JP2002126621A - Method for forming multilayered coating film and multilayered coating film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a multilayered coating film by which a multilayered coating film having excellent shock resistance, particularly chipping resistance comparable to a conventional three-coating film and having good appearance without changing into yellow can be formed. SOLUTION: The method for forming a multilayered coating film includes a process (I) to apply an electrodeposition coating material on the objective body to be coated and then to heat and harden to form an electrodeposition coating film, a process (II) to apply a water-based intermediate coating material on the electrodeposition coating film to form an unhardened intermediate coating film, a process (III) to apply a water-based base coating material on the intermediate coating film to form an unhardened base coating film, a process (IV) to apply a clear coating material on the base coating film to form an unhardened clear coating film, and a process (V) to heat and harden the intermediate coating film, base coating film and clear coating film at a time to form a multilayered coating film. The electrodeposition coating film contains particles (A) and particles (B). The water-based intermediate coating material and the water-based base coating material contain pigment dispersion pastes compounded therein.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of applying an aqueous intermediate coating, an aqueous base coating, and a clear coating on a workpiece on which an electrodeposition coating film has been formed by wet-on-wet, and performing heat curing at a time. The present invention relates to a method for forming a multilayer coating film using a three-wet coating system, and more particularly, to a method for forming a multilayer coating film capable of obtaining a multilayer coating film having excellent chipping resistance, and a method for forming a multilayer coating film. It relates to a multilayer coating.

[0002]

2. Description of the Related Art In recent years, in the field of paints, especially in the field of automotive coatings, resource saving, cost saving and environmental burden (VOC and HOC) have been observed.
(APs etc.) In order to solve the problem of reduction, there is a strong demand for shortening the coating process. That is, in the conventional automobile coating finishing procedure, the electrodeposition coating, the intermediate coating, and the topcoat have been performed by a three-coat three-bake coating method in which the coating is baked after each coating. After baking the electrodeposition coating film, the three coating processes of intermediate coating, base coating and clear coating are performed on the wet coating on a wet basis, and a three-wet coating system for baking these wet coating films collectively is performed. It is required to reduce the number of baking steps and to maintain coating performance equivalent to that of a three-coat film obtained by a conventional three-coat three-bake coating method.

Among the above-mentioned coating film performances, with respect to the impact resistance, particularly the so-called chipping resistance due to the collision of obstacles such as pebbles against a running automobile body, the conventional three-coat, three-bake coating method requires chipping resistance. Although the chipping resistance could be ensured by providing a special intermediate coating film having a property, the conventional intermediate coating material used in the above-described three-wet coating system was adapted to the obtained coating film, It cannot be used because inferior appearance such as inversion occurs and the appearance is inferior, and the coating film obtained by the three-wet coating system has a drawback that the impact resistance and the chipping resistance are low.

Japanese Patent Application Laid-Open No. 62-65765 discloses that a resin layer (so-called chipping-resistant primer layer) having an ability to absorb impact on a coating film is formed during the formation of a multilayer film, especially between an electrodeposition coating film and an intermediate coating film. Is disclosed. However, further incorporation of such a step into the painting process of an automobile body does not meet the market needs for the above-mentioned step saving and cost saving.

On the other hand, in the field of paints, especially in the field of automobile coating, in recent years, water-based paints have attracted attention in order to reduce environmental burden (VOC, etc.). Aqueous paints are generally used to coat a water-based film-forming resin having a hydrophilic functional group and a curing agent, and a pigment-dispersed paste obtained from a pigment and a pigment-dispersed resin with low pollution such as water and alcohol. In a hydrophilic medium. As the pigment-dispersed resin, it is common to use a part of the above-mentioned water-based film-forming resin to be mixed with the water-based paint. In many cases, a resin in which a functional group is introduced is combined with a basic substance such as an amine as a counter ion. However, in the above-mentioned three-wet coating system, when an aqueous coating material containing a pigment-dispersed paste containing a pigment-dispersed resin in which a basic substance such as an amine is used as the counter ion is used, the resulting coating film is yellowed. There was a point.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a three-wet, one-bake coating method aiming at shortening the coating process, reducing costs and reducing environmental impact, and has excellent impact resistance comparable to conventional three-coat films, especially An object of the present invention is to provide a method for forming a multilayer coating film having chipping resistance and capable of forming a multilayer coating film having a good appearance without yellowing.

[0007]

According to the present invention, there is provided a step (I) of forming an electrodeposition coating film by coating an electrodeposition coating material on an object to be coated and then heating and curing the same to form an electrodeposition coating film. A water-based intermediate coating to form an uncured intermediate coating (I)
I), a step of applying an aqueous base coating on the intermediate coating film to form an uncured base coating film (III); and applying a clear coating material on the base coating film, A multi-layer coating comprising a step (IV) of forming a cured clear coating and a step (V) of simultaneously heating and curing the intermediate coating, the base coating and the clear coating to obtain a multi-layer coating In the film forming method, the electrodeposition coating composition comprises particles A containing a resin (a) having a solubility parameter δa and particles B containing a resin (b) having a solubility parameter δb and a curing agent. (1) (δb−δ
The value of a) is 1.0 or more, and (2) the dynamic glass transition temperature of the resin film formed from the particles A in the electrodeposition coating film formed from the electrodeposition paint is −110 to 110. The elongation percentage of the coating film obtained by forming a film with only the particles A is 200% or more. (3) Among the electrodeposition coating films formed from the electrodeposition coating material, the particles B Has a dynamic glass transition temperature of 60 to 150 ° C., and the aqueous intermediate coating and / or the aqueous base coating substantially contains a pigment and a volatile basic substance. And a pigment dispersion paste obtained by previously dispersing a pigment dispersant that is not present. The present invention also provides a multilayer coating film formed by the above-described method for forming a multilayer coating film. Hereinafter, the present invention will be described in more detail.

The method for forming a multilayer coating film of the present invention comprises the steps of: (a) coating an electrodeposition coating material on an object to be coated, and then heating and curing to form an electrodeposition coating film; A water-based intermediate coating to form an uncured intermediate coating (I)
I), a step of applying an aqueous base coating on the intermediate coating film to form an uncured base coating film (III); and applying a clear coating material on the base coating film, A step (IV) of forming a cured clear coating film, and a step (V) of simultaneously heating and curing the intermediate coating film, the base coating film, and the clear coating film to obtain a multilayer coating film.

[0009]Step (I)  In the method for forming a multilayer coating film of the present invention, the step (I)
After applying the electrodeposition paint on the object to be coated,
To form an electrodeposition coating film.

[0010]Electrodeposition paint  The electrodeposition paint is a resin having a solubility parameter of δa.
(A) and the solubility parameter is δb.
Containing resin (b) and particles B containing a curing agent
And (1) the value of (δb−δa) is 1.0 or more.
(2) An electrodeposition coating film formed from the above electrodeposition coating material.
That is, the dynamic glass transition of the resin film formed from the particles A
The temperature is −110 to 10 ° C., and the temperature is
The elongation percentage of the coating film obtained by filming is 200% or more,
(3) Of the electrodeposition coating films formed from the above electrodeposition coating materials,
Dynamic glass transition temperature of resin film formed from particles B
Is 60 to 150 ° C.

The above-mentioned electrodeposition paint forms an electrodeposition coating film having a multilayer structure by using two kinds of resin components which are incompatible with each other. A resin layer having an anti-corrosion property is formed, and a resin layer having an impact resistance (chipping resistance) is formed on the side in contact with the air, so that the corrosion resistance and the impact resistance can be highly compatible.

The above electrodeposition paint has a solubility parameter of δ.
It contains particles A containing the resin (a) which is a, and particles B containing the resin (b) having a solubility parameter of δb and a curing agent. In the present specification, the particles A and the particles B are each prepared as separate emulsions, and both emulsions are mixed in the preparation of the electrodeposition paint, but are separated without fusing each other in the paint. Means those that exist as particles.

In the above electrodeposition paint, the resin (a)
Is greater than or equal to 1.0 (δb−δa) between the solubility parameter δa of the resin (b) and the solubility parameter δb of the resin (b). An electrodeposited coating film having a multilayer structure can be formed by selecting two types of resin components incompatible or hardly compatible with each other, in which the value of (δb−δa) is 1.0 or more. .

It is generally considered that if the difference in solubility parameter between resins is 0.5 or more, the resin loses compatibility and the coating film exhibits a separated structure. However, in the above-mentioned electrodeposition paint, it is necessary to form a coating film structure in which the layers are clearly separated from each other, so that a difference in solubility parameter of at least 1.0 or more is required. If it is less than 1.0, a clear layered coating structure will not be formed in the case of electrodeposition coating, and the level of compatibility between impact resistance, particularly chipping resistance and corrosion resistance, will not be sufficient.

The above-mentioned solubility parameter δ is generally S
It is also called P (solubility parameter) and is a scale indicating the degree of hydrophilicity or hydrophobicity of the resin, and is also an important scale in judging the compatibility between resins. The solubility parameters are numerically quantified based on turbidity measurement methods known to those skilled in the art (KW Suh, DH Clarke, J. Pol).
ymer. Sci. , A-1,5,1671 (196
7)).

Regarding the resin (a) and the resin (b), those having a large solubility parameter, that is, the resin (b) has a higher affinity for a conductive substrate surface such as a metal having a high surface polarity. Since it is high, the electrodeposition coating film formed from the particles B containing the resin (b) is formed on the side in contact with the conductive substrate made of a metal material or the like during heating and curing. On the other hand, the particles A containing the resin (a) move to the air layer side to form a resin layer. Thus, the difference between the solubility parameters of the two resins is considered to be the driving force that causes the separation of the resin layers.

In order to confirm the separation state of the resin layer, a method of visually observing the cross section of the electrodeposited film with a video microscope or observing the cross section with a scanning electron microscope (SEM observation) can be mentioned. Further, in order to identify the resin component constituting each resin layer, for example, a total reflection type Fourier transform infrared photometer (FTIR-ATR) can be used.

The dynamic glass transition temperature of the resin film formed from the particles A containing the resin (a) in the electrodeposition coating film formed from the electrodeposition paint is -110 to 10 ° C. 1
When the temperature exceeds 0 ° C, the flexibility and impact resistance of the coating film obtained from the particles A are inferior. Preferably, -100 to -30
° C. The measurement of the dynamic glass transition temperature is performed by applying an electrodeposition coating on the substrate using the above electrodeposition coating, and then peeling off the electrodeposition coating formed by curing using mercury, and using Leo Vibron (manufactured by Orientec). And a dynamic viscoelasticity measurement device such as a rheometrics dynamic analyzer (manufactured by Rheometrics).

The particle A containing the resin (a) has an elongation of 200% or more in a coating film obtained by forming a film using only the particle A. If it is less than 200%, the resulting coating film will have poor elasticity. Preferably, it is at least 500%. The elongation can be measured according to JIS K6301.

The type of the resin (a) is not particularly limited as long as it is a resin having the above characteristics. For example, a homopolymer of a conjugated diene monomer such as butadiene, isoprene and chloroprene can be used. Or a conjugated diene monomer and ethylene, propylene, ethylidene, norbornene, dicyclopentadiene, 1,4-hexadiene, vinyl acetate, vinyl chloride, styrene, acrylonitrile, isobutylene, (meth) acrylic acid (ester), etc. Random or block copolymer with monomer; Polyurethane thermoplastic elastomer synthesized by polyaddition reaction of diisocyanate and diol; Ester made from dimethyl terephthalate, 1,4-butanediol, poly (tetramethylene) glycol, etc. Exchange and polycondensation reactions Therefore polyester thermoplastic elastomers are synthesized; lactams, dicarboxylic acids, polyether diol as a raw material, and polyamide-based thermoplastic elastomers that are synthesized by transesterification and polycondensation reactions.

In the above electrodeposition paint, the resin (a)
Is an elastomer (rubber) obtained by polymerizing a monomer component composed of a conjugated diene-based monomer in an amount of 50% by weight or more in view of the possibility of expressing an impact resistance level, economy (cost) and versatility. Preferably, there is. If it is less than 50% by weight,
It becomes difficult to form a resin layer having the above-mentioned glass transition temperature and elongation at the time of forming a coating film, and as a result, impact resistance and chipping resistance are reduced. More preferably 60
An elastomer obtained by polymerizing a monomer component composed of a conjugated diene-based monomer in an amount of at least 65% by weight, more preferably at least 65% by weight.

In the molecule of the resin (a), a reactive group or a polar group such as a hydroxyl group, an amino group, a vinyl group, a carboxyl group, a urethane group or a urea group is provided in the middle and / or at the end of the molecular structure. May be included. The reactive group or the polar group is obtained by copolymerizing a monomer component containing a monomer having a reactive group or a polar group when preparing the resin (a), or obtained by copolymerization. It can be introduced into the resin (a) by a known method.

The above copolymerization is preferably carried out in the presence of a radical polymerization initiator. Examples of the radical polymerization initiator include 2,2′-azobisisobutyronitrile,
Azo initiators such as 2,2'-azobis (2,4-dimethylvaleronitrile); and peroxide initiators such as benzoyl peroxide, lauryl peroxide, and t-butyl peroctoate. These initiators are used in an amount of 0.1 to 100 parts by weight of the total polymerizable monomer.
2 to 10 parts by weight, preferably 0.5 to 5 parts by weight are preferred.

When the resin (a) is an oligomer (liquid rubber) having a number-average molecular weight of less than 10,000, the adhesiveness is high and the impact resistance is low as it is. It is necessary to cause a curing reaction at the time of forming a coating film in order to make this manifest. In this case, the hydroxyl number is 2
It is preferable to contain a hydroxyl group so as to be in the range of 0 to 200. If the hydroxyl value is less than 20, poor curing of the coating film is caused, and rubber properties such as sufficient elongation are not exhibited. 2
If it exceeds 00, as a result of excess hydroxyl groups remaining in the coating film after curing, the water resistance may decrease. Furthermore, as a result of an increase in the hardness of the cured coating film, a sufficient elongation cannot be achieved.

When the resin (a) has a high molecular weight having a number average molecular weight of 10,000 or more, if the resin has low tackiness even without being cured and shows sufficient impact resistance, it is particularly cured in the formation of a coating film. There is no need to react. In the case above,
It is not necessary to provide a reactive group and a polar group in the resin structure in advance.

The molecular weight of the resin (a) is not particularly limited, but the number average molecular weight is 1,000.
Those having a range of 0 to 200,000 are preferred. 1,00
If it is less than 0, it is difficult to obtain a coating film having an elongation of more than 200%, even if the crosslinking reaction is effectively performed during the formation of the coating film. If it exceeds 200,000, not only is the handling of the obtained resin emulsified and dispersed difficult due to the high viscosity of the resin solution, but also the film appearance of the obtained electrodeposition coating film is significantly reduced. There is. In addition, due to the high viscosity, layer separation at the time of baking a coating film may be difficult.

The resin (a) forms particles A by emulsification and dispersion in an aqueous medium separately from the resin (b).

The resin (a) may be used as such after introducing a cationic group such as an amino group among the reactive group and the polar group by a urethanization reaction or the like, or in an aqueous medium with a neutralizing agent. May be in a form capable of self-emulsifying and dispersing. Alternatively, it is also possible to separately apply a cationic emulsifier to emulsify and disperse in an aqueous medium. At that time, if necessary, for example, an appropriate amount of a curing agent may be included in the resin and emulsified and dispersed. Examples of the neutralizing agent include inorganic acids such as hydrochloric acid, nitric acid, and phosphoric acid; formic acid, acetic acid, lactic acid, sulfamic acid,
Organic acids such as acetylglycinic acid can be mentioned.

In the above electrodeposition paint, since the hydrophobicity of the whole particles A including the resin (a) increases and a multilayer structure in which the layers are clearly separated can be obtained, the resin (a) is prepared by using a cationic emulsifier. Is preferably emulsified and dispersed in an aqueous medium.

The cationic emulsifier is not particularly limited as long as it contains a cationic group, but preferably has a number average molecular weight of 1,000 to 200,000. If it is less than 1,000, the water resistance of the coating film may be adversely affected. If it exceeds 200,000, layer separation may be hindered due to high viscosity of the system during coating film baking.

In order to ensure the emulsifying dispersibility of the resin (a), the content of the cationic group in the cationic emulsifier, that is, the content of the amino group, ammonium base and sulfonium base in the emulsifier, is equivalent to the amine value. 30 to 15
It is preferably about 0. When it is less than 30, the emulsifying dispersibility with respect to the resin (a) is inferior.
The water resistance of the coating film may be adversely affected.

The amount of the cationic emulsifier to be blended is 1 to 100 parts by weight of the solid content of the resin (a) in terms of solid content.
The range of 0 to 50% by weight is preferred. When the amount is less than 10% by weight, the dispersion stability of the emulsion becomes poor. When the amount exceeds 50% by weight, not only the water resistance of the coating film deteriorates, but also the characteristics such as impact resistance based on the resin (a) are hardly sufficiently exhibited. .

The above cationic emulsifier can be prepared by subjecting the resin main chain to an appropriate reaction by a known method to give a cationic group. The resin skeleton of the cationic emulsifier is not particularly limited,
For example, acrylic resin, epoxy resin, liquid rubber (elastomer), polyurethane, polyether, and modified resins based on these can be used.

When the acrylic resin is used as a resin skeleton, it can be synthesized, for example, by a ring-opening addition reaction between an acrylic copolymer containing a plurality of epoxy groups in a molecule and an amine. That is, with respect to an epoxy group-containing acrylic resin obtained by copolymerizing an acrylic monomer having an epoxy group such as glycidyl (meth) acrylate with another monomer, all of the epoxy groups are converted to amines. The ring is opened by the reaction with the above to obtain a cationic acrylic resin.

The above amines are not particularly restricted but include, for example, butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine, N, N-dimethylethanol. Primary, secondary or tertiary amine salts such as amine salts can be mentioned. Further, a secondary amine containing a ketimine block primary amino group such as aminoethylethanolamine methyl isobutyl ketimine can also be used.
These amines need to be reacted in at least an equivalent amount to the epoxy ring in order to open all the epoxy rings.

The above-mentioned cationic acrylic resin can also be obtained by a method of directly synthesizing by copolymerizing an acrylic monomer having an amino group with another monomer. Examples of the acrylic monomer having an amino group include N, N-dimethylaminoethyl (meth) acrylate and N, N-di-t-butylaminoethyl (meth) acrylate.

The other monomer to be copolymerized with the acrylic monomer having an epoxy group or the acrylic monomer having an amino group is not particularly limited. Examples thereof include a hydroxyl group-containing acrylic monomer, Other acrylic monomers and non-acrylic monomers can be exemplified. The hydroxyl group-containing acrylic monomer is preferably used because it can improve curing reactivity.

In the case where the epoxy resin has a resin skeleton, a cationic group can be introduced by modifying the epoxy group in the resin in the same manner as described above. For those having a resin skeleton of the above-mentioned liquid rubber (elastomer), polyurethane and polyether, a hydroxyl group, a carboxyl group, an epoxy group, etc. existing at the molecular terminal and / or in the middle of the molecular structure are reacted with a urethane-forming reaction of an amine or A cationic group can be introduced by an addition reaction.

The cationic emulsifier may be a long-chain alkyl group such as a stearyl group, a dodecyl group or an octyl group for introducing a primary hydroxyl group for imparting curing reactivity or improving the adsorptivity to the resin (a). May be introduced. These include a secondary amine having a hydroxy group and a secondary amine having a long-chain alkyl group with respect to the functional group in the main chain.
The reaction can be carried out by reacting a secondary amine, or can be introduced by copolymerization using a monomer having such a group.

In the above cationic emulsifier, the above cationic group plays a role as a hydrophilic group. Furthermore, the adsorption interaction with the resin (a) can be ensured by the flexible main chain portion and the hydrophobic portion such as an alkyl group and a benzene structure existing in the cationic emulsifier. The cationic emulsifier can be dissolved or dispersed in an aqueous medium as it is.

The particles A may contain a curing agent. As the curing agent, isocyanate curing agent,
Melamine curing agents, amide curing agents and the like can be mentioned. Preferably, it is a blocked polyisocyanate. Examples of the polyisocyanate which is a raw material of the blocked polyisocyanate include aliphatic diisocyanates such as hexamethylene diisocyanate, tetramethylene diisocyanate, and trimethylhexamethylene diisocyanate; isophorone diisocyanate, and 4,4'-
Alicyclic polyisocyanates such as methylenebis (cyclohexyl isocyanate); aromatic diisocyanates such as 4,4'-diphenylmethane diisocyanate, tolylene diisocyanate, and xylylene diisocyanate, and polymers thereof. The blocked polyisocyanate can be obtained by blocking these with an appropriate sealing agent.

Examples of the sealing agent include monohydric alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenol carbinol, and methylphenyl carbinol. Cellosolves such as ethylene glycol monohexyl ether and ethylene glycol mono-2-ethylhexyl ether; phenols such as phenol, para-t-butylphenol and cresol; dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime and cyclohexanone Oximes such as oximes; lactams such as ε-caprolactam and γ-butyrolactam. Oximes and lactams are preferred from the viewpoint of resin curability since they dissociate at low temperatures. The blocking ratio by the above sealing agent is preferably set to 100% in order to secure the storage stability of the paint. The polyisocyanate and the sealant may be used alone,
More than one species may be used in combination. Further, the obtained blocked polyisocyanate may be used in combination of two or more kinds depending on circumstances such as adjustment of physical properties of the coating film and degree of curing.

In the step (I), the resin (a)
When the resin layer composed of the particles A containing a curing agent is cured, at least one solubility parameter (δ
i) is an intermediate value between the solubility parameter δa of the resin (a) and the solubility parameter δb of the resin (b), that is, δa
It is preferable to set <δi <δb. This makes it possible to distribute and dissolve the blocked polyisocyanate in each layer after the two-layer separation, and to achieve both curing of the layer containing the resin (a) and simultaneous curing of the layer containing the resin (b). Therefore, it is possible to improve the interlayer adhesion in the multilayer film and further improve the multilayer appearance after overcoating. Further, as means for accelerating the distribution and dissolution of the blocked polyisocyanate in the resin layer composed of the particles A containing the resin (a), a blocked polyisocyanate partially having an unblocked isocyanate group and the resin are used. The hydroxyl group of (a) is reacted in advance, and the resin (a) and the curing agent are separated during the layer separation accompanying the simultaneous curing of the layer containing the resin (a) and the layer containing the resin (b). It is also possible to devise such a transition.

The mixing ratio of the above-mentioned blocked polyisocyanate to the resin (a) varies depending on the degree of crosslinking required for the purpose of use of the cured coating film and the like. The solid content is preferably in the range of 10 to 50% by weight based on 100 parts by weight of the solid content of the resin (a). If the amount is less than 10% by weight, poor coating properties may be caused, resulting in poor physical properties of the coated film such as mechanical strength, and a poor appearance such as a coating film being damaged by a paint thinner during overcoating. If it exceeds 50% by weight, the curing proceeds excessively, which may result in poor coating film properties such as impact resistance.

The dynamic glass transition temperature of the resin film formed from the particles B containing the resin (b) in the electrodeposition coating film formed from the electrodeposition paint is 60 to 150 ° C. 60
When the temperature is lower than 0 ° C, the difference from the solubility parameter δa of the resin (a) cannot be set to 1.0 or more, and the obtained coating film is inferior in corrosion resistance. If the temperature exceeds 150 ° C., the resulting coating film may be too hard and cracks may occur. Preferably, it is 80 to 140 ° C. The measurement of the dynamic glass transition temperature can be performed according to the method described above.

The resin (b) is preferably a cation-modified epoxy resin from the viewpoint of exhibiting excellent rust-preventive properties with respect to a conductive substrate. The cation-modified epoxy resin can be produced by opening the epoxy ring in the starting material resin molecule by reaction with an amine such as a primary amine, a secondary amine or a tertiary amine salt. The starting material resin is bisphenol A, bisphenol F, bisphenol S,
It is preferably a polyphenol polyglycidyl ether type epoxy resin which is a reaction product of a polycyclic phenol compound such as phenol novolak and cresol novolak with epichlorohydrin. Examples of other starting material resins include oxazolidone ring-containing epoxy resins described in JP-A-5-306327. This epoxy resin is obtained by reacting a diisocyanate compound or a bisurethane compound obtained by blocking the NCO group of a diisocyanate compound with a lower alcohol such as methanol or ethanol, and epichlorohydrin.

The starting material resin is prepared by preparing a bifunctional polyester polyol, polyether polyol, bisphenol,
The chain can be extended by using a basic carboxylic acid or the like. Similarly, before the ring-opening reaction of the epoxy ring by the amines, 2-ethylhexanol, nonylphenol, ethylene glycol, A monohydroxy compound such as mono-2-ethylhexyl ether and propylene glycol mono-2-ethylhexyl ether can be added and used.

Examples of the amines include those exemplified for the cationic emulsifier. As a method for introducing a cationic group into the epoxy resin, it is also preferable to modify the epoxy ring to a sulfonium salt according to the production method described in JP-A-11-209663.

The number average molecular weight of the cation-modified epoxy resin is preferably in the range of 1,500 to 5,000.
If it is less than 1,500, physical properties such as solvent resistance and corrosion resistance of the cured film may be poor. When it exceeds 5,000, not only is the viscosity of the resin solution difficult to control and the synthesis is difficult, but also the viscosity of the obtained resin becomes high, and handling in operation such as emulsification and dispersion sometimes becomes difficult. Furthermore, the flow property during heating and curing is poor, and the appearance of the coating film may be significantly impaired.

The resin (b) has a hydroxyl value of 50.
It is preferable to design the molecule so as to be in the range of ~ 250. If the hydroxyl value is less than 50, poor curing of the coating film may be caused. On the other hand, if it exceeds 250, excessive hydroxyl groups may remain in the coating film after curing, and the water resistance may decrease.

The particles B containing the resin (b) contain a curing agent. The type of the curing agent is not particularly limited as long as the resin component can be cured at the time of heating, and examples thereof include those exemplified above. Among them, a blocked polyisocyanate suitable as a curing agent for the electrodeposition resin is exemplified. Examples of the amount of the curing agent include those described above.

The above resin (b) is used together with the above curing agent,
Emulsified and dispersed in water as it is,
Alternatively, the resin is neutralized with an amount of neutralizing agent capable of neutralizing amino groups in each resin, and emulsified and dispersed in water as a cationized emulsion. In preparing the emulsion, the cationic emulsifier exemplified above can be used. Examples of the method of the emulsification and dispersion include those described above.

The above electrodeposition paint can be prepared by mixing the particles A and B obtained as described above. The mixing ratio of the resin (a) constituting the particles A and the resin (b) constituting the particles B is preferably 5/95 to 70/30 by weight based on the solid content. Outside the above range, the electrodeposition coating, the cured coating film after baking does not have a multilayer structure, the resin having a higher mixing ratio forms a continuous phase, and the lower resin forms a dispersed phase, a sea-island structure. (Or a microdomain structure). Further, even in the case of a layered structure, the thickness of one of the multiple layers becomes extremely thin, so that either impact resistance (chipping resistance) or corrosion resistance is extremely poor, which is not preferable. More preferably, 10/90 to 6
It is in the range of 0/40.

The dry thickness of the resin layer formed from the particles A is preferably 1 to 20 μm. If it is less than 1 μm, the resulting coating film cannot be expected to have impact absorption resistance. 2
When the thickness exceeds 0 μm, the surface roughness increases, and the appearance of the coating film deteriorates. More preferably, it is 3 to 15 μm.
The resin layer formed from the particles B preferably has a dry film thickness of 5 to 40 μm in order to secure the rust prevention, the appearance of the coating, and the concealing property required for the conventional electrodeposition coating. If it is less than 5 μm, the corrosion resistance of the coating film is insufficient. 40 μm
Exceeding the surface roughness will increase the surface roughness, resulting in a decrease in the appearance of the coating film, and the occurrence of coating defects such as armpits will be remarkable. More preferably, it is 10 to 30 μm.

The above electrodeposition paint usually contains a pigment. The pigment is not particularly limited as long as it is commonly used in paints, for example, azochelate pigments, insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments,
Organic color pigments such as dioxane pigments, quinacridone pigments, isoindolinone pigments, metal complex pigments;
Inorganic coloring pigments such as yellow iron oxide, red iron oxide, carbon black, titanium dioxide, and graphite; calcium carbonate,
Extenders such as barium sulfate, kaolin, aluminum silicate (clay), and talc; and rust preventive pigments such as aluminum phosphomolybdate, lead silicate, lead sulfate, zinc chromate and strontium chromate. Of these, carbon black, titanium dioxide, aluminum silicate (clay) and aluminum phosphomolybdate are particularly important as pigments for forming a multilayer cured film after electrodeposition coating. The above titanium dioxide is most suitable for an electrodeposition coating film because it has high concealing properties as a coloring pigment and is inexpensive. The above pigments can be used alone, but a plurality of pigments are generally used according to the purpose. The pigment can be previously dispersed in a generally used cationic pigment dispersion resin, and after adjusting the pigment dispersion paste, an appropriate amount can be blended in preparing the electrodeposition coating.

The mixing amount of the pigment is a ratio P / V of the weight (V) of all the vehicle components other than the pigment in the electrodeposition paint to the total pigment weight (P), and is in the range of 1/10 to 1/3. It is preferable that The term "all vehicle components other than the pigments" means all solid components other than the pigments constituting the paint. If the ratio is less than 1/10, the shortage of pigments causes the barrier properties of the coating film against corrosion factors such as moisture to be excessively reduced, so that a practical level of corrosion resistance may not be exhibited. If it exceeds 1/3, the viscosity may be increased at the time of curing due to an excessive amount of the pigment, and the flowability may be reduced, and the appearance of the coating film may be significantly deteriorated.

The electrodeposition paint may contain an appropriate amount of additives such as a rust preventive and a surfactant (antifoaming agent). As the rust inhibitor, rare earth metal organic acid salts such as zinc, cerium, neodymium, and praseodymium are water-soluble and easy to use because of recent market demands for eliminating harmful heavy metals such as lead. . For example, zinc acetate, cerium acetate, neodymium acetate, etc. may be blended when preparing the particles B, and an appropriate amount may be added in the form of inclusion or adsorption by a resin emulsion.

The above electrodeposition paint has a solid content of 15 to 25.
It is preferable to adjust so as to be in the range of% by weight. The solid content concentration is adjusted using an aqueous medium, for example, water alone or a mixture of water and a hydrophilic organic solvent. Also, a small amount of additives may be introduced into the electrodeposition paint. Examples of the additive include an ultraviolet absorber, an antioxidant, a surfactant, a coating film surface smoothing agent, and a curing accelerator such as an organotin compound.

[0059]Electrodeposition coating method  The method for forming an electrodeposition coating film in the step (I) includes the step of:
Step (1) of obtaining an electrodeposited film by electrodeposition coating on an object to be coated;
And, the obtained electrodeposited film is cured by heating,
It comprises a step (2) of obtaining a coating film. The above process
In general, the electrodeposition coating of (1) is a conductive
Connect the cathode (cathode electrode) terminal to the conductive base
Bath temperature 15-35 ° C, load voltage 100-400V
Can be done in the matter.

The electrodeposited film obtained in the above step (1) is oriented according to the solubility parameter unique to each resin and undergoes layer separation by heating in the step (2) to form a layer formed from the particles A. Is the side directly in contact with the air, and the layer formed of the particles B becomes the electrodeposition cured film having a multilayer structure in which the layer is in direct contact with the object to be coated. The heating in the step (2) is generally performed at 140 to 200 ° C., preferably 1 to 200 ° C.
Performed at 60-180 ° C. for 10-30 minutes.

In order to improve the layer separation property, preheating may be performed after the step (1). By performing the preheating at the same temperature as the heating in the step (2), the preheating can be performed continuously with the step (2). However, in the present invention, the preheating is performed at a temperature lower than the curing temperature of the electrodeposition paint. Is preferred. As a result, layer separation can be improved without impairing the appearance of the coating film. The heating temperature in this case is, for example, 60 to 130 ° C., and the heating time varies depending on the heating temperature or the like, but is about 1 to 10 minutes. As the heating method in the above steps (1) and (2), there are a method of putting a coated object into a heating facility adjusted to a target temperature from the beginning, and a method of increasing the temperature after putting the coated object.

The object to be coated is not particularly limited, and examples thereof include iron, copper, aluminum, tin, zinc and the like; alloys and castings containing these metals. In particular,
Automobile bodies and parts, such as passenger cars, trucks, motorcycles, and buses. It is particularly preferable that these metals are subjected to a chemical conversion treatment with a phosphate, a chromate or the like before the electrodeposition coating is performed.

In the above electrodeposition paint, since the resin (a) is emulsified and dispersed separately from the resin (b), the paint can be stabilized without considering the compatibility between the resin (a) and the resin (b). Property can be ensured. JP-A-5-230402
JP, JP-A-7-207196 and JP-A-9-207196
As described in JP-A-208865, when a polar functional group such as an epoxy group is introduced into the resin (a) in order to ensure compatibility between the resin components, the elongation percentage of the resulting coating film and Although there is a problem that the modulus of elasticity is reduced, such a modification is not required in the above-mentioned electrodeposition paint, so that the electrodeposition coating film can be imparted with high impact absorption performance.

[0064]Step (II)  In the step (II), the electrodeposition formed as described above is performed.
Apply an aqueous intermediate coating on the coating film,
It forms a coating film. Method of forming multilayer coating film of the present invention
In the method, in the aqueous solution used in the above step (1),
Coating paint and water-based used in the above step (2)
At least one of the base paints comprises a pigment and a volatile
A pigment dispersant substantially containing no basic substance is dispersed in advance.
Containing the pigment dispersion paste obtained by
You.

[0065]Waterborne intermediate coating  The above-mentioned waterborne intermediate coating composition contains a pigment and a volatile basic substance.
Face obtained by pre-dispersing a pigment dispersant substantially free of
It is preferable that it contains a filler dispersion paste
No. In this specification, volatile basic substances are substantially
Does not include volatile bases in the solids of the pigment dispersant
No or no more than 3% by weight
Is meant.

In the present invention, the volatile base substance in the intermediate coating film formed from the aqueous intermediate coating composition can be obtained by substantially eliminating the volatile basic substance in the pigment dispersant. The amount of the active substance is reduced, and yellowing of the obtained multilayer coating film can be suppressed. Therefore, when the content exceeds 3% by weight, the obtained multilayer coating film turns yellow and the finished appearance is inferior. Preferably, no volatile basic substance is contained at all, that is, no amine-neutralized pigment-dispersing resin which is conventionally generally used is used.

The volatile basic substance has a boiling point of 30.
It means a basic substance at 0 ° C. or lower. For example,
Mention may be made of inorganic and organic nitrogen-containing basic substances. Examples of the inorganic basic substance include, for example, ammonia, and examples of the organic basic substance include:
C1-C20 linear or branched alkyl group-containing primary to tertiary amines such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, isopropylamine, diisopropylamine and dimethyldodecylamine; monoethanolamine; C1-20 linear or branched hydroxyalkyl group-containing primary or tertiary amines such as diethanolamine and 2-amino-2-methylpropanol; C1-20 linear or such as dimethylethanolamine and diethylethanolamine; Primary to tertiary amines containing a branched alkyl group and a C1-20 linear or branched hydroxyalkyl group; C1-C1 such as diethylenetriamine and triethylenetetramine;
20 substituted or unsubstituted linear polyamines; morpholine, N
C1 such as -methylmorpholine, N-ethylmorpholine, etc.
20 substituted or unsubstituted cyclic monoamines; piperazine, N
-Methylpiperazine, N-ethylpiperazine, N, N-
Examples thereof include amines such as C1-20 substituted or unsubstituted cyclic polyamines such as dimethylpiperazine.

The pigment dispersant is a resin having a structure containing a pigment affinity portion and a hydrophilic portion, and does not substantially contain the volatile basic substance. Examples of the pigment affinity portion and the hydrophilic portion include, for example, nonionic, cationic and anionic functional groups,
One molecule of the pigment dispersant may have two or more of the above functional groups. Examples of the nonionic functional group include a hydroxyl group, an amide group, and a polyoxyalkylene group. Examples of the cationic functional group include an amino group, an imino group, and a hydrazino group. In addition, examples of the anionic functional group include a carboxyl group, a sulfonic group, and a phosphate group. Such a pigment dispersant can be manufactured by a method well known to those skilled in the art.

The pigment dispersant is not particularly limited as long as it does not substantially contain a volatile basic substance, but is preferably a pigment dispersant that can efficiently disperse the pigment with a small amount of the pigment dispersant. .

As the pigment dispersant, commercially available pigment dispersants can be used. Examples of the commercially available products include Disperbyk 190 and Disperby.
k 182, Disperbyk 184 (both by Big Chemie), EFKAPOLYMER 4550
(Manufactured by EFKA), Solsperse 27000, Solsperse 41000, Solsperse 53095 (all manufactured by Abisia), and the like.

The number average molecular weight of the pigment dispersant is 100
It is preferably from 0 to 100,000. If it is less than 1000, the dispersion stability is not sufficient, and if it exceeds 100,000, the viscosity is too high and handling becomes difficult. More preferably,
2000 to 50,000, more preferably 4000 to 5
It is ten thousand.

The pigment dispersant is mixed and dispersed together with the pigment according to a known method to obtain a pigment dispersion paste. The mixing ratio of the pigment dispersant in the pigment dispersion paste,
The content is preferably 1 to 20% by weight based on the solid content of the pigment dispersion paste. If it is less than 1% by weight, the pigment cannot be dispersed stably, and if it exceeds 20% by weight, the physical properties of the coating film may be poor. Preferably, it is 5 to 15% by weight.

The pigment is not particularly limited as long as it is a pigment used in a usual intermediate coating, and examples thereof include those exemplified for the electrodeposition coating. From the viewpoint of improving weather resistance and securing hiding properties, it is preferable that the pigment is a coloring pigment. In particular, titanium dioxide is more preferable since it is excellent in white color hiding properties and is inexpensive.

As the above-mentioned pigment, a standard gray intermediate paint containing carbon black and titanium dioxide as main pigments may be used, or a combination of a top paint and set gray or various colored pigments having the same lightness or hue may be used. A so-called color intermediate coating can also be used.

The pigment preferably has a weight ratio (PWC) of the pigment to the total weight of the pigment and the resin solids (PWC) of 10 to 60% by weight in the aqueous intermediate coating composition. If it is less than 10% by weight, the concealing property may be reduced due to insufficient pigment. If the amount exceeds 60% by weight, the viscosity at the time of curing is increased due to an excessive amount of the pigment, whereby the flowability is reduced and the appearance of the coating film may be reduced.

The aqueous intermediate coating composition can be prepared by mixing the pigment dispersion paste, a film-forming resin and a curing agent. The content of the pigment dispersant in the aqueous intermediate coating composition is 0.5 to 1 based on the solid content when a pigment dispersant containing substantially no basic substance is used.
It is preferably 0% by weight. If the amount is less than 0.5% by weight, the dispersion stability of the pigment may be poor due to a small amount of the pigment dispersant. If it exceeds 10% by weight, the properties of the coating film may be poor. Preferably, it is 1 to 5% by weight.

The film-forming resin is not particularly restricted but includes, for example, acrylic resins, polyester resins, alkyd resins, epoxy resins, urethane resins and the like.
From the viewpoint of pigment dispersibility and workability, a combination of an acrylic resin and / or a polyester resin and an amino resin and / or a blocked isocyanate resin as a curing agent is preferable. The film-forming resin and the curing agent are each 1
Although only the species may be used, two or more species may be used to balance the coating film performance.

The film-forming resin may be water-soluble, or may be emulsified and dispersed by applying a dispersing agent such as a dispersing resin or a surfactant to stabilize the aqueous intermediate coating composition. It can exist. The aqueous intermediate coating composition may further contain additives such as an ultraviolet absorber, an antioxidant, an antifoaming agent, a surface conditioner, and an anti-bake agent.

[0079]Intermediate coating film formation method  The above-mentioned aqueous intermediate paint is a coating material on which the above-mentioned electrodeposition coating film is formed.
A coating is applied on the garment to form an uncured intermediate coat.
The method of applying the aqueous intermediate coating is not particularly limited.
For example, air static called "react gun"
Electrospray; commonly known as “micro / micro (μμ)
"", "Micro (μ) bell", "meta bell", etc.
By using a rotary atomizing electrostatic coating machine
be able to. Preferably, a rotary atomizing type electrostatic coating machine or the like
It is a method using.

The dry film thickness of the intermediate coating film varies depending on the application, but is preferably 5 to 50 μm. If the amount exceeds the upper limit, the sharpness may be reduced, or sagging may occur during coating or baking may occur. If the amount is below the lower limit, the appearance may be reduced.

In the present invention, to form a coating film in an uncured state with respect to the aqueous intermediate coating, the aqueous base coating and the clear coating means that the aqueous intermediate coating, the aqueous base coating and the clear coating are wet-on-wet in this order. Means painting. In the present specification, the term “uncured” is a concept including, for example, a state after preheating. The preheating is, for example, a step of leaving or heating at room temperature to less than 100 ° C. for 1 to 10 minutes after application. For the purpose of obtaining a good finished appearance, it is preferable to perform preheating after applying the aqueous intermediate coating and after applying the aqueous base coating.

[0082]Step (III)  The above-mentioned step (III) includes the step of forming
Apply an aqueous base paint on the cured
It forms a cured base coating.

[0083]Water-based paint  In the present invention, a pigment may be used as the aqueous base paint.
And a pigment dispersant substantially free of volatile basic substances
The pigment dispersion paste obtained by pre-dispersing
Can be used. The volatile basic substance
Quality, and as the pigment dispersant, the aqueous intermediate coating
Examples of the materials can be given.

As the pigment contained in the above-mentioned aqueous base paint, the above-mentioned colored pigments and extender pigments can be used. In addition, a glitter pigment can be blended and used as a metallic base paint, or a glitter pigment can be blended. Instead, a color pigment such as red, blue or black and / or an extender may be blended and used as a solid base paint.

The glitter pigment is not particularly limited.
For example, a non-colored or colored metallic glitter such as a metal or an alloy, and a mixture thereof, interference mica powder, colored mica powder, white mica powder, graphite, and a colorless colored flat pigment can be used. A non-colored or colored metallic brilliant material such as a metal or an alloy, and a mixture thereof are preferable since a coating film having excellent dispersibility and high transparency can be formed. Specific examples of the metal include aluminum, aluminum oxide, copper, zinc, iron, nickel, and tin.

The shape of the brilliant pigment is not particularly limited.
Further, it may be colored, but for example, has an average particle size (D ₅₀ ) of 2 to ₅₀ μm and a thickness of 0.1 to 5 μm
Is preferable. Average particle size 10-35μ
Those having a range of m are excellent in glitter and are more preferable.

One or more of the above-mentioned pigments can be used. One or more of the pigments and extender pigments and, if necessary, the flat pigments and the glittering pigments are used in combination. Can be used.

In the case of the above-mentioned aqueous base paint, the mixing ratio of the pigment dispersant in the pigment dispersion paste is preferably 3 to 50% by weight based on the solid content of the pigment dispersion paste. If it is less than 3% by weight, the pigment cannot be dispersed stably, and if it exceeds 50% by weight, the physical properties of the resulting coating film may be reduced.

The aqueous base paint can be prepared by mixing the pigment dispersion paste, a film-forming resin and a curing agent. The pigment concentration (PWC) in the aqueous base paint, including the above-mentioned brilliant pigment and all other pigments, is as follows:
It is generally 0.1 to 50% by weight, preferably 0.1 to 50% by weight.
5 to 40% by weight, more preferably 1 to 30% by weight
It is. If it exceeds 50% by weight, the appearance of the coating film will be reduced.

The content of the pigment dispersant in the aqueous base paint may be 1 to 20% by weight based on the solid content when a pigment dispersant containing substantially no basic substance is used.
It is preferable that If the amount is less than 1% by weight, the dispersion stability of the pigment may be poor due to a small amount of the pigment dispersant. If it exceeds 20% by weight, the physical properties of the resulting coating film may be reduced.

The film-forming resin, curing agent and other additives used in the above-mentioned aqueous base paint are not particularly restricted but include those described above. From the viewpoint of pigment dispersibility and workability, a combination of an acrylic resin and / or a polyester resin with a melamine resin is preferred. Also,
Regarding the method of preparing the aqueous base coating, the method exemplified for the above-mentioned aqueous intermediate coating can be used.

[0092]Base coating film formation method  The base paint is the uncured material formed as described above.
Apply to form an uncured base coating on top of the coating
You. As the above coating method, apply an aqueous intermediate coating
The method exemplified at that time can be mentioned. Base coating
When applying paint to automobile bodies, etc.,
Multistage coating with air electrostatic spray
Coating, preferably in two stages, or air
Combine the electrostatic spray with the above-mentioned rotary atomizing type electrostatic coating machine.
It is preferable to carry out by a combined coating method.

The dry film thickness of the base coating film varies depending on the application, but is preferably from 5 to 35 μm. Exceeding the upper limit may result in poor clarity and / or inconveniences such as unevenness and flow during coating. Below the lower limit, color unevenness may occur.

[0094]Step (IV)  The step (IV) includes the step of forming the uncured
Apply a clear paint on the base coating film to
Form a rear coating.Clear paint  The clear coating film contains a glitter pigment as a base paint.
Due to glitter pigments when using tallic base paint
Smoothes the unevenness and flicker of the base coating film.
It is also formed to protect the base coating.
You. There is no particular limitation on the clear paint, for example,
For example, from film-forming resins, curing agents and other additives.
Can be mentioned.

The above-mentioned film-forming resin is not particularly restricted but includes, for example, acrylic resins, polyester resins, epoxy resins, urethane resins and the like, which are combined with a curing agent such as an amino resin and / or a blocked isocyanate resin. Used. From the viewpoint of transparency or acid etching resistance, etc., a combination of an acrylic resin and / or a polyester resin and an amino resin, or a carboxylic acid.
It is preferable to use an acrylic resin and / or a polyester resin having an epoxy curing system.

The above-mentioned clear paint contains a viscosity control agent as an additive in order to apply it in an uncured state after the above-mentioned base paint is applied and to prevent break-in, inversion or sagging between layers. Is preferred. The amount of the viscosity control agent to be added is 0.01 to 10 parts by weight, preferably 0.1 to 100 parts by weight, based on 100 parts by weight of the resin solid content of the clear paint.
The amount is from 02 to 8 parts by weight, more preferably from 0.03 to 6 parts by weight. If the amount is more than 10 parts by weight, the appearance deteriorates. If the amount is less than 0.1 part by weight, the effect of controlling the viscosity cannot be obtained, which causes problems such as sagging.

The form of the clear paint may be any of an organic solvent type, an aqueous type (water-soluble, water-dispersible, emulsion), a non-aqueous dispersion type, and a powder type. An adjusting agent or the like can be used.

[0098]Clear coating film forming method  The preparation method and coating method of the above clear paint are as follows.
It can be done according to the following method. Above clear coating
The dry film thickness varies depending on the application, but is 10 to 70 μm
It is. When the dry film thickness exceeds the upper limit, the sharpness deteriorates.
Troubles such as unevenness and flow during painting
Yes, if it is below the lower limit, the appearance may be reduced.

[0099]Step (V)  In the step (V), the intermediate coating film, the base
Heat-curing simultaneously the clear coating film and the clear coating film,
Obtain a multilayer coating. The temperature for heat curing is 1
Perform at 10 to 180 ° C, preferably 120 to 160 ° C
By doing so, it is possible to obtain a cured coating film with a high degree of crosslinking.
You. When the temperature exceeds 180 ° C., the coating film becomes hard and brittle,
If the temperature is lower than ℃, curing is not sufficient. Curing time depends on curing temperature
It changes more, but it takes 10-60 minutes at 120-160 ° C
Appropriate.

The multilayer coating film obtained by the coating film forming method of the present invention has a thickness of usually 30 to 300 μm, preferably 5 to 300 μm.
0 to 250 μm. If it exceeds 300 μm, the physical properties of the film such as a thermal cycle decrease, and if it is less than 30 μm, the strength of the film itself decreases.

The electrodeposition paint applied in the above step (I) is as follows:
Since the functions are shared by forming a multilayer coating film, an electrodeposition coating film having both high impact resistance (chipping resistance) and corrosion resistance as coating film performance can be obtained. Therefore,
The intermediate coating, the base coating and the clear coating are applied on the electrodeposition coating film obtained in the above step (I) by the above steps (II) to (IV) in a wet-on-wet manner. In the so-called 3 wet coating in which the intermediate coating film, the base coating film, and the clear coating film are simultaneously baked, a conventional three-coat, 3 baking method of baking and curing the conventional electrodeposition coating, the intermediate coating, and the top coating respectively. A multilayer coating film having excellent corrosion resistance and impact resistance (chipping resistance) comparable to a coating film can be obtained.

Further, the above step (II) and the above step (I
In II), a pigment dispersing paste using a pigment dispersing agent substantially free of a volatile basic substance is added to the aqueous intermediate coating composition and the aqueous base coating composition, so that the coating film turns yellow. Without this, an excellent finished appearance can be obtained. Further, the three-wet one-bake coating method of the present invention can omit the baking step of the intermediate coating paint as compared with the conventional three-coat two-bake method, so that the process can be shortened, the cost can be reduced, and the energy consumption can be reduced. A new coating system aiming at reduction and environmental load reduction can be constructed.

[0103]

EXAMPLES The present invention will be described in detail with reference to specific examples, but the present invention is not limited to the following examples. Parts and% mean parts by weight and% by weight, respectively.

[0104]Preparation Example 1 (Preparation of electrodeposition paint)  1-1 (Preparation of blocked polyisocyanate curing agent)
Put 222 parts of isophorone diisocyanate in the reaction vessel,
After diluting with 50 parts of methyl isobutyl ketone,
Add 0.2 parts of urate, and after heating to 50 ° C, add methyl
Ethyl ketoxime 17 parts, content temperature exceeds 70 ℃
Not added. And by the infrared absorption spectrum
70 ° C. until the absorption of isocyanate residues is substantially eliminated
For 1 hour, then dilute with 10 parts of n-butanol
The desired blocked polyi with a solid content of 80%
Socyanate (solubility parameter δi = 11.8)
Obtained.

1-2 (Preparation of Blocked Polyisocyanate Curing Agent) A trimer of hexamethylene diisocyanate 19 was placed in a reaction vessel equipped with a stirrer, a nitrogen inlet tube, a cooling tube and a thermometer.
Add 9 parts, dilute with 39 parts of methyl isobutyl ketone, add 0.2 parts of butyltin laurate, raise the temperature to 50 ° C., and then add 44 parts of methyl ethyl ketoxime and 87 parts of ethylene glycol mono 2-ethylhexyl ether. Material temperature was added so as not to exceed 70 ° C. Then, the mixture was kept at 70 ° C. for 1 hour until the absorption of isocyanate residues substantially disappeared according to the infrared absorption spectrum, and then diluted with 43 parts of n-butanol to obtain the desired blocked polyisocyanate (solid solubility: 80%). Parameter δ
i = 10.7).

1-3 (Preparation of Cation-Modified Epoxy Resin Emulsion [Particle B]) In a reaction vessel equipped with a stirrer, a decanter, a nitrogen inlet tube, a thermometer and a dropping funnel, a bisphenol A type epoxy resin having an epoxy equivalent of 188 (commercially available) Name DER-331
J, Dow Chemical Company) 2,400 parts and methanol 14
1 part, 168 parts of methyl isobutyl ketone and 0.5 part of dibutyltin dilaurate were charged and stirred at 40 ° C. to dissolve uniformly, and then 2,4- / 2,6-tolylene diisocyanate (80/20 weight ratio) When 320 parts of the mixture were dropped over 30 minutes, heat was generated and the temperature rose to 70 ° C. 5 parts of N, N-dimethylbenzylamine was added thereto, the temperature in the system was raised to 120 ° C., and the reaction was continued at 120 ° C. for 3 hours until the epoxy equivalent reached 500 while distilling off methanol. Further, 644 parts of methyl isobutyl ketone, 341 parts of bisphenol A, 413 parts of 2-ethylhexanoic acid
After maintaining the temperature in the system at 120 ° C. and reacting until the epoxy equivalent becomes 1070, the temperature in the system is 1
Cooled to 10 ° C. Next, a mixture of 241 parts of diethylenetriaminediketimine (73% solids in methyl isobutyl ketone solution) and 192 parts of N-methylethanolamine was added and reacted at 110 ° C. for 1 hour to obtain a cation-modified epoxy resin. This resin had a number average molecular weight of 2,100 and a hydroxyl value of 160. Measurements of infrared absorption spectrum and the like confirmed that the resin had an oxazolidone ring (absorption wave number: 1750 cm ^-1 ). Further, the solubility parameter δb was 11.4.

Into the cation-modified epoxy resin thus obtained, 1834 parts of the blocked polyisocyanate curing agent prepared in Preparation Example 1-1 (38 wt% of blocked polyisocyanate with respect to 100 parts by weight of the cation-modified epoxy resin) ), 90 parts of acetic acid, 2 parts of zinc acetate and 2 parts of cerium acetate as rust preventive agents were added, and the mixture was diluted with ion-exchanged water to a nonvolatile content of 32%. An aqueous emulsion mainly composed of a modified epoxy resin (hereinafter referred to as E1) was obtained.

1-4 (Preparation of Cationic Dispersant for Resin (a)) A reaction vessel equipped with a stirrer, a cooler, a decanter, a nitrogen inlet tube and a thermometer was charged with methyl isobutyl ketone 114.
After heating to 50 ° C., 75 parts of 4,4′-diphenylmethane diisocyanate and 0.1 part of dibutyltin dibrauronate as a reaction catalyst were charged, and the mixture was charged under a nitrogen atmosphere.
The temperature was kept at 0 ° C. Further, 110 parts of R-15HT (1,4-polybutadiene-α, ω-diol, number average molecular weight = 1,200, hydroxyl value = 103, manufactured by Idemitsu Petrochemical Co., Ltd.) was added dropwise from the dropping funnel over 30 minutes. Stirring was continued for minutes. Next, N-methyldiethanolamine 2
4 parts, 20 parts of ethylene glycol mono-2-ethylhexyl ether and 36 parts of a solution of diethylenetriaminediketimine in methyl isobutyl ketone (solid content 73%) were charged and reacted at 80 ° C. for 30 minutes.
In the chart, isocyanate groups (wave number: 2220c
The end of the reaction was confirmed when the absorption of m ^-1 ) was substantially eliminated. The obtained resin solution had a solid content of 70%, a number average molecular weight of 3,000, and an amine value of 85.

1-5 (Preparation of Resin Emulsion [Particle A]) As the resin (a), R-45HT (1, a product of Idemitsu Petrochemical Co., Ltd.)
4-polybutadiene-α, ω-diol, number average molecular weight = 2,800, hydroxyl value = 47, butadiene content = 9
9%, solubility parameter δa = 9.5) 70 parts, 38 parts of the blocked polyisocyanate curing agent solution prepared in Preparation Example 1-2, 40 parts of the cationic dispersant prepared in Preparation Example 1-4, and acetic acid After adding 2.5 parts, the mixture was diluted with ion-exchanged water to a non-volatile content of 32%, and then concentrated under reduced pressure to a non-volatile content of 36%. I got

1-6 (Preparation of Pigment Dispersion Resin) A bisphenol A type epoxy resin having an epoxy equivalent of 198 (trade name: Epon 829, manufactured by Shell Chemical Co., Ltd.) was placed in a reaction vessel equipped with a stirrer, a cooling pipe, a nitrogen introduction pipe, and a thermometer. 710 parts,
289.6 parts of bisphenol A was charged and reacted at 150 to 160 ° C. for 1 hour under a nitrogen atmosphere.
After cooling to room temperature, 406.4 parts of a 2-ethylhexanol-modified half-blocked tolylene diisocyanate solution in methyl isobutyl ketone (solid content: 95%) was added. After keeping the reaction mixture at 110 to 120 ° C. for 1 hour, 1584.1 parts of ethylene glycol mono n-butyl ether was added. And it cooled to 85-95 degreeC, and made it uniform.

In parallel with the preparation of the above-mentioned reaction product, 384 parts of a 2-ethylhexanol-modified half-blocked tolylene diisocyanate solution of methyl isobutyl ketone (95% solids) was added to 384 parts of dimethylethanolamine 10
The mixture containing 4.6 parts was stirred at 80 ° C. for 1 hour, then charged with 141.1 parts of 75% lactic acid aqueous solution, and further mixed with 47.0 parts of ethylene glycol mono n-butyl ether.
After stirring for a minute, a quaternizing agent (solid content: 85%) was prepared. Then, 62.46 parts of this quaternizing agent was added to the reaction product, and the mixture was maintained at 85 to 95 ° C. until the acid value reached 1. Pigment-dispersed resin varnish (resin solid content: 56%, average molecular weight: 2,200) I got

1-7 (Preparation of Pigment Dispersion Paste) Using a sand mill, a pigment paste containing the pigment dispersion resin obtained in Preparation Example 1-6 and having the following composition was prepared. Pigment-dispersed resin varnish of Preparation Example 1-6 53.6 parts Titanium dioxide 88.0 parts Carbon black 2.0 parts Aluminum phosphomolybdate 10.0 parts

1-8 (Preparation of Electrodeposition Paint) Cation-modified epoxy resin emulsion [Particle B] (E1) obtained in Preparation Example 1-3, resin emulsion [Particle A] obtained in Preparation Example 1-5 (E2), using the pigment-dispersed paste obtained in Preparation Example 1-7 and deionized water,
Resin (a) / resin (b) compounding ratio (resin solid content ratio, not including curing agent weight) 50/50, pigment / resin vehicle (total vehicle weight, including curing agent weight)
Was prepared such that the ratio P / V became 1/4 (solid content concentration was 20%). An emulsified emulsion paste of dibutyltin oxide as a curing accelerator was blended in the above electrodeposition paint in a tin amount of 1.5% of the solid content of the paint.

[0114]Preparation Example 2 (Preparation of aqueous intermediate coating)  2-1 (Production of water-soluble polyester resin A-1) 200.0 parts of isophthalic acid and 1 of phthalic anhydride were placed in a reaction vessel.
79.0 parts, adipic acid 176.0 parts, trimethylol
150.0 parts of propane, neopentyl glycol 29
5.0 parts and 2 parts of dibutyltin oxide were charged, and nitrogen
Mixing and stirring after heating in the air stream to melt the raw materials
The temperature was gradually raised to 170 ° C. Then spend another 3 hours
Deesterification while heating to 220 ° C.
Was. When the acid value reaches 10, cool to 150 ° C
Was. Further, 114.0 parts of hexahydrophthalic acid was added.
The reaction was completed for 1 hour. Up to 100 ° C
After cooling, add 112.0 parts of butyl cellosolve and add
A ester resin A was obtained. The obtained polyester resin A
Is a solid content acid value of 50, a hydroxyl value of 65, GPC (gel
Weight obtained by luminescence chromatography)
The average molecular weight was 10,000. This polyester tree
The fat was cooled to 60 ° C and dimethylethanolamine was added.
0 parts and ion-exchanged water to add 50% non-volatile
The ester resin A-1 was obtained.

2-2 (Polystyrene resin emulsion A)
Production of -2) 33.4 parts of Newcol 1120 (manufactured by Nippon Emulsifier) and 6 parts of ethylene glycol mono-n-hexyl cellosolve were dispersed in 94 parts of ion-exchanged water. While stirring 111.1 parts of the polyester resin A, the polyester resin emulsion A was coated with a nonionic dispersant having a non-volatile content of 47.5% by dropping the dispersion into the dispersion.
-2 was obtained.

2-3 (Production of Colored Pigment Paste C-1) 9.4 parts of a commercially available pigment dispersant Disperbyk 190 (manufactured by BYK-Chemie; solid content: 40% by weight), 36.8 parts of ion-exchanged water, rutile-type dioxide After preliminarily mixing 34.5 parts of titanium, 34.4 parts of barium sulfate, 6 parts of talc, and 0.1 part of carbon black, add a glass bead medium in a paint conditioner and mix at room temperature until the particle size becomes 5 μm or less. This was dispersed to obtain a color pigment paste C-1. In the production of the pigment paste C-1, the time required until the particle size became 5 μm or less was 15 minutes.

2-4 (Preparation of Aqueous Intermediate Paint) Colored paste C-1 obtained in 2-3, water-soluble polyester resin A-1 obtained in 2-1 and polyester resin emulsion A obtained in 2-2 -2, a melamine resin was blended so as to have a solid content as shown in Table 1. As melamine resin,
Cymel 235 (manufactured by Mitsui Cytec) was used.

[0118]

[Table 1]

[0119]Preparation Example 3 (Preparation of aqueous base paint)  3-1 (Preparation of water-soluble acrylic resin) Dipropylene glycol methyl ether 2
3.9 parts and propylene glycol methyl ether 1
6.1 parts were added and mixed and stirred in a nitrogen stream for 120 minutes.
The temperature was raised to ° C. Then, 54.5 parts of ethyl acrylate,
12.5 parts of methyl methacrylate, 2-hydroacrylic acid
14.7 parts of xylethyl, 10.0 parts of styrene, methacrylic acid
A mixed solution of 8.5 parts of phosphoric acid and dipropylene glycol methyl
Toluene, 10.0 parts, t-butylperoxy-2-e
3 parts of an initiator solution consisting of 2.0 parts of tyl hexanoate
It was dropped into the reaction vessel in parallel over time. Drip end
Thereafter, aging was performed at the same temperature for 0.5 hour. Furthermore, dipropi
5.0 parts of lenglycol methyl ether and t-butyl
From 0.3 parts of peroxy-2-ethylhexanoate
The initiator solution is dropped into the reaction vessel over 0.5 hour.
Was. After completion of dropping, aging was performed at the same temperature for 1 hour. Next
Then, under reduced pressure (70 Torr) 110
After distilling off 16.1 parts of the solvent at ℃, dimethylethanol
Add amine and ion-exchanged water to add 31% non-volatile
Water-soluble A with an acid value of 56 mg KOH / g and a hydroxyl value of 70
A krill resin was obtained.

3-2-1 (Preparation of Color Pigment Paste 1) 100.0 parts of the above water-soluble acrylic resin, 28.9 parts of ion-exchanged water, 0.3 part of dimethylaminoethanol, Degussa Carbon FW-285 (Degussa After pre-mixing 5.1 parts of AG, a glass bead medium was added in a paint conditioner and mixed and dispersed at room temperature for 1 hour to obtain a color pigment paste 1 having a particle size of 5 μm or less.

3-2-2 (Preparation of Color Pigment Paste 2) 67.0 parts of the above water-soluble acrylic resin, ion-exchanged water 2
After premixing 3.2 parts and 67.0 parts of rutile-type titanium dioxide, a glass bead medium was added in a paint conditioner, and the mixture was mixed and dispersed at room temperature for 1 hour to obtain a color pigment paste 2 having a particle size of 5 μm or less. .

3-3-1 (Preparation of Aqueous Base Coating Material 1) 134.3 parts of the coloring paste 1, 118.8 parts of a water-soluble acrylic resin, and melamine resin (Cymel 204, manufactured by Mitsui Cytec) 29.1 Parts, 16 parts of ion exchange water
Aqueous base paint 1 was prepared by mixing at 1.3 parts.

3-3-2 (Preparation of Water-Based Base Coating Material 2) The above water-soluble acrylic resin, Cymel 204 (manufactured by Mitsui Cytec), and the above-mentioned colored paste 2 were used to mix the solid content as follows. . Rutile type titanium dioxide 20.0 parts Water-soluble acrylic resin 48.0 parts Cymel 204 32.0 parts

Example 1 Using the electrodeposition coating composition obtained in Preparation Example 1, electrodeposition coating was performed on a dull steel plate treated with zinc phosphate at a voltage such that the dry film thickness became 30 μm. And then baked at 160 ° C. for 15 minutes. The plate was coated with the aqueous intermediate coating composition obtained in Preparation Example 2 by air spray coating at 20 μm, and dried at 80 ° C. for 5 minutes. Thereafter, the aqueous base paint 1 of Preparation Example 3 was applied by air spray coating at 18 μm, and dried at 80 ° C. for 5 minutes. Further, MacFlow O-1801W Clear (clear paint made by Nippon Paint Co., Ltd.) is applied to the coated plate by air spraying to a thickness of 35 μm.
After coating, baking was performed at 140 ° C. for 30 minutes. On the other hand, the aqueous intermediate coating obtained in Preparation Example 2 was applied to the plate on which the electrodeposition coating film was formed by air spray coating at 20 μm, and dried at 80 ° C. for 5 minutes. Thereafter, the aqueous base paint 2 of Preparation Example 3 was applied by air spray coating at 18 μm,
Dry at 0 ° C. for 5 minutes. Furthermore, Mac Flow O-
1801W clear (clear paint made by Nippon Paint Co., Ltd.) was applied by air spraying at 35 μm and baked at 140 ° C. for 30 minutes to obtain a yellowing evaluation plate. Similarly, baking was performed at 160 ° C. for 60 minutes to evaluate yellowing. I got a board.

(Evaluation Method) (1) Evaluation of Electrodeposited Paint Table 2 shows properties and performance evaluation results of the obtained electrodeposited coating film. In Table 2, for convenience, a layer directly in contact with air is an "upper layer", and a layer directly in contact with the conductive substrate is "lower layer".
That. (1-1) Paint stability After the electrodeposition paint was stirred at 30 ° C. for 1 month, 1 liter thereof was filtered through a 400 mesh wire mesh.
It was determined to be good if the amount of residual solids on the wire mesh was 5 mg or less.

(1-2) Layer Separation State of Electrodeposited Coating The cross section was visually observed with a video microscope. In the case of a multilayer electrodeposition film, the main resin constituting each layer is FTIR.
-Identified by ATR analysis. (1-3) Layer Thickness of Each Layer Measured from the results of cross-sectional observation by the video microscope. (1-4) Elongation of Upper Layer Forming Resin Using the emulsion containing the resin (a) obtained in Preparation Example 1-5, a tensile test sample was separately prepared and measured in accordance with JIS K6301. The curing conditions were the same as those for curing the coating film.

(1-5) Tg (glass transition temperature) of upper and lower layers The multilayer electrodeposited film formed on the tin plate was peeled and cut using mercury to prepare a sample for measurement, and a rheometric dynamic analyzer RDA was used. After freezing the sample with liquid nitrogen once using a -II tester (manufactured by Rheometrics, USA), the temperature was raised at a rate of 2 ° C. and a frequency of 1
By applying vibration to the sample at 0 Hz and measuring its viscoelasticity, calculating the ratio (tan δ) of the loss elastic modulus (E ″) to the storage elastic modulus (E ′), and obtaining the inflection point, Each dynamic Tg was determined. (1-6) Surface Roughness of Electrodeposited Film The surface roughness Ra of the obtained coated plate was measured using Handy Surf E-30A (manufactured by Tokyo Seimitsu Co., Ltd.) according to JIS B0601 (cutoff 0.8 m).
m).

(1-7) A cross cut reaching the substrate with a knife was placed on the SDT coated plate, and a salt water immersion test (5% saline, 55 ° C.) was performed for 240 hours. Indicated by the maximum width. (1-8) A cross cut reaching the substrate with a knife was placed on the SST coated plate, and a salt spray test (5% saline) was performed for 240 hours, and the maximum width of rust generated from the cross cut portion was indicated. (1-9) Impact resistance test 1kg weight at room temperature using a DuPont impact tester
g was dropped from a height of 50 cm, and the presence or absence of cracking and peeling of the coating film was examined.

[0129]

[Table 2]

(2) Evaluation of the obtained multilayer coating film (2-1) Evaluation of yellowing The coated plate obtained by using the aqueous base coating material 2 as the base coating material was baked at 140 ° C. for 30 minutes.
When baked at 60 ° C. for 60 minutes, using SM color computer SM-4 (manufactured by Suga Test Instruments Co., Ltd.)
The b value of the baked coating film was measured. The results are shown in Table 1.

(2-2) Visual Evaluation of Appearance The appearance of the coating film after baking at 140 ° C. for 30 minutes was visually evaluated. ：: no yellowing is observed Δ: slight yellowing is observed ×: yellowing is observed

(2-3) Chipping resistance This was carried out on a coated plate obtained by using the aqueous base paint 1 as the base paint. Stepping Stone Testing Machine (Suga Test Machine Co., Ltd.)
The coated plate cooled to −30 ° C. was attached to the sample holder so that the entering angle of the stone was 90 °, and 100 g of No. 7 crushed stone was sprayed at an air pressure of 3 kg / cm ² to collide the crushed stone with the coated plate. At that time, the degree (number, size, destruction place) of peeling was evaluated on a five-point scale. The results are shown in Table 1. 1: peeling on the entire surface, peeling off from the substrate 2: peeling on the entire surface, no peeling off from the substrate 3: partly peeling, peeling off from the substrate 4: partly peeling No peeling from substrate 5: Almost no destruction

Example 2 A coated plate having a multi-layer coating film was prepared and evaluated in the same manner as in Example 1 except that the aqueous intermediate coating composition was prepared according to the formulation shown in Table 1. The results are shown in Table 1.

Comparative Example 1 A coated plate on which a multilayer coating film was formed was prepared in the same manner as in Example 1 except that Powertop U-50 (cationic electrodeposition paint, manufactured by Nippon Paint Co., Ltd.) was used as the electrodeposition paint. Created and evaluated. The results are shown in Tables 1 and 2.

From Table 2, it can be seen that the conventional electrodeposition paint forms a single-layer coating film, whereas the electrodeposition paint used in the present invention has a high Tg in the lower layer, a low Tg in the upper layer, and a resin elongation percentage. It was found that a high multilayer electrodeposition coating film was formed. From Table 1, it was found that the multilayer coating films obtained in the examples were excellent in chipping resistance and appearance.

[0136]

From the electrodeposition coating material used in the present invention, a multi-layer electrodeposition film having a shock absorbing layer formed on an electrodeposition coating layer having corrosion resistance as a main function at the time of forming the coating film. Can be obtained. Further, since a pigment dispersing paste containing a pigment dispersant substantially free of a volatile basic substance is blended as an aqueous intermediate coating and / or an aqueous base coating, the coating film may turn yellow. And an excellent finished appearance can be obtained. Therefore, the multilayer coating film obtained by the present invention has excellent corrosion resistance, impact resistance (chipping resistance) and appearance comparable to conventional three-coat films. The multilayer coating forming method of the present invention plays an important role in the coatings industry, especially in the field of automotive coatings, in building a new 3 wet coating system aiming at shortening the baking process, reducing costs and reducing environmental burdens (VOCs and HAPs). It fulfills.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C09D 5/00 C09D 5/00 D Z 5/44 5/44 Z 7/12 7/12 109/00 109 / 00 175/04 175/04 201/00 201/00 C25D 13/00 307 C25D 13/00 307D 308 308C 13/06 13/06 BE // C09D 163/00 C09D 163/00 F-term (reference) 4D075 AE06 BB26Z BB89X DB02 DC12 EA06 EA43 EB53 EB54 EC11 4J038 CA021 CA081 CA091 CB031 CB091 CC021 CD041 CF021 CG081 CG141 CH031 CM021 CQ011 DB061 DB071 DB151 DB161 DD071 GA03 GA01 DG001 DG161 DG001 DG161 DG001 DG161 GA03 PA19 PB07

Claims (7)

[Claims] After coating an electrodeposition paint on an object to be coated,
Heat curing to form an electrodeposition coating (I), applying an aqueous intermediate coating on the electrodeposition coating to form an uncured intermediate coating, (II) Step (III) of applying an aqueous base paint on the intermediate coating film to form an uncured base coating film, and applying a clear paint on the base coating film to form an uncured clear coating film. A method of forming a film, and a step (V) of simultaneously heating and curing the intermediate coating film, the base coating film and the clear coating film to obtain a multilayer coating film. In addition, the electrodeposition paint has particles A containing a resin (a) having a solubility parameter of δa, and a solubility parameter of δb.
(B) and particles B containing a curing agent, wherein (1) the value of (δb−δa) is 1.0 or more, and (2) formed from the electrodeposition paint. Among the electrodeposition coating films, the dynamic glass transition temperature of the resin film formed from the particles A is −110 to 10 ° C., and the elongation percentage of the coating film obtained by forming the film only with the particles A is: 200% or more,
(3) Among the electrodeposition coating films formed from the electrodeposition coating material, the dynamic glass transition temperature of the resin film formed from the particles B is 60 to 150 ° C. And / or wherein the aqueous base paint contains a pigment dispersion paste obtained by previously dispersing a pigment and a pigment dispersant substantially free of a volatile basic substance. Film formation method. 2. The method for forming a multilayer coating film according to claim 1, wherein the particles A further contain a curing agent, and at least one of the curing agents has a solubility parameter δi of δb>δi> δa. . 3. The method according to claim 1, wherein the weight ratio of the resin (a) to the resin (b) based on the solid content is 5/95 to 70/30. 4. The method for forming a multi-layer coating film according to claim 1, wherein the resin (a) is a resin obtained by polymerizing a monomer component comprising 50% by weight or more of a conjugated diene monomer. . 5. The content of the pigment dispersant in the aqueous intermediate coating composition is 0.5 to 10% by weight based on the solid content.
5. The method for forming a multilayer coating film according to 2, 3, or 4. 6. The composition according to claim 1, wherein the content of the pigment dispersant in the aqueous base paint is 1 to 20% by weight based on the solid content.
6. The method for forming a multilayer coating film according to 3, 4, or 5. 7. A multilayer coating film formed by the method for forming a multilayer coating film according to claim 1, 2, 3, 4, 5, or 6.