JP2011020085A - Method for forming electrodeposition coating film and method for forming multilayer coating film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming an electrodeposition coating film in which the yellowing of the electrodeposition coating film is prevented and not only excellent rustproof properties but also functions such as weather resistance, light resistance, impact resistance and chipping resistance are imparted to the electrodeposition coating film to omit an intermediate coating step and to provide a method for forming a multilayer coating film which includes no intermediate coating film and has an excellent color-developed finish coating film and the external appearance excellent in color development and hues or the like. <P>SOLUTION: The method for forming the electrodeposition coating film comprises the steps of: applying an electrodeposition coating composition onto an object to be coated by an electrodeposition coating method; separating the applied electrodeposition coating composition into multiple layers while heating the applied electrodeposition coating composition; and curing the separated electrodeposition coating composition to form a multilayer cured film comprising at least two layers. The electrodeposition coating composition includes coating film-forming resin compositions (a) and (b), blocked polyisocyanates (c1) and (c2), a pigment and a yellowing inhibitor. The coating film-forming resin compositions (a) and (b) are incompatible with each other so that the coating film-forming resin composition (a) forms an emulsion particle A including the blocked polyisocyanate (c1) and the coating film-forming resin composition (b) forms another emulsion particle B including the blocked polyisocyanate (c2). The yellowing inhibitor of 0.5-5.0 parts weight based on 100 parts weight of all resin solid contents of the emulsion particles A and B is incorporated in the electrodeposition coating composition. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrodeposition coating film forming method capable of forming a multilayer electrodeposition coating film applicable to coating of an automobile body and the like, and further has excellent heat resistance, weather resistance, light resistance, etc. It is related with the electrodeposition coating-film formation method which can provide the function of and can prevent yellowing of an electrodeposition coating film. The present invention also relates to a method of forming a so-called intermediate coating-less multilayer coating film in which an intermediate coating step is omitted in the coating of an automobile body or the like.

  In the field of painting such as automobile bodies, in general, an electrodeposition film is formed as an undercoat film on a coated object such as a steel plate for the purpose of improving rust prevention, and then an intermediate coating film is formed thereon. Further, a colored base coating film, a clear coating film, and the like are sequentially formed thereon as a top coating film to complete the coating.

However, in recent years, in the field of painting such as automobile bodies, there has been a demand for shortening and simplifying the painting process for the purpose of resource saving, energy saving, cost saving, and environmental load reduction (for example, low VOC and low CO ₂ ). Yes.

  In general, a three-coat one-bake (3C1B) method is well known as a simplified coating method. In the 3C1B method, intermediate coating, base coating, and clear coating are sequentially applied on the electrodeposition coating film by wet-on-wet, and these coating films are baked and cured simultaneously. By doing so, a multilayer coating film including these three layers in the order of the intermediate coating film, the base coating film, and the clear coating film can be easily formed on the electrodeposition coating film at a time. The 3C1B method is very advantageous from the viewpoint of energy saving and cost saving because a multilayer coating film can be formed by only one baking and curing after the formation of the cured electrodeposition coating film. In addition, in the 3C1B method, since the multilayer coating film to be formed generally includes an intermediate coating film, functions such as excellent base concealing property, weather resistance, light resistance, impact resistance, and chipping resistance are imparted to the coating film. can do.

  In recent years, as a more simplified coating method, a method for forming a so-called intermediate coating-less multilayer coating film in which the intermediate coating step is omitted has been studied. In the case of omitting the intermediate coating film in a multilayer coating film applied to an automobile body or the like, generally, the electrodeposition coating film needs to play the role of the intermediate coating film. Therefore, the electrodeposition coating film must have not only excellent rust prevention properties but also functions such as excellent weather resistance, light resistance, impact resistance, and chipping resistance to the object to be coated. In this case, even if the intermediate coating film is omitted, it is necessary to provide a coating film appearance equivalent to or higher than that of the coating film formed by the 3C1B method.

  For example, Japanese Patent Publication No. 2-333069 (Patent Document 1) discloses a two-layer coating film-forming thick film electrodeposition coating composition and an electrodeposition coating method. The electrodeposition coating composition disclosed in Patent Document 1 contains a cationic acrylic resin (A) containing 50% by weight or less of a monomer having a benzene nucleus in the composition and having a softening point of 80 ° C. or higher, and a softening point of It contains a cationic phenol type epoxy resin (B) having a temperature of 75 ° C. or less, and the weight ratio of (A) to (B) is 1 to 30: 1. In the electrodeposition coating composition disclosed in Patent Document 1, a two-layer electrodeposition curing comprising a lower layer made of a phenol type epoxy resin layer excellent in rust prevention and an upper layer made of an acrylic resin layer excellent in weather resistance A coating film can be formed.

  However, when the top coat is applied directly on such a two-layered electrodeposition-cured coating film, that is, when the intermediate coating step is omitted, the top coat does not color well, that is, the desired hue cannot be obtained. There is a case. In particular, when a large and complicated object to be coated such as an automobile body is applied by omitting the intermediate coating, the intended hue of the top coating may not be obtained. It is fatal in manufacturing industrial products such as automobiles.

  The reason why the desired hue of the top coat is not obtained is that the electrodeposition coating is locally yellowed. Even if the electrodeposition coating is baked and cured at a constant temperature, there is a part where the heating is not uniform or the heat is inevitably stored in a large and complicated object such as an automobile body. In such places, the electrodeposition coating is strongly yellowed to form a brown spot-like mottled pattern. It may be reduced, and is fatal in automobile bodies and the like where design is regarded as important.

  Conventionally, even if yellowing occurs in the electrodeposition coating film in the electrodeposition coating process, the undercoat electrodeposition coating film can be concealed by the intermediate coating film, and thereafter, rays such as ultraviolet rays are blocked. I was able to. However, when the intermediate coating film is omitted, that is, in the intermediate coating-less system, as described above, yellowing of the electrodeposition coating film becomes a very serious problem.

  In JP-A-5-320546 (Patent Document 2) and JP-A-7-166112 (Patent Document 3), an electrodeposition coating composition is blended with an ultraviolet absorber or a hindered amine compound to provide weather resistance of the electrodeposition coating film. However, all of the formed electrodeposition coating films consist of one layer, and are inferior in weather resistance and light resistance. Further, in the present invention, yellowing of the electrodeposition coating film cannot be sufficiently prevented in the intermediate coating-less system.

  Japanese Unexamined Patent Publication No. 2000-345394 (Patent Document 4) discloses a method for forming a multilayer electrodeposition coating film, wherein the upper layer of the multilayer electrodeposition coating film provides weather resistance, and the lower layer is rustproof. This eliminates the intermediate coating process. Patent Document 4 discloses that conventional additives such as ultraviolet absorbers and antioxidants may be blended in the electrodeposition coating composition, but there are no specific examples thereof. The additive is simply added for the purpose of preventing the photodegradation of the coating film, and as described above, when the top coating is performed without the intermediate coating, the electrodeposition coating is significantly yellowed. May be a problem.

  Also in JP-A-2002-126616 (Patent Document 5), conventional additives such as ultraviolet absorbers and antioxidants are blended in an electrodeposition coating composition capable of forming a multilayer electrodeposition coating film. However, there is no specific example, and these additives cannot sufficiently prevent yellowing of the electrodeposition coating film. In the invention disclosed in Patent Document 5, an intermediate coating process is essential to obtain excellent impact resistance (chipping resistance), and the intermediate coating process cannot be omitted.

  In JP-A-2002-126619 (Patent Document 6), additives such as an ultraviolet absorber and an antioxidant may be blended in an electrodeposition coating composition capable of forming a multilayer electrodeposition coating film. However, there is no specific example, and even in Patent Document 6, these additives cannot sufficiently prevent yellowing of the electrodeposition coating film. Moreover, in patent document 6, in order to obtain the outstanding impact resistance (chipping resistance), the intermediate coating process is essential and an intermediate coating process cannot be skipped. In addition, although patent document 6 has description about yellowing prevention, yellowing prevention here is not intending prevention of yellowing of an electrodeposition coating film itself, but intermediate coating film and base coating film The amount of the basic substance is controlled to prevent yellowing of the entire coating film.

Japanese Examined Patent Publication No. 2-333069 JP-A-5-320546 JP-A-7-166112 JP 2000-345394 A JP 2002-126616 A JP 2002-126619 A

  The present invention aims to prevent yellowing of the electrodeposition coating film, and further, the electrodeposition coating film has not only excellent rust prevention properties, but also weather resistance, light resistance, impact resistance, chipping resistance, etc. An object of the present invention is to provide a method for forming an electrodeposition coating film that can provide the above function and omit the intermediate coating step. Another object of the present invention is to provide a method for forming a multilayer coating film that does not include an intermediate coating film and has an excellent coating film appearance such as excellent color development and hue of the top coating film.

  As a result of intensive studies, the present inventors have added a specific yellowing inhibitor to the electrodeposition coating composition when forming a multilayer electrodeposition cured coating film from an electrodeposition coating composition containing at least two types of emulsion particles. By blending, it is possible to prevent yellowing of the electrodeposition coating film, and not only excellent anticorrosion properties to the electrodeposition coating film, but also excellent weather resistance, light resistance, impact resistance, chipping resistance As a result, it has been found that the intermediate coating process, which has been conventionally required, can be omitted. Accordingly, the present invention provides the following.

Electrodeposition coating formation comprising electrodeposition coating of electrodeposition coating composition on substrate, then separating layers while heating, and then curing to form a multilayer cured film consisting of at least two layers An electrodeposition coating composition comprising:
Film-forming resin components (a) and (b),
Blocked polyisocyanates (c1) and (c2),
Pigments, and yellowing inhibitors,
The film-forming resin components (a) and (b) are incompatible with each other;
The film-forming resin component (a) forms emulsion particles A containing blocked polyisocyanate (c1),
The film-forming resin component (b) forms emulsion particles B containing blocked polyisocyanate (c2),
The yellowing inhibitor is contained in an amount of 0.5 to 5.0 parts by weight with respect to 100 parts by weight of the total resin solids of the emulsion particles A and B.
Electrodeposition coating formation method.

  In the electrodeposition coating film forming method of the present invention, preferably, the yellowing inhibitor is selected from the group consisting of phenolic, phosphorus, hydroxylamine and sulfur yellowing inhibitors.

In the electrodeposition coating film forming method of the present invention, preferably,
The film-forming resin component (a) is a cation-modified acrylic resin,
The film-forming resin component (b) is a cation-modified epoxy resin,
The solubility parameter (δa) of the coating film-forming resin component (a) and the solubility parameter (δb) of the coating film-forming resin component (b) satisfy the relationship {δb−δa} ≧ 0.4.

  In the electrodeposition coating film forming method of the present invention, the blending ratio ((a) / (b)) of the film-forming resin components (a) and (b) is preferably 50/50 to 50% by weight in terms of solid content. 40/60.

  In the electrodeposition coating film forming method of the present invention, preferably, the lightness (L * value) of the multilayer cured film is 55 or more.

  In the electrodeposition coating film forming method of the present invention, preferably, the modified epoxy resin (d) having an onium group is a film-forming resin component (a) and (b) of emulsion particles A and B and a blocked polyisocyanate. 1 to 10 parts by weight is included with respect to 100 parts by weight in total of (c1) and (c2).

  In the electrodeposition coating film forming method of the present invention, preferably, the onium group of the modified epoxy resin (d) having an onium group is selected from the group consisting of an ammonium group and a sulfonium group.

In the electrodeposition coating film forming method of the present invention, preferably,
Blocked polyisocyanate (c1) is an aliphatic polyisocyanate blocked with a sealant,
The blocked polyisocyanate (c2) is obtained by blocking an alicyclic or aromatic polyisocyanate with a sealing agent.

Further, the present invention further comprises a step of applying an overcoating base coating composition on the electrodeposition coating film formed by the above electrodeposition coating film forming method to form an uncured topcoating base coating film,
A step of applying an overcoat clear coating composition on the uncured topcoat base coating to form an uncured topcoat clearcoat; and an uncured topcoat basecoat and an uncured topcoat clear coat The present invention also relates to a method for forming a multilayer coating film, comprising a step of simultaneously heating and curing.

Conventionally, an electrodeposition coating film formed on an object to be coated is covered with an intermediate coating film having excellent base concealing properties. The yellowing of the electrodeposition coating film caused by the influence of the above was not a problem in terms of quality. In addition, conventionally, since the intermediate coating film has further functions such as weather resistance, light resistance, impact resistance, and chipping resistance, the electrodeposition coating film existing as the base is It was well protected by an intermediate coating film. Therefore, conventionally, the electrodeposition coating film only has to have excellent antirust properties. However, in recent years, from the viewpoint of resource saving, energy saving, cost saving, and environmental load reduction (for example, low VOC and low CO ₂ ), it is strongly required to omit the intermediate coating film. As such, it has become necessary to have not only excellent rust resistance but also functions such as weather resistance, light resistance, impact resistance, and chipping resistance. When the intermediate coating film is omitted, yellowing of the electrodeposition coating film is particularly problematic because the top coating composition is directly applied onto the electrodeposition coating film.

  When electrodepositing large and complex objects such as automobile bodies, the outer plate part (the part visible from the outside of the car body) is the inner plate part of the car body when the electrodeposition coating is baked and cured. Yellowing is likely to occur because it is exposed to a higher temperature than the part not visible from the outside. In addition, in the case of a large and complicated object to be coated such as an automobile body, a part where heat is inevitably generated during heat curing of the electrodeposition coating film (for example, a bag-like structure part) is generated. Then, it is difficult to avoid yellowing of the electrodeposition coating film, and brown yellowing spots may occur. In addition, the electrodeposition coating film may be heated excessively (for example, a worker's lunch break, break time, etc.) in the process control of electrodeposition coating. Yellowing is likely to occur due to proper heating, and it was necessary to prevent yellowing of the electrodeposition coating film.

  However, in the present invention, yellowing that occurs during heat curing of the electrodeposition coating film can be prevented by blending a specific yellowing inhibitor into the electrodeposition coating composition. Further, in the present invention, even when the top coating is applied directly on the electrodeposition coating without the intermediate coating step, the top coating is excellent by preventing yellowing of the electrodeposition coating. Color development and desired hue can be sufficiently provided.

  In the present invention, the film-forming resin component (a) forms the blocked polyisocyanate (c1) and the emulsion particles A, and the film-forming resin component (b) is the blocked polyisocyanate (c2). And at least two types of emulsion particles A and B form separate layers at the time of heat curing, thereby forming a multilayer electrodeposition-cured coating film consisting of at least two layers. The bottom layer of the multi-layer coating provides excellent rust resistance to the object to be coated, the top layer exhibits weather resistance and light resistance, and functions such as impact resistance and chipping resistance. Can be provided, so that the intermediate coating process, which has been conventionally required, can be omitted.

  The present invention includes an electrodeposition coating process comprising electrodeposition coating an electrodeposition coating composition on an object to be coated, then separating the layers while heating and then curing to form a multilayer cured film comprising at least two layers. The electrodeposition coating composition that can be used in the present invention relating to a method for forming a coating film comprises at least two types of coating-forming resin components (a) and (b) that are incompatible with each other, and at least two types of blocks. It is characterized by comprising depolyisocyanate (c1) and (c2), a pigment, and a specific yellowing inhibitor. In the present invention, a multi-layer electrodeposition cured coating film is formed from an electrodeposition coating composition containing a specific anti-yellowing agent to prevent yellowing of the electrodeposition coating film. Not only rust prevention, but also functions such as weather resistance, light resistance, impact resistance, and chipping resistance are added, and the intermediate coating process that has been conventionally required is omitted.

  In general, yellowing of an electrodeposition coating is a phenomenon that appears prominently when excessive heat is locally applied to the electrodeposition coating during heat curing of the electrodeposition coating. Yellowing is usually a mottled pattern. As a brown spot, it appears strongly on the substrate. In particular, when electrodeposition is applied to a large and complex object such as an automobile body and the electrodeposition coating is heat-cured, the outer plate (the part visible from the outside of the automobile body (eg, roof, engine hood, trunk) The lid and the like) are exposed to a higher temperature than the inner plate portion (the portion that cannot be seen from the outside of the automobile body), and therefore yellowing is likely to occur. In addition, when electrodepositing large and complex objects such as automobile bodies, heat tends to accumulate in bag-like structures such as the inside of fenders, pillars, and trunks when the electrodeposition coating is heated and cured. Even in these places, brown yellow spots are likely to occur. Moreover, an electrodeposition coating film may be excessively heated in the process control of electrodeposition coating (for example, a worker's lunch break, break time, etc.). Unfortunately, the article passing through the line during this time zone is unfortunately exposed to excessive heating, and therefore yellowing is likely to occur.

  The yellowing of the electrodeposition coating film can be evaluated by the value of chromaticity b * of the L * a * b * color system. Yellow increases as b * increases, blue decreases as b * decreases, and negative (-b *). In this invention, it is judged that remarkable yellowing has arisen that the value of b * of an electrodeposition coating film is 5 or more.

  Moreover, the hue of an electrodeposition coating film can be evaluated by the value of the lightness L * of the L * a * b * color system. The color of the electrodeposition coating film approaches white (L * = 100) as the value of L * increases, and approaches black (L * = 0) as the value of L * decreases. In this invention, the value of L * of an electrodeposition coating film becomes like this. Preferably it is 55 or more, More preferably, it is 55-60. When the value of L * of the electrodeposition coating film is less than 55, when a top coating is directly applied thereon, the color of the base electrodeposition coating film adversely affects the top coating film, and the desired hue of the top coating paint May not be able to get.

  As a coated object to which the method of the present invention can be applied, there is no particular limitation as long as it is a large and complicated shaped coated object such as an automobile body, and a conductive substrate that can constitute them. For example, metals (for example, iron, steel, copper, aluminum, magnesium, tin, zinc, etc. and alloys containing these metals), iron plates, steel plates, aluminum plates and surface treatments (for example, phosphates, Examples thereof include those subjected to chemical conversion treatment using chromate and the like, and molded products thereof.

Anti-yellowing agent The anti-yellowing agent that can be used in the present invention is an anti-yellowing agent selected from the group consisting of phenol, phosphorus, hydroxylamine and sulfur, and these are used alone. However, two or more yellowing inhibitors may be used in combination.

  The phenol-based yellowing inhibitor preferably has a bulky bulky substituent such as a t-butyl group at the ortho position of the phenol. More preferred are those having a bulky substituent at the ortho positions on both sides.

  Examples of phenolic yellowing inhibitors include 2,6-di-t-butylphenol, 2,4-di-t-butylphenol, 2-t-butyl-4,6-di-methylphenol, 2,6. -Di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, 2,4,6-tri-t-butylphenol, 2,6-t-butyl-4-hydroxy Methylphenol, 2,6-di-t-butyl-2-dimethylamino-p-cresol, n-octadecyl-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) propionate, styrene Nate phenol, 2,2′-methylene-bis- (4-methyl-6-tert-butylphenol), 2,2′-methylene-bis- (6-cyclohexyl-4-methylphenol), 2,2 ′ Butylidene-bis- (2-tert-butyl-4-methylphenol), 4,4′-methylene-bis- (2,6-di-tert-butylphenol), 1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate], tris (3,5-di-tert-butyl-4-hydroxybenzyl) isocyanurate, tetrakis- {methylene-3- (3 ′, 5′-di-tert-butyl-4′-hydroxyphenyl) propionate} methane, 1,1,3-tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane, 3,9-bis [ 1,1-di-methyl-2- {β- (3-tert-butyl-4-hydroxy-5-methylphenyl) propionyloxy} ethyl] -2,4,8,10-tetraoxaspiro [5,5 ] An undecane etc. are mentioned.

  Preferred phenolic yellowing inhibitors are 2,6-di-t-butylphenol, 2-t-butyl-4,6-di-methylphenol, 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, 2,4,6-tri-t-butylphenol, styrenate phenol, 2,2'-methylene-bis- (4-methyl-6-t-butylphenol ), 2,2′-methylene-bis- (6-cyclohexyl-4-methylphenol), 4,4′-methylene-bis- (2,6-di-t-butylphenol), tetrakis- {methylene-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) propionate} methane.

  Examples of commercially available phenolic yellowing inhibitors include Sumitizer BHT, Sumilizer S, Sumizer BP-76, Summarizer MDP-S, Summarizer BP-101, Summarizer GA-80, and Summarizer BBM manufactured by Sumitomo Chemical Co., Ltd. -S, Sumilizer WX-R, Summarizer NW, Summarizer GM, Summarizer GS, and ADK STAB AO-20, ADK STAB AO-30, ADK STAB AO-40, ADK STAB AO-50, ADK STAB AO-60 and ADK STAB AO manufactured by Asahi Denka Co., Ltd. -75, ADK STAB AO-80, ADK STAB AO-330, ADK STAB AO-616, ADK STAB AO-635, ADK STAB AO-658, ADK STAB AO-15, ADK STAB AO-18, ADK STAB 28 include ADK STAB AO-37 or the like. In the present invention, sulfur-containing phenolic yellowing inhibitors such as IRGANOX 565 manufactured by Ciba Japan may be used.

  As the phosphorus-based yellowing inhibitor, phosphite-based yellowing inhibitors are preferable. For example, tris (isodecyl) phosphite, tris (tridecyl) phosphite, phenyldiisodecylphosphite, diphenylisooctylphosphite, triphenylphosphine Phyto, tris (nonylphenyl) phosphite, 4,4′-isopropylidene-diphenol alkyl phosphite, tris (mono and dimixed nonylphenyl) phosphite, tris (2,4-di-t-butylphenyl) phos Phyto, distearyl pentaerythritol diphosphite, di (2,4-di-t-butylphenyl) pentaerythritol diphosphite, di (nonylphenyl) pentaerythritol diphosphite, phenyl-bisphenol A pentaerythritol di Sulphite, tetratridecyl-4,4′-butylidenebis- (3-methyl-6-tert-butylphenol) -di-phosphite, hexatridecyl 1,1,3-tris (2-methyl-4-hydroxy-5 -T-butylphenyl) butane triphosphite and the like.

  Further preferred phosphorus-based yellowing inhibitors are tris (isodecyl) phosphite, phenyl diisodecyl phosphite, diphenyl isooctyl phosphite, triphenyl phosphite, distearyl pentaerythritol diphosphite, di (nonylphenyl) pentaerythritol di Phosphite, phenyl-bisphenol A pentaerythritol diphosphite, tetratridecyl-4,4′-butylidenebis- (3-methyl-6-tert-butylphenol) -di-phosphite.

  Examples of commercially available phosphorous yellowing inhibitors include “IRGAFOS 168” manufactured by Ciba Japan, Sumilizer TNP, Sumitizer TPP-R, and Sumilizer P-16 manufactured by Sumitomo Chemical, and Asahi Denka Co., Ltd. ADK STAB PEP-2, ADK STAB PEP-4C, ADK STAB PEP-8, ADK STAB PEP-8F, ADK STAB PEP-8W, ADK STAB PEP-11C, ADK STAB PEP-24G, ADK STAB PEP-36, ADK STAB HP-10, ADK STAB 2112, ADK STAB 260 , Adekas tab P, Adekas tab QL, Adekas tab 522A, Adekas tab 329K, Adekas tab 1178, Adekas tab 1500, Adekas tab C, Adekas tab 135A, Adekas tab 517, Adekas tab 3010, Adekas tab 3010 Stub TPP, and the like.

  As the hydroxylamine-based yellowing inhibitor, dialkylhydroxylamine is preferable, and as a commercially available hydroxylamine-based yellowing inhibitor, “IRGASTAB FS 042” manufactured by Ciba Japan Co., Ltd. may be mentioned.

  As the sulfur yellowing inhibitor, a thioether yellowing inhibitor is preferable. For example, dilauryl-3,3′-thiodipropionate, ditridecyl-3,3′-thiodipropionate, dimyristyl-3,3. '-Thiodipropionate, distearyl-3,3'-thiodipropionate, bis (2-methyl-4- {3-n-alkylthiopropionyloxy} -5-tert-butylphenyl) sulfide, pentaerythritol- Examples include tetrakis- (β-lauryl-thiopropionate) and 2-mercaptobenzimidazole.

  More preferred sulfur-based yellowing inhibitors are dilauryl-3,3′-thiodipropionate, ditridecyl-3,3′-thiodipropionate, distearyl-3,3′-thiodipropionate, bis ( 2-methyl-4- {3-n-alkylthiopropionyloxy} -5-tert-butylphenyl) sulfide.

  Examples of commercially available sulfur-based yellowing inhibitors include Sumitizer TPL-R, Summarizer TPM, Summarizer TPS, Sumiser TP-D, Sumiser TL, and Sumiser MB manufactured by Sumitomo Chemical, and Adeka Stub AO- manufactured by Asahi Denka Co., Ltd. 23, ADK STAB AO-412S, ADK STAB AO-503A and the like.

  The yellowing inhibitor may be blended in any of the emulsion particles A and B, and is 0.5 to 5 with respect to 100 parts by weight of the total resin solid content of the emulsion particles A and B contained in the electrodeposition coating composition. It is included in an amount of 0.0 parts by weight, preferably 1.0 to 3.0 parts by weight. If the yellowing inhibitor is less than 0.5 parts by weight, there is no effect in preventing yellowing. If it exceeds 5.0 parts by weight, the yellowing prevention level is improved, but the appearance of the coating film is lowered. May occur.

  In the present specification, “the total resin solid content of emulsion particles A and B” means the film-forming resin component (a) that forms emulsion particles A, the blocked polyisocyanate (c1) and emulsion contained therein. It means the total solid content contained in the film-forming resin component (b) that forms the particles B and the blocked polyisocyanate (c2) contained therein.

Film-forming resin component The electrodeposition coating composition that can be used in the method of the present invention contains at least two types of film-forming resin components (a) and (b). At least two types of emulsion particles are formed from the conductive resin components (a) and (b), and after application of the electrodeposition coating film, a multi-layer electrodeposition curing coating film comprising at least two layers is formed by heat curing. In this specification, for convenience, the uppermost layer of the multilayer electrodeposition coating film, that is, the resin component contained in the coating film in contact with air is defined as the film-forming resin component (a), and the multilayer electrodeposition coating film The resin component contained in the lowermost layer, that is, the coating film that comes into contact with the object to be coated, is defined as a film-forming resin component (b).

  The film-forming resin components (a) and (b) that can be used in the present invention are incompatible with each other. In the present invention, the solubility parameter (δa) of the film-forming resin component (a) And the solubility parameter (δb) of the film-forming resin component (b) satisfy the relationship: {δb−δa} ≧ 0.4. The solubility parameter (δ) is generally called SP (solubility parameter), and is a measure indicating the degree of hydrophilicity and hydrophobicity of the resin component. Further, the solubility parameter (δ) is an important measure for judging the compatibility between resin components. The solubility parameter (δ) is numerically quantified based on, for example, the following turbidity measurement method (reference document: kW Suh, DH Clarke J. Polymer). Sci., A-1, 5, 1671 (1967).).

  In general, it is considered that if the difference in solubility parameter (δ) of a plurality of resin components contained in the electrodeposition coating composition is 0.4 or more, the compatibility is lost and the formed coating film exhibits a multilayer separation structure. It has been. In the present invention, the two types of resin components (a) and (b) can ensure sufficient incompatibility by satisfying the above formula relating to the above-described solubility parameter. An electrodeposition coating film having a structure can be formed. In the present invention, the difference in solubility parameter between the resin components (a) and (b), that is, {δb−δa} is preferably 0.4 to 1.2, more preferably 0.8 to 1.0. .

  As described above, the solubility parameter (δa) of the resin component (a) and the solubility parameter (δb) of the resin component (b) satisfy the relationship {δb−δa} ≧ 0.4. It shows that the component (b) has a relatively high solubility parameter with respect to the resin component (a), and that the polarity of the resin component (b) is relatively high. Therefore, it shows that the resin component (b) has an excellent affinity for an object to be coated such as a conductive substrate having a high polarity, and the resin component (b) It is contained in the lowermost layer, and the resin component (a) is contained in the uppermost layer. Thus, the difference in the solubility parameter of the resin component is considered to be a driving force that causes layer separation in the formation of the multilayer electrodeposition coating film.

  In the present invention, the structure of the multilayer electrodeposition coating film can be confirmed by visually observing the cross section of the electrodeposition coating film with a video microscope or with a scanning electron microscope (SEM). . Moreover, identification of the resin component which comprises each resin layer can be performed by using a total reflection type Fourier-transform infrared spectrophotometer (FTIR-ATR), for example.

  The resin component (a) is particularly preferably a cation-modified acrylic resin from the viewpoint of weather resistance, light resistance, impact resistance, chipping resistance, and the like.

  The cation-modified acrylic resin is not particularly limited, but can be synthesized by a ring-opening addition reaction between an acrylic copolymer containing a plurality of oxirane rings and a plurality of hydroxyl groups in the molecule and an amine. Such an acrylic copolymer includes a glycidyl (meth) acrylate and a hydroxyl group-containing acrylic monomer (for example, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, or 2-hydroxyethyl (meth) acrylate). Such as a hydroxyl group-containing (meth) acrylic ester and an addition product of ε-caprolactone) and other acrylic monomers and / or non-acrylic monomers.

  Examples of other acrylic monomers include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, Examples include t-butyl (meth) acrylate, cyclohexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, and isobornyl (meth) acrylate. Examples of non-acrylic monomers include styrene, vinyl toluene, α-methyl styrene, (meth) acrylonitrile, (meth) acrylamide and vinyl acetate.

  An acrylic resin containing an oxirane ring based on the above glycidyl (meth) acrylate is a cation-modified acrylic resin which is opened by reaction with a primary amine, a secondary amine or a tertiary amine salt of the oxirane ring of the epoxy resin. It can be a resin.

  There is also a method of directly synthesizing a cation-modified acrylic resin by copolymerizing an acrylic monomer having an amino group with another monomer. In this method, an amino group-containing acrylic monomer such as N, N-dimethylaminoethyl (meth) acrylate or N, N-di-t-butylaminoethyl (meth) acrylate is used instead of the above glycidyl (meth) acrylate. Then, a cation-modified acrylic resin can be obtained by copolymerizing this with a hydroxyl group-containing acrylic monomer and other acrylic monomers and / or non-acrylic monomers.

  The cation-modified acrylic resin thus obtained is introduced with a blocked isocyanate group by an addition reaction with a half-blocked diisocyanate compound as required, as described in the above-mentioned JP-A-8-333528, and self-crosslinking cation-modified. An acrylic resin can also be used.

  The resin component (a) is preferably molecularly designed so that the hydroxyl value is in the range of 50 to 150. When the hydroxyl value is less than 50, the coating film is poorly cured. On the other hand, when it exceeds 150, excessive hydroxyl groups remain in the coated film after curing, and as a result, the water resistance of the cured coating film may be lowered. The number average molecular weight is preferably in the range of 1000 to 20000. If the number average molecule is less than 1000, the cured coating film has poor physical properties such as solvent resistance. On the other hand, if it exceeds 20000, it is difficult to handle the operation of the obtained resin, such as emulsification and dispersion, because the viscosity of the resin solution is high, and the appearance of the obtained electrodeposition coating film may be significantly reduced. is there. In addition, although only 1 type can also be used for the resin component (a), in order to balance the coating-film performance, 2 types or more types can also be used.

  The resin component (a) is a cation having a glass transition temperature (Tg (a)) of 30 to 50 ° C., preferably 30 to 40 ° C., and a number average molecular weight of 7000 to 12000, preferably 7000 to 10,000. A modified acrylic resin is particularly preferred. The glass transition temperature (Tg (a)) of the resin component (a) can be measured by a differential scanning calorimeter, and the number average molecular weight can be measured by GPC (gel permeation chromatography).

  The resin component (b) needs to be a resin component that exhibits excellent rust prevention properties with respect to the conductive base material to be coated. As an example of such a resin component, a cation-modified epoxy resin is well known in the field of cationic electrodeposition coating, and can be suitably used in the present invention. Generally, the cation-modified epoxy resin is not particularly limited, but is produced by opening an epoxy ring in a starting material resin molecule by a reaction with an amine such as a primary amine, a secondary amine, or a tertiary amine acid salt. A typical example of the starting material resin is a polyphenol polyglycidyl ether type epoxy resin which is a reaction product of a polycyclic phenol compound such as bisphenol A, bisphenol F, bisphenol S, phenol novolak, cresol novolak, and epichlorohydrin. Examples of other starting material resins include oxazolidone ring-containing epoxy resins described in JP-A-5-306327. This epoxy resin is obtained by a reaction between a diisocyanate compound or a bisurethane compound obtained by blocking an NCO group of a diisocyanate compound with a lower alcohol such as methanol or ethanol, and epichlorohydrin.

  The starting material resin can be used by extending the chain with a bifunctional polyester polyol, polyether polyol, bisphenol, dibasic carboxylic acid or the like before the ring opening reaction of the epoxy ring with amines. In addition, prior to the ring-opening reaction of the epoxy ring with amines, for the purpose of adjusting the molecular weight or amine equivalent, adjusting the heat flow property, flexibility (softness), glass transition temperature, etc., some epoxy rings On the other hand, monohydroxy compounds such as 2-ethylhexanol, nonylphenol, ethylene glycol mono-2-ethylhexyl ether, ethylene glycol mono n-butyl ether, propylene glycol mono-2-ethylhexyl ether, stearic acid, 2-ethylhexanoic acid, dimer It can also be used by adding an acid component such as an acid.

  Examples of amines that can be used for opening an epoxy ring and introducing an amino group include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine acid Mention may be made of primary, secondary or tertiary amineates such as salts, N, N-dimethylethanolamate. In addition, ketimine block primary amino group-containing secondary amines such as diethylenetriamine diketimine, aminoethylethanolamine ketimine, aminoethylethanolamine methylisobutyl ketimine can be used. These amines must be reacted at least in an equivalent amount with respect to the epoxy ring in order to open all the epoxy rings.

  The number average molecular weight of the cation-modified epoxy resin is preferably in the range of 1500 to 5000. If the number average molecular weight is less than 1500, the cured coating film may have poor physical properties such as solvent resistance and rust prevention. On the other hand, when it exceeds 5000, the viscosity of the resin solution is difficult to control and the synthesis is difficult, and handling such as emulsification and dispersion of the obtained resin may be difficult. Furthermore, because of its high viscosity, the flowability during heating and curing is poor, and the appearance of the coating film may be significantly impaired.

  The resin component (b) is preferably molecularly designed so that the hydroxyl value is in the range of 50 to 250. When the hydroxyl number is less than 50, the coating film is poorly cured. On the other hand, when it exceeds 250, excessive hydroxyl groups remain in the coating film after curing, and as a result, the water resistance of the cured coating film may be lowered.

  The resin component (b) has a glass transition temperature (Tg (b)) of 20 to 35 ° C., preferably 28 to 33 ° C., and a number average molecular weight of 2000 to 5000, preferably from the viewpoint of rust prevention. Is particularly preferably a cation-modified epoxy resin having a molecular weight of 2500 to 3500. The glass transition temperature (Tg (b)) of the resin component (b) can be measured by a differential scanning calorimeter, and the number average molecular weight can be measured by GPC (gel permeation chromatography).

  The compounding ratio ((a) / (b)) of the resin components (a) and (b) is 50/50 to 40/60, preferably 50/50, in terms of solid content weight ratio. When the blending ratio is out of the range of 50/50 to 40/60, the cured coating after electrodeposition coating and baking does not have a multilayer structure, and the resin having the higher blending ratio forms a continuous phase. The resin having a lower ratio may have a sea-island structure (or microdomain structure) that forms a dispersed phase.

  Resin components (a) and (b) are either emulsified and dispersed in water as an emulsion as they are, or neutralized with an organic acid such as acetic acid, formic acid, lactic acid or the like that can neutralize the amino groups in each resin. The emulsion may be emulsified and dispersed in water. As the curing agent, any kind of curing agent can be used as long as each resin component can be cured at the time of heating. Among them, a curing agent for a film-forming resin of a general electrodeposition coating composition. Suitable blocked polyisocyanates are recommended. As the yellowing prevention agent, a phenolic yellowing prevention agent is particularly preferable.

  In a preferred embodiment, the emulsion particles A are formed using a cation-modified acrylic resin as the resin component (a), and the emulsion particles B are formed using a cation-modified epoxy resin as the resin component (b). Multi-layer electrodeposition curing including an epoxy coating with excellent anti-rust properties on the coating, and an acrylic coating with excellent weather resistance, light resistance, impact resistance, and chipping resistance. A coating film can be formed. By forming such an epoxy (lowermost layer) -acrylic (uppermost layer) -based multi-layer electrodeposition cured coating film, when applied to an automobile body, etc., excellent weather resistance due to the acrylic coating film on the outer plate part, Light resistance, impact resistance, and chipping resistance are obtained, and the inner plate part has excellent anti-corrosion properties due to the epoxy coating, and both the outer plate part and the inner plate part have an appropriate film thickness. A coating can be achieved and is very beneficial. In addition, such an epoxy (lowermost layer) -acrylic (uppermost layer) -based multi-layer electrodeposition-cured coating film has excellent impact resistance, chipping resistance, and the like, and conventionally an intermediate coating film is responsible for it. It is particularly useful as an alternative to the intermediate coating film, and the intermediate coating film can be omitted. In addition, a film thickness of about 15 μm, preferably 14 to 15 μm, can be achieved in the outer plate portion of the automobile body, and a film thickness of about 10 μm, preferably 7 to 10 μm, can be achieved in the inner plate portion. It is very useful from the viewpoint of resource saving, energy saving, cost saving and environmental load reduction. Moreover, in this invention, the surface of an electrodeposition coating film can be smoothed and the outstanding coating-film external appearance can be obtained.

  For example, the smoothness of the surface of the electrodeposition coating film can be evaluated by measuring the arithmetic average roughness (Ra) in accordance with JIS-B0601, and the smaller the value of Ra, the more uneven the surface. Less, indicates that the smoothness and coating film appearance are excellent. In the present invention, the value of Ra is 0.20 to 0.30 μm, preferably 0.20 to 0.25 μm.

Curing Agent In the present invention, the electrodeposition coating composition contains blocked polyisocyanate as a curing agent.

  Examples of the polyisocyanate that is a raw material of the blocked polyisocyanate include aliphatic polyisocyanates such as hexamethylene diisocyanate (including trimer), tetramethylene diisocyanate, and trimethylhexamethylene diisocyanate; isophorone diisocyanate, 4,4 ′ -Cycloaliphatic polyisocyanates such as methylene bis (cyclohexyl isocyanate); aromatic polyisocyanates such as 4,4'-diphenylmethane diisocyanate, tolylene diisocyanate, xylylene diisocyanate, and the like. The blocked polyisocyanate can be obtained by blocking these with a suitable sealant.

  Examples of the sealant include monovalent alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenol carbinol, methylphenyl carbinol, ethylene glycol mono Cellosolves such as hexyl ether, ethylene glycol mono 2-ethylhexyl ether, phenols such as phenol, para-t-butylphenol, cresol, dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime, cyclohexanone oxime, etc. Oximes and lactams represented by ε-caprolactam and γ-butyrolactam are preferably used. In particular, sealants of oximes and lactams are suitable from the viewpoint of resin curability because they dissociate at a low temperature.

  The blending ratio of the blocked polyisocyanate to the resin components (a) and (b) varies depending on the degree of crosslinking required for the purpose of use of the cured coating film, but considering the coating film properties and topcoat coating compatibility A range of 15 to 40% by weight is preferred. If the blending ratio is less than 15% by weight, the coating film may be poorly cured. As a result, the physical properties of the coating film such as mechanical strength may be lowered, and the coating film may be damaged by the paint thinner at the time of top coating. There is a case. On the other hand, if it exceeds 40% by weight, it may be excessively cured, resulting in poor coating properties such as impact resistance. In addition, blocked polyisocyanate may be used in combination of a plurality of types for the convenience of coating film properties and adjustment of the degree of curing.

  The solubility parameter of the blocked polyisocyanate is important in securing the curability necessary for developing the coating performance of the upper layer and the lower layer after the separation of the two layers. That is, the solubility parameter (δc1) of the blocked polyisocyanate (c1) designed for the upper layer is designed to be about the same as or lower than the solubility parameter (δa) of the resin component (a) constituting the upper layer. More preferably, [(δa) − (1.0)] ≦ δc1 ≦ [(δa) + (0.2)]. Also, the solubility parameter (δc2) of the blocked polyisocyanate (c2) designed for the lower layer is designed to be about the same as or higher than the solubility parameter (δb) of the resin component (b) constituting the lower layer. More preferably, [(δb) − (0.2)] ≦ δc2 ≦ [(δb) + (1.0)].

  In the present invention, the electrodeposition coating composition preferably contains at least two types of blocked polyisocyanates (c1) and (c2). More preferably, the blocked polyisocyanate (c1) is obtained by blocking an aliphatic polyisocyanate with the above-mentioned sealing agent, and the blocked polyisocyanate (c2) is an alicyclic or aromatic type. Polyisocyanate is blocked with a sealing agent.

  The blocked polyisocyanate (c1) is preferably an aliphatic polyisocyanate blocked with the above-mentioned sealant, and in particular an upper layer of two-layer separation electrodeposition in which weather resistance, smoothness and chipping properties are required. Valid for parts.

  The blocked polyisocyanate (c2) is preferably a cycloaliphatic or aromatic polyisocyanate blocked with the above-mentioned sealant, and in particular, a two-layer separation electrodeposition requiring rust prevention. It is effective for the lower layer part of

Modified Epoxy Resin Having Onium Group In the present invention, the electrodeposition coating composition may further contain a modified epoxy resin (d) having an onium group.

  The onium group contained in the modified epoxy resin (d) having an onium group is selected from the group consisting of an ammonium group and a sulfonium group.

Modified epoxy resin (d) having an onium group includes resin components (a) and (b) of emulsion particles A and B and blocked polyisocyanates (c1) and (c2) [hereinafter, all resins of emulsion particles A and B It is contained in an amount of 1 to 10 parts by weight, preferably 3 to 5 parts by weight with respect to a total of 100 parts by weight of the solid content] .

  In the electrodeposition coating composition that can be used in the present invention, the modified epoxy resin (d) having an onium group may form emulsion particles A together with the resin component (a) and the blocked polyisocyanate (c1). it can. The modified epoxy resin (d) having an onium group can form emulsion particles B together with the resin component (b) and the blocked polyisocyanate (c2).

  The modified epoxy resin (d) having an onium group is a resin that assists in emulsifying (emulsifying) the binder resin. In the case of the emulsion particles A, the binder resin here means the resin component (a) and the blocked polyisocyanate (c1), and in the case of the emulsion particles B, the resin component (b) and the blocked polyisocyanate (c2). is there.

  Examples of the modified epoxy resin (d) having an onium group include a modified epoxy resin having a quaternary ammonium group and a modified epoxy resin having a tertiary sulfonium group.

  The modified epoxy resin having a quaternary ammonium group is a resin obtained by reacting an epoxy resin with a tertiary amine.

  As the epoxy resin, polyepoxide is generally used. This epoxide has an average of 2 or more 1,2-epoxy groups in one molecule. These polyepoxides preferably have an epoxy equivalent of 180 to 1000, in particular an epoxy equivalent of 375 to 800. When the epoxy equivalent is less than 180, no film can be formed at the time of electrodeposition, and a coating film cannot be obtained. If it exceeds 1000, the amount of onium groups per molecule is insufficient, and sufficient water solubility cannot be obtained.

  Useful examples of such polyepoxides include epoxy resins that can be used in the production of the resin component (b). Furthermore, as the epoxy resin, an oxazolidone ring-containing epoxy resin described in JP-A-5-306327 can be used.

  In particular, an epoxy resin containing a hydroxyl group may be a urethane-modified epoxy resin in which a blocked isocyanate group is introduced by reacting a half-blocked isocyanate with the hydroxyl group.

  The half-blocked isocyanate used to react with the epoxy resin described above is prepared by partially blocking the organic polyisocyanate. The reaction between the organic polyisocyanate and the blocking agent is preferably performed by cooling to 40 to 50 ° C. while dropping the blocking agent with stirring in the presence of a tin-based catalyst as necessary.

  The polyisocyanate is not particularly limited as long as it has two or more isocyanate groups on average in one molecule. As a specific example, a polyisocyanate that can be used in the preparation of the above-mentioned blocked polyisocyanate curing agent can be used.

  Suitable blocking agents for preparing the above half-blocked isocyanate include lower aliphatic alkyl monoalcohols having 4 to 20 carbon atoms. Specific examples include butyl alcohol, amyl alcohol, hexyl alcohol, 2-ethylhexyl alcohol, heptyl alcohol and the like.

  The reaction between the epoxy resin and the half-blocked isocyanate is preferably carried out by maintaining at 140 ° C. for about 1 hour.

  As said tertiary amine, a C1-C6 thing is preferable and may have a hydroxyl group. Specific examples of the tertiary amine include dimethylethanolamine, trimethylamine, triethylamine, dimethylbenzylamine, diethylbenzylamine, N, N-dimethylcyclohexylamine, tri-n-, as well as the tertiary amine that can be used above. Butylamine, diphenethylmethylamine, dimethylaniline, N-methylmorpholine and the like can be used.

  Further, the neutralizing acid used by mixing with the tertiary amine is not particularly limited, and specifically, inorganic acids or organic acids such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, and lactic acid are used. The reaction between the neutralized salt of tertiary amine thus obtained and the epoxy resin can be carried out by a conventional method. For example, the epoxy resin is dissolved in a solvent such as ethylene glycol monobutyl ether, the resulting solution is heated to 60 to 100 ° C., and a neutralized acid salt of a tertiary amine is added dropwise to the acid value of 1. The reaction mixture is kept at 60-100 ° C. until

  A modified epoxy resin having a tertiary sulfonium group can be obtained by reacting in the same manner as above except that a sulfide is used instead of a tertiary amine. Examples of sulfides that can be used include aliphatic sulfides, mixed aliphatic-aromatic sulfides, aralkyl sulfides, and cyclic sulfides. Specifically, diethyl sulfide, dipropyl sulfide, dibutyl sulfide, diphenyl sulfide, dihexyl sulfide, ethyl phenyl sulfide, tetramethylene sulfide, pentamethylene sulfide, thiodiethanol, thiodipropanol, thiodibutanol, 1- (2-hydroxy And ethylthio) -2-propanol.

  The modified epoxy resin (d) having an onium group may be used alone or in combination of two or more.

  Due to the onium group of the modified epoxy resin (d) having an onium group, the emulsification performance is improved when the emulsion particles A and B are prepared.

Pigments The pigments that can be used in the method of the present invention can be used without particular limitation as long as they are usually used in paints. Examples include colored pigments such as carbon black, titanium dioxide, and graphite, extender pigments such as kaolin, aluminum silicate (clay), and talc, aluminum phosphomolybdate, lead silicate, lead sulfate, zinc chromate, strontium chromate, Examples thereof include rust preventive pigments such as silicon dioxide. Among these, titanium dioxide, aluminum silicate (clay), and aluminum phosphomolybdate are particularly important as pigments having a dispersion concentration gradient in the multilayer cured film after electrodeposition coating. In particular, titanium dioxide is most suitable as an electrodeposition coating film because it has high concealability as a color pigment and is inexpensive. In addition, although the said pigment can also be used independently, it is common to use multiple according to the objective.

  In the multilayer cured film, the pigment is represented by a ratio P / V of the weight (V) of all vehicle components other than the pigment forming the multilayer cured film to the total pigment weight (P). A range of 1/3 is preferable. Here, the total vehicle component other than the pigment means all solid components (main resin components that are incompatible with each other, their respective curing agents, and other resin components, etc.) that constitute the paint other than the pigment. When the P / V is less than 1/10, the barrier property against corrosion factors such as light and moisture with respect to the coating film is excessively reduced due to insufficient pigment, and weather resistance and rust resistance at a practical level may not be exhibited. On the other hand, if P / V exceeds 1/3, an excessive pigment may cause an increase in viscosity at the time of curing, the flowability may be lowered, and the coating film appearance may be remarkably deteriorated. This ratio is substantially the same as the weight of all vehicle components relative to the total pigment weight in the electrodeposition coating composition used in the present invention.

  As the pigment dispersion resin (e), it is generally preferable to use a cationic polymer such as a cationic or nonionic low molecular weight surfactant or a modified epoxy resin having a quaternary ammonium group and / or a tertiary sulfonium group. The solubility parameter (δe) is preferably designed to be about the same as or higher than the solubility parameter (δb) of the resin component (b) constituting the lower layer, and more preferably, [(δb) − ( 0.2)] ≦ δe ≦ [(δb) + (1.0)]. Moreover, the suitable compounding quantity of the dispersion resin with respect to a pigment is 5-40 solid content weight% (vs. pigment weight). When the blending amount of the dispersion resin is less than 5, it is difficult to ensure the pigment dispersion stability, and when it exceeds 40, it may be difficult to control the curability of the coating film.

Preparation of Electrodeposition Coating Composition An electrodeposition coating composition that can be used in the method of the present invention comprises at least two types of film-forming resin components that are incompatible with each other, a curing agent, a pigment, and yellowing prevention A curing agent adapted to each resin component separately, and a yellowing inhibitor if necessary, and, if necessary, other modified epoxy resin (d) having an onium group In this method, the emulsion is blended so as to satisfy the above-mentioned blending ratio after being emulsified in an aqueous medium containing a neutralizing agent together with the components. Examples of the neutralizing agent include inorganic acids such as hydrochloric acid, nitric acid and phosphoric acid, and organic acids such as formic acid, acetic acid, lactic acid, sulfamic acid and acetylglycine acid.

  In the electrodeposition coating composition that can be used in the present invention, the coating film-forming resin component (a) is modified with a blocked polyisocyanate (c1), a yellowing inhibitor, and if necessary, an onium group The emulsion particle A containing the epoxy resin (d) is formed, and the coating film-forming resin component (b) is modified with a blocked polyisocyanate (c2), a yellowing inhibitor, and if necessary, an onium group More preferably, the emulsion particles B containing the epoxy resin (d) are formed, the emulsion particles A and B are separately prepared in advance and blended with the other components in the electrodeposition coating composition. Is particularly preferred. In the present invention, the yellowing inhibitor may be added to only one of the emulsion particles A and B, but particularly preferably, it is added to the emulsion particles A existing on the surface layer when the coating film is formed. That is.

  The electrodeposition coating composition that can be used in the present invention is preferably adjusted so that the solid content concentration is in the range of 15 to 25% by weight. The solid content is adjusted using an aqueous medium (water alone or a mixture of water and a hydrophilic organic solvent). A small amount of additives may be introduced into the coating composition. Examples of the additive include an ultraviolet absorber, an antioxidant, a surfactant, a coating film surface smoothing agent, a curing accelerator (such as an organic tin compound), and the like.

Formation of Multi-layer Electrodeposition Coating In the present invention, the multi-layer electrodeposition coating film comprises a conductive substrate as a cathode as a cathode, and a cathode (cathode electrode) terminal connected to the object to be coated. Generally, a coating film having an amount of a dry film thickness of 14 to 15 μm is applied by electrodeposition under conditions of a bath temperature of the coating composition of 15 to 35 ° C. and a load voltage of 100 to 400 V. Thereafter, baking is performed at 140 to 200 ° C., preferably 160 to 180 ° C. for 10 to 30 minutes. Resin component (a), emulsion particles A containing blocked polyisocyanate (c1) and resin component (b), blocked, contained in the electrodeposition-coated electrodeposition coating composition by heating for the purpose of baking. The emulsion particles B containing the polyisocyanate (c2) are oriented according to their inherent solubility parameters. When the coating is cured after baking, an electrodeposition cured film having a multilayer structure is formed in which emulsion particles A form the uppermost layer in direct contact with air and emulsion particles B form the lowermost layer in direct contact with the object to be coated. To do. In addition, the baking heating method includes a method of putting a coated material in a heating facility adjusted to a target temperature from the beginning and a method of raising the temperature after putting the coated material.

Formation of multilayer coating film A multilayer coating film excellent in weather resistance and appearance can be formed by further applying and baking a top coating on the multilayer electrodeposition-cured coating film formed by the above method. The top coat may be any of solvent type, aqueous type, and powder.

In the present invention, two types of top coats, that is, the top coat base paint composition and the top coat clear paint composition, can be used to obtain a more excellent coating appearance. In this case, the multilayer coating film forming method of the present invention is a so-called two-coat one-bake (2C1B) method including the following steps (1) to (3).
A step (1) of further applying an overcoating base coating composition on the multilayer electrodeposition-curing coating film formed by the above-described electrodeposition coating film forming method to form an uncured topcoating base coating film;
A step (2) of forming an uncured top clear film by applying a top clear composition on the uncured top base film, and an uncured top base film and an uncured top coat Step of curing the clear coating film by heating simultaneously (3)

  The coating in the above step (1) or (2) is, for example, an air electrostatic spray commonly called “react gun”, commonly called “micro-microbell (μμbell)”, “microbell (μbell)”, “metallic”. It can be applied by spraying using a rotary atomizing electrostatic coating machine or the like called “bell”. The coating amount can be appropriately changed according to the type and application of the coating composition to be used.

  After the step (1) or (2), a time interval called an interval may be performed. By this interval, the solvent contained in the coating film can be sufficiently volatilized. Appearance is improved. The interval is, for example, 10 seconds to 15 minutes.

  Moreover, you may perform drying operation called preheating within the time of an interval, and volatilization of the solvent contained in a coating film can be performed efficiently in a short time by this preheating. This drying operation does not actively cure the coating film. Therefore, as said drying conditions, it is 1 to 10 minutes at room temperature-100 degreeC, for example. Preheating can be performed using, for example, a warm air heater or an infrared heater.

  The heat curing step (3) can be performed using a conventional heat curing furnace (for example, a gas furnace, an electric furnace, an IR furnace, an induction heating furnace, or the like). The heat curing temperature can be appropriately set according to the coating composition to be used, and is, for example, 120 to 160 ° C., and the heat curing time is, for example, 10 to 30 minutes.

  Hereinafter, the present invention will be described in more detail with reference to production examples, examples and comparative examples. In each example, “part” represents “part by weight”, and “%” represents “% by weight”.

Production Example 1: Production of cation-modified acrylic resin (film-forming resin component (a)) Into a reaction vessel equipped with a stirrer, a cooler, a nitrogen introduction tube, a thermometer and a dropping funnel, 50 parts of methyl isobutyl ketone was charged. Heated and maintained at 115 ° C. in a nitrogen atmosphere. Further, 2.49 parts of methyl methacrylate, 41.76 parts of ethylhexyl methacrylate, 17.75 parts of isobornyl methacrylate, 11.56 parts of hydroxypropyl methacrylate, 10.44 parts of hydroxyethyl methacrylate, N, N-dimethylaminoethyl A mixture of 16.00 parts of methacrylate and 3.00 parts of t-butylperoxy 2-ethylhexanoic acid was dropped from the dropping funnel over 3 hours, and then 0.50 parts of t-butylperoxy 2-ethylhexanoic acid was further added. Was added dropwise and held at 115 ° C. for 1.5 hours. The obtained cation-modified acrylic resin had a solid content of 65%, a solubility parameter (δa) of 10.5, a glass transition temperature (Tg) of 30 ° C., and a number average molecular weight of 8,000.

Production Example 2: Production of blocked polyisocyanate (c1) 199 parts of hexamethylene diisocyanate trimer was placed in a reaction vessel equipped with a stirrer, nitrogen introduction tube, cooling tube and thermometer, and diluted with 39 parts of methyl isobutyl ketone. Thereafter, 0.2 part of butyltin laurate was added, and after raising the temperature to 50 ° C., 44 parts of methyl ethyl ketoxime and 87 parts of ethylene glycol mono-2-ethylhexyl ether were added so that the temperature of the contents did not exceed 70 ° C. Then, the infrared absorption spectrum is maintained at 70 ° C. for 1 hour until the absorption of the isocyanate residue substantially disappears, and then diluted with 43 parts of n-butanol, thereby blocking 80% solid content of blocked polyisocyanate (c1) (dissolved) Sex parameter (δc1) 10.7) was obtained.

Production Example 3: Production of modified epoxy resin (d) having sulfonium group
Preparation of 2-ethylhexanol half-blocked IPDI (MIBK solution ) 222.0 parts of isophorone diisocyanate (hereinafter abbreviated as IPDI) was placed in a reaction vessel equipped with a stirrer, cooling tube, nitrogen introduction tube and thermometer, and MIBK39 After diluting with 1 part, 0.2 part of dibutyltin dilaurate was added thereto. Then, after heating this to 50 degreeC, 131.5 parts of 2-ethylhexanol was dripped over 2 hours in dry nitrogen atmosphere, stirring. The reaction temperature was maintained at 50 ° C. by cooling appropriately. As a result, 2-ethylhexanol half-blocked IPDI (resin solid content: 90.0%) was obtained.
A reaction vessel was charged with 382.2 parts of bisphenol A type epoxy resin having an epoxy equivalent of 188 (manufactured by Dow Chemical Company) and 117.8 parts of bisphenol A, and heated to 150 to 160 ° C. in a nitrogen atmosphere. The reaction mixture was reacted at 150-160 ° C. for about 1 hour, then cooled to 120 ° C., and 209.8 parts of 2-ethylhexanol half-blocked IPDI (MIBK solution) prepared above was added. After reacting at 140 to 150 ° C. for 1 hour, 205 parts of a polyalkylene oxide compound (trade name BPE-60, manufactured by Sanyo Chemical Co., Ltd.) was added and cooled to 60 to 65 ° C. Thereto was added 408.0 parts of 1- (2-hydroxyethylthio) -2-propanol, 144.0 parts of deionized water and 134 parts of dimethylolpropionic acid, and the reaction was carried out at 65 to 75 ° C. until the acid value reached 1. Then, a tertiary sulfonium group was introduced into the epoxy resin, and 1595.2 parts of deionized water was added to terminate the tertiaryization to obtain a varnish of a modified epoxy resin (d) having a tertiary sulfonium group ( 30% solids).

Production Example 4-1: Production of cation-modified acrylic emulsion (A-1) 247.1 parts of cation-modified acrylic resin obtained in Production Example 1 and blocked polyisocyanate (c1) 120.0 obtained in Production Example 2 53.0 parts of the modified epoxy resin (d) having a sulfonium group obtained in Production Example 3 and 0.6 part of Sumilizer GA-80 (manufactured by Sumitomo Chemical Co., Ltd.) as a phenol-based yellowing inhibitor were added. Stir for 30 minutes. Thereafter, 5 parts of acetic acid was added and diluted with ion-exchanged water to a non-volatile content of 30%, and then concentrated under reduced pressure to a non-volatile content of 38% to obtain a cation-modified acrylic emulsion (A-1).

Production Example 4-2: Production of cation-modified acrylic emulsion (A-2) 247.1 parts of cation-modified acrylic resin obtained in Production Example 1 and blocked polyisocyanate (c1) 120.0 obtained in Production Example 2 30 parts by adding 53.0 parts of the modified epoxy resin (d) having a sulfonium group obtained in Production Example 3 and 60 parts of Sumilizer GA-80 (manufactured by Sumitomo Chemical Co., Ltd.) as a phenol-based yellowing inhibitor. Stir. Thereafter, 5 parts of acetic acid was added and diluted with ion-exchanged water to a non-volatile content of 30%, and then concentrated under reduced pressure to a non-volatile content of 38% to obtain a cation-modified acrylic emulsion (A-2).

Production Example 4-3: Production of cation-modified acrylic emulsion (A-3) 247.1 parts of cation-modified acrylic resin obtained in Production Example 1 and blocked polyisocyanate (c1) 120.0 obtained in Production Example 2 Part, 53.0 parts of the modified epoxy resin (d) having a sulfonium group obtained in Production Example 3, and 6.0 parts of Sumilizer GA-80 (manufactured by Sumitomo Chemical Co., Ltd.) as a phenol-based yellowing inhibitor. Stir for 30 minutes. Thereafter, 5 parts of acetic acid was added and diluted with ion-exchanged water to a non-volatile content of 30%, and then concentrated under reduced pressure to a non-volatile content of 38% to obtain a cation-modified acrylic emulsion (A-3).

Production Example 4-4: Production of cation-modified acrylic emulsion (A-4) 247.1 parts of cation-modified acrylic resin obtained in Production Example 1 and blocked polyisocyanate (c1) 120.0 obtained in Production Example 2 30 parts by adding 53.0 parts of the modified epoxy resin (d) having a sulfonium group obtained in Production Example 3 and 6.0 parts of Sumilizer TPM (manufactured by Sumitomo Chemical Co., Ltd.) as a sulfur-based yellowing inhibitor. Stir. Thereafter, 5 parts of acetic acid was added and diluted with ion-exchanged water to a nonvolatile content of 30%, and then concentrated under reduced pressure to a nonvolatile content of 38% to obtain a cation-modified acrylic emulsion (A-4).

Production Example 4-5: Production of cation-modified acrylic emulsion (A-5) 247.1 parts of cation-modified acrylic resin obtained in Production Example 1 and blocked polyisocyanate (c1) 120.0 obtained in Production Example 2 Part, 53.0 parts of the modified epoxy resin (d) having a sulfonium group obtained in Production Example 3, and 6.0 parts of IRGANOX 565 (manufactured by Ciba Japan) as a sulfur-containing phenolic yellowing inhibitor And stirred for 30 minutes. Thereafter, 5 parts of acetic acid was added and diluted with ion-exchanged water to a non-volatile content of 30%, and then concentrated under reduced pressure to a non-volatile content of 38% to obtain a cation-modified acrylic emulsion (A-5).

Production Example 4-6: Production of cation-modified acrylic emulsion (A-6) 247.1 parts of cation-modified acrylic resin obtained in Production Example 1 and blocked polyisocyanate (c1) 120.0 obtained in Production Example 2 Part, 53.0 parts of the modified epoxy resin (d) having a sulfonium group obtained in Production Example 3, and 6.0 parts of IRGASTAB FS 042 (manufactured by Ciba Japan) as a hydroxylamine-based yellowing inhibitor And stirred for 30 minutes. Then, after adding 5 parts of acetic acid and diluting to 30% of non volatile matter with ion-exchange water, it concentrated to 38% of non volatile matter under reduced pressure, and obtained the cation modified | denatured acrylic emulsion (A-6).

Production Example 4-7: Production of cation-modified acrylic emulsion (A-7) 247.1 parts of cation-modified acrylic resin obtained in Production Example 1 and blocked polyisocyanate (c1) 120.0 obtained in Production Example 2 30 parts by adding 53.0 parts of the modified epoxy resin (d) having a sulfonium group obtained in Production Example 3 and 6.0 parts of IRGAFOS 168 (manufactured by Ciba Japan) as a phosphorus-based yellowing inhibitor. Stir for minutes. Thereafter, 5 parts of acetic acid was added and diluted with ion-exchanged water to a non-volatile content of 30%, and then concentrated under reduced pressure to a non-volatile content of 38% to obtain a cation-modified acrylic emulsion (A-7).

Production Example 5: Production of cation-modified epoxy resin (coating film-forming resin component (b)) A flask equipped with a stirrer, a cooler, a nitrogen injection tube and a dropping funnel was attached. DER-331J, manufactured by Dow Chemical Co., Ltd.) 680.94 parts, bisphenol A 237.12 parts, stearic acid 105.27 parts, dimer acid (trade name Tsunodim 216, manufactured by Tsukuno Food Industry Co., Ltd.) 68.62 parts, methyl isobutyl After 115.54 parts of ketone and 0.10 parts of dibutyltin dilaurate were added and dissolved uniformly, the temperature was raised from 130 ° C. to 142 ° C., and the reaction was continued until an epoxy equivalent of 1150 was reached.
Thereafter, 71.37 parts of methyl isobutyl ketone was added and cooled to 100 ° C., 49.93 parts of N-methylethanolamine and 87.84 parts of diethylenetriamine diketimine (73% methyl isobutyl ketone solution) were added, and 110 ° C. The reaction was performed for 2 hours. The obtained cation-modified epoxy resin had a solid content of 85%, a solubility parameter (δb) of 11.4, and a glass transition temperature (Tg) of 26 ° C.

Production Example 6 Production of Blocked Polyisocyanate (c2) 222 parts of isophorone diisocyanate was placed in a reaction vessel equipped with a stirrer, nitrogen introduction tube, condenser tube and thermometer, diluted with 56 parts of methyl isobutyl ketone, and then butyltin laurate. After adding 0.2 parts and raising the temperature to 50 ° C., 17 parts of methyl ethyl ketoxime was added so that the content temperature did not exceed 70 ° C. Then, the infrared absorption spectrum is kept at 70 ° C. for 1 hour until the absorption of the isocyanate residue substantially disappears, and then diluted with 43 parts of n-butanol, whereby the blocked polyisocyanate (c2) (c2) (70% solid content) Solubility parameter (δc2) 11.8) was obtained.

Production Example 7: Production of cation-modified epoxy emulsion (B-1) 308.8 parts of the cation-modified epoxy resin obtained in Production Example 5, 112.5 parts of the blocked polyisocyanate (c2) obtained in Production Example 6, And 58.8 parts of the modified epoxy resin (d) having a sulfonium group obtained in Production Example 3 was added and stirred for 30 minutes. Thereafter, 6 parts of acetic acid was added, diluted with ion-exchanged water to a non-volatile content of 30%, and concentrated under reduced pressure to a non-volatile content of 38% to obtain a cation-modified epoxy emulsion (B-1).

Production Example 8 Production of Pigment Dispersion Resin (e) Bisphenol A type epoxy resin having an epoxy equivalent of 198 (trade name Epon 829, manufactured by Shell Chemical Co., Ltd.) 710 in a reaction vessel equipped with a stirrer, a cooling tube, a nitrogen introduction tube, and a thermometer And 289.6 parts of bisphenol A were added and reacted at 150 to 160 ° C. for 1 hour under a nitrogen atmosphere. After cooling to 120 ° C., a methyl isobutyl ketone solution of 2-ethylhexanolated half-blocked tolylene diisocyanate (solid 406.4 parts). After maintaining the reaction mixture at 110-120 ° C. for 1 hour, 1584.1 parts of ethylene glycol mono n-butyl ether was added. And it cooled to 85-95 degreeC and made it uniform.
In parallel with the production of the reaction product, 104.6 parts of dimethylethanolamine was added to 384 parts of methyl isobutyl ketone solution (solid content 95%) of 2-ethylhexanolated half-blocked tolylene diisocyanate in another reaction vessel. The mixture was stirred at 80 ° C. for 1 hour, and then 141.1 parts of 75% aqueous lactic acid was added. Further, 47.0 parts of ethylene glycol mono-n-butyl ether was mixed and stirred for 30 minutes to form a quaternizing agent (solid content: 85% ). Then, 620.5 parts of this quaternizing agent is added to the above reaction product, and the mixture is kept at 85 to 95 ° C. until an acid value of 1 is reached. A pigment dispersion resin (resin solid content 56%, average molecular weight 2200, solubility Parameter (δe) 11.3) was obtained.

Production Example 9: Production of pigment dispersion paste 120 parts of the pigment dispersion resin of Production Example 8 in a sand grind mill, 2.0 parts of carbon black, 100.0 parts of kaolin, 80.0 parts of titanium dioxide, aluminum phosphomolybdate 18. 0 parts, 15 parts of silicon dioxide and 188.1 parts of ion-exchanged water were added and dispersed until the particle size became 10 μm or less to obtain a pigment dispersion paste (solid content 60%).

Production Example 10-1: Production of cationic electrodeposition coating composition ( 1) 211.6 parts of cation-modified acrylic emulsion (A-1) obtained in Production Example 4-1 and cation-modified epoxy obtained in Production Example 7 Cation electrodeposition paint by mixing 190.0 parts of emulsion (B-1), 75.5 parts of pigment dispersion paste obtained in Production Example 9, 3.5 parts of dibutyltin oxide, and 519.4 parts of ion-exchanged water A composition (1) was obtained. The solid content of this cationic electrodeposition coating composition (1) was 20%.

Production Example 10-2: Production of Cationic Electrodeposition Coating Composition ( 2) 228.6 parts of cation-modified acrylic emulsion (A-2) obtained in Production Example 4-2, cation-modified epoxy obtained in Production Example 7 Cation electrodeposition paint by mixing 173.0 parts of emulsion (B-1), 75.5 parts of pigment dispersion paste obtained in Production Example 9, 3.5 parts of dibutyltin oxide, and 519.4 parts of ion-exchanged water A composition (2) was obtained. The solid content of the cationic electrodeposition coating composition (2) was 20%.

Production Example 10-3: Production of cationic electrodeposition coating composition ( 3) 213.3 parts of cation-modified acrylic emulsion (A-3) obtained in Production Example 4-3, cation-modified epoxy obtained in Production Example 7 Cation electrodeposition paint by mixing 188.3 parts of emulsion (B-1), 75.5 parts of pigment dispersion paste obtained in Production Example 9, 3.5 parts of dibutyltin oxide, and 519.4 parts of ion-exchanged water A composition (3) was obtained. The solid content of the cationic electrodeposition coating composition (3) was 20%.

Production Example 10-4: Production of cationic electrodeposition coating composition (4)
213.3 parts of the cation-modified acrylic emulsion (A-4) obtained in Production Example 4-4, 188.3 parts of the cation-modified epoxy emulsion (B-1) obtained in Production Example 7, and obtained in Production Example 9 75.5 parts of pigment dispersion paste, 3.5 parts of dibutyltin oxide, and 519.4 parts of ion-exchanged water were mixed to obtain a cationic electrodeposition coating composition (4). The solid content of the cationic electrodeposition coating composition (4) was 20%.

Production Example 10-5: Production of cationic electrodeposition coating composition (5)
213.3 parts of the cation-modified acrylic emulsion (A-5) obtained in Production Example 4-5, 188.3 parts of the cation-modified epoxy emulsion (B-1) obtained in Production Example 7, and obtained in Production Example 9 75.5 parts of pigment dispersion paste, 3.5 parts of dibutyltin oxide, and 519.4 parts of ion-exchanged water were mixed to obtain a cationic electrodeposition coating composition (5). This cationic electrodeposition coating composition (5) had a solid content of 20%.

Production Example 10-6: Production of cationic electrodeposition coating composition (6)
213.3 parts of the cation-modified acrylic emulsion (A-6) obtained in Production Example 4-6, 188.3 parts of the cation-modified epoxy emulsion (B-1) obtained in Production Example 7, and obtained in Production Example 9 75.5 parts of pigment dispersion paste, 3.5 parts of dibutyltin oxide, and 519.4 parts of ion-exchanged water were mixed to obtain a cationic electrodeposition coating composition (6). The solid content of the cationic electrodeposition coating composition (6) was 20%.

Production Example 10-7: Production of cationic electrodeposition coating composition (7)
213.3 parts of the cation-modified acrylic emulsion (A-7) obtained in Production Example 4-7, 188.3 parts of the cation-modified epoxy emulsion (B-1) obtained in Production Example 7, and obtained in Production Example 9 75.5 parts of pigment dispersion paste, 3.5 parts of dibutyltin oxide, and 519.4 parts of ion-exchanged water were mixed to obtain a cationic electrodeposition coating composition (7). The solid content of the cationic electrodeposition coating composition (7) was 20%.

  Using the cationic electrodeposition coating compositions (1) to (7), a zinc phosphate-treated steel sheet (JIS G3141 SPCC-SD Surfdyne SD-5000 (manufactured by Nippon Paint Co., Ltd.) steel sheet) is used as the object to be coated. Then, electrodeposition coating was performed under the following electrodeposition coating conditions 1 to 3, respectively.

Electrodeposition condition 1 (equivalent to the condition of the inner plate of the car body)
Electrode coating composition temperature: 28 ° C
Deposition conditions: 200 V, 180 seconds, coating film thickness: 15 μm
Heat curing conditions: 160 ° C., 20 minutes

Electrodeposition condition 2 (equivalent to the condition of the outer plate of the car body)
Electrode coating composition temperature: 28 ° C
Deposition conditions: 200 V, 180 seconds, coating film thickness: 15 μm
Heat curing conditions: 180 ° C., 20 minutes

Electrodeposition condition 3 (overheating condition)
Electrode coating composition temperature: 28 ° C
Deposition conditions: 200 V, 180 seconds, coating film thickness: 15 μm
Heat curing conditions: 190 ° C, 40 minutes

  The occurrence of yellowing was evaluated by the value of chromaticity b * of the L * a * b * color system. The value of b * was measured using a color difference meter “CR-300” manufactured by Minolta.

In the electrodeposition coating condition 1, when the value of b * is in the range of −0.5 to +0.5, it is determined that an excellent yellowing prevention effect is obtained.
In the electrodeposition coating condition 2, when the value of b * is within the range of +1.0 to +3.0, it is determined that an excellent yellowing prevention effect is obtained.
In the electrodeposition coating condition 3, when the value of b * is within the range of +5.0 to +10.0, it is determined that an excellent yellowing prevention effect is obtained.

Moreover, the arithmetic average roughness (Ra) of the surface of the electrodeposition coating film after hardening was measured based on JIS-B0601, and the surface of the electrodeposition coating film was evaluated. The arithmetic average roughness (Ra) was measured using an evaluation type surface roughness measuring machine (manufactured by Mitsutoyo, SURFTEST SJ-201P).
If the value of Ra is 0.25 μm or less, it is judged that an electrodeposition coating film appearance excellent in smoothness was obtained.

  Also, an aqueous base coating composition (“AQUAREX AR-3100 Base”, manufactured by Nippon Paint Co., Ltd., acrylic resin / melamine resin-based coating) is applied by air spray coating onto the electrodeposition coating film obtained in Example 1. The film was coated to a dry film thickness of 12 μm and preheated at 80 ° C. for 3 minutes. Further, a clear coating composition ("Polyure Excel O-3100 Clear", manufactured by Nippon Paint Co., Ltd., acrylic resin / isocyanate compound-based paint having a urethane crosslinking system) is applied to the coating plate by air spray coating to a dry film thickness of 35 μm. When this two-layer coating film was heated at 140 ° C. for 30 minutes and cured at the same time, a good finished appearance was obtained.

The evaluation results are summarized in the following table.

  In the table, the blending amount (% by weight) of the yellowing inhibitor was calculated based on 100 parts by weight of the total resin solid content of the emulsion particles A and the emulsion particles B.

  In Comparative Example 1, the blending amount of the yellowing inhibitor is 0.1% by weight and deviates from the specified amount of 0.5 to 5.0% by weight in the present invention. A marked yellowing was also observed. In particular, under the electrodeposition coating condition 3 (excessive heating condition), the value of b * was 17.2, and a lot of yellowing occurred.

  In Comparative Example 2, the blending amount of the yellowing inhibitor is 10.0% by weight and deviates from the specified amount of 0.5 to 5.0% by weight in the present invention. The yellowing level was within an acceptable range. However, in Comparative Example 2, since the yellowing inhibitor was added in a large amount, the appearance of the coating film was remarkably lowered (Ra = 0.43 μm).

  As described in this specification, it is generally determined that significant yellowing has occurred when the b * value of the electrodeposition coating film is 5 or more. In Comparative Example 1, significant yellowing is observed even in the electrodeposition coating condition 2 (corresponding to the condition in the outer plate portion of the automobile body). It turns out that it is not suitable for wearing.

  On the other hand, in Examples 1 to 5 of the present invention, the value of b * indicating the yellowing level is within an allowable range in any of the electrodeposition coating conditions 1 to 3, and exhibits an excellent yellowing prevention effect. In Examples 1 to 5, in particular, in the electrodeposition coating condition 2 (corresponding to the condition in the outer plate portion of the automobile body), the value of b * is less than 3, and the present invention is particularly It can be seen that it is suitable for electrodeposition coating of the outer plate. Surprisingly, in Examples 1 to 5 of the present invention, the value of b * is less than 10 even under the electrodeposition coating condition 3 (overheating condition), because it is an overheating condition. Although yellowing occurred, the value was about half that of Comparative Example 1.

  Moreover, in Examples 1-5 of this invention, the outstanding coating-film external appearance (Ra = 0.25 micrometer or less) can be obtained, and it turns out that this invention is not especially suitable for the electrodeposition coating of a motor vehicle body. .

The present invention can prevent yellowing of an electrodeposition coating film, and forms an electrodeposition coating film that is excellent not only in rust prevention but also in weather resistance, light resistance, impact resistance, chipping resistance, etc. Therefore, it is very useful when painting an object having a large and complicated shape such as an automobile body. In addition, according to the method of the present invention, it is possible to omit an intermediate coater that has been conventionally required, and to achieve resource saving, energy saving, cost saving, and reduction of environmental load (for example, low VOC and low CO ₂ ). Can do.
Furthermore, when the method of the present invention is applied to electrodeposition coating of an automobile body or the like, yellowing can be significantly prevented in both the inner plate portion and the outer plate portion. Further, even under excessive heating conditions, yellowing of the electrodeposition coating film can be prevented, which is very useful for industrial production.

Claims (9)

Electrodeposition coating formation comprising electrodeposition coating of electrodeposition coating composition on substrate, then separating layers while heating, and then curing to form a multilayer cured film consisting of at least two layers An electrodeposition coating composition comprising:
Film-forming resin components (a) and (b),
Blocked polyisocyanates (c1) and (c2),
Pigments, and yellowing inhibitors,
The film-forming resin components (a) and (b) are incompatible with each other;
The film-forming resin component (a) forms emulsion particles A containing blocked polyisocyanate (c1),
The film-forming resin component (b) forms emulsion particles B containing blocked polyisocyanate (c2),
The yellowing inhibitor is contained in an amount of 0.5 to 5.0 parts by weight with respect to 100 parts by weight of the total resin solids of the emulsion particles A and B.
Electrodeposition coating formation method.   The electrodeposition coating film forming method according to claim 1, wherein the yellowing inhibitor is selected from the group consisting of phenolic, phosphorus, hydroxylamine and sulfur yellowing inhibitors. The film-forming resin component (a) is a cation-modified acrylic resin,
The film-forming resin component (b) is a cation-modified epoxy resin,
The solubility parameter (δa) of the coating film-forming resin component (a) and the solubility parameter (δb) of the coating film-forming resin component (b) satisfy the relationship {δb−δa} ≧ 0.4.
The electrodeposition coating film forming method according to claim 1 or 2.   The blending ratio ((a) / (b)) of the film-forming resin components (a) and (b) is 50/50 to 40/60 in terms of solid content weight ratio. 2. The electrodeposition coating film forming method according to item 1.   The electrodeposition coating-film formation method of any one of Claims 1-4 whose brightness (L * value) of the said multilayer cured film is 55 or more.   The modified epoxy resin (d) having an onium group is based on 100 parts by weight of the total of the film-forming resin components (a) and (b) of the emulsion particles A and B and the blocked polyisocyanates (c1) and (c2). The electrodeposition coating film forming method according to any one of claims 1 to 5, which is contained in an amount of 1 to 10 parts by weight.   The electrodeposition coating film forming method according to claim 6, wherein the onium group of the modified epoxy resin (d) having an onium group is selected from the group consisting of an ammonium group and a sulfonium group. Blocked polyisocyanate (c1) is an aliphatic polyisocyanate blocked with a sealant,
The blocked polyisocyanate (c2) is obtained by blocking an alicyclic or aromatic polyisocyanate with a sealing agent.
The electrodeposition coating-film formation method of any one of Claims 1-7. An overcoating base coating composition is further applied on the electrodeposition coating film formed by the electrodeposition coating film forming method according to any one of claims 1 to 8, and an uncured top coating base coating film is applied. Forming a process,
A step of applying an overcoat clear coating composition on the uncured topcoat base coating to form an uncured topcoat clearcoat; and an uncured topcoat basecoat and an uncured topcoat clear coat The method of forming a multilayer coating film including the process of heating and hardening simultaneously.