JP2008155172A - Coating formation method using precoat steel, and coating article - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To dispense with a sealer coating which has been time consuming in the coating process of an automobile. <P>SOLUTION: A coating formation method and a coating article obtained thereby are characterized by forming a predetermined shaped article to be coated using a precoat steel with one side precoated and setting the precoat surface on its outside, and by coating the unprecoated inner plate part by an electrodeposition coating. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a coating film forming method, particularly to a coating film forming method in which electrodeposition coating is applied to a surface other than the precoated surface of a precoated steel sheet, and a coated article obtained therefrom.

  The automobile painting process includes various processes, and is basically classified into cationic electrodeposition coating, intermediate coating, and top coating. Usually, there is a sealer coating process between electrodeposition coating and intermediate coating. Sealer coating may be integrated with intermediate coating. In any case, sealer painting is the most labor-intensive and expensive process.

  The most manual part of sealer coating is the joint between steel plates such as doors, trunks and bonnets. These parts differ slightly depending on the type of vehicle, and complicated and delicate work contents are required. Therefore, automation, particularly robotization, has not been achieved.

  In FIG. 1, the schematic cross section of the fitting part of a steel plate is shown. Although the steel plates to which electrodeposition coating has been applied are combined, as is apparent from this figure, there is always an edge portion of the steel plate, that is, a tip portion, and this edge portion is shown in the enlarged view of FIG. In addition, the electrodeposition coating film is thin and has a portion with insufficient rust prevention. The lack of rust prevention is compensated by the aforementioned sealer coating.

  If the coating film viscosity of the electrodeposition coating film is increased without applying the sealer coating, an electrodeposition coating film with a sufficient film thickness at the edge portion will be formed, and the sealer coating will be unnecessary. On the contrary, when the viscosity of the electrodeposition coating film is increased, unevenness is produced in the flat portion, and the appearance of the coating film is deteriorated. Therefore, the method for increasing the viscosity of the electrodeposition coating film cannot be used as it is.

Japanese Patent Application Laid-Open No. 8-332454 (Patent Document 1) describes that a precoated steel plate is shaped and welded, and then the portion of the precoated steel plate that has been peeled off is electrodeposited. In this method, the electrodeposition coating is electrodeposition coating on the part where the coating on the pre-coated surface of the pre-coated steel plate is peeled off by welding, and no processing is performed on the surface other than the pre-coated surface. Patent Document 1 describes that another steel plate that is not a pre-coated steel plate that is welded and adhered to the pre-coated steel plate is electrodeposited at the same time as the electrodeposition coating of the film peeling portion. It is not an electrodeposition coating on a surface other than the surface.
JP-A-8-332454

  It is an object of the present invention to eliminate the need for time-consuming sealer coating in an automobile painting process.

  The inventors of the present invention have found that a sealer coating step is not required by using a pre-coated steel plate and performing cationic electrodeposition coating on a surface other than the pre-coated surface of the pre-coated steel plate, thereby achieving the present invention.

  That is, the present invention uses a pre-coated steel plate that has been pre-coated on one side to form an object having a predetermined shape with the pre-coated surface outside, and the inner plate portion that is not pre-coated is applied by electrodeposition coating. A coating film forming method is provided.

  The present invention is also a coated article comprising an outer plate portion having a cosmetic appearance and exposed to the outside, and an inner plate portion other than the outer plate portion, and the coated article is formed from a pre-coated steel plate on which one side has been previously coated, There is provided a coated article characterized in that the precoated surface of the precoated steel sheet is an outer plate portion, and the other inner plate portion is formed by electrodeposition coating by electrodeposition coating.

  The electrodeposition paint used for the electrodeposition coating of the inner plate preferably has a minimum coating film viscosity of 10,000 Pa · S or more upon curing.

  In this invention, it is preferable that the inner-plate part which is not precoated does not form coating films other than an electrodeposition coating film.

  The pre-coated surface of the pre-coated steel sheet may be further subjected to finish coating.

  In the coating film forming method of the present invention, at least one layer or all of the surface coating is simplified by using a pre-coated steel sheet pre-coated on one side for manufacturing an automobile and applying electrodeposition coating to a surface other than the pre-coated surface. be able to. In addition, the surface other than the pre-coated surface can be electrodeposited to form an inner plate portion with a strong anti-rust coating, and the inner plate portion can be coated other than electrodeposition coating. Therefore, the painting process can be greatly simplified in this respect as well.

  Further, in the present invention, the electrodeposition paint has a minimum coating viscosity of 10,000 Pa · S or more at the time of curing in the electrodeposition coating, so that the anticorrosion property at the edge portion is increased, and the sealer coating is not required at the mating portion. Even if the pre-coating surface is subjected to intermediate coating or top coating, since the sealer coating is unnecessary, the most laborious process can be omitted, and the process simplification ability is very high.

Various pre- coated steel sheets already exist, and existing ones may be selected according to the respective purposes. Here, although the coating material which forms the coating film of a general precoat steel plate is demonstrated, it is not limited to these. Moreover, in this invention, the coating form of a precoat surface changes greatly depending on whether a coating film is further formed in the precoat surface of a precoat steel plate. When a pre-coated surface of the pre-coated steel sheet is not formed, it is necessary to form a film with a certain degree of appearance. However, when further pre-coating is applied on the pre-coated surface, it is called so-called intermediate coating. It may be about the same as a coated film.

  As the main component of the resin for forming the precoat film, it is preferable to use a polyester resin comprising a polyester polyol and an amino resin and / or an isocyanate compound. For example, a part of the polyester polyol is modified with epoxy or the like. The so-called modified polyester resin is also included. In addition to the above polyester-based resins, epoxy-based, acrylic-based, vinylidene fluoride-based, silicon polyester-based, vinyl chloride-based resins, and the like can be considered as coating film forming resins. Since the resin and the silicon polyester resin are poor in flexibility, the coating film is easily cracked or peeled off during the molding process. Moreover, since vinylidene fluoride resin is non-adhesive, the interlayer adhesion with the top coat film is poor. Furthermore, since vinyl chloride resin has a soft coating film, workability at the time of molding is poor, and it is not preferable from the viewpoint of environmental pollution.

  The polyester polyol used in the present invention preferably has at least two hydroxyl groups in one molecule and has a number average molecular weight in the range of 1000 to 30000, preferably in the range of 2000 to 20000. Moreover, the hydroxyl group in the molecule | numerator of polyester polyol may exist in either the terminal in a molecule | numerator, or a side chain. If the number average molecular weight of the polyester polyol is less than 1000, the processability is remarkably lowered, which is not preferable. On the other hand, if the number average molecular weight exceeds 30000, the coating film hardness becomes insufficient and the viscosity becomes high, so a large amount of dilution solvent is required. It becomes. In addition, the number average molecular weight of this polyester polyol is a polystyrene conversion molecular weight measured by gel permeation chromatography.

  Polyester polyol is a copolymer obtained by heat-reacting a polybasic acid component and a polyhydric alcohol component by a known method. As the polybasic acid component, for example, phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, maleic acid, adipic acid, fumaric acid and the like can be used. Examples of the polyhydric alcohol component include ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, triethylene glycol, Glycerin, pentaerythritol, trimethylolpropane, trimethylolethane and the like can be used. Examples of commercially available polyester polyols include Almatex manufactured by Mitsui Toatsu Chemical Co., Ltd., alkynol and desmophene manufactured by Sumitomo Bayer Urethane Co., Ltd., and Byron manufactured by Toyobo Co., Ltd.

  Polyester polyol forms a cured coating film by reaction with a curing agent. An amino resin and / or an isocyanate compound can be used for the curing reaction of the polyester polyol. The amino resin used in the present invention is a resin obtained by a reaction of urea, benzoguanamine, melamine or the like with formaldehyde, and an alkyl etherified with an alcohol such as methanol or butanol can also be used. Specifically, methylated urea resin, n-butylated benzoguanamine resin, methylated melamine resin, n-butylated melamine resin, iso-butylated melamine resin and the like can be mentioned. Examples of commercially available amino resins include Cymel manufactured by Mitsui Cyanamid Co., Ltd., Uban manufactured by Mitsui Toatsu Chemical Co., Ltd., Sumimar manufactured by Sumitomo Chemical Co., Ltd., Melan manufactured by Hitachi Chemical Co., Ltd., etc. It is done. The blending ratio of the polyester polyol and amino resin is preferably 95/5 to 65/35 in terms of the weight ratio of the polyester polyol / amino resin in the solid content.

  The isocyanate compound is a compound having an isocyanate group in the molecule, but when used as a curing agent for a precoated steel sheet as in the present invention, the isocyanate group is phenol, cresol, aromatic secondary amine, tertiary. It is desirable to use an isocyanate compound blocked with a blocking agent such as alcohol, lactam, or oxime. These blocking agents are released from the isocyanate compound by being heated at a predetermined temperature during baking. Such blocked isocyanate compounds include hexamethylene diisocyanate and its derivatives, tolylene diisocyanate and its derivatives, 4,4′-diphenylmethane diisocyanate and its derivatives, xylylene diisocyanate and its derivatives, isophorone diisocyanate and its derivatives, trimethylhexa And methylene diisocyanate and derivatives thereof.

  Examples of commercially available isocyanate compounds include Sumidur, Death Module manufactured by Sumitomo Bayer Urethane Co., Ltd., Coronate manufactured by Nippon Polyurethane Co., Ltd., and the like. The isocyanate compound and the polyester polyol are preferably blended so that the molar ratio (NCO / OH) between the isocyanate group of the isocyanate compound and the hydroxyl group of the polyester polyol is in a ratio of 0.7 to 1.4. When the molar ratio (NCO / OH) is less than 0.7, the coating film is not sufficiently cured, and the expected hardness and strength of the coating film may not be obtained. On the other hand, if the molar ratio (NOC / OH) exceeds 1.4, side reactions between excess isocyanate groups may occur, and the processability of the coating film may be reduced. In addition, as a hardening | curing agent, it is also possible to use together an amino resin and an isocyanate compound in the range of said compounding ratio.

  It is preferable to add a lubricant to the precoat coating film for the purpose of reducing the frictional resistance of the coating film during molding. Examples of the lubricant include hydrocarbon waxes such as paraffin and low molecular weight polyethylene, silicone waxes, amide waxes, and polyester waxes. The addition amount of the lubricant is preferably 0.02 to 2.0 parts by weight, more preferably 0.03 to 0.5 parts by weight with respect to 100 parts by weight of the resin component. When the addition amount of the lubricant is out of the above range, the lubricity is insufficient and the molding processability is inferior.

  In addition, there are curing catalysts (such as tin octoate, dibutyltin dilaurate, lead 2-ethylhexoate), inorganic pigments (calcium carbonate, kaolin, clay, alumina, silica, oxidation) depending on the purpose and application. Titanium, talc, barium sulfate, mica, etc.), organic pigments (Bengara, manganese blue, carbon black, chrome yellow, chrome green, quinacridone red, etc.) and other various additives can be blended as necessary. In preparing the coating composition, the above-mentioned components can be blended using a normal disperser or kneader such as a sand grind mill, a ball mill, or a blender.

  Although the film thickness of a precoat coating film is not specifically limited, It is preferable to set it as the range of 5-30 micrometers. If the film thickness is less than 5 μm, the sharpness after top coating is insufficient. On the other hand, if it exceeds 30 μm, the surface of the coating tends to be cracked during baking and curing, and the enamel is used during cutting and punching. Peeling of the coating called hair is likely to occur. The pre-coated steel sheet of the present invention may be subjected to final coating after part of the forming process. For the purpose of homogenizing the coated surface and imparting interlayer adhesion, the pre-coated steel sheet may be used. A pretreatment process such as polishing may be added to the film surface.

  In addition, the coating method of the precoat coating film is not particularly limited, but it is preferably applied by a method such as roll coater coating or curtain flow coating. After applying the coating, heating means such as hot air drying, infrared heating, induction heating, etc. By baking, the resin is crosslinked to obtain a cured coating film. It is preferable that the baking temperature (final plate temperature) at the time of heat curing is 200 to 250 ° C., and the baking time is about 40 seconds to 3 minutes. In addition, it cannot be overemphasized that the coating | coated structure of the precoat steel plate of this invention can be applied only to the single side | surface of a steel plate, In that case, the other side must be the surface which can receive electrodeposition coating mentioned later, such as a base steel plate surface and a plating surface. I must.

The steel sheet material to be coated (substrate) which is a film-forming target used in the steel sheet present invention, galvanized steel, zinc alloy-plated steel plate, aluminum plate, aluminum alloy plate, copper, brass, iron, stainless steel, etc. A metal material can be mentioned. In these, a galvanized steel plate, a stainless steel plate, a zinc alloy plating steel plate etc. can be used preferably. In addition, as the steel plate, a pre-coated steel plate on which a coating film made of a resin or the like is already formed can be used. Further, the steel sheet can be pretreated on the surface as necessary. Examples of the pretreatment include water washing, hot water washing, pickling, alkali degreasing, grinding, polishing, chromate treatment, and phosphate treatment. The composite oxide film treatment or the like can be performed alone or in combination.

Forming In the present invention, the precoated steel sheet is formed into a predetermined shape with the precoated surface facing outside. The molding method is not particularly limited, and a molding method usually used in the assembly of an automobile or the like is used. Examples of the object to be formed in this molding step include not only automobile bodies but also various coated articles. Of course, not only finished products but also parts are included.

The article formed from the electrodeposition coated precoated steel sheet is then subjected to electrodeposition coating. Since the precoated steel sheet has a coating film, that is, an insulating film, formed on the precoated surface, no electrodeposition film is formed by electrodeposition coating. On the other hand, since there is no insulating coating on the surface other than the precoat surface, an electrodeposition coating is formed by electrodeposition coating. In the electrodeposition coating, an electrodeposition coating film is formed by immersing an object to be coated in an electrodeposition bath containing an electrodeposition coating and applying a voltage. The electrodeposition paint may be either an anionic type or a cationic type, but it is generally preferable to use a cationic type electrodeposition paint having excellent corrosion resistance. Hereinafter, a cationic electrodeposition paint will be described, but there is no problem even if an anion electrodeposition paint is used.

Cationic electrodeposition coating composition In the coating film forming method of the present invention, the commonly used cationic electrodeposition coating compositions include those containing a cationic epoxy resin, a curing agent, and, if necessary, pigments and additives. It is done.

Cationic Epoxy Resin The cationic epoxy resin used in the present invention includes an amine-modified epoxy resin. Cationic epoxy resins typically open all of the epoxy rings of a bisphenol-type epoxy resin with an active hydrogen compound capable of introducing a cationic group, or some epoxy rings with other active hydrogen compounds. It is produced by opening the ring and opening the remaining epoxy ring with an active hydrogen compound capable of introducing a cationic group.

  A typical example of the bisphenol type epoxy resin is a bisphenol A type or bisphenol F type epoxy resin. As the former commercial product, there are Epicoat 828 (manufactured by Yuka Shell Epoxy Co., Epoxy Equivalent 180-190), Epicoat 1001 (Same, Epoxy Equivalent 450-500), Epicoat 1010 (Same, Epoxy Equivalent 3000-4000), etc. Examples of the latter commercially available product include Epicoat 807 (same as above, epoxy equivalent 170).

  The following formula described in JP-A-5-306327

[Wherein R represents a residue excluding the glycidyloxy group of the diglycidyl epoxy compound, R ′ represents a residue excluding the isocyanate group of the diisocyanate compound, and n represents a positive integer. An oxazolidone ring-containing epoxy resin represented by the above formula may be used as the cationic epoxy resin. This is because a coating film having excellent heat resistance and corrosion resistance can be obtained.

  As a method for introducing an oxazolidone ring into an epoxy resin, for example, a block polyisocyanate blocked with a lower alcohol such as methanol and a polyepoxide are heated and kept in the presence of a basic catalyst, and a by-product lower alcohol is introduced from the system. Obtained by distilling off.

  Particularly preferred epoxy resins are oxazolidone ring-containing epoxy resins. This is because a coating film having excellent heat resistance and corrosion resistance and further excellent impact resistance can be obtained.

  It is known that an epoxy resin containing an oxazolidone ring can be obtained by reacting a bifunctional epoxy resin with a diisocyanate blocked with a monoalcohol (ie, bisurethane). Specific examples and production methods of the oxazolidone ring-containing epoxy resin are described in, for example, paragraphs 0012 to 0047 of JP-A No. 2000-128959 and are publicly known.

  These epoxy resins may be modified with suitable resins such as polyester polyols, polyether polyols, and monofunctional alkylphenols. In addition, the epoxy resin can be chain-extended using a reaction between an epoxy group and a diol or dicarboxylic acid.

  These epoxy resins are ring-opened with an active hydrogen compound so that an amine equivalent of 0.3 to 4.0 meq / g is obtained after ring opening, and more preferably 5 to 50% of them are occupied by primary amino groups. It is desirable to do.

  Active hydrogen compounds that can introduce a cationic group include primary amines, secondary amines, tertiary amine acid salts, sulfides and acid mixtures. In order to prepare a primary, secondary or / and tertiary amino group-containing epoxy resin, an acid salt of a primary amine, secondary amine or tertiary amine is used as an active hydrogen compound capable of introducing a cationic group.

  Specific examples include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine hydrochloride, N, N-dimethylethanolamine acetate, diethyl disulfide / acetic acid mixture, etc. In addition, there are secondary amines in which primary amines such as aminoethylethanolamine ketimine and diethylenetriamine diketimine are blocked. A plurality of amines may be used in combination.

Curing Agent The curing agent used in the present invention is preferably a blocked polyisocyanate obtained by blocking polyisocyanate with a blocking agent, where polyisocyanate refers to a compound having two or more isocyanate groups in one molecule. . The polyisocyanate may be, for example, any of aliphatic, alicyclic, aromatic and aromatic-aliphatic.

  Specific examples of polyisocyanates include aromatic diisocyanates such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), p-phenylene diisocyanate, and naphthalene diisocyanate; hexamethylene diisocyanate (HDI), 2,2,4- C3-C12 aliphatic diisocyanates such as trimethylhexane diisocyanate and lysine diisocyanate; 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI), 4,4′-dicyclohexylmethane diisocyanate (hydrogenated MDI) , Methylcyclohexane diisocyanate, isopropylidene dicyclohexyl-4,4'-diisocyanate, and 1,3-diisocyanatomethylcyclo Carbon such as xane (hydrogenated XDI), hydrogenated TDI, 2,5- or 2,6-bis (isocyanatomethyl) -bicyclo [2.2.1] heptane (also referred to as norbornane diisocyanate). Aliphatic diisocyanates having a number of 5 to 18; aliphatic diisocyanates having an aromatic ring such as xylylene diisocyanate (XDI) and tetramethylxylylene diisocyanate (TMXDI); modified products of these diisocyanates (urethanes, carbodiimides, Uretdione, uretoimine, burette and / or isocyanurate modified product); and the like. These may be used alone or in combination of two or more.

  Adducts or prepolymers obtained by reacting polyisocyanates with polyhydric alcohols such as ethylene glycol, propylene glycol, trimethylolpropane and hexanetriol at an NCO / OH ratio of 2 or more may also be used as curing agents.

  The polyisocyanate is preferably an aliphatic polyisocyanate or an alicyclic polyisocyanate. This is because the formed coating film is excellent in weather resistance.

  Preferred specific examples of the aliphatic polyisocyanate or alicyclic polyisocyanate include hexamethylene diisocyanate, hydrogenated TDI, hydrogenated MDI, hydrogenated XDI, IPDI, norbornane diisocyanate, dimer (biuret) and trimer thereof. (Isocyanurate) etc. are mentioned.

  The blocking agent is added to a polyisocyanate group and is stable at ordinary temperature, but can regenerate a free isocyanate group when heated to a temperature higher than the dissociation temperature.

  As a blocking agent, when low temperature curing (160 ° C. or lower) is desired, lactam blocking agents such as ε-caprolactam, δ-valerolactam, γ-butyrolactam and β-propiolactam, and formaldoxime, acetoald It is preferable to use an oxime blocking agent such as oxime, acetoxime, methyl ethyl ketoxime, diacetyl monooxime, and cyclohexane oxime.

  The binder containing the cationic epoxy resin and the curing agent is generally contained in the electrodeposition coating composition in an amount that occupies 25 to 85% by mass, preferably 40 to 70% by mass of the total solid content of the electrodeposition coating composition. The

Crosslinked Resin Particles Crosslinked resin particles can be blended in the cationic electrodeposition coating composition of the present invention as necessary to increase the viscosity of the coating film. The crosslinked resin particles are preferably obtained by emulsion polymerization of an α, β-ethylenically unsaturated monomer mixture using an acrylic resin having an ammonium group as an emulsifier. This α, β-ethylenically unsaturated monomer mixture contains poly (meth) acrylate having two or more α, β-ethylenically unsaturated bonds in the molecule for crosslinking the resin particles. The content of such poly (meth) acrylate is 5 to 20% by weight in the α, β-ethylenically unsaturated monomer mixture. If this amount is less than 5% by weight, the crosslinking of the resin particles does not proceed sufficiently, and if it exceeds 20% by weight, the crosslinking of the resin particles proceeds too much, which may cause a problem in the physical properties of the coating film obtained by electrodeposition. There is.

  The poly (meth) acrylate is a compound having a structure in which a plurality of (meth) acrylic acids are ester-bonded to a divalent or higher alcohol, such as ethylene glycol di (meth) acrylate, di (meth) acrylic. Examples include triethylene glycol acid, neopentyl glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, and trimethylolpropane tri (meth) acrylate. Two or more of these may be mixed and used.

  The α, β-ethylenically unsaturated monomer mixture may contain a general α, β-ethylenically unsaturated monomer in addition to the poly (meth) acrylate. Such common unsaturated monomers include hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, allyl alcohol, methacrylic alcohol, hydroxyethyl (meth) acrylate and ε -What has a hydroxyl group, such as an adduct with caprolactone, and what has epoxy groups, such as a glycidyl (meth) acrylate, can be mentioned. The unsaturated monomer having such a reactive functional group is preferably contained in the mixture in an amount of 20% by weight or less, and both the hydroxyl value or the epoxy value are preferably 20 or less. Other examples of α, β-ethylenically unsaturated monomers that may be included include (meth) acrylic acid esters such as methyl (meth) acrylate, polymerizable amide compounds such as (meth) acrylamide, and styrene. And polymerizable aromatic compounds such as acrylonitrile, α-olefins such as ethylene and propylene, vinyl esters such as vinyl acetate, and dienes such as butadiene.

  When the crosslinked resin particles obtained in the present invention are used in a cationic electrodeposition coating, if the above-described block polyisocyanate is further included in the acrylic resin as an emulsifier, the coating appearance and rust prevention properties of the resulting coating film are further improved. Can be made.

  The acrylic resin having an ammonium group is easily obtained by the same method as that of the cation-modified epoxy resin already described, that is, by adding a tertiary amine compound and an organic acid to an acrylic resin having an epoxy group and quaternizing. be able to. In addition, it is preferable that the SP value of the monomer used for obtaining the acrylic resin which has this epoxy group is 9-12. When the SP value of the monomer exceeds 12, the coating appearance and rust prevention properties of the coating film obtained when used for the cationic electrodeposition coating are lowered, and when it is less than 9, adhesion failure with the top coat occurs. The SP value can be determined by a method well known to those skilled in the art such as a turbidity method.

  The acrylic resin having an epoxy group can be obtained by a conventional method such as solution polymerization of the above monomer using a generally well-known initiator. The number average molecular weight is preferably 5,000 to 20,000. When the number average molecular weight exceeds 20000, the problem of an increase in the viscosity of the emulsifier occurs. When the number average molecular weight is less than 5000, the problem of poor edge rust prevention property may occur. Examples of the emulsifier include Antox MS-60 (trade name, manufactured by Nippon Emulsifier Co., Ltd.), Eleminol JS-2 (trade name, manufactured by Sanyo Kasei Kogyo Co., Ltd.), and Adekaria Soap NE-20 (trade name, Asahi). Commercially available emulsifiers such as Akaron HS-10 (trade name, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) can be used in combination.

  In order to obtain crosslinked resin particles using the above-mentioned emulsifier, a generally well-known emulsion polymerization method is used. An example of a preferred polymerization method is to dissolve an emulsifier in an aqueous medium, drop a polymerization initiator under heating and stirring, and then drop some of the α, β-ethylenically unsaturated monomer first, There is a method of dropping the remaining α, β-ethylenically unsaturated monomer mixture emulsified in advance using an emulsifier and water. According to this polymerization method, variation from the target particle diameter is reduced, and preferable crosslinked resin particles can be obtained.

  Examples of the polymerization initiator include azo oily compounds such as azobisisobutyronitrile, water soluble compounds such as 4,4′-azobis (4-cyanovaleric acid), and redox oily peroxides such as benzoyl peroxide. Oxides and water-soluble peroxides such as potassium persulfate and ammonium persulfate. The amount of initiator relative to the total amount of α, β-ethylenically unsaturated monomer mixture is generally 0.1 to 5% by weight, preferably 0.2 to 2% by weight.

  The weight ratio of the α, β-ethylenically unsaturated monomer mixture and the emulsifier is preferably 95: 5 to 70:30. If it exceeds 95: 5, the rust prevention property cannot be maintained, and if it is less than 70:30, there is a risk that it will aggregate and generate flaws. In order to adjust the molecular weight of the crosslinked resin particles, mercaptans such as lauryl mercaptan and chain transfer agents such as α-methylstyrene dimer may be used. The reaction temperature at the time of polymerization is preferably 60 to 90 ° C., for example, when an azo initiator is used, and 30 to 70 ° C. for a redox initiator. The reaction time is 1 to 8 hours.

  The average particle size of the crosslinked resin particles obtained in this manner is preferably in the range of 0.05 to 0.30 μm. When the particle diameter is less than 0.05 μm, the effect of improving workability is small, and when it exceeds 0.30 μm, the appearance of the obtained coating film may be deteriorated. The particle size can be adjusted, for example, by adjusting the monomer composition and emulsion polymerization conditions.

Pigment The electrodeposition coating composition used in the present invention may contain a commonly used pigment. Examples of pigments that can be used include commonly used inorganic pigments, for example colored pigments such as titanium white, carbon black and bengara; extender pigments such as kaolin, talc, aluminum silicate, calcium carbonate, mica and clay; Zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate and aluminum phosphomolybdate, phosphomolybdic acid Examples include rust preventive pigments such as aluminum zinc.

  The pigment is generally contained in the electrodeposition coating composition in an amount of 1 to 35% by mass, preferably 10 to 30% by mass, based on the total solid content of the electrodeposition coating composition.

  Since the electrodeposition coating composition of the present invention is lead-free, corrosion resistance imparting agents containing lead, for example, lead-based anticorrosive pigments such as basic lead silicate, basic lead sulfate, lead red, and cyanamide lead are used. It should not be used or should be used in such an amount that the lead ion concentration of the diluted paint (added to the electrodeposition bath) is 50 ppm or less. This is because a high lead ion concentration is harmful to the environment.

Pigment-dispersed paste When a pigment is used as a component of an electrodeposition paint, generally the pigment is dispersed in an aqueous medium at a high concentration in advance together with a resin called a pigment-dispersed resin to form a paste. This is because the pigment is in a powder form, and it is difficult to disperse in a single step in a low concentration uniform state used in the electrodeposition coating composition. Such a paste is generally called a pigment dispersion paste.

  The pigment dispersion paste is prepared by dispersing a pigment in an aqueous medium together with a pigment dispersion resin varnish. As the pigment-dispersed resin varnish, 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 is generally used. As the aqueous medium, ion-exchanged water or water containing a small amount of alcohol is used. Generally, the pigment-dispersed resin varnish is used at a solid content ratio of 5 to 40 parts by mass, and the pigment is used at a solid content ratio of 10 to 30 parts by mass.

  After the pigment dispersion resin varnish and the pigment are mixed in an amount of 10 to 1000 parts by mass with respect to 100 parts by mass of the resin solid content, a ball mill or a sand grind is used until the pigment in the mixture has a predetermined uniform particle size. A pigment dispersion paste is obtained by dispersing using a normal dispersing device such as a mill.

Preparation of an electrodeposition coating composition An electrodeposition coating composition is prepared by dispersing a cationic epoxy resin, a curing agent, and a pigment dispersion paste in an aqueous medium. Further, the aqueous medium usually contains a neutralizing agent in order to improve the dispersibility of the cationic epoxy resin. Neutralizing agents are inorganic or organic acids such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, lactic acid. The amount is that which achieves a neutralization rate of at least 20%, preferably 30-60%.

  The amount of curing agent is sufficient to react with active hydrogen-containing functional groups such as primary, secondary or / and tertiary amino groups and hydroxyl groups in the cationic epoxy resin during curing to give a good cured coating film. In general, it is generally in the range of 90/10 to 50/50, preferably 80/20 to 65/35, expressed as a solid mass ratio of the cationic epoxy resin to the curing agent.

  The electrodeposition paint can contain a tin compound such as dibutyltin dilaurate and dibutyltin oxide, and a usual urethane cleavage catalyst. Since what does not contain lead substantially is preferable, it is preferable that the quantity shall be 0.1-5 mass% of a block polyisocyanate compound.

  The electrodeposition paint can contain conventional paint additives such as water-miscible organic solvents, surfactants, antioxidants, UV absorbers, and pigments.

  In the cationic electrodeposition coating composition of the present invention, the minimum coating viscosity at curing is 10,000 Pa · S or more, preferably 50,000 or more, more preferably 50,000 to 500,000 Pa · S. Even when the minimum coating viscosity at the time of curing is less than 10,000 Pa · S, the effect is sufficiently exhibited, but the rust prevention property of the edge portion may not necessarily be sufficient. Therefore, in the present invention, it is preferable to control the minimum coating film viscosity at the time of curing to 10,000 Pa · S or more in order to improve the rust prevention property of the edge portion and sufficiently exhibit the effect of eliminating the need for sealer coating. . As described above, the minimum coating film viscosity at the time of curing can be controlled by blending the crosslinked resin particles. In addition, the minimum coating viscosity at the time of curing can be determined by other methods such as increasing the molecular weight of the epoxy paint, increasing the molecular weight of the curing agent, increasing the effect speed by changing the catalyst, increasing the pigment content, or using the UV curing agent. But you can control it. On the other hand, if it exceeds 500,000 Pa · S, the flowability of the coating film is insufficient.

  As described above, the object to be coated when the electrodeposition coating is performed using the electrodeposition coating composition is an object formed from a pre-coated steel plate, and, if necessary, phosphoric acid by a dipping, spraying method or the like in advance. Although surface treatment such as zinc treatment may be performed, the surface treatment may not be performed.

The conditions under which electrodeposition is performed are generally the same as those used for other types of electrodeposition coating. The applied voltage may vary greatly and may range from 1 to several hundred volts, preferably 50 to 250 volts. The current density is typically about 10 amps / m ^{2 to} 160 amps / m ² and tends to decrease during electrodeposition.

  After electrodeposition by the electrodeposition coating method of the present invention, the electrodeposition coating film is baked at an elevated temperature in a usual manner, for example, in a baking furnace, a baking oven or an infrared heat lamp. The baking temperature may vary but is usually about 140 ° C to 180 ° C. The coated product coated by the electrodeposition coating method of the present invention is dried and baked after the final water washing to form a cured electrodeposition coating film, thereby completing the coating process.

  The following examples further illustrate the present invention, but the present invention is not limited thereto. In the examples, “parts” and “%” are based on weight unless otherwise specified.

Production Example 1
Cationic resin having a hydroxyl group (A)
In a flask equipped with a stirrer, cooler, nitrogen inlet tube, thermometer and dropping funnel, 92 parts of 2,4- / 2,6-tolylene diisocyanate (mass ratio = 8/2), methyl isobutyl ketone (MIBK) 95 parts and 0.5 parts of dibutyltin dilaurate were added, and 21 parts of methanol was further added dropwise while stirring them. The reaction started from room temperature, but was heated to 60 ° C. due to exotherm. Then, after continuing reaction for 30 minutes, 57 parts of ethylene glycol mono-2-ethylhexyl ether was dripped from the dropping funnel, and also 42 parts of bisphenol A-propion oxide 5 mol adducts were added. The reaction was mainly carried out in the range of 60 to 65 ° C., and continued until the absorption based on the isocyanate group disappeared while measuring the infrared spectrum.

  To the block polyisocyanate thus obtained, 365 parts of an epoxy resin having an epoxy equivalent of 188 synthesized from bisphenol A and epichlorohydrin was added, and the temperature was raised to 125 ° C. Thereafter, 1.0 part of benzyldimethylamine was added and reacted at 130 ° C. until an epoxy equivalent of 410 was reached. Subsequently, 87 parts of bisphenol A was added to the flask and reacted at 120 ° C., resulting in an epoxy equivalent of 1190. Thereafter, the mixture was cooled, 11 parts of diethanolamine, 24 parts of N-methylethanolamine and 25 parts of a ketimine compound of aminoethylethanolamine (79% by mass MIBK solution) were added and reacted at 110 ° C. for 2 hours. Then, it diluted with MIBK until it became non-volatile content 80%, and the cationic resin (A) which has a hydroxyl group was obtained.

Production Example 2
Curing agent (B)
To a flask equipped with a stirrer, a cooler, a nitrogen inlet tube, a thermometer, and a dropping funnel, 723 parts of isophorone diisocyanate, 333 parts of methyl isobutyl ketone and 0.01 part of dibutyltin dilaurate were added, and the temperature was raised to 70 ° C.

  And after the content melt | dissolved uniformly, 610 parts of methyl ethyl ketone oximes were dripped over 2 hours. After completion of the dropping, the reaction was continued until the absorption based on the isocyanate group disappeared while measuring the infrared spectrum while maintaining the reaction temperature of 70 ° C. to obtain a curing agent (B).

Production Example 3
Main emulsion (C)
67 parts (converted to solid content) of the cationic resin (A) having a hydroxyl group obtained in Production Example 1 and 33 parts (converted to solid content) of the curing agent (B) prepared in Production Example 2 were mixed uniformly, and then ethylene 2% of glycol mono-2-ethylhexyl ether and 2% of dibutyltin dilaurate were added to the solid content. Acetic acid was added thereto so that the neutralization rate was 50.0% (neutralization rate with respect to the cationic group of the resin), and ion-exchanged water was added to dilute until the solid content became 37.0% by mass.

  Thereafter, the mixture of MIBK and water was removed under reduced pressure until the solid content was 41.9% by mass to prepare a main emulsion (C).

Production Example 4
Main emulsion (D)
67 parts of the cationic resin (A) having a hydroxyl group obtained in Production Example 1 (solid content conversion), 33 parts of the curing agent (B) prepared in Production Example 2 (solid content conversion), pentaerythritol triacrylate (A- TMM-3L (manufactured by Shin-Nakamura Chemical Co., Ltd.) 10 parts and Glocure 1173 (manufactured by Ciba-Geigy) were mixed uniformly, and then 2% ethylene glycol mono-2-ethylhexyl ether and dibutyltin dilaurate were solid. 2% was added to the minute. Acetic acid was added thereto so that the neutralization rate was 50.0% (neutralization rate with respect to the cationic group of the resin), and ion-exchanged water was added to dilute until the solid content became 37.0% by mass.

  Thereafter, the mixture of MIBK and water was removed under reduced pressure until the solid content was 41.9% by mass, thereby preparing a main emulsion (D).

Production Example 5
Production of Crosslinked Resin Particles (E) Production of acrylic resin having ammonium groups 120 parts of butyl cellosolve was placed in a reaction vessel and heated to 120 ° C. with stirring. A solution prepared by mixing 2 parts of t-butylperoxy-2-ethylhexanoate and 10 parts of butyl cellosolve, 15 parts of glycidyl methacrylate, 50 parts of 2-ethylhexyl methacrylate, 20 parts of 2-hydroxyethyl methacrylate and n- A monomer mixture consisting of 15 parts of butyl methacrylate and having an SP value of 10.1 was added dropwise over 3 hours. After aging for 30 minutes, a solution in which 0.5 part of t-butylperoxy-2-ethylhexanoate and 5 parts of butyl cellosolve were added dropwise in 30 minutes, and after aging for 2 hours, the mixture was cooled. . The number average molecular weight of the acrylic resin having an epoxy group determined by GPC in terms of polystyrene was 12,000, and the weight average molecular weight was 28,000. To this, 7 parts of N, N-dimethylaminoethanol and 15 parts of 50% lactic acid aqueous solution were added, followed by quaternization by heating and stirring at 80 ° C. When the acid value became 1 or less and the viscosity increase stopped, heating was stopped to obtain an acrylic resin having an ammonium group. The number of ammonium groups per molecule of the acrylic resin having ammonium groups was 6.0.

  To the reaction vessel, 20 parts of the acrylic resin having an ammonium group obtained above and 270 parts of deionized water were added, and the mixture was heated and stirred at 75 ° C. Thereto was added dropwise 1.5 parts of 2,2'-azobis (2- (2-imidazolin-2-yl) propane) 100% neutralized aqueous acetic acid solution over 5 minutes. After aging for 5 minutes, 30 parts of methyl methacrylate was added dropwise over 5 minutes. After further aging for 5 minutes, 170 parts of methyl methacrylate, 40 parts of styrene, 30 parts of n-butyl methacrylate, 5 parts of glycidyl methacrylate and neoplastic were added to a solution obtained by mixing 70 parts of acrylic resin having an ammonium group and 250 parts of deionized water. A pre-emulsion obtained by adding and stirring an α, β-ethylenically unsaturated monomer mixture consisting of 30 parts of pentyl glycol dimethacrylate was added dropwise over 40 minutes. After aging for 60 minutes, the mixture was cooled to obtain a dispersion of crosslinked resin particles. The resulting dispersion of the crosslinked resin particles had a non-volatile content of 36%, a pH of 5.0, and an average particle size of 100 nm.

Production Example 6
Pigment dispersion paste (F)
A bisphenol-type epoxy resin having an epoxy equivalent of 450 is reacted with a half-blocked isophorone diisocyanate of 2-ethylhexanol, and tertiary sulfonium is formed with 1- (2-hydroxyethylthio) -2-propanol and dimethylolpropionic acid to form tertiary sulfonium. A resin varnish for pigment dispersion having a conversion rate of 70.6% by mass and a resin solid content of 60% by mass was prepared.

  Disperse 50.0 parts of the pigment-dispersing resin varnish, 100.0 parts of ion-exchanged water and 100.0 parts of the granular mixture shown in Table 1 below with a sand grind mill, and further pulverize until the particle size becomes 5 μm or less. A pigment dispersion paste (D) (solid content 52.0% by mass) was obtained.

Example 1
100 parts of the main emulsion (D) was mixed with 85 parts of ion-exchanged water to obtain a cationic electrodeposition paint having a solid content of 20%. Using this electrodeposition paint, the minimum coating film viscosity was obtained by the following method and shown in Table 1.

Next, a model of the door hem (the part where the inner plate of the door part is folded and surrounded) is created by stacking two iron plates (manufactured by Nihon Coatings Co., Ltd.) pre-coated on one side as shown in FIG. After degreasing, surface adjustment, and chemical conversion treatment, electrodeposition coating was performed using an electrodeposition paint at an applied voltage of 200 V and 30 ° C. for 3 minutes. The obtained coated product was irradiated with UV (ultraviolet light) at an intensity of 5.0 J / cm ² to be semi-cured, and then baked and cured at 150 ° C. for 20 minutes. The corrosion resistance of the obtained coated product was evaluated by the following method, and the results are shown in Table 1.

Method for Measuring Minimum Film Viscosity An electrodeposition coating film was formed on the article to be coated to a film thickness of 15 μm, and this was washed with water to remove excess electrodeposition coating material. Next, after removing excess moisture on the surface of the coating film, the coating film was taken out without drying to prepare a sample. The sample thus obtained was measured under a time-dependent measurement condition using Rheosol-G3000 (manufactured by UPM Co., Ltd.) which is a rotary dynamic viscoelasticity measuring apparatus. The setting conditions for the time-dependent measurement were set at a strain of 2.0 deg and a frequency of 2.00 Hz, and the temperature raising program entered the time-temperature conditions in the actual electrodeposition baking process.

  Generally, as the temperature rises, the viscosity decreases and gradually increases from around 120 ° C. where the curing reaction starts. Accordingly, the viscosity of the coating film becomes the lowest in the range of 100 to 120 ° C. This lowest viscosity was defined as the lowest coating film viscosity.

The corrosion-resistant salt spray was dried at 35 ° C. for 6 hours, dried at 60 ° C. for 1 hour, and then subjected to wet conditions at 50 ° C. for 1 hour. This cycle is defined as one cycle, and the ratio of the rust generation region in the electrodeposition coating region at the time of 180 cycles and 360 cycles is evaluated under the following evaluation conditions.
XX: Rust generation is 50% or more in 180 cycles.
X: Rust generation is 50% or less in 180 cycles.
Δ: Rust generation is 50% or more in 360 cycles.
○ Δ: Rust generation is 10 to 50% in 360 cycles.
◯: Rust generation is 10% or less in 360 cycles.

Example 2
100 parts of the main emulsion (C) and 25 parts of the pigment dispersion paste (F) were mixed with 125 parts of ion-exchanged water to obtain a cationic electrodeposition paint having a solid content of 20%. Cross-linked resin particles (E) were added to this cationic electrodeposition coating so that the minimum coating film viscosity was 10,000 Pa · S. Using this electrodeposition paint, electrodeposition was applied in the same manner as in Example 1, and baked and cured at 150 ° C. for 20 minutes without UV irradiation. The corrosion resistance of the coating film was evaluated, and the results are shown in Table 1.

Example 3
100 parts of the main emulsion (C) and 25 parts of the pigment dispersion paste (F) were mixed with 125 parts of ion-exchanged water to obtain a cationic electrodeposition paint having a solid content of 20%. Cross-linked resin particles (E) were added to this cationic electrodeposition coating so that the minimum coating viscosity was 130,000 Pa · S. Using this electrodeposition paint, electrodeposition was applied in the same manner as in Example 1, and baked and cured at 150 ° C. for 20 minutes without UV irradiation. The corrosion resistance of the coating film was evaluated, and the results are shown in Table 1.

Comparative Example 1
100 parts of the main emulsion (C) and 25 parts of the pigment dispersion paste (F) were mixed with 125 parts of ion-exchanged water to obtain a cationic electrodeposition paint having a solid content of 20%. The minimum coating film viscosity of this cationic electrodeposition paint was measured, and the results are shown in Table 1. Using this electrodeposition paint, electrodeposition was applied in the same manner as in Example 1, the corrosion resistance of the coating film was evaluated, and baked and cured at 150 ° C. for 20 minutes without UV irradiation. The results are shown in Table 1.

Comparative Example 2
100 parts of the main emulsion (C) and 25 parts of the pigment dispersion paste (F) were mixed with 125 parts of ion-exchanged water to obtain a cationic electrodeposition paint having a solid content of 20%. Cross-linked resin particles (E) were added to this cationic electrodeposition coating so that the minimum coating film viscosity was 2,000 Pa · S. Using this electrodeposition paint, electrodeposition was applied in the same manner as in Example 1, and baked and cured at 150 ° C. for 20 minutes without UV irradiation. The corrosion resistance of the coating film was evaluated, and the results are shown in Table 1.

Comparative Example 3
100 parts of the main emulsion (D) was mixed with 85 parts of ion-exchanged water to obtain a cationic electrodeposition paint having a solid content of 20%. The minimum coating film viscosity of this cationic electrodeposition paint was measured, and the results are shown in Table 1. Using this electrodeposition paint, electrodeposition was applied in the same manner as in Example 1, and baked and cured at 150 ° C. for 20 minutes without UV irradiation. The corrosion resistance of the coating film was evaluated, and the results are shown in Table 1.

FIG. 1 is a schematic cross-sectional view of a mating portion of steel plates. FIG. 2 is an enlarged schematic view of the cross section of the edge tip of the steel plate. FIG. 3 is a diagram showing how to stack two precoated iron plates used in the examples and comparative examples.

Claims (8)

  A pre-coated steel sheet that has been pre-coated on one side is used to form an object to be coated with the pre-coated surface outside, and the inner plate part that is not pre-coated is applied by electrodeposition coating. Method.   The method for forming a coating film according to claim 1, wherein the electrodeposition coating used for electrodeposition coating of the inner plate has a minimum coating film viscosity of 10,000 Pa · S or more upon curing.   The coating film forming method according to claim 2, wherein the inner plate portion not pre-coated does not form a coating film other than the electrodeposition coating film.   The coating film forming method according to claim 3, wherein the precoated surface of the precoated steel sheet is further subjected to finish coating.   A pre-coated surface of a pre-coated steel sheet, wherein the coated article is made of a pre-coated steel sheet that is pre-painted on one side in a coated article having a cosmetic exterior and exposed outer plate portion and the other inner plate portion. A coated article characterized in that is an outer plate portion and the other inner plate portion is formed by electrodeposition coating by electrodeposition coating.   The coated article according to claim 5, wherein the electrodeposition paint used for electrodeposition coating of the inner plate has a minimum coating film viscosity of 10,000 Pa · S or more upon curing.   The coated article according to claim 6, wherein the inner plate portion that is not pre-coated does not form a coating film other than the electrodeposition coating film.   The coated article according to claim 7, wherein the precoated surface of the precoated steel sheet is further subjected to finish coating.

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