JP2011167614A - Laminated coating film forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain energy saving while ensuring excellent coating film finishing property and suppressing increase of the use amount of a solvent for a base coating material, in formation of laminated coating film. <P>SOLUTION: In the multi-layer coating film forming method, a solvent type base coating material containing a polyol resin and a curing agent is applied on an electrodeposition film 2 of a material 1 to be coated, and a two liquid urethane clear coating material containing a polyol resin and isocyanate is applied by wet-on-wet process to form a base coating film 3 and a clear coating film 4, which are baked and cured at the same time, wherein the application of the solvent type base coating material is divided into several stages, and in the stage of the application of the two liquid urethane clear coating material right before the application, the coating material having a molecular weight of the polyol resin larger than that in the other stage is used. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for forming a laminated coating film.

  Conventionally, in car body painting, undercoating (electrodeposition coating), intermediate coating, and top coating (base coating and clear coating) are performed in this order, and solvent-based coatings are used for the intermediate coating and base coating. It has been. Although base coating and clear coating are performed by wet-on-wet, it is necessary to bake and harden the coating film for each step of electrodeposition, intermediate coating and top coating. On the other hand, Patent Document 1 describes that intermediate coating, base coating, and clear coating are sequentially performed by wet on wet, that is, saving energy by omitting baking after intermediate coating. Yes.

  In addition, regarding the base coating, in recent years, from the viewpoint of reducing environmental burden (reducing the amount of organic solvent used), conversion from a solvent-based base coating to a water-based base coating has also been performed. For example, in Patent Document 2, a water-based paint is used for the base coating, and the water-based base coating is divided into two layers, a first layer and a second layer, and the UV transmittance of the first layer is lowered. It is described that the painting is omitted. However, in the case of a water-based base coating material, a pre-drying process for removing moisture after the base coating and an air-conditioning system for controlling the drying state of the base coating film are necessary for the clear-on-wet coating. Therefore, even if the intermediate coating is omitted, it is not so effective from the viewpoint of energy saving.

  In addition, Patent Document 3 relates to top coating of automobiles (base and clear wet-on-wet coating). When a low molecular weight polyol is used for the clear paint, the finished appearance is caused by a mixture of the base coat layer and the clear coat layer. In order to solve this problem, it is described that a clear coating material containing a polyol having a specific hydroxyl value and a number average molecular weight and a polyisocyanate is used.

JP 2007-75791 A Special table 2008-529766 gazette JP 2009-149825 A

  In the formation of the above-mentioned laminated coating film, from the viewpoint of energy saving, it is effective to be able to omit the intermediate coating and to eliminate the need for the preliminary drying process and the air conditioning equipment by using the base coating as a solvent type. . However, the intermediate coating film has a role of mitigating the impact against external force, and if this is omitted, the chipping resistance (the peeling resistance of the coating film against stepping stones) decreases.

  As a countermeasure against this, in the present invention, a two-component urethane clear coating having high impact absorption is adopted for the clear coating. In that case, the problem was the transfer of isocyanate (curing agent) from the clear coating film applied wet-on-wet to the base coating film. That is, when a part of isocyanate is transferred from the clear coating film to the base coating film, the curing rate of the base coating film varies during heat baking. That is, in the base coating film, the base coating surface side begins to cure from a lower temperature than the inside due to the isocyanate transferred from the clear coating film. Subsequently, internal curing starts, and the base coating film contracts due to the elimination of at least one of alcohol and blocking agent generated by the reaction between the polyol and the curing agent (at least one of melamine resin and blocked isocyanate resin). This is a problem that fine irregularities are generated on the surface and the finish (particularly the gloss of the coating film surface) is lowered.

  Then, this invention makes it a subject to aim at energy saving, suppressing the increase in the solvent usage-amount of a base coating material, obtaining favorable coating-film finish.

  According to the inventor's research, the transition of the clear coating isocyanate to the base coating is a phenomenon in which the organic solvent of the clear coating partially dissolves the polyol resin of the base coating. I understood it. And when the molecular weight of the polyol resin of the base coating film was increased, the migration of the isocyanate was suppressed, and it was found that the finish was not lowered. However, if the molecular weight of the polyol resin is increased, the viscosity of the base paint increases accordingly, and thus it is necessary to increase the amount of solvent in order to ensure good paintability. It leads to an increase in environmental load. Therefore, the present invention prevents the above-mentioned isocyanate migration while suppressing an increase in the amount of solvent used.

  Hereinafter, specifically described, a preferred embodiment of the present invention includes a polyol resin and a curing agent that reacts with a hydroxyl group of the polyol resin on the electrodeposition coating film of the article on which the electrodeposition coating film is formed. A solvent-based base coating is applied, and a two-component urethane clear coating containing a polyol resin and an isocyanate is applied thereon by wet-on-wet to form a base coating and a clear coating, and the base coating and A method for forming a multi-layer coating film in which a clear coating film is baked and cured at the same time, wherein the application of the solvent-based base coating is performed in a plurality of stages, and the stage immediately before the application of the two-component urethane clear coating is different from the other stages. Is also characterized in that a paint having a high molecular weight of the polyol resin is used.

  According to this method, since the molecular weight of the polyol resin on the surface of the base coating film is large, the transition of the isocyanate that is the curing agent of the clear coating material to the base coating film can be suppressed. Accordingly, it is possible to avoid a decrease in finish due to variations in the curing rate (timing) of the base coating film. Regarding the solvent amount of the base paint, the molecular weight of the polyol resin is large at the stage immediately before the application of the clear paint, so the amount of solvent for adjusting the viscosity of the paint increases, but at other stages, the molecular weight of the polyol resin is relative. Therefore, the amount of solvent used can be reduced. That is, as a whole, an increase in the amount of solvent used can be suppressed, and an increase in environmental load can be avoided. In addition, the adoption of a two-component urethane clear paint makes it possible to omit intermediate coating (no need for a solvent for intermediate coating), and the base paint is solvent-based. On the other hand, a preliminary drying process and air conditioning equipment are unnecessary, which is advantageous from the viewpoint of energy saving.

  As the curing agent for the solvent-based base paint, various types can be adopted, but it is preferable to use at least one of a melamine resin and a blocked isocyanate resin.

  The solvent-based base coating can be applied in two stages: a first stage using a paint having a relatively low molecular weight of the polyol resin and a second stage using a paint having a relatively high molecular weight of the polyol resin. That is, since base coating is usually performed in two stages, the method does not cause a substantial increase in work man-hours.

  When applying the above solvent-based base coating in three or more stages, use a coating with a high molecular weight of the polyol resin only in the stage immediately before the application of the two-component urethane clear coating, and the other stages have a relative molecular weight of the polyol resin. A small paint can be used.

  The mass average molecular weight of the polyol resin of the solvent-based base paint is preferably 50,000 to 200,000 in the stage immediately before the application of the two-component urethane clear paint, and 5000 to 30,000 in the other stages. This is advantageous for ensuring the suppression of the transition of the clear coating to the base coating film of the isocyanate, and also advantageous for suppressing an increase in the amount of solvent used as a whole.

  Examples of the object to be coated include an automobile body, and the present invention can be applied to other objects to be coated.

  According to the present invention, a solvent-based base paint containing a polyol resin and a curing agent that reacts with a hydroxyl group of the polyol resin is applied on the electrodeposition coating film of the object to be coated, and the polyol resin and the isocyanate are coated thereon. In the method for forming a multi-layer coating film, the base coating film and the clear coating film are formed by applying the contained two-component urethane clear coating composition wet-on-wet, and the base coating film and the clear coating film are simultaneously baked and cured. The mold base coating is divided into a plurality of stages, and the paint immediately before the application of the two-component urethane clear paint is a paint having a higher molecular weight of the polyol resin than the other stages. While suppressing an increase in the amount of solvent used, it is possible to suppress the migration of isocyanate from the clear coating to the base coating. While obtaining the finishing property, which is advantageous in terms of saving energy.

It is sectional drawing which shows the coating-film structure which concerns on embodiment of this invention. It is explanatory drawing of the example of this invention which shows the state by which the transfer of the isocyanate from a clear coating film to a base coating film is suppressed. It is explanatory drawing of a prior art example which shows a mode that isocyanate transfers to a base coating film from a clear coating film.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

  FIG. 1 shows a laminated coating film structure according to the present invention. In the figure, reference numeral 1 denotes a steel object, an electrodeposition coating 2 is formed thereon, a base coating 3 is formed thereon, and a clear coating 4 is formed thereon. . The base coating film 3 is formed by applying a base coating in two stages. The lower first base coating layer 3a by the first stage base coating and the upper second coating by the second stage base coating. It consists of the base coating layer 3b. The base coating film 3 is not clearly divided into two layers of the first base coating film layer 3a and the second base coating film layer 3b. For convenience of explanation, the portion of the first stage base coating is the first base coating layer 3a, and the portion of the second stage base coating is the second base coating layer 3b.

  Each of the first base coating layer 3a and the second base coating layer 3b of the base coating 3 is formed by application of a solvent-based base coating containing a polyol resin and a curing agent that reacts with the hydroxyl group of the polyol resin. Yes. However, the first base coating film layer 3a and the second base coating film layer 3b are formed of the same solvent-based base coating material, but the molecular weights of the polyol resins of the base coating materials used for each of them are different. That is, the first base coating film layer 3a is formed of a base paint containing a polyol resin having a relatively low molecular weight, and the second base coating film layer 3b is a base containing a polyol resin having a relatively high molecular weight. It is made of paint.

  The clear coating film 4 is formed by applying a two-component urethane clear paint containing a polyol resin and an isocyanate.

  The base coating film 3 and the clear coating film 4 are formed by applying a clear paint on the base coating film 3 by wet-on-wet and simultaneously baking and curing.

<Laminated coating film forming method>
In the method for forming a laminated coating film according to the present invention, first, an electrodeposition coating is applied to an object 1 such as an automobile body treated with zinc phosphate, and a baking and drying process is performed to form an electrodeposition coating film 2. A solvent-based base coating is applied on the electrodeposition coating 2 in a plurality of stages to form a base coating 3. Next, a two-component urethane clear coating is applied on the base coating 3 by wet on wet to form a clear coating 4. And the base coating film 3 and the clear coating film 4 are baked and hardened simultaneously.

-About electrodeposition coating-
The object 1 is immersed in a cationic electrodeposition paint, the object 1 is used as a cathode, and the electrode plate in the electrodeposition tank is used as an anode. Precipitation can be formed. The cationic electrodeposition coating contains a cationic epoxy resin, a curing agent, a pigment and an additive.

  Cationic epoxy resins include epoxy resins modified with amines. As the epoxy resin, those modified with a resin such as polyester polyol, polyether polyol and alkylphenol, or those obtained by extending the chain length of the epoxy resin can be used.

  As the curing agent, block polyisocyanate obtained by blocking polyisocyanate with a blocking agent can be used. The polyisocyanate may be any of aliphatic, alicyclic, aromatic-aliphatic and the like.

  The amount of the curing agent is generally preferably in the range of 80/20 to 50/50, expressed as a solid weight ratio of the cationic epoxy resin to the curing agent, and the amount of the cationic epoxy resin and the curing agent is generally electrodeposition. A range of 30 to 80% by weight of the total solid content of the coating composition is preferred.

  Electrodeposition paints generally contain pigments as colorants. Examples of color pigments include titanium oxide, carbon black and iron oxide, examples of extender pigments include kaolin, talc, aluminum silicate, calcium carbonate, mica and clay, examples of rust preventive pigments include zinc phosphate, Examples thereof include iron phosphate, aluminum phosphate, calcium phosphate, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, and calcium molybdate. The amount of the pigment can be in the range of 10 to 30% by weight of the total solid content of the electrodeposition coating composition.

-About base coating-
After the cationic electrodeposition coating / baking and drying treatment, a solvent-based base coating is applied onto the electrodeposition coating 2 by air spray coating, airless spray coating, rotary atomization coating, curtain coating, or the like. Electrostatic application may be performed during coating.

  The base paint contains the polyol resin and a curing agent that reacts with the polyol resin. As the polyol resin, an acrylic polyol resin (a polymer having a hydroxy group in a side chain obtained by polymerizing methacrylic acid esters) can be preferably used, but is not limited thereto, and is not limited to a polyether polyol or a polyester polyol. Other polyol resins can be used, or different types of polyol resins can be mixed and used. Moreover, a polyol resin and other film forming resin can be mixed and used. Examples of the curing agent include melamine resin, amino resin, and isocyanate curing agent. From the viewpoint of pigment dispersibility and workability, it is preferable to combine, for example, an acrylic polyol resin and / or a polyester polyol resin with a melamine resin and / or a blocked isocyanate resin.

  Examples of organic solvents include hydrocarbon solvents, ester solvents, ketone solvents, alcohol solvents, ether solvents, aromatic petroleum solvents and the like.

  If necessary, the base paint may contain pigments, non-aqueous dispersion resins, polymer fine particles, curing catalysts, ultraviolet absorbers, light stabilizers, coating surface conditioners, antioxidants, fluidity conditioners, waxes, etc. as appropriate. Can be contained.

  This base coating is performed in multiple stages. In this multi-stage coating, a base paint containing a polyol resin having a relatively high molecular weight is used in the stage immediately before clear coating, and a base paint containing a polyol resin having a relatively low molecular weight is used in the other stages. Use.

  Regarding the base coating immediately before the clear coating, the polyol resin preferably has a mass average molecular weight of 50,000 or more and 200,000 or less. One of the problems of the present invention is to suppress the transition of the isocyanate (curing agent) from the clear coating film 4 to the base coating film 3 and improve the finish of the coating film. The organic solvent 4 dissolves the polyol resin of the base coating film 3. In order to prevent the migration, the molecular weight of the polyol resin is increased in advance. If the mass average molecular weight is less than 50000, it is insufficient for inhibiting the migration of the isocyanate, and if the mass average molecular weight exceeds 200,000, a large amount of solvent is required for adjusting the viscosity of the base paint, and the environment. It is not preferable from the viewpoint of reducing the load. This base paint is Ford Cup #No. In a four viscometer, it is preferable to prepare so as to have a viscosity of about 15 seconds at 20 ° C.

  On the other hand, in the case of base paints at other stages, the polyol resin preferably has a mass average molecular weight of 5000 or more and 30000 or less. Another object of the present invention is to suppress an increase in the amount of organic solvent used in the base paint. As described above, in the base paint immediately before the clear coating, the amount of solvent used increases because of the use of a polyol resin having a large molecular weight. Therefore, in the other stages, by reducing the molecular weight of the polyol resin, the desired viscosity of the paint can be obtained even with a small amount of solvent, and the amount of the solvent used for forming the base coating film 3 should not be increased as a whole. Is. However, when the mass average molecular weight is less than 5000, there is a concern that the physical properties of the coating film such as weather resistance deteriorate. Moreover, when the said mass mean molecular weight exceeds 30000, it will become disadvantageous at the point of the increase suppression of the above-mentioned solvent usage-amount. This base paint is Ford Cup #No. In a four viscometer, it is preferable to prepare so as to have a viscosity of about 15 seconds at 20 ° C.

  The dry film thickness of the base coating film 3 can be set to, for example, 10 μm or more and 35 μm or less, and preferably 15 μm or more and 25 μm or less. Among them, the second base coating film layer 3b of, for example, 4 μm or more and 20 μm or less, preferably 5 μm or more and 15 μm or less is formed on the stage immediately before clear coating. When the film thickness of the base coating film 3 is increased, the sharpness may be deteriorated, or unevenness or flow may occur in the coating film. Neither of these is preferable because a discontinuous state of the film may occur.

-Clear paint-
A two-component urethane clear paint is applied on the base coating film 3 by an airless spray, air spray, a rotary atomizing coating machine or the like. Electrostatic application may be performed during coating.

  The two-component urethane clear coating contains a polyol resin and an isocyanate as a curing agent. For example, it contains a hydroxyl group-containing acrylic resin and a polyisocyanate compound. Examples of the hydroxyl group-containing acrylic resin include a hydroxyl group-containing polymerizable unsaturated monomer, or other polymerizable unsaturated monomers. Examples of the hydroxyl group-containing polymerizable unsaturated monomer include polyhydric alcohols and acrylic acid or methacrylic acid. A compound obtained by ring-opening polymerization of ε-caprolactone to a monoesterified product of the polyhydric alcohol and acrylic acid or methacrylic acid, and other polymerizable unsaturated monomers include acrylic acid or methacrylic acid. Alkyl esters of acids, carboxyl group-containing polymerizable unsaturated monomers, aminoalkyl acrylates, aminoalkyl methacrylates, acrylamides, methacrylamides or derivatives thereof, quaternary ammonium base-containing monomers, polyvinyl compounds, UV absorption or UV stability Polymerizable unsaturation Nomar and the like.

  Examples of the polyisocyanate compound include aliphatic diisocyanates, cycloaliphatic diisocyanates, aromatic diisocyanates, organic polyisocyanates themselves, cyclized polymers of organic polyisocyanates, and isocyanate / biuret bodies.

  Examples of organic solvents include hydrocarbon solvents, ester solvents, ketone solvents, alcohol solvents, ether solvents, aromatic petroleum solvents and the like.

  If necessary, the clear paint contains pigments, non-aqueous dispersion resins, polymer fine particles, curing catalysts, ultraviolet absorbers, light stabilizers, coating surface conditioners, antioxidants, fluidity conditioners, waxes, etc. as appropriate. Can be contained.

  Regarding the simultaneous baking and curing of the base coating film 3 and the clear coating film, the baking temperature may be, for example, 60 ° C. to 140 ° C., and the baking time may be, for example, 10 minutes to 40 minutes.

<Examples and Comparative Examples>
-Preparation of acrylic resin for base paint-
In an organic solvent in which 85 parts of xylol and 15 parts of n-butanol are mixed, monomers of 30 parts of styrene, 40 parts of n-butyl methacrylate, 10 parts of 2-ethylhexyl acrylate, 18 parts of 2-hydroxyethyl acrylate and 2 parts of acrylic acid are added. By reacting, respective solutions of six types of hydroxyl group-containing acrylic resins A-1, A-2, A-3, B-1, B-2, and B-3 having different mass average molecular weights were prepared. Each molecular weight is as follows, and the resin solid content of any hydroxyl group-containing acrylic resin solution is 50% by mass.

Hydroxyl-containing acrylic resin A-1; mass average molecular weight 5000
Hydroxyl-containing acrylic resin A-2; mass average molecular weight 15000
Hydroxyl group-containing acrylic resin A-3; mass average molecular weight 30000
Hydroxyl-containing acrylic resin B-1; mass average molecular weight 50,000
Hydroxyl group-containing acrylic resin B-2; mass average molecular weight 80000
Hydroxyl group-containing acrylic resin B-3; mass average molecular weight 200000

-Preparation of base paint-
In each solution of the hydroxyl group-containing acrylic resins A-1, A-2, A-3, B-1, B-2, B-3, Uban 20SE (Butylated melamine curing agent manufactured by Mitsui Toatsu Chemical Co., Ltd .: solid) 60 mass%), an aluminum paste (pigment) was added and stirred, and Swazol 1000 (a petroleum aromatic solvent manufactured by Cosmo Oil Co., Ltd.) was further added. The paints A-1, A-2, A-3, B-1, B-2, and B-3 shown in Table 1 that were diluted and adjusted to have a viscosity of 15 seconds / 20 ° C. with a 4 Ford cup were obtained.

-Preparation of test piece-
The dull steel plate was coated with a cationic electrodeposition paint PN-1020 (manufactured by Nippon Paint Co., Ltd.) so as to have a dry film thickness of 20 μm, and the electrodeposition coating film was baked at 160 ° C. for 30 minutes to be cured. Next, the first base paint and the second base paint shown in Table 2 selected from the paints A-1, A-2, A-3, B-1, B-2, and B-3 shown in Table 1 are respectively dried. The base coating film was formed by air spray coating on the electrodeposition coating film so that the thickness was 10 μm. After standing at room temperature for 5 minutes, KINO # 6800 (two-component urethane clear paint manufactured by Kansai Paint Co., Ltd.) was applied by air spraying so that the dry film thickness was 30 μm to form a clear coating film (wet on wet). After standing at room temperature for 10 minutes, the base coating and the clear coating were baked at 140 ° C. for 30 minutes to be cured. Thus, test pieces of Examples 1 to 9 and Comparative Examples 1 to 7 shown in Table 2 were obtained.

-Coating film evaluation method-
Using a WaveScan DOI manufactured by BYK, the degree of waviness of the coating film surface when the test piece was applied vertically was measured by the structural spectra Wa (0.1 to 0.3 mm) and Wd (3.0 to 10.0 mm). The results are shown in Table 2. The measured value Wa represents the glossiness of the coating film. Wd represents the smoothness of the coating film. It can be said that the finished values Wa and Wd have better finish as the numerical values are smaller.

-Laminated coating film evaluation-
According to Table 2, it can be seen that Examples 1 to 9 all have small Wa values and Wd values and good finishing properties. Moreover, Examples 1-9 have comparatively high average solid content of a base coating material, and it turns out that the usage-amount of a solvent is suppressed. On the other hand, in Comparative Examples 1 to 4, although the average solid content of the base paint is high, the Wa value and the Wd value are large, which is difficult in terms of finish. Moreover, although Comparative Example 5-7 has small Wa value and Wd value, the average solid content of a base coating material is low, and there are many solvent usage-amounts.

  FIG. 2 is an explanatory diagram of the embodiment and shows a state in which the migration of the isocyanate 12 from the clear coating film 4 to the base coating films 3a and 3b is suppressed. That is, in the case of the example, the second base coating film layer 3b made of the second base paint has a large molecular weight of the acrylic resin 13, so that the dissolution of the acrylic resin 13 by the solvent of the clear coating film 4 does not proceed so much. Therefore, it is recognized that the transition of the clear coating 4 to the base coating side of the isocyanate 12 is suppressed, the entire base coating is uniformly cured, and the finish is improved. Moreover, since the acrylic resin 14 of the 1st base coating film layer 3a has a small molecular weight, there is little solvent usage-amount.

  FIG. 3 is an explanatory diagram of the case of Comparative Example 2, for example. That is, in Comparative Example 2, the acrylic resin 15 of each of the first base paint and the second base paint has a mass average molecular weight of 30000. In this case, as the acrylic resin 13 of the base coating film 3 is dissolved by the solvent of the clear coating film 4, the isocyanate 12 moves from the clear coating film 4 to the base coating film 3 in a relatively large amount. Therefore, it is recognized that the base coating film 3 is unevenly cured at the time of heating and baking, that is, the surface side of the base coating film 3 is cured first, and the finish is deteriorated.

  2 and 3, reference numeral 11 denotes a resin of the clear coating film 4.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Electrodeposition coating 3 Base coating 3a First base coating 3b Second base coating 4 Clear coating 12 Clear coating isocyanate 13 Base coating acrylic resin (polyol resin)

Claims (5)

A solvent-based base paint containing a polyol resin and a curing agent that reacts with a hydroxyl group of the polyol resin is applied on the electrodeposition coating film of the object on which the electrodeposition coating film is formed, and the polyol resin is coated thereon. And forming a base coating film and a clear coating film by applying wet-on-wet two-component urethane clear paint containing isocyanate and isocyanate, and simultaneously baking and curing the base coating film and the clear coating film. ,
Lamination characterized in that the application of the solvent-based base paint is performed in a plurality of stages, and the paint immediately before the application of the two-component urethane clear paint uses a paint having a higher molecular weight of the polyol resin than the other stages. Coating film forming method. In claim 1,
A method for forming a laminated coating film, comprising using at least one of a melamine resin and a blocked isocyanate resin as a curing agent for the solvent-based base paint. In claim 1 or claim 2,
A method for forming a laminated coating film, wherein the solvent-based base coating is applied in two stages. In any one of Claim 1 thru | or 3,
The multilayer coating film characterized by having a mass average molecular weight of the polyol resin of the solvent-based base coating in the stage immediately before the application of the two-component urethane clear coating in the range of 50,000 to 200,000 and in the other stages of 5,000 to 30,000. Forming method. In any one of Claims 1 thru | or 4,
A method for forming a laminated coating film, wherein the article to be coated is a car body of an automobile.

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