Patent Publication Number: US-2010116673-A1

Title: Cationic electrodeposition coating and application thereof

Description:
TECHNICAL FIELD 
     The present invention relates to a cationic electrodeposition coating composition superior in smoothness and edge coatability and a method for satisfying both of the smoothness and the edge coatability of a cationic electrodeposition film using the same. 
     Further, the present invention relates to a cationic electrodeposition coating composition superior in smoothness and edge coatability, specifically a cationic electrodeposition coating composition superior in smoothness and edge coatability which comprises a specific crosslinked resin particle, and a method for satisfying both of the smoothness and edge coatability of a cationic electrodeposition film using the same. 
     BACKGROUND OF THE INVENTION 
     Electrodeposition coating is a coating process carried out by immersing an article to be coated in an electrodeposition coating composition and applying a voltage. Since the electrodeposition coating process can automatically and continuously coat an article to be coated having a complicate shape to a nicety, it has been widely and practically used as a process for primarily coating a large size article having complicate shape such as, in particular, an automobile body. 
     Since the electrodeposition coating is a coating on an article, it is naturally desirable that coated surface is smooth. Further, the perforation portion of metal and the like have sharp edge and unless a coated film is adequately coated on the edge portion, anticorrosive performance is deteriorated. Consequently, both of surface smoothness and edge coatability are performances required for the electrodeposition coating. On the other hand, the surface smoothness is obtained by lowering the viscosity of the uncured coating film at curing by baking to be fluidized, but the edge coatability is obtained by keeping so as not lowering the viscosity of the uncured coating film. Namely, the edge coatability requires the suppression of sagging of coating film at curing the coating film and the coating film remains also at sharp edge. Namely, the surface smoothness and the edge coatability are conflicting performances. 
     Technology relating to the coating film viscosity of the electrodeposition film is described in Japanese Patent Application Publication No. 2002-285077 (Patent document 1) and it describes an electrodeposition coating composition for an electric wire wherein the minimum coating film viscosity at the curing process of coating film is between 30 to 150 PaS (claim  3 ). The patent document 1 describes edge coatability and the like can be improved without sagging at melt by adjusting the minimum coating film viscosity at the curing process of coating film. 
     Japanese Patent Application Publication No. 6-65791 (Patent document 2) discloses a process for coating an anti-chipping primer on an uncured coating film surface formed by coating a cationic electrodeposition coating composition, further carrying out an intermediate coating and a top coating, and curing the three layers simultaneously, wherein the minimum melt viscosity during curing the coating film of the cationic electrodeposition coating composition is 10 4  to 10 8  cps. It discloses that since the three layers of the coating films are baked only at once, coating steps are shortened, it is superior in edge covering property and the resulting coating film consisting of a plurality of layers is superior in finishing property and anti-chipping property. The publication discloses the finishing property and edge covering property in the coating film consisting of a plurality of layers, but does not study the finishing property and the edge covering property on an electrodeposition film itself. On the other hand, it has been conventionally carried out in general coating compositions including the cationic electrodeposition coating composition of the present invention that the viscosity of the coating film is controlled using a particle described later. 
     By the way, the reduction of ash contents in the electrodeposition coating composition has been recently promoted. The reduction of ash contents is that the amount of solid components with a high specific gravity such as an inorganic pigment is reduced and that sedimentation is designed not to occur in the solid contents of the electrodeposition coating composition. The reduction of ash contents reduces energy and labor for stirring an electrodeposition bath hitherto for prevention of sedimentation. Accordingly, when the content of an inorganic pigment is reduced in order to correspond the request of the above-mentioned reduction of ash contents, the quantity of resin contents in the coating composition is relatively enhanced, the viscosity of the uncured coating film obtained by the electrodeposition coating cannot be appropriately increased, and as a result, the control of sagging at an edge portion cannot be suitably adjusted to the lower edge coatability. 
     On the other hand, since solid concentration of about 20% by weight is used in the current cationic electrodeposition coating composition, rinsing with water is carried out at several steps separately after the electrodeposition coating, and a baking step is carried out after completely removing the electrodeposition coating composition adhered on the article unnecessarily, in particular, its solid contents. Accordingly, a large quantity of rinsing water is used, the rinsing step with water is elongated and the reduction of rinsing water and the shortening of the rinsing steps with water has been recently desired. As the means for shortening the rinsing step with water, the further lowering of solid concentration in the coating composition of 20% by weight, so-called low solid content is required. However, when such low solid content is simply carried out, the sedimentation of solid contents in the electrodeposition coating composition occurs easily because of the lowering of the coating composition viscosity, and the like. When the content of an inorganic pigment is further reduced as described above, the sedimentation of solid contents occur further easily. Consequently, the stirring in an electrodeposition bath must be carried out in order to prevent the sedimentation, and the reduction of energy load is difficult. Namely, a cationic electrodeposition coating composition capable of controlling viscoelasticity so as to easily carry out the edge coatability, and superior in surface smoothness and prevent the sedimentation, has been desired, even if low solid content is realized for energy saving and the shortening of steps. 
     In relation to a means for obtaining such coating composition, namely a coating composition improved in thixotropy, there exist several technologies adding crosslinked resin particles to the cationic electrodeposition coating composition. Japanese Patent Application Publication No. 2005-23232 (Patent document 3) discloses that minute resin particles with a particle size of 0.01 to 0.2 μm whose inside was crosslinked are added to a cationic electrodeposition coating composition (Patent document 3, claim 6). It has been conventionally existed as improving thixotropy that resin particles with such small sizes are added in the electrodeposition coating composition. 
     Japanese Patent Application Publication No. 2002-212488 (Patent document 4) discloses a cationic electrodeposition coating composition that comprises crosslinked resin particles obtained by carrying out the emulsion polymerization of α,β-ethylenically unsaturated monomer mixture using an acryl resin having an ammonium group as an emulsifier, in order to improve the anticorrosive property of an edge portion of an article. The resin particles obtained herein is small with a particle size of 0.05 to 0.3 μm. However, when a crosslinked resin particles with an average particle size of 1.0 μm or less are added in an electrodeposition coating composition, the smoothness of the resulting coating film is lowered. 
     Patent document 1: Japanese Patent Application Publication No. 2002-285077
 
Patent document 2: Japanese Patent Application Publication No. 6-65791
 
Patent document 3: Japanese Patent Application Publication No. 2005-23232
 
Patent document 4: Japanese Patent Application Publication No. 2002-212488
 
     SUMMARY OF THE INVENTION 
     Disclosure of the Invention 
     Problem to be Solved by the Invention 
     It is the object of the present invention to provide a method for satisfying both of the conflicting performances of the surface smoothness and edge coatability in a cationic electrodeposition coating composition, as described above. 
     Further, it is the object of the present invention to provide a method for lowering the solid concentration in a cationic electrodeposition coating composition, preventing the sedimentation of the coating composition for reduction of ash contents, and satisfying both of the conflicting performances of the surface smoothness and edge coatability in a cationic electrodeposition coating composition, as described above. 
     Means for Solving Problem 
     Accordingly, the present invention provides a cationic electrodeposition coating composition, which provides an uncured electrodeposited film having storage elasticity modulus (G′) at 140° C. within a range of from 80 to 500 dyn/cm 2  and loss elasticity modulus (G″) at 80° C. within a range of from 10 to 150 dyn/cm 2 , and which is superior in smoothness and edge coatability. 
     The cationic electrodeposition coating composition preferably comprises a cationic epoxy resin, a blocked isocyanate curing agent, and if necessary, a resin particle (preferably a crosslinked resin particle) and/or a pigment (preferably an inorganic pigment). 
     The present invention further provides a method for producing a cationic electrodeposition film having established smoothness and edge coatability, wherein the cationic electrodeposition film is prepared by applying a voltage to an article immersed in a cationic electrodeposition coating composition, which includes steps of: 
     adjusting storage elasticity modulus of an uncured electrodeposited film of the cationic electrodeposition coating composition (G′) at 140° C. within a range of from 80 to 500 dyn/cm 2 , and 
     adjusting loss elasticity modulus of an uncured electrodeposited film of the cationic electrodeposition coating composition (G″) at 80° C. within a range of from 10 to 150 dyn/cm 2 . 
     In order to adjust storage elasticity modulus and loss elasticity modulus, addition of a crosslinked resin particle or an inorganic pigment is preferable. The crosslinked resin particles preferably have an average particle size within a range of from 1.0 to 3.0 μm. The content of the crosslinked resin particles is preferably 3 to 15% by weight relative to weight of resin solid contents in the cationic electrodeposition coating composition. 
     The inorganic pigment is added to the cationic electrodeposition coating composition, wherein content of the inorganic pigment is preferably 10 to 20% by weight relative to weight of solid contents in the cationic electrodeposition coating composition, in order to adjust storage elasticity modulus and loss elasticity modulus. 
     In order to adjust storage elasticity modulus and loss elasticity modulus, both of an inorganic pigment and crosslinked resin particles having an average particle size within a range of from preferably 1.0 to 3.0 μl can be added to the cationic electrodeposition coating composition, wherein content of the inorganic pigment is preferably 0.5 to 10% by weight relative to weight of solid contents in the cationic electrodeposition coating composition, 
     In the case that both of an inorganic pigment and crosslinked resin particles are added to the cationic electrodeposition coating composition in order to adjust storage elasticity modulus and loss elasticity modulus, it is preferable that the content of the crosslinked resin particles is 3 to 15% by weight relative to weight of resin solid contents in the cationic electrodeposition coating composition. 
     The present inventors have investigated a method for establishing both of surface smoothness and edge coatability in a cationic electrodeposition coating composition with low solid and low ash content. The present inventors found that addition of a certain crosslinked resin particle to a cationic electrodeposition coating composition easily and facilely could solve the problem and reached to the present invention. 
     Accordingly, the present invention provides a cationic electrodeposition coating composition comprising crosslinked resin particles having an average particle size within a range of from 1.0 to 3.0 μm and thermal softening temperature within a range of from 120 to 180° C., which is superior in smoothness and edge coatability. 
     The content of the crosslinked resin particles is preferably 3 to 15% by weight relative to weight of resin solid contents in the cationic electrodeposition coating composition. 
     The present cationic electrodeposition coating composition is preferably a cationic electrodeposition coating composition with low solid and low ash content comprising no inorganic pigment or a inorganic pigment no more than 7% by weight relative to weight of the solid contents in the cationic electrodeposition coating composition. 
     The present cationic electrodeposition coating composition has a solid concentration within a range of from preferably 0.5 to 9% by weight. 
     In the present invention, the crosslinked resin particles may be prepared from (a) a compound preferably having two or more unsaturated double bonds in the molecule and (b) a (meth)acrylate by a known method such as a suspension polymerization, emulsion polymerization, etc. 
     The present invention further provides an uncured electrodeposited film of a cationic electrodeposition coating composition, which has storage elasticity modulus (G′) at 140° C. within a range of from 80 to 500 dyn/cm 2  and loss elasticity modulus (G″) at 80° C. within a range of from 10 to 150 dyn/cm 2 . 
     The present invention further provides a cured cationic electrodeposition film having no more than 0.25 μm of Ra value (as an index of smoothness of a coating film), which is obtained by curing the cationic electrodeposition coating composition. 
     The present invention further provides a method for producing a cationic electrodeposition film having established smoothness and edge coatability, wherein the cationic electrodeposition film is prepared by applying a voltage to an article immersed in a cationic electrodeposition coating composition, and wherein the cationic electrodeposition coating composition comprises crosslinked resin particles having an average particle size within a range of from 1.0 to 3.0 μm and thermal softening temperature within a range of from 120 to 180° C. 
     The present invention further provides a method for producing a cationic electrodeposition film having improved smoothness and edge coatability from a cationic electrodeposition coating composition with low ash and low solid content, which includes steps of: 
     adjusting storage elasticity modulus of an uncured electrodeposited film (G′) at 140° C. within a range of from 80 to 500 dyn/cm 2 , and 
     adjusting loss elasticity modulus of an uncured electrodeposited film (G″) at 80° C. within a range of from 10 to 150 dyn/cm 2 , 
     wherein the cationic electrodeposition coating composition comprises crosslinked resin particles having an average particle size within a range of from 1.0 to 3.0 μM and thermal softening temperature within a range of from 120 to 180° C. and content of the crosslinked resin particles is 3 to 15% by weight relative to weight of resin solid contents in the cationic electrodeposition coating composition. 
     EFFECT OF THE INVENTION 
     According to the present invention, both of the smoothness and edge coatability can be established by simultaneously adjusting loss elasticity modulus G″ and storage elasticity modulus G′ among the dynamic viscoelasticities of an uncured electrodeposited coating film during the electrodeposition coating. In a conventional technology, smoothness has been secured only by managing the lowest melt viscosity by controlling complex viscosity coefficient η* in the measurement of dynamic viscoelasticity, but it was grasped that the compatibility of the above-mentioned smoothness and the edge coatability was impossible by only viscosity merely. In the present invention, it has been found that it is important to control loss elasticity modulus: G″ (viscosity item) within a specified range at controlling the smoothness, in dynamic viscoelasticities of an uncured coating film of a cationic electrodeposition coating composition. 
     Further, it has been found that it is important to control the storage elasticity modulus G′ (elastic item) in a specified range at controlling the edge coatability. Further, in the present invention, it has been found that it is important to control the loss elasticity modulus G″ in a specific range and to simultaneously control the storage elasticity modulus G′ in a specific range in order to secure the both of the smoothness and edge coatability of the electrodeposition film, that has been conventionally considered as a contradictable event. The both of the established smoothness and the edge coatability of the resulting electrodeposition film have been achieved by considering these G″ and G′ as independent parameters and controlling these parameters within respective specific ranges. 
     According to the present invention, both of the established surface smoothness and edge coatability can be evaluated only by controlling both of the loss elasticity modulus and storage elasticity modulus of an uncured electrodeposited film by an electrodeposition coating. A method for performance test or performance management useful for a cationic electrodeposition coating composition can be provided. 
     Further, according to the present invention, both of the established surface smoothness and edge coatability are possible by adding crosslinked resin particles with an average particle size within a range of from 1.0 to 3.0 μm and thermal softening temperature within a range of from 120 to 180° C. to a cationic electrodeposition coating composition. Since increase of the viscosity in a coating film cannot be achieved by an inorganic pigment in case of a low ash type cationic electrodeposition coating composition, it is anticipated that the edge coatability is deteriorated, but the edge coatability is also improved by adding the specific crosslinked resin particles to the cationic electrodeposition coating composition according to the present invention and it is effective as a means for keeping or improving the coating film performance in the low ash type cationic electrodeposition coating composition. Herein, the low ash type cationic electrodeposition coating composition means that an inorganic pigment is not contained at all in the solid contents in the cationic electrodeposition coating composition or even if it is contained, it is up to 7% by weight relative to the weight of the solid contents in the composition (i.e., a cationic electrodeposition coating composition with low ash content). Further, in the present invention, there is provided the low solid type cationic electrodeposition coating composition that is superior in an ability of preventing sedimentation than the conventional one and enables the establishment of the surface smoothness and the edge coatability as described above. Herein, the low solid type cationic electrodeposition coating composition means that the solid content concentration of the cationic electrodeposition coating composition is lower than the conventional 20% by weight, and within a range of from specifically 0.5 to 9% by weight (i.e., a cationic electrodeposition coating composition with low solid content). 
     In the study by the present inventors, the establishment of both of the surface smoothness and edge coatability can be correlated with measurement of dynamic viscoelasticities of the resulting electrodeposited film by the electrodeposition coating. In particular, when the loss elasticity modulus G″ at 80° C. and the storage elasticity modulus G′ at 140° C. are within specific ranges, namely, the loss elasticity modulus G″ at 80° C. is  10  to  150  dyn/cm 2 , and the storage elasticity modulus G′ at 140° C. is  80  to  500  dyn/cm 2 , both of the surface smoothness and the edge coatability are established, but in the present invention, it has been found, as an solving means, that crosslinked resin particles with an average particle size within a range of from 1.0 to 3.0 μm and thermal softening temperature of 120° C. or more are added to the cationic electrodeposition coating composition. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a graph showing the behaviors of the loss elasticity modulus (G″) values in the dynamic viscoelasticities in five coating compositions. 
         FIG. 2  is a graph showing the behaviors of the storage elasticity modulus (G′) values in the dynamic viscoelasticities in five coating compositions. 
         FIG. 3  is a graph showing the behaviors of the complex viscosity coefficient (η*) values in the dynamic viscoelasticities in five coating compositions. 
         FIG. 4A  is a graph showing a relation between the storage elasticity modulus (G′) at 80° C. and electrodeposition texture in several coating compositions. 
         FIG. 4B  is a graph showing a relation between the complex viscosity coefficient (η*) at 80° C. and electrodeposition texture in several coating compositions. 
         FIG. 4C  is a graph showing a relation between the loss elasticity modulus (G″) at 80° C. and electrodeposition texture in several coating compositions. 
         FIG. 5A  is a graph showing a relation between the storage elasticity modulus (G′) at 140° C. and electrodeposition texture in several coating compositions. 
         FIG. 5B  is a graph showing a relation between the complex viscosity coefficient (η*) at 140° C. and electrodeposition texture in several coating compositions. 
         FIG. 5C  is a graph showing a relation between the loss elasticity modulus (G″) at 140° C. and electrodeposition texture in several coating compositions. 
         FIG. 6A  is a graph showing a relation between the storage elasticity modulus (G′) at 80° C. and edge coatability in several coating compositions. 
         FIG. 6B  is a graph showing a relation between the complex viscosity coefficient (η*) at 80° C. and edge coatability in several coating compositions. 
         FIG. 6C  is a graph showing a relation between the loss elasticity modulus (G″) at 80° C. and edge coatability in several coating compositions. 
         FIG. 7A  is a graph showing a relation between the storage elasticity modulus (G′) at 140° C. and edge coatability in several coating compositions. 
         FIG. 7B  is a graph showing a relation between the complex viscosity coefficient (η*) at 140° C. and edge coatability in several coating compositions. 
         FIG. 7C  is a graph showing a relation between the loss elasticity modulus (G″) at 140° C. and edge coatability in several coating compositions. 
         FIG. 8  is a graph showing a relation between temperature and storage elasticity modulus G′ for illustrating the thermal softening temperature. 
         FIG. 9  is a view schematically showing a part in a distance of 30 microns from an edge of a cutter knife blade. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The dynamic viscoelasticity is an elasticity modulus observed when vibrational (periodical) strain or force (stress) is applied to a linear viscoelastic body, and depends on a vibrational number and temperature. The description related to the dynamic viscoelasticity below refers to contents described in Rheology (edited by Japan Rheology Academy), the second section: Polymer liquid rheology, pages 31 to 39; and Polymer Chemistry, Introduction (edited by Seizo Okamura, Akio Nakajima, Shigeharu Onogi, Yasunori Nishijima, Toshinobu Higashimura and Norio Ise), the fourth section: Various performances of polymer substances, Viscoelasticities, pages 149 to 155. 
     Stress and strain at an angular velocity [ω( 2 π×frequency F)] are provided by the following formulae. 
       Strain γ( t )=γ 0   e   iωt (dyn/cm 2 ) 
       Stress σ( t )=σ 0   e   i(ωt+δ) (dyn/cm 2 ) 
     wherein γ(t) is a strain at a time (t), σ(t) is a stress at a time (t), γ 0  is a strain at t=0, σ 0  is a stress at t=0, and δ represents phase contrast. 
     The complex elasticity modulus G* is represented by the equation: 
         G *=(σ 0 /γ 0 ) e   iδ =(σ 0 /γ 0 )(cos δ−i sin δ) 
     The complex viscosity coefficient [η*=G*/ω(poise)] generally used as a viscosity control factor of a coating composition is obtained by quantifying viscoelasticity having properties in combination of both of viscosity and elasticity of the coating composition. 
     Namely, in the present invention, viscosity and elasticity are grasped separately, and the establishment of both of the smoothness and edge coatability was enabled by controlling them respectively. It is necessary for securing smoothness to control the flowability of the coating composition at the baking process. Viscous properties are related to the flowability, and this is represented by the following formula according to a relation between stress and strain. 
       Loss elasticity modulus (viscosity)  G″=G * sinδ(dyn/cm 2 ) 
     On the other hand, it is necessary for the securing the edge coatability to control a force going to remain at the site at the baking process, and the force is related with elastic properties. This is represented by the following formula according to a relation between stress and strain. 
       Storage elasticity modulus (elasticity)  G′=G * cos δ(dyn/cm 2 ) 
     In case of a general coating composition including a cationic electrodeposition coating composition, the viscosity item dominates an uncured coating film at the initial stage of a baking process, and the composition is greatly subjected to an influence of the loss elasticity modulus G″. At the posterior stage, the uncured coating film is reached to a gelation point (apparently, in a continuous state in both ends) by a fusing and a pseudo-crosslinking. The elastic item dominates thereafter, and the film is greatly subjected to an influence of the storage elasticity modulus G′. The gelation point is a temperature at which a relation between the loss elasticity modulus G″ (the viscosity item) and the storage elasticity modulus G′ (the elastic item) as viscoelasticity-behaviors during the baking process is (Loss elasticity modulus G″)&lt;(Storage elasticity modulus G′). Namely, it means a point at which the domination by the viscosity item is changed to a domination by the elastic item. 
     In the present invention, it has been found that the establishment of both of the smoothness and edge coatability is enabled by the control of the loss elasticity modulus G″ at temperature (80° C.) no more than the gelation point and the control of the storage elasticity modulus G′ at temperature (140° C.) no less than the gelation point, and the present invention was achieved thereby. 
     Herein, the present invention is described by an explanation of the process by which the present invention has been achieved. Firstly, the followings were carried out as preliminary experiments. 
     Viscoelastic behaviors were observed for several coating compositions, specifically, a conventional coating composition to which components such as a pigment were added, a coating composition without them, and a coating composition comprising a crosslinked resin particle. The viscosity of the coating compositions begins to be lowered at 40 to 80° C. in accordance with the rising of the temperature, the viscosity is slightly raised between about 80 and about 100° C., and when it exceeds 100° C., the viscosity is greatly decreased to be flown. After the flowing, the curing reaction starts to raise the viscosity again, to raise it gradually till nearby 150° C. and then, the viscosity is abruptly raised to complete the curing. In order to investigate the dynamic viscoelasticities of the coating composition while confirming it, five coating compositions were measured using Rheosol-G3000 by UBM Corporation, and strain values γ(t) for stress values ρ(t) applied and phase contrast δ between stress and strain were measured under conditions of a strain of 0.5 degree, a frequency of 0.02 Hz and a temperature rising rate of 2° C./min. The storage elasticity modulus (G′), the loss elasticity modulus (G″) and the complex viscosity coefficient (η*) are calculated according to the above formulae from the relations between the resulting stress values σ(t), strain values γ(t) and phase contrast δ, and are respectively shown in  FIG. 1  to  FIG. 3 . The coating compositions used in  FIGS. 1 to 3  are as followings: “STD” is PN-310 (a cationic electrodeposition coating composition: manufactured by Nippon Paint Co., Ltd.); “Pigment free” is a coating composition without any pigment components in the PN-310 (PWC=0%); “Resin particle 1” is a coating composition in which 15% by weight of crosslinked resin particles (with average particle size of 1 to 3 μm) were added to the “Pigment free”; “Resin particle 2” is a coating composition in which 5% by weight of crosslinked resin particles (with average particle size of 100 nm) were added to the “Pigment free”; and “Resin particle 3” is a coating composition in which 10% by weight of crosslinked resin particles (with average particle size of 100 nm) were added to the “Pigment free”, which particles are different from those in the “Resin particle 2”. 
     As seen from  FIGS. 1 to 3 , it is grasped that the behaviors are considerably different depending on the respective coating compositions. Almostly, it is divided into 3 modes (40 to 80° C., 80 to 100° C. and 100° C. or more), but it can be grasped that the behaviors of the dynamic viscoelasticities are greatly changed depending on the formulations of the coating compositions, in particular, in the presence of the components such as the particles, and that these graphs depicted by five coating compositions are different. Accordingly, it is also grasped that the behaviors of the dynamic viscoelasticities can be optimally controlled by changing the formulation. 
     In particular, viewing  FIGS. 1 to 3 , it can be understood that great differences between respective coating compositions are based on the behavior of the viscoelasticity nearby 80° C. and the behavior of the viscoelasticity nearby 140° C. 
     Further, the following experiments were carried out based on these bases. Several coating compositions, such as PN-310 (a cationic electrodeposition coating composition: manufactured by Nippon Paint Co., Ltd.); a coating composition in which the amount of the inorganic pigment component in the PN-310 coating composition was changed; a coating composition in which an inorganic pigment component was removed from the PN-310; and a coating compositions in which the kind and amount of the crosslinked resin particles to be added in the last composition were changed, were prepared, and viscosity behavior for each of them was measured. From the results of their viscoelasticities, three viscoelasticity behaviors at 80° C., namely, all of G′ values and electrodeposition texture ( FIG. 4A ), η* values and electrodeposition texture ( FIG. 4B ) and G″ values and electrodeposition texture ( FIG. 4C ) were displayed in  FIG. 4  so that changes at the respective temperatures are easily grasped from the result of those viscoelasticities. Similarly, three viscoelasticity behaviors at 140° C., namely, all of G′ values and electrodeposition texture ( FIG. 5A ), values and electrodeposition texture ( FIG. 5B ) and G″ values and electrodeposition texture ( FIG. 5C ) were displayed in  FIG. 5 . Further, the electrodeposition texture is represented by a surface roughness (Ra). The electrodeposition texture evaluated herein means the appearance of electrodeposition film described later, namely smoothness, and that represented by the measurement value of the arithmetic average roughness (Ra) of a roughness carve. Namely, the relation between the electrodeposition texture and the viscoelasticity behavior is observed by evaluating the above-mentioned smoothness by the electrodeposition texture. 
     As seen from the behaviors of  FIGS. 4 and 5 , there is a correlation with the electrodeposition texture at 80° C. in the relation between the viscoelasticity change and electrodeposition texture at the respective measurement points and in the measured coating compositions (see  FIG. 4C ). Further, similarly, the measuring results of the behaviors of the edge coatabilities and three viscoelasticity behaviors are described in  FIGS. 6A to 6C  and  FIGS. 7A to 7C . As seen from  FIGS. 6 and 7 , it is grasped that the relation between the storage elasticity modulus (G′) at 140° C. and the edge coatability exhibits a correlation (see  FIG. 7A ). Namely, it means that the change of the viscosity value and the electrodeposition texture (smoothness) or the edge coatability have a correlation. Herein, the above-mentioned edge coatability can be determined by an evaluation method described later. Further, the “coatability” represented in  FIGS. 6 and 7  is the same meaning as the “edge coatability” mentioned here. 
     From these measuring results, it is found that, as an evaluation basis, the present invention can employ the loss elasticity modulus (G″) at 80° C. for the electrodeposition texture (smoothness) and the storage elasticity modulus (G′) at 140° C. for the edge coatability. The present invention has been completed thereby. Further, the preferable ranges of the storage elasticity modulus (G′) and the loss elasticity modulus (G″) can be selected referring to the appended  FIGS. 4 and 7 . Namely, G′ at 140° C. is within a range of from preferably 80 to 500 dyn/cm 2  referring to  FIG. 7A , and G″ at 80° C. can be selected within a range of from 10 to 150 dyn/cm 2  referring to  FIG. 4C  (the smaller electrodeposition texture Ra means the better smoothness). The storage elasticity modulus (G′) is within a range of from preferably 90 to 500 dyn/cm 2  and more preferably from 100 to 500 dyn/cm 2 . Further, the loss elasticity modulus (G″) at 80° C. is within a range of from preferably 10 to 120 dyn/cm 2  and more preferably from 10 to 100 dyn/cm 2 . 
     When the storage elasticity modulus (G′) is lowered than the desirable lower limit of the storage elasticity modulus (G′), there is a fear that the edge coatability of the electrodeposition film obtained is deteriorated, and when the storage elasticity modulus (G′) exceeds the desirable upper limit, there is a fear that smoothness is lowered. When the loss elasticity modulus G″ is lowered than the desirable lower limit of the loss elasticity modulus G″, there is a fear that although the smoothness is improved, the edge coatability of the electrodeposition film obtained is deteriorated, and when the loss elasticity modulus G″ exceeds a desirable upper limit, there is a fear that smoothness is lowered. 
     Herein, the storage elasticity modulus G′ and the loss elasticity modulus G″ are relate to the elasticity modulus of uncured electrodeposited film. The “uncured” means a state in which an electrodeposited coating film obtained by carrying an electrodeposition coating of a cationic electrodeposition coating composition is not cured yet by baking. 
     The cationic electrodeposition coating composition, as described above, contains or comprises a crosslinked resin particle and/or an inorganic pigment, but further contains an aqueous medium; a binder resin containing a cationic epoxy resin and a blocked isocyanate curing agent dispersed or dissolved in an aqueous medium; a neutralizing acid; and an organic solvent. 
     In order to adjust the above-mentioned viscoelasticity behaviors, there is a process of adding a crosslinked resin particle, as a first process. The average particle size of the crosslinked resin particles is within a range of from preferably 1.0 to 3.0 μm. When the average particle size is smaller than 1.0 μm, the proportion of the surface area is increased, and interaction with a cationic epoxy resin or the like, as binder resin components, contained in the cationic electrodeposition coating composition is increased, and the viscosity of the deposited coating film is abruptly raised; therefore the above-mentioned adjustments of viscoelasticity behaviors become difficult. On the other hand, when the particle size is larger than 3.0 μm, the lowering of smoothness caused by the sedimentation of the electrodeposition coating composition at no stirring and by the accumulation of particles applied on a horizontal plane upon coating occurs. 
     Further, the crosslinked resin particles used in the present invention have preferably an average particle size within a range of from 1.0 to 3.0 μm and a thermal softening temperature of 120° C. or more and within a range of from preferably 120 to 180° C. for establishing both of the surface smoothness and the edge coatability of a cationic electrodeposition coating composition with low ash and low solid content. Although a proposal of an addition of crosslinked resin particles in cationic electrodeposition coating composition is carried out also in a. conventional technology, the resin particles are almost those having an average particle size of less than 1.0 μm. Since resin particles are added for merely controlling the viscosity in a conventional technology, the resin particles with an average particle size of less than 1.0 μm are required, but in the present invention, the crosslinked resin particles having a larger average particle size than that in the conventional technology and a thermal softening temperature of 120° C. or more and within a range of from preferably 120 to 180° C. are preferably added for the achievement of establishing both of the surface smoothness and the edge coatability, from the view point of the dynamic viscoelasticities, in particular, from the view points of the loss elasticity modulus (G″) at 80° C. and the storage elasticity modulus (G′) at 140° C. 
     The average particle size of the crosslinked resin particles used in the present invention is within a range of from 1.0 to 3.0 μm, as described above, but the lower limit is preferably 1.2 μm and further preferably 1.5 μm. On the other hand, the upper limit is preferably 2.5 μm and further preferably 2.2 μm. As described above, when it is less than 1.0 μm, it is within the range of the average particle size of resin particles in a conventional technology, and it is not preferable because the surface smoothness is deteriorated. The crosslinked resin particles having an average particle size of more than 3.0 μm provide the lowering of smoothness caused by the sedimentation in an electrodeposition coating composition at no stirring and by the accumulation of particles on a horizontal plane at an electrodeposition coating caused by dropping. The average particle size herein can be measured by the method below. 
     The average particle size of the resin particles is measured by a granular particle transmission measurement method using MICROTRAC9340UPA manufactured by Nikkiso Co., Ltd. Further, the particle size distribution of the resin particles is measured in a measurement device, and the average particle size at cumulative relative frequency F(x)=0 is calculated from the measurement values. These measurements and calculations employ the refractive index of 1.33 of solvent (water) and the refractive index of 1.59 of the resin content. 
     The crosslinked resin particle used for the present invention have a thermal softening temperature within a range of from 120 to 180° C., as described above, for establishing both of the surface smoothness and the edge coatability in a cationic electrodeposition coating composition with low ash and low solid content, but the upper limit value is preferably 140° C. and more preferably 160° C. 
     When the thermal softening temperature is lower than 120° C., the storage elasticity modulus G′ is not a given value at baking the uncured electrodeposition film, and the edge coatability cannot be secured. On the other hand, a material in which the thermal softening temperature of the crosslinked resin particle exceeds 180° C. cannot substantially be synthesized. 
     The thermal softening temperature is a temperature at which the crosslinked resin particle starts to be softened. Namely, G′ values at the respective temperatures of the objective crosslinked resin particles are determined. The temperature at a point at which the changes of G′ values for the temperature changes are abruptly changed is called as a thermal softening temperature. It can be determined according to the followings. The storage elasticity modulus G′ of a sample obtained by adjusting the concentration of the crosslinked resin particles to 30% by weight (as a solid content) is measured from 90° C. under conditions of a strain of 0.5 degree, a frequency of 0.02 Hz and a rising temperature rate of 4.0° C./min in a temperature dependent measurement with Rheosol-G3000 (manufactured by UBM Corporation) that is a rotational type dynamic viscoelasticity measurement device. The measurement results are shown in a graph in  FIG. 8 . As seen in  FIG. 8 , although the storage elasticity modulus G′ of the crosslinked resin particle keeps a constant viscosity at an initial temperature region (about 90 to 140° C. in  FIG. 8 ), the lowering of the storage elasticity modulus G′ begins to occur at a temperature (temperature exceeding 140° C. in  FIG. 8 ). The tangential line in an area at which viscosity is a constant and the tangential line in an area at which the lowering of viscosity occurs are drawn, and the temperature at the cross point is defined as a thermal softening temperature. 
     In order to increase the thermal softening temperature of the resin particle, the crosslinking degree of the resin particle is required to be increased. It is necessary for securing the thermal softening temperature area in the present invention that the resin particle is a crosslinked resin particle. Glass transition temperature is also an index of softening of a resin, but when the glass transition temperature (Tg) is measured in the crosslinked resin particle, it reaches at a level of several hundred order (° C.); therefore the thermal decomposition of the resin is frequent at the temperature, and the softening property of particle itself cannot be observed. Accordingly, the thermal softening temperature is employed in the present invention. 
     Further, the crosslinked resin particle is required to have a crosslinking structure. In case of no crosslinking structure, the value of the above-mentioned storage elasticity modulus G′ at 140° C. is less than 80 dyn/cm 2 , and it is not preferable because the edge coatability cannot be secured. The crosslinked resin particle is preferably used in an amount of 3 to 15% by weight relative to the weight of the solid resin contents of the cationic electrodeposition coating composition. When the content of the crosslinked resin particles is less than 3% by weight, the establishment of both of the surface smoothness and the edge coatability is difficult, and when it exceeds 15% by weight, there is a fear that the lowering of a coating film performance such as anticorrosion property is provided. Herein, the “solid resin content(s)” mean(s) all of the solid content(s) weight of the resin components (including the crosslinked resin particles) contained in the cationic electrodeposition coating composition. 
     The content of the crosslinked resin particles in the present invention is within a range of from preferably 3 to 15% by weight relative to the weight of the solid resin contents in the cationic electrodeposition coating composition with low ash and low solid content for establishment of both of the surface smoothness and the edge coatability, but its lower limit is preferably 4% by weight and further preferably 5% by weight. On the other hand, its upper limit is preferably 10% by weight and further preferably 8% by weight. 
     Considering that the average particle size of the crosslinked resin particles is within a range of from 1.0 to 3.0 μm, they are preferably produced by a suspension polymerization. Although it is also possible to produce them by other process such as an emulsion polymerization if their particle size and the thermal softening temperature satisfy the above-mentioned range, but the suspension polymerization is preferable from the aspect of arranging the particle size within a desired range. 
     The crosslinked resin particles include, but are not specifically limited to, for example, resin particles containing a resin having a crosslinking structure obtained by mainly using an ethylenically unsaturated monomer, resin particles containing a urethane resin internally crosslinked, fine resin particles containing a melamine resin internally crosslinked, and the like. 
     The above-mentioned resin having a crosslinking structure obtained by mainly using an ethylenically unsaturated monomer includes, but is not specifically limited to, for example, resin particles internally crosslinked that are obtained by carrying out a suspension polymerization of a monomer composition containing a crosslinking monomer as an essential component and an ethylenically unsaturated monomer, in an aqueous medium, to prepare an aqueous dispersion, and substituting the above-mentioned aqueous dispersion with a solvent; resin particles internally crosslinked obtained by a NAD method of dispersing resin particles internally crosslinked that are obtained by carrying out the copolymerization of a monomer composition containing a crosslinking monomer as an essential component and an ethylenically unsaturated monomer, in a non-aqueous organic solvent that dissolves a monomer but does not dissolve a polymer such as a low SP organic solvent such as an aliphatic hydrocarbon, a high SP organic solvent such as an ester, a ketone and an alcohol, or by a sedimentation-precipitation method, or the like. 
     The above-mentioned ethylenically unsaturated monomer includes, but is not specifically limited to, for example, the alkyl esters of acrylic acid or methacrylic acid such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl(meth)acrylate, isobutyl (meth)acrylate and 2-ethylhexyl (meth)acrylate; styrene, α-methylstyrene, vinyl toluene, t-butylstyrene, ethylene, propylene, vinyl acetate, vinyl propionate, acrylonitrile, methacrylonitrile, dimethylaminoethyl (meth)acrylate, and the like. Two or more of the above-mentioned ethylenically unsaturated monomers may be used in combination. 
     The above-mentioned crosslinking monomer includes, but is not specifically limited to, for example, a monomer having 2 or more of ethylenically unsaturated bonds, that are radically polymerizable, in the molecule, a monomer having 2 or more of ethylenically unsaturated groups respectively supporting mutually reactive groups, etc. 
     The monomer having 2 or more of ethylenically unsaturated bonds, that are radically polymerizable, in the molecule, that can be used for the production of the above-mentioned internally crosslinked fine resin particles includes, but is not specifically limited to, for example, the polymerizable unsaturated monocarboxylic acid esters of polyalcohols such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, 1,4-butanediol diacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, 1,6-hexanediol diacrylate, pentaerythritol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, pentaerythritol dimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, glycerol dimethacrylate, glycerol diacrylate, glycerolaryloxy dimethacrylate, 1,1,1-trishydroxymethylethane diacrylate, 1,1,1-trishydroxymethylethane triacrylate, 1,1,1-trishydroxymethylethane dimethacrylate, 1,1,1-trishydroxymethylethane trimethacrylate, 1,1,1-trishydroxymethylpropane diacrylate and 1,1,1-trishydroxymethylpropane dimethacrylate; polymerizable unsaturated alcohol esters of polybasic acids such as triallyl cyanurate, triallyl isocyanurate, triallyl trimellitate, diallyl terephthalate and diallyl phthalate; aromatic compounds substituted with 2 or more of vinyl groups such as divinyl benzene, etc. 
     The combination of mutually reactive functional groups existing in the above-mentioned monomer having 2 or more of ethylenically unsaturated groups respectively supporting mutually reactive groups includes, but is not specifically limited to, for example, the combinations of an epoxy group and a carboxyl group, an amino group and a carbonyl group, an epoxy group and a carboxylic anhydride group, an amino group and a carboxylic acid chloride group, an alkyleneimino group and a carbonyl group, an organoalkoxysilane group and a carboxyl group, a hydroxyl group and isocyanate glycidyl acrylate group, and the like. Among others, the combination of an epoxy group and a carboxyl, group is more preferable. 
     The above-mentioned resin particles containing a urethane resin internally crosslinked are fine resin particles composed of polyurethane polymer that is obtained by reacting a polyisocyanate component with an active hydrogen containing component having diol having a hydroxy group at a terminal and diol or triol having a carboxyl group to form a polyurethane prepolymer containing an isocyanate terminal group having a carboxylic acid salt at a side chain, and successively reacting the prepolymer with a chain elongating agent containing an active hydrogen. 
     The polyisocyanate component used for the above-mentioned prepolymer includes aromatic diisocyanates such as diphenylmethane-4,4′-diisocyanate, tolylene diisocyanate and xylylene diisocyanate; aliphatic diisocyanates such as hexamethylene diisocyanate and 2,2,4-trimethylhexane diisocyanate; alicyclic diisocyanates such as 1-cyclohexane diisocyanate, 1-isocyanato-3-isocyanatomethyl-3,5-trimethylcyclohexane (isophorone diisocyanate), 4,4′-dicyclohexylmethane diisocyanate and methylcyclohexylene diisocyanate; and the like. The above-mentioned polyisocyanate component is more preferably hexamethylene diisocyanate and isophorone diisocyanate. 
     The above-mentioned diol having a hydroxy group at a terminal includes, but is not specifically limited to, for example, polyether diol, polyester diol or polycarbonate diol having a molecular weight of 100 to 5000 and the like. The diol having a hydroxy group at a terminal includes, but is not specifically limited to, for example, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polybutyrene adipate, polyhexamethylene adipate, polyneopentyl adipate, polycaprolactone diol, poly-3-methylvalerolactone diol, polyhexamethylene carbonate, and the like. 
     The above-mentioned diol containing a carboxyl group includes, but is not specifically limited to, for example, dimethylol acetate, dimethylol propionate, dimethylol lactate, and the like. Among others, dimethylol propionate is preferable. 
     The above-mentioned triol includes, but is not specifically limited to, for example, trimethylol propane, trimethylol ethane, glycerine polycaprolactone triol, and the like. The inside of urethane resin particles has a crosslinking structure by using a triol. 
     The above-mentioned fine resin particles containing a melamine resin internally crosslinked include, but is not specifically limited to, for example, melamine resin particles internally crosslinked that are obtained by dispersing a melamine resin and a polyol, in the presence of an emulsifier, in water, and then, carrying out the crosslinking reaction of the melamine resin and the polyol in the particles formed by dispersing; and the like. 
     The above-mentioned melamine resin includes, but is not specifically limited to, for example, di-; tri-, tetra-, penta- and hexa-methylol melamines and alkyl ethers thereof (alkyl is methyl, ethyl, propyl, isopropyl, butyl or isobutyl), and the like. As the above-mentioned melamine resin that is commercially available, for example, resins such as CYMEL 303, CYMEL 325, CYMEL 1156 (manufactured by Mitsui Cytec Industries Inc.) can be mentioned. 
     The above-mentioned polyol includes, but is not specifically limited to, for example, triol or tetrol having a molecular weight of 500 to 3000, and the like. The above-mentioned polyol is more preferably polypropylene ether triol and polyethylene ether triol. 
     The above-mentioned crosslinked resin particles may be those obtained by isolating the internally crosslinked fine resin particles by methods such as filtration, spray drying and freeze drying, and pulverizing them to an appropriate particle size, as they are or using a mill, to be used in a state of powder; an aqueous dispersion obtained as they are; or those in which medium is replaced with solvent replacement to be used. 
     As the second process adjusting the above-mentioned viscoelasticity behaviors, there is a process by which an inorganic pigment is used at an amount within a range of from 10 to 20% by weight (hereinafter, occasionally called as “PWC”) relative to the weight of the solid contents of a cationic electrodeposition coating composition. In a conventional cationic electrodeposition coating composition, the above-mentioned PWC exceeds 20% by weight, and is set as 25% by weight or less; therefore both of the smoothness and edge coatability could not be established, but both of the smoothness and edge coatability can be established by using the PWC within a range of from 10 to 20% by weight. Herein, the PWC means a proportion for all of the solid contents of the resin components and pigment components contained in the cationic electrodeposition coating composition. When the PWC of the inorganic pigment is less than 10% by weight, the content of a resin is much, and the resin is softened by the rising of temperature; therefore objective high viscosity cannot be obtained, and the above-mentioned viscosity behaviors cannot be adjusted. On the other hand, when the PWC exceeds 20% by weight, pigments become adversely much, fusing effects by the resin cannot be obtained, and as a result, high viscosity is not expressed; therefore the control of viscoelasticity is difficult. Further, as described above, the PWC for the inorganic pigment affects the viscosity behaviors, but the particle size does not affect the viscosity behaviors so much. 
     The inorganic pigment, as used herein, is not specifically limited so far as it is a pigment usually used for an electrodeposition coating composition. The example of the pigment includes inorganic pigments usually used, for example, coloring pigments such as titanium white and colcothar; filler pigments such as kaolin, talc, aluminum silicate, calcium carbonate, mica and clay; anticorrosive pigments such as zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate, aluminum phosphomolybdate, aluminum zinc phosphomolybdate, bismuth compounds and cerium compounds, etc. 
     The third process adjusting the above-mentioned viscoelasticity behaviors is a process of using the above-mentioned crosslinked resin particle and inorganic pigment in a combination. In this case, the average particle size of the above-mentioned crosslinked resin particles is within a range of from 1.0 to 3.0 μm, and its amount to be used is within a range of from 3 to 15% by weight relative to the weight of the solid contents in a coating composition. On the other hand, the amount of inorganic pigment to be used (PWC) can be reduced to within a range of from 0.5 to 10% by weight relative to the weight of the solid contents in the cationic electrodeposition coating composition. Its lower limit is preferably 1% by weight, and further preferably 2% by weight. On the other hand, its upper limit is preferably 7% by weight, and further preferably 5% by weight. When it is used in an amount exceeding 10% by weight, the pigment amount is much more than the necessary amount, and there is a fear of the deterioration of the planar appearance caused by the sedimentation of the pigment. Further, when it is less than 0.5% by weight, there is a fear of lowering color-hiding property. 
     The amount of the inorganic pigments can be further reduced by using both of the inorganic pigment and the crosslinked resin particle, and as a result, the reduction of energy and labor for preventing the sedimentation of the solid contents in the electrodeposition coating composition can be expected. Further, when the viscoelasticity behaviors are adjusted only by using the crosslinked resin particle without using the inorganic pigment, the above-mentioned energy and labor for preventing the above-mentioned sedimentation of the solid contents can be greatly reduced. Further, when the inorganic pigment is not contained or when an extremely small amount of the inorganic pigment is contained even if the inorganic pigment is contained, the water rinsing step is greatly shortened although the rinsing of a coated article with water is carried out after the electrodeposition coating; therefore great effects for the simplification of the facilities and the reduction of using resources are provided. 
     Then, components used for a general cationic electrodeposition coating composition are described. 
     Cationic Electrodeposition Coating Composition 
     The cationic electrodeposition coating composition comprises an aqueous media; a binder resin comprising a cationic epoxy resin dispersed or dissolved in an aqueous media and a blocked isocyanate curing agent; a neutralizing acid; and an organic solvent. The cationic electrodeposition coating composition may further comprise an inorganic pigment. Content of the inorganic pigment is preferably no more than 7% by weight relative to weight of the solid contents of the cationic electrodeposition coating composition. As stated above, in order to realize the low ash content, the composition may comprise no inorganic pigments. As stated above, in the present invention, in order to provide a low ash and/or low solid type cationic electrodeposition coating composition having both of the surface smoothness and edge cotability, the cationic electrodeposition coating composition may comprise the particular crosslinked resin particles. 
     Cationic Epoxy Resin 
     The cationic epoxy resin which may be employed in the present invention includes an epoxy resin modified with an amine. The cationic epoxy resin is typically produced by opening all of the epoxy rings of a bisphenol type epoxy resin with an active hydrogen compound which can introduce a cationic group, or by opening a portion of epoxy rings with other active hydrogen compound and then opening the residual epoxy rings with an active hydrogen compound which can introduce a cationic group. 
     A typical example of the bisphenol type epoxy resin includes a bisphenol A type epoxy resin and a bisphenol F type epoxy resin. The commercially available product of the former includes YD-7011R (manufactured by Tohto Kasei Co., Ltd., epoxy equivalent: 460 to 490), Epikote 828 (manufactured by Yuka-Shell Epoxy Co., Ltd., epoxy equivalent: 180 to 190), Epikote 1001 (the same manufacturer, epoxy equivalent: 450 to 500), Epikote 1010 (the same manufacturer, epoxy equivalent: 3000 to 4000) and the like, and the commercially available product of the latter includes Epikote 807 (the same manufacturer, epoxy equivalent: 170) and the like. 
     An oxazolidone ring-containing epoxy resin which is represented by the following formula and disclosed in JP-A-5-306327: 
     
       
         
         
             
             
         
       
     
     wherein R means a residual group formed by removing a glycidyloxy group of a diglycidylepoxy compound, R′ means a residual group formed by removing an isocyanate group of a diisocyanate compound, and n means a positive integer, may be used as the cationic epoxy resin. This is because the resulting coating film is superior in heat resistance and corrosion resistance. 
     An example of the method for introducing an oxazolidone ring into an epoxy resin includes reacting a polyepoxide with a blocked isocyanate curing agent which has been blocked with a lower alcohol such as methanol, in the presence of a basic catalyst, with heating and keeping its temperature, and distilling off a lower alcohol as a resulting by-product from the system to give the product. 
     It is known that a reaction of a bifunctional epoxy resin with a monoalcohol-blocked diisocyanate (i.e., bisurethane) gives an oxazolidone ring-containing epoxy resin. Examples of the oxazolidone ring-containing epoxy resin and preparation thereof are known and disclosed in JP-A-2000-128959, paragraphs 0012 to 0047. 
     Such epoxy resin may be modified with an appropriate resin such as polyester polyol, polyether polyol and monofunctional alkylphenol. Furthermore, the epoxy resin can extend its chain by utilizing the reaction of an epoxy group with a diol or a dicarboxylic acid. 
     It is desirable that the ring of the epoxy resin is opened with an active hydrogen compound so that an amine equivalent is 0.3 to 4.0 meq/g, after ring opening, and the primary amino group occupies more preferably 5 to 50% therein. 
     The active hydrogen compound which can introduce a cationic group includes the acid salts of primary amine, secondary amine and tertiary amine, sulfide and an acid mixture. The acid salts of primary amine, secondary amine or/and tertiary amine(s) are used as the active hydrogen compound which can introduce a cationic group in order to prepare an epoxy resin containing primary amino, secondary amino or/and tertiary amino group(s). 
     Specific examples include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methyl-ethanolamine, triethylamine hydrochloride, N,N-dimethyl-ethanolamine acetate, a mixture of diethyldisulfide and acetic acid, and secondary amine, which is a blocked primary amine, such as ketimine of aminoethylethanolamine and diketimine of diethylenetriamine, etc. One or more amines are available in a combination. 
     Blocked Isocyanate Curing Agent 
     As a polyisocyanate for a blocked isocyanate curing agent to be employed in the present invention means a compound having 2 or more of isocyanate groups in a molecule. An example of the polyisocyanate includes any type of polyisocyanates, such as an aliphatic type, an alicyclic type, an aromatic type, an aromatic-aliphatic type, etc. 
     Specific example of the polyisocyanate includes aromatic diisocyanates such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), p-phenylene diisocyanate and naphthalene diisocyanate; aliphatic diisocyanates having 3 to 12 carbon atoms, such as hexamethylene diisocyanate (HDI), 2,2,4-trimethylhexane diisocyanate and lysine diisocyanate; alicyclic diisocyanates having 5 to 18 carbon atoms, such as 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI), 4,4′-dicyclohexylmethane diisocyanate (hydrogenated MDI), methylcyclohexane diisocyanate, isopropylidene dicyclohexyl-4,4′-diisocyanate and 1,3-diisocyanatomethylcyclohexane (hydrogenated XDI), hydrogenated TDI and 2,5- or 2,6-bis(isocyanatomethyl)-bicyclo[2.2.1]heptane (also called as norbornane diisocyanate); aliphatic diisocyanates having an aromatic ring, such as xylylene diisocyanate (XDI) and tetramethylxylylene diisocyanate (TMXDI); the modified products of these diisocyanates (e.g., urethanated product, carbodiimides, urethodione, urethoimine, biuret and/or isocyanurate modified product), etc. These can be used alone or 2 or more thereof can be used in combination. 
     An adduct or a prepolymer which is obtained by reacting a polyisocyanate with a polyalcohol such as ethylene glycol, propylene glycol, trimethylolpropane or hexanetriol at a ratio NCO/OH of 2 or more may be also used as a blocked isocyanate curing agent. 
     The blocking agent is added to a polyisocyanate group, stable at ambient temperature, but can regenerate a free isocyanate group when it is heated to the dissociation temperature or more. 
     The blocking agent includes conventional blocking agents, such as ε-caprolactam, butyl cellosolve, etc. 
     The cationic electrodeposition coating composition comprises crosslinked resin particles as an component, the crosslinked resin particles may be added to the electrodeposition coating composition at any stage of the preparing process. Preferably, the crosslinked resin particles may be directly added to the previously prepared cationic electrodeposition coating composition. 
     Inorganic Pigment 
     The electrodeposition coating composition used in the present invention may contain a conventional inorganic pigment. When it is used in a low ash type, the content of the pigment, in particular, inorganic pigment may be reduced or the pigment may not be added. The example of the inorganic pigment includes conventional inorganic pigments, for example, coloring pigments such as titanium white and colcothar; filler pigments such as kaolin, talc, aluminum silicate, calcium carbonate, mica and clay; anticorrosive pigments such as zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate, aluminum phosphomolybdate, aluminum zinc phosphomolybdate, bismuth oxide, bismuth hydroxide, basic bismuth carbonate, bismuth nitrate and bismuth sulfate, and the like. 
     The content of the inorganic pigment is 7% by weight or less, and preferably 5% by weight or less and more preferably 3% by weight or less, relative to the weight of the solid resin contents in the cationic electrodeposition coating composition. Further, the “percent weight” relative to the weight of the solid resin contents is called as PWC. When the concentration of the inorganic pigment exceeds 7% by weight, low ash cannot be adequately attained; therefore energy load for the prevention of the sedimentation is increased. 
     When the pigment is used as a component of the electrodeposition coating composition, these pigments are generally dispersed in an aqueous medium at a high concentration preliminarily to be a paste (i.e., pigment dispersed paste). Since the pigment is a powder, it is difficult to disperse the powder at one step in an uniform state at a low concentration to be used for the electrodeposition coating composition. Such paste is generally called as a pigment dispersed paste. 
     The pigment dispersed paste is prepared by dispersing the pigments together with a pigment dispersing resin in an aqueous medium. As the pigment dispersing resin, a cationic or nonionic low molecular weight surfactant, or a cationic polymer such as a modified epoxy resin having a quaternary ammonium group and/or a tert-sulfonium group is generally used. As the aqueous medium, ion exchanged water, water containing a small amount of an alcohol, and the like are employed. 
     In general, the pigment dispersing resin is used in an amount of 20 to 100 parts by weight based on 100 parts by weight of the pigments (as a basis of the solid content). After the pigment dispersing resin is mixed with a pigment, the pigment is dispersed using a usual dispersion device such as a ball mill or a sand grind mill until the particle size of the pigment in the mixture becomes a certain uniform particle size to give a pigment dispersed paste. 
     The cationic electrodeposition coating composition used in the present invention may contain an organotin compound such as dibutyltin laurate, dibutyltin oxide and dioctyltin oxide; amines such as N-methylmorpholine; and metal salts such as strontium salts, cobalt salts and copper salts, as a catalyst, in addition to the above-mentioned components. These can act as a catalyst for dissociation of the blocking agent from the curing agent. The concentration of the catalyst is preferably 0.1 to 6 parts by weight based on 100 parts by weight of the solid contents in the total of the cationic epoxy resin and the curing agent in the electrodeposition coating composition. 
     Preparation of Cationic Electrodeposition Coating Composition 
     The cationic electrodeposition coating composition of the present invention can be prepared by dispersing the above-mentioned cationic epoxy resin and a blocked isocyanate curing agent, and if necessary, the crosslinked resin particles and/or a pigment-dispersed paste and a catalyst, in aqueous medium. Further, the aqueous medium usually contains a neutralizing acid for neutralizing the cationic epoxy resin to improve the dispersibility. The neutralizing acid includes inorganic acids or organic acids, such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, lactic acid, sulfamic acid and acetylglycine. The aqueous medium, as used herein, is water or a mixture of water with an organic solvent. Ion exchanged water is preferably used as water. The example of the usable organic solvent includes hydrocarbons (for example, xylene or toluene), alcohols (for example, methyl alcohol, n-butyl alcohol, isopropyl alcohol, 2-ethylhexyl alcohol, ethylene glycol and propylene glycol), ethers (for example, ethyleneglycol monoethyl ether, ethyleneglycol monobutyl ether, ethyleneglycol monohexyl ether, propyleneglycol monoethyl ether, 3-methyl-3-methoxybutanol, diethyleneglycol monoethyl ether and diethyleneglycol monobutyl ether), ketones (for example, methyl isobutyl ketone, cyclohexanone, isophorone and acetylacetone), esters (for example, ethyleneglycol monoethyl ether acetate and ethyleneglycol monobutyl ether acetate), and a mixture thereof. 
     The cationic electrodeposition coating composition of the present invention may contain the crosslinked resin particle. As a method for the addition, the crosslinked resin particle may be added at any stage during the production stages of the electrodeposition coating composition, and it is preferable to directly add the crosslinked resin particle to the previously produced cationic electrodeposition coating composition. 
     The amount of the blocked isocyanate curing agent must be adequate for the curing reaction with a functional group containing an active hydrogen, such as the primary amino group, secondary amino group or a hydroxyl group in the cationic epoxy resin, to provide a good cured coating. In general, the weight ratio of the solid contents in the cationic epoxy resin to the solid contents in the blocked isocyanate curing agent is generally within a range of from 90/10 to 50/50 and preferably 80/20 to 65/35 (epoxy resin/curing agent). The amount of neutralizing acid is an amount adequate for neutralizing at least 20% and preferably 30 to 60% of the cationic group of the cationic epoxy resin. 
     The organic solvent is an essential as a solvent for preparing the resin components such as the cationic epoxy resin and the blocked isocyanate curing agent. The complex operations are necessary for completely removing the solvent. 
     Further, when an organic solvent is contained in the cationic epoxy resin as a binder resin component, the fluidity of a coating film during the film formation is improved, and the smoothness of the coating film is improved. 
     The organic solvent usually contained in the coating composition includes ethyleneglycol monobutyl ether, ethyleneglycol monohexyl ether, ethyleneglycol monoethylhexyl ether, propyleneglycol monobutyl ether, dipropyleneglycol monobutyl ether, propyleneglycol monophenyl ether, and the like. 
     The cationic electrodeposition coating composition can contain a conventional additive for a coating composition, such as a plasticizer, a surfactant, an antioxidant and an ultraviolet absorbent, in addition to the above-mentioned components. 
     According to the present invention, in the case of the cationic electrodeposition coating composition with low solid content, the solid content concentration is set at 20% by weight or less. The conventional content is 20% by weight. Specifically, the solid content concentration of the coating composition is within a range of from preferably 0.5 to 9% by weight, and its lower limit value is preferably 2% by weight and more preferably 4% by weight. On the other hand, its upper limit value is preferably 7% by weight and more preferably 6% by weight. When the solid content concentration is less than 0.5% by weight, the appropriate coating film cannot be formed, and when it is higher than 9% by weight, effects such as the removal of a rinsing step with water and the simplification of the facilities, these are effects caused by low solid content, cannot be obtained in the cationic electrodeposition coating process. Herein, the solid content concentration means a concentration relative to the total weight of the pigment(s) component and the resin component(s) (also including the crosslinked resin particle component) (as a basis of the solid content) in a cationic electrodeposition coating composition. Thus, the low solid content has fear of lowering the electric conductivity of the cationic electrodeposition coating composition. Accordingly, it is preferable to separately add an electroconductivity controlling agent. 
     The electroconductivity controlling agent used for the present invention is not specifically limited so far as it is a material adjusting the electroconductivity of the cationic electrodeposition coating composition within a desired range, but the electroconductivity controlling agent composed of an amino group-containing containing compound having an amine value of 200 to 500 mmol/100 g is preferable. When the amine value is adjusted for the electroconductivity controlling agent for the cationic electrodeposition coating composition of the present invention within the above-mentioned range, it may be any compound containing an amino group, but generally, the electroconductivity controlling agent is preferably an amine modified epoxy resin or an amine modified acryl resin. Further, the electroconductivity controlling agent for the cationic electrodeposition coating composition of the present invention may be neutralized by an acid, if necessary. The amine value is preferably 250 to 450 mmol/100 g and most preferably 300 to 400 mmol/100 g. When the amine value is less than 200 mmol/100 g, addition amount necessary for adjusting the electroconductivity of the cationic electrodeposition coating composition with low solid content concentration to an optimum value is increased, and there is a fear of loosing anticorrosive property. Further, when it exceeds 500 mmol/100 g, it has defects that depositability is lowered and the desired throwing power is not obtained. Further, the adaptability to a zinc steel plate is also lowered. 
     The above-mentioned electroconductivity controlling agent includes an amino-group containing compound having from a low molecular weight to a high molecular weight, such as a conventional high molecular weight resin such as amine modified epoxy resins and amine modified acryl resins. The example of the low molecular weight compound containing an amino group includes monoethanolamine, diethanolamine, dimethylbutylamine, and the like. 
     The high molecular weight compound containing an amino group is preferable, and in particular, the amine modified epoxy resins and the amine modified acryl resins are preferable. The amine modified epoxy resin is obtained by modifying an epoxy group of an epoxy resin with an amine compound. As the epoxy resin, general epoxy resins can be used, and a bisphenol type epoxy resin, a t-butylcathecol type epoxy resin, a phenolnovolak type epoxy resin and a cresolnovolac type epoxy resin, that have a molecular weight of 500 to 20000, are preferable. Among these epoxy resins, a phenolnovolak type epoxy resin and a cresolnovolac type epoxy resin are most desirable. In particular, these epoxy resins are commercially available. Example of the epoxy resin includes a phenolnovolak type epoxy resin DEN-438 manufactured by Dow Chemical Japan Co., Ltd.; a cresolnovolac type epoxy resin YDCN-703 manufactured by Tohto Kasei Co., Ltd., etc. 
     These epoxy resins may be modified with resins such as polyester polyol, polyether polyol and monofunctional alkylphenol. Further, the epoxy resin can extend its chain utilizing a reaction of an epoxy group with a diol or a dicarboxylic acid. 
     As the amine modified acryl resin, for example, the homopolymer of dimethylaminoethyl methacrylate that is a monomer containing an amino group, or a copolymer of dimethylaminoethyl methacrylate with other polymerizable monomer may be used as it is, and it can be obtained by modifying the glycidyl group of the homopolymer of glycidyl methacrylate or the glycidyl group of a copolymer of glycidyl methacrylate with other polymerizable monomer, with an amine compound. 
     The compound introducing an amino group to the epoxy resin or the acryl resin containing an epoxy group includes primary amines, secondary amines, tertiary amines, and the like. Their specific example includes butylamine, octylamine, diethylamine, butylamine, dimethylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine hydrochloride, N,N-dimethylethanolamine hydrochloride, a mixture of diethyldisulfide and acetic acid, and additionally, secondary amines that are blocked primary amines such as the diketimine of aminoethylethanolamine and the diketimine of diethylhydroamine. A plurality of the amines may be used. 
     As described above, the number average molecular weight of the amine modified epoxy resin or the amine modified acryl resin is within a range of from preferably 500 to 20000. When the number average molecular weight is smaller than 500, there is a fear of losing anticorrosive property, and although the reason is not clear, the throwing power is lowered and the adaptability to a zinc steel plate is lowered. When the number average molecular weight is larger than 20000, there is a fear of providing the deterioration of the finishing appearance. 
     The amine modified epoxy resin and/or the amine modified acryl resin can be also used by being preliminarily neutralized by a neutralizing acid. Acid used for neutralization includes inorganic and organic acids such as hydrochloric acid, nitric acid, phosphoric acid, sulfamic acid, formic acid, acetic acid and lactic acid. 
     Application of Cationic Electrodeposition Coating Composition 
     The above-mentioned cationic electrodeposition coating composition is applied on an article by an electrodeposition to form an electrodeposition film. The article includes, but is not specifically limited to, so far as it is electroconductive, for example, an iron plate, a steel plate, an aluminum plate and a surface treated article thereof, and a molded article thereof, etc. 
     The electrodeposition coating with the cationic electrodeposition coating composition is usually carried out by applying a voltage within a range of from 50 to 450 V between an anode and a cathode which is an article to be coated. When the applied voltage is less than 50 V, the electrodeposition is inadequate, and when it exceeds 450 V, the coating film is broken and the appearance is abnormal. During the electrodeposition coating, the temperature of the liquid coating composition in a bath is usually adjusted within a range of from 10 to 45° C. 
     The electrodeposition coating includes a step of immersing an article in a cationic electrodeposition coating composition, and a step of applying a voltage between an anode and a cathode, which is an article to be coated, to form an electrodeposited film. Further, the time for applying a voltage can be varied depending on the electrodeposition conditions and generally 2 to 4 min. 
     The thickness of the resulting electrodeposition film can be generally within a range of from 5 to 25 μm. When the film thickness is less than 5 μm, there is a fear of inadequate anticorrosive property, and when the film thickness exceeds 25 μm, the thickness is sufficient to provide the required coating film performances. Further, the film resistance of the electrodeposition film is within a range of from preferably 1000 to 1600 kΩ/cm 2  at a film thickness of 15 p.m. When the film resistance of the coating film is less than 1000 kΩ/cm 2 , it is a state in which adequate electric resistance is not obtained, and there is a fear of inferior throwing power. Further, when it exceeds 1600 kΩ/cm 2 , there is a fear of inferior coating film appearance. The film resistance of the coating film is within a range of from more preferably 1100 to 1500 kΩ/cm 2 . 
     The film resistance value of the coating film can be determined by the following formula according to the residual electric current value (A) of the coating film at the final coating voltage (V). 
       Film resistance value (FR)= V/A    
     After the electrodeposition coating, thus obtained electrodeposition film as it is or rinsed with water, baked at 120 to 260° C. and preferably 140 to 220° C. for 10 to 30 min to give a cured electrodeposition film. 
     The cured electrodeposition film of the present invention has an excellent surface smoothness or Ra value as an evaluation index of the surface smoothness, preferably 0.25 μm or less and more preferably 0.20 μm or less. Further, its lower limit value is preferably zero. Ra value is measured with an evaluation type surface roughness measuring machine (SURFTEST SJ-201P manufactured by Mitsutoyo Corporation) according to JIS-B0601. The smaller Ra value provides the better coating film appearance having a suppressed concavo-convex. 
     Further, in the present invention, there is provided a method for establishing both of the smoothness and the edge coatability of the cationic electrodeposition coating composition characterized in that the cationic electrodeposition coating composition comprises the crosslinked resin particles having an average particle size within a range of from 1.0 to 3.0 μm and a thermal softening temperature within a range of from 120 to 180° C. in a process of forming a cationic electrodeposition film by immersing an article in the cationic electrodeposition coating composition and applying a voltage. Further, in the present invention, even if the cationic electrodeposition coating composition is low solid type and low ash type, the ability of preventing the sedimentation of the solid contents in the electrodeposition coating composition can be improved by adding the specific crosslinked resin particle in the cationic electrodeposition coating composition as an additive, and both of the surface smoothness and edge coatability can be established. The amount in that case is within a range of from 3 to 15% by weight relative to the weight of the solid contents in the cationic electrodeposition coating composition. 
     EXAMPLES 
     The present invention is further specifically described below according to the Examples, but the present invention is not limited to these Examples. Further, the term “part(s)” represent(s) part(s) by weight unless otherwise noticed. 
     Production Example 1A 
     Production of Blocked Isocyanate Curing Agent 
     In a flask equipped with a stirrer, a cooler, a nitrogen charging tube, a thermometer and a dropping funnel, 199 parts of the trimer of hexamethylene diisocyanate (CORONATE HX: manufactured by Nippon Polyurethane Industry Co., Ltd.), 32 parts of methyl isobutyl ketone and 0.03 part of dibutyltin dilaurate were weighed, and 87.0 parts of methyl ethyl ketone oxime was added dropwise thereto from the dropping funnel over 1 hr, while stirring and bubbling nitrogen. Temperature was raised from 50° C. to 70° C. Thereafter, the reaction was continued for 1 hr, and the reaction was continued until the absorption of NCO group was extinguished by an infrared spectrometer. Then, 0.74 part of n-butanol and 39.93 parts of methyl isobutyl ketone were added to prepare a mixture with a non-volatile content of 80%. 
     Production Example 2A 
     Production of Amine Modified Epoxy Resin Emulsion 
     In a flask equipped with a stirrer, a cooler, a nitrogen charging tube and a dropping funnel, 71.34 parts of 2,4-/2,6-tolylene diisocyanate (80/20% by weight), 111.98 parts of methyl isobutyl ketone and 0.02 part of dibutyltin dilaurate were weighed, and 14.24 parts of methanol was added dropwise from the dropping funnel over 30 min while stirring and bubbling nitrogen. The temperature was raised from room temperature to 60° C. by exothermic heat. Then, after the reaction was continued for 30 minutes, 46.98 parts of ethyleneglycol mono-2-ethylhexyl ether was added dropwise from the dropping funnel over 30 min. Temperature was raised to 70 to 75° C. by exothermic heat. After the reaction was continued for 30 min, 41.25 parts of the adduct of bisphenol A with propylene oxide (5 mol) (BP-5P manufactured by Sanyo Kasei Co., Ltd.) was added to the mixture, temperature was raised to 90° C., and the reaction was continued while measuring IR spectrum until NCO group was extinguished. 
     Successively, 475.0 parts of a bisphenol A type epoxy resin having an epoxy equivalent of 475 (YD-7011R manufactured by Tohto Kasei Co., Ltd.) was added to be homogeneously dissolved, and then temperature was raised from 130° C. to 142° C., and water was removed from the reaction system by azeotrope with MIBK. After the reaction mixture was cooled to 125° C., 1.107 parts of benzyldimethylamine was added, and reaction of forming an oxazolidone ring by demethanolation was carried out. The reaction was continued until the epoxy equivalent was 1140. 
     Then, the mixture was cooled to 100° C., and 24.56 parts of N-methylethanolamine, 11.46 parts of diethanolamine and 26.08 parts of ketimine of aminoethylethanolamine (78.8% methyl isobutyl ketone solution) were added thereto to be reacted at 110° C. for 2 hrs. Then, 20.74 parts of ethyleneglycol mono-2-ethylhexyl ether and 12.85 parts of methyl isobutyl ketone were added to the mixture to be diluted, and non-volatile content was adjusted to 82%. Number average molecular weight (by GPC method) was 1380 and amine equivalent was 94.5 meq/100 g. 
     145.11 Parts of ion exchanged water and 5.04 parts of acetic acid were weighed in another container, a mixture of 320.11 parts (75.0 parts as solid content) of the above-mentioned amine modified epoxy resin and 190.38 parts (25.0 parts as solid content) of the blocked isocyanate curing agent of Production Example 1A, which was heated to 70° C., was added thereto dropwise gradually, and the mixture was stirred to be homogeneously dispersed. Then, ion exchanged water was added thereto to adjust the solid content to 36%. 
     Production Example 3A 
     Production of Pigment Dispersing Resin 
     In a flask equipped with a stirrer, a cooler, a nitrogen charging tube, a thermometer and a dropping funnel, 382.20 parts of a bisphenol A type epoxy resin having an epoxy equivalent of 188 (under product name: DER-331J) and 111.98 parts of bisphenol A were weighed, temperature was raised to 80° C. to dissolve the mixture homogeneously, then 1.53 parts of 1% solution of 2-ethyl-4-methylimidazole was added, and reaction was carried out at 170° C. for 2 hrs. After cooling the mixture to 140° C., 196.50 parts of 2-ethylhexanol-half blocked isophorone diisocyanate (non-volatile content: 90%) was added to the mixture, and the reaction was carried out until NCO group was extinguished. Thereto, 205.00 parts of dipropylene glycol monobutyl ether was added, successively, 408.00 parts of 1-(2-hydroxyethylthio)-2-propanol and 134.00 parts of dimethylol propionate were added, 144.00 parts of ion exchanged water was added, and the mixture was reacted at 70° C. The reaction was continued until acid value was 5 or less. The obtained resin varnish obtained was diluted to a non-volatile content of 35% with 1150.50 parts of ion exchanged water. 
     Production Example 4A 
     Production of Pigment Dispersed Paste 
     In a sand grind mill, 120 parts of the pigment dispersing resin varnish obtained in Production Example 3A, 100.0 parts of kaolin, 92 parts of titanium dioxide, 8.0 parts of dibutyltin oxide and 184 parts of ion exchanged water were charged, and dispersed until particle size was 10 μm or less to obtain a pigment dispersed paste (solid content: 48%). 
     Production Example 5A 
     Production of Crosslinked Resin Particles 
     In a reaction container, 120 parts of butylcellosolve was charged, and it was heated to 120° C. with stirring. Thereto, a solution which was a mixture of 2 parts of t-butylperoxy-2-ethylhexanoate and 10 parts of butylcellosolve, and a monomer mixture containing 15 parts of glycidyl methacrylate, 50 parts of 2-ethylhexyl methacrylate, 20 parts of 2-hydroxyethyl methacrylate and 15 parts of n-butyl methacrylate whose SP value was 10.1 were added dropwise over 3 hrs. After aging for 30 min, a solution which was a mixture of 0.5 part of t-butylperoxy-2-ethylhexanoate and 5 parts of butylcellosolve was added dropwise for 30 min, and after aging for 2 hrs, the mixture was cooled. Quartenization was carried out by adding 7 parts of N,N-dimethylaminoethanol and 15 parts of 50% aqueous lactic acid solution to the mixture with heating at 80° C. and stirring. When acid value was 1 or less and the rising of viscosity was stopped, heating was terminated to obtain an acryl resin having an ammonium group. The number of the ammonium group per one molecule of the acryl resin having an ammonium group was 6.0. 
     To the reaction container, 120 parts of the acryl resin having an ammonium group and 270 parts of deionized water were added, and the mixture was stirred with heating at 75° C. Thereto, the 100% neutralized aqueous solution of 1.5 parts of 2,2′-azobis(2-(2-imidazolin-2-yl)propane) with acetic acid was added dropwise over 5 min. After aging for 5 min, 30 parts of methyl methacrylate was added dropwise over 5 min. After aging further 5 min, an α,β-ethylenically unsaturated monomer mixture containing 170 parts of methyl methacrylate, 40 parts of styrene, 30 parts of n-butyl methacrylate, 5 parts of glycidyl methacrylate and 30 parts of neopentylglycol dimethacrylate was added to a solution which was a mixture of 170 parts of the acryl resin having an ammonium group and 250 parts of deionized water with stirring to give a pre-emulsion, and the pre-emulsion was added dropwise over 40 min. After aging for 60 min, it was cooled to give a dispersion of crosslinked resin particles  1 . The non-volatile content in the dispersion of the crosslinked resin particles was 35%, pH was 5.0 and an average particle size was 100 nm. 
     Production Example 6A 
     Production of Non-Crosslinked Resin Particles 
     2 Parts of lauroyl peroxide was dissolved in a solution which was a mixture of 104 parts of styrene, 20 parts of 2-ethylhexyl methacrylate and 76 parts of lauryl methacrylate. This was added in 497 parts of aqueous solution in which 8 parts of polyvinyl alcohol (GOUSENOL GH-17, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) was dissolved in deionized water, while stirring, and a dispersion was produced at 3500 rpm with a HOMOMIC LINE FLOW  30  type machine (high speed dispersing machine manufactured by TOKUSYU KIKA KOUGYOU Co., Ltd.). 
     The suspension polymerization of the suspension was carried out at a stirring speed of 150 rpm and a reaction temperature of 81 to 83° C. over 5 hrs using a usual batch wise reaction container, and after cooling, the resulted dispersion was filtered with a 200 mesh net to give non-crosslinked resin particles. The non-volatile content in the dispersion of the non-crosslinked resin particles was 30% and an average particle size was 3 μm. 
     Example 1A 
     2222 Parts of the emulsion obtained in Production Example 2A, 417 parts of the pigment dispersed paste obtained in Production Example 4A and 2361 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 16.5%, the content of the crosslinked resin particles was zero % by weight, and the solid content was 20% by weight. 
     Comparative Example 1A 
     738 Parts of the emulsion obtained in Production Example 2A, 4 parts of dibutyltin oxide and 4598 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was zero % by weight, and the solid content was 5% by weight. 
     Comparative Example 2A 
     702 Parts of the emulsion obtained in Production Example 2A, 38 parts of the crosslinked resin particles obtained in Production Example 5A, 4 parts of dibutyltin oxide and 4596 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 5% by weight, and the solid content was 5% by weight. 
     Comparative Example 3A 
     665 Parts of the emulsion obtained in Production Example 2A, 76 parts of the crosslinked resin particles obtained in Production Example 5A, 4 parts of dibutyltin oxide and 4596 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 10% by weight, and the solid content was 5% by weight. 
     Comparative Example 4A 
     665 Parts of the emulsion obtained in Production Example 2A, 89 parts of the non-crosslinked resin particles obtained in Production Example 6A, 4 parts of dibutyltin oxide and 4582 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the non-crosslinked resin particles was 10% by weight, and the solid content was 5% by weight. 
     Comparative Example 5A 
     389 Parts of the emulsion obtained in Production Example 2A, 125 parts of the pigment dispersed paste obtained in Production Example 4A and 3486 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 25%, the content of the crosslinked resin particles was 0% by weight, and the solid content was 5% by weight. 
     Example 2A 
     702 Parts of the emulsion obtained in Production Example 2A, 42 parts of the crosslinked resin particles (crosslinked resin particles in which methyl methacrylate was a main component; TAFTIC® F-200: manufactured by Toyobo Co., Ltd.), 4 parts of dibutyltin oxide and 4592 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 5% by weight, and the solid content was 5% by weight. 
     Example 3A 
     665 Parts of the emulsion obtained in Production Example 2A, 84 parts of the crosslinked resin particles (crosslinked resin particles in which methyl methacrylate was a main component; TAFTIC® F-200: manufactured by Toyobo Co., Ltd.), 4 parts of dibutyltin oxide and 4587 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 10% by weight, and the solid content was 5% by weight. 
     Example 4A 
     628 Parts of the emulsion obtained in Production Example 2A, 127 parts of the crosslinked resin particles (crosslinked resin particles in which methyl methacrylate was a main component; TAFTIC® F-200: manufactured by Toyobo Co., Ltd.), 4 parts of dibutyltin oxide and 4581 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 15% by weight, and the solid content was 5% by weight. 
     Example 5A 
     628 Parts of the emulsion obtained in Production Example 2A, 40 parts of the crosslinked resin particles (crosslinked resin particles in which Styrene monomer was a main component; CHEMISNOW SX500H: manufactured by Soken Chemical &amp; Engineering Co., Ltd., which had an average particle size of 3 μm), 4 parts of dibutyltin oxide and 4668 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was. 15% by weight, and the solid content was 5% by weight. 
     Example 6A 
     567 Parts of the emulsion obtained in Production Example 2A, 54 parts of the pigment dispersed paste obtained in Production Example 4A, 40 parts of crosslinked resin particles (crosslinked resin particles in which styrene monomer was a main component; CHEMISNOW SX500H: manufactured by Soken Chemical &amp; Engineering Co., Ltd., which had an average particle size of 3 μm) and 4739 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 8%, the content of the crosslinked resin particles was 15% by weight, and the solid content was 5% by weight. 
     With respect to thus prepared cationic electrodeposition coating compositions, the loss elasticity modulus at 80° C. and the storage elasticity modulus at 140° C. in dynamic viscoelasticities, and the smoothness and the edge coatability were evaluated by the methods below. 
     Measurement of Loss Elasticity Modulus and Storage Elasticity Modulus of Electrodeposition Film 
     A tin plate was immersed in the cationic electrodeposition coating composition prepared as described above. An electrodeposition film was formed by applying a voltage so that the film thickness after baking was 15 μm, and then the plate was rinsed with water to remove the excessive electrodeposition coating composition. Then, after removing moisture, without drying, the plate having the uncured coating film was immediately taken out to prepare a sample. The dynamic viscoelasticities of the sample were measured depending on the temperature with Rheosol-G3000 (manufactured by UBM Corporation) that was a rotational type dynamic viscoelasticity measurement device (under measurement conditions of a strain of 0.5 deg and a frequency of 0.02 Hz), wherein the sample was set, and the measurement temperature was kept at 50° C. After starting the measurement, the measurement of the viscosity of the coating film was carried out when the electrodeposition film was uniformly spread in a cone plate. 
     Evaluation of Appearance (Smoothness) of Electrodeposition Film 
     Evaluation of an appearance of an electrodeposition film was carried out by measuring an arithmetic average roughness (Ra) on a roughness curve. A cold rolling steel plate treated with zinc phosphate was immersed in the cationic electrodeposition coating composition prepared as described above. An uncured electrodeposition film obtained by applying a voltage, so that the film thickness after baking was 15 μm, was baked at 160° C. for 10 min. Then, the Ra value of the cured electrodeposition film was measured with an evaluation type surface roughness measuring machine (SURFTEST SJ-201 P manufactured by Mitsutoyo Corporation) in accordance with JIS-B0601. Measurement was repeated 7 times on the sample with cut-offs in a width of 2.5 mm (partition number was 5), and the Ra value was an average of the measured values without the maximum and minimum values. The results are shown in Table 1. It can be understood that the smaller Ra value provides the better coating film appearance with a suppressed concavo-convex. 
     Evaluation Method for Edge Coatability 
     A cutter knife blade (LB-50K manufactured by OLFA Co.) treated with zinc phosphate, as an article to be coated, was immersed into a cationic electrodeposition coating composition. A voltage was applied between an anode and a cathode, which is the above-described article, to give an electrodeposition film, wherein the above-mentioned electrodeposition conditions on the applying voltage and time were adjusted so that the thickness of the film electrodeposited on the knife blade was 15 μm. The resulted electrodeposition film was rinsed with water, and then baked at 160° C. for 10 min to give a cured electrodeposition film. 
     The cutter knife blade coated with the electrodeposition film was folded off in the center. The thickness of the electrodeposition film applied on the cutter knife blade was measured with a digital microscope (VH-8000 manufactured by KEYENCE Corporation) in a distance (30 microns) from the (sharp) edge of the cutter knife blade.  FIG. 9  schematically shows the point of the cutter knife blade in the distance, 30 microns, from the edge of the blade. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                 Comp. 
                 Comp. 
                 Comp. 
                 Comp. 
                 Comp. 
                   
               
               
                   
                 Ex. 1A 
                 Ex. 2A 
                 Ex. 3A 
                 Ex. 4A 
                 Ex. 5A 
                 Ex. 1A 
               
               
                   
               
               
                 Resin particle 
                 None 
                 Production 
                 Production 
                 Production 
                 None 
                 None 
               
               
                   
                   
                 Ex. 5A: 
                 Ex. 5A: 
                 Ex. 6A: 
               
               
                   
                   
                 Crosslinked 
                 Crosslinked 
                 Non-crosslinked 
               
               
                   
                   
                 Average 
                 Average 
                 Average particle 
               
               
                   
                   
                 particle 
                 particle 
                 size: 
               
               
                   
                   
                 size: 0.1 μm 
                 size: 0.1 μm 
                 1 to 5 μm 
               
               
                 Resin particle content (%) 
                 None 
                 5 
                 10 
                 10 
                 0 
                 None 
               
               
                 Ash (pigment) content (%) 
                 0 
                 0 
                 0 
                 0 
                 25 
                 16.5 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Melt 
                 Loss 
                 23 
                 156 
                 228 
                 50 
                 152 
                 89 
               
               
                 viscosity 
                 elasticity 
               
               
                   
                 modulus 
               
               
                   
                 G″ (dyn/cm 2 ) 
               
               
                   
                 Storage 
                 21 
                 73 
                 110 
                 50 
                 136 
                 155 
               
               
                   
                 elasticity 
               
               
                   
                 modulus 
               
               
                   
                 G′ (dyn/cm 2 ) 
               
               
                 Evaluation 
                 Smoothness 
                 0.19 
                 0.30 
                 0.37 
                 0.20 
                 0.33 
                 0.18 
               
               
                 results 
                 Ra (C/O = 2.5) 
               
               
                   
                 Edge 
                 3.6 
                 6.0 
                 8.3 
                 5.1 
                 8.0 
                 7.9 
               
               
                   
                 coatability 
               
               
                   
                 (μm) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Ex. 2A 
                 Ex. 3A 
                 Ex. 4A 
                 Ex. 5A 
                 Ex. 6A 
               
               
                   
                   
               
               
                   
                 Resin particle 
                 TAFTIC 
                 TAFTIC 
                 TAFTIC 
                 CHEMISNOW 
                 CHEMISNOW 
               
               
                   
                   
                 F-200: 
                 F-200: 
                 F-200: 
                 SX500H 
                 SX500H 
               
               
                   
                   
                 Average 
                 Average 
                 Average 
                 Average 
                 Average 
               
               
                   
                   
                 particle 
                 particle 
                 particle 
                 particle size: 
                 particle size: 
               
               
                   
                   
                 size: 
                 size: 
                 size: 
                 3 μm 
                 3 μm 
               
               
                   
                   
                 2 μm 
                 2 μm 
                 2 μm 
               
               
                   
                 Resin particle content (%) 
                 5 
                 10 
                 15 
                 15 
                 15 
               
               
                   
                 Ash (pigment) content (%) 
                 0 
                 0 
                 0 
                 0 
                 12 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Melt 
                 Loss 
                 88 
                 113 
                 142 
                 121 
                 144 
               
               
                   
                 viscosity 
                 elasticity 
               
               
                   
                   
                 modulus 
               
               
                   
                   
                 G″ (dyn/cm 2 ) 
               
               
                   
                   
                 Storage 
                 85 
                 107 
                 125 
                 116 
                 136 
               
               
                   
                   
                 elasticity 
               
               
                   
                   
                 modulus 
               
               
                   
                   
                 G′ (dyn/cm 2 ) 
               
               
                   
                 Evaluation 
                 Smoothness 
                 0.21 
                 0.21 
                 0.23 
                 0.24 
                 0.24 
               
               
                   
                 results 
                 Ra (C/O = 2.5) 
               
               
                   
                   
                 Edge 
                 7.5 
                 7.8 
                 8.2 
                 8.1 
                 8.3 
               
               
                   
                   
                 coatability 
               
               
                   
                   
                 (μm) 
               
               
                   
                   
               
            
           
         
       
     
     As seen in the Table 1, it is understood that the electrodeposition coating compositions having the loss elasticity modulus (G″) and the storage elasticity modulus (G′) within the defined ranges as dynamic viscoelasticities provide excellent performances in the smoothness and the edge coatability. Specifically, in the Comparative Example 1A, the electrodeposition coating composition having a storage elasticity modulus (G′) out of the defined range in the present invention does not provide a good edge coatability. In the Comparative Example 2A, the electrodeposition coating composition comprising the crosslinked resin particles of the Production Example 5A and having a loss elasticity modulus (G″) and a storage elasticity modulus (G′), both of which are out of the defined ranges in the present invention, does not provide a good smoothness and a good edge coatability. Similar to the Comparative Example 2A, in the Comparative Example 3A, the electrodeposition coating composition comprising the crosslinked resin particles of the Production Example 5A, wherein the crosslinked resin particles have a small average particle size of 100 nm, and having a loss elasticity modulus (G″) out of the defined range in the present invention, does provide a poor smoothness. In the Comparative Example 4A, the electrodeposition coating composition comprising the non-crosslinked resin particles, and having a storage elasticity modulus (G′) out of the defined range in the present invention, does provide a poor edge coatability. In the Comparative Example 5A, the electrodeposition coating composition comprising the inorganic pigment without any resin particles, and having a loss elasticity modulus (G″) out of the defined range in the present invention, does not provide a good smoothness. In the Example 1A, the electrodeposition coating composition comprising the pigment of the Production Example 4A, wherein all the parameters are within the present defined ranges, provides an excellent smoothness and an excellent edge coatability. In the Examples 2A to 6A, each of the electrodeposition coating compositions comprises a certain particle, wherein the storage elasticity modulus (G′) and the loss elasticity modulus (G″) are controlled within the present defined ranges, and provides an excellent smoothness and an excellent edge coatability. 
     Production Example 1B 
     Production of Blocked Isocyanate Curing Agent 
     In a flask equipped with a stirrer, a cooler, a nitrogen charging tube, a thermometer and a dropping funnel, 199 parts of the trimer of hexamethylene diisocyanate (CORONATE HX: manufactured by Nippon Polyurethane Industry Co., Ltd.), 32 parts of methyl isobutyl ketone and 0.03 part of dibutyltin dilaurate were weighed, and 87.0 parts of methyl ethyl ketone oxime was added dropwise thereto from the dropping funnel over 1 hr while stirring and bubbling nitrogen. Temperature was raised from 50° C. to 70° C. initially. Thereafter, the reaction was continued for 1 hr, and the reaction was continued until the absorption of NCO group was extinguished by an infrared spectrometer. Then, 0.74 part of n-butanol and 39.93 parts of methyl isobutyl ketone were added to prepare a mixture with a non-volatile content of 80%. 
     Production Example 2B 
     Production of Emulsion Containing Amine Modified Epoxy Resin and Blocked Isocyanate Curing Agent 
     In a flask equipped with a stirrer, a cooler, a nitrogen charging tube and a dropping funnel, 71.34 parts of 2,4-/2,6-tolylene diisocyanate (80/20% by weight), 111.98 parts of methyl isobutyl ketone and 0.02 part of dibutyltin dilaurate were weighed, and 14.24 parts of methanol was added dropwise from the dropping funnel over 30 min while stirring and bubbling nitrogen. The temperature was raised from room temperature to 60° C. by exothermic heat. Then, after the reaction was continued for 30 minutes, 46.98 parts of ethyleneglycol mono-2-ethylhexyl ether was added dropwise from the dropping funnel over 30 min. Temperature was raised to 70 to 75° C. by exothermic heat. After the reaction was continued for 30 min, 41.25 parts of the adduct of bisphenol A with propylene oxide (5 mol) (BP-5P manufactured by Sanyo Kasei Co., Ltd.) was added to the mixture, temperature was raised to 90° C., and the reaction was continued while measuring IR spectrum until NCO group was extinguished. 
     Successively, 475.0 parts of a bisphenol A type epoxy resin having an epoxy equivalent of 475 (YD-7011 R manufactured by Tohto Kasei Co., Ltd.) was added to be homogeneously dissolved, and then, temperature was raised from 130° C. to 142° C., and water was removed from the reaction system by azeotrope with MIBK. After the reaction mixture was cooled to 125° C., 1.107 parts of benzyldimethylamine was added and reaction of forming an oxazolidone ring by demethanolation was carried out. The reaction was continued until the epoxy equivalent was 1140. 
     Then, the mixture was cooled to 100° C., and 24.56 parts of N-methylethanolamine, 11.46 parts of diethanolamine and 26.08 parts of ketimine of aminoethylethanolamine (78.8% methyl isobutyl ketone solution) were added thereto to be reacted at 110° C. for 2 hrs Then, 20.74 parts of ethyleneglycol mono-2-ethylhexyl ether and 12.85 parts of methyl isobutyl ketone were added to the mixture to be diluted, and non-volatile content was adjusted to 82%. An amine modified epoxy resin in which number average molecular weight (by GPC method) was 1380 and amine equivalent was 94.5 meq/100 g was obtained. 
     145.11 Parts of ion exchanged water and 5.04 parts of acetic acid were weighed in another container, a mixture of 320.11 parts (75.0 parts as solid content) of the above-mentioned amine modified epoxy resin and 190.38 parts (25.0 parts as solid content) of the blocked isocyanate curing agent of Production Example 1B, which was heated to 70° C., was added thereto dropwise gradually, and the mixture was stirred to be homogeneously dispersed. Then, ion exchanged water was added thereto to adjust the solid content to 36%. 
     Production Example 3B 
     Production of Pigment Dispersing Resin Varnish 
     In a flask equipped with a stirrer, a cooler, a nitrogen charging tube, a thermometer and a dropping funnel, 382.20 parts of a bisphenol A type epoxy resin having an epoxy equivalent of 188 (under product name: DER-331J) and 111.98 parts of bisphenol A were weighed, temperature was raised to 80° C. to dissolve the mixture homogeneously, then 1.53 parts of 1% solution of 2-ethyl-4-methylimidazole was added, and reaction was carried out at 170° C. for 2 hrs. After cooling the mixture to 140° C., 196.50 parts of 2-ethylhexanol-half blocked isophorone diisocyanate (non-volatile content: 90%) was added to the mixture, and the reaction was carried out until NCO group was extinguished. Thereto, 205.00 parts of dipropylene glycol monobutyl ether was added, successively, 408.00 parts of 1-(2-hydroxyethylthio)-2-propanol and 134.00 parts of dimethylol propionate were added, 144.00 parts of ion exchanged water was added, and the mixture was reacted at 70° C. The reaction was continued until acid value was 5 or less. The obtained resin varnish was diluted to a non-volatile content of 35% with 1150.50 parts of ion exchanged water. 
     Production Example 4B 
     Production of Pigment Dispersed Paste 
     In a sand grind mill, 120 parts of the pigment dispersing resin varnish obtained in Production Example 3B, 100.0 parts of kaolin, 92.0 parts of titanium dioxide, 8.0 parts of dibutyltin oxide and 184 parts of ion exchanged water were charged, and dispersed until particle size was 10 μm or less to obtain a pigment dispersed paste (solid content: 48%). 
     Production Example 5B 
     Production of Crosslinked Resin Particles for Comparison 
     In a reaction container, 120 parts of butylcellosolve was charged, and it was heated to 120° C. with stirring. Thereto, a solution which was a mixture of 2 parts of t-butylperoxy-2-ethylhexanoate, and 10 parts of butylcellosolve, and a monomer mixture containing 15 parts of glycidyl methacrylate, 50 parts of 2-ethylhexyl methacrylate, 20 parts of 2-hydroxyethyl methacrylate, and 15 parts of n-butyl methacrylate were added dropwise over 3 hrs. After aging for 30 min, a solution which was a mixture of 0.5 part of t-butylperoxy-2-ethyl hexanoate and 5. parts of butylcellosolve was added dropwise for 30 min, and after aging for 2 hrs, the mixture was cooled. Thereto, 7 parts of N,N-dimethylaminoethanol and 15 parts of 50% aqueous lactic acid solution were added to the mixture with heating at 80° C. and stirring. When an acid value was 1 or less and the rising of viscosity was stopped, heating was terminated to obtain an acryl resin having an ammonium group. The number of the ammonium group per one molecule of the acryl resin having an ammonium group was 6.0. 
     To the reaction container, 120 parts of the acryl resin having an ammonium group and 270 parts of deionized water were added, and the mixture was stirred with heating at 75° C. Thereto, the 100% neutralized aqueous solution of 1.5 parts of 2,2′-azobis(2-(2-imidazolin-2-yl)propane) with acetic acid was added dropwise over 5 min. After aging for 5 min, 30 parts of methyl methacrylate was added dropwise over 5 min. After aging further 5 min, an ethylenically unsaturated monomer mixture containing 170 parts of methyl methacrylate, 40 parts of styrene, 30 parts of n-butyl methacrylate, 5 parts of glycidyl methacrylate and 30 parts of neopentylglycol dimethacrylate was added to a solution which was a mixture of 170 parts of the acryl resin having an ammonium group and 250 parts of deionized water with stirring to give a pre-emulsion, and the pre-emulsion was added dropwise over 40 min. After aging for 60 min, it was cooled to give a dispersion of crosslinked resin particles  1 . The non-volatile content in the dispersion of the crosslinked resin particles was 35%, pH was 5.0 and an average particle size was 0.1 μm. Herein, the average particle size was measured according to the followings. 
     The average particle size of the resin particles was measured by a granular particle transmission measurement method with MICROTRAC9340UPA manufactured by Nikkiso Co., Ltd. Further, the particle size distribution of the resin particles was measured by the measurement device, and an average particle size at a cumulative relative frequency [F(x)=0.5] was calculated from the measurement values. In these measurements and calculations, the employed refractive index of a solvent (water) was 1.33, and the employed refractive index of the resin content was 1.59. 
     Production Example 6B 
     In a flask equipped with a reflux cooler and a stirrer, 295 parts of methyl isobutyl ketone (hereinafter, abbreviated as “MIBK”), 37.5 parts of methylethanolamine and 52.5 parts of diethanolamine were charged, and the mixture was kept at 100° C. with stirring. Thereto, 205 parts of cresol novolac epoxy resin (under product name; YDCN-703, manufactured by Tohto Kasei Co., Ltd.) was gradually added. After complete addition, the reaction was carried out for 3 hrs. When its molecular weight was measured, it was 2100. When the amine value (MEQ(B)) of the amino modified resin was measured, it was 340 mmol/100 g. 
     5.5 Parts of formic acid and 1254.5 parts of deionized water were added to 140 parts of the amino modified resin solution, and the mixture was stirred for 30 min while keeping it at 80° C. The organic solvent was removed under reduced pressure to give an electroconductivity-controlling agent with a solid content of 5.0%. 
     Example 1B 
     628 Parts of the emulsion obtained in Production Example 2B, 127 parts of the crosslinked resin particles (crosslinked resin particles in which methyl methacrylate monomer was a main component; GM-0105 (under product name): manufactured by GANZ Chemical Co., Ltd.), 4 parts of dibutyltin oxide and 4581 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the resin particles was 15% by weight, and the solid content was 5% by weight. 
     Example 2B 
     628 Parts of the emulsion obtained in Production Example 2B, 127 parts of the crosslinked resin particles (crosslinked resin particles in which methyl methacrylate was a main component; TAFTIC®F-200: manufactured by Toyobo Co., Ltd.), 4 parts of dibutyltin oxide and 4581 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 15% by weight, and the solid content was 5% by weight. 
     Example 3B 
     561 Parts of the emulsion obtained in Production Example 2B, 19 parts of the pigment dispersed paste obtained in Production Example 4B, 114 parts of the crosslinked resin particles (crosslinked resin particles in which methyl methacrylate monomer was a main component; TAFTIC® F-200: manufactured by Toyobo Co., Ltd.), 3 parts of dibutyltin oxide and 4303 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 3%, the content of the crosslinked resin particles was 10% by weight, and the solid content was 5% by weight. 
     Example 4B 
     578 Parts of the emulsion obtained in Production Example 2B, 360 parts of the electroconductivity-controlling agent (solid content: 5%) obtained in Production Example 6B, 127 parts of the crosslinked resin particles (crosslinked resin particles in which methyl methacrylate monomer was a main component; TAFTIC® F-200: manufactured by Toyobo Co., Ltd.), 4 parts of dibutyltin oxide and 4331 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 15% by weight, and the solid content was 5% by weight. 
     Comparative Example 1B 
     2444 Parts of the emulsion obtained in Production Example 2B, 250 parts of the pigment dispersed paste obtained in Production Example 4B, 2346 parts of ion exchanged water and 10 parts of dibutyltin oxide were mixed to give a cationic electrodeposition coating composition in which the solid content was 20% by weight. 
     Comparative Example 2B 
     738 Parts of the emulsion obtained in Production Example 2B, 4 parts of dibutyltin oxide and 4598 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0% (ash content was not contained), the content of the crosslinked resin particles was 0% by weight, and the solid content was 5% by weight. 
     Comparative Example 3B 
     702 Parts of the emulsion obtained in Production Example 2B, 38 parts of the crosslinked resin particles obtained in Production Example 5B, 4 parts of dibutyltin oxide and 4596 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 5% by weight, and the solid content was 5% by weight. 
     Comparative Example 4B 
     665 Parts of the emulsion obtained in Production Example 2B, 76 parts of the crosslinked resin particles obtained in Production Example 5B, 4 parts of dibutyltin oxide and 4596 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 10% by weight, and the solid content was 5% by weight. 
     Comparative Example 5B 
     579 Parts of the emulsion obtained in Production Example 2B, 38-parts of the crosslinked resin particles (crosslinked resin particles in which styrene monomer was a main component; CHEMISNOW® SX130M: manufactured by Soken Chemical &amp; Engineering Co., Ltd.), 4 parts of dibutyltin oxide and 4388 parts of ion exchanged water were mixed to give a cationic electrodeposition coating composition in which PWC was 0%, the content of the crosslinked resin particles was 15% by weight and the solid content was 5% by weight. 
     With respect to thus prepared cationic electrodeposition coating compositions, the loss elasticity modulus at 80° C. and the storage elasticity modulus at 140° C. in dynamic viscoelasticities, and the smoothness and the edge coatability, and the like were evaluated by the methods below. 
     Measurement of Loss Elasticity Modulus and Storage Elasticity Modulus of Electrodeposition Film 
     A tin plate was immersed in the cationic electrodeposition coating composition prepared as described above. An electrodeposition film was formed by applying a voltage so that the film thickness after baking was 15 μm, and then the plate was rinsed with water to remove the excessive electrodeposition coating composition. Then, after removing moisture, without drying, the plate having the uncured coating film was immediately taken out to prepare a sample. The dynamic viscoelasticities of the sample, i.e., storage elasticity modulus (G′) and loss elasticity modulus (G″) were measured depending on the temperature with Rheosol-G3000 (manufactured by UBM Corporation) that was a rotational type dynamic viscoelasticity measurement device (under measurement conditions: a strain of 0.5 deg; a frequency of 0.02 Hz, and a raising rate of 2.0° C./min). 
     Evaluation of Appearance (Smoothness) of Electrodeposition Film 
     Evaluation of an appearance of an electrodeposition film was carried out by measuring an arithmetic average roughness (Ra) on a roughness curve. A cold rolling steel plate treated with zinc phosphate was immersed in a cationic electrodeposition coating composition. An uncured electrodeposition film obtained by applying a voltage, so that the film thickness after baking was 15 μm, was baked at 160° C. for 10 min. Then, the Ra value of the uncured electrodeposition film was measured with an evaluation type surface roughness measuring machine (SURFTEST SJ-201 P manufactured by Mitsutoyo Corporation) in accordance with JIS-B0601. Measurement was repeated 7-times on the sample with cut-offs in a width of 2.5 mm (partition number was 5), and the Ra value was an average of the measured values without the maximum and minimum values. The results are shown in Tables 2 and 3. It can be understood that the smaller Ra value provides the better coating film appearance with a suppressed concavo-convex. Specifically, an acceptable range of the Ra value is no more than 0.25 μm. 
     Evaluation Method for Sedimentability (Planer Appearance) 
     A cold rolling steel plate treated with zinc phosphate was immersed into each of the cationic electrodeposition coating compositions obtained in Production Examples and Comparative Examples, in a horizontal direction, and an uncured electrodeposition film was obtained by applying a voltage so that the film thickness after baking was 15 μm. After baking of the uncured electrodeposition film at 160° C. for 10 min, the arithmetic average roughness (Ra) on a roughness curve was measured in a similar manner to that in the above-mentioned evaluation for an appearance of an electrodeposition film with a surface roughness measuring machine. 
     If sedimentability of an electrodeposition coating composition is inferior, a horizontal (planar) appearance (smoothness in a horizontal direction) of the electrodeposition film is deteriorated in comparison with a vertical appearance (smoothness in a vertical direction) of the electrodeposition film, since the sedimentable components are sedimented on a horizontal plane upon the electrodeposition coating. The sedimentability can be evaluated from the Ra values of the horizontal appearance and the vertical appearance, as follows, if the sedimentability is acceptable or not acceptable. 
     Evaluation Basis for Sedimentability 
     Acceptable (O): Horizontal Ra value−Vertical Ra value=less than 0.05 μm 
     Not acceptable (X): Horizontal Ra value−Vertical Ra value=no less than 0.05 μm 
     Measurement of Thermal Softening Temperature 
     The storage elasticity modulus G′ of a sample obtained by adjusting the concentration of the crosslinked resin particles to 30% by weight (as a solid content) is measured from 90° C. under conditions of a strain of 0.5 degree, a frequency of 0.02 Hz and a rising temperature rate of 4.0° C./min in a temperature dependent measurement with Rheosol-G3000 (manufactured by UBM Corporation) that is a rotational type dynamic viscoelasticity measurement device. The measurement results are shown in a graph in  FIG. 8 . The tangential line in an area at which viscosity is a constant and the tangential line in an area at which the lowering of viscosity occurs are drawn, and the temperature at the cross point is the thermal softening temperature. 
     Evaluation Method for Edge Coatability 
     As described above, the edge coatability was evaluated.  FIG. 9  is a view schematically showing a point in a distance of 30 microns from the edge of a cutter knife blade. If the thickness of the film on this point is no less than 7.8 μm, the edge coatability is acceptable. 
     The Measurement Method of Average Particle Size of Crosslinked Resin Particles 
     The average particle size of the crosslinked resin particles employed in each of the above-described Examples and Comparative Examples was measured according to the followings. The average particle size of the crosslinked resin particles was measured by a granular particle transmission measurement method with MICROTRAC9340UPA manufactured by Nikkiso Co., Ltd. Further, the particle size distribution of the crosslinked resin particles was measured by the measurement device, and an average particle size at cumulative relative frequency F(x)=0.5 was calculated from the measurement values. In these measurements and calculations, the refractive index of a solvent (water) was 1.33, and the refractive index of resin component was 1.59. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Example 1B 
                 Example 2B 
                 Example 3B 
                 Example 4B 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Content of inorganic pigment (%) 
                 0 
                 0 
                 3 
                 0 
               
            
           
           
               
               
               
               
               
               
            
               
                 Crosslinked resin 
                 Species 
                 Crosslinked resin 
                 Crosslinked resin 
                 Crosslinked resin 
                 Crosslinked resin 
               
               
                 particles 
                   
                 particles #3 
                 particles #4 
                 particles #4 
                 particles #4 
               
               
                   
                 Content (%) 
                 15 
                 15 
                 10 
                 15 
               
               
                   
                 Degree of 
                 Middle 
                 Large 
                 Large 
                 Large 
               
               
                   
                 crosslinking 
               
               
                   
                 Thermal 
                 120 
                 140 
                 140 
                 140 
               
               
                   
                 softening 
               
               
                   
                 temperature 
               
               
                   
                 Particle size (μm) 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
            
           
           
               
               
               
               
               
            
               
                 Electroconductivity controlling agent 
                 — 
                 — 
                 — 
                 ◯ 
               
            
           
           
               
               
               
               
               
               
            
               
                 Melt viscosity 
                  80° C./G″ value 
                 113 
                 107 
                 90 
                 99 
               
               
                   
                 140° C./G′ value 
                 125 
                 475 
                 222 
                 188 
               
            
           
           
               
               
               
               
               
            
               
                 Evaluation of sedimentability 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Smoothness Ra (C/0 = 2.5) 
                 0.21 
                 0.23 
                 0.23 
                 0.22 
               
               
                 Edge coatability (μm) 
                 7.8 
                 8.0 
                 7.9 
                 7.8 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 Example 1B 
                 Example 2B 
                 Example 3B 
                 Example 4B 
                 Example 5B 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Content of inorganic 
                 23 
                 0 
                 0 
                 0 
                 0 
               
               
                 pigment (%) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Crosslinked 
                 Species 
                 — 
                 — 
                 Crosslinked 
                 Crosslinked 
                 Crosslinked 
               
               
                 resin 
                   
                   
                   
                 resin particles 
                 resin 
                 resin particles 
               
               
                 particles 
                   
                   
                   
                 #1 
                 particles #1 
                 #2 
               
               
                   
                 Content (%) 
                 — 
                 — 
                 5 
                 10 
                 15 
               
               
                   
                 Degree of 
                 — 
                 — 
                 Large 
                 Large 
                 Small 
               
               
                   
                 crosslinking 
               
               
                   
                 Thermal 
                 — 
                 — 
                 140 
                 140 
                 107 
               
               
                   
                 softening 
               
               
                   
                 temperature 
               
               
                   
                 Particle size 
                 — 
                 — 
                 0.1 
                 0.1 
                 1.5 
               
               
                   
                 (μm) 
               
            
           
           
               
               
               
               
               
               
            
               
                 Electroconductivity 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 controlling agent 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Melt 
                 80° C./G″ 
                 89 
                 23 
                 156 
                 228 
                 94 
               
               
                 viscosity 
                 value 
               
               
                   
                 140° C./G′ 
                 155 
                 21 
                 73 
                 110 
                 75 
               
               
                   
                 value 
               
            
           
           
               
               
               
               
               
               
            
               
                 Evaluation of 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 sedimentability 
               
               
                 Smoothness Ra (C/0 = 2.5) 
                 0.18 
                 0.19 
                 0.30 
                 0.37 
                 0.22 
               
               
                 Edge coatability (μm) 
                 7.9 
                 3.6 
                 6.0 
                 8.3 
                 5.6 
               
               
                   
               
            
           
         
       
     
     The degree of the crosslinking was described idepending on the thermal softening temperature and according to the measurement of the thermal softening temperature. 
     Degree of crosslinking (Large): Thermal softening temperature of 140° C. or more.
 
Degree of crosslinking (Middle): Thermal softening temperature of 120° C. or more and less than 140° C.
 
Degree of crosslinking (Small): Thermal softening temperature of 120° C. or less
 
Crosslinked resin particles #1: Crosslinked resin particles obtained in the Production Example 5B.
 
Crosslinked resin particles #2: CHEMISNOW SX-130M (under product name) manufactured by Soken Chemical &amp; Engineering Co., Ltd.
 
Crosslinked resin particles #3: GM-0105 (under product name) manufactured by GANZ Chemical Co., Ltd.
 
Crosslinked resin particles #4: F-200 (under product name) manufactured by Toyobo Co., Ltd.
 
     As seen from the above Tables 2 and 3, it is understood that the electrodeposition coating composition with a low ash content and a low solid content, which comprises the crosslinked resin particles having an average particle size within a range of from 1.0 to 3.0 μm and a thermal softening temperature within a range of from 120 to 180° C., could provide excellent performances superior in both of the smoothness and the edge coatability. The performances are in a similar extent to that of the Comparative Example 1B which is a conventional coating composition. In the Comparative Example 1B, the electrodeposition coating composition comprises a conventional inorganic pigment without any resin particles, which can provide a good surface smoothness and a good edge coatability. The electrodeposition coating composition has a low sedimentability evaluation degree, since the ash content therein is high. In the Comparative Example 2B, the electrodeposition coating composition comprising no inorganic pigments and no resin particles can provide a good smoothness and a highly deteriorated edge coatability. With respect to the Comparative Examples 3B to 5B, the electrodeposition coating composition comprises resin particles. In the Comparative Examples 3B and 4B, the particle size is small. In the Comparative Example 5B, the thermal softening temperature is low. Therefore, the Comparative Examples 3B to 5B would provide a poor edge coatability and a poor surface smoothness.