Method of forming coating films

Abstract: The invention provides a method of forming a coating film by forming in sequence a pigmented base coat and a clear top coat on a substrate followed by finishing by the two-coat one-bake technique, the method being characterized by using, as a coating composition for pigmented base coat formation, a composition comprising, as essential components thereof, (1) an OH-containing resin, (2) an amino resin, (3) a polyorganosiloxane which has, on an average, at least two groups, per molecule, each selected from the class consisting of a silanol group and an alkoxysilane group and has a number average molecular weight of at least 1,000, (4) a flaky metal powder and/or a mica powder, and (5) an organic solvent, and using, as a coating composition for clear top coat formation, a composition comprising, as essential components thereof, (1) an OH-containing base resin which further contains at least one group selected from the class consisting of a silanol group and a hydrolyzable group bound directly to a silicon atom, (2) an amino resin, and (3) an organic solvent.

Description: The present invention relates to a novel method of forming coating films and, more particularly, to a method of forming a coating film excellent in finished appearance. Top coats, particularly those on automotive outer body, are required to be improved almost constantly in finished appearance and coating film performance characteristics. Furthermore, the advent of low-temperature curable coating films is eagerly awaited for reducing energy costs and for insuring that an automotive outer body consisting of steel and plastic materials can be integrally coated with one and the same coating composition. Coating compositions based on thermosetting acrylic resin/melamine resin, those based on thermosetting polyester resin/melamine resin and the like are now mainly used as coating compositions for top coat in the automobile industry. In terms of the degree of levelness and smoothness, these compositions have almost reached their limit, while there is still much room for improvement in these and other film performance characteristics. Another problem is that their low-temperature curability is not fully satisfactory. Furthermore, such problems as ebullition of coating films due to condensation byproducts (formalin etc.) generated in the curing step and environmental pollution remain to be solved. The inventors of the present invention previously proposed an automotive coating composition excellent in low-temperature curability in JP-A-160879/1990. This composition is a silicone-based composition comprising a copolymer obtained by polymerizing a silanol- and/or alkoxysilane group-containing polysiloxane macromonomer and an oxirane-containing vinyl monomer, and a metal chelate compound. However, this composition has a drawback in that when it is used as both a metallic base coat composition (containing an aluminum flake pigment) and a clear top coat composition in a two-coat one-bake system, the metallic base coat is adversely affected by the clear top coat, that is to say the orientation of the aluminum flakes is disturbed, with the result that a coating film having an aluminum gloss (glitters, white reflections) cannot be obtained. When a thermosetting polyester resin/melamine resin-based coating composition is used as the metallic base coat composition, the clear top coat begins to cure before the start of curing of said metallic base coat, and this disadvantageously causes such defects as ebullition and shrinkage of the coat. When a finish coat film has a defect, the defect is generally repaired by recoating with a metallic base coat and a clear top coat each of the same kind as before. With the silicone-based coating composition mentioned above, there remains a problem that the mutual adhesion between coating films (i.e. between top clear coat and metallic base coat for recoating) is insufficient. It is an object of the present invention to provide a method of forming coating films excellent in finish and recoatability. Other objects as well as features of the invention will become apparent from the description which follows. The invention provides a method of forming coating films by forming in sequence a pigmented base coat and a clear top coat on a substrate followed by finishing by the two-coat one-bake technique, the method being characterized by, as a coating composition for pigmented base coat formation, a composition (A) comprising, as essential components thereof, (1) a certain OH-containing resin as specified hereinafter, (2) an amino resin, (3) a polyorganosiloxane which has, on an average,at least two groups, per molecule, each selected from a silanol group and an alkoxysilane group and has a number average molecular weight of from 1,000 to 50,000, (4) a flaky metal powder and/or a mica powder, and (5) an organic solvent, in the proportions of components (1) to (3) as given below; and using, as a coating composition for clear top coat formation, a composition (B) comprising, as essential components thereof, (1) a certain OH-containing base resin which further contains at least one group selected from a silanol group and certain hydrolyzable groups bound directly to a silicon atom, as specified in more detail below; (2) an amino resin in a certain amount as given below, and (3) an organic solvent. The invention is now described more detailedly in the following. Pigmented base coat In the coating method of this invention, the pigmented base coat is formed from a base coat composition (A) which comprises, as essential components thereof, (1) a certain OH-containing resin (hereinafter sometimes referred to as "OH-containing base coat resin"), (2) an amino resin, (3) a polyorganosiloxane having, on an average, at least two groups each selected from a silanol group and an alkoxysilane group and having a number average molecular weight of from 1,000 to 50,000 (hereinafter sometimes referred to as "polyorganosiloxane") for short), (4) a flaky metal powder and/or a mica powder (hereinafter sometimes referred to as "metal flake" for short) and (5) an organic solvent. The OH-containing base coat resin is either a OH-containing polyester resin or a OH-containing vinyl resin, both resins having certain molecular weights and hydroxyl values. The OH-containing polyester resins have a hydroxyl value of 20 to 200, preferably 50 to 150. When the hydroxyl value is smaller than 20, the rate of curing of the base coat film becomes slower than that of the top coat film, so that the curing of the top coat film proceeds before that of the base coat film. As a result, the top coat film tends to develop defects such as shrinkage, leading to impairment of the appearance of the finished film. Furthermore, the curing can hardly be complete, so that the performance characteristics (e.g. water resistance, impact resistance) of the coating film will unfavorably be deteriorated. On the other hand, hydroxyl values exceeding 200 are undesirable since, in that case, a large number of unreacted hydroxyl groups remain within the coating film, leading to deteriorated performance characteristics (e.g. water resistance, weather resistance) of the coating film. Said OH-containing polyester resins may contain one or more carboxyl groups within the molecule. In particular, the carboxyl group can increase the rate of reaction between the hydroxyl group and the amino group, between the hydroxyl group and the silanol or alkoxysilane group, or between one silanol or alkoxysilane group and another silanol or alkoxysilane group, to thereby effectively improve the finish of the coating film. Recommendably, the carboxyl group content should correspond to about 0 to 50, preferably about 5 to 20 in an acid value of resin. As examples of the OH-containing polyester resins, there may be mentioned oil-free alkyd resins derived from at least one polybasic acid and at least one polyhydric alcohol and, if necessary, an aromatic monobasic acid; and alkyd resins derived from at least one polybasic acid and at least one polyhydric alcohol and, if necessary, an aromatic monobasic acid plus an aliphatic monobasic acid. The polybasic acid, polyhydric alcohol, aromatic monobasic acid and aliphatic monobasic acid that are to be used in the production of said (oil-free) alkyd resins are more specifically mentioned below. As the polybasic acid, there may be mentioned, for example, phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, tetrahydroterephthalic acid, hexahydrophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, chlorendic acid, trimellitic acid, hexahydrotrimellitic acid, pyromellitic acid, cyclohexanetetracarboxylic acid, methyltetrahydrophthalic acid, methylhexahydrophthalic acid, endomethylenehexahydrophthalic acid, methylendomethylenetetrahydrophthalic acid, maleic acid, fumaric acid, itaconic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, suberic acid, pimelic acid, dimer acids (dimers of tall oil fatty acid), tetrachlorophthalic acid, naphthalenedicarboxylic acid, 4,4'-diphenylmethanedicarboxylic acid and 4,4'-dicarboxybiphenyl, acid anhydrides of these, and dialkyl esters of these, in particular dimethyl esters. As the polyhydric alcohol, there may be mentioned, among others, ethylene glycol, propylene glycol, 1,4-butanediol, 1,3-butanediol, 2,3-butanediol, 1,2-butanediol, 1,5-pentanediol, 1,4-pentanediol, 2,4-pentanediol, 2,3-dimethyltrimethylene glycol, 3-methylpentane-1,5-diol, 3-methyl-4,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 2,5-hexanediol, 1,4-cyclohexanedimethanol, diethylene glycol, dipropylene glycol, triethylene glycol, neopentyl glycol, hydroxypivalic acid neopentyl glycol ester, polyalkylene oxides, bishydroxyethyl terephthalate, (hydrogenated) bisphenol A-alkylene oxide adducts, glycerol, trimethylolpropane, trimethylolethane, diglycerol, pentaerythritol, dipentaerythritol and sorbitol. Monoepoxy compounds such as Cardura® E 10 (Shell Chemical Company), α-olefin epoxides and butylene oxide may also be used as a kind of glycol. Furthermore, compounds containing both carboxyl and hydroxyl groups within the molecule may also be used. As such compounds, there may be mentioned, for example, dimethylolpropionic acid, pivalic acid, 12-hydroxystearic acid and ricinolic acid. Lactones such as ε-caprolactone and γ-valerolactone also fall under the category of the compounds mentioned above since they are cyclic ester compounds. The aromatic monobasic acid includes, among others, benzoic acid and p-tert-butylbenzoic acid. The aliphatic monobasic acid includes, among others, caproic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, isononanoic acid and coconut oil (fatty acids). The (oil-free) alkyd resins mentioned above can be produced by any of the conventional methods, for example by subjecting a mixture of the above-mentioned polybasic acid and polyhydric alcohol and, if necessary, monobasic acid to esterification or transesterification in the presence of an esterification catalyst (e.g. dibutyltin dilaurate). Said polybasic acid and polyhydric alcohol are desirably used in proportions such that the number of moles of the polybasic acid component(s) is within the range of about 0.7 to 0.99, preferably about 0.8 to 0.98, per mole of the polyhydric alcohol component(s). The esterification catalyst is desirably used in an amount of about 0.1 to 1.0 part by weight, preferably about 0.2 to 0.5 part by weight, per 100 parts by weight of the total sum of the polyhydric alcohol and polybasic acid components. As for the reaction conditions, the reaction temperature is generally about 160° to 280°C, preferably about 180° to 260°C, and the reaction period is generally about 5 to 12 hours, preferably about 6 to 8 hours. As the OH-containing polyester resin, vinyl-modified alkyd resins, for instance, may also be used. Usable as said vinyl-modified alkyd resins are, for example, reaction products from an OH-containing (oil-free) alkyd resin and a COOH- or NCO-containing vinyl resin, reaction products derived from an OH- and COOH-containing (oil-free) alkyd resin and an epoxy-containing vinyl resin, products of radical polymerization of a vinyl monomer [e.g. a polymerizable unsaturated monomer (b) mentioned below] in the presence of an (oil-free) alkyd resin having a radical-polymerizable unsaturated group [e.g. an alkyd resin containing a drying oil as an essential component, an alkyd resin derived from an OH-and COOH-containing (oil-free) alkyd resin by reaction with glycidyl (meth)acrylate], and products of polymerization of a vinyl monomer [e.g. a polymerizable unsaturated monomer (b) mentioned below] in the presence of an (oil-free) alkyd resin or vinyl-modified alkyd resin such as mentioned above, for instance, as a suspension stabilizer, in an organic solvent in which said monomer and suspension stabilizer can be dissolved but the polymer particles obtained from said monomer will not be dissolved. The molecular weight of the OH-containing polyester resin, as expressed in terms of number average molecular weight, is within the range of 500 to 10,000 in the case of an (oil-free) alkyd resin, or within the range of 2,000 to 80,000 in the case of a vinyl-modified alkyd resin. When said molecular weight is below the above range, the melt viscosity of the base coat decreases during baking, whereby the orientation of metal flakes changes, making it difficult to obtain coating films with a metallic tone. A moleuclar weight exceeding the above range is also undesirable since the workability in coating generally decreases. The OH-containing vinyl resins have a hydroxyl value of 20 to 200, preferably 50 to 150. When the hydroxyl value is below 20, the rate of curing of the base coat film becomes slower than that of the top coat film, so that the curing of the top coat film precedes the curing of the base coat film. As a result, the top coat film tends to develop defects such as shrinkage, causing the finished coating film to have an impaired appearance. Furthermore, the curing can hardly be complete, so that the performance characteristics (e.g. water resistance, impact resistance) of the coating film will unfavorably be deteriorated. Hydroxyl values higher than 200 are also undesirable since, in that case, a large number of unreacted hydroxyl groups remain in the coating film, leading to deteriorated performance characteristics (e.g. water resistance, weather resistance) of the coating film. Said OH-containing vinyl resins may contain one or more carboxyl groups within the molecule. In particular, the carboxyl group can increase the rate of reaction between the hydroxyl group and the amino group, between the hydroxyl group and the silanol or alkoxysilane group, and between one silanol or alkoxysilane group and another silanol or alkoxysilane group, to thereby effectively improve the finish of the coating film. The carboxyl group content should recommendably correspond to about 0 to 50, preferably about 5 to 20 in an acid value of resin. Usable as the OH-containing vinyl resins are, for example, (co)polymers obtained by radical polymerization of at least one of the OH-containing vinyl monomers (a) mentioned below, optionally together with at least one of other polymerizable unsaturated monomers (b) mentioned below. As said OH-containing vinyl monomers (a), there may be mentioned, for example, the following (a-1) to (a-5). (a-1) Hydroxyalkyl vinyl ethers, such as hydroxybutyl vinyl ether; (a-2) Allyl alcohol and methallyl alcohol; (a-3) (Meth)acrylic acid hydroxyalkyl esters, such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate; (a-4) (Poly)alkylene glycol monoacrylates, such as ethylene glycol monoacrylate, and polyethylene glycol monoacrylate; (a-5) Adducts of (a-1) to (a-4) with a lactone (e.g. ε-caprolactone, γ-valerolactone). As other polymerizable unsaturated monomers (b), there may be mentioned, for example, C₁₋₂₄ alkyl or cycloalkyl esters of (meth)acrylic acid, such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, lauryl (meth)acrylate and cyclohexyl (meth)acrylate; COOH-containing compounds such as (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid (anhydride), fumaric acid and 2-carboxyethyl (meth)acrylate; aromatic vinyl compounds such as styrene and vinyltoluene; perfluoroalkyl (meth)acrylates such as perfluorobutylethyl (meth)acrylate, perfluoroisononylethyl (meth)acrylate and perfluorooctylethyl (meth)acrylate; (meth)acrylonitrile; olefins; fluoroolefins; vinyl esters; cyclohexyl or alkyl vinyl ethers; and allyl ethers. Said OH-containing vinyl resins can be produced in the conventional manner, for example by subjecting the above monomer or monomers (a), optionally together with one or more other monomers (b), to polymerization in a substantially inert organic solvent in the presence of a radical polymerization initiator at about 80° to 220°C for about 4 to 18 hours. As the organic solvent, there may be mentioned, for example, aromatic hydrocarbons such as xylene and toluene; esters such as ethyl acetate, propyl acetate and butyl acetate; ketones such as acetone and methyl ethyl ketone; and ethers such as ethylene glycol, cellosolve, butylcellosolve and cellosolve acetate. Such organic solvents may be used either singly or as a mixture of two or more of them. Radical polymerization initiators which can be used include, among others, azo type initiators such as 2,2'-azobisisobutyronitrile and 2,2'-azobis(2,4-dimethylvaleronitrile), and peroxide type initiators such as benzoyl peroxide, lauryl peroxide and tert-butyl peroctoate. The OH-containing vinyl resins have a number average molecular weight of 2,000 to 80,000, preferably 4,000 to 20,000. When the molecular weight is smaller than 2,000, the melt viscosity of the base coat decreases excessively during baking, allowing the orientation of metal flakes to change. As a result, coating films with a metallic tone can hardly be obtained. Molecular weights higher than 80,000 are also undesirable since the workability in coating decreases. The amino resin is used as a crosslinking agent and includes methylol amino resins obtained by reacting an aldehyde with an amino component such as melamine, urea, benzoguanamine, acetoguanamine, stearoguanamine, spiroguanamine or dicyandiamide. The aldehyde is, for example, formaldehyde, paraformaldehyde, acetaldehyde, or benzaldehyde. Products of etherification of these methylol amino resins with an alcohol may also be used. Examples of the alcohol to be used for etherification are methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, 2-ethylbutanol and 2-ethylhexanol, among others. Highly etherified melamine resins, namely melamine resins having, on an average, at least three methyl ether moieties per triazine ring or melamine resins derived therefrom by partial transetherification of the methoxy groups with an alcohol containing 2 or more carbon atoms, can be used as said amino resins. Among these, low-molecular-weight melamines having an average condensation degree of not more than 2 with single nucleus forms accounting for at least 50% by weight are preferred since high solids contents of the base coat comosition can be attained with them. When such melamines are used, a conventional curing catalyst such as p-toluenesulfonic acid should preferably be added. The polyorganosiloxane is used as a crosslinking agent for the OH-containing base coat resin, like the amino resin mentioned above, and further as a self-curing resin, and contains, on an average, at least two groups, preferably 2 to 10 groups, per molecule, each selected from among silanol and alkoxysilane groups. It has a number average molecular weight of from 1,000 to 50,000. The incorporation of a polyorganosiloxane leads to improved curability of the interface layer (mixed layer) between the pigmented base coat and the clear top coat, whereby such coating film performance characteristics as finish appearance and solvent resistance can be improved. The polyorganosiloxane preferably contains a ladder structure portion and can be prepared, for example by subjecting a silane compound essentially comprising a trifunctional silane of the general formula R¹ x Si(OR²) 4-x (I) wherein R¹ and R² may be the same or different and each is a hydrocarbon group containing 1 to 13 carbon atoms and x is 1, to hydrolysis condensation. As the R¹ and R² groups mentioned above, there may be mentioned, for example, methyl, ethyl, propyl, butyl and phenyl. The silane compound to be subjected to hydrolysis condensation may be composed of a trifunctional silane of formula (I) alone or may contain, in addition to the trifunctional silane, a bifunctional or monofunctional silane in which x in formula (I) has a value of 2 or 3, respectively, or both. In said silane compound, the content of the trifunctional silane of formula (I) should preferably be not less than 30% by weight, more preferably not less than 80% by weight. As typical examples of the trifunctional silane of formula (I), there may be mentioned methyltrimethoxysilane, methyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, isobutyltrimethoxysilane, isobutyltriethoxysilane and ethyltrimethoxysilane. As the bifunctional or monofunctional silane which can be used in admixture with the trifunctional silane of formula (I), there may be mentioned, for example, dimethyldimethoxysilane, diphenyldimethoxysilane, diphenyldiethoxysilane, diisobutyldimethoxysilane, diisobutyldipropoxysilane and trimethylmethoxysilane. In the practice of the invention, the polyorganosiloxane should preferably contain, on an average, not only at least two groups per molecule each selected from among silanol and alkoxysilane groups but also at least one epoxy group, preferably 1 to 30 epoxy groups, per molecule. By using an epoxy-containing polyorganosiloxane, the curability of the mixed layer can be further improved to give coating films very excellent in finish appearance, solvent resistance and so forth. The epoxy-containing polyorganosiloxane can be prepared by subjecting to hydrolysis cocondensation a silane representable by the general formula (I) wherein x is 1, 2 or 3 and an epoxy-containing silane of the general formula wherein R³ and R⁴ may be the same or different and each is a hydrocarbon group containing 1 to 13 carbon atoms, y is 1, 2 or 3, G is a group of the formula (III) or (IV) given below. In the above formulas (III) and (IV), R⁵ is a bivalent hydrocarbon group containing 1 to 13 carbon atoms and the R⁶ groups may be the same or different and each is a hydrogen atom or a methyl group. As the R³ and R⁴ groups mentioned above, there may be mentioned, for example, methyl, ethyl, propyl, butyl and phenyl. As the R⁵ group mentioned above, there may be mentioned, for example, methylene, ethylene, propylene, butylene and hexamethylene. The proportions of the silane of formula (I) in which x is 1, 2 or 3 and the silane of formula (II) to be subjected to cocondensation are not critical but, generally, the silane (I):silane (II) weight ratio is within the range of 95:5 to 10:90. It is preferable that, among the silane species to be charged, the silane of formula (I) in which x is 1 and the silane of formula (II) in which y is 1 combinedly account for not less than 30% by weight, more preferably not less than 80% by weight. Typical examples of the silane of formula (I) which is to be used for cocondensation with the epoxy-containing silane of general formula (II) include, in addition to the above-mentioned silanes in which x is 1, silanes in which x is 2 or 3, such as dimethyldimethoxysilane, diphenyldimethoxysilane, diphenyldiethoxysilane, diisobutyldimethoxysilane, diisobutyldipropoxysilane and trimethylmethoxysilane. Typical examples of the silane of formula (II) are γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane and β-(3,4-epoxycyclohexyl)ethyltriethoxysilane. The hydrolysis condensation or cocondensation of the silane compounds mentioned above is carried out by mixing the silane compound(s) with a water-soluble solvent (e.g. alcohol type or cellosolve type solvent) as necessary and allowing the hydrolysis and condensation reactions to proceed in the presence of an inorganic acid such as hydrochloric acid, sulfuric acid or phosphoric acid or an organic acid such as formic acid or acetic acid and in the presence of water, preferably at a pH of not more than 6, at a temperature of about 20° to 100°C with stirring for about 30 minutes to 20 hours. The molecular weight of the polyorganosiloxane can suitably be controlled by adjusting or selecting the amount of water, the kind and amount of catalyst, the reaction temperature and the reaction time, among others. When, in the above-mentioned polyorganosiloxane, the average number of silanol and/or alkoxysilane groups per molecule is below 2, the rate of curing of the base coat film becomes slower than that of the top coat film and the curing of the top coat film begins before curing of the base coat film, so that the top coat film develops defects such as shrinkage and the finished coating film presents an unsatisfactory appearance. When the polyorganosiloxane resin has a number average molecular weight of less than 1,000, the melt viscosity of the base coat becomes excessively low during baking and, as a result, the orientation of metal flakes is disturbed, so that coating films with a metallic tone can hardly be obtained. In the base coat composition, the proportions of the OH-containing base coat resin, amino resin and polyorganosiloxane, which are to serve as binders, based on the total amount (resin solids content) of said three components, are as follows: OH-containing base coat resin 5 to 90% by weight, preferably 30 to 60% by weight, amino resin 5 to 50% by weight, preferably 10 to 40% by weight, and polyorganosiloxane 1 to 40% by weight, preferably 5 to 30% by weight, more preferably 5 to 20% by weight. When the OH-containing base coat resin is used in an amount of less than 5% by weight, the adhesion to substrates becomes insufficient, while, when said amount is above 90% by weight, the appearance and performance characteristics (e.g. water resistance, processability) of the finished coating film become unsatisfactory. When the proportion of the amino resin is below 5% by weight, the crosslink density of the OH-containing resin becomes low and the water resistance and impact resistance become markedly poor. When said proportion exceeds 50% by weight, the mechanical characteristics and the adhesion to substrates are compromised. When the proportion of the polyorganosiloxane is below 1% by weight, the accelerating effect on the rate of curing of the base coat film is slight, hence the finished coating film cannot be improved in appearance, whereas, when said proportion exceeds 40% by weight, the adhesion to substrates becomes unsatisfactory. The metal flakes are used to give a metallic tone to coating films. As the flaky metal, there may be mentioned, for example, aluminum flakes, nickel flakes, copper flakes, brass flakes and chromium flakes. As the mica powder, there may be mentioned pearl mica and pigmented pearl mica, among others. In the base coat composition, metal flakes are incorporated in an amount of about 1 to 20 parts by weight per 100 parts by weight of resin solids. As examples of the organic solvent, there may be mentioned aromatic hydrocarbons such as xylene and toluene, esters such as ethyl acetate, propyl acetate and butyl acetate, ketones such as acetone and methyl ethyl ketone, and ethers such as ethylene glycol, cellosolve, butylcellosolve and cellosolve acetate. Such organic solvents may be used either singly or in the form of a mixture of two or more of them. From the standpoint of finishing performance, the boiling point should preferably be not higher than about 150°C but this is not critical. Other additives for coating composition than those mentioned above which can be incorporated in the base coat composition as necessary include organic pigments, inorganic pigments, pigment dispersing agents, finely divided polymers, ultraviolet absorbers, coated surface modifiers, curing catalysts, cellulose acetate (and derivatives thereof), etc. As pigments which may be incorporated in said coating composition, there may be mentioned, for example, organic pigments (e.g. quinacridone pigments such as quinacridone red, azo pigments such as pigment red, phthalocyanine pigments such as phthalocyanine blue and phthalocyanine green), inorganic pigments (e.g. titanium oxide, barium sulfate, calcium carbonate, baryta, clay, silica), and carbon-based pigments (e.g. carbon black). The base coat composition to be used in the practice of the invention generally has a resin solids content of about 10 to 50% by weight. Clear top coat The top coat is formed from a clear coating composition (B) (top coat composition) comprising, as essential components thereof, a certain OH-containing base resin (hereinafter referred to as "top coat base resin") which further contains one or more groups each selected from a silanol group and certain hydrolyzable groups bound directly to a silicon atom (hereinafter referred to as "hydrolyzable silyl group"), an amino resin and an organic solvent. The hydrolyzable group in the above top coat base resin is a group capable of undergoing hydrolysis in the presence of water to form a silanol group. Said group is selected from C₁₋₅ alkoxy groups; aryloxy groups such as phenoxy, tolyloxy, p-methoxyphenoxy, p-nitrophenoxy and benzyloxy; acyloxy groups such as acetoxy, propionyloxy, butanoyloxy, benzoyloxy, phenylacetoxy and formyloxy; and residues of the formula -N(R⁷)₂, -ON(R⁷)₂, -ON=C(R⁷)₂ or -NR⁸COR⁷ (in which the R⁷ groups are the same or different and each is C₁₋₈ alkyl, aryl or aralkyl and R⁸ is H or C₁₋₈ alkyl). The top coat base resin to be used in the practice of the invention may be an OH-containing resin which further contains at least one group selected from a silanol group and a hydrolyzable silyl group as an additional essential functional group component within the same molecule (hereinafter said resin being sometimes referred to as "single resin") or a mixed resin composed of a resin having at least one group selected from the class consisting of a silanol group and a hydrolyzable silyl group [hereinafter said resin being sometimes referred to as "resin (or polymer) A"] and an OH-containing resin [hereinafter sometimes referred to as "resin (or polymer) B"]. In kind, said resins are selected from vinyl polymers, polyester resins and polyether polyester resins. These resins have a number average molecular weight of 2,000 to 100,000 preferably 4,000 to 80,000, in the case of vinyl polymers, 500 to 20,000, preferably 1,000 to 5,000, in the case of polyester and polyether polyester resins. When the number average molecular weight is below the lower limit mentioned above, the paint film surface tends to develop such defects as shrinkage. When the number average molecular weight is above the upper limit, the workability in painting will be inferior and the paint film surface tends to develop such defects as orange peel. Among the resins mentioned above, vinyl polymers, which are excellent in performance characteristics such as weather resistance, acid resistance and mar resistance, are preferred as the top coat base resin to be used in the practice of the invention. Such vinyl polymers are mentioned below in further detail. As examples of the single resin of the vinyl polymer type, there may be mentioned vinyl copolymers produced by copolymerizing a silanol- and/or hydrolyzable silyl-containing vinyl monomer (c) and an OH-containing vinyl monomer (a), optionally together with some other polymerizable unsaturated monomer (b), and polymers produced by first preparing a functional group-containing vinyl polymer and then reacting said vinyl polymer with a silanol- and/or hydrolyzable silyl-containing silane compound which further has a group complementarily reacting with the functional group of said vinyl polymer (the silanol and/or hydrolyzable silyl group may be the complementarily reacting group), for example polymers produced by reacting a copolymer of glycidyl (meth)acrylate and an OH-containing vinyl monomer (a) and some other vinyl monomer (b) with γ-aminopropyltrimethoxysilane and polymers produced by reacting a copolymer of (meth)acrylic acid, an OH-containing vinyl monomer (a) and some other polymerizable unsaturated monomer (b) with γ-glycidoxypropyltrimethoxysilane. Said vinyl polymers should desirably have, on an average, at least one silanol and/or hydrolyzable silyl group per molecule. When the average number of silanol and/or hydrolyzable silyl groups is less than one per molecule, the curability will be unsatisfactorily low and coating films excellent in weather resistance, acid resistance, mar resistance and other characteristics can hardly be formed. Furthermore, said vinyl polymers should desirably have hydroxyl groups in a content such that the hydroxyl value amounts to about 30 to 300, preferably about 50 to 200. When the hydroxyl value is below about 30, the curability will be low and coating films excellent in weather resistance, acid resistance, mar resistance and other characteristics will not be obtained. Conversely, when the hydroxyl value exceeds 300, a large number of unreacted hydroxyl groups will remain in the coating film, unfavorably leading to decreased water resistance, for instance. As examples of the mixed resin of the vinyl polymer type, there may be mentioned, as polymer A, vinyl polymers produced by (co)polymerizing a silanol- and/or hydrolyzable silyl-containing vinyl monomer (c), optionally with another polymerizable unsaturated monomer (b) and polymers produced by first preparing a functional group-containing vinyl polymer and then reacting said vinyl polymer with a silanol- and/or hydrolyzable silyl-containing silane compound which further has a group complementarily reacting with the functional group of said vinyl polymer (the silanol and/or hydrolyzable silyl group may be the complementarily reacting group), for example polymers produced by reacting a copolymer of glycidyl (meth)acrylate and some other polymerizable unsaturated monomer (b) with γ-aminopropyltrimethoxysilane and polymers produced by reacting a copolymer of (meth)acrylic acid and some other polymerizable unsaturated monomer (b) with γ-glycidoxypropyltrimethoxysilane, and, as polymer B, polymers produced by polymerizing an OH-containing vinyl monomer (a) and copolymers of said vinyl monomer (a) and some other polymerizable unsaturated monomer (b). Said polymer A should desirably have, on an average, at least one silanol and/or hydrolyzable silyl group per molecule. When the average number of silanol and/or hydrolyzable silyl groups is less than one per molecule, the curability will be low and it will be impossible to form coating films excellent in weather resistance, acid resistance, mar resistance, etc. Said polymer B should preferably have a hydroxyl value of not less than about 30. When the hydroxyl value is less than about 30, the curability will be low and paint films excellent in weather resistance, acid resistance, mar resistance and other characteristics can hardly be formed. When the hydroxyl value is excessively high, a large number of unreacted hydroxyl groups will remain in the coating film, causing the water resistance and other properties of the coating film to decrease. Hence, the hydroxyl value should preferably be not more than about 300. Said polymer A and polymer B are admixed in a polymer A/polymer B weight ratio of 90:10 to 10:90, preferably 80:20 to 20:80. Among the single and mixed resins mentioned above, single resins, which can readily form coating films excellent in appearance and performance characteristics with uniform curing, are preferred. As the monomers (a) and (b), there may be mentioned those already mentioned hereinbefore. As the vinyl monomer (c) mentioned above, there may be mentioned, for example, acrylic silane compounds such as γ-(meth)acryloxyethyltrimethoxysilane, γ-(meth)acryloxypropyltrimethoxysilane, γ-(meth)acryloxypropyltriethoxysilane, γ-(meth)acryloxypropyltripropoxysilane, γ-(meth)acryloxypropylmethyldimethoxysilane, γ-(meth)acryloxypropylmethyldiethoxysilane, γ-(meth)acryloxypropylmethyldipropoxysilane, δ-(meth)acryloxybutylphenyldimethoxysilane, δ-(meth)acryloxybutylphenyldiethoxysilane, δ-(meth)acryloxybutylphenyldipropoxysilane, γ-(meth)acryloxypropyldimethylmethoxysilane, γ-(meth)acryloxypropyldimethylethoxysilane, γ-(meth)acryloxypropylphenylmethylmethoxysilane, γ-(meth)acryloxypropylphenylmethylethoxysilane and γ-(meth)acryloxypropyltriacetoxysilane; vinylsilane compounds such as vinyltrimethoxysilane and vinyltriethoxysilane; allylsilane compounds such as allyltriethoxysilane; and styrylsilane compounds such as 2-styrylethyltrimethoxysilane. The top coat base resin to be used in the practice of the invention may be a COOH-containing one. A method of introducing the carboxyl group into said resin comprises, for instance, using a COOH-containing vinyl monomer as the other vinyl monomer (b) component in the vinyl polymer production. Such monomer, when employed, is used generally in an amount such that the COOH-containing vinyl monomer amounts to about 5 to 50% by weight, preferably about 10 to 40% by weight, on the vinyl polymer basis. The amino resin to be used in combination with the above-mentioned top coat base resin includes, among others, those methylol amino resins and etherified amino resins (derived from methylol amino resins by etherification with an alcohol) mentioned above for the pigmented base coat paint composition. Said amino resin is used in an amount of 2 to 100 parts by weight per 100 parts by weight of the base resin. When its amount is less than 2 parts by weight, the water resistance and other characteristics will be unsatisfactory, while, when its amount is above 100 parts by weight, the weather resistance will be adversely affected. Useful as the organic solvent are, for example, hydrocarbon solvents such as heptane, toluene, xylene, octane and mineral spirit; ester solvents such as ethyl acetate, n-butyl acetate, isobutyl acetate, methyl-cellosolve acetate and butylcarbitol acetate; ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone and diisobutyl ketone; alcohol solvents such as ethanol, isopropanol, n-butanol, sec-butanol and isobutanol; and ether solvents such as n-butyl ether, dioxane, ethylene glycol monomethyl ether and ethylene glycol monoethyl ether. The clear coating composition to be used in clear top coat formation in accordance with the invention can be prepared by dissolving or dispersing the above-mentioned top coat base resin and amino resin in an organic solvent. Said composition may be in the form of a nonaqueous dispersion coating composition. Said clear coating composition may contain, when necessary, a curing catalyst for the silanol and/or hydrolyzable silyl group, in addition to the above-mentioned base resins and organic solvent. As the curing catalyst, there may be mentioned, for example, acidic compounds such as p-toluenesulfonic acid, dodecylbenzenesulfonic acid, trichloroacetic acid, phosphoric acid, mono-n-propyl phosphate, monoisopropyl phosphate, mono-n-butyl phosphate, monoisobutyl phosphate, mono-tert-butyl phosphate, monooctyl phosphate, monodecyl phosphate, other monoalkyl phosphates, di-n-propyl phosphate, diisopropyl phosphate, di-n-butyl phosphate, diisobutyl phosphate, di-tert-butyl phosphate, dioctyl phosphate, didecyl phosphate, other dialkyl phosphates, β-hydroxyethyl (meth)acrylate phosphate, mono-n-propyl phosphite, monoisopropyl phosphite, mono-n-butyl phosphite, monoisobutyl phosphite, mono-tert-butyl phosphite, monooctyl phosphite, monodecyl phosphite, other monoalkyl phosphites, di-n-propyl phosphite, diisopropyl phosphite, di-n-butyl phosphite, diisobutyl phosphite, di-tert-butyl phosphite, dioctyl phosphite, didecyl phosphite, other dialkyl phosphites; tetraisopropyl titanate, tetrabutyl titanate, other titanium-containing compounds; tin octanoate, dibutyltin diacetate, dibutyltin dioctanoate, dibutyltin dilaurate, dibutyltin dimaleate, other tin-containing compounds; and basic compounds such as butylamine, tert-butylamine, dibutylamine, hexylamine, ethylenediamine, triethylamine, isophoronediamine, imidazole, lithium hydroxide, sodium hydroxide, potassium hydroxide and sodium methylate. At least one of these is used. These curing catalysts may be used in an amount of about 0.05 to 20 parts by weight, preferably about 0.1 to 10 parts by weight, per 100 parts by weight of the top coat base resin. When hexamethoxymethylmelamine or an etherified melamine resin derived therefrom by partial or complete substitution of the methoxy groups with a C₃ or higher alcohol is used as the amino resin, a strongly acidic catalyst, such as p-toluenesulfonic acid or dodecylbenzenesulfonic acid, is generally used as the curing catalyst for the reaction of such resin with the hydroxyl group. Said strongly acidic catalyst is neutralized (blocked) with an amine compound such as triethylamine, diethanolamine or 2-amino-2-methylpropanol for providing storage stability and enabling the use of the coating composition as a one-pack type paint. This neutralized (blocked) strongly acidic catalyst can serve as a curing catalyst for alkoxysilane groups as well. Thus, at a baking temperature of 100°C or above, the neutralized (blocked) strongly acidic catalyst serves as a common catalyst for the reaction of resins and the reaction of alkoxysilanes. The clear coating composition may contain, as necessary, ultraviolet absorbers, antioxidants, light stabilizers, fine polymer particles and various other additives for coating compositions. The clear coating composition (B) to be used in the practice of the invention should generally have a resin solids content of about 35 to 70% by weight. The method of this invention can be practiced by applying the pigmented base coat composition (A) to a substrate such as a coated surface prepared by electrodeposition coating of a steel sheet (after chemical conversion treatment) followed by or without a subsequent intercoating, or a coated surface prepared by applying a primer to any of various plastic materials with or without subsequent intercoating, and then applying the top clear coat composition (B). The electrodeposition coating composition and the intercoating composition are baked generally at 140° to 190°C for 30 to 90 minutes, although the baking conditions may vary depending on the composition types. The pigmented base coat composition and top clear coat composition can be applied in the conventional manner, for example using an electrostatic or non-electrostatic coating machine or equipment. The thickness of the pigmented base coat film is preferably about 10 to 50 µm (after curing). After application of base coat composition, the coating film is dried by allowing to stand at room temperature for several minutes or by forced drying at about 50° to 80°C for several minutes, and then the clear coat composition is applied. The clear coat film preferably has a thickness of 20 to 100 µm (after curing). The coated material is heated at about 120° to 180°C for about 30 to 90 minutes, whereby curing can be attained. In the method of this invention, the pigmented base coat composition (A) to be finished by the two-coat one-bake technique contains, as a binder component, a polyorganosiloxane having a number average molecular weight of 1,000 to 50,000 and having at least two groups, per molecule, which are each selected from a silanol group and an alkoxysilane group, so that, upon heating, the reaction between silanol groups (inclusive of silanol groups resulting from hydrolysis of alkoxysilane groups) and the reaction of a silanol group, an amino group in the amino resin and a hydroxyl group in the OH-containing resin occur rapidly. Said reactions proceed faster than the curing reaction of the clear coating composition (B), so that, in the early stage of baking, the viscosity of the base coat composition (A) increases more rapidly than that of the clear coat composition (B). It is presumable that, for this reason, a base coat film durable to the shrinking force of the clear coat film as resulting from curing of the clear coating composition and volume change thereof due to solvent evaporation is formed and, as a result, the coating film acquires an improved finish appearance. Furthermore, the coating film formed shows excellent coating film performance characteristics (solvent resistance, weather resistance, mar resistance etc.). In addition, even when said pigmented base coat composition (A) is applied again to the cured clear coat film for recoating, an improved adhesion between both the coating films can be observed. The following examples are further illustrative of the present invention. In the examples, "part(s)" and "%" are "part(s) by weight" and "% by weight", respectively. Production Example 1 A reaction vessel was charged with 0.29 mole of isophthalic acid, 0.23 mole of phthalic acid, 0.43 mole of hexahydrophthalic acid, 0.4 mole of trimethylolpropane, 0.6 mole of neopentyl glycol and 0.1 mole of coconut oil fatty acid and the condensation polymerization was carried out at 200-230°C to give a coconut oil-modified polyester resin with an acid value of 8 and a hydroxyl value of 72. A 60% resin solution (a-1) was prepared by adding 43 parts of xylene to 100 parts of said polyester resin. Its viscosity was Y (25°C, Gardner bubble viscosity). Production Example 2 An OH-containing acrylic resin solution (a-2) was prepared by reacting 30 parts of methyl methacrylate, 53 parts of n-butyl methacrylate, 15 parts of 2-hydroxyethyl acrylate and 2 parts of acrylic acid in an organic solvent composed of 85 parts of xylene and 15 parts of n-butanol. The number average molecular weight of the resin was 6,600, the resin solids content 50%, and the Gardner bubble viscosity V. Production Example 3 An OH-containing acrylic resin solution (a-3) was prepared by reacting 30 parts of methyl methacrylate, 45 parts of n-butyl methacrylate and 25 parts of 2-hydroxyethyl acrylate in an organic solvent composed of 85 parts of xylene and 15 parts of n-butanol. The number average molecular weight was 5,000, the resin solids content 50%, and the Gardner bubble viscosity T. Production Example 4 Phenyltrimethoxysilane 198 parts Deionized water 54 parts 98% Sulfuric acid 0.002 part The above materials were mixed up and subjected to reaction at 60°C for 5 hours, the byproduct methanol was then distilled off under reduced pressure, and xylene was added to the remaining mixture to give a polyorganosiloxane solution (b-1) with a solids content of 50% and a Gardner viscosity of AB. The polyorganosiloxane obtained had a number average molecular weight of about 5,000 and had, on an average, 6 silanol groups per molecule. Production Example 5 Methyltrimethoxysilane 136 parts Diphenyldimethoxysilane 182 parts Deionized water 90 parts 60% Phosphoric acid 1 part The above materials were mixed up and subjected to reaction at 60°C for 10 hours, the byproduct methanol was distilled off under reduced pressure, and butyl acetate was added to the remaining mixture to give a polyorganosiloxane solution (b-2) with a solids content of 50% and a Gardner viscosity of D. The polyorganosiloxane obtained had a number average molecular weight of about 15,000 and had, on an average, 10 silanol groups per molecule. Production Example 6 Diphenyldimethoxysilane 182 parts β-(3,4-Epoxycyclohexyl)ethyltrimethoxysilane 186 parts Deionized water 108 parts 60% Phosphoric acid 1 part The above materials were mixed up and subjected to reaction at 60°C for 15 hours, the byproduct methanol was then distilled off under reduced pressure, and butyl acetate was added to the remaining mixture to give a polyorganosiloxane solution (b-3) with a solids content of 50% and a Gardner viscosity of G. The polyorganosiloxane obtained had a number average molecular weight of about 2,000 and had, on an average, 10 silanol groups and 7 epoxy groups per molecule. Production Example 7 Phenyltrimethoxysilane 198 parts γ-Glycidoxypropyltrimethoxysilane 236 parts Deionized water 108 parts 60% Sulfuric acid 0.1 part The above materials were mixed up and subjected to reaction at 60°C for 10 hours, the byproduct methanol was distilled off under reduced pressure, and xylene was added to the remaining mixture to give a polyorganosiloxane solution (b-4) with a solids content of 50% and a Gardner viscosity of DE. The polyorganosiloxane obtained had a number average molecular weight of about 8,000 and had, on an average, 6 silanol groups and 20 epoxy groups per molecule. Pigmented base coat composition Base coat composition 1 A mixture of 100 parts of the resin solution (a-1) obtained in Production Example 1 (60 parts as solids), 33.3 Parts of U-Van® 20SE (Note 1) (20 parts as solids), 40 parts of the polyorganosiloxane solution (b-1) obtained in Production Example 4 (20 parts as solids), 20 parts of an aluminum paste and 1 part of Reibo® #3 (Note 2) was stirred and then a mixed solvent composed of Swazol® #1000 (Note 3) and ethyl acetate (20/80 by weight) was added to adjust the viscosity of the coating composition to 15 seconds (Ford cup #4/20°C). The coating composition thus prepared was subjected to testing. (Note 1) U-Van® 20SE, trademark of Mitsui Toatsu Chemicals, a butyl etherified melamine resin solution with a solids content of about 60%. (Note 2) Reibo® #3, trademark of Reibo Chemical, a silicone-based surface modifier solution (active ingredient content about 1%). (Note 3) Swazol® #1000, trademark of Cosmo Oil Co., a petroleum-derived aromatic solvent. Base coat composition 2 Base coat composition 2 was prepared according to the same formulation as mentioned above for base coat composition 1 except that the polyorganosiloxane solution (b-2) was used in lieu of the polyorganosiloxane solution (b-1). Base coat composition 3 Base coat composition 3 was prepared according to the same formulation as mentioned above for base coat composition 1 except that the polyorganosiloxane solution (b-3) was used in lieu of the polyorganosiloxane solution (b-1). Base coat composition 4 Base coat composition 4 was prepared according to the same formulation as mentioned above for base coat composition 1 except that the polyorganosiloxane solution (b-4) was used in lieu of the polyorganosiloxane solution (b-1). Base coat composition 5 A mixture of 112 parts of the resin solution (a-2) obtained in Production Example 2 (56 parts as solids), 40 parts of U-Van® 20SE (24 parts as solids), 40 parts of the polyorganosiloxane solution (b-1) obtained in Production Example 4 (20 parts as solids), 20 parts of an aluminum paste and 1 part of Reibo® #3 was stirred and then a 30:70 (by weight) mixture of Swazol® #1000 and ethyl acetate was added to adjust the viscosity of the coating composition to 15 seconds (Ford cup #4/20°C). The resultant composition was subjected to testing. Base coat composition 6 Base coat composition 6 was prepared according to the same formulation as mentioned above for base coat composition 5 except that the resin solution (a-3) was used in lieu of the resin solution (a-2). Base coat composition 7 Base coat composition 7 was prepared according to the same formulation as mentioned above for base coat composition 5 except that the polyorganosiloxane solution (b-2) was used in lieu of the polyorganosiloxane solution (b-1). Base coat composition 8 Base coat composition 8 was prepared according to the same formulation as mentioned above for base coat composition 5 except that the polyorganosiloxane solution (b-3) was used in lieu of the polyorganosiloxane solution (b-1). Base coat composition 9 Base coat composition 9 was prepared according to the same formulation as mentioned above for base coat composition 5 except that the resin solution (a-3) was used in lieu of the resin solution (a-2) and that the polyorganosiloxane solution (b-3) was used in lieu of the polyorganosiloxane solution (b-1). Base coat composition 10 Base coat composition 10 was prepared according to the same formulation as mentioned above for base coat composition 5 except that the polyorganosiloxane solution (b-4) was used in lieu of the polyorganosiloxane solution (b-1). Base coat composition 11 Base coat composition 11 was prepared according to the same formulation as mentioned above for base coat composition 1 except that the polyorganosiloxane solution (b-1) was not used. Base coat composition 12 Base coat composition 12 was prepared according to the same formulation as mentioned above for base coat composition 5 except that the polyorganosiloxane solution (b-1) was not used. Production Example 8 Styrene 100 parts n-Butyl acrylate 450 parts 2-Ethylhexyl methacrylate 200 parts 2-Hydroxyethyl methacrylate 150 parts γ-Methacryloxypropyltrimethoxysilane 100 parts Azobisisobutyronitrile 40 parts A mixture composed of the above-materials was added dropwise to the same quantity (as said mixture) of xylene at 110°C over 3 hours and maturation was effected at the same temperature for 2 hours to give a polymer solution. The clear polymer obtained had a number average molecular weight of 6,000. Production Example 9 Styrene 150 parts n-Butyl methacrylate 500 parts 1,4-Butanediol monoacrylate 200 parts γ-Methacryloxypropyltrimethoxysilane 150 parts Azobisisobutyronitrile 40 parts A mixture composed of the above materials was added dropwise to a mixed solvent composed of 500 parts of xylene and 500 parts of n-butanol at 120°C over 3 hours, followed by 2 hours of maturation at the same temperature, to give a polymer solution. The clear polymer obtained had a number average molecular weight of 6,000. Clear top coat composition Clear top coat composition 1 Solution of Production Example 8 (solids content 50%) 140 parts Cymel® 303 (note 4) 30 parts Nacure® 5225 (note 5) 1.5 parts Surface modifier (BYK Chemie's BYK-300® solution; hereinafter the same shall apply) 0.1 part Ultraviolet absorber (Ciba-Geigy's Tinuvin® 900; hereinafter the same shall apply) 1.0 part A mixture composed of the above materials was diluted with Swazol® #1000 to thereby adjust the viscosity (Ford cup #4, 20°C) to 25 seconds. (Note 4) Cymel® 303: fully methoxylated melamine resin, available from Mitsui Cyanamid Ltd. (Note 5) Nacure® 5225: dimethyloxazolidine-neutralized dodecylbenzenesulfonic acid, available from King Industries. Clear top coat composition 2 Solution of Production Example 9 (solids content 50%) 160 parts Cymel® 303 20 parts Nacure® 5225 2 parts Surface modifier 0.1 part Ultraviolet absorber 1.0 part The procedure for clear top coat composition 1 was followed using the above materials. Clear top coat composition 3 Solution of Production Example 9 (solids content 50%) 140 parts 60% U-Van® 20SE 50 parts Dibutyltin dilaurate 0.5 part Surface modifier 0.1 part Ultraviolet absorber 1.0 part The procedure for clear top coat composition 1 was followed using the above materials. Examples 1 to 30 and Comparative Examples 1 to 6 Preparation of coated sheets An epoxy resin-based cationic electrodeposition coating film (25 µm) was formed on dull steel sheets (after chemical conversion treatment) and, after curing thereof by heating at 170°C for 30 minutes, Luga Bake® AM (trademark of Kansai Paint, an automotive coating composition of the polyester resin/melamine resin type) was applied to a film thickness of 30 µm (after drying) for intercoating, which was followed by 30 minutes of baking at 140°C. The coated surface of each sheet was subjected to wet sanding using a #400 sand paper and, after draining and drying, wiped using petroleum benzine, and used as a substrate. Using the air spray coating technique, the pigmented base coat composition was applied and, three minutes thereafter, the clear top coat composition was immediately applied. The film thicknesses were 15 to 20 µm and 35 to 45 µm, respectively, after drying. Then, after 10 minutes of standing at room temperature, baking was carried out at 140°C for 30 minutes. In recoatability testing, the baking was carried out at 160°C for 30 minutes and the recoat layer was baked at 120°C for 30 minutes. Typical performance characteristics and the appearance of each coating film thus obtained are shown in Table 1.

Claims: A method of forming a coating film by forming in sequence a pigmented base coat and a clear top coat on a substrate, followed by finishing by the two-coat one-bake technique, the method being characterized by using, as a coating composition for pigmented base coat formation, a composition (A) comprising, as essential components thereof, (1) an OH-containing resin selected from OH-containing polyester resins having a hydroxyl value of from 20 to 200 and a number average molecular weight of from 500 to 80,000 and OH-containing vinyl resins having a hydroxyl value of from 20 to 200, and a number average molecular weight of from 2,000 to 80,000, (2) an amino resin, (3) a polyorganosiloxane which has, on an average, at least two groups, per molecule, each selected from a silanol group and an alkoxysilane group and has a number average moelcular weight of from 1,000 to 50,000, (4) a flaky metal powder and/or a mica powder, and (5) an organic solvent, the proportions of the OH-containing resin, amino resin and polyorganosiloxane being 5 to 90% by weight, 5 to 50% by weight and 1 to 40% by weight, respectively, based on the total amount (resin solids) of the OH-containing resin, amino resin and polyorganosiloxane; and using, as a coating composition for clear top coat formation, a composition (B) comprising, as essential components thereof, (1) an OH-containing base resin, which further contains at least one group selected from a silanol group and a hydrolyzable group bound directly to a silicon atom, said OH-containing base resin being selected from vinyl polymers having a number average molecular weight of from 2,000 to 100,000, polyester resins having a number average molecular weight of from 500 to 20,000 and polyether polyester resins having a number average molecular weight of from 500 to 20,000, and said hydrolyzable group being selected from C₁₋₅ alkoxy groups, aryloxy groups, acyloxy groups and residues of the formula -N(R⁷)₂, -ON(R⁷)₂, -ON=C(R⁷)₂ and -NR⁸COR⁷ (in which the R⁷ groups are the same or different and each is selected from C₁₋₈ alkyl, aryl or aralkyl and R⁸ is H or C₁₋₈ alkyl), (2) an amino resin, and (3) an organic solvent, the proportion of the amino resin being 2 to 100 parts by weight per 100 parts by weight of the base resin. A method of forming coating films as claimed in Claim 1, wherein the polyorganosiloxane contained in the pigmented base coat composition further contains, on an average, at least one epoxy group per molecule. A method of forming coating films as claimed in Claim 1, wherein the base resin contained in the clear top coat composition is (i) on OH-containing resin which further contains at least one group selected from a silanol group and a hydrolyzable group bound directly to a silicon atom within the same molecule or (ii) a mixed resin composed of a resin containing at least one group selected from a silanol group and a hydrolyzable group bound directly to a silicon atom and an OH-containing resin.

Inventors: ITO SATORU, KASARI AKIRO, NAKAMURA SHIGERU, ITO, SATORU, KASARI, AKIRO, NAKAMURA, SHIGERU
Applicants: KANSAI PAINT CO LTD, KANSAI PAINT CO. LTD.
Application year: 1992
Main classification: C09D201
IPC codes: B05D5, C09D167, C08L83, C08L61, C09D133, C09D143, B05D7