Laminated coating film and method for forming the same

Abstract

To appropriately transit polyisocyanate to a base coating film 3 from a clear coating film 4, and improve chipping resistance.SOLUTION: There is provided a laminated coating film in which a base coating film 3 containing a polyol-derived resin component and a polyurethane-based clear coating film 4 are laminated immediately above an electro-deposition coating film 2 on a material 1 to be coated 1. The base coating film 3 has a double layer structure of a first base coating film 5 containing aluminum flake as a brilliant material, and a second base coating film 6 containing a coloring material. Light transmittance of the second base coating film 6 is 85% or more and 99% or less at a wavelength of 700 nm, and 2% or less at a wavelength of 480 nm. The clear coating film 4 contains 0.9 or more and less than 1.3 of polyol and polyisocyanate, in terms of NCO / OH. The base coating film 3 contains 16 mass% or more of polyisocyanate transited from the clear coating film 4 with respect to a coating film solid content of the base coating film 3, when the polyisocyanate is not transited. Dry film thickness of the base coating film 3 is less than 20 μm, and dry film thickness of the clear coating film 4 is less than 45 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a multilayer coating film preferably used for automobile bodies and a method for forming the same. [Background technology]

[0002] A known method for forming a coating film on an automobile body is, for example, to apply electrodeposition coating to the substrate, heat-cure the coating, and then apply an intermediate coat, a base coat, and a clear coat in that order, and bake and cure the coatings to form a multilayer coating film using a 3-coat, 1-bake method. Another known method is to apply a base coat and a clear coat in that order without applying an intermediate coat, and then bake and cure the coatings. Another known method is to apply a first base coat, a second base coat, and a clear coat in that order, and then bake and cure the coatings, without applying an intermediate coat. These coating film formation methods are called wet-on-wet methods because the clear coat is applied to an uncured, i.e., wet, base coat.

[0003] For example, Patent Document 1 describes a method in which a solvent-based base paint containing a polyol resin and a curing agent that reacts with the hydroxyl groups of the polyol resin is applied onto an electrodeposition coating film of an object to be coated, and then a two-component urethane clear paint containing a polyol resin and a polyisocyanate is applied onto the base paint in a wet-on-wet manner to form a base coating film and a clear coating film, and the base coating film and the clear coating film are simultaneously baked and cured.

[0004] The intermediate coating film plays a role in absorbing impact from external forces, and omitting it reduces chipping resistance (resistance of the coating film to peeling caused by flying stones). As a countermeasure, Patent Document 1 employs a two-component urethane clear paint with high impact absorption properties for the clear coating. Furthermore, in the case of the wet-on-wet method, some of the polyisocyanate migrates from the clear coating film to the base coating film. Patent Document 1 addresses the problem of polyisocyanate migrating from the clear coating film degrading the finish of the base coating film, and employs a high-molecular-weight star-shaped polyol resin in the base coating to suppress polyisocyanate migration. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-200791 Summary of the Invention [Problem to be solved by the invention]

[0006] The present inventors have found through experimental research into various multilayer coating films that when a portion of the polyisocyanate migrates from the clear coating film to the base coating film, the fracture energy of the base coating film increases in some cases, and furthermore, when the polyisocyanate reaches the interface between the base coating film and the electrodeposition coating film, the adhesive strength between the base coating film and the electrodeposition coating film increases in some cases. On the other hand, if the migration of polyisocyanate from the clear coating film to the base coating film results in an excess or deficiency of polyisocyanate relative to the polyol of the clear coating film, the clear coating film will not achieve the desired physical properties.

[0007] Therefore, the present invention makes the migration of polyisocyanate from the clear coating film to the base coating film appropriate from the viewpoint of improving the physical properties of the clear coating film, the breaking energy of the base coating film, and improving the adhesive strength with the base coating film. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention controls the film thickness of each of the base coating film and the clear coating film, the amount of polyisocyanate that migrates from the clear coating film to the base coating film, and the amount of polyisocyanate remaining in the clear coating film.

[0009] The multilayer coating film disclosed herein is a multilayer coating film in which a base coating film containing a polyol-derived resin component is laminated directly on an electrodeposition coating film on a substrate, and a polyurethane-based clear coating film is laminated on the base coating film, The base coating film has a two-layer structure consisting of a first base coating film containing a luster material and a second base coating film laminated on the first base coating film and containing a coloring material, The first base coating film contains aluminum flakes as the luster material, The light transmittance of the second base coating film is 85% or more and 99% or less at a wavelength of 700 nm, and 2% or less at a wavelength of 480 nm, The clear coating film contains polyol and polyisocyanate in a molar ratio of NCO groups to OH groups (NCO / OH) of 0.9 or more and less than 1.3, the base coating film contains polyisocyanate migrated from the clear coating film in an amount of 16% by mass or more relative to the coating film solid content when no migration of the polyisocyanate occurs in the base coating film; The dry film thickness of the base coating film is less than 20 μm, The clear coating film is characterized in that the dry film thickness is less than 45 μm.

[0010] An example of a conventional coating film is a coating film in which a first base coating film, a second base coating film, and a clear coating film, which do not contain aluminum flakes as a lustrous material, are layered in this order on top of an electrodeposition coating film. Of these, the first base coating film layered on top of the electrodeposition coating film is thought to have the greatest effect on chipping resistance. In the case of a conventional coating film, since the first base coating film on top of the electrodeposition coating film does not contain aluminum flakes, it is designed to have sufficient flexibility from the beginning, ensuring sufficient chipping resistance.

[0011] In this regard, the multilayer coating film according to the present invention contains aluminum flakes as a luster material in the first base coating film, which results in the multilayer coating film having lower flexibility than conventional coating films, which is disadvantageous in terms of chipping resistance.

[0012] According to the above configuration, the chipping resistance of the laminated coating film is improved by the penetration of the isocyanate from the clear coating film side to the base coating film side. That is, since the ratio of polyol to polyisocyanate constituting the clear coating film, converted into a molar ratio (NCO / OH), is in the range of 0.9 or more and less than 1.3, a clear coating film having high breaking energy and the desired viscoelastic properties is obtained. A more preferable range for this molar ratio is 1.0 or more and less than 1.2. Meanwhile, since the base coating film is formed containing 16% by mass or more of polyisocyanate derived from the clear coating film, it has high flexibility, resulting in high breaking energy and high adhesive strength to a substrate on which an electrodeposition coating or the like is formed.

[0013] The improvement in chipping resistance is particularly noticeable in reddish multilayer coating films in which the light transmittance of the second base coating film falls within the above numerical range.

[0014] Here, the dry film thickness of the base coating film is preferably less than 20 μm. This is because, if the dry film thickness of the base coating film is large, the internal stress (tensile stress) of the coating film due to its cure shrinkage increases. In this case, stress concentrates at the point where external force is applied, making it more likely to crack, and the effect of improving chipping resistance achieved by including 16% by mass or more of polyisocyanate derived from the clear coating film cannot be obtained. The dry film thickness of the base coating film is more preferably less than 17 μm. From the viewpoint of adding pigments or the like to the base coating film to obtain the desired optical properties (design) of the laminated coating film, the dry film thickness of the base coating film is preferably 7 μm or more.

[0015] Furthermore, the dry film thickness of the clear coating film is preferably less than 45 μm. As the dry film thickness of the clear coating film increases, the internal stress (tensile stress) associated with the cure shrinkage of the coating film increases, as in the case of the base coating film. This makes it more susceptible to cracking when external force is applied, and the effect of improving chipping resistance achieved by setting the ratio of polyol to polyisocyanate constituting the clear coating film, converted into a molar ratio (NCO / OH) of 0.9 or more and less than 1.3, cannot be obtained. The dry film thickness of the clear coating film is more preferably less than 30 μm. From the viewpoint of ensuring the weather resistance and durability of the multilayer coating film, the dry film thickness of the clear coating film is preferably 10 μm or more.

[0016] The method for forming a multilayer coating film disclosed herein comprises applying a first base coating containing a polyol, a curing agent reactive with the OH group of the polyol, and a luster material directly onto a cured electrodeposition coating film on an object to be coated, applying a second base coating containing a polyol, a curing agent reactive with the OH group of the polyol, and a coloring material wet-on-wet onto the first base coating, and applying a two-component curing polyurethane clear coating wet-on-wet onto the second base coating to form an uncured first base coating film, an uncured second base coating film, and an uncured clear coating film, and baking and curing the first base coating film, the second base coating film, and the clear coating film simultaneously, in a method for forming a multilayer coating film, The first base paint contains aluminum flakes as the lustrous material, the light transmittance of the baked and cured second base coating film is 85% or more and 99% or less at a wavelength of 700 nm and 2% or less at a wavelength of 480 nm; The two-component curing polyurethane clear coating contains polyisocyanate in a molar ratio (NCO / OH) of NCO groups to OH groups of a polyol in the two-component curing polyurethane clear coating of 1.4 or more, the amount of polyisocyanate in the two-component curing polyurethane clear paint transferred from the clear coating film to the base coating film is 16% by mass or more relative to the coating film solid content of the base coating film, and the amount of polyisocyanate remaining in the clear coating film is 0.9 or more and less than 1.3 in terms of the molar ratio; the base coating film consisting of the first base coating film and the second base coating film has a dry film thickness after the baking and curing of less than 20 μm; The clear coating film is characterized in that the dry film thickness after baking and curing is less than 45 μm.

[0017] When a two-component curing urethane clear paint is applied to the surface of a base paint, a portion of the polyisocyanate in the clear paint migrates from the clear paint film to the base paint film. Because this clear paint contains polyisocyanate in a molar ratio (NCO / OH) of 1.4 or more, the amount of polyisocyanate that migrates to the base paint film can be set to 16 mass% or more, and the amount of polyisocyanate remaining in the clear paint film can be set to a molar ratio of 0.9 or more to less than 1.3.

[0018] By controlling the amount of polyisocyanate transferred to the base coating film to 16% by mass or more, a base coating film with high flexibility and therefore high breaking energy and high adhesive strength to a substrate on which an electrodeposition coating or the like has been formed can be obtained. Furthermore, since the amount of polyisocyanate remaining in the clear coating film is at least 0.9 but less than 1.3 in terms of the molar ratio, a clear coating film with high breaking energy and the desired viscoelastic properties can be obtained. A more preferred range for the molar ratio is at least 1.0 but less than 1.2.

[0019] Since the base coating film has a dry film thickness of less than 20 μm after baking and curing, the increase in internal stress (tensile stress) of the coating film due to its cure shrinkage can be avoided. The dry film thickness of the base coating film is more preferably less than 17 μm. From the viewpoint of adding pigments or the like to the base coating film to obtain the desired optical properties (design properties) of the laminated coating film, the dry film thickness of the base coating film is preferably 7 μm or more.

[0020] Furthermore, since the dry film thickness of the clear coating film after baking and curing is less than 45 μm, it is possible to avoid the increase in internal stress (tensile stress) that accompanies cure shrinkage of the coating film. The dry film thickness of the clear coating film is more preferably less than 30 μm. From the viewpoint of ensuring the weather resistance and durability of the multilayer coating film, it is preferable that the dry film thickness of the clear coating film is 10 μm or more.

[0021] Therefore, according to the above-mentioned method for forming a multilayer coating film, a multilayer coating film having high chipping resistance can be obtained.

[0022] In the multilayer coating film and the method for forming a multilayer coating film according to the present invention, it is preferable that the first base coating film further contains chromium oxide flakes as a luster material.

[0023] The second base coating preferably contains an organic pigment as a coloring material, and the organic pigment is preferably perylene red. [Effects of the Invention]

[0024] According to the present invention, the clear coating film contains polyol and polyisocyanate in a molar ratio (NCO / OH) of 0.9 or more and less than 1.3, and the base coating film contains polyisocyanate that has migrated from the clear coating film in an amount of 16 mass% or more relative to the solid content of the base coating film when there is no migration of the polyisocyanate.The dry film thickness of the base coating film is less than 20 μm, and the dry film thickness of the clear coating film is less than 45 μm.This makes it possible to form a laminate structure of a base coating film with high breaking energy and high adhesive strength to the substrate, and a clear coating film with high breaking energy and desired viscoelastic properties, which is advantageous for improving chipping resistance. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a cross-sectional view showing a coating film configuration according to an embodiment of the present invention. [Figure 2] FIG. 2 is a graph showing a calibration curve for determining the amount of polyisocyanate migration. DETAILED DESCRIPTION OF THE INVENTION

[0026] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description of preferred embodiments is merely exemplary in nature and is not intended to limit the scope of the present invention, its applications, or uses.

[0027] The present embodiment relates to a multilayer coating film preferably used for automobile bodies and a method for forming the same.

[0028] <Configuration of multilayer coating film> In the multilayer coating shown in Figure 1, reference numeral 1 denotes a steel substrate that forms the body of an automobile or the like, with an electrodeposition coating film 2 formed on the surface of this substrate 1, on which is layered a base coating film 3, and on which is layered a clear coating film 4. The base coating film 3 has a two-layer structure consisting of a first base coating film (metallic base coating film containing a luster material) 5 formed on the electrodeposition coating film 2, and a second base coating film (color base coating film containing a colorant) 6 layered on top of the first base coating film 5.

[0029] The electrodeposition coating film 2 is formed using a cationic electrodeposition paint. Both the first base coating film 5 and the second base coating film 6 contain a polyol-derived resin component. Specifically, the first base coating film 5 and the second base coating film 6 are formed by applying a one-component curing first base paint and a one-component curing second base paint, respectively, which contain a polyol resin and a curing agent that reacts with the OH groups of the polyol resin. The clear coating film 4 is formed by applying a two-component curing urethane clear paint containing a polyol resin and a polyisocyanate. The first base coating film 5, the second base coating film 6, and the clear coating film 4 are formed using a 3-coat-1-bake process in which the first base paint, the second base paint, and the clear paint are applied wet-on-wet and then simultaneously baked and cured.

[0030] The clear coating film 4 is formed by containing polyol and polyisocyanate in a molar ratio of NCO groups to OH groups (NCO / OH) of 0.9 or more and less than 1.3. That is, the clear coating film 4 contains the polyol-derived resin constituent component of the clear paint and the polyisocyanate-derived resin constituent component of the clear paint in a molar ratio of 0.9 or more and less than 1.3. The dry film thickness of the clear coating film 4 after baking and curing is 10 μm or more and less than 45 μm.

[0031] The base coating film 3 is formed by the wet-on-wet process, containing 16% by mass or more of the polyisocyanate migrated from the clear coating film 4 relative to the total coating film solids content of the first base coating film 5 and the second base coating film 6 combined when there is no migration of the polyisocyanate. The total dry film thickness of the first base coating film 5 and the second base coating film 6 combined after bake curing is 7 μm or more and less than 20 μm.

[0032] The light transmittance of the first base coating film 5 at a wavelength of 700 nm is, for example, 20% or less, preferably 5% to 15%, and at a wavelength of 480 nm is, for example, 0.1% or less, preferably 0.05% or less.

[0033] The light transmittance of the second base coating film 6 is 85% to 99%, preferably 85% to 97%, at a wavelength of 700 nm, and 2% or less, preferably 0.1% to 0.5% at a wavelength of 480 nm.

[0034] <Multilayer coating film formation method> In the method for forming a multilayer coating film according to the embodiment, first, electrodeposition coating is performed on a substrate 1, such as a zinc phosphate-treated automobile body, using a cationic electrodeposition paint, and then a baking and drying process is performed to form an electrodeposition coating film 2. A one-component curing first base paint is applied on top of this electrodeposition coating film 2 to form a first base coating film 5. Next, a one-component curing second base paint is applied wet-on-wet on top of the first base coating film 5 to form a second base coating film 6. Next, a two-component curing urethane clear paint is applied wet-on-wet on top of the second base coating film 6 to form a clear coating film 4. The base coating film 3 and the clear coating film 4 are then simultaneously baked and cured.

[0035] Wet-on-wet refers to applying an upper layer of paint while the lower layer is still in an uncured state, and also includes a method in which a preheat treatment is performed to the extent that the solvent or water in the lower layer is evaporated before the upper layer of paint is applied.

[0036] -Electrodeposition coating- The substrate 1 is immersed in cationic electrodeposition paint, and the substrate 1 serves as the cathode and the electrode plate in the electrodeposition tank serves as the anode. A direct current is passed between them to deposit the paint on the substrate 1, forming an electrodeposition coating film 2. The electrodeposition coating film 2 is then washed with water and baked for hardening. The cationic electrodeposition paint contains a cationic epoxy resin, a curing agent, pigments, and additives.

[0037] The cationic epoxy resin includes an amine-modified epoxy resin, which may be modified with a resin such as polyester polyol, polyether polyol, or alkylphenol, or may have an extended chain length.

[0038] As the curing agent, a blocked polyisocyanate obtained by blocking a polyisocyanate with a blocking agent can be used. The polyisocyanate may be any of aliphatic, alicyclic, aromatic-aliphatic, etc.

[0039] The amount of curing agent, expressed as the solid mass ratio of cationic epoxy resin to curing agent, is generally preferably in the range of 80 / 20 to 50 / 50, and the amount of cationic epoxy resin and curing agent is generally preferably in the range of 30% by mass to 80% by mass of the total solid content of the electrodeposition coating composition.

[0040] The electrodeposition coating may contain, as pigments, coloring pigments, extender pigments, and anti-rust pigments. Examples of coloring 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 anti-rust 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 may be in the range of 10% by mass to 30% by mass of the total solids content of the electrodeposition coating composition.

[0041] -Base paint- After the electrodeposition coating film 2 has been baked and hardened, a one-component curing first base paint is applied to the electrodeposition coating film 2 by rotary atomization coating, air spray coating, airless spray coating, curtain coating, or the like to form a first base coating film 5. Electrostatic force may be applied to the paint during coating. After the first base coating film 5 has been preheated, a one-component curing second base paint is applied to the first base coating film 5 by rotary atomization coating, air spray coating, airless spray coating, curtain coating, or the like to form a second base coating film 6. Electrostatic force may be applied to the paint during coating. The first base coating film may not be preheated, and after its formation, it may be left at room temperature for an appropriate period of time before the second base coating film is applied thereon.

[0042] The first base paint and the second base paint (collectively referred to as "base paints") contain the polyol resin and a curing agent that reacts with the OH groups of the polyol resin. The polyol resin is preferably an acrylic polyol resin (a polymer with OH groups in the side chains obtained by polymerizing methacrylic acid esters), but is not limited thereto. Other polyol resins such as polyether polyols and polyester polyols can also be used, or different types of polyol resins can be mixed. Polyol resins can also be mixed with other film-forming resins. Examples of curing agents include melamine resins, amino resins, and isocyanate-based curing agents. From the standpoint 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. The base paint may contain resin components other than the polyol resin and the curing agent, specifically, urethane-modified polyester, depending on the physical properties and design required for the laminated coating film.

[0043] The content of resin components in the total solid content of each base paint (the content of resin components in each coating film after curing) is determined appropriately depending on the physical properties, design, etc. required for the laminated coating film, and although not intended to be limiting, it can specifically be, for example, 50 mass% or more and 95 mass% or less.

[0044] The content of the curing agent in the resin component of each base coating material is preferably in the range of 10% by mass to 50% by mass.

[0045] The first base paint and the second base paint may be solvent-based or water-based. Examples of organic solvents include hydrocarbon solvents, ester solvents, ketone solvents, alcohol solvents, ether solvents, and aromatic petroleum solvents.

[0046] If necessary, the base coating may contain additives such as coloring pigments (coloring materials), luster materials, and flat pigments such as talc, as well as other additives such as non-aqueous dispersion resins, polymer microparticles, dyes (coloring materials), curing catalysts, ultraviolet absorbers, light stabilizers, coating surface conditioners, antioxidants, flowability adjusters, and waxes.

[0047] The content of pigments in the total solid content of each base paint (the content of pigments in each coating film after curing) is determined appropriately depending on the physical properties, design, etc. required for the multilayer coating film, and although not intended to be limiting, can be, for example, 1% by mass or more and 40% by mass or less. The content of additives in the total solid content of the base paint is also determined appropriately depending on the physical properties, design, etc. required for the multilayer coating film, and although not intended to be limiting, can be, for example, 1% by mass or more and 15% by mass or less.

[0048] In this embodiment, a lustrous material is added to the first base paint to form the first base paint film 5 into a metallic base paint film. The first base paint film 5 may contain a color pigment. A color pigment, preferably an organic pigment, is added to the second base paint to form a translucent color base paint film. The second base paint may or may not contain pigments, but it is desirable that the second base paint not contain pigments in order to ensure that the light transmittance of the second base paint film 6 falls within the above-mentioned numerical range.

[0049] The first base coating film 5 contains aluminum flakes as the above-mentioned lustrous material, and may further contain chromium oxide flakes, chromium flakes, stainless steel flakes, etc. These may be blended alone or in combination of two or more.

[0050] The color pigment may be an organic pigment or an inorganic pigment. Examples of organic pigments include red organic pigments such as perylene red, dibromoanzathrone red, azo red, anthraquinone red, quinacridone red, and diketopyrrolopyrrole, and it is preferable to use perylene red, which has excellent weather resistance. Examples of inorganic pigments include black inorganic pigments such as carbon black, chromium oxide, iron oxide, manganese oxide, and black indigoid pigments. These may be used alone or in combination of two or more.

[0051] The average particle size, average thickness, aspect ratio (average particle size / average thickness) of the luster pigment, and the average particle size of various color pigments are not particularly limited and are appropriately determined depending on the physical properties and design desired for the multilayer coating film. Specifically, for example, luster pigments having an average particle size of 5 μm to 30 μm, an average thickness of 10 nm to 500 nm, and an aspect ratio of 30 to 300 can be used. Furthermore, for example, color pigments having an average particle size of 2 nm to 160 nm can be used. The average particle size of pigments is, for example, the number-average particle size, which can be obtained by determining the D50, which is the 50% value of the particle size distribution measured using a laser diffraction particle size distribution analyzer. The average thickness of pigments can be obtained by, for example, observing the luster pigments with a scanning electron microscope, measuring the thickness of multiple (e.g., 50) luster pigments, and calculating the average value.

[0052] The content of the metallic pigment in the total solid content of the first base paint is not intended to be limited, but specifically can be, for example, 5% by mass or more and 15% by mass or less.

[0053] When the first base paint contains a color pigment, the content of the color pigment in the total solid content of the first base paint is not intended to be limited, but can specifically be, for example, 5% by mass or more and 20% by mass or less.

[0054] The content of the color pigment in the total solid content of the second base paint is not intended to be limited, but specifically can be, for example, 1% by mass or more and 10% by mass or less.

[0055] In addition, in the base coating, the application amounts of the first base paint and the second base paint are controlled so that the total dry film thickness of the first base coating film 5 and the second base coating film 6 after baking hardening is 7 μm or more and less than 20 μm.

[0056] -Clear coating- After preheating the second base coating film 6, a two-component curing polyurethane clear paint is applied to the second base coating film 6 by rotary atomization coating, air spray coating, airless spray coating, curtain coating, or the like to form the clear coating film 4. Electrostatic force may be applied to the paint during coating. The second base coating film may not be preheated, and after its formation, it may be left at room temperature for an appropriate period of time before the clear coating film is applied thereon.

[0057] The two-component curing polyurethane clear coating contains a polyol resin as a base resin and a polyisocyanate as a curing agent, for example, an OH group-containing acrylic resin and a polyisocyanate compound.

[0058] Examples of OH group-containing acrylic resins include OH group-containing polymerizable unsaturated monomers and other polymerizable unsaturated monomers. Examples of OH group-containing polymerizable unsaturated monomers include monoesters of polyhydric alcohols with acrylic acid or methacrylic acid, and compounds obtained by ring-opening polymerization of the monoesters of polyhydric alcohols with acrylic acid or methacrylic acid with ε-caprolactone. 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, acrylamide, methacrylamide or derivatives thereof, quaternary ammonium base-containing monomers, polyvinyl compounds, and ultraviolet-absorbing or ultraviolet-stable polymerizable unsaturated monomers.

[0059] Polyisocyanate compounds are compounds having two or more free isocyanate groups in one molecule. Examples include tolylene diisocyanate (TDI), diphenylmethane diisocyanate, xylylene diisocyanate (XDI), metaxylylene diisocyanate, hexamethylene diisocyanate (HDI), lysine diisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, isophorone diisocyanate (IPDI), trimethylhexamethylene diisocyanate, dimer acid diisocyanate, adduct of TDI and trimethylolpropane (TDI adduct) (molar ratio 3:1), T Examples of suitable polyisocyanate compounds include aliphatic, alicyclic, and aromatic polyisocyanate compounds such as dimers or trimers of DI, adducts of HDI and trimethylolpropane (HDI adducts) (molar ratio of 3:1), reaction products of HDI and water, adducts of XDI and trimethylolpropane (XDI adducts) (molar ratio of 3:1), adducts of TDI and HDI (molar ratio of 3:2), adducts of IPDI and trimethylolpropane (IPDI adducts), biuret forms of HDI, isocyanurates of HDI, biuret forms of IPDI, and isocyanurates of IPDI. These compounds can be used alone or in combination of two or more. Among these, biuret forms, adducts, or isocyanurates of HDI or IPDI are preferably used alone or in combination of two or more from the viewpoint of coating film properties.

[0060] The clear coating may be a solvent-based coating or a water-based coating. Examples of organic solvents include hydrocarbon solvents, ester solvents, ketone solvents, alcohol solvents, ether solvents, and aromatic petroleum solvents.

[0061] The clear coating, like the base coating described above, may contain resin components other than the polyol resin and polyisocyanate as needed. Also, like the base coating described above, the clear coating may contain, as needed, pigments such as coloring pigments (coloring materials), luster materials, and flat pigments such as talc, as well as other additives such as non-aqueous dispersion resins, polymer particles, dyes (coloring materials), curing catalysts, ultraviolet absorbers, light stabilizers, coating surface conditioners, antioxidants, flowability adjusters, and waxes.

[0062] The amount of clear paint applied is controlled so that the dry film thickness of the clear coating film 4 after baking and hardening is 10 μm or more and less than 45 μm.

[0063] - Bake hardening - The first base coating 5, the second base coating 6, and the clear coating 4 are simultaneously baked and cured at a baking temperature of, for example, 80° C. to 160° C., and a baking time of, for example, 10 minutes to 40 minutes.

[0064] <About polyisocyanate migration> As shown in Figure 1, in the above-mentioned wet-on-wet coating, before baking and curing, part of the polyisocyanate, which is the curing agent for the clear coating film 4, penetrates into the base coating film 3. In this embodiment, the amount of polyisocyanate blended in the clear paint is increased so that part of the polyisocyanate from the clear coating film 4 penetrates and migrates to the second base coating film 6, and further to the first base coating film 5, and reaches the interface between the electrodeposition coating film 2 and the first base coating film 5.

[0065] Specifically, the clear paint contains polyisocyanate in a molar ratio (NCO / OH) of NCO groups to OH groups of the polyol in the clear paint of 1.4 or more, and the amount of polyisocyanate that migrates from the clear paint film 4 to the base paint film 3 is 16 mass% or more relative to the total paint film solids of the base paint film 3 when no polyisocyanate migrates, and the amount of polyisocyanate remaining in the clear paint film 4 is such that the molar ratio (NCO / OH) of NCO groups to OH groups of the polyol in the clear paint is 0.9 or more and less than 1.3.

[0066] Therefore, since the base coating film 3 is baked in a state in which it contains 16% by mass or more of polyisocyanate that has migrated from the clear coating film 4, a coating film is formed by the reaction of the polyol resin as the main component, the curing agent, and the polyisocyanate. In other words, the base coating film 3 is formed so that it contains 16% by mass or more of polyisocyanate that has migrated from the clear coating film 4, relative to the coating film solids content of the base coating film 3 in the absence of migration of the polyisocyanate. Furthermore, since the dry film thickness of the base coating film 3 after baking and curing is less than 20 μm, it is possible to avoid large internal stresses (tensile stresses) associated with cure shrinkage.

[0067] This results in a base coating film 3 that is highly flexible and has a large breaking energy. In addition, the polyisocyanate migrated from the clear coating film 4 reaches the interface between the electrodeposition coating film 2 and the first base coating film 5, and the reaction hardening proceeds in this state, resulting in high adhesive strength between the first base coating film 5 and the electrodeposition coating film 2.

[0068] On the other hand, the clear coating film 4 is formed by the reaction of polyol and polyisocyanate at a molar ratio (NCO / OH) of 0.9 or more and less than 1.3. Therefore, the clear coating film 4 is a coating film containing polyol and polyisocyanate in a molar ratio of NCO groups to OH groups (NCO / OH) of 0.9 or more and less than 1.3. Furthermore, since the dry film thickness of the clear coating film 4 after baking and curing is less than 45 μm, it is possible to avoid an increase in internal stress (tensile stress) due to cure shrinkage.

[0069] As a result, a clear coating film 4 having a large breaking energy and desired viscoelastic properties is obtained.

[0070] <Examples and Comparative Examples> The formulations of the first base paint, second base paint, and clear paint used to prepare each of the multilayer coating films in the Examples and Comparative Examples are as shown in Tables 1 to 3. However, the formulation of the clear paint shown in Table 3 is a basic formulation (Comparative Example 3) in which the molar ratio (NCO / OH) of the OH group of the acrylic resin (polyol) to the NCO group of the isocyanate resin (polyisocyanate) is 1.0. In the Examples and Comparative Examples described below, the respective molar ratios were adjusted by changing the ratio of the amount of isocyanate resin to the amount of acrylic resin based on the basic formulation in Table 3.

[0071] [Table 1]

[0072] [Table 2]

[0073] [Table 3]

[0074] Example 1 A dull steel plate was coated with cationic electrodeposition paint PN-1020 (manufactured by Nippon Paint Co., Ltd.) to a dry film thickness of 20 μm, and the electrodeposition coating was baked and cured for 30 minutes at 160°C. A first base paint shown in Table 1 and a second base paint shown in Table 2 were sprayed onto the electrodeposition coating using a wet-on-wet air spray method to form a base coating film, with dry film thicknesses of 6 μm and 8 μm, respectively, after bake curing. After leaving the coating at room temperature for 5 minutes, a clear coating with a molar ratio (NCO / OH) of 1.4, adjusted based on the clear coating shown in Table 3, was sprayed onto the base coating using a wet-on-wet air spray method to form a clear coating film with a dry film thickness of 28 μm after bake curing. After leaving the coating at room temperature for 10 minutes, the base coating and clear coating were baked and cured for 30 minutes at 140°C to produce a laminate coating sample according to Example 1.

[0075] Examples 2 to 6, Comparative Examples 1 to 3 For Examples 2 to 6 and Comparative Examples 1 to 3, multilayer coating samples were prepared in the same manner as in Example 1, with the thickness of each of the base coating film and clear coating film, and the molar ratio (NCO / OH) of the clear paint set to the values ​​shown in Table 4.

[0076] [Table 4]

[0077] - Dry film thickness measurement - The dry film thickness of each coating film of the laminated coating film samples of the Examples and Comparative Examples was measured using an eddy current film thickness meter / magnetic induction film thickness meter specified in JIS K 5600-1-7:2014 (general testing method for paints).

[0078] -Measurement of light transmittance- The light transmittance of the first base coating film and the light transmittance of the second base coating film were measured by the following method.

[0079] After applying the first base paint or the second base paint to a glass plate, a cut is made with a cutter knife approximately 1 to 2 mm inward from the outer edge of the coating, and the coated glass plate is immersed in warm water at 40 to 50°C and left for 0.5 minutes. The coating film peeled off from the glass plate is sandwiched between clean gauze and thoroughly dried in a dryer maintained at 40 to 50°C. Using a spectrophotometer, the transmittance at each wavelength is calculated using the following formula, with the state without the coating being considered as 100% transmittance. Light transmittance (%) = (Transmittance value at each wavelength / 100% transmittance value at each wavelength) x 100 -Measurement of residual polyisocyanate content- Each of the clear coatings of the above Examples and Comparative Examples was air spray painted onto a PP (polypropylene) plate so that the dry film thickness was 30 μm to form a clear coating film. After leaving it at room temperature for 10 minutes, the clear coating film was baked at 140 ° C for 30 minutes to harden. IR measurement of the clear coating film surface was performed, and the 1720 cm -1 and 1680cm -1 Measure the height from the baseline (1680cm -1 / 1720cm-1 ) 2 ...(A) was calculated. 1720cm -1 The absorption at 1680cm is due to the ester bond of the urethane resin. -1 The absorption is due to the urethane bond resulting from the reaction between the NCO group and the OH group.

[0080] Subsequently, IR measurement was carried out on the surface of the clear coating film of each of the multilayer coating films of the above Examples and Comparative Examples, and the IR spectrum (1680 cm -1 / 1720cm -1 ) 2 ...(B) was calculated. Then, ((B) / (A)) x 100 was calculated to obtain the residual isocyanate ratio. From this residual ratio, the amount of polyisocyanate remaining in the clear coating was calculated as a molar ratio (NCO / OH).

[0081] -Measurement of polyisocyanate migration amount- The isocyanate curing agent for the clear paint (isocyanate resin in Table 3) was added to 100g of diluted first base paint, and after thorough stirring, the paint was hand-sprayed onto a PP plate. Five amounts of isocyanate curing agent were added: approximately 5g, approximately 10g, approximately 15g, approximately 20g, and approximately 30g, as shown in Table 5. Each of these five amounts was baked at 140°C for 30 minutes, left for one day, and then the coating film was peeled off from the PP plate and IR measurement was performed on the backside. From the chart obtained, the IR (1680cm) -1 / 1720cm -1 ) 2 The calibration curve shown in Figure 2 was created by drawing a regression line between the amount of curing agent and the solid content of the coating film.

[0082] [Table 5]

[0083] The first base paint, second base paint and clear paint used in Examples 1-6 and Comparative Examples 1-3 were applied wet-on-wet to a PP plate so that the film thickness of each of the first base paint film, second base paint film and clear paint film would be the values ​​shown in Table 4, and then baked at 140°C for 30 minutes to cure (no electrocoating). After leaving it for one day, the coating film was peeled off from the PP plate and IR measurement was carried out on the backside of the first base paint film. From the obtained chart, -1 / 1720cm -1 ) 2 The amount of transferred isocyanate curing agent (mass % / coating solids content) was calculated based on the above calibration curve. The results are shown in Table 4.

[0084] -Measurement of the fracture energy of clear coatings- Each clear coating of Examples 1-6 and Comparative Examples 1-3 was applied to a PP plate. After leaving it at room temperature for 10 minutes, it was baked at 140°C for 30 minutes to cure. After peeling the coating from the PP plate, the coating was stretched at 23°C so that the strain rate (1 / s) was 0.1, and the stress and strain until breakage were measured. From the resulting chart, the tensile strength and elongation at break were calculated, and the product of these values ​​multiplied by 0.5 was taken as the breaking energy (kgf cm). The results are shown in Table 4.

[0085] -Method for evaluating chipping resistance of multilayer coatings- At a temperature of -20°C, each of the multilayer coating samples of Examples 1-6 and Comparative Examples 1-3 was hit with No. 6 crushed stone weighing 0.7 to 0.8 g at a speed of 80 km / h at an impact angle of 45°. The area of ​​the coating peeled off due to the impact was measured using an image processing device, and the average value (average value for 10 samples for each) was calculated. Chipping resistance was evaluated using the following three-level scale.

[0086] ◎(Excellent): Peeling area 1.3mm 2 less than ○ (Good): Peeling area 1.3 mm 2 More than 1.5mm 2 less than △ (bad): Peeling area 1.5 mm 2 End In Examples 1-3, the NCO / OH molar ratio of the clear paint increases in the order of Examples 1, 2, and 3, but the thickness of each base coating film is changed in the range of 14 to 16, and the thickness of the clear coating film is changed in the range of 27 to 29. Then, the molar ratio of the clear coating film after isocyanate migration is 0.99 to 1.16, and the amount of isocyanate migration is 21 to 22 mass% ((1680 cm) of the backside of the first base coating film). -1 / 1720cm -1 ) 2 As a result, the breaking energy of the clear coating film is relatively large, and the chipping resistance is good, with a small peeling area.

[0087] Example 4 and Comparative Example 3 differ only in the NCO / OH molar ratio of the clear coating (the molar ratio before isocyanate migration) (the former is 1.4, the latter is 1). As a result, in Comparative Example 3, the NCO / OH molar ratio of the clear coating film after isocyanate migration was 0.83, which was smaller, and the amount of isocyanate migration was also 15% by mass (1680 cm on the backside of the first base coating film). -1 / 1720cm -1 ) 2 The value was 0.7), which is small. Thus, in Comparative Example 3, the breaking energy of the clear coating film was small and the chipping resistance was poor compared to Example 4. Therefore, it is preferable to increase the NCO / OH molar ratio (the molar ratio before isocyanate migration) of the clear paint so that the molar ratio of the clear coating film after isocyanate migration is 0.9 or more, and even 1.0 or more. In addition, it is preferable to increase the amount of isocyanate migration to 16% by mass or more ((1680 cm) of the backside of the first base coating film). -1 / 1720cm -1 ) 2 It is preferable to set the value to 0.8 or more.

[0088] In Example 3 and Comparative Example 4, the NCO / OH molar ratio of the clear paint (the molar ratio before isocyanate migration) was both 1.6, and only the film thickness of the clear coating film differed (the former was 29 μm, the latter was 43 μm). As a result, in Comparative Example 4, the NCO / OH molar ratio of the clear coating film after isocyanate migration was 1.32, which was larger, and the amount of isocyanate migration was also 29 mass% ((1680 cm) of the backside of the first base coating film). -1 / 1720cm -1 ) 2 The value is 1.6, which is large. Thus, the chipping resistance of Comparative Example 4 is worse than that of Example 3. Therefore, it is necessary to make the NCO / OH molar ratio of the clear coating film after isocyanate migration less than 1.3, and to make the amount of isocyanate migration less than 28 mass % ((1680 cm) of the backside of the first base coating film). -1 / 1720cm -1 ) 2 It is preferable to set the value to less than 1.5.

[0089] Regarding the molar ratio of the clear coating film after isocyanate migration, if this molar ratio is 1.3 or more, crosslinking of the clear coating film will be incomplete and the coating film strength will decrease. Therefore, the molar ratio is preferably less than 1.3, and even more preferably less than 1.2. Furthermore, if the amount of isocyanate migration is 28% by mass or more, crosslinking of the base coating film will be incomplete and the coating film strength will decrease. Therefore, it is preferable that the amount of isocyanate migration is less than 28% by mass.

[0090] The only difference between Examples 4, 5, 6, and Comparative Example 2 is that the film thickness of the clear coating film is greater than 30 μm, in that order. Due to this difference, the molar ratio of the clear coating film after isocyanate migration varies from 0.99 to 1.18, and the amount of isocyanate migration varies from 20 to 24 mass%. Thus, although there is no significant difference between Examples 4, 5, 6, and Comparative Example 2 in the molar ratio and amount of isocyanate migration of the clear coating film after isocyanate migration, the poor chipping resistance of Comparative Example 2 is believed to be due to the clear coating film's large film thickness of 45 μm. In other words, the large film thickness of the clear coating film increases the internal stress (tensile stress) of the clear coating film due to its cure shrinkage, and as a result, stress concentrates at locations where external force is applied, making it more likely to crack. Therefore, it is preferable that the film thickness of the clear coating film be less than 45 μm.

[0091] Furthermore, when comparing Example 2-3 and Example 4-6, although there is not much difference in the molar ratio of the clear coating film after isocyanate migration and the amount of isocyanate migration between the two, the former (Example 2-3) in which the film thickness of the clear coating film is less than 30 μm has better chipping resistance. In other words, it is more preferable that the film thickness of the clear coating film is less than 30 μm.

[0092] Furthermore, in Comparative Example 1, the only difference from Example 4 is that the base coating film has a larger thickness of 22 μm, but the chipping resistance is poor. This is believed to be due to the fact that the thick base coating film increases the internal stress (tensile stress) of the clear coating film due to its cure shrinkage. Therefore, it is preferable that the thickness of the base coating film is less than 20 μm, and even more preferably less than 17 μm. [Explanation of symbols]

[0093] 1 Object to be coated 2 Electrodeposition coating 3 Base coating 4 Clear coating 5 First base coat 6 Second base coat

Claims

1. A laminated coating film in which a base coating film containing a polyol-derived resin component is laminated directly on an electrodeposition coating film on a substrate, and a polyurethane-based clear coating film is laminated on the base coating film, The base coating film has a two-layer structure consisting of a first base coating film containing a luster material and a second base coating film laminated on the first base coating film and containing a coloring material, The first base coating film contains aluminum flakes as the luster material, The light transmittance of the second base coating film is 85% or more and 99% or less at a wavelength of 700 nm, and 2% or less at a wavelength of 480 nm; The clear coating film contains a polyol and a polyisocyanate in a molar ratio of NCO groups to OH groups (NCO / OH) of 0.9 or more and less than 1.3, the base coating film contains polyisocyanate migrated from the clear coating film in an amount of 16% by mass or more relative to the coating film solid content when no migration of the polyisocyanate occurs in the base coating film; The dry film thickness of the base coating film is less than 20 μm, A multilayer coating film characterized in that the dry film thickness of the clear coating film is less than 45 μm.

2. In claim 1, A multilayer coating film characterized in that the dry film thickness of the base coating film is less than 17 μm.

3. In claim 1 or claim 2, A multilayer coating film characterized in that the dry film thickness of the clear coating film is less than 30 μm.

4. In claim 1 or claim 2, A multilayer coating film characterized in that the molar ratio is 1.0 or more and less than 1.

2.

5. In claim 1 or claim 2, A laminated coating film characterized in that the first base coating film further contains chromium oxide flakes as the luster material.

6. In claim 1 or claim 2, A laminated coating film characterized in that the second base coating film contains an organic pigment as the coloring material.

7. In claim 6, A multilayer coating film characterized in that the organic pigment is perylene red.

8. A method for forming multilayer coating films, comprising: applying a first base coating containing a polyol, a curing agent reactive with the OH group of the polyol, and a lustrous material directly onto a cured electrodeposition coating film on an object to be coated; applying a second base coating containing a polyol, a curing agent reactive with the OH group of the polyol, and a coloring material wet-on-wet on top of that; and applying a two-component curing polyurethane clear coating wet-on-wet directly on top of that to form an uncured first base coating film, an uncured second base coating film, and an uncured clear coating film; and simultaneously baking and curing the first base coating film, the second base coating film, and the clear coating film. The first base paint contains aluminum flakes as the lustrous material, The baked and cured second base coating film has a light transmittance of 85% or more and 99% or less at a wavelength of 700 nm and 2% or less at a wavelength of 480 nm; The two-component curing polyurethane clear coating contains polyisocyanate in a molar ratio (NCO / OH) of NCO groups to OH groups of a polyol in the two-component curing polyurethane clear coating of 1.4 or more, the amount of polyisocyanate in the two-component curing polyurethane clear paint transferred from the clear coating film to the base coating film is 16% by mass or more relative to the coating film solids content of the base coating film, and the amount of polyisocyanate remaining in the clear coating film is 0.9 or more and less than 1.3 in terms of the molar ratio; the base coating film consisting of the first base coating film and the second base coating film has a dry film thickness after the baking and curing of less than 20 μm; The method for forming a multilayer coating film is characterized in that the dry film thickness of the clear coating film after the baking and curing is less than 45 μm.

9. In claim 8, A method for forming a multilayer coating film, characterized in that the dry film thickness of the base coating film is less than 17 μm.

10. In claim 8 or claim 9, The method for forming a multilayer coating film is characterized in that the dry film thickness of the clear coating film is less than 30 μm.

11. In claim 8 or claim 9, A method for forming a multilayer coating film, characterized in that the residual amount of the polyisocyanate is 1.0 or more and less than 1.2 in terms of the molar ratio.

12. In claim 8 or claim 9, A method for forming a multilayer coating film, wherein the first base coating film further contains chromium oxide flakes as the luster material.

13. In claim 8 or claim 9, A method for forming a multilayer coating film, characterized in that the second base coating film contains an organic pigment as the coloring material.

14. In claim 13, A method for forming a multilayer coating film, characterized in that the organic pigment is perylene red.

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