JP2013000710A - Method for forming multilayer coating film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a multilayer coating film, capable of achieving excellent smoothness in a 3 wet coating process of applying an aqueous coating material onto an automobile body, even if its electrodeposition coating film is of poor surface smoothness.SOLUTION: The method for forming a multilayer coating film comprising: applying an aqueous intermediate coating material onto an automobile body coated with an electrodeposition coating film and having been subjected to a baking treatment (a process (i)); carrying out setting (a process (ii)), and carrying out predrying (a process (iii)); subsequently applying aqueous base coating material thereonto; further applying clear coating film thereonto; and heating and curing the resulting intermediate coating film, base coating film and clear coating film at the same time, is characterized in that the viscosity of the coating film formed of the aqueous intermediate coating material and the content of nonvolatile components present therein are each within a specified range respectively, immediately after the process (i) and immediately before the process (iii).

Description

  The present invention relates to a method for forming a multilayer coating film, and more particularly to a method for forming an aqueous intermediate coating film and a top coating film on an automobile body using a three-coat one-bake method.

  The coating of an automobile body is performed by sequentially forming an electrodeposition coating film, an intermediate coating film, a base coating film and a clear coating film on a steel plate as an object to be coated. Conventionally, these coating films have been formed by baking and curing each coating film after applying each coating film. When multiple coatings are applied, if the upper layer coating is not applied after the lower layer coating is completely cured, the adjacent coating layers interfere with each other, and the lower layer irregularities are reflected in the upper layer, resulting in a multilayer coating. This is because the appearance of the film is deteriorated.

  However, in recent years, the emphasis on environmental performance has increased, and in order to meet the demands for energy saving, reduction of global warming gas emissions, and reduction of environmental pollution in the painting process, the painting method for automobile bodies uses multiple paints in a state where they are not baked and cured. A method for forming a multi-layer coating film in which coatings are applied and then cured simultaneously has been studied. Such a coating method is referred to as a wet-on-wet method. Various aspects of the wet-on-wet method in the coating of automobile bodies have been proposed. For example, electrodeposition-intermediate wet-on-wet method, intermediate-base-base coating, in which an electrodeposition coating and an intermediate coating are baked and dried simultaneously. An intermediate top coat wet-on-wet method has been proposed in which three layers of a film-clear coating film are simultaneously baked and dried.

  Patent Document 1 proposes an electrodeposition-intermediate wet-on-wet method in which an electrodeposition coating is applied, an aqueous intermediate coating is applied, and both are baked and dried simultaneously. However, the coating film formed by this method cannot obtain a high level of smoothness and does not satisfy the coating film appearance standard required in automobile painting. Patent Document 2 describes an intermediate top coating wet-on-wet method in which three layers of an intermediate coating, a base coating, and a clear coating are simultaneously baked and dried. In this method, a method is described in which the coating film non-volatile content of the undried coating film is controlled within a specified range in each coating step of intermediate coating and top coating. However, there is no particular description about the coating composition used in such a method, and as a result, the coating film appearance standard required in automobile painting cannot be satisfied. Patent Document 3 describes a wet-on-wet method for an intermediate top coat, which suppresses a mixed layer between the intermediate coat and the top coat by controlling the neutralization value of the intermediate coat and the base paint within a specified range, and is good It is stated that a good coating appearance can be obtained. According to this method, when the smoothness of the coating film formed by electrodeposition coating is good, for example, when the value of the surface roughness Ra measured at the cutoff value 25 is less than 2.5, it is good. When coating on an electrodeposition coating with poor surface smoothness with a surface roughness Ra value of 2.5 or more, the coating film appearance is obtained, but the smoothness of the formed composite coating is still required in automobile coating. The coating film appearance standard cannot be satisfied.

JP 2000-96295 A JP 2004-267834 A JP-A-8-290102

  The present invention has been made in order to solve the above-described problems of the prior art, and the purpose thereof is even in the case where the smoothness of the electrodeposition coating film formed in the lower layer is inferior in the intermediate top wet on wet coating method. Another object of the present invention is to provide a method for forming a multilayer coating film having excellent smoothness.

That is, the gist of the present invention is the following (1) to (4).
(1) A water-based intermediate coating is applied on an automobile body that has been coated with an electrodeposition coating and has been baked (step (i)), setting (step (ii)), and preliminary drying (step (iii)). Subsequently, a water-based base coating is applied, a clear coating is further applied, and the water-based intermediate coating, the water-based base coating and the clear coating are simultaneously heated and cured to form a multilayer coating. The water-based intermediate coating film has a viscosity of 10 to 200 Pa. At a measurement temperature of 23 ° C. and a shear rate of 1.0 sec ⁻¹ immediately after the step (i). S and a non-volatile content of 45 to 55% by weight, the conditions are 5 to 35 ° C., and after the step (ii) is performed for 2 to 20 minutes, and immediately before the step (iii), the measurement temperature is 23 ° C., Viscosity of 10 to 1000 Pa. Under a shear rate of 1.0 sec- ¹ . S, The non-volatile content is 46 to 60% by weight.
(2) The multi-layer coating film according to (1), wherein the aqueous intermediate coating material comprises a polyester resin, a urethane resin, an aminoplast resin, a water-soluble solvent having a boiling point of 200 ° C. or higher, and a colorant. Forming method.
(3) The polyester resin comprises (A) an acid value of 20 mgKOH / g or less, a glass transition point of 50 to 80 ° C., an aqueous polyester resin having an average particle size of 50 to 500 nm, and (B) an acid value of 10 to 90 mgKOH / g, The method for forming a multilayer coating film according to (2), which is a mixture of a water-soluble polyester resin having a glass transition point of 0 to 40 ° C. and an average particle diameter of 20 to 100 nm.
(4) The method for forming a multilayer coating film according to (2) or (3), wherein the water-soluble solvent having a boiling point of 200 ° C. or higher is a glycol ether solvent.

  According to the method for forming a multilayer coating film of the present invention, in the intermediate top coating wet-on-wet coating method, even when the smoothness of the electrodeposition coating film formed in the lower layer is inferior, the multilayer coating having excellent smoothness. A film can be formed.

  In the method for forming a multilayer coating film of the present invention, an electrocoating film is applied, and a water-based intermediate coating is applied on the automobile body after baking (step (i)) and setting (step (ii)). , Pre-drying (step (iii)), followed by coating with an aqueous base paint and further a clear paint, and simultaneously heating and curing the aqueous intermediate coating film, aqueous base coating film and clear coating film. The step (i) is a step of applying a water-based intermediate coating onto the object to be coated on which the electrodeposition coating is formed, and the step (ii) aims at leveling the applied intermediate coating. Step (iii) is a step of pre-drying the formed uncured intermediate coating film for the purpose of removing excess volatile components contained in the coating film.

As a result of earnest research on a method for forming a multilayer coating film having excellent smoothness even when the electrodeposition coating film formed in the lower layer is inferior, the present inventors have determined that an uncured intermediate coating It has been found that the object of the present invention can be achieved by defining the viscosity and nonvolatile content of the film as follows. That is, the viscosity of the intermediate coating film immediately after the step (i) is 10 to 200 Pa.s under the conditions of a measurement temperature of 23 ° C. and a shear rate of 1.0 second− ¹ . The intermediate coating immediately before the step (iii) after the step (ii) is carried out with the non-volatile content in the range of 45 to 55% by weight and the conditions being 5 to 35 ° C. and 2 to 20 minutes. The viscosity of the film was measured at 10 to 1000 Pa.s under the conditions of a measurement temperature of 23 ° C. and a shear rate of 1.0 sec- ¹ . It has been found that the object of the present invention can be achieved by setting the non-volatile content in the range of 46 to 60% by weight in the range of S.

Here, the viscosity can be measured using a commercially available shear rate variable viscosity measuring device, and for example, it can be measured using “Haake RS-600 type” (thermoelectron viscoelasticity measuring device). it can. The non-volatile content can be calculated from the amount of weight loss before and after drying the uncured coating film. The uncured coating film used for the viscosity and non-volatile content tests is a scraper that is applied under the same conditions as the automobile body coating using a coating device that is usually used to paint the outer panel of an automobile body. Use the one collected by using the instrument. In order to correlate the viscosity of the uncured coating film thus collected with the smoothness of the cured film obtained, it is effective for the inventors to define the viscosity at a shear rate of 1.0 sec ⁻¹ . It is clear from research. The reason is that the viscosity at a shear rate of 1.0 sec- ¹ indicates the degree of leveling of the uncured coating film in the process from application of the paint to drying.

  The viscosity immediately after step (i) is 10 Pa. When it is lower than S, the unevenness of the lower electrodeposition coating film cannot be concealed, which is not preferable. Further, the viscosity immediately before step (iii) is 10 Pa. If it is lower than S, the unevenness of the lower electrodeposition coating film cannot be concealed, which is not preferable. Furthermore, the viscosity immediately after the step (i) is 200 Pa. Higher than S or a viscosity immediately before step (iii) is 1000 Pa.s. When it is higher than S, the smoothness of the intermediate coating film itself is inferior, which is not preferable. Concerning the non-volatile content of the coating film, the unevenness of the lower electrodeposition coating film should be concealed both when it is lower than 45% by weight immediately after step (i) and when it is lower than 46% by weight immediately before step (iii). This is not preferable. Further, when it is higher than 55% by weight immediately after the step (i) or higher than 60% by weight immediately before the step (iii), the smoothness of the intermediate coating film itself is unfavorable.

  By prescribing the viscosity and non-volatile content of the aqueous intermediate coating film as described above, it is possible to prevent the uncured intermediate coating film from being excessively leveled and hiding the unevenness of the electrodeposition coating film. By maintaining an appropriate leveling state in which the coating film itself does not generate irregularities, a target smooth composite coating film can be formed. The conditions of process (ii) are 5-35 degreeC and the range for 2 to 20 minutes, The viscosity of the said coating film and the non volatile matter of a coating film can be obtained by performing the setting in such conditions.

Next, the waterborne intermediate coating material used in the present invention will be described.
The water-based intermediate coating is applied to conceal the unevenness of the base on which electrodeposition coating has been applied, to ensure surface smoothness after top coating, and to provide film properties such as impact resistance and chipping resistance. Is.

  The aqueous intermediate coating material used in the present invention is preferably a coating material in which a polyester resin, a urethane resin, an aminoplast resin, a water-soluble solvent having a boiling point of 200 ° C. or higher, and a colorant are dispersed in water. Accordingly, various additives, various solvents other than those described above, and the like can be blended.

  The polyester resin used here is one or a mixture of two or more reaction products of polyhydric alcohols and polycarboxylic acids and / or their anhydrides in an amount greater than the stoichiometric equivalent. When used in an aqueous intermediate coating, the carboxyl group contained in the obtained resin is neutralized with a neutralizing agent to produce an ionic group, and the dissolved or dispersed state is maintained in water. Dissolution refers to a form in which the resin is completely dissociated into a molecular state in water, and dispersion refers to a system in which the resin is stably present in water as particles having a particle diameter of about 20 to 1000 nm.

  Examples of polyhydric alcohols include, for example, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, isomeric pentanediols, and isomeric hexanediol. Or octanediols such as 2-ethyl-1,3-hexanediol, 1,2-, 1,3- and 1,4-bis (hydroxymethyl) -cyclohexane, trimethylolpropane, glycerol, 1,4- Examples thereof include dihydroxybenzene and 1,4-dihydroxymethylbenzene. Examples of polycarboxylic acids and polycarboxylic anhydrides include divalent carboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, fumaric acid, maleic acid, and itaconic acid. Acid, trimellitic acid and other trivalent carboxylic acids, phthalic anhydride, tetrahydrophthalic anhydride, maleic anhydride, trimellitic anhydride and other polycarboxylic acid anhydrides, dimer or trimer fatty acids such as castor oil Mention may be made of fatty acid trimers. The production of the polyester resin can be carried out by a known method. For example, the polyester resin can be obtained by dehydration condensation of polyhydric alcohol and polyhydric carboxylic acid or polyhydric carboxylic acid anhydride at a temperature of 200 to 230 ° C. in a reaction vessel. . The acid value of the resulting polyester resin can be increased or decreased by controlling the ratio between the starting polyhydric alcohol and the polyvalent carboxylic acid and the dehydration condensation rate. The acid value is measured by neutralizing and titrating the polyester resin with potassium hydroxide and displaying the weight of potassium hydroxide required for neutralization. This titration method is publicly known, and standards such as JIS can be referred to.

  The glass transition point of the obtained polyester resin is determined by the type of polyhydric alcohol and polycarboxylic acid used, and is generally synthesized as a main structure having a linear structure, for example, a structure containing many long-chain alkylene groups. A polyester resin has a low glass transition point, and a polyester resin containing a cyclic structure such as an aromatic ring has a high glass transition point. Examples of the polyhydric alcohol containing a long chain alkylene group include 1,5-pentanediol, 1,6-hexanediol, and the like. Examples of the polyvalent carboxylic acid containing a long chain alkylene group include adipine. Examples thereof include acid and sebacic acid. Examples of the polyhydric alcohol containing a cyclic structure such as an aromatic ring include 1,4-dihydroxybenzene and 1,4-dihydroxymethylbenzene. Examples of the divalent carboxylic acid or carboxylic acid anhydride include phthalic acid, isophthalic acid, terephthalic acid, and anhydrides thereof. In general, the glass transition point is known to affect the thermal properties of a coating film, and when used as a coating material, it is known to affect the leveling properties of an uncured coating film before baking. That is, when the glass transition point of the polyester resin is low, the leveling property is good, and when it is high, the leveling property tends to be inferior. The glass transition point can be measured by a known method using, for example, dynamic viscoelasticity measurement or a differential operation calorimeter. Generally, the inflection point of tan δ obtained by dynamic viscoelasticity measurement is the glass It is defined as a transition point. The dynamic viscoelasticity measuring apparatus is commercially available. For example, a DDV-25FP-W type identification year elasticity measuring apparatus manufactured by Orientec Co., Ltd. can be used. As measurement conditions, a frequency of 11 Hz and a heating rate of 3 ° C./min are generally employed.

  In order to use a polyester resin in the aqueous intermediate coating, it is necessary to make the polyester resin water-soluble or water-dispersed, and various water-solubilizing or water-dispersing methods have been proposed, and any method can be applied. It is. Among these methods, a method in which a carboxyl group is neutralized with a basic neutralizing agent to produce an ionic functional group, and water-soluble or water-dispersed with the ionic functional group is preferable. As the neutralizing agent, an amine compound is used. For example, methylamine, ethylamine, n-propylamine, isopropylamine, n-butylamine, isobutylamine, t-butylamine, n-amylamine, isoamylamine, dimethylamine, diethylamine, Di-n-propylamine, diisopropylamine, di-n-butylamine, diisobutylamine, di-t-butylamine, di-n-amylamine, diisoamylamine, trimethylamine, triethylamine, tri-n-propylamine, triisopropyl Amine, tri-n-butylamine, triisobutylamine, tri-t-butylamine, tri-n-amylamine, triisoamylamine, methyldiethylamine, dithymeethylamine, dimethyl-n-propylamine, Ethanolamine, diethanolamine, triethanolamine, N-methylethanolamine, N-ethylethanolamine, Nn-propylethanolamine, N-isopropylethanolamine, N-isobutylethanolamine, N, N-dimethylethanolamine, N , N-diethylethanolamine, N, N-di-n-propylethanolamine, N, N-di-n-butylethanolamine, N, N-diisopropylethanolamine, N, N-diisobutylethanolamine, N-methyl Diethanolamine, N-ethyldiethanolamine, Nn-propyldiethanolamine, N-isopropyldiethanolamine, N-isobutyldiethanolamine, morpholine, N-methylmorpholine, N-ethylmorph Phosphorus, diethylenetriamine, triethylenetetramine, ethylenediamine, tetramethylenediamine, can be mentioned amines such as hexamethylene diamine. In particular, it is preferable to use amines such as triethylamine, N, N-dimethylethanolamine, N-methylmorpholine. Whether the neutralized polyester resin is water-soluble or water-dispersed is often determined by the acid value of the polyester resin, and neutralizes the polyester resin in the acid value range of 10 to 40 mgKOH / g. Thus, an aqueous dispersion is obtained and becomes water-soluble by neutralizing a polyester resin having an acid value in the range of 40 to 120 mgKOH / g. The neutralization rate with amine can be appropriately selected within the range of 40 to 120 mol%.

  The polyester resin used in the intermediate coating is composed of a water-dispersed resin (aqueous polyester resin A) composed of a polyester resin having an acid value of 20 mgKOH / g or less, a glass transition point of 50 to 80 ° C., and an average particle size of 50 to 500 nm, and an acid. It is preferably a mixture of a water-soluble resin (aqueous polyester resin B) made of a polyester resin having a valence of 10 to 90 mgKOH / g, a glass transition point of 0 to 40 ° C., and an average particle size of 20 to 100 nm. The aqueous polyester resin A exhibits an effect of suppressing an increase in the viscosity of the coating film at a stage immediately before coating and pre-drying are started by adjusting the acid value, glass transition point, and average particle diameter to the above ranges. Moreover, the water-based polyester resin B exhibits the effect of keeping the viscosity of the uncured coating film low at 1.0 minutes after the coating by setting the acid value, the glass transition point, and the average particle diameter within the above ranges. Both of these effects are achieved by adjusting the acid value, glass transition point, and average particle size to the above ranges, and any of the acid value, glass transition point, and average particle size is the upper limit of the above value or If it deviates from the lower limit, the desired viscosity cannot be obtained after 1.0 minute after coating or just before the preliminary drying, and as a result, an excellent multilayer coating film appearance cannot be obtained.

  Further, by using both the water-based polyester resin A and the water-based polyester resin B, the viscosity and non-volatile content of the coating film at the time of 1.0 minute after the above-mentioned coating and the time immediately before the preliminary drying are specified values. Can be obtained.

  The mixing ratio of the aqueous polyester resin A and the aqueous polyester resin B is not particularly limited, but is preferably in the range of 30 to 250 parts by weight of the aqueous polyester resin B with respect to 100 parts by weight of the aqueous polyester resin A, more preferably the aqueous polyester resin A100. It is the range of 40-200 weight part of water-based polyester resin B with respect to a weight part.

  The urethane resin used in the aqueous intermediate coating is one or two polyurethane resins obtained by reacting polyester polyols and optionally polyhydric alcohols or other polyols with a stoichiometric little diisocyanate to extend the chain. It is a mixture of seeds and more. Examples of polyester polyols include, for example, one or a mixture of two or more reaction products of a polyhydric alcohol and a polycarboxylic acid and / or anhydride thereof in an amount greater than the stoichiometric equivalent. Examples of the polyhydric alcohol used here include, for example, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, isomer pentanediols, isomerism Hexanediols or octanediols such as 2-ethyl-1,3-hexanediol, 1,2-, 1,3- and 1,4-bis (hydroxymethyl) -cyclohexane, trimethylolpropane, glycerin, etc. Can be mentioned. Examples of the polyvalent carboxylic acid and polyvalent carboxylic acid anhydride include divalent oxalic acid, succinic acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, fumaric acid, maleic acid, itaconic acid and the like. Carboxylic acid, trivalent carboxylic acid such as trimellitic acid, phthalic anhydride, tetrahydrophthalic anhydride, polyhydric carboxylic acid anhydride such as maleic anhydride, trimellitic anhydride, dimer or trimer fatty acid, for example Mention may be made of trimers of castor oil fatty acids.

  Examples of the polyhydric alcohol used as desired include, for example, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, and isomeric pentane. Diols, isomeric hexanediols or octanediols such as 2-ethyl-1,3-hexanediol, 1,2-, 1,3- and 1,4-bis (hydroxymethyl) -cyclohexane, trimethylolpropane And glycerin.

  Examples of other polyols used as desired include polyether polyols, polyester polyols, polycaprolactone polyols, polycarbonate polyols, and the like.

  Examples of polyether polyols include polytetramethylene glycol obtained by ring-opening polymerization of tetrahydrofuran and alkylene oxide adducts of polyhydric alcohols. Examples of the polyhydric alcohol used here include, for example, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, isomer pentanediols, isomerism Hexanediols or octanediols such as 2-ethyl-1,3-hexanediol, 1,2-, 1,3- and 1,4-bis (hydroxymethyl) -cyclohexanone, trimethylolpropane, glycerin, etc. Examples of the alkylene oxide include ethylene oxide, propylene oxide, 1,2-, 1,3- or 2,3-butylene oxide, tetrahydrofuran, styrene oxide, epichlorohydrin, and the like. It is also possible to use it.

  Examples of the polycaprolactone polyol include compounds obtained by ring-opening polymerization of lactones such as ε-caprolactone and γ-valerolactone in the presence of the aforementioned polyhydric alcohol such as ethylene glycol. Furthermore, examples of the polycarbonate polyol include compounds produced from the aforementioned polyhydric alcohols such as ethylene glycol and carbonates such as dimethyl carbonate and diethyl carbonate.

  Next, examples of the diisocyanate include 2,4- and / or 2,6-diisocyanate toluene, 2,4-diisocyanate-dicyclohexylmethane, 4,4-diisocyanate-dicyclohexylmethane, hexamethylene diisocyanate, 1-isocyanate- 3,3,5-trimethyl-5-isocyanatomethylcyclohexane and the like. These can be used individually by 1 type or can be used in mixture of 2 or more types.

  The polyurethane resin having a free isocyanate group can be obtained by heating and mixing polyester polyols, optionally the above-mentioned polyhydric alcohols or polyols, and diisocyanate in the presence or absence of a catalyst. Examples of catalysts include organometallics such as organotin compounds such as dibutyltin dilaurate, tertiary amines such as triethylamine, carboxylates such as naphthenates such as calcium naphthenate and tin naphthenate, cobalt octylate, octyl Mention may be made of octylates such as lead acid.

  The mixing ratio of the polyester polyols, the polyhydric alcohols or polyols used as desired, and the diisocyanate is the ratio of the total hydroxyl value of the polyester polyols, polyhydric alcohol or polyols, and the isocyanate value of the diisocyanate. Are preferably in the range of 1: 0.8 to 1: 0.95. The reaction step is usually completed in the presence of a diluent such as an organic solvent at 60 to 140 ° C. for 2 to 4 hours.

  When a urethane resin is used for the aqueous intermediate coating, the aqueous dispersion resin can be obtained by neutralizing the carboxyl group contained in the urethane resin with a neutralizing agent such as an amine, and then dispersing in water. The types and amounts of amines that can be used are the same as those in the examples of the aqueous polyester resin solution or water-dispersed resin described above. Moreover, you may use an emulsifier together in order to stabilize water dispersion resin. Examples of emulsifiers include an anion selected from a compound having a hydrocarbon group having 6 or more carbon atoms and a hydrophilic moiety such as a carboxylate, sulfonate, or sulfate partial ester in the same molecule. And nonionic emulsifiers. Among these, as anionic emulsifiers, alkali metal salts or ammonium salts of alkylphenols or sulfuric alcohol half esters; alkyl or allyl sulfonate alkali metal salts or ammonium salts; polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl ethers Or the alkali metal salt or ammonium salt of the sulfuric acid half ester of polyoxyethylene allyl ether is mentioned. Examples of the nonionic emulsifier include polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ether, and polyoxyethylene allyl ether. In addition to these general-purpose anionic and nonionic emulsifiers, the molecule has a radically polymerizable unsaturated double bond, that is, acrylic, methacrylic, propenyl, allyl, allyl ether, maleic acid. Various anionic and nonionic reactive emulsifiers having a group such as a system are also used singly or in combination of two or more.

  Next, the aminoplast resin used for the aqueous intermediate coating is obtained by methylolating an amino group-containing compound such as melamine, benzoguanamine, cyclohexylguanamine, urea, etc., and etherifying this with an alkanol or cyclohexanol having 1 to 6 carbon atoms. It is a crosslinkable resin that can be used alone or in combination of two or more. Representative aminoplast resins include alkyl etherified melamine resins, such as butyl etherified melamine resins, methyl etherified melamine resins, and butylmethyl mixed etherified melamine resins. Moreover, the alkyl etherified melamine resin can also use a commercial item. As such melamine resins, those having a methoxy group alone, Cymel 325, Cymel 327, Cymel 370, Mycoat 723; those having both a methoxy group and a butoxy group, Cymel 202, Cymel 204, Cymel 232, Cymel 235, Cymel 236, Cymel 238, Cymel 254, Cymel 266, Cymel 267 (all trade names, manufactured by Nippon Cytec Industries, Inc.); Mycoat 506 (trade name, Nippon Cytec, a single product having a butoxy group) Industries, Inc.), Uban 20N60, Uban 20SE (both trade names, manufactured by Mitsui Chemicals) and the like. These may be used alone or in combination of two or more.

  The mixing ratio of the polyester resin, the urethane resin, and the aminoplast resin in the aqueous intermediate coating is not particularly limited, but polyester resin: urethane resin: aminoplast resin = 100: 10-50: 5-20 by weight ratio. The range of is preferable.

  A water-soluble solvent having a boiling point of 200 ° C. or higher used for an aqueous intermediate coating has a structure in which a hydroxyl group on one or both sides of glycols such as ethylene glycol and propylene glycol is substituted with an alkoxy group, a cycloalkoxy group or an arylalkoxy group, A glycol ether solvent having a boiling point of 200 ° C. or higher that is miscible in a free ratio is preferable. If the boiling point is less than 200 ° C., the non-volatile content immediately before the preliminary drying does not fall within the range of 46 to 60% by weight, which is not preferable. Examples of such solvents include diethylene glycol monoethyl ether, diethylene glycol mono-n-butyl ether, triethylene glycol mono-n-butyl ether and the like, and these can be used alone or in combination of two or more. The addition amount is not limited, but is generally in the range of 0.5 to 10.0% by weight in the intermediate coating.

  The aqueous intermediate coating used in the present invention contains a colorant in addition to these resins and solvents. In addition to adjusting the color tone of the top coat paint, the colorant also plays an auxiliary role in adjusting the viscosity and non-volatile content of the uncured coating film of the intermediate coat, and generally contains a large amount of colorant. The viscosity of the intermediate coating film increases.

  Examples of the colorant include organic or inorganic paint color pigments, flake-like aluminum, copper, mica, coating mica, bright metallic pigments such as mica-like iron oxide, and the like. Or 2 or more types can be used. Specifically, titanium dioxide, azo chelate pigment, insoluble azo pigment, condensed azo pigment, phthalocyanine pigment, indigo pigment, perinone pigment, perylene pigment, dioxane pigment, quinacridone pigment, isoindolinone pigment , Organic color pigments such as diketopyrrolopyrrole pigments, benzimidazolone pigments, metal complex pigments, inorganic color pigments such as yellow lead, yellow iron oxide, Bengala, carbon black, aluminum flakes, natural or synthetic mica flakes, etc. Is mentioned. These pigments may be used in combination with extender pigments such as calcium carbonate, barium sulfate, clay and talc. The content of the colorant is adjusted according to the color tone, the viscosity of the uncured coating film, and the non-volatile content as described above, but is in the range of 1 to 100 parts by weight with respect to 100 parts by weight of the resin. When it mix | blends exceeding 100 weight part, an uncured coating-film viscosity becomes high too much and is unpreferable.

  In addition to these components, the aqueous intermediate coating used in the present invention, if desired, thickeners, antifoaming agents, leveling agents, stabilizers, UV absorbers, pigment dispersants, neutralizing agents, catalysts and other additives, alcohol , Glycols, glycol ethers, glycol esters, glycol ether esters, ketones, esters, aromatic hydrocarbons, aliphatic hydrocarbons, alicyclic hydrocarbons and mixtures of these hydrocarbons, etc. These organic solvents and other components can be used in combination.

  In particular, the thickener can be used to adjust the aforementioned uncured intermediate coating film viscosity to a specified value. Such a thickener is not particularly limited, and examples thereof include viscose, methylcellulose, ethylcellulose, hydroxyethylcellulose, and commercially available cellulose such as Tyroze MH and Tyroze H (both manufactured by Hoechst, trade names). Examples of products, sodium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, and commercially available products (hereinafter all trade names) include Primal ASE-60, Primal TT-615, Primal RM-5 (all of which are Rohm & Haas) Manufactured by Co., Ltd.), Alkali thickened type such as Euker Polyphobe (manufactured by Union Carbide Co., Ltd.), polyvinyl alcohol, polyethylene oxide, and commercially available products (hereinafter referred to as trade names), Adecanol UH-420, Adecanol UH 462, Adecanol UH-472, UH-540, Adecanol UH-814N (Asahi Denka Kogyo), Primal RH-1020 (Rohm & Haas), Kuraray Poval (Kuraray), etc. are listed. be able to. These may be used alone or in combination of two or more.

  As the water-based base paint used in the present invention, a paint composition usually used as a water-based intermediate coating for automobile bodies can be used. Examples thereof include water-dispersed resins, crosslinkable resins, glitter pigments, pigments such as colored pigments and extender pigments, and water-based paints containing various additives. Examples of water-dispersed resins that can be used include polyester resins, acrylic resins, urethane resins, carbonate resins, and epoxy resins. As the crosslinkable resin, pigment, and various additives, those usually used can be used.

  As the clear paint used in the present invention, a paint composition usually used as a clear paint for automobile bodies can be used. For example, organic solvent-based one-pack or two-pack clear paint, water-based clear paint using water-dispersed resin or water-soluble resin, powder clear paint obtained by pulverizing solid powder resin at room temperature, powder clear paint A powder slurry type clear coating material in which a resin for use is dispersed in a medium such as water can be used. Of these, one-component or two-component organic solvent-based clear coatings are most suitable, and examples of the resin component used in these clear coatings include acrylic resins, polyester resins, epoxy resins, and urethane resins. These are used in combination with crosslinkable resins such as aminoplast resins, isocyanate resins and acid anhydride group-containing resins, and various types have been put to practical use. For example, before using a one-component acrylic melamine paint composed of a main resin composed of an acrylic resin and a crosslinkable resin composed of an aminoplast resin, a main component composed of an acrylic and / or polyester resin, and a crosslinkable resin composed of a polyisocyanate. Two-component urethane paints that are mixed and used, and one-component acid / epoxy paints composed of a mixture of an oxirane group-containing acrylic resin and / or polyester resin and an acid anhydride group-containing resin have been put into practical use. Among these, from the viewpoints of appearance, acid etching resistance, etc., a one-component type clear composed of a mixture of an oxirane group-containing acrylic resin and / or polyester resin and an acid anhydride group-containing resin is preferable, but is not limited thereto. It is not a thing.

  In the method for forming a multilayer coating film of the present invention, an electrodeposition coating film is formed, and coating is performed on an object to be coated which has been baked and dried. The electrodeposition coating film can be formed by applying a known cationic electrodeposition coating material to an object to be coated and baking and curing under the conditions specified for the electrodeposition coating material to be used. The smoothness of the electrodeposition coating film is an important factor that affects the appearance of the coating film after completion of intermediate coating and top coating, but in the present invention, it is not necessary to limit the smoothness of the electrodeposition coating film. The Ra value of the applicable electrodeposition coating film can be approximately 0.5 (when measured at a cutoff of 2.5) or less.

  Next, an aqueous intermediate coating is applied onto the electrodeposition coating that has been baked and cured to form an intermediate coating. The intermediate coating can be applied by a known method, and can be applied using, for example, a rotary atomizing electrostatic coating machine commonly called a bell electrostatic coating apparatus. The film thickness is not particularly limited, but is adjusted to be 10 to 50 μm, preferably 15 to 25 μm.

  By using the above-mentioned aqueous intermediate coating material in the coating process of the present invention, the above-mentioned viscosity and nonvolatile content can be obtained. The temperature and humidity of the booth where the painting is performed can be adjusted as appropriate, but generally the temperature of the coating booth is kept at 15 to 40 ° C. and the humidity is kept at 40 to 90%. Preferably, the temperature is 20-30 ° C. and the humidity is 60-70%. The applied intermediate coating is set for 2 to 20 minutes in an atmosphere of 5 to 35 ° C. and then pre-dried. If the setting conditions are different from these, it is not preferable because the desired viscosity and non-volatile content of the coating film cannot be obtained.

  Although the preliminary drying method is not particularly limited, it is usually carried out by blowing hot air of 50 to 100 ° C. over the article to be coated. The preliminary drying time is 1 to 10 minutes, but is not limited thereto.

  Next, a water-based base coating and a clear coating are sequentially applied wet-on-wet on the intermediate coating after completion of the preliminary drying to form a base coating / clear coating. The coating method of the water-based base paint and the clear paint can be applied using a rotary atomizing electrostatic coater generally called a bell electrostatic coater. The aqueous base coating film can be applied to a thickness of 10 to 30 μm, and the clear coating film can be applied to a thickness of 10 to 50 μm.

  Further, the intermediate coating, base coating and clear coating are simultaneously baked and cured. Baking is usually performed by heating to a temperature of 110 to 180 ° C., preferably 130 to 160 ° C., and appropriate conditions may be selected depending on the material of the clear coating film to be used.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited only to these Examples. In the examples, “parts” means “parts by weight” unless otherwise specified.

(Production of water-based polyester resin [A])
In a four-necked flask equipped with a stirrer, thermometer, reflux condenser and water separator, 33.0 parts of isophthalic acid, 16.7 parts of adipic acid, 16.5 parts of 1,4-cyclohexanedimethanol, trimethylolpropane 20.9 parts and 18.4 parts of 2-butyl-2-ethyl-1,3-propanediol were blended and heated. Next, the content was heated from 160 ° C. to 230 ° C. over 3 hours, and then kept at 230 ° C., while the condensed water produced was distilled off by a rectifying tower, the acid value was reduced to 3 mgKOH / g or less. The reaction was continued until Next, after adding 5.7 parts of trimellitic anhydride to the product, 4.4 parts of dimethylethanolamine was added for neutralization, and then 144.7 parts of water was added to form an aqueous polyester having a nonvolatile content of 40% by weight. Resin [A] was obtained. The acid value of the aqueous polyester resin [A] was 12.5 mgKOH / g, the glass transition point was 65 ° C., and the average particle size was 100 nm. The inflection point of tan δ was obtained from a chart measured at a frequency of 11 Hz by an acid value by a potassium hydroxide titration method and a glass transition point by a dynamic viscoelasticity measurement method and used as a glass transition point.

(Production of water-based polyester resin [B])
In a four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen gas inlet tube, 20.0 parts of isophthalic acid, 10.0 parts of trimellitic anhydride, 9.3 parts of adipic acid, hexadecenyl succinic anhydride 25.0 parts, 20.8 parts of 1,4-butanediol and 14.9 parts of trimethylolpropane were charged and subjected to esterification reaction at 220 ° C. for 2 hours and then at 190 ° C. for 2 hours. A water-soluble polyester resin having a value of 120 and a number average molecular weight of 1,600 was obtained. The temperature of the resulting resin reaction product was lowered to 100 ° C., 28.0 parts of trimellitic anhydride was added thereto, and then 9 parts of 10% by weight dimethylaminoethanol water, 31 parts of distilled water and 10 parts of butyl cellosolve were added. An aqueous polyester resin [B] having a nonvolatile content of 40% by weight was obtained. The acid value of the aqueous polyester resin [B] was 80 mgKOH / g, the glass transition point was 10 ° C., and the average particle size was 70 nm.

(Production of water-based polyester resin [C])
In a four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen gas inlet tube, 20.0 parts of isophthalic acid, 10.0 parts of trimellitic anhydride, 18.0 parts of adipic acid, 1,4- 14.0 parts of butanediol and 20.0 parts of trimethylolpropane were charged and subjected to an esterification reaction at 220 ° C. for 2 hours and then at 190 ° C. for 2 hours to give an acid value of 100, a hydroxyl value of 60, and a number average molecular weight of 2,600. Water-soluble polyester resin was obtained. The temperature of the obtained resin reaction product was lowered to 100 ° C., and 9 parts of 10% by weight dimethylaminoethanol water, 31 parts of distilled water and 10 parts of butyl cellosolve were added thereto, and an aqueous polyester resin [C] having a nonvolatile content of 40% by weight was added. Got. The acid value of the aqueous polyester resin [C] was 40 mgKOH / g, the glass transition point was 45 ° C., and the average particle size was 100 nm.

(Manufacture of urethane aqueous dispersion resin [D])
In a four-necked flask equipped with a stirrer, thermometer, Dimroth reflux condenser and nitrogen gas inlet tube, 566 parts of polypropylene glycol having a number average molecular weight of 2,000 and polyoxytetramethylene glycol 283 having a number average molecular weight of 1,000 And 85 parts of dimethylolbutanoic acid (DMBA) were charged, and nitrogen gas was passed through, and the temperature was raised to 90 ° C. while stirring at 30 rpm. Subsequently, 345 parts of tetramethylxylylene diisocyanate were added and reacted at 115 ° C. until the isocyanate group content (NCO%) reached 1.8% by weight to obtain 1279 parts of a urethane prepolymer, followed by 217 parts of isopropyl alcohol. And stirred and mixed at 30 rpm. Thereafter, after stirring for 5 minutes, an isopropyl alcohol solution of a linear urethane prepolymer having tertiary aliphatic isocyanate groups at both ends was obtained. Next, in another four-necked flask equipped with a stirrer, a thermometer, a Dimroth type reflux condenser tube and a nitrogen gas introduction tube, 842 parts of water, 11 parts of 25 wt% aqueous ammonia, and 9.0 parts of aminoethylethanolamine were added. The mixture was charged with nitrogen gas and stirred at 25 ° C. Subsequently, 488 parts of the isopropyl alcohol solution of the linear urethane prepolymer was added and reacted while stirring at 35 ° C. for 3 hours to obtain an aqueous urethane aqueous dispersion resin [D] having a nonvolatile content of 31% by weight.

(Preparation of aqueous intermediate coating materials of Examples 1 to 9 and Comparative Examples 1 to 11)
The aqueous polyester resin [A], [B], or [C] obtained as described above was mixed in the ratios shown in Tables 1 and 2 to prepare a total of 100 parts of resin liquid. Further, 10 parts of titanium dioxide and 25 parts of distilled water were mixed to prepare a white pigment paste. Next, the urethane aqueous dispersion resin [D] obtained as described above, aminoplast resin (Cymel 325 (manufactured by Nippon Cytec Industries)), and water-soluble solvent (diethylene glycol mono-n-butyl ether, ethylene glycol mono-n) -Butyl ether or diethylene glycol) were mixed in the proportions shown in Table 1 to produce each water-based intermediate coating.

(Creation of test plate)
Cathodic electrodeposition coating was performed on a zinc phosphate-treated dull steel plate so that the dried coating film had a thickness of 20 μm, followed by heating and curing at 160 ° C. for 30 minutes to prepare an object to be coated. Subsequently, after applying the water-based intermediate coatings of Examples 1 to 9 and Comparative Examples 1 to 11 under the conditions shown in Tables 1 and 2, setting and preliminary drying were performed, followed by water-based base coating (HW- manufactured by DuPont Shinto Automotive Systems). 10-199) and clear paint (HC-10 manufactured by DuPont Shinto Automotive Systems) were applied, and these coating films were simultaneously baked at 140 ° C. for 20 minutes to form a multilayer coating film.

The finish of the multilayer coating film obtained on each steel plate was evaluated by visual appearance and PGD value. The details of the visual appearance and the evaluation method of the PGD value are as follows.
Visual appearance A: Both gloss and clearness are extremely good.
○: Good gloss and sharpness.
X: Low gloss and remarkably lacking in sharpness.
PGD value Based on the method prescribed in ASTM D523-08, the PGD value was measured using a PGD-2 type sharpness glossiness meter manufactured by Japan Color Research Laboratory.

  Tables 1 and 2 show the compositions, properties, application conditions, and evaluation results of the aqueous intermediate coating materials of Examples 1 to 9 and Comparative Examples 1 to 11.

  As is clear from Tables 1 and 2, Examples 1 to 9 were excellent in visual appearance and PGD value, but Comparative Examples 1 to 11 were inferior in these evaluation items.

  According to the method for forming a multilayer coating film of the present invention, in the intermediate top coating wet-on-wet coating method, even when the smoothness of the electrodeposition coating film formed in the lower layer is inferior, the multilayer coating having excellent smoothness. A film can be formed.

Claims (4)

Applying a water-based intermediate coating (step (i)), setting (step (ii)), and pre-drying (step (iii)) on the automobile body on which the electrodeposition coating has been applied and baking has been completed, Subsequently, a water-based base coating is applied, and then a clear coating is applied, and the water-based intermediate coating film, the water-based base coating film and the clear coating film are simultaneously heated and cured to form a multilayer coating film. The aqueous intermediate coating film has a viscosity of 10 to 200 Pa. Immediately after the step (i) under the conditions of a measurement temperature of 23 ° C. and a shear rate of 1.0 sec- ¹ . S and a non-volatile content of 45 to 55% by weight, the conditions are 5 to 35 ° C., and after the step (ii) is performed for 2 to 20 minutes, and immediately before the step (iii), the measurement temperature is 23 ° C., Viscosity of 10 to 1000 Pa. Under a shear rate of 1.0 sec- ¹ . S, The non-volatile content is 46 to 60% by weight.   The method for forming a multilayer coating film according to claim 1, wherein the aqueous intermediate coating material comprises a polyester resin, a urethane resin, an aminoplast resin, a water-soluble solvent having a boiling point of 200 ° C or higher, and a colorant.   The polyester resin comprises (A) an acid value of 20 mgKOH / g or less, a glass transition point of 50 to 80 ° C., an average polyester particle size dispersion resin of 50 to 500 nm, and (B) an acid value of 10 to 90 mgKOH / g, a glass transition point. The method for forming a multilayer coating film according to claim 2, which is a mixture of a water-soluble polyester resin having an average particle diameter of 20 to 100 nm at 0 to 40 ° C.   The method for forming a multilayer coating film according to claim 2 or 3, wherein the water-soluble solvent having a boiling point of 200 ° C or higher is a glycol ether solvent.