JPWO2009081943A1 - Multi-layer coating formation method - Google Patents

Abstract

In the present invention, a base coating is applied to a substrate to form an uncured base coating (1), and a clear coating is applied to the uncured base coating obtained in the above step (1). And forming the uncured clear coating film (2), and the uncured base coating film and the uncured clear coating film obtained in the above steps (1) and (2) are cured by heating to form a multilayer coating. A method for forming a multilayer coating film comprising the step (3) of forming a film, wherein (a) the L value of the uncured base coating film is 70 to 105 at 25 degrees reflection and 75 degrees reflection (B) The ΔL value, which is a value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film by 25-degree reflection, is −0.3 to 0.00. (C) L value of the uncured base coating film from the L value of the multilayer coating film by 75 degree reflection. The ΔL value, which is a value obtained by subtracting γ, is from −0.2 to 0.4, and the clear coating material is a long chain (meta) having a linear hydrocarbon group having 9 to 15 carbon atoms in the ester portion. ) Acrylic ester monomer is obtained from a monomer mixture containing 10-15% by mass with respect to the total amount of monomer solids, and containing a carboxyl group-containing (meth) acrylic monomer and a hydroxyl group-containing (meth) acrylic monomer, and the number average molecular weight is The present invention relates to a method for forming a multilayer coating film, which is a clear coating composition comprising 4000 to 6000, an acrylic resin having a solid content acid value of 1 to 5 mgKOH / g, a polyisocyanate compound, and crosslinked resin particles. According to the present invention, it is possible to provide a method for forming a multilayer coating film capable of forming a multilayer coating film having excellent design properties, acid resistance, and scratch resistance.

Description

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

  In painting automobile bodies and the like, it is widely practiced to form a top coat film containing a base coat film and a clear coat film. In particular, since the clear coating film constitutes the outermost layer in the coating film of an automobile body or the like, various physical properties such as design properties, acid resistance, and scratch resistance are required.

  Conventionally, acrylic / melamine resin thermosetting paint has been used as a clear paint for automobile painting. However, since the coating film obtained by using such a melamine resin as a curing agent is inferior in acid resistance, it is likely to be deteriorated by acid rain, and there is a risk of causing a problem in appearance.

Examples of such clear paints other than the acrylic / melamine resin thermosetting type include urethane-based paints (for example, see Patent Document 1).
JP-A-2005-787

  However, although a urethane-based paint can obtain a coating film excellent in acid resistance and scratch resistance, a clear coating film is formed on the uncured base coating film using a urethane-based paint, and the base coating film and the clear coating film are formed. When a top coat film is formed by so-called two-coat one-bake in which the film is heated and cured at a temperature of 120 ° C. or more at the same time, the acrylic resin (acrylic polyol) in the clear coating composition penetrates into the uncured base coat film. May affect the appearance of the coating film.

In particular, when the base paint composition is a metallic paint having a glitter pigment, the penetration of the components in the clear paint composition disturbs the arrangement of the glitter pigment in the uncured base coating, resulting in highlights. A phenomenon called color reversion of the base coating film in which the shade (bright surface) becomes bright while the (front surface) becomes dark may occur, and the coating film appearance may deteriorate.
Therefore, urethane-based paints are difficult to obtain coating films having excellent design properties as clear paints for automotive coatings, especially clear paints for automotive coatings for markets where high design properties are required. There was a problem.

  In view of the above-mentioned present situation, the present invention suppresses the occurrence of color reversion when forming a multilayer coating film including a base coating film with high brightness such as a metallic base coating film, and has a high design characteristic that the base coating film originally has. In addition, an object of the present invention is to provide a multilayer coating film forming method capable of forming a multilayer coating film having excellent acid resistance and scratch resistance.

The present invention includes a step (1) of applying a base paint to a base material to form an uncured base coating film,
A step (2) of forming an uncured clear coating film by applying a clear coating on the uncured base coating film obtained in the step (1); and
A multilayer coating film forming method comprising the step (3) of heating and curing the uncured base coating film and the uncured clear coating film obtained in the steps (1) and (2) to form a multilayer coating film. There,
(A) The L value of the uncured base coating film is 70 to 105 in 25 degree reflection and 20 to 50 in 75 degree reflection,
(B) The ΔL value, which is a value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film by 25-degree reflection, is −0.3 to 0.15,
(C) The ΔL value, which is a value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film by 75-degree reflection, is −0.2 to 0.4,
And,
The clear coating material contains 10 to 15% by mass of a long-chain (meth) acrylic acid ester monomer whose ester part is a straight-chain hydrocarbon group having 9 to 15 carbon atoms, and contains a carboxyl group An acrylic resin obtained from a monomer mixture containing a (meth) acrylic monomer and a hydroxyl group-containing (meth) acrylic monomer, having a number average molecular weight of 4000 to 6000 and a solid content acid value of 1 to 5 mgKOH / g, a polyisocyanate compound, A method for forming a multilayer coating film, which is a clear coating composition containing crosslinked resin particles.

The clear coating composition preferably contains 0.1 to 10% by mass of crosslinked resin particles with respect to the total solid content of the coating composition.
The multilayer coating film forming method may include a step (P) of forming an intermediate coating film by applying an intermediate coating before the step (1).

  According to the present invention, it is possible to form a multi-layer coating film having high brightness, having a high design appearance with suppressed color reversion, and excellent in acid resistance and scratch resistance.

FIG. 1 is a diagram schematically showing a method for measuring the L value.

  The present invention is described in detail below.

In the multilayer coating film forming method of the present invention, (1) after forming an uncured base coating film satisfying the above (a), (2) applying the clear coating material on the uncured base coating film and uncuring A clear coating film is formed, and (3) the uncured base coating film and the uncured clear coating film are cured by heating to form a multilayer coating film that satisfies the above (b) and (c).
An uncured base coating film having high brightness, such as a metallic base coating film, is generally a film having excellent design properties because the color return of the base coating film is likely to be manifested in a 2-coat 1-bake coating method. Is difficult to form.
However, according to the method for forming a multilayer coating film of the present invention, an uncured base coating film is formed so that the L value satisfies the above (a), and the clear coating composition is applied to the uncured base coating film. By forming a cured clear coating film and curing by heating, it is possible to form a multilayer coating film having high appearance and a high design appearance with suppressed color reversion.

  The reason why it is possible to suppress the occurrence of color reversion and maintain the high designability inherent in the base coating film by the method for forming a multilayer coating film of the present invention is that the acrylic resin contained in the clear coating composition is relatively long. Since it has a straight chain hydrocarbon group having 9 to 15 carbon atoms as a side chain, when a clear coating is applied in the step of forming a top coat film by 2 coats and 1 bake, acrylic resin is removed due to the steric hindrance of the straight chain hydrocarbon group. This is considered to be because the resin can be prevented from penetrating into the uncured base coating film. Furthermore, it can be considered that making the acrylic resin have a high acid value is effective in preventing the penetration of the acrylic resin into the uncured base coating film.

The paint colors can be classified into three types according to the L value of the highlight of the cured base single film. First, a deep color having an L value of less than 40, a medium color of 40 to 80, and a light color of 80 or more. Here, in the base single film containing the glitter pigment, the intermediate color to the light color with a large L value of the highlight can be obtained by laminating a clear coating film so that the flip-flop property and the design property are high. A membrane can be obtained.
However, the L value fluctuates even with a slight disturbance in the orientation of the glitter pigment due to the mixed layer of the base coating and the clear coating, so that the L value of the highlight and the L value in the shade are likely to increase. The flip-flop property is also likely to deteriorate. Further, in the dark color where the L value of the highlight of the cured base coating film is small, the L value in the original highlight is small, so the orientation of the glitter pigment is disturbed by the mixed layer of the base coating film and the clear coating film. If it is slight, it is difficult to appear as a change in the L value.
Therefore, there is a need for a clear coating composition that can control the mixed layer with the base layer, particularly for medium to light colors.

The multilayer coating film forming method of the present invention includes a step (1) of applying a base coating to a base material to form an uncured base coating film.
As the base paint, a brightener base paint containing a base resin component, a bright pigment, a colored pigment, an extender pigment, a solvent and the like can be used. This base coating composition is of an aqueous or organic solvent type including an aqueous dispersion or an organic solvent dispersion. The aqueous solvent and organic solvent are not particularly limited, and conventionally known solvents can be used.

  The base resin component contained in these base paints is composed of a base resin and, if necessary, a base curing agent. Using the base resin component, the glitter pigment and, if necessary, the color pigment are dispersed in the glitter base coating.

  Examples of the base resin include acrylic resin, polyester resin, polyurethane resin, alkyd resin, epoxy resin, and polyether resin. Of these resins, acrylic resins, polyester resins, and polyurethane resins are preferably used. These resins may be used alone or in combination of two or more.

  The base resin generally has a curable type and a lacquer type, but a curable type is preferably used. When a curable type is used, a base curing agent such as a melamine resin, a (block) isocyanate compound, an oxazoline compound and a carbodiimide compound is used in combination with the base resin. This base curing agent can be included in the base resin component to allow the curing reaction to proceed later under heating or at room temperature. Further, a curable type base resin and a non-curable type resin can be used in combination.

  When a base hardening | curing agent is contained, the preferable mass ratio of the base resin and the base hardening | curing agent in paint solid content is 90 / 10-50 / 50, More preferably, it is 85 / 15-60 / 40. If the amount of the base curing agent is so small that the mass ratio between the base resin and the base curing agent deviates from 90/10, sufficient crosslinking in the coating film may not be obtained. On the other hand, when the amount of the base curing agent is so large that this ratio is deviated from 50/50, the storage stability of the coating material is lowered and the curing rate is increased, which may deteriorate the appearance of the coating film.

  As the base curing agent, an etherified melamine resin can be used in addition to those described above. The etherified melamine resin is obtained by etherifying melamine with an alcohol such as methanol or butanol.

As a pigment contained in the base paint, a non-colored or colored scale-like metallic luster material such as aluminum, copper, zinc, iron, nickel, tin, aluminum oxide or an alloy, or a mixture thereof, or interference mica powder And bright pigments such as non-colored colored bright materials such as colored mica powder, white mica powder, graphite and glass flakes.
In addition to the glitter pigment, for example, extender pigments such as barita powder, precipitated barium sulfate, barium carbonate, gypsum, clay, silica, talc, magnesium carbonate, and alumina white, and colored pigments. Examples of coloring pigments include azo lake pigments, phthalocyanine pigments, indigo pigments, perylene pigments, quinophthalone pigments, dioxazine pigments, quinacridone pigments, isoindolinone pigments, diketopyrrolopyrrole pigments, and benzimidazolones. Organic pigments such as system pigments and metal complex pigments, and inorganic pigments such as yellow lead, yellow iron oxide, bengara, titanium dioxide, and carbon black. The amount of the pigment can be arbitrarily set according to the desired performance and hue. These pigments may be used alone or in combination of two or more.

  In addition to the above components, the base paint is a wax such as a polyamide wax which is a lubricating dispersion of an aliphatic amide or a polyethylene wax which is a colloidal dispersion mainly composed of oxidized polyethylene, a curing catalyst, an ultraviolet absorber, and an antioxidant. , Leveling agents, surface conditioners such as silicone and organic polymers, anti-sagging agents, thickeners, antifoaming agents, lubricants, crosslinkable polymer particles (microgel), and the like can be appropriately added. By blending these additives at a ratio of 15 parts by mass or less with respect to 100 parts by mass (based on solid content) of the base resin component, the performance of the paint or coating film can be improved.

  The upper limit of the pigment concentration (PWC) with respect to the solid content of the base coating is preferably 65% by mass. Moreover, it is preferable that the minimum is 5 mass%. Further, the solid content concentration of the base coating is preferably in the range of a lower limit of 15% by mass and an upper limit of 60% by mass.

  Specific examples of the method of applying the base paint include multi-stage coating by air electrostatic spraying or rotary atomizing electrostatic coating machine, preferably two-stage coating. By performing this coating method, the designability can be improved. Or the coating method which combined the air electrostatic spray and the rotary atomization type electrostatic coating machine as an application method of a base coating material is mentioned. The dry film thickness of the topcoat base coating film is preferably 5 to 50 μm, more preferably 10 to 30 μm per coat.

  The obtained uncured base coating film may be preheated. By applying pre-heating after applying the base coating material to form an uncured base coating film, a multilayer coating film having a better finished appearance can be finally obtained. In the present invention, “uncured” refers to a state where the film is not completely cured, and includes a state of a coating film that has been preheated after coating. “Preheating” can be performed by leaving or heating at room temperature to 100 ° C., which is lower than the temperature used in the heat curing (baking) treatment, for 1 to 10 minutes.

The uncured base coating film obtained by performing the above step (1) has an L value of 70 to 105 at 25 degrees reflection and 20 to 50 at 75 degrees reflection.
In the present invention, after forming an uncured base coating film having an L value within the range, an uncured clear coating film is formed on the uncured base coating film by a clear coating described later, and is cured by heating. Thus, it is possible to form a multilayer coating film that has high brightness, suppresses color reversion, and maintains the high design properties inherent in the base coating film.
In the present invention, the L value (brightness) is an index used to represent the color of the object to be measured by the Lab color system (hunter color system), and the value of the object to be measured increases as the value increases. It means that the whiteness increases, and the blackness of the object to be measured increases as the numerical value decreases. In the present invention, as shown in FIG. 1, the L value in the 25-degree reflection is from the light source at an angle of 25 degrees when the light-receiving part at a position perpendicular to the coating film to be measured is 0 degrees. It means L value of light received by irradiating light. Similarly, the L value in 75-degree reflection means the L value of light received by irradiating light from the light source at an angle of 75 degrees from the light receiving unit. The L value can be determined, for example, by measuring using a variable angle color difference meter (for example, “CM-512m3” manufactured by Minolta).

After forming an uncured base coating film in the step (1), a clear coating is applied on the uncured base coating film to form an uncured clear coating film.
The clear coating composition includes a long-chain (meth) acrylic acid ester monomer having an ester part 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. It is obtained from a monomer mixture containing a monomer, and contains an acrylic resin having a number average molecular weight of 4000 to 6000 and a solid content acid value of 1 to 5 mgKOH / g, a polyisocyanate compound, and crosslinked resin particles.

  The acrylic resin comprises a long-chain (meth) acrylic acid ester monomer, a carboxyl group-containing (meth) acrylic monomer, and a hydroxyl group-containing (meth) acrylic monomer whose ester part is a linear hydrocarbon group having 9 to 15 carbon atoms. It is obtained from the monomer mixture containing.

  The long chain (meth) acrylic acid ester monomer is, for example, the following general formula (A)

  (Wherein R represents hydrogen or a methyl group, and n represents a natural number of 8 to 14).

  The long-chain (meth) acrylic acid ester monomer is not particularly limited as long as the ester portion is a straight chain hydrocarbon group having 9 to 15 carbon atoms. For example, lauryl (meth) acrylate, (meta ) Tridecyl acrylate, decyl (meth) acrylate, and nonyl (meth) acrylate. These may use only 1 type and may use 2 or more types together.

Content of the long-chain (meth) acrylic acid ester monomer in the monomer mixture is 10 to 15% by mass with respect to the total amount of monomer solids.
If it is less than 10% by mass, it is difficult to sufficiently suppress the influence of the clear coating film on the base coating film side. On the other hand, when it exceeds 15 mass%, there exists a possibility that the adhesiveness in the case of repainting may fall.

The carboxyl group-containing (meth) acrylic monomer is not particularly limited, and examples thereof include acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, isocrotonic acid, and maleic acid. These may use only 1 type and may use 2 or more types together. Of these, acrylic acid and methacrylic acid are preferable.
By using a carboxyl group-containing (meth) acrylic monomer, a coating film having excellent design properties can be formed. Such an effect is presumed to be because the apparent molecular weight of the acrylic resin can be increased by the association of carboxyl groups, and thus the penetration of the acrylic resin into the base coating film can be prevented.

  The hydroxyl group-containing (meth) acrylic monomer is not particularly limited. For example, hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, allyl alcohol, methacryl alcohol, Examples include adducts of hydroxyethyl (meth) acrylate and ε-caprolactone. These may use only 1 type and may use 2 or more types together.

  The acrylic resin can be obtained from the monomer mixture containing the long-chain (meth) acrylic acid ester monomer, the carboxyl group-containing (meth) acrylic monomer, and the hydroxyl group-containing (meth) acrylic monomer. The monomer mixture may contain other unsaturated monomers.

  Examples of the other unsaturated monomer include, for example, (meth) acrylic acid ester, methyl (meth) acrylate, ethyl (meth) acrylate, i-propyl (meth) acrylate, and n-butyl (meth) acrylate. (Meth) acrylic acid alkyl ester having 1 to 8 carbon atoms in the ester moiety such as i-butyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate; (Meth) acrylic acid cyclic hydrocarbon esters such as phenyl acrylate, cyclohexyl (meth) acrylate; (meth) acrylate isobornyl; (poly) ethylene glycol mono (meth) acrylate, polyethylene glycol mono ( (Meth) acrylic acid polyalkylene glycol esters such as (meth) acrylates; and In addition to alkoxyalkyl (meth) acrylates having 1 to 3 carbon atoms, (meth) acrylamide; vinyl compounds such as styrene, α-methylstyrene, vinyl acetate, vinyl propionate, vinyl benzoate, vinyltoluene, acrylonitrile; and Examples thereof include diesters of unsaturated dibasic acids such as crotonic acid esters; maleic acid diesters and itaconic acid diesters. The other unsaturated monomers may be used alone or in combination of two or more.

  The polymerization method for obtaining the acrylic resin is not particularly limited, and known methods such as solution polymerization, dispersion polymerization, and emulsion polymerization can be used.

The acrylic resin has a number average molecular weight (Mn) of 4000 to 6000. When the number average molecular weight (Mn) is less than 4,000, the curability of the coating is not sufficient, and when it exceeds 6000, the viscosity becomes high and it may be difficult to form a coating composition having a high solid content.
In addition, in this specification, a number average molecular weight (Mn) is a number average molecular weight of polystyrene conversion measured by GPC (gel permeation chromatography).

The acrylic resin preferably has a solid content acid value of 1 to 5 mgKOH / g.
By making the solid content acid value within the above range, the apparent molecular weight can be increased by the association of carboxyl groups, so that the penetration of the acrylic resin into the base coating film can be prevented, and the excellent design properties It is because a coating film can be formed.
If the solid content acid value is less than 1 mg KOH / g, the number of points of association decreases and it becomes difficult to increase the apparent molecular weight, which may make it difficult to form a coating film having excellent design properties. . When the solid content acid value exceeds 5 mgKOH / g, the viscosity becomes too high and it may be difficult to obtain a coating composition having a high solid content.

  The acrylic resin preferably has a solid content hydroxyl value of 50 to 140 mgKOH / g. There exists a possibility that sclerosis | hardenability may fall that solid content hydroxyl value is less than 50 mgKOH / g. When the solid content hydroxyl value exceeds 140 mgKOH / g, the water resistance of the coating film may decrease. The solid content hydroxyl value is more preferably 80 to 140 mgKOH / g. In addition, the said solid content acid value and hydroxyl value can be adjusted with the compounding ratio of a raw material monomer.

  The acrylic resin preferably has a glass transition temperature (Tg value) of −30 to 50 ° C. If the Tg value is less than −30 ° C., the coating film hardness may be reduced, and the coating film thickness formed may vary due to fluctuations in the coating liquid temperature. On the other hand, when it exceeds 50 ° C., the viscosity becomes too high and it may be difficult to form a coating composition having a high solid content. The acrylic resin preferably has a Tg value of -10 to 30 ° C.

  The Tg value of the acrylic resin can be calculated, for example, by calculating from the known Tg value and the composition ratio of the constituent monomer or homopolymer.

The clear coating composition contains a polyisocyanate compound.
The polyisocyanate compound is not particularly limited as long as it is a compound having two or more isocyanate groups. For example, trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate (HMDI), trimethylhexamethylene diisocyanate and the like. Aliphatic isocyanates such as 1,3-cyclopentane diisocyanate, 1,4-cyclohexane diisocyanate, 1,2-cyclohexane diisocyanate, and the like, xylylene diisocyanate (XDI), 2,4-tolylene diisocyanate (TDI) ), Aromatic isocyanates such as 2,6-tolylene diisocyanate, isophorone diisocyanate (IPDI), norbornane diisocyanate Alicyclic isocyanates such Tomechiru, these isocyanurate body, can be given burette body, multimers of the adduct and the like and mixtures. Among these, a nurate body is preferable from the viewpoint of weather resistance.

  The functional group amount ratio between the NCO group of the polyisocyanate compound and the carboxyl group and / or hydroxyl group of the acrylic resin is preferably from 1 / 1.5 to 1.5 / 1. When the functional group amount ratio is less than 1 / 1.5, curability becomes insufficient, and when it exceeds 1.5 / 1, the cured film becomes too hard and brittle. The functional group amount ratio is more preferably 1.4 / 1 to 1 / 1.4.

The crosslinked resin particles have a rheology control effect, exhibit a high effect on sagging at the time of coating and heat curing, and at the same time contribute to stabilization of the L value of the coating film. Such crosslinked resin particles are not particularly limited and can be appropriately selected from those obvious to those skilled in the art.
The content of the crosslinked resin particles is preferably 0.01 to 10% by mass, more preferably 0.1 to 10% by mass with respect to the resin solid content of the clear coating composition, More preferably, it is -5 mass%. When the content is less than 0.01% by mass, the rheology control effect may not be obtained. When the content exceeds 10% by mass, the smoothness of the resulting coating film may be adversely affected.

  The clear coating composition may contain an ultraviolet absorber, a hindered amine light stabilizer, an antioxidant, a surface conditioner, and the like.

  The clear coating composition is preferably a two-component curable coating composition comprising the acrylic resin-containing composition and the polyisocyanate compound.

  Specific examples of a method for applying the clear coating composition include a micro-microbell and a coating method using a rotary atomizing electrostatic coater called a microbell. In the case where the clear coating composition is a two-component curable type, the clear coating composition can be applied by the above method after mixing them before use. The coating film thickness of the clear coating composition is preferably in the range of a lower limit of 20 μm and an upper limit of 60 μm in terms of dry film thickness.

  The base uncured coating film and the clear uncured coating film formed by the above method are simultaneously heated and cured to form a multilayer coating film. The heating temperature is preferably performed within a range of a lower limit of 100 ° C. and an upper limit of 180 ° C. More preferably, the lower limit is 120 ° C and the upper limit is 160 ° C. The heat-curing time varies depending on the curing temperature or the like, but when it is performed at the above-mentioned heat-curing temperature, it is appropriate to be 10 to 30 minutes.

  The multilayer coating film thus obtained has a ΔL value that is obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film by 25-degree reflection, and is −0.3. The ΔL value, which is obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film by 75-degree reflection, is −0.2 to 0.4. is there. A multilayer coating film having a ΔL value within the above range has an appearance with a high design property that suppresses the occurrence of color reversion.

  The thickness of the multilayer coating film is preferably in the range of a lower limit of 30 μm and an upper limit of 80 μm. The multilayer coating film obtained by the above method is excellent in coating film properties such as scratch resistance, acid resistance and solvent resistance, and has high design properties.

  As the above-mentioned base material, it can be used for, for example, metal molded products, plastic molded products, foams, etc., but as base materials for forming a multilayer coating film for automobiles, iron, aluminum, zinc-based substrates and these Examples thereof include metal molded products such as alloys, plastic molded products, and the like. It is preferable to apply to metal molded products that can be subjected to cationic electrodeposition coating. The base material is preferably subjected to a chemical conversion treatment on the surface. Furthermore, an electrodeposition coating film may be formed on the substrate. As the electrodeposition paint, the cation type and the anion type can be used. From the viewpoint of anticorrosion, a cation type electrodeposition paint is preferable.

Moreover, in the multilayer coating-film formation method of this invention, it is preferable to include the process (P) of apply | coating an intermediate coating material to the said base material and obtaining an intermediate coating film before a process (1).
As the intermediate coating material that can be used in the step (P), a conventionally used intermediate coating material can be used.

  The intermediate coating may be a water-based coating or a solvent-based coating. Examples of the components of these paints include intermediate coating resin components, colored pigments, extender pigments, and aqueous and / or organic solvents. The intermediate coating resin component is composed of an intermediate coating resin and, if necessary, an intermediate coating curing agent. As the intermediate coating resin component, intermediate coating curing agent, coloring pigment, extender pigment, various additives and solvent contained in the intermediate coating, any of those described for the above-mentioned base coating can be used.

  The pigment concentration (PWC) with respect to the solid content of the intermediate coating is preferably in the range of a lower limit of 30% by mass and an upper limit of 65% by mass. The upper limit is more preferably 50% by mass. The solid content concentration of the intermediate coating is preferably in the range of 35% by mass to 65% by mass.

  Examples of the method for applying the intermediate coating include those using a spray method, a roll coater method, and the like. Specifically, it uses air electrostatic sprays called “react guns” or rotary atomizing electrostatic coatings called “micro-micro (μμ) bell”, “micro (μ) bell”, “metabell”, etc. It is preferable to paint using a machine. Among these, it is particularly preferable to perform the coating using a rotary atomizing electrostatic coating machine. A preferable dry film thickness of the intermediate coating film is 5 to 80 μm, and more preferably 10 to 50 μm.

  The uncured intermediate coating film thus formed can be baked and cured at 120 to 160 ° C. for a predetermined time to obtain a cured intermediate coating film. The base paint and the clear paint can be applied on the cured intermediate coating film thus obtained.

  In addition, the above base coating and clear coating are applied wet-on-wet on the obtained uncured intermediate coating, and the uncured base coating and clear coating are applied on the uncured intermediate coating. Are preferably formed sequentially. In this way, when applying base paint and clear paint in wet-on-wet, by applying pre-heating after applying the intermediate coating, an uncured intermediate coating film is formed, and finally a better finished appearance Can be obtained.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited only to these examples. In the examples, “parts” means “parts by mass” unless otherwise specified.

[Synthesis Example a]
448 parts of propylene glycol monomethyl ether acetate was added to a reaction vessel equipped with a thermometer, a stirring blade, a nitrogen introducing tube, a cooling condenser and a dropping funnel, and heated to 120 ° C. in a nitrogen atmosphere. In the container, using a dropping funnel, 100 parts of propylene glycol monomethyl ether acetate, 105 parts of tert-butyl peroxy 2-ethylhexanoate, 200 parts of styrene as a monomer, 67 parts of acrylic acid-n-butyl, methacryl A monomer mixed solution consisting of 100 parts of alkyl acid, 270 parts of isobornyl methacrylate, 360 parts of 4-hydroxybutyl acrylate and 3 parts of methacrylic acid was added dropwise at a constant rate over 3 hours. Thereafter, the mixture was kept at 120 ° C. for 0.5 hour, and 10 parts of tert-butylperoxy 2-ethylhexanoate dissolved in 50 parts of propylene glycol monomethyl ether acetate was added dropwise at a constant rate over 30 minutes. Furthermore, heating was continued at 120 ° C. for 1 hour.
As a result, calculated Tg 5.3 ° C., solid content acid value 2 mgKOH / g, hydroxyl value 140 mgKOH / g, number average molecular weight (Mn) 4600, weight average molecular weight (Mw) 11300 in terms of standard polystyrene obtained using GPC, An acrylic resin A having a resin solid content of 62.5% was obtained. In addition, as alkyl methacrylate, acrylate ester SL (manufactured by Mitsubishi Rayon Co., Ltd.) having a mixing ratio (mass basis) of lauryl methacrylate / tridecyl methacrylate of 4/6 was used.

[Synthesis Examples b and c]
Acrylic resins B and C shown in Table 1 were obtained in the same manner as in Synthesis Example a except that the monomer mixture and the post-drop solution shown in Table 1 were used. The calculated Tg, solid content acid value, hydroxyl value, number average molecular weight and resin solid content of each resin are shown.

  *) Acrylic ester SL (manufactured by Mitsubishi Rayon Co., Ltd.) having a mixing ratio of 4/6 (mass basis) of alkyl methacrylate: lauryl methacrylate / tridecyl methacrylate.

[Synthesis Example e] Synthesis of Crosslinked Resin Particles In a glass container equipped with a thermometer, stirring blade, nitrogen inlet tube, cooling condenser and decanter, 213 parts of bishydroxyethyl taurine, 208 parts of neopentyl glycol, 296 parts of phthalic anhydride, 376 parts of azelaic acid and 30 parts of xylene were charged and the temperature was raised. Water produced by the reaction was removed by azeotropy with xylene. The reaction liquid temperature was adjusted to 210 ° C. over about 3 hours from the start of refluxing, and stirring and dehydration were continued until the solid content acid value corresponding to carboxylic acid reached 135 mgKOH / g. After cooling the liquid temperature to 140 ° C., 500 parts of “Cardura E10” (Versic acid glycidyl ester manufactured by Shell) was dropped in 30 minutes, and then the stirring was continued for 2 hours to complete the reaction. An amphoteric ion group-containing polyester resin having a solid content acid value of 55 mgKOH / g, a hydroxyl value of 91 mgKOH / g and a number average molecular weight of 1250 was obtained.
A monomer suspension was prepared by vigorously stirring 10 parts of this zwitterionic group-containing polyester resin, 140 parts of deionized water, 1 part of dimethylethanolamine, 50 parts of styrene and 50 parts of ethylene glycol dimethacrylate in a stainless steel beaker. . In addition, an aqueous initiator solution was prepared by mixing 0.5 part of azobiscyanoyoshinoic acid, 40 parts of deionized water, and 0.32 part of dimethylethanolamine.
A glass container equipped with a thermometer, stirring blades, nitrogen inlet tube, cooling condenser and dropping funnel is charged with 5 parts of the above zwitterion group-containing polyester resin, 280 parts of deionized water and 0.5 part of dimethylethanolamine, and 80 ° C. The temperature was raised to. To this, 251 parts of the monomer suspension and 40.82 parts of the aqueous initiator solution were simultaneously added dropwise over 60 minutes, and the reaction was further continued for 60 minutes, and then the reaction was terminated. An emulsion containing crosslinked resin particles having a particle diameter of 55 nm measured by a dynamic light scattering method was obtained. Butyl acetate was added to this emulsion, water was removed by azeotropic distillation under reduced pressure, and the medium was replaced with butyl acetate to obtain a crosslinked resin particle solution having a resin solid content of 20%.

[Production of Clear Coating Compositions A to D]
Each of the acrylic resins A to C is 68.68 parts by solid resin content, Sumidur N3300 (isocyanurate-type isocyanate compound) manufactured by Sumika Bayer Urethane Co., Ltd., 31.32 parts, Tinuvin 928 (UV manufactured by Ciba Specialty Chemicals) Agent) 2 parts, Tinuvin 292 (Company light stabilizer) 1 part, Cross-linked resin particles 4 parts, Modaflow (Monsanto surface conditioner) 0.1 part, blended with a disperser to clear the paint Compositions A to C were obtained. The clear coating composition A that did not contain the crosslinked resin particles was designated as clear coating composition D.
Each clear coating composition was subjected to No. 1 using a thinner composed of propylene glycol monomethyl ether acetate / 3-ethyl ethyl ethoxypropionate = 1/2 (mass ratio). Dilute to 25 seconds / 20 ° C. with 4 Ford cups.

[Examples 1 and 2, Comparative Examples 1 and 2]
Cathode electrodeposition paint “Power Top U-50” and gray intermediate paint “Orga P-30” (both trade names) manufactured by Nippon Paint Co., Ltd. on the phosphated steel sheet so that the dry film thicknesses are 25 μm and 40 μm, respectively. 1F7 (silver-based), 4P7 (beige-based), 4R3 (beige-based), 1D2 (water-based paint “AQUAREX AR-2000” (trade name) manufactured by Nippon Paint Co., Ltd. Each coating color (gray) was applied.
On top of this, the clear coating compositions A to D were applied by wet-on-wet, and baked and dried at 140 ° C. for 30 minutes to prepare a 2-coat 1-bake coating test plate.
In addition, the multilayer coating film by the said base coating material and clear coating composition was apply | coated so that the dry film thickness might be 15 micrometers and 40 micrometers, respectively. Separately, the silver metallic base paint was applied to the test plate so that the dry film thickness was 15 μm, and baked and dried at 140 ° C. for 30 minutes to produce a base single-layer coating test plate.

[Evaluation methods]
Evaluation was performed by the following methods (1) and (2) using the coating test plate obtained by the method described above. The evaluation results are shown in Table 2.

(1) L value The color difference with the metallic coating film obtained by 2 coat 1 baking was measured on the basis of the corresponding metallic base single layer coating film.
As schematically shown in FIG. 1, measurement is performed with CM-512m3 manufactured by Minolta Co., Ltd., L values at light receiving angles of 25 ° (highlight) and 75 ° (shade) are measured, and ΔL value is calculated by the following equation. did.
ΔL value = (L value of multilayer coating film) − (L value of uncured base single layer coating film)

(2) Visual observation The flip-flop property of the highlight and shade of the obtained multilayer coating film was evaluated as the coating film appearance by visual judgment. The evaluation criteria are as follows.
○: The difference in L value between the highlight and the shade is large, and the flip-flop property is strong.
(Triangle | delta): The difference of L value is not so large by highlight and a shade, and flip-flop property is weak.
X: The base coating film and the clear coating film are mixed, and the flip-flop property is very weak.
XX: The mixed layer of the base coating film and the clear coating film is severe, and there is no flip-flop property.

In Examples 1 and 2, even if the base coating film is made of any one of the base paints of 1F7, 4P7, 4R3, and 1D2, a multilayer coating film having a high design appearance that suppresses the appearance of color return is obtained. I was able to.
On the other hand, in Comparative Example 1 using a clear coating composition containing an acrylic resin that does not contain alkyl methacrylate as a raw material, color reversion of the base coating film occurs, and a multilayer coating film with high design properties is obtained. I couldn't. Further, in Comparative Example 2 using the clear coating composition in which the crosslinked resin particles were not blended, the base coating film was recolored, and it was not possible to obtain a multi-layer coating film with high design properties. In particular, it has been found that color reversion occurs more markedly as the base coating film has a higher L value.

  INDUSTRIAL APPLICABILITY The present invention is suitable as a method for forming a multi-layer coating film for automobile painting with a high brightness such as a metallic color, particularly as a method for forming a multi-layer coating film for automobile coating for a market that requires high designability. Can be used.

Claims (3)

A step (1) of applying a base coating to a substrate to form an uncured base coating;
A step (2) of applying a clear coating on the uncured base coating obtained in the step (1) to form an uncured clear coating; and
A multilayer coating film forming method comprising the step (3) of heating and curing the uncured base coating film and the uncured clear coating film obtained in the steps (1) and (2) to form a multilayer coating film. There,
(A) The L value of the uncured base coating film is 70 to 105 in 25 degree reflection, and 20 to 50 in 75 degree reflection,
(B) The ΔL value, which is a value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film by 25 ° reflection, is −0.3 to 0.15,
(C) The ΔL value, which is a value obtained by subtracting the L value of the uncured base coating film from the L value of the multilayer coating film by 75-degree reflection, is −0.2 to 0.4,
And,
The clear paint contains 10 to 15% by mass of a long-chain (meth) acrylic acid ester monomer whose ester part is a linear hydrocarbon group having 9 to 15 carbon atoms, and contains a carboxyl group An acrylic resin obtained from a monomer mixture containing a (meth) acrylic monomer and a hydroxyl group-containing (meth) acrylic monomer, having a number average molecular weight of 4000 to 6000 and a solid content acid value of 1 to 5 mgKOH / g, a polyisocyanate compound, A method for forming a multilayer coating film, which is a clear coating composition containing crosslinked resin particles.   The said clear coating composition is a multilayer coating-film formation method of Claim 1 which contains 0.1-10 mass% of crosslinked resin particles with respect to the total solid of a coating composition.   The method for forming a multilayer coating film according to claim 1 or 2, comprising a step (P) of forming an intermediate coating film by applying an intermediate coating material before the step (1).

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