JP2006008936A - Clear coating composition for automobile, and method for forming multi-layered coating film by using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a clear coating composition for automobile forming a coating film which has an excellent scratch resistance sustained for long time in a high level and excellent stain resistance such as water stain, etc. <P>SOLUTION: The invention relates to the clear coating composition for automobile comprising a polymer having hydroxyl group and a polyfunctional isocyanate compound, wherein the clear coating composition comprises 25-50 % by mass of a soft segment represented by general formula (1): -(CH<SB>2</SB>)<SB>n</SB>- (1), (In the formula n is an integer of more than 4), per total solid content of the polymer having hydroxyl group and the polyfunctional isocyanate composition, and are prepared to have 60-80°C of dynamic glass transition point and 0.8×10<SP>-3</SP>mol/cc of crosslink density. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a clear coating composition for automobiles and a method for forming a multilayer coating film using the same.

  As a binder used in a top coating for automobiles, it is common to use a combination of a hydroxyl group-containing polymer and a melamine polymer curing agent. However, the cured coating film obtained by using such a melamine polymer as a curing agent is generally inferior in acid resistance, and is particularly easily deteriorated by acid rain which has become a problem in recent years, resulting in problems in appearance. Had. Since the coating film obtained by using melamine polymer as a curing agent in this way is inferior in acid resistance due to the triazine ring in the melamine polymer, the disadvantage is inferior in acid resistance as long as melamine polymer is used as a curing agent. Was not resolved.

  In order to solve such drawbacks, JP-A-2-45577 (Patent Publication 1) and JP-A-3-287650 (Patent Publication 2) describe coating compositions that do not use a melamine polymer. ing. Such a coating composition has an ester bond generated by reacting a carboxylic acid group and an epoxy group as a cross-linking point, and therefore has good acid resistance and sufficient weather resistance as a top coat film for automobiles. ing.

Further, JP 2003-253191 A (Patent Publication 3) discloses a half ester acid group-containing acrylic copolymer, an epoxy group-containing acrylic copolymer, a carboxyl group-containing polyester polymer, and a carboxyl group-containing acrylic polymer. consists of at least one carboxyl group-containing polymer is selected from the group consisting of the formula :-( CH 2) _n - _(wherein, n own the soft segment portion represented by represents the integer of 4 or more) A clear coating composition having a fixed amount is described. Such a clear coating composition is capable of forming a coating film having physical properties with a good balance in all properties of scratch resistance, acid resistance and solvent resistance.
JP-A-2-45577 JP-A-3-287650 JP 2003-253191 A

  The present invention is an automobile capable of forming a coating film that is based on the above-described conventional technology and that has excellent scratch resistance maintained at a high level for a long period of time and at the same time has excellent stain resistance against water stains and the like. It is an object of the present invention to provide a clear coating composition for use, and a method for forming a multilayer coating film using the same.

  As a result of intensive studies to achieve the above object, the inventors of the present invention can obtain a soft segment portion with a predetermined amount in a clear coating composition containing a hydroxyl group-containing polymer and a polyfunctional isocyanate compound. The present inventors have found that the above object can be achieved by setting the dynamic glass transition point and the crosslink density of the coating film to a predetermined range, and have completed the present invention.

That is, the clear paint composition for automobiles of the present invention is a clear paint composition containing a hydroxyl group-containing polymer and a polyfunctional isocyanate compound, and the clear paint composition has the following general formula (1):
_{- (CH 2) n - (} 1)
(In the formula, n represents an integer of 4 or more.)
In the solid content of the hydroxyl group-containing polymer and the polyfunctional isocyanate compound, and the resulting glass film has a dynamic glass transition point of 60-80. It is characterized by being prepared so as to have a crosslinking density of 0.8 × 10 ⁻³ mol / cc or more at a temperature of 0 ° C.

  In the automotive clear coating composition of the present invention, the hydroxyl group-containing polymer is preferably at least one selected from the group consisting of a hydroxyl group-containing acrylic copolymer and a hydroxyl group-containing polyester copolymer.

  The method for forming a multilayer coating film of the present invention is a method for forming a multilayer coating film having a top coat on an object to be coated, wherein the clear coating composition for automobiles of the present invention is applied to the top coat. It is the method characterized by painting as.

  In the method for forming a multilayer coating film of the present invention, a base coating composition is applied to the object to be coated to obtain a base uncured coating film, and then the automobile clear coating composition is applied to the base uncured coating film. It is preferable that a clear uncured coating film is obtained by coating the base uncured coating film and the clear uncured coating film is heated and cured simultaneously.

  The dynamic glass transition point (Tm) according to the present invention is determined by the following method. That is, using a forced stretching vibration type viscoelasticity measuring device (Orientec Co., Ltd., Vibron), the tan δ at each temperature is obtained from the phase difference generated between the stress generated during the temperature rise and the vibration strain. The temperature giving the value was Tm. The measurement frequency was 11 Hz.

  Moreover, the crosslinking density concerning this invention is calculated | required with the following method. That is, the dynamic elastic modulus (E ′) at the time of temperature rise was obtained under the same conditions as those for obtaining Tm, and the temperature was calculated from the minimum value and the temperature at which E ′ was minimized by the following equation.

      E ′ = 3 nRT (n: crosslink density, R: gas constant, T: absolute temperature)

  According to the present invention, an automotive clear coating composition capable of forming a coating film having excellent scratch resistance maintained at a high level over a long period of time and at the same time having excellent stain resistance against water stains, In addition, it is possible to provide a method for forming a multilayer coating film using the same.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

First, the clear coating composition of the present invention will be described. That is, the automotive clear coating composition of the present invention is a clear coating composition containing a hydroxyl group-containing polymer as a film-forming resin and a polyfunctional isocyanate compound as a curing agent. The product has the following general formula (1):
_{- (CH 2) n - (} 1)
(In the formula, n represents an integer of 4 or more.)
The soft segment portion is represented by 25 to 50% by mass with respect to the total solid content of the hydroxyl group-containing polymer and polyfunctional isocyanate compound, and the obtained coating film has a Tm of 60 to 80 ° C. and is crosslinked. The density is adjusted to 0.8 × 10 ⁻³ mol / cc or more.

  The curing system in the clear coating composition of the present invention containing the hydroxyl group-containing polymer and the polyfunctional isocyanate compound is as follows. That is, the isocyanate group of the polyfunctional isocyanate compound reacts with the hydroxyl group in the hydroxyl group-containing polymer by heating to form a crosslinking point, and the hydroxyl group-containing polymer is crosslinked through the polyfunctional isocyanate compound to cure. Proceeding to achieve high crosslink density. And in the coating film heat-cured and shape | molded after coating-film formation using the clear coating composition of this invention, the said soft segment part will exist in a principal chain or a crosslinked chain. Thus, the presence of the soft segment portion in the main chain or the crosslinked chain effectively imparts excellent scratch resistance to the coating film.

  The amount of the soft segment part represented by the general formula (1) is a numerical value representing the mass ratio of the soft segment part contained in the total solid content of the hydroxyl group-containing polymer and the polyfunctional isocyanate compound. The soft segment part represented by the general formula (1) needs to have n in the general formula (1) of 4 or more. This is because when n is 3 or less, it does not have sufficient properties as a soft segment. N represents an integer of 4 or more, and is more preferably 4 or 5. When n = 4 or 5, it is easy to produce the monomer and polymer having the soft segment represented by the general formula (1), and the handling property tends to be excellent. Further, two or more types of soft segment portions having different values of n may be used in combination.

  In the clear coating composition of the present invention, the effect of improving the scratch resistance of the coating film is obtained by having the soft segment portion in the above ratio. When the proportion of the soft segment portion is less than 25% by mass, the effect of scratch resistance cannot be sufficiently obtained. On the other hand, when the proportion of the soft segment exceeds 50% by mass, there is a problem that sufficient acid resistance and solvent resistance cannot be obtained. Moreover, it is preferable that the minimum of the ratio of the said soft segment part is 30 mass%, and on the other hand, it is preferable that the upper limit of the ratio of the said soft segment part is 45 mass%.

  The soft segment part only needs to be present in at least one of the hydroxyl group-containing polymer and the polyfunctional isocyanate compound, and the reaction between the free isocyanate group after baking and curing of the coating film and water reduces the elasticity of the coating film, resulting in scratch resistance. It is preferable that it exists in a hydroxyl-containing polymer at least from a viewpoint that there exists a tendency for property to fall. When the soft segment part is present in the hydroxyl group-containing polymer, it is introduced into the hydroxyl group-containing polymer by blending the monomer having the soft segment part represented by the general formula (1) during the polymerization reaction. be able to.

  The content of the soft segment portion in the hydroxyl group-containing polymer is theoretically calculated based on the blending amount of the monomer composition used for polymerization and the amount of the soft segment portion contained in the monomer. It can be calculated by

  The hydroxyl group-containing polymer used as the film-forming resin in the clear coating composition of the present invention is not particularly limited as long as it is a polymer containing a hydroxyl group. For example, a hydroxyl group-containing acrylic copolymer, a hydroxyl group-containing polyester copolymer is used. Examples thereof include a polymer, a hydroxyl group-containing alkyd resin, and a hydroxyl group-containing silicon resin. Such a hydroxyl group-containing polymer has only to have a hydroxyl group, and may further have a carboxyl group, an epoxy group, or the like. The hydroxyl group-containing polymer according to the present invention preferably has a hydroxyl value of 50 to 300 mgKOH / g, an acid value of 5 to 32 mgKOH / g, and a number average molecular weight (Mn) of 1000 to 10,000. When the hydroxyl value and the acid value are less than the lower limit, the curability of the coating film tends to decrease, and when the upper limit is exceeded, the water resistance of the coating film tends to decrease. Further, if the Mn is less than the lower limit, the strength of the coating film tends to decrease. On the other hand, if the Mn exceeds the upper limit, the viscosity of the coating tends to increase and the appearance of the coating film tends to decrease. In addition, the hydroxyl value and acid value in this specification are values in terms of solid content. Moreover, it is preferable that the content rate of the said soft segment part in the hydroxyl-containing polymer which concerns on this invention is 7-50 mass% with respect to the solid content whole quantity. Such a hydroxyl-containing polymer may be used independently and may use 2 or more types together.

  Suitable hydroxyl group-containing acrylic copolymers for the present invention include those obtained by copolymerizing a hydroxyl group-containing ethylenically unsaturated monomer and other ethylenically unsaturated monomers. The hydroxyl group-containing acrylic copolymer according to the present invention preferably has a number average molecular weight (Mn) of 1000 to 10,000, and more preferably 1100 to 8000. If the number average molecular weight is less than the above lower limit, the coating workability and the layering property with the clear coating film tend to be reduced. On the other hand, if the upper limit is exceeded, the non-volatile content during coating tends to be low, and the workability tends to deteriorate. . The number average molecular weight of the hydroxyl group-containing acrylic copolymer is particularly preferably in the range of 1200 to 7000 from the viewpoint of the appearance of the coating film. The hydroxyl value of the hydroxyl group-containing acrylic copolymer according to the present invention is preferably 50 to 280 mgKOH / g, and more preferably 70 to 260 mgKOH / g. If the hydroxyl value exceeds the upper limit, the water resistance tends to decrease when the coating film is formed. On the other hand, if the hydroxyl value is less than the lower limit, the curability of the coating film tends to decrease. Furthermore, the acid value of the hydroxyl group-containing acrylic copolymer according to the present invention is preferably 7 to 32 mgKOH / g, and more preferably 10 to 27 mgKOH / g. When the acid value exceeds the above upper limit, the water resistance tends to decrease when the coating film is formed. On the other hand, when the acid value is less than the lower limit, the curability of the coating film tends to decrease. Moreover, it is preferable that the content rate of the said soft segment part in the hydroxyl-containing acrylic copolymer which concerns on this invention is 7-50 mass% with respect to the solid content whole quantity.

  The hydroxyl group-containing acrylic copolymer according to the present invention can be obtained by copolymerizing a hydroxyl group-containing ethylenically unsaturated monomer and another ethylenically unsaturated monomer. Based on the total amount of the ethylenically unsaturated monomer used to produce the copolymer, the hydroxyl group-containing ethylenically unsaturated monomer is 5 to 60% by mass (more preferably 8 to 50% by mass), and other ethylenically unsaturated monomers. The saturated monomer is preferably 95 to 40% by mass (more preferably 92 to 50% by mass). If the content of the hydroxyl group-containing ethylenically unsaturated monomer is less than the lower limit, the production stability tends to be lowered. On the other hand, if the content exceeds the upper limit, the water resistance tends to be lowered when the coating film is formed.

  Specific examples of the hydroxyl group-containing ethylenically unsaturated monomer include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, allyl alcohol, methacryl alcohol, and These and lactones (β-propiolaclone, dimethylpropiolactone, butyllactone, γ-valerolactone, ε-caprolactone, γ-caprolactone, γ-caprolactone, crotolactone, δ-valerolactone, δ-caprolactone, etc. ) And the like. Further, from the viewpoint of further improving the scratch resistance of the resulting coating film, it is preferable to use one having the soft segment portion as at least a part of the hydroxyl group-containing ethylenically unsaturated monomer. It is particularly preferable to use an adduct of (meth) acrylate, 2-hydroxyethyl (meth) acrylate and ε-caprolactone. Such hydroxyl group-containing ethylenically unsaturated monomers can be used alone or in admixture of two or more.

  Although it does not specifically limit as said other ethylenically unsaturated monomer, First, the ethylenically unsaturated monomer which has a carboxyl group can be mentioned. Examples thereof include (meth) acrylic acid derivatives {e.g., acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, acrylic acid dimer, α-hydro-ω-((1 -Oxo-2-propenyl) oxy) poly (oxy (1-oxo-1,6-hexanediyl)) etc.}; and unsaturated dibasic acids, their half esters, half amides and half thioesters {eg maleic acid, Fumaric acid, itaconic acid, its half ester, half amide, half thioester, etc.}. Furthermore, examples of ethylenically unsaturated monomers other than those having a carboxyl group include (meth) acrylate ester monomers {eg, methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl acrylate , T-butyl acrylate, 2-ethylhexyl (meth) acrylate, lauryl methacrylate, phenyl acrylate, isobornyl (meth) acrylate, cyclohexyl methacrylate, t-butylcyclohexyl (meth) acrylate, dicyclopentadienyl (meth) acrylate, dihydro Dicyclopentadienyl (meth) acrylate, etc.}, polymerizable aromatic compounds (eg, styrene, α-methylstyrene, vinyl ketone, t-butylstyrene, parachlorostyrene) Vinyl naphthalene, etc.), polymerizable nitriles (eg, acrylonitrile, methacrylonitrile, etc.), α-olefins (eg, ethylene, propylene, etc.), vinyl esters (eg, vinyl acetate, vinyl propionate, etc.), dienes (eg, butadiene) , Isoprene, and the like) and, if necessary, an isocyanate group-containing monomer. Such other ethylenically unsaturated monomers can be used alone or in admixture of two or more.

  A hydroxyl group-containing acrylic copolymer suitable for the present invention can be obtained by copolymerizing the above hydroxyl group-containing ethylenically unsaturated monomer and other ethylenically unsaturated monomers, but the polymerization method is not particularly limited, Ordinary methods described in known literature such as solution radical polymerization can be used. For example, a method of stirring while dropping a suitable radical polymerization initiator and a monomer mixed solution into a suitable solvent at a polymerization temperature of 60 to 160 ° C. over 2 to 10 hours can be mentioned. The radical polymerization initiator that can be used here is not particularly limited as long as it is usually used in polymerization, and an azo compound (for example, dimethyl-2,2′-azobisisobutyrate), a peroxide (for example, t-butylperoxy-2-ethylhexanoate) and the like. The amount of such initiator is generally from 0.1 to 10% by weight, preferably from 0.5 to 8% by weight, based on the total amount of unsaturated monomers. Moreover, the solvent which can be used here will not be specifically limited if it does not have a bad influence on reaction, For example, alcohol, a ketone, hydrocarbon solvent (for example, propylene glycol monomethyl ether acetate, xylene) etc. are mentioned. Furthermore, in order to adjust the molecular weight, a mercaptan such as lauryl mercaptan or a chain transfer agent such as α-methylstyrene dimer can be used as necessary.

  Examples of the hydroxyl group-containing polyester copolymer suitable for the present invention include those obtained by polycondensation of polyvalent carboxylic acid and / or acid anhydride and polyhydric alcohol. A polymer obtained by adding can also be used. The hydroxyl group-containing polyester copolymer according to the present invention preferably has a number average molecular weight (Mn) of 1000 to 4500. If the number average molecular weight is less than the lower limit, sufficient curing tends not to be obtained. On the other hand, if the number average molecular weight exceeds the upper limit, the smoothness becomes insufficient and a good appearance cannot be obtained, and at the same time the viscosity at the time of coating becomes too high. There is a tendency. Moreover, it is preferable that the hydroxyl value of the hydroxyl-containing polyester copolymer concerning this invention is 70-300 mgKOH / g. If the hydroxyl value exceeds the upper limit, the elasticity tends to decrease and chipping resistance tends to be poor, whereas if it is less than the lower limit, curability tends to be poor. Furthermore, the acid value of the hydroxyl group-containing polyester copolymer according to the present invention is preferably 5 to 20 mgKOH / g. When the acid value exceeds the upper limit, the water resistance tends to decrease, and when it is less than the lower limit, the curability of the coating film tends to decrease. Moreover, it is preferable that the content rate of the said soft segment part in the hydroxyl-containing polyester copolymer which concerns on this invention is 7-50 mass% with respect to the solid content whole quantity.

  The hydroxyl group-containing polyester copolymer according to the present invention can be obtained by polycondensation of polyvalent carboxylic acid and / or acid anhydride and polyhydric alcohol. The molar ratio of acid groups and / or acid anhydride groups is preferably 1.105-2. If the hydroxyl group is less than the lower limit, the number average molecular weight (Mn) becomes too large, so that the crosslinking density is lowered and sufficient curability cannot be obtained. On the other hand, if the upper limit is exceeded, the water resistance tends to be lowered.

  The polyvalent carboxylic acid and / or acid anhydride is not particularly limited. For example, phthalic acid, isophthalic acid, phthalic anhydride, tetrahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalic anhydride, Methyltetrahydrophthalic acid, methyltetrahydrophthalic anhydride, hymic anhydride, trimellitic acid, trimellitic anhydride, pyromellitic acid, pyromellitic anhydride, terephthalic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, adipine Acid, azelaic acid, sebacic acid, succinic acid, succinic anhydride, dodecenyl succinic acid, dodecenyl succinic anhydride and the like. Such polyvalent carboxylic acids and / or acid anhydrides may be used alone or in combination of two or more.

  The polyhydric alcohol is not particularly limited, and examples thereof include ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, neopentyl glycol, 1,2-butanediol, and 1,3-butane. Diol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,4-cyclohexanediol, 2,2-dimethyl-3-hydroxypropyl-2, 2-dimethyl-3-hydroxypropionate, 2,2,4-trimethyl-1,3-pentanediol, polytetramethylene ether glycol, polycaprolactone polyol, glycerin, sorbitol, annitol, trimethylol eta , Trimethylol propane, trimethylol butane, hexanetriol, pentaerythritol, dipentaerythritol and the like. Such polyhydric alcohols can be used alone or in admixture of two or more.

  In obtaining a hydroxyl group-containing polyester copolymer suitable for the present invention, monocarboxylic acid, hydroxycarboxylic acid may be used as a reaction component other than the polyvalent carboxylic acid and / or acid anhydride and the polyhydric alcohol component. , Lactones and the like may be contained. As such other reaction components, lactones can also be used from the viewpoint of increasing the amount of the soft segment. Such lactones can be subjected to ring-opening addition to a polyester chain of a polyvalent carboxylic acid and a polyhydric alcohol to form a graft chain, such as β-propiolaclone, dimethylpropiolactone, butyllactone, Examples include γ-valerolactone, ε-caprolactone, γ-caprolactone, γ-caprolactone, crotolactone, δ-valerolactone, and δ-caprolactone, with ε-caprolactone being most preferable.

  Furthermore, from the viewpoint of further improving the scratch resistance of the resulting coating film, it is preferable to use one having the soft segment portion as at least a part of the component used when obtaining the hydroxyl group-containing polyester copolymer. Of these, ε-caprolactone, 1,4-butanediol, and 1,6-hexanediol are particularly preferable.

  In addition, the polyhydric alcohol used for obtaining a polymer obtained by adding a lactone compound to a low molecular polyhydric alcohol is not particularly limited, and for example, those having at least 3 hydroxyl groups in one molecule are preferable. , Trimethylolpropane, trimethylolethane, 1,2,4-butanetriol, ditrimethylolpropane, pentaerythritol, dipentaerythritol, glycerin and the like. The lactone compound is not particularly limited as long as it is a lactone compound capable of causing a ring-opening addition reaction to the low molecular weight polyhydric alcohol compound. Since the ring-opening addition reaction is likely to occur, the lactone compound preferably has 4 to 7 carbon atoms, such as ε-caprolactone, γ-caprolactone, γ-valerolactone, δ-valerolactone, and γ-butyrolactone. Can be mentioned. This may be used alone or in combination of two or more. Among these, ε-caprolactone and δ-valerolactone are more preferable, and ε-caprolactone is more preferable from the viewpoint of reactivity.

  Examples of the polyfunctional isocyanate compound used as a curing agent in the clear coating composition of the present invention include alicyclic, aromatic group-containing aliphatic or aromatic polyfunctional isocyanate compounds, and preferred examples include diisocyanate. Or the isocyanurate (triisocyanate of diisocyanate) can be mentioned.

  As said diisocyanate, what contains generally 5-24, preferably 6-18 carbon atoms can be used. Examples of such diisocyanates include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, 1,6-diisocyanatohexane (HDI), 2,2,4-trimethylhexane diisocyanate, undecane diisocyanate- (1,11 ), Lysine ester diisocyanate, cyclohexane-1,3- and 1,4-diisocyanate, 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (IPDI), 4,4'-diisocyanatodisi Chlorocyclomethane, ω, ω′-dipropyl ether diisocyanate, thiodipropyl diisocyanate, cyclohexyl-1,4-diisocyanate, dicyclohexylmethane-4,4′-diisocyanate, 1,5-dimethyl-2,4-bis (isocyanatomethyl) benzene, 1,5-trimethyl-2,4-bis (ω-isocyanatoethyl) -benzene, 1,3,5-trimethyl-2,4 -Bis (isocyanatomethyl) benzene, 1,3,5-triethyl-2,4-bis (isocyanatomethyl) benzene, dicyclohexyldimethylmethane-4,4'-diisocyanate, 2,4-toluene diisocyanate, 2,6 -Toluene diisocyanate, diphenylmethane-4,4'-diisocyanate and the like. 2,4-diisocyanatotoluene and / or 2,6-diisocyanatotoluene, 4,4′-diisocyanatodiphenylmethane, 1,4-diisocyanatoisopropylbenzene, cyclohexyl-1,4-diisocyanate, Aromatic diisocyanates such as toluene diisocyanate and hexamethylene diisocyanate can also be used. Furthermore, as said isocyanurate, the trimer of the diisocyanate mentioned above can be mentioned. Such polyfunctional isocyanate compounds can be used alone or in admixture of two or more. Moreover, it can also be used as a mixture of said diisocyanate and isocyanurate (trimer).

  In addition, the said soft segment part may exist in the above-mentioned polyfunctional isocyanate compound, The content rate of the said soft segment part in the polyfunctional isocyanate compound which concerns on this invention is 60 mass% with respect to the solid content whole quantity. The following is preferable.

  The clear coating composition for automobiles of the present invention contains a hydroxyl group-containing polymer as a film-forming resin and a polyfunctional isocyanate compound as a curing agent, and the hydroxyl group-containing polymer and the polyfunctional isocyanate compound It is preferable that the isocyanate group in a polyfunctional isocyanate compound exists in the range of 0.5-1.5 with respect to the number 1 of the hydroxyl group in a hydroxyl-containing polymer. When the blending ratio of the polyfunctional isocyanate compound is less than the lower limit, sufficient curing tends not to be obtained.On the other hand, when the upper limit is exceeded, unreacted isocyanate groups react with moisture in the air and the Tm of the coating film is reduced. It tends to increase and the scratch resistance tends to deteriorate.

  The clear coating composition may have an organotin compound curing catalyst. The organotin compound catalyst is not particularly limited, and examples thereof include dibutyltin dilaurate, dibutyltin diacetate, and dibutyltin dioctate. The blending amount of the curing catalyst is preferably 0.005 parts by weight for the lower limit and 0.05 parts by weight for the upper limit with respect to 100 parts by weight of the total amount of polymer solids in the clear coating composition.

  In addition to the polymer, the clear coating composition may contain an ultraviolet absorber, a hindered amine light stabilizer, an antioxidant, crosslinked resin particles, a surface conditioner, and the like. When the crosslinked resin particles are used, it is preferable that the lower limit is 0.01% by mass and the upper limit is 10% by mass with respect to the resin solid content of the clear coating composition of the present invention. The lower limit is more preferably 0.1% by mass, and the upper limit is more preferably 5% by mass. When the added amount of the crosslinked resin particles exceeds 10% by mass, the appearance of the obtained coating film tends to deteriorate, and when it is less than 0.01% by mass, the rheology control effect tends to be not obtained.

The clear coating composition of the present invention described above has a dynamic glass transition point (Tm) of the obtained coating film of 60 to 80 ° C. (particularly preferably 65 to 75 ° C.) and a crosslinking density of 0.8 × 10 ⁻ It is prepared to be ³ mol / cc or more (particularly preferably 1.0 × 10 ^{−3 to} 2.5 × 10 ⁻³ mol / cc). When the dynamic glass transition point of the obtained coating film is less than the above lower limit, the stain resistance against water spots and the like is not sufficiently improved, and the maintenance of scratch resistance is also lowered, whereas the above upper limit is exceeded. Will deteriorate the initial scratch resistance. Moreover, also when the crosslinking density of the obtained coating film is less than the said minimum, the stain resistance with respect to a water spot etc. will not fully improve, but the flaw resistance maintenance property will also fall.

  As a condition for satisfying the dynamic glass transition point and crosslinking density of the obtained coating film and achieving scratch resistance, the amount of the soft segment portion is a solid of the hydroxyl group-containing polymer and the polyfunctional isocyanate compound. It is important that the amount is 25 to 50% by mass (preferably 30 to 45% by mass) based on the total amount.

  The clear coating composition of the present invention can be applied by spray coating, brush coating, dip coating, roll coating, flow coating, or the like. The clear coating composition of the present invention is also advantageous for any substrate, such as wood, metal, glass, cloth, plastic, foam, etc., especially plastic and metal surfaces (eg, steel, aluminum and alloys thereof). It can be used suitably as a clear paint for automobiles.

  Next, the formation method of the multilayer coating film of this invention is demonstrated. That is, the method for forming a multilayer coating film of the present invention is a method for forming a multilayer coating film having a top coat on an object to be coated, and the clear coating composition for automobiles of the present invention is applied to the top coat. It is the method characterized by painting as.

  As the object to be coated, it can be used for various substrates such as metal molded products, plastic molded products, foams, etc., but as the objects to be coated to form a multilayer coating film for automobiles, iron, aluminum And metal molded products such as these alloys, plastic molded products, and the like. It is preferable to apply to metal molded products that can be subjected to cationic electrodeposition coating. The surface of the object to be coated is preferably subjected to chemical conversion treatment. Furthermore, an electrodeposition coating film may be formed on the object to be coated. As the electrodeposition paint, a cation type and an anion type can be used. From the viewpoint of anticorrosion, a cation type electrodeposition paint is preferable.

  Furthermore, an intermediate coating film may be formed as necessary. An intermediate coating is used to form the intermediate coating film. The intermediate coating is not particularly limited, and examples thereof include water-based or organic solvent-type ones well known by those skilled in the art.

  In the method for forming a multilayer coating film of the present invention, a base coating composition is applied to the object to be coated to obtain a base uncured coating film, and then the vehicle clear coating composition of the present invention is applied to the base uncured coating film. It is preferable to apply the composition to obtain a clear uncured coating film, and simultaneously heat and cure the base uncured coating film and the clear uncured coating film. It is also possible to apply the automobile clear of the present invention as a second clear on the base and clear cured coating film, and to cure by heating.

  It does not specifically limit as said base coating material, For example, coloring pigments, extender pigments, etc., such as a film-forming resin, a hardening | curing agent, an organic type, an inorganic type, or a luster material, may be included. The form of the base paint is not particularly limited, and examples thereof include an aqueous or organic solvent type.

  The method of applying the base paint to the object to be coated is not particularly limited, and examples thereof include spray coating and rotary atomizing coating. From the viewpoint of improving the appearance, many of these methods are used. It is preferable that the coating method is stage coating or a combination thereof.

  It is preferable that the coating film thickness by the said base coating material in the formation method of the multilayer coating film of this invention is a dry film thickness in the range whose lower limit is 10 micrometers and whose upper limit is 20 micrometers. In the method for forming a multilayer coating film of the present invention, when the base coating material is water-based, the base uncured coating is applied before the clear coating composition is applied in order to obtain a good finished coating film. It is desirable to heat the membrane at 40-100 ° C. for 2-10 minutes.

  As a method for applying the clear coating composition of the present invention in the method for forming a multilayer coating film of the present invention, specifically, a micro-microbell, a coating method using a rotary atomizing electrostatic coater called a microbell Can be mentioned.

  In the method for forming a multilayer coating film of the present invention, the coating thickness of the clear coating is preferably a dry film thickness with a lower limit of 30 μm and an upper limit of 45 μm. Moreover, it is preferable that the base uncured coating film and the clear uncured coating film formed by the above method are simultaneously heated and cured, thereby forming a multilayer coating film. It is preferable to perform the said heating temperature within the range whose lower limit is 100 degreeC and whose upper limit is 180 degreeC. More preferably, the lower limit is 120 ° C and the upper limit is 160 ° C. The heat-curing time varies depending on the curing temperature or the like, but when it is performed at the above-mentioned heat-curing temperature, it is appropriate to be 10 to 30 minutes.

  Thus, it is preferable that the film thickness of the obtained multilayer coating film exists in the range whose lower limit is 40 micrometers and whose upper limit is 65 micrometers. The multi-layer coating film obtained by the method for forming a multi-layer coating film according to the present invention maintains excellent scratch resistance at a high level over a long period of time, and at the same time is resistant to contamination against water spots and the like. Is also excellent.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example. In this example, when “%” is simply described, it means “mass%”.

Synthesis Example 1 Synthesis of Hydroxyl-Containing Acrylic Copolymer a 224.0 g of propylene glycol monomethyl ether acetate was added to a 1 L glass container equipped with a stirring blade, a nitrogen introduction tube, a cooling condenser and a dropping funnel, and the nitrogen atmosphere was 130. Warmed to ° C. In the container, 50.0 g of propylene glycol monomethyl ether acetate, 50.0 g of tert-butylperoxy-2-ethylhexanoate, 50.0 g of styrene, 50.0 g of 2-ethylhexyl methacrylate, 50.0 g of 2-hydroxyethyl acrylate 200.0 g of 4-hydroxybutyl acrylate and 150.0 g of Plaxel FM-2D (1: 2 adduct of 2-hydroxyethyl methacrylate and ε-caprolactone; manufactured by Daicel Industries) were added dropwise at a constant rate over 3 hours. Thereafter, the mixture was kept at 130 ° C. for 0.5 hour, and 5.0 g of tert-butylperoxy 2-ethylhexanoate dissolved in 25.0 g of propylene glycol monomethyl ether acetate was added dropwise at a constant rate in 30 minutes. Furthermore, the objective hydroxyl-containing acrylic copolymer a was obtained by continuing a heating at 130 degreeC for 1.0 hour.

  Regarding the synthesized hydroxyl group-containing acrylic copolymer a, the molecular weight values in terms of standard polystyrene obtained using GPC were Mn = 5100, Mw = 12000, and Mw / Mn = 2.35. The hydroxyl value was 251 mgKOH / g, the calculated Tg was −23 ° C., and the resin solid content was 61.5%. Furthermore, the content rate of the said soft segment part in the hydroxyl-containing acrylic copolymer a was 27.29% with respect to the solid content whole quantity. Table 1 shows the composition and various physical properties of the hydroxyl group-containing acrylic copolymer a.

(Synthesis Example 2) Synthesis of hydroxyl group-containing acrylic copolymer b 224.0 g of propylene glycol monomethyl ether acetate was added to a 1 L glass container equipped with a stirring blade, a nitrogen introduction tube, a cooling condenser, and a dropping funnel, and the nitrogen atmosphere was 130. Warmed to ° C. In the container, 50.0 g of propylene glycol monomethyl ether acetate, 50.0 g of tert-butylperoxy 2-ethylhexanoate, 10.0 g of styrene, 90.0 g of 2-ethylhexyl acrylate, 50.0 g of 2-hydroxyethyl acrylate, 200.0 g of 4-hydroxybutyl acrylate and 150.0 g of Plaxel FM-2D (1: 2 adduct of 2-hydroxyethyl methacrylate and ε-caprolactone; manufactured by Daicel Industries) were added dropwise at a constant rate over 3 hours. Thereafter, the mixture was kept at 130 ° C. for 0.5 hour, and 5.0 g of tert-butylperoxy 2-ethylhexanoate dissolved in 25.0 g of propylene glycol monomethyl ether acetate was added dropwise at a constant rate in 30 minutes. Furthermore, the objective hydroxyl-containing acrylic copolymer b was obtained by continuing a heating at 130 degreeC for 1.0 hour.

  Regarding the synthesized hydroxyl group-containing acrylic copolymer b, the molecular weight values in terms of standard polystyrene obtained using GPC were Mn = 4300, Mw = 9900, and Mw / Mn = 2.30. The hydroxyl value was 251 mgKOH / g, the calculated Tg was −40 ° C., and the resin solid content was 62.2%. Furthermore, the content rate of the said soft segment part in the hydroxyl-containing acrylic copolymer b was 27.29% with respect to the solid content whole quantity. Table 1 shows the composition and physical properties of the hydroxyl group-containing acrylic copolymer b.

(Synthesis Example 3) Synthesis of hydroxyl group-containing acrylic copolymer c 224.0 g of propylene glycol monomethyl ether acetate was added to a 1 L glass container equipped with a stirring blade, a nitrogen introduction tube, a cooling condenser and a dropping funnel. Warmed to ° C. In the container, 50.0 g of propylene glycol monomethyl ether acetate, 50.0 g of tert-butylperoxy 2-ethylhexanoate, 10.0 g of styrene, 47.5 g of 2-ethylhexyl acrylate, 50.0 g of 2-hydroxyethyl acrylate, 217.5 g of 4-hydroxybutyl acrylate, Plaxel FM-2D (1: 2 adduct of 2-hydroxyethyl methacrylate and ε-caprolactone; manufactured by Daicel Industries), 35.0 g of Plaxel FM-5 (with 2-hydroxyethyl methacrylate) 140.0 g of ε-caprolactone 1: 5 adduct (manufactured by Daicel Kogyo Co., Ltd.) was added dropwise at a constant rate over 3 hours. Thereafter, the mixture was kept at 130 ° C. for 0.5 hour, and 5.0 g of tert-butylperoxy 2-ethylhexanoate dissolved in 25.0 g of propylene glycol monomethyl ether acetate was added dropwise at a constant rate in 30 minutes. Furthermore, the objective hydroxyl-containing acrylic copolymer c was obtained by continuing a heating at 130 degreeC for 1.0 hour.

  About the synthesized hydroxyl group-containing acrylic copolymer c, the molecular weight values in terms of standard polystyrene obtained using GPC were Mn = 4700, Mw = 11300, and Mw / Mn = 2.40. The hydroxyl value was 251 mgKOH / g, the calculated Tg was −40 ° C., and the resin solid content was 63.3%. Furthermore, the content rate of the said soft segment part in the hydroxyl-containing acrylic copolymer c was 33.65% with respect to the solid content whole quantity. Table 1 shows the composition and various physical properties of the hydroxyl group-containing acrylic copolymer c.

(Synthesis Example 4) Synthesis of hydroxyl group-containing acrylic copolymer d In a 1 L glass container equipped with a stirring blade, a nitrogen introduction tube, a cooling condenser and a dropping funnel, 112.0 g of propylene glycol monomethyl ether acetate and 112.0 g of xylene In addition, it was heated to 130 ° C. in a nitrogen atmosphere. In the container, 50.0 g of propylene glycol monomethyl ether acetate, 50.0 g of tert-butylperoxy 2-ethylhexanoate, 75.0 g of styrene, 175.0 g of n-butyl acrylate, 18.0 g of n-butyl methacrylate, 2 -50.0 g of 4-hydroxyethyl acrylate, 50.0 g of 4-hydroxybutyl acrylate, 132.0 g of Plaxel FM-2D (1: 2 adduct of 2-hydroxyethyl methacrylate and ε-caprolactone; manufactured by Daicel Industries) over 3 hours Was dropped at a constant speed. Thereafter, the mixture was kept at 130 ° C. for 0.5 hour, and 5.0 g of tert-butylperoxy 2-ethylhexanoate dissolved in 25.0 g of propylene glycol monomethyl ether acetate was added dropwise at a constant rate in 30 minutes. Furthermore, the objective hydroxyl-containing acrylic copolymer d was obtained by continuing a heating at 130 degreeC for 1.0 hour.

  With respect to the synthesized hydroxyl group-containing acrylic copolymer d, the molecular weight values in terms of standard polystyrene obtained using GPC were Mn = 4800, Mw = 11800, and Mw / Mn = 2.46. The hydroxyl value was 129 mgKOH / g, the calculated Tg was −24 ° C., and the resin solid content was 62.5%. Furthermore, the content rate of the said soft segment part in the hydroxyl-containing acrylic copolymer d was 14.21% with respect to the solid content whole quantity. Table 1 shows the composition and various physical properties of the hydroxyl group-containing acrylic copolymer d.

(Synthesis Example 5) Synthesis of hydroxyl-containing acrylic copolymer e In a 1 L glass container equipped with a stirring blade, a nitrogen introducing tube, a cooling condenser and a dropping funnel, 200.0 g of propylene glycol monomethyl ether acetate and 24.0 g of xylene In addition, it was heated to 130 ° C. in a nitrogen atmosphere. In the container, 50.0 g of propylene glycol monomethyl ether acetate, 50.0 g of tert-butylperoxy 2-ethylhexanoate, 100.0 g of styrene, 95.0 g of methyl methacrylate, 80.0 g of 2-hydroxyethyl acrylate, Hydroxyethyl methacrylate (225.0 g) was added dropwise at a constant rate over 3 hours. Thereafter, the mixture was kept at 130 ° C. for 0.5 hour, and 5.0 g of tert-butylperoxy 2-ethylhexanoate dissolved in 25.0 g of propylene glycol monomethyl ether acetate was added dropwise at a constant rate in 30 minutes. Furthermore, the objective hydroxyl-containing acrylic copolymer e was obtained by continuing a heating at 130 degreeC for 1.0 hour.

  Regarding the synthesized hydroxyl group-containing acrylic copolymer e, the molecular weight values in terms of standard polystyrene obtained using GPC were Mn = 5200, Mw = 12600, and Mw / Mn = 2.42. The hydroxyl value was 271 mgKOH / g, the calculated Tg was 56.9 ° C., and the resin solid content was 63.0%. Furthermore, the content rate of the said soft segment part in the hydroxyl-containing acrylic copolymer e was 0% with respect to the solid content whole quantity. Table 1 shows the composition and physical properties of the hydroxyl group-containing acrylic copolymer e.

Synthesis Example 6 Synthesis of Hydroxyl-Containing Polyester Copolymer f In a 2 L glass container equipped with a stirring blade, a nitrogen introduction tube, and a cooling condenser, 254.0 g of dipentaerythritol, 974.7 g of ε-caprolactone and dibutyltin oxide 0 .25 g was added and heated to 170 ° C. Thereafter, the mixture was held at 170 ° C. for 3.0 hours, and 409.3 g of ethyl 3-ethoxypropionate was added to obtain the desired hydroxyl group-containing polyester copolymer f.

  Regarding the synthesized hydroxyl group-containing polyester copolymer f, the molecular weight values in terms of standard polystyrene obtained using GPC were Mn = 2200, Mw = 2800, and Mw / Mn = 1.27. The hydroxyl value was 274 mgKOH / g, and the resin solid content was 75.0%. Furthermore, the content rate of the said soft segment part in the hydroxyl-containing polyester copolymer f was 48.71% with respect to the solid content whole quantity. Table 1 shows the composition and physical properties of the hydroxyl group-containing polyester copolymer f.

(Synthesis Example 7) Synthesis of hydroxyl group-containing acrylic copolymer g 201.5 g of butyl acetate was added to a 1 L glass container equipped with a stirring blade, a nitrogen introduction tube, a cooling condenser and a dropping funnel, and heated to 130 ° C. under a nitrogen atmosphere. Warm up. In the container, 90.0 g of propylene glycol monomethyl ether acetate, 90.0 g of tert-butylperoxy 2-ethylhexanoate, 100.0 g of styrene, 66.0 g of 2-ethylhexyl acrylate, 132.0 g of 2-ethylhexyl methacrylate, 2 -197.0 g of hydroxyethyl methacrylate and 5.0 g of methacrylic acid were added dropwise at a constant rate over 3 hours. Thereafter, the mixture was kept at 130 ° C. for 0.5 hour, and 10.0 g of tert-butyl peroxy 2-ethylhexanoate dissolved in 25.0 g of propylene glycol monomethyl ether acetate was added dropwise at a constant rate in 30 minutes. Furthermore, the objective hydroxyl-containing acrylic copolymer g was obtained by continuing a heating at 130 degreeC for 1.0 hour.

  Regarding the synthesized hydroxyl group-containing acrylic copolymer g, the molecular weight values in terms of standard polystyrene obtained using GPC were Mn = 2700, Mw = 5000, and Mw / Mn = 1.85. The hydroxyl value was 170 mgKOH / g, the calculated Tg was 19.5 ° C., and the resin solid content was 57.0%. Furthermore, the content rate of the said soft segment part in the hydroxyl-containing acrylic copolymer g was 0% with respect to the solid content whole quantity. Table 1 shows the composition and physical properties of the hydroxyl group-containing acrylic copolymer g.

(Synthesis Example 8) Synthesis of hydroxyl group-containing polyester h In a 2 L glass container equipped with a stirring blade, a nitrogen introduction tube, a cooling condenser, a water separator and a rectifying tower, 566 g of 1.6-hexanediol, 437 g of trimethylolpropane, 467 g of adipic acid, 308 g of hexahydrophthalic anhydride and 0.81 g of dibutyltin oxide were added and heated to 180 ° C. When the reaction water began to come out, the temperature was slowly raised to 230 ° C. over 3 hours while distilling off the reaction water, and kept at 230 ° C. The mixture was cooled after the acid value became 7 or less, and 596 g of ethyl 3-ethoxypropionate was added at 100 ° C. or less to obtain the desired hydroxyl group-containing polyester h. For the synthesized hydroxyl group-containing polyester h, the molecular weight values in terms of standard polystyrene obtained using GPC were Mn = 1800, Mw = 3960, and Mw / Mn = 2.2. The hydroxyl value was 289 mgKOH / g, and the resin solid content was 70.0%. Table 1 shows the composition and various physical properties of the hydroxyl group-containing polyester h.

(Examples 1-4 and Comparative Examples 1-7)
According to the formulation shown in Table 2, the components were mixed and stirred with a disper to obtain the clear coating compositions of Examples 1 to 4 and Comparative Examples 1 to 7. The clear coating composition was subjected to No. 1 using a thinner composed of propylene glycol monomethyl ether acetate / 3-ethyl ethyl ethoxypropionate = 1/2 (mass ratio). Each was diluted with a 4 Ford cup to 25 ° C / 20 ° C. In addition, Sumika Bayer Urethane Co., Ltd. Sumidur N-75 is used as the bullet type isocyanate curing agent, and Sumika Bayer Urethane Co., Ltd. Sumidur N3300 is used as the isocyanurate type isocyanate curing agent. It was.

  Next, power top U-50 (cationic electrodeposition paint manufactured by Nippon Paint Co., Ltd.) is electrodeposited onto a 150 × 300 × 0.8 mm dull steel plate treated with zinc phosphate so that the dry film thickness is 20 μm. No. 1 was applied to the coated plate baked at 160 ° C. for 30 minutes. Olga P-2 (melamine curable polyester resin-based gray intermediate coating material manufactured by Nippon Paint Co., Ltd.) diluted in advance so as to be 25 seconds / 20 ° C. in a 4 Ford cup is air-dried so that the dry film thickness becomes 35 μm. Two-stage coating was performed by spraying, baking was performed at 140 ° C. for 30 minutes, and then cooling to obtain an intermediate coated substrate. After spray coating Superlac M-260 Black (Nippon Paint Co., Ltd. water-based base paint) on the above-mentioned intermediate coating substrate under conditions of room temperature of 25 ° C. and humidity of 85% to a dry film thickness of 15 μm, 80 Preheating was performed at 3 ° C. for 3 minutes. After preheating, the coated plate is allowed to cool to room temperature, and Superlac O-170 (acrylic melamine curable clear paint manufactured by Nippon Paint Co., Ltd.) is applied as the first clear so as to have a dry film thickness of 30 μm. Heat cured for minutes. After polishing the first clear substrate with # 2000 water research paper, each of the diluted clear coating compositions was spray-coated to a dry film thickness of 35 μm, and then heated at 140 ° C. for 25 minutes in a drying furnace. A multilayer coating film was formed on the substrate.

  The dynamic glass transition point and crosslinking density of the coating films obtained using the clear coating compositions of Examples 1 to 4 and Comparative Examples 1 to 7 were measured, and the results obtained are shown in Table 2. Moreover, the content rate (content rate (mass%) with respect to solid content whole quantity of a hydroxyl-containing polymer and a polyfunctional isocyanate compound) of the said soft segment part in the clear coating composition of Examples 1-4 and Comparative Examples 1-7 is the following. As shown in Table 2.

<Abrasion resistance test>
The multilayer coating films obtained using the clear coating compositions of Examples 1 to 4 and Comparative Examples 1 to 7 were evaluated for scratch resistance by the following evaluation methods.

  That is, first, after spreading a test dust composition consisting of 15 g of test dust (7 types mixed, particle size 27 to 31 μm) and 100 g of water on a mini car wash table, the mini car wash machine is run without flowing water. It was rotated (45 rpm) and reciprocated once to allow dust to adhere to the car wash brush. Thereafter, a coated plate (70 mm × 150 mm) was fixed on the table, and about 5 g of foundry sand was sprinkled on the coated plate, and then the mini car washer was rotated (96 rpm) and reciprocated three times without flowing water. After the test, washing and drying were performed. The 20 ° gloss of the coated plate was measured, and the gloss retention (initial 20 ° GR (%)), which is the ratio of the 20 ° gloss to the 20 ° gloss before the test, was calculated.

Moreover, the xenon lamp light (irradiation intensity: 180 W / m < ² >) was irradiated to the multilayer coating film obtained using the clear coating composition of Examples 1-4 and Comparative Examples 1-7 for 800 hours, and then With respect to the multi-layer coating film, the 20 ° gloss of the coated plate was measured by the same method as described above, and the gloss retention ratio (20 ° GR (%) after 800 hours of xenon), which is the ratio of the 20 ° gloss to the 20 ° gloss before the test. ) Was calculated.

Then, the maintenance ratio of scratch resistance after promotion of xenon was calculated as a ratio of 20 ° GR after 800 hours of xenon to the initial 20 ° GR, and evaluated based on the following criteria. The obtained results are shown in Table 2.
◎: Scratch resistance maintenance ratio of 95% or more ○: Scratch resistance maintenance ratio of 80% or more and less than 95% △: Scratch resistance maintenance ratio of 70% or more and less than 80% ×: Scratch resistance maintenance ratio Is less than 70%.

<Contamination resistance test>
The multilayer coating films obtained using the clear coating compositions of Examples 1 to 4 and Comparative Examples 1 to 7 were evaluated for contamination resistance by the following evaluation methods.

That is, the outdoor exposure test is conducted under the following conditions in accordance with JIS K5600-7-6.
Exposure field: Kagoshima exposure platform angle: The test was carried out horizontally, and the presence or absence of water spots after the coated plate was washed with flannel cloth after 3 months was evaluated based on the following criteria. The obtained results are shown in Table 2.
No rain spots: No rain spots. Rain spots: Rain spots.

  As is apparent from the results shown in Table 2, the multilayer coating film obtained using the clear coating composition of the present invention has excellent scratch resistance maintained at a high level over a long period of time, and at the same time, The multi-layer coating film obtained by using the clear coating composition of the comparative example satisfies all of scratch resistance, scratch resistance maintenance and stain resistance. Nothing existed.

  As described above, according to the present invention, an automobile capable of forming a coating film having excellent scratch resistance maintained at a high level over a long period of time and at the same time having excellent stain resistance against water stains and the like. It becomes possible to obtain a clear coating composition for use. Therefore, according to the method of forming a multilayer coating film of the present invention using the clear coating composition of the present invention, excellent scratch resistance is maintained at a high level over a long period of time, and at the same time, stain resistance against water stains and the like. In addition, it is possible to efficiently and reliably form a coating film for automobiles that is also excellent.

Claims (4)

A clear coating composition containing a hydroxyl group-containing polymer and a polyfunctional isocyanate compound, wherein the clear coating composition has the following general formula (1):
_{- (CH 2) n - (} 1)
(In the formula, n represents an integer of 4 or more.)
In the solid content of the hydroxyl group-containing polymer and the polyfunctional isocyanate compound, and the resulting glass film has a dynamic glass transition point of 60-80. A clear coating composition for automobiles, which is prepared at a temperature of 0 ° C. and a crosslinking density of 0.8 × 10 ⁻³ mol / cc or more.   The clear coating composition for automobiles according to claim 1, wherein the hydroxyl group-containing polymer is at least one selected from the group consisting of a hydroxyl group-containing acrylic copolymer and a hydroxyl group-containing polyester copolymer.   A method for forming a multi-layer coating film having a top coat on an object to be coated, wherein the multi-layer coating composition comprises applying the clear coating composition for automobiles according to claim 1 or 2 as the top coat. Method for forming a coating film.   After applying the base coating composition to the object to be coated to obtain a base uncured coating film, the base uncured coating film is coated with the automotive clear coating composition to obtain a clear uncured coating film, 4. The method for forming a multilayer coating film according to claim 3, wherein the base uncured coating film and the clear uncured coating film are simultaneously heated and cured.