Method for forming laminated coating films

Abstract

To form a coating film on a sealer part without applying dry processing of the sealer part when laminating the coating film on the sealer part, improve adhesion between the sealer part and the coating film, and suppress slipping of the coating film.SOLUTION: A method according to the present invention comprises: a process in which an electro-deposition film 2 is formed on a coated object 1; a process in which a sealer coating film 3 containing a plasticizer 8 is formed on the electro-deposition film; and a process in which an undercoat coating film 4 is formed on the sealer coating film in a non-dry state. In the process in which the undercoat coating film is formed on the sealer coating film, a paint for forming the undercoat coating film contains a dilution solvent, the dilution solvent has a functional group same as the plasticizer which the sealer coating film contains, an SP value, which is a solubility parameter, falls within a range of ±0.6 with respect to an SP value of the plasticizer, and an evaporation rate (a relative evaporation rate when a standard butyl acetate evaporation rate is 100) is 100 or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for forming a laminated coating film. For example, it relates to a method for forming a laminated coating film in which an intermediate coating layer and a colored coating film are sequentially formed on a sealer portion, and a clear coating film is formed thereon.

Background Art

[0002] An automotive coating film is generally formed by sequentially laminating a primer coating film 51 (electrodeposited film) mainly for rust prevention, an intermediate coating film 52, and a top coating film 60 on an object to be coated 50, as schematically shown in a cross-sectional view in FIG. 3. The top coating film 60 is composed of a base coating film 53 (coloring layer) containing a pigment for coloring and a clear coating film 54 thereon. In addition to the purpose of coloring, the base coating film 53 aims to improve the adhesion and smoothness between the intermediate coating film 52 and the clear coating film 54.

[0003] In addition to the purpose of improving the adhesion and smoothness with the primer coating film 51 and the top coating film 60, the intermediate coating film 52 aims to enhance light resistance deterioration resistance, chipping resistance, and coloring properties. In particular, when the primer coating film 51 formed by an epoxy-based cationic electrodeposition paint is irradiated with a large amount of ultraviolet rays, the surface layer portion thereof deteriorates, and the coating film above it peels off. Therefore, the primer coating film 51 is protected from ultraviolet rays by the intermediate coating film 52 to enhance light resistance deterioration resistance.

[0004] By the way, in parts constituting an automotive body as an object to be coated, such as doors, bonnet hoods, roofs, etc., there are joints or seams where two plate-shaped steel plates are overlapped with each other. There are gaps at the boundaries of these joints and seams. Such gaps are difficult to cover only by painting using ordinary paint compositions such as an intermediate coating paint composition, a top base coating paint composition, and a top clear coating paint composition. Furthermore, foreign substances such as water and dust easily enter the gaps at the boundaries existing in the joints or seams, and there is also a problem that these foreign substances cause rust generation. Therefore, in such joints or seams, a sealer layer (sealer coating) is generally provided by applying and filling with a sealant. The application and filling of the sealant is generally performed on steel plates on which an electrodeposited coating has been formed. Then, an intermediate coating and a topcoat are formed on the object to which the sealer layer has been provided.

[0005] However, when forming an intermediate coating on the sealer portion, there was a problem in that the adhesion between the sealer portion and the intermediate coating was reduced due to the influence of plasticizers contained in the sealer portion. To address this issue, for example, Patent Document 1 discloses a method for forming a coating film in which a sealer portion formed on a substrate using a polyvinyl chloride sealant containing diisononyl phthalate (DINP) as a plasticizer is applied, and then an aqueous basecoat coating composition containing a hydroxyl group-containing polyurethane resin with a solubility parameter (SP) value of 10.0 or higher of the resin component is applied to form an aqueous basecoat coating film. Patent Document 1 states that the measured SP value of the sealant is 10.6, and that the SP value of the hydroxyl group-containing polyurethane resin is 10.0 or higher for sufficient adhesion. This improves the adhesion between the sealer and the undercoat paint. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-100574 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In the embodiment described in Patent Document 1, after applying a sealant to the object to be coated, the sealant is formed by, for example, drying at room temperature for 2 hours or forced drying at 80°C for 5 minutes. In other words, the invention disclosed in Patent Document 1 is a method for forming a coating film that provides an effective effect on a dried sealer portion, and is based on the premise that a drying process is carried out after applying a sealant to the coated object.

[0008] However, in the formation of the coating film, there is a need to reduce energy consumption and the number of steps involved, and it is desirable to apply an intermediate coat of paint over the applied sealant (sealer portion) without drying it first. However, as mentioned above, sealants contain plasticizers, and these plasticizers bleed (seep out) onto the surface of the sealer. As a result, when an intermediate coating is formed on the surface of the sealer, the plasticizers prevent the intermediate coating from adhering to the sealer and cause it to slide in the direction of the surface. In particular, when the sealer is located on the edge of an automobile body, the intermediate coating flows due to its own weight, resulting in a thin, translucent appearance of the paint (hereinafter referred to as "transparency").

[0009] The present invention has been made in view of the above circumstances, and aims to provide a method for forming a laminated coating film that, when laminating a coating film on a sealer portion, forms a coating film on the sealer portion without drying the sealer portion, improves the adhesion between the sealer portion and the coating film, and suppresses slippage of the coating film. [Means for solving the problem]

[0010] To solve the aforementioned problems, the present invention provides a method for forming a laminated coating film on a workpiece, comprising the steps of: forming an electrodeposited film on the workpiece; forming a sealer coating film containing a plasticizer on the electrodeposited film; and forming a primer coating film on the undried sealer coating film, wherein the paint for forming the primer coating film contains a diluent, and the diluent solvent has the same functional groups as the plasticizer contained in the sealer coating film, its solubility parameter SP value is within ±0.6 of the SP value of the plasticizer, and its evaporation rate (relative evaporation rate when the evaporation rate of butyl acetate, a reference, is set to 100) is 100 or more. Furthermore, in the step of forming a primer film on the sealer film, it is desirable that the ratio of the diluting solvent to the total solvent in the primer film be 20% or more. Furthermore, in the step of forming a sealer coating containing a plasticizer on the electrodeposited film, it is desirable to use a phthalate ester as the plasticizer, and in the step of forming a primer coating on the sealer coating, it is desirable to use ethyl acetate or toluene as the diluent solvent contained in the paint for forming the primer coating.

[0011] Thus, according to the present invention, a primer coating is formed on an undried sealer coating containing a diluent solvent that has a high evaporation rate, the same functional groups as the plasticizer contained in the sealer coating, and an SP value (solubility parameter) that is similar to that of the plasticizer. While the plasticizer contained within the sealer film bleeds (seeps out) onto the upper surface of the undried sealer film, the solvent contained in the primer film evaporates rapidly. As a result, the plasticizer is incorporated into the primer film by the upward force of the solvent, and the solvent evaporates from the upper surface of the primer film. Consequently, the plasticizer contained in the sealer film is incorporated into the lower layer of the primer film from the surface, thereby maintaining a non-slip surface against the sealer film. This prevents the paint film from slipping on the sealer film and suppresses the occurrence of skew, even when the sealer film is formed on the edges of the vehicle body. [Effects of the Invention]

[0012] According to the present invention, when laminating a coating film on a sealer portion, it is possible to form a coating film on the sealer portion without drying the sealer portion, thereby improving the adhesion between the sealer portion and the coating film and suppressing slippage of the coating film. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a schematic cross-sectional view showing a laminated coating structure according to an embodiment of the present invention. [Figure 2] Figures 2(a) and 2(b) are schematic cross-sectional views illustrating the incorporation of plasticizer from the sealer portion into the undercoat film in embodiments of the present invention. [Figure 3] Figure 3 is a schematic cross-sectional view showing an example of a conventional laminated coating structure. [Modes for carrying out the invention]

[0014] The following describes embodiments of the laminated coating film formation method according to the present invention, based on the drawings. The following description of preferred embodiments is essentially illustrative and is not intended to limit the present invention, its applications, or its uses.

[0015] Figure 1 is a cross-sectional view showing a laminated coating structure formed by a laminated coating formation method according to an embodiment of the present invention. In FIG. 1, the object to be coated 1 is a steel object to be coated used, for example, as an outer panel of an automobile body, and shows a portion where there is a joint or seam (referred to as seam 1a for the purpose of explanation) formed by overlapping two plate-shaped steel plates with each other. On the seam 1a of this object to be coated 1, a primer coating film (electro-deposited film) 2 as an undercoat film is formed, and a sealer portion 3 (sealer coating film) is formed on this primer coating film 2. Further, on the sealer portion 3, an undercoat coating film 4, an intermediate coating film 5, a color coating film 6, and a clear coating film 7 are sequentially laminated.

[0016] The object to be coated 1 is an iron plate, a steel plate, aluminum, stainless steel, etc., and is, for example, an iron plate constituting the vehicle body of an automobile. The primer coating film 2 is formed by immersing the object to be coated 1 in a cationic electro-deposition paint, using the object to be coated 1 as the cathode and the electrode plate in the electro-deposition tank as the anode, and flowing a direct current therebetween to deposit and form on the object to be coated 1 side. The thickness of this primer coating film 2 is formed, for example, to be 20 μm.

[0017] The sealer portion 3 is formed, for example, by a polyvinyl chloride sealing agent. The polyvinyl chloride sealing agent contains diisononyl phthalate (DINP) as a plasticizer. Phthalic acid ester is blended into the sealing agent as a plasticizer that gives flexibility, has durability, and is of low cost.

[0018] The undercoat coating film 4 is formed by a film formation using an oil-based (solvent type) paint added with a pigment. Specifically, as the oil-based paint, for example, there are a melamine-curing type oil-based paint containing a polyester resin, a melamine resin, and a coloring pigment, etc. This primer coating film 4 is provided to improve the adhesion between the sealer part 3 and the intermediate coating film 5. In the interior (lower layer part) of the primer coating film 4, the plasticizer 8 contained in the sealer part 3 is taken in from the surface of the sealer part 3. The primer coating film 4 is formed of a composition with a high coating viscosity having a viscosity of about 1000 Pa·s after 90 seconds. By including a solvent with a high evaporation rate such as ethyl acetate in the primer coating film 4, the film thickness during drying is 5 μm, and with a temperature of 23°C and a relative humidity of 65%, a large amount of the solvent evaporates, and the solid content concentration rapidly increases, resulting in a high viscosity. For example, the film thickness is formed to be 3 μm or more and 20 μm or less. If it is thinner than 3 μm, the primer coating film 4 becomes discontinuous on the surface of the sealer part 3 and cannot cover it. If it is thicker than 20 μm, the amount of solvent contained in the primer coating film 4 increases, so the coating viscosity becomes low, and it becomes easy to slip against the sealer part 3.

[0019] Also, the intermediate coating film 5 and the color-developing coating film 6 are formed by an oil-based (solvent-based) intermediate coating paint to which a pigment is added as a colorant. Specifically, as the oil-based intermediate coating paint, for example, there are melamine-curing type oil-based paints containing a polyester resin, a melamine resin, and a coloring pigment, etc. The film thicknesses of the intermediate coating film 5 and the color-developing coating film 6 are each formed to be 5 μm or more and 40 μm or less.

[0020] Also, for the clear coating film 4, the resin forming it is not particularly limited, but it is formed by a combination of an acrylic resin and / or a polyester resin and an amino resin, or an acrylic resin and / or a polyester resin having a carboxylic acid-epoxy curing system, etc. The film thickness of the clear coating film 4 is formed to be, for example, 30 μm.

[0021] As described above, in the lower layer part of the primer coating film 4, the plasticizer 8 contained in the sealer part 3 is taken in from the surface of the sealer part 3, thereby maintaining a state where it is difficult to slip against the sealer part 3. Furthermore, since the primer coating film 4 is formed with a high coating viscosity, the coating film is difficult to move, and the incorporated plasticizer is prevented from moving to the upper layer. As a result, even if the area where the sealer portion 3 is formed is an edge of a vehicle or the like, the paint film laminated on the sealer portion 3 will not slide off, and the occurrence of paint film bleed is suppressed.

[0022] Next, the method for forming a laminated coating film according to the present invention will be described. The present invention relates to a method for forming a laminated coating film on a joint or seam formed by overlapping two plate-shaped steel plates.

[0023] (Formation of primer coating) As described above, the object to be painted 1 is, for example, an iron plate, steel plate, aluminum, and stainless steel. For example, if the object to be painted is an iron plate of an automobile, a primer layer may be formed on the iron plate in advance. The primer layer is a film of paint or resin formed on the surface of the vehicle body by electrodeposition coating, and plays a role in ensuring corrosion resistance and adhesion with the intermediate coating film. This primer coating film 2 is deposited on the object to be painted 1 by immersing the object to be painted 1 in cationic electrodeposition paint, with the object to be painted 1 as the cathode and the electrode plate in the electrodeposition tank as the anode, and passing a DC current between them. The thickness of this primer coating film 2 is formed to, for example, 20 μm.

[0024] Cationic electrodeposition coatings include cationic epoxy resins, curing agents, and pigments and additives. Cationic epoxy resins include epoxy resins modified with amines. Epoxy resins that have been modified with resins such as polyester polyols, polyether polyols, and alkylphenols, as well as epoxy resins with extended chain lengths, can be used.

[0025] Compounds into which cationic groups can be introduced include salts of primary amines, secondary amines, and tertiary amines, sulfides, and acid mixtures. Examples include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine hydrochloride, N,N-dimethylethanolamine acetate, diethyldisulfide-acetic acid mixtures, ketimine of aminoethylethanolamine, and diketimine of diethylenetriamine, which are secondary amines in which primary amines have been blocked.

[0026] As a curing agent, blocked polyisocyanate obtained by blocking polyisocyanate with a blocking agent can be used. The polyisocyanate may be any of the following: aliphatic, alicyclic, aromatic-aliphatic, etc.

[0027] Examples of aromatic isocyanates among polyisocyanates include tolylene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI). Examples of aliphatic isocyanates include hexamethylene diisocyanate (HDI) and 2,2,4-trimethylhexane diisocyanate. Examples of alicyclic isocyanates include 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (hydrogenated MDI), 1,3-diisocyanatomethylcyclohexane (hydrogenated XDI), hydrogenated TDI, and norbornane diisocyanate. Examples of aromatic-aliphatic isocyanates include xylylene diisocyanate (XDI) and tetramethylxylylene diisocyanate (TMXDI). Other examples include modified isocyanates such as urethanes, biuretes, and modified isocyanurates. These can be used individually or in combination of two or more.

[0028] Examples of blocking agents include lactam-based blocking agents such as ε-caprolactam, and oxime-based blocking agents such as formaldehyde. The amount of curing agent is generally expressed as the solids weight ratio of the cationic epoxy resin to the curing agent, and is generally in the range of 80 / 20 to 50 / 50. The amounts of cationic epoxy resin and curing agent are generally in the range of 30 to 80% by weight of the total solids of the electrodeposition coating composition.

[0029] Electrodeposition coatings generally contain pigments as colorants. Examples of coloring pigments include titanium dioxide, carbon black, and iron oxide; examples of extender pigments include kaolin, talc, aluminum silicate, calcium carbonate, mica, and clay; and examples of rust-preventive pigments include zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, and calcium molybdate. The amount of pigment can be in the range of 10 to 30% by weight of the total solids of the electrodeposition coating composition.

[0030] (Formation of the sealant layer) Next, a polyvinyl chloride sealant containing diisononyl phthalate (DINP) as a plasticizer is applied to the primer coating 2 of the object to be coated 1 to form an undried sealer portion 3.

[0031] (Formation of the undercoat film) Next, an oil-based (solvent-type) primer paint with added pigment is formed on the undried sealer portion 3 to create a primer film 4. As mentioned above, an oil-based primer paint is, for example, a melamine-curing type oil-based paint (40% solvent) containing polyester resin, melamine resin, and coloring pigment. To this melamine-curing type oil-based paint (40% solvent), a diluent solvent such as ethyl acetate is added so that the ratio of the diluent solvent to the total solvent is, for example, 20% or more, more preferably 33% or more. The diluent solvent must have a high evaporation rate (relative evaporation rate of 100 or more when the evaporation rate of butyl acetate, a reference, is set to 100), and to improve affinity with the plasticizer contained in the sealer portion 3, it must have the same functional group as the plasticizer and have a solubility parameter (SP value) close to that of the plasticizer (within ±0.6). Specifically, as mentioned above, ethyl acetate (SP value 9.1), an ester-based solvent, or toluene (SP value 8.9), an aromatic hydrocarbon-based solvent, can be used.

[0032] Here, as shown in Figure 2(a), the plasticizer 8 contained in the sealer portion 3 bleeds (seeps out) onto the upper surface of the undried sealer portion 3. This plasticizer 8 is then incorporated into the primer film 4 as shown in Figure 2(b) by the upward force of the solvent 9 contained in the primer film 4 evaporating. The solvent 9 evaporates from the upper surface of the primer film 4. As a result, the plasticizer contained in the sealer 3 is incorporated into the lower layer of the undercoat film 4 from the surface of the sealer 3, thereby maintaining a non-slip state against the sealer 3. Furthermore, because the undercoat film 4 is formed with high adhesion viscosity (for example, viscosity of about 1000 Pa·s 90 seconds after application), the film is less likely to move, and the incorporated plasticizer is less likely to move to the upper layer.

[0033] (Formation of the intermediate coating and the colored coating) Next, on the undercoat film 4, an intermediate coat film 5 and a coloring film 6 are sequentially formed by methods such as air spray painting, airless spray painting, rotary atomization painting, or curtain coat painting, so that the dry film thickness (when the solvent has completely evaporated) is 5 μm or more. Electrostatic application may also be performed during painting. If the dry film thickness of the intermediate coating 5 and the coloring coating 6 is less than 5 μm, the strength will be insufficient, which is undesirable.

[0034] Here, the intermediate coating film 5 and the colored coating film 6 can be formed by applying an oil-based (solvent-type) intermediate coating paint. Specifically, the oil-based (solvent-type) intermediate coating paint can be, for example, a melamine-curing oil-based paint containing polyester resin, melamine resin, coloring pigment, extender pigment, talc, and surface modifiers and dispersants. For example, hydrocarbon solvents, ester solvents, ketone solvents, alcohol solvents, ether solvents, aromatic petroleum solvents, etc., may be used as solvents.

[0035] In addition, pigments are added as colorants to oil-based colored paints. Examples of pigments include coloring pigments, extender pigments, and luminescence pigments. Examples of coloring pigments include organic azochelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolon pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments. Inorganic pigments include lead yellow, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. Furthermore, extender pigments such as calcium carbonate, barium sulfate, clay, and talc may also be used.

[0036] (Formation of a clear coating) Then, a clear coating 7 is applied to the colored coating 6 using an airless spray, air spray, rotary atomizer, or the like to form a clear coating 7. Electrostatic discharge may be applied during painting. After painting to a dry film thickness of 20-40 μm, the coating is heated at 140 degrees Celsius for 20 minutes to cure. The resin used to form the clear coating film 7 is not particularly limited, but examples include a combination of acrylic resin and / or polyester resin and amino resin, or an acrylic resin and / or polyester resin having a carboxylic acid-epoxy curing system.

[0037] For example, a two-component urethane clear coating contains a hydroxyl group-containing acrylic resin and a polyisocyanate compound. Examples of hydroxyl group-containing acrylic resins include hydroxyl group-containing polymerizable unsaturated monomers or other polymerizable unsaturated monomers. Examples of hydroxyl group-containing polymerizable unsaturated monomers include monoesters of polyhydric alcohols with acrylic acid or methacrylic acid, compounds obtained by ring-opening polymerization of the monoester of the polyhydric alcohols with acrylic acid or methacrylic acid with ε-caprolactone, and other polymerizable unsaturated monomers. Examples of other polymerizable unsaturated monomers include alkyl esters of acrylic acid or methacrylic acid, carboxyl group-containing polymerizable unsaturated monomers, aminoalkyl acrylates, aminoalkyl methacrylates, acrylamides, methacrylamides or their derivatives, quaternary ammonium base-containing monomers, polyvinyl compounds, and UV-absorbing or UV-stable polymerizable unsaturated monomers.

[0038] Examples of polyisocyanate compounds include aliphatic diisocyanates, cyclic aliphatic diisocyanates, aromatic diisocyanates, organic polyisocyanates themselves, cyclized polymers of organic polyisocyanates, and isocyanate biuret compounds. Examples of organic solvents include hydrocarbon solvents, ester solvents, ketone solvents, alcohol solvents, ether solvents, and aromatic petroleum solvents.

[0039] Clear coatings may contain, as needed, pigments, non-aqueous dispersion resins, polymer microparticles, curing catalysts, UV absorbers, light stabilizers, surface conditioners, antioxidants, flow modifiers, waxes, etc. Examples of curing catalysts include organotin compounds, triethylamine, and diethanolamine. Examples of UV absorbers include benzophenone-based, benzotriazole-based, cyanoacrylate-based, salicylate-based, and oxalic acid anilide-based compounds, as well as UV stabilizers such as hindered amine compounds.

[0040] As described above, according to the embodiment of the present invention, a primer coating 4 containing a diluent solvent having a high evaporation rate, the same functional groups as the plasticizer contained in the sealer portion 3, and an SP value similar to that of the plasticizer is formed on the undried sealer portion 3. On the upper surface of the undried sealer portion 3, the plasticizer 8 contained within the sealer portion 3 bleeds (seeps out), but because the solvent 9 contained in the undercoat film 4 evaporates rapidly, the plasticizer 8 is incorporated into the undercoat film 4 by the force that moves it upwards from the solvent 9, and the solvent 9 evaporates from the upper surface of the undercoat film 4. As a result, the plasticizer contained in the sealer portion 3 is incorporated into the lower layer of the undercoat film 4 from the surface of the sealer portion 3, thereby maintaining a non-slip state relative to the sealer portion 3. This prevents the paint film on the sealer portion 3 from slipping and suppresses the occurrence of skew, even when the sealer portion 3 is formed on the edge of the vehicle body.

[0041] In the above embodiment, a configuration was described in which an intermediate coating 5, a color-developing coating 6, and a clear coating 7 are sequentially laminated on the undercoat coating 4. However, the present invention is not limited to this configuration and can be broadly applied to configurations in which the undercoat coating 4 is formed on the sealer portion 3. For example, the intermediate coating 4 may be omitted, and a configuration in which the color-developing coating 6 and the clear coating 7 are sequentially laminated on the undercoat coating 4 may be used. [Examples]

[0042] The method for forming a laminated coating film according to the present invention will be further described based on examples. (Experiment 1) Experiment 1 investigated the conditions of the solvent contained in the primer coating. In Example 1, the sealer consisted of 15% PVC (polyvinyl chloride) as the main resin, 50% calcium carbonate or the like as a filler, and 25% phthalate ester (ester, aromatic hydrocarbon type, SP value 8.9) as a plasticizer. In addition, 10% each of polyamide isocyanate, stabilizer, and high-boiling point solvent were included. The viscosity of the phthalate ester plasticizer after 1.5 minutes of application was 0.03 Pa·s. The SP value of the phthalate ester plasticizer was 8.9. The primer film was formed by adding ethyl acetate (SP value 9.1, evaporation rate 391 (relative evaporation rate when the evaporation rate of butyl acetate, which serves as a reference, is set to 100)), an ester-based solvent, at a concentration of 33% of the total solvent to a melamine-curing oil-based paint (40% solvent) containing polyester resin, melamine resin, and coloring pigment.

[0043] In Example 1, the undercoat film was formed on the undried sealer portion that had been deposited on the edge portion of the vehicle body, which was the object to be coated. Then, an intermediate coat film, a coloring film, and a clear coat film were formed in order on top of the undercoat film. Afterward, the edges of the dried car body were observed to evaluate whether or not skew had occurred in the paint film (skewness evaluation).

[0044] In Example 2, a melamine-curable oil-based paint (40% solvent) for forming a primer film was mixed with toluene (SP value 8.9, evaporation rate 196 (relative evaporation rate when the evaporation rate of butyl acetate, which serves as a reference, is set to 100)) as a diluent, at a concentration of 33% of the total solvent. Other conditions were the same as in Example 1.

[0045] Example 3 Next, to form the undercoat film, a melamine-curing oil-based paint (40% solvent) was used, and 33% of isobutyl acetate (SP value 8.3, evaporation rate 150 (relative evaporation rate when the evaporation rate of standard butyl acetate is set to 100)), an ester-based solvent, was added as a diluent. All other conditions were the same as in Example 1.

[0046] Example 4 Next, to form the undercoat film, 33% of butyl acetate (SP value 8.5, evaporation rate 100 (relative evaporation rate when the evaporation rate of standard butyl acetate is set to 100)), an ester-based solvent, was added to a melamine-curable oil-based paint (40% solvent) as a diluent. All other conditions were the same as in Example 1.

[0047] Comparative Example 1 Next, to form the undercoat film, a melamine-curable oil-based paint (40% solvent) was used, and 33% of the total solvent was added as a diluent, methyl ethyl ketone (SP value 9.3, evaporation rate 452 (relative evaporation rate when the evaporation rate of butyl acetate, the standard, is set to 100)), which is a ketone-based solvent. All other conditions were the same as in Example 1.

[0048] Comparative Example 2 Next, to form the undercoat film, a melamine-curable oil-based paint (40% solvent) was used, and 33% of the total solvent was added as a diluent, methyl isobutyl ketone (SP value 8.6, evaporation rate 150 (relative evaporation rate when the evaporation rate of butyl acetate, the standard, is set to 100)), which is a ketone-based solvent. All other conditions were the same as in Example 1.

[0049] Comparative Example 3 Next, to form the undercoat film, a melamine-curable oil-based paint (40% solvent) was used, and xylene (SP value 8.8, evaporation rate 59 (relative evaporation rate when the evaporation rate of butyl acetate, which serves as the standard, is set to 100)), an aromatic hydrocarbon solvent, was added at a concentration of 33% of the total solvent to form the undercoat film. All other conditions were the same as in Example 1.

[0050] Comparative Example 4 Next, to form the undercoat film, a melamine-curable oil-based paint (40% solvent) was used, and 33% of the total solvent was added as a diluent, using solvent naphtha (SP value 8.8, evaporation rate 17 (relative evaporation rate when the evaporation rate of butyl acetate, the standard, is set to 100)), which is an aromatic hydrocarbon solvent. All other conditions were the same as in Example 1.

[0051] Comparative Example 5 Next, to form the undercoat film, a melamine-curing oil-based paint (40% solvent) was used, and acetone (SP value 9.9, evaporation rate 560 (relative evaporation rate when the evaporation rate of butyl acetate, which serves as the standard, is set to 100)), a ketone-based solvent, was added at a concentration of 33% of the total solvent to form the undercoat film. All other conditions were the same as in Example 1.

[0052] Comparative Example 6 Next, to form the undercoat film, a melamine-curing oil-based paint (40% solvent) was used, and 33% of the total solvent was added as a diluent, butanol (SP value 11.4, evaporation rate 45 (relative evaporation rate when the evaporation rate of butyl acetate, the standard, is set to 100)), which is an alcohol-based solvent. All other conditions were the same as in Example 1.

[0053] Comparative Example 7 Next, to form the undercoat film, a melamine-curing oil-based paint (40% solvent) was used, and 33% of isopropyl alcohol (SP value 11.5, evaporation rate 150 (relative evaporation rate when the evaporation rate of butyl acetate, which serves as the standard, is set to 100)), an alcohol-based solvent, was added as a diluent. All other conditions were the same as in Example 1.

[0054] Comparative Example 8 Next, to form the undercoat film, 33% of ethanol (SP value 12.7, evaporation rate 203 (relative evaporation rate when the evaporation rate of butyl acetate, which serves as the standard, is set to 100)), an alcohol-based solvent, was added to the total solvent as a diluent. All other conditions were the same as in Example 1.

[0055] The results of Experiment 1 are shown in Table 1. In Table 1, the evaluation of transparency was as follows: ◎ indicates no transparency at all, and ○ indicates slight, barely noticeable transparency. slightly Items with noticeable bleed were marked with a triangle (△), while those with obvious bleed were marked with a cross (×).

[0056] [Table 1]

[0057] As shown in Table 1, for example, Example 3 So, although it has the same functional groups as plasticizers and a fast evaporation rate, its SP value is 0.6 different from that of plasticizers. Compared to Example 1 The transparency rating was somewhat poor. Also, Example 4 So, it has the same functional groups as plasticizers, and its SP value is close to that of plasticizers, but its evaporation rate is slow, Compared to Example 1 The transparency rating was somewhat poor. Also, Comparative Example 1 Therefore, although the SP value was close to that of the plasticizer and the evaporation rate was fast, the scalability evaluation was poor because it did not have the same functional groups as the plasticizer. Based on these results, and in Examples 1 and 2, 3、4 From the results, it was found that when the plasticizer (phthalate ester) has the same functional group as the main functional group (ester, aromatic hydrocarbon), the SP value is approximate (within ±0.6), and the evaporation rate is 100 or higher (relative evaporation rate when the evaporation rate of butyl acetate, the reference, is set to 100) (Examples 1 and 2, Examples 3 and 4 In this case, it was confirmed that the occurrence of schafts can be prevented.

[0058] (Experiment 2) In Experiment 2, the same ethyl acetate as in Example 1 was used as the diluent, and the preferred ratio of ethyl acetate to the total solvent for good transparency evaluation was investigated. Example 5 Next, the ratio of ethyl acetate to the total solvent was set to 13%. All other conditions were the same as in Example 1.

[0059] Example 6 Next, the ratio of ethyl acetate to the total solvent was set to 20%. All other conditions were the same as in Example 1. Example 7 Next, the ratio of ethyl acetate to the total solvent was set to 33%. All other conditions were the same as in Example 1. Example 8 Next, the ratio of ethyl acetate to the total solvent was set to 38%. All other conditions were the same as in Example 1.

[0060] Example 9Next, the ratio of ethyl acetate to the total solvent was set to 50%. All other conditions were the same as in Example 1. Example 10 Next, the ratio of ethyl acetate to the total solvent was set to 55%. All other conditions were the same as in Example 1. Example 11 Next, the ratio of ethyl acetate to the total solvent was set to 70%. All other conditions were the same as in Example 1.

[0061] Comparative Example 9 The ratio of ethyl acetate to the total solvent was set to 0%. All other conditions were the same as in Example 1.

[0062] The results of Experiment 2 are shown in Table 2. In Table 2, the evaluation of transparency was as follows: ◎ indicates no transparency at all, and ○ indicates slight, barely noticeable transparency. slightly Items with noticeable bleed were marked with a triangle (△), while those with obvious bleed were marked with a cross (×).

[0063] [Table 2]

[0064] As shown in Table 2, it was confirmed that the ratio of the diluent solvent (ethyl acetate) to the total solvent in the primer coating film is preferably 20% or more, and more preferably 33% or more. [Explanation of Symbols]

[0065] 1 Object to be coated 2. Primer coating (electrodeposited film) 3. Sealer section (sealer coating) 4. Primer coating 5. Intermediate coating 6. Color-developing coating 7 Clear coating 8 Plasticizers 9 Solvents

Claims

1. A method for forming a laminated coating film on an object to be coated, A process of forming an electrodeposited film on the object to be coated, The process comprises the steps of forming a sealer coating containing a plasticizer on the electrodeposited film and forming an undercoat coating on the undried sealer coating. In the step of forming a primer film on the sealer film, The paint for forming the aforementioned undercoat film contains a diluent solvent, A method for forming a laminated coating film, characterized in that the diluting solvent has the same functional groups as the plasticizer contained in the sealer coating film, its solubility parameter SP value is within ±0.6 of the SP value of the plasticizer, and its evaporation rate (relative evaporation rate when the evaporation rate of butyl acetate, which serves as a reference, is set to 100) is 100 or more.

2. In the step of forming a primer film on the sealer film, The method for forming a laminated coating film according to claim 1, characterized in that the ratio of the diluent solvent to the total solvent in the undercoat coating film is 20% or more.

3. In the step of forming a sealer coating film containing a plasticizer on the electrodeposited film, Using a phthalate ester as the plasticizer, In the step of forming a primer film on the sealer film, The method for forming a laminated coating film according to claim 1, characterized in that ethyl acetate or toluene is used as the diluent in the paint for forming the aforementioned undercoat coating film.

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