JP2008001036A - Top coat, coating method and car exterior component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-layer coating film which is energy-saving and has a highly aesthetic finish appearance. <P>SOLUTION: The coating method is based on forming a base coating film 12 on a to-be-coated substrate 10 and then a multi-layer coating film consisting of a first clear coating film 14 and a second clear coating film 18, formed in this order, and comprises applying a base coating to the substrate 10, then applying, in order, the first clear coating by the wet-on-wet method and the second clear coating by the wet-on-wet method, followed by baking drying. The first clear coating is a two-liquid type polyurethane coating comprising an isocyanate compound and a polyhydric alcohol, and the blend ratio of the isocyanate compound to the polyhydric alcohol is 0.5-1.0, in a NCO/OH mol ratio. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a top coating material, a coating method, and an automotive exterior part, and more particularly, to a top coating material and a coating method capable of improving interlayer adhesion in a top coating film.

  For example, conventionally, as a method of forming a top coat film on an automobile body outer plate portion, a two-coat one-bake (two-layer coat film is formed by applying a colored base paint, and then applying a two-clear paint, followed by heat curing to form a two-layer paint film ( 2C1B) method has been used (for example, Patent Document 1).

  However, from the viewpoint of improving the smoothness, sharpness, etc. of the coating film, a three-coating system in which the top coating is applied twice has recently been adopted. For example, as a three-coat system, after applying a colored base paint and a first clear paint, heat and cure both paints, then apply a second clear paint, and heat cure again to form a three-layer coating film A 3-coat 2-bake (3C2B) method (for example, Patent Documents 2 and 3), a colored base paint, a first clear paint, and a second clear paint are applied by wet-on-wet, and this three-layer coating film is applied simultaneously. A three-coat one-bake (3C1B) system for curing is known.

  Since the above-described 3-coat 2-bake method has two heating steps, the number of steps is one more than that of the 3-coat 1-bake method, which is not suitable for energy saving. Also, in the 3-coat 2-bake method, since the first clear paint is applied and then heated, and then the second clear paint is applied, the interlayer adhesion between the first clear paint film and the second clear paint film is reduced. There was also a risk of doing so.

  In view of this, a method of performing top coating by a 3-coat 1-bake method has been proposed. For example, Patent Document 4 discloses a hydroxyl group-containing acrylic resin having a number average molecular weight of 10,000 to 12,000, a hydroxyl value of 100 to 110 mgKOH / g, and an acid value of 5 mgKOH / g, as well as a first clear paint and a second clear paint. It has been proposed to be used as

Japanese Patent Laid-Open No. 10-85659 JP 2003-277678 A JP 2000-136345 A Japanese Patent Laid-Open No. 10-244218

  However, in recent years, further improvement in the smoothness and sharpness of the coating film has been desired, and more robust interlayer adhesion between the first clear coating film and the second clear coating film is also desired from the viewpoint of durability. ing.

  The present invention has been made in view of the above problems, and provides a top coating material and a coating method for improving interlayer adhesion in a top coating film, and an automotive exterior part obtained therefrom.

  The top coat, the coating method and the automotive exterior part of the present invention have the following characteristics.

  (1) An overcoat paint for forming a multilayer coating film on a substrate to be coated, comprising a base paint, a first clear paint and a second clear paint, wherein the first clear paint comprises an isocyanate compound and a polyhydric alcohol. The second clear paint is a thermosetting paint containing a carboxyl group-containing polymer and an epoxy group-containing polymer, and the blending amount of the isocyanate compound and the polyhydric alcohol is: The NCO / OH ratio is 0.5 to 1.0 in terms of molar ratio.

  Since the two-component polyurethane paint consisting of the isocyanate compound and polyhydric alcohol that constitutes the first clear paint can react at two temperatures, the first clear paint and the second clear paint are applied wet-on-wet. However, the smoothness of the first clear coating film can be secured. Furthermore, the first clear paint and the second clear paint are wet-on by setting the blending amount of the isocyanate compound and the polyhydric alcohol in the first clear paint to a molar ratio of NCO / OH ratio = 0.5 to 1.0. When wet coating and baking are performed simultaneously, many residual hydroxyl groups (OH groups) that promote adhesion are present on the coated surface of the first clear coating film. As a result, interlayer adhesion between the first clear coating film and the second clear coating film after baking is improved, and a multi-layer top coating film having high appearance and durability can be obtained.

  (2) In a coating method for forming a base coating film on a substrate to be coated and further forming a multilayer coating film having a first clear coating film and a second clear coating film in order, A coating method in which a base paint is applied, then a first clear paint is applied by wet-on-wet, and then a second clear paint is applied by wet-on-wet, followed by baking and drying. The clear coating is a two-component polyurethane coating composed of an isocyanate compound and a polyhydric alcohol, and the blending amount of the isocyanate compound and the polyhydric alcohol is such that the NCO / OH ratio is 0.5 to 1.0 in molar ratio. is there.

  As described above, by using a two-component polyurethane paint composed of an isocyanate compound and a polyhydric alcohol as the first clear paint, two liquids can react even at room temperature, so the first clear paint and the second clear paint Even when wet-on-wet is applied, a multilayer clear coating film composed of two layers can be formed while ensuring the smoothness of the first clear coating film. Therefore, compared with the 3-coat 2-bake method, one-time baking can be omitted, and the economy is excellent. Moreover, the first clear paint and the second clear paint are wet-on by setting the blending amount of the isocyanate compound and the polyhydric alcohol in the first clear paint to a molar ratio of NCO / OH ratio = 0.5 to 1.0. When wet coating and baking are performed simultaneously, many residual hydroxyl groups (OH groups) that promote adhesion are present on the coated surface of the first clear coating film. Therefore, the interlayer adhesion between the first clear coating film and the second clear coating film after baking is improved, and a multi-layer top coating film having high appearance and durability can be obtained.

  (3) An automotive exterior part formed by applying the top coating composition described in (1) above.

  (4) An automotive exterior part that is painted using the coating method described in (2) above.

  According to the present invention, interlayer adhesion of a two-layer clear coating film having smoothness and sharpness can be improved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Top coat]
The top coating material of this embodiment is a top coating material for forming a multilayer coating film on a substrate to be coated, including a base coating material, a first clear coating material, and a second clear coating material. Is a two-component urethane paint comprising an isocyanate compound and a polyhydric alcohol, and the second clear paint is a thermosetting paint containing a carboxyl group-containing polymer and an epoxy group-containing polymer, The blending amount of the polyhydric alcohol is such that the NCO / OH ratio is 0.5 to 1.0 in terms of molar ratio. In the present invention, “polyhydric alcohol” and “polyol” are synonymous.

<Base paint>
The base paint of the present embodiment contains a base resin and a crosslinking agent. Examples of the base resin include acrylic resins having a functional group such as a hydroxyl group, a carboxyl group, and an epoxy group, a polyester resin, and an alkyd resin. be able to. Moreover, as a crosslinking agent, the melamine resin which can react with the functional group in base resin, a urea resin, a polyisocyanate compound, a block isocyanate compound, a carboxyl group containing compound etc. can be mentioned, for example. The ratio of the base resin and the crosslinking agent is 50 to 90% by weight, preferably 60 to 80% by weight, and 50 to 10% by weight, preferably 40 to 40% by weight, based on the total amount of both components. 20% by weight. A one-component curable coating such as melamine-acrylic resin is preferable as the base coating.

  When the base paint is a colored base paint, a color pigment is added. As the color pigment, for example, a solid color pigment, a metallic pigment, a pearl pigment, and the like can be used. Examples of solid color pigments include inorganic pigments such as titanium oxide, zinc white, carbon black, cadmium red, molybdenum red, chrome yellow, chromium oxide, Prussian blue, cobalt blue; azo pigments, phthalocyanine pigments, quinacridone pigments, isoindoline pigments, Examples thereof include organic pigments such as selenium pigments and perylene pigments. Examples of metallic pigments include flaky aluminum and mica-like iron oxide. Further, examples of the pearl-like pigment include mica, mica whose surface is coated with a metal oxide, and the like.

  These color pigments can be used alone or in combination of two or more. The blending amount of these coloring pigments can be arbitrarily selected according to the target color tone, metallic feeling, and pearl tone, and is not particularly limited.

  The organic solvent is not particularly limited as long as it can dissolve or disperse the base resin, the cross-linking agent, and the color pigment. For example, a hydrocarbon solvent, an ester solvent, an ether solvent, an alcohol solvent, a ketone solvent, and the like can be used. Can be used.

  The colored base paint can be prepared by mixing and dissolving or dispersing the base resin, the crosslinking agent, and the colored pigment in an organic solvent by a conventional method.

<First clear paint>
The first clear paint of the present embodiment is a low-temperature curable clear paint, and this low-temperature curable clear paint is preferably at least sufficient under preheat drying conditions such as 70 to 80 ° C. for 10 to 30 minutes. It is of a type that exhibits curability and is cured by a curing agent or a crosslinking agent that satisfies various performances such as glossiness, weather resistance, hardness and the like suitable as the outermost surface layer in multilayer coating. Examples of the low temperature curable clear paint as described above include a two-component polyurethane paint, particularly a polyol curable polyurethane paint.

  The polyol curable polyurethane coating is a coating that cures with a urethane bond by chemically reacting a hydroxyl group of a polyol such as a polyester, polyether, or (meth) acrylic resin having a hydroxyl group with an isocyanate prepolymer.

  The polyol component may be any of them, but considering the adhesion to the base paint, the same system as the base paint is desirable. For example, in terms of organic solvent resistance, a polyester-based one is From the viewpoint of alkali resistance and water resistance, a polyether type is preferable, and from the viewpoint of stain resistance and quick drying, a (meth) acrylic type is preferable.

  The polyhydric alcohol in the two liquids of the first clear paint of the present embodiment is an alkylene (linear, side chain, alicyclic) polyhydric alcohol or an alkylene ether polyhydric alcohol. It has 2 or more hydroxyl groups. For example, dihydric alcohols such as ethylene glycol, propylene glycol, trimethylene glycol, tetraethylene glycol, dipropylene glycol, butanediol, pentanediol, 2,3-dimethyltrimethylene glycol, hexanediol, cyclohexanedimethanol; glycerin, trimethylol Trihydric or higher polyhydric alcohols such as propane, trimethylolethane, diglycerin, hexanetriol, pentaerythritol, dipentaerythritol, sorbitol mannitol; and these dihydric alcohols and / or trihydric or higher polyhydric alcohols as ethers And alkylene ether polyols obtained by conversion.

  In addition, the shorter the carbon chain length of the polyhydric alcohol, the higher the solidity of the first clear paint, and a clear feeling is exhibited.

  On the other hand, as the isocyanate compound in the two liquids of the first clear paint of the present embodiment, for example, tolylene diisocyanate (TDI), hexamethylene diisocyanate (HMDI), 4,4-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate , Triphenylmethane triisocyanate, tolidine diisocyanate (TODI), xylene diisocyanate, dicyclohexylmethane diisocyanate, isophorone diisocyanate (IPDI) and the like, and these hydrogenated products can also be used. Of these, hexamethylene diisocyanate is more preferable.

  In addition, the isocyanate prepolymer used in the polyol curable polyurethane coating material may be various types constituted by using isocyanate alone as a raw material as exemplified above.

When the first clear paint is applied, a polyurethane coating film can be formed by a two-component reaction as described below. In particular, coating performance such as smoothness and clearness can be ensured even by low-temperature baking (for example, 80 ° C.).

  In this Embodiment, the compounding quantity of the said polyhydric alcohol and an isocyanate compound is NCO / OH ratio 0.5-1.0 in molar ratio. Here, when the NCO / OH ratio is less than 0.5, there are few hydroxyl groups which are functional groups remaining on the first clear coating film surface after baking and curing, and interlayer adhesion with the second clear coating film described later is low. May be inferior. On the other hand, when the NCO / OH ratio exceeds 1.0, there is a high possibility that unreacted NCO reacts with moisture in the coating film and has water absorption. Therefore, moisture easily accumulates in the coating film, and as a result, problems such as blistering (blowing) occur.

<Second clear paint>
It is desirable that the second clear paint of the present embodiment has a particularly high aesthetic appearance and weather resistance. Accordingly, although not limited to the following, preferably, it is a paint for forming a transparent coating film containing a carboxyl group-containing resin and an epoxy group-containing compound, and the cured or uncured first clear paint It is a paint to paint on the painted surface.

  The carboxyl group-containing resin used in the second clear paint is a resin having two or more carboxyl groups in one molecule, and one or more selected from the following groups can be used.

  For example, it is possible to use one in which two or more carboxyl groups are introduced by ring-opening addition of a polybasic acid anhydride to a hydroxyl group in an acrylic resin by a half esterification reaction.

  As the acrylic resin, for example, as an acrylic monomer, for example, a monoester product of acrylic acid or methacrylic acid and a monoalcohol having 1 to 20 carbon atoms can be used, and the monomer includes an epoxy group, a hydroxyl group and It does not have a functional group such as a carboxyl group. Moreover, as said acrylic monomer, glycidyl acrylate, glycidyl methacrylate, methyl glycidyl acrylate, methyl glycidyl methacrylate, etc. can also be used, for example. Also included are copolymers in which other monomers such as styrene, vinyltolueneacrylonitrile, methacrylonitrile, vinyl acetate, vinyl chloride are used in combination with the above acrylic monomers. The amount is preferably equal to or less than that of the acrylic monomer. The acrylic resin has a number average molecular weight of about 3,000 to 100,000, particularly 4,000 to 50,000.

  Furthermore, a polybasic acid anhydride may be added to some or all of the hydroxyl groups of the hydroxyl group-containing acrylic resin obtained by copolymerizing the monomer component containing the acrylic monomer and the hydroxyl group-containing monomer exemplified for the acrylic resin. Is added by a half esterification reaction, and the anhydrous ring is opened to introduce a free carboxyl group, whereby a carboxyl group-containing resin is obtained.

  The hydroxyl group-containing acrylic resin before the half esterification reaction has two or more hydroxyl groups in one molecule, and the hydroxyl equivalent is 10 to 200 mgKOH / g, particularly 20 to 120 mgKOH / g.

  Examples of the polybasic acid anhydride to be half-esterified include phthalic anhydride, maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, het anhydride, trimellitic anhydride, pyromellitic anhydride, succinic anhydride, etc. Is mentioned.

  The half-esterification reaction can be carried out by a known method, and by this reaction, an anhydrous ring is opened, one carboxyl group is ester-reacted with a hydroxyl group in the acrylic resin, and the remaining carboxyl group is bonded to the acrylic resin in a free state. Yes.

  A carboxyl group can also be obtained by subjecting a polybasic acid anhydride and a hydroxyl group-containing monomer to a half esterification reaction in advance and then copolymerizing it with an acrylic monomer and, if necessary, a hydroxyl group-containing monomer. A containing resin is obtained.

  Therefore, the carboxyl group-containing resin has two or more carboxyl groups in one molecule, and a hydroxyl group may coexist. The acid value is 10 to 200 mgKOH / g, particularly 20 to 120 mgKOH / g, and the hydroxyl value is 190 mgKOH / g or less, preferably 5 to 120 mgKOH / g, particularly preferably 10 to 100 mgKOH / g.

  On the other hand, examples of the epoxy group-containing compound include alicyclic epoxy compounds such as 7-oxabicyclo (4,1,0) hept-3-ylmethyl ester, glycidyl ethers of hydrogenated bisphenols, and one molecule. In addition, compounds containing two or more epoxy groups in one molecule such as glycidyl ether of aliphatic polyhydric alcohol having two or more hydroxyl groups and glycidyl ether of bisphenol can also be used.

The reaction of the two liquid components in the second clear paint is as follows.

  In addition, the second clear paint further includes diluents (including reactive diluents), and other additives such as stabilizers, antioxidants, ultraviolet absorbers, viscosity modifiers, and the like as necessary. Yes.

  In addition, the first and second clear paints described above can be made into so-called color clear paints by blending clear paints with the same or different pigments contained in the color base paint as long as they do not impair transparency. It is. In this case, it is desirable that the pigment blended in the clear paint has a smaller particle size than the pigment contained in the color base paint. The low-temperature curable clear paint may be in any form such as a general solution type or a high solid type, and among these, a general solution type is particularly preferable.

[Coating method]
The coating method of the present embodiment is a coating method for forming a base coating film on a substrate to be coated, and further forming a multilayer coating film having a first clear coating film and a second clear coating film in order, The base paint is applied to the substrate to be coated, then the first clear paint is applied with wet-on-wet, then the second clear paint is applied with wet-on-wet, and then baking and drying is performed. The first clear paint is a two-component polyurethane paint composed of an isocyanate compound and a polyhydric alcohol, and the NCO / OH ratio in the molar ratio of the isocyanate compound and the polyhydric alcohol is 0.00. 5 to 1.0, which is a 3 coat 1 bake system.

  In the coating method of the present embodiment, examples of the substrate to be coated include steel plates or metal parts such as automobile body outer plates, and resin parts such as resin bumpers. In addition, the automobile body skin includes an automobile body.

  The coating method of this embodiment will be specifically described with reference to FIGS. FIG. 1 shows an outline of a method for painting an automobile body, and FIG. 2 shows an outline of a method for painting an automobile body door opening. In FIGS. 1 and 2, as an example of a substrate 10 to be coated, an automobile body outer plate that has been subjected to intermediate coating on an electrodeposited steel plate and held at 140 ° C. for about 18 minutes and baked is taken as an example. explain.

  The electrodeposition coating film can be obtained by applying a cationic electrodeposition coating as an undercoat on the steel sheet surface. Here, as the cationic electrodeposition coating, a known one is used in which an aqueous solution or dispersion of a salt of a cationic polymer compound is blended with a crosslinking agent, a pigment and various additives as necessary. The kind is not particularly limited. Examples of the cationic polymer compound include those obtained by introducing a cationic group such as an amino group into an acrylic resin or epoxy resin having a crosslinkable functional group, which is neutralized with an organic acid or an inorganic acid. Can be water-soluble or water-dispersed. As a crosslinking agent for curing these polymer compounds, a block polyisocyanate compound, an alicyclic epoxy resin, or the like can be used.

  The electrodeposition coating film is preferably in the range of 10 to 40 μm based on the cured coating film, and the coating film can be heated and cured at 140 to 220 ° C. for 10 to 40 minutes.

  As the intermediate coating composition for forming the intermediate coating film, the thermosetting resin composition used as a vehicle component basically comprises a base resin and a crosslinking agent. Examples of the base resin include a hydroxyl group and an epoxy. Acrylic resin, polyester resin, alkyd resin, etc. having two or more crosslinkable functional groups such as a group, isocyanate group, carboxyl group in one molecule. Examples of the crosslinking agent include melamine resin and urea resin. An amino resin such as, a polyisocyanate compound which may be blocked, a carboxyl group-containing compound and the like.

  The intermediate coating film is applied with a film thickness of 30 to 40 μm. If necessary, the intermediate coating film may contain pigments such as titanium oxide pigments and aluminum flakes.

  Furthermore, for base coating formation using a base paint, as described above, the car body outer plate is divided into the case of a general body part of an automobile and the case of a door opening as described below. explain.

  In addition, as the base paint for forming the base coating film 12 shown in FIG. 1, there are a solvent-based base paint and a water-based base paint. Then, as described above, the water-based base paint, particularly the thermosetting water-based paint contains a pigment, a resin that can be dissolved or dispersed in water, a cross-linking agent as necessary, and water as a solvent. As a resin that can be dissolved or dispersed in water, for example, a resin containing a hydrophilic group such as a carboxyl group and a crosslinkable functional group such as a hydroxyl group in one molecule, specifically, an acrylic resin, a polyester resin, A polyurethane resin etc. are mentioned. Examples of the crosslinking agent include hydrophobic or hydrophilic alkyl ether melamine resins and blocked isocyanate compounds. On the other hand, the solvent-based base paint contains a scaly titanate pigment, a resin similar to the above, and a cross-linking agent and a solvent as necessary.

  The base coating film 12 is applied with a film thickness of 10 to 15 μm when a water-based base paint is used, and is applied with a film thickness of 12 to 18 μm when a solvent-based base paint is used.

  Then, using the top coat of this embodiment as described above, the base paint is applied to the substrate 10 to be coated, preheated at 80 ° C. for about 10 minutes, and then wet-on-wet. 1 Apply clear paint and preheat at 80 ° C. for about 10 minutes. By this preheating, the coating surface residual functional group 16 derived from a hydroxyl group can be present on the coating surface of the first clear coating film 14.

  Moreover, the 1st clear coating film 14 is painted by the film thickness of 20-50 micrometers.

  Next, the second clear paint is applied on the first clear coating surface by wet-on-wet and baked and dried at 130 to 160 ° C., preferably 140 ° C. for 15 to 20 minutes, preferably 18 minutes. The second clear coating film 18 is applied with a film thickness of 20 to 50 μm.

  With the above configuration, the base paint, the first clear paint, and the second clear paint are only wet-on-wet and preheated at low temperatures. Each of the coating surface residual functional groups 16 exists. Therefore, the interlayer adhesion between the base coating film 12 and the first clear coating film 14 and the interlayer adhesion between the first clear coating film 14 and the second clear coating film 18 are high. Moreover, since it is a 3 coat 2 preheat 1 bake system, energy saving can be achieved compared with the 3 coat 1 preheat 2 bake system of FIG. 3 mentioned later. Furthermore, since it is only preheated at a low temperature after the application of the first clearing material, the thermal contraction of the first clear coating film can be suppressed, and the smoothness of the coating surface of the first clear coating film 14 can be improved. it can. Therefore, the sharpness of the two-layer clear coating film is also improved.

  On the other hand, since the door opening of the body does not become an automobile body appearance when the door is closed, even the single layer clear coating does not impair the aesthetics of the automobile body appearance. By only forming a single-layer clear coating film in this way, the number of painting steps can be reduced, and the door is opened and closed multiple times for clear coating as in the case of forming a multi-layer clear coating film. There is no need for workers and energy saving can be achieved.

  The door opening of the automobile body is performed as part of the painting of the automobile body. Therefore, the method of painting the door opening in the present embodiment omits the painting of the first clear paint shown in FIG. It becomes. That is, as shown in FIG. 2, the base paint is applied to the substrate 10 to be coated, pre-heated at 80 ° C. for about 10 minutes, and then the second clear paint is applied by wet-on-wet. Bake and dry at ~ 160 ° C, preferably 140 ° C for 15-20 minutes, preferably 18 minutes. The second clear coating film 18 is applied with a film thickness of 20 to 50 μm.

  With the above configuration, after the base paint is applied and preheated at a low temperature, the second clear paint is applied wet-on-wet. Therefore, the interlayer adhesion between the base coating film 12 and the second clear coating film 18 is high. Moreover, since it is a 2 coat 1 preheat 1 bake system, energy saving can be achieved compared with the 2 coat 2 bake system of FIG. 4 mentioned later. Furthermore, since it is only preheated at a low temperature after the base coating is applied, the thermal contraction of the base coating 12 can be suppressed, and the smoothness of the coated surface of the base coating 12 can be improved. Therefore, even if it is a single-layer clear coating film, if it is a door opening part, sufficient aesthetics can be ensured.

  Next, a case where the second clear paint used in the present embodiment is used as the first clear paint will be described as a reference example with reference to FIGS.

  As described above, the second clear paint is a two-component paint composed of a carboxyl group-containing polymer having two or more carboxyl groups in one molecule and an epoxy group-containing polymer. Therefore, since the reaction does not start unless the temperature is higher than 120 ° C., it is necessary to perform baking at 140 ° C., for example, as shown in FIG. That is, as shown in FIG. 3, after applying a two-component first clear paint comprising a carboxyl group-containing polymer having a carboxyl group and an epoxy group-containing polymer, the first clear paint is baked at 140 ° C. The coating surface residual functional group 36 is not expressed on the coating surface of the film 24. Further, by applying the same second clear paint as described above to the coating surface of the first clear coating film 24 and baking and drying it, the interlayer adhesion of the first and second clear coating films can be sufficiently obtained. However, as compared with the process shown in FIG. 1, since it is 3 coats and 2 bake, two baking processes are necessary, and if the baking time is taken into consideration, the coating time may be prolonged, which is not preferable.

  Furthermore, in the case of single-layer clear coating of the door opening shown in FIG. 4, since the coating process proceeds with the automobile body, the base coating film 12 is baked and dried at, for example, 140 ° C. as shown in FIG. The coating surface residual functional group 26 of the 12 coating surfaces disappears. Therefore, there is a possibility that the interlayer adhesion with the second clear coating film 18 formed on the base coating film 12 is deteriorated, which is not preferable.

  Below, the top coat coating method of this invention is demonstrated concretely using an Example.

Example 1.
In this example, the body of an automobile was used as a base material, and the general body part shown in FIG. 1 and the door opening part shown in FIG. 2 were simultaneously coated using the following paints. That is, after performing electrodeposition coating, a paint using a polyester resin and a melamine resin as a base resin was used as an intermediate coating. Further, as the base paint, a paint containing an acrylic resin and a melamine resin as a base resin and containing a titanium oxide pigment at a content rate PWC of 20% was used. Further, as the first clear paint, a two-component urethane paint R254 (manufactured by Nippon Bee Chemical Co., Ltd.) having an NCO / OH ratio (molar ratio) adjusted to 1.0 was applied and preheated at 80 ° C. for 10 minutes. Further, as the second clear paint, a clear paint (“Mac Flow O-80 Clear”, manufactured by Nippon Paint Co., Ltd.) containing a carboxyl group-containing polymer and an epoxy group-containing polymer was used.

  Furthermore, the coating method of a present Example is demonstrated using FIG. First, an intermediate coating without pigment was applied to an electrodeposited steel sheet so as to have a film thickness of 35 μm, and baked at a maximum temperature of 140 ° C. for 18 minutes. Subsequently, the pigment-containing water-based base paint was applied with a film thickness of 13 μm. Here, since a water-based base paint is used, preheating is performed for 10 minutes at a maximum temperature of 80 ° C., and then the first clear paint is applied to the general body so that the film thickness is 35 μm on the base coating film. Preheat for 10 minutes at an ultimate temperature of 80 ° C, and apply a second clear paint to the general body and door opening to a film thickness of 35μm and hold at the maximum ultimate temperature of 140 ° C for 18 minutes for baking. went.

Example 2
A multilayer coating film according to Example 1 except that as the first clear paint, a two-component urethane paint R254 (manufactured by Nippon Bee Chemical) with an NCO / OH ratio (molar ratio) adjusted to 0.5 was used. Formed.

Comparative Example 1
A multilayer coating film was formed according to Example 1 except that the second clear paint of Example 1 was used instead of the first clear paint of Example 1.

Comparative Example 2
In the painting process shown in FIG. 3, the second clear paint of Example 1 was used instead of the first clear paint of Example 1, and the first clear paint was baked and dried at 140 ° C. for 18 minutes after painting. Except for the above, a multilayer coating film was formed according to Example 1.

Comparative Example 3
A multilayer coating film was formed according to Example 1 except that the first clear paint was baked and dried at 140 ° C. for 18 minutes after coating.

Comparative Example 4
A multilayer coating film according to Example 1 except that as the first clear paint, a two-component urethane paint R254 (manufactured by Nippon Bee Chemical) with an NCO / OH ratio (molar ratio) adjusted to 0.3 was used. Formed.

Comparative Example 5
A multilayer coating film according to Example 1 except that as the first clear paint, a two-component urethane paint R254 (manufactured by Nippon Bee Chemical) having an NCO / OH ratio (molar ratio) adjusted to 1.1 was used. Formed.

<Evaluation method>
Under the above conditions, the finished appearance of the multilayer coating film was visually evaluated.
◎: Finished appearance is extremely good ○: Finished appearance is good △: Finished appearance is slightly poor and adhesion was evaluated by a gobang test. The results are shown in Table 1.
The Gobang eye test is a test in which 100 squares of 2 mm square are drawn on the coating film with a cutter, and cellophane tape is pressed and peeled off instantaneously from the top to check the detached state of the 100 square coating film. is there. The evaluation criteria are as follows.
○: 0/100
X: 1/100 to 100/100
Moreover, after the water-resistant adhesion test immersed the coating-film test piece in 40 degreeC water for 10 days, the said goban adhesion test was done and evaluated.

  As shown in Table 1, by using the first clear paint of the present invention, a 3-coat 1-bake method can be adopted, and furthermore, the thermal contraction of the first clear paint film is suppressed. The finished appearance is improved. Furthermore, the interlayer clear adhesion of the first and second clear coatings is increased by the NCO / OH ratio (molar ratio) of the two liquids of the first clear coating, and the sharpness that more reflects the smoothness of the first clear coating. High coating film can be obtained.

  The top coating composition and the coating method of the present invention are effective for any application as long as the coating film requiring more aesthetics is formed, for example, can be used for the formation of a coating film for vehicles, In particular, the coating film for a vehicle can be used for forming a coating film for a vehicle exterior.

It is process schematic explaining an example of the coating process of the general part of a motor vehicle body in the coating method of this invention. It is process schematic explaining an example of the coating process of the door opening part of a motor vehicle body in the coating method of this invention. It is process schematic which explains the reference example of the painting process of the general part of the motor vehicle body for demonstrating the coating method of this invention. It is process schematic explaining the reference example of the coating process of the door opening part of the motor vehicle body for demonstrating the coating method of this invention.

Explanation of symbols

  10 substrate to be coated, 12 base coating film, 14 first clear coating film, 16 coating surface residual functional group, 18 second clear coating film.

Claims (4)

In a top coating for forming a multilayer coating film on a substrate to be coated, including a base coating, a first clear coating, and a second clear coating,
The first clear paint is a two-component polyurethane paint composed of an isocyanate compound and a polyhydric alcohol,
The second clear paint is a thermosetting paint containing a carboxyl group-containing polymer and an epoxy group-containing polymer,
The top coating composition, wherein the isocyanate compound and the polyhydric alcohol are mixed in an NCO / OH ratio of 0.5 to 1.0 in molar ratio. In a coating method for forming a base coating film on a substrate to be coated, and further forming a multilayer coating film having a first clear coating film and a second clear coating film in sequence,
Apply the base paint to the substrate to be coated,
Next, apply the first clear paint with wet-on-wet,
Next, apply the second clear paint with wet-on-wet,
After that, it is a painting method for baking and drying
The first clear paint is a two-component polyurethane paint composed of an isocyanate compound and a polyhydric alcohol,
The coating method according to claim 1, wherein the isocyanate compound and the polyhydric alcohol are mixed at an NCO / OH ratio in a molar ratio of 0.5 to 1.0.   An exterior part for automobiles, which is formed by applying the top coating composition according to claim 1. An exterior part for an automobile, which is coated using the coating method according to claim 2.

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