JP7099994B2 - How to paint plastic molded products - Google Patents

Description

The present invention relates to a method for painting a plastic molded product.

Plastic molded products such as propylene resin are lightweight and have excellent mechanical strength and are widely used as automobile parts such as bumpers. The plastic molded product is usually coated with a primer paint, a base paint and a clear paint. Of these, the primer paint is a paint having a function of improving the adhesion between the plastic molded product and the base paint.

Recently, from the viewpoint of environmental protection, it is desired to use a water-based paint as a primer paint and a base paint. However, since the water-based paint cannot be expected to have an effect such as swelling of the plastic material due to the organic solvent, there is a problem that the adhesiveness to the plastic material is inferior and the water resistance is lowered. Further, there is a problem that the adhesiveness to the topcoat coating film is poor and it is necessary to separately prepare a solvent-type primer when recoating to the topcoat is required.

Regarding such a problem regarding water-based paints, Patent Document 1 below discloses an aqueous primer composition for plastic molded products in which an active methylene block polyisocyanate compound is further contained in a system in which a chlorinated polyolefin and a polyurethane dispersion are combined. It improves the adhesion to the plastic material and the adhesion to the top coat such as the base paint.

Japanese Unexamined Patent Publication No. 2004-307648

However, when the primer composition of Patent Document 1 is used, the adhesion between the plastic molded product and the topcoat coating film is improved, but the adhesion between the painted plastic molded product and the glass member such as the windshield and the rear glass is improved. When the required adhesion strength was taken into consideration, the performance was not sufficient, and the adhesion to the glass member was insufficient, especially under high temperature and water resistance. In addition, the finished appearance of the formed coating film was not sufficient, and there was still room for improvement.

Therefore, the present invention provides a coating method for a plastic molded product capable of forming a multi-layer coating film having high adhesion to a glass member and an excellent finished appearance while achieving low VOC by using a water-based paint. The purpose is to do.

As a result of diligent studies, the present inventors have formed a plastic molded product to which glass is adhered by sequentially coating a water-based primer (I), a water-based base coat paint (II), and a clear coat paint (III). We have found that the above problems can be solved by specifying the composition of the aqueous primer (I) in the coating method using 3 coats and 1 bake in which the coating film of the layer is simultaneously heat-cured, and the present invention has been completed.

That is, the present invention
(1) A step of applying an aqueous primer (I) to a plastic molded product to which glass is adhered to form a primer layer.
(2) A step of applying the water-based base coat paint (II) on the primer layer to form a base coat layer.
(3) A step of applying a clear coat paint (III) on the base coat layer to form a clear coat layer.
(4) A step of simultaneously heating and curing the three-layer coating film formed by the above steps (1) to (3).
It is a method of painting plastic molded products including
The aqueous primer (I) comprises an aqueous polyolefin resin (A) having a melting point in the range of 85 to 110 ° C. and a weight average molecular weight in the range of 80,000 to 120,000 in its total solid content. 25 to 50% by mass, water-based acrylic resin (B) 26 to 50% by mass, polyester resin (C) 4 to 10% by mass, melamine resin (D) 2.5 to 10% by mass, blocked polyisocyanate compound (E) is contained in an amount of 4 to 20% by mass and the diester compound (F) represented by the following general formula (1) is contained in an amount of 2.5 to 10% by mass.
The viscosity of the aqueous primer (I) when heated at 60 ° C. for 3 minutes is in the range of 100 to 1000 Pa · s (share rate 0.1 sec ^-1 ), and the primer layer is water at 20 ° C. for 2 minutes. The present invention relates to a method for coating a plastic molded product, which comprises a water absorption rate of 50% by mass or less after immersion.

[In the formula, R ¹ and R ² independently represent a hydrocarbon group having 4 to 18 carbon atoms, R ³ represents an alkylene group having 2 to 4 carbon atoms, m is an integer of 3 to 20, and m. The ^R3s may be the same or different from each other]

According to the present invention, a water-based primer (I), a water-based base coat paint (II), and a clear coat paint (III) are sequentially applied to a plastic molded product to which glass is adhered, and the formed three-layer coating film is heated at the same time. In the coating method using 3 coats and 1 bake to be cured, the aqueous primer (I) has an aqueous polyolefin resin (A), an aqueous acrylic resin (B), and a polyester resin (C) having a specific range of melting point and weight average molecular weight. , Melamine resin (D), blocked polyisocyanate compound (E), and diester compound (F) are contained in a specific amount, and the viscosity of the aqueous primer (I) and the plastic molded product are coated with the aqueous primer (I). Since the water absorption rate of the primer layer formed in the above process is set in a specific range, even if a coating method using 3 coats and 1 bake is adopted while achieving low VOC using a water-based paint, high adhesion to the glass member is achieved. , And a method for coating a plastic molded product capable of forming a multi-layer coating film having an excellent finished appearance can be provided.

In particular, since the coating method of the present invention uses a water-based paint, it is possible to reduce VOC.
Further, the water-based polyolefin resin used for the water-based primer (I) has a high melting point of 85 to 110 ° C. and a weight average molecular weight of 80,000 to 120,000, which is higher than that of the glass member. Adhesion, especially high adhesion under high temperature and water resistance, can be achieved.
Further, since the viscosity of the aqueous primer (I) when heated at 60 ° C. for 3 minutes is in the range of 100 to 1000 Pa · s (share rate 0.1 sec ^-1 ), it is said that the flowability of the primer layer is excellent. For that reason, an excellent finished appearance can be achieved. Further, since the water absorption rate of the primer layer after being immersed in water at 20 ° C. for 2 minutes is 50% by mass or less, water transfer from the aqueous base coat paint (II) is suppressed, which causes an excessive decrease in viscosity of the primer layer and an excessive decrease in viscosity. An excellent finished appearance can be achieved because the excessive increase in viscosity of the water-based base coat layer is suppressed.

The method for painting a plastic molded product of the present invention (hereinafter, also simply referred to as “the method of the present invention”) is a process in which the following steps (1) to (4) are sequentially performed.
(1) A step of applying an aqueous primer (I) to a plastic molded product to which glass is adhered to form a primer layer.
(2) A step of applying the water-based base coat paint (II) on the primer layer to form a base coat layer.
(3) A step of applying the clear coat paint (III) on the base coat layer to form a clear coat layer, and (4) a three-layer coating film formed by the above steps (1) to (3). The process of heating and curing at the same time.

In the method of the present invention, in particular, the aqueous primer (I) has a melting point in the range of 85 to 110 ° C. and a weight average molecular weight of 80,000 to 120 in the total solid content of the components (A) to (F). 25 to 50% by mass of the water-based polyolefin resin (A), 26 to 50% by mass of the water-based acrylic resin (B), 4 to 10% by mass of the polyester resin (C), and melamine resin (melamine resin) in the range of 000. D) is contained in an amount of 2.5 to 10% by mass, a blocked polyisocyanate compound (E) is contained in an amount of 4 to 20% by mass, and a diester compound (F) represented by the following general formula (1) is contained in an amount of 2.5 to 10% by mass. The viscosity of the aqueous primer (I) when heated at 60 ° C. for 3 minutes is in the range of 100 to 1000 Pa · s (share rate 0.1 sec ^-1 ), and the primer layer is 20 ° C. for 2 minutes. It is characterized in that the water absorption rate after immersion in water is 50% by mass or less.

[In the formula, R ¹ and R ² independently represent a hydrocarbon group having 4 to 18 carbon atoms, R ³ represents an alkylene group having 2 to 4 carbon atoms, m is an integer of 3 to 20, and m. The ^R3s may be the same or different from each other]

Hereinafter, the method of the present invention will be described in detail.

(Plastic molded product)
The plastic molded product in the method of the present invention is not particularly limited as long as it is bonded to glass, but is used, for example, for an automobile outer panel such as a back door or a roof, or an outer panel of a household electric appliance. Various plastic molded products and the like can be mentioned.

As the material of the plastic molded product, for example, a polyolefin obtained by polymerizing at least one of olefins having 2 to 10 carbon atoms such as ethylene, propylene, butylene, and hexene is particularly preferable, but the material is not limited thereto. , Polycarbonate, ABS resin, urethane resin, nylon and the like may be used. Further, these plastic molded products can be appropriately subjected to degreasing treatment, washing treatment and the like in advance by a method known per se.

(Aqueous primer (I))
The aqueous primer (I) used in the method of the present invention is an aqueous polyolefin resin (A), an aqueous acrylic resin (B), a polyester resin (C), a melamine resin (D), and a blocked polyisocyanate compound (E). , And the diester compound (F) represented by the above general formula (1).

[Aqueous polyolefin resin (A)]
As the aqueous polyolefin resin (A), either a polyolefin or a modified polyolefin can be used. The polyolefin is (co) polymerized with one or more of olefins having 2 to 10 carbon atoms such as ethylene, propylene, butylene, and hexene, and the modified polyolefin is non-polyolefin. Included are saturated carboxylic acids or acid anhydride modified products, acrylic modified products, chlorinated products, or modified polyolefins obtained by using a combination of these modifications.

As the polyolefin used in the present invention, one containing propylene as a polymerization unit is particularly preferable, and the weight fraction of the propylene unit in the polyolefin or modified polyolefin is compatible with other components and the formed coating film. From the viewpoint of adhesiveness and the like, those in the range of 0.5 or more, particularly 0.6 to 1, and more particularly 0.7 to 0.95 are preferable.

As the polyolefin, as long as it has a melting point and a weight average molecular weight in a specific range described later, a polyolefin known per se can be used without particular limitation, but the obtained polyolefin has a narrow molecular weight distribution and is also suitable for random copolymerizability and the like. From the viewpoint of superiority and the like, those produced by (co) polymerizing an olefin using a single-site catalyst as a polymerization catalyst are preferable. The single-site catalyst is a polymerization catalyst having a uniform active point structure (single-site), and among the single-site catalysts, a metallocene-based catalyst is particularly preferable. The metallocene-based catalyst is a metallocene (bis (cyclopentadienyl)) which is a transition metal compound of Group 4 to 6 or 8 of the periodic table having at least one conjugated five-membered ring ligand and a rare earth transition metal compound of Group 3. ) A metal complex and its derivative), an co-catalyst such as an aluminoxane or a boron system that activates the metal complex), and an organic aluminum compound such as trimethylaluminum can be prepared. The (co) polymerization of olefins can be carried out according to a method known per se, for example, by continuously adding alkylaluminum and metallocene while supplying olefins such as propylene and ethylene and hydrogen to the reaction vessel.

The unsaturated carboxylic acid or acid anhydride-modified polyolefin can be produced, for example, by graft-polymerizing an unsaturated carboxylic acid or an acid anhydride thereof with a polyolefin according to a method known per se. As the unsaturated carboxylic acid or its acid anhydride that can be used for modification, an aliphatic carboxylic acid having at least one, preferably one polymerizable double bond in one molecule and having 3 to 10 carbon atoms is used. Specific examples thereof include (meth) acrylic acid, maleic acid, fumaric acid, itaconic acid, maleic anhydride and the like, and maleic acid or an acid anhydride thereof is particularly preferable. The amount of graft polymerization of the unsaturated carboxylic acid or its acid anhydride on the polyolefin can be changed according to the physical properties desired for the modified polyolefin, but generally, 1 to 20 weight based on the solid content weight of the polyolefin. %, Preferably 1.5 to 15% by weight, more preferably 2 to 10% by weight.

Acrylic-modified polyolefins can be produced by graft-polymerizing at least one acrylic unsaturated monomer to the polyolefin by a suitable method known per se. Examples of the acrylic unsaturated monomer that can be used for this acrylic modification include methyl (meth) acrylic acid, ethyl (meth) acrylic acid, propyl (meth) acrylic acid, butyl (meth) acrylic acid, and (meth) acrylic acid. C ₁ to C ₂₀ alkyl esters of (meth) acrylic acids such as hexyl, cyclohexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate; C1 to _C21 hydroxyalkyl esters of (meth) acrylic acids such as ₂ -hydroxyethyl acrylate, 3-hydroxypropyl (meth) acrylic acid; (meth) acrylic acid, glycidyl (meth) acrylate, (meth) acrylamide. , Other (meth) acrylic monomers such as (meth) acrylonitrile, and further styrene and the like, which can be used alone or in combination of two or more.

In addition, in this specification, "(meth) acrylic" means "acrylic or methacrylic", and "(meth) acrylate" means "acrylate or methacrylate".

Acrylic modification of a polyolefin is, for example, first, for example, an acrylic unsaturated monomer having a reactivity with a carboxyl group in an unsaturated carboxylic acid or an acid anhydride-modified polyolefin produced as described above, for example, (meth) acrylic. A polymerizable unsaturated group is introduced into the polyolefin by reacting with glycidyl acid or the like, and then the acrylic unsaturated monomer is polymerized alone or in combination of two or more of the polymerizable unsaturated group (co). It can be carried out. The amount of the acrylic unsaturated monomer used in the acrylic modification of the polyolefin can be changed according to the physical properties desired for the modified polyolefin, but from the viewpoint of compatibility with other components and adhesion of the formed coating film. In general, it is desirable that the content is 30% by weight or less, particularly 0.1 to 20% by weight, and more preferably 0.15 to 15% by weight based on the solid content weight of the obtained modified polyolefin.

Chlorinated polyolefins can be produced by chlorinating polyolefins. Chlorination of the polyolefin can be performed, for example, by blowing chlorine gas into an organic solvent solution or dispersion of the polyolefin or a modified product thereof, and the reaction temperature can be 50 to 120 ° C. The chlorine content in the chlorinated (solid content) of the polyolefin can be changed according to the physical properties desired for the chlorinated polyolefin, but in general, chlorine of the polyolefin is generally considered from the viewpoint of adhesion of the formed coating film. It is desirable that the content is 35% by weight or less, particularly 10 to 30% by weight, and more preferably 12 to 25% by weight based on the weight of the compound.

It is important that the water-based polyolefin resin (A) used in the present invention has a melting point in the range of 85 to 110 ° C. When the melting point of the aqueous polyolefin resin (A) is in the above range, the adhesion to the glass member can be improved. It is presumed that this is because the crystallinity of the aqueous polyolefin is increased and the adhesion of the coating film to the plastic molded product is improved. The melting point is preferably 90 ° C. or higher, more preferably 95 ° C. or higher. Further, it is preferably 105 ° C. or lower, more preferably 100 ° C. or lower.

The melting point of the aqueous polyolefin resin (A) is determined by using a differential scanning calorimetry device "DSC-5200" (manufactured by Seiko Denshi Kogyo Co., Ltd., trade name) using 20 mg of the aqueous polyolefin resin (A) and raising the temperature at 10 ° C. It was obtained by measuring the amount of heat at / min. The melting point of the aqueous polyolefin resin (A) can be adjusted by changing the monomer composition of the polyolefin, particularly the amount of the α-olefin monomer.

It is important that the aqueous polyolefin resin (A) used in the present invention has a weight average molecular weight in the range of 80,000 to 120,000. When the weight average molecular weight of the aqueous polyolefin resin (A) is in the above range, the adhesion to the glass member can be improved. It is presumed that this is because the breaking strength of the coating film is improved. The weight average molecular weight is preferably 85,000 or more, more preferably 90,000 or more. Further, it is preferably 115,000 or less, more preferably 110,000 or less. The weight average molecular weight of the aqueous polyolefin resin (A) can be adjusted by appropriately adjusting the monomer composition of the polyolefin, the polymerization reaction conditions, and the like.

The weight average molecular weight of the aqueous polyolefin resin (A) is a value obtained by converting the weight average molecular weight measured by gel permeation chromatography based on the weight average molecular weight of polystyrene, and is referred to as "HLC / HLC / It was measured at a column temperature of 135 ° C. and a flow rate of 1.0 ml / min using "GPC150C" (manufactured by Water, 60 cm x 1) and o-dichlorobenzene as a solvent. The injection sample was prepared by dissolving at 140 ° C. for 1 to 3 hours at a solution concentration of 5 mg of polyolefin in 3.4 ml of o-dichlorobenzene. As a column for gel permeation chromatography, "GMH _HR -H (S) HT" (manufactured by Tosoh Corporation, trade name) can be used. The number average molecular weight can also be obtained in the same manner as described above.
In addition, in this specification, the weight average molecular weight or the number average molecular weight other than the aqueous polyolefin resin (A) can be obtained in the same manner.

Further, in the obtained aqueous polyolefin resin (A), it is preferable to neutralize a part or all of the introduced carboxyl groups with an amine compound in order to facilitate water solubilization or water dispersion. Examples of the amine compound that can be used for neutralization include tertiary amines such as triethylamine, tributylamine, dimethylethanolamine and triethanolamine; secondary amines such as diethylamine, dibutylamine, diethanolamine and morpholine; propylamine and ethanolamine. And the like, primary amines and the like can be mentioned. It is also possible to use a surfactant in combination with neutralization with these amine compounds for water solubilization or water dispersion.

[Aqueous acrylic resin (B)]
As the aqueous acrylic resin (B), a hydroxyl group-containing acrylic resin can be preferably used. Further, it is preferable to have a carboxyl group because of its solubility or dispersibility in water, crosslinkability and the like.

The hydroxyl group-containing acrylic resin is obtained by polymerizing a hydroxyl group-containing monomer, a (meth) acrylic acid alkyl ester-based monomer, and if necessary, other monomers by a known polymerization method, for example, a solution polymerization method. Can be obtained by

The hydroxyl group-containing monomer is a compound having a hydroxyl group and a polymerizable unsaturated group, and is, for example, with (meth) acrylic acid such as hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, and hydroxybutyl (meth) acrylate. Examples thereof include a monoesterified product with an alkylene glycol having 2 to 10 carbon atoms.

Examples of the (meth) acrylic acid alkyl ester-based monomer include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, and hexyl (meth) acrylate. , (Meta) -2-ethylhexyl acrylate, (meth) lauryl acrylate, (meth) stearyl acrylate and the like, and examples include monoesters of (meth) acrylic acid and monoalcohol having 1 to 20 carbon atoms. can.

Examples of the other monomer include compounds having a polymerizable unsaturated bond other than the hydroxyl group-containing monomer and the (meth) acrylic acid alkyl ester-based monomer, such as (meth) acrylic acid and maleic acid. Carboxyl group-containing monomer; epoxy group-containing monomer such as (meth) glycidyl acrylate; (meth) acrylamide, acrylonitrile, styrene, vinyl acetate, vinyl chloride and the like can be mentioned.

The hydroxyl group-containing acrylic resin usually has a hydroxyl value of about 10 to 100 mgKOH / g, preferably about 30 to 70 mgKOH / g, and an acid value of about 10 to 100 mgKOH / g, preferably about 20 to 50 mgKOH / g. It is appropriate that the number average molecular weight is about 2,000 to 100,000, preferably about 10,000 to 70,000.

[Polyester resin (C)]
The polyester resin (C) used in the method of the present invention can usually be obtained by an esterification reaction between a polybasic acid and a polyhydric alcohol. A polybasic acid is a compound having two or more carboxyl groups in one molecule (including anhydride), and a polyhydric alcohol is a compound having two or more hydroxyl groups in one molecule, each of which is a compound having two or more hydroxyl groups. Those commonly used in the field can be used. Further, it can be modified with a monobasic acid, a higher fatty acid, an oil component, or the like.

The polyester resin can have a hydroxyl group, and its introduction can be carried out by using a divalent alcohol and a trihydric or higher alcohol in combination. Further, the polyester resin may have a carboxyl group together with a hydroxyl group, and generally has a weight average molecular weight in the range of about 1,000 to 100,000, preferably about 1,500 to 70,000. It is preferable to do.

[Melamine resin (D)]
Specifically, the melamine resin (D) is a methylolated melamine resin formed by reacting melamine with formaldehyde; a moiety obtained by reacting the methylolated melamine resin with a monoalcohol having 1 to 10 carbon atoms or a fully etherified melamine. Examples include resin. As these melamine resins, those in which an imino group coexists can also be used. These may be either hydrophobic or hydrophilic. In particular, the weight average molecular weight is preferably 400 to 6,000, more preferably 500 to 4,000, and even more preferably 600 to 2,000.

[Blocked polyisocyanate compound (E)]
The blocked polyisocyanate compound (E) is obtained by adding a blocking agent to the isocyanate group of the polyisocyanate compound to block it.

Examples of the polyisocyanate compound include hydrophilic polyisocyanate compounds having an unreacted isocyanate group, for example, tolylene diisocyanate (TDI), 4,4'-diphenylmethane diisocyanate (MDI), xylylene diisocyanate (XDI), and the like. Aromatic diisocyanates such as mexylylene diisocyanate (MXDI); aliphatic diisocyanates such as hexamethylene diisocyanate (HDI); alicyclic diisocyanates such as isophorone diisocyanate (IPDI) and hydrogenated MDI; these diisocyanates are made non-volatile. Compounds in a form with low toxicity; burettes, uretdiones, isocyanurates or adducts of these diisocyanate compounds; relatively low molecular weight urethane prepolymers; polyisocyanate compounds made hydrophilic. be able to.

To make the polyisocyanate compound hydrophilic, for example, a hydrophilic group such as a carboxyl group, a sulfonic acid group or a tertiary amino group is introduced into the compound, and a neutralizing agent such as a hydroxycarboxylic acid such as dimethylolpropionic acid is introduced. It can be carried out by neutralizing with ammonia, a tertiary amine or the like. Further, for example, a polyisocyanate compound may be mixed and emulsified with a surfactant to be used as a so-called self-emulsifying polyisocyanate compound.

As the hydrophilic polyisocyanate compound, a commercially available product can be used. Examples of commercially available products include "Bayer Jules 3100" (trade name, manufactured by Sumika Bayer Urethane Co., Ltd., hydrophilic hexamethylene diisocyanurate) and the like.

Examples of the blocking agent include phenols such as phenol, cresol, xylenol, nitrophenol, ethylphenol, hydroxydiphenyl, butylphenol, isopropylphenol, nonylphenol, octylphenol and methyl hydroxybenzoate; ε-caprolactam, δ-valerolactam and γ. -Lactams such as butyrolactam and β-propiolactam; aliphatic alcohols such as methanol, ethanol, propyl alcohol, butyl alcohol, amyl alcohol and lauryl alcohol; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether , Diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether, methoxymethanol and other ethers; benzyl alcohol, glycolic acid, methyl glycolate, ethyl glycolate, butyl glycolate, lactic acid, methyl lactate, ethyl lactate, butyl lactate. , Ethylol urea, methylol melamine, diacetone alcohol, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate and other alcohols; formamide oxime, acetamide oxime, acetoxime, methyl ethyl ketooxime, diacetyl monooxime, benzophenone oxime, cyclohexane oxime and the like. Oxime-based; active methylene-based such as dimethyl malonate, diethyl malonate, ethyl acetoacetate, methyl acetoacetate, acetylacetone; butyl mercaptan, t-butyl mercaptan, hexyl mercaptan, t-dodecyl mercaptan, 2-mercaptobenzothiazole, thiophenol , Methylthiophenol, mercaptans such as ethylthiophenol; acidamides such as acetoanilide, acetoaniside, acetotolide, acrylamide, methacrylicamide, acetate amide, stearate amide, benzamide; Imid-based; amine-based such as diphenylamine, phenylnaphthylamine, xylidine, N-phenylxylidine, carbazole, aniline, naphthylamine, butylamine, dibutylamine, butylphenylamine; imidazole-based such as imidazole, 2-ethylimidazole; urea, thiourea , Oxime urea, Oxime thiourea, Diphenylurea Urea-based compounds such as N-phenylcarbamic acid phenyl-carbamic acid ester-based; imine-based such as ethyleneimine and propyleneimine; sulfite-based compounds such as sodium bisulfite and potassium bisulfite; azole-based compounds and the like can be mentioned. Examples of the azole compound include pyrazole, 3,5-dimethylpyrazole, 3-methylpyrazole, 4-benzyl-3,5-dimethylpyrazole, 4-nitro-3,5-dimethylpyrazole, 4-bromo-3, Pyrazole or pyrazole derivative such as 5-dimethylpyrazole, 3-methyl-5-phenylpyrazole; imidazole or imidazole derivative such as imidazole, benzimidazole, 2-methylimidazole, 2-ethylimidazole, 2-phenylimidazole; 2-methylimidazoline , 2-Pyrazole and other imidazoline derivatives such as phenylimidazolin.
Among them, preferable blocking agents include active methylene-based blocking agents, pyrazoles or pyrazole derivatives.

Blocking of the polyisocyanate compound can be carried out by, for example, water-dispersing with an appropriate emulsifier and / or protective colloid agent because the compound is generally hydrophobic after being blocked with a blocking agent. ..

[Diester compound (F)]
The diester compound (F) is represented by the following general formula (1).

[In the formula, R ¹ and R ² independently represent a hydrocarbon group having 4 to 18 carbon atoms, R ³ represents an alkylene group having 2 to 4 carbon atoms, m is an integer of 3 to 20, and m. The ^R3s may be the same or different from each other]

The diester compound (F) particularly suppresses the transfer of water from the upper aqueous base coat paint (II) to the primer layer, thereby suppressing an excessive decrease in viscosity of the primer layer and an excessive increase in viscosity of the aqueous base coat layer. For this reason, it exhibits the effect of being able to form a multi-layer coating film having high brightness, excellent finished appearance, sagging resistance and armpit resistance.

In the above formula (1), as the hydrocarbon group represented by R ¹ or R ² , an alkyl group having 5 to 11 carbon atoms is preferable, an alkyl group having 5 to 9 carbon atoms is more preferable, and an alkyl group having 6 to 8 carbon atoms is more preferable. Alkyl group of is more preferred. In particular, when R ¹ and R ² are branched alkyl groups having 6 to 8 carbon atoms, even when the paint is applied after being stored for a relatively long period of time, the formed coating film has excellent film forming properties. Can be given. Further, R ³ is preferably ethylene, and m is particularly preferably an integer of 4 to 10.
The diester compound (F) can be obtained, for example, by subjecting a polyoxyalkylene glycol having two terminal hydroxyl groups to a monocarboxylic acid having a hydrocarbon group having 4 to 18 carbon atoms in an esterification reaction.

Examples of the polyoxyalkylene glycol include polyethylene glycol, polypropylene glycol, block copolymers of polyethylene glycol and polypropylene glycol, polybutylene glycol, and the like, and among them, polyethylene glycol is particularly preferable. From the viewpoint of water resistance and the like, these polyoxyalkylene glycols generally have a weight average molecular weight in the range of about 120 to about 800, particularly about 150 to about 600, and more preferably about 200 to about 400.

Examples of the monocarboxylic acid having a hydrocarbon group having 4 to 18 carbon atoms include pentanoic acid, hexanoic acid, 2-ethylbutanoic acid, 3-methylpentanoic acid, benzoic acid, cyclohexanecarboxylic acid, and heptanic acid, 2. -Ethylpentanoic acid, 3-ethylpentanoic acid, octanoic acid, 2-ethylhexanoic acid, 4-ethylhexanoic acid, nonanoic acid, 2-ethylheptanoic acid, decanoic acid, 2-ethyloctanoic acid, 4-ethyloctanoic acid, Dodecanoic acid, hexadecanoic acid, octadecanoic acid and the like can be mentioned. Among these, hexanoic acid, heptanic acid, 2-ethylpentanoic acid, 3-ethylpentanoic acid, octanoic acid, 2-ethylhexanoic acid, 4-ethylhexanoic acid, nonanoic acid, 2-ethylheptanic acid, decanoic acid, 2- Monocarboxylic acids having an alkyl group having 5 to 9 carbon atoms such as ethyl octanoic acid and 4-ethyl octanoic acid are preferable, and heptanoic acid, 2-ethylpentanoic acid, 3-ethylpentanoic acid, octanoic acid and 2-ethylhexanoic acid are preferable. , 4-Ethylhexanoic acid, nonanoic acid, 2-ethylheptanoic acid and other monocarboxylic acids having an alkyl group having 6 to 8 carbon atoms are more preferable, 2-ethylpentanoic acid, 3-ethylpentanoic acid and 2-ethylhexane. More preferably, a monocarboxylic acid having a branched alkyl group having 6 to 8 carbon atoms, such as an acid, 4-ethylhexanoic acid, and 2-ethylheptanoic acid.

The diesterization reaction between the polyoxyalkylene glycol and the monocarboxylic acid can be carried out by a method known per se. The polyoxyalkylene glycol and the monocarboxylic acid can be used alone or in combination of two or more.

The resulting diester compound (F) generally preferably has a molecular weight in the range of about 320 to about 1,000, particularly about 400 to about 800, and more particularly preferably about 500 to about 700.

The aqueous primer (I) used in the method of the present invention has a melting point in the range of 85 to 110 ° C. and a weight average molecular weight of 80,000 to 80,000 in the total solid content of the components (A) to (F). The water-based polyolefin resin (A) in the range of 120,000 is 25 to 50% by mass, the water-based acrylic resin (B) is 26 to 50% by mass, and the polyester resin (C) is 4 to 10% by mass. , 2.5 to 10% by mass of the melamine resin (D), 4 to 20% by mass of the blocked polyisocyanate compound (E), and 2.5 of the diester compound (F) represented by the general formula (1). It is necessary to contain ~ 10% by mass.
If even one of these components does not satisfy the quantitative relationship, the effect of the present invention cannot be achieved.

The aqueous primer (I) preferably contains 30 to 45% by mass of the aqueous polyolefin resin (A) and preferably the aqueous acrylic resin (B) in the total solid content of the components (A) to (F). Is 27 to 49% by mass, the polyester resin (C) is preferably 5 to 9% by mass, the melamine resin (D) is preferably 4 to 7% by mass, and the blocked polyisocyanate compound (E) is preferably 8 to 8 to 9% by mass. It is preferable to contain 16% by mass and 4 to 7% by mass of the diester compound (F) represented by the above general formula (1).

Further, the aqueous primer (I) is more preferably 33 to 40% by mass of the aqueous polyolefin resin (A) and the aqueous acrylic resin (B) in the solid content of the components (A) to (F). More preferably 32 to 44% by mass, more preferably 6 to 8% by mass of the polyester resin (C), more preferably 4 to 6% by mass of the melamine resin (D), and the blocked polyisocyanate compound (E). It is particularly preferable to contain 10 to 14% by mass and more preferably 4 to 6% by mass of the diester compound (F) represented by the above general formula (1).

Further, the aqueous primer (I) preferably further contains a pigment.
As the above-mentioned pigment, a coloring pigment, an extender pigment, a brilliant pigment and the like can be used. Above all, it is preferable that the aqueous primer (I) contains a coloring pigment as at least one of the above pigments.

Examples of the coloring pigment include titanium oxide, zinc oxide, carbon black, molybdenum red, prusian blue, cobalt blue, azo pigment, phthalocyanine pigment, quinacridone pigment, isoindoline pigment, slene pigment, perylene pigment and the like. However, titanium oxide and carbon black can be preferably used.

Examples of the extender pigment include clay, kaolin, barium sulfate, barium carbonate, calcium carbonate, talc, silica, alumina white and the like, and among them, barium sulfate and / or talc is preferably used.

Glittering pigments include, for example, aluminum (including vaporized aluminum), copper, zinc, brass, nickel, aluminum oxide, mica, titanium oxide or iron oxide coated aluminum oxide, titanium oxide or iron oxide coated. Examples include mica, glass flakes, hologram pigments, etc. These bright pigments can be used alone or in combination of two or more. Aluminum pigments include non-reefing type aluminum and leaving type aluminum, both of which can be used.

The solid content of the aqueous primer (I) is usually preferably about 10 to 80% by mass. Since the primer (I) is an aqueous type, it is suitable from the viewpoint of low VOC and the like.

(Aqueous base coat paint (II))
As the water-based base coat paint (II) used in the method of the present invention, any thermosetting water-based base coat paint known for painting automobile bodies and the like can be used. For example, a substrate resin such as an acrylic resin, a polyester resin, an alkyd resin, a urethane resin, or an epoxy resin having a crosslinkable functional group such as a carboxyl group or a hydroxyl group, and a polyisocyanate compound, a melamine resin, or a urea resin which may be blocked. It is possible to use a resin component made of a curing agent such as, which is dissolved or dispersed in water together with a pigment and other additives to form a paint.

Further, as the above-mentioned pigment, a coloring pigment, an extender pigment, a brilliant pigment and the like can be used. Among them, it is preferable that the water-based base coat paint contains a brilliant pigment as at least one of the above pigments.
As the coloring pigment, the extender pigment, and the brilliant pigment, for example, the coloring pigment, the extender pigment, and the brilliant pigment described in the explanation column of the aqueous primer (I) can be used.

(Clear coat paint (III))
As the clear coat paint (III) used in the method of the present invention, any paint known per se can be used as the paint for the top coat clear coat, and for example, acrylic having a crosslinkable functional group such as a carboxyl group or a hydroxyl group. A base resin such as a resin, a polyester resin, an alkyd resin, a urethane resin, or an epoxy resin and a cross-linking agent such as a polyisocyanate compound, a blocked polyisocyanate compound, a melamine resin, or a urea resin are not dissolved in water and / or an organic solvent. A dispersion and a paint can be preferably used.

The clear coat paint (III) has transparency so that the paint film of the underlying water-based base coat paint (II) and the paint film of the water-based primer (I) can be observed through the paint film, but if necessary. , Coloring pigments, metallic pigments, extender pigments, dyes, ultraviolet absorbers and the like can be appropriately contained.

Next, each step of the method of the present invention will be described.
The step (1) in the method of the present invention is a step of applying the above-mentioned aqueous primer (I) to a plastic molded product to form a primer layer.

The primer layer can be formed by painting the above-mentioned aqueous primer (I) by a known painting method. Examples of the coating method include air spray coating, airless spray coating, rotary atomization coating, curtain coat coating and the like. The coating film thickness is usually in the range of about 5 to 45 μm, preferably about 25 to 40 μm as the dry film thickness.

From the viewpoint of preventing the occurrence of coating film defects such as armpits, the formed primer layer is preferably preheated, air blown or the like under heating conditions in which the coating film is not substantially cured. The preheat temperature is preferably about 40 to 100 ° C, more preferably about 45 to 90 ° C, and even more preferably about 50 to 80 ° C. The preheating time is preferably about 30 seconds to 15 minutes, more preferably about 1 to 10 minutes, and even more preferably about 2 to 5 minutes. Further, the air blow can usually be performed by blowing air heated to room temperature or a temperature of about 25 ° C. to 80 ° C. on the coated surface of the object to be coated for about 30 seconds to 15 minutes. Further, the setting may be appropriately applied before the preheating.

Further, in the method of the present invention, the viscosity of the aqueous primer (I) when heated at 60 ° C. for 3 minutes is in the range of 100 to 1000 Pa · s (share rate 0.1 sec ^-1 ), and the primer layer is the same. It is important that the water absorption rate after immersion in water at 20 ° C. for 2 minutes is 50% by mass or less.

If the viscosity is less than 100 Pa · s, the sagging resistance and armpit resistance are deteriorated, and conversely, if it exceeds 1000 Pa · s, the finished appearance is deteriorated. The viscosity is more preferably 500 to 1000 Pa · s.
The viscosity of the aqueous primer (I) is measured at 60 ° C. when the measurement temperature is changed from 20 ° C. to 60 ° C. in 3 minutes at a share rate of 0.1 seconds ^-1 . The measurement can be performed using the device "HAAKE RheoStress RS150" (trade name, manufactured by HAAKE).

Further, when the water absorption rate of the primer layer after being immersed in water at 20 ° C. for 2 minutes exceeds 50% by mass, the water transfer from the aqueous base coat paint (II) to the primer layer is not suppressed, and the effect of the present invention is exhibited. I can't. The water absorption rate is more preferably 35 to 45% by mass.

The water absorption rate can be measured by the following method. The aqueous primer (I) was coated on a tin plate whose mass (W1) had been measured in advance so that the dry film thickness was 30 μm, preheated at 60 ° C. for 3 minutes, and then the aqueous primer (I) was applied. Measure the mass (W2) of the tin plate coated with. Then, the tin plate coated with the aqueous primer (I) was immersed in deionized water at 20 ° C. for 2 minutes and then taken out, and the excess water adhering to the tin plate was lightly wiped with a filter paper. The mass (W3) is measured. The water absorption rate was calculated by the following formula (1) from the above mass measurement results of W1 to W3.
Water absorption rate (%) = {(W3-W1) / (W2-W1) -1} x 100 (1)

In order to adjust the viscosity, there are methods such as adjusting the blending amount and the molecular weight of the aqueous acrylic resin (B).
In order to adjust the water absorption rate, there is a method such as adjusting the blending amount of the diester compound (F).

The step (2) in the method of the present invention is a step of coating the water-based base coat paint (II) on the primer layer to form a base coat layer.
The film thickness of the base coat layer is usually in the range of about 5 to 30 μm, preferably about 10 to 20 μm as the cured film thickness. After painting, settings may be applied as appropriate.

The step (3) in the method of the present invention is a step of applying the clear coat paint (III) on the base coat layer to form the clear coat layer. The coating can be performed after preheating the base coat layer, if necessary.
The film thickness of the clear coat layer is usually in the range of about 10 to 40 μm, preferably about 20 to 35 μm as the cured film thickness. After painting, settings may be applied as appropriate.

The step (4) in the method of the present invention is a step of simultaneously heating and curing the three-layer coating film formed by the above steps (1) to (3).

The heating conditions for the three-layer coating film are usually preferably about 80 to 130 ° C. for about 5 to 60 minutes. Thus, a multi-layer coating film having an excellent finished appearance can be suitably formed by the 3-coat 1-bak method.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following examples. Both "part" and "%" are based on the mass standard, and the film thickness of the coating film is based on the cured coating film.

In the examples and comparative examples, the following components were used.
<Aqueous polyolefin resin (A)>
-Aqueous polyolefin resin (A-1) Melting point: 95 ° C., Weight average molecular weight (Mw): 90000
-Aqueous polyolefin resin (A-2) Melting point: 80 ° C., Weight average molecular weight (Mw): 90000
-Aqueous polyolefin resin (A-3) Melting point: 80 ° C., Weight average molecular weight (Mw): 120,000
-Aqueous polyolefin resin (A-4) Melting point: 95 ° C., Weight average molecular weight (Mw): 75000
-Aqueous polyolefin resin (A-5) Melting point: 125 ° C., Weight average molecular weight (Mw): 75000

<Aqueous acrylic resin (B)>
40 parts of ethylene glycol monobutyl ether and 30 parts of isobutyl alcohol are charged in a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, etc., heated and stirred, and after reaching 100 ° C., a mixture of the following monomers and the like is added. It was added dropwise over 3 hours.

Styrene 10 parts Methyl methacrylate 38 parts n-Butyl acrylate 25 parts 2-Hydroxyethyl methacrylate 20 parts Acrylic acid 7 parts 2,2'-azobisisobutyronitrile 1 part Isobutyl alcohol 5 parts

After completion of the dropping, the mixture was kept at 100 ° C. for another 30 minutes, and then an additional catalyst solution, which was a mixture of 0.5 part of 2,2'-azobisisobutyronitrile and 10 parts of ethylene glycol monobutyl ether, was required for 1 hour. Dropped. After further stirring at 100 ° C. for 1 hour, the mixture was cooled, 15 parts of isobutyl alcohol was added, 4 parts of N, N-dimethylaminoethanol was added at 75 ° C., and the mixture was stirred for 30 minutes to have a solid content of 50%. A solution of the aqueous acrylic resin (B) was obtained. The hydroxyl value of this aqueous acrylic resin (B) was 86 mgKOH / g, the acid value was 54.5 mgKOH / g, and the number average molecular weight was 20,000.

<Polyester resin (C)>
The esterification reaction was carried out by a conventional method using trimethylolpropane, cyclohexanedimethanol, isophthalic acid and adipic acid to obtain a polyester resin having a number average molecular weight of 4500, a hydroxyl value of 120 and an acid value of 10.

<Melamine resin (D)>
A methylbutyl mixed etherified melamine containing an imino group having a weight average molecular weight of 1200 was used.

<Blocked polyisocyanate compound (E)>
A compound in which hexamethylenediisocyanate was fully blocked with dimethyl malonate was used.

<Diester compound (F) represented by the above general formula (1)>
Diester compound (F-1): A diester compound of polyoxyethylene glycol and n-hexane acid was used. In the above general formula (1), R ¹ and R ² are each a pentyl group, R ³ is an ethylene group, m is 5, and the molecular weight of the compound is 434.
Diester compound (F-2): A diester compound of polyoxyethylene glycol and 2-ethylhexanoic acid was used. In the above general formula (1), R ¹ and R ² are 2-ethylpentyl groups, R ³ is an ethylene group, m is 7, and the molecular weight of the compound is 578.

<Pigment>
White pigment: Rutile-type titanium oxide (trade name "JR-806", manufactured by TAYCA)
Black pigment: carbon black (trade name "MA-100", manufactured by Mitsubishi Chemical Corporation)

Each of the above components (A) to (F) and the pigment are mixed at the blending ratios (mass% of solid content) shown in Tables 1 and 2 below, and sufficiently stirred with a mixer to obtain various aqueous primers (I). rice field. The viscosity of the aqueous primer (I) when heated at 60 ° C. for 3 minutes and the water absorption rate of the aqueous primer (I) coating film after being immersed in water at 20 ° C. for 2 minutes were measured by the above method. In Comparative Examples 15 and 16, polypropylene glycol (trade name “Sanniks GP-1000” manufactured by Sanyo Chemical Industries, Ltd.) was used instead of the diester compound (F) or together with the diester compound (F).

(Examples 1 to 3, 5 to 18, comparative examples 1 to 17)
Subsequently, the step (1) of the method of the present invention was carried out.
First, black polypropylene was used as a plastic molded product, degreased, and then air-spray coated with an aqueous primer (I) so as to have a dry film thickness of 30 μm. The obtained coating film was left at room temperature for 3 minutes for setting, and then preheated at 60 ° C. for 3 minutes to form a primer layer.

Next, as step (2), a water-based heat-curable transparent colored paint (trade name "WBC-713T No. 062"), which is a water-based base coat paint (II), is placed on the primer layer obtained in step (1). , Kansai Paint Co., Ltd.) was non-electrostatic coated so that the cured film thickness was 15 μm, left at room temperature for 2 minutes to set, and then preheated at 80 ° C. for 3 minutes to form a base coat layer. ..

Subsequently, as the step (3), the acrylic resin / urethane resin-based heat-curable clear paint (trade name "Soflex 7175"), which is the clear coat paint (III), is placed on the base coat layer obtained in the step (2). , Kansai Paint Co., Ltd.) was non-electrostatic coated so that the cured film thickness was 25 μm, and left at room temperature for 7 minutes for setting.

Next, as step (4), the three-layer coating film formed by steps (1) to (3) was simultaneously heat-cured. The curing conditions were 120 ° C. for 30 minutes.
As described above, the plastic molded body was coated to obtain samples of Examples 1 to 3, 5 to 18 and Comparative Examples 1 to 17.

(Example 4)
In the step (1), the water-based primer (I) was set to a dry film thickness of 15 μm, and in the step (2), the water-based thermosetting red paint (trade name “WBC-713T No.”) which is the water-based base coat paint (II). Steps (1) to (4) were carried out in the same manner as in Example 1 except that "3R3" (manufactured by Kansai Paint Co., Ltd.) was used. As described above, the plastic molded body was painted to obtain a sample of Example 4.

For each of the obtained samples, the following items were evaluated with respect to the adhesion to the glass member and the finished appearance.

[Evaluation of adhesion with glass members]
<Adhesive compatibility for window glass>
Urethane-based adhesive (trade name "3740", manufactured by Sunstar Co., Ltd., windshield agent for automobiles) was further applied to each sample so that the coating shape had a width of 20 mm, a thickness of 3 mm, and a length of 100 mm or more. After covering with the release paper, it was pressed evenly with a flat plate. After removing the flat plate, it was left to cure at a temperature of 23 ± 2 ° C. and a humidity of 50 ± 5% for 72 hours. Then, the release paper was peeled off to prepare a test plate A for an adhesive compatibility test for window glass.

(initial)
The following peeling test was performed on each of the prepared test plates A.
While pulling the cured adhesive layer by hand in a direction of 90 degrees or more with respect to the coating film, use a cutter knife to reach the coating film surface at an angle of about 60 degrees with respect to the coating film at intervals of 2 to 3 mm. Make a cut. The peeled state after peeling the adhesive layer was evaluated according to the following criteria. In addition, according to the following criteria, 〇 or higher is considered as a pass.
⊚: Cohesive failure of the adhesive layer occurred, and no exposure of the coating film surface and cohesive failure in the coating film were observed.
◯: No exposure of the coating film surface was observed, and the adhesive layer was mainly cohesive and fractured, but the cohesive fracture of the coating film was observed when the peeling length parallel to the pulling direction was within 2 mm.
Δ: The coating film causes cohesive failure and peels off.
X: Peeling occurs at the interface between the coating film and the adhesive layer, and the surface of the coating film is exposed.

(After soaking in water)
Each of the prepared test plates A was immersed in a constant temperature water bath set at 60 ° C. for 240 hours, then immersed in water at 23 ° C. for 1 hour to cool, and then the above peeling test was performed for evaluation.

[Evaluation of finished appearance]
Preparation of Finished Appearance Evaluation Test Plate (Examples 1 to 3, 5 to 18, Comparative Examples 1 to 17)
A polypropylene plate having a size of 11 cm × 45 cm was provided with 21 punch holes having a diameter of 5 mm in a row at 2 cm intervals in a portion 3 cm from the end on the long side, and the polypropylene plate was placed on the polypropylene plate. The aqueous primer (I) was coated with a film thickness gradient so as to obtain a film thickness of approximately 30 μm to 60 μm in the long direction, left for 3 minutes, and then preheated at 60 ° C. for 3 minutes.
Next, on the uncured primer layer, a water-based interference color base coat paint "WBC-713T No. 062" (product name, manufactured by Kansai Paint Co., Ltd.) was non-electrostatic coated so as to have a dry film thickness of 15 μm. After leaving it for 2 minutes, it was preheated at 80 ° C. for 3 minutes.
Further, a clear coat paint "Soflex 7175" (product name, manufactured by Kansai Paint Co., Ltd.) is non-electrostatically coated on the interference color base coat layer so that the dry film thickness is 25 μm, and the coated plate is made almost vertical. A test plate was prepared by curing the aqueous primer layer, the interference color base coat layer and the clear coat layer by heating at 120 ° C. for 30 minutes after 7 minutes had passed after coating.

(Example 4)
A polypropylene plate having a size of 11 cm × 45 cm was provided with 21 punch holes having a diameter of 5 mm in a row at 2 cm intervals in a portion 3 cm from the end on the long side, and the polypropylene plate was placed on the polypropylene plate. The aqueous primer (I) was coated with a film thickness gradient so as to obtain a film thickness of approximately 15 μm to 30 μm in the long direction, left for 3 minutes, and then preheated at 60 ° C. for 3 minutes.
Next, on the uncured primer layer, a water-based red base coat paint "WBC-713T No. 3R3" (product name, manufactured by Kansai Paint Co., Ltd.) was non-electrostatic coated so as to have a dry film thickness of 15 μm. After leaving for a minute, preheating was performed at 80 ° C. for 3 minutes.
Further, a clear coat paint "Soflex 7175" (product name, manufactured by Kansai Paint Co., Ltd.) is non-electrostatically coated on the colored base coat layer so that the dry film thickness is 25 μm, and the coated plate is placed almost vertically. A test plate was prepared by standing upright, 7 minutes after coating, and heating at 120 ° C. for 30 minutes to cure the aqueous primer layer, the colored base coat layer, and the clear coat layer.

<Sauce resistance>
Investigate the position where sagging of the coating film of 5 mm is observed from the lower end of the punch hole of each test plate obtained above, measure the film thickness [sauce limit film thickness (μm)] at that position, and use the following criteria. The sagging resistance was evaluated. The larger the sagging limit film thickness, the better the sagging resistance.
(Examples 1 to 3, 5 to 18, comparative examples 1 to 17)
⊚: Sauce limit film thickness is 40 μm or more ○: Sauce limit film thickness is 35 μm or more and less than 40 μm ×: Sauce limit film thickness is less than 35 μm (Example 4)
⊚: Sauce limit film thickness is 25 μm or more ○: Sauce limit film thickness is 20 μm or more and less than 25 μm ×: Sauce limit film thickness is less than 20 μm

<Axillary resistance>
The position where the armpit of each test plate obtained above was observed was investigated, the film thickness [armpit limit film thickness (μm)] at that position was measured, and the armpit resistance was evaluated according to the following criteria. The larger the armpit limit film thickness, the better the armpit resistance.
(Examples 1 to 3, 5 to 18, comparative examples 1 to 17)
⊚: Armpit limit film thickness is 40 μm or more ○: Armpit limit film thickness is 35 μm or more and less than 40 μm ×: Armpit limit film thickness is less than 35 μm (Example 4)
⊚: Armpit limit film thickness is 25 μm or more ○: Armpit limit film thickness is 20 μm or more and less than 25 μm ×: Armpit limit film thickness is less than 20 μm

<Smoothness>
The test plate obtained above was evaluated using the Wd value measured by "Wave Scan DOI" (trade name, manufactured by BYK Gardener). The smaller the Wd value, the higher the smoothness of the coated surface. The smaller the Wd value, the better, but at least the condition of less than 27 must be satisfied.
A: Less than 21 B: 21 or more and less than 24 C: 24 or more and less than 27 D: 27 or more and less than 30 E: 30 or more

<Water resistance>
The test plate obtained above is immersed in warm water at 40 ° C. for 240 hours, pulled up, dried at 20 ° C. for 12 hours, and then the multi-layer coating film of the test plate is cut in a grid pattern with a cutter so as to reach the substrate. Make 100 2mm x 2mm goban eyes. Subsequently, an adhesive cellophane tape was attached to the surface thereof, and the residual state of the Goban-mesh coating film after the tape was rapidly peeled off at 20 ° C. was examined, and the water resistance was evaluated according to the following criteria.
⊚: 100 Goban-grained coating films remain and no small borders of the coating film occur at the edge of the notch of the cutter. ○: 100 Goban-grained coating films remain, but the coating film is small at the edge of the notch of the cutter. Borders are generated Δ: 90 to 99 Goban-mesh coating films remain ×: The number of Goban-mesh coating films remaining is 89 or less.

The results are also shown in Tables 1 and 2.

As shown in Tables 1 and 2, in the examples, a three-layer coating film formed by sequentially coating a plastic molded product with a water-based primer (I), a water-based base coat paint (II), and a clear coat paint (III). In the coating method using 3 coats and 1 bake, which are simultaneously heat-cured, the composition of the water-based primer (I) is specified as described above. On the other hand, it was found that even if the coating method using 3 coats and 1 bake was adopted, a multi-layer coating film having high adhesion to glass and an excellent finished appearance was formed.

Claims (1)

(1) A step of applying an aqueous primer (I) to a plastic molded product to which glass is adhered to form a primer layer.
(2) A step of applying the water-based base coat paint (II) on the primer layer to form a base coat layer.
(3) A step of applying a clear coat paint (III) on the base coat layer to form a clear coat layer.
(4) A step of simultaneously heating and curing the three-layer coating film formed by the above steps (1) to (3).
It is a method of painting plastic molded products including
The aqueous primer (I) comprises an aqueous polyolefin resin (A) having a melting point in the range of 85 to 110 ° C. and a weight average molecular weight in the range of 80,000 to 120,000 in its total solid content. 25 to 50% by mass, water-based acrylic resin (B) 26 to 50% by mass, polyester resin (C) 4 to 10% by mass, melamine resin (D) 2.5 to 10% by mass, blocked polyisocyanate compound (E) is contained in an amount of 4 to 20% by mass and the diester compound (F) represented by the following general formula (1) is contained in an amount of 2.5 to 10% by mass.
The viscosity of the aqueous primer (I) when heated at 60 ° C. for 3 minutes is in the range of 100 to 1000 Pa · s (share rate 0.1 sec ^-1 ), and the primer layer is water at 20 ° C. for 2 minutes. A method for painting a plastic molded product, characterized in that the water absorption rate after immersion is 50% by mass or less.

[In the formula, R ¹ and R ² independently represent a hydrocarbon group having 4 to 18 carbon atoms, R ³ represents an alkylene group having 2 to 4 carbon atoms, m is an integer of 3 to 20, and m. The ^R3s may be the same or different from each other]