Method for forming a multilayer coating film

Abstract

A method for forming a multilayer coating film, comprising: Applying a primer coating composition to a substrate to form an uncured primer coating film (step 1), Applying a clear coating composition to the uncured base coating film obtained in step 1 to form an uncured clear coating film (step 2), and Baking and curing the uncured base coat film and the uncured clear coat film of steps 1 and 2 to form a multi-layer coating film (step 3), wherein (a) the L value of the uncured primer coating film is 70 to 105 at 25° reflection and the L value of the uncured primer coating film is 20 to 50 at 75° reflection, (b) the ΔL value obtained by subtracting the L value of the uncured base coating film from the L value of the multi-layer coating film at 25° reflection is -0.3 to 0.15, and (c) the ΔL value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film at 75° reflection is -0.2 to 0.4, and wherein the clear coating composition includes: an acrylic resin having a number average molecular weight of 4,000 to 6,000 and a solid acid value of 1 to 5 mg KOH / g, which is obtained from a monomer mixture comprising 10 to 15 wt%, based on the total monomer solids, of a long-chain (meth)acrylic ester monomer, a carboxy group-containing (meth)acrylic monomer and a hydroxy group-containing (meth)acrylic monomer, wherein the long-chain (meth)acrylic ester monomer has an ester portion of a linear hydrocarbon group having 9 to 15 carbon atoms; a polyisocyanate compound; and cross-linked resin particles.

Description

FIELD OF THE INVENTION:

[0001] The present invention relates to a method for producing a multilayer coating film. BACKGROUND OF THE INVENTION:

[0002] Generally, coatings on vehicle bodies, such as automobile bodies, involve the formation of a coating film comprising a base coat film and a clear coat film thereon, i.e., the formation of a topcoat film. In particular, the clear coat film forms the outermost surface layer among these coatings on the vehicle body, such as the automobile body. Therefore, the clear coat film must have various properties, such as design properties, acid resistance, scratch resistance, etc.

[0003] Conventional clearcoat compositions for automotive application include coating compositions with a heat-curing system that utilizes an acrylic melamine resin. This system uses a melamine resin as the curing agent. Therefore, the resulting coating film exhibits poorer acid resistance. Consequently, the resulting coating film would be susceptible to acid rain and brittle. Accordingly, this can lead to coating failure across the outer surface.

[0004] In addition to the clear coating compositions in the heat-curing system using an acrylic melamine resin, urethane coating compositions, for example, are known in the art (see Patent Literature 1).

[0005] Patent Literature 1: Japanese Patent Application Laid-Open (JP-A-2005-000787) DISCLOSURE OF THE INVENTION:Problem to be solved by the invention:

[0006] The urethane coating compositions can provide a coating film with excellent acid resistance and scratch resistance. However, the urethane coating compositions can provide inferior film appearance due to the migration of acrylic resin (acrylic polyol) of the clear coating compositions into an uncured base coat film when forming an outermost coating film by the so-called two-coat, one-bake coating procedure, which includes a step of baking above 120°C and curing an uncured base coat film and simultaneously curing an uncured clear coat film.

[0007] In certain cases where the basecoat composition comprises glossy color pigments, i.e., where the basecoat composition is a so-called basecoat composition for metallic colors, the migration of some components from the clearcoat composition into the uncured basecoat film disrupts the orientation of the glossy color pigments therein. As a result, "color inversion" occurs in / above the basecoat film. Color inversion is a phenomenon in / on the basecoat film in which glossy (top) (highlight) areas are darkened and intermediate (graduated) areas are brightened. Therefore, the design properties may be impaired.

[0008] Accordingly, the urethane coating compositions fail to provide a coating film having excellent design properties, and therefore, the urethane coating compositions are not suitable for forming a clear coating film on a vehicle body, particularly on an automobile where excellent design properties are required in the market.

[0009] An object of the present invention is based on the above problem and is to provide a method for forming a multi-layer coating film having excellent acid resistance and scratch resistance, as well as having excellent design properties, including the inherent excellent design properties of the base coating film without color reversal in the case that the multi-layer coating film comprises a base coating film having a high lightness value, such as a metallic base coating film. Means to solve the problems:

[0010] The present invention relates to a method for forming a multilayer coating film, which comprises: Applying a primer coating composition to a substrate to form an uncured primer coating film (step 1), Applying a clear coating composition to the uncured base coating film obtained in step 1 to form an uncured clear coating film (step 2), and Baking and curing the uncured base coat film and the uncured clear coat film of steps 1 and 2 to form a multi-layer coating film (step 3), wherein (a) the L value of the uncured primer coating film is 70 to 105 at 25° reflection and the L value of the uncured primer coating film is 20 to 50 at 75° reflection, (b) the ΔL value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film at 25° reflection is from -0.3 to 0.15, and (c) the ΔL value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film at 75° reflection is from -0.2 to 0.4, and wherein the clear coating composition includes: an acrylic resin having a number average molecular weight of 4,000 to 6,000 and a solid acid value of 1 to 5 mg KOH / g, obtained from a monomer mixture comprising 10 to 15 wt%, based on the total monomer solids, of a long-chain (meth)acrylic ester monomer, a carboxy group-containing (meth)acrylic monomer and a hydroxy group-containing (meth)acrylic monomer, wherein the long-chain (meth)acrylic ester monomer has an ester portion of a linear hydrocarbon group having 9 to 15 carbon atoms; a polyisocyanate compound; and cross-linked resin particles.

[0011] The clear coating composition may comprise crosslinked resin particles within the range of 0.1 to 10 wt.% based on the total solids weight of the clear coating composition.

[0012] In addition, the method for forming a multilayer coating film may comprise applying an intermediate coating composition to a substrate to form an intermediate coating film prior to step 1 (step P). ADVANTAGEOUS EFFECT OF THE INVENTION:

[0013] The present invention can provide a multilayer coating film having increased brightness, suppressed color reversal, and excellent design properties, as well as excellent acid resistance and scratch resistance. SHORT DESCRIPTION OF THE DRAWINGS: Fig. Figure 1 is a schematic view illustrating the measurement of L values. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

[0014] The present invention will be described in detail below.

[0015] A method according to the invention for forming a multilayer coating film comprises the steps: Applying a base coating composition to a substrate to form an uncured base coating film that satisfies the above requirement (a) (step 1), Applying a clear coating composition to the uncured base coating film obtained in step 1 to form an uncured clear coating film (step 2), and Baking and curing the uncured base coat film and the uncured clear coat film from steps 1 and 2 to form a multi-layer coating film that meets the above requirements (b) and (c) (step (3)).

[0016] A non-cured base coating film with higher brightness, such as a metallic base coating film, generally tends to exhibit color inversion in / over the coating film in the two-coat and one-bake coating procedure, which makes the coating film formation with outstanding design properties difficult.

[0017] On the other hand, the present invention comprises: forming an uncured base coat film satisfying the above requirement (a); applying a clear coat composition to the uncured base coat film; and baking and curing the uncured coating films; which can provide a multi-layer coating film having increased brightness, suppressed color reversal, and excellent design properties.

[0018] The method for forming a multilayer coating film of the present invention can provide a multilayer coating film exhibiting excellent design properties, which is caused by the clear coating composition containing an acrylic resin having a long chain of a linear hydrocarbon group with 9 to 15 carbon atoms. This can prevent the migration of acrylic resin in the clear coating composition into the uncured base coating film during the formation of the topcoat film according to the two-coat and one-bake coating method due to the steric hindrance of the linear hydrocarbon groups present therein. Furthermore, the higher acid value of the acrylic resin appears to effectively contribute to preventing the migration of the acrylic resin into the uncured base coating film.

[0019] The colors of coating films are divided into three groups based on the L value in the highlight of a cured base coating film (single film): highly saturated colors with an L value of less than 40 (L < 40), medium saturated colors with an L value of greater than or equal to 40 and less than 80 (40 ≤ L < 80) and light saturated colors with an L value of greater than or equal to 80 (80 ≤ L).

[0020] Applying a clear coating film over a base coating film containing a metallic effect pigment (luster pigment) and having a higher L value (medium saturated color or light saturated color) can provide a multi-layer coating film exhibiting significant flip-flop properties and outstanding design characteristics.

[0021] On the other hand, the L value may change slightly due to a slight misalignment of luster pigments resulting from a mixing layer between a base coat film and a clear coat film, which may result in a decrease in the L value in the highlight, an increase in the L value in the shade, and a degradation of flip-flop properties. In contrast, a dark-saturated color with a small L value in the highlight of its cured base coat film will exhibit a slight change in the L value due to a slight misalignment of luster pigments resulting from a mixing layer between a base coat film and a clear coat film because it has a smaller L value in the highlight.

[0022] Thus, a clear coating composition with improved mix layer properties over a base coating film was required in the formation of multi-layer coating films, especially in the color range of medium-saturated colors or light-saturated colors.

[0023] The method for forming a multi-layer coating film according to the present invention comprises a step of applying a base coating composition to a substrate to form an uncured base coating film (step 1). As the base coating composition, a color luster pigment-containing base coating composition comprising a resin component for forming a base coating film, a color luster pigment, a coloring pigment, a filler pigment, a solvent, and the like can be used. The base coating composition may be of an aqueous solvent type (i.e., water-based) or an organic solvent type, including an aqueous solvent dispersion or an organic solvent dispersion.The aqueous solvent and the organic solvent include, but are not particularly limited to, such conventional solvents known to those skilled in the art.

[0024] The resin component to be added to the basecoat composition may be a resin for forming a basecoat film, which may be used in combination with a curing agent for the resin, if necessary. In the basecoat composition containing a color luster pigment, the resin component may disperse the color luster pigment and, if necessary, the coloring pigment.

[0025] The resin for forming a base coat film includes, for example, acrylic resins, polyester resins, polyurethane resins, alkyd resins, epoxy resins, polyether resins, etc. Of these resins, acrylic resins, polyester resins, and polyurethane resins are preferred. One of these resins can be used alone. Alternatively, two or more of these resins can be used in combination.

[0026] Generally, the resin for forming a base coat film includes curable-type resins and varnish-type resins. The curable-type resin is preferred. The curable-type resin may be used in combination with a curing agent for the resin for forming a base coat film, such as a melamine resin, a (blocked) isocyanate compound, an oxazoline compound, a carbodiimide compound, or the like. The curing agent for the resin may be added to the resin component and then subjected to a curing reaction with the resin component with or without heating, that is, at room temperature. Herein, the curable-type resin may be used in combination with a resin type other than the curable-type resin.

[0027] When the resin is used in combination with the curing agent for the resin, the weight ratio of the resin to the curing agent (resin / curing agent) based on the solid content of the coating composition may preferably be within the range of 90 / 10 to 50 / 50, more preferably within the range of 85 / 15 to 60 / 40. If the weight ratio of the resin to the curing agent is greater than 90 / 10, in which the content of the curing agent is less than 10, the resulting coating film may have insufficient crosslinking. If the weight ratio is less than 50 / 50, in which the content of the curing agent is more than 50, the resulting coating composition may have reduced storage stability and an increased curing rate, and may therefore provide a coating film with deteriorated appearance.

[0028] Instead of the curing agent described above, an etherified melamine resin can be used. The etherified melamine resin can be obtained by etherifying a melamine resin with an alcohol such as methanol, butanol, or the like.

[0029] Examples of pigments in the base coating composition include scaly luster color pigments made of a metal such as aluminum (or aluminum oxide), copper, zinc, iron, nickel, tin, or an alloy thereof, or the like, which may be colored, and mixtures thereof; and non-colored color luster pigments which, however, have an inherent color, such as interference color mica powder, colored mica powder, white mica powder, graphite, glass flakes, or the like; etc.

[0030] Other pigments besides the above-described color luster pigments include, for example, filler pigments such as baryta powder, precipitated barium sulfate, barium carbonate, gypsum, clay, silica, talc, magnesium carbonate, and aluminum oxide white; and coloring pigments, etc. The coloring pigments include, for example, organic pigments such as azo mordant pigments, phthalocyanine pigments, indigo pigments, perylene pigments, quinophthalone pigments, dioxazine pigments, quinacridone pigments, isoindolinone pigments, diketopyrrolopyrrole pigments, benzimidazole pigments, and metal complex pigments; and inorganic pigments such as chrome yellow, yellow iron oxide, red iron oxide, titanium dioxide, and carbon black. The content of the pigment can be appropriately determined depending on the desired properties and the development of the desired color. One of these pigments may be used alone. Alternatively, two or more of these pigments may be used in combination.

[0031] If necessary, the base coating composition may suitably comprise, in addition to the above-described component(s), a wax such as a polyamide wax including a lubricant dispersion comprising an aliphatic amide, and polyethylene wax including a colloidal dispersion comprising a polyethylene oxide as the main component; a curing catalyst; a UV absorber; an antioxidant, a leveling agent; a surface conditioner such as silicones and organic polymers; an anti-sagging agent; a thickener, an anti-foaming agent; a lubricant; crosslinkable polymer particles (e.g., microgels); etc. The additive may be used in an amount of 15 parts by weight or less, based on the solid content, relative to 100 parts by weight.-parts of the resin component, in order to improve the properties of the coating composition and thus the properties of the resulting coating film.

[0032] The pigment content in the basecoat composition, relative to the solids content of the coating composition, ie, the pigment weight content (PWC), may preferably be up to 65 wt.%. It is more preferred that the PWC be not less than 5 wt.%. Herein, the solids content of the basecoat composition may preferably be within the range of 15 to 60 wt.%.

[0033] The application procedure of the basecoat composition comprises, for example, a multi-step, preferably a two-step, coating by means of electrostatic painting (air electrostatic spraying) or by means of an electrostatic spray coater with a rotary atomizer. These application procedures can be performed to improve the design properties of the resulting coating film. Additionally, the basecoat composition can be applied in a combination of electrostatic painting and the electrostatic spray coater with a rotary atomizer. The thickness of the resulting dried basecoat film can preferably be within the range of 5 to 50 µm per coating film, and more preferably within the range of 10 to 30 µm.

[0034] The resulting uncured base coating film may be subjected to preheating. The applied base coating composition may be subjected to preheating to form an uncured base coating film, which can result in a multi-layer coating film having an excellent final appearance. Herein, the term "uncured" refers to a state in which the applied coating composition is not completely cured, which further includes a state in which the applied coating composition has been subjected to preheating. The "preheating" may be performed without heating, i.e., by allowing to stand, or with heating for 1 to 10 minutes at a temperature within the range of room temperature to 100°C, which is lower than the temperature in the subsequent heating (i.e., baking) and curing step.

[0035] The uncured basecoat film obtained by the above step 1 has an L value of 70 to 105 at 25° reflection and an L value of 20 to 50 at 75° reflection. The present invention comprises: forming an uncured basecoat film having L values within the above range; applying a clearcoat composition described below to the uncured basecoat film; and baking and curing the uncured coating films, which can provide a multilayer coating film with increased brightness, suppressed color reversal, and excellent design properties. In the present application, the L value (brightness index) is a brightness index in the Lab color difference formula (Hunter's color difference formula) and is recognized as higher whiteness when a higher L value is exhibited and higher blackness when a lower L value is exhibited.In the present invention, “L value at 25° reflection” means an L value of incident light when a coating film to be measured is irradiated with light from 25° with the incident position at the vertical position opposite to the coating film being set at “0°” as shown in FIG. Fig. 1. Similarly, "L value at 75° reflection" refers to the L value of incident light when a coating film to be measured is irradiated with light at 75°. The L value can be measured using an adjustable-angle spectrophotometric colorimeter, such as a CM-512m3 manufactured by Minolta Inc.

[0036] After forming the uncured base coating composition in the above step 1, a clear coating composition is applied to the uncured base coating film to form an uncured clear coating film.

[0037] The clear coating composition includes: an acrylic resin having a number average molecular weight of 4,000 to 6,000 and a solid acid value of 1 to 5 mg KOH / g, obtained from a monomer mixture comprising a long chain (meth)acrylic acid ester monomer having an ester portion of a linear hydrocarbon group having 9 to 15 carbon atoms, a carboxy group-containing (meth)acrylic monomer and a hydroxy group-containing (meth)acrylic monomer; a polyisocyanate compound; and cross-linked resin particles.

[0038] The acrylic resin is obtainable / obtained from a monomer mixture comprising a long-chain (meth)acrylate monomer having an ester portion of a linear hydrocarbon group having 9 to 15 carbon atoms, a carboxyl group-containing (meth)acrylic monomer, and a hydroxyl group-containing (meth)acrylic monomer.

[0039] The long-chain (meth)acrylate monomer described above includes, for example, a monomer represented by the formula (A): wherein R is a hydrogen atom or a methyl group; and n is an integer of 8 to 14.

[0040] The long-chain (meth)acrylate monomer includes, but is not particularly limited to, lauryl (meth)acrylate, tridecyl (meth)acrylate, decyl (meth)acrylate, nonyl (meth)acrylate, and the like, provided that the monomer has an ester portion composed of a linear hydrocarbon group having 9 to 15 carbon atoms. One of these (meth)acrylate monomers may be used singly. Alternatively, two or more of these (meth)acrylate monomers may be used in combination.

[0041] The content of the long-chain (meth)acrylate monomer can preferably be within a range of 10 to 15 wt.% relative to the total weight of the monomer solids in the monomer mixture. If the content is less than 10 wt.%, it may be difficult to adequately control the influence of the clear coat film on the base coat film. If the content is more than 15 wt.%, this may lead to the problem of poor adhesion when recoating.

[0042] The carboxyl group-containing (meth)acrylic monomer described above includes, but is not particularly limited to, acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, isocrotonic acid, and maleic acid. One of the carboxyl group-containing (meth)acrylic monomers can be used singly. Alternatively, two or more of the carboxyl group-containing (meth)acrylic monomers can be used in combination. Of these carboxyl group-containing (meth)acrylic monomers, acrylic acid and methacrylic acid are preferred.

[0043] The use of the carboxyl group-containing (meth)acrylic monomer can provide a coating film with outstanding design properties. This appears to be due to the potential prevention of acrylic resin migration into the basecoat film, as association of the carboxyl group with the acrylic resin can increase the apparent molecular weight of the acrylic resin.

[0044] The hydroxyl group-containing (meth)acrylic monomer described above includes, but is not limited to, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methacrylic alcohol, an adduct of hydroxyethyl (meth)acrylate and ε-caprolactone, etc. One of the hydroxyl group-containing (meth)acrylic monomers can be used singly. Alternatively, two or more of the hydroxyl group-containing (meth)acrylic monomers can be used in combination.

[0045] The above-described monomer mixture comprising the long-chain (meth)acrylate monomer, the carboxyl group-containing (meth)acrylic monomer, and the hydroxyl group-containing (meth)acrylic monomer, each of which is described above, can provide the acrylic resin. The monomer mixture may further comprise other unsaturated monomer(s).

[0046] The other unsaturated monomer(s) include, for example, (meth)acrylates, comprising

[0047] Alkyl (meth)acrylates wherein the alkyl radical has 1 to 8 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate;

[0048] Cyclohydrocarbyl (meth)acrylates such as phenyl (meth)acrylate and cyclohexyl (meth)acrylate;

[0049] Isoboronyl(meth)acrylate;

[0050] Polyalkylene glycol (meth)acrylates, such as (poly)ethylene glycol mono(meth)acrylate, polyethylene glycol (with a degree of polymerization in the range of 2 to 10) mono(meth)acrylate;

[0051] Alkoxyalkyl (with 1 to 3 carbon atoms) (meth)acrylates; etc.

[0052] The other unsaturated monomer(s) further comprises

[0053] (Meth)acrylamides;

[0054] Vinyl compounds such as styrene, α-methylstyrene, vinyl acetate, vinyl propionate, vinyl benzoate, vinyl toluene and acrylonitrile;

[0055] Crotonates;

[0056] Diesters of unsaturated dibasic acids, such as maleic acid diesters and itaconic acid diesters; etc.

[0057] One of the other unsaturated monomers may be used alone. Alternatively, two or more of the other unsaturated monomers may be used in combination.

[0058] The polymerization process for providing the acrylic resin includes, but is not limited to, polymerization processes known to those skilled in the art, such as solution polymerization, dispersion polymerization, and emulsion polymerization.

[0059] The acrylic resin has a number-average molecular weight (Mn) in the range of 4,000 to 6,000. If the number-average molecular weight (Mn) is less than 4,000, the resulting coating composition may have insufficient curability. If the number-average molecular weight (Mn) is more than 6,000, the resulting coating composition may have a higher viscosity, and therefore, the resulting coating composition may not allow for a higher solid content.

[0060] Herein, the number-average molecular weight (Mn) can be determined by gel permeation chromatography (GPC) as a calculated value using a polystyrene standard.

[0061] The acrylic resin preferably has an acid value in the range of 1 to 5 mg KOH / g based on the solid content. If the acid value is within this range, the association of the acrylic resin with the carboxyl group can be increased, and thus the apparent molecular weight of the acrylic resin can also be increased. This can prevent migration of the acrylic resin into the base coat film and therefore provide a coating film with outstanding design properties.

[0062] If the acid value is less than 1 mg KOH / g, such an association will be reduced, and it will be difficult to increase the apparent molecular weight of the acrylic resin. Therefore, in this case, it may be difficult to form a coating film with outstanding design properties. If the acid value is more than 5 mg KOH / g, the resulting coating composition may exhibit excessive viscosity, and a higher solid content may not be possible.

[0063] The acrylic resin may preferably have a hydroxyl value in the range of 50 to 140 mg KOH / g based on the solid content. If the hydroxyl value is less than 50 mg KOH / g, there may be a problem that the curability is reduced. If the hydroxyl value is greater than 140 mg KOH / g, there may be a problem that the resulting coating film has inferior water resistance. It may be more preferable that the hydroxyl value is in the range of 80 to 140 mg KOH / g. Herein, the acid value and the hydroxyl value can be adjusted by varying the mixing ratio of the starting monomers.

[0064] The acrylic resin may preferably have a glass transition temperature (hereinafter referred to as Tg) in the range of -30 to 50°C. If the Tg is less than -30°C, there may be a problem that the resulting coating film has reduced hardness, and there may be a problem that the thickness of the resulting coating film varies depending on the temperature change of the coating composition. If the Tg is greater than 50°C, the resulting coating composition may have excessive viscosity, and therefore, the resulting coating composition may not allow for a higher solid content. It may be more preferable for the acrylic resin to have a Tg in the range of -10 to 30°C.

[0065] The Tg value of the acrylic resin can be determined, for example, by means of a calculation with known Tg values of the constituent monomers or the homopolymers thereof and the formulation thereof.

[0066] The clearcoat composition comprises a polyisocyanate compound. Examples of the polyisocyanate compound include, but are not limited to, compounds having at least two isocyanate groups, which include: aliphatic isocyanates, such as trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate (HMDT) and trimethylhexamethylene diisocyanate; aliphatic cyclic isocyanates, such as 1,3-cyclopentane diisocyanate, 1,4-cyclohexane diisocyanate and 1,2-cyclohexane diisocyanate; aromatic isocyanates, such as xylylene diisocyanate (XDI), 2,4-trilene diisocyanate (TDI) and 2,6-trilene diisocyanate; alicyclic isocyanates, such as isophorone diisocyanate (IPDI) and norbornane diisocyanate methyl; and Multimers, such as nurates, biurates and adducts thereof, and mixtures thereof; etc.

[0067] Of these polyisocyanate compounds, polyisocyanates in nurate forms are preferred, which can provide good weather resistance.

[0068] The stoichiometric ratio of the total of the carboxyl groups and / or the hydroxyl groups in the acrylic resin to the NCO groups in the polyisocyanate compound (total of the carboxyl group(s) and the hydroxyl group(s) in the acrylic resin / NCO group(s) in the polyisocyanate compound) may preferably be in the range of 1 / 1.5 to 1.5 / 1. If the stoichiometric ratio is less than 1 / 1.5, sufficient curability may not be achieved. If the stoichiometric ratio is greater than 1.5 / 1, the resulting cured coating film may be excessively cured and brittle. It may be more preferable that the stoichiometric ratio be in the range of 1.4 / 1 to 1 / 1.4.

[0069] The crosslinked resin particles exhibit a rheology-controlling effect, providing effective prevention of sagging during coating or heating, as well as a regulated L value of the coating film. Examples of crosslinked resin particles are not particularly limited, and they can be selected from conventional crosslinked resin particles in the art. The crosslinked resin particles can be added to the composition within a preferable range of 0.01 to 10 wt.% based on the solid resin content of the clear coating composition. More preferably, the range may be 0.1 to 10 wt.%, and most preferably, the range may be 0.5 to 5 wt.% If the amount is less than 0.01 wt.%, the desired rheology-controlling effect may not be obtained. If the amount of the crosslinked resin particles to be added is more than 10 wt.%% the resulting coating film may have a deteriorated appearance.

[0070] The clear coating composition may include a UV absorber, a hindered amine light stabilizer, an antioxidant, a surface conditioner, etc.

[0071] The clear coating composition of the present invention may preferably be a two-pack curing system coating composition comprising a composition comprising the above-described acrylic resin and the polyisocyanate compound.

[0072] In the method for producing the multi-layer coating film according to the present invention, the process for applying the clear coating composition includes, for example, application with an electrostatic spray coating device with a rotary atomizer, referred to as a so-called Micro-Micro-Bel or Micro-Bel. When the clear coating composition is a two-pack curing system coating composition, the two packed compositions may be mixed before application, and then the resulting clear coating composition may be applied according to the coating procedure described above. According to the present invention, the clear coating composition may be applied such that the thickness of the dried coating film formed with the clear coating composition in the resulting multi-layer coating film may preferably be within the range of 20 to 60 µm.

[0073] The uncured base coat film and the uncured clear coat film formed thereon according to the above-described coating procedures are simultaneously heated (i.e., baked) and cured to form a multi-layer coating film. Heating can be carried out at a temperature preferably in the range of 100 to 180°C, and more preferably in the range of 120 to 160°C. The heating time and curing may vary depending on the conditions, such as the curing temperature. If the above-defined temperature is used for heating and curing, the time is suitably in the range of 10 to 30 minutes.

[0074] Thus, the obtained multilayer coating film has a ΔL value of -0.3 to 0.15, which is obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film at 25° reflection, and a ΔL value of -0.2 to 0.4, which is obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film at 75° reflection. The multilayer coating film with a ΔL value within the above range exhibits excellent design properties and suppressed color reversal.

[0075] The total thickness of the multilayer coating film may preferably be in the range of 30 to 80 µm. The multilayer coating film obtainable / obtained by the method described above can provide excellent film properties, such as scratch resistance, acid resistance, and solvent resistance, and can therefore provide excellent design properties.

[0076] The usable substrate may be, for example, a metal molding, a plastic molding, a foamed article, or the like. The substrate on which the multilayer coating film for an automobile can be formed includes molded metal articles made of a substrate made of iron, aluminum, or zinc or an alloy thereof, or the like, and molded plastic articles and the like. In forming the multilayer coating film of the present invention, a molded metal article on which a cationic electroplating coating composition may be applied is employable and preferable. It is preferable that the substrate be chemically treated on its surface. The substrate may further comprise a plating coating film thereon comprising a plating coating composition.The electroplating coating composition includes cationic electroplating coating compositions and anionic electroplating coating compositions. The cationic electroplating coating compositions are preferred because of their corrosion resistance.

[0077] Herein, the method for forming the multilayer coating film according to the invention may further comprise, prior to the above-described step 1, a step p of applying an intermediate coating composition to the substrate to form an intermediate coating film. The intermediate coating composition that can be used in step p includes conventional intermediate coating compositions known to those skilled in the art.

[0078] The intermediate coating composition includes a water-based coating composition and a solvent-based coating composition. The intermediate coating composition may include, as components therein, a resin component for forming an intermediate coating film, a coloring pigment, a filler pigment, and an aqueous solvent and / or an organic solvent. The resin component includes a resin for forming an intermediate coating film, and it may be used in combination with a curing agent for the resin for forming the intermediate coating film, if necessary. The resin component, the curing agent, the coloring pigment, the filler pigment, and the other additives and the solvent that can be added to the intermediate coating composition can be appropriately selected from those specified in the above-described base coating composition.

[0079] The pigment content in the intermediate coating composition, based on the solids content of the coating composition, ie, as pigment weight content (PWC), may preferably be in the range of 30 to 65 wt.%. It may be more preferred that the PWC be up to 50 wt.%. Herein, the solids content of the intermediate coating composition may preferably be in the range of 35 to 65 wt.%.

[0080] The intermediate coating composition can be applied by spraying, roller coating, or the like. For example, coating methods such as electrostatic painting using a so-called "REACT" and an electrostatic spray coating device with a rotary atomizer using a so-called "Micro-Micro-Bel (µµ-Bel)," "Micro-Bel (µ-Bel)," "Meta-Bel," or the like are preferred. Of these methods, coating with an electrostatic spray coating device with a rotary atomizer is particularly preferred. The thickness of the dried intermediate coating film may preferably be in the range of 5 to 80 µm, and more preferably in the range of 10 to 50 µm.

[0081] The resulting uncured intermediate coating film can be subjected to baking and curing at a temperature in the range of 120 to 160°C for a given period of time to form a cured intermediate coating film. The above-described base coating composition and the above-described clear coating composition can be applied to the thus-obtained cured intermediate coating film.

[0082] Herein, the base coating composition may preferably be applied to the resulting uncured intermediate coating film by wet-on-wet coating, and then the clear coating composition may be applied thereon to form the uncured intermediate coating film, the uncured base coating film, and the uncured clear coating film in this order. If the clear coating composition is applied to the uncured base coating composition by wet-on-wet coating, the applied intermediate coating composition may be subjected to preheating to form an uncured intermediate coating film. As a result, a multi-layer coating film with an excellent final appearance can be provided. EXAMPLES

[0083] The present invention will be further described in detail with reference to the following examples, but it is not limited to these examples. In the examples, the terms "part(s)" indicate "part(s) by weight" unless otherwise specified. MANUFACTURING EXAMPLE (a)

[0084] Propylene glycol monomethyl ether acetate (448 parts) was added to a reaction vessel equipped with a thermometer, a stirring blade, a nitrogen inlet tube, a condenser, and a dropping funnel, and then heated to 120°C under a nitrogen atmosphere. 100 parts of propylene glycol monomethyl ether acetate, 105 parts of tert-butyl peroxy-2-ethylhexanoate, and a monomer mixture consisting of 200 parts of styrene, 67 parts of n-butyl acrylate, 100 parts of alkyl methacrylate, 270 parts of isobornyl methacrylate, 360 parts of 4-hydroxybutyl acrylate, and 3 parts of methacrylic acid were added dropwise to the vessel through the dropping funnel at a constant rate over 3 hours. The mixture was then left at 120°C for 0.5 hour. Then, 10 parts of tert-butylperoxy-2-ethylhexanoate dissolved in 50 parts of propylene glycol monomethyl ether acetate were added dropwise to the mixture at a constant rate over 30 minutes.The mixture was left under heating at 120°C for 1 hour.

[0085] Consequently, the resulting product was an acrylic copolymer, hereinafter referred to as acrylic copolymer “A”, with calculated Tg: 5.3°C, acid value: 2 mg KOH / g and hydroxyl value: 140 mg KOH / g (based on solid content), number average molecular weight (Mn): 4,600 and weight average molecular weight (Mw): 11,300 (calculated by GPC with a polystyrene standard) and solid resin content: 62.5%.

[0086] The alkyl methacrylate used was ACRYLESTER SL (manufactured by Mitsubishi Rayon Co., Ltd.), in which the mixing ratio of lauryl methacrylate to tridecyl methacrylate was 4 / 6 (lauryl methacrylate / tridecyl methacrylate) by weight. MANUFACTURING EXAMPLES (b) AND (c)

[0087] As shown in the following Table 1, the acrylic copolymers (B) and (C) shown in the following Table 1 were prepared according to Preparation Example (a) using their formulations shown in Table 1. TABLE 1 Acrylic resin A B C Monomer formulation Styrene 200,0 200,0 100,0 n-butyl acrylate 67,0 17,0 167,0 Alkyl methacrylate* 100,0 150,0 0,0 Isobornyl methacrylate 270, 0 270, 0 370, 0 4-Hydroxybutylacrylate 360,0 360,0 360,0 Methacrylic acid 3,0 3,0 3,0 Content of long-chain (meth)acrylate monomer (in wt%) 10,0 15,0 0,0 Resin parameters Calculated Tg (°C) 5,3 4, 7 7,2 Acid value (mg KOH / g) 2 2 2 Hydroxyl value (mg KOH / g) 140 140 140 Number average molecular weight (Mn) 4.600 4.400 4.500 Weight-average molecular weight (Mw) 11.300 10.500 10.800 Solid resin content (%) 62, 5 62, 5 62, 6 * Alkyl methacrylate: ACRYLESTER SL (manufactured by Mitsubishi Rayon Co., Ltd.), wherein the mixing ratio of lauryl methacrylate to tridecyl methacrylate was 4 / 6 (lauryl methacrylate / tridecyl methacrylate) by weight MANUFACTURING EXAMPLE (e)Production of cross-linked resin particles:

[0088] In a glass vessel equipped with a thermometer, a stirrer, a nitrogen inlet tube, a condenser, and a decanter, 213 parts of bishydroxyethyltaurine, 208 parts of neopentyl glycol, 296 parts of phthalic anhydride, 376 parts of azelaic acid, and 30 parts of xylene were added. The ingredients were then heated. The water formed by the reaction was removed azeotropically along with xylene.

[0089] The solution was gradually heated to 210°C for about 3 hours after the start of reflux, and stirring and dehydration were continued until the acid value, equivalent to carboxylic acid, reached 135 mg KOH / g. The solution was then cooled to 140°C. Next, 500 parts of Cardula E10 (glycidyl varsatate, manufactured by Shell Company) were added dropwise over 30 minutes. Stirring was then continued for 2 hours to complete the reaction. The obtained polyester resin, which had an amphoteric ion, had an acid value of 55 mg KOH / g (based on the solid content), a hydroxyl value of 91, and a Mn of 1250.

[0090] In a corrosion-resistant beaker, 10 parts of the resulting polyester resin containing an amphoteric ion, 140 parts of deionized water, 1 part of dimethylethanolamine, 50 parts of styrene, and 50 parts of ethylene glycol dimethacrylate were vigorously stirred to form a monomer suspension. Separately, 0.5 part of azobiscyanovaleric acid, 40 parts of deionized water, and 0.32 part of dimethylethanolamine were stirred to form an aqueous initiator solution.

[0091] In a glass vessel equipped with a thermometer, a stirrer, a nitrogen inlet tube, a condenser, and a decanter, 5 parts of the obtained polyester resin containing an amphoteric ion, 280 parts of deionized water, and 0.5 part of dimethylethanolamine were added and heated to 80°C. Then, 251 parts of the monomer suspension and 40.82 parts of the aqueous initiator solution were simultaneously added dropwise over 60 minutes, and the mixture was reacted for another 60 minutes to complete the reaction. An emulsion containing crosslinked resin particles with a particle size of 55 nm, measured by a dynamic scattering method, was obtained. Butyl acetate was added to the emulsion, and the water was azeotropically removed under reduced pressure to replace the solvent with butyl acetate. A solution containing crosslinked resin particles (resin solid content: 20%) was obtained. Preparation of clear coating compositions (A) to (D):

[0092] Each of the acrylic copolymers (A) to (C) (68.68 parts), 31.32 parts of Sumidule N-3300 manufactured by Suika Bayer Urethane Co., Ltd. (an isocyanate compound in an isocyanurate form), 2 parts of Tinuvin 928 (UV absorber manufactured by Ciba Specialty Chemicals Co., Ltd.), 1 part of Tinuvin 292 (light stabilizer manufactured by Ciba Specialty Chemicals Company), 4 parts of crosslinked resin particles, and 0.1 part of Modaflow (surface conditioner manufactured by Monsanto Company) were added and stirred with a stirrer to obtain each of the clear coating compositions (A) to (C). The clear coating composition (D) was prepared in the same manner as the clear coating composition (A), except that no crosslinked resin particles were included.

[0093] Each of the obtained clear coating compositions (A) to (D) was diluted with a thinner composed of propylene glycol monomethyl ether acetate and 3-ethoxyethyl propionate (weight ratio: 1:2) so that they had a viscosity of 25 seconds (measured at 20°C using the No. 4 Ford Cup). EXAMPLES 1, 2 AND COMPARISON EXAMPLES 1, 2

[0094] A cationic electroplating coating composition available under the trade name "POWERTOP U-50" (manufactured by Nippon Paint Co., Ltd.) was applied to a phosphate-treated steel sheet, and then heated and cured so that the thickness of the resulting dried electroplating coating film was 25 μm. Subsequently, a gray intermediate coating composition available under the trade name "ORGA P-30" (manufactured by Nippon Paint Co., Ltd.) was applied to the coating film, and then heated and cured so that the thickness of the resulting dried intermediate coating film was 40 μm to prepare a test sheet.

[0095] Each of the water-based primer coating compositions “AQUAREX AR-2000 1F7” (trademark, manufactured by Nippon Paint Co., Ltd.) with silver color, “AQUAREX AR-2000 4P7” (trademark, manufactured by Nippon Paint Co., Ltd.) with beige color, “AQUAREX AR-2000 4R3” (trademark, manufactured by Nippon Paint Co., Ltd.) with beige color, and “AQUAREX AR-2000 1D2” (trademark, manufactured by Nippon Paint Co., Ltd.) with gray color were applied to the test panel to form an uncured primer coating film.

[0096] Then, each of the clear coating compositions (A) to (D) was applied to the uncured base coat film by wet-on-wet coating. The panel was baked at 140°C for 30 minutes and dried to prepare a coated test panel with the multi-layer coating film obtained by the so-called two-coat, one-bake (2C1B) coating procedure.

[0097] Herein, the multi-layer coating film having the base coating film and the clear coating film thereon was applied to the sheet such that the thickness of the dried base coating film was 15 µm and the thickness of the dried clear coating film was 40 µm.

[0098] Separately, the base coating composition in silver metallic color was applied to the other test panel so that the thickness of the dried base coating film was 15 µm, and then baked and dried at 140°C for 30 minutes to prepare a coated test panel with a single base coating film. Evaluation:

[0099] The coating test panels thus obtained were subjected to the following tests (1) and (2). The evaluation results obtained are shown in Table 2. TABLE 2 Example 1 L-value of a single, uncured primer coating film Base coat composition 1F7 4 P7 4R3 1D2 Color range slightly saturated color medium saturated color Highlight 100, 92 93, 31 88,24 78,71 halftone 44, 56 40, 54 34, 01 29, 90 Highlight / Halftone 56,36 52,77 54,23 48,81 Clearcoat Composition A L-value of the multilayer coating film Highlight 100,71 93,18 88,23 78,83 halftone 44, 88 40, 75 34,20 29,93 ΔL value Highlight -0,21 -0,13 -0,01 0,12 halftone 0,32 0,21 0,19 0,03 Design characteristics of the multilayer coating film O O O O TABLE 2 CONTINUED Example 2 L-value of a single, uncured primer coating film Base coat composition 1F7 4 P7 4R3 1D2 Color range slightly saturated color medium saturated color Highlight 100, 92 93,31 88,24 78,71 halftone 44,56 40, 54 34,01 29,90 Highlight / Halftone 56,36 52,77 54,23 48,81 Clearcoat composition B L-value of the multilayer coating film Highlight 100, 93 93,22 88,13 78,74 halftone 44,83 40,72 34,12 29,91 ΔL value Highlight 0,01 -0,09 -0,11 0,03 halftone 0,27 0,18 0,11 0,01 Design characteristics of the multilayer coating film ◯ ◯ ◯ ◯ TABLE 2 CONTINUED Comparison example 1 L-value of a single, uncured primer coating film Base coat composition 1F7 4 P7 4R3 1D2 Color range slightly saturated color medium saturated color Highlight 100, 92 93,31 88,24 78,71 halftone 44,56 40, 54 34,01 29, 90 Highlight / Halftone 56,36 52,77 54,23 48,81 Clearcoat Composition C L-value of the multilayer coating film Highlight 98, 60 91,76 87, 68 78,38 halftone 47,16 43,02 35,68 30,52 ΔL value Highlight -2,32 -1,55 -0,56 -0,33 halftone 2,60 2,48 1,67 0,62 Design characteristics of the multilayer coating film ×× ×× × Δ TABLE 2 CONTINUED Comparison example 2 L-value of a single, uncured primer coating film Base coat composition 1F7 4P7 4R3 1D2 Color range slightly saturated color medium saturated color Highlight 100, 92 93,31 88,24 78,71 halftone 44,56 40,54 34,01 29,90 Highlight / Halftone 56,36 52,77 54,23 48,81 Clearcoat composition D L-value of the multilayer coating film Highlight 100,17 93,03 88,05 78,53 halftone 45, 44 41,07 34,47 30,33 ΔL value Highlight -0,75 -0,28 -0,19 -0,18 halftone 0,88 0,53 0,46 0,43 Design characteristics of the multilayer coating film × Δ Δ Δ (1) L-value:

[0100] The difference between the metallic color of the multi-layer coating film formed according to the 2C1B coating procedure and the metallic color of the corresponding single base coating film was measured.

[0101] As in the attached Fig. As schematically illustrated in Figure 1, the measurement was performed using a CM-512m3, manufactured by Minolta Inc. The L values were measured at angles of incidence of 25° (hereinafter referred to as "highlight") and 75° (hereinafter referred to as "halftone"). The ΔL value was calculated according to the following equation: ΔL value = (L value of the multilayer coating film) − (L value of the single base coating film) (2) Visually observed appearance:

[0102] The design properties of the multilayer coating film were visually observed and evaluated. The evaluation basis was as follows.

[0103] Valuation basis: good (O) has a large ΔL in the highlight and halftone and significant flip-flop properties not good (Δ) has a small ΔL in the highlight and halftone and weak flip-flop properties poor (×) shows miscibility between the base coat film and the clear coat film and hardly any flip-flop properties very poor (××): has very poor miscibility between the base coat film and the clear coat film and no flip-flop properties.

[0104] Each of Examples 1 and 2 provided a multilayer coating film with suppressed color reversal and excellent design properties for all base coating films 1F7, 4P7, 4R3, and 1D2.

[0105] In contrast, Comparative Example 1, which used a clear coating composition comprising an acrylic resin without alkyl methacrylate, provided a multilayer coating film with color reversal of the base coating film and inferior design properties.

[0106] Comparative Example 2, which used a clear coating composition without crosslinked resin particles, also provided a multilayer coating film with color reversal of the base coating film and inferior design properties.

[0107] Comparative Examples 1 and 2 show that a higher L value of the base coating film, such as a slightly saturated color, resulted in a stronger color reversal. INDUSTRIAL APPLICABILITY:

[0108] The present invention can be used to form a multi-layer coating film on vehicles such as automobiles, particularly with increased brightness such as metallic colors, particularly to form a multi-layer coating film on automobiles where outstanding design properties are desired in the market.

Claims

[1] A method for forming a multilayer coating film, comprising: Applying a primer coating composition to a substrate to form an uncured primer coating film (step 1), Applying a clear coating composition to the uncured base coating film obtained in step 1 to form an uncured clear coating film (step 2), and Baking and curing the uncured base coat film and the uncured clear coat film of steps 1 and 2 to form a multi-layer coating film (step 3), wherein (a) the L value of the uncured primer coating film is 70 to 105 at 25° reflection and the L value of the uncured primer coating film is 20 to 50 at 75° reflection, (b) the ΔL value obtained by subtracting the L value of the uncured base coating film from the L value of the multi-layer coating film at 25° reflection is -0.3 to 0.15, and (c) the ΔL value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film at 75° reflection is -0.2 to 0.4, and wherein the clear coating composition includes: an acrylic resin having a number average molecular weight of 4,000 to 6,000 and a solid acid value of 1 to 5 mg KOH / g, which is obtained from a monomer mixture comprising 10 to 15 wt%, based on the total monomer solids, of a long-chain (meth)acrylic ester monomer, a carboxy group-containing (meth)acrylic monomer and a hydroxy group-containing (meth)acrylic monomer, wherein the long-chain (meth)acrylic ester monomer has an ester portion of a linear hydrocarbon group having 9 to 15 carbon atoms; a polyisocyanate compound; and cross-linked resin particles. [2] The method for forming a multi-layer coating film according to claim 1, wherein the clear coating composition comprises crosslinked resin particles in the range of 0.1 to 10 wt% based on the total solid weight of the clear coating composition. [3] A method for forming a multi-layer coating film according to claim 1 or 2, wherein the method comprises applying an intermediate coating composition to a substrate before step 1 to form an intermediate coating film (step P).

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