Method for producing particles and method for producing molded product

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 a fluororesin to the surface of a substrate by a method such as electrostatic coating, fluidized bed dipping, or rotational molding, a coating film that protects the substrate surface and has excellent chemical resistance can be formed. In recent years, there has been a demand for fluororesins with a low content of low boiling components. If particles containing fluororesins 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 demand for a method for producing particles containing fluororesin that is excellent in removing low boiling components and 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 inventors have found that if particle A containing a melt-formable fluororesin 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, it is possible to achieve excellent removal of low-boiling components and suppress fibrillation even when a crushing process is performed, and have arrived at the present invention.

[0007] That is, the inventors discovered that the above problems can be solved by the following configuration. [1] A method for producing particles, comprising heat-treating a particle A containing a melt-formable 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 a particle B containing the fluororesin. [2] The method for producing particles according to [1], wherein the heat treatment temperature 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 time is 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-fluorinated 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 is an integer of 2 to 8.) [8] The method for producing particles according to any one of [1] to [7], wherein the average particle size of the particles A is 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 a low boiling point component, and the content of the low boiling point component 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 a reduction rate of the amount of low boiling components before and after the heat treatment calculated using the contents of low boiling components contained in the particles A and the particles B is 20 to 100% by the following formula: 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 particles C obtained by pulverizing the particles B have an average particle size of 10 to 1000 μm.

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

[11] or

[12] ,

[14] A method for producing a molded product, comprising 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 product containing the fluororesin. Effect 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, which are excellent in terms of the removal efficiency of low boiling components and can suppress fibrillation even when a crushing treatment is performed. 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 densifies the fluororesin particles, thereby suppressing the formation of fissile particles. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The terms used in the present invention have the following meanings. A numerical range expressed using "~" means that the range includes the numerical values ​​before and after "~" as the lower and upper limits. "Melt moldable" means that it 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 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). Both the midpoint glass temperature and the extrapolated melting start temperature are values ​​(°C) calculated in accordance with the method of JIS K7121 (1987) and are obtained based on a DSC curve obtained by a differential scanning calorimetry (DSC) method. mg " and the extrapolated melting temperature is "T im "It is also called. The term "unit" refers collectively to an atomic group derived from 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 part of the above-mentioned atomic group. In the following, in some cases, a unit derived from an individual monomer will be referred to by the name of the monomer with "unit" added. 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-moldable fluororesin-containing particles A at a temperature of 1000° C. to 1000° C. mg ) and the extrapolated melting onset temperature (T im ) or lower (hereinafter also referred to as "specific heat treatment") to obtain 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 when particle A is heat-treated at the above temperature, the molecular chain motion of the fluororesin is facilitated, and low boiling components can be sufficiently removed from particle A. im It is presumed that heat treatment of particle A at the following temperatures can suppress fusion between particles caused by partial melting of the fluororesin, thereby enabling the low boiling components to be sufficiently removed from the particles. Also, T im Heat treatment of particles A at the following temperatures has the advantages of facilitating the pulverization process 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. From the viewpoint of excellent moldability, however, it is preferable that the specific fluororesin has units based on a fluorine-containing monomer and units based on a non-fluorine 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. The fluorine-containing monomer may be used alone or in combination of two or more kinds. 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 from the viewpoint of excellent moldability, and ETFE is particularly preferred.

[0014] In ETFE, the ratio of the content of E units to the total content of E units and TFE units is preferably from 20 to 70 mol %, more preferably from 25 to 60 mol %, particularly preferably from 35 to 55 mol %. When the content of the E units is at least the lower limit of the above range, the mechanical strength is excellent, and when it is no more than 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 monomer units other than these. 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 is an integer of 2 to 8 (preferably 3 to 7, more preferably 4 to 6). A specific example of the monomer F1 is 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 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 in which the total of E units and TFE units is 100 mol. When the content of the other monomer units is equal to or more than the lower limit of the above range, the crack resistance is good, and when it is equal to or less than 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 of only TFE units and perfluoroalkyl vinyl ether units, and may contain one or more other monomer units other than these. 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 a molar ratio in which the total of TFE units and perfluoroalkyl vinyl ether units is 100 mol.

[0019] In FEP, the ratio of the content of TFE units 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 monomer units other than these. A preferred example of the other monomer is a perfluoroalkyl vinyl ether. 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 based on 100 mol of the total of TFE units and hexafluoropropylene units.

[0020] In ECTFE, the ratio of the content of E units 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 besides these. A preferred example of the other monomer is a perfluoroalkyl vinyl ether. 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 based on 100 mol of the total of E units and chlorotrifluoroethylene units.

[0021] The particles A may contain a low boiling point component. Specific examples of the low boiling point component include, but are not limited to, an unreacted monomer among the monomers used for polymerization of the specific fluororesin, a low molecular weight fluorine-containing polymer (oligomer, etc.) generated during polymerization of the specific fluororesin, and a polymerization solvent used for polymerization of the specific fluororesin. Low boiling components that may be contained in the particles A are at least partially removed by the particle production method of the present invention. When particle A contains a low boiling component, the content of the low boiling component is preferably more than 0 mass% and not more than 0.5 mass%, more preferably more than 0 mass% and not more than 0.3 mass%, and particularly preferably more than 0 mass% and not more than 0.2 mass%, relative to the total mass of particle A. The content of low boiling components is calculated using the following formula based on the masses of particles A before and after treatment by heating 60 mg of particles A in dry air at a rate of 10°C / min from 50°C using a simultaneous differential thermal and thermogravimetric analyzer (e.g., an apparatus similar to TG / DTA7200 (manufactured by Hitachi High-Technologies Corporation)) and holding the temperature at a temperature 30°C lower than the melting point of the specific fluororesin for 30 minutes. Content of low boiling components 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) or a columnar shape (for example, a cylindrical shape). 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 equal to or greater than the lower limit of the above range, handling properties are excellent, and when it is equal to or less than the upper limit, grindability is excellent.

[0023] <Method of manufacturing specific fluororesin> Examples of the method 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 is a method for polymerizing units based on a fluorine-containing monomer and units based on a non-fluorine monomer in the presence of a hydrofluorocarbon or a hydrofluoroether. The method for producing the specific fluororesin will be described below based on this preferred embodiment. The fluorine-containing monomer, the non-fluorine-containing monomer and the 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 fluorine-containing 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 the 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 of such components include a polymerization solvent (e.g., an organic solvent other than hydrofluorocarbons and hydrofluoroethers, and water), a chain transfer agent (a chain transfer agent other than hydrofluorocarbons and hydrofluoroethers, such as an alcohol or a hydrocarbon), a polymerization initiator, and a polymerization inhibitor. 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 above-mentioned polymerization solvent and in a suspended state. The content of the specific fluororesin is preferably from 1 to 40% by mass, particularly preferably from 3 to 35% by mass, based on the total mass of the slurry, from the viewpoint of excellent granulation properties. The content of the polymerization solvent is preferably from 60 to 99 mass %, particularly preferably from 65 to 97 mass %, based on the total mass of the slurry, from the viewpoint of excellent granulation properties.

[0027] The slurry is preferably used after mixing with water because it has excellent granulation properties. When water and the slurry are used in combination, 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, from the viewpoint of excellent granulation properties.

[0028] The granulation process may be a known granulation process. A specific example of the granulation process may be a process in which the 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, it is preferably 10 to 130°C, and particularly preferably 20 to 110°C. 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 the particles A, it is preferable that the particles A is not heated at a temperature exceeding the extrapolated melting onset temperature of the specific fluororesin contained in the particles A during the specific heat treatment to obtain the particles B described later. This can prevent a part of the particles A from melting, and thus can prevent the decrease in crushability caused by the melted part, so that the particle size after crushing tends to be uniform. Specifically, it is more preferable that the particles A are not melt-kneaded. This can prevent the generation of fibril-like crushed material when the particles B described later are crushed.

[0030] [Specific heat treatment] The specific heat treatment is performed at the midpoint glass transition temperature (T mg ) and the extrapolated melting onset temperature (T im ) or lower. This 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. In addition, when the particles B are crushed, the crushing process is facilitated, and the generation of fibrillated crushed material can be suppressed. 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≦the heating temperature of the specific heat treatment [° C.]), and more preferably, the specific heat treatment is performed 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≦heating temperature for specific heat treatment [℃]). 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< Heating temperature for specific heat treatment [℃]≦T im It refers to a temperature in the range. In addition, the specific heat treatment is preferably performed 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, the specific heat treatment 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 of the specific heat treatment are also included. For example, the specific heat treatment is preferably carried out at a temperature 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≦Temperature of specific heat treatment [℃]≦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≦Temperature of specific heat treatment [℃]≦T im -20), and it is particularly preferred to carry out the process at a temperature 50°C or more higher than the midpoint glass transition temperature and 30°C or more lower than the extrapolated melting onset temperature (i.e., T mg +50≦Temperature of specific heat treatment [℃]≦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 starting 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 starting 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 them, 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, if the heating temperature is equal to or higher than a temperature 60°C lower than the extrapolated melting starting temperature, the specific heat treatment time is preferably 1 to 20 hours, more preferably 5 to 10 hours. In the case of an indirect heating method, even if part of particle A is broken during the specific heat treatment, particle B can be almost entirely recovered because it is not blown away by wind as in the case of a direct heating method.

[0034] The specific heat treatment is preferably carried out so that the reduction rate of the low boiling components before and after the specific heat treatment, represented by the following formula, is 20 to 100%, more preferably 40 to 100%, and particularly preferably 50 to 100%, in order to further reduce the generation of white smoke due to evaporation of low boiling components during molding and to obtain a molded product with few foaming 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 low boiling components can be removed more effectively. As the heat treatment device, either a direct heating method or an indirect heating method can be used. As the direct heating method, a shelf oven (hot air circulation thermostat), a solid-phase polymerization device such as a heat treatment tank, or a fluidized bed dryer can be used. As the indirect heating method, a conical dryer such as a vacuum tumble dryer, a paddle dryer, a steam tube dryer, a Ribocone, a Nauta mixer, etc. can be used. The direct heating method and the indirect heating method can also be used in combination. In addition to the above-mentioned heating via hot air or a jacket, an infrared heater, etc. 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. However, the content of the low boiling components is less than that of the particles A because at least a part of the low boiling components has been removed from particles A by the specific heat treatment. When particles B contain a low boiling component, the content of the low boiling 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 the low boiling components that may be contained in the particles B is the same as the method for calculating the content of the low boiling components that may be contained in the particles A, except that the particles B are used.

[0037] The shape and average particle size of the particles B are not particularly limited and are similar to those of the particles A, so the 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 a mechanical pulverization treatment, and the generation of fibrillated pulverized material during pulverization can be suppressed. The crushing 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 particle B to crush the particle B by applying a shear force. A specific example of a method using cutting is a method in which particle B is crushed by cutting the particle B 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 with heating.

[0039] [Particle C] Particles C are obtained by subjecting particles B to a pulverization treatment, and contain a specific fluororesin. The average particle size of the 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 the particles C is at least the lower limit of the above range, the particles C have excellent handleability, and when the average particle size is at most the upper limit, the particles C have excellent surface smoothness after melting. The apparent density of particle C is 0.3 to 1.2 g / cm 3 is preferably 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 more 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 a 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 flowability improvers (e.g., silica, alumina), reinforcing materials (e.g., inorganic fillers), heat stabilizers (e.g., cuprous oxide, cupric oxide, cuprous iodide, cupric iodide), ultraviolet absorbers, and acid acceptors (e.g., composite metal oxides, metal compounds). The component other than the fluororesin is used, for example, by mixing 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 product of the present invention, the particles B obtained through the specific heat treatment described above or the particles C obtained by pulverizing the particles B are used, so that the generation of white smoke during the production of the molded product can be suppressed. Also, the molded product can be obtained with suppressed foaming marks due to low boiling components.

[0042] The melting of the particles B or C can be carried out 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 having through holes rotate in the same direction, or a counter-rotating extruder in which two screws rotate in opposite directions. As the twin-screw extruder, a co-rotating twin-screw extruder is preferred because it has excellent conveying capacity, melting / kneading capacity, and separation (dehydration) capacity, is capable of continuous resin processing, and is also excellent in the efficiency of the processing process.

[0043] The meshing 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. In the screw elements, a specific function is generated according to the number of flights, which means the number of flights, and the twist angle at which the cross-sectional shape in the direction perpendicular to the axis rotates around the shaft. The screw elements include rotary elements, kneading elements, and mixing elements according to their functions. The rotary element may be a screw element having a helix angle that rotates continuously about a shaft and has a conveying capacity. The kneading element may be a screw element composed of a plurality of plate-shaped disks having no twist angle. The mixing element may be a screw element having a notch formed in a full-flight element of a positive thread, or a screw element having a notch formed in a full-flight element of a reverse thread. The mixing element may or may not have a self-cleaning property. 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 provided in the twin screw extruder, for example, by using a barrel block equipped with a vacuum vent. Vacuum vents may also be provided in multiple barrel blocks.

[0046] The die preferably has a structure capable of extruding the specific fluororesin to form strands. The number of the discharge opening in the die may be one or more. The die preferably has several to several tens of discharge openings, from the viewpoint of forming a plurality of strands and improving productivity.

[0047] The melt kneading temperature is preferably from 10 to 150° C. higher than the melting point of the specific fluororesin, more preferably from 20 to 130° C. higher, and particularly preferably from 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 (e.g., 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 product containing the specific fluororesin may be any shape such as pellets or a film. When pellets are produced as the molded product, a method can be used in which the molten specific fluororesin is extruded from a die attached to the discharge port of a twin-screw extruder to form strands, and then the strands are cut by a pelletizer to obtain pellets. In addition, when a film is produced as a molded product, a method in which the specific fluororesin in a molten state is extruded from a die attached to the discharge port of a twin-screw extruder and molded into a film can be mentioned. The pellets obtained as described above may be used to produce a film.

[0049] Although the method of obtaining a molded article by melting particles B or particles C in a twin-screw extruder has been described above, a molded article 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 material, the powder may be applied (e.g., electrostatically applied) onto a substrate to form a coating layer composed of the powder, and the coating layer may then be melted by heating to form a coating film (one embodiment of a molded body) on the substrate. EXAMPLES

[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] [Unit Percentage] 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] [Mid-point 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. This value was regarded as the melting point of the fluororesin.

[0054] [Low boiling point content] Using a differential thermal / thermogravimetric simultaneous analyzer (TG / DTA7200, Hitachi High-Technologies 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 the fluororesin's temperature of 230°C for 30 minutes, and the mass was calculated using the following formula based on the mass of the particles 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 in low boiling components before and after heat treatment] Reduction rate of low boiling components in particles before and after heat treatment [%] = 100 x (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 a 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 of the Examples and Comparative Examples were filled into a rectangular mold of 2 cm x 5 cm placed on a slide glass so that the film thickness after baking would be about 200 μm. After gently removing the mold, the film was baked in an electric furnace at 300 ° C for 60 minutes, and the foaming state of the baked coating film 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] The inside of a polymerization tank with a stirrer and an internal volume of 430 L was degassed. 418.2 kg of CF3(CF2)5H, 2.12 kg of PFBE, and 3.4 kg of methanol were placed in the polymerization tank, and the temperature was raised to 66°C while stirring. A mixed gas of TFE / ethylene = 84 / 16 (molar ratio) was introduced into the polymerization tank until the pressure in the polymerization tank reached 1.5 MPa [gage]. A solution of 26 g of a 50 mass% tert-butyl peroxypivalate CF3(CF2)5H solution and 4974 g of CF3(CF2)5H was injected into the polymerization tank to start 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 in the polymerization tank reached 1.5 MPa [gage]. After 34 kg of TFE / ethylene mixed gas was charged, the polymerization vessel was cooled, and the residual gas was purged to terminate the polymerization. In this manner, 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 monomers, obtaining a granule. The granule was dried at 150°C for 5 hours to obtain 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 type, 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 to 2] Particles B2 to B8 after heat treatment and particles C2 to C8 after pulverization were obtained in the same manner as in Example 1, except that the heat treatment conditions (heat treatment temperature, heat treatment time) of particle 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 Nikka 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 was 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 small hot air circulation type incubator TRO-52DPA manufactured by Kato at the hot air temperature and time shown in Table 4. After 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 under the following conditions for particles C1 to C6 and C9 to C21, 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. Particles were placed in a steel pipe with an outer diameter of 20 mm, and lining was performed by direct flame heating. After a certain period of time had passed, a part of the flange was 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 a melt-moldable 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 have excellent removal properties of low boiling components (Examples 1 to 21). In contrast, when particles containing a melt-moldable fluororesin were heat-treated at a temperature exceeding the extrapolated melting onset temperature, the particles fused together and could not be crushed, and various evaluations could not be carried out (Comparative Example 1). Furthermore, it was confirmed that when particles containing a melt-moldable fluororesin were heat-treated at a temperature lower than 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: heat-treating a particle A containing a 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 a particle B containing 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 material containing the fluororesin, and then subjecting the granulated material to a treatment involving heating, The method for producing particles, wherein the particles A contain a low boiling point component, and the content of the low boiling point 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. The method for producing particles according to claim 1 or 2, wherein the heat treatment time is from 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; The method for producing particles according to claim 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 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 from 2 to 8.)

7. The method for producing particles according to any one of claims 1 to 6, wherein the average particle diameter of the particles A is 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 particle A−content of low boiling components contained in particle B) / (content of low boiling components contained in particle A)

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

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 the 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 product, comprising melting particles produced by the method according to any one of claims 1 to 11 at a melting point of the fluororesin or higher to obtain a molded product containing the fluororesin.