an infrared furnace or an electric induction heating oven, etc. , to form the desired hardened paint film. There are no specific restrictions as to objects which can be painted using the method of the present invention for forming a paint film; examples of materials include iron, aluminum, zinc and alloys thereof, metals surface treated, etc. , by iron phosphate treatment, zinc phosphate or chromate treatment, etc. , plastics such as polyurethane, polypropylene and polycarbonate, etc., and wood, etc. These objects to be painted can also have been painted with undercoat paint and middle coat paint, etc. There are no specific restrictions as to the fields within which the method of the present invention for forming a paint film can be applied; however, its application for forming finishing coat paint films on the outer shells of automobiles is particularly preferred. Painted objects of the present invention are objects on which a paint film has been formed by the aforementioned method; they have outstanding acid resistance, appearance, water resistance and adhesion, etc. Benefits of the invention With the present invention it is possible to form paint films of outstanding acid resistance, finished appearance, water resistance and adhesion, etc. , even when painting using a water-based base finishing coat and a solvent-based top finishing coat wet-on-wet. It is also possible to obtain painted objects which have such a paint film. Modes of implementing the invention The present invention will next be explained in more detail by means of preparation examples, embodiments and comparison examples; however, the present invention is not restricted in any way by these examples. In the different examples parts and percentages are based on weight. Preparation Example 1 Making a polyurethane dispersion (A-l) for a water- based base finishing coat 798 parts of Polypol 1009 (trade name Unichema International: dimerized fatty acid with an average molecular weight of 1400, synthesized from hexane-1,6- diol and phthalic anhydride), 12.5 parts of hexane-1,6- diol, 65 parts of dimethylolpropionic acid and 529.5 parts of methyl ethyl ketone were loaded into a 4- mouthed flask fitted with a stirrer, a reflux cooler, a thermometer and a tube for introducing nitrogen gas, stirred to mix, and then 329 parts of l,3-bis(2- isocyanatopropyl-2-yl)benzene (American Cyanamid product m-TMXDI) was added. The mixture was stirred and held at 82°C, and the reaction was continued until the quantity of free isocyanate groups reached 1% of the total quantity loaded. Then 31 parts of trimethylol propane was added, and the temperature was held at 82°C. The rise in viscosity of the reaction mixture was monitored, and the reaction was continued until the viscosity reached 5 dPa.s (requiring ca. 5 hours). Viscosity was determined at 23°C using a cone and plate viscometer, with 10 ml of sample dissolved in 10 ml of N- ethylpyrrolidone. The reaction was then stopped by adding 54 parts of propanol. A mixture of 38 parts of dimethylethanolamine and 3254 parts of deionized water was then added, and after stirring for 2 hours the ethyl methyl ketone was distilled off under decreased pressure. This gave a polyurethane dispersion (A-l) of 31% solids. Preparation Example 2 (Making a polyester dispersion (A-2) for a water-based base finishing coat) 729 parts of neopentyl alcohol, 827 parts of hexanediol, 462 parts of hexahydrophthalic anhydride and 1710 parts of polymer fatty acid (minimum 98% dimer, maximum 2% trinter, trace quantities of monomer) were loaded into a 4-mouthed flask fitted with a water separator, and stirred as the temperature was raised. Care was taken that the temperature of the column fitted to the water separator did not rise above 100°C. The esterification reaction was performed at a maximum temperature of 220°C, and continued until the acid value reached 8.5. After cooling to 180°C, 768 parts of trimellitic anhydride was added, and the esterification reaction was continued until the acid value reached 30. The reaction mixture was then cooled to 120°C, and 1410 parts of butanol was dissolved in it. After further cooling to 90°C, 16.2 parts of dimethylethanola ine was added, and then 1248 parts of deionized water was added. This gave a polyester dispersion (A-2) of pH 7.8, 60% solids and an acid value of 30. Preparation Example 3 (Making a water-based base finishing coat paint) Firstly butyl cellosolve 2.7% and melamine resin (American Cyanamid trade name Cy el 327; 90% isopropanolic solution) 6.4% were stirred into a rheology adjusting agent 34.3% in a dissolver. Then the polyurethane dispersion (A-l) made in Preparation Example 1, 35.7%, was stirred in to give a melamine resin/polyurethane resin dispersion. As the rheology adjusting agent above, a 3% aqueous solution of sodium magnesium silicate (Laporte trade name Laponite RD) was used. The electrical conductivity of the rheology adjusting agent solution was 886 μS/cm. Electrical conductivity was determined using a Toa Electronics TOA CM-20S conductivity meter. In parallel with the operation above, a slurry of an aluminum pigment was prepared as follows. A commercial aluminum paste stable in water (mean particle size : 15 μm) 3.8% was mixed with 6.0% of butyl cellosolve. The polyester dispersion (A-2) obtained in the aforementioned Preparation Example 2, 6.7%, was mixed into this mixture to obtain a slurry of an aluminum pigment. This slurry of an aluminum pigment was stirred into the aforementioned melamine resin/polyurethane resin dispersion. Finally 4.4% of deionized water was - 17 - added and the pH was adjusted to the range 8.10-8.30 by using dimethylethanolamine solution (10% solution in deionized water) , to obtain a water-based base finishing coat composition. This water-based base finishing coat composition was adjusted using deionized water to ca. 24% solids, to give a viscosity which would enable spray coating. Preparation Examples 4-7 and Comparison Examples 1 and 2 Making top finishing coat acrylic resin solutions (B-Gl A quantity of xylene recorded in Table 1 or Table 2 was loaded into a flask fitted with a stirrer, a reflux cooler and a thermometer, and the temperature was raised to 140°C. Then the corresponding monomers and polymerization initiator recorded in Table 1 or Table 2 were added dropwise over 2 hours. After completing dropwise addition, the system was held at reflux temperature for 1 hour, and then the contents were cooled to 100°C. After cooling to 100°C, the polymerization initiator recorded in Table 1 or Table 2 was added dropwise over 30 minutes. The polymerization was completed by holding the temperature at 100°C for a further 3 hours, to obtain acrylic resin solutions (B- G) having the properties shown in Table 1 and Table 2. Table l Table 2 - 21 Notes on Table 1 and Table 2 *1 CHMA : Cyclohexyl methacrylate *2 FMl : Plaxel FMl (trade name Daicel Chemical Industry; monomer addition product of 1 mol of 2- hydroxyethyl methacrylate and l mol of ε-capro¬ lactone) *3 HEA : 2-Hydroxyethyl acrylate *4 HEMA : 2-Hydroxyethyl methacrylate *5 t-BMA : t-Butyl methacrylate *6 EHA : 2-Ethylhexyl acrylate *7 MMA : Methyl methacrylate *8 EHMA : 2-Ethylhexyl methacrylate *9 AA : Acrylic acid *10 MA : Methyl acrylate *11 t-BPBz : Polymerization initiator, t-butyl peroxy- benzoate *12 t-BPEH : Polymerization initiator, t-butyl peroxy-2-ethylhexanoate *13 Tg : Calculated by the aforementioned Equation (1) of T.G. Fox *14 Sp : Calculated by the method of Fedors by the aforementioned Equation (2) *15 Mw : Determined by gel permeation chromatography against a polystyrene standard - 11 - Preparation Examples 8-11 and Comparison Preparation Examples 3 and 4 (Making top finishing coat paints) Clear coat paints were obtained using the starting materials and quantities shown in Table 3, by mixing and dispersing. Table 3 Notes Table 3 *1 Superbeckamin L-116-70 (trade name Dainippon Ink & Chemicals; isobutylated melamine resin solution, heating residue 70%) *2 Takenate XB-72-G16 (trade name Takeda Chemical Industries; solution of a non-yellowing blocked isocyanate compound, heating residue 80%) *3 10% xylene solution of Ciba-Geigy product Tinuvin 900 *4 10% xylene solution of Sankyo product Sanol LS292 *5 Modaflow (trade name Monsanto, leveling agent) Embodiment 1-4 and Comparison Examples 1 and 2 Aqua No. 4200 (trade name Japan Oils & Fats, cationic electrodeposition paint) was electrostatically coated onto zinc-phosphate-treated sheet steel to give a dry paint film thickness of 20 μm, and then it was stoved at 175°C for 20 minutes. Haiepiko No. 100 (trade name Nippon Oil & Fats, middle coat paint) was then air spray coated to give a dry paint film thickness of 40 μm, and stoved at 140°C for 30 minutes to make a test sheet. The water-based base finishing coat paint obtained in Preparation Example 3 was air spray coated onto the surface of this test sheet to give a dry paint film thickness of 15 μm, and after drying at 80°C for 10 minutes a top finishing coat obtained in aforementioned Preparation Example 8-11 or Comparison Preparation Example 3-4 was air spray coated to give a dry paint film thickness of 40 μm, and then stoved at 140°C for 30 minutes to make test sheets. The results of tests of paint film performance on these test sheets are shown in Table 4. Table 4 & t Notes on Table 4 *1 ■ Acid resistance : 0.2 ml of 40% sulfuric acid aqueous solution was spotted onto the test sheet, which was heated at 50°C for 15 minutes and then washed with water, and the paint film was investigated for abnormalities in appearance, such as stains, blistering or shrinkage, etc. *2 Finished appearance : The finished appearance of the paint film was assessed using a Suga Test Instruments reflectometer (HA-ICM) . The higher the numerical value the better the finished appearance. *3 Water resistance : The paint film was investigated for abnormalities in appearance such as stains, blistering or shrinkage after immersing the test sheet for 240 hours in warm water at 20°C. From the results of Table 4 it is evident that the painted objects of Embodiments 1-4 obtained by the method of the present invention for forming a paint film all had outstanding acid resistance, finished appearance and adhesion. By contrast, in Comparison Example 1 acid resistance was inferior because the proportion of vinyl monomer having a ring structure in the acrylic resin copolymer used in the top finishing coat was less than 20 wt%. Similarly, in Comparison Example 2 finished appearance and water resistance were inferior because the solubility of the acrylic resin used in the top finishing coat exceeded 11.0.

Claims: -_ _ _ - - 27 - Claims 1. Method for forming a paint film, which is a method for forming a hardened paint film in which a water- based thermosetting base finishing coat is painted onto the surface of an object to be painted, a thermosetting top finishing coat is painted on top of this wet-on- wet, and the resulting paint films are hardened at the same time, characterized in that the aforementioned top finishing coat contains as the principal resin a copolymer which is a copolymer of a vinyl monomer which has a ring structure and other vinyl monomer(s) , in which the proportion in the total quantity of monomers of vinyl monomer having a ring structure is 20-50 wt%, and the proportion of other vinyl monomer(s) is 80-50 wt%, and the glass transition temperature is 0-60°C the solubility parameter is 9.0-11.0, and the weight-average molecular weight 4000-30,000. 2. Painted object obtained by the method of Claim 1.

Inventors: FURUUCHI KAZUYOSHI, MORI SOUICHI, FURUUCHI, KAZUYOSHI, MORI, SOUICHI
Applicants: BASF LACKE & FARBEN, FURUUCHI KAZUYOSHI, MORI SOUICHI, NOF CORP, BASF LACKE + FARBEN AG, FURUUCHI, KAZUYOSHI, MORI, SOUICHI, NOF CORPORATION
Application year: 1996
Main classification: B05D7
IPC codes: B05D1, B05D5, B05D7