Powder composition, powder coating material, coating film, and method for producing powder composition

Abstract

Provided are: a powder composition which can form coating films having excellent surface smoothness and comprises a powder including a fluororesin and a powder including a polyester resin; a powder coating material; a coating film; and a method for producing the powder composition.　This powder composition comprises a powder A including a hydroxylated fluororesin and a specific blocked isocyanate and having an average particle diameter of 25-80 μm and a powder B including a polyester resin.

Description

Powder composition, powder coating, coating film, and method for producing powder composition

[0001] The present invention relates to a powder composition, a powder coating, a coating film, and a method for producing the powder composition.

[0002] Paints containing fluororesin are capable of forming coating films with excellent durability, etc., and have therefore attracted attention from economic and environmental perspectives as materials that can reduce the life cycle costs of coated articles. Among such paints containing fluororesin, powder paints are widely used as one of the paints with a low environmental impact. Patent Document 1 discloses a powder paint containing a powder made of a composition containing a fluororesin and an ultraviolet absorber, and a powder made of a composition containing a thermosetting resin other than the fluororesin, a curing agent, and a light stabilizer.

[0003] International Publication No. 2015 / 016185

[0004] In recent years, the performance requirements for coating films obtained using powder coatings have been increasing, and specifically, further improvements in surface smoothness have been required. When the present inventors evaluated coating films obtained using the powder coating described in Patent Document 1, they found that there was room for improvement in the surface smoothness of the coating films.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to provide a powder composition containing a powder containing a fluororesin and a powder containing a polyester resin, which are capable of forming a coating film with excellent surface smoothness, a powder coating material, a coating film, and a method for producing the powder composition.

[0006] As a result of intensive research into the above-mentioned problems, the present inventors have found that when a powder composition comprising powder A containing a hydroxyl group-containing fluororesin and a specific blocked isocyanate and having an average particle size of 25 to 80 μm, and powder B containing a polyester resin is used as a raw material for a powder coating, the surface smoothness of a coating film formed using the powder coating is excellent, and have arrived at the present invention.

[0007] That is, the inventors have found that the above-mentioned problems can be solved by the following configurations. [1] A powder composition comprising: powder A, which contains a hydroxyl group-containing fluororesin and a blocked isocyanate having a curing initiation temperature of 185°C or higher and has an average particle size of 25 to 80 μm; and powder B, which contains a polyester resin. [2] The powder composition according to [1], wherein the blocked isocyanate has a melting point of 90°C or higher. [3] The powder composition according to [1] or [2], wherein the ratio of the molar amount of blocked isocyanate groups in the blocked isocyanate to the molar amount of hydroxyl groups in the hydroxyl group-containing fluororesin is 40 to 150%. [4] The powder composition according to any of [1] to [3], wherein the average particle size of powder A is 28 to 38 μm. [5] The powder composition according to any of [1] to [4], wherein powder A further contains a surface conditioner. [6] The powder composition according to any one of [1] to [5], wherein the powder A is substantially free of pigment. [7] The powder composition according to any one of [1] to [6], wherein the hydroxyl-containing fluororesin has units based on a fluoroolefin. [8] The powder composition according to any one of [1] to [7], wherein the hydroxyl-containing fluororesin has units based on chlorotrifluoroethylene and units based on a vinyl ether. [9] The powder composition according to any one of [1] to [8], wherein the average particle size of the powder B is 20 to 80 μm.

[10] The powder composition according to any one of [1] to [9], wherein the mass ratio of the content of the powder B to the content of the powder A is 0.8 to 9.0.

[11] A powder coating comprising the powder composition according to any one of [1] to

[10] .

[12] A coating film obtained using the powder coating according to

[11] .

[13] A method for producing a powder composition according to [1], comprising melting and kneading a mixture a1 containing a hydroxyl group-containing fluororesin and a blocked isocyanate having a curing initiation temperature of 185°C or higher to obtain a kneaded mixture a2, pulverizing the kneaded mixture a2 to obtain powder A having an average particle size of 25 to 80 μm, melting and kneading a mixture b1 containing a polyester resin to obtain kneaded mixture b2, pulverizing the kneaded mixture b2 to obtain powder B, and then dry-blending the powder A and the powder B.

[14] A method for producing a powder composition according to

[13] , wherein the blocked isocyanate has a melting point of 90°C or higher.

[0008] According to the present invention, it is possible to provide a powder composition having a powder containing a fluororesin and a powder containing a polyester resin, which can form a coating film with excellent surface smoothness, a powder coating material, a coating film, and a method for producing the powder composition.

[0009] The meanings of terms used in the present invention are as follows. A numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the upper and lower limits. In the numerical ranges described in this specification in stages, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another staged numerical range. Furthermore, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the Examples. In this specification, each component may be used alone or in combination with two or more substances corresponding to the component. Here, when two or more substances are used in combination for each component, the content of that component refers to the total content of the substances used in combination, unless otherwise specified. In this specification, a combination of two or more preferred embodiments is a more preferred embodiment. A unit is a collective term for an atomic group based on one molecule of the above-mentioned monomer that is formed directly by polymerization of the monomer, and an atomic group obtained by chemically converting a portion of the above-mentioned atomic group. The content (mol%) of each unit relative to the total units contained in the resin can be determined by analyzing the resin using nuclear magnetic resonance spectroscopy and can also be determined from the amounts of components used in producing the resin. Hydrolyzable silyl groups refer to groups that can undergo hydrolysis to form silanol groups. The acid value and hydroxyl value are values ​​measured in accordance with JIS K 0070-3 (1992). The glass transition temperature (Tg) is the midpoint glass transition temperature of the resin measured by differential scanning calorimetry (DSC). The melt viscosity is the melt viscosity value of a sample at 170°C when heated from 130°C to 200°C at a heating rate of 10°C / min using a rotational rheometer. The number average molecular weight (Mn) and weight average molecular weight (Mw) are values ​​measured by size exclusion chromatography (gel permeation chromatography) using polystyrene as a standard substance. The ratio of Mw to Mn represents the molecular weight distribution, and is also referred to as Mw / Mn. The average particle size of particles (powder) is the 50% diameter value obtained by calculating the volume average from the particle size distribution measured using a known particle size distribution measuring device (e.g., Helos-Rodos, manufactured by Sympatec) that uses laser diffraction as the measurement principle.The film thickness is a value measured using an eddy current film thickness meter (eg, EDY-5000, manufactured by Sanko Electronics Co., Ltd.).

[0010] [Powder Composition] The powder composition of the present invention includes powder A, which contains a hydroxyl group-containing fluororesin and a specific blocked isocyanate and has an average particle size of 25 to 80 μm, and powder B, which contains a polyester resin. Here, the term "powder composition" refers to a composition containing a powder. The specific blocked isocyanate refers to a blocked isocyanate having a melting point of 90°C or higher (hereinafter also referred to as "blocked isocyanate x1") or a blocked isocyanate having a curing initiation temperature of 185°C or higher (hereinafter also referred to as "blocked isocyanate x2"). Hereinafter, a first embodiment of the powder composition of the present invention will be described, and a second embodiment of the powder composition of the present invention will be described, in which powder A contains blocked isocyanate x1.

[0011] <First embodiment> A powder composition according to a first embodiment of the present invention (hereinafter also referred to as powder composition 1) includes powder A containing a hydroxyl group-containing fluororesin and a blocked isocyanate x1 and having an average particle size of 25 to 80 μm, and powder B containing a polyester resin.

[0012] When powder composition 1 is used as a raw material for a powder coating, the coating film obtained using the powder coating has excellent surface smoothness. The reason for this is not entirely clear, but it is believed to be as follows: When a powder coating containing a powder containing a hydroxyl-containing fluororesin and a powder containing a polyester resin is heated to form a coating, the polyester resin and the hydroxyl-containing fluororesin may undergo layer separation, with the hydroxyl-containing fluororesin being located in the upper layer (air side) of the coating film and the polyester resin being located in the lower layer (substrate side) of the coating film. Here, the curing rate of a conventional powder containing a hydroxyl-containing fluororesin is often higher than that of powder B containing a polyester resin. As a result, during coating film formation, the curing of the hydroxyl-containing fluororesin proceeds more rapidly, reducing the fluidity of the hydroxyl-containing fluororesin located in the upper layer of the coating film. As a result, the surface smoothness of the coating film is thought to be insufficient. To address this problem, the inclusion of blocked isocyanate x1 in powder A reduces the curing rate of powder A compared to that of a conventional powder containing a hydroxyl-containing fluororesin. That is, the curing rate of Powder A becomes close to the curing rate of Powder B. As a result, the fluidity of the hydroxyl group-containing fluororesin constituting the upper layer of the coating film is improved, which is thought to improve the surface smoothness of the coating film. Furthermore, it is thought that the surface smoothness of the coating film is improved by setting the average particle size of Powder A to 25 to 80 μm.

[0013] Powder A contains a hydroxyl group-containing fluororesin and blocked isocyanate x1.

[0014] The average particle size of powder A is 25 to 80 μm, and from the viewpoint of obtaining better effects of the present invention, it is preferably 28 μm or more, more preferably 30 μm or more, and is preferably 60 μm or less, more preferably 50 μm or less, even more preferably 45 μm or less, particularly preferably 40 μm or less, and most preferably 35 μm or less.

[0015] The content of Powder A is preferably 20 to 100% by mass, more preferably 30 to 80% by mass, and even more preferably 50 to 70% by mass, relative to the total mass of Powder Composition 1. When the content of Powder A is equal to or greater than the lower limit, the weather resistance of the coating film is more excellent. When the content of Powder A is equal to or less than the upper limit, the gloss of the coating film is more excellent.

[0016] The hydroxyl group-containing fluororesin contained in powder A is a fluororesin having a hydroxyl group (hydroxy group).

[0017] The hydroxyl group-containing fluororesin preferably has units based on a fluoroolefin. A fluoroolefin is an olefin in which one or more hydrogen atoms have been substituted with a fluorine atom. In the fluoroolefin, one or more hydrogen atoms that are not substituted with a fluorine atom may be substituted with a chlorine atom. The number of carbon atoms in the fluoroolefin is preferably 2 to 8, more preferably 2 to 4. Specific examples of the fluoroolefin include CF ２ =CF ２ (TFE), CF ２ =CFCl (chlorotrifluoroethylene), CF ２ = CHF, CH ２ =CF ２ (VDF), CF ２ =CFCF ３ , C.F. ２ = CHCF ３ , C.F. ３ CH=CHF, CF ３ CF=CH ２ , formula CH ２ =CX ｆ１ (CF ２ ) ｎ１ Y ｆ１ (In the formula, X ｆ１ and Y ｆ１ are each independently a hydrogen atom or a fluorine atom, and n1 is an integer of 2 to 10.) From the viewpoint of superior adhesion, CF ２ =CF ２ , C.H. ２ =CF ２ , C.F. ２ = CFCl, CF ３ CH=CHF, CF ３ CF=CH ２ is preferred, and CF ２ =CF ２ or CF ２ =CFCl is more preferred, CF ２ =CFCl is more preferred.

[0018] Two or more types of fluoroolefins may be used in combination. When the hydroxyl group-containing fluororesin contains units based on fluoroolefin, the content of the units based on fluoroolefin is preferably 30 to 70 mol%, more preferably 40 to 60 mol%, and even more preferably 45 to 55 mol%, based on all repeating units of the hydroxyl group-containing fluororesin. When the content of units based on fluoroolefin is equal to or greater than the lower limit, the weather resistance of the coating film (cured film) is excellent. When the content of units based on fluoroolefin is equal to or less than the upper limit, the adhesion between the fluororesin layer and the polyester resin layer is excellent upon layer separation.

[0019] The hydroxyl group-containing fluororesin preferably has a unit based on vinyl ether. The vinyl ether may be a vinyl ether having a crosslinkable group or may be a vinyl ether without a crosslinkable group. The vinyl ether preferably has a crosslinkable group from the viewpoint of further improving the durability of the coating film obtained using the hydroxyl group-containing fluororesin. Specific examples of the crosslinkable group include at least one group selected from the group consisting of a hydroxyl group (hydroxy group), an amino group, an epoxy group, an oxetanyl group, a hydrolyzable silyl group, a sulfo group, and a carboxy group. From the viewpoint of further improving the hardness of the coating film, a hydroxyl group or a carboxy group is preferred, and a hydroxyl group is more preferred. Note that the sulfo group and the carboxy group can be ionized to form -SO ３ － or -COO － It may be -SO ３ － Na ＋ or -COO － Na ＋ etc. The number of crosslinkable groups may be 1 or 2 or more. Two or more types of crosslinkable groups may be used in combination. Two or more types of vinyl ethers may be used in combination, and among these, an embodiment in which a vinyl ether having a crosslinkable group and a vinyl ether not having a crosslinkable group are used in combination is preferred. The vinyl ether may or may not have a fluorine atom, but it is preferable that it does not have a fluorine atom. Two or more types of vinyl ethers having a crosslinkable group may be used in combination.

[0020] Specific examples of vinyl ethers having a hydroxyl group include CH ２ =CHO-CH ２ -cycloC ６ H １０ -CH ２ OH, CH ２ =CHOCH ２ CH ２ OH, CH ２ =CHOCH ２ CH ２ CH ２ CH ２ OH. ６ H １０ "-" represents a cyclohexylene group, and "-cycloC ６ H １０ The bonding site of "-" is usually 1,4-. Two or more types of vinyl ethers having a hydroxyl group may be used in combination.

[0021] Specific examples of vinyl ethers having a carboxy group include CH ２ = CHO(CH ２ ) ｎ１２ OC(O)CH ２ CH ２ Examples include monomers represented by COOH (where n12 represents an integer of 1 to 10). Two or more types of vinyl ethers having a carboxy group may be used in combination.

[0022] Specific examples of vinyl ethers having no crosslinkable group include ethyl vinyl ether, tert-butyl vinyl ether, 2-ethylhexyl vinyl ether, cyclohexyl vinyl ether, and cyclohexylmethyl vinyl ether. Two or more types of vinyl ethers having no crosslinkable group may be used in combination.

[0023] The content of units based on vinyl ether is preferably 1 to 80 mol%, more preferably 10 to 70 mol%, even more preferably 20 to 60 mol%, and particularly preferably 45 to 55 mol%, based on all repeating units of the hydroxyl-containing fluororesin. When the units based on vinyl ether include units based on vinyl ether having a crosslinkable group (particularly a hydroxyl group), the content of units based on vinyl ether having a crosslinkable group (particularly a hydroxyl group) is preferably 1 mol% or more, more preferably 3 mol% or more, even more preferably 5 mol% or more, particularly preferably 10 mol% or more, based on all repeating units of the hydroxyl-containing fluororesin, from the viewpoint of further improving the durability of the coating film obtained using the hydroxyl-containing fluororesin, and is preferably 30 mol% or less, more preferably 25 mol% or less, and even more preferably 20 mol% or less. When the vinyl ether-based units contain units based on vinyl ether that do not have a crosslinkable group, the content of the units based on vinyl ether that do not have a crosslinkable group is, from the viewpoint of further improving the smoothness and hardness of a coating film obtained using the hydroxyl group-containing fluororesin, preferably 1 mol % or more, more preferably 5 mol % or more, even more preferably 10 mol % or more, particularly preferably 25 mol % or more, and preferably 70 mol % or less, more preferably 60 mol % or less, even more preferably 50 mol % or less, particularly preferably 40 mol % or less, and most preferably 30 mol % or less, based on all repeating units of the hydroxyl group-containing fluororesin.

[0024] The hydroxyl-containing fluororesin may contain units other than fluoroolefin-based units and vinyl ether-based units (hereinafter also referred to as other units), but it is preferable that it is substantially free of other units in order to obtain a coating film having better weather resistance using the hydroxyl-containing fluororesin. Here, "substantially free of other units" means that the content of other units is 0.1 mol% or less based on the total repeating units of the hydroxyl-containing fluororesin. A specific example of other units is a vinyl ester-based unit.

[0025] The hydroxyl group-containing fluororesin preferably does not have a carboxy group, since this can further improve the hardness of the coating film obtained by using the hydroxyl group-containing fluororesin.

[0026] The hydroxyl group-containing fluororesin preferably contains units based on chlorotrifluoroethylene and units based on vinyl ether, more preferably units based on chlorotrifluoroethylene, units based on vinyl ether having a crosslinkable group (preferably a hydroxyl group), and units based on vinyl ether having no crosslinkable group, further preferably a fluorocopolymer essentially consisting of units based on chlorotrifluoroethylene, units based on vinyl ether having a crosslinkable group (preferably a hydroxyl group), and units based on vinyl ether having no crosslinkable group, and particularly preferably a fluorocopolymer consisting only of units based on chlorotrifluoroethylene, units based on vinyl ether having a crosslinkable group (preferably a hydroxyl group), and units based on vinyl ether having no crosslinkable group. "A fluorocopolymer essentially consisting of units based on chlorotrifluoroethylene, units based on vinyl ether having a crosslinkable group, and units based on vinyl ether having no crosslinkable group" means that the content of units other than units based on chlorotrifluoroethylene, units based on vinyl ether having a crosslinkable group, and units based on vinyl ether having no crosslinkable group is 0.1 mol% or less based on all repeating units of the hydroxyl group-containing fluororesin. The hydroxyl group-containing fluororesin preferably contains 30 to 70 mol %, 1 to 30 mol %, and 1 to 70 mol % of units based on chlorotrifluoroethylene, units based on vinyl ether having a crosslinkable group (preferably a hydroxyl group), and units based on vinyl ether having no crosslinkable group, in this order, based on all repeating units of the hydroxyl group-containing fluororesin, and more preferably contains 45 to 55 mol %, 5 to 25 mol %, and 10 to 50 mol %, in this order.

[0027] The Tg of the hydroxyl group-containing fluororesin is preferably 20 to 60°C, more preferably 25 to 60°C, and even more preferably 30 to 55°C, from the viewpoint of further improving the blocking resistance of powder A and the surface smoothness of a coating film obtained using the powder coating material.

[0028] The melt viscosity of the hydroxyl group-containing fluororesin at 170° C. is preferably from 0.1 to 450 Pa·s, more preferably from 1 to 450 Pa·s, and even more preferably from 3 to 400 Pa·s.

[0029] The hydroxyl value of the hydroxyl group-containing fluororesin is preferably more than 0 mgKOH / g and less than 150 mgKOH / g, more preferably from 5 to 120 mgKOH / g, even more preferably from 15 to 110 mgKOH / g, and particularly preferably from 30 to 100 mgKOH / g. When the hydroxyl group-containing fluororesin has an acid value, the acid value of the hydroxyl group-containing fluororesin is preferably more than 0 mgKOH / g and less than 10 mgKOH / g, more preferably from 2 to 8 mgKOH / g, and even more preferably from 3 to 7 mgKOH / g.

[0030] The Mw of the hydroxyl group-containing fluororesin is preferably 10,000 to 60,000, more preferably 12,000 to 55,000, and even more preferably 15,000 to 50,000, from the viewpoint of achieving superior effects of the present invention. The Mn of the hydroxyl group-containing fluororesin is preferably 5,000 to 20,000, more preferably 6,000 to 18,000, and even more preferably 7,000 to 16,000, from the viewpoint of achieving superior effects of the present invention. The Mw / Mn (molecular weight distribution) of the hydroxyl group-containing fluororesin is preferably 2.0 to 3.5, more preferably 2.1 to 3.3, and even more preferably 2.2 to 3.2, from the viewpoint of achieving superior effects of the present invention.

[0031] The content of the hydroxyl group-containing fluororesin is preferably 55 to 80 mass %, more preferably 60 to 75 mass %, and even more preferably 65 to 70 mass %, based on the total mass of powder A. When the content of the hydroxyl group-containing fluororesin is equal to or greater than the lower limit, the blocking resistance is excellent. On the other hand, when the content of the hydroxyl group-containing fluororesin is equal to or less than the upper limit, the impact resistance of the coating film is excellent.

[0032] The blocked isocyanate is a compound containing a blocked isocyanate group. The blocked isocyanate x1 is a blocked isocyanate having a melting point of 90°C or higher. The blocked isocyanate x1 is used as a curing agent. The blocked isocyanate group is a group in which an isocyanate group is blocked with a blocking agent. When the powder A is heated and melted, the blocking agent in the blocked isocyanate group of the blocked isocyanate x1 is eliminated to form an isocyanate group, which reacts with the hydroxyl group of the hydroxyl group-containing fluororesin. This promotes the curing of the resulting coating film.

[0033] The blocked isocyanate x1 preferably has two or more blocked isocyanate groups, more preferably has two to three blocked isocyanate groups, and even more preferably has two blocked isocyanate groups, in order to provide excellent curing properties for the coating film.

[0034] The blocked isocyanate x1 is preferably one produced by reacting an aliphatic, aromatic, or araliphatic diisocyanate with a low molecular weight compound having active hydrogen to obtain a polyisocyanate, and then reacting the resulting polyisocyanate with a blocking agent to perform masking. Specific examples of the diisocyanate include tolylene diisocyanate, 4,4'-diphenylmethane isocyanate, xylylene diisocyanate, hexamethylene diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), methylcyclohexane diisocyanate, bis(isocyanatomethyl)cyclohexaneisophorone diisocyanate, dimer acid diisocyanate, and lysine diisocyanate. Specific examples of low molecular weight compounds having active hydrogen include water, ethylene glycol, propylene glycol, trimethylolpropane, glycerin, sorbitol, ethylenediamine, ethanolamine, diethanolamine, hexamethylenediamine, isocyanurate, uretidione, low molecular weight polyesters containing hydroxyl groups, and polycaprolactone. Specific examples of blocking agents include alcohols (methanol, ethanol, benzyl alcohol, etc.), phenols (phenol, cresol, etc.), lactams (ε-caprolactam, butyrolactam, etc.), and oximes (cyclohexanone, oxime, methyl ethyl ketoxime, etc.), with lactams being preferred and ε-caprolactam being more preferred.

[0035] As the blocked isocyanate x1, a commercially available product can be used, such as Crelan (registered trademark) UI manufactured by Sumika Covestro Urethane Co., Ltd.

[0036] The melting point of the blocked isocyanate x1 is 90°C or higher, and from the viewpoint of obtaining superior effects of the present invention, it is preferably 91°C or higher, more preferably 92°C or higher, and even more preferably 93°C or higher. From the viewpoint of fluidity at curing temperatures, the upper limit of the melting point of the blocked isocyanate x1 is preferably 120°C or lower, more preferably 115°C or lower, and even more preferably 110°C or lower. The melting point of the blocked isocyanate x1 is measured by DSC (differential scanning calorimetry).

[0037] The curing initiation temperature of the blocked isocyanate x1 is preferably 185°C or higher, more preferably 188°C or higher, and even more preferably 191°C or higher. When a powder coating containing a powder containing a hydroxyl-containing fluororesin and a powder containing a polyester resin is heated to form a coating film, the polyester resin and the hydroxyl-containing fluororesin may undergo layer separation, with the hydroxyl-containing fluororesin being located in the upper layer (air side) of the coating film and the polyester resin being located in the lower layer (substrate side) of the coating film. The curing rate of a conventional powder containing a hydroxyl-containing fluororesin is often higher than that of powder B containing a polyester resin. As a result, during coating film formation, the curing of the hydroxyl-containing fluororesin proceeds rapidly, reducing the fluidity of the hydroxyl-containing fluororesin located in the upper layer of the coating film. This is thought to result in insufficient surface smoothness of the coating film. To address this issue, if the curing initiation temperature of the blocked isocyanate x1 is 185°C or higher, the curing rate of powder A is likely to be lower than that of a conventional powder containing a hydroxyl-containing fluororesin. That is, the curing rate of powder A becomes closer to the curing rate of powder B. As a result, it is believed that the fluidity of the fluororesin constituting the upper layer of the coating film is improved, thereby improving the surface smoothness of the coating film. The curing initiation temperature of the blocked isocyanate x1 is preferably 210°C or lower, more preferably 200°C or lower. When the curing initiation temperature of the blocked isocyanate is 210°C or lower, the heating maintenance time during coating film formation can be shortened, thereby further improving productivity. In addition, since the curing of the coating film is completed within a specified time, the solvent resistance of the coating film is further improved.

[0038] The curing initiation temperature of the blocked isocyanate x1 is measured as follows. First, a fluororesin (Lumiflon (registered trademark) LF710F, manufactured by AGC, a hydroxyl group-containing fluororesin) and the blocked isocyanate x1 are mixed to obtain a mixture (the number of moles of hydroxyl groups in the fluororesin: the number of moles of blocked isocyanate groups in the blocked isocyanate x1 = 1:1). Next, 1 g of the mixture is placed in a rheometer (HAAKE MARS rheometer (trade name), manufactured by Thermo Fisher Scientific) and heated from 110°C to 220°C at a heating rate of 10°C / min, and the melt viscosity (Pa s) of the mixture is measured. The melt viscosity decreases with heating, and the temperature at which the melt viscosity starts to increase (i.e., the temperature at which the minimum value is shown in a graph with temperature on the horizontal axis and melt viscosity on the vertical axis) is defined as the curing initiation temperature of the blocked isocyanate x1. The reason why the melt viscosity starts to increase is thought to be that the reaction between the isocyanate groups generated from the blocked isocyanate x1 by the heating and the hydroxyl groups of the fluororesin progresses, causing the molecules to lose fluidity due to crosslinking.

[0039] The ratio of the molar amount of blocked isocyanate groups in the blocked isocyanate x1 to the molar amount of hydroxyl groups in the hydroxyl-containing fluororesin is preferably 40 to 150%, more preferably 50 to 140%, even more preferably 70 to 120%, particularly preferably 90 to 110%, even more preferably 95 to 105%, and more preferably 100%. If this ratio is 40% or more, the solvent resistance of the coating film is better. If this ratio is 150% or less, the non-stickiness is better. Here, the above ratio is calculated based on the charged amounts of the hydroxyl-containing fluororesin and blocked isocyanate x1.

[0040] The content of the blocked isocyanate x1 is preferably 10 to 30 mass %, more preferably 12 to 25 mass %, and even more preferably 13 to 23 mass %, relative to the total mass of powder A, from the viewpoint of excellent curability of the coating film.

[0041] Powder A preferably contains a surface conditioner, which can further improve the surface smoothness of the coating film.

[0042] The term "surface conditioner" refers to a compound that orients on the surface of a coating film during the curing process, thereby uniforming the surface tension of the coating film. Specific examples of surface conditioners include polyacrylate-based surface conditioners and castor oil-based surface conditioners. From the viewpoint of achieving superior effects of the present invention, it is preferable to include a castor oil-based surface conditioner, and it is preferable to include both a castor oil-based surface conditioner and a polyacrylate-based surface conditioner. Commercially available surface conditioners can also be used, and specific examples include BYK-360P (trade name, manufactured by BYK Corporation, polyacrylate-based surface conditioner adsorbed on silicon dioxide), Resiflow P-67 (trade name, manufactured by Estron Chemical Corporation, polyacrylate-based surface conditioner), and RHEOBYK-7590 (trade name, manufactured by BYK Corporation, castor oil-based surface conditioner).

[0043] The surface conditioner may be used alone, but it is preferable to use two or more types in combination. By using two or more types of surface conditioners, the surface smoothness of the coating film is further improved.

[0044] The content of the surface conditioner is preferably 5 to 20 mass %, more preferably 7 to 13 mass %, based on the total mass of powder A, in order to further improve the surface smoothness of the coating film.

[0045] Powder A may further contain additives. Specific examples of additives include catalysts (curing catalysts, etc.), fillers (resin beads, etc.), light stabilizers, ultraviolet absorbers, matting agents, degassing agents, flow agents, heat stabilizers, antistatic agents, rust inhibitors, silane coupling agents, low-staining treatment agents, plasticizers, adhesives, etc. The content of the additives is preferably 5 to 15 mass %, more preferably 5 to 13 mass %, relative to the total mass of Powder A.

[0046] It is preferable that Powder A is substantially free of pigment, which results in excellent gloss of the coating film. Here, "substantially free of pigment" means that the pigment content is 0.001% by mass or less, relative to the total mass of Powder A, and is preferably 0% by mass (i.e., no pigment is contained).

[0047] Powder B contains a polyester resin. The curing initiation temperature of Powder B is preferably 190 to 210° C., in order to improve the surface smoothness of the coating film.

[0048] The average particle size of Powder B is preferably 20 to 80 μm, more preferably 25 μm or more, even more preferably 28 μm or more, and particularly preferably 30 μm or more, from the viewpoint of achieving better surface smoothness and coating efficiency, and is more preferably 70 μm or less, even more preferably 60 μm or less, particularly preferably 50 μm or less, still more preferably 45 μm or less, more preferably 40 μm or less, and even more preferably 35 μm or less.

[0049] The content of Powder B is preferably 50 to 90 mass%, more preferably 60 to 90 mass%, even more preferably 70 to 85 mass%, and particularly preferably 75 to 85 mass%, relative to the total mass of Powder Composition 1. When the content of Powder B is equal to or greater than the lower limit, the color development of the coating film is more excellent. When the content of Powder B is equal to or less than the upper limit, the weather resistance of the coating film is more excellent.

[0050] The mass ratio of the content of powder B to the content of powder A (content of powder B / content of powder A) is preferably 0.8 to 9.0, more preferably 1.0 to 6.0, even more preferably 1.5 to 5.0, particularly preferably 2.3 to 4.5, and most preferably 3.0 to 4.0. When the mass ratio is equal to or greater than the lower limit, the weather resistance of the coating film is more excellent. When the mass ratio is equal to or less than the upper limit, the gloss of the coating film is more excellent.

[0051] As the polyester resin contained in powder B, commercially available products can be used, and specific examples thereof include "CRYLCOAT (registered trademark) 4642-3", "CRYLCOAT (registered trademark) 4890-0", and "CRYLCOAT (registered trademark) 4842-3" manufactured by Daicel Allnex Corporation, "UPICACOAT (registered trademark) GV-250", "UPICACOAT (registered trademark) GV-740", "UPICACOAT (registered trademark) GV-175", and "UPICACOAT (registered trademark) GV-110" manufactured by Japan U-PICA Corporation, "Uralac (registered trademark) P1680" and "Uralac (registered trademark) P6504" manufactured by DSM, and "Vylon (registered trademark) 220" manufactured by Toyobo Co., Ltd.

[0052] The content of the polyester resin is preferably 60 to 90% by mass, more preferably 70 to 90% by mass, and even more preferably 75 to 90% by mass, based on the total mass of powder B. When the content of the polyester resin is equal to or greater than the lower limit, the impact resistance of the coating film is superior. When the content of the polyester resin is equal to or less than the upper limit, the weather resistance of the coating film is superior.

[0053] Powder B may further contain additives. Specific examples of additives include pigments, curing agents, catalysts (such as curing catalysts), fillers (such as resin beads), light stabilizers, ultraviolet absorbers, matting agents, surface conditioners, degassing agents, flow agents, heat stabilizers, antistatic agents, rust inhibitors, silane coupling agents, low-staining treatment agents, plasticizers, adhesives, etc. The content of the additives is preferably 5 to 25% by mass, more preferably 10 to 20% by mass, relative to the total mass of powder B.

[0054] Powder composition 1 may contain components other than powder A and powder B, but preferably contains only powder A and powder B. In powder composition 1, the total content of powder A and powder B is preferably 95 to 100 mass %, and more preferably 100 mass %, relative to the total mass of powder composition 1.

[0055] The method for producing powder composition 1 includes the following steps 1 to 3. Step 1: melt-kneading a mixture a1 containing a hydroxyl group-containing fluororesin and a blocked isocyanate x1 to obtain a kneaded product a2, and pulverizing the kneaded product a2 to obtain powder A having an average particle size of 25 to 80 μm. Step 2: melt-kneading a mixture b1 containing a polyester resin to obtain a kneaded product b2, and pulverizing the kneaded product b2 to obtain powder B. Step 3: dry-blending powder A and powder B. Here, melt-kneading means kneading while melting.

[0056] In addition to the hydroxyl group-containing fluororesin and blocked isocyanate x1, mixture a1 may also contain the above-mentioned surface conditioner and additives. The content of each component contained in mixture a1 may be adjusted appropriately so that the content of each component contained in powder A falls within the above-mentioned range. Mixture a1 is obtained by mixing the components contained in mixture a1. It is preferable that each component used in producing mixture a1 is previously pulverized into powder. Specific examples of equipment used for mixing include a high-speed mixer, a V-type mixer, and a reversing mixer.

[0057] Specific examples of the apparatus used for kneading include a single-screw extruder, a twin-screw extruder, and a planetary gear extruder. After cooling, the kneaded product a2 is preferably pelletized.

[0058] Examples of the apparatus used for pulverizing the kneaded material a2 include pulverizers such as a pin mill, a hammer mill, and a jet mill.

[0059] Step 1 preferably includes a classification treatment. The classification treatment is carried out after pulverization of the kneaded material a2. By carrying out the classification treatment, the average particle size of the powder A can be easily adjusted to fall within the above range. Specific examples of the classification method include a sieving method and an air classification method.

[0060] In addition to the polyester resin, the mixture b1 may contain the additives described above. The content of each component contained in the mixture b1 may be adjusted appropriately so that the content of each component contained in the powder B falls within the above-mentioned range. The mixture b1 is obtained by mixing the components contained in the mixture b1. It is preferable that each component used in producing the mixture b1 is pulverized in advance to form a powder. Specific examples of the apparatus used for mixing are the same as the apparatus used in producing the mixture a1.

[0061] Specific examples of the apparatus used for kneading the mixture b1 are the same as those used for kneading the mixture a1. After cooling, the kneaded product b2 is preferably pelletized.

[0062] The apparatus used for pulverizing the kneaded material b2 is the same as the apparatus used for pulverizing the kneaded material a2.

[0063] The step 2 preferably includes a classification treatment. The classification treatment is carried out after the kneaded material b2 is pulverized. Specific examples of the classification method are the same as those of the classification treatment in the step 1.

[0064] Dry blending refers to the mixing of two or more powders. Specific examples of devices used for dry blending include a high-speed mixer, a double cone mixer, a kneader, a dumpler mixer, a mixing shaker, a drum shaker, and a rocking shaker. The mixing ratio of powder A and powder B used when dry blending can be appropriately adjusted so that the mass ratio of the content of powder B to the content of powder A in the powder composition falls within the above-mentioned range.

[0065] Second Embodiment A powder composition according to a second embodiment of the present invention (hereinafter also referred to as powder composition 2) comprises powder A, which contains a hydroxyl-containing fluororesin and a blocked isocyanate x2 and has an average particle size of 25 to 80 μm, and powder B, which contains a polyester resin. When powder composition 2 is used as a raw material for a powder coating, the coating film obtained using the powder coating exhibits excellent surface smoothness. The reason for this is not entirely clear, but is thought to be as follows. When a coating film is formed by heating a powder coating containing a powder containing a hydroxyl-containing fluororesin and a powder containing a polyester resin, layer separation between the polyester resin and the hydroxyl-containing fluororesin may occur, resulting in the hydroxyl-containing fluororesin being positioned in the upper layer (air side) of the coating film and the polyester resin being positioned in the lower layer (substrate side) of the coating film. Here, the curing rate of conventional powders containing hydroxyl-containing fluororesin is often greater than that of powder B, which contains a polyester resin. As a result, during coating film formation, the curing of the hydroxyl-containing fluororesin proceeds rapidly, reducing the fluidity of the hydroxyl-containing fluororesin disposed in the upper layer of the coating film. As a result, it is believed that the surface smoothness of the coating film will be insufficient. To address this problem, by including blocked isocyanate x2 in powder A, the curing rate of powder A is reduced compared to the curing rate of conventional powders containing hydroxyl-containing fluororesin. In other words, the curing rate of powder A becomes closer to the curing rate of powder B. As a result, it is believed that the fluidity of the hydroxyl-containing fluororesin constituting the upper layer of the coating film is improved, improving the surface smoothness of the coating film. Furthermore, it is believed that the average particle diameter of powder A is 25 to 80 μm, improving the surface smoothness of the coating film.

[0066] Powder composition 2 is similar to the above-described powder composition 1, including preferred embodiments, except that blocked isocyanate x2 is used instead of blocked isocyanate x1 as the blocked isocyanate contained in powder A. Therefore, hereinafter, the blocked isocyanate x2, which is a difference from powder composition 1, will be described, and the description of the components other than blocked isocyanate x2 will be omitted because they are the same as those of powder composition 1. However, in the content described in the above-described first embodiment, the term "powder composition 1" should be read as "powder composition 2."

[0067] The blocked isocyanate is a compound containing a blocked isocyanate group. The blocked isocyanate x2 is a blocked isocyanate having a curing initiation temperature of 185°C or higher. The blocked isocyanate x2 is used as a curing agent. The blocked isocyanate group is a group in which an isocyanate group is blocked with a blocking agent. When the powder A is heated and melted, the blocking agent in the blocked isocyanate group of the blocked isocyanate x2 is eliminated to form an isocyanate group, which reacts with the hydroxyl group of the hydroxyl group-containing fluororesin. This promotes the curing of the resulting coating film.

[0068] The blocked isocyanate x2 preferably has two or more blocked isocyanate groups, more preferably has two to three blocked isocyanate groups, and even more preferably has two blocked isocyanate groups, in order to provide excellent curing properties for the coating film.

[0069] The blocked isocyanate x2 is preferably produced by reacting an aliphatic, aromatic, or araliphatic diisocyanate with a low molecular weight compound having active hydrogen to obtain a polyisocyanate, followed by reaction with a blocking agent for masking. The diisocyanate, the low molecular weight compound having active hydrogen, and the blocking agent are the same as those for the blocked isocyanate x2, including preferred embodiments.

[0070] As the blocked isocyanate x2, a commercially available product can be used, such as Crelan (registered trademark) UI manufactured by Sumika Covestro Urethane Co., Ltd.

[0071] The curing initiation temperature of the blocked isocyanate x2 is 185°C or higher, preferably 188°C or higher, and more preferably 191°C or higher, from the viewpoint of achieving better effects of the present invention. The curing initiation temperature of the blocked isocyanate x2 is preferably 210°C or lower, and more preferably 200°C or lower. When the curing initiation temperature of the blocked isocyanate is 210°C or lower, the heating maintenance time during coating film formation can be shortened, thereby further improving productivity. In addition, since curing of the coating film is completed within a predetermined time, the solvent resistance of the coating film is further improved.

[0072] The curing initiation temperature of the blocked isocyanate x2 is measured in the same manner as the curing initiation temperature of the blocked isocyanate x1, except that the blocked isocyanate x2 is used instead of the blocked isocyanate x1.

[0073] The melting point of the blocked isocyanate x2 is preferably 90°C or higher, more preferably 91°C or higher, even more preferably 92°C or higher, and particularly preferably 93°C or higher. When a powder coating containing a powder containing a hydroxyl-containing fluororesin and a powder containing a polyester resin is heated to form a coating film, the polyester resin and the hydroxyl-containing fluororesin may undergo layer separation, with the hydroxyl-containing fluororesin being located in the upper layer (air side) of the coating film and the polyester resin being located in the lower layer (substrate side) of the coating film. The curing rate of a conventional powder containing a hydroxyl-containing fluororesin is often higher than that of powder B containing a polyester resin. As a result, during coating film formation, the curing of the hydroxyl-containing fluororesin proceeds more rapidly, reducing the fluidity of the hydroxyl-containing fluororesin located in the upper layer of the coating film. As a result, the surface smoothness of the coating film is thought to be insufficient. To address this issue, if the melting point of the blocked isocyanate x2 is 90°C or higher, the curing rate of powder A is likely to be lower than that of a conventional powder containing a hydroxyl-containing fluororesin. That is, the curing rate of powder A becomes closer to the curing rate of powder B. As a result, it is believed that the fluidity of the hydroxyl group-containing fluororesin constituting the upper layer of the coating film is improved, thereby improving the surface smoothness of the coating film. The upper limit of the melting point of the blocked isocyanate x2 is preferably 120°C or lower, more preferably 115°C or lower, and even more preferably 110°C or lower, in terms of fluidity at the curing temperature. The melting point of the blocked isocyanate x2 is measured in the same manner as the method for measuring the melting point of the blocked isocyanate x1, except that the blocked isocyanate x2 is used instead of the blocked isocyanate x1.

[0074] The ratio of the molar amount of blocked isocyanate groups in the blocked isocyanate x2 to the molar amount of hydroxyl groups in the hydroxyl-containing fluororesin is preferably 40 to 150%, more preferably 50 to 140%, even more preferably 70 to 120%, particularly preferably 90 to 110%, even more preferably 95 to 105%, and more preferably 100%. If this ratio is 40% or more, the solvent resistance of the coating film is better. If this ratio is 150% or less, the non-stickiness is better. Here, the above ratio is calculated based on the charged amounts of the hydroxyl-containing fluororesin and the blocked isocyanate x2.

[0075] The content of the blocked isocyanate x2 is preferably 10 to 30% by mass, more preferably 12 to 25% by mass, and even more preferably 13 to 23% by mass, relative to the total mass of powder A, from the viewpoint of excellent curability of the coating film.

[0076] The method for producing powder composition 2 includes the following steps 1 to 3: Step 1: melt-kneading a mixture a1 containing a hydroxyl group-containing fluororesin and a blocked isocyanate x2 to obtain a kneaded product a2, and pulverizing the kneaded product a2 to obtain powder A having an average particle size of 25 to 80 μm; Step 2: melt-kneading a mixture b1 containing a polyester resin to obtain a kneaded product b2, and pulverizing the kneaded product b2 to obtain powder B; Step 3: dry-blending powder A and powder B. The method for producing powder composition 2 is similar to the method for producing powder composition 1 described above, including preferred embodiments, except that blocked isocyanate x2 is used instead of blocked isocyanate x1 in step 1, and therefore, description thereof will be omitted.

[0077] [Powder Coating] The powder coating of the present invention (hereinafter also referred to as the present powder coating) contains the above-mentioned powder composition 1 or powder composition 2. The present powder coating may contain components other than the above-mentioned powder composition 1 or powder composition 2, but preferably contains only the above-mentioned powder composition 1 or powder composition 2. In other words, it is preferable to use the above-mentioned powder composition 1 or powder composition 2 as the present powder coating itself. In the present powder coating, the content of the above-mentioned powder composition 1 or powder composition 2 is preferably 95 to 100 mass %, and more preferably 100 mass %, relative to the total mass of the present powder coating.

[0078] [Coating film] The coating film of the present invention (hereinafter also referred to as the present coating film) is a coating film obtained using the present powder coating material described above. The present coating film is preferably formed by applying the present powder coating material described above to a substrate.

[0079] Specific examples of the material of the substrate include inorganic substances, organic substances, and organic-inorganic composites. Specific examples of inorganic substances include concrete, natural stone, glass, and metals (iron, stainless steel, aluminum, copper, brass, titanium, etc.). Specific examples of organic substances include plastics, rubber, adhesives, and wood. Specific examples of organic-inorganic composites include fiber-reinforced plastics, resin-reinforced concrete, and fiber-reinforced concrete. The substrate may also be subjected to a known surface treatment (chemical conversion treatment, etc.). The surface of the substrate may also have a resin layer (polyester resin layer, acrylic resin layer, silicone resin layer, etc.) formed by applying a primer or the like.

[0080] The substrate is preferably made of a metal, more preferably iron, an alloy containing iron (e.g., carbon steel, stainless steel), or aluminum. Specific examples of the shape of the substrate include a flat plate, a sphere, and a rod.

[0081] The thickness of the coating film is preferably 20 to 1,000 μm, more preferably 20 to 500 μm.

[0082] The present coating film may be a coating film present on a coated article. In this case, the coated article preferably has a substrate and a coating film formed from the present powder coating material disposed on the substrate. The coated article is preferably produced by applying (painting) the present powder coating material to the surface of the substrate to form a coating layer, heating the resulting coating layer, and then cooling it. Methods for forming the coating layer include coating methods such as electrostatic coating, electrostatic spraying, electrostatic immersion, fluidized bed immersion, and spraying, with electrostatic coating using a powder coating gun being preferred. Specific examples of powder coating guns include corona-charged coating guns and tribo-charged coating guns. Corona-charged coating guns are coating guns that spray powder coating material after corona discharge treatment. Tribo-charged coating guns are coating guns that spray powder coating material after tribo-charge treatment. The heating temperature during heat treatment is preferably 120 to 250°C. The heating temperature during the heat treatment is preferably 5 to 80°C higher than the curing initiation temperature of the blocked isocyanate, more preferably 10 to 70°C higher, and even more preferably 10 to 50°C higher. The heating time is usually 2 to 60 minutes. After the heat treatment, it is preferably cooled to 20 to 25°C. The coating layer melts and hardens (melt-hardens) through the heat treatment and cooling, forming a main coating film.

[0083] The present invention will be described in detail below with reference to examples. Examples 1 to 14 are working examples, and Examples 15 to 17 are comparative examples. However, the present invention is not limited to these examples. The blending amounts of each component in the tables below are based on mass.

[0084] Synthesis Example 1 Into a 2,500 mL stainless steel pressure reactor equipped with a stirrer, 20 g of a piperidyl group-containing compound (manufactured by BASF, trade name "TINUVIN292"), a mixture of bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate and methyl 1,2,2,6,6-pentamethyl-4-piperidylsebacate (mass ratio 3:1), and 20 g of hydrotalcite (Kyowa Chemical Kogyo Co., Ltd. (trade name "KW500"), particle size 45 μm or less: 38%, 45-75 μm: 35%, 75-106 μm: 21%, 106-500 μm: 6%), 679 g of xylene, 118 g of 4-hydroxybutyl vinyl ether (hereinafter also referred to as HBVE), and 453 g of cyclohexyl vinyl ether (hereinafter also referred to as CHVE) were charged, and dissolved oxygen in the liquid was removed by degassing with nitrogen. Furthermore, 536 g of chlorotrifluoroethylene (hereinafter also referred to as CTFE) was introduced into the reactor, and the temperature was increased. When the temperature inside the reactor reached 65°C, the pressure indicated 0.59 MPaG. Thereafter, 2 ml of a 5% xylene solution of tert-butyl peroxypivalate (hereinafter also referred to as PBPV) was added to the reactor, and the reaction was initiated. As the pressure decreased, while maintaining the pressure, additional monomers (110 g of CTFE, 93 g of CHVE, and 24 g of HBVE) were continuously added to the reactor, and a radical polymerization initiator, a 5% xylene solution of PBPV, was intermittently added to the reactor to allow polymerization to proceed. After the start of polymerization, the total amount of 5% xylene solution of PBPV intermittently added to the reactor was 55 ml. After 16 hours, the reactor was water-cooled to stop the reaction, and the reaction solution was cooled to room temperature, after which unreacted monomers were purged and the reactor was opened. The resulting reaction solution was transferred to a pressure filter equipped with filter paper No. 63 for viscous liquids, and the hydrotalcite was filtered out at a pressure of 0.05 MPa. Then, at least a portion of the solvent in the reaction solution was distilled off using a vacuum distillation apparatus under reduced pressure heating at 85 ° C and 55 Torr to obtain a solution containing fluororesin a1. Then, xylene was added to the solution containing fluororesin a1 to adjust the concentration, and composition 1 was obtained. The composition of the fluororesin a1 was CTFE units / HBVE units / CHVE units=50 / 11 / 39 (molar ratio).

[0085] Synthesis Example 2 A 2500 mL stainless steel pressure reactor equipped with a stirrer was charged with 8.6 g of a piperidyl group-containing compound (manufactured by BASF under the trade name "TINUVIN 292"), 5.7 g of hydrotalcite, 850 g of xylene, 196 g of ethyl vinyl ether (hereinafter also referred to as EVE), 123 g of HBVE, 198 g of CHVE, and 10 g of PBPV, and dissolved oxygen in the liquid was removed by pressurizing with nitrogen, purging, and degassing. Next, 629 g of CTFE was introduced, and the temperature was gradually raised. The reaction was continued while maintaining the temperature at 65°C. After 12 hours, the reactor was water-cooled to stop the reaction. The reaction liquid was cooled to room temperature, and then the unreacted monomer was purged and the reactor was opened. The obtained reaction solution was transferred to a pressure filter equipped with a No. 63 filter paper for viscous liquids, and the hydrotalcite was filtered off at a pressure of 0.05 MPa. Then, using a vacuum distillation apparatus, at least a portion of the solvent in the reaction solution was distilled off under reduced pressure heating at 85 ° C and 55 Torr to obtain a solution containing fluororesin a2. Then, xylene was added to the solution containing fluororesin a2 to adjust the concentration, and composition 2 was obtained. The composition of fluororesin a2 was CTFE unit / EVE unit / HBVE unit / CHVE unit = 50 / 25 / 10 / 15 (molar ratio).

[0086] [Synthesis Example 3] 579 g of xylene, 145 g of ethanol, 269 g of HBVE, 292 g of CHVE, 10 g of potassium carbonate, and 25 g of PBPV were charged into a 2,500 mL stainless steel pressure reactor equipped with a stirrer. Dissolved oxygen in the liquid was removed by pressurization, purging, and degassing with nitrogen. Next, 539 g of CTFE was introduced, and the temperature was gradually increased. The reaction was continued while maintaining the temperature at 65 °C. After 12 hours, the reactor was water-cooled to stop the reaction. After the reaction liquid was cooled to room temperature, unreacted monomer was purged, and the reactor was opened. The resulting reaction liquid was transferred to a pressure filter equipped with No. 63 filter paper for viscous liquids. The potassium carbonate was filtered off (pre-filtration) at a pressure of 0.05 MPa, and then 0.1 g of hydroquinone monomethyl ether (hereinafter referred to as HQMME) was added. Next, the reaction solution was concentrated under reduced pressure and heating using a vacuum distillation apparatus until the amount of ethanol in the reaction solution became 0.03% by mass or less relative to the polymerization solvent. Next, the solid content of the concentrated solution was measured, and then fresh xylene was added to adjust the solid content to 60% by mass, thereby obtaining a concentration-adjusted solution. Next, the concentration was adjusted to 0.06 g / cm with respect to the filtration area. ２ Diatomaceous earth (median particle size 30.1 μm) was added to the concentration-adjusted liquid, mixed and stirred, and then transferred to a pressure filter equipped with filter paper No. 63 for viscous liquids and filtered twice at a pressure of 0.2 MPa to remove the diatomaceous earth, thereby obtaining composition 3 containing fluororesin a3. The composition of fluororesin a3 was CTFE units / HBVE units / CHVE units = 50 / 25 / 25 (molar ratio).

[0087] Synthesis Example 4 A composition 4 containing a fluororesin a4 was obtained in the same manner as in Synthesis Example 3, except that the ingredients were changed to 576 g of xylene, 144 g of ethanol, 105 g of EVE, 219 g of HBVE, 179 g of CHVE, and 24 g of PBPV. The composition of the fluororesin a4 was CTFE units / EVE units / HBVE units / CHVE units=50 / 15 / 20 / 15 (molar ratio).

[0088] [Production of Fluororesins] At least a portion of the solvent contained in each of Compositions 1 to 4 was distilled off using a vacuum dryer. The resulting mixture was dried under full vacuum at a temperature of 65°C for 3 hours. Thereafter, the resulting mixture was dried under full vacuum at a temperature of 130°C for 20 minutes to obtain Fluororesins a1 to a4.

[0089] [Production of Fluororesin-Containing Powder] Fluororesin-containing powders A-1 to A-17 were produced as follows.

[0090] <Production of Powder A-1> First, fluororesin a1 was powdered using a pulverizer. Next, powdered fluororesin a1, curing agent 1 (manufactured by Sumika Covestro Urethane Co., Ltd., Crelan (registered trademark) UI, melting point 93 ° C., curing initiation temperature 191 ° C.), surface conditioner 1 (manufactured by BYK, BYK-360P), surface conditioner 2 (manufactured by Estron Chemical, Resiflow P-67), surface conditioner 3 (manufactured by BYK, RHEOBYK-7590), benzoin as a degassing agent, Tinuvin 479 (manufactured by BASF) as an ultraviolet absorber, and Tinuvin 111FDL (manufactured by BASF) as a hindered amine light stabilizer were added in the mass ratios shown in Table 1, and then mixed for about 10 to 30 minutes using a high-speed mixer (manufactured by Yuzaki Co., Ltd.) to obtain a powder mixture. Next, the powder mixture was melt-kneaded using a twin-screw extruder (16 mm extruder manufactured by Thermo Prism) at a barrel setting temperature of 120°C to obtain a pellet-like kneaded product. The obtained pellet-like kneaded product was pulverized at room temperature using a pulverizer (Rotor Speed ​​Mill P14 manufactured by FRITSCH) and classified using a 150 mesh sieve to obtain Powder A-1.

[0091] <Production of Powders A-2 to A-17> Powders A-2 to A-17 were obtained in the same manner as in the production of Powder A-1, except that the types and amounts of components used were as shown in Table 1. Details of the components other than those shown in the production of Powder A-1 are as follows.

[0092] Fluorine resins a2 to a4: Fluorine resins produced as described above, used in the same powder form as Fluorine resin a1. Curing agent 2: Vestagon (registered trademark) B1530, manufactured by EVONIK, blocked isocyanate, melting point 65°C, curing initiation temperature 183°C.

[0093]

[0094] [Production of Powder Containing Polyester Resin] Polyester resin-containing powders B-1 to B-9 were produced as follows. The components listed in Table 2 were mixed in powder form using a high-speed mixer. The resulting mixture was melt-kneaded using a twin-screw extruder (16 mm extruder manufactured by Thermoprism) at a barrel setting temperature of 120°C to obtain a pellet-shaped kneaded product. The resulting pellet-shaped kneaded product was pulverized at 25°C using a pulverizer, and the resulting pulverized product was classified using a 150 mesh to obtain powders B-1 to B-7, respectively. Details of the components used in the production of powders B-1 to B-9 are as follows.

[0095] Polyester resin b1: Daicel-Allnex Corporation, CRYLCOAT (registered trademark) 4890-0, hydroxyl value: 30 mg KOH / g, melt viscosity: 5.25 Pa·s (190°C) Polyester resin b2: Daicel-Allnex Corporation, CRYLCOAT (registered trademark) 4842-3, acid value: 36 mg KOH / g, melt viscosity: 1.83 Pa·s (190°C) Polyester resin b3: DSM Corporation, Uralac (registered trademark) P6504, hydroxyl value: 40 mg KOH / g, melt viscosity: 3.06 Pa·s (190°C) Polyester resin b4: DSM Corporation, Uralac (registered trademark) P1680, hydroxyl value: 30 mg KOH / g, melt viscosity: 0.88 Pa·s (190°C) Polyester resin b5: U-Pica Coat (registered trademark) GV-110, manufactured by Japan U-Pica Corporation, hydroxyl value: 49 mg KOH / g, melt viscosity: 2.95 Pa·s (190°C) Polyester resin b6: U-Pica Coat (registered trademark) GV-740, manufactured by Japan U-Pica Corporation, hydroxyl value: 50 mg KOH / g, melt viscosity: 1.69 Pa·s (190°C) Polyester resin b7: Vylon (registered trademark) 220, manufactured by Toyobo Co., Ltd., hydroxyl value: 50 mg KOH / g, melt viscosity: 0.55 Pa·s (190°C)

[0096] Curing agent 2: As described above Curing agent 3: Araldite (trade name) PT910, a glycidyl ester-based curing agent manufactured by HUNTSMAN Curing catalyst: xylene solution of dibutyltin dilaurate (10,000-fold diluted product) Light stabilizer: Tinuvin 111FDL (trade name) manufactured by BASF Degassing agent: Benzoin Surface conditioner 1: BYK-360P (trade name) manufactured by BYK

[0097]

[0098] [Examples 1 to 17 (Production of Powder Coatings)] Powders A-1 to A-17 and powders B-1 to B-9 were mixed in the combinations and blending ratios shown in Table 3 to obtain powder coatings (powder compositions) of Examples 1 to 17, respectively.

[0099] [Production of coated substrates] Using the powder coating material of each example, one side of a chromate-treated aluminum plate was electrostatically coated using an electrostatic coater (GX3600C, manufactured by Onoda Cement Co., Ltd.) to form a coating layer of the powder coating material. Each aluminum plate with a coating layer was then held in an atmosphere of 180°C for 20 minutes. After heating, the aluminum plate with a coating layer corresponding to each example was left to cool to room temperature, yielding an aluminum plate (coated article) with a coating film (cured film) having a thickness of 55 to 65 μm.

[0100] [Evaluation] <Coating rate relative to target film thickness> The film thickness of the coating film on the coated aluminum plate was measured, and the ratio of the measured film thickness to the target film thickness was calculated, and the calculated ratio was taken as the coating rate (%). The film thickness is a value measured using an eddy current film thickness meter (such as EDY-5000, product name, manufactured by Sanko Electronics Co., Ltd.). ○: Coating rate is 80% or more. ×: Coating rate is less than 80%.

[0101] <Surface Smoothness> The surface smoothness of the coating film on the coated aluminum sheet was evaluated according to the following criteria using standard plates for visually judging smoothness provided by the Powder Coating Institute (PCI). There were 10 standard plates, numbered 1 to 10, with the larger the number, the better the surface smoothness. When standard plate n (n represents an integer between 1 and 9) and standard plate n+1 were judged to be better than standard plate n but inferior to standard plate n+1, the PCI value was recorded as n+0.5. ○: PCI value of 6.5 or more. ×: PCI value of 6 or less.

[0102] <Solvent resistance> Absorbent cotton soaked in xylene was applied with 500 g / cm ２ The coating film was rubbed back and forth 100 times under a load of 0.05g, and peeling was visually observed. ◯: No peeling was observed in the coating film. Δ: Peeling was observed in more than 0% but less than 20% of the coating film area. ×: Peeling was observed in 20% or more of the coating film area.

[0103] <Non-adhesiveness> Five sheets of gauze were placed on the coating surface of a coated aluminum plate, a 500 g weight was placed in the center of the gauze, and the sheet was left in the air at 45°C for 24 hours. After leaving the sheet, the gauze was removed from the coating surface, and the degree of adhesion between the coating surface and the gauze and the traces of the texture on the coating surface were checked. ○: No adhesion or texture trace. △: At least one of adhesion or texture trace was present.

[0104]

[0105] As shown in Table 3, it was confirmed that when the powder composition of the present invention was used as a powder coating, a coating film having excellent surface smoothness was obtained (Examples 1 to 14).

[0106] The entire contents of the specification, claims and abstract of Japanese Patent Application No. 2024-154227, filed on September 6, 2024, are incorporated herein by reference as the disclosure of the present invention.

Claims

1. A powder composition comprising: powder A containing a hydroxyl group-containing fluororesin and a blocked isocyanate having a curing initiation temperature of 185°C or higher, and having an average particle size of 25 to 80 μm; and powder B containing a polyester resin.

2. The powder composition according to claim 1, wherein the blocked isocyanate has a melting point of 90°C or higher.

3. The powder composition according to claim 1 or 2, wherein the ratio of the molar amount of blocked isocyanate groups in said blocked isocyanate to the molar amount of hydroxyl groups in said hydroxyl group-containing fluororesin is 40 to 150%.

4. The powder composition according to claim 1 or 2, wherein the average particle size of said powder A is 28 to 38 μm.

5. The powder composition according to claim 1 or 2, wherein said powder A further contains a surface conditioner.

6. The powder composition according to claim 1 or 2, wherein said powder A is substantially free of pigments.

7. The powder composition according to claim 1 or 2, wherein the hydroxyl group-containing fluororesin has units based on fluoroolefin.

8. The powder composition according to claim 1 or 2, wherein the hydroxyl group-containing fluororesin has units based on chlorotrifluoroethylene and units based on vinyl ether.

9. The powder composition according to claim 1 or 2, wherein the average particle size of said powder B is 20 to 80 μm.

10. The powder composition according to claim 1 or 2, wherein the mass ratio of the content of said powder B to the content of said powder A is 0.8 to 9.

0.

11. A powder coating comprising the powder composition according to claim 1 or 2.

12. A coating film obtained using the powder coating material according to claim 11.

13. A method for producing a powder composition according to claim 1, comprising melting and kneading a mixture a1 containing a hydroxyl group-containing fluororesin and a blocked isocyanate having a curing initiation temperature of 185°C or higher to obtain a kneaded product a2, pulverizing the kneaded product a2 to obtain powder A having an average particle size of 25 to 80 μm, melting and kneading a mixture b1 containing a polyester resin to obtain kneaded product b2, pulverizing the kneaded product b2 to obtain powder B, and then dry-blending the powder A and the powder B.

14. The method for producing a powder composition according to claim 13, wherein the melting point of the blocked isocyanate is 90°C or higher.

Images