JP2000325868A - Formation of coating film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a coating film having high appearance quality using high-solid type clear paint by a two-coat-one-bake painting method. SOLUTION: This method comprises applying low-solid type base paint having VOC of 700-800 in coating viscosity and 2,500-10,000 poises viscosity (at 25 deg.C) when the nonvolatile content is 60-70 wt.% and the shear rate is 0.1/sec, setting for a prescribed time and applying high-solid type clear paint containing a carboxylic group-containing resin and a polyepoxide and having VOC of 400-500 in coating viscosity and 20-100 poises viscosity (at 25 deg.C) when the nonvolatile content is 55-75 wt.% and the shear rate is 0.1/sec and being integrally baked/ cured. Thus a mixed layer is hardly formed on the interface and the roughness of the surface is prevented when the clear paint is applied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming a coating film having excellent gloss and appearance by using a two-coat one-bake coating method. The coating film forming method of the present invention can be optimally used for a top coating or coil coating of an automobile.

[0002]

2. Description of the Related Art Metallic coating has been widely used as a top coating for automobiles. This metallic coating is generally applied by a two-coat one-bake coating method. That is, a base paint containing a hydroxyl group-containing polymer such as an acrylic resin or a polyester resin and a melamine resin as main components and containing an aluminum pigment or the like as a glitter material is first applied. After setting for a predetermined time, the surface is coated with a clear paint mainly containing a hydroxyl group-containing polymer such as an acrylic resin or a polyester resin and a melamine resin. Thereafter, the base coating film and the clear coating film are integrally baked and cured by heating at 140 ° C. for about 30 minutes. The metallic coating thus obtained is relatively excellent in gloss, hardness, water resistance and the like, and is widely used as a top coat for automobiles.

[0003] On the other hand, as a top coat for automobiles, it is required to give a remarkably high appearance quality.
Improvements in surface gloss, surface smoothness and the like are further required. However, the conventional clear paint using a low solid type melamine resin as a crosslinking agent generally has a low nonvolatile content of 30 to 50% by weight at the time of application. Therefore, the amount of the organic solvent volatilized at the time of painting or baking hardening is large, and the number of processing steps is large. Further, in the case of a thick film, there is a problem of sagging when coating at a time, so that it is necessary to apply the coating in several steps, and there is also a problem that the number of coating steps increases.

In order to improve surface gloss and smoothness,
Conventionally, additives such as leveling agents have been added. However, the method relying on the additives does not substantially improve, and the addition of the additives may cause a problem that the stability and workability of the paint are impaired. Furthermore, a coating film cured using a conventional melamine resin as a crosslinking agent cannot be said to have high acid resistance due to the presence of a triazine ring, and improvement in resistance to acid rain in recent years is also required.

[0005] JP-A-8-259667 discloses a resin composition containing a carboxyl group-containing resin and a polyepoxide. Since a resin solution obtained by dissolving the resin composition in an organic solvent has a high concentration and a low viscosity, a coating using the resin composition is a high solid coating.
Therefore, if this high solid paint is used, the amount of volatile organic solvent can be suppressed, and a thick coating film can be formed in a single coating step. Also, since there is no triazine ring in the coating film, it is also excellent in acid rain resistance.
That is, this high solid paint is most suitable as a top coating for automobiles.

[0006]

However, when a high-solid base paint and a high-solid clear paint are manufactured from the above resin composition and applied by a conventional two-coat one-bake coating method, a thick film is applied at one time. Although it can be performed, the surface smoothness and gloss are insufficient, and there is a problem that the appearance quality is poor as a top coat for automobiles.

The present invention has been made in view of such circumstances, and a two-coat one-bake coating using a high-solid type clear coating containing a resin composition as disclosed in Japanese Patent Application Laid-Open No. Hei 8-259667. An object of the present invention is to form a coating film having high appearance quality by a method.

[0008]

The features of the coating film forming method of the present invention that solves the above-mentioned problems include a melamine resin as a cross-linking agent, a non-volatile content of 60 to 70% by weight, and a shear rate of 0.1%. A low solid type base coating material having a viscosity of 2500 to 10,000 poise (25 ° C.) at 1 / sec and a VOC of 700 to 800 in a diluted coating material diluted to the coating viscosity is applied to form a wet base coating film. After the process and leaving the wet base coating film for a predetermined time, the wet base coating film contains a carboxyl group-containing resin and a polyepoxide, has a nonvolatile content of 55 to 75% by weight, and has a shear rate of 0.1%. / Viscosity at 20/100
Applying a high solid type clear paint having a poise (25 ° C.) and a VOC of 400 to 500 in the diluted paint diluted to the coating viscosity to the surface of the wet base coating film to form a wet clear coating film; And baking and curing the wet base coating film and the wet clear coating film simultaneously.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method for forming a coating film of the present invention, a melamine crosslinked low solid type base coating and a high solid type clear coating are applied by a two-coat one-bake coating method. That is, first, a low solid type base coating material is applied, and after setting for a predetermined time, a high solid type clear coating material is applied to the surface of the wet base coating film. Then, the base coating film and the clear coating film are integrally baked and hardened by holding at a predetermined temperature for a predetermined time.

That is, in the method for forming a coating film of the present invention, the non-volatile content of the previously formed wet base coating film increases due to the evaporation of the solvent during setting, and the wet base coating film has a higher wet base than when a high solid type base coating material is applied. The viscosity of the coating film is high. Therefore, when a high solid type clear paint is applied on a wet base coating, mixing of the two paints at the coating interface is unlikely to occur, suppressing a decrease in the appearance of the coating and forming a coating excellent in gloss and surface smoothness. It is thought to be done. In addition, since the base paint and the clear paint having different resin components are combined, mixing of both coating films can be suppressed. From this point, it is considered that a good coating film appearance can be formed.

By the way, the top coat for automobiles is usually applied by spray coating to obtain high quality. When the clear paint is spray-coated on the surface of the wet base paint film, a shear stress acts on the wet base paint film when the coated particles of the clear paint collide with the wet base paint film. Therefore, the viscosity of the wet base coating film changes due to the shear stress, and generally, the viscosity decreases due to the shear stress. Even in the case of paint that has been spray-coated, shearing stress acts on the coating particles during flight, and the shearing stress acts even at the moment of application.

On the other hand, when a low solid type base coating used for forming a wet base coating film is applied to an automobile body, air spray coating or a rotary atomization type so-called "bell" is generally used. The paint is applied by an electric coating machine or the like, and the clear paint is applied after a predetermined time interval. In addition, since it is generally said that the appearance of a coating film is governed by the viscosity at a shear rate of 0.1 / sec, in the method of forming a coating film of the present invention, as described above, the particles of the clear paint are coated. The viscosity of the wet base coating film when colliding with the wet base coating film, that is, the non-volatile content in which the solvent is volatilized by setting and the non-volatile content is increased is 6
In the state of 0 to 70% by weight, attention was paid to the viscosity of the wet base paint when the shear rate was 0.1 / sec. The viscosity at a shear rate of 0.1 / sec can be measured by a steady flow measurement method using a conical disk viscometer.

The low solid type base coating material used in the coating film forming method of the present invention has a nonvolatile content of 60 to 70% by weight and a viscosity of 2500 at a shear rate of 0.1 / sec.
It is important that it is between 10,000 and 10,000 poise (25 ° C.). As a result, the viscosity is high even when a shearing stress of the degree caused by the spray coating of the clear paint is applied. Therefore, it is considered that the roughness of the coating film surface due to the mixing of the two paints is further suppressed as described above.

The low solid base paint has a nonvolatile content of 6
When the viscosity at a shear rate of 0.1 / sec is lower than 2500 poise (25 ° C.) in a state of 0 to 70% by weight, the two paints are easily mixed at the interface between the wet base coating film and the wet clear coating film. In addition, the surface of the coating film becomes rough.
On the other hand, if it is higher than 10,000 poise (25 ° C.), it becomes difficult to smooth the wet base coating film, and the appearance is deteriorated.

Further, the low solid type base paint is diluted to a coating viscosity, and the VOC in the diluted paint is 700 to 800.
It is preferable to use a low solid type base paint which is VO in the coating viscosity of the low solid type base paint.
When C is less than 700, the flowability of the wet base coating film is reduced and the surface of the coating film becomes rough. If it exceeds 800, the non-volatile content in the base paint at the coating viscosity becomes too low, and it is difficult to secure a desired film thickness.

[0016] VOC stands for Volatite Organic Compo
It is an abbreviation of und, and refers to a value calculated by actually measuring the nonvolatile content of the diluted paint and the specific gravity of the diluted paint and substituting it into the following equation. Generally, the higher the VOC, the lower the non-volatile content in the paint, and the lower the VOC, the higher the non-volatile content. VOC = 1000 × Dilution paint specific gravity × (1−Dilution paint non-volatile content / 100) Further, in the coating film forming method of the present invention, as described above, the clear coating when the coating particles of the clear paint collide with the wet base coating film is performed. The viscosity of the film, that is, the non-volatile content of the clear paint is 55 to 55
At 75% by weight, attention was also paid to the viscosity of the clear paint when the shear rate was 0.1 / sec.

As a result, the high solid type clear paint used in the method for forming a coating film of the present invention has a nonvolatile content of 55-7.
It has been found that it is important that the viscosity at a shear rate of 0.1 / sec in the state of 5% by weight be 20 to 100 poise (25 ° C.). As a result, during spray coating (when a shear stress is applied), even at high solids, the viscosity is low, and the liquid is applied to the surface of the wet base coating film in a state of low viscosity, so that the flowability after coating is improved. It is considered that excellent surface smoothness is improved.

The nonvolatile content of the high solid type clear paint is 5
When the viscosity at a shear rate of 0.1 / second at 5 to 75% by weight is lower than 20 poise (25 ° C.), sagging is likely to occur, and when the viscosity is higher than 100 poise (25 ° C.), Surface smoothness decreases. Further, it is preferable to use a high solid clear paint, a high solid clear paint having a VOC of 400 to 500 in a diluted paint diluted to a coating viscosity, and a VOC in the coating viscosity of the high solid clear paint of less than 400 is used. The surface smoothness of the formed coating film decreases, and if it exceeds 500, it becomes difficult to secure a desired film thickness.

Hereinafter, the components of the present invention will be described in detail. Low solid type base paint used in the present invention,
It is the same as a conventional melamine crosslinked type base paint, and contains a resin having a hydroxyl group as a resin component and a melamine resin serving as a crosslinking agent. Examples of the resin having a hydroxyl group include an acrylic resin obtained by copolymerizing a hydroxyl group-containing ethylenically unsaturated monomer such as hydroxyethyl acrylate and hydroxyethyl methacrylate, and a polyester resin condensed with a composition in which the hydroxyl group is in excess of the carboxyl group. You. Examples of the melamine resin include a butylated melamine resin and a methylated melamine resin. Although a melamine resin is essential, a resin obtained by co-condensing benzoguanamine or urea may be used. The mixing ratio of the resin having a hydroxyl group to the melamine resin is such that the weight ratio of the resin having the hydroxyl group to the melamine resin is 80:20 to 60:40.
Degree. In addition, other resins can be mixed as long as the physical properties are not impaired.

The low solid type base coating material can be used only with a resin solution obtained by dissolving the above resin components in an organic solvent. However, in the case of a metallic coating, a bright material is generally contained. In the case of solid color coating, a filler is generally contained. Examples of the glittering material include an aluminum pigment, a mica pigment, a pearl mica pigment, a mica-like iron oxide pigment, and the like, and are mixed in an amount according to the intended glitter. As a filler, an extender pigment such as calcium carbonate, barium sulfate, or talc can be used according to the purpose. Further, in the same manner as in the related art, various additives such as a suspending agent, a leveling agent, an antioxidant, and an ultraviolet absorber, or a coloring material such as an organic pigment or an inorganic pigment may be added in addition to the glittering material and the filler. .

As the high solid type clear paint used in the present invention, a paint containing a carboxyl group-containing resin and a polyepoxide as resin components is used, and has an ester bond formed by a reaction between a carboxyl group and an epoxy group by heating. Those that have been used are preferably used. The carboxyl group-containing resin (a) has an average of two or more carboxyl groups in one molecule, and has an acid value of 5 to 300 m.
gKOH / g (solid content) and number average molecular weight 500-8000
Value obtained by reacting 10 to 70 parts by weight of an acrylic polycarboxylic acid having the formula (b) with a polyester polyol having three or more hydroxyl groups and an acid anhydride.
50 mgKOH / g (solid content), number average molecular weight 400-35
It is desirable to use one comprising 00 and 5 to 70 parts by weight of a polyester polycarboxylic acid having a weight average molecular weight / number average molecular weight of 1.8 or less.

As the acrylic polycarboxylic acid (a), a carboxyl group and a carboxylic ester group obtained by reacting (1) an acrylic polyanhydride with (2) a monoalcohol are adjacent. It is desirable to use an acrylic polycarboxylic acid bonded to carbon.
(A)-(1) An acrylic polyanhydride is an ethylenically unsaturated monomer having no acid anhydride group, having 15 to 40% by weight, preferably 15 to 35% by weight of an ethylenically unsaturated monomer containing an acid anhydride group. 60-85% by weight of saturated monomer, preferably 6
And 5 to 85% by weight.
If the amount of the acid anhydride group-containing ethylenically unsaturated monomer is less than 15% by weight, the curability will be insufficient, and if it exceeds 40% by weight, the coating film will be too hard and brittle, and the weather resistance will be insufficient.

Examples of the acid anhydride group-containing ethylenically unsaturated monomer include itaconic anhydride, maleic anhydride, citraconic anhydride and the like. The ethylenically unsaturated monomer having no acid anhydride group is not particularly limited as long as it is an ethylenically unsaturated monomer that does not adversely affect the acid anhydride group, but has 3 to 3 carbon atoms having one ethylenically unsaturated bond.
It is preferred to use 15, especially 3 to 12 monomers.

The ethylenically unsaturated monomer having no acid anhydride group is used to improve the compatibility between resins.
It is also preferable to use a combination of two or more species. Such ethylenically unsaturated monomers having no acid anhydride group include styrene, α-methylstyrene, pt-butylstyrene,
Various (meth) acrylates, VeoV from Shell
versatic acid glycidyl ester such as a-9 and VeoVa-10; When styrene or a styrene derivative is used, it is desirable to use styrene or a styrene derivative in the range of 5 to 40% by weight based on all monomers.

Further, as an ethylenically unsaturated monomer having no acid anhydride group, a monomer having a carboxyl group such as acrylic acid, methacrylic acid, itaconic acid and maleic acid, and an ethylenically unsaturated monomer having a hydroxyl group and an acid An adduct with an anhydride group-containing compound can also be used. Among them, the use of a long-chain carboxyl group-containing monomer having a spacer portion of about 5 to 20 carbon atoms between the ethylenically unsaturated group and the carboxyl group is particularly preferable because the scratch resistance of the coating film is improved. .

The number average molecular weight of the (a)-(1) acrylic polyanhydride obtained by copolymerizing an ethylenically unsaturated monomer having an acid anhydride group with an ethylenically unsaturated monomer having no acid anhydride group is as follows: , 500-8000, and 8
It is preferable to set it to 00-6000, especially 1500-4000. When the number average molecular weight exceeds 8000, the compatibility between the resins is reduced, and the appearance quality of the coating film is reduced, and 500
If it is less than this, the curability becomes insufficient. It is desirable that the polymer obtained has an average of at least 2, preferably 2 to 15 acid anhydride groups in one molecule. If the number of acid anhydride groups is less than 2, the curability will be insufficient, and if it exceeds 15, the coating film will be too hard and brittle, and the weather resistance will be insufficient.

(A)-(2) As the monoalcohol,
It is preferable to use one having 1 to 12, particularly 1 to 8 carbon atoms. This is because the alcohol easily volatilizes during the reaction with the acrylic polyanhydride, which is suitable for regenerating the acid anhydride group. Such monoalcohols include methanol, ethanol, n-
Propanol, i-propanol, n-butanol, i
-Butanol, t-butanol, n-hexyl alcohol, lauryl alcohol, methyl cellosolve, ethyl cellosolve, methoxypropanol, ethoxypropanol, furfuryl alcohol, dimethylaminoethanol, diethylaminoethanol, acetol, allyl alcohol, propargyl alcohol and the like. .

When (a)-(1) an acrylic polyanhydride is reacted with (a)-(2) a monoalcohol to synthesize (a) an acrylic polycarboxylic acid, an acid anhydride group and a hydroxyl group Are in a molar ratio of 1/10 to 1/1, preferably 1 /
5 to 1/1, more preferably 1/2 to 1/1. If the molar ratio is less than 1/10, excess alcohol may cause cracking at the time of curing, and if it exceeds 1/1, unreacted acid anhydride groups remain to deteriorate storage stability.

The acrylic polycarboxylic acid (a) having a carboxyl group and a carboxylic ester group obtained by the above reaction has an acid value of 5 to 300 mgKOH / g, particularly 50
Preferably, it is 〜250 mgKOH / g. Acid value 5 m
If it is lower than gKOH / g, the curability will be insufficient and 300 mgKOH
/ G, the storage stability becomes poor. (A) The acrylic polycarboxylic acid is 10 to 70% by weight, preferably 15 to 50% by weight, based on the total nonvolatile components in the resin composition.
%, More preferably from 20 to 45% by weight. When the amount is less than 10% by weight, the weather resistance of the obtained coating film is reduced, and when it exceeds 70% by weight, the coating film becomes too hard.

The polyester polycarboxylic acid (b), which is another component of the carboxyl group-containing resin, is obtained by half-esterifying (1) a polyester polyol having three or more hydroxyl groups and (2) an acid anhydride. can get. In addition, the polyester polyol refers to a polyhydric alcohol having two or more ester bond chains and reacts with an acid anhydride group to become a polyester polycarboxylic acid having two or more acid functionalities per molecule and the following properties. .

The (b)-(1) polyester polyol has a carbon number of 3 having at least three hydroxyl groups.
It can also be synthesized by adding a lactone compound such as ε-caprolactone to the low-molecular-weight polyhydric alcohol of up to 16 or adding the lactone compound to the low-molecular-weight polyhydric alcohol to extend the chain. By introducing a linear aliphatic group into the low molecular weight polyhydric alcohol, the resulting coating film is given flexibility and the impact resistance is improved.

Examples of the low molecular weight polyhydric alcohol used include trimethylolethane, trimethylolpropane,
1,2,4-butanetriol, ditrimethylolpropane, pentaerythritol, dipentaerythritol,
Glycerin and mixtures thereof are exemplified. Examples of preferably used low molecular weight polyhydric alcohols include trimethylolpropane, ditrimethylolpropane, pentaerythritol, and adducts of these with lactone compounds. Examples of the lactone compound include ε-caprolactone, γ-caprolactone, γ-valerolactone, δ-valerolactone, and γ-butyrolactone, and ε-caprolactone, γ-valerolactone, and γ-butyrolactone are preferred.

Examples of (b)-(2) acid anhydride include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, 4-methylhexahydrophthalic anhydride, trimellitic anhydride, succinic anhydride and the like. Is done.
(B)-(1) Polyester polyol and (b)-
(2) The half esterification reaction with an acid anhydride is carried out at room temperature to 1
The reaction can be performed under ordinary reaction conditions such as 50 ° C. and normal pressure. However, it is not necessary to modify all of the hydroxyl groups of the polyester polyol into carboxyl groups, and the hydroxyl groups may be left. According to the above-described method, the molecular weight distribution becomes sharp, so that further high solidification is possible,
A coating film excellent in weather resistance and water resistance can be formed.

The (b) polyester polycarboxylic acid obtained by the above reaction has an acid value of 50 to 350 mgKOH /
g (solid content), preferably 100 to 300 mg KOH / g
(Solid content), more preferably 150 to 250 mgKOH / g
(Solid content), the number average molecular weight is 400-3500,
Preferably 500 to 2500, more preferably 700 to
The weight average molecular weight / number average molecular weight is 1.8 or less, preferably 1.5 or less, more preferably 1.35 or less.

If the acid value exceeds 350, the viscosity of the resin becomes too high, causing a decrease in the nonvolatile content of the paint. If the acid value is less than 50, the curability of the paint becomes insufficient. In addition, the molecular weight is 3
If it exceeds 500, the resin viscosity becomes too high, causing a decrease in the nonvolatile content of the diluted paint diluted to the coating viscosity, and if it is less than 400, the curability becomes insufficient. Further, when the weight average molecular weight / number average molecular weight exceeds 1.8, the water resistance and weather resistance of the coating film decrease.

(B) Since the polyester polycarboxylic acid has a hydroxyl group in the molecule, a carboxyl group and a hydroxyl group are simultaneously provided on the surface of the coating film, for example, the adhesion at the time of recoating is improved. In this case, the hydroxyl value of the polyester polycarboxylic acid is preferably 150 mgKOH / g (solid content) or less, preferably 5 to 100 mgKOH / g (solid content), and more preferably 10 to 80 mgKOH / g (solid content). .
If the hydroxyl value exceeds 150 mgKOH / g (solid content), the water resistance will decrease.

(B) having a hydroxyl group and a carboxyl group
As described later, the polyestercarboxylic acid reacts with both (c) the polyepoxide and (a) the acrylic polycarboxylic acid, so that a stronger coating film can be formed. Therefore, it is desirable that the polyester carboxylic acid (b) has an average of 0.1 or more hydroxyl groups in one molecule.

In order to obtain such a polyester carboxylic acid, the molar amount of the acid anhydride group of the (b)-(2) acid anhydride with respect to the molar amount of the hydroxyl group of the (b)-(1) polyester polyol is 0.1%. It is preferably 2 to 1.0 times, particularly preferably 0.5 to 0.9 times. If this ratio is less than 0.2 times, the curability of the paint will be insufficient. (B) The polyester polycarboxylic acid component has a content of 5 to 5 based on the weight of all nonvolatile components in the paint.
70% by weight, preferably 5 to 50% by weight, more preferably 10 to 40% by weight.
(B) If the amount of the polyester polycarboxylic acid is less than 5% by weight, the non-volatile content of the coating does not increase, and if it exceeds 70% by weight, the weather resistance of the coating film decreases.

The polyepoxide (c), which is another component of the high solid type clear coating material used in the present invention, has an average of two or more, preferably 2 to 10, more preferably 2 to 10, epoxy groups in one molecule. It is a compound having 3 to 8 compounds. (C) Preferred as polyepoxides are:
(1) Epoxy group-containing ethylenically unsaturated monomer 10
60% by weight, preferably 15 to 50% by weight, and (2) 40 to 9 ethylenically unsaturated monomers having no epoxy group.
An acrylic polyepoxide obtained by copolymerizing 0% by weight, preferably 10 to 60% by weight. If the amount of the epoxy group-containing ethylenically unsaturated monomer is less than 10% by weight, the curability will be insufficient, and if it is more than 60% by weight, it will be too hard and the weather resistance will be insufficient.

(C)-(1) Examples of the epoxy group-containing ethylenically unsaturated monomer include glycidyl (meth) acrylate, β-methylglycidyl (meth) acrylate, and 3,4-epoxycyclohexanyl (meth) acrylate. Is exemplified. It is preferable to use glycidyl (meth) acrylate in order to ensure balanced curability and storage stability.

(C)-(2) As the ethylenically unsaturated monomer having no epoxy group, the above-mentioned ethylenically unsaturated monomer having no acid anhydride group which does not affect the epoxy group can be used. Can be used. The number average molecular weight of the acrylic polyepoxide obtained by copolymerization of the above monomers is 500 to 10,000, preferably 1,000 to 8,000, more preferably 1500 to 500.
0. If the number average molecular weight is less than 500, the curability of the coating will be insufficient, and if it exceeds 10,000, the nonvolatile content of the diluted coating diluted to the coating viscosity will decrease. The epoxy equivalent is 5
It is 0-700, preferably 80-600, more preferably 100-500. When the epoxy equivalent exceeds 700, the curability of the coating becomes insufficient, and when it is less than 50, the coating becomes too hard and the coating film becomes brittle, which is not preferable.

As the (c)-(2) ethylenically unsaturated monomer having no epoxy group, a hydroxyl group-containing ethylenically unsaturated monomer represented by the following formula (1) can also be used.

[0043]

Embedded image

(In the formula, R is a hydrogen atom or a methyl group, and X is an organic chain represented by Formula 2 or Formula 3.)

[0045]

Embedded image

(In the formula, Y is a linear or branched alkylene group having 2 to 8 carbon atoms, m is an integer of 3 to 7, and q is an integer of 0 to 4.)

[0047]

Embedded image

(Wherein, R is a hydrogen atom or a methyl group, and n is an integer of 2 to 50). Specific examples of the hydroxyl group-containing ethylenically unsaturated monomer represented by the formula 1 include (meth) acrylic acid 2 -Hydroxyethyl, 2-hydroxypropyl (meth) acrylate, 2-hydroxybutyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, 6-hydroxyhexyl (meth) acrylate, and their ε-caprolactone With (meth) acrylic acid and a large excess of diols (eg, 1,4 butanediol, 1,6 hexanediol,
Polyethylene glycol, polypropylene glycol)
And the like.

As the hydroxyl group-containing ethylenically unsaturated monomer used herein, those having 5 to 23 carbon atoms, particularly 5 to 13 carbon atoms, can be preferably used. If the chain length of the monomer is too short, flexibility near the cross-linking point is lost, and the monomer becomes too hard. If the chain length is too long, the molecular weight between cross-linking points becomes too large. When the hydroxyl group-containing ethylenically unsaturated monomer is used, the adhesion and recoating property of the obtained coating film are improved. Acrylic polyepoxide having a hydroxyl group and a carboxyl group, as described below,
Since the (a) acrylic polycarboxylic acid having a carboxyl group and a carboxylic acid ester group reacts with and binds to both functional groups of a hydroxyl group and an epoxy group, a stronger coating film can be obtained.

The hydroxyl value of the acrylic polyepoxide is 5
-300 mgKOH / g (solid content), preferably 10-20
0 mgKOH / g (solid content), more preferably 15 to 150
mgKOH / g (solid content). Hydroxyl value is 300 mgKOH
/ G (solid content), the nonvolatile content of the coating material decreases and the water resistance of the coating film is insufficient, and if it is less than 5 mgKOH / g (solid content), the adhesion is poor.

The polyepoxide (c) suitable for use in the present invention comprises 5-70% by weight of a hydroxyl group-containing ethylenically unsaturated monomer having the structure represented by the above-mentioned formulas (1) to (3).
(1) Epoxy group-containing ethylenically unsaturated monomer 10
It is obtained by copolymerizing 60% by weight and optionally 0 to 85% by weight of an ethylenically unsaturated monomer having neither a hydroxyl group nor an epoxy group. In this case, the obtained polyepoxide (c) has an average of 2 to 12, more preferably 3 to 10, epoxy groups and an average of 0.5 to 10, more preferably 1 to 8, epoxy groups in one molecule. Have

(C) The polyepoxide component is 10 to 80% by weight, preferably 20 to 70% by weight, more preferably 30 to 80% by weight, based on the weight of the total solids in the curable resin composition.
It can be blended in an amount of up to 65% by weight. When the amount of the polyepoxide is less than 10% by weight, the curability is reduced, and
If it exceeds 0% by weight, the yellowing resistance is deteriorated. The high solid type clear paint referred to in the present invention comprises (a) an acrylic polycarboxylic acid, (b) a polyester polycarboxylic acid,
(C) It can be produced by mixing three components with polyepoxide. Particularly when using (a) an acrylic polycarboxylic acid having a carboxyl group and a carboxylic ester group and using (c) a polyepoxide having a hydroxyl group and an epoxy group, a coating film having particularly excellent acid resistance is formed. To obtain a high solid type clear paint.

In this case, the molar ratio of the carboxyl group contained in (a) the acrylic polycarboxylic acid and (b) the polyester polycarboxylic acid to the epoxy group contained in (c) the polyepoxide is 1 / 1.4. (A) a carboxy group bonded to a carboxyl group or a carbon adjacent to a carboxyl group-bound carbon contained in an acrylic polycarboxylic acid; The molar ratio of the acid ester group to the hydroxyl group contained in the (b) polyester polycarboxylic acid and (c) polyepoxide is from 1 / 2.0 to 1 / 0.5, more preferably from 1/1.
It is preferable to mix in such an amount as to be 5/1 / 0.7.

When the molar ratio of the carboxyl group contained in (a) the acrylic polycarboxylic acid and (b) the polyester polycarboxylic acid to the epoxy group contained in (c) the polyepoxide exceeds 1 / 0.6. When the curability of the obtained coating material is reduced and becomes smaller than 1 / 1.4, the coating film turns yellow. Further, (a) a carboxyl group contained in the acrylic polycarboxylic acid or a carboxylate ester group bonded to the carbon adjacent to the carboxyl group-bonded carbon, and (b) a hydroxyl group contained in the polyester polycarboxylic acid and (c) the polyepoxide If the molar ratio exceeds 1 / 0.5, the curability of the resulting coating material will decrease, and if it is less than 1 / 2.0, the hydroxyl group will be excessive and the water resistance will decrease.

The curing mechanism of the high solid type clear paint thus obtained is as follows. First, the carboxyl group and the carboxylic acid ester group in the (a) acrylic polycarboxylic acid react by heating to (a) the acrylic polyacrylic acid. An acid anhydride group is formed in the carboxylic acid, and a free monoalcohol is formed. The produced monoalcohol evaporates and is removed out of the system. The acid anhydride groups generated in the (a) acrylic polycarboxylic acid react with the hydroxyl groups contained in the (b) polyester polycarboxylic acid and the (c) polyepoxide to form cross-linking points, and form carboxyl groups again. Form. The carboxyl group and the carboxyl group present in (b) the polyester polycarboxylic acid form a crosslinking point by reacting with the epoxy group present in (c) the polyepoxide. As described above, the three kinds of polymers react with each other, whereby the curing proceeds, and a crosslinked coating film having a high density is formed.

The high solid type clear coating composition according to the present invention may contain a curing catalyst such as a quaternary ammonium salt which is generally used for an esterification reaction between an acid and an epoxy. Examples of the curing catalyst include benzyltriethylammonium chloride or bromide, tetrabutylammonium chloride or bromide, salicylate or glycolate, and paratoluenesulfonate.

The amount of the curing catalyst to be added is generally 0.01 to 3.0% by weight, preferably 0.1 to 1.5% by weight, more preferably 0.4 to 0.4% by weight based on the solid content of the resin composition. 1.
2% by weight. If the amount of the curing catalyst is less than 0.01% by weight, the effect of the addition cannot be obtained, and if the amount exceeds 3.0% by weight, the storage stability decreases. In the high solid type clear coating composition of the present invention, a tin compound disclosed in JP-A-2-51651 or the like can be mixed. Examples of such a tin-based catalyst include dimethyltin bis (methylmalate), dimethyltinbis (ethylmalate), dimethyltinbis (butylmalate), and dibutyltinbis (butylmalate).

The amount of the tin-based catalyst to be added is generally 0.05 to 6.0% by weight, preferably 0.1 to 4.0% by weight, more preferably 0.2 to 2.0% by weight based on the solid content of the resin composition. ~
2.0% by weight. The amount of tin catalyst is 0.05% by weight
If the amount is less than the above, the storage stability decreases, and if the amount exceeds 6.0% by weight, the weather resistance decreases. When a curing catalyst and a tin-based catalyst are used in combination, the weight ratio between the curing catalyst and the tin-based catalyst is preferably 1/4 to 1 / 0.2.

To the high solid type clear coating composition of the present invention, a blocked isocyanate may be added in order to increase the crosslinking density and improve the water resistance. Further, a known ultraviolet absorber, light stabilizer, antioxidant and the like may be added. Further known rheology control agent, a surface conditioner may be added, or a solvent such as an alcohol-based solvent, an aromatic hydrocarbon-based solvent, an ester-based solvent, and a ketone-based solvent may be used for the purpose of adjusting viscosity. it can.

In the method for forming a coating film of the present invention, the low solid base paint and the high solid clear paint are applied by spray coating, brush coating, dip coating, or the like.
Examples include roll coating and sink coating. In the case of automotive topcoats, atomization coating is suitable, air spray coating,
Airless spray coating, electrostatic coating and the like can be used.

The low solid type base paint is generally applied in a range where the dry film thickness is 10 to 30 μm. When the thickness is larger than 30 μm or smaller than 10 μm, the appearance quality is deteriorated. The setting time after application of the low solid type base paint is 5 to 15 at room temperature as in the past.
Minutes. If the setting time is shorter than 5 minutes, a burning may occur at the time of printing, and even if the setting time is 15 minutes or more, the effect is saturated and dust or the like adheres, which is not preferable.

The high solid type clear paint is generally applied in a range where the dry film thickness is 25 to 70 μm. If this film thickness is greater than 70 μm, sagging will occur and 25 μm
When the thickness is smaller than m, the film thickness of the final coating film is insufficient and the appearance quality is deteriorated. The setting time after application of the high solid clear paint is about 3 to 15 minutes. If the setting time is shorter than 3 minutes, armpits are likely to occur, and if the setting time is longer than 15 minutes, the effect is saturated and dust adheres, which is not preferable.

Thereafter, both wet coatings were 100-180.
C., preferably 120-160.degree. C., and are integrally baked and cured to form a coating film. The baking time varies depending on the temperature, but is suitably from 120 to 160 ° C. and from 10 to 30 minutes.

[0064]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) (1) Preparation of low solid type base paint First, the following resin was prepared. Acrylic resin A (Nippon Paint Co., Ltd., nonvolatile content 50
Weight%, hydroxyl value 45 mgKOH / g, acid value 15 mgKOH /
g, number average molecular weight 12000) Acrylic resin B (Nippon Paint Co., Ltd., nonvolatile content 55)
Weight%, hydroxyl value 75 mgKOH / g, acid value 20 mgKOH / g
g, number average molecular weight: 6000) Melamine resin (product name "Uban 20N-60" manufactured by Mitsui Toatsu Co., Ltd., nonvolatile content: 60% by weight) 25 parts by weight of the acrylic resin A and 4 parts of the acrylic resin B
5 parts by weight, 30 parts by weight of a melamine resin, and 10 parts by weight of an aluminum pigment ("Alpaste Al60-600" manufactured by Toyo Aluminum Co., Ltd., solid content: 65% by weight) are stirred and mixed, and an aromatic hydrocarbon solvent and an alcohol solvent are mixed. Paint thinner (Ford Cup No. 3, 20
At 17 ° C. for 17 seconds). At this time, the nonvolatile matter (0.5 ± 0.1 g of the paint was accurately weighed, the mass of the paint after drying at 110 ± 5 ° C. for 60 minutes was precisely weighed, and the value obtained by dividing the mass of the paint before drying by 100 times. Is 18% by weight, and VO
C was 770. The non-volatile content (paint 0.5 ± 0.1) measured with a rotor having a diameter of 4 cm and an angle of 1.3 ° was measured using a solidometer viscometer manufactured by Rheology Co., Ltd.
g was precisely weighed and calculated from the mass of the paint after drying at 110 ± 5 ° C for 60 minutes) and the viscosity at a shear rate of 0.1 / sec was 6,500 poise (25 ° C).

(2) Preparation of high solid type clear paint a. Synthesis of acrylic polycarboxylic acid 700 parts by weight of xylene and 350 parts by weight of Solvesso 100 (Esso aromatic solvent) in a 3 L reaction tank equipped with a thermometer, a stirrer, a cooling pipe, a nitrogen inlet pipe, and a dropping funnel. And the temperature was raised to 130 ° C. and maintained.

On the other hand, 300 parts by weight of a styrene monomer, 109 parts by weight of 2-ethylhexyl methacrylate, 325 parts by weight of isobutyl acrylate, 26 parts by weight of acrylic acid, 240 parts by weight of maleic anhydride, 300 parts by weight of propylene glycol monomethyl ether acetate, And a solution consisting of 150 parts by weight of t-butylperoxy 2-ethylhexanoate and 150 parts by weight of xylene was prepared and added dropwise from the dropping funnel into the above reaction vessel over 3 hours.

After maintaining the temperature at 130 ° C. for 30 minutes after the completion of the dropwise addition, a solution comprising 20 parts by weight of t-butylperoxy 2-ethylhexanoate and 20 parts by weight of xylene was added dropwise over 30 minutes. After the completion of this dropping, 1 hour 1 more
The reaction was continued at 30 ° C., and thereafter, 1100 parts by weight of the solvent was removed to prepare a varnish having a nonvolatile content of 70% by weight containing an acrylic polyanhydride having a number average molecular weight of 2,000.

To 1590 parts by weight of the obtained varnish, 125 parts by weight of methanol was added and reacted at 70 ° C. for 23 hours.
An acrylic polycarboxylic acid having an acid value of 158 mgKOH / g (solid content) was obtained. The infrared absorption spectrum of this acrylic polycarboxylic acid was measured, and it was confirmed that the absorption (1785 cm ^-1 ) of the acid anhydride group disappeared.

B. Synthesis of Polyester Polycarboxylic Acid 278 parts by weight of ethyl 3-ethoxypropionate was placed in a 3 L reaction vessel equipped with a thermometer, a stirrer, a cooling pipe, and a nitrogen inlet pipe.
268 parts by weight of trimethylolpropane, 228 parts by weight of ε-caprolactone, and 0.1 part by weight of dibutyltin oxide were charged and heated to 150 ° C. and maintained.

After maintaining at 150 ° C. for 2 hours, 616 parts by weight of hexahydrophthalic anhydride dissolved by heating is added, and the mixture is maintained at 150 ° C. for 1 hour to obtain a number average molecular weight of 80.
0, weight average molecular weight / number average molecular weight = 1.18, acid value 2
02 mgKOH / g (solid content) and hydroxyl value 101 mgKOH /
A varnish containing 80% by weight of a non-volatile content containing g (solid content) of a polyester polycarboxylic acid was obtained.

C. Synthesis of Polyepoxide 250 parts by weight of xylene and 200 parts by weight of propylene glycol monomethyl ether acetate were charged into a 2 L reaction tank equipped with a thermometer, a stirrer, a cooling tube, a nitrogen inlet tube, and a dropping funnel, and heated to 130 ° C. and maintained. . On the other hand, 450 parts by weight of glycidyl methacrylate, 236 parts by weight of isobornyl methacrylate, 4-hydroxybutyl acrylate 6
A solution consisting of 4 parts by weight, 250 parts by weight of t-butylstyrene, 110 parts by weight of t-butylperoxy-2-ethylhexanoate and 150 parts by weight of xylene was prepared, and the solution was dropped from the dropping funnel into the reactor over 3 hours. It was dropped. The solution temperature was kept at 130 ° C.

After maintaining the temperature at 130 ° C. for 30 minutes after the completion of the dropwise addition, a solution comprising 10 parts by weight of t-butylperoxy-2-ethylhexanoate and 50 parts by weight of xylene was added dropwise over 30 minutes. After the completion of this dropping, 1 hour 1 more
The reaction was continued at 30 ° C., and then 270 parts by weight of the solvent was removed to obtain a number average molecular weight of 2200 and an epoxy equivalent of 31.
6 and a varnish having a nonvolatile content of 72% by weight containing an acrylic polyepoxide having a hydroxyl value of 25 mgKOH / g (solid content).

D. Paint Acrylic polycarboxylate varnish synthesized as described above, polyester polycarboxylate varnish and acrylic polyepoxide varnish were mixed at a nonvolatile content of 24 parts by weight: 25 parts by weight: 51 parts by weight, respectively. 0.5 parts by weight of a butyl ammonium bromide curing catalyst,
0.5 parts by weight of a dibutyltin bis (butylmalate) catalyst, 2 parts by weight of an ultraviolet absorber ("Tinuvin 900" manufactured by Ciba Geigy), 1 part by weight of a light stabilizer ("Sanol LS-440" manufactured by Mitsui) and a surface conditioner ( "Modaflow")
0.1 parts by weight was mixed with a disperser while stirring.

The obtained paint was coated with a thinner consisting of ethyl 3-ethoxypropionate / S-150 (manufactured by Esso Corporation) = 1/1 to obtain a coating viscosity (Ford Cup No. 4, 20 ° C.).
To 35 seconds) to prepare a high solid type clear paint. At this time, the nonvolatile content (calculated from the paint after drying at 110 ± 5 ° C. for 60 minutes) was 53% by weight, and the VOC was 470. Also, using a Rheology Co., Ltd.
A non-volatile content (0.5 ± 0.1 g of paint was weighed accurately and measured by a rotor having a rotor diameter of 4 cm and an angle of 1.3 °,
The viscosity at the time of 65% by weight (calculated from the mass of the paint after drying at ± 5 ° C for 60 minutes) and the shear rate of 0.1 / sec was 50 poise (25 ° C).

(3) Coating On a 0.8 mm-thick phosphoric acid-treated steel sheet, a cationic electrodeposition paint and an intermediate paint were applied so as to have a dry film thickness of 25 μm and 40 μm, respectively. Atomization pressure of 5 kg / cm ² by “Mini Bell” manufactured by Landsberg Gemma
Then, the above-mentioned low solid type base coating material was applied, and after standing at room temperature for 1.5 minutes, the above low solid type base coating material was further applied so that the total dry film thickness was 14 to 18 μm. Thereafter, setting was performed for about 4 minutes to form a wet base coating film. This coating was performed on two types of test plates: a horizontal state and a vertical state.

Then, the above-mentioned high solid type clear paint is applied to an electrostatic coating machine (“Mini Bell” manufactured by Landsberg Gemma).
With the atomization pressure of 5 kg / cm ² , the dry film thickness is about 30 μm and about 50 μm.
After about 7 minutes, baking was performed at 140 ° C. for 25 minutes to cure the base coating film and the clear coating film integrally. The test plate coated with the base paint in a horizontal state was painted in a horizontal state, and the test plate painted with a base paint in a vertical state was painted in a vertical state. Also, at the time of baking, the test plate painted in a horizontal state was baked in a horizontal state, and the test plate painted in a vertical state was baked in a vertical state.

(Comparative Example 1) A low solid type base paint similar to that of Example 1 and "Macflow O-380 (low solid type clear paint made by Nippon Paint Co., Ltd.)" were prepared. In advance, Ford Cup No. 4, diluted at 20 ° C. for 20 seconds, and painted and baked in the same manner as in Example 1.

The characteristic values of the base paint and the clear paint were measured in the same manner as in Example 1, and the results are shown in Table 1. (Comparative Example 2) As a high solid type base coating material, "Super Lac M-1300 (manufactured by Nippon Paint Co., Ltd.)" was previously coated with a thinner composed of butyl acetate / n-hexyl acetate = 1/1 to obtain a coating viscosity (Ford Cup No. .
3, 20 ° C. for 20 seconds).

Using the above high solid type base coating material and the same high solid type clear coating material as in Example 1, coating and baking were performed in the same manner as in Example 1. The characteristic values of the base paint and the clear paint were measured in the same manner as in Example 1, and the results are shown in Table 1. (Evaluation)

[0080]

[Table 1]

The appearance of the coating film surface was measured using a GT meter (manufactured by Tokai Rika Co., Ltd.) for each of the four types of coated plates obtained in Examples and Comparative Examples. According to this GT meter, a numerical value can be evaluated in a state where the surface gloss of the coating film and the degree of skin roughness are combined, and the higher the numerical value, the better the appearance quality. Table 1 shows the results. According to Table 1, according to the method of forming a coating film of Example 1 of the present invention, the GT value of the coating, particularly in the vertical state, was higher than that of each of the comparative examples, which is the effect of the configuration of the present invention. Is evident.

On the other hand, in Comparative Example 1, when the thickness of the clear paint is small, the GT value is low. This is due to sagging. In Comparative Example 2, no sagging occurred even when the clear paint was applied thickly, but the GT value was lower than in Example 1. This is presumably because the phenomenon of mutual dissolution at the interface when the clear paint was applied became large, and thus the surface of the clear paint film was roughened.

Since the high solid type clear paint used in Example 1 has a low VOC and a high nonvolatile content in the coating viscosity, the amount of volatile organic solvent is small and the number of exhaust treatment steps can be reduced. In addition, even when the coating is applied to a degree that does not cause sagging, the coating film obtained is thick and exhibits high appearance quality. Further, the high solid type clear paint used in Example 1 does not contain a triazine ring, and thus has high resistance to acid rain. The method for forming a coating film of Example 1 is most suitable for top coating of automobiles.

[0084]

According to the coating film forming method of the present invention, a coating film having an extremely high quality coating film appearance can be surely formed by a two-coat one-bake coating method. In addition, since a high solid type clear coating is used, a thick coating film can be formed while suppressing the amount of organic solvent discharged, and the formed coating film has excellent acid rain resistance. Therefore, the method for forming a coating film of the present invention is extremely suitable for use as a top coat for automobiles.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuyuki Kuwano 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Naohiro 3-5-5 Nittacho Takahama City, Aichi Japan Paint Co., Ltd. In-house (72) Inventor Yu Takagi 3-5-1-5 Nittacho, Takahama-shi, Aichi Japan Paint Co., Ltd. F-term (reference) 4D075 AE06 BB26Z CB04 DA06 DB02 DC12 EB22 EB33 EB45 EB52 EC30

Claims (1)

[Claims] 1. A melamine resin as a crosslinking agent, having a non-volatile content of 60 to 70% by weight and a viscosity of 2500 to 10,000 poise (25 ° C.) at a shear rate of 0.1 / sec.
A process of applying a low solid type base paint having a VOC of 700 to 800 in the diluted paint diluted to the coating viscosity to form a wet base coating film, and after leaving the wet base coating film for a predetermined time, On the wet base coating film, containing a carboxyl group-containing resin and polyepoxide, and having a nonvolatile content of 55 to 75% by weight,
A high solid type clear paint having a viscosity of 20 to 100 poise (25 ° C.) at a shear rate of 0.1 / sec and a VOC of 400 to 500 in a diluted paint diluted to the coating viscosity is applied to the wet base coating. A method for forming a coating film, comprising: a step of forming a wet clear coating film by applying to a film surface; and a step of simultaneously baking and curing the wet base coating film and the wet clear coating film.