Method for producing particles and method for producing compacts

Abstract

To provide a method for producing particles excellent in removability of low boiling components and capable of suppressing fibrillation even when pulverization treatment is carried out, and a method for producing a molded product.SOLUTION: A method for producing particles of the present invention is a method of heat-treating particles A comprising a melt-moldable fluororesin at a temperature of at least the midpoint glass transition temperature and at most the extrapolated melting initiation temperature to obtain particles B comprising the fluororesin.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing particles and a method for producing a molded article containing a fluororesin. [Background technology]

[0002] Fluorine resins, which have excellent heat resistance, chemical resistance, weather resistance, etc., are used in various fields such as the semiconductor industry, the automobile industry, the chemical industry, etc. For example, by applying a powder composed of particles containing fluororesin to the surface of a substrate by methods such as electrostatic coating, fluidized bed coating, and rotational molding, it is possible to form a coating film that protects the surface of the substrate and has excellent chemical resistance. In recent years, there has been a demand for fluororesins with a low content of low-boiling components. If particles containing fluororesin contain a large amount of low-boiling components, white smoke may be generated due to evaporation of the low-boiling components when these particles are used to produce a molded product such as a coating film. For this reason, Patent Document 1 discloses a method for removing the low-boiling components contained in the fluororesin by controlling the melt volume flow rate of the fluororesin when melt-kneading the fluororesin such as ETFE using a twin-screw extruder. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 209133 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when the present inventors subjected pellets of fluororesin obtained by the method described in Patent Document 1 to a pulverization treatment, fibrillated pulverized material was generated, and particles usable as powder were not obtained in some cases. Therefore, there is a need for a method for producing particles containing fluororesin that is excellent in removing low-boiling components and that suppresses fibrillation even when a pulverization treatment is performed.

[0005] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a method for producing particles and a method for producing molded bodies that are excellent in removing low-boiling components and can suppress fibrillation even when a crushing process is performed. [Means for solving the problem]

[0006] As a result of intensive research into the above-mentioned problems, the present inventors have found that if particles A containing a melt-formable fluororesin are heat-treated at a temperature equal to or higher than the midpoint glass transition temperature and equal to or lower than the extrapolated melting onset temperature, the particles can be excellently removed with low boiling components, and fibrillation can be suppressed even when a crushing treatment is performed, thereby arriving at the present invention.

[0007] That is, the inventors have found that the above problems can be solved by the following configuration. [1] A method for producing particles, comprising heat-treating particles A containing a melt-moldable fluororesin at a temperature equal to or higher than the midpoint glass transition temperature and equal to or lower than the extrapolated melting onset temperature, to obtain particles B containing the fluororesin. [2] The method for producing particles according to [1], wherein the temperature of the heat treatment is at least 10°C higher than the midpoint glass transition temperature and at least 10°C lower than the extrapolated melting onset temperature. [3] The method for producing particles according to [1] or [2], wherein the heat treatment is carried out for 30 minutes to 10 hours. [4] The method for producing particles according to any one of [1] to [3], wherein the fluororesin is obtained by polymerizing a fluorine-containing monomer and a non-fluorine-containing monomer in the presence of a hydrofluorocarbon or a hydrofluoroether. [5] The fluorine-containing monomer is at least one selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, perfluoro(alkyl vinyl ether), chlorotrifluoroethylene, vinylidene fluoride, and vinyl fluoride; The method for producing particles according to [4], wherein the non-fluorine-containing monomer is at least one selected from the group consisting of ethylene, propylene, itaconic anhydride, and vinyl acetate. [6] The method for producing particles according to any one of [1] to [5], wherein the fluororesin is any one of a copolymer containing units based on tetrafluoroethylene and units based on ethylene, a copolymer containing units based on chlorotrifluoroethylene and units based on ethylene, a copolymer containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), and a copolymer containing units based on tetrafluoroethylene and units based on hexafluoropropylene. [7] The method for producing particles according to any one of [1] to [6], wherein the fluororesin is a copolymer containing units based on tetrafluoroethylene and units based on ethylene, and the copolymer further contains a monomer F1 represented by the following formula (F1): CH2=CX(CF2) n Y (F1) (X and Y each independently represent a hydrogen atom or a fluorine atom, and n represents an integer of 2 to 8.) [8] The method for producing particles according to any one of [1] to [7], wherein the particles A have an average particle size of 100 μm to 10.0 mm. [9] The method for producing particles according to any one of [1] to [8], wherein the particles A contain low-boiling components, and the content of the low-boiling components is more than 0 mass% and 0.5 mass% or less relative to the total mass of the particles A.

[10] The method for producing particles according to [9], wherein the reduction rate of the low boiling components before and after the heat treatment calculated using the content of the low boiling components contained in the particles A and the particles B by the following formula is 20 to 100%. Reduction rate of low boiling components before and after heat treatment [%] = 100 × (content of low boiling components contained in particle A - content of low boiling components contained in particle B) / (content of low boiling components contained in particle A)

[11] The method for producing particles according to any one of [1] to

[10] , wherein the particles B are pulverized to obtain particles C.

[12] The method for producing particles according to

[11] , wherein the particles C obtained by pulverizing the particles B have an average particle size of 10 to 1000 μm.

[13] The apparent density of particles C obtained by pulverizing the particles B is 0.3 to 1.2 g / cm 3 The method for producing particles according to

[11] or

[12] , wherein

[14] A method for producing a molded article, characterized by melting particles produced by the method according to any one of [1] to

[13] at a temperature equal to or higher than the melting point of the fluororesin to obtain a molded article containing the fluororesin. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method for producing particles and a method for producing a molded product that are excellent in the ability to remove low-boiling components and that can suppress fibrillation even when a crushing treatment is carried out. Although the mechanism by which the above-mentioned effects of the present invention are obtained is not entirely clear, one possible reason is that the removal of low boiling components makes the fluororesin particles denser, thereby suppressing the formation of fibrillation. DETAILED DESCRIPTION OF THE INVENTION

[0009] The terms used in the present invention have the following meanings. A numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. "Melt-moldable" means that the material exhibits melt fluidity. "Exhibiting melt fluidity" means that there exists a temperature at which the melt volumetric flow rate is 0.1 to 1000 g / 10 min at a temperature 20°C or more higher than the melting point of the resin under a load of 49 N. The "melt volumetric flow rate" refers to the melt mass flow rate (MFR) specified in JIS K 7210:1999 (ISO 1133:1997). The midpoint glass temperature and the extrapolated melting onset temperature are both values (°C) calculated in accordance with the method of JIS K7121 (1987) and are determined based on a DSC curve obtained by differential scanning calorimetry (DSC). mg " and the extrapolated melting onset temperature is "T im "It is also called ". The term "unit" refers collectively to an atomic group derived from one molecule of the monomer that is formed directly by polymerizing the monomer, and an atomic group obtained by chemically converting a part of the atomic group. In the following, units derived from individual monomers may be referred to by the name of the monomer followed by "unit." The "average particle size of particles" is a volume-based median size obtained by measurement using a laser diffraction / scattering particle size distribution measuring device.

[0010] The method for producing particles of the present invention is to melt-mold particles A containing a fluororesin at a temperature of midpoint glass transition temperature (T mg ) or higher than the extrapolated melting onset temperature (T im ) or lower (hereinafter also referred to as "specific heat treatment") to obtain the particles B containing the fluororesin. The method for producing particles of the present invention is excellent in removing low boiling components. The reason for this is not clear, but mg It is presumed that heat treatment of particle A at the above temperature facilitates the molecular chain motion of the fluororesin, allowing the low boiling components to be sufficiently removed from particle A. im It is presumed that heat treatment of particle A at the following temperatures can prevent the particles from fusing together due to partial melting of the fluororesin, thereby allowing the low boiling components to be sufficiently removed from the particles. Also, T im Heat-treating particles A at the following temperatures has the advantages of facilitating the pulverization of the resulting particles B and suppressing the generation of fibrillated pulverized material.

[0011] [Particle A] The particles A contain a melt-moldable fluororesin (hereinafter also referred to as a "specific fluororesin.") The specific fluororesin is preferably solid at room temperature (25°C).

[0012] The specific fluororesin is not particularly limited as long as it is melt-moldable, and any known fluororesin can be used. However, from the viewpoint of excellent moldability, it is preferable that the specific fluororesin has units based on a fluorine-containing monomer and units based on a non-fluorine-containing monomer.

[0013] Specific examples of the fluorine-containing monomer include tetrafluoroethylene (hereinafter also referred to as "TFE"), hexafluoropropylene (hereinafter also referred to as "HFP"), perfluoro(alkyl vinyl ether), chlorotrifluoroethylene (hereinafter also referred to as "CTFE"), vinylidene fluoride (hereinafter also referred to as "VdF"), and vinyl fluoride. One type of fluorine-containing monomer may be used alone, or two or more types may be used in combination. Specific examples of the non-fluorine monomer include ethylene, propylene, itaconic anhydride, and vinyl acetate. The non-fluorine monomer may be used alone or in combination of two or more. Specific examples of the specific fluororesin include a copolymer (ETFE) containing units based on TFE (hereinafter also referred to as "TFE units") and units based on ethylene (hereinafter also referred to as "E units"), a copolymer (ECTFE) containing units based on CTFE and units based on ethylene, a copolymer (PFA) containing TFE units and units based on perfluoro(alkyl vinyl ether) (hereinafter also referred to as "perfluoroalkyl vinyl ether units"), and a copolymer (FEP) containing TFE units and units based on hexafluoropropylene (hereinafter also referred to as "hexafluoropropylene units"), of which ETFE and ECTFE are preferred, with ETFE being particularly preferred, due to their excellent moldability.

[0014] In ETFE, the proportion of the content of E units relative to the total content of E units and TFE units is preferably from 20 to 70 mol %, more preferably from 25 to 60 mol %, and particularly preferably from 35 to 55 mol %. When the content of E units is at least the lower limit of the above range, the mechanical strength is excellent, and when it is at most the upper limit, the chemical resistance is excellent.

[0015] ETFE may be a copolymer consisting of only E units and TFE units, or may contain one or more types of other monomer units. A preferred example of the other monomer is a monomer F1 represented by the following formula (F1).

[0016] CH2=CX(CF2) n Y (F1) In formula (F1), X and Y each independently represent a hydrogen atom or a fluorine atom, and n represents an integer of 2 to 8 (preferably 3 to 7, more preferably 4 to 6). Specific examples of the monomer F1 include CH2=CF(CF2) n F, CH2=CF(CF2) n H, CH2=CH(CF2) n F, CH2=CH(CF2) n H, CH2=CH(CF2) n F is preferred, and CH2=CH(CF2)2F and CH2=CH(CF2)4F ((perfluorobutyl)ethylene, hereinafter referred to as PFBE) are more preferred.

[0017] When ETFE contains other monomer units, the total content thereof is preferably 0.1 to 10 mol, more preferably 0.1 to 5 mol, and particularly preferably 0.2 to 4 mol, in terms of the molar ratio where the total of E units and TFE units is 100 mol. When the content of other monomer units is at least the lower limit of the above range, the crack resistance is good, and when it is at most the upper limit, the melting point of the fluororesin is lowered and the moldability is good.

[0018] In PFA, the ratio of the content of TFE units to the total content of TFE units and perfluoroalkyl vinyl ether units is preferably 9 to 99 mol%, more preferably 80 to 99 mol%, and particularly preferably 90 to 99 mol%. PFA may be a copolymer consisting only of TFE units and perfluoroalkyl vinyl ether units, or may contain one or more other monomer units. A preferred example of the other monomer is hexafluoropropylene. When the other monomer unit is contained, the content thereof is preferably 0.1 to 10 mol, more preferably 0.1 to 6 mol, and particularly preferably 0.2 to 4 mol, in terms of the molar ratio relative to 100 mol of the total of TFE units and perfluoroalkyl vinyl ether units.

[0019] In FEP, the proportion of the content of TFE units relative to the total content of TFE units and hexafluoropropylene units is preferably from 70 to 99 mol %, more preferably from 80 to 99 mol %, and particularly preferably from 90 to 99 mol %. The FEP may be a copolymer consisting of only TFE units and hexafluoropropylene units, or may contain one or more types of other monomer units. Preferred examples of the other monomer include perfluoroalkyl vinyl ethers. When other monomer units are contained, the total content thereof is preferably 0.1 to 10 mol, more preferably 0.1 to 6 mol, and particularly preferably 0.2 to 4 mol, in terms of a molar ratio relative to 100 mol of the total of TFE units and hexafluoropropylene units.

[0020] In ECTFE, the proportion of the content of E units relative to the total content of E units and chlorotrifluoroethylene units is preferably from 2 to 98 mol %, more preferably from 10 to 90 mol %, and particularly preferably from 30 to 70 mol %. ECTFE may be a copolymer consisting of only E units and chlorotrifluoroethylene units, or may contain other monomer units. Preferred examples of the other monomer include perfluoroalkyl vinyl ethers. When other monomer units are contained, the total content thereof is preferably 0.1 to 10 mol, more preferably 0.1 to 5 mol, and particularly preferably 0.2 to 4 mol, in terms of a molar ratio where the total of E units and chlorotrifluoroethylene units is 100 mol.

[0021] The particles A may contain a low-boiling component. Specific examples of the low-boiling component include, but are not limited to, unreacted monomers among the monomers used for polymerization of the specific fluororesin, low-molecular-weight fluorine-containing polymers (oligomers, etc.) produced during polymerization of the specific fluororesin, and polymerization solvents used for polymerization of the specific fluororesin. At least a portion of the low boiling components that may be contained in the particles A is removed by the particle production method of the present invention. When particles A contain low-boiling components, the content of the low-boiling components is preferably more than 0% by mass and not more than 0.5% by mass, more preferably more than 0% by mass and not more than 0.3% by mass, and particularly preferably more than 0% by mass and not more than 0.2% by mass, relative to the total mass of particles A. The content of low boiling components is calculated using the following formula based on the mass of particle A before and after treatment by using a simultaneous differential thermal and thermogravimetric analyzer (for example, an apparatus similar to TG / DTA7200 (manufactured by Hitachi High-Technologies Corporation)) to heat 60 mg of particle A in dry air from 50°C at a rate of 10°C / min and then holding the temperature at a temperature 30°C lower than the melting point of the specific fluororesin for 30 minutes. Content of low boiling points in particle A [mass%] = 100 × (mass of particle A before treatment - mass of particle A after treatment) / mass of particle A before treatment)

[0022] The shape of the particles A is not particularly limited, and may be any shape such as a sphere (including an ellipsoid), a columnar shape (for example, a cylindrical shape), or the like. The average particle size of particles A is preferably 100 μm to 10.0 mm, more preferably 0.5 mm to 10.0 mm, and particularly preferably 0.8 mm to 8.0 mm. When the average particle size of particles A is at least the lower limit of the above range, handling is excellent, and when it is at most the upper limit, grindability is excellent.

[0023] <Method of manufacturing specific fluororesin> Examples of methods for producing the specific fluororesin include known methods such as suspension polymerization, solution polymerization, emulsion polymerization, and bulk polymerization, with suspension polymerization and solution polymerization being preferred, and solution polymerization being particularly preferred. A preferred embodiment of the method for producing the specific fluororesin includes a method of polymerizing units based on a fluorine-containing monomer and units based on a non-fluorine-containing monomer in the presence of a hydrofluorocarbon or a hydrofluoroether. Hereinafter, the method for producing the specific fluororesin will be described based on this preferred embodiment. The fluorine-containing monomer, non-fluorine-containing monomer and specific fluorine resin are as described above, and therefore the description thereof will be omitted.

[0024] Hydrofluorocarbons and hydrofluoroethers are used as polymerization solvents and chain transfer agents. Hydrofluorocarbons and hydrofluoroethers are more environmentally friendly than chlorine-containing fluorine solvents such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Specific examples of hydrofluorocarbons include 1,1,2,2-tetrafluorocyclobutane, CF3CFHCF2CF2CF3, CF3(CF2)4H, CF3CF2CFHCF2CF3, CF3CFHCFHCF2CF3, CF2HCFHCF2CF2CF3, CF3(CF2)5H, CF3CH(CF3)CF2CF2CF3, CF3CF(CF3)CFHCF2CF3, CF3CF(CF3)CFHCFHCF3, CF3CH(CF3)CFHCF2CF3, CF3CF2CH2CH3, and CF3(CF2)3CH2CH3. Specific examples of hydrofluoroethers include CF3CH2OCF2CF2H, (CF3)2CFCF(CF2CF3)OCH3, (CF3)2CFCF2OCH2CH3, (CF3)2CFCF2OCH3, CF3(CF2)3OCH2CH3, CF3(CF2)3OCH3, and C3F7OCH3. The amount of hydrofluorocarbon or hydrofluoroether used as the polymerization solvent is preferably 20 to 99.99 parts by mass, particularly preferably 99 to 99.9 parts by mass, based on the total mass (100 parts by mass) of the polymerization solvent and chain transfer agent. The amount of hydrofluorocarbon or hydrofluoroether used as the chain transfer agent is preferably 0.01 to 80 parts by mass, particularly preferably 0.1 to 1 part by mass, based on the total mass (100 parts by mass) of the polymerization solvent and the chain transfer agent.

[0025] In the polymerization of the specific fluororesin, known components other than the above-mentioned monomers, hydrofluorocarbons, and hydrofluoroethers may be used, and examples thereof include polymerization solvents (e.g., organic solvents other than hydrofluorocarbons and hydrofluoroethers, and water), chain transfer agents (chain transfer agents other than hydrofluorocarbons and hydrofluoroethers; e.g., alcohols and hydrocarbons), polymerization initiators, and polymerization inhibitors. As the polymerization conditions for the specific fluororesin (for example, reaction temperature, reaction time), known conditions can be adopted.

[0026] <Method of manufacturing particle A> The particles A can be obtained, for example, by a granulation process using the slurry containing the specific fluororesin obtained as described above. The slurry is a solution in which the specific fluororesin is dissolved or swollen in the polymerization solvent and in a suspended state. The content of the specific fluororesin is preferably 1 to 40 mass %, particularly preferably 3 to 35 mass %, based on the total mass of the slurry, in terms of excellent granulation properties. The content of the polymerization solvent is preferably from 60 to 99% by mass, particularly preferably from 65 to 97% by mass, based on the total mass of the slurry, in terms of excellent granulation properties.

[0027] The slurry is preferably used by mixing with water because it has excellent granulation properties. When water and the slurry are mixed and used, the content of water is preferably 100 to 700 parts by mass, particularly preferably 150 to 550 parts by mass, per 100 parts by mass of the slurry, in terms of excellent granulation properties.

[0028] The granulation process may be a known granulation process. A specific example of the granulation process is a process in which a slurry is stirred in a granulation tank. The granulation process may be carried out while heating. The temperature in the granulation treatment (hereinafter also referred to as "granulation temperature") may be lower than the melting temperature of the specific fluororesin, and specifically, 10 to 130°C is preferred, and 20 to 110°C is particularly preferred. The granulation treatment time and stirring conditions (for example, the number of revolutions of the stirring blade) are not particularly limited and are appropriately set in accordance with known conditions.

[0029] After the production of particles A, it is preferable that particles A are not heated at a temperature exceeding the extrapolated melting onset temperature of the specific fluororesin contained in particles A until the specific heat treatment to obtain particles B described below is performed. This prevents a portion of particles A from melting, thereby preventing a decrease in grindability that may occur due to the melted portion, and as a result, particle diameters after grinding tend to be uniform. Specifically, it is more preferable that particles A are not melt-kneaded. This prevents the generation of fibrillar ground material when particles B described below are ground.

[0030] [Specific heat treatment] The specific heat treatment is performed at the midpoint glass transition temperature (T mg ) or higher than the extrapolated melting onset temperature (T im ) or less. This treatment results in the production of particles B. By carrying out the heat treatment within this heating temperature range, low-boiling components can be effectively removed from particles A for the reasons described above. Furthermore, when the resulting particles B are pulverized, this treatment has the advantages of facilitating the pulverization treatment and suppressing the generation of fibrillated pulverized material. The specific heat treatment is carried out at a temperature equal to or higher than the midpoint glass transition temperature, but is preferably carried out at a temperature 10° C. or higher than the midpoint glass transition temperature (i.e., T mg +10≦heating temperature of specific heat treatment [°C]), and more preferably, it is carried out at a temperature 30°C or higher than the midpoint glass transition temperature (i.e., T mg +30≦heating temperature of specific heat treatment [°C]), and it is particularly preferred to carry out the heat treatment at a temperature 50°C or higher than the midpoint glass transition temperature (i.e., T mg +50≦ specific heat treatment temperature [℃]). The specific heat treatment is carried out at a temperature equal to or lower than the extrapolated melting onset temperature, but is preferably carried out at a temperature close to the extrapolated melting onset temperature, particularly preferably at the extrapolated melting onset temperature, in order to more effectively remove low boiling components from the particles A. Here, the temperature close to the extrapolated melting onset temperature is, for example, T im -10< specific heat treatment temperature [℃] ≦ T im Refers to temperatures within the range. In addition, the specific heat treatment is preferably carried out at a temperature 10°C or more lower than the extrapolated melting onset temperature (i.e., the heating temperature of the specific heat treatment [°C] ≦ T im -10), and more preferably, it is carried out at a temperature 20°C or more lower than the extrapolated melting onset temperature (i.e., the heating temperature of the specific heat treatment [°C] ≦ T im -20), and it is particularly preferable to carry out the specific heat treatment at a temperature 30°C or more lower than the extrapolated melting onset temperature (i.e., the heating temperature of the specific heat treatment [°C] ≦ T im -30). Other suitable ranges for the specific heat treatment are also possible. For example, the specific heat treatment is preferably carried out at a temperature that is 10°C or higher than the midpoint glass transition temperature and 10°C or lower than the extrapolated melting onset temperature (i.e., T mg +10≦Specific heat treatment temperature [℃]≦T im -10), and more preferably at a temperature 30°C or more higher than the midpoint glass transition temperature and 20°C or more lower than the extrapolated melting onset temperature (i.e., T mg +30≦Specific heat treatment temperature [℃]≦T im -20), and it is particularly preferred to carry out the process at a temperature at least 50°C higher than the midpoint glass transition temperature and at least 30°C lower than the extrapolated melting onset temperature (i.e., T mg +50≦Specific heat treatment temperature [℃]≦T im -30).

[0031] The midpoint glass transition temperature and extrapolated melting onset temperature of ETFE vary depending on the content of each unit contained in ETFE, but typically, the midpoint glass transition temperature of ETFE is 40 to 100°C, and the extrapolated melting onset temperature of ETFE is 160 to 250°C.

[0032] The time of the specific heat treatment (heat treatment time of particles A) is appropriately set depending on the heating temperature in the specific heat treatment, but is preferably 30 minutes to 10 hours, more preferably 40 minutes to 6 hours, and particularly preferably 60 minutes to 3 hours, from the viewpoints of more effectively removing low-boiling components from particles A, facilitating the pulverization process when pulverizing the obtained particles B, and suppressing the generation of fibrillated pulverized material.

[0033] The time for the specific heat treatment is preferably 30 minutes to 10 hours, more preferably 60 minutes to 5 hours, and even more preferably 60 minutes to 3 hours, if the heating temperature in the specific heat treatment is at least a temperature 40° C. lower than the extrapolated melting onset temperature. In the case of a direct heating method such as specific heat treatment hot air drying, if the heating temperature is equal to or higher than a temperature 60°C lower than the extrapolated melting onset temperature, the specific heat treatment time is preferably 30 minutes to 10 hours, more preferably 60 minutes to 8 hours, and even more preferably 60 minutes to 5 hours. Among these, the hot air drying method in which hot air passes between particles A can efficiently remove low-boiling components even if the heating temperature is low and the heat treatment time is short. In the case of an indirect heating method such as a conical dryer, the specific heat treatment time is preferably 1 to 20 hours, more preferably 5 to 10 hours, if the heating temperature is at least 60°C lower than the extrapolated melting onset temperature. In the case of an indirect heating method, even if some of the particles A are broken during the specific heat treatment, they are not blown away by the wind as in the case of the direct heating method, so almost all of the particles B can be recovered.

[0034] The specific heat treatment is preferably carried out so that the reduction rate of low boiling components before and after the specific heat treatment, expressed by the following formula, is 20 to 100%, more preferably 40 to 100%, and particularly preferably 50 to 100%, because this can further reduce the generation of white smoke due to evaporation of low boiling components during molding and can produce a molded product with few foam marks on the surface. Reduction rate of low boiling components before and after specific heat treatment [%] = 100 × (content of low boiling components contained in particle A - content of low boiling components contained in particle B) / (content of low boiling components contained in particle A)

[0035] The specific heat treatment can be carried out using a known heat treatment device. The specific heat treatment may be carried out while stirring the particles A, since this allows for more effective removal of low boiling components. The heat treatment device can be either a direct heating method or an indirect heating method. Direct heating methods include shelf ovens (hot air circulating incubators), solid-state polymerization devices such as heat treatment tanks, and fluidized bed dryers. Indirect heating methods include conical dryers such as vacuum tumble dryers, paddle dryers, steam tube dryers, Ribocones, and Nauta mixers. Direct and indirect heating methods can also be used in combination. In addition to the above-mentioned heating via hot air or a jacket, infrared heaters and the like can also be used.

[0036] [Particle B] Particles B are particles containing a specific fluororesin and are obtained by subjecting particles A to a specific heat treatment. Particles B may contain the above-mentioned low-boiling components, but the content of the low-boiling components is less than that of particles A because at least a portion of the low-boiling components has been removed from particles A by the specific heat treatment. When particles B contain a low boiling point component, the content of the low boiling point component in particles B is preferably 0 to 0.1 mass %, more preferably 0 to 0.05 mass %, particularly preferably 0 to 0.03 mass %, based on the total mass of particles B. The method for calculating the content of low boiling components that may be contained in particles B is the same as the method for calculating the content of low boiling components that may be contained in particles A, except that particles B are used.

[0037] The shape and average particle size of the particles B are not particularly limited and are the same as those of the particles A, so a description thereof will be omitted.

[0038] [Crushing process] The method for producing particles of the present invention may include a pulverization treatment for pulverizing particles B. This results in particles C having a smaller particle size than particles B. Since the particles B are particles obtained by the specific heat treatment described above, they can be easily pulverized by mechanical pulverization, and the generation of fibrillated pulverized material during pulverization can be suppressed. The pulverization is preferably carried out by shearing or cutting. A specific example of a method using shearing is a method in which pressure is applied to particles B to apply shear force to pulverize them. A specific example of a method using cutting is a method in which particles B are pulverized by cutting them with a cutting tool such as a knife. The pulverization can be carried out, for example, by mechanical pulverization using a pulverizer, etc. Examples of the pulverizer include a turbo mill, a cross bitter mill, a rotor bitter mill, and a cutting mill. The grinding may be carried out at room temperature or under heating.

[0039] [Particle C] Particles C are obtained by pulverizing particles B and contain a specific fluororesin. The average particle size of particles C is preferably 10 to 1000 μm, more preferably 20 to 500 μm, and particularly preferably 25 to 400 μm. When the average particle size of particles C is at least the lower limit of the above range, handling is excellent, and when it is at most the upper limit, surface smoothness after melting is excellent. The apparent density of particle C is 0.3 to 1.2 g / cm 3 is preferred, and 0.4 to 1.1 g / cm 3 More preferably, 0.5 to 1.0 g / cm 3When the apparent density of the particles C is equal to or greater than the lower limit of the above range, the particles have excellent handleability, and when it is equal to or less than the upper limit, the particles have excellent surface smoothness.

[0040] An aggregate composed of a large number of particles C is also called a powder, and is used, for example, as powder coating in the paint industry. The powder may contain a component other than the fluororesin contained in the particles C. Specific examples of the component other than the fluororesin include additives such as a flowability improver (e.g., silica, alumina), a reinforcing material (e.g., inorganic filler), a heat stabilizer (e.g., cuprous oxide, cupric oxide, cuprous iodide, cupric iodide), an ultraviolet absorber, and an acid acceptor (e.g., a composite metal oxide, a metal compound). The component other than the fluororesin is used, for example, by mixing it with the particles C.

[0041] [Method of manufacturing molded body] The method for producing a molded article of the present invention is a method for obtaining a molded article containing the specific fluororesin by melting particles (specifically, particles B or particles C) produced by the above-mentioned particle production method at a temperature equal to or higher than the melting point of the specific fluororesin. In the method for producing a molded body of the present invention, particles B obtained through the specific heat treatment described above or particles C obtained by pulverizing the particles B are used, so that the generation of white smoke during the production of a molded body can be suppressed. In addition, a molded body can be obtained in which foaming marks due to low boiling components and the like are suppressed.

[0042] The particles B or C can be melted using a known melt-kneading device, such as a twin-screw extruder. The twin-screw extruder preferably has two screws, a barrel incorporating the two screws, a vacuum vent provided in the barrel, a raw material supply port provided in the barrel, and a die provided at the downstream end of the barrel. The twin-screw extruder may be a co-rotating twin-screw extruder in which two screws inserted into a cylinder of a barrel formed with through-holes rotate in the same direction, or a counter-rotating extruder in which two screws rotate in opposite directions. As a twin-screw extruder, a co-rotating twin-screw extruder is preferred because it has excellent conveying capacity, melting / kneading capacity, and separation (dehydration) capacity, and is capable of continuous resin processing and is also excellent in terms of efficiency of the processing process.

[0043] The intermeshing of the two screws may be of a non-intermeshing type, a partial intermeshing type, or a complete intermeshing type. The screw may be a screw having a plurality of screw elements attached to a shaft. The screw elements preferably have the same cross-sectional shape in the direction perpendicular to the axis. The screw elements have specific functions depending on the number of flights, which indicates the number of flights, and the helix angle at which the cross-sectional shape in the direction perpendicular to the axis rotates around the shaft. Screw elements can be categorized by function as rotary elements, kneading elements, and mixing elements. The rotary element may be a screw element that has a helix angle that rotates continuously around a shaft and has a conveying capacity. Examples of the kneading element include a screw element composed of a plurality of plate-shaped disks with no twist angle. Examples of mixing elements include screw elements with notches formed in a full-flight element of a right-handed thread, and screw elements with notches formed in a full-flight element of a reverse-handed thread. The mixing elements may or may not have self-cleaning properties. As the screw of the twin-screw extruder, a screw composed of a rotary element, a kneading element and a mixing element is preferably used.

[0044] The barrel preferably has a structure in which a plurality of barrel blocks are connected in series. The barrel block preferably has a through-hole formed therein that corresponds to the cross-sectional shape of the screw.

[0045] The vacuum vent can further remove low boiling components that may be contained in the specific fluororesin when the particles B or C are melt-kneaded by the screws of the twin-screw extruder. A vacuum vent can be installed in a twin screw extruder, for example, by using a barrel block equipped with a vacuum vent. Vacuum vents may also be installed in multiple barrel blocks.

[0046] The die preferably has a structure that allows the specific fluororesin to be extruded to form strands. The number of discharge ports in the die may be one or more. The die preferably has several to several tens of discharge ports, as this allows a plurality of strands to be formed and improves productivity.

[0047] The melt-kneading temperature is preferably 10 to 150°C higher than the melting point of the specific fluororesin, more preferably 20 to 130°C higher, and particularly preferably 30 to 100°C higher. The melting point of the specific fluororesin in the present invention is the temperature (°C) corresponding to the maximum value of the melting peak of the specific fluororesin when the specific fluororesin is heated at a rate of 10°C / min using a differential scanning calorimeter (for example, an apparatus similar to DSC 7020 (manufactured by Seiko Instruments Inc.)).

[0048] The specific fluororesin is extruded from the die of the twin-screw extruder and molded into a desired shape. The shape of the molded article containing the specific fluororesin may be any shape such as pellets or film. When pellets are produced as the molded article, a method can be used in which the molten specific fluororesin is extruded from a die attached to the outlet of a twin-screw extruder to form strands, and then the strands are cut by a pelletizer to obtain pellets. When a film is produced as a molded article, the specific fluororesin in a molten state is extruded through a die attached to the outlet of a twin-screw extruder to form a film. The pellets obtained as described above may also be used to produce a film.

[0049] Although the method of obtaining a molded product by melting particles B or particles C in a twin-screw extruder has been described, a molded product may also be produced by the following method. For example, when a powder composed of a large number of particles C is used as a powder coating, the powder may be applied (e.g., electrostatically applied) to a substrate to form a coating layer composed of the powder, and then the coating layer may be melted by heating to form a coating film (one embodiment of a molded body) on the substrate. [Example]

[0050] The present invention will be described in detail below with reference to 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.

[0051] Percentage of units The proportion of each unit in the fluororesin was calculated from data measured by melt NMR analysis, fluorine content analysis, and infrared absorption spectroscopy.

[0052] [Midpoint glass temperature and extrapolated melting onset temperature] The midpoint glass temperature and extrapolated melting onset temperature of the fluororesin were determined based on a DSC curve measured using a differential scanning calorimeter (DSC 7020, manufactured by Seiko Instruments Inc.) in accordance with the method of JIS K7121 (1987).

[0053] [Melting point] Using a differential scanning calorimeter (DSC 7020, manufactured by Seiko Instruments Inc.), the melting peak was recorded when the temperature of the fluororesin was raised at a rate of 10°C / min, and the temperature (°C) corresponding to the maximum value of the melting peak of the fluororesin was determined, and this value was taken as the melting point of the fluororesin.

[0054] [Low boiling point component content] Using a differential thermal and thermogravimetric simultaneous analyzer (TG / DTA7200, Hitachi High-Tech Corporation), 60 mg of particles (particles containing fluororesin) were heated in dry air from 50°C at a rate of 10°C / min and then held at 230°C (the temperature of the fluororesin) for 30 minutes. The mass of the particles was calculated using the following formula based on the mass before and after the treatment. Content of low boiling components in particles [mass%] = 100 × (mass of particles before treatment - mass of particles after treatment) / mass of particles before treatment)

[0055] [Reduction rate of low boiling points before and after heat treatment] Reduction rate of low boiling components in particles before and after heat treatment [%] = 100 × (content of low boiling components in particles before heat treatment - content of low boiling components in particles after heat treatment) / content of low boiling components in particles before heat treatment

[0056] [Average particle size] The particles were dispersed in a 0.1% by mass aqueous solution of surfactant (Newcol 1308FA(90), manufactured by Nippon Nyukazai Co., Ltd.), and the volume-based median diameter was measured using a laser diffraction / scattering particle size distribution analyzer (LA-920, manufactured by Horiba, Ltd.).

[0057] [Apparent Density] The apparent density of the particles was measured in accordance with JIS K6819.

[0058] [Pile-up test] The crushed particles from the Examples and Comparative Examples were filled into a 2 cm x 5 cm rectangular mold placed on a glass slide so that the film thickness after baking would be approximately 200 μm. After gently removing the mold, the film was baked in an electric furnace at 300°C for 60 minutes. The foaming state of the baked coating was visually observed and evaluated according to the following criteria. The results are shown in Table 1. ⊚: No foaming marks were observed. ○: Almost no foaming marks were observed. ×: Bubbling marks were observed over the entire surface.

[0059] [Fibrillation test] After pulverization, each particle was visually observed and evaluated according to the following criteria. Yes: Fibrillated None: Not fibrillated

[0060] [Production of particles A1 containing fluororesin 1] A 430L polymerization vessel equipped with a stirrer was degassed. 418.2 kg of CF3(CF2)5H, 2.12 kg of PFBE, and 3.4 kg of methanol were placed in the vessel and heated to 66°C while stirring. A mixed gas of TFE / ethylene = 84 / 16 (molar ratio) was introduced into the vessel until the pressure inside the vessel reached 1.5 MPa [gage]. A solution containing 26 g of a 50 mass% tert-butyl peroxypivalate CF3(CF2)5H solution and 4974 g of CF3(CF2)5H was injected into the vessel to initiate polymerization. During polymerization, a mixed gas of TFE / ethylene = 54 / 46 (molar ratio) and PFBE in an amount equivalent to 1.4 mol% relative to 100 mol% of the mixed gas were continuously introduced so that the pressure inside the vessel reached 1.5 MPa [gage]. After charging 34 kg of TFE / ethylene mixed gas, the polymerization vessel was cooled, and the remaining gas was purged to terminate the polymerization. In this way, a slurry containing fluororesin 1 was obtained. Note that fluororesin 1 was a melt-moldable resin.

[0061] The slurry in the polymerization tank was transferred to an 850 L granulation tank, 340 L of water was added, and the mixture was heated to 105°C while stirring to remove the solvent and unreacted monomer, yielding a granule. The granule was dried at 150°C for 5 hours to yield 34 kg of particles A1 (average particle size 1.6 mm) containing fluororesin 1. The molar ratio of ethylene-based units to TFE-based units (E units / TFE units) contained in Fluororesin 1 was 45.0 / 55.0 (molar ratio), and the content of PFBE-based units was 1.7 mol% relative to the total units of the polymer constituting the fluororesin. Fluororesin 1 had a midpoint glass temperature of 90°C, an extrapolated melting onset temperature of 240°C, and a melting point of 260°C.

[0062] [Example 1] 25 kg of particles A1 containing fluororesin 1 were stored in a heat treatment tank (hot air method, manufactured by Osaka Reiken Co., Ltd.) and heat-treated at 150°C for 1 hour to obtain particles B1 containing fluororesin 1. The obtained particles B1 were pulverized using a turbo mill pulverizer (manufactured by Turbo Kogyo Co., Ltd.) to obtain particles C1 containing fluororesin 1.

[0063] [Examples 2 to 6, Comparative Examples 1 and 2] Heat-treated particles B2 to B8 and pulverized particles C2 to C8 were obtained in the same manner as in Example 1, except that the heat treatment conditions (heat treatment temperature, heat treatment time) of particles A1 were changed as shown in Table 1.

[0064] [Table 1]

[0065] [Examples 7 to 14] 20 kg of particles A1 containing fluororesin 1 were placed in a vacuum tumble dryer (50 L) manufactured by Nikku Kogyo Co., Ltd., and heat-treated at a rotation speed of 10 rpm and a vacuum degree of 5 torr at the temperature and time shown in Table 2. After heat treatment, the particles were pulverized in the same manner as in Example 1.

[0066] [Table 2]

[0067] [Examples 15 to 17] 15 kg of particles A1 containing fluororesin 1 were heat-treated using a batch-type fluidized bed dryer (B-FBD) manufactured by Nara Machinery Works, Ltd. at a heat treatment temperature of 200°C for the time shown in Table 3. After the heat treatment, the particles were pulverized in the same manner as in Example 1.

[0068] [Table 3]

[0069] [Examples 18 to 21] 15 kg of particles A1 containing fluororesin 1 were heat-treated using a Kato small hot air circulation incubator TRO-52DPA at the hot air temperature and for the time shown in Table 4. After the heat treatment, the particles were pulverized in the same manner as in Example 1.

[0070] [Table 4]

[0071] [Example 22] Lining tests were carried out on particles C1 to C6 and C9 to C21 under the conditions below, and it was confirmed that the white smoke derived from low boiling components was significantly reduced compared to particle C8. Lining test: The rock and roll method was used. The particles were placed in a steel pipe with an outer diameter of 20 mm and lined with direct flame heating. After a certain period of time had passed, the flange was partially opened and white smoke from low boiling components was visually observed.

[0072] As shown in Tables 1 to 4, it was confirmed that when particles containing melt-moldable fluororesin were heat-treated at a temperature equal to or higher than the midpoint glass transition temperature and equal to or lower than the extrapolated melting onset temperature, excellent removal of low-boiling components was achieved (Examples 1 to 21). In contrast, when particles containing melt-moldable fluororesin were heat-treated at a temperature exceeding the extrapolated melting onset temperature, the particles fused together and could not be crushed, making it impossible to carry out various evaluations (Comparative Example 1). Furthermore, it was confirmed that when particles containing a melt-moldable fluororesin were heat-treated at a temperature below the midpoint glass transition temperature, the removal of low boiling components was poor (Comparative Example 2).

[0073] The entire contents of the specifications, claims and abstracts of Japanese Patent Application No. 2018-245705 filed on December 27, 2018 and Japanese Patent Application No. 2019-168193 filed on September 17, 2019 are hereby incorporated by reference as the disclosure of the specification of the present invention.

Claims

1. A method for producing particles comprising a fluororesin, wherein a particle A is heat-treated at a temperature equal to or higher than the midpoint glass transition temperature and equal to or lower than the extrapolated melting onset temperature to obtain a particle B comprising said fluororesin, The fluororesin is melt-moldable, the fluororesin is any one of a copolymer containing units based on tetrafluoroethylene and units based on ethylene, a copolymer containing units based on chlorotrifluoroethylene and units based on ethylene, a copolymer containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), and a copolymer containing units based on tetrafluoroethylene and units based on hexafluoropropylene, the particles A are particles obtained by drying a slurry containing the fluororesin to obtain a granulated product containing the fluororesin, and then subjecting the granulated product to a treatment involving heating, The method for producing particles, wherein the particles A contain a low-boiling component, and the content of the low-boiling component is more than 0 mass % and 0.5 mass % or less with respect to the total mass of the particles A.

2. 2. The method for producing particles according to claim 1, wherein the temperature of the heat treatment is at least 10°C higher than the midpoint glass transition temperature and at least 10°C lower than the extrapolated melting onset temperature.

3. 3. The method for producing particles according to claim 1, wherein the heat treatment is carried out for 30 minutes to 10 hours.

4. The method for producing particles according to any one of claims 1 to 3, wherein the fluororesin is obtained by polymerizing a fluorine-containing monomer and a non-fluorine-containing monomer in the presence of a hydrofluorocarbon or a hydrofluoroether.

5. the fluorine-containing monomer is at least one selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, perfluoro(alkyl vinyl ether), chlorotrifluoroethylene, vinylidene fluoride, and vinyl fluoride; 5. The method for producing particles according to claim 4, wherein the fluorine-free monomer is at least one selected from the group consisting of ethylene, propylene, itaconic anhydride, and vinyl acetate.

6. The method for producing particles according to any one of claims 1 to 5, wherein the fluororesin is a copolymer containing units based on tetrafluoroethylene and units based on ethylene, and the copolymer further contains units based on a monomer F1 represented by the following formula (F1): CH ２ =CX(CF ２ ) ｎ Y (F1) (X and Y are each independently a hydrogen atom or a fluorine atom, and n is an integer of 2 to 8.)

7. The method for producing particles according to any one of claims 1 to 6, wherein the particles A have an average particle size of 100 µm to 10.0 mm.

8. The method for producing particles according to any one of claims 1 to 7, wherein a reduction rate of the low boiling components before and after the heat treatment calculated by the following formula using the contents of the low boiling components contained in the particles A and the particles B is 20 to 100%. Reduction rate of low boiling components before and after heat treatment [%]=100×(content of low boiling components contained in particles A−content of low boiling components contained in particles B) / (content of low boiling components contained in particles A)

9. The method for producing particles according to any one of claims 1 to 8, wherein particles C are obtained by pulverizing the particles B.

10. The method for producing particles according to claim 9, wherein the particles C obtained by pulverizing the particles B have an average particle size of 10 to 1000 μm.

11. The apparent density of particles C obtained by pulverizing the particles B is 0.3 to 1.2 g / cm ３ The method for producing particles according to claim 9 or 10,

12. A method for producing a molded article, comprising melting particles produced by the method according to any one of claims 1 to 11 at a temperature equal to or higher than the melting point of the fluororesin to obtain a molded article containing the fluororesin.

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