Epoxy resin powder coating and coating method

Abstract

To provide an epoxy resin powder coating material which uses a bisphenol F type epoxy resin, has excellent storage stability, and can form a coating film excellent in coating film appearance, water resistance and solvent resistance.SOLUTION: The epoxy resin powder coating material contains an epoxy resin (A), a curing agent (B), and a pigment (C). The epoxy resin (A) is a bisphenol F type epoxy resin (A1). The curing agent (B) contains an imidazoline derivative (B1), an imidazole derivative (B2) having no triazine skeleton, and an imidazole derivative (B3) having a triazine skeleton. The melting point of the imidazole derivative (B3) having a triazine skeleton is in the range of 150°C or higher and 200°C or lower.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to epoxy resin powder coatings and coating methods. [Background technology]

[0002] Many metal products and parts used as structural materials in buildings, home appliances, automobiles, etc., require aesthetic appeal and corrosion resistance, and therefore their surfaces are coated with paint compositions. In certain fields, considering increased productivity and reduced environmental impact, dry powder coatings that harden quickly without solvents are preferred over wet coatings that use solvents. For example, powder coatings, which offer excellent safety and work efficiency, are widely used to coat metal parts used in infrastructure such as water pipes.

[0003] Resins used in powder coatings include epoxy resins, which have excellent storage stability, meaning that the coating performance does not change much even after long-term storage after manufacturing, and which have excellent adhesion to metal surfaces. More recently, from the perspective of greater safety, there has been a demand for powder coatings using bisphenol F type epoxy resin, which has excellent adhesion to metal surfaces. However, bisphenol F type epoxy resin powder coatings have problems with storage stability, as their performance tends to change easily between manufacturing and use as a coating. Furthermore, when used for water pipes and their components, a coating surface with minimal irregularities, no air bubbles remaining, and long-term water resistance are required. In addition, solvent resistance of the coating film is also required because solvent rubbing may be performed after painting to repair the coating on water pipes or to check the curing properties.

[0004] Therefore, in order to solve these conventional problems, for example, Reference 1 discloses an epoxy resin powder coating composition containing a bisphenol A or F type epoxy resin and an imidazole derivative having a melting point of 170°C or higher. Furthermore, Reference 2 discloses an epoxy resin powder coating composition for the inner surface of cast iron pipes, which is a mixture of bisphenol F type epoxy resin, epoxy resin curing agent, and quartz-based powder in a certain proportion. Furthermore, Reference 3 discloses an epoxy resin powder coating that mainly contains bisphenol F type epoxy resin and a curing agent with a melting point of 130-200°C.

[0005] While Patent Documents 1 to 3 all describe improvements in storage stability, they had problems with insufficient coating appearance, water resistance, and solvent resistance. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 10-204329 [Patent Document 2] Japanese Patent Publication No. 2015-048454 [Patent Document 3] Japanese Patent Publication No. 2021-169593 [Overview of the project] [Problems that the invention aims to solve]

[0007] Therefore, the object of the present invention is to provide an epoxy resin powder coating and a coating method that can form a coating film using a bisphenol F type epoxy resin that has excellent storage stability and excellent coating film appearance, water resistance and solvent resistance. [Means for solving the problem]

[0008] The features of the present invention are listed below. (1) An epoxy resin powder coating comprising an epoxy resin (A), a curing agent (B), and a pigment (C), wherein the epoxy resin (A) is a bisphenol F type epoxy resin (A1), the curing agent (B) contains an imidazoline derivative (B1), an imidazole derivative (B2) having no triazine skeleton, and an imidazole derivative (B3) having a triazine skeleton, and the melting point of the imidazole derivative (B3) is in the range of 150°C or higher and 200°C or lower. (2) The epoxy resin powder coating according to (1), wherein the content of the imidazoline derivative (B1) is 33% by mass or more and 99% by mass or less based on the total amount of the imidazoline derivative (B1), the imidazole derivative (B2), and the imidazole derivative (B3). (3) The epoxy resin powder coating according to (1) or (2), which is used for coating straight pipes or profiled pipes. (4) A coating method in which a powder coating is applied to the inner surface of a straight pipe or a profiled pipe workpiece preheated within the range of 120°C or higher and 280°C or lower, and then cured or heated to form a coating film on the inner surface of the workpiece. The powder coating is an epoxy resin powder coating containing an epoxy resin (A), a curing agent (B), and a pigment (C), wherein the epoxy resin (A) is a bisphenol F type epoxy resin (A1), the curing agent (B) contains an imidazoline derivative (B1), an imidazole derivative (B2) having no triazine skeleton, and an imidazole derivative (B3) having a triazine skeleton, and the melting point of the imidazole derivative (B3) is in the range of 150°C or higher and 200°C or lower.

Advantages of the Invention

[0009] Therefore, according to the present invention, it is possible to provide an epoxy resin powder coating and a coating method that can form a coating film with excellent storage stability, excellent coating film appearance, water resistance, and solvent resistance by using a bisphenol F type epoxy resin.

Embodiments for Carrying Out the Invention

[0010] The epoxy resin powder coating of the present invention will be described in detail below.

[0011] The present invention relates to an epoxy resin powder coating comprising an epoxy resin (A), a curing agent (B), and a pigment (C), wherein the epoxy resin (A) is a bisphenol F type epoxy resin (A1), and the curing agent (B) contains as essential components an imidazoline derivative (B1), an imidazole derivative without a triazine skeleton (B2), and an imidazole derivative having a triazine skeleton (B3), and the melting point of the imidazole derivative (B3) is in the range of 150°C to 200°C.

[0012] (Epoxy resin powder coating) The epoxy resin powder coating in this invention is a powder coating that does not contain organic solvents or water, and consists only of a film-forming component containing epoxy resin. By making it a powder, handling in storage and inventory, and work efficiency in painting and other operations can be improved.

[0013] <Bisphenol F type epoxy resin (A1)> In this invention, bisphenol F type epoxy resin (A1) is used as the epoxy resin (A). Bisphenol F type is known to be safer for humans and other organisms than bisphenol A type. The bisphenol F type epoxy resin (A1) is obtained by the reaction of bisphenol F and epichlorohydrin. The bisphenol F type solid epoxy resin of the present invention may also have substituents. The substituents are preferably alkyl groups, phenyl groups, or α-methylbenzyl groups, and particularly preferably methyl groups or α-methylbenzyl groups.

[0014] The epoxy equivalent (g / eq.) of the bisphenol F type epoxy resin (A1) is preferably in the range of 700 to 3000, more preferably in the range of 900 to 2500, and even more preferably in the range of 1000 to 2300. If the epoxy equivalent is too low, the storage stability of the powder coating may be poor. If the epoxy equivalent is too high, the melt viscosity will be high, resulting in poor fluidity of the powder coating, which may lead to an uneven appearance and other defects, as well as an increase in the retention of air bubbles in the coating film. If the epoxy equivalent is in the range of 700 to 3000, the powder coating will have no problems with storage stability, a good coating film appearance, and good water resistance and solvent resistance.

[0015] The softening point of the bisphenol F type epoxy resin (A1) is preferably in the range of 70 to 130°C, more preferably in the range of 80 to 120°C, and even more preferably in the range of 85 to 110°C. If the softening point is too low, the storage stability of the powder coating will be poor, and if the softening point is too high, the fluidity of the powder coating will be poor, and the smoothness of the coating surface after application may be impaired, potentially reducing water resistance and solvent resistance. If the softening point is in the range of 70 to 130°C, when used as a powder coating, a powder coating with no storage stability problems can be obtained, and a coating film with good appearance, water resistance, solvent resistance, etc., can be obtained.

[0016] <Hardening agent (B)> The epoxy resin powder coating of the present invention contains, as a curing agent (B), an imidazoline derivative (B1), an imidazole derivative without a triazine skeleton (B2), and an imidazole derivative having a triazine skeleton (B3) as essential components. By simultaneously containing these three types, the epoxy resin powder coating of the present invention can exhibit excellent storage stability, coating film appearance, water resistance, and solvent resistance. Each component is described below.

[0017] <Imidazolin derivative (B1)> The imidazoline derivative (B1) is a curing agent that has an imidazoline skeleton within the compound. The imidazoline derivative (B1) is represented by the following formula (1). [ka]

[0018] In formula (1), R 1 , R 2 and R 3 Each of these may independently be a hydrogen atom, an aliphatic group, or an aromatic group, preferably an aliphatic group (e.g., an aliphatic hydrocarbon group), such as an alkyl group, an alkenyl group, an aryl group, or a hydroxyalkyl group. 4 The organic group in may be a divalent aliphatic group or a divalent aromatic group, and is preferably an aliphatic group. Also, R 4 The organic group in this may have one or more nitrogen atoms or oxygen atoms.

[0019] Specifically, examples of imidazoline derivatives (B1) include 1-(2-hydroxy-3-phenoxypropyl)-2-phenylimidazoline, 1-(2-hydroxy-3-butoxypropyl)-2-methylimidazoline, 2-methylimidazoline, 2,4-dimethylimidazoline, 2-ethylimidazoline, 2-ethyl-4-methylimidazoline, 2-benzylimidazoline, 2-phenylimidazoline, 2-(o-tolyl)-imidazoline, tetramethylene-bis-imidazoline, and 1,1,3-trimethyl-1,4-tetramethylene-bis-imidazoline. Examples include tramethylene-bis-imidazoline, 1,3,3-trimethyl-1,4-tetramethylene-bis-imidazoline, 1,1,3-trimethyl-1,4-tetramethylene-bis-4-methylimidazoline, 1,3,3-trimethyl-1,4-tetramethylene-bis-4-methylimidazoline, 1,2-phenylene-bis-imidazoline, 1,3-phenylene-bis-imidazoline, 1,4-phenylene-bis-imidazoline, and 1,4-phenylene-bis-4-methylimidazoline, as well as modified versions thereof. These may be used individually or in combination of two or more. From the viewpoint of storage stability, imidazoline derivatives do not need to have a hydroxyl group.

[0020] In addition, the imidazoline derivative (B1) lowers the melt viscosity in the epoxy resin powder coating by heating during the painting operation, making it easier for air bubbles to escape when the powder coating is melted during the painting operation and less likely for air bubbles to remain in the coating film, resulting in excellent coating appearance after painting. The imidazoline derivative (B1) is contained in the epoxy resin powder coating in the range of 0.1 to 2.0% by mass, preferably in the range of 0.5 to 2.0% by mass. If it is less than 0.1% by mass, air bubbles are difficult to escape, so air bubbles remain in the coating film and the coating film appearance deteriorates. In particular, in the painting of ductile cast iron pipes, the adhesion between the coating film and the surface of the ductile cast iron pipe may decrease due to the remaining air bubbles. Also, if it exceeds 2.0% by mass, there is a concern that the change in the paint properties during long-term storage is large, deteriorating the coating film appearance and performance.

[0021] <Imidazole derivative (B2) having no triazine skeleton> The imidazole derivative (B2) having no triazine skeleton is a curing agent (B) that does not have a compound having a triazine skeleton in the imidazole skeleton within the compound. The imidazole derivative (B2) is represented by the following formula (2).

Chemical formula

[0022] In formula (2), R 11 , R 12 and R 13 The organic groups in may each independently be an aliphatic group or an aromatic group, preferably an aliphatic group (for example, an aliphatic hydrocarbon group), such as an alkyl group, an alkenyl group, an aryl group or a hydroxyalkyl group, more preferably an alkyl group or an alkenyl group. R 11 , R 12 and R 13 At least one (for example, one or two, particularly one or both of R 12 and R 13 ) may be a hydrogen atom, and all may be hydrogen atoms. R 14The organic group in may be a divalent aliphatic group or a divalent aromatic group, preferably an aliphatic group. 14 The organic group in R may have one or more nitrogen atoms or oxygen atoms, or it may not have any. 14 The number of carbon atoms in the organic group can be 1 to 20, for example, 1 to 15.

[0023] Furthermore, imidazole derivatives without a triazine skeleton (B2) represent compounds in which the main imidazole skeleton does not contain a compound with a triazine skeleton. Therefore, R 11 , R 12 , R 13 and R 14 This represents compounds that do not have a triazine skeleton in any of the above. The imidazole derivative (B2) without a triazine skeleton is contained in the epoxy resin powder coating in an amount of 0.01 to 1.0% by mass, preferably in an amount of 0.01 to 0.5% by mass. If the amount is less than 0.01% by mass, a coating film with high crosslinking density cannot be obtained, and solvent resistance decreases.

[0024] Examples of imidazole derivatives (B2) that do not have a triazine skeleton include imidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-undecylimidazole, 2-phenylimidazole, 1-aminoethyl-2-methylimidazole, 1-(2-hydroxy-3-phenoxypropyl)-2-methylimidazole, 1-(2-hydroxy-3-phenoxypropyl)-2-ethyl-4-methylimidazole, 1-(2-hydroxy-3-butoxypropyl)-2-methylimidazole, 1-(2-hydroxy-3-butoxypropyl)-2-ethyl-4-methylimidazole, and 1-cyanoethyl-2-undecylimidazolium trimellitate, as well as modified versions thereof. These may be used individually or in combination of two or more types. From the viewpoint of curability, the imidazole derivatives do not need to have a hydroxyl group.

[0025] <Imidazole derivative having a triazine skeleton (B3)> Furthermore, the curing agent (B) of the present invention contains an imidazole derivative (B3) having a triazine skeleton. From the viewpoint of storage stability, the imidazole derivative (B3) having a triazine skeleton may have at least one (e.g., one, two, or three) amino groups directly bonded to the triazine skeleton, and preferably two. The imidazole derivative (B3) having triazine may not have a hydroxyl group directly bonded to the triazine skeleton.

[0026] The triazine skeleton is a compound represented by the following formula (3). [ka]

[0027] In formula (3), R 21 , R 22 and R 23 Each of these may independently be a hydrogen atom, an aliphatic group, and an aromatic group. 21 , R 22 and R 23 At least one (preferably one) of the groups is a group containing an imidazole skeleton. 21 , R 22 and R 23 It is preferable that at least one, preferably two, of these are amino groups (alkylamino groups, -NH2 groups). 22 and R 23 If R is an amino group, 21 R may be a group containing an imidazole skeleton or an imidazoline skeleton, and preferably a group containing an imidazole group. 21 The group may or may not have a hydroxyl group. Examples of groups containing a triazine skeleton include the 2,4-diamino-1,3,5-triazine-6-ylethyl group.

[0028] For example, an example of a curing agent having an imidazole derivative (B3) having a triazine skeleton is shown in formula (4) below. [ka] Here, R in the imidazole skeleton 14 The triazine skeleton may be bonded to it via, for example, an aliphatic hydrocarbon group and a nitrogen-containing aliphatic hydrocarbon group, or it may be directly bonded to it.

[0029] Furthermore, the imidazole derivative (B3) having a triazine skeleton has a melting point in the range of 150°C to 200°C. Imidazole (C3H4N2) has a melting point of 89-91°C, 11 , R 12 and R 13 The organic groups in each are compounds of aliphatic or aromatic groups, and R 14 By bonding it with a compound having a triazine skeleton, an imidazole derivative (B3) having a triazine skeleton with a melting point in the range of 150°C to 200°C can be obtained. By containing this imidazole derivative (B3) having a triazine skeleton, the epoxy resin powder coating of the present invention can be made to have storage stability. However, if the melting point is below 150°C, long-term storage becomes difficult and storage stability may decrease. If it exceeds 200°C, the resulting coating film will have many irregularities, reducing the appearance of the coating film and potentially reducing water resistance.

[0030] The imidazole derivative (B3) having a triazine skeleton is contained in the epoxy resin powder coating in an amount of 0.01 to 1.0% by mass, preferably in an amount of 0.01 to 0.5% by mass. If the amount is less than 0.01% by mass, it may act as a curing agent when unused before use, which may reduce storage stability.

[0031] Examples of imidazole derivatives (B3) having a triazine skeleton include 2,4-diamino-6-[2'-undecylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-ethyl-4'-methylimidazolyl-(1')]-ethyl-s-triazine, and modified versions thereof. These may be used individually or in combination of two or more. From the viewpoint of storage stability, the imidazole derivatives do not need to have a hydroxyl group.

[0032] Furthermore, the content of imidazoline derivative (B1) is preferably 33% to 99% by mass, and more preferably 50% to 90% by mass, relative to the total amount of imidazoline derivative (B1), imidazole derivative (B2), and imidazole derivative (B3). In the epoxy resin powder coating of the present invention, if a large amount of imidazoline derivative is incorporated, the melt viscosity decreases. In the case of a combination of only imidazoline (B1) and imidazole (B2) without a triazine skeleton, the storage stability of the epoxy resin powder coating decreases, and in the case of a combination of only imidazoline (B1) and imidazole (B3) having a triazine skeleton, the solvent resistance deteriorates. Therefore, by using these three types of curing agents (B1), (B2), and (B3) together, it is possible to obtain the epoxy resin powder coating of the present invention, which has excellent storage stability, coating film appearance, water resistance, and solvent resistance. Furthermore, if the content of (B1) is less than 33% by mass relative to the total amount of the three types of hardeners (B1), (B2), and (B3), the appearance of the coating film and the ability to suppress bubbles in the coating film may be reduced. If it exceeds 99% by mass, the water resistance and solvent resistance may be reduced.

[0033] Unless there are any particular problems with the epoxy resin powder coating of the present invention, a commonly used catalyst / curing agent (B4) can be used in combination with the curing agent (B). In particular, examples include imidazole compounds, imidazoline compounds, dicyandiamides, acid anhydrides, polycarboxylic acid hydrazides and their derivatives, phenol compounds and their derivatives. Examples of polycarboxylic acid hydrazides include adipic acid dihydrazide, sebacate acid dihydrazide, dodecanedione dihydrazide, isophthalic acid dihydrazide, salicylic acid hydrazide, and the like.

[0034] <Pigment (C)> The epoxy resin powder coating of the present invention contains a pigment (C). As for the pigment (C), it is necessary to contain at least an extender pigment (C1) in order to form a coating film that has excellent coating film appearance, storage stability, water resistance, and solvent resistance, and may further contain a coloring pigment (C2), a rust-preventive pigment (C3), a gloss pigment (C4), etc., as needed.

[0035] The extender pigment (C1) is a white or colorless pigment (C). Because of its low refractive index, it has little effect on opacity when mixed with a binder, and is therefore used as an extender to adjust coloring power, gloss, strength, and feel. The extender pigment (C1) can be any known material, such as precipitated barium sulfate, silica, cristobalite, calcium carbonate, alumina, alum, white clay, magnesium hydroxide, and magnesium oxide. The extender pigment may also be a thin, flat pigment (flak-like pigment) such as glass flakes, talc, mica, and kaolin clay. In particular, when used in various water supply materials, precipitated barium sulfate, silica, and calcium carbonate are preferred.

[0036] Coloring pigments (C2) are broadly classified into inorganic pigments and organic pigments based on their composition. Examples of inorganic pigments include titanium dioxide, red iron oxide, yellow iron oxide, and carbon black, while examples of organic pigments include phthalocyanine blue, phthalocyanine green, naphthol red, quinacridone red, benzimidazolone yellow, Hansa yellow, benzimidazolone orange, and dioxazine violet. Among these, titanium dioxide, red iron oxide, yellow iron oxide, and carbon black are preferred when used in various water supply materials.

[0037] As the rust-preventive pigment (C3), known materials can be used, including zinc powder, zinc oxide, barium metaborate, calcium silicate, aluminum phosphate, condensed aluminum phosphate, aluminum tripolyphosphate, zinc phosphate, zinc phosphite, potassium phosphite, calcium phosphite, aluminum phosphite, calcium zinc phosphate, aluminum zinc phosphate, zinc phosphomolybdate, aluminum phosphomolybdate, magnesium phosphate, vanadic acid / phosphate mixed pigment, etc.

[0038] Furthermore, in the epoxy resin powder coating of the present invention, the content of pigment (C) in the solid content of the film-forming component is preferably 5% by mass or more and 60% by mass or less, and preferably 10% by mass or more and 50% by mass or less. These pigments may be used individually or in combination of two or more. By setting the pigments within this range, it is possible to obtain the epoxy resin powder coating of the present invention, which has excellent storage stability and film performance, including film appearance, water resistance, and solvent resistance.

[0039] <Other additives (D)> The epoxy resin powder coating of the present invention may contain, as appropriate, other additives (D) such as matting agents, flexibility modifiers, surface modifiers, lubricants, wetting agents, dispersants, emulsifiers, viscosity modifiers, settling inhibitors, anti-skinning agents, anti-sagging agents, defoaming agents, color separation inhibitors, leveling agents, drying agents, plasticizers, film-forming aids, antifungal agents, antibacterial agents, antiviral agents, insecticides, light stabilizers, UV absorbers, electrostatic lubricants, antistatic agents, and conductivity modifiers, within limits that do not impair the properties of the coating. In particular, when used in various water supply materials, it is preferable to use a surface modifier.

[0040] <Other resins> Furthermore, the epoxy resin powder coating of the present invention can be used by mixing bisphenol F type epoxy resin (A1) with other resins as the resin component. Commonly used resins can be used as the resin to be mixed, but examples include acrylic resin, silicone resin, acrylic silicone resin, styrene acrylic copolymer resin, polyester resin, fluororesin, rosin resin, petroleum resin, coumarone resin, phenolic resin, urethane resin, melamine resin, urea resin, epoxy resin, cellulose resin, xylene resin, alkyd resin, aliphatic hydrocarbon resin, butyral resin, maleic acid resin, fumaric acid resin, vinyl resin, amine resin, ketimine resin, etc. These resins may be used individually or in combination of two or more types.

[0041] (Method of manufacturing epoxy resin powder coating) The epoxy resin powder coating of the present invention is produced by first mixing the raw materials at room temperature using a mixer such as a Nauter mixer or a Henschel mixer, and then melt-kneading them using a melt-kneader commonly used in powder coating production, such as a single-screw or twin-screw extruder. After cooling, the formed pellets are crushed using a pulverizer such as a pin mill or jet mill, the resulting fine powder is sieved, and the particle size distribution is adjusted to the desired level using a classifier or the like to produce the powder coating.

[0042] In the epoxy resin powder coating of the present invention, the volume-average particle size is preferably 20 to 150 μm. If the volume-average particle size is less than 20 μm, the total surface area per unit mass of the powder coating particles becomes large, which can significantly worsen the fluidity of the powder and greatly reduce the workability of the coating. If the volume-average particle size exceeds 150 μm, the surface irregularities of the coated film become large, which reduces the appearance, water resistance, and solvent resistance of the coated film. The volume-average particle size is measured using a particle size distribution measurement method utilizing laser diffraction scattering, a Coulter counter that uses changes in electrical resistance in a liquid, or the like.

[0043] (Application method) The present invention provides a method for applying powder coatings, which involves preheating straight or irregularly shaped water pipes and their component materials to a constant temperature within the range of 120°C to 280°C, applying the powder coating, and then curing or heating it to form a coating film on the inner surface of the object to be coated.

[0044] The materials to be coated are not particularly limited, but specifically include iron plates, steel plates, aluminum plates, ceramic plates, etc., and those with surface treatments. Furthermore, from a safety standpoint, it is applied to various water supply materials. In particular, water pipes using ductile cast iron pipes, which have strength and ductility due to the spherical shape of the graphite, are a good example. Ductile cast iron pipes contain many shrinkage cavities formed during casting on the inner surface of the cast iron pipe and have the shape of a straight pipe or a shaped pipe.

[0045] Methods for coating ductile cast iron pipes include rotary spraying, electrostatic coating, fluidized bed immersion, and thermal spraying. However, the coating is applied to the inner surface of a hollow object to be coated, preferably the inner surface of a pipe. In this case, methods such as the Provac method and suction method may also be used. Specifically, a predetermined straight or irregularly shaped pipe is preheated to a constant temperature within the range of 120 to 280°C, preferably 170 to 250°C, and more preferably 180 to 230°C. After preheating, it is placed on a stand, the inside of the pipe is depressurized, and powder coating is drawn into the pipe from a paint supply tank such as a paint fluidized bed tank, thereby coating the inner surface of the pipe. This method is suitable for coating a hardened coating film that forms at the preheated temperature. In the case of ductile cast iron pipes, which are the objects to be coated, the pipes are preheated to 120-280°C, placed on a coating stand, and while straight pipes are rotated or irregularly shaped pipes are suspended, powder coating is sprayed onto the inner surface of the pipes using air transport or reduced pressure suction to form a coating layer. A cured coating is then obtained by post-heating at a temperature that allows for sufficient curing, or by allowing it to cool without post-heating. Unapplied powder coatings are returned to the coating tank and reused. In this coating method, the force with which the powder coating adheres to the surface of the object to be coated is due to electrostatic coating or adhesion to a molten layer formed on the surface of the object to be coated. Therefore, in order to obtain a good cured coating film, it is important to control the curing rate at the preheating temperature, as well as the particle size distribution and bulk density of the powder coating. The epoxy resin powder coating of the present invention has basic coating film properties such as uniform thickness and corrosion resistance, and a good cured coating film can be obtained without the generation of air bubbles, and it can also be recycled. The film thickness of the coating on ductile cast iron pipes is 300 μm or more and 1500 μm or less, preferably 300 to 1200 μm, and more preferably 300 μm or more and 800 μm or less. The film thickness of the epoxy resin powder coating of the present invention can be varied depending on the type of substrate to which it is applied and its application. [Examples]

[0046] Examples of the present invention are described below. The present invention can be in various forms and is not limited to the following examples.

[0047] (Preparation of Examples 1-10 and Comparative Examples 1-7) The raw materials listed in Table 1 were combined and mixed in a high-speed mixer for 1 minute. Then, the mixture was kneaded using a single-screw mixer (manufactured by BUSS) with the temperature adjusted to 120°C. The extruded mixture was cold-rolled with a cooling roll, then pulverized using a pin mill, and classified in a classifier to obtain powder coating examples 1 to 10 and comparative examples 1 to 7. The raw materials listed in Table 1 are as follows:

[0048] Table 1 shows details of the raw materials used and examples of formulations. (1) Epoxy resin (A): (A1-1) Bisphenol F type epoxy resin (jER4005P, manufactured by Mitsubishi Chemical Corporation, epoxy equivalent: 1075 g / eq, softening point: 87°C): 60 parts by mass (2) Hardener (B) (B1-1) Imidazolin derivative (Trade name: Curesol 2PZL (2-phenylimidazoline), manufactured by Shikoku Chemicals Co., Ltd., melting point 95~104℃) (B1-2) Imidazolin derivative (product name: Curesol 2MLZ-F (2-methylimidazoline), manufactured by Shikoku Chemicals Co., Ltd., melting point 91-106℃) (B2-1) Imidazole derivatives that do not have a triazine skeleton (Trade name: Curazole C11Z (2-undecylimidazole), manufactured by Shikoku Chemicals Co., Ltd., melting point 69-74°C) (B2-2) Imidazole derivatives that do not have a triazine skeleton (Trade name: Curesol 2MZ (2-methylimidazole, manufactured by Shikoku Chemicals Co., Ltd., melting point 137~145℃)) (B2-3) Imidazole derivatives that do not have a triazine skeleton (Trade name: Curesol 2P4MZ (2-phenyl-4-methylimidazole, manufactured by Shikoku Chemicals Co., Ltd., melting point 174~184℃) (B3-1) Imidazole derivative having a triazine skeleton (Trade name: Curesol C11Z-A (2,4-diamino-6-[2'-undecylimidazolyl-(1')]-ethyl-s-triazine, manufactured by Shikoku Chemicals Co., Ltd., melting point 187~195℃) (B3-2) Imidazole derivative having a triazine skeleton (Trade name: Curezol 2E4MZ-A (2,4-diamino-6-[2'-ethyl-4'-methylimidazolyl-(1')]-ethyl-s-triazine, manufactured by Shikoku Chemicals Co., Ltd., melting point 215~225℃) (B4-1) Hydrazide derivative (Trade name: ADH (Dihydrazide adipate, manufactured by Otsuka Chemical Co., Ltd., melting point 185℃) (3) Pigment (C) (C1-1) Extender pigment (Silica (Product name: SB903, Sibelco Japan Co., Ltd.)) (C2-1) Coloring pigment (Titanium dioxide (product name: Ti-Pure R-960, manufactured by Chemours)) (C2-2) Coloring pigment (carbon black (product name: MA100, manufactured by Mitsubishi Chemical Corporation)) (4) Other additives (D) (D1-1) Surface modifier (acrylic copolymer PF-95 (product name: Polyflow No. 95, manufactured by Kyoeisha Chemical Co., Ltd.)) (D2-1) Lubricant (Amorphous silicon dioxide S358 (product name: Silicea 358, manufactured by Fuji Silicea Chemical Co., Ltd.))

[0049] Examples 1-10 and Comparative Examples 1-7 were prepared according to the formulations shown in Table 1.

[0050] [Table 1]

[0051] <Evaluation of coating film performance> The coating performance of the test boards (coated bodies) in Examples 1-10 and Comparative Examples 1-7 was evaluated. In the powder coatings prepared in Examples 1-10 and Comparative Examples 1-7, a 2.0 mm thick shot blast plate preheated to 210°C was suspended vertically, and test plates were prepared by electrostatic coating using a corona-charging electrostatic powder coating machine (Asahi Sanac PG-1 model) at a voltage of -60 kV to achieve film thicknesses of 300-400 μm and 800-1000 μm. After coating, the test plates were heated at 200°C for 10 minutes. The coating performance was evaluated based on its appearance, storage stability, water resistance, and solvent resistance. The evaluation method is described below. The evaluation results are shown in Table 2.

[0052] <<Visual evaluation of appearance (finish)>> The appearance of the paint film after painting was visually inspected. ○: A smooth coating film is obtained, resulting in a good appearance. ×: A smooth coating cannot be obtained, resulting in a poor appearance.

[0053] <<Storage Stability>> The prepared powder coating was stored at 40°C for 7 days, and then the stored powder coating was used for painting. The appearance of the painted film after painting was visually evaluated. ○: A smooth coating film is obtained, resulting in a good appearance. ×: A smooth coating cannot be obtained, resulting in a poor appearance.

[0054] <<Water resistance>> After immersion in boiling tap water for 24 hours, the samples were left in an environment of 23°C / 80RH% for 7 days, and the appearance of the coating was visually inspected. ○: No blistering on the coating, and no visible change in the appearance of the coating compared to before immersion. △: Although there was no blistering of the coating, a change in hue was observed between the appearance of the coating before immersion and the visually observed color. ×: The coating had blistering, and a change in the appearance of the coating was observed visually compared to before immersion.

[0055] <<Solvent resistance>> An appropriate amount of xylene was applied to gauze, and the surface of the coating was wiped back and forth 10 times with a force of approximately 500 gf. The condition of the coating was then visually inspected. ○: No visible change or only minor change. △: A slight decrease in gloss or slight softening was observed visually. ×: The coating softened or dissolved.

[0056] [Table 2]

[0057] As shown in Table 2, the powder coatings of Examples 1 to 10 all received a "○" or "△" rating for coating film appearance, storage stability, water resistance, and solvent resistance, indicating no practical problems.

[0058] The epoxy resin powder coatings in Comparative Examples 1 and 2 had a poor coating appearance ("×") because they did not contain a hardener (B1).

[0059] The epoxy resin powder coating in Comparative Example 3 did not contain a hardener (B2), resulting in a "×" rating for solvent resistance.

[0060] The epoxy resin powder coating in Comparative Example 4, lacking a curing agent (B2), exhibited poor storage stability and water resistance.

[0061] The epoxy resin powder coating of Comparative Example 5 did not contain the curing agents (B2) and (B3), resulting in poor storage stability and water resistance.

[0062] The epoxy resin powder coating in Comparative Example 6 had poor water resistance ("×") because it contained a curing agent (B3) with a melting point exceeding 200°C.

[0063] The epoxy resin powder coating of Comparative Example 7 contained a different curing agent than those used in (B1), (B2), and (B3), resulting in a "×" rating for the appearance of the coating film.

[0064] The results from Examples 1-10 and Comparative Examples 1-7 show that the epoxy resin powder coating of the present invention has no practical problems in terms of coating film appearance, storage stability, water resistance, and solvent resistance.

Claims

1. An epoxy resin powder coating comprising epoxy resin (A), a curing agent (B), and a pigment (C), The epoxy resin (A) is a bisphenol F type epoxy resin (A1), The curing agent (B) is Imidazolin derivative (B1), An imidazole derivative (B2) that does not have a triazine skeleton, It contains an imidazole derivative (B3) having a triazine skeleton, The imidazoline derivative (B1) is contained in the epoxy resin powder coating in an amount of 0.1 to 2.0% by mass. The content of the imidazoline derivative (B1) is 33% by mass or more and 99% by mass or less, relative to the total amount of the imidazoline derivative (B1), the imidazole derivative (B2), and the imidazole derivative (B3). An epoxy resin powder coating wherein the melting point of the imidazole derivative (B3) is in the range of 150°C to 200°C.

2. The epoxy resin powder coating according to claim 1, wherein the imidazole derivative (B2) and the imidazole derivative (B3) are each contained in the epoxy resin powder coating in an amount of 0.01 to 1.0% by mass.

3. The epoxy resin powder coating according to claim 1 or 2, wherein the epoxy resin powder coating is used for coating straight pipes or irregularly shaped pipes.

4. A coating method comprising applying powder coating to the inner surface of a straight or irregularly shaped pipe preheated to a temperature of 120°C or higher and 280°C or lower, and then curing or heating to form a coating film on the inner surface of the pipe, The powder coating is an epoxy resin powder coating comprising an epoxy resin (A), a curing agent (B), and a pigment (C), The epoxy resin (A) is a bisphenol F type epoxy resin (A1), The curing agent (B) is Imidazolin derivative (B1), An imidazole derivative (B2) that does not have a triazine skeleton, It contains an imidazole derivative (B3) having a triazine skeleton, The imidazoline derivative (B1) is contained in the epoxy resin powder coating in an amount of 0.1 to 2.0% by mass. The content of the imidazoline derivative (B1) is 33% by mass or more and 99% by mass or less, relative to the total amount of the imidazoline derivative (B1), the imidazole derivative (B2), and the imidazole derivative (B3). A coating method wherein the melting point of the imidazole derivative (B3) is in the range of 150°C to 200°C.

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