JP2008214705A - Cation electrodeposition coating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To uniform the thickness of an electrodeposition film as much as possible in a cation electrodeposition coating process into which various shaped and sized materials (particularly parts) to be coated are transported. <P>SOLUTION: In the cation electrodeposition coating method by which the electrodeposition coating is carried out in a condition where the materials to be coated each having different shape and size are mixed in a cation electrodeposition coating material bath, the electrodeposition film formed to have 15 μm thickness in the cation electrodeposition coating material bath is adjusted to have 700-1,800 KΩ cm<SP>2</SP>film resistance. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a cationic electrodeposition coating method, and more particularly, to a cationic electrodeposition coating method in which a difference in film thickness is small between parts even when the cationic electrodeposition coating is performed while components having different sizes and shapes are mixed.

  Electrodeposition coating can be applied to the details even if the object has a complicated shape, and it can be applied automatically and continuously. It is widely used not only for the objects to be coated but also for the rust-proofing undercoating of small parts.

  When painting small parts by electrodeposition coating, as schematically shown in FIG. 1, a part (object to be coated) 3 to be painted is suspended by a hanging means called a hanger 4, and the conveying means 5. The electrode 3 is immersed in the electrodeposition paint 2 of the electrodeposition bath 1, and a voltage is applied to the component 3 and the counter electrode in the meantime to perform electrodeposition coating.

  In the case of electrodeposition coating of small parts rather than large objects, generally, various shapes or sizes of parts are successively suspended from the hanger 4 and applied. In the electrodeposition coating, ideally, the coated film thickness should be uniform no matter what shape and size of the component is immersed in the electrodeposition bath as an object to be coated. In practice, however, the coating film thickness varies depending on various conditions such as the difference in the hanging position of the object to be coated, the difference in energization time, the difference in the immersion depth, or the difference in the coating area of the object to be coated. . The difference in film thickness occurs between the objects to be coated or within one object to be coated.

  Currently, taking into account the difference in film thickness of the electrodeposition coating film as described above, the electrodeposition time and the amount of energization are such that the film thickness of the thinnest part becomes the predetermined film thickness. It is controlling. However, although the portion with good conditions has an excessive film thickness, the coating conditions are determined in accordance with the portion where the thin film is formed as it is.

  In electrodeposition coating, many studies have been made on making the film thickness uniform. For example, in Japanese Patent Laid-Open No. 9-249994 (Patent Document 1), even when a dissimilar metal joint is present on the object to be coated, the object to be coated enters the electrodeposition bath in order to reduce the film thickness difference. A method is described in which current concentration during the reduction is reduced by applying a pulse voltage. This method inevitably complicates the apparatus, and cannot cope with a case where a large number of parts having different sizes and shapes are coated with one electrodeposition bath.

  Japanese Patent Application Laid-Open No. 2004-83824 (Patent Document 2) discloses a cation having a specific composition in order to obtain a uniform coating property without a difference between an outer plate thickness and an inner plate thickness of an article having a bag structure. The use of an electrodeposition paint is disclosed. Since this method is intended only for the difference in film thickness between the inner plate portion and the outer plate portion, it cannot cope with the film thickness of component coating in which components of various shapes or sizes are conveyed.

Japanese Patent Application Laid-Open No. 7-18495 (Patent Document 3) describes each of the objects to be coated even when there is a site where the paint hardly deposits and a site where the paint easily deposits depending on the distance from the electrode immersed in the electrodeposition bath. In order to make the film thickness of the coating film uniform in a part, it has been proposed to set the coating temperature higher in the vicinity of the portion where precipitation is difficult and set the coating temperature lower in the portion where precipitation tends to occur. The partial control of the liquid temperature is technically complicated and at the same time depends only on the distance from the electrode, and the film thickness cannot be controlled in accordance with the form of the object to be coated.
Japanese Patent Laid-Open No. 9-249994 JP 2004-83824 A Japanese Unexamined Patent Publication No. 7-18495

  An object of the present invention is to make the film thickness of an electrodeposition coating film uniform in cationic electrodeposition coating in which articles (particularly parts) having various shapes and sizes are conveyed.

In order to achieve the above object, the present inventors have studied a cationic electrodeposition coating material and have come to make the present invention. That is, the present invention relates to a cationic electrodeposition coating method in which electrodeposition coating is performed in a state where mixed parts having different sizes and / or shapes are mixed in a cationic electrodeposition coating bath.
Cationic electrodeposition characterized in that the electrodeposition coating film formed when electrodeposition is applied to a thickness of 15 μm in the cationic electrodeposition paint bath has a membrane resistance of 700 to 1,800 KΩ · cm ^2. Provide a painting method.

  The film thickness difference of the part to be coated is preferably within 5 μm.

  When the cationic electrodeposition coating method of the present invention is used, a wide variety of objects to be coated (particularly parts) are mixed and conveyed to the cationic electrodeposition paint bath, and formed on the object to be coated. The film thickness of the electrodeposition coating film is uniform, and there is not much difference in film thickness between the painted objects or within one painted object.

  In conventional cationic electrodeposition coating, the film thickness of the electrodeposition coating was not uniform, and the electrodeposition coating conditions were selected so that a sufficient electrodeposition coating was formed on the thin part. In this part, the film thickness was inevitably thick. In the present invention, since the film thickness of the cationic electrodeposition coating film can be made uniform, the portion where the film thickness becomes thick is reduced, and the amount of the paint to be adhered is reduced, so that resource saving can be achieved.

  The cationic electrodeposition paint used in the present invention is known as an electrodeposition paint having a high throwing power, but the term “throwing power” is used only for painting a car body. Which is not used in the coating of objects, particularly parts, intended by the present invention. In the present invention, there is an advantage that is not bound by common sense in that paints and concepts that have not been used for electrodeposition coating of parts and the like have been applied to parts and the like.

Definition of terms

  In this specification, “applying electrodeposition coating in a mixed state of parts to be coated of different sizes and / or shapes” means electrodeposition coating between different kinds of articles. Including not only when different parts are suspended on a hanger and introduced into the electrodeposition bath, but also when parts of the same shape and size are suspended on the hanger and introduced into the electrodeposition bath . Here, “size” means a range of an area where the article physically occupies the space, and “shape” means a shape formed when the article exists in the space. Since the size is a component, it is not a large size but can be hung with a hanger or the like. The shape may be any shape, and may be a coil shape, a rod shape, a flat plate shape, or a complicated shape other than those. Specific examples of the parts to be painted include automobile wheels, motorcycle handles, automobile doors, cross members, lower arms, lateral rings, and engine mounts. In the present invention, the difference in film thickness formed by electrodeposition is reduced between these different kinds of articles or within the same article. When several articles are hung on one hanger, the difference in film thickness formed between articles in one hanger is reduced, or the film between articles hung on different hangers Includes both cases where the difference in thickness decreases.

  The present invention will be described in detail with reference to FIG. FIG. 1 is a schematic view of an electrodeposition bath part of an electrodeposition coating line for parts. The electrodeposition bath 1 is filled with the electrodeposition paint 2, and the parts 3 that are the coatings are successively carried into the electrodeposition bath 1. The component 3 is suspended by a suspension means called a hanger 4, and the hanger 4 is moved at a predetermined speed in the direction of the arrow 6 by the conveying means 5. When each component 3 is present in the electrodeposition coating 2, a predetermined voltage is applied so that the component 3 becomes a cathode, and the cationic electrodeposition coating, that is, the coating component in the cationic electrodeposition coating is applied to the object to be coated. As a result, an electrodeposition coating film is formed.

In the present invention, the cationic electrodeposition coating used for the cationic electrodeposition coating has a film resistance of 700 to 1,800 KΩ · cm ² of a coating film electrodeposited to a thickness of 15 μm with respect to an object to be coated. Is. Hereinafter, the cationic electrodeposition paint will be described.

  The cationic electrodeposition coating composition used in the present invention usually contains an aqueous medium, an additive such as a binder resin, a neutralizing acid, an organic solvent, a pigment, and a metal catalyst dispersed or dissolved in the aqueous medium. . The binder resin contains an amine-modified epoxy resin, a sulfonium-modified epoxy resin, and a block polyisocyanate curing agent. As the aqueous medium, ion exchange water or the like is generally used.

In the cationic electrodeposition coating composition used in the amine-modified epoxy resin present invention include bisphenol type epoxy resin modified with an amine. The amine-modified epoxy resin may be a conventionally known one as described in, for example, JP-A Nos. 54-4978 and 56-34186. The amine-modified epoxy resin is typically produced by opening the epoxy ring of a bisphenol type epoxy resin with an amine.

  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).

  An oxazolidone ring-containing epoxy resin as described in the formula in the paragraph 0004 of JP-A No. 5-306327 and Chemical Formula 3 may be used as the amine-modified 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.

  When the epoxy resin is an oxazolidone ring-containing epoxy resin, 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 this oxazolidone ring-containing epoxy resin are described, for example, in paragraphs 0012 to 0047 of JP-A No. 2000-128959.

  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.

  Examples of the active hydrogen compound into which a cationic group can be introduced include primary amine, secondary amine, and tertiary amine acid salts. Of these amines, secondary amines are particularly preferred. When an epoxy resin and a secondary amine are reacted, an amine-modified epoxy resin having a tertiary amino group is obtained.

  Specific examples of amines include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine hydrochloride, N, N-dimethylethanolamine acetate, aminoethylethanolamine There are secondary amines that are blocked from primary amines such as diketimine of diethylenetriamine. A plurality of amines may be used in combination.

Sulfonium-modified epoxy resin The cationic electrodeposition coating composition used in the present invention may contain a sulfonium-modified epoxy resin. The sulfonium-modified epoxy resin is a resin in which a sulfonium base is introduced at the same time as the epoxy group is opened by reacting an epoxy resin with a sulfide compound and a neutralizing acid. The sulfonium-modified epoxy resin may be a conventionally known one as described in, for example, JP-A-6-128351 and JP-A-7-206968. The sulfonium-modified epoxy resin is typically produced by opening the epoxy ring of a bisphenol type epoxy resin with a sulfide compound and a neutralizing acid.

  The sulfide compound to be reacted with the epoxy resin includes all sulfide compounds that react with the epoxy group and do not contain an interfering group. The reaction between the epoxy resin and the sulfide compound needs to be performed in the presence of a neutralizing acid, and as a result, a sulfonium group is introduced into the epoxy resin.

  Specific examples of the sulfide compound may be aliphatic sulfide, mixed aliphatic-aromatic sulfide, aralkyl sulfide, or cyclic sulfide. Examples of sulfide compounds that can be used include diethyl sulfide, dipropyl sulfide, ethylphenyl sulfide, tetramethylene sulfide, pentamethylene sulfide and the like.

［式中、Ｒ及びＲ'はそれぞれ独立して炭素数２〜８の直鎖又は分枝鎖アルキレン基である。］
で表されるチオジアルコールである。かかるスルホニウム変性エポキシ樹脂は電着開始直後の短時間（約１０秒間）塗膜抵抗の形成を遅くする機能を有し、かつバインダー樹脂に水分散安定性を付与する。 Particularly preferred sulfide compounds are of the formula

[Wherein, R and R ′ each independently represents a linear or branched alkylene group having 2 to 8 carbon atoms. ]
It is a thiodialcohol represented by Such a sulfonium-modified epoxy resin has a function of delaying the formation of coating film resistance for a short time (about 10 seconds) immediately after the start of electrodeposition, and imparts water dispersion stability to the binder resin.

  Examples of thiodialcohols include thiodiethanol, thiodipropanol, thiodibutanol, 1- (2-hydroxyethylthio) -2-propanol, 1- (2-hydroxyethylthio) -2,3-propanediol, 1- (2-hydroxyethylthio) -2-butanol and 1- (2-hydroxyethylthio) -3-butoxy-1-propanol. Most preferably, the sulfide compound is 1- (2-hydroxyethylthio) -2-propanol.

Blocked isocyanate curing agent The polyisocyanate used in the blocked isocyanate curing agent of the present invention 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-diisocyanatomethyl cyclohexyl 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 the block isocyanate curing agent.

  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 the blocking agent, those usually used such as ε-caprolactam and butyl cellosolve can be used. However, among these, many volatile blocking agents are regulated as targets of HAPs, and it is preferable that the amount used is minimized.

Pigment The electrodeposition coating composition of the present invention may contain a commonly used pigment. Examples of pigments that can be used include 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, Rust preventive pigments such as aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate and aluminum phosphomolybdate, zinc phosphomolybdate And silicic acid compounds such as zinc silicate, calcium silicate, and silica.

  The amount of the pigment is such that the mass ratio (P / V) between the pigment and the resin solid content contained in the coating composition is 1/4 or less. If the amount of the pigment in the coating composition exceeds 1/3 of the mass ratio with respect to the resin solid content, the depositability of the coating solid content decreases, and the throwing power decreases. The mass ratio (P / V) of 1/4 to 1/3 of the pigment and resin solid content contained in the coating composition is preferable.

Pigment-dispersed paste When a pigment is used as a component of an electrodeposition paint, generally the pigment is dispersed in advance in an aqueous medium at a high concentration 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 dispersion. The pigment dispersion resin dispersion is obtained by dispersing a pigment dispersion resin in an aqueous medium. As the pigment dispersion resin, 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.

Metal Catalyst The cationic electrodeposition coating composition of the present invention may contain a metal catalyst as a metal ion as a catalyst for improving the corrosion resistance of the coating film. As the metal ions, cerium ions, bismuth ions, copper ions, and zinc ions are preferable. These are contained in the electrodeposition coating composition as an eluate from a pigment containing a salt or metal ion combined with an appropriate acid. The acid may be any of inorganic acids or organic acids such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid and lactic acid, which will be described later as neutralizing acids for neutralizing sulfonium-modified epoxy resins and amine-modified epoxy resins. I just need it. A preferred acid is acetic acid.

Electrodeposition Coating Composition The cationic electrodeposition coating composition of the present invention comprises the above-described metal catalyst, amine-modified epoxy resin, sulfonium-modified epoxy resin (optional component), blocked isocyanate curing agent, and pigment dispersion paste in an aqueous medium. It is prepared by dispersing in. Further, the aqueous medium usually contains a neutralized acid in order to neutralize the amine-modified epoxy resin and the sulfonium-modified epoxy resin and improve the dispersibility of the binder resin emulsion. The neutralizing acid is an inorganic or organic acid such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, lactic acid.

  In the electrodeposition coating composition used in the present invention, the number of milliequivalents of amine base contained in a total of 100 g of the amine-modified epoxy resin is 10 to 25, preferably 15 to 20. Further, the number of milliequivalents of sulfonium base contained in 100 g of the sulfonium-modified epoxy resin is 7 to 45, preferably 10 to 35. If the number of milliequivalents of the amine base is less than 10 milliequivalents, the affinity for water is not sufficient and cannot be dispersed in water, or the stability deteriorates. The performance decreases and the throwing power is inferior. If the number of milliequivalents of the sulfonium base is less than 7 milliequivalents, the hydrophilicity of the sulfonium-modified epoxy resin becomes insufficient, and the dispersion stability of the paint cannot be maintained, and if it exceeds 45 milliequivalents, the throwing power of the paint is poor. It will be.

  When the amount of neutralizing acid contained in the coating composition is increased, the neutralization rate of the sulfonium-modified epoxy resin and amine-modified epoxy resin is increased, the affinity of the binder resin particles to the aqueous medium is increased, and the dispersion stability is increased. To do. This means that the binder resin hardly deposits on the object to be coated during electrodeposition coating, and the depositability of the solid content of the paint is lowered.

  Conversely, if the amount of neutralizing acid contained in the coating composition is small, the neutralization rate of the sulfonium-modified epoxy resin and amine-modified epoxy resin will be low, the affinity of the binder resin particles to the aqueous medium will be low, and dispersion stability will be reduced. Sex is reduced. This means that the binder resin is likely to be deposited on the object to be coated at the time of coating, and the depositability of the solid content of the paint is increased.

  Therefore, in order to reduce the difference in the film thickness of the electrodeposition coating between different types of articles and improve the throwing power of the electrodeposition paint, the amount of neutralizing acid contained in the paint composition is reduced and the sulfonium modification It is preferable to suppress the neutralization rate of the epoxy resin and the amine-modified epoxy resin to a low level.

  Specifically, the amount of the neutralizing acid is 10 to 25 mg equivalent, preferably 15 to 20 mg equivalent, based on 100 g of the binder resin solid content including the sulfonium-modified epoxy resin, the amine-modified epoxy resin and the blocked isocyanate curing agent. If the amount of the neutralizing acid is less than 10 mg equivalent, the affinity for water is not sufficient and the dispersion in water cannot be performed or the stability is extremely poor. If the amount exceeds 25 mg equivalent, the amount of electricity required for precipitation increases. In addition, the precipitation of the solid content of the paint is lowered and the throwing power is inferior, and there is a possibility that the film thickness difference between different kinds of articles cannot be reduced.

  In the present specification, the “amount of neutralizing acid” is the amount of acid used to neutralize the sulfonium-modified epoxy resin and the amine-modified epoxy resin, and is a binder contained in the coating composition. Expressed as the number of mg equivalents relative to 100 g of resin solids, it is expressed as MEQ (A).

  For the method of blending a sulfonium-modified epoxy resin, an amine-modified epoxy resin, and a blocked isocyanate as a curing agent and dispersing them in an aqueous medium, each of the sulfonium-modified epoxy resin and the amine-modified epoxy resin, or any one of them is blocked isocyanate. They may be mixed in a solution state to make each emulsion, and then each emulsion may be mixed, or a mixed solution in which a sulfonium-modified epoxy resin and an amine-modified epoxy resin are previously mixed in a solution state and a blocked isocyanate is added thereto. May be made into an emulsion.

  The amount of the block isocyanate curing agent reacts with active hydrogen-containing functional groups such as primary, secondary amino groups, and hydroxyl groups in the sulfonium-modified epoxy resin and amine-modified epoxy resin at the time of curing to give a good cured coating film. Generally, it is generally expressed as 90/10 to 50/50 in terms of a solid content mass ratio (epoxy resin / curing agent) of the sum of the sulfonium-modified epoxy resin and the amine-modified epoxy resin and the blocked isocyanate curing agent. The range is preferably 80/20 to 65/35.

  The mixing ratio of the sulfonium-modified epoxy resin and the amine-modified epoxy resin is 25/75 to 50/50, preferably 40/60 to 50/50, in mass ratio. If the mass ratio of the sulfonium-modified epoxy resin is less than the mixing ratio 25/75, the gas pin resistance of the paint is inferior. If the mixing ratio exceeds 50/50, the appearance defect of the coating film is difficult to be solved.

  The coating composition may contain a tin compound such as dibutyltin dilaurate and dibutyltin oxide, and a normal urethane cleavage catalyst. However, since the cationic electrodeposition coating composition of the present invention does not substantially contain lead, the amount is preferably 0.1 to 5% by mass of the resin solid content.

  Organic solvents are indispensable as solvents when synthesizing resin components such as sulfonium-modified epoxy resins, amine-modified epoxy resins, blocked isocyanate curing agents, and pigment-dispersed resins, and complicated operations are required for complete removal. . Moreover, when the organic solvent is contained in binder resin, the fluidity | liquidity of the coating film at the time of film forming will be improved, and the smoothness of a coating film will improve.

  Examples of the organic solvent usually contained in the coating composition include ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol monoethyl hexyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, propylene glycol monophenyl ether and the like.

  In addition to the above, the coating composition may contain conventional coating additives such as a plasticizer, a surfactant, an antioxidant, and an ultraviolet absorber.

  The conductivity of the cationic electrodeposition coating composition of the present invention is preferably 1500 to 2000 μS / cm. If it is less than 1500, the throwing power will decrease and the film thickness difference between different types of articles will increase, and if it exceeds 2000, the appearance of the coating film surface will deteriorate. The conductivity can be measured using a commercially available conductivity meter.

  The cationic electrodeposition coating composition of the present invention is electrodeposited on an object to be coated to form an electrodeposition coating film (uncured). The object to be coated is not particularly limited as long as it has conductivity, but in the present invention, a large object such as an automobile is not an object, and a relatively small object such as a part is an object to be coated. As the parts, automobile parts and industrial parts are most suitable.

  Electrodeposition coating is usually performed by applying a voltage of 50 to 450 V between the object to be coated as a cathode and the anode. When the applied voltage is less than 50V, electrodeposition is insufficient, and when it exceeds 450V, the coating film is destroyed and an abnormal appearance is obtained. At the time of electrodeposition coating, the bath temperature of the coating composition is usually adjusted to 10 to 45 ° C.

  The electrodeposition coating composition of the present invention needs to have a minimum precipitation pH of 11.90 to 12.00 in electrodeposition coating. If it is less than 11.90, the stability of the electrodeposition bath decreases, and if it exceeds 12.00, the throwing power decreases. Here, the minimum precipitation pH refers to a pH based on the hydroxide ion concentration required for the binder resin to precipitate in cationic electrodeposition coating.

Said minimum precipitation pH can be calculated | required by the electrodeposition behavior in constant current electrodeposition coating, ie, the electrodeposition coating which made current density (mA / cm < ² >) constant. In constant current electrodeposition coating, when deposition of a resin on the surface to be electrodeposited begins, a higher applied voltage is required depending on the increase in electrical resistance due to the deposition of the resin. Here, from the energization time until the electrical resistance increases, the hydroxide ion concentration (C _OH− ) required for the precipitation of the resin can be obtained by the following equation.

The minimum precipitation pH can be determined by the following formula.

  Moreover, the graph which shows the relationship between the applied voltage and the electricity supply time in minimum precipitation pH is shown in FIG.

  The electrodeposition process includes a process of immersing an object to be coated in a cationic electrodeposition coating composition, and a process of applying a voltage between the object to be coated as a cathode and an anode to deposit a film. Moreover, although the time which applies a voltage changes with electrodeposition conditions, generally it can be made into 2 to 4 minutes. In the present specification, the “electrodeposition coating film” refers to an uncured coating film after electrodeposition coating after the above-described process of depositing the coating film and before baking hardening.

The film thickness of the electrodeposition coating film is preferably 5 to 25 μm, more preferably 20 μm. When the film thickness is less than 5 μm, the rust prevention property is insufficient, and when it exceeds 25 μm, the paint is wasted. The film resistance of the electrodeposition coating film is preferably 700 to 1,800 kΩ · cm ² at a film thickness of 15 μm. When the film resistance of the coating film is less than 700 kΩ · cm ^2, it is impossible to obtain a uniform film thickness, and when it exceeds 1,800 kΩ · cm ² , the appearance of the coating film is remarkably deteriorated. Film resistance of the coating, more preferably 900~1500kΩ · cm ^2, and most preferably 1000~1300kΩ · cm ^2.

  In the present invention, the difference in the film thickness of the electrodeposition coating film of each article to be coated is small, specifically, the difference in the film thickness of the electrodeposition coating film is within 5 μm, preferably within 3 μm. In this case, the film thickness means an average film thickness. If the difference in film thickness is larger than 5 μm, it will not be different from the conventional one. Here, “film thickness” refers to a value measured with a contact-type film thickness meter. The above film thickness measures the film thickness near the center on the surface of the article to be coated, but since the article is not so large, the film thickness near the center is considered to represent the film thickness of the part. .

  The film resistance of the electrodeposition coating film can be adjusted by controlling the charge transfer medium amount and viscosity of the deposited film. For example, the viscosity is controlled by adjusting the amount of solvent to control the membrane resistance.

  The electrodeposition coating film obtained as described above is cured by baking at 120 to 260 ° C., preferably 140 to 220 ° C. for 10 to 30 minutes after completion of the electrodeposition process or after washing with water.

  The present invention will be described in detail by the following examples, but the present invention is not limited thereto. In the examples, “parts” and “%” are based on mass unless otherwise specified.

Production Example 1
Production of Blocked Isocyanate Curing Agent 1250 parts of diphenylmethane diisocyanate and 266.4 parts of methyl isobutyl ketone (hereinafter referred to as “MIBK”) were charged into a reaction vessel and heated to 80 ° C., and then 2.5 parts of dibutyltin dilaurate was added. . A solution prepared by dissolving 226 parts of ε-caprolactam in 944 parts of butyl cellosolve was added dropwise at 80 ° C. over 2 hours. Furthermore, after heating at 100 degreeC for 4 hours, in the measurement of IR spectrum, it confirmed that the absorption based on an isocyanate group disappeared, and after standing to cool, MIBK 336.1 parts was added and the block isocyanate hardening | curing agent was obtained.

Production Example 2
Production of sulfonium-modified epoxy resin In a flask equipped with a stirrer, a cooling tube, a nitrogen introducing tube, a thermometer and a dropping funnel, 87 parts of 2,4- / 2,6-tolylene diisocyanate (mass ratio = 8/2), MIBK85 Part and 0.1 part of dibutyltin dilaurate were charged. While stirring the reaction mixture, 32 parts of methanol was added dropwise. The reaction was started from room temperature and heated to 60 ° C. due to heat generation. The reaction was mainly carried out in the range of 60 to 65 ° C. and continued until absorption based on the isocyanate group disappeared in the measurement of IR spectrum.

  Next, 550 parts of epoxy resin having an epoxy equivalent of 188 synthesized from bisphenol A and epichlorohydrin by a known method was added to the reaction mixture, 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 330 was reached.

  Subsequently, 100 parts of bisphenol A and 36 parts of octylic acid were added and reacted at 120 ° C., resulting in an epoxy equivalent of 1030. Thereafter, 107 parts of MIBK was added to cool the reaction mixture, 52 parts of SHP-100 (1- (2-hydroxyethylthio) -2-propanol, Sanyo Chemical Co., Ltd.), 21 parts of ion-exchanged water, and 39 parts of 88% lactic acid were added. The reaction was performed at 80 ° C. The reaction was continued until the acid value was below 5, and an epoxy resin having a tertiary sulfonium base (resin solid content: 80%) was obtained.

  The obtained resin was mixed with the blocked isocyanate curing agent obtained in Production Example 1 so as to be uniform at a solid content ratio of 60/40. Thereafter, ion-exchanged water was slowly added for dilution. By removing MIBK under reduced pressure, a blocked isocyanate-containing sulfonium-modified epoxy resin emulsion having a solid content of 36% was obtained. Further, the milliequivalent of the base per 100 g of resin solid content of this emulsion was 10.

Production Example 3
Production of amine-modified epoxy resin In a flask equipped with a stirrer, a condenser tube, a nitrogen inlet tube, a thermometer and a dropping funnel, 87 parts of 2,4- / 2,6-tolylene diisocyanate (mass ratio = 8/2), MIBK85 Part and 0.1 part of dibutyltin dilaurate were charged. While stirring the reaction mixture, 32 parts of methanol was added dropwise. The reaction was started from room temperature and heated to 60 ° C. due to heat generation. The reaction was mainly carried out in the range of 60 to 65 ° C. and continued until absorption based on the isocyanate group disappeared in the measurement of IR spectrum.

  Next, 650 parts of epoxy resin having an epoxy equivalent of 188 synthesized from bisphenol A and epichlorohydrin by a known method was added to the reaction mixture, 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 300 was reached.

  Subsequently, when 165 parts of bisphenol A and 29 parts of octylic acid were added and reacted at 120 ° C., the epoxy equivalent was 1160. Thereafter, 107 parts of MIBK was added, the reaction mixture was cooled, 85 parts of diethanolamine was added, and the mixture was reacted at 110 ° C. for 2 hours. Then, it diluted with MIBK until it became 80% of non volatile matters, and the epoxy resin (resin solid content 80%) which has a tertiary amino base was obtained.

  The obtained resin was mixed with the blocked isocyanate curing agent obtained in Production Example 1 so as to be uniform at a solid content ratio of 60/40. Thereafter, formic acid was added so that the number of milliequivalents of acid was 25 per 100 g of resin solids, and further ion-exchanged water was slowly added for dilution. MIBK was removed under reduced pressure to obtain a blocked isocyanate-containing amine-modified epoxy resin emulsion having a solid content of 36%.

Production Example 4
Production of amine-modified epoxy resin In a flask equipped with a stirrer, a condenser tube, a nitrogen inlet tube, a thermometer and a dropping funnel, 87 parts of 2,4- / 2,6-tolylene diisocyanate (mass ratio = 8/2), MIBK85 Part and 0.1 part of dibutyltin dilaurate were charged. While stirring the reaction mixture, 32 parts of methanol was added dropwise. The reaction was started from room temperature and heated to 60 ° C. due to heat generation. The reaction was mainly carried out in the range of 60 to 65 ° C. and continued until absorption based on the isocyanate group disappeared in the measurement of IR spectrum.

  Next, 492 parts of an epoxy resin having an epoxy equivalent of 188 synthesized from bisphenol A and epichlorohydrin by a known method was added to the reaction mixture, and the temperature was raised to 125 ° C. Thereafter, 1.0 part of benzyldimethylamine was added and reacted at 130 ° C. until the epoxy equivalent was 360.

  Subsequently, when 70 parts of bisphenol A and 29 parts of octylic acid were added and reacted at 120 ° C., the epoxy equivalent was 850. Thereafter, 107 parts of MIBK was added and the reaction mixture was cooled. 48 parts of methylethanolamine and 70 parts of ketiminized diethylenetriamine were added and reacted at 110 ° C. for 2 hours. Then, it diluted with MIBK until it became 80% of non volatile matters, and the epoxy resin (resin solid content 80%) which has a tertiary amino base was obtained.

  The obtained resin was mixed with the blocked isocyanate curing agent obtained in Production Example 1 so as to be uniform at a solid content ratio of 60/40. Thereafter, formic acid was added so that the number of milliequivalents of acid was 25 per 100 g of resin solids, and further ion-exchanged water was slowly added for dilution. MIBK was removed under reduced pressure to obtain a blocked isocyanate-containing amine-modified epoxy resin emulsion having a solid content of 36%.

Production Example 5
Production of amine-modified epoxy resin In a flask equipped with a stirrer, a condenser tube, a nitrogen inlet tube, a thermometer and a dropping funnel, 87 parts of 2,4- / 2,6-tolylene diisocyanate (mass ratio = 8/2), MIBK85 Part and 0.1 part of dibutyltin dilaurate were charged. While stirring the reaction mixture, 32 parts of methanol was added dropwise. The reaction was started from room temperature and heated to 60 ° C. due to heat generation. The reaction was mainly carried out in the range of 60 to 65 ° C. and continued until absorption based on the isocyanate group disappeared in the measurement of IR spectrum.

  Next, 834 parts of epoxy resin having an epoxy equivalent of 188 synthesized from bisphenol A and epichlorohydrin by a known method was added to the reaction mixture, and the temperature was raised to 125 ° C. Thereafter, 1.0 part of benzyldimethylamine was added and reacted at 130 ° C. until the epoxy equivalent was 270.

  Subsequently, when 194 parts of bisphenol A and 29 parts of octylic acid were added and reacted at 120 ° C., the epoxy equivalent was 1540. Thereafter, 107 parts of MIBK was added, the reaction mixture was cooled, 85 parts of diethanolamine was added, and the mixture was reacted at 110 ° C. for 2 hours. Then, it diluted with MIBK until it became 80% of non volatile matters, and the epoxy resin (resin solid content 80%) which has a tertiary amino base was obtained.

  The obtained resin was mixed with the blocked isocyanate curing agent obtained in Production Example 1 so as to be uniform at a solid content ratio of 60/40. Thereafter, formic acid was added so that the number of milliequivalents of acid was 25 per 100 g of resin solids, and further ion-exchanged water was slowly added for dilution. MIBK was removed under reduced pressure to obtain a blocked isocyanate-containing amine-modified epoxy resin emulsion having a solid content of 36%.

Production Example 6
Manufacture of pigment-dispersed resin dispersion First, 222.0 parts of isophorone diisocyanate (hereinafter abbreviated as IPDI) is placed in a reaction vessel equipped with a stirrer, a cooling pipe, a nitrogen introduction pipe and a thermometer, and diluted with 39.1 parts of MIBK. After that, 0.2 parts 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.

  Next, 87.2 parts of dimethylethanol, 117.6 parts of 75% lactic acid aqueous solution and 39.2 parts of ethylene glycol monobutyl ether are added in order to a suitable reaction vessel, and the mixture is stirred at 65 ° C. for about half an hour to add the quaternizing agent. Prepared.

  Next, 710.0 parts of EPON 829 (bisphenol A type epoxy resin manufactured by Shell Chemical Company, epoxy equivalent 193 to 203) and 289.6 parts of bisphenol A were charged into a suitable reaction vessel, and the reaction was conducted under a nitrogen atmosphere. When heated to 150 to 160 ° C., an initial exothermic reaction occurred. The reaction mixture was allowed to react at 150-160 ° C. for about 1 hour, then cooled to 120 ° C., and then 498.8 parts of previously prepared 2-ethylhexanol half-blocked IPDI (MIBK solution) was added.

  The reaction mixture is kept at 110-120 ° C. for about 1 hour, then 463.4 parts of ethylene glycol monobutyl ether are added, the mixture is cooled to 85-95 ° C. and homogenized, and then the quaternizing agent 196 prepared above is used. .7 parts were added. After maintaining the reaction mixture at 85 to 95 ° C. until the acid value becomes 1, 964 parts of deionized water is added to finish quaternization in the epoxy-bisphenol A resin, and a pigment dispersion having a quaternary ammonium salt portion A resin dispersion was obtained (resin solid content 50%).

Production Example 7
Production of pigment dispersion paste 120 parts of pigment dispersion resin dispersion obtained in Production Example 6 in a sand grind mill, 2.0 parts of carbon black, 90 parts of calcined kaolin, silicic acid compound (silica particles, silicia manufactured by Fuji Silysia Chemical Co., Ltd.) 10 parts, 80.0 parts of titanium dioxide, 18.0 parts of aluminum phosphomolybdate and 221.7 parts of ion-exchanged water were added and dispersed until the particle size became 10 μm or less to obtain a pigment paste (solid content 48%). .

Example 1
The sulfonium-modified epoxy resin emulsion obtained in Production Example 2 and the amine-modified epoxy resin emulsion obtained in Production Example 3 were mixed to obtain a resin solid content ratio of 25/75, and then the pigment dispersion obtained in Production Example 7 The paste was mixed. Further, 1% by mass of dibutyltin oxide and ion exchange water were added to the resin solid content to obtain a cationic electrodeposition coating composition having a solid content of 20%. The obtained electrodeposition coating composition has a MEQ (A) (amount of neutralizing acid: mg equivalent number per 100 g of binder resin solid content contained in the coating composition) of 19.5. The mass ratio (P / V) between the contained pigment and the resin solid was 1 / 4.5.

<Membrane resistance of electrodeposition coating film>
Electrodeposition paint containing zinc phosphate-treated steel sheet (JIS G 3141 SPCC-SD surfdyne SD-2500 treatment) (dimensions: 70 mm x 150 mm, thickness 0.7 mm) in an electrodeposition bath containing a cationic electrodeposition paint composition Was immersed in 10 cm. A voltage was applied to the steel sheet, the voltage was increased to 200 V over 30 seconds, and electrodeposition was performed for 150 seconds. The coating voltage with a coating thickness of 15 μm at a bath temperature of 28 ° C. and the residual current at the end of electrodeposition were measured to calculate the coating resistance value (kΩ · cm ² ). The coating film resistance value of Example 1 was 1200 kΩ · cm ² . The results are shown in Table 1.

  Using this electrodeposition paint, nine iron plates with a thickness of 0.5 mm, a length of 20 mm and a width of 50 mm are hung on the hanger shown in FIG. 2, and electrodeposition is applied at a voltage of 200 V at 30 ° C. for 3 minutes. The film thickness of each object was measured with a contact-type film thickness meter (Fischer, Deltascope). Table 1 shows the film thickness measurement results. Table 1 also describes the maximum film thickness and the minimum film thickness of the object to be coated, and the difference between the two.

Comparative Example 1
Electrodeposition coating was performed using the hanger described in FIG. 2 under the same conditions using an electrodeposition paint (trade name Powernics 120M) manufactured by Nippon Paint. The film thickness of each object was measured in the same manner, and the results are shown in Table 1. Coating resistance of the electrodeposition paint of Comparative Example 1 was 650kΩ · cm ^2.

  As apparent from the above results, in Example 1, the difference in film thickness between the objects to be coated is 2.5 μm, which is much smaller than the difference in film thickness in Comparative Example 1 5.8 μm.

It is a schematic diagram of the electrodeposition bath part of the electrodeposition coating line of components. It is a schematic diagram which shows the hanging state of the hanger and to-be-coated object (iron plate) used in the Example and the comparative example.

Explanation of symbols

1 ... Electrodeposition bath,
2 ... Electrodeposition paint,
3 ... Parts,
4 ... hangers,
5 ... Conveying means,
6 ... Arrow.

Claims (2)

In the cationic electrodeposition coating method in which electrodeposition coating is performed in a state in which parts to be coated having different sizes and / or shapes are mixed in a cationic electrodeposition paint bath,
Cationic electrodeposition characterized in that the electrodeposition coating film formed when electrodeposition is applied to a thickness of 15 μm in the cationic electrodeposition paint bath has a membrane resistance of 700 to 1,800 KΩ · cm ^2. How to paint.   The cationic electrodeposition coating method according to claim 1, wherein the film thickness difference of the electrodeposition coating film formed on the part to be coated is within 5 μm.

Images