A multilayer coating film forming method and a coated product using said method

[Technical Field] [0001]

The present invention concerns a method for forming a multilayer coating film having outstanding long-term corrosion resistance and outstanding chipping resistance and flexibility. More specifically, the invention concerns a multilayer coating film forming method that allows the formation of a coated product having a film coating that is used to form films on automotive underbody components, and not only has outstanding long-term corrosion resistance, but also shows resistance to chipping resulting from rocks, etc. that bounce up while driving, and outstanding flexibility and adhesion with respect to deformation of automobile components. [Prior Art] [0002]

Conventionally, epoxy resin powder coatings have been widely used to coat automotive components requiring long-term durability. Moreover, epoxy resin powder coatings containing zinc powder have been proposed as epoxy resin powder coating compositions for undercoating use showing improved long-term corrosion resistance (cf. Patent Documents 1, 2, and 3) . However, when coated products are obtained by coating using these powder coating compositions or by applying commonly used coatings as overcoatings, this causes problems with the chipping resistance, flexibility, and adhesion of the coating film. [0003] Moreover, a powder coating composition showing improved chipping resistance that blends in polyolefin

powder has been proposed (cf. Patent Document 4) . However, coated products in which this powder coating composition is applied show problems from the standpoint of long-term corrosion resistance of the coating film.

Multilayer coating film forming methods have also been proposed as methods in which epoxy resin powder coatings containing zinc are used as undercoatings or overcoatings for commonly used powder coatings (cf. Patent Documents 5, 6, 7) . However, in these multilayer coating film forming methods, the performance requirements involving long-term durability of automotive components subjected to repeated vibration and stress have not been met . [0004]

[Patent Document 1] Japanese Unexamined Patent Application No. 2001-146567

[Patent Document 2] Japanese Unexamined Patent Application No. 2004-99808

[Patent Document 3] Japanese Unexamined Patent Application No. 2004-189907

[Patent Document 4] Japanese Unexamined Patent Application No. 2004-292573

[Patent Document 5] Japanese Unexamined Patent Application No. H10-113613

[Patent Document 6] Japanese Unexamined Patent Application No. 2005-218998

[Patent Document 7] Japanese Unexamined Patent Application No. 2005-218998

[Presentation of the Invention] [Problems to be Solved by the Invention] [0005]

The purpose of the present invention is to provide a multilayer coating film forming method in order to obtain

a coated material having a coating film showing outstanding long-term corrosion resistance as well as outstanding chipping resistance, flexibility, and adhesion.

[Means for Solving Problems] [0006]

The inventors of the present invention conducted thorough research in an effort to achieve the above purpose, discovering that when a film was formed as an undercoating using a powder coating containing zinc, and a heat-curable powder coating composed of a resin having crosslinkable functional groups, a curing agent capable of reacting with said crosslinkable functional groups, a fibrous filler, and heat-expandable resin particles was applied thereto, by means of this multilayer coating film forming method, a coating film was obtained showing outstanding long-term corrosion resistance, as well as outstanding chipping resistance and flexibility, thereby meeting long-term durability requirements for automotive components subjected to repeated vibration and deformation, thus arriving at the present invention. [0007]

Specifically, the present invention provides a multilayer coating film forming method, characterized by comprising a method in which, with respect to a metal coated material that has a first layer coating film composed of a first powder coating made up of a resin composition containing zinc, a second layer coating film composed of a second powder coating is formed on said material, and characterized in that said second powder coating is a heat-curable powder coating composition composed of a resin containing crosslinkable functional groups that are solid at room temperature (A) , a curing agent capable of reacting with said crosslinkable

functional groups (B) , a fibrous filler (C) , and heat- expandable resin particles (D) . [0008] Moreover, the present invention provides a multilayer coating film forming method, characterized in that the first powder coating contains 50-500 parts by mass of zinc with respect to 100 parts by mass of the resin component . It also provides a multilayer coating film forming method, characterized in that the resin component of the first powder coating is composed of an epoxy resin and a curing agent capable of reacting with its crosslinkable functional groups. Furthermore, it provides a multilayer coating film forming method, characterized in that the curing agent of the first powder coating is at least one substance selected" from among an amine, polyamine, dihydrazide, dicyandiamide, imidazole, phenol resin, carboxyl group- containing polyester resin, and a dibasic acid or acid anhydride . [0009]

The present invention also provides a multilayer coating film forming method, characterized in that the resin composition of the first powder coating contains 1- 50 parts by mass of an extender pigment that has been surface-treated using a silane coupling agent with respect to 100 parts by mass of the resin component.

The present invention also provides a multilayer coating film forming method, characterized in that the first and second powder coatings are applied by means of at least one method selected from the electrostatic powder coating method and the flow immersion coating method. It also provides a coated product, characterized by

having a multilayer coating film obtained by the above multilayer coating film forming methods formed on it. Moreover, the present invention also provides a coated product, characterized in that the product is a helical spring for automotive use.

[Effect of the Invention] [0010] According to the multilayer coating film forming method of the present invention, one achieves the effect of being able to form a multilayer coating film having outstanding corrosion resistance as well as outstanding chipping resistance and flexibility. Moreover, using the multilayer coating film forming method of the present invention as a coating for automotive underbody components has the effect of preventing rust due to chipping and peeling caused by rocks that bounce up during driving in cold areas in which snow melting agents such as rock salt are used, thus making it possible to protect the lower components of the automobile body over a long period of time.

[Optimum Mode of Embodiment of the Invention] [0011]

The following is an explanation of the composite film forming method of the present invention in greater detail.

The first powder coating is a powder material that forms a film containing zinc, and it is applied to the metal coated product before the second powder coating.

As an example of metal coated products, one can mention metal automotive components, and these should preferably be subjected in advance to surface treatments such as degreasing and chemical conversion treatment .

In the first powder coating, the resin composition contains zinc as its essential component, and an extender pigment should also preferably be added as a coupling agent. As necessary, moreover, one may add coloring pigments, stabilizers, matting agents, defoaming agents, leveling agents, thixotropic agents, ultraviolet absorbers, surface control agents, curing accelerators, dispersants, viscosity control agents, waxes, etc. in order to obtain an optimum heat-curable powder coating. [0012]

The resin used in the resin composition contains crosslinkable functional groups that are solid at room temperature, and it is therefore in a solid state at room temperature (25 ⁰ C) . Preferably, its softening point should be 160 ⁰ C, with a softening point of 150 ⁰ C being even more preferable. The lower limit is ordinarily 60 °C. There are no particular restrictions on this resin, provided that it is a resin for use as a heat-curable powder coating conventionally used in prior art.

For example, one may use a basic resin such as epoxy resin containing glycidyl groups, acrylic resin, or polyester resin containing hydroxyl groups in combination with a curing agent such as an amine, acid, or block isocyanate capable of reacting with said functional groups, with powder coatings containing epoxy resin as a basic resin being optimum examples. [0013]

Examples of this epoxy resin include aliphatic epoxy resins such as bisphenol A epoxy resin, bisphenol F epoxy resin, phenol novolac or cresol novolac epoxy resin, cyclic epoxy resin, hydrogenated bisphenol A or AD epoxy resin, propylene glycol diglycidyl ether, pentaerythritol polyglucidyl ether, epoxy resins obtained from aliphatic or aromatic carboxylic acids and epichlorohydrin, epoxy

resins obtained from aliphatic or aromatic amines and epichlorohydrin, heterocyclic epoxy resins, spiro ring- containing epoxy resins, and epoxy modified resins. As needed, one may blend liquid epoxy resins into the epoxy resin within a range that does not cause the composition obtained to undergo blocking. The epoxy equivalent amount of said epoxy resin should be 150-3000 g/eq, and preferably 170-2500 g/eq, with an amount of 200-2000 g/eq being particularly preferred. [0014]

Moreover, examples of said epoxy resin curing agent include amines, polyamide, dycyandiamide, hydrazide, imidazole, phenol, carboxyl group-containing polyester resin, amide imide, dibasic acids, and anhydrides, with dihydride adipate, dicyandiamide, phenol resin, carboxyl group-containing polyester resin, and dibasic acids being preferred.

There are no particular restrictions on the carboxyl group-containing polyester resin, with specific examples including a polyester resin containing 2 or more carboxylic acid groups per molecule, and examples include an acid constituent having as its main component a polyvalent carboxylic acid and an alcohol constituent having as its main component a polyhydric alcohol as raw materials used in condensation polymerization by a common method. [0015]

There are no particular restrictions on the aforementioned acid components, with examples including aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and their anhydrides, 2 , 6-naphthalene dicarboxylic acid, and 2 , 7-naphthalene dicarboxylic acid and their anhydrides, saturated

aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid, alicyclic dicarboxylic acids such as 1,4-dichlorohexane dicarboxylic acid and their anhydrides, lactones such as γ-butyrolactone and ε-caprolactone, aromatic oxymonocarboxylic acids such as p-hydroxyethoxy benzoic acid and hydroxycarboxylic acids corresponding thereto. The acidic component may be used either individually or in combinations of 2 or more . [0016]

There are no particular restrictions on the aforementioned alcohol component, with examples including aliphatic glycols having a side chain such as ethylene glycol, 1,3 -propane diol, 1,4 -butane diol, 1,5-pentane diol, 1,5-hexane diol, diethylene glycol, triethylene glycol, 1, 4-cyclohexane diol, 1, 4-cyclohexane dimethanol, bisphenol A-alkylene oxide adducts, bisphenol S-alkylene oxide adducts, 1,2-propane diol, neopentyl glycol, 1,2- butane diol, 1,3 -butane diol, 1,2-pentane diol, 2,3- pentane diol, 1,4-pentane diol, 1,4-hexane diol, 2,5- hexane diol, 3 -methyl-1, 5-pentane diol, 1,2-dodecane diol, and 1, 2-octadecane diol, and polyvalent alcohols having a valence of 3 or above such as trimethylol propane, glycerin, and pentaerythritol . These alcohol components may be used either individually or in combinations of 2 or more. [0017]

The number-average molecular weight of the aforementioned carboxyl group containing polyester resin should be 1500-6000, with a figure of 2000-5000 being even more preferable. If the aforementioned number- average molecular weight is less than 1500, the performance of the coating film obtained will decrease, and on the other hand, if it exceeds 6000, the smoothness

of the coating film obtained will be decreased. [0018]

From the standpoints of blocking resistance and external appearance of the film obtained, the glass transition temperature (Tg) of the aforementioned carboxyl group-containing polyester resin should be 35- 100 ⁰ C, and preferably 50-70°C. The glass transition temperature of the present invention may be determined by using a differential scanning calorimeter (DSC) . The curing agent contained in the first powder coating may be used individually or in combinations of 2 or more.

Looking at the ratio of the curing agent used, it should be 0.5-1.5 eq, based on equivalents of the functional groups in the curing agent, per 1 eq of the functional groups in the resin containing crosslinkable functional groups that are solid at room temperature, with an amount of 0.7-1.2 eq being preferred. [0019] The average particle diameter of the zinc powder used in the first powder coating is 1-20 μm, and preferably 3-10 μm. The average particle diameter of the zinc powder can be measured using a microtrack particle size distribution indicator. The zinc content should be 50-500 parts by mass, and preferably 100-300 parts by mass, with respect to 100 parts by mass of the resin composition.

Examples of extender pigments include talc, alumina powder, mica powder, metal silicate, molten silica, silicon dioxide, feldspar, wet silica, dry silica, barium sulfate powder, diatomaceous earth, titanate salt, quartz, and potassium carbonate. Particularly preferred extender pigments are metal silicate, wet type silica, dry type silica, and calcium carbonate. Moreover, it is preferable to treat the surface of said extender pigment

with a silane coupling agent. Examples of said silane coupling agents include titanate coupling agents and aluminate coupling agents. [0020]

The amount of the coupling agent used should be 0.1- 10 parts by mass, and preferably 0.5-5 parts by mass, with respect to 100 parts by mass of the extender pigment . In this coupling agent, the extender pigment should be surface-treated according to a method known in the art.

The amount of the extender pigment used here should be 1-50 parts by mass of the extender pigment subjected to surface treatment with a silane coupling agent with respect to 100 parts by mass of the resin composition, with an amount of 10-40 parts by mass being particularly preferred. [0021] Moreover, there are no particular restrictions on the aforementioned coloring pigments, with examples including titanium dioxide, carbon black, graphite, iron oxide, lead oxide, chrome yellow, phthalocyanine blue, phthalocyanine green, quinacridone, perilene, aluminum powder, alumina powder, bronze powder, copper powder, tin powder, mica, and natural and synthetic mica. [0022]

The second powder coating is a heat-curable powder coating composition containing a resin having crosslinkable functional groups, a curing agent that reacts with these groups, a fibrous filler, and heat- expandable resin particles.

Moreover, an optimum heat-curable powder coating can be obtained by including extender pigments, coloring pigments, stabilizers, matting agents, defoaming agents,

leveling agents, thixotropic agents, ultraviolet absorbers, surface control agents, curing accelerators, dispersants, viscosity control agents, waxes, etc. in order to obtain an optimum heat-curable powder coating. [0023]

The resin (A) containing crosslinkable functional groups that are solid at room temperature used in the heat-curable powder coating composition of the second powder coating is in a solid state at room temperature

(25°C) . Preferably, it has a softening point of 160 ⁰ C or below, with a point of 150° C or below being particularly preferred. The lower limit is 60 °C. If the softening point exceeds 160 °C, the external appearance of the film will be impaired, and if the softening point is less than

60 °C, the storage stability of the powder coating

(blocking resistance) will be insufficient. There are no particular restrictions on this resin for powder coating use, provided that it is a resin for use in heat-curable powder coatings commonly used in prior art . As representative resins having crosslinkable functional groups, epoxy resins, polyester resins, and acrylic resins can be mentioned, with epoxy resin being particularly preferred. [0024]

Examples of this epoxy resin include aliphatic epoxy resins such as bisphenol A epoxy resin, bisphenol F epoxy resin, phenol novolac or cresol novolac epoxy resin, cyclic epoxy resin, hydrogenated bisphenol A or AD epoxy resin, propylene glycol diglycidyl ether, pentaerythritol polyglucidyl ether, epoxy resins obtained from aliphatic or aromatic carboxylic acids and epichlorohydrin, epoxy resins obtained from aliphatic or aromatic amines and epichlorohydrin, heterocyclic epoxy resins, spiro ring- containing epoxy resins, and epoxy modified resins.

[0025]

As needed, one may blend in liquid epoxy resins with the epoxy resin in a range such that the composition obtained will not undergo blocking during storage. The epoxy equivalent of said epoxy resin should be 150-3000 g/eq, and preferably 170-2500 g/eq, with a figure of 200- 2000 g/eq being particularly preferred. [0026] As the epoxy resin, a polymer microparticle dispersion-type epoxy resin having a core-shell structure, in which polymer microparticles having a core- shell structure are dispersed in the epoxy resin, is preferred. By evenly dispersing polymer microparticles having a core-shell structure in the epoxy resin, one can further impart the properties of high adhesion, low internal stress, and durability to the heat-curable powder coating composition. In particular, this contributes toward improving chipping resistance at low temperatures . By first dispersing in the epoxy resin polymer microparticles having a core-shell structure, the above properties can be more easily achieved, as one obtains more uniform dispersibility than in cases where polymer microparticles having a core-shell structure are added as is to the powder coating composition during manufacturing thereof . [0027]

As an example of polymer microparticles having a core-shell structure, one can mention polymer microparticles having a core-shell structure composed of a rubber core layer and a hardened shell layer. The average particle diameter of the polymer particles having a core-shell structure should preferably be 0.1-1 μm.

An example of a rubber material composed of a core layer is a copolymer of glycidyl group-containing

ethylenically unsaturated monomers and other ethylenically unsaturated monomers.

Moreover, an example of hard substances having shell structures include a copolymer of a hydroxyl group containing ethylene unsaturated monomer and other ethylene unsaturated monomers and a copolymer composed of carboxylic group-containing ethylene unsaturated monomers and other ethylene unsaturated monomers. The amount of the polymer particles having a core- shell structure in 100 parts by mass of a polymer microparticle dispersion-type epoxy resin having a core- shell structure should be 1-50 parts by mass, and preferably 5-40 parts by mass, with a content of 10-20 parts by mass being particularly preferred. Examples of a commercial product of this type of polymer microparticle dispersion-type epoxy resin having a core-shell structure include Epotohto YR-628 and YR-693, manufactured by Tohto Kasei Co., Ltd., etc. [0028]

Furthermore, the content ratio of the polymer microparticles having a core-shell structure in the total amount of the epoxy resin should be 1-50 parts by mass with respect to 100 parts by mass of the total epoxy resin, and preferably 1.5-30 parts by mass, with an amount of 2-20 parts by mass being particularly preferred, and an amount of 3-20 parts by mass being even more preferred.

Examples of the curing agent (B) used in the heat- curable powder coating composition of the second powder coating include curing agents such as polyester resins containing amines, polyamide, dicyandiamide, hydrazide, imidazole, phenol, and carboxyl groups, amidoimides, dibasic acids, and anhydrides, with dihydrazide adipate, dicyandiamide, phenol resin, carboxyl group-containing

polyester resin, and dihydrochloric acid, etc., being preferred, and dihydrazide adipate, dicyandiamide, and phenol resin are particularly preferred. [0029]

Moreover, there are no particular restrictions on the carboxyl group-containing polyester resin, with specific examples including a polyester resin having 2 or more carboxylic acid groups per molecule, such as resins obtained by condensation polymerization according to the usual method using an acid constituent having a polyvalent carboxylic acid as its main component and an alcohol constituent having a polyhydric alcohol as its main component as raw materials. [0030]

There are no particular restrictions on the aforementioned acid components, with examples including aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and their anhydrides, 2, 6-naphthalene dicarboxylic acid, and 2 , 7-naphthalene dicarboxylic acid and their anhydrides, saturated aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid, and dodecane dicarboxylic acid and their anhydrides, alicyclic dicarboxylic acids such as 1, 4-dichlorohexane dicarboxylic acid and their anhydrides, lactones such as γ-butyrolactone and ε-caprolactone, aromatic oxymonocarboxylic acids such as p-hydroxyethoxy benzoic acid and hydroxycarboxylic acids corresponding thereto. The acidic component may be used either individually or in combinations of 2 or more. [0031]

There are no particular restrictions on the aforementioned alcohol component, with examples including aliphatic glycols having a side chain such as ethylene

glycol, 1,3-propane diol, 1,4-butane diol, 1,5-pentane diol, 1,5-hexane diol, diethylene glycol, triethylene glycol, 1, 4-cyclohexane diol, 1,4-cyclohexane dimethanol, bisphenol A-alkylene oxide adducts, bisphenol S-alkylene oxide adducts, 1,2-propane diol, neopentyl glycol, 1,2- butane diol, 1,3 -butane diol, 1,2-pentane diol, 2,3- pentane diol, 1,4-pentane diol, 1,4-hexane diol, 2,5- hexane diol, 3 -methyl- 1, 5-pentane diol, 1,2-dodecane diol, and 1, 2-octadecane diol, and polyvalent alcohols having a valence of 3 or above such as trimethylol propane, glycerin, and pentaerythritol . These alcohol components may be used either individually or in combinations of 2 or more. [0032]

The number-average molecular weight of the aforementioned carboxyl group-containing polyester resin should be 1500-6000. A number-average molecular weight of 2000-5000 is even more preferable. If the aforementioned number-average molecular weight is less than 1500, the performance of the coating film obtained will decrease, causing problems with storage stability of the powder coating. On the other hand, if the number-average molecular weight exceeds 6000, the smoothness of the coating film obtained will be decreased.

From the standpoints of blocking resistance and external appearance of the film obtained, the glass transition temperature (Tg) of the aforementioned carboxyl group-containing polyester resin should be 35- 100 ⁰ C, and preferably 50- 70 "C. The glass transition temperature of the present invention may be determined by using a differential scanning calorimeter (DSC) . [0033]

The curing agent contained in the heat-curable powder coating composition of the second powder coating

may be used individually or in combinations of 2 or more.

The amount of the curing agent used should be 0.5-1.5 eq of the functional groups of the curing agent per 1 eq of the functional groups of the resin containing the crosslinkable functional groups that are solid at room temperature of component (A), and preferably 0.7-1.2 eq.

As the fibrous filler (C) used in the heat-curable powder coating composition of the second powder coating, one may use a fibrous filler when the aspect ratio of 5- 500, and preferably 10-250, with a ratio of 10-100 being even more preferable. The term "aspect ratio" used here refers to the ratio of average fiber length to average fiber diameter (D) of the fibrous filler (L/D) . [0034]

If the aspect ratio is less than 5, sufficient chipping resistance will not be seen, and in order to prevent this, the amount of the filler added must be markedly increased. Moreover, if the aspect ratio exceeds 500, it becomes impossible to achieve uniform dispersion, there is a tendency for the external appearance of the film to decrease,, and long-term corrosion resistance also decreases. The average fiber diameter and average fiber length of the fibrous filler can be measured using an optical microscope equipped with a micrometer eyepiece, the average fiber diameter should be 1-20 μm, with a diameter of 3-15 μm being particularly preferred. The average fiber length should be 50-300 μm, with a length of 100-200 μm being particularly preferred. [0035]

There are no particular limits on the fibrous filler, provided that it is composed of an insulator, with examples including inorganic fibrous fillers and organic fibrous fillers. Specific examples of inorganic fibrous fillers (inorganic compounds) include calcium

metasilicate, potassium titaπate, magnesium sulfate, sepiolite, zonolite, aluminum borate, rock wool, and glass fibers. Moreover, specific examples of organic fibrous fillers (organic compounds) include polyoxybenzoyl (PO30B) , polyoxynaphthoyl (PON) , polyacrylonitrile fibers, aramid fibers, etc. The fibrous filler may be used individually or in combinations or 2 or more . [0036]

The content of the fibrous filler (C) should be within the range of 1-100 parts by mass with respect to a total of 100 parts by mass of the resin containing crosslinkable functional groups that are solid at room temperature (A) and the curing agent capable of reacting with said crosslinkable functional groups (B) . If the amount is less than 1 part by mass, the improvement in chipping resistance will not be sufficient. Moreover, if it exceeds 100 parts by mass, the external appearance of the film will be impaired, and its long-term corrosion resistance will decrease. It is particularly preferable to add an amount of 5-50 parts by mass of the fibrous filler.

In order to maximize the effect of the fibrous filler (C) an effective means is coupling treatment of the filler interface, particularly in the case of inorganic fibrous fillers. Examples of coupling agents include silane coupling agents, titanate coupling agents, and aluminate coupling agents. In the case of organic fibrous fillers, treatments such as plasma treatment are preferred. [0037]

As an example of the heat-expandable resin particles (D) used in the heat-curable powder coating composition of the second powder coating, one can mention

microspheres composed of a thermoplastic resin shell enclosing a liquefied gas, which are characterized by the fact that when they are heated, the gas pressure inside the shell increases, the thermoplastic resin shell softens and expands, and hollow spherical particles are formed. The average particle diameter of the heat- expandable resin particles (D) should be 5-30 μm. Moreover, the volume of the heat-expandable resin particles (D) after expansion should preferably be increased by a factor of 30-150.

Examples of commercial heat-expandable resin particles (D) include Expancel 092DU40, Expancel 092DU80, and Expancel 009DU80, manufactured by Japan Fillite Co., Ltd., and M520 and M520D microspheres manufactured by Dainichiseika Color and Chemicals Mfg. Co., Ltd. [0038]

The heat-expandable resin particles (D) may be used individually or in combinations of 2 or more. The content of the heat-expandable resin particles (D) should be within the range of 0.1-20 parts by mass with respect to a total of 100 parts by mass of the resin containing crosslinkable functional groups that are solid at room temperature (A) and the curing agent capable of reacting with said crosslinkable functional groups (B) . A particularly preferable content of the heat-expandable resin particles (D) is 0.5-15 parts by mass. If the content of the heat-expandable resin particles (D) is less than 0.1 part by mass, the improvement in chipping resistance will be insufficient. Moreover, if it exceeds 20 parts by mass, too many hollow portions will be formed inside the coating film, conversely reducing its chipping resistance. [0039] In order to enhance the effect of the heat-expandable

resin particles, prefoamed organic hollow resin particles and inorganic hollow particles (hollow balloons) may be included in the heat-curable coating composition of the second powder coating.

Examples of such hollow particles include polyacrylonitrile resin-type hollow particles, phenol resin-type hollow particles, and silica resin-type hollow particles. The heat-curable powder coating composition of the second powder coating may also contain plasticizers, coloring pigments, thermal stabilizers, optical stabilizers, matting agents, defoaming agents, leveling agents, thixotropic agents, ultraviolet absorbers, surface control agents, curing accelerators, dispersants, viscosity control agents, antistatic agents, waxes, etc. [0040]

There are no particular restrictions on the aforementioned coloring pigments, with examples including titanium dioxide, carbon black, graphite, iron oxide, lead oxide, chrome yellow, phthalocyanine blue, phthalocyanine green, quinacridone , perilene, aluminum powder, alumina powder, bronze powder, copper powder, tin powder, mica, and natural and synthetic mica. [0041]

In order to control the heat-curable powder coating composition of the second powder coating, one may carry out manufacturing by the so-called dry method using melt kneaders such as hot rollers or extruders or by the so- called wet method, which involves melt dispersion in a solvent, followed by removal of the solvent by vacuum distillation or thin film distillation and pulverization.

The heat-curable powder coating composition of the present invention may be obtained by any method commonly known in the art, such as the electrostatic coating

method or the flow immersion method to obtain a coating film thickness on the surface of the coated object of 50- 800 μm, and preferably 100-400 μm, and by carrying out baking, ordinarily at a temperature of 140-180 ⁰ C for a period of 5 minutes to 2 hours, one can obtain a sufficiently cured foamed film. [Working Examples] [0042] We will now explain the invention in detail by means of working examples, but the invention is by no means limited by these examples.

(Manufacturing Examples P-I through 14, S-I through 14) Heat-curable powder coating mixtures having the compositions shown in Tables 1 and 5 were uniformly mixed for one minute in a dry blender (product name: Henschel mixer, manufactured by Mitsui Mining Co., Ltd), after which melt kneading was carried out at a temperature of 80-100 ⁰ C in an extrusion kneader (product name: Busco Kneader PR46, manufactured by Coperion Corp,) and after cooling, the product was pulverized into fine particles using a hammer-type impact pulverizer. After this, it was filtered through a 150-mesh screen to obtain the first undercoating powder coatings P-I through P-14 and the second overcoating powder coatings S-I through S-14. [0043]

(Working Examples 1-14, Comparison Examples 1-4) First, the first undercoating powder coatings P-I through P-14 obtained were applied in a film thickness of 50-200 μm to a soft steel plate with a thickness of 2.3 mm subjected to lead phosphite treatment by means of electrostatic coating with a charge of -80 KV, and after baking for 20 minutes at 160 ⁰ C, this was taken as the first undercoating powder film.

Next, with the compositions shown in Tables 6-8, the second overcoating powder coatings S-I through S-14 were applied in a film thickness of 200-400 μm by means of electrostatic application with a charge of -80 KV to the first undercoating powder film, and baking was carried out for 20 minutes at 160 ⁰ C to obtain a multilayer coating film.

Performance evaluation was then carried out by means of Working Examples 1-14 and Comparison Examples 1-4.

Tables 6-8 show evaluation results for the multilayer coating film compositions. [0044]

The performance evaluations of the films shown in the table were conducted as follows .

(1) Adhesion (according to JIS K5600 5-6)

100 notches were made in the coated surface using a knife at intervals of 1 mm, cellophane tape was applied to the surface and then vigorously peeled off, and the number of remaining pieces of coating film was counted and evaluated.

O: Number of remaining pieces of coating film after tape peeling is 100/100.

δ: Number of remaining pieces of coating film after tape peeling is 70-99/100. x: Number of remaining pieces of coating film after tape peeling is 69 or less/100. [0045]

(2) Impact resistance (according to JIS K5600 5-3) The test piece was positioned with the coated surface facing upward, a 500 g weight was dropped onto it from a height of 50 cm, and the extent of cracking of the film was evaluated. O: No cracking δ: Slight cracking

x: Pronounced cracking [0046]

(3) Saltwater-spray resistance (according to JIS K5600 7-1)

A coated plate crosscut in advance was placed for 960 hours in a saltwater-spray testing unit under conditions of 35 °C and 5% NaCl, and after removal, the width of unilateral swelling from the crosscut surface and the width of unilateral peeling caused by cellophane tape were evaluated.

•: Width of unilateral swelling and peeling is 1 mm or less.

0: Width of unilateral swelling and peeling is 1-3 mm.

δ: Width of unilateral swelling and peeling is 3-5 mm. x: Width of unilateral swelling and peeling exceeds 5 mm.

[0047]

(4) Moisture resistance (according to JIS K5600 7-2) A coated plate was placed for 960 hours in a moisture-resistant testing unit under conditions of 50 ⁰ C and 95% RH, and the adhesion of the material to the film was evaluated based on the number of remaining pieces of coating film using cellophane tape.

0 : Number of remaining pieces of coating film after tape peeling is 100/100. δ : Number of remaining pieces of coating film after tape peeling is 70-99/100. x : Number of remaining pieces of coating film after tape peeling is 69 or less/100. [0048] (5) Low-temperature chipping resistance

A coated test piece was placed for 6 hours or more in a low temperature, constant temperature unit at -30 ⁰ C, chipping was carried out using a gravelometer, and the extent of peeling was evaluated. Chipping was carried out with No. 6 crushed stone (200 g) at an air pressure of 0.5 MPa.

*: No peeling reaching the substrate.

•: Peeling reaching the substrate, with peeling area of 1 mm ² or less .

O: Peeling reaching the substrate, with peeling area greater than 1 mm ² and less than 3 mm ² .

δ: Peeling reaching the substrate, with peeling area greater than 3 mm ² and less than 10 mm ² . x: Peeling reaching the substrate, with peeling area exceeding 10 mm ² .

[0049]

[Table 1]

[0050]

1) Product name: manufactured by Japan Epoxy Resin Co., epoxy resin, epoxy equivalent 925 g/eg, softening point 97°C

2) Product name: manufactured by Japan Epoxy Resin Co., epoxy resin, 750 g/eq, softening point 89°C

3) Product name: manufactured by Japan Epoxy Resin Co., phenol resin, phenolic OH 4.0 meq/g 4) Product name: manufactured by Shikoku Kasei K. K., 2-methylimidazole, melting point 137 ⁰ C

5) Product name: 2-heptadecylimidazole, melting point 86 ⁰ C

6) Product name: manufactured by Nippon Paint

Corrosion Coatings K. K., zinc powder, average particle diameter 7-8 μm

7) Product name: manufactured by Honso Chemical K. K., zinc powder, average particle diameter 7-8 μm

8) Product name: manufactured by Nyco Minerals, Inc., calcium metasilicate

9) Product name: manufactured by Illinois Minerals, Inc . , silica

10) Product name: manufactured by Mitsui Chemicals, Inc., acrylic surface control agent

11) Product name: manufactured by Monsanto, acrylic surface control agent

[0051]

[Table 2]

[ 0052 ] [Table 3 ]

[0053]

12) Product name: manufactured by Tohto Kasei Co., Ltd. , polymer microparticle dispersion-type epoxy resin having a core-shell structure, epoxy equivalent 910 g/eg, softening point 97 °C, average particle diameter of polymer microparticles having a core-shell structure of 0.5 μm, content of polymer micropores having core-shell structure 12.5% by mass

13) Product name: manufactured by Ube Industries, Ltd., 1,10-dodecane dicarboxylic acid

14) Product name: manufactured by DSM, carboxyl group-containing polyester resin, acid value 85 mg/KOH/g, number-average molecular weight 2200, glass transition

temperature 71 ⁰ C.

15) Product name: manufactured by Taiheyo Material Corp., rock wool, average fiber length 135 μm, average particle diameter 5 μm, aspect ratio 27

16) Product name: manufactured by Nyco Minerals, Inc., calcium metasilicate, average fiber length 156 μm, average fiber diameter 12 μm, aspect ratio 13

17) Product name: manufactured by Dainichiseika Color and Chemicals Mfg. Co., Ltd., heat-expandable resin beads, particle diameter 14 μm

18) Product name: manufactured by Japan Fillite Co., Ltd., heat-expandable resin beads, average particle diameter 13 μm 19) Product name: manufactured by Aj inomoto Fine- Techno Co., Inc., amine adduct curing accelerator

[0054]

[Table 4]

[0055]

[Table 5]

[0056]

[Table 6]

[0057] [Table 7]

[0058]

[Table 8]

[0059] As shown in Table 6 and Table I ₁ in Working Examples 1-14, multilayer coating films were obtained showing outstanding adhesion, moisture resistance, and corrosion resistance, and chipping resistance was also favorable. In particular, the materials of Working Examples 5 and 13, polymer microparticles dispersion-type epoxy resins having a core-shell structure at a specified ratio, showed highly outstanding chipping resistance.

On the other hand, as shown in Table 8, in Comparison Example 1, the amount of the zinc powder blended in as an undercoating was less than 50 parts by

mass with respect to a total of 100 parts by mass of the resin and the curing agent capable of reacting with its crosslinkable functional groups, and the overcoating did not have fibrous filler mixed in, so saltwater-spray resistance was poor, and major peeling was observed in the chipping resistance test. In Comparison Example 2, the amount of zinc powder blended in as an undercoating was 500 parts or more by mass with respect to a total of 100 parts by mass of the resin and the curing agent capable of reacting with its crosslinkable functional groups, and this was an example in which fibrous filler was not mixed in with the overcoating; although saltwater-spray resistance was favorable, impact resistance was poor, and in chipping resistance, major peeling was observed. In Comparison Example 3, the amount of the moisture-resistant pigment surface treated with a silane coupling agent of the undercoating was 50 parts by mass or more with respect to a total of 100 parts by mass of the resin and the coupling agent capable of reacting with its crosslinkable functional groups, and this was a case in which heat-expandable resin particles were blended in with the overcoating, moisture resistance was poor, and in chipping resistance, major peeling was observed. In Comparison Example 4, the amount of the moisture-resistant pigment surface treated with a silane coupling agent mixed into the undercoating was less than 1 part by mass with respect to a total of 100 parts by mass of the resin and the coupling agent capable of reacting with its crosslinkable functional groups, and this was a case in which no heat-expandable resin particles were mixed in with the overcoating, so adhesion and moisture resistance were poor, and insufficient chipping resistance was achieved.