JP2009062468A - Powder coating composition - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a powder coating composition capable of forming a paint film that satisfies both chemical resistance and flexibility in a satisfiable level when applied to inside coating of drum. <P>SOLUTION: The powder coating composition comprises an o-cresol novolac-type resin (A), a bisphenol-A-type epoxy resin (B), and at least one sort of curing agent selected from the group consisting of a phenolic curing agent, amine-based curing agent, hydrazide-based curing agent, and carboxylic acid-based curing agent, and the compounding ratio [(A)/(B):weight ratio] of the o-cresol novolac-type resin (A) and the bisphenol-A-type epoxy resin (B) is 95/5 to 60/40. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a powder coating composition. Specifically, it is related with the powder coating composition used suitably for the inner surface coating of metal containers.

  A metal container such as a drum can is stored for a long period of time after filling and transporting various contents such as monomers, surfactants, resins, and their aqueous solutions and foods. Therefore, the inner surface of such a container is required to have high chemical resistance. On the other hand, it is known to impart chemical resistance by coating the inner surface of the container and covering it with a coating film. In order to impart such chemical resistance, it is necessary to increase the mechanical strength of the coating film. On the other hand, the coating film to be formed needs to have sufficient flexibility so as not to be broken by bending or impact / deformation (for example, formation of irregularities) of the painted surface itself. That is, it is required that the paint used for the inner surface coating of the container be formed with a coating film having both high chemical resistance and sufficient flexibility.

However, a conventionally known powder coating material (for example, Patent Document 1) can form a highly flexible coating film, but has insufficient chemical resistance, and therefore has both chemical resistance and flexibility. It is impossible to achieve both at a satisfactory level. Moreover, there was no example which used the powder coating for the inner surface coating of such a metal container.
JP-A-8-323288

  The present invention has been made to solve the above-mentioned problems, and the object of the present invention is to satisfy the chemical resistance and the flexibility when used for the inner surface coating of metal containers. The object is to provide a powder coating composition capable of forming a compatible coating film.

  That is, the powder coating composition of the present invention comprises an o-cresol novolac type epoxy resin (A), a bisphenol A type epoxy resin (B), a phenolic curing agent, an amine curing agent, a hydrazide curing agent, and And a blending ratio of the o-cresol novolac type epoxy resin (A) and the bisphenol A type epoxy resin (B) [(a). A) / (B): weight ratio] is 95/5 to 60/40.

  In a preferred embodiment, the curing agent is a bisphenol A type phenolic curing agent (excluding those having an epoxy group).

  In a preferred embodiment, the powder coating composition further includes an inorganic filler.

  In a preferred embodiment, the powder coating composition comprises the inorganic filler at a pigment volume concentration of 2.3 to 8.9%.

  In a preferred embodiment, the powder coating composition further includes a curing accelerator.

  In preferable embodiment, the said powder coating composition is used for the inner surface coating of metal containers.

  According to another aspect of the present invention, a metal container is provided. The metal container has the powder coating composition coated on the inner surface.

  In a preferred embodiment, the metal container is a steel drum.

  According to still another aspect of the present invention, a coating method is provided. The coating method includes a step of applying the powder coating composition to the inner surface of the metal container and a step of heating the inner surface of the metal container to which the powder coating composition is applied.

  According to the powder coating composition of the present invention, since it contains o-cresol novolac type epoxy resin as a main resin component and bisphenol A type epoxy resin as a sub resin component, it has high chemical resistance and sufficient flexibility. It is possible to form a coating film that achieves both.

A. Powder coating composition The powder coating composition of the present invention comprises an o-cresol novolac epoxy resin (A), a bisphenol A epoxy resin (B), a phenolic curing agent, an amine curing agent, and a hydrazide curing. And at least one curing agent selected from the group consisting of a carboxylic acid curing agent, and a blend of the o-cresol novolac epoxy resin (A) and the bisphenol A epoxy resin (B) The ratio [(A) / (B): weight ratio] is 95/5 to 60/40. Preferably, the powder coating composition of the present invention may further contain an inorganic filler and / or a curing accelerator. Hereinafter, the present invention will be described in detail.

A-1. o-Cresol Novolac Type Epoxy Resin As the o-cresol novolac type epoxy resin, any appropriate one can be used. Preferably, a resin that is solid at room temperature is used. When using a resin that is not solid at room temperature, there may be a problem that fusion between powder particles is likely to occur during storage of the powder coating composition, and it may not be solid at room temperature and may not form a powder coating body. . In the present specification, “solid at room temperature” means, for example, solid at a temperature range of less than 50 ° C., preferably a softening point of 60 ° C. or higher, and more preferably a softening point of 60 It means that it is ℃ -128 ℃.

  As said o-cresol novolak-type epoxy resin, what has 1.5 or more epoxy groups in 1 molecule is preferable.

  The o-cresol novolak type epoxy resin can be obtained, for example, by reacting o-cresol novolak, which is a reaction product of o-cresol and formaldehyde, with an epihalohydrin such as epichlorohydrin.

  A commercial product may be used as the o-cresol novolac type epoxy resin. Examples of commercially available products include Epototo YDCN-701 (epoxy equivalent of 195 to 220 g / eq, softening point of about 60 to 70 ° C., manufactured by Tohto Kasei Co., Ltd.), Epototo YDCN-702 (epoxy equivalent of 195 to 220 g / eq, softening point of about 70 to 80 ° C., manufactured by Toto Kasei Co., Ltd., Epototo YDCN-703 (epoxy equivalent 195 to 220 g / eq, softening point of about 75 to 85 ° C., manufactured by Toto Kasei Co., Ltd.), Epototo YDCN-704 (epoxy equivalent 195 to 220 g / eq) , Softening point of about 85 to 95 ° C, manufactured by Tohto Kasei Co., Ltd.), Epicoat 180S65 (epoxy equivalent of 205 to 220 g / eq, softening point of about 67 ° C, manufactured by Japan Epoxy Resin Co., Ltd.) These may be used alone or in combination of two or more.

A-2. Bisphenol A-type epoxy resin Any appropriate resin can be used as the bisphenol A-type epoxy resin. Preferably, a resin that is solid at room temperature is used. When a resin that is not solid at room temperature is used, there may be a problem that fusion between powder particles is likely to occur during storage of the powder coating composition, or it may not be solid at room temperature and may not form a powder coating body. .

  As said bisphenol A type epoxy resin, what has 1.5 or more epoxy groups in 1 molecule is preferable.

  The bisphenol A type epoxy resin is prepared by, for example, reacting bisphenol A [2,2-bis (4-hydroxyphenyl) propane] with an epihalohydrin such as epichlorohydrin to produce a low molecular weight epoxy resin, and then further bisphenol. It can be obtained by a two-stage method in which A is subjected to addition polymerization to adjust to a desired molecular weight.

  A commercial product may be used as the bisphenol A type epoxy resin. Examples of commercially available products include Epototo YD-014 (epoxy equivalent 900-1000 g / eq, softening point 91-102 ° C., manufactured by Tohto Kasei Co., Ltd.), Epototo YD-017 (epoxy equivalent 1750-2100 g / eq, softening point 117- 127 ° C, manufactured by Tohto Kasei Co., Ltd.), Epototo YD-904 (epoxy equivalent 900-1000 g / eq, softening point 96-107 ° C, manufactured by Toto Kasei Co., Ltd.), Epototo YD-907 (epoxy equivalent 1300-1700 g / eq, softening point) 117-127 ° C., manufactured by Tohto Kasei Co., Ltd.), Epicoat 1003F (epoxy equivalent 700-800 g / eq, softening point about 96 ° C., manufactured by Japan Epoxy Resin), Epicoat 1004F (epoxy equivalent 875-975 g / eq, softening point about 103) ° C, manufactured by Japan Epoxy Resin Co., Ltd.), Epicoat 1005F ( Poxy equivalent 950-1050 g / eq, softening point about 107 ° C., manufactured by Japan Epoxy Resin Co., Ltd., Araldide XAC5007 (epoxy equivalent 600-700 g / eq, softening point about 90 ° C., manufactured by Ciba Geigy Japan), Araldide GT7004 (epoxy equivalent 730) ˜830 g / eq, softening point of about 100 ° C., manufactured by Ciba-Geigy Corporation of Japan), Araldide GT7097 (epoxy equivalent 1650-2000 g / eq, softening point of about 120 ° C., manufactured by Ciba-Geigy Corporation of Japan) and the like. These may be used alone or in combination of two or more.

  In the powder coating composition of the present invention, the o-cresol novolac type epoxy resin (A) and the bisphenol A type epoxy resin (B) are 95/5 to 60/40, preferably 95/5 to 65/35. More preferably, it is contained at a blending ratio [(A) / (B): weight ratio] of 95/5 to 70/30. When the blending ratio [(A) / (B): weight ratio] is greater than 95/5, the blending amount of the bisphenol A type epoxy resin is reduced, and sufficient flexibility cannot be obtained. On the other hand, when the blending ratio [(A) / (B): weight ratio] is less than 60/40, the blending amount of the o-cresol novolac type epoxy resin is small and sufficient chemical resistance cannot be obtained. That is, the powder coating composition of the present invention comprises an o-cresol novolac type epoxy resin as a main component of an epoxy resin and a bisphenol A type epoxy resin (B) as a subcomponent at a predetermined blending ratio, thereby providing a high resistance. A coating film having both chemical properties and flexibility can be formed.

A-3. Curing Agent The curing agent used in the present invention includes a phenolic curing agent, an amine curing agent, a hydrazide curing agent, and a carboxylic acid curing agent. The curing agent is preferably a phenolic curing agent. A hardening | curing agent may be used independently and may use 2 or more types together.

（式中、ｍは、１〜４の整数を表す。） Arbitrary appropriate things can be used as a phenolic hardening | curing agent. Preferably, a bisphenol A type phenolic curing agent having no epoxy group is used. As a bisphenol A type phenolic hardening | curing agent which does not have an epoxy group, the compound represented by following General formula (1) is mentioned, for example. By using such a bisphenol A type phenolic curing agent, a coating film with more excellent flexibility can be formed.

(In the formula, m represents an integer of 1 to 4.)

  In the above formula (1), when m is less than 1, it cannot be synthesized using bisphenol A as a raw material as described in detail below. When m exceeds 4, the reaction proceeds too much during synthesis, and synthesis may be difficult.

  The phenolic hydroxyl group equivalent of the compound represented by the general formula (1) is preferably 200 to 800 g / eq. If it is less than 200 g / eq, the softening point of the powder coating composition is lowered, and fusion between powder particles is likely to occur during storage of the powder coating composition, which may reduce storage stability. is there. When it exceeds 800 g / eq, the reactivity is lowered, so that the chemical resistance may be lowered.

  The compound represented by the general formula (1) can be obtained, for example, by a reaction between a bisphenol A type epoxy resin and bisphenol A. Commercial products may also be used. Examples of commercially available products include TH-4100 (phenolic hydroxyl group equivalent of about 725 g / eq, softening point of about 110 ° C., manufactured by Tohto Kasei Co., Ltd.), EpiCure 171 (phenolic hydroxyl group equivalent of 200 to 286 g / eq, softening point of about 80 ° C. And Epoxy 170 (phenolic hydroxyl group equivalent of 286 to 400 g / eq, softening point of about 90 ° C., manufactured by Japan Epoxy Resin Co., Ltd.).

  The amount of the phenolic curing agent used can be appropriately set according to the material of the object to be coated, the stored contents, and the like. Preferably, the phenolic curing agent is used in an amount of the phenolic hydroxyl group of the phenolic curing agent with respect to 1 equivalent of the epoxy group of the epoxy resin component (the o-cresol novolac type epoxy resin and the bisphenol A type epoxy resin). The amount is 0.4 equivalent or more, more preferably 0.9 to 1.1 equivalent. When the phenolic hydroxyl group equivalent is less than 0.4, the epoxy resin is not sufficiently polymerized, and the hardness and chemical resistance of the resulting coating film may be lowered.

  Any appropriate curing agent can be used as the amine curing agent. Specific examples include dicyandiamide, diaminodiphenylmethane (DDM), an adduct of DDM, and metaphenylenediamine. The usage-amount of an amine type hardening | curing agent is 1-10 weight part with respect to 100 weight part of epoxy resin components, for example, Preferably it is 2-5 weight part.

  Any appropriate hydrazide-based curing agent can be used. Specific examples include adipic acid dihydrazide, sebacic acid dihydrazide, and isophthalic acid dihydrazide. The usage-amount of a hydrazide type | system | group hardening | curing agent is 5-50 weight part with respect to 100 weight part of epoxy resin components, for example, Preferably it is 10-30 weight part.

  Any appropriate carboxylic acid curing agent can be used. Specific examples include adipic acid, sebacic acid, and isophthalic acid. Preferably, the amount of the carboxylic acid-based curing agent used is such that the carboxyl group of the carboxylic acid-based curing agent is based on 1 equivalent of the epoxy group of the epoxy resin component (the above o-cresol novolac type epoxy resin and bisphenol A type epoxy resin). 0.4 equivalent or more, more preferably 0.9 to 1.1 equivalent.

A-4. Inorganic filler The powder coating composition of the present invention preferably further comprises an inorganic filler. The inorganic filler contributes to the blocking of the corrosion factor and improves the chemical resistance and can improve the flexibility of the coating film.

  Examples of the inorganic filler include extender pigments such as alumina, silica, precipitated barium sulfate, calcium carbonate, clay, talc and mica; colored inorganic pigments such as titanium dioxide, red iron oxide, yellow iron oxide and carbon black; phosphorus Examples thereof include rust preventive pigments such as zinc acid and aluminum phosphate. Preferably, colored inorganic pigments such as titanium dioxide, red iron oxide, yellow iron oxide, and carbon black are used. This is because a powder coating composition having excellent concealability can be obtained.

  As a usage-amount of an inorganic filler, pigment volume concentration (PVC = Pigment Volume Concentration) is 2.3-8.9%, for example, Preferably it is 3.5-8.5%, More preferably, it is 3.5- The amount is 7.3%. By using an inorganic filler within the preferred range, a coating film having better corrosion resistance can be formed. In addition, PVC calculates | requires the volume by remove | dividing the weight compounding quantity of each component by each density, and calculates | requires it by remove | dividing the volume sum total for a pigment by the total volume sum total.

A-5. Curing Accelerator The powder coating composition of the present invention preferably further comprises a curing accelerator. Examples of the curing accelerator include imidazole compounds and imidazoline compounds.

  Examples of the imidazole compounds include 2-ethyl-4-methylimidazole, 1-methylimidazole, 1,2-dimethylimidazole, 2-methylimidazole, 2-ethylimidazole, 2-undecylimidazole, and 2-heptadecyl. Alkylimidazoles such as imidazole and 2-isopropylimidazole, carbamylalkyl-substituted imidazoles such as 1- (2-carbamylethyl) imidazole, cyanoalkyl-substituted imidazoles such as 1-cyanoethyl-2-methylimidazole, 2-phenyl Aromatic substituted imidazoles such as imidazole, 2-phenyl-4-methylimidazole, 1-benzyl-2-methylimidazole, alkenyl substituted imidazoles such as 1-vinyl-2-methylimidazole, 1-allyl-2 Allyl-substituted imidazoles such as ethyl-4-methylimidazole and poly and imidazole. Of these, alkyl imidazoles and aromatic substituted imidazoles are preferably used.

  Examples of the imidazoline compounds include 2-phenylimidazole, 2-methylimidazoline, 2-undecylimidazoline, 2-heptadecylimidazoline, and the like.

  The amount of the curing accelerator used is preferably 0.05 to 15 parts by weight, and more preferably 0.1 to 10 parts by weight with respect to 100 parts by weight of the curing agent. If the amount used is less than the preferred range, the curing time may be long. In addition, when the amount used is larger than the preferable range, the reactivity of the powder coating composition is increased, so that blocking may occur even in a normal temperature range, resulting in poor storage stability.

A-6. Other Components The powder coating composition of the present invention may further contain additives and auxiliaries such as any appropriate organic pigment, leveling agent, fluidizing aid, and degassing agent as necessary.

A-7. Manufacturing method of powder coating composition The powder coating composition of this invention may be manufactured using arbitrary appropriate manufacturing methods. For example, after preparing raw materials composed of the above components, the raw materials are preliminarily mixed using a super mixer, Henschel mixer, etc., and then the raw materials are melt-kneaded using a kneader such as a kneader or an extruder. To do. The melt kneading is performed at a temperature at which at least a part of the raw material is melted and the whole can be kneaded. The temperature during melt kneading is generally 80 to 120 ° C. The obtained melt is cooled and solidified by a cooling roll, a cooling conveyor or the like, and pulverized to a desired particle size through coarse pulverization and fine pulverization steps. The volume average particle size of the powder coating composition of the present invention thus obtained is preferably 5 to 50 μm.

B. The object to be coated of the powder coating composition of the present invention is typically the inner surface of a metal container. The metal container is preferably an aluminum, stainless steel, steel, or iron container, more preferably a steel or iron container, and particularly preferably a steel drum. The inner surface of the metal container may be subjected to a phosphate treatment such as zinc phosphate or iron phosphate or undercoating. That is, according to another aspect of the present invention, there is provided a metal container, preferably a steel drum can, on which the powder coating composition is coated on the inner surface. The embodiment with a steel drum can is a painting with 3 pieces (top plate, body, ground plate), but it is also possible to paint with 2 pieces (ground plate + body, cup, top plate). Further, in the case of a steel drum, a welding step is generated because the steel sheet is wrapped in the body seam weld. Since this part is not uniformly coated, the corrosion resistance is likely to be insufficient. Therefore, it is desirable that the fuselage seam welded portion is subjected to a rolling process to uniformly coat the coating film.

C. Coating method The coating composition can be obtained by applying the powder coating composition of the present invention to an object to be coated (typically, the inner surface of a metal container) and then heating. That is, according to another aspect of the present invention, the step of applying the powder coating composition to the inner surface of the metal container, and the step of heating the inner surface of the metal container to which the powder coating composition is applied. Including, a coating method is provided. According to the coating method, excellent chemical resistance can be imparted to the metal container.

  Any appropriate method can be used as a method of applying the powder coating composition. Specific examples include an electrostatic powder coating method, a spray coating method, and a fluidized immersion method. Of these, the electrostatic powder coating method is preferably used from the viewpoint of coating efficiency. The coating film thickness is appropriately adjusted according to the purpose. Generally, the coating film thickness is set to 20 to 150 μm. In the case of a steel drum can, the wound portion is subjected to severe plastic deformation, and thus the coating film is easily peeled off. In order to prevent this, it is preferable to reduce the coating thickness of the tightening portion to about 1/3 to 2/3 of the normal thickness. Corresponding deterioration of corrosion resistance is not a problem in practice. As another countermeasure, it is also effective to apply a repair agent to the winding part.

  The heating conditions after applying the powder coating composition can be appropriately set according to the kind of the epoxy resin component, the blending ratio, and the like. For example, the heating temperature is 150 to 280 ° C, preferably 180 to 250 ° C. The heating time can be appropriately set according to the heating temperature. The heating time is, for example, 1 to 20 minutes.

[Example 1]
(1) Preparation of powder coating composition The raw materials shown in Table 1 were premixed for about 3 minutes with a super mixer (manufactured by Kawata Corporation). Subsequently, melt-kneading extrusion was performed on the conditions of about 100 degreeC with the Kneader (made by Busus). The extruded blended product was cooled to room temperature and coarsely pulverized, and then finely pulverized with an atomizer (manufactured by Fuji Powder Co., Ltd.) to obtain a powder coating composition having an average particle size of 35 μm.

(2) Preparation of test piece Electrostatic powder coating machine using the powder coating composition prepared in (1) on a zinc phosphate-treated steel plate (0.8 t x 70 x 150 mm SPCC-SD (dull steel plate)) (GX3300, manufactured by Onoda) and electrostatic powder gun (GX107, manufactured by Onoda) were used for electrostatic coating. Subsequently, the test piece in which the coating film was formed on the surface was obtained by heating at 230 degreeC for 15 minute (s). The thickness of the coating film was 60 to 80 μm.

(3) Flexibility evaluation [Copping resistance test]
The test piece obtained in the above (2) was subjected to a cupping resistance test in accordance with JIS K5600-5-2. Those having a minimum indentation depth of 5 mm or more where defects (cracking, peeling, etc.) occur in the coating film were evaluated as “◯”, and those having a depth of less than 5 mm were evaluated as “x”. The results are shown in Table 1.

[Weight drop resistance test]
The test piece obtained in the above (2) was subjected to a weight drop resistance test in accordance with JIS K5600-5-3. A film where cracking / peeling of the coating film was not recognized due to impact deformation at a height of 50 cm was evaluated as “◯”, and the others were evaluated as “×”. The results are shown in Table 1.

(4) Chemical resistance evaluation After immersing the test piece obtained in (2) above in various compound solutions at 20 to 60 ° C. for 30 days, JIS K5600-5-6 (cross cut tape method; 2 mm interval) Based on the above, the resistance evaluation to various compounds was performed. Based on the classification described in JIS K5600-5-6, a classification of “0” is evaluated as “◯”, a classification of “1” or “2” is evaluated as “△”, and Those with a classification exceeding “3” were evaluated as “x”. The results are shown in Table 1.

[Examples 2-7, Comparative Examples 1-5]
Using the raw materials listed in Table 1, a powder coating composition was prepared in the same manner as in Example 1, and a test piece was prepared. The obtained test piece was subjected to a cupping resistance test, a weight drop resistance test, and a chemical resistance test in the same manner as in Example 1. The results are shown in Table 1.

[Test Example 1]
Powder coating composition prepared in Example 1, comparative coating composition 1 (manufactured by Toyo Ink Co., product number “519S”; solvent-based epoxy phenol coating composition), and comparative coating composition 2 (manufactured by Toyo Ink Co., Ltd.) The product was coated on a steel drum using product number “526”; solvent-based phenol paint composition), and the performance of the coating film was evaluated. The results are shown in Table 2.

(1) Coating of steel drum can Each paint composition was hand-painted on the inner surface of a steel drum can (inner diameter: 567 mm, inner height: 830 mm) using a pole gun. Subsequently, the steel drum which the coating film with a thickness of about 70 micrometers was formed in the inner surface was produced by heating at 230 degreeC for 15 minutes. The following flexibility evaluation was performed about the formed coating film using the side part (10 cm x 10 cm) of this drum can. Further, various compound solutions were sealed in the drum, and chemical resistance evaluations 1 and 2 of the formed coating film were performed.

(2) Flexibility evaluation [Copping resistance test]
In accordance with JIS K5600-5-2, a cupping resistance test was performed, and the minimum indentation depth at which defects (cracking, peeling, etc.) occurred in the coating film was measured.

[Weight drop resistance test]
In accordance with JIS K5600-5-3, a weight drop resistance test was performed, and the minimum height at which the coating film was cracked or peeled off due to impact deformation was measured.

[Pencil hardness measurement]
Based on JIS K5600-5-4, the pencil hardness of the coating film was measured.

[Fundamental eye test]
A base eye test was conducted in accordance with JIS K5600-5-6 (cross cut tape method; 2 mm spacing). Based on the classification described in JIS K5600-5-6, a classification of “0” is evaluated as “◯”, a classification of “1” or “2” is evaluated as “△”, and Those with a classification exceeding “3” were evaluated as “x”.

[Solvent rubbing test]
The surface of the coating film was rubbed with absorbent cotton soaked with methyl ethyl ketone, and the number of rubs until the substrate was exposed was measured.

(3) Chemical resistance evaluation 1
In the same manner as in Example 1 (4), the chemical resistance of the coating film was evaluated.

(4) Chemical resistance evaluation 2
The drum painted in (1) above was filled with a 10% by weight Igepearl aqueous solution and left at room temperature for 6 months. After the start of standing, the state of the coating film was visually observed after 1 month, 3 months, and 6 months.

  As shown in Tables 1 and 2, the powder coating composition of the present invention contains o-cresol novolac type epoxy resin and bisphenol A type epoxy resin in a predetermined ratio, so that chemical resistance and flexibility are achieved. It is possible to form a coating film that satisfies both of these requirements. Moreover, the coating film which was further excellent in the flexibility can be formed by containing an inorganic filler moderately.

  Since the powder coating composition of the present invention is suitable for inner surface coating of metal containers, it can be suitably used in the paint field.

Claims (9)

o-cresol novolac type epoxy resin (A),
Bisphenol A type epoxy resin (B);
Including at least one curing agent selected from the group consisting of a phenolic curing agent, an amine curing agent, a hydrazide curing agent, and a carboxylic acid curing agent,
The blending ratio [(A) / (B): weight ratio] of the o-cresol novolac type epoxy resin (A) and the bisphenol A type epoxy resin (B) is 95/5 to 60/40.
Powder coating composition.   The powder coating composition according to claim 1, wherein the curing agent is a bisphenol A type phenolic curing agent (excluding those having an epoxy group).   The powder coating composition according to claim 1 or 2, further comprising an inorganic filler.   The powder coating composition according to claim 3, comprising the inorganic filler at a pigment volume concentration of 2.3 to 8.9%.   The powder coating composition according to any one of claims 1 to 4, further comprising a curing accelerator.   The powder coating composition according to any one of claims 1 to 5, which is used for coating an inner surface of a metal container.   A metal container in which the powder coating composition according to claim 1 is coated on an inner surface.   The metal container according to claim 7, which is a steel drum. Applying the powder coating composition according to any one of claims 1 to 6 to an inner surface of a metal container; and heating the inner surface of the metal container to which the powder coating composition is applied. How to paint.