JP2013010248A - Method for forming multilayer coating film - Google Patents

Abstract

The present invention provides a method for forming a multi-layer coating film that is capable of obtaining a multi-layer coating film having a sufficient bending resistance suitable for a plastic material with few restrictions on selection of a coating material.
A method for forming a multilayer coating film of a molded article made of a plastic material, the step of applying an aqueous primer on a substrate (1), the step of applying an aqueous color base coating composition (2), It comprises a step (3) of applying a mica base coating composition and a step (4) of applying a clear coating composition, and the coating elongation of the aqueous color base coating composition is 10 to 120%. The rate is 2 to 8%, the coating elongation of the aqueous mica base coating composition is 10 to 20%, the coating elongation of the clear coating composition is 10 to 50%, and the aqueous color base coating is A method for forming a multilayer coating film comprising a carbodiimide compound.
[Selection] Figure 1

Description

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

Examples of members used in automobiles include steel plates constituting bodies and the like, and plastic materials constituting bumpers and the like. Since these two members have large differences in strength and expansion coefficient against heat, the present situation is that coating is performed using a coating composition suitable for each member. Since plastic materials are flexible materials, it is necessary to form a coating film with excellent bending resistance that can follow the plastic material, and bending resistance has become an important quality requirement for coating films. . In addition, water-based paints are often used to protect the environment. In particular, primer, color base, and mica base use water-based paint.

Examples of the plastic material coating system include a coating system composed of three layers of primer coating, base coating, and clear coating, and a coating system composed of four layers of primer coating, color base coating, mica base coating, and clear coating.
In a conventional plastic material coating system, in order to form a multilayer coating film having excellent bending resistance, a method of forming a coating film having a high elongation rate at all coating stages has been performed. For this reason, there is a problem that the selection of the paint is restricted, and it is necessary to pay attention to the fact that the selection range of the paint is further narrowed in the coating system composed of the four layers as described above, because a lot of paint is used. It was.

As a method of sequentially performing primer coating, color base coating, mica base coating, and clear coating, Patent Document 1 discloses a method of sequentially performing white conductive primer coating, color base coating, interference color base coating, and clear coating. However, Patent Document 1 does not discuss the elongation percentage of the coating film.

Patent Document 2 describes the use of a coating composition containing carbodiimide in a multilayer coating film for painting an automobile body. When such a coating composition is used, there is an advantage that mixing between the multilayer coating films can be prevented. However, in the coating of plastic products, the above-mentioned bending resistance is required. The coating film formed by the coating composition containing carbodiimide may not be sufficiently high in bending resistance, and it may be difficult to use the one for automobile body as it is for coating plastic products. .

Japanese Patent Laid-Open No. 2004-262988 JP 2009-262002 A

In view of the above, the present invention is capable of obtaining a multilayer coating film having sufficient bending resistance suitable for a plastic material while using a coating composition containing carbodiimide as a color base coating, with few restrictions on coating selection. A method for forming a multilayer coating film is provided.

The present invention
A method for forming a multilayer coating film of a molded product made of a plastic material,
A step of applying a water-based primer (1), a step of applying a water-based color base coating composition (2), a step of applying a water-based mica base coating composition (3), and a step of applying a clear coating composition on a substrate. (4)
The coating rate of the aqueous primer is 10 to 120%
The coating film elongation of the aqueous color base coating composition is 2 to 8%,
The coating film elongation of the aqueous mica base coating composition is 10 to 20%,
The clear coating composition has a coating film elongation of 10 to 50%,
The water-based color base paint contains a carbodiimide compound, and is a method for forming a multilayer coating film.

The carbodiimide compound has the following general formula (1):

(Wherein n represents 1 to 5, R ¹ is the same or different and represents an aliphatic, aromatic or alicyclic alkylene group, and R ² represents a general formula R ³ — (— O—CH ₂ -CH ₂ -. _in) p -OH (wherein, p represents an integer of having 6 to 20, ^{R 3} is polyethylene glycol monoalkyl ether represented by) represents an alkyl group having 1 to 3 carbon atoms and / Or the following general formula (2):

(In the formula, q represents an integer of 15 to 60, and R ⁴ represents an alkyl group having 1 to 8 carbon atoms.) The residue of the mixture with the polypropylene glycol monoalkyl ether represented by It is preferable that it is a compound represented by this.

In the method for forming a multilayer coating film of the present invention, in a four-layer coating system, the coating film elongation of the water-based primer, the water-based color base coating composition, the water-based mica base coating composition, and the clear coating composition is within a certain range. Furthermore, by using a water-based color base coating composition containing a carbodiimide compound, a multilayer coating film having excellent performance in various physical properties such as sufficient bending resistance, smoothness, and chipping resistance Can be formed. That is, according to the present invention, it is not always necessary to use a paint having a high coating film elongation rate in all steps, and it is an excellent method capable of satisfying the required quality suitable for plastic materials while relaxing the restrictions on paint selection. is there.

Sample preparation method for measuring film elongation

The present invention is described in detail below.
The present invention includes a water-based primer, a water-based color base coating composition, a water-based mica base coating composition, and a clear coating composition having a coating elongation percentage within a certain range and containing a carbodiimide compound as the water-based color base coating composition. By using such a coating, it is possible to obtain a multilayer coating film having a high degree of freedom in selecting a color base coating film, having sufficient bending resistance, and excellent chipping performance.

In the present invention, the elongation percentage of each coating composition is an important characteristic. Here, the coating film elongation rate in this specification is measured by the following method.
The water-based primer coating composition is dried by spray coating on a peelable polytetrafluoroethylene sheet plate, the water-based color base coating composition, the water-based mica base coating composition, and the clear coating composition on a peelable PP plate. After coating to a film thickness of 30 μm, the film is dried at 120 ° C. for 35 minutes to obtain a coating film. In addition, we shall not evaluate items with defects such as dust, debris, repellency, excavation and scratches.
The obtained coating film was allowed to stand at room temperature for 24 hours, then cut into a 10 mm wide rod shape as shown in FIG. 1, masking tape (30 mm width) was applied to both ends, and then folded back to the back with half of the masking tape Was used as a sample. Using the AUTO GRAPH AG-IS manufactured by Shimadzu Corporation, 5 samples were measured per sample at 20 ° C., 65% RH, and a pulling speed of 5 mm · min, and the average value was calculated.

In the present invention, a flexible coating film having a high elongation rate is formed by the aqueous primer and the clear coating composition, and a hard coating film having a low elongation rate is formed in the aqueous color base paint. By doing in this way, the multilayer coating film with favorable chipping resistance can be obtained. As a result, a multi-layer coating film that has a hard coating layer having a low elongation rate in part, has excellent bending resistance, can improve chipping resistance, and has well-balanced physical properties. It is presumed that it can be formed. Moreover, by using a carbodiimide compound, it is possible to prevent a mixed layer between coating films and to obtain excellent performance in appearance.

This also widens the range of paint selection and allows existing paints to be combined more freely than before.

Furthermore, it is also characterized by using the thing containing a carbodiimide as an aqueous color base coating composition. Since carbodiimide has a fast curing speed, the use of the carbodiimide in a color base paint allows the lower layer to be cured quickly, thereby improving the appearance characteristics. In addition, there is an advantage that the range of workability is widened by suppressing the mixed layer of color base and mica base by the interface control.

The multilayer coating film formed by the method for forming a multilayer coating film of the present invention has a coating film elongation of 10 to 120%. If the elongation percentage of the primer is less than 10%, it is not preferable in that the flexibility may be deteriorated. When the elongation percentage of the primer exceeds 120%, it is not preferable in that the adhesion may be deteriorated.
The lower limit is more preferably 15%, still more preferably 20%. The upper limit is more preferably 100%, still more preferably 90%.

The multilayer coating film formed by the method for forming a multilayer coating film of the present invention has a coating film elongation percentage of 2 to 8%. When the elongation percentage of the water-based color base paint is less than 2%, it is not preferable in that the flexibility may be deteriorated. When the elongation percentage of the water-based color base paint exceeds 8%, it is not preferable in that the finished appearance may be deteriorated.
The lower limit is more preferably 3%, still more preferably 4%.
The upper limit is more preferably 7%, still more preferably 6%.

The multilayer coating film formed by the method for forming a multilayer coating film of the present invention has a coating film elongation percentage of 10 to 20% of the aqueous mica base paint. If the elongation percentage of the water-based mica base paint is less than 10%, it is not preferable in that the flexibility may be deteriorated. When the elongation percentage of the water-based mica base paint exceeds 20%, it is not preferable in that the chipping resistance may be deteriorated.
The lower limit is more preferably 11%, and still more preferably 12%.
The upper limit is more preferably 19%, still more preferably 18%.

The multilayer coating film formed by the method for forming a multilayer coating film of the present invention has a coating film elongation of 10 to 50%. If the clear coating material has an elongation of less than 10%, it is not preferable in that the flexibility may be deteriorated. If the clear paint has an elongation percentage of more than 50%, it is not preferable in that the solvent resistance may be deteriorated.
The lower limit is more preferably 15%, still more preferably 20%.
The upper limit is more preferably 45%, still more preferably 40%.

As a result, for example, when an automobile body and a bumper are integrally coated, it is possible to perform a suitable coating adapted to both the steel plate and the bumper in the steps after the coating of the water-based color base paint. Moreover, since the coating composition with high hardness can also be used, the tensile strength of a multilayer coating film increases and chipping resistance can also be improved.

In the present invention, the coating film elongation of each coating composition can be adjusted to a predetermined range by adjusting the composition of the coating composition and adjusting the composition of the resin used by a method well known to those skilled in the art. . Moreover, it can also adjust using softeners, such as a coating alkyd resin, a coating polyester resin, polytetramethylene glycol, polypropylene glycol, polyethylene glycol, and its mixture.

Hereinafter, the water-based primer, the water-based color base coating composition, the water-based mica base coating composition, and the clear coating composition that are applied in the steps (1) to (4) of the present invention will be described in detail.

(Water-based primer)
Although it does not specifically limit as said aqueous primer, The thing which can provide electroconductivity to a plastic raw material is preferable. Among them, an aqueous white conductive primer is preferable, and examples include a primer resin and a conductive agent (antimony-doped tin oxide-treated titanium oxide), and those containing a white pigment and other raw materials as necessary. Can do.

The mixing ratio of water in the aqueous white conductive primer is preferably 45 to 90% by mass, more preferably 50 to 80% by mass with respect to the entire white conductive primer. When the blending ratio of water is less than 45% by mass, the viscosity becomes high, and the storage stability and the coating workability deteriorate. On the other hand, when the blending ratio of water exceeds 90% by mass, the ratio of the non-volatile content is decreased, the coating efficiency is deteriorated, and abnormal appearance such as sagging and peeling is likely to occur. The aqueous white conductive primer may further contain an organic solvent, and the blending ratio is usually 40% by mass or less based on the contained water.

As the primer resin component of the aqueous white conductive primer, it is preferable to use acid-modified chlorinated polyolefin, epoxy resin, polyurethane resin; pigment dispersion resin such as aqueous alkyd resin and water-soluble acrylic resin. It may contain all of these.

In the present invention, the step (1) is a step of applying the aqueous primer to the substrate surface. The primer can be applied by a technique such as spray painting or bell painting. You may wash | clean and degrease the said base material as needed.

The method for forming a multilayer coating film of the present invention preferably includes a preheating step after the step (1) and before the step (2). In addition, what is necessary is just to set the heating temperature in the case of preheating suitably, but 40-100 degreeC is preferable and 40-90 degreeC is more preferable. There is no restriction | limiting in particular about the method of preheating, For example, what is necessary is just to employ | adopt well-known methods, such as a hot-air drying method and an infrared rays drying method.

The primer coating film is preferably 5 to 30 μm in dry film thickness. If the thickness is less than 5 μm, the hiding property is insufficient, and if it exceeds 30 μm, the cracks and sagging are likely to occur. Preferably it is 10-20 micrometers. The dry film thickness can be measured by using SDM-miniR manufactured by SANKO.

(Water-based color base paint composition)
In the present invention, a paint composition containing a carbodiimide compound is used as the aqueous color base paint.
A carbodiimide compound is a compound having one or more structures represented by the structural formula of —N═C═N— in the molecule.

The carbodiimide compound of the present invention is preferably one represented by the following general formula (1).

(Wherein n represents 1 to 5, R ¹ is the same or different and represents an aliphatic, aromatic or alicyclic alkylene group, and R ² represents a general formula R ³ — (— O—CH ₂ -CH ₂ -. _in) p -OH (wherein, p represents an integer of having 6 to 20, ^{R 3} is polyethylene glycol monoalkyl ether represented by) represents an alkyl group having 1 to 3 carbon atoms and / Or the following general formula (2):

(In the formula, q represents an integer of 15 to 60, and R ⁴ represents an alkyl group having 1 to 8 carbon atoms.) The residue of the mixture with the polypropylene glycol monoalkyl ether represented by It is preferable that it is a compound represented by this.

In the formula, R ¹ is the same or different and represents an aliphatic, aromatic, or alicyclic alkylene group, and is usually an alkyl group of a polyisocyanate compound that is a raw material for producing the carbodiimide compound. . Therefore, aromatic compounds such as tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, xylylene diisocyanate and metaxylene diisocyanate, which are widely used diisocyanate compounds; alicyclic compounds such as isophorone diisocyanate; hexamethylene diisocyanate, etc. Among these aliphatic groups, the structure of the residue excluding the isocyanate group can be exemplified.
Especially, the following general formula (3)

In view of physical properties, the 4,4'-dicyclohexylmethane group represented by

Moreover, it is preferable to use the carbodiimide compound which has the above-mentioned polyethyleneglycol group or a polypropyleneglycol group in a molecule | numerator from the point that the mixing | blending in an aqueous coating is easy for the said water-based color base coating material.

Method for producing the carbodiimide compound is not particularly limited, for example, to perform a combination of the isocyanate-terminated polycarbodiimide by condensation reaction accompanying decarboxylation of organic diisocyanate and then, having the end groups R ² groups A synthesis method in which a compound is reacted with the isocyanate-terminated polycarbodiimide can be mentioned.

The above-mentioned isocyanate-terminated carbodiimide is basically produced by a conventional method for producing polycarbodiimide (US Pat. No. 2,941,956, Japanese Patent Publication No. 47-33279, J. Org. Chem., 28, 2069). -2076 (1963), Chemical Review 1981, vol. 81, No. 4, 619-4, 621).

The condensation reaction involving decarbonization of the diisocyanate compound proceeds in the presence of a carbodiimidization catalyst. Examples of the catalyst include 1-phenyl-2-phospholene-1-oxide and 3-methyl-2-phospholene. -1-oxide, 1-ethyl-2-phospholene-1-oxide, 1-ethyl-3-methyl-2-phospholene-1-oxide, 3-methyl-1-phenyl-2-phospholene-1-oxide or these Phosphorene oxide, such as 3-phospholene isomer, can be used, and 3-methyl-1-phenyl-2-phospholene-1-oxide is preferable in terms of reactivity.

The reaction temperature in the condensation reaction is preferably in the range of 80 ° C. to 180 ° C. When the reaction temperature falls below this range, the reaction time becomes extremely long, and when the reaction temperature exceeds the above range, side reactions occur. Occurs and no good carbodiimide is obtained, which is undesirable in either case.

Further, the degree of condensation is preferably 1 to 5, and when the degree of condensation exceeds 5, the dispersibility when the carbodiimide represented by the above general formula (1) is added to the water-based paint is lowered, and the general formula (1 When the carbodiimide represented by) is previously used as an aqueous solution or aqueous dispersion, a good aqueous solution or aqueous dispersion cannot be obtained due to low dispersibility. In order to complete the reaction promptly, the reaction of the carbodiimide represented by the general formula (1) is preferably performed under an inert gas stream such as nitrogen.

The carbodiimide compound represented by the above general formula (1) is obtained by adding the general formula R ³ — (— O—CH ₂ —CH ₂ —) _p —OH to the isocyanate-terminated polycarbodiimide synthesized by the above-described method. Represents an integer of 6 to 20, and R ³ represents an alkyl group having 1 to ³ carbon atoms.), A polyethylene glycol monoalkyl ether represented by the general formula:

(In the formula, q represents an integer of 15 to 60, and R ⁴ represents an alkyl group having 1 to 8 carbon atoms.) It is obtained by reacting with a polypropylene glycol monoalkyl ether or a mixture thereof. It is In addition, in said formula (1), n represents the integer of 1-5.

It is represented by the general formula R ³ — (— O—CH ₂ —CH ₂ —) _p —OH (wherein p represents an integer of 6 to 20 and R ³ represents an alkyl group having 1 to ³ carbon atoms). Specific examples of the polyethylene glycol monoalkyl ether include poly (ethylene oxide) monomethyl ether, poly (ethylene oxide) monoethyl ether, poly (ethylene oxide) monopropyl ether and the like, and particularly poly (ethylene oxide). ) Monomethyl ether is preferred.

General formula:

(In the formula, q represents an integer of 15 to 60, and R ⁴ represents an alkyl group having 1 to 8 carbon atoms.) Specifically, the polypropylene glycol monoalkyl ether represented by poly (propylene oxide) monomethyl Examples include ether, poly (propylene oxide) monoethyl ether, poly (propylene oxide) monophenyl ether and the like, and poly (propylene oxide) monomethyl ether is particularly preferable.

In the present invention, the mixing molar ratio of the polyethylene glycol monoalkyl ether and the polypropylene glycol monoalkyl ether is preferably mixed at a ratio of 60:40 to 40:60, and used at a ratio of 50:50. More preferably. When the ratio is outside the above range, when the proportion of polyethylene glycol monoalkyl ether decreases (the proportion of polypropylene glycol monoalkyl ether increases), the hydrophilicity decreases, and it does not disperse in water. May not be enough. On the other hand, when the proportion of polyethylene glycol monoalkyl ether increases (the proportion of polypropylene glycol monoalkyl ether decreases), the hydrophilicity increases, it becomes easier to have affinity with water, and the carbodiimide group may be deactivated. .

When the polyethylene glycol monoalkyl ether and the polypropylene glycol monoalkyl ether are used in combination, they may be reacted with an isocyanate-terminated polycarbodiimide simultaneously or sequentially. .

A catalyst is generally used for the addition reaction of the isocyanate-terminated carbodiimide, a mixture of the polyethylene glycol monoalkyl ether and the polypropylene glycol monoalkyl ether.

The reaction temperature for the above reaction is in the range of 60 ° C. to 160 ° C., preferably in the range of 100 ° C. to 150 ° C. When the reaction temperature falls below this range, the reaction time becomes extremely long. When the above range is exceeded, side reactions occur and a high-quality carbodiimide composition cannot be obtained, which is not preferable in any case.

The carbodiimide compound as described above can be isolated from the reaction system according to a usual method, and the structure is represented by the general formula (1) because the infrared absorption (IR) spectrum and nuclear magnetic resonance absorption (NMR) ) Supported by spectrum.

In addition to the carbodiimide compound described above, the water-based color base paint preferably further contains a hydroxyl group- and carboxyl group-containing acrylic resin and a curing agent.

The hydroxyl group and carboxyl group-containing acrylic resin can be in the form of a water-soluble resin, a water-dispersible resin, a resin emulsion, etc., among which a resin emulsion is preferable.

The hydroxyl group and carboxyl group-containing acrylic resin emulsion emulsifies a monomer mixture containing (meth) acrylic acid alkyl ester (i), carboxyl group-containing ethylenically unsaturated monomer (ii), and hydroxyl group-containing ethylenically unsaturated monomer (iii). It can be obtained by polymerization. In addition, you may use the compound illustrated below as a component of a monomer mixture, combining suitably 1 type (s) or 2 or more types. In the present specification, “meth (acrylic)” represents both acrylic and methacrylic.

The (meth) acrylic acid alkyl ester (i) is used for constituting the main skeleton of the acrylic resin emulsion.

Specific examples of (meth) acrylic acid alkyl ester (i) include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, and (meth) acrylic. Isobutyl acid, t-butyl (meth) acrylate, hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, ( Examples thereof include dodecyl (meth) acrylate and stearyl (meth) acrylate.

The carboxyl group-containing ethylenically unsaturated monomer (ii) improves various stability such as storage stability, mechanical stability, stability against freezing of the resulting acrylic resin emulsion, Used to accelerate the curing reaction with the curing agent.

Examples of the carboxyl group-containing ethylenically unsaturated monomer include acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, ethacrylic acid, propylacrylic acid, isopropylacrylic acid, itaconic acid, maleic anhydride and fumaric acid.

The hydroxyl group-containing ethylenically unsaturated monomer (iii) imparts hydrophilicity based on hydroxyl groups to the acrylic resin emulsion and increases the workability and stability against freezing when it is used as a coating material, as well as melamine resin and isocyanate-based curing. Used to impart curing reactivity with the agent.

Examples of the hydroxyl group-containing ethylenically unsaturated monomer (iii) include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, N-methylolacrylamide, allyl alcohol, Examples include ε-caprolactone-modified (meth) acrylic monomers.

Specific examples of the ε-caprolactone-modified acrylic monomer include “Placcel FA-1”, “Placcel FA-2”, “Placcel FA-3”, “Placcel FA-4”, “Placcel” manufactured by Daicel Chemical Industries, Ltd. FA-5 "," Placcel FM-1 "," Placcel FM-2 "," Placcel FM-3 "," Placcel FM-4 "," Placcel FM-5 "and the like.

The monomer mixture may contain, as an optional component, at least one monomer selected from the group consisting of a styrene monomer, (meth) acrylonitrile, and (meth) acrylamide. Examples of the styrenic monomer include α-methylstyrene in addition to styrene.

Emulsion polymerization can be carried out by heating the monomer mixture in an aqueous liquid with stirring in the presence of a radical polymerization initiator and an emulsifier. The reaction temperature is about 30 to 100 ° C., for example, and the reaction time is preferably about 1 to 10 hours, and the reaction temperature is adjusted by batch addition or temporary dropping of the monomer mixture or monomer pre-emulsified liquid in a reaction vessel charged with water and an emulsifier. It is good to do.

As the radical polymerization initiator, known initiators usually used in emulsion polymerization of acrylic resins can be used. Specifically, as a water-soluble free radical polymerization initiator, for example, a persulfate such as potassium persulfate, sodium persulfate, or ammonium persulfate is used in the form of an aqueous solution. In addition, so-called redox initiators in which an oxidizing agent such as potassium persulfate, sodium persulfate, ammonium persulfate or hydrogen peroxide and a reducing agent such as sodium bisulfite, sodium thiosulfate, Rongalite or ascorbic acid are combined are aqueous solutions. Used in the form of

As an emulsifier, an anionic system selected from a micelle compound having a hydrocarbon group having 6 or more carbon atoms and a hydrophilic moiety such as a carboxylate, sulfonate, or sulfate partial ester in the same molecule Alternatively, a nonionic (nonionic) emulsifier is used. Among these, as anionic emulsifiers, alkali metal salts or ammonium salts of sulfuric acid half esters of alkylphenols or higher alcohols; alkali metal salts or ammonium salts of alkyl or allyl sulfonates; polyoxyethylene alkylphenyl ethers, polyoxyethylenes Examples thereof include alkali metal salts or ammonium salts of alkyl ether or sulfuric acid half ester of polyoxyethylene allyl ether. Examples of nonionic emulsifiers include polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ether, and polyoxyethylene allyl ether. In addition to these general-purpose anionic and nonionic emulsifiers, the molecule has a radically polymerizable unsaturated double bond, that is, acrylic, methacrylic, propenyl, allyl, allyl ether, maleic acid. Various anionic and nonionic reactive emulsifiers having a group such as are also used singly or in combination of two or more.

In the emulsion polymerization, the use of an auxiliary agent (chain transfer agent) for adjusting the molecular weight such as a mercaptan compound or a lower alcohol promotes the smooth and uniform formation of the coating film from the viewpoint of promoting the emulsion polymerization. From the viewpoint of improving the adhesion to the material, there are many cases where it is preferable, and it is carried out depending on the situation as appropriate.

Emulsion polymerization can be performed by any one of the usual one-step continuous monomer uniform dropping method, core-shell polymerization method which is a multi-stage monomer feed method, and power feed polymerization method which continuously changes the monomer composition fed during the polymerization. Can be legal.

In this way, a hydroxyl group- and carboxyl group-containing acrylic resin emulsion is prepared. Although the weight average molecular weight of the obtained acrylic resin is not particularly limited, it is generally about 50,000 to 1,000,000, for example, about 100,000 to 800,000. In addition, the number average molecular weight and the weight average molecular weight in this specification are styrene standard conversion values by gel permeation chromatography.

The glass transition temperature (Tg) of the hydroxyl group- and carboxyl group-containing acrylic resin is preferably -50 ° C to 20 ° C. More preferably, it is in the range of −40 ° C. to 10 ° C., and further preferably in the range of −30 ° C. to 0 ° C. By setting the Tg of the resin in this range, when an aqueous color base paint containing an acrylic resin emulsion is used in a wet-on-wet system, the affinity and adhesion to the aqueous mica base paint and the clear paint become good, and the wet state It is preferable in terms of being able to suppress inversion at the interface with the upper coating film and prevent reversal. Moreover, the moderate softness | flexibility of the coating film finally obtained is acquired, and chipping resistance is improved. As a result, a multilayer coating film having a very high appearance can be formed. If the Tg of the resin is less than −50 ° C., the mechanical strength of the coating film is insufficient and the chipping resistance is weak. On the other hand, if the Tg of the resin exceeds 20 ° C., the coating film becomes hard and brittle, so that it lacks impact resistance and weakens chipping resistance. The type and amount of each monomer component are selected so that the Tg of the resin falls within the above range.

The acid value of the hydroxyl group- and carboxyl group-containing acrylic resin is preferably 2 to 60 mgKOH / g. More preferably, it is the range of 5-50 mgKOH / g. By setting the acid value of the resin within this range, various stability such as storage stability, mechanical stability and stability against freezing of the resin emulsion and the base coat paint using the resin emulsion are improved. Curing reaction with a curing agent such as melamine resin sufficiently occurs, and various strengths, chipping resistance, and water resistance of the coating film are improved. When the acid value of the resin is less than 2 mgKOH / g, the above-mentioned various stability is inferior, and the curing reaction with a curing agent such as a melamine resin is not sufficiently promoted, and the coating film has various strengths, chipping resistance, and water resistance. Inferior. On the other hand, when the acid value of the resin exceeds 60 mgKOH / g, the polymerization stability of the resin is deteriorated, the above-mentioned various stability is deteriorated, or the water resistance of the obtained coating film is inferior. The type and amount of each monomer component are selected so that the acid value of the resin falls within the above range. As described above, it is important to use a carboxyl group-containing monomer among the acid group-containing ethylenically unsaturated monomers (b), and among the monomers (b), the carboxyl group-containing monomer is preferably 50% by mass or more, More preferably, it is contained by 80% by mass or more.

The hydroxyl value of the acrylic resin is preferably 10 to 120 mgKOH / g. More preferably, it is the range of 20-100 mgKOH / g. By setting the hydroxyl value of the resin within this range, the resin has moderate hydrophilicity, and when used as a coating composition containing a resin emulsion, the workability and stability against freezing are increased, and the melamine resin or isocyanate type is used. The curing reactivity with the curing agent is also sufficient. When the hydroxyl value is less than 10 mgKOH / g, the curing reaction with the curing agent is insufficient, the mechanical properties of the coating film are weak, chipping resistance is poor, and water resistance and solvent resistance are also poor. On the other hand, when the hydroxyl value exceeds 120 mgKOH / g, the water resistance of the obtained coating film is decreased, the compatibility with the curing agent is poor, the coating film is distorted, and the curing reaction occurs unevenly. As a result, the various strengths of the coating film, particularly chipping resistance, solvent resistance and water resistance are inferior. The type and amount of each monomer component are selected so that the hydroxyl value of the resin falls within the above range.

A basic compound may be added to the obtained acrylic resin emulsion in order to neutralize part or all of the carboxylic acid and maintain the stability of the acrylic resin emulsion. As these basic compounds, ammonia, various amines, alkali metals and the like can be appropriately used.

A hydroxyl group-containing polyester resin may be blended with the water-based color base paint as necessary. This hydroxyl group-containing polyester resin is added to adjust the flow properties and physical properties of the coating film.

As the hydroxyl group-containing polyester resin, an oil-free polyester resin obtained by condensing a polyhydric alcohol component and a polybasic acid component, or in addition to a polyhydric alcohol component and a polybasic acid component, castor oil, dehydrated castor oil, tung oil, In addition to the above-mentioned acid component and alcohol component, safflower oil, soybean oil, linseed oil, tall oil, coconut oil, and the like, and an oil component that is a mixture of two or more of these fatty acids, An oil-modified polyester resin obtained by reacting can be mentioned. Moreover, the polyester resin which grafted acrylic resin and vinyl resin can also be used. Furthermore, a urethane-modified polyester resin obtained by reacting a polyisocyanate compound with a polyester resin obtained by reacting a polyhydric alcohol component and a polybasic acid component can also be used.

The number average molecular weight (Mn) of the hydroxyl group-containing polyester resin is preferably 800 to 10,000. More preferably, it is 1000-8000. If Mn is less than 800, the stability when the polyester resin is dispersed in water may decrease, and if it exceeds 10,000, the viscosity of the resin will increase. May decrease.

The hydroxyl value of the hydroxyl group-containing polyester resin is 35 to 170, preferably 50 to 150. When the hydroxyl value is less than 35, the curability of the resulting coating film decreases, and when it exceeds 170, the chipping resistance of the coating film decreases.

The hydroxyl group-containing polyester resin preferably has an acid value of 15 to 100 mgKOH / g. Preferably it is 20-80. When the acid value is less than 15, the water dispersion stability of the polyester resin is lowered, and when it exceeds 100, the water resistance of the coated film is lowered.

Moreover, it is preferable that the glass transition temperature of the said hydroxyl-containing polyester resin is -40-50 degreeC. When the said glass transition temperature is less than -40 degreeC, there exists a possibility that the hardness of the coating film obtained may fall, and when it exceeds 50 degreeC, there exists a possibility that base concealment property may fall. More preferably, it is -40-10 degreeC. The glass transition temperature can be measured with a differential scanning calorimeter (DSC) or the like.

Examples of the polyhydric alcohol component include, for example, ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2, 2-diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, hydroxypivalic acid neopentyl glycol ester, 2-butyl-2-ethyl-1,3-propane Diols such as diol, 3-methyl-1,5-pentanediol, 2,2,4-trimethylpentanediol and hydrogenated bisphenol A, and trivalent or more such as trimethylolpropane, trimethylolethane, glycerin, pentaerythritol Polyol component of And hydroxycarboxylic acids such as 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolpentanoic acid, 2,2-dimethylolhexanoic acid, 2,2-dimethyloloctanoic acid, etc. An acid component can be mentioned.

Examples of the polybasic acid component include, for example, aromatic polyvalent carboxylic acids such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrabromophthalic anhydride, tetrachlorophthalic anhydride, pyromellitic anhydride, etc. And acid anhydrides; alicyclic polycarboxylic acids and anhydrides such as hexahydrophthalic anhydride, tetrahydrophthalic anhydride, 1,4- and 1,3-cyclohexanedicarboxylic acid; maleic anhydride, fumaric acid, succinic anhydride And polybasic acid components such as aliphatic polycarboxylic acids such as adipic acid, sebacic acid and azelaic acid and anhydrides, and anhydrides thereof. A monobasic acid such as benzoic acid or t-butylbenzoic acid may be used in combination as necessary.

When preparing a polyester resin, as reaction components, monoepoxide compounds such as Cardura E (trade name: manufactured by Ciel Chemical), and lactones (β-propiolactone, dimethylpropiolactone, butyrolactone, γ- Valerolactone, ε-caprolactone, γ-caprolactone, γ-caprolactone, crotolactone, δ-valerolactone, δ-caprolactone, etc.) may be used in combination. In particular, lactones serve to improve the chipping resistance of a color base coating composition by ring-opening addition to a polyester chain of a polyvalent carboxylic acid and a polyhydric alcohol to form a polyester chain itself. These may be contained in 3 to 30%, preferably 5 to 20%, in particular 7 to 15% of the total mass of all reaction components.

The hydroxyl group-containing polyester resin can be easily made aqueous by adjusting its acid value and neutralizing the carboxyl group with a basic substance (for example, 50% or more). Examples of basic substances used here include ammonia, methylamine, ethylamine, dimethylamine, diethylamine, trimethylamine, triethylamine, dimethylethanolamine, diethanolamine, and triethanolamine. Among these, diethanolamine, dimethylethanolamine, and triethanolamine are included. Ethanolamine and the like are preferred. Moreover, the neutralization rate in the case of the said neutralization is not specifically limited, For example, it is 80 to 120%.

The curing agent that can be blended in the aqueous color base paint is not particularly limited as long as it causes a curing reaction with the acrylic resin or the hydroxyl group-containing polyester resin, and examples thereof include a melamine resin and an isocyanate resin. These 1 type (s) or 2 or more types are used in combination as appropriate. It is preferable to contain a melamine resin on the performance of the coating film.

The melamine resin is not particularly limited as long as it can be cured with an acrylic resin or a hydroxyl group-containing polyester resin and can be blended in the base coat paint. Specifically, an imino melamine resin is preferable, for example, Cymel 211 (Mitsui Cytec Co., Ltd., imino melamine resin, trade name), Cymel 327 (Mitsui Cytec Co., Ltd. imino melamine resin, trade name), and the like.

Moreover, the alkyl etherified melamine resin alkylated is preferable, and the melamine resin substituted by the methoxy group and / or butoxy group is more preferable. As such melamine resins, those having a methoxy group alone, Cymel 325, Cymel 327, Cymel 370, Mycoat 723; those having both a methoxy group and a butoxy group, Cymel 202, Cymel 204, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 254, Cymel 266, Cymel 267 (all trade names, manufactured by Mitsui Cytec); My Coat 506 (trade name, Mitsui Cytech, Inc.) having a butoxy group alone Product), Uban 20N60, Uban 20SE (both trade names, manufactured by Mitsui Chemicals) and the like. These may be used alone or in combination of two or more.

The aqueous color base paint may further contain an additional resin component, a pigment dispersion paste, a thickener, other additive components, and the like.

Although it does not specifically limit as said additional resin component, For example, a urethane resin emulsion, an acrylic resin, a carbonate resin, an epoxy resin etc. can be mentioned.

The pigment dispersion paste is obtained by previously dispersing a pigment and a pigment dispersant in a small amount of an aqueous medium. It is preferable that the solid content of the pigment dispersant does not contain any volatile basic substance or contains 3% by mass or less. In the water-based color base paint used in the present invention, by using such a pigment dispersant, the amount of volatile basic substances in the paint film formed from the water-based color base paint is reduced, and the resulting multilayer paint film Yellowing can be suppressed. Therefore, if the volatile basic substance is contained in the solid content of the pigment dispersant in an amount of more than 3% by mass, the resulting multilayer coating film tends to turn yellow and the finished appearance tends to deteriorate. Absent.

The volatile basic substance means a basic substance having a boiling point of 300 ° C. or less, and examples thereof include inorganic and organic nitrogen-containing basic substances. As an inorganic basic substance, ammonia etc. are mentioned, for example. Examples of the organic basic substance include linear or branched alkyl having 1 to 20 carbon atoms such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, isopropylamine, diisopropylamine, and dimethyldodecylamine. Group-containing primary to tertiary amines; monoethanolamine, diethanolamine, 2-amino-2-methylpropanol and other linear or branched hydroxyalkyl group-containing primary to tertiary amines having 1 to 20 carbon atoms; dimethylethanolamine A primary or tertiary amine containing a linear or branched alkyl group having 1 to 20 carbon atoms and a linear or branched hydroxyalkyl group having 1 to 20 carbon atoms such as diethylethanolamine; diethylenetriamine , Substitution of 1 to 20 carbon atoms such as triethylenetetramine Unsubstituted chain polyamine; substituted or unsubstituted cyclic monoamine having 1 to 20 carbon atoms such as morpholine, N-methylmorpholine, N-ethylmorpholine; piperazine, N-methylpiperazine, N-ethylpiperazine, N, N-dimethylpiperazine Examples thereof include amines such as substituted or unsubstituted cyclic polyamines having 1 to 20 carbon atoms.

The pigment dispersant is a resin having a structure including a pigment affinity part and a hydrophilic part. Examples of the pigment affinity portion and the hydrophilic portion include nonionic, cationic and anionic functional groups. The pigment dispersant may have two or more of the above functional groups in one molecule.

Examples of the nonionic functional group include a hydroxyl group, an amide group, and a polyoxyalkylene group. Examples of the cationic functional group include an amino group, an imino group, and a hydrazino group. Examples of the anionic functional group include a carboxyl group, a sulfonic acid group, and a phosphoric acid group. Such pigment dispersants can be produced by methods well known to those skilled in the art.

The pigment dispersant is not particularly limited as long as it does not contain a volatile basic substance in the solid content or has a content of 3% by mass or less, but it is efficient with a small amount of the pigment dispersant. Those that can disperse the pigment are preferably used. For example, commercially available products (hereinafter, trade names) can also be used. Specifically, Disperbyk, an anionic / nonionic dispersant manufactured by Big Chemie, Inc.
190, Disperbyk 181, Disperbyk 182 (polymer copolymer), Disperbyk 184 (polymer copolymer), EFKAPOLYMER 4550, an anionic nonionic dispersant manufactured by EFKA, Solsperse, a nonionic dispersant manufactured by Abycia 27000, Solsperse 41000 which is an anionic dispersant, Solsperse 53095, and the like.

The number average molecular weight of the pigment dispersant is preferably a lower limit of 1000 and an upper limit of 100,000. If it is less than 1000, the dispersion stability may not be sufficient, and if it exceeds 100,000, the viscosity may be too high and handling may be difficult. More preferably, the lower limit is 2000 and the upper limit is 50,000, and even more preferably, the lower limit is 4000 and the upper limit is 50,000.

The pigment dispersion paste is obtained by mixing and dispersing a pigment dispersant and a pigment according to a known method. It is preferable that the ratio of the pigment dispersant at the time of manufacturing the pigment dispersion paste is a lower limit of 1% by mass and an upper limit of 20% by mass with respect to the solid content of the pigment dispersion paste. If it is less than 1% by mass, it is difficult to disperse the pigment stably, and if it exceeds 20% by mass, the physical properties of the coating film may be inferior. Preferably, the lower limit is 5 mass% and the upper limit is 15 mass%.

The pigment is not particularly limited as long as it is a pigment used in ordinary paints, but it is a colored pigment from the viewpoint of obtaining a coating film with a high appearance and improving weather resistance and ensuring concealment. It is preferable. Examples of the colored pigment include azo chelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, and isoindolinone pigments. Organic color pigments such as diketopyrrolopyrrole pigments, benzimidazolone pigments, metal complex pigments; inorganic color pigments such as titanium oxide, yellow lead, yellow iron oxide, Bengala, and carbon black. These pigments may be used in combination with extender pigments such as calcium carbonate, barium sulfate, clay and talc.

Although it does not specifically limit as a thickener, Cellulose type | system | groups, such as viscose, methylcellulose, ethylcellulose, hydroxyethylcellulose, and what is marketed, such as Tyroze MH and Tyroze H (all are the Hoechst company make, brand name), etc. Things; Sodium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, and commercially available products (all are trade names below) include Primal ASE-60, Primal TT-615, Primal RM-5 (all manufactured by Rohm & Haas) ), Alkali thickening type such as Euker Polyphobe (manufactured by Union Carbide); polyvinyl alcohol, polyethylene oxide, and commercially available products (hereinafter referred to as trade names) include Adecanol UH-420 and Adecanol UH- 4 2, Adecanol UH-472, UH-540, Adecanol UH-814N (Asahi Denka Kogyo Co., Ltd.), Primal RH-1020 (Rohm & Haas), Kuraray Poval (Kuraray Co., Ltd.) be able to. These may be used alone or in combination of two or more.

By containing a thickener, the viscosity of the water-based color base paint can be increased, and the occurrence of dripping can be suppressed when the water-based color base paint is applied. Moreover, the mixed layer between a color base coating film and a mica base coating film can be suppressed more. As a result, compared to the case where no thickener is included, the coating workability during coating is improved, and the finished appearance of the resulting coating film can be made excellent.

Examples of other additives include additives usually added in addition to the above-mentioned components, such as ultraviolet absorbers; antioxidants; antifoaming agents; surface conditioners; pinhole inhibitors. These compounding amounts are within the range known to those skilled in the art.

The aqueous color base paint is preferably prepared by mixing a carbodiimide compound, an acrylic resin emulsion, a curing agent, a pigment dispersion paste, and the like.

The above curing agent has a lower limit of 2% by mass and an upper limit of 50% by mass, preferably a lower limit of 4% by mass and an upper limit of 40% by mass, more preferably a lower limit of 5% by mass and an upper limit of 30% by mass with respect to the solid content of the curing agent and the acrylic resin emulsion. % To use. When it is less than 2% by mass, the water resistance of the resulting coating film tends to decrease. Moreover, when it exceeds 50 mass%, there exists a tendency for the peeling property of the coating film obtained to fall.

Appropriate amounts of the additional resin component, pigment dispersion paste and other additives may be mixed. However, the additional resin component is preferably blended in a proportion of 50% by mass or less based on the solid content of all the resins contained in the color base coating composition. When it exceeds 50% by mass, it is difficult to increase the solid content concentration in the paint, which is not preferable.

The pigment preferably has a pigment concentration (PWC: pigment weight content) of 10 to 60% by mass with respect to the total mass of the solid content and pigment of all the resins contained in the aqueous color base paint. If it is less than 10% by mass, the concealability may be lowered. When it exceeds 60 mass%, viscosity increase at the time of hardening will be caused, flow property may fall and a coating-film external appearance may fall.

It is preferable that content of the said carbodiimide compound is 1-30 mass parts with respect to 100 mass parts of resin solid content (solid content of all the resin contained in a color base paint) of the said water-based color base coating material, 5-15 More preferably, it is part by mass. If it is less than 1 part by mass, the interface may be disturbed when the mica base paint is applied wet-on-wet. When it exceeds 30 mass parts, there exists a possibility that various performances of an external appearance and the coating film obtained may fall.

The order of adding these components may be before or after adding the curing agent to the hydroxyl group- and carboxyl group-containing acrylic resin. The form of the water-based color base paint of the present invention is not particularly limited as long as it is aqueous. For example, the form may be water-soluble, water-dispersed, water-based emulsion, or the like.

About the application | coating method of the said water-based color base coating composition, it is preferable to carry out by electrostatic coating. It is preferable in that electrostatic coating can be performed efficiently and a sufficient film thickness can be obtained by one-stage coating.

The color base coating film preferably has a dry film thickness of 10 to 30 μm, and more preferably 15 to 25 μm. When the dry film thickness is 10 μm or more, a colorful appearance can be given to the surface of the molded product. If the thickness exceeds 30 μm, problems such as sagging and peeling occur.

(Water-based mica base coating composition)
In the present invention, next, the step (3) of applying the aqueous mica base coating composition is performed. The aqueous mica base coating composition is not particularly limited as long as it contains a base resin and a mica pigment.

The base resin is not particularly limited, and examples thereof include water-soluble acrylic resins, water-dispersible acrylic resins, emulsion acrylic resins, water-soluble polyester resins, and water-dispersible polyester resins. Moreover, it is preferable to use together hardening agents, such as a melamine resin and a polyisocyanate compound.

By blending the above mica pigment, it is possible to obtain a pearly glossy coating film that is more excellent in design and design. The mica pigment is not particularly limited, and examples of commercially available products include “Iriodin” and “Silary” manufactured by Merck. The content is preferably 1 to 20% by mass in the total solid content (resin solid content and other solid content such as pigment) in the paint.

As the method for applying the water-based mica base coating composition, it is preferable to use electrostatic coating as in the methods mentioned as the method for applying the water-based color base coating composition. The mica base coating film preferably has a dry film thickness of 5 to 20 μm, and more preferably 7 to 15 μm. If the dry film thickness is less than 5 μm, the target color may not be exhibited. On the other hand, when the thickness exceeds 20 μm, defects such as sagging and flares may occur.

It is preferable to perform a preheating process after the said process (3). The said preheating process can be performed on the conditions similar to the preheating process after the process (1) mentioned above.

(Clear paint composition)
The method for forming a multilayer coating film of the present invention further comprises a step (4) of applying a clear coating composition. The clear coating composition is a coating used to form the top layer (uppermost layer) of a four-layer coating film, and imparts excellent physical properties such as weather resistance and solvent resistance to the cured coating film.

Although it does not specifically limit as said clear coating composition, A conventionally well-known thing should just be used, For example, the polyol resin containing a hydroxyl group is used as a main ingredient, and the hardening | curing agent is isocyanate 2 liquid clear coating (for example, 2 Liquid curable urethane paints) are preferred. This is because the resulting clear coating film has a good appearance and excellent acid resistance. The polyol resin used as the main agent is not particularly limited, and for example, polyester polyol, acrylic polyol, polycarbonate polyol, polyurethane polyol and the like can be used.

Examples of the isocyanate used as the curing agent include non-yellowing type compounds having two or more isocyanate groups in the molecule (for example, adducts such as hexamethylene diisocyanate and isophorone diisocyanate, nurate and burette). Can be mentioned. Examples of commercially available curing agents include Dismodule 3600 and Sumidur 3300 manufactured by Sumika Bayer, Coronate HX manufactured by Nippon Polyurethane, Takenate D-140NL and D-170N manufactured by Mitsui Takeda Chemical, and Asahi Kasei. DURANATE 24A-90PX, THA-100, etc. can be mentioned.
Examples of commercially available clear paints include R2500-1 manufactured by Nippon Bee Chemical Co., which is a two-component curable urethane paint.
Also, in order to increase the tensile elongation relative to commercially available products, softeners such as coating alkyd resins, coating polyester resins, polytetramethylene glycol, polypropylene glycol, polyethylene glycol and mixtures thereof are added and used. You can also

Furthermore, conventionally known additives such as curing accelerators, antifoaming agents, rheology control agents, lubricants, UV absorbers, and organic solvents are used as necessary.

The method of applying the clear coating composition is not particularly limited, and for example, air spray coating, airless spray coating, bell coating, or the like can be employed.

The dry film thickness of the clear coating film is preferably 20 to 50 μm. If it is out of the above range, appearance deterioration such as rough skin and poor workability such as sagging and peeling may occur.

In the method for forming a multilayer coating film of the present invention, an aqueous primer, an aqueous color base coating composition, an aqueous mica base coating composition, and a clear coating composition are applied in this order on the surface of a substrate to form a multilayer uncoated layer. You may form a cured film and perform a baking process (5).
In selecting each coating composition, it is necessary to select a coating that can be sufficiently cured and dried in the baking process. If the drying is insufficient and water or solvent remains in the cured coating film, performances such as water resistance and solvent resistance are likely to deteriorate in the cured coating film.

In the baking step (5), the four uncured films are simultaneously baked to form a cured coating film composed of four layers of an aqueous primer coating film, an aqueous color base coating film, an aqueous mica base coating film, and a clear coating film. It is a process of forming.

The baking temperature is preferably set to 110 to 130 ° C., for example, in consideration of rapid curing and prevention of deformation of the plastic molded product. Preferably, it is 120-130 degreeC. The baking time is usually 10 to 60 minutes, preferably 15 to 50 minutes, and more preferably 20 to 40 minutes. When the baking time is less than 10 minutes, the coating film is not sufficiently cured, and the performance such as water resistance and solvent resistance of the cured coating film is deteriorated. On the other hand, if the baking time exceeds 60 minutes, it will be hardened too much and the adhesion in the recoating will be reduced, the total time of the coating process will be prolonged, and the energy cost will be increased. This baking time means the time during which the surface of the substrate actually keeps the target baking temperature. More specifically, the time until the target baking temperature is reached is not considered, and the target baking temperature is not considered. It means the time when the temperature has been maintained since reaching the temperature.

Examples of the heating apparatus used for simultaneously baking the uncured film of the paint include a drying furnace using a heating source such as hot air, electricity, gas, and infrared, and two or more of these heating sources are used in combination. It is preferable to use a drying furnace because the drying time is shortened.

The multilayer coating film obtained by the present invention has sufficient bending resistance that can be applied to a plastic molded product. Here, sufficient bending resistance means the property shown below.

(Bending strength)
After forming a multilayer coating film on a polypropylene material, the multilayer coating film was peeled off by drying at 120 ° C. for 35 minutes. When the obtained multilayer coating film was bent at 90 ° at a uniform speed and uniform pressure around a φ20 mm mandrel with the coating surface of the test piece facing outside at room temperature 25 ± 5 ° C. In the present specification, “having flex resistance” means that the coating film does not crack.

The plastic molded product to which the multilayer coating film forming method of the present invention can be applied is not particularly limited, and examples thereof include automobile parts such as bumpers, spoilers, grills, and fenders. Also, the plastic material is not particularly limited. For example, polyolefin such as polyethylene and polypropylene, polystyrene, ABS, vinyl chloride, polycarbonate, polyacetal, polyester, polyamide, polyurethane, PPO, polymethyl methacrylate, epoxy resin, phenol resin, melamine Resin etc. are mentioned.

EXAMPLES Hereinafter, although an Example is hung up and demonstrated in more detail, this invention is not limited only to these Examples. In the examples, “parts” and “%” mean “parts by mass” and “% by mass” unless otherwise specified.

Production Example 1 Production of acrylic resin particle aqueous dispersion (for aqueous primer)
Methacrylic acid as an acrylic monomer component in a mixture of 118 parts of deionized water, 6 parts of PVA218EE (polyvinyl alcohol manufactured by Kuraray Co., Ltd.) and 3 parts of New Coal 714 (anionic surfactant manufactured by Nippon Emulsifier Co., Ltd.) A mixture of 30 parts of glycidyl, 13 parts of lauryl methacrylate, 1 part of n-butyl acrylate and 56 parts of styrene, and 2 parts of lauryl peroxide as a radical polymerization initiator was added. This was stirred at 8000 rpm for 30 minutes using a homogenizer to obtain a raw material dispersion. The average particle size of the resin particles in the raw material dispersion measured by using SALD-2200 (Shimadzu Corporation laser diffraction particle size distribution analyzer) was 0.5 μm. This raw material dispersion was added dropwise to 137 parts of deionized water heated to 80 ° C. over 2 hours with stirring, and stirring was continued for 4 hours after the completion of the addition. After cooling, the mixture was filtered through a 400 mesh sieve to obtain an acrylic resin particle aqueous dispersion a. It was 0.8 micrometer when the average particle diameter of the acrylic resin particle in this water dispersion was measured using SALD-2200.

Production Example 2 Production of acid anhydride-modified chlorinated polyolefin (for aqueous primer)
In a reaction vessel equipped with a stirring blade, a thermometer, a dripping device, a temperature control device, a nitrogen gas introduction tube and a cooling tube, maleic anhydride-modified chlorinated polyolefin “Supercron 892LS” (manufactured by Nippon Paper Industries Co., Ltd., chlorine content 22%) , 288 parts of a weight average molecular weight of 70,000 to 80,000), 62 parts of a surfactant “Emulgen 920” (manufactured by Kao Corporation), 74 parts of an aromatic hydrocarbon solvent “Sorvesso 100” (manufactured by Exxon), and 32 parts of carbitol acetate. The temperature was raised to 110 ° C., heated at this temperature for 1 hour to dissolve the resin and the like, and then cooled to 100 ° C. or lower. Subsequently, 710 parts of ion-exchanged water in which 6 parts of dimethylethanolamine was dissolved was added dropwise over 1 hour while cooling, and phase inversion emulsification was performed. Then, it cooled to room temperature (25 degreeC), and filtered with the 400 mesh metal-mesh, and obtained the maleic anhydride modified chlorinated polyolefin emulsion. The nonvolatile content of this emulsion was 30% by weight.

Production Example 3 Production of polyurethane dispersion (for aqueous primer)
1100 parts of adipic acid and 3-methyl-1,5 while passing nitrogen gas through a pressure-resistant reaction vessel equipped with a stirring blade, a thermometer, a dropping device, a temperature control device, a nitrogen gas introduction tube, a sample collection tube and a reflux device with a cooling tube -Charge 900 parts of pentanediol and 0.5 part of tetrabutyl titanate, set the reaction temperature of the liquid in the container to 170 ° C, conduct esterification reaction by dehydration, and the acid value becomes 0.3 mgKOH / g or less Continued until. Next, the reaction was carried out under reduced pressure conditions of 180 ° C. and 5 kPa or less for 2 hours to obtain a polyester having a hydroxyl group content of 112 mgKOH / g and an acid value of 0.2 mgKOH / g. Then, in another reaction vessel equipped with the same apparatus as the above reaction vessel, 500 parts of this polyester polyol, 134 parts of dimethyl 5-sulfosodium isophthalate and 2 parts of tetrabutyl titanate were charged in the same manner as above. While nitrogen gas was passed, the reaction vessel was set at 180 ° C. for esterification, and finally a sulfonic acid group-containing polyester having a weight average molecular weight of 2117, a hydroxyl value of 53 mgKOH / g, and an acid value of 0.3 mgKOH / g was obtained.

280 parts of the sulfonic acid group-containing polyester, 200 parts of polybutylene adipate, 35 parts of 1,4-butanediol, 118 parts of hexamethylene diisocyanate and 400 parts of methyl ethyl ketone are mixed with a stirring blade, a thermometer, a temperature controller, a dropping device, and a sample. Nitrogen gas was charged into a reaction vessel equipped with a mouth and a cooling tube, and the urethanization reaction was carried out while maintaining the liquid temperature at 75 ° C. with stirring to obtain a urethane polymer having an NCO content of 1%. Subsequently, the liquid temperature in the reaction vessel was lowered to 40 ° C., and 955 parts of ion-exchanged water was uniformly added dropwise with sufficient stirring to perform phase inversion emulsification. Next, the internal temperature was lowered, and an adipic acid hydrazide aqueous solution in which 13 parts of adipic acid hydrazide and 110 parts of ion-exchanged water were mixed was added to carry out amine extension. Next, the solvent is removed by raising the temperature to 60 ° C. in a slightly reduced pressure state. When the solvent is finished, ion-exchanged water is added so that the solid content of the polyurethane dispersion is 35%, and the sulfonic acid group-containing polyurethane is added. I got a dispersion. The acid value of the polyurethane resin in the dispersion was 11 mgKOH / g.

Production Example 4 Production of pigment dispersion paste
Production example 4-1 Production of pigment-dispersed resin for aqueous primer 55 parts of propylene glycol monomethyl ether was charged into a reaction vessel equipped with a stirring blade, a dropping device, a temperature control device, a nitrogen gas introduction tube and a cooling tube, While introducing the gas, the temperature was raised to 120 ° C. with stirring. Next, 12 parts of 2-hydroxymethyl methacrylate, 9 parts of methacrylic acid, 35 parts of isobutyl methacrylate, 44 parts of n-butyl acrylate, and 1 part of t-butyl peroxy-2-ethylhexanate are mixed with propylene. A solution dissolved in 8 parts of glycol was added dropwise over 3 hours with internal stirring. Next, after completion of the dropwise addition, after aging reaction at 120 ° C. for 1 hour, a solution in which 0.1 part of t-butylperoxy-2-ethylhexanate was dissolved in 4 parts of propylene glycol was further added for 1 hour. Over the reaction vessel. In either case, the internal stirring state and the liquid temperature of 120 ° C. were maintained. Thereafter, the mixture was aged at 120 ° C. for 2 hours with stirring, then the internal temperature was cooled to 70 ° C., and 9.5 parts of dimethylaminoethanol was added dropwise and stirred for 30 minutes. Furthermore, 167 parts of ion-exchanged water was slowly dropped while stirring while maintaining the internal temperature at 70 ° C., and cooled to room temperature (25 ° C.) to obtain a water-soluble acrylic resin solution. Using ion-exchanged water, the non-volatile content was adjusted to 30%, and this was used as a pigment dispersion resin for a primer in the following pigment dispersion paste. The obtained pigment dispersion resin (water-soluble acrylic resin solution) had a pH of 8.2, and the acrylic resin had a weight average molecular weight of 42,000.

Production example 4-2 Production of pigment dispersion paste 1 In a stainless steel cylindrical stirring vessel equipped with a stirrer, 8.4 parts of the pigment dispersion resin for primer obtained as described above was charged and stirred. 20 parts of ion exchange water was added. Subsequently, 1.2 parts of a pigment dispersant “SURFYNOL GA” (manufactured by Air Products, nonvolatile content 78%) was added with stirring. While sufficiently stirring, 0.3 part of an antifoaming agent “Nopco 8034-L” (manufactured by San Nopco, nonvolatile content 100%) was added. Next, while continuing stirring, 64.7 parts of titanium oxide “ET-500W” (Ishihara Sangyo Co., Ltd., average primary particle size 0.2 to 0.3 μm) as a white conductive agent, 2.6 parts of ion-exchanged water Then, 2.4 parts of melamine resin “Cymel 701” (manufactured by Nippon Cytec Co., Ltd., non-volatile content: 82%) 2.4 parts was added in order, and stirring was continued for 15 minutes until the whole became uniform. did. This had a non-volatile content of 70.4% and a pigment concentration (PWC) of 91.9%.

Production example 4-3 Production method of pigment dispersion paste 2 A pigment dispersion paste 2 was prepared in the same manner as in Production Example 4-2, except that the amount of each component was changed as shown in Table 1. This had a non-volatile content of 61.6% and a pigment concentration (PWC) of 87.9%.

Production Example 5 Production of aqueous white conductive primer Each component shown in Table 2 was sequentially added to a stainless steel container equipped with a stirrer and mixed to obtain a white conductive primer. About the coating film formed by the method mentioned above, the coating-film elongation rate of the obtained white conductive primer was measured by Shimadzu Corporation AUTOGRAPH AG-IS, and the result was shown in Table 2.

(Unit: parts by mass)

Production Example 4 Production of aqueous acrylic resin emulsion A (for aqueous color base)
In a reaction vessel for producing an ordinary acrylic resin emulsion equipped with a stirrer, a thermometer, a dropping funnel, a reflux condenser, a nitrogen introducing tube, etc., 445 parts of water and 5 parts of New Coal 293 (manufactured by Nippon Emulsifier Co., Ltd.) The temperature was raised to 75 ° C. while charging and stirring. A mixture of MAA 0.38 parts, HEA 2.04 parts, St 11.07 parts, NBA 5.35 parts, EA 5.77 parts, water 240 parts and New Coal 293 30 parts was emulsified using a homogenizer, and the prepolymer emulsion was The reaction vessel was added dropwise with stirring over 3 hours. In parallel with the dropwise addition of the monomer pre-emulsified solution, an aqueous solution in which 1 part of APS (ammonium persulfate) as a polymerization initiator is dissolved in 50 parts of water is uniformly dropped into the reaction vessel until the completion of the dropping of the monomer pre-emulsified solution. did. After completion of the dropwise addition of the monomer pre-emulsion, the reaction was further continued at 80 ° C. for 1 hour, and then cooled. After cooling, an aqueous solution in which 2 parts of dimethylaminoethanol was dissolved in 20 parts of water was added to obtain a hydroxyl group-containing acrylic resin emulsion A having a nonvolatile content of 40.6% by weight. The pH of the acrylic resin emulsion A was adjusted to 7.2 using a 30% dimethylaminoethanol aqueous solution.

Production Example 5 Production of polyester resin In a reactor, 25.6 parts of isophthalic acid, 22.8 parts of phthalic anhydride, 5.6 parts of adipic acid, 19.3 parts of trimethylolpropane, 26.7 parts of neopentyl glycol Part, 17.5 parts of ε-caprolactone and 0.1 part of dibutyltin dioxide were added, and the temperature was raised to 170 ° C. with mixing and stirring. Thereafter, while the temperature was raised to 220 ° C. over 3 hours, water produced by the condensation reaction was removed until the acid value reached 8. Subsequently, 7.9 parts of trimellitic anhydride was added and reacted at 150 ° C. for 1 hour to obtain a polyester resin having an acid value of 40. Furthermore, after cooling to 100 ° C., 11.2 parts of butyl cellosolve was added and stirred until uniform, and after cooling to 60 ° C., 98.8 parts of ion-exchanged water and 5.9 parts of dimethylethanolamine were added, and the solid content was 50% by weight. A polyester resin having a solid content acid value of 40, a hydroxyl value of 110, and a number average molecular weight of 2870 was obtained.

Production Example 6 Preparation of carbodiimide compound To 250 parts of 4,4-dicyclohexylmethane diisocyanate, 2.5 parts of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidization catalyst was added, and an NCO equivalent at 170 ° C. It was made to react until it became 300. The reaction product had an average of 2.8 carbodiimide groups per molecule. To this, 106 parts of polyethylene glycol monomethyl ether with an average of 9 repeats, 295 parts of polypropylene glycol monobutyl ether with 19 repeats and 0.18 part of dibutyltin dilaurate were added, and the reaction was continued at 90 ° C. until there was no NCO absorption by IR. went. To this, 1525 parts of deionized water was added and stirred to obtain an aqueous dispersion of a carbodiimide compound having a resin solid content of 30% by mass.

Production Example 7 Preparation of pigment dispersion paste Disperbyk 190 (by Big Chemie) 4.5 parts, BYK-011 (by Big Chemie) 0.5 parts, ion-exchanged water 22.9 parts, Tyco CR-97 (Ishihara Sangyo Co., Ltd.) After premixing 72.1 parts of titanium), a glass bead medium was added and mixed and dispersed in a paint conditioner to obtain a pigment dispersion paste.

Production Example 8 Production of white pigment paste for aqueous color base paint Aqueous acrylic resin A 15.2 parts, deionized water 17.3 parts, and Taipei CR-97 (Ishihara Sangyo Titanium Dioxide) 67.5 parts Were added sequentially, and stirring was continued for 15 minutes until the whole became uniform with sufficient stirring to obtain a white pigment paste.

Production Example 9 Preparation of aqueous color base paint composition The components shown in Table 3 were mixed and stirred to obtain an aqueous color base paint.

Production Example 10 Preparation of water-based mica base paint composition Water-based base coat AR2000 7A21 (manufactured by Nippon Paint Co., Ltd.) was used as the water-based mica base paint.

Production Example 11-1 Preparation of Clear Coating Composition 1 “R2500-1” (acrylic clear main ingredient manufactured by Nippon Bee Chemical Co., Ltd.) 100.0 parts and isocyanate curing agent “Desmodur Z4470” 44.0 parts manufactured by Bayer Were mixed to prepare a clear coating composition 1.

Production examples 11-2 to 4 Production of clear coating compositions 2 to 4 The components shown in Table 4 were mixed and stirred to obtain clear coating compositions 2 to 4.

(Example 1)
The above-mentioned aqueous white conductive primer is spray-coated on the surface of a polypropylene material (70 mm × 150 mm × 3 mm) wiped with isopropyl alcohol with “Wider-71” (manufactured by Anest Iwata) in an environment of 25 ° C./70% RH ( The film was dried at 80 ° C. for 5 minutes. Thereafter, electrostatic coating (gun distance: 200 mm, gun speed: 900 mm / s, applied voltage) using a new cartridge bell (trade name: new cartridge bell manufactured by ABB Co., Ltd.) under the same environment as the aqueous color base coating composition. Spray coating (dry film thickness: 15 μm) was performed under the conditions of −60 kV, rotation speed: 35000 rpm, and shaping air pressure: 0.15 MPa. Next, a water-based mica base coating composition (AR2000 # 7A21 manufactured by Nippon Paint Co., Ltd.) was spray-coated (dry film thickness 13 μm) under the same conditions as the water-based color base coating composition. After drying at 80 ° C. for 5 minutes, the clear coating composition was electrostatically coated thereon using a Robobell 951 (gun distance: 200 mm, gun speed: 700 mm / s, applied voltage: −60 kV, rotation speed: 25000 rpm). , Shaping air pressure: 0.07 MPa) spray coating (dry film thickness 25 μm). Then, after setting for 10 minutes, it dried at 120 degreeC for 35 minutes, and formed the multilayer coating film.
In addition, the coating-film elongation rate of the said water-based mica base coating composition was 15%.

(Examples 2-7, Comparative Examples 1-6)
In the same manner as in Example 1, a multilayer coating film was formed using the paint shown in Table 5.

The following evaluation was performed about the multilayer coating film of each Example and the comparative example. The results are shown in Table 5.
[Flexibility]
The bending resistance of the obtained multilayer coating film was evaluated based on the following criteria.
○: No cracking of the coating film occurred. Good: Cracking of the coating film occurred.

[Xylene rubbing resistance]
A load of 500 g was applied to the final coated plate with xylene-containing gauze, and an 8-reciprocating rubbing test was performed to evaluate the presence or absence of abnormalities such as tackiness, wrinkles, blistering, peeling. In the case where no abnormality was observed, it was marked as “◯”, and in the case where any abnormality was observed, it was marked as “X”.

[Initial adhesion]
Evaluation is made according to JIS-K-5600-5-6. Specifically, 100 square grids of 2 mm are made on the coating film with a cutter knife, and a cellophane adhesive tape is completely adhered thereon, and one end of the tape is lifted and peeled upward. This peeling operation is performed three times at the same location, and the number of square meshes in which the coating film is peeled off by 50% or more in the area of the first square is shown. 0 was accepted (O), and 1 or more was rejected (X).

[finish]
Wave-scan
When measured using T (BYK company), the case where W3 (glossiness) was 30 or less was marked as ◯, and the case where it was larger than 30 was marked as x.

[Chip resistance]
The evaluation of chipping resistance of the overall coating film was conducted by cooling the obtained coated plate to −30 ° C., and then allowing the stone to enter the sample holder of the stepping stone testing machine (manufactured by Suga Test Instruments Co., Ltd.) at 90 °. Attach, 100 g of No. 7 crushed stone was sprayed at an air pressure of 3 kg / cm 2, and the crushed stone collided with the coated plate. At that time, the degree of scratches (number, size, location of destruction) was evaluated. A case where there was a small peeling scratch but there was no peeling from the substrate was judged as pass (◯), and a case where there was a large peeling scratch on the entire surface and peeling from the substrate was judged as unacceptable (x).

From Table 5, the multilayer coating film obtained by the method for forming a multilayer coating film of the present invention does not cause a decrease in design properties due to the mixed layer of the coating film, has excellent bending resistance, and has excellent chipping resistance. It was shown to be excellent.

The method for forming a multilayer coating film according to the present invention can be suitably used, for example, as a method for forming a multilayer coating film on a substrate surface made of a plastic material such as a bumper.

Claims (2)

A method for forming a multilayer coating film of a molded product made of a plastic material,
A step of applying a water-based primer (1), a step of applying a water-based color base coating composition (2), a step of applying a water-based mica base coating composition (3), and a step of applying a clear coating composition on a substrate. (4)
The coating rate of the aqueous primer is 10 to 120%
The coating film elongation percentage of the aqueous color base coating composition is 2 to 8%,
The coating film elongation percentage of the aqueous mica base coating composition is 10 to 20%,
The clear coating composition has a coating film elongation of 10 to 50%,
The method for forming a multilayer coating film, wherein the water-based color base paint contains a carbodiimide compound. The carbodiimide compound has the following general formula (1):
(Wherein n represents 1 to 5, R ¹ is the same or different and represents an aliphatic, aromatic or alicyclic alkylene group, and R ² represents a general formula R ³ — (— O—CH ₂ -CH ₂ -. _in) p -OH (wherein, p represents an integer of having 6 to 20, ^{R 3} is polyethylene glycol monoalkyl ether represented by) represents an alkyl group having 1 to 3 carbon atoms and / Or the following general formula (2):
(In the formula, q represents an integer of 15 to 60, and R ⁴ represents an alkyl group having 1 to 8 carbon atoms.) The residue of the mixture with the polypropylene glycol monoalkyl ether represented by The method for forming a multilayer coating film according to claim 1, wherein the compound is represented by the formula: